Evaluation of thermionic emitter surfaces

Advanced Energy Conversion.

Vol. 7, pp. 201-218.

P e r g a m o n Press Ltd., 1968.

Printed in Great Britain

E V A L U A T I O N OF T H E R M I O N I C EMITTER SURFACES L. VAN SOMEREN,~"D. LIEB,~" G. MISKOLCZY* and S. S. KITRILAKIS~"

(Received21 January 1966) Abstract--This paper describes an experimental evaluation of cesiated tungsten and cesiated rhenium thermionic emitter surfaces prepared by different methods. Two sorts of evaluation are presented. In the first, the average emission characteristics of these surfaces are described by graphs of cesiated work function vs the ratio of surface temperature T to cesium reservoir temperature Tn. Details of the work function distribution for certain surfaces are also obtained and related to the average characteristics. In the second method, surfaces are evaluated in terms of the electrical output of thermionic converters utilizing these surfaces as emitters (cathodes). For certain materials curves of power density, and of current density, vs. output voltage are presented, where each curve has cesium reservoir temperature and interelectrode spacing optimized, and emitter temperature appears as a parameter. Other curves show output power density as a function of emitter temperature, at specified output voltage and spacing. INTRODUCTION SIx VARIABLESinfluence the performance of a cesiated thermionic converter--the condition of the emitter and collector surfaces (including both the material and its treatment), the temperatures of emitter and collector, TE and Tc, the interelectrode spacing, d, and the cesium reservoir temperature, TR. The converter designer can usually control Tc, TR, and d. The emitter temperature is determined by the heat source in use. The collector surface condition has been studied little, and its effects are not yet clear. This paper is concerned with the way in which emitter surface condition enters into converter performance as measured by electrical power production. In dealing with a multivariate system such as this, it is often useful to reduce the number of independent variables by combining some of them into a dimensionless parameter. RASOR and WARNER [1] among others showed that, to a good approximation, the work function of a particular cesium-covered surface is a single-valued function of the ratio of its temperature T to the cesium reservoir temperature Tn over a wide range of T/TR. The results of this work will therefore be presented partly in the form of plots of work function vs. this temperature ratio, or ¢ vs. T/Tn, which will be called Rasor Plots. Subject to certain precautions, such plots can be made with data from a simple thermionic converter in which the interelectrode spacing, collector temperature, and collector work function need not be known accurately. By comparing plots for different emitter materials, it is possible to rank the materials in the order of the performance they are capable of giving. We show below that materials having Rasor Lines lying at higher values of T/TR for a given ¢ are capable of higher performance. The effect of two surface preparations on each of two kinds of rhenium is also described. Work function distribution curves are obtained for various T/TR, and "Rasor Bands" defined by the highest and lowest work functions found on a specimen are given. Electroetching of the emitter surface diminishes the width of these bands, and therefore increases the uniformity of the emitter. * Present address: Avco Corporation, Space Systems Division, Wilmington, Massachusetts. t Thermo Electron Corporation, Waltham, Massachusetts. 201

202

L.

VAN SOMEREN, D. LIEB,

G. MISKOLCZYand S. S. KITRILAKIS

P R E P A R A T I O N OF EMITTERS

1. Rhenium (a) Fabrication. Wrought powder-metallurgy material and chemical vapor-deposited (CVD) material were examined. No consistent difference in purity between the two classes of material was discovered, and each was better than 99.98 per cent pure. Each material had a preferred orientation, with basal planes parallel to the specimen surface, and that of the vapor-deposited material was more pronounced. [2] (b) Preparation. All specimens were ground and polished on silicon carbide abrasive paper to No. 600. They were then electropolished using a slight modification of the method of GEACH et al. [3] One specimen of each material was annealed and examined, while another specimen of each material was electroetched. The latter process, again using the electrolyte of Geach, but at a potential of 5 V for 1 min., removes material from the surface at a rate strongly dependent on the crystal orientation. [2] Its effect is to produce a rough jagged surface consisting largely of basal planes of rhenium, by dissolving away the other highindex planes. The process is advantageous not only because it reduces the range of crystal planes on the surface, but also because the residual basal planes, being close-packed, give particularly good electron emission when cesiated. After electrochemical preparation, each surface was heat-treated at a pressure less than 10-6 torr and at a temperature between 1800°C and 2400°C in order to achieve thermal stability during testing. Because the basal planes revealed by etching are also those with minimum specific surface free energy, annealing further develops these planes; it also causes some rounding and smoothing of the sharp edges created by etching. Details of the heat-treatments are summarized in the Heat Treatment Index of Table 1, which is discussed below. The treatments applied to CVD material were moderated to avoid adhesion problems between deposit and substrate. The designations of the various rhenium surfaces examined are shown in Table 1. TABLE 1. EVALUATION OF RHENIUM EMITTER SURFACES

