Sintered Powder Samples Research Articles

Exploring microstructure and caloric effects in gas-atomized Ni–Mn–Sn–Co precursor for additive manufacturing

Recently, additive manufacturing (AM) of intrinsically brittle Ni–Mn–X alloys, in which obtaining high-quality powders is imperative, has received widespread attention. Here, a batch weight of about 34 kg Ni–Mn–Sn–Co powder was prepared using industrial gas atomization equipment. To explore the sinterability of the gas-atomized powder and provide a direct comparison in caloric effects for binder-based AM techniques, we sintered Ni–Mn–Sn–Co alloy powder samples at 1173–1273 K for 2–24 h by using a powder metallurgy process with air cooling. The sintered powder samples had a relative density of about 60.3–80.1 %, and 5M martensite and L21–ordered austenite were found to coexist at room temperature. For all sintered samples, martensitic transformation temperature width of about 11–17 K and hysteresis of about 21–23 K were obtained, and strong magneto-structural coupling was observed. The magnetocaloric and elastocaloric effects of the sample sintered at 1273 K for 2 h were examined. A magnetic entropy change of about 23.0 J kg−1·K−1 and an adiabatic temperature change (ΔTad) of −0.97 K were achieved for magnetic fields of 5.0 and 1.25 T, respectively. Furthermore, a ΔTad of −4.71 K was achieved upon unloading a compressive stress of 350 MPa. These competitive caloric effects in the gas-atomized powder lay the foundation for the AM of Ni–Mn–Sn–Co alloys with complex structures. This study provides a reference for the reliable manufacturing of composition-sensitive Ni–Mn–X Heusler alloys.

Thermal and magnetic properties study of NiCo2O4/graphene and NiFe2O4/graphene

The article explores thermal stability studies and magnetization properties of spinel ferrites with graphene. Different spinel ferrites such as CoFe2O4, Ni0.25Zn0.75Fe2O4, NiFe2O4, NiCoFe2O4 were prepared by a co-precipitation method, followed by sintering at 900 °C, 1200 °C, 1450 °C, and graphene was prepared by Hummers method. The magnetic polarization after particle bonding was found higher for sintered powder samples. The saturation magnetization (Ms) of 50.57 emu/g for NiCoFe2O4 powder and 63.1 emu/g for CoFe2O4 specimen sintered at 1200 and 1400 °C. This could be by the weakening of spin canting with a variation in the increase of sintering temperature. The addition of graphene to nickel–cobalt improved the saturation magnetization from 40.8 to 43.8 emu/g with an increase in graphene content 5 to 20 wt%. An increase in magnetization of 6.8 % was gained by the influence of graphene. While nickel-ferrite with 26 mg graphene without sintering gained the Ms value as 30.79 emu/g. Thermal stability enhanced upon the increase in graphene concentration for NiCo and NiFe due to higher crystallite size as a higher stage 2 slope than stage 1 of DTG denoting a faster crystallite growth.

