Microwave ignition and combustion synthesis of composites

Materials Science and Engineering, A 144 ( 1991 ) 91-97

91

Microwave ignition and combustion synthesis of composites D. E. Clark, I. Ahmad and R. C. Dalton Department of Materials Science and Engineering, University of Horida, Gainesville, Fir 32611-2066 ¢U.S.A.)

Abstract Microwave processing and combustion synthesis have been united to fabricate composite materials. The internal heating with microwave energy initiates the ignition in the center of the sample and the combustion wave front propagates radially outward. Under suitable processing conditions the wave front can be controlled in contrast with the self-propagation in conventional combustion synthesis. The exothermic energy released during the combustion reaction assists in microwave heating (microwave absorption increases with increasing temperature). Thus microwave ignition, controlled combustion and sintering of dense compacts allows fabrication of composites which are difficult to obtain with conventional self-propagating high-temperature synthesis.

I. Introduction Microwave energy is unique in its rapid and internal heating. This distinct feature has been exploited to ignite reactions in the interior of the sample. A relatively uniform combustion reaction wave front travels radially outward until it reaches the surface of the sample, converting the reactants into the product phase. Conventionally, two or more combustible powders are mixed together and shaped into the desired form. To ignite the sample, one surface of the sample is exposed to a conventional heat source. The heat source may be a heating element, a sparker or a laser. The surface temperature is raised to the ignition temperature where the materials react exothermically. The heat generated at the surface allows the adjacent material to react, and a combustion wave front propagates through the entire volume. During propagation of the combustion wave front, the reactants are converted to products, and most impurities w)latilize owing to high reaction temperatures. The volatile impurities greatly expand the pre-form, producing highly porous ceramics and composites with low mechanical strengths. Dense monoliths can be produced only when pressure is applied during the combustion reaction [1]. The conversion of the reactants to products increases with decreasing combustion wave front velocity. A slower wave front velocity 0921-5093/91/$3.50

also allows the volatile impurities to be removed in a more controllable manner, which reduces the expansion and increases the density of the product. The velocity of the combustion wave depends on a number of factors. These include the heat of formation, density, thermal conductivity, particle size distribution, combustion temperature and kinetics of the reaction. The velocity of the combustion wave front decreases with an increase in density, increase in thermal conductivity, increase in particle size, decrease in heat of formation and decrease in combustion temperature. The available heat can be controlled by adding diluents in the form of reinforcements or product phases [1, 2]. The pre-reacted compact density is reported to be optimized between 55 and 70% of the theoretical bulk density. Reports indicate that compacts with a theoretical bulk density out of this range tend either not to ignite or to burn out before the process can be completed [2]. Bulk densities greater than 80% could not be ignited because of the threefold increase in thermal conductivity. The high thermal conductivity does not allow the surface temperature to reach the ignition temperature [3, 4]. The purpose of this paper is to highlight the important features of microwave heating that can be employed in combustion synthesis. However, first we shall provide a brief but relevant over© Elsevier Sequoia/Printed in The Netherlands

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view of the use of microwave energy for processing materials. 2. Microwave processing of materials Microwave heating is fundamentally different from conventional heating. With microwave energy the heat is generated internally through material-microwave interaction instead of originating from an external heating source. This internal volumetric heating results in thermal gradients that are the reverse of"those observed in conventional heating. Consequently, microwave processing makes it possible to heat both small and large shapes very rapidly and relatively uniformly. This can help in efficient removal of volatile constituents (binders, moisture etc.) and can significantly reduce the thermal stresses that cause cracking during processing. Microwave processing has been employed in a number of areas. A comprehensive paper summarizing almost all the work done in this area was published by Sutton [5]. Microwave heating, until recent years, has dealt almost exclusively with food processing [6], but this is now expanding to include almost all materials. Industrial microwave heating applications currently being explored, include curing of polymers, medical diagnostics, electronics, mineral processing, asphalt recycling and ceramic processing. Ceramic processing includes drying, binder burnout, calcining, sintering, annealing, nucleation-crystallization and joining of ceramics.

Many ceramics are transparent to microwaves at ambient temperatures. Generally, dielectric materials become more receptive to microwave energy with increase in temperature and as a consequence absorb a greater amount of energy. Coupling agents can be added to materials that cannot be efficiently heated with microwave energy. The coupling agent interacts with the microwave and pre-heats the sample. At higher temperatures the sample itself couples with microwave radiation. Alternatively, susceptor materials can be used around the transparent ceramic which pre-heats the sample to a temperature where it starts to couple with microwave energy. An advantage of the susceptor is that it too reduces the temperature gradient that usually exists in the sample, thus allowing the sample to be heated uniformly. The extent of susceptor heating can be varied to get the optimum processing conditions. We refer to the use of a susceptor to aid in microwave heating as hybrid heating. The set-up for hybrid heating is shown in Fig. 1. Here the susceptor material is silicon carbide (SIC) which is coated on the inside of an insulation which retains the heat. Figure 2 shows the uniformity achieved in an alumina sample with hybrid heating [7]. The porosity vs. position across the width (points A-E in Fig. 2(a)) of the sample is almost a straight line (Fig. 2(b)) for the microwave hybrid heating case. The conventionally sintered sample displays a higher porosity in the interior of the sample, with the lowest values still higher than those for the

