r

NATIONAL BUREAU OF STANDARDS REPORT

' 8300

THE CURRENT STATUS OF

THERMAL CONDUCTIVITY REFERENCE STANDARDS

AT THE NATIONAL BUREAU OF STANDARDS

Complementary Report

March 1964

by

H. E. Robinson And D. R. Flynn

to the

Bureau of Ships

Department of the Navy

Washington, D. C.

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards is a principal focal point in the Federal Government for assuring

maximum application of the physical and engineering sciences to the advancement of technology in industry and commerce. Its responsibilities include development and maintenance of the national stand- ards of measurement, and the provisions of means for making measurements consistent with those standards; determination of physical constants and properties of materials; development of methods

for testing materials, mechanisms, and structures, and making such tests as may be necessary, particularly for government agencies; cooperation in the establishment of standard practices for incorporation in codes and specifications; advisory service to government agencies on scientific and technical problems; invention and development of devices to serve special needs of the Government; assistance

to industry, business, and consumers in the development and acceptance of commercial standards and

simplified trade practice recommendations; administration of programs in cooperation with United

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practice; and maintenance of a clearinghouse for the collection and dissemination of scientific, technical, and engineering information. The scope of the Bureau’s activities is suggested in the following

listing of its four Institutes and their organizational units. Institute for Basic Standards. Electricity. Metrology. Heat. Radiation Physics. Mechanics. Applied Mathematics. Atomic Physics. Physical Chemistry. Laboratory Astrophysics.* Radio Standards Laboratory: Radio Standards Physics; Radio Standards Engineering.** Office of Standard Ref- erence Data. Institute for Materials Research. Analytical Chemistry. Polymers. Metallurgy. Inorganic Materials. Reactor Radiations. Cryogenics.** Office of Standard Reference Materials. Central Radio Propagation Laboratory.** Ionosphere Research and Propagation. Troposphere

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* NBS Group, Joint Institute for Laboratory Astrophysics at the University of Colorado. ** Located at Boulder, Colorado.
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1006-20-10461 3 March 1964 8500

THE CURRENT STATUS OF

THERMAL CONDUCTIVITY REFERENCE STANDARDS

AT THE NATIONAL BUREAU OF STANDARDS

by

H„ E. Robinson and D. R. Flynn

Heat Transfer Section

Building Research Division

Complementary Report to the

Bureau of Ships

Department of the Navy

Washington, D. C,

INDEX NO. S-R007 12 01, Task No. 800

(BuShips 1700S-716-62). Code 2781

IMPORTANT NOTICE NATIONAL BUREAU OF STA

for use within the Government. B and review. For this reason, the | whole or in part, is not authorizi Bureau of Standards, Washington

the Report has been specifically pi

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST) on October 9, 2015.

accounting documents intended

ibjected to additional evaluation

sting of this Report, either in Office of the Director, National

le Government agency for which

ies for its own use.

II. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

THE CURRENT STATUS OF

THERMAL CONDUCTIVITY REFERENCE STANDARDS

AT THE NATIONAL BUREAU OF STANDARDS

TABLE OF CONTENTS

PAGE

FORWARD

ABSTRACT 1

INTRODUCTION 1

THERMAL INSULATORS AND ORGANIC SOLIDS 3

AMORPHOUS INORGANIC SOLIDS 3

POLYCRYSTALLINE SOLIDS 5

ALLOYED METALS 8

PURE METALS 13

SUMMARY OF RETURNED QUESTIONNAIRES 14

LIST OF TABLES

Table 1. 2

Table 2. Thermal Conductivity of a Sample of Pyroceram Code 9606 7
TABLE OF CONTENTS (Continued)

Figure 1

Figure 2

Figure 3

Figure 4

PAGE

LIST OF FIGURES

Thermal Conductivity and Thermal Resistivity

of a Sample of Pyroceram Code 9606. Curve

A: Data from Absolute Cut-bar Apparatus,

Short Cylinder; Curves B, C, and D: Data

from Long-bar Metals Apparatus, with Diatomaceous Earth Insulation (B) ; Opacified

