电场处理对碳化硅/聚合物复合材料电导特性的影响

通过在碳化硅(SiC)/低密度聚乙烯(LDPE)的热压成型和SiC/硅橡胶的硫化过程中施加不同形式的直流电场,研究了电场处理对SiC/聚合物复合非线性绝缘材料电导特性的影响.研究结果表明,在SiC/LDPE的热成型过程中待到模具中的物料流动结束后施加均匀电场仅使复合材料的电导率发生微弱变化,而在SiC/LDPE的热压流动过程中施加均匀电场导致复合材料的电导率明显增加;SiC/硅橡胶共混物的粘度较SiC/LDPE的粘度低,故均匀电场处理导致前者电导率增加的趋势较后者明显;非均匀电场处理导致复合材料的电导率明显增大,电导非线性特性明显得到改善,其作用效果明显好于均匀电场.     

微纳米SiC/环氧树脂复合材料的界面和非线性电导特性

以微米和纳米SiC为填料,制备了不同填料配比的微纳米SiC/环氧树脂(EP)复合材料。测试了微纳米SiC/EP复合材料的玻璃化转变温度、室温介电谱和直流电导特性。分析了填料与基体之间的界面对玻璃化转变温度、介电谱及直流电导特性的影响。实验结果表明,在微米和纳米SiC填料的共同掺杂下,随着纳米SiC填料含量的增加,微纳米SiC/EP复合材料的玻璃化转变温度先降低后升高。在相同频率下,微纳米SiC/EP复合材料具有更低的相对介电常数和低频损耗峰幅值。与EP相比,微纳米SiC/EP复合材料具备显著的非线性电导特性。与微米SiC/EP复合材料相比,微纳米SiC/EP复合材料具有更高的非线性指数和阈值电场强度。微纳米SiC/EP复合材料的非线性电导特性与SiC颗粒和EP基体之间的界面区密切相关。      

制备工艺对热压烧结SiC/SiC复合材料结构与性能的影响

采用纳米SiC和亚微米SiC粉料作为基体形成原料,通过热压烧结技术制备了SiC/SiC 复合材料.研究了粉料颗粒、烧结温度、烧结压力对复合材料显微结构和各种性能的影响.结果显示,采用纳米碳化硅粉体可有效降低烧结温度,促进复合材料的致密化过程,在1780℃、20MPa条件下可获得性能优良的复合材料.而采用亚微米SiC粉体,复合材料的致密化过程需要较高的温度,但随着密度的增加,基体与纤维之间的作用力增强,不利于性能的提高.     

碳化硅／硅橡胶复合材料的非线性电导特性

为研究碳化硅(SiC)/硅橡胶复合材料的非线性电导特性,测试了填加不同种类、掺量、晶型及粒径SiC的SiC/硅橡胶复合材料的直流伏安特性.研究结果表明:SiC/硅橡胶复合材料的电导率随SiC掺量的增加而增大,且当SiC掺量超过一定值后,复合材料的电导机理发生变化;当SiC掺量相等时,在相同强度电场作用下,黑SiC、纳米SiC复合材料的直流电导率分别大于绿SiC、微米SiC复合材料的电导率,且前者的电导非线性特性明显优于后者;与α-SiC/硅橡胶复合材料相比,β-SiC/硅橡胶复合材料的电导非线性系数值发生变化的电场强度低,其最大非线性系数值明显大于前者,且在场强增大到一定值后载流子浓度的增加趋于饱和,其非线性系数值又变小.     

高压扭转法对SiC_p-Al基复合材料颗粒分布的影响

采用高压扭转(High-Pressure Torsion,HPT)工艺制备SiCp-Al基复合材料。通过显微组织的定性分析及样方法的定量计算,深入研究不同工艺参数对SiC颗粒分布的影响规律,结果表明:采用高压扭转法可以直接将8.75%(体积分数)SiC-Al混合粉末制备成金属基复合材料。通过金相分析得出:SiC颗粒在试样不同扭转半径处分布情况具有差异:工艺参数(温度、压力、圈数)对SiC颗粒分布有重要影响,结合样方法对颗粒分布情况的定量分析得出:随着扭转圈数、压力、扭转半径的增大,剪切作用增强,SiC颗粒分布均匀性提高;变形温度升高,基体流动性提高,颗粒分布均匀性得到改善。     

