36 Ë 2 Superconducting materials
heat transfer, faster oxygenation and less microcracking, possibility of reinforcement
and of interlocking connections, etc. may result from the development of processing
methods of the perforated and textured Y123 with a high performance which can open
new pathways towards practical applications. Lousberg et al. [123] have reported the
effects of filling the holes of drilled HTS samples with a soft ferromagnetic powder. The
magnetic properties of the trapped field magnet were measured, and the experiments
demonstrated an increase in flux trapping ability.
Sawh et al. [124] recently reported the results of studies on 53 melt-textured YBCO
trapped field magnets (TFMs), 2 cm in diameter. The average trapped field on the
seed-side surface was 2.04 T at 77 K.
2.5 Thermal properties of HTS bulk
It is well known that the critical temperatureTc, critical current densityJc, and
upper critical fieldHc2of HTS bulks are the most important parameters for high-
field applications of bulk superconductors, which have been discussed above. HTS
bulks have a series of unique properties, which are very important for the applicati-
ons. These problems should be mainly for the physicists and materials scientists to
investigate. In addition to the three critical parametersTc,Jc, andHc2, some thermal
and mechanical properties of HTS bulk materials associated with the applications will
be briefly discussed in the section. Details of the HTS bulk properties can be found
elsewhere [125].
2.5.1Specific heat
The specific heat capacity, often simply called specific heat, is the heat capacity per
unit mass of a material. An object’s heat capacityCis defined as the ratio of the amount
of heat energy transferred to an object and the resulting increase in temperature of the
object,
Q=Cm훥T, (2.1)whereQis the amount of heat,Cis the specific heat capacity,mis the mass, and훥Tis
the temperature change after absorption or release of heat. In the International System
of Units, heat capacity has the unit J/(kg⋅K).
The specific heatCin a normal conductor consists of two contributions: from
electrons in the conduction bandCeland from the lattice or phononsCph. The elec-
tronic specific heatCelis defined as the ratio of that portion of the heat used by the
electrons to the rise in temperature of the system. The free electron contribution to the
specific heat is typically less than 1% of the phonon specific heat at room temperature.