Neutron powder-diffraction studies of the crystal and magnetic structures of the magnetocaloric compound Mn1.1Fe0.9(P0.8Ge0.2) have been carried out as a function of temperature, applied magnetic field, and pressure. ...
详细信息
Neutron powder-diffraction studies of the crystal and magnetic structures of the magnetocaloric compound Mn1.1Fe0.9(P0.8Ge0.2) have been carried out as a function of temperature, applied magnetic field, and pressure. The data reveal that there is only one transition observed over the entire range of variables explored, which is a combined magnetic and structural transformation between the paramagnetic (PM) and ferromagnetic phases (Tc≈255 K for this composition). The structural part of the transition is associated with an expansion of the hexagonal unit cell in the direction of the a and b axes and a contraction of the c axis as the FM phase is formed, which originates from an increase in the intralayer metal-metal bond distance. The application of pressure is found to have an adverse effect on the formation of the FM phase since pressure opposes the expansion of the lattice and hence decreases Tc. The application of a magnetic field, on the other hand, has the expected effect of enhancing the FM phase and increasing Tc. We find that the substantial range of temperature/field/pressure coexistence of the PM and FM phases observed is due to compositional variations in the sample. In situ high-temperature diffraction measurements were carried out to explore this issue, and reveal a coexisting liquid phase at high temperatures that is the origin of this variation. We show that this range of coexisting phases can be substantially reduced by appropriate heat treatment to improve the sample homogeneity.
Nonresonant anisotropic metamaterials (AMMs), such as nanotube arrays, can be applied to achieve broadband absorption of electromagnetic (EM) waves. However, the AMMs should be very thick (∼1000 wavelengths) to attai...
详细信息
Nonresonant anisotropic metamaterials (AMMs), such as nanotube arrays, can be applied to achieve broadband absorption of electromagnetic (EM) waves. However, the AMMs should be very thick (∼1000 wavelengths) to attain high absorption. Here, we propose that using a slanted or corrugated metallic mirror, a thin AMM (∼3 wavelengths) is sufficient to obtain high absorption in a wide wavelength range (absorption >87% for wavelengths of 0.5–3 μm) and over a wide angular range. Unlike common absorptions, EM waves in an ambient medium (air) is first coupled with a propagating wave in the AMM and is then converted into a lossy wave by mirror reflections.
Normal state optical spectroscopy on single crystals of the new iron arsenide superconductor Ba0.55K0.45Fe2As2 shows that the infrared spectrum consists of two major components: a strong metallic Drude band and a well...
详细信息
Normal state optical spectroscopy on single crystals of the new iron arsenide superconductor Ba0.55K0.45Fe2As2 shows that the infrared spectrum consists of two major components: a strong metallic Drude band and a well-separated midinfrared absorption centered at 0.7 eV. It is difficult to separate the two components unambiguously but several fits using Lorentzian peaks suggest a model with a Drude peak having a plasma frequency of 1.6 to 2.1 eV and a midinfrared peak with a plasma frequency of 2.5 eV. Detailed analysis of the frequency dependent scattering rate shows that the charge carriers interact with a broad bosonic spectrum extending beyond 100 meV with a very large coupling constant λ=3.4 at low temperature. As the temperature increases this coupling weakens to λ=0.78 at ambient temperature. This suggests a bosonic spectrum that is similar to what is seen in the lower Tc cuprates.
Quartz fiber reinforced silicon nitride composites were prepared with quartz fabric and polysilazane precursor by precursor infiltration and pyrolysis (PIP) method. The preparation method and dielectric, ablative and ...
详细信息
ISBN:
(纸本)9781615671601
Quartz fiber reinforced silicon nitride composites were prepared with quartz fabric and polysilazane precursor by precursor infiltration and pyrolysis (PIP) method. The preparation method and dielectric, ablative and mechanical performance of the composites were focused on. It has been found that the composites have stable dielectric constants at room temperature and still have high values at high temperatures. In addition, to overcome the brittle rupture caused by strong bonding between interfaces, a boron nitride was designed and the mechanical properties were improved. The composites obtained by this method are one of excellent high temperature microwave transparent materials.
Mo-Cu composite powders were prepared by the Sol-Gel method using citric acid as chelating reagent. The composition, morphology and particle size of the powders were studied by the XRD and TEM. TEM observation result ...
