NISTIR 88-3901

NEW NIST PUBLICATIONS

February 7> 1989

MICROPOROUS FUMED-SILICA

INSULATION BOARD AS A CANDIDATE

STANDARD REFERENCE MATERIAL OF

THERMAL RESISTANCE

David R. Smith

Jerome G. Must

National Institute of Standards and Technology

(formerly National Bureau of Standards)

U.S. Department of Commerce

Boulder, Colorado 80303-3328

October 1988

Stimulating America’s Progress 1913-1988

NISTIR 88-3901

MICROPOROUS FUMED-SILICA

INSULATION BOARD AS A CANDIDATE

STANDARD REFERENCE MATERIAL OF

THERMAL RESISTANCE

David R. Smith

Jerome G. Must

Chemical Engineering Science Division

Center for Chemical Engineering

National Engineering Laboratory

National Institute of Standards and Technology

Boulder, Colorado 80303-3328

October 1988

Sponsored, in part, by

U.S. Department of Energy

Oak Ridge National Laboratory

Oak Ridge, Tennessee 37830

U.S. DEPARTMENT OF COMMERCE, C. William Verity, Secretary

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY, Ernest Ambler, Director

CONTENTS

Page

LIST OF TABLES iv

LIST OF FIGURES v

ABSTRACT 1

1. INTRODUCTION 2

2. CHARACTERIZATION OF MATERIAL 3

3. MEASUREMENTS 3

4. DATA ANALYSIS AND DISCUSSION 5

A. Thermal Conductivity as a Function of Temperature. . . 5

B. Thermal Conductivity as a Function of Pressure 6

6. COMPARISIONS 8

6. RECOMMENDATIONS 10

7. REFERENCES 11
LIST OF TABLES

Table 1. Thermal conductivity of a pair of microporous fumedsilica insulation boards with a mean density of

301 kg/m3 , at mean temperatures from 321 to 723 K

(48 to 450 oC). Measurements were performed in air

at ambient humidity and at a pressure of 83.6 kPa

(627 Torr), using the high-temperature guarded hot

plate at the National Institute of Standards and

Technology in Boulder, Colorado. Temperature differences through the specimens were chosen to be about

10 percent of the value of the mean temperature within the specimens. The bias in values of Kmeas is

estimated to be 1.5 percent at 330 K, and 2.5

percent at 720 K. Deviations compare measured thermal

conductivity to values of Kcaic computed from eq (1).... 12

Table 2. Thermal conductivity of a pair of microporous fumedsilica insulation boards with a mean density of 301

kg/m3 , at a mean temperature of 318 K (45 oC). Measurements were performed in air at ambient humidity

and at pressures ranging from 40 to 83.5 kPa (626

Torr) using the NIST/B high-temperature guarded hot

plate. Temperature differences through the specimens

were chosen to be about 10 percent of the value of

the mean temperature within the specimens. The bias

in values of Kmeas is estimated to be 1.5 percent

at 330 K. Deviations compare measured thermal conductivity to values of Kcaic, computed from eq (2).

The highest pressure (83.5 kPa; 626 Torr) is the

local ambient atmospheric pressure at Boulder,

Colorado 13

iv
LIST OF FIGURES

Figure 1. Thermal conductivity of a pair of microporous

fumed-silica insulation boards with a mean density

of 301 kg/m3 , at mean temperatures from 321 to 723

K (48 to 450 oC). Measurements were performed in

air at ambient humidity and at a pressure of 83.6

kPa (627 Torr) using the NIST/B high-temperature

guarded hot plate. The measurement bias in values

of Kmeas is estimated to be 1.5 percent at 330 K,

and 2.5 percent at 720 K. The solid curve represents values of Kcaic, obtained using eq (1) 14

Figure 2. Deviations of thermal conductivity of microporous

fumed-silica specimens from values calculated with

eq (1) 15

Figure 3. Thermal conductivity of a pair of microporous

fumed-silica insulation boards with a mean density

of 301 g/m3 , at a mean temperature of 318 K (45 oC).

Measurements were performed in air at ambient humidity and at pressures varying from 40 to 83.5 kPa

(300 to 626 Torr). The solid line was calculated

from eq ( 2 ) 16

Figure 4. Deviations of thermal conductivity of microporous

fumed-silica specimens from values calculated with

eq (2) 17

Figure 5. Extrapolated correlation of thermal conductivity

with pressure, eq (2), estimating thermal conductivity of microporous fumed-silica insulation at sealevel atmospheric pressure (101.3 kPa) as indicated

