US1793672A - Crystals and their manufacture - Google Patents

Description

Feb. 24, 1931.

P. W. BRIDGMAN CRYSTALS AND THEIR MANUFACTURE Filed Feb. is, 1926 Patented set. 24, 1931 UNITED [STATES PATENT OFFICE rune! w, rmemnn, or cmnamor, massacnusnrrs CRYSTALS THEIR MANUFACTURE Applicationfiled February 16, 1926. Serial 110. 88,650.

ticularly, molded large single crystals, there-- by produced. The present application is a m continuation in part of applications, Serial No. 687,147 filed January 18, 1924, and Serial No. 36,640, filed June 12, 1925.

To the layman, the word crystal connotesa regular, external form, comprising 16 plane surfaces arranged along the crystal axes according to some law of symmetry. Not all substanceshaving such regular form, however, are crystals; and a. crystal is none the less a crystal because it has lost its ex- 20 ternal' form, as by machining, or because it 7 never had such form to start with. The dis tinguishing characteristic of a crystal is its internal structure. It will conduce to an understanding of the present invention, there- 2 fore, to bear in mind that in a crystal of any substance, the molecules are arranged throughout that particular body according to a definite law of regularity that is characteristic of the particular substance in question;

though the fact should not be lost sight of that some substances may crystallize according to more than one law. Many crystals are weak along their planes oi cleavage or other characteristic plane s,\. so -much so that the mere act of machimng,

to change their exterior contour, is liable to cause the parts of the crystal to slide or otherwise move relatively to each other, thereby destroying the regularity of the crys- 4 talline arrangement, and the unitary nature of the crystal. It is therefore fortunate that many crystalline substances, like the metals,

are constituted of small crystal grains that V are intimately bound together, helter-skelter,

with their axes differently oriented, and bit.

ing into the impurities that are bound together with them; for this arrangement enables the metals to resist successfully the stresses .to which they are subjected, in use.

60 In the form of a single, large crystal, made in.

accordance with the present invention, the same metal will be found to'be, unable to withstand such stresses, but will yield along 1 its'gliding planes. A metal crystal of circularly cylindrical form, for example, if subjected to compression, may become shorter in length, but elliptical in cross section, or of some other cross-sectionalshape, depending upon the nature of the crystalline substance; Althou h weak mechanically and, therefore, not so esirable as an ordinary, multicrystalline metal where mechanical strength is necessary, a single crystal has many properties which multicrys'talline metal does not have.

Among these are lower electrical resistance,

much greater thermo-electric power in one direction than others, and much greater thermal expansion. It is, therefore, desirable, for many purposes, to be able to produce large, singlecrystals; and according to the purpose for which these crystals-are to be used, it is desirable to be able to give them any external form, and, also, sometimes, to orient the crystal axes in any desired way with respect to the external form. Since, in general, a single crystal is exceedingly weak mechanically, it can not be easily or economically machined to give it the desired external form, as machining tends to destroy the regularity of the. internal crystalline arrangement. It is therefore convenient to give it the external form desired by some operation of molding.

'A single crystal of the substance will naturally contain fewer impurities; for there is greater opportunity for impurities to become lodged between many irregularly small crystal grains than for them to become crystallized as part of a single crystal. A single crystal, furthermore will not have the piipc.

ily .no

and blow holes that are ordinarily so rea formed. in the metals, as ordinarily manufactured.

vention to provide an improved methodlof, and apparatus for, making crystals. It is a further object to provide an improved method of and apparatus for urifying substances, particularly metals. Xnother object is to improve u on present-day methods of, and apparatus or, manufacturing moldings,

It is therefore an object of the present insuch as moldings of the metals, so as to avoid the formation of pipes and blow holes. It is a further object to control the orientation of the crystal axes with respect to the mold, where this may be desirable. Other and further objects will be explairi ed hereinafter in connection with the accompanying drawings, and will be particularly pointed out in the appended claims. I

In the accompanying drawings, Fig. 1 is a diagrammatic view, partly in section, of apparatus constructed according to a preferred embodiment of the present invention; Fig. 2 is a detail view of a modified mold; Fig. 3 is a fragmentary view of a crystal made with the mold shown in Fig. 1; Fig. 4 is a section takenupon the line 4-4 of Fig. 3, looking .in the direction of the arrows; Fig. 5 is a view of a further modified mold; Fig. 6 is a diagrammatic view of circuits and apparatus for controlling the motor of Fig. 1; Figs. 7 and 8 are views of still further modified molds; and Fig. is a view of a modified apparatus.

