AECD-2682

UNITED STATES ATOMIC ENERGY COMMISSION

LIFE TESTS OF TUNGSTEN AND TANTALUM FILAMENTS IN ION GAUGES

by

W. E. Bush

University of CaliforniaRadiation Laboratory

Date of Manuscript:Date Declassified:

January 8, 1944August 26, 1949

Issuance of this document does not constituteauthority for declassification of classifiedcopies of the same or similar content and title

and by the same author.

This copy is reproduced direct from copyas submitted to this office.

Technical Information Branch, ORE, Oak Ridge, TennesseeAEC, Oak Ridge, Tenn., 2-14-50--850-A14019

PRINTED IN USAPRICE 10 CENTS

metadc100793

LIFE TESTS OF TUNGSTEN AND TANTALUM FILAMENTS IN ION GAUGES

By W. E. Bush

Purpose

This paper presents the results of an investigation carried on for the following purposes:1. To determine the relative life of filaments when operated at low emission (low temperatures)

as compared to those operated at high emission values.2. To determine the nature of the dependence of life upon the initial diameter of the filament.3. To determine the effect of varying concentrations of water vapor and air upon the life of the

filaments.4. To determine the relative life characteristics of tantalum as compared to tungsten.

Method

A large tank (1180 liters) equipped with a suitable vacuum system was arranged to mount sevenion gauges. A leak was prepared to independently admit air or water vapor or both into the tank.

The gauges used for pressure determinations were calibrated for air by the rate of rise method.Calibration of the gauges for water vapor was accomplished by opening a water leak of considerable size(after a good vacuum had been obtained) and maintaining a trap between the gauge and the tank at thetemperature of solid CO2 (dry ice). This gives a water pressure in the gauge which is that of water atthe solid CO2 temperature, namely 0.50 microns. (This value varies slightly with the barometric pres-sure, for the CO 2 point is -78.51*C at 760 mm Hg. Tables may be consulted to find the vapor pressureof water at temperatures corresponding to different pressures on the Co2 ')

Gauges to be tested were put on the tank and pumped out for some time before being operated. Then,under high vacuum, they were turned on and allowed to out-gas for an hour or so. (This latter procedurewas not followed in the case of tantalum, since outgassing took but a very few minutes.)

The time during the rate of rise experiment was then counted from the instant the leaks were opened.An occasional check of the relative air to water vapor ratio was made by putting liquid air on the

traps to eliminate the water. In the case of the tungsten filaments, a coiled filament 10 cm long wasused (except in tests of tetrode-type gauges, in which case it was 6 cm long and coiled), while in thecases of the tantalum a hairpin filament 3.8 cm long was used in a Distillation Products Type VG-1Agauge.

The grid voltage was approximately 130 volts and the plate voltage was -22 volts in all cases.A total of 18 filaments was tested, 7 of these being tungsten and the rest tantalum.

Discussion

Figure 1 presents the life history of four of the tungsten filaments, started all at the same time andunder identical circumstance except as noted. Numbers 1, 2, and 3 were 10 mil diameter coiled fila-ments, while No. 4 was a 15 mil coil. Until Nos. 1, 2 and 3 failed, and for some time afterward, theatmosphere was held constant at a pressure equal to 1 micron of water plus 1 micron of air. At first,some difficulty was experienced in doing this and a scattering of points resulted, as is shown in Fig. 1.However, this was repeated with gauges No. 5 and No. 6, this time with well regulated control through-out the experiment. The results are nearly identical to those presented on Figure 1.

AECD-2682 1

As time went on, it was apparent that the low emission (0.010 ma) filament was becoming consider-ably tapered in dimension, so when the tests were finally finished the remains were carefully savedand measured with a micrometer. The results of these measurements are shown in Figure 2. It willbe noted that only in the very low emission tube did tapering seriously enter into the destruction of thefilament. This tube, operating at an emission 1/500 that of the hottest one, started at a lower wattageinput and ran to very nearly zero, the last readings being well below one watt.

It is well to note that during the tapering of filament No. 1, the calibration did not materially change.From tube No. 4 on Figure 1 (which has not yet failed after some 90 hours and retains about a

2 mil filament), it can be seen that the decay rate responds immediately to changes in concentration ofair and water vapor. Tubes Nos. 5 and 6 show this same phenomena, as do Figures 4, 5, and 6. Theyalso indicate that there is nearly a threshold concentration value (around 2 x 10-4 mm Hg) for waterbelow which decay is very slow. This may also be true in the case of air.

Further, it seems true that this threshold decreases as the operating temperature is lowered. Thisis again shown in all of the decay curves for tantalum and tungsten, for in all cases excepting one ofeighteen, the life is longer as the emission is increased. This may be due to the fact that though thegauge pressure is more or less the same in each case, the higher the temperature, the lower the gaseousconcentration.

