Thermionic work function of refractory intermetallic compounds and their electronic and crystal structure

Miller, C. S.

Plunkett, J. D.

English

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I 1 
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university of Denver 
COLORADO S E M I N A R Y  
D E N V E R  R E S E A R C H  INSTITUTE U N I V E R S I T Y  PARK, D E N V E R ,  COLORADO aozio 
I 
II N 65- 32 034 
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30 June 1965GPO PRICE $ 
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National Aeronautics and Space Administration 
1520 H Street, N. W.  Microfiche (MF) 
Washington 25, D. C. 
Attention: Office of Grants and Research Contracts, Code SC 
ff 653 July 65 
Gentlemen: 
This letter i s  a report of progress  on Research Grant NsG-365, 
entitled "Theoretical Studies on the Relationships between the Thermionic 
Work Function of Refractory Intermetallic Compounds and their Elec - 
tronic and Crystal Structures for the period 1 January to 30 June 1965. 
1 . 0  Int r o duct ion 
The group VI-A metals, chromium, molybdenum, and tungsten 
combine with rhenium to form intermediate phases of interest .  
u -phase is present in all of the group VI-A-Re systems. 
and W-Re systems also contain an additional intermediate phase having 
the a-Mn structure.  
ionic work functions of the five intermediate phases to discover what 
relationships may exist between the work functions and the electronic 
and crystal structures.  
A single 
The Mo-Re 
The objective of the study i s  to measure the therm-  
A t  the time of the las t  report, considerable difficulties were being 
The major encountered in the fabrication of the diodes shown in Figure 1. 
problem at that time was in the design of a spring which would keep the 
diode filament concentric with the collector elements from ambient tem - 
perature to 2000°C.  This problem was surmounted during the report  
period just passed, and the diodes presently being produced appear to 
be satisfactory. A l l  of the diodes received previously have been returned 
to the manufacturer for replacement of the springs with those of the latest  
design. 
. 
https://ntrs.nasa.gov/search.jsp?R=19650022433 2020-03-17T01:01:10+00:00Z
~~ 
F i g u r e  1. The rmion ic  Diode 
2 . 0  Measurement of the Work Function 
A major portion of the report period was devoted to measure-  
ment of the current-voltage characterist ics for the various mater ia ls .  
A s  described in a previous report, the work function is given by the 
R ichar ds on - L aue - Du shm an e quati on: 
j = A, (1-F) T2  exp ( - @ / l a )  
where j = current density 
A, = constant = 120 amp - cm-2 - deg” - r = reflection coefficient of the surface for electrons 
T = absolute temperature 
@ = work function 
k = Boltzmann constant 
If the work function i s  independent of temperature, a plot of log j /T2  
vs .  1/T yields a straight line having a slope equal to -@/2.303 k. 
In the absence of an applied field, electrons emerging from the 
emitter surface a r e  influenced by various retarding fields. 
cases ,  only a fraction of the emitted electrons reach the collector, and 
the current that would be measured i s  not the t rue emission current.  
Therefore, one commonly used method is to provide an applied field so 
a s  to overcome the retarding fields and cause all of the emitted electrons 
to travel to the collector. A diode operating in this condition i s  said to 
be saturated. 
In such 
According to theSchottky theory, of which an excellent review 
i s  given by Herring and Nichols’ the logarithm of current density in 
the saturated region varies linearly as the square root of the applied 
field. 
applied field, the value obtained is taken to be the value that would be 
obtained in the absence of all electric fields. 
Schottky plot constructed from current-voltage characterist ics of a tung- 
sten emitter at 11 66C. 
If this linear portion of the Schottky plot i s  extrapolated to zero 
Figure 2 shows a typical 
2 .1  Measurement of the Work Function of the Tungsten Reference 
A diode having a pure tungsten filament was prepared for the 
purpose of checking the equipment and procedures used in  the determination 
C .  Herring and M. H. Nichols, Rev. Mod. Phys.  - 21 (1949). 
4 
10-7 1 
9 
I I I I I I I I I I 
6 
I I I I I 1 I 1 I I 
I O 0  0 20 60 80 40 
VF ( V / c r n P 2  - 
Figure 2 .  Extrapolation of Linear Portion of Schottky Plot to Obtain 
Zero-Field Current Density 
*.- 
5 
of the work function. 
spring of the proper design, and some deflection of the filament was 
encountered at incandescent temperatures.  Nevertheless, the current-  
voltage characterist ics of the diode were measured at various tempera- 
tu res  between 1150 and 1600C, before returning the diode to the manu- 
facturer for replacement of the spring. 
As originally supplied, the diode did not have a 
The diode characteristics were determined by measuring the 
