Journal of Electrostatics, 7 (1979) 1--11 1 © Elsevier Scientific Publishing Company , Amste rdam -- Printed in The Netherlands

HIGH FIELD CONDUCTION AND BREAKDOWN IN DIELECTRIC LIQUIDS

Y. INUISHI

Faculty of Engineering, Osaka University, Yamada-Kami, Suita-shi, Osaka-fu,

Japan

ABSTRACT

The review of the work done in the author's group on high field conduction

and breakdown in liquid dielectrics is presented. Time of flight measurement

of the carrier mobility at higher field and the formative time lag measurement

of breakdown at nano-second range to obtain streamer propagation velocity

constitute two main subjects discussed. Especially the importance of anode

asperities is suggested to trigger the start of positive streamers and to

decrease breakdown strength in nano-second range.

From the change of breakdown strength at the transition from liquid to solid

due to,ray induced polymerization, the effect of bubble in the liquid breakdown

for longer pulse width is recognized. At high pressure, and shorter pulse width

( < i00 ns) the breakdown seems to occur through avalanche streamer mechanism.

INTRODUCTION

In this lecture, I would like to review our works on high field conduction

and breakdown of liquids. ~he experimental results obtained from measurements

such as dc and pulse conductivity, carrier mobility and life time, time lag of

breakdown as well as laser induced breakdown at optical frequencies and the

effect of viscosity on breakdown field constitute the bases of this discussion.

Since the current density J observed under the application of an electric

field E is proportional to the product of mobility ~ (or drift velocity ~E)

and carrier density n, the nature of carrier density and mobility have to be

investigated separately in order to clarify the mechanism of conduction and

breakdown in liquids. In ordinary, impure liquids ions with lower mobility are

the main charge carriers due to the short life time of free electrons. The ion

mobility can be characterized by an extension of Walden's rule I) .

~ffi K ~-n

(n = i~2) (I)

However, in carefully purified liquids electrons become the important charge

carriers and their mobility has been studied by many workers such as W.E. Schmidt 2# . ~

The relatively high mobilities observed in these cases were attributed to free

electrons or hopping polarons depending on the molecular structure. It should

be emphasized here that the mobility of electronic carriers increases with

increasing applied electrical field from hopping polarons to free electrons.

However, high mobility free electrons sometimes show a decrease in mobility at

higher fields due to the hot electron effect as for example in the case of liquid

argon.

The carrier density has to be determined from a balance between generation

due to injection, extraction, dissociation, impact ionization and annihilation

due to recombination, trapping (attachment). Unfortunately direct measurements

of the ionization coefficient in liquids have not been successful untill now.

The mechanism of dielectric breakdown in liquid has long been the target of

intense discussion. The avalanche multiplication with some positive feed back s)

mechanism , and bubble formation 4) due to electron injection are two frequently

involved mechanisms. Recently, optical observations of avalanche or streamer

propagations have been reported by several workers. For example Forster et al 5J ~

and Yamashita et al 6) reported velocities of l~5x10 5 cm/sec for avalanche or

negative streamer propagation. Devins et al 7~ ~ obtained much lower values of about

2x10 4 cm/sec. The role of bubbles is favoured by the former authors but not by

the later ones.

High field conduction and mobility measurement

Dc conduction in n~Hexane up to the prebreakdown range was measured 8'9)

using a diverter system as shown in Fig.l. As the number of breakdowns increases,

the current in the low field region decreases possibly due to the disappearance

of the larger asperities on the cathode and/or degassing of the liquid at the

electrode surfaces. At much higher fields prior to breakdown, electron injection

from the cathode asperities increases the negative carrier (ion) density leading

to a steep rise of the current just prior to breakdown. This steep rise component

is usually erratic and noisy. After a large number of breakdowns~ in the same

system the prebreakdown current increases due to the damage of the electrode

surfaces.

Figure 2 shows the effect of an intentional impurity (ethyl alcohol) on the

conduction in n-Hexane I0) . Although ionic current in the lower field range

increases due to the dissociation of impurities, the breakdown field also

increases possibly due to an electron scavenger action and cathodic field reduction

by the impurities. This means that the breakdown field is not necessarily

determined by the ionic conduction but by the instability in the electronic system.

8

The Kerr effect and pulse conduction measurements showed the enhancement of the

cathodic field due to the transport of ionic impurities II) .

Fig.l. Effect of presparking by

the diverter on dc conduction

current in n-hexane

i,,.

r =

._g

U

~6 8

5

"" 2 <C v

u 5

g

2 8 do

5;

a.o.

