NASA CR-134549

NBSIR 74-447

HIGH RELIABILITY SHEATHED,

BERYLLIA INSULATED,

TUNGSTEN-RHENIUM ALLOY

THERMOCOUPLE ASSEMBLIESTHEIR FABRICATION AND EMF

STABILITY

by

G. W. Bums, W. S. Hurst and M. G. Scroger

prepared for

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INTERAGENCY AGREEMENT

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U.S. DEPARTMENT OF COMMERCE

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Washington, D.C. 20234
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This report was prepared as an account of Government sponsored

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1. Report No. NASA CR-134549

2. Government Accession No. 3. Recipient's Catalog No.

4. Title and Subtitle HIGH RELIABILITY SHEATHED, BERYLLIA INSULATED, TUNGSTEN-RHENIUM

ALLOY THERMOCOUPLE ASSEMBLIES - THEIR FABRICATION AND EMF STABILITY

5. Report Date June, 1974

6. Performing Organization Code

7. Author(s)

G. W. Burns, W. S. Hurst, and M. G. Scroger

8. Performing Organization Report No. NBoiR l^—^^l

10. Worl< Unit No.

9. Performing Organization Name and Address

U. S. Department of Commerce

National Bureau of Standards

Institute for Basic Standards

Washington, D. C. 20234

11. Contract or Grant No.

0— Oi J't J—

D

13. Type of Report and Period Covered

Contractor Report

12. Sponsoring Agency Name and Address National Aeronautics and Space Administration

Washington, D. C. 20546

14. Sponsoring Agency Code

15. Supplementary Notes Project Manager, I. Warshawsky, Instrument Consultant, Physical Science Division, National

Aeronautics and Space Administration, Lewis Research Center, Cleveland, Ohio, and G. E. Glawe,

Technical Monitor

16. Abstract

1.6 mm diameter tantalum sheathed, BeO insulated, W-3% Re/W-25% Re thermocouple assemblies

have been fabricated and their emf drift determined during 2059 hours of exposure at 2073 K in

a gaseous helium environment. The sheathed thermocouple assemblies were constructed from aged

thermoelements, specially heat-treated BeO insulators, and specially cleaned and etched tantalum

sheaths. Their thermal emf drifts ranged from the equivalent of only -0.3 to -0.8 K drift per

1000 hours of exposure at 2073 K. No evidence of any gross chemical attack or degradation of the component materials was found. The emf drift and material behavior of some unsheathed, BeO

insulated, W-3% Re/W-25% Re thermocouples at 2250 and 2400 K were also determined. Unsheathed

thermocouples tested in an argon environment at 2250 K for 1100 hours and at 2400 K for 307 hours

exhibited changes in thermal emf that typically ranged from the equivalent of a few degrees K

to as much as +11 K. Post-test examinations of these thermocouples revealed some undesirable

material degradation and interaction which included erosion of the BeO insulators and contamination of the themoelements by tantalum from the tantalum blackbody enclosure in which the thermocouples were contained. Preliminary tests to examine the chemical compatibility of sintered BeO insulators with Ta sheaths in an argon environment at 2073, 2250, and 2400 K

Were also conducted. They revealed gross erosion of the insulator after only 50 hours ex- posure at 2400 K, A 50% reduction in the insulator diameter at the open end of the sheath

was typical. Well inside the sheath, where gaseous reaction products were confined, the erosion was considerably less. Similar behavior occurred with long exposure (1100 hours) at 2250 K. Serious problems with the BeO insulators were not apparent in tests at 2073 K.

17. Key Words (Suggested by Author(s)

)

Beryllium Oxide

Emf Drift

Sheathed Thermocouples

Tantalimi Temperature Measurements

Tungsten-Rhenium Alloys

18. Distribution Statement

Unclassified - Unlimited

19. Security Classif. (of this report) Unclassified

20. Security Classif. (of this page) Unclassified

21. No. of Pages

36

22. Price*

3.00

* For sale by the National Technical Information Service, Springfield, Virginia 22151

NASA-C-168 (Rev. 6-71)
TABLE OF CONTENTS

Page

SUMMARY 1

INTRODUCTION 2

EXPERIMENTAL PROCEDURES AND APPARATUS 3

DRIFT TEST WITH SHEATHED THERMOCOUPLE ASSEMBLIES 3

Preparatory heat treatments and chemical cleaning of materials 3

Fabrication and Installation of the thermocouple assemblies 3

Initiation of the drift test and test procedures 4

DRIFT TESTS WITH UNSHEATHED THERMOCOUPLE ASSEMBLIES 6

COMPATIBILITY STUDIES OF BeO INSULATORS WITH TANTALUM SHEATHS 6

MATERIALS TESTED 6

EXPERIMENTAL RESULTS AND DISCUSSION 7

TESTS WITH SHEATHED THERMOCOUPLE ASSEMBLIES 7

Thermal emf drift . 7

Post-test examination of materials 14

TESTS OF UNSHEATHED BeO-INSULATED THERMOCOUPLES 22

Thermal emf drift 22

Material behavior 22

STUDIES OF BeO-Ta SHEATH COMPATIBILITY 29

CONCLUDING REMARKS 32

BeO-Ta COMPATIBILITY TESTS 32

TESTS WITH UNSHEATHED BeO-INSULATED THERMOCOUPLES 34

TESTS WITH SHEATHED THERMOCOUPLE ASSEMBLIES 34

REFERENCES 36

ii
HIGH RELIABILITY SHEATHED, BERYLLIA INSULATED,

TUNGSTEN-RHENIUM ALLOY THERMOCOUPLE ASSEMBLIES—

THEIR FABRICATION AND EMF STABILITY

by

G. W. Burns, W. S. Hurst, and M. G. Scroger

National Bureau of Standards

Temperature Section

SUMMARY

This report presents information on the fabrication and the performance of 1.6 mm

diameter tantalum sheathed, beryllium oxide (BeO) insulated, W-3% Re vs W-25% Re thermocouple assemblies. The thermocouple assemblies were tested at 2073 K for 2059 hours.

Tests were also performed with unsheathed, BeO insulated W-3% Re vs W-25% Re thermocouples.

Tnese thermocouples were exposed at 2250 and 2400 K for periods up to 1100 hours. In addition, studies of the chemical compatibility of sintered BeO insulators with tantalum

sheaths at 2073, 2250, and 2400 K are also presented. All of the investigations were

conducted in gaseous environments of high purity argon or helium at a pressure of

; nominally 1 atm. Material characterization included metallographic examination and chemical

analyses both before and after the high temperature exposures.

The sheathed thermocouple assemblies were constructed from aged W-3% Re and W-25% Re

thermoelements, specially heat treated sintered BeO insulators, and specially cleaned and

etched tantalum sheaths. Four such assemblies were constructed. After evacuating, baking,

and helium leak testing the assemblies, they were filled with high purity argon gas and

hermetically sealed. The thermal emf drift of the assemblies was then determined during

an exposure in a gaseous helium environment at 2073 K for 2059 hours. The measured thermal

emf drifts were smaller than the estimated measurement uncertainties and ranged from the

- equivalent of only 0.3 to 0.8 K downward drift per 1000 hours of exposure at 2073 K. Rapid cooling (at a rate of about 400 K/min during the first 2 minutes) of the thermocouple

assemblies to room temperature after exposure for 88, 252, 553, and 1680 hours did not

i appreciably alter their values of thermal emf at 2073 K. Post test examinations of the

j component materials revealed no evidence of any gross chemical attack. However, some minor

i contamination at the surface of the thermoelements was observed along those portions that were in a steep temperature gradient during the test.

The unsheathed, BeO insulated W-3% Re vs W-25% Re thermocouples were tested in argon

at 2250 K for 1100 hours and at 2400 K for 307 hours. Their values of thermal emf at the

test temperature typically increased by the equivalent of a few degrees K to as much as 11 K. While these changes are small, they nevertheless are substantially larger than the

changes observed in the test of the sheathed thermocouple assemblies. The unsheathed

( thermocouples exhibited undesirable material degradation and interaction effects in both

I tests. These effects included erosion and a reduction in the diameter of BeO insulators

in the hot-zone of the furnace, coatings of tantalum on bared portions of the thermoelements, coatings of beryllium on the BeO insulators in the temperature-gradient zone of

the furnace and microcracking of the BeO insulators in cases where the exposure temperature

exceeded the phase transformation temperature of the BeO.

