Emitter corrosion in thermionic converters

Advanced Energy Conversion.

Vol. 3, pp. 137-156.

Pergamon Press, 1963.

Printed in Great Britain

EMITTER CORROSION IN THERMIONIC CONVERTERS* R. L. McKISSON~" Summary--It has been observed that under certain circumstances a relatively large amount of material has been transported from a molybdenum thermionic emitter to the collector. Because of the serious implications of this transfer on diode life, an analysis of the chemical system within a cesium diode has been made. The various chemical species expected, and their interactions under various conditions, are described. It is shown that a gross transfer of molybdenum is not likely in a system having only oxygen as an impurity. However, if hydrogen is also present within the diode, a mechanism for gross transfer of molybdenum is established, and a serious transfer problem will be produced. The analysis is extended to consideration of other emitter metals. The external corrosion of the molybdenum emitter sheath is a major detriment to the use of a fossil fuel flame as a thermionic generator heat source. However, since a flame-heated thermionic generator appears to be an attractive solution for certain specialized military tasks, this corrosion problem is currently being examined. Various protective coatings for molybdenum are described and discussed. Experimental results of tests of the silicon-base Durak-B coating are presented and analyzed.

INTRODUCTION THE PERFORMANCE characteristics o f a t h e r m i o n i c d i o d e are largely d e t e r m i n e d b y r a t h e r c o m p l e x interactions o f emission a n d p l a s m a physics p h e n o m e n a . The p e r f o r m a n c e life o f a t h e r m i o n i c d i o d e is determined, to a great extent, b y chemical a n d m e c h a n i c a l p h e n o m e n a . This study considers certain o f the chemical interactions which have a direct b e a r i n g on d i o d e service life. T w o areas will be investigated. The first o f these is the chemistry in the cesium v a p o r space; the second is the chemistry o f a m o l y b d e n u m disilicide protective c o a t i n g in a flame environment. CHEMICAL

BEHAVIOR

IN THE CESIUM

VAPOR SPACE

In some cases, the o p e r a t i o n o f a cesium thermionic d i o d e has led to the transfer o f a relatively large a m o u n t o f m o l y b d e n u m f r o m the emitter to the collector. In fact, this transfer has been so great t h a t the d i o d e elements were shorted o u t b y a foil o f the deposited metal. Since this p h e n o m e n o n is n o t regularly observed, it has been suggested t h a t the cause o f such transfer m i g h t be the presence o f extraneous gases within the affected diodes. The a m o u n t s o f m o l y b d e n u m transferred have been such that it is very unlikely that the t r a n s p o r t could have been a c c o m p l i s h e d b y a simple, once-through, transfer process; so t h a t some sort o f cyclic process is a l m o s t certain to have been the cause. Since b o t h oxygen a n d h y d r o g e n are the m o s t likely d i o d e c o n t a m i n a n t s , the following t h e r m o d y n a m i c analysis was m a d e to investigate the chemical b e h a v i o r o f the C s - O - M o a n d C s - O - H - M o systems u n d e r typical thermionic d i o d e conditions.

The C s - O - M o system First, let us consider the c e s i u m - o x y g e n subsystem with reference to its b e h a v i o r in a thermionic diode. The b u l k o f the cesium is present as liquid in the Cs reservoir, a n d a * Work supported in part by U.S. Office of Naval Research and by U.S. Army Signal Research and Development Laboratory. t Atomics International, Canoga Park, California. 137

138

R . L . McKISSON

small portion of it is present as vapor in the vapor space. The cesium vapor is primarily monatomic [1], with about 1 per cent Cs2(g) at 600°K. At higher temperatures, the Cs2(g) becomes relatively less stable, so that for practical purposes the vapor can be considered to be monomeric Cs(g) over the entire temperature range. At low temperatures, this vapor is an excellent getter for oxygen, and presumably much of the free oxygen present in the diode will find its way into the cesium (or into a more powerful "getter", if one is present). Oxygen, which is present in liquid cesium, exists in one of two forms. First, at low con30 20

-I0

oE - 2 0

"6 I

"6

-30 -40 -50 -6C -7C -8C -9C 50O

IO00 Temperature,

1500

2000

*K

FIG. 1. Free energies of formation.

centrations, it is probably dissolved as oxygen ions in the cesium solvent. As the concentration is increased, the solubility limit will be reached, and the oxygen will then be present as a cesium oxide phase in the metal. In his oxide review article, Brewer [2] reports the existence of several cesium suboxide species which are stable at low temperatures. These are CsTO, Cs40, Cs702, and Cs30. These oxide species have been identified by X-ray techniques, but all decompose to Cs20 below 450°K. Coughlin [3] gives thermodynamic data for 4 solid cesium oxides, Cs20, Cs~O2, Cs2Oa, and CsO2. Of these, Cs20 is the most stable and its equilibrium with Cs(g) and O2(g) will, in part, regulate the oxygen pressure within the diode. Its freeenergy of formation, shown on Fig. 1, indicates Cs(g) to be ineffective as an oxygen getter above 1500°K because of its low affinity for oxygen.

Emitter Corrosion in Thermionic Converters

139

With respect to the gaseous species over Cs20(s), Brewer [2] states that there is no evidence of any important gaseous alkali metal oxides, except possibly Li20 and LiO. The other M20 oxides vaporize by decomposition to the metal gas and oxygen. Therefore, the expected vapor species over a Cs-O mixture are Cs, with small amounts of Cs2, 02, and O. In cesium-vapor diode systems the cesium vapor pressure is regulated by the temperature of the cesium reservoir, which is normally held at 600 ° ± 25°K to give a nominal Cs vapor pressure of 5 × 10-3 atm. With this effective Cs pressure, the equilibrium partial pressure of 02 at 600°K is 10 89 atm. However, at 1000°K, the equilibrium 02 pressure is 4 × 10-5 atm; and at 2000°K it is 1-4 × 10+17 atm. Although these figures seem a little startling, they merely mean that at the relatively low temperatures of the Cs reservoir, there can be essentially no oxygen in the gas phase. At emitter temperatures, however, the Cs(g) present does not tend to react with oxygen, so that as far as the Cs interaction is concerned, a large amount of oxygen may be present in the gas phase. The molybdenum-oxygen subsystem is characterized by the presence of at least five solid molybdenum oxide phases, MOO2, Mo4Oll, MosO2a, Mo9026, and MoOa. Of these, only MoO2 and MoO3 are thermodynamically stable above 1300°K. MoO~ is the more stable, but at about 2000°K it disproportionates to Mo(s) and a (MoO3)x(g) species [2]. MoOa(s) is much less refractory, and melts at 1068°K. The solid is reported to vaporize as a mixture of polymeric species, (MoOa)a, (MoOa)4, and (MoOa)5 [4, 5]. In addition to these polymers. the gaseous species MoOa(g), MoO2(g), and MoO(g) are known. In general, only one (or perhaps two) of these will be major gaseous species at any given temperature and oxygen partial pressure. The free energies of formation of the more pertinent molybdenum oxide species are given in Fig. 1. The most stable Mo-O compound below 2000°K is MoOz(s). Up to 1500°K, MoOa(s) is the second most stable species; but above 1500°K, the gaseous trimer's stability is second to that of MoO2(s) and approaches it. At about 2000°K, their chemical stabilities (i.e. their -AFI's per g-atom of oxygen) are equal. These relations among the AFI's control the behavior of the Mo-O system, in that above 1500°K, MoO3(s) readily evaporates as (MoOs)n(g) and at temperatures above about 2000°K, MoO2(s) disproportionates to (MoOa)a(g) and Mo(s). This disproportionation is enhanced in vacuum, and because of it, molybdenum metal can be "outgassed" by heating in vacuum to 2000 ° to 2300°K. In a thermionic diode, then, the molybdenum-oxygen species of interest are MoO2(s), (MoOa)a(g), MoOs(g), and possibly MoO2(g). The partial pressure of 02 in the vicinity of a thermionic emitter will be controlled by the reaction Mo(s) 4- O~(g) = MoO~(s),

