WO2007019332A1

WO2007019332A1 - Apparatus and methods for combustion synthesis of nano-powders

Info

Publication number: WO2007019332A1
Application number: PCT/US2006/030486
Authority: WO
Inventors: Alexander Mukasyan; Peter Dinka
Original assignee: University Of Notre Dame Du Lac
Priority date: 2005-08-04
Filing date: 2006-08-04
Publication date: 2007-02-15
Also published as: WO2007019332A9

Abstract

Apparatus and methods for the combustion synthesis of a nano-powder are disclosed. The apparatus includes a carrier substrate (14) an a solution applicator (21) for impregnating the carrier (14) with a reaction solution. A dryer (22) is adapted to remove at least a portion of a free fluid (e.g. water) from the impregnated paper carrier (14). The dried impregnated carrier (14) is then conveyed to a combustion chamber 926) wherein the impregnated paper (14) is ignited, and a combustion reaction is initiated and maintained. The resultant combustion synthesis of the impregnated carrier (14) forms a nano-powder (48) with a high surface area for use as desired.

Description

APPARATUS AND METHODS FOR COMBUSTION SYNTHESIS OF NANO-POWDERS

Cross Reference to Related Application

[0001] This application is a non-provisional application claiming priority from U.S. Provisional Application Serial No. 60/705,197, filed August 4, 2005, entitled "Combustion Synthesis of Nano-Powders" and incorporated herein by reference in its entirety.

Field of the Disclosure

[0002] The present disclosure relates generally to the synthesis of materials using impregnated paper combustion and more particularly to apparatus and methods for combustion synthesis of nano-powders.

Background of Related Art

[0003] Self-Propagating, High-Temperature Synthesis (SHS) is a known process and generally includes a mixture of metals or compounds that will react exothermically in a self- propagating reaction to form a final compound or alloy. In a typical SHS system, the initial reaction medium is a mixture of particles with a size scale in the approximate range of 1 to 100 microns (μm). After reaction initiation, this mixture burns at a relatively high temperature, for example, greater than 2000°K, in a self-sustained manner to form the desired compound. While providing a rapid technique for the synthesis of powder with well-defined crystalline structures, the particle size and high temperature generally make it difficult to obtain nano-scale (e.g., high surface area) products.

[0004] One example process utilized to attempt to create nano-scale products is the combination of SHS and a reactive solution. This process in commonly known as a solution (or aqueous) combustion synthesis (SCS) method. The SCS process typically involves a reaction in liquid solution of metal nitrates and different fuels, which can be classified based on their chemical structure, i.e., their type of reactive groups (e.g., amino, hydroxyl, carboxyl, etc.) bonded to the hydrocarbon chain. These fuels reacting with oxygen containing species, formed during nitrates decomposition, provide rapid high temperature interaction in the system. More specifically, after preheating to relatively moderate temperatures, for example, approximately 150 to 200°C, the reaction may be initiated in the aqueous solution leading to the formation of fine solid particles.

[0005] In the aqueous SCS method, however, the high combustion temperatures and relatively long post-combustion zone (e.g., approximately seconds), typically result in the synthesis of particles on the order of approximately 50 to 500 nanometers (nm). Furthermore, it is oftentimes difficult to continuously produce such particles by reaction in an aqueous solution. Finally, a variety of low-exothermic solution systems, which lead to the formation of unique phases, cannot be produced in a self-propagating combustion mode by utilizing the SCS method.

Brief Description of the Drawings

[0006] FIG. 1 is a front elevational view of an example apparatus for continuous synthesis of a nano-powder.

[0007] FIG. 2. is a schematic illustration of the example apparatus for continuous synthesis of a nano-powder of FIG. 1.

[0008] FIG. 3 is an illustration of an example combustion synthesis process, [0009] FIG. 4 is an illustration of an example nano-powder formed by the example apparatus and methods.

[0010] FIG. 5 is a flowchart of an example method that may be performed during operation of the apparatus of FIG. 1.

Detailed Description

[0011] The following description of the disclosed embodiment is not intended to limit the scope of the invention to the precise examples detailed herein. Instead the following description is intended to be illustrative of the principles of the invention so that others may follow its teachings.

