Synthesis of a polytitanocarbosilane and its conversion into inorganic compounds

The novel polytitanocarbosilane, formed by the cross-linking of polycarbosilane with titanium tetra-alkoxide, was synthesized to examine the process of converting a multielement organometallic polymer into an inorganic compound. The chemical structure of this polymer was investigated by the techniques of infra-red spectroscopy (IR), gel permeation chromatography (GPC), number average molecular weight measurements and²⁹Si nuclear magnetic resonance (NMR) measurements. The pyrolysis products in N₂ gas at 1400° C and 1700° C were the microcrystalline and crystalline states of silicon carbide and titanium carbide, respectively.     

Synthesis of thermosetting copolymer of polycarbosilane and perhydropolysilazane

A copolymer of polycarbosilane and perhydropolysilazane was obtained by reacting polycarbosilane with titanium n-butoxide and perhydropolysilazane. Titanium n-butoxide and perhydropolysilazane were essential for the polymer to show a thermosetting property. The thermosetting copolymers were converted into silicon carbide-based ceramics by pyrolysis in a stream of nitrogen to 1000 °C with about 80 wt% ceramic yield. The main phase of the pyrolysis product at 1500 °C in nitrogen was small crystallite -SiC. Elemental carbon, based on rule-of-mixtures composition, in the final ceramics could be reduced by varying the ratio of polycarbosilane/perhydropolysilazane. The copolymer was dry spun and pyrolysed to produce ceramic fibre. Pyrolysis in nitrogen to 1500 °C yielded a silicon carbide-based fibre with low oxygen and low elemental carbon content. A tensile strength of 1.8 GPa and an elastic modulus of 220 GPa were obtained for the fibre which ranged from 10–12 m in diameter. Crystallization to -Si₃N₄, -SiC, and -Si₃N₄ proceeded on annealing in nitrogen at 1700 °C for 1 h.     

Production mechanism of polytitanocarbosilane and its conversion of the polymer into inorganic materials

The reaction of polycarbosilane with tetra-alkyltitanate proceeded at 300° C in nitrogen atmosphere by the condensation of Si-H bonds in polycarbosilane and the substituent groups of the tetra-alkyltitanate accompanied by evolution of alkan gas, and then the formation of Si-O-Ti bonds occurred. In this condensation reaction using tetra-isopropyl titanate, tetra-n-butyl titanate and tetra-2-ethylhexyl titanate, activation energies of the initial rate of the increase in molecular weight were 17.04, 20.07 and 31.07 kcal mol^–1 respectively, and thus the more bulky the substituent group of tetra-alkyltitanate, the lower the reactivity became. Of these alkyltitanates, tetra-2-ethylhexyl titanate was found to be the most advantageous reactant for obtaining polytitanocarbosilane with a narrow molecular weight distribution, low gel fraction and high titanium concentration. Polytitanocarbosilane with high titanium concentration was converted into the densified amorphous inorganic material with high Si-C bonding energy in high yield. Titanium contained in the pyrolysed polytitanocarbosilane was furthermore found to have the effect of inhibiting crystalline grain growth of -type SiC up to high temperature.     

Structure and properties of a moulded carbon derived from rice hull

Rice hull was moulded into a tube (outer diameter: 54 mm; inner diameter: 17 mm, length: c. 170 mm) by use of an extruder and then carbonized in nitrogen atmosphere below 1000 °C. Ash content of the hull was 16 wt%, of which c. 94 and 4 wt% were SiO₂ and K₂O, respectively. Carbon yield and shrinkage of the mould after carbonization at 1000 °C were 42 wt%, and 43 vol%, respectively. The bulk density increased with rising of carbonization temperature to reach to 0.93 g ml^–1 at 1000 °C via 0.82 g ml^–1 at 500 °C. The largest compressive strength of 3.6 MPa was obtained after carbonization at 1000 °C. No micropore was developed after carbonization, and the total pore volume measured by a mercury porosimeter was 0.25–0.31 ml g^–1 after carbonization. These data were compared with those of charcoal.     