by the open circle at the top. Mean temperature of

specimens is 318 K 18

V

Microporous Fumed-Silica Insulation Board

as a Candidate Standard Reference Material

of Thermal Resistance

David R. Smith

and

Jerome G. Hust

Chemical Engineering Science Division

National Engineering Laboratory

National Institute of Standards and Technology

Boulder, CO 80303-3328

Measurements of apparent thermal conductivity of microporous

fumed-silica insulation board are reported in order to provide a

basis for certifying it as a Standard Reference Material (SPd4) of

thermal resistance. These data, for a pair of specimens having a

mean density of 301 kg/m^ , encompass a range of temperature from

321 to 723 K and environmental gas pressures at and below ambient

atmospheric pressure (40 to 83.7 kPa). Detailed analyses and

intercomparisons of previously published data are given. Correlations of thermal conductivity with temperature and with pressure

are given which represent the data within a standard deviation of

0.2%. This fumed-silica material has a thermal conductivity of

19.68 mW/(m- K) at 300 K and is suitable for use as an SRM of very

low conductivity from room temperature up to temperatures beyond

720 K (450 c»C). Great care in handling this material is necessary

because of its fragility.

Key words: apparent thermal conductivity ; density; microporous

fumed silica; pressure; Standard Reference Material; temperature;

thermal insulation; thermal resistance.

This work was funded, in part, by DOE/ORNL under contract ORNL/IA21428.

1
1. INTRODUCTION

The Office of Standard Reference Materials of the U.S. National

Institute of Standards and Technology, NIST, (formerly the National

Bureau of Standards, NBS) establishes Standard Reference Materials

(SPiMs) to improve reliability in measurement of physical properties.

Specifically, SRMs of thermal resistance are used by industrial,

academic, and government laboratories to verify the correct operation of guarded hot plates (GHPs) and to calibrate instruments such

as heat-flow meters. These instruments measure the rate of heat

flow under given temperature differences through specimens of known

thickness. From such data one can obtain the thermal resistance (Rvalue) of the measured specimen, as well as the thermal conductivity

(or its reciprocal, the resistivity) of the homogeneous material.

SRMs can also be used to indicate the accuracy of measurement of

apparent conductivity of thermally conducting materials over a range

of parameters such as bulk density, temperature, and environmental

air pressure. The Properties of Solids Group within the Chemical

Engineering Science Division of NIST in Boulder, Colorado, (NIST/B)

has participated for about 25 years in establishing SRMs of thermal

resistance over a broad range of thermal conductivity, pressure, and

temperature. The status of this effort has been summarized by Hust

[ 1 ].

During the mid-1970's, the American Society for Testing and

Materials (ASTM) recognized the great need for thermal insulation

SRMs. Consequently, a task-group was established under the auspices

of ASTM subcommittee C16.30 on thermal measurements. Recommendations for establishing thermal insulation SRMs were published in

1978 [2J.

No SRM of thermal resistance for use at temperatures above 330

K is available at the present time from the Office of Standard Reference Materials of NIST in Gaithersburg, Maryland (NIST/G), in

spite of the very strong need for such SRMs.

We report measurements of apparent thermal conductivity of one

pair of specimens of microporous fumed-silica insulation board.

This pair was obtained from the Heat Transfer Group of the Center

for Building Technology at NIST/G, which is certifying one lot of

this material as an SliM at room temperature. This material was

investigated because of the recommendations of the ASTM C16.30

committee and of an advisory panel composed of representatives from

government and industry. Additional measurements are currently

being performed at NIST/B and NIST/G on several other specimen pairs

of similar material, more fully to characterize its properties.

Workers at NIST/G are measuring additional specimens with their 1-

meter GHP, at environmental pressures very near 101.3 kPa, and at a

temperature of 297 K. Work at NIST/B involves study of a selected

subset of these specimens, with measurements of thermal conductivity performed over a greater range of temperature and pressure than

is possible with the 1-meter apparatus.

2
2. CHARACTERIZATION OF MATERIAL

This insulation material is produced as rigid boards of bonded

silica-aerogel composite made of very fine particles of synthetic

amorphous silica, opacifier, fine ceramic fibers, and a binder

[3,4], The size of the silica particles has been reported by the

manufacturer to be approximately 10"® m. The fine ceramic fibers

increase the cohesiveness of the board, and the binder cements the

surfaces of the particles and fibers together into a cellular structure by sintering when the boards are cured at high temperatures

(900^0)

Ilmenite (FeO- Ti02

)

is the opacifier added to reduce

radiative heat transmission through the board for use at high temperatures. The fractions of silica particles, opacifier, and ceramic fiber stand approximately in the ratios 60:35:5. All organic

matter is burned out in curing.

Microporous fumed-silica insulation board in its raw-product

form is very fragile and must be handled with great care. The edges

of the slabs are easily chipped during installation and removal from

the measurement apparatus. Boards having an edge dimension longer

than about 30 cm are easily damaged during handling and shipping

unless great care is taken to support them over a whole face and to

protect them from localized impacts. However with proper cushioning

of the specimens when they are packed for shipping, and with proper

support of the broad faces when they are installed in an apparatus,

a careful worker can probably measure them 20 or 30 times without

significant degradation of their surfaces due to chipping or

breaking. Great care will be necessary in handling these specimens.

