Structural analysis of plate-type fuel assemblies and development of a non-destructive method to assess their integrity

This work is concerned with the structural behaviour and the integrity of parallel plate-type nuclear fuel assemblies. A plate-type assembly consists of several thin plates mounted in a box-like structure and is subjected to a coolant flow that can result in a considerable drag force. A finite element model of an assembly is presented to study the sensitivity of the natural frequencies to the stiffness of the plates' junctions. It is shown that the shift in the natural frequencies of the torsional modes can be used to check the global integrity of the fuel assembly while the local natural frequencies of the inner plates can be used to estimate the maximum drag force they can resist. Finally a non-destructive method is developed to assess the resistance of the inner plates to bear an applied load. Extensive computational and experimental results are presented to prove the applicability of the method presented. © 2013 Elsevier B.V. All rights reserved.

Investigation on the structural origin of n-type conductivity in InN films

This work presents a study of the correlation between the electrical properties and the structural defects in nominally undoped InN films. It is found that the density of edge-type threading dislocations (TDs) considerably affects the electron concentration and mobility in InN films. The Hall-effect measured electron concentration increases, while the Hall mobility decreases with the increase in the edge-type TD density. With the combination of secondary ion mass spectrometry and positron annihilation analysis, we suggest that donor-type point defects at the edge-type TD lines may serve as dominant donors in InN films and affect the carrier mobility.

Band-gap bowing and p-type doping of (Zn, Mg, Be)O wide-gap semiconductor alloys: a first-principles study

Using a first-principles band-structure method and a special quasirandom structure (SQS) approach, we systematically calculate the band gap bowing parameters and p-type doping properties of (Zn, Mg, Be)O related random ternary and quaternary alloys. We show that the bowing parameters for ZnBeO and MgBeO alloys are large and dependent on composition. This is due to the size difference and chemical mismatch between Be and Zn(Mg) atoms. We also demonstrate that adding a small amount of Be into MgO reduces the band gap indicating that the bowing parameter is larger than the band-gap difference. We select an ideal N atom with lower p atomic energy level as dopant to perform p-type doping of ZnBeO and ZnMgBeO alloys. For N doped in ZnBeO alloy, we show that the acceptor transition energies become shallower as the number of the nearest neighbor Be atoms increases. This is thought to be because of the reduction of p-d repulsion. The N-O acceptor transition energies are deep in the ZnMgBeO quaternary alloy lattice-matched to GaN substrate due to the lower valence band maximum. These decrease slightly as there are more nearest neighbor Mg atoms surrounding the N dopant. The important natural valence band alignment between ZnO, MgO, BeO, ZnBeO, and ZnMgBeO quaternary alloy is also investigated.

MBE growth of very short period InAs/GaSb type-II superlattices on (001) GaAs substrates

First, GaSb epilayers were grown on (001) GaAs substrates by molecular beam epitaxy. We determined that the GaSb layers had very smooth surfaces using atomic force microscopy. Then, very short period InAs/ GaSb superlattices (SLs) were grown on the GaSb buffer layer. The optical and crystalline properties of the superlattices were studied by low-temperature photoluminescence spectra and high resolution transition electron microscopy. In order to determine the interface of SLs, the samples were tested by Raman-scattering spectra at room temperature. Results indicated that the peak wavelength of SLs with clear interfaces and integrated periods is between 2.0 and 2.6 mu m. The SL interface between InAs and GaSb is InSb-like.

Interplay between s-d exchange interaction and Rashba effect: Spin-polarized transport

The authors investigate the spin-polarized transport properties of a two-dimensional electron gas in a n-type diluted magnetic narrow gap semiconductor quantum well subjected to perpendicular magnetic and electric fields. Interesting beating patterns in the magnetoresistance are found which can be tuned significantly by varying the electric field. A resonant enhancement of spin-polarized current is found which is induced by the competition between the s-d exchange interaction and the Rashba effect [Y. A. Bychkov and E. I. Rashba, J. Phys. C 17, 6039 (1984)]. (c) 2006 American Institute of Physics.

Effects of edge dislocations and intentional Si doping on the electron mobility of n-type GaN films

The effects of dislocations and Si doping on the electrical properties of n-type GaN grown by metal organic chemical vapor deposition (MOCVD) are investigated. It is found that both electron mobility and carrier concentration are strongly influenced by edge dislocations. A moderate Si doping during the GaN growth improves the electron mobility, but the best doping effect depends on the dislocation density of the sample. High quality about 4-mu m-thick MOCVD-grown GaN film with a room temperature electron mobility as high as 1005 cm(2)/V s is obtained by optimizing growth conditions. (c) 2006 American Institute of Physics.