Polycarbosilane derived Ti3SiC2

Titanium silicon carbide (Ti₃SiC₂) ceramic was synthesized by in-situ reaction of metal titanium and polycarbosilane. Reaction mechanisms which lead to the formation of Ti₃SiC₂ were suggested on the basis of XRD analysis. The content of Ti₃SiC₂ reached 93% in products obtained from heating the Ti/polycarbosilane green compact at 1400 °C in Ar. The morphology and compositions of the products were examined by SEM equipped with EDX. The typical laminate structure of Ti₃SiC₂ particles with 1-4 μm in thickness and 4-15 μm in length was observed. EDX results showed that the atomic ratio of Ti:Si:C of grains is close to 3:1:2, which agrees with Ti₃SiC₂ composition.     

Structural study of lithium titanium mixed oxides prepared by sol-gel process

A structural study, using TGA-DSC analysis, X-ray diffraction, Raman scattering and FT Infrared absorption, is performed on mixed titanium lithium oxide with 20% of lithium prepared by sol-gel process. The structure is investigated as a function of the annealing temperature. At low temperatures the sample is in the anatase phase and transforms to the rutile phase near 500°C. The crystallite size of rutile TiO₂ increases from 40 to 100 nm as the temperature increases. However the size increase presents some discontinuity at temperature around 600°C. At thermal treatment temperatures from 500°C to 850°C the presence of LiTi₂O₄ in the sample is clearly observed. Finally at 1000°C the sample is composed by a mixture of rutile TiO₂ and Li₂Ti₃O₇.     

Growth process from amorphous GaN to polycrystalline GaN on Si (111) substrates

Amorphous GaN (a-GaN) films on Si (111) substrates have been deposited by RF magnetron sputtering with GaN powder target. The growth process from amorphous GaN to polycrystalline GaN is studied by XRD, SEM, PL and Raman. XRD data mean that annealing under flowing ammonia at 850-950 °C for 10 min converts a-GaN into polycrystalline GaN (p-GaN). The growth mechanism can be mostly reaction process through N³⁻ in amorphous GaN replaced by N³⁻ of NH₃. Annealing at 1000 °C, the appearance of GaN nanowires can be understood based on the vapor-liquid-solid (VLS) mechanism. In addition, XRD, PL and Raman measurement results indicate that the quality of GaN films increases with increasing temperature. The tensile stress in the films obtained at 1000 °C is attributable to the expansion mismatch between GaN and Si, with the gallium in the film playing a negligible role.     

The effect of Tb doping on the structure and spectroscopic properties of MgAl2O4 nanopowders

In this paper, a modified sol-gel method was employed to prepare nanostructured MgAl₂O₄ spinel powders doped with Tb³⁺ ions and thermally treated at 700 and 1000 °C for 3 h. The structural properties of the prepared at 700 and 1000 °C powders where characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). According to obtained XRD patterns the formation of single-phase spinels after calcination was confirmed. The XRD analyses demonstrated that the powders were single-phase spinel nanopowders with high crystallite dispersion. The Rietveld method was applied to calculate lattice parameters. The averaged spinel particle size was determined to be ∼10 nm for calcination at 700 °C and ∼20 nm at 1000 °C. The emission and excitation spectra measured at room and low temperature (77 K) for the samples calcined at 700 and 1000 °C demonstrated characteristic spectra of Tb³⁺ ions. The effect of MgAl₂O₄:Tb³⁺ grain sizes on luminescence properties was noticed.     

