Grain boundary phases

The structures of phases such as the chiral nematic, the blue phases and the twist grain boundary phases are known to result from the presence of chiral interactions between the constituent molecules [3]. It should be possible, therefore, to explore the properties of such phases with computer simulations by introducing chirality into the pair potential and this can be achieved in two quite different ways. In one a point chiral interaction is added to the Gay-Berne potential in essentially the same manner as electrostatic interactions have been included (see Sect. 7). In the other, quite different approach a chiral molecule is created by linking together two or more Gay-Berne particles as in the formation of biaxial molecules (see Sect. 10). Here we shall consider the phases formed by chiral Gay-Berne systems produced using both strategies. 

Bayon, R., Coco, S. and Espinet, P. (2002) Twist-Grain Boundary Phase and Blue Phases in Isocyanide Gold(I) Complexes. Chemistry of Materials, 14, 3515-3518. 

Lubensky TC, Renn SR (1990) Twist-grain-boundary phases near the nematic smectic-A smectic-C point in liquid crystals. Phys Rev A 41 4392-4401... 

Table 11. Grain boundary phases in Si3N4 ceramics [101]... 

Fig. 24. Driving force for an amorphous grain boundary phase in equilibrium with /tes as a function of composition in the system Si-N-O-Al and strength of interaction [330]...

The specific heat of Si3N4 ceramics is in the temperature range 293 up to 1200 K [Cp (293 K) = 0.67 KJ (K kg)-1] nearly independent of the composition of the additives. The isobaric specific heat values agree well with the isochoric specific heat calculated by Debye s theory. Also the Dulong Petit s rule can applied as an approximation of the Cv values [25 J(K mol)-1] at temperatures >1100 K [371]. From the Cp values at around 100 K the amount of the amorphous grain boundary phase can be calculated [371]. 

In principle, the amount, composition and degree of crystallisation of the grain boundary phase are key factors which must be considered for successful development of Si3N4 ceramics for applications at elevated temperatures. It is... 

The strong dependence of the oxidation on composition of the ceramic as well as the amount and state of the grain boundary phase (composition and crystallisation ability) is the reason for the scatter of the oxidation constants shown in Fig. 33, i.e., small amounts of impurities may change the oxidation rate by several orders of magnitude [455, 459]. Especially cation impurities... 

In Table 13 the interactions of Si3N4 ceramics with common metals are summarised (see also [18, 472, 473]). For application in metallurgy not only the interaction with the metal but also the interaction of oxide slags on the surface of the metals has to be taken into account. At higher temperatures most metal oxides react with the grain boundary phase. For example, V205,... 

No wetting of pure Mg at 850-950 °C in vacuum or air 90-150° [482, 484] Wetting, 10° at melting point [484] Reaction of the oxides with the glassy grain boundary phase... 

CuO, and PbO react at temperatures >600-700 °C quite strongly with the grain boundary phase and accelerate the oxidation and degradation. At temperatures below the transition temperature Tg of the glassy phase this interaction can be neglected because of the low ion diffusion into the grain boundary. 

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