Thermal Spreading Research Articles

Numerical Model of Thermoelectric Topping Cycle of Coal-Fired Power Plant

Traditional fossil fuel power generation process typically has low efficiency. Large amount of the energy loss in Rankine cycle steam turbines (ST) is due to the temperature difference between the combustion flame temperature ∼2250 K (adiabatic) and the high pressure steam temperature up to 900 K. However, some of this energy can be harvested using solid-state thermoelectric (TE) power generators which are placed into the gap between the flame temperature and the steam temperature that produce additional electrical power. This study investigates the potential placement of TE on water tube wall inside a boiler at a coal-fired power plant. Three-dimensional (3D) numerical model of a simplified TE module is developed, and hot gas temperature and steam temperature from the boiler are used as boundary conditions at the hot side and cold side of the TE. The numerical results are compared with analytical calculations. The 3D effects of the thermal spreading in the TE module are investigated. Parameters such as TE leg cross section area and TE fill factor are examined in order to maximize the electrical power production of the TE without sacrificing the boiler efficiency (i.e., reducing the steam temperature). The study also looks into the various locations inside the boiler that have good potential for TE installation.

Thermal simulation of joints with high thermal conductivities for power electronic devices

The thermal properties of new power modules joined by materials with high thermal conductivities, such as Ag or Cu nanoparticle joints, can differ from those of current modules joined by ordinary solders with low thermal conductivities. However, these properties have not been thoroughly investigated thus far. The overall thermal resistance of a simple simulation module was calculated by the 3-dimensional finite element method to study the correlation between the thermal conductivity of the joint layer and the thermal properties. The calculation results identified an optimal thickness to achieve the minimum thermal resistance when the thermal conductivity of the joint layer is much higher than that of the heatsink. This is presumed to occur because the thermal resistance decreases in the heatsink much more than it increases in the joint layer, owing to the increased uniformity of thermal spreading as the joint-layer thickness increases to the optimal value. This effect of thermal resistance reduction with thickening of the joint layer is seen when the thermal conductivity of the joint layer is sufficiently higher than that of the heatsink and the area of the joint layer is sufficiently smaller than that of the heatsink. The same effect is also expected in an actual module with a joint between a silicon carbide chip and a direct bonded copper substrate. This study reveals that the design concept for power modules should change to preliminarily estimate the optimal thickness to achieve the minimum thermal resistance when the thermal conductivity of the joint layer is much higher than that of the heatsink.

Optimal rectangular microchannel design, using simulated annealing, unified particle swarm and spiral algorithms, in the presence of spreading resistance

This article focuses on the solution of the mathematical model of a rectangular microchannel through three algorithms: SA (Simulated Annealing), UPSO (Unified Particle Swarm Optimization) and SO (spiral Optimization). The criterion of minimum entropy generation was used for constructing the model, which includes the effect of the thermal spreading resistance. This type of thermal resistance occurs whenever a smaller heat source (e.g. the chip) comes in contact with the base of a larger heat transfer element (e.g. the heat sink). This simulation work was split into two stages. The first one considered different scenarios for a silicon heat sink using air as a working fluid. In this case, volume flow rates between 4.0 and 4.5 × 10−3 m3/s exhibited lower values of entropy generation rate. The second stage considered three materials (silicon, aluminum, and copper) and two working fluids (air and ammonia gas). We found, for this particular set of conditions, that entropy generation becomes minimum, for a given material and working fluid, as long as a volume flow rate of 4.5 × 10−3 m3/s is used. Even so, the best practical configuration found was an aluminum heat sink with ammonia gas. For all the simulations, a marked effect due to the spreading resistance was found, typically ignored in real life applications.

Impact of carrier dynamics on the photovoltaic performance of quantum dot solar cells

The study presents a theoretical investigation of the impact of individual electron and hole dynamics on the photovoltaic characteristics of InAs/GaAs quantum dot solar cells. The analysis is carried out by exploiting a model which includes a detailed description of quantum dots (QD) kinetics within a drift-diffusion formalism. Steady-state and transient simulations show that hole thermal spreading across the closely spaced QD valence band states allows to extract the maximum achievable photocurrent from the QDs; on the other hand, slow hole dynamics turns QDs into efficient traps, impairing the short circuit current despite the extended light harvesting provided by the QDs.

