Controlled Firing of Reaction‐Bonded Aluminum Oxide (RBAO) Ceramics: Part I, Continuum‐Model Predictions

Abstract

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.

Andalusite‐Based High‐Alumina Castables

“…The addition of nonhydraulic bonding materials led to further mullite formation. 9 Metallic aluminum was used to assess if it might form a reaction-bonded aluminum oxide (RBAO) product, 10,11 and silicon carbide was used to compensate for shrinkage during sintering and to potentially form mullite through its oxidation in the presence of alumina. 12,13 The nonhydraulic bonding agent contents were fixed at 0, 2, and 4 wt% for the aluminum powder (Grade 120, Alcoa, Pittsburgh, PA) and at 2, 4, and 6 wt% for the silicon carbide (Exolon-ESK, Tonawanda, NY).…”

Preparation and properties of high toughness RBAO macroporous membrane support

“…Falamaki et al [11] used alumina powder with an average size of 1.6 mm and aluminum with an average size of 38 mm to synthesis the supports with an average pore size of 0.2 mm. The obtained porous RBAO material is largely superior to the porous Al 2 O 3 , such as high bending strength, high permeability and other advanced properties [12][13][14]. The supports with small size pores are usually used as the support for gas separation membranes.…”

A phenomenological description of the rate of the aluminum/oxygen reaction in the reaction-bonding of alumina

“…The addition of zirconia improves the microstructure and mechanical properties of the final ceramic. 2,3 The oxidation of aluminum occurring during the RBAO process is given by the reaction 4Al + 3O 2 → 2Al 2 O 3 (1) This reaction is extremely exothermic and results in volume expansion and weight gain. Due to the weight gain, the reaction rate can be determined using themogravimetry (TG).…”

“…It is the uneven volume gains and sharp thermal gradients associated with the ignition phenomena that cause stresses which result in sample cracking. [2][3][4]6,7 Cracking can be avoided by using low heating rates and isothermal temperature holds that avoid reaction runaway. [2][3][4]6,7 Knowledge of the reaction kinetics of the oxidation of aluminum can be used to avoid uncontrolled reactions.…”