Material

Electropolished Electroetched, and annealed Electropolished and annealed Ground, lapped, and annealed

Letter

Heat Treatment Index, cm~ c

Reservoir Pressure Index, torr Work function ~ , range,eV Scanner Converter

Wrought

A

6" 6

2.5

1"8*

0' 13

CVD Wrought

D C B E F

4" 1 5.9 5- 0 4.2 (4" 2)

5 -3 8 --

3 4 3 -3.5

0" 15 -0" 26 0'17 --

CVD Wrought

Ideal surface

Minimum

0

* Believed to be low because of the presence of oxygen. The reservoir pressure and work function range indices are evaluated for Te = 2000°K and J = 20 A/cmL Note: The tungsten emitter surface W was prepared from wrought material by electropolishing, followed by heat treatment in vacuum for 1 hr at 2300°C.

2. Tungsten (a) Fabrication. A specimen was prepared from wrought powder-metallurgy bar stock better than 99.98 per cent pure. No X-ray examination of the material was made, but it is likely to show a slight preferred orientation with (110) planes parallel to the emitter surface.

Evaluation of Thermionic Emitter Surfaces

203

(b) Preparation. The specimen was ground and polished on silicon carbide paper to No. 600. It was then electropolished in 5 per cent aqueous sodium hydroxide at l0 V for 30 sec. Following electropolishing it was annealed for 1 hr at 2300°C at a pressure less than 10 -6 torr. EXPERIMENTAL

PROCEDURE

Emitters A, B, and D were examined in two ways. A thermionic scanning device was used to give a m a p of local emission and to study work function distribution. Studies of power production were made using the specimen under investigation as the emitter in a variable-parameter converter.

1. Thermionic scanning device Specimens were examined in a thermionic scanning device shown schematically in Fig. 1 and in detail elsewhere. [4] In this device the saturation current from the specimen is collimated and directed by a variable magnetic field, and a part of it passes through the main collector on to an auxiliary collector, which is biased positively. mitter

Collector

__

_---

_

B3

ry

collector

1. Schematic diagram of thermionic scanning device showing magnetic fields and auxiliary collector.

Qualitative data are obtained on an oscilloscope screen as a m a p of emission intensity. Quantitative data are obtained from an X - Y plotter, which plots emission intensity as a function of position along a diameter of the emitter. For the fine-grained emitters examined, the diametric sample is taken to be representative of the emitter. A number of such "cross scans" is taken on each emitter, with emitter and reservoir temperatures adjusted to cover a wide range of T/TR.

2. Parametric converter Measurements of current density vs. voltage were made using a planar guard-ringed converter with an emitter area of 3 cm z and a collector of 2 cm 2, variable spacing, and independent control of emitter, collector, and guard-ring temperatures and potentials. A cross-section is shown in Fig. 2. In normal operation collector and guard ring are maintained at the same potential by an external circuit, and are within 20°K of the same temperature.

204

L. VAN

D. LIEB,G. M.ISKOLCZYand S. S. KITRILAKIS

SOMEREN,

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Emitter

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Vacuum envelope FIG. 2. Schematic cross-section of planar guard-ringed converter for parametric studies. Current-voltage curves were obtained for the emitter temperature range 1630 °K-1950 °K over a wide range of other parameters. The effect of collector temperature on these curves is very slight in the range around 1000°K where the data are obtained. Work function values were derived from saturation current data for each emitter. The saturation current is defined as that current observed to flow when the collector is made positive ( 2 4 V) with respect to the emitter. Measurements were made in conditions of ion richness at the emitter, so that a well defined voltage-independent current was observed, (Fig. 3). However, the ion-richness requirement restricted the range in which data were available to that above ~ ~ 2.8 eV. Another important requirement for accurate work function measurement is that the area from which the collector receives current be well defined. In the present work this was

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current.

