Qixing Xia

Effect of PEO power modes on structure and corrosion resistance of ceramic coatings on AZ91D Mg alloy

Abstract The aim of this work is to investigate the effects of different power modes [constant current (CC), constant voltage (CV) and constant power (CP)] on the structure and corrosion resistance of the plasma electrolytic oxidation ceramic coatings containing Ca and P on AZ91D Mg alloy. The phase composition, morphology and element distribution of the coatings were studied by X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. The corrosion resistance of the coatings in the simulated body fluid was examined by electrochemical impedance spectroscopy and polarisation curve methods. The results showed that the coatings were of porous structure and mainly composed of MgO. The thickness, surface roughness, the size of the surface micropores and the amount of MgO were all increased with the cell voltage under the same power mode or increased in the sequence CV>CC>CP under the different power modes. The corrosion resistance of coatings was related to the structure characteristics, which were determined by different power modes. Among the different power modes, the corrosion resistance of the coatings is generally increased in the sequence CV>CP>CC, and under the same mode, the proper increase in the cell voltage is liable for improving the corrosion resistance of the coatings.

Investigation of absorptance and emissivity of thermal control coatings on Mg-Li alloys and OES analysis during PEO process.

Thermal control ceramic coatings on Mg–Li alloys have been successfully prepared in silicate electrolyte system by plasma electrolytic oxidation (PEO) method. The PEO coatings are mainly composed of crystallized Mg2SiO4 and MgO, which have typical porous structure with some bulges on the surface; OES analysis shows that the plasma temperature, which is influenced by the technique parameters, determines the formation of the coatings with different crystalline phases and morphologies, combined with “quick cooling effect” by the electrolyte; and the electron concentration is constant, which is related to the electric spark breakdown, determined by the nature of the coating and the interface of coating/electrolyte. Technique parameters influence the coating thickness, roughness and surface morphology, but do not change the coating composition in the specific PEO regime, and therefore the absorptance (αS) and emissivity (ε) of the coatings can be adjusted by the technique parameters through changing thickness and roughness in a certain degree. The coating prepared at 10 A/dm2, 50 Hz, 30 min and 14 g/L Na2SiO3 has the minimum value of αS (0.35) and the maximum value of ε (0.82), with the balance temperature of 320 K.

Study on coating growth characteristics during the electrolytic oxidation of a magnesium–lithium alloy by optical emission spectroscopy analysis

The aim of this study is to analyze the composition, structure and growth characteristics of plasma electrolytic oxidation (PEO) coatings through optical emission spectroscopy (OES). The PEO coatings were prepared on a magnesium–lithium alloy in a phosphate system at various frequencies. The composition and structure of the coatings were examined using X-ray diffraction (XRD), X-ray photo-electron spectroscopy (XPS) and scanning electron microscopy (SEM), as well as energy-dispersive X-ray (EDX). The discharge sparks of the PEO process were measured by optical emission spectroscopy. The results show that the PEO coating prepared at 50 Hz is composed of crystalline MgO and crystalline Mg3(PO4)2, and that the coating at 500 Hz is composed of crystalline MgO and amorphous Mg3(PO4)2. The coating prepared at 50 Hz has a greater degree of roughness than that prepared at 500 Hz, and the sizes of the micropores on the coating prepared at 50 Hz are considerably larger than that at 500 Hz, whereas the numbers of the micropores at various frequencies change in opposition to the pore sizes. The plasma temperature (Te) calculated with OES at 50 Hz is about 3100 K higher than that at 500 Hz. This means that more energy generated per cycle was applied to the electrode surface at 50 Hz than 500 Hz, which consequently influenced the structure and composition of the coatings. Based on the OES analysis, the growth characteristic of the PEO coatings was proposed to explain the changes of the coating roughness and the formation mechanism of crystalline or amorphous Mg3(PO4)2 at various working frequencies by the Te and the liquid-cooling effect, which was further proven by the experiments designed by changing the electrical parameters of the PEO process. This study also illustrates that the adjustment of the phase composition and structure by the electrical parameters can be well explained by OES. Besides, the corrosion resistance of the MAO coatings was evaluated by the polarization curves in 3.5 wt% NaCl solution. The corrosion resistance of the coatings is mainly determined by thickness and roughness, and the coatings prepared under 500 Hz generally present better corrosion resistance than those prepared under 50 Hz.

Hydrothermal synthesis of a uniform sub-micrometer-spherical Zn0.83Cd0.17S photocatalyst with high activity for photocatalytic hydrogen production

In this work, a series of Zn0.83Cd0.17S with high photocatalytic activity were hydrothermally synthesized and the effects of the hydrothermal temperature on the structural, chemical, morphological properties of the samples were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), particle size analyzer and X-ray photoelectron spectroscopy (XPS), respectively. The photoabsorption properties were measured using a UV-vis diffused reflectance spectrophotometer and the photocatalytic activities of the samples for hydrogen production were evaluated under 300 W Xe lamp irradiation. The results show that the sub-micro sized spherical Zn0.83Cd0.17S particles are uniform and mainly composed of cubic zinc-blende phase. An increase of temperature improves the crystallinity of the samples and the ratio of Cd and Zn in the solid solution and consequently makes the absorption edges gradually shift monotonically to longer wavelengths. Also, the hydrothermal temperature influences the particle size and distribution of sulfide solid solution. The sample synthesized at 160 °C exhibits the best photocatalytic activity for H2 evolution with a hydrogen production rate of 45.97 mmol h−1 g−1 when the amount of the sample is 0.03 g in a 200 ml aqueous solution containing 0.35 M Na2S and 0.25 M Na2SO3.