ASTM D6745-01
(Test Method)Standard Test Method for Linear Thermal Expansion of Electrode Carbons
Standard Test Method for Linear Thermal Expansion of Electrode Carbons
SCOPE
1.1 This test method covers the determination of the coefficient of linear thermal expansion (CTE) for carbon anodes and cathodes used in the aluminum industry, in baked form, by use of a vitreous silica dilatometer.
1.2 The applicable temperature range for this test method for research purposes is ambient to 1000°C. The recommended maximum use temperature for product evaluation is 500°C.
1.3 This test method and procedure is based on Test Method E 228, which is a generic all-encompassing method. Specifics dictated by the nature of electrode carbons and the purposes for which they are used are addressed by this procedure.
1.4 Electrode carbons in the baked form will only exhibit primarily reversible dimensional changes when heated.
1.5 The values stated in SI units are to be regarded as standard.
1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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An American National Standard
Designation: D 6745 – 01
Standard Test Method for
Linear Thermal Expansion of Electrode Carbons
This standard is issued under the fixed designation D6745; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope 3.1.2 mean coeffıcient of linear thermal expansion (CTE)—
The linear thermal expansion per change in temperature; the
1.1 This test method covers the determination of the coef-
mean coefficient of linear thermal expansion is represented by:
ficient of linear thermal expansion (CTE) for carbon anodes
andcathodesusedinthealuminumindustry,inbakedform,by DL/L 1 DL 1 L 2 L
0 1 0
a 5 5 · 5 (1)
T
DT L DT L T 2 T
use of a vitreous silica dilatometer.
0 0 1 0
1.2 The applicable temperature range for this test method
3.1.2.1 Thishastobeaccompaniedbythevaluesofthetwo
forresearchpurposesisambientto1000°C.Therecommended
temperatures to be meaningful; the reference temperature (T )
maximum use temperature for product evaluation is 500°C.
is 20°C, and the notation may then only contain a single
1.3 ThistestmethodandprocedureisbasedonTestMethod
number, such as a¯ , meaning the mean coefficient of linear
E228, which is a generic all-encompassing method. Specifics
thermal expansion between 20 and 200°C.
dictatedbythenatureofelectrodecarbonsandthepurposesfor
3.2 Definitions of Terms Specific to This Standard:
which they are used are addressed by this procedure.
3.2.1 reference specimen—a particularly identified or pedi-
1.4 Electrode carbons in the baked form will only exhibit
greed material sample, with well-characterized behavior and
primarily reversible dimensional changes when heated.
independently documented performance.
1.5 The values stated in SI units are to be regarded as
3.2.2 specimen—a representative piece of a larger body
standard.
(anode, cathode, and so forth) that is considered to be fairly
1.6 This standard does not purport to address all of the
typical of a portion or of the entire piece.
safety concerns, if any, associated with its use. It is the
3.2.3 vitreous silica dilatometer—a device used to deter-
responsibility of the user of this standard to establish appro-
mine linear thermal expansion, by measuring the difference in
priate safety and health practices and determine the applica-
linear thermal expansion between a test specimen and the
bility of regulatory limitations prior to use.
vitreous silica parts of the dilatometer.
2. Referenced Documents
4. Summary of Test Method
2.1 ASTM Standards:
4.1 Arepresentativespecimenisplacedintoavitreoussilica
E228 Test Method for Linear Thermal Expansion of Solid
dilatometer and heated, while its linear expansion is continu-
Materials with a Vitreous Silica Dilatometer
ously recorded.The change of the specimen length is recorded
as a function of temperature. The coefficient of linear thermal
3. Terminology
expansion is then calculated from these recorded data.
3.1 Definitions:
3.1.1 linear thermal expansion—the change in length per 5. Significance and Use
unit length resulting from a temperature change. Linear ther-
5.1 Coefficients of linear thermal expansion are used for
malexpansionissymbolicallyrepresentedby DL/L ,where DL
design and quality control purposes and to determine dimen-
isthelengthchangeofthespecimen(L −L ), L and L arethe
1 0 0 1
sional changes of parts and components (such as carbon
specimens lengths at reference temperature T and test tem-
0 anodes, cathodes, and so forth) when subjected to varying
perature T , respectively. Linear thermal expansion is often
temperatures.
expressed as a percentage or in parts per million (such as
6. Apparatus
µm/m).
6.1 Dilatometer—Thedilatometerconsistsofthefollowing:
6.1.1 Specimen Holder and Push-rod,bothmadeofvitreous
This test method is under the jurisdiction of ASTM Committee D02 on
silica. The design of the device shall ensure that the push-rod
Petroleum Products and Lubricants and is the direct responsibility of Subcommittee
load on the specimen by itself is not causing deformation. The
D02.05.0E on Properties of Fuels, Petroleum Coke and Carbon Materials.
