Reactive diffusion in the system vanadium–silicon

The diffusion-controlled growth of vanadium silicides (V₃Si, V₅Si₃, V₆Si₅, VSi₂) was studied on bulk V–Si diffusion couples annealed for 2–36 h at 1150–1390°C. The layer growth kinetics was parabolic for all of the silicides. Only at 1150 and 1200°C was an induction period observed before the formation of a continuous V₆Si₅ layer. The parabolic growth constants of the II kind for the exclusive growth of each silicide from the adjacent phases were calculated from the parabolic constants of the I kind measured on the V–Si diffusion couples. The rate constants of the II kind were in turn related to the diffusion properties of the silicides. As a result, the interdiffusion coefficient, taking into account the diffusion of both elements, was obtained for each phase. The resulting activation energies were 240 kJ mol⁻¹ for V₃Si, 250 kJ mol⁻¹ for V₅Si₃ and 190 kJ mol⁻¹ for VSi₂. The activation energies scale well with the melting point of the compounds. For V₆Si₅, the activation energy is strongly dependent on the set of thermodynamic data used in the calculation owing to the uncertainty in the decomposition temperature of this phase.     

Gradients in elastic modulus for improved contact-damage resistance. part ii: the silicon nitride–silicon carbide system

In an effort to create elastic-modulus (E) graded materials for contact-damage resistance—free of substantial amounts of glass—silicon nitride (Si₃N₄)-silicon carbide (SiC) graded materials were processed. The structure of these graded materials is such that Si₃N₄ (E=300 GPa) is at the contact surface and SiC (E=400 GPa) is in the interior, with a stepwise gradient in composition existing between the two over a depth of 1.6 mm. A pressureless, liquid-phase co-sintering method, in conjunction with a powder-layering technique, was used to achieve this structure. The liquid phase used was yttrium aluminum garnet (YAG). Under spherical indentation, cone-cracks did not form in the graded material, but some inelastic shear deformation was observed. Cone cracks formed in both the monolithic Si₃N₄ and the monolithic SiC end member materials under identical indentation conditions. Finite element analysis (FEA) of the stresses associated with indentation revealed that the maximum principal tensile stresses outside the Hertzian contact circle, which drive the classical cone-cracks, are reduced by approximately 12% in the graded material relative to the monolithic silicon nitride case. This reduction is significantly lower than what was calculated for the Si₃N₄-glass case (Part I), owing to the shallower, linear E-gradient over a 1.6 mm depth in Si₃N₄-SiC, as compared with the power-law, steeper E-gradient over 0.4 mm depth in the Si₃N₄-glass. It appears that, in addition to the E-gradient, the inelastic deformation contributes to the suppression of cone cracks in the Si₃N₄-SiC graded material. It is suggested that compressive residual stresses may be present in the Si₃N₄-SiC graded material, which are also likely to aid in the suppression of cone-cracks.     

Selective growth of carbon nanotube on silicon substrates

The carbon nanotube （CNT） growth of iron oxide-deposited trench-patterns and the locally-ordered CNT arrays on silicon substrate were achieved by simple thermal chemical vapor deposition（STCVD） of ethanol vapor. The CNTs were uniformly synthesized with good selectivity on trench-patterned silicon substrates. This fabrication process is compatible with currently used semiconductor-processing technologies, and the carbon-nanotube fabrication process can be widely applied for the development of electronic devices using carbon-nanotube field emitters as cold cathodes and can revolutionize the area of field-emitting electronic devices. The site-selective growth of CNT from an iron oxide nanoparticle catalyst patterned were also achieved by drying-mediated self-assembly technique. The present method offers a simple and cost-effective method to grow carbon nanotubes with self-assembled patterns.     

Synthesis of GaN films on porous silicon substrates

A novel and simple method was employed to synthesize GaN films on porous silicon (PS) substrates. GaN films were obtained through the reaction between NH3 and Ga2O3 films deposited on the substrates with magnetron sputtering. Since GaN and PS are all good materials for luminescence, it is expected to obtain some new properties from GaN on PS. The samples were analyzed with X-ray diffraction (XRD) to identify crystalline structure. Fourier transmit infrared (FTIR) spectrum was used to analyze the chemical state of the samples. The films were observed with scanning electron microscopy (SEM) and were found to consist of many big crystal grains. Photoluminescence (PL) spectrum was used to illuminate the optical property of the GaN films.     

