The preparation of boron-doped silicon carbide powder by the carbothermal reduction of oxides derived from the hydrolyzed methyltriethoxysilane

A precursor prepared from a methyltriethoxysilane ((CH₃)Si(OC₂H₅)₃, MTES)-B(OC₂H₅)₃-polyacrylonitrile (PAN) composition was used for the synthesis of boron-doped SiC by carbothermal reduction. Initially, MTES was hydrolyzed with HCl (MTES: H₂O: HCl = 1 : 1 : 0.01) for 1 h in a sealed vessel. The number-average molecular weight measurement and a NMR spectrum revealed that the species in the solution were mainly oligometric. With the addition of B(OC₂H₅)₃, no significant changes in the distribution of the Si containing species were detected. The mixing of the hydrolyzed solution with a PAN-dimethylsulfoxide solution led to a homogeneous solution. After the evaporation of the solvents and the subsequent heat treatment at 220° C, brown precursor powder was obtained. By the heat treatment at 1600° C in Ar, it was converted to boron-doped SiC powder.     

Hydrolysis and condensation reactions of Si(OC2H5)4 related to silica fiber drawing

The hydrolysis and condensation reactions of Si(OC₂H₅)₄ (TEOS) at 80°C in the TEOS---H₂O---C₂H₅OH---HCl solutions with H₂O/TEOS molar ratios (r) from 1.0 to 2.0 were followed by gas chromatography (GC) and measurement of molecular weight ( ) of the hydrolyzates, in order to explain the viscosity change of the solutions. It has been found that the siloxane oligomers with average polymerization degree (n) from 2 to 7 are formed in the early stage of reaction and undergo condensation to form higher polymers. The n and the number of silanol groups of the oligomers are increased with increasing r. The fact that the solution viscosity increased once rapidly around the so-called gelation point followed by a sluggish increase with the increase of reaction time for an r of 1.7, while it increased rapidly without a break till gelation for an r of 2.0, was attributed to fewer silanol groups and higher steric hindrance as a result of more −OC₂H₅ groups remaining in the oligomers for an r of 1.7.     

Sodium borosilicate glasses prepared by the sol-gel process

A sodium borosilicate gel of composition 80SiO₂·15B₂O₃·5Na₂O (wt%) was prepared from tetraethyl orthosilicate, trimethyl borate, sodium methylate, H₂O, and HCl as the catalyst. Variation of specific surface area and porosity as a function of heating temperature indicated that closed pores were opened at temperatures lower than 400°C and collapsed above 450°C. From TG and DTA curves, about 19% Si and B atoms are evaluated to have −OH bonds. X-ray diffraction patterns indicated crystallization of low-cristobalite out of the gel when it was heated at 700°C for 5 h, showing a difference from a melt-quenched glass of the same composition.     

Fabrication of fluorine-doped silica glasses by the sol-gel method

Fluorine-doped silica glasses are produced by the sol-gel method for optical fiber preforms. In order to dope fluorine into silica glass, fluorinated silicon alkoxide, Si(OC₂H₅)₃F, is titrated into SiO₂ sol solutions. The fluorine content in silica glass depends on: the fluorine concentration in the gel, the specific surface area of SiO₂ particles and the heating rate in the sintering process. Fluorine-doped silica glass with a maximum relative refractive index difference of −0.93% is obtained. Using this technique, optical fibers with a triangular refractive index profile are fabricated with a minimum optical loss of 1.6 dB/km at 1.69 μm wavelength.     

Influence of oxygen contamination during Si low presure vapour phase epitaxy on epitaxial layer quality

The influence of oxygen contamination on Si low pressure vapour phase epitaxy (LPVPE) at 800°C in the SiCl₂H₂−H₂ system has been investigated. O₂ was added intentionally with partial pressures between 10⁻⁸ and 2×10⁻⁴ mbar. The quality of the epitaxially grown silicon layers was determined by comparing surface morphology, defect density, Schottky diode characteristics and SIMS measurements. These four parameters are correlated and they reveal a drastic decrease in epitaxial layer quality for O₂ pressures above 15×10⁻⁶ mbar. The critical oxygen pressure which has been until now considered as a limit for epitaxial growth can therefore be exceeded by one order of magnitude.     

