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.     

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.     

短切SiCf/LAS复合材料的介电性能

讨论了纤雏长度与热压保温时间对含量为25%(体积分数)的短切SiCf增强LAS玻璃陶瓷复合材料介电性能的影响.在8～12GHz频率范围内,介电性能测试结果表明,随纤维长度由2mm增加到4mm,复合材料的复介电常数实部ε′增大,而其虚部ε″及介电损耗tgδ减小.当保温时间由10min延长到20min时,复合材料的ε′增大,而其ε″与tgδ均减小;且保温20min时,其ε′、ε″与tgδ均已接近适合损耗微波能量的数值.复合材料有望成为电损耗型宽带微波损耗材料.     

活性碳纤维/树脂复合吸波材料的设计

研究了长度为1—2mm的活性碳纤维的介电特性。发现随频率的增大，介电常数的实部减小，虚部增大，具有频响效应。介电常数的实部和虚部均随复合材料中纤维质量百分含量的升高而增大。依据纤维的介电常数，通过阻抗匹配设计方法，优化设计两层和四层活性碳纤维吸波复合材料。根据优化结果制备了含有四个结构层的吸波复合材料，材料-10dB以下的频带宽度为8GHz，最大反射衰减-39．3dB。     

热压烧结短切SiCf/LAS复合材料的介电性能

采用热压烧结法制备出致密的短切SiCf增强IAS玻璃陶瓷复合材料,讨论了热压保温时间与纤维长度对复合材料介电性能的影响.结果表明,测试频率在8～12GH2之间,复合材料的复介电常数实部ε'由基体的7.6上升到10～100,虚部ε"由基体的0.34上升到60～140,介电损耗tgδ由基体的0.04上升到1～40,并具有明显的频散效应.当保温时间由10min增加到20min时,复合材料ε'增大,ε"与tgδ减小.保温时间10min时,随着纤维长度由2mm增加到4mm,复合材料ε'减小,ε"先减小后增大,而tgδ增大;保温时间20min时,随着纤维长度由2mm增加到4mm,复合材料ε'先减小后增大,ε"与tgδ则先增大后减小.复合材料具有成为电损耗型宽带吸波材料的潜力.     

微波铁氧体吸收剂复磁导率和复介电常数的温度特性研究

本文对（Ｚｎ＿（１－ｘ）Ｃｏ＿ｘ）＿２－ｗ型六角晶系铁氧体吸波材料的复磁导率μ＿ｒ和复介电常数ε＿ｒ的温度特性进行了研究。实验发现当温度从－２５℃升至１００℃时，复磁导率的实部μ＇＿ｒ值从１．５１降至１．３０，虚部μ″＿ｒ值从０．１９降至０．１４，复介电常数的实部ε′＿ｒ值从６．５０升至７．１０，虚部ε″＿ｒ值从１．７０升至１．８０，理论上对电磁参数随温度变化特性进行了分析讨论。     

Nd掺杂对钛酸钡电磁及微波吸收特性影响研究

采用溶胶-凝胶法制备稀土Nd掺杂钛酸钡纳米粉体.采用X射线衍射(XRD)、矢量网络分析仪等手段分别分析表征了样品的微结构和高频电磁特性.结果表明,Nd离子掺杂的钛酸钡样品在750℃温度下退火1h,结晶良好.随着Nd掺杂量的提高,晶粒细化.Nd离子掺杂后,在高频下钛酸钡纳米粉体的介电常数的实部略有减小,虚部峰值得到提高,...     

热压烧结短切εSiCf／LAS复合材料的介电性能

采用热压烧结法制备出致密的短切SiCf增强LAS玻璃陶瓷复合材料，讨论了热压保温时间与纤维长度对复合材料介电性能的影响。结果表明，测试频率在8～12GHz之间，复合材料的复介电常数实部ε′由基体的7．6上升到10～100，虚部，由基体的0．34上升到60～140，介电损耗tgδ由基体的0．04上升到1～40，并具有明显的频散效应。当保温时间由10min增加到20min时，复合材料ε′增大，ε″与tgδ减小。保温时间10min时，随着纤维长度由2mm增加到4mm，复合材料ε′减小，ε″先减小后增大，而tgδ增大；保温时间20min时，随着纤维长度由2mm增加到4mm，复合材料ε′先减小后增大，ε″与tgδ则先增大后减小。复合材料具有成为电损耗型宽带吸波材料的潜力。     

三折叶形截面碳化硅纤维的制备及其电磁性能

通过熔融纺丝制备出三折叶形聚碳硅烷原纤维,然后将原纤维不熔化及高温烧成得到平均当量直径为27μm、拉伸强度为1.7GPa的三折叶形碳化硅纤维.以环氧树脂为粘结剂,分别研究了有规排列长纤维及无规排列短切纤维在复合材料中的电磁性能,在X波段,有序排列长纤维的介电常数实部ε'为6.52～6.88,虚部ε"为3.02～4.03,无规排列短切纤维的介电常数实部ε'为10.5～12.0,虚部ε"为11.1～14.2,在2mm厚度下,无规排列短切纤维具有更高的介电损耗和更好的频散效应.     

