T H E STRUCTURE AND ELECTRICAL PROPERTIES OF INSULATING MATERIALS BY J O H N WARREN WILLIAMS
The importance of making available information concerning the chemical structure of commercial dielectrics is becoming increasingly evident as the mechanisms of conduction and loss of energy in them are being described. These mechanisms usually depend upon the presence of ions, either free or adsorbed, but may also sometimes depend upon the presence of polar molecules if alternating current behavior is being studied. Important engineering studies of the electrical characteristics of solid and liquid dielectrics have been made in recent years and it can be said that in general the difficulties due to lack of exact chemical knowledge of the materials have been recognized. For example, crystals have been subjected to test because of their relative freedom from impurities, and the breakdown processes in highly purified liquids such as hexane, heptane and xylene have been carefully investigated. The highly purified liquids behave quite differently from the ordinary insulating oils in that the latter always show a residual conductivity, which, incidentally, has been traced to the presence of colloidal particles in the oils. The belief seems to be growing that in most solid dielectrics the conduction of the electric current does not take place uniformly through the material as a whole but rather along paths of higher conductivity. I n the case of crystalline dielectrics in which there are ionic conductors Smekal’ believes that the mechanism of electrical conductance is essentially bound up with the deviation of the actual crystal structure from that of the ideal lattice. The ions which take part in conduction are assumed to be concentrated in positions in the crystal where these lattice imperfections are present and move in an adsorbed condition along the paths formed by these crystalline fissures. Smekal has estimated that the ideal part of the lattice in a crystal unit contains something of the order of magnitude 1 0 4 to 106 atoms so that the “mosaics” or blocks are truly colloidal in dimension. This idea of Smekal has found favor with many investigators who have studied the electrical properties of crystals and it is supported by others in their considerations of the thermal, optical and mechanical properties of these materials. I t has been considered favorably in an interesting article by Murphy and Lowry2 on the complex nature of dielectric absorption and dielectric loss. While such a mechanism is questionable in the case of crystalline substances it appears to give a correct description of the conduction processes taking place in such moisture absorbing dielectrics as cellulose, cotton, silk, rubber, the resins, and other similar and related materials. The ability of l
Z. Elektrochemie, 34, 472 (1928); Ann. Physik, (1)83,
* J. Phys
Chem., 34, 598 (1930).
IZOZ
(1927).
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JOHX WARREN WILLIAMS
forms of these solids to give definite and interpretable X-ray diagrams, observations concerning the manner in which they swell in suitable solvent media, the viscosity of their solutions and their ability to form homogeneous films on water and mercury all indicate what may be termed a fiber-like structure for them. A number of theories have been proposed to explain the mode of formation of these substances which for purposes of discussion will be classified as gels or as highly polymerized organic substances. It is more or less generally agreed that they are heterogeneous in the sense that there is present both a continuous and a disperse phase, with the two phases forming a network. Studies of their elasticity and rigidity can best be interpreted on the assumption that the disperse phase is made up of particles which have aggregated to form chains or fibrils which will be arranged regularly in some cases and randomly in others. But although the existence of these chains had been suspected for many years it is not until rather recently that their existence seems definitely established. Discussion with regard to the actual constitution of these high molecular (or aggregate) weight compounds has centered around two theories known in the literature as the association theory of Hess and Pringsheim and the macromolecular theory of Staudinger. I n the association theory smaller molecular units or residues are held together to form the aggregates through secondary valences, while in the macromolecular theory the fundamental groups are chains or fibrils of such residues which are held together by primary valence forces. Their lengths, molecular weight and other physical properties will depend upon the degree of polymerization or condensation, while their chemical properties will depend in a large measure upon the groups which happen to be present at the end of the chains. We shall favor the explanation of the macromolecular theory in this article. It has been mentioned that the results of a number of physical studies on cellulose, silk, tissue and stretched rubber indicate the presence of long primary valence chain macromolecules. I n the space available it will not be possible to completely outline these results, but the manner in which they have been obtained may be suggested. Foremost among them are the X-ray diffraction studies which have been made by Meyer and Mark, Hengstenberg, Sponsler and Dore, Herzog and Jahnke, Polanyi, Weissenberg, Clark, Hauser, and others. The point of view has gradually develpped that it is not necessary, as had