High voltage SnO<Subscript>2</Subscript> varistors prepared from nanocrystalline powders

In this study, SnO₂-based varistors were prepared from mechanically activated nanocrystalline powders. Nanocrystalline powders were derived by subjecting the initial powders to intensive high-energy activation with different times and ball to powder ratio. The effect of activation parameters on the powder properties and sintering temperature, as well as microstructural, micro-electrical and macro-electrical properties of the final specimens was evaluated. Varistors derived from high-energy mechanical activation exhibit a higher density (98.3% relative density) and more refined microstructure upon sintering at 1,300 °C in comparison varistors prepared from conventional powders. Breakdown voltage and nonlinear coefficient were increased up to 24 kV/cm and 45 respectively.     

Thermal stability of nanocrystalline CeO<Subscript>2</Subscript> prepared through freeze drying

The growth of ceria nanoparticles during annealing at high temperatures (up to 700°C) has been studied by X-ray diffraction, transmission electron microscopy, and low-temperature nitrogen adsorption measurements. The results indicate that CeO₂ nanopowders prepared using freeze drying offer enhanced thermal stability.     

Properties of nanocrystalline ZrO<Subscript>2</Subscript>-Y<Subscript>2</Subscript>O<Subscript>3</Subscript>-CeO<Subscript>2</Subscript>-CoO-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> powders

We have studied the properties of nanocrystalline ZrO₂-Y₂O₃-CeO₂-CoO-Al₂O₃ powders prepared via hydrothermal treatment of a mixture of coprecipitated hydroxides at 210°C. A number of general trends are identified in the variation of the properties of the synthesized powders during heat treatment at temperatures from 500 to 1200°C. Our results demonstrate that the addition of 0.3 mol % CoO to nanocrystalline ZrO₂-based powders containing 1 to 5 mol % Al₂O₃ allows one to obtain composites with good sinterability at a reduced temperature (1200°C).     

Effect of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> on the properties of nanocrystalline ZrO<Subscript>2</Subscript> + 3 mol % Y<Subscript>2</Subscript>O<Subscript>3</Subscript> powder

We have studied the properties of nanocrystalline ZrO₂〈3 mol % Y₂O₃〉 and 90 wt % ZrO₂〈3 mol % Y₂O₃〉-10 wt % Al₂O₃ powders prepared via hydrothermal treatment of coprecipitated hydroxides at 210°C. The results demonstrate that Al₂O₃ doping raises the phase transition temperatures of the metastable low-temperature ZrO₂ polymorphs and that the structural transformations of the ZrO₂ and Al₂O₃ in the doped material inhibit each other.     

Synthesis and cycling performance of double metal doped LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> cathode materials for rechargeable lithium ion batteries

In order to improve the cycling performance of LiMn₂O₄, the spinel phases LiCo_0.15Mn_1.85O₄ and LiCo_0.05M_0.1Mn_1.85O₄ (M = Ni, Zn, Cu) were prepared by the sol-gel method. Their structures have been investigated by x-ray diffraction. Electrochemical studies were carried out using the Li | Li _x Mn₂O₄ (x = 1.05, 1.1), LiCo_0.15Mn_1.85O₄, and LiCo_0.05M_0.1Mn_1.85O₄ (M = Ni, Zn, Cu) cells. The capacity loss of Li | Li _x Mn₂O₄ (x = 1.05, 1.1) cells is about 21.7 and 6.4% after 30 cycles, whereas that for Co, Co-Ni, Co-Zn, and Co-Cu doped spinel materials is about 4.0, 2.0, 1.0, and 1.9%, respectively. The good capacity retention of LiCo_0.05M_0.1Mn_1.85O₄ (M = Ni, Zn, Cu) electrodes is attributed to stabilization of spinel structure by double metal doping for Mn ion sites. Double substituted spinels display better performance in terms of cycle-life compared with LiMn₂O₄. The text was submitted by the authors in English.     