Experimental study on the transient thermal characteristics of an integrated deflector under the periodic impingement of a supersonic flame jet

The flow deflector effectively facilitates thermal spreading and the split of high-temperature gas flow from jet impingement. In this study, we developed two samples of integrated deflectors with four layers. The first sample consisted of a composite of silicon carbide reinforced with carbon fiber (C/SiC), carbon aerogel, silica aerogel, and a sandwich corrugated plate. The second sample replaced carbon aerogel with lithium fluoride (LiF), a high-temperature composite material of phase change. Under the periodic impingement of a supersonic flame jet, the transient thermal response of the two deflectors was investigated experimentally. The thermal image of the impinged surface was obtained by an infrared camera. After four periods, the periodic temperature variation in this surface stabilized. The thermal disturbance propagated and attenuated gradually from the impinged surface to the corrugated plate back surface. The back surface temperature of corrugated plate increased monotonically with impingement period without any fluctuation. The entire temperature of the deflector was effectively controlled in the range from 3400K (the total gas temperature) to 480K at the centerpoint of the back surface of the sandwich corrugated plate without incurring thermal damage. The deflector temperature level and the ablation rate of the impingement surface were lowered further using LiF as a thermal shield layer because of its high storage of latent energy.

Study of Phase Change Materials Applied to CPV Receivers

There are lots of factors which can directly affect output efficiency of photovoltaic device. One of them is high temperature which would cause adverse effect to solar cell. When solar cell is operated in high temperature, the cell’s output efficiency will become low. Therefore, improving thermal spreading of solar cell is an important issue. In this study, we focused on finding new materials to enhance the thermal dispreading and keep the temperature of solar cell as low as possible. The new materials are different from conventional metal ones; they are called “phase change materials (PCMs)” which are mainly applied to green buildings. We chose two kinds of PSMs to study their thermal dispreading ability and to compare them with traditional aluminum material. These two kinds of PCMs are wax and lauric acid. We made three aluminum‐based cuboids as heat sinking units and two of them were designed with hollow space to fill in the PCMs. We applied electric forward bias on solar cells to simulate the heat contributed from the concentrated sunlight. Then we observed the thermal distribution of these three kinds of thermal spreading materials. Two levels of forward biases were chosen to test the samples and analyze the experiment results.

Role of side groups and temperature dependent studies in a molecular device

The quantum transport through Tour Wires (TWs) functionalized with different side groups was studied using nonequilibrium Green's function formalism combined with extended Huckel theory. Au---TW---Au junctions were constructed with functional groups $$\hbox {NO}_{2}$$NO2 and $$\hbox {NH}_{2}$$NH2. The transmission spectrum and the isosurface of transmission eigen channel at the HOMO resonance and the LUMO resonance shows that the resonant transmission peaks are related to the delocalized nature of the $$\pi $$?-orbitals of the TWs that was not much affected by the functionalization at room temperature. Furthermore, the influence of the temperature effect on the transport characteristics have been emphasized, and the result shows that for the TW and TW---$$\hbox {NH}_{2}$$NH2 systems conductance increase with increasing temperature indicating the dominating transport mechanism which is due to thermionic emission. The temperature dependence arises from the thermal spreading in the leads but also from a thermal average over the different configurations. In particular, negative differential resistance nature was observed for TW---$$\hbox {NO}_{2}$$NO2 at the temperature of 100 K in the positive and the negative bias region.

