Thermal conductivity in mullite/ZrO2 composite coatings

Mullite-based multilayered structures have been suggested as promising environmental barrier coatings for Si₃N₄ and SiC ceramics. Mullite has been used as bottom layer because its thermal expansion coefficient closely matches those of the Si-based substrates, whereas Y–ZrO₂ has been tried as top layer due to its stability in combustion environments. In addition, mullite/ZrO₂ compositions may work as middle layers to reduce the thermal expansion coefficient mismatch between the ZrO₂ and mullite layers. Present work studies the thermal behaviour of a flame sprayed mullite/ZrO₂ (75/25, v/v) composite coating. The changes in crystallinity, microstructure and thermal conductivity of free-standing coatings heat treated at two different temperatures (1000 and 1300 °C) are comparatively discussed. The as-sprayed and 1000 °C treated coatings showed an almost constant thermal conductivity (K) of 1.5 W m⁻¹ K⁻¹. The K of the 1300 °C treated specimen increased up to twice due to the extensive mullite crystallization without any cracking.     

Deposition behavior and characteristics of hydroxyapatite coatings on Al2O3, Ti,and Ti6Al4V formed by a chemical bath method

A simple chemical bath method was used to deposit hydroxyapatite (HA) coatings on Al₂O₃, Ti, and Ti6Al4V substrates at ambient pressure by heating to 65–95 °C in an aqueous solution prepared with Ca(NO₃)₂·4H₂O, KH₂PO₄, KOH, and EDTA. The deposition behavior, morphology, thickness, and phase of the coatings were investigated using scanning electron microscopy and X-ray diffractometry. The bonding strength of the coatings was measured using an epoxy resin method. The HA coatings deposited on the three kinds of substrates were fairly dense and uniform and exhibited good crystallinity without any additional heat treatment. A coating thickness of 1–1.8 μm was obtained for the samples coated once. By repeating the coating process three times, the thickness could be increased to 4.5 μm on the Al₂O₃ substrate. The bonding strength of these coatings was 18 MPa.     

Composite Ni/Al2O3 coatings and their corrosion resistance

Composite coatings Ni/Al₂O₃ were electrochemically deposited from a Watts bath. Al₂O₃ powder with particle diameter below 1 μm was codeposited with the metal. The obtained Ni/Al₂O₃ coatings contained 5-6% by weight of corundum. The structure of the coatings was examined by scanning electron microscopy (SEM). It has been found that the codeposition of Al₂O₃ particles with nickel disturbs the nickel coating's regular surface structure, increasing its microcrystallinity and surface roughness. DC and AC electrochemical tests were carried out on such coatings in a 0.5 M solution of Na₂SO₄ in order to evaluate their corrosion resistance. The potentiodynamic tests showed that the corrosion resistance of composite coating Ni/Al₂O₃ is better than that of the standard nickel coating. After 14 days of exposure the nickel coating corrodes three times faster than the Ni/Al₂O₃ coating. The electrochemical behaviour of the coatings in the corrosive solution was investigated by electrochemical impedance spectroscopy (EIS). An equivalent circuit diagram consisting of two RC electric circuits: one for electrode, nickel corrosion processes and the other for processes causing coating surface blockage, were adopted for the analysis of the impedance spectra. The changes in the charge transfer resistance determined from the impedance measurements are comparable with the changes in corrosion resistance determined from potentiodynamic measurements.     

Directionally solidified CeO2 (or GDC)/CoO eutectic ceramics as cermet precursors for SOFCs anodes: Microstructure cross-over

Rods of CeO₂ and gadolinium-doped CeO₂ (GDC)-CoO eutectics were prepared by directional solidification using a laser heated floating zone (LFZ) technique. The microstructure has been studied as a function of the growth rate from V = 10 to 750 mm/h. Regular eutectic microstructures are obtained except for the highest growth rate. The interspacing follows the λ²V = C law with C = 4.1(3) × 10⁻¹⁷ and 2.6(3) × 10⁻¹⁷ m³/s for CeO₂-CoO and GDC-CoO eutectics, respectively. A cross-over between fibrous and lamellar eutectic microstructures was observed depending on the growth rate. The crystallography of the eutectics was studied by Electron Backscatter Diffraction (EBSD). The growth directions [1 1 0]_GDC ∼ //[110]_CoO, and the interfacial planes (200)_GDC//(111)_CoO, were identified. Solubility of Co in the ceria matrix was determined by Energy Dispersive X-ray (EDX) Spectroscopy after Co was leached out from the matrix. Co solubility in ceria at 1650 °C was found to be less than 1 mol%.     

