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原ASTM D5470-2006薄的热导性固体电绝缘材料传热性能的测试标准先更新为ASTM D5470-2012
发布日期:[2015/5/22 8:50:26] 共阅[4738]次
 Designation: D5470 – 12 An American National Standard
Standard Test Method for
Thermal Transmission Properties of Thermally Conductive
Electrical Insulation Materials1
This standard is issued under the fixed designation D5470; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the Department of Defense.
1. Scope*
1.1 This standard covers a test method for measurement of
thermal impedance and calculation of an apparent thermal
conductivity for thermally conductive electrical insulation
materials ranging from liquid compounds to hard solid materials.
1.2 The term “thermal conductivity” applies only to homogeneous
materials. Thermally conductive electrical insulating
materials are usually heterogeneous and to avoid confusion this
test method uses “apparent thermal conductivity” for determining
thermal transmission properties of both homogeneous and
heterogeneous materials.
1.3 The values stated in SI units are to be regarded as
standard.
1.4 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.
2. Referenced Documents
2.1 ASTM Standards:2
D374 Test Methods for Thickness of Solid Electrical Insulation
E691 Practice for Conducting an Interlaboratory Study to
Determine the Precision of a Test Method
E1225 Test Method for Thermal Conductivity of Solids by
Means of the Guarded-Comparative-Longitudinal Heat
Flow Technique
3. Terminology
3.1 Definitions of Terms Specific to This Standard:
3.1.1 apparent thermal conductivity (l), n—the time rate of
heat flow, under steady conditions, through unit area of a
heterogeneous material, per unit temperature gradient in the
direction perpendicular to the area.
3.1.2 average temperature (of a surface), n—the areaweighted
mean temperature.
3.1.3 composite, n—a material made up of distinct parts
which contribute, either proportionally or synergistically, to the
properties of the combination.
3.1.4 homogeneous material, n—a material in which relevant
properties are not a function of the position within the
material.
3.1.5 thermal impedance (u), n—the total opposition that an
assembly (material, material interfaces) presents to the flow of
heat.
3.1.6 thermal interfacial resistance (contact resistance),
n—the temperature difference required to produce a unit of
heat flux at the contact planes between the specimen surfaces
and the hot and cold surfaces in contact with the specimen
under test. The symbol for contact resistance is RI.
3.1.7 thermal resistivity, n—the reciprocal of thermal conductivity.
Under steady-state conditions, the temperature gradient,
in the direction perpendicular to the isothermal surface
per unit of heat flux.
3.2 Symbols Used in This Standard:
3.2.1 l = apparent thermal conductivity, W/m·K.
3.2.2 A = area of a specimen, m2.
3.2.3 d = thickness of specimen, m.
3.2.4 Q = time rate of heat flow, W or J/s.
3.2.5 q = heat flux, or time rate of heat flow per unit area,
W/m2.
3.2.6 u = thermal impedance, temperature difference per
unit of heat flux, (K·m2)/W.
4. Summary of Test Method
4.1 This standard is based on idealized heat conduction
between two parallel, isothermal surfaces separated by a test
specimen of uniform thickness. The thermal gradient imposed
on the specimen by the temperature difference between the two
contacting surfaces causes the heat flow through the specimen.
This heat flow is perpendicular to the test surfaces and is
uniform across the surfaces with no lateral heat spreading.
1 This test method is under the jurisdiction of ASTM Committee D09 on
Electrical and Electronic Insulating Materials and is the direct responsibility of
Subcommittee D09.19 on Dielectric Sheet and Roll Products.
Current edition approved Jan. 1, 2012. Published February 2012. Originally
approved in 1993. Last previous edition approved in 2011 as D5470 – 11. DOI:
10.1520/D5470-12.
2 For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website.
1
*A Summary of Changes section appears at the end of this standard.
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4.2 The measurements required by this standard when using
two meter bars are:
T1 = hotter temperature of the hot meter bar, K,
T2 = colder temperature of the hot meter bar, K,
T3 = hotter temperature of the cold meter bar, K,
T4 = colder temperature of the cold meter bar, K,
A = area of the test surfaces, m2, and
d = specimen thickness, m.
