Thermionic work function of refractory metallic compounds and electronic and crystal structure - emissivity measurement and diode fabricatio

Miller, C. S.

Plunkett, J. D.

English

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UN IVE R S ITY "OF DE N\fE R D e n v e r R e s e a i c h I n Bf'i t u t e K- 1 
U n i v e r s i t y  P a r k ,  D e n v e r  IO, C o l o r a d o  
31 December 1964 
COLOPADO SEMINARY 
National Aeronautics and Space Administration 
1520 H Street N. W. 
Washington, D. C. 20546 
N65 - L 8 8 5 2 
Attention: Office of Grants and Research Contracts, Code SC 
Gentlemen: 
This letter is a report of progress on Research Grant NsG-365, COT- 33 
entitled "Theoretical Studies on the Relationship Between the Thermionic 
Work Function of Refractory Intermetallic Compounds and Their Electronic 
and Crystal Structures," for the period 1 July 1964 to 31 December 1964. 
1 . 0  Introduction 
Intermetallic compounds exist for all  the group VI-A metals and it is 
the purpose of this investigation to measure the thermionic work function of 
the u-compounds and the intermediate phase having the a - M n  structure of the 
molybdenum-rhenium and tungsten-rhenium systems as well as the u-phase 
of the chromium-rhenium. 
thermionic work functions of these five intermetallic phases and determine 
their relationship to electronic and crystal structure. 
The objective of this study is to measure the 
2. 0 Equipment 
A. High Temperature Vacuum Furnace 
The high temperature vacuum furnace is comple2  and has worked 
comple&y satisfactorily to produce the annealing treatment required for the 
intermetallic phases. 
cables overheat slightly, but this is a minor difficulty which can be overcome 
easily at a convenient time. 
Above 2000°C the furnace electrode electrical supply 
B. Emissivity Apparatus 
Previous investigators have, in general, measured the temperature 
of the electron emitting cathode by attaching a thermocouple head. 
questioned this practice and initial heat transfer calculations indicate that 
a three mil thermocouple wire attached to a ten mil cathode causes reduction 
in the temperature of the cathode at the thermocouple bead as much a s  25°C 
We have 
G P O  
https://ntrs.nasa.gov/search.jsp?R=19650019251 2020-03-17T02:07:06+00:00Z
at 1600°K. 
lower its apparent temperature. 
large that this source of e r ro r  cannot be neglected, but must be reduced or 
this method, though commonly used, will introduce large e r r o r s  into the 
derived thermionic data. 
The thermocouple wires conduct heat away from the cathode and 
The magnitude of this effect i s  sufficiently 
Two possible methods exist which might be useful to eliminate this 
First, AC electrical heating of the thermocouple wire to source of e r ror .  
approximately a matching temperature with the cathode. 
be eliminated and the depressed thermocouple bead temperature e r r o r  would 
approach zero. 
AC fields inside the collector rings is a very dubious practice. Secondly the 
use of optical methods completely sidesteps the difficulty. 
The heat loss would 
This method was not used since the presence of additional I 
Optical observation of the cathode i s  complicated by the fact that no 
accurate and useful emittance data a re  available for the five refractory inter- 
metallics under study. Therefore, since the optical temperature measurement 
emittances of these materials will be undertaken. 
I 
method is the most satisfactory method experimental measurement of the ~ 
I 
The emittance method selected was a modified Worthing tube method. 
The apparatus 
Near the midpoint of'the tube i s  drilled a small 
This is a simple but reliable procedure of obtaining the data. 
consists of a hollow tube electrically heated with an axial flow of current to 
incandescent temperatures. 
blackbody hole. 
specimen surface and the blackbody hole with an optical pyrometer. 
Emittance data are obtained by alternately observing the 
To measure the thermal emittance of the five intermetallic phases 
under study, apparatus has been constructed and tested which uses samples 
0.25" OD with a 10 mil wall thickness and 10" long. The intermetallics a re  
electrophoretically applied as coatings to the tantalum tubes. 
stainless steel and gold coated stainless for which reasonably good data a r e  
available indicate the apparatus to be performing in a perfectly satisfactory 
manner. 
Initial work on 
c. High Voltage Pulse Generator 
. A   15 K V  pulse generator, patterned after the design of Haas, capable 
of producing pulses of a wide range of widths and pulsing rates has been 
constructed -and is currently being debugged. 
