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。     

瓷质砖坯体的导热系数及其影响因素

探讨了温度、成型压力、含湿率、添加剂对坯体导热系数的影响。试验结果表明：对于相同化学组成的瓷质砖坯体,在保证同一实验平均温度的情况下,其导热系数随着成型压力和含湿量的增加而非线地增大;在低温段,其导热系数随着温度的升高而减小;添加物对坯体导热系数影响比较复杂。     

C/C坯体对C/SiC复合材料组织结构和导热性能的影响

以无涂层、C+SiC复合涂层处理的炭布/网胎预制体,分别经过化学气相渗透、树脂浸渍/炭化制备了3种C/C坯体,熔融渗硅后获得不同的C/SiC复合材料,对其组织结构和导热性能进行了研究。结果表明:热解炭坯体的C/SiC复合材料存在集中分布的Si,混合基体炭的C/SiC中可见较多微裂纹,C+SiC涂层的材料中残留Si含量少,基体组织均匀;热解炭坯体的C/SiC复合材料热扩散率和导热系数最大;混合基体炭的坯体,纤维经过C+SiC涂层,可明显提高材料的热扩散率和导热系数,且随温度的升高,导热系数的下降速率增大。     

新型陶瓷坯体增强剂的研究

本文对陶瓷坯体增强剂在现代陶瓷坯体配方中遇到的问题进行了概述和分析。坯体的强度是建陶行业发展的一个瓶颈因素。除了找寻优质原料,在增加坯体强度方面,企业应同时另辟蹊径。     

大规格建陶坯体对流干燥热质耦合传递的数值模拟

以Whitaker的体积平均方程为基础,推导出多孔介质内部热质传递的等效耦合扩散模型,并对瓷质砖坯体干燥过程进行了数值分析和实验测定。探讨了干燥介质参数（湿度、温度和流速）和坯体物性参数（有效导热系数和有效扩散系数）对干燥过程的影响。结果表明,温度高、流速快、湿度小的热空气有利于干燥,其中,改变风速对干燥速率的影响很小,介质的温度和湿度对干燥影响较大,但有可能造成干燥缺陷。有效扩散系数对坯体内湿度的分布影响很大,而有效导热系数对坯体内湿度的分布几乎没有影响。     

物性参数对连铸结晶器传热模型计算结果的影响

针对某方坯连铸实际情况,建立了二维非稳态传热模型,研究了导热系数、比热和密度对结晶器传热模型计算结果的影响规律.研究结果表明,其他条件一定,导热系数越大,铸坯表面温度下降越缓慢,铸坯中心温度则下降越快;增大密度或比热,出结晶器的铸坯表面中心及铸坯中心温度均增大.相比较而言,导热系数对结晶器内铸坯凝固传热过程的影响最为显著,比热的影响次之,密度的影响最小.     

侧面散热对导热系数测量的影响研究

稳态平板法测量导热系数因方法简单直接在测量材料的导热系数中广泛应用。但通过数值模拟和实验发现侧面散热对导热系数测量影响明显,采用大空间自然对流换热实验关联式对实验数据处理公式进行修正,在实验中得到较满意结果。利用数值模拟和实验对影响侧面散热的主要因素如环境温度、热源温度、样品厚度、试样导热系数进行研究,发现环境温度降低、热源温度增加、试样厚度的增加以及试样导热系数的减小,都会导致传热温差增加,侧面散热的绝对量和占总传热量的比例也随之增加。     

石墨改性PA46导热复合材料的性能

对聚酰胺基导热材料进行了研究。采用石墨对聚己二酰丁二胺(PA46)进行填充改性,通过熔融共混及注射成型工艺制备导热复合材料。研究表明:随着石墨填充量的增加,复合材料的导热率显著增加,拉伸强度、冲击强度稍有降低。SEM形貌分析表明:随着石墨含量的增加,石墨在基体中更易形成导热链、导热网络等通路。当石墨的质量分数为50%时,导热率达到了3.743 W·m-1·K-1,是纯基体的13.91倍;热扩散系数为2.344 mm2/s,为纯基体的15.95倍。实验值与3种导热模型对比的结果表明:与Y Agari导热模型匹配的比较好,适用于低填充量的情况。     

