WO1993021109A1

WO1993021109A1 - Method of making materials using vapor phase transport and reactive gases

Info

Publication number: WO1993021109A1
Application number: PCT/US1993/003594
Authority: WO
Inventors: John J. Moore; Dennis W. Readey
Original assignee: Moore John J; Readey Dennis W
Priority date: 1992-04-15
Filing date: 1993-04-15
Publication date: 1993-10-28
Also published as: AU4105193A

Abstract

A method of making materials using gas coupled with vapor phase transport. One embodiment of the method includes the steps of providing a reaction mixture including a plurality of reactive chemical species capable of reacting exothermically and heating the mixture in an environment including a reactive gas such that the plurality of reactive chemical species will react, wherein sufficient heat is generated from an exothermic reaction between the plurality of reactive species to generate a volatile intermediate reaction species which subsequently reacts to form the at least one chemical product. In another embodiment, the reaction mixture, reactive gas and vapor phase transport are employed in the presence of a nucleating agent to produce a desired product form. The materials include ceramics, metals, and combinations thereof in varying morphologies, including fine particles (32c), whiskers (32a and 32b), hollow shells, platelets and combinations thereof, and composite materials containing the same which have been synthesized in situ.

Description

METHOD OF MAKING MATERIALS USING VAPOR PHASE TRANSPORT AND REACTIVE GASES

FIELD OF THE INVENTION

The present invention relates to the production of powders, whiskers, hollow shells, platelets, bodies and composite materials made of ceramics, metals and mixtures thereof using vapor phase transport in a reactive, gaseous medium. The technique may optionally be combined with combustion synthesis.

BACKGROUND OF THE INVENTION As used herein, "combustion synthesis" refers to the reaction of two or more reactants wherein the reaction is exothermic and results in the formation of a solid product such as a powder or a coherent body. In a preferred embodiment, combustion synthesis involves a self- propagating, high temperature reaction which generates sufficient heat once ignited to be self-sustaining. This can be achieved by local ignition such as using a heated tungsten-resistance wire which heats the surface of a green pellet of compressed powdered reactants. Once the reactant mixture is ignited locally, the reaction will become self- propagating and a reaction front will propagate through the pellet of the reactant powder mix. This is called the propagating mode of combustion. An alternative method of igniting these pellets of compressed powdered reactants is to place the green pellet in a furnace at a temperature which is controlled above the ignition temperature for the exothermic reaction to be started. Under this mode of combustion, i.e., bulk mode or thermal explosion or simultaneous combustion, the whole of the pellet is heated to above the ignition temperature and, therefore, the whole of the pellet reactant mix goes through the exothermic reaction at approximately the same time. No obvious propagating wave is observed as in the propagating mode described above.

Vapor phase sintering has been used to produce porous ceramic bodies from pressed green bodies of ceramic powder. Vapor phase sintering includes the sintering or heating of a compressed powdered ceramic mixture in a reactive gas medium, such as hydrogen chloride. The ceramic mixture reacts with the reactive gas to produce volatile compounds which provide the vapor transport medium for the sintering process. The vapor transport sintering mechanism typically provides a sintered ceramic body having a high porosity and low strength.

SUMMARY OF THE INVENTION The present invention provides a method of making materials using reactive gas coupled with vapor phase transport.

One embodiment of the method according to the present invention employs combustion synthesis and includes the steps of providing a reaction mixture including a plurality of reactive chemical species capable of reacting exothermically» The plurality of reactive species will react exothermically under appropriate conditions such as a temperature at or above the ignition temperature (T_ig) . The reaction mixture is heated in an environment including a reactive gas such that the plurality of reactive chemical species react exothermically and generate a volatile intermediate reaction species which subsequently reacts to form the at least one different chemical species. The volatile intermediate reaction species is an intermediate transition product of reactions between the reactive gas and at least one of the plurality of reactive chemical species. The volatile intermediate reaction species preferably has a partial pressure of about 10^"6 atmospheres or greater and more preferably about 10^"4 atmospheres or greater and most preferably 10^"3 atmospheres or greater. In preferred embodiments, the present invention includes various materials made using the present method. The products can include ceramics, metals, and combinations thereof in varying morphologies, including fine particles, whiskers, platelets and hollow shells, and combinations thereof. Coherent bodies of material can also be produced. For example, composite materials containing whiskers which are synthesized in situ can be produced.

