National Institute Of Materials Physics - Romania

Spray frozen, freeze dried nitrate powder with composition Bi:Pb:Sr:Ca:Cu 1.7:0.3:2.0:2.5:3.5 has been thermally decomposed (up to 870 degreesC). In situ neutron diffraction investigations during heating and cooling of the powder have been performed at DN2 instrument from the IBR2 reactor at JINR, Dubna, Russia. Phase formation and reformation and domains of stability for different phases were observed and discussed.

Thin films of YBa2Cu3O7-x have been obtained on NdGaO3 substrate by pulsed laser deposition with an excimer KrF* (lambda =248 nm, tau (FWHM)greater than or equal to 20 ns) laser source from a stoichiometric target. In the present paper we report details of the technology of pulsed laser deposition of high-temperature superconductor thin films, substrates' requirements and high-temperature superconductor thin film characterisation. The study of some parameters' intercorelations for optimisation of the technology of the pulsed laser deposition of high-temperature superconductor thin films is also analysed Moreover, the contacting technology and techniques of characterisation are described.

Quaternary Ti53Zr21Ni13Fe13 alloys were prepared by melt-spinning using peripheral wheel speeds omega =19.5, 24.2 and 30m/s. The icosahedral (i-) phase formation was studied using high resolution synchrotron radiation powder diffraction. An amorphous phase component is present irrespective of the rapid solidification conditions, its volume fraction decreasing towards lower quenching speed values. The amorphous phase coexists with a b.c.c. rational approximant phase (W-phase). For omega below 20 m/s the formation of an i-phase was also noticed. Transmission Fe-57 Mossbauer spectroscopy measurements indicate that the number of asymmetrical Fe positions decreases with decreasing w in agreement with the improved structural perfection of the W-phase and with the formation of an additional i-phase. (C) 2000 Elsevier Science B.V. All rights reserved.

In this review are presented the main results of a complex research project devoted to preparation and investigations of physical properties of [(BaCuO2)(n)/(CaCuO2)(m)](N) artificial superconducting superlattices. The paper starts with the layer-by-layer pulsed laser deposition technique developed for this project and discuss the structure of this new material. The superconducting properties and the effects of structural disorder on them are also presented.

In a freeze-dried nitrate precursor powder with cation composition Bi:Pb:Sr:Ca:Cu = 1.7:0.3:2.0:2.5:3.5, Sc2O3 was added. The stoichiometry for the mixed precursor powder was Bi:Pb:Sr:Sc:Ca:Cu = 1.7:0.3:2.0:0.25:2.5:3.5. The two nitrate precursor powders with and without Sc were thermally decomposed, pressed, and sintered in the same conditions. They show different behavior, as revealed by thermal analysis. X-ray diffraction (XRD), Scanning electron microscopy (SEM), and chi'(T) measurements. Addition of Sc slows down synthesis processes, including 2223-phase during reactive sintering. After 70 h of sintering, 43% of 2223-phase formed in the pellet with Sc, whereas in the Sc-free pellet, 51% of the 2223-phase was attained. Moreover, addition of Sc leads to a certain morphology, resulting in lower superconducting characteristics.

Electrical resistivity and calorimetric measurements on Dy3Co show that below the Neel temperature (T-N = 44 K) the non-collinear antiferromagnetic structure exhibits: field-induced magnetic phase transitions of a first-order type along all principal axes, accompanied by a strongly anisotropic giant magnetoresistance and by a change of the Sommerfeld coefficient of the specific heat. Quantum tunnelling of the magnetization appears to be possible for T < 0.6 K.

The Ca0.45CU0.55O phase was successfully produced from a mixture of Ca and Cu nitrate powders with cation ratio Ca:Cu = 1:1. The heat treatment for decomposition and the synthesis of the indicated phase were pet-formed at 710 degrees C in air and flowing oxygen, respectively. The Ca0.45Cu0.55O phase concentration increases with synthesis time. After 209 h of heat treatment, the concentration of the Ca0.45Cu0.55O phase determined by X-ray diffraction (XRD) analysis has reached 87% and the kinetic data have indicated that the material enters the saturation stage of the reaction, The secondary phases are CaO and CuO. (C) 2000 Elsevier Science B.V. All rights reserved.

A series of compounds YbxM4Sb12, M = Fe, Co, FeCo, Rh,Ir, were synthesised by reaction sintering. From Rietveld refinements isotypism was determined in all cases with the LaFe4P12-(skutterudite)-type, space group Im (3) over bar - No. 204. These refinements also served to derive the Yb-content in the samples. There is a systematic trend for the Yb-occupancy in the parent lattice M4Sb12; revealing a gradual decrease of the Yb-content from x = 0.8 (M = Fe), x = 0.5 (FeCo), x = 0.2 (Co); x = 0.1 (Rh) to x approximate to 0.02 (Ir). This dependency seems to correlate with the thermal stability of the ternary compounds: a true ternary compound forms for M = Fe, whilst for M = Co, Rh, Ir stable binary skutterudite compounds MSb3 already exist. Measurements of various bulk properties revealed the absence of any long range magnetic order in this series of compounds. While the samples rich in Yb behave metallic like, the Rh and Ir based skutterudites show a semiconducting-like resistivity which at lower temperatures is characterised by variable range hopping in the presence of strong Coulomb interaction. Although Yb0.1Rh4Sb12 exhibits a Seebeck coefficient up to about 150 mu V/K, figures of merit ZT generally are below 0.1 near room temperature, primarily due to the large resistivities of the sintered material.

A far-from-equilibrium preparation route and using a particular combination of starting materials led us to obtain nonstoichiometric Y(Fe,V)(12) and its interstitial carbide. Structural analysis and EDAX results reveal the formation of the main 1:12 phase with the ThMn12-type structure, with 13-15% vacancies at the 3d metal sites. Insertion of interstitial carbon increases the T-C and magnetization of the 1:12 phase, whereas the unit cell volume remains unchanged. The effects of vacancies and interstitial carbon on the Mossbauer hyperfine fields and isomer shifts are discussed with reference to the calculated quantities. (C) 2000 Elsevier Science S.A. All rights reserved.