the choice of cathode material in a hot cathode

8/2/2019 The Choice of Cathode Material in a Hot Cathode

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The choice of cathode material in a hot cathodeionization gaugereceived in final form 8 August 1975P E Gear. The Science Research C ouncil. Rutherford Laboratory, Chilton. Oxfordshire, Engla nd

A paper in our Education Series: The Theory and Pract ice of Vacuum Science and Technologyin Schools and Colleges.The paper outlines the problems encountered in using a hot cathode ionization gauge withparticular reference to the tungsten cathode. The theory of thermionic emission from metals andsemiconductors is mentioned as is activation and poisoning of surfaces.Low temperature cathodes are discussed with emphasis on the monolayer cathodes, oxidecathodes and the refractory compound cathodes.

1. IntroductionIn a solid of macroscopic dimensions the fraction of atoms ofthe total at the surface is of the order of 10e8 and the contri-bution of these atoms to the normal bulk properties of the solidis negligible. However, the properties of the surface canbecome extremely important under certain circumstances. Itis through the surface that charge must flow in thermionicemission and it i s on the surface that chemical reac tions withthe gaseous environment occurs. One of the most importanttechnica l applications of the properties of surfaces is theemission of electrons from the surface of metals. This effect,known as thermionic emission is very important for all devicesrequiring electron guns e.g. thermionic valves , TV tubes,electron mic roscopes etc. and thermionic ionization gauges.However the use of hot cathodes in ionizat ion gauges in-troduces problems in the measurement of low pressures. Theseproblems are due to:

(a) Chemical interaction of the residual gases at the hotcathode resulting in changes in gas composition.(b) Emission of positive ions and neutrals from the cathode.(c) Thermal effects resulting from heating of electrodes andgauge walls by radiation from the hot cathode.Many different types of ionization gauges (see for exampleLeek or Redhead et a12) and various cathode materials havebeen developed to try and reduce the effects of the aboveproblems.

2. Principles of thermionic emissionAt the surface of any material is a potential barrier whichprevents electrons escaping unless they acquire sufficientenergy to overcome the barrier. The electron energies in a puremetal have a Fermi -Dirac distribution (Figure 1) whichis strongly dependent on temperature. At absolute zeroall electrons lie below the Fermi level EF and the height ofthe potential barrier above this level is known as the workfunction 4. As the temperature is raised so the distribution ismodified, as shown, until some of the electrons have acquiredenough kinetic energy to escape over the barrier into thevacuum, and thermal emission occurs.

These electrons con escapeowx potentlot borrler \

Dlstr l butlon at T=b

\NlEl -

Metal Vacuum

Figure 1. The potential diagram of a metal-vacuum interface, withFermi. Diract energy distributions. At T = 0 all electrons ie belowthe Fermi level and there can be no thermal emission .At T > 0 thedistribution is modified so that some electronshave enough energyto escapeover the potential barrier and thermal emissioncan occur.

The current drawn from the metal can be calculated from theRichardson-Dushman equation.I, = AT(l - r)exp -2( >where 1, is the emission current

A is the Richardson constant = (4rmek2/h3)$ is the work function (usua lly expressed in electronvolts)k is Boltzmanns constantT is temperature (absolute)I is a reflection coefficient to take into account reflectionof electrons approaching the metal surface.

Taking logs of equation (1) gives:In $ = In A(1 - r) - $TA plot of In IO /T2 against 1/ T givesa strait&t liqe frc$ $iir# {heconstant A and the work function 4 can be determined. Eigureishows such a plot for tungsten.3

Vacuum/volume 26/number 1. Pergam on Press LtdJPrinted in Great Britain 3


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P 15 Gear:The choice of cathode material in a hot cathode ionization gauge

Pigure2. Richardson plot for tungsten, from which the work functionand the Richardson constant can be determined.From equation (1) the temperature T for a given emissioncurrent la is approximately proportional to 4 so that a low valueof 4 is needed for a low temperature cathode.The above is a simpl ified equation which fails to take intoaccount a number of difIicult ies namely,(a) The temperature dependence of 4.(b) Effect of electric fields at the surface on the work function.(c) Non uniformity of the work function.Simple theory also suggests that the work function might beexpected to vary between different crystal planes. The atoms inlattice planes with low c rystal indices are tightly arranged andgives a higher work function than the less closely packed

planes. Differences of up to 10% from the mean work functionfor polycrystalline materials have been measured betweendifferent crystal planes.Thermionic emission from an n-type semiconductor layerdeposited on a metal substrate is the basis of an importanttype of cathode-the oxide cathode. The potential diagram isshown in Figure 3. Electron equilibrium between the metalSemi-conductor Vacuum

