Synthesis of beaded poly(vinyl ether) solid supports with unique solvent compatibility

Poly(vinyl ether) gels SLURPS (Superior Liquid Uptake Resin for Polymer-supported synthesis) with low cross-linking levels have been synthesized for the first time in beaded form using a non-aqueous inverse suspension polymerisation approach. The synthetic protocol was optimized with regards to several parameters including reactions conditions, type and concentration of suspension stabilizer and controlled low temperature addition of co-initiator. Particle size measurements confirm the production of beads with average diameters of 700-950 μm. Optimization of the monomer composition of the poly(vinyl ether) gels resulted in a novel beaded polymer support with considerably improved as well as unique swelling characteristics in solvents ranging from hexane to water. The synthetic utility of the new gel was confirmed by carrying out a set of transformations with complete conversion leading to a useful amino and hydroxy terminated solid-phase precursor resin. Reaction progress could be monitored easily by ¹H and ¹³C gel-phase NMR.     

Electrothermal properties of perovskite ferroelectric films

The electrothermal properties of the perovskite oxides barium titanate (BTO), lead titanate (PTO), and strontium titanate (STO) are computed near the temperatures of their ferroelectric and/or ferroelastic phase transitions. The computations are performed using a modified 2-4-6 Ginzburg–Landau–Devonshire polynomial as functions of applied electric field and temperature for mechanically free monodomain crystals and for epitaxial thin films subject to perfect lateral clamping. For BTO and PTO, which display weak first-order ferroelectric phase transitions at their Curie points, the application of a bias field exceeding the electrical critical point reduces the dependence of the electrocaloric (EC) response on temperature and automatically reduces its magnitude. Under conditions of perfect lateral clamping, the weak first-order phase change is transformed into second-order phase change. In this instance the electrical critical point is coincident with the Curie temperature and a lower bias field is required to produce a comparable reduction in temperature sensitivity. Comparison of the electrothermal behaviors of BTO and PTO with that computed for STO near the temperature of the second-order ferroelastic phase transition provides insight concerning the EC properties of ferroelectric solid solution systems wherein the Curie temperature and the first-order character of the paraelectric to ferroelectric transition both may change subject to a change in composition. The results illustrate how electrical and mechanical boundary conditions can be adjusted, in conjunction with composition, in altering the EC properties of ferroelectric materials selected for use in a particular temperature range.     

Underground mining technique selection by multicriterion optimization methods

The authors propose a number of modified versions of well-known multicriterion optimization methods and discuss their application to select an optimal underground chromite mining technique. __________ Translated from Fiziko-Tekhnicheskie Problemy Razrabotki Poleznykh Iskopaemykh, No.4, pp. 78–89, July–August, 2008.     

Hysteresis Offset and Dielectric Response of Compositionally Graded Ferroelectric Materials

Ferroelectrics with a spatial variation in the polarization exhibit behavior and properties that are not routinely observed from homogenous bulk or thin film ferroelectrics. A systematic spatial variation in the polarization in a ferroelectric material can be achieved by variations in the composition of the material, impressing temperature gradients across the structure, or by imposing non-uniform external stress fields. In this article, a Landau-Ginzburg thermodynamic model is used to develop a methodology for analyzing compositionally graded ferroelectric materials. The composition gradient is shown to give rise to a corresponding polarization gradient. The misfit due to variations in the self-strain tends to flatten the polarization gradient and leads to the bending of the ferroelectric. The polarization gradients result in asymmetric hysteresis responses with “up” or “down” charge offsets which are strong functions of composition. Analysis shows that the compositional grading results in a broadening in the dielectric response as a function of temperature.     

Minimization of imaging gradient effects in diffusion tensor imaging

A new, sample independent optimization criterion for minimizing the effect of the imaging gradients, including the directional awareness they create, is defined for diffusion tensor imaging (DTI) experiments. The DTI linear algebraic framework is expanded to a normed space to design optimal diffusion gradient schemes (DGS) in an integral fashion without separating the magnitude and direction of the DGS vectors. The feasible space of DGS vectors, for which the estimation equations are determinate, thus a hard constraint for the optimization, is parametrized. Newly generated optimal DGSs demonstrate on an isotropic sample and an ex-vivo baboon brain that the optimization goals are reached as well as a significant increase in estimation performance.     

Defect microstructures in epitaxial PbZr0.2Ti0.8O3 films grown on (001) SrTiO3 by pulsed laser deposition

Transmission electron microscopy has been used to investigate the character and distribution of the microstructural features in epitaxial (001) ferroelectric PbZr_0.2Ti_0.8O₃ films grown on (001) SrTiO₃ substrates by pulsed laser deposition. The TEM observations revealed that the films were predominantly c-oriented with embedded a ₁- and a ₂-oriented domains lying on {101} planes. The substrate/film interfaces contained arrays of edge-type misfit dislocations and there were extraordinarily high densities ( ) of threading dislocations in the films. The character and distribution of these features are consistent with the following relaxation sequence. Firstly, the lattice misfit between the phases is accommodated at the growth temperature by the introduction of misfit dislocations at the edges of island nuclei, and some of these dislocations are forced away from the interface to form threading segments upon island coalescence. Next, the film adopts the c-orientation upon cooling through the Curie temperature with a ₁- and a ₂-oriented domains being formed to ameliorate the self-strain of the transformation. Finally, some redistribution of the embedded domains and misfit dislocations occurs in response to stresses caused by expansion coefficient differences. The impact of these defects on the electrical and electromechanical properties of epitaxial ferroelectric properties is discussed.