Synthesis of TiC via polymeric titanates: the preparation of fibres and thin films

Polymerization of titanium isopropoxide via its transesterification with o-xylene-,-diacetate enables the formation of a polymeric titanate that can be thermally converted into carbon-deficient TiC after inert atmosphere pyrolysis at 800 °C. The physical and rheological properties of this polymeric titanate allow for the production of crystalline TiC fibres and thin films. The synthesis of this polymeric precursor and the structural transformations leading to the formation of TiC are discussed.     

Synthesis of silicon carbide foams from polymeric precursors and their blends

Several polysilanes with different overall functionalities have been synthesized and pyrolyzed to produce porous silicon carbide. The polysilanes and their ceramic products have been characterized using gel permeation chromatography, Fourier transform-infrared spectroscopy, thermogravimetry, X-ray diffractometry and microscopy. Some products were foams while others were micro-porous ceramics. The effect of the final pyrolytic yield on the Type of ceramic produced, its pore structure and shape retention are discussed. Two polysilanes were blended in various ratios to control the pyrolysis process more precisely. This allowed the Type, shape and pore-structure of the silicon carbide produced to be controlled more efficiently. There exists a relationship between the composition and structure of the precursors and their final pyrolytic yield and this determines the Type, shape retainability and pore structure of the ceramics produced. In this work, precursors or their blends which gave a final pyrolytic yield of 50–60 wt % produced the best silicon carbide foams.     

Preparation of NbC, TaC and Mo2C fibres and films from polymeric precursors

Polymeric precursors for carbothermal reactions were prepared from niobium alkoxide, tantalum alkoxide and molybdenum ethylene glycolate, respectively, by simultaneous reaction with a chelating reagent (acetylacetone) and organic compounds having two or more reactive OH groups, such as ethylene glycol, saccharose, tartaric acid or dihydroxybenzenes. The precursors exist in common polar and non-polar solvents mostly as linear polymers. The viscous solutions show rheological properties that allow for the preparation of polymer fibres and films. At temperatures up to 1500 °C, bulk precursors as well as fibres and films were thermally converted into carbide powders, fibres and coatings. The structural transformations of the polymeric materials into carbides were investigated using simultaneous thermogravimetric–differential thermal analyses (TGA–DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM).     

Carbothermal reduction synthesis of nanocrystalline zirconium carbide and hafnium carbide powders using solution-derived precursors

Zirconium carbide (ZrC) and hafnium carbide (HfC) powders were produced by the carbothermal reduction reaction of carbon and the corresponding metal oxide (ZrO₂ and HfO₂, respectively). Solution-based processing was used to achieve a fine-scale (i.e., nanometer-level) mixing of the reactants. The reactions were substantially completed at relatively low temperatures (<1500°C) and the resulting products had small average crystallite sizes (50–130 nm). However, these products contained some dissolved oxygen in the metal carbide lattice and higher temperatures were required to complete the carbothermal reduction reactions. Dry-pressed compacts prepared using ZrC-based powders with 100 nm crystallite size could be pressurelessly sintered to 99% relative density at 1950°C.     

A simple way to prepare precursors for zirconium carbide

A precursor for zirconium carbide was obtained by just blending zirconium butoxide Zr(OC₄H₉)₄ (ZTB) and divinylbenzene (DVB). This precursor satisfied the requirements for use in ceramic matrix composites fabrication via precursor infiltration and pyrolysis (PIP) process, that is, it was a solution, cross-linked at 150 °C for 2 h, and transformed to ZrC matrix upon heat treatment at 1,600 °C with a ceramic yield around 40%. The cross-linking behavior, pyrolysis process, and optimal molar ratio (ZTB and DVB) of the precursor were investigated by IR, DSC–TGA, and XRD analysis. ZTB and DVB decomposed into ZrO₂ and carbon, respectively, at 400–500 °C, and ZrO₂ and carbon reacted with each other via carbo-thermal reaction at higher temperature to form ZrC.     