High-pressure structural change in the ferroelectric layered perovskite Sr2Nb2O7

The structural changes of sintered powder samples of perovskite-slab layered polycrystalline ${\mathrm{Sr}}_{2}{\mathrm{Nb}}_{2}{\mathrm{O}}_{7}$ (SNO) ferroelectric compound subjected to high pressures are here investigated. The samples were prepared using a solid-state reaction in the presence (SNOE) or absence (SNO) of an applied electric field. Density functional theory (DFT) calculations including hydrostatic pressure indicate that SNO's ferroelectricity remains up to 25 GPa in the structure of space group $Cmc{2}_{1}$ derived from the condensation of one zone-center soft phonon. The predicted DFT theoretical structural changes are discussed and compared with the results of the experimental Raman spectra as a function of pressure. The pressure-dependent spectra were recorded from atmospheric pressure up to 11.5 and 13.5 GPa for SNOE and SNO, respectively. Both samples underwent a pressure induced phase transition from an incommensurate to a commensurate state at room temperature, SNO at ${P}_{i\ensuremath{-}c}=6.5\ifmmode\pm\else\textpm\fi{}0.2$ GPa, and SNOE at ${P}_{i\ensuremath{-}c}=6.9\ifmmode\pm\else\textpm\fi{}0.3$ GPa. The DFT calculations enable the identification of the change in phase to the orthorhombic structure with the space group $Cmc{2}_{1}$. The experimental values for ${P}_{i\ensuremath{-}c}$ are in reasonably good agreement with the theoretical predicted value of \ensuremath{\sim}7.3 GPa. After the critical pressures, the number of observable phonons decreases, that is, when the compound adopts a higher symmetry structure, several phonons vanish abruptly in both the SNO and SNOE samples, as expected. The Raman spectra for both samples show hysteresis effects, that is, after the pressure is removed, a few extra lines remain visible, as well as many relative intensity changes and broadenings for some phonon bands. The bulk moduli of the $\mathrm{S}{\mathrm{r}}_{2}\mathrm{N}{\mathrm{b}}_{2}{\mathrm{O}}_{7}$ before and after ${P}_{i\ensuremath{-}c}$ are calculated resulting in 117.0 and 147.8 GPa, respectively. The Gr\uneisen parameters of the phonons observed are finally calculated and discussed.

Radiophotoluminescence and thermoluminescence characteristics of undoped and Mg doped LiF phosphor in the high dose region

Radiophotoluminescence (RPL) and thermoluminescence (TL) from undoped and Mg-doped LiF powder samples were investigated for high dose dosimetric applications. F-aggregate centres in LiF are created by radiolysis. The RPL emission at 530 nm and 650 nm from F3+ and F2 centres respectively (on excitation at 450 nm) in undoped commercial LiF powder enhanced with increasing sintering temperature. Its TL glow curve showed peaks at 145, 290 and 365 °C. The RPL-TL correlation studies carried out for the first time showed that the thermal annihilation of F3+ and F2 centes following Fi0 - Va (interstitial-vacancy) recombination cause 290 °C and 365° TL peaks respectively. This showed that F2 centres are thermally more stable than F3+ centres. Its integrated TL response kept increasing nearly linearly up to 14 kGy, in contrast to the response of the TL peaks in dosimetry grade LiF:Mg, Ti (TLD-100). However, the TL from LiF suffers from fading problems. A 100 °C, 15 min pre-read annealing treatment erased most of the 145 °C TL peak without disturbing the other high temperature peaks. A 750° C, 22 h sintering treatment in carbon atmosphere shifted the major TL glow peak of undoped LiF to 200 °C but reduced its TL sensitivity by more than a factor of 10. Mg-doped LiF samples exhibit a TL glow curve peaking at ∼240 °C with satellite peaks typical of Mg doped LiF whose shape did not change significantly with gamma dose. However it showed lower TL and RPL sensitivities as compared to those of undoped LiF. All LiF except LiF alfa (750 °C, 1 h, CB) showed sublinear increase in TL with dose and therefore will cause inaccuracy in dose measurements especially at high doses due to reduced sensitivity. LiF alfa 750 °C, 22 h and 1000 °C, 6 min sintered powder samples are better than others because their RPL sensitivity increases nearly linearly with dose. However, a low fading observed in the latter sample supports its application. The bright yellow RPL emission seen through a 550 nm cut on filter illuminated with a 450 nm blue LED or He-Cd laser light could quickly confirm an irradiated consignment on which LiF samples are affixed. The increase in green to red emission ratio with dose and post-irradiation storage period might be related to slow migration of anion vacancies created by one secondary electron track to F centres created by another track and consequent conversion of F2 → F3+ centres.