4.3 Based on the idealized test configuration, measurements
are taken to compute the following parameters:
TH = the temperature of the hotter isothermal surface, K,
TC = the temperature of the colder isothermal surface, K,
Q = the heat flow rate between the two isothermal surfaces,
W,
thermal impedance = the temperature difference between the
two isothermal surfaces divided by the heat flux through them,
K·m2/W, and
apparent thermal conductivity = calculated from a plot of
specimen thermal impedance versus thickness, W/m·K.
4.4 Interfacial thermal resistance exists between the specimen
and the test surfaces. These contact resistances are
included in the specimen thermal impedance computation.
Contact resistance varies widely depending on the nature of the
specimen surface and the mechanical pressure applied to the
specimen by the test surfaces. The clamping pressure applied to
the specimen should therefore be measured and recorded as a
secondary measurement required for the method except in the
case of fluidic samples (Type I, see section 5.3.1) where the
applied pressure is insignificant. The computation for thermal
impedance is comprised of the sum of the specimen thermal
resistance plus the interfacial thermal resistance.
4.5 Calculation of apparent thermal conductivity requires an
accurate determination of the specimen thickness under test.
Different means can be used to control, monitor, and measure
the test specimen thickness depending on the material type.
4.5.1 The test specimen thickness under test can be controlled
with shims or mechanical stops if the dimension of the
specimen can change during the test.
4.5.2 The test specimen thickness can be monitored under
test with an in situ thickness measurement if the dimension of
the specimen can change during the test.
4.5.3 The test specimen thickness can be measured as
manufactured at room temperature in accordance with Test
Methods D374 Test Method C if it exhibits negligible compression
deflection.
5. Significance and Use
5.1 This standard measures the steady state thermal impedance
of electrical insulating materials used to enhance heat
transfer in electrical and electronic applications. This standard
is especially useful for measuring thermal transmission properties
of specimens that are either too thin or have insufficient
mechanical stability to allow placement of temperature sensors
in the specimen as in Test Method E1225.
5.2 This standard imposes an idealized heat flow pattern and
specifies an average specimen test temperature. The thermal
impedances thus measured cannot be directly applied to most
practical applications where these required uniform, parallel
heat conduction conditions do not exist.
5.3 This standard is useful for measuring the thermal
impedance of the following material types.
5.3.1 Type I—Viscous liquids that exhibit unlimited deformation
when a stress is applied. These include liquid compounds
such as greases, pastes, and phase change materials.
These materials exhibit no evidence of elastic behavior or the
tendency to return to initial shape after deflection stresses are
removed.
5.3.2 Type II—Viscoelastic solids where stresses of deformation
are ultimately balanced by internal material stresses
thus limiting further deformation. Examples include gels, soft,
and hard rubbers. These materials exhibit linear elastic properties
with significant deflection relative to material thickness.
5.3.3 Type III—Elastic solids which exhibit negligible deflection.
Examples include ceramics, metals, and some types of
plastics.
5.4 The apparent thermal conductivity of a specimen can be
calculated from the measured thermal impedance and measured
specimen thickness if the interfacial thermal resistance is
insignificantly small (nominally less than 1 %) compared to the
thermal resistance of the specimen.
5.4.1 The apparent thermal conductivity of a sample material
can be accurately determined by excluding the interfacial
thermal resistance. This is accomplished by measuring the
thermal impedance of different thicknesses of the material
under test and plotting thermal impedance versus thickness.
The inverse of the slope of the resulting straight line is the
apparent thermal conductivity. The intercept at zero thickness
is the sum of the contact resistances at the two surfaces.
5.4.2 The contact resistance can be reduced by applying
thermal grease or oil to the test surfaces of rigid test specimens
(Type III).
TEST METHOD
6. Apparatus
6.1 The general features of an apparatus that meets the
requirements of this method are shown in Figs. 1 and 2. This
apparatus imposes the required test conditions and accomplishes
the required measurements. It should be considered to
be one possible engineering solution, not a uniquely exclusive
implementation.
6.2 The test surfaces are to be smooth within 0.4 microns
and parallel to within 5 microns.
