BACKGROUND OF THE INVENTION

Combustion synthesis describes a method whereby two or more reactants are mixed and ignited to yield a new material. This method may also be referred to as "self-propagating high-temperature synthesis" (SHS) or "self-propagating synthesis" (SPS) . The reactants may be in the form of solids, gases or liquids and various combinations of these. The present invention relates to the use of microwave energy to achieve ignition and controlled combustion to yield a wide variety of useful materials including ceramics, cermets and ceramic matrix composites.

Combustion synthesis is a process by which the reaction between two or more materials form a new product phase. The reaction is exothermic and the energy given off by the reaction allows the remaining reactants to be heated to a state where formation of the products is thermodynamically favorable. Other investigations have shown that high purity materials can be produced by this method quickly and at low cost. The high purity is due to the volatilization of impurities during the reaction. Energy costs are low because the exothermic reactions.

under the right conditions, produce the necessary heat to continue the process. The time to synthesize the product is extremely short _. once ignition has been induced.

Conventionally, the reactants are ignited with a heating coil at one surface. The solid reactants may be in the form of loose powders, whiskers, platelets or a shaped body. The reaction occurs at one surface and a reaction wavefront propagates towards the other end of the mass. A problem that is encountered with coil ignition is the formation of a nonuniform wavefront. Heat radiated from the ignition coil does not uniformly raise the temperature of the entire surface on the exposed side of the sample. Therefore the reaction wavefront is not always a single planar wave.

Alternatively, the material may be placed directly in a conventional radiant-heated furnace and ignited. In this process, the surface of the sample is more uniformly heated than with the use of a heating coil to ignite at a spot. Even so the combustion is not uniform throughout the material (the exterior ignites first and the combustion wavefronts move towards the center) and severe thermal gradients may develop which can result in thermal shock and ¹ nonuniform material properties.

The reactants have also been ignited by a laser. A laser beam ignites the reactants at a small spot on the surface of the sample. This ignition creates the reaction front. Similar types of problems as encountered with ignition using a heating coil are also encountered with laser ignition.

Specific examples of conventional combustion synthesis processes can be found in U.S. Patent No. 2,886,454 and U.S. Patent No. 4,678,760, for example. U.S. Patent 2,886,454 describes a combustion synthesis process wherein reactants are ignited and an electric current is employed for ignition. U.S. Patent No. 4,678,760 describes combustion synthesis processes employing various methods of ignition. For example, U.S. Patent No. 4,678,760 describes igniting reactants using a radio frequency (RF) coil. However, use of a RF coil for ignition results in the formation of a non-uniform wavefront, and suffers from the disadvantages discussed above with respect to a heating coil. Finally, U.S. Patent No. 4,481,091 describes the use of electromagnetic energy (laser, ultraviolet and visible) to promote various chemical reactions such as photocatalysis, chemical synthesis and purification. The wavelength of the electromagnetic energy employed in U.S. Patent No. 4,481,091 is from about 0.5 to 60 microns and is

substantially smaller than the wavelength of microwave energy. U.S. Patent No. 4,481,091 does not specifically describe combustion synthesis methods.

Therefore, it is an object of the present invention to provide a combustion synthesis method which does not suffer from the disadvantages of prior processes.

It is yet another object of the present invention to provide a method of igniting and sustaining controlled combustion of reactants using microwave energy.

It is a further object of the present invention to provide a method of combustion synthesis wherein a uniform reaction wavefront is propagated.

Yet another object of the present invention is to provide a combustion synthesis method using microwave energy whereby reaction products having superior characteristics and properties are produced.

These and other objects of the present invention will be further understood and apparent from the following description and drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS

Figures la-Id illustrate combustion wave propagation for different ignition methods;

Figure 2a shows a temperature profile of a sample versus time for a process employing microwave ignition in accordance with the present invention and Figure 2b shows a temperature profile versus time for a process employing conventional ignition;

Figure 3 shows the absorption of microwave energy versus temperature in a combustion synthesis process of the present invention;

Figure 4 illustrates the temperature versus time relationship in a combustion synthesis process using microwave energy;

Figure 5a is the X-ray diffraction pattern of an unreacted mixture of titanium and carbon black powders;

Figure 5b is the X-ray diffraction pattern of the product phase titanium carbide produced by a combustion synthesis reaction;

Figure 6a is the X-ray diffraction pattern of an unreacted mixture of aluminum, titania and graphite powders; and

Figure 6b is the X-ray diffraction pattern of a composite material of Al ₂ 0 ₃ -TiC formed by a combustion synthesis reaction.

