Changes In Thermal Conductivity Research Articles

Radiative convective heat transfer and magneto-hydrodynamic flow in a viscous fluid-saturated porous channel with variable thermal conductivity

The proposed model offers a wide range of practical use, including nuclear energy plants, oil recovery, geothermal extraction, solar systems, dispersing fog, electrical power generation, hydro-metallurgical industry and advancements in medical sciences. The article explores the impact of radiative-convective heat transfer(HT) and magneto-hydrodynamic flow of a viscous fluid in a vertical porous channel with varying thermal conductivity and viscosity. The developing mathematical model analyses fluid flow and temperature distribution profiles. The nonlinear coupled differential equations (DEs) have been solved analytically and numerically. The numerical solution has been obtained from the Chebyshev spectral collocation method and a significant agreement is found with the analytical solution in the special case. The obtained solution offers a graphical representation of velocity, and temperature profiles have been found with respect to involving various parameters. The authors also study the behaviour of fluid flow near the surface and heat transfer from high to low temperatures at the walls. Correlations have been established for skin friction and Nusselt number in relation to involving parameters. The study reveals that the velocity profile is more pronounced when thermal conductivity changes compared to variable viscosity. The purpose of the study is to enhance the efficiency of oil recovery, geothermal extraction, etc., models.

Thermal and Mechanical Performances Optimization of Plaster–Polystyrene Bio-Composites for Building Applications

Polystyrene is renowned for its excellent thermal insulation due to its closed-cell structure that traps air and reduces heat conduction. This study aims to develop sustainable, energy-efficient building materials by enhancing the thermal and mechanical properties of plaster–polystyrene bio-composites. By incorporating varying amounts of polystyrene (5% to 25%) into plaster, our research investigates changes in thermal conductivity, thermal resistance, and mechanical properties such as Young’s modulus and maximum stress. Meticulous preparation of composite samples ensures consistency, with thermal and mechanical properties assessed using a thermal chamber and four-point bending and tensile tests. The results show that increasing the polystyrene content significantly improved thermal insulation and stiffness, though maximum stress decreased, indicating a trade-off between insulation and mechanical strength.

Comparison of Thermal Conductivity and Long-Term Change of Building Insulation Materials According to Accelerated Laboratory Test Methods of ISO 11561 and EN 13166 Standard

The insulation performance of a building is one of the most fundamental factors in reducing its energy use. The insulation of the building envelope is the main factor that directly affects the cooling and heating loads, which is responsible for the largest portion of the building’s energy consumption. Recently, in South Korea, building insulation materials such as PIR and PF, which have not only fire resistance but also high insulation performance, are becoming mainstream in the industry as fire safety standards have been strengthened. However, there are concerns about the long-term change of high-performance insulation materials. In particular, in contrast with existing plastic insulation materials such as EPS and XPS, the evaluation of the reliability of the long-term change of insulation materials with a high closed-cell rate is becoming an issue. This is because, unlike the existing EPS and XPS, there are insufficient international standard test methods for evaluating the long-term thermal performance of composite insulation materials such as polyurethane and phenol. They are combined with layers of other types of thermal insulation and/or one or both of the surfaces are covered with coatings, facings, membranes or other surface skins, all serving some functional purpose depending on the product application. In this study, the long-term change of building insulation materials was tested and analyzed based on the standard of the accelerated aging test. Four types of building insulation materials were used in the experiment: EPS, XPS, PF and PIR. The accelerated aging tests were conducted based on ISO 11561 and BS EN 13166 standards (accelerated laboratory test methods). The results of the accelerated aging tests for the materials were analyzed and the thermal resistance of the insulation materials was also compared. From the test results according to the slicing test method specified in the ISO 11561, the thermal resistance of EPS, XPS, PF, and PIR with long-term changes decreased by 2.4%, 8.5%, 16.4%, and 18.0%, respectively, compared to the initial thermal resistance. From the test results according to the 70 °C heat acceleration test method specified in the EN 13166, the thermal resistance decreased by 2.0% for EPS, 6.7% for XPS, 8.7% for PF and 13.6% for PIR, compared to their respective initial thermal resistance values. The EPS showed highly similar thermal resistance changes for the two different test methods. It was found that there was no effect from the slicing of the insulation material, and the accelerated aging test showed minimal changes in thermal resistance. In contrast, the other insulation materials exhibited different thermal resistance changes depending on the test method, with each material showing a thermal resistance change of 8.8% to 15.9% when subjected to the accelerated aging test.

