SiC/SiC复合材料高温力学性能研究

以聚碳硅烷为连续SiC陶瓷基体相的先驱体,三维四向SiC纤维预制体为增强相,采用聚合物先驱体浸渍裂解工艺制备了SiC纤维增强SiC陶瓷基(SiC/SiC)复合材料,分析表征了复合材料的组成、结构和力学性能.结果表明,SiC/SiC复合材料室温弯曲强度和断裂韧性分别为400 MPa和16.5 MPa·m1/2,优异的室温力学性能可以保持到1350℃.随着温度增加,弯曲强度基本不变,1350℃时因界面层受到破坏而断裂韧性稍有下降.     

热压烧结制备AlN/BN复相陶瓷及其性能研究

以氮化铝(AlN)粉和高活性六方氮化硼(h-BN)粉为原料,不添加烧结助剂,采用热压烧结法制备了AlN/BN(20vol;)复相陶瓷.研究了烧结温度(1750～1900℃)对复相陶瓷相对密度、物相组成、显微结构、力学性能、热导率及介电性能的影响.结果表明,在1850℃以上可以制备出相对密度大于98.6;的致密AlN/BN复相陶瓷.试样显微结构均匀,晶粒细小,晶界干净,无明显杂质相,h-BN未形成明显的卡片房式结构.随着烧结温度的提高,试样的相对密度、力学性能、热导率及介电性能(1 MHz)均显著提高.1900℃烧结的试样性能最优,相对密度99.3;,抗弯强度482±42 MPa、断裂韧性4.4±0.4 MPa·m1/2、维氏硬度8.56±0.33GPa、热导率47.2 W·m-1·K-1、介电常数7.64,介电损耗4.62×10-4.     

MgO-Al2O3-Re2O3(Re=Lu,Y)对无压烧结Si3N4陶瓷显微结构和性能的影响

研究了MgO-Al2O3-Re2O3(Re=Lu,Y)三元烧结助剂体系对无压烧结Si3N4陶瓷显微结构和力学性能的影响.研究结果表明,添加MgO-Al2O3-Lu2O3三元助剂制备的Si3N4陶瓷显微结构具有明显的双峰分布,晶粒较粗化,致密度、硬度、弯曲强度、断裂韧性分别为96.4;、14.59 GPa、964 MPa、7.64 MPa·m1/2;而添加MgO-Al2O3-Y2O3三元助剂制备的Si3N4陶瓷具有细化的显微结构,致密度、硬度、弯曲强度、断裂韧性分别为99.9;、15.29 GPa、758 MPa、6.60 MPa·m1/2.     

AlN含量对SiC基复相陶瓷性能的影响

以MgO-CeO2为烧结助剂,采用热压烧结工艺在1850℃下制备了SiC基复相陶瓷.研究了不同AlN含量对复相陶瓷致密性与导热性能的影响.结果表明:不添加AlN时,试样致密性最差,气孔率和体积密度分别为4.71;和2.43 g/cm3.AlN含量升高至5wt;时,试样致密性有所提高.AlN含量进一步升高至10wt;～20wt;,试样完全致密,气孔率和体积密度分别保持在0.20;和3.31 g/cm3.在AlN含量为10wt;时,样品具有最高的热导率51.62 W·m-1·K-1,同时弯曲强度和断裂韧性达到顶点,分别为731.3 MPa和7.3 MPa·m1/2.     

Yb2O3-Al2O3烧结助剂对气压烧结氮化硅陶瓷性能的影响

以Yb2O3-Al2O3体系为烧结助剂,采用气压烧结法制备了氮化硅陶瓷.研究了烧结温度对气压烧结氮化硅陶瓷的致密度、失重率、物相、力学性能与显微结构的影响及材料的烧结机理.结果表明:随着烧结温度的升高,氮化硅的致密度、抗弯强度、断裂韧性和硬度均呈现先增加后降低的趋势,而失重率呈现一直升高的趋势;当烧结温度为1780℃、烧结气压为6 MPa时,所得氮化硅烧结体的体积密度(3.31 g·cm-3)、抗弯强度(967.2)、断裂韧性(8.9 MPa·m1/2)和硬度(17.1 GPa)达最大值,晶粒以长柱状的β相为主;烧结温度高于或等于1700℃时,材料中的α相可完全转化为β相,β-Si3 N4晶粒的平均长径比达12.31.     