Figure 7 is a photograph of a sample synthesized by microwave ignition and controlled combustion in accordance with the present invention.

SUMMARY OF THE INVENTION

The present invention relates to the use of microwave energy to initiate an exothermic reaction between reactants in a combustion synthesis process. More specifically, the invention relates to the use of microwave energy to ignite a mixture of reactants, which reactants react exothermically to produce a reaction product. The invention further relates to the use of microwave energy to control combustion of reactants.

DETAILED DESCRIPTION OF THE INVENTION

Microwave energy is an alternate, yet distinct way of igniting and sustaining controlled combustion of the reactants. Microwave energy, because of its novel internal heating mechanism, tends to heat an entire sample nearly uniformly. The surface of the sample radiates energy, resulting in a higher temperature at the interior of the sample.

Referring now to Figs, la-d, a comparison of combustion wave propagation for different ignition methods is shown. Figs, la-c illustrate that, with combustion synthesis methods other than microwave, ignition and combustion are initiated at the surface and propagate inward. Fig. Id shows the outward, radial wave propagation in combustion synthesis using microwave

energy. In the inventive process of Microwave Ignition and Combustion (MICOM) , because of the higher temperatures, the sample ignites in the center and a combustion wavefront propagates outward in a radial manner as shown in Fig. Id.

Figs. 2a and 2b give the temperature profile within a sample at different times. Conventional external ignition and combustion result in a severe thermal gradient as indicated in Fig. 2b. In contrast to conventional ignition, MICOM can be better controlled throughout the entire mass of the sample to yield more uniform propagation with less severe thermal gradients as shown in Fig. 2a.

Alternatively MICOM can be used to produce controlled thermal gradients that can be advantageous in subsequent processing steps. For example, if the sample is to be crushed into a fine powder after controlled combustion, thermal gradients can be used to yield controlled stresses/microcracks in the compact that will be beneficial in the crushing process.

In conventional ignition, the propagation of the combustion wavefront is strongly dependent on the thermal conductivity, the reaσtant-powder-compact density, the composition and the _* surroundings of the sample. At high compaction densities, the propagation

rate may decrease or terminate due to self-extinction. In certain cases it may even fail to ignite. Thus, one expects an unstable wavefront or no wavefront at all. In contrast, with microwave ignition, energy is constantly absorbed within the material. This absorption ensures that the ignition temperature is sustained. As the temperature of the material is increased, the absorption of microwave energy by the material is increased as shown in Fig. 3. Higher absorption leads to increased dissipation of energy, resulting in a corresponding increase in temperature. Ignition temperature (T ) is reached first in the center of the sample, leading to an even higher combustion temperature (T _c ) , which further increases the absorption of microwave energy. So, microwave energy and combustion synthesis assist each other in sustaining the reaction. The high temperature in the interior of the sample forces propagation of a relatively uniform radial combustion wavefront to the exterior of the sample, which ensures complete combustion of the materials as long as the microwave energy is maintained. Thus, with microwave ignition and combustion, the dependence of the reaction on the thermal conductivity and the density of the compact is greatly reduced, compared to those samples

ignited according to conventional methods. Samples pressed to compact densities in excess of 80% have been easily ignited using a higher power level microwave oven. Further, as mentioned earlier, the volumetric heating with microwave energy results in better control of thermal gradients and consequently better control of the properties of the final product.