Mo-Cu composite powders were prepared by the Sol-Gel method using citric acid as chelating reagent. The composition, morphology and particle size of the powders were studied by the XRD and TEM. TEM observation result shows the particle size of Mo-Cu powders is about 150nm after reduction and the Mo grains are covered homogeneously by Cu on the surface. The reduction behavior and mechanism were analyzed systematically. Temperature programmed reduction (TPR) results indicate that the appropriate two-step reduction temperatures are 500°C and 700°C, respectively. The results from FT-IR show that the bond of Mo=O is weakened due to the bond of Cu-O-Mo, which leads to the increase of the reduction capability of the Mo-Cu composite oxide powders.
Neutron-diffraction and magnetization measurements have been carried out on a series of samples of the magnetorefrigerant Mn1+yFe1−yP1−xGex. The data reveal that the ferromagnetic and paramagnetic phases correspond to...
详细信息
Neutron-diffraction and magnetization measurements have been carried out on a series of samples of the magnetorefrigerant Mn1+yFe1−yP1−xGex. The data reveal that the ferromagnetic and paramagnetic phases correspond to two very distinct crystal structures, with the magnetic-entropy change as a function of magnetic field or temperature being directly controlled by the phase fraction of this first-order transition. By tuning the physical properties of this system we have achieved a magnetic-entropy change [magnetocaloric effect (MCE)] for the composition Mn1.1Fe0.9P0.80Ge0.20 that has a similar shape for both increasing and decreasing field, with the maximum MCE exceeding 74 J/kg K—substantially higher than the previous record. The diffraction results also reveal that there is a substantial variation in the Ge content in the samples which causes a distribution of transition temperatures that reduces the MCE. It therefore should be possible to improve the MCE to exceed 100 J/kg K under optimal conditions.
Leucite particles were synthesized from feldspar mixed with 0% to 52% potassium nitrate fired from 800 ℃ to 1 200 ℃ by solid state method. The X-ray Diffraction (XRD) patterns show that in the temperature range fr...
详细信息
Leucite particles were synthesized from feldspar mixed with 0% to 52% potassium nitrate fired from 800 ℃ to 1 200 ℃ by solid state method. The X-ray Diffraction (XRD) patterns show that in the temperature range from 800 ℃ to 1 200 ℃, the leucite can be removed as the single crystalline phase. Kalsilite may be crystallized with leucite at 800 ℃, but can be eliminated after prolonged heating. The scanning electron Microscopy (SEM) images clearly display the that crystals of micrometer scale leucite, and the leucite crystals distribute evenly in the matrix. The Thermal expansion coefficient (TEC) of the samples fabricated is as high as 20.52×10^-6 ℃^-1 measured from 20 ℃ to 500 ℃. The mechanism of transformation from feldspar to leucite was proposed.
Ni-BaCe0.9Y0.1O3-δ mixed protonic-electronic conductor can be used to separate hydrogen from syngas. Considering that water exists in syngas, it is necessary to evaluate the effect of moisture on chemical stability a...
详细信息
Ni-BaCe0.9Y0.1O3-δ mixed protonic-electronic conductor can be used to separate hydrogen from syngas. Considering that water exists in syngas, it is necessary to evaluate the effect of moisture on chemical stability and hydrogen permeability of the cermet. In this paper, hydrogen permeation rates of Ni- BaCe0.9Y 0.1O3-δ (40:60 in volume ratio) in different water partial pressures were measured at intermediate temperatures (600-750δC). It is found that hydrogen permeation rate of the cermet is highest at an appropriate water partial pressure. Samples after experiment were analyzed by XRD, ICP and pH value measurement, in which Ba(OH)2 and doped CeO2 were found. The reaction of water with samples resulting in insulating Ba(OH)2 and doped CeO2 is contributed to the decrease of hydrogen permeation in excessive water partial pressure.
The high temperature properties of hydrogenated Ti50Zr25Co25 ribbons were investigated up to 380°C. As indicated by tensile tests at room temperature, hydrogen-induced strengthening occurred until ~21 at.% hydrog...
The high temperature properties of hydrogenated Ti50Zr25Co25 ribbons were investigated up to 380°C. As indicated by tensile tests at room temperature, hydrogen-induced strengthening occurred until ~21 at.% hydrogen. At this range of hydrogen concentration (Hconc) varying from 0 to ~21 at.%, tensile tests conducted at high temperatures (300 to 400°C; strain rate: 10-4s-1) showed that in addition to enhancement in strength, elongation larger than 20% was obtained. The temperature range at which such property improvements occurred depends on the hydrogen concentration. The change in the mechanical properties in this range of Hconc and temperature is attributed to the corresponding changes in the thermal behaviour of the alloy, along with the inherent structural modifications induced by hydrogen addition.
暂无评论