Conduction bond P-r X

Fermi level

Donor impurity levelSL L z A

Figure 3. The potential diagram of an n-type semiconductor-vacuuminterface.

and semiconductor can only be achieved if the Fermi level liesapproximately midway between the donor and conductionlevels.Only electrons in the conduction band with energies greaterthan x the electron affinity can be thermally emitted. The currentdrawn is given byIO = AT2 exp - yThis is the Richardson-Dushman equation with the meta llicwork function + replaced by (x + U). Hence (x + U) is oftencalled the semiconductor work function.3. Activation and poisoning of surfacesThe work function of a metal such as tungsten can be loweredby surface adsorption of alkali or alkaline earth atoms. Thelowest value of the work function is obtaine d when there is amonolayer of adsorbed atoms.The reason for such a change can be shown by consideringthe potential energy of an atom as a function of its distancefrom the metallic surface (Figure 4). The curve ABC is theattraction due to Van der Waals forces with D, the energyrequired to dissociate the adsorbed atom from the metal

//R/

//

/IFigure 4. Potential energy as a function of distance from a metalsurface.ABC corresponds o an atom, PQR to an ion.surface. If the atom i s ionized the electron is taken into themetal yield ing a gain in energy equal to the work function 4of the metal. Hence the potential energy curve for the ionstarts (I - 4) above the atomic curve at point P , (where I isthe ionization energy). On approaching the metal the ion is underthe influence of an image force, i.e. the attraction potential is acoulomb attraction proportional to the reciprocal of the dis-tance from the metal. As the potential curve for the atom i s ofthe Van der Waals type it varies with a higher power of thedistance. Hence the potential curve for the ion in tersects theatomic curve and if Q is lower than B, the atom is adsorbed asa positive ion rather than as an atom. The condition for ionicadsorption isDi-I+~>D,where D , is the binding energy of the ion.

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P E Gear: The choice of cathode material in a hot cathode ionization gaugeAtoms with a low ionization energy (such as alkali andalkaline earth atoms) will be adsorbed as ions on a metal witha relat ively high work function like tungsten.If atoms are electropositive i.e. they are adsorbed as positiveions on the metal surface, then a negative surface charge isbuilt up and a double layer exists . Thus electrons emitted fromthe interior of the metal into vacuum see an extra potentialenergy drop resulting from the double layer equal to

A$ = 4nNe2dwhere n is the distance between positive and negative chargesand N is the number of ions adsorbed per unit area. It is there-fore easier for the electrons to escape than without the doublelayer and the effective work function of the metal is lowered byA# (Figure 5a). This process is known as activation. A reduc-tion in the work function results in a particula r thermionicemission leve l being obtained at a lower operating temperaturewith a consequent reduction in gas-cathode surface reactions.This activation process wil l be looked at further when consider-ing low temperature monolayer emitters.

-Monolayerac uum Meta l Mono l ay e r V ac uum(0 1 (b )Figure5. Poten tial diagrams of metal surfaces,4 is the work function

of a clean surface, 9, is the work function when the surface s con-taminated with an electropositive layer and 4. the work functionwhen contaminated with an electronegative ayer.The opposite effect known as poisoning occurs when atomsare adsorbed as negative ions (electronegative contamination).In this case the work function is increased (Figure 5b) resulting