Fig. 4 depicts the temperature versus time relationship when synthesizing combustible material with microwave energy. At ignition temperature ( J) , the constituents react to produce exothermic energy that raises the temperature of the sample. As indicated in Fig. 4, at higher temperatures the samples absorb more microwave energy, thus generating more heat. If the microwave power is left on after combustion, temperature (T _s ) higher than the ignition temperature (T-) can be sustained due to high microwave absorption at the combustion temperature (T ) . It has been reported elsewhere that diffusion rates are higher in the presence of microwave energy. Thus, with more controlled temperatures and enhanced diffusion rates, improved homogeneity and more dense products are produced than that which is produced by conventional ignition.

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One additional feature of MICOM is that the removal/retention of gases released during reaction can be better controlled than with conventional methods. This is because of the internal heating and combustion from the inside propagating outward. In certain cases, gas retention may be desirable, (i.e., with making powders or foamed ceramics used for insulation and filters) while in other cases it may be undesirable (i.e. , dense materials used for structural applications) .

Combustion synthesis has been reported to produce borides, carbides, suicides, aluminides, hydrides, chalσogenides, germanides, and nitrides. These include, but are not limited to borides of Ti, Zr, Nb, Ta, Mo and La; carbides of B, Al, Ti, Hf, Ta, Si and W; suicides of Ti, Zr, Nb and Mo; nitrides of Ti, Zr, Hf, Tb, Ta, Si, B, Al and Be; chalcogenides of Mg, Nb, Ta, Mo and W; hydrides of Ti, Zr, Cs, Pr, I and Nd; and aluminides of Cu, Ni, Nb and Fe

Based on our work, many metals as well as boron, carbon (graphite) and silicon powders absorb microwave energy very well and can reach high temperature in a short interval of time, as indicated in Table I. Thus, microwave ignition and combustion of the powders to form the borides, carbides and suicides is possible. Several metallic powders have been heated with microwave

energy to reasonably high temperatures (Table I) . Even higher temperatures are possible if a higher power microwave oven is used (see footnote below Table I) .

TABLE I

Microwave Heating of Various Elements at a Frequency of 2.45 GHz and Power of 700W.

Element Temperature* Time ( ^β C) (Sec.)

Boron 1100 40

Carbon (graphite) 1200 60

Silicon 1000 70

Titanium 1150 60

Nickel 500 195

Aluminum 600 90

Tantalum 700 160

Zirconium 710 160

Molybdenum 650 150

Niobium 700 190

* The temperatures reported are those obtained when heated in a microwave oven having a power level of only 700 watt. Materials heated to a temperature of 250 ^β C in 8 minutes in this oven, have been successfully heated to 1200*C in less than 2 minutes in a higher power microwave oven (Manufactured by Raytheon Co. , 6.4 kW maximum 2.45 GHz) .

This ability to heat metals is completely unexpected in view of the literature which suggests that metals reflect microwave energy and may not be used in microwave processing. It is believed that the reason the powders can be heated efficiently with microwave energy is that the powders provide much more internal surface area that interacts with the microwave energy and dissipates energy, resulting in high temperatures. It has also been observed in our laboratory that dense (non- porous) alumina does not heat very efficiently in microwave energy, whereas low-density alumina (porous) can be heated much more rapidly. Thus, the presence of pores in a material improves the heating e ficiency in microwave energy. This explains the heating behavior of metallic powders, providing the potential for forming the nitrides, hydrides and aluminides of almost all the elements mentioned.

In addition to porosity, particle size of the reactants is also important. Higher heating rates are obtained with smaller particle size, larger specific surface area powders of metals, non-metals and compounds. Microwave energy and combustion synthesis assist each other to sustain the reaction. Thus all

I known materials formed by combustion synthesis and combinations of these materials are believed to be

.3 suitable for synthesis with MICOM or Microwave Ignition and Controlled Combustion (MICROCOM) in accordance with the present invention. This includes, in addition to the binary compounds, composites of two or more compounds, cermets and Platelet Reinforced Composites (PRC T ^J " ^l M ⁱ ) similar to those fabricated by the Lanxide process.