TEMPERATURE DISPLAYS AND CONTROLLERS FEEDBACK SIGNAl, PY ROME'I'ER

THERMO,

INSULATION

SAMPLE

INSULATION WITH SUSCEPTOR LINING

Fig. 1. Experimental set-up using the Raytheon multimode microwave oven (6.4 kW maximum, 2.45 GHz).

93

(a)

minerals and elements including metal powders. Ho and Kramer [10] have studied the microwave dielectric properties of metal-filled particulate composites (metal particles in an epoxy matrix). They have shown that composites with smaller particle sizes have higher values for the dielectric constant (real part) and that the dielectric constant increases with the metal volume percentage.

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3. Microwave ignition and combustion

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sample heated with microwave energy. These microstructural improvements translate into improved mechanical performance (Fig. 2(c)). It is generally believed that metals reflect microwave energy and hence metals should not be placed in microwave ovens because they result in arcing. This is true for solid metal bodies and other conductors. However, metal powders have been effectively heated to their melting points in our laboratory [8]. Walkiewicz and McGill [9] at the Bureau of Mines have listed heating various

Combustion synthesis has been performed very effectively using microwave energy. Because of its novel internal heating, microwave energy heats the entire sample nearly uniformly. The surface of the sample radiates energy, resulting in a higher temperature at the interior of the sample. Thus in microwave ignition and combustion, because of higher temperatures, the sample ignites in the center and a combustion wave front propagates outward in a radial manner. In conventional ignition, the propagation of the combustion wave front is strongly dependent on a number of parameters outlined above. At high compaction densities, the propagation rate may decrease or terminate owing to self-extinction, or samples may fail to ignite in certain cases. Thus one expects a non-uniform wave front or no wave front at all. However, microwave energy can be used to overcome this limitation. When using microwaves for ignition, energy is absorbed constantly within the material. This ensures that the ignition temperature is sustained. As the temperature of the material increases, the absorption of microwave energy by the material increases. Higher absorption leads to increased dissipation of energy. This results in a corresponding increase in temperature. The ignition temperature T~ is reached first in the center of the sample, leading to an even higher combustion temperature 7~. 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 wave front to the exterior of the sample, which ensures complete combustion of the material. With microwave ignition and combustion, the dependence of the reaction on the thermal conductivity and the density of the compact is greatly reduced in comparison with those samples ignited with conventional methods. Furthermore, after the reaction is complete, the microwave power can be left on to

94

densify the sample further, because at higher temperatures it can absorb more microwaves to help to maintain these temperatures. This is not possible in conventional synthesis because, once the combustion has taken place, the temperature decreases rapidly. It also has been reported elsewhere that diffusion rates are higher in the presence of microwave energy [11]. With more controlled temperatures and enhanced diffusion rates, one expects more homogeneous and more dense products than those obtained by conventional synthesis.

wave front propagation. Further, a high value for density reduces microwave heating rates of the sample. These two factors combine to provide control of the propagation of the combustion wave front. The rate of propagation of the wave front also can be controlled by the incident power. Turning the power off can terminate the propagation, whereas pulsing the incident power (by altering the duty cycle) will give even more precise control on the velocity of propagation of the combustion wave front.

4. Microwave ignition and controlled combustion

5. Experimental details

In addition to the advantages of using the microwave energy mentioned above, control of the propagation of the combustion wave front is also possible. Having control on the wave front propagation allows for a gradual release of volatiles, which is very important for fabricating dense products. In conventional combustion synthesis, ignition takes place on the surface of the sample. If the density and the thermal conductivity are high, the combustion wave front may not be able to propagate at all. The use of microwave energy is especially favorable under these circumstances, because the wave front can be forced to propagate by constantly dissipating microwave energy in the sample. Let us consider a sample exposed to microwave energy. The power absorbed Pab by the sample is given by [12] Pab =

aE2 V= 2gfE0eeffE 2 V

( 1)

and the increase in temperature is

AT =°E2t

(2)

where a is the conductivity, f is the frequency, e0 is the permittivity of free space, eeefis the effective relative dielectric loss factor, E is the electric field in the sample, p is the density, V the volume and Cpis the specific heat of the sample. The increase A T in temperature is obviously dependent on the density p of the sample. An increase in the density (and the corresponding increase in thermal conductivity) of the sample results in a lower heating rate. As mentioned above, high compact density and high thermal conductivity limit, or inhibit, the

