Laser Sintering of SnO2 : Sb Sol-Gel Coatings

Sb doped sol-gel SnO2 single layers (thickness 100 nm) were prepared from alcoholic solution and deposited via a dip coating process on fused silica substrates. The coatings have been sintered at a typical rate of 10–15 cm2/s by CO2 laser irradiation. The laser spot was scanned in one direction at a speed of 15,000 cm/s and the sample was moved in a perpendicular one at a speed up to 250 mm/s. The temperature of the topmost 10 m layer was monitored by a fast pyrometer (s resolution). The following properties of the coatings have been determined: the electrical resistivity , the carrier density n, and mobility , the structure, the thickness, the mesoscopic and micromorphology and the density. The sintering by CO2 laser radiation is mainly a thermal driven process. At T 500°C it allows to obtain coatings with a smaller resistivity (6.8×10–3 cm) than those produced by conventional furnace firing (el2.9×10–2 cm). The results are discussed in terms of particle size and packing density.     

Structural Changes in Sol-Gel Derived SiO2 and TiO2 Films by Exposure to Water Vapor

Structural changes in sol-gel derived thin films by exposure to water vapor were investigated using Fourier transform infrared absorption spectroscopy, ellipsometry and x-ray diffraction. We found that SiO₂ gel films were densified with a decrease in OH groups by the exposure at 60°–180°C. The shrinkage and the peak frequency of ₄ (TO) for the exposed films were comparable to those heated in a dry atmosphere at temperatures above 500°C. However, OH groups in the films were not completely removed by the water exposure. A subsequent annealing above 300°C changed the structure of the water-exposed SiO₂ films with condensation of the remaining OH groups. Although the exposed SiO₂ gel films were amorphous, TiO₂ gel films were transformed to anatase with a decrease in OH groups by the treatments at 80°–180°C.     

Effects of thermo-hygro-mechanical densification on the surface characteristics of trembling aspen and hybrid poplar wood veneers

The effect of thermo-hygro-mechanical (THM) densification temperature on the surface color, roughness, wettability, and chemical composition of trembling aspen (Populus tremuloides) and hybrid poplar (Populus maximowiczii × P. balsamifera) veneers was investigated. Veneers were subjected to four THM densification temperatures (160 °C, 180 °C, 200 °C, and 220 °C). Veneer color darkened with increasing THM densification temperature. Surface roughness decreased between 160 °C and 200 °C. Wettability decreased after THM densification, but no significant difference was found between treated specimens. ATR-FTIR and XPS results confirmed that THM densification caused major chemical changes in veneer surfaces, and more pronounced at temperatures higher than 160 °C.     

Effect of nitrogen pressure on the combustion synthesis of titanium nitride

Abstract

Titanium nitride, TiN, has attractive physical and chemical properties such as hardness, chemical stability and electrical conductivity. It is a typical material with a wide range of stoichiometry. It can be synthesised by high pressure combustion synthesis. The composition and microstructures can vary with the experimental conditions especially with thermal treatment and nitrogen pressure.     

氧化钇对氧化铝陶瓷致密化的影响

采用湿法球磨方法将不同含量氧化钇粉末添加到氧化铝粉末中,经冷等静压成型,1550℃常压烧结.通过研究发现,当氧化钇含量低于0.25wt;时,晶粒长大,存在封闭晶内气孔,相对密度变小,致密化程度低;当氧化钇含量介于0.25wt;～0.75wt;时,随着氧化钇含量增加,封闭气孔减少,晶粒减小,致密化程度增高;当氧化钇含量为1.0wt;时,在晶界生成第二相钇铝石榴石,相对密度较小,致密化程度降低.     

AlN含量对SiC基复相陶瓷性能的影响

以MgO-CeO2为烧结助剂,采用热压烧结工艺在1850℃下制备了SiC基复相陶瓷.研究了不同AlN含量对复相陶瓷致密性与导热性能的影响.结果表明:不添加AlN时,试样致密性最差,气孔率和体积密度分别为4.71;和2.43 g/cm3.AlN含量升高至5wt;时,试样致密性有所提高.AlN含量进一步升高至10wt;～20wt;,试样完全致密,气孔率和体积密度分别保持在0.20;和3.31 g/cm3.在AlN含量为10wt;时,样品具有最高的热导率51.62 W·m-1·K-1,同时弯曲强度和断裂韧性达到顶点,分别为731.3 MPa和7.3 MPa·m1/2.     

