Gas-Solid Reactive Sintering

In some cases, the reaction between a gas and a solid gives a ceramic of interest, and this reaction can be used to densify the green body. The classic example is reaction sintered SisN4, also called reaction bonded Si3N4. The gas—solid reaction used to make reaction bonded silicon nitride [132] is between silicon metal powder and nitrogen  

As the reaction takes place the pores in the initial silicon powder compact are filled by fibers of a-SigN4 at 1200 C. At 1400 C silicon melts and the reaction produces j8-Si3N4. The reaction causes a 21.7% change in molar volume, filling the pores in the initial powder compact. 

FIGURE 16JS0 Schematic of the reactive sintermg of SKb) + N2(g) - Si3N4(s). 

The initial green density of the Si compact is, therefore, tailored to be just that necessary to give the theoretical density SisN4 after the reaction is complete. 

Assuming that the effective diffusion coefficient is constant (i.e., the porosity, e, and the pore radius, r, do not change) and that we have pseudo-steady state, the governing equation becomes 

---

Gas-solid reactive sintering can also be used in the oxidation of most metals or, more important, mixtures of metals. After oxidation, solid state sintering takes place. With mixtures of metals, a mixed oxide is produced which can be sintered by either solid state or liquid phase sintering. One example of alloy oxidation to produce a ceramic is in the synthesis of the superconductor YBa2Cu3Q. ... 

---