Studies of the chemical compatibility of the BeO insulators with the tantalum sheaths were performed by exposing open-ended tantalum sheaths that contained a specimen of BeO

tubing. Tests were run at 2073, 2250, and 2400 K in an argon environment. Gross erosion

of the BeO insulator occurred with only 50 hours exposure at 2400 K, particularly at the

open end of the sheath (greater than 50% erosion was typical) . Erosion was markedly less

inside the sheath, where the gaseous reaction products were confined. Microcracking of
the BeO also occurred. With 1100 hours of exposure at 2250 K similar erosion of the BeO

was observed but no microcracking was evident. Serious problems with the BeO were not

apparent in tests at 2073 K. At this temperature the BeO insulators exhibited shrinkage of about 10 percent in diameter. This was also observed in earlier tests (ref. 1) in

which BeO insulators were exposed to high temperatures in the presence of tantalum. The

shrinkage results from further densif ication of the sintered insulator and is accompanied

by a reduction in the impurity level.

INTRODUCTION

This report presents results of a continuing study (refs. 2 and 3), which has been

directed towards improving the general reliability of metal-sheathed, ceramic-insulated,

W-Re type thermocouple sensors . Reliable sensor operation for prolonged periods of time

(for many thousands of hours) at temperatures above 1800 K has been of primary concern. Useful criteria have been sought regarding the selection, preparation, and performance of

materials that might be applied in the design and construction of suitable sensors.

Previous studies were concerned with the behavior of bare (non- insulated) commercially

available W-Re alloy thermoelements and with the behavior of these thermoelements when

fabricated as a thermocouple and insulated with high purity BeO. In the initial studies

(ref. 4) 0.25 mm diameter W-3% Re and W-25% Re thermoelements were exposed at temperatures

of 2200 to 2600 K in environments of high purity argon or hydrogen (nominally 1 atm

pressure) or in high vacuum of less than 1.3 x 10~^ Pa (1 x 10~^ torr)^ for periods up to 1000 hours. With exposure in the gaseous environments, initial changes^ (shifts) in the

emf-temperature relationship of the thermoelements were found to occur usually within 50

hours or less of exposure^. With continued high temperature exposure of the thermoelements

in the gaseous environments, the subsequent long term changes (drift) in the emf-temperature relationship were negligible. Conventional metallographic and chemical analyses were performed to more fully characterize the thermoelements. The thermoelements exposed

in vacuum at temperatures above 2200 K exhibited a continual drift in their emf-temperature

relationship as a result of the preferential evaporation of Re.

Subsequent work (ref. 1) extended the bare-wire studies to include other commercially

available W-Re alloy thermoelements. larameters for aging were developed that would

minimize the initial shift in the thermal emf and yet leave the thermoelements with

sufficient room temperature ductility to permit subsequent fabrication of thermocouple

sensors. In addition, studies were initiated to examine the behavior of thermoelements and

thermocouples when insulated with high purity sintered BeO tubing. W-3% Re vs W-25% Re

thermocouples, fabricated from thermally aged thermoelements and insulated with well degassed sintered BeO tubing, exhibited drifts of only 2 to 3 K during exposure in argon at 2073 K for 1029 hours.

1 torr = (1.01325 x 10^/760) Pa. All values of pressure are indicated equivalent

nitrogen pressures.

The change in the emf-temperature relationship of a thermoelement is defined as the

emf-temperature relationship of a thermocouple that is comprised of that thermoelement

versus an unheated "as received" thermoelement from the same spool (lot) of wire, when

the reference junctions are maintained at 273.15 K and the unheated "as received"

thermoelement is designated as the negative leg of the thermocouple.

Efforts to minimize the shift have been reported. See for example, Tseng et al.,

ref. 8.

2
This work presents further studies of the thermoelectric stability of BeO- insulated

W-3% Re versus W-25% Re thermocouples during exposure in argon at 2250 K for 1100 hours

and at 2400 K for 307 hours. Studies of the compatibility of BeO with Ta sheaths at 2073,

2250 and 2400 K are presented. Finally, using carefully prepared and cleaned materials,

Ta sheathed, BeO insulated W-3% Re vs W-25% Re thermocouples were fabricated and tested in

helium at 2073 K for 2059 hours. The thermal emf test results and the post-test analyses

of the component materials are presented.

EXPERIMENTAL PROCEDURES AITO APPARATUS

DRIFT TEST WITH SHEATHED THERMOCOUPLE ASSEMBLIES

Preparatory heat treatments and chemical cleaning of materials . Four 1.6 mm diameter

tantalum sheathed, BeO- insulated , W-3% Re versus W-25% Re thermocouple assemblies were

fabricated for testing at 2073 K. Prior to the fabrication, the component materials were

subjected to preparatory heat treatments and cleaning procedures.

The thermoelements were cleaned with ether and then heated for about 1 hour at 1570 K

in vacuum (<5 x 10~^ torr)'*, using test chambers described previously (refs. 2 and 4).

This was followed by aging at 2400 K in high purity argon (<10 ppm total impurities) in

order to essentially remove the initial emf shift (refs. 1 and 3). The W-3% Re thermoelements were aged for 1 hour and the W-25% Re thermoelements were aged for 10 minutes.

The BeO insulators were vacuum degassed in an ultra-high-vacuum (UHV) , nigh temperature furnace. The furnace, described previously, employs a tungsten wire mesh heating

element, tungsten radiation shields and a copper cold wall, and is pumped with an ion

pump and an optional titanium sublimation pump (refs. 1 and 2). For the degas, the BeO

insulators were suspended in the 30 cm long hot-zone of the furnace in a W-Re basket. The

furnace temperature was increased to about 1525 K and held for about 12 hours, and then increased to about 1700 K where it was held for 9 hours. The pressure in the furnace was maintained at less than 5 x 10"^ torr by continuous pumping during the degas.

The tantalum sheaths were cut to approximately the desired length with a 0.15 mm

thick Ail203 cut-off wheel. The sheatis were rinsed in methanol and trichloroethylene,

and then etched for about 2.5 minutes in a solution of 33% HNO3 , 33% HF and 33% lactic acid.

Following the acid etch, the sheaths were rinsed for 5 minutes in tap water and then rinsed

again in the above solvents. The sheaths were cut to final size (about 29 cm long) by

cutting off one end with a tube cutter that had a hardened steel cutting wheel and was modified so that its steel rolling surfaces were replaced with a strip of Ta, The cut end

of the sheath that was partially closed by the tube cutter was then welded closed with a tungsten inert gas welder under cover of argon gas. The sheaths were then put in a Ta

holder and placed in the UHV furnace for vacuum degassing. For degassing, the furnace

temperature was increased slowly over a period of 10 hours to about 1875 K, held at this

temperature for about 16 hours and then increased to about 2175 K where it was maintained

for about 1 hour. The furnace was then cooled in steps to room temperature in about 2 hours. During the degas, the pressure was maintained at less than 5 x 10"^ torr.

Fabrication and installation of the thermocouple assemblies . Following the heat

treatments of the component materials, the components were stored in a vacuum until the

construction of the thermocouple assemblies was undertaken. For assembly, the measuring

junctions of the four thermocouples were formed by welding with a dc arc in a helium

atmosphere. The double-bore BeO insulators were then gently slid onto the thermoelements

and the thermocouples were slipped into the tantalum sheaths until the measuring junctions were approximately 5 mm from the closed-end of the sheath. All material handling was done

with polyethylene gloves and chemically-cleaned tools.

1 torr = (1.01325 x 10^/760)Pa. All values of pressure are indicated equivalent

nitrogen pressures.

3
The four tantalum sheathed thermocouple assemblies were installed in a tantalum

blackbody enclosure that was suspended in the hot-zone of the UHV high-temperature furnace j as shown in Fig. 1. The apparatus is designed so that, after testing, the blackbody and | top radiation shield pack can be removed along with the 15 cm diameter top access flange j in such a manner as to not load any of the sheaths. Dismantling of the sheaths after

testing can then be done outside of the furnace in as non-destructive a manner as possible.

The blackbody enclosure closely approximates an ideal blackbody if the walls of the en- closure are at a uniform temperature. Using calculations derived by DeVos (ref. 5) , it

is estimated that the effective emittance of the enclosure is greater than 0.995.

