Chemical size effect on the magnetic and electrical properties in the (Tb(1-)(x)Eu(x))MnO(3) (0 < or = x < or = 1.0) system

The effect of isovalent chemical substitution of Eu3+ into the Tb3+ sites on the magnetic and electrical properties of (Tb1-xEux)MnO3 (0 相似文献   

Stable oxide nanoparticle clusters obtained by complexation

We report on the electrostatic complexation between polyelectrolyte-neutral copolymers and oppositely charged 6-nm crystalline nanoparticles. For two different dispersions of oxide nanoparticles, the electrostatic complexation gives rise to the formation of stable nanoparticle clusters in the range 20-100 nm. It is found that inside the clusters, the particles are "pasted" together by the polyelectrolyte blocks adsorbed on their surface. Cryo-transmission electronic microscopy allows visualization of the clusters and determination of the probability distribution functions in size and in aggregation number. The comparison between light scattering and cryo-microscopy results suggests the existence of a polymer brush around the clusters.     

Experimental observation of spatially localized dynamo magnetic fields

We report the first experimental observation of a spatially localized dynamo magnetic field, a common feature of astrophysical dynamos and convective dynamo simulations. When the two propellers of the von Kármán sodium experiment are driven at frequencies that differ by 15%, the mean magnetic field's energy measured close to the slower disk is nearly 10 times larger than the one close to the faster one. This strong localization of the magnetic field when a symmetry of the forcing is broken is in good agreement with a prediction based on the interaction between a dipolar and a quadrupolar magnetic mode.     

Measurement techniques for unsteady flows in turbomachines

 The growing interest for unsteady flows in turbomachines over the last two decades has led to an intensive development of fast response measurement techniques, capable of resolving with high frequency phenomena related to inlet distortion, rotating stall and blade row interference effects with blade passing frequencies ranging from 3 to 30 kHz. This development was favoured by major advances in sensor technology and data acquisition systems. The paper reviews the progress in fast response measurement techniques for high speed turbomachinery and application with emphasis on fast response pressure and temperature probes and blade surface sensors including pressure, heat transfer and shear stress determination. Received: 9 November 1998/Accepted: 21 September 1999     

Electron ionization mass spectral fragmentation of derivatized 4,5- and 5,6-epoxysterols

The electron ionization (EI) mass spectral fragmentation of derivatized 4,5- and 5,6-epoxysterols was investigated. Interesting fragmentation processes involving a transannular cleavage of the epoxide ring after transfer of the trimethylsilyl group are significant in the case of 4,5-epoxysterol trimethylsilyl ethers (affording abundant fragment ions at m/z 403 and 404). Different pathways, which have been substantiated by deuterium labelling, are proposed in order to explain the formation of these ions. In contrast, this transfer is not significant in the case of 5,6-epoxysterol trimethylsilyl ethers. The EI mass spectra of these latter compounds appear to be very complex and to differ slightly according to the stereochemistry of the epoxy group. Acetate and trifluoroacetate derivatives of 4,5-epoxysterols display interesting EI mass spectra dominated by a fragment ion at m/z 332 resulting from cleavage of the steroid ring A.     

Precipitation-redispersion of cerium oxide nanoparticles with poly(acrylic acid): toward stable dispersions

We exploit a precipitation-redispersion mechanism for complexation of short chain polyelectrolytes with cerium oxide nanoparticles to extend their stability ranges. As synthesized, cerium oxide sols at pH 1.4 consist of monodisperse cationic nanocrystalline particles having a hydrodynamic diameter of 10 nm and a molecular weight of 400 000 g mol(-1). We show that short chain uncharged poly(acrylic acid) at low pH when added to a cerium oxide sols leads to macroscopic precipitation. As the pH is increased, the solution spontaneously redisperses into a clear solution of single particles with an anionic poly(acrylic acid) corona. The structure and dynamics of cerium oxide nanosols and their hybrid polymer-inorganic complexes in solution are investigated by static and dynamic light scattering, X-ray scattering, and chemical analysis. Quantitative analysis of the redispersed sol gives rise to an estimate of 40-50 polymer chains per particle for stable suspension. This amount represents 20% of the mass of the polymer-nanoparticle complexes. This complexation adds utility to the otherwise unstable cerium oxide dispersions by extending the range of stability of the sols in terms of pH, ionic strength, and concentration.     

