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排序方式: 共有311条查询结果,搜索用时 15 毫秒
21.
InxGa1 − xN/GaN heterostructures and quantum wells (QWs) are particularly important in the application of III-V nitride materials for light emitting diodes and laser diodes. The photoluminescence (PL) emissions from InxGa1 − xN/GaN QW structures have been reported, where, for successive annealing operations, the PL peak suffers a primary red shift, followed by a blue shift. The observed phenomenon remains unexplained because of its complexity. This paper is intended towards a proper explanation of the observed experimental results through suitable quantum mechanical models and computations, whether the band gap of InN is 1.95 eV or 0.7 eV. 相似文献
22.
Anish Kumar K. Laha T. Jayakumar K. Bhanu Sankaro Rao Baldev Raj 《Metallurgical and Materials Transactions A》2002,33(6):1617-1626
Modified 9Cr-1Mo ferritic steel (T91/P91) has been subjected to a series of heat treatments consisting of soaking for 5 minutes
at the selected temperatures, starting from the α-phase region (1073 K) to the γ + δ-phase region (1623 K), followed by oil quenching. Hardness measurements, microstructural features, and grain-size measurements
by the linear-intercept method have been used for correlating them with the ultrasonic parameters. Ultrasonic velocity and
attenuation measurements, and spectral analysis of the first backwall echo have been used for characterization of the microstructures
obtained by various heat treatments. As the soaking temperature increased above Ac
1, the ultrasonic velocity decreased because of the increase in the volume fraction of martensite in the structure. There were
sharp changes in the ultrasonic velocities corresponding to the two critical temperatures, Ac
1 and Ac
3. Ultrasonic longitudinal- and shear-wave velocities were found to be useful in identifying the Ac
1 and Ac
3 temperatures and for the determination of hardness in the intercritical region. However, ultrasonic attenuation and spectral
analysis of the first backwall echo were found to be useful to characterize the variation in the prior-austenitic grain size
and formation of δ ferrite above the Ac
4 temperature. The scattering coefficients have been experimentally determined for various microstructures and compared with
the theoretically calculated value of the scattering coefficients for iron reported in literature. 相似文献
23.
24.
Tapas Gangopadhyay Raj Kumar Ohdar Dilip Kumar Pratihar Indrajit Basak 《The International Journal of Advanced Manufacturing Technology》2011,53(1-4):301-312
Three-dimensional finite element analyses has been carried out using DEFORM 3D software on multi-stage hot forming of railway wheels involving the processes of upsetting, forging, and punching of wheels. Thermal analysis related to heating the blank in furnace and all intermediate heat transfer stages between deforming operations have been conducted. Rigid viscoplastic finite element method has been utilized for coupled thermo-mechanical analysis of the processes. Modeling of punching the wheel bore has been carried out using Cockcroft and Latham fracture criterion. Evolution of thermo-mechanical parameters at selected points within the workpiece has been studied in detail. The method of simulating the effects of various process parameters has been explained using relevant mathematical relations. This study shows that design, optimization, and analysis of process perturbations for multi-stage railway wheel manufacturing process can be done efficiently in three-dimensional finite element simulations instead of conventional time and cost intensive trials. It might be necessary to use the results of finite element analysis in shop-floor to enhance productivity and reduce wheel rejection. 相似文献
25.
Microwave-assisted chemical bath deposition (MACBD) is an emerging route for rapid synthesis of films and nanostructured particles. In this paper we report MACBD of ZnO rod-array films on bare glass substrates from an aqueous bath of tetra ammonium zinc complex. The deposition time is reduced to about 1 min as compared to around 60 min for conventional CBD. X-ray diffraction study shows that as-deposited films are uniaxially out-of-plane textured along the c-axis. Scanning Electron Microscopy reveals that the films consist of elongated elliptical tapered rods of diameters 250 to 350 nm. Atomic Force Microscopy shows that the films consist of about 350 nm grains. The RMS roughness is about 60 nm. The energy band gap is 3.27 eV as estimated from optical data. The films are n-type with electrical conductivity of 1 × 10− 4 S/cm. 相似文献
26.
Water-dispersible graphene was prepared by reacting graphite oxide and 6-amino-4-hydroxy-2-naphthalenesulfonic acid (ANS). X-ray diffraction study showed that the basal reflection (002) peak of graphite oxide was absent in the ANS-functionalized graphene (ANS-G), indicating crystal layer delamination. Ultraviolet-visible spectral data were recorded to assess the solubility of the ANS-G in water. Fourier transform infrared spectral analysis suggested the attachment of ANS molecules to the surface of graphene. Raman and x-ray photoelectron spectroscopy revealed that oxygen functionality in the graphite oxide had been removed during reduction. Atomic force microscopy found that the thickness of ANS-G in water was about 1.8 nm, much higher than that of single layer graphene. Thermal stability measurements also indicated successful removal of oxygen functionality from the graphite oxide and the attachment of thermally unstable ANS to the graphene surfaces. The electrical conductivity of ANS-G, determined by a four-point probe, was 145 S m(-1) at room temperature. 相似文献
27.