Evaluation of Thermionic Emitter Surfaces

205

achieved using a guard ring around the collector. In the absence of a guard ring, emission from areas adjacent to the emitter which are below TE may reach the collector; such areas will have higher cesium coverage and lower work function than-the emitter, and will contribute disproportionately to the saturation current. To put the work function onto a Rasor Plot requires accurate knowledge of TR, but small errors in TE are not critical, because they enter into calculating ¢ and plotting TB/TR in opposite senses, and tend to cancel. Measurement of Tn in this work is discussed under Results lb.

3. Data reduction (a) Average work functions are calculated from the Richardson-Dushman Equation J = 120 AT~ exp ~ by submitting the current density J obtained from the emitter at temperature TE, to give ¢. In the thermionic scanner, the current from a point on the emitter is calculated from the intensity vs. position plot at that point, taking into account the scanning hole size. This current and the emitter temperature are then substituted in the above equation. The Rasor Plot of ¢ vs. TE/TR is constructed from the ~ data and knowledge of the emitter temperature TE and corresponding reservoir temperature. (b) Power data are computed by first making a family of JV curves for various TR, holding all other parameters constant. These curves cross each other, and their envelope defines the performance available in those conditions with reservoir temperature optimized. This process of eliminating a variable by allowing it to vary and define an envelope on the JV plot is repeated for many families of JV curves with different spacing d. This gives a set of JV envelopes for various TE, representing the best performance obtainable when TR and d are optimized. A correction for resistive losses in the leads is applied to produce a map of optimized current density versus electrode voltage, with TE as a parameter. From this it is straightforward to produce a map of power density versus electrode voltage, at optimum reservoir temperature and spacing. (c) Work function distributions or histograms are constructed from scanner data by plotting on each cross scan a set of parallel lines corresponding to saturation currents for surfaces differing by 0.02 eV in work function. Then the sum of the lengths on the cross scan falling within a range of current is taken to represent the area of the emitter having the corresponding mean work function. Such data are plotted as histograms (normalized to unit area) showing the fraction of emitter area falling in each work function range (Fig. 5). A uniform surface would appear as a column of unit height. (d) A Rasor Band can be defined for each surface scanned by plotting the maximum and minimum work function found on each histogram at the appropriate T/Tn. Smooth curves through the maximum and minimum points then define a band which shows clearly how the spread of work functions found on an emitter varies with T/TR. In summary, a converter provides a Rasor Line and power map characterizing its emitter, and a scanner provides detailed information about the work function distribution found on the emitter. This information may take the form of a distribution histogram for the work functions at a particular T/TR, or a Rasor Band showing the range of work functions present.

206

L. VAN SOMEREN,D. LmB, G. MISKOLCZYand S. S. KITRILAKIS

The range of useful data from thermionic scanners extends up to about ¢ = 3.1eV. Above this ¢, the current emitted at practical TB is reduced by the 2 × 10 -5 cm 2 scanning hole to a very small value at the auxiliary collector. A calibration signal is always used when taking quantitative measurements, but drift in the amplifiers imposes this upper limit on the work function data, obtained from scanners. Near and below the neutralization line, about 2.8 eV, converter data from the ignited mode may need a correction for plasma losses before they can give accurate work function results. The conditions in which a correction is needed, and its magnitude, have been discussed by KITRILAKISand RUFEH. [5] It is found that no such correction is needed for the data obtained from the thermionic scanner. D E F I N I T I O N OF I N D I C E S To facilitate comparisons between the surfaces studied, the following numerical indices have been defined:

