Recoil distance lifetime measurements in 26Mg and 26Al

Levels in ²⁶Mg and ²⁶Al were excited with the ²³Na + α reaction, and the recoil-distance method was used to determine the mean lives of the following levels: ²⁶Mg, 3.59 MeV (9.6 ± 1.2ps) and 3.94 MeV (1.38 ± 0.11 ps); ²⁶Al, 1.76 MeV (6.4 ± 0.4 ps) and 2.072 MeV (1.00 ± 0.10ps). The deduced values are compared to previous measurements by the Doppler-shift attenuation method.     

Synthesis and characterization of stable Co and Cd doped nickel hydroxide nanoparticles for electrochemical applications

The present paper describes the physical-chemical characterization and electrochemical behavior of a new nanomaterial formed by the addition of cadmium and cobalt atoms into the structure of nickel hydroxide nanoparticles, these ones synthesized by an easy sonochemical method. Particles of about 5nm diameter were obtained and characterized by high resolution transmission electron microscopy (HRTEM), X-ray diffraction and Raman spectroscopy. Different nickel hydroxide nanoparticles were immobilized onto transparent conducting substrates by using electrostatic layer-by-layer providing thin films at the nanoscale and the electrochemical behavior was investigated. The formation of a mixed hydroxide was corroborated by observation of very interesting properties as redox potential shifting to less positive potentials and high stability when submitted to long electrochemical cycling or high times of ultrasonic synthesis, suggesting practical applications.     

Direct observations of single particle fragmentation in the early stages of combustion under dry and wet conventional and oxy-fuel conditions

This article investigates the single particle fragmentation of three solid fuels in the early stages of combustion under dry and wet conventional and oxy-fuel conditions. The three solid fuels studied were a low rank sub-bituminous Colombian coal, a low-rank/high-ash sub-bituminous Brazilian coal, and a charcoal residue from black acacia. Particles, with size in the range 125–150 µm, were burned in a drop tube furnace with a constant wall temperature of 1475?K, under six different mixtures of O₂/N₂/CO₂/H₂O, which allowed simulating dry and wet conventional and oxy-fuel combustion conditions. A high-speed camera was used to record the fragmentation process during the early stages of combustion and the collected images were treated to characterize the fragmentation mode, probability and time. The observed fragmentation modes are characterized by the occurrence of exfoliation, radial fragmentation or a combination of both. The results disclose that the fragmentation mode is strongly affected by the fuel type, but less affected by the atmosphere; the fragmentation probability is strongly affected by both the fuel type and the atmosphere; and, finally, fragmentation in air occurs significantly dispersed after ignition, but it tends to cluster closer to the ignition under simulated oxy-fuel conditions.     

Alloy formation of supported gold nanoparticles at their transition from clusters to solids: does size matter?

Gold nanoclusters of a size approaching the molecular limit (<3 nm) were prepared on Si substrates in order to study alloy formation on the nanometer scale. For this purpose, indium atoms are deposited on top of the gold particles at room temperature and the formation of AuIn(2) is studied by x-ray photoelectron spectroscopy in situ. It is observed that the alloy formation takes place independent of whether the particles electronically are in an insulating molecular or in a metallic state. Most important, however, closed packed full-shell clusters containing 55 Au atoms are found to exhibit an outstanding stability against alloying despite a large negative heat of formation of the bulk Au-In system. Thus, Au(55) clusters may play a significant role in the design of nanoscaled devices where chemical inertness is of crucial importance.     

Heteroepitaxial growth of cubic boron nitride films on single-crystalline (001) diamond substrates

Heteroepitaxial cubic boron nitride (c-BN) films of significantly improved crystalline quality have successfully been deposited on (001) diamond single crystals using an ion beam assisted preparation method. The results of various characterization techniques prove that films containing 100% pure c-BN phase were nucleated directly on top of diamond without any intermediate hexagonal BN layer. Epitaxially grown, 500-nm-thick c-BN films are mechanically stable even under ambient conditions, though they still exhibit a compressive stress of 5 GPa. Their rocking angles of 0.2°, as observed by X-ray diffraction, point to a hitherto unprecedented quality of the films. Their surface smoothness, the magnitude of their Youngs modulus as well as their ultrahardness corroborate the outstanding quality of these epitaxially grown c-BN films on single-crystalline diamond. PACS 68.55.Jk; 81.15.Jj; 62.20.Qp; 81.05.Je     

