成形压力对SiC/TiB2复合材料组织与性能的影响

基于熔融Si浸渗法制备出较致密的SiC/TiB₂复合材料, 并研究了坯体成形压力对SiC/TiB₂复合材料致密度、相组成、显微组织和力学性能的影响。实验结果表明, 复合材料由TiB₂、SiC和Si相组成。SiC/TiB₂复合材料的显微组织特征为: TiB₂相和SiC相均匀分布, 游离Si填充在TiB₂相和SiC相的空隙处, 且形成了连续相。随成形压力的增大, 复合材料中游离Si含量降低, TiB₂颗粒尺寸减小, 复合材料的力学性能先增加后降低。坯体最佳成形压力为200 MPa, 对应SiC/TiB₂复合材料的体积密度、开口气孔率、抗弯强度、断裂韧性和维氏硬度分别为3.63 g/cm³、0.90%、(354±16) MPa、(6.8±0.2) MPa·m^1/2和(21.0±1.1) GPa。     

Phase formation process of IrxSi1−x thin films Structure and electrical properties

Experimental data on the phase formation process of amorphous Ir_xSi_1−x thin films with 0.30 ≤ x ≤ 0.41 are presented and discussed in relation to electric transport properties. The structure formation process at temperatures from 300 K up to 1223 K was investigated by means of X-ray diffraction. Distinct phases were observed in the final stage in dependence on the initial composition: Ir₃Si₄, Ir₃Si₅, and IrSi₃. An unknown metastable phase was found in films with a silicon concentration of 61 at.% to 64 at.% after annealing above the crystallization temperature T = 970 K. The crystal structure of this phase was determined by the combined use of X-ray diffraction and electron diffraction. It was found to be monoclinic, basic-face centred with lattice constants a = 1.027 nm, b = 0.796 nm, c = 0.609 nm, and γ = 113.7°. Additionally, microstructure and morphology of the films were investigated by transmission electron microscopy (TEM). The annealing process was studied by means of mechanical stress investigations as well as by electrical resistivity and thermopower measurements. Correlations between the structure, the phase formation and the electrical transport behaviour are discussed on the basis of conduction mechanism.     

Microwave power dependence of the optical and structural properties of boron and phosphorus-doped SiC:H films prepared using ECR-CVD

Hydrogenated silicon carbide films (SiC:H) were deposited using the electron cyclotron resonance chemical vapour deposition (ECR-CVD) technique from a mixture of methane, silane and hydrogen, and using diborane and phosphine as doping gases. The effects of changes in the microwave power on the deposition rate and optical bandgap were investigated, and variations in the photo- and dark-conductivities were studied in conjunction with film analysis using the Raman scattering technique. In the case of boron-doped samples, the conductivity increased rapidly to a maximum, followed by rapid reduction at high microwave powers. The ratio of the photo- to dark-conductivity (σ_ph/σ_d) peaked at microwave power of 600 W. Under conditions of high microwave power, Raman scattering analysis showed evidence of the formation and increase in the silicon microcrystalline and diamond-like phases in the films, the former of which could account for the rapid increase and the latter the subsequent decrease in the conductivity. In the case of phosphorus-doped SiC:H samples, it was found that increase in the microwave power has the effect of enhancing the formation of the silicon microcrystalline phase in the films which occurred in correspondence to a rapid increase in the conductivity. The conductivity increase stabilised in samples deposited at microwave powers exceeding 500 W probably as a result of dopant saturation. Results from Raman scattering measurements also showed that phosphorus doping has the effect of enhancing the formation of the silicon microcrystals in the film whereas the presence of boron has the effect of preserving the amorphous structure.     

以YbH2-MgO体系为烧结助剂制备高热导率高强度氮化硅陶瓷

以YbH₂-MgO体系为烧结助剂, 采用两步法烧结制备了高热导率高强度氮化硅陶瓷, 研究了YbH₂-MgO对氮化硅致密化行为、相组成、微观形貌、热导率和抗弯强度的影响。在预烧结阶段, YbH₂在还原SiO₂的同时原位生成了Yb₂O₃, 进而形成“缺氧-富氮”液相。该液相不仅有利于晶粒的生长, 更有利于阻碍晶格氧的生成, 相较于Yb₂O₃-MgO助剂体系, β-Si₃N₄晶粒尺寸更大, 晶格缺陷更少, 低热导晶间相更少, 在1900 ℃保温24 h后, 热导率最优可达131.15 W·m^-1·K^-1, 较Yb₂O₃-MgO体系提升13.7%。用YbH₂代替Yb₂O₃, 在低温条件下烧结制备得到的氮化硅抗弯强度有所改善, 在1800 ℃保温4 h的抗弯强度可达(1008±35) MPa; 但在高温烧结时强度略有下降, 这与微观结构的变化密切相关。研究表明, YbH₂-MgO体系是制备高热导率高强度氮化硅陶瓷的有效烧结助剂。     

