Iamze Kvartskhava

Chemical doping with different elements and compounds at various amounts represents the most suitable approach to improve the superconducting properties of bismuth-based superconductors for technological applications. In this paper, the influence of partial substitution of Sr(BO2)2 for SrO on the phase formation kinetics and transport properties of (Bi,Pb)-2223 HTS has been studied for the first time. Samples with nominal composition Bi1.7Pb0.3Sr2-xCa2Cu3Oy[Sr(BO2)2]x, x=0, 0.0375, 0.075, 0.15, 0.25, were prepared by the standard solid state processing. The appropriate mixtures were calcined at 845 oC for 40 h. The resulting materials were pressed into pellets and annealed at 837 oC for 30 h in air. Superconducting properties of undoped (reference) and Sr(BO2)2-doped (Bi,Pb)-2223 compounds were investigated through X-ray diffraction (XRD), resistivity (ρ) and transport critical current density (Jc) measurements. The surface morphology changes in the prepared samples were examined by scanning electron microscope (SEM). XRD and Jc studies have shown that the low level Sr(BO2)2 doping (x=0.0375-0.075) to the Sr-site promotes the formation of high-Tc phase and leads to the enhancement of current carrying capacity in (Bi,Pb)-2223 HTS. The doped sample with x=0.0375 has the best performance compared to other prepared samples. The estimated volume fraction of (Bi,Pb)-2223 phase increases from ~25 % for reference specimen to ~70 % for x=0.0375. Moreover, strong increase in the self-field Jc value was observed for this dopant amount (Jc=340 A/cm2), compared to an undoped sample (Jc=110 A/cm2). Pronounced enhancement of superconducting properties of (Bi,Pb)-2223 superconductor can be attributed to the acceleration of high-Tc phase formation as well as the improvement of inter-grain connectivity by small amounts of Sr(BO2)2 dopant.

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.

Proper Doping of a high-temperature Bi(Pb)-2223 superconductor is an effective route to prepare Bi(Pb)-2223 HTS with enhanced transport characteristics. In this paper, we studied the impact of Ca(BO2)2 doping on high-Tc phase formation kinetics and transport critical current density of Bi(Pb)-2223. Samples with a nominal composition of Bi1.7Pb0.3Sr2Ca2-x[Ca(BO2)2]xCu3Oy, x = 0÷0.15 were prepared by the solid-state reaction method. The phase evolution of the prepared materials was studied by X-ray diffraction. The microstructure was analyzed using a scanning electron microscope. The resistivity and critical current density were measured by the standard four-probe method. The obtained results show that Ca(BO2)2 doping leads to the promotion of the high-Tc phase formation and enhancement of the transport critical current density in Bi(Pb)-2223 HTS.

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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