Effect of Phenolic Resin Pyrolysis on Thermal Properties of SiFRP Composites under High Heating Rates

Abstract During the ablation process of resin-based composite materials, residual carbon generated from pyrolysis of phenolic resin change the thermal performance of the material. Phenolic resin pyrolysis process depends on temperature and heating rate. Based on the mass loss curves of phenolic resin under high heating rates (5K/s, 50K/s, 100K/s), the study analyzes the variations in thermal performance parameters of SiFRP composite materials with temperature assuming phenolic resin and silica fibres are in parallel in heat transferring. As temperature rising, the pyrolysis conversation ratio of phenolic resin increases, thermal conductivity and specific heat capacity of the composites increase accordingly. The greater the heating rates, the smaller the change of thermal conductivity and specific heat capacity of composites at the same temperature. The complete pyrolysis temperature of phenolic resin is positively correlated with the temperature rise rate. When the temperature rise rate is very large (such as 100K/s), the influence of pyrolysis on the thermal properties of composites can be ignored at medium and low temperature.

Experimental Investigation of the Influence of Different Thermal Aging Conditions on the Thermal Conductivity of Battery Electrodes

A thermal management system is required to ensure the safe and optimized operation of a lithium-ion battery systems in electric vehicles. The design of the thermal management systems is thereby commonly based on the respective cell behavior only at the Begin of Life (BoL). During operation, various aging mechanisms occur on both the anode and cathode side, depending on the cell chemistry. These mechanisms change the properties of the battery cells. For a more profound understanding and design of thermal management systems the dependency of the thermal cell behavior on the State of Health (SoH) until the End of Life (EoL) is crucial.The aging mechanisms themselves depend on the temperature during aging. The changes in the microstructure at the electrode level lead to changes in the effective thermal material properties, resulting in reduced heat transfer among other changes. Alterations of the thermal cell behavior will influence the electrical and electrochemical behavior. In literature, few studies have been carried out investigating the changes in the integral thermal conductivity of pouch cells on cell level, which have shown an influence of the SoH on the thermal conductivity (for example Tendera et al. 2023). However, from these studies it is not directly possible to assign the individual aging effects and the observed changes in thermal conductivity to the individual processes at the anode and cathode.Therefore, in this work, the influence of different thermal aging conditions on the thermal conductivity of cathode and anode stacks is investigated on electrode level to gain more insight on the individual underlying mechanisms. An experimental method for the thermal characterization at electrode level (electrode stack, current collector and coating) was developed. In combination with measurements of the specific heat capacity with differential scanning calorimetry, the density with gas pycnometer and the temperature diffusivity with laser flash analysis, regarding the microstructure, the effective thermal conductivity can be determined.The thermal aging conditions for the investigated cell were chosen over a wide temperature range (from 0 °C to 50 °C) to trigger different aging mechanisms within the cells. The investigated cell consists of a graphite anode and a NCA/LCO blend cathode. For lower temperatures, for example, lithium plating occurs increasingly on the anode side. At higher temperatures, particle cracks can occur in the cathode particles and side reactions resulting in cover layers are enhanced. In addition, various inhomogeneous temperature distributions were considered, such as the change in temperature during charging / discharging and inhomogeneous temperature fields over the geometric dimensions of the cell.The main focus of this contribution is the influence of the inhomogeneous temperature distribution during aging and the comparison to homogenous aging conditions. The aging mechanisms are investigated through various post-mortem analysis methods and compared to the effective thermal conductivity for the different samples. Several samples are taken from aged cells of each aging condition and the corresponding effective thermal material properties are determined using the described methodology. Through comparison with the results of the post-mortem analysis and the homogenously aged samples the aging effects responsible for the changes of the thermal properties are investigated.

