Cohesive energy and physical properties of nanocrystals

Recently, a lattice-type-sensitive model, free of any adjustable parameter, for the size dependence of the cohesive energy of nanocrystals (nanodisks, -films, -wires and -particles) has been developed, taking into account the effects of the averaged structural and energetic properties of their surface and volume. These effects are related to the first- and second-nearest-neighbor atomic interactions. Now, considering the intimate relation between cohesive energy and other physical properties of materials, the recently obtained formula for the cohesive energy of nanocrystals has been applied to the cases of melting point (In, Bi, Si and Ag), evaporation temperature (Ag and Au), vacancy formation energy (Au), diffusion activation energy (Au), surface energy (Au, Al and Na), liquid–vapor interfacial energy (Al and Na), Curie temperature (Pb), Debye temperature (Au and Fe) and band gap energy (Si) of nanocrystals. In general, good agreement between the present model and the data has been obtained. Moreover, the surface-area-difference (SAD) model has been derived as a first-order approximation of the present model.     

Hydrogen chemisorption on Al,Mg and Na surfaces — calculation of adsorption sites and binding energies

Chemisorption properties for H on Al, Mg and Na surfaces have been computed. Starting with the results from a self-consistent, Kohn-Sham calculation for a H-jellium system, the effects of the substrate pseudopotential lattice have been included by first order perturbation theory. An improved method, namely inclusion of part of the pseudopotentials in a self-consistent way and only the remainder in perturbation theory, has also been used. Binding energies, bond lengths, vibrational frequencies and activation energies have been calculated. It is found that the three substrates behave qualitatively different, as Al is a segregator, Na is an absorber and Mg is in between. Calculated bond lengths agree in general well with the ones for the corresponding molecules. The binding energy is predicted to be below or close to the dissociation energy of H₂ for Al and Na, while it is slightly larger for Mg.     

A型沸石中Lwenstein规则的能学研究

通过模拟退火方法,使用协合分子力学场对Ｓｉ,Ａｌ分布分别为４：０序列,两种３：１序和随机分布的ＮａＡ型沸石结构进行了能量最小化计算,获得了不同结构的位能及其生成热大小,计算结果表明,４：０序结构的位能和生成热在所讨论的几种序结构中最低,从而在理论上证实了Ｌｏｅｗｅｎｓｔｅｉｎ规则是分子筛结构中能量最小化的自然结果。     

An Auger and X-ray photoelectron spectroscopic study of sodium metal and sodium oxide

Photoelectron and Auger electron measurements have been made on polycrystalline films of sodium metal evaporated in ultra high vacuum, and on Na₂O produced by in-situ oxidation by dry oxygen. Most of the spectra were recorded using Mg Kα (1254 eV) radiation but excitation by 5 keV electrons or monochromatized Al Kα (1487 eV) X-rays was used for specific purposes. Core and valence electron binding energies, photoionization cross-sections relative to Na 1s, KLL and KLV Auger energies and transition probabilities are reported. Energy losses in the metal and oxide are discussed and the relative intensities of surface and bulk plasmon losses have been used to calculate mean electron escape depths in the metal. When corrections were made for experimental geometry, escape depths of 10 Å at 180 eV and 31 Å at 1200 eV were obtained. An escape depth of 23 Å at 980 eV was obtained by Na 1s-Na K-Auger intensity correlation and this is consistent with the plasmon data. Data on Auger satellite lines are presented and, in particular, evidence has been obtained which indicates that a high energy satellite should not be attributed to a plasmon gain mechanism. Valence band influences on the KLV Auger spectra are discussed with reference to the XPS spectrum and other sources of valence band information. Unexpected structure was found in the KLV spectra of the metal which, pending thorough interpretation, offsets the sensitivity and resolution advantages which these spectra otherwise offer for valence band studies.     

半Heusler型Na_(1-x)Cs_xAlGe(0≤x≤1)合金能带反转结构的研究

采用第一性原理的计算方法,研究了全部由轻元素构成的半Heusler型NaAlGe合金中,掺杂元素对合金拓扑能带结构的影响.发现利用同族的Cs元素掺杂替换Na元素,能够诱导Na1-x Csx AlGe合金的能带结构由原本正常带序(0 x0.125)转换为反转带序(0.125 x 1).基于对这一现象的深入讨论,我们提出在几乎没有自旋-轨道耦合作用的材料中,掺杂元素(Cs)是通过其离子半径的不同,进而影响晶格参数变化导致另外两种近邻原子间杂化作用发生变化,来诱导拓扑反带结构形成的.     

