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In this article, silicone rubber (SiR) is filled with industry preferred micro-silica and specialty hexagonal boron nitride (h-BN) nanosheets, by varying the ratio between the two fillers, while keeping the filler to matrix ratio the same in all three composites. The fillers are added to SiR using an electrostatic disperser which aids in homogenizing the filler-polymer mixture by means of shearing force along with elongation. The performance of synthesized silicone composites are evaluated using thermal, erosion resistance, morphological, mechanical, and high-temperature dielectric properties. Addition of as low as 5 wt% h-BN fillers significantly improved the thermal properties; 6%–20% improvement in thermal conductivity and in less than half the weight loss compared to only silica filled composites with laser erosion. The morphological and mechanical analyses showed improved filler dispersion and filler to silicone matrix interaction owing to good filler dispersion achieved through the electrostatic dispersing method. The dielectric properties measured in wide frequency range, between 10−4 and 103 Hz and at elevated temperatures (up to 175 °C) showed a negligible effect of increase in h-BN content. Thus, this article implies that low amounts of h-BN in silica-silicone composites improve thermal conductivity and erosion resistance significantly and not altering the dielectric responses of these hybrid composites even at elevated temperatures. Hence these composites can find applications both in the high voltage industry and as electronic packaging materials. [ABSTRACT FROM AUTHOR]

This article aims to improve nanofiller dispersion in epoxy resins by using an electrostatic disperser (ED), as well as reduce costs by using a combination of microfillers and nanofillers. The ED method aids in homogenized mixing of nanofillers in epoxy resin, due to its jet elongation and shearing effect on a polymer mix. In this article, two concentrations (7.5 wt% and 5 wt%) of nanosilica are used with resin, and the rheological properties of hybrid composites are tested before and after the addition of 50 wt% microsilica. It is observed that the electrostatic dispersion method aids in the network formation of nanosilica, unlike the high shear (HS) method, wherein clusters of nanosilica are formed. Hence, the ED method aids in reducing the viscosity values with the addition of microsilica, which is helpful in the product molding process. A plausible mechanism of filler dispersion is explained for ED and HS methods. The morphological analysis showed that the composites are brittle due to the high silica content, and that the silica fillers are dispersed well within the epoxy matrix. The flexural strength lies in the same range for both composites prepared using ED and HS methods. Thermal conductivity depends both on nanosilica and epoxy matrix dispersion, while heat deflection temperature, thermal diffusivity and specific heat depend on the dispersion method. [ABSTRACT FROM AUTHOR]

Thermal interface materials (TIMs) used to fill the gaps between chip and heat sink are one of the most important materials for electronic packaging. Low thermal resistance is the key to achieving high heat dissipation efficiency. However, most studies only focused on how to enhance the thermal conductivity of the gel TIMs and ignored the thermal resistance, which is significantly influenced by the rheological properties. In this work, we aimed to investigate the effects of in situ modification of aluminum fillers on the rheological properties and thermal resistance of the gel TIMs. Six modifiers were compared for their ability to improve the compatibility between aluminum fillers and silicone oil. The rheological results showed that the gel TIM modified with 0.170 wt% dodecyl trimethoxysilane had the best rheological properties, including the lowest viscosity and yield stress, the weakest Payne effect, and the shortest relaxation time. The thermal resistance measurements further verified that the bond line thickness and thermal resistance of gel TIMs were determined by the rheological properties. This work provides a reference for the formulation design and process optimization of gel TIMs. [ABSTRACT FROM AUTHOR]

This article explores the potential of epoxy resin/silicon carbide whisker (EP/SiCw) composites (1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%) as the field-dependent conductivity (FDC) packaging materials for the high-voltage power modules by investigating their thermal and electrical properties at different temperatures. The cross-sectional morphology of the prepared materials is characterized by field emission-scanning electron microscopy (FE-SEM). The thermal analysis consists of differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The electrical properties, such as dc conductivity, dielectric spectroscopy, thermally stimulated discharge current (TSDC), space charge distribution, and dc breakdown strength, are measured. The molecular orbital energy levels of the EP and SiCw monomers are calculated by quantum chemical calculation (QCC) to analyze the effect of EP interfacial trap sites on the electrical properties. The effect of the EP/SiCw interfacial charges and electrical field distortion on the electrical properties of the EP/SiCw composites with different filler content is investigated by the finite-element method (FEM) simulation. It is found that the 2 wt% EP/SiCw composite has a better thermal performance. The 1 wt% has lower real permittivity, lower trap density, lower space charge accumulation, and higher breakdown strength. The EP/SiCw composites have a higher trap level than the pure EP. The nonlinearity percolation threshold of the EP/SiCw composites is 3 wt%. As the filler content increases, the nonlinear conductivity coefficient of the EP/SiCw composites becomes higher, whereas the breakdown strength decreases due to the electric field distortion near the electrodes caused by the apparent hetero charge accumulation at high filler content. [ABSTRACT FROM AUTHOR]

This work reports on the significant enhancement of the thermal properties of the FR3 natural ester dielectric oil after the addition of pentyl-graphene nanosheets, as confirmed by thermal diffusivity and specific heat studies at different concentrations and temperatures (30 °C–90 °C). Experimental results of the dielectric constant demonstrated a constant value in the kHz region while increased in the low-frequency region, decreased with increased temperature, and saturated at 0.008% w/w concentration. In addition, light absorption is used for a better understanding of the properties variation upon changing graphene’s concentration as a method to estimate the agglomeration level. The optimum concentration for its best performance in terms of thermal and dielectric properties is 0.008% w/w, whereby the thermal diffusivity and the dielectric constant increased by 43.04% and 6.18%, respectively. [ABSTRACT FROM AUTHOR]

