Huisheng Wang

Humification degrees of peat in Qinghai-Xizang Plateau and palaeoclimate change

In this article we report a new and sensitive palaeoclimate proxy indicator-humification degrees of peat. Based on the comparison of humification degrees with other climate proxy records, such as λ13C time series of theC. mulieensis remains cellulose in the same peat profile, we suggest that humification degrees of peat in Qinghai-Xizang Plateau can served as a palaeoclimate proxy indicator. The higher the humification degrees of peat, the warmer-wetter the climate; on the contrary, the lower the humification degrees, the colder-drier the climate. Due to the simple method of determination, humification degree of peat is worthy studying and applying further.

Crystallographic and microstructural studies of BaTiO3 thin films grown on SrTiO3 by laser molecular beam epitaxy

A series of trial tests were carried out to grow BaTiO3 thin films on SrTiO3 substrates in our newly built laser molecular beam epitaxial system. The phase, crystallographic orientation and crystalline quality of the films were investigated by x-ray diffraction θ/2θ scan, symmetric and asymmetric rocking curves (ω scans) and φ scan. The interfacial microstructures of the BaTiO3/SrTiO3 and the surface morphology of BaTiO3 films were analyzed by cross-section transmission electron microscopy and atomic force microscopy. Under appropriate growth conditions, epitaxial, single-crystal films with fully c-axis oriented tetragonal structure were obtained. The tetragonality was calculated by using {303} asymmetric rocking curve for a BaTiO3 thin film to be 1.027, which is larger than the bulk value of BaTiO3. The BaTiO3/SrTiO3 interface is very abrupt with no transitional layer and the BaTiO3 film exhibits an atomically smooth surface.

Atomic scale epitaxial growth of BaTiO3 thin films by laser molecular beam epitaxy

By using laser molecular beam epitaxy (L-MBE), atomic scale epitaxid growth of BaTiO3(BTO) thin films on SrTiO3 (STO) substrates is achieved. Measurements of reflection high energy electron diffraction (RHEED), X-ray diffraction (XRD), scanning electron microscopy, and transmission electron microscopy reveal that the BTO films arec-axis oriented single crystals with smooth surface. The multi-layer ferroelectric/superconducting heterostructures are also prepared and the ferroelectric properties of BTO films are studied. The results show that by using L-MBE technique, high quality BTO films and improved device performance can be obtained.

Microstructure and mechanical properties of nanoscale SiC/Ca α-SiAlON composites

Ceramic composites comprising nanoscale (less than 200 nm) silicon carbide particles distributed in a matrix of calcium α-SiAlON (α′) have been prepared by uniaxial hot pressing, and the reaction sequence, during densification of a sample containing 20 wt% SiC, has been investigated in the temperature range 1400–1800°C. Samples containing up to 20 wt% SiC were produced to near full density by pressure sintering at temperatures as low as 1600°C for 1 h. For samples with higher SiC contents subjected to a similar treatment, there was residual porosity which was detrimental to the mechanical properties. The SiC was preferentially distributed intergranularly within the α-SiAlON and was effective in controlling grain growth of the α-SiAlON during processing. There was an increase in the uniformity of the grain structure and a significant refinement of the grain size of the composite microstructures with increasing SiC content. The hardness and the three-point bending strength of the composite samples increased markedly with increasing SiC content up to a level of 20 wt%. For a sample containing 20 wt % SiC, the bending strength was about ∼1.5 times that of single-phase α-SiAlON. For samples with higher SiC contents, the rate of increase in hardness was diminished and the bending strength decreased because of incomplete densification. The initial improvement in fracture strength with increasing SiC content is plausibly attributable to the uniform refined grain structure of the composite materials, assuming that the maximum flaw size scales with the grain size.

ROOM-TEMPERATURE DEPOSITION OF YTTRIA-STABILIZED ZIRCONIA BUFFER LAYER ON METALLIC SUBSTRATES BY LASER ABLATION

ConclusionWe investigated the deposition of YSZ buffer layers on polycrystalline metallic substrate at low temperature and low oxygen pressure. The (100) preferred orientation can be obtained in the YSZ films and was explained by the model of growth-based orientation selection. Optimizations of oxygen partial pressure and target distance were done to offer referable values for IBAD process.

Two-dimensional growth and structural characterization of oxide ceramic thin films grown by laser molecular beam epitaxy

Atomically regulated unit-cell by unit-cell homoepitaxial SrTiO3 (STO) and heteroepitaxial BaTiO3 (BTO) films were fabricated on STO (100) substrates by laser molecular beam epitaxy. The fine streak patterns and more than 1000 cycles undamping intensity oscillation were obtained by in situ reflection high-energy electron diffraction (RHEED). The films were examined by atomic force microscopy (AFM), X-ray diffraction (XRD), x-ray photoelectron spectrometer (XPS), (phi) scan, and the cross-section high-resolution TEM. The root-mean-square surface roughness of the films is about 0.1 nm. The FWHM of the XRD (omega) -rocking curve for the (200) diffraction peak of BTO film is 0.235 degrees. The results indicate that the films have a high degree of c-oriented epitaxial crystalline structure and the surfaces of films are atomically smooth.

Dependence of the laser-induced voltages on the oxygen content in normal state YBa2Cu3O7−δ films

Abstract Our investigation on the relation between oxygen content and the laser-induced voltages of the YBa 2 Cu 3 O 7−δ films in normal-state shows that deoxygenation of the superconducting films reduces the laser-induced voltages greatly, even reverses the sign of the signals at large oxygen deficiency. The absolute value of the negative signal at large oxygen deficiency can be greater than that of the positive signal.

Epitaxial growth of infinite layer MCuO2 (MBa, Sr, Ca) thin films and their superlattices by laser molecular beam epitaxy

Abstract Laser molecular beam epitaxy has been used to grow the artificial infinite layer MCuO 2 (M Ba, Sr, Ca) structures and their superlattices. Electron and X-ray diffraction measurements evidence the formation of the expected structures and the high quality of hetero-epitaxy.

Structural and ferroelectric properties of BaTiO3/YBa2Cu3O7 heterostructures prepared by laser molecular beam epitaxy

Abstract Heteroepitaxial BaTiO3(BTO)/YBa2Cu3O7(YBCO) thin films were grown on (100) SrTiO3(STO) substrates by ozone assistant laser molecular beam epitaxy (L MBE). The results show that by using this technique, high quality ferroelectric/superconductor heterostructures with high crystalline quality and desirable device performance can be obtained.

Laser molecular beam epitaxy of BaTiO3 thin films

Atomic-scale epitaxial BaTiO3 (BTO) thin films were grown on SrTiO3 (STO) substrate by computer-controlled laser molecular beam epitaxial system. The crystal quality, surface and interface microstructure of the laser MBE BTO thin film has been investigated by in-situ reflection high- energy electron diffraction, x-ray diffraction, transmission electron microscope and atomic force microscope. We find that the laser MBE BTO thin film prepared at ozone pressure of 10-6 Torr and at STO substrate temperature of 750 degrees C has highly c-axis oriented tetragonal phase and single crystal structure, the BTO/STO interface is abrupt, and the surface of the BTO thin film shows atomically smooth.