氙(Xe)原子在铂(Pt)表面吸附与脱附的分子动力论方法研究

运用分子动力论方法研究单原子气体分子(Xe)在固壁表面(Pt)的吸附与脱附。在构造了合理的气体分子与固壁表面相互作用势模型之后,运用随机经典轨道方法求解气体分子及固壁原子的运动方程,得到气体分子的运动轨迹及状态。结果表明,气体分子在固壁表面的吸附几率与分子的入射平动能量、壁面温度及气-固相互作用势模型的选取等因素有关,气体分子在固壁表面的脱附速率基本满足Arrhenius关系式。     

气体-表面相互作用的分子动力学模拟研究

采用分子动力学模拟方法研究了气体分子Ar在光滑和粗糙Pt表面上的散射规律.提出了一种速度抽样方法,计算了不同温度条件下气体分子对光滑和粗糙表面的切向动量适应系数和吸附概率.结果显示：光滑表面条件下,气体分子的切向动量系数和吸附概率都随着温度的升高而降低;粗糙度对气体分子切向动量与表面的适应具有极大的促进作用,当粗糙度足够大时,切向动量适应系数的大小趋近于1.0,对温度的敏感性也逐渐降低.采用粒子束方法对气体分子在光滑和粗糙表面上的散射规律进行了定量分析.总结了散射过程中气体分子的典型轨迹和动量变化规律,将气体分子在光滑表面的散射分为两种类型：单次碰撞后散射和多次碰撞后散射.单次碰撞后散射的气体分子平均切向动量有所减小,而经过多次碰撞后散射的气体分子则倾向于保持原有的平均切向动量.对于粗糙表面,粗糙度的存在使气体分子与表面间的动量和能量适应更加充分,导致气体分子在较粗糙表面上散射后的平均切向动量大幅减小并接近于0,且气体分子在表面上经历的碰撞次数越多,其散射后的能量损失越严重.     

硫酸钠溶液中方解石表面油膜脱附机理研究

摘要：本文重点研究油湿型方解石表面形成的过程和方解石表面油膜在SO42-作用下脱附的过程,从分子尺度探究油膜形成机理和SO42-作用机理。首先,为模拟碳酸盐岩储层实际而建立油湿型碳酸盐岩表面。范德华力和静电作用下油分子运动到方解石表面形成双层结构的油膜,方解石由水湿变为油湿。结果表明,与第二层相比,第一层排列更有序,结构更稳定,对储层润湿性影响更大。而后,研究方解石表面油膜在SO42-作用下脱附的过程和作用机理。方解石表面油膜的脱附可以分为两步：静电作用下水通道的形成;氢键作用下水膜的形成。最终,方解石表面由油湿变为水湿。研究表明：SO42-参与了方解石表面油膜脱附过程中重要的静电力作用和氢键作用,加速了水通道和水膜的形成,加速了方解石表面油膜的脱附,对储层润湿性影响较大。     

释气对介质沿面闪络击穿影响的粒子模拟

为研究释气下的高功率微波介质沿面闪络击穿物理机制,首先建立了理论模型,包括:动力学方程、粒子模拟算法、次级电子发射、蒙特卡罗碰撞模型以及碰撞退吸附气体分子模型;其次,基于理论模型,编制了1D3V PIC-MCC程序,分别研究了弱退吸附、强退吸附以及释气分子运动速率对沿面闪络击穿的影响.研究结果表明:介质沿面闪络击穿本质是沉积功率的持续增加.弱退吸附下,次级电子倍增占优,随着退吸附系数的增加,碰撞电离效应对次级电子倍增有促进作用,主要表现为介质窗表面静电场、表面碰撞电子平均能量以及表面碰撞电子数目的增加,此处的表面碰撞电子主要是次级电子倍增形成的;释气分子运动速率高导致介质面附近气压下降,不利于次级电子与气体分子间碰撞电离过程形成.强退吸附下,气体碰撞电离效应占优,随着退吸附系数的增加,离子数增加速度表现为电离频率增加的指数增长形式,碰撞电离效应对次级电子倍增有抑制作用,主要表现为介质窗表面静电场为负、表面碰撞电子平均能量的降低,但是表面碰撞电子数目却得以增加,此处的表面碰撞电子主要是贴近介质面的气体碰撞电离形成的;释气分子运动速率高导致气体厚度增加,扩大了气体碰撞电离作用区域,有利于气体碰撞电离.     

