Thermal Boundary Resistance

Electric-field-induced heating is studied using noise measurements in n-type GaN grown on sapphire substrates. The measured electron temperature is found to be an order of magnitude higher than what is expected based on calculations of electron–phonon coupling via acoustic deformation potential scattering processes in GaN. The discrepancy may be explained by a large thermal boundary resistance between the GaN film and the sapphire substrate.

Achieving robust mechanical and thermal properties in GaN-on-diamond is critically important for reliable next-generation high-power electronics based on this material system. The work described here demonstrates that excellent stress management and interfacial strength has been achieved, as well as homogeneous interfacial thermal properties across the wafer for the latest GaNon-diamond technology. INTRODUCTION Over the past decade significant effort and progress has been made to integrate diamond with GaN-based electronics for improved thermal management. To date, GaN-ondiamond HEMTs have been demonstrated with outstanding performance for high-power microwave device applications [1,2]. Of the various GaN-diamond integration technologies, the Element-Six approach considered in this work involves removing the substrate from a standard GaN-on-Si wafer, followed by the deposition of a thin dielectric layer and the subsequent growth of CVD diamond on the exposed GaN surface. Stress mana...

Reliable thermophysical property values of thin films are important to develop advanced industrial technologies such as highly-integrated electric devices, optical disks, magneto-optical disks, and thermoelectric devices. In order to meet this requirement, the National Metrology Institute of Japan of the National Institute of Advanced Industrial Science and Technology (NMIJ/AIST) has developed thermoreflectance methods by picosecond pulse heating and nanosecond pulse heating under the same geometrical configuration to the laser flash method which is the standard method to measure thermal diffusivity of bulk materials. These light pulse heating methods observe one-dimensional heat diffusion across well-defined length of the specimen thickness. Thermal diffusivity values across thin films were measured with small uncertainty. Since the geometry is very simple, thermal diffusivity can be determined reliably with uncertainty evaluation based on Guide to the Expression of Uncertainty in ...

Thermoreflectance methods by picosecond pulse heating and by nanosecond pulse heating have been developed under the same geometrical configuration as the laser flash method by the National Metrology Institute of JAPAN, AIST. Using these light pulse heating methods, thermal diffusivity of each layer of multilayered thin films and boundary thermal resistance between the layers can be determined from the observed transient temperature curves based on the response function method. The measurement results of various thin films as transparent conductive films used for flat panel displays, hard coating films and multilayered films of the next generation phase-change optical disk will be presented.

There has came into existence some practical instruments to measure the thermophysical properties of thin films, and those instruments have been recognized to be of benefit to R&D in the thin film technology. However, there are no systems to calibrate such instruments by standards that are guaranteed to trace up to national standards or reference materials. Accordingly, NMIJ/AIST has established a high speed laser flash apparatus as a national standard in Japan. The apparatus can measure the thermal diffusivity of thin films normal to the surface with small uncertainty using a thermoreflectance technique. Based on this apparatus, a calibration service of the thermal diffusion time for user's specimen has started at the end of 2007 FY. We have moved to develop a thin film reference material for thermal diffusivity, since the reference material is more useful to calibrate the practical instrument. The reference material is made of a titanium nitride (TiN) thin film with 700 nm in thickness. The TiN film was deposited on a synthesized silica wafer (76.2 mm in diameter, 0.525 mm in thickness, Shin-Etsu Chemical Co., Ltd.) using a reactive dc magnetron sputtering. The target metal is pure Ti and the sputtering gas is pure N 2 gas. The TiN film on the wafer was patterned with each individual chips using a photo-resist and metal-resist coatings by lithography technique. The chip is 10mm x 10 mm, and an etched line of 1 mm x 100 µm is positioned at the center of the film. This etched line is for a surface profiler in order to measure the film thickness. The thermal diffusion time of a typical film is 145.9 × 10 -9 s, and its expanded uncertainty is 3.6 %. We are planning to establish the thin film reference materials ｂｙ end of 2008 FY in Japan.

