Solid-liquid phase change materials (PCMs) are attractive candidates for thermal energy storage and electronics cooling applications, but once all the PCMs have completely phase-changed and approximate their thermal storage limit, they will become the bottleneck for heat dissipation on the contrary and the electronics have to stop working. In this paper, a modularized thermal storage unit (MTSU) was proposed to overcome such fatal drawback. Once the PCMs reach their limit, the completely phase-changed MTSU will be replaced by a new one due to the modularization. Such online thermal charging and offline thermal discharging working characteristics enable the continuous working of electronics. The proposed MTSU is fabricated by encapsulating paraffin with epoxy resin, and the paraffin is thermally enhanced via copper or nickel foams. Theoretical and experimental validations reveal that the ETC is increased by 376% via copper foam with the porosity of 95.52%, and by 205% for nickel foam with the porosity of 95.61% due to the relatively lower skeleton thermal conductivity of nickel foam. The cycled test revealed that the proposed MTSU has good thermal stability. Compared with the conventional TSU, the proposed MTSU avoids the slow re-solidification process and exhibits potential for continuous thermal storage over long periods of time. The proposed MTSU is expected to be applied in the field of driving batteries and solar-thermal conversion system. (C) 2018 Published by Elsevier Ltd.

Organic phase change materials (PCMs) have drown continuous attentions over time due to their large latent heat and constant-temperature solid-liquid phase transition with promising applications in thermal energy storage. Nevertheless, they suffer from the relatively low intrinsic thermal conductivity. Filling the PCMs with graphite-nanosheets (GNs) by ultrasonic exfoliating could alleviate this problem, and GNs with longer ultrasonic exfoliation time is reported to possess larger effective thermal conductivity (ETC) monotonously. In this paper, we discover a non-monotonous variation of ETC for the first time, when enhancing the ETC of paraffin with ultrasonic exfoliated GNs. The mechanism behind this phenomenon is explained by the variation of GNs morphologies over time in the paraffin. Experimental results reveal that longer exfoliation time can increase the aspect ratio and ETC, but over a critical time, the bending stiffness of GNs decrease and the particles tend to be folded with increased interfacial thermal resistance and decreased ETC. The ETC as a function of ultrasonic time shows an obvious peak value at similar to 2 min, and the ETC could be increased from 0.3 to 3.0 W/(m.K) at GNs loading of 4 wt% with negligible effect on the phase change characteristics. In addition, the GNs/paraffin composite exhibits the quick thermal response and longer working time. The present non-monotonous discovery reveals the underlying mechanism and provides suggestions on the improvement of ultrasonic exfoliation process.

Phase Change Materials (PCMs) have been widely investigated as a cooling solution due to their significant latent heat capacity. However, the current PCMs generally suffer a low thermal conductivity, thus hindering the application of PCMs. Composite Phase Change Materials (CPCMs) filling with high thermal conductivity materials have been proposed to solve this issue. Nevertheless, the latent heat of the CPCMs decreases with the mass fraction of fillings, thus leading to a lower allowable working time under safe operating temperature. Therefore, an optimal filling mass fraction of CPCMs is in urgent needed to improve the application of CPCMs. In this study, we developed a one-dimensional conduction heat transfer model of CPCMs to predict the optimal filling mass fraction of CPCMs to realize the maximum allowable working time. The filling mass fraction was introduced into the model and the relationship between the thermal conductivity and latent heat was built. We adopted paraffin as the matrix material and Expanded Graphite (EG) as the thermal conductivity enhancer. The allowable working time of the CPCMs as the function of filling mass fraction was obtained. Based on the principle of the maximum allowable working time, the optimal filling mass fraction was calculated. Comparative experiments were also conducted to validate the accuracy of the prediction model. The parameters which affect the maximum allowable working temperature were also investigated, including input heat flux, safe temperature, and height of CPCMs. The results show that a higher heat flux and height requires a larger filling mass fraction, and it's opposite for the safe temperature.

