ZrB2粒径对Si3N4-ZrB2陶瓷相组成、显微结构及电阻率的影响

以20vol; ZrB2粗粉和细粉为导电相,以3vol; MgO-2vol; YB2O3烧结助剂,通过热压烧结在1500℃制备了Si13N4-ZrB2复相陶瓷,研究了ZrB2粒径对致密度、相组成、显微结构以及电阻率的影响.结果表明,不依赖于Zrl2粒径,通过引入MgO-YB2O3烧结助剂,均可以获得高致密Si3N4-ZrB2陶瓷.以Zrl2粗粉为原料时,Si3N4-ZrB2陶瓷包含主要的αt-Si3N4 、β-Si3N4和ZrB2相以及微弱的Yb4 Si2N2O7相,由于ZrB2晶粒保持孤立状态,样品电阻率较高,为9.5×103 Ω·m;而以ZrB2细粉为原料时,其与Si3N4发生轻微的高温反应,除了包含主要的d-Si3N4、β-Si3N4和ZrB2相及微弱的Yb4Si2N2O7相之外,Si3 N4-ZrB2陶瓷还含有新生成的微弱ZrSi2和ZrN导电相,由于ZrB2晶粒保持连通状态,样品电阻率显著降低,仅有6.8 Ω·m.     

有机前驱体法裂解制备SiBCN陶瓷及其表征

采用固态核磁共振(NMR)为主要研究手段,FT-IR,XRD等作为辅助方法对陶瓷前驱体高温裂解制备的SiBCN陶瓷的结构进行分析和表征.研究表明SiBCN陶瓷是非晶结构,并且SiBCN陶瓷结构中存在Si四面体结构,与非晶的氧化硅中硅氧四面体的结构相类似;SiBCN陶瓷前驱体中C-C键,Si-C键,Si-N键以及C-B-C键经过高温裂解后保留在SiBCN陶瓷结构中;SiBCN陶瓷中的硼原子与其它原子形成的是平面三角结构,但是前驱体向陶瓷转化过程中,由于季碳的产生,空间位阻增大,围绕中心碳发生了偏转以降低产生季碳造成的应力增大,这样的偏转改变了一部分硼的空间结构,使硼原子不在一个平面内而使化学位移发生改变.     

粉体粒径及烧结工艺对ZrB2陶瓷致密化行为与晶粒长大的影响

利用热压烧结(HP)和放电等离子烧结(SPS)制备了ZrB2陶瓷,研究了粉体粒径和烧结工艺对ZrB2陶瓷致密化行为和晶粒长大的影响.结果表明,相同工艺下以平均粒径为200 nm的ZrB2粉体为原料替代平均粒径为2μm的ZrB2粉体可以明显促进粉体的致密化烧结,采用SPS替代HP工艺可以显著降低粉体的致密化温度,采用平均粒径为200 nm的ZrB2粉体在1900℃进行SPS工艺烧结即可实现ZrB2陶瓷的致密化烧结.     

ZrB_2-B_4C复相陶瓷的SPS烧结过程研究

采用碳化硼还原法,利用SPS烧结技术烧结制备了ZrB2-B4C复相陶瓷。采用热力学计算、示差扫描量热分析,结合不同温度下产物的物相分析,探讨了两个体系的SPS反应过程;对烧结过程中样品收缩数据进行分析,探讨了ZrB2-B4C体系的烧结机理。结果表明,原位制备ZrB2-B4C复相陶瓷过程中,反应温度与热力学理论计算的结果基本一致;在烧结初期收缩符合粘性流动传质方程,其烧结是表面熔融烧结机制。     

新型SiC/SiBCN复合陶瓷的析晶性能

通过热解聚碳硅烷(PCS)和聚硼硅氮烷(PBS)两种有机先驱体混合物,制得非晶态SiC/SiBCN复合陶瓷,然后对复合陶瓷进行高温热处理,研究其析晶规律及影响因素.利用热分析(TG-DTA)、X射线衍射(XRD)及透射电子显微镜(TEM)分别研究先驱体的热解机理,先驱体的组成、热解温度和时间对SiC/SiBCN复合陶瓷结晶性的影响,以及复合陶瓷的微观结构.结果表明,先驱体的组成和热处理温度对复合陶瓷的析晶行为有重要影响.当PBS/PCS=1(w/w),1100 ℃/2 h制得非晶态SiC/SiBCN复合陶瓷;1400 ℃/4 h PCS热解转变的非晶SiC开始结晶,晶粒尺寸～3 nm;1500 ℃/4 h 得到产物的相结构为～10 nm 的SiC纳米晶均匀弥散在PBS热解得到的非晶态SiBCN中;1700 ℃/2 h SiC的晶粒尺寸～16 nm;1800 ℃/2 h非晶态SiBCN开始析晶,SiC晶粒尺寸～35 nm.     