[0012] FIG. 1 illustrates an example apparatus 10 shown in a front elevational view. FIG. 2 similarly illustrates the apparatus 10 of FIG. 1 with slight differences as will be described below. Referring to both FIGS. 1 and 2, the apparatus 10 provides a sustainable combustion synthesis of nano-powders, for example inorganic nano-powders, by utilizing impregnated paper combustion. The apparatus 10 includes three basic operational portions: impregnation; drying; and combustion. In one example, a reaction solution is impregnated into a substrate by spraying a controlled quantity of aqueous reaction solution, for example, metal nitrate and glycine, onto a carrier substrate, for example paper. At least a portion of any free fluid (e.g., water) portion of the solution is removed from the substrate by heating and evaporating in a controlled drying process, such as with a convection process. The impregnated and dried substrate is then ignited and subsequently burns in a sustainable combustion process. As a result of combustion, the substrate is substantially completely burned out to produce a gas phase product, such as for example, carbon monoxide and/or carbon dioxide, while combustion of the reaction solution results in the formation of a nano-powder with desired compositions.

[0013] In the illustrated example of FIGS. 1 and 2, the impregnation portion of the apparatus 10 includes a carrier substrate, such as for example, a paper 14, a substrate conveyor 16, and a solution application, which in the example of FIG. 1, comprises a solution reservoir 18, a pump 19, and a spray applicator 20. The paper 14 is provided as a paper roll and is mounted to a spindle for free rotation, such that the conveyor 16 may withdraw the paper from the roll at a controlled rate and speed. The conveyor 16 may be any suitable conveyor adapted to transfer the paper 14 from the roll through the apparatus 10. In this example, the conveyor 16 includes a drive portion and a support surface (not shown). The drive portion of the example conveyor 16 includes two continuous parallel lengths of steel roller drive chains, a pair of drive sprockets, numerous idler sprockets, and a DC motor, although any suitable drive mechanism may be utilized. The drive sprockets include a common shaft and are driven by a fractional horsepower DC motor at, in this example, approximately 0 to 3 meters per second. A slip clutch may also be provided to protect the operator and/or the apparatus 10 in the event of a material blockage. The support surface of the example conveyor 16 is a foraminous surface formed by a plurality of steel rods lying between parallel chain links to provide support to the paper 14 through the drying portion as described below.

[0014] The reservoir 18 is adapted to store a desired aqueous reaction solution 21 for application on the paper 14. In this example, the solution 21 comprises a metal nitrate and glycine mixture, however, persons of ordinary skill in the art will appreciate that various oxidizer and fuel combinations may be utilized. An optional heater 25, such as for instance, a hot-plate may maintain the solution 21 at an elevated temperature if desired. The pump 19 supplies pressurized solution 21 to the spray applicator 20 for delivery of the solution 21 to the paper 14. In this example, the pump 19 is a peristaltic pump, although any pump may be utilized.

[0015] The example spray applicator 20 of FIG. 1 includes a pressure regulating circuit 23, comprising a pressure regulating valve, a pressure switch, and a reservoir return line. The pressure regulating circuit 23 maintains a substantially constant pressure in the fluid system as well as providing a solution agitation to assist in maintaining the mixture of the solution 21. The pressure regulating circuit 23 also includes a shut-off valve, flow control valve, and spray nozzles. Accordingly, the circuit 23 provides consistent, repeatable control of the rate of fluid application to the paper 14. [0016] The illustrated example of FIG. 2 includes an alternative example of the impregnation portion of the apparatus 10, wherein the impregnation portion does not include the pump 19 or the spray applicator 20, but rather impregnates the paper 14 by submersion. In particular, the example reservoir 18 of FIG. 2 is adapted to store the aqueous reaction solution 21 for application on the paper 14. In this example, the reservoir 18 is sized so that the conveyor 16 and the paper 14 are submersible into the solution 21. Similar to FIG 1, the reservoir 18 may include a heater 25 to maintain the solution 21 at an elevated temperature. [0017] Referring again to both FIGS. 1 and 2, the drying portion of the apparatus 10 includes a dryer 22 for removing at least a portion of the free fluid (e.g., water) from the paper 14. The example dryer 22 is a convection dryer including an enclosure 31, an exhaust plenum and damper (not shown), with an HVAC style electric furnace 33 comprising a blower motor, resistance heating element, and thermostatic control components. In operation, ambient air is heated as it passes through the electric furnace and is directed through the enclosure 31 in a direction perpendicular to that of the movement of the paper 14 by a plurality of fans 35. It will be appreciated by persons of ordinary skill in the art that the dryer 22 may be any type of dryer, capable of evaporating or removing fluid from the impregnated paper 14 without initiating a combustion reaction in the paper 14. After the impregnated paper 14 is sufficiently dry, the paper 14 proceeds to the combustion portion of the apparatus 10 for creation of the desired product.

