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FACTORS INFLUENCING DIELECTRIC PROPERTIES
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FACTORS INFLUENCING DIELECTRIC PROPERTIES
The following factors influences the dielectric properties i.e Frequency, Nature of Fibres, Moisture in
Fibres, Temperature and Impurities.
FREQUENCYY :
Frequency has a most important influence on dielectric properties. At low frequency, the dipoles line up
in the field, reverse direction when the field reverses, and so contribute to a high dielectric constant.
The dipole takes certain time to reverse direction due to their inertia and restraints in the structure; this
characterized as their relaxation time. When the frequency becomes so high that the reversal of the
field take place at intervals comparable to the relaxation time, then the dipoles cease to follow changes
of field completely and dielectric constant will decrease. At higher frequencies still, the dipoles will not
follow the changes at all, and there will be no contribution to dielectric constant.
NATURE OF FIBRES:
Nature of fibres also influence the dielectric properties , such as, cotton, viscose rayon, acetate, wool,
and nylon over the frequency range between 50c/s and 10Mc/s showing influence of dielectric
constant are progressively marked as the damper the specimen – indicating that a range of relaxation
times is involved.
In several other fibres(viscose rayon, acetate, dry cotton) the power factor begins to increase with an
increase in frequency in the region of 1 Mc/s which suggests that there will be a maximum in the power
factor and a corresponding drop in the dielectric constant.
In wool a decreasing power factor is found as the frequency increased from 3,000 Mc/s to 26,000 Mc/s.
It is to be noted that relaxation effects occur in the mechanical behavior of fibers at similar frequencies
to those found in dielectric properties. Dielectric constant at Optical Frequencies (1015 c/s) of some
fibers are given below:-
DIELECTRIC CONSTANTS AT OPTICAL FREQUENCIES
є = 2, with light vibration
Fibre
Parallel to fibre axis Perpendicular to fibre axis
Cotton 2.50 2.34
Viscose rayon 2.37 2.31
Acetate 2.19 2.16
Wool 2.40 2.37
Casein 2.37 2.37
Nylon 2.50 2.31
Terylene 2.96 2.37
Orlon 2.25 2.25
Polyethylene 2.43 2.28
Glass 2.40 2.40
MOISTURE:
The moisture has a marked effect on the dielectric properties. At higher frequencies the dielectric
properties of cellulosic fibres are consistent with the assumption that the water molecules are
restrained in a manner similar to ice. For wool, the dielectric properties is lower, indicating that the
absorbed water molecules are more tightly held , and can not line up in the field.
TEMPERATURE:
Arise in temperature reduces the restraints on the dipoles causing a increase in dielectric constant in
solid materials. In terylene it has been found that there is a maximum in dissipation factor occurring at
about 1 Mc/s at room temperature and moving to lower frequencies at lower temperatures. At higher
temperatures, in the low frequency-range and in high-temperature region the dissipation factor
increases rapidly. When the material is wet the low-temperature ridge is unaltered in position, but rises
to higher values of dissipation factor.
IMPURITIES:
The presence of impurities would alter the dielectric properties; in particular the ionic impurities would
probably have a considerable effect at low frequencies as shown in table 19.3,Morton) where the effect
of removal of surface dressings from some synthetic fibres by extracting with methanol and benzene.
Only in Dacron there was a large change in dielectric properties.
EFFECTS OF EXTRACTION:
The effect of extraction on Dielectric properties at 65% R.H. and 1 kc/s is given below Table 19.3 (of
Morton).
EFFECTS OF EXTRACTION ON DIELECTRIC PROPERTIES AT 65% R.H. AND 1 KC/S.
Un extracted Extracted
D Density of Dielectric Dielectric
Material packing(%) Constant єm Power factor Cos Power Constant єm factor Cos
Nylon 50 2.34 0.054 2.43 0.063
Orlon 40 2.28 0.123 1.73 0.044
Acrilan 50 2.00 0.076 1.94 0.043
Dacron 50 39.40 0.773 1.66 0.007
EFFECTS OF MOISTURE ON DIELECTRIC PROPERTIES:-
The effects of Moisture on Dielectric Properties (due to Hearle) is given below:-
EFFECTS OF MOISTURE ON DIELECTRIC PROPERTIES.