The effect of rapid thermal annealing on structural and electrical properties of TiB2 thin films

This work reports on the effect of post-deposition rapid thermal annealing on the structural and electrical properties of deposited TiB₂ thin films. The TiB₂ thin films, thicknesses from 9 to 450 nm, were deposited by e-beam evaporation on high resistivity and thermally oxidized silicon wafers. The resistivity of as-deposited films varied from 1820 μΩ cm for the thinnest film to 267 μΩ cm for thicknesses greater than 100 nm. In the thickness range from 100 to 450 nm, the resistivity of TiB₂ films has a constant value of 267 μΩ cm.

A rapid thermal annealing (RTA) technique has been used to reduce the resistivity of deposited films. During vacuum annealing at 7 × 10⁻³ Pa, the film resistivity decreases from 267 μΩ cm at 200 °C to 16 μΩ cm at 1200 °C. Heating cycles during RTA were a sequence of 10 s. According to scanning tunneling microscopy analysis, the decrease in resistivity may be attributed to a grain growth through polycrystalline recrystallization, as well as to an increase in film density.

The grain size and mean surface roughness of annealed films increase with annealing temperature. At the same time, the conductivity of the annealed samples increases linearly with grain size. The obtained results show that RTA technique has a great potential for low resistivity TiB₂ formation.     

A simple model of radiation swelling of silicon

The radiation swelling of silicon is explained as a diffusion-like process where the flux of interstitials out of the plane is defined by the gradient of the concentration of interstitials and the gradient of mechanical stresses in the ion-implanted region of the solid. This model was applied to describe the dynamics and the main regularities (dependence of strain on the ion flux density, ion energy, substrate temperature) of ion implanted silicon (Ni⁺, E = 40–160 keV, j = 5–180 μA cm⁻²). It is demonstrated that suppression of radiation swelling at high temperatures of the substrate or high ion beam current density can be explained by the annihilation of radiation defects.     

In:Ga2O3氧化物半导体晶体的生长与性能研究

β-Ga₂O₃晶体是一种新型宽禁带氧化物半导体材料, 本征导电性差。为了在调控导电性能的同时兼顾高的透过率和结晶性能, 离子掺杂是一种有效的途径。采用光学浮区法生长出ϕ8 mm×50 mm蓝色透明In:Ga₂O₃晶体, 晶体具有较高的结晶完整性。In³⁺离子掺杂后, β-Ga₂O₃晶体在红外波段出现明显的自由载流子吸收, 热导率稍有减小。室温下, In:Ga₂O₃晶体的电导率和载流子浓度分别为4.94×10^-4 S/cm和1.005×10¹⁶ cm^-3, 其值高于β-Ga₂O₃晶体约1个数量级。In:Ga₂O₃晶体电学性能对热处理敏感, 1200℃空气气氛和氩气气氛退火后电导率降低。结果表明, In³⁺离子掺杂能够调控β-Ga₂O₃晶体的导电性能。     

Fabrication of SiCf/SiC composites by chemical vapor infiltration and vapor silicon infiltration

Three-dimensional (3D) silicon carbide fiber reinforced silicon carbide matrix (SiC_f/SiC) composites, employing KD-1 SiC fibers (from National University of Defense Technology, China) as reinforcements, were fabricated by a combining chemical vapor infiltration (CVI) and vapor silicon infiltration (VSI) process. The microstructure and properties of the as prepared SiC_f/SiC composites were studied. The results show that the density and open porosity of the as prepared SiC_f/SiC composites are 2.1 g/cm³ and 7.7%, respectively. The SiC fibers are not severely damaged during the VSI process. And the SiC fibers adhere to the matrix with a weak interface, therefore the SiC_f/SiC composites exhibit non-catastrophic failure behavior with the flexural strength of 270 MPa, fracture toughness of 11.4 MPa·m^1/2 and shear strength of 25.7 MPa at ambient conditions. Moreover, the flexural strength decreases sharply at the temperature higher than 1200 °C. In addition, the thermal conductivity is 10.6 W/mk at room temperature.     