As is apparent from Table II, the ignition time is dependent on the mass and density of the samples. The greater the mass for a given density, the shorter the time for samples to ignite. Similarly, as the density increases, the time for ignition increases. In relatively high density samples for the Al + C + Ti0 ₂ system, ignition was not as violent as for low density samples and the controlled combustion wavefront did not propagate to the surface of the samples during the time indicated in Table II. A series of samples has been sectioned to examine the extent of combustion. With the increase in time allowed for combustion in the microwave oven the reacted area (in cross-section) has increased

(Table II) . The mass of all the samples was 3 grams and the density was approximately 65% of the theoretical value. This suggests that, at relatively higher densities, the combustion can be controlled by the time allowed in the microwave oven for combustion. It should be understood that microwave processing conditions that

lead to controlled ignition and combustion depend on density, thermal conductivity and the chemical and physical properties of the reactants and these will be different for each system.

TABLE II Ignition time and extent of combustion (diameter of reacted area) as a function of sample mass, density, total microwave time and the mass of silicon carbide (as a blanket) , for the synthesis reaction: 4A1 + 3Ti0 ₂ + 3C ^ΔH > 2A1 ₂ 0 ₃ + 3TiC + Exothermic energy

Mass Density Ignition Time Extent of mass of time in MW reaction Sic blanket (g) (% Th) (m:sec) (m:sec) (Dia in mm) (g)

1 54 4:27 8:00 12.9 55

1 60 5:28 8:00 12.9 55

1 63 Undetectable** 8:00 7.9 55

2 56 3:39 8:00 12.9 55

2 61 5:31 8:00 12.9 55

2 63 Undetectable** 8:00 5.2 55

3 59 2:50 8:00 12.9 55

3 63 3:42 8:00 12.9 55

3 65 Undetectable** 8:00 8.8 55

3 65 Undetectable** 12:00 12.9 55

3 65 Undetectable** 4:00 0.0 29

3 65 Undetectable** 5:00 6.6 29

3 65 Undetectable** 6:00 7.9 29

3 65 Undetectable** 7:00 8.2 29

* Ignition time refers to the time taken by the sample to ignite violently, which is visible for uncontrolled reactions.

** Undetectable means that is this sample violent ignition does not occur and ignition cannot be visibly observed. Although ignition time is referred to as undetectable, ignition does not take place and the combustion wavefront propagates in a controlled matter.

Another parameter that can play a role in controlling the rate of combustion is the amount of microwave absorbing material, e.g., silicon carbide, around the crucible containing the samples to be synthesized. This is useful for providing a blanket around the sample to control thermal gradients. As shown in Table II, increasing the amount of silicon carbide granules around the crucible increases the rate of combustion because of the extra heat provided by the jacket that itself, absorbs a portion of the microwave energy. Reducing the silicon carbide effectively lowers the rate of propagation for small-mass (1 to 3 gram) samples. However, when processing high-mass samples the reverse trends have been observed. Thus there are a number of parameters that one may use to control the rate and extent of combustion with microwave energy. These are the sample mass, sample density, microwave power, microwave time and the amount of silicon carbide or other blanket material around the sample.

The controlled synthesis is termed icrowave Ignition and Controlled Combustion (MICROCOM) . Figure 7 is a photograph of a sample synthesized by MICROCOM. It can be seen from Fig. 7 that ignition and combustion originate in the center and propagate towards the outside. The black area corresponds to products

(Al ₂ 0 ₃ /TiC) and the gray area to the unreacted material (Al + Ti0 ₂ + C) .

It has been reported in the literature by American as well as Russian authors that it is difficult to ignite samples having compact densities greater than 80% of the theoretical density. We have used microwave energy to successfully ignite samples with compact densities greater than 80% using the materials system Ti + C. Not only are we able to ignite these samples, but we are able to carry out the combustion in a controlled manner. However, it was not possible to ignite Ti + C samples in the same microwave oven (700 watts) that had been used for the other experiments. The microwave oven used for ignition and combustion of high compact density samples has a much higher power (6.4 kW maximum) . It was operated at 3.2 kW and a duty cycle of 40% for only 5 minutes.

A major advantage of the controlled combustion is that the samples were less likely to cleave. They retained their original shape. This controlled combustion is very advantageous for fabricating monolithic structures, which are almost impossible to produce by any other ignition/combustion method.