反应热压烧结制备SiC/ZrB2复相陶瓷及其性能表征

以二硼化锆、硅和活性碳为原材料,在1850℃、20 MPa条件下,采用反应热压烧结工艺制备出了SiC/ZrB2陶瓷基复合材料.研究了添加剂(硅和活性碳)含量对ZrB2陶瓷烧结行为和力学性能的影响.借助X射线衍射和扫描电镜分析了复合材料的物相组成和微观结构.研究结果表明:添加剂可以显著提高复合材料的烧结致密度和力学性能.复合材料的XRD衍射图谱中只有ZrB2和SiC的衍射峰.当添加剂含量为12wt;时,复合材料的弯曲强度和断裂韧性分别达到584MPa和7.25MPa ·m1/2.显微结构分析表明,致密度的提高、晶粒粒径的减小以及断裂模式的转变是复合材料力学性能提高的主要原因.     

Si对热压烧结B4C陶瓷材料显微结构与性能的影响

采用Si作为烧结助剂,利用热压烧结技术烧结制备了SiB6-B4C陶瓷复合材料.采用热力学计算、XRD物相分析,结合SEM图片,探讨了Si-B4C陶瓷的烧结过程和机理.结果表明:Si有助于促进B4C陶瓷的致密化烧结,原位生成的SiB6有助于B4C陶瓷机械性能的提高;Si的最佳加入量为10wt;;预烧处理对Si-B4C陶瓷烧结有利,1000 ～ 1400℃预烧8h后制备的B4C陶瓷弯曲强度447.3 MPa,断裂韧性4.42 MPa· m1/2,HRA硬度为94.     

YF3/氧化物烧结助剂对碳化硅陶瓷热导率的影响

利用YF3/氧化物体系作为烧结助剂热压烧结碳化硅陶瓷,研究了此体系中氧化物的种类、含量对样品的致密度、导热性能、物相成分、微观形貌的影响.实验结果表明,在烧结温度1900 ℃、压力50 MPa条件下,YF3/氧化物体系烧结助剂对碳化硅陶瓷热导率有所提升,其中同时添加5wt;YF3+3wt;MgO双相烧结助剂的SiC陶瓷性能最优,其致密度为98.93;,热扩散系数为71.40 mm2/s,热导率为154.29 W/(m· K).     

烧结温度对96Al2O3陶瓷力学性能及热导率的影响

采用CaO-MgO-SiO2为烧结助剂,采用无压烧结技术,研究了烧结温度对96Al2O3陶瓷热导率及力学性能的影响.采用阿基米德排水法、三点弯曲法、激光脉冲法、扫描电子显微镜(SEM)及X射线衍射(XRD)等分析手段和设备,对烧结后陶瓷样品的密度、力学性能、热导率和微观组织结构进行了分析研究.研究结果表明:1600℃烧结的Al2O3样品的具有较好的导热性和力学性能,其热导率、密度、维氏硬度和抗弯强度分别为24.9W/(m·K),3.82g/cm3,(13.8±0.2)GPa,(362.9±26.9)MPa.     

低温烧结3Y-TZP陶瓷的研究

以不同质量分数的MnO2-TiO2(质量比为1:1)为烧结助剂,在1300～1500 ℃下低温烧结制备了3Y-TZP陶瓷.对3Y-TZP陶瓷的相对密度、物相及显微结构、显微硬度、抗弯强度及断裂韧性进行了测试分析,并对烧结助剂的基本性能进行了表征.探究了烧结助剂及烧结温度对3Y-TZP陶瓷性能的影响.实验结果表明:在3Y-TZP陶瓷中加入烧结助剂MnO2-TiO2(质量比为1:1)可以实现低温烧结.试样的相对密度、显微硬度、抗弯强度、断裂韧性随烧结温度的升高先增大后降低.在烧结助剂为0.5wt;,烧结温度为1350 ℃时,试样的相对密度及力学性能都达到最大,在此条件下,试样的相对密度达97.16;,显微硬度为2032.8 HV,抗弯强度为300 MPa,断裂韧性为8.35 MPa· m1/2,且试样的断裂方式为晶粒拔出及晶粒断裂遵循着穿-沿晶断裂的模式,且晶粒极小.     