Zr催化剂对NaAlH4和Na3AlH6可逆储氢性能的影响

采用基于密度泛函理论的平面波赝势方法,分别计算纯净的以及掺杂Zr的NaAlH₄和Na₃AlH₆的晶格结构常数、能量、电子局域函数和电子态密度.结果表明：NaAlH₄和Na₃AlH₆分别是带隙为46和31 eV的绝缘体;NaAlH₄和Na₃AlH₆中Al—H键是共价键,Na—H键是离子键;Zr原子替代Na原子 关键词： 储氢 4')" href="#">NaAlH₄ 3AlH₆')" href="#">Na₃AlH₆ Zr掺杂     

Electronic structure of Al2O3 from electron energy loss spectroscopy

The electronic structure of Al₂O₃ has been studied by electron energy loss spectroscopy (ELS), and an energy level model of both filled and empty states has been constructed from the ELS and available optical data. For the high temperature pyrolytic α-polycrystalline Al₂O₃ films, the transitions are assumed to originate at the two principal peaks in the valence band density of states and the O(2s) core state, and to terminate on two peaks within the conduction band density of states. We also report energy loss spectra due to excitations out of the deeper Al(2p), Al(2s), Al(1s), and O(1s) core levels. The excitations originating at the Al(2p), Al(2s), and Al(1s) core levels terminate on levels in the conduction band and on an exciton lying about 1 eV below the conduction-band edge.     

The (d,p) Reactions below 3.0 MeV

Excitation functions for a number of proton groups for the reactions ²³Na(d, p)²⁴Na and ²⁷Al(d, p)²⁸Al were measured in the energy range E_d = 1.5 to 3.0 MeV. The angular distributions for a number of proton groups were measured at a number of deuteron energies below 3.0 MeV for the three reactions ¹²C(d, p)¹³C, ²³Na(d, p)²⁴Na and ²⁷Al(d, p)²⁸Al. The theory of deuteron stripping reactions at incident energies below the Coulomb barrier has been considered. A closed analytical form for the differential cross-section has been obtained considering three cases according to the incident deuteron and outgoing proton energies. An attempt has been made to fit the angular distribution measurements at a number of deuterons bombarding energies accordingly.     

激光烧蚀Al热原子与CF4反应中C2的形成及其发光光谱研究

脉冲激光烧蚀金属平面铝靶产生的热原子与气相CF4碰撞反应中，在400—600nm之间观测到激发态C2分子的发光光谱，它们可归属为Swan带的d3Πg-a3Πu跃迁中Δv＝２，1，0，-1，-2五个振动序列（v'≤6）.光谱强度分析表明，C2激发态的振动温度达6340K左右.与激光烧蚀Al＋O2反应生成AlO的实验结果以及激光烧蚀Cu＋CF4的光谱比较，对比Al(2P1/2-2S1/2,3944nm）和C2的d—a跃迁（0—0）带带头（5165nm)的飞行时间轮廓，认为激发态的Al(2S1/2)原子通过 关键词： 激光烧蚀 发光光谱 C2分子     

Si掺杂Al2O3电子结构的第一性原理计算

本文采用第一性原理对纯Al2O3和Si掺杂的Si 0.167Al0.833O1.5, Si 0.25Al0.75O1.5晶体体系的能带结构、态密度进行了计算分析. 结果发现：随着Si在Al2O3晶体中所占比例的增加,体系能隙变小,在Si 0.25Al0.75O1.5晶体体系中能隙已降到2.5eV,表明该体系为半导体材料;而在掺杂的体系中有数条分散的能带穿过了费米能级,即可以预测该掺杂体系有特别的光电性质;同时对比纯Al2O3和Si掺杂的Si 0.167Al0.833O1.5, Si 0.25Al0.75O1.5晶体体系的总态密度,发现掺杂体系的价带和导带向低能区域移动.     