添加碳对ZrB2-SiC陶瓷的低温致密化和力学性能影响

以纳米ZrB2粉体为原材料,采用热压烧结的方法制备了ZrB2-SiC超高温陶瓷,研究了碳含量对ZrB2-SiC陶瓷致密度、微结构和力学性能的影响.结果表明:纳米粉体显著降低了ZrB2-SiC陶瓷的烧结温度,在1500℃即可获得95.1;的致密度,在1800℃实现了超高温陶瓷的致密化烧结;添加碳能够进一步促进ZrB2-SiC陶瓷的低温致密化,同时抑制晶粒长大.当碳的含量为1.0wt;时,材料表现出最佳的烧结性能,在1700℃热压烧结的ZrB2-SiC陶瓷致密度达到99;以上,而且烧结后的材料表现出优异的力学性能.     

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.     

ZrB2-SiC-Csf超高温陶瓷复合材料碳纤维损伤抑制研究

采用放电等离子烧结和热压烧结制备了短切碳纤维(Csf)增韧ZrB2-SiC超高温陶瓷复合材料(ZrB2-SiC-Csf),研究了制备工艺对ZrB2-SiC-Csf复合材料微结构演变、力学性能和抗热冲击性能的影响.结果表明:烧结温度是导致碳纤维结构损伤的主要因素,降低烧结温度能有效抑制碳纤维的结构损伤.采用纳米ZrB2粉体在1450 ℃低温热压烧结制备的ZrB2-SiC-Csf复合材料在断裂过程中表现出纤维拔出、纤维侨联和裂纹偏转增韧机制,其临界热冲击温差高达741 ℃,表现出良好的力学性能和优异的抗热冲击性能.从热力学的角度阐明了ZrB2-SiC-Csf复合材料中碳纤维结构损伤的机理,并揭示了该类材料的烧结温度应低于1500 ℃.     

添加聚碳硅烷(PCS)对ZrB2-SiC超高温陶瓷微结构和力学性能的影响

采用聚碳硅烷(PCS)和纳米ZrB2粉体为原料在不同温度下热压烧结制备了ZrB2-SiC超高温陶瓷,对比了PCS和颗粒状SiC的引入对ZrB2陶瓷微结构和力学性能的影响.结果表明:通过PCS替代颗粒状SiC制备ZrB2-SiC超高温陶瓷可以形成SiC均匀包覆基体ZrB2晶粒的微观结构,明显促进了材料的低温致密化并抑制了晶粒长大.但力学性能略有降低,其原因可能是PCS裂解产生的微量碳遗留在基体ZrB2的晶界处,弱化了晶界结合强度.本文验证了采用PCS和纳米ZrB2粉体进行热压烧结是实现ZrB2-SiC超高温陶瓷低温致密化的有效手段.     

熔盐法低温合成ZrB2-ZrC-SiC复合粉体的研究

以ZrSiO4、Na2B4O7、Mg粉及C粉为原料,MgCl2为熔盐介质,采用熔盐法制备了ZrB2-ZrC-SiC复合粉体,研究了熔盐温度(900～1200℃)、原料配比对熔盐法合成ZrB2-ZrC-SiC复合粉体的物相组成及含量的影响.结果表明:当MgCl2∶反应物=4∶1(wt;),ZrSiO4∶Na2B4O7∶Mg∶C=2∶1∶18.2∶2(mol;)时,经1150℃反应3h所制备的复合粉体中ZrB2-ZrC-SiC的相对含量最高,约为78wt;.     