(1)

and the partial pressure of (MoO3)z(g) will be controlled by a reaction equivalent to 9 MoO2(s) = 3 Mo(s) + 2(MoO3)z(g).

(2)

In addition to the trimer, monomeric MoOn(g) will be present. The relative amounts of these species will depend upon the partial pressure of 02, so that as the p(O2) decreases, the relative amount of the monomer increases. The partial pressures of these species at thermionic emitter temperatures for the p(O2) determined by equation (1), are shown on Fig. 2. A ten-fold decrease in p(O2) is enough that the MoOa(g) species is more prevalent than the trimer. It should be noted that the vapor pressure measurements [4, 5] from which the stability of (MoOz)3(g) was computed were made below 1000°K. Therefore, a substantial

140

R.L. McKiSSON

uncertainty must be assigned t o the partial pressure of the trimer at the higher temperatures. In spite of this, it does seem likely that the trimer's stability is greater than that o f the monomer at emitter temperatures. Another effect of decreasing p(Oz) is that the lower molybdenum oxide species become more prominent. If the p(O2) is reduced a hundred-fold from the values predicted from equation (1), the trimer becomes unimportant, and MoO2(g) becomes a major molybdenum gaseous species. However, the partial pressures o f these species are substantially lower than those shown on Fig. 2, so that as the O2(g) in the system is reduced, the rate of transfer o f molybdenum is also reduced.

g_

It..

500

I000 Ternperot ure,

FIG. 2.

15OO

2000

°K

Equilibrium partial pressures of gaseous species.

In addition to its effect on the prominence of the various gaseous molybdenum oxide species, a decreasing oxygen pressure also markedly affects the equilibrium pressure of monatomic oxygen. For equilibrium conditions defined by equation (1), the oxygen is nominally 50 per cent monatomic. If the total oxygen pressure were reduced ten-fold, the major oxygen species would be monatomic oxygen. The conditions existing within a thermionic diode system containing oxygen can be deduced from the foregoing as follows. First, the cesium reservoir temperature controls the partial pressure of Cs(g) in the diode. The reservoir is normally held at 600°K, and at this temperature the Cs is an excellent getter for oxygen. Any oxygen which finds its way into the reservoir is permanently trapped there. The cesium vapor, at 5 × 10-3 atm pressure, acts as an oxygen pressure regulator in the low temperature regions of the diode. In the

Emitter Corrosion in Thermionic Converters

141

vicinity of the 1000°K collector, the maximum p(O2) which can exist is 6 × 10 -s atm. At emitter temperatures, the stability of Cs20(1) is so low that the Cs(g) does not react with oxygen. In the vicinity of the emitter, the molybdenum controls the oxygen partial pressure. At 1900°K, the maximum p(O2) is 10-8 atm (by equation (1)). Oxygen in excess o f this amount is converted to MoO2(s), which disproportionates to Mo(s) and (MoO3)8(g). At t900°K, the computed equilibrium partial pressure of (MoO3)8(g) over MoOz(s) is 10 2 atm, and p(MoOa)(g) is 10-6 atm. These pressures can be attained only if a separate MoOz(s) phase is present at the emitter. In the usual case, there is not enough oxygen present to exceed the solubility of 02 in the metal, and the developed partial pressures of the molybdenum trioxide species will be lower than the maximum values quoted, the MoO3(g) becoming the more important. Because of its chemical inertness at 1900°K, the adsorbed Cs on the emitter does not interact, and oxygen transfer from the emitter takes place only via (MoO3)3(g) or MoOa(g) evaporation. A stainless steel collector, operating at 1000°K, will be covered with an adsorbed monolayer of Cs atoms. This Cs can reduce either (MoO3)3(g) or MoOa(g) to MoO~(s) rather readily, and to Mo(s) to a small extent. Therefore, the collector can act as a sink for gaseous Mo species; so that if there is a source of oxygen in the system, the transfer o f Mo from the hot emitter to the collector would occur quite readily. The MoO2(s) which is produced may adhere to the collector surface, and may itself become covered with adsorbed Cs, so that the effective collector surface is the Cs atom layer. Although the MoO~(s) is not readily reduced by Cs, it can be at least partially reduced by the iron and chromium in stainless steel. If a molybdenum collector were used, the MoOz(s) would adhere to the surface and the oxygen would tend to slowly diffuse into the base metal. Thus, in this system the (MoO3)3(g) and MoO3(g) formed at the emitter can migrate to the collector and be reduced by cesium to MoO2(s). The Cs20 formed has an oxygen decomposition pressure of about 6 × 10-s atm, and some oxygen can therefore diffuse back to the emitter to react again. However, no more than one-third of the oxygen is able to follow this circuit, because that which remains on the collector as MoO2(s) is trapped. If the conditions were such that a significant fraction of the gaseous molybdenum species were MoO2(g), this species would be expected to deposit on the collector as MoO2(s) and would remain. In this case very little oxygen could be released to return to the emitter. Because of this major oxygen-capturing action of the collector, plus that of the cesium reservoir, it would seem highly unlikely that oxygen within the diode could itself enter into any extensive cyclic transport process. Only if there were a rather major source of oxygen present could the observed molybdenum transport be attributed to oxygen alone. In the usual diode system, there are two potentially significant sources of oxygen: one is the alumina insulator, the other is the oxygen content of the molybdenum emitter material. One can readily show that alumina is much more stable than Cs20(s, 1), so that, except for adsorbed gas on the ceramic at the outset, only a vanishingly small amount of oxygen could be removed from the insulator. However, if an intergranular corrosion e f A12Oz by Cs were to occur, the insulator might be penetrated with only a very small amount o f material being involved. If the emitter were the oxygen supply, there would have to be more than 5000 p.p.m. 02 in the metal to account for the observed 1 mil thick deposit. Since the emitters are made from arc-cast molybdenum stock having only a 10-20 p.p.m, quoted oxygen level, the observed molybdenum transport cannot reasonably be attributed to an oxygen cycle.