Measurements of the Kapitza conductance o. z made using a picosecond optical technique at tempera- tures between 50 and 300 K are presented for interfaces between metals and dielectrics. The Debye tem- peratures 8D of the metals (Pb, Au, Al, and Ti) were in the range from 102 to 426 K, while those of the dielectrics (BaF2, sapphire, and diamond) varied from 287 to 2200 K. Conductances measured between materials with OD differing by less than about a factor of 5 were found to be in reasonable agreement with calculations based on a lattice dynamical theory. However, for more widely mismatched solids the measured conductances were found to be greatly in excess of the lattice dynamical calculations. In some of these cases the conductances exceeded even the phonon radiation limit, indicating that much of the heat flow between the solids was via an inelastic channel. It was demonstrated experimentally that the inelastic channel does not involve an interaction between the phonons in the dielectric and electrons in the metal. We show that the anomalously large conductance can be understood in terms of a model in which the anharmonicity of the metal plays an important role.

We report new measurements in four cells of the thermal boundary resistance R between copper and 4 He below but near the superfluid-transition temperature T λ. For 10 −7 ≤ t ≡ 1 − T /T λ ≤ 10 −4 fits of R = R0t x b + B0 to the data yielded x b ≃ 0.18, whereas a fit to theoretical values based on the renormalization-group theory yielded x b = 0.23. Alternatively, a good fit of the theory to the data could be obtained if the amplitude of the prediction was reduced by a factor close to two. The results raise the question whether the boundary conditions used in the theory should be modified.

We report new measurements in four cells of the thermal boundary resistance R between copper and ^4He below but near the superfluid-transition temperature T_lambda. For 10-7 <= t ≡ 1 - T/T_lambda <= 10-4 fits of R = R0 t^xb + B0 to the data yielded xb ~= 0.18, whereas a fit to theoretical values based on the renormalization-group theory

We report new measurements in four cells of the thermal boundary resistance R between copper and 4 He below but near the superfluid-transition temperature T λ. For 10 −7 ≤ t ≡ 1 − T /T λ ≤ 10 −4 fits of R = R0t x b + B0 to the data yielded x b ≃ 0.18, whereas a fit to theoretical values based on the renormalization-group theory yielded x b = 0.23. Alternatively, a good fit of the theory to the data could be obtained if the amplitude of the prediction was reduced by a factor close to two. The results raise the question whether the boundary conditions used in the theory should be modified.

WARDLE, MIT, UNIS. OF STAN-FORD, OKLAHOMA, TOKYO TEAM -It is very significant to experimentally and numerically examine thermal properties of aligned CNT polymeric nanocomposites (PNCs) with variable volume fraction (vol%) and controlled morphology. MWNTs having 200-1000um length synthesized by CVD are densified mechanically to achieve 1-20vol% Thermal conductivities of MWNT-epoxy along the MWNT axis with different vol% are measured by the temperature gradient with an infrared microscope. The developed random walk model with taking into account the thermal boundary resistance (TBR) at the CNT-epoxy and/or CNT-CNT interface is validated by experimental results The different vol% and CNT aligned in PNCs allows extracting the TBR values between MWNTs and epoxy or even between MWNTs by combining the developed model and experiment results. Further numerical investigation is conducted to compare systematically the thermal conductivity of both MWNT-and SWNT-epoxy with different CNT orientations under the wide range of the CNT vol% with different TBR between the CNTs and the CNT-matrix and various inter-CNT contact degrees.

