NASA-CR- 135055

NBSIR 76-1026

STUDIES OF NOBLE-METAL THERMOCOUPLE STABILITY

AT HIGH TEMPERATURES

p. D. Freeze and D. B. Thomas

Prepared for NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

Interagency Agreement

NASA Order No. C-67546-B, Modification 5

Issued July, 1976

U.S. Department of Comnnerce

National Bureau of Standards

Institute for Applied Technology

Washington, D. C. 20234
NOTICE

This report was prepared as an account of Government sponsored

work. Neither the United States, nor the National Aeronautics

and Space Administration (NASA), nor any person acting on behalf of NASA:

A. ) Makes any warranty or representation, expressed or implied, with respect to the accuracy, completeness,

or usefulness of the information contained in this

report, or that the use of any information, apparatus,

method, or process disclosed in this report may not

infringe privately owned rights; or

B. ) Assumes any liabilities with respect to the use of, or for damages resulting from the use of any information, apparatus, method or process disclosed in

this report.

As used above, "person acting on behalf of NASA" includes

any employee or contractor of NASA, or employee of such con- tractor, to the extent that such employee or contractor of NASA,

or employee of such contractor prepares, disseminates, or provides access to, any information pursuant to his employment

or contract with NASA, or his employment with such contractor.

ii
1. Report No. NA3A-CR-lj503p NBSIR- 76-IO26

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

Title and Subtitle STUDIES OF NOBLE-METAL THERMOCOUPLE STABILITY AT HIGH TEMPERATURES5. Report Date July 1976 (Published)

6. Performing Organization Code

7. Author(s) P. Freeze and D. Thomas

8. Performing Organization Report No. NBSIR 76-1026

10. Work Unit No. Performing Organization Name and Address U.S. Department of Commerce

National Bureau of Standards

Institute of Applied Technology

Washington, D.C. 20234

11. Contract or Grant No. C-67546-B, Modification 5

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

Washington, D.C. 205^*6

13. Type of Report and Period Covered Contractor Report

14. Sponsoring Agency Code

15 Supplementary Notes Project Manager, I. Warshawsky, Technical Consultant, Instrument Development

and Applications Office, National Aeronautics and Space Administration, Lewis Research Center, Cleveland, Ohio, and G. E. Glawe, Technical Monitor

16, Abstract This report describes two investigatory studies on performance characteristics of noblemetal thermocouples: (1) thermoelectric stability as affected by preferential oxidation

of iridium in the system iridium-40% rhodium versus iridium, and (2) the effects of tem- perature gradients on the emf stability of the systems platinum-13^ rhodium versus platinum and iridium-'40% rhodium versus iridium, operating in air. The stability investigation was carried out at three temperatures — 1700, I85O, and 2000°C —by comparing the output of the test thermocouple in air with the output of an identically

constructed reference thermocouple in nitrogen. The results show that no calibration shift

was observed producing a change in output greater than that corresponding to a 2.0^ change

in the indicated temperature for all samples tested.

The investigation of gradient effects was carried out by subjecting test thermocouples to both severe and mild gradients for periods up to 200 hours. For the platinum system, the operating temperature was 1500°C with gradients of 1^75 and 700°C/cm; for the iridium system, 2000°C with gradients of 700, 1500, and 1975°C/cm. Exposure to temperature gradients

was found to introduce significant changes in calibration for both systems.

In both investigations, the thermoelements were examined by means of electron-probe

analysis and by metal lographic methods to detect chemical and structural changes. Data and micrographs are presented.

17. Key Words (Suggested by Autfior(s)

)

drift temperature gradient

high temperature thermocouples

i rid i um-rhod i urn alloy

p 1 at i num- rhod i um alloy

preferential oxidation

stab i 1 i ty

18. Distribution Statement

Unc 1 ass i f i ed-Un 1 i mi ted

19 Security Classif. (of this report) Unci ass i f i ed20. Security Classif. (of this page) Unclassified21. No. of Pages 57

22. Price' $4o50

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

\AS-\-C -1 'i.^ i R. . r,-71
TABLE OF CONTENTS

Page

Cover Page i

Disclaimer Statement ii

Title Page Hi

Table of Contents iv

Acknowl edament V

Summary 1

1 . I nt roduct ion 3

2. Thermoelectric Stability of the lridium-40% Rhodium

Versus iridium Thermocouple ^ "2.1 Background ^ 2.2 Summary of Test Method 5

2.3 Apparatus 5

2.3.1 Furnace 5

2.3-2 Furnace Control Instrumentation 6

2.3.3 Potentiometer Assembly 7 2.3.^ Optical Pyrometers 7

2.4 Test Thermocouple Materials 7

2.5 Test Procedure 8

2.6 Mi crostructural Examination and Chemical Analysis .... 10

2.7 Results and Interpretation 10

2.7.1 Thermoelectric Observations 10

2.7-2 Mi crostructural and Chemical Studies 11 2.7-3 Interpretation of Combined Thermoelectric and

Microstructural Results \k

2.8 Stability Test Conclusions \h

3. Gradient Studies 15

3. 1 Background 15

3.2 Summary of Test Method 15

3.3 Apparatus 16

3-3-1 Test Chamber 16

3-3-2 Laser Heat Source 16

3-3-3 Optical Pyrometer Measurements 17

3-3-4 Thermocouple EMF Measurements 17

3-4 Test Thermocouple Materials 17

3-5 Test Procedure I8

3.6 Surface and Microstructural Examination and Chemical

Analysis 20

3.7 Results and Interpretation 20

3.7.1 Thermoelectric Observations 20

3.7.2 Surface, Microstructural, and Chemical Studies . . 21

3.8 Gradient Test Conclusions 23

4. Remarks and Recommendations 23

5. References 24

6. List of Figures 25

iv
Acknowledgment

The work described was carried out at the National Bureau of Standards

under the overall direction of J. Stern, Chief, Instrumentation Applications Section. The Section is a component of the Electronic Technology

Division in the Institute of Applied Technology.

The sponsor Project Manager was I. Warshawsky, Chief, Instrument Research

Branch , NASA Lewis Research Center. The sponsor Technical Monitor was

G.E. Glawe.

V

STUDIES OF NOBLE-METAL THERMOCOUPLE STABILITY AT HIGH TEMPERATURES

SUMMARY

1 This report describes two investigatory studies on performance charactert istics of noble-metal thermocouples: (l) thermoelectric stability as affected by preferential oxidation of Iridium in the system iridium-40^

rhodium versus iridium, and (2) the effects of temperature gradients on the performance of the systems platinum-13^ rhodium versus platinum and

I iridium-^0% rhodium versus iridium, operating in air.

Preferential Oxidation

; The stability investigation was carried out at three temperatures — 1700,

1850, and 2000°C — by comparing the output of the test thermocouple in

air with the output of an identically constructed reference thermocouple

In nitrogen. All thermocouple reference junctions were maintained at

0°C. The test thermocouple and the reference thermocouple were mounted

In test cells Incorporated in a high-temperature furnace and designed so that the thermocouple hot junctions would be In an isothermal zone. The

results show that no calibration shift was observed producing a change in

I output greater than that corresponding to a 2.0^ change In the indicated

temperature for all samples listed. The changes in calibration were found

to be affected by test temperature, duration of exposure to elevated temperature, and thermoelement diameter.

Observations tended to confirm that changes in calibration result from

I preferential oxidation of the Iridium component of the alloy element.

The results of meta 1 1 ograph i c studies and qualitative chemical analyses

of sample lengths from exposed and unexposed thermoelements tend to cor- roborate that this mechanism is responsible. Observations showed that

the alloy elements experienced considerable grain growth after thermal

exposure and became brittle. It should be noted that these studies also

revealed the presence of silicon, which, acting as a contaminant, might

have contributed in part to the observed changes in thermocouple calibrat ion

Gradient Effects

The investigation of gradient effects on thermocouple emf stability was carried out by subjecting test thermocouples to both severe and mild gradients for periods of up to 200 hours in an air atmosphere. For the severegradient test, platinum-13% rhodium versus platinum thermocouples were exposed to a gradient of l475°C/cm with the measuring junction at 1500°C.

A water-cooled thermocouple mount was used to achieve this gradient. For

the mild-gradient test, thermocouples of the same system mounted in an Insulating sleeve were exposed to a gradient of 700°C/cm, with the junction at 1500°C. The investigation was successfully extended to the higher

junction temperature of 2000°C for the IrIdium-40^ rhodium versus iridium

thermocouple system, and thermocouples of this system were exposed to gradients of 1975, 1500, and 700°C/cm. Thermocouple reference junctions were

maintained at 0°C.

1
A Nd:YAG laser was used for all tests as the heat source to raise the

thermocouple junction temperature to either 1500 or 2000°C. Thermocouple

wire sizes selected for test had diameters of 0.25, 0.51, and 0.8l mm. Thermocouple junctions were formed into either stirrup-type or wedge-type

configurations, as appropriate.

Measurements indicate the effects of temperature gradients on thermocouple

output are sizable, i.e., exceeding a change in output corresponding to a 2.0^ change in the test temperature. The test results indicated that the

calibration shifts observed after exposure were affected by junction con- figuration, wire diameter, and gradient.

