WO1998031197A1 - Improved coatings for electrical, metal sheathed heating elements - Google Patents
Improved coatings for electrical, metal sheathed heating elements Download PDFInfo
- Publication number
- WO1998031197A1 WO1998031197A1 PCT/US1998/000438 US9800438W WO9831197A1 WO 1998031197 A1 WO1998031197 A1 WO 1998031197A1 US 9800438 W US9800438 W US 9800438W WO 9831197 A1 WO9831197 A1 WO 9831197A1
- Authority
- WO
- WIPO (PCT)
- Prior art keywords
- coating
- tantalum
- heating element
- refractory metal
- sheath
- Prior art date
Links
- 238000000576 coating method Methods 0.000 title claims abstract description 116
- 238000010438 heat treatment Methods 0.000 title claims abstract description 106
- 229910052751 metal Inorganic materials 0.000 title claims description 28
- 239000002184 metal Substances 0.000 title claims description 28
- 239000003870 refractory metal Substances 0.000 claims abstract description 32
- 239000000919 ceramic Substances 0.000 claims abstract description 24
- 229910000765 intermetallic Inorganic materials 0.000 claims abstract description 16
- 230000007797 corrosion Effects 0.000 claims abstract description 15
- 238000005260 corrosion Methods 0.000 claims abstract description 15
- 239000000203 mixture Substances 0.000 claims abstract description 15
- 229910044991 metal oxide Inorganic materials 0.000 claims abstract description 13
- 150000004706 metal oxides Chemical class 0.000 claims abstract description 13
- 239000011248 coating agent Substances 0.000 claims description 85
- GUVRBAGPIYLISA-UHFFFAOYSA-N tantalum atom Chemical compound [Ta] GUVRBAGPIYLISA-UHFFFAOYSA-N 0.000 claims description 42
- 229910052715 tantalum Inorganic materials 0.000 claims description 40
- 238000009792 diffusion process Methods 0.000 claims description 15
- HBMJWWWQQXIZIP-UHFFFAOYSA-N silicon carbide Chemical compound [Si+]#[C-] HBMJWWWQQXIZIP-UHFFFAOYSA-N 0.000 claims description 14
- 229910010271 silicon carbide Inorganic materials 0.000 claims description 12
- PXHVJJICTQNCMI-UHFFFAOYSA-N Nickel Chemical compound [Ni] PXHVJJICTQNCMI-UHFFFAOYSA-N 0.000 claims description 10
- 238000005498 polishing Methods 0.000 claims description 10
- 238000009835 boiling Methods 0.000 claims description 8
- 230000004580 weight loss Effects 0.000 claims description 7
- 238000007654 immersion Methods 0.000 claims description 6
- 238000004544 sputter deposition Methods 0.000 claims description 6
- 229910052759 nickel Inorganic materials 0.000 claims description 5
- 206010010144 Completed suicide Diseases 0.000 claims description 4
- 229910003468 tantalcarbide Inorganic materials 0.000 claims description 4
- WFKWXMTUELFFGS-UHFFFAOYSA-N tungsten Chemical compound [W] WFKWXMTUELFFGS-UHFFFAOYSA-N 0.000 claims description 4
- 229910052721 tungsten Inorganic materials 0.000 claims description 4
- 239000010937 tungsten Substances 0.000 claims description 4
- 239000003929 acidic solution Substances 0.000 claims description 3
- 229910052735 hafnium Inorganic materials 0.000 claims description 3
- 238000009413 insulation Methods 0.000 claims description 3
- 229910052702 rhenium Inorganic materials 0.000 claims description 3
- WUAPFZMCVAUBPE-UHFFFAOYSA-N rhenium atom Chemical compound [Re] WUAPFZMCVAUBPE-UHFFFAOYSA-N 0.000 claims description 3
- 239000000758 substrate Substances 0.000 claims description 3
- VYZAMTAEIAYCRO-UHFFFAOYSA-N Chromium Chemical compound [Cr] VYZAMTAEIAYCRO-UHFFFAOYSA-N 0.000 claims description 2
- 238000009713 electroplating Methods 0.000 claims description 2
- VBJZVLUMGGDVMO-UHFFFAOYSA-N hafnium atom Chemical compound [Hf] VBJZVLUMGGDVMO-UHFFFAOYSA-N 0.000 claims description 2
- 238000010884 ion-beam technique Methods 0.000 claims description 2
- 229910052758 niobium Inorganic materials 0.000 claims description 2
- 239000010955 niobium Substances 0.000 claims description 2
- GUCVJGMIXFAOAE-UHFFFAOYSA-N niobium atom Chemical compound [Nb] GUCVJGMIXFAOAE-UHFFFAOYSA-N 0.000 claims description 2
- 238000007740 vapor deposition Methods 0.000 claims description 2
- 229910018487 Ni—Cr Inorganic materials 0.000 claims 1
- 238000005524 ceramic coating Methods 0.000 claims 1
- 229910052804 chromium Inorganic materials 0.000 claims 1
- 239000011651 chromium Substances 0.000 claims 1
- VNNRSPGTAMTISX-UHFFFAOYSA-N chromium nickel Chemical compound [Cr].[Ni] VNNRSPGTAMTISX-UHFFFAOYSA-N 0.000 claims 1
- 238000000034 method Methods 0.000 description 27
- 239000007789 gas Substances 0.000 description 23
- 239000010410 layer Substances 0.000 description 19
- XKRFYHLGVUSROY-UHFFFAOYSA-N Argon Chemical compound [Ar] XKRFYHLGVUSROY-UHFFFAOYSA-N 0.000 description 18
- 229910000831 Steel Inorganic materials 0.000 description 18
- 239000010959 steel Substances 0.000 description 18
- 238000000151 deposition Methods 0.000 description 16
- 230000008569 process Effects 0.000 description 16
- 230000008021 deposition Effects 0.000 description 15
- 239000000463 material Substances 0.000 description 14
- 238000007747 plating Methods 0.000 description 13
- 239000000243 solution Substances 0.000 description 11
- 229910052786 argon Inorganic materials 0.000 description 10
- 239000012298 atmosphere Substances 0.000 description 9
- 150000001875 compounds Chemical class 0.000 description 8
- 238000002360 preparation method Methods 0.000 description 8
- 239000000126 substance Substances 0.000 description 8
- 238000005229 chemical vapour deposition Methods 0.000 description 7
- 230000007547 defect Effects 0.000 description 7
- 239000007788 liquid Substances 0.000 description 7
- 239000002253 acid Substances 0.000 description 5
- 238000006243 chemical reaction Methods 0.000 description 5
- 239000002243 precursor Substances 0.000 description 5
- 239000007787 solid Substances 0.000 description 5
- 238000004140 cleaning Methods 0.000 description 4
- -1 for example Chemical compound 0.000 description 4
- 150000004820 halides Chemical class 0.000 description 4
- 239000008188 pellet Substances 0.000 description 4
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 4
- VYPSYNLAJGMNEJ-UHFFFAOYSA-N Silicium dioxide Chemical compound O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 description 3
- 230000002378 acidificating effect Effects 0.000 description 3
- 150000007513 acids Chemical class 0.000 description 3
- 230000008901 benefit Effects 0.000 description 3
- 239000012159 carrier gas Substances 0.000 description 3
- 230000015556 catabolic process Effects 0.000 description 3
- 230000006378 damage Effects 0.000 description 3
- 230000007812 deficiency Effects 0.000 description 3
- 238000006731 degradation reaction Methods 0.000 description 3
- 238000005530 etching Methods 0.000 description 3
- 239000001257 hydrogen Substances 0.000 description 3
- 229910052739 hydrogen Inorganic materials 0.000 description 3
- 150000002739 metals Chemical class 0.000 description 3
- 238000005240 physical vapour deposition Methods 0.000 description 3
- 230000008439 repair process Effects 0.000 description 3
- 239000000523 sample Substances 0.000 description 3
- 238000000859 sublimation Methods 0.000 description 3
- 230000008022 sublimation Effects 0.000 description 3
- PBCFLUZVCVVTBY-UHFFFAOYSA-N tantalum pentoxide Inorganic materials O=[Ta](=O)O[Ta](=O)=O PBCFLUZVCVVTBY-UHFFFAOYSA-N 0.000 description 3
- OEIMLTQPLAGXMX-UHFFFAOYSA-I tantalum(v) chloride Chemical compound Cl[Ta](Cl)(Cl)(Cl)Cl OEIMLTQPLAGXMX-UHFFFAOYSA-I 0.000 description 3
- 238000005979 thermal decomposition reaction Methods 0.000 description 3
- QGZKDVFQNNGYKY-UHFFFAOYSA-N Ammonia Chemical compound N QGZKDVFQNNGYKY-UHFFFAOYSA-N 0.000 description 2
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 description 2
- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 description 2
- GRYLNZFGIOXLOG-UHFFFAOYSA-N Nitric acid Chemical compound O[N+]([O-])=O GRYLNZFGIOXLOG-UHFFFAOYSA-N 0.000 description 2
- NBIIXXVUZAFLBC-UHFFFAOYSA-N Phosphoric acid Chemical compound OP(O)(O)=O NBIIXXVUZAFLBC-UHFFFAOYSA-N 0.000 description 2
- 229910052581 Si3N4 Inorganic materials 0.000 description 2
- QAOWNCQODCNURD-UHFFFAOYSA-N Sulfuric acid Chemical compound OS(O)(=O)=O QAOWNCQODCNURD-UHFFFAOYSA-N 0.000 description 2
- 229910026551 ZrC Inorganic materials 0.000 description 2
- OTCHGXYCWNXDOA-UHFFFAOYSA-N [C].[Zr] Chemical compound [C].[Zr] OTCHGXYCWNXDOA-UHFFFAOYSA-N 0.000 description 2
- 125000004429 atom Chemical group 0.000 description 2
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 2
- 230000015572 biosynthetic process Effects 0.000 description 2
- 230000008859 change Effects 0.000 description 2
- PMHQVHHXPFUNSP-UHFFFAOYSA-M copper(1+);methylsulfanylmethane;bromide Chemical compound Br[Cu].CSC PMHQVHHXPFUNSP-UHFFFAOYSA-M 0.000 description 2
- 238000005238 degreasing Methods 0.000 description 2
- 239000008367 deionised water Substances 0.000 description 2
- 229910021641 deionized water Inorganic materials 0.000 description 2
- 229910003460 diamond Inorganic materials 0.000 description 2
- 239000010432 diamond Substances 0.000 description 2
- 238000011010 flushing procedure Methods 0.000 description 2
- 239000007792 gaseous phase Substances 0.000 description 2
- WHJFNYXPKGDKBB-UHFFFAOYSA-N hafnium;methane Chemical compound C.[Hf] WHJFNYXPKGDKBB-UHFFFAOYSA-N 0.000 description 2
- 239000001307 helium Substances 0.000 description 2
- 229910052734 helium Inorganic materials 0.000 description 2
