热管理领域用碳纳米管/铝复合材料的组织与热学性能研究

采用金属有机化学气相沉积工艺在碳纳米管(CNTs)表面包覆了W金属层。采用磁力搅拌和放电等离子体烧结工艺制备了镀钨碳纳米管(W-CNTs)与CNTs增强的铝基复合材料。组织观察结果表明钨金属层有效的加强了CNTs与Al基体的界面结合。随着W-CNTs含量增加,W-CNTs/Al复合材料的热导率先增加后降低,且当W-CNTs含量体积分数为1.5%时,复合材料获得最大热导率;W-CNTs/Al复合材料的热导率高于CNTs/Al复合材料。热膨胀系数结果表明随CNTs含量增加,复合材料热膨胀系数降低,且W-CNTs/Al热膨胀系数低于CNTs/Al复合材料。     

SiC的添加对AlN/Mo/SiC复合陶瓷的制备和性能影响探究

以氮化铝(AlN),钼(Mo),碳化硅(SiC)为原料,采用放电等离子烧结的方式,在1700℃,30 MPa制备出致密度高达98.5％的SiC添加的AlN/Mo复合陶瓷.采用XRD,FESEM,Agilent4284型LCR自动测试仪,网络分析仪及激光导热仪对样品的微观形貌,导电性能,介电性能和热导率进行了测试分析.研究结果表明:采用放电等离子烧结工艺,在较低的温度下可以制备出组织致密均匀的AlN/Mo/SiC复合材料,但随着SiC添加含量的增加,当SiC的体积含量占到总体的60％时,复合材料的致密度又会出现明显的下降.添加SiC后,相对于AlN/Mo复合陶瓷,复合材料的电阻率得以进行有效的控制,不会出现非线性突变-渗流现象,而是在一定的范围内缓慢下降.在26.5～40.0 GHz下复合材料的介电常数和介电损耗均随着SiC含量的增加而增加.复合材料的热导率随着SiC含量的增加而下降:当致密度高于某临界值时,其热导率主要受SiC含量及其分布状态的影响,随着SiC含量的增加呈缓慢下降的趋势;而当致密度低于临界值后,致密度成为热导率的决定性因素,使得复合材料的热导率随着致密度的降低快速下降.     

挤压温度和挤压比对6063铝合金导热性能的影响

研究了挤压温度和挤压比对6063铝合金组织及导热性能的影响。结果表明：通过挤压能有效地改善该合金的组织,使得强化相Mg2 Si均匀弥散分布在α-Al基体上。随着挤压温度的增大,材料的热导率下降。当挤压比小于50时,材料的热导率随着挤压比的增大而增大;当挤压比大于50时,材料热导率下降。当挤压比为50,挤压温度为380℃时,材料有最大热导率221.2 W/（m.K）。     

导电颗粒Mo对AlN/Mo复合陶瓷性能的影响

以氮化铝、钼为原料,CaF2为烧结助剂,在氮气气氛下,采用无压烧结方式制备了致密度高于98%的AlN/Mo复合微波衰减陶瓷。采用XRD,FESEM,网络分析仪及激光导热仪对样品的微观结构、介电性能及热导率进行测试分析。结果表明:样品的致密度随着Mo含量的增加逐渐增大。电阻率的变化符合渗流理论,渗流阈值在Mo含量为23%附近。介电常数和介电损耗均随着Mo含量的增加而增加。复合陶瓷的热导率受致密度和Mo含量两个因素的影响,当致密度低于某临界值时,热导率随着致密度的增加而增加;当致密度超过临界值后,复合陶瓷的热导率主要受Mo含量的影响,随着Mo含量的增加,热导率逐渐减小。在烧结温度为1720℃时,Mo含量为5%的复合陶瓷的热导率为114.6 W.m-.1K-1。分析了复合陶瓷的显微结构与导热理论。     

耐火材料对热工设备节能环保作用的探讨

耐火材料对热工设备节能环保作用大。当阻止热量损失时,用热导率低的隔热保温材料;当隔火焰加热及余热回收热交换时,用高热导率耐火材料;当蓄热储能时,用热容量大的高热导率耐火材料;电能转换的发热体是特种耐火材料高温陶瓷。     