A Thermal Impedance Model for IGBT Modules Considering the Nonlinear Thermal Characteristics of Chips and Ceramic Materials

The traditional method of calculating junction temperature does not consider the dependence of a material’s thermal conductivity on temperature, in which the thermal conductivity changes with temperature. However, with an increase in junction temperature, the temperature sensitivity (TS) will have a more significant impact on the actual temperature of chips. This study established an improved IGBT equivalent thermal impedance model that considers the nonlinear characteristics of the TS of chips and ceramic materials. The Fourier series analysis method was used to obtain the heat flux density curve, and then the heat diffusion angles of each layer were solved. Moreover, iterations were performed until the thermal conductivity and temperature of the chip and ceramic layers matched the nonlinear characteristics of the TS. When the power loss was less than 200 W, the maximum error of the junction temperature calculated by the proposed method considering TS was 3%, while the maximum error of the method without considering TS was 9.5%. Compared with the finite element simulation, the proposed method has a faster solving speed and high accuracy. The proposed method only requires the input material parameters, size parameters, and boundary conditions to solve the junction temperature, which has strong practicality and high accuracy.

Corrugated veneer panel thermophysical properties

The article substantiates the need to develop new mechanisms for the hardwood use in modern conditions of Republic of Karelia timber industry. One of the potential uses of birch wood in wooden house construction is building materials production from veneer and slab materials based on it. A large amount of associated waste from processing birch wood into veneer stands out as one of the key problems. A new slab joinery and construction material made of corrugated birch veneer is considered. The purpose of this study is to evaluate the thermophysical properties of a corrugated board made of birch wood. To achieve this goal, the tasks and methods of research are defined. An experimental device has been developed to conduct an experiment to determine the values of thermophysical characteristics. DS18B20 temperature sensors were used to measure the surface temperature, as well as to monitor device operation and the room air temperature. The sensors are connected to the Arduino microcontroller platform, which was used to record and transmit sensor readings. Additionally, the course of the experiment was monitored using a thermal imager Testo 875-1i. During the experiment, more than 1000 measurements were carried out. As a result of data processing, a diagram of the dependence of the density of the heat flux passing through the sample on time, as well as diagrams of the dependence of thermal conductivity and thermal resistance on the temperature difference on the sample surfaces, was obtained. The diagrams show the regression dependences of changes in heat flux density, thermal conductivity and thermal resistance during measurements. The values of the heat flux density, thermal conductivity coefficient and thermal resistance calculated on the basis of regression equations and the values obtained experimentally are determined. The directions of further research of the material under consideration are determined.

Competitive influence of interface effect, scale effect and mixed salt ratio on thermal conductivity of mesoporous complex nitrate

The rising prominence of energy security and climate change issues necessitates the advancement of sustainable energy development and energy storage technologies. Enhancing the capabilities of mesoporous composite phase change materials (CPCMs) has gained considerable research interest. CPCM’s heat storage and release performance is largely determined by its thermal conductivity. While the existing literature has documented the individual effects of the composition ratio of mixed molten salts, scale effects, and interfacial effects on thermal conductivity, no studies have been found that investigate the competitive interplay among these three factors with respect to their impact on thermal conductivity. This study aims to gain a deeper insight into the thermal conductivity changes of CPCM consisting of mixed molten salts as the phase change material (PCM) and mesoporous skeleton. A combination of molecular dynamics simulations and experimental investigations was employed to explore how interface effects, scale effects, and the proportion of mixed salts contribute to the thermal conductivity of CPCM. The findings indicate that augmented thermal conductivity, resulting from interface effects, supersedes the diminishing effects of interionic interactions. Compared to mixed salt ratios, interface effects primarily result in variations in the thermal conductivity of CPCM. The thermal conductivity of mixed nitrates escalates alongside scale increases. In the 3–4 nm range, the scale effect and mixed nitrate proportions do not notably compete in terms of influence on thermal conductivity, interface effects are more profound than scale effects. In the 4–9.5 nm range, the scale effect is more profound than mixed nitrate ratios. If the skeleton transitions from SiO2 to Al2O3, the impact of the interface effect on thermal conductivity is greater influential than the scale effect. While transitioning the interface from Al2O3 to ceramic, the effect of the interface is less than or equal to that of the scale effect on the thermal conductivity.

Both edge substitution effects on thermal conductivity of armchair graphene nanoribbons under tensile strain: From equilibrium molecular dynamics simulations

Thermal conductivity and phonon spectra change of graphene nanoribbon as function of both side edges substitution and strain is investigated using equilibrium molecular dynamics. Under small strain, the edge’s partial substitution by graphane plays effect role to change the phonon mode of GNR. Under large tensile strain, reduction ratio of thermal conductivity is the largest in case of fluorographane hybrid and the smallest in case of graphane hybrid. Especially, under 16 % strain, the thermal conductivity of GNR is reduced until 13 % by edges substitution of graphane. The phonon spectra shift peaks towards low frequency at high frequency range (>50 THz).