- SWQJXJOGLNCZEY-UHFFFAOYSA-N helium atom Chemical compound [He] SWQJXJOGLNCZEY-UHFFFAOYSA-N 0.000 description 2
- 150000002500 ions Chemical class 0.000 description 2
- 150000001247 metal acetylides Chemical class 0.000 description 2
- UNASZPQZIFZUSI-UHFFFAOYSA-N methylidyneniobium Chemical compound [Nb]#C UNASZPQZIFZUSI-UHFFFAOYSA-N 0.000 description 2
- NFFIWVVINABMKP-UHFFFAOYSA-N methylidynetantalum Chemical compound [Ta]#C NFFIWVVINABMKP-UHFFFAOYSA-N 0.000 description 2
- 229910017604 nitric acid Inorganic materials 0.000 description 2
- 150000004767 nitrides Chemical class 0.000 description 2
- 239000001301 oxygen Substances 0.000 description 2
- 229910052760 oxygen Inorganic materials 0.000 description 2
- BPUBBGLMJRNUCC-UHFFFAOYSA-N oxygen(2-);tantalum(5+) Chemical compound [O-2].[O-2].[O-2].[O-2].[O-2].[Ta+5].[Ta+5] BPUBBGLMJRNUCC-UHFFFAOYSA-N 0.000 description 2
- 239000012071 phase Substances 0.000 description 2
- 229920005989 resin Polymers 0.000 description 2
- 239000011347 resin Substances 0.000 description 2
- HQVNEWCFYHHQES-UHFFFAOYSA-N silicon nitride Chemical compound N12[Si]34N5[Si]62N3[Si]51N64 HQVNEWCFYHHQES-UHFFFAOYSA-N 0.000 description 2
- ZOXJGFHDIHLPTG-UHFFFAOYSA-N Boron Chemical compound [B] ZOXJGFHDIHLPTG-UHFFFAOYSA-N 0.000 description 1
- PZNSFCLAULLKQX-UHFFFAOYSA-N Boron nitride Chemical compound N#B PZNSFCLAULLKQX-UHFFFAOYSA-N 0.000 description 1
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 1
- 229910000975 Carbon steel Inorganic materials 0.000 description 1
- ZAMOUSCENKQFHK-UHFFFAOYSA-N Chlorine atom Chemical compound [Cl] ZAMOUSCENKQFHK-UHFFFAOYSA-N 0.000 description 1
- XFXPMWWXUTWYJX-UHFFFAOYSA-N Cyanide Chemical compound N#[C-] XFXPMWWXUTWYJX-UHFFFAOYSA-N 0.000 description 1
- YCKRFDGAMUMZLT-UHFFFAOYSA-N Fluorine atom Chemical compound [F] YCKRFDGAMUMZLT-UHFFFAOYSA-N 0.000 description 1
- 229910020781 SixOy Inorganic materials 0.000 description 1
- NINIDFKCEFEMDL-UHFFFAOYSA-N Sulfur Chemical compound [S] NINIDFKCEFEMDL-UHFFFAOYSA-N 0.000 description 1
- RTAQQCXQSZGOHL-UHFFFAOYSA-N Titanium Chemical compound [Ti] RTAQQCXQSZGOHL-UHFFFAOYSA-N 0.000 description 1
- 229910045601 alloy Inorganic materials 0.000 description 1
- 239000000956 alloy Substances 0.000 description 1
- 229910000147 aluminium phosphate Inorganic materials 0.000 description 1
- 229910021529 ammonia Inorganic materials 0.000 description 1
- 229910021417 amorphous silicon Inorganic materials 0.000 description 1
- 229910052796 boron Inorganic materials 0.000 description 1
- 230000001680 brushing effect Effects 0.000 description 1
- 238000003490 calendering Methods 0.000 description 1
- 229910052799 carbon Inorganic materials 0.000 description 1
- 239000010962 carbon steel Substances 0.000 description 1
- 125000002915 carbonyl group Chemical group [*:2]C([*:1])=O 0.000 description 1
- 239000003518 caustics Substances 0.000 description 1
- 238000003486 chemical etching Methods 0.000 description 1
- 239000000460 chlorine Substances 0.000 description 1
- 229910052801 chlorine Inorganic materials 0.000 description 1
- KRVSOGSZCMJSLX-UHFFFAOYSA-L chromic acid Substances O[Cr](O)(=O)=O KRVSOGSZCMJSLX-UHFFFAOYSA-L 0.000 description 1
- 238000009833 condensation Methods 0.000 description 1
- 230000005494 condensation Effects 0.000 description 1
- 239000004020 conductor Substances 0.000 description 1
- 238000001816 cooling Methods 0.000 description 1
- 230000007423 decrease Effects 0.000 description 1
- 238000003618 dip coating Methods 0.000 description 1
- 238000010494 dissociation reaction Methods 0.000 description 1
- 208000018459 dissociative disease Diseases 0.000 description 1
- 238000004070 electrodeposition Methods 0.000 description 1
- 238000005868 electrolysis reaction Methods 0.000 description 1
- 238000005566 electron beam evaporation Methods 0.000 description 1
- 238000010894 electron beam technology Methods 0.000 description 1
- 239000012530 fluid Substances 0.000 description 1
- 229910052731 fluorine Inorganic materials 0.000 description 1
- 239000011737 fluorine Substances 0.000 description 1
- AWJWCTOOIBYHON-UHFFFAOYSA-N furo[3,4-b]pyrazine-5,7-dione Chemical compound C1=CN=C2C(=O)OC(=O)C2=N1 AWJWCTOOIBYHON-UHFFFAOYSA-N 0.000 description 1
- 229910000449 hafnium oxide Inorganic materials 0.000 description 1
- WIHZLLGSGQNAGK-UHFFFAOYSA-N hafnium(4+);oxygen(2-) Chemical compound [O-2].[O-2].[Hf+4] WIHZLLGSGQNAGK-UHFFFAOYSA-N 0.000 description 1
- 150000002431 hydrogen Chemical class 0.000 description 1
- 239000012535 impurity Substances 0.000 description 1
- 230000006698 induction Effects 0.000 description 1
- 239000011261 inert gas Substances 0.000 description 1
- 150000002484 inorganic compounds Chemical class 0.000 description 1
- 238000007689 inspection Methods 0.000 description 1
- 238000007735 ion beam assisted deposition Methods 0.000 description 1
- 239000012705 liquid precursor Substances 0.000 description 1
- FUJCRWPEOMXPAD-UHFFFAOYSA-N lithium oxide Chemical compound [Li+].[Li+].[O-2] FUJCRWPEOMXPAD-UHFFFAOYSA-N 0.000 description 1
- 229910001947 lithium oxide Inorganic materials 0.000 description 1
- 238000001755 magnetron sputter deposition Methods 0.000 description 1
- 229910001507 metal halide Inorganic materials 0.000 description 1
- 150000005309 metal halides Chemical class 0.000 description 1
- 229910052987 metal hydride Inorganic materials 0.000 description 1
- 239000007769 metal material Substances 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- 229910000484 niobium oxide Inorganic materials 0.000 description 1
- URLJKFSTXLNXLG-UHFFFAOYSA-N niobium(5+);oxygen(2-) Chemical compound [O-2].[O-2].[O-2].[O-2].[O-2].[Nb+5].[Nb+5] URLJKFSTXLNXLG-UHFFFAOYSA-N 0.000 description 1
- 229910052757 nitrogen Inorganic materials 0.000 description 1
- 229910052755 nonmetal Inorganic materials 0.000 description 1
- 150000002843 nonmetals Chemical class 0.000 description 1
- 150000002894 organic compounds Chemical class 0.000 description 1
- 125000002524 organometallic group Chemical group 0.000 description 1
- 238000013021 overheating Methods 0.000 description 1
- DYIZHKNUQPHNJY-UHFFFAOYSA-N oxorhenium Chemical compound [Re]=O DYIZHKNUQPHNJY-UHFFFAOYSA-N 0.000 description 1
- 238000001020 plasma etching Methods 0.000 description 1
- 238000007750 plasma spraying Methods 0.000 description 1
- 230000010287 polarization Effects 0.000 description 1
- 238000007517 polishing process Methods 0.000 description 1
- 238000012545 processing Methods 0.000 description 1
- 239000011253 protective coating Substances 0.000 description 1
- 230000001681 protective effect Effects 0.000 description 1
- 238000000197 pyrolysis Methods 0.000 description 1
- 238000006722 reduction reaction Methods 0.000 description 1
- 229910003449 rhenium oxide Inorganic materials 0.000 description 1
- 238000007761 roller coating Methods 0.000 description 1
- 235000012239 silicon dioxide Nutrition 0.000 description 1
- 239000000377 silicon dioxide Substances 0.000 description 1
- 239000002356 single layer Substances 0.000 description 1
- 239000002904 solvent Substances 0.000 description 1
- 238000005507 spraying Methods 0.000 description 1
- 239000011593 sulfur Substances 0.000 description 1
- 229910052717 sulfur Inorganic materials 0.000 description 1
- 229920003002 synthetic resin Polymers 0.000 description 1
- 239000000057 synthetic resin Substances 0.000 description 1
- 150000003481 tantalum Chemical class 0.000 description 1
- MZLGASXMSKOWSE-UHFFFAOYSA-N tantalum nitride Chemical compound [Ta]#N MZLGASXMSKOWSE-UHFFFAOYSA-N 0.000 description 1
- 229910001936 tantalum oxide Inorganic materials 0.000 description 1
- CZDYPVPMEAXLPK-UHFFFAOYSA-N tetramethylsilane Chemical compound C[Si](C)(C)C CZDYPVPMEAXLPK-UHFFFAOYSA-N 0.000 description 1
- 238000005382 thermal cycling Methods 0.000 description 1
- 238000002207 thermal evaporation Methods 0.000 description 1
- 238000007751 thermal spraying Methods 0.000 description 1
- 230000000930 thermomechanical effect Effects 0.000 description 1
- 239000010936 titanium Substances 0.000 description 1
- 229910052719 titanium Inorganic materials 0.000 description 1
- 238000012546 transfer Methods 0.000 description 1
- 238000004506 ultrasonic cleaning Methods 0.000 description 1
- 238000011179 visual inspection Methods 0.000 description 1
Classifications
-
- H—ELECTRICITY
- H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
- H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
- H05B3/00—Ohmic-resistance heating
- H05B3/40—Heating elements having the shape of rods or tubes
- H05B3/42—Heating elements having the shape of rods or tubes non-flexible
- H05B3/48—Heating elements having the shape of rods or tubes non-flexible heating conductor embedded in insulating material
-
- H—ELECTRICITY
- H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
- H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
- H05B3/00—Ohmic-resistance heating
- H05B3/62—Heating elements specially adapted for furnaces
- H05B3/64—Heating elements specially adapted for furnaces using ribbon, rod, or wire heater
Definitions
- the present invention relates to improved coatings for electrical, metal sheathed heating elements and, more particularly, coatings for such heating elements comprised of a refractory metal, refractory metal oxide, ceramic, inter-metallic, or mixtures thereof.