The mechanical properties of carbon fibres coated with titanium carbide

Journal of Materials Science -     

Synthesis and characterization of titanium and zirconium oxynitride coatings

Thin films of zirconium oxynitride (ZrNO) and titanium oxynitride (TiNO) have been deposited onto Si(100) substrates at room temperature by radiofrequency magnetron sputtering in an argon-oxygen-nitrogen atmosphere. Single oxynitride layers have been stacked to obtain a multilayer structure. The film structure has been determined by X-ray diffraction while compositional analysis has been performed by X-photoelectrons spectroscopy. Structural analysis has shown that TiNO can be represented as a cubic structure where oxygen atoms replace nitrogen ones while ZrNO can be described as a cubic ZrO₂ where nitrogen atoms replace oxygen ones. Besides the main peak, the multilayer films show satellite peaks, proving the formation of the stacked structure. The final films stoichiometry has been explained by a growth model. It establishes that in TiNO films the nitrogen vacancies filling by reactive reactions with oxygen atoms is favourite while for ZrNO films the oxygen vacancies filling by energetic nitrogen atoms is more likely to happen. The different behaviour between TiNO and ZrNO is further confirmed during the multilayer growth.     

Hafnium and zirconium tetramethylnonanedionates as new MOCVD precursors for oxide films

New bulky Zr and Hf β-diketonates (2,2,8,8-tetramethyl-4,6-nonanedionates, tmnd) were synthesized and characterized by elemental analyses, ¹H NMR, FT-IR and mass spectrometry. A volatile copper compound Cu(tmnd)₂, an intermediate product of ligand synthesis, was isolated and characterized as well. The M(tmnd)₄ (M=Zr, Hf) compounds were tested as precursors for MOCVD of ZrO₂ and HfO₂ films. Preferentially (001)/(010)/(100) textured and in-plane oriented films of monoclinic oxides have been deposited by pulsed liquid injection MOCVD on R plane sapphire. Smooth films could be grown, especially on sapphire and at low temperature (500 °C). The films on Si(100) were polycrystalline and had rougher surface. XPS study showed 3–4 and 7–8 at.% of carbon in HfO₂ and ZrO₂ films, respectively. Zr(tmnd)₄ and Hf(tmnd)₄ lead to significantly higher growth rates of ZrO₂ and HfO₂ films at low temperature than conventional Zr(thd)₄ and Hf(thd)₄ precursors (thd=2,2,6,6-tetramethyl-3,5-heptanedionate) and are attractive precursors for oxide films.     

锆钒合金非晶膜的制备及性能研究

采用磁控溅射法制备出V/Zr比为2:1的非晶态锆钒合金膜,系统研究了合金膜的物相、表面成分、生长形貌和动态力学性能。研究结果表明,在533～773K的沉积温度区间内,合金膜呈现非晶态,表面存在约26nm厚的氧化层,其组成与膜基体显著不同;合金膜在钼基片上以球形岛状模式生长;膜的动态硬度DHT115和弹性模量Eit/(1-υ2)随沉积温度的升高在583K处出现极大值,分别达到436.8GPa和1.13E+5N/mm2。     

Synthesis and characterization of vanadium carbide nanoparticles by thermal refluxing-derived precursors

Vanadium carbide (VC) nanoparticles were synthesized by a novel refluxing-derived precursor. The organic/inorganic hybrid precursor was prepared by a two-step refluxing method using hydrous V₂O₅ as vanadium source and n-dodecane as carbon source. The reaction process, phase composition, microstructure, and element composition of VC were investigated by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy (XPS). The results showed that VC nanoparticles could be obtained at 900 °C for 1 h in flowing Argon (Ar), which was much lower than those of conventional synthesis methods. XRD pattern indicated that the product was face-centered cubic VC with a lattice constant a = 4.1626 Å and average crystallite size of 22.3 nm. Raman spectra indicated that long time refluxing resulted in alkane dehydrogenation and the formation of coke on V₂O₅ nanoparticles. XPS investigations confirmed oxygen presence in VC lattice. Electron microscopy photographs showed the particle size ranged from 20 to 50 nm. All these results confirmed that the two-step refluxing method was a novel and feasible method to synthesize VC nanoparticles.     