Coupling combustion synthesis with vapor phase transport by conducting the combustion synthesis reaction, i.e., the exothermic reaction, in a reactive gas provides for the formation of volatiles which are intermediate chemical species. These volatile intermediate reaction species (e.g., chlorides) subsequently react to produce the desired reaction products. In another embodiment of the present invention, products having certain desired morphologies, in particular whiskers, are produced using a reactive gas and a vapor phase transport technique. In order to achieve the desired results, three elements are necessary: (1) a source of materials capable of forming the desired final product,

(2) a reactive gas capable of forming volatile intermediate reaction species which functions as a transport species, and (3) a nucleating agent on which the product is formed in the desired morphology. This embodiment can be combined^' with combustion synthesis or can be performed without employing combustion synthesis. A specific example of a system employed to produce ceramic products of certain morphologies includes a source of aluminum, a source of oxygen, a hydrogen chloride reactive gas and a nucleating agent such as titanium diboride (TiB₂) . With this system, aluminum oxide-containing material is produced and, in particular, the product can be produced in the form of whiskers. Another system includes: (1) a source of silicon and a source of carbon, (2) a reactive gas (HCl) , and (3) a nucleating agent to produce silicon carbide-containing materials in desired morphologies, such as whiskers. Further, silicon nitride (Si₃N₄) material having different morphologies such as whiskers can be produced by heating a source of silicon, a reactive gas comprising NH₃ or NH₃ and HCl, and a nucleating agent. Alternatively, silicon nitride powder can be sintered in a reactive atmosphere such as HCl in the presence of a nucleating agent. In addition, the desired morphologies can be produced in situ, such as alumina whiskers produced within a TiB₂ matrix.

Since the desired products will form from intermediate volatile gaseous species, there is an opportunity to speed up the synthesis reaction while at the same time provide for controlled nucleation and growth of the required products in a morphology which may not be easily produced by conventional processing reactions.

In one embodiment of the present invention, the increased temperature created by a combustion synthesis reaction, coupled with the presence of a reactive gas, can result in increased partial pressures of the volatile intermediate reaction species, which facilitates the vapor phase transport mechanism. As the exothermic combustion synthesis reaction subsides, the volatile intermediate reaction species can react and can allow the synthesis reaction to be completed.

Some examples of the resulting product morphology include the production of hollow shells, fine particles, platelets and whiskers out of ceramics, metals and combinations thereof, other forms of products can also be produced such as composites containing whiskers, fine particles, platelets and hollow shells and combinations thereof. Therefore, the present invention is capable of producing the required ceramic and/or metal products in the form of fine particles, hollow shells, whiskers, platelets, composites containing these materials and morphologies or various combinations thereof. Advantages of the present invention include the ability to control the morphology of the ceramic and/or metallic products and to achieve very rapid reaction rates. A further advantage of the present invention is the ability to produce in situ whiskers within a ceramic and/or metallic composite material. Typically, such whiskers are incorporated separately into the product. Whiskers can be useful for reinforcing the matrix material. Fine whiskers, however, are a considerable health hazard and require elaborate precautions in handling when trying to incorporate these as a reinforcing medium into a ceramic and/or metallic matrix composite material. Therefore, synthesizing these fine whisker reinforcements in situ is advantageous in overcoming the problem of handling of these hazardous materials. Further, the ability to produce platelets is advantageous because platelets do not appear to present the same health hazards as whiskers. Also, the ability to produce hollow shells of fine-grain or variable- grain ceramic and/or metallic products which can subsequently be comminuted to produce fine particles provides a useful method for the manufacture of fine particulate ceramic and/or metallic materials which, until now, have been very difficult or impossible to produce. Fine particulate ceramic and/or metallic particles can also be produced in situ within the reaction system by control of temperature, partial pressure of volatile intermediate phases, heating and cooling rates. A further advantage according to one embodiment of this invention is the heat generated by the combustion synthesis reaction coupled with the reaction with the reactive gas produces increased partial pressures of the intermediate volatile species which will facilitate the formation of different ceramic and/or metallic material product morphologies.

These and various other advantages and features of novelty which characterize the present invention are pointed out with particularity in the claims annexed hereto and forming a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof and to the accompanying descriptive material, in which there is illustrated and described preferred embodiments of the present invention.

BRIEF DESCRIPTION OF DRAWINGS Fig. 1 is a photocopy of a photomicrograph in which TiB₂ ceramic particles have been produced by the reaction Ti + 2B = TiB₂ combustion synthesis in (a) argon gas atmosphere and (b) HCl gas environment. The titanium diboride particles produced by combustion synthesis in inert argon gas are solid whereas the titanium diboride particles produced by combustion synthesis in hydrogen chloride gas are hollow. Fig. 1 indicates the difference in product morphology by conducting this combustion synthesis reaction in argon compared with reactive hydrogen chloride gas. It should be noted that the hollow shell formation is clearly evident when conducting this combustion synthesis reaction in a hydrogen chloride reactive gas environment, but no hollow shells are formed when this reaction is conducted in an inert argon gas environment.

Fig. 2 are photocopies of photomicrographs of titanium diboride produced by combustion synthesis in a hydrogen chloride gas environment in which the reaction was conducted at (a) a low heating rate, approximately 32°C/min and (b) a higher heating rate, approximately 50°C/min. The higher heating rate produced titanium diboride hollow shells with larger grain sizes of the TiB₂ grains in the walls of the hollow shells. Therefore, heating rate is a further parameter that can be used to control the final particle size that can be achieved by subsequent ball milling of the combustion synthesized product conducted in a reactive gas medium. This shows the effect of heating rate on the grain size of the hollow shells of titanium diboride. Using a heating rate of 50°C per minute produces larger grains in the walls of the hollow shells compared with using a heating rate of 32°C per minute. Subsequent grinding of these hollow shells will produce fine particles from these hollow shells of material produced in HCl at the lowest heating rate.

