M. A. Ansari

PHASE FORMATION AND SUPERCONDUCTIVITY OF Fe-TUBE ENCAPSULATED AND VACUUM-ANNEALED MgB2

We report optimization of the synthesis parameters viz. heating temperature (TH), and hold time (thold) for vacuum-annealed (10-5 Torr) and LN2 (liquid nitrogen) quenched MgB2 compound. These are single-phase compounds crystallizing in the hexagonal structure (space group P6/mmm) at room temperature. Our XRD results indicated that for phase-pure MgB2, the TH for 10-5 Torr annealed and LN2-quenched samples is 750°C. The right stoichiometry i.e., MgB2 of the compound corresponding to 10-5 Torr and TH of 750°C is found for the hold time (thold) of 2.30 hours. With varying thold from 1–4 hours at fixed TH (750°C) and vacuum (10-5 Torr), the c-lattice parameter decreases first and later increases with thold (hours) before a near saturation, while the a-lattice parameter first increases and later decreases beyond a thold of 2.30 hours. The c/a ratio versus thold plot showed an inverted bell-shaped curve, touching the lowest value of 1.141, which is the reported value for perfect stoichiometry of MgB2. The optimized stoichimetric MgB2 compound exhibited superconductivity at 39.2 K with a transition width of 0.6 K. In conclusion, the synthesis parameters for phase pure stoichimetric vacuum-annealed MgB2 compound are optimized and are compared with widely-reported Ta tube encapsulated samples.

Resistivity and thermoelectric power of NaxCoO2 (x=1.0, 0.7, and 0.6) system

Results of thermoelectric power (S) and electrical resistivity (ρ) measurements are reported on NaxCoO2 compounds with x=1.0, 0.7, and 0.6. These are single-phase compounds crystallizing in the hexagonal structure (space-group P63∕mmc) at room temperature. Thermoelectric power values at 300K (S300K) are ≅80, 39, and 37μV∕K for the x=1.0, 0.7, and 0.6 samples, respectively. The samples with x=0.7 and 1.0 are metallic down to 5K, while the x=0.6 sample is semiconducting. The value of ρ300K for the x=1.0 sample is ∼0.895mΩcm and the power factor (S2∕ρ) is ∼7.04×10−3W∕mK2 which qualifies it as a good thermoelectric material. In the x=1.0 sample, S(T) is positive throughout the 300–5-K temperature range and decreases monotonically to zero as temperature T→0. In contrast, S(T) of the x=0.7 and 0.6 samples changes sign and shows negative values between 90 and 16K before approaching zero as T→0. The anomalous S(T) behavior of the x=0.6 and 0.7 samples, which are coincidentally the precursor materials to the repor...

Revival of superconductivity by Y3+/Ca2+ substitution in YBa2Cu2.7Co0.3O7 without reported phase transformation

Results of phase formation, resistivity (ρ), and thermoelectric power (S), are reported on Y1−xCaxBa2Cu2.7Co0.3O7 compounds with x=0.1 and 0.2. Pristine compound, i.e., without Co or Ca substitution crystallizes in orthorhombic structure with space-group P∕mmm. The Cu-site Co substituted compound, i.e., YBa2Cu2.7Co0.3O7 is tetragonal. With simultaneous doping of Ca at the Y site in Co substituted compound, i.e., Y1−xCaxBa2Cu2.7Co0.3O7 the tetragonal nature still remains. ρ(T) measurements showed superconducting transition temperature (Tc) to decrease from 90K (YBa2Cu3O7) to 33K for YBa2Cu2.7Co0.3O7, which with further Ca substitution increases from 33to53K (Y0.9Ca0.1Ba2Cu2.7Co0.3O7) and 67K for Y0.8Ca0.2Ba2Cu2.7Co0.3O7. Tc decreases first with Cu-site Co substitution by hole filling and later recovers by simultaneous hole creation by Y site Ca substitution. Room temperature thermoelectric power S (300K), which is an indirect measure of mobile carriers shows the decrease of carriers with Co doping and crea...

Systematic induction of superconductivity in the Cu (Y1−xCax)Ba2Cu2O6+δ (\delta \approx 0.3 ) system by Ca substitution

Series of samples of the composition Cu (Y1−xCax)Ba2Cu3O6+δ () have been prepared by a solid-state synthesis route. Precise oxygen content was determined for all the samples by iodometric titration. The c lattice parameter increases with increasing x, indicating successful substitution of Y3+ by the bigger Ca2+ ion. Resistivity (R) versus temperature (T) measurements show that the pristine sample (x = 0.0) is semiconducting down to 5 K. Induction of superconductivity is seen with increasing x. The x = 0.10 sample exhibits the onset of superconducting transition (Tconset) at 42 K without attaining a zero resistance superconducting transition temperature (TcR = 0) state above 5 K. For the x = 0.15 and 0.20 samples TcR = 0 is observed at 25 and 35 K with Tconset at 52 and 61 K, respectively. DC magnetic susceptibility measurements corroborated the findings from the resistivity measurements. Thermoelectric power (S) measurements in the temperature range of 5–300 K exhibit TcS = 0 at 25 and 35 K respectively for x = 0.15 and 0.20 samples. The room-temperature S value decreases with increasing x, indicating an enhanced number of mobile holes. Magneto-transport measurements in applied magnetic fields of 3 and 6 T show a broadening of the superconducting transition. These results will be useful in constructing a more complete phase diagram of the Cu (Y1−xCax)Ba2Cu3O6+δ system in terms of x and δ.

IMPACT OF Zn SUBSTITUTION ON PHASE FORMATION AND SUPERCONDUCTIVITY OF Bi1.6Pb0.4Sr2Ca2Cu3-xZnxO10-δ WITH x = 0.0, 0.015, 0.03, 0.06, 0.09 AND 0.12

Samples of series Bi1.6Pb0.4Sr2Ca2Cu3-xZnxO10-δ with x = 0.0, 0.015, 0.03, 0.06, 0.09 and 0.12 are synthesized by solid-state reaction route. All the samples crystallize in tetragonal structure with majority (> 90%) of Bi-2223(Bi2Sr2Ca2Cu3O10) phase (c-lattice parameter ~ 36 A). The proportion of Bi-2223 phase decreases slightly with an increase in x. The lattice parameters a and c of main phase (Bi-2223) do not change significantly with increasing x. Superconducting critical transition temperature (Tc) decreases with x as evidenced by both resistivity [ρ(T)] and ac magnetic susceptibility [χ(T)] measurements. Interestingly the decrement of Tc is not monotonic and the same saturates at around 96 K for x > 0.06. In fact Tc decreases fast (~ 10 K/at%) for x = 0.015 and 0.03 samples and later nearly saturates for higher x values. Present results of Zn doping in Bi-2223 system are compared with other Zn-doped HTSC (high temperature superconducting) systems, namely the RE-123 (REBa2Cu3O7) and La-214 ((La, Sr)2CuO4).