Metallurgical and chemical tests were made on selected sample lengths

from exposed and unexposed thermoelements. Significant changes were observed in the composition of the alloy elements of both thermocouple systems. These changes are of sufficient magnitude to account for the calibration changes measured, but further work is required to demonstrate a one-to-one correspondence between composition change and calibration shift.
INTRODUCTION

Noble-metal thermocouples used to measure temperatures of exhaust gases in

advanced propulsion systems are subject to a number of effects resulting

from the severity of the operating environment that tend to degrade thermocouple performance. Three such effects have been investigated at the request

of and with sponsorship by the National Aeronautics and Space Administration

Lewis Research Center. Two of these investigations (NASA Contract Order Number C-675A6-B, Modification 5, September 26, 1972) are reported here: (1)

thermoelectric stability as affected by preferential oxidation of iridium

in the system iridium-40% rhodium versus iridium and (2) the effects of temperature gradients on the performance of the systems platinum-13% rhodium

versus platinum and iridium-^0% rhodium versus iridium, operating in air.

(The third investigation concerned catalysis effects. [1]'0

j The stability study was conducted in accordance with the following guidelines:

1 1. The thermocouple system to be studied is to consist of a positive component of 60% iridium, kO% rhodium paired with

a negative component of Iridium (irIdium-40^ rhodium versus

i r I d i urn-''" ) .

2. A determination is to be made of the change in calibration

of a thermocouple as a result of preferential oxidation of

the Iridium constituent in the alloy thermoelement (IridiumkQ% rhodium) as a result of exposure to long-term heating.

3. The tests are to be made at the highest temperature attainable up to 2000°C.

4. In order to maximize the ratio of surface area to crosssectional area, the tests are to be conducted with thermocouple wires of the smallest practical diameter available

with reliable calibration data.f

5. The test duration Is to be 1000 hours of exposure to high

temperature, or the time required for a calibration drift

equal to the change in thermocouple output (thermal emf)

corresponding to a 2.0% change in test temperature, or

until failure of the sample.

i The gradient study was conducted in accordance with specific guidelines,

jl with modifications suggested by early work and agreed to by the sponsor.

' The guidelines as revised follow:

1. The thermocouple system to be used in gradient tests is to

consist of a positive component of S7% platinum, 13% rhodium

paired with a negative component of platinum (p 1 at i num- 1 3%

Figures in brackets refer to references In section 5-

In this convention for identifying thermocouple systems, the positive

element is given first.

t "Reliable" in the sense that composition Is homogeneous along the length

of the wi re

3
rhodium versus pi at i num) wi th the hot junction to be at

1500°C in air. If significant calibration changes are

found, further tests are to be conducted with the system

iridium-40% rhodium versus iridium in air with the hot

junction at the highest temperature attainable up to 2000°C,

2. A comparison is to be made of the effect on thermocouple

calibration of a severe temperature gradient to that of a mild temperature gradient, with the hot junction at the same temperature in both instances. The specified system

is to be platinum-13^ rhodium versus platinum.

3. Studies are to be made with thermocouples constructed from

wires of the smallest diameter practicable, with an air

atmosphere, and at a gas pressure of one atmosphere (10^ Pa).

A. Test durations are to be 200 hours of exposure at 1500°C

(or at 2000°C for the i r i d i um- rhod i urn system), or the time

required for a calibration drift equal to the change in

thermocouple output corresponding to a 2.0^ change in test

temperature, or until failure of the sample.

The number of test samples used in both studies was limited. Analysis of the

data yields results that may be used as guidelines for estimating the magnitude of the measurement errors that would occur with exposure of thermocouples

composed of the systems used in these studies to conditions similar to the

test conditions.

2. THERMOELECTRIC STABILITY OF THE iRIDIUM-40% RHODIUM

VERSUS IRIDIUM THERMOCOUPLE

2. 1 Background

Feussner in 1933 [2] proposed the use of i r i d i um- rhod i um versus iridium

thermocouples "for very high temperature." Thermocouples of these materials

appeared suitable for use at temperatures up to 2000°C, in oxidizing media.

Several alloy combinations were later proposed by various experimenters for

the positive component, with the greatest interest being shown in iridiumrhodium alloys containing 40, 50, and 60% rhodium. Thermal emfs of thermocouples made from wires of these compositions versus iridium do not differ

widely as a function of temperature. The output of iridium-50% rhodium

versus iridium is slightly greater than that of either iridium-40% rhodium

or iridium-60^ rhodium versus iridium. Carter [3] predicted that exposure

to oxygen-rich gases at high temperatures would volatilize iridium preferentially from the i r i d i um- rhod i um alloy, which would become relatively richer

in rhodium. A change in thermocouple emf results from the loss of iridium

from the iridium-40% rhodium alloy and reaches a maximum when the composition

becomes iridium-50% rhodium. Additional loss of iridium slightly decreases

the thermocouple output, and thus the output of iridium-AO^ rhodium versus iridium remains more nearly in calibration for a longer time of exposure to

high temperatures in the presence of oxygen than that of the other common thermocouple systems based on i r i d i um- rhod i um alloys. It is also expected

that the high-temperature limit will be somewhat greater for the alloy con- taining more iridium because of its higher melting temperature.

4
There is little information available on the calibration drift of thermocouples of i r i d i um-rhod i um alloys versus iridium in oxidizing atmospheres.

The studies of Rudnitshkil and Tyurin [k] reported changes in thermal emfs

for the iridium-60% rhodium versus iridium system for an exposure of 10 hours

at 1800°C. The total change did not exceed a change in output corresponding

to a change of 0.8% in the measured temperature. Aleksalin et al [5] observed that "at 2000°C the iridium-60% rhodium versus iridium thermocouple

allows us with sufficient accuracy to measure the temperature, at least

briefly, for a period of 10-20 hours."

2.2 Summary of Test Method

I The stability investigation was carried out at three temperatures — 1700,

1850, and 2000°C — by comparing the output of the test thermocouple in air

with the output of an identically constructed reference thermocouple in

n i t rogen

.

I

I The thermocouple under test and the reference thermocouple were mounted In test cells incorporated in a high-temperature furnace in such a manner that

both thermocouple hot junctions were in an isothermal zone. The design of

the furnace was Intended to provide a gas flow through the test cells sufI ficient to prevent any significant impurity concentration from developing

around the test thermocouples. The nitrogen used was certified by the

supplier to contain less than 2.0% oxygen by weight. The air was certified to contain a total hydrocarbon level of less than 0.5 ppm. The outputs

of test and reference thermocouples were measured potent i omet r i ca 1

1 y , and

the calibration change (also referred to as "thermal emf drift") was calculated.

2.3 Apparatus

Figure 1 Is an overall view of the stability-test apparatus. The cabinet

on the left housed controls for the specially constructed furnace which

' contained the thermocouple test cells. A separate cabinet housed the furnace power supply and associated water-cooling system. The furnace itself

was mounted in a supporting structure made from pipe. An o i 1 -d i f f us ion

' pump with cold trap and valve, which formed part of the vacuum system

used to purge the furnace, was also supported by this structure. Two opI tical pyrometers used to measure furnace temperatures are shown swung away

1' to reveal the upper and lower furnace ports. A precision potentiometer

console together with a null -balance galvanometer were used to measure

thermocouple emfs (outputs).

2.3.1 Furnaoe — The furnace is shown in cross section in figure 2. As

shown in figure 1, the actual orientation of the furnace was vertical.

I The furnace was electrically heated by a split tantalum tube element 10.2-cm

fin diameter and 80-cm long. This heating element and all other tantalum comI ponents were maintained in an atmosphere of helium to prevent oxidation.

The thermocouple test cells were positioned inside the heating element and

were suspended through a removable top plate. Each cell was in the form

of a zirconia tube, 80-cm long with an outer diameter of 2.5 cm. The tubes

were closed at both ends, with removable plugs at the upper ends. A small

5
zirconia gas-supply tube passed through a seal at the top of each cell and

extended nearly to the bottom closure. Gas entering each cell through

these tubes provided the means by which a controlled atmosphere was established within each cell. For clarity, the gas outlet tubes near the top

of each cell are omitted in figure 2.

Positioned between the test cells along the center axis of the furnace and

also suspended through the top plate was a control thermocouple well in the

form of a tantalum tube, 80-cm long with an outer diameter of 1.3 cm. The

bottom end was closed, and provisions for supplying helium were incorporated

(not shown in figure 2).

A small tantalum tube (1.9~cm long, 4.8-mm outer diameter, 0.8-mm wall

thickness) was mounted transversely through the well just above the bottom

closure. This tube was closed at one end; the open end was aligned with a viewing port (sight window) in the furnace wall. In use, the tube served

as a blackbody target for the lower optical pyrometer, which was mounted in

line with the viewing port.

A second viewing port mounted in the furnace wall made it possible to ob- serve a point on the tantalum well approximately halfway along its length.

An optical pyrometer was mounted outside the furnace in line with this

viewing port also. Both upper and lower optical pyrometers were arranged

to swing aside to permit visual observation through the ports.

To reduce radiation heat losses, the tantalum heating element was surrounded

by a shield assembly consisting of six spaced layers of tantalum sheet. This

assembly was made in three parts: a cylindrical main shield and top and bottom shields.

All the furnace components were enclosed in a double-wall, cylindrical,

stainless-steel pressure vessel, or chamber, with stainless-steel end

plates. Water circulated between the walls provided cooling.

A vacuum system was used to purge the furnace chamber prior to gas fill.

Major components of this system included a mechanical roughing pump, an

oil diffusion pump, a liquid-nitrogen cold trap, and a high vacuum manifold and valve assembly.

