Work functions of rhenium and tungsten determined by thermionic and mass spectrometric measurements

423 International Journal of Mass Spectrometry and Ion Physics, 12 (1973) 323432 @j)Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

WORK FUNCTIONS OF RHENIUM AND TUNGSTEN DETERMINED BY THERMIONIC AND MASS SPECTROMETRIC MEASUREMENTS

BRUCE YK CHAO AND FREDERICK A. WHITE Division of Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, N-Y. It181

(U.S.A.)

(First received 11 May 1973; in final form 18 July 1973)

ABSTRACT

of

Measurements have been made of the changes in the efTective work function polycrystalline tungsten, rhenium, and tungsten-rhenium alloys at high

temperatures,

as a function

of the ambient

oxygen

pressures

in the ranse

of lo-’

to lo-’ Torr. The observed work function changes were determined by two techniques: (1) the standard thermionic emission method, utilizing Richardson’s equation and (2) a mass spectrometric method wherein the positive ion emission rates of the specimen were monitored as a function of specimen temperature. In this latter method, calculations were made on the basis of the Saha-Langmuir equation that relates ion emission rates to the ionization potential, work function, and absolute temperature of the sample. Experimental data from both methods in the temperature range 1700 K to 2150 K yielded consistent work function values for the pure metals. A maximum work function increase about 0.7 eV was noted for both tungsten and rhenium at 1710 K at low5 Torr, compared to a lo-’ Torr reference. In the case of the alloys, there is a large difference in the effective work function change as determined by thermionic and mass spectrometric measurements. Ion emission is dominated by the high work function element, while electron emission is dominated by the low work function element.

INTRODUCTION

The fact that changes in the work formance of diodes, ization sources used

a small amount .of an adsorbate may produce significant function of a surface has been used for improving the perthe efficiency of electron multiplier tubes, and surface -ionin mass spectrometry. Interest in the effect of oxygen o$ the

424

effective work function’ of a metallic surface arises tram the significant increase in the number of ions that can be emitted from a surface ionization source. Specific interest in this laboratory relative to the work function of metals has been generated by the need for making iqotopic assays of trace metals that are found in the environment, both air and water. The increase or decrease in the concentration of these elements is often a direct measure of the improvement or degradation of the environment, and any increase in analytical sensitivity for detecting trace metals provides an increased capability in environmental monitoring. It is known that metallic ions are usually more easily ionized from a surface ionization source if the hot metallic surface from which ions are generated is operated so that ions leave an oxide-coated surface rather than a pure metallic surface. In fact, an order of magnitude increase in ion production can often be realized from an oxide vs. a pure metal hot surface. Nevertheless, there are practical limits to the ambient oxygen pressure that can be used in such an ion source, because such pressures must be below the break-down potentials of the electrodes in the ion gun. In order to investigate the quantitative effect of partial pressures of oxygen on several surface ionization filaments, apparatus was devised so that a change in the work function could be observed by bcth electron emission and ion emission. In the first method, Richardson’s equation was employed to obtain the effective work function of polycrystalline filaments as a function of temperature. The Richardson equation is expressed as follows J =

AT’e-dkT

where J is the thermionic-emission cmrent density, A is a constant, k is the Boltzmann constant, T is the absolute temperature, and cp is the work function of the filament. In the second method, the ion emission was detected in a mass spectrometer and a correlation was made with reference to the Saha-Langmuir equation.

EXPERIMENTAL

APPARATUS

Thermionic diode

The thermionic diode used for electron current measurements is shown in Fig. 1, a schematic diagram of the whole system. It is comprised of three 2$-inch I.D. stainless steel “crosses”, that can be outgassed with heating tapes. The filament holder is supported by three adjustable set-screws, located within the stainless steel cylinder .which served for maintaining a symmetric distribution of the electric field. A disc with a O.Ol&inch diameter hole which was mounted-on top

425

Collector

Fig.