The performance and reliability of downhole electronics will degrade in high temperature environments. Various active cooling techniques have been proposed for thermal management of such systems. However, these techniques require additional power input, cooling liquids and other moving components which complicate the system. This study presents a passive Thermal Management System (TMS) for downhole electronics. The TMS includes a vacuum flask, Phase Change Material (PCM) and heat pipes. The thermal characteristics of the TMS is evaluated experimentally. The results show that the system maintains equipment temperatures below 125 degrees C for a six-hour operating period in a 200 degrees C downhole environment, which will effectively protect the downhole electronics. (C) 2017 Elsevier Ltd. All rights reserved.

Photoluminescence quantum dots (QDs) have been considered as a kind of promising light converting materials with high luminous efficiency, tunable spectrum, and narrow emission. However, the current light conversion efficiency (LCE) of QD films is at a relative low level, which will result in many problems, such as inferior luminous performance, severe self-heating, etc. To enhance the LCE of QD films, SiO2 particles were doped into QD films by physical blending, for their remarkable light scattering effect. The LCE enhancement after adding SiO2 particles were studied by experiments. Experimental results showed that the SiO2 modified QD films improved the LCE up to 63.45%, which was 103.88% higher than conventional QD films. Besides, as the SiO2 particles diameters increased, the optimal mass fractions of SiO2 particles for top LCE would decrease.

The concentrically layered thermal cloaks with isotropic materials could realize the equivalent thermal cloaking effect with Pendry’s cloak,while the effectiveness is scarcely investigated quantitatively.Here we examine the cloaking effectiveness quantitatively by evaluating the standard deviation of the temperature difference between the simulated plane with the layered thermal cloak and Pendry’s thermal cloak.The design rules for the isotropic materials in terms of thermal conductivity and layer thickness are presented.The present method could quantitatively evaluate the cloaking effectiveness,and could open avenues for analyzing the cloaking effect,detecting the（anti-） cloaks,etc.

For light-emitting diode (LED) packages, temperatures, especially junction and phosphor temperature, can greatly affect the optical performance. Thus, measuring the temperatures accurately is of significance for the performance estimation. Among the methods of measuring temperature, physical contact method, represented by thermocouple method, is most convenient and economical In this paper, we measured the junction and phosphor temperature of LED packages by thermocouple. In the experiments to measure junction temperature of a blue LED module, we simplified the judgment by measuring the air temperature above the bare chip and it was found that the test result was much higher than that obtained by another recognized method. The causes were ascribed to two factors, i.e. thermal resistance of the air gap and the light energy absorption. In the experiments to measure phosphor temperature, plug-in measurement was adopted to eliminate the influence of air gap's thermal resistance. However, the tested value turned out to be even higher. Then the main cause of the inaccurate measurement by thermocouple was attributed to the light energy absorption and conversion. And an experiment was carried out to support the explanation. As a whole, to accurately measure the LED's temperatures by thermocouple, the light absorption effect needs to be considered.

Phase change materials (PCMs), due to their extraordinary latent heat storage characteristic, have been widely investigated in the thermal management of electronic devices. However, the current PCM-based heat sinks generally suffer a long heat dissipation time after heat absorption. To address this issue, the modularized thermal storage unit (MTSU) was proposed in this paper. The online charging and offline discharging working modes of the MTSU allow it to cool the intermittent electronic devices with a high duty cycle. In this paper, the MTSU samples were fabricated with paraffin wax as latent heat storage core unit and polymer encapsulation shell. The specific latent heat storage and morphology of the MTSU samples were studied. Afterward, the thermal storage performance of each individual MTSU sample was specially characterized with the sample subjected to the constant heat flux. The MTSU samples showed desirable performance on encapsulating the PCMs and stabilizing the heat source temperature. The safe operation time of the heat source was prolonged by 181% with 16 g of PCM in the vacuum environment. Furthermore, the performance of the MTSU samples on cooling of the electronic devices with a relatively high duty cycle was investigated. The results show that owing to the modularization and replaceability of the samples, the ON/OFF working time ratio of the heat source was greatly enlarged. When cooled by the MTSU sample with 11.42 g of paraffin, the heat source could work with an ON/OFF time ratio up to 6.7 at 4 W and 14.9 at 3 W.

