Mingyuan Gu

Preparation and characterization of silica-doped titania photocatalyst through sol-gel method

Silica-doped nanocrystalline titania powders were prepared using the sol–gel technique. The influence of dopant concentration on the phase transition and grain growth was studied. Subsequently the sample was studied at a constant dopant concentration to investigate the effect of calcination temperature on the photoactivity of titania. It was found that 30SiO2/TiO2 (30 denotes the weight percent of SiO2) fired at 800 °C has a higher photoactivity than other samples with different silica content and different calcination temperature. The effect of addition of silica was to increase the photoactivity by suppressing the phase transformation of titania from anatase to rutile and forming oxygen vacancies in titania.

Optical properties of silver-dispersed PVP thin film

Abstract Silver-dispersed PVP (Ag/PVP) thin films were prepared by spin coating from an alcohol solution of polyvinylpyrrolidone (PVP) in which silver nitrate were dissolved. A peak formed at 430–450 nm in the UV/VIS-spectra resulted from the surface plasma resonance of the silver nanoparticles. With the increase of annealing temperature, the plasma resonance absorption peak was broadened and intensities increased. The peak position of the surface plasma resonance also shifted to a high wavelength. XRD results confirmed the presence of silver particles in thin films. The absorption peak position calculated from the absorption spectrum is in very good agreement with Maxwell-Garnet (MG) theory using the dielectric constant of silver particles to calculate results of the absorption coefficient, α, of the silver particles.

Preparation, structure and properties of TiO2–PVP hybrid films

Abstract The organic–inorganic hybrid films of TiO 2 –PVP have been prepared by the sol-gel process. The TiO 2 was obtained by the hydrolysis of titanium n -butoxide (TNBT) under the catalysis of HCl. The hybrid film has a smooth surface with average roughness less than 3 nm. The thermal stability and structure of the gels have been studied by TGA and FTIR spectra, respectively. The TiO 2 –PVP films are transparent and their transmittance is high up to 93% in the visible light range. The microhardness and dielectric properties for the hybrid material films are dependent on the annealing temperature. The microhardness of the film reaches 2.65 GPa after heat treatment at 270°C for 0.5 h.

Superhardness effects of heterostructure NbN/TaN nanostructured multilayers

Although superhardness effects have been extensively investigated for epitaxial ceramic nanomultilayer films with the same crystal structures in the last decade, those for multilayers with different crystal structures have been seldom studied. In this article, NbN/TaN nanomultilayers have been designed and deposited by reactive magnetron sputtering. The results showed that the crystal structures of NbN and TaN are face-centered cubic and hexagonal in superlattice films, respectively, and the lattice plane (111) of NbN is coherent with the (110) of TaN, i.e., {111}fcc-NbN∥{110}h-TaN. The results of microhardness measurement showed that the superhardness effects of NbN/TaN multilayers exist in a wide range of modulation period from 2.3 to 17.0 nm. This phenomenon is different from that of epitaxial ceramic multilayers where the maximum hardness usually takes place at a modulation period of 5.0–10.0 nm. It is proposed that the coherent stresses and the structural barriers (fcc/hexagonal) to dislocation motio...

Microstructure and mechanical properties of boron carbide thin films

Abstract Boron carbide thin films were deposited on single-crystal silicon substrates with magnetron sputtering method at different temperatures using a sintered B 4 C target. Auger electron spectroscopy, Fourier transform infrared spectroscopy and transmission electronic microscopy were employed to characterize their chemical compositions and microstructure. Mechanical properties of the films were evaluated using a nanoindenter. The results show the as-deposited boron carbide films are amorphous or microcrystalline with high hardness and high modulus. With the increase of the substrate temperature, boron carbide films show a tendency of crystallization, and accordingly the hardness and modulus reach 50.4 and 420 GPa from 42.5 and 300 GPa, respectively.

