“G. E. REVIEW”-LIQUID

LIQUID DIELECTRICS

ACKNOWLEDGMENT Dr. A. H. Sharbaugh and Dr. R. W. Crowe, Research Associates in the Physical Chemistry Section of the Knolls Laboratory, are the authors of this article which was published in the May 1954 issue of General Electric Review.

Although solid insulators play an electric capacity per unit volume, and all-important role in the electrical in- so on. dustry-that of isolating canductors from one another and from ground What electrical properties determine while supporting them physically- the performance of a liquid ? liquid insulators are equally important Primarily they are: 1) ability to resist in many electrical products. Trans- breakdown under electrical stress, 2) formers, circuit breakers, , the electric capacitance per unit of and cables are some of the more com- volume determined by the dielectric mon devices that utilize liquid dielec- constant, and 3) loss factor-the en- trics. In the industry ergy loss per unit volume per cycle. alone, insulating oil by the tens of These properties depend on the liquid’s millions of gallons is used annually. operating temperature and frequency as well as on the structure of its con- stituent molecules. Through a better understanding of how liquid dielectrics behave in elec- Let’s look first at electrical phenom- tric fields, engineers have the oppor- ena in liquids at low voltages, then tunity to further increase the oper- proceed to the less understood phe- ating efficiency of their products. For nomenon of at the development of liquids with better much higher voltages. (We’ll confine electrical properties will yield trans- our attention to electrical properties formers of greater power capacity per alone-not the physical ones such as unit volume, capacitors of greater heat transfer and mechanical forces.)

DIELECTRIC CONSTANT

In deciding what liquids may be of the liquid.) On this basis, for ex- classed as insulators, you can arbitrar- ample, you would class water as a liq- ily select those having resistivities uid , although its 1.esistivity greater than a certain value, say, 10- is a million times smaller than other million ohm-centimeters. (An ohm- commonly used insulating liquids. centimeter is the resistance between opposite faces of one cubic centimeter Actually, the application dictates the

702 “G. E. REVIEW”-LIQUID DIELECTRICS

permissible lower limit of resistivity. What do we mean by a dielectric Thus water, with a high dielectric con- constant? Perhaps the most familiar stant, can be a perfectly respectable property of liquid dielectrics is the dielectric in some applications and an capacitance increase observed when extremely poor one in others. an air is immersed in a liq-

-@-e -+4e - -e 0

Fig. 1. INDUCED DIPOLES are formed in a neutral atom (left) when placed in an electric field (right). Arrows indicate the magnitude and direction of polarization.

uid. In fact, this ratio of the liquid- to for liquid helium to 78 for water; most air-filkd capacitance is often Used to commonly used liquids have dielectric define the dielectric constant of the constants between 2 and 5. On the 0th- liquid in question* in this er hand, the dielectric constank of all ma@itude becomes a Prop- way, gases are very nearly unity. And erty of the liquid, independent of the so, measuring voltage or geometry of the gases do not Offer the larger capac- test capacitor. Extremes of such ratios itances per unit volume that liquids for liquid dielectrics range from 1.05 do.

POLARIZATION

The dielectric constant varies with called poralization, common to all liq- the temperature and molecular struc- uid dielectrics. true of a liquid and with the frequen- cy of the applied voltage wave. To un- When a unit positive charge +q derstand the reason for this variation and a unit negative charge -q coin- and why liquids differ broadly in their cide in space, their electric fields can- dielectric properties, you need to be- cel one another and their net charge is come acquainted with a phenomenon zero. If they are displaced a distance

703 “G. E. REVIEW”-LIQUID DIELECTRICS

H CI f I

H-C C-H C-H

H-C C-H C-H

c c

Fig. 2. ELECTRICAL BALANCE of a neutral benzene molecule with six dipoles (left) is upset when an atom of hydrogen is interchanged with an atom of chlorine.