The experimental set-up used for this work is shown in Fig. 1. A high power industrial multimode microwave oven (6.4 kW maximum; 2.45 GHz) manufactured by Raytheon Co. was used for the synthesis of composites. However, a simple kitchen model microwave oven (Hardwick; 700 W; 2.45 GHz) was also used to ignite Ti-C samples and some composites with lower green densities. Samples of Ti-C powder mixtures were enclosed in an insulation and placed inside the microwave oven. At full power (700 W), ignition occurred within several minutes. The reactants transformed into the product phase according to the following reaction: Ti + C ~ T i C + heat

(3)

The higher power industrial multimode microwave oven was used successfully to ignite samples with densities greater than 80%. The product showed little or no expansion following combustion. The composite system presented in this paper is aluminum oxide (A1203)-titanium carbide (TIC). Titania (TiO2), aluminum metal and graphite were used to synthesize the AI203-TiC composite. The reactants in powder form were weighed in the appropriate molar ratios given in 4AI + 3C + 3TiO 2 ~ 2A1203 + 3TiC + heat

(4)

After dry mixing, the mixture was pressed into cylindrical samples with diameters of 12.9 mm and heights ranging from 5 to 15 mm depending on the mass of the sample. Pre-ignition densities of the samples were typically in the range 60-72% of theoretical values.

95 6. Results and discussion

6.1. The titanium carbide system The X-ray diffraction pattern for the T i - C mixture (Fig. 3(a)) has titanium peaks labeled t. Carbon peaks are not present because of the amorphous structure of carbon black. Figure 3(b) is the X-ray diffraction pattern of the material obtained from the T i - C sample after microwave ignition and reaction. The transformation of T i - C to TiC is evident by the presence of TiC. The peaks have been labeled T for TiC and M for sillimanite (AlzSiOs). (Sillimanite is an alien material introduced during sample preparation for X-ray diffraction analysis.) Approximately 3-5 min were required for samples of mass 1-4 g to reach ignition temperatures in the microwave oven. Samples of mass about 7-8 g required less than 30 s to reach the ignition temperature.

the product after microwave ignition and reaction. The transformation of 4AI + 3C + 3TIO2 to 2A1203 + 3TiC is evident by the disappearance of the aluminum, graphite and TiO 2 diffraction patterns and the appearance of the A1203 A and TiC T diffraction patterns. Samples with higher bulk densities and smaller mass took longer to ignite than those with lower densities and larger mass (Fig. 5). Ignition times for samples of various masses and densities, syn-

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700 W; 2.45 GHz; 55 g of susceptor). Larger masses and low bulk density samples require less time for ignition and combustion in the microwaveoven.

96 TABLE 1 Ignition time and extent of combustion (diameter of reacted area) as a function of sample mass, density, total microwave time and the mass of susceptor (SIC) for the synthesis reaction 4AI + 3TiO 2 + 3C ~ 2A1203 + 3TiC Mass (g)

Density (% of the theoretical density)

Ignition time

1 1 1

54 60 63

4 m i n 27 s 5 min 28 s Undetectable

2 2 2

56 61 63

3 3 3 3 3 3 3 3

Diameter of extent of reaction (mm)

Mass of SiC susceptor (g)

8 8 8

12.9 12.9 7.9

55 55 55

3 min 39 s 5 min 31 s Undetectable

8 8 8

12.9 12.9 5.2

55 55 55

59 63 65 65

2 min 50 s 3 rain 42 s Undetectable Undetectable

8 8 8 12

12.9 2.9 8.8 2.9

55 55 55 55

65 65 65 65

Undetectable Undetectable Undetectable Undetectable

4 5 6 7

0.0 6.6 7.9 8.2

29 29 29 29

thesized using microwave energy, were studied (Table 1). The time to ignite a sample was observed to be dependent on the mass and density of the samples. The greater the mass for a given density, the shorter is the time required for samples to ignite. Similarly, with an increase in the density, the time for the ignition increased. In relatively high density samples for the system given in Table 1, the time taken by the sample to ignite was not detectable and the combustion wave front did not propagate to the surface of the samples. This indicated that, at relatively high densities, the combustion can be controlled by the time allowed in the microwave. A series of samples was sectioned to examine the extent of combustion. As the time allowed for combustion in the microwave oven was increased, the reacted area increased (cross-sectional diameter given in Table 1). Figure 6 shows a series of samples in cross-section. It indicates that ignition initiates at the center of the sample and that combustion propagates radially outward in a controlled manner. (The dark area corresponds to the product and the grey area to the unreacted material.) As the wave front propagates radially outward, more surface (4zrr 2, which increases with increasing r) is encountered, which absorbs the heat generated at the smaller adjacent surface (smaller r). This is another factor contributing to controlled propagation. In planar wave front propagation (from one end of the sample to the other)

Time in microwave oven (min)

Fig. 6. Cross-section of a number of samples showing the reaction at different times. Samples indicate the radial propagation and controlled combustion.

the cross-sectional area remains constant, and the heat absorbed per unit area is larger than that of radial propagation. Thus radial wave front propagation is slower, allowing for more control than planar wave front propagation. Another parameter that plays a role in controlling the rate of combustion is the susceptor (in this case SiC) around the crucible containing the samples which helps to reduce the thermal gradients. Thus a number of parameters were used to control the rate and extent of combustion with microwave energy. These include the sample mass, the sample density, the microwave power, the duty cycle of microwave power and the amount of susceptor around the sample.

