Journal of Electrostatics, 7 (1979) 93--101 93 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

LASER-INDUCED BREAKDOWN IN DIELECTRIC LIQUIDS

H. FUJII, K. YOSHINO and Y. INUISHI

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

Japan

ABSTRACT

In linear hydrocarbon liquids and dimethyl silicone oil, the threshold fields

of laser-induced breakdown decrease with increasing chain length (molecular weight)

and with decreasing temperature, but those for d.c. impulse breakdown show the

opposite characteristics. In alcohols, a similar molecular chain length dependence

of the laser-induced breakdown threshold is observed but the threshold field is

lower. The threshold fields of laser-induced breakdown in aromatic hydrocarbon

liquids are lower than for saturated hydrocarbon liquids. From these results,

laser-induced breakdown in dielectric liquids is also explained in terms of

microwave breakdown theory as in the case of solids and gases.

INTRODUCTION

Since the appearance of 'lasers', damage to optical materials by intense laser

light has become a serious problem which has attracted the interest of many workers.

The mechanism of this optical breakdown (damage) in solids and gaseous dielectrics

is now understood in terms of the avalanche multiplication of free electrons I)2)

On focusing a high power laser beam into dielectric liquids? also, the dielectric

breakdown which occurs is accompanied by a strong shock wave and bright spark

light emission. At the same time, small bubbles are formed which lead to cavitation

Though the occurrence of a shock wave and the formation of small bubbles have

been studied 3) only recently, the detailed mechanism of laser-induced dielectric

breakdown in liquids was first studied by the authors 4)5)6)

In this paper, experimental results of laser-induced breakdown in various organic

liquids (dimethyl silicone oil, saturated hydrocarbon liquids, alcohols, and

aromatic hydrocarbon liquids etc.) will be presented.

EXPERIMENTAL PROCEDURE

Although the detailed experimental procedure has already been described in our

previous papers 5)6) , a scheme of the experimental set-up of laser-induced breakdown

is shown in Fig.l.

94

Lens Attenuator f= 3crn __, ~-switch:~Las.eL~. J~-. --Spl.il t,~ (~.-.

I IOsciUoscope

~ Pyrex Tube

Fig.l. Experimental set-up for laser-induced breakdown.

The lasers used were a ruby laser Q-switched by saturable absorber (time width

T = 20 ns and wavelength % = 0.6943 Hm) and a Nd + glass laser Q-switched by a

rotating prism ( %= 40 ns and %= 1.06 Hm). The laser beam was focused into a

liquid sample sealed in the Pyrex tube with a lens of 3-cm focal length. The

occurrence of breakdown was confirmed by the observation of a bright light emission

of the spark at the focal point and the formation of small bubbles. Laser power

was controlled by changing the concentration of a CuSO 4 solution filter and

monitored using a biplanar phototube.

RESULTS AND DISCUSSION

Saturated hydrocarbon liquids and silicone oil

Figure 2 shows the threshold field of ruby laser-induced breakdown against the

chain length (n) of the linear hydrocarbon liquids CnH2n+2 at 26°C.

:0

" " \T n-CnH2n+2

" (a) •

o .~

. . . . . . . . . . . . . . . . . 0 ]0 0 5 10 15 20

Carbon Number

Fig.2. The threshold field of ruby laser-induced breakdown as a function of chain length of n-C H at room temperature. The dotted curves indicate the ionization

2n 2 potentials (a~ an~ electron mobilities (b) of linear hydrocarbon liquids.

95

The threshold field decreases remarkably with increasing molecular chain length.

Similar molecular weight dependence of the threshold breakdown field by glass laser

irradiation (at 20°C) is also shown in Fig.3. On the contrary, the threshold

fields under pulsed d.c. conditions increase with increasing molecular weight or

chain length as shown in Fig.4 (linear hydrocarbon liquids) and Fig.5 (silicone

oil). Viscosity (centi-poise)

10 5 102 5 103 510 ~ 510 s

4 I- Silicone Oil I " Room Tern.

"o I:

E 0

O / . . . . r , 10 3 2 5 10 ~ 2 5 10 s 2

Mean Molecular Weight

Fig.3. The threshold field of glass laser-induced breakdown in dimethyl silicone oil as a function of mean molecular weight at room temperature.

'~2.0

v _ 1.5

LE c 1.0

~0.5 D.C. Impulse

('~=100nse n-Cn H2n.~ Room Teml~ d=50pm

. . . . i . . . . i . . . . i

5 10 15 Carbon Number

Fig.4. The threshold breakdown fleld under pulsed condition (T= i00 ns) in linear hydrocarbon liquids as a function of carbon number n at room temperature.

