Recent progress in material characterization of α-HgI2 crystals for X- and γ-ray detectors

α-HgI₂ is presently the most promising material for room temperature X- and γ-ray detectors. A certain drawback, however, is the low mobility of holes which is attributed to defects in the crystals. We have investigated organic impurities and nonstoichiometry as possible sources of defects. Mass spectrometric investigation of a molecular beam formed by evaporating HgI₂ crystals shows clearly the existence of nonstoichiometry. Both excess of Hg or excess of I are possible. After thermal treatment, pure nonstoichiometric samples give stoichiometric crystals. However, if hydrocarbons are also dissolved in the lattice the nonstoichiometry becomes fixed. Furthermore, the evaporation measurements show a change in the state of the surface of α-HgI₂ crystals at 67°C. Our investigations up to now indicate the existence of a surface reconstruction at this temperature. As this temperature is appreciably lower than the crystal growth temperature by sublimation (about 110°C) this effect should be responsible for the high concentration of defects found up to now near the surface of the as-grown crystals.     

Preparation of α-Fe_2O_3 Nanodisks by Blocking the Growth of (001) Plane

Based on the difference of hydroxy group configuration on different planes of α-Fe2O3 nanoparticles, using the special adsorption and coordination of phosphate on the (001) plane of α-Fe2O3, well-crystallized and well dispersed α-Fe2O3 nanodisks with diameter of 150–200 nm and thickness of 40–80 nm were synthesized via a hydrothermal method. The magnetic properties of synthesized nanodisks were investigated. It was found that the nanodisks possessed a saturation magnetization (Ms) of 0.38 emu/g, a remanent ...     

Two-step solvothermal synthesis of α-MnS spheres: Growth mechanism and characterization

Stable α-MnS spheres have been successfully synthesized by two-step solvothermal method and traditional solvothermal technique without any surfactant assistance. The XRD patterns and FE-SEM images show that the as-prepared samples are cubic α-MnS micro-spheres with different morphologies. The formation of spheres was studied under different experimental conditions. The two-step solvothermal method helps us cognize the growth mechanism in the solvothermal synthesis better. The result shows that template-direct growth and Ostwald ripening for the formation of the α-MnS spheres are approved. In addition, both mechanisms can be effective at the same time. The UV-Vis absorption spectra reveal that the samples synthesized by different methods have the maximal absorption at the same wavelength (around 365 nm).     

Growth of TiO2 Nanorods by Sol-gel Template Process

In this work, TiO2 nanorods with uniform diameter of about 100 nm and a length of several micrometers were successfully prepared by the sol-gel template method. Also the influence of molar ratios of precursor on the morphology and structure of TiO2 nanorods has been investigated. The prepared samples were characterized by Fourier transform infrared spectroscopy （FTIR）, X-ray diffraction （XRD） and scanning electron microscopy （SEM）. The XRD results indicated that the TiO2 nanorods were crystallized in the anatase and rutile phases, after annealing to 400-700℃ up to 2 h.     

Ion exchange of layered α-NaFeO2

Ion exchange of α-NaFeO₂ was performed at 270°C, the melting point of LiNO₃. The exchanged product had a composition of Li₂O-1.2Fe₂O₃. Its crystal structure was assumed to be the disordered rock-salt type. Cubic closest packing of oxygen was preserved but the distribution of alkali and ferric ions changed during the reaction.     

Alcohol Precipitation for the Separation of α1 and α2 Components of Gelatin

Abstract

Optimal conditions of alcohol precipitation for the separation of α1 and α2 components were investigated. Under the optimal conditions, hot aqueous alcoholic solutions of gelatin on slow cooling, gave precipitates in which most of the α2 component was removed. Similarly, gelatin solutions of a little higher alcohol concentration gave, after centrifligation, supernatants with no α1 component.

Size exclusion chromatography of these samples, α2-removed precipitate and α1-removed supernatant, gave fractions in which the impurities were largely removed. The results indicate that the isolation of α1 and α2 components with little impurity is possible by the combination of the two techniques.     

Growth of Carbon Nanocoils by Porous α-Fe2O3/SnO2 Catalyst and Its Buckypaper for High Efficient Adsorption

High-purity(99%)carbon nanocoils(CNCs)have been synthesized by using porousα-Fe2O3/SnO2 catalyst.The yield of CNCs reaches 9,098%after a 6 h growth.This value is much higher than the previously reported data,indicating that this method is promising to synthesize high-purity CNCs on a large scale.It is considered that an appropriate proportion of Fe and Sn,proper particle size distribution,and a loose-porous aggregate structure of the catalyst are the key points to the high-purity growth of CNCs.Benefiting from the high-purity preparation,a CNC Buckypaper was successfully prepared and the electrical,mechanical,and electrochemical properties were investigated comprehensively.Furthermore,as one of the practical applications,the CNC Buckypaper was successfully utilized as an efficient adsorbent for the removal of methylene blue dye from wastewater with an adsorption efficiency of 90.9%.This study provides a facile and economical route for preparing high-purity CNCs,which is suitable for large-quantity production.Furthermore,the fabrication of macroscopic CNC Buckypaper provides promising alternative of adsorbent or other practical applications.     