Further construction details and a discussion of furnace design considerations

are given in [6]. For the work described here, two rather than four test

cells were used, although the furnace was designed to accommodate four.

2.3.2 Fumaoe Control Instrumentation — The furnace control system provided

manual and automatic modes of operation. In the automatic mode, a proportional-control unit was capable of maintaining the temperature at the refer- ence thermocouple junction to within ± 0.5% of the set temperature. At temperatures between 1700 and 2000°C, this degree of control established a A6-cmlong hot zone uniform in temperature to approximately ± 1%.

The control unit was an SCR type and supplied heating current to a combination Scott-connected and step-down transformer. The unit received its control signal from the control thermocouple and incorporated a device to limit

6
the maximum current supplied to the furnace to a preselected value. Once

this current value was reached during a run, the unit continued to supply

it until the limiter was reset.

The tungsten-25% rhenium versus tungsten-3% rhenium control thermocouple

output was recorded on a strip-chart recorder.

2.5.3 Potentiometer Assembly — Thermocouple potentials were measured with

a five-decade precision potentiometer.- Measurements were taken with a least count of 1 yV. A provision was made to switch the emf of the test

thermocouple, the reference thermocouple, or the control thermocouple to

the potentiometer assembly or to the console strip-chart recorder.

2.3.4 Optical Pyrometers — Calibrated optical pyrometers were used to

infer the zirconia test-cell temperatures at two stations (calibrating and

test). Corrections were applied for transmission losses through each of

the viewing-port windows. Measurements made at the upper port were corrected to allow for the spectral emittance of the outer surface of the

tantalum reference thermocouple well. The measurement precision of these

instruments varies with measurement range and depends in part on operator

performance as well as on instrument capability. Experience has shown that

the following are conservative values for a competent operator: 700 to

1100°C, ± 2°C; 1100 to 1500°C, ± 3°C; and 1500 to 2000°C, ± 5°C.

2.4 Test Thermocouple Materials

The wire from which the test thermocouples were constructed was provided

by the selected manufacturer to a specification developed by the sponsor,

the NBS, and the manufacturer in consultation. State-of-the-art procedures

were to be used In producing three sizes of pure iridium wire for the negative components and of 60% iridium-40% rhodium alloy wire for the positive

components. The wires were to be as free from mechanical and structural defects as possible, and the decision was made not to use doping."-

The three wire diameters selected were 0.25, 0,51, and O.8I mm. To provide

a maximized ratio of surface to cross-sectional area consistent with reliable calibration data, there was concern that thermocouples constructed

from 0.25-mm-d iameter wire might not survive the intended aging temperatures

Accordingly, the two larger sizes of wire were also used. The use of three

wire diameters also offered the opportunity of detecting in the results any

tendency for a functional dependence on wire size.

J. The manufacturer's statement of uncertainty is ± (0.005^ of reading + 0.1 yV)

[low range] or ± (0.005% of reading + 0.3 yV) [middle range]. All measurements were taken in these two ranges.

J- j- Doping refers to the addition of carefully chosen impurities in controlled

amounts to the wire. It is often used to tailor thermocouple outputs so

that existing, well-established tables of thermocouple emf-versus-temperature

may be used. For the study reported here, the use of doped wire would have

introduced an uncontrolled factor, as the thermoelectric-stability effects

on such wires of the high temperatures used are not known.

7
Immediately before thermocouple fabrication, wires were immersed in a solution of soapy water and then rinsed in ethyl alcohol. The wires then

were annealed for approximately one hour by heating them to a temperature

100°C above the intended aging temperature for the given thermocouple. The

wires were heated by passing a current through them while they hung suspended

in catenary form in the laboratory. The annealing temperature was monitored

with an optical pyrometer. Following annealing, the wires were welded together in an oxygen-hydrogen flame to form thermocouple junctions.

2.5 Test Procedure

The test procedure may conveniently be described as a series of steps, as

f o 1 1 ows

1. The zirconia test cells were heated in position in the furnace

to 1600°C (measured by the control thermocouple in the well)

to remove carbonaceous materials, and the furnace was then permitted to cool slowly'" to room temperature. The outside walls

of the cells were immersed in an atmosphere of helium, and the

inside cell surfaces exposed to a positive flow of air. Prior

to heating, the furnace chamber was first vacuum purged and

then the helium supply for the chamber and for the control

thermocouple well was turned on.

2. Reference and test thermocouples were then installed into

the test cells in the "up", or calibrating, position with

the junctions in line with the center of the upper viewing

port (32 cm from the bottom of each cell).

3. Following another furnace purging, the gas flows for the test

cells were turned on: nitrogen into the test cell with the

reference thermocouple, and air into the other test cell. As

before, helium was circulated through the furnace chamber and

the control thermocouple well. The test cell flow rate was adjusted to be 1 cm^/s.

k. The reference thermocouple (in nitrogen) and the test thermocouple (in air) were annealed in the furnace to minimize any

effects on thermocouple output that might have resulted from

cold working of the thermocouple wires during fabrication or installation. Annealing was accomplished by heating the fur- nace to the intended aging temperature for approximately 2

minutes and then permitting it to cool slowly to 1200°C.

5. Starting at 1200°C, measurements of the outputs of the thermocouples were taken at 200°C increments, with an upper limit of

1700°C, 1850°C, or 2000°C, depending on the intended aging temperature of the test. The average duration of a calibration

over a temperature range of 1200 to 2000°C was approximately

^ to 5 hours. The data from these measurements were regarded

as the base calibration for each thermocouple. The temperatures of the test cells were inferred from direct opticalpyrometer measurements of the outside wall of the reference

thermocouple well. A curve was fitted to the base-calibration

At a rate no greater than 400°C/hour.

8
data for the reference thermocouple; the equation of this curve permitted computation of tables of thermocouple output

versus temperature for any desired temperature intervals over

the temperature-measurement range. The procedure is described

I n deta i 1 in [7]

.

6. The reference and test thermocouples were slowly-'- moved to the

"down", or aging, position with the junctions near the plane

of the blackbody (K5 cm from the bottom of each cell). [Both

calibration and aging positions of the junctions were well within

the isothermal hot zone of the furnace. The purpose of the deeper aging position compared to the calibration position was to en- sure that when calibration measurements were made, those portions

of thermocouple wires subject to temperature gradients during aging would be outside the furnace at room temperature and therefore not contributing to thermocouple output. As a result, mea- sured changes in output following aging may be attributed to aging alone.

]

7. The reference and test thermocouples were maintained at the

selected aging temperature for a period originally Intended to

be about l6 hours, although various experimental difficulties

tended to vary the duration of an aging period considerably.

The temperature of the junctions was inferred from opticalpyrometer measurements of the blackbody temperature. Measurements of both thermocouple outputs were made at selected intervals.

8. The reference and test thermocouples were slowly-"" withdrawn

from the aging position to the calibrating position. Since

iridium and I r i d i um-a 1 1 oy wires are extremely fragile when at

temperatures above 1700°C, great care was required to avoid

jerking the wires, which were retracted and inserted manually.

Also, any contact between the thermocouple wires and the zirconia tube at high temperatures resulted in instant destruction

of the tube and samples. The wires were withdrawn at a rate

that permitted them to cool to a few hundred degrees Celsius

before emerging into the laboratory air.

9. Measurements of the reference and test thermocouple outputs

were made and the calibration shifts were calculated In terms

of an equivalent change In test temperature.

10. If the calibration shift was less than an equivalent 2.0% change

in test temperature, steps 6 through 9 were repeated until a calibration shift greater than an equivalent 2.0% change in test temperature was detected or until failure of either the reference or

the test thermocouple. If the calibration shift was greater than

an equivalent 2.0% change in test temperature, the test was terminated.

At a rate of approximately 2 cm/min.

At a rate of approximately 1 cm/min.

9
2.6 Microstructural Examination and Chemical Analysis

The work reported here was carried out to test the hypothesis that hightemperature exposure of the iridium-40^ rhodium versus iridium system to

an oxidizing atmosphere would result in preferential loss of iridium by

oxidation and thus a change in alloy composition. Accordingly, selected

segments of various thermocouple wires were sectioned, polished, and etched

using conventional metallurgical techniques. These specimens were then ex- amined at magnifications from 80X to 1 200X with a metallurgical microscope.

Specimens were selected from wires of all three diameters, exposed at the

three aging temperatures. In addition, specimens were taken from thermocouples exhibiting large changes in thermoelectric output, or obvious ex- ternal changes in appearance, or both.

An electron microprobe was used to provide quantitative information about

the alloy composition of, and the presence of contaminants in, selected

areas of some of the specimens studied by microscope. The microprobe was calibrated using a specimen of irldium-40^ rhodium alloy thermocouple wire

as received from the wire manufacturer. Since the stability study was con- cerned with indicative changes, this spectrum was accepted as being 60%

iridium, kO% rhodium according to the manufacturer's analysis, and thus the

decision was made not to attempt a more precise measurement of composition

by other techniques. Considerable difficulty was experienced by the operators of the microprobe at the time ths thermocouple specimen composition

measurements were made. Because of this difficulty, the microprobe results are reported to the nearest percent.

2.7 Results and Interpretation

2.7.1 Thermoelectrio Observations —Table 1 presents a summary of the cumulative relative changes in thermoelectric emf with aging time at 1700°C in an oxidizing atmosphere (air) for an irldlum-40% rhodium versus Iridium thermocouple of 0 . 81 -mm-d 1 ameter wire. The reference is the thermal emf of the

similar reference thermocouple exposed to the same aging program in a nitrogen atmosphere. Results are given for six calibration temperatures from

1200 through 1700°C for up to 359 hours of aging.

