除草剂苄嘧磺隆在针铁矿表面吸附的红外光谱研究

文章用FTIR光谱,探讨了热带、亚热带地区水稻土中普遍存在的针铁矿对近年来广泛使用于水稻田的磺酰脲类除草剂苄嘧磺隆的吸附机理。结果表明:(1)在针铁矿对苄嘧磺隆的吸附过程中,游离羟基(A型)及C型羟基均没有明显作用。(2)针铁矿表面的水合羟基峰在吸附前后的图谱中由3158移到3129cm~(-1),缔合程度增高,表明在该吸附过程中针铁矿表面的水合羟基峰起主要作用。(3)在吸附后的针铁矿图谱中出现了波数增高了的苄嘧磺隆的羰基及磺酰基弱峰。其波数分别由1712和1161cm~(-1)增至1789和1164cm~(-1),由此判断,苄嘧磺隆被针铁矿表面吸附是由该农药分子中的羰基和磺酰基起主要作用。(4)由以上结果推断:苄嘧磺隆在针铁矿表面的吸附是苄嘧磺隆分子中的羰基和磺酰基与针铁矿表面的水合羟基形成了电荷-偶极键以氢键结合的吸附。     

氮化铀热中子截面的第一性原理计算

氮化铀（UN）因其较好的热物性和耐事故容错性成为先进动力堆的候选燃料,但目前热能区缺少可靠的UN热中子截面数据,这对于热中子反应堆物理计算是很不利的.本文基于量子力学的第一性原理,利用VASP/PHONON软件模拟计算了UN的声子态密度,以此为积分得到UN的定容比热容,并基于新制作的声子态密度,采用核截面处理程序NJOY/LEAPR,利用热中子散射理论,得到UN的S（α,β）数据,进而研究UN的热中子散射截面,并与传统压水堆的二氧化铀（UO₂）进行对比.结果表明：优化的晶格参数与数据库符合较好,UN声子态密度的声子项和光子项较UO₂的分隔更加明显,定容比热容计算结果与实验值一致,基于该声子态密度计算得到的UN中²³⁸U的非弹性散射和弹性散射截面比相同温度下UO₂中²³⁸U小,UN中N仅考虑了非相干散射部分,随着温度升高,UN弹性散射截面变小,非弹性散射变大,并在高能段趋于自由核散射截面.本文的研究结果填补了UN热中子截面数据的缺失,为下一步系统研究UN燃料在轻水堆中的中子学性能奠定了基础.     

铀氢化反应过程中氢扩散行为的第一性原理研究

铀氢化反应动力学及产物形式受氢在铀氧化物及氢化物中的固溶、扩散等过程控制.本文利用第一性原理计算方法,研究氢在UO_2、α-UH_3、β-UH_3、PdH_x等介质中的固溶、扩散行为,从微观角度解释氢化反应各个阶段氢化反应速率的主要特征,并对α/β相生成的条件作一番探讨.研究表明,UO_2中较高的氢扩散势垒导致氢扩散到达氧化物-金属界面非常困难,这是氢化反应起始阶段存在孕育期的主要原因;与此相反,β-UH_3的氢具有非常低的溶解能和扩散势垒,且实际状态下β-UH_3的表面积非常大,对氢化反应具有显著的促进作用;PdH_x具有和α-UH_3相近且均略高于β-UH_3的溶解能和扩散势垒,这使得氢在其中的扩散速率较低,是Pd膜覆盖下铀氢化主要产物为α-UH_3的主要原因.     

La3Co29-xFexSi4B10的择优占位、电子结构和晶格振动性质的理论研究

分别用第一性原理及原子间相互作用势对初始模型进行几何优化, 所得稳定结构的晶格参数均与实验值符合较好. 通过第一性原理密度泛函理论, 计算了稳定结构La₃Co_29-xFe_xSi₄B₁₀化合物择优占位情况, 计算结果表明, Fe原子最择优替代Co 原子的2c晶位, 择优占位顺序为2c > 8j1 > 8i2 > 8j2 > 8i3 > 16k > 8i1, 这与实验结果非常符合. Fe原子每次只替代不同晶位的一个Co原子时, La₃Co_29-xFe_xSi₄B₁₀ 体系的晶格常数几乎不变, 磁矩却发生了有趣的变化. 当Fe原子沿着择优顺序依次替代不同晶位的所有Co原子时, 随着La₃Co_29-xFe_xSi₄B₁₀体系中Fe原子含量的增多, 其电子态密度整体向左移动. Fe原子完全替代Co时, 与未掺杂时相比体系的总磁矩增加. 最后, 利用原子间相互作用势进一步预测了La₃Co_29-xFe_xSi₄B₁₀体系的晶格振动及热力学性质. 在低频部分, 振动模式主要由质量较大的Co, Fe和La元素作贡献; 随着掺杂原子Fe的增多, 体系的截止频率先减小后增多, 中频部分由Si元素引起的振动模式减少; B-B强相互作用引起了高频部分的振动模式. 基于声子态密度预测了不同数量Fe掺杂后体系的比热、熵和德拜温度的变化, 当Fe 含量大于Co时, 德拜温度明显升高.     