Magnetism in V-/Mn-doped ZnO layers fabricated on sapphire

Doping ZnO with transition metals (TM) is an obvious approach to produce diluted magnetic semiconductors for magnetoelectronic and spintronic applications. We have carried out experimental studies on the fabrication and characterisation of Mn-doped ZnO layers and V-doped ZnO layers and nanorods, the results of which are reviewed in this paper. From SQUID measurements, both epitaxial and implanted ZnMnO layers show paramagnetic behaviour. Epitaxial ZnVO layers show ferromagnetic SQUID signals, but the presence of any secondary phases in the ZnVO layers may not be ruled out. We also show that the used Al₂O₃ substrates produce a ferromagnetic SQUID signal, that complicates the analysis of magnetisation data and hence the confirmation of ferromagnetism only from SQUID results. PACS 75.50.Pp; 85.75.-d; 81.05.Dz; 75.30.-m; 75.60.Nt; 75.70.-i     

Interface reactions in [Fe/B]<Subscript>n</Subscript> multilayers: a way to tune from crystalline/amorphous layer sequences to homogeneous amorphous Fe<Subscript>x</Subscript>B<Subscript>100-x</Subscript> films

[Fe/B]_{n ≥2} multilayers were prepared by thermal evaporation, ion-beam sputtering and laser ablation. By applying in situ electron spectroscopies (UPS, XPS) and monitoring the electrical resistance during layer growth, evidence could be provided for the occurrence of interface reactions within the range of studied deposition temperatures (77 K ≤T ≤300 K). These reactions result in amorphous Fe_xB_100-x phases, which are spatially restricted to a width of less than 3 nm at the original interface. The amorphicity of the reacted interlayers was unequivocally proven by additional high-resolution electron microscopy (HRTEM) and their characteristically changed magnetic properties. Due to the well-defined width of the interface reaction, homogeneous amorphous Fe_xB_100-x films can be obtained by reducing the individual Fe and B layer thicknesses to below the above reaction depth, while for larger thicknesses layer sequences of the crystalline/amorphous/crystalline type will result. Received: 30 January 2002 / Accepted: 31 January 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +49-731/502-2963, E-mail: hans-gerd.boyen@physik.uni-ulm.de     

T c-depression of superconducting diluteInMn-alloys produced by ion implantation

Well annealed In-films were implanted at low temperatures (T<7 K) with Mn⁺-ions and the superconducting transition temperatureT _c of these alloys was determined in situ. The results can be described best by an Abrikosov-Gorkov-likeT _c-depression with an initial slope of –50 K/at. %. From this we conclude that Mn has a long-lived magnetic moment in In. In addition, a comparison to quench condensedInMn-alloys shows that theT _c-depression is not influenced by the much higher degree of disorder in these latter samples. This is contrasted by In-films, which were irradiated with high fluences of Ar⁺-ions before the Mn-implantation. Here a linearT _c(C _Mn) behavior was found with a slope of –39 K/at. %. In a complementary experimentInMn-alloys were irradiated with Ar⁺ after the Mn-implantation. The results indicate a vanishing pair breaking of the Mn-atoms. The observations of both irradiation experiments can be interpreted as changes of the magnetic moment due to specific local environments of the Mn-atoms produced by the Ar⁺-irradiation.     

原子簇模型晶体带隙的计算

本文以金刚石的原子簇模型化合物MC_4H_(12)为例,使用CNDO、INDO、MNDO(对M=C)和ab initio(对M=C,B,N)方法,对原子簇模型晶体带隙和电子结构进行了计算。将原子簇模型的晶体带隙定义为原子簇体系的激发能,其结果普遍地比基态的前线轨道处理法要好得多。另外,还对C_5H_(12)分子的光电子能谱以及N杂质引起的晶格畸变和顺磁共振实验结果作了合理解释。