Die-bonding materials are essential packaging materials for high-power light-emitting diodes (LEDs), which affects their heat dissipation and reliability. In this work, a die-bonding structure with high strength and low thermal resistance was proposed for high-power LEDs. A Cu-foam/Sn composite preform was prepared as the die-bonding material in this die-bonding structure. The composite preform is composed of high-melting-point Cu skeleton and low-melting-point Sn. The Cu-skeleton structure accelerates the bonding process and forms intermetallic compounds (IMCs) with high remelting point. Consequently, the die-bonding structure displays robust joints from the transient-liquid-phase (TLP) interconnection layer and high thermal conductivity from the Cu skeleton. The effects of bonding temperature on the mechanical and thermal properties of LEDs were investigated. After bonded with the Cu-foam/Sn composite preform, the LED achieves a low thermal resistance of 6.75 K/W, a low junction temperature change of 6.5 °C, and a high shear strength of 37.8 MPa. Moreover, the proposed die-bonding structure has a lower thermal resistance and higher light output power, which are 27.2% lower and 7.2% higher than the traditional die-bonding structure, respectively. [ABSTRACT FROM AUTHOR]

An accurate and efficient algorithm is presented, which allows deriving the structure function of a discretized 3-D heat conduction problem. In this approach, the partial thermal conductances and capacitances in the structure function are computed in terms of weighted spatial averages of thermal resistivity and volumetric heat capacity. As a result, the exact influence of all materials and geometric details on each part of the structure function is determined. The approach is validated on a state-of-the-art SiGe heterojunction bipolar transistor (HBT) for high-frequency applications. [ABSTRACT FROM AUTHOR]

Thermal and structural properties of aged silicone rubber composite insulators widely used in high-voltage transmission power systems were characterized by photothermal radiometry (PTR) to evaluate the degree of aging. A two-layer PTR theoretical model was developed to extract the thermal diffusivity and the aged layer thickness from the PTR frequency-scan data. Results show that aging generated an aged layer with a reduced thermal diffusivity at the surface of the composite insulator. Further investigation into the depth dependence of the thermal diffusivity of aged composite insulators found a monotonically increasing thermal diffusivity along the depth direction, approximating to a hyperbolic tangential profile, which indicates a spatially continuous variation from surface to bulk during the aging process. The PTR depth-profiling of thermal and structural properties provides a deeper understanding of the aging process in composite insulators. [ABSTRACT FROM AUTHOR]

Epoxy resin (EP) insulations are widely considered as potential candidates for various applications. However, it is not suitable for high frequency (HF) and high temperature (HT) applications due to its low thermal conductivity. This research presents the tuning of thermal and electrical properties of EP via resin stoichiometry and incorporation of surface-modified hexagonal boron nitride (h-BN) nanofillers. In this research, the curing agent concentration for pure EP is optimized first based on high thermal stability and sufficiently reliable electrical properties. Second, the optimized pure EP is used as a base material to prepare epoxy/h-BN nanocomposites. Various physicochemical, thermal, and electrical characterizations are performed to analyze the properties of prepared materials. Results show that the higher curing agent concentrations offer a good balance between the thermal and electrical performance of the EP. However, the HF and HT stress exhibits detrimental effects on the breakdown strength and dielectric properties of the materials. Also, the addition of nanofillers further enhances the thermal properties while exhibiting sufficient electrical properties. By the control of resin stoichiometry and nanofiller addition, suitable composites for the HF and HT environment can be realized. [ABSTRACT FROM AUTHOR]

Rare-earth barium copper oxide (REBCO) tape is used for resistive type superconducting fault current limiters (RSFCLs) owing to its quick response and simple structure. To shorten the recovery time, film boiling on the REBCO tape should be suppressed as it exhibits poor heat transfer performance. To address the aforementioned issue, this study utilized low thermal conductivity materials as the thermal/electrical interface material for porous-stabilized REBCO tape. The experimental results showed that the cooling and recovery performances were dramatically improved by utilizing an insulating epoxy, which has a thermal conductivity of 0.6 W/mK. Based on the visualization of the boiling behavior, the liquid-solid contact between liquid nitrogen and the REBCO tape was promoted by the insulating epoxy, in which the film boiling was easily broken. [ABSTRACT FROM AUTHOR]

To suppress space charge accumulation and minimize the antioxidant migration, polyethylene (PE) grafted with reactive hindered amine antioxidant MC prepared by melt radical grafting is investigated in this work. Fourier transform infrared spectroscopy is used to investigate the grafting reaction, which shows that the antioxidant MC is successfully introduced onto PEe Space charge distribution, DC conductivity, DC breakdown strength and thermally stimulated current are measured and the electronic structure is calculated from first-principles. The results show that PE grafted with antioxidant MC can effectively suppress space charge accumulation, reduce DC electrical conductivity and enhance DC breakdown strength. The polar functional groups of antioxidant MC anchored on PE macromolecular chain form immobile deep traps at the grafted MC sites, which can capture space charges and scatter charge carriers to reduce the amount, mobility and energy of charge carriers. Seen from the oxidative induction time of the specimens before and after acetone extraction, the level of retention of the grafted antioxidant MC are significantly higher than those of conventional antioxidant 300 and antioxidant 4020, which indicates that PE grafted with antioxidant MC has excellent long-term thermal oxidation stability. [ABSTRACT FROM AUTHOR]