Silica Aerogel (C) ; and Powdered Alumina (D) ... 6

Age-hardening Treatment of Inconel

702 Specimen 9

The Thermal Conductivity of Inconel 702

Alloy, Solution-annealed (Curves C and D)

and Age-hardened (Curves A and B) , as Determined by Two Different Methods:

Long-bar Apparatus for Metals (Curves

A and C) , and Absolute Cut-bar Apparatus

(Curves B and D) . 10

Thermal Conductivity Changes of Inconel

702 on Holding Steadily at the Temperatures Shown in the Top Plot for From 90

to 127 Hours. The Lower Plot Shows the

Changes Found in a Following Test at

Lower Temperatures, Solutionannealed and ——- - Age -hardened 12
The Current Status of

Thermal Conductivity Reference Standards

at the National Bureau of Standards

FORWARD

This report is an unmodified version of a paper

given by the authors at the 1963 Thermal Conductivity Conference sponsored by Oak Ridge National Laboratory and held

at Gatlinburg, Tennessee, on 16-18 October 1963. It is

being made available as an NBS Report in the belief that the

contents may be of general interest.

In this report, thermal conductivity data are presented which have not been formally published. All such

data are subject to review, editorially and otherwise, prior

to formal publication, and their preliminary status should

be recognized. The contents of this report may not be referenced.

THE CURRENT STATUS OF THERMAL CONDUCTIVITY

REFERENCE STANDARDS AT THE NATIONAL BUREAU OF STANDARDS

H. E. Robinson and D„ R. Flynn*

Heat Transfer Section

National Bureau of Standards

Washington, D. C.

ABSTRACT

To provide thermal conductivity reference standards to cover

the wide range of conductivities exhibited by materials means that a number of different reference materials are needed, each occupying a niche or range of conductivities more or less peculiar to a particular

kind of material. Six niches are suggested. Current work at the Heat

Transfer Section of the National Bureau of Standards, aiming at development of reference standards in several of these niches, is discussed.

Experimental data are given in regard to several materials under

consideration.

A summary of information on needs for reference standards, obtained from replies to the questionnaire circulated at Ottawa, is appended.

INTRODUCTION

In the wide spectrum of values of thermal conductivity exhibited

by materials, which ranges at room temperatures from under 0.1 to over

5000 mW/cm deg, and which may be of interest at temperatures ranging from

the cryogenic to over 2000°C, depending on the material, it is evident

that to provide suitable thermal conductivity reference standards, several

reference standards are needed, each suitable for its niche. At least six

general niches seem discernible, each being more or less the domain of a particular kind of material.

Table 1 gives a breakdown along these lines, with possible

reference materials for each niche and kind of material. The materials

marked with an asterisk are those which are being used or investigated as reference materials at the National Bureau of Standards.

* Chief, and Physicist, respectively.
TABLE 1.

Niche

k at

mW/cra

o°c,

deg Kind of material

Possible

reference material

1 0.2 0.5 Thermal

insulation

Fibrous glass*

2 1 5 Organic solid Rubber,* plastic

3 5 20 Amorphous

inorganic solid

Pyrex,* fused

silica

4 20 - 100 Polycrystalline

solid

Pyroceram*

5 100 - 500 Alloyed metals Inconel 702,*

stainless steel

6 500 - 5000 Pure metal Iron, platinum,

tungsten

The steps which appear to be essential in developing a reference standard include: (a) choosing a material in each niche which

appears satisfactory for use over an adequately wide temperature range;

(b) estimating whether its probable uniformity and reproducibility of

conductivity are ascertainable by means of other properties, or whether

its use as a standard requires setting up a uniform stock or making

individual measurements on each reference specimen; (c) determining if

the stability of conductivity of the material is significantly affected

by age or by environmental or temperature experience, and, if so, if

adequate stability can be attained by a suitable pretreatment (e.g., a heat treatment). After these steps are satisfactorily taken, the determination of the thermal conductivity with minimum uncertainty follows.

