介孔二氧化硅球形孔内近场辐射换热

介孔二氧化硅基材内含不连续且均匀分布的球形孔。由于孔径小于热辐射特征波长,近场辐射作用不容忽视。本文基于涨落耗散理论和并矢格林函数,计算介孔二氧化硅球形孔内的近场辐射换热,由此得到的近场辐射的当量导热系数,将进一步用来修正介孔二氧化硅的有效导热系数。采用稀介质孔隙率加权模型耦合球形孔内近场辐射当量导热系数、孔内受限气体导热系数及介孔二氧化硅基材导热系数,得到介孔二氧化硅的有效导热系数,并进一步考察了孔径和温度的影响。研究结果表明,在介观尺度下,其辐射热流比宏观尺度下要高2~7个数量级。球形孔内近场辐射的热流及当量导热系数随着孔径的增加呈指数衰减,随着温度的升高而增大。介孔二氧化硅的有效导热系数随着孔隙率的增加逐渐减小,随着温度的升高缓慢增加。孔径越小,近场辐射的作用越显著,不容忽视。当孔径大于50 nm时,尺寸效应逐渐消失。     

Lattice thermal conductivity of boron nitride nanoribbon from molecular dynamics simulation

The lattice thermal conductivity of boron nitride nanoribbon(BNNR) is calculated by using equilibrium molecular dynamics(EMD) simulation method. The Green–Kubo relation derived from linear response theory is used to acquire the thermal conductivity from heat current auto-correlation function(HCACF). HCACF of the selected BNNR system shows a tendency of a very fast decay and then be followed by a very slow decay process,finally,approaching zero approximately within 3 ps. The convergence of lattice thermal conductivity demonstrates that the thermal conductivity of BNNR can be simulated by EMD simulation using several thousands of atoms with periodic boundary conditions. The results show that BNNR exhibit lower thermal conductivity than that of boron nitride(BN) monolayer,which indicates that phonons boundary scatting significantly suppresses the phonons transport in BNNR. Vacancies in BNNR greatly affect the lattice thermal conductivity,in detail,only 1% concentration of vacancies in BNNR induce a 60% reduction of the lattice thermal conductivity at room temperature.     

Effect of sintering parameters on the microstructure and thermal conductivity of diamond/Cu composites prepared by high pressure and high temperature infiltration

Pure Cu composites reinforced with diamond particles were fabricated by a high pressure and high temperature (HPHT) infiltration technique. Their microstructural evolution and thermal conductivity were presented as a function of sintering parameters (temperature, pressure, and time). The improvement in interfacial bonding strength and the maximum thermal conductivity of 750 W/(m·K) were achieved at the optimal sintering parameters of 1200℃, 6 GPa and 10 min. It is found that the thermal conductivity of the composites depends strongly on sintering pressure. When the sintering pressure is above 6 GPa, the diamond skeleton is detected, which greatly contributes to the excellent thermal conductivity.     

中厚板精轧过程的高精度温度预测模型

从设定模型角度结合中厚板精轧过程的工艺特点,分析了热辐射和对流、高压水除鳞、轧辊的热传导和塑性功对钢板温度变化的影响,得出如下结果:①钢板热辐射和对流过程可以简化成一维热传导方程,钢板的黑度可考虑成钢板厚度的函数;②高压水除鳞过程可以简化成半无限体平板的瞬态热传导模型;③轧辊的热传导过程可简化成两个半无限体之间的热传导过程,接触热阻的影响通过修正系数进行调节;④塑性功造成的温度变化必须考虑热功转化效率的影响·通过与实际数据的比较可以看出该模型具有很好的预测精度·     

大温差测试条件下热防护材料高温导热系数试验方法

热防护材料高温导热系数是进行高超声速飞行器设计不可或缺的参数。通过热防护材料导热系数理论分析以及传统稳态法试验原理数学推导,说明了热防护材料导热系数物理本质。探讨了试验温差对热防护材料导热系数测试的重要性和传统稳态法在测试热防护材料高温导热系数时的技术局限性。基于热防护材料内部温度分层特征以及导热系数-温度非线性关系函数形式直接假设,提出并分析证明了两种适用于大温差测试条件的热防护材料高温导热系数试验方法。对热防护材料高温导热系数获取有重要的参考价值。     

Effects of boron on the microstructure and thermal properties of Cu/diamond composites prepared by pressure infiltration

Diamond reinforced copper (Cu/diamond) composites were prepared by pressure infiltration for their application in thermal management where both high thermal conductivity and low coefficient of thermal expansion (CTE) are important. They were characterized by the microstructure and thermal properties as a function of boron content, which is used for matrix-alloying to increase the interfacial bonding between the diamond and copper. The obtained composites show high thermal conductivity (>660 W/(m·K)) and low CET (<7.4×10-6 K-1) due to the formation of the B₁₃C₂ layer at the diamond-copper interface, which greatly strengthens the interfacial bonding. Thermal property measurements indicate that in the Cu-B/diamond composites, the thermal conductivity and the CTE show a different variation trend as a function of boron content, which is attributed to the thickness and distribution of the interfacial carbide layer. The CTE behavior of the present composites can be well described by Kerner’s model, especially for the composites with 0.5wt% B.     

用多尺度形态学方法实现成像测井电导率图像的缝洞参数表征

裂缝和溶蚀孔洞为缝洞型储层提供了主要的储集空间和渗流通道,其分布的随机性和复杂性严重影响了对缝洞储层的定量评价。基于高覆盖率和高分辨率的电成像测井数据,采用多尺度数学形态学方法提取电导率图像中的缝洞孔隙度谱。选择不同尺度和形状的结构元素,构造不同种类的形态学滤波算子,实现井壁裂缝和溶蚀孔洞的电导率异常边缘检测。针对缝洞异常的边缘检测结果,用椭圆形函数拟合溶蚀孔洞,用多项式插值函数拟合裂缝边缘,继而提取缝洞参数并获得缝洞孔隙度谱。实验结果表明,用多尺度数学形态学方法对电导率图像的边缘检测有效地实现了缝洞的自动识别,验证了该方法计算缝洞孔隙度谱的准确性。     

Impedance and ferroelectric properties of Sr2+ modified PZT-PMN ceramics

Sr^2＋ modified polycrystalline PZT-PMN ceramics were synthesized by a semi-wet route. Impedance spectroscopy studies indicate the bulk and grain boundary effects of PZT-PMN material along with the negative temperature coefficient of resistance. The bulk conductiv-ity exhibits an Arrhenius-type thermally activated hopping process which is supported by the AC conductivity behavior as a function of fre-quency and temperature. It is observed that the remnant polarization increases with an increase in the Sr2＋content in PZT-PMN.     

Zn_(0.95)Co_(0.05)O的介电性质研究

通过在高温下(473~1 073 K)对离子电导率、交流电导率以及介电常数和损耗的测量,来研究Zn0.95Co0.05O体系的电学及介电性质,获得了体系的离子导电和交流导电的活化能,以及介电常数和损耗随温度和频率的变化关系.     

导电高分子复合材料的有效电导率模型

基于Maxwell理论和平均极化理论,建立了导电高分子复合材料的有效电导率模型。有效电导率描述为各组成成分的体积、形状和颗粒尺寸的函数。利用该模型,讨论了渗流阈值与导电颗粒的大小及轴长比的关系,得到了导电高分子复合材料的有效电导率随导电颗粒轴长比和半径变化的规律。用该模型计算了碳纤维聚脂树脂复合材料的有效电导率,计算结果与实验结果符合较好。