The crystal of the present invention may be made in many ways. According to the preferred method of the above-designated applications, the first step is tc obtain, in molten form, the substance a molded crystal of which is desired or which it is desired to purify. The invention may, however, in its broader features, be practiced by first dissolving the substance in a suitable solvent. Most metals must be heated to melt them. and a vertically disposed mold 2, within which the substance is adapted 'to be contained, is therefore diagrammatically shown within a vertically disposed, cylindrical, electric furnace 4. The

mold 2 must, of course, be constituted of a refractory substance such that it shall withstand the heat of the furnace, while permitting the melting of the metal, and the refractory substance must .not be such as to become dissolved in the molten substance, or otherwise impart impurities thereto. For metals that liquefy at comparatively low temperatures, about 600 (3., more or less, such as lead, tin, zinc, cadmium, tellurium, bismuth, antivmony and, the like, the mold 2 may be constituted of glass. If the crystal is to be,

molded to a complicated shape, it may be desirable to make the mold of graphite, which can be more readily formed to complicated shapes than glass. For substances that liquefy above the softening point of glass, the mold 2 must be constituted of some other material suitable to the purpose in hand. Before the furnace 4 is heated, the position of the illustrated mold 2 therein is suitably adjusted so that the furnace, after heating, may melt, and maintain melted, all parts of the substance contained in the mold 2. If a very long mold is used, longer than the furnace, metal may be fed into the top of the mold as the mold is passed through the furnace, meltously. In either case, the metal will be thoroughly melted above the surface of dep osition so that crystallization may take place from a liquid from which all nuclei of the solid have been removed by thorou h melting, as will be understood from the escription to follow. Any well known means may be employed for controlling thetemperature of the furnace, such as a heating coil 6, in series with a variable resistor 8 and a source of energy 10. The mold'is now gradually lowered into the cooler, outside air (Fig. 1) through an opening 12 in ,the bottom of the furnace 4, without affecting the operation of the furnace. In practice, to avoid convection air currents, the mold may be protected from direct contact with the outside air by a pipe 14 or the like that is maintained tightly against the bottom of the furnace 4 in any desired way, as by means'of wedges 16. The

bottom 15 of the pipe 14 is closed'to prevent air currents from streaming upward through the pipe from below. The layers of the molten substance in the lower portions of the mold 2 will become solidified or crystallized upon leaving the furnace 4. A saturated solution may be used, instead, of a substance such that its solubility decreases as the temperature falls. The solute substance will'then crystallize out as the mold leaves the furnace,

depositing the crystal of the desired substance on the surface already crystallized. The deposition takes place at that surface whose temperature is the saturation temperature. Thus, sugar may be crystallized out of a saturated solution of sugar in some solvent, and salt out of a saturated salt solution. The solute will crystallize out of the solvent. Of course, the solvent may also be crystallized out, yielding a purified substance. An ad-.

vantage of crystallizing from solution is that it enables one to obtain crystals of some substances that can not be conveniently obtained from the melt, such as salt and sugar. Sugar decomposes before melting, and the melting point of salt is inconveniently' high, and, furthermore, is liable to become shattered by thermal contraction, as will be mentioned later.

If the bottom of the mold is of considerable magnitude, the metal will usually start crystallizing from a number of different nuclei at various places on the bottom,.as, for example, at 62 and 64, Fig. 3, and therefore the casting may consist of several grains, with a plane of separation 66 between them. In the case of some metals, however, crystallization may be more rapid from grains whichstart with a particular orientation, so that some grains grow at the expense of others, and it may result that in the upper part of the mold, one grain has gained the u per hand. When this happens, the casting isflieyond a certain point, all one crystal grain. If it is desired merely swam through the entire lengt of thecasting at small angles to the axis .68 of the mold, if they ever'get started; or it may be necessary to have the entire contents of the mold of one grain, without discardin any part. Hence;vv

1n most cases, it is desira le to rely on somethingemore positive than mere chance to compel the crystallization to start froma single nucleus. Thismay'be done in a'variety of ways. It is found that if the'lo'wer end of the mold is drawn out to a sharp point, as at 18, Fig. 1, crystallization, in the caseof many metals, is much more likely to start from a single nucleus at the sharp end. In order to function properly, the metal must run down cleanly into the sharp point; this may be conveniently accomplished by filling the mold in a vacuum, running the molten metal through a properly'disposed filter to remove stray particles of mechanical dirt. In fact, with any method, it is necessary to have the metal mechanically clean, because it is found that'particles of dirt are very apt to act 'as centers of new nuclei. If a mold with a pointed end 18 is used, the taper above the point must be so gradual that, when the mold emerges from the furnace, the temperature increases smoothly at greater and greater distances from the point, else new nuclei will be formed above the original surface of crystallization.

