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LEGAL NOTICE
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83564
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Pimyo This note 18 an addendim to RN -3556 and was prepared to ailow presenta-
tion of the most rece!. rad: seat flow apparatus measurements at the
Nationa i Prys :51 Labor-story fer .cc on Thermal Conductivity, Teddington,
Engia :., Juny J.., 962.
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THERMO :38
ES OF GRADE CGB CEAPHITE*
J. D Mourc, I. G. Gorlfer and D. L MCEIRO
Metals Cer' s Divin.
Tak R: .. Jati di laboratory
as. Piqe, nessee, USA
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Facsimile Price s
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Microfilm Price $_
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LEGAL NOTICE -
This report as prepared sa un acoma of Government sponsored work. Nothout the United
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racy, complement, or weters of the winnmathow contain the report of that when we
of way lubormation, manau, worth a proces declared that this report may not matters
printly a nd rights;
n. A way that staa wa rast to the web, w for tempo read the tren the
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Am vend then the whova, sperma actiung an d of the commen" besteden wury we
shery o choc ad the Commis, et v oy a cha cunctor, When out that
a employee or contractor of the Contest w staplegue at wwch cpretrun tor prepares,
docrinales, or awhing across any motormation par le do employment or coorrect
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Available from the
Office of Technical Services
Department of Commerce
Washington 25, D. C.
..;) Selim Rikom
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*Research sponsored by
contract with the Union Carc.
U. ). Atomic Energy Commission und
Cc :oration.
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THERMAL PROPERTIES OF GRADE CGB GRAPHITE
J. P. Moore, T. G. Godfrey, and D. L. McElroy
SPECIMEN CHARACTERIZATION
Grade CGB graphite is a nuclear graphite which is basically an
extruded petroleum coke bonded with coal tar pitch. No carbon blacks are
used and the low-permeation graphite is finished through a series of
impregnations and heat treatments with a final heat treatment of all
components to 2800°C. The graphite is produced by the Carbon Products
Division of Union Carbide Corporation as 2 1/4-in.-square by 72-in.-long
bars with an average bulk density of 1.85 g/cm3. One bad feature of the
graphite is its cracked condition which 18 caused by the structure of the
graphite being 80 dense that hydrocarbons produced through pyrolysis of
the impregnants cannot escape via the pores during fabrication. In
general, these cracks are 0.001 to 0.002 in. thick, 1/8 to i 1/4 in. wide,
and less than 3 in. long. The bar stock was machined into 2 1/8-in.-diam
by l-in.-thick disks for the radial heat flow specimens and radiographs
or these were used to select two crack-free measuring disks. These speci-
mens, which had a measured density of 1.824 g/cm", permitted thermal con-
ductivity measurements with heat flow normal to the extrusion axis, which
is the low k directior.. The axial core heater hole was made only 3/8 in.
in diameter in order to allow the largest practical value of lo rz/rı
(12 was 0.969 in. and i was 0.315 in.) and hence the largest possible st.
A 1/4-in.-diam spectrographic carbon rod was used for the core heater and
70% Pt-30% Rh wires were placed in shailow grooves to serve as voltage taps.
THERMAL CONDUCTIVITY DATA
A listing of the results obtained is given in Table 1. T. results
at 51°C are considered questionable since the measured ar was jéis than
1/2°C. The results obtained at 704 and 602°C after heating to 910°C were
1% higher than those of the first cycle. This increase was caused by a
slight contamination of the 90% Pt 10% Rh-Pt thermocouples at 910°C but
was not sufficient to doubt the validity of the 910°C results. However,
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Table 1. Total Thermal Conductivity of Grade CB Graphite
Normal to the Extrusion Axd8 28 Measured
in the Radial Heat Now Apparatus
t(°C)
T(°K)
(w oppi cm-2)
(-762 cm)
51
1.0878
0.9610
101
2014
0.8786
295
397
500
324.2
374.2
477.2
568.2
670.2
773.2
875.2
977.2
1183.2
1288.2
876.2
0.920
1.040
1.138
1.247
1.403
1.522
1.611
1.769
2.023
1.924
1.368
0.8019
0.7129
0.6571
0.6020
0.5612
0.4944
0.51966
0.73210
602
704
910
· 1015
603
& Accuracy of 51°C data is questionable because of very
small at across specimen.