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The inventive method of combustion synthesis using microwave energy can be employed to fabricate large monolithic structures, powders, and composites using whiskers, platelets or fibers. This combustion synthesis method is also useful for fabricating coatings of different compositions on various substrates.

The combustion synthesis process or controlled combustion process can be controlled by either pulsing the microwave energy source or continuously applying the microwave energy. Pulsing may be achieved by switching the microwave oven on or off at specified time intervals or by controlling the duty cycle of the oven.

In addition, the process can be controlled by the use of microwaves together with the incorporation of a diluent phase into the reactants. Suitable diluent phases include those previously employed in combustion synthesis processes.

Oxidizing and reducing agents may be incorporated as additions into the reaσtant material mixture or system in order to increase or decrease the combustion rate and increase the mass transfer. Suitable oxidizing agents include KC10 ₄ , KMn0 ₄ , KC10 ₃ , KN0 ₃ , HgCl, NaN0 ₃ , NaN ₃ , NH ₄ N0 ₃ , NH ₄ C10 ₄ , metal halides of Group I- III metals, and the like. Suitable reducing agents include NH ₄ F, NH ₄ N ₃ , NH ₂ NH ₂ , ammonium salts, thiourea and

IB,

the like. Other additives such as ammonium persulfate, urea, sodium sulfate and sodium thiosulfate may also be incorporated in the reaσtant material system.

In addition to ignition and combustion of the reactant powders using microwave energy, these powders also can be cleaned of physically and chemically bound impurities using microwaves prior to ignition.

A complete process employing the microwave ignition or combustion in accordance with the present invention may include the following steps:

(1) preprocessing the reactant powders to remove adsorbed gases or other volatile impurities;

(2) ignition/combustion of the reactant powders using microwave energy and control of the combustion rate and combustion wavefront velocity cycling and (3) further densification of the resultant reactant material using microwave energy.

It is also within the scope of the present invention to locally ignite a portion of the reactant mixture using microwave energy in a microwave oven by deliberately incorporating on the surface or in the bulk of the reactant mixture a small amount of microwave coupling materials that would heat up and then ignite the remaining reactant mixture. Further, local ignition can be achieved by bringing a metal wire in close proximity

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to the reactants inside the microwave oven. This would result in a high temperature in the vicinity of the tip of the wire, resulting in ignition and combustion. These methods of local ignition can be carried out in single or multiple spots to achieve desired effects.

The inventive process can be carried out in an atmosphere of a variety of gases, including air, argon and nitrogen.

Microwave wavelengths are defined to be between lmm and lm. The microwave energy useful in the process of the present invention, therefore, has a wavelength between lmm and lm. Further, it is believed that RF waves having wavelength between lm and 10m could be used in the inventive combustion synthesis process in place of microwaves. In this context, it is pointed out that the use of RF waves would require a special tube similar to the magnetron tubes used in microwave ovens. A RF oven would provide advantageous wavefront propagation characteristics similar to those of microwaves. Use of a RF oven would not require the use of a RF coil and a combustion synthesis process employing a RF oven would not suffer from the disadvantages of a process employing a RF coil as set forth above.

Finally, it is pointed out that the inventive process may be viewed as either microwave-assisted combustion synthesis or combustion synthesis-assisted microwave processing. As an example of combustion synthesis-assisted microwave processing, it is pointed out that some materials do not couple very well with microwaves at room temperature. However, as their temperatures increase, these materials couple very well, thus, illustrating the concept of combustion synthesis- assisted microwave processes.

EXAMPLES

By way of example only, microwave ignition can be used to fabricate TiC (From Ti and C) and to fabricate a composite of Al ₂ 0 ₃ /TiC (from Al, C and Ti0 ₂ ) . Titanium metal and carbon black powders are used to synthesize titanium carbide (TiC) . Titania (Ti0 ₂ ) , aluminum metal (Al) and graphite (C) were used to synthesize the aluminum oxide-titanium carbide composite.

