Thermal-Conductivity Apparatus for Steady-State,Comparative Measurement of Ceramic Coatings

An apparatus has been developed to measure the thermal conductivity of ceramic coatings. Since the method uses an infrared microscope for temperature measurement, coatings as thin as 20 μm can, in principle, be measured using this technique. This steady-state, comparative measurement method uses the known thermal conductivity of the substrate material as the reference material for heat-flow measurement. The experimental method is validated by measuring a plasma-sprayed coating that has been previously measured using an absolute, steady-state measurement method. The new measurement method has a relative standard uncertainty of about 10 %. The measurement of the plasma-sprayed coating gives 0.58 W·m⁻¹·K^−l which compares well with the 0.62 W·m⁻¹·K^−l measured using the absolute method.     

液相剥离法制备石墨烯及在导热硅橡胶中的应

采用高速剪切机液相剥离法, 在胆酸钠的水溶液中将鳞片石墨剥离, 离心得到石墨烯分散液。AFM、TEM、Raman表征结果发现, 剥离出的石墨烯厚度小于4层, 尺寸大约在2~3 μm, 高质量缺陷少(I_D/I_G≈0.15)。将石墨烯分散液冷冻干燥后与银粉共同添加到硅橡胶中, 制备出导热硅橡胶。利用稳态热流法测试导热硅橡胶的导热系数发现, 当添加3vol%石墨烯时, 复合材料的导热系数由未添加石墨烯时的4.900 W/(m·K)提高到12.367 W/(m·K)。综上所述, 通过液相剥离法成功制备出缺陷较少的少层石墨烯, 能够与银粉协同提高导热硅橡胶的导热系数。     

海藻酸钠离子凝胶法制备直通孔氧化铝多孔陶瓷

利用海藻酸钠的离子凝胶过程, 采用溶剂置换结合冷冻干燥的工艺, 成功制备了具有高度有序六方排列的直通孔多孔氧化铝陶瓷, 整个工艺过程及所使用的原料都是环境友好的。研究结果表明, 1500℃烧结2 h样品的孔径尺寸在200 μm左右, 且与固相含量的关系不大, 而孔壁上存在0.3 μm~0.5 μm的小孔。通过控制浆料中氧化铝的固相含量可以对材料的性能进行有效地调控, 研究表明, 随着固相含量从5wt%提高到15wt%, 材料的密度从0.87 g/cm³提高到1.16 g/cm³, 渗透率从2.57×10^-11 m²下降到2.16×10^-11 m², 而抗压强度从(18.9±3.2) MPa提高到(44.2±5.4) MPa, 平行孔道方向的热导率从2.1 W/(m·K)提高到3.1 W/(m·K), 而垂直孔道方向的热导率从1.3 W/(m•K)提高到1.7 W/(m·K), 并且平行孔道方向热导率的增加幅度要明显大于垂直孔道方向。     

Thermal Conductivity of Magnesium Oxide From Absolute,Steady-State Measurements

The thermal conductivity of polycrystalline magnesium oxide has been measured over the temperature range from 400 K to 1300 K using a modified guarded-hot-plate design. Three different thicknesses of specimens having 93 % of theoretical density were tested to verify the operation, accuracy, and reproducibility of our apparatus. The measured thermal conductivity ranges from 30 W · m⁻¹ · K⁻¹ down to 8 W · m⁻¹ · K⁻¹ and has an inverse-temperature functionality. The results agree well with literature values for this material.     

Thermally conductive polyamide 6/carbon filler composites based on a hybrid filler system

We explored the use of a hybrid filler consisting of graphite nanoplatelets (GNPs) and single walled carbon nanotubes (SWCNTs) in a polyamide 6 (PA 6) matrix. The composites containing PA 6, powdered GNP, and SWCNT were melt-processed and the effect of filler content in the single filler and hybrid filler systems on the thermal conductivity of the composites was examined. The thermal diffusivities of the composites were measured by the standard laser flash method. Composites containing the hybrid filler system showed enhanced thermal conductivity with values as high as 8.8 W (m · K)⁻¹, which is a 35-fold increase compared to the thermal conductivity of pure PA 6. Thermographic images of heat conduction and heat release behaviors were consistent with the thermal conductivity results, and showed rapid temperature jumps and drops, respectively, for the composites. A composite model based on the Lewis–Nielsen theory was developed to treat GNP and SWCNT as two separate types of fillers. Two approaches, the additive and multiplicative approaches, give rather good quantitative agreement between the predicted values of thermal conductivity and those measured experimentally.     