分子动力学模拟研究方解石表面油膜脱附机理

本文重点研究油湿型方解石表面的形成过程和方解石表面油膜在SO_4~(2-)作用下脱附的过程,从分子尺度探究油膜形成机理和SO_4~(2-)作用机理.首先,为模拟碳酸盐岩油藏实际而建立油湿型方解石表面.范德华力和静电作用下油分子运动到方解石表面形成双层结构的油膜,方解石表面由水湿变为油湿.结果表明,与第二层相比,第一层排列更有序,结构更稳定,对润湿性影响更大.而后,研究方解石表面油膜在SO_4~(2-)作用下脱附的过程和作用机理.方解石表面油膜的脱附可以分为两步:静电作用下水通道的形成;氢键作用下水膜的形成.最终,方解石表面由油湿变为水湿.研究表明:SO_4~(2-)参与了方解石表面油膜脱附过程中重要的静电力作用和氢键作用,加速了水通道和水膜的形成,加速了方解石表面油膜的脱附,对储层润湿性影响较大.     

球状胶束中表面活性剂脱附的离子调控机制

建立耦合伞状采样的粗粒度分子动力学方法,研究球状胶束中表面活性剂分子的脱附过程,揭示表面活性剂聚集数、盐种类及浓度对表面活性剂脱附过程的影响机制。发现球状胶束半径及偏心率均随聚集数增加而增大,盐浓度的影响主要取决于抗衡离子的半径和吸附特性,半径更大、吸附更强的水杨酸根离子对胶束结构的影响更为显著;基于伞状采样方法获得了表面活性剂脱附自由能、脱附时间等关键参数,发现球状胶束中表面活性剂脱附自由能和脱附时间均随聚集数和盐浓度呈非单调变化,揭示其主要机制为离子吸附引起的静电屏蔽作用;发现自由能在表面活性剂脱附过程中起主导作用,结合胶束热力学理论发展了临界胶束浓度预测方法,获得了临界胶束浓度下胶束尺寸的分布范围。     

真空表面的绝缘特性

绝缘子的真空表面绝缘特性研究真空下的高压沿绝缘子表面击穿物理过程，称真空表面闪络过程。影响该过程的因素主要包括绝缘材料结构、空间电场分布、表面处理方法、所加电压特征，脉冲宽度等。研究了真空表面闪络过程，有两类理论：二次电子发射崩溃和电子引发极化松弛。SEEA理论以绝缘子表面在电子轰击下发射二次电子为基础，包含了电子诱发脱附和脱附气体离子化，并且对闪络过程产生的影响等过程，对表面闪络现象进行了解释。     

密度泛函理论研究纯的及不同掺杂原子石墨烯和CaH_2分子相互作用

本研究采用密度泛函理论研究了纯的及V,Fe,Ni,Pd,Si,P,S和Cl掺杂原子的石墨烯和CaH_2分子之间相互作用.研究结果发现CaH_2分子与所有石墨烯表面均具有较大的相互作用,而CaH_2分子与掺杂石墨烯相互作用都大于与纯石墨烯的相互作用,在所有掺杂原子中,其中与Pd掺杂石墨烯具有最大的相互作用,S次之,其它掺杂石墨烯与CaH_2分子相互作用能力相差不大.这些结果表明虽然所有石墨烯均有助于CaH_2中H原子的脱附,但掺杂石墨烯脱附能力仍然大于纯的石墨烯.在掺杂原子中,Pd和S掺杂石墨烯对CaH_2中H原子的脱附效果最好,其它的掺杂原子脱附效果相差不明显.此研究结果将有望为CaH_2分子在石墨烯基材料中吸氢-脱氢行为提供有用的理论参考价值.     

密度泛函理论研究纯的及不同掺杂原子石墨烯和CaH2分子相互作用

本研究采用密度泛函理论研究了纯的及V, Fe, Ni, Pd, Si, P, S和Cl掺杂原子对石墨烯和CaH2分子之间相互作用. 研究结果发现CaH2分子与所有石墨烯表面均具有较大的相互作用, 而CaH2分子与掺杂石墨烯相互作用都大于与纯石墨烯的相互作用, 在所有掺杂原子中, 其中与Pd掺杂石墨烯具有最大的相互作用, S次之, 其它掺杂石墨烯与CaH2分子相互作用能力相差不大. 这些结果表明虽然所有石墨烯均有助于CaH2中H原子的脱附, 但掺杂石墨烯脱附能力仍然大于纯的石墨烯. 在掺杂原子中, Pd和S掺杂石墨烯对CaH2中H原子的脱附效果最好, 其它的掺杂原子脱附效果相差不明显. 此研究结果将有望为CaH2分子在石墨烯基材料中吸氢-脱氢行为提供有用的理论参考价值.     

