Plasma enhanced aerosol–gel deposition of Al2O3 coatings

The new plasma enhanced aerosol–gel technique has been used for alumina films preparation, in this work. This process integrates aerosol–gel deposition of films and their plasma treatment in one reactor. The alumina films deposited by aerosol–gel method on Si substrate were plasma or thermally treated. The influence of deposition and condensation conditions on properties of the films was studied. Produced coatings were characterized in terms of surface morphology (SEM, AFM) as well as crystalline and chemical structure (FTIR, XRD). Plasma discharge used for modification of the substrates prior to the deposition process improved homogeneity of produced coatings. Coatings obtained at room temperature exhibit boehmite structure which was transformed into γ-Al₂O₃ after annealing. A similar transformation was induced by low temperature oxide plasma discharge treatment for sufficiently thin coatings.     

Preparation of Al2O3–ZrO2 nanocomposite films by laser chemical vapour deposition

Alumina (Al₂O₃)–zirconia (ZrO₂) nanocomposite films were prepared by laser chemical vapour deposition. α-Al₂O₃–ZrO₂ and γ-Al₂O₃–t-ZrO₂ nanocomposite films were prepared at 1207 and 1000 K, respectively. In the nanocomposite films, 10-nm-wide t-ZrO₂ nanodendrites grew inside the α- or γ-Al₂O₃ columnar grains. The γ-Al₂O₃–t-ZrO₂ nanocomposite films exhibited high nanoindentation hardness (28.0 GPa) and heat insulation efficiency (4788 J s^−1/2 m⁻² K⁻¹).     

Growth of Al2O3–PTFE composite film at room temperature by aerosol deposition method

To fabricate a ceramic-based substrate for 3-dimensional integration modules with a thick film coating process at room temperature, aerosol deposition method was employed. Al₂O₃ was chosen as a main coating material for the requirements of low permittivity and dielectric loss. Especially to give a functionality of plasticity, composite film with polytetrafluoroethylene (PTFE) was also studied. The effects of PTFE, which was incorporated in the film, were investigated by the microstructural characterization. It was confirmed that Al₂O₃–PTFE film with the grain size of 100–200 nm were grown at room temperature using Al₂O₃–0.5 wt% PTFE mixture powders. Dielectric constant and dielectric loss of Al₂O₃–PTFE film were 4.5 and 0.005 at 1 MHz, respectively.     

Photocurable cyanate ester containing hybrid coatings by an anhydrous sol–gel technique

This work reports the synthesis and characterization of hybrid coatings obtained by UV curable anhydrous sol–gel process. Chemical structure of the products was confirmed using spectroscopic methods such as infrared and nuclear magnetic resonance spectroscopy. The properties of the hybrid coatings such as thermal and mechanical properties were investigated in detail by scanning electron microscope, thermogravimetric analysis, and mechanical measurement. Cure kinetics of the coating formulations was investigated by differential scanning photo-calorimetry (Photo-DSC). Cross cut adhesion, pendulum hardness, gloss, pencil hardness, MEK rub test were also performed to measure the coating performance of the hybrid coatings. The results indicate that the addition of the appropriate amount of CPM and sol–gel can effectively improve the thermal and mechanical properties.     

Crystallization characteristics of Li2O–Nb2O5 compound films on sapphire substrates and formation of crack-free and thick LiNbO3 epitaxial films

The formation of crystal phases in Li₂O–Nb₂O₅ compound films deposited on sapphire C-plane and A-plane substrates was studied by X-ray diffraction. Sufficiently oxidized samples with Li-deficient or stoichiometric compositions were prepared by co-sputtering from LiNbO₃ (LN) and Li₂O targets. Crystallization during deposition at elevated temperatures and solid-phase crystallization (SPC) of deposited amorphous films were investigated. For films on sapphire C-planes, nucleation into Li₃NbO₄(222) domains occurred at the onset temperature of crystallization. In the case of stoichiometric films, the LN(006) signal indicating epitaxial growth was the primary one for crystallization during deposition above 460 °C and SPC above 750 °C. Misoriented LN(104) domains tended to coexist with LN(006) domains. In the case of Li-deficient films, LiNb₃O₈(_602) domains coexisted with LN(006) at temperatures above 750 °C as a result of Li₂O loss. For films on sapphire A-planes, epitaxial LN(110) domains were predominant. Li₃NbO₄(222) domains were totally absent and the signal intensity of LiNb₃O₈(212) was less than 10% of that of LN(110) even for the Li-deficient films, which reflected fast crystallization of LN(110) domains. The SPC rate of stoichiometric film was considerably lower than that of Li-deficient film. As-crystallized LN film on the sapphire C-plane was strained with a narrow domain width. Thick film cracked as a result of stress caused by lattice mismatch with the substrate. In contrast, LN film processed by SPC was not strained and had large domains with flat film surface. Based on these results, crack-free 1-μm-thick LN epitaxial films on a sapphire C-plane were achieved. First, an amorphous LN buffer was subject to SPC to obtain a relaxed buffer layer. Subsequently, a stress-free thick LN overlayer was grown by co-sputtering from Li₂O and LN targets at 530 °C. The relaxed buffer layer effectively mitigated the strain caused by the lattice mismatch with the substrate.     