1 apart, a resultant field arises. Such a kinds of atoms have different amounts pair of charges now constitutes a di- of attraction for electrons, called val- pole, and has a moment p equal to the ence. Thus, when a molecule is formed product of their displacement and from its constituent atoms, it may not magnitude of either charge, or 1s. be completely in electrical balance. As an example, take first the molecular For example, an atom (Fig. 1, left) structure of benzene (Fig. 2, left), a with four electrons symmetrically 10- common dielectric liquid. Here the in- cated about a nucleus of four protons dividual dipoles arising from the dif- is the equivalent of four dipoles. The ferent electron affinities of carbon and moment of each pair, electron and pro- hydrogen are in complete electrical ton, is represented by the length and balance. If, however, a chlorine atom direction of the arrows. You cgn see in is substituted for one of the hydrogens this instance that all the individual di- (Fig. 2, right) there is no longer com- poles balance each other and that the plete neutralization of the individual atom as a whole is electrically neutral. dipoles, and the molecule is out of But place the same atom in an electric electrical balance. This kind of mole- field between the plates of a capacitor cule has a permanent dipole moment and see what happens: Now the indi- because it doesn’t owe its existence to vidual dipoles no longer mutually can- any external electric field. cel each other (Fig. 1, right) because the electrons are attracted to the posi- The general term polarization refers tive plate. Conversely, the positive nu- to the movement of electric charges cleus is slightly displaced toward the within a molecule in response to an negative plate. The resultant dipoles applied field. You may further classify are called induced dipoles because it according to the kind of body that they exist only by virtue of the exter- carries the charge-electronic, atomic, nally applied field. or dipole. Electronic polarization, ex- plained in connection with the atom, You may remember from your involves the relative displacement of studies of chemistry that different electrons with respect to their atomic

704 “G. E. REVIEW”-LIQUID DIELECTRICS nucleus. Atomic polarization, on the different kinds of polarization that can other hand, is caused by shifts in the form. Although all liquids will have equilibrium positions of atoms within electronic and atomic polarizations the molecule with respect to each oth- that contribute to their dielectric con- er. Dipole polarization (not illustrat- stants. the amount of dipole polariza- rd) arises when the randomly oriented tion will vary greatly according to the permanent dipoles of a molecule are magnitude of the dipole moment. If the oriented by an applied electric field. molecule is electrically symmetrical, it will be Lero. The dielectric constant At a given frequency and tempera- of a liquid may therefore be large or ture, the value of the dielectric con- small, according to the arrangement stant will depend on the number of of atoms within its molecules.

. .. mow~~ uL*o WI’UIBD WyU€ INCREASING FREQUENCY - INCREASING FREQUENCY - Fig. 3. FREQUENCY DEPENDENCE of the dielectric constant, resistivity, and d!electric loss (left) is depicted for one type of polarization. Superimposing curves, shown in heavy black, for the different polarizations result in the composite picture.

FREQUENCY DEPENDENCE - - -

If you use a low frequency or d-c charge carriers give a clue to the time voltage to measure the dielectric con- required for the different types of pol- rtant of a liquid having permanent di- arization to form. Electronic polariza- poles, all the polarizations will be com- tion (Fig. 3, right), involving the ex- plete, and you’ll obtain the highest tremely light and small electron, per- possible value of dielectric constant. sists from power frequencies through If, however, you increase the frequen- the visible frequencies. Atomic polar- cy of the applied a-c voltage, the di- ization, requiring the movement of the rection of the electric field may be re- heavier atom, usually does not persist versed before polarization has time to beyond the infrared range. Dipole pol- form. This leads to a decrease of di- arization needs even longer times be- electric constant from some steady- cause it requires the movement of en- state value c’,, (Fig. 3, left) at low fre- tire molecules, and usually it con- quencies to another steady-state val- tributes to the dielectric constant only ue e’- at some higher frequency. at power and radio frequencies. The size and weight of the various One by one the permanent dipole

705 “G. E. REVIEW”-LIQUID DIELECTRICS

and atomic polarization cease to con- reduced to an extremely small value tribute to the total dielectric constant in the case of organic fluids. For ex- as the frequency is increased from d-c ample, carefully purified water has a or low-power frequencies to the high- d-c resistivity of about lo’ ohm-centi- er visible frequencies. For this reason meters. Thus it represents a highly water has a dielectric constant of 78 at “lossy” liquid dielectric. On the other power frequencies (60 cps) and only hand, a hydrocarbon oil-a liquid such 2 at visible frequencies (10’’ through as petroleum containing only carbon 10lScps) . and hydrogen-may have a d-c resist- ivity of 1015 ohm-centimeters, making The heat generated in a liquid di- it almost a perfect insulator. electric under stress is determined by the combined effect of: 1) a frequency- Referring back to Fig. 3 (left), YOU independent resistivity measured with can see how the measured resistivity d-c voltage and 2) a frequency-depen- starts at its d-c value and decreases dent resistivity that contributes when to some limiting value pm.The loss fac- a-c voltage is applied. Determined tor C’’ is a measure of the amount of principally by the number of free ions heat generated per cycle as a result of in the liquid, the d-c resistivity can be this resistivity.

