Composite materials made from a porous 2D-carbon-carbon preform densified with boron nitride by chemical vapour infiltration

2D-C-C/BN composites (with a BN volume fraction (V _BN) up to 0.60) have been obtained by chemical vapour infiltration of hexagonal-BN from a BF₃-NH₃ mixture, within the pores of a 2D-C-C preform made of a stacking of carbon fabric layers which has been weakly consolidated with pyrocarbon. They were tested in compression on samples with the carbon fabric layers directed either parallel or perpendicular to the load axis. In the first case (p-direction), the compression behaviour is mainly elastic (at least for high enoughV _BN). Bothσ _∥ ^R andE _∥ increase regularly with risingV _BN, as the BN-layer deposited within the pores of the preform (mainly located between adjacent carbon layers) becomes thicker,ε _∥ ^R is weak and progressively decreases with increasingV _BN. In the second case (o-direction), the stress-strain curve exhibits both elastic and pseudoplastic domains whose respective extension depends onV _BN (or onV _p). The pseudoplastic behaviour is related to an irreversible microstructural damage of the interlayer C/BN filling.σ _⊥ ^R ,σ _⊥ ^E andE _⊥ increase with increasingV _BN (or decreasingV _p) according to parabolic laws,ε _⊥ ^R is much higher thanε _∥ ^R . The materials remain anisotropic even at highV _BN. Oxidation tests in air (or oxygen/argon mixtures) have shown that 2D-C-C/BN, incompletely densified (V _p ∼ 0.10) by BN, exhibit a weight loss (oxidation of the carbon skeleton) at low temperatures and a weight increase (oxidation of BN) above 900° C. Oxidation resistance is enhanced by increasingV _BN (which results in aV _p decrease) and decreasing oxygen partial pressure. It is predicted that a good oxidation protection of the carbon skeleton requires a full densification by BN (V _p ∼ 0) and will be effective at medium temperatures. The results of the present study could be easily extended to 2D-BN-BN composites.     

BCl3-NH3-H2-N2前驱体化学气相沉积法制备氮化硼涂层

以BCl₃-NH₃-H₂-N₂为前驱体系统, 在垂直放置的热壁反应器中利用化学气相沉积工艺制备氮化硼(BN)涂层, 分析了工艺参数对沉积速率的影响, 通过扫描电子显微镜(SEM)、X射线光电子能谱(XPS)和X射线衍射技术(XRD)分析了碳化硅纤维表面BN涂层的形貌和微观结构, 提出了BN沉积过程中主要的气相和表面反应, 以及关键气相组分。研究结果显示：在600~850℃的范围内, 随着沉积温度的升高, BN沉积速率逐渐加快, 同一温度下, 沉积区域内BN沉积速率沿气流方向逐渐减缓, 表明气相组分在气流方向逐渐消耗; 随着系统压力的提高, BN沉积速率先加快后减缓, 表明沉积过程由表面反应控制转变为质量传输控制; 随滞留时间延长, 距气体入口1~3 cm处, BN的沉积速率逐渐增大, 而距气体入口4~5 cm处, 沉积速率先增大后逐渐变小。SEM照片显示碳化硅纤维表面BN涂层光滑致密, XPS结果表明主要成分为BN及氧化产物B₂O₃, XRD图谱表明热处理前BN为无定形态, 1200℃热处理后BN的结晶度提高, 并向六方形态转变。BN的沉积是由BCl₃和NH₃反应所生成的中间气相组分Cl₂BNH₂、ClB(NH₂)₂和B(NH₂)₃来实现的。     

Structure and morphology of titanium nitride films deposited by laser-induced chemical vapour deposition

Results on the deposition of titanium nitride on AISI M2 tool steel-type substrates by pyrolytic laser chemical vapour deposition are reported. Spots of TiN were deposited from a gas mixture of TiCl₄, nitrogen and hydrogen using a continuous wave quasi-TEM_oo CO₂ laser beam. The morphology and the structure of the deposited material were investigated by optical microscopy, scanning electron microscopy and X-ray diffraction. The chemical composition was studied with a scanning electron microscope with an energy dispersive spectrometer, and with an electron probe microanalyser. The topography of the coating was analysed with a stylus profilometer and different thickness profiles were measured depending on the laser-power densities and irradiation times. The morphology of the films showed a strong dependence on the laser-power density, interaction time and partial pressure of TiCl₄.     