Thermal performance of thin loop-type vapor chamber

This experimental study examines the thermal performances of a newly devised thin loop-type vapor chamber (VC) of 0.5mm interior height at the controlled evaporator heat fluxes (q̇) and condenser thermal resistances (Rth,con) with vertical and horizontal orientations. The capillary layer over the entire heated VC endwall is constructed by the 0.05mm deep parallel/staggered square grooves with a copper woven wire mesh sintered above the staggered grooves on the evaporator endwall. The vapor–liquid flow images and their responsive overall thermal resistances (Rth) and effective spreading thermal conductivities (keff) are simultaneously detected with the objective to study the effects of q̇, Rth,con and VC orientation on the vapor–liquid two-phase flow phenomena and the associated heat transfer performances. At various test conditions, the responsive Rth and keff results to the interfacial flow structures, the comparative Rth and keff properties as well as the relative thermal performance improvements from the copper plate for the test VC are examined.

Thermal Spreading with Flat-Plate Oscillating Heat Pipes

The thermal performance of a flat-plate oscillating heat pipe operating as a heat spreader was experimentally investigated. The copper flat-plate oscillating heat pipe had overall dimensions of , possessed two layers of minichannels, and was charged with acetone to a filling ratio of approximately 80%. The flat-plate oscillating heat pipe was centrally heated with a square heat source and had a copper pin-fin heat sink attached to its opposite side for crossflowing air at speeds of either 1, 2, or . The results clearly indicated that the flat-plate oscillating heat pipe is capable of operating in the thermal spreading mode, providing for a 10–15% reduction in total thermal resistance (relative to pure copper) and managing heat inputs up to 230 W () with the maximum surface temperature not exceeding 100°C. In addition, the temperature uniformity of the heat sink was greater when attached to the flat-plate oscillating heat pipe, especially for lower Reynolds numbers (airspeeds). A critical heat input was required to initiate the flat-plate oscillating heat pipe, and this heat input increased with Reynolds number. This study further confirms the heating mode directly affects the heat transfer performance of a flat-plate oscillating heat pipe.

Thermal Spreading Resistance Inside Anisotropic Plates with Arbitrarily Located Hotspots

Graphite-based anisotropic materials are becoming the key component of next-generation cooling systems in electronics and telecommunication industries. Proper use of these materials in the form of thermal spreaders, compared to conventional metallic ones, can significantly reduce the thermal stress and thermal resistance in the system. In this study, a new analytical model for temperature distribution inside anisotropic rectangular plates subjected to multiple sources and sinks on the top and bottom surfaces is presented. All lateral faces are assumed insulated. The solution is first justified for the case with single hotspots on each side and then using the superposition principle, it is extended into the general form to cover multihotspot cases. The model is validated by numerical simulation data and a perfect agreement is observed. Thermal spreading resistance is defined for the anisotropic plate and a comprehensive parametric study for optimization purpose is performed. The influence of both anisotropy and geometrical parameters on the resistance is discussed in detail and critical values are evaluated.

Comparative Effect of Nano-Sized ZrO2 and TiO2 Additional Supports in Silica-Supported Tungsten Catalysts on Performance in Metathesis of Ethylene and 2-Butene to Propylene

The concept of thermal spreading has been adapted in this work to obtain special performance catalyst where the increase of conversion and selectivity were observed beyond the increase of coking. Nano-sized ZrO2 and nano-sized TiO2 were employed as an additional support to be physically mixed with WO3/SiO2 catalysts and heated under thermal condition before being investigated in the metathesis reaction of ethylene and trans-2-butene to produce propylene. The presence of tungsten was found on the separated nano-sized ZrO2 and TiO2 supports evidenced by ICP-OES, confirming thermal spreading of tungsten oxide species. The dispersion of WOx examined by FT-Raman and UV–Vis for WO3/SiO2 catalysts mixed with ZrO2 was better than the ones mixed with TiO2. The nano-sized ZrO2 chosen as an additional support having higher surface energy than the nano-sized TiO2 was shown to provide differentiating the catalytic performances where trans-2-butene conversion could be boosted up together with propylene selectivity with lesser amount of coke even higher acidity.