UHMWPE纤维增强LDPE复合材料的界面形态研究

为实现聚乙烯单聚合物复合材料(PE SPC)的嵌件注射成型,研究基体与增强体间的界面非常关键.本文采用超高分子量聚乙烯(UHMWPE)纤维增强低密度聚乙烯(LDPE)基体,对纤维和基体进行了差示扫描量热仪测试,在偏光显微镜下模拟了基体与纤维的复合过程,研究不同因素对复合材料界面结晶形态的影响.根据DSC确定了UHMWPE和LDPE复合的温度范围在110.98~147.14℃;合适的温度和剪切作用都有利于界面横晶的产生,从而使基体和纤维产生更好的粘结,提高复合材料的力学性能;温度比剪切的影响更大,注射温度设置在125~135℃可在保证纤维与基体复合的情况下不破坏纤维的增强作用;纤维丝之间会相互影响界面结晶形态,部分界面有横晶产生,说明在实际注射成型过程中纤维束或纤维布的结构对基体渗透和界面形成有较大影响.     

两种典型星用聚合物介质抗内带电改性防护技术研究

空间高能电子辐射易造成星用聚合物介质内带电水平过高,是卫星运行可靠性的潜在威胁因素。对航天器介质材料进行非线性电导改性是提高航天器介质材料自释电荷能力,进而降低内带电水平的有效方法。实验选用半导电无机添加剂对典型空间聚合物材料聚四氟乙烯、聚酰亚胺进行改性工艺和电导特性研究,测量了常态体电导率与添加剂含量关系,不同含量添加剂下复合材料体电导率随温度与电场的变化规律以及复合材料的导热性能参数。实验表明,添加剂含量会显著影响复合材料的体电导特性及导热特性,特别是复合材料的非线性电导特性阈值电场降低最为明显。这种既能保持高绝缘性能又具有良好非线性电导特性的新型复合介质材料,有希望成为从根本上解决星用聚合物介质深层带电问题的有效措施。     

碳化硅/低密度聚乙烯复合材料的直流伏安特性

为探讨碳化硅（SiC）/低密度聚乙烯（LDPE）复合材料的电导非线性特性,结合扫描电镜观察和X射线衍射分析研究了SiC的掺量、种类、晶型及粒径对SiC/LDPE复合材料直流伏安特性的影响。研究结果表明:SiC掺量增加可导致SiC/LDPE复合材料电导率增大和电导非线性系数发生改变的临界场强值降低;当外施电场强度相等时,在相同的SiC掺量下,纳米α-SiC、绿β-SiC、黑α-SiC复合材料的电导率分别大于微米α-SiC、绿α-SiC、绿α-SiC复合材料的电导率,且前者的电导非线性特性明显优于后者;当黑 α-SiC的掺量达到37.5wt%时,SiC粒径的增大可导致双对数坐标下的电导率与场强关系曲线的拐点向低场强方向移动。      

MoS2/Ti3SiC2层状复合材料的制备

采用热压烧结方法制备MoS2/Ti3SiC2(MoS2质量分数为2%)的层状复合材料.研究了不同烧结温度对烧结试样性能的影响.研究表明,在1 400℃,30 MPa压力和保温2 h条件下,可以得到致密度达99%以上的MoS2/Ti3SiC2复合材料;在Ti3SiC2中添加MoS2后,烧结温度越高维氏硬度越大;在1 400℃,烧结试样维氏硬度达6 220 MPa,高于纯Ti3SiC2材料的4 000 MPa;MoS2有良好的导电性能,使得烧结试样的电导率比较高,在1 400℃,烧结试样电导率达9.68×106 S·m-1,是纯Ti3SiC2材料的2倍.     

高温处理对3D C/SiC复合材料热膨胀性能的影响

研究了不同高温处理前后3D C/SiC复合材料热膨胀系数(CTE)的变化规律,从材料内部热应力变化及结构改变的角度定性地分析了其变化机理。研究发现,3D C/SiC复合材料的热膨胀系数受界面热应力的影响,其变化规律是纤维和基体相互限制、相互竞争的结果;高温处理可提高材料的热稳定性,并通过改变界面热应力及材料内部结构,来影响材料热膨胀系数的变化规律;通过增加基体裂纹来降低复合材料的低温热膨胀,但不影响其变化规律;通过改变材料内部结构,使热应力发生变化并重新分布,对复合材料的高温热膨胀产生显著影响。但高温处理没有改变3D C/SiC复合材料的基体裂纹愈合温度(900℃)。      