Elaboration of lead-free Na0.5Bi0.5TiO3–BaTiO3 (NBT-BT) thick films by aerosol deposition method (ADM)

Thick and dense ceramic films of lead-free 0.94Na_0.5Bi_0.5TiO₃-0.06 BaTiO₃ (NBT-BT) composition were elaborated by aerosol deposition method (ADM) at room temperature. A powder of suitable grain size was elaborated by solid state reaction. Using this powder, two samples were elaborated by ADM respectively on glass and metallic substrates, in order to check for microstructure and electrical properties. This process allowed obtaining a thick film (3.2 µm) with dense microstructure. Measurement of electrical properties revealed a lossy dielectric behavior indicating interfacial phenomena at the electrode–film interface. The measurement of the ferroelectric hysteresis cycle does not show any characteristics of a ferroelectric behavior, but corresponds well to the one of a lossy non-linear dielectric. The absence of ferroelectricity is probably due to the low grain size of the obtained thick film (130 nm). Further experiments are in progress in order to try to obtain ferroelectric properties.     

Al2O3-based binders for corrosion resistance optimization of Al2O3–MgAl2O4 and Al2O3–MgO refractory castables

This work addresses the main aspects related to the use of alternative binders [hydratable (HA) or colloidal alumina (ColAlu)] in castables containing different spinel sources (pre-formed or in situ generated), in order to point out: (i) the features that control the corrosion behavior of these materials, and (ii) the key factors to better select a refractory composition. Thermodynamic calculations, corrosion cup-test and SEM analyses were carried out in order to evaluate the slag attack of the designed refractory compositions. According to the attained results, the alumina-based binders (HA or ColAlu) induced a more effective sintering process due to their high specific surface area, improving the physical properties and the binding level of the generated microstructure. The spinel grain size also played an important role in the corrosion behavior of these refractories, as the finer the particles, the greater their dissolution was into the molten liquid, leading to further precipitation of spinel in the solid–liquid interface as a continuous and thick layer. Among the evaluated compositions and considering the presence of silica fume, the most suitable formulation with optimized corrosion resistance was the one with in situ spinel generation and HA as a binder.     

Electrohydrodynamic atomization deposition of PZT sol–gel slurry and sol infiltration on the films

This paper reports the use of a deposition technique, called electrohydrodynamic atomization, for producing PZT films from a composite sol–gel slurry. PZT slurries of different powder concentrations were prepared, and then atomization deposition of them was carried out and resultant films examined. It was observed that large splats with well-mixed PZT particles in sol and fine PZT clusters with dry PZT particle agglomeration can be deposited using this technique. In particular, intermediated sol infiltration process on the EHDA deposited film under optimized atomization condition was also studied. It was found that the film with sol infiltration process become more dense and the electrical properties (P_r, ?_r) were improved compared with that without sol infiltration after EHDA deposition.     

The corrosion resistance of microsilica-containing Al2O3–MgO and Al2O3–spinel castables

Microsilica addition in Al₂O₃–MgO and Al₂O₃–spinel castables helps to improve their flowability and partially accommodate their residual expansion after firing. Nevertheless, there is a lack of conclusive statements in the literature regarding the effects of microsilica on one of the main requisites for steel ladle refractories: corrosion resistance. In the present work, the performance of alumina–magnesia and alumina–spinel with or without microsilica when in contact with a steel ladle slag was evaluated based on three aspects: the material's physical properties, its chemical composition and the microstructural features before the slag attack. According to the attained results, microsilica induced liquid formation and pore growth during sintering, favoring the physical slag infiltration. Moreover, due to this liquid, CA₆ was formed in the matrix, mainly for the Al₂O₃–spinel composition, which also favored the castable dissolution into the molten slag.     