An evaluation lot comprising 10 specimens of this material was

originally obtained by NIST/G [4]The specimens as received were

about 2.5 cm in thickness and 60 cm square. After measurements of

thermal conductivity were performed in the 1 -meter GHP at NIST/G, a

selected pair of specimens was cut into 30 cm squares and shipped to

NIST/B. Two specimens for which thermal conductivity measurements

are reported here had a mean density of 301 kg/m® (18.8 Ibm/ft®).

3. MEASUREMENTS

For brevity, we shall use the term “thermal conductivity" to

denote the term “apparent thermal conductivity" in the following

text. Analysis of our data in the following section suggests that

radiative heat transfer in this microporous fumed-silica board is

not negligible, but, near room temperature, contributes only a small

amount in comparison to conductive heat transfer. Because of the

density and microporosity of this material, convective heat transfer

is totally negligible. Thermal conductivity is therefore an

appropriate descriptor of the heat-transfer behavior of this

material over the temperature range studied.

3
The thermal conductivity data reported here were obtained with

the high-temperature guarded hot plate described by Bust, Filla,

Hurley, and Smith [5]. The diameter of this circular hot-plate

stack is 25 cm (10 in), and the area of the metered main heater has

a diameter of 12.5 cm (5 in).

The specimens were received as square slabs nominally 300 mm on

a side. The measurement apparatus requires disks 250 mm in

diameter, so disks were cut from the square slabs by using a bandsaw

set to a low cutting speed. The material is very easily cut and the

disk was not damaged during the cutting. The annular material

making up the rest of the square is sometimes split during the

cutting, however. Because the material generates air-borne dust

during machining operations, prudence requires the use of a dust

mask by the person cutting this material. Although the material

from which the insulation is manufactured is not toxic, the dust is

very fine and is borne by stray air currents over distances of

several meters.

We measured the thickness of each specimen using a digital

electronic caliper with a precision of 0.03 mm (0.001 in). Measurements of thickness were made at eight locations equally spaced

around the perimeter of the disk, and the measurements were

averaged. The standard deviation of the thickness measurements is

0.08 mm for a nominal specimen thickness of 25 mm (relative standard

deviation of 0.3%).

Using a steel ruler that had a precision of 0.25 mm, we measured the diameter of each specimen along four different directions,

and the results averaged. The standard deviation of these measurements was 0.8 mm for a nominal diameter of 255 mm (relative standard

deviation of 0.3%).

Masses of the specimens were measured on a laboratory trip balance with a sensitivity of 0.1 g for a typical mass of about 400 g

(relative imprecision of 0.03%).

The density can be determined from these measurements with a

precision of + 0.5%. The mass of each specimen decreased by about

3 g from conditioning in an oven at 90° C before the thermal conductivity was measured, and then increased by about 2 g during the measurement, as revealed by the mass of the specimen upon removal from

the apparatus. These changes in mass due to the rigors of conditioning and measurement amount to about 0.8% uncertainty in the

density.

4
After the specimens were installed in the apparatus they were

heated to the highest temperature of measurement before the thermal

conductivity was measured. There is no evidence in the thermal

conductivity data that phase changes occur in the material below

4750 C, the highest temperature to which the specimens (that is,

their sides adjacent to the hot main plate) were exposed during

measurements. There does seem to be some adsorption of moisture,

which must be driven off before measurements are stable. We found

it necessary to repeat measurements two or more times at a given

mean temperature until the values obtained did not vary by more than

the experimental reproducibility (0.5% for this apparatus).

The thermal conductivity of this pair of specimens was measured at mean temperatures up to 723 K (450<>C) with air at ambient

pressure and humidity in the environmental chamber. Absolute temperatures and temperature differences were stabilized and measured

with a precision of 0.02%. The precision of the measurement of

thermal conductivity was estimated to be 0.5% at 330 K, and 0.7% at

720 K. The bias was estimated to be 1.5% at 330 K and 2.5% at 720 K.

In table 1 we list 38 measurements of thermal conductivity for

this pair of microporous fumed-silica insulation boards in air at a

pressure of 83.6 kPa (627 Torr) and at mean temperatures varying

from 321 to 723 K (48 to 450oC).

In order to obtain the effect of the environmental gas pressure

on the thermal conductivity, we measured this pair at selected environmental air pressures evenly spaced over the range from 40.0 kPa

(300 Torr) to 83.5 kPa (626 Torr). The measurements of thermal conductivity at these pressures were made at a constant mean temperature of 318 K (45oC). Gas pressure was measured with a precision of

0.13 kPa (0.1 Torr; a relative precision of 0.3% at the lowest pressure). The inaccuracy of pressure measurements is about 1%. The 20

measurements from these experiments are listed in table 2.

