ვახტანგი ჟღამაძე

 A series of compounds with nominal compositions Bi2Sr2Ca1Cu2BxOy and Bi1.8Pb0.2Sr2Ca1Cu2BxOy (х=0, 0.10, 0.25, 0.50, 0.75) were synthesized. Both slow cooling in furnace and quenching into liquid nitrogen were applied after annealing. B2O3 addition enhances the Tc of furnace cooled Pb-free (x=0.10-0.25) and Pb-substituted 2212 (x=0.10-0.75) HTSs.

The influence of boron doping on the superconducting properties of YBa2Cu3BxO7-y ceramics prepared by solid state reaction has been investigated. Results show that addition of B2O3 in YBa2Cu3O3-y superconducting ceramics promotes the formation of Y-123 phase and enhances the intergrain (transport) critical current density.

The Influence of lithium substitution for cuprum and boron addition on the superconducting properties of bismuth-based 2223 and 2212 phases has been investigated. It was found that the Bi-2223 samples doped with Li and B transform to the low-Tc Bi-2212 phase. Lithium and boron-doping improves the characteristics of the superconducting transition in furnace-cooled Pb-containing samples of 2212 phase.

The nominally pure and boron-doped YBa2Cu3BxO7-y samples with B-doping level x varying between 0 and 0.15 were prepared by the solid state reaction. X-ray diffraction analysis shows that all the obtained compounds are single YBa2Cu3O7-y phase. Temperature dependence of resistivity was measured by the four-probe method. Superconducting transition and grain boundary critical current density measurements were performed using contactless method of high harmonics. The small additives of boron in YBa2Cu3BxO7-y (x= 0.025 and 0.05) does not essentially affect the critical temperature Tc and it remains near 90 K. The high-level boron doping causes degradation of Tc and tail remains on the (T) curve for x=0.15 at 65K. Lowest-level boron doping applied leads to a significant improvement in Jc compared to the undoped sample (from 100 A cm-2 for a control sample to 147 A cm-2 for a B-doped sample with x=0.025). Grain boundary critical current density decreases with increasing boron doping amount. The obtained results indicate possibility of added dopant being entered into the lattice-sites.

In this paper, the combined effects of B4C-doping and planetary ball milling on the phase evolution, microstructure and transport properties of Bi1.7Pb0.3Sr2Ca2Cu3Oy(B4C)x HTS with x=0÷0.125 were studied through X-ray diffraction (XRD), scanning electron microscopy (SEM), resistivity and critical current density measurements. Obtained results have shown that B4C additive leads to the strong acceleration of high-Tc phase formation and substantial enhancement in Jc. High-energy ball milling seems to produce a more homogeneous distribution of refined doped particles in the (Bi,Pb)-2223 HTS which results in an improved intergranular flux pinning and better self-field Jc performance.

Controllable modification of superconducting properties of Bi(Pb)-2223 phase by incorporation of various additives that can act as the effective pinning centers and/or accelerate superconducting phase formation, is an efficient tool to prepare Bi(Pb)-2223 materials with enhanced characteristics. In this paper, the influence of BN additive on the phase formation and transport properties of Bi(Pb)-2223 superconductor has been studied. Two sets of samples with nominal composition Bi1.7Pb0.3Sr2Ca2Cu3Oy[BN]x, x=0÷0.25 were prepared by the solid state reaction technique. Two different doping methods have been used: incorporation of microsized BN particles at the initial stage of synthesis and addition of high energy ball milled BN into the material before pelletizing. The precursor powders were calcined at 845 oC for 40 h on alumina plates. The samples were pressed into pellets and annealed at 845 oC for 30 h in air. Superconducting phase evolution of prepared materials were analyzed by X-ray diffraction (XRD). Resistivity (ρ) and critical current density (Jc) measurements were performed by a standard four-probe method. The surface morphology of samples were examined by scanning electron microscope (SEM). Incorporation of microsized and ball-milled BN additives leads to the enhancement of Bi(Pb)-2223 phase formation. Addition of microsized BN particles at the initial stage of synthesis increases Jc. The critical current and density of samples decrease monotonically with increasing content of ball milled BN additive. The preliminary results obtained in this study show that BN may be considered as a suitable additive for enhancing the formation and transport properties of Bi(Pb)-2223 superconductor.

