Page 250 - Electrical Properties of Materials

232 Dielectric materials

Table 10.1 Dielectric constant and refractive index of some non-polar,
weakly polor, polar, and semiconducting materials

Material Refractive (Refractive Dielectric constant
3
index index) 2 measured at 10 Hz
Non-polar
C (diamond) 2.38 5.66 5.68
H 2 (liquid) 1.11 1.232 1.23
Weakly polar
polythene 1.51 2.28 2.30
ptfe (poly-tetra 1.37 1.89 2.10
fluoro-ethylene)
CCl 4 (carbon- 1.46 2.13 2.24
tetrachloride)
Paraffin 1.48 2.19 2.20
Polar
NaCl (rocksalt) 1.52 2.25 5.90
TiO 2 (rutile) 2.61 6.8 94.0
SiO 2 (quartz) 1.46 2.13 3.80
Al 2 O 3 (ceramic) 1.66 2.77 6.5
Al 2 O 3 (ruby) 1.77 3.13 4.31
Sodium carbonate 1.53 2.36 8.4
Ethanol 1.36 1.85 24.30
Methanol 1.33 1.76 32.63
Acetone 1.357 1.84 20.7
Soda glass 1.52 2.30 7.60
Water 1.33 1.77 80.4
Semiconductors
Si 3.42 11.70 11.9
GaAs 3.3 10.89 13.2
Envelope perturbed Gaussian sphere GaP 3.2 15.68 17.7
by field, centre C – of radius d
InSb 3.96 15.68 17.7

For an optical property we need to consider only electronic polarizability as
ionic, and molecular responses are too slow. Let us suppose that each atomic
3
C – volume, (4/3)πr is uniformly occupied by the total electronic charge, Ze.
d When an electric field, E , is applied, the centre of charge of the electronic
–
C cloud shifts a distance d to C from the nucleus at C (Fig. 10.8). To find the
restoring force attracting the electrons back towards the nucleus, we can con-
–
struct a Gaussian surface of radius d about C , so that C is just excluded.
The negative charge inside the Gaussian sphere is, according to the uniform
3
charge approximation, equal to Ze(d/r) . So the attractive force, F, towards
the nucleus is
Envelope of electron
cloud about centre C 2
(Ze) d
F = . (10.24)
Fig. 10.8 4π 0 r 3
Displacement of electron cloud about
an atom centred on C by a distance d, This must be balanced by the field force causing the charge displacement,
by an applied electric field.
F = ZeE . (10.25)