Li diffusion in LiNi0.5Mn0.5O2 thin film electrodes prepared by pulsed laser deposition

Kinetic and transport parameters of Li ion during its extraction/insertion into thin film LiNi_0.5Mn_0.5O₂ free of binder and conductive additive were provided in this work. LiNi_0.5Mn_0.5O₂ thin film electrodes were grown on Au substrates by pulsed laser deposition (PLD) and post-annealed. The annealed films exhibit a pure layered phase with a high degree of crystallinity. Surface morphology and thin film thickness were investigated by field emission scanning electron microscopy (FESEM). The charge/discharge behavior and rate capability of the thin film electrodes were investigated on Li/LiNi_0.5Mn_0.5O₂ cells at different current densities. The kinetics of Li diffusion in these thin film electrodes were investigated by cyclic voltammetry (CV) and galvanostatic intermittent titration technique (GITT). CV was measured between 2.5 and 4.5 V at different scan rates from 0.1 to 2 mV/s. The apparent chemical diffusion coefficients of Li in the thin film electrode were calculated to be 3.13 × 10⁻¹³ cm²/s for Li intercalation and 7.44 × 10⁻¹⁴ cm²/s for Li deintercalation. The chemical diffusion coefficients of Li in the thin film electrode were determined to be in the range of 10⁻¹²-10⁻¹⁶ cm²/s at different cell potentials by GITT. It is found that the Li diffusivity is highly dependent on the cell potential.     

Carbon coated Li3V2(PO4)3 cathode material prepared by a PVA assisted sol-gel method

Carbon coated Li₃V₂(PO₄)₃ cathode material was prepared by a poly(vinyl alcohol) (PVA) assisted sol-gel method. PVA was used both as the gelating agent and the carbon source. XRD analysis showed that the material was well crystallized. The particle size of the material was ranged between 200 and 500 nm. HRTEM revealed that the material was covered by a uniform surface carbon layer with a thickness of 80 Å. The existence of surface carbon layer was further confirmed by Raman scattering. The electrochemical properties of the material were investigated by charge-discharge cycling, CV and EIS techniques. The material showed good cycling performance, which had a reversible discharge capacity of 100 mAh g⁻¹ when cycled at 1 C rate. The apparent Li⁺ diffusion coefficients of the material ranged between 9.5 × 10⁻¹⁰ and 0.9 × 10⁻¹⁰ cm² s⁻¹, which were larger than those of olivine LiFePO₄. The large lithium diffusion coefficient of Li₃V₂(PO₄)₃ has been attributed to its special NASICON-type structure.     

Evaluation of hot corrosion behavior of CSZ,CSZ/micro Al2O3 and CSZ/nano Al2O3 plasma sprayed thermal barrier coatings

Hot corrosion is one of the main destructive factors in thermal barrier coatings (TBCs) which come as a result of molten salt effect on the coating–gas interface. Hot corrosion behavior of three types of plasma sprayed TBCs was evaluated: usual CSZ, layer composite of CSZ/Micro Al₂O₃ and layer composite of CSZ/Nano Al₂O₃ in which Al₂O₃ was as a topcoat on CSZ layer. Hot corrosion studies of plasma sprayed thermal barrier coatings (TBCs) were conducted in 45 wt% Na₂SO₄+55 wt% V₂O₅ molten salt at 1050 °C for 40 h. The graded microstructure of the coatings was examined using scanning electron microscope (SEM) and X-ray diffractometer (XRD) before and after hot corrosion test. The results showed that no damage and hot corrosion products was found on the surface of CSZ/Nano Al₂O₃ coating and monoclinic ZrO₂ fraction was lower in CSZ/Micro Al₂O₃ coating in comparison with usual CSZ. reaction of molten salts with stabilizers of zirconia (Y₂O₃ and CeO₂) that accompanied by formation of monoclinic zirconia, irregular shape crystals of YVO₄, CeVO₄ and semi-cubic crystals of CeO₂ as hot corrosion products, caused the degradation of CSZ coating in usual CSZ and CSZ/Micro Al₂O₃ coating.     