Even after successful completion of all of these steps, it may be necessary to recommend precautions on the care of the reference material, such

as,

,s Do not pound with hammers," or "Do not anoint with oil," or "Do not

heat in a sulfurous atmosphere."

The current status of the work in our group at the National

Bureau of Standards in respect to the asterisked items of the table is

presented below. In addition, a summarization of the information from

twelve returns of the questionnaire on reference standards circulated at

the Ottawa meeting is included as an Appendix.
-3

“

THERMAL INSULATORS AND ORGANIC SOLIDS

For a number of years, we have furnished, on a fee cost basis,

samples of insulating materials, of thickness from 1 in. downward, and

8 in. or more square. The samples consist of two like pieces of a material, the average conductivity of which is determined by a test in a guarded hot-plate apparatus (ASTM C 177). The tests currently cover a limited temperature range (mean temperatures typically are 0, 25, and

50°C) ; there is now an increasing demand for higher temperatures, up to

about 500°C, which currently we are not able to satisfy, partly because

of the problem of a suitable material.

The material most used is one we keep in stock--a dense, semirigid resin-bonded fiber-glass insulating board--from which stock are taken the samples to be measured and furnished. The material is very

little hygroscopic, has a conductivity of about 0.33 mW/cm deg with a positive temperature coefficient [ (1/k) dk/dt = 0.0035 per deg C] and is

slightly compressible so that it should be used at nearly the same thick- ness as that during our measurements. The uncertainty in our measured

conductivities is believed to be about one percent; reproducibility in

successive measurements on the same samples is well within 0.5 percent.

These reference samples are fairly widely used--our traffic in them is

from 30 to 40 samples per year--both to check absolute measurements in

other apparatus and by other methods, and to calibrate comparative methods

using some form of heat flow meter.

Occasionally requests are received for reference samples of

greater conductivity than the fiber-glass material. Generally, these are satisfied by means of specially-purchased samples of gum rubber (k ~ 1.6

mW/cm deg) or of silicone rubber (k ~ 3.5). Necessary temperature limits on gum rubber (65°G max.) for stability, and the relatively high cost of

silicone rubber, tend to restrict their wide use.

At present, it does not appear feasible to supply samples from

a stock without individual measurement, if the uncertainty is to be

limited to one percent. The variability among different samples, which

may vary also in density, is such that the uncertainty would be on the

order of three percent if the samples were not individually measured.

AMORPHOUS INORGANIC SOLIDS

We have been asked at times to measure the conductivity of rigid

slabs of a material such as Pyrex for use as a reference at around room

temperature. The only available equipment for the desired measurement

was the guarded hot-plate apparatus, and it soon became evident that mea- suring the temperature drop through the rigid high conductance specimen

was a major problem. Setting fine thermocouples in fine grooves in the
-4-

surfaces was not allowable in soma instances, and in any case this does

not obviate the effect of variations of plate -to -specimen contact re- sistance which arise from imperfect flatness and contact of the specimen

and the plates of the apparatus. For some rigid materials, dishing or warping of the specimen as a result of the temperature gradient set up

in it may also impair uniform contact with the plates. The conductivity

results obtained in the guarded hot-plate apparatus on 1-inch-thick Pyrex

samples by the usual temperature-drop measuring techniques were sometimes

definitely lower than available literature values, by as much as 20 percent.

A technique was developed which improved matters considerably,

and which may be of interest here. A test is made using on each side of

the hot plate a composite specimen consisting of the principal specimen

sandwiched between two sheets of resilient material. In most applications

so far, the resilient material has been soft gum rubber (30 to 45 Durometer

floating stock Fed. Spec. MIL R-88QA) 1/8-inch thick, which is soft enough

to mold to minor irregularities of contacting surfaces, but is practically

incompressible as an 8-inch square sheet under the compressive load

applied. The thermal resistance of the composite specimens is determined

using the temperature drop from the hot to the cold plate, which is measured by permanent ly- inset thermocouples. A second test is then made at

the same mean temperature, and approximately the same plate temperatures,

on only the two sheets of resilient material on each side, the principal

specimens being removed. The thermal resistance of the sheets is calculated on the basis of the temperature drop indicated by the same plate

thermocouples. The thermal resistance of the principal specimen is then

calculated as the difference of that of the composite specimen and that

of the sheets. The overall thicknesses of the composite and sheet speci- mens in the apparatus are also observed, and the deduced thicknesses of

the specimen and sheets are compared with direct measurements; generally

the agreement is excellent.

