RHEED Intensity Oscillations for the Stoichiometric Growth of SrTiO3 Thin Films by Reactive Molecular Beam Epitaxy

The growth of high quality multicomponent oxide thin films by reactive molecular beam epitaxy (MBE) requires precise composition control. We report the use of in situ reflection high-energy electron diffraction (RHEED) for the stoichiometric deposition of SrTiO₃ (1 0 0) from independent strontium and titanium sources. By monitoring changes in the RHEED intensity oscillations as monolayer doses of strontium and titanium are sequentially deposited, the Sr:Ti ratio can be adjusted to within 1% of stoichiometry. Furthermore, the presence of a beat frequency in the intensity oscillation envelope allows the adjustment of the strontium and titanium fluxes so that a full monolayer of coverage is obtained with each shuttered dose of strontium or titanium. RHEED oscillations have also been employed to determine the doping concentration in barium- and lanthanum-doped SrTiO₃ films.     

A molecular dynamics study of bubble nucleation in liquid oxygen with impurities

The present study investigates bubble nucleation in liquid oxygen with dissolved impurities (nitrogen or helium molecules) using molecular dynamics simulations. When the mole fraction of impurities is 0.05, there is a fundamental difference in the bubble nucleation mechanism between the two dissolved impurities cases; vaporization in the homogeneous bulk makes a bubble in the case of a nitrogen‐dissolved liquid while phase separation of impurities and liquid molecules makes a nucleus in the case of a helium‐dissolved liquid. Fluctuations can cause local voids, which in turn can grow to be bubbles, and this effect is stronger in the case of a helium‐dissolved liquid with a lower mole fraction (0.01) than in the case of a nitrogen‐dissolved liquid with a higher mole fraction (0.05). From these results, we conclude that helium molecules have a much stronger action to raise the bubble formation pressure compared with nitrogen. In this paper, the kinetically‐defined critical nucleus, which is a very important factor in quantitatively evaluating the nucleation mechanism, is also estimated through the calculation of the size change rate of each nucleus. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(7): 514–526, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20082     

FX系列催化剂在乙苯合成中活化温度的研究

研究了活化温度对烷基化反应催化剂（ＦＸ－０２）及烷基转移反应催化剂（ＦＸ－０６）反应活性的影响，结果表明：在乙苯合成过程中，２种催化剂的适宜活化温度均为３００℃；同时本还利用吡啶脉冲吸附及程序升温脱附（ＴＰＤ）测定了催化剂的总酸量、Ｂ酸量及酸分布的酸性表征，支持了这个结果。     

氧化石墨烯复合水泥基材料的分子动力学模拟

氧化石墨烯（GO）边缘的大量含氧官能团具有更好的反应活性，能够有效的增强水泥基材料的性能。本文为探究氧化石墨烯中各含氧官能团对水泥基材料性能的影响，建立了GO/C-S-H、GO羧基/C-S-H、GO羟基/C-S-H和GO环氧基/C-S-H模型。采用分子动力学的方法对复合材料的力学参数、界面吸附能、动力学特性进行研究。结果表明：GO的加入使复合材料的杨氏模量提高了10.7%，泊松比提高了12%。GO中的环氧基有助于复合材料界面的稳定性，羟基有助于复合材料内部原子间作用更紧密。含氧官能团与钙原子间的钙氧键是粘聚力的来源之一，钙氧键键长的顺序为Ca-OH相似文献   

Nonequilibrium molecular dynamics calculation of the shear viscosity of carbon dioxide

Nonequilibrium molecular dynamics calculations of the shear viscosity of supercritical carbon dioxide along the 313 K isotherm are reported. Three different intermolecular potential models of increasing complexity are considered: a spherically symmetric Lennard-Jones potential, a two-site Lennard-Jones potential, and a three-site potential which includes a quadrupole-quadrupole moment. Results for the three potentials are compared with experimental data.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.     

Influence of gel and molecular weight on the properties of natural rubber

The influence of gel and molecular weight on the properties of natural rubber has been described. Crystallization, stress relaxation and orientation properties of different samples have been studied. Samples having different gel contents but similar molecular weights and samples having the same gel content with varying molecular weights have been prepared by extracting whole natural rubber. The gel content was varied from 1.5 to 29% and molecular weight from 1 × 10⁵ to 9.9 × 10⁵ g mol⁻¹. The gels reduce the overall crystallinity measured by differential scanning calorimetric experiments after freezing the samples for different time periods at −15°C. They also have a stiffening effect on the tensile relaxation modulus and decrease the rate of relaxation significantly. Birefringence, which measures the orientation and stress-induced crystallization, increases with time at high elongation for samples containing gels. At lower elongation, however, the birefringence of unvulcanized samples decreases with time. This decrease is less for gel samples. Natural rubbers of high molecular weight behave similarly in the above properties when compared to low molecular weight samples. The effect of gel is more predominant than that of molecular weight.     

Synthesis of carbon molecular sieves from palm shell by carbon vapor deposition

A series of experiments were conducted to produce carbon molecular sieves (CMS) through carbon deposition from a locally available palm shell of Tenera type for separating gaseous mixtures. The process involves three stages; carbonization, physical activation with steam, and carbon deposition by using benzene cracking technique. Carbonization of the dried palm shells was occurred at 900°C for duration of 1 h followed by steam activation at 830°C for 30–420 min to obtain activated carbons with different degree of burn-offs. The highest micropore volume of activated carbon obtained at 53.2% burn-off was used as a precursor for CMS production. Subsequent carbon deposition of the activated sample at temperature range from 600 to 900°C for 30 min has resulted in a series of CMSs with different selectivities of CO₂/CH₄ and O₂/N₂. The kinetic adsorption isotherm of CO₂, CH₄, O₂ and N₂ at room temperature also presented in this work.     

Structural, photoluminescent and electrical properties of CdTe films with different compositions fabricated by CMBD

CdTe films with different compositions (Cd-rich, Te-rich and stoichiometric) were fabricated by a novel and low cost chemical molecular beam deposition method (CMBD) in atmospheric pressure hydrogen flow. Cd and Te granules were used as precursors. The films were deposited on ceramic (SiO₂:Al₂O₃) substrates at 580 °C and 600 °C. The growth rate was varied in the range of 9-30 Å/s. The composition (Cd/Te) of the samples was changed by controlling the molecular beam intensity (MBI) ratio. Three samples fabricated at MBI ratios Cd/Te = 0.5, 1.0 and 1.16 were investigated by XRD, AFM, EDX, SEM, photoluminescence (PL) and Hall methods.