Oxidation kinetics of SiC deposited from CH3SiCl3/H2 under CVI conditions

Oxidation tests were performed on SiC deposits prepared from CH₃SiCl₃/H₂ under chemical vapour infiltration conditions, at temperatures ranging from 900–1500 °C under a flow of pure oxygen at 100 kPa (passive oxidation regime). The kinetics of growth of the silica layer were established from thickness measurements performed by spectroreflectometry. They obey classical parabolic laws from which rate constants are calculated. Within 1000–1400 °C, the oxidation process is thermally activated with an apparent activation energy of 128 kJ mol^–1. Above 1400 °C and below 1000 °C, an increase in the activation energy is observed which is thought to be related to a change in the mechanism of the oxygen transport across the silica layer forT>1400 °C and tentatively to stress effects forT<1000 °C. The kinetics data are compared to those measured on silicon single crystals (used as a standard) and to other reported data on SiC.     

Investigating microstructural evolution during homogenization of the equiatomic NiTi shape memory alloy produced by vacuum arc remelting

The vacuum arc remelting process is widely used for the commercial production of NiTi shape memory alloys. Due to the absence of electromagnetic forces in this method, several remelting and long-time homogenizing are required. In this work, a Ti-50 at.% Ni alloy was prepared using the non-consumable vacuum arc melting technique in a water-cooled copper crucible. After four times of remelting process, specimens were subjected to homogenization at 1000 °C. Micro/macrostructural changes during homogenization were investigated by optical microscope and SEM equipped with EDS analyzer. The results showed that the as-cast specimen consisted of mostly Ni₃Ti, Ti₂Ni and monoclinic (B1^9′) phases with high segregation. By increasing the holding time during the homogenization process at 1000 °C, the amount of austenite (B₂) phase was increased, while segregation and unfavorable phases, and accordingly, hardness were decreased. After 4 h of homogenization, austenite (B₂) was the only phase maintained in the microstructure of Ti-50 at.% Ni. In addition, macrostructure of the alloy was turned into polygonal structure after such a homogenization treatment.     

A study of the oxidation curing mechanism of polycarbosilane fibre by solid-state high-resolution nuclear magnetic resonance

The chemical structure of oxidation-cured polycarbosilane fibres has been studied by IR and chemical analysis, but its structure has not been identified in detail. In this work, the molecular structure was examined by chemical analysis, and solid state¹³C and²⁹Si nuclear magnetic resonance (NMR) spectroscopy. Si-O-Si, Si-O-C(I) and Si-O-C(II) bonds were formed by the oxidation curing process. The six chemical bonds (Si-C, Si-H, Si-Si, Si-O-Si, Si-O-C(I) and Si-O-C(II)) in oxidation-cured polycarbosilane were determined quantitatively, and the chemical structural model was shown. Solid state²⁹Si resolution NMR spectroscopy has proved to be a powerful tool for investigating the curing mechanism of oxidation-cured polycarbosilane fibres.     

Ultralow expansion ceramics in the HfO2-TiO2 system synthesized by an hydrolysis and polycondensation technique

Ultra-refractory ceramics from the HfO₂-TiO₂ system in the range 30–40 mol% TiO₂, with a near-zero thermal expansion, have been synthesized by hydrolysis and polycondensation of titanium alkoxide and hafnium dichloride alcoholic solutions and sintered at moderate temperature. Thermal stability, crystallization, density and microstructure of these materials have been examined. The as-prepared powder, amorphous at room temperature, crystallized quickly when heated at 500 ° C. Entire crystallization occurred after treatment at 1000 °C. Sintering at 1500 °C on cold-pressed samples led to ceramics with weak porosity (7%), low expansion coefficient <1×10^–6 °C^– with a minimum for 30 mol% TiO₂ content. SEM examination on sintered materials at 1500 °C reveals a grain size from 2–6 m, increasing with TiO₂ content.     