The constituent reactants in powder form were weighed in the appropriate ratios with a 0.1 weight percent excess addition of the metal species. The reactants were dry mixed for five minutes. Following mixing, the reactants were pressed into cylindrical samples with a diameter of 12.9 mm and height varying

Si

from 5mm to 15 mm (depending on the mass of the sample) . A sample was placed into a microwave oven which was used to perform a combustion reaction. For most of the results reported herein the microwave oven used was a domestic model, manufactured by Hardwick, which has an operating frequency of 2.45 GHz and a power rating of 700 Watts. Approximately 3-5 minutes were required to reach ignition temperatures for samples weighing 3-4 grams. As the mass of the sample is increased, the time for ignition decreases. Samples weighing about 7-8 grams required less than 30 seconds to reach T.. Thus, this process appears to be exceptionally advantageous for igniting larger masses. Pre-ignition density of the samples was in the range of 60 - 72% of theoretical values. Samples with higher densities took longer to ignite than those with lower densities.

A higher power industrial microwave (Raytheon 2.45 GHz and a power of 6.4 kW maximum) was used to ignite samples with densities greater than 80%.

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Example 1: The Titanium Carbide System

A 3 gram sample consisting of Ti and C powders was placed inside the microwave oven at full power (700 watts) . Ignition occurred within several minutes, and the reactants were transformed into the product phase of titanium carbide according to the following reaction:

ΔH Ti + C > TiC + Exothermic Energy

The formation of titanium carbide was proven by X-ray diffraction analysis of the material extracted from the product phase. Figures 5a and 5b are the X-ray diffraction patterns of the reactants, (Ti + C) , and the product phase, (TiC) , respectively.

The X-ray diffraction pattern for the mixture of Ti and C (Fig. 5a) has titanium peaks labeled "t". Carbon peaks are not present because of the amorphous structure of carbon black. Figure 5b is the X-ray diffraction pattern of the material obtained from the Ti + C sample after microwave ignition and reaction. The diffraction pattern indicates the presence of TiC. The peaks are labeled "T" for TiC and "M" for sillimanite (Al ₂ Si0 ₅ ) . (The sillimanite is an alien material introduced during sample preparation for X-ray diffraction analysis.) The transformation of Ti + C to TiC is evident by the disappearance of the Ti + C

a* diffraction pattern and the appearance of the TiC diffraction pattern after ignition and reaction.

Other samples of different masses and densities were also synthesized using MICOM/MICROCOM with comparable results.

Example 2: The Aluminum Oxide - Titanium Carbide Composite System

Samples consisting of the reactants (aluminum, titania, and carbon (this time in the form of graphite,

G) ) were transformed into the product phases (aluminum oxide and titanium carbide) according to the following reaction:

ΔH 4A1 + 3C + 3Ti0 ₂ > 2A1 ₂ 0 ₃ + 3TiC + Exothermic Energy

The formation of aluminum oxide and titanium carbide was proven by X-ray diffraction analysis of the material extracted from the product phases. Figures 6a and 6b are the X-ray diffraction patterns for the reactants (Al, G and Ti0 ₂ ) and the products (A1 ₂ 0 ₃ and TiC) , respectively. This sample weighed approximately 4 grams and required about 3-5 minutes to ignite.

The X-ray diffraction pattern for the mixture of Al, C and Ti0 ₂ (Fig. 6a) has the aluminum peaks labeled "Al", the graphite peaks labeled "G" and the

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titania peaks labeled "AN". Figure 6b is the X-ray diffraction pattern of the material obtained from the 4A1 + 3C + 3Ti0 ₂ sample after microwave ignition and reaction. The diffraction pattern indicates the presence of aluminum oxide and titanium carbide. The peaks are labeled "A" for aluminum oxide and "T" for titanium carbide. The transformation of 4A1 + 3C + 3Ti0 ₂ to 2 1 ₂ 0 ₃ + 3TiC is evident by the disappearance of the aluminum, graphite and titania diffraction pattern and the appearance of the aluminum oxide and titanium carbide diffraction patterns after ignition and reaction.

Ignition times for samples of various masses and densities synthesized using MICOM/MICROCOM are also listed in Table II.

Although the invention has been described with reference to particular embodiments and examples thereof, it is to be understood that modifications may be made without departing from the scope of the invention as described above and as defined in the claims which follow.