Molecular insights of DGEBA/MeTHPA three-dimensional resin network from design to heat transfer mechanism: Dynamic simulation and experimental research

The epoxy resin composed of bisphenol A diglycidyl ether (DGEBA) and methyl tetrahydrophthalic anhydride (MeTHPA) has been found to enhance its thermal conductivity by increasing its molecular weight and crosslinking density. This was achieved through experiments and all-atom molecular dynamics simulations. Molecular dynamics was employed to calculate the root-mean-square displacement, free volume fraction, potential energy, density, and quantity of hydrogen bonds. The results showed the influence of molecular weight and crosslinking density on the system's thermal conductivity. The results showed that the high molecular weight epoxy resin system exhibited high thermal conductivity at elevated crosslinking densities. During the crosslinking process, the epoxy resin's thermal conductivity increased, starting with the center portion weakening and then strengthening. In addition, a simulation-verified free volume was found to have an inversely proportional connection with thermal conductivity. This study explored the thermal conductivity mechanism of epoxy resin by analyzing the structural changes at the micro level and correlating them with the changes in thermal conductivity at the macro level to enhance guidance for industrial production and practical application.

Modelling of oxide fuel restructuring during first hours after a reactor reaches its power

There is a substantial amount of experimental data that confirm the peculiarity of the oxide fuel behavior during the first hours after a reactor reaches its power. With an increase in temperature during the said period, oxide fuel pellets crack because of a significant temperature gradient. Further developments occur due to the accumulation and redistribution of fission products, and manifest themselves as changes in the fuel matrix porosity and the formation (or diameter increase) of a central hole. The zones that differ in their microstructure, density and thermal conductivity are formed along the fuel pellet radius. As the oxide fuel composition and restructuring result in its thermal conductivity change, it is important to pay attention to the formulas used in the stress-strain state calculations of a fuel element. The methodology for calculating changes in the oxide fuel porosity and the pellet’s internal diameter is proposed and based on the published research papers dedicated to the study of oxide fuel properties and behavior during the first hours after a reactor reaches its power. Тhe methodology was tested using real experimental data on the porosity redistribution along the fuel pellet radius. The presence and the size of the pellet’s inner hole, as well as changes in the fuel matrix porosity, have a noticeable effect on the maximum temperature value. Taking into account the pellet structure evolution while performing computational simulation of the fuel rod operation under irradiation allows assessing the fuel element’s operability more accurately. The proposed methodology can be used in computer codes designed to calculate the stress-strain state of cylindrical fuel elements of fast reactors.

Effects of lattice instability on the thermoelectric behavior of kagome metal ScV6Sn6

Kagome metal ScV6Sn6 has been a subject of interest due to the emergence of a first-order structural phase transition with intriguing charge density wave behavior below the transition temperature Tc ∼ 92 K. To explore the thermoelectric properties and provide experimental insights into the nature of the phase transition, we have carried out a combined study by means of the electrical resistivity, Seebeck coefficient, and thermal conductivity measurements on single crystalline ScV6Sn6. Pronounced features near Tc have been characterized by all measured physical quantities. In particular, the Seebeck coefficient exhibits a marked reduction as lowering temperature across Tc, attributed to an imbalance of the contribution from different type of carriers induced by the structural phase transition. From the examination of the electronic and lattice thermal conductivities, we obtained a confirmation that the observed enhancement at Tc is essentially caused by the change of the lattice thermal conductivity, demonstrating the primary importance of lattice distortions for the heat transport of ScV6Sn6. In addition, the lattice thermal conductivity above Tc was found to increase monotonically with temperature. We associated the peculiar phenomenon with lattice fluctuations, highlighting the essence of structural instability in the kagome lattice ScV6Sn6. These results add to the knowledge about the thermal transport properties in kagome materials with a hexagonal HfFe6Ge6-type structure.