The calibration of the reference thermocouple was relatively stable during

the tests, changing by no more than 7 yV at 1700°C and by no more than 10 yV

at either I85O or 2000°C. As described in 2.k, the calibration temperature

was measured with an optical pyrometer. In contrast, the test thermocouple

emf Increased during the first II6 hours of aging and decreased thereafter.

Two calibration measurements were taken at 71.5 and 76.5 hours, but only at

1700°C, because of experimental difficulties. At 259 and 359 hours, calibration measurements were also made only at the aging temperature, again be- cause of experimental difficulties. Figure 3, which is a graphic representation of the data In table 1, shows the plot of AE versus time at a calibration

temperature of 1700°C. It is seen that AE rises to a maximum of about 73 yV

(~1^°C) at 116 hours and then decreases thereafter at the rate of 0.12 yV per hou

(0.02°C per hour) to the end of the test at 359 hours. Examination following

the test showed that both thermocouples had undergone structural changes which

resulted in the wires becoming brittle. This embr 1 tt 1 ement was the probable

cause of the mechanical failures which occurred as the junctions were being

repos i t ioned

10
The data from a single test have been presented in detail as being typical

of the results of all the tests. The number of tests successfully conducted

for each wire size and aging temperature is given in table 2.

Figure h consists of plots of relative change in thermal emf for three thermocouples of 0 . 81 -mm-d i ameter wire as a function of aging time at l850°C. The

plots are for calibration at l850°C as determined by the optical pyrometer.

Relative change in thermal emf is defined as the difference between the thermal

emfs of the test thermocouple in air and the reference thermocouple in nitrogen.

In general, the thermal emfs of the reference thermocouples changed little, so that the change in the relative thermal emf may be considered to reflect the

behavior of the test thermocouples. As shown, after the first 10 hours all

three test thermocouples experienced greater changes in thermal emf than did

the reference thermocouples. The observed changes in thermal emf corresponded

to a change in the l850°C aging temperature of less than ] .2%, before failure.

The maximum spread in the three relative emfs occurred at 70 hours and was 33 yV,

corresponding to 6.1°C. The average rate of drift during the first 70 hours

was 0.2°C per hour.

Figure 5 is a plot of relative change in thermal emf of a single thermocouple of 0 . 5

1 -mm-d i ameter wire as a function of aging time at the highest

test temperature of 2000°C. The plot is of five calibrations made at 2000°C.

Relative change in thermal emf has the same meaning as for figure 4. After

about an hour at 2000°C, the test thermocouple experienced greater changes

in thermal emf than did the reference thermocouple. The observed maximum

relative thermal emf of 85 yV at 8 hours corresponded to a change in the

2000°C aging temperature of 15.7°C, or less than 0.8%. The rate of drift

during 8 hours of heating was 2°C per hour, ten times higher than that for

the 0. 81 -mm-d i lameter wire at 1850°C. Failure occurred in the subsequent

aging period.

Figure 6 consists of plots of changes in thermal emf of three thermocouples

of the three different wire sizes as a function of aging time at l850°C.

The calibration temperature was also l850°C. Failure occurred in the thermocouple made from the smallest wire first, at less than 10 hours. The thermocouple made from the largest wire lasted beyond 100 hours. These data, together with the results of other tests, indicate that thermocouples made from

smaller-diameter wire have shorter lifetimes in high-temperature environments.

There also is a tendency for such thermocouples to experience a higher rate of

change of thermal emf compared to thermocouples made from larger wires. The

rates of drift are 2.9, 1-7, and 0.2°C per hour, respectively, for the 0.25-,

0.51", and O.8I -mm-d i ameter wires, for the aging times indicated.

Figure 7 consists of plots of changes in the thermal emfs of three thermocouples of 0 . 81 -mm-d i ameter wire as a function of aging time at three temperatures. Each thermocouple was calibrated at its aging temperature. The

changes in thermoelectric output occur at a greater rate and the lifetime is

markedly reduced as the aging temperature increases from I7OO to 2000°C. The

average drift rates we.re 0.1, 0.2, and 1.5°C per hour at temperatures of 1700,

1850, and 2000°C, respectively.

2.7.2 Miorostructural and Chemical Studies — Two thermocouples of 0.8l-mmdiameter wire, for which test results were presented in table 1, were among

1
those chosen for microstructural examination and chemical analysis. As

described, these thermocouples withstood 359 hours of exposure to an aging

temperature of 1700°C, the longest exposure time for any set of thermocouples, prior to failure. Specimen lengths of wire were cut from each

element of the two thermocouples for microstructural examination starting

about 0.3 mni from each junction and extending to about 2.^ cm from the junction. Such samples are identified as "near junction". Adjacent to the

junction of the thermocouple exposed to air, the alloy component showed

evidences of oxidation and melting along the wire surface. About 7-5 cm from the junction, a reddish-brown coating was also observed on the surface

of this component.

Figure 8 presents micrographs of the sectioned iridium-^0^ rhodium component

of the test thermocouple. Photographs a, b, and c were taken with a magnification of 100X; d, at 1200X, The changes in structure appear greater than

those experienced by the other components in either atmosphere and suggest

the possibility that this alloy may have experienced a change in composition

as well as structural deterioration. Photograph 8a shows a longitudinal section of the alloy wire mi crost ructure in regions not exposed to any high temperature, for reference. Photograph 8b shows a longitudinal section of the

alloy wire that was exposed to high temperatures in an air atmosphere. The

mi crostructure reveals a grain-boundary phase and large, smooth-cornered

grains, whose presence indicates that melting had occurred. Photograph 8c

is a longitudinal section of the same wire about 2.h cm from the thermocouple junction showing erosion of the surface and voids near the surface.

Considerable grain growth is also shown. Photograph 8d is an enlargement of

the grain-boundary phase in 8b.

Table 3 gives the results of composition analyses of the specimens shown in

figure 8. The specimen shown in 8a was taken as a reference, with a composition of 60% iridium and hO% rhodium by weight.

The changes in composition of the alloy component in the nitrogen atmosphere

were found to be negligible, and only a slight enlargement of grain size was observed. However, the alloy component in the oxidizing atmosphere changed

considerably. The composition analyses of phases enclosed by the grain

boundaries near the thermocouple junction of the sample heated in air, photographs 8b and 8d, showed no great change in composition except in the areas near the wire surface, where the iridium content dropped to about 20% and the

rhodium content increased to 75%. In these regions the presence of a significant amount of silicon was detected. From the sample taken farther from the

junction, shown in 8c, voids were observed near the surface. There was an accompanying enrichment of rhodium near the wire surface, which is consistent

with the hypothesis that iridium had been oxidized and removed from the surface.

A possible source of silicon contamination was the magnesia-stabilized zirconia test cells. Spectrochemical analysis of the wire specimens revealed an estimated silicon content of 0.1 to ] .0% by weight. However, a greater con- centration, approximately 5-0%, was found (by microprobe analysis) near the

grain boundaries of wire samples closest to thermocouple junctions. The pres- ence of silicon will result in the formation of a low-melting eutectic with

either iridium or rhodium [8].

12
Figure 9 consists of four micrographs at a magnification of 80X of specimens

that experienced some structural deterioration. These specimens are longitudinal sections, and the widths of the polished surfaces shown in the figures do not necessarily correspond to the product of the wire diameter

and the magnification. Following exposure to high temperature, some specimens were found to be considerably deformed and enlarged, especially near

the tip. Figure Sa is a photograph of an iridium specimen taken from a reference thermocouple aged for 88 hours at 1850°C in nitrogen; photograph

9b is of an iridium specimen taken from a test thermocouple aged for 88 hours

at 1850°C in air; photograph 9c Is of an alloy specimen from the reference

thermocouple; and photograph 9d is of an alloy specimen from the test thermocouple. The wire diameter was 0.81 mm. Photograph 3a shows a very large

grain size, which indicates that grain growth had occurred. A slight sur- face erosion of metal is evident near the tip of the sample, at the right.

These factors could have contributed to a structural weakening of the wire.

Photograph 9b shows larger grains at the tip than at the shank. Some sur- face erosion is also In evidence. Photograph 9c shows erosion of metal at

the surface. This photograph also shows a grain-boundary second phase and

fine precipitate throughout the structure. It Is probable that the grainboundary phase was a liquid contaminant or an oxygen-deficient oxide at the

test temperature, and that the precipitate in the grains was a sub-oxide

precipitated during cooling. This is suggested by the rounded appearance of

the grain-boundary corners. The lighter coloration of the oxide phase Indicates a lack of oxygen.

Photograph 9d shows considerable surface erosion, a second phase at the

grain boundaries, and fine precipitate throughout the structure. It seems probable from the coloration of the oxide phase that the oxide liquid was

less oxygen-deficient than that for the specimen shown in 9c.

The possibility also exists that the observed structural deterioration of

specimens from the reference thermocouple resulted from contamination of

the nitrogen atmosphere with oxygen. During the final cooling phase of the

test, immediately prior to thermocouple failure, the test cells were subjected to severe thermal shock, resulting In cracks In the reference cell

and permitting the entry of air. The precipitate areas found In the grain

structure of the specimens from the reference thermocouple probably resulted

from certain oxides that solidified during the rapid cooling which followed

failure of the test cell.