In this work, we analyze the impact of self-heating effects (SHEs) on 14-nm negative capacitance (NC)-FinFET performance from device to the circuit level. The 3-D thermal TCAD simulations, after careful calibration with measurements, are performed to analyze the impact of SHE in a broad range of frequency. Furthermore, we use the TCAD calibrated BSIM-CMG model to analyze the impact of SHE in NC-FinFET at the circuit level, after including a physics-based model to capture the NC effect. For the first time, we analyze the impact of a nonuniform distribution of temperature dissipated from the channel region to gate-stack in NC-FinFETs. On account of the thermal insulating properties of the gate-stack, the ferroelectric (FE) layer is found to be cooler than the channel region under the impact of SHE. We demonstrate that neglecting that and, hence, using the channel temperature to evaluate the temperature-dependent parameter α (in the Landau–Khalatanikov model of NC effect) of the FE layer result in a significant overestimation of SHE-induced degradations, such as in the NC voltage gain. Based on our TCAD analysis, we propose a relation between gate-stack temperature and the channel temperature and use this to accurately model the α parameter and, hence, SHE in NC-FinFETs. The SHE is found to dominate for both FinFET and NC-FinFET in the gigahertz range, which eventually degrades the performance at the circuit level, which is further confirmed using ring oscillator (RO) simulations. [ABSTRACT FROM AUTHOR]

The traditional cup-shaped LED packaged with epoxy molding compound materials (EMC-LED) and chip-scale packaged LED (CSP-LED) were prepared and analyzed in this article. It is found that high temperature can result in the spectrum red shift and the photoelectric performance degradation for these LEDs due to the degradation of chip and packaging materials. Compared with EMC-LED, the CSP-LED device has a lower thermal resistance of 3.31 °C/W due to the shorter thermal dissipation path and has better stability under the different temperature and drive current conditions. All the simulation results agree well with experimental and theoretical results, indicating that the photoelectric and thermal properties of the CSP-LED can be improved by introducing ceramic substrate and optimizing the material of the interconnected surface and the size of the conduction area. These results have guiding significance for the future CSP-LED design and application. [ABSTRACT FROM AUTHOR]

Objective: Tissue electroporation is achieved by applying a series of electric pulses to destabilize cell membranes within the target tissue. The treatment volume is dictated by the electric field distribution, which depends on the pulse parameters and tissue type and can be readily predicted using numerical methods. These models require the relevant tissue properties to be known beforehand. This study aims to quantify electrical and thermal properties for three different tissue types relevant to current clinical electroporation. Methods: Pancreatic, brain, and liver tissue were harvested from pigs, then treated with IRE pulses in a parallel-plate configuration. Resulting current and temperature readings were used to calculate the conductivity and its temperature dependence for each tissue type. Finally, a computational model was constructed to examine the impact of differences between tissue types. Results: Baseline conductivity values (mean 0.11, 0.14, and 0.12 S/m) and temperature coefficients of conductivity (mean 2.0, 2.3, and 1.2 % per degree Celsius) were calculated for pancreas, brain, and liver, respectively. The accompanying computational models suggest field distribution and thermal damage volumes are dependent on tissue type. Conclusion: The three tissue types show similar electrical and thermal responses to IRE, though brain tissue exhibits the greatest differences. The results also show that tissue type plays a role in the expected ablation and thermal damage volumes. Significance: The conductivity and its changes due to heating are expected to have a marked impact on the ablation volume. Incorporating these tissue properties aids in the prediction and optimization of electroporation-based therapies. [ABSTRACT FROM AUTHOR]

The relevance of anisotropic heatsinks for heat-assisted magnetic recording (HAMR) is investigated. First, 3-D thermal modeling is used to demonstrate that the heatsink performance on a glass substrate is determined by the heatsink in-plane thermal conductivity. The out-of-plane thermal conductivity has almost no impact on the heatsink ability to carry heat away from the hot spot. As a result, an anisotropic heatsink can be a good alternative to some conventional isotropic heatsink materials when its in-plane thermal conductivity is larger than that of the isotropic material. Second, time-domain thermo-reflectance (TDTR) measurements are presented to extract both the in-plane and the out-of-plane thermal conductivities of a (Cu/Ta) multilayer heatsink. The sensitivity to in-plane thermal conductivity is achieved by using small laser beam sizes and by combining measurements at multiple modulation frequencies on both a low and a high thermally conductive substrate. The fitting of all TDTR spectra gives for the (Cu[5.1]/Ta[1.4]) multilayers an effective out-of-plane thermal conductivity of 11.5 W/mK and an in-plane thermal conductivity of 69 W/mK. These thermal properties make such multilayer material an interesting candidate as HAMR heatsink, equivalent to a 70 W/mK isotropic material. [ABSTRACT FROM AUTHOR]

Research on material characterization has received an increasing amount of attention recently. In several application scenarios, it is essential to effectively estimate the physical properties of objects without coming into contact with them (e.g., tele-operated or autonomous robotics). This article presents the Haptic Eye: a framework using active thermography that uses a custom multichannel neural network approach to perform classification between samples and regression toward their thermal properties. This neural network structure is uniquely suited for effective processing of thermographic data. The framework is realized, implemented, and evaluated with a set of ten samples with diverse thermal/physical properties. Experimental results on a realization of the framework validate this approach, with a 92.20% classification accuracy using multichannel neural network with majority vote, as well as more than 99.6% R2-fit with respect to three different thermal properties, namely thermal conductivity, thermal diffusivity, and thermal effusivity (which is very useful to define thermal exchange during physical interaction). [ABSTRACT FROM AUTHOR]