Synthesis and Conductivity Investigation of Cu Doped Apatite Type Lanthanum Silicate Electrolyte

In this paper, Cu-doped lanthanum silicate electrolyte (La9.33Si6-xCuxO26-x) precursor was synthesized by urea-nitrate combustion method using La2O3, CuO and TEOS as raw materials. The as-prepared precursor was lighted at 600 °C and sintered at 800 °C. The sintered powder samples were grinded and mixed absolutely by ball milled. Finally, we preformed and sintered powder samples to synthesis Cu-doped lanthanum silicate ceramics. Ac impedance method and analysis was used to test conductivity of as-prepared electrolyte, investigated the influence of balling time, sintering temperature and doping content on conductivity property of LSO sintered product. The final result shows that best balling time is 3 h, secondary sintering temperature is 1500 °C and the doping content is 0.3. Under this condition, the conductivity of Cu-doped lanthanum silicate electrolyte could reach 7.28×10-3 s/cm at 700 °C.

Influence of annealing on the structure of nanoporous oxide films on the surface of titanium‒aluminum powder alloy

Oxide films obtained during anodization of Ti‒40% Al sintered powder samples in fluorine-containing electrolytes are investigated. With scanning electron microscopy and X-ray phase analysis, it is demonstrated that an X-ray amorphous nanoporous anodic oxide film is formed on the surface of the powder microparticles under optimal anodization conditions. After annealing at T = 1093 K in air and vacuum (10‒2 Pa), the oxide films are revealed to crystallize with its regular porous structure retained. The composition of the polycrystalline anodic-oxide films annealed in air is a mixture involving TiO2 (anatase and rutile) and α- and γ-Al2O3 phases and Ti2O3 and Al2TiO5 traces. The vacuum annealing process makes it possible to identify TiO2, in which anatase is the main phase, α- and γ-Al2O3, and Ti2O3 and TiO traces. However, rutile is not revealed. The presented results indicate that the application of the anodic nanostructuring of Ti‒40% Al powders is promising for the obtainment of new photocatalytic active nanomaterials.

Suppression of geometric frustration by magnetoelastic coupling in AuCrS2

We studied the structural, magnetic, and electronic properties of the geometrically frustrated layered AuCrS${}_{2}$ system by means of x-ray and neutron powder diffraction, specific heat, dc magnetization, and dc electrical resistivity measurements. The room-temperature structural refinement is consistent with a hexagonal centrosymmetric $R$-3$m$ symmetry and with formal valence states Au${}^{+}$ and Cr${}^{3+}$, where the Cr${}^{3+}$ ions form a regular triangular lattice within the hexagonal planes. On cooling, we observe a first-order structural phase transition to a monoclinic $C$2/$m$ symmetry concomitant to an antiferromagnetic order at ${T}_{N}$ $=$ 47 K. The atomic displacements associated with this transition stretch the triangular lattice, thus suppressing the geometric frustration. This accounts for the magnetic order observed and gives evidence of a large magnetoelastic coupling. The refined magnetic structure is commensurate and consists of double ferromagnetic chains along the stretching direction with \ensuremath{\mu} $=$ 2.54 ${\ensuremath{\mu}}_{B}$/Cr${}^{3+}$; the residual frustration stabilizes an elegant pattern of alternate ferromagnetic and antiferromagnetic intra- and interplane couplings between adjacent chains. The electrical transport of our sintered powder samples is found to be semiconducting-like with ${\ensuremath{\rho}}_{300\mathrm{K}}$ \ensuremath{\sim} 157 \ensuremath{\Omega}cm and an activation energy of 0.38 eV.

Superconducting property and Fe valence state of FeSe thick films grown from high temperature solution

Thick FeSe films (1–2 μm) were grown from high temperature solution with SeSn as the flux. Electron backscatter diffraction confirmed the films of tetragonal β phase with high crystallinity. Superconducting transition was observed by magnetic measurements, with the onset T c of 6.1 K for the as-grown films and rising to 6.9 K after post-growth annealing at 400 °C, which was still 1.5 K lower than the sintered powder samples. X-ray photoelectron spectroscopy showed that the Fe 2p 3/2 binding energy in the FeSe compound was composed of two peaks at 707.8 eV and 706.6 eV, respectively. The former was close to the value of Fe in polarized ionic bonds, while the later had the typical value in metallic bondings. The ratio of the two bondings was 1.56 and 1.94 for the films and sintered powders, respectively. The critical temperature may have some correlation with the ratio of the two bondings. A lower average Fe valence was probably the cause for the lower T c observed in thick films.

