Ž .Applied Surface Science 146 1999 105–108

Work function of refractory metals and its dependence uponworking conditions

H. Kawano ), T. Takahashi, Y. Tagashira, H. Mine, M. MoriyamaFaculty of Science, Ehime UniÕersity, 2-5 Bunkyo, Matsuyama 790-8577, Japan

Abstract

Ž . Ž .To investigate the stability of work function f of polycrystalline refractory metals Nb, Mo, Ta, W and Re , theŽ . y7thermal electron current J from a thin wire heated at a residual gas pressure of P f1=10 Torr was measured as ar

Ž . Ž .function of wire temperature Tf1200–2200 K , elapsed time tf0–20 min after flashing at T)2000 K or the pressureŽ y7 y5 . Ž .P or P f10 –10 Torr of introduced gas air or oxygen . Theoretical analysis of experimental data yields thea O

Ž . Ž .following results. 1 At P , the lowest boundary temperature T keeping each metal surface virtually clean ranges fromr bŽ .;1500–1900 K, above which the clean surface work functions f of Nb, Mo, Ta, W and Re are kept constant at 4.02,c

Ž .4.39, 4.28, 4.54 and 4.96 eV, respectively. 2 Below T , f is increased by ;0.1–0.9 eV, depending upon metal species.bŽ . Ž . Ž .3 This increase is caused by adsorption of residual gases especially of oxygen . 4 When air or oxygen is introduced up to;2.5=10y5 or 5=10y6 Torr, f of Re at 1700 K, for example, is increased from ;5.0 to 5.5 eV, which is governed not

Ž . Ž .by P but by P r5 equivalent to P . 5 The lowest temperature T keeping f in O depends upon P according toa a O c c 2 OŽ .T saqb log P , where as2795 K and bs115 K in the case of W. 6 Among the five metals, Re is highest in f , butc O c

chemically most stable against oxidation and also mechanically strongest, thereby most stable in J and longest in durabilityas a thermal electron source metal. q 1999 Elsevier Science B.V. All rights reserved.

PACS: 73.20-r; 82.65.-i

Keywords: Polycrystalline refractory metal; Thermal electron emission; Work function; Oxygen adsorption

1. Introduction

As a very simple and convenient source of ther-mal electrons, a heated thin wire of polycrystalline

Ž .refractory metals W, Re, etc. has long been em-ployed in many fields of basic and applied scienceand technology. For example, W is widely used as ahot cathode for an electron impact type ion source ina mass spectrometer and for an ionization type vac-

) Corresponding author. Tel.: q81-89-927-9597; Fax: q81-89-927-9590; E-mail: kawano@dpc.ehime-u.ac.jp

uum gauge. However, the stability of work functionŽ .f has not yet fully been investigated with anymetals. Our previous work on thermal positive ionemission from polycrystalline metal surfaces heated

w xin easily attainable high vacua 1,2 indicates that theŽ q.work function f effective for ion emission is

very different from f and also that fq has a strongŽ .dependence upon surface temperature T , elapsed

Ž .time t after high temperature flashing, residual gasŽ .pressure P , incident sample beam flux and sampler

species. Therefore, it may be very interesting andimportant to examine the stability of f and itsdependence upon various working conditions.

0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0169-4332 99 00033-1

( )H. Kawano et al.rApplied Surface Science 146 1999 105–108106

Fig. 1. Richardson plot obtained with Re at P f1=10y7 Torr.r

From the above point of view, f of Nb, Mo, Ta,W and Re has been measured under various condi-tions in this work, which shows that f as well as

q Žf depends upon T , t, P and upon the pressure Pr a.or P of air or oxygen introduced in the electronO

source region without stopping evacuation.

2. Experiments

The experimental apparatus used previously forw xthermal ion emission 1 was again employed here.