It seemed surprising that the power required for a given emission is a linear function of the timeas long as the conditions are constant, but all cases indicate this is true for tungsten. No linear re-lationships were found for tantalum.

The surmise was made that in the case of tungsten (if we assume no poisoning or activating) thepower for a given emission is constant for a given area and a decrease in power means a decrease inradius. To check this, the filaments of gauges Nos. 1, 2, and 3 were measured and the results com-pared with those computed for the ratio of final to initial power. These results are here shown:

Tube Measured Diameter Calculated

1 0.0017 (from R) 0.0025 " (from R)2 0.0021 0.00228 "3 0.0047 0.00508"

This indicates that if tapering is not serious, the relation is essentially true.The case of tantalum presented unexpected results inasmuch as it was reported to be superior in

performance to tungsten. For this reason observations on the first pair of tubes was inadequate, as isshown in Figure 8. Also in this case, coiled filaments were used similar to the tungsten ones. Failureseemed to be due to sagging and consequently hairpin filaments were adopted for later tests. Subsequentresults, however, indicate that the sagging was merely incidental with burnout.

The results of the several tests made may be summarized by the following table:

Tube Atmosphere Emission Life

No. 7 (coiled fil10mil) 1air + 1 H2 0 0.5 ma 1hr. 50 min.

No. 8 (coiled fil10 mil) 1 air + 1 H2 0 5.0 ma 2 hrs. 30 min.

TetrodetubeNo. 9A (coiled fil

20mil) 1air + 1 H 2 0 0.5 ma 2 hrs. 44 min.No. 10 hairpin 10 mil 1 air + 1 H 20 0.5 ma 1 hr. 50 min.No. 11 hairpin 10 mil 1 air + 1 H 2 0 5.0 ma 2 hrs. 0.4 spin.No. 12 hairpin 10 mil 1 air + 1 H2 0 0.5 ma 2 hrs. 39 min.No. 13 hairpin 10 mil 1 H 2 0 5.0 ma 2 hrs. 55 min.No. 15 hairpin 10 mil 1 H 2 0 0.5 ma 6 hrs. 48 min.No. 16 hairpin 10 mil 1 air 5.0 ma 7 hrs. 21 min.No. 17 hairpin 10 mil 1 H2 0 0.5 ma 3 hrs. 18 min.No. 18 hairpin 10 mil 1 H2 0 5.0 ma 3 hrs. 26 min.

2 A ECD -2682

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The most noticeable characteristic of the tantalum filaments was their irregular behavior as com-pared to that of the tungsten. At burnout the sketches (Figure 7) illustrate typical appearance. Thewhole filament mass seems to become sponge-like and quite irregular in size. This has been observedin other cases of the use of tantalum for filaments. This swelling, resulting in increased surface, isreflected in the sometimes rather continuous increase required in wattage. Different individual fila-ments showed different properties so far as details were concerned, but the life seemed to be moreor less specifically determined by the emission and the atmosphere.

With tantalum, as in the case of tungsten, somewhat better life results at high rather than lowtemperatures. Gauge No. 18 disproved this rule.

During the experiments, reports from F. R. Elder of the General Electric Company indicated thatvery long life was being obtained from tantalum with an "atmosphere of water at 1 micron". Since thisin no way checked the results herein obtained, an analysis was requested of the wire being used here.Also, the G. E. Research Lab. sent a sample of the 10 mil Ta that they were using. Gauges #17 and#18 were constructed from this wire (which also is undergoing analysis); so far, no report has beenmade concerning these determinations. However, since the results obtained with the G. E. sample are

very similar to those of the previously used filaments under similar conditions (#12 and #13), the as-sumption seems good that both samples actually are tantalum. The wattage behavior of the two samplesis different, but the life is essentially the same.

Conclusions

From the foregoing it would seem that the following several conclusions are justified.1. For the concentrations of air and water vapor tried, there is no advantage to be gained in operating

tungsten filaments at excessively low emissions in order to reduce the temperature. This seems likelyfor tantalum also.

2. In the case of tungsten, the wattage required for a given emission is a linear function of the time

of operation, and the ratio of the final to the initial power is the same as the ratio of the final to theinitial diameters of the filaments.

3. Tapering of tungsten filaments at usual emission values is negligible.4. The life of a tungsten filament is proportional to the initial diameter.

5. No great changes (more than 10%) of calibration were noted during the life of any filament ex-cepting in tantalum a few minutes before burnout.