炭黑用量及硫化对橡胶导热系数的影响

利用闪光导热分析仪LFA 447 Nanoflash^TM测量了橡胶的导热系数,研究了炭黑用量及硫化对橡胶导热系数的影响。实验结果表明：当炭黑用量在高范围变动时,混炼胶的导热系数随其用量的增加而降低;混炼胶的导热系数大于硫化橡胶。     

零ODP值发泡剂对硬质聚氨酯泡沫的泡孔结构和导热系数的影响

以聚醚多元醇、聚合MDI为基础原料体系,用ODP值为零的不同发泡剂制备了密度约为31 kg/m3的喷涂聚氨酯泡沫。研究了4种ODP值为零的物理发泡剂HFC-365mfc、HFC-365/227、CP和化学发泡剂K10对泡沫泡孔结构和导热系数的影响,并与HCFC-141b、水发泡剂及其混合体系进行了对比。通过泡沫导热系数、闭孔率测定与扫描电子显微镜等测定,探讨了导热系数与泡孔结构之间的内在关系。研究结果表明,较小的泡孔直径和均匀的泡孔结构有助于降低泡沫导热系数。导热系数随着发泡剂气相导热系数增加而增大。     

Numerical simulation of the effect of moisture on the thermal conductivity of porous ceramic materials

A simulation model to analyze the influence of moisture content on the thermal conductivity of porous ceramic materials is developed based on the numerical integration of the energy equation. The experimental technique employed for thermal conductivity measurements is the hot wire parallel technique. The numerical model proposed is checked by evaluating the thermal conductivity of a hypothetical porous ceramic material containing different concentrations of water in its structure. The behaviour of the thermal conductivity as a function of temperature, as predicted by the model proposed in this work was experimentally verified by the authors for an unfired refractory concrete. ©     

热探针法测量陶瓷粉末导热系数的实验研究

以Al2O3粉末为例,在分析探针法测量原理的基础上,用热探针法对Al2O3粉末的导热系数进行测量,研究在不同含水率和温度下Al2O3粉末导热系数的变化情况。试验结果发现,Al2O3粉末的导热系数随着秸秆密度,温度和含水率的增加而增加,并基本呈线性关系。     

Computer model of drying behaviour of ceramic green bodies with particular reference to moisture content dependent properties

A numerical model was developed to predict the drying behavior of ceramic green bodies. Resolution of the simultaneous heat and mass transfer equations involved finite elements and the Backward Euler method. Based on experimental data, the model uses equivalent moisture diffusivity, water activity, thermal conductivity and heat capacity as input parameters which depend on moisture content. In particular, the equivalent moisture diffusivity is a key parameter controlling water transport from the body interior to the surface. A simple method was used to estimate the effect of shrinkage on drying rate during the initial drying stage. Predictions of the internal moisture distribution, drying rate and surface temperature as a function of time gave good agreement to experiment for green bodies of alumina paste. External conditions of convection coefficient and relative humidity are shown to sensitively control drying rate and surface temperature evolution during the constant rate period.     

Enhanced anti‐deliquescent property and ultralow thermal conductivity of magnetoplumbite‐type LnMeAl11O19 materials for thermal barrier coating

Magnetoplumbite‐type LaMgAl₁₁O₁₉ ceramic has been proposed as one of promising candidates for the next generation thermal barrier coatings (TBCs) due to its low thermal conductivity. However, LaMgAl₁₁O₁₉ shows poor water‐resistance with significant weight loss at elevated temperatures in water‐containing atmosphere. In this work, we revealed that the essential reason for the poor water‐resistance of magnetoplumbite‐type LaMgAl₁₁O₁₉ ceramic is Mg²⁺ migration from the intrinsic site under moisture environment. And then an effective approach was proposed to improve its anti‐deliquescent property by completely substituting divalent alkaline earth ions Mg²⁺ with Zn²⁺. Finally, a panoscopic strategy was proposed to further lower thermal conductivity through co‐substituting La and Zn sites in LaZnAl₁₁O₁₉ with trivalent and divalent transition metal ions. The mechanism for the lowered thermal conductivity is due to the panoscopic approach, which providing all‐scale hierarchical architectures of phonon scattering mechanisms. The excellent anti‐moisture performance and ultralow thermal conductivity endow the LaZnAl₁₁O₁₉ based ceramics as a kind of promising candidates for advanced thermal barrier coatings.     