Two electrical methods and one optical method are used, in order to measure the temperature of the hot spot of a 150nm, 2x150µm GaN HEMT. One of the electrical methods is based on Gate Resistance Thermometry (GTR) [1] and the other one is based on the On State resistance Ron obtained from Id(Vgs,Vds) drain current characterization [2]. The optical one uses the reflectivity variation principle to measure the surface temperature and is based on CCD-thermoreflectance [3]. The measurement results are supported by nonlinear simulation, using Finite Element Method (FEM). The use of the GTR electrical method allows a characterization of the temperature along the gates close to the hot spot region and averages the temperature over the gate width. This method demonstrates a relative linearity in the variation of thermal resistance as function of the applied power dissipation and notably a higher thermal resistance value compared to the conventional way of RON extraction. We may notice that with the On State resistance, the average is realized over the Source-Drain length. The optical method based on the ThermoReflectance (TR) allows a measurement of the surface temperature of the component at the hot spot. The values obtained are consistent with those found by electrical methods and FEM simulation up to 10W/mm.

In this paper an analysis of thermal behavior of microwave power AlGaN/GaN HEMTs has been carried out through pulsed current-voltage (PIV) measurements and small signal [S] parameters. A special care about trapping effects has been followed where it is shown that the thermal resistance of the device can be accurately determined provided that some assumptions on the trapping behavior of the device are verified. The values obtained have been checked by three dimensional finite element (3D-FE) simulations. Finally, the thermal boundary resistance (TBR) between GaN/SiC has been extracted and compared to literature. The results we have obtained are in line with what can be found. In addition to this first set of results, a second 3D-FE analysis of a larger transistor has been performed and checked thanks to liquid crystal measurements.

We have measured shifting of the energy gap of a Nb Josephson junction under direct optical illumination. The response is linear with optical input power over more than 5 case we expect the orders of magnitude, and is nearly independent of temperature from 4 to 8 K. The rise time of this signal is faster than the 2 * dp e (iT ps rise time of the chopped light signal. These direct signals are 500 to 1500 times larger than those obtained when the where e is the same optical power is focused elsewhere on the substrate. function. This enhanced direct response is interpreted as resulting from thermal isolation of the Josephson junction from the substrate due to thermal boundary resistance. in these experiments, we response due to constant i dV-2 dA R~ charge* a Junction Illumination

In dealing with transport in composites systems, high contrast materials pose a special problem for numerical simulation: the time scale or step size in the high thermal conductivity material must be much smaller than in the low conductivity material. High conductivity inclusions can be treated as having an infinite conductivity, removing the need to model transport within the high conductivity inclusions. We develop a random walk algorithm to model thermal transport in composites with high conductivity. We observed the standard random walk algorithm leads to non uniform temperature distribution at the vicinity of the high conductivity inclusion violating the second law of thermodynamics. We show how a standard random walk algorithm can be altered to improve speed while still preserving the second law of thermodynamics. We demonstrate the algorithm in 1D and 3D systems.

The thermal boundary resistance between copper and superfiuid helium was measured near Tx for heat fluxes Q from 0.04 to 130pW/cm ~ in three cells of similar construction. In the low-power limit the results are consistent with the singularity reported by Duncan et al.. The excess finite-power contribution to the boundary resistance near Tx varies significantly for different cells.

In this paper, the assumptions implicit in Leveque's approximation are re-examined, and the variation of the temperature and the thickness of the boundary layer were illustrated using the developed solution. By defining a similarity variable the governing equations are reduced to a dimensionless equation with an analytic solution in the entrance region. This report gives justification for the similarity variable via scaling analysis, details the process of converting to a similarity form, and presents a similarity solution. The analytical solutions are then checked against numerical solution programming by FORTRAN code obtained via using Runge-Kutta fourth order (RK4) method. Finally, others important thermal results obtained from this analysis, such as; approximate Nusselt number in the thermal entrance region was discussed in detail.

