SiC微粉添加量对SiC耐磨材料结构和性能的影响

以紧密堆积的三级配SiC颗粒(粒径为325 μm、212 μm、80 μm,质量比为17∶7∶1)为基础配方,将Owt;、1wt;、2wt;、3wt;和4wt;且粒径为5μm的SiC微粉添加到SiC耐磨材料中,经1600℃保温3h烧制,研究了SiC微粉添加量对SiC耐磨材料结构和性能的影响.结果表明:SiC微粉可促进SiC耐磨材料的烧结致密化,并改善其力学性能,当其添加量为3wt;时,试样的综合性能较优,其体积密度和显气孔率分别为2.63 g/cm3和7.62;,硬度、抗折强度和磨损量分别为2458 HV、183 Mpa和0.26 g/min.SiC耐磨材料烧结性能和力学性能的提高可归因子SiC微粉充填在SiC颗粒间,缩短了扩散传质路径,且较小粒径的SiC微粉具有较大的表面能,烧结时易于晶粒重排,保证了烧结网络的连续性,增大了颗粒间的结合程度.     

Y2O3-SiO2助剂对常压烧结SiC-AlN复相材料烧结性能的影响

SiC∶AlN以质量比1∶1,添加不同质量分数Y2O3-SiO2(摩尔比为2.9∶7.1)复合烧结助剂,分别在氢气和氩气气氛下常压烧结制备SiC-A1N复相材料.研究烧结气氛、烧成温度和Y2O3-SiO2含量对该复相材料烧结性能的影响.结果表明,与氢气气氛相比,氩气气氛下烧结体更易致密;1650 ～1850℃,随着烧结温度升高,烧结致密性明显提高.氩气气氛1850C下保温1h,Y2O3-SiO2复合助剂含量为9.09wt;,烧结体的显气孔率可低于0.15;,晶粒尺寸均匀且连接紧密.烧结过程中,Y2O3、SiO2烧结助剂与AlN表面的Al2O3在一定温度下形成液相有助于样品致密化.Y2O3与AlN表面的Al2O3反应生成钇铝石榴石(Y3Al5O12),SiO2高温下主要形成玻璃相.SiC-AlN复相材料是由主晶相6H-SiC和AlN,次晶相Y3Al5O12组成.     

温度对Al2O3-C材料中β-Sialon晶须形成的影响研究

本文研究了温度对Al3O3-C耐火材料中β-Si3Al3O3N5晶须形成的影响规律.研究发现:在弱还原性气氛下,Al2O3-Si-Al-C稳定存在的非氧化物物相为Al4C3 、AlN、SiC和p-Sialon相.其中,催化作用下,Al2O3-C耐火材料中的金属Al在1000℃时转化为AlN,金属Si在1200℃时转化成SiC,在1400℃时有β-Sialon相生成;而无催化剂存在时,生成的物相仅为SiC和AlN相.催化作用下,AlN形貌呈短柱状;SiC呈晶须状,直径为亚微米级,且晶须有液点存在;β-Sialon相呈纳米晶须状.SiC和β-Sialon晶须的生长都符合气-液-固机理.     

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.     

Al粉粒度及其添加量对SiC陶瓷材料结构和性能的影响

以Al粉为烧结助剂,采用无压烧结工艺于1600℃下保温3 h烧制SiC陶瓷材料,研究了不同Al粉粒度及其添加量对SiC陶瓷材料结构和性能的影响.结果表明:Al粉可促进SiC陶瓷材料的烧结和力学性能的提高,同时起抗氧化的作用,且粒度较小的Al粉对其性能提升的幅度较大.当添加4wt;粒度为48μm的Al粉时,SiC陶瓷材料的性能较佳,体积密度和显气孔率分别为2.69 g/cm3和5.8;,显微硬度和抗折强度分别为2790 HV和189 MPa.SiC陶瓷材料烧结性能和力学性能的提高可归因于Al粉的促烧结作用,及其氧化产物Al2 O3和原位生成的少量莫来石分布在SiC颗粒间所起的强化作用.     