18"C t B.D d : 0 4 . . ~ . .

~../jo

o

/.//r~ 5% Ethyl alcohol

# I I I I I I I

0.1 0.2 0.3 0,4 0.5 0.6 0.7 08 field strength (MVIcm)

Fig.2. Effects of ethyl alcohol on dc conduction current of hexane

4

Time of flight measurements of negative carriers8'10)in n hexane and benzene

were done by shining u.v. light on the cathode surface. The carrier transit

time was obtained from an analysis of the charge signal trace. The carrier

mobility at 20°C, was estimated to be ixlO -3 in n-hexane and 0.45xi0 -3 cm2/V'sec

in benzene, respectively. These mobilities were field independent up to 0.5 MV/cm

and in both cares the activation energy was 0.16 eV.

I/) >

E u

v

lo-S

r~

2

5

2 I 2

~ ~ n - H e x a n e

\ d = O lmm ,~ V : 1 kV

"\\~ 0

I I I i ' , 0 5 10-2 2 5 10-1

viscosity r~ (poise)

Fig.3. Viscosity dependence of carrier mobility in the n-hexane=silicone

oil system

Figure 3 shows the relation of viscosity and mobility of negative carriers

in a n-hexane-silicone mixture. Walden's rule (n=l, dotted line) is valid except

for pure n-hexane which shows larger value. The magnitude of the induced charge

signal in these experiment saturates (Hecht curve) at higher field in conditioned

n-hexane, but shows an increase in unconditioned systems as shown in Fig.4.

This means that the carrier multiplication does not exist untill fields greater

than 0.5 MV/cm are reached in degassed n-hexane. It may exist in systems

containing dissolved gasses possibly due to impact ionization. From the Hecht

curve, the life time of the negative carrier has been estimated to be 500 ~sec.

All these facts suggest that in n-hexane the negative carrier in fields up to

0.5 MV/cm is of ionic nature.

10_

5 .6

K, ,

0 ~I, 2 - ol L _

o ._=

n- Hexane 20°C

.,,.4~(a) before conclitioning

/ of electrode

I I I i i

0.1 0.2 0.3 0.4 0.5 field strength (MVIcm)

Fig.4. Hecht curves of n-hexane (a) before conditioning of electrodes (b)

after conditioning of electrodes

On the other hand in extremely purified systems, free electrons

(10-v400 cm2/V.sec) and hopping electrons or small polarons (10-110 -3 cm2/V.sec)

were observed for short periods by W.F. Schmidt 2) " even in these high field ranges

due to the longer life time of the free electrons. However under breakdown

conditions, due to the detachment of ions and continuous impact ionization,

free electrons predominate even in impure liquids as will be shown in the next

section.

Time lag of breakdown and streamer propagation velocity

By using a Fletcher type nano second pulse generator, the time lag of the

breakdown in various liquids has been investigated 12'13) . As is well known the

ratio of the number of trials with observed time lag larger than T, N(T), among

the total number of trials, N, can be expressed as

N(T)/N = exp[.(T-TF)/Ts] , and (2)

T F = d/v = H/~-E, T = I/[~(E)P(E)] (3) F s s

6

where TF, Ts, Vs, ~ (E), P(E) and E are formative, statistical time lag t streamer

velocity, initial electron appearing frequency, breakdown probability and

applied field respectively.

100

50

20

z 10

5

~urninum

~ cart:~rum~n

L\ field

I 1 I I I 20 40 60 80 100 120

time lag t ( n ~ : )

100

50

v

2O

"-' 10 Z ,

" o4! MV/cm

(b) L.Ar

"d =400 }Jm

I 20 40

time lag

I I ............ 60 80 100

t (nsec)

Fig.5. Laue plot for (a) silicone oil and (b) liquid Argon

Figure 5 shows examples of a Laue plot [IogN(T)/N vs.T] for silicone oil and

liquid argon. From eq. (3) the formative time lag T F and statistical time lag

T were obtained under various condition in silicone, liquid argon, liquid S

nitrogen, liquid helium and others. TFS at the same field condition are found

to be proportional to the electrode separation provided a uniform field is used.

For example propagation velocity of negative streamers (or avalanche) and

apparent mobility of breakdown electrons obtained from formative time lag

T F in silicone under nearly uniform field conditions are shown in Fig.6.

