L. Y. Cao

Preparation of a SiC/Cristobalite-AlPO4 Multi-layer Protective Coating on Carbon/Carbon Composites and Resultant Oxidation Kinetics and Mechanism

In order to improve the oxidation resistance of carbon/carbon (C/C) composites, a SiC/C-AlPO 4 multi-layer coating was fabricated on the C/C composites by a simple and low-cost method. The internal SiC bonding layer was prepared by a two-step pack cementation process and the external C-AlPO 4 coating was deposited by hydrothermal electrophoretic deposition process. Phase compositions and microstructures of the as-prepared multi-layer coating were characterized by X-ray diffraction (XRD), scaning electron microspocy (SEM) and energy dispersive spectrometer (EDS). Anti-oxidation properties, oxidation behavior and the failure behavior of the coated composites were investigated. The results indicate that the multi-layer coating exhibits obviously two-layer structure. The inner layer is composed of β-SiC, α-SiC phase with a scale of silicon phase. The outer layer is composed of cristobalite aluminum phosphate (C-AlPO 4 ) crystallites. The SEM observation shows the good bonding between the inner and outer layers. The multi-layer coating displays an excellent oxidation resistance in air in the temperature range from 1573 to 1773 K, and the corresponding oxidation activation energy of the coated C/C composites is calculated to be 117.2 kJ/mol. The oxidation process is predominantly controlled by the diffusion of O 2 through the C-AlPO 4 coating. The failure of the multi-layer coating results from the generation of the microholes that may be left by the escape of the oxidation gases.

Hydrothermal electrophoretic deposition of yttrium silicate coating on SiC–C/C composites

Abstract Yttrium silicate coatings were deposited on the surfaces of SiC–C/C composites by a hydrothermal electrophoretic process using yttrium silicate nanocrystallites, isopropanol and iodine as source materials, solvent and charging agent respectively. The yttrium silicate nanocrystallites were preprepared by a sonochemical process. The influence of deposition temperatures on the microstructures and degree of crystallisation of yttrium silicate coatings were investigated. The as prepared coatings were characterised by XRD and SEM. Results show that the coatings are composed of yttrium silicate crystallites with Y2Si2O7 as the main phase and smaller amounts of Y2SiO5. The thickness and density of the coatings increase with increasing deposition temperature; but the coating process has little influence on the phase compositions of the coatings.

Synthesis and characterisation of ZnS optical thin films through cathodic electrodeposition technique

Abstract Zinc sulphide (ZnS) thin films were deposited on iridium tin oxide glass by a cathodic electrodeposition technique. The phase compositions, morphologies and optical properties of the prepared thin films were characterised. Both X‐ray diffraction and atomic force microscopy analyses show the growth of ZnS thin films is highly preferential along (200) orientation and the crystallisation of the thin films improves with increasing the solution pH values. The ultraviolet absorption spectra of the films reveal a broad absorption peak ∼300 nm and the bandgap of the as deposited films ranges from 3·42 to 3·71 eV.

Effect of heating rate on microstructure of (Ba0 ˙ 99Bi0 ˙ 01)TiO3 thin film prepared by sol–gel deposition process

Abstract The (Ba0˙99Bi0˙01)TiO3 ferroelectric film were successfully prepared with sol–gel deposition process. The effects of heating rate on c/a ratio and microstructure of the thin films were studied with X-ray diffractometry (XRD) and scanning electron microscopy (SEM). Calculation of c/a ratio with XRD analysis revealed that the c/a ratio of the films was increased with increasing heating rate. The SEM analysis indicated that columnar character of the film was increased with increasing heating rate.

Influence of pH value on PbS thin films prepared by electrodeposition

Abstract PbS thin films were prepared on indium–tin oxide transparent conductive film glass substrates using a constant voltage cathodic electrodeposition method. The as deposited thin films were characterised by XRD, AFM and Fourier transform infrared spectrometer. The influence of pH value on the phase compositions, surface morphologies and optical properties of the films has been particularly investigated. Results show that cubic PbS thin films with oriented growth along (111) and (200) direction can be obtained at pH=2·4, deposition voltage=3 V, deposition time=20 min and adding EDTA as a complexing agent. The as deposited thin films exhibit a dense surface morphology. The compressive stress and optical bandgap of the PbS thin films first decrease and then increase with increasing the pH value, and the bandgap of the thin films is calculated to be 0·38–0·39 eV.