Transesterification of dimethyl oxalate with phenol over a MoO3/SiO2 catalyst prepared by thermal spreading

MoO3/SiO2 catalysts for the transesterification of dimethyl oxalate (DMO) with phenol were prepared by both the thermal spreading (TS) and incipient wetness impregnation methods. The results showed that the 10%MoO3/SiO2 catalyst prepared by TS (10%MoO3/SiO2-TS) exhibited higher catalytic performance compared with the 10%MoO3/SiO2 catalyst prepared by incipient wetness impregnation (10%MoO3/SiO2-C). The catalysts were characterized by X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, pyridine-IR spectroscopy, and NH3 temperature- programmed desorption. These analyses indicated that weak Lewis acid sites were formed on the catalyst surfaces and that the Mo species were present as monomeric MoO3 rather than as isolated molybdenum oxide or polymolybdate species on both catalysts, although the 10%MoO3/SiO2-TS exhibited better dispersion of MoO3 and a higher surface Mo content than the 10%MoO3/SiO2-C. Under the optimal transesterification reaction conditions (1.2 g 10%MoO3/SiO2-TS, T = 180 °C, n(DMO)/n(phenol) = 2, t = 4 h), the conversion of phenol was 70.9%, and the yields of methyl phenyl oxalate and diphenyl oxalate were 63.1% and 7.7%, respectively.

A numerical and analytical study of the effect of aspect ratio on the behavior of a round thermal

A round thermal is formed when an element of buoyant fluid is released instantaneously into a quiescent ambient. Although the thermal spreading rate is of primary importance to mathematical modeling, the reported values in the literature vary greatly. To identify possible factors affecting the thermal spreading rate, we investigated the effect of different initial conditions numerically by solving the unsteady Reynolds-averaged Navier–Stokes equations with a two-equation turbulence closure. The initial aspect ratio (i.e. length-to-diameter ratio) of the thermal was varied between 0.125–4.0, and the initial density differences was changed from 1 to 10 %. Results show that the spreading rate is greatly affected by the initial aspect ratio, which also explains the variations in earlier reported values. Following the numerical study, an analytical model using buoyant vortex ring theory is developed to predict the spreading rate of a thermal. The predictions show good agreement with the results from both the numerical simulations and previous experimental studies. Another simple analytical model is also presented to approximate the thermal induced flow, and is validated using the numerical simulations.

Influence of micro- and nano-sized SiO2 excess support on the metathesis of ethylene and trans-2-butene to propylene over silica-supported tungsten catalysts

Silica-supported tungsten catalysts with various tungsten oxide loadings were physically mixed with micro- or nano-sized SiO2 excess supports and investigated in the metathesis of ethylene and trans-2-butene to propylene. Adding nano-sized SiO2 excess support led to higher trans-2-butene conversion and propylene yield than those of micro-one and also removed the induction period. It was indicated from FT-Raman spectra and in situ UV–vis spectra that silica-supported tungsten catalysts mixed with nano-sized SiO2 excess support had the higher ratio of isolated surface tetrahedral tungsten oxide species to WO3 crystal and could be well-dispersed than micro-one. The characterization results of the separated nano-sized SiO2 excess support from the spent catalyst by in situ Raman spectroscopy clearly exhibited the presence of isolated surface tetrahedral tungsten oxide species, the active site precursor of metathesis reaction, on the separated nano-sized SiO2 excess support, confirming the thermal spreading of tungsten from the original catalysts to the nano-sized SiO2 and creating more isolated surface tetrahedral tungsten oxide species.

Thermal spreading resistance characteristics of a high power light emitting diode module

In this study, effects of the dimensions and the thermal conductivity of the substrate on the heat transfer characteristics of a LED module are investigated. The total thermal resistance corresponding to a LED module operating at different power levels is measured using a method following JESD51-1 and JESD51-14 standards. In addition, a finite element method (FEM) numerical simulation is carried out to analyze the heat transfer phenomena in the LED module. It is found that, for the current experimental conditions, the importance of the thermal spreading resistance effect increases with decreasing substrate thickness and/or increasing input power of the LED module, which corresponds to an increase in the total thermal resistance and correspondingly a higher chip temperature. Experimental and numerical results show that the thermal spreading resistance and thus the chip temperature can be reduced by increasing the substrate thickness or by utilizing materials with high lateral thermal conductivities (directionally-dependent) for the substrate. In consequence, for LED modules with the same substrate thickness, using graphite composite to replace aluminum as the substrate material reduces the spreading resistance by nearly 14% in the current study.