Effect of lightly Si doping on the minority carrier diffusion length in n-type GaN films

We investigate the effects of lightly Si doping on the minority carrier diffusion length in n-type GaN films by analyzing photovoltaic spectra and positron annihilation measurements. We find that the minority carrier diffusion length in undoped n-type GaN is much larger than in lightly Si-doped GaN. Positron annihilation analysis demonstrates that the concentration of Ga vacancies is much higher in lightly Si-doped GaN and suggests that the Ga vacancies instead of dislocations are responsible for the smaller minority carrier diffusion length in the investigated Si-doped GaN samples due to the effects of deep level defects. (c) 2006 American Institute of Physics.

Aluminium doping induced enhancement of p-d coupling in ZnO

Valence-band type Auger lines in Al doped and undoped ZnO were comparatively studied with the corresponding core level x-ray photoelectron spectrography (XPS) spectra as references. Then the shift trend of energy levels in the valence band was that p and p-s-d states move upwards but e and p-d states downwards with increasing Al concentration. The decreased energy of the Zn 3d state is larger than the increased energy of the 0 2p state, indicating the lowering of total energy. This may indicate that Al doping could induce the enhancement of p-d coupling in ZnO, which originates from stronger Al-O hybridization. The shifts of these states and the mechanism were confirmed by valence band XPS spectra and 0 K-edge x-ray absorption spectrography (XAS) spectra. Finally, some previously reported phenomena are explained based on the Al doping induced enhancement of p-d coupling.

Self-organized type-II In0.55Al0.45As/Al0.50Ga0.50As quantum dots realized on GaAs(311)A

Self-organized In0.55Al0.45As/Al0.50Ga0.50As quantum dots are grown by the Stranski-Krastanow growth mode using molecular beam epitaxy on the GaAs(311)A substrate. The optical properties of type-II InAlAs/AlGaAs quantum dots have been demonstrated by the excitation power and temperature dependence of photoluminescence spectra. A simple model accounting for the size-dependent band gap of quantum dots is given to qualitatively understand the formation of type-II In0.55Al0.45As/Al0.50Ga0.50As quantum dots driven by the quantum-confinement-induced Gamma --> X transition. The results provide new insights into the band structure of InAlAs/AlGaAs quantum dots. (C) 2000 American Institute of Physics. [S0003-6951(00)00725-7].

Homoepitaxial growth and MOS structures of 4H-SiC on off oriented n-type (0001)Si-faces

Homoepitaxial growth of 4H-SiC on off-oriented n-type Si-face (0001) substrates was performed in a home-made hot-wall low pressure chemical vapor deposition (LPCVD) reactor with SiH4 and C2H4 at temperature of 1500 C and pressure of 20 Torr. The surface morphology and intentional in-situ NH3 doping in 4H-SiC epilayers were investigated by using atomic force microscopy (AFM) and secondary ion mass spectroscopy (SIMS). Thermal oxidization of 4H-SiC homoepitaxial layers was conducted in a dry O-2 and H-2 atmosphere at temperature of 1150 C. The oxide was investigated by employing x-ray photoelectron spectroscopy (XPS). 4H-SiC MOS structures were obtained and their C-V characteristics were presented.