Si掺杂Al_2O_3电子结构的第一性原理计算

本文采用第一性原理对纯Al2O3和Si掺杂的Si0.167Al0.833O1.5,Si0.25Al0.75O1.5晶体体系的能带结构、态密度进行了计算分析.结果发现:随着Si在Al2O3晶体中所占比例的增加,体系能隙变小,在Si0.25Al0.75O1.5晶体体系中能隙已降到2.5 e V,表明该体系为半导体材料;而在掺杂的体系中有数条分散的能带穿过了费米能级,即可以预测该掺杂体系有特别的光电性质;同时对比纯Al2O3和Si掺杂的Si0.167Al0.833O1.5,Si0.25Al0.75O1.5晶体体系的总态密度,发现掺杂体系的价带和导带向低能区域移动.     

26g,mAl production cross sections from the 23Na(α, n)26Al reaction

Cross sections have been determined for the production of ^{26g, m}Al from the ²³Na(α, n) reaction. Total ²⁶Al production cross sections were obtained from measurements of the thick-target neutron yield. ^26mAl cross sections were measured using an activation technique. ^26gAl cross sections were deduced by subtracting the ^26mAl cross sections from the total (α, n) cross sections. The principle of detailed balance has been applied to the low-energy data to obtain cross sections for the astrophysically interesting ^26gAl(n, α₀)²³Na reaction. These results are compared with the results of Hauser-Feshbach calculations.     

Validation of CASCADE code using the monitor reactions

Production cross-sections of monitor reaction ²⁷Al(x, y)²⁴Na for proton, neutron and deuteron projectiles have been estimated from the CASCADE code from tens of MeV/n to tens of GeV/n energy and compared them with the available experimental data. It has also been shown that using the CASCADE code production cross-section of ²⁷Al(d, y₂)²⁴Na reaction can be obtained from the proton and neutron projectiles. Implicitly, this provides an alternative way of knowing production cross-section of ²⁷Al(n, y ₁)²⁴Na reaction which may be used to monitor the neutron flux in a wide range of energy. However, in the paper need of experimental determination of cross-sections for neutron projectile has been stressed. Similarly, cross-section of other monitor reactions like ²⁷Al(p, y ^′)²²Na and ²⁷Al(n, y ^′ ₁)²²Na are also calibrated as function of energy up to several GeV.     

Messung von Wirkungsquerschnitten für die Reaktionen 27Al(N,P) 27Mg, 27Al(N, α)24Na und 27Al(N, α)24MNa im Energiebereich zwischen 6,3 MeV und 8,3 MeV

Measurements of Cross Sections for the Reactions ²⁷Al(N, P) ²⁷Mg, ²⁷Al(N, α)²⁴Na and ²⁷Al(N, α)^24mNa in the Energy Region between 6,3 MeV and 8,3 MeV Using the activation method cross-sections for the reactions ²⁷Al(n, α₀)²⁴Na, ²⁷Al(n, α₁)^24mNa and ²⁷Al(n, p)²⁷Mg have been measured in the energy range 6.3 MeV to 8.3 MeV. Fast neutrons were produced at the Stuttgart University 4 MV Dynamitron via the reaction ⁹Be(α, n)¹²C irradiating 0.52 μm beryllium layers on high pressure water-cooled copper backings. The energy of the He-ions was varied between 1.86 MeV and 2.76 MeV, while the average current was 200 μAmp. The neutron flux has been measured with a 2″ × 2″ NE 213 liquid scintillator cell.     

RF溅射的SiO_2和SiO_2/Al/SiO_2薄膜的喇曼散射光谱研究

用射频溅射(RF Sputtering)法制成了SiO_2和SiO_2/Al/SiO_2薄膜。应用喇曼光谱研究了薄膜结构。结果表明:RF溅射制成的SiO_2薄膜是含有大量环结构缺陷的玻璃态;SiO_2/Al/SiO_2层状薄膜的喇曼光谱中观察到Al_2O_3的特征峰,证实了Al/SiO_2薄膜界面确有氧化还原反应发生;从喇曼光谱中Al_2O_3的特性峰的位置和相对强度可推断出,SiO_2/Al/SiO_2薄膜界面处的Al_2O_3是非晶γ-Al_2O_3。     