current a s  the electric field was increased by increments f rom 0 to 
3700 V/cm. 
power supply, and the current was measured with a Keithley Model 
414 micro-microammeter.  
measured with a micro-optical pyrometer and corrected for the emit-  
tance of the material  and for the transmittance of the pyrex envelope. 
The zero-field current densities were determined by constructing a 
Schottky plot for each temperature a t  which the current-voltage meas -  
urements were made. A s  described in section 2 .0 ,  the zero-field 
values were used in a plot of log jo/T2 vs.  1/T. The work function for  
tungsten determined in this way was 4.58 e .v .  a s  shown in  Figure 3 .  
Published values2 range from 4.25 to 4.69, depending on the crystal  
orientation of the emitting surface. 
The voltage was supplied by a Lambda Model 1500M 
The temperature of the emitter was 
The work function will be measured again after the diode has  
been fitted with the correct  spring. 
however, due to the cylindrical geometry of the diode, and because the 
deflection of the filament was small. 
Only a minor change i s  anticipated, 
2 . 2  High Voltage Pulser  Svstem 
The 15KV pulser system which i s  a substantial copy of the ap- 
paratus constructed by Haas of the U. S. Naval Research Laboratory 
is  still not operating satisfactorily. 
being generated; however, the current pulse is not obtained. 
resolve this problem is still underway. 
Figure 4. 
A satisfactory voltage pulse i s  
Work to  
The pulser system is shown in 
2.3 Measurement of the Current-Voltage Characterist ics of 
Intermediate Phases  in the Mo-Re and W-Re Systems 
Four diodes incorporating the new spring design have been r e -  
ceived to date, of which one diode was returned because of an inadequate 
H. B.  Michaelson, J .  A.pp1. Phys. 21, 536 (1950). 2 
I 0-9 
10-10 
t 
(u 
I- 
\ 10-11 
0 .- 
10-12 
9 = 2.303 k (slope) = 4.58 ev. 
I 1 1 I I I 
5.00 6.00 
1 0 ~ 1 ~ -  
I 
7.00 
6 
Figure 3 .  Richardson Plot f o r  Determination of the Work Function of 
Tungsten 
F i g u r e  4. 15 K V  P u l s e r  System 
7 
, 
vacuum. 
three satisfactory diodes at various temperatures has been accomplished, 
and Schottky plots have been constructed to obtain the zero-field values 
shown in Table I. The values shown were obtained graphically from the 
original Schottky plots. A simple regression analysis program for the 
Burroughs B5500 computer has  been adapted to find the best values, and 
these correct  values will be included in the final report .  
Measurement of the current-voltage characterist ics of the 
Table I. Zero Field Currents at Various Temperatures Obtained 
from Extrapolation of the Linear Portion of the Schottky Plots 
Diode #1 
T("C) 10 (amp) 
1297 2.7X 1 0 - ~  
1335 9 . 2 5 ~  
1440 1.17X 
1390 4.25X lo-' 
1490 3.68 X 
1540 2.35X 
1096 2.35 X 10'" 
1157 1.83 X lo-'' 
1198 6.30X l o - "  
1245 3 . 9 5 ~  
1295 1.12 x lo- '  
1341 7 . 2 2 x  10- 
1458 7.70X io-' 
1501 2.23 X 
1554 6.30X 
2 .4  Emittance Measurements 
Diode #4 
1154 2 . 2 x  lo-'' 
1256 2.05X 1 0 ' ~  
1303 1 . 2 0 x  lo- '  
1432 5 . 0 5 ~  
1196 4.15X lo-'' 
1356 7 . 6 0 X  lo- '  
1498 2.84X 
1534 6.05 X 
Since temperature is measured with an optical pyrometer, it 
is necessary to correct  the observed temperatures for the emittance of 
the material  being observed. 
the correct  temperature could be obtained by solution of the Planck 
radiation equation. 
values is available. 
Such data a r e  available for tungsten,3 and 
A tabulation of the solutions for various emittance 
4 
For the mater ia ls  of the present study, no such published data 
a r e  available, but a r e  presently being determined by the Worthing tube 
method, a s  discussed in the last  report .  
Handbook of Thermophysical Propert ies  of Solid Materials, Volume I 3 
- Elements. 
D. E .  Poland, Green, J. W . ,  andMargrave, J. L . ,  CorrectedOptical 
Pyrometer  Readings, NBS Monograph 30. 
. 
. 
3 .0  Discussion of Future Work 
In the two-month period from 1 July 1965 to 31 August 1965, 
measurement of the current-voltage characterist ics for the remaining 
two mater ia ls  should be completed, as well a s  the measurement of the 
emittances for all of the materials of interest .  These data will permit 
calculation of all of the work functions, a s  well a s  the temperature 
dependence of the work functions. 
Respectfully submitted , 
- @zif%/- 
C. Samuel Miller 
Graduate Research Assistant 


<theoretical studies on the relationships be- tween the thermionic work function of refrac- tory intermetallic compounds and their electronic and crystal structures<  progress report, 1 jan. - 30 jun. 1965

<theoretical studies on the relationships be- tween the thermionic work function of refrac- tory intermetallic compounds and their electronic and crystal structures< progress report, 1 jan. - 30 jun. 1965

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