Vacuum distillation refining of metallurgical grade silicon （Ⅰ） ——-Thermodynamics on removal of phosphorus from metallurgical grade silicon

The removal of impurity phosphorus from metallurgical grade silicon is one of the major problems on purification of metallurgical grade silicon for solar grade silicon preparation. The thermodynamics on vacuum refining process of the metallurgical grade silicon was studied via separation coefficient of impurity phosphorus in the metallurgical grade silicon and vapor-liquid equilibrium composition diagram of Si-P binary alloy at different temperatures. The behaviors of impurity phosphorus in the vacuum distillation process were examined. The results show that the vacuum distillation should be taken to obtain silicon with less than 10^-7 P, and the impurity phosphorus is volatilized easily by vacuum distillation in thermodynamics. Phosphorus is distilled from the molten silicon and concentrated in vapor phase.     

Effects of light-mass structural forms on silicon carbide mirror

To reduce the mass of the mirror, silicon carbide was used as the material and four light-mass structures were designed. The properties of SiC mirror open back structure with triangular cell, open back structure with hexagonal cell, sandwich structure with triangular cell, sandwich structure with hexagonal cell, were analyzed by using finite element method. The results of the static, dynamic, and thermal properties of the four kinds of SiC mirror indicate that the surface figures of the SiC mirrors are all satisfactory with the design requirements. The properties of the mirrors with sandwich structure are better than those with open back structure, except the high cost. And the mirror with triangular cell has better combination properties than the mirror with hexagonal cell. Considering the overall performance and the cost, open back structure with triangular cell is the most suitable for the SiC mirror.     

Preparation and dielectric properties of porous silicon nitride ceramics

Porous silicon nitride ceramics with difference volume fractions of porosity from 34.1% to 59.2% were produced by adding different amount of the pore-forming agent into initial silicon nitride powder. The microwave dielectric property of these ceramics at a frequency of 9.36 GHz was studied. The crystalline phases of the samples were determined by X-ray diffraction analysis. The influence of porosity on the dielectric properties was evaluated. The results show that α-Si3N4 crystalline phase exists in all the samples while the main crystalline phase of the samples is β-Si3N4, indicating that the αβ transformation happens during the preparation of samples and the transformation is incomplete. There is a dense matrix containing large pores and cavities with needle-shaped and flaky β-Si3N4 grains distributing. The dielectric constant of the ceramics reduces with the increase of porosity.     

Microstructure of flow pattern defects in boron-doped Czochralski-grown silicon

The morphology and microstructure of flow pattern defects （FPDs） in lightly boron-doped Czochralski-grown silicon （Cz-Si） crystals were investigated using optical microscopy and atomic force microscopy. The experimental results showed that the morphology of FPDs was parabola-like with several steps. Single-type and dual-type voids were found on the tip of FPDs and two heaves exist on the left and right sides of the void. All the results have proved that FPDs were void-type defects. These results are very useful to investigate FPDs in Cz-Si wafers further and explain the annihilation of FPDs during high-temperature annealing.     

Preparation and properties of continuous Al-containing silicon carbide fibers

Continuous SiC(OA1) fibers, named KD-A fibers, were prepared by the melt-spinning of ceramic precursor polyaluminocarbosilane, air-curing, and pyrolizing at 1 300℃. These fibers contained small amount of aluminum and 7%-9% oxygen. The KD-A fibers were converted into sintered SiC(A1) fibers, named KD-SA, by sintering at 1 800℃. The fibers were characterized by chemical analysis, tensile strength test, SEM and XRD. The tensile strength, elastic modulus and diameter of the KD-A fibers are 2.6 GPa, 210 GPa, 12- 14μm, respectively. The KD-A fibers have higher thermal stability, more excellent oxidation resistance than the Nicalon fibers. The properties of the KD-A fibers have reached the level of Hi-Nicalon fibers. The tensile strength, elastic modulus and diameter of the KD-A fibers are 2.1 GPa, 405 GPa, 10 - 12μm, respectively. The KD-SA fibers with nearly stoichiometric component have stable performance at high temperature, and better creep resistance than the Tyranno SA fibers.     