142

R.L. McKIssoN

The Cs-O-H-Mo system The addition of hydrogen to the Cs-O-Mo system introduces the possibility of forming several species in addition to those already examined. These are: CsH(s), CsH(g), CsOH(1), CsOH(g), HzO(g), and H2(g). No new molybdenum species are expected Solid cesium hydride is only moderately stable (Fig. 1) and has an equilibrium hydrogen partial pressure of 0.12 atm at 600°K [6]. CsH(g) is unstable [7] and will not be a significant gaseous species. The behavior of CsOH(s, 1) is more difficult to assess. The solid phase itself is quite stable, AF~ (298) = --87 kcal/mol CsOH [7]; and, in the absence of excess cesium, CsOH would be a major solid phase. However, if one considers the behavior of CsOH in Cs(1) to be analogous to that of NaOH in Na(1) [16], then any CsOH in the Cs reservoir should decompose to form Cs~O and CsH (or oxygen and hydrogen dissolved in liquid Cs). Thus, any CsOH(s, 1) in the cesium reservoir will probably act as a mixture of oxide and hydride, with the result that the oxygen would remain trapped in the reservoir, but the hydrogen would be at equilibrium with that in the diode space. Although no reports of any (CsOH)x(g) species were found in the literature, it is likely that both the dimer, Cs~(OH)2(g), and the monomer, CsOH(g), are stable at low temperatures. This conclusion is based upon the observations of Porter and Schoonmaker [8] that the species NaOH(g), Na~(OH)2(g), and Na(g) are found in the vapor over NaOH(1) at 600°K and the species KOH(g), K~(OH)2(g), and K(g) are found in the vapor over KOH(I) at 600°K. At higher temperatures, the monomer becomes relatively more stable and the amount of dissociation increases. It is very difficult to predict the chemical behavior of CsOH(g), but it seems likely that it will be less stable than either NaOH(g) or KOH(g). Since both NaOH(g) and KOH(g) show slight evidence of decomposition even at 600°K, one might reasonably conclude the CsOH(g) would be thermodynamically unstable at emitter temperatures. Estimates of the free energies of formation of CsOH(g) and CsOH(1) from Cs(g), H~(g), and O2(g) have been made and are shown in Fig. 1. From these data, one can predict the composition of the vapor over a system containing excess cesium at 600°K. The maximum p(H2) will be 0-12 atm, corresponding to a hydrogen-saturated cesium solution, and the p(Cs) will be 5 × 10-a atm. The interaction between Cs(g) and H20(g) to form CsOH(1) will tend to maintain a rather low partial pressure of water vapor in the vicinity of the reservoir (p(H20) is 2 × 10-9 atm for p(H~) = 0" 12 atm, but will be less for p(H2) _< 0-12). The CsOH(I) formed will decompose in the presence of large excesses of liquid Cs. At higher temperatures the stability of the CsOH species decreases markedly relative to that of H20(g) so that at collector temperatures, the equilibrium partial pressure of water vapor can approach 0.45 atm. Although the presence of hydrogen (or water vapor) affects some of the Mo-O equilibria, no new gaseous molybdenum species are expected. Hydrogen can readily reduce MoO3(s, I) and can partially reduce MoO2(s) at collector temperatures. This suggests that a cyclic process could be set up consisting of the variation in the reactions, MoO2(s) + 2H2(g) = MoO3(s) + 3H2(g) = Mo(s) + 3H20(s) = 3Mo(s) + 9HeO(g) =

2H20(g) ÷ Mo(s), 3H20(g) + Mo(s), 3H2(g) + MoO3(g), 9Hz(g) + (MoO3)a(g).

(3) (4) (5) (6)

At the emitter, reaction (6) proceeds as written above, and reaction (5) proceeds to a slight

Emitter Corrosion in Thermionic Converters

143

extent. At the collector, condensed molybdenum oxide species react with hydrogen by reactions (3) and (4). These reactions are all similar in that molybdenum oxide species are shown interacting with hydrogen, but the direction of the interaction at the different temperatures is dependent upon differences in the equilibrium constants for the reactions. The temperature dependence of these K-values for these four reactions is shown in Fig. 3. In order that the ratios o f p ( H 2 0 ) / p ( H 2 ) be of the same order for all reactions, the equations are written for the reaction of 9 moles of hydrogen to produce 9 moles of water as shown I 016

i014

i012

i0 'o

,o' Y

,o~ c o

Jo'

ou E

cy i,i

DO

tO-2

tO-4

10 -6

10-8

Note that R x ( 5 ) ~ a n d R x ( 6 ) ~ a r e to Rx(5) and R x ( 6 ) I 1000

I 1500 Temperature,

Rx(3)*~MoOz(s) + 9H2(g) -+ 9HzO(g) + ~Mo(s) K : pg(H20)/pg(H2)

opposite

I 2000 °K

Rx(5)*3MoOa(g)+ 9H2(g) --+ 9HzO(g) + 3Mo(s) K ~ p9(H20)/pg(H2)pa(Mo03)

Rx(4)*3MoOa(s) + 9Ha(g) ~ 9HzO(g) + 3Mo(s) ]{x(6)*(MoO3)3(g)+ 9Ha(g) ~ 9H~O(g) + 3Mo(s) K = pg(H20)/p9(Hz) K = pg(H20)/pg(H2)p [(MOO3)3] FIG. 3. Equilibrium constants of reactions. on Fig. 3. Note that this introduces an exponent of 9 on the water/hydrogen partial pressure ratio but does not introduce any fractional powers on the other species. Therefore, a K-value of 109 for reactions (3*) and (4*) means the p(H20)~p(H2) ratio is really 10. Inspection of Fig. 3 then shows that at the collector, reaction (4) is favorable and MoO3(s) should be readily reduced by hydrogen. This tends to increase the (HzO)/(Hz) ratio in the interelectrode space. Reaction (3) is much less favorable, but could proceed if the local (HzO)/(He) ratio were low. At the emitter, reaction (6) applies and will tend to reduce the (HzO)/(Hz) ratio to be compatible with its K-value. Of course, the extent of the reaction which actually occurs in any given system will depend upon the total amount of hydrogen and;or oxygen available. As the water vapor reacts at the emitter, it forms H2(g) so that the ratio

