Characterization and microstructural evolution of SiC(OAl) fibers to SiC(Al) fibers derived from aluminum-containing polycarbosilane

The SiC(OAl) fibers and the SiC(Al) fibers were fabricated by the use of aluminum-containing polycarbosilane (Al–PCS) precursor. The two types of fibers have been characterized. Chemical element analysis, AES, SEM, XRD, RMS and NMR have been employed. The chemical formula of SiC(OAl) fibers is SiC_1.31O_0.25Al_0.018 with C and O rich on the surface. The microstructure of SiC(OAl) fibers is a mixture of β-SiC nanocrystals, free carbon, and an amorphous silicon oxycarbide (Si–C–O phase), which have been confirmed by an amount of SiC₂O₂, SiCO₃, SiO₄ and SiC₃O units in the ²⁹Si MAS NMR spectrum. A small quantity of aluminum is embedded uniformly in the Si–C–O amorphous continuous phase. For SiC(Al) fibers, nearly stoichiometric composition was confirmed as chemical composition of SiC_1.03O_0.013Al_0.024. The fiber is composed of a large number of β-SiC crystallites, a small amount of -SiC crystalline and SiC amorphous phase. The aluminum in the SiC(Al) fibers mainly exists in two manners: Al–C bonds connected with the surfaces of the β-SiC grains and Al–O bonds, or Al₂O₃, to the amorphous phase.     

KH-570接枝改性碳纤维/乙烯-醋酸乙烯共聚物复合泡沫材料的制备及表征

采用浓H₂SO₄/浓HNO₃混合酸对碳纤维(CF)进行表面氧化处理得到氧化碳纤维(OCF),再利用γ-甲基丙烯酰氧丙基三甲氧基硅烷(KH-570)与OCF进一步反应得到KH-570接枝改性碳纤维(KCF),随后将其应用于乙烯-醋酸乙烯共聚物(EVA)复合泡沫材料中。利用FTIR、XPS、Raman、FESEM和电子万能试验机等考察了碳纤维的表面改性效果以及碳纤维/EVA复合材料的结构与性能。结果表明:氧化和接枝反应均可以增加碳纤维表面的活性官能团含量和粗糙度,从而改善碳纤维与EVA基体之间的相容性,使碳纤维/EVA复合泡沫材料的物理性能得到改善。相同条件下,KH-570接枝改性碳纤维/EVA复合泡沫材料的物理性能更优异。      

Electrochemical manufacturing of nanocarbons from carbon dioxide in molten alkali metal carbonate salts: roles of alkali metal cations

One simple and fast way to manufacture a useful product from CO₂ is to capture the gas by, and then carry out electrolysis in molten alkali metal carbonates. Carbon electro-deposition in molten Li₂CO₃-Na₂CO₃-K₂CO₃ (molar ratio: 43.5:31.5:25.0) has been widely reported in literature. However, studies in each of the individual alkali metal carbonates either have received less attention or are simply lacking in literature. Electrochemical studies of these molten carbonates are important to understand their underlying processes and reactions during the electrolysis. In this work, cyclic voltammograms (CVs) were recorded in each of the above-mentioned molten alkali carbonate salts using a 0.25 mm diameter Pt wire working electrode. In molten Na₂CO₃ and K₂CO₃, the main cathodic reaction was likely the formation of alkali metal, while that in Li₂CO₃ was carbon deposition. The results also suggest that other competing reactions such as CO and alkali metal carbide formation are possible as well in different molten salts. On the CVs, the anodic current peaks observed are mostly associated with the oxidation of cathodic products. Flake/ring/sheet-like structures and quasi-spherical particles were observed in the produced carbon. The morphology of the carbon contained both amorphous and graphitic structures, which varied with different electrolysis variables.     