4. DATA ANALYSIS AND DISCUSSION

A. Thermal Conductivity as a Function of Temperature

The 38 data points in table 1 were analyzed to determine the

correlation of thermal conductivity with mean specimen temperature.

The thermal conductivity over the temperature range 321 < T < 723 K,

for the pair of specimens with a density of 301 kg/m^ and in an

environment of air at a pressure of 83.6 kPa and ambient humidity,

was represented by

k = 18.46 + 2.283*10-3 T + 1.983*10-8 T3 , (1)

5
where k is in mW/(m- K) and T is in K. This correlation was obtained

using a weighted least-squares fit and the thermal conductivityintegral (TCI) method [6].

The curve in figure 1 representing the thermal conductivity as a

function of temperature is seen to have very little curvature. Also,

relatively small temperature differences were imposed on the specimens during these measurements. Thus it would not be necessary to

use the TCI method to fit these data. The TCI method is indicated

for use whenever measurements are made with large differences in

the boundary temperatures of the specimen and with appreciable

curvature in the thermal conductivity function over the included

range of temperature. However, we always use the TCI method as a

matter of course, whether or not the data seem to have enough curvature to require the method. This ensures that no additional errors

are ever introduced by our fitting procedure due to nonlinearity in

the fitted function.

The 38 thermal conductivity data kmeas used to determine this

dependence of k on temperature are plotted as open circles in figure

1; the correlation given by eq(l) is plotted in figure 1 as the

solid line. The deviations of the data from values of kcaic

computed from eq(l) are plotted in figure 2 and are listed in the

last column of table 1 . The standard deviation of the data from the

fitted curve is 0.21%.

The curvature of the solid line in figure 1 is correlated with

the T^ term in eq(l), which, being nonlinear, grows larger relative

to the other two terms with increasing temperature. This T3 term is

related to the contribution of radiative heat transfer to the total

heat transfer through the material. At 297 K the magnitude of this

term is 0.5 mW/(m* K) out of a total apparent conductivity of 19.7

mW/(m-K), or 2.6% of the total. At 700 K the magnitude of the T^

term is 6.8 mW/(m- K) out of a total of 26.9 mW/(m-K), or 25.3% of

the total. Thus in this range of temperature, the radiative contribution to the total heat transfer is becoming appreciable.

B. Thermal Conductivity as a Function of Pressure

The correlation of thermal conductivity with pressure P was obtained from the 20 data points listed in table 2, for a mean specimen temperature of 318 K (45oC), a temperature difference within the

specimen no greater than 30 K, and a density of 301 kg/m^ . By use

of a weighted least-squares fit the correlation was represented by

k = 9.337 + 0.17436 P - 5.6095*10-4 p2 (2)

6
where k is in mW/(m- K) and P is in kPa; this relation is valid over

the range of pressure from 40 to 83.5 kPa.^ Because the pressure is

constant throughout the specimen, it is not necessary to use the TCI

method to fit this correlation with pressure. For the relatively

small temperature difference used (30 K), any error made by

associating the thermal conductivity with the mean temperature of

the specimen is inappreciable.

The thermal conductivity data used to determine this dependence

of k on pressure are plotted as open squares in figure 3; the correlation given by eq(2) is plotted in figure 3 as the solid line.

Deviations of the data from values of kcaic computed from eq(2) are

plotted in figure 4 and are also listed in the last column of table

2. The standard deviation of the data from this curve is also 0.21%.

The maximum value of kcaic, 19.98 mW/(m- K)

,

given in table 2,

represents the value of thermal conductivity kB at the ambient

atmospheric pressure at Boulder, Colorado. At this elevation, approximately 1650 m, or 5400 ft, above sea level, the atmospheric

pressure is 83.3, with normal fluctuations, due to local weather, of

+0.5 kPa. By extrapolation of eq(2) to a condition of one standard

atmosphere of pressure, 101.3 kPa (corresponding to sea level), the

thermal conductivity ksi is estimated to be 21.245 mW/(m-K). This

value is compared with the data for pressure dependence by representing it as the circle in figure 5. This value is 6.3% higher

than the value kB = 19.98, obtained for our laboratory in Boulder.

That is, the ratio of the two values is ksi/kB = 1.063. This factor

will be of use in the next section comparing these data with the

results of measurements on this same lot of material by workers at

NIST/G, at sea level.