The effects of microstructural features on the performance gap in corrosion resistance between bulk and HVOF sprayed Inconel 625

It is commonly observed that there is a performance gap between the corrosion resistance of thermally sprayed coatings and the equivalent bulk material. This is attributed to the significantly modified microstructure of the sprayed coatings. However, currently there is no detailed understanding of which aspects of microstructural modification are primarily responsible for this performance gap. In this work several deliberately microstructurally modified versions of the Ni-based superalloy Inconel 625 were produced. These were subjected to potentiodynamic electrochemical testing in 0.5 M H 2SO 4 to investigate the links between specific microstructural features and electrochemical behaviour. Samples were prepared by high-velocity oxy-fuel (HVOF) thermal spraying, laser surface remelting using a high power diode laser and conventional powder sintering. Microstructural features were examined by optical and scanning electron microscopy and X-ray diffraction. Potentiodynamic testing was carried out on the following forms of Inconel 625: wrought sheet; HVOF sprayed coatings; sintered powder compacts; laser melted wrought sheet and HVOF sprayed coatings. Using the corrosion behaviour, i.e. passive current density, of the wrought sheet as a baseline, the performance of different forms of Inconel 625 was compared. It is found that a fine dendritic structure (with associated microsegregation) produced by laser remelting wrought sheet has no significant effect on corrosion performance. Up to 12% porosity in sintered powder samples increases the passive current density by a factor of only around 2. As observed previously, the passive current density of HVOF sprayed coatings is 20–40 times greater. However, HVOF coatings subjected to laser surface remelting are found to have a passive current density close to that of wrought material. It is concluded that, whilst porosity in coatings produces some decrease in corrosion resistance, the main contributing factor is the galvanic corrosion of localised Cr-depleted regions which are associated with oxide inclusions within HVOF sprayed samples.

Constraints on the Fe–S melt connectivity in mantle silicates from electrical impedance measurements

The connectivity of FeS melts in olivine and in a fertile peridotite matrix has been addressed through in situ electric impedance spectroscopy (IS) measurements at 1 GPa. A first series of experiments used sintered powder samples of a fertile peridotite xenolith mixed with 5–15 vol.% Fe 70S 30 of eutectic composition. The sheared high-T garnet peridotite with Mg# ∼ 0.90 is composed of 60 vol.% olivine, 15% orthopyroxene, 5.3% clinopyroxene and 19% garnet, the powder grain size was 20–30 μm, similar to the one employed by Yoshino et al. (2003). For a second series, San Carlos olivine aggregates were used as solid matrix and 10–20 vol.% of eutectic Fe 70S 30 were added. For these, the average grain size was 3 μm, much smaller than in the experiments by Yoshino et al. (2003). The powder mixtures of peridotite + Fe 70S 30 and olivine aggregates + Fe 70S 30 were first annealed for 2–5 days in a conventional piston cylinder at 1 GPa and 950–970 °C. The electrical conductivity of samples has been measured using the impedance spectroscopy method in a BN–graphite–CaF 2 pressure cell with concentric cylindrical electrodes made from Mo- or Re-foil (the estimated oxygen fugacity was close to the IW-buffer). The results indicate that up to 15 vol.% of Fe 70S 30 the melt phase does not built a stable interconnected network in a peridotite matrix, as was recently indicated by Walte et al. (2007). The percolation threshold for a stable FeS network in olivine matrix lies at 17.5 vol.%, much higher the 6 vol.% found by Yoshino et al. (2003). Our result is in line with the high dihedral angles of typically 70–100° for Fe–S melts in mantle materials. The higher interconnectivity threshold of this study, as compared to previous studies (Yoshino et al., 2003, 2004; Roberts et al., 2007) is a result of our smaller starting grain sizes (for olivine) in combination with much longer run durations. Both these experimental conditions result in enhanced grain growth and thus to a higher degree of textural equilibration, leading to the occurrence of the time depending pinging off of Fe–S melt films in our texturally more mature experiments.