Magnetic and photocatalytic properties of nanocrystalline ZnMn<Subscript>2</Subscript>O<Subscript>4</Subscript>

The present study describes the synthesis of ZnMn₂O₄ nanoparticles with the spinel structure. These oxide nanoparticles are obtained from the decomposition of metal oxalate precursors synthesized by (a) the reverse micellar and (b) the coprecipitation methods. Our studies reveal that the shape, size and morphology of precursors and oxides vary significantly with the method of synthesis. The oxalate precursors prepared from the reverse micellar synthesis method were in the form of rods (micron size), whereas the coprecipitation method led to spherical nanoparticles of size, 40–50 nm. Decomposition of oxalate precursors at low temperature (∼ 450°C) yielded phase pure ZnMn₂O₄ nanoparticles. The size of the nanoparticles of ZnMn₂O₄ obtained from reverse micellar method is relatively much smaller (20–30 nm) as compared to those made by the co-precipitation (40–50 nm) method. Magnetic studies of nanocrystalline ZnMn₂O₄ confirm antiferro-magnetic ordering in the broad range of ∼ 150 K. The photocatalytic activity of ZnMn₂O₄ nanoparticles was evaluated using photo-oxidation of methyl orange dye under UV illumination and compared with nanocrystalline TiO₂. Dedicated to Prof. C N R Rao on his 75th birthday     

Synthesis and microstructure of nanocrystalline Yb<Subscript>2</Subscript>O<Subscript>3</Subscript> powders

Nanocrystalline ytterbia powders have been synthesized using different precursors prepared by precipitation from nitrate solutions: ytterbium carbonates, oxalates, and hydroxides. The powders have been characterized by X-ray diffraction and scanning electron microscopy. The nature of the precursor has no effect on the crystallization temperature of ytterbia but influences its microstructure. The particles range in shape from spherical to platelike. The average crystallite size of the Yb₂O₃ powders is 20–25 nm. Raising the heat-treatment temperature from 600 to 1000°C increases the crystallite size to 33–46 nm. The highest thermal stability is offered by the ytterbia powders prepared through carbonate decomposition.     

Stress reduction during phase change in Ge<Subscript>2</Subscript>Sb<Subscript>2</Subscript>Te<Subscript>5</Subscript> by capping TiN film

It has been one of the most important issues to minimize the stress reduction during phase change in GST (Ge₂Sb₂Te₅) alloy for PRAM (Phase-change Random Access Memory) applications, because the alloy has been reported to face the significant stress during the phase change. We fabricated GST/oxide/substrate as a basic structure, and then added two more structures by capping an adhesion layer (Ti) or a barrier metal (TiN) on GST layer, respectively. We report that TiN-capped structure shows about 40% stress reduction during the phase change compared with that of the basic structure. The stress reduction is considered to be due to the intrinsic compressive stress in TiN film itself.     

Fabrication and magnetic properties of NiFe<Subscript>2</Subscript>O<Subscript>4</Subscript> nanocrystalline nanotubes

Aligned NiFe₂O₄ polycrystalline nanotubes have been successfully fabricated inside the nanochannels of porous anodic aluminum oxide (AAO) templates by wetting chemical deposition. A mixture of Fe nitrate and Ni nitrate, which was thermally decomposed at no less than 400 °C, was used to yield NiFe₂O₄ tubes. By varying the deposition conditions and the parameters of the templates, we could tailor the lengths and the outer as well as the inner diameters of the tubes. Transmission electron microscopy (TEM) images reveal that the nanotubes are uniform and well isolated. X-ray diffraction (XRD) and selected area electron diffraction (SAED) demonstrate that the as-obtained nanotubes can be indexed to polycrystalline cubic spinel. The Mössbauer spectra show that the magnetic hyperfine field is reduced with the decrease of the metrical temperature as well as the decrease of the size of nanoparticles.     