1. Schematic

diagram

of thermionic

Flange

diode.

of this cylinder served as the anode. The electrons emitted from the hot f3ament were accelerated through the anode hole and impinged on the collector. The current from the impinging electrons was measured by a vibrating capacitor electrometer. The distance between the collector and anode was adjustable with the built-in vacuum bellows. The temperature of the filament was measured with an opticai pyrometer and the pressure of the vacuum system was determined by a Hughes “nude” ionization gauge and controlled by a Granville-Phillips ionization gauge controller. Research grade oxygen was permitted to flow through a Varian leak valve with a leak rate as low as lo- lo Torr-liters per second. Four-stage

mass specfromefer

A Four-Stage Mass Spectrometer was employed to verify the surface ionization phenomena by the alternative method. This instrument has already been reported by White2, and Forman and White3. The analyzing components consist of two s-ton 20-inch radius of curvature magnets, two 20-inch radius of curvature electrostatic lenses, a source ion gun, and a 20-stage electron multiplier. The data were either plotted by an X-Y plotter or recorded through an ion counter connected to a PDP-8/I computer.

EXPERIMEZNTAL DAT_4 FOR TUNGSTEN

AND RHENIUM

The thermionic diode, along with the Hughes nude ionization gauge was sealed into the stainless steel system, which consisted of anionization pump backed by a liquid nitrogen operated sorption pump which in turn was backed by a

426 mechanical pump. Between .ihe sorption and mechanical pumps, a foreline trap was used to prevent cil vapor from entering the system. All the flanges are metal to metal sealed with copper gaskets. The whole system can be baked, including the ion pump. Each ribbon filament, 0.030 x 0.001 inchZ, was cleaned by a procedure foflowing that reported by Engelmaier and Stickney4 joutgassingat an oxygenpressure of 10m6 Torr at temperature 2200 I( for 63 hrs.). Eowever, an additional thermal

4x6

, I.0

0

OlSTANCE

Fig. 2. Characteristic

Fig. 3. Richardson

1

20 FROM

ANODE

. TO

3.0

CCLLECTDR

(CM)

curve of distance change from anode to col!ector

plot for rhenium.

at T =

1831 K.

427 cycling was undertaken to remove filament impurities and to stabilize the grain structure. This procedure was repeated until consistent sets of data could be obtained. The tube characteristics and the apparent emission constants in vacua were determined. Figure 2 is a plot of collector current vs. distance from anode to collector D, with a comparison of different anode voltages of Y = 2.0 kV to V = 0.5 kV at the filament temperature T = 1821 K. As D is increased, the collector current at V = 2.0 kV begins to rise and comes to a maximum and then decreases slightly; but at V = 0.5 kV, the current is decreasing slightly throughout the 2-centimeter distance change. At Y = 0.5 kV, many elecuons fait to arrive at the collector because of the inadequate field strength; at V = 2.0 kV, the electrons

impinging on the collector surface have sufEcient kinetic energy to produce a significant number of secondary electrons. The secondary electrons are accelerated back to the anode and thus subtract from the primary current. Figure 3 shows the Richardson plot obtained for rhenium. Subsequent to these measurements, oxygen was introduced into the system. Emission data taken at various 0, pressures and filament temperatures are shown in Fig. 4 for rhenium and F’ig. 5 for tungsten_ As expected, the work function increases with increasing O2 pressure. Figure 6 is a plot of the relative rhenium ion emissions measured by the mass spectrometer at different oxygen pressures. It is apparent that the higher the oxygen pressure: in the system, the greater is the relative number of rhenium atoms ionized. The ion current was measured as a function of temperature of the rhenium filament, and oxygen pressure of the source housing. Let us assume a constant efficiency for the mass spectrometer and a constant flow of neutral particles, No_ This assumption is justified for temperatures below 2000 K”. Using the Saha-Langmuir equation at the same teinperature but with

Fig.

4.

Work function changes of rhenium as a function of temperature and oxygen pressure.

428 0.7 0.

k

0.

TLJNGSTEN 0 v a -

T=l7!0 T-1765 ~=I821 ?=I876

v Tsl932

$0.

’ T=I908

-i

0.

0

0

4i

*

-

.

0 0

.

-

I

,

D

(

3

.

-

-

.

.*--

I

I

-5

PRkURE OF’;(TORR)

function. changes

’ -4

,S5 Re

I

.

-

-0

-9

of tungsten

(1) P =42Xld’ w

-

.

7

Iti6

Fig. 5. Work

.

.

O.0

.

0 .

=

m

.

-

.

D q

aa

n.

0

. 0

0

a

0

0.1

-0

=

‘0

n

0

rci5

as a function

L

iCP 01 temperature

and oxygen

pressure.