- - - ANDTEMERATUREEFFECTS Why does the dielectric constant generally lowers viscosity, the mole- change with temperature? Remember cules experience less molecular fric- that its value at any particular tem- tion as they rotate among their neigh- perature is determined by the com- bors. They undergo an increased mo- bined contributions of the different tion as the temperature rises. This ac- kinds of polarization. In the instance of tion promotes a random orientation of electronic and atomic polarization, the the dipoles that opposes the ordering influence of temperature is slight be- influence of the applied electric field. cause these polarizations are affected And so, the net effect of temperature only as the density of the liquid increase on dipole polarization is a changes. Hence, they become slightly lowering of the measured dielectric smaller as the liquid’s temperature is constant. raised, because the number of con- (Because the dielectric “constant” tributing atoms per unit volume be- of a liquid may depend strongly on comes smaller. Conversely, they be- frequency and temperature, you might come slightly larger as the tempera- ccxnclude that the name isn’t too de- ture is lowered. scriptive. This is true. However it has gained rather universal acceptance, al- ,A much stronger temperature de- though various other terms are occa- pendence is experienced in dipole pol- sionally found in literature. One that arization: the entire molecule rotates is sometimes used is specific inductive instead of just its charges being dis- capacity, and another is dielectric co- placed. Because a rise in temperature efficient.

HIGH-VOLTAGE PHENOMENA When liquid dielectrics are utilized that associated with the polarization as insulating material in high-voltage of molecules. This difference arises equipment, they are important because from their ability to withstand high of a property differing entirely from electrical stresses for long periods of

706 “G. E. REVIEW”-LIQUID DIELECTRICS time. For these purposes, engineers When designing high-voltage equip- usually prefer liquids that are therm- ment using liquid insulation, the engi- ally stable and nonflammable. neer wants to know what he can ex- pect of a liquid exposed to high elec- If the voltage applied to any dielec- tric fields. Unfortunately, this isn’t al- tric insulator placed between two met- ways predictable. Any property that al electrodes is increased sufficiently, deals with the violent destruction of an electric spark will pass through it. matter is often complicated and diffi- When this occurs the insulator has cult to reproduce. As a result, the phe- nomenon of electrical breakdown in broken down electrically. And the heat liquids is not well understood as yet. accompanying such a breakdown Surprisingly enough, the great influ- spark usually results in the accumu- ence of extraneous factors not asso- lation of a large quantity of decompo- ciated with the liquid itself makes it sition products along its path. Because difficult to correlate breakdown with such products weaken the dielectric any known molecular properties. The even more, an initial breakdown often engineer, then, is forced to use abnor- renders a piece of equipment perma- mally large safety factors when de- nently inoperative. signing his equipment.

WHAT IS ELECTRIC AL BREAKDOWN?

The appearance of an electric spark trodes, free electrons emitted by the is certainly evidence of electrical cathode-or alreadiy present in the liq- breakdown; but it doesn’t tell you why uid itself-are acoderated toward the breakdown occurred. Extremely large anode. This acceleration will continue currents in sparks do, however, sug- until the rate at which electrons lose gest this: Electrical breakdown in- energy by collision with molecules just volves the failure of the insulator to equals the rate at which they gain en- retain its initially high electrical re- ergy from the electric field. Under sistance. Hence, a general definition of these conditions the electric current a liquid’s breakdown voltage is that will remain small. If, however, you in- amount of voltage required to convert it rapidly from an insulator to a con- ductor.

How does such a conversion take place? And what does the magnitude of the voltage have to do with electri- cal breakdown? Perhaps the most _I+ widely accepted theory of explaining this phenomenon deals with the elec- tron avalanche.

Consider a system (Fig. 4) composed of a liquid dielectric placed between two parallel-plate metal electrodes. Fig. 4. ELECTRON AVALANCHE be- Choose the system SO that the electric gins when a free electron gains enough field will be homogeneous throughout. energy to knock another electron from a Whetl you apply a voltage to the elec- molecule.

707 “G. E. REVIEW’’-LIQUID DIELECTRICS crease the voltage so that each free same process, the result is an electron electron gains sufficient energy to avalanche. Mathematically it works knock another electron from a mole- like this: If a single starting electron cule of the liquid-called ionization- makes n ionizing collisions while the number of current carriers, and crossing the gap, then 2 n electrons the current, is multiplied by two. will reach the anode. When n reaches some critical value, the electric current To take it a step further, suppose you through the liquid will be high enough increase the voltage until each start- to lead to a runaway process culmina- ing ekctron can multiply in this man- ting in the spark. Thus, you can define ner a number of times as it crosses the electric strength as the voltage that gap. Since each new electron produced will just cause breakdown divided by by collision is also free to undergo the the distance between electrodes.

