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Fundamentals of Thermal Conductivity Measurement via ASTM 5470
by
Dr. John W. Sofia
Analysis Tech Inc.
2010
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Definition of Apparent Thermal Conductivity
A
L*
TTk
21
app
Q
Conductivity defined only for heat flow between parallel, isothermal surfaces (ASTM 5470)
ASTM 5470 is Ohms law applied to one-dimensional
heat flow.
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Sample
Apparent versus Actual Thermal Conductivity
Q
TsampR
ARRR
L
*)(k
bsamptapp
sampactual *
kRA
L
Actual conductivity of sample equals apparent conductivity only if contact resistances are zero
Hot Test Surface
Cold Test Surface
Contact Resistance = Rt
Contact Resistance = Rb
L
kactual = kapp If Rt & Rb = 0
Q
Q
Q
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Best Method for Conductivity Measurement
ARRR
L
RA
L
RA
L
*)(**k
bsamptsampsampactual
Sample Thickness
RA
slope
1 kactual
Intercept = (Rt + Rb)*A
- best
-fit data
line -
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Best Method for Conductivity MeasurementRequirements:
Sum of Contact resistances must be the same for all samples
(stacked samples have higher total contact resistance)
Fractional composition must be the same all samples
(supporting matrices or meshes cause problems here)
Variable thicknesses must have uniform bulk properties
(fabrication can yield thickness-dependent properties)
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Test Error: Failure to Use Constant Pressure (Type 2 Material)
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Contact Impedance Versus Pressure (Type 3 material, hard rubber, dry contact)
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Characteristics of Surface Contact Resistances
Behavior
Inversely related to contact pressure
Time and pressure dependent
Unpredictable if untreated
Causes
Foreign substances trapped on surfaces
Surface roughness details
Surface flatness details
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Treating Surface Contact Resistances
Treatments
Liquid to eliminate air on surfaces
Thermal grease to lower surface resistance
Control of contact pressure & time
Goals
Minimize contact resistances
Make them consistent & repeatable
Determine time / pressure dependency
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Type 1 Samples (ASTM 5470)
Test Method
Controlled thickness test mode
Care for elimination of air bubbles
Avoid very thin samples
Remove excess material
Character
Fluidic: greases, gels, liquids
No deformation limits; viscosity
Near zero contact resistances
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Best Method for Conductivity Measurement(Type 1 Materials)
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Sample
Testing of Thin Type I Samples(“bond-line” testing)
)*ARRRRA bsampt (
A
LR
*ksamp
Hot Test Surface
Cold Test Surface
Top-Contact Rt
BottomContact Rb
L
Q
Q
Q
a) For small L, uncertainty, ε, dominates Rsamp measurement.
b) For small L and large k, Rsamp is small: yields low delta T and poor measurement accuracy
c) Hidden information about the test surfaces dominates RA
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Difficulties with Thin Type I Sample Testing
Accuracies
Sample thickness rivals measurement error
Higher power to maintain ΔT accuracy
Hidden Information
Test surface flatness & coplanarity
Filtration of grease: k change
Application details in production
Surface micro-details dominate results
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Type 2 Samples (ASTM 5470)
Character
Elastic & plastic deformations combined
Elasticity increasing with deformation
Low contact resistances
Test Method
Use controlled contact pressure
Check pressure and time effects
Fixed-thickness for very soft samples
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Best Method for Conductivity Measurement(Type 2 Materials)
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Type 3 Samples (ASTM 5470)
Character
Very Stiff / hard; near-zero deformation
Surface preparation is critical
High contact resistance without treatment
Test Method
Use high contact pressures
Use oil or grease surface treatment
Smooth, flat, and parallel surface prep.
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Best Method for Conductivity Measurement(Type 3 Materials)
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Repeatability of TIM Measurements
Contact Resistances
Variations due to sample mounting
Variation with time and pressure
Variation in surfaces of samples
Others Sources
Convergence of final result
Inherent measurement variation
Bulk material variations
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Accuracy of TIM Measurements
Dependencies
Δ-temperature across sample (low RA)
Heat flow measurement (high RA)
Environment temperature stability
Impedance variation with time
Sample area measurement & mounting
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TIM Tester 1300, 1400 Features
Automatic operation under WinTIM Software
Electronic sample thickness measurement - "as-tested"
Sample-temperature control & batch testing
Automatic pressure control & batch testing
Controlled-thickness & controlled-pressure test modes
Automatic estimation of measurement accuracy
In-house calibration procedures
English or metric unit selection
Requires chiller, compressed air, & PC computer
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TIM Tester 1300
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TIM Tester Pressure Range Options
Kit 1: 5 to 95 psi (60 to 650 kPa) +/- 3 psiKit 2: 10 to 170 psi (100-1100 kPa) +/- 5 psi Kit 3: 10 to 380 psi (100-2600 kPa) +/- 10 psi
Kit 3: recommended for mostly type 3 materials
Kits 1 & 2: recommended for mostly type 1 & 2 materials
Pressure accuracy is +/-2.7% of maximum pressure
Kits can be changed by end-user and are available in complete set of 3 kits.
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