in a decrease n emission at a particular operating temperature.This can occur whenever oxidiz ing gases such as 01, HZ0 orCO2 are present.4. Pure metal cathodesFor a clean metal to be useful as a cathode it must g ive therequired emission and have long life, i.e. it should not evaporatetoo rapid ly at the operating temperature. Table 1 gives theemission characteristics of various pure metals.4 As can be seenonly tungsten, tantalum and rhenium give useful emissionlevels . These metals have an advantage that due to their highoperating temperature contaminating electronegative gaseswhich would increase the work function and reduce emissionleve ls are rapidly evaporated. Tantalum and rhenium havedisadvantages that wil l be discussed later, and it is tungstenthat is in genera1used in ionization gauges operating in medium-high vacuum range. However a tungsten cathode is run attemperatures between 2250-2500 K to give adequate emissionleve ls and at these temperatures chemically active gases reactat the filament to produce other gases. Hence use of a tungstencathode can cause considerable changes in the gas compositionof an ultra high vacuum system.4.1. Reaction of oxygen with tungsten. Oxygen moleculesincident on a hot tungsten cathode can be adsorbed for asufficient time to enable dissociation to take place. The degreeof dissociation has been calculated to be 99 % at a temperatureof 2000 K.5 The chemically active atomic oxygen produced atthe cathode surface then reacts with carbon impurities in thecathode to produce CO and CO1. The production of COoccurs at the surface of the cathode and the replacement ofsurface carbon, by diffusion from the bulk, results in a con-tinuous generation of CO. If the cathode is run at temperaturesin excessof 2000 K, in oxygen pressures greater than 10S6 thenthe diffusion process is rapid enough to produce depletion ofthe carbon impurities with subsequent reduction in CO*production.6 If the time of the reaction is long enough (10-60 hdepending on the cathode) then the carbon impurities areeliminated. If carbon free tungsten cathodes are used no CO orCOZ formation occurs. However atomic oxygen can evaporatefrom the cathode and combine with the carbon impurities on

Table 1. The emissioncharacterist ics f various pure metals; only tungsten, tantalum andrhenium give useful emmission evelsMctnl Melt ing point Richardso n Work functio n Usable

OK constant A eV emission-2 -2Acm T A cm -2

w 3640 80 4.54 4 x 10-lTa 3270 60 4.10 6 x IO-'RI.2 3440 700 4.7 2.6 x IO-'PlO 2890 55 4.15 5 IO -3xPt 2050 170 5.40 2 -8x IONi 1730 60 4.1 5 x 1o-gBa 1120 60 2.11 I x lo-

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P E Gear:The choice of cathode material in a hot cathode ionization gaugethe gauge walls to give CO. The atomic oxygen also interactswith the tungsten at the surface of the cathode forming gaseoustungsten oxides, the amounts desorbed depending on cathodetemperature. The principal products of oxidation are WOz andW03, the trioxide being most important at low temperaturesand the dioxide the dominant product at high temperatures. Attemperatures in excess of 2600 K, WO is produced. Thesechemical processes result in the removal of oxygen from thesystem, therefore an ion gauge acts as a pump for oxygen.An analysis of the oxygen pumping effect by tungsten hasbeen carried out using a quasiequ ilibrium treatment of gas-solid reactions. The general shape of the predicted curves (thefraction of incident oxygen molecules that are pumped againstfilament temperature), is very simi lar to those obtained from anexperimental study of the oxygen-tungsten interaction5 (Thesame analysis provides a semiquantitative explanation ofexperimental data on the rate of impurity emission from hottungsten.)4.2. Reaction of hydrogen with tungsten. Hydrogen is dissociatedby a hot tungsten cathode at temperatures above 1000 K, theprobabil ity of dissociation being dependent on cathodetemperature. The atomic hydrogen evolved is readily adsorbedon most surfaceswithin the ion gauge and hence an anomalouslyhigh pumping speed for hydrogen is obtained. The contaminantgases ormed from the interaction of atomic hydrogen with thechamber wa lls and electrodes are CO, CH4 and H20. Thecarbon is present in the gauge or in the cathode and the oxygenin the gauge walls. Table 2 shows the relative ion current

for contaminant gases as a function of tungsten cathodetemperature.8 As can be seen the most commonly evolved gasis carbon monoxide.Many complex actions take place with hydrogen and it hasbeen shown that the pumping effect i s due to a complexfunction of both cathode and grid temperatures.g Undernormal operation the cathode dissolves hydrogen, while thegrid adsorbs hydrogen. Experimenta l data has shown thatthere i s a variation in the sensitivity of Bayard-Alpert gauges ofthe order of 10% when operated in hydrogen.O This has beeninterpreted as being due to the adsorption of the atomichydrogen on the grid, causing a change in the reflectidh coeffi-

cient of the electrons at the grid. These reflected electronsoscillate about the grid structure and are indistinguishablefrom primaries coming directly from the cathode. Theycontribute direc tly to the gauge collector current increasingthe sensitivity by a factor (I -II y -1-yz t- . . .), where y is thereflection coefficient. Similar experiments have shown thatBayard-Alpert gauges with low temperature cathodes have amore stable sensitivity in hydrogen.A monolayer coverage of hydrogen increases the work func-tion of tungsten by 0.35 eV resulting in a decrease in theemission current.

