Jannie Marfaing

Deep metastable eutectic condensation in Al-Fe-SiO-H2-O2 vapors : Implications for natural Fe-aluminosilicates

Abstract Vapors of Al-Fe-SiO-O2-H2 having two different compositions produced ferroaluminosilica grains as a function of agglomeration and fusion along mixing lines in the Al2O3-FeO-SiO2 system that are defined by the predictable, deep metastable eutectic (DME) compositions of the smallest condensate grains. Disorder of these amorphous grains is higher than in quenched glass of identical composition, which is the very property of dissipative structures (Prigogine 1978, 1979) that are states of organization of matter where disequilibrium becomes a source of order. Iron-oxidation states control ferrosilica condensate compositions. We present the first magnetic measurements showing a high Fe3+ content in condensed ferrosilica grains. The Fe-cordierite grain composition is primarily the result of predictable non-equilibrium condensation, not the bulk gas phase composition. Natural terrestrial and anthropogenic (e.g., smelters, coal fly ash) Fe-cordierite might well be a metastable phase due to kinetically controlled processes. Amorphous Mg,Fe-bearing aluminosilica dust in chondritic interplanetary dust aggregates and (rare) Mg,Fe-aluminosilicates in meteorites might have condensed via similar processes.

Formation of a hexagonal silicon phase after pulsed laser excitation

Abstract The formation, via pulsed laser excitation, from amorphous material of a new crystalline silicon phase with a hexagonal structure is described. The lattice image obtained by high-resolution electron microscopy and the selected-area diffraction patterns provide direct information on this amorphous-to-crystalline transition. It is proposed that laser-induced electronic excitation of amorphous material plays a dominant role in this metastable phase formation.

Insulating-(super) conducting transitions in new ferroelectric-superconductor composites: Pb2(ScTa)O6 YBa2Cu3O7−δ

Abstract We studied the structural and electrical properties of ferroelectric-superconductor composites with variable volume fractions. Specifically, the insulating-(super)conducting transition is examined. Our results show that compatibility between Pb 2 (ScTa)O 6 ferroelectric and YBa 2 Cu 3 O 7−δ superconducting materials is strong, as superconducting composites with 56% of ferroelectric material in volume have a zero resistance critical temperature at 87 K. From resistivity measurements, the critical fraction of YBCO, for which the insulating-superconducting transition occurs at low temperature, and the percolation threshold for the insulating-conducting transition at room temperature are evaluated. For this second transition, we estimate that the transport critical exponent, t = 2.0 ± 0.3, is in excellent agreement with the percolation theory in three dimensions (3D). This gives strong evidence that transport in the composites takes place in 3D and indirectly confirms that the interfacial reactivity between the two materials is very weak, as shown by the X-ray diffraction patterns.

Effect of Mn inclusion in superconducting YBCO-based composites

Abstract The doping effect of Mn in polycrystalline YBa 2 Cu 3 O 7 − δ material (YBCO) is carefully analyzed considering the transport properties of composites formed by small inclusion particles of Mn embedded whether in a YBCO matrix or in superconducting Fe-YBCO composites. In both cases, the improvement of their respective electrical resistivity was evidenced. The use of double element doping clearly shows that an a priori degradation of the YBCO electrical properties by Fe atoms can be greatly restored by appropriate Mn doping. This character can be very useful for technological applications. This improvement is mainly due to the reduction of the secondary and/or insulating phases formed at the grain boundaries, limiting the current transport. The involved mechanism is the probable existence of free charged Mn-based species which are responsible for an optimal oxygenation in the material contributing to the electrical response improvement. These species can be located in the composite, whether entering the CuO chains of the YBCO lattice or complexing and reducing the secondary phases formed during the sintering process, especially Fe 2 O 3 and Y 2 BaCuO 5 in the case of Fe-YBCO composites.

Temperature dependence of the resistivity in Au-YBa2Cu3Ox sintered composites

Abstract We combine experimental and numerical techniques to investigate the temperature dependence of the resistivity ρ in metal–superconductor (M─S) Au─YBa2Cu3Ox (Au─YBCO) sintered composites. The experimental data for ρ are studied as a function of the Au volume fraction φ. Below the superconductor critical temperature T < 92 K, a systematic shift of the M─S transition threshold φS is observed as T varies. Up to T = 200 K, the resistivity curves present a maximum. Both the amplitude of the maximum and the corresponding composition vary with T. Our experimental results are well described by a random conductor network model which incorporates porosity and interface resistances. The interface resistance computed from the model is consistent with a simple picture of local modifications of the oxygen concentration at the YBCO─YBCO and Au─YBCO contacts.