Dielectric constant
Material Extrapolated from P% 0% r.h. 65% r.h.
1 kc/s 100 kc/s 1 kc/s 100 kc/s
Cotton 44 3.2 3.0 18 6.0
Viscose 44 3.6 3.5 8.4 5.3
Viscose c.f 73 15 7.1
Acetate staple 45 2.6 2.5 3.5 3.3
Acetate c.f 79 4.0 3.7
Wool 53 2.7 2.6 5.5 4.6
Nylon staple 53 2.5 2.4 3.7 2.9
Nylon c.f 87 4.0 3.2
Orlon staple 42 2.8 2.3 4.2 2.8
Orlon staple (extracted) 38 2.8 2.5
Vinyon staple 46 2.7 2.5 3.0 2.6
Saran c.f 70 2.9 2.4 2.9 2.4
Dacron staple (extracted) 48 2.3 2.3 2.3 2.3
Fibre glass c.f 63 3.7 3.4 4.4 3.6
Ardil staple 38 2.7 2.6 3.8 3.3
EFFECTS OF DIRECTION OF ELECTRIC FIELD:-
With the change of direction of applied electric field, the permeability of a textile material changes.
Some examples are:-
EFFECT OF PERMEABILITY ON ELECTRIC FIELDS-
Fibre Permeability
When electric field When electric field
is parallel to is perpendicular to
fibre axis fibre axis
Cotton 2.5 2.34
Wool 2.19 2.16
Rayon 2.37 2.31
The cellulosic fibres have the highest dielectric constant, followed by the protein fibres, with the
synthetic non-hygroscopic fibres having lowest values. The power factors follow a similar order.
ELECTRICAL RESISTANCE OF TEXTILES
CONDUCTORS, SEMICONDUCTORS AND INSULATORS:
The electrical conduction properties of different elements and compounds are explained in terms of the
electrons having energies in the valance and conduction bands. The electrons laying in the lower energy
bands, which are normally filled plays no part in the conduction process.
INSULATORS:
Insulators are those materials, textiles are among them, in which valence electrons are bound very
tightly to their parent atoms, thus requiring very large electric field to remove them from the attraction
of the nuclei.
In terms of energy bands, it means that insulators
i) have a full valence band.
ii) have an empty conduction band.
iii) have a large energy gap (of several eV ) between them, and
iv) at all ordinary temperatures, the probability of electrons from full valence band gaining
sufficient energy so as to surmount energy gap and becoming available for conduction in the
conduction band is slight.
This is shown in Fig. 1a. For conduction to take place, electrons must be given sufficient energy to jump
from the valence band to the conduction band. Increase in temperature
enables some electrons to go to the conduction band which fact accounts for the negative resistance-
temperature coefficient of insulators.
INSULATORS – THEIR PROPERTIES AND USES:
Insulators are those materials which, on application of a potential difference pass such extremely small
current that is generally considered negligible. The main function of insulators is to confine the path of
electric current along conductors. It is achieved either by covering these conductors or else by
supporting them by an insulating material.
The different properties desired in a good insulation material are as under:
a) permanence – it is most important quality but one least easily obtained.
b) high dielectric strength, i.e. ability to resist electrical break-down under considerable electrical
stress.
c) mechanical strength, and
d) fairly high insulation resistance.
Some of the commonly used insulators along with their special fields of application are given below:
i) Asbestos – for covering wires in high-power machines.
ii) Bitumen(vulcanized) – for low-voltage mining cables and also as a cable box filling
compound.
iii) Cotton, Jute and other cellulosic fibres – for covering electric wires.
iv) Ebonite – for making covers for resistance boxes.
v) Empire cloth – for wrapping armature coils.
vi) Gutta percha – for covering submarine cables.
vii) Micanite – for insulating commutator segments and as slot lining for high voltage machines.
viii) Paper and paraffin wax – for cable insulation when oil impregnated and for covering
transformer conductors,
ix) Porcelain – for insulators and use in overhead transmission lines.
x) Shellac – for use in the form of insulating varnish.