A review of measurement of thermophysical properties of silicon melt

Measurements of thermophysical properties of Si melt and supplementary study of X-ray scattering/diffraction by the authors' group were reviewed. The values obtained differed variously from those of literature. Density was 2–3% larger, surface tension 20–30% smaller, viscosity up to 40% larger, electrical conductivity 8% smaller, spectral emissivity more or less in good agreement with literature values, and thermal diffusivity a few percent larger. An anomalous density jump was found near the melting point. Surface tension and viscosity also showed anomaly. A strange time-dependent change of density was observed over 3 h after melting. X-ray analyses suggested a slight change in local atom ordering, but showed no sign of cluster formation. An addition of 0.1 at% gallium caused the density jump to disappear, while that of boron caused no change. An EXAFS study of the former melt indicated a strong interaction between Ga and Si atoms as if molecules of GaSi₃ existed. The implications of the measured properties are a possibility of soft-turbulence in an Si melt in a relatively large crucible, a more complicated manner of intake of oxygen depleted molten Si from the free surface region to underneath the growing crystal, and a relaxation of the melt after melting arising from trapped gas species.     

电子显微分析在表征纳米晶体化学组分中的应用

纳米材料的化学组分及含量影响其光、电、声、热、磁等物理性能,电子显微分析是表征纳米晶体化学组分的重要方法之一.本文综述了X-射线能谱(EDS)、X-射线波谱(WDS)、电子能量损失谱(EELS)和选区电子衍射(SAED)等现代电子显微分析技术在表征纳米晶体化学组分、形貌、尺寸和结构等方面的应用及其研究进展,并比较了这些分析方法存在的差异,提出了其应用中存在的不足及今后的研发方向.     

原位氮化法制备TiN纳米粉体

用溶胶凝胶法合成的纳米TiO2粉体作为原料,将该粉体在氨气中进行原位氮化制备了TiN纳米粉体.用XRD,TEM,化学分析等手段对合成的TiN纳米粉体的物相组成、形貌、成分进行了分析.实验分析表明:在1000℃和1100℃下分别氮化5h,可以制备粒径大约为40nm和80nm的TiN粉体,其TiN的含量分别为95.40;和98.37;;而在1000℃条件下氮化时间减少到2h时,TiN的含量仅为58.36;.氮化温度和氮化时间是合成纳米TiN的重要因素,提高合成温度和延长氮化时间均可形成纯度较高的TiN纳米粉体,但延长氮化时间更有利于获得粒径小的氮化钛粉体.     

On the influence of structural features of DNA on the possibility of metabolic transfer of electrons

Abstract

A fragment of a DNA molecule is considered as one of the channels of metabolic electron transfer. The heterogeneity of the complementary chains is effectively taken into account. This made it possible to find the speed of the electron injected into the DNA conduction band and the current density that it creates. Estimates of electron mobility in nucleic acid chains are made. They were an order of magnitude smaller than that of typical semiconductors. For the specific conductivity of nucleic acid chains, estimates provide a conductivity of one to two orders of magnitude lower than in graphite.     

Mesomorphism Dependence on the Position of Substitution of a Pair of Esters

A novel homologous series of ethylene derivatives of thermotropic liquid crystals has been synthesized. The methoxy to octyloxy derivatives are nematogenic, the decyloxy to tetradecyloxy derivatives are smectogenic, in addition to nematogenic, and the hexadecyloxy homologue is smectogenic only. All the members of the series are enantiotropically mesogenic. Thermotropic behavior was determined by an optical polarizing microscope equipped with a heating stage and Differential Scanning Calorimetry (DSC) study. Analytical and spectral data confirm the molecular structures of homologues (infrared, nuclear magnetic resonance, mass spectra, X-ray, and DSC data). Textures of the nematic phase are threaded or Schlieren and that of smectic phase are focal conic fan-shaped of smectic A or C. Transition curves of the phase diagram behave in a normal manner except one or two deviations from the normal trend. The mesophase range (Sm+N) varies from 3°C to 44°C. The average thermal stability for smectic is 93°C and that for nematic 117.4°C. The LC behavior of the novel series is compared with a structurally similar known series.     

