SiC/Al复合材料的热导率研究

用无压浸渗法制备了高导热的SiC/Al电子封装材料.采用光学显微镜、X射线衍射仪、扫描电镜和激光热导仪对复合材料导热率、晶体结构和微观形貌进行了分析,研究了SiC颗粒大小、形状、体积分数、基体中Mg的含量和预氧化等参数对SiC/Al复合材料的导热率的影响.结果表明,选择适当的原料参数和工艺参数可制得导热率高达172.27 W/(m·k)的SiC/Al复合材料,满足电子封装材料的要求.     

高速正交切削SiCp/Al复合材料的切屑形成及边界损伤仿真研究

针对SiCp/Al复合材料颗粒增强相与基体相不同的机械力学性能,对照SiCp/Al复合材料金相照片中的SiC颗粒形状与分布状态建立仿真模型,并对SiCp/Al复合材料的颗粒和基体进行分别定义.使用ABAQUS有限元软件仿真研究了高速切削复合材料切屑的形成、工件已加工表面的缺陷特征、边界损伤的形成机理及切削用量对边界损伤程度的影响规律.结果表明:在第1变形区切削变形主体表现为沿剪切角方向Al合金基体的剪切滑移,滑移主要发生在SiC颗粒与Al合金基体的边界邻近区域;对刀刃直接作用的SiC颗粒,由于其压力较大,会发生破碎现象;SiCp/A1复合材料的切屑形态以单元状切屑和由几个单元构成的节状切屑为主;SiC颗粒的破碎和整体剥落是造成已加工表面缺陷的重要因素;负剪切角是产生边界损伤的标志;切削速度对边界缺损几何尺寸大小的影响较小,切削深度对边界缺损几何尺寸大小的影响较大;随着切削深度的增大,边界缺损长度呈直线上升趋势,边界缺损高度先显著增加后缓慢增加.     

FeNi@CNOs/AlN复合材料的制备及其吸波性能

核壳结构的碳材料具有优异的吸波性能,但是在陶瓷基体中难以分散均匀.本研究通过化学气相沉积法在氮化铝陶瓷机体中引入磁性纳米洋葱碳,制备了FeNi@CNOs/AlN复合材料,研究了其相组成、微观形貌和吸波性能.结果表明,1100 K下基于Al粉与C粉,FeNi催化剂的存在可原位生成FeNi@CNOs/AlN复合材料;10wt;FeNi@CNOs/AlN复合材料在8.29～15.32 GHz范围内,其RL值均低于-10 dB,在13.2 GHz处达到最大值为-23 dB.     

SiCp/Al复合材料切削仿真及实验研究

对SiCp/Al复合材料的颗粒和基体分别进行定义建立有限元仿真模型,使用ABAQUS有限元软件从微观角度分析了不同切削速度和切削深度对切削力的影响以及应力场分布情况,研究切屑的形成过程、基体和颗粒的内部应力分布、刀具和颗粒之间的相互作用以及简要分析了材料表面缺陷的成因,通过车削实验进行验证.研究表明:微观仿真模型中由于高硬度SiC颗粒的存在会产生大量的微裂纹以及单一小空洞和不连续空洞,形成表面缺陷,且剪切区域微裂纹的扩展是产生切屑的重要因素;通过对比,实验获得的切削力变化及其波动和形成机制与仿真结果一致.     

离心-凝胶注模成型技术制备梯度碳纤维/HA复合材料

以经过低温氧化和提拉HA溶胶改性处理的碳纤维为增强体,采用离心-凝胶注模成型技术制备梯度碳纤维/HA复合材料.观察了碳纤维改性处理后的形貌,研究了碳纤维/HA浆料的特性,以及浆料的凝胶固化过程.分析了离心转数、碳纤维含量对梯度复合材料生坯密度梯度的影响.观察了烧结后梯度复合材料的显微组织,并测量了不同烧结温度和碳纤维分布的HA复合材料的抗弯强度和断裂韧性.研究结果表明改性处理碳纤维表面形成一层致密的、结合性能较好的膜层,烧结后可以很好地连接基体和碳纤维.在pH =9,分散剂含量为5wt;,浆料固相含量为40wt;,碳纤维含量为2wt;时,碳纤维/HA浆料具有良好的分散性和稳定性.当催化剂和引发剂含量均为0.9wt;时,起始凝胶和终止固化时间与离心成型工艺相匹配.当离心转数为1000 r/min,碳纤维含量为2wt;时,离心-凝胶注模成型所得试样具有良好的密度梯度,1100℃烧结2h后,该复合材料的抗弯强度达到最大值101.7 MPa;断裂韧性为1.98 MPa·m1/2,分别比干压成型制备的均匀碳纤维/HA复合材料提高了29.7;和25.3;.     