previously been believed, for the unit crystal cell to contain an integral number of whole molecules or its equivalent in ions. In the substances with distinctly fibrous structure the chains linked by primary valences pass through the unit cell in such a way that only two links in each chain are found in the unit crystal cell. The m$cromolecules themselves have been shown to be sometimes as long as 500 A while the single molecular unitsofrom which the chain is formed will be of the order of magnitude of I O A in height. I n cases where the chains are built in spiral form one complete turn is indicated by a distinctive periodicity in the X-ray diagram from which the height of the unit cell is defined. A certain number of these chains are held together by the secondary valence forces to form bundles or micelles. The micelles are
STRUCTURE OF INSULATING MATERIALS
43 9
probably held together by means of amorphous cementing materials which would be called the dispersion medium in the language of colloid chemistry. Of course the micelles will not always be oriented with their long dimension parallel (or nearly so) to the fiber axis but it is perhaps surprising how common this arrangement is. It has been established for cellulose, silk, tissue, stretched rubber, stretched gelatin and certain of the silicates. It is not difficult to demonstrate that the forces acting in a direction parallel to the chains are much stronger than those acting perpendicular to them. In those cases where the micelles are already oriented, the fibers subjected to tensile strength tests are very strong in the direction of the fibers and much weaker in other directions. Measurements of the coefficient of expansion in the different directions also indicate the presence of this orderly arrangement. Parallel and random arrangements of the micelles may also be differentiated by experiments in which the swelling of the materials is studied. When a section of regular arrangement saells it expands a t right angles to the direction of the fibers but is not elongated. On the other hand, a section built of micelles arranged in random fashion swells not only uniformly but also much more rapidly. In the oriented structures the micelles are more tightly packed and liquids penetrate more slowly. One of the most difficult points in connection with these theories has been the explanation of the nature of the secondary valence forces in these high molecular weight substances. In the cases where it has been possible to obtain X-ray diagrams it appears that two carbon at:ms joined by the primary valences are separated by a distance of 1.5 to 1.6A,and the carbon to nitrogen .7~ less. I n the case of the macromolecular distance of separation is perhaps 1 theory of the composition of these materials it is assumed that the primary valence bonds act in the direction of the chains so that their strength for directions other than parallel to these chains must be explained. This may be accomplished according to Meyer and Mark3 by assuming the secondary valences to be cohesive forces of a van der Waals nature acting between atoms in different chains which will be separated by distances of the order of magnitude 4 A. We could be more satisfied with this explanation if the mechanism of these forces could be more exactly described. The existing classical theories we owe to Debye’ and Keesom5 who have assumed them to be due to the electrostatic action of fixed dipoles or quadrupoles, and to the modification of existing dipoles by a distortion effect. In the case of interaction between fixed dipoles the forces will diminish with increasing temperature but in the case of induced dipoles the effect will be independent of temperature, according to the now well known dipole theory of Debye. The method of the theories has been to calculate the dipole and quadrupole moments from known values of the van der Waals constant. However, it appears from quantum mechanical calculations6 too recent to be included in the book of Meyer and hfark that “Der Aufbau der hochpolymeren organischen Naturstoffe” (1930). 4Physik. Z., 21, 178 (1920); 22, 302 (1921). 6 Physik. Z., 22, 129 (1921). BEisenschite and London: Z. Physik, 60, 491 (1930);London: 63, 245 (1930). 3
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JOHN WARREN WILLIAMS
hydrogen has a quadrupole moment which according to the classical theory would have given a value of the van der Waals constant which is much too low. Therefore, these theories are in need of some revision. The method of the new theory is to take the mutual perturbations of the periodic electronic motions into account with the result that there may be calculated both the primary and secondary valence forces for very simple molecules. The primary forces act over very short distances only (order of magnitude 1.5 but the van der Waals or secondary forces diminish with distance much less rapidly and in addition the magnitudes of the latter seem to be of the right order of magnitude. I t still remains to be seen whether or not this kind of calculation can be extended to the complicated systems under discussion. It is desired to ascertain whether or not the electrical properties of t,hese materials will depend upon their capillary structure. The results of a number of investigations have shown that the conduction of the electric current by these substances is not due to moisture condensed on outside surfaces but takes place because of the presence of moisture and ionizable materials in them. The form of conductance vs. electrolyte