CaCl2-NaCl熔盐中槽电压对SiO2电还原影响机制研究

以SiO2为原料,采用熔盐电解法制备得到了纳米尺度的单质Si。研究了槽电压对SiO2电还原过程和单质Si微观结构的影响。在槽电压1.7~2.0V范围内,较高的槽电压有利于SiO2的电解还原,在1.8~2.0V槽电压下,SiO2能够被电解完全生成单质Si。当槽电压为1.8V时,电解产物Si的微观结构为40nm左右宽的纳米线;当槽电压为1.9V时,微观结构包括50nm左右宽纳米线和粒径100nm左右的纳米颗粒;当槽电压为2.0V时,微观结构为100~300nm的纳米颗粒堆积。     

Ar气流量对石墨表面CVD TaC涂层生长与表面形貌的影响

用C3H6作为碳源气,Ar作为稀释气体和载气,TaCl5为钽源,采用化学气相沉积法(chemical vapor deposition,CVD)在高纯石墨表面制备TaC涂层。采用X射线衍射(XRD)和扫描电镜(SEM)等对涂层进行表征,研究1 000℃下稀释气体(Ar)流量对TaC涂层成分、织构及表面形貌的影响。结果表明:随着稀释气体流量增大,表面均匀性和光滑度提高,晶粒尺寸减小,晶体择优取向降低,沉积速率减小,涂层中C含量增多。当稀释气体流量为100 mL/min时,TaC涂层晶粒尺寸与沉积速率分别为32.5 nm和0.60μm/h;而当稀释气体流量增大到600 mL/min时,涂层晶粒尺寸与沉积速率分别下降到21 nm和0.25μm/h。     

热冲击对金刚石/铜复合材料的热学性能影响

采用压力浸渗和超高压熔渗法制备不同界面状态的金刚石/铜复合材料,分析界面状态对热学性能的影响,重点研究在-65~125℃和-196~85℃两种热冲击载荷下,循环100周次后材料的热导率和热膨胀系数的变化规律。结果表明:通过添加Cr元素的Dia/CuCr和使用超高压制备的EHV-Dia/Cu,材料的界面状态得到了改善;界面强度的提高,有利于获得高热导率,低热膨胀系数的复合材料。Dia/Cu的热导率仅有459.1 W·m-1·K-1,而EHV-Dia/Cu高达678.2 W·m-1·K-1,Dia/CuCr则为529.7 W·m-1·K-1。-55~125℃的热冲击条件下,Dia/Cu,Dia/CuCr,EHV-Dia/Cu的热导率保持良好的稳定性,变化在2.5%以内。而在-196~85℃的热冲击条件下,Dia/Cu由于界面结合力弱,在热应力的作用下热导率急剧下降;Dia/CuCr和EHV-Dia/Cu则表现出了良好的抗热冲击能力,循环后热导率仅下降3%左右。Dia/Cu和Dia/CuCr的初始热膨胀系数分别为8.45×10-6K-1和6.93×10-6K-1,Cr元素的添加使得界面结合强度提高,低膨胀系数的金刚石对高膨胀系数的基体约束力增加,使得热膨胀系数明显下降。在两种热冲击实验条件下,Dia/Cu的热膨胀系数基本保持不变,Dia/CuCr分别上升6.64%和7.22%。     

镀铬金刚石粒度、品级和膜厚对金刚石/铜复合材料热物性能的影响

对不同品级和粒度的金刚石进行表面镀膜改性,用熔渗法制备了金刚石/铜复合材料。研究了金刚石品级、粒度与膜厚对复合材料热导率以及热膨胀特性的影响。结果表明:金刚石/铜复合材料的热导率随金刚石粒度增大和品级提高而增大;在其他条件相同的情况下,金刚石表面铬膜厚度为210 nm时复合材料的热导率为787 W/(m·K),当铬膜厚度为150 nm时复合材料的热导率为633 W/(m·K),表明镀铬膜太薄会降低复合材料的热导性能;在30~200℃范围内,金刚石/铜复合材料的热膨胀系数在(3.00~8.00)×10~(-6)K~(-1)之间,且随温度升高而升高。     

LiF-[CaF2/YbF3]熔盐体系热物性研究

以热分析数据为基础，采用外推切线法分析和确定LiF-CaF2及LiF-YbF3熔盐体系的共晶点成分，试验测定体系的热导率和热膨胀系数随温度的变化规律，并与预测模型计算结果进行了对比。结果表明，LiF-22%CaF2及LiF-21%YbF3（摩尔比）的体系更接近共晶成分点；两个共晶体系热导率在950~1 200 K内随温度的升高而降低，模型估算结果为线性变化，试验值为非线性变化，模型估测值在液态时与试验值符合较好；同时，体系热膨胀系数在低于熔点的固态时随温度的升高而上升，达到熔点后开始下降，完全融化的液态熔盐热膨胀系数近似为线性减少。     

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.      

