Watergel in Liquid Helium

Semitransparent soft water clouds forming in superfluid He-II by condensation of a gaseous mixture of ⁴He with water impurities transform with time to more rigid, highly porous icebergs. The icebergs suspended in the bulk of He-II are stable at a constant temperature T1.6 K, and they can beak down to small ice pieces on heating the liquid above T. The temperature of decomposition of the icebergs in He-I depends strongly on the vapor pressure above the surface of the liquid: at P0.2 atm they start to decompose at T_d2.5 K, but increasing the pressure to 1 atm causes T_d to rise to 4 K. When withdrawn from He-II the dry icebergs segregate to an ice powder and He on heating them above 1.8 K in He gas atmosphere. The total content of the water in the bulk of the icebergs, estimated from the ratio by volume of icebergs and powder, is 10²⁰ molecules/cm³. From observations of acoustic oscillations in the cell filled with He-1 (the ratio of amplitudes of vibrations of the iceberg and He-I level is about 0.2–0.3) it can be estimated that the density of the iceberg is only a few percent higher than the density of the surrounding liquid. We suggest that the highly porous water condensate (watergel) is composed of water nanoclusters, coated with 1-2 layers of solidified helium, which form a dispersed system of gel, and of liquid Helium, filling the pores between these van-der-Waals complexes, which serves as the dispersion medium of the gel.     

Synthesis and study of organically capped ultra small clusters of cadmium sulphide

Ultra small clusters of cadmium sulphide are synthesized using non-aqueous and aqueous chemical methods. Thiophenol has been used as a capping agent for non-aqueous synthesis whereas various reagents such as mercaptoethanol, hexametaphosphate, ethylene glycol and ethanol have been used as additives for an aqueous method of synthesis. Properties of the clusters synthesized are discussed based on optical absorption, X-ray diffraction, transmission electron diffraction and photoelectron spectroscopy. Particles as small as 0.7 nm diameter could be synthesized with thiophenol and mercaptoethanol as additives. The effect of varying the molarities of the different additives on the properties of the CdS nanoclusters synthesized are discussed. Systematic ageing studies of the nanoclusters showed that larger particles age faster than the smaller clusters. Ageing also leads to better crystallization of the particles. It has been observed that the smallest particles (0.7 nm diameter) possess tetrahedrally bonded fragments of CdS and intercluster structural long range order does not exist. However, bigger particles (2.0 nm diameter) show bulk cubic structure. X-ray photoelectron spectroscopy studies have been done to study the purity and stoichiometry of the clusters synthesized and strongly support the existing proposal of the formation and stability of CdS nanoclusters.     

Phase formation kinetics of nanoparticle-seeded strontium bismuth tantalate powder

Strontium bismuth tantalate (SBT) having composition of Sr_0.7Bi_2.3Ta₂O₉ has been prepared through sol-gel method using their corresponding metal alkoxides as precursors. Seeded SBT powder was prepared by the addition of 5 wt% of nanometer-sized SBT particles to the sol followed by pyrolysis. Differential thermal analyses (DTA) were performed on the unseeded and seeded powder and Aurivillius phase formation temperatures were found to be reduced by 60°C in the seeded ones. Non-isothermal kinetic analyses were applied to the DTA results to obtain activation energy and Avrami exponent values for the Aurivillius phase formation in unseeded and seeded samples. The activation energy for the Aurivillius phase formation was found to be 268 kJ/mol for the seeded ones, while 375 kJ/mol for the unseeded ones, which plays a major role for the enhanced kinetics in the seeded ones. The Avrami exponent values for the Aurivillius phase formation in unseeded and seeded ones were determined as 2.80 and 1.15, respectively, revealing different nucleation and crystal growth mechanisms.     

Adsorption of polyvinylpyrrolidone (PVP) and its effect on the consolidation of suspensions of nanocrystalline CeO2 particles

CeO₂ particles with an average size of 9 nm were synthesized under hydrothermal conditions. The adsorption of polyvinylpyrrolidone (PVP) on to the particle surfaces was measured in aqueous suspensions in the pH range of 3.7 to 11.5. The amount of adsorbed PVP decreased significantly with increasing pH value. For suspensions prepared at a pH value of 3.7, complete adsorption occurred for 2.5 wt% of PVP added to the suspension. Further additions of PVP produced a gradual increase in the adsorption until a limiting value was reached when the total amount of PVP added to the suspension was 10 wt%. At this PVP concentration, 6 wt% of the PVP was adsorbed and 4 wt% remained free in solution. The effect of the adsorbed PVP on the microstructural homogeneity of films deposited by spin coating of suspensions was investigated. With no addition of PVP, crack-like voids were prevalent in the dried and sintered films. Crack-free films were obtained from suspensions containing 10 wt% of PVP. Higher PVP additions (25 wt%) produced an increase in the viscosity of the suspension but no observable change in the microstructural homogeneity of the films. The use of adsorbed polymers for steric stabilization coupled with data from the adsorption isotherms is shown to provide a rational approach to the deposition of homogeneous films from suspensions of nanocrystalline particles.     