TiO2表面氧空位对NO分子吸附的作用

采用程序升温热脱附(TPD)实验方法测定了NO在TiO2表面吸附后的脱附谱，利用分子轨道理论研究了TiO2吸附NO的原子簇模型及吸附前后的原子簇能级变化．结果表明，NO在TiO2表面吸附后可在两个峰值温度450和980K脱附出N2．TiO2表面经预覆氧处理后，N2的脱附量降低．吸附时NO中的O能够占据TiO2表面氧空位并与N脱离，而N原子则相互结合成为N2脱附．分子轨道理论计算证明在TiO2(110)表面能够存在氧空位并具备吸附NO的结构条件．     

Hydrogen isotope replacement reactions on rhenium

The existence of a mobile equilibrium at room temperature between part of the hydrogen adsorbed on rhenium and gaseous hydrogen is demonstrated by the easy exchange of isotopes between the adsorbed layer and the gas phase. The adsorbed gas is desorbed as a mixture of homonuclear molecules (of H₂ or D₂)and of the isotopically mixed species (HD). However, the replacement reactions are not symmetrical; there is a greater proportion of HD in the desorbed gas when deuterium is replaced by hydrogen than in the converse reaction. This kinetic isotope effect is attributed to differences between the zero-point energies of the various hydrogen containing species.Quantatitive agreement between the shapes of the experimentally observed desorption curves and calculated curves is obtained if the zero-point energy of the bond between a surface rhenium atom and deuterium is assigned the value 2.6 kcal mole^?1.     

Concentration-dependent self-diffusion of adsorbates in mesoporous materials

The pulsed-field gradient NMR method has been applied to study self-diffusion of liquids in mesoporous materials with different pore sizes and morphologies as a function of pore loading. It is found that the effective diffusivities of adsorbate molecules in mesopores at partial loadings are related to two mechanisms, the Knudsen diffusion through the gaseous phase in the pore space and the diffusion within the layer of molecules adsorbed on the pore walls. The relative contributions of these modes, which are determined by the details of the interphase equilibrium, change with variation of the pore loading, leading to a complex behavior of the effective self-diffusion coefficient. The impact of the pore size and the adsorbate-surface interaction on self-diffusion is elucidated. Possible reasons for an experimentally obtained hysteresis in the diffusivities measured on adsorption and desorption in mesopores are discussed.     

Kinetics of desorption from adsorbed layers formed by two-dimensional islands and by a two-dimensional gas

The kinetics of desorption from the adsorbed layers formed by two-dimensional (2D) islands and by a 2D gas of single atoms is considered for the case when the flux of adsorbate atoms from the gaseous phase reaches the surface. The effect of this flux on the desorption kinetics is analyzed. It is shown that this effect can be disregarded when the islands are in equilibrium with the surrounding 2D gas. The possibilities of determining the energy required for the removal of atoms from 2D islands to a vacuum are considered for the case of the absence of equilibrium in adsorbed layers. The conditions under which such a determination is possible are formulated, and the corresponding recommendations for experimental studies are worked out.     

Dissociation of carbon monoxide on stepped nickel surfaces

Field emission microscopy with photometric measurements has been applied to investigate the adsorption of carbon monoxide on various crystal planes of a nickel field emitter. Upon heating the CO-covered surface, the work function drastically decreased during desorption of CO molecules into the gas phase and exhibited almost the value of the clean surface at 450 K. However, part of the CO molecules adsorbed on the stepped planes such as (510) and (310) were found to dissociate upon heating at 450–470 K, which was accompanied by an increase of the work function of ～0.2 eV.     

The surface chemistry of ZnO nanoparticles applied as heterogeneous catalysts in methanol synthesis

Zinc oxide has a variety of applications in catalysis both as support and as active phase for hydrogenation or dehydrogenation reactions. This review provides an overview of the surface chemistry of ZnO nanoparticles concerning the interaction with small molecules such as CO, CO₂, H₂, H₂O, and CH₃OH, which are relevant for the catalytic synthesis of methanol and the water gas shift reaction. These interactions were studied by combining surface-sensitive methods such as infrared spectroscopy, temperature-programmed desorption, and adsorption calorimetry. A thorough understanding of the processes occurring on the different exposed facets of the ZnO particles in an atmosphere of reactive gases was achieved based on the comparison with results obtained in ultra-high vacuum with single-crystalline surfaces, i.e. under well-defined conditions, and by using first-principles calculations.     

