氧缺失对Nd_(0.7)Sr_(0.3)MnO_3外延膜结构和输运性的影响

不同厚度的Nd0.7Sr0.3MnO3(NSMO)外延膜是由脉冲激光沉积生长在(LaAlO3)0.3(Sr2AlTaO6)0.7(LSAT)(001)衬底上的。X射线衍射(XRD)和电阻率测量结果显示,沉积氧压为21.333Pa时薄膜c轴参数随着膜厚的减小而增加,同时金属-绝缘体的转变温度TP下降,电阻率增大;另一组生长在27.999Pa氧压下,厚度为120nm的薄膜单胞体积随着退火温度的升高而增大,同时电阻率升高,TP下降。上述结果归因于低的原位沉积氧压和真空退火引起的氧缺失导致n(Mn3+)/n(Mn4+)的增大以及MnO6八面体的畸变。结果表明,对超薄的应变薄膜,要获得较高的TP值,较高的沉积氧压是必需的,同时应仔细考虑真空退火对薄膜性能的影响。

Preparation of ultrafine rhenium powders by CVD hydrogen reduction of volatile rhenium oxides

A novel CVD process for the preparation of ultrafine rhenium powders was investigated using ammonium perrhenate as starting materials. In the process, volatile rhenium oxides, such as ReO4 and Re2O7, were vaporized under a controlled oxidizing atmosphere via the pyrolysis of ammonium perrhenate, and carried into reduction zone by carrier gas, and there reduced into rhenium powders by hydrogen gas. Thermodynamic calculations indicated that Re207 could be prevented from further decomposition through controlling the oxygen partial pressure higher than 10 1.248 Pa in the pyrolysis of ammonium perrhenate. This result was further validated via DSC-TGA analysis of ammonium perrhenate. The typical rhenium powders prepared by the CVD method proposed show irregular polyhedron morphology with particle size in the range of 100-800 nm and a Ds0 of 308 nm. The specific surface area and oxygen content were measured to be 4.37 m^2/g and 0.45%, respectively.

Formation mechanism and oxidation behavior of MoSi_2-SiC protective coating prepared by chemical vapor infiltration/reaction

In order to protect C/C composites from oxidation, SiC-MoSi2 composite coating was synthesized by chemical vapor infiltration /reaction （CVI/CVR） technology. A porous Mo layer was prefabricated on SiC coated C/C composites, and then MoSi2 and SiC were subsequently prepared in a CVI /CVR process using methyltrichlorosilane （MTS） as precursor. The deposition and reaction mechanism of the MoSi2-SiC composite coating was investigated by X-ray diffraction （XRD） and scanning electron microscopy （SEM）. The oxidation behavior of SiC-MoSi2 coated specimens was tested. The results show that the porous Mo layer can be densified with SiC phase decomposed from MTS, and transformed into SiC-MoSi2 by reacting with MTS as well. A dense composite coating was prepared with optimized deposition parameters. The coated specimen exhibits a good oxidation resistance with a little mass loss of 1.25% after oxidation at 1500 °C for 80 h.

Recent Advances in ZnO Films Prepared by Pulsed Laser Deposition

Pulsed laser deposition(PLD) is emerging as the most rapid and efficient technique for fabricating the many compound films. ZnO thin films can be prepared under various deposition conditions by PLD. The effects of various substrate temperature, oxygen partial pressure, annealing temperature, substrate, buffer layers thickness and film thickness on micro-structural, optical and electrical properties of ZnO films grown by PLD technology are reviewed. ZnO films with special function can grow under proper conditions by PLD.

Influence of deposition voltage on phase, microstructure and antioxidation property of cristobalite aluminum phosphate coatings

Cristobalite aluminum phosphate (C-AlPO_4) coatings were prepared by a hydrothermal electrophoretic deposition process on SiC-coated C/C composites. Phase compositions and microstructures of the as-prepared coatings were characterized by XRD and SEM analyses. The influence of deposition voltage on the phase, microstructure and antioxidation property of the cristobalite aluminum phosphate coatings was investigated. Results show that the as-prepared coatings are composed of cristobalite aluminum phosphate crystallites. The thickness and density of cristobalite aluminum phosphate coatings are improved with the increase of deposition voltage. The deposition amount and bonding strength of the cristobalite aluminum phosphate coatings also increase with the increase of deposition voltage. The deposition mass per unit area of the coatings and the square root of the deposition time at different hydrothermal voltages satisfy linear relationship. The antioxidation property of the coated C/C composites is improved with the increase of deposition voltage. Compared with SiC coatings prepared by pack cementation, the multilayer coatings prepared by pack cementation with a later hydrothermal electrophoretic deposition process exhibit better antioxidation property. The as-prepared multi-coatings can effectively protect C/C composites from oxidation in air at 1 773 K for 37 h with a mass loss rate of 0.53%.

A new sediment type of coated grain: Oolitic sinter

As a special sedimentary grain type, the coated grain(with an ooid model) has been known for two centuries due to its fascinating special fabric and genesis developments. The leading factors in forming the coated grain consist mainly of:(1) microorganism movement field;(2) chemical sedimentary effect;(3) hydrodynamic force environment and topography condition;(4) abundant core material supply;(5) embedding condition; and(6) humic acids condition in water medium. With the development of the coated grain genesis, the single factor theory cannot reasonably explain the exact formation of the surface sediment of coated grain. Here, we find a new way to study the coated grain on the basis of traditional research methods. The Wenquan area on the northeast edge of the Qiangtang Basin is one of the few areas where the coated grain is developing, and is a rare "natural laboratory" for the study of the coated grain and the thermal spring sediment. The oolitic sinter of the area has the triaxiality modality of pea polymer, and is obviously different from the karst travertine and the normal lacustrine ooid. We found that the hot spring water in the Wenquan area has higher partial pressure of CO2(PCO2) and saturation index of the calcite(SIc) than normal. Macrocosmically, the oolitic sinter is shaped like a pea, and its grains and gap fillings are light yellow. Microcosmically, the sinter grain forms six types of fundamental lamina, and those six types are developed to be four grain types with different combinations. The C-axis of the mineral grain of sinter cement(calcite) is normal to the lamina face, and grows on it with distinct generation formations. In short, the grain type of oolitic sinter is the oncoid, with the grain development caused by the factors such as the shallow water of strong hydrodynamic force, the special hydrochemistry condition, and the extensive algae activities(diatom).