content and conductance vs. moisture content curves may be considered proof of this statement. An excellent example of the effect of the presence of ionizable materials is shown in the recent studies of Kemp7 who has demonstrated that if rubber is purified with respect to the nitrogenous constituents always present its electrical characteristics may be considerably improved. Textile materials to be used as covering for wire are now washed in water to remove inorganic impurities. I t is suggested here that the conductance is determined by the capillary structure of the insulating materials, it being due to the ionic processes operating between individual fibers or chains. The objection may be raised that such an ionic process in conducting paths makes the presence of the complex electric currents which are always found in these substances impossible of explanation. The interionic attraction theory of electrical conductance nom widely accepted indicates that free ions such as exist in the conducting paths of moisture absorbing dielectrics should behave to some extent like a dielectric, owing to the ionic atmosphere surrounding each ion. The free ions then carry not only the ordinary conduction current but also a complex current with its displacement current and conduction current components. Also in addition to the constant conduction current with its I?R heat loss free ions may produce dielectric loss in cases where the resistance of the path is variable, and conducting paths instead of being pure resistances become equivalent to resistance in series with a condenser, giving a greater alternating current conductance. Ions, in addition to being free, may be adsorbed along the conducting paths. If these ions are at all mobile they may oscillate due to an impressed alternating field giving rise to a corresponding absorption of energy. Such a movement, of ions may in certain cases be equivalent to a condenser charging and discharging current. Any mechanism which describes the process of conduction in a. dielectric must account for the fact that the conductivity is increased when the strength
a)
Hell System T e c h . J., 10, ~ j 119311 z
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of the applied field is increased. In this case it seems reasonable to assume that the effect of increased voltage will be to increase the number of dissolved ions because those least strongly adsorbed will be set free to carry the current in the ordinary ways. Another possible explanation of the effect produced by the use of high field strengths may be found in the deviations from Ohm’s Law first observed by W e n 8 and explained by Joos and B l ~ m e n t r i t t . To ~ discuss this effect we shall have to describe the ionic atmosphere more exactly. According to the newer theories dealing with the behavior of strong electrolytes in dilute aqueous solution only ions are present, furthermore, since Coulomb’s Law is assumed to express the forces between them, there will be more ions of unlike than of like sign around a given ion, in other words any given ion will in effect be surrounded by a kind of space lattice arrangement of oppositely charged ions called its ionic atmosphere. I t is similar in all respects to the double layer so commonly referred to in colloid chemistry. This atmosphere has a radius which may be calculated and it requires a definite time to be either formed or destroyed. Furthermore, it will always be symmetrically built about a stationary ion. But because of its finite time of relaxation the atmosphere can no longer be built symmetrically if the ion is caused to move and it will become unsymmetrical in the direction of the motion. Before the ion there will be more ions of like charge and behind it more ions of opposite charge so that each moving ion, positive or negative, is subjected to a force which decreases its mobility. If we consider an interval of time over which the atmosphere can be regarded as relatively stationary, and if during this interval the ion is removed to a distance much greater than the thickness of the atmosphere, the influence of the latter will become small and we are left with an increased conductance because the interionic forces which decrease the ionic mobilities have been overcome. Gemant,lo in a recent book, has suggested an explanation of the effect of high field strengths which is quite different. Accordingly to this investigator it seems more logical to assume that undissociated molecules may also take part in the conduction, their decomposition being caused by the attraction of the poles for the several parts of the molecule. I t has been noted that Smekal and others believe the conduction in crystals to be due to the presence of lattice imperfections in which the ions move in an adsorbed or free condition along paths formed by these fissures and that the Smekal point of view is supported by indirect evidences provided by the mechanical and optical properties of crystals. However, the writer believes the conductance in these systems to be a volume process of the normal ionic type in which the dielectric losses can be accounted for by the Joule heat law. This conclusion has been previously drawn by Joff6,ll Phipps,12and by others. In the first place Joffe has shown that a crystal has a characteristic specific bAnn. Physik, (4) 83, 327 (1927); 85, 795 (1928). Physik. Z., 28, 836 (1927). l o “Elektrophysik der Isolierstoffe” (1930). I’ “The Physics of Crystals” (1928). Phipps, Lansing and Cooke: J. Am. Chem. Soc., 48, I I Z (1926).