- Heating elements are widely employed in order to raise the temperature of such media as liquids and gases.
- the element When utilizing a heating element to raise the temperature of a medium, the element is typically immersed in the liquid or gas which is desired to be heated. The immersed element is then heated via a power source such that the generated heat is radiated to the liquid or gas and the temperature of the liquid or gas rises.
- the liquid or gas to be heated is itself corrosive to the heating element, contains corrosive substances, or contains substances which become corrosive upon heating. Corrosion of the heating element often results which necessitates excess repair and replacement costs to the unit which contains the heating element.
- heating elements are often encapsulated with a thick, i.e. from about 0.32 to about 0.65 centimeters, layer of a substance such as a synthetic fluorine containing resin.
- synthetic resins for example, TeflonTM
- TeflonTM are well-known coating agents for many items, including cookware, because of their "non-stick" properties.
- the resin coating protects the heating element somewhat from corrosion but, unfortunately, has many deficiencies.
- TeflonTM TeflonTM
- the coating decreases the efficiency of the heating element by reducing the transfer of heat from the element to the media to be heated.
- a related problem is that due to its non-stick properties, TeflonTM does not adhere well to the heating element surface. Therefore, the heating element must be encapsulated by the TeflonTM. This requires sleeving a thick pre-formed, open-ended TeflonTM tube around the element. Often during such sleeving the heating element may be damaged. Additionally, the open-end of the TeflonTM sleeving allows for the possibility that corrosive materials may come into contact with part of the heating element because condensation is trapped between the TeflonTM sleeve and the surface of the heater.
- TeflonTM protective sleeves Yet another deficiency of TeflonTM protective sleeves is that above about 260°C in air, the TeflonTM coating will often degrade. Such degradation may be difficult to detect before significant corrosion of the heating element occurs. However, even if the degradation is detected, the sleeve is not easily repaired and often must be replaced in its entirety via the sleeving process described above.
- Another manner of protecting heating elements from corrosion is to use a sheath made entirely of ceramic rather than coating a metal sheath with TeflonTM. Such ceramic sheathed elements are disclosed in US Patent No. 5,084,606. Unfortunately, when the sheath is made of solid ceramic, the heating element is fragile due to the brittleness of the ceramic. Such elements are also expensive due to the amount of ceramic.
- ceramic is not very conductive and thus heat is not readily transferred by the element. It would be desirable to discover a coating for an outer metallic shell of a heating element which is significantly thinner than the TeflonTM coating currently employed. If such a coating could be discovered then the heating elements would be efficient, as well as, less expensive and fragile than a ceramic sheathed heating element. It would further be desirable if such a coating would adhere to the heating element such that it could be applied via a simple process which would not harm the heating element and such that the coating did not leave an opening for corrosive materials to attack the heating element. It would yet further be desirable if such a coating would offer protection to the heating element at high temperatures and in corrosive environments and could be repaired easily if such repair becomes necessary.
- a refractory metal, refractory metal oxide, ceramic, inter-metallic. or a mixture thereof may be utilized to coat heating elements.
- Such coatings offer excellent corrosion protection in many different environments including acidic environments.
- the coatings are very energy efficient in that heat is readily conducted through the coating.
- the numerous processes which may be utilized to apply the refractory metal, refractory metal oxide, ceramic, inter-metallic, or mixture thereof to the heating element offer significant advantages.
- One advantage is that the processes do not damage the heating element.
- the coatings do not require sleeving the heating element with an open-ended tubular structure such as TeflonTM, but rather, coat the entire heating element. In this manner, if the coating becomes damaged, it may be repaired at the point of damage and replacement of the entire coating is not necessary. Also, by not having an open end, the heating element is better protected from corrosion.
- Figure 1 illustrates a typical heating element which is coated with a corrosion resistant coating in accordance with the instant invention.
- FIG. 2 illustrates the coatings of the instant invention. Detailed Description of the Invention
- heating element refers to any electrical resistance heating element used to raise the temperature of a liquid such as water or a gas such as air.
- electrical resistance heating elements are of many different shapes and sizes depending upon the intended application. Among such shapes include disks, tubes, filaments, and cartridges. The sizes may vary from a small heating element suitable for a personal hair dryer to a large heating element suitable for a commercial water heater.
- useful elements to be coated in this invention typically produce watt density of at least 5, preferably at least 50, more preferably at least 200 watts per square inch.
- a typical heating element which may be utilized in the instant invention is illustrated in Figure 1.
- the heating elements of this invention are useful in immersion heaters which are subject to corrosive environments. Such heaters are often used in heating chemicals and thus subject to the corrosive environments thereof.
- Such immersion heaters typically are comprised of an electrically conductive resistive heating element and a metallic sheath covering the element, the sheath being insulated from the element by conventional insulation.
- the coatings of the instant invention are useful to cover the sheath and protect the element from corrosion even when the current flow through the element produces a watt- density of at least 5, preferably at least 50, more preferably at least 200 watts per square inch.
- refractory metals refers to those metals capable of withstanding temperatures to which a heating element may be subjected. Typically, such metals may withstand temperatures up to 100, preferably up to 500, more preferably up to 1000 °C.
- refractory metals those found most suitable for this invention include tantalum, niobium, hafnium, and rhenium. The most preferable refractory metal is tantalum.
- refractory metal oxide refers to a compound which contains oxygen and a refractory metal in any ratio sufficient for use as a coating in the instant invention.
- useful oxides of this invention include tantalum oxide, niobium oxide, hafnium oxide, and rhenium oxide such as, for example, Ta 2 0 5 , Nb 2 0 5 , Hf 2 0 5 , and Re 2 0 5 .
- ceramics generally refers to metallic or non-metallic compounds which are capable of withstanding temperatures to which a heating element may be subjected. Typically, such materials may withstand temperatures up to 100, preferably up to 500, more preferably up to 1500 °C.
- Ceramics are described generally in, for example, Electric Process Heating by Maurice Orfeuil, Batelle Press, (1987) pp. 60-61, which is incorporated herein by reference.
- the term ceramics includes “carbides”, “nitrides” and “oxides”.
- the term “carbides” refers to a compound, metallic or non- metallic, associated with carbon such as silicon carbide (SiC), tantalum carbide (TaC), hafnium carbide (HfC), aluminum nitride (A1N), niobium carbide (NbC), rhenium carbide (ReC), and zirconium carbide (ZrC), as well as, diamond.
- nitrides refers to a compound, metallic or non-metallic, associated with nitrogen such as boron nitride (BN), tantalum nitride (TaN or Ta N), silicon nitride (Si 3 N 4 ), and hafnium nitride (HfN).
- oxides refers to a compound, metallic or non-metallic, associated with oxygen such as silicon dioxide (Si0 2 ), tantalum pentoxide (Ta 2 0 5 ) or lithium oxide.
- inter-metallics is a two or more component system which has a semiconducting metal.
- a typical inter-metallic compound for use in the present invention includes nickel suicide (Ni 3 Si).
- the term "coating” means a film or thin layer applied to an outer metallic shell, i.e., sheath, of a heating element.
- the sheath often encompasses components of an element which are in need of protection from corrosion. Such components may include thermocouples, resistance wires, and connectors.
- the thickness of such films or layers varies depending upon the particular coating material and the surface smoothness of the heating element.
- the coatings of the instant invention are at least 1000, preferably at least 10,000, more preferably at least 50,000 Angstroms thick.
- the coatings are not so thick that ductility is lost or the coating becomes fragile.
- the coating are less than 100, preferably less than 10, more preferably less than 1 microns thick.
- the coatings of the instant invention are substantially pinhole and stress free.
- the coating adheres strongly to the surface of a metallic sheath of the heating element at temperatures ranging from - 20° to over 1000° C. After multiple thermal cycling, the coatings also conserves its strong adherence.
- the undercoat may include any of the materials used for coatings herein.
- a particularly preferred undercoating material is tungsten.
- the thickness of the undercoat is typically less than 5, preferably less than 1, and more preferably less than 0.1 micrometers.