Measurements of thermocouple output made after 88 hours of aging of these

two thermocouples showed changes that were in reasonable agreement with

those obtained with other thermocouples tested under like conditions.

Figure 10 consists of four micrographs at a magnification of 80X and Illustrates changes in mi crostructure that may occur following exposure to a

h i gh- temperature oxidizing environment. Photograph 10a Is of a specimen

taken from an unheated portion of a thermocouple alloy component; photograph

10b is of a specimen taken from the same thermocouple component, but from a portion exposed to 2000°C, in air. Similarly, photograph 10c is of a specimen taken from an unheated portion of the Iridium component of the same thermocouple, and photograph lOd is of a specimen taken from a portion of the same iridium component exposed to 2000°C, in air. The aged specimens were

13
both taken from portions of wire from 2 to 4 cm from the junction. Wire

diameter was 0.81 mm. Gross surface erosion is evidenced in 10b compared

with 10a. Photograph 10b also shows a grain-boundary second phase and a fine precipitate throughout the structure.

To detect any changes in alloy composition, and specifically to determine

if the phases shown in 10b were rhodium rich (as might have been expected

if iridium had been depleted by oxidation), an electron microprobe traverse

was carried out from the wire surface to the center. Along this distance of

some O.kO mm, measurements showed that the iridium concentration (by weight)

had varied from 28% at the wire surface to 63% at the center. The corre- sponding change in the rhodium concentration varied from G\% to 36%.

Comparison of photographs 10c and lOd shows little effect on the iridium

component from exposure to a h i gh- temperature oxidizing atmosphere. There

is an indication that a slight increase in grain size occurred, but as the

specimens are from widely separated portions of the iridium wire, this con- clusion remains tentative.

2.7.3 Interpretation of Combined Thermoeteotrio and Miorostruaturat Results — Changes in thermocouple calibration observed following aging of

thermocouples at temperatures of 1700, I85O, and 2000°C in an oxidizing

atmosphere indicate some instability of the iridium-40% rhodium versus

iridium thermocouple at these temperatures. The mechanism that has been

proposed to account for these calibration shifts has been that of preferential oxidation of iridium in the alloy component. An initial loss of

iridium would be expected to increase thermocouple output until the alloy

component composition became about 50-50; any further loss of iridium

would then decrease output, as the alloy became effectively more and more

rhodium enriched. The direction of the observed thermoelectric changes is

consonant with this hypothesis. No calibration shift was observed producing

a change in output greater than that corresponding to a 2.0% change in test

temperature. Unfortunately, no set of test and reference thermocouples

withstood an aging time of more than 36O hours, and that performance was exceptional (the thermocouples involved were made from 0. 8I -mm-d iameter

wire; aging temperature was 1700°C). Premature mechanical failures of

thermocouples suggested that the wires were embrittled because of structural changes such as grain growth and the formation of new phases and

boundaries. Subsequent mi crostructural examination of related specimens

verified that, in some tests, such changes had in fact taken place. Microchemical analysis by means of an electron microprobe was also carried

out for some specimens. The resulting composition data suggest that the

pref erent i a 1 -ox i dat i on hypothesis may be correct, but are not conclusive.

2.8 Stability Test Conclusions

The following conclusions, based on results with the limited number of

thermocouples tested, are presented for the stability investigation for

the iridium-40% rhodium versus iridium thermocouple system in a high-temperature oxidizing environment.

1. No calibration shift was observed producing a change in output greater than that corresponding to a 2.0% change in temperature during the lifetime of the thermocouple.

1^
2. Calibration changes and thermocouple lifetime appear to be

functionally dependent on. aging temperature, aging time, and

wire size. The rate of calibration change is seen to be

strongly dependent on aging temperature: a higher temperature corresponds to a greater rate,

3. Under the test conditions, no test thermocouple pair had a lifetime exceeding 36O hours.

k. Observed calibration shifts and structural and chemical

changes are consonant with the preferent i a 1 -ox i dat ion hypothesis.

5. The structural changes observed included marked grain growth

which may account for the considerable degree of embrittlement that occurred.

3. GRADIENT STUDIES

3 . 1 Background

A number of practical measurement applications, such as the measurement of

gas temperatures in aircraft turbines, require that the transducer sensing

element be exposed to thermal gradients of up to several thousand degrees

Celsius per centimeter. The thermally generated potential, or output, developed by a thermocouple constructed from homogeneous components is in

principle a function of the thermocouple materials and of the junction

temperature. However, i nhomogene i t i es along thermocouple wires may be developed, for example, by cold working that may occur in the course of fabrication and in normal use, both in the laboratory and elsewhere. The

thermal emf , or output, generated by a thermocouple with i nhomogeneous

components depends not only on the component materials and the junction

temperature, but also on any temperature gradients that may exist along

the wires. Effects that may be negligible as a result of the presence of

mild gradients may become appreciable for severe gradients and seriously

limit measurement accuracy. The purpose of the work reported here was to

explore the effects of gradients on the emf stability of selected thermocouple systems from 700°C/cm to nearly 2000°C/cm, with junction temperatures

up to 2000°C.

3.2 Summary of Test Method

Test thermocouples were fabricated in either stirrup or wedge configuration,

as shown in figure 13- Thermocouples of the system platinum-13% rhodium

versus platinum were prepared for tests with gradients of 700 and l475°C/cm,

with a junction temperature of 1500°C. For tests extending the gradient

severity to nearly 2000°C/cm, another thermocouple system was required.

Thermocouples of iridium-40% rhodium versus iridium were prepared for tests

with gradients of 700, 1500, and 1975'^C/cm, with a junction temperature of

2000°C. To investigate effects of wire size, thermocouples of platinum-13%

rhodium versus platinum were fabricated from wires 0.25, 0.51, and O.8I mm

in diameter. lridium-40^ rhodium versus iridium thermocouples were all made

from 0 . 5

1 -mm-d i ameter wire.

15
The test thermocouple was mounted in a test chamber open to the laboratory

atmosphere. A continuous infrared laser beam was focused onto the thermocouple junction, and the resulting gradient could be controlled by adjusting

the distance between the water-cooled mount and the junction and by regulating the cooling-water flow through the mount. Thermocouple temperatures

were measured with a calibrated optical pyrometer, and the thermocouple emf was measured with a precision potentiometer assembly. The number of tests

successfully conducted for each thermocouple system, wire size, gradient,

and configuration is given in table k.

3.3 Apparatus

Figure 11 is an overall view of the gradient-test apparatus. The cabinet

on the left constituted the precision potentiometer console that was used

to measure test thermocouple emfs and, indirectly, temperatures in the test

chamber by means of the optical pyrometer. An interference filter prevented

reflected laser radiation from entering the pyrometer. The test thermocouple junction was mounted in the test chamber and positioned to be at the

focus of the laser beam by means of an x-y translation stage. The beam from

the 6O-W, continuous, Nd:YAG laser was focused by a glass lens mounted in a tube. Dual laser controls were mounted in a small console; a separate cabinet housed the laser power supply and cooling system.

5.2.2 Test Chamber — The test chamber is shown in cross section in figure

12. The chamber, constructed of brass, was double walled, with provisions

for cooling water to flow between the walls. Overall, the chamber was 5 cm

in diameter and 20 cm in height. The support for the chamber also carried

an x-y translation stage with micrometer adjustments. Both horizontal and

vertical adjustments were provided to position the test-thermocouple junction

into the focused laser beam. The beam was focused by a lens mounted in a two-piece tube that was supported at one end by the chamber and at the other

end by the laser head. The test thermocouple was mounted in an aluminum oxide insulator, and the insulator mounted in what is identified in figure 12 as the "thermocouple probe".- The mount was water cooled. The lower ends of

both the brass mount and the aluminum oxide insulator are shown in cross section in figure 13- Ports in the chamber gave optical access to the mounted

junction for the focused laser beam and for the optical pyrometer.

The test chamber was vented at top and bottom so that laboratory air could

flow freely into and out of the chamber. During tests, convective flow was established which ensured that a continuous supply of fresh air surrounded

the junction and wires.

3.3.2 Laser Heat Source — The test-thermocouple junction was heated by the

focused beam of an infrared (wavelength of 1,065 ym) Nd:YAG continuous laser

with a maximum beam power of 60 W. Major components of the laser system included the laser head, with lamps, elliptical cavity, and laser rod; an optical bench to support the head; a dual laser control station; lamp power

supplies; and water-cooling equipment.

The design of the mount simulates that of thermocouple probes used with

aircraft propulsion systems.

16
3.3.2 Optical PyrometeT Measuvements —The thermocouple junction temperature

was measured with an optical pyrometer. The measuring scale of the instrument was not used. Instead, to improve the precision of the pyrometer for

temperatures up to 2000°C, the pyrometer was calibrated and used on a basis

of temperature versus pyrometer filament current. A 2.0-n rheostat was con- nected in series with the pyrometer slide wire to provide a more sensitive

means of setting filament current. The filament current was measured by

using a standard resistor with a nominal value of 1 and a precision potentiometer assembly." Measurements were taken with a least count of 1 yV.

Appropriate corrections were applied to the optical pyrometer measurements

to compensate for the spectral emittance of the thermocouple materials, for

window transmission losses, and for transmission losses imposed by the interference filter. The spectral emittance values used for the thermocouple materials were obtained from [9]. Although the actual spectral emittances of the

thermocouple materials during testing may have deviated slightly from the ref- erence [9] values (due to alloy composition changes or surface structure

changes), these deviations were probably quite small. Also, no oxide or con- tamination coatings were observed on the thermocouple junctions during or after testing.