TORR

P,=2_OXtO= TORR

(3) P*=4ClXI@

TORR

(4) PeCWXIOa

TORR

MASS UNIT

Fig. 6. Ion emissions

from rhenium filament at different oxygen pressures at T =

1988 K.

two different oxygen pressures, one gets the change of work function as

where IV,, lV2 are the numbers of positive ions emitted per unit time at the average work functiqns qSl, 42 corresponding to the oxygen pressures pl, p2, respectively; K is the Bol_~mann constant, and T is the absolute temperature.

429 With respect to the polycrystalline structure, the principal contribution to the thermionic electron current is made by the faces with low work functions, whereas the ion current is influenced (via positive surface ionization of ‘atoms) primariiy by the faces with high work functions. In other words, one might expect to notice a small diffkrence in work functions as measured by the two techniques. This is evident in the data shown in Table 1. TABLE

i

THE CHANGES TEMPERATURE

OF EFFECTIVE

WORK

FUNCTIONS

OF RHENIUM

1988 K MEMUREi> BY MASS SPECTROMETER

The basic pressures of the spectrometer

AS A FUNCTION AND

OF OXYGEN

BY THERMIONIC

and the diode are 4.2~ IO-’

PRESSURE

AT

DIODE

and 1.55 x lo-*

Torr,

respectively. Pressure (Torr) 4.0 x 10-6 5.5 x 10-C 7.0 >: 10-e 9.0x10-6

Accrage counts per second

Standard det-iation

FI& measured by spectrometer (e V)

L?U&measured by diode (eV)

1591.5

33.7

0.0231 f0.0017

0.026f0.0021

2086.8 2283.4 2436.6

29.7

0.049210.0009 0.0646;0.0007 0.0757~0.0008

0.044~0.0017

11.6 31.2

0.060~0.0018 0.075 &-0.0020

EXPERME3’ZTA.L DATA FOR TUXGSTEN-RHENIUM ALLOYS

The W-i6 % Re material data measured by the thermionic eIectron method is shown in Fig. 7. Because the lower work function element dominates electron emission, this dia,oram more closely resembles pure tungsten than pure rhenium. The mass spectrometer data shows that the rhenium content of the filament of W-5 oA Re decreased with each test; eventually only the tungsten peaks were

TUNGSTEN

- 26

VT=1765 0 T= 1821 . T= 1931 n T =I988

, .*i0"

: IO’ PRESSURE

I

Iti6 OF

02

Fig. 7. Work functionchages of W-26 %-Re as a function of temperature and oxygen pressure.

430

detected as shown in Fig. 8. The same phenomenon was observed by Abey6. He used an electron-beam probe to detect the Re concentration of the filament, and after his measurements were completed he found that rhenium at the filament

MASS

UNIT

Fig. 8. Ion emission changes of W-5 y0 Re as a function of temperature and oxygen pressure.

II! P

-,Fig. 9. &I

4oxd

Toa

I

emissiansfrom W-26 % Re filament at different oxygen pkssures at T = 1988 R :

431 surface had diminished due to sublimation and diffusion efiizcts. in the case of W-26 y0 Re, a substantial amount of Re remained on the surface. Figure 9 is a plot showing the metallic spectra of Re, i.e., isotopes 187 and 185 No m&al ions of tungsten were observed, but the oxides of tungsten were detected_ This result is expected due to the high work function that is required to emit sizable beams of rhenium ions, and the greater probability of oxide formation for tungsten. Table 2 provides data for a tungsten-rhenium ahoy consisting of 26 “/o rhenium. TABLE THE

2

FUNCTION OF W-26 0/0 Re ALLOY AS A FUNCTION OF OXYGEN K MEASURED BY MASS SPECTROMETFiR hhiD BY THERMIONIC DIODE The basic pressures of the spectrometer and the diode are 2.5 x IO-’ and 1.20x 30-* Torr, CH~XGES

PRESSURE

AT

OF

EFFECTIVE

TEMPERATURE

WORK

1988

respectively. Pressure (Ton)

Acerage counts per secor;d

Standard

A& measured bv

A& measured

deciation

spectrometer (eV)

by diode (e V)

