Magnetocaloric Properties of (Gd_(1-x)Tb_x)_3Al_2 Compounds Near Room Temperature

Magnetic and thermal properties of the (Gd 1- x Tb x ) 3Al 2 compounds were studied as potential magnetic refrigerant materials which are used in magnetic refrigeration near room temperature at low magnetic field. The compounds (Gd 1- x Tb x ) 3Al 2 with x =0, 0.1, 0.2 and 0 3 exhibit a second order magnetic transition. Curie temperature varies from 255 K for x =0.3 to 280 K for x =0. The maximum of the isothermal magnetic entropy change Δ S increases by substituting Tb element for Gd element. Δ S max =18.9 kJ·m -3 ·K -1 for x =0.1 by changing the magnetic field from 0 to 1 T.

Magnetocaloric effect in Gd_6Co_(1.67)Si_3 compound with a second-order phase transition

The magnetic properties and the magnetic entropy change AS have been investigated for Gd6Co1.67Si3 compounds with a second-order phase transition. The saturation moment at 5 K and the Curie temperature TC are 38.1μB and 298 K, respectively. The AS originates from a reversible second-order magnetic transition around TC and its value reaches 5.2 J/kg.K for a magnetic field change from 0 to 5T. The refrigerant capacity （RC） of Gd6Co1.67Si3 are calculated by using the methods given in Refs.[12] and [21], respectively, for a field change of 0 5T and its values are 310 and 440 J/kg, which is larger than those of some magnetocaloric materials with a first-order phase transition.

Magnetic entropy change of Ce6Ni2Si3-type and large refrigerant capacity GdCoSiGe compound＊

Magnetic entropy change （△SM） and refrigerant capacity （RC） of Ce6Ni2Si3-type Gd6Co1.67Si2.5Geo.5 compounds have been investigated. The Gd6Col.67Si2.5Geo.5 undergoes a reversible second-order phase transition at the Curie temperature Tc = 296 K. The high saturation magnetization leads to a large ASM and the maximal value of △SM is found to be 5.9 J/kg. K around TC for a field change of 0-5 T. A broad distribution of the △SM peak is observed and the full width at half maximum of the △SM peak is about 101 K under a magnetic field of 5 T. The large RC is found around TC and its value is 424 J/kg.

Influence of the Erbium Substitution for Gd in Gd_5Si_(1.8)Ge_(2.2) Alloys on the Magnetocaloric Effect in Low-Field

The phases and magnetocaloric effect in the alloys (Gd1-xErx)5Si1.8Ge2.2 with x=0, 0.1, 0.2 and 0.3 were investigated by X-ray diffraction analysis and magnetization measurement. The samples were single phase with the monoclinic Gd5Si2Ge2-type structure. With the increase of Er content, the Curie temperature (Tc) decreased obviously from 253 K of the alloy with x=0 to 114 K with x=0.3. The maximum magnetic entropy changed in the samples of (Gd1-xErx)5Si1.8Ge2.2 with x=0.0, 0.1, 0.2 and 0.3 were 6.88, 8.32, 9.59 and 10.24 J·kg-1·K-1 respectively in the applied field change of 0～2.0 T.

Thermal expansion anomaly and spontaneous magnetostriction of Gd_2Fe_(17) compound

Materials with negative thermal expansion have many practical applications. However, these materials are known in only several oxide systems, and when the negative thermal expansion occurs, the contraction is usually small and limited to a narrow temperature range beyond room temperature. For obtaining a compound with negative thermal expansion in broad temperature range, the structural and magnetic properties of Gd2Fe17 compound were investigated by means of X-ray diffraction and magnetization measurements. The Gd2Fe17 compound annealed at 1050 oC had a Th2Zn17-type structure. There existed an anisotropic strong spontaneous magnetostriction and a negative thermal expansion in Gd2Fe17 compound. The average thermal expansion coefficients was =–7.40×10–6/K in the temperature range of 294–453 K and =–1.80×10–5/K in 453–534 K, respectively. The spontaneous magnetostrictive deformation ωS decreased from 4.34×10–3 to near zero with temperature increasing from 294 to 572 K. The spontaneous linear deformation λc was much larger than λa at the same temperature below about 500 K.

Anomalous thermal expansion of Gd_2Fe_(15.5)Cr_(1.5) compound

The structure and magnetic properties of Gd2Fe15.5Cr1.5 compound were investigated by means of X-ray diffraction and magnetization measurements.The Gd2Fe15.5Cr1.5 compound had a rhombohedral Th2Zn17-type structure.The Curie temperature of Gd2Fe15.5Cr1.5 compound was about 570 K.This value was about 60 K higher than that of the mother compound Gd2Fe17.Negative thermal expansion was found in Gd2Fe15.5Cr1.5 compound in a broad temperature range 294-572 K by X-ray dilatometry.The coefficient of the average thermal expansion was =-3.87×10-6 K-1 in 294-512 K,and-1.58×10-5 in 512-572 K.The magnetostriction deformations from 294 to 532 K were calculated by means of the differences between the experimental values of the lattice parameters and corresponding values extrapolated from the paramagnetic range.The result showed that the spontaneous volume magnetostrictive deformation ωS decreased linearly from 4.73×10-3 to 0.11×10-3 with the temperature increasing from 294 to 532 K.The analysis showed that the ωS mainly came from the contribution of the spontaneous linear magnetostrictive λc along the c axis.