预氧化时间对中空多孔炭纤维介电常数的影响

以中空多孔聚丙烯腈( polyacrylonitrile,PAN)原丝为原料,通过预氧化和炭化工艺制备了中空多孔炭纤维(Hollow-porous carbon fibres,HPCFs).研究了预氧化时间对其微观结构、元素组成、电导率和介电常数的影响.结果表明:预氧化时间对所获HPCFs微观结构的影响不大;HPCFs的碳含量随预氧化时间增加而减少,而氮含量则随预氧化时间增加而增加.随着预氧化时间增加,所得HPCFs电导率和介电常数依次降低.预氧化时间60 min、炭化温度800℃条件下所制HPCFs在2GHz处的介电常数实部和虚部分别为17.20和11.90,对应的电导率为564.50Ω1·m-1.可通过热处理工艺调节HPCFs的电导率和介电常数.     

预氧化温度对中空多孔炭纤维介电常数的影响

以中空多孔聚丙烯腈(PAN)原丝为原料,通过预氧化和炭化工艺制备了中空多孔炭纤维。研究了预氧化温度对其微观结构、元素组成和介电常数的影响。结果表明,预氧化温度对中空多孔炭纤维微观结构的影响不大;碳含量随预氧化温度的升高而减少,而氮、氧含量则相反。随预氧化温度的升高,所得中空多孔炭纤维的介电常数依次降低。在2GHz处,预氧化温度为250℃时ε′和ε″分别为23.12和73.51,而预氧化温度为310℃时ε′和ε″分别为19.32和9.87。因此,可通过预氧化温度来调节中空多孔炭纤维的介电常数。     

高比表面积聚苯乙烯基球状活性炭的制备研究

以聚苯乙烯基大孔吸附树脂球为炭前驱体,经空气预氧化、炭化和活化制备了高比表面积球状活性炭.系统考察了不同氧化和活化条件对氧化球和活化球的物理性能的影响.结果表明:升温速率、氧化温度和氧化时间分别为0.25℃/min、300℃和3h时所得到的氧化球的CCl4吸附值最高,可达970mg/g.此外,当活化温度和活化时间分别为850℃和4h时,球状活性炭的CCl4吸附值最高,为2700mg/g,相应的比表面积为1759m2/g.     

Si—B掺杂沥青基炭纤维的制备及其抗氧化性能

为了提高炭纤维的高温抗氧化性能,提出了一种制备Si—B掺杂沥青基炭纤维的方法。通过聚硼硅氮烷(PSNB)和石油沥青低温共裂解合成了Si—B掺杂沥青,Si—B掺杂沥青经熔融纺丝、原丝预氧化和炭化得到Si—B掺杂沥青基炭纤维。研究了Si—B掺杂沥青及其炭纤维的组成、微观结构和低温抗氧化性能。结果表明,随原料沥青中PSNB掺杂比例的提高,Si—B掺杂炭纤维的拉伸强度和杨氏模量逐渐降低,抗氧化性能逐渐增强。1 400℃炭化得到的Si—B掺杂炭纤维在600℃氧化240 min失重率为25%,650℃氧化140 min失重率为60%。未掺杂炭纤维在相同条件下的氧化失重率分别为46%和99%。Si—B掺杂炭纤维氧化形成的B_2O_3具有较好的流动性,可以在纤维表面形成连续的玻璃膜,有效地抑制基体炭的氧化。     