The mounting of the sheath into the stainless steel access flange is shown in Fig. 1. Underneath the flange, the Ta sheath is held in a stainless steel three-piece compression

type fitting. A 0.15 mm thick gold 0-ring is inserted into the fitting and provides the

primary vacuum seal at the end of the Ta sheath. The stainless steel tee mounted on the

top of the flange is an UHV coupling that employs copper gaskets. The thermoelements are brought out through a Kovar-seal header, and the thermoelements are sealed with a solventfree epoxy resin to the Kovar feedthrough tubes in the header. A connection from each tee

is made to a manifold through a 6.3 mm diameter stainless steel bellows and a stainless

steel UHV valve. The manifold is also of stainless steel, of UHV construction, and is

equipped with a 200 liter/s ion pump, an ultra-high purity argon gas supply, and a partialpressure analyzer.

Initiation of the drift test and test procedures . After installing the blackbody

enclosure with the thermocouple assemblies in the UHV furnace, the furnace and manifold i were evacuated and baked for about 10 hours at about 525 K. The thermocouple assemblies

were then vacuum degassed iri situ : The furnace temperature was increased to about 975 K

over a 3-hour period, held at this temperature for about 22 hours, and then increased to about 1325 K and held for 2 hours. The pressure in the thermocouple manifold was maintained at less than 2 x 10~^ torr during these processes. Following the in situ degas,

the thermocouple p-^semblies were cooled to room temperature and the furnace chamber was | backfilled with helium gas. The thermocouple manifold was backfilled with argon gas. The

j temperature of the furnace was then slowly increased (over a 4-hour period) to about 2073 K

and stabilized to initiate the drift test. The thermocouples were calibrated against a visual optical pyrometer at about 1195, 1395, 1590 and 1795 K on bringing them to test

temperature. (With the furnace at 2073 K, the pressure in the furnace chamber was about ' 840 torr and the pressure in the thermocouple manifold was about 780 torr.) The isolation { valves to each thermocouple assembly were closed after reaching the test temperature.

During the 2059 hours of test, temperature measurements were made about every 24 hours

with an automatic photoelectric optical pyrometer and about every 70 to 100 hours with a visual optical pyrometer (using two observers). Corrections were applied to account for the

transmittance of the view port of the furnace. A mechanical shutter was used to shield this

view port from the blackbody whenever temperature measurements were not being made. The

pyrometers used were of commercial design and were calibrated^ by relating the pyrometer

lamp current to the brightness or blackbody temperature. In use, the pyrometer lamp current

was determined by measuring the voltage drop across a 1 ohm standard resistor in series with

the lamp. The reference junctions of the thermocouples were maintained at 273.15 K in an

ice bath during the test. The values of thermal emf were measured with a digital voltmeter

(DVM) having an absolute voltage accuracy of ±(0.004% of voltage measured +2 yV) . The DVM

was intercompared with a standard cell and also with a precision potentiometer after every

several hundred hours of test. Following procedures similar to those used in previous

tests (ref.l), the measured values of emf were adjusted to correspond to a temperature of

2073 K by making a small correction equivalent to the difference between the temperature

determined with the optical pyrometer and 2073 K.

Calibration was performed by the Optical Radiation Section of the NBS.

4
STAINLESS STEEL TEE HEADER FOR THERMOELEMENT

FEEDTHROUGH

STAINLESS STEEL

UHV VALVE

STAINLESS STEEL

BELLOWS

STAINLESS STEEL^^

UHV FLANGE

COPPER GASKETS

STAINLESS STEEL

COMPRESSION FITTING

TANTALUM RADIATION SHIELDS

ELECTRODE

TO MANIFOLD LEADING

TO ION PUMP AND TO

GAS SUPPLY

TUNGSTEN WIRE-MESH

HEATING ELEMENT

TUNGSTEN RADIATION

SHIELDS

TANTALUM

SUPPORT TUBE

TANTALUM SHEATHED

THERMOCOUPLE

ASSEMBLY

TANTALUM BLACKBODY

ENCLOSURE

BLACKBODY OPENING

(2mm diam.)

Figure 1. Features of experimental apparatus illustrating the installation of the sheathed

thermocouple assemblies for drift testing. Four assemblies were installed.

5
DRIFT TESTS WITH UNSHEATHED THERMOCOUPLE ASSEMBLIES

Tests were conducted to evaluate the performance of unsheathed BeO-insulated W-3% Re versus W-25% Re thermocouples in argon at temperatures above 2073 K. Test temperatures of

2250 and 2400 K were employed. The preparation of the W-Re thermoelements and the BeO

insulators was the same as that described for the sheathed thermocouple drift test. For

the tests, the thermoelements were inserted into the BeO, and the assembly was placed into

a tantalum blackbody (similar to that in Fig. 1) so that the measuring junction was within

the blackbody enclosure. The insulated thermocouples were inserted into the blackbody en- closure through the 6.35 mm diameter tantalum supporting tube (as described in more detail

in ref. 1). Usually, four thermocouples were inserted into the blackbody. Prior to testing, the thermocouples were vacuum degassed in_ situ with 2 hour exposures at temperatures

of 1000, 1100 and 1300 K. The procedure for testing of the thermocouples was very similar

to that described for the sheathed thermocouple assemblies in the previous section.

COMPATIBILITY STUDIES OF BeO INSULATORS WITH TANTALUM SHEATHS

A series of tests were run exposing the BeO insulators contained in the tantalum

sheaths to high temperatures. In preparation for these tests, the BeO insulators were

degassed in the same manner as that described for the drift tests. The procedures for

cleaning and vacuum-degassing of the tantalum sheaths were also similar to that previously

described, except that the sheaths were not etched. (Later, similar tests were performed

in which etched sheaths were used and virtually identical results were obtained.) Short

lengths (typically 5 to 6 cm) of the Ta sheathing were cut with the A2,203 cut-off wheel,

cleaned, vacuum degassed, B^i then pinched closed at one end. The degassed BeO insulators were broken into lengths aoout 1 cm longer than the Ta sheaths, and placed in the sheaths.

Hence, about 1 cm of the insulator extended beyond the open end of the sheath. The BeO-Ta

sheath assemblies were then strapped with Ta wire on the outside of a Ta blackbody. The

blackbody, which was similar to the one shown in Fig. 1, was then placed in the UHV furnace,

and the usual bake-out of the furnace and a low temperature degas of the assemblies (at

about 1000 to 1300 K for 3 to 6 hours at pressures of less than 5 x 10~^ torr) were performed. After cooling the furnace to ambient temperature, it was back-filled with ultrahigh-purity argon gas in preparation for the high temperature exposure.

MATERIALS TESTED

The W-3% Re and W-25% Re thermoelements for the tests were taken from a matched lot of

thermocouple wire that was obtained from a leading commercial supplier. The thermoelements were nominally 0.25 mm in diameter. They were given a special surface cleaning treatment

by the supplier in order to more thoroughly remove surface contaminants introduced during

the manufacturing processes. The cleaning treatment included ultrasonic degreasing,

abrading, and electro-etching, and is described in detail by Toenshoff, et al. (ref. 6).

The W-3% Re thermoelement is an alloy which has been "doped" by adding small amounts of

potassium, silicon, and aluminum compounds to the tungsten oxide prior to its reduction to

a metal. The residual doping elements, which remain after the wire forming process, act to produce metallurgical structural changes in the wire when it is exposed to high temperatures that result in improvements in the mechanical properties (ref. 7). The W-25% Re

thermoelement is not "doped".

Typical mass spectrographic analyses for specimens from these lots of thermoelements were reported previously (ref. 1). For the W-3% Re thermoelement the major impurities

detected were K (100 ppm) , Mo (100 ppm) , and Fe (50 ppm) , while impurities of Na, Mg, AZ,

Si, CI, Ca, V, Cr, Ni, Cu, Ge, and Ta were detected at the 1 to 20 ppm level. For the

W-25% Re thermoelement. Mo (100 ppm) and Fe (50 ppm) were the major impurities detected,

with Na, P, K, Cr, Ni, Ge, and Ta detectable at the 10 to 20 ppm level.

Representative thermocouples from the matched lot of thermocouple wire were calibrated

in an argon atmosphere by intercomparison with Pt-Rh type thermocouples in the 673 to 1473 K range and by intercomparison with an automatic photoelectric optical pyrometer at

temperatures to 2400 K. The emf -temperature relationship of the thermocouples, as derived

6
from these calibrations, deviated by less than ±1% from the tabular values of emf and

temperature that were reported for the W-3% Re vs W-25% Re thermocouple by Tseng et al.

(ref. 8).