气体压力熔渗制备电子封装用金刚石/铝复合材料的研究

金刚石颗粒增强铝基(diamond/Al)复合材料是最受关注的新一代电子封装材料.本文采用气体压力熔渗法制备了金刚石体积分数达到65;的diamond/Al复合材料;通过对相组成及断口形貌的分析,明确了复合材料的界面优化机制及破坏方式,在此基础上,系统研究了复合材料的热物理性能和力学性能随金刚石粒度的变化规律.气体压力熔渗法制备diamond/Al复合材料可以依靠基体铝与金刚石间的扩散反应实现界面优化,显著改善界面形貌,提高结合强度,复合材料断口微观形貌则呈现出典型的塑性断裂特征;依靠两相原位反应优化后的界面可以更加有效的实现热载流子的耦合移动和应力的传递,从而保证复合材料可以获得更为优异的热导率和力学性能;随着金刚石颗粒直径减小,两相界面的影响得以加强,因此复合材料热导率降低,热膨胀系数小幅减小,而力学性能提高,在本文选择的粒度范围内(30～ 150 μm),复合材料的热导率、热膨胀系数以及抗拉、抗压、抗弯强度的相应变化区间分别为400～760 W·m-1·K-1、4.5 ～5.3×10-6 K-1、143～ 94 MPa、603 ～ 363 MPa和429 ～ 277 MPa.     

TiAl-B4C复合材料耐海水腐蚀性研究

对所制备的TiAl/B4C复合陶瓷材料耐海水腐蚀性能进行了研究分析,将10wt;～40wt; TiAl含量的TiAl/B4C复合材料在天然海水中进行60天的全浸实验,通过SEM对试样的腐蚀形貌进行观察表征,XRD分析相结构对腐蚀过程进行推测,结合电化学阻抗测试对试样耐腐蚀能力的强弱进行具体定量化的分析.结论表明,TiAl/B4C具有良好的抗腐蚀性能,且腐蚀性能的差异受TiAl金属间化学物含量的影响,含量为10wt;的TiAl/B4C复合材料耐腐蚀性能最优.     

溶胶-凝胶法改性碳纤维增强纳米HA复合材料的制备及性能研究

先对碳纤维进行对氨基苯甲酸预处理,然后通过溶胶-凝胶技术在预处理后碳纤维表面涂覆HA涂层,随后利用粉末冶金技术制备改性碳纤维增强纳米HA复合材料.研究碳纤维的改性工艺,观察改性后碳纤维表面的微观形貌,测量小同碳纤维含量下复合材料的抗弯强度和断裂韧性.结果表明对氨基苯甲酸处理后碳纤维表面形成大量的纵向凹槽,表面粗糙度增加,将其在HA溶胶中提拉5次后可以在表面获得一层致密的、结合性能较好的膜层.烧结产物中HA过渡层可以很好地连接基体和碳纤维,提高纳米HA复合材料的力学性能,当碳纤维含量为3vol;时,溶胶-凝胶改性碳纤维/纳米HA复合材料的抗弯强度达到最大值84.6 MPa,是基体抗弯强度的3.45倍.当碳纤维含量为4vol;时,溶胶-凝胶改性碳纤维/纳米HA复合材料的断裂韧度达到最大值1.92 MPa·m1/2,是基体断裂韧度的2.43倍.     

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.     