AlN表面处理及级配填充对覆铜板绝缘层性能的影响规律与机制研究

在覆铜板绝缘层基体中添加导热陶瓷填料是提高其导热性能的一种有效方法。AlN是一种导热率高、绝缘性好的陶瓷填料, 但其易水解的性质限制了实际应用。此外, 相比于陶瓷填料-树脂基体复合材料体系, 有关填料填充型覆铜板产品性能的系统研究较少。本研究通过对AlN进行磷酸酸洗, 获得了抗水解性能优异的pAlN, 进一步研究了不同pAlN粒径和填充量对覆铜板导热性、剥离强度、介电性能和其他性能的影响。为了获得更有效的填料分布网络, 采取了不同粒径pAlN级配填充策略, 探究了多种级配方案对覆铜板性能的影响, 获得了最优级配和综合性能优异的覆铜板。在最优级配为pAlN-50 μm60%-5 μm5%时, 覆铜板绝缘层的热导率增大至0.757 W/(m·K), 相比纯树脂覆铜板提高160%, 具有优异的力学性能(剥离强度为1.012 N/mm, 弯曲强度为335 MPa)和介电性能(介电常数为4.499, 介电损耗为6.668×10^-3), 同时吸水率低至0.53%。同时探讨了AlN填料在覆铜板应用中存在的问题和解决方法, 系统研究了不同填充方案对覆铜板绝缘层性能的影响, 对其实际应用具有指导意义。     

Ni-P-PTFE化学复合镀层导热性能的研究

采用Ni-P-PTFE化学复合镀对铜管进行表面处理能有效减少污垢在换热表面上形成。然而,在实际应用上,复合镀层对铜管导热性能的影响是必须考虑的问题。实验利用热阻法对铜基Ni-P-PTFE复合镀层的导热系数进行测量,并利用Wilson plot方法处理数据最终得到Ni-P-PTFE复合镀层的导热系数。分析了镀层各组分镍(Ni),聚四氟乙烯(PTFE),碳(C),磷(P)的质量分数对其导热性能的影响规律。结果表明,Ni-P-PTFE复合镀层的导热系数随PTFE和P的质量分数增大而降低,随C的质量分数增加而增大。当PTFE,C以及P的质量分数w(PTFE)=1.76%,w(C)=3.82%和w(P)=10.81%时,最大值为23.12 W/(m.K)。尽管复合镀层的导热系数不高,但由于其厚度很小,镀层产生的热阻仅为9.91×10-5~1.6×10-4(m2.K)/W,所以复合镀层铜管仍保持很高的导热系数值314.88~357.55W/(m.K)。     

Improved thermoelectric performance of highly-oriented nanocrystalline bismuth antimony telluride thin films

Improved thermoelectric performance of highly-oriented nanocrystalline bismuth antimony telluride thin films is described. The thin films are deposited by a flash evaporation method, followed by annealing in hydrogen. By optimizing the annealing conditions, the resulting thin films exhibit almost perfect orientation with the c-axis normal to the substrate, and are composed of nano-sized grains with an average grain size of 150 nm. The in-plane electrical conductivity and Seebeck coefficient were measured at room temperature. The cross-plane thermal conductivity of the thin films was measured by a 3ω method, and the in-plane thermal conductivity was evaluated by using an anisotropic factor of thermal conductivity based on a single crystal bulk alloy with almost the same composition and carrier concentration. The measured cross-plane thermal conductivity is 0.56 W/(m K), and the in-plane thermal conductivity is evaluated to be 1.05 W/(m K). Finally, the in-plane power factor and figure-of-merit, ZT, of the thin films are 35.6 μW/(cm K²) and 1.0 at 300 K, respectively.     

石墨表面镀Si对石墨/Al复合材料热物理性能的影响

研究了石墨粒径及表面镀Si处理对石墨/Al复合材料热物理性能的影响。结果表明:在盐浴过程中石墨表面形成了SiC层,这不仅增强了石墨-Si/Al复合材料的界面结合力,而且抑制了Al4C3相的产生。随着石墨鳞片体积分数从50%增加到70%,复合材料X-Y方向的热导率从492 W/(m·K)增加到654 W/(m·K),而且体积分数为50%的镀Si石墨/Al复合材料抗弯强度达到了81 MPa,相比未镀覆的提高了53%,是理想的定向导热电子封装材料。随着石墨粒径从500μm减小到150μm,石墨-Si/Al复合材料X-Y面方向的热导率由654 W/(m·K)降低到445 W/(m·K),但Z方向的热导率和复合材料抗弯强度变化不明显。     

