fundamentals of thermal conductivity measurement via astm 5470

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Fundamentals of Thermal Conductivity Measurement via ASTM 5470

byDr. John W. Sofia

Analysis Tech Inc.2016


Definition of Apparent Thermal Conductivity

AL*

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.


Sample

Apparent versus Actual Thermal Conductivity

QT

sampR

ARRRL

*)(k

bsamptapp

sampactual *

kRAL

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


Best Method for Conductivity Measurement

ARRRL

RAL

RAL

*)(**k

bsamptsampsampactual

Sample Thickness

RA

slope1 kactual

Intercept = (Rt + Rb)*A

- best-fit

data lin

e -


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)


Test Error: Failure to Use Constant Pressure (Type 2 Material)


Contact Impedance Versus Pressure (Type 3 material, hard rubber, dry contact)


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


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


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


Best Method for Conductivity Measurement(Type 1 Materials)


Sample

Testing of Thin Type I Samples(“bond-line” testing)

)*ARRRRA bsampt (

ALR

*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


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



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



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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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 Thickness measurement accuracy


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


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 materialsKits 1 & 2: recommended for mostly type 1 & 2 materialsPressure accuracy is +/-2.7% of maximum pressureKits can be changed by end-user and are available in complete set of 3 kits.

fundamentals of thermal conductivity measurement via astm 5470

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