Electrical breakdown of water vapour

Volume

33A, number

PHYSICS

2

LETTERS

across the unit and a variable load resistor up to 10 kS1was used. This further

letter details

We would

is only are like

a preliminary

under

1970

References [l] S. R. Gvshinsky. Phys. Rev. Letters 21 (1968) 1450. [2] M. Hansen and K. Anderko, Constitution of binary alloys (McGraw Hill, New York, 1958).

information,

investigation.

to thank Miss

5 October

A. Ten Bosch for

helpful discussions.

*****

ELECTRICAL

BREAKDOWN

OF WATER

VAPOUR

R. HACKAM Department

of Electronic The University Received

and Electrical of Sheffield.

Engineering. UK

26 August 1970

Electrical breakdown potentials of water vapour are measured between nickel electrodes in the pressure range 20.6-123.5 torr and the temperature range 297-333 OK. A minimum sparking potential 2550 V is observed in the range 17-19 torr cm.

The electrical breakdown of gases and vapours has been widely studied for its practical applications and for providing important data on collision properties. In this letter measurements of the breakdown voltages of water vapour made under very pure and carefully controlled conditions are reported. As far as is known the only published study of breakdown potentials of water vapour is that of [l] which was made between platinum electrodes. In the present study the breakdown potential measurements of water vapour are made between planar parallel electrodes of diameter 5.2 cm and a fixed gap length of 0.507 cm. The electrodes are made of nickel and polished to 0.25 X 10m6m with a diamond paste and thoroughly degreased with the aid of an ultrasonic vibrator. The discharge tube is evacuated at about 4000C for a few days. To remove the occluded gases from the electrodes surfaces they are radio frequency heated to a high temperature (about 7500C) and the evolved gases monitored during bakeout using an Omegatron mass spectrometer. After the background pressure has been levelled at about 10-V torr for some time, the heating is turned off and a water globule is distilled into the tube and sealed. The water was previously distilled, under vacuum, into a Pyrex glass capsule containing a breakable seal. The dissolved atmospheric gases have been meticulously and carefully removed by a

of

combination of vibrating, freezing and distillation of the water. The electrical circuit arrangement for measuring the breakdown voltage is similar to that described previously [2]. The breakdown potential is defined as that required to give a current of the order of low6 - 10S5 A (depending on the water vapour pressure) and which would give a large increase in the current

I

I

I

I

I

I

I

I

I 20

I

I 40

I

I 60

I

6000 L

0

L

Pd

(tOiT

I 00

cm)

Fig. 1. Breakdown potential of saturated water vapour against pd (Pressure not normalized).

65

Volume

33A.

number

I 20

1’ I-11’ S 1 C S I >I<‘1’‘1‘E R S

2

I 40 pd (tOrr

/

I 60

---

1-JRC‘

cm:!

b’ig. 2. Breakdown potential of saturated water v:rpo~l against pod (pressure normnlized to 273°KJ.

with only a slight change in the applied voltage. The breakdown currents are restricted ( 50 ~AI to reduce the fragmentation of the water vapour. In figs. 1 and 2 the breakdown potential 1’ is plotted, respectively, against pd and pocl. The vapour pressure is varied in the range 20.6123.5 torr and the ambient temperature of the saturated vapour in the range 297 - 333 OK. The temperature over the discharge tube is maintained within iO.25oC. All vapour pressures are reduced to 273oK according to the relation p. 7. p (273, rg). where p. is the reduced vapour pressure, /, the saturated pressure deduced from standard tables [3] and Tg the ambient temperature of the water vapour. the electrodes and the walls of the discharge tube. It will be observed that for values of Pod below about 18 torr cm. 1’ increases with decreasing pres-

66

., October

1970

sure. As pocI increases above about 22 torr cm the breakdown potential starts to increase with increasing pocl. The rate of rise of 1: with inc.reasing pool tends to saturate when PotI is above 32 torr cm. The breakdown potential has ;I minimum value of 2550 V which occurs in the range /)ot/ 17-19 torr cm. The present breakdown potential values are higher than those of ref. 11j. It is thought that this is due to the different electrode material employed in the present work. Nickel electrodes are known to give higher sparking potentials be-~ cause of its lower secondary electron yield conpared to that of platinum [ 4 I. The minimum sparking potential in water vapour appears to bc higher than that reported in most of the common gases [2, 5-71 or in saturated vapour of merc’ury [Sj. The present results lead to the conclusion that the addition of water vapour to other gases in an enclosed system, either from surface desorption or through external leaks, does not re-sult in lowering the holding voltage of the devict.

161 11. Hnckam. 171 R.Haclwm, (1969) 216.

Intern. .I_ I~.lectronics 28 (1970) 2li3. .J. Phys. TX. (Atom. hlolec. i’hys., L’