Low temperature synthesis of monolithic silica glass from the system Si(OC2H5)4---H2O---HCl---HOCH2CH2OH by the sol-gel method

To avoid breaking of the gel during drying any organic solvent such as ethanol has not been used. When the amount of HCl is increased and the mixture is stirred vigorously, a transparent silica sol can be prepared immediately with only Si(OC₂H₅)₄ and H₂O. By adjusting the amount of HCl and the temperature, the sol may gel at any time, even in 5 min at room temperature. Such a gel may be dried without cracking under completely open conditions. The addition of glycol as a complex agent can apparently increase the pore size of the gel and decrease its capillary force and surface tension. So, large dried gels can be made in a rather short time. For example, a dried gel of diameter 68 mm and thickness 13 mm can be made in only 4 days. Because of the large pores, the dried gels can be sintered to monolithic silica glasses of practical size with a rather fast heating rate. The mechanisms in the whole process have been explored.     

基于5-磺基水杨酸的单、双核铜(Ⅱ)三元混配配合物的合成及晶体结构

以5-磺基水杨酸和咪唑衍生物为配体合成了单核、双核结构的三元混配Cu(Ⅱ)配合物:[Cu(H2 biim)(Hssal)(H2O)2]·H2O(1)和{[Cu(MeHbiim)(Hssal)(H2O)]·0.5H2O}2(2)(H3ssal=5-磺基水杨酸,H2biim=2,2'-联咪唑,MeHbiim=N-甲基-2,...     

Calculation of NMR shieldings and other properties for three and five coordinate Si, three coordinate O and some siloxane and boroxol ring compounds

Valuable information on the geometric and electronic structure of both predominant and defect species in the bulk and on the surface of amorphous SiO₂ can be obtained from solid state ²⁹Si and ¹⁷O NMR. Most previous NMR.studies have made assignments of structure types to NMR signals by relying on a comparison to crystalline reference materials. Using the quantum mechanical technique of Coupled Hartree-Fock Perturbation Theory (CHFPT) we can directly calculate NMR shielding tensors, σ, for Si and O and electric field gradients tensors, q for O in silicates. Previous CHFPT calculations on (SiH₃)₂O at Si---O---Si = 180° to 140° semiquantitatively reproduced observed trends in σ for SiO₂ polymorphs. These calculations have been improved and extended to Si---O---Si = 120° in (SiH₃)₂O and to the small siloxane rings H₄Si₂O₂, H₆Si₃O₃, H₈,Si₄O₄ which are models for the ring structures which may occur in the bulk and surface of amorphous SiO₂. Thus, predictions can be made of the ¹⁷O and ²⁹Si NMR characteristic of such species. We have also calculated NMR shieldings and other properties for three and five coordinate Si and for three coordinate O. The ²⁹Si NMR shielding increases with coordination, as expected, and five coordinate Si shows an anisotropy which although large is still smaller than that inferred from spin-lattice relaxation times in silicate melts. O q values are also calculated for (BH₂)₂O and H₃B₃O₆. The results are consistent with O EFGs and Raman frequencies observed in B₂O₃ glass, supporting a boroxol ring model.     

TiO2-SiO2 monolithic glass formation from sol-gel

This study demonstrates that hydrolysis should be carried out in a step manner in gel synthesis. The key to the increase in the amount of water added is the control of the hydrolysis rate of Ti(OC₄H₉)₄. The hydrolysis of Si(OC₂H₅)₄ can be carried out at about 75°C. The amount of added water (γ_WI), which varied with TiO₂ content (in mol%), was about 64–88% of the total amount of added water. The hydrolysis reaction should be performed at room temperature while Ti(OC₄H₉)₄ is added. The total amount of added water (γ_W) is related to the amount of solvent (R). For example, if TiO₂ is 40mol%, γ_W will vary from 3.2 to 8.0 when R varies from 0.8 to 2.0. The amount of added water was affected by the distribution of solvent in the metal alkoxides. The amount of added water can be increased when R_Si(OC2H5)4 = 1, R_Ti(OC4H9)4 > 1. The rate of rise in temperature of the thermolysis of the dried gel should be less than 10°C per hour, and the heat treatment temperatyre is related to the TiO₂ content (in mol%). Gel glasses without devitrification can only be obtained by thermolysis at 600°C from the gel with no less than 20 mol% TiO₂.     