Synthesis and characterization of corrosion-resistant and biocompatible Al2O3–TiB2 nanocomposite films on pure titanium

Alumina is widely used as a coating on a metal implant due to its favorable mechanical and biological properties. In this research, in order to improve mechanical and biological properties of alumina, a composition of nanoparticles of alumina (instead of microparticles) and titanium diboride micro powder is introduced. The atmospheric plasma spray (APS) technique was applied to deposit Al₂O₃–TiB₂ on the pure titanium substrate. The properties of Al₂O₃–TiB₂ nanocomposite coatings with various weight percent of TiB₂ (20, 30 and 40 wt%) were experimentally studied. The characteristics of nanocomposite films of TiB₂ (20, 30 and 40 wt%) were analyzed using Field Emission Scanning Electron Microscopy (FE-SEM), energy dispersive electron spectroscopy (EDX) and X-Ray Diffraction (XRD) tests. The XRD spectra exhibited that in addition to alumina and titanium diboride, the films contained titania. Thickness and morphology of the films were calculated from FE-SEM images and the thickness of the optimized coating (Al₂O₃-30 wt% TiB₂) was about 30–45 μm. Also, the roughness, corrosion resistance, hardness and cytotoxicity (MTT) tests were studied. The highest of hardness and roughness of the samples were obtained from Al₂O₃-30 wt% TiB₂. According to the obtained results from the polarization test, Al₂O₃-30 wt% TiB₂ coating had the highest corrosion resistance (222558.9962 Ω cm²). Therefore, the toxicity of Al₂O₃-30 wt% TiB₂ was investigated as the optimized coating and the results confirmed its non-toxicity and biocompatibility.     

Mechanical properties and thermal shock in thin ZrO2–Y2O3–Al2O3 films obtained by the sol-gel method

Thin multilayer coatings of ZrO₂–Y₂O₃–Al₂O₃ were prepared using the sol-gel method and dip-coating technique in order to advance in the study of what influence the incorporation of Al₂O₃ has on films of Y₂O₃-doped ZrO₂, investigating its role in the synthesis of the solutions and in the characteristics and properties of the coatings. After the characterization of the solutions used in the process, the microstructure of the films was studied and their mechanical behaviour and resistance to thermal shock were determined so as to optimize the characteristics and functionality of these coatings. With increased alumina content, 3YSZ-Al₂O₃ (20 mol%), the cubic phase of the zirconia disappeared completely at the sintering temperature used (700 °C), resulting in the tetragonal phase with Al in solution. There was also a decrease in the coatings' hardness and Young's modulus, and an increase in toughness and resistance to thermal shock. These results allow guidelines to be established for the design of multilayer structures that are, tougher, more resistant, and have improved surface properties.     

Electrical resistivity of Cr–Al2O3 and ZrxAly–Al2O3 composites with interpenetrating microstructure

AC and DC resistivity of Cr–Al₂O₃ and Zr_xAl_y–Al₂O₃ composites with varying metal content were measured. A strong percolation behavior was observed in the Cr–Al₂O₃ system, where the AC resistivity varied nine orders of magnitude close to the percolation threshold of 28 vol.%. AC measurements were less dependant on the contact resistance than DC measurements. The best reproducibility was obtained at a frequency of 100 kHz. AC resistivity values of insulating composites differed from DC values and may also be frequency-dependant. DC measurements up to 600 °C indicate that the intermetallic phases ZrAl₃ and ZrAl are PTC conductors. The electrical properties of Zr_xAl_y–Al₂O₃ samples with a metal content of 29 vol.% were anisotropic, with a much higher resistivity in the pressing direction.     

Electrical humidity response of sol–Gel processed undoped and alkali-doped TiO2–Al2O3 thin films

Al₂O₃-TiO₂ thin films were prepared by a sol–gel processing. Sols were prepared having different TiO₂/Al₂O₃ molar ratios, i.e., 9:1, 8:2, and 6:4, without and with the addition of 10 at% of K ions. The films were prepared by dipping the substrates (silicon wafers, alkali-free glass or alumina with comb-type Au electrodes) in the sols. The films were fired in air for 1 h at 300, 500, 650 and 800°C. The films were amorphous, at any composition, up to the firing temperature of 500°C. Crystallization of the films was inhibited by larger contents of Al₂O₃ and K. The humidity-sensitive electrical properties of the thin films were studied using d.c. and a.c. measurements. The addition of K dramatically improved the relative humidity response of the films.     