Effect of aluminum substitution on microwave absorption properties of barium hexaferrite

Aluminum substituted barium hexaferrites were prepared by the self-propagating combustion method and subsequent calcination at 850u2009°C. The crystalline structure, complex permittivity, complex permeability, and hyperfine parameters of BaFe12−xAlxO19 (x varies from 1.5 to 2.3 in steps of 0.2) were measured with x-ray diffraction (XRD), vector network analyzer and Mossbauer spectroscopy. The XRD results show that all Al3+ ions enter into the lattice of hexagonal barium ferrite. The substitution of Al3+ ions can greatly affect the complex permittivity and permeability of barium ferrite. With increasing substitution, the real part of complex permittivity increases gradually, and the peaks of the imaginary part of complex permeability shift into higher frequency band. When the substitution amount x is 1.9, the largest movement of the peaks is 1.95 GHz, which indicates that the ferromagnetic resonant frequency of barium ferrite increases by 1.95 GHz. The Al3+ ions preferentially occupy the 4f2, 2a, 4f1, and 12k ...

Effects of PVP on structure of TiO2 prepared by the sol-gel process

Abstract The synthesis of nanocrystalline TiO 2 powders obtained from the hydrolysis of Ti(OBu n ) 4 in the presence of polyvinylpyrrolidone (PVP) was investigated. Porous titania with a Brunauer–Emmett–Teller surface area up to 65 m 2 g −1 and distribution of pores in the range of mesopores were obtained after heat treatment at 500 °C devoted to the removal of PVP from TiO 2 /PVP composites. The influence of PVP on the phase transformation of TiO 2 from anatase to rutile was investigated by using X-ray diffraction and Raman spectra. It was found that PVP could noticeably enhance the transformation by reducing the onset transformation temperature. The obtained powders were also characterized by differential scanning calorimeter, transmission electron microscopy and high-resolution electron microscopy.

Coherent growth and mechanical properties of AlN/VN multilayers

The growth condition of metastable cubic AlN (c-AlN) in AlN/VN multilayers and the effect of c-AlN on the mechanical properties of multilayers were investigated. A series of AlN/VN multilayers with different modulation periods were prepared by reactive magnetron sputtering. The microstructure and mechanical properties of multilayers were characterized with low-angle x-ray diffraction, high-resolution transmission electron microscopy, and nanoindentation. The results show that AlN exists as a metastable cubic phase in multilayers at small modulation periods due to the “template effect” and forms a superlattice with VN through coherent epitaxial growth. Correspondingly, multilayers show the superhardness effect with the enhancement of hardness and elastic modulus. With the increase of modulation periods, c-AlN transforms to the stable hexagonal structure (h-AlN) and multilayers demonstrate a “brick-wall” structure with nanometer grains. The hardness and elastic modulus of multilayers with large modulation p...

Two-step penetration: a reliable method for the measurement of mechanical properties of hard coatings

A two-step penetration method using the microindentation technique is presented to investigate the mechanical properties of hard coatings in this article. This method employs a large-load indentation step, first to exhibit the influences of substrate deformation and coating thickness on the measured hardness values, and then selects a small load, based on the results of the first step to perform the measurement. Since with this small load, the measurement will not be affected by the substrate, the hardness and elastic modulus of the coatings can be obtained precisely. Experiments with TiN coatings deposited on high-speed steel substrate shows that by using this two-step penetration method, the mechanical properties of hard coatings can be measured accurately and reliably.

Effect of PVP on the photocatalytic behavior of TiO2 under sunlight

Abstract TiO 2 prepared by the sol–gel method and adding 30 wt.% of polyvinylpyrrolidone (PVP) has been proved to be efficient in the photodegradation of methylene red under ultraviolet (UV) light. The same powder was also found to be photoactive under sunlight in this research. TiO 2 powders with various contents of added PVP were prepared and characterized by XRD, diffuse reflectance spectroscopy (DRS), etc. It was found that the presence of PVP in the sol–gel process can reduce the transformation temperature from anatase to rutile phase and the crystallite size of TiO 2 . At the same time, a unique coexistence structure of the anatase and rutile phases was obtained in these materials.