以中间相沥青为粘结剂的低密度高导热炭纤维网络体的研究

以中间相沥青为粘结剂, 采用500 ℃低温炭化炭纤维, 经低压模压成型、炭化和石墨化后得到低密度高导热炭纤维网络体。与以1300 ℃炭化炭纤维为原料和以酚醛为粘结剂制备的炭纤维网络体进行了比较。对粘结剂炭收率(热重分析)、样品微观形貌(扫描电子显微分析)、石墨化度及微晶尺寸(X射线衍射分析)等进行了表征。研究结果表明: 由于高炭收率和高片层取向度的中间相沥青与500 ℃低温炭化处理炭纤维共同经历后续热处理时呈现出相近的热收缩率, 因而具备良好的相互粘结性和石墨片层铆接效应, 其制备的炭纤维网络体经石墨化后密度为0.317 g?cm ^-3, 由此制备的相变复合材料的面内热导率为19.30 W·m ^-1·K ^-1, 较纯相变材料(石蜡)提升了80倍, 明显高于以1300 ℃炭化炭纤维为原料, 以中间相沥青和酚醛分别为粘结剂制备样品的面内热导率(17.03和14.47 W·m ^-1·K ^-1)。     

真空碳化处理对聚丙烯腈基碳纤维介电性能的影响

将聚丙烯腈基预氧化纤维编织布在真空烧结炉中以10℃/min速率升温到650℃、700℃、750℃、800℃和850℃进行2h保温碳化处理.在固定碳化温度为650℃的情况下,保温10min、2h和4h后随炉冷却至室温.研究了真空碳化处理温度和保温时间对聚丙烯腈基预氧化碳纤维结构、电导率和介电性能的影响,结果表明,经真空碳化处理后,纤维电导率随温度的升高先增大后减小.当碳化温度为650～800℃时,纤维的介电常数实部(ε′)和虚部(ε″)都随碳化处理温度的升高而增大,且虚部的增大速度高于实部的增大速度.当碳化温度高于750℃后,在8.2～12.4GHz内纤维介电常数随频率的增大而减小.当碳化处理温度达到850℃时,纤维的介电常数相对于800℃时有所减小.在650℃,经过不同保温处理的纤维布,随保温时间的延长,纤维的介电常数实部和虚部都增大.根据以上研究结果,可以为制定以经适当碳化处理的纤维布为吸收体的吸波材料提供有效参考.     

激光诱导预氧化PAN炭化的反应分子动力学模拟

聚丙烯腈(PAN)分子向类石墨结构的转变过程在PAN基炭纤维的炭化过程中起着关键作用,研究激光诱导PAN分子的炭化过程对于进一步认识激光诱导PAN基炭纤维炭化机理有很大帮助。本文利用反应分子动力学方法模拟研究在2 500 K下,激光诱导预氧化PAN分子炭化过程与传统热处理诱导炭化的差异,分析了两种热处理方法过程中体系的能量变化、化学反应速率、原子波动状态及原子受力情况。结果表明,激光诱导过程由于激光具有使物体快速升温的特性,给辐射的炭纤维带来一定的热冲击应力,致使纤维内部原子波动加剧,活性原子数量增加,原子活性增大,与周边原子发生碰撞和化学反应的几率增大,从而促进石墨结构的形成。使用ReaxFF反应力场研究PAN分子的炭化过程,可以提供详细的反应分子动力学过程,有助于理解炭化过程中石墨结构的形成机理,同时也表明激光辐照PAN基炭纤维能得到更好的炭化效果。     

沥青基炭纤维的制备及其表征

石油沥青在氮气氛中420℃热缩聚7h,制得软化点为295℃的炭纤维前驱体沥青.此前驱体沥青在单孔纺丝器中熔纺获得沥青纤维.将沥青纤维于空气中320℃稳定化处理,最后在氮气流中1000℃炭化制成炭纤维.应用SEM、TGA、FTIR和XRD对石油沥青、前驱体沥青、沥青纤维、预氧化纤维和炭纤维分别进行表征.发现:前驱体沥青中含有质量分数70.5%中间相组分,炭纤维具有径核结构,其最大抗拉强度为650MPa.     