Use of this technique doubles the work of a measurement, but it

has brought results into much closer concordance with literature values.

Trials in hot-plate apparatus in three different laboratories, on the

same 1-inch Pyrex specimens and rubber sheets, have yielded results in

close agreement. The conductivities at 22°C obtained were 11.30, 11.24

and 11.45 tnW/cm deg.

All of these measurements were made on the Pyrex slabs as received. Later it was found through refractive index measurements that

the material had considerable unrelieved strain. Accordingly the slab

specimens were annealed at 570°C and cooled very slowly. Upon testing

the annealed specimens in the hot-plate apparatus, using the composite

technique, the conductivity at 22 °C was found to be 11.14 mW/cm deg as compared to the unannealed value of 11.45. It is felt that the decrease

of 3 percent in conductivity is not all due to experimental error difference

.
-5-

In the paper on a cut-bar thermal conductivity apparatus to be

given later in this conference, D. R. Flynn reports results obtained

therein on a piece of Pyrex (stated by the supplier to be from the same sheet as the slab specimens) which had been annealed as indicated above.

The results, extrapolated to 22°C, yield a conductivity value of 11.94

mW/cm deg, about 8 percent higher than that obtained on the annealed slab

specimens. Further work to resolve the difference, or to pursue the effect of strain or annealing, has not been feasible up to this time, although the matter needs investigation.

The usefulness of Pyrex as a reference material is unfortunately

somewhat limited by its relatively low softening temperature, as well as

its significantly increasing radiant transmittance above about 350°C.

Other possible candidates for this conductivity niche may be fused silica,

or a material like Vycor, provided that they can be adequately opacified

to reduce radiant transmittance. Some of the people at Corning Glass

Works are cooperating with us in exploring the feasibility of this

approach

.

POLYCRYSTALLINE SOLIDS

The conductivity niche from about 20 to 100 mW/cm deg is one of

considerable present interest in connection with semi-conductor materials

and refractory solids. At present, it appears that Pyroceram Code 9606

may be a suitable thermal conductivity reference material for this niche,

for temperatures up to about 1000°C.

Figure 1 shows the thermal conductivity and resistivity results

that have been obtained on a sample of Pyroceram 9606 using two different

methods of measurement. Curve A represents data obtained from 200°C to

1000°C using our absolute cut-bar apparatus, with a specimen 1 in. in

diameter and 1/2 in. long. These results were reported in the paper by

D. R. Flynn at the last conference. It is pertinent to note that the

specimen was held at 1000°C for about 275 hours with no perceived change

in conductivity at this or lower temperatures as a result of the prolonged

high temperature treatment.

Curves B and C were obtained using the low temperature model of our long-bar apparatus for metals (see 1961 Conference, pp. 13-16), with

a cylindrical Pyroceram specimen 4.44 cm in diameter and 31 cm long. The

powder insulation surrounding the specimen inside the guard-tube was diatomaceous earth for Curve B, and opacified silica aerogel for Curve C.

Chrome 1 -a lumel 26-gage thermocouples were used, butt-welded and tightly

fitted in transverse slits in the cylindrical surface of the specimen.

