American Journal of Physical Medicine & Rehabilitation: March 1995 - Volume 74 - Issue 2 - p 179

Reaction kinetics for oxygen uptake into partially substituted YBa2Cu3Ox are investigated. Materials studied include the Y1−yLayBa2Cu3Ox system, YBa2Cu2.5Fe0.5Ox and Y0.5Na0.5Ba2Cu3Ox. The reaction kinetics are found to decrease strongly on substitution at the copper site, but only vary mildly on substitution at the rare earth site. For Y1−yLayBa2Cu3Ox a change in the rate law governing the oxygenation reaction is observed with y. We observe generally that Y1−yLayBa2Cu3Ox with increasing y in a confined temperature range (around 400C) behaves similarly to YBa2Cu3Ox with increasing temperature. This pertains to the change in rate law from linear to parabolic, as well as to initial decrease in activation energy, decrease in superconducting transition temperature and decrease in orthorhombic splitting. Correlations are accordingly drawn between structural detail, superconductivity, and kinetic parameters.

YBa2Cu3Ox type structures are obtained by partial substitution according to Y1−yLayBa2Cu3Ox over the entire range of composition. However, structures and superconducting transition temperatures, Ts, depend delicately on initial and subsequent heat treatment and O2 pressures. For conventionally prepared materials (initial calcining at 1220 K, denoted C 1220 K) structural transformations as a function of increasing annealing temperature Ta (under air) encompass a change from an orthorhombic modification with b ⋍ c3 (Obc phase) to an orthorhombic modification with a ≠ b ≠ c3 (Oa,b,c phase) for low y. At higher values of y only the degree of a - b splitting decreases with increasing Ta. This splitting however decreases with increasing y. C 1220 K materials can be prepared under air with superconducting transition temperatures Ts ⋍ 90 K up to y = 0.5 but Ts decreases sharply to 60 K thereafter. For these materials, the activation energies of O uptake decrease somewhat with y. Initial calcining of YBa2Cu3Ox at relatively low temperature (e.g., at 1020 K from the nitrates) results in a tetragonal modification which is stable versus further annealing at lower temperatures. The situation is similar with LaBa2Cu3Ox, however, different thermal treatment is needed to achieve an orthorhombic phase and Ts approaching 90 K. Data are discussed within a model according to which a Ts ~ 60 K is connected with localized holes (or valency > 2) on Cu chains which experience stronger lattice pressure due to the absence of CuO buckling. When Cu planes also become mixed valent, Ts ~ 90 K.

A theoretical and experimental study is reported on how to modify the temperature dependence of coercivity, Hc(T), in R2F14B type compounds (R=rare earth) through modifications of the temperature dependence of anisotropy field, HA(T). This is accomplished by partial substitutions on R sites. Experimentally it is found that for Y2Fe14B fine particles ( approximately 1- mu m), values of Hc are in the range of 1 kOe. For these materials, a slightly positive Hc(T) is obtained near 300 K. Stable HA(T) and HC(T) near 300 K are obtained, e.g., with Nd0.2Y0.8Fe14B, while strongly increasing HA(T) and Hc(T) are obtained with small substitutions of Er for Y in Y2Fe14B. More complex HA(T) and Hc(T) can be obtained in more complex systems.

Indications for new phases with superconducting properties are reported in Bi-Cu-Sr-Ca oxides. One of these is a tetragonal compound of approximate composition CaSr2Bi2Cu2Ox with a=b=3.82Å, c=30.7 Å. It shows onset of diamagnetism near Ts = 110K. The strength of the signal increases with annealing near the melting temperature ( ≈ 880°C). Titration gives a composition near x = 8.25. Somewhat different structures are obtained at different compositions and heat treatment.

The stability of fine particles of Nd 2 Fe 14 B type materials visavis O 2 is investigated through Arrhenius plots. It is found that partial Cr substitution for Fe increases oxidation resistance. Conditions for obtaining relatively high coercivities (H c ) in fine particles are discussed. It is shown that for fine particles the temperature dependence of H c is positive and negative for materials based on R 2 Fe 14 B with R = Y and Nd respectively.