Crystallization from a single nucleus may also be effected by leading through the bottom wall of the mold. 2 a pointed metal or other wire 20, Fig. 2-(depending upon the temperature of the molten substance that it is desired to crystallize), of better thermally conducting material than thatofwhich1the mold 2 is constituted. This effects cooling of the molten metal around the pointed wire 20 by oondilction'of'the heatfrom the interior of the mold 2 to the outside, resulting in the -fi formation of -a single nucleus :at the point 1 where the wire 20 is positioned. If desired, the wire 20 may be artificially cooled from outside, so as to start the formation of a crys- -tallized nucleus before it is commenced to lower the mold 2 oujl; of the furnace 4. When the wire 20 is emp oyed-,'the bottom of the mold need not necessarilyibe sharp pointed,

as shown at 18, but mav be. rounded, as illus trated at 22 in Fig. 2.

forming a si le, unita lar e c al 24 having the sh p ie of the 131d. lhe dh d of the mold thus determines the shape 0 the crystal. All that is neoessaryto effect this result is to lower the mold 2 out of the furnace 4 at a speed so low as to afford each successively lowered horizontal layer of the molten or dissolved substance time to crys-- tallize before the next higher layer commences to crystallize. All parts of the substance 76 having the crystallizing temperature will then lie ona single, horizontal surface of deposition that advances into the molten or dis.- solved substance at a speed less than the crystallizing velocity. The lowering is deter- 80 mined by twofactors. the first place, the speed must be less than (or, possibly, equal to) the maximum velocity of crystallization of the substance. This is a charact'eristicof the substance and the particular (orientation of the crystal axes in the nucleus from which the crystallizing growth commenced. In the secorrd place, the speed must be low enough so that the heat set free by the crystallizing substance, due to its latent heat of melting, may be conducted away into the surrounding atmosphere, so that the crystal surface on which the new crystal molecules become deposited will not tend to rise to a higher temperature than the melting point. This speed therefore depends upon the size, the design and the materials of the apparatus and will, in general, be less for large molds and for molds made of poorly conducting substances. Both of these fac- 10o tors may, for brevity, be included in the general statement that the speed-must be lower than the-crystallizing velocity of the substance. Similar considerations ap l to crystals deposited from solution, as eunder- 1 5 stood by persons skilled in the art. I

'There'1s, of course, no harm done in low- 'ering the mold more slowly than the said crystallizing velocity; but a greater speed of lowering will cause the higher layers of the liquidto commence crystallizing from new nuclei while the lower layers are still liquid, with the result that the finally solidid mass may be constituted of a number of crystals, and not of a single 0 stal. As thelayers of the liquid in the conical end portion 18 of the mold 2,if a mold of var mg diameter is' employed,'will natural y crystallize more rapidly than the up erlayers, because being'of smaller area, t e mold 2 may be lowered at greater speed, to start with. The speed of lowering may be con trolled in any desired way, as by clock-work,

or by means of a variable-speed motor 25 having an armature 26, and the speed of which '1 worm 34 to a gear 36. The gear 36 is concentric with a worm 38 that meshes with a gear 40. This double worm-gear arrangement provides for a lowering speed of the mold 2 as smallas desired; say, one-half inch per hour. The conditions thus established are highly favorable to a similar orientation of the molecular crystal axes throughout the mold, and, therefore, to the formation of a single crystal in the mold. The gear 40 is rlgid with a shaft 42 upon which are slidably mounted two ears 44 and 46. The gears 44 and 46 are keyed to the shaft 42 so as to rotate therewith, andare adapted selectively to mesh with gears 48 and 50, respectively, thus providing for a further range in the lowering speed of the mold 2. The gears 48 and 50 are mounted upon a threaded shaft 52, the threads of-which engage in a nonrotating nut 54. The rotation of the shaft 52, therefore, effects the raising and lowering of an arm 56 that is integral with the nut 54, and from which the mold 2 is'suspended by alink or cord 58 that passes through an opening 60 at the top of the furnace 4. An extension 62 of the'nut 54is provided with an opening 64 within which a rod 67 is slidably mounted to guide the arm 56, and the mold 2 that is. suspended therefrom, against swinging movement.

It has been found by experience that, in some cases, and with some materials, 'a nucleus of one orientation will grow at the expense of nuclei of other orientations, so that that nucleus will eventually prevail, even I though there may have been several nuclei initially present.

