[en] Bi-Sr-Ca-Cu-O (BSCCO) superconducting fibres were grown using the laser floating zone technique assisted by a dc electrical current (electrically assisted laser floating zone-EALFZ process). The simultaneous effect of pulling rate and applied current intensity was evaluated on the premises of the: (i) elemental partition coefficient between solid and molten zone; (ii) phase development; (iii) crystal size and alignment degree of the primary and superconducting phases; (iv) phase transformation during heat treatment and (v) superconducting properties. The highest critical current density value (J_C = 2240 A cm^-2) was obtained for the fibres pulled at intermediate pulling rates (75 mm h^-1) grown under a maximum dc electric current intensity (200 mA).

[en] Investigations of crystal structure and magnetic properties of Bi_0.8(Gd_1-xBa_x)_0.2FeO₃ (x = 0, 0.5, 1) samples have been performed. The Bi_0.8Gd_0.2FeO₃ and Bi_0.8Ba_0.2FeO₃ compounds have been shown to crystallize in the polar space groups Pn2₁a and R3c, respectively. It has been found that no continuous series of solid solutions is formed in the Bi_0.8(Gd_1-xBa_x)_0.2FeO₃ system: the crystal structure of the Bi_0.8Gd_0.1Ba_0.1FeO₃ sample is characterized by a coexistence of Pnma and R3c structural phases which differ in their chemical compositions. All of the Bi_0.8(Gd_1-xBa_x)_0.2FeO₃ (x = 0, 0.5, 1) compounds have been found to possess a spontaneous magnetization at room temperature. For Gd-containing samples, a significant enhancement of the magnetization takes place with decreasing temperature.

[en] Bi_1-xA_xFeO₃ ceramics (A=Ca,Sr,Pb) were sintered by conventional mixed oxide route. The crystallographic structure of all samples is characterized by the rhombohedral symmetry (space group R3c). The existence of switchable ferroelectric polarization is verified by piezoresponse force microscopy that is proven to be a useful technique in semi-insulating ferroelectrics. Magnetic properties of Ca and Sr-doped ceramics are found to reproduce the antiferromagnetic behavior of undoped BiFeO₃ without any enhancement of the magnetization. On the contrary, Pb-doped compound demonstrates appearance of a weak ferromagnetism. It is thus shown that Pb doping of BiFeO₃ is a promising way for preparing multiferroic materials

[en] Highly-texturized polycrystalline fibres of the Bi-Sr-Ca-Cu-O system have been grown by the laser floating zone technique at seven different pulling rates: (1.1, 2.2, 4.17, 8.3, 16.7, 33.3, 60.5) x 10^-6 m s^-1. The assessment of the cation segregation at the solid/liquid interface allowed us to calculate their equilibrium and effective distribution coefficients. The equilibrium distribution coefficients (k_0,Bi = 0.55, k_0,Sr = 0.97, k_0,Ca = 1.67, k_0,Cu = 1.10) were estimated using the Burton, Primm and Slichter (BPS) theory by taking into account the determined effective values. The effective distribution coefficients tend to unity as long as the pulling rate increases. The composition profiles along the initial transient region of the solidified fibres show a fast approach to the nominal composition as the pulling rate increases. The outstanding effect of the growth speed on superconducting phase type development is explained based on the solute trapping phenomena. The sequence of crystallization for superconducting phases ('2212' → '4413' → '2201') with pulling rate is a spontaneous effect of the system thermodynamics in order to balance the Bi trapping. This phase sequence corresponds to the smallest change of Bi chemical potential from the liquid phase to the solid phase. A diagram of free energy curves of the interdendritic superconducting phases illustrates the partitionless solidification phenomena at the highest growth speed

[en] The electrical assisted laser floating zone (EALFZ) solidification process makes the tailoring of fibre microstructures possible. The application of a dc electrical current of ∼8 A cm^-2 during the solidification process of Bi₂Sr₂Ca₂Cu₄O₁₁ nominal composition fibres strongly modified phase development, crystal shapes and effective distribution coefficients. Growth conditions with the solidification interface positively polarized deviate the system from metastability, leading to the development of the equilibrium cuprate (Sr_xCa_1-x)₁₄Cu₂₄O₄₁ (14/24) as primary phase dendrites. Compared to the morphology of the Sr_xCa_1-xCuO₂ (1/1) primary crystals of the conventional LFZ process, the 14/24 crystals are aligned higher along the fibre axis, with half the thickness and twice the extension. One of the major effects of EALFZ is the control of the effective distribution coefficients, k. At equal values of the fibre pulling rate, R, the copper partition between the liquid and the solid is the most affected, the k_Cu increasing from 1 to 1.22 due to the ionic drift from the zone melt to the negative polarized feed rod. Bismuth and calcium effective distribution coefficients present the lowest values (k_Bi = 0.69 and k_Ca = 1.13) in these conditions, according to the field-modified BPS theory. When the reverse current is applied, the dendritic morphology disappears and a globular structure of completely new phases develops