High purity sintered beryllium oxide (BeO) double-bore insulating tubing with 0.3 mm

diam. bores, 1.1 mm outer diameter and 0.13 ±0.03 mm web size (space between bores) was

used in the experiments. All the tubing was from the same production batch and was supplied by the manufacturer in random lengths between 10 and 25 cm long. Quantitative

spectrochemical analyses of representative "as received" samples from this batch of tubing were reported previously (refs.~ 1 and 3). The major impurities detected were kl

(200 to 300 ppm)^, C (120 to 230 ppm) , Si (120 to 150 ppm) , Mg (90 ppm) , and Ca (30 to 50

ppm) , vvhile the total detectable impurities (metallic and carbon) were less than 1000 ppm.

The grain size of the "as received" tubing, as determined from examinations of polished

and etched specimens, ranged from about 1 to 5 ym.

Tantalum tubing with a nominal 1.6 mm o.d. and 1.3 mm i.d. was used in the experiments.

The tubing was all taken from the same production lot, which was made by a seamless

method from electron-beam melted raw material. A variation of about 10 percent was observed in the inner diameter of tubing from this lot. The tubing was supplied by the

manufacturer in an as-drawn, straightened, and cleaned condition. The manufacturer re- ported that the cleaning process consisted of vapor degreasing in hot trichloroethylene

after drawing to final size. A typical mass spectrographic analysis of the tantalum tubing

is given in the Experimental Results and Discussion section. Niobium (50 ppm) was the

principal metallic impurity detected, while other impurities such as F, K, Cu, Zn, and W

were detected at the 1 to 10 ppm level. A report of chemical analysis furnished by the

manufacturer of the tantalum tubing indicated carbon at 25 ppm, O2 at < 50 ppm, H2 at

< 5 ppm and N2 at < 10 ppm.

The argon and helium gases used in the various tests were taken from cylinders of

ultra-high purity compressed gas. The gases were certified by the commercial supplier to contain less than 10 ppm by volume total impurities^ and within detectable limits this was supported by chemical analyses performed by the Analytical Chemistry Division of the NBS

Concurrent with these studies, a program was sponsored by NASA for development of

commercial thermocouple assemblies. This has been reported by Toenshoff, Zysk and

Fleischner (ref. b) . The thermocouples they produced had the same general design as those

described in this report. The materials used in both programs were of similar quality.

EXPERIMENTAL RESULTS AND DISCUSSION

TESTS WITH SHEATHED THERMOCOUPLE ASSEMBLIES

Thermal emf drift . The thermal emf drift of four 1.6 mm o.d. tantalum sheathed, BeO

insulated, W-3% Re versus W-25% Re thermocouple assemblies was determined during exposure

at 2073 K for 2059 hours in helium (1 atm) . The methods used during the test to obtain

the experimental data are discussed in the section on experimental procedures and apparatus.

The four thermocouple assemblies exhibited virtually identical behavior during the test.

The values of thermal emf determined for one of the thermocouple assemblies are plotted

against time of exposure at 2073 K in Fig. 2. The data presented are based upon measurements made with the automatic photoelectric optical pyrometer. A straight line is fitted

to the data by the method of least squares to show the general trend of the data (the

drift). The slope of the line is -0.013 yV/h, which is equivalent to about -0.8 K per

1000 hours. While some systematic variation of the data points about the straight line is

The values in parentheses give the range in the composition of the element in parts per

million by weight as determined from analyses of several specimens.

This assumes the helium gas to be free of neon.

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evident, the maximum deviation of any data point from the line is equivalent to only 1.2 K. The standard deviation of residuals is equivalent to 0.5 K and the standard deviation of

the slope is equivalent to 0.09 K per 1000 hours. The use of higher order terms to achieve

a more precise fit of the experimental data seemed unwarranted. The oscillatory nature of

the experimental data is thought to have resulted principally from some instability in the

automatic photoelectric optical pyrometer.

Such oscillations were also apparent in the data obtained for the other three thermocouple assemblies. Similar treatment of the experimental data for the other thermocouple

assemblies gave values for the "drift at 2073 K of -0.5, -0.7, and -0.8 K per 1000 hours.

Measurements with the visual optical pyrometer yielded values for the drift that agreed

within 0.2 K per 1000 hours with those derived from the automatic photoelectric pyrometer

measurements. Typical results, based upon measurements with the visual optical pyrometer,

are shown in Fig. 3. The data of both observers are plotted against exposure time at

2073 K for the same thermocouple assembly as the one used in Fig. 2. The straight line

fitted to the data has a slope of -0.010 pV/h, which is equivalent to -0.6 K per 1000

hours. The standard deviation of residuals is equivalent to 0.7 K and the standard deviation of the slope is equivalent to 0.15 K per 1000 hours. The test results for all

four thermocouple assemblies are summarized in Table I.

As indicated in Figs. 2 and 3 the thermocouple assemblies were cooled to room temperature ( ~ 295 K) on four occasions during the test; after about 88, 252, 553, and 1680 h of

exposure. The rate of cooling on all four occasions was approximately the same and is

shown in Fig. 4. The average rate of cooling during the first 2 minutes was about 400 K/min.

After cooling, the thermocouple assemblies were heated to 2073 K over a period 1 to 4 hours

in order to resume the test. It is evident from the results presented in Figs. 2 and 3 that the thermal cycling had no appreciable effect upon the thermal emf of the thermocouple

at 2073 K. During the thermal cycles which occurred after 252 and 553 h of exposure, the

thermocouple assemblies were calibrated by intercomparison with the visual optical pyrometer at about 1195, 1395, 1590, 1795, and 2073 K. The calibrations were performed during

the heating portion of the cycle and two observers made readings with the pyrometer. A

similar calibration was also performed both before and after the test. The differences

between the calibrations that were performed after 252, 553 and 2059 hours of exposure and

the calibration performed before the test are shown in Fig. 5 for two of the thermocouple

assemblies. They are the assemblies which had the largest and the smallest measured

drifts at 2073 K (see Table I, thermocouple Nos . 2 and 3, respectively). These data indicate the drift was also less than the equivalent of 1 K per 1000 hours for calibration

temperatures below 2073 K. While a small deviation between the initial calibration and

the subsequent calibrations is apparent at the lower temperatures, the calibrations after

252, 553 and 2059 hours of exposure agree within estimated observer precision.

Before any significance can be attached to the small drifts that were derived from

the experimental data, the measurement uncertainties must be considered. Drift in the

calibrations of the optical pyrometers and changes in the spectral transmittance of the

furnace window during the test are the principal factors contributing to the measurement

uncertainties. The calibrations of both pyrometers were checked at about 2060 K before, near the middle, and after the 2059 hour drift test by the Optical Radiation Section at

the NBS. The pyrometers were intercompared with the NBS standard photoelectric pyrometer

(ref. 9) using a vertical resistive-heated graphite blackbody as a transfer source. A

small change, equivalent to about 1.8 K, was detected in the calibration of the automatic photoelectric pyrometer and a small correction was applied to the experimental data

to account for it. It is estimated that this reduced the uncertainty in the measurements,

due to any drift in the calibration of the automatic photoelectric pyrometer, to less

than the equivalent of 1 K. There was no significant change ( 4 equivalent of 1.5 K)

detected in calibration of the visual optical pyrometer. The spectral transmittance of

the furnace window was also checked before and after the test and no significant change

was apparent in it ( 4 equivalent of 0.5 K) . The calibration of the DVM was checked

periodically during the test by the method described in the section on experimental procedures and apparatus. These checks indicated that uncertainties in the measurements

arising from any drift in its calibration were less than the equivalent of 0.2 K. Systematic errors introduced by other means, such as a gradual change in blackbody

conditions, are thought to be small ( 4 equivalent of 0.2 K)

9
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If the above allowances for systematic error are added, one gets 1.9 K for measurements with the automatic photoelectric pyrometer and 2.4 K for measurements with the visual

optical pyrometer. When the imprecision of the experimental data is considered together

with possible systematic errors, the observed changes, which were less than 2 K during the

2059 hour test, are small by comparison and are therefore concluded to be insignificant.

It is interesting to compare the results of this test with those reported recently by

Heckleman and Kozar (ref. 10) for an out-of-pile evaluation of some sheathed W~3% Re versus W-25% Re thermocouples, heckleman and Kozar measured the thermal emf drift of six

tantalum sheathed, BeO insulated, 0.157 cm diameter, grounded junction W-3% Re versus W-25% Re thermocouples during exposure to temperatures in the 1900 to 2100 K range for in excess of 8000 hours. The thermocouples were built commercially to rigorous specifications

and supposedly represented the current state of the art as far as fabrication techniques were concerned. During the test they were inserted in a tantalum blackbody, which was heated in a high purity helium gas environment. A negative drift of 55 to 60 K at 2073 K

occurred during the first 2000 hours of testing but little change occurred thereafter.