This work investigates the effect of graphene filler in epoxy resin with the purpose of developing an improved thermally conductive epoxy encapsulation material for electronic devices. The findings in this work show that the thermal conductivity increases by a factor of about 1.6 times with 0.1 wt% loading of graphene in epoxy resin. The increase in the rate of surface charge verifies an improved anti-static property. On examining the phase resolved partial discharge patterns, it is observed that the inception voltage and the number of partial discharge pulses is lower for the filled epoxy compared to that of the neat epoxy. The increase in the rate of surface charge decay indicates an improved anti-static property. The findings in this paper provide a reference in the development of materials with improved thermal conductivity and anti-static properties for graphene/epoxy materials for use in the packaging of electronic devices. [ABSTRACT FROM AUTHOR]

This study introduces silicone elastomers with micro and nano-sized boron nitride (BN) and silicon carbide (SiC) particles with various doping levels to improve thermal properties and partial discharge resistance. The effect of micro and nano-fillers on thermal conductivity, coefficient of thermal expansion (CTE), and thermal stability are investigated. Polarization and depolarization current and partial discharge are measured to investigate the non-linear conductivity, trap energy density distribution, and partial discharge resistance. Experimental results show higher thermal conductivity, lower CTE, and better thermal stability than the original silicone elastomer. Large size fillers dominate the thermal conductivity when the doping level is low, whereas the composite microstructure plays a significant role in the thermal conductivity when the doping level is high. The combination of different filler type and size has less effect on thermal stability and CTE compared to the effect of the doping level. According to the depolarization current, the composite's trap depth is generally shallower than that of the original silicone elastomer. With increased doping level, the shallow trap density increases, providing hopping sites for carriers, whereas the deep trap density decreases, reducing the number of trapped charges. The partial discharge inception voltage is higher than that of the original silicone elastomer, and it increases with the doping level, which may be because the electrostatic field generated by diffused charges reduces the local electric field near the high voltage electrode. [ABSTRACT FROM AUTHOR]

The increase in voltage level and compactness of electrical equipment result in demands for electrical insulations that have high breakdown strength, high thermal conductivity, and high electrical resistivity. Use of dielectric polymer nanocomposites is a promising approach that has great advantages over the traditional materials. This article explores the influence of alumina nanofillers, both treated and untreated, on properties of silicone rubber-based nanocomposites. Composite samples made of 10 wt% and 20 wt% filler loading are prepared using high shear (HS) and electrostatic disperser (ES) techniques with the aim of achieving maximum dispersion of fillers in the silicone matrix. Effects of filler type, filler concentration and mixing method on morphological changes, thermal, erosion, and electrical properties are analyzed. Scanning electron micrographs (SEMs) showed better filler dispersion in composites prepared using ES than using HS mixer. Thermogravimetric analysis and thermal conductivity measurements revealed enhanced thermal stability and conductivity with increasing filler loadings. Additionally, ES composites showed high erosion resistance. Composites containing treated alumina performed better than those containing untreated alumina. In total, composites prepared with treated alumina using the ES method showed marked improvement in thermal properties and erosion resistance due to homogeneous filler dispersion imparting high number of filler-matrix interfaces and stronger bonding as visualized from SEMs and dielectric spectroscopy data. [ABSTRACT FROM AUTHOR]

This paper introduces a new nonlinear multidisciplinary design approach (NMDA) for axial-flux (AF) eddy current brakes (ECBs). The proposed NMDA is developed by nonlinear magnetic–thermal–structural coupled modeling. The nonlinear behaviors of the AF-ECB covering the BH curve, resistivity, heat capacity, thermal conductivity, and the temperature are jointly analyzed in time domain to investigate the actual brake properties and to determine brake operating range. In this paper, the nonlinear magnetic modeling based on two-dimensional (2-D) reluctance network is studied by a modified Newton–Raphson method. Furthermore, the nonlinear thermal modeling is carried out by lumped-parameters considering the change of heat capacity, thermal conductivity, and the temperature and updating the data in the evaluation process. Finally, the nonlinear structural modeling is performed to obtain the deflection and the mechanical safety factor of the brake. The nonlinear modeling methods in the proposed NMDA are validated by independent 3-D electromagnetic, thermal, and structural finite element analyses (FEAs), and the proposed NMDA is tested with two different AF-ECB prototypes. The experimental results confirm that the proposed NMDA has high accuracy, and compared to 3-D-FEA approaches, it provides a fast solution to predict the AF-ECB performance. [ABSTRACT FROM AUTHOR]

Research on material characterization has received an increasing amount of attention recently. In several application scenarios, it is essential to effectively estimate the physical properties of objects without coming into contact with them (e.g., tele-operated or autonomous robotics). This article presents the Haptic Eye: a framework using active thermography that uses a custom multichannel neural network approach to perform classification between samples and regression toward their thermal properties. This neural network structure is uniquely suited for effective processing of thermographic data. The framework is realized, implemented, and evaluated with a set of ten samples with diverse thermal/physical properties. Experimental results on a realization of the framework validate this approach, with a 92.20% classification accuracy using multichannel neural network with majority vote, as well as more than 99.6% R2-fit with respect to three different thermal properties, namely thermal conductivity, thermal diffusivity, and thermal effusivity (which is very useful to define thermal exchange during physical interaction). [ABSTRACT FROM AUTHOR]

To reduce the electric field distortion and increase the thermal conductivity of the dielectric materials in cable accessories, this paper proposes a method to prepare composites with good nonlinear electrical conductivity and high thermal conductivity. Silver nanoparticles grown on hexagonal boron nitride (AgNPs/BN) were prepared as a powder and introduced into ethylene propylene diene monomer (EPDM) to obtain AgNPs/BN/EPDM composites. The nonlinear electrical conductivity, thermal conductivity and breakdown field strength of the composites were investigated at different temperatures. The nonlinear electrical conductivity and thermal conductivity of the composites increased with increasing AgNPs content grown on BN. Moreover, the breakdown field strength decreased with increasing AgNPs, and exhibited a weaker temperature dependence than neat EPDM. Simulations showed that the composites could improve the electric field distribution in a cable accessory. [ABSTRACT FROM AUTHOR]