Viscosity (centi-poise) 10 5 102 5 103 5 1 0 ~ 5 1 0 s

D.C. Impulse(~c=6usec) 0.5 ~ SiLicone 0}[

i Room Temp. d=lOOpm

O/ , , I t

103 2 5 10 ~ 2 5 105 2 Mean Molecular Weight

Fig.5. The threshold breakdown field under pulsed d.c. conditions (T= 6~s) in dimethyl silicone oil as a function of mean molecular weight at room temperature

96

According to microwave breakdown theory, the rate of energy input from the

optical electrical field E (r.m.s.) is approximately represented by the following 7)

equation ,

dE e2E 2 Vm dt m 2 (w >> ~m ) (i)

w

, where E, e, m, co and v are electron energy, electron charge, electron mass, m

angular frequency of optical electric field and mean collision frequency between

electrons and molecules, respectively.

Accordingly, the energy input rate is proportional to the collision frequency

, which should increase with increasing molecular length 6). Therefore, the m

optical breakdown field decreases with increasing molecular chain length as shown

in Figs.2 and 3.

Simple semiclassical calculation of the collision cross-section between electrons

and molecules by the method of Adamczewski 8) shows that the total effective cross-

section in linear hydrocarbon liquids increases with increasing carbon number and

becomes nearly constant for a carbon number greater than 12 as shown in Fig.6.

Yoshino et al. 9) have also demonstrated that the collision cross-section between

electrons and molecules increases with increasing carbon number n. These facts

support the above-mentioned speculation.

xlO'

ts

uu')

~'o ~ 0 05 Linear Hydrocarbon m Liquids( CnH2n.2 "6 I - -

;. i 1'2 16 20 Carbon Number

Fig.6. The total effective cross-section per unit volume (cm 3) between electron ~nd linear hydrocarbon liquids calculated by means of Adamczewski's method.

Figure 7 shows the temperature dependence of the breakdown threshold field for

ruby-laser irradiation in n-decane C12H26. A similar temperature dependence of

the threshold field of glass-laser induced breakdown is observed in dimethyl

silicone oils; for example, in the sample having a viscosity of 3000 cP as shown

in Fig.8. However, the sample having viscosity of i00 cP indicates temperature-

insensitive dependence. On the contrary, the threshold breakdown fields under

pulsed conditions decrease with increasing temperature as shown in Fig.9 (n-dodecane)

9?

and Fig.10 (silicone oils).

E ~8 :E

"o6

hE

~2

C

Fig.7.

Ruby Laser n- dodecane

,~ 8'0 ,2o Temperature (°C)

The threshold field of ruby laser-induced breakdown in n-dodecane (C12H26) as a function of temperature.

E 3 4

~3 LE e- ~2 "(3

• Nd-Glass Laser • ""-, Silicone Oil

"", th~ lOOcp(a) .,"-\'~' 3000cp(b)

-/-~ 0 40 80 120 160 201 Tempe rat u re (°C)

] 0 3 .~ s ~

I0

Fig. 8. oils [(a) lOOcP and (b) 3000 cP] as a function of temperature. indicate the viscosities of the liquids.

~201 D.C. Impulse ('c=100nsec)

0.5 111

0 "

The threshold fields of glass laser-induced breakdown in dimethyl silicone The dotted curves

Fig. 9. in n-dodecane as a function of temperature.

4~ 85 1~o ,oo Temperature ('C)

The threshold breakdown field under pulsed d.c. conditions (T = i00 ns)

98

E [ DC. Impulse ~. 1.5 [ ('C'= 6psec)

""%,

N 1.0 [L

= Slimone Oil 0,5 "" 200cp ' " " " " " ....

d=lOOpm

" o Zo 8b 16o- Temperature ( ° C)

5

o

1o 2 .5 E

5 ~

o

10 ~ .~_ >

Fig.10. in dimethyl silicone oil (200 cP) as a function of temperature. indicates the viscosity.

The threshold breakdown field under pulsed d.c. conditions (T= 6 ps) The dotted curve

Since bubble formation becomes easier at temperatures near the boiling point, the

d.c. voltage breakdown field decreases at higher temperatures due to a lower

avalanche starting voltage in the gas phase. However, this is not the case in

optical breakdown, because the threshold field of optical breakdown of the gaseous

phase with smaller collision frequency is larger than that of the liquid phase as

seen from the eq.(1) where ~ >> ~ . The decrease of density at higher temperature m

may play a role in the laser-induced breakdown process in liquids. The total

effective cross-section between electrons and molecules per unit volume of liquid

will decrease with decreasing density of liquids, as is evident from the simple

calculation of Adamczewski8)(Fig.ll), resulting in a smaller energy input and

higher optical breakdown field at higher temperatures as seen in Fig.7.

u 2

% O.

o=

u

0

Fig.ll. and n-dodecane molecules as a function of temperature.

n-dodecane

Temper~ure (°C)

The total effective cross-section per unit volume (cm 3) between electron

99

The optical breakdown fields by ruby laser irradiation in branched hydrocarbon

liquids (C6H14) at room temperature are shown in Fig.12. The optical breakdown

fields of 2-methylpentane and 3-methylpentane were found to be higher than that

of n-hexane. The breakdown field of 2-methylpentane lies beyond the maximum

intensity of our ruby laser. Since electron mobility in liquids depends on the

molecular shape even with the same carbon number, these situations are not unexpected

Breakdown FieLd (MV/cm) 0 2 4 6 8 ]0

- j

Ruby Laser Room Temp.

n-Hexane 2-Methylpentane 3-MethyLpentane

2,2 -Dimethyl bu tane 2,3-Dimethyibutane p ; :

Fig.12. The threshold fields of ruby laser-induced breakdown in branched hydro- carbon liquids (C6H14) at room temperature.