Reactive ion beam sputtering of thin films of lead,zirconium and titanium

Thin films of lead, zirconium and titanium were reactively sputtered in an oxygen atmosphere by a focused ion beam sputtering technique. With this technique, the plasma is contained in the ion gun so that neither the target nor the substrate is in a plasma environment. As a result, several factors which can interfere with the interpretation of the mechanism of compound formation (high energy secondary electron bombardment, high substrate temperature, high deposition pressure) are virtually eliminated.Properties such as deposition rate, resistivity and internal stress as a function of the partial pressure of oxygen were investigated. A decrease in the deposition rate was observed as the partial pressure of oxygen was increased from 1 × 10^-6 to 2 × 10^-4 Torr. This decrease was attributed to a build-up of a surface layer of adsorbed oxygen on the target. A model is proposed to predict the partial pressure of oxygen at which a dielectric film forms, based on the thermodynamic properties of the compound formed. All films deposited are in tensile stress. This suggests that compound formation occurs at the target and that the oxide is sputtered in the collision process.     

Hot-pressing kinetics of zirconium carbide

The hot-pressing kinetics of zirconium carbide were studied between 1700 and 2400° C in argon. The validity of different theoretical models due to Murray, Koval'chenko, Skorokhod, Scholz and Lersmacher was tested. For temperatures exceeding 2200° C, there is reasonably good agreement between kinetics and the whole set of models, but it has not been possible to classify them in order to draw conclusions on the sintering mechanism. The activation energy of ZrC hot-pressing was calculated, starting from the viscosity calculated by Murray's formula, as 41 kcal mol^–1 (171.7 kJ mol^–1).     

Carbon fibres coated with titanium carbide using the liquid metal transfer agent technique

Protective coatings of titanium carbide were applied to PAN type carbon fibres by a liquid metal transfer agent (LMTA) technique using tin as a transfer agent. The effect of the coating on the strength of the fibres was evaluated by performing single fibre tensile tests. The coatings were examined metallographically, by X-ray diffractometry, and by scanning electron microscopy. Carbide coating thicknesses obtained ranged from approximately 0.05 to 0.5 μm and the coatings were found to be uniform and adherent to the fibres. It was found that wetting of the fibres by the tin alloy is associated with the spontaneous formation of a carbide layer with a thickness dependent upon the melt temperature, after which the carbide layer was found to grow parabolically with time and with an apparent activation energy of 187 kJ mol⁻¹. The strength of the carbon fibres decreased with increasing coating thickness according to a Griffith relation.     

Properties of aminosilane precursors for the preparation of Si-C-N films

R _n Si(NEt₂)_{4 − n} aminosilanes have been synthesized by reacting the R _n SiCI_{4 − n} (R = H, CH₃; n = 1, 2) organylchlorosilanes with diethylamine. Using IR, UV, and NMR (¹H, ¹³C, and ²⁹Si) spectroscopies and elemental analysis, we have identified the reaction products and determined their spectroscopic characteristics. Vapor pressure measurements have been used to determine the saturated vapor pressure as a function of temperature and calculate the thermodynamic characteristics of vaporization. Thermodynamic modeling of chemical vapor deposition processes has been performed with the aim of predicting the phase composition of the deposit as a function of the nature of the precursor and process conditions.     

Aluminium nitride coatings on silicon carbide fibres,prepared by pyrolysis of a polymeric precursor

Metallic aluminium was anodically dissolved in an organic electrolyte. A viscous solution, containing a polyiminoalane precursor was obtained. Nicalon (SiC) fibres were thermally pretreated and then coated with this fluid. The coated fibres were dried and calcined at 900 ° C in anhydrous ammonia. Thermal pretreatment of the fibres in nitrogen at 1400 ° C and dip coating with solutions of relatively low aluminium concentrations resulted in dense, nearly uncracked aluminium nitride layers.