We studied the thermal conductivity of superfluid 3 He in a 2.5 mm effective diameter and 0.15 m long channel connecting the two volumes of our experimental assembly. The main volume contained pure solid 4 He, pure liquid 3 He and saturated liquid 3 He-4 He mixture at varying proportions, while the separate heat-exchanger volume housed sinter and was filled by liquid 3 He. The system was cooled externally by a copper nuclear demagnetization stage, and, as an option, internally by the adiabatic melting of solid 4 He in the main volume. The counterflow effect of superfluid just below the transition temperature T c resulted in the highest observed conductivity about five times larger than that of the normal fluid at the T c. Once the hydrodynamic contribution had practically vanished below 0.5T c , we first observed almost constant conductivity nearly equal to the normal fluid value at the T c. Finally, below about 0.3T c , the conductivity rapidly falls off towards lower temperatures.

Measurement of the thermal boundary conductance (TBC) by use of a nondestructive optical technique, transient thermoreflectance (TTR), is presented. A simple thermal model for the TTR is presented with a discussion of its applicability and sensitivity. A specially prepared sample series of Cr, Al, Au, and Pt on four different substrates (Si, sapphire, GaN, and AlN) were tested at room temperature and the TTR signal fitted to the thermal model. The resulting TBC values vary by more than a factor of 3 0.71×108-2.3×108 W/m2 K. It is shown that the diffuse mismatch model (DMM) tended to overpredict the TBC of interfaces with materials having similar phonon spectra, while underpredicting the TBC for interfaces with dissimilar phonon spectra. The DMM only accounts for diffuse elastic scattering. Other scattering mechanisms are discussed which may explain the failure of the DMM at room temperature.

In this paper, the assumptions implicit in Leveque's approximation are reexamined , and the variation of the temperature and the thickness of the boundary layer were illustrated using the developed solution. By defining a similarity variable, the governing equations are reduced to a dimensionless equation with an analytic solution in the entrance region. This report gives justification for the similarity variable via scaling analysis, details the process of converting to a similarity form, and presents a similarity solution. The analytical solutions are then checked against numerical solution programming by FORTRAN code obtained via using Runge-Kutta fourth order (RK4) method. Finally, others important thermal results obtained from this analysis, such as; approximate Nusselt number in the thermal entrance region was discussed in detail.

This article aims at drawing strategic priorities of the main researches challenges related to carbon nanotubes (CNTs) materials and their related polymer nanocomposites. The fundamental structure/property correlations, manufacturing techniques and applications are discussed in-depth. Many reported breakthroughs in terms of CNT's dispersion in polymer matrices, are amongst the highlighted promising iteration toward the true

The objective of this thesis is to master quantitative aspects when using nearfield thermal microscopy by using the scanning thermal microscopy technique (SThM). We start by taking an in-depth look into the work performed previously by other scientist and research organizations. From there, we understand the progress the SThM probes have made through the decades, understand the probe sensitivity to the range of conductivity of the materials under investigation, verify the resistances encountered when the probe comes in contact with the sampl and the applications of SThM.Then we look into the equipment necessary for performing tests to characterize material thermal properties. The SThM we use is based on atomic force microscope (AFM) with a thermal probe attached at the end. The AFM is described in this work along with the probes we have utilized.For the purpose of our work, we are only using thermoresistive probes that play the role of the heater and the thermometer. These probes al...

Aligned carbon nanotube morphogenesis predicts physical properties of their polymer nanocomposites Tomography-driven three-dimensional morphology of aligned carbon nanotube (CNT) polymer nanocomposites is utilized with modelling and simulation to elucidate the underlying mechanisms governing their electrical, thermal, and mechanical behaviour. We fi nd that elastic modulus is governed by local CNT curvature evolution with packing proximity and that transport properties scale with number of CNT-CNT junctions but to diff erent degrees: thermal conductivity increases linearly and electrical conductivity to the ½ power with CNT-CNT junction density, respectively. These fi ndings could enable independent property tuning of these materials for a variety of high value applications.