PbTiO3 sol-gel process studied by207Pb-NMR,IR, DTA and XRD

PbTiO₃ film and powder materials have been obtained from a sol prepared at room temperature by mixing lead(II) acetate trihydrate (LAT) and tetraisopropylorthotitanate (TPOT) in methoxyethanol. The sol-gel process was studied by means of²⁰⁷Pb nuclear magnetic resonance and infrared spectroscopy, differential thermal analysis and X-ray diffraction measurements. This investigation shows that an interaction between LAT and TPOT occurs immediately on mixing in solution and, as a result of the interaction, the lead and titanium compounds are indistinctly linked togetherthrough bidentate acetate bridging. This linkage remains unchanged during solution reactions and gelation, Pb-O-Ti bonding being absent at this stage. Pb-O-Ti bonding in the sol-gel materials is believed to be first formed between 200 and 350 °C during the decomposition of the OAc group of LAT and its removal from the gel. The gel retains its amorphous state on heating at 370 °C for 1 h. An unidentified primary crystal phase appears after heating the gel at 400 °C for 1 h, and it is transformed into perovskite PbTiO₃ crystals at 500 °C and higher temperatures.     

Synthesis and structural and optical properties of metastable ZrO2 nanoparticles with intergranular Cr3+/Cr4+ doping and grain surface modification

Nanoparticles of stabilized ZrO₂ in a single cubic (c) phase are obtained with an intergranular doping of 4 to 20 at.% Cr³⁺/Cr⁴⁺ additives by a chemical method using a high energy amorphous precursor with polymer molecules of sucrose and polyvinyl alcohol. In the polymer, the metal cations disperse and rearrange in a specific network structure with local symmetry probably similar to that in c-ZrO₂. On heating at 250 to 800°C in air, the polymer network decomposes and burns out spontaneously (with a strong exothermic peak over 350 to 500°C in thermal analysis) in a refined microstructure in 10 to 20 nm diameter particles of near spherical shape. Those are identified to be of c-ZrO₂ by x-ray diffraction. A modified microstructure of 15 to 30 nm crystallites of dispersed tetragonal (t) and/or monoclinic (m) phases in stabilized c-ZrO₂ develops on a prolong heating at 900 to 1000°C from a polymer precursor for 2 h or longer. Particles in t-ZrO₂ are in acicular shape, as long as 450 nm with aspect ratio 5 to 20, while in the shape of platelets in m-phase in an average 300 nm size. It is found that the Cr³⁺/Cr⁴⁺ additives promote formation of c-ZrO₂ by a controlled decomposition and combustion of precursor in small particles at 250 to 800°C temperature. Part of the additives form a thin amorphous surface layer in individual c-ZrO₂ grains so that it prevents them to grow or transform in the equilibrium m-ZrO₂ bulk structure as long as the temperature lies below 900°C. The x-ray diffraction in light of the optical spectrum reveals that part of the Cr⁴⁺ cations occupy Zr⁴⁺ sites in a distorted c-ZrO₂ lattice.     

Silicon carbide fibre reinforced glass-ceramic matrix composites exhibiting high strength and toughness

Silicon carbide fibre reinforced glass-ceramic matrix composites have been investigated as a structural material for use in oxidizing environments to temperatures of 1000° C or greater. In particular, the composite system consisting of SiC yarn reinforced lithium aluminosilicate (LAS) glass-ceramic, containing ZrO₂ as the nucleation catalyst, has been found to be reproducibly fabricated into composites that exhibit exceptional mechanical and thermal properties to temperatures of approximately 1000° C. Bend strengths of over 700 MPa and fracture toughness values of greater than 17 MN m^–3/2 from room temperature to 1000° C have been achieved for unidirectionally reinforced composites of 50 vol% SiC fibre loading. High temperature creep rates of 10^–5 h^–1 at a temperature of 1000° C and stress of 350 MPa have been measured. The exceptional toughness of this ceramic composite material is evident in its impact strength, which, as measured by the notched Charpy method, has been found to be over 50 times greater than hot-pressed Si₃N₄.     

Vapor-phase synthesis of uniform silica spheres through two-stage hydrolysis of SiCl4

We report, for the first time, a vapor-phase synthesis of nearly monodispersed silica spheres 250-300 nm in size through a two-stage hydrolysis of SiCl₄. In the first stage, SiCl₄ vapor was partially hydrolyzed with water vapor in a batch reactor at 150 °C to form silicon oxychloride particles, nearly monodispersed and spherical. In the second stage, these oxychloride particles were converted into silica particles through further hydrolysis at 1000 °C in a tubular reactor, while the morphology and size after the first-stage reaction remained virtually unchanged.     