n型SnS热电材料的制备与性能研究

SnS由低毒、廉价、高丰度的元素组成, 在热电研究领域受到广泛关注。采用机械合金化(MA)结合放电等离子烧结(SPS)工艺制备了n型SnS_1-xCl_x(x=0, 0.02, 0.03, 0.04, 0.05, 0.06)多晶块体热电样品, 并研究了Cl^-掺杂量对SnS物相、微观结构以及电热输运性能的影响。结果表明: Cl^-的引入会提高电子浓度, 使SnS由本征p型转变为n型半导体。随着Cl^-掺杂量的增加, n型SnS半导体室温下的霍尔载流子浓度从6.31×10¹⁴ cm^-3 (x=0.03)增加到7.27×10¹⁵cm^-3 (x=0.06)。x=0.05样品在823 K取得最大的电导率为408 S·m^-1, 同时具有较高的泽贝克系数为-553 μV•K^-1, 使其获得最大功率因子为1.2 μW·cm^-1·K^-2。Cl^-的掺入会引入点缺陷, 散射声子, 使晶格热导率κ_lat由0.67 W·m^-1·K^-1(x=0)降至0.5 W·m^-1·K^-1 (x=0.02)。x=0.04样品在823 K获得了最大ZT为0.17, 相比于x=0样品(ZT~0.1)提高了70%。     

Atomic layer deposition of calcium oxide and calcium hafnium oxide films using calcium cyclopentadienyl precursor

Calcium oxide and calcium hafnium oxide thin films were grown by atomic layer deposition on borosilicate glass and silicon substrates in the temperature range of 205–300 °C. The calcium oxide films were grown from novel calcium cyclopentadienyl precursor and water. Calcium oxide films possessed refractive index 1.75–1.80. Calcium oxide films grown without Al₂O₃ capping layer occurred hygroscopic and converted to Ca(OH)₂ after exposure to air. As-deposited CaO films were (200)-oriented. CaO covered with Al₂O₃ capping layers contained relatively low amounts of hydrogen and re-oriented into (111) direction upon annealing at 900 °C. In order to examine the application of CaO in high-permittivity dielectric layers, mixtures of Ca and Hf oxides were grown by alternate CaO and HfO₂ growth cycles at 230 and 300 °C. HfCl₄ was used as a hafnium precursor. When grown at 230 °C, the films were amorphous with equal amounts of Ca and Hf constituents (15 at.%). These films crystallized upon annealing at 750 °C, showing X-ray diffraction peaks characteristic of hafnium-rich phases such as Ca₂Hf₇O₁₆ or Ca₆Hf₁₉O₄₄. At 300 °C, the relative Ca content remained below 8 at.%. The crystallized phase well matched with rhombohedral Ca₂Hf₇O₁₆. The dielectric films grown on Si(100) substrates possessed effective permittivity values in the range of 12.8–14.2.     

Oxide rate characterization of high dose krypton- implanted silicon

A technique for removing thin silicon layers by anodization was used in an investigation of the depth profiles of heavily disordered regions in silicon which had undergone krypton ion implantation. The method is based on the fact that the oxidation rate of high dose ion-implanted silicon depends strongly on the concentration of easy paths with a high oxygen conductivity. It is speculated that the highly damaged regions in implanted silicon are new phase regions resulting from dopant precipitation. Because of their different chemical and physical properties from those of silicon, the new phase regions after transformation into oxide may act as easy paths for oxygen atoms migrating during anodization. A set of profiles produced by implanting krypton ions of energy 10–300 keV to a total dose of (1–2) × 10¹⁷ ions cm^-2 was obtained and was analysed by the method of moments. The average range, straggling, skewness and kurtosis of the measured profiles were compared with theoretical results for ion and damage distributions calculated using the method of Winterbon.     

