Characteristics investigation on heat transfer growth of sonochemically synthesized ZnO-DW based nanofluids inside square heat exchanger

Journal of Thermal Analysis and Calorimetry - In recent decades, the growth of heat transfer using nanomaterials in the conventional base fluid has caught the attention of researchers...     

The effect of rapid rotation on a vertical bridgman furnace at large Rayleigh number

In quasi-steady operation, convection currents in a Bridgmandevice, used for producing a semi-conductor crystal, createinhomogeneities that may make the crystal unusable. It has oftenbeen suggested that additional forces due to rotation or magnetismmight be efficacious in reducing the segregation of the elementsof the alloy. It has been found that, over a wide range of rotationrates, there is no improvement in performance due to rotationabout the vertical axis. However, numerical results that havebeen obtained previously (Lee & Pearlstein, J. Crys. Growth240, 2002) indicate that, when effects of centrifugal buoyancyare introduced, a substantial reduction in segregation is achieved.In the work reported here, by contrast, in which we extend previouslarge-Rayleigh-number asymptotic analysis to include centrifugalbuoyancy, we find no improvement in radial segregation, butrather increasing segregation with increasing rotation rate.     

Controlled hydrothermal growth of ZnO nanostructures by sequestering the Zn metal ions with the chelating agent EDTA

In the present work, a controlled growth of ZnO nanostructures by manipulating Zn metal ion concentration by the chelating action of ethylene diaminetetra acetic acid in hydrothermal method is studied. EDTA produces metal–chelate complex by the formation of bidentate ligand with Zn²⁺ in the solution and diminishes the reactivity of Zn metal cations. Concentration of EDTA in the mother solution was varied in different ranges like 3, 5 and 10 mM while retaining the zinc metal salt and the NaOH concentration the same. Three different morphologies of wurtzite structured ZnO nanostructures such as nanorods-bunch, separate/discrete uniformly sized hexagonal nanorods and tapered flower petals like shapes are achieved by 3, 5 and 10 mM strengths of EDTA, respectively. The medium concentration 5 mM of EDTA is found to have moderate control over producing ZnO nanostructures of uniform diameter and a high aspect (length to diameter) ratio. An array of vertically aligned free standing ZnO nanorods with uniform spacing is successfully achieved by the addition of 5 mM of EDTA in the mother solution and the same is studied for its fluorescence property at an excitation of 325 nm and it has exhibited a characteristic UV emission of ZnO around 383 nm.     

An investigation of photo-quenching properties of LEC GaAs by using optical and electrical techniques

We have performed a series of novel measurements on n-type GaAs, involving photo-quenching of Schottky capacitance and optical absorption on samples grown from melts of various stoichiometry. Decrease of Schottky capacitance by photo-quenching at temperatures below 130 K, which is normally ascribed to the photo-quenching of EL2 centres, is accompanied by optical absorption bleaching at 1 μm wavelength. After complete bleaching of both Schottky capacitance and optical absorption by EL2 centres at a measurement wavelength of 1 μm, the near-band-edge absorption extending to about 50 meV below E_g, was still present. Photo-quenching of this absorption could be observed but only at temperatures below 45 K. However, no capacitance change related to this absorption bleaching was observed. It follows that the photo-quenching of the near-band-edge absorption cannot be due to EL2 centres and that the absorption mechanisms at 1 μm and close to the band-edge are due to different defects.     

Photoluminescence of chromium doped and chromium diffused GaAs

Photoluminescence (PL) measurements on the 0.84 eV Cr-related PL line have been made on both Cr diffused and Cr doped GaAs. In the former case, no luminescence is observed below a diffusion temperature of 800°C. The form of the PL spectrum is found to be the same, independent of whether the majority donor species is group IV(Si) or group VI(Te). PL profiling experiments are used to show that the PL intensity is proportional to the concentration of chromium. Although it is possible that a group VI donor is involved, the insensitivity of the spectrum to different donor species and its linear dependence on chromium concentration suggest that the luminescent centre is a [Cr_Ga-V_As] complex.Measurements on growth doped material are typified by the low intensity near the seed and the scatter in PL intensity in samples where the measured [Cr] is nearly constant. By comparison with the results on the diffused material, the anomalously low PL intensity near the seed end is deduced to be indicative of the high crystal stoichiometry in this region.     

New ideas in neutrino detection

What is new in the field of neutrino detection? In addition to new projects probing both the low and high ends of the neutrino energy scale, an inexpensive, effective technique is being developed to allow tagging of antineutrinos in water Cherenkov (WC) detectors via the addition to water of a solute with a large neutron cross-section and energetic γ daughters. Gadolinium is an excellent candidate since in recent years it has become very inexpensive, now less than $8 per kilogram in the form of commercially available gadolinium trichloride. This non-toxic, non-reactive substance is highly soluble in water. Neutron capture on gadolinium yields an 8.0 MeV gamma cascade easily seen in detectors like Super-Kamiokande. The uses of GdCl₃ as a possible upgrade for the Super-Kamiokande detector — with a view toward improving its performance as an antineutrino detector for supernova neutrinos and reactor neutrinos — are discussed, as are the ongoing R&;D efforts which aim to make this dream a reality within the next two years.     

Dislocation density, infrared absorption and cathodoluminescence mapping of microstructure associated with dislocation cells in semi-insulating LEC GaAs

We have investigated the microstructure that occurs both inside and closely associated with dislocation cells in undoped, semi-insulating LEC GaAs. Using A/B etching, we have identified regions containing large volumes of this microstructure for examination by transmission electron microscopy. A precipitate with a possible associated dislocation loop has been observed in such a region. Cathodoluminescence and reverse contrast images show excellent agreement and demonstrate correlation between regions of high precipitate (and dislocation) density and low concentrations of particular unidentified point defects. In-situ, optical scattering imaging by A/B etched surfaces and reverse contrast absorption imaging demonstrate directly the excellent correlation between these techniques for the first time.     

The incorporation of Cr into gallium arsenide grown by the LEC method

To investigate the incorporation of Cr into LEC GaAs, we have grown nine Cr-doped GaAs crystals where the dopant is labelled with radio-tracer ⁵¹Cr. A thermodynamic model is developed to describe the segregation behaviour in which the distribution coefficient of the Cr is a function of the arsenic activity in the melt. The analysis shows that either the congruent melt is ˜ 2% richer in Ga than the stoichiometric composition or, much more probably, that a significant amount of Ga is lost into the B₂O₃ encapsulant. The latter postulate is shown to be consistent with recent studies by Emori et al. [Japan. J. Appl. Phys. 24 (1985) L291].     

有机硅活性中间体的研究 II. 一些含环丙基的有机硅化合物的合成及其结构

用锂有机物的方法合成了四种含有环丙基有机硅化合物,对上述每种化合物中可能存在着的立体异构进行了分离和和构型测定。