Economical bolometers, using YBaCuO thin films sputtered on polycrystalline zirconia substrates, have been fabricated and tested at h =10 pm with a cw C02 laser. A 2D thermal model for the bolometric response has been developed to include the film to substrate thermal boundary resistance, and take into consideration the various heat flow regimes inside the film and the substrate. This model has been found successful to predict both the amplitude and phase components of the bolometer response with respect to the modulation signal frequency. In particular, the amplitude response, which contains a succession of f-I andplJ2 sections separated by well-identified corner frequencies, could be quantitatively predicted in the 0.5 Hz to 100 kHz experimental frequency range.

The implementation of thermal barriers in thermoelectric materials improves their power conversion rates effectively. For this purpose, material boundaries are utilized and manipulated to affect phonon transmissivity. Specifically, interface intermixing and topography represents a useful but complex parameter for thermal transport modification. This study investigates epitaxial thin film multilayers, so called superlattices (SL), of TiNiSn/HfNiSn, both with pristine and purposefully deteriorated interfaces. High-resolution transmission electron microscopy and X-ray diffractometry are used to characterize their structural properties in detail. A differential 3 ω -method probes their thermal resistivity. The thermal resistivity reaches a maximum for an intermediate interface quality and decreases again for higher boundary layer intermixing. For boundaries with the lowest interface quality, the interface thermal resistance is reduced by 23% compared to a pristine SL. While an uptake of...

Measurements of the Kapitza conductance o. z made using a picosecond optical technique at temperatures between 50 and 300 K are presented for interfaces between metals and dielectrics. The Debye temperatures 8D of the metals (Pb, Au, Al, and Ti) were in the range from 102 to 426 K, while those of the dielectrics (BaF2, sapphire, and diamond) varied from 287 to 2200 K. Conductances measured between materials with OD differing by less than about a factor of 5 were found to be in reasonable agreement with calculations based on a lattice dynamical theory. However, for more widely mismatched solids the measured conductances were found to be greatly in excess of the lattice dynamical calculations. In some of these cases the conductances exceeded even the phonon radiation limit, indicating that much of the heat flow between the solids was via an inelastic channel. It was demonstrated experimentally that the inelastic channel does not involve an interaction between the phonons in the dielectric and electrons in the metal. We show that the anomalously large conductance can be understood in terms of a model in which the anharmonicity of the metal plays an important role.

A micro-structurally informed two scale unit cell analysis has been presented for the prediction of thermal conductivities of 3D hybrid and 4D in-plane carbon/carbon composites. Realistic micro-structural features such as bundle cross sections, alignments, and macro voids have been included directly in the unit cells geometry through reconstructed X-ray computed tomography images of the composite. Asymptotic homogenization technique has been used on both scales (bundle and composite) along with periodic boundary conditions to obtain effective thermal conductivities. The imperfectly bonded conditions at the fiber/matrix and fiber bundle/matrix interfaces have also been modeled by considering the contact and equivalent gap conductance between their surfaces. The model has been validated through the experiment conducted in the in-plane direction of 3D hybrid composite. Finally, the effective thermal conductivities of 3D hybrid composite in out-of-plane directions and of 4D in-plane composite in x, y, and z-directions have been predicted for full temperature range (300-1500K).

Achieving robust mechanical and thermal properties in GaN-on-diamond is critically important for reliable next-generation high-power electronics based on this material system. The work described here demonstrates that excellent stress management and interfacial strength has been achieved, as well as homogeneous interfacial thermal properties across the wafer for the latest GaNon-diamond technology.

The effect on the thermal parameters of superconducting transition edge bolometers produced on a single crystalline SrTiO3 (STO) substrate with and without a CeO2 buffer layer was investigated. Metal organic deposition was used to deposit the 20 nm CeO2 buffer layer, while RF magnetron sputtering was applied to fabricate 150 nm thick superconducting YBa2Cu3O 7−δ (YBCO) thin film. The critical transition temperature for both of the YBCO films was 90 K and the transition width was ∼1.9 K. The bolometers fabricated from these samples were characterized with respect to the voltage phase and amplitude responses, and the results were compared to that of simulations conducted by applying a one-dimensional thermophysical model. It was observed that adding the buffer layer to the structure of the bolometer results in an increased response at higher modulation frequencies. Results from simulations made by fitting the thermal parameters in the model with and without an additional CeO2 layer were found to be in agreement with the experimental observations.