THEORY AND PRACTICE From what has been said you might proceed through the entire gap before expect the electric strength of a liquid they are snuffed out. (The occurrence to be controlled by its ability to resist of ionization leads nonetheless to a the acceleration of electrons to those gradual decomposition of the liquid energies causing ionization. This would and in time may cause its failure. En- be true only under ideal conditions. gineers commonly call pre-breakdown However, as mentioned earlier, many avalanches of this kind corona-ob- extraneous factors often overshadow served electrically as small bursts of the properties of the liquid itself. Usu- current. ) ally they prevent measurement of the liquid’s maximum electric strength. Closely associated with the effect of electrode shape are small particles on Of these complications, one of the the electrode surfaces or suspended in most serious is the shape of electrodes. the liquid. Having a dielectric con- Seldom do you find smooth, flat elec- stant higher than the surrounding me- trodes in actual equipment. Irregulari- dium, these particles will tend to move ties on electrode surfaces-such as into the regions of highest electric sharp edges or points-intensify the field. Their effect is to create intense electric field in their immediate vicin- localized fields and cause premature ity. Such a situation often gives rise breakdown. to localized electric fields far more in- tense than the uniform field through- But perhaps the most difficult exter- out the liquid. And if their intensity nal factor to control in the laboratory is great enough, electron avalanches is the electrode material itself. Here’s can be sufficiently large to cause why: The electric fields involved in the breakdown. You would thus measure breakdown of liquids are of the order an apparent electric strength far be- of one-million volts per centimeter. low that realized in the uniform field. At these voltages, most metals emit Often, however, electrode irregular- electrons profusely by a process ities give rise to electron avalanches known as field emission, or cold emis- of limited magnitude-too small to sion. The electric field at which this cause complete breakdown directly. emission becomes extensive has a high This is so because the field strength sensitivity not only to the kind of elec- throughout most of the liquid is low, trode metal but also to the nature of and localized avalanches are unable to its surface. For example, if a highly

708 “G. E. REVIEW”-LIQUID DIELECTRICS polished electrode is partially covered they can be swept to the anode by the with an oxide film, its emission char- electric field, there results a large elec- acteristics may vary a considerable tronic space charge near the cathode. amount from one part of its surface to This gives rise to an intense field in another. the vicinity of the anode. And such be- havior invariably results in low val- Again, if electrons are emitted by ues of the apparent electric strength of this mechanism at a faster rate than theliquid.

THERMAL BREAKDOWN

Although the distortion caused by happen if the liquid itself possesses a intense localized fields may lower the relatively low electrical resistance at apparent electric strength of a liquid, low fields. it doesn’t alter the general mechanism by which disruption of the dielectric This rise in temperature is usually occurs. For example, another compli- observed in liquids that require long- cation that may not be considered ex- time exposure to . If it be- ternal doesn’t depend on field distor- comes high enough, the liquid may de- tion at all. If ionic impurities are pres- compose thermally. More often, how- ent in the liquid, the application of an ever, bubbles of once-dissolved gases electric stress may cause a large -or vaporized liquid-that are ejected enough current to flow even without at the elevated temperatures cause ionization to cause a definite increase premature failure. This phenomenon in temperature. The same thing can is called thermal breakdown.

LABORATORY TRICKS

You might infer from what has been pared to the gap spacing between them, said that any measurement of the elec- a small region in the center of the gap tric strength of a liquid has little or no will have a configuration approaching meaning in terms of the liquid’s struc- that of two parallel-plane electrodes. ture and composition. This isn’t neces- This electrode curvature, reducing the sarily true. For if you make a concen- electric field in all other parts of the trated effort to .either fix or remove gap, limits breakdown to the small extraneous factors, it’s possible to ob- center region of the electrodes where serve significant differences in the the field is nearly uniform. And by electric strengths of liquid dielectrics. using rectangular voltage pulses of ap- You can do this by carefully purifying proximately one microsecond’s dura- and filtering the liquids before testing tion, the danger of interference from and by carefully selecting and prepar- thermal breakdown is easily removed. ing electrodes. (When pulses of such short duration are applied singly to the breakdown Hemispherical electrodes are usually cell, there’s not enough time for harm- used to test liquid dielectrics in the ful temperature changes to take place.) laboratory for a number of reasons. For one, they are relatively easy to With these techniques it’s possible to fabricate and polish. For another, if measure the electric strengths of liquid their radii or curvature are large com- hydrocarbons within an accuracy of