TEM observation of iron oxide pillars in montmorillonite

The microstructures of iron oxide-pillared montmorillonite have been investigated by highresolution transmission electron microscopy. The iron oxide is incorporated in the interlayer space of montmorillonite as Fe₂O₃ crystallites. The crystallites are present in two different morphologies: isolated particles 10 nm long and 4 nm thick, and bands (possibly plates) with a thickness 1 nm running parallel to the basal plane.     

Chemical doping and improved flux pinning in Hg-based superconductors

Chemical doping of a small amount (0.020.3) of rhenium (Re) ion into HgBa₂Ca_n–1Cu_nO_y(n=14) has been studied. The flux pinning strength in these Hg-based superconductors is significantly enhanced compared to the undoped parent materials. Neutron diffraction analysis of representative samples (n=3) revealed that Re partially substitutes the Hg-site with octahedrally fully coordinated oxygen atoms, giving rise to the stabilization of Hg(Re)O layer and reduction of this blocking layer thickness. Associated with the expected hybridization of Re-5d and O-2p orbitals and possible metallization of the Hg(Re)O layer, the improved flux pinning behavior is considered to be caused by the decrease of the electromagnetic anisotropy and the enhanced interlayer superconducting coupling strength.     

Carbothermal reduction synthesis of nanocrystalline zirconium carbide and hafnium carbide powders using solution-derived precursors

Zirconium carbide (ZrC) and hafnium carbide (HfC) powders were produced by the carbothermal reduction reaction of carbon and the corresponding metal oxide (ZrO₂ and HfO₂, respectively). Solution-based processing was used to achieve a fine-scale (i.e., nanometer-level) mixing of the reactants. The reactions were substantially completed at relatively low temperatures (<1500°C) and the resulting products had small average crystallite sizes (50–130 nm). However, these products contained some dissolved oxygen in the metal carbide lattice and higher temperatures were required to complete the carbothermal reduction reactions. Dry-pressed compacts prepared using ZrC-based powders with 100 nm crystallite size could be pressurelessly sintered to 99% relative density at 1950°C.     

CO/pH2: A Molecular Thermometer

We utilize reversible temperature dependent changes in the IR absorption spectrum of CO molecules isolated in solid parahydrogen (pH₂) to probe bulk temperature changes during rapid vapor deposition. The intensity of a well resolved feature near 2135 cm^–1 increases monotonically with temperature over the 2 to 5 K range. The thermally populated initial state of this transition lies 12 K above the CO/pH₂ ground state. During the deposition of 100 ppm CO/pH₂ samples, we detect temperature gradients 10 K/cm in 0.1 cm-thick samples subjected to heat loads 10 mW/cm². The resulting estimated thermal conductivity (TC) is 3(±2) mW/cm-K, averaged over the 2 to 5 K region. This value is 1000 times lower than the TC of single crystal solid pH₂, and 10 times lower than previously measured for pH₂ solids doped with 100 ppm concentrations of heavy impurities [Manzhelii, Gorodilov, and Krivchikov, Low Temp. Phys. 22, 131 (1996)]. We attribute this abnormally low TC to the known mixed fcc/hcp structure of the rapid vapor deposited solids.     

Sol-gel prepared Ni-alumina composite materials

The microstructure of sol-gel prepared Ni-alumina ceramic-metal composites containing up to 50% Ni has been studied with X-ray diffraction, the scanning electron microscope and the transmission electron microscope. The influence of processing temperature upon the size distribution of Ni was established. It was found that increasing the total amount of Ni increases only the number of micrometre-size Ni inclusions in the alumina, whereas the hot-pressing temperature determines the size distribution of Ni. When temperatures much higher than the melting temperature of Ni are used, a large number of Ni inclusions of the order of 10 nm can be found mainly within alumina grains; only a few are formed in grain boundaries and in triple points. When a temperature close to the melting point is used, there are fewer nanometre-size Ni inclusions and a larger number of Ni inclusions of the order of 100 nm to 1 m. In this case, the large ( 100 nm) and small ( 10 nm) Ni inclusions are found in grain boundaries and triple points.     

Pyrosol deposition of fluorine-doped tin dioxide thin films

Fluorine-doped tin dioxide (SnO₂F) films were deposited from a tin tetrachloride solution in methanol utilizing a pyrosol deposition process. It is shown from thermodynamic calculations that the atmosphere during deposition is oxygen-rich and also suggested that chlorine and hydrogen chloride, which are produced during the deposition reaction, influence crystal growth. Detailed electrical, optical and structural properties of the material with respect to varying film thickness and substrate temperature are presented and discussed. Resistivity of the films deposited at 450 °C decreased from 6×10^–4 to 2×10^–4 cm, while the mobility increased from 14 to 45 cm²V^–1s^–1, respectively, when the film thickness was varied from 100 to 1650 nm. The carrier concentration was relatively unchanged for film thicknesses higher than 200 nm. Optimized SnO₂F films (600 nm) having a resistivity of 6×10^–4 cm, a carrier mobility of 20 cm²V^–1s^–1, a carrier concentration of 8×10²⁰ cm^–3 and a transmittance in excess of 80% are quite suitable as electrodes for amorphous silicon solar cells.