Ni-ZrO2纳米复合电铸层的制备及纳米颗粒分布均匀性研究

研究了在复合电铸过程中,当其它工艺参数一定时,复合电铸层的沉积速率及其厚度随时间的变化趋势, 以及复合电铸层表面微观形貌随电沉积时间的变化趋势.测定了复合电铸层的组织成分,并就纳米颗粒在复合电铸层表面和横截面上分布的均匀性进行了评价.结果表明,复合电铸层表面平整,组织均匀致密,其组成主要是镍和所复合的纳米颗粒,纳米颗粒较为均匀地分散在复合电铸层中.     

Effect of alumina on enthalpy of mixing of mixed alkali silicate glasses

The enthalpy of mixing of mixed alkali (Na₂O and K₂O) silicate glasses containing various concentrations of alumina was determined using an ion-exchange equilibrium method. For glasses with a constant alkali concentration, the enthalpy of mixing was found to become less negative with alumina addition. Consistent with our previous results on the enthalpy of mixing of alumina-free mixed alkali silicate glasses, the magnitude of enthalpy of mixing exhibited a good correlation with the molar volume mismatch of the corresponding two single alkali glasses as well as with the extent of conductivity mixed alkali effect, e.g. excess activation energy of conductivity, ΔE. The reduction of the magnitude of the enthalpy of mixing with alumina addition can be attributed to the reduction of non-bridging oxygen and ionic field strength. Combining the present results with results obtained earlier, the magnitude of the enthalpy of mixing for all mixed alkali (Na₂O and K₂O) silicate glasses with and without alumina was expressed by a simple function of a modified Tobolsky parameter, which takes into account the alkali concentration and the difference in cation-to-effective anion distances. The enthalpy of mixing data of the mixed alkali glasses was then compared with reported experimental data on the conductivity of mixed alkali aluminosilicate glasses. What appears to be conflicting experimental data can be understood in terms of the magnitude of the enthalpy of mixing and we can conclude that the mixed alkali effect is closely correlated with the negative enthalpy of mixing.     

PEG聚合度对氧化铝造粒粉的性能影响

为制备适用于干压成型的氧化铝造粒粉,研究了PEG聚合度对氧化铝造粒粉微观形貌、流动性和松装密度的影响.结果表明PEG的聚合度对氧化铝浆料粘度影响显著,PEG2000-6000是较为理想的粘结剂选择,造粒粉的流动性与环境温度及湿度相关.采用正交实验设计,以造粒粉的流动性和松装密度为评价指标,对PEG聚合度、粘结剂添加量和固含量进行了优选,其影响顺序为PEG聚合度＞固含量＞粘结剂添加量.以优选参数PEG6000、添加量为4wt;、固含量为80wt;,制备了性能优良的氧化铝喷雾造粒粉.     

近化学计量比铌酸锂晶体组分测定与缺陷观察

采用助熔剂提拉法生长得到近化学计量比LiNbO3晶体.用多种方法测定了晶体组分,结果表明生长得到的晶体中[Li2O]含量为49.80;摩尔分数;对晶体缺陷的研究表明晶体质量有待提高,并分析了晶体中出现包裹物的原因.     

Some peculiarities of spectral proprieties of leukocytes

This article presents the results of spectral investigations of white blood human cells including absorption, fluorescence and phosphorescence (using low-temperature measurements). For this research, the main optical centres were identified and local distribution of emitting and absorbing centres into white blood human cells was determined. In addition, the spectra of normal and pathological (B-cell chronic lymphocytic leukemia, B-CLL) cells were compared.