Coarsening in Sintering: Grain Shape Distribution,Grain Size Distribution,and Grain Growth Kinetics in Solid-Pore Systems

Sintering occurs when packed particles are heated to a temperature where there is sufficient atomic motion to grow bonds between the particles. The conditions that induce sintering depend on the material, its melting temperature, particle size, and a host of processing variables. It is common for sintering to produce a dimensional change, typically shrinkage, where the powder compact densifies, leading to significant strengthening. Microstructure coarsening is inherent to sintering, most evident as grain growth, but it is common for pore growth to occur as density increases. During coarsening, the grain structure converges to a self-similar character seen in both the grain shape distribution and grain size distribution. Coarsening behavior during sintering conforms to classic grain growth kinetics, modified to reflect the evolving microstructure. These modifications involve the grain boundary coverage due to pores, liquid films, or second phases and the altered grain boundary mobility due to these phases. The mass transport rates associated with each of these interfaces are different, with different temperature and composition dependencies. Hence, the coarsening rate during sintering is not constant, but changes with the evolving microstructure. Core aspects treated in this review include models for coarsening, grain shape, grain size distribution, and how pores, liquids, dispersoids, and other phases determine microstructure coarsening during sintering.     

Laser densification of medium pore size gel-silica glass

Type VI sol-gel silica monoliths with 32 Å and 45 Å pore radii were made using tetramethylsiloxane (TMOS) with HF as a catalyst. The pore texture was characterized using nitrogen absorption/desorption isotherms and BET analysis. Monoliths thermally stabilized to specific bulk densities of 1.67 g/cc to 2.00 g/cc were irradiated with a CO₂ laser to create microlenses. The densified spots were characterized using microscopic Fourier transform infrared (FTIR) spectroscopy. Full density microlenses were achieved; the peak position of the Si-O-Si vibrational mode was at 1122 cm^–1, equivalent to commercial optical silicas. The refractive index in the center of the densified spots was 1.46 at a wavelength of 632.8 nm with a gradient in refractive index of 0.07–0.08. The combinations of laser power and substrate bulk density that yield full density microlenses is expanded when compared with conditions previously established for small pore, 12 Å, gel-silica substrates. Dimensions of the laser densified region, magnitude of the index gradient, and shape of the GRIN profile are easier to control with the medium pore size gel-silica substrates.     

Raman Spectroscopic Investigations of the Effects of Ag+ and Ce3 + Doping on the Densification of Nanoporous Silica Xerogels

Doped and undoped 50 Å porous silica xerogels were heat-treated at various annealing temperatures ranging from 800°C to 1150°C. The heating was performed in air for 1 hour at each temperature. Raman spectroscopy was used to follow the structural changes occurring at various stages of the gel-to-glass transformation, as well as to investigate the effects of metal ions on the densification of these nanoporous silica xerogels. Raman data and density measurements showed that while densification is completely achieved at 1050°C for undoped xerogels, it occurs at 950°C for Ag⁺-doped samples. On the other hand, Ce^{3 +} doping was found to slow down the densification process, with complete densification occurring at 1100°C.     

GNSS clock corrections densification at SHAO: from 5 min to 30 s

High frequency multi-GNSS zero-difference applications like Precise Orbit Determination(POD)for Low Earth Orbiters(LEO)and high frequency kinematic positioning require corresponding high-rate GNSS clock corrections.The determination of the GNSS clocks in the orbit determination process is time consuming,especially in the combined GPS/GLONASS processing.At present,a large number of IGS Analysis Centers(AC)provide clock corrections in 5-min sampling and only a few ACs provide clocks in 30-s sampling for both GPS and GLONASS.In this paper,an efficient epoch-difference GNSS clock determination algorithm is adopted based on the algorithm used by the Center for Orbit Determination in Europe(CODE).The clock determination procedure of the GNSS Analysis Center at Shanghai Astronomical Observatory(SHAO)and the algorithm is described in detail.It is shown that the approach greatly speeds up the processing,and the densified 30-s clocks have the same quality as the 5-min clocks estimated based on a zero-difference solution.Comparing the densified 30-s GNSS clocks provided by SHAO with that of IGS and its ACs,results show that our 30-s GNSS clocks are of the same quality as that of IGS.Allan deviation also gives the same conclusion.Further validation of the SHAO 30-s clock product is performed in kinematic PPP and LEO POD.Results indicate that the positions have the same accuracy when using SHAO 30-s GNSS clocks or IGS(and its AC)finals.The robustness of the algorithm and processing approach ensure its extension to provide clocks in 5-s or even higher frequencies.The implementation of the new approach is simple and it could be delivered as a black-box to the current scientific software packages.