- the instant invention provides a tenacious, stress, and pinhole-free coating suitable for protecting heating elements
- the invention is particularly applicable to heating elements which are in need of such protection.
- Typical heating elements in need of such protection include those heating elements used in immersion and gas heaters subjected to a corrosive environment.
- the coatings of the instant invention offer protection because the coatings are substantially impermeable to many corrosive environments which the heating element may be subjected.
- a corrosive environment to which an immersion heater may be subjected often includes those environments where concentrated acids exist.
- the corrosive environment has a pH of from about less than 1 to about 6 and may comprise sulfuric acid, nitric acid, chromic acid, phosphoric acid, or other acids harmful to unprotected heating elements, as well as, mixtures of the aforementioned acids.
- a heating element may be subjected to a corrosive gaseous environment, such as one comprising cyanide, chlorine, sulfur, or mixtures thereof.
- a corrosive environment may include high temperatures, for example, temperatures from about 100 to about 1000°C, pressures from about 0 to about 1 atmospheres, as well as, acidic pH's, for example, from about 0.01 to about 6, or basic pH's, for example, from about 9 to about 14.
- acidic pH's for example, from about 0.01 to about 6, or basic pH's, for example, from about 9 to about 14.
- Proper surface preparation fosters adhesion between the element and coating.
- Surface preparation also eliminates foreign objects and debris which could cause cracks or holes in the coating and allow corrosion of the heating element.
- Typical surface preparation may include a number of different steps or combination of steps.
- Typical surface preparation steps comprise, for example, one or more of the following: electropolishing, mechanical polishing, chrome plating, deburring, electroless nickel plating, acid or chemical etching, plasma etching, utrasonical cleaning, or vapor degreasing.
- a particularly preferred surface preparation procedure involves mechanically polishing the surface of the metal sheath of the heating element onto which the coating will be deposited.
- the polishing may include removing from about 0.1 to about 30 micrometers or more of the surface of the metal sheath.
- the above polishing be accomplished by use of an automated polishing process.
- the polishing is highly precise and efficient, for example, often on the order of seconds for a typical polished tube of dimensions from about 35 to about 200 centimeters in length and from about 4 to about 6 centimeters in outside diameter.
- Electropolishing is another preferred method of preparing the surface of the heating element. This method is of particular importance when minimizing the roughness profile of the metal surface.
- a mirror-like surface of the metal sheath may be achieved with an immersion time of from about 5 to about 15 minutes or longer.
- the process used to coat the heating element with the refractory metal, refractory metal oxide, ceramic, inter-metallic, or mixture thereof is not particularly critical so long as the coating is impermeable to corrosive fluids and adheres strongly to the surface of the metal sheath. It is preferable for most applications that the coating be of uniform thickness and adhere via physical means or chemical means to the heating element. Often the coating process utilized is based upon the size and shape of the heating element to be coated.
- coating methods as electrolysis, electroplating, electrodeposition, conventional chemical vapor deposition such as the deposition of tantalum via a liquid or solid precursor, gas and solid diffusion such as pack cementation and chemical vapor deposition, physical vapor deposition such as RF and DC sputtering including both reactive and non-reactive processes (non-reactive processes generally consists of using inert or non-reactive gases such as Argon or Helium), plasma enhanced, or ion beam- assisted vapor deposition, as well as, vacuum or mechanical coating means including brushing, spraying such as thermal or plasma spraying, calendering, dip coating, roller coating, or any combination thereof may be utilized.
- conventional chemical vapor deposition such as the deposition of tantalum via a liquid or solid precursor, gas and solid diffusion such as pack cementation and chemical vapor deposition
- physical vapor deposition such as RF and DC sputtering including both reactive and non-reactive processes (non-reactive processes generally consists of using inert or non-reactive gases such as Ar
- a particularly preferred coating process is that of pack cementation.
- This process consists of placing the heating element to be coated into a semi-permeable enclosure, for example, a box.
- the enclosure contains, or is subsequently filled, with the refractory metal, refractory metal oxide, ceramic, inter-metallic, or mixture thereof which is to become the coating.
- the enclosure also contains, or is subsequently filled with, a halide which is volatile at the temperature of heat treatment.
- the pack cementation process of coating includes thermal decomposition of the metallic halide followed by a thermal deposition and diffusion of the refractory metal or oxide onto the surface of, and into the surface of, the outer metal sheath of the heating element.
- Typical deposition stages may be represented as follows:
- Another particularly preferable coating process is that of physical vapor deposition. As described above, it is preferable to surface treat the heating element and then clean it by a means such ultrasonic cleaning or vapor degreasing before coating by a means such as physical vapor deposition.
- the surface of the heating element After cleaning, it is preferable to plasma etch the surface of the heating element in argon under ultra-high vacuum and then sputter coat the surface with reactive or non-reactive gases in conjunction with the desired coating material.
- the operation of the aforementioned process at ultra high vacuum avoids the introduction of most impurities onto the heating element's surface.
- low to high voltage biasing can be applied to promote a strong adhesion of the coating to the surface of the metal sheath of the heating element.
- the coating will also usually become highly dense when this biasing method is employed.
- Pulsing technique can also be utilized for coating of semi-conductive material such as silicon carbide. The above process often allows the heating element to be processed at low temperatures, preferably less than 400 degrees Celsius.
- the chemical vapor deposition technique is based upon a chemical reaction, such as thermal decomposition (pyrolysis), between a gaseous phase and a heated surface of the element to be coated.
- the surface of the heating element is first directly or indirectly heated via induction heating.
- a gaseous phase comprised of a non-chlorinated organo-metallic, metal carbonyls or metal hydrides compound comprising a refractory metal. It is necessary to select a non-chlorinated precursor, such as tetramethylsilane. which can generate the desired coating at reaction temperatures below 750° C due to the large mismatch in thermal expansion between the coatings and the materials of the heating element.
- deposition temperatures below 750° Celsius eliminate formation of microcracks in the coatings during the cooling process.
- Typical deposition pressures are usually between 0.0132 atmosphere (10 torr) and 0.131 atmosphere (100 torr).
- Yet another preferred method of applying the coating to the surface of the metal sheath of the heating element is by gas diffusion via chemical vapor deposition. In this process, a halide of the solute metal is passed in vapor form over the metallic surface of the heating element to be coated, the surface being heated such that it is at a temperature at which diffusion can take place. Such temperatures are generally from about 400 to about 1000 degrees Celsius or more, depending on the precursor for the coating.
- the carrier gas for the halide vapor can be a reducing gas such as hydrogen, cracked ammonia, or the like or inert gases such as helium or argon.
- a reducing gas such as hydrogen, cracked ammonia, or the like or inert gases such as helium or argon.
- the metal halide BX 2 in gaseous form is reduced to metal B which then diffuses into the solvent metal A.
- the three major chemical reactions for this gas phase diffusion process can be summarized as follows: Interchange reaction: A + BX 2 (gas) — > AX 2 + B Reduction reaction: BX 2 + H 2 — > 2HX + B
- the interchange reaction implies the removal of one atom of A at the surface for each atom of B deposited.
- the deposition of tantalum can also be processed via gas or liquid precursor such as tantalum pentachloride or other derivatives of tantalum, provided that the thermodynamics for thermal decomposition of tantalum precursor can occur at temperatures less than 1000 degrees Celsius or preferably less than 500 degrees Celsius.
- the pressure used for this process can range from about full vacuum to about 0.98 atmospheres.
- the heating elements of this invention are particularly useful in corrosive environments. The heating elements of the invention will typically not show any pits, crevices, holes, or cracks indicating degradation of the coating even when subjected to pH's of less than about 1.0 (such as concentrated chromic plating solution and nitric acid) at boiling temperatures for more than 60, preferably more than 100 hours.
- the coatings of this invention may be modified by addition of such substances such as metals, non-metals, alloys, as well as, inorganic and organic compounds so long as such substance does not destroy the functionality of the protective coating.
- Such modifications may often be desirable in order to alter one or more of the properties, for example electrical, thermo-mechanical, or chemical properties, of the coating for a particular application.
- the coating may be doped with boron or titanium to promote such ductility.
- materials such as inter-metallic substances, for example, nickel suicide, which have a coefficient of thermal expansion which is relatively equivalent to the thermal expansion coefficient of a base material of the heater. In this manner, efficient heating is accomplished.
- the coatings of this invention may have more than one layer. This may be desirable if the heating element is to be subjected to a particularly corrosive environment for an extended length of time or if the heating element is unusually fragile.
- the use of layered coatings may reinforce the failure of the outer layers in case of a severe electrochemical attack. Often if multiple layers are to be utilized, the number of layers will range from one (1) to one hundered (100) or more, preferably one (1) to ten (10), depending on the specific application.
- the thickness of each layer may generally vary from about 100 to 5000 Angstroms and it may be desirable to use a material such as tungsten between the layers to enhance the adhesion of the layers to each other. Each layer may be of different or identical materials and optimum performance may be achieved via routine experimentation.
- the coated heating elements of this invention may be easily inspected for defects such as microcracks, cracks, holes, or other failures in the coating. Such inspection may be accomplished in numerous ways.
- leakage current is tested by immersing the coated area of the heating element in deionized water and acidic solution in conjunction with a metallic electrode probe. A current, for example 12 volts, is applied to the uncoated area of the heater and metallic electrode probe. If the probe detects a voltage change, then a defect exists within the coating. Typically, the magnitude of the voltage change increases as the size of the defect.
- a second method useful for coatings which are conducting, semi-conducting, or insulating is that of potentiodynamic polarization.
- a coated heating element may be repaired in a similar manner to which it was coated.
- the repair steps include surface preparation, cleaning, and then reapplying the coatings onto the clean surface of the heating element as described above.
- FIG. 1 A schematic of a resistance heating device of the instant invention is described in Figure 1.
- the internal elements of the heating device comprise of a resistance wire (1), refractory or ceramic insulation (2), a Chromalox patented connector (3) used to control the overheating of the resistance wire (1) and a terminal block (4).