Al production cross sections from the Na(α, n)Al reaction

Cross sections have been determined for the production of ^{26g, m}Al from the ²³Na(α, n) reaction. Total ²⁶Al production cross sections were obtained from measurements of the thick-target neutron yield. ^26mAl cross sections were measured using an activation technique. ^26gAl cross sections were deduced by subtracting the ^26mAl cross sections from the total (α, n) cross sections. The principle of detailed balance has been applied to the low-energy data to obtain cross sections for the astrophysically interesting ^26gAl(n, α₀)²³Na reaction. These results are compared with the results of Hauser-Feshbach calculations.     

Finite temperature model-potential theory of metals

The well-known model-potential theory of simple metals is extended to finite temperature range and applied to Na, K and Al equation of state calculation up to a few eV. An approximate expression of temperature dependent exchange-correlation effects has been derived. The results are quite encouraging.     

Cross sections for monitor reactions 27Al((p, x)24Na, 27Al(p, x)22Na, and 27Al(p, x)7Be at proton energies in the range 0.04–2.6 GeV

The cross sections for the monitor reactions ²⁷Al(p, x)²⁴Na, ²⁷Al(p, x)²²Na, and ²⁷Al(p, x)⁷Be at 12 proton energies, 2605, 1598, 1199, 799, 600, 400, 249, 147.6, 97.2, 66.0, 44.6, and 40.8 MeV, have been determined with 72 × 72-mm square and 10.5-mm-diameter round aluminum foils. The rates of the reactions of the production of ²⁴Na, ²²Na, and ⁷Be in the foils in each irradiation run have been determined by γ spectrometry, whereas the number of protons transmitted through these foils has been determined using calibrated fast current transformers. The cross sections have been determined as the ratios of the corresponding reaction to the average proton fluence.     

在硅酸盐矿物,玻璃和熔体中Si和Al的配位与局部结构：K—边 …

本文采用步辐射的ＳｉＫ－边Ｘ－射线吸收近边结构（ＸＡＮＥＳ）谱研究了Ｓｉ在ＳｉＯ２－Ｐ２Ｏ５和Ｎａ２Ｏ－ＳｉＯ２－Ｐ２Ｏ５的低压磷硅酸盐玻璃中结构与配位,以及Ｓｉ的配位几何随玻璃中Ｐ２Ｏ５含量而变化：同步辐射的Ａｌ Ｋ－边ＸＡＮＥＳ谱研究了Ａｌ在铝硅酸盐成分为ＮａＡｌＳｉ２Ｏ６－ＮａＡｌＳｉ３Ｏ８的玻璃和熔体中的配位和局部结构,并提供了直接的实验证据该成分的玻璃体系中由于压力的变化所诱导Ａｌ配位的     

掺钕镉铝硅酸盐玻璃的Judd-Ofelt理论分析与光谱特性

以掺杂光子晶体光纤为介质的光纤激光器一直受到科研工作者的广泛关注,应用于光子晶体光纤纤芯的掺稀土元素玻璃的制备成为研制掺杂光子晶体光纤的关键问题。利用高温熔融工艺制备钕离子掺杂的40SiO_2-14Al_2O_3-(40-x)CdO-2Li2O-2K2O-2Na_2O-xNd_2O_3(x=0.07,0.14,0.21,0.35,0.42,0.56mol)重金属硅酸盐玻璃系统,测试了其吸收光谱和荧光光谱。采用Judd-Ofelt理论,计算了玻璃样品的强度参数Ωt(t=2,4,6)以及钕离子的自发辐射概率、荧光分支比、荧光辐射寿命等参数。利用测得的荧光光谱计算了钕离子能级跃迁4 F3/2→4 I11/2的受激发射截面及荧光有效线宽。结果表明:当掺Nd_2O_3的摩尔分数为0.42时,制备的镉铝重金属硅酸盐玻璃具有较大的受激发射截面和比较宽的荧光有效线宽,且与相关文献中的钕离子掺杂玻璃相比,具有良好的激光性能和增益性能,有望在研制掺杂光子晶体光纤中得到应用。