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Fundamentals Fundamentals of Thermal Conductivity Measurement via ASTM 5470 by Dr. John W. Sofia Analysis Tech Inc. 2010
Fundamentals Definition of Apparent Thermal Conductivity Conductivity defined only for heat flow between parallel, isothermal surfaces (ASTM 5470) ASTM 5470 is Ohms law applied to one-dimensional heat flow.
Fundamentals Sample Apparent versus Actual Thermal Conductivity Actual conductivity of sample equals apparent conductivity only if contact resistances are zero Hot Test Surface Cold Test Surface Contact Resistance = R t Contact Resistance = R b L k actual = k app If R t & R b = 0 Q Q Q
Fundamentals Best Method for Conductivity Measurement Sample Thickness RA Intercept = (R t + R b )*A - best-fit data line -
Fundamentals Best Method for Conductivity Measurement Requirements: 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)
Fundamentals Test Error: Failure to Use Constant Pressure (Type 2 Material)
Fundamentals Contact Impedance Versus Pressure (Type 3 material, hard rubber, dry contact)
Fundamentals 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
Fundamentals 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
Fundamentals 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
Fundamentals Best Method for Conductivity Measurement (Type 1 Materials)
Fundamentals Sample Testing of Thin Type I Samples (bond-line testing) Hot Test Surface Cold Test Surface Top-Contact R t BottomContact R b L Q Q Q a)For small L, uncertainty, ε, dominates R samp measurement. b)For small L and large k, R samp is small: yields low delta T and poor measurement accuracy c)Hidden information about the test surfaces dominates RA
Fundamentals 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
Fundamentals 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
Fundamentals Best Method for Conductivity Measurement (Type 2 Materials)
Fundamentals 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.
Fundamentals Best Method for Conductivity Measurement (Type 3 Materials)
Fundamentals 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
Fundamentals Accuracy of TIM Measurements Impedance variation with time Sample area measurement & mounting
Fundamentals 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
Fundamentals TIM Tester 1300
Fundamentals TIM Tester Pressure Range Options Kit 1: 5 to 95 psi (60 to 650 kPa) +/- 3 psi Kit 2: 10 to 170 psi ( kPa) +/- 5 psi Kit 3: 10 to 380 psi ( 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.