- the internal elements are protected by an outer metallic shell or sheath (5).
- the base (6) of the metallic sheath comprises a heavy-gauge disk which is securely welded.
- a corrosion-resistant coating of the instant invention (7) surrounds the sheath.
- Figure 2 shows as an embodiment of this invention, the high performance corrosion resistant coating (7) of refractory metals, refractory metal oxides, ceramics, inter-metallics. or mixture thereof which is applied onto and into the outer surface of the metallic sheath of the heating device is illustrated in 2a of Figure 2.
- the coating can be single (2b of Figure 2) or layered (2c of Figure 2). According to this invention, the coating diffuses into the surface or grain boundaries of the metallic sheath or tube to promote a strong adhesion.
- the coating is 100 percent (%) dense.
- Each layer of the multi-layer coatings (2c of Figure 2) can be of identical or different materials selected from refractory metals, refractory metal oxides, ceramics, inter-metallics, or a mixture thereof.
- RF Bias sputtering was used to apply 100% dense amorphous silicon carbide to the surface of four steel tubes.
- an undercoating layer of tungsten of 100 Angstroms thick was deposited prior to the silicon carbide coating to enhance adhesion.
- Silicon carbide was then deposited on each of the four steel tubes.
- the first steel tube was coated with a single layer of silicon carbide which was 3 micrometers in thickness.
- the second steel tube was coated with 10 layers of silicon carbide, each layer being 1000 Angstroms.
- the third steel tube was coated with 20 layers of silicon carbide, each layer being 1000 Angstroms.
- the fourth steel tube was coated with 30 layers of silicon carbide, each layer being 1000 Angstroms. Upon visual inspection, each steel tube was found to be free of microcracks and pinholes.
- the silicon carbide coated steel tubes were tested by immersing them in chromic plating solution at boiling temperature for 48 hours.
- the chromic plating solution had a pH near 0.0 at boiling temperature.
- the average weight loss was 0.1 percent (%).
- DC Bias magnetron sputtering was used to apply 100% dense tantalum to the surface of steel disks, tubes, and shells of cartridge and tubular type heaters.
- a typical heating element having a tantalum coating is shown in Figure 2.
- the sputtering atmosphere was argon.
- the temperature of the surface of the substrate was 250°C.
- the DC bias voltage was 500 volts to 4 kilovolts.
- the argon pressure was 1.31 x 10 " atmospheres.
- the sputtering time ranged from 60 to 90 minutes.
- the resulting tantalum coatings were about 3 micrometers in thickness.
- the tantalum coated steel disks, tubes, and shells of cartridge and tubular type heaters were examined for defects under a microscope at a magnification of 500 times and 5000 times. No defects- were observed and the tantalum coating adhered strongly to the surface of the metallic sheath..
- the tantalum coated steel disks, tubes, and shells of cartridge and tubular type heaters were tested by immersing them in chromic plating solution at 67° C for 100 hours.
- the chromic plating solution had a pH of 1.0 at 67° C.
- the average weight loss was 0.005 percent (%).
- the tantalum coated steel disks, tubes, and shells of cartridge and tubular type heaters were tested by immersing them in chromic plating solution at boiling temperature for 100 hours.
- the chromic plating solution had a pH near 0.0 at boiling temperature.
- the average weight loss was 0.09 percent (%).
- Ion beam-assisted deposition in particular, electron beam evaporation is useful to apply 100% dense tantalum to the surface of steel disks, tubes, and shells of cartridge and tubular type heaters.
- the metal substrate was biased to enhance the adhesion of the coating.
- the bias voltage was 500 volts to 2 kilovolts.
- Low temperature processing of less than 250 degrees Celsius is useful for the heat sensitive surfaces of the outer sheaths. Tantalum chips, pellets, or a mixture thereof are evaporated under high vacuum by an electron beam.
- An ion beam of argon via an ion gun bombards the tantalum vapors onto and into the surface to be coated. Tantalum films which are 100 percent (%) dense, about 3 micrometers thick, defect-free, and have strong adhesion to the outer metal sheath are obtained using this deposition method of coating.
- a gas diffusion method was used to apply 100 percent (%) dense tantalum to the surface of steel disks, tubes, and shells of cartridge and tubular type heaters.
- the surface was first heated to temperatures ranging from 300 to 500 degrees Celsius. Tantalum chips or pellets were then heated to their sublimation temperature. Tantalum vapor was then transported to the deposition chamber with a carrier gas such as Argon. Hydrogen was also introduced directly into the deposition chamber during the gas diffusion process. A typical in-depth diffusion of 0.5 to 1 micrometers of tantalum into the surface of the shell heater was achieved within 30 minutes to two hours. Tantalum was also introduced into the deposition chamber by using tantalum pentachloride (gas) as a precursor associated with tantalum. Hydrogen was used to decompose tantalum pentachloride into tantalum at temperatures of less than 900 degrees Celsius.
- Typical deposition parameters useful for high density Tantalum coating onto and into carbon steel sheath via gas diffusion - chemical vapor deposition are as follows: Etching gas: Hydrochloric (HC1)
- Etching temperature 400 degrees Celsius
- Flushing gas and pressure Argon @ 0.026 atmosphere (20 torr) Flushing time: 15 minutes
- Tantalum deposition parameters Total Pressure: 0.013 atmosphere (10 torr)
- Tantalum chips to sublimation temperature Deposition temperature 500 degrees Celsius
- Example Three were examined for defects under a microscope at a magnification of 500 times and 5000 times. No defects were observed.
- the tantalum coated steel disks, tubes, and shells of cartridge and tubular type heaters were tested by immersing them in chromic plating solution at 67° C for 100 hours.
- the chromic plating solution had a pH of 1.0 at 67° C.
- the average weight loss was 0.1 percent (%).
- the tantalum coated steel disks, tubes, and shells of cartridge and tubular type heaters were tested by immersing them in chromic plating solution at boiling temperature for 100 hours.
- the chromic plating solution had a pH near 0.0 at boiling temperature.
- the average weight loss was 0.5 percent (%).
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Abstract
Heating elements having novel coatings of a refractory metal, refractory metal oxide, ceramic, inter-metallic, or a mixture thereof have been discovered. Such coatings provide the heating element with excellent corrosion protection and therefore, are especially useful for heating elements which will be subjected to highly corrosive environments. In addition, the coatings are easily applied and may be repaired if necessary.
Description
IMPROVED COATINGS FOR ELECTRICAL. METAL SHEATHED HEATING ELEMENTS
Field of the Invention The present invention relates to improved coatings for electrical, metal sheathed heating elements and, more particularly, coatings for such heating elements comprised of a refractory metal, refractory metal oxide, ceramic, inter-metallic, or mixtures thereof.
Background of the Invention Heating elements are widely employed in order to raise the temperature of such media as liquids and gases. When utilizing a heating element to raise the temperature of a medium, the element is typically immersed in the liquid or gas which is desired to be heated. The immersed element is then heated via a power source such that the generated heat is radiated to the liquid or gas and the temperature of the liquid or gas rises. Unfortunately, in many instances the liquid or gas to be heated is itself corrosive to the heating element, contains corrosive substances, or contains substances which become corrosive upon heating. Corrosion of the heating element often results which necessitates excess repair and replacement costs to the unit which contains the heating element.
Because of the corrosion problem described above, heating elements are often encapsulated with a thick, i.e. from about 0.32 to about 0.65 centimeters, layer of a substance such as a synthetic fluorine containing resin. Such synthetic resins, for example, Teflon™, are well-known coating agents for many items, including cookware, because of their "non-stick" properties. The resin coating protects the heating element somewhat from corrosion but, unfortunately, has many deficiencies.
One such deficiency with a Teflon™ coating is that due to its thickness and relatively poor thermal conductivity, the coating decreases the efficiency of the heating element by reducing the transfer of heat from the element to the media to be heated. A related problem is that due to its non-stick properties, Teflon™ does not adhere well to the heating element surface. Therefore, the heating element must be encapsulated by the Teflon™. This requires sleeving a thick pre-formed, open-ended Teflon™ tube around the element. Often during such sleeving the heating element may be damaged. Additionally, the open-end of the Teflon™ sleeving allows for the possibility that corrosive materials may come into contact with part of the heating element because condensation is trapped between the Teflon™ sleeve and the surface of the heater.
Yet another deficiency of Teflon™ protective sleeves is that above about 260°C in air, the Teflon™ coating will often degrade. Such degradation may be difficult to detect before significant corrosion of the heating element occurs. However, even if the degradation is detected, the sleeve is not easily repaired and often must be replaced in its entirety via the sleeving process described above. Another manner of protecting heating elements from corrosion is to use a sheath made entirely of ceramic rather than coating a metal sheath with Teflon™. Such ceramic sheathed elements are disclosed
in US Patent No. 5,084,606. Unfortunately, when the sheath is made of solid ceramic, the heating element is fragile due to the brittleness of the ceramic. Such elements are also expensive due to the amount of ceramic. Additionally, ceramic is not very conductive and thus heat is not readily transferred by the element. It would be desirable to discover a coating for an outer metallic shell of a heating element which is significantly thinner than the Teflon™ coating currently employed. If such a coating could be discovered then the heating elements would be efficient, as well as, less expensive and fragile than a ceramic sheathed heating element. It would further be desirable if such a coating would adhere to the heating element such that it could be applied via a simple process which would not harm the heating element and such that the coating did not leave an opening for corrosive materials to attack the heating element. It would yet further be desirable if such a coating would offer protection to the heating element at high temperatures and in corrosive environments and could be repaired easily if such repair becomes necessary.