~j

g

I I >

0

10 ! uniform field BO Hm /

- - v e l o c i t y / '

. . . . . mobility . /

x 103 c s / / o 104 cs

2 I 2

f 2 ~ilicone - oil

0 I I I 0 1 3 4

field strength (MV/cm)

3.2

A

U

¢M

E v

3.1 ,,a

II

.Q 0 E

3

Fig.6. Propagation velocity versus uniform applied field in silicone oil

This velocity lies between 105 and 106 cm/sec in agreement with Forster's and

Yamashita's values. The apparent mobility of avalanche electrons is several

orders of magnitude higher than that of low field ionic and electronic mobilitie;

as was mentioned previously.

TABLE 1

Propergation velocity of negative streamer or avalanche, 50% breakdown field

at 50 nsec pulse and mobility in various liquids

liquid negative strearm er propagation 3 x 105 velo~ity(cm/s)

effective mol~li- 0.15 ty (cm2/V-s)

50°1o breakdown 2 field,5Ons(MV/cm)

low field mobility 10-5 (cm2/V.s)

'silicone liq. Ar Iiq.N2 liq.He n-l-lexane!

6xi05 1.5xi05 5xi05 4.7xi05

2 0.02 0.3 0.14

0.3 6 1.5 3.5

450 8x10 -3 2x10- 2 9x I(~ 2

Table i shows the propagation velocity of negative streamers or avalanches

in various liquids as 50% breakdown field and 50 nsec pulse duration. In the

nagative needle to plane electrode system, nagative streamer propagates simiiar

in speed to that observed under uniform field conditions at similar electrode

separation and average field. In the positive needle to plane electrode system,

on the other hand, positive streamer propagation velocities obtained from T F

are about one order of magnitude higher than that of the nagative streamer

obtained in negative needle and uniform field cases as shown in Fig.7 for silicone

oil and in Fig.8 for liquid argon.

Fig.7. Propagation velocity

vs. mean applied field for

various electrode systems

in silicone oil

14

e-

10

E 8 u

£ 6

13

II >

4

2

0

0

f

positive 1100 )Jm

• 100 pm negat ive 6 0 p m /

100 pm /. , ~ / / /

10 3 cs , y

,Y" .d ./

~,~ "J J" / i / #/ •

/ / silicone-oil I I I 2 3

negative field E (MV/cm) I I I

0.1 0.2 0.3 positive field E (MVlcm)

14

12 .-- 8.

10~

8 5 ~.O o

6

I I >

2 > ,

u O

Fig.8. Propagation velocity

vs. mean applied field for

various electrode systems

in liquid Argon

15

I/)

U c.D

x ~ g

positive needle

liq. Ar

I 0.1

m e a n

negat i v e ~ - - ~ /

/f®,,or

I I I 0-2 0-3 0.4 field ( MV/cm )

15 A

U

,o E u

5 x

O 0.5

When using carefully polished parallel electrodes, breakdown starts from the

cathode as a negative streamer directed toward the anode. It is then followed by

a faster positive streamer propagating backward to complete breakdown.

However, if we scratch the anode to create asperities, the breakdown directly

starts from the anode and the breakdown time lag is shortend 12) to the positive

streamer value as seen in Fig.5. This tendency is enhanced when aromatic

hydrocarbon is added possibly because the extraction of electrons is facilitated

at anode thus helping to nucleate positive streamer. The ratio of statistical

time lag Ts to formative time lag T F is smaller in liquid rare gases than in

other liquids, suggesting an abundance of initial electrons in the former liquid.

Effect of bubble formation and optical breakdown

To clarify the effect of bubble formation on dielectric breakdown of liquids,

the change of breakdown field at the liquid to solid phase transition and laser

induced optical breakdown were investigated. As shown in Fig.914), the lO~sec

pulse breakdown field observed with the MMA-->PMMA system increases steeply at

solidification upon co6U~ irradiation in vacuum. In the liquid phase the breakdown

field increases with increasing pressure, showing the importance of bubble

mechanism at longer pulse duration, but this is not the case in the solid phase.

The effect of pulse width in the ~sec range is seen in the liquid phase but not

in the solid one. This observation further supports the bubble mechanism.