Comparative study of pore structure evolution during solvent and thermal debinding of powder injection molded parts

The solvent debinding process has been widely accepted in the powder injection molding (PIM) industry due to its short debinding cycle. In the current study, specimens were immersed in a heptane bath for different lengths of time, and the pore structure evolvement in the compact was analyzed. Mercury porosimetry analyses and scanning electron micrographs showed that the binder extraction started from the surface and progressed toward the center of the compacts. As the debinding contin-ued, the pores grew and were widely distributed in size. This pore structure evolvement was different from that of straight thermal debinding in which the pore size distribution was quite narrow and the mean pore diameter shifted toward smaller sizes as debinding time increased. After the soluble binders were extracted, parts were subjected to a subsequent thermal debinding during which these pores served as conduits for decomposed gas to escape. Concurrently, the remaining binder became fluidlike and was redistributed within the compact due to capillarity. This pore structure, as observed from the mercury intrusion curves, showed a sharp increase in the pore volume at the 0.8-μm size, followed by a series of fine pores, which is different from the pore structure of straight thermal debinding. The difference in the pore structure evolvement between solvent and thermal debinding and its effect on the debinding rate are discussed.     

介孔二氧化硅基复合相变材料研究进展

相变储能技术的发展对于促进新能源开发和提高能源利用效率具有非常重要的意义。相变材料由于具有高储能密度和小体积变化等优势引起了人们的广泛关注。然而，相变材料在固–液相转变过程中易发生液体泄漏而限制了其应用。因此，人们选择用多孔支撑材料来解决相变材料的泄露问题。介孔二氧化硅材料由于具有良好的物理化学稳定性、生物相容性、阻燃性能、低毒性、耐腐蚀性、尺寸可控、表面形貌可调和高比表面积等优点，其作为载体材料能综合提高相变复合物的各方面性能并拓宽相变储能材料的应用空间。对近年来国内外关于介孔二氧化硅载体的孔尺寸、孔结构和孔表面性质对相变材料结晶行为的影响等方面进行了综合分析，并对今后提高介孔二氧化硅相变材料储能效率的研究方法的前景做了展望。      

Simulation of microporosity formation in modified and unmodified A356 alloy castings

In order to comprehensively model both the performance and inspectability of early design stage safety critical aluminum castings, the size, shape, and location of defects such as pores should be determined by simulation. In this article, a two-dimensional (2-D) model to predict grain size, pore size, pore morphology, and location is presented. The proposed model couples hydrogen gas evolution and microshrinkage pore formation mechanisms with a grain growth simulation model. The nucleation and growth of grains are modeled with a probabilistic method that uses the information from a macroscale heat transfer simulation to determine the rules of transition for grain evolution. Microshrinkage pores and the combination of microshrinkage and gas pores are addressed. The proposed model and postprocessing can provide direct simulated views of the microstructure of the solidifying casting. In the present work, the effect of Sr modifier and hydrogen content on pore size and morphology for equiaxed aluminum alloy A356 is modeled. The simulation results correlate well with the experimental observation of cast structures and other published data. In addition, Sievert’s law and the conditions for spontaneous growth of a gas pore are derived from first principles in the Appendix.     

On the removal of pores from castings by sintering

The causes, sizes, and distribution of porosity in castings have been reviewed and quantitatively evaluated for several important modes of alloy solidification. In general, gas exsolution is found to be the most probable cause of porosity in castings which solidify in either a cellular or dendritic fashion. On the other hand, solidification alone may cause porosity creation if the interdendritic liquid metal cannot feed the solidification shrinkage. This effect may be enhanced by gas exsolution. Removal of porosity by “sintering” after solidification requires that the grain size be of the order of, or smaller than, the pore spacing, and that the pores be small (>1 μ) for removal within reasonable times (tens of hours). When gas exsolution is the cause of pore creation, the gas must be diffused out of the sample to permit pore shrinkage. Small ingot sizes (>10 cm) and rapidly diffusible gases (H₂) are required for pore elimination within reasonable times (tens of hours). The application of low pressure (>20 atm) during sintering increases the rate, or the size (to >10 mμ) of the pores which can be eliminated within >20 hr. This paper is based on an invited talk presented at a symposium on Homogenization of Alloys, sponsored by the IMD Heat Treatment Committee, and held on May 11, 1970, at the spring meeting of The Metallurgical Society of AIME, in Las Vegas, Nev.     