Chemical vapour deposition of molybdenum carbides: aspects of phase stability

Thin films of different molybdenum carbides (δ-MoC_1−x, γ′-MoC_1−x and Mo₂C) have been deposited from a gas mixture of MoCl₅/H₂/C₂H₄ at 800°C by CVD. The H₂ content in the vapour has a strong influence on the phase composition and microstructure. Typically, high H₂ contents lead to the formation of nanocrystalline δ-MoC_1−x films while coarse-grained γ′-MoC_1−x is formed with an H₂-free gas mixture. This phase has previously only been synthesized by carburization of Mo in a CO atmosphere and it has therefore been considered as an oxycarbide phase stabilized by the presence of oxygen in the lattice. Our results, however, show that γ′-MoC_1−x films containing only trace amounts of oxygen can be deposited by CVD. Stability calculations using a FP-LMTO method confirmed that the γ′-MoC_1−x phase is stabilized by oxygen but that the difference in energy between e.g. δ-MoC_0.75 and oxygen-free γ′-MoC_0.75 is small enough to allow the synthesis of the latter phase in the absence of kinetic constraints. Annealing experiments of metastable δ-MoC_1−x and γ′-MoC_1−x films showed two different reaction products suggesting that kinetic effects play an important role in the decomposition of these phases.     

KOH／K2ZrF6对TC4钛合金微弧氧化膜层形貌和结合强度的影响

研究电解液中添加KOH及K₂ZrF₆对TC4钛合金微弧氧化膜层形貌及结合强度的影响。选用硅酸盐-磷酸盐复合盐为主盐,添加甘油作为辅助试剂,分别配制两种均不添加、只添加KOH、只添加K₂ZrF₆和两种物质均添加的4种电解液对TC4钛合金进行微弧氧化处理。通过膜层厚度、膜层表面粗糙度、膜层硬度、膜层与基体的结合强度等检测,对比分析不同膜层的综合性能,并通过扫描电子显微镜、能谱仪及X射线衍射仪等对膜层物相进行对比分析。结果表明:在硅酸盐-磷酸盐复合盐为主盐的电解液中,氧化30 min后,添加KOH试剂使微弧氧化反应起弧电压从350 V降低至300 V,膜层表面粗糙度Ra为1.66μm,结合强度为29 MPa,膜层硬度866HV,厚度19μm。添加K₂ZrF₆试剂使微弧氧化的起弧电压提高至380 V,膜层表面粗糙度Ra为2.47μm,结合强度为18 MPa,膜层硬度992HV,厚度为26μm,同时,氧化反应生成ZrO₂,可大幅提高膜层硬度。添加KOH和K₂ZrF₆试剂可以影响微弧氧化反应起弧电压及工作电压,进而影响膜层的组织形貌,从而改善膜层的表面粗糙度和厚度以及结合强度,硬度等性能。     

Electrical properties of sol-gel processed barium titanate films

The sol-gel technique has been used to prepare ferroelectric barium titanate (BaTiO₃) films. The electrical properties of the films have been investigated systematically. The room temperature dielectric constant (ε) and loss tangent (tanδ) at 1 kHz were respectively found to be 370 and 0.012. Both ε and tanδ showed anomaly peaks at 125°C. The room temperature remanant polarization (P_r) and coercive field (E_c) were found to be 3.2 μC/cm² and 30 kV/cm, respectively. The capacitance–voltage (C–V) and conductance–voltage (G–V) characteristics also showed hysteresis effect. The temperature variation of C–V and G–V characteristics also confirms the ferroelectric to paraelectric phase transition at 125°C.     