低压铸造升液管用Si3N4/SiC复相非致密陶瓷材料显微结构和力学性能

以低压铸造用升液管为研究目的,磨切单晶硅废料Si粉和SiC为原料,Y2O3-Al2O3-Fe2O3为复合烧结助剂,反应烧结法制备Si3N4/SiC复相陶瓷.研究了Y2O3含量对复合材料结构和力学性能的影响,采用XRD、SEM对复合材料的相组成、微观形貌进行分析.结果表明,反应烧结后试样生成Si3N4结合SiC晶粒为主相的烧结体,并含有少量Sialon晶须及未反应的Si.Y2O3含量对复相陶瓷力学性能影响很大,在分析稀土Y2O3作用机理的基础上,得到2.5wt; Y2O3优化试样的力学性能优良,相对密度达到88;,维氏硬度达到1.1 GPa,常温抗弯强度50 MPa.     

晶硅切割废粉为原料的反应烧结Si3N4结合SiC复相陶瓷的工艺研究

以晶硅切割废料Si粉和SiC为原料,Y2O3-Al2O3-Fe2O3为复合烧结助剂,反应烧结法制备低压铸造升液管用Si3N4/SiC复相陶瓷材料.设计L9(34)正交实验,研究了原料中Si、助剂Al2O3、Y2O3和Fe2O3的含量对陶瓷材料力学性能的影响和优化.采用X射线衍射(XRD)和扫描电镜(SEM)对复合材料的相组成、断口形貌进行分析.结果表明,反应烧结后试样生成Si3N4结合SiC晶粒为主相的烧结体,并含有少量SiALON及未反应的Si.Si含量对力学性能的影响最为显著,通过对正交试验的验证,20wt; Si、3.2wt; Al2O3、0.8wt;Fe2O3和2wt; Y2O3时烧结体抗弯强度最高.     

Al2O3加入量对AlN-Al2O3复相陶瓷制备及性能的影响

以AlN和Al2O3为原料,Y2O3为烧结助剂,N2气氛下在1650 ℃下热压烧结制备出了AlN-Al2O3复相陶瓷;采用X射线衍射、扫描电子显微镜和透射电子显微镜等对复相材料的相组成、显微结构进行表征.研究了Al2O3加入量对AlN-Al2O3复相材料的强度、热导率和介电性能的影响.结果表明,Al2O3加入量的增加可促进材料的烧结致密化和抑制AlN基体晶粒的生长.Al2O3含量为20 wt;和30 wt;时可制得致密的AlN-Al2O3复相陶瓷.Al2O3利用其高强度和弥散强化作用对AlN基体起到了明显的增强效果.随着Al2O3加入量的增加,试样的抗弯强度显著提高,热导率和介电性能也得到改善.加入30 wt; Al2O3的复相陶瓷的抗弯强度和热导率达到最大值457 Mpa和57 W/(m·K),介电常数和介电损耗达到最低值9.32和2.6×10-4.当Al2O3含量达到40 wt;以后,材料中部分AlN与Al2O3反应生成AlON,从而使材料的抗弯强度、热导率和介电性能又明显下降.     

三聚甲醛存在下EMT分子筛的合成及表征

以硅酸钠为硅源、铝酸钠为铝源,以三聚甲醛(TOX)为有机添加剂,室温下悬浮聚集陈化合成了EMT分子筛.考察了凝胶组成、晶化温度、晶化时间等因素对EMT分子筛合成的影响.采用XRD、FT-IR、SEM、BET、ICP等手段对EMT分子筛进行了表征.结果表明,合成EMT分子筛适宜的条件为:凝胶摩尔组成nNa2O∶nAl2O3∶nSiO2∶nH2O∶nTOX=18.5∶1∶5∶240∶0.5,晶化温度30 ℃,晶化时间72 h,此条件下合成的EMT分子筛,XRD特征峰较强,与典型的EMT分子筛的特征衍射峰相吻合,相对结晶度为80.1;,经ICP分析计算Si/Al=2.01,BET比表面积为80.57 m2/g,孔容和孔径分别为0.03 cm3 /g和2.07 nm,SEM照片显示EMT为球形晶体颗粒团聚状生长,大小均匀,排列紧凑.     

