Retardation factors, liquid crystals

In the study of dielectric relaxation, temperature is an important variable, and it is observed that relaxation times decrease as the temperature increases. In Debye s model for the rotational diffusion of dipoles, the temperature dependence of the relaxation is determined by the diffusion constant or microscopic viscosity. For liquid crystals the nematic ordering potential contributes to rotational relaxation, and the temperature dependence of the order parameter influences the retardation factors. If rotational diffusion is an activated process, then it is appropriate to use an Arrhenius equation for the relaxation times ... 

The rate of the supercooled liquid transition to the crystalline state at T < Tm depends on two factors the number of the crystallization centres formed per unit time in unit volume, nc, and the linear rate of the crystal growth, vc. At T < Tm, v,. at first increases as the temperature decreases. This is connected with the fact that the heat which is liberated upon formation of the crystals and retards their formation dissipates more efficiently the lower is the temperature of the solution. Then, vc reaches its maximum and,... 

At the same time, if a solid has strong initial plastic character (e.g., metal single crystal), the transition into a brittle state with reduced strength may not occur, even at low a. For example, liquid bismuth does not cause an embrittlement of copper single crystals. The grain boundaries in polycrystalline solids, while acting as a strengthening factor, can cause a reduction in plasticity due to the retardation of the dislocation movement. For this reason, for a polycrystalline solid, the reduction in 0 may be already sufficient for a transition to the brittle state to take place. Indeed, liquid bismuth is known to cause an anbrittlement of polycrystalline copper. 

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