scholarly journals Large Low-Field Reversible Magnetocaloric Effect in Itinerant-Electron Hf1-xTaxFe2 Alloys

Materials ◽  
2021 ◽  
Vol 14 (18) ◽  
pp. 5233
Author(s):  
Zhao Song ◽  
Zongbin Li ◽  
Bo Yang ◽  
Haile Yan ◽  
Claude Esling ◽  
...  
Keyword(s):  
Magnetocaloric Effect ◽  
High Performance ◽  
Thermal Hysteresis ◽  
Second Order ◽  
Order Transition ◽  
Itinerant Electron ◽  
First Order ◽  
Second Order Transition ◽  
Magnetic Field Change ◽  
Materials Used

First-order isostructural magnetoelastic transition with large magnetization difference and controllable thermal hysteresis are highly desirable in the development of high-performance magnetocaloric materials used for energy-efficient and environmental-friendly magnetic refrigeration. Here, we demonstrate large magnetocaloric effect covering the temperature range from 325 K to 245 K in Laves phase Hf1-xTaxFe2 (x = 0.13, 0.14, 0.15, 0.16) alloys undergoing the magnetoelastic transition from antiferromagnetic (AFM) state to ferromagnetic (FM) state on decreasing the temperature. It is shown that with the increase of Ta content, the nature of AFM to FM transition is gradually changed from second-order to first-order. Based on the direct measurements, large reversible adiabatic temperature change (ΔTad) values of 2.7 K and 3.4 K have been achieved under a low magnetic field change of 1.5 T in the Hf0.85Ta0.15Fe2 and Hf0.84Ta0.16Fe2 alloys with the first-order magnetoelastic transition, respectively. Such remarkable magnetocaloric response is attributed to the rather low thermal hysteresis upon the transition as these two alloys are close to intermediate composition point of second-order transition converting to first-order transition.

scholarly journals Reversibility of the Magnetocaloric Effect in the Bean-Rodbell Model

2021 ◽  
Vol 7 (5) ◽  
pp. 60
Author(s):  
Luis M. Moreno-Ramírez ◽  
Victorino Franco
Keyword(s):  
Magnetic Field ◽  
Thermal Hysteresis ◽  
Entropy Change ◽  
Second Order ◽  
Zero Field ◽  
First Order ◽  
Magnetic Field Change ◽  
Magnetocaloric Materials ◽  
Moderate Magnetic Field ◽  
The Magnetic Field

The applicability of magnetocaloric materials is limited by irreversibility. In this work, we evaluate the reversible magnetocaloric response associated with magnetoelastic transitions in the framework of the Bean-Rodbell model. This model allows the description of both second- and first-order magnetoelastic transitions by the modification of the η parameter (η<1 for second-order and η>1 for first-order ones). The response is quantified via the Temperature-averaged Entropy Change (TEC), which has been shown to be an easy and effective figure of merit for magnetocaloric materials. A strong magnetic field dependence of TEC is found for first-order transitions, having a significant increase when the magnetic field is large enough to overcome the thermal hysteresis of the material observed at zero field. This field value, as well as the magnetic field evolution of the transition temperature, strongly depend on the atomic magnetic moment of the material. For a moderate magnetic field change of 2 T, first-order transitions with η≈1.3−1.8 have better TEC than those corresponding to stronger first-order transitions and even second-order ones.

Nematic/Smectic-A Transition (NSA), Location of Tri Critical Point (TCP) in nO.m Series – A Birefringence Study

2010 ◽  
Vol 65 (4) ◽  
pp. 335-341 ◽  
Author(s):  
Venkata G.K. M. Pisipati ◽  
Divi Madhavi Latha ◽  
Boddapati T. P. Madhav ◽  
Potapragada V. Datta Prasad
Keyword(s):  
Critical Point ◽  
Homologous Series ◽  
Order Parameter ◽  
Second Order ◽  
The Body ◽  
Order Transition ◽  
Refractive Indices ◽  
First Order ◽  
Smectic A ◽  
Second Order Transition

The tri critical point (TCP), where the second-order transition transforms to first order has been located in nO.m homologous series. The order parameter has been estimated from the birefringence δn, from the refractive indices and from birefringence data available in literature and from those obtained at our laboratory on a number of nO.m compounds. The compounds in the nO.m series exhibit both second and first-order nematic/smectic-A (NSA) transition depending on the McMillan ratio (TNA/TIN) which in turn depends on the nematic and smectic-A thermal ranges. The data presented are compared with the body of the data available on this homologous series obtained with other techniques.

NMR · NQR and DTA · DSC Studies of Phase Transitions in Pyridinium Tetrachloropalladate(II) and Pyridinium Tetrachloroplatinate(II)

1998 ◽  
Vol 53 (6-7) ◽  
pp. 419-426 ◽  
Author(s):  
Tetsuo Asaji ◽  
Keizo Horiuchi ◽  
Takehiko Chiba ◽  
Takashige Shimizu ◽  
Ryuichi Ikeda
Keyword(s):  
Phase Transitions ◽  
Low Temperatures ◽  
Structural Phase ◽  
Orientational Order ◽  
Second Order ◽  
Order Transition ◽  
Spin Lattice Relaxation ◽  
Potential Wells ◽  
H Nmr ◽  
Second Order Transition

Abstract From the measurements of DTA • DSC and the temperature dependences of 35Cl NQR frequencies, phase transitions were detected at 150 K, 168 K, and 172 K for (pyH)2 [PtCl4], and at 241 K for (PyH)2 [PdCl4]. In order to elucidate the motional state of the constituent ions in the crystals in connection with the structural phase transitions, the 35Cl NQR and 1H NMR spin-lattice relaxation times and the second moment of the 1H NMR line were measured as functions of temperature. For both compounds, the potential wells for the cationic reorientation are suggested to be highly nonequivalent at low temperatures. Above 168 K, the pyridinium ions in (pyH)2[PtCl4] are expected to reorient between almost equivalent potential wells. As for (pyH)2[PdCl4], it is expected that the orientational order of the cation still remains even above the second order transition at 241 K. A change of the potential curve from two-unequal to three-unequal wells is proposed as a possible mechanism of the second order transition. The activation energies for the cationic motion in the respective model potential are derived for both compounds at high and low temperatures.

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