A High-Temperature Transient Hot-Wire Thermal Conductivity Apparatus for Fluids

A new apparatus for measuring both the thermal conductivity and thermal diffusivity of fluids at temperatures from 220 to 775 K at pressures to 70 MPa is described. The instrument is based on the step-power-forced transient hot-wire technique. Two hot wires are arranged in different arms of a Wheatstone bridge such that the response of the shorter compensating wire is subtracted from the response of the primary wire. Both hot wires are 12.7 µm diameter platinum wire and are simultaneously used as electrical heat sources and as resistance thermometers. A microcomputer controls bridge nulling, applies the power pulse, monitors the bridge response, and stores the results. Performance of the instrument was verified with measurements on liquid toluene as well as argon and nitrogen gas. In particular, new data for the thermal conductivity of liquid toluene near the saturation line, between 298 and 550 K, are presented. These new data can be used to illustrate the importance of radiative heat transfer in transient hot-wire measurements. Thermal conductivity data for liquid toluene, which are corrected for radiation, are reported. The precision of the thermal conductivity data is ± 0.3% and the accuracy is about ±1%. The accuracy of the thermal diffusivity data is about ± 5%. From the measured thermal conductivity and thermal diffusivity, we can calculate the specific heat, C_p, of the fluid, provided that the density is measured, or available through an equation of state.     

热处理对热障涂层孔隙率及热导率的影响

热障涂层高温使用过程中不可避免发生烧结,引起涂层失效。为探索烧结过程对涂层可靠性的影响,采用等离子喷涂制备氧化钇稳定氧化锆(YSZ)涂层,并对其进行1 000,1 100,1 200,1 300℃高温烧结试验,研究其高温烧结过程中的微观结构及热导率演变规律。结果表明:高温热处理引起热障涂层组织结构和热导率均发生变化,稳定状态的孔隙率为12%~14%,在热障涂层服役温度范围内热导率增加到1.25~1.45 W/(m·K)。     

微波烧结制备ZrO2-AlN复合陶瓷的微观结构与性能研究

氧化锆(ZrO₂)陶瓷具有出色的机械性能, 但其应用受到低热导率(Thermal Conductivity, TC)的限制。本研究设计并通过微波烧结制备了高热导率氧化锆-氮化铝(AlN)复合陶瓷, 优化制备条件后, 抑制了两种物质之间的反应, 获得了致密的复合陶瓷(相对密度>99%), 详细研究了该复合陶瓷的组织演变、热学性能和力学性能。研究结果表明, 随着AlN含量的增加, 复合陶瓷的室温下热导率、热扩散系数和热容增加, 分别达到41.3 W/(m·K)、15.2 mm²/s和0.6 J/(g·K)。这种具有高热导率和抗热震性的ZrO₂-AlN复合复合陶瓷在能源系统的高温热交换材料领域具有广阔的应用前景。     

氧化锆陶瓷四方相以及晶界热导率的定量计算

氧化钇部分稳定的氧化锆(YSZ)涂层是应用广泛的热障涂层材料。为了更好地研究各种因素对热障涂层热导率的影响, 使用压制烧结的方法制备基本致密的氧化锆陶瓷, 研究相组成和晶粒大小对热导率的定量影响。在不同的烧成制度下制备出不同晶粒大小的氧化锆陶瓷。用电子背散射衍射(EBSD)图像研究氧化锆陶瓷材料的相组成以及晶界的分布情况。综合有限元模拟的方法以及傅立叶传导方程, 计算出四方相和晶界的热导率分别为2.65 W/(m·K)和1.54 W/(m·K)。研究表明, 四方相的热导率比氧化锆陶瓷的热导率高, 而晶界的热导率比氧化锆陶瓷的低。     

AlN-BN复相陶瓷的热等静压制备与性能研究

以氮化铝(AlN)和氮化硼(BN)为原料, 无烧结助剂、热等静压烧结制备了AlN-BN复相陶瓷, 研究了热等静压温度和压强对两种不同原料配比(摩尔比)烧结试样的微观结构和性能的影响。结果表明: 增加BN的添加量对复相陶瓷的烧结致密化影响较小, 但逐渐降低硬度和热导率、增大体积电阻率。相同原料配比下, 复相陶瓷的密度越高, 其热导率、体积电阻率、硬度越高。热导率和体积电阻率的实测值与两相复合模型方程较为符合。当n_AlN:n_BN=75:25时, 在温度为1600℃、压强为90 MPa、保温3 h的热等静压工艺下可以制备出相对密度达98.03%、热导率为77.29 W/(m·K)、体积电阻率为1.35×10¹⁵ Ω·cm的复相陶瓷。     

Low Electrolytic Conductivity Standards

Standards of low electrolytic conductivity were developed to satisfy the demands of the U.S. Navy and American industry for the measurement of high quality water. The criteria for the selection of appropriate solvent and solutes, based on the principles of equivalent conductivity and Onsager’s limiting law, are described. Dilute solutions of potassium chloride and benzoic acid in 30 % n-propanol–water have been chosen as standards. The electrolytic conductivity of both sets of these solutions as a function of molality was determined. Solutions of potassium chloride and of benzoic acid are recommended for use as 5 μS/cm, 10 μS/cm, 15 μS/cm, 20 μS/cm, and 25 μS/cm conductivity standards. Solutions prepared from potassium chloride in 30 % n-propanol–water have been certified as Standard Reference Materials (SRMs). SRM 3198 and SRM 3199 are certified nominally at 5 μS/cm and 15 μS/cm, respectively, at 25.000 °C.     