Increased demand on the mechanical and thermal properties on the thermal interface and die attach material creates a demand for materials with tailored material properties. Solder matrix fiber composites (SMFCs) have been shown to address these challenges, but have, so far, required complicated procedures and components. In this paper, we present the fabrication of a new type of SMFC based on commercially available fiber networks infiltrated with Sn–Ag–Cu alloy (SAC305) or indium using equipment for large-volume production. The composite material exhibits similar thermal properties compared to pure solder, and mechanical properties that can be tailored toward specific applications. We also show that the handling properties of the SMFC allows it to be used in process flows where multiple reflow cycles are required and can achieve a well-defined bond line thickness (BLT) and good bonding using fluxless reflow under pressure. [ABSTRACT FROM AUTHOR]

In nanocomposites, different surface states of nanoparticles can potentially provide different interactions with the base polymer and in turn change the bulk properties. Aluminium nitride (AlN) nanoparticles were surface functionalised with three different silane coupling agents (SCAs) with varying organofunctional and hydrolysable groups. The effects of the filler surface chemistry on the resulting AlN/polypropylene (PP) nanocomposites were examined and compared with an unfilled reference system. It is observed that different organofunctional groups can provide different nucleating effects and the dispersion states of nanoparticles while the hydrolysable group is not the dominant factor. The dielectric spectroscopy results show the hydrolysable group of SCA will also result in a difference of the interphase since the trimethoxy silane treated systems show much higher imaginary permittivity than the triethoxy silane treated systems when the frequency is below 1 Hz. The grafted organofunctional layer on the particle surface can provide a significant improvement of the thermal conductivity of the composite materials, e.g. 15 % improvement in thermal conductivity was observed when adding 10 wt% methacrylate silanes treated nano-AlN into PP, while the untreated counterpart only has 5 % improvement. [ABSTRACT FROM AUTHOR]

The thermal response test (TRT) is the standard method for characterizing the thermal properties of the ground and those of a borehole heat exchanger (BHE). During the TRT, the inlet and outlet temperatures of the BHE are monitored. However, this test typically considers the ground as a homogeneous, isotropic, and infinite media, and therefore, it only determines the bulk and effective parameters, such as effective thermal conductivity and thermal borehole resistance. Hence, the enhanced TRT protocols are necessary where the depth-dependent temperatures are measured to estimate depth-specific thermal properties. Thus, a novel instrument with a data logger to automatically obtain the temperature measurements along the BHE is introduced. This device is based on a Zynq-7000 all programmable system on a chip. It has a dual-core central processing unit and a field-programmable gate array on one chip, thus providing a versatile architecture that reduces cost and improves efficiency in comparison with other systems of similar characteristics. This paper describes the implemented hardware and software developments that range from user interface application to a free-distribution operating system based on an embedded Linux. The proposed instrument can be easily incorporated throughout a TRT, and the nonspecialized staff can remotely manage or visualize the results through a menu-driven interface. The device is tested in a specific BHE installation and validated with standard Pt100-temperature-sensors. The results are comparable and, therefore, demonstrate the applicability of this novel instrument called Geowire. [ABSTRACT FROM AUTHOR]

Epoxy resin is widely used in magnetic confinement fusion reactors, and high thermal conductivity is also required to dissipate heat rapidly under the cooling and quench condition. In this paper, epoxy/BN/SiC composites are cured under ac and dc electric fields. An electric field is used to induce nanoparticle orientation to obtain higher thermal conductivity. Thermal and electrical properties of the prepared materials were tested. The results show that electric field induction can improve the thermal conductivity of composite. The orientation of SiC particles along the electric field could cause the increase of bulk conductivity. Charge dissipation of the filled samples becomes slower compared with the pure epoxy. The effect of ac electric field induction is larger than that of the dc electric field, which would affect the conductivity, charge, and trap characteristics of composites. [ABSTRACT FROM AUTHOR]

An optimized two-zone model based on the critical degree of saturation is proposed in this paper for ampacity calculations of underground power cables. Moisture migration is fully considered based on geological soil models. Comprehensive finite-element method (FEM) simulations are performed to study the effect of the moisture content and its effects on soil thermal resistivity and critical temperature rise. The critical degree of saturation (Scr) is used in the improved two-zone model instead of the critical temperature rise (Δθx) used in the IEC Standard 60287. The comparative analysis presented between the new method and the IEC Standard shows that IEC is more susceptible to changes in the ambient conditions. The use of a fixed critical temperature rise may yield significant errors. Shortcomings of numerical solutions are discussed highlighting the advantages of the new (optimized) two-zone model. The optimized method is useful for the design of new installations as it accounts for changes in soil thermal resistivity and critical temperature rise. The new method has been validated with available test results. [ABSTRACT FROM AUTHOR]

Thermal interface materials (TIMs) play a major role in the performance of semiconductor devices by optimizing the thermal contact between device and heatsink. Their influence is further increasing with the usage of novel chip materials such as SiC and GaN. In this methodology study, we compared five of the most established evaluation methods for solid TIMs with each other: transient plane source, LaserFlash, DynTIM, TIMA, and an application-oriented $R_{\mathrm{ th}}$ measurement system. We investigated a wide range of typical TIMs in order to explore the limits of the different measurement systems. The results show that, despite existing norms, the used characterization method has a significant influence on the measured thermal conductivity. We also show that the temperature and pressure dependence has a significant influence on the thermal performance of TIMs and that these data need to be included in device specifications. Additionally, detailed error analysis and discussion about sample selection, error influence, and measurement effort for the presented methods are given. [ABSTRACT FROM AUTHOR]

