  Monocore powder-in-tube MgB2 strands were cold-drawn and heat-treated at 600C
and 700C for times of up to 71 hours and structure-property relationships
examined. Drawing-induced elongation of the Mg particles led, after HT, to a
textured macrostructure consisting of elongated polycrystalline MgB2 fibers
separated by elongated pores. The superconducting Tc, Jc and Fp were correlated
with the macrostructure and grain size. Grain size increased with HT time at
both 600C and 700C. Jc and hence Fp decreased monotonically but not linearly
with grain size. Overall, it was observed that at 700C, the MgB2 reaction was
more or less complete after as little as 30 min; at 600C, full reaction
completion did not occur until 71 h. into the HT. Transport, Jct(B) was
measured in a perpendicular applied field, and the magnetic critical current
densities, Jcm\bot(B) and Jcm{\phi}(B), were measured in perpendicular and
parallel (axial) applied fields, respectively. Particularly noticeable was the
premature dropoff of Jcm\bot(B) at fields well below the irreversibility field
of Jct(B). This effect is attributed to the fibrous macrostructure and its
accompanying anisotropic connectivity. Magnetic measurements with the field
directed along the strand axis yielded a critical density, Jcm\bot(B), for
current flowing transversely to the strand axis that was less than and dropped
off more rapidly than Jct(B). In the conventional magnetic measurement, the
loop currents that support the magnetization are restricted by the lower of
Jct(B) and Jcm{\phi} (B). In the present case the latter, leading to the
premature dropoff of the measured Jcm(B) compared to Jct(B) with increasing
field. This result is supported by Kramer plots of the Jcm{\phi} (B) and Jct(B)
data which lead to an irreversibility field for transverse current that is very
much less than the usual transport-measured longitudinal one, Birr,t.