3.3.4 Thermocouple EMF Measurements — Thermocouple emfs were measured using

the precision potentiometer assembly. A mechanical switch permitted either

the thermocouple output or the pyrometer current to be selected for measurement.

3. A Test Thermocouple Materials

The wire from which the test thermocouples were constructed was obtained

from a commercial manufacturer. The thermocouple system first used was platinum-13% rhodium versus platinum. The sponsor had stipulated thermocouple construction from wires of the smallest "practicable" size. As with

the stability study, there was concern that thermocouples fabricated from

very small wires might not survive the tests, and therefore the same selection of wire diameters was made. Accordingly, thermocouples were fabricated

In either stirrup or wedge type (as shown in figure 13) from wire with diameters of 0.25, 0.51, and 0.81 mm. Fabrication procedures were those described

In 2.^. The use of three wire diameters offered the opportunity of detecting

In the results any tendency for a functional dependence on wire size. The two

thermocouple configurations are typical of those used in practical applications-", and their incorporation in the test program was intended to detect

any marked performance advantage one design had over the other when subject

to gradients of 700 to 1975°C/cm.

Since early test results revealed that significant calibration changes were

occurring, the range of gradients tested was extended to nearly 2000°C/cm

The manufacturer's statement of uncertainty is ± (0.01% of reading + 0.2 yV)

[low range] or ± (0.01% of reading + 2.0 yV) [middle range]. All measurements

were taken in these two ranges.

J- J- The stirrup type provides improved heat transfer from the surrounding gas

to the junction, compared to the wedge type; the wedge type provides greater

mechanical strength in a high-velocity stream.

17
with thermocouples of Iridium-^0^ rhodium versus iridium, as the sponsor

had requested. For this work, only one wire size was to be used. Experi- ence in the stability study had shown that 0 . 25-mm-d i ameter wires had a short lifetime as components of thermocouples subjected to 2000°C. Therefore, the choice was made to use 0 . 5

1 -mm-d i ameter wire for the extended

tests. A slight modification of both stirrup and wedge thermocouple con- figurations was required to permit a 2000°C junction to be achieved. A

tapered hole was u 1 t rason i ca 1

1 y machined into the welded junction through

the center of the bead until the diameter of the hole at the surface

reached 0.4 mm. Depending on the exact form of the junction, the hole

penetrated about two thirds of the way through it. Such holes are indicated by the small irregular circles in the junctions in figure 13- Multiple reflections of the focused laser beam in the hole increased the energy

transfer from the beam to the junction. Other than the machining of the

hole, fabrication procedures were the same as those described in 2.k.

Before testing, all thermocouples were annealed in air to improve calibration stability with heating, Platinum-13% rhodium versus platinum thermocouples were heated to 1450°C for one hour, and allowed to cool; iridiumkO% rhodium versus iridium thermocouples were heated to 2100°C for two

minutes, and allowed to cool.

3.5 Test Procedure

As with the stability study, the test procedure may conveniently be described

as a series of steps, as follows:

1. The test thermocouple was mounted in an aluminum oxide Insulator.

2. The insulator was installed in the water-cooled mount for all

tests except those for intended gradients of 700°C/cm. The

junction was adjusted to project approximately 1 cm beyond

the lower end of the mount.

An additional control parameter for setting gradients was cooling water flow through the mount. For tests with Intended gradients of 700°C/cm, the insulator was installed In a mount that had no water-cooling. The thermocouple junction

was adjusted to extend approximately 2.5 cm beyond the lower

end of the insulator.

For all gradients, the exact distance was determined by trial

and error for each combination of junction temperature and w 1 re d i ameter

3. The mounted test thermocouple was Installed into the test

chamber in such a manner that the plane of the wires was perpendicular to the laser-beam path. If the thermocouple junction had a conical hole, the thermocouple was oriented so that

the hole faced the laser.

4. A check was made to ensure that the tube enclosing the laserbeam path was in place and that the interference filter was also in place.

5. The laser was turned on at a low power setting, and the focused

beam spot observed through the optical pyrometer. The spot size was on the order of a few tenths of a millimeter in diameter.

18
6. Vertical and horizontal adjustments were made to bring the

center of the thermocouple junction into the focused beam.

If the conical hole was present in the junction, fine adjustments were made to bring the beam into the hole.- [No lens

focusing was ordinarily necessary. The initial focus adjustment was made on a trial -and-error basis.]

7. The laser-beam power was adjusted to bring the junction temperature to either 1500 or 2000°C (depending on the test) as determined by the optical pyrometer.

8. At a convenient distance from the junction, the temperature

(T-low) of the wire was measured with the optical pyrometer,

and the gradient computed. If the desired gradient had not

been achieved, adjustments were necessary. After each adjustment, the junction temperature and T-low were measured and the

gradient computed. This procedure was iterated until the desired gradient was achieved within ± 10°C/cm for gradients of

700°, 1475, and 1500°C/cm, and within ± 15°C/cm for a gradient

of 1975°C/cm. For Intended gradients other than 700°C/cm, adjusting the cooling-water flow through the mount provided a means for achieving the desired gradient. If the adjustment

required was greater than that provided by the cooling-water

flow or for gradients of 700°C/cm, the thermocouple had to

be remounted with either a greater or a lesser projection of

the junction beyond the mount, and steps 3 through 8 repeated.

9. Thermocouple emf was monitored to determine when the thermocouple achieved thermal stability. An opt I ca 1 -pyrometer mea- surement of junction temperature was taken to provide an initial calibration for the thermocouple.

10. Measurements of junction temperature were taken at intervals

of time and adjustments in laser power made if needed. In some tests, particularly those with a junction temperature of

2000°C and with gradients of 1975°C/cm, the junction bead was partially eroded. In other tests, heating of the junction re- sulted In a bending of the thermocouple wires with a consequent

displacement of the junction. These changes usually required

either vertical or horizontal adjustment of the junction position, or both.

11. The operations of step 8 were repeated, except that no adjustment was made of the thermocouple projection beyond the mount.

Once a test was underway, this adjustment would have terminated

the test. Tests with gradients of 700°C/cm, with no watercooling adjustment available, tended to be less sensitive to

projection distance than tests with more severe gradients, and

therefore the gradient tended to remain within the given limits.

If the change in thermocouple emf corresponded to an equivalent change in

junction temperature of less than 2.0%, the test continued with iterations of

steps 10 and 11. If the change in emf was equal to or greater than an equi vain some instances it was necessary to focus the beam on the side of the conical

surface to achieve a temperature of 2000°C.

19
2.0% change in junction temperature, the test was terminated, as specified

by the sponsor. The test was also terminated after 200 hours of thermocouple exposure to heating or by thermocouple failure. The heating periods

for the test thermocouples were not continuous (that is, not longer than a seven-hour period) because of restrictions imposed by laser operation.

3.6 Surface and Microstructural Examination and Chemical Analysis

Mechanisms judged likely to account for calibration changes in the thermocouples tested in this work include gross mechanical damage, resulting from

exfoliation of surface layers and from hot-gas erosion; microstructural

changes, Such as the formation of new phases and boundaries; and chemical

changes in alloy-component composition or micro-composition.

Three types of examination and analysis were suggested by these possibilities:

(l) Selected thermocouple junctions, with a few centimeters of component wire

attached, were examined under a low-power microscope, with magnifications on the order of 10 to 15X. (2) Specimens were prepared from some junctions and

wires whose external appearance or condition suggested the possibility of

internal changes. These specimens were sectioned, polished, and etched using conventional metallurgical techniques and then examined in a metallurgical

microscope at intermediate magnifications on the order of lOOX. (3) As with

the stability study, an electron microprobe was used to provide quantitative

information about alloy composition and the presence of contaminants for se- lected areas of some of the specimens studied by microscope. The remarks

given in 2.6 concerning the calibration of the microprobe instrument apply

here also, except that comparative measurements were possible to 0.]%.

3.7 Results and Interpretation

3.7.1 Thermoeleatria Observations — Table 5 presents a summary of the cumulative changes in thermoelectric output with exposure to a junction temperature of 1500°C and to either a "mild" gradient of 700°C/cm or to a "severe"

gradient of l475°C/cm in an oxidizing atmosphere (air) for eight platinum13^ rhodium versus platinum thermocouples made from wire diameters of 0.25,

0.51, and 0.81 mm. Four thermocouple junctions were of the stirrup configuration, and four were of the wedge configuration. The data show that two

of the thermocouples experienced emf changes corresponding to more than an equivalent 2.0% change in the junction temperature.

Figure 1^ presents a comparison between the test performance of two platinum13^ rhodium versus platinum thermocouples, one stirrup, one wedge, alike in

all respects other than junction configuration. The test conditions were a junction temperature of 1500°C and a gradient of l475°C/cm; wire size was

0.51 mm. The plots are typical of the results in that the wedge thermocouple experienced a continuous decrease in thermoelectric emf at an average rate of 0.5°C per hour (corresponding to an equivalent \ .S% change in

the junction temperature after 52 hours), while the emf of the stirrup

thermocouple decreased about 0.2°C per hour for the first 50 hours, and

then decreased rapidly at about 2.^°C per hour for the next 10 hours. At

60 hours, the emf had changed by the equivalent of 2.0^ of the temperature.