Silicon nitride films deposited from SiF4/NH3 gas mixtures

Silicon nitride films have been deposited from SiF₄/NH₃/H₂ gas mixtures. The deposition reaction at high pressure (52 torr), takes place only for temperatures above 800°C. In the temperature range 800–1000°C the reaction is controlled by a surface process. The increase in H₂ and SiF₄ partial pressures enhances the deposition rate. The SiF₄ molecules provide a high concentration of available silicon atoms, while the hydrogen molecules inhibit the etching effect of the free fluorine atoms. Finally, the effect of an r.f. plasma in the chemical vapour deposition reaction has been evaluated.     

Chemical stability of hydrogen-containing boron nitride films obtained by plasma enhanced chemical vapour deposition

The purpose of this paper is the investigation of the dehydrogenation kinetics of boron nitride films during thermal annealing. BN_x:H films on silicon substrates were prepared by remote plasma enhanced chemical vapour deposition at 473 K using a mixture of borazine and helium. IR spectroscopy and ellipsometry were used to characterize the film properties and composition. The films contain a certain amount of hydrogen in B---H and N---H bonds. The breakage kinetics of these bonds is different. The breakage of N---H bonds determines the hydrogen annealing kinetics at 973–1073 K. The low-temperature annealing (673–873 K) of B---H bonds is sensitive to the generation of hydrogen from N---H bonds. Heat treatment leads to ordering of the films.     

Hybrid plasma chemical vapour deposition of aluminium nitride from Al and NH3/N2 mixtures

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Properties of silicon nitride films prepared by plasma-enhanced chemical vapour deposition of SiH4-N2 mixtures

The refractive indices and IR absorption spectra are measured for silicon nitride films plasma deposited from SiH₄-N₂-H₂ gas mixtures. The composition of the film (N:Si ratio) is derived from the value of the refractive index and the concentration of bonded hydrogen as Si-H and N-H in the film is estimated from the absorption intensities in the IR spectrum. The optimum deposition conditions for giving excellent insulating silicon nitride films are confirmed to be same as the conditions for giving films with stoichoimetric composition and the lowest amount of incorporated hydrogen.     

Laser physico-chemical vapour deposition of cubic boron nitride thin films

A laser physico-chemical vapour deposition (LPCVD) technique was developed based on the interaction of an ultraviolet laser beam with a boron nitride target and borazine gas to synthesize cubic boron nitride (CBN) thin films on silicon substrates. The process involved a hybrid of pulsed laser ablation (PLA) of a solid HBN target and chemical vapour deposition (CVD) using borazine as a feed stock. The films were characterized with scanning electron microscopy, X-ray diffraction and infrared spectroscopy. Results indicate that the thin films consisted of almost single-crystalline CBN structures and that the film quality in terms of adherence, particulate density and smoothness was excellent. The purity and crystal structure of target material, laser beam wavelength and energy fluence were the key variables that controlled the film characteristics. In contrast to LPCVD, the conventional PLA method did not generate CBN films.     

Characterization by XPS and SEM of reactive chemical vapour deposited boron carbide on carbon fibre

A process and apparatus used in the large-scale deposition of boron-based films on carbon fibres is described. The deposition occurs when the heated moving fibres react with a BCl₃-H₂ mixture. Among other advantages, such a process provides for continuous processing at atmospheric pressure. The properties of the as-deposited fibres, ex-Pan based, are discussed in terms of their use in material composites under ambient atmosphere. X-ray photoelectron spectroscopy shows that the filament surface consists of a B₄C-BN mixture.     