97 TABLE 2

J. W. McCauley (eds.), Materials Processing by Self-Propa-

Various materials synthesized using microwave energy for ignition and combustion

gating High-Temperature Synthesis (SHS), Defense Advanced Research Projects Agency-U.S. SHS Symp. Proc., Daytona Beach, FL, 1985.

Carbides Boride Silicide Nitrides and cermets Composites Superconducting ceramic

TiC, SiC, B4C TiB 2 TisSi3 AIN-AI, TiN-Ti AI203-TiC, AI203-TiB2, TiC-TiB 2 Y-Ba-Cu oxide

Numerous materials, listed in Table 2, have been synthesized using microwave energy. These include composites and various ceramics including the superconducting ceramics. 7. Conclusions It has been shown that combustion synthesis can be carried out using microwave energy. Combining microwave processing and combustion synthesis is especially useful for high bulk density where it is possible to control propagation of the combustion wave front. The control allows a gradual release of the volatiles, resulting in little or no expansion in the product. This offers great potential for the fabrication of dense ceramics and composites. References 1 Z. A. Munir, Synthesis of high temperature materials by self-propagation combustion, Am. Ceram. Soc., BulL, 67 (2) (1988) 342-349. 2 W. Frankhouser, M. C. Kieszek, K. W. Brendley and S. T. Sullivan, Gasless combustion synthesis of refractory compounds, Noyes, Park Ridge, N J, 1985. 3 T. Kottke and A. Niiler, Effects of thermal conductivity on SHS reaction kinetics, in K. A. Gabriel, S. G. Wax and

4 R.W. Rice, G. Y. Richardson, J. M. Kunetz, T. Schroeter and W. J. McDonough, Effects of self propagating synthesis reactant compact character on ignition, propagation and resultant microstructure, in K. A. Gabriel. S. G. Wax and J. W. McCauley (eds.), Materials Processing by

Self-Propagating High-Temperature Synthesis (SHS), Defense Advanced Research Projects Agemy-U.S. SkIS Symp. Proc., Daytona Beach, FL, 198.5. 5 W. H. Sutton, Microwave processing of ceramic materials, Am. Ceram. Soc., Bull., 68(2)(1989) 376-386. 6 R.V. Decareau and R. A. Peterson, Microwave Processing and Engineering, Ellis Horwood, Chichester, Sussex; Verlag Chemie, New York, 1986. 7 A. S. De, I. Ahmad, E. D. Whitney and D. E. Clark, Effect of green microstructure and processing variables on microwave sintering of alumina, in W. B. Snyder, Jr., W. H. Sutton, M. E Iskander and D. L. Johnson (eds.), Microwave Processing of Materials IL Vol. 189, Materials Research Society, Pittsburgh, PA, 1991, pp. 283-288. 8 1. Ahmad, R. C. Dalton and D. E. Clark, Unique application of microwave energy to the processing of ceramic materials, Proc. 24th Annu. International Microwave

Power Institute Meet., Stamford, CT, August 21-23, 1989, in J. Microwave Power Electromagn. Energy, in press. 9 J. W. Walkiewicz and S. L. McGill, The heating and processing of minerals using microwave energy, Proc.

89th Annu. Meet. of the American Ceramic Society, Pittsburgh, PA, 1987, American Ceramic Society, Columbus, OH, 1987. 10 Y. S. Ho and J. J. Kramer, Microwave dielectric properties of metal filled particulate composites, in W. H. Sutton, M. H. Brooks and I. J. Chabinsky (eds.), Micro-

wave Processing of Materials, Materials Research Society Syrup. Proc., Vol. 124, Materials Research Society, Pittsburgh, PA, 1988, pp. 161-166. 11 M. A. Janney and H. D. Kimery, Microstructure evolution in microwave sintered alumina, in C. A. Handwerker, J. E. Blendall and W. A. Kaysser (eds.), Sintering of Advanced Ceramics, American Ceramic Society, Columbus, OH, 1989, p. 382. 12 R. C. Metaxas and R. J. Meredith, Industrial Microwave Heating, Peter Peregrinus, Hitchin, Herts., 1983.