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51.
Vibrational spectroscopy and periodic DFT studies of LaMg2PdH7: A material with two types of hydride
Stewart F. Parker Jon W. Taylor Henryk Herman Jean-Philippe Rapin Nicolas Penin Klaus Yvon 《Journal of Alloys and Compounds》2009,470(1-2):80-84
The need for a safe, reliable and cheap method for hydrogen transportation has prompted an intense effort in the synthesis of novel hydrides. To direct the search for better materials, it is essential to understand the bonding present in these systems. A newly synthesized material is LaMg2PdH7 which is best formulated as {La3+}{Mg2+}2{[PdH4]4−}{H−}3. Thus hydrogen is formally present both as a free hydride ion and also as a covalently bound ligand. The combination of inelastic neutron scattering (INS), infrared and Raman spectroscopies and periodic density functional theory (DFT) has been used to characterise the material. We find that the material is less ionic than Ba2[PdH4], which also contains [PdH4]4−. The charges carried by both the coordinating and free hydrides are similar which hints at a degree of covalency in the interactions between the interstitial hydrides and the lanthanum and magnesium ions. 相似文献
52.
为探究微缺陷对五唑阴离子盐冲击响应、化学分解及损伤演化的影响,采用从头算分子动力学模拟方法研究了完美型Mn(N5)2晶体及含有3%空位浓度的Mn(N5)2晶体在不同速度(8,9,10,11,12 km·s-1)c轴冲击波作用下的动力学演化和初始分解反应机理。冲击雨贡纽线的计算结果表明,含空位的Mn(N5)2体系比完美型体系在高压条件下表现出更大的体积压缩比。分子动力学模拟结果显示,当冲击波速vshock<10 km·s-1时,完美型及含有空位的体系在5000 fs内均未出现分解反应,而仅出现了不足10%的体积压缩;当vshock=10 km·s-1时,完美型体系于512.8 fs时刻开始在晶体内均匀地出现N—N键断裂的现象,而含空位体系的初始反应时间则提前至281.6 fs,并且N—N键的断裂集中发生在空位附近;当vshock继续增加至11 km·s-1和12 km·s-1时,两种体系的初始反应时间不断提前,反应进程不断加快,但空位对体系冲击波感度的提升作用和对分解反应进程的加速作用随着冲击波速的提升而不断减弱。研究结果表明空位是热点的早期成核结构之一,空位的存在促进周围的五唑分子发生级联分解,使损伤不断演化成长和传播,进而引起含能材料的点火。 相似文献
53.
目的原核表达并纯化慢性粒细胞白血病(CML)Bcr/Abl基因OD域融合蛋白。方法将TAT、OD和HA基因片段顺次克隆入pET32a(+)原核表达载体,构建重组原核表达质粒pTAT-OD-HA,经PCR、双酶切和测序鉴定正确的重组质粒转化E.coliBL2(lDE3),IPTG诱导表达,表达产物经SDS-PAGE及Westernblot分析后,用镍离子亲和层析柱纯化。结果所构建的重组质粒pTAT-OD-HA经PCR、双酶切及测序鉴定正确;表达的重组蛋白相对分子质量约为30000,诱导6h蛋白表达量达最高,约为10%;Westernblot分析显示,该蛋白可与鼠抗HA单克隆抗体发生特异性反应;纯化后纯度约为95%。结论已成功原核表达并纯化了TAT-OD-HA融合蛋白,为进一步研究其在CML中的作用奠定了基础。 相似文献
54.