In this paper,experiments were conducted on effects on heat storage performance and heat storage density of paraffin using aluminum plate-fin and copper metal foam.Temperature-time curves of heat source of pure paraffin,aluminum plate-fin and copper metal foam were obtained,respectively.The results show that the temperature difference between heat source and paraffin was reduced by 76％ and 18％,and the average temperature of heat source during heat storage process was reduced by 22.2℃ and 9.5℃ with the use of aluminum plate-fin and copper metal foam,respectively.From that point,aluminum plate-fin is better than copper metal foam.On the other hand,the total heat storage density was reduced by 40.3％ and 4.2％ using aluminum plate-fin and copper metal foam,respectively.From that point,copper metal foam is better than aluminum plate-fin.Therefore.the suitable heat storage enhancement of paraffin is chosen based on the combined effects on heat storage performance and heat storage density.

Conventional studies on the design of a (carpet) thermal cloak are based on the coordinate transformation techniques, which are complicated and hard to realize with natural materials. Here we show another feasible approach to design the carpet thermal cloak through the rotated alternatively stacking materials based on the refraction law of heat flux in analogy to that of light. To make the consequent bending of heat flux parallel to the profile of the cloaking region, three design rules to realize the carpet cloaking effect by such configurations are provided and validated. The boundary condition tolerances of the carpet thermal cloak are examined as well. Copyright (C) EPLA, 2015

In this paper, a self-heating model was developed based on Run Hu's light scattering model to calculate the heat generation and an integrated thermal resistance model was established to predict the temperature of phosphor silicone. It is found that the trends of highest temperature along with the phosphor concentration and particles diameters are both non-monotonic with a wave crest. While the temperature varies monotonically with the changes of thermal conductivity of matrix and thickness of phosphor silicone, separately. Then the physical mechanism behind these phenomena are discussed respectively.

Many high-power electronic devices such as high-power light-emitting diodes and aerial devices work intermittently. If some of the heat generated by the chips could be stored in the thermal storage medium during the working time and then be released to the ambient in the nonworking time, the heat dissipation load of the heat sinks could be diminished and a better thermal characteristic could be achieved. Inspired by this idea, we proposed a thermal storage substrate and investigated its thermal storage properties by experiment in this study. First, the composite phase-change material (CPCM) was prepared as the thermal storage medium. Second, the thermal and phase-transition properties of the CPCM were studied through differential scanning calorimeter tests. Third, the thermal conductivity of the CPCM was measured for the analysis of thermal performance. Afterward, the CPCM-based substrate was fabricated and several experiments were conducted to examine its thermal storage performance. The results showed that the thermal storage substrate could store the heat, as much as 55,773.80 J, which accounts for 32% of the heat generated by the heat source approximately. With so much heat stored in the CPCM, the temperature of the heat source went up much more slowly. To accelerate the heat conduction inside the CPCM, five aluminum pillars were added into the substrate. As a result, the temperature of the heat source and the substrate wall decreased by 3.5°C and 4.5°C, respectively.

For multiple-chip high power LED arrays, a high temperature uniformity among LED chips is the guarantee of optical performance, thermal stability and reliability of the devices. Single-layer microchannel heat sink (SMHS) is a good alternative to reach this goal, but its performance is limited because the water temperature may rise at the downstream of the channel. In this paper, a double-layer microchannel heat sink (DMHS) with counter water flow arrangement was proposed to decrease temperature difference among the LED chips. And a compact thermal model (CTM) was established to model thermal characteristics of the DMHS. By analogizing the thermal resistance to electrical resistance, the equations of the thermal networks were established following the Kirchhoff Current Law (KCL). The DMHS structure was optimized with the pump power fixed. Under the condition of inlet water temperature Tin=20°C, we obtained the case temperature of three chips, which were TC1=32.54°C, TC2=32.81°C and TC3=33.40°C respectively. The maximum temperature difference was 0.86°C among chips. Meanwhile, a CFD numerical simulation was conducted to examine the optimized structure. Simulation results showed that the temperature of three chips were TC1=32.14°C, TC2=32.39°C and TC3=32.52°C respectively, which matched well with the CTM. Compared to the SMHS under the same operating conditions, which achieved a temperature difference of 2.3°C, the DMHS significantly improve the temperature uniformity.