不同填料复配对尼龙6/石墨烯复合材料导热性能的影响

高分子材料的绝热特性极大地限制了其作为导热材料在工业中的应用。选用多层石墨烯作为导热填料,并分别与导热填料氧化铝(Al_2O_3)和碳化硅(SiC)复配,探究导热填料的复配对尼龙6(PA6)复合材料导热性能的影响。加入质量分数为3%石墨烯时,PA6复合材料的热导率为0.548W·m^-1·K^-1,相比PA6基体提高161%。通过调节石墨烯与Al_2O_3和SiC复配的比例以及复合填料量,PA6复合材料的热导率可控在0.653~4.307W·m^-1·K^-1之间,最高是PA6基体的20倍。为拓展石墨烯在导热材料方面的应用及PA6导热材料在工业上应用提供了有价值的实验依据。     

Effect of filament and substrate temperatures on the structural and electrical properties of SiC thin films grown by the HWCVD technique

The effect of varying filament and substrate temperatures on the structure and electrical conductivity of crystalline SiC films prepared by HWCVD technique are described in this paper. At a constant filament temperature, the electrical conductivity of the SiC films increases with increasing substrate temperature. However, TEM studies show that there is no change in the size of the SiC columnar grains. On the other hand, a significant variation in filament temperature at constant substrate temperature leads to a variation of structure and conductivity. Raman spectroscopy and TEM studies reveal that crystallinity improves with increase in filament temperature. Furthermore, a μc-Si phase exists alongside SiC at low filament temperature (1750 °C).     

KD-II型碳化硅纤维热输运性质的实验研究

SiC纤维是复合材料SiC_f/SiC中的组分材料, 其力学性能已经得到实验验证, 但热学性能尚未见报道。本研究采用综合T型法测量了不同温度热处理的KD-II型SiC纤维在80~300 K温度范围内的电导率、热导率和塞贝克系数, 热处理温度分别为1400、1500和1600℃。研究发现, 在实验设定的热处理温度范围内, SiC纤维电导率不随热处理温度改变而改变, 但其热导率随热处理温度升高有显著的变化。在环境温度为290 K时, 1600℃热处理的SiC纤维热导率为11.6 W•m^-1•K^-1, 比未热处理的材料提高了42%以上。     

Pressure infiltration casting process and thermophysical properties of high volume fraction SiCp/Al metal matrix composites

Abstract

SiC_p/Al composites containing high volume fraction SiC particles were fabricated using a pressure infiltration casting process, and their thermophysical properties, such as thermal conductivity and coefficient of thermal expansion (CTE), were characterised. High volume fraction SiC particulate preforms containing 50–70 vol.-%SiC particles were fabricated by ball milling and a pressing process, controlling the size of SiC particles and contents of an inorganic binder. 50–70 vol.-%SiC_p/Al composites were fabricated by high pressure infiltration casting an Al melt into the SiC particulate preforms. Complete infiltration of the Al melt into SiC preform was successfully achieved through the optimisation of process parameters, such as temperature of Al melt, preheat temperature of preform, and infiltration pressure and infiltration time after pouring. Microstructures of 50–70 vol.-%SiC_p/Al composites showed that pores resided preferentially at interfaces between the SiC particles and Al matrix with increasing volume fraction of SiC particles. The measured coefficients of thermal expansion of SiC_p/Al composites were in good agreement with the estimated values based on Turner's model. The measured thermal conductivity of SiC_p/Al composites agreed well with estimated values based on the 'rule of mixture' up to 70 vol.-% of SiC particles, while they were lower than the estimated values above 70 vol.-% of SiC particles, mainly due to the residual pores at SiC/Al interfaces. The high volume fraction SiC_p/Al composite is a good candidate material to substitute for conventional thermal management materials in advanced electronic packages due to their tailorable thermophysical properties.     

In Situ Thermocouples in Macro-Components Fabricated Using SALD and SALDVI Techniques. II. Evaluation of Processing Parameters

In order to fabricate well-control led in situ SiC/C thermocouples embedded within macro-structural SiC components using an integrated selective area laser deposition (SALD) and the selective area laser deposition and vapor Infiltration (SALDVI) technique, the major processing parameters affecting the crystal structure, the deposition rate, surface morphology of deposits, and shapes and sizes of the cross section of deposited lines are evaluated. It is found that the growth rate of SiC deposits increases with temperature and tetramethy Isilane (TMS) gas pressure over the temperature and pressure range studied. The apparent activation energy for depositing SiC from TMS is 61 kJ/mole in the temperature range from 700 to 1200°C and independent of the TMS gas pressure ranging from 20 to 60 torr. The shape and size of the cross section of SiC lines depend strongly on the deposition temperature. XRD examination indicates that the deposition product using a C₂H₂ precursor at 900°C is crystalline graphite. The crytallinity of Si₃N₄ deposits is affected by the substrate material even though the deposition temperature and other process parameters are the same. These phenomena have been explained in terms of the growth controlling mechanisms of deposits, the temperature distribution induced by an incident laser beam, and the thermal conductivity of the substrate.     