which presented a safety hazard are being overcome. In general, these 
consist of changes in the types of connectors used at cable terminations. 
Certain de sign deficiencies 
? 3 
c 
Cables a r e  required to car ry  the high voltage pulse from the generator to the 
diodes. We believe that all difficulties will be rectified within a short time. 
3.  0 Diode Fabrication 
Diodes suitable for thermionic work function determinations by direct 
electrical measurement require three distinct steps. These are:  1) Inter- 
metallic alloy preparation; 2)  Electrophoretic coating of the cathodes; and 
3)  Assembly of the diodes. 
A. Intermetallic Alloy Preparation 
Four of the five intermetallics have been prepared by a r c  melting and 
annealing treatments. The chromium-rhenium phase has not been prepared 
by a r c  melting due to the difficulty of obtaining the required narrow compo- 
sition range due to the large difference in the volatility of chromium with 
respect to rhenium in the arc .  Currently it is planned to react these mater- 
ials using standard powder metallurgy methods at much lower temperatures. 
B. Electrophoretic Cathode Coating 
Since the electrophoretic coating of cathode wires of 5 to 10 mils 
diameter is difficult due to the high density of refractory intermetallics 
which causes a rapid vertical concentration gradient within the coating vessel 
very fine particles a re  required. 
particles of intermetallics f ree  of contamination proved to be a major task. 
No commercial grinding device was found which could perform this task with- 
out contamination. 
which would introduce a tramp material which could be removed easily and 
completely. 
crushed to minus 60 mesh in an iron mortar.  
for approximately 24 hours in a vibratory grinder, also constructed of iron. 
After grinding the major tramp iron was extracted with a strong magnet. 
Approximately 50% of the ground powder proved to be iron. 
ten consecutive hydrochloric acid treatments were used to chemically leach 
out all iron smeared over the surface of the intermetallic particles. Chemical 
analysis indicated less  than 0. 005% Fe  for all except one sample which showed 
0.020% Fe  and it was purposely acid leached only a few times to provide a 
known high iron content sample. 
The obtaining of five to ten micron diameter 
Therefore, it was decided to select a grinding system 
The selected procedure was as follows: the a r c  button was 
This coarse powder was ground 
Subsequently 
The four samples were supplied to Indelco Corporation of Beverly, 
Massachusetts and coating was completed by Mid-October. A one to two mil 
coating was applied to a 5 mil  tungsten wire to produce the finished cathodes. 
.. 
4 
A heat treatment a t  1700°C bonded the coatings to the wire but this tempera- 
ture is sufficiently low to prevent significant interdiffusion of the coating and 
substrate. 
C. Diode Fabrication 
Diode fabrication requires the use of "clean room" facilities of a type 
which the University of Denver does not have. Therefore, a contract was let 
to the ITT Electron Tube Division, International Telephone and Telegraph 
Corporation located at Easton, Pennsylvania. The first diode with a pure 
tungsten filament has been received and one serious defect found. 
design used by ITT is such that the spring used to hold the cathode straight 
by taking up thermal dilation possesses such a large spring constant and such 
a short length that as the cathode heats up the tension is completely relaxed 
in the spring and the cathode bows out of its central position. 
The diode 
ITT has been notified of this defect and we have supplied them with 
technical references on the proper design of the cathode support spring. 
ITT will reopen the diode and insert the proper spring. Delivery of the 
modified diode is supposed to be made within three weeks. 
4 . 0  Discussion of Future Work 
Diode fabrication delays and defects have made it impossible to keep 
the planned schedule and another grant extension will be required. 
Respectfully submitted, 
D 
--t C. Samuel Miller 
Graduate Research Assistant 
% r o&<t Super vis or  


<theoretical studies on the relationship between the thermionic work function of re- fractory intermetallic compounds and their electronic and crystal structures<  progress report letter, 1 jul. - 31 dec. 1964

<theoretical studies on the relationship between the thermionic work function of re- fractory intermetallic compounds and their electronic and crystal structures< progress report letter, 1 jul. - 31 dec. 1964

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