We report on the response of a monolithic high Tc transition-edge bolometer to about 3-mm-wave for the first time. The detector structure consisting of 400 nm YBa2Cu3O7−x (YBCO) film on buffered Yttria Stabilized Zirconia substrate without any coupled antenna, shows bolometric type responsivity to the 3-mm-waves radiation at its transition temperature. The YBCO thin film and Ce0.9La0.1O2 buffer layer are both fabricated by the metal-organic deposition method. The meander line pattern of the bolometer is designed for obtaining maximum absorption and responsivity possible when the polarized radiation of the source is aligned with the pattern. Meander lines are 50 micrometers wide and 1.5 mm long. We have measured amplitude and phase of the response versus modulation frequency of the detector to the linearly polarized 95 GHz source, and the detector was biased at 5 distinct temperatures at the transition corresponding to five different electrical conductivities of the YBCO film. When the meander lines of the device are parallel to the incident beam polarization, the YBCO pattern is speculated to act as a dissipative antenna resulting in higher absorption leading to high magnitude of the response as observed. The results from the measured phase of the response versus modulation frequency are also in agreement with the discussed absorbed mechanism. The absorption of the YBCO pattern is also measured to depend on the electrical conductivity of the YBCO film and our results show that there is an optimum electrical conductivity for having maximum absorption for this detector. Simulation results for this structure confirm the experiments showing that at electrical conductivity value of 1.33 × 10 5 S/m we have the maximum absorption for our device. These observations promise design of versatile THz and millimeter-wave detectors with potentials for applications in medical and security imaging.

Impressive results have been published for GaN-based transistors for large frequency range. Therefore, both single chip and complete amplification system reliability demonstration is becoming an important subject of concern. In this paper a 3000 hour DC life test is described and the last results derived from the data treatment of this test are presented. The transistor parameters show an evolution strictly related to the biasing point. The High Forward Gate Current test does not present any particular degradation of the transistor characteristics. The most important degradation is observed on the drain saturation current during the High Temperature Operating test. The effect of hotcarriers seems to be the main cause for device degradation.

Heat removal from III-Nitride-based devices into a substrate depends also on an acoustic coupling at III-Nitride/substrate interface. We investigate thermal boundary resistance (TBR) and its effects on temperature distribution for GaN layers on Si, SiC, or sapphire substrates. Micro-Raman method is used for the investigation of TBR at the GaN/Si interface while the transient interferometric mapping (TIM) method is used for investigation of GaN/SiC and GaN/sapphire systems. Thermal modeling is used to analyze the experimental data. We found TBR to be ∼7×10−8 m2 K/W for GaN/Si and ∼1.2×10−7 m2 K/W for GaN/SiC interfaces. The role of TBR at the GaN/sapphire interface in the poor heat transfer from GaN to substrate is found to be less important. It is suggested that the substrate cooling efficiency may be improved if fewer defects are present at the interface to the GaN epistructure.

Submitted for the MAR11 Meeting of The American Physical Society Thermal boundary resistance between carbon nanotubes in nanocomposites with Monte Carlo simulations 1 KHOA BUI, BRIAN GRADY, DIMITRIOS PAPAVASSILIOU, The University of Oklahoma-Enhancing the thermal conductivity of composites by incorporating carbon nanotubes (CNTs) has been an area of vigorous research recently. Measurements of the effective thermal conductivity (keff) for CNT-polystyrene composites at high CNT %wt found that the ratio (keff/kpolymer) at high concentration of CNTs is not as good as that at low CNT concentration [1]. It appears that the CNT dispersion pattern becomes worse, resulting in the formation of CNT bundles. In this work, we apply Monte Carlo simulations to investigate the keff at different weight fractions taking into account the bundle size and orientation, as well as the thermal boundary resistance. By validating with the experiment data, we found that the phonon transmission probability at the interface decreases by temperature. In addition, the poor enhancement of keff at high CNT concentration is because of the CNT-CNT contact resistance and because of the bundle geometry itself, which is equivalent to the presence of one low aspect ratio nanotube. References [1] Peters