CONDUCTORS :
The conducting materials are those in which plenty of free electrons are available for electric
conduction.
In terms of energy bands, it means that electrical conductors are those which have overlapping valence
and conduction bands as shown in Fig. 1b. In fact there is no physical distinction between the two
bands, hence, the availability of a large number of conduction electrons.
Another point to be noted is that the absence of forbidden energy gap in good conductors, there is no
structure to establish holes. The total current in such conductors is simply a flow of electrons .It is
exactly for this reason that the existence of holes was not discovered until semiconductors were studied
thoroughly.
SEMICONDUCTORS:
A semiconductor material is one whose electrical properties lie in between those of insulators and good
conductors.
In terms of energy bands, semiconductors can be defined as those materials which have almost an
empty conduction band and almost filled valence band with a very narrow energy gap (of the order of 1
eV) separating the two.
At 0 degree K, there are no electrons in the conduction band and the valence band is completely filled.
But with the increase of temperature, width of the forbidden energy band is decreased so that some of
the electrons are liberated into the conduction band. In other words, conductivity of semiconductors
increases with temperature. Moreover, such departing electrons leave behind positive holes in the
valence band (Fig. 1d). Hence semiconductor current is the sum of electrons and hole currents flowing in
opposite directions.
\
CONDUCTION OF ELECTRICITY IN TEXTILES:
In a consideration of the mechanism of conduction of electricity, the first questions to be answered are :
‘where is the current flowing?’ and ‘what is carrying the current?’. Both of these problems have to be
solved mainly by circumstantial evidence.
Hersh and Montgomery have shown that for nylon filaments the resistance is inversely proportional to
the area of cross-section. This indicates that conduction is predominantly a volume effect, with the
current flowing through the bulk of the material. As if conduction had been a surface effect, the
resistance would have been inversely proportional to the circumference i.e. square root of the area of
cross-section . Different types of cottons also have the same specific resistance, though here the range
of fineness covered is smaller; different qualities of wool fibre differ to an appreciable extent only at low
moisture content, when the impurities is great. Further indirect evidence that conduction is a volume
effect is provided by the lack of hysteresis between resistance and moisture content (which is a volume,
not a surface, quantity), despite hysteresis between moisture content and relative humidity. Thus in
hygroscopic fibres, it appears that volume conduction is the dominant effect, surface conduction being
negligible in comparison.
Both the close association between resistance and moisture content and the relation between the
resistances of cotton and viscose rayon indicate that the current will be flowing in the amorphous
regions of the fibre. Indeed, the ordered arrangement of cellulose molecules in a crystalline region
would be expected to be highly insulating.
In synthetic fibres, with higher resistance and negligible moisture absorption, surface conduction is likely
to be more important and may be dominant mechanism. Certainly when conducting surface finishes are
applied, the current flow will be almost entirely on the surface.
Current may be carried either by electrons or by ions. Baxter in 1943, put forward a theory that
conduction in wool was by electrons, the water molecules acting as impurity centers in an electronic
semi-conductor, but most workers have assumed that conduction is by ions as the products of
electrolysis have been directly observed (by O’Sullivan, using cellulose film, and by King and Medley,
using keratin film), the current must be ionic. The variation of resistance with electrolyte content and
the polarization effects also support this view. Thus with the possible exceptions the general picture is
that, in textiles, it is ionic conduction taking place through the bulk of the material. However, it is to be
noted that there is enormous variation of resistance with moisture content, large variation with
temperature, and other effects, such as the higher resistance of protein fibres and low conductivity of
bivalent ions.
For a specimen having v ions per unit length available for conduction, with z as the valence of the ions
and e electronic charge, and assuming that the ions move with an average velocity u under a potential
difference V between the ends of the specimen, the current, I, is given by:
I = vzeu;
and the resistance, R, by:
R = V/I = V/vzeu.