Sandra Kurko Igor MilanovićJasmina Grbović Novaković Nenad IvanovićNikola Novaković 《International Journal of Hydrogen Energy》2014
Desorption of hydrogen atoms from the (110) surface of rutile magnesium hydride (MgH2) was investigated using density functional theory (DFT) and pseudopotential method. System was represented by (110) (2×2) slab MgH2 supercell with 12 atomic layers along the z-axis. The H-desorption was modeled by the successive release of the four two-fold bonded H atoms from the (110) surface of MgH2. Dependence of the H-desorption energy on number and configuration of remaining surface hydrogen atoms has been determined. The features of the H atoms diffusion from the bulk towards the surface have been investigated, too. The results suggest that decrease in number of surface H atoms generally lowers the H-desorption energy in each desorption step and that both the H–H and the Mg–H interatomic interactions strongly influence the H-desorption process. The hydrogen vacancy formation energy in the first three sub-surface layers also exhibits a pronounced dependence on concentration. These findings lead to the conclusion that tendency of the MgH2 (110) surface to preserve a maximum possible surface H concentration in its most stable configuration is the limiting factor for the H-desorption kinetics. In principle, the obtained results allow us to determine preferred paths of surface and sub-surface H-diffusion for a wide range of H concentrations and the principle features of the MgH2 dehydrogenation process, at least for the H-rich region. Being rather comprehensive, the approach is applicable for other metal hydrides, as well. 相似文献
55.
Stefano Curtarolo Wahyu Setyawan Renee D. Diehl 《Israel journal of chemistry》2011,51(11-12):1304-1313
To commemorate the awarding of the Nobel Prize for Chemistry to Daniel Shechtman for his discovery of quasicrystals, this paper reviews our recent studies of the interaction of rare gases and hydrocarbon gases with the tenfold surface of quasicrystalline decagonal Al-Co-Ni. 相似文献
56.
MgH2 is one of the most promising materials for hydrogen storage. However, its rather slow hydrogen absorption and desorption kinetics and high dissociation temperature essentially limit its application in this field. Nevertheless mixing Mg or MgH2 with small amount of transition metals or their oxides remarkably accelerates the hydrogen kinetics. Recently a series of new hydrides Mg7TiHx, Mg6.5NbHx and Mg6VHx of Ca7Ge type structure has been synthesized. The hydrogen desorption properties have been found to be better than for pure MgH2. Here, we report on the results of our theoretical study of the electronic structure of these new hydrides carried out within the framework of the full-potential, self-consistent linearized augmented plane-wave method. We use these results, along with calculations of the heat of formation and relative stability, to discuss the bonding of these materials and their hydrogen-storage properties. 相似文献
57.
AbstractNb-silicide based alloys could be used at T > 1423 K in future aero-engines. Titanium is an important additive to these new alloys where it improves oxidation, fracture toughness and reduces density. The microstructures of the new alloys consist of an Nb solid solution, and silicides and other intermetallics can be present. Three Nb5Si3 polymorphs are known, namely αNb5Si3 (tI32 Cr5B3-type, D8l), βNb5Si3 (tI32 W5Si3-type, D8m) and γNb5Si3 (hP16 Mn5Si3-type, D88). In these 5–3 silicides Nb atoms can be substituted by Ti atoms. The type of stable Nb5Si3 depends on temperature and concentration of Ti addition and is important for the stability and properties of the alloys. The effect of increasing concentration of Ti on the transition temperature between the polymorphs has not been studied. In this work first-principles calculations were used to predict the stability and physical properties of the various Nb5Si3 silicides alloyed with Ti. Temperature-dependent enthalpies of formation were computed, and the transition temperature between the low (α) and high (β) temperature polymorphs of Nb5Si3 was found to decrease significantly with increasing Ti content. The γNb5Si3 was found to be stable only at high Ti concentrations, above approximately 50 at. % Ti. Calculation of physical properties and the Cauchy pressures, Pugh’s index of ductility and Poisson ratio showed that as the Ti content increased, the bulk moduli of all silicides decreased, while the shear and elastic moduli and the Debye temperature increased for the αNb5Si3 and γNb5Si3 and decreased for βNb5Si3. With the addition of Ti the αNb5Si3 and γNb5Si3 became less ductile, whereas the βNb5Si3 became more ductile. When Ti was added in the αNb5Si3 and βNb5Si3 the linear thermal expansion coefficients of the silicides decreased, but the anisotropy of coefficient of thermal expansion did not change significantly. 相似文献
58.