Ž .The thermal electron current J emitted from a thinŽwire ;0.1–0.2 mm in diameter and ;2 cm in.length at an electron extraction voltage of 300 V

Ž .was collected mainly by a cylinder and partly ;1%Ž .by a Faraday cup. The wire temperature T was

determined with an optical pyrometer. In the finalŽ y7 .vacuum P f1=10 Torr attained with an oilr

diffusion pump through a liquid nitrogen trap, theresidual gases consisted of H O, CO, O , CO and2 2 2

w xhydrocarbons 3 . To examine the effect of gasesupon electron emission, air or oxygen was intro-duced up to a pressure of P f3=10y5 Torr ora

P f5=10y6 Torr under continuous evacuation.O

Occasionally, a molecular beam of metal halide wasdirected onto the wire at the incidence rate of less

than ;1=1014 molecules cmy2 sy1 in order tocheck the adsorption effect upon J.

3. Results and discussion

3.1. Temperature dependence

A typical Richardson plot obtained with Re at Pr

is shown in Fig. 1. The slope of a linear part of lineŽ . Ž .1 above a boundary temperature T yields theb

Ž .work function f s4.96 eV of the essentially cleancŽ .surface of Re. The difference between curve 1 and

Ž .extrapolated line 2 yields the work function in-Ž . Ž .crease Df , as shown by curve 3 . It is readily

evaluated from

DfskT ln J rJ , 1Ž .2 1

where J and J are the currents corresponding to1 2Ž . Ž .curve 1 and line 2 , respectively. Such an increase

Ž .is observed also with other metals see Fig. 2 . Thevalues of f and T are summarized in Table 1.c b

Each f is in good agreement with literature valuescw x4 within the limit of "4%, thereby indicating thatthe present experiment is free from systematic errors.The values of T and Df depend upon the speciesb

of metal, strongly suggesting that f is increased byresidual gas adsorption at temperatures below T andb

Fig. 2. Temperature dependence of the work function at P f1=r

10y7 Torr.

( )H. Kawano et al.rApplied Surface Science 146 1999 105–108 107

Table 1Important results achieved with polycrystalline refractory metals

Ž . Ž . Ž . Ž . Ž . Ž . Ž .Metal f eV T K Df eV t min Df eV a K b Kc b 1 2

Nb 4.02"0.05 ;1500 ;0 ;0 – – –Mo 4.39"0.04 ;1960 ;0.6 ;3 ;0.7 2750"43 100"42Ta 4.28"0.05 ;1800 ;0.1 ;1 – – –W 4.54"0.06 ;1950 ;0.7 ;3 ;0.9 2795"83 115"52Re 4.96"0.04 ;1740 ;0.4 ;2 ;0.7 3275"67 209"18

also that the adsorption effect upon f has a strongdependence upon both T and the metal species.

3.2. Time Õariation

To examine the above hypothesis, J, at constantvalues of both T and P , was measured as a functionr

Ž .of the time t elapsing after flashing above 2000 K.Analysis of the data obtained with W yields Fig. 3,where f is evaluated from

fsf qkT ln J rJ . 2Ž .c 0 t

Here, J is the current at ts0. At T)T f1950 K,0 b

f of W is kept constant at f f4.54 eV, as showncŽ . Ž .by lines 1 and 2 in Fig. 3. Below T , however, fb

Ž .increases with t and reaches a constant value f att

Ž .a certain time t . As T decreases, both f and tt

become larger, and each f agrees with each ft

Fig. 3. Work function of W at P f1=10y7 Torr vs. timer

elapsing after high temperature flashing to make the tungstensurface virtually clean at ts0.

Ž Ž . .determined as a function of T curve 2 in Fig. 2 .A similar tendency was found with other metals, too.In a separate experiment employing flash desorptionmass spectrometry where the gases desorbed fromthe wire were converted into positive ions by elec-tron impact, the desorption amount of oxygen de-tected as Oq at a given value of t increased as Tdecreased, showing a similar pattern as that in Fig. 3.

The above results indicate that the work functionincrease after ts0 or below T is caused mainly byb

adsorption of oxygen included in the residual gases.