4.3. Reaction of other gases with tungsten. Reactions with thecathode is not restricted to oxygen and hydrogen but takesplace readily with gases containing either of these elements.The following reactions with water vapour can occur with ahot tungsten cathode.7 WOJ

H,O+W+C + COI H

The atomic hydrogen and tungsten oxide recombine at the glasswalls of the gauge to form tungsten and water vapour. Watervapour therefore acts as a mechanism for transporting tungstenfrom the cathode to glass walls with a consequent reduction inthe life of the tungsten cathode.Methane and other hydrocarbons are found in most vacuumsystems due to back-streaming of diffusion pump o ils andflashing of getters and these can dissociate on the cathode toform carbon monoxide. A small degree of dissociation ofnitrogen can occur but the effect is very small. Sim ilarly there isnegligible dissociation of carbon diox ide, although it does reactto form metal oxides and CO. In the casesof both nitrogen andcarbon dioxide there is a change in the work function oftungsten in the presence of a monolayer coverage. In thepresence of COz the work function is increased by 0.55 eVgiving a decrease in emission. While in the presence of Nz thework function is decreased by 0.5 eV giving an increase in theemission current. 4.4. Emission of positive ions and neutrals from tungsten. Theemission of positive ions and neutrals from heated surfaces is acommon occurrence and can affect the performance of hotcathode ionization gauges.

Table2. The relative on current (1+/I +(H~)A) for contaminant gases sa functionof tungsten cathode temperature, the most commonly evolved gas is carbonmonoxide

Filament TemperatureOK H202

co H2 0 CH49. x x %300 0.26 0.56 0.33 0.02

1010 0.25 0.65 0.28 0.011550 0.44 0.82 0.44 0.04la40 0.78 3.1 0.95 0.461975 0.66 11.3 0.65 2.5


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P E Gear: The choice of cathode material in a hot cathode ionization gaugeWhen metals are heated close to their melting point signi ti-cant ion emission can occur. Impurity ions are also emittedusually at lower temperatures.The current of positive ions emitted at a temperature T isgiven by

where C, is the speci fic heat at constant pressure of an ion in thecondensed state.r is the ion reflection coefficient++ is the work function for positive ions

and M is the ion mass.This equation is dentica l in form to the Richardson-Dushmanequation for thermionic electron emission except for a factor of2 and the specific heat term. As with thermionic emission ofelectrons the emitted ions have a Maxwellian velocity distri-bution.Impurities leaving a hot cathode come off as both neutralparticles and ions as predicted by the Langmuir-Saha equation.

This equation gives the ratio of positive ions to neutral atomsevaporated per unit time as2 = A+ exp[(s-kTy,)e]where A+ is a constant involving the statistical weights of ionsand atoms

+- is the electron work functionand V , is the ionization potential of the atom.It has been shown that tungsten (which contains impurities ofthe alkali metals Na, K, Rb and Cs) emits alka li metal ions inpulses at temperatures above 1300 K. These pulses contain

approximately lo6 ions and the pulse rate varies from 102-lo4 s-l. The cause of this ion pulsing has been shown to be dueto the uncovering of impurity pockets of defects and grainboundaries in the crystal structure. 13 In the presenceof chemic-ally active gases 02, COZ and HZ0 the pulse rate increases,while the presence of N2, CO and A, which do not react withtungsten, have little effect.Although the positive ions cannot normally reach thecollector because of the grid potential, the Langmuir-Sahaequation predicts that the emission of positive ions is accom-panied by a large flux of neutrals. These can reach all parts ofthe gauge including the grid. By the process of electron induceddesorption a fraction of these neutrals are ionized. These ionsare then able to reach the collector giving an anomalous ioncurrent not related to gas pressure.Thy ion current from a heated cathode var ies as shown inFigure 6. As can be seen, at low pressures (