Analytical Solution for Temperature Rise in Complex Multilayer Structures With Discrete Heat Sources

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 ~100 μm of the heat source.

Thermal management of electronic devices by composite materials integrated in silicon

As the power of electronic systems is increasing, thermal fluxes are getting higher, up to more than 100W/cm2 in the more critical cases. They result in hot spots with various consequences, especially performance reduction and reliability issues. Most of the prior research has been focused on active liquid cooling and on reducing hot spots by the implementation of thermal interface materials (TIMs) and spreading solutions.The approach presented here is based on the implementation in silicon of nanocomposite structures including carbon nanotubes (CNTs) and phase change materials (PCMs). The simulation model presented here shows how the composite CNTs/PCM structure efficiently reduces the temperature excursion at the silicon surface compared to the implementation of PCM only or a thicker silicon. A fabrication process flow is presented with a special focus on the assembly of silicon top and bottom parts with CNTs. Process conditions are explored to insure mechanical adhesion and thermal contact quality. This thermal interposer concept provides a new solution for thermal management and reliability improvement of devices. It is of great interest for electronic and optical devices, MEMS and 3D integration.

Thermal Analysis and Optimization of 6.4 W Si-Based Multichip LED Packaged Module

Multichip packaging was achieved the best solution to significantly reduce thermal resistance at the same time, to increase luminance intensity in LEDs packaging application. For the packaging, thermal spreading resistance is an important parameter to get influence the total thermal performance of LEDs. In this study, silicon-based multichip light emitting diodes (LEDs) packaged module has been examined for thermal characteristics in several parameters. Compared to the general conventional single LED packaged chip module, multichip LED packaged module has many advantages of low cost, low density, small size, and low thermal resistance. This analyzed module is comprised of multichip LED array, which consists of 32 LED packaged chips with supplement power of 0.2 W at every single chip. To realize the extent of thermal distribution, the computer-aided design model of 6.4 W Si-based multichip LED module was designed and was performed by the simulation basis of actual fabrication flow. The impact of thermal distribution is analyzed in alternative ways both optimizing numbers of fins and the thickness of that heatsink. In addition, a thermal resistance model was designed and derived from analytical theory. The optimum simulation results satisfies the expectations of the design goal and the measurement of IR camera results. tart after striking space key 2 times.

The optimal design of the thermal spreading on high power LEDs

New energy and energy saving are the two basic energy issues. This study proposes an innovative optimal method to design LEDs with high thermal spreading holder. The optimization uses the FEM combined with the simplified conjugated gradient method (SCGM). The minimal temperature of the MCPCB backside of the LEDs is obtained throughout this design. The simulations have been proofed by the experiment of IR and thermal couple measurement. The optimal temperature decreases at about 4°C compared with the original temperature without any active cooling device. This design can prolong the LEDs׳ life to benefit the energy saving obviously. In addition, this design combined with the active cooling device will achieve better heat dissipation on cooling of the electronic component.

Thermal Spreading Analysis of Rectangular Heat Spreader

A unique nondimensional scheme that employs a source-to-substrate “area ratio” (e.g., footprint), has been utilized for analytically determining the steady-state temperature field within a centrally-heated, cuboidal heat spreader with square cross-section. A modified Laplace equation was solved using a Fourier expansion method providing for an infinite cosine series solution. This solution can be used to analyze the effects of Biot number, heat spreader thickness, and area ratio on the heat spreader's nondimensional maximum temperature and nondimensional thermal spreading resistance. The solution is accurate only for low Biot numbers (Bi < 0.001); representative of highly-conductive, two-phase heat spreaders. Based on the solution, a unique method for estimating the effective thermal conductivity of a two-phase heat spreader is also presented.