Testing And Study Of The Thermal Conductivity Of Thin-walled Materials Using The Transient Plane Source Technique

The miniaturization,thinning,and lightening of electronic devices have created an increasing need for cooling in confined spaces with high heat fluxes.According to a rough survey,thinwalled materials having a sub-millimeter thickness from 0.1 to 1 mm form the major components of modern electronic devices(e.g.,cellphones,laptops,and security cameras).These include a variety of materials such as metals/alloys,polymers,glasses,and composites,and are mainly present in the form of thermal interface materials,heat sink modules,and external casing.The thin-walled materials serve as the primary heat rejection paths to the ambient,and their thermal conductance has a direct impact on the cooling effectiveness,operation performance,and service life of electronic devices.Therefore,the thermal conductivity of thin-walled materials is an indispensable thermophysical parameter for thermal design of electronic devices,and the accuracy of its values has a critical influence on the reliability of the thermal design.Based on the heat conduction principle,the thermal conductivity of sub-millimeter thin-walled materials is still the bulk thermal conductivity,as the size effect down to the micro-/nanometer length scale has not yet played a role.However,due to the small thickness,great discrepancy of the thermal conductivity among various materials,as well as the inhomogeneity that is often present,it is challenging to accurately measure the thermal conductivity of thin-walled materials using commercialized instruments towards real-world applications.As of now,there is a lack of holistic solutions to such measurements.In this thesis,on the basis of a comprehensive screening and comparison among the available thermal conductivity measurement techniques in terms of their principles,applicable ranges,and measuring conditions,the Hot Disk 3500 S Thermal Constants Analyzer,which is based upon the transient plane source(TPS)technique,was chosen as the instrument.A variety of common thinwalled material samples,having different thermal conductivities and thicknesses,were measured and investigated using this instrument with the auxiliary plate module,thin-film module,and inhomogeous module.The TPS results were compared to those measured by another common commercial instrument – the laser flash apparatus – that is also based on a transient technique as well as a customized steady-state apparatus constructred by ourselves.Based on this comparative analysis,the applicability and accuracy of using the TPS technique to measure the thermal conductivity of thin-walled materials were explored for practical reference.In the first part of this paper,thermal conductivity of three thin plate materials such as 304 stainless steel,AlN ceramic,and 1060 aluminum with thickness of 0.1~1 mm were tested based on the TPS plate module.and test guidance is given.For thin plate specimens with a thermal conductivity of 10 to 100 W/m·K,a 5501 probe is recommended,with initial heating power and time of 0.4 W and 4~5 s,respectively.For thin plate specimens with a thermal conductivity of 100 to 200 W/m·K,it is recommended to use 8563 probe with initial heating power and time of 1.5~ 4.0 W and 1 s,respectively.For thin plate samples with thermal conductivity up to 200~ 300W/m·K,it is recommended to use 4922 probe with heating power and time of 2.0~4.0 W and 1 s,respectively.The better the thermal conductivity,the larger the probe size,the higher the heating power,and the shorter the heating time are recommended.Under above test conditions,the deviations of the thermal conductivity and thermal diffusion test results between samples of different thicknesses are basically within 2% and 5%.And the thermal conductivity and thermal diffusion deviations of the 304 stainless steel thin plate sample and the block standard sample are also within 2% and 5%,which indicates that the TPS plate module is reliable for testing thin plate samples with high thermal conductivity.In addition,after stacking multiple samples,the TPS anisotropic module was used to measure thinner plates,and the test results were found in good agreement with the test results of the flat plate module.And the relative deviation between the thermal diffusion measured by the laser flash method and the TPS method does not exceed 5%.In the second part of the work,the TPS thin film module was used to test the thermal conductivity of phase change material(PCM)film and polytetrafluoroethylene(PTFE)film.The effects of loading pressure and sample thickness changes on the test results were studied.It was found that the test result increased with the increase of the loading pressure and thickness,which is caused by the change of the thermal contact resistance(TCR)during the test.The nominal thermal resistance corresponding to the test value has a linear relationship with the thickness of the sample,where the slope of the curve is the inverse of the thermal conductivity of the sample,and the intercept is the value related to the contact thermal resistance during the test.Based on this principle,the TPS method test results were modified.After the correction,the maximum deviation between the corrected value and the test value of PCM35 in solid state was 80%,and the maximum deviation for PTFE was 37.5%.According to the correction curve,it was found TCR changes exponentially with the loading pressure.The test results of the PCM41 film were corrected according to the PCM film correction rule,and the deviation from the test value was 73.7%.The steady-state thermal conductivity tester designed and built by ourselves was also used to measure the thermal conductivity of thin film samples,and results were compared with the corrected values of the TPS method.The deviation between the two results of PCM35 is about 30%,which is caused by the thickness test error caused by the deformation of the PCM film under loading pressure;the PCM41 thermal conductivity value corrected according to the PCM film correction rule is only 6.6% deviated from the steady state test result,and the deviation for PTFE film is only 1.3%,which indicates the feasibility of the above-mentioned correction method based on the test value of the TPS thin film module.In sum,it was revealed that the TPS-based Hot Disk 3500 S instrument,with its auxiliary modules,can be employed as an efficient method to measure the thermal conductivity of engineering thin-walled materials,which is applicable to a wide range of materials(inherent thermal conductivities)and thicknesses.As a transient method,the TPS technique is rapid,making it rather suitable for massive tests in practical engineering applications.The rapid measurements also lead to a pronounced advantage for the application scenario when measurements at multiple temperature points are required to draw the temperature-dependent thermal conductivity variation curves.The recommended parameter setting ranges and the proposed correction for thermal contact resistance in this thesis are of great practical significance.