Flow channel inserts (FCIs) are the key elements in the high-temperature dual-coolant lead-lithium blanket, since in this concept the flowing PbLi reaches temperatures near 700 °C and FCIs should provide the necessary thermal and electrical insulations to assure a safe blanket performance. In this paper, the use of a SiC-sandwich material for FCIs consisting of a porous SiC core covered by a dense chemical vapor deposition-SiC layer is studied. A fabrication procedure for porous SiC is proposed and the resulting materials are characterized in terms of thermal and electrical conductivities (the latter before and after being subjected to ionizing radiation) and flexural strength. SiC materials with a wide range of porosities are produced; in addition, preliminary results using an alternative route based on the gel-casting technique are also presented, including the fabrication of hollow samples to be part of future lab-scale FCI prototypes. Finally, to study the corrosion resistance of the material in hot PbLi, corrosion tests under static PbLi at 700 °C and under flowing PbLi at ~10 cm/s and 550 °C, with and without a 1.8–2T magnetic field, were performed to materials coated with a 200–400- $\mu \text{m}$ -thick dense SiC layer, obtaining promising results. [ABSTRACT FROM AUTHOR]

Electrosurgical vessel joining is commonly performed in surgical procedures to maintain hemostasis. This process requires elevated temperature to denature the tissue and while compression is applied, the tissue can be joined together. The elevated temperature can cause thermal damages to the surrounding tissues. In order to minimize these damages, it is critical to understand how the tissue properties change and how that affects the thermal spread. The purpose of this study is to investigate the changes of tissue thermal conductivity and how the changes correlate to thermal dose during the joining process. We propose a hybrid method combining experimental measurement with inverse heat transfer analysis to determine thermal conductivity of thin tissue sample. Porcine aorta arterial tissues were used to investigate tissue thermal conductivity with variable thermal dose. Different joining times were used to create different amounts of thermal dose. A 36% decrease in tissue thermal conductivity was found when the thermal dose reaches the threshold for second-degree burn. When thermal dose is beyond the threshold of third-degree burn, the tissue thermal conductivity does not decrease significantly. A regression model was also developed and can be used to predict tissue thermal conductivity based on the thermal dose. [ABSTRACT FROM AUTHOR]

The combination of polymer materials, such as silicone rubber (SiR) and ethylene propylene diene monomer (EPDM), has become one of the potential developments of new polymer insulator with superior characteristics. SiR/EPDM with a ratio of 50:50 weight percentages had produced the optimal electrical and mechanical properties of polymer composites. However, the study of these balanced blends with inclusion nanofillers remains limited and has yet to be completely explored. In the current research, SiR/EPDM blends were prepared with two types of nanofillers, namely, Al2O3 and TiO2. The loading concentrations of the nanofillers for each specimen were 1, 2, 3, 4, and 5 Vol%. The effects of the Al2O3 and TiO2 nanofillers with different loading concentrations on the surface-tracking characteristics were investigated. Experimental results revealed that the addition of nanofillers to the SiR/EPDM composite significantly improved the performance of the electrical surface-tracking resistance, thereby decelerating the aging process or minimizing damage to the surface. Moreover, experimental results showed that SiR/EPDM filled with 1 Vol% of the Al2O3 nanofiller had an improved tracking time performance than the SiR/EPDM filled with 2 Vol% of the TiO2 nanofiller had. The tracking time and thermal conductivity characteristics of both nanocomposites were higher than those of the unfilled SiR/EPDM. [ABSTRACT FROM PUBLISHER]

A reliable thermal interface material (TIM) typically has a low storage modulus ( \textG^\prime ) and a tan( $\delta$ ) less than one; these mechanical properties are crucial for minimizing contact resistance ( \mathbf {R}_{\mathbf {c}} ). This paper demonstrates the correlation between storage modulus ( \textG^\prime ) and the crosslink density of silicone gels. Formulations span between 0.30 and 1.00 hydride to vinyl (H:Vi) ratios for the vinyl terminated and hydride pendent polymers used in the gel. These gels are tailored for TIM applications by adding highly conductive particles to improve thermal conductivity. During this study, when 80 wt% conductive filler was added to gels with a 0.6 H:Vi ratio, the \textG^\prime increased by 6025%. In order to minimize \textG^\prime , the crosslinker in the formulation was replaced with a chain extender. With this formula substitution, the modulus of the filled gel can be successfully lowered to the original value of the unfilled gel while maintaining the crucial properties of a reliable TIM. In effect, it is possible to independently optimize the thermal conductivity of a TIM while creating a compliant gel that maintains surface contact and minimizes contact resistance. [ABSTRACT FROM AUTHOR]

The thermal conductivity between winding and lamination is one of the most critical parameters in electrical machine thermal analysis. Its value depends on the insulation material and on the manufacturing process of the winding system, and for this reason cannot be computed using analytical equations. In this paper, a practical investigation of equivalent thermal conductivity of electrical machines stator winding is presented and discussed. The fully experimental approach is based on a fast transient thermal test to evaluate the winding equivalent thermal conductivity. The test campaign is carried out on two sets of Total Enclosed Fan Cooled (TEFC) industrial induction motors produced by two different manufacturers; the rated power of the devices under test ranges from 1.5 to 55 kW. [ABSTRACT FROM PUBLISHER]

