A new class of magnetic regenerator materials for 4 K cryocoolers has been developed. A hexagonal gadolinium oxisulfide has been found to provide a very high volumetric heat capacity of more than at 5.2 K. Polycrystal ceramic GOS particles have been fabricated with diameters between 0.35 mm and 0.45 mm. The achieved smooth surface on the spherical particles and a Vickers hardness of ~900 make the material very suitable for use in regenerators.The measured data on the thermal and magnetic properties of the GOS also show excellent characteristics as a regenerator material; no change in heat capacity is observed when applying a magnetic field up to 1 T, and the high thermal conductivity of ~0.05 W/cmK at 6 K is twice that of ers where a portion of the in the 2nd regenerator was replaced with GOS. At a 50% volumetric ratio of GOS to the cooling power increased up to ~25 % at 4.2 K. The cooling power did not drop above 4.5 K, where it significantly decreases when GAP is used. The new ceramic regenerator material GOS is expected to not only contribute to increased cooling performance at 4.2 K, but also to decreased fabrication cost.
Our group has reported several candidates for magnetic regenerator materials in the helium temperature range, such as, (Erl-xDYx)Ni2• Er(Nil-xCox)2• ErNi and Er3Ni. However, those Curie temperatures Tc's are more than ~s.s K and the specific heats C(T)'s of those systems become very small near 4.2 K. In the present investigation we have succeeded in developing a new low Tc material, Ero.gYbo.1Ni2 whose Tc is ~4 K. Moreover, other new regenerator materials, such as Erl-xYbxNi and Er1-xHoxNi2 have also been investigated and usefulness of those compounds was verified. We have also studied the origin of the small C(T) of some Er compounds near 4.2 K and it has been made clear that the Schottky type specific heat reduces the absolute value of C(T) in low temperature range.
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