Powder X-ray diffraction, infrared and conductivity studies of AgSbMP3O12 (M = Al, Ga, Fe and Cr)

New Nasicon type of compounds of composition AgSbMP₃O₁₂ (M = Al, Ga, Fe and Cr) are synthesized by solid-state method. All the compounds crystallize in the hexagonal lattice with space group . The infrared spectra of these compounds show characteristic bands due to PO₄ group. The frequency independent conductivity of these compounds shows Arrhenius type behavior and the activation energy for conduction is in the range 0.40–0.55 eV. Frequency independent conductivity (σ_dc) studies and frequency dependent (σ_ac) impedance measurements correlate well. The Cole–Cole plots do not show any spikes on the lower frequency side indicating negligible electrode effects. The activation energies obtained from the plots of log σ_dcT versus 1/T, log σ_ac(0) versus 1/T and log τ versus 1/T are approximately the same. The peak width at half height for electric modulus (M″) plot is 1.24 decades for all samples, which is close to 1.14 decades observed for Debye solid. The height of electric modulus (M″) obtained from the experimental plots are close to that of M″ (max) = C₀/2C indicating the Debye nature of the samples.     

Dielectric properties, ac conductivity and thermal behaviour of flux grown cadmium titanate crystals

The dependence of loss tangent (tan δ) and both real and imaginary parts of the dielectric constant (′ and ″) on temperature in the range 298–923 K and frequency in the range 10³–10⁶ Hz for flux grown CdTiO₃ single crystals is reported. The ln σ_ac versus T plots suggest the conduction mechanism to be ionic hopping conduction. From ln σ_ac versus frequency curves, it can be seen that the slope decreases with the rise in temperature, suggesting that the ionic hopping conduction diminishes with the rise in temperature. The activation energy at various fixed frequencies is calculated from the slope of the graph between ln σ_ac versus 1/T (×10³ K⁻¹). Thermal behaviour of flux grown CdTiO₃ crystals using thermoanalytical techniques including TG, DTA and DTG is discussed. Thermal analysis suggests decomposition of CdTiO₃ in the temperature interval of 1386–1693 K leading to the formation of TiO₂ as the final product. Results obtained on application of TG based models viz. Horowitz–Metzger, Coats–Redfern and Piloyan–Novikova are reported. The results of kinetics of thermal decomposition suggest contracting cylinder model as the one that is relevant to the decomposition of CdTiO₃. The kinetic parameters viz. the order of reaction, activation energy, frequency factor, and entropy of activation using the above mentioned models are computed.     

两种连续SiC纤维的高温介电及吸波性能对比

采用XPS、XRD、Raman光谱等方法分析了两种典型SiC纤维的微观结构, 研究了微观结构对纤维电导性能的影响, 并揭示了其高温介电性能和吸波性能的演变规律, 对高温结构吸波复合材料的研究具有重要意义。研究表明: KD-I SiC纤维表面富碳, 而SLF SiC纤维以Si-C-O结构为主, 前者具有较高的电导率和复介电常数, 从而导致KD-I纤维与空气阻抗匹配差, 而SLF纤维的损耗较小, 因此两者在X波段的室温反射率仅为-3.2 dB和-0.3 dB。KD-I纤维的复介电常数随着温度的升高显著增大, SLF纤维的实部增幅较大而虚部略小, 两者在700℃时的复介电常数分别达到20.9-j25.0和5.0-j0.37, 高温条件下由于阻抗匹配均变差, 吸波性能无明显改善。     

Spectroscopic properties of Yb in heavy metal contained fluorophosphate glasses

A new series of 20Bi(PO₃)₃–10Sr(PO₃)₂–35BaF₂–35MgF₂ doped with Yb³⁺ is introduced for fiber and waveguide laser applications. The stimulated emission cross-section σ_emi, which was found to be 1.37 pm² at the lasing wavelength of 996 nm, is the highest one among fluorophosphate glasses. It has been found that an extremely high gain coefficient of G = 1.65 ms pm⁴ and high quantum efficiency of η = 93% for 1 wt.% Yb₂O₃ doped systems. The various concentration effects on laser performance properties including minimum pumping intensity I_min, the minimum fraction of excited ions β_min and the saturation pumping intensity I_sat are analyzed as a function of Yb₂O₃ concentration. Those results obtained in current system had advantage over some fluorophosphate glasses reported.     

