Electrical Properties of Liquid-Crystal Materials for Display Applications
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charged particles in LC materials can be characterized by the diffusion constant and the number density very well by means of curve fitting between measured and calculated permittivity dispersion [29]. Here, the effect of electric double layers are shown to be separated clearly in far low frequency range [36], and, moreover, the charged particles can be characterized by ranging according to the order of their size [30]. In the case when the charge carriers are neither electrons nor holes but ions, Walden's rule will be valid [18]:
(1)
Aur = const.,
where y, is the mobility of the charge carrier and r is the viscosity of the matrix material. The p, values can be obtained from the above measurements of the diffusion constant D by applying the Nernst-Einstein-Townsend (NET) equation;
p, = qD/kBT,
(2)
where q is the electric charge, k, the L.OE-I0 Boltzmann constant and T the absolute temperature. The r values can be measured by using a ]on( 1) conventional micro viscometer. Measurements were reported on the system composed of LC matrix materials and doped electrolyte, lon(2) tetra-n-butylammonium iodide IOE- I I (TBAI) [30]. The results shown in 0.00305 0.0031 0.00315 0.0032 0.00325 0.0033 0.00335 0.0034 0.00345 Fig. I clearly exhibit the validity of I/T (K") Walden's rule for ions (1) and (2), Fig.1 Temperature dependence of the product of presumed to be I - and TBA + mobility of ions /tI and viscosity of matrix LC respectively. material 71(Ions (1) and (2) are presumed to be IFor LC materials, validity of and TBA'respectively.) Walden's rule was confirmed first JE-lO for 5CB (see Table I) by transient photocurrent measurement, Ion( I) providing a conclusion that the lon(2) charge carriers are positive ions JE-11II [14]. Detailed measurements were A n_ lon(3) reported by ranging the charged particles contained in PCH-5 into three categories, according to their IE-12 0.0028 0.00285 0.0029 0.00295 0.003 00()305 size [37]. As is shown in Fig. 2, I/T (K') major charge-carriers in PCH-5 (see Table I) also follow Walden's Fig.2 Temperature dependence of the product of rule and can be judged as ions. mobility y, of charge carriers and viscosity 17of matrix LC material PCH-5
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Table II shows measured results of the Table I. Structure and acronym of LC characteristics of the ions contained in several LC substances Structure Acronym substances (Table I) in an isotropic phase at 807C. The typical ions in LC materials are characterized by the CH2,+1-@ýJCN n CB diffusion coefficient in the order of 101-2 m2/s at R.T. F 2 Due to these relatively small values, the mobile ions LCN -C-O3YF-nCBE C.H2n., contained in LC materials affect the dielectric 0 1H_ PC-_ permittivity and the electric conductivity only in a PCH-n iCnH 2n~ -Q)-CN 1 very low frequency range. Table II. Measured attributes of ions in LC materials (809C) Diffusion Single
Viscosity
coefficient D
Mobility itl
Density n
Compound
[cSt]
[m2 s-S-
[m2 -V- 1 s-1]
[nm-3 ]
3CB
5.1
1.lxlO0-
3.6x 10-9
5.4x 1020
5CB
6.0
5.8x10-1
1.9x10-9
4.9x10
7CB
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