On the chemical vapour deposition of Ti3SiC2 from TiCl4-SiCl4-CH4-H2 gas mixtures

In the ternary system Ti-Si-C, the ternary compound Ti₃SiC₂ seems to exhibit promising thermal and mechanical properties. Its synthesis as a thin film from the vapour phase is very difficult owing to the complexity of the system. A contribution to the knowledge of the CVD of Ti₃SiC₂ from a TiCl₄-SiCl₄-CH₄-H₂ gas mixture is proposed on the basis of a thermodynamic approach. This approach is based on a reliable estimation of Ti₃SiC₂ thermodynamic data in good accordance with recent experimental results on its thermal stability. A first equilibrium calculation for the deposition on an inert substrate shows the influence of the experimental parameters on the composition of both the deposit and the gas phase. As a result, the deposition of Ti₃SiC₂ can be favoured by an excess of TiCl₄ ( 45%), a rather low pressure (10–20 kPa), high temperature ( 1273 K) and low H₂ dilution ratio. On the basis of equilibrium calculations for various reactive substrates, complex mechanisms of Ti₃SiC₂ deposition are pointed out, with intermediate steps of substrate consumption, e.g. the formation of TiC from a carbon substrate or TiSi₂ from a silicon substrate.     

Characterization of titanium nitride films deposited onto silicon

Films of titanium nitride were prepared by reactive evaporation and r.f. sputtering and were characterized from the optical, electrical, and chemical and structural points of view. The effect of oxygen, introduced during the deposition process as well as in a subsequent thermal annealing, on the properties of the films is reported. The applicability of titanium nitride to silicon solar cells as a transparent conducting material is briefly discussed.     

On the chemical vapour deposition of Ti3SiC2 from TiCl4-SiCl4-CH4-H2 gas mixtures

An experimental study of the deposition of Ti₃SiC₂-based ceramics from TiCI₄-SiCI₄-CH₄-H₂ gaseous precursors is carried out under conditions chosen on the basis of a previous thermodynamic approach, i.e. a temperature of 1100°C, a total pressure of 1 7 kPa, various initial compositions and substrates of silicon or carbon. Ti₃SiC₂ is deposited within a narrow composition range and never as a pure phase. A two-step deposition process is observed, in agreement with the thermodynamic calculations. For a silicon substrate, TiSi₂ is formed as an intermediate phase from consumption of Si by TiCI₄ and then is carburized by CH₄ into Ti₃SiC₂. For a carbon substrate, the first step is the formation of TiC_x either from consumption of carbon by TiCI₄ or by reaction between TiCI₄ and CH₄ and then TiC_x reacts with the gaseous mixture to give rise to Ti₃SiC₂. In most cases, Ti₃SiC₂ is obtained as small hexagonal plates oriented perpendicular to the substrate surface. These nano- or micro-crystals are usually co-deposited with TiC_x and to a lesser extent SiC, and their size is increased by increasing the dilution of the gaseous mixture in hydrogen.     

Thermal oxidation characteristics of chemical vapour deposited diamond films

Thermal oxidation characteristics of chemical vapour deposited diamond films were studied at 973 K and at different oxygen potentials by analysing the samples before and after partial oxidation using optical and scanning electron microscopy, X-ray diffraction, and Raman and luminescence spectroscopy. On oxidation, diamond films attached to the silicon wafers undergo a, colour change. Oxidation proceeds by etching pits on the diamond grains in the films. There is no evidence of any phase transformation of diamond to non-diamond carbon forms. The concentration of defects, particularly neutral vacancies, increase on oxidation. Possible routes for the reaction between diamond and oxygen are postulated.     

Enhanced field emission from ZnO nanoneedles on chemical vapour deposited diamond films

ZnO nanoneedles were coated on hot filament chemical vapour deposited diamond thin films to enhance the field emission properties of ZnO nanoneedles. The virgin diamond films and ZnO nanoneedles on diamond films were characterized using scanning electron microscopy, X-ray photoelectron spectroscopy and Raman spectroscopy. The field emission studies reveal that the ZnO nanoneedles coated on diamond film exhibit better emission characteristics, with minimum threshold field (required to draw a current density ~ 1 μA/cm²) as compared to ZnO needles on silicon and virgin diamond films. The better emission characteristic of ZnO nanoneedles on diamond film is attributed to the high field-enhancement factor resulting due to the combined effect of the ZnO nanoneedles and diamond film.     

