Search

The magnetic phase transitions of the ternary compounds TbIrGe 2 and HoIrGe 2 have been studied by magnetic, specific heat and neutron diffraction measurements. Both compounds crystallize in the orthorhombic YIrGe 2 -type structure (space group Immm ) in which the rare earth atoms occupy two non-equivalent crystallographic sites. The rare earth magnetic moments at different crystallographic sites order independently with two different types of magnetic ordering. The Tb and Ho moments at 4 i site form a collinear commensurate antiferromagnetic structure (Tb moments lie in bc -plane while those of Ho in ab -plane ). The magnetic moments at 4 h site also form a collinear antiferromagnetic structure at low temperature but the orientation of magnetic moments is different: Tb moments are aligned along the c-axis while Ho moments lie in ac -plane (in case of TbIrGe 2 the magnetic unit cell is four times larger than the crystallographic one). In TbIrGe 2 the magnetic order of moments at 4 h site at T t = 9 K turns into a sine modulated one with k ⇒ = [ 0.45 , 0 , 0 ] . The Neel temperatures for the different sublattices are: for 4 h site ∼ 12 K (Tb) and 5 K (Ho) and for 4 i site: 10 K (Tb) and 2.3 K (Ho). The above results suggest that interactions between the moments at 4 h site are stronger than those for 4 i site. Each sublattice has an uniaxial antiferromagnetic moment arrangement but different reorientation processes and different ordering temperatures reflect the presence of two distinct order parameters.

4 pags., 5 figs., 2 tabs., Ultra-microindentation was used to measure the microhardness and modulus of silk (Bombyx mori) membranes, cast from 20 to 80°C. The microhardness and modulus were determined from the loading/unloading curves. The membranes exhibit microhardness of about 400 MPa which is larger than the values for most common synthetic polymers (50-300 MPa) implying a greater scratch resistance. The moduli are of the order of those measured by the other means for B. mori silk membranes (5 GPa), and fibers (7-11 GPa). There is some correlation between microhardness and the dimensions of the grains/nanofibrils, but none with surface roughness. The results extend the range of an empirical correlation between microhardness and modulus. The present data together with previous data from other polymers fit the equation, H=0.55E0.74, with a correlation coefficient of 0.94. Finally, it is shown that elastic recovery of the silk membranes is an increasing function of the maximum load applied. © 2004 Elsevier Ltd. All rights reserved., This work was supported in part by Air Force Office of Scientific Research (AFOSR) Award number F49620-03-1-0169. Grateful acknowledgement is due to MCYT (grantBFM2000-1474) for the support of this investigation. Oneof us, I.P.O., also acknowledges the support of the FPIProgramme of MCYT (Spain).

17 pags., 7 figs., The present work is concerned with the study of the development of the crystalline structure of poly(ethylene terephthalate) in multilayered films of poly(ethylene terephthalate)/polycarbonate (PET/PC) prepared by means of layer multiplying coextrusion. Small angle X-ray scattering patterns were recorded during isothermal crystallization experiments and evaluated by means of Ruland's interface distribution function. Thus, structural parameters describing the thickness distribution of crystalline and amorphous layers were determinated. It is shown that the crystallization of PET is delayed with increasing confinement. However when the crystallization process comes to an end, the values of the nanostructural parameters of the lamellar system are nearly the same for the confined and non-confined PET. © 2009 Elsevier Ltd.