The larger emf instability exhibited by the wedge thermocouple, compared to

that of the stirrup thermocouple, may have resulted from dissimilar tempera20
ture profiles along the wires for the two configurations." Because of the

effects of convective heating, the temperature gradient was probably more

severe near the water-cooled region of the wedge thermocouple as compared to

the respective gradient for the stirrup thermocouple. Following the test,

impurities and volatilized materials were observed on the thermocouple wires

at distances of approximately 0.7 to 1.0 cm from the measuring junction of

both thermocouple configurations. The presence of these contaminants may

have contributed to the greater emf instability in the wedge thermocouple,

since the gradient along the contaminated portion of this thermocouple was more severe than the gradient along the contaminated portion of the stirrup

thermocouple. Small fluctuations, such as are shown in figure \k, may have

resulted from thermocouple instabilities as well as from experimental error.

Table 6 presents the test results from three platinum-13% rhodium versus platinum thermocouples of different wire sizes exposed to a junction temperature of 1500°C and a gradient of lA75°C/cm in an oxidizing atmosphere

(air). Each thermocouple was of the stirrup configuration. The emf of the

thermocouple of 0. 8l -mm-d iameter wire did not show fluctuations of more than

the equivalent of a change of 6.8°C after 200 hours of heating, except for

two deviations corresponding to changes of 11.3 and 8.6°C at 25 and 30 hours,

respectively. The thermocouple of 0 . 5 1 "Tim-d i ameter wire showed no major emf

drift for the first 50 hours, but then showed a change corresponding to 31.2°C

after 60 hours of heating. The thermocouple of 0 . 25-mm-d i ameter wire showed

a drift of 0.8°C per hour until failure after 3^ hours. The net emf change

at 3^ hours was equivalent to 26.2°C. For both junction configurations, the

emfs of thermocouples fabricated from smaller wires consistently changed more

rapidly than the emfs of thermocouples fabricated from larger wires.

Figure 15 presents plots of the test performance of four iridium-^0% rhodium

versus iridium thermocouples fabricated from 0 . 5

1 ~mm-d i ameter wire and ex- posed to a junction temperature of 2000°C and to gradients of 700°C/cm (two

thermocouples), 1500°C/cm (one thermocouple), and 1975°C/cm (one thermocouple) .

One of the thermocouples (A) exposed to the gradient of 700°C/cm behaved in

a peculiar manner not typical of others. This thermocouple experienced a nearly steady decrease in thermoelectric output, with the thermoelectric

emf decreasing sharply before failure, which occurred at the junction center.

The other three thermocouples showed behavior similar to each other. During

the first few hours of the test, the emfs of these thermocouples increased

relative to the initial calibration. Then some fluctuation in output occurred

around a more-or-less stable value significantly above the initial calibration;

and finally the emfs decreased to below the initial calibration value until

fa 1 1 ure occurred

2.7.2 Surfaaej Micros truatural^ and Chemioal Studies — Figure 16 presents

photographs at a magnification of 15X of the two thermocouples of figure 1^.

The gradient was intended to be the same for both configurations. The average gradient calculated from the measuring junction to the water-cooled re- gion was in fact the same for the two configurations within the limits of

experimental error; however, the rate-of -change of gradient along the wires

differed for the two configurations.

21
The wedge-type thermocouple experienced more severe crystallization changes,

as evidenced by the recrystal 1 i zat ion along the wires leading from the junction, although some recrystal 1 izat ion may be detected on the stirrup wires

also. The greater changes in crystalline structure may contribute to the

less stable thermoelectric behavior of the wedge configuration.

Figures 17 and l8 present micrographs (at 90X) of specimens taken from the

alloy component (figure 17) and the platinum component (figure 18) of a platinum-13% rhodium versus platinum thermocouple exposed to a junction

temperature of 1500°C and a gradient of lA75°C/cm. Wire diameter was 0.51

mm. The exposure time before failure was 60 hours. The emf values of this

thermocouple are given in table 6. In each figure, photograph a^ shows a section taken 1.0 cm from the junction; b^, 0.5 cm from the junction; and £,

0.05 cm from the junction. The temperatures at these points along the wires

may be estimated to be 25, 760, and l425°C, respectively, asssuming a some- what linear gradient. The photographs of the specimens closest to the junction show marked grain growth compared to the grain size in cooler areas. There is no evidence of severe surface erosion.

The alloy component of this thermocouple was appropriately sectioned and

analyzed with an electron microprobe to determine if composition changes

had taken place. Table 7 gives the results of two traverses, one longitudinally along the wire extending from the junction bead to approximately

2.5 fTim away, the other, within a few micrometers of the bead across a radius starting at the wire surface and moving toward the wire center, with

one point at the opposite surface for comparison. Also shown in table 7 are two surface composition measurements made at the same time from an un- tested thermocouple. Significant changes occurred in the composition of

the alloy component, with an effective platinum enrichment. The radial

traverse showed a continuous decrease in rhodium content from the wire center to the wire surface, although the low concentration of 7-5% rhodium measured at one surface (at the start of the traverse) was not repeated at the

opposite surface (3.5%). Both values represent a large change from the

12.6% rhodium content measured in the untested wire.

At times during the tests described in 3-7-1 and figure 15 with iridiumkO% rhodium versus iridium thermocouples, a considerable amount of dark

smoke was observed. Probably the smoke was evidence of significant iridium

volatilization. If iridium were continuously lost from the alloy component,

the thermocouple output might be expected to increase initially, stabilize

for a short period, and then decrease to below its initial value, according

to the preferent ial -oxidat ion hypothesis discussed in connection with the

stability study. The thermoelectric emf changes shown in figure 15 for

three thermocouples were, in general, consonant with the predicted behavior.

Failure for these thermocouples occurred in the iridium component, close to

the junction. Figure 19 presents four photographs at a magnification of

12X of the components of two of these thermocouples, one exposed to a gradient of 700°C/cm, the other, to a gradient of 1975°C/cm. The junction temperature was 2000°C. All photographs show that the wires had been severely

eroded near the junction. The photographs also show that cooler portions of

the wire were covered with a dark, spiny deposit. Analysis of this deposit

showed that its major constituent was iridium, with traces of less than ]%

of rhodium, copper, aluminum, and zinc.

22
3.8 Gradient Test Conclusions

The following conclusions, based on results with the limited number of

thermocouples tested, are presented for the investigation of the effects

of selected thermal gradients on thermocouple performance in an oxidizing

atmosphere for two thermocouple systems: platinum-13% rhodium versus

platinum, and iridium-40% rhodium versus Iridium.

1. The maximum calibration shifts of some of the test

thermocouples were greater than a corresponding change

in the test temperature of 1.0%.

2. Calibration changes and thermocouple lifetime appear

to be functionally dependent on wire size, junction

temperature, and, to a lesser extent, gradient severity.

Thermocouples fabricated from larger wires tend to change

less and to have longer lifetimes compared to thermocouples fabricated from smaller wires.

3. Thermocouples with stirrup configuration tended to be more stable with exposure to a specific gradient than those

with wedge configuration; however, there was no significant difference In lifetime.

h. Observed thermoelectric changes In thermocouples of the

system IrIdium-40% rhodium versus iridium are consonant

with the preferential-oxidation hypothesis.

5. Significant structural and chemical changes were observed

near the junction of platinum-13^ rhodium versus platinum

thermocouples. Measured rhodium depletion amounted to as much as hO% at one site of one specimen (from 12.6% by

weight to 7- 5%)

k. REMARKS AND RECOMMENDATIONS

Inherent and severe problems exist in noble-metal thermocouple applications

involving exposure of junction and component wires to high-temperature oxidizing environments. in particular, for irIdium-40% rhodium versus iridium

thermocouples, these problems include lack of thermoelectric stability, loss

of mechanical strength as embr i tt 1 ement occurs, and short lifetime at high

temperatures and with severe gradients present along wires.

Some of these deleterious effects may be counteracted by operational procedures which minimize thermocouple exposure to adverse environments, but

a more satisfactory solution is required, especially for the expanding field

of hot-gas measurements.

Two lines of attack, are proposed for future work.

1. Protective coatings offer a means for shielding thermocouple

junctions and wires from chemical attack, such as preferential oxidation, and from mechanical erosion. Performance

criteria for coatings need to be developed, including con- ceptual methods of test. Experimental work would include

23
test-method development and evaluation of the effectiveness of specific coatings.

2. A thermocouple system having only pure-element (unalloyed) components would not be subject to thermoelectric

instability because of preferential volatilization or oxidation. Work is required to investigate new thermocouple

systems with pure-element components or with alloy com- ponents that are oxidation resistant at high temperatures.

The gradient study has also revealed the presence of serious problems for

thermocouples of the systems studied in applications involving exposure

in oxidizing atmospheres to thermal gradients of the order developed in

the tests. The tests indicate the presence of large structural and com- positional changes, but correlations between these changes and the emf

stability of the thermocouples have not been demonstrated. The following

further work is recommended:

3. Continue tests with enough thermocouples for each system,

wire size, and configuration to provide data for reasonable

statistical evaluation.

k. Extend tests to include a non-oxidizing atmosphere to

eliminate changes resulting from preferential oxidation.

Other test atmospheres may also be of interest with re- spect to specific thermocouple applications.

5. REFERENCES

[l] Freeze, P.D., Thomas, D.B., Edelman, S., and Stern, J., Study of

Performance Characteristics of Noble Metal Thermocouple Materials

to 2000°C, NASA-CR- 120990 (August 1972).

