The reaction‐bonded aluminum oxide (RBAO) process is an attractive alternative to conventional processing of ceramics, because of advantages such as lower costs, enhanced green machinability, near‐net‐shape forming, and broad microstructural variability. However, various problems are still encountered in the production of RBAO ceramics. Part I of the paper presented model predictions that may allow for the controlled firing of RBAO ceramics. The current work investigates the reaction behavior of RBAO ceramics under the model‐predicted conditions (i.e., for varying oxygen content, heat loss, heating cycles, and scale) via thermogravimetry, differential thermal analysis, and analysis of samples that have been fired in a box furnace. By controlling the reaction, one can produce large, crack‐free RBAO ceramics.
The general reaction behavior of the 3A process under the thermal explosion mode of synthesis has been investigated via a continuum model. The continuum model uses mass and energy balances to predict temperature difference (T s,avg ؊ T f ) curves, as well as profile curves of the reactant conversions and sample temperature. In particular, the effect of the dimensionless parameters associated with the rate of local heat generation (, the thermicity factor), the activation energy (␥, the Arrhenius number), the rate of heat redistribution (␣, the modified thermal diffusivity), the rate of heat transfer by convection (Bi, the Biot number or convective heat transfer parameter), and the rate of heat transfer by radiation (⍀, the radiative heat transfer parameter) were investigated. Conditions to control the reaction process, which should produce high-density final products, were determined. It was found that the overall maximum temperature may be reduced for high ␥, low , high ␣, and high Bi and ⍀. In terms of processing conditions, this may be obtained by reducing the initial reactant concentrations, optimizing the particle size, using small sample sizes and high compaction pressure, and increasing the heat loss by using a high thermal conductivity inert gas.
The reaction sintering of Ti x Al y -Al 2 O 3 composites from TiO 2 /Al starting powder mixtures has been characterized by thermogravimetry and differential thermal analysis (TG/ DTA), in situ temperature measurements, and predictions via a continuum model. In order to model the TiO 2 /Al reaction system, it was necessary to first determine the postmill reactant concentrations and the dominant reaction. The postmill reactant concentrations were obtained from TG/DTA measurements in air, while X-ray diffraction (XRD) was used to gain insight into the reaction mechanisms. A continuum model of the process was fitted to in situ temperature measurements by adjusting two parameters. The model was then used to investigate the effects of various processing conditions on the reaction behavior.
The reaction‐bonded aluminum oxide (RBAO) process is a novel, reaction‐forming technique for producing monolithic, alumina‐based ceramics. Although there has been extensive work on the RBAO process, it is often difficult to reproduce the process and avoid sample cracking. To solve the problems that are associated with the RBAO process, it is necessary to have a fundamental understanding of the reaction‐bonding process and the effects of various processing parameters on the reaction behavior. To gain some insight into the process, a continuum model has been developed. The model, which considers the interaction between the macroscopic material and energy balances, is used to predict conditions under which RBAO bodies may be fired in a controlled manner, i.e., avoiding the runaway reaction. In particular, the effects of the oxygen content of the atmosphere, the heat loss by convection and radiation, the heating cycle, and scale (sample size) have been investigated. For small sample sizes, model predictions indicate that the reaction may be controlled by reducing the oxygen content of the atmosphere, increasing the heat loss, and/or incorporating an isothermal hold into the heating cycle at a temperature just below the ignition temperature. For larger sample sizes, model predictions indicate the need for multiple low‐temperature holds at increasing temperatures. It is believed that firing RBAO bodies in a controlled manner will allow one to avoid sample cracking. Part II of this work presents a complementary experimental study that investigates the reaction behavior and structural integrity of samples that have been fired under the predicted conditions.
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