66 research outputs found
Thermal conductivity and diffusivity of Cu-Y alloys produced by different powder metallurgy routes
Full density Cu-1%Y and Cu-0.8%Y alloys have been produced by different powder metallurgy routes and subsequent hot isostatic pressing. Some of the alloys have been subjected to equal channel angular pressing (ECAP) via BC route up to 8 passes. ECAP deformation homogenizes and refines the microstructure up to attaining a sub-micron grain structure. Thermal properties have been characterized by the laser flash method in the temperature range 373–773 K. The ECAP process, irrespective of the production route, enhanced the thermal conductivity to values similar to those for CuCrZr (ITER grade). The linear thermal expansion coefficient was temperature independent for all materials
Variational Mote Carlo Study of Flat Band Ferromagnetism -- Application to CeRh_3 B_2
A new mechanism for ferromagnetism in CeRh_3B_2 is proposed on the basis of
variational Monte Carlo results. In a one-dimensional Anderson lattice where
each 4f electron hybridizes with a ligand orbital between neighboring Ce sites,
ferromagnetism is stabilized due to a nearly flat band which is a mixture of
conduction and 4f electron states. Because of the strong spin-orbit interaction
in 4f electron states, and of considerable amount of hybridization in the
nearly flat band, the magnetic moments from 4f and conduction electrons tend to
cancel each other. The resultant ferromagnetic moment becomes smaller as
compared with the local 4f moment, and the Fermi surface in the ferromagnetic
ground state is hardly affected by the presence of 4f electrons. These
theoretical results are consistent with experimental observations in CeRh_3B_2
by neutron scattering and dHvA effects.Comment: to be published in J.Phys.Soc.Jp
Damage threshold of CuCrFeTiV high entropy alloys for nuclear fusion reactors
A CuCrFeTiV high entropy alloy was prepared and irradiated with swift heavy ions in order to check its adequacy for use as a thermal barrier in future nuclear fusion reactors. The alloy was prepared from the elemental powders by ball milling, followed by consolidation by spark plasma sintering at 1178 K and 65 MPa. The samples were then irradiated at room temperature with 300 keV Ar+ ions with fluences in the 3 × 1015 to 3 × 1018 Ar+/cm2 range to mimic neutron-induced damage accumulation during a duty cycle of a fusion reactor. Structural changes were investigated by X-ray diffraction, and scanning electron microscopy and scanning transmission electron microscopy, both coupled with X-ray energy dispersive spectroscopy. Surface irradiation damage was detected for high fluences (3 × 1018 Ar+/cm2) with formation of blisters of up to 1 μm in diameter. Cross-sectional scanning transmission electron microscopy showed the presence of intergranular cavities only in the sample irradiated with 3 × 1018 Ar+/cm2, while all irradiation experiments produced intragranular nanometric-sized bubbles with increased density for higher Ar+ fluence. The Williamson-Hall method revealed a decrease in the average crystallite size and an increase in residual strain with increasing fluence, consistent with the formation of Ar+ bubbles at the irradiated surface.publishedVersio
Thermodynamic Studies on Non Centrosymmetric Superconductors by AC Calorimetry under High Pressures
We investigated the non centrosymmetric superconductors CePtSi and UIr by
the ac heat capacity measurement under pressures. We determined the pressure
phase diagrams of these compounds. In CePtSi, the N\'{e}el temperature
= 2.2 K decreases with increasing pressure and becomes zero at the
critical pressure 0.6 GPa. On the other hand, the
superconducting phase exists in a wider pressure region from ambient pressure
to 1.5 GPa. The phase diagram of CePtSi is very
unique and has never been reported before for other heavy fermion
superconductors. In UIr, the heat capacity shows an anomaly at the Curie
temperature = 46 K at ambient pressure, and the heat capacity
anomaly shifts to lower temperatures with increasing pressure. The present
pressure dependence of was consistent with the previous studies by
the resistivity and magnetization measurements. Previous ac magnetic
susceptibility and resistivity measurements suggested the existence of three
ferromagnetic phases, FM1-3. shows a bending structure at 1.98,
2.21, and 2.40 GPa .The temperatures where these anomalies are observed are
close to the phase boundary of the FM3 phase.Comment: This paper was presented at the international workshop ``Novel
Pressure-induced Phenomena in Condensed Matter Systems(NP2CMS)" August 26-29