Curve D was obtained in the high temperature model of the metals apparatus, using the same specimen but with 0.38 mm platinum- 107, rhodium: platinum

thermocouples. The results represented by the smoothed curves B, C and D

have been corrected for the estimated longitudinal heat flow in the powder
THERMAL

CONDUCTIVITY

,

W/cm

deg

- 6 ~

FICUSS I. IHHMAL CONDUCTIVITY AND THERMAL RESISTIVITY OF A SAMPLE OF

PYS0CEMM CODE 9606. CURVE A: DATA FROM ABSOLUTE CUT-BAR

APPARATUS, SHORT CYLINDER; CURVES B, C s AND D: DATA FROM

LOIG-BAR METALS APPARATUS, WITH DIATOMACEOUS EARTH INSULATION

(B)

;

OPACIFIED SILICA AEROGEL (C)

;

AND POWDERED ALUMINA (D)

THERMAL

RESISTIVITY

, cm

deg

/W
-7-

insulation surrounding the specimen, on the assumption that the relative

contributions from the specimen and guard are proportional to their

cylindrical surface areas (i.e., to their radii). The correction increased with temperature, ranging from -0.4 to -1.8 percent for Curve B,

from -0.2 to -0.3 percent for Curve C, and from -2.3 to -4.8 percent for

Curve D.

Curve B is in good agreement with Curve A at their juncture at

200°C; there is a maximum difference of about 3 percent between Curves B and C in their common temperature range from -100°C to -180°C. Curve D,

obtained in the high temperature metals apparatus with alumina powder

insulation, departs rather widely (from +5 to -2 percent) from Curves A

and B. It is believed that the error in Curve D is considerably larger

than that of the other curves, because of the greater ratio of the con- ductivity of the powder insulation to that of the specimen, for alumina.

Conductivity values taken from the smooth curves A and B of

Figure 1 are given in Table 2. It is worth noting that the thermal re- sistivity of the sample increases linearly with temperatures above 200°C.

TABLE 2. THERMAL CONDUCTIVITY OF A SAMPLE OF

PYROCERAM CODE 9606

T k T k

°C W/cm deg

-180 0.0527 200 0.0350

-158 approx. .0568 (peak) 400 .0327

-100 .0514 600 .0308

0 .0413 800 .0290

100 .0372 1000 .0275

In view of the apparent promise of Pyroceram 9606 as a con- ductivity reference material, as indicated by the information given here

obtained on the single sample available to us, it was decided to purchase

a stock of about two cubic feet--large enough to enable all necessary

measurements and tests, and yet to leave enough material for distribution

as conductivity reference samples for the proximate future _i_f the material proves satisfactorily uniform and stable for such use. The stock was received a few months ago, in the form of seven 18-in. diameter disks

about 2 1/4 in. thick, all poured from one melt, and all given the same eeramming heat treatment. Currently, seventeen core-drill samples have

been taken from the seven disks for density determinations to provide a first, and statistical, estimate of uniformity. If this appears satisfactory, the stability of the conductivity of the material in the face of

temperature exposure will be investigated by successive measurements at

temperatures near that at which the peak of conductivity occurs (approximately -158®C), in the comparative apparatus mentioned earlier in our
- 8 -

Progress Report, to be made on one or more samples subjected to various

heat treatments. The same apparatus can also be used to investigate

uniformity of thermal conductivity, as compared to uniformity of density.

If the material passes well enough through these gantlets, the final step

will be to determine its conductivity as accurately as possible.

ALLOYED METALS

A reference material in the niche or conductivity range 100 to

500 mW/cm deg seems especially desirable in view of the current importance

of heat resistant alloys and stainless steels, which are relatively low in

conductivity for metals.

Because a moderate stock of Inconel 702 all from one billet was

in our possession, which had the desirable qualities of excellent oxidation resistance to over 1200°C, and an appropriate conductivity for this

niche, it appeared worthwhile to investigate its possible usefulness as a thermal conductivity reference material. Experimental data on the con- ductivity of this nickel -chromium alloy were reported to you at the Ottawa

conference; since then a few additional measurements have been made, at our laboratory, and at least at one other which is scheduled to report at

this conference.

