Diffusion sintering and

In this volume discussions are presented on a variety of aspects of ionic solid systems kinetics of reaction, of diffusion and sintering, of crystallization, of nucleation and crystal growth, of precipitation, and of the destruction of crystals by evaporation or by thermal decomposition of a solution. Unquestionably such studies will provide valuable aid in furthering the practical utilization of reactions in ionic systems. However, as pointed out in the chapter by Stringer et al % theoretical models are still, in general, inadequate for... 

Fig. 4. Thermal diffusivity of silicon-based stmctural ceramics (a) reaction-bonded SiC (b) hot-pressed and sintered SiC (c) hot-pressed (1% MgO,...

Sintering consists of heating a mixture of fine materials to an elevated temperature without complete fusion. Surface diffusion and some incipient fusion cause the soHd particles in contact with one another to adhere and form larger aggregates. In the processing of hematite, Fe202, or magnetite,... 

To produce wear-resistant or hardened surfaces, thin layers of borides can be prepared on metal surfaces by reaction and diffusion (see Metal SURFACE treatments). Boride powders can be formed iato monolithic shapes by cold pressing and sintering, or by hot pressiag. 

Ashby pointed out diat die sintering studies of copper particles of radius 3-15 microns showed clearly the effects of surface diffusion, and die activation energy for surface diffusion is close to the activation energy for volume diffusion, and hence it is not necessarily the volume diffusion process which predominates as a sintering mechanism at temperatures less than 800°C. 

Ashby also constructed sintering maps in which x/a is plotted versus the homologous temperature T/Tm where Tm is the melting point. These maps, which must be drawn for a given initial radius of the sintering particles, use the relevant diffusion and vapour pressure data. Isochrones connect values of x/a which can be achieved in a fixed time of annealing as a function of the homologous temperature. 

Because only relatively small variations in cy occur in typical specimens undergoing sintering and diffusional creep (Chapter 16), we prefer to carry out the analyses of surface smoothing, sintering, and diffusional creep in terms of atom diffusion and the diffusion potential using Eq. 3.72. In this approach, the boundary conditions on a can be expressed quite simply.15... 

The sintering rate due to boundary diffusion and crystal diffusion will be proportional to Db and DXL, respectively. The scaling laws show that the sintering rate due to boundary diffusion will decrease by the factor A 4 when the particle size is increased by the factor A. The corresponding factor for sintering by crystal... 

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