Fig. 3 is a photocopy of the photomicrographs of titanium diboride produced by combustion synthesis in (a) argon inert gas and (b) a hydrogen chloride reactive gas environment. The titanium diboride product combustion synthesized in a reactive gas clearly produces a finer particle size of the ball milled TiB₂ product, i.e., submicron particles of TiB₂. The products shown are products of ball milling for 15 hours in an appropriate liquid medium using 12 alumina balls in a porcelain ball mill for the TiB₂ powder produced by combustion synthesis in (a) argon and (b) a reactive hydrogen chloride gas. This example demonstrates that the product produced in the reactive hydrogen chloride gas can be ball milled to much finer particle sizes.

Fig. 4 is a photocopy of the photomicrographs of the extensive formation of aluminum oxide-containing whiskers which are products of the reaction:

3Ti0₂ + 3B₂0₃ + 10A1 = 3TiB₂ + 5Al-,0₃ This combustion synthesis reaction was conducted in hydrogen chloride reactive gas. The titanium dioxide, boron oxide and aluminum powders were pressed to a certain green density into a pellet form and ignited in a hydrogen chloride gas environment by placing inside a furnace controlled at a temperature above the ignition temperature for this exothermic reaction. The photomicrographs in Fig. 4 clearly shows the voluminous production of aluminum oxide-containing whiskers. Fig. 4b is an enlargement of Fig. 4a taken from the outlined rectangle in Fig. 4a.

Fig. 5 is a photocopy of the photomicrographs taken from the same product sample as in Fig. 4a, but from a different position on the product sample. The outlined rectangle in Fig. 5a indicates the compressed aluminum oxide whiskers produced by the combustion synthesis reaction conducted in a reactive gas environment. Fig. 5b is an enlargement of Fig. 5a taken from the area indicated by the rectangle in Fig. 5a. Even the denser portions of this composite indicate compressed whiskers. The compressed whiskers are clearly evident in Fig. 5b. The compression of these A1₂0₃ whiskers occurred within the reaction system itself, and no external pressure or hot pressing was used.

Fig. 6 is a representation of the x-ray -diffraction analysis patterns taken from the total product produced by the reaction of titanium dioxide, boron oxide, and aluminum powders reacted in a hydrogen chloride reactive gas environment. This analysis shows that the combustion synthesis of this reaction, conducted under a reactive hydrogen chloride gas environment produces the following ceramic products: titanium diboride and aluminum oxide- containing material. It is clear from Fig. 6 that the products synthesized are indeed the products desired, i.e. , titanium diboride and aluminum oxide-containing material.

Fig. 7 is a photocopy of the photomicrograph of the synthesized ceramic composite material of titanium diboride and aluminum oxide-containing material. The white particles are titanium diboride and the darker whiskers are aluminum oxide-containing material. This coupled combustion synthesis reaction conducted in a reactive gas environment has produced a combination of aluminum oxide- containing whiskers embedded in fine titanium diboride particles. This composite product could be subsequently or simultaneously hot pressed to produce the desired final density in the composite material.

The remaining figures represent certain examples of the present inventive subject matter.

Fig. 8 provides the introduction in that one aspect of the research explores the manufacture of ceramic and/or metallic and/or composite materials by coupled combustion synthesis and gas transport. In this particular case, the reactive gas medium is hydrogen chloride gas. The applications for this particular invention are to develop innovative techniques for the manufacture of ceramic and/or metallic materials such as powders, hollow shells and reinforcing media such as whiskers and also to produce ceramic and/or metal matrix composites of these components. A further application is to produce materials which have repeatable and controlled microstructures, i.e., powders, hollow shells, fine particles, whiskers, platelets and combinations of these.

Fig. 9 provides an overview of example model combustion synthesis reactions which are being studied in this particular case using a reactive gas (hydrogen chloride) or an inert gas (argon) . The advantages and characteristics of combustion synthesis reactions are highlighted and the model reaction systems are set forth. Fig. 10 is an overview of the vapor transport systems which are used in the model reaction systems described.

Hydrogen chloride gas is the vapor transport medium which provides volatile halide intermediates in the particular example.

Fig. 11 describes the vapor transport mechanisms that can occur between a small particle with a large surface free energy and a large particle with a small surface free energy. The degree of coarsening of a particle is characterized by the formula in Fig. 11 where R is the average particle radius; D the diffusivity; Omega is the molar volume; R the gas constant; T the temperature; P₀ is the vapor pressure of the gaseous intermediate transport species; and t the time in seconds that the vapor transport species exists.

Fig. 12 outlines the expectations for the synergy of coupling combustion synthesis with vapor transport.