铀高压状态方程的第一原理研究

利用基于密度泛函理论的赝势平面波方法,研究了铀的状态方程。基态计算结果表明：在100GPa的压强范围内,α-U结构是最稳定的相。利用量子分子动力学方法(QMD),计算了有限温度下的状态方程,在此基础上计算了冲击Hugoniot点,并与实验数据及其他理论计算结果进行了比较,结果表明：在100GPa以下的压力区域内,QMD计算结果与实验结果符合得很好,而用快速计算状态方程(QEOS)方法计算的结果偏软。

     

由29Si NMR谱看高岭土与针铁矿的胶结本质

通过对高岭土与针铁矿的人工胶结样的²⁹Si NMR谱的分析,认为氢键生成是胶结本质.     

由27Al的核磁共振谱看高岭土与针铁矿的胶结本质

通过对高岭土与针铁矿的人工胶结样的²⁷ Al核磁田共振谱的分析,认为氢键生成是胶结本质。     

铀的弹性、能带结构和光学常数的第一性原理计算

采用密度泛函理论,赝势平面波方法计算了金属铀a相的晶体结构,弹性常数,体模量,电子能带结构和光学常数(折射率n和消光系数k)等.其中,铀的晶格参数,弹性常数和体模量等与实验及其它第一性原理计算结果十分吻合.计算得到了铀的光学常数,与实验结果作了对比并进行了分析说明.     

由￣（27）Al的核磁共振谱看高岭土与针铁矿的胶结本质

通过对高岭土与针铁矿的人工胶结样的￣（２７）Ａｌ核磁田共振谱的分析，认为氢键生成是胶结本质。     

铀高压状态方程的第一原理研究北大核心CSCD

利用基于密度泛函理论的赝势平面波方法,研究了铀的状态方程。基态计算结果表明:在100GPa的压强范围内,α-U结构是最稳定的相。利用量子分子动力学方法(QMD),计算了有限温度下的状态方程,在此基础上计算了冲击Hugoniot点,并与实验数据及其他理论计算结果进行了比较,结果表明:在100GPa以下的压力区域内,QMD计算结果与实验结果符合得很好,而用快速计算状态方程(QEOS)方法计算的结果偏软。     

Alloying effects and site occupancies of Re in the C14 Cr-based Laves phases: a first-principles study

First-principles calculations based upon the density functional theory have been carried out to investigate the alloying effects and site occupancies of Re in the C14 XCr₂ (X?=?Nb, Ta, Ti, Zr, Hf) Laves phases. The calculated results indicate that Re tends to facilitate and participate in the formation of the Laves phases, generating X₈Cr₁₅Re accordingly. The partial density of states and charge density di?erence were analysed to re?ect the bonding characteristics. For X₈Cr₁₅Re, the bonding between the doped Re and its nearest neighbour Cr atoms all show covalent characteristics, which contribute to the phase stability. The substitution of Re on X sites is energetically unfavourable due to the weak bonding between Re and its nearest neighbour X atoms.     

荧光粉中激活剂离子掺杂格位分析

荧光粉的发光特性由激活剂离子的电子构型决定,同时也受激活剂离子在基质中的配位环境的影响.配位环境对激活剂离子发光性能的影响主要体现在电子云膨胀效应和晶体场劈裂效应这两个方面.电子云膨胀效应的强弱取决于激活剂离子与配体离子的成键特性(离子键与共价键成分的比例)以及阴离子极化率的大小;而晶体场劈裂效应的大小取决于激活剂离子...     

On the expected number of distinct points in a subset visited by anN-step random walk

Many investigators have calculated asymptotically valid expressions for the expected number of distinct points visited by ann-step random walk on a lattice. In this note we point out that the same formalism can be used to study the expected number of distinct points in a subset of lattice points. We also calculate the expected occupancy of the subset and give sufficient conditions for the ratio of the two calculated quantities to have the same asymptotic time dependence as for the full lattice. Specific examples are considered.     