This mold 74, though of somewhat more complicated construction, is more certain in operation, and more desirable for some metals. This mold consists of two portions, a lower chamber 78, the function of which is to start the crystallization, and which may be cut from the rest of the mold when the casting is completed, and the main part of the mold 74. The chamber 78 is separated from 7 6, and therefore the the part 74 by a narrow neck 76. Crystallization starts at the lower end of the chamber 78 from a number of nuclei but because of the slow rate of deposition, it will be found that the grains which grow from these nuclei are not microscopic in size, as in the case of many ordinary castings, but are larger, perhaps of the order of a millimeter or more in diameter.

If, however, the neck at 76 is made smaller than these grains, perhaps @millimeter in diameter, it will result that, except under especially unfavorable conditions, onlyone of the grains in the chamber 78 is properly situated to grow through the neck 76, so that crystallization in the chamber 74, which is the main crystallizing chamber, apd has the shape to which it is desired to mold the crystal, starts from a single gram at the neck entire main part of the mold crystallizes as a single grain. The mold may be readily filled by first exhausting the air from. the mold, and then allowing the molten metalto flow in, driven by external air pressure. The mold'74, with the substance that it is desired to crystallize, in solid form, is placed in the furnace 4, after which the air is exhausted by a pump from the mold 7 4. The furnace 4 is then heated to melt the substance contained in the mold 74. The

of the mold. This structure, therefore, ren- 80 ders it possible to prevent air becoming trapped in the body of the mold 74, which is particularly necessary when a small mold is used to manufacture small crystals. The

method of erystallizing is the same as before 5 described, the small neck 76 insuring that the substance shall crystallize from a nucleus at that point,into a single crystal.

A modification of the procedure outlined above has been found desirable in thec'a'se of bismuth, which differs from the other metals mentioned above "in'that it expands on solidification, instead of contracting. It has been found that it is much more difiicult to produce single crystalsvof bismuth by the method above than of the other metals. The reason is that the solid bismuth, whlch is deposited on the bottom of the mold, is lighter than the liquid above it, so that particles of the solid tend to detach themselves from the surface of deposition and, rising through the liquid, act as new nuclei of crystallization ofan orientation different from that of the original crystal. 'This difliculty may be avoided by causing the solid to deposit itself on the top of the mold instead of the bottom.

In order to effect this, the mold may be raised through the top of the furnace, instead of through the bottom, using for the purpose a special form of mold. This mold is shown in Fig. 7 and is essentially the mold of Fig. 5 inverted, though the other illustrated molds could equally well be used inverted. At the upper end, is .a small chamber 7 8, connecting I by a narrow neck 76 with the main crystalhzing compartment 74, which is provided at the lower end with a narrow opening 80. The tube is filled with'molten metal by first exhausting the air, and then allowing the molten metal to flow in through the bottom under external air pressure, thus filling the entire tube by barometer action. The neck 80 is so small that air bubbles can not pass into the chamber, and the liquid. is maintained in position by the external air pres- '125 sure. As crystallization proceeds, the volume of metal in the mold increases, and liquid metal drops out through the neck 80 in com-- pensation. Crystallization may start in many nuclei at the extreme upper end of the mold,

but the neck orchannel 7 6 'is so fine that only one of the crystals produced from these nuclei grows into the neck, so that crystallization in'the main crystallizing compartment 74 starts from a single nucleus, and only one crystal grain is formed in the main compartment.

The procedure outlined forthe other metals I is now duplicated for bismuth (or other simisolution, where the crystallized substance is lighter than the liquid solvent. If crystallization starts at haphazard, as

described above, the crystal axes will have a haphazard orientation with respect to' themold. It will be found, in practice, however, that vthere is usually a preferred direc tion of orientation, so that the majority of the castings made by this method will have the axes approximating to some special direction. For the low-melting metals mentioned above, it will be found that the principal cleavage or slip plane prefers to stand perpendicular to the plane on which the crystal is deposited. ,The strength of this preference is much greater in crystals of large than of small diameter cases of highly. oblique incidence of the cleavage planes are not uncommon in the smaller-diameter castings. In order to control the direction of the crystalline axes, the mold illustrated in Fig. 8 may be used. It is, in the first place, necessary to start from a seed crystal of the desired --orientation. This seed may be obtained by selecting from a large number of castings of small diameter one which. has the desired orientation. These castings may, if desired,

. be made of one of the methods already described. This seed is placed in a long slender projection 82 attached to thelower end of the main part of the mold 84. The main part of the mold is now placed in the electric furnace,

with the projection 82 extending out into the air. The mold is now charged with metal and melted. After the charge is completely melted, the mold is drawn up partially into the furnace, so that the upper part of the seed crystal .is melted, but not the lower part. The mold is now-lowered in the regular way by the slow lowering mechanism, and crystallization begins from the upper end of the seed "crystal, which forces the material freshly deposited to have the same crystal orientation as itself, so that the orientation of the crystal form at the freezing formed in the mainpart of themold 84 thereby Controlled;