Precision and accuracy of the heat-pulse calorimetric technique: lowtemperature heat capacities of milligram-sized synthetic mineral samples

Low-temperature heat capacities (cp) for milligram(mg)-sized samples of corundum, fayalite and sanidine were measured with a recently available commercial calorimeter (the heat capacity option of the Physical Properties Measurement System (PPMS), produced by Quantum Design®, which operates on the basis of a heat-pulse technique. The measurements were performed between 5 and 300 K on synthetic single-crystals and powders. Since cp-data for the above minerals are known from low-temperature adiabatic calorimetry (low-TAC) these data were used to constrain the accuracy of the PPMS-calorimeter in measuring their cp on mg-sized samples. A relative uncertainty (100*σcp/cp), as a measure of precision, of ~0.3% for T > 50 K, increasing to ~0.5% for T 100 K, 2-3% at T < 100 K for 10-20 mg powders, and ~ 5% for 5 mg powders). The accuracy was highest for cp measurements on single-crystals and sintered-powder. For these, PPMS heat capacities were in excellent agreement with comparable low-TAC data (deviating from these by a maximum of 0.5 ± 0.8% in the temperature range 100–300 K). Heat capacity measurements on sealed powders did not achieve such a high degree of accuracy, but were systematically lower than low-TAC data by 1-2%. Reference entropies were reproduced with a relative error of ≤ 0.5% using single-crystal and sintered-powder samples, and were 1-2% too low for sealed-powder samples. Our measurements demonstrate that the PPMS-calorimeter is a promising new tool for obtaining low-temperature cp-data and calorimetrically determined standard entropies for mg-sized mineralogical samples which are only available in limited amounts.

Preparation and structural characterization of ferromagneticMn5Si3Cxfilms

Carbon-doped Mn-Si films prepared by magnetron sputtering at elevated substrate temperatures exhibit ferromagnetic order for compositions around ${\mathrm{Mn}}_{5}{\mathrm{Si}}_{3}{\mathrm{C}}_{x}$ $(x&gt;0)$ in contrast to the antiferromagnetic ${\mathrm{Mn}}_{5}{\mathrm{Si}}_{3}$ compound. In these sputtered samples the carbon concentration can be strongly enhanced compared to sintered powder samples. The local structural order around the Mn site in ${\mathrm{Mn}}_{5}{\mathrm{Si}}_{3}{\mathrm{C}}_{x}$ films was investigated for $x=0$ and 0.75 by x-ray-absorption spectroscopy at different temperatures T. At low T, both films retain their hexagonal structure and do not exhibit the orthorhombic distortion observed for bulk ${\mathrm{Mn}}_{5}{\mathrm{Si}}_{3}$ powder. For the carbon-doped film a local distortion of the octahedra formed by six Mn atoms of the ${\mathrm{Mn}}_{5}{\mathrm{Si}}_{3}$ structure is found when surrounding a C atom. This indicates an anisotropic change of the atomic Mn environment by C, in contrast to a simple lattice expansion as inferred from previous structural analysis.

Some important factors in the wet precipitation process of hydroxyapatite

The precipitation process considered here, involved the addition of orthophosphoric acid solution to a calcium hydroxide solution. Several parameters such as the effects of atmosphere controlling, pH, adding extra acid solution, the amount of stirring speed, and post-chemical treatment were studied. In order to study the thermal stability of synthesised powders, they were sintered for 2 h at 1200 °C and the as-precipitated and sintered powder samples were examined using SEM, XRD, FTIR and ICP methods. The results clearly revealed that the final precipitated powder is hydroxyapatite (HA) with good purity, stoichiometry and successful thermal stability. SEM high magnification micrographs showed that the precipitated HA consists of small rod-like particles.