Direct electron-transfer and electrochemical catalysis of hemoglobin immobilized on mesoporous Al2O3

The direct electrochemistry of hemoglobin (Hb) has been achieved by immobilizing Hb on mesoporous Al₂O₃ (meso-Al₂O₃) film modified glassy carbon (GC) electrode. Meso-Al₂O₃ shows significant promotion to the direct electron-transfer of Hb, thus it exhibits a pair of well defined and quasi-reversible peaks with a formal potential of −0.345 V (vs. SCE). The electron-transfer rate constant (k_s) is estimated to be 3.17 s⁻¹. The immobilized Hb retains its biological activity well and shows high catalytic activity to the reduction of hydrogen peroxide (H₂O₂) and nitrite (NO₂⁻). Under the optimized experimental conditions, the catalytic currents are linear to the concentrations of H₂O₂ and NO₂⁻ in the ranges of 0.195-20.5 μM and 0.2-10 mM, respectively. The corresponding detection limits are 1.95 × 10⁻⁸ M and 3 × 10⁻⁵ M (S/N = 3). The resulting protein electrode has high thermal stability and good reproducibility due to the protection effect of meso-Al₂O₃. Ultraviolet visible (UV-vis) absorption spectra and reflection-absorption infrared (RAIR) spectra display that Hb keeps almost natural structure in the meso-Al₂O₃ film. The N₂ adsorption-desorption experiments show that the pore size of meso-Al₂O₃ is about 14.4 nm, suiting for the encapsulation of Hb (average size: 5.5 nm) well. Therefore, meso-Al₂O₃ is an alternative matrix for protein immobilization and biosensor preparation.     

Spontaneously ordered TiO2 nanostructures

Titanium dioxide thin films were deposited on quartz substrates kept at different O₂ pressures using pulsed laser deposition technique. The effects of reactive atmosphere and annealing temperature on the structural, morphological, electrical and optical properties of the films are discussed. Growth of films with morphology consisting of spontaneously ordered nanostructures is reported. The films growth under an oxygen partial pressure of 3 × 10⁻⁴ Pa consist in nanoislands with voids in between them whereas the film growth under an oxygen partial pressure of 1 × 10⁻⁴ Pa, after having being subjected to annealing at 500 °C, consists in nanosized elongated grains uniformly distributed all over the surface. The growth of nanocrystallites with the increase in annealing temperature is explained on the basis of the critical nuclei-size model.     

Supercapacitive properties of a nanowire-structured MnO2 electrode in the gel electrolyte containing silica

Nanowire-structured MnO₂ active materials were prepared by a chemical precipitation method and their supercapacitive properties for use in the electrodes of supercapacitors were investigated by means of cyclic voltammetry in an aqueous gel electrolytes consisting of 1 M Na₂SO₄ and fumed silica (SiO₂). The MnO₂ electrode showed a maximum specific capacitance of 151 F g⁻¹ after 1000 cycles at 100 mV s⁻¹ when using the gel electrolyte containing 3 wt.% of SiO₂, which is higher than 121 F g⁻¹ obtained when using the 1 M Na₂SO₄ liquid electrolyte alone.     

Comparison of AlPO4- and Co3(PO4)2-coated LiNi0.8Co0.2O2 cathode materials for Li-ion battery

Electrochemical and thermal properties of Co₃(PO₄)₂- and AlPO₄-coated LiNi_0.8Co_0.2O₂ cathode materials were compared. AlPO₄-coated LiNi_0.8Co_0.2O₂ cathodes exhibited an original specific capacity of 170.8 mAh g⁻¹ and had a capacity retention (89.1% of its initial capacity) between 4.35 and 3.0 V after 60 cycles at 150 mA g⁻¹. Co₃(PO₄)₂-coated LiNi_0.8Co_0.2O₂ cathodes exhibited an original specific capacity of 177.6 mAh g⁻¹ and excellent capacity retention (91.8% of its initial capacity), which was attributed to a lithium-reactive Co₃(PO₄)₂ coating. The Co₃(PO₄)₂ coating material could react with LiOH and Li₂CO₃ impurities during annealing to form an olivine Li_xCoPO₄ phase on the bulk surface, which minimized any side reactions with electrolytes and the dissolution of Ni⁴⁺ ions compared to the AlPO₄-coated cathode. Differential scanning calorimetry results showed Co₃(PO₄)₂-coated LiNi_0.8Co_0.2O₂ cathode material had a much improved onset temperature of the oxygen evolution of about 218 °C, and a much lower amount of exothermic-heat release compared to the AlPO₄-coated sample.     