热处理对含CSiCTaCC界面C/C复合材料力学性能的影响

以准三维针刺炭纤维毡为预制体, 采用化学气相渗透工艺在预制体中炭纤维/基体炭之间制备C-SiC-TaC-C复合界面, 利用树脂浸渍-炭化工艺对材料进一步增密, 获得含C-SiC-TaC-C界面的C/C复合材料。研究了1400～2500℃不同温度热处理前后复合材料的微观结构和力学性能。结果表明: 热处理前, SiC-TaC界面为管状结构, 复合材料的抗弯强度为241.6 MPa, 以脆性断裂为主; 经1400～1800℃热处理后, TaC界面破坏呈颗粒状, 复合材料的平均抗弯强度下降到238.9～226.1 MPa, 其断裂方式不变, 但断裂位移由0.7 mm增至1.0 mm; 经2000～2500℃热处理后, SiC、 TaC界面均受到破坏, 复合材料平均抗弯强度急剧下降至158.7～131.8 MPa, 断裂方式由脆性断裂转变为假塑性断裂。      

硬质隔热用炭纤维整体毡的制备工艺研究

以预氧化纤维网胎为原料,经过接力层叠针刺工艺制备的预氧化纤维整体毡为预制体,在不同的炭化条件下得到炭纤维整体毡,并以热固性树脂为先驱体,采用浸渍—固化—树脂炭化—石墨化的方法制备了硬质隔热用炭纤维整体毡。研究了针刺工艺、炭化温度、浸渍液浓度和固化时所加压力等因素对材料密度的影响,并借助扫描电镜观察样品的微观结构。结果表明:采用高的针刺密度和针刺深度参数,得到的预制体密度较大;炭化温度对整体毡密度的影响不大,但其含碳量随炭化温度变化明显;浸渍液浓度及预制体密度均对硬质毡的密度有一定影响;随着固化时压力的增大,样品的密度也逐渐提高。     

Quantitative prediction of effective material properties of heterogeneous media

Effective electrical conductivity and electrical permittivity of water-saturated natural sandstones are evaluated on the basis of local porosity theory (LPT). In contrast to earlier methods, which characterize the underlying microstructure only through the volume fraction, LPT incorporates geometric information about the stochastic microstructure in terms of local porosity distribution and local percolation probabilities. We compare the prediction of LPT and of traditional effective medium theory with the exact results. The exact results for the conductivity and permittivity are obtained by solving the microscopic mixed boundary value problem for the Maxwell equations in the quasistatic approximation. Contrary to the predictions from effective medium theory, the predictions of LPT are in better quantitative agreement with the exact results.     

Electrical conductivity characterization and variation of carbon fiber reinforced cement composite

Electrical conductivity of carbon fiber reinforced cement composite (CFRCC) was measured. The conductivity of specimens increased by several orders of magnitude while the volume fraction of fibers reached a higher value than the critic concentration. The microstructure associated with electrical percolation phenomena was observed. The mechanism of conduction was interpreted as being due to fibers touching each other. The changes of electrical conductivity under three different loading levels were investigated. The percolation threshold value decreased with loading. The relative changes of electrical conductivity both under single loading and cyclic loading could sense the stress in non-elastic, elastic and fracture region with sensitive response. The influences of fiber volume fraction and fiber length on the sensitivity of electrical conductivity measurement were discussed. The results provide some new information for the fabrication of conductive and intrinsically smart carbon fiber reinforced cement composites.     

Irreversible anomalies in the electrical properties of oxides with the tetragonal tungsten bronze structure

The anomalies in the electrical properties of (Pb,Sr,Ba)₄Li₂Nb₁₀O₃₀ solid solutions were studied by measuring dielectric permittivity and electrical conductivity as functions of temperature and time (during isothermal holding). The anomalies were shown to occur in defect-rich ferroelectrics. Both the dielectric permittivity and conductivity vary exponentially with time during isothermal holding. The effects of heating rate and UV irradiation time are analyzed. The results can be accounted for in the framework of a fluctuation-cluster model.     

