High Thermal Conductivity Materials Research Articles

CHALLENGES AND METHODS IN ANALYZES OF THE HEAT TRANSFER IN POLYMER DRY BEARINGS

Heat generation and dissipation in dry polymer bearings are important aspects in their design and operation, because the overheating may lead to fast wear or product damage. The estimation of the maximum temperature under defined load conditions is crucial, but it is also a challenging task. Firstly, it is difficult to measure temperature directly at the contact surface between the bearing and the shaft. Secondly, thermocouples that are commonly used as the temperature sensors might create measurement errors. The work presented in this paper utilizes the numerical model of a polymer bearing for the analysis of the internal temperature field. The model is validated with use of experimental data; and, in order to mitigate the measurement errors of the thermocouple sensor, their geometry and properties are included in the simulation model. The achieved agreement between simulation and experimental temperatures is 10% on average, and it is judged that the numerical model may be applied for thermal analysis of the polymer bearing. The obtained results confirm the influence of the thermocouples with metallic sheaths on the temperature distribution inside the tested polymer bearing. It is shown that the value of the measurement errors depends on the layout of thermocouples and might be significantly reduced by their proper arrangement. It is believed that the presented approach for the analysis of thermal performance of dry polymer bearings might be applied to similar cases, which are characterized by large temperature gradients and require temperature sensors, that are made of the materials of high thermal conductivity.

Characterization of a Water Heating System Using Solar Collector With Conical Concentrator

This study aimed to evaluate a solar water heating system for using on residences, using a solar collector with conical concentrator. The principle of light concentration in a solar collector with conical concentrator is the capture and reflection of solar radiation in the center of a tapered concentrator with internal reflective faces. The area of concentration of solar energy is occupied by a receiver with material of high thermal conductivity, properly isolated by transparent surfaces, to form the greenhouse effect, where the thermal energy is transferred to a working fluid. The characterization of the system was done through field tests to determine the efficiency in the water heating. The tests were performed considering different scenarios, which varied according to the heating system (passive and active with different water flow) and solar tracking (manual adjustment and stationary). The results showed that the scenarios with solar tracking presented an average efficiency of 12.63%, which was more efficient than those presented by the fixed orientation, which was 11.44%. Besides that, it was verified that the active solar heating systems were more efficient than the passive ones.

TSV-integrated thermoelectric cooling by holey silicon for hot spot thermal management

The trends toward higher power, higher frequency, and smaller scale electronics are making heat dissipation ever more challenging. Passive thermal management based on high thermal conductivity materials or through-silicon vias (TSVs) may not provide sufficient cooling for hot spots reaching 1 kW cm−2, and active thermal management by thermoelectric cooling (TEC) may require large power consumption or suffer from a large off-state thermal resistance of thermoelectric materials. Here we address these issues by integrating a holey silicon-based TEC with a TSV that directly draws heat from a hot spot to combine active and passive cooling approaches. Our simulations of the TSV-integrated TEC demonstrate exceptional cooling performance, which reduces the hot spot temperature from 154 °C to 68 °C while dissipating a heat flux of 1 k W cm−2 and consuming 0.5 W for TEC operation. The off-state hot spot temperature, 154 °C, is 24 °C lower than that of the same TEC with no TSV, and the on-state hot spot temperature, 68 °C, is 67 °C lower than that of the same TEC with no TSV. We also investigate the cooling prospects of metal-filled holey silicon by modeling the electron–phonon coupling and size dependent transport phenomena, which can further increase the thermal conductivity anisotropy and improve the TEC performance depending on the metal-to-silicon interfacial resistance. These results show the combined passive and active cooling in TSV-integrated TEC offers effective hot spot thermal management solutions for advanced electronics.

Thermal and mechanical properties of micro Cu doped Sn58Bi solder paste for attaching LED lamps