Fig. 13 outlines the experimental summary for one of the model systems, i.e., Ti + 2NiO — Ti0₂ + 2Ni combustion synthesis reaction conducted in an inert argon gas environment and a reactive hydrogen chloride gas environment. The points made on this overview indicate the salient features of this particular system conducted in a hydrogen chloride reactive gas environment. Fig. 14 gives the predicted volatile chloride species that are present using a thermodynamic free energy minimization algorithm (Solgas) as a function of temperature. Since a minimum of 10^"6 atmospheres vapor pressure or partial pressure for each of these species is preferred, it is clear from this data that even various chlorides of titanium are present to above 10^"4 atmospheres. Therefore, this system should be able to benefit from combustion synthesis coupled with a reactive gas environment.

Fig. 15 is for the reaction as outlined at the top of this overview which also incorporates 2.1 moles (70%) of inert or diluent titanium dioxide, i.e., product phase, into the reactant mix. This reduces the combustion temperature and increases the volume fraction of Ti0₂ in the Ni-Ti0₂ product materials, while also helping to control the overall reaction kinetics. This graph is a plot of enthalpy against temperature for both the reactants and the products. Using an ignition temperature, which is typical of this system, of just below 1000°K produces an adiabatic temperature which is slightly in excess of 2000°K. Since this reaction is conducted under non-adiabatic conditions, a combustion temperature of approximately 1600°K is typical as outlined in the diagram. The various phase changes that occur in both the reactants and the products are also indicated on the diagram as vertical lines.

Fig. 16 is a plot of the temperature/time relationship for conducting this particular combustion synthesis reaction with 70% titanium dioxide used as the diluent in a hydrogen chloride gas environment. These data relate to the same conditions as indicated in Fig. 15. Conducting this combustion synthesis reaction in a hydrogen chloride gas environment compared with conducting it in an inert argon gas environment reduces the ignition temperature from 740*C in argon to 670°C in hydrogen chloride gas. At the same time, the maximum combustion temperature achieved is increased from 1160°C in argon to above 1325°C in hydrogen chloride gas. Therefore, another advantage of conducting the combustion synthesis reaction in a reactive gas environment could be to initiate this combustion synthesis reaction at a lower ignition temperature, thereby conserving energy of processing while still achieving a higher exothermocity and a higher combustion temperature for the favorable formation of suitable partial pressures of the required intermediate volatile species.

Fig. 17 tabulates five typical examples of combustion synthesis reactions; two conducted in an inert argon atmosphere and three conducted in a reactive hydrogen chloride gas atmosphere. Those reactions conducted in a reactive hydrogen chloride gas environment clearly indicate a lower ignition temperature and, therefore, some energy savings in conducting this combustion synthesis reaction in a reactive gas environment.

Fig. 18 is a photocopy of photomicrographs of titanium dioxide pressed to a certain green density and sintered in a hydrogen chloride gas environment and alternatively in an argon gas environment. These photomicrographs indicate the advantage of vapor phase transport in increasing the coarsening or growth of titanium dioxide when sintered under a reactive gas environment. This indicates the advantage of producing a volatile species as an intermediate vapor transport mechanism for increasing coarsening or grain growth or particle growth. No combustion synthesis reaction occurred in this case. Fig. 19 is a photocopy of four photomicrographs. Top left-hand corner is the combustion synthesis reaction of titanium and nickel oxide conducted in argon and viewed under the scanning electron microscope. The top right-hand photomicrograph is the same reaction conducted in a reactive hydrogen chloride gas environment. The bottom left-hand photomicrograph is the same as the top left-hand viewed under the scanning electron microscope and conducted in argon, but viewed using the backscattered electron image to indicate the compositions. The light areas in this photomicrograph are nickel and the darker areas are titanium dioxide. The bottom right-hand photomicrograph is the same as the top right-hand photomicrograph, i.e., the combustion synthesis of this reaction conducted in hydrogen chloride gas, but viewed under the scanning electron microscope using backscattered electrons; again, the light areas are nickel particles and the darker areas are titanium dioxide. In general, it will be noticed that conducting the combustion synthesis reaction in hydrogen chloride gas, i.e., a reactive gas, provides coarsening of both the titanium dioxide and nickel phases.

Fig. 20 is a photocopy of a photomicrograph of the same synthesized products given in Fig. 19, i.e., the titanium-nickel oxide reaction with 70% titanium dioxide diluent conducted in either argon inert gas or HCl reactive gas. The left-hand side photomicrograph is for the argon inert gas reaction and the right-hand side photomicrograph is for the HCl reactive gas reaction. For the reaction conducted in hydrogen chloride gas, there is an increase in coarsening of both the titanium dioxide and the nickel phases as indicated in the previous diagram.

Fig. 21 outlines the main features for a second model reaction system studied; that is, Ti + 2B — TiB₂. This reaction system was chosen for its relative simplicity and the ease of generation of gaseous intermediate species with a HCl reactive gas environment.

Fig. 22 provides the predicted volatile chloride species when conducting this reaction in a hydrogen chloride reactive gas environment. As indicated by the data in this diagram, volatile vapor pressures are quite high (well above the 10^"6 atmospheres minimum desired for this type of reaction) .