The chief question now appears to be that of the stability of

thermal conductivity of the alloy in the face of its temperature history

and thermal experience, and whether it may be possible to make it adequately stable by a suitable heat treatment. The alloy was obtained in a "solution-annealed" condition (i.e., heated to 1080°C and air-quenched to room temperature). It is age-hardenable , however, by precipitation of a gamma-prime phase, which occurs at temperatures between about 900° and

650°C. The coarseness of the precipitate, and Its amount, depend on the

thermal history.

At the Ottawa conference, we presented our available results

(Table 3 and Figure 3, pp. 331-332), concluding that from 800° to 1200°C common values of conductivity were satisfactory for both the previously

solution-annealed and the previously age-hardened alloy, but that at and

below 700°C the age-hardened alloy was from one to three percent higher

in conductivity than the solution-annealed alloy. Further evidence in

this connection has been obtained in some measurements just completed,

made in our long-bar apparatus for metals (low temperature model) from

-157°C to 540 S C

,

A bar specimen of the alloy 2.54 cm in diameter and 37 cm long was machined for testing, and solution-annealed by heating to 1Q80°C,

followed by rapid air-cooling in a fan blast. After Its conductivity had

been measured as indicated below, the same specimen was age-hardened by

heating it to 1080°C, and slowly cooling it with two constant-temperature
TEMPERATURE

,

9-

steps as shown in Figure 2. The age-hardened specimen, with new thermocouples, was then measured, undergoing substantially the same thermal

history in the second series of tests as in the first.

FIGURE 2. AGE-HARDENING TREATMENT OF INCONEL 702 SPECIMEN
- 10 -

The tests were conducted using diatomaceous earth powder insulation, and chrome 1 -a lumel thermocouples, and using as coolants both

liquid nitrogen and 40®C water. The temperatures of the specimen, at the same test stages, were substantially duplicated in the two series of

tests; the times elapsed in each stage were roughly the same.

The results are shown in Figure 3, by the smoothed curves A (for

the age-hardened specimen) and C (for the solution-annealed specimen).

FIGURE 3. THE THERMAL CONDUCTIVITY OF INCONEL 702 ALLOY, SOLUTION

ANNEALED (CURVES C AND D) AND AGE“HARDENED (CURVES A AND B)

,

AS DETERMINED BY TWO DIFFERENT METHODS : LONG -BAR APPARATUS

FOR METALS (CURVES A AND C) , AND ABSOLUTE CUT-BAR APPARATUS

(CURVES B AND D)
- 11 -

Curves B and D, respectively, show for comparison the results previously

reported by D. R. Flynn for the age-hardened and the solution-annealed

alloy, as obtained by measurement of a 1-in. diameter by 3-in. long speci- men in the absolute cut-bar apparatus.

The conductivity obtained for the age-hardened alloy is con- sistently greater than that obtained for the solution-annealed alloy, by

about 6 percent at -150°C and about 3 percent at 540°C. The difference is

in good agreement with the difference reported earlier by Flynn.

The difference between the age-hardened and the solutionannealed alloy would not militate against its use as a reference material

if either one or both of the conditions was stable under later thermal

experience. To investigate this point, the specimen, in each condition, was held steadily at the temperatures existing during the measurements at

the highest temperature (i.e., that yielding a mean temperature of 540°C

for the hottest of the six measuring spans of the specimen) for 90 hours

(solution-annealed) and 127 hours (age-hardened), at which times observations were made to re-determine the conductivity at that sustained test

condition. Following this, the specimen in each case was lowered in temperature to the test temperature conditions yielding a mean temperature of

290°C for the hottest span, at which condition the conductivity was again

determined.

The results of these tests are given in Figure 4, which shows

the percentage change of conductivity of each of the six 3.51-cm measuring

spans of the specimen as a result of the thermal experience of the span.

A slight decrease was obtained for both the solution-annealed and the agehardened specimen, in the spans held above 300°C, as a result of sustained

holding of the highest -temperature test condition. However, on lowering

the specimen to the cooler test condition shown at the bottom of Figure 4,

a marked decrease of conductivity, up to 4 percent, was obtained for the

three hotter spans of both specimens. The apparent reduction of more than

one percent in the conductivity of the three cooler spans of the agehardened specimen, not evident for the solution-annealed specimen, may be

spurious. It is difficult to account for a change which, if real, must

have occurred at temperatures below 325°C, and even below 165°C. These

results became available to us less than a week ago, and there has not

been time to scrutinize all data for accidental errors.