Summary of the Invention Surprisingly, it has now been discovered that a refractory metal, refractory metal oxide, ceramic, inter-metallic. or a mixture thereof may be utilized to coat heating elements. Such coatings offer excellent corrosion protection in many different environments including acidic environments. In addition, the coatings are very energy efficient in that heat is readily conducted through the coating. The numerous processes which may be utilized to apply the refractory metal, refractory metal oxide, ceramic, inter-metallic, or mixture thereof to the heating element offer significant advantages. One advantage is that the processes do not damage the heating element. Yet another advantage is that the coatings do not require sleeving the heating element with an open-ended tubular structure such as Teflon™, but rather, coat the entire heating element. In this manner, if the coating becomes damaged, it may be repaired at the point of damage and replacement of the entire coating is not necessary. Also, by not having an open end, the heating element is better protected from corrosion.
Brief Description of the Drawings Figure 1 illustrates a typical heating element which is coated with a corrosion resistant coating in accordance with the instant invention.
Figure 2 illustrates the coatings of the instant invention. Detailed Description of the Invention
As used herein, the term "heating element" refers to any electrical resistance heating element used to raise the temperature of a liquid such as water or a gas such as air. Such electrical resistance heating elements are of many different shapes and sizes depending upon the intended application. Among such shapes include disks, tubes, filaments, and cartridges. The sizes may vary from a small heating element suitable for a personal hair dryer to a large heating element suitable for a commercial water heater. Thus, useful elements to be coated in this invention typically produce watt density of at
least 5, preferably at least 50, more preferably at least 200 watts per square inch. A typical heating element which may be utilized in the instant invention is illustrated in Figure 1.
Preferably, the heating elements of this invention are useful in immersion heaters which are subject to corrosive environments. Such heaters are often used in heating chemicals and thus subject to the corrosive environments thereof. Such immersion heaters, for example as described in US Patent No. 5,013,890 incorporated herein by reference, typically are comprised of an electrically conductive resistive heating element and a metallic sheath covering the element, the sheath being insulated from the element by conventional insulation. The coatings of the instant invention are useful to cover the sheath and protect the element from corrosion even when the current flow through the element produces a watt- density of at least 5, preferably at least 50, more preferably at least 200 watts per square inch.
As used herein, the term "refractory metals" refers to those metals capable of withstanding temperatures to which a heating element may be subjected. Typically, such metals may withstand temperatures up to 100, preferably up to 500, more preferably up to 1000 °C. Among the refractory metals, those found most suitable for this invention include tantalum, niobium, hafnium, and rhenium. The most preferable refractory metal is tantalum.
As used herein, the term " refractory metal oxide" refers to a compound which contains oxygen and a refractory metal in any ratio sufficient for use as a coating in the instant invention. Among useful oxides of this invention include tantalum oxide, niobium oxide, hafnium oxide, and rhenium oxide such as, for example, Ta205, Nb205, Hf205, and Re205. As used herein, the term "ceramics" generally refers to metallic or non-metallic compounds which are capable of withstanding temperatures to which a heating element may be subjected. Typically, such materials may withstand temperatures up to 100, preferably up to 500, more preferably up to 1500 °C. Ceramics are described generally in, for example, Electric Process Heating by Maurice Orfeuil, Batelle Press, (1987) pp. 60-61, which is incorporated herein by reference. Typically, the term ceramics includes "carbides", "nitrides" and "oxides". The term "carbides" refers to a compound, metallic or non- metallic, associated with carbon such as silicon carbide (SiC), tantalum carbide (TaC), hafnium carbide (HfC), aluminum nitride (A1N), niobium carbide (NbC), rhenium carbide (ReC), and zirconium carbide (ZrC), as well as, diamond. The term "nitrides" refers to a compound, metallic or non-metallic, associated with nitrogen such as boron nitride (BN), tantalum nitride (TaN or Ta N), silicon nitride (Si3N4), and hafnium nitride (HfN). The term "oxides" refers to a compound, metallic or non-metallic, associated with oxygen such as silicon dioxide (Si02), tantalum pentoxide (Ta205) or lithium oxide. Other ceramic compounds of use in the instant invention include an oxidized form of the above compounds, for example, SixOyN, TaxOyN, and SiOC wherein x and y represent integers which provide a stable stoichiometric ratio of the formula.
As used herein, the term "inter-metallics" is a two or more component system which has a semiconducting metal. A typical inter-metallic compound for use in the present invention includes nickel suicide (Ni3Si).
As used herein, the term "coating" means a film or thin layer applied to an outer metallic shell, i.e., sheath, of a heating element. The sheath often encompasses components of an element which are in need of protection from corrosion. Such components may include thermocouples, resistance wires, and connectors. The thickness of such films or layers varies depending upon the particular coating material and the surface smoothness of the heating element. Typically, for most applications, the coatings of the instant invention are at least 1000, preferably at least 10,000, more preferably at least 50,000 Angstroms thick. Correspondingly, the coatings are not so thick that ductility is lost or the coating becomes fragile. Generally, the coating are less than 100, preferably less than 10, more preferably less than 1 microns thick.
The coatings of the instant invention are substantially pinhole and stress free. The coating adheres strongly to the surface of a metallic sheath of the heating element at temperatures ranging from - 20° to over 1000° C. After multiple thermal cycling, the coatings also conserves its strong adherence. Occasionally, it may be desirable to apply an undercoat on the surface of the metallic sheath to enhance the adhesion of the coating to the surface of the metallic sheath of the heating elements and/or fill any pinholes or cracks existing in the surface of the sheath. The undercoat may include any of the materials used for coatings herein. A particularly preferred undercoating material is tungsten. The thickness of the undercoat is typically less than 5, preferably less than 1, and more preferably less than 0.1 micrometers. Because the instant invention provides a tenacious, stress, and pinhole-free coating suitable for protecting heating elements, the invention is particularly applicable to heating elements which are in need of such protection. Typical heating elements in need of such protection include those heating elements used in immersion and gas heaters subjected to a corrosive environment. The coatings of the instant invention offer protection because the coatings are substantially impermeable to many corrosive environments which the heating element may be subjected.
A corrosive environment to which an immersion heater may be subjected often includes those environments where concentrated acids exist. Generally, the corrosive environment has a pH of from about less than 1 to about 6 and may comprise sulfuric acid, nitric acid, chromic acid, phosphoric acid, or other acids harmful to unprotected heating elements, as well as, mixtures of the aforementioned acids. Often, in addition to the aqueous, acidic environments described above, a heating element may be subjected to a corrosive gaseous environment, such as one comprising cyanide, chlorine, sulfur, or mixtures thereof. Furthermore, in some instances a corrosive environment may include high temperatures, for example, temperatures from about 100 to about 1000°C, pressures from about 0 to about 1 atmospheres, as well as, acidic pH's, for example, from about 0.01 to about 6, or basic pH's, for example, from about 9 to about 14.
Before coating the heating element with the refractory metal, refractory metal oxide, ceramic, inter-metallic, or mixture thereof, it is often preferable to prepare the surface of the heating element. Proper surface preparation fosters adhesion between the element and coating. Surface preparation also eliminates foreign objects and debris which could cause cracks or holes in the coating and allow corrosion of the heating element. Typical surface preparation may include a number of different steps or combination of steps. These steps, as one skilled in the art will appreciate, will vary depending upon the content and nature of the heating element's surface, as well as, upon the coating material and process to be utilized. Typical surface preparation steps comprise, for example, one or more of the following: electropolishing, mechanical polishing, chrome plating, deburring, electroless nickel plating, acid or chemical etching, plasma etching, utrasonical cleaning, or vapor degreasing.
A particularly preferred surface preparation procedure involves mechanically polishing the surface of the metal sheath of the heating element onto which the coating will be deposited. Depending on the degree of roughness of the metal sheath, the polishing may include removing from about 0.1 to about 30 micrometers or more of the surface of the metal sheath. One may often determine whether the mechanical polishing is sufficient examining the metal sheath under a microscope at 500x magnification. If the surface is substantially free from scratches, pits, and undercuts the mechanical polishing is sufficient. If further surface preparation is desirable for a given application, then a mirror-like surface may be achieved by further polishing the surface of the metal sheath with a polishing compound such as ultrafine diamond or A1 03 grit and paste. While not necessary, it is preferred that the above polishing be accomplished by use of an automated polishing process. In this manner, the polishing is highly precise and efficient, for example, often on the order of seconds for a typical polished tube of dimensions from about 35 to about 200 centimeters in length and from about 4 to about 6 centimeters in outside diameter.
Electropolishing is another preferred method of preparing the surface of the heating element. This method is of particular importance when minimizing the roughness profile of the metal surface.
Often, a mirror-like surface of the metal sheath may be achieved with an immersion time of from about 5 to about 15 minutes or longer.
After such surface preparation it is often preferable to ultrasonically clean or vapor degrease the polished surface of the metal sheath to be coated. This ensures a clean surface for coating the heating element.
The process used to coat the heating element with the refractory metal, refractory metal oxide, ceramic, inter-metallic, or mixture thereof is not particularly critical so long as the coating is impermeable to corrosive fluids and adheres strongly to the surface of the metal sheath. It is preferable for most applications that the coating be of uniform thickness and adhere via physical means or chemical means to the heating element. Often the coating process utilized is based upon the size and shape of the heating element to be coated. Accordingly, such coating methods as electrolysis, electroplating,
electrodeposition, conventional chemical vapor deposition such as the deposition of tantalum via a liquid or solid precursor, gas and solid diffusion such as pack cementation and chemical vapor deposition, physical vapor deposition such as RF and DC sputtering including both reactive and non-reactive processes (non-reactive processes generally consists of using inert or non-reactive gases such as Argon or Helium), plasma enhanced, or ion beam- assisted vapor deposition, as well as, vacuum or mechanical coating means including brushing, spraying such as thermal or plasma spraying, calendering, dip coating, roller coating, or any combination thereof may be utilized.