Low temperature resistivity of Au-YBa2Cu3O7-delta sintered materials: Influence of porosity and secondary phases

The resistivity rho of sintered Au and YBa2Cu3O7-delta(Au and YBCO) mixed powders is measured at T = 60K, as a function of the gold volume fraction phi . The samples retain zero resistivity up to phi = phi c = 0.62. In the range phi [phi c, 1], the curve rho (phi) is bell-shaped, with a maximum at phi 0.75. Scanning electron microscopy observations suggest that the increase of resistivity can be attributed both to porosity and to secondary phases inducing contact resistances between YBCO and Au grains. A random conductor network model is used to test the relative influence of both parameters. The numerical data show that porosity alone cannot account for the resistivity maximum. Conversely, a peak is indeed observed in the simulations when a substantial interface resistivity is imposed.

The effect of the YBCO-PST composite composition on the superconducting carrier concentration determined by microwave studies under high pressure

Abstract The effect of hydrostatic pressure ( p T c in YBa 2 Cu 3 O 7− δ –Pb(Sc 0.5 Ta 0.5 )O 3 (YBCO–PST) composite is measured by the method of magnetically modulated microwave absorption (MMMA). The T c dependence on the PST fraction in weight x (0, 0.25, 0.5 and 0.75) is approximated by an inverted parabola function whereas the influence of pressure on T c is represented by the equation: d T c /d p =0.61(2)−1.72(6) x . The result may be explained assuming that PST phase in YBCO–PST composite influences the superconducting carrier concentration similar to the chemical substitution in YBa 2 Cu 3 O 7 [J.J. Neumeier, H.A. Zimmermann, Phys. Rev. B 47 (1993) 8385]. It is suggested that ions from PST diffuse to YBCO cell during the sintering of the composite.

Magnetic study of YBCO-PST composites

Abstract The magnetic properties of granular superconductor–insulator composites were investigated. The composites were intimate mixture of the YBa 2 Cu 3 O 7− δ (YBCO) high-temperature superconductor and Pb(Sc 0.5 Ta 0.5 )O 3 (PST) ferroelectric insulator. The observed decay of superconductivity, lowering of the critical temperature and increasing of the number of paramagnetic centres as PST content increases are explained assuming interdiffusion among YBCO and PST elements and formation of an additional phase, which did not exist in the initial powders.

Low temperature variation of YBa2Cu3O7 − δ lattice parameters in Fe/YBa2Cu3O7 − δ superconducting composites

In order to complement a previous study of the electrical properties of xFe/(1 − x) YBa2Cu3O7 − δ composites, the thermal dependence of lattice constants and orthorhombic strain (b − a) / (a+b) in the range 15 K < T < 300 K was investigated by high resolution X-ray diffraction. For low Fe contents (x = 2%), the main differences with the pure YBa2Cu3O7 − δ (YBCO) parameters are found at temperatures lower than 100 K only; moreover, for higher concentrations (x = 5%) the orthorhombic strain is found to be about 10 % smaller than in the pure material in the whole temperature range. The quantitative differences of the a and b lattice constants with Fe content, compared with the pure YBCO value, and their temperature dependence suggest that the Cu(1) and Cu(2) positions are preferred for substitution. The substitution, however, is small enough to retain the orthorhombic structure, as the transition to the insulating tetragonal structure is not reached. At T < 100 K, the composite with x = 2% Fe exhibits an anomalous thermal behavior of lattice constants which may be caused by a magnetostrictive response to a magnetic ordering of Fe in the lattice.

Investigation on the low temperature electrical response of ferroelectric-superconducting composites (PST-YBCO) with variable composition

Combination of ferroelectric and superconducting materials, respectively PdSc0.5Ta0.5O3 (PST) and YBa2Cu3O7-δ(YBCO) in the form of composites, with variable concentration, gives the possibility to modulate and diversify the properties of devices over a very wide range of temperature. Comparative results, relative to the electrical response of such composites, with variable composition up to 80% in weight of ferroelectrical material, associated with their cristallographic signature and final microstructures are analyzed considering the local diffusion. In spite of the fact that these composites present a superconducting transition (88K≤Tc≤92 K), a remarquable result is that this character persists even with a high ratio of PST, showing a weak inter-reactivity of materials and a specific microwawe absorption band near 100 KHz.