Enhanced solid-state electrogenerated chemiluminescence of Au/CdS nanocomposite and its sensing to H2O2

Au nanoparticles (AuNPs) are good quenchers once they closely contact with luminophore. Here we reported a simple approach to obtain enhanced electrogenerated chemiluminescence (ECL) behavior based on Au/CdS nanocomposite films by adjusting the amount of AuNPs in the nanocomposite. The maximum enhancement factor of about 4 was obtained at an indium tin oxide (ITO) electrode in the presence of co-reactant H₂O₂. The mechanism of this enhancement was discussed in detail. The strong ECL emission from Au/CdS nanocomposites film was exploited to determine H₂O₂. The resulting ECL biosensors showed a linear response to the concentration of H₂O₂ ranging from 1.0 × 10⁻⁸ to 6.6 × 10⁻⁴ mol L⁻¹ with a detection limit of 5 nmol L⁻¹ (S/N = 3) and good stability and reproducibility.     

Application of EIS and salt spray tests for investigation of the anticorrosion properties of polyurethane-based nanocomposites containing Cr2O3 nanoparticles modified with 3-amino propyl trimethoxy silane

The Cr₂O₃ nanoparticles were modified with 3-amino propyl trimethoxy silane in order to obtain proper dispersion and increment compatibility with the polyurethane coating matrix. The nanocomposites prepared were applied on the St-37 steel substrates. The existence of 3-amino propyl trimethoxy silane on the surface of the nanoparticles was investigated by Fourier transform infrared (FTIR) spectroscopy and thermal gravimetric analysis (TGA). Dispersion of the surface modified particles in the polyurethane coating matrix was studied by a field emission-scanning electron microscope (FE-SEM). The electrochemical impedance spectroscopy (EIS) and salt spray tests were employed in order to evaluate the corrosion resistance of the polyurethane coatings. Polarization test was done in order to investigate the corrosion inhibition properties of the Cr₂O₃ nanoparticle on the steel surface in 3.5 wt.% NaCl solution. The adhesion strengths of the coatings were evaluated by pull-off adhesion tester before and after 120 days immersion in 3.5 wt.% NaCl solution. FT-IR and TGA analyses revealed that surface modification of the nanoparticles with 0.43 silane/5 g pigment resulted in the greatest amount of silane grafting on the surface of particles. Results obtained from FE-SEM analysis showed that the surface modified nanoparticles dispersed in the coating matrix properly. Results obtained from EIS and salt spray analyses revealed that the surface modified particles enhanced the corrosion protection performance of the polyurethane coating considerably. The improvement was more pronounced for the coating reinforced with 0.43 g silane/5 g pigment. Moreover, the adhesion loss decreased in the presence of surface modified nanoparticles with 0.43 silane/5 g pigment.     

Effect of arc current on microstructure,texturing and wear behavior of plasma sprayed CaZrO3 coatings

