Effect of oxygen partial pressure and melting temperature on Ag sheathed Bi-2212 wires
- PDF / 1,094,000 Bytes
- 6 Pages / 576 x 792 pts Page_size
- 34 Downloads / 191 Views
A serious problem for Ag-sheathed Bi-2212 wires produced by a partial melt process is oxygen release which causes effluence of the oxide materials from the Ag sheath. Higher oxygen partial pressure during the melt process cuts the oxygen release. We investigated the effect of the oxygen partial pressure, melting temperature, and cooling rate on superconducting properties of Ag-sheathed wires. It was found that (i) the formation of the Bi-2212 phase under a higher oxygen partial pressure of 1 atm is slower than that with a lower oxygen partial pressure of 0.2 or 0.5 atm; therefore, the cooling rate should be as slow as 0.25 °C/min to get homogeneity of the intragrain superconductivity as well as formation of the Bi-2212 phase, and (ii) melting temperature should be just above the incipient melting point; otherwise, Jc significantly decreases due to the formation and growth of voids together with degradation of superconductivity.
I. INTRODUCTION The B i - S r - C a - C u - 0 system contains two main superconducting phases: the high-r c phase (2223 phase) with a composition of Bi 2 Sr 2 Ca2Cu 3 0 ) , and the low-J c phase (2212 phase) with a composition of Bi2Sr2CaiGi2O>,. It has been reported that a good c-axis alignment of the 2212 phase can be obtained by a partial melt process in doctor-blade cast tape, which gives a high critical current density, Jc, at 4.2 K.1"3 The melt process has been applied to Ag-sheathed wires, and high Jc values of the order of 105 A/cm 2 have been achieved in short samples.4'5 However, it has been difficult to produce long wires or magnets with good superconducting properties due to effluence of the oxide materials from the Ag sheath and/or formation of voids inside the Ag sheath because of oxygen release during the partial melt process. We have reported that (i) the powders release less oxygen when they are calcined at temperatures closer to the incipient melting point, despite the calcining atmosphere, and (ii) the addition of 0.2 mol Ag and a higher oxygen partial pressure in the partial melt process are effective for reducing oxygen release.6 In the present paper, we discuss the effect of the oxygen partial pressure, cooling rate, and melting temperature on superconducting properties of Ag-sheathed wires produced by the partial melt process.
for 10 h in Ar, which gives powders with a small amount of oxygen release.6 A silver tube (6 mm o.d., 4.5 mm i.d.), filled with the powder, was processed by drawing to give a 0.7 mm outer diameter wire. The wire was cut into 3 cm lengths and each sample was partially melted at various oxygen partial pressures at 880 °C for 10 min, cooled down to 815 °C at the rate, R, of 0.05-2.5 °C/min, followed by furnace cooling to room temperature. Then the samples were annealed at 800 °C in A r - 7 % O 2 atmosphere for 20 h. The transport critical current, Jc, was measured by a standard four-probe method at 4.2 K with a 1 /AV • cm"1 criterion. The intragrain superconducting properties were investigated by an ac inductive method as a function of temperature
Data Loading...