A multistage field emitter based on graphene-linked ZnO nanowire array is realized by means of spin-coating a graphene dispersion (reduced graphene oxide) over a nanostructured platform followed by plasma modification. Spin-coating leads to interlinking of graphene sheets between the neighboring nanowires whereas plasma etching in the subsequent step generates numerous ultra-sharp graphene edges at the nanowire tips. The inherent tendency of graphene to lay flat over a plane substrate can easily be bypassed through the currently presented nanostructure platform based technique. The turn-on and threshold field significantly downshifted compared to the individual components in the cascade emitter. Through the facile electron transfer from nanowires to graphene due to band bending at the ZnO-graphene interface together with multistage geometrical field enhancement at both the nanowire and graphene edges remain behind this enriched field emission from the composite cold cathode. This strategy will open up a new direction to integrate the functionalities of both the graphene array and several other inorganic nanostructure array for practical electronic devices. 相似文献
28.
Grate JW O'Hara MJ Farawila AF Douglas M Haney MM Petersen SL Maiti TC Aardahl CL 《Analytical chemistry》2011,83(23):9086-9091
A sample preparation sequence for actinide isotopic analysis by thermal ionization mass spectrometry (TIMS) is described that includes column-based extraction chromatography as the first separation step, followed by anion-exchange column separations. The sequence is designed to include a wet ashing step after the extraction chromatography to prevent any leached extractant or oxalic acid eluent reagents from interfering with subsequent separations, source preparation, or TIMS ionization. TEVA resin and DGA resin materials, containing extractants that consist only of C, N, O, and H atoms, were investigated for isolation of plutonium. Radiotracer level studies confirmed expected high yields from column-based separation procedures. Femtogram-level studies were carried out with TIMS detection, using multiple monoisotopic spikes applied sequentially throughout the separation sequence. Pu recoveries were 87% and 86% for TEVA and DGA resin separations, respectively. The Pu recoveries from 400 μL anion-exchange column separation sequences were 89% and 93% for trial sequences incorporating TEVA and DGA resin. Thus, a prior extraction chromatography step in the sequence did not interfere with the subsequent anion-exchange separation when a simple wet ash step was carried out in between these column separations. The average measurement efficiency for Pu, encompassing the chemical separation recoveries and the TIMS ionization efficiency, was 2.73% ± 0.77% (2σ) for the DGA resin trials and 2.67% ± 0.54% for the TEVA resin trials, compared to 3.41% and 2.37% (average 2.89%) for two control trials. These compare with an average measurement efficiency of 2.78% ± 1.70%, n = 33 from process benchmark analyses using Pu spikes processed through a sequence of oxalate precipitation, wet ash, iron hydroxide precipitation, and anion-exchange column separations. We conclude that extraction chromatography can be a viable separation procedure as part of a multistep sequence for TIMS sample preparation. 相似文献
29.
Kalpana R. Dey Tapas Debnath Claus H. Rüscher Margareta Sundberg Altaf Hussain 《Journal of Materials Science》2011,46(5):1388-1395
Samples of nominal compositions, Cs0.25Nb
y
W1−y
O3 and Cs0.3Nb
y
W1−y
O3 with 0.0 ≤ y ≤ 0.25 and 0.0 ≤ y ≤ 0.3 were synthesized using appropriate amounts of Cs2WO4, WO3 and WO2 in evacuated and closed silica glass tubes at 800 °C. The polycrystalline products contain hexagonal shaped crystals of up
to 15 μm diameter as long as y ≤ 0.15. X-ray powder patterns of the samples reveal the formation of hexagonal tungsten bronze (HTB-I) type phase with y < 0.1. A mixture of HTB-I and an analogous less reduced hexagonal tungsten bronze (HTB-II) type phase is seen when y ≥ 0.1. HTB-II content increases with increasing y, revealing close similarity to bronzoid type phases when y = x. Results of SEM/EDX analysis also support a partial substitution of tungsten by niobium in the HTB-I type phase. Infrared
absorption and optical reflectivity data shows the effect of increasing amount of non-metallic phase for y > 0.1 and the effect of counterdoping by Nb5+/W5+ substitution in the metallic HTB-I type phase for y ≤ 0.1, respectively. Reinvestigations in the system Rb0.3Nb
y
W1−y
O3 (0.0 ≤ y ≤ 0.175) show similar results with increasing content of HTB-II type phase related with y. 相似文献
30.
We propose a new scheme of designing a vector quantizer for image compression. First, a set of codevectors is generated using the self-organizing feature map algorithm. Then, the set of blocks associated with each code vector is modeled by a cubic surface for better perceptual fidelity of the reconstructed images. Mean-removed vectors from a set of training images is used for the construction of a generic codebook. Further, Huffman coding of the indices generated by the encoder and the difference-coded mean values of the blocks are used to achieve better compression ratio. We proposed two indices for quantitative assessment of the psychovisual quality (blocking effect) of the reconstructed image. Our experiments on several training and test images demonstrate that the proposed scheme can produce reconstructed images of good quality while achieving compression at low bit rates. Index Terms-Cubic surface fitting, generic codebook, image compression, self-organizing feature map, vector quantization. 相似文献