等静压成型对氮氧化铝陶瓷透明性的影响

本文采用高纯Al2O3、AlN和Al粉为原料,用Y2O3和MgO作为烧结助剂,在150 MPa下等静压成型,然后在氮气氛下,1850℃保温1 h制备出透明尖晶石型氮氧化铝(AlON)陶瓷.结果表明,透明AlON陶瓷晶粒尺寸细小,晶粒发育完善且分布均匀,晶界平直光滑且无第二相的存在.用等静压成型能明显能明显提高紫外区和可见光区的透明性能,但对红外区影响不明显,其在紫外光区(200～1100 nm)最大透光率为82;,在红外光区(1750～5000nm)最大透光率为98;,密度为3.70 g/cm3,约为其理论密度的99.7;.     

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.     

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.     

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.     

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.     

立方晶系晶体本征铁弹相变序参量的选取

在本征铁弹相变的软模理论的基础上,依据弹性本构关系和居里原理,给出了立方晶系晶体所有可能的自发应变的种类,研究了立方晶系晶体本征铁弹相变序参量的选取问题.以Th群和Oh群为例,阐明了不同类的序参量导致晶体对称性变化相同的的原因,提出了该情况下序参量选取的“就少不就多”的原则.得到了发生本征铁弹相变时所有可能的序参量及晶体的对称性的变化.     

Features of transformations in REE-containing systems of nitrate precursors in preparatory processes of formation of multifunctional oxide materials

Abstract

The complex study provides a reliable idea of ??the trends in the joint behavior of structural components in the water-salt systems of nitrate precursors of REE, alkaline, alkaline earth metals in the preparatory stages of the processes of forming multicomponent oxide polyfunctional materials on their basis with thermal activation. Stages of such transformations are revealed; The regularities of complex and phase formation in systems and factors influencing them are determined; A number of physicochemical properties of the intermediate phases formed - coordination lanthanides nitrates: their composition, types of compounds, atomic-crystalline structure, regularities of transformations during heat treatment were studied.     

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.     

Growth and characterization of semicarbazone of cyclohexanone

A new organic single crystal of semicarbazone of cyclohexanone (SCCH) has been synthesized and grown as a bulk single crystal by low temperature solution growth technique for the first time in the literature. The grown crystal has been confirmed by X‐ray diffraction and proton nuclear magnetic resonance spectral analyses and also characterized by FT‐ir and FT‐Raman studies. Thermal properties of the grown crystals were studied by thermogravimetric and differential thermal analyses. Optical transmittance was studied by ultraviolet‐visible spectrum and the second harmonic generation property was tested by using Q switched Nd: YAG laser as a source. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of density of states on optical properties of GaSe thin film

A systematic investigation on the effect of substrate temperature on the structure, optical absorption and density of states of vacuum evaporated gallium monoselenide (GaSe) thin films is reported. The X‐ray diffraction analysis shows an occurrence of amorphous to polycrystalline transformation in the films deposited at higher‐temperature substrates (573K). The compositional analysis is made with Auger Electron Spectroscopy (AES). The thickness of the film (175nm) is measured by a multiple beam interferometery. Optical characteristics of the GaSe sample have been analyzed using spectrophotometer in the photon energy range of 1.0 ‐ 4 eV. The absorption mechanism has been recognized and the allowed indirect as well as forbidden direct transitions have been found. As‐deposited films show two indirect and allowed transitions due to spin‐orbit splitting of the valence band, as reported here for the first time. Low field conduction have enabled us to determine the density of states in amorphous and poly‐GaSe films. The amorphous and polycrystalline GaSe thin films have localized states density values of N (E_F) = 1.686 × 10¹⁷ cm^‐3 eV^‐1 and 1.257 × 10¹⁵ cm^‐3 eV^‐1 respectively. The experimental results are interpreted in terms of variations in the density of localized states due to progressive decrease of the unsaturated bonds during deposition. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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.