短碳纤维-铜复合材料的研制

研究了用粉未冶金方法制造短碳纤维-铜复合材料,对这种复合材料的力学性能及物理性能进行了测试,证明该复合材料中,碳纤维与铜基体间存在着界面结合。     

碳纤维增强水泥复合材料的电导性能及其应用

研究了碳纤维增强水泥复合材料的电导性能, 用扫描电子显微镜(SEM ) 观察了材料产生电导渗流时的显微结构, 讨论了纤维掺量和纤维长度对电导性能的影响以及受载过程中材料电导率的变化规律。结果表明, 适当控制碳纤维的尺寸、含量, 可以明显提高材料的电导性能; 材料结构中存在电导渗流现象, 渗流阈值随受载过程而变化; 碳纤维增强水泥复合材料能够作为本征机敏材料, 反应试件受载时的应力应变关系。     

多晶铁纤维吸收剂微波复磁导率和复介电常数的理论计算

由麦克斯韦方程出发,导出了多晶铁纤维吸收剂微波复磁导率和复介电常数的理论计算公式;通过对５ＧＨｚ频率点多晶铁纤维吸收剂的数值计算,分析了多晶铁纤维的直径、长径比及电导率等对其电磁参数的影响。     

Induction heating of mastic containing conductive fibers and fillers

The objective of this research is to examine the induction heating of mastic through the addition of electrically conductive fillers and fibers (graphite and steel wool), and to prove that this material can be healed with induction energy. The effect of fibers content, sand–bitumen ratio and the combination of fillers and fibers on the induction heating of mastic was investigated. It was found that there is an optimum content of fibers for each sand–bitumen ratio, above which mastic cannot be heated any more. This optimum seems to coincide with the optimum electrical conductivity of the mixture shown in [1]. It was found that the maximum temperature reached within a certain time period was a function of the sand–bitumen ratio (s–b) and of the volume content of fibers. The mastic could be heated with the addition of a very low volume of conductive fibers. The fastest heating power was obtained with the mix with the maximum electrical conductivity. Gel-Permeation Chromatography (GPC) was also used to show that there is not ageing of bitumen during the heating process.     

SiC纤维表面C涂层的制备及介电性能研究

采用化学气相沉积(CVD)法,在SiC纤维表面沉积了100nm厚的C涂层,研究了制备温度对C涂层微观结构、单丝纤维体电导率及纤维编制体介电性能的影响.采用SEM和RAM显微技术(Raman microscopy)对C涂层的表面形貌和微观结构进行分析.结果表明:保持C涂层厚度一致,当沉积温度由800℃升到900℃后,C涂层的石墨化程度提高,晶粒变大,SiC纤维单丝体电导率由0.745Ω~(-1)·cm~(-1)升到6.289Ω~(-1)·cm~(-1);SiC纤维编制体的复介电常数实部由90升到132,介电损耗由0.95升到1.14,其中虚部由87升到150.实部增大与载流子浓度增大有关,虚部增大与材料漏导电有关.认为这是SiC纤维表面沉积的C层使纤维电导率增大所致.直流电导损耗足其主要损耗机制.     

Optical properties of deposit models for paints: full-fields FFT computations and representative volume element

Abstract

A 3D model of microstructure containing spherical and rhombi-shaped inclusions ‘falling’ along a deposit direction is used to simulate the distribution of nanoscale color pigments in paints. The microstructure’s anisotropy and length scales, characterized by their covariance functions and representative volume element, follow that of transversely isotropic or orthotropic media. Full-field computations by means of the fast Fourier method are undertaken to compute the local and effective permittivity function of the mixture, as a function of the wavelength in the visible spectrum. Transverse isotropy is numerically recovered for the effective permittivity of the deposit model of spheres. Furthermore, in the complex plane, the transverse and parallel components of the effective permittivity tensor are very close to the frontiers of the Hashin–Shtrikman’s domain, at all frequencies (or color) of the incident wave. The representative volume element for the optical properties of paint deposit models are studied. At fixed accuracy, it is much larger for the imaginary part of the permittivity than for the real part, an effect of the strong variations of the electric displacement field, exhibiting hot-spots, a feature previously described in the context of conductivity.