[en] The mechanism of the formation of heterovalent-substitution-induced defects as well as their influence on the magnetic properties of BiFeO₃-based multiferroics has been studied. It has been shown that heterovalent A²⁺ substitution results in the formation of oxygen vacancies in the host lattices of both antiferromagnetic and weak ferromagnetic Bi_1-xA_xFeO₃ (A = Ca, Sr, Pb, Ba; x = 0.2, 0.3) compounds, thus indicating the intrinsic (i.e. not related to defects themselves) mechanism of doping-induced enhancement of magnetization. A correlation between the ionic radius of the substituting element and the value of the spontaneous magnetization of the corresponding solid solution has been found. The experimental results suggest that A-site substitution with the biggest ionic radius ions effectively suppresses the spiral spin configuration of antiferromagnetic BiFeO₃. (fast track communication)

[en] The application of an electric current of 200 mA through the molten zone of BSCCO superconducting fibres during laser floating zone processing constitutes an upgrade for improving the grain alignment. When a direct electric current (positively polarized fibre) passes through the solidification interface, the solidification conditions approach equilibrium, favouring the development of a higher amount of 2212 and 14/24 stable phases. The new electrically assisted laser floating zone (EALFZ) technique also improves the 2223 phase formation in fibres heat treated at high temperatures (860 deg. C). However, the 2223 crystals grow perpendicularly to the fibre axis at the interface between the 2212 and 14/24 phases, crossing the crystals of the main phase responsible for the current transport, cancelling the alignment effect. The resultant current density and critical temperature values were J_c77K = 230 A cm^-2 and T_c = 85 K, respectively. When a lower heat treatment temperature was accomplished (820 deg. C), the 2223 transverse crystals do not develop and a higher current density value of J_c77K = 510 A cm^-2 was achieved, although with a critical temperature of T_c = 90 K

[en] Polycrystalline samples with the nominal composition Bi_0.8Pb_0.2FeO₃ have been studied via x-ray diffraction, Moessbauer spectroscopy, dielectric, magnetic, and local ferroelectric measurements. It has been found that the heterovalent Pb²⁺ substitution in Bi_0.8Pb_0.2FeO_y is realized through the formation of oxygen vacancies. The crystal structure of the compound has been shown to be described by the non-centrosymmetric space group R3c. Investigations of local ferroelectric and magnetic properties have confirmed that spontaneous polarization and weak ferromagnetism coexist in this material at room temperature. The nature of the weak ferromagnetic moment in this compound is discussed in terms of a doping-induced change in the magnetic anisotropy

[en] In the definition of conceptual developments and design of new materials with singular or unique properties, characterisation takes a key role in clarifying the relationships of composition, properties and processing that define the new material. B_4C has a rare combination of properties that makes it suitable for a wide range of applications in engineering: high refractoriness, thermal stability, high hardness and abrasion resistance coupled to low density. However, the low self-diffusion coefficient of B_4C limits full densification by sintering. A way to overturn this constraint is by using an alloy, for example Al-Si, forming composites with B_4C. Multi-carbide B_4C-SiC/(Al, Si) composites were produced by the reactive melt infiltration technique at 1200 – 1350 °C with up to 1 hour of isothermal temperature holds. Pressed preforms made from C-containing B_4C were spontaneously infiltrated with Al-Si alloys of composition varying from 25 to 50 wt% Si. The present study involves the characterisation of the microstructure and crystalline phases in the alloys and in the composites by X-ray diffraction and SEM/EDS with EBSD. Electron backscatter diffraction is used in detail to look for segregation and spatial distribution of Si and Al containing phases during solidification of the metallic infiltrate inside the channels of the ceramic matrix when the composite cools down to the eutectic temperature (577 °C). It complements elemental maps of the SEM/EDS. The production of a flat surface by polishing is intrinsically difficult and the problems inherent to the preparation of EBSD qualified finishing in polished samples of such type of composites are further discussed

[en] Although Cd and Ca ions have the same valence and cation size, their incorporation into vacancy-doped La manganites induce different properties. While the incorporation of Ca leads to high Tc up to 250 K and induces a metallic-like behaviour, the incorporation of Cd severely reduces Tc and promotes insulator-like behaviour. In this work, the Cd hyperfine fields have been measured with the Perturbed Angular Correlations (PAC) technique after implantation and annealing of ^111mCd in La-Cd-MnO₃ samples. The PAC results are compared with measurements of the resistivity and magnetization performed on the same samples. The mixed La and Mn site Cd occupancy is suggested as a possibility to explain the properties of the La-Cd-MnO₃ system