In the case of optical breakdown by Q-switched ruby laser, where the optical

frequency is higher than the electron collision frequency, the energy gain rate

of free electron from the electric field is proportional to the collision

frequency 15) . Accordingly the breakdown field in the gas phase should be larger

than that in the liquid phase at the same pressure. Consequently, the effect

of bubble formation is negligible for pulsed optical breakdown. In fact the

optical breakdown field of solid PMMA of 0.5 MV/cm is lower than that in liquid

MMA of 2.5 MV/cm. From these facts it is suggested that bubble or cavitation

mechanism may play an important role at lower overvoltages and longer pulse

duration. However, at higher overvoltages and shorter pulses, an avalanche-

streamer mechanism seems to predominate. Crowe 16) investigated the relation

between breakdown field and duration of ~sec range pulses in n-hexane by

repeated pulsing at lower overvoltages. The apparent mobility of 0.9xlO-2cm2/V.sec

obtained under these condition indicates a streamer velocity of 2xlO4cm/sec in

agreement with Devins's value. However, in both cases repeated low overvoltage

pulses seem to give an average speed of stepwise progression of partial breakdown

containing bubbles.

10

4

A E

~E v

0:)

uJ 2 "o

0

I

o' q. i EB '~'=19)Js )

CH2C(CH3)COOCH 3

I ~ ' d I I I I 0.6 1.2 1.8 2.4 3.0

- ray dose ( x 10 6 R )

-2

x

O

Fig.9. Breakdown field vs. radiation dose

CONCLUSION

(i) In moderately purified n-hexane negative ion mobility was observed by

time of flight method which is independent of the applied field uptill 0.5 MV/cm.

Carrier multiplication was not observed uptill 0.5 MV/cm in conditioned system,

but apparent multiplication was observed in non-conditioned system. This fact

suggests that ionization in adsorbed and/or absorbed gas in liquid-electrode system

play important role in premature ionization.

(Ii) The importance of cathod asperities at prebreakdown range as the carrier

injector has been shown by the conditioning experiment using diverter system.

(iii) The addition of ethyl-alcohol enhances the low field conduction due to

ionic dissociation but increases breakdown strength. These facts support the

electronic nature of liquid breakdown.

(IV) From the formative time lag measurement at nanosecond range streamer

propagation velocity were estimated for various liquids assuming single avalanche

mechanism for these high overvoltage region. The avalanche or negative streamer

propagation velocities lie between 3~6xl05cm/sec at several MV/cm. The propagatiol

velocity of positive streamer is almost order of magnitude higher (106 107 cm/sec)

(V) In well polished uniform field electrode system breakdown starts from

cathode as avalanche or negative streamer. After arrival of negative streamer

at anode, much faster positive streamer starts from anode to lead to final

breakdown. By scratching anode, the breakdown tends to start from anode as

positive streamer which shorten formative time lag and breakdown strength very

effectively.

(VI) Breakdo~n strength in the process of ~-ray polymerization of MMA to PMMA increases abruptly at solidification point.

11

The decrease of pulse width ~s) and ambient pressure increase the breakdown

strength in liquid phase but not in solid phase. This fact supports the important

role of bubble or cavitation in liquid breakdown and is consistent with the

results of optical breakdown by laser.

REFERENCES

i I. Adamczewskl, "Ionization, Conductivity and Breakdown in Dielectric Liquids." 1969.

2 W.F. Schmidt, "Electron Migration in Liquid and Glasses", HMI, B-156(1974). 3 P. Watson and A. Sharbaugh, J. Electrochem., 107(1960)516. 4 K. Macfadeyn, British J.A.P., 6(1955)1. 5 E. Forster and P. Wong, IEEE Transaction, EI-12(1978)435., Can. J. Chem.,

55(1977)1890., J. Electrostatics, 5(1978)157. 6 H. Yamashita, H. Amano and T. Mori, J. Phys. D. 10(1977)1753. 7 J. Devlns, S. Rzad and R. Schwabe, Appl. Phys. Letters, 31(1977)313. 8 P. Chong, Ph.D. Thesis, Faculty of Eng. Osaka University, (1961). 9 P. Chong, T. Kawarabayashi and Y. Inuishi, Tech. Repts. Osaka University,

10(1960)25. i0 P. Chong and Y. Inuishl, ibid., 10(1960)545. ii P. Chong, C. Yamanaka and T. Suita, ibid., 9(1959)17. 12 K. Arii, K. Hayashi, I. Kitanl and Y. Inuishi, Proc. 5th Inter.,

Conf. and Breakdown in Liquids (1975)163. 13 K. Yoshino, H. Fujil, K. Hayashi, U. Kubo and Y. Inuishi, This Conf., (1978),

J.I.E.E. Japan 98-A(1978)No.5. 14 Y. Inuishi, J.I.E.E. Japan, 78(1958)172. 15 H. Fujli, K. Yoshino and Y. Inuishi, J. Phys. D., 10(1977)1975. 16 R. Crowe, J.A.P., 27(1956)156.