The effective thermal conductivity (K ef f) of carbon nanotube (CNT) composites is affected by the thermal boundary resistance (TBR) and by the dispersion pattern and geometry of the CNTs. We have previously modeled CNTs as straight cylinders and found that the TBR between CNTs (TBR CN T −CN T) can suppress K ef f at high volume fractions of CNTs [1]. Effective medium theory results assume that the CNTs are in a perfect dispersion state and exclude the TBR CN T −CN T [2]. In this work, we report on the development of an algorithm for generating CNTs with worm-like geometry in 3D, and with different persistence lengths. These worm-like CNTs are then randomly placed in a periodic box representing a realistic state, since the persistence length of a CNT can be obtained from microscopic images. The use of these CNT geometries in conjunction with off-lattice Monte Carlo simulations [1] in order to study the effective thermal properties of nanocomposites will be discussed, as well as the effects of the persistence length on K ef f and comparisons to straight cylinder models.

The electronic properties and electron transport of a sawtooth penta-graphene nanoribbon (SSPGNR) under uniaxial strains are theoretically studied by densityfunctional theory (DFT) in combination with the nonequilibrium Green's function formalism. We investigated the electronic structures and the current-voltage (I-V) characteristics of the SSPGNRs under a sequence of uniaxial strains in range from 10% compression to 10% stretch. In this strained range, carbon atoms still keep a pentagon network, but with the changing bond lengths. The CC bond lengths change almost linearly with the tolerable strain. The value of the band gap of SSPGNRs can be depicted as a parabola under uniaxial strain. Our calculations show that the current is monotonous increase with compressive strain at the same applied bias voltage. In case of tensile strain, the variable rule of the current is different that it increases at first and decrease later. The fundamental physical properties (band structure, I-V characteristic) of SSPGNRs seem to be more sensitive to compressive strain than the stretch strain. The current intensity of the compressive-SSPGNR is by 2 orders of magnitude compared to that of the tensile-SSPGNR at the same strain in range from 6% to 10%. The results obtained from our calculations are beneficial to practical applications of these strained structures in SSPGNRs-based electromechanical devices.

In this article we present an inclusive review of research carried out in the field of phase change heat transfer enhancement. First, we discuss about different kinds of conventional heat transfer enhancement techniques performed in convection heat transfer related heat exchangers. Next, we present the advantages of implementing phase change heat transfer and report a brief introduction to the physics behind the phase change (boiling) heat transfer phenomenon. We present a well explained data about different kinds of enhancement techniques using micro and nano scale structures on heat transfer surface/device to increase the limit of boiling heat transfer. The entire review article is broadly divided into two categories: first the investigation related to fluid flow or transport mechanism over the micro/nano structured surface which is of crucial importance, second is the actual computational and experimental methods to achieve higher heat transfer capability in terms of critical hea...

In this paper, we discuss about various methods adopted by researchers around the world for heat transfer enhancement. Three major categories of heat transfer enhancement techniques viz. annihilation of thermal boundary layer, employment of magnetohydrodynamics flow and enhancement of critical heat flux during boiling are studied in brief. We also present about the advantages as well as drawbacks of each one of them. There are various field of application in the real world where these methods can be utilized, but each has its own limitations which we discuss in the current work. Through this work we put forward a well comprehension of the ongoing issues in heat transfer, thermal management and energy savings which are always the centre of attraction for the researchers from wide-ranging field in engineering. Index Terms-heat transfer enhancement, boiling, thermal boundary layer.