Temperature and pressure effects on the crystal structure ofSr3Ru2O7:Evidence for electronically driven structural responses

The crystal structure of the ruthenate Sr{sub 3}Ru{sub 2}O{sub 7} as a function of temperature (5--305 K) and pressure (ambient -0.62 GPa) was studied by neutron diffraction. Upon cooling, the lattice parameter c and the rotation angle of the oxygen octahedra {phi} increase for 305K>T>50K, and decrease for 20K>T>5K. As pressure is applied at room temperature, the outer apical Ru-O bond expands. This remarkable behavior is discussed in terms of modification of the occupation of energy levels of the Ru{sup 4+} ion in the crystalline electric field of the oxygen octahedron and a crossover to Fermi-liquid behavior at low temperature. The very anisotropic thermal expansion leads, in sintered powder samples, to a high degree of strain, which is relieved somewhat with each cooling cycle.

Development of molten carbonate fuel cell using novel cathode material

The slow dissolution of lithiated-NiO cathodes in molten carbonates is the main obstacle for the commercialization of molten carbonate fuel cells. The aim of the present work was to investigate the possibility of producing an electrode based on LiCoO 2. The Li x CoO 2 powder samples (0.8 < x < 1.1) were obtained by thermal decomposition of carbonate, acetate and oxide precursors, in air. The syntheses were monitored by thermal analysis (TGA, DTA). The calcined and sintered powder samples were characterized by X-ray diffraction and atomic absorption spectroscopy The porous electrodes were prepared with different pore-formers by cold pressing and sintering. A bi-modal pore size distribution was observed in all the materials. Conductivity measurements were carried out in the temperature range 500–700 °C. The solubility in molten carbonates was measured. To test the cathodic performance of the materials under study, electrochemical impedance spectroscopy measurements were carried out to investigate the porous electrode/molten carbonate interface.

Muon-spin-rotation studies of the temperature dependence of the magnetic penetration depth in theYBa2Cu3Oxfamily and related compounds

A systematic muon-spin-rotation (${\mathrm{\ensuremath{\mu}}}^{+}$SR) study is presented of the temperature dependence of the London penetration depth in sintered powder samples of the ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{x}}$ system and related compounds. The in-plane penetration depth ${\ensuremath{\lambda}}_{\mathit{a}\mathit{b}}$ is estimated from the ${\mathrm{\ensuremath{\mu}}}^{+}$SR depolarization rate of ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$, ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{4}$${\mathrm{O}}_{8}$, and a series of samples of the ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{x}}$ family, respectively. It is found that not only the low-temperature value ${\ensuremath{\lambda}}_{\mathit{a}\mathit{b}}$(0), but also the temperature behavior ${\ensuremath{\lambda}}_{\mathit{a}\mathit{b}}$(T) is specific to each compound. The form of ${\ensuremath{\lambda}}_{\mathit{a}\mathit{b}}$(T) can be well characterized by a simple power law. In particular, the ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{x}}$ family shows a systematic variation of the form of ${\ensuremath{\lambda}}_{\mathit{a}\mathit{b}}$(T) with the oxygen content x which points to a varying coupling strength, whereas ${\ensuremath{\lambda}}_{\mathit{a}\mathit{b}}$(0) as a function of x suggests a positive charge transfer into the ${\mathrm{CuO}}_{2}$ planes with increasing oxygen doping. Furthermore, our data is consistent with an empirical ansatz which has been proposed in the framework of a Bose-gas picture of high-temperature superconductivity. As a consequence, the pressure and the isotope coefficients can be extracted from the ${\mathrm{\ensuremath{\mu}}}^{+}$SR depolarization rate and compared to direct measurements of these quantities, showing good agreement. Moreover, in the Bose-gas picture the variation of ${\ensuremath{\lambda}}_{\mathit{a}\mathit{b}}$(T) in the ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{x}}$ family may be interpreted as a crossover from a dense (high-${\mathit{T}}_{\mathit{c}}$) to a dilute (low-${\mathit{T}}_{\mathit{c}}$) system of weakly interacting local pairs.