Hot tear susceptibility of aluminium–silicon binary alloys

Abstract

Hot tear susceptibility (HTS) of Al–Si alloys in the 0–5–3·0 wt-%Si has been determined using a constrained rod casting test and a quantitative index. Hot tear susceptibility was seen to decrease with increasing silicon content. Hot tear severity varied directly with solidification shrinkageβ, the non-equilibrium freezing range ΔT′, dendrite arm spacing and the effective grain size d _eff. The highest HTS was seen in the Al–0·5%Si alloy with the largest ΔT′, the highest β, the largest dendrite arm spacing, the lowest amount of eutectic phase f _eu, and, hence, the longest local freezing time t _f. Correlation was observed between HTS and the theoretical hot tear criteria Clyne–Davies CSC, Feurer and Campbell CSC _b indices, with Campbell's criterion showing a very strong correlation as HTS _c≈0·03CSC_b + 6 with R²=0·93, where HTS _c is HTS of rod C.     

Orientation dependence of secondary recrystallisation in silicon–iron

Experiments and analyses have been carried out to reach a better understanding of the mechanism of Goss texture formation during the secondary recrystallisation of silicon steel processed by the single cold reduction route. A new experimental approach demonstrated the effect of misorientation on the growth rates of secondary grains and it is shown that these rates are controlled by the proportion of matrix grains having Σ9 CSL relationships to growing secondary grains. It is considered that the Σ9 boundaries have lower energy than general grain boundaries and so are less strongly inhibited by Zener drag. The relative infrequency of Σ9 boundaries around the periphery of secondary grains is seen as evidence for their sacrificial behaviour. Other experiments involving growth of randomly oriented nuclei provide independent support for the important role of Σ9 boundaries during secondary recrystallisation in this steel.     

Effect of silicon on oxidation of Ni-15Al alloy

1　INTRODUCTIONChromium, aluminum, and silicon can formsatisfactory protective scales on Ni based alloys.Chromium is expensive and not suitable for use attemperatures above 1 000℃ due to the evaporationof CrO3. It has also been well established that theincorporation of Si in many alloy systems has abeneficial effect on their oxidation resistance[1, 2].In addition, silicon is abundant and cheap. More over, Si has one of the largest solubility in Ni3Alwhere it …     

Microstructure of ternary Zn1-xCdxO films on silicon substrate

Ternary Zn1-x CdxO alloying films were deposited on silicon substrates by a reactive magnetron sputtering method. The structures of the films were characterized by transmission electron microscopy(TEM) and X-ray diffraction(XRD) analysis, respectively. The XRD measurement shows that the wurtzite-type structure of Zn1-xCdxO can be stabilized up to Cd content of x=0.53 without a cubic CdO phase separation. The TEM measurement shows that the films have a columnar structure and the grains are highly c-axis oriented perpendicularly on silicon substrate although some grain boundaries are slightly tilted. High resolution TEM observation indicates that a native layer of amorphous SiO2 exists at the ZnCdO/Si interface and that ZnCdO grains with c-axis preferred orientation nucleate directly on substrate surface.     

Microstructure–mechanical properties correlation in siliconized silicon carbide ceramics

The microstructure and high temperature mechanical properties of siliconized silicon carbide ceramics (reaction bonded, reaction formed, and biomorphic SiC) have been investigated. The microstructural differences between these materials have been analyzed. Reaction formed and biomorphic SiC show better, high temperature compressive creep resistance and strength than reaction bonded SiC. Additionally, these two materials show a continuous decrease in the creep rate, which is more pronounced at higher temperature and silicon content. This behavior is explained in detail by using a model of creep controlled by a viscous intergranular phase. The maximum strengths were exhibited by the biomorphic SiC when compressed in the axial direction. The strength of reaction formed SiC is roughly the average between the strength of biomorphic SiC compressed in the axial and radial directions. The dependence of the high temperature compressive strength with the microstructure, and volume fraction of SiC is discussed in terms of the minimum solid area approach.     

