Vitalij K. Pecharsky

New Magnetic Refrigeration Materials for the Liquefaction of Hydrogen

Five heavy lanthanide ferromagnetic intermetallic compounds were studied as potential magnetic refrigerants for the liquefaction of hydrogen gas. (Dy0.5Er0.5)Al2 and TbNi2 appear to be better refrigerants than GdPd for a Joule-Brayton cycle refrigerator, while (Gd0.54Er0.46)AlNi seems to be a suitable refrigerant for an Ericsson cycle refrigerator.

Preparation, crystal structure and magnetocaloric properties of Tb5(SixGe4−x)

The crystal structure and magnetothermal properties of the potential magnetic refrigerant materials, Tb5(SixGe4−x) alloys where x⩾2, have been studied. The crystal structure of the Tb5(SixGe4−x) alloys varies from the orthorhombic Gd5Si4-type to the monoclinic Gd5(Si2Ge2)-type as silicon concentration decreases from x=4 to x=2. The dc magnetic susceptibility and magnetization data indicate that a paramagnetic to ferromagnetic phase transition occurs in these alloys. The heat capacity data measured in magnetic fields varying from 0 to 100 kOe show that a second order (x=4,3) or a first order phase transition (x=2) occurs in the alloys. The magnetic entropy change and the adiabatic temperature change calculated from the heat capacity data indicate that the Tb5(SixGe4−x) alloys are promising materials for magnetic refrigeration from about 100 to 250 K when their chemical composition varies from x=2 to x=4.

Performance of a new regenerator material in a pulse tube cooler

A new class of alloys has been developed at Ames Laboratory in Ames, Iowa. These alloys exhibit heat capacities that exceed those of all other materials, including lead, over a wide range in temperature (15 K<T<85 K). An effort is underway to employ these alloys in a two-stage pulse tube cooler driven by a linear compressor to achieve cooling at 20 K. The first stage of the cooler will have the conventional stainless steel screen regenerator matrix. The matrix for the second stage regenerator (<60 K) will be made from the newly developed alloys. The performance of one such alloy is being tested in an apparatus that consists of a single-stage pulse tube cooler pre-cooled by a liquid nitrogen stage. The regenerator material used in the tests is in spherical powder from having particle sizes ranging from 90 μm to 106 μm. Preliminary results from tests performed in the pulse tube cooler employing the new regenerator material are presented.

Formation of Co Moment in the Paramagnetic Phase of RCo 2

The formation of Co magnetic moment in the paramagnetic phase of GdCo2 has been studied by means of first principles calculations. The introduction of impurities in the transition metal-rare-earth Laves phases leads to changes in the density of states of the transition metal 3d band. In this paper, we calculate the electronic structure of GdCo2 system (a C15 unit cell) with an interstitial impurity (Ti). Calculations show that local disorder introduced by the impurity shifts the Fermi level in such a way that Co 3d up and down states split. This result suggests that local disorder may be responsible for the formation of Co moment in this system. To deepen this phenomenon, we have considered the antisite substitution of a Gd atom by a Co within the unit cell, which is the most common defect in the Laves phases. Calculations show the formation of magnetic moment in the Co atom at the substituted site as well as in their nearest Co atoms, similarly to the observed in the systems with the interstitial impurity. Our results are compatible with the reported x-ray magnetic circular dichroism measurements performed in the RCo2 family of compounds.

Magnetothermal properties of single crystal dysprosium

The magnetocaloric properties (the adiabatic temperature change) of the high purity single crystalline dysprosium have been measured directly over the temperature range from 78 to 220 K in magnetic fields from 0 to 14 kOe applied along the easy magnetization direction (a-axis). These results are in good to excellent agreement, except for two regions (105 to 127 K, and 179 to 182 K), with the previous magnetocaloric effect data reported on lower purity dysprosium samples. The magnetic phase diagram of Dy has been refined based on the results of these measurements and two new high magnetic field phases have been identified.

Effects of praseodymium additions on the magnetothermal properties of erbium

The effects of the Pr additions (10, 20, 25, and 30 at.%) on the magnetothermal properties of polycrystalline metallic Er have been studied using low temperature, high magnetic field heat capacity (4∼120 K, 0∼75 kOe), dc magnetization and ac magnetic susceptibility. These properties include the volumetric heat capacity, the magnetocaloric effect (MCE), the magnetic transition temperatures, the paramagnetic Curie temperatures and the effective magnetic moments. It has been found that Pr additions increase the Curie temperature and decrease the Neel temperature of Er, as well as wipe-out the 26 K and 52 K transitions apparently by merging with the former two transitions. This gives rise to only one MCE peak in each of the Er-Pr alloys, from the broad table-like shape for the Er90Pr10 alloy to the typical narrow caret-like shape for the Er70Pr30 alloy. Pr additions also increase the heat capacity of Er significantly below 18 K. The implications of this higher heat capacity as potential cryocooler regenerator...

Production and Testing of Microchannel of Magnetocaloric Regenerators

Prepared ~1.1 kg of CoMn{sub 1{+-}x}Ge{sub 0.95{+-}y}Sn{sub 0.05{+-}z} (x=0, 0.01, 0.02, 0.04, y=0, 0.005, 0.01, 0.03, z=0, 0.005, 0.01, 0.03) samples by using arc melting and induction melting techniques. The Curie temperatures of the majority of the samples are near the room temperature with a deviation of {+-}20K. Prepared ~4.5 kg of Gd with the dimensions and in the shape required by MER. Characterized CoMn{sub 1{+-}x}Ge{sub {0.95{+-}y}Sn{sub 0.05{+-}z} (x=0,0.01, 0.02, 0.04, y=0, 0.005, 0.01, 0.03, z=0, 0.005, 0.01, 0.03) samples prepared by arc melting, induction melting, and hot pressing techniques. The characterization techniques include powder x-ray diffraction, magnetometry, calorimetry, and Scanning Electron Microscopy (SEM) equipped with Energy Dispersive Spectroscopy (EDS).