L. Titelman

Effect of porous support fabric on osmosis through a Loeb-Sourirajan type asymmetric membrane

Abstract Commercially available asymmetric membranes of the Loeb-Sourirajan (L-S) type comprise a support fabric, bonded to the porous substructure. The influence of this fabric on osmotic permeation flux was examined, mostly with a Toray CA-3000 membrane from which, with care, it was possible to remove the support fabric. In osmosis experiments with 12% MgCl2 solution on one side (either side) and 6% solution on the other, the permeation flux (J1) was of the order of 0.01 and 0.06 m3/m2 d with and without fabric, respectively. These results could be generalized by considering the resistivity to solute diffusion in the non-skin part of the membrane. This resistivity term averaged 104 and 17 d/m for membranes with and without fabric, respectively, and in further tests without fabric, it was between 15 and 25 d/m over a wide range of MgCl2 concentrations. Four other L-S membranes, all with support fabric, were tested in osmosis experiments. Their resistivity values were similar to or higher than those of the Toray membrane with fabric, but, with one of the four, the results were affected by switching the location of the high and low concentration solutions. It was concluded that existing commercially available L-S membranes are not appropriate for large-scale osmosis applications because their support fabric decreases permeation flux excessively.

Magnetic properties of nanocrystalline La1- xMnO3+δ manganites : Size effects

The magnetic properties of nanocrystalline manganites La1?xMnO3+? with particle size of 20 (LMO20), 25 (LMO25), and 30 nm (LMO30), prepared by the citrate method, have been investigated in the temperature range 5?320?K, magnetic field up to 90?kOe and under quasi-hydrostatic pressures up to 14.5?kbar. The studies involve sequential zero-field-cooled magnetization (M) measurements followed by magnetization measurements during cooling in the same magnetic field (H) and complementary measurements of ac susceptibility. Additional measurements of M versus H were carried out at ambient and applied pressures. All nanoparticles exhibit a paramagnetic to ferromagnetic transition (PFT) at a Curie temperature TC>200?K. It was found that the relative volume of the ferromagnetic phase increases for larger particle size and approaches a value of about 93% for LMO30. The real part of the ac susceptibility of sample LMO20 exhibits strong frequency dependence in a wide temperature range below TC, whereas for sample LMO30 only relatively weak frequency dependence was observed. The magnetization of sample LMO30 exhibits a PFT of second order; the type of transition could not be established for the smaller particles. It was found that an applied pressure enhances the TC of La1?xMnO3+? nanoparticles with a pressure coefficient of dTC/dP?1.9?K?kbar?1 for LMO20 and dTC/dP?1.4?K?kbar?1 for LMO25 and LMO30 samples. Peculiar magnetic memory effects observed for sample LMO20 are discussed.

Magnetotransport in granular LaMnO3+δ manganite with nano-sized particles

Transport and magnetic properties of compacted LaMnO3+? manganite nanoparticles of an average size of 18?nm have been investigated in the temperature range 5?300?K. The nanoparticles exhibit a paramagnetic-to-ferromagnetic (FM) transition at the Curie temperature TC ~ 246?K. However, the spontaneous magnetization disappears at a higher temperature of about 270?K. It was found that at low temperatures the FM core occupies about 50% of the particle volume. The temperature dependence of the resistivity shows a metal?insulator transition and a low-temperature upturn below the resistivity minimum at T ~ 50?K. The transport at low temperatures is controlled by the charging energy and spin-dependent tunnelling through grain boundaries. It has been found that the charging energy decreases monotonically with increasing magnetic field. The low temperature I?V characteristics are well described by an indirect tunnelling model while at higher temperatures both direct and resonant tunnelling dominates. The experimental features are discussed in the framework of a granular ferromagnet model.

Size-dependent spin state and ferromagnetism in La0.8Ca0.2CoO3 nanoparticles

Magnetic and structural properties of nanocrystalline low-doped La0.8Ca0.2CoO3 cobaltites with particle size of 8, 13, 23, and 50 nm, prepared by the glycine-nitrate method, were investigated in temperature range 5–320 K, magnetic field up to 50 kOe and under hydrostatic pressure up to 10 kbar. With particle downsizing, a noticeable expansion of unit cell, with concomitant changes in the rhombohedral structure toward the cubic one was observed. It was found that the increased surface-disorder effect strongly suppresses the ferromagnetic state in La0.8Ca0.2CoO3 nanoparticles leading to a decrease, by factor of about 2, both in spontaneous magnetization, MS, and Curie temperature, TC, when particle’s size decreases from 23 to 8 nm. The effective magnetic moment μeff was found also to decrease distinctly due to the strong interdependence between Co–O–Co interactions and Co spin state. The size-induced magnetic disorder drives the La0.8Ca0.2CoO3 nanoparticles to a dominant glassy behavior for 8 nm particles. This is evidenced by the fact that the freezing temperature varies with magnetic field in a strict conformity with the de Almeida–Thouless law for spin glasses and also by the observation of characteristic slowing down in the spin dynamics. The applied pressure suppresses TC, MS, and coercive field HC, like it is observed for bulk La0.8Ca0.2CoO3. Nevertheless, in nanoparticles the pressure effect on TC is noticeably stronger, while HC diminishes with pressure much slower then in bulk material.

Size-driven magnetic transitions in La1/3Ca2/3MnO3 nanoparticles

Magnetic properties of electron-doped La1/3Ca2/3MnO3 manganite nanoparticles with average particle size ranging from 12 to 42 nm, prepared by the glycine-nitrate method, have been investigated in temperature range 5–300 K and in magnetic fields up to 90 kOe. Reduction in the particle size suppresses antiferromagnetism and decreases the Neel temperature. In contrast to bulk crystals, the charge ordering does not occur in all studied nanoparticles, while a weak ferromagnetism appears above 200 K. Low temperature magnetic hysteresis loops indicate upon exchange bias effect displayed by horizontal and vertical shifts in field cooled processes. The spontaneous and remanent magnetization at low temperature shows a relatively complex variation with particle size. The size-induced structural/magnetic disorder drives the La1/3Ca2/3MnO3 nanoparticles to a pronounced glassy behavior for the smallest 12 nm particles, as evidenced by large difference between zero field cooled and field cooled magnetization, frequency ...

Effect of nanoporous ZrO2 crystal size on the surface sulphur capacity and performance of sulfated zirconia as an acidic catalytic material

Three sulfated zirconia materials were prepared with crystal size of tetragonal ZrO 2 of 2.5–15 nm and sulphur content corresponding to the full surface sulphur capacity of this phase. They were synthesized by sulfation of amorphous Zr hydroxide precipitated in presence and absence of block-copolymer non-ionic surfactant and of tetragonal zirconia preformed inside mesopores of SBA-15 silica matrix. It was demonstrated that while the crystal size of tetragonal ZrO 2 phase and its surface are determine the maximal amount of sulfate ions and concentration of acid sites in the material the acidity strength and catalytic activity in three different reactions is a function of preparation history.

Thermostable sulfated 2–4 nm tetragonal ZrO2 with high loading in nanotubes of SBA-15: a superior acidic catalytic material

The high-loaded (48-60 wt.%) 2-4 nm tetragonal ZrO2 phase inserted in mesostructured silica SBA-15 by chemical solution decomposition (CSD) of Zr(n-PrO)4 and activated at 873 K displayed approximately 3 times higher capacity for surface sulfate ions and, respectively, 1.5-2.2 times higher catalytic activity per gram of SO4-ZrO2/SBA-15 composite in condensation of MeOH with t-BuOH and dehydration of isopropanol compared with the regular bulk sulfated zirconia material.