Enhanced stability of Pd/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> during aqueous oxidation reaction via SiH<Subscript>4</Subscript> treatment

γ-Al₂O₃ is widely present as a support in catalytic application. However, the transformation of γ-Al₂O₃ into the undesired hydrated boehmite (γ-AlOOH) under the aqueous reaction conditions usually results in an irreversible inactivation of supported Al₂O₃ catalysts. Toward suppressing the hydration of γ-Al₂O₃ in the process of catalytic reactions, herein we have devised a new strategy by exploring the SiH₄ treatment for successful preparation of Si–Pd/Al₂O₃ catalysts. SiH₄ treatment enables a significant improvement in the poor stability of Pd/Al₂O₃ for selective oxidation of toluene because SiH₄ could effectively anchor palladium nanoparticles and inhibit the formation of boehmite by reacting with unsaturated aluminum sites to reduce the intensity of Lewis acid sites on the surface of γ-Al₂O₃. Pd/Al₂O₃ pretreated with SiH₄, that is Si–Pd/Al₂O₃, provides high catalytic activity and stability in the aqueous oxidation of toluene even at high temperature. Moreover, Si–Pd/Al₂O₃ is reusable without obvious loss of the catalytic activity and selectivity, compared to Pd/Al₂O₃.     

Properties of CdSe films produced via spray pyrolysis of [Cd((NH<Subscript>2</Subscript>)<Subscript>2</Subscript>CSe)<Subscript>2</Subscript>Cl<Subscript>2</Subscript>]

Data are presented on microwave photoconductivity kinetics in CdSe films prepared via spray pyrolysis of aqueous solutions of thiourea complexes on quartz and glass-ceramic substrates at temperatures from 300 to 600°C. The films were characterized by x-ray diffraction and optical absorption and reflection spectroscopies. Microwave photoconductivity (excitation with 8-ns laser pulses at 337 nm) was measured at 295 K by a resonator method in the 9-and 36-GHz ranges. The results indicate that the photoresponse decay kinetics depend on deposition temperature and incident intensity. In the films deposited below 400°C, the photoresponse decay follows a first-order rate law. At deposition temperatures above 450°C, the photoresponse decay cannot be represented by a first-or second-order rate law at low excitation intensities and approaches second-order kinetics at high intensities (> 10¹⁵ photons/cm² per pulse). Analysis of the photoresponse decay kinetics in terms of the interaction between free and trapped electrons, holes, and ions allowed us to formulate a model for the processes involved and to evaluate the rate constant of electron-hole recombination in CdSe: (4–6) × 10^?11 cm³/s.     

Effect of Au and NiO catalysts on the NO<Subscript>2</Subscript> sensing properties of nanocrystalline SnO<Subscript>2</Subscript>

Nanocrystalline tin dioxide has been synthesized, and its surface has been modified with Au and NiO. Their distributions in the nanocrystalline tin dioxide have been examined by X-ray diffraction and transmission electron microscopy. The NO₂ sensing properties of the materials have been studied in the range 100–1000 ppb. Both gold and nickel enhance the NO₂ response of SnO₂. Codoping with Au and NiO markedly enhances its sensing response and, in addition, lowers the peak response temperature. The observed effect of NO₂ concentration in dry air on the sensing response of the SnO₂〈Au, NiO〉 nanocomposite can be understood in terms of the sequence of processes that take place on the SnO₂ surface upon nitrogen dioxide adsorption in the presence of chemisorbed oxygen.     

Microstructure and H<Subscript>2</Subscript>S gas sensing properties of nanocrystalline perovskite-type La<Subscript>0.6</Subscript>Co<Subscript>0.4</Subscript>Mn<Subscript>0.5</Subscript>Ni<Subscript>0.5</Subscript>O<Subscript>3</Subscript>

