Modeling of sequential reactions during micropyretic synthesis
- PDF / 1,135,440 Bytes
- 12 Pages / 594 x 774 pts Page_size
- 78 Downloads / 218 Views
I.
INTRODUCTION
MICROPYRETIC synthesis or combustion synthesis has in the recent years received considerable attention as an energy and time saving processing technique for the manufacture of various ceramics (TiC and TiB2), intermetallies (NiA1 and MoSi2), and composites (TiB2-AI203 and MoSi2-A1203).U~l The principle in this technique involves the use of exothermic reactions between elemental powders. A compact containing the reactive powders can be ignited at one end, and the heat of the reaction is sufficient to allow the combustion wave to propagate throughout the sample, leading to product formation. The time required for synthesis is low, typically a few minutes. The combustion reaction can be characterized by the velocity of propagation and the combustion temperature. These characteristics determine the microstructure and the properties of the micropyretically synthesized samples. The velocity and temperature are influenced by various factors. These factors include processing variables such as green density, particle size, amount of diluent, and type of diluent, as well as thermophysical and chemical properties such as activation energy, heat of the reaction, thermal conductivity, pre-exponential constant, density, and specific heat of the reactant and product. Previous studies have reported on the numerical simulation of micropyretic synthesis.t7-z2] The investigations have considered systems in which two reactants give rise to the product in a single step. This is applicable for many systems such as NiA1, TiC, and TiB z. Numerical simulation of systems where the reaction takes place in stages has also been studied. These studies, however, were conducted for gas reactions513,'41 One important class of reactions forming composites is
the aluminothermic reaction. A typical reaction is between A1, an oxide (MoO 3 or TiO2), and a third oxide or element (SiO2, Si, or B203) , giving rise to aluminum oxide and a second compound (MoSi2 or TiB2).u5,~61 In this study, the numerical modeling of an aluminothermic reaction will be studied. The aluminothermic reaction is the one between molybdenum oxide, aluminum, and silicon. The overall reaction can be written as MoO~ + 2A1 + 2Si = MoSi 2 + AI203
Aluminothermic reactions, in general, have a reaction sequence which involves first the reduction of the oxide by A1 to form the element followed by the interaction of the reduced element with the third reactant to form the compound, v7,181Micropyretic synthesis of MoO3 and AI has also been performed experimentally,rIg,z~ No fast exothermic reaction is expected between M o O 3 + Si and A1 + Si. Therefore, for the MoO3-A1-Si mixture, it may be assumed that the reduction of M003 by AI takes place first. This is followed by the reaction between Mo (from the reduced MOO3) and Si. Even though the reduction reaction itself may proceed in more than one step, a single step is often assumed. The two-step sequence can be represented as MoO3 + 2A1 = Mo + A1203 Mo + 2Si= MoSiz
METALLURGICAL AND MATERIALSTRANSACTIONS A
[2a] [2b]
In
Data Loading...
