Dielectric Heating and Relaxations in Nematic Liquid Crystals

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Abstract
Novel experiments on dielectric heating were made to induce changes of temperature ΔT in nematic liquid crystal layers. Measurements of the dependence of ΔT on cell voltage, driving frequency, electrode spacings, external cell temperature and liquid crystal material parameters are correlated with molecular relatxation proceses occurring in the frequency range 0-10 MHz. Temperature increases ΔT up to 40°C were achieved at low driving voltages in nematics belonging to two different liquid crystal classes which exhibit large positive static dielectric anisotropies. It could be shown that homeotropic wal alignment leads to a markedly different dependence of ΔT(w) in th vicinity of the cross-over frequency ωc, of the lowest dielectric relaxation process, compared with unoriented or parallel aligned layers. The material requirements and limits for a possible application of dielectric heating in liquid crystal displays are discussed.