Magnetoimpedance effect at various temperatures for manganite La0.7Ca0.3MnO3−δ

In the present work, the AC magnetoimpedance effect in La_0.7Ca_0.3MnO_3−δ at various temperatures are investigated. The peak of the metal–insulator transition occurs in the temperature dependence of impedance. Negative magnetoimpedance effect in the La_0.7Ca_0.3MnO_3−δ is obtained at frequencies f≤10 MHz. In the magnetoimpedance effect of manganites, the magnetic field not only decreases the permeability μ_t, but also reduces the resistivity ρ by aligning the local spins and varying the transfer integral t_ij. The AC magnetoimpedance participated by the DC colossal magnetoresistance (CMR) in manganites, should be connected with the combined effects of double exchange interaction, electron–phonon coupling and skin effect.     

Fabrication and mechanical properties of carbon short fiber reinforced barium aluminosilicate glass–ceramic matrix composites

Dense BaSi₂Al₂O₈ (BAS) and Ba_0.75Sr_0.25Si₂Al₂O₈ (BSAS) glass–ceramic matrix composites reinforced with carbon short fibers (C_sf) were fabricated by hot pressing technique. The microstructure, mechanical properties and fracture behavior of the composites were investigated by X-ray diffraction, scanning and transmission electron microscopies, and three-point bend tests. The carbon fibers had a good chemical compatibility with the glass–ceramic matrices and can effectively reinforce the BAS (or BSAS) glass–ceramic because of associated toughening mechanisms such as crack deflection, fiber bridging and pullout effects. Doping of BAS with 25 mol% SrSi₂Al₂O₈ (SAS) can accelerate the hexacelsian to celsian transformation and result in the formation of pure monoclinic celsian in C_sf/BSAS composites, which can avoid the undesirable reversible hexacelsian to orthorhombic transformation at 300 °C and reduce the thermal expansion mismatch between the fiber and matrix.     

Study on the yield behavior of Al2O3–SiO2(sf)/Al–Si metal matrix composites

As the yield behavior of Al₂O₃–SiO₂(sf)/Al–Si MMCS is concerned, effects of heat treatment and parameters of short fiber, including volume fraction, size, distribution mode, were investigated. Dislocation configurations adjacent to interface between matrix and fiber were observed by TEM. Macro-yield stress (σ_0.2) and micro-yield stress (σ_MYS) vary with parameters of short fiber, and effects of these parameters on σ_0.2 appear to be opposite to those on σ_MYS. This phenomenon was interpreted by thermal residual stress in matrix and dislocation configuration. Suitable quenching followed aging treatment is an effective method to enhance the σ_MYS and the σ_0.2 simultaneously. For the specimen with heat treatment of 550 °C/1 h WQ + 170 °C/6 h (T6) AC, σ_0.2 and σ_MYS reach 200 and 58 MPa, respectively, and they are almost as twice as those as-cast.     

The development of compression damage zones in fibrous composites

Recent experimental work (Narayanan S, Schadler LS. Mechanisms of kink band formation in graphite/epoxy compsites: a micromechanical experimental study. Comp Sci Technol 1999; 59:2201-13) suggests that kink bands in unidirectional continuous carbon fiber reinforced polymer composites initiate from damage zones formed under axial compressive loads. A damage zone consists of a cluster of locally crushed fibers and broken fibers, that are often fractured at an angle, θ > 0°, normal to the fiber axis. Typically, under compressive loads, fiber breaks in damage zones form roughly along a plane at an angle φ, normal to the fiber axis. These damage zones produce stress concentrations which can lead to instabilities in the nearby fiber and matrix and initiate microbuckling and kink bands. This paper extends a micromechanical influence function technique based on earlier shear lag fiber composite models. Our modified technique calculates the fiber axial and matrix shear stress concentrations due to multiple angled and crushed fibers in arbitrary configurations. Modeling reveals that angled or ‘shear’ breaks (θ > 0°) can lead to higher shear stress concentrations in the matrix than transverse breaks (θ=0°). Also we find that the damage zone is more likely to form at an angle φ, which is greater than that of its individual fiber breaks, θ. When φ is slightly greater than θ, the shear stress in the surrounding matrix regions within the damage zone achieves a maximum, potentially weakening the matrix and interface and consequently leading to kink band formation. Monte Carlo simulations incorporating this stress analysis predict that the initiation and propagation of crushed and angled breaks progress roughly along an angle, φ ≈ 17° in a linear elastic system. When possible, our model results are compared to strain measurements of fiber composites under compression obtained by Narayanan and Schadler using micro-Raman spectroscopy (MRS).     

