多孔陶瓷热导率的影响因素及其有效热导率的数值计算方法

多孔陶瓷因具有孔隙率高、体积密度小、比表面积大等独特的表面物理特性而被广泛应用于保温材料、炉膛材料、热障涂层材料、高温烟气过滤材料等,研究多孔陶瓷导热机制并给出其有效热导率的计算方法既是重点又是难点。本文总结了国内外研究的多孔陶瓷热导率的影响因素,概述了多孔陶瓷有效热导率的计算方法,并重点分析了不同显微结构的不同计算方法。针对不同的应用领域对材料热导率的不同要求,提出通过控制显微结构控制热导率是今后多孔陶瓷热导率研究得发展趋势。     

稳态平板法测导热系数的补充实验研究

以建陶坯体为研究对象,针对目前平板法测量圆饼状固体材料导热系数的常见问题,采用蜡封方法避免水分沿圆周方向的逸出,分析湿度对导热系数的影响。试验结果表明:坯体导热系数随温度和含水率的升高而升高,并首次针对不符合尺寸要求的实验样品采用简化的实验方法,推导出其导热系数的计算公式。     

HOMOTOPY ANALYSIS OF FREE CONVECTION BOUNDARY LAYER FLOW WITH HEAT AND MASS TRANSFER

A steady two-dimensional laminar boundary layer flow of an incompressible viscous fluid over a semi-infinite surface is considered to investigate the accuracy of the homotopy analysis method. The governing coupled nonlinear system of differential equations is solved by means of the HAM approach. Explicit analytical series solutions are obtained and compared with numerical solutions. Good agreement is observed between the numerical results and HAM analytical solutions.     

Chebyshev配置点谱方法直接求解离散坐标辐射传递方程

针对三维长方形炉内具有吸收-发射介质的辐射换热,基于Chebyshev配置点谱方法和Schur分解开发了直接求解辐射离散坐标方程的求解器。针对离散后所得到的三维矩阵方程,分别用两种方法进行求解,一种是用张量积将三维转变成二维然后直接用Schur分解求解;另一种是自行开发三维Schur分解直接求解。数值实验表明,在相同的输入参数下,新求解器具有很好的精度,尤其相比于标准离散坐标法,新求解器能节省大量计算时间。特别是基于三维Schur分解的直接求解器,在相同的输入参数下,计算时间只有标准离散坐标法的10%～1%。     

Experimental and theoretical study for the determination of thermal conductivity of porous building material made with pumice and tragacanth

In this study, porous solid materials were produced with mixture combinations of materials such as tragacanth, cement, and pumice aggregate. Thermal conductivities of the produced materials were determined using hot wire methodology. The thermal conductivities of the produced samples ranged from 0.433 to 0.177 W/mK, depending on pumice diameter, pumice, tragacanth, and cement rates. A new model was developed to determine the effective thermal conductivities of the produced samples. The thermal conductivities obtained by the measurements were compared to the ones predicted by the model. The predicted values deviated as 1–31% from the measured values. It was finally shown that the experimental results were in good agreement to the predicted results.     

Highly porous nano-SiC with very low thermal conductivity and excellent high temperature behavior

Highly porous nano-SiC is fabricated by partial sintering and decarburizing process using SiC nano-powders as starting materials and graphite flakes as pore forming agents. The prepared porous nano-SiC ceramics possess multiple pore structures, including well-distributed meso-pores in the skeleton and interconnected flakelike micro-pores. The samples prepared at 1800 °C have relatively low thermal conductivities of 5.61 ～ 0.25 W m^?1 K^?1 with porosities of 55.5–76.1%. While the samples sintered at 1500 °C with porosities between 54.0% to 76.3% show very low thermal conductivities of 0.74 ～ 0.14 W m^?1 K^?1, which is attributed to the integrated nano-scale phonon-scattering mechanisms and duplex pore structures. Porous nano-SiC ceramics also show good retention of elastic stiffness up to 1350 °C and low thermal conductivity at 1400 °C. Our results shed light on porous nano-SiC as a promising thermal insulator used in extreme thermal and chemical environments.     

Porous γ-(Y1-xHox)2Si2O7 thermal insulator with excellent high-temperature strength retention and very low thermal conductivity

High-temperature thermal insulation materials challenge extensive candidates with good mechanical, thermal and chemical reliability at high temperatures. Recently, porous γ-Y₂Si₂O₇ was indicated a promising thermal insulator in harsh environment; however, its strength at 1300?°C reduced to 34% of that at room temperature. In this work, we significantly improved its high-temperature strength by doping Ho. Highly porous γ-(Y_1-xHo_x)₂Si₂O₇ solid solution was fabricated by in-situ foam-gelcasting method. Especially, porous γ-(Y_2/3Ho_1/3)₂Si₂O₇ demonstrated the optimal high-temperature strength, for instance 65% retention at 1300?°C, as well as high compressive strength (13.9?MPa) and low thermal conductivity (0.186?W/(m?K)) at room temperature, at the porosity of 79.3%. Interestingly, porous solid solution sample displayed obviously lower thermal conductivity than the two end pure-phase porous materials. Porous γ-(Y_1-xHo_x)₂Si₂O₇ solid solution is clearly highlighted as a promising high-temperature thermal insulator with outstanding high-temperature strength retention and optimal low thermal conductivity.     