Novel silicon carbide/polypyrrole composites; preparation and physicochemical properties

Novel silicon carbide/polypyrrole (SiC/PPy) conducting composites were prepared using silicon carbide as inorganic substrate. The surface modification of SiC was performed in aqueous solution by oxidative polymerization of pyrrole using ferric chloride as oxidant. Elemental analysis was used to determine the mass loading of polypyrrole in the SiC/PPy composites. Scanning electron microscopy showed the surface modification of SiC by PPy. PPy in composites was confirmed by the presence of PPy bands in the infrared spectra of SiC/PPy containing various amounts of conducting polymer. The conductivity of SiC/PPy composites depends on PPy content on the surface. The composite containing 35 wt.% PPy showed conductivity about 2 S cm⁻¹, which is in the same range as the conductivity of pure polypyrrole powder prepared under the same conditions using the same oxidant. PPy in the composites was clearly detected by X-ray photoelectron spectroscopy (XPS) measurements by its N1s and Cl2p peaks. High resolution scans of the C1s regions distinguished between silicon carbide and polypyrrole carbons. The fraction of polypyrrole at the composite surface was estimated from the silicon and nitrogen levels. The combination of XPS and conductivity measurements suggests that the surface of the SiC/PPy composites is polypyrrole-rich for a conducting polymer mass loading of at least 12.6 wt.%.     

Thermal conduction in Cr3C2/SiC composite

Thermal conduction behaviour of Cr₃C₂/SiC composite is investigated in terms of temperature and SiC content. Experimental results showed that thermal diffusivity of the composite increases with SiC content up to 20 vol%, corresponding to a conductivity maximum, then decreases with further increase of SiC. The reduction in diffusivity and conductivity at higher SiC content may due to formation of small amounts of solid solution at the interface and/or of interfacial gaps due to lack of perfect contact among SiC aggregates leading to increased phonon scattering. The thermal conductivity demonstrates a positive temperature dependence but becomes temperature-independent when SiC content is above 30 vol%. A correlation with composite theory is present.     

Fine SiC fiber synthesized from organosilicon polymers: relationship between spinning temperature and melt viscosity of precursor polymers

A very fine silicon carbide (SiC) fiber with diameter of 6 m, about a half of that of a commercially available SiC fiber, was synthesized from a polymer blend of polycarbosilane (PCS) and polyvinylsilane (PVS). The fine SiC fiber was obtained by optimizing the composition and the spinning temperature of PCS-PVS polymer blends. In order to determine these optimum conditions, the relationship between temperature and melt viscosities of the polymer blends was investigated. As a result, it was found that the optimum spinning temperature range was within a temperature range where the melt viscosity is 5–10 Pa · s. Moreover, by blending PVS with PCS, the spinning temperature of the polymer blends was lowered, the spinnability of polymer system was improved, and finer polymer fiber was obtained compared with PCS. The optimum content of PVS in the polymer blend was 15–20 wt%.     

On the thermal conductivity of bimodal SiC/A356 composites fabricated via powder metallurgy route

The present study investigates the thermal conductivity of bimodal SiC particulate distribution in aluminum matrix composites fabricated via powder metallurgy route. The effects of the SiC_p reinforcement size distribution and processing parameters such as sintering time and temperature on the thermal conductivity have been examined. The Box–Behnken experimental array was employed to identify the effects of selected variables on the thermal conductivity of the composite. A reasonable augmentation in the thermal conductivity was observed with an increase in sintering time and %volume fraction of fine SiC particulates. It has been demonstrated that the matrix doped with fine SiC particulates (37?µm) occupied interstitial positions and formed continuous SiC–matrix network resulting in minimizing the micropores that contributed for good thermal conductivity, that is, 235?W/mK. Scanning electron microscopy (SEM) and x-ray diffraction (XRD) were conducted to evaluate the microstructure architecture and interfacial phase formation.     

SiC换热器材料热物理性质的研究

本文采用ＴＬＰ－１８型激光热常数仪和岛津ＴＭＡ－３０热分析仪研究了温度对等静压ＳｉＣ换热器材料的导热系数和热膨胀系数的影响，并对影响ＳｉＣ抗热震性能的各因素进行了分析。