Experimental studies and computer simulation of the preparation of nanoporous silicon-carbide membranes by chemical-vapor infiltration/chemical-vapor deposition techniques

Preparation of asymmetric nanoporous silicon-carbide membranes using chemical-vapor infiltration/chemical-vapor deposition (CVI/CVD) is described. We use macroporous SiC disks and tubes as supports, and tri-isopropylsilane as the precursor. Experimental data for the permeation and selectivity of the membranes are presented. We also develop two dynamic models to describe the preparation of the membranes. The models are shown to provide accurate predictions for the experimental data for the permeation characteristics of the membranes, as a function of the preparation conditions.     

熔融挤压快速成型中支撑工艺的研究

针对单喷头的熔融挤压快速成型制作成型件的特点,必须保证能很好地去除支撑才能得到制件的要求,对熔融挤压快速成型中支撑工艺进行了研究。支撑关系到零件的加工成败、加工时间和表面质量等,提出了支撑的一些生成原则和设置辅助支撑的作用。根据挤出丝的截面形状,建立了支撑丝宽的数学模型,对生成支撑的重要工艺参数进行了分析,结果表明:栅格宽度w较出料速度v对支撑间的间隙L0影响要大,但当层高h由0.2 mm变为0.1 mm时,栅格宽度对支撑间间隙的影响基本不变,而出料速度对支撑间间隙的影响成倍增长,并进行了实验验证。     

In-situ Fourier transform infrared spectroscopy gas phase studies of vanadium (IV) oxide coating by atmospheric pressure chemical vapour deposition using vanadyl (IV) acetylacetonate

This paper describes the use of in-situ Fourier transform infrared spectroscopy to monitor the gas phase reactions of the formation of VO₂ thin films from VO(acac)₂ under atmospheric pressure chemical vapour deposition conditions. In the absence of O₂, it is found that anhydride species may form, while there is also some evidence of ester species. In the presence of O₂, the spectra obtained are almost identical to those in the absence of O₂. However in this case, there is also some indication for the enhanced production of CO and the suppression of the formation of C-H species. A possible mechanism for the formation of VO₂ is proposed, which involves the release of two C₃H₄ molecules and the decomposition of vanadyl (IV) acetylacetonate into VO(CH₃COO)₂, which then further decomposes to yield (CH₃CO)₂O and VO₂. However, while spectroscopic evidence for the formation of these species is presented, the mechanism proposed cannot be confirmed on the basis of these data alone.     

TiO2 thin films using organic liquid materials prepared by Hot-Wire CVD method

TiO₂ thin films prepared by Hot-Wire CVD method have been studied as a protecting material of transparent conducting oxide (TCO) against atomic hydrogen exposures for the fabrications of Si thin film solar cells. It was found that electrical conductivity of the films at room temperature reached a value of 0.4 S/cm. This value is 2-3 orders of magnitude higher than that of TiO₂ films prepared by RF magnetron sputtering and electron-beam evaporation methods in our previous works. The conductivity improvement seems to be partly due to the enlargement of TiO₂ crystallites.     

Plasma-assisted deposition of lithium phosphorus oxynitride films: Substrate bias effects

Lithium phosphorus oxynitride (Lipon) films have been synthesized by a plasma-assisted directed vapor deposition (PA-DVD) approach. In this approach, a hollow cathode technique was used to create an argon plasma through which was propagated an electron-beam generated Li₃PO₄ vapor entrained in a N₂-doped helium gas jet. Without plasma assistance, amorphous, mud cracked and highly porous Li₃PO₄ films were formed. When plasma-assistance was used, nitrogen was incorporated creating a Lipon film whose composition, morphology, structure, and deposition rate could be manipulated by modifying the substrate bias. Films with spiral or very smooth surfaces could be made in this way. Fully amorphous films or films with locally crystallized regions in an amorphous matrix could be synthesized by varying the bias voltage. The presence of these local regions of crystallinity within a Lipon film decreased the Li-ion conductivities from the 10⁻⁷ S cm⁻¹ to 10⁻¹⁰ S cm⁻¹ range.     