Disregard because of severe thermocouple instabilities.
the results obtained at 1015°C should be disregarded because of severe
thermocouple instabilities. In addition, the electrical resistance of
the core heater at 603°C indicated the thermocouples bad a -10 to -15°C
error which is sufficient justification to disregard the 603°C data.
po.
AUXILIARY MEASUREMENTS
In addition to the thermal conductivity measurements in the radial
heat flow apparatus, electrical resistivity measurements and low-temperature
thermal conductivity measurements were made normal and parallel to the
extrusion axis of the same CGB graphite stock.
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Electrical Resietivity Measurements
Electrical resistivity measurements were made by the four-probe
potentiometric method under a vacuum of 1005 to 10-6 torr, between 23 and
1000°C. The results were reproducible to 10.1% and accurate to +0.25%.
The results listed in Table 2 were taken from a smooth curve drawn through
the data plotted in Fig. 1. This figure also shows the temperature varia-
tion of the ratio of the electrical resistivities in the normal and parallel
directione. The electronic portion of the thermal conductivity was calcu-
lated using the Wiedemann-Franz-Lorenz relation.
Table 2. Electrical Resistivity and Electronic Portion
of Thermal Conductivity of CGB Graphite
Perpendicular to
Extrusion Axis
Parallel to
Extrusion Axis
p . ke
(uohm-cm) (w ºr em 2)
p
t(90)
T("K)
(uohn-cm)
(w ºk?cm-2)
23.
100
200
300
400
296.2
373.2
473.2
573.2
673.2
773.2
873.2
973.2
1073.2
1173.2
1273.2
1232
1132
1045
993.2
962.7
945.4
937.3
936.5
941.5
948.8
960.8
0.0059
0.0081
0.0112
0.0142
0.0171
0.0200
0.0228
0.0254
0.0279
0.0303
0.0325
583.4
538.0
501.5
480.5
469.0
463.5
463.5
467,8
474.0
482.0
490.0
0.0124
0.0170
0.0231
0.0292
0.0352
0.0409
0.0461
0.0510
0.0555
0.0596
0.0637
2.11175
.. 2.10408
2.08375
2.06701
2.05266
2.03969
2.02222
2.00192
1.98629
1.96846
1.96082
60
700
800
900
1000*
Thermal Conductivity Measurements
Thermal conductivity measurements were made in the range of 34 to
75°C in a longitudinal canparative type heat flow apparatus. This apparatus
consisted of two l-in.-diam gold-plated Armco iron meter bars mounted
coaxially in a vacuum chamber with the cylindrical disk specimen compressed
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between them. A temperature gradient was established along the meter bar's
8.nd the specimen by a Nichrome heater at the top of one meter bar and a
water-cooled heat sink at the bottom of the other mater bar. The tempera-
ture at known positions along the meter bars was determined with
Chranel-P/Constantan thermocouples. The temperature gradient along each
meter bar was extrapolated to the meter bar specimen interface to determine
the temperature drop across the specimen. The measured t:"nperature gradi-
ents along the meter bars were used with the known thermal conductivity of
the iron meter bars to calculate the amount of ineat flowing through the
specimen. Interracial resistance was minimized with l-mil-thick indium
foils and by careful lapping of the specimen and meter bar surfaces. This
apparatus was evaluated using an Armco iron specimen and yielded data with
a repeatability of better than 1/4% and a probable mccuracy of +3%.
The thermal conductivity measurements listed in Table 3 were made
with this apparatus on a 0.8-in.-high by 1.00-in. -diam .CGB graphite speri-
men cut so that the heat flow wes normal to the extrusion ads. Also
listed in Table 3 are similar measurements on an 0.8-in.-high by 1.00-in.-
diam CGD graphite specimen cut so that the heat flow was parallel to the
extrusion axis.