CsI-LiCl共晶材料的坩埚下降法生长与闪烁性能研究

本文采用坩埚下降法,在真空密封的石英坩埚中成功生长出CsI-LiCl与CsI-LiCl:Na共晶闪烁体。通过扫描电子显微镜(SEM)观察晶体微结构表明该共晶中LiCl相与CsI相存在周期性的层状排列,CsI相的厚度在5 μm左右。共晶样品的X射线激发发射谱显示在CsI-LiCl和CsI-LiCl:Na共晶样品存在缺陷发光,在CsI-LiCl样品中还观察到了纯CsI的自陷激子(STE)发光。CsI-LiCl样品在α粒子激发下的多道能谱中观察到明显的全能峰,这一结果证明CsI-LiCl共晶可用于热中子探测的潜力。     

静电纺丝法制备CeO2微纳米纤维及其除氟性能

以聚丙烯腈(PAN)为载体,六水合硝酸铈[Ce(NO₃)₃·6H₂O]为原料,采用静电纺丝法制备了Ce(NO₃)₃/PAN纤维,在空气中热处理得到CeO₂微纳米纤维,通过XRD、BET和SEM对CeO₂微纳米纤维进行表征。采用静态吸附实验探讨了CeO₂微纳米纤维去除水溶液中氟离子的性能,考察了溶液pH值、初始氟离子浓度及共存阴离子等对吸附性能的影响。结果表明,pH=3时,CeO₂微纳米纤维对F^-的吸附性能最佳,CeO₂吸附量随着F^-浓度的增大呈上升趋势。CeO₂微纳米纤维对F^-的吸附等温线遵循Langmuir模型,二级动力学模型能很好地描述CeO₂微纳米纤维对F^-的吸附过程。CeO₂微纳米纤维的除氟性能优良,可为其实际应用提供理论参考。     

X-ray analysis of sideroxol from <Emphasis Type="Italic">Sideritis leptoclada</Emphasis>

Sideroxol (1), a kaurane diterpene which has the ent-7α,18-dihydroxy-15β,16β-epoxykaurane structure (MW = 320.47, C₂₀H₃₂O₃) was obtained from the acetone extract of Sideritis leptoclada plant as well as from some other Sideritis species. It crystallizes in the orthorhombic space group P2₁, 2₁, 2₁ with a = 10.967(3), b = 24.555(5), c = 6.372(4) Å, Dc = 1.240 g cm⁻³, Z = 4, and refines to R = 0.065 for 721 independent reflections. The skeleton consists of three fused six-membered rings and a five-membered ring with fused epoxide. The six membered rings exhibited slightly distorted chair conformation. In addition to sideroxol, two kaurane and five kaurene diterpenes were isolated from the hexane and acetone extracts of the studied plant.     

New zeotype borophosphates with chiral tetrahedral topology: (H)0.5M1.25(H2O)1.5[BP2O8]·H2O (M = Co(II) and Mn(II))

Two new isostructural open‐framework zeotype transition metal borophosphate compounds, (H)_0.5M_1.25(H₂O)_1.5[BP₂O₈]·H₂O (M = Co(II) and Mn(II)) were synthesized by mild hydrothermal method. The structure of compounds were characterized by single‐crystal X‐ray diffraction which have ordered, alternating, vertex‐sharing BO₄, PO₄, and (MO₄)OM(H₂O)₂ groups with hexagonal, P 6₁ 2 2 (No 178) space group and unit cell parameters for Co a = 9.4960(6) Å, c = 15.6230(13) Å, for Mn a = 9.6547(12) Å, c = 15.791(3) Å, Z = 1 for both of them. TGA/DTA analysis, IR spectroscopy were used for characterization. Magnetic susceptibility measurements for both of the compound indicate strong antiferromagnetic interaction between metal centers. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Photoluminescence characteristics of Mg- and Si-doped GaN thin films grown by MOCVD technique

We have studied the optical, structural and surface morphology of doped and undoped GaN thin films. The p- and n-type thin films have been successfully prepared by low-pressure MOCVD technique by doping with Mg and Si, respectively. The different carrier concentrations were obtained in the GaN thin films by varying dopant concentrations. Photoluminescence (PL) studies were carried to find the defect levels in the doped and undoped GaN thin films at low temperature. In the undoped GaN thin films, a low intensity and broad yellow band peak was observed. The donor–acceptor pair (DAP) emission and its phonon replicas were observed in both the Si or Mg lightly doped GaN thin films. The dominance of the blue and the yellow emissions increased in the PL spectra, as the carrier concentration was increased. The XRD and SEM analyses were employed to study the structural and surface morphology of the films, respectively. Both the doped and the undoped films exhibited hexagonal structure and polycrystalline nature. Mg-doped GaN thin films showed columnar structure whereas Si-doped films exhibited spherical shape grains.     