Analysis of interface states and series resistance at MIS structure irradiated under Co γ-rays

In this research, we investigated the effect of ⁶⁰Co γ-ray exposure on the electrical properties of Au/SnO₂/n-Si (MIS) structures using current–voltage (I–V) measurements. The fabricated devices were exposed to γ-ray doses ranging from 0 to 300 kGy at a dose rate of 2.12 kGy h⁻¹ in water at room temperature. The density of interface states N_ss as a function of E_c–E_ss is deduced from the forward bias I–V data for each dose by taking into account the bias dependence effective barrier height and series resistance of device at room temperature. Experimental results show that the γ-irradiation gives rise to an increase in the zero bias barrier height Φ_BO, as the ideality factor n and N_ss decrease with increasing radiation dose. In addition, the values of series resistance were determined using Cheung's method. The R_s increases with increasing radiation dose. The results show that the main effect of the radiation is the generation of interface states with energy level within the forbidden band gap at the insulator/semiconductor interface.     

MgO/SrO/La2O3多元添加对(Zr0.8Sn0.2)TiO4微波陶瓷结构性能影响

基于液相促进固相反应烧结机制, 设计MgO/SrO/La₂O₃多元复合添加(Zr_0.8Sn_0.2)TiO₄(ZST)体系, 探究复合添加剂对ZST陶瓷的物相组成、微观结构、烧结特性以及高频介电性能等参数的影响。实验结果表明: 陶瓷的主晶相均为ZST相; 适量添加MgO/SrO/La₂O₃可以有效地降低ZST陶瓷的烧结温度, 获得较优的微波介电性能; 但MgO添加量的增多对材料的综合性能有小幅度的影响; SrO的添加量过大会造成晶粒的不完全生长、瓷体不致密和气孔的增多, 从而导致材料的密度、介电常数和Q×f值的下降; 此外, 添加剂对陶瓷的频率温度系数(τ_f)影响不大。在复合添加0.2wt%MgO、0.6wt%SrO、1.0wt%La₂O₃时, 1300℃保温5 h的ZST陶瓷综合性能优异: ρ=5.14 g/cm³, ε_r=40.11, Q×f=51000 GHz (f=5.61 GHz), τ_f=-2.85×10^-6℃^-1。     

高Q值(1-x)(Sr0.2Nd0.208Ca0.488)TiO3-xNd(Ti0.5Mg0.5)O3微波陶瓷的微结构及介电性能研究

采用固相法制备了(1-x)(Sr_0.2Nd_0.208Ca_0.488)TiO₃-xNd(Ti_0.5Mg_0.5)O₃(0.3≤x≤0.4, SNCT-NTMx)系微波介质陶瓷材料, 并研究了该体系的相组成、显微结构、烧结性能和微波介电性能之间的关系。结果表明: 在x = 0.3~0.35范围内, SNCT-NTMx陶瓷形成了正交钙钛矿固溶体, 并伴随有少量未知第二相; 当x增至0.4时, 第二相含量有所增加。介电性能研究结果显示: 随着x的增加, 体系介电常数(ε_r)减小, 但品质因子(Q×f)得到改善; 此外, 体系谐振频率温度系数(τ_f)随NTM含量的增加逐渐向负值方向移动。当x = 0.35, 陶瓷样品在1520℃烧结4 h 得到的微波介电性能较优: ε_r=50.1, Q×f =44910 GHz, τ_f= -1.7×10^-6/℃。     