6.3 The heat sources are either electrical heaters or temperature
controlled fluid circulators. Typical electrical heaters are
made by embedding wire wound cartridge heaters in a highly
conductive metal block. Circulated fluid heaters consist of a
metal block heat exchanger through which a controlled temperature
fluid is circulated to provide the required heat flow as
well as temperature control.
6.4 Heat flow through the specimen can be measured with
meter bars regardless of the type of heater used.
6.4.1 Electrical heaters offer convenient measurement of the
heating power generated but must be combined with a guard
heater and high quality insulation to limit heat leakage away
from the primary flow through the specimen.
D5470 – 12
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6.4.2 Heat flow meter bars can be constructed from high
conductivity materials with well documented thermal conductivity
within the temperature range of interest. The temperature
sensitivity of thermal conductivity must be considered for
accurate heat flow measurement. The thermal conductivity of
the bar material is recommended to be greater than 50 W/m·K.
6.4.3 Guard heaters are comprised of heated shields around
the primary heat source to eliminate heat leakage to the
environment. Guard heaters are insulated from the heat source
and maintained at a temperature within 60.2 K of the heater.
This effectively reduces the heat leakage from the primary
heater by nullifying the temperature difference across the
insulation. Insulation between the guard heater and the heat
source should be at least the equivalent of one 5 mm layer of
FR-4 epoxy material.
6.4.4 If the heat flow meter bars are used on both the hot and
cold surfaces, guard heaters and thermal insulation is not
required and the heat flow through the test specimen is
computed as the average heat flow through both meter bars.
6.5 Meter bars can also be used to determine the temperature
of the test surfaces by extrapolating the linear array of
meter bar temperatures to the test surfaces. This can be done
for both the hot side and cold side meter bars. Surface
temperatures can also be measured with thermocouples that are
located in extreme proximity to the surfaces although this can
be mechanically difficult to achieve. Meter bars can be used for
both heat flow and surface temperature measurement or for
exclusively one of these functions.
6.6 The cooling unit is commonly implemented with a metal
block cooled by temperature controlled circulating fluid with a
temperature stability of 60.2 K.
6.7 The contact pressure on the specimen can be controlled
and maintained in a variety of ways, including linear actuators,
lead screws, pneumatics, and hydraulics. The desired range of
forces must be applied to the test fixture in a direction that is
perpendicular to the test surfaces and maintains the parallelism
and alignment of the surfaces.
7. Preparation of Test Specimens
7.1 The material type will dictate the method for controlling
specimen thickness. In all cases, prepare specimens of the same
area as the contacting test surfaces. If the test surfaces are not
of equal size, prepare the specimen equal to the dimension of
the smaller test surface.
FIG. 1 Test Stack Using the Meter Bars as Calorimeters
FIG. 2 Guarded Heater Test Stack
D5470 – 12
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7.1.1 Type I—Use shims or mechanical stops to control the
thickness of the specimen between the test surfaces. Spacer
beads of the desired diameter can also be used in approximately
2 % volumetric ratio and thoroughly mixed into the
sample prior to being applied to the test surfaces.
7.1.2 Type II—Use an adjustable clamping pressure to
deflect the test specimen by 5 % of its uncompressed thickness.
This represents a trade-off between lower surface contact
resistance and excessive sample deflection.
7.1.3 Type III—Measure the sample thickness in accordance
with Test Method C of Test Methods D374.
7.2 Prepare specimens from material that is in original,
as-manufactured condition or as noted otherwise. Remove any
contamination and dirt particles. Do not use solvent that will
react with or contaminate the specimens.
8. Procedure
8.1 Determination of test specimen thickness.
8.1.1 Machines with in situ thickness measurement apparatus.
8.1.1.1 Close the test stack and apply the clamping pressure
required for the specimen to be tested.
8.1.1.2 Turn on the heating and cooling units and let
stabilize at the specified set points to give an average sample
temperature of 50°C (average of T2 and T3), unless otherwise
specified.
8.1.1.3 Zero the thickness measuring device (micrometer,
LVDT, laser detector, encoder, etc.).
8.1.2 Machines without an in situ thickness measuring
apparatus.
8.1.2.1 At room temperature, measure the specimen thickness
in accordance with Test Method C of Test Methods D374.