Because the form of the k dependence given by eq(2) is nonlinear (quadratic dependence on P), the rate of change of k with

pressure changes with pressure. Figure 2 shows that the slope of k

versus P is steeper at lower pressures and less steep at‘ higher

pressures. For this fumed-silica insulation the rate of change of k

with pressure P, or slope of the k(P) function plotted in figure 2,

is given by the mathematical first derivative of the k(P) function:

dk/dP = 0.17436 - 2 * 5.6095 * 10-4 p. (3)

At sea level, where Pel = 101.325 kPa, the rate of change of k with

pressure takes the value 0.0607, which is also the slope of the k(P)

relation, eq(2), at Psi . At the elevation of our laboratory at

Boulder, where the pressure is Pb = 83.5 kPa, the rate of change of

k with pressure is 0.0807, which is the slope of the k(P) relation,

eq(2), at Pb

But because these slopes are different, neither of

them is valid for extrapolating values of k to a different range of

pressure. Rather, the k(P) relation itself, given by eq(2), should

be used.

7
5. COMPARISONS

A similar microporous material of low thermal conductivity,

with a nominal density of about 310 kg/m3 , was studied by Tye [7].

Its thermal conductivity ranged from 26.5 mW/(m- K) at 25^0 to 48.5

mW/(m- K) at 650oC. He recommended in 1970 that it be considered as

a possible SRM.

I Microporous fumed-silica insulation board has been investigated ! for use as a possible SRM of low thermal conductivity by Rennex, I Somers, Faison, and Zarr [3], and by Zarr, Somers and Ebberts [4] at J NIST/G. All but two of the latter set of measurements were perform- | ed at a mean specimen temperature of 297 K (24oC) and at a pressure

of 101.33 kPa (760 Torr), on specimens having densities ranging from

310 to 333 kg/m3 . The apparatus used at NIST/G, a GHP with a diameter of 1 m, is enclosed in a rectangular environmental chamber

with dimensions larger than 1 m. Thus this apparatus is limited by

its size and geometry to a very small range of environmental gas

pressure, around ambient barometric pressure, that can be safely

| used within it. The present measurements extend the range of corre- ^ lation of thermal conductivity for microporous fumed silica insula- j tion to the temperature interval from 318 to 723 K (45 to 450oC). I The correlation is also extended to environmental gas pressures over ^ the range from 40 to 83 kPa (300 to 626 Torr).

The microporous insulation material had a somewhat lower con-

| ductivity than that of the material studied by Tye. Rennex, Somers,

Faison and Zarr [3], found that the thermal conductivity of this i material at a temperature of 297 K could be represented by the

correlation

k = 21.94 + 0.03076 (D-301.4) + 0.09936 (P-101.33) (4)

where k is conductivity in mW/(m* K) , D is density in kg/m^ , and P is

pressure in kPa. || The coefficient, 0.09936, of the pressure term in eq(4), is the

rate of change of thermal conductivity with pressure, or slope of a I plot of k versus P, for a specimen at a temperature of 297 K and an > environmental gas pressure of 101.3 kPa. This coefficient was

obtained over the range of pressures from 99.3 to 101.2 kPa. This

value compares with the value of 0.0607 obtained by extrapolating

eq(3), the relation for the slope of k against P, to Po = 101.3 kPa.

A full understanding of the disagreement between the two slopes of

k(P) obtained by the two different laboratories has yet to be

obtained. According to the NIST/G group, however, moisture content

has a slight but measurable effect on thermal conductivity at 297 K,

and this variable was not controlled by either lab for the results

| reported here. j

8
However, extrapolating eq(l) to a value of T = 297 K, only

slightly below the lowest temperature used by us in obtaining this

correlation, we estimate that k 19.66 mW/(m* K) is the value of

thermal conductivity for our fumed-silica specimens at 297 K, that

would be measured in the Boulder lab. We estimated in the previous

section that the effect of increasing the environmental air pressure

on these specimens to that at sea level (the air pressure at NIST/G)

would be to increase k by a factor of 1.063 from the values measured

at the elevation of Boulder. Combining these two results, we

estimate that kB* = 1.063 * 19.66 = 20.9 mW/(m* K) for a temperature

of 297 K and an ambient environmental pressure equal to that at sea

level. Under the same conditions, Rennex, Somers, Faison and Zarr

[3] found kG = 21.8 mW/(m- K) at an environmental air pressure of

100.1 kPa and a temperature of 297 K, for specimens having a density

of 301 kg/m^ . The difference is about 1.9% and is less than the

combined estimates for the experimental biases of the NIST/B

(± 1.5%) and NIST/G (+ 0.9%) apparatus. This agreement is satisfactory. Recent work by NIST/B, to be published separately, indicates

that the different densities used in this comparison have little or

no effect on the conclusion.

This analysis has assumed that the pressure dependence of the

thermal conductivity , given by eq(2) and obtained for a specimen

temperature of 318 K, was valid for estimating the pressure

dependence of k at 297 K in order to compare results obtained by

different labs at different environmental pressures. While the two

temperatures used are not very different, so the comparison made

should be valid, the effect of temperature on the k(P) correlation

is presently under investigation.