Current edition approved Dec. 10, 2001. Published February 2002.
Annual Book of ASTM Standards, Vol 14.02.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D6745–01
use of pressure distribution quartz plates on top of the 6.3 Furnace—Thefurnaceisusedforuniformlyheatingthe
specimen is permissible. specimen over the temperature range of interest, but not above
1000°C. Temperature uniformity shall be at least 6 0.5°C per
NOTE 1—Dilatometers are usually constructed in horizontal or vertical
3 50 mm of sample length. The temperatures shall be controlled
configurations (1) Vertical devices are preferred for very large samples
as a function of time. The furnace may have a muffle (quartz,
and when extensive shrinkage is expected. Horizontal configurations
usually afford better temperature uniformity over the specimen, but are mullite, alumina, inconel, monel, or stainless steel are most
subject to drooping when large specimens are employed. Horizontal
common) or other provisions to provide a protective atmo-
devices, when used with very large specimens, require special provisions
sphere for the specimen. The furnace shall have provisions for
to reduce friction between the specimen and the dilatometer tube to
continuous purging with an inert gas at a sufficient rate, and
minimizepush-rodpressurerequiredtokeepthespecimenincontactwith
exclude air from the specimen while a purge is maintained.
the end plate. For this application, either configuration is acceptable.
6.4 Caliper—Thecaliper(micrometerorVerniertype)isfor
NOTE 2—For basic construction details and fused silica annealing
measuring the initial length of the specimen, L , with an
schedule, consult Test Method E228.
NOTE 3—Multiplerodssupportingaplatforminplaceoflargediameter accuracy within 6 25 µm, and a capacity to open to the length
tubes have been also used successfully in the vertical configuration.
of the specimen plus 1 mm.
6.1.2 Transducer or Indicator, for measuring the difference
7. Test Specimen
in length between the specimen and the dilatometer with an
7.1 Specimens shall be cylindrical, preferably with a 50 6
accuracy within 6 2 µm. The transducer shall translate these
2mmdiameter.Slightlysmallerorlargerdiameterscanalsobe
movements into an electrical signal suitable for displaying or
accommodated without degradation of data. The length of the
recording. The non-linearity of this conversion must be less
specimensshallbebetween50and130mminlengthandhave
than0.25%ofthefullscalevalueoftheoutput.Thetransducer
flat and parallel ends to within 6 25 µm.
shall be protected or mounted so that the maximum tempera-
7.2 It is permissible to stack up to five disks of smaller
ture change observed in the transducer during a test will affect
lengths to obtain a proper length specimen. The interfaces,
the transducer readings by less than 1 µm.
however, must be flat and parallel within 6 25 µm to prevent
6.1.3 Temperature Sensors, for determining the mean tem-
rocking.
perature of the specimen with an accuracy within 6 0.5°C.
7.3 The dimensions of the specimens should be ordinarily
When a thermocouple is used, it shall be referenced (cold
measured as received.
junction compensated) to the ice point with an ice-water bath
7.4 If water was used in conjunction with their preparation,
or an equivalent system.
each specimen must be kept in an oven at 110 6 5°C for at
6.1.3.1 Duetothelargesizeofthespecimen,aminimumof
least 6 h and allowed to cool down thereafter, prior to testing.
one thermocouple per 40 mm specimen length must be
If any heat or mechanical treatment is applied to the specimen
employed. It is permissible to read the output of each thermo-
prior to testing, this treatment should be noted in the report.
couple independently and average the readings or to connect
them in series and divide the single reading by the number of
8. Calibration
thermocouples to obtain the average. In the latter case, inter-
connections must be made at or beyond the point of cold 8.1 Thetransducershouldbecalibratedbyimposingaseries
of known displacements with a precision screw micrometer,
junction compensation.
6.1.3.2 The temperature sensors shall be in close proximity gageblocks,orequallyaccuratedevice.Forabsolutetransduc-
ers (such as digital encoders, and so forth), this procedure is
to the specimen, preferably in between the quartz dilatometer
tube and the specimen. The temperature sensors shall not be omitted and periodic verification is sufficient.
8.2 Verificationofthecalibrationofthetemperaturesensors
directly exposed to the furnace walls.
6.2 Readout or Recording of Data: separately from the dilatometer is to be performed periodically
or when contamination of the junction is suspected.
6.2.1 Manual recording of expansion and temperature val-
8.3 Regardlessofindependentcalibrationsofthetransducer
uesindicatedatselectedtemperaturepointsmaybemadeifthe
and the thermocouples, the dilatometer, as a total system, shall
transducer is equipped with or connected to a suitable display
be calibrated by determining the thermal expansion of at least
and the thermocouples outputs are determined with a potenti-
one reference material of known thermal expansion. Recom-
ometer or millivolt meter.
mended reference materials are listed in Annex A1.
