O X Y G E N GAS A N A L Y Z E R

L. A . B o r e i m a g o r s k a y a a n d V . M. S i t n i e h e n k o UDC 543.272.1

Oxygen content must be carefully monitored in the gases of a number of chemical , metallurgical, and other industries . Generally ser ious emergencies occur if oxygen content deviates from the standard. This requires the development of different types of sensors or recorders to quickly and accurately deter- mine oxygen content.

Investigations showed that luminescence of the T a - - Ta205 -- SnO 2 e lectrolumiscent sy s t em is a strong function of oxygen content. This sy s t em is a tantalum foil with a 10-4-10 -s cm thick tantalum oxide layer (produced e lectrochemical ly in an aqueous solution of oxalic acid) with a thin layer of light transparent SnO 2 (deposited by pyrolitic decomposit ion of SnC12). The two electrodes of the sy s t em are metallic tantalum and the SnO 2 layer.

Luminescence of an electroluminescent cel l (about 1 cm 2 in area), placed in a hermeticaIly sealed cryostat with a builtin furnace, was investigated in air, oxygen, carbon dioxide, and nitrogen at 10-i-1023 mm Hg vacuum and atmospheric pressure . Electroluminescence was activated by a 7-200-V sound fre- quency alternating or direct current. Luminescence was measured by a photomultiplier. A galvanometer or osci l lograph measured the output of the photomultiplier.

Luminescence monotonically increases as air is removed from the eryostat (Fig. 1). It reaches a maximum at 5"10 .2 mm Hg pressure . Further decrease in pressure results in montonic reduction of luminescence .

Cell luminescence is 1-1.5 orders of magnitude less in air at atmospheric pressure than at 5 . 1 0 .2 mm Hg at constant potential difference. It is only 50% as strong in oxygen as in air.

Luminescence is practically the same in a cell filled with carbon dioxide or nitrogen (air was pre- viously evacuated). Water vapor exerts the same effect.

Results of these experiments were compared with those of [1, 2]. It c a n b e concluded that lumines- cence of the e lectroluminescent cell is mainly determined by adsorption of oxygen rather than other gases present in the atmosphere .

Current flowing through the sy s t em was recorded in studying the luminescence of the T a - - T a 2 0 5 - SnO 2 sys tem as a function of atmosphere composition. More current flows through the sys tem in air at

TABLE I

Pressure. mm Hg

t o t a l

5 �9 10 - 2 7 �9 10 - 2 I �9 I0 - I 2 �9 I0 - 1 5 * 10 - 1

I 2 5

10 50

100 760

O x y g s n c o n - L u m i n e s c e n c e tent I n a i r , i n r e l a t i v e

p a r t i a l ~ / m ~ u n i t s ( o x y g e n )

I , I �9 10 - 2 1,6 �9 10 - 2 2,3 �9 10 - 2 4,6 �9 10 - 2 1,1 , lO - 1

0,23 0,46 1,1 2,3

11,8 22,0

172

2 �9 10 - 2 2,8 - 10 - 2

4 - 10 - 2 8 - 10 - 2

0,2 0,4 0,8

4 20 40

300

2500 240O 2300 1970 1560 1180 930 650 430 110 45 10

Translated from Khimicheskoe i Neftyanoe Mashinostroenie, NO. 4, pp. 47-48, April, 1976.

�9 76 Plenum Publishing Corporation, 22 7 West 17th Street, New York, N. Y. 10011. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission of the publisher. A copy o f this article is available from the publisher for $15.00.

388

V

~oo

$oo / 300

0

Fig. i.

la- p, mmHg

Fig. 2. Fig. I . Luminescence V (in relat ive units) of the tantalum oxide film as a function of p r e s su re p.

Fig. 2. Block schematic of an oxygen gas ana lyze r : 1) exci- tation voltage source; 2) oxygen sensing element (electro- luminescent ceil); 3) emiss ion r ece ive r (photomultiplier); 4) power source ; 5) measur ing instrument (galvanometer, r e - corder , etc.).

6.10 -3 mm Hg pressure than through air or oxygen at atmospheric pressure at a constant potential dif- ference. The relationship of current (cell luminescence) to atmosphere composition can be explained by adsorption of oxygen on the oxide film surface. Adsorbed oxygen atoms or molecules are captured by the free current carriers reducing the concentration of free electrons. This reduces luminescence and elec- trical conductivity of the Ta205 system [3, 4].

A sensor for measuring oxygen in air was developed based on this relationship of the Ta-- Ta205 - - SnO 2 system luminescence to oxygen content. This oxygen gas analyzer (Fig. 2) is much simpler in construction than existing analyzers and can be used for measurements at atmospheric pressure as well as low and intermediate vacuums.

The instrument was tested under laboratory conditions. The measurements were car r ied ou~ in a i r with 400 Hz, 50 V excitation current for the e lectrolumineseent cell. Results a re shown in Table 1.

The feasibility of measurements under a tmospher ic p r e s su re with different oxygen contents was a l so checked. The e lectroluminescent cell was energized by 400 Hz, 120 V current . Measurements are listed below:

Oxygen content, % 2 10 21 45 70 98 Luminescence in relat ive units 450 140 80 56 44 35

The sensit ivity of the e lect roluminescent oxygen sensor can be i~mreased by changing the voltage or frequency of the excitation cur ren t . Tests were car r ied out to investigate cell luminescence at 20-2000 Hz frequency excitation current . Luminescence monotonically increases with higher frequencies (20 to 1500 Hz) at a constant potential differential on the cell : increase is rapid in the 20-400 Hz range; increase is s lower above 400 Hz. At even higher excitation current frequencies luminescence dec reases . Cell luminescence as a function of cur rent voltage at a given frequency is adequately represen ted by the fo r - n-lu la

where B 0 and b a re constants, and U is the excitation voltage.

The best operating conditions, for monitoring high and low oxygen concentrat ions, can be selected by varying ~(oltage and frequency.

The excitation potential of the ana lyzer depends on the excitation current frequency and oxygen con- tent in the gas mixture. Thus, 20 V is required for sample luminescence at 50 Hz frequency in air at a tmospher ic p re s su re , 7 V at 5 �9 10 -2 mm Hg p ressu re , and 60 V in oxygen at a tmospher ic p res su re . The excitation potential of the cell is reduced at higher current f requencies . Measurement e r r o r in the inves- t igated concentrat ion and p re s su re range of the analyzed mixture did not exceed 3-4%.

The cell can be easily replaced when it fails.

389

L I T E R A T U R E C I T E D

1. L . A . Boreimagorskaya and V. V~ Mikho, "The effect of oxygen on the electroluminescence of tantalum oxide fi lms," Zh. Fiz. Khim., 44, No. 10, 2598-2599 (1970).

2. L . A . Boreimagorskaya and V. V. Mikho, "The attenuation effect of oxygen on the electrolumines- cence of metal oxide fi lms," Zh. Fiz. Khim., 47, No. 11, 2852-2854 (1973).

3. K . V . Tagantsev and A. N. Terenin, "The effect of gas adsorption on zinc oxide luminescence," Izv. Akad. Nauk SSSR, Ser. Fiz., 21, No. 4, 525-527 (1957).

4. A . N . Gorban and V. A. Sokolov, Luminescence and Adsorption [in Russian], Nauka, Moscow (1969).

390