A method for the formation of monodisperse polystyrene (PS) and polymethylmethacrylate (PMMA) spheres, in the micron diameter range, was developed. The polymerisation reaction was carried out in a series of alcohols, using homo‐ and co‐polymers as steric stabilisers in combination with a quaternary ammonium salt which probably acts as an electrostatic co‐stabiliser. Monodisperse PS spheres were formed in the particle size range of 1–6 μm, by a single step process. The size range could be extended by further addition of monomer to the reaction mixture during the polymerisation. The solubility parameter of the alcohol used and the nature of the surfactant determine the diameter of the resulting spheres. However, the concentration of surfactant did not seem to affect strongly the size of the spheres but only the dispersity.
Layered metal disulfides-MS(2) (M = Mo, W) in the form of fullerene-like nanoparticles and in the form of platelets (crystallites of the 2H polytype) have been intercalated by exposure to alkali metal (potassium and sodium) vapor using a two-zone transport method. The composition of the intercalated systems was established using X-ray energy dispersive spectrometer and X-ray photoelectron spectroscopy (XPS). The alkali metal concentration in the host lattice was found to depend on the kind of sample and the experimental conditions. Furthermore, an inhomogeneity of the intercalated samples was observed. The product consisted of both nonintercalated and intercalated phases. X-ray diffraction analysis and transmission electron microscopy of the samples, which were not exposed to the ambient atmosphere, showed that they suffered little change in their lattice parameters. On the other hand, after exposure to ambient atmosphere, substantial increase in the interplanar spacing (3-5 A) was observed for the intercalated phases. Insertion of one to two water molecules per intercalated metal atom was suggested as a possible explanation for this large expansion along the c-axis. Deintercalation of the hydrated alkali atoms and restacking of the MS(2) layers was observed in all the samples after prolonged exposure to the atmosphere. Electric field induced deintercalation of the alkali metal atoms from the host lattice was also observed by means of the XPS technique. Magnetic moment measurements for all the samples indicate a diamagnetic to paramagnetic transition after intercalation. Measurements of the transport properties reveal a semiconductor to metal transition for the heavily K intercalated 2H-MoS(2). Other samples show several orders of magnitude decrease in resistivity and two- to five-fold decrease in activation energies upon intercalation. These modifications are believed to occur via charge transfer from the alkali metal to the conduction band of the host lattice. Recovery of the pristine compound properties (diamagnetism and semiconductivity) was observed as a result of deintercalation.
Unique occurrence of magnetism is shown, in which magnetism appears ex nihilo, when organic molecules are self-assembled as monolayers on gold substrate. The molecules as well as the substrate, when they stand alone, are diamagnetic. Using a superconducting quantum interference device type magnetometer we obtained direct evidence that close-packed organized thio-organic films adsorbed on gold substrates possess magnetic properties at room temperature. The films studied show very high specific magnetization, up to many tens Bohr magnetons per adsorbed molecule, with a very small hysteresis. It is highly anisotropic and shows almost no temperature dependence. The magnetism observed is related to charge transfer between the organic layer and the metal substrate. Yet, the uniqueness here is that many spins are polarized per adsorbed molecules. The magnetic effect is related to the two dimensional organization of the organic molecules on the metal substrate which might explain the high anisotropy.
Films of polystyrene‐poly(vinylmethyl ether) blends of various compositions are formed by a dip‐coating procedure, the thickness of the film being controlled by the concentration of the solution. The substrates used are glass and gold. The phase separation process is followed by a laser light scattering experiment in which the total forward scattering intensity is monitored as a function of temperature. Morphological examination shows that phase separation occurs by a spinodal decomposition mechanism. A thickness effect on the phase separation temperature is noticeable when film thickness is smaller than 1 μm. This effect is substrate dependent. In all films formed on gold the spinodal temperature increases as film thickness decreases. Films formed on glass exhibit a destabilizing effect on decreasing film thickness. This effect is slight in films of composition poorer in polystyrene than the critical composition, and is enhanced in films richer in polystyrene. The stabilizing effect of decreasing the thickness of films formed on the gold substrate is considered to reflect mainly a purely geometrical effect. The decreasing dimensionality is shown by simple theoretical considerations to increase the phase‐separation temperature. However, the phase separation behavior of thin films on glass appears to be the result of two kinds of substrate‐polymer interactions in addition to the geometrical effect: (a) electrostatic interaction of the charged glass surface (a destabilizing effect at all film compositions) and (b) selective adsorption of polystyrene on glass.
WO3 crystals with a surface composition of Na0.05WO3 were grown. These crystals exhibit a sharp diamagnetic step in magnetization at 91 K, and a magnetic hysteresis below this temperature. As the temperature is lowered below 100 K in transport measurements, a sharp metal to insulator transition is observed, this is followed by a sharp decrease in the resistivity when the temperature is lowered to about 90 K. When the surface of the crystals was covered by gold the depth of the diamagnetic step had decreased considerably. These results indicate a possible nucleation of a superconducting phase on the surface of these crystals. This is a non cuprate system exhibiting a critical temperature in the HTS range. PACS. 74.70.-b Superconducting materials (excluding high-Tc compounds) -74.20.-z Theories and models of superconducting state -74.60.Ec Mixed state, critical fields, and surface sheathTwo dimensional and quasi 2D superconductivity are common phenomena. The first example goes back to 1963 when Saint-James and de Gennes [1] showed that in an ideal sample with anhomogenous order parameter the nucleation of superconducting regions in decreasing field should always occur near the surface of the sample. For a superconductor of the second kind this implies that in fields H : H 2 < H < H 3 there remains a superconducting sheath on some parts of the sample confined to a thickness of the order of the correlation length ξ. In HTS perovskite cuprates the order parameter is spatially modulated along the c axis as the superconductivity is quasi 2D in character and is confined to the CuO planes coupled by Josephson junctions. A question arises about a possible system in which the structure is modulated in such a way so that superconductivity will develop on the surface only, not a type III superconductivity, and will not propagate into the bulk. In this short paper we suggest that WO 3 single crystals doped on the surface with sodium may constitute such a superconductor. This is a non cuprate system with a critical temperature of 91 K.The 5d-transition-metal oxides WO 3 and Na x WO 3 are similar in electronic structure to 3d oxides. They have nearly the ABO 3 perovskite crystal structure, with the W ions occupying the octahedral B cation sites. In WO 3 the A cation site is vacant, while in Na x WO 3 the Na + ions occupy that site. Stoichiometric WO 3 is an insulator, since the W 5d band is empty; when Na ions are added to WO 3 , they donate their 3s electron to the W 5d band, resulting in bulk metallic behavior for x ≥ 0.3 [2,3]. These materials in tetragonal or hexagonal form exhibit also bulk superconductivity at sub-liquid helium temperature [4,5]. Sheet superconductivity at 3 K in twin walls of a non superconducting tetragonal WO 3−x was reported by Aird et al. in 1998 [6,7]. For x < 0.3 the Na x WO 3 sodium tungsten bronzes, formed by doping the insulating host WO 3 with Na + ions, are n-type semiconductors.We prepared single crystals in which the surface is doped with Na + ions following the method described in reference...
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