Solubility of Ag2O into the Na2O–B2O3–Al2O3 system

The solubility of Ag₂O was measured for the Na₂O–B₂O₃ and Na₂O–B₂O₃–Al₂O₃ system with the rotating crucible method and static method, respectively, under air atmosphere at temperatures ranging from 1273 to 1423 K. The contamination of melts from crucibles could be avoided by the rotating crucible method, with which it became possible to measure the solubility of Ag₂O for the Na₂O–B₂O₃ system above the melting point of Ag for the first time. It was found that the addition of Na₂O decreases the solubility of Ag₂O while the addition of Al₂O₃ had little effect on the solubility. The effect of Na₂O and Al₂O₃ on the solubility of Ag₂O is expressed by interaction coefficients and is analyzed in terms of the basicity of melts. The solubility of Ag₂O in Na₂O–B₂O₃–Al₂O₃ melts increased with increased temperature. This phenomena was explained by a small enthalpy change in oxidation of silver.     

Considerations on the structure and physical properties of B2O3-SiO2 and GeO2-SiO2 glasses

Data of density, refractive index and thermal expansion coefficient for B₂O₃-SiO₂ and GeO_2-SiO₂ glasses have been analyzed. The volumes of the structural units are the same found for the vitreous B₂O₃, GeO₂ and SiO₂. The volume of any structural unit is constant over the entire composition region of the glass system. The same has been found for the differential refraction and unit refraction of the structural units in these glasses. Different features are observed for the differential expansion of the structural units. There is a considerable change with composition in the differential expansion of BO₃, GeO₄ and SiO₄ units. The effect is attributed to a change in the asymmetry of vibrations with the number of Si-O-B or Si-O-Ge linkages in the matrix. The thermal expansion coefficient is mainly determined by the contribution of B₂O₃ or GeO₂ in the concerned glasses.     

Molecular structure and crystal packing of uncomplexed 1,6-bis(N-cyano-p-methoxy-anilino)-2,4-hexadiyne, C22H18N4O2

The X-ray crystal structure of 1,6-bis(N-cyano-p-methoxy-anilino)-2,4-hexadiyne, C₂₂H₁₈N₄O₂, is determined. The crystal packing is dominated by phenyl stacking interactions. Weak C–H···N hydrogen bonds help align the molecules. C–H··· hydrogen bonding is not apparent.     

Structural and electrical properties of vapour phase grown Cd1 − xFexTe single crystals

Cd_1 − xFe_xTe single crystals were prepared by vapour phase growth method in the composition range of 0 ≤ x ≤ 0.03. Chemical analysis, surface morphology, structural investigations and electrical properties were carried out by EDAX, SEM, XRD, TEM and transport technique, respectively. Microscopic variations between the target and actual compositions were noticed. Morphology studies revealed that dislocation aided growth is active in the present crystals. TEM and XRD studies confirmed that the samples of all compositions crystallized in zinc blende structure, and the lattice parameters varied almost linearly decreases with Fe content. At room temperature, the resistivity of the Cd_1 − xFe_xTe crystals of all compositions (x = 0.01, 0.015, 0.02, 0.025 and 0.03) lies in the range of 3.5-6.5 M Ω, the activation energies lie in the range of 63-133 meV, and the samples were show the ‘p’ type conductivity.     

空位浓度对纤锌矿CdS电子结构和光学性质影响的第一性原理研究

本文基于密度泛函理论的平面波超软赝势方法,采用第一性原理研究了含Cd空位缺陷CdS和含S空位缺陷纤锌矿CdS的几何结构、能带结构、电子态密度及光学性质。通过计算分析可知,含Cd空位缺陷的CdS体系均为p型半导体,含S空位缺陷的CdS体系跃迁方式均由直接跃迁变为间接跃迁。Cd、S空位缺陷的CdS体系的态密度总能量降低。空位CdS体系相较于本征CdS体系的静介电常数均有提高,并随着空位浓度的增大而增大,Cd空位缺陷体系更为明显,极化能力得到显著提升。空位Cd的CdS体系相较于本征CdS体系在红外波段存在明显的吸收,空位S的CdS体系相较于本征CdS体系在可见光波段存在明显的吸收。