9
6. RECOMMENDATIONS

The thermal conductivity of this material is somewhat lower

than that of the presently established SRM 1450b (fibrous glass

board with organic binder), and the material can withstand much

higher temperatures. The limiting high temperature used in this

study was set by the apparatus and not by the physical properties of

this material. When a suitable apparatus becomes available this

material should be characterized at even higher temperatures.

The most serious disadvantage of this material in its proposed

use as an SRM is its high mechanical fragility. If this material is

adopted as an SRM, users will have to be cautioned to exercise great

care in handling it when measuring its mass and physical dimensions,

when conditioning it to proper moisture content, and when installing

it in, and removing it from, measurement apparatus. The material is

made from extremely fine particles, and fine airborne dust is easily

generated during sawing or other machining operations. Use of a

dust mask should be required of those machining the material when

preparing to install it into a measurement apparatus.

The relatively low thermal conductivity of this material, its

apparent stability under exposure to temperatures as high as 500© C,

its freedom from phase changes, and its inorganic composition are

useful and desirable properties for a new SRM. While the correlation of thermal conductivity with temperature seems to indicate the

presence of some radiative heat transfer, this radiative component

seems to be relatively small in comparison to the total apparent

thermal conductivity. This suggests that the total heat transfer is

due mostly to conduction through the microporous solid and the

entrapped gas within the pores

10
7. REFERENCES

[1] Bust, J.G. “Status of Thermal Conductivity Standard Reference

Materials at the National Bureau of Standards", THERMAL CONDUCTIVITY

18: Proceedings of the 18th International Conference on Thermal Conductivity . Ashworth, T. and Smith, David R. , Eds.; pp. 327-338

(Plenum, New York, 1985).

[2] ASTM Subcommittee C16.30. "Reference Materials for Insulation

Measurement Comparisons", Thermal Transmission Measurements of Insulation. ASTM STP 660 . Tye, R.P., Ed.; pp. 7-29 (ASTM, Philadelphia,

1978)

[3] Rennex, B.G., Somers, T.A.

,

Faison, T.K., and Zarr, R.R. "Evaluation of Fumed-Silica Insulation for a Thermal Conductivity Standard Reference Material", ASTM/DoE Symposium on Insulation Materials.

Testing and Applications . ASTM STP, to be published (ASTM, Philadelphia, 1989 )

[4] Zarr, R.R.

,

Somers, T.A. and Ebberts, D.F. "Room-Temperature

Thermal Conductivity of Fumed-Silica Insulation for a Standard Reference Material", NBSIR 88-3847, (U.S. National Institute of

Standards and Technology, June 1988).

[5] Bust, J.G., Filla, B.J., Hurley, J.A. and Smith, D.R. "An

Automated High Temperature Guarded-Hot-Plate Apparatus for Measuring Thermal Conductivity of Insulation Between 300 and 750 K", NBSIR

88-3089 (U.S. National Institute of Standards and Technology, May

1988)

[6] Bust, J.G., and Lankford, A.B. "Comments on the Measurement of

Thermal Conductivity and Presentation of a Thermal Conductivity

Integral Method", Int. J. of Thermophysics, 3/1 . 67-76 (1982).

[7] Tye, R.P. "The Thermal Conductivity of MINK-2000 Thermal Insulation in Different Environments to High Temperatures", 9th Conference on Thermal Conductivity . CONF-691002, pp. 341-351 (U.S. Atomic

Energy Commission, Div. of Technical Information, March 1970)

11
Table 1 Thermal conductivity of a pair of microporous fumed-silica

insulation boards with a mean density of 301 kg/m , at mean temperatures from 321 to 723 K (48 to 450 ®C). Measurements

were performed in air at ambient humidity and at a pressure

of 83.6 kPo (627 Torr), using the high-temperature guarded

hot plate ot the National Institute of Standards and

Technology in Boulder, Colorado. Temperature differences

through the specimens were chosen to be about 10 percent of

the value of the mean temperature within the specimens. The

bias in values of Kmeas is estimated to be 1.5 percent at

330 K and 2.5 percent at 720 K. Deviations compare measured

thermal conductivi

Th1 i Tl 0

(K) (K)

443. 256 403. 184

441 . 168 405. 093

443. 168 403. 099

445. 166 401 . 159

603. 281 543. 078

708. 294 637. 800

758. 146 688. 102

737. 959 658. 086

735. 060 661128

733. 157 663. 205

713. 031 633. 139

711 . 104 635. 156

708. 141 638. 187

706. 160 640. 261

681 . 099 615. 150

653. 024 593. 135

653..150 593. 229

653,.151 593. 201

628..057 568.,134

605,,207 541.133

603,.127 543..159

601 ,125 545.,221

578 .078 518..148

550 .107 496.,166

548,.151 498..170

546 .152 500,.169

523 .120 473 .165

500 .134 446,.159

498 .134 448 .130

496 .097 450 . 108

391 .242 355 .150

393 .187 353 .153

395 .141 351 .120

418 .205 378 .168

341 .115 306 .888

339 .181 307 .182

340 .177 306 .394

337 .172 305 .412

ty to va 1 ues of Kca 1 c

Tmeon Kmeas

0<) mW/( m *K)