Carlos Sabater María José Caturla Juan José Palacios Carlos Untiedt 《Nanoscale research letters》2013,8(1):257
We have studied experimentally jump-to-contact (JC) and jump-out-of-contact (JOC) phenomena in gold electrodes. JC can be observed at first contact when two metals approach each other, while JOC occurs in the last contact before breaking. When the indentation depth between the electrodes is limited to a certain value of conductance, a highly reproducible behaviour in the evolution of the conductance can be obtained for hundreds of cycles of formation and rupture. Molecular dynamics simulations of this process show how the two metallic electrodes are shaped into tips of a well-defined crystallographic structure formed through a mechanical annealing mechanism. We report a detailed analysis of the atomic configurations obtained before contact and rupture of these stable structures and obtained their conductance using first-principles quantum transport calculations. These results help us understand the values of conductance obtained experimentally in the JC and JOC phenomena and improve our understanding of atomic-sized contacts and the evolution of their structural characteristics. 相似文献
59.
Mechanisms for formation,chlorination, dechlorination and destruction of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) 总被引:1,自引:0,他引:1
Mohammednoor Altarawneh Bogdan Z. DlugogorskiEric M. Kennedy John C. Mackie 《Progress in Energy and Combustion Science》2009
Polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) constitute a group of persistent organic pollutants that form almost inexorably in all thermal and combustion operations. This review focuses on mechanisms that govern their formation, chlorination, dechlorination and destruction. As a consequence of their extreme toxicity and propensity to bioaccumulate, PCDD/Fs have been subjected to much scientific research, designed to understand mechanisms and conditions that govern their emission rates and congener distribution (fingerprints). Consensus of opinions in the literature points to heterogeneous pathways contributing substantially more in the total yield of PCDD/Fs in combustion systems than the gas phase pathway. However, in our view, a great complexity of both homogeneous and heterogeneous routes and uncertainties in many thermochemical and rate parameters enable no conclusive statement about the contribution of each route. Chlorination patterns of precursors appear to play a major role in final congener profiles of PCDD/F emissions. According to the most recent theoretical studies, these congener profiles seem consistent with thermodynamic stabilities of dioxins and furans produced in thermal processes, however, further theoretical investigations at more accurate levels are needed to clarify this matter further. Theoretical studies along with experimental findings reveal that the PCDD/PCDF ratio remains very sensitive to the operating conditions, with pyrolytic conditions favouring the formation of PCDFs. A number of reaction mechanisms has been proposed to answer many of the most intriguing questions about the formation of PCDD/Fs. These mechanisms include models of gaseous and heterogeneous reactions, often inferred from theoretical quantum chemical calculations studies, which incorporate steps responsible for formation, chlorination, dechlorination and destruction of dioxins and furans. The review identifies gaps in our present understanding of the reaction mechanisms and suggests that further progress in the field needs to be facilitated by development of reliable mechanistic models for (i) catalytic pathways, (ii) chlorination/dechlorination reactions including the Deacon reaction and the direct transfer of chlorine from solid surfaces into the aromatic moieties, and (iii) formation of PCDD/Fs from precursors other than chlorophenols, especially pesticides. 相似文献
60.
N. Novakovi N. Ivanovi V. Koteski I. Radisavljevi J. Beloevi-avor B. Ceki 《Intermetallics》2006,14(12):1403-1410
In order to investigate Hf-TM (TM = Fe, Co, Rh, Ru) phase diagrams in the region of 50:50% atomic ratio, we performed ab initio Full-Potential Linearized Augmented Plane Waves calculations of the most stable Hf and TM elemental phases and HfTM compounds of the CsCl and CuAu structure types. The obtained electronic structures, cohesive energies and enthalpies of formation are discussed and compared to some of the existing models and available experimental data. The non-existing compound HfFe is found to be at least metastable, and the reason for its absence from the phase diagram is discussed. 相似文献