无压烧结制备透辉石/AlTiB增韧补强氧化铝基结构陶瓷材料

采用温度梯度无压烧结工艺制备了透辉石/AlTiB增韧补强Al2O3基结构陶瓷材料,探讨了其致密化烧结特性,并对其力学性能进行了测试和分析.研究了透辉石/AlTiB增韧补强Al2O3基结构陶瓷材料的微观结构,并分析了其力学性能和微观结构与透辉石含量的关系.结果表明:与纯Al2O3相比,透辉石/AlTiB增韧补强Al2O3基结构陶瓷材料的力学性能得到明显提高,其中添加6;(体积百分含量,下同)透辉石和4;AlTiB的Al2O3基结构陶瓷材料获得较好的综合力学性能,其硬度、抗弯强度和断裂韧性分别达到16.02 GPa、370 MPa和5.11 MPa·m1/2.力学性能提高的主要原因为:添加相与Al2O3基体之间界面反应的发生以及透辉石和AlTiB对复合材料的协同晶粒细化效应.     

Peculiarities of resonant interaction in paired impurity centers of 2-fluoronaphthalene in naphthalene

Abstract

Fluorescence and absorption spectra of the 2-fluoronaphthalene admixtures in naphthalene were studied at low temperature (T?=?4.2 К). Two types of pairwise impurity centers were formed at admixture concentrations of more than 1?wt%. Polarization of absorption bands was detected; these spectra were determined by resonance interactions between molecules of the impurity center. Resonant splitting of electronic levels for the translationally nonequivalent molecules in the unit cell of the naphthalene crystal was analyzed for the case, when one molecule was in the similar phase with the incident light wave and the other one was in antiphase.     

Temperature dependencies of intensities of f-f transitions in Pr and Dy in glasses

Absorption spectra of Pr³⁺ and Dy³⁺ embedded in three glass matrices (SiO₂-P₂O₅-GeO₂, Al₂O₃-B₂O₃-SiO₂ and LiB₃O₅) have been studied in detail in the near infrared spectral range. The temperature related behavior of the intensity, width and position of the observed absorption lines were studied. It has been shown that f-f transitions in the studied glasses are mainly allowed by static odd distortions of the ligand environment, and these distortions decrease with a rise in temperature. Comparative analysis of the parameters of the absorption lines and their temperature dependencies in different materials have enabled us to ascertain the difference of size, symmetry and dispersion of distortions of the nearest environment of these rare earth ions in different glasses.     

Role of sublattices in the formation of electron density in metal nitrites

The self-consistent valence electron densities of NaNO₂, AgNO₂, and their constituent sublattices are calculated on the basis of the theory of the local-density functional. The quantity characterizing the relation between different sublattices is introduced as the difference density resulting from the subtraction of the densities of the individual sublattices from the total electron density. The role of metal in the formation of electron density is established, and, in particular, it is shown that, in AgNO₂, anionic bonds have the covalent component formed at the expense of the electron density of the cation. It is also shown that, qualitatively, the difference density in NaNO₂ corresponds to the experimental deformation density.     

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.     

Diffusion of vanadium in GaAs

The diffusion of Vanadium has been studied in V-doped GaAs layers (GaAs:V) grown by Metal-Organic Chemical Vapour Deposition (MOCVD) using secondary ion mass spectroscopy (SIMS). The vanadium (V) concentration profiles of sandwiched structures made of alternatively undoped and V doped GaAs layers have shown a concentration independent diffusion coefficient (D_V) for varying V doping levels from 10¹⁸ to 10¹⁹ cm⁻³. Measurements of D_V at 550, 615 and 680 °C indicate that the temperature dependence of D_V can be represented by the Arrhenius equation:  cm² s⁻¹. It is suggested that V diffuses via interstitial sites.     

Diffusion of chromium in chrysoberyl

Cr³⁺ diffusion in chrysoberyl (BeAl₂O₄) irradiated by H⁺ ions and electrons has been studied and compared with diffusion in non-irradiated samples. Chrysoberyl crystals were irradiated with 6 MeV H⁺ ions to fluencies of 1×10¹⁶ cm^–2 for 25 min and with 10 MeV electrons to fluencies of 2×10¹⁷ cm^–2 for 1 h. Three different types of samples, which were doped with Cr³⁺, were annealed in horizontal alumina tube furnaces by 50 K intervals in the temperature range from 1773 to 1923 K for 200 h. Scanning electron microscope–energy dispersive X-ray spectrometer (SEM–EDX) was used to measure the diffusion. Arrhenius equations for the diffusion coefficient for Cr³⁺ in the temperature range 1773–1923 K were developed:     

