Oleksiy V. Khavryuchenko
,
Volodymyr D. Khavryuchenko
,
苏党生
催化学报
doi:10.1016/S1872-2067(15)60948-2
当初始态和过渡态之间存在能量差时，通常会考虑用催化的方法来增加反应速率，即经由能量更低过渡态的反应路径，或增加反应物初始态能量，如改变溶剂或在特殊表面吸附分子。然而，对于某些类型的反应，如H2和O2反应生成水， NO的氧化等，仅仅采用这些方法还不够，因为反应物和产物具有不同的自旋态，而自旋守恒定律一般会禁止该过程的发生。即使克服了能垒，该体系也不能转化成稳定的产物，且不改变其电子自旋多重态。但在特殊的非热力学条件下，该类反应仍可发生。这通常指自旋不守恒的过程。改变体系电子自旋最直接的方法是与原子核自旋有作用的电子自旋翻转。它可由自旋-轨道耦合来实现，因此，其实现的几率取决于元素的原子数。从3d金属开始，发生这种情况的可能性甚至更大。这是由于与d轨道耦合的效率更高所致，所以采用自旋交叉就能方便地解释自旋禁止反应发生的原因。然而，轻原子发生自旋翻转的可能性很低，因此，当面对无金属化学时，我们应该考虑自旋禁止反应发生的第二个方法—自旋催化。通过帮助克服自旋禁止的物质以促进化学反应，或通过顺磁催化剂诱导自旋解耦以降低活化能垒，这类现象就可定义为自旋催化。简而言之，为了得到想要的和热力学更有利的自旋态，一个反应粒子可以与自旋催化剂交换磁矩；该自旋催化剂需有拟自旋-简并基态或低激发态。因此，体系整个的自旋仍守恒，但可发生反应。值得强调的是，自旋“催化剂”指的是分子状态和固体状态，通常后者应用于工业中则方便得多。自旋催化是作为自旋化学的范例来研究的，或在外磁场中化学诱导动态核极化和反应，主要是指采用EPR光谱研究以液相为主的过程。本文简略介绍了自旋催化的理论。实际上，自旋催化与常规催化之间并不相互抵触—相同的材料既可作为自旋催化剂，也可用作常规催化剂，都起着降低反应能垒和提供自旋交换的自由基中心的作用。另外，许多现代材料包含着常规催化剂和自旋催化剂的特性。有些发生在多核磁性金属中心上的过程可能本身就包含着常规催化(活化底物)、自旋交叉(与d金属相互作用)和自旋催化(金属-氧簇合物中低能的铁磁-反铁磁转化)效应。气相自旋催化在实际应用中产生的主要问题是在稀释的体系中三粒子碰撞几率低，常规催化在这方面则有很多优势。我们首先列出固体成为自旋催化剂的条件：(1)活性中心中至少2个自旋态的能量差要小于反应温度下热运动的能量，这样无需额外的能量活化催化剂；(2)较大的比表面积以提供更多的自旋中心用于相互作用；(3)最好能够自旋导电，在此情况下反应无需三粒子碰撞即可发生。这是因为自由基可通过催化剂的电子系统交换自旋，使得自旋-自旋相互作用的几率迅速增加，从而有利于自旋催化反应的进行。另外，如果固体或载体具有自旋导电性，则无需外加电场或磁场，即有可能遥控催化中心的电荷和自旋态，从而避免使用外电场或磁场。实际上，许多反应过程本身就包含着自旋催化，它还可使反应过程的许多不足变成优势，如对于不需要的链式自由基反应；通过引入自旋捕获剂使得自旋禁止反应成为有用的自旋催化；磁性粒子的浓度以及外加磁场和电场均可导致自旋催化/禁止反应。由于均相自旋催化的研究较早，在此不再赘述。本文详细介绍了多相体系中的自旋催化的应用，如燃烧、温和氧化、环化、开环、非极性小分子的活化和不稳定自由基的耦合等。可能用作自旋催化剂的材料有：含有不同顺磁性的金属离子的磁性氧化物或多核磁性簇合物；嫁接在一些载体上的多核络合物或过渡金属(低温催化)；带有非整数氧化价态的导电金属；各种具有半金属性和导电性的碳材料。研究自旋催化反应的一个突出问题就是这些过程大部分是自由基式的，趋于非线性区域，因而很难预测，也没有一个便利的工具用以描述，甚至预测自旋催化反应，但也许这些研究的特点就是不可预测性。自旋催化除了可调变反应的进行，我们还可从中获得一个独特的调节手段：通过外加磁场或电场触发基元过程，在非热力学上控制反应的进行。该手段在控制化学反应方面具有明显优势，因为现在大都是通过调节温度、压力和流量等参数来实现对化学反应的控制，且滞后严重。同时，通过添加外加磁场可立刻改变反应速率。另外，通过降温可精细控制基元反应，从而开辟了一种抑制副反应的方法，因而也使得反应的随机性降低。
关键词:
催化
,
自旋化学
,
化学键均裂
,
氧化过程
,
自由基耦合
Acta Chimica Sinica
Passive film formed on 304 stainless steel in 2.5 mol/L H2SO4 solution by using alternating voltage (A. V.) passivation have been investigated by measuring capacitance and photoelectrochemical parameters. The investigation of the effect of measure frequency on the slope of Mott-Schottky curve has been carried out. The photoelectrochemical measurements were consistent with the capacitance measurement. Analysis of the experimental results showed that the passive film formed 304 stainless steel by using A. V. passivation exhibits semiconducting properties. Using the simple model of semiconductor rather than the multi-donor level model can explain satisfactorily the semiconducting behavior of the film formed on 304 stainless steel by using A.V. passivation.
