IV Transport Critical Current and n Number of MgB2 Conductors as a Function of Temperature

IV data of MgB2 conductors were obtained over more than 3 voltage decades from 0.1 to 400 μV/cm and currents up to 600 A, revealing the characteristic
n
number to increase rapidly with decreasing temperature from
n
= 20 around 33 K to above 60 by 25 K. Constant temperature, current-voltage, (
IV
) characteristics of magnesium diboride conductors over a sufficient voltage range to determine
n
are not available in the literature and remain a serious experimental challenge. Conductors, instrumented with voltage and temperature sensors, were mounted on a cryofree, variable-temperature-controlled platform within a custom built cryostat. To achieve constant temperature measurement, the stability had to be improved by reducing the thermal path either longitudinally or laterally. Both dc current and transient measurements were implemented and refined until good agreement was obtained. At the highest stable voltage reached, for a given temperature, any further increase led to a rapid thermal propagation until the sample was fully resistive. In carbon-doped samples, the temperature-dependent transport critical current density is best described by two linear functions with different
T
_c
's. The lower
T
_c
function has the steeper increase in transport critical current with decreasing temperature.