Determination of the elemental composition and changes in thermal and electrical conductivity of “Hanford” and low-ash medium-grained graphite’s grade graphites depending on the fast neutrons fluence

Studying the properties of graphite recovered from operating nuclear reactors is vital for predicting the properties and integrity of graphite as part of assessing the continued operation and life extension of nuclear reactors. The purpose of the study is to determine the electrical conductivity and thermal conductivity of low-ash medium-grained graphite’s grade graphite in the thermal column masonry of the VVR-SM research reactor in the measurement temperature range corresponding to the conditions of normal operation of the reactor to determine the service life. In this work, the change in thermal conductivity and electrical conductivity of GMZ graphite was studied, as well as for comparison of Hanford grade graphite depending on the fluence of fast neutrons and the measurement temperature. The dependence of electrical conductivity and thermal conductivity on dose and temperature has been established. It has been shown that the greater the neutron fluence, the more both the thermal conductivity and electrical conductivity of the material decreases. The service life of the thermal column has been determined.

Accurately Measuring the Infrared Spectral Emissivity of Inconel 601, Inconel 625, and Inconel 718 Alloys during the Oxidation Process.

Accurate measurement of the infrared spectral emissivity of nickel-based alloys is significant for applications in aerospace. The low thermal conductivity of these alloys limits the accuracy of direct emissivity measurement, especially during the oxidation process. To improve measurement accuracy, a surface temperature correction method based on two thermocouples was proposed to eliminate the effect of thermal conductivity changes on emissivity measurement. By using this method, the infrared spectral emissivity of Inconel 601, Inconel 625, and Inconel 718 alloys was accurately measured during the oxidation process, with a temperature range of 673-873 K, a wavelength range of 3-20 μm, and a zenith angle range of 0-80°. The results show that the emissivity of the three alloys is similar in value and variation law; the emissivity of Inconel 718 is slightly less than that of Inconel 601 and Inconel 625; and the spectral emissivity of the three alloys strongly increases in the first hour, whereafter it grows gradually with the increase in oxidation time. Finally, Inconel 601 has a lower emissivity growth rate, which illustrates that it possesses stronger oxidation resistance and thermal stability. The maximum relative uncertainty of the emissivity measurement of the three alloys does not exceed 2.6%, except for the atmospheric absorption wavebands.

Water Thermal Conductivity in Nanoscale and Its Effect on Dropwise Condensation Heat Transfer

The present study employs equilibrium molecular dynamics simulation based on the Green-Kubo approach to investigate the thermal conductivity of liquid water at the nanoscale. Specifically, the discussion centers around the influence of the size and surface wettability of two-dimensional nanochannels on the confined water thermal conductivity. The simulation results reveal a significant impact of nanochannel size on the thermal conductivity of confined water, with a decrease in channel spacing leading to a reduction in thermal conductivity below its bulk value. However, the thermal conductivity of water confined within a surface is not influenced by the water surface wettability. Additionally, the study investigates the influence of water thermal conductivity on droplet formation. The results indicate that incorporating changes in droplet thermal conductivity during the nucleation process can partially address the issue of overestimating the heat transfer coefficient for dropwise condensation.

Modeling the thermal behavior of multifunctional syntactic foams containing phase change materials for heat management applications

AbstractSyntactic foams are lightweight porous composites obtained by the addition of hollow glass microspheres (HGMs) to a polymer matrix. This work experimentally and numerically investigates the thermal behavior of epoxy‐based syntactic foams with enhanced multifunctionality produced by incorporating microencapsulated phase change materials (PCMs) as functional fillers, providing thermal energy storage and temperature stabilization due to its large latent heat of melting. Foams are fabricated using varying ratios of HGMs (0–40 vol%) and PCM (0–40 vol%) to achieve tuned densities and thermal properties with a total filler content of up to 40 vol%. A one‐dimensional numerical heat transfer model based on the enthalpy method is presented to simulate the thermal behavior of these materials. The model accurately incorporates the specific heat and thermal conductivity of the foam components, as well as the latent heat absorption during PCM melting (up to 68 J/g). The model is experimentally validated by demonstrating good agreement with the measurements of transient temperature profiles during heating tests on the foam samples. The validated tool is then leveraged to model the thermal response of the foams under a sinusoidally varying heat source, similar to the possible real applicative scenarios. These results can help optimize foam compositions balancing energy storage density, heat transfer properties, and structural support for lightweight energy storage systems. Potential applications include high‐efficiency thermal management in battery packs, electronic cooling, solar energy storage, and sustainable temperature‐controlled buildings.Highlights Foams with 0–40 vol% PCM and 0–40 vol% HGM show up to 68 J/g latent heat. 1D numerical model accurately captures thermal conductivity and PCM phase change. Validated model simulates thermal response under sinusoidal heat, optimizing foam composition. Foams act as thermal reservoirs, maintaining up to 5°C lower surface temperatures than epoxy.