Fig. 23 provides the enthalpy/temperature relation- ships for both the reactants and the products for this particular reaction system for the synthesis of titanium diboride from titanium and boron. Using an ignition temperature of approximately 1200°K produces an adiabatic temperature slightly in excess of 3500°K. However, typical combustion temperatures of 2600°K indicate that this reaction is conducted under non-adiabatic conditions. The phase changes for both the reactants and the products are indicated on this diagram and appear as vertical lines. Fig. 24 indicates the relationship between the ignition temperature and the sample mass used for each of these compressed pellets pressed to a load of 10,000 psi and a heating rate of approximately 95°C per minute. It is evident that conducting this combustion synthesis reaction in a reactive hydrogen chloride gas environment lowers the ignition temperature compared with conducting this combustion synthesis reaction in an argon inert gas environment. Fig. 25 illustrates the postulated reaction mechanism for the combustion synthesis reaction of Ti + 2B = TiB₂ in a hydrogen chloride gas environment. The hollow shell formation produced using this reactive gas condition is clearly explained in these four steps. Fig. 26 provides evidence of the hollow shell forming around the reacting boron particle when conducting the combustion synthesis reaction in a reactive hydrogen chloride gas environment at a heating rate of 74°C per minute. Fig. 27 gives the x-ray diffraction analysis data for the combustion synthesis of titanium diboride in the hydrogen chloride reactive gas environment using two heating rates. The product is clearly titanium diboride for both heating rates used. Fig. 28 gives a third model system examined using this technique, i.e., 3Ti0₂ + 3B₂0₃ + 10A1 = 3TiB₂ + 5A1₂0₃. This combustion synthesis reaction is conducted in either an inert argon gas environment or a reactive hydrogen chloride gas environment. This particular reaction system is chosen because of its economy in production and because thermodynamics predict high conversion to volatile chloride intermediate species. Extensive whisker production can also be expected from this system due to the high vapor pressures generated with aluminum chlorides and chloride of boron.

Fig. 29 provides the predicted volatile chloride species from this reaction system which are seen to be high.

Fig. 30 provides the enthalpy temperature relationship for this reaction system for both the reactants and the products. The phase changes occurring in both the reactants and products are also indicated on this Figure and are represented by a vertical line. Using a typical ignition temperature of just over 1000°K will produce an adiabatic temperature in excess of 3000°K. Typical non- adiabatic or combustion temperatures of about 2300°K coincide with the aluminum oxide melting point. Fig. 31 provides the conclusions for this particular example of these three model reaction systems which have combustion synthesis reactions conducted in either an argon inert atmosphere or hydrogen chloride gas reactive atmosphere. Three reaction mechanisms which can account for microstructure observations using the coupled combustion synthesis and vapor phase transport systems are outlined. Fig. 32 illustrates a photocopy of a series of photomicrographs illustrating the effect of the nucleating agent on the growth of whiskers in a reactive gas atmosphere. Twenty-nine weight percent TiB₂ was added to an alumina composition and the result is illustrated in Fig.

32a. It is seen that many whiskers are growing from the

TiB₂ nucleating sites. Fig. 32b illustrates that the number of whiskers appears to decrease as the amount of nucleating agent is reduced to five weight percent TiB₂. Finally, Fig. 32c illustrates that no whisker^' growth occurs when the alumina is sintered in a reactive gas atmosphere without a nucleating agent.

DETAILED DESCRIPTION OF THE INVENTION In accordance with the present invention, common features of the various embodiments disclosed include the use of reactive gas and vapor phase transport system. In one embodiment, the reactive gas is employed together with a source of materials which form the product and a nucleating agent to provide for the production of ceramic and/or metallic materials having desired morphologies. A preferred morphology is whiskers. In accordance with another embodiment of the present invention, combustion synthesis and vapor phase transport are combined to provide for the production of ceramic and/or metallic materials.

A mixture of reactive chemical species which react exothermically is provided. Reactive gas is also provided which reacts to form volatile intermediate reaction species which react to form the product.

One advantage of the present invention is the ability to control the morphology of the products. Examples of morphologies that can be produced include hollow shells, whiskers, platelets and fine powders. In addition, the various morphologies can be produced in situ in a composite material.

The hollow shells can be subsequently ball milled or ground to produce very small particles, preferably submicron particles of the required ceramic and/or metallic products. These particles will preferably be. less than about 30 microns, preferably less than about 20 microns, more preferably less than about 10 microns, even more preferably less than about 5 microns, even more preferably less than about 1 micron, and most preferably less than about 0.5 micron in diameter.