Electrochemical Investigations of Polycrystalline H xNb 2O 5

Polycrystalline samples of H x Nb 2O 5 were prepared by electrochemical reduction of pressed and sintered powder samples of the H-Nb 2O 5 modification. The hydrogen content of the bronze was determined by redox titration. The electrode potential showed that the bronze is thermodynamically unstable with respect to decomposition into Nb 2O 5 and gaseous hydrogen. The hydrogen loss obeys a second-order rate law. The electrode potential fulfills Nernst's law with a square dependence on the hydrogen content. The specific resistance of H 0.18Nb 2O 5 determined by four-point dc measurements amounts to ρ(30°C) = (0.0110 ± 0.0015) Ω cm. The protonic conductivity of the bronze was substantiated by polarization experiments and the diffusion coefficient of H ( D ≈ 5 × 10 -9 cm 2/sec) was estimated from the rate law of the depolarization voltage.

Structure and phase transitions of low-dimensional thallium vanadium bronze Tl xV 2O 5 (0.44 ≤ x ≤ 0.48)

Tl 0.48V 2O 5 belongs to the solid solution Tl x V 2O 5 (0.44 ≤ x ≤ 0.48). Its crystal structure has been refined from X-ray powder diffraction data in space group C2 m . The framework of this lamellar compound is built up from double-sheet slabs of edge-sharing distorted octahedra, which can preferably be depicted as square pyramids. The highly polarizable Tl + cations adopt a nearly cubic coordination between these slabs. A detailed analysis of the π-bonds shows that the electrons d 1 are localized in the d xy -orbital of vanadium ions V(1). The migration of these self-trapped but mobile electrons, via a hopping process, confers on Tl 0.48V 2O 5 the properties of a two-dimensional polaronic semiconductor 2D-(SC) between 165 and 655 K. Below 165 K, which can be viewed as a three-dimensional ordering temperature T 3D c, transport properties are analyzed by considering that the small polarons progressively become trapped. This trapping, which is evidenced in E.P.R. data, allows one to explain, at least qualitatively, the spin-glass-like behavior observed in Tl 0.48V 2O 5. Between 655 and 710 K, this compound undergoes a phase transition either to a (SC) state or a metallic state. From the evolution of the conductivity curve, the (SC) state seems to be the most probable. However, because of the highly anisotropic nature of Tl 0.48V 2O 5 and since transport measurements have been performed on sintered powder samples only, a transition to a metallic state cannot be definitively excluded.

Flux creep after thermal neutron irradiation of uranium-doped oriented compacts of YBa2Cu3Ox

Fission fragment damage was introduced by thermal neutron irradiation into aligned, sintered powder samples of YBa2Cu3Ox with uranium additions of 0.08 wt % UO2. This resulted in an increase in the intragranular critical current and a decrease in flux creep. Flux creep of the magnetization M exhibited simple log time dependence and was characterized by absolute and normalized slopes as functions of magnetic field and temperature. Before irradiation, the slopes were strongly field dependent and showed peaks as a function of temperature. After irradiation the slopes were much less field dependent and the peaks were suppressed. Both the magnitude and the field dependence of the creep were improved. Apparent pinning potentials at 0.8 T increased by a factor of about 2 on irradiation, for temperatures from 10 to 35 K. These increases in the apparent pinning potentials are attributed to the damage caused by the travel of the fission fragments through the crystal.

Magnetism in electron-doped copper oxides

We report muon spin relaxation/rotation measurements on sintered powder samples of Nd2−xCexCuO4−y and a large single crystal of Nd2CuO4−y. We find an electronic phase diagram which is quite similar to that of hole-doped superconductors such as La2−xSrxCuO4−y, although the doping of electrons into the system is less efficient in destroying the static moments on the copper spins. Static magnetic order in Nd2CuO4−y appears below about 250 K, and two spin reorientations are seen atT=75 K andT=35 K. Measurements of the magnetic field penetration depth have been unsuccessful due to the rare-earth paramagnetism of these materials.