Nanocrystalline La_1−x Co _x Mn_1−y Ni _y O₃ (x = 0.2 and 0.4; y = 0.1, 0.3, and 0.5) thick films sensors prepared by sol–gel method were studied for their H₂S gas sensitivity. The structural and morphological properties have been carried out by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Average particle size estimated from XRD and TEM analyses was observed to be 30–35 nm. The gas response characteristics were found to depend on the dopants concentration and operating temperature. The maximum H₂S gas response of pure LaMnO₃ was found to be at 300 °C. In order to improve the gas response, material doped with transition metals Co and Ni on A- and B-site, respectively. The La_0.6Co_0.4Mn_0.5Ni_0.5O₃ shows high response towards H₂S gas at an operating temperature 250 °C. The Pd-doped La_0.6Co_0.4Mn_0.5Ni_0.5O₃ sensor was found to be highly sensitive to H₂S at an operating temperature 200 °C. The gas response, selectivity, response time and recovery time were studied and discussed.     

Direct synthesis and hydrogen storage behaviors of nanocrystalline Na<Subscript>2</Subscript>LiAlH<Subscript>6</Subscript>

Nanocrystalline Na₂LiAlH₆ was directly synthesized by mechanical milling 2NaH/LiH/Al mixture with TiF₃ catalyst under hydrogen pressure of 3.0 MPa. The synthesized Na₂LiAlH₆ exhibits a dehydriding capacity of 3.09 wt% in the first cycle, which is higher than that of Na₃AlH₆. Because of the complexity of mass transfer, the rehydrogenation process of the dehydrided Na₂LiAlH₆ is more intricate than that of the dehydrided sodium alanate, causing the formation of Na₃AlH₆ and the reduction of rehydriding capacity in the following cycles. As temperature increases from 70 to 120 °C, hydrogen absorption kinetics is extremely enhanced. The dehydrided material can reabsorb 80% of the reversible hydrogen capacity within 5 min when the temperature is above 100 °C with an initial hydrogen pressure of 4 MPa. The scanning electron microscopy and energy dispersive X-ray spectroscopy show that the as-synthesized Na₂LiAlH₆ along with the catalyst form a much homogeneous composite with a spherical particle size of 200 nm–2 μm.     

Shape-selective synthesis of CeO<Subscript>2</Subscript> via an EDTA-assisted route

Nanoparticles and sub-microrods of cubic CeO₂ were selectively prepared via an ethylendiamine tetraacetate (EDTA)-assisted route. They were characterized by transmission electron microscopy (TEM), X-ray powder diffraction (XRD), and infrared (IR) techniques. Controlling the reaction process, we obtained CeO₂ with different shapes. Sub-microrods were formed via an incomplete reaction while uniform nanoparticles were formed through a complete reaction between NaClO₃ and Ce–EDTA complexes. The addition of EDTA was critical to obtain CeO₂ sub-microrods and nanoparticles. Other experimental conditions, such as EDTA/Ce³⁺ molar ratio and the reaction time were of importance in the final product morphology. A possible formation mechanism of CeO₂ was discussed. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

A facile method to enhance electrochemical performance of high-nickel cathode material Li(Ni<Subscript>0.8</Subscript>Co<Subscript>0.1</Subscript>Mn<Subscript>0.1</Subscript>)O<Subscript>2</Subscript> via Ti doping

The cycle stability of Li(Ni_0.8Co_0.1Mn_0.1)O₂ is enhanced obviously by titanium doping via a facile solid-state method. The property of crystal structure is evaluated by XRD, which illustrates the samples possessed a layered α-NaFeO₂ structure with R-3m space group. According to the charge/discharge studies, the capacity retention of pristine sample is around 51% after 125 cycles at 5 C, and the sample with Ti dopant displays a good cyclic stability, after 125 cycles, the capacity retention increases to 75% under 5 C, suggesting it could be possibly applied in fast charge Lithium-ion battery area. The superb electrochemical performance might be attributed to the Ti⁴⁺ occupy the layer structure to broaden the Lithium-ion channel, which is benefit to lithium intercalation and deintercalation during cycling.     