A particularly preferred coating process is that of pack cementation. This process consists of placing the heating element to be coated into a semi-permeable enclosure, for example, a box. The enclosure contains, or is subsequently filled, with the refractory metal, refractory metal oxide, ceramic, inter-metallic, or mixture thereof which is to become the coating. The enclosure also contains, or is subsequently filled with, a halide which is volatile at the temperature of heat treatment. The pack cementation process of coating includes thermal decomposition of the metallic halide followed by a thermal deposition and diffusion of the refractory metal or oxide onto the surface of, and into the surface of, the outer metal sheath of the heating element. Typical deposition stages may be represented as follows:
MC12 + H2 -» M(solid) Formation + 2HC1 -» M(solid) via Diffusion + 2HC1 Another particularly preferable coating process is that of physical vapor deposition. As described above, it is preferable to surface treat the heating element and then clean it by a means such ultrasonic cleaning or vapor degreasing before coating by a means such as physical vapor deposition.
After cleaning, it is preferable to plasma etch the surface of the heating element in argon under ultra-high vacuum and then sputter coat the surface with reactive or non-reactive gases in conjunction with the desired coating material. The operation of the aforementioned process at ultra high vacuum avoids the introduction of most impurities onto the heating element's surface. In addition, low to high voltage biasing can be applied to promote a strong adhesion of the coating to the surface of the metal sheath of the heating element. The coating will also usually become highly dense when this biasing method is employed. Pulsing technique can also be utilized for coating of semi-conductive material such as silicon carbide. The above process often allows the heating element to be processed at low temperatures, preferably less than 400 degrees Celsius. Yet another preferable method of coating the heating element is by use of chemical vapor deposition. The chemical vapor deposition technique (CVD) is based upon a chemical reaction, such as thermal decomposition (pyrolysis), between a gaseous phase and a heated surface of the element to be coated. The surface of the heating element is first directly or indirectly heated via induction heating. A gaseous phase comprised of a non-chlorinated organo-metallic, metal carbonyls or metal hydrides compound comprising a refractory metal. It is necessary to select a non-chlorinated precursor, such as tetramethylsilane. which can generate the desired coating at reaction temperatures below 750° C due to
the large mismatch in thermal expansion between the coatings and the materials of the heating element. Additionally, deposition temperatures below 750° Celsius eliminate formation of microcracks in the coatings during the cooling process. Typical deposition pressures are usually between 0.0132 atmosphere (10 torr) and 0.131 atmosphere (100 torr). Yet another preferred method of applying the coating to the surface of the metal sheath of the heating element is by gas diffusion via chemical vapor deposition. In this process, a halide of the solute metal is passed in vapor form over the metallic surface of the heating element to be coated, the surface being heated such that it is at a temperature at which diffusion can take place. Such temperatures are generally from about 400 to about 1000 degrees Celsius or more, depending on the precursor for the coating. The carrier gas for the halide vapor can be a reducing gas such as hydrogen, cracked ammonia, or the like or inert gases such as helium or argon. In this manner, the metal halide BX2 in gaseous form is reduced to metal B which then diffuses into the solvent metal A. The three major chemical reactions for this gas phase diffusion process can be summarized as follows: Interchange reaction: A + BX2 (gas) — > AX2 + B Reduction reaction: BX2 + H2 — > 2HX + B
Thermal dissociation reaction: BX2 — > X2 + B
The interchange reaction implies the removal of one atom of A at the surface for each atom of B deposited.
A typical gas phase diffusion of tantalum onto and into the metallic surface of the heating element can typically be processed at temperatures of less than about 800 degrees Celsius. Tantalum can be introduced into the deposition chamber by reacting heated tantalum chips or pellets with hydrochloric gas. Another preferred method of introducing tantalum into the deposition chamber consists of heating the tantalum chips or pellets to its sublimation temperature to form tantalum vapor. The tantalum vapor is then transported into the deposition chamber with an inert carrier gas such as argon. The deposition of tantalum can also be processed via gas or liquid precursor such as tantalum pentachloride or other derivatives of tantalum, provided that the thermodynamics for thermal decomposition of tantalum precursor can occur at temperatures less than 1000 degrees Celsius or preferably less than 500 degrees Celsius. The pressure used for this process can range from about full vacuum to about 0.98 atmospheres. As described above, the heating elements of this invention are particularly useful in corrosive environments. The heating elements of the invention will typically not show any pits, crevices, holes, or cracks indicating degradation of the coating even when subjected to pH's of less than about 1.0 (such as concentrated chromic plating solution and nitric acid) at boiling temperatures for more than 60, preferably more than 100 hours. The coatings of this invention may be modified by addition of such substances such as metals, non-metals, alloys, as well as, inorganic and organic compounds so long as such substance does not
destroy the functionality of the protective coating. Such modifications may often be desirable in order to alter one or more of the properties, for example electrical, thermo-mechanical, or chemical properties, of the coating for a particular application. For example, if a more ductile coating is desirable, then the coating may be doped with boron or titanium to promote such ductility. As another example, it may be desirable for some applications to add materials such as inter-metallic substances, for example, nickel suicide, which have a coefficient of thermal expansion which is relatively equivalent to the thermal expansion coefficient of a base material of the heater. In this manner, efficient heating is accomplished. In addition to being modifiable by the addition of other substance, the coatings of this invention may have more than one layer. This may be desirable if the heating element is to be subjected to a particularly corrosive environment for an extended length of time or if the heating element is unusually fragile. The use of layered coatings may reinforce the failure of the outer layers in case of a severe electrochemical attack. Often if multiple layers are to be utilized, the number of layers will range from one (1) to one hundered (100) or more, preferably one (1) to ten (10), depending on the specific application. The thickness of each layer may generally vary from about 100 to 5000 Angstroms and it may be desirable to use a material such as tungsten between the layers to enhance the adhesion of the layers to each other. Each layer may be of different or identical materials and optimum performance may be achieved via routine experimentation.
The coated heating elements of this invention may be easily inspected for defects such as microcracks, cracks, holes, or other failures in the coating. Such inspection may be accomplished in numerous ways. In one method used most often for oxide coatings, leakage current is tested by immersing the coated area of the heating element in deionized water and acidic solution in conjunction with a metallic electrode probe. A current, for example 12 volts, is applied to the uncoated area of the heater and metallic electrode probe. If the probe detects a voltage change, then a defect exists within the coating. Typically, the magnitude of the voltage change increases as the size of the defect. A second method useful for coatings which are conducting, semi-conducting, or insulating is that of potentiodynamic polarization.
If a coated heating element has a defect then it may be repaired in a similar manner to which it was coated. Briefly, the repair steps include surface preparation, cleaning, and then reapplying the coatings onto the clean surface of the heating element as described above. When cleaning the surface of a damaged heating element, it is preferable to ultrasonically clean the element with deionized water in order to neutralize any acid which may have become trapped in the element.
A schematic of a resistance heating device of the instant invention is described in Figure 1. The internal elements of the heating device comprise of a resistance wire (1), refractory or ceramic insulation (2), a Chromalox patented connector (3) used to control the overheating of the resistance wire (1) and a terminal block (4). The internal elements are protected by an outer metallic shell or sheath (5). The base
(6) of the metallic sheath comprises a heavy-gauge disk which is securely welded. A corrosion-resistant coating of the instant invention (7) surrounds the sheath.
Figure 2 shows as an embodiment of this invention, the high performance corrosion resistant coating (7) of refractory metals, refractory metal oxides, ceramics, inter-metallics. or mixture thereof which is applied onto and into the outer surface of the metallic sheath of the heating device is illustrated in 2a of Figure 2.
The coating can be single (2b of Figure 2) or layered (2c of Figure 2). According to this invention, the coating diffuses into the surface or grain boundaries of the metallic sheath or tube to promote a strong adhesion. The coating is 100 percent (%) dense. Each layer of the multi-layer coatings (2c of Figure 2) can be of identical or different materials selected from refractory metals, refractory metal oxides, ceramics, inter-metallics, or a mixture thereof. Example One
RF Bias sputtering was used to apply 100% dense amorphous silicon carbide to the surface of four steel tubes. On each steel tube, an undercoating layer of tungsten of 100 Angstroms thick was deposited prior to the silicon carbide coating to enhance adhesion. Silicon carbide was then deposited on each of the four steel tubes. The first steel tube was coated with a single layer of silicon carbide which was 3 micrometers in thickness. The second steel tube was coated with 10 layers of silicon carbide, each layer being 1000 Angstroms. The third steel tube was coated with 20 layers of silicon carbide, each layer being 1000 Angstroms. The fourth steel tube was coated with 30 layers of silicon carbide, each layer being 1000 Angstroms. Upon visual inspection, each steel tube was found to be free of microcracks and pinholes.
The silicon carbide coated steel tubes were tested by immersing them in chromic plating solution at boiling temperature for 48 hours. The chromic plating solution had a pH near 0.0 at boiling temperature. The average weight loss was 0.1 percent (%). Example Two
DC Bias magnetron sputtering was used to apply 100% dense tantalum to the surface of steel disks, tubes, and shells of cartridge and tubular type heaters. A typical heating element having a tantalum coating is shown in Figure 2. The sputtering atmosphere was argon. The temperature of the surface of the substrate was 250°C. The DC bias voltage was 500 volts to 4 kilovolts. The argon pressure was 1.31 x 10" atmospheres. The sputtering time ranged from 60 to 90 minutes. The resulting tantalum coatings were about 3 micrometers in thickness.
The tantalum coated steel disks, tubes, and shells of cartridge and tubular type heaters were examined for defects under a microscope at a magnification of 500 times and 5000 times. No defects- were observed and the tantalum coating adhered strongly to the surface of the metallic sheath..
The tantalum coated steel disks, tubes, and shells of cartridge and tubular type heaters were tested by immersing them in chromic plating solution at 67° C for 100 hours. The chromic plating solution had a pH of 1.0 at 67° C. The average weight loss was 0.005 percent (%).
The tantalum coated steel disks, tubes, and shells of cartridge and tubular type heaters were tested by immersing them in chromic plating solution at boiling temperature for 100 hours. The chromic plating solution had a pH near 0.0 at boiling temperature. The average weight loss was 0.09 percent (%).