ZBAS对BaAl2Si2O8结构与微波介电性能的影响

采用固相反应工艺, 按化学计量百分比BaAl₂Si₂O₈-x(ZnO-Al₂O₃-SiO₂-B₂O₃)(x=0, 1%, 2%, 3%, 4%)制备样品, 研究了不同含量ZnO-Al₂O₃-SiO₂-B₂O₃(ZBAS)玻璃相对BaO-Al₂O₃-SiO₂系介电材料显微结构及微波介电性能的影响。结果表明: 添加ZBAS玻璃相可以适当降低烧结温度, 促进六方钡长石转变为单斜钡长石。当x≥3%时, 六方钡长石可以完全转变为单斜钡长石。随着ZBAS玻璃相含量的增多, 样品的密度、介电常数(ε_r)、品质因数(Q×f)和谐振频率温度系数(τ_f )增大。在x=3%, 烧结温度为1360℃时, 可以获得综合性能相对较好的单斜钡长石, 其介电性能: ε_r=6.72, Q×f=28058 GHz, τ_f =-29.79×10^-6℃^-1。     

钛酸钾晶须-玻璃纤维/苯并噁嗪混杂复合材料的摩擦磨损性能

利用六钛酸钾晶须（K₂O·6TiO₂）对苯并噁嗪（BOZ）树脂及玻璃纤维/苯并噁嗪（GF/BOZ）复合材料的摩擦磨损性能进行改性,分析了K₂O·6TiO₂/BOZ复合材料以及K₂O·6TiO₂-GF/BOZ混杂复合材料的摩擦磨损性能以及改性机理。结果表明:因K₂O·6TiO₂的加入,K₂O·6TiO₂/BOZ复合材料以及K₂O·6TiO₂-GF/BOZ混杂复合材料的摩擦系数和比磨损率较纯BOZ树脂和GF/BOZ复合材料的明显降低。K₂O·6TiO₂显著减轻了BOZ树脂和GF/BOZ复合材料的摩擦粘着状况,使摩擦系数降低,同时磨粒磨损情况也大为减轻。GF/BOZ复合材料在摩擦过程中摩擦应力通过率先破坏GF和BOZ树脂的界面,进而诱发破坏GF束内、GF束间和层间BOZ树脂,使得GF/BOZ复合材料的摩擦系数比BOZ树脂降低,但比磨损率较BOZ树脂升高,而K₂O·6TiO₂的加入,使K₂O·6TiO₂-GF/BOZ混杂复合材料的比磨损率有效降低。BOZ树脂、GF/BOZ、K₂O·6TiO₂-GF/BOZ 3种材料的摩擦系数和比磨损率分别为0.34和0.66×10-6 mm3·（N·m）-1,0.19和1.2×10-6 mm3·（N·m）-1,0.09和0.69×10-6 mm3·（N·m）-1。      

Characterization of Cat-CVD grown Si–C and Si–C–O dielectric films for ULSI applications

Si–C films with the Si compositions ranging from 40 to 70% have been grown by Cat-CVD using dimethylsilane [DMSi, Si(CH₃)₂H₂] compounds. Tetraethoxysilane [TEOS, Si(OC₂H₅)₄] and dimethyldimethoxysilane [DMDMOS, Si(CH₃)₂(OCH₃)₂] gas source gave us Si–C–O (C-doped SiO_x) films with wide ternary alloy compositions. The dielectric constant of a Si–C film has been evaluated by C–V measurements (at 1 MHz) using Al/Si–C/n-Si(001)/Cu MIS structure. The relative dielectric constant value of a Si–C film was estimated to be 3.0. The resistivity of the Si–C layer with 1 mm diameter and 0.24 μm thickness was estimated to be more than 24.5 Gohm·cm. These results gave us promising characteristics of Si–C and Si–C–O films grown by alkylsilane- and alcoxysilane-based Cat-CVD.