Alternatively, whiskers can be produced by this technology in order to be used subsequently as a reinforcing medium in other ceramic, ceramic metal, or metal matrix composite materials. Whiskers are fine, single crystal fibers, preferably having diameters of less than about 30 microns, preferably less than about 20 microns, more preferably less than about 10 microns, even more preferably less than about 5 microns, even more preferably less than about 1 micron, and most preferably less than about 0.5 micron in diameter. The aspect ratio

(length to diameter) of the whiskers will preferably be greater than about 10:1, more preferably about 25:1, even more preferably about 50:1, and most preferably about 60:1. A further alternative is to produce the whiskers in situ within the reaction system such that the reinforcing whiskers are produced alone or together with fine particles, hollow shells, platelets and/or whiskers of other materials within a composite system. Each of these products, i.e., whiskers, hollow shells, platelets, fine particles, and composite materials containing such products can be hot pressed into the required net or near-net shape within the overall processing system. The high temperatures achieved in the combustion synthesis reaction will provide for relatively easy hot pressing into required near-net or net shapes during or immediately following the completion of the synthesis reaction.

In accordance with an embodiment of a process of the present invention, reactant powder mixtures are mixed in the desired stoichiometry to produce the exothermic reaction products. The reactant mixtures are thoroughly mixed in order to avoid segregation and subsequently pressed to a certain green density into a desired pellet net or near-net shape. The reactant mixture is then heated in a reactive gas to above the ignition temperature of the combustion synthesis reaction. The reactive gas is a gas that will provide and produce intermediate volatile species of the reactants which will subsequently react to produce the desired ceramic and/or metallic product materials. One example of a reactive gas is hydrogen chloride in which the reactants will produce volatile intermediate chlorides as reactants. Relatively little partial pressure is required from these intermediate gaseous species. For example, 10^"6 atmospheres of a volatile halide is generally sufficient to produce an improvement in the reaction kinetics and to provide for substantial changes in morphology of the required ceramic and/or metallic products in either hollow shells, particles, platelets or whisker forms.

The reactive gas can be any gas which produces intermediate volatile species when reacted with the reactants such that the volatile intermediate species are preferably in amounts sufficient to generate partial pressures of present is at least about 10^"6 atmospheres or higher. On cooling from the maximum temperature or the combustion temperature achieved during the reaction, the intermediate volatile species can react to produce the required ceramic and/or metallic product materials.

Some typical examples of reactive gases include all the halide gases, e.g., chlorine, bromine, iodine, fluorine, etc., and the hydrogen—halide gases, e.g., HCl, HI, HBr, HF, etc. However, any gas can be used which satisfies the above criteria.

The intermediate species include any volatile species produced from the reaction between the chemical reactants or reactive species and the reactive gas. These intermediate species can be generally predicted using the thermodynamic free energy considerations as outlined in Figs. 14, 22 and 29 hereof. For instance, when halides or hydrogen halides are used as the reactive gas, the intermediate volatile species can include such halides, e.g., A1C1, BC1, and the like and oxyhalides, e.g., BC10 and the like. Other examples of reactive gases include oxygen, carbon monoxide, carbon dioxide and water vapor in which the intermediate volatile species is typically oxides and suboxides (with 0₂, H₂0, and/or C0₂) , carbonyls or even reduced elemental species (with C0₂) or the like. Various combinations of these reactive gases can be used.

Once the combustion synthesis reaction is initiated within the gaseous medium, the exothermic reaction takes very little time to complete, e.g., on the order of 2-3 seconds maximum. Previously, combustion synthesis reactions were typically conducted in either vacuum or inert gas, e.g., argon atmospheres. Therefore, one difference between one embodiment of the present invention and prior processes is that the present invention provides for the production of intermediate gaseous species which improve the rate at which the reaction takes place and also provides for controlled nucleation and growth of unusual or specific product ceramic and/or metallic material morphologies. The combustion synthesis reaction can be controlled by green density; that is, the density to which the green compact is pressed prior to reacting in the reactive gas environment, and/or the use of diluent or inert materials as a means of controlling the reaction temperature. An example of an inert or diluent would be the addition of a certain amount of the product phase into the reactant mix which will simply take up heat and not take part in the reaction. However, any material or compound can be used as a diluent or inert material providing it does not react with the other reactants. It can react with the reactive gas to produce intermediate volatile species. Inert and/or diluent materials are added to the reactants to control the reaction or combustion temperature, i.e., lower it, and to affect the volume fractions and morphologies and sizes of the product phases and components. Further process parameters that can be varied include reactant stoichiometry, particle size of the reactants, green mass and green density, heating rate to the reaction temperature, and cooling rate from the reaction temperature to final temperature. It is believed that the growth of whiskers continues after the synthesis reaction has substantially completed. Therefore, it is possible to control the size, shape and amount of whiskers by controlling the time, temperature and partial pressure of reactive gases present after the substantial completion of the synthesis reaction.