Some substances, particularly various salts,

become torn into a numberof pieces as the mold is lowered out of the furnace and into the outside air. This is becausethe temperature gradient between the furnace and the,

outside air is so great that the newly solidified substance, which is less strong that the metals, can not withstand the differential thermal expansion resulting from their pass-- ing through this region. Single crystals 01 these substances may, however, be produced, by lowering the mold from the furnace 4, not into the outside air, but into a second furnace *86, as shown in Fig, 9. The furnace 86 preferably comprises two windings, eachwith its own control, as illustrated, with no separating material between the two parts of the double furnace. The furnace dis maintained heated to a' temperature slightly above the melting temperature, and the furnace 86 at a temperature slightly below. After the mold has been completely lowered into the furnace 86, the mold is held stationary and the temperature ofthe furnace 86 is slowly reduced to that of the outside air. Large thermal distortions resulting from steep temperature gradients are thus avoided and the 1 crystals are thus prevented from becoming shattered. It will be readily understood that multicrystalline, unshattered solids may similarly be produced at a more rapid rate by increasing the speed of lowering of the mold.

It has been stated that crystals ma be produced either from themolten state or rom solutions. There is still another method of production. Some metals, like zinc and tin, crystallize from the melt in one crystallme in the case of'tin at, on further cooling, as the mold is lowered further from the furnace (to 161 in the case of tin) the crystal formed from the melt is-spontaneously transformed to another sort of crystal. If the crystal resulting. from direct solidification out of the melt were to be maintained in the form thus produced, as by means of the apparatus illustrated in Fig. 9, it would be of I uite a different character from that obtain le by lowering the mold directl from the, furnace 4 into the outsideair. .T e space lattices of the two crystals are in general quite differtemperature (232. G.

cut. The final crystal of such a substance therefore the product of two transitions, first from the melt to the one form of solid, and

then, at a temperature between the melting point and 'ordina temperatures, from this as .it isto control the first transition if a single crystal is to be produced; and exactly the same methods of control ahovedescribed form of solid to the other form of solid. It a 'is' just as necessary tocontrol the second transition, from the high-temperature form v of solid to the lowgtemperature modification,

are applicable. In the apparatus of Fig. 1, the two transitions take place successively and automatically at the appropriate temperatures as the mold is lowered out of the furnace and into the outside air. The first transition may, for example, take place at 88, Fig. 1; and the -second transition when the mold leaves thefurnace 4. v

In the case of zinc and tin, in which there is little internal resistance to the change from the high-temperature form to the low-temperature form, the final crystal is always of one form at room temperature. Some substances, however, in which the internal resistance to the transition is greater, may exist atroom or other temperatures in two or more distinct forms. The production of one or another of the desired forms may be controlled in the apparatus 9) by suitably controlling the lowerin of the furnace 86 to room temperature a ter a single crystal has been produced from the melt.

There is also another way to handle such substances. When a bar of ordinary tin, which has a transition at 161 C., is heated to between 161 C. and its melting temperature, 232 C., it will be transformed into the high-temperature modification. If this bar is'now placed in an electric furnace. maintained at 200 C., and slowly lowered, as above-described in connection with the lowering of molds, thelower end of the bar will begin to become transformed into the lowtemperature modification at 161 C. The lower endof the bar may be made pointed, like the lower end of the mold shown in Fig. 1, and if'the lowering is slow enough, a single crystal will be produced in the same manner as before described. It is thus possible to produce crystals of some substances without melting and withoutlusingmolds.

In all cases, the substance is moved from a higher-temperature region to a lower-temperature region'in which the molecules have a different arrangement. On passing fre n the region in which the higher temperature prevails into the lower-temperature region, the molecules rearrange themselves. If the operation is slow enough, they will so rearrange themselves that they shall all form parts of the same crystal lattice, or one large crystal grain. This is true whether the hightemperature condition is that of molten metal, or a solution or another crystal modification. In the two first-named cases, a mold is necessary; in the third, it is not.