The junction temperature is a key factor in determining the reliability of light emitting diode (LED) package, which can be lowered using high thermal conductive soldering material. The effects of Cu micro particles on the thermal and mechanical properties as well as microstructures of Sn58Bi solder paste were investigated in this paper. The results indicated that the thermal conductivity of Sn58Bi-5 wt% Cu solder layer is 26.60 W/(m·K), which is much higher than the Sn58Bi solder layer of 18.89 W/(m·K). According to the thermal simulation analysis, the temperature on chip is the highest area inside the whole LED package. The maximum junction temperature of soldered LED package decreases with 5 wt% Cu particles addition and then increases. The maximum junction temperature of LED package increases as the input power increases from 1 to 3 W. The lowest junction temperature is obtained in Sn58Bi-5 wt% Cu soldered package. There is no significant degradation in shear strength of 5 wt% Cu micro particles doped Sn58Bi solder joint when comparing with the pure Sn58Bi soldered ones. The shear strength of solder joint decreases approximately 50% when the content of added Cu micro particles increases from 5 to 15 wt%, and weak bonding presents in 20 wt% Cu doped solder joint. Comparing with the Sn58Bi solder, the solder layer with 5 wt% Cu micro particles exhibited more uniform microstructure and the number of coarse Bi-rich phases is reduced. The Cu micro particles facilitate the heat dissipation process of Sn58Bi solder layer by adding highly conductive cores in the layer. The 5 wt% Cu micro particles doped Sn58Bi solder exhibits remarkable thermal and mechanical performance in attaching LED packages.

Silver Nanoparticle-Deposited Aluminum Oxide Nanoparticle as Fillers for Epoxy Composites with High Thermal Conductivity

With the development of polymer-filled composites, the demand of high thermal conductivity materials is much attractive than ever. However, the process of a common method to improve thermal conductivity of composites is considerably complicated. The aim of this study is to investigate thermal conductivity of epoxy filled silver nanoparticle deposited aluminum nitride nanoparticles with relatively convenient process. We found that the thermal conductivities of composites filled with AlN/Ag nanoparticles are effectively enhanced, which is enormously increased from 0.48 Wm-1K-1(1.88 vol%) to 3.66 Wm-1K-1 (19.54 vol%). This can be ascribed to the bridging connections of silver nanoparticle among aluminum nitride nanoparticles. In addition, the thermal contact resistance of the epoxy composites filler with AlN/Ag nanoparticles is decreased, which is proved by the fitting measured thermal conductivity of epoxy composite with one physical model. We believe the finding has great potential for any microelectronic application.

Introducing highly conductive materials into a fin for heat transfer enhancement

The straight fin is a common choice for enhancing heat transfer in cooling of planar systems such as the electronic chips that are becoming smaller and smaller every day. However, there is still effort required to increase the heat transfer rate per unit mass of a straight fin. With this design point of view, in this paper, a novel method is proposed by using highly conductive materials embedded in a straight fin. While the amount of such high thermal conductivity materials (‘inserts’) is considered as a constraint, the geometric shape and configuration of inserts are optimized to reach the maximum heat transfer. The numerical results indicate that the highly conductive materials reduce the thermal resistance of the fin and therefore, enhance the heat transfer. Also, it is shown that there exists an optimal insert aspect ratio that maximizes the insert effectiveness, depending on the insert volume fraction, Biot number, thermal conductivity ratio and the fin aspect ratio. For example, for Biot = 0.1, thermal conductivity ratio = 10 and the fin aspect ratio = 0.1, heat transfer enhancement by the fin is increased about 175% by using highly conductive materials of 10% volume fraction compared with the case of absent highly conductive materials.

The effect of embedded high thermal conductivity material on combustion performance of catalytic micro combustor

A novel micro combustor embedded with high thermal conductivity material has been proposed for micro-thermophotovoltaic system application. The effect of the materials, which is graphite and nickel with widths 1 mm and 3 mm on the thermal and radiant performance of the new micro combustor, was experimentally and numerically investigated. It was found that high and uniform temperature distribution along the walls of the combustor was obtained by embedding graphite with width of 1 mm because of the enhanced heat transfer, which is desirable for the micro-thermophotovoltaic system. The available radiation energy increased from 4.6 W for the conventional combustor to 5.2 W for the novel combustor when the inlet velocity was 1.0 m/s and the hydrogen/oxygen equivalence ratio was 0.5 under the experimental conditions. The available radiation efficiency increased from 8.9% to 9.8% and the effect of the embedded material could be enhanced by increasing the chemical energy input. Based on the combustor and fuel in this study, the conditions for good thermal performance are graphite with width of 1 mm, inlet velocity of 1.0 m/s and 0.5 equivalence ratio of hydrogen/oxygen by considering the reliability and durability of the combustor.