144

R.L. McKISSON

p(H20)]p(H2) rapidly decreases as (MoOa)s(g) is formed. However, the gross thermodynamic calculations indicate the direction of the chemical reactions, and there seems to be no doubt that a "water vapor cycle" is thermodynamically possible in this system. The conditions existing within a thermionic diode system containing hydrogen and oxygen impurities can now be deduced from the foregoing discussion. The cesium reservoir temperature again controls p(Cs) and the associated equilibria, as described previously. In addition, the cesium reservoir acts as a source or sink for hydrogen. When a diode is first operated and hydrogen impurity is released into the vapor space, it quickly diffuses into the liquid cesium. Until the cesium is saturated with hydrogen, the hydrogen partial pressure will be suppressed. When saturated, the Cs reservoir becomes a hydrogen pressure regulator; and at 600°K, the saturation p(H2) is 0.12 atm. With hydrogen and oxygen present, water vapor is quickly formed. Its pressure will be controlled in the vicinity of the cesium reservoir by, its reaction with Cs(g), and the maximum p(H20) will be 2 × 10-9 atm in this region. In the higher temperature regions, the p(H20) will be controlled by p(H2) and the various molybdenum reactions involved in the water-vapor cycle. The emitter will react with water vapor to form MoO2(s), (MoOa)a(g), and hydrogen until the (H20)/(H2) ratio is reduced. Then the H2(g) and the (MoOs)3(g) will diffuse to the collector. The (MoOa)3(g) will be cooled and will tend to condense to MoOa(s). Then reaction (4) can begin to produce Mo(s) and water vapor. (Reaction (6) could also occur in the gas phase because of its large difference in K between the emitter and collector temperatures.) Although this cycle is described as a sequential operation, a steady-state process will be established and diffusion gradients of all the gaseous species will be produced. Hydrogen gas and the (MoOa)n species will diffuse from the emitter to the collector, and H20(g) will diffuse in the opposite direction. The result will be a steady transfer of molybdenum from the emitter to the collector, and since there is no loss of oxygen or hydrogen in the water vapor cycle sequence, small amounts of these gases can transport large amounts of molybdenum. One of the interferences in this cycle is the Cs20-H20(g) reaction. Although the product, CsOH(I), will tend to exchange with H2(g) to form Cs(g) and H~O(g) it is expected that the decomposition of the hydroxide will be slow. The formation of Cs~O by the cesium reduction of, say, MoOs(s) will then almost certainly result in the immediate absorption of water to form CsOH(1). Although this compound forms and persists in areas away from the Cs reservoir, its effect on the system is that a steady-state concentration will be set up and maintained. The partial pressure of water vapor will be depressed, but it is virtually impossible to predict the extent of the depression. It appears that the only processes which would be effective in quenching the cyclic process would be those which transport oxygen into the liquid cesium, or product CsOH(1) at 800°K or lower. The most effective oxygen-removal process thus seems to be the diffusion of water vapor toward the cesium reservoir. However, the usual close-spaced diode geometry appears to minimize the effect of this oxygen-loss process, and to favor the cyclic water vapor process. There seems to be no trap for hydrogen once the liquid cesium is saturated, so the limit on the amount of molybdenum transferred is the average number of cycles an oxygen atom makes before it is trapped. Since the source of the oxygen is almost certainly the emitter and the source of the hydrogen is probably the collector, and these structures are quite closely spaced, it seems likely that many cycles could take place before a water vapor molecule escaped the interelectrode space.

Emitter Corrosion in ThermionicConverters

145

Expected behavior of other emitter materials Although this analysis has been directed toward a molybdenum emitter, its results provide a reference upon which to base predictions of the behavior of other emitter materials. For example, a tungsten emitter, for which the high-temperature chemistry is quite similar to that of molybdenum, would be expected to behave as does molybdenum, with equivalent transport rates at 100°-200°C higher temperatures. Tantalum and niobium emitters would not be expected to show serious material transport behavior because their solid oxides are much more stable than is water vapor. Therefore, the hydrogen reduction process should not take place readily at the collector, so that the oxides which arrive at the collector will tend to condense and remain there as oxide. Rhenium and iridium both appear to have relatively unstable solid and gaseous oxides. Because of this, the metals cannot readily react with water vapor to form volatile oxides at the emitter. Therefore, one should not expect to find a serious metal transport problem with either a rhenium or an iridium emitter. It therefore appears that of the most desirable thermionic emitter materials, only molybdenum and tungsten can present a serious emitter metal transport problem. If a getter is employed to absorb hydrogen, and if the diode parts are well outgassed before assembly, the amount of transport by this process can be held to a negligibly low value. This technique has been found to be successful, and is currently in use at Atomics International. Since its inception, the transfer of molybdenum in our diodes has not been observed. CHEMICAL BEHAVIOR OF MOLYBDENUM DISILICIDE IN FLAMES During the past few years, a variety of protective coatings for molybdenum has been investigated. Several coatings have proven satisfactory for service temperatures up to 1400°K (about 2000°F), but only the molybdenum disilicide-base coatings have shown promise for higher temperatures. Siliconized molybdenum, or molybdenum disilicide, is reported to protect molybdenum in air for substantial periods of time [10]. When silicon is applied to the molybdenum surface at high temperature, it forms a series of Mo-Si intermetallic phases. The silicon first converts the outer surface to MoSi2 and then diffuses inward to form a layer of MosSia, and finally MoaSi as the innermost layer. When this layered structure is exposed to air, the MoSi2 oxidizes to MoOa(g) and SiO2(g,1). The MoOa(g) evaporates, but the SiO2 glass layer builds up until the rate of diffusion of oxygen to the underlying metal is very low. When the SiO2 glassy layer is lost or becomes thin, it can repair itself. Although silicon has often been used alone as a coating agent, a small amount of boron is sometimes used as a ternary additive. Tests in quiet, dry air indicate that the Si-B mixture is superior to plain Si [10]. Silicide coatings are readily produced by vapor decomposition and by the pack method. Coating thicknesses of 1-3 mils are normally used, and the protection-period is nominally proportional to the coating thickness. Vapor deposited coatings are reported to protect molybdenum in stagnant air for 3700 hr at 1375°K (2000°F), 200 hr at 1650°K (2500°F), I00 hr at 1925°K (3000°F), and 30 hr at 2125°K (3360°F) [10]. One of the best of the pack-method coatings is the Durak-B coating developed by the Chromizing Corp. of Hawthorne, Calif. [9]. This coating is reported to offer 4000 hr protection in stagnant air at 1375°K (2000°F), and 2000+ hr protection at 1775°K (2700°F) [ll]. Because of its superior high-temperature performance, the Durak-B coating has been 6