In order to 'avoid circum'locution of language the term freezing temperature or its equivalent will be employed in the claims to denote freezingor transition temperature, including the change from one crystal solid to another, including also, separation from solution. v I I Crystals made in accordance with the present invention are much purer than the ordinary manner. It should be noted, however, that'when the described method is used for purposes-of purification, the speed of lowering depends not only upon the two factors already'explained, butalso upon the rapidity with which the impurity will dif fuse away from the surface of deposition, towardsthe top of the mold, through the molten substance. For this reason, the speed may be less than when the only. object in view is to form a single crystal, whether or not relatively pure. On the other'lrand, if the sole object aimed at is purification, and if the impurity is such that it diffuses rapidly, the lowering may be more rapidthan is required forthe production of a single crystal. Under some circumstances, it may be desirable to stir the liquid to-hasten the segregation of the impurities. v

Not only is it possibleto obtain very pure, single crystals, of any desired shape and size, by the method ofthe present invention, but the molded crystals so produced will be free from pi es, blow holes, or other flaws. The unitary c aracter of the crystals is readi- 1y demonstrated in various ways, for the crystals possessproperties that are not possessed by similar masses of the same substances that are solidified in the ordinary way. v

It will be understood that the invention-is susceptible ofconsiderable modification and change by persons skilled in'the art, andall such are considered to fall within the spirit and sco of the invention. It is therefore intende to express in the appended claims all the novelty that the invention may possess.

The molded crystal made in' accordance with the present invention is not claimed herein, as 1t forms the subject matter of the aforesaid application, Serial No. 36,640. I -What is claimed is 1. A method of solidifying a molten substance that comprises placing a seed crystal adjacent to the substance, melting a portion stance that comprises placing a seed crystal adjacent to the substance with the axis of the seed crystal oriented in a predetermined diloo = the molten substance, limiting the speed of rection, advancing the surface of crystallization of the substance from the seed crystal seed crystal driented in a predetermined direction, melting a portion of. the seed crystal adjacent to the substance, advancing the'surface of deposition from the seed crystal into the substance, whereby the axis of the resulting crystal will have the same orientation as the seed crystal, and controlling the speed of advance of the surface. 4'. A machine for crystallizing a substance,

having, in combination, a mold having a body portion and a slender projecting portion, the.

projecting portion being adapted to contain a seed crystalwith the axis of the seed crystal oriented in a predetermined direction, and the bodyportion being adapted to contain a substance, means for melting a portion of the seed crystal adjacent to the substance, and

means for crystallizing the substance from the seed crystal outward with the substance,

whereby the axis of the resulting crystal will have the same orientation as a seed crystali I 5. A machine of the character described having, in combination, a mold having a body P0113011 adapted to contain a substance and a slender (projecting portion adapted to contain a see crystal, means for relatively movmg a portion of the projecting portion from a region that is maintained below the freezing temperature of the substance into a region that is maintained at or above the freezing.

temperature of the substance, whereby a portion of the seed crystal is melted, and means for moving the mold from the second-named region into the first-named region to crystallize the substance.

6. A method .of crystallizing a molten substance that comprises advancing the sur-- face of crystallization of the substance into advance of the said surface into the molten substance to a value smaller than the crystal-V hzmg velocity of the substance, and controlling the orientation of the crystaljaxes.

7. A machine for crystallizinga substance having, in combination, a furnace maintained at a temperatureabove the freezing temperature of the substance, a furnace maintained at or slightly below the freezing temperature of the substance, a mold adapted to. contain a substance, means for moving the mold from the firstrnamed furnace into the second f.nam6d-fl1m8$, and means for changing the ing temperature of the substance into a region that is maintained at or slightly below the freezing temperature of the substance, and

changing the temperature. of the secondnamed region. a

9. A'method' of crystallizing a substance that comprises relatively moving a mold con-.

taining the substance from a furnace that .is

maintained at a temperature above the freezing temperature of the substanceinto a furnace that is maintained at or slightly below the freezing temperature of the substance,-

controlling the speed of movement of the mold, and gradually changing the temperature of the second-named furnace.

10. A method ofcrystallizinga substance that comprises advancing the surface of deposition of the substance, controlling the speed of advance of'the surface to produce a single crystal of the substance, then redepositing the substance along new sufifa'cs of deposition to produce a crystal of the substance. of different form fromithe first-named crystal. 11. A method of solidifying a substance in a mold that comprises placing a seed crystal adjacent to the substance with the axis of the seed crystal oriented in a predetermined direction with respect to the 'mold, melting a 1 portion of the seed crystal adjacent to the substance, and advancing the surface of solidification from the see crystal into the-substance. whereby the axis of theresulting crystal will have. the same orientation with respect to the mold as the seed crystal.

. 12. A method of solidif 'nga substance that is in liquid form that comprises crystal lizing a nucleus of the substance, successively subjecting successive portions of the substance while in liquid form and that are successively farther and farther removed from the nucleus to a temperature at or below the freezing temperature of the substance, and

controlling the speed at which the said suc-' cessive portions of the substance are subjected to the said temperature.