D−A型有机近红外材料的设计、合成及表征

近年来，随着近红外领域研究的深入，特别是光通讯工业的发展，迫切需要一大批具有良好发光性质的近红外材料。有机材料由于成本低、工艺简单和结构易调控等优点而备受关注。目前，对近红外有机发光材料的研究主要集中在两大类：一是稀土元素配合物；二是有机离子染料。但由于稀土元素的4f-4f跃迁是宇称禁阻的，其分子的激发需通过配体与中心离子的能量转移，发光效率低。同时有机离子染料由于静电相互作用，分子容易因聚集而导致发光萃灭，其器件结构主要采用主客体掺杂技术，存在着较强的主体材料发光和器件发光效率低等缺点。传统有机非离子型材料则不受上述因素的限制，可得到聚集态下高效的发光效率。本论文通过将电子给体与受体共轭连接，设计并合成了系列非离子型的窄带隙有机小分子，研究了它们在近红外电致发光等器件中的应用，主要工作内容和结果如下： (1)在分子内同时引入电子给体和受体，将它们共轭连接，设计并合成了系列D-π-A-π-D型的有机小分子，利用分子内电荷转移，实现近红外区域的吸收和发射。我们选择三苯胺和芴为电子给体，苯并双噻二唑及其衍生物为电子受体，苯、噻吩和吡咯为连接基团，得到的系列化合物的光谱及电化学性质能在很大的范围内进行调节。化合物的吸收光谱可从600nm至1400nm，发射光谱从900nm至1600nm范围内调节。化合物的带宽可从1.19eV减小到0.56eV。由于分子间的强相互作用，化合物在混合溶剂中可自组装成带状结构。同时该系列化合物在隔绝氧气的环境中有良好的光化学稳定性，可应用于电致发光或光伏器件中。 (2)多层器件结构是提高有机电致发光效率的有效方法，但前提是发光材料最好是可真空蒸镀。我们在前部分工作的基础上，合成了系列热稳定的可蒸镀型有机小分子。通过改变受体及给体结构可调节发光波长及效率。利用“掺杂剂/主体材料的思想”制备的电致发光器件，发光波长覆盖700nm到1500nm的范围，最长中心波长为1115nm。发光波长752nm时，器件最高外量子效率为1.12%。基于化合物IV-5制备的非掺杂器件，发光波长为1080nm，外量子效率为0.28%。该效率比基于有机离子染料的器件提高了近10倍。基于化合物IV-7的器件最大发射波长为1220nm，为迄今为止非掺杂有机电致发光器件中的最长发射波长。以上结果证明，该系列分子是良好的近红外电致发光材料。 (3)我们研究了含苯并双噻二唑分子与常见阴离子的相互作用关系，发现其只对CN-离子和F-离子有响应，而对其它阴离子如Cl , Br , I , AcO , H2PO4 , HSO4 , 和NO3 等则没有任何响应。并且通过将其中噻二唑环换成硝基，可实现对CN-离子和F-离子的区分。化合物V-2可高选择性的检测氰离子，其它离子的存在不会干扰检测信号，包括氟离子。检测信号为外观颜色的改变，可作为显色传感器。化合物V-3可定量检测CN-离子，检测极限为1μM。并且检测信号包括近红外发射的萃灭、可见发射的增强和吸收光谱的改变，多重检测信号可增加检测的可靠性。由于响应信号处于近红外区的生物波段，可应用于生物领域

SPECTROSCOPIC INVESTIGATION OF HYDROGEN-DOPANT COMPLEXES IN BULK P-TYPE AND IMPLANTED N-TYPE CRYSTALLINE SILICON

Infrared absorption experiments have been performed on hydrogenated and deuterated bulk boron- and aluminum-doped-Si and implanted P, As, and Sb donors in silicon. A first evidence of complex formation in bulk p-type Si is obtained and the spectra confirm the anomalous 3.3-cm-1 deuterium frequency shift with respect to boron isotopes. The ratio of the D-B-11 and D-B-10 peak areas is found to be the same as that of the two boron isotopes natural abundance. In donor-implanted silicon, a quantitative analysis of the obtained data has allowed a rough estimate of the passivating rate due to diffusing deuterium. While the frequencies of the various vibrational lines are found to be in agreement with those reported in the literature, the data on the broad line at 1660 cm-1 (H) or 1220 cm-1 (D) seem to suggest an assignment of this peak to a complex in the bulk involving some type of defect due to the implantation process.

Type I-type II transition of self-assembled In0.55Al0.45As/Al0.5Ga0.5As quantum dots

The photoluminescence (PL) of In0.55Al0.45As/Al0.5Ga0.5As self-assembled quantum dots has been measured at 15 and 80 K under hydrostatic pressure. The lateral size of the dots ranges from 7 to 62 nm. The emissions from the dots with 26, 52 and 62 nm size have a blue shift under pressure, indicating that these quantum dots have the normal type-I structure with lowest conduction band at the Gamma -valley. However, the PL peak of dots with 7 nm diameter moves to lower energy with increasing pressure. It is a typical character for the X-related transition. Then these small dots have a type-II structure with the X-valley as the lowest conduction level. An envelope-function calculation confirms that the Gamma -like exciton transition energy will rise above the X-like transition energy in the In0.55Al0.45As/Al0.5Ga0.5As structure if the dot size is small enough.