In order to enhance the biocompatibility of metallic implants, various ceramic coatings are currently in vogue. CaZrO₃, a promising candidate material, was deposited through plasma spraying on stainless steel (316L) substrates at arc currents of 400, 500 and 600 A. The coatings were characterized using a SEM, XRD, surface profilometers and a tribometer. It was found that the arc current had profound effects on the thickness, microstructure, phase evolution, crystallinity and wear behavior of the coatings. The cross-sectional images and fractographic analysis showed that a denser coating with better inter-splat fusion was produced at arc current of 600 A. The average roughness (Ra) of the coatings increased from 3.62 to 6.68 μm as the arc current was increased from 400 to 600 A. The feedstock (powder) and the coatings were predominantly composed of CaZrO₃ along with a minor amount of CaZr₄O₉ phase. The rise in the arc current resulted in a slight increase in the relative proportion of the CaZrO₃ phase. Also, the coating produced at arc current of 600 A exhibited highest crystallinity. The detailed XRD analysis of (002) and (200) reflections of the ferroelectric CaZrO₃ revealed the preferred orientation of crystals in the coatings. The presence of this texture is explained on the basis of shifting the unstable Zr⁴⁺ ion in oxygen octahedral cage preferably in one direction. The increase in the arc current decreased the coefficient of friction and, as a result, relatively better wear resistance was observed for the coating produced using higher arc current. Moreover, the coating fabricated using arc current of 600 A reduced the volumetric weight loss by 13 times during the wear test as compared to the substrate. Plasma sprayed CaZrO₃ coating not only enhanced the wear resistance of the stainless steel but also showed the potential to furnish a bioactive surface.     

Amorphous-to-crystalline transition of silicon-incorporated anodic ZrO2 and improved dielectric properties

Sputter-deposited zirconium and Zr-16 at.% Si alloy have been anodized to various voltages at several formation voltages in 0.1 mol dm⁻³ ammonium pentaborate electrolyte at 298 K for 900 s. The resultant anodic films have been characterized using X-ray diffraction, transmission electron microscopy, Rutherford backscattering spectroscopy, glow discharge optical emission spectroscopy, and electrochemical impedance spectroscopy. The anodic oxide films formed on Zr-16 at.% Si are amorphous up to 30 V, but the outer part of the anodic oxide films crystallizes at higher formation voltages. This is in contrast to the case of sputter-deposited zirconium, on which the crystalline anodic oxide films, composed mainly of monoclinic ZrO₂, are developed even at low formation voltages. The outer crystalline layer on the Zr-16 at.% Si consists of a high-temperature stable tetragonal phase of ZrO₂. Due to immobile nature of silicon species, silicon-free outermost layer is formed by simultaneous migrations of Zr⁴⁺ ions outwards and O²⁻ ions inwards. An intermediate crystalline oxide layer, in which silicon content is lower in comparison with that in the innermost layer, is developed at the boundary of the crystalline layer and amorphous layer. Capacitances of the anodic zirconium oxide are highly enhanced by incorporation of silicon due to reduced film thickness, even though the permittivity of anodic oxide decreases with silicon incorporation.     

Simulation of SiO2/S coating deposition in a pilot plant set-up for coking inhibition

The anti-coking SiO₂/S coating was prepared on the inner surface of HK40 alloy tube in a pilot plant set-up by atmospheric pressure chemical vapour deposition (APCVD). The coating deposition was simulated using the computational fluid dynamics code, Fluent. The reaction parameters of the surface reaction for SiO₂ formation were determined based on the comparison between the experimental and the calculated values. Further, the influences of the inlet flow rate and mass concentration of source materials on the coating deposition rate were investigated. The simulated results showed that an increase of inlet flow rate led to the decrease of mass conversion of gas intermediates. The coating deposition rate along the reactor tube increased by 1–5 times as the inlet flow rate increased from 10 to 80 g min⁻¹. The mass conversion rate of the gas intermediate, Si(OH)₄, changed little at different inlet mass concentrations of source materials when the inlet flow rate was 30 g min⁻¹, and it had an increase for sulphide intermediates. The coating deposition rate along the reactor tube increased by about 10 times with increasing the inlet mass concentration from 0.2% to 2%. In the conditions we studied, SiO₂/S coating deposition was surface reaction rate limited. When the inlet flow rate was 30–40 g min⁻¹ with the resource material concentration of 1–1.6%, the SiO₂/S coating was about 15 μm at the tube outlet with the silicon-containing intermediate conversion rate of above 30% and a good uniformity of S along the reactor. This work provides a theoretical basis for optimisation of operational parameters of the anti-coking SiO₂/S coating preparation in the pilot plant set-up.     