13. 'A method of crystallizing a substance ing to a definite law of regularity into ac'rystal the molecules of .which are-arranged according to another definite law of regularity, the said method comprising crystallizing a .11 the molecules of which are arranged accordthe substance, and controlling-the speed at whichth'e saidsuccessive .portQons oftlie 'subff I stance are subjected to thesa d temperature temperature of the second-named furnace. to maintain each'ofthe. sard'successlvepor-iis tions of the substance in condition such that the molecules thereof are arranged according to the first-named definite law of regularity until after the next preceding portion of the substance has become crystallized according to the-said other definite law of regularity.

14. A method of thecharacter described .that comprises successively solidifying successive surface layers of successively increasmg area of a molten substance at a speed such that each of the said successive surface layers becomes solidified before the next succeeding mg area of a substance to a temperature at or substance having, in combination, means for i from thenucleus, means for controlling the below' the freezing temperature of the substance at a speed such that each of the said successive surface layers become solidified before the next succeeding surface layer becomes solidified.

17. A method of the character described that comprises successively subjecting, successive layer of successively increasing area of a substance the molecules of which are arranged according to adefinite law of regular- 1tyto a temperature at or below the freezing:

temperature of the substance to cause the molecules to become arranged according to another definite law of regularity and maintaining each of the said successive layers in condition such that the molecules are arranged according to the first-named definite law of regularity until after the molecules of ,the next preceding layer have become arranged according to the said other definite law of regularity.

18. A method of the character described that comprises relatively moving a mold containing a substance from a region that is maintained above the freezing temperature of the substance into a region that'is maintained at or below the freezing temperature of the substance at a. speed of movement less than the crystallizing velocity of the substance.

19. A machine for crystallizing a molten crystallizing a nucleus of 'the substance and for advancing the surface of deposit on into the molten substance from the nucleus along adjacent portions of the substance that are successively farther and farther removed speed of advance of the surface to maintain each of the said successive portions of the substance in molten condition until after the next preceding portion of the substance has become crystallized, so as to aiford each successive layer of the substance time to crystallize before the next surface layer commences to crystallize, and means for maintaining the successive portions of the substance in molten condition until the next preceding portion of the substance has become crystallized.

20. A machine for crystallizing a molten substance having, in combination, means for advancing the surface of deposition into the .molten substance along successive portions of the substance, means for controlling the speed of advance of the surface to permit each of the said successive portions to crystallize to the next previously crystallized portion of the crystal in the crystal arrangement already prevailing inthe said portion of the crystal, so as to afford each successive layer of the substance time to crystallize before the next surface layer commences to crystallize, and means for maintaining the successive portions of the substance in molten condition until the next preceding portion of the substance has become crystallized.

21. A machine for solidifying a molten substance having, in combination, means for crystallizing a nucleus of the substance and for successively subjecting successive portlons of the substance that are successlvely farther and farther removed from the nucleus to a temperature at or below the freezing or transition temperature of the substance, and means for controlling the speed at which the said successive portions of the substance are subjected to the said temperature.

22. A machine for crystallizing a substance having, in combination, means for crystallizing a nucleus of the substance and for successively subjecting succcessive portions of the substance that are successively farther and farther removed from the nucleus to a tem-- perature at or belowthe freezing temperature of the substance, and means for controlling 23. A machine of the character'described having, in combination, a mold adapted to contain asubstance, means for relatively moving the mold from a region that is maintained above the freezing temperature of'the substanceinto a region that is m "ntained at orv below the freezingtemperature of the substance, and means for limiting the movement of the moldto a speed comparable-to the crystallizing velocity of the substance. I

24. A machine of the character described having, in combination, amold adapted to contain a substance, and means for relatlvely moving the mold from a region that is mamtained above the freezmg' temperature of the the substance.

26. A machine of the class described having, in combination, a furnace having an opening in its bottom, a vertically disposed mold adapted to contain a'substance, means for relatively moving the mold out of the furnace through the opening, and means for limiting the movement of the moving means to a speed comparable to the crystallizing velocity of the substance.

27. A machine of the class described having, in combination, a furnace, a mold adapted to contain a substance, means for moving the mold into and out of the furnace, and

means for limiting the rate of movement of the mold to a speed comparable to the crystallizing velocity of the substance.

'28. A machine of the class described having, in combination, a furnace, means for controlling the temperature of the furnace, a

mold adapted to contain a substance, means for moving the mold into and out of the furnace, and means for limiting the rate of movement of the mold to a speed comparable to the crystallizing velocity of the substance.