146

R.L. McKIssoN

chosen for use in our flame-heated thermionic program, and has undergone a series of evaluation tests to determine its service life. In addition, a thermodynamic evaluation of the MoSi2-flame component system has been made, and is summarized in the following section. Thermodynamic

analysis

The chemical interactions of possible importance in the corrosion of MoSiz-protected molybdenum are of two types. First, there are the external reactions by which the SiO~ layer is attacked by the components of the gas phase. These are listed below with their estimated standard free energies, and the resultant idealized equilibrium partial pressures of the silicon-bearing gaseous species. SiO2(gl) -----SiO(g) + ½02(g) A F ° (1800°K) = + 7 6 kcal; p(SiO) = 6 × 10 -9 atm A F ° (2000°K) ---- + 6 4 kcal; p(SiO) = 9 × 10 -a atm (in an atmosphere with p(O2) = 10 -z atm)

(7)

SiO2(gl) + H20(g) = (SiO-H20) (g) + ½02(g) A F ° (1800°K) = + 5 9 kcal; p ( S i O - H 2 0 ) = 5 × 10 - s a t m A F ° (2000°K) = + 5 4 kcal; p ( S i O - H 2 0 ) = 1 × 10 -e atm (in a gas flame with p(HzO) = 0. 135 atm, and p(Oz) = 10 -2 atm)

(8)

SiO2(g,1) + CO(g) = SiO(g) + COs(g) A F ° (1800°K) = + 4 6 kcal; p(SiO) = 3 x 10 -7 atm A F ° (2000°K) = + 3 8 kcal; p(SiO) = 6 x 10 -6 atm (in a gas flame with p ( C O z ) / p ( C O ) = 11)

(9)

SiO2(gl) + CO(g) = H20(g) = SiO-HzO(g) + COz(g) (1800°K) = + 2 9 kcal; p ( S i O - H 2 0 ) = 4 x 10 -° atm A F ° ( 2 0 0 0 ° K ) = + 2 8 kcal;p(SiO-H20)----- 1 × 10 -5 atm (in a gas flame with p ( C O 2 ) / p ( C O ) = 11, and p(H20) ----0. 135 atm)

(10)

AF °

The second group of reactions are those by which the silica glass may interact with the underlying metal phase. These are listed with their estimated free energies and the resultant equilibrium partial pressures of the silicon-bearing gaseous species. 3 MoSiz(s) + 15 SiO2(g,l) ----21 SiO(g) + (MoOa)3(g) (1800°K) = + 5 8 4 kcal; p(SiO) = 7 × 10 -4 atm A F ° (2000°K) = + 4 0 4 kcal; p(SiO) = 0.012 atm

(11)

5 MoSiz(s) + 7 SiO2(gl) = 14 SiO(g) + MosSi3(s) A F ° (1800 °K) = + 192 kcal; p(SiO) = 0.022 atm A F ° (2000°K) = + 8 7 kcal; p(SiO) = 0-21 atm

(12)

3 MosSi3(s) + 54 SiO2(g,l) = 63 SiO(g) + 5 (MoOa)8(g) AF o (1800°K) = +2346 kcal; p(SiO) = 8 × 10 -5 atm A F ° (2000°K) = +1737 kcal; p(SiO) = 2 × 10.8 atm

(13)

3 MosSi3(s) + 4 SiO2(gl) = 8 SiO(g) + 5 MoaSi(s) A F ° (1800 °K) = +158 k c a l ; p ( S i O ) = 4 × 10-a atm A F ° (2000°K) = + i 0 0 kcal;p(SiO) = 4 × 10 -z atm

(14)

AF °

Emitter Corrosion in Thermionic Converters

MosSi(s) + 10 SiO2(g,1) = 11 SiO(g) + (MoO3)3(g) AF ° AF °

147

(15)

(1800°K) = +434 kcal; p(SiO) = 5 × 10 -5 atm (2000°K) = +326 kcal; p(SiO) = 1 × 10-3 atm

Mo3Si(s) + SiO2(g,l) = 2 SiO(g) + 3 Mo(s)

(16)

AF ° (1800°K) = + 4 2 kcal; p(SiO) = 3 × 10-a atm A F ° (2000°K) = + 2 7 kcal; p(SiO) = 0.032 atm Inspection of these reactions shows several interesting things about the protection mechanism. Reaction (7), which is the most likely path for the direct evaporation of silica, suggests that while silica will evaporate under mildly oxidizing conditions, the rate would not be expected to be too severe (a partial pressure of 10- s a t m would give an observable loss rate under Langmuir conditions in a vacuum, but, under the suppressive influence of an atmosphere, the rate would not be important in terms of a few hundred hours protection life). Reaction (8) suggests that the tendency to form a hydrated gaseous silicon species will markedly decrease the protection-life of a SiO2 glass film. At present there is some doubt regarding the thermodynamic properties of the (SiO-H20) species. The free energy values derived here are based upon the work of Elliott [12] who investigated the properties of several hydrated gaseous molecules. His work on SiO-H20(g) was not exhaustive, however, and the A F ' s must be assigned a large uncertainty. That silica and compounds of silica do exhibit enhanced volatility in the presence of water vapor is well known, so that the partial pressure of the gaseous hydrate would be expected to be greater than that of the vaporization product SiO as given by reaction (7). The question here is whether the difference should be as large as it appears. In any event, reaction (8) appears as one probable mechanism for the loss of silica from the glassy phase of these coatings. In a flame-heated system, one can reasonably assume that, as a first approximation, the rate-determining step in the mechanism of the loss o f SiO2 is the diffusion of the SiO-HzO(g) species through the gas boundary layer into the bulk flame stream. On this basis, one can obtain an estimate of the rate of loss of SiO2, using the technique described by McKisson [13]. The predicted loss rate of SiO2 for the 1800°K conditions of reaction (8) is 7000 hr/mil. Reactions (9) and (I0) illustrate the effects of a reducing condition upon the SiO2. Because of the stability of SiO(g) and SiO-HzO(g), SiO2(gl) can be attacked by reducing agents such as CO. The expected partial pressure of SiO-HzO(g) is higher if reaction (10) can proceed, and one predicts that the reduction of SiOz by CO in a flame will be an important corrosion reaction. For the conditions p(CO) = 0.01 atm, p(CO2) = 0.11 atm, and p(H20) = 0. 135 atm, the equilibrium partial pressure of SiO-H20(g) at 1800°K is 4 × I0 -0 atm. For this partial pressure, the corrosion rate based upon the boundary layer diffusion concept is estimated to be about 150 hr/mil. However, equation (10) probably does not take place as written (3-body reaction), but is more likely to occur as a reduction reaction (reaction (9)) followed by a hydration of the SiO(g). Under these conditions it is difficult to define the diffusion system, so that the corrosion rate noted above is quite uncertain. However, this equilibrium does suggest that service lives of these coatings may be limited to a few hundred hr at 1800°K in a reducing flame. The computed corrosion rate from reaction (9) is about 2000 hr/mil coating thickness. The second set of equations compares the expected behavior of the three Mo-Si compounds in contact with SiO2(gl). According to Searcy and Tharp [14], MoSi2(s) is the least stable of the three molybdenum silicides. This, of course, is reflected in the higher p(SiO)