1. Heat treatment index U p o n exposure to temperatures typical of thermionic converters, the atoms of the emitter acquire significant mobility, and tend to rearrange themselves in a configuration of minimum free energy. This leads to changes on the surface of the emitter which may alter the thermionic emission significantly. It is therefore important to be able to evaluate the extent of the changes taking place, and to compare the changes on emitters exposed to different conditions. To this end, a Heat Treatment Index was devised to provide a measure of the changes occurring. The extent of the changes depends on time, temperature, and the principle mechanism producing changes, which on these emitters is surface diffusion. The effects of time and temperature are combined as follows: The extent of changes is given by the solution of a diffusion equation involving (in addition to numerical constants) a function of the product tDT, where DT is the (surface) diffusion coefficient at temperature T, and t is the time the surface is exposed to temperature T during heat-treatment The product tDT is the basis for the index. DT is defined as DT = Do exp (-- Q/RT) where Q is the activation energy for surface diffusion, and Do is a rate factor. Values of Q = 100 kcal/mole and Do = 1 cm2/sec were chosen here, by analogy with other refractory metal systems, and these are felt to be adequately accurate for the approximate index. So tDT is numerically t exp ( - - 105/RT). This function was evaluated for each heat treatment of each surface. These were in the range of 10 -6±z cmL The values of different treatments of a surface were summed, and to produce a more tractable index the logarithm of the result was taken and added to 10 to give the Heat Treatment Index. The resulting factor, larger for more intense heat treatments, is felt to be a reasonable relative index of the combined time and temperature effects on rhenium surfaces.

2. Cesium Pressure Index F r o m the work of Rasor and Warner we see that, within broad limits, the work function of a surface can be reduced by raising the reservoir temperature, and so the cesium pressure in the converter. This suggests that a converter designer can achieve an arbitrarily high emission current at will. However, the over-all performance of the converter is also strongly dependent on transport phenomena between the electrodes, and these in turn depend on the

FIo. 4. Composite assembly of types of data from vapor deposited electropolished rhenium, ReE. (a) Photomicrograph of typical area. (b) Thermionic emission picture. (c) Cross scan. [Facing p. 206]

Evaluation of Thermionic Emitter Surfaces

207

cesium pressure: Losses or inefficiencies are greater at higher cesium pressures. So, broadly speaking, higher cesium pressures produce favorable emission conditions and unfavorable transport conditions, and a satisfactory balance of these two competing factors must be achieved in converter design. According to Rasor and Warner the parameter best defining the optimum emission behavior of a surface is the bare work function. Their theory predicts that, for the range of interest in converter design, high-bare-work-function surfaces should have lines parallel to, and shifted up the T/TR scale from, the lines for lower ¢0 surfaces. For reasons which will become apparent later, we find it is not convenient and accurate to distinguish emitter surfaces by their bare work functions alone. Therefore we defined the Cesium Pressure Index as the cesium pressure in torr required to produce a saturation current of 20 A/cm 2 from a surface at 2000 °K. (To do so requires a work function of 2.94 eV.) The Rasor Plot for a particular surface is used to determine the TE/TR corresponding to 2.94 eV, and, using TE -=--2000°K, the Tn, and so the cesium pressure, can be determined. This is a practical figure of merit for the emitter as a whole: lower cesium pressures are preferred. 3. The Work Functon Band Index Rasor Plots for the lowest and highest work-function areas of the emitter define the limits of a band, whose height is the range of work functions found on the emitter. We define the Work Function Band Index as the height of this band in the conditions previously chosen--i.e. TE = 2000°K and J ---- 20 A/cmL (More exactly, it is the height of the band when the weighted mean work function is 2.94 eV.) It is an unweighted measure of the scatter of work functions on the emitter surface. RESULTS I. Rhenium (a) The following sorts of data were obtained from the four rhenium surfaces with different preparations designated A, B, D and E" photomicrographs, qualitative emission displays, quantitative cross scans. Examples of such data for one surface, that of CVD electropolished rhenium, ReE, are shown in Fig. 4. Examples of the quantitative data obtained from cross scans on this emitter are shown in Figs. 5 and 6. Similar data were obtained for the other three surfaces studied, but are not shown here. It is from these data that the indices in Table 1 are obtained. Figure 7 is a composite Rasor Plot of data obtained from these rhenium surfaces. (b) The preferred Rasor Plot for wrought electropolished rhenium (surface C) is shown in Fig. 8, together with the theoretical line for ¢0 ---- 5.0 eV. (c) Optimized current density and power density data from one wrought electropolished rhenium surface (surface C) are shown in Figs. 9 and 10. 2. Tungsten (a) The Rasor Plot for the wrought electropolished tungsten surface studied is shown in Fig. 11, together with the line for rhenium. 15

208

L. VAN SOMEREN, D. LIEB, G. MISKOLCZY and S. S. KITRILAKIS

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2"8

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FXG. 5. Histogram of work function distributions from vapor-deposited electropolished rhenium, ReE derived from scanner data.