关键词:
A. V. passivation;passive film;semiconducting properties;stainless;steel;model;electrodes
Physics Letters A
We perform first-principles calculations of spin-dependent quantum transport in V doped boron nitride nanotube: the junction of pristine (6, 0) boron nitride nanotube in contact with V doped (6, 0) boron nitride nanotube electrodes. Large tunnel magnetoresistance and perfect spin filtration effect are obtained. The zero bias tunnel magnetoresistance is found to be several thousand percent, it reduces monotonically to zero with a voltage scale of about 0.65 V, and eventually goes to negative values after the bias of 0.65 V. The ratio of spin injection is above 95% till the bias of 0.85 V and is even as large as 99% for the bias from 0.25 eV to 0.55 eV when the magnetic configurations of two electrodes are parallel. The understanding of the spin-dependent nonequilibrium transport is presented by investigating microscopic details of the transmission coefficients. (C) 2008 Elsevier B.V. All rights reserved.
关键词:
first-principles;one-dimensional;tunnel magnetoresistance;spin filter;large magnetoresistance;ferromagnetic-films;sic nanowires;bn;nanotubes;systems;conductance;junctions
Journal of Physics-Condensed Matter
The segregation effects of Nb and V on bcc FeSigma3[110](111) grain boundary cohesion are studied by the first-principles DMol method within the framework of density functional theory. The calculated segregation energy difference between the grain boundary and the corresponding free surface is -0.51 eV (-0.58 eV) for solute Nb (V), which indicates that both Nb and V could enhance the grain boundary cohesion in body-centred-cubic Fe. We found that the chemical effect and the geometry effect of Nb (V) play crucial but opposite roles in determining whether a material is brittle or ductile. The chemical effect is dominant and advantageous for grain boundary cohesion. Also, Nb and V show very different behaviours: in chemical effect, Nb is more conducive to ductility than V; while in the geometry effect, Nb is less conducive to ductility and more conducive to embrittlement than V.