148

R.L. McKiSSON

computed for reaction (I1) than for reactions (13) and (15). Furthermore, all six of the reactions in this group yield a higher computed p(SiO) than those in the previous group. This merely means that the intermetallic compounds are better reducing agents than CO, but does not necessarily mean that the pressure of SiO will reach the computed values. The most probable effect will be that some SiO-type species and some MoO3 will dissolve in the glassy phase and tend to suppress further reaction. However, as the thermodynamic activity of these species becomes equivalent to a p(SiO) of 10-2 atm at the interface, one would expect that a diffusion gradient would be set up across the glass layer from this value to the 10-9 value at the outer glass surface. Concurrently, there will also be an oxygen pressure gradient from the outer surface of the glass to the interface. Presumably then, these two species could interact to produce additional SiOz(g,1) within the body of the glass. The extent of this reaction will depend upon the partial pressure of oxygen at the surface and the relative magnitudes of the diffusion coefficients. In oxidizing flames, the "SiO" diffusion would be suppressed; but, in reducing flames, it would be expected to be appreciable. As a result, the effective partial pressure of SiO at the glass-reducing flame interface could be greater than the 3 × 10-7 atm resulting from reaction (9). In such a case, the rate of corrosion would be increased, perhaps ten-fold, so that the rate might be as high as 200'hr/mil MoSi2. In view of the relative thermodynamic stabilities of the molybdenum silicides, it is interesting that the least stable of them presumably offers the best protection. The reason for this anomoly may be that the ratio of SiO2 to (MOO3) or (MoOa)3 produced in the oxidation is greatest for MoSi2. The slightly volatile silica can presumably form an adherent coating when only a relatively small amount of (MoOa)n must be allowed to evaporate. In the case of the oxidation of MoaSi, there is six times as much of the (MoOa)n gas to be evaporated as for MoSi2, so that the sheer volume of gas which must leave the surface prevents the adequate adhesion or wetting by the SiO~(g,1). PERFORMANCE DATA With the foregoing as a basis, it is of interest to examine and compare the corrosion lives of various molybdenum disilicide coatings reported in the literature. Figure 4 shows a plot of these data. Because of the scarcity of data and the lack of a definitive characterization of most of the coatings, the individual points are plotted without differentiation. In general, the coatings ranged from 1-3 mils in thickness, but, in general, the nominal coating thickness is a less important factor than is the uniformity of the coating, and the absence of imperfections. Many early failures are attributed to edge-effects, and it is quite important that all corners have a minimum radius of about ~ in. On smaller radii, the coatings on the edge surfaces tend to expand toward the corner to meet and tend to lift one another away from the fiat surfaces. On larger radii, the coating follows the curve and has less tendency to separate. The data shown in Fig. 4 generally follow a reaction-rate trend (i.e., there is a suggestion that a straight line "fits" the data, when plotted as reciprocal hours to failure vs. °K-l). This curve suggests that the service life should be about 100 hr at 2000°K, and about 400 hr at 1600°K. Until additional data are made available, the single point for the boron additive (1400 hr at 1900°K) must be regarded with hopeful caution. Figure 5 shows the air-exposure data taken at Atomics International for the Chromizing Corporation's Durak-B coatings. The samples were i in. diameter molybdenum wire about 5 in. long. Two coating thicknesses were tested, a nominal 1½rail coating, and a

Emitter Corrosion in Thermionic Converters

]49

nominal 2½ mil coating. The samples were electrically heated by their own resistance using a Variac and stepdown transformer power supply. The sample temperatures were read using an optical pyrometer, and the observed temperatures corrected for an assumed emissivity of 0.6. The test apparatus used is shown in Fig. 7. The data presented on Fig. 5 are shown with the identifying symbol located at the nominal test temperature and a horizontal line to an X at the temperature of the sample region at which failure occurred. Consequently, those samples failing at the center do not have a line. The test data show a definite grouping as a function of coating thickness, and it is encouraging that the 2½ mil Durak-B coating life appears to be rather uniformly superior to the previously reported air-exposure data. oK

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Protection life of MoSi~ coatings, exposure in air.

However, since our particular needs are for a coating which can withstand a flame environment, the data of Fig. 6 are being developed. Thus far, there appears to be no difference in the performance in a flame between the two coating thicknesses, and all of the samples tested in CO and in a Fischer-burner flame (with supplemental electrical resistance heating) show essentially the same protection-life as was observed for the 1½ mil airexposure samples. N o reason for the lack of a difference in performance between the 1½ and the 2½ mil samples in these tests is not now apparent. However, the obviously shorter service-lives do substantiate the results of the theoretical treatment. During the testing, one interesting phenomenon occurred with rather exasperating regularity. This was that the samples set up for testing at a nominal 1850°-2150°K would fail

150

R.L. McKassoN

more often than not in a lower temperature region. (This has been accounted for in plotting the data in Figs. 5 and 6.) In most cases the failures would occur in the 1650°-1750°K zones. The reason for these "premature" failures is not clear, but it is postulated that the glassy silica protective layer is at fault. This temperature range corresponds to that in which the silica glass-to-cristobalite transition is known to be relatively rapid. If such a change were to occur, the crystalline cristobalite would be much more susceptible to mechanical and thermal shocks than is the glassy silica. It would also be expected to be more permeable to oxygen than is the glassy silica. At higher temperatures, the recrystallization rate would be °K 104

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Fzo. 5. Protection life of Durak-B coating in air. greater, but the rate of formation of new silica would also be more rapid, so that the deleterious effects of a mostly crystalline protective phase would not appear. At the highest temperatures, the silica is above its melting point, and ultimately the problem will become one of preventing the flow of the glass from thinning the coating too much in the hot zone. At the lower temperatures, the transition rates and the diffusion rates are lower, and apparently do not cause difficulty. WachteU has also observed a tendency for a minimum in the service life vs. temperature curve [9]. He reports the minimum in the curve for the W-2 coating exposure-life in air to be at 1700°K (2600°F), but his highest test temperature was 1750°K; so that it is difficult to establish a good picture of the higher temperature performance.