0.65Pb(Mg1/3Nb2/3)O3–0.35PbTiO3 thick films prepared by electrophoretic deposition from an ethanol-based suspension

We have investigated the processing of 0.65Pb(Mg_1/3Nb_2/3)O₃–0.35PbTiO₃ (denoted PMN–PT) thick films using an electrophoretic deposition process (denoted EPD), with the PMN–PT particles suspended in an ethanol-based suspension. The zeta-potential and the viscosity were measured to identify the conditions for the preparation of a stable suspension suitable for the EPD. The applied voltage, the deposition time, the chemical composition of the suspension and the concentration of the powder were investigated in order to obtain a high-quality PMN–PT deposit with a target thickness of about 50 μm. The PMN–PT thick films prepared from stoichiometric and PbO-excess suspensions by sintering at 950 and 1100 °C were examined by X-ray powder-diffraction analysis and scanning electron microscopy. The highest functional response was obtained for a homogeneous, dense, crack-free PMN–PT thick film prepared from a PMN–PT suspension with excess PbO. The film was deposited at a constant voltage of 10 V and for a time of 120 s, followed by sintering at 1100 °C in a PbO-rich atmosphere. The film's properties were as follows: a dielectric permittivity ? of 20,250 at a T_m of 172 °C, a remanent polarization of 17 μC/cm² and a coercive field of 5 kV/cm.     

Surface and bulk crystallization in Nd2O3–Al2O3–SiO2–TiO2 glasses

Neodymium aluminosilicate (Nd₂O₃–Al₂O₃–SiO₂; NdAS) glasses have been investigated for the effect of concentration of TiO₂ on the crystallization mechanism and for the effect of surface condition on crystal growth. NdAS glasses with 0–10 wt.% TiO₂ were heat-treated for nucleation and crystal growth and were examined for phase separation and morphology of surface crystals as well as for crystal growth rate. All the glasses exhibit surface crystallization, however, the glass having 8 wt.% TiO₂ also exhibits internal crystallization after a two-stage heat treatment. Surface crystallization was dependent on the condition of the glass surface and the amount of TiO₂. The crystal growth on the cut surface was faster than on the fractured surface and the growth rate in surface increased with increasing TiO₂. The phase separation found in NdAS glasses containing above 8 wt.% TiO₂, was confirmed to be an important factor controlling the internal crystallization process in Nd₂O₃–Al₂O₃–SiO₂–TiO₂ glasses.     

Integrated experimental phase equilibria study and thermodynamic modelling of the binary ZnO–Al2O3, ZnO–SiO2, Al2O3–SiO2 and ternary ZnO–Al2O3–SiO2 systems

Phase equilibria of the ZnO–SiO₂, Al₂O₃–SiO₂ and ZnO–Al₂O₃–SiO₂ systems at liquidus were characterized at 1340–1740 °C in air. The ZnO–Al₂O₃ subsolidus phase equilibria were derived from the experiments with the SiO₂- and CaO + SiO₂-containing slags. High-temperature equilibration on silica or platinum substrates, followed by quenching and direct measurement of Zn, Al, Si and Ca concentrations in the phases with the electron probe X-ray microanalysis (EPMA) was used to accurately characterize the system. Special attention was given to zincite phase that was shown to consist of two separate ranges of compositions: round-shaped low-Al zincite (<2 mol.% AlO_1.5) and platy high-Al zincite (4–11 mol.% AlO_1.5). A technique was developed for more accurate measurement of the ZnO solubility in the low-ZnO phases (corundum, mullite, tridymite and cristobalite) surrounded by the ZnO-containing slag, using l-line for Zn instead of K-line, avoiding the interference of secondary X-ray fluorescence. Solubility of ZnO was found to be below 0.03 mol.% in corundum and cristobalite, and below 0.3 mol.% in mullite. Present experimental data were used to obtain a self-consistent set of parameters of the thermodynamic models for all phases in this system using FactSage computer package. The modified quasichemical model with two sublattices (Zn²⁺, Al³⁺, Si⁴⁺) (O^2?) was used for the liquid slag phase; the compound energy formalism was used for the spinel (Zn²⁺,Al³⁺)[Zn²⁺,Al³⁺,Va]₂O^2-₄ and mullite Al³⁺₂(Al³⁺,Si⁴⁺) (O^2?,Va)₅ phases; the Bragg-Williams formalism was used for the zincite (ZnO, Al₂O₃); other solid phases (tridymite and cristobalite SiO₂, corundum Al₂O₃, and willemite Zn₂SiO₄) were described as stoichiometric. Present study is a part of the research program on the characterization of the multicomponent Pb–Zn–Cu–Fe–Ca–Si–O–S–Al–Mg–Cr–As–Sn–Sb–Bi–Ag–Au–Ni system.     