Thermal management for white light-emitting diode (LED) packages is of great importance. Conventional thermal management methods mainly focus on the outside of LED package, but we, in this study, focused on the thermal management inside the LED package. Besides the heat generated in LED chips, we also consider the phosphor heat generation, which is calculated by Kubelka-Munk theory. We analyzed the effect of phosphor particle distribution on the thermal behaviors of LED packages. Two kinds of phosphor sizes, i.e. 5 μm and 7 μm, were applied for configuration optimization. Finite-element method (FEM) was adopted to simulate three cases. It is found that when the upper phosphor layer has larger concentration, the phosphor temperature is higher but the light extraction efficiency (LEE) is lower; when the lower phosphor layer has larger concentration, the phosphor temperature is lower but the LEE is higher. The phosphor temperature uniformity can be enhanced by 0.22% when the concentration of the lower phosphor layer is larger. Phosphor sedimentation, although deteriorates the optical performance, benefits the thermal behaviors on the contrary. There is a compromise that the thermal behaviors and optical performance seem to be inconsistent for different phosphor configurations.

In this paper, a phase change material (PCM) based heat sink was proposed and its geometry was optimized for heat transfer enhancement. The thermal resistance network between the heat source and the control volume was applied to analyze the influence on heat storage performance of the phase change material based heat sink (PCMBHS) and the optimization suggestions were proposed. Besides, the heat storage performance of the optimized fined PCMBHS was examined experimentally. The results showed that the temperature difference of the PCM inside each control volume reduced by 67% and the heat storage rate increased by 14% compared to those of the uniformly fined PCMBHS during the latent heat storage period.

Thermal management of the electronic devices who work intermittently with the method of latent heat storage is regarded as one of the most prospective alternative passive thermal management solutions. In this paper, the thermal management performance of the hybrid heat sink with the bottom of the fins immersed in the phase change material (PCM) is studied experimentally. The effect of the melting temperature and the quantity of the PCM on the performance of the hybrid heat sink is examined. The results show that the high-melting temperature based hybrid heat sink can help the chip maintain a safe temperature for a longer time and the time increases as the quantity of the PCM increases. However, when applied for the thermal management of the devices who work with a low power, the hybrid heat sink shows no advantages.

Transformation thermodynamics, as one of the important branches among the extensions of transformation optics, has attracted plentiful attentions and interests recently. The result of transformation thermodynamics, or called as "thermal cloak", can realize isothermal region and hide objects from heat. In this paper, we presented the concept of "reverse thermal cloak" to correspond to the thermal cloak and made a simple engineering definition to identify them. By full-wave simulations, we verified that the reverse thermal cloak can concentrate heat and realize local heating. The performance of local heating depends on the anisotropic dispersion of the cloaking layer's thermal conductivity. Three-dimensional finite element simulations demonstrated that the reverse thermal cloak can be used to heat up objects. Besides pre-engineered metamaterials, such reverse thermal cloak can even be realized with homogenous materials by alternating spoke-like structure or Hashin coated-sphere structure.

In this study, we modeled the transmitted and reflected light of a phosphor layer in light-emitting diode packages by coupling the revised Kubelka-Munk and Mie-Lorenz theories. Through analyzing the transmitted and reflected light separately, it is found that the transmitted and reflected blue light vary monotonically with the changes of phosphor particle size, concentration and thickness. While the trends of the transmitted and reflected yellow light are non-monotonic, which are influenced by the interactions among the phosphor particle size, concentration and thickness. The light extraction efficiency through a phosphor layer was discussed. The reasons behind these phenomena were also presented.

In this paper, a phase change heat storage substrate for cooling high-power electronic devices was proposed and the paraffin/expended graphite composite phase change material was used in the heat storage substrate. The heat storage performance of the substrate was examined experimentally. The results show that the experimental heat storage of the substrate is consistent with the theoretical heat storage and the ratio between the heat stored in the substrate and the heat generated by the heat source is about 32%. When the time elapses, the heat dissipation load of the unit time reduces. Furthermore, the temperature rising rates of the heat source and the substrate diminish.