Thermal Performance Optimization in Electric Vehicle Power Trains by Locally Orthotropic Surface Layer Design

In this paper, the application of orthotropic material orientation optimization for controlling heat flow in electric car power trains is presented. The design process is applied to a case model, which conducts heat while storing heat-sensitive electronic components. The core of the case is designed using a low thermal conductivity material on order to focus the heat flow into the surface layer, which is designed using a high thermal conductivity material. Material orthotropy is achieved in the surface layer of the case by removing the material at points determined by the optimization analysis. For this purpose, an orthotropic material orientation optimization method was extended to calculate optimal material distribution. This is achieved by transforming the initially obtained optimal orientation vector field into a scalar field through the use of coupled time-dependent nonisotropic Helmholtz equations. Multiple parameters allow the control of the scalar field and therefore the control over material distribution in accordance to the optimal orientation. This allows the material distribution pattern to be scaled depending on the desired manufacturing method. The analysis method is applied to divert heat flow from a specific section of the model while focusing the heat flow to another section. The results are shown for a model with a 0.1 mm thick surface layer of copper and are compared to those results from several other materials and layer thicknesses. Finally, the manufactured design is presented.

Condensation heat transfer in horizontal three dimensional two-layer two-side enhanced tubes

Condensation heat transfer characteristics of near-azeotropic refrigerant R410A inside the horizontal copper and stainless steel EHT series tubes were measured. All three dimensional two-layer two-side enhanced tubes have deep-textured surface structures composed of petal-shaped background patterns and dimpled protrusions or boat-shaped cavities. Tests were conducted at an average saturation temperature of 45 °C; over a mass flux range of 100–450 kg/(m2·s); with an inlet quality of 0.8 and outlet quality of 0.2. Condensation heat transfer coefficient of the copper 1EHT1 tube is about 39–47% higher than that of a stainless steel one, showing a beneficial effect of high thermal conductivity material. Effects of saturation temperature and tube diameter on the copper 1EHT tubes were also discussed. Compared with a smooth tube, the heat transfer enhancement ratio of these stainless steel EHT tubes is up to 1.19. As expected, the 1EHT1 tube exhibits the highest condensation heat transfer coefficient due to its larger dimpled protrusions. In addition, the heat transfer performance of 1EHT2 and 3EHT tubes are similar. An explanation could be the condensate retention in boat-shaped cavities on the bottom of the 3EHT tube.

Construction of conductive pathways using Genetic Algorithms and Constructal Theory

In present study, two methodologies are proposed for the construction of high conductivity pathways inserted in a square plate of low conductivity and with volumetric internal heat generation. Constructal Design and Genetic Algorithms were used to reach main goal of this work that is to minimize global thermal resistance that occurs in the volume, facilitating access to the heat flow through the conductive pathways, which are connected to a heat sink. Total volume of the plate and that occupied by the material of high thermal conductivity are fixed as well as the thermal ratio between the conductivities of the materials. First methodology facilitates the construction of the paths in an evolutionary way using all available material, while second methodology, also using the idea of evolution, gradually distributes (a percentage to each evolution) the material, incrementally. Forms found for the paths have great similarity to the tree-shaped branched forms found in nature. When ratio of high to low conductivity material is small, the high conductive material tends to accumulate closer to the heat sink creating thicker paths, on the other hand, using a larger conductivity ratio, these paths are thinner and tend to move away from the heat sink. Although there is no universal optimal form for this problem, results presented offer an efficient performance, characterized by the constant minimization of temperature as the material is added and/or the Genetic Algorithm (GA) is applied. The use of GA offers great computing advantages. In addition, some of obtained results presented better performance, compared with I-shape case studied in the literature.

Estimation of thermal performance and design optimization of finned multitube latent heat thermal energy storage

Abstract This paper presents the effectiveness of finned multi-tube latent heat thermal energy storage system (LHTES) for medium temperature (∼200 °C) solar thermal power plant in reducing the fluctuations in heat transfer fluid (HTF) temperature caused due to the intermittent solar radiation. Commercially available phase change material (PCM) of melting temperature 168.7 °C is used as the storage material in the shell of LHTES, whereas a thermic oil based HTF passes through the tubes. Majority of the available PCMs have very low thermal conductivity (∼0.2–0.5 W/m.K), which drastically affects the thermal performance of the thermal storage system. Hence, in this study, thermal conductivity enhancer (TCE) in the form of fin is used to enhance heat transfer in the PCM. The fluid flow and heat transfer behaviour latent heat thermal energy storage system in the LHTES is studied by using a numerical model coupled with the enthalpy technique to account for the phase change process in the PCM. The developed numerical model, which is validated with the lab-scale experimental setup, is used to investigate the effect of number of fin, fin thickness and fin height on the HTF temperature at the outlet of the storage system. It is found that the number of fins and fin thickness significantly affect the thermal performance of the storage system, whereas the enhancement in heat transfer for high thermal conductivity material fin is marginal. Further, optimization of LHTES is performed for a defined objective function to identify the best configuration.