~icohols

Figure 13 shows the threshold field of the optical breakdown (ruby laser) against

the carbon number (n) of the normal alcohols CnH2n+IOH at room temperature. In

this case, a similar trend to that of linear hydrocarbon liquids (Fig.2) was

observed. These facts can also be interpreted in terms of increasing collision

frequency with increasing carbon number just as in the case of n-CnH2n+2.

However, the magnitude of the threshold field in alcohols was lower and the change

of the threshold field against carbon number n was smaller in comparison with

linear hydrocarbon liquids. The lower threshold field of breakdown of CnH2n+IOH

seems to be due to the higher densitie~ to the larger refractive index of

CnH2n+IOH and to the difference in boiling points. The difference in the bond

energy between C-H and C-O may also play some role.

n-CnH2n~OH

Ruby Laser Room Terrp. ~ 7.~

~5,C

2 4 6 8 Carbon Number

Fig.13. The threshold field of ruby laser-induced breakdown as a function of chain length of n-CnH2n+lOH at room temperature.

100

Aromatic hydrocarbon liquids

The threshold fields of ruby laser-induced breakdown of several aromatic hydro-

carbon liquids including cyclohexane (C6H12) are shown in Fig.14. Except for

cyclohexane (having nearly an equal optical breakdown field strength to n-hexane),

ruby laser-induced breakdown fields were nearly constant and lower than that of

aliphatic hydrocarbon liquids. This lower threshold breakdown field of aromatic

hydrocarbon liquids seems to be related to the existence of ~ electrons.

The optical absorption wavelength of liquids containing ~ electrons tends to be

longer in comparison wlth that of linear hydrocarbon liquids, resulting in the

higher probability of multiphoton excitation in aromatic hydrocarbon liquids.

Therefore, the supply of the initial electron by laser irradiation becomes much

easier in the aromatic hydrocarbon liquids, which may be one reason for the

observed lower breakdown strength by laser irradiation in these materials.

Breakdown Field (MVIcrn) 0

Benzene Toluene

o-Xylene m-Xylene p-Xylene

Cyclohexane

2 4

I-0-1

I-0-1

6 8 Ruby Laser Room Temp.

Fig. 14. The threshold fields of ruby laser-induced breakdown in several aromatic hydrocarbon liquids including cyclohexane, saturated hydrocarbon liquid, at room temperature.

Mixture of hydrocarbon liquid and alcohol with aromatics

As shown in Fig.15, the ruby laser-induced breakdown field in ethylalcohol

(C2H50}I) decreases with increasing the concentration of doped N,N-dimethylaniline.

The decrease of the threshold breakdown field becomes remarkable around 10 -3 mol/l

The breakdown threshold field of 2-methylpentane doped with benzene also much

decreases. N,N-dimethylaniline and benzene have ~ electrons as already mentioned.

The energy necessary to excite ~ electron of aromatics like N,N-dimethylaniline

and benzene is much lower than saturated hydrocarbons. Therefore, the decrease

of breakdown field in these mixtures may be due to the increase of initial

electron supply from the molecules of aromatic hydrocarbon liquids.

101

~ 4

_~3 ,_¢, u._ c

"(D

n,~

E0'~lalcohol • N,N-Dirne~,21an~ine Ruby Laser Room Temp,

!

Concentration of N,N-Dimethylaniline (mol]l)

Fig.15. The threshold field of ruby laser-induced breakdown in ethylalcohol as a function of the concentration of doped N,N-dimethylanlline.

REFERENCES

1 Y.Yasoj ima, M. Takeda and Y . I n u i s h i , Japan . J . Appl. Phys. 7(1968)552. 2 N.Bloembergen, IEEE J. Quantum Electron. QE-10(1974)484. 3 for instance, W.Lauterborn and K.J.Ebeling, Appl. Phys. Lett. 31(1977)663. 4 K.Yoshino, H.Fujii and Y.Inuishi, Japan. J, Appl. Phys. 15(1976)1409. 5 H.Fujii, K.Yoshino and Y.Inuishi, Teeh. Rpts. Osaka Univ. 26(1976)461. 6 H.Fujii, K.Yoshino and Y.Inuishi, J. Phys. D: Appl. Phys. 10(1977)1975. 7 S.C.Brown, Basic Data of Plasma Physics (Cambridge, Mass.: MIT Press and New

York: Wiley) (1959)142. 8 I.Adamczewski, Ionization, Conductivity and Breakdown in Dielectric Liquids

(London: Taylor & Francis) (1969). 9 K.Yoshino, U. Sowada and W.F.Schmidt, Phys. Rev. A 14(1976)438.