Table 3. Thermal Conductivity of Type CGB Graphite
as Determined from Measurements in the
Longitudinal Heat Flow Apparatus
Direction of Heat Flow
+(°C)
T°K)
(w or I com-2)
Normal to extrusion axis
Normal to extrusion axis
Parallel to extrusion axis
Parallel to extrusion axis
Parallel to extrusion axis
307.2
348.2
307.2
326.2
348.2
1.087
1.058
1.942
1.921
1.884
DISCUSSION OF RESULTS
The total thermal resistivity, 1/k, values for the axis normal to
extrusion that are tabulated in Table 1 are plotted as a function of the
absolute temperature in Fig. 2. The lattice component, ktor of the thermal
conductivity was calculated by subtracting the electronic component, ke,
as determined from appropriate electrical resistivity data and the
Wiedemann-Franz-Lorenz relation, from the total thermal conductivity.
These calculated lattice component values are also plotted in Fig. 2.
Above its Debye temperature, the lattice component of the thermal
resistivity of an electrically insulating solid should depend linearly on
the absol'te temperature. The lattice component values in Hg. 3 are all
within 14% of the linear equation,
1/1 = R1 = 0.48812 + 1.4060 x 10-3 T,
where
k = w ºk?cm-, and
T - K.
The electronic component of the thermal conductivity is shown in Fig. 2
with the appropriate scale on the right. The numbers in parentheses above
the ke line at 200°K increments indicate the percentage of the total
thermal conductivity which is electronic in nature. Since the thermal
conductivity of graphites varies so widely with composition and manu-
facturing technique, no attempt was made to selectively compare these
results with those of other investigators.
The results obtained from the longitudinal heat flow apparatus are
limited in temperature range but are very useful since they provide a
check on the radial results for k in the normal direction and allow com-
parison between the values of k along the two principal axes of the graphite.
The results from this apparatus are tabulated in Table 3 and are plotted
in Fig. 3. The results from the radial apparatus are also shown on this
figure and the intercomparison of the two methods is very good. At these
temperatures, the thermal conductivity ratio of the axis parallel to
extrusion and the normal axis is 1.74.
CONCLUSIONS
Theoretical considerations based on three-phonon umklapp processes
predict an inverse relationship between the lattice component of the
thermal conductivity and the absolute temperature for temperatures above
the Debye temperature of the solid. The results from this work show that
this inverse relationship holds for graphite over the wide temperature
range from 323 to 1183°K. This entire temperature span 18 well below the
Debye temperature for graphite.
e,
1300 .
UNCLASSIFIED
ORNL-DWG 6:1-5879
12.16
ELECTRICAL RESISTIVITY COB GRAPHITE
Tod TO EXTRUSION DIRECTION
2.14
::|| TO EXTRUSION DIRECTION
PllPro
2.12
.-
-
-
-.
---
--
2...
-
2.10
--
-
-.
.
2.08
P. ELECTRICAL RESISTIVITY (M.2-cm)
....--
...
12.06
lid, To
...
--
..
.
2.04
..-.
.-
2.02

2.00
400
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+
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o
1.96
200 400 600 800 1000
TEMPERATURE (°C)
Fig. 1. The Electrical Resistivity of Grade CGB Graphite Along
the Principa). Axes.
UNCLASSIFIED
ORNL-DWG 64--5877
--
.....
• Vkl
o Vk
1.8 ----------- A ke
VK THERMAL RESISTANCE (w-Ok cm)
Ke, ELECTRONIC THERMAL CONDUCTIVITY (W OK-4 cm)

(6.5%)
(4.7%)
(3.3%)
oom, L:6199) —
(4.9%) ...!
(0.9%)
-
0.8l.
300
400
500
600
700 800 900
TEMPERATURE (°K)
1000
1100
1200
1300
Fig. 2. Therma). Resistivity in the Normal to Extrusion Direction
of Grade CGB Graph::from 300 to 1200^K us Measured in the Radial lleut
Flow Apparatus.
UNCLASSIFIED
ORNL-DWG 64-5878

A PARALLEL TO EXTRUSION AXIS
O NORMAL TO EXTRUSION AXIS
O RESULTS FROM RADIAL APPARATUS
K, THERMAL CONDUCTIVITY (w OK-'cm')
0.8
300
310
320
360
370
380
330 340 350
TEMPERATURE (°K).
Fig. 3. Thermal Conductivity Results on Grade CGB Graphite
Obtained in a Low-Temperature longitudinal Heat Flow Apparatus and
From the Radial lcat Flow Apparat:18.

END