Magnetic levitation above the superconducting YBa2Cu3O7−x /polymer 0-3 and 2-2 composites

Superconducting YBa₂Cu₃O_7–x /polyurethane 0-3 composites have been fabricated with several volume fraction fillers. Magnetic forces between a small permanent magnet and the composites have been measured at 77 K and show hysteretic behaviour as a function of their separation distances. Such behaviour indicates multiple equilibrium heights of the magnet levitating above the composites. It is shown that the levitation height increases with increasing volume fraction of superconductor filler. The parallel YBa₂Cu₃O_7–x /polyurethane and series YBa₂Cu₃O_7–x /polypropylene 2-2 composites were also prepared and only tested for magnetic levitation.     

Nanostructured Thermoelectric Materials and High-Efficiency Power-Generation Modules

For thermoelectric applications, the best materials have high electrical conductivity and thermopower and, simultaneously, low thermal conductivity. Such a combination of properties is usually found in heavily doped semiconductors. Renewed interest in this topic has followed recent theoretical predictions that significant increases in performance are possible for nanostructured materials, and this has been experimentally verified. During exploratory synthetic studies of chalcogenide-based bulk thermoelectric materials it was discovered that several compounds spontaneously formed endotaxially embedded nanostructures. These compounds have some of the best known properties for bulk thermoelectric materials in the 500–800 K temperature range. Here we report our continued efforts to better understand the role of the nanostructures while concurrently furthering the development of these new materials (for example n-type lead–antimony–silver–tellurium, and p-type lead–antimony–silver–tin–tellurium) into thermoelectric power-generation devices.     

Resolution cannot polynomially simulate compressed-BFS

Many algorithms for Boolean satisfiability (SAT) work within the framework of resolution as a proof system, and thus on unsatisfiable instances they can be viewed as attempting to find proofs by resolution. However it has been known since the 1980s that every resolution proof of the pigeonhole principle (PHP_n^m), suitably encoded as a CNF instance, includes exponentially many steps [18]. Therefore SAT solvers based upon the DLL procedure [12] or the DP procedure [13] must take exponential time. Polynomial-sized proofs of the pigeonhole principle exist for different proof systems, but general-purpose SAT solvers often remain confined to resolution. This result is in correlation with empirical evidence. Previously, we introduced the Compressed-BFS algorithm to solve the SAT decision problem. In an earlier work [27], an implementation of a Compressed-BFS algorithm empirically solved instances in (n⁴) time. Here, we add to this claim, and show analytically that these instances are solvable in polynomial time by Compressed-BFS. Thus the class of tautologies efficiently provable by Compressed-BFS is different than that of any resolution-based procedure. We hope that the details of our complexity analysis shed some light on the proof system implied by Compressed-BFS. Our proof focuses on structural invariants within the compressed data structure that stores collections of sets of open clauses during the Compressed-BFS algorithm. We bound the size of this data structure, as well as the overall memory, by a polynomial. We then use this to show that the overall runtime is bounded by a polynomial.     

Terahertz pulsed spectroscopy as a new tool for measuring the structuring effect of solutes on water

Absorption spectra of aqueous solution of 'chaotropes' (structure maker) and 'kosmotropes' (structure breaker) have been recorded in the mid-infrared (MIR) and terahertz (THz) spectral region. A different impact of the two groups of solutes on the absorption spectrum of water was found in the recorded THz spectra. A concentration-dependent increased absorption across the investigated THz spectral region (0.04-2 THz, 1.3-66 cm(-1), respectively) has been recorded for all studied chaotropic solutions, whereas the opposite has been obtained for kosmotrope containing solutions. In the case of ionic solutes a further increase in absorption towards higher frequencies was measured. The distinction between chaotrope and kosmotrope solutes was, as expected, also possible in the MIR spectral region. Depending on the structure-forming effect of the solute the OH stretch vibration of the water (around 3400 cm(-1)) was slightly shifted. A red shift has been observed for solution of kosmotropes, whereas a blue shift was observed in the case of solutions containing chaotropes. Compared to the MIR spectral region the structure influencing effect of solutes can be more efficiently studied in the THz spectral region, which provides information from interactions between neighboring water molecules.     

Glass-recrystallization of ferroelectric Pb5Ge3O11

Thin plate-like crystals of ferroelectric Pb₅Ge_3?xSi_xO₁₁ (0 ≤ x ≤ 1.5) were prepared by a glass-recrystallization technique during which the glass was heat-treated at 600 to 720°C. The crystals show preferred orientation with the polar c-axis perpendicular to the sample surface, an ideal geometry for pyroelectric infrared detectors.     

Polar glass ceramics — A new family of electroceramic materials: Tailoring the piezoelectric and pyroelectric properties

Grain oriented multicomponent polar glass-ceramics have been prepared by crystallizing the glasses in a temperature gradient. Inexpensive, large area piezoelectric and pyroelectric devices can be fabricated by this method, and by adjusting the composition of the glasses and crystallization conditions, it is possible to tailor the properties to meet device requirements. Based on the growth characteristics and the connectivity pattern of the crystallites, the piezoelectric, pyroelectric and dielectric properties of glass-ceramic composites can be predicted. Two examples discussed in this paper are piezoelectric glass-ceramics which are not pyroelectric, and pyroelectric glass-ceramics which are not piezoelectric.     

Flexible composite transducers

Flexible PZT/polymer composite transducers have been fabricated with a novel microstructural configuration. The concept of connectivity has been applied in the evaluation of the type of structure needed to optimize the properties of the composite. Properties of several kinds of piezoelectric transducers are compared.     

Transverse honeycomb composite transducers

Composites with 3-1 connectivity were fabricated by impregnating an extruded, sintered honeycomb configuration of PZT with epoxy. The composites had lower density (≈ 3000 Kg/m³) and lower dielectric constant (≈ 400) than that of solid PZT. The maximum piezoelectric d?₃₃ coefficient of the composites was 350 pC/N, and the maximum hydrostatic d?_h 220 pC/N. ?_h and d?_h?_h of the composites were an order of magnitude higher than that of solid PZT. Considering the symmetry and phase connectivity of the individual phases in the composite, an explanation is given for the improved piezoelectric properties of the composites.     

Effects of pH and H2O2 upon coprecipitated PbTiO3 powders

The properties of precipitated materials are highly dependent upon the complex ionic equilibria of the species in the solutions used for precipitation. Concentration, temperature, and pH dictate the complex species present within aqueous systems, and therefore affect the final precipitate properties. This paper discusses the effect of pH on the properties of PbTiO₃ precursor powders prepared by adding stoichiometric mixtures of TiCI₄ and Pb(NO₃)₂, in aqueous solution, to NH₄OH solutions. Several powders were prepared between pH 8.00 and 10.50. The pH does not affect the amorphous structure, but does have a pronounced effect upon the specific surface area and growth mechanisms of the precipitates.Since previous studies indicated that hydrogen peroxide (H₂O₂) affects the hydroxylation of the precipitated powders, the effect of (H₂O₂) concentration on the precipitate properties was also studied. Several precipitates were prepared from solutions containing (H₂O₂): PbTiO₃ ratios between 0:1 and 6:1. When (H₂O₂) was not added to the solutions used for precipitation, atmospheric CO₂ dissolved in solution caused precipitation of carbonate species. Thus, addition of the (H₂O₂) to the solutions inhibited precipitation of the carbonates.