Al2O3-SiO2复合气凝胶的制备及改性剂对其结构与隔热性能的影响

以AlCl₃·6H₂O为前驱体,采用离子交换工艺和溶胶-凝胶法,在正硅酸四乙酯（TEOS）乙醇溶液中浸泡实现Al₂O₃和SiO₂的复合,经表面改性和常温常压干燥制备出低成本、无杂质离子、低热导率的Al₂O₃-SiO₂复合气凝胶。探索了不同有机硅烷改性剂对Al₂O₃-SiO₂复合气凝胶结构和隔热性能的影响。结果表明,在改性剂为三甲氧基甲基硅烷（MTMS）,改性环境为中性（pH为7）时,Al₂O₃-SiO₂复合气凝胶表现出最均匀的微观结构,SiO₂和Al₂O₃主要以无定形形式存在。MTMS可有效减少Al₂O₃-SiO₂湿凝胶表面的-OH基团,形成Si-O-Si和Al-O-Si基团。Al₂O₃-SiO₂气凝胶比表面积和孔体积分别达到574 m2/g和2.3 cm3/g,热导率低至0.029 W（m·K）-1。以上研究为促进气凝胶材料在隔热领域的应用提供了支持。      

Structural and optical characterization of β-FeSi2 layers on Si formed by ion beam synthesis

Structural and optical properties have been investigated for surface β-FeSi₂ layers on Si(100) and Si(111) formed by ion beam synthesis using ⁵⁶Fe ion implantations with three different energies (140–50 keV) and subsequent two-step annealing at 600 °C and up to 915 °C. Rutherford backscattering spectrometry analyses have revealed Fe redistribution in the samples after the annealing procedure, which resulting in a Fe-deficient composition in the formed layers. X-ray diffraction experiments confirmed the existence of /gb-FeSi₂ by annealing up to 915 °C, whereas the phase transformation from the β to phase has been induced at 930 °C. In photoluminescence measurements at 2 K, both β-FeSi₂/Si(100) and β-FeSi₂/Si(111) samples, after annealing at 900–915 °C for 2 h, have shown two dominant emissions peaked around 0.836 eV and 0.80 eV, which nearly coincided with previously reported PL emissions from the sample prepared by electron beam deposition. Another β-FeSi₂/Si(100) sample has shown sharp emissions peaked at 0.873 eV and 0.807 eV. Optical absorption measurements at room temperature have revealed the allowed direct bandgap of 0.868–0.885 eV as well as an absorption coefficient of the order of 10⁴ cm⁻¹ near the absorption edge for all samples.     

Structural, electrical, and optical studies on rapid thermally processed ferroelectric BaTiO3 thin films prepared by metallo-organic solution deposition technique

Polycrystalline BaTiO₃ thin films having the perovskite structure were successfully produced on platinum coated silicon, bare silicon, and fused quartz substrate by the combination of the metallo-organic solution deposition technique and post-deposition rapid thermal annealing treatment. The films exhibited good structural, electrical, and optical properties. The electrical measurements were conducted on metal-ferroelectric-metal (MFM) and metal-ferroelectric-semiconductor (MFS) capacitors. The typical measured small signal dielectric constant and dissipation factor at a frequency of 100 kHz were 255 and 0.025, respectively, and the remanent polarization and coercive field were 2.2 μC cm⁻² and 25 kV cm⁻¹, respectively. The resistivity was found to be in the range 10¹⁰–10¹² Ω·cm, up to an applied electric field of 100 kV cm⁻¹, for films annealed in the temperature range 550–700 °C. The films deposited on bare silicon substrates exhibited good film/substrate interface characteristics. The films deposited on fused quartz were highly transparent. An optical band gap of 3.5 eV and a refractive index of 2.05 (measured at 550 nm) was obtained for polycrystalline BaTiO₃ thin film on fused quartz substrate. The optical dispersion behavior of BaTiO₃ thin films was found to fit the Sellmeir dispersion formula well.     

Microstructures and mechanical properties of silicon nitride bonded silicon carbide ceramic foams

Silicon nitride bonded silicon carbide foams have been produced by nitridation of the foamed compacts containing silicon carbide and silicon powders. When no nitridation additive was used the ceramic foams nitrided at all temperatures studied contained a significant amount of whisker phase α-Si₃N₄ formed both inside and outside the cell walls leading to a loose microstructure and a low mechanical strength. When the Al₂O₃ and Y₂O₃ were used as nitridation additives, the ceramic foams nitrided at temperatures of 1360 and 1395 °C containing certain amount of Si₂N₂O and whisker α-Si₃N₄ phases that are bonded by a glassy phase and behave as reinforcements for the ceramic foams exhibited a much higher mechanical strength. At nitridation temperature of 1430 °C, the ceramic foam showed the locally formed β-Si₃N₄ as the main nitrided phase that caused no increase in bonding area between the nitrided phase and the silicon carbide particles. Thus, a relatively lower mechanical strength was observed for the ceramic foam.     