2006, Fukuoka Japa
Non-Centrosymmetric Heavy-Fermion Superconductors
In this chapter we discuss the physical properties of a particular family of
non-centrosymmetric superconductors belonging to the class heavy-fermion
compounds. This group includes the ferromagnet UIr and the antiferromagnets
CeRhSi3, CeIrSi3, CeCoGe3, CeIrGe3 and CePt3Si, of which all but CePt3Si become
superconducting only under pressure. Each of these superconductors has
intriguing and interesting properties. We first analyze CePt3Si, then review
CeRhSi3, CeIrSi3, CeCoGe3 and CeIrGe3, which are very similar to each other in
their magnetic and electrical properties, and finally discuss UIr. For each
material we discuss the crystal structure, magnetic order, occurrence of
superconductivity, phase diagram, characteristic parameters, superconducting
properties and pairing states. We present an overview of the similarities and
differences between all these six compounds at the end.Comment: To appear in "Non-Centrosymmetric Superconductors: Introduction and
Overview", Lecture Notes in Physics 847, edited by E. Bauer and M. Sigrist
(Springer-Verlag, Berlin Heidelberg, 2012) Chap. 2, pp. 35-7
Critical Scaling of the Magnetization and Magnetostriction in the Weak Itinerant Ferromagnet UIr
The weak itinerant ferromagnet UIr is studied by magnetization and
magnetostriction measurements. Critical behavior, which surprisingly extends up
to several Tesla, is observed at the Curie temperature K and is
analyzed using Arrott and Maxwell relations. Critical exponents are found that
do not match with any of the well-known universality classes. The
low-temperature magnetization below 3 T
rises towards higher fields and converges asymptotically around 50 T with the
magnetization at . From the magnetostriction and magnetization data, we
extract the uniaxial pressure dependences of , using a new method
presented here, and of . These results should serve as a basis for
understanding spin fluctuations in anisotropic itinerant ferromagnets.Comment: 4 pages, 3 figure
The effects of mechanical alloying on the physical and thermal properties of CuCrFeTiV alloy
The present work reports the production and key properties of the CuCrFeTiV high entropy alloy synthetized mechanical alloying and spark plasma sintering. The milled powders and the as-sintered samples were analysed through scanning electron microscopy, coupled with energy dispersive X-ray spectroscopy and particle induced X-ray emission. Magnetic properties together with electrical resistivity, thermal conductivity, specific heat differential thermal analysis were also evaluated on the consolidated samples. The powders reveal an increasing content in iron as the millings are prolonged up to 20 h. The elemental composition of the sintered alloy, determined through particle induced X-ray emission, confirms the final composition after mechanical alloying with an increase of iron and a decrease in the remaining elements. Furthermore, although the alloy presents electrical resistivity typical of a high entropy alloy, a ferromagnetic behaviour was found, consistently with major Fe content as detected in prior observations. Finally, thermal measurements show that this CuCrFeTiV entropy alloy possesses thermal properties suitable for its potential use as thermal barriers.info:eu-repo/semantics/publishedVersio
Influences of Dispersions’ Shapes and Processing in Magnetic Field on Thermal Conductibility of PDMS–Fe3O4 Composites
Composites of magnetite (Fe3O4) nanoparticles dispersed in a polydimethylsiloxane (PDMS) matrix were prepared by a molding process. Two types of samples were obtained by free polymerization with randomly dispersed particles and by polymerization in an applied magnetic field. The magnetite nanoparticles were obtained from magnetic micrograins of acicular goethite (α-FeOOH) and spherical hematite (α-Fe2O3), as demonstrated by XRD measurements. The evaluation of morphological and compositional properties of the PDMS:Fe3O4 composites, performed by SEM and EDX, showed that the magnetic particles were uniformly distributed in the polymer matrix. Addition of magnetic dispersions promotes an increase of thermal conductivity compared with pristine PDMS, while further orienting the powders in a magnetic field during the polymerization process induces a decrease of the thermal conductivity compared with the un-oriented samples. The shape of the magnetic dispersions is an important factor, acicular dispersions providing a higher value for thermal conductivity compared with classic commercial powders with almost spherical shapes
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