超级导热型沥青基碳纤维

比金属银、铜热导率(TC)还要高的中间相沥青基碳纤维倍受世人关注。这种超级热导率中间相沥青基碳纤维可广泛用于宇航工业和电子产业,如卫星的太阳能电池基板、集成电路(IC)基板、印刷线路的基板、高能密度电子仪器的散热片等。这种轻质、柔软、高导热的复合材料将随着宇航工业、信息产业和电子工业的需求而得到迅速发展。     

Modeling on lithium insertion of porous carbon electrodes

MCMBs with different crystal structure were tested for an anode of lithium ion batteries (LIB) and the model describing the behavior of porous anodes was simulated numerically by using orthogonal collocation method (OCM). Kinetic parameters such as diffusion coefficients, exchange current densities, and transfer coefficient, describing electrochemical intercalation system of lithium ions, were estimated by fitting the experimental cyclic voltametry (CV) results with the theoretical ones. It was investigated that the theoretical cyclic voltamograms obtained using above parameters fitted well with the experimental curves for the various scan rates from 1 mV s⁻¹ to 5 μV s⁻¹. The parameters were then evaluated on their extended application in various C-rate-charge/discharge cycling tests with showing good agreements between experiments and simulations. As the results show, it was found that numerical simulations based on both potentiometry and galvanometry experimental data resulted in more accurate parameters of electrochemical system. Simulations indicate there exist the optimum design conditions of electrode and separator to obtain the good performance of lithium ion batteries.     

The effective thermal conductivity of three-dimensional reticulated foam materials

In this paper, the effective thermal conductivity k _eff of three-dimensional (3D) reticulated SiC foams were investigated through experimental and numerical methods. The results showed that the k _eff of SiC foams increases as the volume fraction f increases from 30% to 50%. However, there are no systematic changes detected in k _eff when the cell size of the foam varies at a fixed volume fraction. The k _eff of SiC foams as a function of f was obtained. Compared the experimental results with the calculated ones, it indicated that the outcome can be widely applied in estimating the effective thermal conductivity of other foam materials.     

Synthesis of hierarchically porous mullite ceramics with improved thermal insulation via foam-gelcasting combined with pore former addition

ABSTRACT

To further improve the thermal insulation performance of porous mullite ceramics used in important industrial sectors, a combined foam-gelcasting and pore-former addition approach was investigated in this work, by which hierarchical porous mullite ceramics with excellent properties, in particular, thermal insulation property, were prepared. Both mesopores (2–50?nm) and macropores (117.8–202.7?μm) were formed in porous mullite ceramics resultant from 2?h firing at 1300°C with various amounts of submicron-sized CaCO₃ pore former. The former mainly arose from the decomposition of CaCO₃, and the latter from the foam-gelcasting process. The porous samples prepared with CaCO₃ addition had low linear shrinkage of 2.35–4.83%, high porosity of 72.98–79.07% and high compressive strength of 5.52–14.82?MPa. Most importantly, they also exhibited a very low thermal-conductivity, e.g. 0.114?W?m^?1?K^?1 at 200°C, which was much lower than in the cases of their counterparts prepared via the conventional foam-gelcasting route.     

多孔固体催化剂的有效导热系数

建立了测定多孔固体催化剂骨架导热系数的实验装置和由此计算催化剂有效导热系数的简化方法，测定并计算了若干工业过程催化剂的有效导热系数，同时，计算了甲烷蒸汽转化催化剂的非等温效率因子。     

数值方法在平板准稳态法测量有机玻璃热物性实验中的应用

利用数值方法对平板准稳态法实验获得的有机玻璃试样的导热率及比热容进行了修正。在修正的过程中,使用了步进法,以计算曲线的热、冷端温差及其热、冷端温升速率与实验曲线相等为计算终止条件,最终获得修正的导热率及比热容。其修正幅度分别为1.7%和2. 8%,且修正后的数值均在相关文献的报道范围之内。     

炼钢用辅料中氮元素检测

随着生产控制要求的提高,氮元素检测在钢铁冶炼中的重要性愈发体现。常用的热导法检测氮元素虽有准确度高、分析时长短的优点,但也存在检测费用高的缺点。通过对多种炼钢原辅料进行试验,初步确定了凯氏法定氮的试样消解方法、碱液用量。对比凯氏法和热导法定氮的检测结果,二者基本一致,可用凯氏法代替热导法用于炼钢原辅料定氮。     

导热系数测试方法的综述

本文介绍了导热系数的五种测试方法,描述各种方法的测试原理及其计算方法。材料导热系数测试方法各有其特点,在选择时。应该充分考虑测试材料的性质、导热系数范围、测试温度等。     

测油脂导热系数的研究新进展

简要介绍测定油脂导热系数在科学研究和工程设计方面的意义,系统综述目前测定液体导热系数的方法,如:理论推导、稳态法及非稳态法等,并对这些方法的优缺点进行了论述和分析;最后指出了油脂导热系数测定方法存在的问题及其发展趋势。测量实践表明:瞬态平面热源法测量准确可靠,具有巨大的应用前景。     

双热流计法测定聚氨酯保温材料的导热系数

简要介绍了几种测定固体材料导热系数的方法,重点讨论了稳态双热流计法测定聚氨酯保温材料导热系数的方法原理、导热仪的结构、影响测试结果的因素,并对提高检测结果的准确性、减少误差提出了建议。     

导热塑料及其加工研究进展

摘要：综述了结构型导热塑料和填充型导热塑料的导热机理,介绍了填充型导热复合材料的几种成型设备,包括双螺杆挤出、注塑机和模压机,并详细介绍了影响填充型导热塑料导热性能的几个因素,包括填料的填充量、形状尺寸、表面处理以及基体材料的粒径尺寸等。指出需要研究开发新型导热填料或改善加工工艺,在不影响材料力学性能且制备方法简单的情况下提高复合材料的导热系数,从而满足导热塑料在不同领域的应用要求。