高定向石墨块的控制制备及其导热性能影响因素研究

以廉价易得的高结晶度天然鳞片石墨(NG)和中间相沥青为原料, 采用中温热模压一次成型再高温炭化、石墨化处理可以制备高密度、高定向、高导热石墨块体材料。XRD、SEM和PLM分析表明该石墨块具有高度择优取向结构, 其内部石墨片垂直热压方向有序堆积排列。原料中鳞片石墨和沥青粘结剂的组成和配比以及制备工艺参数等对所制石墨材料的面向导热性能有显著影响。采用86wt%+32目鳞片石墨和14wt%AR中间相沥青在500℃、10 MPa下热模压成型的炭块经1000℃炭化、2800℃石墨化后样品的热物理综合性能较好, 其体积密度达到1.91 g/m³以上, 室温面向热导率为550 W/(m·K), 3000℃石墨化室温面向热导率高达620 W/(m·K)。     

利用稳态法测量聚合物薄膜纵向热导系数

在一维稳态热传导模型的基础上,设计了一套用于测量聚合物薄膜纵向热导系数的实验装置,并利用Comsol软件对该测量装置进行数值模拟并优化设计。同时利用本文设计的实验装置,测量得到了不同温度下聚酰亚胺(PI)膜、聚四氟乙烯(PTFE)膜以及混合纤维素酯(MCE)膜的热导系数。在35℃~60℃的温度范围内热导系数测量值分别维持在0.21 W/(m.K),0.26 W/(m.K),0.13 W/(m.K)左右,标准不确定度在9.5%以下。测量结果与参考值相符,验证了实验装置的测试精度。     

碳化硅纳米线增强多孔碳化硅陶瓷基复合材料的制备

构建多孔碳化硅纳米线(SiCNWs)网络并控制化学气相渗透(CVI)过程,可设计并获得轻质、高强度和低导热率SiC复合材料。首先将SiCNWs和聚乙烯醇(PVA)混合,制备具有最佳体积分数(15.6%)和均匀孔隙结构的SiCNWs网络;通过控制CVI参数获得具有小而均匀孔隙结构的SiCNWs增强多孔SiC(SiCNWs/SiC)陶瓷基复合材料。SiC基体形貌受沉积参数(如温度和反应气体浓度)的影响,从球状颗粒向六棱锥颗粒形状转变。SiCNWs/SiC陶瓷基复合材料的孔隙率为38.9%时,强度达到(194.3±21.3) MPa,导热系数为(1.9 ± 0.1) W/(m∙K),显示出增韧效果,并具有低导热系数。     

SiC纤维/YSZ复合热障厚涂层韧性及热震性能研究

采用大气等离子喷涂(APS)技术, 以ZrO₂-8wt%Y₂O₃(8YSZ)和团聚的P7216(8YSZ和珍珠岩粉)粉末为原料, 在基体上制备了厚度大于4 mm的SiC纤维/YSZ(SFY)复合厚热障涂层, 通过扫描电子显微镜(SEM)分析了涂层的显微结构, 发现SFY涂层具有钢筋混凝土结构, 这种结构能够防止因为涂层厚度增加而引起的失效。此外, 基于计算机的断层成像技术分析热障涂层孔隙率的变化, 考察了SFY涂层和YSZ 热障涂层的抗热震性能、断裂韧性以及热导率性能, 并探讨了纤维的增韧机制。研究结果表明, SFY涂层具有更高的断裂韧性值和更好的抗热震性能, 25℃时SFY涂层的热导率为0.632 W/(m·K), 大约是传统YSZ热障涂层热导率的一半。SiC纤维对涂层内部裂纹的偏转和截止作用, 防止了裂纹扩散长大, 形成网状微裂纹结构, 有效提高了涂层的抗热震性能和断裂韧性。     

新型多腔孔陶瓷复合保温材料的制备及性能研究

通过原料及配方的创新,以硅酸铝纤维、玻化微珠等为原料制备了一种新型多腔孔陶瓷复合保温材料。研究了材料的导热性能和显微结构。结果表明:材料导热系数低,热面温度200℃时导热系数仅为0.050 W/(m·K),热面温度600℃时导热系数为0.084 W/(m·K);材料内部结构疏松,存在多级配的孔隙结构,孔隙尺寸在微米级以下。利用马弗炉进行保温性能测试,保温材料内表面温度600℃,厚度仅为139mm时,稳态时外表面温度即可低于46℃,散热损失仅为158 W/m~2,远远低于标准规定的最大散热损失266 W/m~2。将材料制成1cm厚度的块材时,材料能产生较大弯曲而不损坏,有利于对电厂高温管道进行包覆。