Melting and glass transitions in paraffinic and naphthenic oils

Naphthenic and paraffinic oils were analyzed by modulated differential scanning calorimetry (MDSC). The results showed several improvements in the analysis of thermal properties when compared with standard DSC. The glass transition temperature (T_g), the enthalpy relaxation at T_g, and the melting endotherms could be deconvoluted, and reversible melting could be identified. This allowed for an easier interpretation of the thermal properties of the oils. With MDSC, the T_gs in mineral oils were found to coincide with endothermic enthalpy relaxation, which is generally regarded as a melting endotherm with standard DSC. A decrease in heat capacity after T_g was attributed to the existence of rigid amorphous material. From Δc_p at T_g and the oil molecular weight, the number of repeat units in the oil chains was estimated at less than 20. The T_g of a hypothetical pure aromatic oil was found to be similar to that for petroleum asphaltenes, and that for a naphthenic oil of infinite molecular weight to be similar to that of petroleum resins.     

Influence of substrate hydrophobicity on the adsorption of an amphiphilic diblock copolymer

The adsorption of poly(tert-butylmethacrylate)-block-poly(2-(dimethylamino-ethyl) methacrylate) (PtBUMA-b-PDMAEMA) was studied by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) analysis performed on dried samples. The copolymer was dissolved in toluene at concentrations below (0.01 wt%) and above (0.05 and 1 wt%) the CMC; silicon (SiOH) and CH(3)-grafted silicon (SiCH(3)) were used as substrates. Whatever the concentration and the substrate, a layer of individual copolymer molecules, 1.5-3 nm thick, formed rapidly. The adsorbed amount was slightly higher and the resistance to AFM tip scraping was stronger on SiOH than on SiCH(3). This is attributed to hydrogen bonding between the PDMAEMA block and the OH groups of the silicon surface, leading to polarization of the adsorbed layer. Above the CMC, on SiOH, randomly scattered dot-like features (about 5 nm high) observed by AFM were attributed to individual micelles, which were not displaced by drying. On SiCH(3), the particles found on the top of the adsorbed layer were micelle aggregates, about 50 nm thick, the lateral size of which was strongly influenced by the rate of drying. This further difference between SiCH(3) and SiOH is tentatively attributed to the exposure of PDMAEMA by the adsorbed layer formed on SiCH(3), while only PtBUMA would be exposed by the layer adsorbed on SiOH. The red blood cell shape and the size of the micelles observed in single layers indicate that the PtBUMA corona was not made compact as a result of drying.     

Differences between tethered polyelectrolyte chains on bare mica and hydrophobically modified mica

This study investigates the structures of layers of amphiphilic diblock copolymers of poly(t-butyl styrene)-poly(styrene sulfonate) (PtBS-PSS) adsorbed on both the bare mica surface (hydrophilic) and an octadecyltriethoxysilane (OTE)-modified mica surface (hydrophobic). When the surface is rendered hydrophobic, the nonsoluble block exhibits stronger interaction with the surface and higher adsorbed masses are achieved. Interaction forces between two such adsorbed layers on both substrates were measured using the surface forces apparatus. The effect of salt concentration (Cs) and molecular weight (N) on the height of the self-assembled layers (L0) was examined in each case. The resulting scaling relationship is in good agreement with predictions of the brush model, L0 proportional to N(1.0) in the low-salt limit and L0N(-1) proportional to (Cs/sigma)(-0.32) in the salted regime, when adsorption takes place onto the hydrophobized mica surface. For adsorption on the bare mica surface, L0N(-0.7) proportional, variant Cs(-0.17) agrees with the scaling prediction of the sparse tethering model. The results suggest that, on the hydrophilic bare mica surface, the adsorbed amount is not high enough to form a brush structure and only very little intermolecular stretching of the tethered chains occurs; in contrast, the presence of the hydrophobic OTE layer increases the tethering density such that the polyelectrolyte chains adopt a brush conformation.     

Collective dynamics of interacting molecular motors

The collective dynamics of N interacting processive molecular motors are considered theoretically when an external force is applied to the leading motor. We show, using a discrete lattice model, that the force-velocity curves strongly depend on the effective dynamic interactions between motors and differ significantly from those of a simple approach where the motors equally share the force. Moreover, they become essentially independent of the number of motors if N is large enough (N> or approximately 5 for conventional kinesin). We show that a two-state ratchet model has a very similar behavior to that of the coarse-grained lattice model with effective interactions. The general picture is unaffected by motor attachment and detachment events.