Quantification and Taxonomy of Pores in Thermal Spray Coatings by Image Analysis and Stereology Approach

Porosity is one of the most important microstructural features in thermal spray coatings and has been actively studied and measured by many methods. Image analysis techniques have become popular techniques in determining porosity in coatings because of simplicity, accessibility, and an ability to measure both open and closed porosities as well as pore characteristics such as size, shape, orientation, and spatial distribution. In the current study, an image analysis technique has been complemented by several stereology procedures to determine the porosity level and characteristics of pores within coatings. Stereology protocols such as Delesse, DeHoff, and Cruz-Orive analyses were used to derive the porosity level, pore size, and shape distributions, and the effectiveness of each stereology protocol was compared. Standoff distance (SOD) and annealing process did not alter the distribution trend of number of pores but influenced the distribution of pore volume fractions significantly. The bivariate size–shape distribution of the pores was used to predict the dominant pore type and fractions of pores that arose from different formation mechanisms. It was found that nearly spherical pores that originated from gas bubbles and entrapped gas pockets dominate at shorter SOD, while the different types of pores become more evenly distributed when the SOD was increased.     

The thermal conductivity of slags used in electroslag remelting

Measurements of thermal diffusivities of solid ESR slags from the system CaF₂ + Al₂O₃ + CaO for temperatures between 200 and 1200°C using laser flash method. Thermal conductivities derived from these data. In the polycrystalline specimens used in this study the radiation conduction is shown to be negligible, heat transfer occurs predominantly by phonon conduction. An appreciable contribution to the thermal conductivity arises from gaseous conduction across pores and microcracks in the specimen. Thermal conductivity decreases with decreasing CaF₂ content.     

添加红柱石对高炉炭砖孔径分布与热导率影响

采用X射线衍射仪、压汞仪、激光导热仪、扫描电子显微镜和能谱分析仪等测试手段,研究了不同红柱石加入量的焙烧炭砖的孔径分布及热导率;并借助灰色关联理论分析了气孔孔径区间与炭砖热导率的相关性.结果表明:炭砖的气孔孔径分布受红柱石的分解和SiC生成量共同影响.其提高微孔化的机理是:引入的红柱石在1300℃开始原位分解可提高微孔...     

内置高导C/C材料的疏导式热防护结构防热效果

本文建立了内置高导C/C材料的疏导式热防护结构原理模型,通过实验的方法给出了高导C/C材料与耐热三维编织C/C材料间的接触热阻,并利用数值仿真针对影响结构热防护效果的若干关键参数进行了参数影响研究.研究结果表明:减小耐热层厚度是一种降低驻点温度的有效方法,但是必须同时考虑由此引起的强度问题;界面接触热阻对热防护效果影响很大,必须通过工艺处理降低界面热阻才能实现有效的热防护.      

原料处理工艺及气孔演化对AZO靶材烧结致密化的影响

以ZnO和Al_2O_3粉体为原料,按照98∶2(质量比)球磨混合制备氧化锌铝(aluminum zinc oxide,AZO)粉体,通过扫描电镜和透射电镜观察粉体形貌及均匀性,确立最佳混合工艺;将混合均匀的粉体进行煅烧预处理后进行热压烧结致密化试验,通过差热法分析原料性能,阿基米德法分析靶材密度以及压汞实验分析气孔演化对AZO靶材烧结致密化的影响。结果表明:微纳米粉体在球磨4 hr时Al元素分布均匀性最好,煅烧前的混合粉体在差热分析过程中失重0.43%并伴随放热效应;煅烧处理后粉体形貌类球形,在差热分析实验0℃-1000℃无失重,表明挥发组分排除干净且无异常放热效应,烧结性能得到优化。采用预处理的AZO粉体在不同热压温度和保温时间下制备AZO靶材,分析热压致密过程中平均孔径和闭孔率的变化规律。由此发现:在热压致密化初级阶段(温度900℃~1100℃时),连通气孔发生合并与生长,平均孔径先稍有收缩随后迅速增大为158 nm,同时闭孔率维持在0.5%以下,靶材密度不断上升;当温度升高至1150℃时,连通孔径逐渐缩小至108 nm,大量连通气孔转为闭合状态导致闭孔率迅速上升至7.2%以上,表明体系已经进入到热压深度致密化阶段。在1150℃、18 MPa下,保温时间延长到70 min时,连通气孔合并生长同时闭孔率异常增加至7%,孔径分布范围变宽导致出现反致密化现象;当保温保压时间延长至90 min时,气孔尺寸减小到136 nm,闭孔率下降至3.7%,靶材相对密度上升到96.2%,在较低压力和较短保温时间下实现了AZO靶材的深度致密化。