6.2.2 Chart or data logger recording of the expansion and
8.3.1 The calibration should be done using approximately
temperature signals may be accomplished using a device
the same thermal cycle as that used for testing (see 9.7 and
whoseresolutionisatleast1000timeshigherthantheexpected
9.8).
maximum output signal. All calculations and corrections (see
8.3.2 The calibration constant may be derived as follows:
Section 10) must be done externally based on the recorded
values.
DL DL
A 5 2 (2)
S D S D
6.2.3 Computerized recording may be used with similar
L L
0 0
t m
restriction to 6.2.2. Calculations and corrections may be done
where:
using suitable software.
m = measured expansion of the reference material, and
t = true or certified expansion of the reference material.
8.4 Ifthecalibrationspecimenisconsiderablysmallerinthe
Hidnert, P. and Krider, H.S., “Thermal Expansion Measurements,” Journal of
Research, National Bureau of Standards, Vol 48, 1952, p. 209. cross section than the specimen, it is necessary to provide a
D6745–01
thermaljacketaroundittopreventerrorscausedbyconvective 9.2 Place the specimen into the dilatometer after making
currents. A thermal jacket may be produced by drilling a hole certain that all contacting surfaces are free of foreign material.
in the axis of a carbon specimen and loosely fitting the It is important to have good seating of the specimen in a stable
calibrationspecimenintoit.Thecarbonjacketmustbeabout1 position. (Warning—Alkali contamination will adversely af-
mm shorter than the calibration specimen. fect fused silica parts. Avoid touching them with hands.)
8.5 The use of published values of thermal expansion for 9.3 Ensure that the temperature sensors shall not restrict
quartz may not be used to compute a correction factor. movement of the specimen in the dilatometer. Do not allow an
Accounting for the expansion of the dilatometer parts through exposed junction to contact any carbonaceous materials.
calculationsinplaceofacalibrationproceduredescribedabove 9.4 Make certain that the push-rod is in stable contact with
is not permitted. thespecimen.Apressuredistributionplatemadeoffusedsilica
8.6 Materials usable for calibration or verification of opera- may be used between the push-rod and the specimen.
tion fall into four categories. 9.5 Insert the loaded dilatometer into the furnace (at ambi-
8.6.1 Standard Reference Materials—These are actual ent temperature) and allow the temperature of the specimen to
specimens supplied with a certificate by NIST , or a similar come to equilibrium.
5,6
national standards organization of another country. 9.6 Record the initial readings of the temperature sensors,
T , and the transducer, X .
0 0
NOTE 4—These materials are very few and NIST supplies have been
9.7 Heat the furnace to 300 or 500 6 10°C at a rate not
exhausted in some cases. To determine the absolute accuracy of a device,
exceeding 10°C/min. Allow the furnace and specimen to
materials in this category are the most preferred.
stabilizeatthattemperaturefor60min.Recordreadingsofthe
8.6.2 Traceable Reference Materials—These are exten-
temperaturesensors, T ,andthetransducer, X .Ifdataobtained
1 1
sively investigated materials, substantially described in pub-
during ramping is to be used, heating rates above 1°C/min are
lished literature and considered stable. Often they are generi-
not permitted.
cally identical to Standard Reference Materials. Specific lots
9.8 Alternate to 9.7. Heat the furnace at rates up to 10°C/
when tested in a systematic fashion using a dilatometer
min. Hold the furnace and specimen at a single or series of
calibrated with a certified Standard Reference Material can be
constant temperatures until the transducer reading reaches a
referred to as Traceable Reference Materials. They may also
constant value (variation < 6 2.6 µm), At that point, the
serve well in comparing equipment or test procedures at
indicated temperature of the specimen shall not vary by more
different laboratories and to arbitrate disputes and differences.
than 6 2°C and the temperature gradient in the specimen shall
8.6.3 Reference Materials—These are widely investigated,
not exceed 2% of its actual temperature. The dwell time is a
well-characterized materials that were found to be stable with
function of the thermal mass of the dilatometer and the
time and temperature exposure and performance data are
specimen. It will vary with temperature and heating rate. The
readilyavailableintheliterature;forexample,platinum.These
length of the dwell shall be sufficient as to limit consecutive
materials are well suited for round-robin, day-to-day verifica-
length change readings taken in 5 min intervals to less than 4
tion (working reference) of equipment performance and peri-
µm.Readingsoftemperature, T,andspecimenlengthchanges,
i
odic verification programs. Purity and physical parameters
X,needtoberecordedateachconstanttemperature.Whenthis
i
(density, electrical resistivity, and so forth) must be reasonably
alternate procedure is used, it is important that the cal
...
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