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Evaluation of Thermionic Emitter Surfaces

4"4

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Rhenium emitter

A Wrought electroetched B ~ C-Wrought electropolished D Vop. dep. electroetched E Vap. dep. electropoUshed

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T /TR FIG. 7. Composite Rasor Plot of results from rhenium. Lines are converter data and bands are scanner data.

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210

L. VAN SOMEREN,D. LIEB, G. MISKOLCZYand S. S. KnXlLAmS

-0

02

04. 06 08 Electrode output voltage,

I-0

I-2

V

FIo. 9. Optimized current density vs. output voltage for wrought electropolished rhenium, ReC.

(b) Optimized current density and power density data from this wrought electropolished Tungsten surface are shown in Figs. 12 and 13.

3. Additional data for wrought electropolished rhenium and tungsten are presented in the form of curves for power vs. emitter temperature at selected combinations of voltage and spacing, in Figs. 14-18. DISCUSSION

1. Rhenium (a) Effect of surface preparation. From Fig. 7 and Table 1 we see that (1) electroetching reduces the work function spread (and so increases the uniformity) of wrought material significantly, and reduces the work function spread of vapor-deposited material; (2) of the two non-electroetched (and similarly heat-treated) surfaces, the wrought one showed a substantially greater work function spread than did the vapor-deposited material; this is consistent with the lesser preferred orientation of the former material; (3) the work function bands for different preparation methods on the same material overlap, but the bands for the two materials do not overlap, and the wrought-material bands lie wholly to the right (greater T/TR) of those for vapor-deposited material. That is, the wrought materials have more favorable cesium pressure indices than the vapor-deposited material. The maximum and minimum work functions occurring on each histogram in Fig. 5 are used to define H and L points on Fig. 6. The weighted mean work function for each histogram is plotted between the H and L lines on Fig. 6. Note that the histograms are skewed towards low work functions, and consequently the weighted mean work function is

Evaluation of thermionic emitter surfaces 6C 5C

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04 06 08 Electrode output voltage,

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FIG. 10. Optimized electrode power vs. output voltage for wrought electropolished rhenium, ReC.

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Electropolished tungsten ~,-thermionic scanner data O-converterdata electropolished

> 3-6

]

i ~ " ~ f "

rhenium

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converter data Re C

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2.8

2.4

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T / TR

FIG. l 1. Composite Rasor Plot for wrought electropolished tungsten W with preferred line for

wrought electropolished rhenium, ReC, both from converter data.

212

L. VAN SOMEREN, D. LIEB, G. MISKOLCZY and S. S. KITRILAKIS

Electrode voltage,

V

FIG. 12. Optimized current density vs. output voltage for wrought electropolished tungsten, W. 6C 5C

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Evaluation of Thermionic Emitter Surfaces

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1400

1500

1800

Emitter temperature, *C

FIG. 14. Output power density as a function of emitter temperature at an electrode output voltage of 0.5 V and spacing of 6 mil (0"15 ram). seen to be nearer the L than the H values. This is presumably because the vapor-deposited electropolished material from which this data was taken has a very strong preferred orientation, and there are few grains with orientations far from the basal plane, and so the weighted mean work function differs little from the minimum (cesiated) work function represented by the basal planes. (b) Consistency of data. The consistency between scanner and converter results for wrought polished rhenium is evident in Fig. 7. The scanner and converter data for vapordeposited etched emitter D are not consistent. However, the scanner data showed an unusual amount of scatter, and this inconsistency cannot be regarded as significant. Converter data for wrought etched material clearly do not have the same slope on the Rasor Plot as do the other surfaces. This converter produced extraordinarily high power, and after operation the emitter was found to have developed hexagonal etch pits never observed on rhenium emitters before. A leak was also found in the converter. This emitter was therefore subjected to cesium, with oxygen as an additive. We hypothesize that the change in slope of the Rasor Plot for this surface is due to the presence of oxygen as an additive. As a result of experience with the design of converter used to obtain work function data from emitters A, B, and D, we suspect that there may have been a systematic error in the measurement of reservoir temperature, owing to the particular location of the thermocouple relative to the liquid cesium surface. This would lead to the measured TR being low, and T/Tn plots determined on this vehicle being displaced up the T/TR scale (B, Fig. 7). Modifications were made on a subsequent converter, and a Rasor Plot was again determined for wrought electropolished rhenium. This is shown in Fig. 8 and as line C on Fig. 7, and is