3.3. Gas pressure dependence

To check the effect of residual gases upon f,gradually, P was increased by admission of air. Asr

exemplified with Re in Fig. 4, f tends to increasewith an increase in P even at T)T f1740 Ka b

Fig. 4. Work function of Re vs. gas pressure increased by airintroduction.

( )H. Kawano et al.rApplied Surface Science 146 1999 105–108108

Fig. 5. Work function of Re vs. partial or total oxygen pressure.

Ž Ž . .curve 1 in Fig. 2 . Fig. 5 shows that curvesŽ . Ž . Ž X. Ž X.1 – 3 well overlap with those 1 – 3 , respec-tively. This result indicates that the increase in f isgoverned by P r5 equivalent to P and, hence, thata O

O , not N , in air is responsible for the increase. At2 2

1925 K, for example, f of Re begins to increase atcy7 Ž .P f3=10 Torr Fig. 5 . Above the critical tem-O

Ž .perature T , the surface in an oxygen atmosphere isc

kept virtually clean at f . The relation between Tc c

and P is expressed byO

T saqb log P in Torr , 3Ž . Ž .c O

where a and b are the constants with each metal, assummarized in Table 1. Here, T is the lowestb

temperature above which each metal surface is keptvirtually clean at f in an easily attainable highc

Ž y7 .vacuum P f1=10 Torr , Df is the workr 1

function increase caused by adsorption of residualŽ .gases especially of oxygen at P and Ts1500 K, tr

is the time elapsed during the increase in f by Dfc 1

owing to the gas adsorption, and Df is the work2

function increase due to oxygen adsorption at P sO

4=10y7 or P s2=10y6 Torr and at 1500 K.a

Incidence of a molecular beam of salt such as LiCl,NaI, RbBr, CsF, InI or TlCl has little effect upon f ,c

Df and Df so long as T is above 1500 K and the1 2

incident beam flux is less than ;1=1014 moleculescmy2 sy1. This flux corresponds to that of O at2

y7 y6 ŽP f3=10 Torr or P f1.5=10 Torr Fig.O a.5 .

ŽBoth Nb and Ta are lowest in f less than 4.3c.eV , but J from them heated at a constant heating

voltage is subject to a considerable change in timeowing to the attack by oxygen in residual gases. Anywire of Nb especially becomes thinner by rapidoxidation, thereby making it impossible to keep Tconstant at a fixed heating voltage. Wires of Tabecome very fragile in long heating. With respect toNb and Ta, therefore, it is very difficult to determine

Ž .the correct values of a and b in Eq. 3 . Owing torecrystallization by long heating at high tempera-tures, any wire of Mo is usually more fragile than

Žthat of W. Both Mo and W are highest in T seeb.Table 1 , thereby making it impossible to keep their

surfaces clean at T-2000 K. In contrast, Re isŽ .highest in f 4.96 eV but relatively low in Tc b

Ž .;1740 K and chemically most stable against oxi-dation and also mechanically strongest, thereby moststable in J and longest in durability as a thermal

Želectron source metal. In usual high vacua P fry6 y7 .10 –10 Torr , it may be recommended that Re

should be heated at ;2000 K to keep the surfaceclean at f without depending upon changes in Pc r

and, hence, to keep J both strong and constant for along time.

References

w x1 H. Kawano, S. Matsui, N. Serizawa, Rev. Sci. Instrum. 67Ž .1996 1193.

w x2 H. Kawano, K. Ogasawara, H. Kobayashi, A. Tanaka, T.Ž .Takahashi, Y. Tagashira, Rev. Sci. Instrum. 69 1998 1182.

w x3 H. Kawano, S. Itasaka, S. Ohnishi, Y. Hidaka, Int. J. MassŽ .Spectrom. Ion Processes 70 1986 195.

w x4 V.S. Fomenko, Handbook of Thermionic Properties, Plenum,New York, 1966.