Synthesis of nanocrystalline TiB<Subscript>2</Subscript> powder from TiO<Subscript>2</Subscript>, B<Subscript>2</Subscript>O<Subscript>3</Subscript> and Mg reactants through microwave-assisted self-propagating high-temperature synthesis method

In this research work, microwave-assisted self-propagating high-temperature synthesis (SHS) process was employed for the fabrication of titanium diboride (TiB₂) compound from TiO₂–B₂O₃–Mg mixtures. Thermodynamic evaluations of this system and its relevant subsystems revealed that TiB₂–MgO composite powder can be easily produced by a SHS reaction. However, experimental results of a TiO₂ : B₂O₃ : 5Mg mixture heated in a domestic oven showed the formation of some intermediate compounds such as Mg₃B₂O₆, presumably due to some degree of Mg loss. The optimum amount of Mg in TiO₂ : B₂O₃ : xMg mixtures, yielding the highest amount of TiB₂ phase, was found to be around 7 mol, i.e., 40 mol% more than the stoichiometric amount. Experimental results revealed that a pure TiB₂ compound could be obtained by leaching the unwanted by-products in an HCl acid solution. Scanning electron microscopic observations and Scherrer calculations showed that the produced TiB₂ contains sub-micron (150–200 nm) particles, where each particle consists of a number of nanosized (32 nm) crystallites.     

Effect of heating rate on the microstructural and magnetic properties of nanocrystalline Fe<Subscript>81</Subscript>Si<Subscript>4</Subscript>B<Subscript>12</Subscript>P<Subscript>2</Subscript>Cu<Subscript>1</Subscript> alloys

The effect of heating rate on the structural and magnetic properties of the nanocrystalline Fe₈₁Si₄B₁₂P₂Cu₁ alloy has been investigated. Amorphous Fe₈₁Si₄B₁₂P₂Cu₁ alloy was annealed at 753 K for 180 s at different heating rates ranging from 0.05 to 5 K/s in protective argon atmosphere. The structural and magnetic properties of the as-quenched and annealed alloys were studied using X-ray diffractometer (XRD), differential scanning calorimeter (DSC), vibrating sample magnetometer (VSM), and B–H loop tracer, respectively. Amorphous precursor prepared by industry-grade raw materials is obtained. The increase of heating rate is found to be significantly effective in decreasing the grain size of α-Fe(Si) phase, but the grain size increases at higher heating rate. The volume fraction of α-Fe(Si) phase shows a monotonic decrease with the increase of the heating rate. The coercivity H _c markedly decreases with increasing heating rate and exhibits a minimum at the heating rate of 0.5 K/s, while the saturation magnetization, M _s, shows a slight decrease. These results suggest that the effect of heating rate on H _c and M _s is originated from the changes of grain size and the volume fraction of α-Fe(Si) phase.     

Enhanced thermal stability in K<Subscript>2</Subscript>O-metakaolin-based geopolymer concretes by Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and SiO<Subscript>2</Subscript> fillers addition

Based on the principle of stability of geopolymer gel as refractory binder, a geopolymeric paste in the K₂O–Al₂O₃–SiO₂ system was developed and used to produce refractory concretes by adding various amount of α-quartz sand (grain size in the range 0.1 μm to 1 mm) and fine powder alumina (grain size in the range 0.1–100 μm). The consolidated samples were characterized before and after sintering using optical dilatometer, DSC, XRD and SEM. The total shrinkage in the range of 25–900 °C was less than 3%, reduced with respect to the most diffused potassium or sodium based geopolymer systems, which generally records a >5% shrinkage. The maximum shrinkage of the basic geopolymer composition was recorded at 1000 °C with a 17% shrinkage which is reduced to 12% by alumina addition. The temperature of maximum densification was shifted from 1000 °C to 1150 or 1200 °C by adding 75 wt% α-quartz sand or fine powder alumina respectively. The sequences of sintering of geopolymer concretes could be resumed as dehydration, dehydroxylation, densification and finally plastic deformation due to the importance of liquid phase. The geopolymer formulations developed in this study appeared as promising candidates for high-temperature applications: refractory, fire resistant or insulating materials.