In this paper, we propose a new method for estimating the peak junction temperature and thermal resistance in modern heterojunction bipolar transistors (HBTs). The proposed method uses the temperature dependence of thermal conductivity of the material. The method is analytic in nature and does not require any iteration as opposed to the existing state-of-the-art model. This analytic method can easily include the available scaling relations relevant to specific technology to estimate the junction temperatures and thermal resistances of the corresponding transistors. The analytic model is tested against iterative self-consistent solutions for simple structures without any trench isolation and for structures corresponding to the ST Microelectronics B9MW technology that includes shallow and deep trench isolations. The model is slightly modified in order to include the effects from the back-end-of-line metal layers. The resulting analytic model is validated against the measured results for silicon germanium HBTs fabricated in ST Microelectronics B9MW technology. [ABSTRACT FROM PUBLISHER]

This work explores the use of p-diamond back-barriers (BBs) and cap layers to enhance the performance of GaN-based high electron mobility transistors (HEMTs). Diamond can offer a heavily doped p-type layer, which is complementary to GaN electronics. Self-consistent electrothermal simulations reveal that the use of p-diamond BBs and cap layers can increase the breakdown voltage of GaN-based HEMTs by fourfold, at the same time that they enhance the 2-D-electron-gas confinement and reduce short channel effects. These results highlight that p-diamond layers can improve the performance of GaN HEMTs for high-power and high-frequency applications beyond the thermal improvements pursued until now. [ABSTRACT FROM PUBLISHER]

Thermal performance characteristics of fin-shaped FETs (FinFETs) are studied and analyzed in this paper for sub-22-nm technologies using the well-calibrated TCAD simulations. In this paper, we show that bulk FinFETs have a relatively better thermal performance as compared with SOI FinFETs. In order to understand the isothermal characteristics of these devices because of thermal effects, we use pulse rise-time as well as ac conductance methods. We demonstrate that the ac conductance method fails to accurately capture thermal time constants for FinFETs, as self-heating and gate resistance regions are indistinguishable from each other. A pulse rise-time method gives isothermal characteristics of these devices. As verified from both these independent techniques, only at high frequencies (>1 GHz), FinFETs show a suppression of thermally induced degradation, which can be attributed to their higher surface-to-volume ratio. It is observed that bulk FinFETs will perform better than SOI FinFETs for small effective fin heights. However, as we show in this paper, increased body doping in bulk FinFETs will increase the self-heating effects. Channel length scaling in FinFETs, which shows a decrease in drain current degradation with heating, will be an important design parameter for sub-22-nm devices. [ABSTRACT FROM AUTHOR]

Liquid helium is used as the coolant for high field superconducting magnets or samples tested in the high magnetic field at the National High Magnetic Field Laboratory. When the magnetic field is ramping up, the boil-off helium gas generated by heat losses in the coil and from external environment of the cryostat can be trapped in the liquid helium bath in a region where \Bz\cdot\dBz/dz is less than $-$2100 \T^2/\m due to the weakly diamagnetic property of helium. It results to the surfaces of the magnet or samples being covered partially or even fully with the accumulated helium gas instead of liquid helium. The very low thermal conductivity of helium gas may cause an inefficient cooling for the magnets or samples. In the beginning stage of YBCO coils development, the trapped gas bubble caused a temperature increase in the region of trapped gas bubble and prevented the test coils ramped up to a higher current. This raised an issue for the cooling of the 32-T magnet, which is to be cooled in a liquid helium bath, because the trapped gas bubble may cause inefficient cooling of the magnet in this gas bubble region. Relevant tests were performed, and the trapped gas bubble was observed in recent tests of the 32-T magnet prototype coils. The impact of the gas bubble on the cooling of the 32-T YBCO prototype coils was tested, and the test results are presented. [ABSTRACT FROM PUBLISHER]

Transformer oil is widely used in electric transformers serving the dual functions as insulating medium and cooling material. However, safety and stability of power system is interrupted due to the operation failure of electric transformers in certain case. Therefore efforts should be focused on improving the property of transformer oil for the stable operation of insulating system. The purpose of this paper is to obtain a new type of nanomodified transformer oil with improved dielectric and thermal properties. Boron nitride (BN) nanoparticles with high thermal conductivity were dispersed into transformer oil to form the nanooil. Dielectric and thermal properties of the samples before and after modification were measured. Dielectric property experiments were performed in accordance with IEC standard. Discharge phenomenon in breakdown process was captured by a high speed CCD camera. In order to investigate the effect of BN nanoparticles on thermal property, temperature change and heat distribution of the samples in heat transfer process were measured by an infrared thermal imager and a temperature sensor. It is found that the nanooil has higher dielectric breakdown strength and lower dissipation factor which indicate better dielectric property compared with nonmodified oil. Obtained results show significant improvement in heat transfer process with increasing the concentration of BN nanoparticles. It is proposed that the nanoparticles create an interfacial region which contributes to the enhancement of dielectric strength. For thermal property the ballistic phonon transport of the nanoparticles acts as the key factor in the remarkable improvement. Furthermore, the improved thermal property of the nanooil has an inhibitory effect on the breakdown process according to the bubble theory. [ABSTRACT FROM AUTHOR]

Electrical power switching devices based on widebandgap (WBG) semiconductors have the potential for transformative impact on a wide range of energy conversion applications. Significantly improved electrical and thermal conductivities of WBG semiconductors compared with the semiconductor silicon have the potential for more efficient, compact, and robust power conversion systems. However, to offset inherently higher manufacturing cost of WBG power devices and obtain system-level benefits, power converters need to be operated at higher semiconductor chip junction temperatures and/or at higher switching frequencies. This paper discusses the future prospects of WBG-based power electronics by considering the current state of the art of WBG chip manufacturing, packaging, and thermal management technologies. [ABSTRACT FROM AUTHOR]