Synthesis and electrochemical performance of Li2FeSiO4/carbon/carbon nano-tubes for lithium ion battery

Li₂FeSiO₄/carbon/carbon nano-tubes (Li₂FeSiO₄/C/CNTs) and Li₂FeSiO₄/carbon (Li₂FeSiO₄/C) composites were synthesized by a traditional solid-state reaction method and characterized comparatively by X-ray diffraction, scanning electron microscopy, BET surface area measurement, galvanostatic charge-discharge and AC impedance spectroscopy, respectively. The results revealed that the Li₂FeSiO₄/C/CNT composite exhibited much better rate performance in comparison with the Li₂FeSiO₄/C composite. At 0.2 C, 5 C and 10 C, the former composite electrode delivered a discharge capacity of 142 mAh g⁻¹, 95 mAh g⁻¹, 80 mAh g⁻¹, respectively, and after 100 cycles at 1 C, the discharge capacity remained 95.1% of its initial value.     

Microwave dielectric properties of SnO2-doped CaSiO3 ceramics

SnO₂-doped CaSiO₃ ceramics were successfully synthesized by a solid-state method. Effects of different SnO₂ additions on the sintering behavior, microstructure and dielectric properties of Ca(Sn_1−xSi_x)O₃ (x=0.5–1.0) ceramics have been investigated. SnO₂ improved the densification process and expanded the sintering temperature range effectively. Moreover, Sn⁴⁺ substituting for Si⁴⁺ sites leads to the emergence of Ca₃SnSi₂O₉ phase, which has a positive effect on the dielectric properties of CaO–SiO₂–SnO₂ materials, especially the Qf value. The Ca(Sn_0.1Si_0.9)O₃ ceramics sintered at 1375 °C possessed good microwave dielectric properties: ε_r =7.92, Qf =58,000 GHz and τ_f=−42 ppm/°C. The Ca(Sn_0.4Si_0.6)O₃ ceramics sintered at 1450 °C also exhibited good microwave dielectric properties of ε_r=9.27, Qf=63,000 GHz, and τ_f=−52 ppm/°C. Thus, they are promising candidate materials for millimeter-wave devices.     

Electrochemical impedance spectroscopy and corrosion behaviour of Al2O3-Ni nano composite coatings

In this paper, the results on the electrochemical impedance spectroscopy and corrosion properties of electrodeposited nanostructured Al₂O₃-Ni composite coatings are presented. The nanocomposite coatings were obtained by codeposition of alumina nanoparticles (13 nm) with nickel during plating process. The coating thickness was 50 μm on steel support and an average of nano Al₂O₃ particles inside of coatings at 15 vol.% was present. The structure of the coatings was investigated by scanning electron microscopy (SEM). It has been found that the codeposition of Al₂O₃ particles with nickel disturbs the nickel coating's regular surface structure. The electrochemical behavior of the coatings in the corrosive solutions was investigated by polarization potentiodynamic and electrochemical impedance spectroscopy methods. As electrochemical test solutions 0.5 M sodium chloride and 0.5 M potassium sulphate were used in a three electrode open cell. The corrosion potential is shifted to more negative values for nanostructured coatings in 0.5 M sodium chloride. The polarization resistance in 0.5 M sodium chloride decreases in 24 h, but after that increases slowly. In 0.5 M potassium sulphate solution the polarization resistance decreases after 2 h and after 30 h of immersion the polarization resistance is higher than that of the beginning value. The corrosion rate calculated by polarization potentiodynamic curves obtained after 30 min from immersion in solution is smaller for nanostructured coatings in 0.5 M potassium sulphate (4.74 μm/year) and a little bit bigger in 0.5 M sodium chloride (5.03 μm/year).     

Synthesis of spinel LiMn2O4 with manganese carbonate prepared by micro-emulsion method

Micro-spherical particle of MnCO₃ has been successfully synthesized in CTAB-C₈H₁₈-C₄H₉OH-H₂O micro-emulsion system. Mn₂O₃ decomposed from the MnCO₃ is mixed with Li₂CO₃ and sintered at 800 °C for 12 h, and the pure spinel LiMn₂O₄ in sub-micrometer size is obtained. The LiMn₂O₄ has initial discharge specific capacity of 124 mAh g⁻¹ at discharge current of 120 mA g⁻¹ between 3 and 4.2 V, and retains 118 mAh g⁻¹ after 110 cycles. High-rate capability test shows that even at a current density of 16 C, capacity about 103 mAh g⁻¹ is delivered, whose power is 57 times of that at 0.2 C. The capacity loss rate at 55 °C is 0.27% per cycle.