Improving thermal conductivity of polymer composites in embedded LEDs systems

Polymer embedding of LEDs increases safety and waterproof levels in LED based lighting systems. The embedding allows for mechanical flexibility of these systems. The increase of polymer thermal conductivity has been a research challenge for decades. Here, we suggest materials for enhancing thermal conductivity in polymer embedded LED systems. We demonstrate that thermally conductive fillers into the polymer matrix to form a composite improved heat transfer from the LEDs to the environment. Non metallic boron nitride with a high intrinsic thermal conductivity is a good candidate. Thermal conductivity of basic polymer PDMS with various filler size and polymer ratios is reported here. Here, an in situ measurement tool to fast evaluate the quality of the composites in LED applications is demonstrated. Future work will focus on further increasing the thermal conductivity of the composites by using different mixtures.     

Investigation of the thermal properties of thin solid materials at different temperature levels using a set of microresistors

The measurement of thermal properties of solid materials at different temperatures above ambient is investigated using a set of microresistors. Samples consisted of suspended films with sets of long, parallel resistive wires deposited on their surfaces. One resistive wire was heated by an alternating current. Surface temperature changes in DC and AC regimes were then detected by measuring the change in electrical resistance of the other wires deposited on the surface. The length of wires was chosen so that they may be assumed isothermal and such that heat diffusion acts perpendicularly to their axes. By measuring the dependence of the surface alternating temperature oscillation on the modulation frequency f and on the separation between the heating wire and the probing wires, the thermal diffusivity of the sample was determined. Through adjustment of the alternating current amplitude in the source wire, the temperature at which the thermal diffusivity of the sample was evaluated was finely controlled. For the validation of the method, pure silicon samples were first studied. An experimental bench was set up and resistive source and probes were experimentally characterized. Results obtained from ambient temperature to 500 K for pure silicon are in accordance with reference data found in the scientific literature.     

The influence of minor titanium addition on thermal properties of diamond/copper composites via in situ reactive sintering

A minor amount of titanium addition is proposed to improve interfacial bonding between diamond particles and copper matrix for diamond/copper composites. The volume fractions of diamond and minor titanium in the sintering process are optimized. The microstructures, thermal properties, interface reaction production, and the effects of minor amount of titanium on the properties of the composites are investigated. The results show that the interfacial bonding of the composites could be strengthened by changing the volume fraction of titanium and diamond. The 45 vol%-diamond/copper composites with 3 vol% titanium at 945 °C for 5 min exhibits thermal conductivity as high as 670 W/(m K), which is 90% of the theoretical prediction value. High thermal conductivity is achieved by forming the titanium carbide (TiC) and intermetallic compounds (Cu₃Ti₂) at the diamond/copper interface to obtain a good interface.     

固相反应法制备NiO-Co3O4复合物及其超级电容器性能

采用固相反应法合成了摩尔比为4∶1的NiO-Co3O4复合物,用XRD和SEM表征了样品的相结构和形貌,采用循环伏安(CV)法和恒流充放电测试两种方法研究了纯NiO、纯Co3O4和NiO-Co3O4复合物电极的电化学性能。结果表明:所得复合物样品为立方相结构,Co3O4的掺入增加了NiO样品的孔穴率。在6mol.L–1的KOH电解液中,复合物的比电容达到了476F.g–1,远高于纯NiO(约93F.g–1)和纯Co3O4的比电容(约148F.g–1)。在充放电1000次后,NiO-Co3O4的比容量仅衰减了3.4%,显示出该复合物的电化学性能明显优于纯NiO和纯Co3O4。     

Finite element analysis of low temperature thermal nanoimprint lithography using a viscoelastic model