烧结温度对相分离锰氧化物中磁相变的影响

采用标准固态反应法,在不同的烧结温度下制备了Pr0.6Ca0.38Ba0.02MnO3和Pr0.6Ca0.4Mn0.98Ga0.02O3,以研究烧结温度对样品颗粒尺寸以及磁相变的影响。通过X射线衍射测试,我们发现对于这两个系列的样品,无论是Ba对Ca的替代还是Ga对Mn的替代,各样品均为菱面体对称结构,且没有任何杂相。扫描电镜实验表明,随着烧结温度的升高,两个系列样品的颗粒尺寸都将增大。研究低温下样品磁化强度随外加磁场变化(M-H)的实验结果后,我们发现随烧结温度的升高即颗粒尺寸的增大,由反铁磁向铁磁发生磁相变的临界磁场减小,而场致铁磁性将增加,这说明颗粒长大对铁磁态的发展是有利的。对比两个样品的磁性质后,我们发现Ba2+替代在破坏电荷轨道有序导致场致铁磁性的能力上比Ga3+替代要强。     

Morphology Change in Crystallization of α L-Glutamic Acid in the Presence of L-Phenylalanine

Abstract

Effect of L-phenylalanine(L-Phen) on the precipitation behavior and morphology of a L-glutamic acid (L-Glu) crystals, and inclusion of L-Phen in L-Glu crystals were investigated. L-Phen suppresses both nucleation rate and growth rate of L-Glu. The morphology of L-Glu was observed to change depending on L-Phen concentration. Two predominant morphologies were obtained, which have the large (111) face and the new face of (110), respectively. Irregular increase of the molar ratio of L-Phen to L-Glu in nucleated crystals was detected and it was attributed to the morphology change of L-Glu crystals. Dependence of R on L-Phen concentration appeared to be different between the nucleated crystals and seed crystals.     

Role of oxygen vacancies in the origin of ferromagnetism in Mn‐doped ZnO

The relationship between the oxygen vacancy and ferromagnetism in Mn‐doped ZnO has been studied based on the first‐principles calculations. Three possible charge states of oxygen vacancies, i.e., neutral (V_O⁰), 1+ (V_O¹⁺) and 2+ (V_O²⁺) are considered. Results show that the lattice relaxations around oxygen vacancies are large difference under different charge states. It is found that V_O¹⁺ and V_O²⁺ oxygen vacancies induce ferromagnetism. However, Mn‐doped ZnO system shows ferromagnetism with V_O⁰ oxygen vacancies in hydrogenated environment, the ferromagnetism is attributed to the interstitial H, which forms a bridge bond and mediates d ‐d coupling and stabilizes the ferromagnetic state. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Role of pore structure in salt crystallisation in unsaturated porous stone

Laboratory driven crystallisation of sodium sulphate and sodium chloride from concentrated solution in unsaturated porous stones has been performed. This contributes to a better understanding of the mechanisms by which salts crystallise and as a consequence limit the durability of porous materials which has an impact on buildings, civil constructions, and historical monuments. The identification of minerals in porous materials has been performed by scanning electron microscopy (SEM), Environmental Scanning Electron Microscopy (ESEM) and sequential profiles of X-ray diffraction (XRD) under temperature control of sample. The study of porous stones has been combined with experiments in capillary tubes. Data from SEM show that halite tends to precipitate on the surface of the stone with a similar distribution in all samples. However, the mirabilite–thenardite precipitation takes place preferably inside the stone and its depth from the surface and its relative concentration depends on the pore size distribution. In addition, mirabilite (Na₂SO₄·10H₂O) crystallises homogeneously, whereas thenardite (Na₂SO₄) and halite (NaCl) tend to nucleate heterogeneously. To explain the precipitation sequence from concentrated solutions in unsaturated porous materials, a detailed analysis of the thermodynamic equations has been carried out by establishing a simple model. The proposed model shows the influence of the pore structure both on the water activity and saturation degree of involved salts.