[0018] The combustion portion of the apparatus 10 is where the impregnated paper 14 is ignited and burns in a sustained combustion process producing a gas phase product and a synthesized nano-powder. The example combustion portion includes a pair of rollers 24 positioned at the discharge end of the dryer enclosure 31 and a combustion chamber 26. The example rollers 24 operate at a rotational speed to match that of the conveyor 16 and the paper 14. The rollers 24 additionally provide a seal against the combustion chamber 26 to isolate the inside of the chamber 26. By maintaining contact between the paper 14 and the roller 24, combustion of the paper 14 is confined to the interior of the chamber 26. [0019] The illustrated combustion chamber 26 is a fully enclosed chamber, with an inlet port 38 and an outlet port 40. The inlet port 38 and outlet port 40 cooperate to maintain a customizable environment within the chamber 26. In particular, the inlet port 38 allows for the introduction of gases, such as atmospheric air, or other gases, while the outlet port allows for the extraction of byproducts of combustion (e.g., CO, CO₂, etc.). In this manner, the combustion environment, such as the percentage of oxygen, ambient temperature, humidity, and/or percentage of carbon dioxide or other gases may be varied as desired. [0020] The combustion chamber 26 also includes an ignition element 42 to initiate combustion of the paper 14. In this example, the ignition element 42 is a hot tungsten wire. Once ignited, the environment of the chamber 26 and the solution 21 impregnated in the paper 14 react to sustain a stable synthesis reaction. The product of the combustion synthesis may be collected in a product collection area 28 located in and/or below the chamber 26. The product collection are 28 may be integrated within the chamber 26, or may be a separate area coupled to the chamber 26.

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[0021] An example of the sustained combustion synthesis is illustrated in FIG. 3. In the illustrated example, the paper 14 maintains a stable reaction front 46, where the oxidizer and fuel react to form a product 48. FIG. 4, meanwhile, illustrates an example product 48 formed by the disclosed combustion synthesis process. In the disclosed example, the product 48 of the combustion synthesis of the metal nitrate and glycine solution 21 impregnated within the paper 14, is a metal oxide nano-powder with a high surface area.

[0022] To assist in the automation of the process, the apparatus 10 may optionally include a controller 30 for monitoring and controlling the apparatus 10 during operation. For example, the controller 30 may provide data collection allowing for the adjustment (either automatically or manually) of the speed of the conveyor 16, the temperature of the dryer 22, the flow rate of the solution 21, the temperature of the combustion chamber 26, the environment within the chamber 26, the quality of the product 48, etc. Each of the monitored parameters may be automatically adjusted by the controller 30, or may be communicated to an operator and/or other component such that the operator or components may take corrective actions as needed.

[0023] It will be appreciated by persons of ordinary skill in the art that while the apparatus 10 is described in relation to various components, the implementation of the apparatus 10 may include any suitable arrangement. Further, it will be appreciated that the size, location and order of the components of the apparatus 10 may be varied based upon desired design considerations.

[0024] FIG. 5 is a flowchart of an example method 100 that may be performed by the apparatus 10 for the combustion synthesis of a nano-powder. In the example method 100, a solution 21 is prepared for use in the apparatus 10 (block 102). In one example, as noted above, the solution may include at least one metal nitrate in combination with glycine. More particularly, in one example, the solution may be an iron nitrate and glycine combination with a ratio between fuel and oxidizer (i.e., glycine to iron nitrate) in the approximate range of 0.5:1 to 3:1.

[0025] Once prepared, the solution 21 is impregnated into the paper 14 (block 104). As in the above described examples, the impregnation of the solution 21 may be performed by the spray applicator 20 and/or by the submersion of the paper 14 into the solution 21. It will be appreciated by persons of ordinary skill in the art, however, that the impregnation of the solution 21 into the paper 14 may be accomplished by any impregnation process. [0026] The impregnated paper 14 is then dried to evaporate at least a portion of any free fluid remaining in the paper 14 (block 106). In the example apparatus 10, the drying process is performed by a convection dryer. However, the drying time, temperature, method, etc. may be performed by any appropriate dryer.

[0027] Once dried, the impregnated paper 14 is then ignited to initiate a sustained combustion of the paper 14 and solution 21 (block 108). In the example apparatus 10, the impregnated paper 14 is fed into the combustion chamber 26 and is ignited by the ignition element 42 (e.g., a hot tungsten wire). Once ignited, the solution 21 impregnated in the paper 14 sustains a stable synthesis reaction, and that reaction may be monitored to maintain its stable nature (block 110). For example, the apparatus 10 (e.g., the controller 30) and/or the operator may monitor the system to ensure a friendly environment to combustion synthesis exists (e.g., temperature, ambient air, humidity, etc.)

[0028] The product of the combustion synthesis may then be collected for use as desired (block 112). For example, the product may fall toward the product collection area 28 located below or at the bottom of the chamber 26, where it can be harvested. It will be understood that the combustion reaction may be maintained indefinitely, as long as the combustion reaction is provided with sufficient material (e.g., solution, paper, environment, etc) to maintain a stable combustion synthesis reaction.