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P E Gesr: The choice of cathode material n a hot cathode ionization gaugeTable 3. The partial pressures bserved when a Bayard-Alperl gauge (BAG) is oper-ated at equilibr ium bulb temperature (-50C) and corresponding pressure when thebulb is cooled with an air blast

Ga s Partial Pressure % Partial Pressure %

H 2Hcco

at 5OC (torr)I.1 x IO -10

1.3 x IO -103 x lo-11

with bulb cooled36 2 -I Ix IO 14,642.6 9 x IO-II 65.5

9.9 2 x 1o- l 14 .6cty

3.5 I i lo- lCH4)

11.5 7. 5 x lo-l2 5. 3

Tota l prcssurc 3 x 10 -10 I.4 x IO -10

of hydrogen when heated in a hydrogen atmosphere andcooled to room temperatureI (as can happen to an ion gaugefilament). On reheating about 30% of the hydrogen remainswithin the structure of the material causing it to become verybrittle due to crysta l formation. A further effect of the hydrogenis to increase the resistance of the cathode by as much as 100%which would cause severe problems for any emission controlcircuit.Rhenium has a number of advantages over tungsten as acathode material. It is more resistant to water cycling and doesnot form stable nitrides and carbides. Hence oxygen interactionwith carbon impurities produces lesscarbon monoxide. It doesnot become brittle at high temperatures and because of itshexagonal close-packed crystalline structure is an ideal basematerial for lanthanum hexaboride coatings. The high workfunction is a disadvantage as the evaporation rate is about150 times higher than tungsten for the same emission level.The life time of a rhenium cathode is sti ll long enough howeverto be useful in ionization gauges.Iridium and rhodium have high work functions, low meltingpoints and high vapour pressuresat relatively low temperatures.This makes them unsuitable as cathode materials. Howeverthey are extremely resistant to oxidation and are used as basematerials in both thoriated and oxide coated cathodes.Most of the problems outlined above in respect of pure metalcathodes can be reduced by lowering the operating temperatureof the cathode. This has led to a search for suitable cathodematerials with low work functions to replace the conventionaltungsten cathode. The cathode materials that have been in-vestigated can be grouped into three main sections.

(a) Monolayer emitters.(c) Oxide cathodes.(d) Refractory compounds.

5. Monolayer emittersIn the previous discussion on cathode materials only cleanmetal surfaces have been considered. It was stated in section 3,that in the process known as activation a smal l amount offoreign material adsorbed onto a metal surface can cause alarge reduction in the work function of the surface. This

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process s made use of in producing cathodes collectively knownas monolayer emitters. Table 4 gives values for the workfunctions of tungsten contaminated with a monolayer ofvarious electropositive materials.

A reduction in the work function means that a cathode canbe run at a much lower temperature for a given emissioncurrent. In general the operating temperature for a givenemission is reduced by approximately the same ratio as thereduction in work function.

Table4. Values for the work functions of tungsten contaminated witha monolayer of various electropositive materials

Adsorbed materialCarsiumOxygen-Qesiutn

BariumOxygen- BariumThoriumZi 1:coniumClean tungsten

Work function eV1. 51. 41. 61. 32. 73. 14.54

For a monolayer cathode to be suitable, the evaporation rateof the contaminant at the operating temperature must be lowand some means for its replenishment must be found. A mono-layer gives the maximum possible emission and is at the sametime inherently stable, and so any excess contaminant must bequickly removed. Fortunately, the binding energy of themonolayer atoms to the substrate is usually greater than thebinding energy of any subsequent layer deposited on themonolayer. So providing the rate of supp ly of atoms is sufficientto maintain the monolayer and any contaminants are evapor-ated there will be very little reduction in emission.The types of monolayer emitters are limited due to theproblem of replenishment of the monolayer. The most commontypes are thoriated tungsten and the barium dispenser cathodes.