Emitter Corrosion in Thermionic Converters

15 !

A technique by which this intermediate-temperature failure may be controlled is currently under study. Basically, it consists of using the beneficial effect of having a molten (but highly viscous) protective coating in the critical region. A series of glasses of varying melting points are being screened in an effort to prevent or regulate these premature failures. No definitive results are presently available. Since, at present, the desired coating protection life of a nominal 500 hr in a flame is not attainable, and, since the service life of a flame-heated diode should not be too deoK I0'

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FIG. 6. Protection life of Durak-B coating in flame components. pendent upon the corrosion life of the emitter's protective sheath, the present solution is to use an expendable silicon carbide liner upon which the flame will impinge. By preventing direct flame contact on the MoSi2, we hope to extend the service life of the Durak-B coating and the diode. Figure 8 illustrates the nature of the problem and Fig. 9 shows a Durak-B coated surface which was protected by a SiC wafer. Another process by which the protection-life of a MoSi2-coated surface may be limited is the diffusion of the silicon from the coating layers into the base metal. This phenomenon is discussed by Wachtell [9], who shows electron microprobe analyses of the Chromalloy Corporation's W-2 MoSi2-base coating. The results of his analysis are summarized in Fig. 15. Upon heating, the original MoSi2 disappears and a MosSia layer forms between the

152

R.L. McKissoN

MoSi2 and the molybdenum. After a 22 hr exposure at 1750°K, only a small amount of the original disilicide remains. These data clearly indicate the inward transfer of the silicon from the MoSiz to form MosSis, but the data show only a very slight indication that the silicon is diffusing into the substrate molybdenum. Since diffusion is time and temperature dependent, one must conelude that longer and higher temperature exposures would show an increase in the inward movement of "dissolved silicon". Some indirect evidence o f this sort of behavior is illustrated in the photomicrographs shown in Figs. I0, 11, 12, 13, and 14, and in the summary of Fig. 15. Figure 10 shows a

I

¸

A Pr p ° . coating. 9

Microprobe Analysis, ~achtell g'-2 Coating Exposed 10 hr in Air at 1750~K, Microprobe Analysis, Wachtell9 ¢~-2Coating Expoled 22 hr in Air at 1750~K, Microprobe

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LEGEND OF PHASES

Base (Ho3Si) molybdenum

(HosSi3)

HoSi 2

SiO 2

FIG. 15. Summary of MoSi2 coating behavior after air exposures.

photomicrograph of an as-received 1½ mil Durak-B coated molybdenum wire sample. The coating appears as a single layer, and the hardness profile of the specimen is relatively uniform across the molybdenum core at a K H N (50 g) value of 355 -q- 32. Figure 11 shows a typical result of a 50 hr air exposure at 1850°K on the 1½ mil coating. Four layers appear with thicknesses and compositions as shown in Fig. 15. The layers are tentatively identified as the outer SiO2 protective layer, the remainder of the MoSi2 layer, a layer of columnar crystals which is probably the MosSi3 layer reported by Wachtell [9], and a fourth layer which appears in samples heated more than about 25 hr. This lowest-lying layer is thought to be a MosSi phase. The hardness profile of the core is rather uniform at a K H N (50 g) value of 235 ~2 11. These observations indicate that, up to 50 hr exposure at 1850°K, the silicon continually moves inward to form the various Mo-Si compounds in turn. The next step would be the

FIG. 7. Corrosion tcst apparatus.

FIG. 8. Simulated cathode, after 3 hr exposure at 1335 K, plus 1 hr at 1725 K in a reducing flame. []ktcinL, p

152]

Fits. 9. Simulated cathode, protected by a SiC wafer, after 16 hr above 1675 'K in a reducing flame.

Fic. 10. As-received 1½ mil Durak-B coating.

Flta. 11. 1½ mil Durak-B coating after 50 hr exposure in air at 1850 'K.

FIc. 12. As-received 2~ mil Durak-B coating.

F~c;. 13.2½ rail Durak-B coating after 155 hr exposure in air at J850 K,

F~(;. 14, 2½ rail Durak-B coating after 420 hr exposure in air at 2100 'K

Emitter Corrosion in ThermionicConverters

153

formation of a Mo-Si solution in the core, but the hardness profile suggests that this process has not begun in this sample. Figure 12 shows a photomicrograph of an as-received 2½ mil Durak-B coated molybdenum wire sample. The coating again appears as a single layer, and the hardness profile of the specimen core is uniform at a KHN (50 g) value of 275 ± 27. Figure 13 shows the effects of heating the 2½ rail coating in air for 155 hr at 1850°K. Three coating layers appear. The outer layer is the protective SiO2 glass, which in this sample is rather thin; the middle layer is the columnar crystal phase, MosSi3; and the third layer is presumed to be a MosSi phase. The hardness profile of the core shows a somewhat higher KHN (50 g) hardness value (290) near the interface than is found for the bulk of the core (220 ~ 11) which suggests that the silicon has begun to diffuse into the base metal in 155 hr at 1850°K. Figure 14 shows the effects of heating the 2½ mil coating in air for 420 hr at 2100°K. Two coating layers appear. The outer one is bubble-filled silica. The inner layer appears to be the Mo3Si layer as is seen in Fig. 11. This two-layer structure is typical of the samples heated above 2000°K. The most likely explanation for the many bubbles in the silica layer is that the metal-silica reaction products are gaseous, and have a total pressure high enough to produce bubbles at the interface. At lower test temperatures, the partial pressures of the gaseous species are lower, and the applied atmospheric pressure prevents bubble formation. Above 2000°K, however, the bubbles are obviously able to form, and then they slowly work their way out of the glass. Thus, the silica layer has the observed lava-like appearance. The hardness profiles of these high temperature samples are quite different from those exhibited by the 1850°K samples, and are characteristically hardest at the surface of the core and become softer toward the center. As the exposure times increase, the diffusion shell thickness increases. For the sample shown in Fig, 14, the centerline KHN (50 g) hardness value is 230; the value at the interface is 630; and at the quarter-point, it is 280. Figure 15 summarizes the protection coating phase data, and suggests a mechanism for the protective behavior of the molybdenum disilicide coatings. First, the techniques for depositing silicon lead to the immediate formation of the disilicide on the surface. As the samples are put into service, the MoSi~. oxidizes, the molybdenum escapes as (MoO3)• and the silica forms a continuous sheath. The thickness of the sheath increases to a steady state value at which there is a steady loss of silicon as SiO and SiO-H20, and an equal rate of production of the SIO2. Concurrently, the silicon in the MoSt2 diffuses into the underlying molybdenum and the MoiSt3 phase forms. This process is thermodynamically favorable and it proceeds so that after a time there is no MoSt2 phase remaining. The movement of silicon continues, however, and the Mo3Si phase begins to appear after several tens of hours. At the lower temperatures, the rate of development of this phase is rather slow; but, at the higher temperatures at which the diffusion of silicon is more rapid, the MoaSi phase develops early, and in time may be the only intermetallic phase remaining. Once Mo3Si is formed, the silicon continues to move into the core by diffusion. No additional phases can be formed so that the silicon then appears as a solute. This explanation is consistent with all the known facts. However, Searcy [17] reports that the Mo-Si compositions of lower Si content than MoSt2 and MosSi3 are markedly less protective than is MoSt2; and indeed they may be, if exposed bare. The relatively rapid formation of MoOz species could very likely prevent a properly adherent SiO2 layer from developing. However, in the sample of Fig. 14, we see no evidence of any Mo-Si protective layer other than Mo3Si. This suggests that if a protective silica layer is formed over either MoSiz or MosSi3, which intermetallics are then converted to MoaSi, the existing silica layer can be maintained.