(Invited) Materials Properties and Their Impact on Beta Gallium Oxide for High Power Electronics Applications

Beta gallium oxide represents a promising material for high power device applications. However, the incorporation of this material into device structures is not as well understood as materials which have been used previously for high power device applications. Those materials (for example silicon, SiC, and GaN) possess crystal structures with higher symmetry. The monoclinic structure of beta gallium oxide leads to non-uniform thermal stresses in bulk and epitaxial layers and significant differences in reactivity due to surface orientations and doping levels. The low thermal conductivity of beta gallium oxide can be compensated by integrating it with other high conductivity materials. We address the crystalline perfection in state-of-the-art beta gallium oxide using high resolution x-ray scattering and topography techniques, the role of chemical mechanical polishing on sub-surface damage, the impact of the differences in thermal expansion coefficients along different crystallographic directions on gallium oxide structures in epitaxial systems, and initial studies into large scale exfoliation techniques for materials integration of gallium oxide with high thermal conductivity materials.

Decoupled trends for electrical and thermal conductivity in phase-confined CNT co-continuous blends

In the present work, the morphology and the electrical and thermal conduction properties of co-continuous poly(vinylidene fluoride) (PVDF), maleated polypropylene (PPgMA) and multiwall carbon nanotubes (CNT) nanostructured blends are investigated. CNT preferentially locates in the PPgMA phase and clearly causes a refinement in the co-continuous structure. Electrical conductivity experiments show that nanocomposites are well above the percolation threshold and evidence for one order of magnitude enhancement in conductivity for the co-continuous nanocomposites compared to the monophasic nanocomposites with the same CNT volume fraction. On the other hand, thermal diffusivity enhancement for the co-continuous blends is found lower than that for the monophasic nanocomposites at the same CNT volume fraction. An explanation is proposed in terms of large interfacial area, causing phonon scattering at the interface between immiscible PVDF and PPgMA domains. Results described in this paper open the way to the preparation of high electrical and low thermal conductivity materials with possible application as thermoelectrics.

Efficient fabrication of gradient nanostructure layer on surface of commercial pure copper by coupling electric pulse and ultrasonics treatment

Severe plastic deformation can be easily produced on metal surfaces by coupling the micro thermal shock from high peak pulse current and the micro mechanical shock from ultrasonics. Moreover, an efficient method for preparing a gradient nanostructured metal surface by coupling electric pulse and ultrasonics treatment (CEPUT) is developed in this study. The variation in microstructure and hardness of the specimen are investigated by electron backscatter diffraction, transmission electron microscope, X-ray diffraction, and nano-indentation measurement. Results showed that on the treated copper surface with CEPUT, the original grain boundaries are no longer recognized, the average grain size decreases from 48.77 μm to 39.22 nm, and the thickness of severe plastic deformation layer reaches to approximately 500 μm. Moreover, the hardness reaches to 2.105 GPa, and CEPUT also reduces the texture in the sample surface. A computational model is developed and the grain refinement mechanism is proposed to describe the electrical-thermal-mechanical phenomena during CEPUT. The proposed simple and cost-effective method of grain refinement and to produce the graded materials is effective, especially in the materials of high thermal and electrical conductivity.

Prediction Models for Thermal Conductivity of Cement-based Composites

Abstract Cement-based materials are the most consumed materials in the construction industry. Low or high thermal conductive cement-based materials are of interest in applications such as embedded floor heating systems, building envelopes or structural elements. This paper describes prediction models for thermal conductivity of cementitious composites by considering different variables such as constituent materials, porosity and moisture content. The presented prediction models may be used for thermal conductivity based mix design of cementitious materials. Based on the desired accuracy, different solutions are proposed.

A novel experimental-numerical method for studying the thermal behaviors of phase change material in a porous cavity

