Densification rate

Figure 7.36 Temperature dependence of grain growth and sintering (densification) rates. The slope of each cnrve represents the activation energy. Adapted from J. S. Reed, Principles of Ceramics Processing, 2nd ed. Copyright 1995 by John Wiley Sons, Inc. This material is used by permission of John WUey Sons, Inc.

Fig. 16. Typical shrinkage dL/dLo (a) and densification rate dL/dt (b) curves of Si3N4 ceramics. Additives Y2O3/AI2O3 mixture of weight ratio 2 1 and contents of 3.5, 6.0 and 8.5 vol%...

As the average particle size in green compact is reduced, the specific surface area increases and the diffusion distance is reduced. Both effects produce higher densification rates. According to the Herring scaling law [22],... 

While the observed dependence of the densification rate on particle size, applied pressure, and temperature follows qualitatively the predictions of the sintering models, factors such as particle packing, sintering atmosphere, and heating schedule can have a decisive effect on the sintering process. 

The discussion in Section 5.4 clearly indicates that a temperature gradient has a significant influence on the densification of the preform. In order to increase the densification rate and improve the density, it is necessary to establish a very steep thermal gradient within the preform. In 1984 Houdayer el al. [52] described a method for rapid densification by immersing the preform into a liquid precursor at high temperatures to fabricate the C/C composites. This densification process is also called liquid-immersion CVI (LI-CVI). 

Table 5.2. Relationship between densification rate and processing temperature [53]...

The in-depth model of the deposition process is another important aspect of gaining a deep understanding of an LI-CVI process as well as a TG-CVI process. Under a strong thermal gradient the relationship between the deposition rate on a fibre (udep) and the densification rate inside the preform (u/ront) can be calculated by using the one-dimensional model [42],... 

Considering that the temperature gradient is constant in the thickness of the densification front the precursor starts to be pyrolysed between the deposition temperature Tmax and the lowest temperature Tmm. Then the front densification rate is the function of Tmax and temperature gradient V7 This is expressed as... 

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