Post-test examinations of the thermocouples directed towards identifying the mechanisms

responsible for the drift were planned, but could not be reported, because of termination

of the reactor program. Heckleman and Kozar conjectured that the drift was probably caused

by chemical composition changes resulting from contamination by impurities that were present in the insulator, sheath, or on the wires. They concluded that after about 2000 hours

the impurities had apparently been gettered or had diffused to some other location where

they were no longer effective in promoting further drift.

The thermocouples tested by Heckleman and Kozar used crushable insulators that were compacted by swaging and had grounded junctions, while those tested in the present work

had hard, sintered double-bore insulators and ungrounded junctions. This might possibly

account for some difference in behavior. We attribute the highly stable behavior of the

thermocouple assemblies that were fabricated at NBS primarily to the careful selection,

preparation and assembly of the component materials. While the type of construction may

also be a factor, its importance is thought to be secondary. In any event, from the

results of these two experiments it is clear that highly reliable tantalum-sheathed W-Re

type thermocouple sensors can be fabricated. Satisfactory performance for periods in excess of 8000 hours at temperatures to 2100 K without any appreciable emf drift is

possible with suitably fabricated sensors in environments that are not intrinsically

deleterious to the sensor materials

.

Post-test examination of materials . At the conclusion of the 2059 h test, the

thermocouple assemblies and tantalum blackbody were removed from the furnace as a unit

and inspected. Some warpage of the tantalum sheaths was evident and each of the sheaths was fused to the tantalum blackbody at the point where it entered the 3.2 cm diam. en- closure (see Fig. 1). The sheath on thermocouple No. 2 was also fused to one of the

tantalum radiation shields at the top of the furnace hot-zone and it had a small crack

at this point. It was not clear whether the crack occurred during the test (probably on thermal cycling) or whether it occurred when the assemblies were removed from the furnace.

In any event, the performance of thermocouple No. 2 was not significantly different from

that of the other three thermocouples. To check on gas tightness, the sheaths on the

other three thermocouple assemblies were evacuated and helium leak tested. Uo leaks were

detected in any of the sheaths. Visual inspection revealed no evidence of any serious

chemical attack of the tantalum sheaths or the tantalum blackbody enclosure.

The thermocouple assemblies were then removed from the blackbody enclosure and dismantled. By making transverse cuts at several places along the thermocouple assemblies

the BeO insulators were easily removed from the tantalum sheaths. The thermocouple wires were free within the bores of the insulators and were also easily removed for examination.

No gross chemical attack of the thermoelements, BeO insulators, or the tantalum sheaths

was apparent.

Photomicrographs of transverse cross-sections that were taken at about 2 cm and at about 28 cm from the sheath tip of one thermocouple assembly are shown in Fig. 6. The

microstructures of the materials, as illustrated in the cross-section at 2 cm, are typical

of that exhibited by materials which were taken from locations up to 15 cm from the sheath

14

/
Figure 6. Photomicrographs showing transverse cross-sections of a 1.6 mm diameter tantalum sheathed, BeO-insulated, W-3% Re/W-25% Re thermocouple (a) at 28 cm from the sheath

tip where no high temperature exposure occurred, and (b) at 2 cm from the sheath tip

where exposure for 2059 hours at 2073 K occurred.

15
tip. The cross-section at 28 cm from sheath tip was taken at a location along the thermocouple assembly that was not heated. Therefore, the microstructures are typical of the

materials prior to testing (and subsequent to the preparatory heat treatments and assembly)

.

Those portions of the W-25% Re thermoelements that were in the hot-zone region of the

furnace experienced substantial grain growth, with the wires exhibiting equiaxed grains

roughly 50 ym up to one-half wire diameter in size. In contrast, the exposed portions of

the W-3% Re thermoelements experienced virtually no grain-growth. They had a fine-grained

structure similar to that of the aged "starting" material. These results are to be ex- pected from material behavior observed in earlier work (refs. 1 and 4), where bare thermoelements were exposed at temperatures up to 2400 K for periods as long as 1000 hours.

As shown in Fig. 7, a temperature gradient of approximately 400 to 500 K/cm existed

along the thermocouple assemblies during the test in the region between 15 and 18 cm from

the sheath tips . Examination of the thermoelements in this region revealed that some chemical interactions had occurred at the wire surfaces. The appearance of the surfaces

of both thermoelements is shown by the photomicrographs in Fig. 8. For purposes of com- parison, photomicrographs showing the typical appearance of the surfaces of thermoelements

in the high-temperature zone (extending from the sheath tip to 15 cm from tip) and in the

unheated zone are included in Fig. 8. The interaction is more apparent in the W-25% Re

thermoelement. The surface of the W-25% Re thermoelement is shown at higher magnification

by the photomicrograph in Fig. 9. The formation of an apparent second phase at the grain

boundaries can be seen. A longitudinal cross-section taken very near the surface of the W-25%

Re thermoelement is given in Fig. 10 and it clearly shows the presence of a second phase

at the grain boundaries. The second phase presumably resulted from some attack of the

wire surface by chemical contaminants. While some diffusion from the wire surface along

the grain boundaries is evident, it is limited to a depth of about 10 ym. Transverse

cross-sections of the thermoelements in this region revealed no evidence of other phases

at the interior of the wire. A positive identification of the chemical composition of the

second phase has not been made. However, spectrochemical analyses of specimens that were

taken from the thermoelements in this region indicated a small increase in the Mg and Ca

impurity levels. No significant increases in the impurity levels of A£, Be, Fe, Si, or Ta were detected.

The BeO tubing that was in the high temperature zone had a translucent appearance. As

shown in Fig. 6 appreciable grain-growth and about a 10% reduction in the tubing diameter

are evident. The grain size of the exposed BeO tubing ranges from about 30 to 90 ym. By

comparison, the grain size of the "as received" BeO tubing was typically 1 to 5 ym and

virtually no change in the grain size occurred as a result of the preparatory heat treatments (vacuum degassing) . The diameter reduction is thought to be a result of densification. In other tests (see section on Studies of BeO-Ta Sheath Compatibility and also

ref. 1) where tubing from the same lot was exposed at temperatures in the 2000 to 2250 K

range for prolonged periods in the presence of tantalum similar behavior was noted. In addition, in earlier work (refs. 1 and 3) a substantial increase in the purity of the

exposed insulators was observed; sizable reductions were detected in the impurity levels

of principally A£ , C, Ca, Mg and Na. The residual impurities were suspected to have been

gettered by the tantalum. The results of mass spectrographic analyses on exposed tantalum

specimens from the present test further support this assumption. Typical chemical analyses

are given in Table II for a specimen of the chemically cleaned, etched, and vacuum degassed

tantalum tubing and for a specimen of tantalum tubing that was taken from one of the

thermocouple assemblies at a point between 2 and 6 cm from the sheath tip.

Ail, Ca, Fe, Mg, Na, Si and Ti, which are residual impurities in the BeO tubing, are detectable in the exposed thermocouple sheath at a level at least ten-fold greater than in

the unexposed tantalum tubing. The most notable increase is that for A£. This is not

surprising since AH is the principal impurity in the BeO tubing, where it is detectable at

the 200 to 300 ppm level (refs. 1 and 3). The BeO tubing is also the most probable source

of the contaminants which were responsible for the attack noted at the surface of the

thermoelements in the temperature gradient zone.

16
17
W-25%Re THERMOELEMENTS W-3%Re THERMOELEMENTS

figure 8. Photomicrographs showing the typical appearances of the surfaces of the thermoelements in various regions along the tantalum sheathed, BeO-insulated, W-3% Re/W-25% Re

thermocouple assemblies that were exposed for 2059 hours at 2073 K: (a) in the unheatedzone greater than 25 cm from the sheath tip; (b) in the temperature-gradient zone between 15 and 18 cm from the sheath tip; and (c) in the high temperature zone extending

from the sheath tip to 15 cm from the tip.

18
Figure 9. Photomicrograph showing the surface of the W-25% Re thermoelement in Fig. 8(b)

at higher magnification.

Figure 10. Photomicrograph exhibiting a longitudinal cross-section just below the surface

of the W-25% Re thermoelement, whose surface is shown in Figs. 8(b) and 9.