4.0 x 10-S

532.4

5.5 x 10-G 7.0x 10-S 9.0 x 10-b

635.5 743.2 862.4

38.6 27.0 39.1 43.7

0.0100~0.0041 0.0404~0.0014 0.0672~0.0005 0.0926f0.0002

0.048&0.0065 0.080&0.0038 0.10~0.0037 0.13&0.0040

DISCUSSION

AND

CONCLUSIONS

The effect of O2 on the thermionic emission from tungsten WFSfirst studied by Kingdon’ in 1924. A similar investigation was performed by Johnson and Vick’ in 1937. The most recent values of the change in work function of tungsten as a function of oxygen pressure were reported by Greaves and Stickneyg. These investigators also used the effective work function vs. oxygen pressure technique, obtaining data very similar to that shown in Fig. 5. With respect to rhenium, because no direct data could be found for comparison, a surface ionization technique was employed for verifying the change of work function due to the presence of oxygen. Owing to the limitations of the pressure ranges (2 x lo-’ to 10m5Torr) imposed by the mass spectrometer, only a very restricted region (2 x 10s6 to 9 x 10e6 Torr) was utilized. Table 1 shows the good agreement between the two methods. According to Kaminslcy lo , the thermal emission of ions does not yield the same average value for the work function of the polycrystalline surface as would be obtained for measurements of thermoelectron emission. He suggests that ion emission is dominated by the patches having &,,, so that the average value of work function 6 is large. Conversely, electron emission is dominated by the patches having &Fn, so the average 6 is small. The measurements of changes in work function of binary alloys have always been difficult because of preferential depletion of one of the two constituents.

432 For example, the vapor pressure of tungsten is given byl’ log,,P,, where P,,

= 12.02-

48400 +O.l32logJ-0.000162’ T

is the pressure in mm Hg and T is the absolute temperature in Kelvin.

The vapor pressure of rhenium is

log,,P,,

= 10.4038-

40865 -. T

At the temperature about 2000 K, the vapor pressure of tungsten is approximately 2 orders of magnitude less than that of rhenium. Therefore, the depletion of rhenium will be much faster than that of tungsten. As shown in Fig. 8, the rhenium content of the W-5 oA Re filament was completely depleted from the surface due to the repeated flashing and operatin g in oxygen atmosphere_ The depletion of rhenium content was caused by diffusion, sublimation and oxidation at the surface. The changes of effective work function of W-26 % Re alloy measured by using both methods - mass spectrometer and thermionic d&de - are shown in Figs. 9 and 7, respectively. Table 2 lists numerically the summary of the changes of effective work function of W-26 % Re alloy measured by the two methods at a temperature of 1988 X and pressures from P,, = 2.0 x 10B6 to PO2 = 9.0 x 10M6 Torr. Comparing the last two columns of this Table, one finds large discrepancies between work function changes measured by the two methods. However, if one compares Figs. 7 and 4 and Figs. 7 and 5 at high temperatures, it is easy to see that the electron emission is dominated by the low work function tungsten; but no tungsten ions were detected by the mass spectrometer (see Fig. 9). This situation is explained on the basis of Kaminsky’s comments’ ‘. Thus, the measurement of thermal ions does not yield the same work function for an alloy as would be obtained from the measurement of thermal electrons.

REFERENCES

1 2 3 4 5 6 7 8 9 10 11

E. B. HENSLEY,J. Appl. Phys., 32 (‘961) 301. F. A. WHIR, Mass Spectrometry in Science and Techna~ogy, Wiley, New York, 1968. L. FORMANAND F. A. WHITE, Rec. Sci. Instram., 38 (1867) 355. W. ENCELMAIERAND R. E. STICKNEY,Su& Sci., 11 (1968) 370. W. WEIRHAUSEN,in R. M. ELLIOT (editor), Advances in Mass Spectrometry, Vol. 2, The Macmillan Co., London, 1963, p. 38. A. E. ABEY, Appl, Phys., 39 (1968) 120. K. H. KINGDON, Phys. Reo., 24 (1924) 510. M. C. JOHNSONA&?) E A. VICK, Proc. Roy. Sot. London, A151 (1937) 308. W. GRE~VE~AND R. E. STICKXEY, Surfi Sci-, 11 (1968) 39.5. M. KAMINSKY, Atomic and Ionic Impact Phenomena on Metal Swfaces, Academic Press, New York, 1965. A. N. NESMEYANOV, Vapor Pressure of the Elements, Academic Press, New York, 1963.