Preparation and physical properties of amorphous V2O5 containing TiO2, Na2O or Li2O

V₂O₅ gels containing up to 18 mol% of TiO₂ were obtained through the simultaneous hydrolysis of alkoxides in ethanol solution. V₂O₅ gels containing Na₂O or Li₂O were obtained through the ion exchange method. The crystallization temperature, T_cr, of the gels increased and the H₂O content of the gels decreased by the addition of TiO₂ or Na₂O. These additives seem to stabilize the amorphous state of the gels. On the other hand, T_cr and the H₂O-content slightly varied with the addition of Li₂O. No ionic polarization was observed in coating films of the gels dried at temperatures below T_cr. The dc conductivity of the films was anisotropic, and increased with the addition of Li₂O or Na₂O. However, it decreased with increasing TiO₂ content. The fiber-like structure of gels was observed by TEM. The gels obtained from alkoxides were thin and short in comparison with the gels obtained through the ion exchange method.     

RHEED studies of the growth of Si(001) by gas source MBE from disilane

The growth of Si(001) from a gas source molecular beam epitaxy system (Si-GSMBE) using disilane (Si₂H₆) was investigated using reflection high-energy electron diffraction (RHEED). The surface reconstructions occurring between 100 and 775°C were studied as a function of both substrate temperature and surface coverage. We report the first observation of (2x2) and c(4x4) reconstructions during growth at substrate temperatures near 645°C using Si₂H₆. All growth was found to be initiated by the formation of three-dimensional (3D) islands which coalesced at substrate temperatures above 600°C. The surface reconstruction was found to change from a disordered to an ordered (2x1)+(1x2) structure at 775°C via intermediate (2x2) and c(4x4) phases. Thereafter, growth was found to proceed in a 2D layer-by-layer fashion, as evidenced by the observation of RHEED intensity oscillations. This technique has been used, for the first time, to calibrate growth rates during Si-GSMBE. The intensity oscillations were measured as a function of both substrate temperature and incident beam flux. Strong and damped oscillations were observed between 610 and 680°C, in the two-dimensional growth regime. At higher temperatures, growth by step propagation dominated while at lower temperatures, growth became increasingly three-dimensional and consequently oscillations were weak or absent. Similarly, there was a minimum flux limit ( <0.16 SCCM), below which no oscillations were recorded.     

Ca(H2PO4)2-H3PO4-K2SO4体系中石膏晶型及形貌调控

为将Ca(H₂PO₄)₂制备KH₂PO₄过程中的石膏资源化利用,以H₃PO₄与CaCO₃反应制备Ca(H₂PO₄)₂溶液,并与K₂SO₄溶液反应,进行Ca(H₂PO₄)₂-H₃PO₄-K₂SO₄体系中石膏晶型和形貌调控研究。结果表明:通过改变反应时间、反应温度、SO^2-₄过量系数和CaO含量等参数可对Ca(H₂PO₄)₂-H₃PO₄-K₂SO₄体系中石膏晶型和形貌进行调控,制得短柱状α-CaSO₄·0.5H₂O。体系在温度高于95 ℃和CaO含量为3.0%~5.0%(质量分数,下同)时形成α-CaSO₄·0.5H₂O,在CaO含量为5.5%主要形成CaSO₄·2H₂O;反应时间长于20 min和SO^2-₄过量系数大于1.4将形成K₂SO₄(CaSO₄)₅·H₂O,导致石膏晶体表面缺陷增加。本实验条件下,适宜反应条件为:反应时间10 min、反应温度95 ℃、SO^2-₄过量系数1.2和CaO含量5.0%,此条件下可制得长度42~70 μm、直径13~24 μm的短柱状α-CaSO₄·0.5H₂O,其抗折和抗压强度分别可达5.61 MPa和33.74 MPa,滤液中钾收率和脱钙率分别可达94.23%和83.80%。     