Growth of Superconducting Whiskers

The(1112)and(2212)amorphous strips ofBi(Pb)-Sr-Ca-Cu oxide were prepared bynon-equilibrium solidification of melt-quenchingmethod.A lot of superconducting whiskers havebeen grown up on the strip at 840-860°C in astream of oxygen for 50-150 h,which are usuallyseveral to 10 odd mm in length,several μm in thick-ness and more than 10μm in width.These whiskershave very excellent flexibility.     

Growth,morphology and slip system of α-Si3N4 single crystal

Under the conditions of growth temperatures 1500 to 1700° C and total gas pressure 10 to 50 Torr, -Si₃N₄ single crystals have been grown by chemical vapour deposition from a mixture of NH₃, SiCl₄ and H₂. The crystals were transparent and brownish-red to colourless. The effects of the growth conditions on the crystal morphology, growth habit and growth direction have been investigated. On the basal and prismatic planes, the variation in Knoop hardness with orientation of the indenter long-axis has been measured at temperatures up to 1500° C; maximum hardness values were obtained along the 1 0 ¯1 0 direction for the basal plane and along the [0 0 0 1] directions for the prismatic planes. Hardness anisotropy analysis suggests that the active slip systems of -Si₃ N₄ are {1 0 ¯1 0} [0 0 0 1] from room temperature to 1500° C.     

Growth of skyrmionic MnSi nanowires on Si： Critical importance of the SiO2 layer

MnSi in the B20 structure is a prototypical helimagnet that forms a skyrmion lattice, a vortex-like spin texture under applied magnetic field. We have systematically explored the synthesis of single crystal MnSi nanowires via controlled oxide-assisted chemical vapor deposition and observed a characteristic signature of skyrmion magnetic ordering in the MnSi nanowires. The thickness of the SiO2 layer on the Si substrate plays the key role in obtaining a high yield of B20 MnSi skyrmion nanowires. A growth mechanism was proposed that is consistent with the existence of an optimum SiO2 thickness. A growth phase diagram was constructed based on the extensive studies of various growth conditions for various MnSi nanostructures. The persistence of both the helicoidal and skyrmion magnetic ordering in the one-dimensional wires was directly revealed by ac and dc magnetic measurements.     

Auto ignition processing of nanocrystalline α-Al2O3

This paper reports the results of an investigation aiming at synthesizing nanocrystalline α-alumina using the “Auto Ignition” processing technique. The process utilizes in-situ exothermic reactions occurring between a suitable oxidizer (e.g., aluminum nitrate) and a fuel (urea). Due to generation of high temperatures for a short time, the resulting crystalline phase(s) is(are) able to nucleate but cannot grow. This work systematically examines and characterizes the resulting powders obtained from various ratios of the oxidizer and fuel. Both X-ray diffraction and transmission electron microscopy results confirm that under certain conditions it is possible to obtain pure α-alumina in nanocrystalline size. It is expected that sinterability of such powders will be better because the question of γ to α phase transformation does not arise.     

Hydrothermal synthesis of β-Ni(OH)2 platelets and their thermal conversion to NiO

The β-Ni(OH)₂ platelets and nanorods with length ranging from 2 to 6 μm have been successfully synthesized via a hydrothermal method, and no templates or additives were employed. The products were characterized by X-ray powder diffraction and scanning electron microscopy techniques. The dosage of sodium hydroxide and temperature were found to play important roles in controlling the size and morphology of β-Ni(OH)₂. The shape of β-Ni(OH)₂ platelets was successfully sustained during the thermal transformation to NiO. Optical property of the NiO platelets was investigated by means of UV–Vis spectrum, exhibiting small red shift compared to the bulk counterpart. Such β-Ni(OH)₂ and NiO microstructures may find potential applications in micro/nano-devices.     

Hydrothermal synthesis and magnetic properties of α-MnO2 nanowires

In the present study, one-dimensional (1D) α-MnO₂ nanowires with width of 50–60 nm, length about several micrometers have been successfully prepared under hydrothermal conditions in the presence of sodium carboxymethyl cellulose. The samples were characterized by X-ray diffraction, scanning electron microscope, superconducting quantum interference device and N₂ adsorption–desorption experiment. The magnetic measurement reveals that the α-MnO₂ nanowires exhibit a ferromagnetic behavior at 5 K and a paramagnetic behavior at 300 K. The N₂ adsorption–desorption experiment shows that surface area is 160.4 m² g^?1, which is even larger than those of mesoporous nanostructures. At the same time, the possible formation mechanism for the formation of α-MnO₂ nanowires has been proposed according to the experimental results.     