423. 220 20. 999

423. 131 20. 926

423. 134 20. 953

423. 163 20. 950

573. 180 23. 604

673. 047 25. 958

723. 124 27. 713

698. 023 26. 779

698. 094 26. 855

698. 181 26. 860

673. 085 26. 058

673. 130 26. 071

673. 164 26. 085

673. 211 26. 061

648. 125 25. 364

623. 080 24. 651

623.,190 24. 625

623,.176 24.,687

598..096 23..998

573,,170 23.,523

573,.143 23,.521

573..173 23,.534

548,.113 22 .962

523..137 22 .431

523,.161 22 ,509

523 .161 22 .551

498 .143 21 .992

473 .147 21 .650

473 .132 21 .652

473 .103 21 .621

373 .196 20 .375

373 .170 20 .312

373 .131 20 .364

398 .187 20 .693

324 .002 19 .788

323 .182 19 .806

323 .286 19 .898

321 .292 19 .886

Stainda rd

computed from eq(l).

Kco1 1 c Dev i cit i on mW/( m.K) (Percent)

20. 932 0. 32

20. 930 -0. 02

20. 931 0. 1

20. 932 0. 09

23. 512 0. 39

26. 058 -0. 38

27. 625 0. 32

26. 819 -0. 15

26. 818 0. 14

26. 818 0. 15

26. 064 -0. 02

26. 063 0.,03

26. 061 0. 09

26. 061 0. 00

25. 351 0.,05

24. 689 -0,,16

24. 692 -0,.27

24. 692 -0..02

24. 078 -0..33

23. 513 0,,04

23. 511 0..04

23,,511 0,.10

22.,986 -0 .10

22..500 -0 .31

22,,499 0 .04

22,,498 0 .23

22 ,054 -0 .28

21 .647 0 .02

21 .645 0 .03

21 .644 -0 .11

20 .344 0 .15

20 .345 -0 .16

20 .345 0 .09

20 .623 0 .34

19 .875 -0 .44

19 .868 -0 .31

19 .869 0 .14

19 .852 0 .17

Dev i (3t i on = 0 .21

12
Table 2. Thermal conductivity of a pair of microporous fumed-eilica

insulotion boards with a mean density of 301 kg/m , ot a mean

temperature of 318 K (45 ®C). Measurements were performed in

air at ambient humidity and at pressures ranging from 40 to

83.5 kPa (626 Torr) using the NIST/B high-temperature guarded

hot plate. Temperoture differences through the specimens were chosen to be about 10 percent of the value of the mean temperature within the specimens. The bias in values of kmeas is

estimated to be 1.5 percent at 330 K. Deviations compare

measured thermal conductivity to values of Kcalc computed

from eq (2). The highest pressure (83.5 kPa; 626 Torr) is

the local ambient atmospheric pressure at Boulder, Colorado.

Thi TIo Tmean Kmeas Kcalc Pressure Deviation

(K) (K) (K) mW/(m.K) mW/(m#K) (kPo) (Percent)

331 . 151 305,.119 318,,135 20. 070 19. 982 83. 5 0. 44

333. 152 304. 224 318,,688 20. 047 19. 982 83.,5 0. 32

334. 130 304.,259 319,,195 19. 972 19, 982 83..5 -0. 05

331 .,143 305.,150 318,,147 19..913 19, 950 83,, 1 -0. 19

331 . 095 305,,076 318,.086 19..626 19. 696 80..0 -0. 35

332, 209 304,.294 318,,252 19, 099 19,,107 73,,3 -0. 04

333. 133 304..090 318.,612 19,,054 19,. 107 73,.3 -0. 28

334.,140 304..205 319.,173 19,.089 19., 107 73 .3 -0. 09

335. 229 304,,469 319,,849 18.,447 18.,468 66,.7 -0,.11

332.,142 304,,149 318,.146 17,.811 17.,779 60..0 0..18

331 .,164 305..155 318,.160 17,,083 17,.041 53 .3 0, CM in 332,,147 304..151 318.,149 17.,059 17.,041 53,,3 0.. 1

331 .,167 305., 178 318..173 17,.016 17..041 53..3 -0.,14

332,,141 304,.142 318,.142 16..275 16,.252 46 .7 0.,14

331 ..121 305,,109 318.. 115 16,.245 16.,252 46,.7 -0..04

332,,185 304 . 187 318,. 186 15..443 15,,414 40,.0 0.,19

333,.123 303,.818 318,,471 15..376 15..414 40 .0 -0,,25

332,.142 304 .138 318..140 15 .395 15 .414 40 .0 -0,.12

331 ,.152 305,,167 318,.160 17,.768 17,,779 60 .0 -0,.06

332..125 304,.120 318,.123 17..801 17,,779 60 .0 0.,12

Standard Deviation = 0.21

13
flPPRRENT

THERMAL

CONDUCTIVITY,

mW/(m.K)

Temperature^ K

Figure 1. Thennal conductivity of a pair of microporous fumedsilica insulation boards with a mean density of 301 kg/m3

,

at mean temperatures from 321 to 723 K (48 to 450oC).