Y2Si2O7 Whisker Reinforced MoSi2 Multi-composition Coating for SiC Pre-coated Carbon/Carbon Composites

Y2Si2O7–MoSi2/SiC multi-composition coatings were deposited on the surface of SiC pre-coated carbon/carbon (C/C) composites using a hydrothermal electrophoretic process. The prepared coatings were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Influences of Y2Si2O7 whisker on the morphologies and anti-oxidation properties of coatings were particularly investigated. Results show that Y2Si2O7 whisker has a strong influence on the microstructure and oxidation resistant performance of the coated composites. Compared with MoSi2/SiC coating, Y2Si2O7–MoSi2/SiC coating exhibits dense, uniform, and homogeneous morphologies without any microcracks. Y2Si2O7 whisker can effectively decrease the thermal expansion coefficient of MoSi2 and prevent coatings from cracking, which results in improved oxidation resistant performance of the coated composites. The as-prepared multi-layer coating can protect C/C composites from oxidation for 100 h at 1773 K with a weight loss of 1.22×10–3g/cm2.

Preparation and antioxidation properties of glass/yttrium silicates/SiC multilayer coating in both air and combustion atmosphere

Abstract Yttrium silicate coating for SiC precoated C/C composite was prepared by a hydrothermal electrophoretic deposition process. The yttrium silicate coating was sealed with a borosilicate glass outer layer. Phase compositions, surface and cross-section microstructures of the as prepared multilayer coatings were characterised by X-ray diffractometer and scanning electron microscopy. The influences of hydrothermal electrophoretic deposition factors on the phase, microstructure and oxidation resistance of the multilayer coated C/C composites were particularly investigated. Results show that the hydrothermal electrophoretic resulting coating is composed of crystallites with a main phase of Y2Si2O7 and Y2SiO5. The thickness and density of the coatings are improved with the increase in deposition voltage and hydrothermal temperature. Compared with the pack cementation SiC monolayer coating, the as prepared multilayer coatings exhibit better antioxidation property. When oxidation in air and combustion atmosphere, the coated C/C composites lose weight by a linear and a parabolic law respectively. The prepared multilayer coating can effectively protect the C/C composites in combustion atmosphere at 1773 K for 85 h. The flexural strength of the coated specimen arrives at the lowest value at about 1123 K in combustion atmosphere.

Influence of preheating on crystalline anisotropy and particle size of BaTiCoFe10O19 sol–gel powders

Abstract Ultrafine substituted M type barium ferrite BaTiCoFe10O19 powders were synthesised by a sol–gel method. The hydroxide precursor particles were formed in gel solution containing ethanol and water at a ratio of 1:1 and NaOH was used as the co-precipitation agent. The effects of preheating the precursor on the formation and crystalline anisotropy (an indication of magnetocrystalline anisotropy) of BaTiCoFe10O19 powders were studied using X-ray diffraction (XRD) and SEM. The XRD analysis indicated that single phase BaTiCoFe10O19 powders of average particle size 40 nm were formed. Ti–Co substitution and preheating were found drastically to affect the lattice anisotropy (c/a ratio) of hexaferrite powders. Average c/a values increased from 3·9347 for BaFe12O19 powders to 3·9415 for the substituted powder with no preheating and to 3·9392 and 3·9432 with preheating for 1 h at 300 and 400°C respectively. SEM analyses revealed that the particles had plate-like morphology. The particles had higher aspect ratios after no preheating and treatment at 400°C than after treatment at 300°C.

Influence of deposition voltage on phases and microstructure of hydroxyapatite coatings prepared on C–C composites by hydrothermal electrodeposition

Abstract Hydroxyapatite (HAp) coatings were prepared on the surfaces of carbon–carbon (C–C) composites by a novel hydrothermal electrodeposition method. The as prepared HAp coatings were characterised by X-ray diffraction and scanning electron microscopy. The degree of crystallinity, density and homogeneity of the HAp coatings were found to increase with increasing deposition voltage. The deposition rate also increased with increasing deposition voltage.

Preparation of SnO2 nanocrystallites by hydrothermal liquid–solid–solution process

Abstract SnO2 nanocrystallites with regular shape have been successfully prepared by the liquid–solid–solution (LSS) process under hydrothermal conditions. The influences of hydrothermal reaction temperature and [Sn4+] concentrations on the phase composition and particle size of SnO2 crystallites have been particularly investigated. The as obtained crystallites were characterise by X-ray diffraction (XRD), nanoparticle size analyses (NSA) and field beaming scanning electron microscopy (SEM). Results show that the size of the as prepared sphericity SnO2 crystallite is about 9–11 nm. The agglutination of SnO2 nanocrystallites has been observed and the average grain size of the conglomeration was ∼300 nm. The size of as obtained SnO2 nanocrystallites increases with the increase in hydrothermal reaction temperature and [Sn4+] concentrations.