Anomalous effect of temperature on atomic layer deposition of titanium dioxide

A strong influence of growth temperature on the deposition rate and refractive index of TiO₂ thin films, grown by atomic layer deposition from TiCl₄ and H₂O, was observed. The growth rate increased from 0.05 to 0.09 nm per cycle while the refractive index decreased from 2.63 to 2.00 with the increase of growth temperature from 150°C to 225°C. The effect was due to crystallization processes starting at these temperatures. The substrate temperature range, in which the growth rate most significantly changed, depended on the TiCl₄ pulse time.     

基于有机羧酸配体的Co(Ⅲ)/Cu(Ⅱ)配合物的合成、结构及表征

在溶剂热条件下,以含卤素有机羧酸3-溴-吡啶-2,6-二甲酸(H₂L)为配体,以硝酸钴、硝酸铜为金属源,合成了两例配合物:[Co(L′)₃](1)和[Cu(L′)₂]_n(2)(HL′=5-溴-吡啶-2-甲酸),通过元素分析(EA)、X射线单晶衍射(SXRD)、X射线粉末衍射(PXRD)、红外光谱(IR)和热重分析(TGA)进行结构表征。X射线单晶衍射结果表明,配体H₂L在反应过程中发生脱羧现象,生成单羧酸配体5-溴-吡啶-2-甲酸。在配合物1中,每个Co(Ⅲ)都位于略微扭曲的八面体几何构型中,不对称单元中含有两个单核单元,单核单元通过C—H…O氢键形成三维超分子结构。配合物2的不对称单元中含有一个Cu(Ⅱ),两个脱质子的L′^-配体,每个Cu(Ⅱ)都是六配位的,位于扭曲的八面体几何构型中。Cu(Ⅱ)由配体连接生成1D链结构,通过C—H…O氢键形成三维超分子结构。此外,研究了两例配合物的热稳定性能。     

The photoluminescence of ZnO thin films grown on Si (1 0 0) substrate by plasma-enhanced chemical vapor deposition

High-quality ZnO thin films have been grown on a Si(1 0 0) substrate by plasma-enhanced chemical vapor deposition (PECVD) using a zinc organic source (Zn(C₂H₅)₂) and carbon dioxide (CO₂) gas mixtures at a temperature of 180°C. A strong free exciton emission with a weak defect-band emission in the visible region is observed. The characteristics of photoluminescence (PL) of ZnO, as well as the exciton absorption peak in the absorption spectra, are closely related to the gas flow rate ratio of Zn(C₂H₅)₂ to CO₂. Full-widths at half-maximum of the free exciton emission as narrow as 93.4 meV have been achieved. Based on the temperature dependence of the PL spectra from 83 to 383 K, the exciton binding energy and the transition energy of free excitons at 0 K were estimated to be 59.4 meV and 3.36 eV, respectively.     

Flow characteristics and spinnability of sols prepared from silicon alkoxide solution

Structural evolution of the solution of tetraethoxysilane hydrolyzed with [H₂O]/[Si(OC₂H₅)₄] = 2 under acidic condition has been investigated by rheological measurements. It was found that the solution behaves as a Newtonian liquid up to a highly viscous state and shows spinnability before gelation when the evaporation of the solvent is allowed during gelation reaction. On the other hand, the solution behaved as a non-Newtonian liquid in the viscous state, showing no spinnability when the evaporation of the solvent was not allowed during the reaction. It was indicated from the Casson plots that the spinnable solution prepared in the open system has no yield stress up to highly viscous states, whereas the non-spinnable solution prepared in the closed system has a yield stress at viscous states. It was proposed that the increase in viscosity of the open system solution is attributable to the concentration of the sol particles whereas that of the closed system solution is attributable to the formation of the three dimensional network in the solution.     