Any reaction system that exhibits a reasonable exothermocity, e.g., about 5, preferably about 10, more preferably about 20, most preferably about 40 kilojoules per mole or higher, can be utilized in the combustion synthesis embodiment of the system described here. The reactant powders are mixed in the required stoichiometry and compressed to the required green pellet density preferably using either uniaxial die pressing systems or cold isostatic pressing systems to produce a cylindrical pellet or a compressed reactant mixture in the required near-net or net shape formation. Prior to compaction into the required green density, the reactant powders are thoroughly mixed, for example, in a porcelain ball mill using porcelain balls to grind and mix the reactants. Generally, the finer the particle size of the reactants, the more favorable will be the reaction kinetics. Typically, reactant particle sizes should be 40 microns or less. Preferably, the compressed reactant powder pellet or shape is dried for approximately 1 hour or more in an oven using a controlled atmosphere inert gas at a temperature just slightly above 100°C in order to remove moisture. Alternatively, the whole of the powder preparation and pellet preparation can be conducted in an inert glove box. A reactive gas environment is chosen such that the reactants will produce volatile species with a partial pressure of typically about 10^"6, or preferably 10^"4, or more preferably 10^"3 atmospheres or higher. Therefore, a further control process parameter would be the reactive gas and the partial pressure of the volatile species generated and the time and temperature of the furnace operation. A further process parameter would be the heating rate at which the reactant compressed pellet or shape is heated to the ignition temperature of the combustion synthesis reaction. The heating rate will also help to control the morphology of the required ceramic and/or metallic product. The morphology of the required ceramic and/or metallic product will very much be dependent upon the vapor pressures produced of each particular species, the time that the particular species exists within the reaction cycle and the reaction kinetics from the volatile species to the required ceramic and/or metallic products. Also, the nucleation and growth kinetics of the required ceramic and/or metallic products are important parameters in order to control the final morphology of the ceramic and/or metallic product materials. Since different reactants and product species as well as the intermediate volatile species will have different reaction kinetics, thermodynamics, and nucleation and growth characteristics, the process parameters of particle size, heating rate, reactant stoichiometry, reactant green density, vapor pressure of the intermediate volatile species and their time of existence within the reaction cycle, and the ^* nucleation and growth characteristics of the decomposing volatile species to the ceramic and/or metallic products are all important process parameters that will provide variation of the product morphologies.

It is believed, but not relied upon, that the higher the vapor pressure, the more likely it will be to produce whiskers from that intermediate species. Lower and intermediate vapor pressures of the intermediate species are more likely to produce hollow shells which can be subsequently ball milled to produce fine powdered products of the required ceramic materials. Fine ceramic and/or metallic particles such as nickel particles in Ti0₂ can also be produced by control of these parameters, and the reaction kinetics in the reaction 2NiO + Ti = 2Ni + Ti0₂. Using a combination of reactants that produce a range of vapor pressures of the intermediate species is therefore also likely to produce a mixture of whiskers, hollow shells, platelets and fine powder particles which can be incorporated into a ceramic and/or metallic composite materials. To determine the effect of the nucleating agent on the formation of alumina-containing products having a whisker morphology, three different compositions were prepared. The first composition comprised alumina with zero weight percent TiB₂. The second mixture contained 95 weight percent alumina and five weight percent TiB₂. Finally, a third sample contained 29 weight percent TiB₂ and 71 weight percent alumina.

The mixed TiB₂ and A1₂0₃ powders were pressed at 10,000 psi into green pellets. The green pellets were placed in individual silica ampules. The ampules were repeatedly purged with argon and then with HCl. Finally, each ampule was filled with approximately 250 millimeter HCl. An ampule was then sealed with an oxygen-acetylene torch, and heated to about 1020°C in a tube furnace for three hours, after which time it was cooled to room temperature and examined using the SEM. The resulting microstructures are illustrated in Fig. 32. As is evidenced by Fig. 32, the mixture containing zero weight percent TiB₂ contained no whisker-like particles. The composition containing five weight percent TiB₂ contains some whiskers. The sample containing 29 weight percent TiB₂ contained a substantial number of whiskers. Therefore, it is apparent that the amount of nucleating agent can affect the amount of whiskers formed during processing. it is apparent the whiskers are alumina-containing, however, the exact composition is not clear. There is evidence that some mullite (3Al₂0₃.2Si0₂) is present in the whiskers.

The morphology of product phases (i.e., particle size, whisker aspect ratio and size, platelet size) produced during combined combustion synthesis and vapor phase processing can be controlled via the reaction process parameters, e.g., particle size, amount of diluent in the reactant mix, heating rate, cooling rate, size and geometry of reactant mixture, reactive gas partial pressure, reaction stoichiometry, time and temperature in which reactants or products are in the environment of the reactive gas.

Two examples of how these process parameters can be used to control particle size are (1) in the reaction Ti + 2B ■»^» TiB₂ in HCl gas environment, and (2) control of A1₂0₃ whiskers in the reaction 3Ti0₂ + 3B₂0₃ + 10 Al ≠ 3TiB₂ +

