The Use of Exothermic Reactions in the Synthesis and Densification of Ceramic Materials
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This article will provide information about chemical processes which rely on the heat evolved during reaction to synthesize and, in some cases, to simultaneously density single-phase or composite ceramic materials. Although the basic concept underlying these processes is simple, the high temperature reactions are complex and require careful study with individual systems before their potential as fabrication processes can be fully realized. Many reactions between solids involving elements and/or compounds, or between solids and gases are highly exothermic. Listed in Table I is a selected group of typical chemical reactions accompanied by their calculated adiabatic temperatures. As a general rule any reaction with an adiabatic temperature ~2000°C or over can be reacted under combustion conditions. For example, suppose that a cold-pressed cylindrical compact of titanium and boron powder (Eq. 1) is ignited at the top surface with a convenient source of heat such as a laser. From the igniting surface a combustion wave rapidly self-propagates down the compact, transforming the reactants into the TiB2 product. Figure 1 shows a Ti and C powder compact where the combustion wave has progressed about halfway down the compact. Another variation of this combustion reaction occurs when a reactant mixture is rapidly heated to the ignition temperature by an external furnace. On reaching the ignition temperature, combustion takes place simultaneously throughout the volume of the compact, instantaneously converting the reactants into the product. Combustion reactions of this type are referred to as "thermal explosions." These combustion reactions have unique features which form the basis for their projected use in the synthesis of ceramic materials. The main feature is the rapidly moving combustion wave with velocities ranging from 0.1 to —15 cm/s depending on the chemical system. The total elapsed time for combustion reactions will be very short compared with those of more conventional processes. The temperature profile at the combustion front rises abruptly from room temperature (if there is no preheating) to the maximum temperature of the combustion product. Included in the
Table 1: Typical chemical reactions and their calculated adiabatic temperatures. Reaction Ti + 2B Zr + 2B Ti + C Hf + C Si + C Al + 1/2N2 3Si + 2N2 B + 1/2N2 Ti + 1/2N2 5Ti + 3Si Si0 2 + 2Mg + C Ti0 2 + 4AI + C
Product TiB2 ZrB2 TiC HfC SiC AIN Si3N4 BN TiN Ti5Si3 SiC + 2MgO TiC + 2AI203
Adiabatic Temp. °C 2930 3040 2940 3630 1500 2230 1900 (Sublilme) 3000 (Sublime) 4600 2200 2300 2050
Melting Temp. °C 2930 3040 2940 3630 2700 2600 4000 3400 2950 2120
Figure 1. Cold-pressed compact of titanium and carbon powders undergoing combustion. The combustion front has self-propagated approximately halfway down the compact. The maximum temperature measured was 2760°C.
PAGE 60, MRS BULLETIN, OCTOBER 1/NOVEMBER 15, 1987
temperature profile is the leading "heating zone" where the temperature of the reactants is rising, followed by the "reaction zone" where the reac
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