214

L. VAN SOMEREN,D. LIEB,G. MISKOLCZYand S. S. KITRILAKIS 25

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1500

,

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1600

,

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1700

Emitter temperature, *C

FIG. 15. Output power density as a function of emitter temperature at an electrode output voltage of 0"7 V and spacing of 2 mil (0'05 mm).

about 0.1 eV above that shown earlier. It is noteworthy that the scatter of experimental points defining this line is substantially less than the scatter found on the more primitive converter. We now feel confident about this Rasor Plot for wrought electropolished rhenium. The same Fig. 8 shows the line predicted by Rasor for a surface with bare work function of 5.0 eV. Note that the experimental line converges slightly with the theoretical line. Therefore, it is not possible to identify this experimental line with a unique theoretical line corresponding to a certain bare work function, and we cannot describe this surface in terms of a single parameter. We feel that the Rasor Plot is the best description of the thermionic properties of the surface. (The modified converter design was used to obtain the performance data shown in Fig. 9 on.)

2. Comparison of rhenium and tungsten data The Rasor Plot shown in Fig. 11 for the tungsten surface examined is convergent with that for rhenium and slightly above it (--~ 0.1 eV) in the range of converter operation. If we compare the power maps of Figs. 10 and 13, which are optimized with respect to spacing as well as reservoir temperature, we find the following.

Evaluation of Thermionic Emitter Surfaces 2O

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Emitter temperature, "C

FIG. 16. Output power density as a function of emitter temperature at an electrode output voltage of 0.7 V and spacing of 6 mil.

(a) At 1950°K the electropolished rhenium emitter is giving the maximum power output of 35 W/cm 2, 1.4 times that from the tungsten. However, this is produced at a current of 100 A/cm 2, which may be inconveniently high to handle. At 1.0 V, where efficiencies are high, the rhenium gives 11 W/cm 2, still 40 per cent better than the tungsten. (b) At 1750°K, the maximum power from rhenium is 15 W/cm ~, and the more useful power at 0.7 V is 3 W/cm 2. Here the maximum power for tungsten is only a half that for rhenium. However a converter designer may be constrained to work at a particular voltage. To elaborate the complex relations between spacing, power, and voltage, Figs. 14-18 show power density as a function of emitter temperature for certain emitters, each graph having a different combination of voltage and spacing. The emitters considered were studied opposite Molybdenum collectors and prepared as follows: ReC ReF W

Wrought electropolished rhenium, heat-treated 3 hr at 2380°C. H.T.I. = 5.9. Wrought ground and lapped rhenium, heat-treated 24 hr at 1750°C. H.T.I. = 4.2. Wrought electropolished tungsten, heat-treated 1 hr at 2300°C.

From these we see that when voltage and spacing are specified rhenium is not always the best emitter material. For instance at 6 mil (0.15 mm) spacing, tungsten is more attractive at voltages below 0-7 V. The choice of material is less dependent on temperature of operation than on voltage. The beneficial effect of electropolishing and heat treatment of rhenium is clear in each graph.

216

L. VAN SOMEREN,D. L]E8, G. MISKOLCZYand S. S. K1TRILAKIS 25

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Emitter temperature, *C

FIG. 17. Output power density as a function of emitter temperature at an electrode output voltage of 0"9 V and spacing of 2 mil.

Less detailed comparison shows the following. (a) The higher performance comes from the emitter whose Rasor Plot lies below and to the right. (b) The performance cannot be simply correlated with the displacement of the Rasor Plot, but the distinction in performance appears to diminish at higher temperatures. SUMMARY

1. Electroetching and heat-treating a polycrystalline wrought rhenium surface reduces the spread of work functions present on it, and increases its uniformity. 2. Wrought rhenium material performs better than vapor-deposited rhenium, for reasons at present unknown. 3. Thermionic scanners and converters have produced mutually consistent Rasor Plot lines defining electropolished surfaces of wrought rhenium and tungsten. 4. The relation between the position of Rasor lines and corresponding thermionic power output is shown for electropolished surfaces of wrought rhenium and tungsten.

Evaluation

I 800

1500

217

of Tb&mionic Emitter Surfaces

I

1600

I

I 1700

I

1600

Emitter trmprroture,*C

FIG. 18.