Analysis of charging performance of thermal energy storage system with graded metal foam structure and active flip method

The latent heat thermal energy storage (LHTES) technology based on solid-liquid phase change material (PCM) is characterized by high energy storage density, small volume change, and constant operation temperature, which is widely employed in waste heat recovery, solar thermal utilization, and equipment thermal management. This paper introduces active flipping and graded porous to strengthen natural convection and heat conduction to alleviate the issue of low thermal conductivity and non-uniform phase change of PCM. Based on the fixed grid system, the enthalpy-porosity method combined with time-dependent gravity and non-Darcy porous effect is employed to solve the solid-liquid phase change problem under different porosity gradients, dimensionless flipping times (t*), and modified Ra (Ra*) numbers. Furthermore, the effects of the Ra* on the optimal t* and porosity gradient are also discussed. The results show that the flipping method can significantly enhance flow and heat transfer within the container, improve temperature field uniformity, and increase the proportion of latent heat storage. With the increase of porosity gradient and t*, the melting time initially decreases and then increases and the optimal thermal performance is achieved at a 6 % porosity gradient and t* = 0.4. Compared to the uniform porous structure without flipping, it can shorten melting time by 49.37 % and improve thermal energy storage rate (TESR) by 76.23 %. Additionally, the optimal porosity gradient is 6 % under different Ra*, but the optimal t* decreases with the increase of Ra*. This paper explores the charging performance of the thermal energy storage system with the graded metal foam structure and active flip method, which can contribute to the study of heat transfer enhancement in LHTES technology.

First-principles study on thermal transport properties of GaN under different cross-plane strain

With the development of power devices towards high integration and power, the electrical and thermal stresses per unit area become more concentrated and intensified. The impact of cross-plane strain resulting from inverse piezoelectric effect and thermal expansion on power devices cannot be ignored. Cross-plane strain has a substantial influence on the thermal properties of GaN. However, the research on the influence of cross-plane strain on the thermal conductivity of GaN has not been reported in the literature. Based on the first-principles calculation method and the phonon Boltzmann transport equation, the influence of cross-plane strain on the thermal conductivity and phonon characteristics of the GaN lattice is systematically studied in this study. The thermal conductivity of GaN has anisotropy and increases significantly with the decrease in temperature. The thermal conductivity of GaN at room temperature under the free state is calculated to be 257 and 275 W/(m·K) for in-plane (k⊥) and cross-plane (k∥) directions, respectively. Compared with previous theoretical reports, our calculation results are more consistent with the existing experiment values. Under the state of cross-plane strain, the lattice thermal conductivity changes remarkably. In detail, the average thermal conductivity at room temperature decreased by 35 % under a 5 % cross-plane tensile strain state, while it increased by 11.5 % under a 5 % cross-plane compressive strain state. According to the calculation results, the influence of cross-plane on lattice thermal conductivity is mainly due to the change in phonon lifetime. The analysis of two mechanisms of phonon lifetime suppression indicates that the cross-plane strain will significantly change the frequency of the high-frequency optical acoustic branch. This result leads to a change in the phonon scattering process and thus affects the phonon lifetime. Besides, the anisotropy of thermal conductivity changes under different strain values, which may be due to the weakened piezoelectric polarization effect induced by strain.

Phase change material properties identification for the design of efficient thermal management system for cylindrical Lithium-ion battery module