8.2 Load the specimen on the lower test stack.
8.2.1 Dispense Type I grease and paste materials onto the
lower test stack surface. Melt phase change compounds to
dispense onto the stack.
8.2.2 Place Type II and III specimens onto the lower test
stack.
8.3 Close the test stack and apply clamping pressure.
8.3.1 Type I materials being tested with shims to control the
test thickness require only enough pressure to squeeze out
excess material and contact the shim but not too much pressure
that will result in the shim damaging the surfaces of the test
stacks.
8.3.1.1 For machines with screw stops, electromechanical,
or hydraulic actuators controlling the position of the upper test
stack, the magnitude of the clamping pressure is not critical.
8.3.1.2 Raise the temperature of the test stack above the
specimen melting point to enable phase change materials to
flow and permit closing of the test stacks. After the material has
flowed, return the heating and cooling units to the required set
points to maintain an average specimen temperature of 50°C
before beginning the test, unless otherwise specified.
8.3.2 Type II materials require enough pressure to coalesce
stacked specimens together and minimize interfacial thermal
resistances. Too much pressure can damage the specimens.
This can be as low as 0.069 MPa (10 psi) for softer specimens
or as high as 3.4 MPa (500 psi) for harder specimens.
Alternatively, screws or linear actuators can be used to control
the specimen thickness under test for easily deformable Type II
materials.
8.3.3 Type III materials require enough pressure to exclude
excess thermal grease from the interface and to flatten specimens
that are not flat. This can be as low as 0.69 MPa (100 psi)
for flat specimens with low viscosity thermal grease or as high
as 3.4 MPa (500 psi) for non-flat specimens or when using high
viscosity thermal grease.
8.4 Record the temperatures of the meter bars and the
voltage and current applied to electrical heaters at equilibrium.
Equilibrium is attained when, at constant power, 2 sets of
temperature readings taken at 5 minute intervals differ by less
than 60.1°C, or if the thermal impedance has changed by less
than 1 % of the current thermal impedance over a 5 minute
time span.
8.5 Calculate the mean specimen temperature and the thermal
impedance. Label the calculated thermal impedance for the
single-layer specimen as the “thermal impedance” of the
sample.
8.6 Determine the thermal impedance of at least 3 specimen
thicknesses. Maintain the mean temperature of the specimens
at 50 6 2°C (unless otherwise specified) by reducing the heat
flux as the specimen thickness is increased.
8.6.1 For specimens that need to be stacked to get different
thicknesses, first measure the thermal impedance of one layer
alone, then measure the thermal impedance of 2 layers stacked
together, and then measure the thermal impedance of 3 layers
stacked together.
8.6.2 For specimens of 3 different thicknesses A, B, and C,
first measure the thermal impedance of specimen A alone, then
measure the thermal impedance of specimen B alone, then
measure the thermal impedance of specimen C alone.
9. Calculation
9.1 Heat Flow:
9.1.1 Heat Flow When Using the Meter Bars For
Calorimeters—Calculate the heat flow from the meter bar
readings as follows:
Q12 5
l12 3 A
d 3 @T1 – T2# (1)
Q34 5
l34 3 A
d 3 @T3 – T4# (2)
Q 5
Q12 1 Q34
2 (3)
where:
Q12 = heat flow in hot meter bar, W,
Q34 = heat flow in cold meter bar, W,
Q = average heat flow through specimen, W,
l12 = thermal conductivity of the hot meter bar material,
W/(m·K),
l34 = thermal conductivity of the cold meter bar material,
W/(m·K),
A = area of the reference calorimeter, m2,
T1 – T2 = temperature difference between temperature sensors
of the hot meter bar, K,
T3 – T4 = temperature difference between temperature sensors
of the cold meter bar, K, and
D5470 – 12
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d = distance between temperature sensors in the
meter bars, m.
9.1.2 Heat Flow When Not Using the Meter Bars for
Calorimeters—Calculate the heat flow from the applied electrical
power as follows:
Q 5 V 3 I (4)
where:
Q = heat flow, W,
V = electrical potential applied to the heater, V, and
I = electrical current flow in the heater, A.