6. As already known, filament disintegration is in a way proportional to the concentration of eitherair or water, the water being more effective. This seems to be true of tantalum as well as tungsten.

7. Tantalum filaments of size equal to tungsten filaments can be expected to have from 5 to 10%

the life of tungsten in the same atmosphere of air and water.

Further Remarks

Gauges Nos. 9, 9A, and 14 were tetrodes. These showed no particular variation in characteristicsover the triode design. However, the tests made were all at constant emission, the emission being con-trolled by the filament current rather than the control grid. This nullifies any possible conclusion thatmight be drawn about them.

In the case of large filaments, a large amount of deposited material must be allowed for. This meansadequate shielding but such is easily furnished, the VG-1A and others being satisfactory in this respect.

It might also be noted that low temperatures increase the sensitivity of the gauges. Thus, for

example, the gauges when run at 0.010 ma were about 25% more sensitive (positive ions per electron)than the same tube run at 5 ma. This is most likely due to alteration of inter-electrode gas concentra-

tion as a result of difference in temperature.

Further work should be done to obtain more quantitative measures of probable life in the atmospheres

described and also in chlorine and combinations of chlorine, water, and air.

3

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UCRL VAG RESEARCH DEPT4-7-44 )

f0.0OH MS

7o

GO

POWER-GAUGE NO.4 RESISTANCE-GAUGE NO. 3 RESISTANCE-GAUGE NO.4SLOPE=-4.05 WATT/ HR.

50

GAS PRESSURE

PWER-GUEN.34.0 AIR + 4.0 H2OLsOPE=-0.98 WAT T/ HR, o

40

--- POWE R-GAUGE NO. 2

SLOPE=- 4.43 WAT T/HR30 0

BURN-OUT

(DUE TO VOLTAGE SURGE)o POWER-GAUGE NO. 4

SLOPE=-0.65 WATT/ HR.20

(+0.040 AIR)

40>BURN-OUT -4.0 AIR -

(+0.046 p H20) 0.46 p .

BURN -OU T_________- -------___________ _____________ -------________ --- --_______ ____________ __________________________40

OPERATING TIME IN HOURS

Figure 1. Tungsten Filaments in air and water vapor. Power consumption-resistance as functions of (1) emission (temperature); (2) gas concentration; (3) origin al filament size.

Filament Specifications

1. Length: 10 cm.2. Form: 8 turn coil3. Size: Gauges 1, 2, and 3: 10 mil;

gauge 4: 15 mil

Operation

Gauge 1Gauge 2Gauge 3Gauge 4

Ep Eg

Emission 0.010 maEmission 0.50 ma

Emission 5.00 maEmission 0.50 ma

- 22V-22V-22V-22V

Concentration of Gases

Time

From To128V128V128V128V

0 hrs 00 min41 hr 40 min51 hr 00 min58 hr 20 min

41 hr 40 min51 hr 00 min58 hr 20 min81 hr 55 min

Concentration

Air Water

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1.0 A1.0 AL40.016 u0.16 AL

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AECD-2682

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GAUGE NO. 1

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00

0 1 2 3 4 5 6 7CM FROM UPPER END OF 10-CM FILAMENT

8 9 10

Figure 2. Tapering of tungsten filaments. Coiled (8 turn) 10 mil filaments at various emissions useduntil failure in an atmosphere of 1 air + 1 y H 2 0. No. 1 = emission 0.010 ma; No. 2 = emission0.50 ma; No. 3 = emission 5.0 ma. Note: #3 failed as result of voltage surge.

5

AECD-2682

50

40

30

UCRL VAC RESEARCH DEPT12/29/43 A

20

ATMOSPHERE1. AIR + 1 H20

-A C(OO

x

5 A

x___ __V.

A

00,x

0 5 10 15 20 25 30 5TIME IN HOURS

5

Figure 3. Resistance of tungsten filaments 10 mil, 10 cm long. G)= No. 1 emission 0.010 ma; A = No. 2emission 0.50 ma; x = No. 3 emission 5.0 ma.

0

zw0ZHCo

w

6

oC __ X WATER (2 /Lm) (TAKEN FROM AVERAGE AFTER 23 HR PERIOD)

AIR (,O a)x

0 43WATE(55 67a)8

._.WATER + AIR

PRESSURES Oo 40 /L, H2O + ,0/La. AIR WATER (40 a,)0 40 pm H2O t 0.25#a. AIRX 40,u. AIR + 0.20,uaH2O0 p ,hH2 + 0.4 paAIR

TRIODE UCRL 4/5/44

TIME IN HOURS

Figure 4. Ion gauge No. 5. Tungsten filament 10 mil, 10 cm long. Emission 0.5 ma; Eg = 130 volts;Ep = -23 volts.