Phonon resistance can dominate the interfacial thermal resistance between hard and soft solids. Using ab initio calculation, molecular-dynamics simulation and the diffuse mismatch model for Si/In as example, we decompose phonon interfacial resistance into boundary and interfacial-region resistances. These show that the interfacial atomic restructuring as well as the cross-boundary interactions reduce the phonon boundary resistance by providing additional transport channels altering their phonon density of states and cause extra interfacial-region resistances due to additional phonon scattering.

The main purpose of the present paper is to study the effect of thirdand fourth-order anharmonicity on the heat conductivity in the high-temperature (i.e., classical or saturation) regime of disordered systems. We show that the fourthand third-order contributions, respectively, enhance and lower the conductivity, which can be understood by a strengthening or weakening of the stiffness of the bonds in the system. On comparing the heat conductivity in percolating and compositionally disordered systems, we find that disorder loosening the structure (percolating systems) favors the effect of anharmonicity.

We develop an analytical model for the thermal boundary conductance between a solid and a gas. By considering the thermal fluxes in the solid and the gas, we describe the transmission of energy across the solid/gas interface with diffuse mismatch theory. From the predicted thermal boundary conductances across solid/gas interfaces, the equilibrium thermal accommodation coefficient is determined and compared to predictions from molecular dynamics simulations on the model solid-gas systems. We show that our model is applicable for modeling the thermal accommodation of gases on solid surfaces at non-cryogenic temperatures and relatively strong solid-gas interactions (εsf ≳ kBT).

We present direct measurements of the thermal resistance between a saturated dilute 3He-4He solution and sintered silver powder between 5 and i50mK. Measurements obtained using different sinter geometries allow us to distinguish the contribution due to the thermal boundary resistance from that of the size-limited thermal resistance of the 3 IIe-4He solution within the pores of the sinter. The thermal boundary resistance per inverse unit volume is found to vary as T-3 and is insensitive to the sinter particle size.

A novel 3ω thermal conductivity measurement technique called metal-coated 3ω is introduced for use with liquids, gases, powders, and aerogels. This technique employs a micron-scale metal-coated glass fiber as a heater/thermometer that is suspended within the sample. Metal-coated 3ω exceeds alternate 3ω based fluid sensing techniques in a number of key metrics enabling rapid measurements of small samples of materials with very low thermal effusivity (gases), using smaller temperature oscillations with lower parasitic conduction losses. Its advantages relative to existing fluid measurement techniques, including transient hot-wire, steady-state methods, and solid-wire 3ω are discussed. A generalized n-layer concentric cylindrical periodic heating solution that accounts for thermal boundary resistance is presented. Improved sensitivity to boundary conductance is recognized through this model. Metal-coated 3ω was successfully validated through a benchmark study of gases and liquids spanning two-orders of magnitude in thermal conductivity.

We determine the thermal conductivities of a, b, and g graphyne nanotubes (GNTs) as well as of carbon nanotubes (CNTs) using molecular dynamics simulations and the Green-Kubo relationship over the temperature range 50e400 K. We find that GNTs demonstrate considerably lower thermal conductivity than CNTs with the same diameter and length. Among a, b, and g-GNTs, g-GNT has the highest thermal conductivity at all temperatures. By comparing the phonon transport properties of GNTs with CNTs, we find that as the fraction of acetylene bonds in the atomic network increases, the population of highenergy optical phonons increases. This enhances phonon-phonon scattering, and reduces the mean free path, adversely affecting the thermal conductivity of GNTs relative to CNTs. Also reducing the thermal conductivity of GNTs relative to CNTs is the considerably lower acoustic phonon group velocities for the former as well as the lower volumetric heat capacity of GNTs. Optical phonons in a-GNT are high in energy (0.26 eV) with a high population number, making them more energetic than the electronic direct band gap and significantly more energetic than the thermal energy at room temperature. Therefore, we suggest a-GNT as a potential candidate for phonovoltaic energy conversion applications.