Electronic structure and magnetic and optical properties of double perovskite Bi2 FeCrO6 from first-principles investigation

Double perovskite Bi2 FeCrO6 , related with multiferroic BiFeO3 , is very interesting because of its strong ferroelectricity and high magnetic Curie temperature beyond room temperature. We investigate its electronic structure and magnetic and optical properties by using a full-potential density-functional method. Our optimization shows that it is a robust ferrimagnetic semiconductor. This nonmetallic phase is formed due to crystal field splitting and spin exchange splitting, in contrast to previous studies. Spin exchange constants and optical properties are calculated. Our Monte Carlo magnetic Curie temperature is 450 K, much higher than any previously calculated value and consistent with experimental results. Our study and analysis reveal that the main magnetic mechanism is an antiferromagnetic superexchange between Fe and Cr over the intermediate O atom. These results are useful in understanding such perovskite materials and exploring their potential applications.     

Electronic structures and magnetocrystalline anisotropy energies of ordered Co1-xNix alloys: a first principles study

The electronic structures and magnetocrystalline anisotropy(MA) of ordered hexagonal close-packed(hcp) Co1-xNix alloys are studied using the full-potential linear-augmented-plane-wave(FLAPW) method with generalized gradient approximation(GGA).Great changes of magnetocrystalline anisotropy energy(MAE) are gained with different Ni compositions.Also,in-plane magnetocrystalline anisotropy is obtained for Co 15 Ni in which the Snoek’s limit is exceeded.It is found that the changes of the symmetry of the crystal field on Ni induce small variations in band structures around the Fermi level under different compositions,which plays an important role in modulating the magnetization direction,where the hybridization between Co-3d and Ni-3d orbits is of special importance in deciding the magnetocrystalline anisotropy of itinerant states.The rigid-band model is inapplicable to explain the evolution of magnetocrystalline anisotropy energy with Ni composition,and it is also inadequate to predict the magnetocrystalline anisotropy energy through the anisotropy of the orbital magnetic moment.     

杂质对Nb2GeC性质影响的第一性原理研究

为进一步研究Nb₂GeC在辐照环境中的稳定性, 本文研究了O, H和He杂质在Nb₂GeC中的稳定情况. 所有杂质的研究都是从替代和间隙两个方面来进行的, 计算得到了替代和间隙的形成能, 存在替代和间隙时Nb₂GeC的晶格常数, 以及单胞体积, 并且与完美的晶胞进行了比较. 此外, 通过电荷密度分布和Mulliken 布居, 分析了O, H, He杂质对Nb₂GeC 的电子性质的影响.     

氮掺杂(1120) ZnO 薄膜磁性质研究

采用基于密度泛函理论(DFT)和局域密度近似(LDA)的第一性原理分析了氮掺杂(1120) ZnO 薄膜的磁性质.首先,研究了一个N原子掺杂ZnO薄膜的磁性质,结果表明N 2p,O 2p和Zn 3d 发生自发自旋极化.其次,研究了二个N原子掺杂ZnO薄膜的磁性质,9个不同几何结构的计算结果表明N原子之间具有FM耦合稳定性,而且具体分析了N掺杂ZnO铁磁稳定性的产生原因.最后,讨论了氮关键词：第一性原理半导体铁磁性     

Site occupancy behaviours of ternary elements (Zr,Mo, Cr) in the Laves phase of C15 NbCo2: a first-principles study

Abstract

First-principles method based upon the density functional theory has been carried out to investigate the site occupancy behaviours of typical ternary elements Zr, Cr and Mo in the C15 NbCo₂ Laves phase. The calculated equilibrium lattice constant of the Nb₈Co₁₆ unit cell is in good agreement with the previous experimental results. The calculated results indicate that all the considered ternary elements show the preference to occupy the Co sites, and the site preference of these elements in the Laves phase is in the order of Cr, Mo and Zr. Mo and Cr atoms tend to participate in the formation of the Laves phase, generating Nb₇MoCo₁₆, Nb₈Co₁₅Mo and Nb₈Co₁₅Cr, respectively. Zr atoms are not prone to concentrate in the Laves phase. The partial density of states was also calculated to investigate how atoms interact, and the charge density difference is analysed to reflect the bonding characteristics.