Nanoimprint lithography (NIL) is an emerging technology that enables cost-effective and high-throughput nanofabrication. Nevertheless, there are some disadvantages to this method, especially for thermal NIL. A major disadvantage of thermal NIL is the thermal cycle, which requires a significant amount of processing time and limits the throughput. One method to overcome this disadvantage is to reduce the processing temperature. Accordingly, it is necessary to determine the effects on the processing parameters for thermal NIL at reduced temperatures and to optimize the parameters. This requires a clear understanding of the behavior of the polymer material during the thermal NIL process. This work focuses on a temperature range of T_g < T < T_g + 40 °C, in which the polymer displays a semi-molten state behavior; this temperature range is lower than conventionally used for thermal NIL. To understand how the processing conditions of temperature, pressure, pattern density, and initial thickness of the polymer resist are related to the quality of a nanoimprinted pattern, simulations of thin polymer films squeezing into nanocavities during thermal NIL were performed using a two-dimensional viscoelastic finite element analysis taking into account stress relaxation behaviors.     

气敏元件室温光激发气敏性能研究

研究了WO3掺杂的ZnO基气敏元件在紫外(UV)光激发下,对乙醇气体的室温气敏性能。结果表明:在UV光照射下,各元件在室温下对体积分数为100×10–6的乙醇气体显示了很好的光敏、气敏性能,响应、恢复时间均在8s以内,其中以掺杂X(WO3)为1%的元件W(1)为最佳,从而实现了室温下的气敏测试。     

基于独立成分分析的拉曼光纤温度传感系统去噪算法研究

针对拉曼光纤温度传感系统中拉曼散射信号被噪声 淹没的问题,利用快速独立成分 分析(ICA)法对单模光纤(SMF)中红外激光产生的拉曼散射信号进行去噪处理,并在拉曼散射 信号前端添加一定幅值和 相位的正弦信号,根据正弦信号处理前后幅值和相位的变化校正拉曼散射信号的幅值和相 位,从而完全 恢复拉曼散射信号。在20km SMF拉曼温度传感仿真系统中,拉曼散 射信号的 信噪比(SNR)由14.90dB提高 到20.99dB,提高了6.09dB。结果表明,本 文方法能有效提高拉曼测温系统 的SNR,是一种针对拉曼光纤温度传感系统有效的去噪算法。     

Influence of the metallic absorption layer on the quality of thermal conductivity measurements by the transient thermo-reflectance method

This work investigates numerically the influence of a metallic absorption layer on the laser-based measurements of the thermal conductivity of dielectric (SiO₂) and semiconductor (Si) electronic materials. The validity of the approach and the obtained results are assessed by comparison with experimental measurements obtained for gold-covered silicon dioxide samples. The results reveal the presence of behaviors associated with thermally thin and thermally thick absorption layers, depending on the ratio between the thickness of the absorption layer and the heat penetration depth. Optimal performance of the transient thermo-reflectance method was found to exist for thicknesses of metal layers falling between the identified thermally thin and thermally thick layers.     

脉冲激光大气传输热晕数值分析

由于大气通道上介质对激光能量的吸收,改变了大气折射率,形成自散焦负透镜,从而产生热晕。热晕使激光发生畸变,限制了激光能量在大气中的有效传输。以准直高斯光束为例,模拟单脉冲和序列脉冲激光大气传输,对均匀风场和无风两种情况下激光在不同靶程光束截面的光强分布进行了数值分析。结果表明：长脉冲比短脉冲受到非线性热晕效应影响大;序列脉冲存在一个最优化脉冲间隔,使上靶光强存在20%~30%的涨幅。最后,在没有采用自适应光学系统对畸变进行补偿的情况下,提出了采用脉冲间隔适当的序列短脉冲传输来提高激光传输能量的方法。     

端接非线性负载的不等长传输线瞬态分析

对时域有限差分(FDTD)法应用于不等长多导体传输线端接非线性负载的情况进行了介绍.首先给出了多导体传输线电报方程和差分公式;然后介绍了不等长传输线的仿真模型;在此基础上,最后通过建立端接非线性负载的不等长多导体传输线模型,对该情况下传输线两端的电压响应进行了分析.数值仿真结果说明了FDTD法解决此类问题的正确性和有效性,为不等长传输线瞬态分析的进一步研究打下了基础.