基于2,5-二溴对苯二甲酸配体的锌/钴配合物合成、晶体结构及性质

将有机物2,5-二溴对苯二甲酸(H₂L₁)和2,2′-联吡啶(L₂)作为双配体,使用溶剂热法和七水合硫酸锌(ZnSO₄·7H₂O)、六水合硝酸钴(Co(NO₃)₂·6H₂O)分别反应,得到配合物[Zn(L₁)(L₂)(H₂O)]_n(1)和配合物[Co(L₁)(L₂)(H₂O)]_n(2)。采用单晶X射线衍射、元素分析、红外光谱、紫外光谱、荧光光谱、热重分析等测试方法对这两种物质进行分析研究。单晶测试结果表明配合物1是单斜晶系,以Zn²⁺配位连接L^2-₁与L₂形成一维链状结构,各条链在分子间氢键和π…π共轭作用下有规律地堆叠形成三维网络结构。配合物2是三斜晶系,Co1离子和Co1ⁱ离子由H₂L₁上的羧酸氧原子O4和O4ⁱ连接,形成双齿螯合的配位结构单元,以Co²⁺配位连接 L^2-₁和L₂形成二维网格结构,各层在O—H…O分子间氢键和范德瓦耳斯力作用下有规律的堆叠形成三维网络结构。配合物1和2均含有芳香杂环、羧基杂环和氮杂环,具有良好的荧光性质和热稳定性,最大发射波长分别为345 nm和333 nm。     

A SAXS study of the gelation process of silicon and titanium alkoxides

The gelation process of silicon ethoxide and titanium iso-propoxide solutions was studied as a function of water content and reaction time by small-angle X-ray scattering (SAXS). Approaching the gelation points, the SAXS intensities for titanium tetra-iso-propoxide solutions start to follow a power-law decay in the Porod region, except for a H₂O/Ti ratio greater than 4. For silicon ethoxide solutions, the fractal dimension, d_f, measured for aggregated clusters increases continually with the H₂O/Si ratio and can be related to the spinnability of the solutions. For solutions of both silicon and titanium alkoxides, a solution of fractal dimension d_f < 1.79 shows spinnability, whereas solutions having d_f > 1.79 and no fractal structures do not show spinnability.     

Preparation of nanoscaled BaTiO3 powders by DSS method near room temperature under normal pressure

A new method of direct synthesis nano-BaTiO₃ powders from solution was developed in this study. Dissolving Ba(OH)₂·8H₂O into water as base solution, nanocrystalline BaTiO₃ powders can be obtained by mixing ethanol solution of tetrabutyl titanate with hot base solution under normal pressure. When the temperature of the base solution was 60°C, we obtained crystalline powders with average particle size of about 25 nm. The activity of the powders can be held up because the calcination at high temperature is not necessary using this method.     

Vapor phase epitaxy of pure VO2 and V1−xCrxO2

The vapor phase epitaxy of thin epilayers of VO₂ and V_1−xCrxO₂ on TiO₂ transparent substrates is described. Chemical vapor deposition occurs by reacting a (VOCL₃/CrO₂Cl₂/H₂O/H₂) mixture at about 800°C using argon as a carrier gas. The preparation of pure VO₂ requires special care to make it homogeneously stoichiometric and to obtain steep concentration profiles at the TiO₂/VO₂ interface. Layers were obtained which had electrical and optical properties comparable to the best bulk crystals grown by other techniques. Homogeneous solid solutions of V_1−xCr_xO₂ epilayers were also grown for the first time in the range o < x < 0.17. Chromium concentration and homogeneity were determined by electron microprobe analysis. The separation coefficient k was also found to vary with x. It is close to unity below x = 0.001 and above this value Cr is incorporated more easily. High quality heteroepitaxial layers (1 cm² area, 1 to 30 μm thickness) of V_1−xCr_xO₂ have for the first time allowed the measurement of the optical absorption coefficient.