关键词:
first-principles determination;local-density;embrittlement;iron;phosphorus;hydrogen
刘贵仲
,
苏彦庆
,
郭景杰
,
丁宏升
,
贾均
,
傅恒志
材料科学与工艺
doi:10.3969/j.issn.1005-0299.2001.02.015
在基于本课题组建立的熔体组元活度系数计算模型的基础上,计算了Ti-6Al-4V熔体中各组元的活度系数及其蒸气压,推导了判断熔体组元挥发趋势的相对挥发系数β.研究结果表明:在熔体温度低于2 000 K时,铝钛相对挥发系数β(Al:Ti)＞17,铝钒相对挥发系数β(Al:V)＞96,钛钒相对挥发系数β(Ti:V)＞5,即熔炼过程中组元挥发趋势从大到小依次为Al,Ti,V.这说明熔体成分变化主要是由于Al元素的挥发所致.
关键词:
Ti-6Al-4v合金
,
活度系数
,
饱和蒸气压
,
挥发趋势
Thin Solid Films
Single and double layers of tungsten oxide and vanadium oxide thin films have been deposited on In2O3:SnO2 (ITO) glass using the pulsed laser deposition (PLD) technique at a substrate heating temperature of 200 degreesC and in rich oxygen pressure. The structural properties and chemical composition of as-deposited WO3(V) and V2O5(Ti) thin films were analyzed using X-ray diffraction (XRD), Raman spectrum (RS), and X-ray photo-electron spectroscopy (XPS). The as-deposited V2O5 doped WO3 thin film on ITO glass showed an amorphous structure. Highly oriented growth nano-crystalline TiO2 doped V2O5 thin films with polycrystalline orthorhombic structure were successfully synthesized on ITO glass using the scanning laser ablation technique at a deposition temperature as low as 200 degreesC. Cyclic voltammograms, at a sweep rate of 50 mV s(-1), show that long-term degradation was noticed for the as-deposited WO3(V) thin film, and no Ion-term degradation was noticed for the as-deposited double layer WO3(V)/V2O5(Ti) thin film. Durability was verified to 8000 cycles in the voltage range between -1.0 and 1.0 V. It is demonstrated that these double layer WO3(V)/V2O5(Ti) thin films on ITO are good candidates for cathode thin films for rechargeable batteries and electrochromic devices. As the additional top layer of V2O5M) possesses nano-crystalline and c-axis oriented structure, which is suitable for Li-ions transport, the cycle stability and reversibility of WO3(V) films have been improved. Double layer WO3(V)/V2O5(Ti) thin films provide a neutral brownish blue electrochromic color. Therefore, this double layer film maybe suitable for most building applications. (C) 2001 Elsevier Science B.V. All rights reserved.
关键词:
V2O5(Ti);WO3(V) thin films;double layers;pulsed laser deposition;technique;electrochromic properties;chemical-vapor-deposition;vanadium-titanium-oxide;electrical-properties;optical-properties;induced coloration;v2o5;wo3;growth;no2
葛鹏
,
吴欢
,
赵永庆
中国材料进展
doi:10.7502/j.issn.1674-3962.2016.10.06
为了研究钛合金中常用β合金元素Mo、V、Cr等对β相合金化的影响,通过设计制备一系列不同元素含量的Ti-Mo、Ti-V、Ti-Cr二元β型钛合金,分析了含单一合金化元素的β型钛合金的显微组织、硬度及室温拉伸性能.结果发现:随着合金中β稳定元素含量的增加,所有二元合金的晶格常数及晶粒尺寸都呈减小的趋势,其中元素Cr具有最强烈的减小晶格常数效应,而元素Mo对晶粒细化的效果最显著.成分为Ti-xV、Ti-xCr的β相的拉伸强度随元素含量增加的变化规律与硬度变化相似,先减小后增加.Ti-xMo拉伸强度则随元素含量的增加强度升高.元素Cr对β相的强化效果高于Mo和V.
关键词:
钛合金
,
合金元素
,
β相
,
强化