Modeling the microstructural evolution of Ni-base superalloys by phase field method combined with CALPHAD and CVM

A quantitative simulation with phase field method (PFM) in a real alloy system was performed with a new strategy of modeling the microstructure evolution of Ni-base superalloys, in which, the calculation of phase diagrams (CALPHAD) method and cluster variation method (CVM) are combined with the PFM. In this strategy, the four sub-lattice model is used to evaluate the chemical free energy density while CVM to calculate some parameters, such as the lattice misfit and gradient coefficients in phase field equations. With this strategy, the microstructure evolution of a Ni–Al binary alloy at the temperature of 1000 K has been simulated; the elastic energy due to the lattice misfit and elastic inhomogeneity between γ and γ′ phases has also been taken into account. Moreover, the directional coarsening phenomenon of Ni–Al alloy has been reoccurred when the various external stress conditions are applied, which shows good agreement with Pineau’s directional coarsening map.     

保温时间对β-Sialon结合镁铝尖晶石-碳材料的影响及其氧化动力学

以电熔尖晶石、Si粉和鳞片石墨为主要原料, 木质磺酸钙溶液(1.25 g/mL)为成型结合剂, 在氮气气氛下1450℃分别保温1、2、3和4 h原位生成β-Sialon结合MgAl₂O₄-C材料, 研究了保温时间对材料的物相组成、β-Sialon的显微结构及常规物理性能的影响, 并对该复合材料进行氧化动力学研究。结果表明: 当保温时间从1 h增加到4 h, 试样的物相变化规律基本相同, Si单质相消失, 完全转化为SiC、Si₃N₄和β-Sialon(Si₃Al₃O₃N₅)。当保温时间为3 h时, 生成的β-Sialon(Si₃Al₃O₃N₅)为完整圆柱状晶粒, 尺寸分布均匀, 交错成网络结构。随着保温时间的增加, 试样内部产生较多的SiO气体, 导致试样的显气孔率增加, 体积密度下降, 耐压强度和抗折强度先增加后下降, 当保温时间为3 h时, 耐压强度和抗折强度达到最大。氧化动力学研究表明, 氧化过程随着时间的推移分为化学反应控制阶段、化学反应和扩散共同控制阶段及扩散控制三个阶段。     

双组元掺杂钛硅再结晶石墨传导性能的研究

用煅烧石油焦作填料,煤沥青作粘结剂,钛粉和硅粉作添加剂,采用热压工艺制备了一系列双组元掺杂再结晶石墨.考察了不同质量配比的添加剂对再结晶石墨的热导率、电阻率和抗弯强度的影响以及微观结构的变化.实验结果表明,与相同工艺条件下制备的纯石墨材料相比较,掺杂15wt％钛粉再结晶石墨的传导以及力学性能有较大幅度的提高.在掺杂钛粉15wt％、硅粉<2wt％时,双组元再结晶石墨的常温热导率随着硅粉的掺杂量的增加有所提高.当掺杂钛粉及硅粉分别为15wt％和2wt％时,再结晶石墨RG-TiSi-152的常温热导率可达494W/m·K.但是当掺杂钛粉15wt％、硅粉>2wt％时,随着硅粉的继续增加,再结晶石墨的常温热导率反而降低.而双组元掺杂钛硅再结晶石墨的导电以及力学性能却随着硅粉的掺杂量的增加而降低.XRD分析表明,对于双组元掺杂钛硅再结晶石墨而言,钛元素最终在材料中以碳化钛形式存在,而硅元素则大都以气态形式被逸出,XRD物相图谱中未发现硅及其碳化物的存在.材料RG-TiSi-152的微晶尺寸La以及晶面层间距d₀₀₂分别为864和0.3355nm.     

Optical and electrical properties of p-type AgInSnxS2−x (x = 0–0.04) thin films prepared by spray pyrolysis

AgInSn_xS_2−x (x = 0–0.2) polycrystalline thin films were prepared by the spray pyrolysis technique. The samples were deposited on glass substrates at temperatures of 375 and 400 °C from alcoholic solutions comprising silver acetate, indium chloride, thiourea and tin chloride. All deposited films crystallized in the chalcopyrite structure of AgInS₂. A p-type conductivity was detected in the Sn-doped samples deposited at 375 °C, otherwise they are n-type. The optical properties of AgInSn_xS_2−x (x < 0.2) resemble those of chalcopyrite AgInS₂. Low-temperature PL measurements revealed that Sn occupying an S-site could be the responsible defect for the p-type conductivity observed in AgInSn_xS_2−x (x < 2) thin films.