Measurements were performed in air at ambient humidity and

at a pressure of 83.6 kPa (627 Torr) using the NIST/B

high-temperature guarded hot plate. The measurement bias

in values of Kmeas is estimated to be 1.5% at 330 K, and

2.5% at 720 K. The solid curve represents values of

Kcalc, obtained using eq(l).

14
1

^

^obs

^caLc ^

^^caLc

0.4

0 .

0 .

0 .

0 .

- 0 .

- 0 .

- 0 .

- 0 .

- 0 .

Figure

1 -

3 -

4 -

300 350 400 450 500 550 600 650 700 750

Temperature^ K

Deviat-ions of t-hermal conducbivit-y of microporous fumedsilica specimens from values calculated with eq(l).

15
RPPRRENT

THERMRL

CONDUCTIVITY

PRESSURE^ kPa

Figure 3. Thermal conductivity of a pair of microporous fumed-silica

insulation boards with a mean density of 301 g/m3 , at a

mean temperature of 318 K (45®C). Measurements were performed in air at ambient humidity and at pressures varying from 40 to 83.5 kPa (300 to 626 Torr)

The solid line

was calculated from eq(2).

16
^

^obs

^coLc ^

^^caLc

0.5

0.4

0.3

0.2

0.1

0.0

- 0.1

- 0.2

-0.3

-0.4

T^-T-T-r^ ,,,,,, '

1

' ' ' 'T '

-

-

-

.

- -

- -

;

~

-

-

_ —

- -

, , 1 , , , 1. J ^ 1 1 1 1 1

. , . 1 . 35 40 45 50 55 60 65 70 75 80 85

PRESSURE^ kPa

•igure 4. Deviations of thermal conductivity of microporous fumedsilica specimens from values calculated with eq(2).

17
RPPRRENT

THERMRL

CONDUCTIVITY,

mN/(m.K)

PRESSURE^ kPo

Figure 5. Extrapolated correlation of thermal conductivity with

with pressure, eq(2), estimating thermal conductivivity of

microporous fumed-silica insulation at sea-level atmospheric pressure (101.3 kPa) as indicated by the open circle at

the top. Mean temperature of specimens is 318 K.

18
U.S. DEPT. OF COMM. 1. PUBLICATION OR 2. Performing Organ. Report No. 3. Publication Date

BIBLIOGRAPHIC DATA

SHEET (See instructions)

REPORT NO.

NISTIR 88-3901 October 1988

4. TITLE AND SUBTITLE

MICROPOROUS FUMED-SILICA INSULATION BOARD AS A CANDIDATE STANDARD REFERENCE

MATERIAL OF THERMAL RESISTANCE

5. AUTHOR(S)

DAVID R. SMITH and JEROME G. HUST

6. PERFORMING ORGANIZATION (If joint or other than NBS, see in struction s) National Institute of Standards and Technology

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

0RNL/IA-2T428

8. Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP)

Sponored, in part, by

U.S. Department of Energy

Oak Ridge National Laboratory

Oak Ridge, NT 37830

10.

SUPPLEMENTARY NOTES

I I Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bi bliography or literature survey, mention it here)

Measurements of apparent thermal conductivity of microporous

fumed-silica insulation board are reported in order to provide a

basis for certifying it as a Standard Reference Material (SRM) of

thermal resistance. These data, for a pair of specimens having a

mean density of 301 kg/m^ , encompass a range of temperature from

321 to 723 K and environmental gas pressures at and below ambient

atmospheric pressure (40 to 83.7 kPa). Detailed analyses and

intercomparisons of previously published data are given. Correlations of thermal conductivity with temperature and with pressure

are given which represent the data within a standard deviation of

0.2%. This fumed-silica material has a thermal conductivity of

19.68 mW/(m- K) at 300 K and is suitable for use as an SRM of very

low conductivity from room temperature up to temperatures beyond

720 K (450 ©C). Great care in handling this material is necessary

because of its fragility.

12.

KEY WORDS (S/x to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) apparent thermal conductivity; density; microporous fumed silica; pressure;

Standard Reference Material; SRM; temperature; thermal insulation; thermal

resistance.

13.

AVAILABILITY

Uni imited

I I For Official Distribution, Do Not Release to NTIS

I I Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

28

15. Price

*U.S. GOVERNMENT PRINTING OFFICE: 19 8 8-0-673-948/501 3 M-DC 6043-P80
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