29. A method of solidifying a molten substance that comprises advancing the surface of solidification into the molten substance at a speed low enough to afford each successive surface layer of the-substance time to crystallize before the next surface layer commences to'crystallize, and maintaining the successive portions of the molten substance in molten condition until the next preceding portion of the substance has become crystallized.

I 30. A method of solidifying a molten substance that comprises crystallizing a nucleus of the substance, advancing the surface of deposition into the molten substance from the nucleus along adjacent surface layers of the substance that are successively. farther and farther removed from the nucleus at a speed low enough to afford each successive surface layer of the substance time to crystallize before the next surface layer commences to crystallize, and maintaining the successive portions of the molten substance in molten condition until the next preceding portion .of the substance has become crystallized.

31. A method of crystallizing a molten sub-- stance that comprises advancing the surface of crystallization of the substance into the molten substance, limiting the speed of advance of the said surface into the molten substance to a value smaller than the crystallizing velocity of the substance, and maintaining the successive portions of the molten substance in molten condition until after the next preceding portion of the substance has become crystallized. v

32. A methodof crystallizing a molten substance that comprises' subjecting successive molecular layers of the molten substance to a temperature at or below the freezing temperature of the substance, and limiting the speed at which the said successive molecular layers are subjected to the said temperature to a value less than the crystallizing velocity of the substance.

A machine for purifying an impure -molten substance by crystallization having, in combination, a furnace, a mold adapted to contain the substance, means for relatively moving, the mold out of the furnace to advance the surface of deposition into the substance and means for controlling the speed of relative movement of the mold to maintain the respective successive portions of the substance in molten condition until the impurities contained in the substance have successively diffused away from the said respective portions. I

34. A method of purifying an impure substance by crystallization that comprises relatively moving a mold containing the substance from a region that is maintained above' the freezing temperature of the substance into a region that is maintained at or below the freezing temperature of the substance,

and controlling the speed of relative movement of the mold to afford the impurities contained in the substance time to diffuse from the surface of deposition through the substance.

35. A method of crystallizing a molten substance that comprises relatively moving the molten substance from a region that is maintained above the freeing temperature of the substance into a region that is maintained at or below the freezing temperature of the substance at a relative speed of movement of the molten substance less than the crystal lizing velocity of the-substance.

36. A method of crystallizing a substance the molecules of which may, at one temperature, be arranged according to a definite law of regularity into a crystal the molecules of which may, at another temperature, be arranged according to another definite law of regularity, the said method comprismg subjecting successive molecular layers of the substance to a temperature at or below the sald other temperature, and limiting the speed at which the said successive molecular layers of the substance are subjected to the said other temperature to maintain each of the said successive molecular layers of the substance in condition such that the molecules thereof are arranged according to the firstnamed definite law of regularit until after the next preceding molecular ayer of the substance has become crystallized according to the said other definite law of regularity.

37. A machine for solidifying a molten substance having, in combination, means for advancing the surface of solidification into the molten substance, means for controlling the speed of advance of the surface so as to afford each successive layer of the substance time to crystallize before the next surface layer commences to crystallize, and means for maintaining the successive portions of the substance in molten condition until the next precedin portion of the substance has become crysta ized.

38. A machine for crystallizing a molten substance having, in' combination, means for advancing the surface'of deposition of the substance, means for limiting the speed of advance of the surface to a value smaller than the crystallizing velocity of the substance,

and means for maintaining the successive portions of the substance in molten condition until the next preceding portion of the substance has become crystallized.

39. A machine for solidifying a molten substance having, in combination, means. for crystallizing a nucleus of the substance and for advancing the surface of deposition into the substance from the nucleus along adjacent surface layers of the substance that are successively farther and r farther removed from the nucleus, means for limiting the speed of advance of the surface to a value low enough to afford each successive surface layer of the substance time tocrystallize before the next surface layer commences to crysist tallize, and means for maintaining the successive portions of the substance in molten condition until the next preceding portion of the substance has become crystallized.

40. A method of purifying an impure molten substance that comprises advancing the surface of deposition into the substance, controlling the speed of advance of the surface of deposition to afford the impurities contained in the substance time to diffuse from PERCY W. BRIDGMAN;

the surface of deposition through the substance, and maintaining the successive portions of the substance in molten condition until after the impurities have diffused from the next preceding portion of the substance.

41..A1 machine for purifying an impure molten substance having, in combination, means for relatively movmg the molten substance from a "region that is maintained above the freezing temperature of the substance into are 'on that ismaintained at or below- 1 the .freezlng temperature of the substance to advance the surface of deposition of the substance, and means for controlling the speed of relative movement of the. substance to maintainthe respective successive portions of the substance in molten condition the

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