5A1₂0₃ in HCl gas environment. In the first example, using reactant particle sizes of approximately 10μm for titanium and boron in a partial pressure of HCl gas of approximately 0.2 atm and using a heating rate to the ignition temperature of 32°C/minute will produce TiB₂ particles of less than lμm. Using the same reactant and environment at a heating rate of 50°C/ minute will produce TiB₂ particles of approximately 5-10μm. In the second example, using the combustion synthesis reaction 3Ti0₂ + 3B₂0₃ + 10A1 = 5A1₂0₃ in which the Ti0₂ particles and alumina whiskers, the alumina whisker aspect^' ratio and diameter can be controlled by appropriate choice of the reaction process parameters, e.g., particle size of reactant, green density, reaction stoichiometry and diluents, heating and cooling rates, post-combustion synthesis time and temperature. One example of the control of the whisker size and diameter using reaction process parameter control is as follows. Whiskers of lOOym or more can be produced by controlling the post-combustion synthesis reaction time and temperature. If the post- combustion synthesis reaction time is controlled at 1050°C in a HCl gas partial pressure of approximately 0.3 atm, the alumina whisker length can be about 100μ.m when held for 30 minutes at this temperature. Increasing the time at this temperature will increase the volume and length of whiskers. In each case, the whisker diameter at this temperature will be approximately l-10μ,m. Changing the volume fraction of TiB₂ will change the volume fraction of whiskers such that an optimum volume fraction of whiskers occurs at approximately 25 volume percent of TiB₂. The diameter of the whiskers can also be varied by control of post-combustion synthesis time and temperature.

Further, a reinforced composite can be formed according to the present invention by integrating a hot pressing step into the process. According to this embodiment, a green body comprising the source materials and nucleating agent is formed and sintered in a reactive gas to promote the formation of reinforcing particles, such as whiskers or platelets. The time, temperature and partial pressure of the reactive gas can be controlled to control the degree of particle growth. Once formed, the body is then hot pressed to densify the body and form a composite. For example, a load can be applied to a graphite die containing the body while the body is held at an elevated temperature. The foregoing process permits the formation of dense composites with reinforcing particles grown in situ.

It is to be understood that even though numerous characteristics and advantages of various embodiments of the present invention have been set forth in the foregoing description, together with details of the structure and function of various embodiments of the invention, this disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size, arrangement, content, and the like, within the principles of the present invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.

Claims

WHAT IS CLAIMED IS:

1. A method of making materials using combustion synthesis coupled with vapor phase transport, said method comprising the steps of: (a) providing a mixture including a plurality of reactive chemical species capable of reacting exothermically; and

(b) heating the mixture in an environment including a reactive gas such that the plurality of reactive chemical species will react exothermically, wherein the plurality of reactive chemical species generate a volatile intermediate reaction species which subsequently reacts to form the at least one product chemical species, wherein the volatile intermediate reaction species are an intermediate transition product of reactions between the reactive gas and at least one of the plurality of reactive chemical species.

2. The method of Claim 1 wherein the partial pressure of the volatile intermediate reaction species is about 10^"6 atmospheres or greater.

3. The method of Claim 1 wherein the volatile intermediate reaction species react to form at least one product chemical species such that hollow shells are formed, said method further comprising the step of comminuting the hollow shells to form a fine powder.

4. The method of Claim 3 wherein the fine powder has an average particle diameter of less than about 1 micron.

5. A fine powder made of ceramic, metal-ceramic or metallic materials made by the method of Claim 1.

6. A fine powder made of ceramic, metal-ceramic or metallic materials made by the method of Claim 3.

7. Ceramic or metallic whiskers or platelets made by the method of Claim 1.

8. Ceramic, metal-ceramic or metallic hollow shells made by the method of Claim 1.

9. A ceramic, metal-ceramic or metallic composite made by the method of Claim 1.

10. The ceramic, metal-ceramic or metallic composites of Claim 9 wherein the composites include ceramic, metal- ceramic or metallic whiskers contained within the composite, wherein the whiskers are formed in situ during a chemical combustion synthesis reaction following the exothermic reaction between the plurality of reactive chemical species.

11. A process for forming a product comprising the steps of:

(a) reacting a source of elements capable of forming a desired product with a reactive gas to form a transport species with at least a portion of at least one of said elementsr

(b) placing a nucleating agent in fluid communication with said transport species; and

(c) forming said desired product in a desired morphological form on said nucleating agent.

12. The method of Claim 11 wherein said product comprises material in the form of a whisker, hollow shell, platelet or fine powder.

13» The method of Claim 11 wherein said nucleating agent is provided in the form of elements which react to form the desired nucleating agent.

14. The method of Claim 11 wherein said nucleating agent is provided in its desired form.

15. The method of Claim 11 wherein said reaction is an exothermic reaction.

16. The method of Claim 11 wherein the elements which form the volatile species, the nucleating agent and the size of the nucleating agent and the time and temperature of the reaction are all controlled to provide the desired product in a desired morphology.

17. A method for forming aluminum oxide-containing whiskers comprising:

(a) providing a source of aluminum;

(b) providing a source of oxygen; (c) providing a nucleating agent or a source of materials which when reacted form the desired nucleating agent;

(d) reacting the starting materials with a reactive gas to form a volatile reactive phase; and (e) forming aluminum oxide containing whiskers on said nucleating agent.

18. The method of Claim 17, wherein said source of aluminum and said source of oxygen comprise alumina.

19. The method of Claim 18, wherein said nucleating agent comprises TiB₂.

20. The method of Claim 19, wherein said alumina and said TiB₂ are provided in a green body.