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P E Gear: The choice of cathode material in a hot cathode ionizat ion gauge5.1. Thoriated tungsten. A monolayer of thorium on tungstenresults in a decrease in the work function from 4.54 eV for thepure metal to 2.7 eV for the contaminated metal, with acorresponding reduction in the operating temperature toapproximate ly 1400 K.Thoriated tungsten in the form of a wire contains approxi-mately 0.5-l y0 thorium oxide, Th02, uniformly dispersedthroughout the tungsten. To prevent poisoning by residualoxidiz ing gases such as 02, H20 and CO,, the cathode iscarburized by heating in a hydrogen or hydrogen-nitrogenmixture containing an organic vapour such as xylene at atemperature of 2200 K. The hydrocarbon is then dissociatedat the hot cathode to produce free carbon which diffuses intothe tungsten.and combines with it to give tungsten carbide,W2C. The degree of carburization is limited by the fact that themechanica l strength of the cathode decreases rapidly withincreasing carbon penetration. In p ractice the thickness ofthe carburized layer is about 10% of the wire radius. Thepresence of the carbon in the surface inhibits the adsorption ofthe oxidizing vapours and they are removed by the formation ofco.

Heating to the operating temperature activates the cathodeby causing the Thor to diffuse through the W2C layer whereit is reduced to give free thorium.2W,C + ThOz --+4W + 2C0 + ThThe thorium then diffuses to the surface and migrates over itto form a monolayer. This type of cathode has a very longlife and can provide a saturated emission of 4 A cm- with alife in excessof 10,000 h.A much improved electron emission from thoriated tungstenhas been obtained by depositing a layer of grain orientatedtungsten over the cathode surface. The effect of this i s toincrease the Richardson constant by factor of 4 while keepingthe work function almost constant.

Thoriated tungsten cathodes have been used by many workers,and commercial ionization gauges are available using thistype of cathode.A cathode of thoriated iridium is not as susceptib le topoisoning as thoriated tungsten at high oxygen pressure. Thismakes it very suitable for high pressure operation (--lOmZ torr)in the ionization gauge of Schulz and Phelps.* Conventionalionization gauges using thoriated iridium cathodes are available,and several have been used success fully n the authors labora-tory.

5.2. Barium dispenser cathodes. A major group of monolayeremitter cathodes is the barium dispenser cathodes, where theemitting surface is a monolayer of barium on a substrate oftungsten. There are various forms of this cathode of which themost important are the 'L' cathode and the impregnated tung-sten cathode. These cathodes are used extensively in highpower microwave valves such as klystrons and magnetronswhere high current densities are required.There are no reports of tungsten based dispenser cathodesbeing used in ionization gauges because of the large evapora-tion rate. The barium would be adsorbed onto the electrodesof any gauge and give rise to large residual currents. Anionization gauge was developed using a platinum based bariumdispenser cathode but high residual currents were found.ig

6. Oxide cathodesThermion ic emission from an II type semiconductor layerdeposited on a metal substrate is the basis of the oxidecathode. There are three main groups, alkali earth oxidecoatings (barium, strontium, calcium), rare earth oxides andthorium oxide coatings. Cathodes of the first group are verysusceptib le to poisoning by oxidizing gases, 02, HZ0 and COand as such are not suitab le for use in ion ization gauges. Theyare used extensively in gas discharge devices such as thyratrons.Of the second group only yttrium oxide has been used as acathode in ionization gauges.* This cathode consisted of aplatinum-rhodium substrate with a yttrium oxide coating andwas used in an ionization gauge operating in the high pressurerange up to 1 torr.The thoria coated cathodes offer several advantages over thealkali earth and rare earth oxide cathodes.

(a) Electrical resistance is lower for same temperature.(b) Does not form interface compounds.(c) Easier to activate.(d) Less easily poisoned.

These cathodes are prepared by depositing a layer of thoria ona base metal of tungsten or i ridium by cataphoresis. The coatingis then activated by operation in a vacuum at about 2000 K.The thoria coated iridium is more commonly used becauseiridium is very resistant to oxidation and hence the cathode isnot sub ject to burn out if exposed to an increase in pressurewhile hot. Also stable emission is obtained in pressure as highas 10m2 torr. The operating temperature is also much lowerthan for the tungsten-based cathode. Because of these advan-tages many ionization gauges are commercially available withthoria coated iridium cathodes.7. Refractory compoundsOf the refractory compounds such as the carbides, nitrides andborides, lanthanum hexaboride is the most important. It has awork function of 2.8 eV and gives adequate emission at atemperature of 1400 K to be of use n ionization gauges.The cathodes are prepared by coating a refractory metal withlanthanum hexaboride powder. At high temperatures howeverthe small boron atoms which form a three-dimensional frame-work around the relatively large lanthanum atom tends todiffuse into the underlying metal substrate, forming interst itialmetal-boron alloys. This leads to a collapse of the boronframework around the lanthanum atom which in turn allowsthe lanthanum to evaporate rapidly thus shortening cathodelife. In addition the base metal becomes embrittled. Carburiza-tion of the surface of the base metal before coating with thelanthanium hexaboride can be used to reduce this diffusion.This process fills interstitial spaces in the base metal withcarbon atoms thus preventing boron atoms diffusing into thebulk. The d iffusion of boron is least with rhenium as the basematerial due to it s close packed hexagonal structure. Investi-gations of the poisoning characteristics shows there is a reason-able resistance to poisoning and an adequate cathode life i f thetemperature is kept below 1400 K. If a LaBs cathode becomespoisoned it can be reactivated by increasing the temperature fora short time. The time required to reactivate increases as the