Output

power density as a function of emitter temperature voltage of 0.9 V and spacing of 6 mil.

at an electrode output

5. A well-substantiated Rasor line for wrought electropolished rhenium is given. 6. Power production data for electropolished and heat treated emitters of wrought tungsten and rhenium are given. Acknowledgments-This work was supported by the Jet Propulsion Laboratory, California Institute of Technology, under contract NAS-7-100 from the National Aeronautics and Space Administration. The authors wish to thank Prof. E. P. Gyftopoulos for his valuable comments on the manuscript.

REFERENCES [I] N. S. RASORand C. K. WARNER,J. uppl. Phys. 33, 2589-600 (1964). [2] L. VAN SOMEREN,Tungsten and Rhenium for Thermionic Emitters, delivered at Fourth Tech. Conf. on Phys. Metallurgy of Refractory Metals, French Lick, Indiana, Oct. (1965). [3] G. A. GEACH, R. A. JEFFERYand E. SMITH,Rhenium, p. 84, Ed. B. W. Gonser, Elsevier, N.Y. (1962). [4] N. S. RASOR, S. S. KITRILAKISand D. P. LIEB, Correlations of Observed Non-Unijbrm Emission from Surfaces in Cesium Vapor, Report of Thermionic Conversion Specialist Conference, Gatlinburg, Tenn., p. 169, published by IEEE (1963). [5] S. S. KITRILAKISand F. RUFEH, Experimental Correlation of Converter Variables in the Ignited Mode, Report on Thermionic Electrical Power Generation Conference, London, Sept. (1965). Rbsumb-Des surfaces de tungstene et de rhenium en presence de vapeur de dsium et prepartes par des methodes differentes sont evaluees experimentalement. On presente deux sortes devaluations: Dam la premiere, les characteristiques moyennes d’emission de ces surfaces sont decrites par des graphiques du travail de sortie 4 en fonction du rapport de la tempkrature de la surface T B celle du rkservoir de dsium TR. On obtient aussi des d&ails sur la distribution du travail de sortie pour certaines surfaces et on les rapporte

218

L. VANSOMEREN, D. LIEB, G. MI~KOLCZYand S. S. KITRILAKIS

aux characteristiques moyennes. Dans la deuxibme methode, les surfaces sont Cvaldes en fonction de la puissance electrique fournie par des convertisseurs thermioniques utilisant ces surfaces comme Cmitteurs (cathodes). Pour certaines surfaces, on presente des courbes de puissance et de courant par unite de surface en fonction de la force Clectromotricc; chaque courbe correspond a une temperature differente de l’emetteur, et chaque point de la courbe est donne pour des valeurs optimum de la temperature du reservoir de &ium et de la distance entre les electrodes. D’autres courbes donnent la puissance en fonction de la temperature de l’tmetteur a des valeurs particulieres de force Clectromotrice et de distance entre les electrodes. Zusannnenfassung-Diese Abhandlung beschreibt eine experimentelle Auswertung thermionischen EmitterOberfllchen aus caesiiertem Wolfram und caesiier tern Rhenium, die nach verschiedenen Methoden prlpariert wurden. Zwei Arten von Auswertung werden dargestellt. In der ersten werden die durchschnittlichen Emissions-Charakteristiken dieser OberSkhen beschrieben durch graph&he Darstellungen der caesiierten Austrittsarbeit als Funktion der Oberflachen-temperatur Tdividiert durch die Caesium-ReservoirTemperatur TB. Es werden such Einzelheiten der Verteilung der Austrittsarbeit auf gewissen Oberfllchen erhalten und auf die durchschnittlichen Charakteristiken bezogen. Bei der zweiten Methode werden die Oberfllchen untersucht in Abhiingigkeit von der elektrischen Ausgangsleistung der thermionischen Converter, die diese Oberflachen als Emitter (Kathoden) benutzen. Ftir gewisse Werkstoffe werden Kurven der Leistungsdichte und der Stromdichte als Funktion der Ausgangsspannung dargestellt, wobei jede Kurve die Temperatur des Caesiumreservoirs besitzt, der Elektrodenabstand optimiert ist, und die Emittertemperatur als Parameter erscheint. Andere Kurven zeigen die Ausgangsleistungsdichte in Abhtingigkeit von der Emittertemperatur, bei spezifizierten Ausgangsspannungen und Abstlnden.