Performance and life of Lithium-ion battery packs in EVs and energy storage applications are limited by the thermal profile of cells during its life. Passive cooling technique based on Phase Change Material (PCM) is employed for mitigating thermal issues associated with Lithium-ion cells. The challenge with PCM materials is the trade-off between its latent heat of fusion, thermal conductivity and specific heat. Detailed CFD analysis is carried out to study the impact of PCM thermal properties on battery pack thermal profile and provide guidelines for PCM properties selection and usage of existing PCM materials. Impact of critical PCM properties such as latent heat (100–250 kJkg−1), thermal conductivity (0.2–23 Wm−1K−1), melting temperature and thickness of PCM material on thermal performance of a battery module during rapid discharge rate of 25 A (3.2C) is investigated. Battery module maximum temperature (Tmax) reaches 54°C and temperature difference (∆T) to 5.5°C while operating at 35°C with 124 kJkg−1and 0.2 Wm−1K−1. It is found that, PCM with higher latent heat (≥ 175 kJkg−1), thermal conductivity (≥ 3 Wm−1K−1), PCM thickness (≥ 3 mm) and appropriate phase change temperature 41-44°C exhibit improved heat dissipation for high discharge rates by effectively reducing the Tmax to 44.4°C and ∆T to 2.4°C (25 A and 35°C ambient). Further, ways to mitigate low latent heat capacity and thermal conductivity of PCM through increasing PCM material usage (i.e. inter cell distance) is emphasized. Results of the present study can be considered as a design tool for PCM development engineers which could be cost effective both in-terms of development time and effort.

Nanocrystalline Diamond Lateral Overgrowth for High Thermal Conductivity Contact to Unseeded Diamond Surface

We demonstrate diamond lateral overgrowth as a way of increasing the thermal conductivity of thin layers of diamond. The technique can be used as a way of growing diamond on top of semiconductors, creating a thin layer of high thermal conductivity diamond in direct contact with semiconductors and allowing for the encasement of GaN in high thermal conductivity diamond.As we move to higher and higher power densities, the requirements for heat removal become extreme. The best passive way to remove heat from a semiconductor is to have a high thermal conductivity heat spreader near the heat source. Having diamond under the substrate is good. Having diamond under the epi is better and having diamond on top and bottom of the epi is better still.As-grown thin layers of diamond on non-diamond substrates have nano-diamond seeds as the starting material. The diamond thermal conductivity is a function of the grain size (Especially for small grain material) [1]. If we can increase the grain of the diamond near the junction, we can increase the thermal conductivity near the junction and more efficiently spread the heat away from the source before rejecting it into the surrounding material.In the experiment, we start with a bare silicon wafer seeded with nano-diamond seeds then grow about one micron of diamond. We coat it with a few nanometers of SiN. Then pattern the SiN layer with 2μm wide features to facilitate the coalescence of LOG-NCD over the SiN. Then we grow a 2μm thick microwave-plasma CVD NCD film (800W, 15Torr, 750°C, 0.3% CH4/H2) over the patterned SiN film initiating directly from the exposed areas of the host NCD. The diamond growth does not initiate on the patterned silicon nitride without seeds. The exposed diamond acts as a nucleating center initiating the diamond regrowth. The regrown diamond spreads from the diamond crystals exposed from the first growth. The edge crystals closest to the SiN patterned area expand over the silicon nitride forming diamond conformal to the SiN surface. This creates large crystals over the unseeded area. Figure 1 is a transmission electron microscope (TEM) image of the sample. The TEM, the image can be split into two regions. The left has no patterning, the called-out grain is 100nm wide. The right is patterned, and the diamond grains are between 1,000 to 2,000nm wide.We measured the thermal conductivity of the diamond on this samples using the TDTR method. This method uses a laser many times larger in diameter than the thickness of the film. As such, we probe the out-of-plane thermal conductivity. The initial diamond growth near the silicon had an average thermal conductivity of 100W/mK. We measured the second micron of growth without patterns to have Tc of 130W/mK. The diamond directly above the patterns had Tc of 260W/mK with grains 10X larger than the un-patterned diamond.Since the diamond grains are columnar, the difference in thermal conductivity between the patterned and un-patterned diamonds was 2X. Lateral measurements would likely show even larger changes in thermal conductivity. With grain sizes of 100 nm to 250 nm (as is the case with seeded diamond), the in-plane thermal conductivity will be dominated by the quality of the grain/grain interfaces and the lateral grain size dimensions rather than for the quality of the lattice. [1] We used lateral overgrowth to increase the grains to microns rather than 100s of nm. This means that we move into a regime where thermal conductivity is dominated by the quality of the diamond in the grains rather than the grain boundaries. As such the thermal conductivity is increased and can be further improved by improving the crystal quality - a parameter which is controlled by the diamond growth conditions. Typically, samples grown with more than 1% methane / hydrogen ratio show lower thermal conductivity [2]. However higher concentrations are important to establish the initial diamond film without damage to the underlying semiconductor. Here, the lateral overgrowth and methane concentration can be balanced to achieve both high thermal conductivity and low damage to the underlying substrate.