Synthesis and alteration of α-LiFeO2 by mechanochemical processes

The effects of grinding on a stoichiometric mixture of LiOH · H₂O and -FeOOH were studied. It was found that, in the course of grinding, losses of structural water occurred and a phase structurally related to disordered -LiFeO₂ was formed. X-ray diffraction data suggest the occurrence of an ordered phase as intermediate and both -Fe₂O₃ and -Fe₂O₃ were undetected during the comminution process. A prolonged mechanical treatment of this mixture originated an elimination of Li⁺ from the -LiFeO₂ structure and the appearance of the spinel phase, -LiFe₅O₈. Additionally, the mechanical activation of a sample of -LiFeO₂ prepared at high temperatures also leads to a similar rearrangement of cations. The structural transformation is explained with the help of a model in which the vacancies of Li⁺ created during grinding promote the migration of the Fe³⁺ ions from octahedral to tetrahedral sites.     

Preparation and characterization of α-Fe2O3–CeO2 composite

In our previous study we attempted to see the effect of cerium doping (Ce/Fe ratio 0.015 to 0.074) on goethite matrix and conversion of doped goethite to hematite. In the present communication, nano-structured α-Fe₂O₃–CeO₂ composite with Fe/Ce weight ratio as 1.1 has been synthesized by calcination of goethite-cerium hydroxide precursor prepared by co-precipitation method. It was observed that co-precipitation of cerium along with iron in hydroxide medium resulted in hindering the formation of crystalline order as the precursor formed showed poorly crystallized goethite and almost no crystallinity in Ce(OH)₄. Calcination of the precursor at 400 °C showed the formation of hematite together with a broad peak corresponding to cerium oxide whereas at 800 °C, two distinct phases of α-Fe₂O₃ and CeO₂ were observed. The Mössbauer spectra showed the presence of a paramagnetic component both for the precursor as well as for the sample calcined at 400 °C but on raising the calcination temperature to 800 °C, the paramagnetic component disappeared and the spectrum corresponding to pure α-Fe₂O₃ phase was observed. The microstructure of the product obtained by calcining at 800 °C showed rod like structure (30 to 50 nm width and 300 to 500 nm length) of α-Fe₂O₃ having equi-dimensional CeO₂ particles on and around the surface. Besides the rods, equi-dimensional particles and agglomerates corresponding to CeO₂ were also observed. The results show that co-precipitation followed by calcinations gives nanorods hematite with CeO₂ particles bonded to its surface.     

Characterization of α-Al2O3 supports modified with CeO2 and ZrO2

α-Al₂O₃ is a catalytic support adequate by its high stability, availability, and low cost, although it presents as principal disadvantage its limited surface area and reactivity. These disadvantages in the support lead to catalysts of low metallic dispersion and with weak interaction metal-support. According to characterization results obtained in this work, the addition of small amounts of Ce and Zr oxides to α-Al₂O₃ leads to Ce_xZr_1 ? xO₂ composites, thus conferring higher reactivity to the surface due to the presence of basic nature sites. This fact together with the well known oxygen storage capacity of these mixed oxides present in composites allow to obtain supports of optimal properties to prevent the sintering and the formation of superficial carbon present in the catalytic deactivation.     

Precipitation of α(2) phase in α+βsolution-treated and air-cooled Ti-Al-Sn-Zr-Mo-Si-Nd alloys

A series of Ti-Al-Sn-Zr-Mo-Si-Nd alloys with various content of Al were solution treated in alpha+beta phase field and air-cooled. The precipitation of α(2) phase in cooling was investigated by transmission electron microscopic analysis. The precipitation characteristic of α(2) phase was discussed. The precipitation of α(2) phase would proceed by the nucleation and growth of α(2) phase dependent on the diffusion of Al atoms. And a comparison on the difference of precipitation of α(2) phase was carried out under the conditions of air-cooling and quenching in water. The investigation showed that the air-cooling and even quenching could supply enough time for the precipitation and growth of α(2) phase when Al content reached a certain value even though far away from the stoichiometric composition of Ti3Al.     

Growth and Properties of CuAlS2x Se2(1 – x) Single Crystals

CuAlS₂, CuAlSe₂, and CuAlS_2x Se_{2(1 – x)} (0 < x < 1) single crystals were grown by chemical vapor transport in a close-spaced geometry and characterized by x-ray diffraction and differential thermal analysis. The results were used to map out the CuAlSe₂–CuAlS₂ phase diagram. The density of the crystals was found to vary linearly with x, while microhardness shows a maximum. The transmission and reflection data obtained near the intrinsic edge were used to determine the band gap E _g of CuAlS₂, CuAlSe₂, and the solid solutions. E _g was found to vary nonlinearly with composition.