Thermal plasma chemical vapour deposition for SiC powders from SiCH3Cl3-H2

Ultrafine -SiC powders were synthesized by introducing trichloromethylsilane and hydrogen into the high temperature RF thermal plasma argon gas. Powders were characterized by XRD, TEM, TGA, FT-IR and wet chemical analysis. Two different positions of reactant gas injection, i.e., upstream and downstream of the plasma flame, were compared in terms of the powder characteristics. The optimum concentration of hydrogen was found out to be about 3 to 4 mol % for the upstream injection. Amorphous SiC with free carbon was formed when the hydrogen concentration was lower than optimum and -SiC with free silicon was formed when it was higher than the optimum. For the downstream injection, free silicon formation was not significant and free carbon formation was suppressed when the hydrogen concentration was higher than 7 mol %. Chemical reaction pathways were suggested which could explain these observations.     

Stability and degradation of plasma deposited boron nitride thin films in ambient atmosphere

Boron nitride (BN) thin films were deposited at 296 K, 398 K, 523 K and 623 K by low power radio frequency plasma enhanced chemical vapor deposition with nitrogen (N₂) and hydrogen diluted diborane (15% B₂H₆ in H₂) source gases. Fourier transform infrared and UV–visible spectroscopies were used to investigate the stability and degradation of BN films under ambient air conditions. The action of moisture on the films is reduced with increasing substrate temperature (T_s) to the detriment of the film growth rate. This has been interpreted as related to the decrease in porosity and relative volume fraction of B–O containing disordered tissue at higher T_s. The thickness of the unstable films increases logarithmically with the air exposure time. Parallel to this, although the E₀₄ gap increases logarithmically with time, the Tauc gap remains the same. The increase of subgap absorptions and the decrease of Tauc slope with time indicate reduction of structural order. Crystallites of ammonium borate hydrates, the main product of the chemical reactions, are initially formed within the bulk. At a later time, as a result of increased porosity and disorder, the film thickness decreases while the islands of micro-crystallites rapidly grow above the surface of the film. Stability dependence on other deposition parameters was also studied: it is found that the 1260/1360 cm⁻¹ (O–B–O/B–N) infrared peak area ratio plays an indicator role to reveal the stability of BN films.     

Pressure-pulsed chemical vapour infiltration of SiC to porous carbon from a gas system SiCl4-CH4-H2

SiC matrix was deposited into porous carbon from a gas system SiCl₄-CH₄-H₂ in the temperature range 900–1200 °C using pressure-pulsed chemical vapour infiltration (PCVI) process. At 1000 °C, silicon single phase, a mixed phase of (Si + SiC), and SiC single phase, were detected by X-ray diffractions for specimens obtained with the reaction time per pulse of 1, 2–3, and 5 s, respectively. At 1100 °C, SiC single phase was obtained with a reaction time of only 0.3s. Between 1050 and 1075 °C, deposition rate accelerated suddenly. The increase of SiCl₄ concentration increased the deposition rate linearly up to 4%–6%. The residual porosity decreased from 29% to 6% after 2×10⁴ pulses of CVI at 1100 °C, and the flexural strength was 110 MPa.     

Growth conditions of the cubic phase cBN in boron nitride films

The nucleation and the subsequent coalescence period of the cubic phase cBN in sputter deposited BN-films is characterized by a shrinking of the film thickness. This is due to the transition of hBN into the denser cBN-phase which occurs inside a highly textured hBN base layer. The corresponding variation of the film thickness with the deposition time is described by a quantitative model. Full BN-stoichiometry in the hBN base layer is shown to be a mandatory condition for the nucleation process and the following growth of the cubic BN-phase. An increase of the substrate temperature fosters the incorporation of nitrogen into the growing film and, thus, the achievement of the stoichiometry condition.