19
TABLE II. MASS SPECTROGRAPHIC ANALYSES OF TANTALUM THERMOCOUPLE SHEATHS

(composition in ppm by weight)

Specimen from

Chemically cleaned, sheathed thermocouple

etched, and vacuum assembly after 2059 h

Element degassed specimen^ at 2073 K

Li 0.001 0.002

Be 0.005 0.2

B 0.01 0.03

F 2. 30.

Na 0.4 6. Mg 0.1 5. Al 0.1 150.

Si 0.2 6. P 0.05 3.

S 0.3 10.

CI 0.4 2. K 2. 5. Ca 0.2 2. So <0.03 0.03

Ti 0.03 7. V 0.05 1. Cr 0.1 0.5

Mn 0.03 0.2

Fe 0.7 7. Co <0.02 0.2

Ni <0.1 0.3

Cu 3. 2. Zn 3. 5. Ga <0.01 <0.01

Ge 10.02 10.04

As <0.04 <0.04

Se <3. <3.

Br <0.1 0.1

Rb <0.005 0.03

Sr <0.005 0.05

Y 11. 10.2

Zr <0.3 0.5

Nb 50. 50.

Mo 0.1 0.2

Ru <0.06 <0.06

Rh <0.02 <0.02

Pd <0.2 <0.2

Ag 10.04 0.4

Cd <0.04 0.4

In <0.06 <0.06

20
TABLE II, continued

(composition in ppm by weight)

Specimen'' from

Chemically cleaned, sheathed thermocouple

etched, and vacuum assembly after 2059 h

Element degassed specimen^ at 2073 K

Sn <0.03 0.3

Sb <0.02 <0.03

Te <0.04 <0.04

I <0.02 10.02

Cs <0.02 <0.02

Ba <0.01 0.4

La <0.01 <0.01

Ce <0.01 <0.02

Pr <0.01 <0.01

Nd <0.03 <0.03

Sm <0.03 <0.03

Eu <0.02 <0.02

6d <0.03 <0.03

Tb <0.01 <0.01

Dy <0.04 <0.04

Ho <0.01 <0.01

Er <0.03 <0.03

Tm <0.01 <0.01

Yb <0.03 <0.03

Lu <0.01 <0.01

Hf <0.1 <0.1

W 10. 20.

Re <0.05 0.1

Os <0.1 <0.1

Ir <0.1 <0.1

Pt <1. <1.

Au <2. <2.

Hg <0.03 <0.03

Tl <0.03 <0.03

Pb <0.1 0.4

Bl <0.03 <0.03

Th <0.2 <0.2

U <0.3 <0.2

Ta Balance Balance

See section on Experimental Procedures and Apparatus for details of

specimen preparation.

'Taken from between 2 and 6 cm from the tip of sheathed thermocouple

assembly.
TESTS OF UNSHEATHED BeO-INSULATED THERMOCOUPLES

Drift tests with BeO-insulated W-3% Re versus W-25% Re thermocouples were performed in

argon at 2250 K for 1100 hours and at 2400 K for 307 hours. Newly constructed thermocouples were used in each test. The test results were as follows:

Thermal emf drift . The value of the thermal emf at 2250 K as a function of the time

of exposure was typically that illustrated in Fig. 11. The data illustrated used the

photoelectric pyrometer as a reference. For each thermocouple, the experimental data were

fit by the method of least squares to either a quadratic or cubic equation. The results

for all four thermocouples are suiranarized in Table III.

For three of the thermocouples (Nos . 2, 3 and 4) the largest change after 1100 hours

was equivalent to only about +3 K. The other thermocouple (No. 1) exhibited a change

equivalent to about +11 K. The changes computed for the same thermocouple from measurements with the photoelectric and the visual pyrometers differed slightly, but agreed to within the combined precision of the measurements (the standard deviation of the residuals

was equivalent to about 0.4 K for the photoelectric pyrometer data and to about 1 K for the

visual pyrometer data) . The calibrations of both pyrometers were checked^ before and after

the drift test and these checks indicated that no appreciable change occurred in the calibration of either instrument (< 1 K for the photoelectric pyrometer and < 1.5 K for the

visual optical pyrometer) . A check of the furnace window before and after the test indicated no significant change (equivalent to < 0.5 K) occurred in its transmittance

.

Considering the possibility of changes in the calibration of the pyrometers and in

the transmittance of the furnace window, together with the scatter in the measurements

and the nature of the drift results, it is perhaps better to describe the thermal emf of

thermocouples 2, 3 and 4 as being stable during the 1100 hour test period to within the

equivalent of about ±2 K at 2250 K.

The value of the thermal emf at 2400 K as a function of time was typically that

illustrated in Fig. 12. Again, the data illustrated were obtained with the photoelectric

pyrometer as a reference, because of a gradual sagging of the Ta blackbody enclosure

(about 6 mm in 307 hours) , the test was terminated at 307 hours rather than risk failure

of the enclosure and damage to the furnace. The results for four of the thermocouples in

the test are summarized in Table IV. The values of the drift were obtained from curves

(either linear or quadratic) that were fit to the experimental data by the method of least

squares. The drift in the four thermocouples varied from about +3 K to about +11 K at

2400 K after 307 hours of exposure. A fifth thermocouple in the test behaved erratically

after approximately 100 hours of exposure, and the results are not included in the

siumnary. Post- test examination revealed a region of gross erosion of the BeO insulators

that bared the thermoelements (as described in more detail in the next section) . It is

likely that electrical shorting of the thermoelements occurred, thus accounting for the

erratic behavior of the fifth thermocouple.

Material behavior . Exposure of the insulated thermocouples at 2250 and 2400 K

resulted in some undesirable material degradation and interaction effects that were not

observed in previous tests at 2073 K (refs. 1 and 3). In both the 2250 K and the 2400 K

test, material loss or erosion of the BeO occurred in the hot zone. At 2400 K, the erosion

was substantial and bared the thermoelements; for a typical thermocouple, this occurred near

the measuring junction of the thermocouple (see Fig. 13) and also in a region 4 to 7 cm above the measuring junction. At 2250 K, erosion of the BeO insulator in comparable

sections of the thermocouples was also evident, but the loss of the BeO was not sufficient

to bare the thermoelements.

Further, those sections of BeO insulator that were located just outside of the high

temperature zone of the furnace exhibited a metallic deposit on the outer surface of the

BeO (see Fig. 14). The deposit could be observed as a thin metal film on the BeO insulator and as beads of metal varying from 5 \im diameter to 400 ym in length. Samples from

Performed by the Optical Radiation Section of the NBS .

22
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27
Figure 14. Deposit of beryllium on the BeO insulator in a region about 18 cm above the

measuring junction of an unsheathed thermocouple that was exposed within a tantalum

blackbody enclosure for 307 hours at 2400 K in argon, as seen by (a) reflected light

microscope, and (b) scanning electron microscope.

28
both the 2400 K and the 2250 K test were examined using electron and ion probe microanalysis. The deposit was identified as Be. l,o evidence of metallic Be in the interior

of the BeO insulator could be observed under the microscope. The bore of the BeO insulator was free of any visible deposit throughout the total length of the insulator.

In addition, in the 2250 K test the Be deposit that was found on thermocouple No. 1 happened to occur in a section where there was about a 3 mm gap between adjacent lengths

of the BeO insulating tubing. Coarse examination did not reveal a metallic deposit on the

exposed thermoelements. Nevertheless, thermocouple No. 1 experienced a relatively large

change (+11 K) in its thermal emf . The possibility that the metallic deposit at the gap

in the insulators resulted in electrical shunting between the thermoelements is discounted

because the emf of the thermocouple increased during the test.

Besides these effects, other undesirable material effects were also observed in the

2400 K test. On one thermocouple, the erosion of the BeO insulators in the hot zone was sufficient to bare several centimeters of the W-3% Re thermoelement. A formation of

large equiaxed grains could be observed on the thermoelement in place of the usual

elongated fine-grained structure. Conventional photomicroscopy of a transverse cross- section revealed an irregular metallic coating of up to 50 um thickness on the wire

(see Fig. 15). i.lectron and ion probe microanalysis revealed this to be Ta. In the

region of the coating, the W-3% Re thermoelement did not retain its usual ductility.

In addition to the material loss of the BeO, those portions of the BeO that were at a temperature higher than the phase transformation temperature of approximately 2320 K

(ref . 11) exhibited micro-cracks and were opaque. Those sections of the BeO that were below the phase transformation temperature exhibited the normal translucence, large grain

size and shrinkage (densif ication) of approximately 10% in the diameter. Microcracking of

BeO at temperatures above the phase transformation temperature is often observed (ref. 11)

and for this reason the material is probably not well suited for use above that temperature.