Pore formation model for direct laser deposition of Al2O3–ZrO2 ceramic

Pores are some of the most common defects that form during direct laser deposition of ceramic materials. A mathematical model of pore formation for Al₂O₃-ZrO₂ ceramic was developed. The pore formation model, which was developed from the bubble escape factor, reveals the relationship between the movement of the solid-liquid interface of the molten pool and that of the bubbles. In the frontier region of the molten pool, with increasing laser power, the proportion of bubbles that escaped and the area from which bubbles escaped increased. With increasing laser power, the actual and theoretical porosity decreased from 80 % to 40 %. At the central region of the molten pool, the bubble escape factor increased with increasing bubble diameter under the same laser power. The theoretical porosity and the actual porosity decreased with increasing laser power. The pore formation model provides a basis for fabricating high-quality Al₂O₃-ZrO₂ ceramics by direct laser deposition.     

Processing and crystallisation of rapidly solidified Al2O3–Y2O3 fibres

Abstract

Amorphous fibres of the Al₂O₃–Y₂O₃ system were prepared by a melt extraction technique, and subjected to crystallisation. The quality of the melt extracted fibres is controlled by the wheel edge and rotational speed, with both having a significant effect on fibre diameter and avoidance of irregularities and instabilities along the fibre length. Tensile strength in the glassy state varied from 0·6 to 1·0 GPa. Crystallisation activation energies calculated from scan-rate dependence of DTA peaks are 741 and 1374 kJ mol^-1 for E1 (Al₂O₃–yttrium aluminium garnet (YAG) eutectic), 390 kJ mol^-1 for YAG, and 438 kJ mol^-1 for E2 (YAG–yttrium aluminium perovskite (YAP) eutectic) by the Kissinger method; and 698 and 1346 kJ mol^-1 for E1, 352 kJ mol^-1 for YAG, and 399 kJ mol^-1 for E2 by the Augis–Bennett method.     

Thermal Stability and Crystallization Kinetics of MgO–Al2O3–B2O3–SiO2 Glasses

To support commercialization of the MgO–Al₂O₃–B₂O–SiO₂-based low-dielectric glass fibers, crystallization characteristics of the relevant glasses was investigated under various heat-treatment conditions. The study focused on the effects of iron on the related thermal properties and crystallization kinetics. Both air-cooled and nucleation-treated samples were characterized by using the differential thermal analysis/differential scanning calorimeter method between room temperature and 1200°C. A collected set of properties covers glass transition temperature (T_g), maximum crystallization temperature (T_p), specific heat (ΔC_p), enthalpy of crystallization (ΔH_cryst), and thermal stability (ΔT=T_p—T_g). Using the Kinssiger method, the activation energy of crystallization was determined. Crystalline phases in the samples having various thermal histories were determined using powder X-ray diffraction (XRD) and/or in situ high-temperature XRD method. Selective scanning electron microscope/energy-dispersive spectroscopy analysis provided evidence that crystal density in the glass is affected by the iron concentration. Glass network structures, for air-cooled and heat-treated samples, were examined using a midinfrared spectroscopic method. Combining all of the results from our study, iron in glass is believed to function as a nucleation agent enhancing crystal population density in the melt without altering a primary phase field. By comparing the XRD data of the glasses in two forms (bulk versus powder), the following conclusions can be reached. The low-dielectric glass melt in commercial operation should be resistant to crystallization above 1100°C. Microscopic amorphous phase separation, possibly a borate-enriched phase separating from the silicate-enriched continuous phase can occur only if the melt is held at temperatures below 1100°C, that is, below the glass immiscibility temperature. The study concludes that neither crystallization nor amorphous phase separation will be expected for drawing fibers between 1200°C and 1300°C in a commercial operation.