Example Three
Ion beam-assisted deposition, in particular, electron beam evaporation is useful to apply 100% dense tantalum to the surface of steel disks, tubes, and shells of cartridge and tubular type heaters. The metal substrate was biased to enhance the adhesion of the coating. The bias voltage was 500 volts to 2 kilovolts. Low temperature processing of less than 250 degrees Celsius is useful for the heat sensitive surfaces of the outer sheaths. Tantalum chips, pellets, or a mixture thereof are evaporated under high vacuum by an electron beam. An ion beam of argon via an ion gun bombards the tantalum vapors onto and into the surface to be coated. Tantalum films which are 100 percent (%) dense, about 3 micrometers thick, defect-free, and have strong adhesion to the outer metal sheath are obtained using this deposition method of coating.
Example Four
A gas diffusion method was used to apply 100 percent (%) dense tantalum to the surface of steel disks, tubes, and shells of cartridge and tubular type heaters. The surface was first heated to temperatures ranging from 300 to 500 degrees Celsius. Tantalum chips or pellets were then heated to their sublimation temperature. Tantalum vapor was then transported to the deposition chamber with a carrier gas such as Argon. Hydrogen was also introduced directly into the deposition chamber during the gas diffusion process. A typical in-depth diffusion of 0.5 to 1 micrometers of tantalum into the surface of the shell heater was achieved within 30 minutes to two hours. Tantalum was also introduced into the deposition chamber by using tantalum pentachloride (gas) as a precursor associated with tantalum. Hydrogen was used to decompose tantalum pentachloride into tantalum at temperatures of less than 900 degrees Celsius.
Typical deposition parameters useful for high density Tantalum coating onto and into carbon steel sheath via gas diffusion - chemical vapor deposition are as follows: Etching gas: Hydrochloric (HC1)
HC1 Pressure: 0.0065 atmosphere (5 torr)
Etching time: 5 minutes
Etching temperature: 400 degrees Celsius
Flushing gas and pressure: Argon @ 0.026 atmosphere (20 torr) Flushing time: 15 minutes
Tantalum deposition parameters:
Total Pressure: 0.013 atmosphere (10 torr)
Argon Pressure: 0.0066 atmosphere (5 torr)
HC1 Pressure: 0.0066 atmosphere (5 torr)
Tantalum chips to sublimation temperature Deposition temperature: 500 degrees Celsius
Deposition time: 30 minutes
Diffusion temperature: 800 degrees Celsius
Diffusion time: 1 hour
Diffusion Pressure: 0.65 atmosphere (500 torr). The tantalum coated steel disks, tubes, and shells of cartridge and tubular type heaters of
Example Three were examined for defects under a microscope at a magnification of 500 times and 5000 times. No defects were observed.
The tantalum coated steel disks, tubes, and shells of cartridge and tubular type heaters were tested by immersing them in chromic plating solution at 67° C for 100 hours. The chromic plating solution had a pH of 1.0 at 67° C. The average weight loss was 0.1 percent (%).
The tantalum coated steel disks, tubes, and shells of cartridge and tubular type heaters were tested by immersing them in chromic plating solution at boiling temperature for 100 hours. The chromic plating solution had a pH near 0.0 at boiling temperature. The average weight loss was 0.5 percent (%).
Claims
1. In a heating device comprising an electrically conductive heating element with an outer metallic shell having a coating encompassing the surface of said outer metallic shell, the improvement which comprises a coating selected from the group consisting of refractory metals, refractory metal oxides, ceramics, inter-metallics, and mixtures thereof.
2. The device of Claim 1 wherein the refractory metal is tantalum, hafnium, niobium, or rhenium.
3. The device of Claim 1 wherein the ceramic coating is silicon carbide.
4. The device of Claim 1 wherein the inter-metallic coating is nickel suicide.
5. The device of Claim 1 wherein the heating device is an immersion heater.
6. The device of Claim 1 wherein the coating is from about 100 Angstroms to about 100 micrometers in thickness.
7. The device of Claim 1 wherein the coating comprises from about 2 to about 10 layers.
8. The device of Claim 7 wherein each layer of the coating is independently selected from the group consisting of refractory metals, refractory metal oxides, ceramics, inter-metallics, and mixtures thereof.
9. The device of Claim 1 wherein the coating is applied by DC Bias or pulse sputtering.
10. The device of Claim 9 wherein the coating is tantalum or nickel suicide.
1 1. The device of Claim 1 wherein the coating is applied by ion-beam assisted vapor deposition with bias substrate.
12. The device of Claim 1 1 wherein the coating is tantalum.
13. The device of Claim 1 wherein the coating is applied by gas diffusion.
14. The device of Claim 13 wherein the coating is tantalum.
15. The device of Claim 1 wherein the coating is applied by RF. DC, or pulse sputtering.
16. The device of Claim 15 wherein the coating is silicon carbide.
17. The device of Claim 1 wherein the weight loss of the coating is less than about 0.005 percent when subjected to an acidic solution at 67┬░ C for 100 hours.
18. The device of Claim 1 wherein the weight loss of the coating is less than about 0.1 percent when subjected to a boiling acidic solution for 100 hours.
19. The device of Claim 1 wherein the surface of the outer metal sheath is prepared by mechanical polishing, electropolishing, electroplating chromium, or a combination thereof prior to coating the sheath with a refractory metal, refractory metal oxide, ceramic, inter-metallic, or mixture thereof.
20. The device of Claim 1 wherein an undercoating is applied to the metallic shell before the coating.
21. The device of Claim 20 wherein the undercoating is tungsten and is from about 100 to about 10,000 Angstroms thick.
22. In a heating device comprising an electrically conductive heating element with an outer metallic shell having a coating encompassing the surface of said outer metallic shell, the improvement which comprises a coating selected from tantalum or silicon carbide wherein said coating is from about 100 Angstroms to about 100 micrometers in thickness.
23. A heating element comprising:
(a) a nickel-chromium resistance wire;
(b) a metal shell which surrounds the resistance wire and has refractory insulation between the resistance wire and the sheath;
(c) a connector operably connected to the resistance wire and adapted to connect to a power source;
(d) a corrosion-resistant coating encompassing the surface of said outer metallic shell wherein said coating is selected from from tantalum or silicon carbide and wherein said coating is from about 100 Angstroms to about 100 micrometers in thickness.
Priority Applications (1)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
AU62392/98A AU6239298A (en) | 1997-01-07 | 1998-01-07 | Improved coatings for electrical, metal sheathed heating elements |
Applications Claiming Priority (4)
Application Number | Priority Date | Filing Date | Title |
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US3577397P | 1997-01-07 | 1997-01-07 | |
US60/035,773 | 1997-01-07 | ||
US88406797A | 1997-06-27 | 1997-06-27 | |
US08/884,067 | 1997-06-27 |
Publications (1)
Publication Number | Publication Date |
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WO1998031197A1 true WO1998031197A1 (en) | 1998-07-16 |
Family
ID=26712487
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
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PCT/US1998/000438 WO1998031197A1 (en) | 1997-01-07 | 1998-01-07 | Improved coatings for electrical, metal sheathed heating elements |
Country Status (2)
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AU (1) | AU6239298A (en) |
WO (1) | WO1998031197A1 (en) |
Cited By (7)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
DE20121115U1 (en) * | 2001-12-21 | 2003-04-24 | Fritz Eichenauer GmbH & Co. KG, 76870 Kandel | Electrical heating arrangement used for heating corrosive liquid, especially a urea solution, in vehicle comprises heating element sealed by metallic housing coated with inorganic non-metallic layer containing boron, carbon and/or silicon |
US8602248B2 (en) | 2011-03-02 | 2013-12-10 | Bose Corporation | Cooking utensil |
US8796598B2 (en) | 2007-09-07 | 2014-08-05 | Bose Corporation | Induction cookware |
US9630206B2 (en) | 2005-05-12 | 2017-04-25 | Innovatech, Llc | Electrosurgical electrode and method of manufacturing same |
IT201800004346A1 (en) * | 2018-04-10 | 2019-10-10 | New systems of protection / coating of materials usable in various applications characterized by chemically or physically aggressive environments through the deposition of nano- and micro-metric layers on the external surface | |
DE102021117913A1 (en) | 2021-07-12 | 2023-01-12 | Lenz Elektronik Gmbh | Evaporator for generating steam for model vehicles |
EP4231777A1 (en) * | 2022-02-21 | 2023-08-23 | Fratini, Roberto | Resistance for washing machine |
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Cited By (10)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
DE20121115U1 (en) * | 2001-12-21 | 2003-04-24 | Fritz Eichenauer GmbH & Co. KG, 76870 Kandel | Electrical heating arrangement used for heating corrosive liquid, especially a urea solution, in vehicle comprises heating element sealed by metallic housing coated with inorganic non-metallic layer containing boron, carbon and/or silicon |
US9630206B2 (en) | 2005-05-12 | 2017-04-25 | Innovatech, Llc | Electrosurgical electrode and method of manufacturing same |
US10463420B2 (en) | 2005-05-12 | 2019-11-05 | Innovatech Llc | Electrosurgical electrode and method of manufacturing same |
US11246645B2 (en) | 2005-05-12 | 2022-02-15 | Innovatech, Llc | Electrosurgical electrode and method of manufacturing same |
US8796598B2 (en) | 2007-09-07 | 2014-08-05 | Bose Corporation | Induction cookware |
US8602248B2 (en) | 2011-03-02 | 2013-12-10 | Bose Corporation | Cooking utensil |
IT201800004346A1 (en) * | 2018-04-10 | 2019-10-10 | New systems of protection / coating of materials usable in various applications characterized by chemically or physically aggressive environments through the deposition of nano- and micro-metric layers on the external surface | |
DE102021117913A1 (en) | 2021-07-12 | 2023-01-12 | Lenz Elektronik Gmbh | Evaporator for generating steam for model vehicles |
DE102021117913B4 (en) | 2021-07-12 | 2023-07-06 | Lenz Elektronik Gmbh | Evaporator for generating steam for model vehicles |
EP4231777A1 (en) * | 2022-02-21 | 2023-08-23 | Fratini, Roberto | Resistance for washing machine |
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