STUDIES OF BeO-Ta SHEATH COMPATIBILITY

Samples of BeO contained in the open-ended Ta sheaths were exposed for 50 hours, at

2073 K, 2250 K and 2400 K in argon. Tests were also performed for 307 hours at 2400 K and

for 1100 hours at 2250 K. The exposed Ta sheaths exhibited no gross differences in behavior from those samples of Ta sheaths, also exposed in these tests, that did not contain

BeO. Typically, large grains of the order of 200 to 500 um formed in the tantalum with

degassing, and little change in the microstructure occurred thereafter. In some regions,

the Ta sheath exhibited pores along the inner surface of the sheath. The pores were

observed also in the degassed assembly (see Fig. 16), and are thought to be the result of

the sheath fabrication process, when the sheath was shrunk over a mandrel. The observations

suggest that with high temperature exposure a pore breaks into several smaller pores.

After exposure of the Ta-BeO assemblies, the BeO insulators could be removed from the

sheaths if the sheath was first cut with a diamond saw a few millimeters above the closed

end of the tube. The BeO from the assemblies exposed for 50 hours at 2073 K and 2250 K ex- hibited similar behavior. The grain size was typically 15 to 60 vm, as compared to 1 to

5 ym for the degassed BeO. In addition, the originally opaque BeO had attained a translucent appearance, which undoubtedly occurred as a result of the increase in the grain size.

A transverse cross-section of a Ta-BeO assembly before and after exposure is shown in

Fig. 17. The BeO insulators were reduced in diameter by about 10% as a result of the high

temperature exposure. The reduction in diameter is essentially the same as that observed,

in an earlier BeO-insulated thermocouple test (ref. 1) which exposed BeO in the presence of

tantalum for 1029 hours at 2073 K (the BeO was contained in a tantalum blackbody enclosure

for this test). in the test of the sheathed thermocouple assemblies, which is described

in a previous section, similar behavior was also noted. Essentially no evidence of erosion

or other material loss of BeO has been observed in any of the tests at 2073 K. It is

therefore concluded that the reduction in diameter is merely shrinkage resulting from

further densif ication of the BeO.

29
100 //m

Figure 15. Photomicrograph showing a transverse cross-section of a W-3% Re thermoelement

from an unsheathed thermocouple that was exposed within a tantalum blackbody enclosure

for 307 hours at 2400 K in argon. The cross-section was taken at about 6 cm above the

measuring junction of the thermocouple where the BeO insulator had eroded away and bared

the thermoelement. The irregular outer coating of up to 50 ym thickness was identified

as tantalum by electron probe microanalysis.

30
(a)

Figure 17, Photomicrographs showing transverse cross-sections of BeO-Ta sheath assemblies

(a) after vacuum degassing at temperatures to 1300 K, and (b) after exposure for 50 hours

at 2073 K in argon. The cross-sections were taken at about 4 cm below the open end of

the sheath.

31
In contrast, the BeO exposed at 2400 K exhibited a number of gross, detrimental

effects. At the open end of the sheath, substantial erosion of the BeO insulators

occurred. In assemblies exposed for 50 hours, a 50% reduction in the insulator diameter

in the region where it exited from the sheath was typical. This reduction was confined to a region a few millimeters in length. One sample of BeO insulator removed from the sheath

also exhibited a longitudinal channel approximately 0.08 mm deep by 0.15 mm wide that extended the entire length of the insulator. A black deposit with a metallic appearance

could be observed on both sides of the channel. In Ta-BeO assemblies exposed for 307 hours

at 2400 K, the BeO insulator was eroded away completely at the open end of the sheath and

the end of the insulator had a conical shape. Gross erosion of the insulator was apparent

at depths of 1 to 2 cm inside the sheath. A typical photomicrograph of one of the Ta

sheath-BeO assemblies exhibiting a transverse cross-section 4 cm below the open end of the

sheath is shown in Fig. 18. Large cracks in the BeO insulator and erosion of the BeO are evident. While very little erosion of the BeO insulator was detected in assemblies ex- posed for 50 hours at 2250 K, assemblies exposed for 1100 hours at this temperature showed

substantial erosion of the insulator in the region where it exited from the sheath. A 70

to 80% reduction in the diameter of the insulator was observed in this region.

The tests at 2400 K were above the BeO phase transformation temperature of 2320 K

(ref. 11). With the phase change, a large anomalous thermal expansion occurs, and the

cracking of the dense, polycrystalline BeO in these tests is not surprising. The loss of

BeO in the tests at 2250 and 2400 K might be explained by a number of different circumstances. However, if we assume that the observed material transport results only from

reactions between Ta and BeO, the conclusions as to what is happening are entirely

reasonable, as taken from a discussion by Droege, et al., (ref. 12). They employed

thermodynamic considerations to deduce the chemical compatibility of high temperature

ceramic oxides with several refractory metals. If the reaction products of BeO with Ta

are confined including the diffusion of oxygen through the Ta sheath, the system is stable

and very little reaction will take place. Near the open end of the sheath, the gaseous

reaction products experience very little confinement, so that rapid loss of gaseous Be,

gaseous tantalum oxides and perhaps some oxygen occurs. Using the data of Droege et al.,

at 2400 K the partial pressure of Be (g) under equilibrium conditions of BeO with Ta is

8 x 10~^ atm (6 x 10~^ torr) . Thus the rapid loss of BeO is not surprising, even under

a cover gas of 1 atm of argon. well inside the sheath, the reaction products are relatively

confined, so that quasi-equilibrium conditions are partially established, and thus the

rate of loss of BeO is greatly reduced.

CONCLUDING REMARKS

This report has presented studies of the effects of high temperature exposure on the

component materials and on the completed assemblies of Ta-sheathed, BeO- insulated W-3%

Re/W-25% Re thermocouples. The exposures were in a high purity argon or helium environment. Some of the principal results follow.

I. BeO-Ta COMPATIBILITY TESTS.

Tests were performed at 2073, 2250 and 2400 K for 50 hours, for 1100 hours at 2250 K

and for 307 hours at 2400 K. No gross physical changes in the tantalum sheaths were observed after the exposures (no tests were performed to detect changes in chemical purity).

In the BeO, gross deleterious effects were observed in those samples exposed at 2400 K. Erosion of the BeO insulators was substantial. Typically greater than 50% of the BeO

insulator was eroded at the top of the sheath. Erosion was markedly less inside of the

sheath, where the gaseous reaction products were partially confined. With exposure at

2400 K, micro-cracking of the BeO occurred since the phase transformation temperature was

exceeded. Serious problems with the BeO were not apparent at 2073 and 2250 K after 50

hours of exposure. At these temperatures the BeO insulators exhibited essentially only

shrinkage (about 10% in diameter) that resulted from the further densification of the

sintered BeO. Longer exposure (1100 hours) at 2250 K resulted in erosion of the BeO at

the open end of the sheath. It was clear from these tests that BeO cannot be used

reliably for long times in the presence of tantalum at temperatures above about 2100 K,

32
M OH

03 iJ H O

1 (U O

QJ 1 PP COcn

cfl o

o

O

co H

•HO *

QJ C

Ul o

1 60 •

tfl CO m 4-1 o 03

!-i c QJ O •H

CO

OJW (U U o

o 4J > <f 0

I t nj +J C3

CO (U

OJ C! O (U

4:; &.

•H 0

;3 0

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05 M

0

U-l

n3 0) rH

J-i Q) bO ;3 O CO0 a

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•H 0

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iJo (U 3 w 0

U-lCO 0) iJ CO 03 -i CI

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3 CO 03 60 03 u

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!ii

33
if the gaseous reaction products are not well confined.

II. TESTS WITH UNSHEATHED, BeO- INSULATED THERMOCOUPLES

Drift tests with BeO-lnsulated W-3% Re/W-25% Re thermocouples were performed In argon

at 2250 K for 1100 hours and at 2400 K for 307 hours. The thermocouples In both tests ex- hibited a positive change in the thermal emf that was small, typically ranging from the

equivalent of a few degrees K up to +11 K. Nevertheless, undesirable material degradation

and interaction effects were observed in both tests, with the effects much greater at 2400 K. In the hot zone of the furnace, the BeO insulators were eroded and reduced by the

Ta; a deposit that was later identified as Be coated the outside of the BeO insulators in

the region of the furnace temperature gradient. At 2400 K, the erosion was sufficient to bare the W-3% Re thermoelement and the thermoelement exhibited a 50 ym thick coating of Ta. In addition, those portions of the insulator above the phase transformation temperature

exhibited microcracking . Comparison with the results of the Ta-BeO compatability tests

suggest that somewhat better material performance may be expected at these temperatures if

the insulated thermocouples are encased and sealed hermetically within a Ta sheath.