[2] Feussner, 0., New Noble Metal Thermoelements for Very High Temperatures, Elektrotech. Z. 5^, pp. 155-156 (February 1933) [in German]

[3] Carter, F.E., Discussion concerning paper: The Tungsten- I r i d i urn Thermocouple for Very High Temperatures by W.C. Troy and G. Stevens, Trans. Am. Soc. Metals }a, pp. 1151-1152 (1950).

[^] Rudnitshkii, A. A., and Tyurin, 1,1., New Alloys for High Temperature

Thermocouples, trans, from U.S.S.R. J. Inorgania Chem. 5^, Part 1 (Feb. i960). Chemical Society (London, England).

[5] Aleksalin, 1 .A. , et al, Problems of Investigating Thermoelectric

Alloys Stable to Oxidation up to 2000°C, trans, from Russian,

Wright Technology Division, AD 662573 (Wright-Patterson Air

Force Base, Ohio, I967).

[6] Freeze, P.D., Thomas, D.B., Edelman, S., and Stern, J., Development of

Test Facilities — Noble Metal Thermocouple Research Program 1000-

2000°C, NASA-CR-7286I (January 1971).

2k
[7] Blackburn, G.F., and Caldwell, F.R., Reference Tables for Thermocouples of I r id i um-Rhod i urn Alloys versus Iridium, J. Res. Nat.

Bur. Stand. (U.S.) 68C, No. 1, pp. ^1-59 (January-March 196^).

[8] Hansen, M, , The Constitution of Binary Alloys, pp. 87^ and 1155,

McGraw-Hill Book Co. (New York, NY, 1958)

[9] Handbook of Physics and Chemistry, 55th ED., p. E229, CRC Press,

inc. (Cleveland, Ohio, 1975)-

6. LIST OF FIGURES

Figure 1 - Overall View of the Stability-Test Apparatus.

Figure 2 - Cross Section of the Stability-Test Furnace.

Figure 3 " Relative Change in Thermal EMF, AE, (yV) , as a Function

of Aging Time at 1700°C.

Figure h - Plots of Relative Change in Thermal EMF for Three Thermocouples of 0 . 81 -mm-D i ameter Wire as a Function of Aging

Time at 1850°C.

Figure 5 - Plot of Relative Change in Thermal EMF of a Single Thermocouple of 0 . 5

1 -mm-D i ameter Wire as a Function of Aging Time

at 2000°C.

Figure 6 - Plots of Changes in Calibration of Three Thermocouples of

Wire Sizes 0.25, 0.51, and O.8I mm as a Function of Aging

Time at 1850°C.

Figure 7 - Plots of Changes in Calibration of Three Thermocouples of

0 . 8

1 -mm-D i ameter Wire as a Function of Aging Time at 2000°C,

1850°C, and 1700°C.

Figure 8 - Micrographs of Sectioned lridium-^0% Rhodium Component of a Test Thermocouple Aged at 1700°C for 359 Hours.

Figure 9 - Micrographs of Sectioned Components of Test and Test-Reference

Thermocouples Aged at l850°C for 28 Hours.

Figure 10 - Micrographs of Sectioned Components of Test Thermocouples Unheated and Aged at 2000°C for 8 Hours.

Figure 11 - Overall View of the Gradient-Test Apparatus.

Figure 12 - Cross Section of Test Chamber Used in Gradient Studies.

Figure 13 - Test Thermocouple Configurations, Stirrup and Wedge.

Figure \h - Comparison of Thermal EMFs of Stirrup-Configuration Thermocouple and Wedge Configuration Thermocouple.

25
Figure 15 - Plots of Thermal EMFs of Four Thermocouples Fabricated from

0 . 5 1 ~nnm-D i ameter Wire and Exposed to a Junction Temperature

of 2000°C and to Gradients of 700°C/cm, 1500°C/cm, and 1975°C/cm.

Figure 16 - Photographs at Magnification of 15X of Wedge-Configuration and

Stirrup-Configuration Thermocouples Used at 1500°C and at a Gradient of l475°C/cm.

Figure 1? ~ Micrographs of Sectioned Alloy Component of a Test Thermocouple

Aged at 1500°C and at a Gradient of l475°C/cm for 60 Hours.

Figure l8 - Micrographs of Sectioned Platinum Component of a Test Thermocouple Aged at 1500°C and at a Gradient of l475°C/cm for 60 Hours.

Figure 19 - Photographs at a Magnification of 1 2X of Components of Two

Thermocouples Aged at 2000°C and at Gradients of 1975°C/cm

and 700°C/cm.

26
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27
TABLE 2

STABILITY TESTS CONDUCTED

(number of test thermocouples for which data were successfully obtained)

Wire Size (mm) 0.25 0.51 0.81

Aging Temperature

1700

1850

2000

Note: All tests were terminated by failure of either the reference

thermocouple or the test thermocouple, or both.

28
TABLE 3

COMPOSITION ANALYSIS OF IRIDIUM-40% RHODIUM SPECIMENS

TAKEN FROM A THERMOCOUPLE MADE OF 0.81 -mm- DIAMETER WIRE

AND AGED FOR 359 HOURS AT 1700°C

Specimen

Description

Shown in

Photograph

Composition by Weight {%)

Iridium Rhodium

Unheated Reference

Heated in Air - Near Junction

Heated in Air - Uniform Zone

Heated in Air - Grain Boundary

Heated in Nitrogen

Near Junction

7a

7b

7c

7d

60

59

58

20

61

40

39

38

75

39

29
TABLE 4

GRADIENT TESTS CONDUCTED

(number of thermocouples for which data were successfully obtained)

Thermocouple

System

Platinum-13% Rhodium vs Platinum* Iridium-40% Rhodium vs Iridium**

Thermocouple

Design Stirrup

1 Wedge Stirrup Wedge

Wire size (mm) 0.25 0.51 0.81

i

1 0.25i 0.51 0.81 0.51 0.51

Gradient

(°C/cm)

700

1475

1500

1975

1 1 2 —

1 1 1 1 1 1 r

i _ 1

* Junction temperature 1 500*^0

** Junction temperature 2000°C.

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TABLE 6

CALIBRATION CHANGES IN PLATINUM-13% RHODIUM VERSUS PLATINUM THERMOCOUPLES

MADE IN STIRRUP CONFIGURATION FROM THREE WIRE SIZES AND EXPOSED TO A TEMPERATURE GRADIENT OF 1475°C/cm

Total Heating Equivalent Temperature Change*from Initial Calibration

Time at 1500°C ^ (^C)

^^^"'^^^ Wire Diameter (mm)

oTsi 0751 oTzs

0 0.0 0.0 0.0

1 - 0.2 -11 .8 - 2.5

-OR - 7 fi -in d

in - 1 R -fin -fin

1

1 J - f, - ftO • 4*T -14 fi

20 - 6.8 -12.4 -20.4

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65.

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100 + 2.4

115 + 6.8

130 + 5.6

150 + 2.9

165 + 3.3

180 0.0

190 + 3.2

202 + 2.8

+ indicates that thermocouple emf became greater than the initial

calibration value at 1500°C; - indicates that thermocouple emf

became less.

Note: A change in calibration of 30°C is 2.0% of the nominal junction

temperature, 1500°C.

i

32
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36
0 20 40 60 80 100 120

AGING TIME AT ISSO^C (hours)

Figure ^: Plots of relative change in thermal emf for three thermocouples OF 3.81-mm-diameter wire as a function of aging

time at 1S50°C. The plots are for calibrations at 1850°C.

Relative change in thermal emf is defined as the difference between the thermal emfs of the test thermocouple in

AIR and the reference THERMOCOUPLE IN NITROGEN. THERMOCOUPLE system: IRIDIUM-^!'0/o rhodium versus IRIDIUM.

37
AGING TIME AT 2000*C (hour)

Figure 5: Plot of relative change in thermal emf of a SINGLE thermocouple OF 0 . 51-MM-DI AMETER WIRE

as a function of aging time at 2000°c. the

plot is of five calibrations made at 2000°c.

Relative change in thermal emf is defined as THE difference BETWEEN THE THERMAL EMFS OF THE

TEST THERMOCOUPLE IN AIR AND THE TEST-REFERENCE

THERMOCOUPLE IN NITROGEN. THERMOCOUPLE SYSTEM

IRIDIUM-^0$^ rhodium VERSUS IRIDIUM.

38
0 20 40 60 80 100 120

AGING TIME AT 1850*0 (hours)

Figure 6: Plots of changes in calibration (thermal emf)

OF three thermocouples of wire sizes (A) 0.25,

(B) 0.51, and (C) 0,81 MM AS A function of aging

TIME AT 1850''C. The calibration temperature was

ALSO 1850°C. Thermocouple system: iridium-^0%

rhodium versus iridium,

39
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43

A -MICROMETER ADJUSTMENT

B-X-Y TRANSLATION STAGE

C- THERMOCOUPLE PROBE

D- WATER COOLED AREAS

E- THERMOCOUPLE JUNCTION

F- VIEWING WINDOW

G-BEAM FOCUSING LENS

H-SPLIT RING

J -LASER BEAM ENTRANCE PORT

Figure 12: Cross section of test chamber used in gralient

STUDii'.s, The chamber is described in detail in

THE TEXT, 3.3.1.

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B

Figure 16: Photographs at magnification of 15X of wedgeconfiguration (a) and stirrup-configuration (b)

thermocouples used at 1500°c and at a gradient of

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TEXT, 3.7.2. Thermocouple system: platinum-13%

rhodium versus platinum.
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