Heat exchangers are important facilities used in data center cooling systems. The effectiveness of heat exchangers strongly influences the thermal performance of cooling systems. Rear door heat exchangers, in-row, overhead coolers, and fully contained cabinets are some examples of executing liquid and hybrid cooling systems used in data centers. A liquid to liquid heat exchanger is an important component of the coolant distribution unit, which supplies chilled water to the aforementioned heat exchangers. Computer room air handler units are also typically liquid to air cross flow heat exchangers. It is important to characterize the dynamic behavior of these heat exchangers for the thermal management of data centers, to improve the design of control strategies, and energy efficiency. The transient response of a 2-D unmixed–unmixed cross flow heat exchanger, with a finite wall capacitance, is investigated. The transient effectiveness concept is used to build the mathematical models that will be used to solve and analyze the transient problems involving cross flow heat exchangers. Several transient cases are investigated numerically for step, ramp, and exponential variations in the inlet temperature of the minimum capacity rate fluid, as well as step and ramp changes in the flow rate of hot and cold fluids. The transient effectiveness characteristics are tested in parametric studies. The results obtained show that transient effectiveness is a useful method of analyzing and predicting the dynamic performance of cross flow heat exchangers. [ABSTRACT FROM PUBLISHER]

We have investigated the impact of surface recombination on the effective carrier injection efficiency and the Joule heating of axial InGaN/GaN nanowire light-emitting diodes (LEDs). The results reveal that the carrier injection efficiency of such devices is extremely low (<;10%), due to the severe carrier loss through nonradiative surface recombination. It is further observed that the thermal resistance of typical nanowire LEDs is comparable with, or lower than that of their planar counterparts, in spite of the reduced thermal conductivity of nanowires. The poor carrier injection efficiency, however, leads to significantly elevated junction temperatures for nanowire LEDs. We have further demonstrated, both theoretically and experimentally, that the carrier injection efficiency can be significantly improved in p-doped nanowires, due to the downward surface band bending, and in InGaN/GaN/AlGaN dot-in-a-wire core-shell nanoscale heterostructures, due to the superior carrier confinement offered by the large bandgap AlGaN shell. This paper offers important insight for the design and epitaxial growth of high-performance nanowire LEDs. [ABSTRACT FROM AUTHOR]

Temperature rise and thermal spreading resistance in multilayered structures are an important research topic in several branches of the thermal-fluid sciences, including thermal management of electronics and contact resistance. Previous work in developing analytical solutions for the temperature rise and thermal spreading resistance has been limited to relatively few layers and simple conditions at the interfaces. Recent development of multilayer epitaxial structures for high power electronics has led to the need for more general and flexible analytical solutions because numerical methods, such as the finite element method, are often computationally inefficient. This paper presents a closed-form, analytical solution for the temperature distribution in a rectangular structure with rectangular isoflux heat sources, any number of layers of arbitrary thermal conductivity, and perfect interfacial contact or finite interfacial conductance. Extensions are also presented for convective boundary conditions in the source and sink planes. The proposed analytical solution is demonstrated and validated to study gallium nitride (GaN)-based epitaxial structures in realistic device configurations. The capability to explore the parametric space in a computationally efficient manner provides the ability to understand the key dependencies of the temperature rise on the properties of the structure, such as the substrate thickness in GaN-on-diamond epitaxial structures. Our results show that reduction of the thickness of diamond substrates may actually increase the device temperature in a realistic device configuration due to the importance of thermal spreading within the first \sim100~\mum of the heat source. [ABSTRACT FROM AUTHOR]

This study investigated the effect of various ceramic micro and nano fillers on the different properties (dielectric and thermal) of a polyethylene (PE) matrix. The fillers used were alumina (Al2O3) and boron nitride (BN). The polymer composites were prepared by ball milling and thick films containing wt. 50% of the inorganic phase were pressed. The encapsulation of the inorganic additives into the PE matrix was successfully achieved. The real part of the dielectric permittivity of a microcomposite containing Al2O3 was found to be close to that of the PE matrix value whereas the losses remained quite low. Thermal conductivity of PE was greatly improved: it increased by about 50% irrespective of the nature and size of the fillers. When the filler was nano BN, the breakdown strength was found to be slightly reduced by only 5%. This could be due to the size of the particles or to the high percentage of additives and the interface behavior of the fillers/matrix. Improvement of the PE surface resistance to discharges was achieved, specially with using nano-BN. [ABSTRACT FROM AUTHOR]

Many factors affect the loading of the underground power cables, such as ambient temperature, depth of laying of cable, number of parallel circuits of cable, and thermal resistivity of the surrounding soil. One important factor, usually ignored, is the formation of dry zones around the underground power cables. Dry zones are usually formed around underground power cables under loading conditions due to the migration of moisture within the soil. In this paper, the effect of the formation of the dry zone on the ampacity of underground power cables is investigated. The derating factor for the formation of dry zones around underground power cables is suggested and calculated for different types of natural backfill soil. IEC 60287-1-3 is taken as a reference. Experimental work was conducted to study the phenomena of the dry zone as related to different types of soils. [ABSTRACT FROM AUTHOR]

Abstract-Within cryogenic applications and the use of superconductors, efficient current leads are needed. Especially the circumstance to feed electrical current from a source at room temperature into the cryogenic system, while minimizing the heat load, is problematic. These special conductors have to be tailored in electrical and thermal properties by their material composition. In this work properties of stacked Bi-2223 superconducting tapes for current lead application are characterized, like critical current under applied tensile load, thermal properties as thermal conductivity and thermal expansion. The results are used to investigate the influence of the different material components on the performance of the conductor. [ABSTRACT FROM AUTHOR]