Report on Fifth European Conference on Thermophysical Properties at High Temperatures, 15-21 May 1976 at the High Temperature Institute, Academy of Sciences of the USSR, Moscow.

Thermophysical properties of materials

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1413&context=mse_pubs

Extraordinarily strong magneto-responsiveness in phase-separated LaFe2Si

The use of a limited magnetic field strength to induce a giant magnetocaloric response is desired towards the potential magnetic refrigeration applications. A method to lower the external magnetic field through taking advantage of the exchange field gradient in the alloys with different component ratios and random magnetic anisotropy under a constant magnetic field is conceived, where the conventional thermodynamic Maxwell formula is modified by replacing the external magnetic field by internal exchange field to calculate magnetic entropy change. The Ni46Co3Mn37-x CuxIn14 (x=0, 1, 2, 3, at.%) alloys are fabricated, and with increasing x, the martensitic transformation temperature increases towards room temperature, meanwhile, the ferromagnetism is weakened and the Curie temperature decreases down to room temperature. Under a constant magnetic field of 1.5 T, it is calculated that an exchange field variation of 1 T can induce a giant positive magnetic entropy change of ~14 J kg-1K-1 near the martensitic transformation and a high relative cooling power ~110 J kg-1 near the paramagnetic-to-ferromagnetic phase transition. Other features of this magnetocaloric effect include that it is weakened under stronger magnetic fields and the relative cooling power is independent of magnetic field. The Monte Carlo simulation is also performed to fit the magnetization and magnetic entropy change behaviors near the magnetic phase transition, and the exchange energy in alloys are quantified from 1.438×10-22 to 1.290×10-22 J with the x variation from 0 to 1 through fitting the linear relative cooling power results. This work not only opens a new avenue to develop a household magnetic refrigerant with low external stimuli and high signal-to-noise ratio, but also provides a method to quantify the intrinsic magnetic parameters using magnetocaloric effect.

Giant Magnetocaloric Response Induced by Tuning the Intrinsic Exchange Field in the Alloys With Random Magnetic Anisotropy Under a Constant Magnetic Field

Effect of annealing on martensite transformation (MT) and magnetocaloric effect (MCE) in a Ni41.3Co6.3Mn42.4Sn10.0 microwire was investigated. After annealing at temperatures 723–1173 K, the MT temperatures and Curie points shifted to higher range; meanwhile, the thermal hysteresis (ΔThys) decreased. However, partial transition arrest appeared when annealed at temperatures > 1073 K, leading to decreased magnetization difference (ΔM) between martensite and austenite phases. The maxima ΔM and minima ΔThys were produced at an annealing temperature 1023 K. Consequently, a large magnetic entropy change ( Δ S M max ) 20.1 J/kg·K, wide working temperature span (ΔTFWHM) 20 K and thus high effective refrigeration capacity (RCeff) 197 J/kg were obtained under a magnetic field 5.0 T after annealing at 1023 K for 1 h. These facts make Ni-Co-Mn-Sn microwires promising for miniature refrigeration devices due to their high specific surface area, low thermal hysteresis and markedly reduced annealing temperature and time.

Martensite transformation behavior and magnetocaloric effect in annealed Ni-Co-Mn-Sn microwires

MnFe-based magnetocaloric materials exist a giant magnetocaloric effect so that they can be used as the working substance for room-temperature magnetic refrigeration. But there are two key problems to be solved before employing them as the working substance: one is that for single MnFe-based material, its giant magnetocaloric effect only arise in a small temperature range and the other is that these materials exist generally thermal hysteresis. For these reasons, a novel composite material based on MnFe-based materials is designed optimally, and a regenerative Ericsson refrigeration cycle using the composite material as the working substance is established. Furthermore, the performance of the refrigeration cycle with the composite is analyzed and evaluated. The influences of thermal hysteresis on main thermodynamic parameters of the refrigeration cycle are revealed by numerical calculation. The research results can provide some significant guidances for the parametric design and performance improvement of room-temperature magnetic refrigerators.

The effect of thermal hysteresis on the performance of a regenerative Ericsson refrigeration cycle with MnFe-based composite material

We report on the impact of magnetoelastic coupling on the magnetocaloric properties of LaFe$_{11.4}$Si$_{1.6}$H$_{1.6}$ in terms of the vibrational density of states, which we determined with $^{57}$Fe nuclear resonant inelastic X-ray scattering measurements and with density-functional-theory based first-principles calculations in the ferromagnetic low-temperature and paramagnetic high-temperature phase. In experiments and calculations, we observe pronounced differences in the shape of the Fe-partial VDOS between non-hydrogenated and hydrogenated samples. This shows that hydrogen does not only shift the temperature of the first-order phase transition, but also affects the elastic response of the Fe-subsystem significantly. In turn, the anomalous redshift of the Fe VDOS, observed by going to the low-volume PM phase, survives hydrogenation. As a consequence, the change in the Fe specific vibrational entropy $\Delta S_\mathrm{lat}$ across the phase transition has the same sign as the magnetic and electronic contribution. DFT calculations show that the same mechanism, which is a consequence of the itinerant electron metamagnetism associated with the Fe subsystem, is effective in both the hydrogenated and he hydrogen-free compounds. Although reduced by 50 % as compared to the hydrogen-free system, the measured change $\Delta S_\mathrm{lat}$ of 3.2\pm1.9 J/kgK across the FM to PM transition contributes with 35 % significantly and cooperatively to the total isothermal entropy change $\Delta S_\mathrm{iso}$. Hydrogenation is observed to induce an overall blueshift of the Fe-VDOS with respect to the H-free compound; this effect, together with the enhanced Debye temperature observed, is a fingerprint of the hardening of the Fe sublattice by hydrogen incorporation. In addition, the mean Debye velocity of sound of LaFe$_{11.4}$Si$_{1.6}$H$_{1.6}$ was determined from the NRIXS and the DFT data.

Influence of hydrogenation on the vibrational density of states of magnetocaloric $\mathrm{LaFe}_\mathrm{11.4}\mathrm{Si}_\mathrm{1.6}\mathrm{H}_{1.6}$

Valentin Brabänder

Papers

Influence of hydrogenation on the vibrational density of states of magnetocaloric $\mathrm{LaFe}_\mathrm{11.4}\mathrm{Si}_\mathrm{1.6}\mathrm{H}_{1.6}$