Soret-‘shifted’ dew-point temperatures for surfaces exposed to hydrocarbon vapors dilute in compressed nitrogen

Soret-driven species transport causes concentration non-uniformities in the immediate vicinity of ‘cold’ surfaces immersed in undersaturated vapor-containing streams. These concentration non-uniformities, in turn, alter corresponding condensation onset temperatures, often by as much as 30 K (i.e., ca. 3%) in previously studied, near-atmospheric pressure combustion systems [see, e.g., Rosner, D.E. and Nagarajan, R., 1985. Chemical Engineering Science 40 (2), 177]. Because high-pressures often cause remarkable increases in the relevant binary Soret factor, α_T,12, we investigate here the importance of these vapor phase ‘transport’ effects for ‘compressed’ N₂ streams containing dilute quantities of an alkane: C₁₂H₂₆ (n-dodecane) or C₈H₁₈ (n-octane). We invoke the virial equation of state (VES) to predict gas phase non-ideality, and its appreciable effect on previously available ideal gas Soret factors. Our illustrative numerical results, valid for, say, nominally 1000 K N₂ streams up to pressures of over 100 atm, reveal that high-pressure Soret ‘shifts’ in T_dp can amount to ca. 80%, even at surface temperatures above the equilibrium freezing points of these condensates. We conclude that these high-pressure vapor phase transport phenomena will not only influence the interpretation of such hot gas/‘cold’ surface ‘dew-point’ measurements, they will significantly raise the temperatures at which containment or immersed surfaces must be maintained to avoid the ravages of corrosive or insulating inorganic condensates [Rosner, D.E., Chen, B.K., Fryburg, G.C., Kohl, F.J., 1979. Combustion Science and Technology 20, 87; Rosner, D.E., 1988a. Invited paper, Benjamin G. Levich Memorial Issue of Journal of Physico-Chemical Hydrodynamics 10 (5/6), 663]. In principle, the present theory could itself be used to study the pressure dependence of the binary Soret factor—at least for systems with well-characterized saturation vapor pressures.     

Helium Adsorption on Lithium Substrates

We have developed a cryogenic pulsed laser deposition (PLD) system to deposit lithium films onto a quartz crystal microbalance (QCM) at 4 K. Adsorption isotherms of ⁴He on lithium were measured in the temperature range between 1.42 K and 2.5 K. The isotherms are qualitatively different from isotherms on strong substrates such as gold and weak substrates such as cesium. There is no evidence of the formation of solid-like layers of helium, and the helium coverage is approximately linear in the pressure over a wide range. By measuring the low coverage slope of the isotherms, the binding energy of helium to lithium was found to be approximately −13.6 K. For lithium substrates less than approximately 100 layers thick, the chemical potential at which the superfluid transition was observed was surprisingly sensitive to the details of lithium deposition. This work was supported by NSF grant DMR 0509685.     

Nanostructure analysis of graded oxidation coated glass prepared by APCVD on-line

A solar-control coated glass was prepared using chemical vapor deposition in the A0 zone of an annealing furnace of a float glass manufacturing line. The surface morphology and microstructure of the coated composite film were analyzed by scanning electron microscopy (SEM) and secondary-ion mass spectrometry (SIMS). This shows that the surface of amorphous silicon is smooth, the size of the nodules is about 100-200 nm, the surface roughness is 9.66 nm and the thickness of the layer is 70-78 nm. An oxidization gradient along the thickness of the silicon layer was observed.     

Effect of Iron Coating on Thermal Properties of Aluminum‐Lithium Alloy Powder

Aluminum‐lithium alloys are widely studied to improve performance in energetic materials. However, their high reactivity causes severe surface oxidation over micro‐Al particles in storage, resulting in significant deterioration in the overall performance. This study deals with the effect of iron coating on thermal properties and aging stability of aluminum‐lithium alloy powder. Gas atomized Al‐3Li (3 wt. %) alloy powder was prepared and then successfully coated with nano‐sized iron film by modified chemical liquid deposition method. The morphology, thermal properties and combustion enthalpies were characterized by SEM/EDX, TG/DTA and oxygen bomb calorimeter. The results showed that Fe/Al‐3Li composite powder presented obvious core‐shell structure and the outer iron film was very uniform and compact. Significantly enhanced thermal reactivities of Al‐3Li alloy powder and Fe/Al‐3Li composite powder were achieved compared with pure Al. In addition, aging studies indicated that, after coating, the reactivity decay rates of alloy powder decreased significantly, and the mass combustion enthalpies remained basically stable, which demonstrated that iron coating indeed prevented pre‐oxidation of the alloy powder and improved its aging stability.     

Thermomechanical Properties of Chemically Vapor Deposited Silicon Carbide Filaments

The thermal expansion of filaments of CVD SiC was measured parallel and perpendicular to the filament axis. A discontinuity in the expansion versus temperature curve near the melting point of Si indicated the presence of free Si. The magnitude of the discontinuity was used to estimate the amount of the Si in the fibers. The presence of free Si is discussed in relation to the design and maximum-use temperature of SiC-filament-reinforced ceramic composites.