III. TESTS WITH SHEATHED THERMOCOUPLE ASSEMBLIES

Four 1.6 mm diameter Ta sheathed, BeO insulated, W-3% Re versus W-25% Re thermocouple

assemblies were constructed from aged thermoelements, heat-treated BeO insulators, and

cleaned and etched Ta sheaths. The assemblies were checked to be gas tight (no indication

of a leak when tested with a partial pressure analyzer) and were filled with high purity

argon gas (1 atm at test temperature) for the test. The test exposed the thermocouple

assemblies for 2059 h at 2073 K in helium (1 atm)

.

The four thermocouple assemblies exhibited virtually identical behavior during the

test. The drift in the thermal emf was small and linear, varying from the equivalent of

-0.3 to -0.8 K per 1000 h of exposure at 2073 K. The thermocouples were rapidly cooled to room temperature four times during the test (the cooling rate was about 400 K/min for the

first two minutes). The thermal cycling had no appreciable effect on the thermal emf at

2073 K. After these coolings, calibrations of the thermocouples on reheating to the test

temperature indicated that the drift in the thermal emf was also less than the equivalent

of -1 K per 1000 hours of exposure for calibration temperatures below 2073 K. The changes

in the thermal emf during the test (equivalent to less than 2 K) were no larger than the

systematic errors (estimated to be 1.9 K for measurements with the automatic photoelectric

pyrometer)

.

At the conclusion of the test, the thermocouple assemblies were removed and dismantled. No gross chemical attack of the thermoelements, BeO insulators or the tantalum

sheaths was apparent. Photomicrographs revealed the usual changes in structure observed

in other tests (this report and ref s . 1 and 4). Very large equiaxed grains (50 ym to one-half wire diameter in size) formed in the W-25% Re thermoelement. Densification of

the BeO insulators that were exposed to high temperatures resulted in a 10% reduction in

their diameter. This was accompanied by the formation of BeO grains of 30 to 90 ym in size

(compared to 1 to 5 ym grain size in the starting material) . In the region of the thermocouple assembly where the temperature rapidly fell towards ambient, a second phase at the

grain boundaries of the W-25% Re thermoelement was observed, and was confined to the surface of the thermoelement (< 10 ym depth) . Spectrochemical analyses of sections of both

W-3% Re and W-25% Re thermoelements taken from this region indicated a small increase in

the Mg and Ca impurity levels. In earlier work (ref. 1), sizeable reductions in the

impurity levels of the BeO insulator were noted after exposure to high temperatures in the

presence of tantalum. Chemical analyses of sections of tantalum sheath in this test indicated at least a ten-fold increase in AH, Ca, Fe, Mg, Na, Si, and Ti, which were major

residual impurities in the BeO tubing.

These tests results indicate that reliable, long term performance of Ta sheathed,

BeO-insulated W-3% Re versus W-25% Re thermocouple assemblies can be obtained for thousands

of hours with exposure at temperatures of 2000 to 2100 K in high purity gaseous environ34
ments . Careful selection, heat treatment, and handling of materials were employed in these

investigations. it is possible that the material specifications and construction techniques

that were used may be more stringent than that which is required. Nevertheless, those who

would desire to obtain thermocouple assemblies that perform reliably at these temperatures

can be furnished with guidelines for their construction.

The W-3% Re and W-25% Re thermoelements used in this investigation are available

commercially. The thermoelements were given a special surface cleaning treatment by the

supplier in order to more thoroughly remove surface contaminants introduced during the wire

manufacturing process. The <;leaning treatment included ultrasonic degreasing, abrading,

and electro-etching. At NBS , the thermoelements were given an additional cleaning with

ether. All materials were handled using clean plastic gloves and specially cleaned tools.

Tests were performed on samples taken from the thermoelement wire lots to confirm that the

emf-temperature relationship of the "as received" thermocouples from the matched lot

complied with the suppliers calibration table to within the equivalent of ±1% of the

temperature. The thermoelements were degassed in vacuum (< 5 x 10~^ torr) for about 1 hour

at 1570 K, and then aged in argon at 2400 K for 1 hour (W-3% Re) or 10 minutes (W-25% Re)

in order to essentially remove the initial emf shift. Recently obtained thermoelements may

exhibit only minimal shifts (ref. 8), and aging of these thermoelements may not be required.

The sintered double-bore BeO insulators were in excess of 99.8% purity. They were

given a preparatory vacuum (< 5 x 10"^ torr) degassing at temperatures up to 1700 K over a 24 hour period. The degassing removed sorbed gases and also was effective in reducing some

of the metallic impurity levels (ref. 1).

Tantalum tubing of greater than 99.97% purity was employed for the sheath. Degreasing

and then etching was performed in order to remove surface contaminants, which are particularly difficult to remove on the inner wall of the tubing. The tubing was then degassed

in vacuum (< 5 x 10"^ torr) at temperatures up to 2175 K.

Fabrication of the thermocouple assemblies was performed in air. The assemblies were

evacuated (< 1 x 10~^ torr) and tested for leaks with a partial pressure analyzer. The

evacuated assemblies were then degassed at temperatures up to 1325 K to remove residual

gas impurities, and back-filled with high purity argon gas (less than 10 ppm total impurities) .

35
REFERENCES

1. Burns, G. W. and Hurst, W. S.: Some Studies on the Behavior of W-Re Thermocouple

Materials at High Temperatures. National Bureau of Standards (NASA CR-72884)

,

Feb. 1972.

2. Burns, G. W. and Hurst, W. S.: A Program in Refractory Metal Thermocouple Research.

J. Research Natl. Bur. Standards Vol. 75C, No. 2, April-June 1971, pp. 99-106.

3. Burns, G. W. and Hurst, W. S.: btudies of the Performance of W-Re Type Thermocouples.

Temperature, Vol. 4, Part 3, (Instrument Society of America, Pittsburgh, 1972),

pp. 1751-1766.

4. Burns, G. W. and Hurst, W. S.: An Investigation of W-3% Re and W-25% Re Thermoelements

in Vacuum, Argon and Hydrogen. National Bureau of Standards (NASA CR-72639)

March 10,1970.

5. Devos, J. C: Evaluation of the Quality of a Blackbody. Physica, Vol. XX, No. 10,

Oct. 1954, pp. 669-689.

6. Toenshoff, D. A., Zysk, E. D., and Fleischner, P. L.: High Temperature Thermocouple

Development Program. Engelhard Industries Division, Final Report, Contract NAS 3-10950

(NASA CR-120898) , Oct. 1972.

7. Pugh, J. W., Amra, L. H. , and Hurd, D. T.: Properties of Tungsten-Rhenium Lamp Wire.

Trans. Am. Soc. Metals, Vol. 55, No. 3, Sept. 1962, pp. 451-461.

8. Tseng, Y.,,Schnatz, S., and Zysk, E. D. : Tungsten 3 Rhenium vs Tungsten 25 Rhenium

Thermocouple—Some Recent Developments. Engelhard Industries Technical Bulletin,

Vol. XI, No. 1, June 1970, pp. 12-18.

9. Lee, R. D.: The NBS Photoelectric Pyrometer and Its Use in Realizing the International Practical Temperature Scale above 1063 °C. Metrologia, Vol. 2, No. 4,

Oct. 1966, pp. 150-162.

10. Heckelman, J. D. and Kozar, R. P.: Measured Drift of Irradiated and Unirradiated

W-3% Re/W25% Re Thermocouples at a Nominal 2000 K. Temperature, Vol. 4, Part 3,

(Instrument Society of America, Pittsburgh, 1972), pp. 1935-1949.

11. Ryshkewitch, E.: Beryllium Oxide Ceramics Processes, Properties and Applications.

Air Force Materials Laboratory Technical Report AFML-TR-65-378, Wright-Patterson

Air Force Base, oYilo , May 1966.

12. Droege, John W., et^ al . : Refractory-Metal Thermocouples in Nuclear and High-Temperature Applications. AEC Contract W-7405-eng-92 , Report No. BMI-X-10246, Nov. 22, 1968.

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