709 “G. E. REVIEW”-LIQUID DIELECTRICS

1.8- STAlNLESS STEEL ELECTRODES

(GAP=0.0051 CM)

1.6 - FANE-2

1.4, I I I I I

PULSE DURATION (MICROSECONDS) Fig. 5. INVERSE RELATIONSHIP of pulse duration to liquid-hydrocarbon strength ends beyond one microsecond. Beyond 10 microseconds, thermal effects complicate matters.

- 2.0 - four per cent or less. From such meas- urements (Fig.5) the electric strength 2 is found to be independent of pulse 6 18 - duration for times greater than one g microsecond. Thus the time-independ- g ent value of electric strength is the 9 same value you would measure under 1.6 - long-time voltage application--if you could prevent thermal effects. (The 5 latter begin to interfere when pulse 1.4 - duration reaches a value greater than u 10 micro-seconds.) E z 12 I I I I 060 0 6. OM1 012 Olb Field emission effects, on the other DM17Y (GRAMS PER CvBlC CENTIMETER) handy can be fixed but not dY Fig. 6. ELECTRIC STRENGTH of a nated. And SO, the time-hdeWndent liquid hydrocarbon, stressed by high- value of electric strength represents a voltage pulses of short duration, is a func- combined property of the liquid and tion of the liquid’s density and molecular the electrode material. structure.

710 “G. E. REVIEW”-LIQUID DIELECTRICS HOW THE LOW-VOLTAGE GOODNESS OF A LIQUID DIELECTRIC IS DETERMINED

That most liquid dielectrics are not perfect insulators has been experimentally proved. But they do have a measurable power loss that’s manifest as heat. This fact is taken into account by the equation

€* =€p- jp where f* is a liquid’s complex dielectric constant, having direction as well as magnitude. Its real component c’ is the absolute value of the dielectric constant and gives rise to the current out of phase with the applied voltage. The imaginary component c”, called the loss factor, is a measure of the in-phase current or the heat generated per cycle of applied voltage; j equals V -1. This is shown graphi- cally- CAPACITIVE CURRENT-€‘

A quantity expressing the merit of a liquid dielectric is the loss tangent, de- fined by the relationship, tan 6-~“//. As you can see from the diagram, the com- plement of the angle 6 is 4; and the cos 4 is defined as the dielectric’s power fac- tor. For low-loss liquids, 6 is almost zero, making 4 nearly 90 degrees; therefore, tan 6=cm 4. And so, the loss tangent equals the power factor when tan 6 is less than 0.1. Liquids having a power factor less than about 0.05 at their operating frequency are usually considered satisfactory for commercial use. To give you an idea of magnitudes, the Table lists values for some commonly used liquid dielectrics, including water.

DIELECTRIC PROPERTIES OF SOME LIQUIDS AT 25 C AND 60 CPS Loss Tangent Power Factor Liquid -€I €” (Tan 6) (COS 4) Pyranolt 5.3 0.005 0.001 0.001 Hydrocarbon oil 2.2 0.002 0.001 0.001 Dibutyl sebacate 4.5 0.040 0.010 0.010 Water 78.0 78,000 1000 1.000 tReg. Trade-mark of General Electric Company 711 “G. E. REVIEW”-LIQUID DIELECTRICS

By applying the pulse technique to electric strength increases in a linear liquid hydrocarbons of different mole- fashion with the density of the liquid. cirlar weight (Fig. 6), it is found that

THE SUMMING UP

In a general way, some of the more The relationship of a liquid’s elec- important electrical properties of liq- tric strength to its molecular proper- uid dielectries have been presented. ties is obviously of value to the engi- And though these properties were di- neer in search of a liquid dielectric to vided into two distinct groups-low fill his specific need. But unfortunately, and high voltage-it doesn’t neces- theories that explain such correlations -or predict the electrical behavior of sarily mean that there is any line liquids in high dielectric fields-are dielectrics in usage. presently qualitative at best. Perhaps For in many applications you’ll find more information of the type discussed liquids with good characteristics at will, in the future, contribute to a bet- both high and low voltages. ter understanding of liquid dielectrics.

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