The impact of precursor thickness and lead borate-Pb(BO2)2 dopant on phase formation, superconducting transport properties and microstructure of Bi1.7Pb0.3-xCa2Sr2Cu3Oy (Bi(Pb)-2223) high-temperature superconductor was investigated. The presented results show that Pb(BO2)2–doping leads to the strong acceleration of phase formation and enhancement of the transport critical current density of Bi(Pb)-2223 superconductor.

In this work, boron nitride-added thermoelectric materials with nominal composition Bi2Sr2Co1.8Oy(BN)x, x =0, 0.10, 0.15, 0.20, and 0.25 (0, 0.30, 0.45, 0.60, and 0.75 wt. %, respectively) were prepared through the solid-state reaction method. Firstly, raw materials Bi2O3, SrCO3, Co3O4, and BN were calcined at 1040 – 1080 K for 20 h with an intermediate manual grindings and ball milling in a planetary mill. Secondly, the resulting powder mixtures were cold isostatically pressed into pellets with 15 mm in diameter at a hydrostatic pressure of 200 MPa. Then the pellets were sintered at 830 °C for 25 h in air. The temperature dependence of Seebeck coefficient and resistivity of bar-shaped samples with dimensions of ~ 14 × 7 × 3.5 mm3 were investigated at T = 300 – 950 К by a setup designed in the laboratory using a Keithley DMM 6500 multimeter. Thermal conductivity in the temperature range from 300 to 570 K was measured by the Hot Disk TPS 500 Thermal Constants Analyzer (Sweden / Canada). Addition of boron nitride (0.60 wt. %)to Bi2Sr2Co1.8Oy thermoelectric ceramics leads to the marked (2.9 – 1.9-fold) decrease of electrical resistivity in the temperature range from 300 to 950 К, respectively, while not affecting the Seebeck coefficient. Thermal conductivity of 0.60 wt. % BNadded Bi2Sr2Co1.8Oy thermoelectrics increases about 2.2-foldcomparedto areference (un-doped) sample in the whole temperature range under investigation (300 to 570 K). Based on obtained results, the values of power factor (PF) and thermoelectric figure of merit (ZT) were calculated. Addition of BN leads to the increase of power factor at 970 K from 0.026 mW/mK2 for the reference sample to the highest value of 0.04 mW/mK2 in the sample with 0.60 wt. % BN. The figure of merit ZT for Bi2Sr2Co1.8Oy thermoelectrics increases slightly with the addition of 0.60 wt. % BN from 0.0065 and 0.0170 to 0.0083 and 0.0180 at 300 and 600 K, respectively.

Thermoelectric materials with composition Bi2-x[Pb(BO2)2]xCa2Co1.7Oy (x=0-0.225) were prepared using a solid-state reaction method within 750-795 °C temperature interval. Samples were characterized by powder XRD analysis and SEM technique. Resistivity and Seebeck coefficient were measured in the temperature range 25-650°C. Thermal conductivity was analyzed from 25 to 400 °C. Doping of Bi2Ca2Co1.7Oy by Pb(BO2)2 leads to the decrease of its electrical resistivity at x = 0.075-0.125 and thermal conductivity at x = 0.075-0.175. The optimal doping level of Bi2Ca2Co1.7Oy by Pb(BO2)2 is close to x = 0.125: sample with composition Bi1.875[Pb(BO2)2]0.125Ca2Co1.7Oy possesses the highest values of power factor (0.094 mW/m×K2 at 650 °C) and figure-of-merit (0.041 at 400 °C) which are 18 and 15 % larger, respectively, than for undoped Bi2Ca2Co1.7Oy ceramics. Obtained results suggest that the doping of Bi2Ca2Co1.7Oy by Pb(BO2)2 has to be considered as a promising route for enhancing its thermoelectric properties.

In this work, thermoelectric materials with composition Bi2-x[Pb(BO2)2]xSr2Co1.8Oy (x=0, 0.075, 0.15, and 0.25) were prepared through the solid-state reaction method. Resistivity and Seebeck coefficient of the synthesized samples were measured in the temperature range 25-650°C, and the power factor was calculated. The preliminary results obtained show that doping of Bi2Sr2Co1.8Oy by lead borate — Pb(BO2)2 leads to the marked decrease of electrical resistivity, while not remarkably affecting the Seebeck coefficient. For x = 0.075-0.15 dopant content, 2.1-2.4 and 1.6-fold decrease of resistivity was observed compared to the reference sample at 25 and 650 °C, respectively. When x = 0.15, Pb(BO2)2-doped sample exhibits a power factor of 0.04 mW/mK2 with a Seebeck coefficient of 175 μV/ K and a resistivity of 80 mohmcm at 650 °C.

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