A new Al2O3 porous ceramic prepared by addition of hollow spheres

A method for making porous ceramic prepared by adding hollow spheres was developed, and the resulting porous ceramic was named as hollow spheres ceramic. Water soluble epoxy resin was used as a gel former in the gelcasting process of the Al₂O₃ hollow sphere and Al₂O₃ powder, the porous ceramic porosity varies from 22.3 to 60.1 %. The influence of amount of Al₂O₃ hollow sphere and sintering temperature on the microstructure, compressive strength and thermal conductivity were investigated. With an increasing amount of hollow sphere in the matrix, the porosity increases, which leads to decreased bulk density, compressive strength and thermal conductivity. The compressive strength of the porous ceramics has a power law relation with the porosity, and the calculated power law index is 4.5. The equations of the relationship between porosity and thermal conductivity of porous ceramics are proposed. The thermal conductivity of samples with 60.1 % porosity is as low as 2.1 W/m k at room temperature.     

Spark plasma sintered BaTiO3/graphene composites for thermoelectric applications

The viability of spark plasma sintered graphene/barium titanate ceramic matrix composites as thermoelectric materials is investigated. The temperature dependence of electrical conductivity, thermal conductivity and Seebeck coefficient was analyzed. The addition of low amounts of graphene oxide combined with the spark plasma sintering process increases electrical conductivity of pure BaTiO₃ several orders of magnitude, whereas the thermal conductivity shows only a moderate enhancement. The composites display a semiconducting behaviour, with the resistivity decreasing with increasing temperature and following a thermally activated temperature dependence at high T. A strong dependence of ZT figure of merit with the graphene concentration and the measurement temperature was found. Optimal values are found for 1.7 wt% graphene oxide at the maximum experimental temperature (600 K).     

A novel way to fabricate fibrous mullite ceramic using sol-gel vacuum impregnation

Inspired by bird's nest structure, fibrous mullite ceramic was fabricated by vacuum impregnation with mullite fibers as raw material and zirconia sol-gel as inorganic binder. The effect of impregnation times on the properties of the fibrous mullite ceramic, such as porosity, microstructure, compressive strength and room-temperature thermal conductivity were investigated. The results showed that low density (0.45–0.66?g/cm³), relative high compressive strength (0.62–3.34?MPa) and low thermal conductivity (0.037–0.125?W/mk) were exhibited for the sample. The toughness of fibrous mullite ceramic was enhanced due to the micro-creaks caused by transformation of zirconia. From the experimental results, it is suggested that it was an optimal method which have the potential use in high-temperature thermal insulation materials to produce fibrous mullite ceramics.     

Thermal properties of polymer-derived ceramic reinforced with boron nitride nanotubes

We report the thermal properties of boron nitride nanotube (BNNT) reinforced ceramic composites using the polymer derived ceramic (PDC) processing route. The nano-composites had a BNNT loading of up to 35.4 vol.%. TGA results showed that nano-composites have good thermal stability up to 900°C in air. BNNTs in nano-composites survived in an oxidizing environment up to 900°C, revealing that nano-composites can be used for high temperature applications. Thermal conductivity of PDC reinforced with 35.4 vol.% BNNT was measured as 4.123 W/(m·K) at room temperature, which is a 2100 % increase compared to that of pristine PDC. The thermal conductivity value increases with the increase of BNNT content. A thermal conductivity percolation phenomenon appeared when the BNNT content increased to 36 ± 5 vol.%. The results of this study showed that BNNTs could effectively improve the thermal conductivity of PDC materials. BNNT reinforced PDC could be used as thermal structural materials in a harsh environment at temperatures up to 900°C.     

Thermophysical Properties of Stone Fruit

The thermophysical properties of the stone fruits plum, peach, and nectarine were modeled from experimental data as functions of moisture content. Samples were dried to preset moistures in a laboratory cabinet dryer, and the thermal conductivity, specific heat, apparent density, bulk density, and porosity of the fruit were determined. The thermal conductivity and specific heat were found to be linear functions of moisture content, whereas apparent bulk density and porosity followed second-order polynomials. Temperature dependence was not found to be significant.