Effects of oxygen partial pressure on the crystallization of amorphous Bi–Sr–Ca–Cu–O and Bi–Sr–Ca–Cu–O + Ag

Abstract

The temperatures and pathways of crystallization for amorphous Bi-Sr-Ca-Cu-O are strongly dependent on oxygen partial pressure {Po2), the oxidation state of the glass, and the presence of Ag. Reducing the POl from 1 to 0 bar lowers the onset of melting 100 °C, but does not change the onset of crystallization. Decreasing POl does inhibit the formation of (CaSr)CuO3 (Oil). Although Ag appears to be immiscible in B i -S r -C a -C u -0 , finely distributed Ag lowers the onset of melting by 20 to 35 °C, depending on POl. In oxygen-deficient glass, two exotherms are observed upon heating. The first exotherm at 460 °C is independent of PO2 and Ag and corresponds with the formation of Bi2Sr2Cu06 (Bi-2201). The second exotherm occurs at 486 °C with Ag or 500 °C without Ag. The onset temperature of the second exotherm does not change with POl, but the exotherm decreases in intensity with decreasing Po2. EXAFS indicates that in the as-quenched amorphous material, Cu is in a reduced state. Annealing the amorphous material in oxygen below the first exotherm (i.e., glass relaxation) increases the oxidation state of the Cu to that necessary for the formation of the Bi2Sr2CaCu2O8 (Bi-2212) without additional oxygen diffusion. This relaxation of the glass increases the crystallization temperature of the Bi-2201 to 500 °C. The crystallization sequence of Bi-2201 to Bi-2212 occurs at all POl 3= 0.1 bar, but the composition of the secondary phases changes in the unrelaxed glass. Relaxing the glass eliminates liquid formation at low temperatures, allowing for rapid formation of the Bi-2212 phase with minimal formation of secondary phases for 0.1 =£ POl =s 0.2 bar.

Formation of NdBa2Cu3O7−δ amorphous phase by combining aero-acoustic levitation and splat quenching

“…4 Working with an amorphous precursor has a number of advantages, especially for peritectic systems, such as Fe-Nd-B. 4,8 On the other hand, in the RE123 system, amorphous RE123 has hardly been reported. 4,8 On the other hand, in the RE123 system, amorphous RE123 has hardly been reported.…”

High temperature phase equilibria studies in the Bi-Sr-Ca-Cu-O-Ag system

“…The first peritectic reaction is depressed to 871°C with 2 wt.% Ag and 867°C with 10 wt.% Ag added. Again, the presence of Ag seems to have a much greater effect on the stability of the (11) and (21) than on the (9115), so that with the addition of 10 wt.% Ag the decomposition order of (9115) and (11) switches creating a new phase region in which (9115), (21), (10), and liquid are in equilibrium. The nucleation temperature of (10) is lowered by SO'C with 10 wt.% Ag added which also causes slight modifications in the decomposition pathway.…”

“…It is well known that the use of Ag as a sheathing material acts to depress the onset of liquid formation by 15-25°C depending on oxygen partial pressure [10]. As a result, material near the Ag/Bi2212 interlace will begin melting at a lower temperature than within the bulk.…”

“…The (9115) melts soon after this, shifting the liquid conqx)sition onto the plane formed by (21), (10), and Bi2212 causing (10) to begin nucleating from the liquid. The (10) is nearly cubical and rarely grows above 10^m on a side (Figure 4c).…”

“…With 10 wt.% Ag added, the decomposition temperature of (9115) is depressed by about 10°C, while the (11) is depressed by 20°C and the (21) by at least 40°C. As a result, there now exists a phase region in the 10 wt.% Ag system in which (9115), (10), and liquid are in equilibrium. In the Ag free system the (21) and (10) remain in equilibrium with the liquid up to 992°C.…”