9.2 Derive the temperature of the hot meter bar surface in
contact with the specimen from the following:
TH 5 T2 –
dB
dA
3 @T1 – T2# (5)
where:
TH = temperature of the hot meter bar surface in contact
with the specimen, K,
T1 = warmer temperature of the hot meter bar, K,
T2 = cooler temperature of the hot meter bar, K,
dA = distance between T1 and T2, m, and
dB = distance from T2 to the surface of the hot meter bar in
contact with the specimen, m.
9.3 Derive the temperature of the cold meter bar surface in
contact with the specimen from the following:
TC 5 T3 1
dD
dC
3 @T3 – T4# (6)
where:
TC = temperature of the cold meter bar surface in contact
with the specimen, K,
T3 = warmer temperature of the cold meter bar, K,
T4 = cooler temperature of the cold meter bar, K,
dC = distance between T3 and T4, m, and
dD = distance from T3 to the surface of the cold meter bar
in contact with the specimen, m.
9.4 Calculate the thermal impedance from Eq 7 and express
it in units of (K·m2)/W:
u 5
A
Q 3 @TH – TC# (7)
9.5 Obtain apparent thermal conductivity from a plot of
thermal impedance for single and multiple layered specimens
against the respective specimen thickness. Plot values of the
specimen thickness on the x axis and specimen thermal
impedance on the y axis.
9.5.1 The curve is a straight line whose slope is the
reciprocal of the apparent thermal conductivity. The intercept
at zero thickness is the thermal interfacial resistance, RI,
specific to the sample, clamping force used, and the clamping
surfaces.
9.5.2 As a preferred alternative, compute the slope and the
intercept using least mean squares or linear regression analysis.
10. Report
10.1 Report the following information:
10.1.1 Specimen identification:
10.1.1.1 Name of the manufacturer,
10.1.1.2 Batch or lot number,
10.1.1.3 Grade designation,
10.1.1.4 Nominal thickness, and
10.1.1.5 Any other information pertinent to the identification
of the material.
10.1.2 Number of layers used in the test.
10.1.3 Average temperature of the specimen, if other than
323 K.
10.1.4 Pressure used during testing,
10.1.5 Thermal transmission properties:
10.1.5.1 Apparent thermal conductivity from 9.5, and
10.1.5.2 Thermal impedance from 9.4 (normalized to nominal
thickness for Type II materials).
11. Precision and Bias
11.1 A round robin was conducted on five Type II materials
having different constructions and thicknesses. Six laboratories
tested specimens from all of the materials using either the
specified test method or additional Test Method B of this
standard, which is now deleted. Table 1, prepared in accordance
with Practice E691, summarizes the results of the round
robin. Data obtained during the round-robin testing are being
made available in a research report.
11.2 From the data used to generate Table 1 the following
conclusion is made:
11.2.1 Thermal conductivity values for the same material
measured in different laboratories are expected to be within
18 % of the mean of the values from all of the laboratories.
11.3 Bias for this test method is currently under investigation
subject to the availability of a suitable reference material.
12. Keywords
12.1 apparent thermal conductivity; guarded heater method;
thermal conductivity; thermal impedance; thermally conductive
electrical insulation
TABLE 1 Precision for Conductivity Measurement
NOTE 1—Values are in units of watt per meter Kelvin.
Material Identity Average Sr
A SR
B rC RD
Material B 0.923 0.0383 0.163 0.107 0.456
Material E 1.245 0.0834 0.175 0.234 0.491
Material C 1.311 0.0423 0.192 0.119 0.536
Material A 2.732 0.2010 0.311 0.563 0.872
Material D 5.445 0.5691 0.711 1.594 1.991
A Sr = within-laboratory standard deviation of the average.
B SR = between-laboratories standard deviation of the average.
C r = within-laboratory repeatability limit = 2.8 3 Sr.
D R = between-laboratories reproducibility limit = 2.8 3 SR.
D5470 – 12
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SUMMARY OF CHANGES
Committee D09 has identified the location of selected changes to this test method since the last issue,
D5470 – 01, that may impact the use of this test method. (Approved April 1, 2006)
(1) The test method was heavily revised throughout to remove
non-mandatory language and to clarify mandatory aspects in
the method, apparatus, specimens, and procedures.
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