Oxidation behaviours of carbon/carbon composite with multi-coatings of LaB6-Si/polycarbosilane/SiO2

The influence of multi-coatings of LaB₆-Si/polycarbosilane/SiO₂ on the oxidation behaviour of carbon/carbon composite materials was investigated in the temperature range from 500 to 1400 °C. The additives of LaB₆-Si offered lower oxidation rates and accelerated increases in oxidation rates at temperatures below 900 °C. The coating of polycarbosilane (PCS) improved the compatibility of the coating on the carbon/carbon composite and lowered the oxidation rates of the LaB₆-Si coated composite below the transition temperature. With the SiO₂ coating, the cracks of the LaB₆-Si/PCS coating was sealed and a good oxidation resistance of the LaB₆-Si/PCS/SiO₂ coated composite was found at temperatures up to 1300 °C.     

27Al and 29Si NMR study of sol-gel derived aluminosilicates and sodium aluminosilicates

Solid state ²⁷Al and ²⁹Si NMR was used to examine the structures of aluminosilicates and sodium aluminosilicates prepared by the sol-gel method from metal alkoxides. In contrast to the borosilicate system, where B-O-Si bonds are not formed until heat treatment above 150° C, Al-O-Si formation appears complete upon gelation. Aluminium occupies tetrahedral [AlO₄]^– sites in the polymer network and octahedral [Al(H₂O)₆]³⁺ (or similar) sites in the intersticies for charge balance. When sodium is added as a counter ion the octahedral aluminium is converted to tetrahedral aluminium in the oxide network. In gels of high aluminium content prepared from (Bu^sO)₂Al-O-Si(OEt)₃, some aluminium in five coordinate environments is also observed. All gels remain amorphous on heating to 800° C.     

Synthesis of polycrystalline zirconia fibre with organozirconium precursor

Polycrystalline zirconia fibre was successfully synthesized by pyrolysis of preceramic fibre formed from an organozirconium compound. Dibutoxybis(2, 4-pentadionato)zirconium (BPZ) was polymerized at 150° C and 10² Pa, yielding a viscous polymeric product. The infrared absorption bands of the Zr-O bond changed from separate to coalesced bands after polymerization. The signals of the¹³C NMR spectrum of BPZ changed from sharp singlets to multiplets after polymerization. The molecular weight of the polymer was between 400 and 1000. The viscosity of polymer was 580 Pa sec at 30° C and a shear rate of 1.0 sec^–1. The polymer viscosity decreased with increased temperature from 30 to 60° C. The precursor polymer pyrolysed at 400° C in air was amorphous to X-rays, and crystallized in a mixture of monoclinic and tetragonal phases at 450° C. Tetragonal zirconia was synthesized from the polymer including 4.3 mol % yttrium compound (2.2 mol % yttria) after heat treatment at 1200° C for 1 h. The precursor fibres were pyrolysed to yield fine-grained fibres of tetragonal zirconia at 1200° C for 1 h.     

Growth morphology and phase analysis of titanium-based coating produced by thermochemical method

In the present study, high-speed tool steel was used to coat titanium by pack cementation technique. Coatings show a growth morphology similar to that of the chemical vapor deposition method. Time and temperature of the coating affects its growth morphology. Coating obtained at low temperature (900 °C) yields morphology with growth of tiny particles while coating produced at high temperature (1000 °C) has a morphology with coarser particles. Phase structure of the layers also varies depending on the process time and temperature. Short coating duration yields TiC_0.3N_0.7 phase structure, whose composition is close to TiN whereas long coating duration combined with high temperature yields TiC_0.7N_0.3, whose composition is close to TiC. Mechanical properties such as hardness and resistance to abrasion also reflect changes in phase structures of different types of coating.