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JOHN WARREN WILLIAMS
conductance, that is, its conductance is a property of the chemical substance (KCl, SiOz etc.) rather than of the crystal and it is independent of crystal imperfections. This conclusion could be drawn only after the most extensive purifications of the crystals had been carried out. According to this point of view, the electrical conductance of a solid salt depends only upon the number of free ions in the lattice a t the given temperature. Furthermore, if the logarithm of the specific conductance is plotted against the reciprocal of the absolute temperature, a straight line is obtained which is of great significance. This has been expressed in the following way by Phipps: dlnk- q dT RTi I n k = - - +qc . RT where k is the specific conductance of the crystal T is the absolute temperature R is the gas constant c is a constant and q is the heat of liberation of a gram ion in the crystal lattice, that is, the work necessary to produce a mole of ions in the interior of a crystal. It is peculiar to these systems that frequently only one kind of the ions is liberated and they carry all of the current. In a series of simple salts with a common anion, the chlorides of K, Ag, Na, etc., in which the positive ion is the carrier, the slopes of the In k vs. r/T curves will all be equal. In other words the energy necessary to liberate a positive ion from a chloride lattice is always the same If, on the other hand, one deals with a series of simple salts with a common cation, NaF, NaC1, NaBr, and NaI, it is found that the slope of the curve becomes progressively less in the order given, indicating that the energy necessary to liberate Na+ ions becomes less the greater the atomic weight of the anion. The work of liberation of an ion is also related t o other properties. The following table, adapted from the article of Phipps, Lansing and Cooke, shows it to be closely related to the natural quantum of the crystal as derived from specific heat data.
TABLE I Relation between Heat of Liberation of a Gram Ion and the Natural Quantum of the Lattice Crystal
q (CaW
qlhv
NaF NaCl NaBr NaI
32,800
38.2 36.4 42.4 38.5
20,200
18,400 13,800
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The following sentence is quoted from the article of Phipps, Lansing and Cooke. “As the heat of liberation decreases the quantum of energy decreases correspondingly, so that the number of quanta necessary to activate the N a t ion is practically constant for such a series.” The observed conductance effect is not a dielectric displacement (except possibly at very low temperatures), because in that event the effect would change but little with temperature. The current actually transfers charges through distances incomparably greater than atomic and molecular distances. Another important fact to be considered is that many experiments of Joff6 and of TubandtI3 have shown Faraday’s Law of electrolysis to be quantitatively obeyed. It can be predicted that with a systematic study the number of quanta required to activate the several ions will in each case be constant, that periodic regularities will appear, and further that the laws based upon an ionic conduction in a homogeneous medium will be obeyed. In other words, the electrical behavior of a crystal appears to be quite different from that of the type of substance which has been described as a moisture absorbing dielectric, although many believe the mechanisms in the two cases to be quite similar. I n order to definitely decide such questions as have been discussed, it seems necessary to continue the careful studies of the compositions, sizes and shapes, and arrangements of the aggregated molecules which form the highly polymerized organic substances. It is of more than passing interest that as these scientific problems are being solved, means are indicated by which the electrical characteristics of our ordinary dielectric materials may be improved. Laboratory of Colloid Chemistry, Unzversity of Wisconsan, June 1, 1931. l3
Z. anorg. Chern., 115,
IOj (1921), et.
al.