Efficient and stable perovskite–silicon two-terminal tandem solar cells

正In the past several years, perovskite solar cells (PSCs)exhibited unexpected breakthrough and rapid evolution,and the power conversion efficiency (PCE) of singlejunction PSCs has been increased significantly from 3.81 to25.2%[1, 2], resulting from materials engineering, interface engineering, crystallization engineering, fabrication engineering, etc.[3–5]. In the case of silicon solar cells, the     

Aluminum doping and dielectric properties of silicon carbide by CVD

Cubic β-SiC coating was grown onto the graphite substrate by the normal pressure chemical vapor deposition using CH3SiCl3 （MTS） as a source precursor at 1 150 ℃. But the hexagonal Al4SiC4 phase was generated in the doped process with trimethylaluminium （TMA） as the dopant. Microstructure of the deposit coating as-prepared was characterized by scanning electron microscope （SEM）, which consists of spherical particles with a very dense facet structure. The real component of permittivity ε＇ and dielectric loss tang of the coatings undoped and doped by TMA were carried out by a vector network analyzer in the microwave frequency ranges from 8.2 GHz to 12.4 GHz. The results show that both of them have low values, and doped coating has lower ε＇ and tan δ than undoped one due to the existence of Al4SiC4 impurity phase, which indicates that the desired Al/SiC solid solution at 1 150 ℃ in a normal argon atmosphere is not produced.     

Dielectric properties of doped silicon carbide powder by thermal diffusion

The doped SiC powders were prepared by the thermal diffusion process in nitrogen atmosphere at 2 000℃. Graphite film with holes was used as the protective mask, The dielectric properties of the prepared SiC powders at high frequencies were investigated. The complex permittivity of the undoped and doped SiC powders was measured within the microwave frequency range from 8.2 to 12.4 GHz. The XRD patterns show that before and after heat treatment no new phase appears in the samples of undoped and nitrogen-doped, however, in the aluminum-doped sample the AIN phase appears. At the same time the Raman spectra indicate that after doping the aluminum and nitrogen atoms affect the bond of silicon and carbon. The dielectric real part （ε） and imaginary part （ε＂） of the nitrogen-doped sample are higher than those of the other samples. The reason is that in the nitrogen-doped the N atom substitutes the C position of SiC crystal and induces more carriers and in the nitrogen and aluminum-doped the concentration of carriers and the effect of dielectric relaxation will decrease because of the aluminum and nitrogen contrary dopants.     

Low-temperature synthesis of novel ZnO nanowire microspheres on silicon substrates

Microspheres covered with ZnO nanowires were fabricated by oxidative evaporation of pure zinc powder without catalyst at 450℃. X-my diffraction （XRD） demonstrates that the as-obtained sample can be indexed to high crystaUinity with wurtzite structure. The structural features associated with different growth stages were monitored using scanning elec- tron microscope （SEM）, which described the direct observation nucleation and growth process. Meanwhile, room temperature photoluminescence （PL） spectrum showed a UV emission at -388 nm and a broad green emission at -505 nm. The ZnO nanowires with the self-catalyzed growth mechanism were discussed in detail.     

The role of strontium in modifying aluminium–silicon alloys

Small amounts of strontium can transform the morphology of the eutectic silicon phase present in Al–Si casting alloys from coarse plate-like to fine fibrous networks. In order to understand this industrially important but hitherto insufficiently understood effect, the strontium distribution was studied in atomic resolution by atom probe tomography and in nanometre resolution by transmission electron microscopy. The combined investigations indicate that Sr co-segregates with Al and Si within the eutectic Si phase. Two types of segregations were found: (i) nanometre-thin rod-like co-segregations of type I are responsible for the formation of multiple twins in a Si crystal and enable its growth in different crystallographic directions; (ii) type II segregations come as more extended structures, restrict growth of a Si crystal and control its branching. We show how Sr enables both kinds of mechanisms previously postulated in the literature, namely “impurity-induced twinning” (via type I) and growth restriction of eutectic Si phase (via type II).