For further study, it is convenient to separate the factors by taking logarithms:
log R = log V – log v - log ze - log u.
INFLUENCE OF DIELECTRIC CONSTANT ON DISSOCIATION OF ION-PAIRS:
Strong electrolytes are completely ionized, and the ions can only be held together in molecules by
electrostatic forces. In solutions in liquids of high dielectric constant, such as water, these forces are so
weak that there is no close association of ions, but even weak inter-ionic forces prevent the ions from
acting as completely free particles.
If the dielectric constant of the solvent is lower, the electrostatic forces will be stronger, and we may
consider an equilibrium between ion pairs and free ions as :
+ - + -
A B ↔ A + B
The variation in this equilibrium offers a possible explanation of variations in resistance. A rise in
dielectric constant would cause more dissociation, making more free ions available for conduction and
consequently lowering the resistance. This is illustrated diagrammatically in Fig. 20.18 of Morton.
NORMAL, EXCITED AND IONIZED ATOM:
Let us consider the case of the simplest atom i.e hydrogen atom. When its only electron is in the
innermost orbit ( n=1), then the atom is said to be in its normal (or unexcited) state. Generally, it is this
condition in which most of the free hydrogen atoms in a gas are found to exist at normal room
temperature and pressure. However, if spark is passed through hydrogen gas contained in a vessel, then
high-speed electrons produced by the spark collide with hydrogen atom and may either completely
remove the n=1 electron from hydrogen atom or raise it to higher permitted orbits having n=2,3,4 etc.
When the electron is completely removed from the atoms, the atom is said to be ionized . .If , however,
the electrons is forced into an outer or higher n-value orbit, the atoms is said to be excited (or in an
excited state). The atom does not remain in the excited state longer than 10 second because of the
electron under the attractive force of the nucleus jumps to the lower permitted orbit .In so doing, the
electron loses the energy it had earlier gained during collision. However, the electron may return by
several jumps, thereby emitting many different radiations by different frequencies.
The orbital model of hydrogen was extended by Bohr and Stoner to include all the chemical elements.
As shown in the Figure below , each atom consists of a positively-charged nucleus with a certain number
of electrons revolving round it in discrete and definite orbits.
The positive charge carried by the nucleus is numerically equal to the atomic number (Z) and it
determines the number of planetary electrons an atom can have. A helium atom,
with atomic number Z = 2, has two positive charges on the nucleus and two electrons outside the n = 1
orbit. A beryllium atom have (atomic number Z = 4) has 4 positive
charge on the nucleus and four electrons outside, two revolving in n = 1 orbit and other two in the n= 2
orbit. Similarly, sodium atom has 11 positive charges on the nucleus and 11 electrons outside and 2 in n
= 1 orbit, 8 in n = 2 orbit and 1 in n = 3 orbit.
The orbits to which electrons are confined are called electron shells and are denoted by K-shell, L-shell,
M-shell, N-shell etc.
For K-shell, n = 1
E1 = -13.6 eV
For L-shell, n = 2
E2 = -13.6/2 = -3.4 eV
For M-shell, n = 3
E3 = -13.6/3 = -1.51 eV , etc.
The electrical resistance of a specimen, i.e., the voltage across the specimen divided by the current
through it, is determined both by the properties of the material and the dimensions of the specimen.
For most substances, the property of the material is best
given by the specific resistance, p,(in ohm-cm), which is defined as the resistance between opposite
faces of a 1 -cm cube ,but for textiles it is more convenient with fibres to have a definition ON MASS
PER UNIT LENGTH than on area of cross-section. Therefore, a mass specific resistance, Rs , is defined as
the resistance in ohms between the ends of the specimen 1 cm long and of mass 1 g. The units are ohm-
g/cm. The two quantities are related as follows:
R= pρ ……………. (1).
where ρ = density of material in g/cm .
For most textile materials, R will differ from p by less than 50%
The resistance , Rs , of an arbitrary specimen is given by the relation