Experiment and numerical simulation were carried out in this work to investigate the thermal transport behaviors of Newtonian fluids, phase change material (PCM), and composite PCM in a cavity, filled with porous media for thermal energy storage (TES) applications. Herein a novel experimental system was developed by using numerous long and parallel steel wires embedded in the cavity to construct a two-dimensional (2-D) porous structure, which provided an accessible way for numerical simulation by directly meshing the porous structure. As Rayleigh number (Ra) over than 1.0E8, the flow was simulated by standard k-ε turbulent model. The effects of heat conduction through metal structure, natural convection of liquid PCM, and geometric configuration of porous medium and porosity on improving the thermal transport efficiency were discussed. For Newtonian fluid flow and heat transfer, higher Rayleigh number led to stronger natural convection, and the deviation between experimental and numerical is less than 10% that confirmed the reliability of numerical method for further application in phase change process; for phase change behavior of PCM in the cavity with wires, natural convection was apparently weakened as porosity (ε) decreased to 80% and heat conduction started to dominate the heat transfer. The full charging time (tfull, wire) of PCM mixed with wires was then compared to that (tfull, EG) of PCM mixed with expanded graphite (EG) under the same porosity. The results showed that EG significantly improved the heat transfer efficiency that tfull,EG for four cases (ε = 95%, 90%, 85%, 80%) is only 24.4%, 9.5%, 5.7% and 4.0% of tfull,wire, respectively. This result indicates that when mixing PCM with material of high thermal conductivity to improve the thermal conductivity of composite PCM, the porosity and geometric configuration of the material are essentially needed to be considered. Furthermore, the cross-linked structure of material plays a more important role than porosity in enhancing thermal transport efficiency through heat conduction. This work brings deep insights into sensibly improving the thermal conductivity of PCM by adding materials with high thermal conductivity.

Assembly of graphene-aligned polymer composites for thermal conductive applications

Abstract As one important member of graphene-based composites, graphene-aligned composites have drawn large amount of attention and made a great progress in academia and industry. Hence, we present a survey of the literature about graphene-aligned composites. Various their preparations are reviewed and the effects of interfacial interactions between graphene and polymer on the preparation and properties of aligned composites are also discussed. As high thermal conductive materials are becoming necessary, graphene-aligned composites show enormous potential in the applications of thermal conduction.

Filamentous Virus-based Assembly: Their Oriented Structures and Thermal Diffusivity

Organic polymers are generally regarded as thermal insulators because amorphous arrangement of molecular chains reduces the mean free path of heat-conducting phonons. However, recent studies indicated that single chains of polymers with highly oriented structures could have high thermal conductivity than bulk polymers because stretched polymer chains effectively conduct phonons through polymeric covalent bonds. Here, we demonstrated the possibility of non-covalent virus assembly prepared by simple flow-induced methods toward high thermal conductive polymeric materials. Films with high thermal diffusivity composed of non-covalent bond-based assemblies of liquid crystalline filamentous viruses were prepared using a simple flow-induced orientation method. Structural and thermal characterization demonstrated that highly oriented structures of the viruses in the film were attributed to the high thermal diffusivity. Our results will open attractive opportunities for biomolecular-based thermally conductive soft materials even though the assemblies are based on non-covalent bonds.

Effect of air confinement on thermal contact resistance in nanoscale heat transfer

Here, we report a detailed analysis of thermal contact resistance (Rc) of nano-size contact formed between a Wollaston wire thermal probe and the used samples (fused silica and titanium) as a function of air pressure (from 1 Pa to 105 Pa). Moreover, we suggest an analytical model using experimental data to extract Rc. We found that for both samples, the thermal contact resistance decreases with increasing air pressure. We also showed that Rc strongly depends on the thermal conductivity of materials keeping other parameters the same, such as roughness of the probe and samples, as well as the contact force. We provide a physical explanation of the Rc trend with pressure and thermal conductivity of the materials: Rc is ascribed to the heat transfer through solid–solid (probe-sample) contact and confined air at nanoscale cavities, due to the rough nature of the materials in contact. The contribution of confined air on heat transfer through the probe sample contact is significant at atmospheric pressure but decreases as the pressure decreases. In vacuum, only the solid–solid contact contributes to Rc. In addition, theoretical calculations using the well-known acoustic and diffuse mismatch models showed a high thermal conductivity material that exhibits high heat transmission and consequently low Rc, supporting our findings.

Thermal conductivity model for nanofiber networks

Understanding thermal transport in nanofiber networks is essential for their applications in thermal management, which are used extensively as mechanically sturdy thermal insulation or high thermal conductivity materials. In this study, using the statistical theory and Fourier's law of heat conduction while accounting for both the inter-fiber contact thermal resistance and the intrinsic thermal resistance of nanofibers, an analytical model is developed to predict the thermal conductivity of nanofiber networks as a function of their geometric and thermal properties. A scaling relation between the thermal conductivity and the geometric properties including volume fraction and nanofiber length of the network is revealed. This model agrees well with both numerical simulations and experimental measurements found in the literature. This model may prove useful in analyzing the experimental results and designing nanofiber networks for both high and low thermal conductivity applications.