During these measurements we were able also to make measurements

of the average electrical resistivity and thermoelectric power (against

chromel) of the several spans of the specimens at their various mean temperatures as the tests progressed. We have not had time to analyse these

results fully, or to prepare them for presentation here. The electrical

resistivity changes appear consistent with the thermal conductivity

changes, though opposite in magnitude. In connection with the lower plot

of Figure 4, the resistivity changes for all spans of the age-hardened

specimen were substantially less than the corresponding changes for the

solution-annealed specimen, suggesting that the indicated thermal
- 12-

FIGURE 4. THERMAL CONDUCTIVITY CHANGES OF INCONEL 702 ON HOLDING STEADILY

AT rm TEMPERATURES SHOW IN THE TOP PLOT FOR FROM 90 TO 127

HOURS. THE LOWER PLOT SHOWS THE CHANGES FOUND IN A FOLLOWING

TEST AT LOWER TEMPERATURES, ----- SOLUTION -ANNEALED AND —AGE -HARDENED

conductivity changes for the age-hardened material may be in error, and

ought to be less than those of the solution-annealed alloy.

The change of the conductivity of the hotter spans indicated by

the lower temperature test is great enough to be very seriously considered

as unfavorable to use of Inconel 702 as a reference material. For the

present, we are reluctant to dismiss Inconel 702 at ©nee from further con- sideration, although it may be necessary in the end. A considerable

amount of the last data obtained needs further scrutiny and perhaps additional measurements are in order. We hope also to receive at this conference information developed by others which may illuminate the path to a conclusion. A practical consideration which concerns us is that if a metallic alloy suitable as a reference for this particular conductivity

niche is to be found, it is more than likely to contain as a constituent

one of the transition elements, which may be the cause of the instability

that appears so far to be indicated. In such case, a suggestion for an alternative material may not be easy..
-IMPURE METALS

We have not yet undertaken any serious work on development of

a pure metal reference standard, partly because R. W. Powell's work, and

evaluation of data, on Armco iron have made the latter an available

standard for conductivities from 500 to 1000 mW/cm deg. Aside from the

measurements undertaken in connection with the B.M.I. round-robin on Armco iron, reported at the last conference (Ottawa, pp. 325-329), and

some later electrical resistivity measurements, we have not worked further

with Armco iron.

At the last conference we expressed a desire to make some absolute determinations of the thermal conductivity of NBS freezing point

standard lead, but we have not been able to undertake them thus far.

Also, we have considered from time to time the practicability of using

pure platinum as a thermal conductivity reference standard material, es- pecially for high temperatures, but the matter has not as yet progressed

to the stage of undertaking it.
-14-

SUMMARY OF (11) RETURNED QUESTIONNAIRES

Number of Replies in Various Categories

Desired

Category Test Materials Reference Standards

Thermal Conductivity , mW/em deg

0.1 - 1 ......

1 - 20

20 - 70

70 - 500 ......

500 - 5000

Thermal Dlffusivity , cm2

/ sec < 0.01

0.01 - 2

Temperatures , °C - 270 to 0 ......

0 to 1200 . 1200 to 2200 ......

2200 to 3000

Materials

Ceramics ........

Metals

Semi-Conductors .....

Powders

Limit of Test Uncertainty , %

<1 . 1-3 . . 3-5

5-15 .........

Use of Reference Standard

To check apparatus . . . , To calibrate apparatus . , To train operator . . . . ,

Types of Apparatus

Absolute--along bar .... 0

--radial ..... 4

--flat hot plate . . 1 Comparative--meter .... 4

--Powell ball . . 1 Diffusivity 4

9

6

1 16

Atmospheres

Air .......

Vacuum . . . . . Special

Chemical problems .

0

10

6

5

14