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4

5

6

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AVERAGE VALUEWATER (2pa) (FOR 23 HOURS)

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3

PRESSURES

4 0 aO H2O + O1a AIR

A 1Opa H2 0 + 0.25pa AIRX 10 a AIR + 0.20 a H20

TRIODE UCRL 1/1/44 0 1.6pa H2O + 0.4MAIR

5

60 12 3 4 5 6 07 8 9

TIME IN HOURS

Figure 5. Ion gauge No. 6. Tungsten filament 10 mil, 10 cm long. Emission 5.0 ma; Eg = 130 volts;Ep = -23 volts.

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6 7 8 9

Figure 6. Ion gauge No. 14. Tungsten filament 10 mil, 6 cm long. Emission 0.5 ma; Eg = 130 volts.

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AECD-2682

F

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Figure 7. Tantalum filaments.

A - 0.0098" Dia.B - 0.0143C - 0.0072D - 0.0119E - 0.0151F - 0.0111G - 0.0135H - 0.0072I - 0.0135 nI - 0.0103

A - 0.0103" Dia.B - 0.0123 "C - 0.0052D - 0.0123E - 0.0142F - 0.0088G - 0.0097H - 0.0124I - 0.0108

Filament No.10 Filament No. 11

10

35

BU0RN-OUT 1 HR 50M

25BURN-OUT

2 HR 30 M

20

45

TUBE- GLENTA NO. 2 (COIL FILAMENT)UCRL VAC RESEARCH DEPT1/T/44

10"0 2

TIME IN HOURS

Figure 8. Tantalum filaments. 10 mil, 10 cm long. E Gauge No. 7;0.5 ma; gauge No. 8 at 5.0 ma; pressure 10 ya H2 0 + 10 a air.

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Figure 9. Tantalum filaments. Tube VG-1A (Nc. 10 and No. 11 only) 10 mil, 3.8 cm long. o Gauge No.10, 0.5 ma emission; A Gauge No. 11, 5.0 ma emission; x Gauge 9A, 0.5 ma emission. Note: No. 9 is atetrode designed by Frank Kirby. Filament 6 cm, 20 mil Ta.

UCRL VAC RESEARCH DEPT12/30/43

NO. 10

BURN-OUT 41HR 50 M

NO.1

NO. 9 (CURRENT) \,-BURN-OUT 2 HR 04M

BURN-OUT 2 HR 44 M

NOTE: ATMOSPHERE OF 1.O AIR + 1.O H2 0

0

0

21%

2

U- 1

Q-3

0.4

2TIME IN HOURS

3

Figure 10. Life tests of four tantalum filaments. Tube used: DPI-VG-1A. Filament length = 3.8 cm;Ep = -23 volts; Eg = 130 volts.

5

0UCRL VAC RESEARCH DEPT

N0.14 12/30/43

NO.10NO. 1

NO.12

NO. 43

GAUGE SYMB. FIL. EMISSION PRESSURE LIFEAIR WATER

5 NO. 10 -O- 10 MIL Ta 0.5 10~3 MM 10 3 MM 1 HR 50M11 -A- 5.0 10~3 MM 10-3 MM 2 HR 04 M

12 -- X- 0.5 2X10 5 MM 10-3MM 2HR 39M43 -D- 5.0 2X105 MM 10 3 MM 2HR 55M

0

a~

0

i

I

0 NO.15 RUN AT 0.5 mae NO. 16 RUN AT 5.0 ma

UCRL VAC RESEARCH DEPT

GAUGE NO.15

GAUGE NO. 16

0 p-BURN-OUT 6 HR 48M

______________ _______________ ______________ _____________ ______________ ______________ ________ ______________I ___________________ ____ ____ ____ _____ NA__0____ ____

2 4 5TIME IN HOURS

BURN-OUT

6

7 HR 21M-\

7 8 9

Figure 11. Tantalum filaments. Tube VG-1A: 10 mil Ta, 3.8 cm long in atmosphere of 10 sa air.

25

20

j~15

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-NO.18 BURN-OUT (3 HR 18 M)

25

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BURN-OUT (3 HR 26 M)

P NO. 17

15

O GAUGE NO.17

A GAUGE NO.18

TANTALUM FURNISHED BY GENERAL10 ELECTRIC CO.

UCRL VAC RESEARCH DEPT1/7/44

O 1 2 3 4 5 6TIME IN HOURS

Figure 12. Tantalum filaments. Tube VG-1A: 10 mil, 3.8 cm long. Gauge No. 17 at 0.5 ma. GaugeNo. 18 at 5.0 ma. Pressure: 10 ga H 2 0 + 0.4 a air.

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