[0029] The above described examples of fuel and oxidizer combustion synthesis reactions are but one of many example reaction solutions suited for use with the disclosed apparatus and methods. For example, the reaction solution 21 may be any suitable fuel and oxidizer mixture, in any sustainable ratio. Further, the drying times, temperature, combustion environment, etc., may be varied widely to further modify the product 48. For further examples of various reaction solutions suitable for combustion synthesis, one may look to K. Deshpande, A. Mukasyan, and A. Vara, "Direct Synthesis of Iron Oxide Nanopowders by the Combustion Approach: Reaction Mechanism and Properties^'", Chem. Mater 2004, 16, pages 4896-4904 (2004), incorporated herein by reference in its entirety. [0030] Although the teachings of the invention have been illustrated in connection with various examples, there is no intent to limit the invention to the disclosed examples. On the contrary, the intention of this application is to cover all modifications and embodiments fairly falling within the scope of the appended claims either literally or under the doctrine of equivalents.

Claims

We claim:

1. An apparatus for the synthesis of a nano-powder comprising: a carrier substrate; a solution applicator adapted to impregnate an aqueous reaction solution into the carrier substrate; a dryer adapted to evaporate at least a portion of a free fluid portion of the aqueous reaction solution from the impregnated carrier substrate; and a combustion chamber including an ignition source for igniting the impregnated carrier substrate to cause the impregnated carrier substrate to initiate a combustion synthesis, wherein the combustion synthesis of the impregnated carrier substrate forms at least a nano- powder.

2. An apparatus as defined in claim 1, further comprising a substrate conveyor for conveying the impregnated carrier substrate at least through the dryer and to the combustion chamber.

3. An apparatus as defined in claim 2, wherein the substrate conveyor includes a foraminous surface.

4. An apparatus as defined in claim 1, further comprising a collection chamber for collecting the formed nano-powder.

5. An apparatus as defined in claim 1, wherein the carrier substrate is a paper.

6. An apparatus as defined in claim 1 , wherein the solution applicator comprises a spray applicator circuit.

7. An apparatus as defined in claim 6, wherein the spray applicator circuit includes a reservoir to hold the aqueous reaction solution, a spray nozzle for impregnating the aqueous reaction solution into the carrier substrate, and a pump to deliver pressurized aqueous reaction solution to the spray nozzle.

8. An apparatus as defined in claim 1 , wherein the solution applicator comprises a reservoir to hold the aqueous reaction solution, and wherein the carrier substrate is submersible in the reservoir to impregnate to aqueous reaction solution into the carrier substrate.

9. An apparatus as defined in claim 1 , further comprising a heater coupled to the solution applicator to heat the aqueous reaction solution prior to impregnation.

10. An apparatus as defined in claim 1, further comprising an agitator coupled to the solution applicator to agitate the aqueous reaction solution prior to impregnation.

11. An apparatus as defined in claim 1 , wherein the dryer comprises a convection dryer.

12. An apparatus as defined in claim 1, wherein the aqueous reaction solution comprises an oxidizer and a fuel.

13. An apparatus as defined in claim 12, wherein the oxidizer comprises at least one metal nitrate.

14. An apparatus as defined in claim 12, wherein the fuel includes glycine.

15. An apparatus as defined in claim 1, further comprising a controller for receiving data related to the operation of at least one of the solution applicator, the dryer, or the combustion chamber, and for providing an output indicative of the operation of the t received data.

16. A method of synthesizing a nano-powder comprising: providing an aqueous reaction solution; impregnating a carrier substrate with the aqueous reaction solution; drying the impregnated carrier substrate; igniting the impregnated carrier substrate to initiate a combustion synthesis, wherein the combustion synthesis of the impregnated carrier substrate forms at least a nano-powder.

17. A method as defined in claim 16, further comprising collecting the nano- powder.

18. A method as defined in claim 16, further comprising conveying the carrier substrate while at least one of impregnating the carrier substrate or drying the impregnated carrier substrate.

19. A method as defined in claim 16, wherein impregnating a carrier substrate with the aqueous reaction solution comprises spraying the aqueous reaction solution onto the carrier substrate.

20. A method as defined in claim 16, wherein impregnating a carrier substrate with the aqueous reaction solution comprises introducing the carrier substrate into a reservoir containing the aqueous reaction solution.

21. A method as defined in claim 16, further comprising heating the aqueous reaction solution prior to impregnating the carrier substrate.

22. A method as defined in claim 16, further comprising monitoring the combustion synthesis of the impregnated carrier substrate, and adjusting one of providing the aqueous reaction solution, impregnating, drying, or igniting to sustain the combustion synthesis.