154

R . L . McKISSON CONCLUSIONS

That the transfer of molybdenum from the emitter to the collector of a thermionic diode can readily occur in a system having both oxygen and hydrogen impurities appears a certainty [18]. The extent to which this phenomenon will occur will be strongly dependent upon the amounts of the impurity gases present. Either gas alone is not expected to cause an extensive molybdenum transfer, although really large amounts of oxygen could cause difficulty. Of the other interesting emitter materials, only tungsten is expected to show significant material transport. Its chemistry is similar to that of molybdenum, but its transfer rate is expected to be lower than that of molybdenum. Tantalum, niobium, rhenium, and iridium are not expected to enter into a cyclic transport process in oxygen and hydrogen. One method of minimizing the transport of molybdenum is to install a hydrogen getter (titanium or zirconium) in the diode and to thoroughly outgas each diode part during fabrication. The corrosion of a molybdenum disilicide protective coating by flame components is found to be rather severe at thermionic diode temperatures [19]. The difficulties are that the silica glass, which develops as a protective layer, can react with the underlying molybdenum and with the flame components to form volatile decomposition products. Further, there is a tendency for the silicon in the intermetallic compounds to diffuse into the underlying molybdenum at high temperatures. However, while all of these processes are undoubtedly sources of failure, the most prevalent failure-locations in the high temperature tests are the moderate temperature regions. The prevention of these failures is being attempted by the application of multicomponent glasses of high viscosity in the critical regions, but no definitive results are now available. Until a suitable technique is found, and the molybdenum disilicide coated surfaces can be exposed to the flame with confidence, a silicon carbide liner can be used in the areas of direct flame impingement. Acknowledgements--The author wishes to acknowledge the support of this work by the Office of Naval Research, under Contract Nonr-3192(00), and by the U.S. Army Signal Research and Development Laboratory, under Contract DA-36039 SC-88982. I wish to acknowledge the contribution of B. D. Pollock, whose comments on the text are greatly appreciated. I am also greatly indebted to W. R. Martini for his helpful discussions and comments during the course of these studies, and in the preparation of this report. In addition, I would like to acknowledge the contributions of E. V. Clark, not only in carrying out much of the laboratory work, but also in the evaluation and interpretation of the corrosion data. REFERENCES [1] D. R. STULL and G. C. SINKE, Advances in Chemistry Series No. 18, Amer. Chem. Soc., Washington (1956). [2] L. BREWER, Chem. Rev. 52, 1 (1953). [3] J. P. COUGHLIN,Bull. Bur. Min. No. 542 (1954). [4] R. J. ACgERMANN, R. J. THORN, C. ALEXANDERand M. TETENBAUM,J. Phys. Chem. 64, 350 (1960). [5] P. E. BLACK~URN,M. HOCH, and H. L. JOHNSTON,J. Phys. Chem. 62, 769 (1958). [6] A. HEROLO, C.R. Acad. SeL, Paris 228, 686 (1949). [7] F. D. RossiNi, et. al., Selected Values of Chemical Thermodynamic Properties, NBS Circular No. 500 (February 1952). [8] R. F. PORTEntand R. C. SCHOONMAKER,J. Chem. Phys. 28, 168 (1958); J. Phys. Chem. 62, 234 (1958); and J. Phys. Chem. 62, 486 (1958). [9] R. WACHTELL,Protective Coatings for Molybdenum, from the Ceramics and Composites, Coatings and Solid Bodies Sympos., Soc. Aerospace Materials and Process Engrs Meeting, Nov. 14, 15, 1961, Dayton, Ohio. [10] E. S. BARTLEa-r,H. R. OGDEN, and R. I. JAFFEE,Coatings for Protecting Molybdenum from Oxidation at Elevated Temperature, DMIC Report 109 (ASTIA AD 210978; OTS PB 151064) (March 1959). [11] A. FLETCHER,Chromizing Corp. Sales Engineer, private communication. [12] G. R. ELLTOTT,Gaseous Hydrated Oxides, Hydroxides, and other Hydrated Molecules, UCRL 1831 (June 1952).

Emitter Corrosion in Thermionic Converters

155

[13] R. L. McKISSON,An Evaluation of the Beryllia-Water Vapor Reaction in an Open-Cycle Air-Cooled Reactor, NAA-SR-3619 (August 1959). [14] A. W. SEAnCYand A. G. TnA~, J. Phys. Chem. 64, 1539 (1960). [15] W. D. KLOOP,Review of Recent Developments on Oxidation-Resistant Coatings for Refractory Metals, DMIC Memo 102 (April 1961). [16] C. B. JACKSON,Ed. Liquid Metals Handbook, Sodium-NaK Supplement, p. 19. TID 5277 (July 1 1955). [17] A. W. SEARCY,J. Amer. Ceram. Soc. 40, 431 (1957). [18] Additional details may be found in: N. S. RASO~, Ed., First Summary Report of Basic Research in Thermionic Energy Conversion Processes for Nonr-3192(00), Paper B-6, AI-6799 (November 1961). [19] Additional details may be found in: W. R. MARTINI,R. L. McKIssoN, and R. G. HO~, Flame Heated Thermionic Converter Research, First Quarterly Report, AI-6815 (April 1962); and W. R. MAI~a~rNt and R. L. McKISSO~, Flame Heated Thermionic Converter Research, Second Quarterly Report, A16981 (April 1962).