退火处理对ZnO薄膜压敏性能的影响

采用射频磁控溅射技术在Si(111)衬底上生长ZnO基陶瓷薄膜,分别在650℃,750℃,850℃和900℃下退火,研究了退火温度对ZnO基陶瓷薄膜压敏性能的影响。结果表明,随着退火温度的升高,薄膜的压敏电压逐渐增大,非线性系数先增大后减小,漏电流密度先减小后增大。850℃退火处理后的薄膜具有较为理想的综合电性能,其非线性系数为14.93,压敏电压为4.82 V,漏电流密度为0.36μA/mm~2。     

Estimation of Parameters of Universal Distribution of Moments of Failure of Products

A method is proposed for estimating the parameters of a mixture of exponential and Weibull distributions for which the accuracy of preliminary estimates obtained by graphical analysis is refined in accordance with the criterion of maximum likelihood. The efficiency of the proposed method is supported by the results of statistical modeling.     

Evaluation of optical properties of highly scattering media by moments of distributions of times of flight of photons

A novel method for the determination of the optical properties of tissue from time-domain measurements is presented. The data analysis is based on the evaluation of the first moment and the second centralized moment, i.e., the mean time of flight and the variance of the measured distribution of times of flight (DTOF) of photons injected by short (picosecond) laser pulses. Analytical expressions are derived for calculation of absorption and of reduced scattering coefficients from these moments by application of diffusion theory for infinite and semi-infinite homogeneous media. The proposed method was tested on experimental data obtained with phantoms, and results for absorption and reduced scattering coefficients obtained by the proposed method are compared with those obtained by fitting of the same data with analytical solutions of the diffusion equation. Furthermore, the accuracy of the moment analysis was investigated for a range of integration limits of the DTOF. The moment analysis may serve as a comparatively fast method for evaluating optical properties with sufficient accuracy and can be used, e.g., for on-line monitoring of optical properties of biological tissue.     

Estimation of potentialities of methods of summation of direct measurement errors

Conventional methods for summation of direct measurement errors are considered. Conditions under which these methods provide consistent results are stated. It is proposed to utilize for characteristic of the total error, its interval estimator or the half-sum of the value obtained by adding up its arithmetic and geometric random errors.Translated from Izmeritel'naya Tekhnika, No. 4, pp. 10–12, April, 1995.     

空压机轴承箱蜗壳温度探头油封改造

由于空压机进气侧轴承箱蜗壳温度探头密封效果不好,使空压机及其后续管道受到油污染。后将温度探头的O形圈密封改为骨架油封,杜绝了轴承箱漏油问题。文章简介空压机的结构和技术参数,详细介绍了骨架油封的制作和优点。     

Characteristics of mechanical properties of materials in studies of conditions of attainment of marginal states

It is shown that conditions of attainment of marginal states in the material of structural components are formulated on the basis of analysis of force and deformation criteria of formation and development of destruction. The parameters of these criteria are data on types of structural materials, standard and special characteristics of their mechanical properties, structural forms of considered elements of the equipment, and their operating stress loading. In this case, the construction of a system of state equations for describing interrelation of current damage, external actions, and responses to them is based on the concept of calculating variations in properties of materials at all stages of the design life cycle.     

Determination of parameters of microstructural inhomogeneity of deformation of metals by the method of modeling

The article suggests a method of evaluating the microstructural inhomogeneity of deformation of metals by modeling. The method includes the measurement of deformations of metal according to meshes of a dividing grid with small base and the calculation of the distribution parameters of microdeformations in model representation. It is shown that the modeling method makes it possible objectively to evaluate the microinhomogeneity of deformation of metals independently of the chosen mesh size of the dividing grid. New structural characteristics are introduced: the base and wave of variation expressing the degree of crumbling or amalgamation of metal grains by inhomogeneity of deformation.Translated from Problemy Prochnosti, No. 7, pp. 40–44, July, 1991.     

Analysis of the rate of propagation of the wave of change of boiling modes

An approximate analytical expression is derived for the velocity of the front of the wave of change of boiling modes on a rod and on a plate. The impact of the Thomson effect and of the heater orientation in the gravity field is included. Adequate agreement is demonstrated between the derived relations and experimental data.     

水三相点复现的不确定度评定

水三相点是开尔文热力学温度的唯一基准点，也是ITS-90国际温标重要的定义固定点。因此，水三相点不确定度分析对整个温标的建立、温度量值传递起着至关重要的作用。近3年来，中国计量科学研究院研制出一系列高质量的水三相点容器，加强了水三相点的研究，为不确定度的分析提供了更为可靠的实验依据。同时，不确定度的分析也是客观评价新研制容器性能的一个重要指标。因此，根据实验结果对新研制容器所复现的水三相点进行了不确定度评定。评定结果表明，其扩展不确定度为0．16mK(k=2．69，P=0．99)。