Structural and reactive properties at the metal-halogen interface: The interaction of bromine with chromium (110)

The chemisorption and subsequent reaction of bromine on Cr(110 has been studied by Auger spectroscopy, LEED, Δφ, and thermal desorption measurements. For gas doses of < 7.5 × 10¹⁸ molecules m^?2, very efficient dissociative chemisorption leads to a series of well-ordered, out-of-registry compression structures. Uniquely, however, the overlayer falls back into registry at saturation coverage; at this point the appearance of glide symmetry indicates that the three-fold coordinated adsorption sites are occupied exclusively. Bromine → metal charge transfer occurs during adsorption (in contrast to Cr(100)). On raising the temperature at low coverages, the surface phase decomposes by evaporation as CrBr molecules; at higher coverages the desorption product switches to CrBr₂. Continuous growth of bulk CrBr₂ sets in at high gas exposures, this corrosion reaction proceeding at a rate which is ten times slower than the rate of overlayer formation. The chromium dibromide layer also evaporates as CrBr₂(g). Structural relationships with related metal-halogen systems are discussed.     

高场强下半导体沿面闪络现象的研究进展

 综述了国内外半导体沿面闪络的研究现状，阐述了电极结构的类型和影响闪络的主要因素；根据自定义的等值电导参数将闪络的发展划分为欧姆电导、局部闪络和贯通闪络3个阶段；通过对表面细丝电流的热过程进行实验和仿真研究，认为表面细丝电流通道的温度可以接近材料熔点，这一结果同时也支持了表面发热导致气体分子解吸附的假设；进一步通过气体解吸附的实验初步确定了存在外部的闪络通道。     

Gaseous Fuel Reactor Research

Gaseous Fuel Nuclear Reactors are externally moderated and contain the fissile material inside a cavity where it is suspended by fluid mechanics forces. The gaseous phase of the nuclear fuel permits operation of the reactor at temperatures much higher than the melting point of all materials. NASA has originally supported relevant research for space propulsion. The continuation of this work includes now research on power generation on Earth for improved economy and environmental acceptability. In reactor experiments with enriched uranium hexafluoride, UF6, a critical mass of 6 kg is determined. Pressurized UF6 remains chemically stable at temperatures up to 2000 kelvins. The interaction of fission fragments with their gaseous environment causes preferential excitation and ionization, leading to non-equilibrium optical radiation. Powerful fluxes of photons are expected to become a superior mechanism of energy extraction from the fissioning gas or plasma in the reactor. The pumping of lasers solely by fission fragments is realized in a variety of lasants. A near term objective of the NASA gaseous fuel reactor program is a benchmark experiment at 100 kw power and at a gas temperature of 1600 kelvins, demonstrating the feasibility of major advances in reactor technology. A concerted research effort is leading to this experiment. A plasma core cavity reactor for high specific impulse propulsion in space reminas a long range goal.     

State selective enhanced production of excited fragments and ionic fragments of gaseous Si(CH3)2Cl2 and solid-state analogs following core-level excitation

State-selective fragmentation dynamics for excited fragments and ionic fragments of gaseous and condensed Si(CH3)2Cl2 following Cl 2p and Si 2p core-level excitations have been characterized. The Cl 2p-->15a1* excitation of Si(CH3)2Cl2 induces significant enhancement of the Cl+ desorption yield in the condensed phase and the Si(CH3)+2 and SiCH+3 yields in the gaseous phase. The core-to-Rydberg excitations at both Si 2p and Cl 2p edges lead to enhanced production of the excited fragments. These complementary results provide deeper insight into the origin of state-selective fragmentation of molecules via core-level excitation.     

A transient TGA study on oxygen permeation properties of perovskite-type ceramic membrane

This paper reports transient weight curves of La_0.2Sr_0.8CoO_3−δ (LSC) membrane (crashed in small grains) subject to an instantaneous change of oxygen activity in the gas phase. When the surrounding gas changes between pure oxygen and helium, the LSC sample experiences a slow weight loss in the oxygen desorption period and a rapid weight gain in the oxygen absorption period due mainly to the difference in the driving force. The LSC sample undergoes phase transformation/chemical reaction with the surrounding gas changing between the pure oxygen and methane. A mathematical model considering surface reaction as the rate-limiting step is presented to describe the oxygen transport through the membrane material, and employed to obtain the surface reaction rate constants. A simple correlation between the oxygen permeation flux and rate constants measured by the transient thermogravimetric analysis (TGA) method is given. The oxygen permeation fluxes calculated from the data measured by the present TGA method are consistent with the data measured by the permeation method. The TGA method is found particularly useful for studying the surface reaction rates of oxygen permeation through thin mixed-conducting ceramic membranes.