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P E Gear: The choice of cathode material in a hot cathode ionization gaugenumber and severity of poisonings increases varying from afew seconds for a new cathode to several minutes towards theend of its life.Ionization gauges using LaB6 have been constructed usingboth rhenium and tantalum as the base material.

8. ConclusionsIonization gauges constructed with low temperature electronemitters are preferable to gauges constructed with tungstencathodes. With relatively low operating temperatures of1200-1500 K not only are gas-surface interactions reduced to aminimum but the gauges give a much more stable reading.For use in an uhv gauge a cathode should have a very lowevaporation rate and a low operating temperature. Figure 7shows the maximum emission density available from various

cathodes as a function of temperature.2 LaB6 has a highevaporation rate at temperatures above 1400 K and i s notsuitable for uhv use except where the emission density is keptbelow 10e4 A cmm2. Cathodes of Th02 on tungsten or iridiumhave reasonable emission efficiencies, ow evaporation rates andare relatively unaffected by chemically active gases. ThOz oniridium has the advantage that it can be exposed to air whenhot without damage. These factors make this cathode verypopular, and many ionization gauges using Th02 on iridiumcathodes are commercially available.

AcknowledgementThe author wishes to acknowledge the assistance given by theRutherford Laboratory in the preparation of this article.

Filament temperature. OKFigure 7. Maximum electronemissiondensityasa function of cathodetemperature for different cathode materials.

References J H Leek,Pressure Measuremen/ in Vacuum SyssIems,Chapman andHall Ltd, London (1964).2 P A Redhead, J P Hobson and E V Kornelsen, ThePhysical Busisof Ultrahigh Vacuum , Chapman and Hall Ltd. London (I 967).3 G Herrmann and S Wagener,The Oxide Coaled Cathode, Chapmanand Hall (1951).4 R 0 Jenkins, Vacuum, 19, 1969, 353.5 J H Singleton,J C/iernPhys, 45, 1966, 2819.6 J A Becker,E JBeckerand R G Brandes, Appl Phys, 32, 1960,411. A E Dabin and R E Stickney,J Vat Sci Technol, 9, 1972, 1032.s T W Hickmott, J Appl P hyi, 31, 1960, 128.9 L A Petermann and F A Baker, Br J Applphys, 16, 1965,487.I0 J G Werner and J H Lock, Vacuum, 19, 1969, 317.I D E Abey, J Appl Phys, 40, 1969, 284.I* S Datz. R E Minturn and E H Tavlo r. J Aool Phvs. 31. 1960.880.I3 fi F Winters, D R Denison, D G Bil1sand.EE Donaldson, j ApplPhys, 34, 1963, 1810.I4 J W Ackley, C F Lothrop and W R Wheeler,9th An Vat Sym p AmVa t Sot, Los Angeles I 962).Is P A Redhead, Trans A. VS. Vat Symp, 7, 1960, 108.I6 M Pirani, Hecrrochem, 11, 1905, 555:* I Weissman. Aool Phvs. 36. 1965.406.I8 G J Schulzand k V Phelps,Reu i Instrum, 28, 1957, 1051.I9 R W Lawson, Br J Appl P hys, 18, 1967, 1763.2oJ S Cleaves, Sci Instrum, 44, 1967, 969.2LJ D Buckingham, Er J Appl P hys, 16, 1965, 1821.

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the choice of cathode material in a hot cathode

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