Magnetocaloric effect of Pr2Fe17−x Mn x alloys

Polycrystalline Pr2Fe17-xMnx（x = 0, 1, and 2）alloys were studied by X-ray diffraction（XRD）, heat capacity, ac susceptibility, and isothermal magnetization measurements. All the alloys adopt the rhombohedral Th2Zn17-type structure. The Curie temperature increases from 283 K at x = 0 to 294 K at x = 1, and then decreases to 285 K at x = 2. The magnetic phase transition at the Curie temperature is a typical second-order paramagnetic–ferromagnetic transition. For an applied field change from0 to 5 T, the maximum-△SM for Pr2Fe17-xMnxalloys with x = 0, 1, and 2 are 5.66, 5.07, and 4.31 J·kg^-1·K^-1,respectively. The refrigerant capacity（RC） values range from 458 to 364 J·kg^-1, which is about 70 %–89 % that of Gd. The large, near room temperature △SM and RC values,chemical stability, and a high performance-to-cost ratio make Pr2Fe17-xMnxalloys be selectable materials for room temperature magnetic refrigeration applications.     

The standard state of cerium

Based on two studies carried out about 20 years apart, it was found that the double close-packed hexagonal βCe phase slowly transformed to the face-centered cubic γCe phase at room temperature and ambient pressure. This indicates that γCe is the stable modification at standard temperature and pressure (i.e., 298 K and 1 atm). The βCe↔γCe equilibrium temperature at 1 atm, the heat of transformation, the temperature/pressure phase diagram, and the heat capacities of βCe and γCe are presented. The Ames Laboratory is operated by the U.S. Department of Energy of Iowa State University under contract No. W-7405-ENG-82. This work was supported by the Office of Basic Energy Sciences, Division of Materials Sciences.     

Wavefront sensor based on the Talbot effect with the precorrected holographic grating

A holographic wavefront sensor based on the Talbot effect is proposed. Optical wavefronts are measured by sampling the light amplitude distribution with a two-dimensional (2D) precorrected holographic grating. The factors that allow changing an angular measurement range and a spatial resolution of the sensor are discussed. A comparative analysis with the Shack-Hartmann sensor is illustrated with some experimental results.     

Investigation of the thermochemical transformations in the LiAlH4-LiNH2 system

The thermal transformations in the lithium alanate-amide system consisting of lithium aluminum hydride (LiAlH₄) and lithium amide (LiNH₂), mixed in a 1:1 M ratio, were investigated using the pressure-composition-temperature analysis, solid-state nuclear magnetic resonance, X-ray powder diffraction, and residual gas analysis. Below 250 °C, the alanate decomposes into Al, LiH and H₂, through the formation of Li₃AlH₆, whereas the amide remains largely intact. The release of gaseous hydrogen corresponds to approximately 5 wt%. Above 250 °C, additional ∼4 wt% of hydrogen is produced through solid-state reactions among LiNH₂, LiH and metallic Al, through the formation of intermetallic Li-Al binary alloy and an unidentified intermediate. The overall reaction of the thermochemical transformation of the LiAlH₄-LiNH₂ mixture results in the production of Li₃AlN₂, metallic Al, LiH and the release of 9 wt% of gaseous hydrogen. The reaction mechanism of the thermal decomposition is different from one identified earlier during mechanical treatment of the same system. Rehydrogenation of the thermally-decomposed products of LiAlH₄-LiNH₂ mixture using high hydrogen pressure (180 bar) and heating (275 °C) yields LiNH₂ and amorphous aluminum nitride (AlN).     

High-power bipolar multilevel pulsed electroporator

A novel, compact, high-voltage, bipolar electroporator is reported that produces single or multiple, symmetrical or asymmetrical, high-power square wave pulses up to ±1 kV and 100 A. A wide 1 μs to 10 ms pulse duration is provided with high resolution of 1 V and 0.5 μs. The device provides increased flexibility due to enhanced control of the pulse shape and delivered energy.     

Structure fragmentation in Fe-based alloys by means of cyclic martensitic transformations of different types

The effect of martensite transformations of different types on the misorientation of austenite crystalline lattice, which characterizes the degree of structure fragmentation, was investigated for Fe-Ni and Fe-Mn alloys. As a result of multiple face-centered cubic (f.c.c.)-body-centered cubic (b.c.c.)-f.c.c. transformations, an austenite single-crystalline specimen is transformed in a polycrystalline one due to progressive fragmentation. It was shown that the degree of fragmentation depends on the magnitude of volume change and the density of dislocations generated on martensitic transformations.     

Synthesis, structure, thermal and mechanical properties of nanocomposites based on linear polymers and layered silicates modified by polymeric quaternary ammonium salts (ionenes)

Polymer–clay composites has been prepared by melt blending an organo-bentonite with linear polymers (polyamide, polysterene and polypropylene) in a disk-screw extruder. In first time organo-clay was prepared by surface treatment of Na-forms bentonite with polymeric quaternary ammonium salts (PQAS). XRD indicated that organo-bentonite layers were exfoliated and dispersed into polyamide and polystyrene. Addition of 2 wt% organo-bentonites (optimal concentration) to polyamide increased tensile strength by 53% and Sharpy impact by 140%. With the incorporation of 2 wt% organo-bentonites (optimal concentration) into polystyrene the tensile strength increased to 28% and the Sharpy impact increased to 25%. For polypropylyne–organo-bentonites composites we did not observe delamination of layered structure, and as result absence of reinforcements. TGA showed that the polyamide and polystyrene nanocomposites have higher decomposition temperature in comparison with the original polymers.     

A family of ductile intermetallic compounds

Stoichiometric intermetallic compounds have always been touted for their attractive chemical, physical, electrical, magnetic and mechanical properties, but few practical uses have materialized because they are brittle at room temperature. Here we report on a large family of fully ordered, stoichiometric binary rare-earth intermetallic compounds with high ductility at room temperature. Although conventional wisdom calls for special conditions, such as non-stoichiometry, metastable disorder or doping to achieve some ductility in intermetallic compounds at room temperature, none of these is required in these unique B2 rare-earth compounds. Ab initio calculations of YAg, YCu and NiAl crystal defect energies support the observed deformation modes of these intermetallics.