Adjusting of the performance characteristics of the La(Fe,Si)13 compounds and their hydrides for multi-stimuli cooling cycle application

Magnetic refrigeration is an emerging and promising technology to substitute of conventional vapor-compression analogs. It is based on the utilization of the magnetocaloric effect (MCE) [1], [2], [3]. There are several obstacles to market entry of this technology. The reasons are not sufficiently high values of the adiabatic temperature change in a limited range of magnetic fields (up to 1 T), a narrow operating temperature range of materials demonstrating a first-order phase transition (FOPT), temperature and field hysteresis. These factors reduce the cooling capacity of magnetic refrigerators.

There are several approaches to overcome the abovementioned limitation, including: i) the use of active magnetic regenerator (AMR) cooling cycles, which can significantly increase the temperature span [4], but at the same time it becomes necessary to create working bodies, possessing FOPT, in the form of a set of materials with a variable phase transition temperature. Moreover, the cooling capacity of these cycles is negligible compared to classical refrigeration cycles [5]; ii) mastering the hysteresis characteristics of materials by varying the microstructure, texture and chemical composition in order to accelerate the kinetic of transition [6] and reach the critical point [7], [8], [9], [10], at which the material has sufficiently high values of the MCE but its hysteresis behavior under the action of fields and temperatures disappears; iii) the most promising approach is the use of multi-stimulus materials and the construction of new cooling cycles based on them[11], [12], [13], [14]. This approach involves the simultaneous action of several generalized forces (temperature, external field, and pressure) on the material near the phase transition. The practical implementation of magnetic cooling cycles with multi-stimulus materials will not only expend the range of operating temperatures by shifting the Curie point under the action of the field and pressure, but will also allow to increase the cooling capacity of magnetic refrigerators. The latter is governed by the efficient use of the interpolar volume of the magnetic system by maintaining the peak values of the MCE of the working bodies over a wide operating temperature range. In addition, the application of external pressure will increase the values of the adiabatic temperature change in these materials due to raising the abruptness of the transition [15].

It should be noted that approximate models for describing the behavior of magnetocaloric materials near phase transitions under the simultaneous action of temperature, external field, and pressure have already been created [16], [17], [18]. They have shown that for the La(Fe,Mn,Si)13Hδ alloy system that it is necessary to apply a pressure of 10 MPa to reduce the phase transition temperature by one degree. La(Fe,Mn,Si)13Hδ alloy is considered as the most promising material for the working bodies of magnetic refrigerators [19], [20], [21]. This increment of linear pressure dependence of Curie point is maintained in the pressure range far from critical pressure, at which it becomes impossible to induce a metamagnetic phase transition from a paramagnetic state to a ferromagnetic state by means of a magnetic field or temperature. Hence, it is necessary to provide an operating temperature span of 40 K during the development of magnetic refrigerators operating near room temperature and utilizing multi-stimuli cooling cycles. The implementation of such an operating temperature span requires the application of a pressure of 400 MPa to the working bodies. Therefore, it becomes extremely important to study the evolution of performance characteristics, such as yield strength, during the hydrogenation procedure. The latter is necessary to shift the phase transition temperatures in these compounds to the room temperature range.

Recent research [22] demonstrated that the internal strength limit for La(Fe,Mn,Si)13 compounds is 6 GPa. Moreover, it is strongly size-dependent. As a consequence, the strength limit differs at several times for micro- to macrosized specimens, which is associated with the features of their microstructure. Hydrogenation degrades the mechanical properties even further. Therefore as a starting point for optimization we should elaborate the certain geometry of the refrigerant to exanimate its yield strength. The former is dictated by the provision of sufficient values of the thermal diffusivity of heat exchanger elements [23].

In the majority of articles, package bed heat exchangers with a particle size of 50–300 µm are utilized [24]. The use of this type of heat exchanger makes the application of external pressure and the simultaneous implementation of heat exchange between the refrigerant and the heat transfer medium quite challenging. In contrast, theoretical simulations carried out in [25], [26], [27] indicate that the most efficient heat exchangers made of gadolinium are assemblies of plates or rods. This is due to several reasons. Firstly, an increase in the heat transfer surface and mass of the material due to the control of the void fraction of the heat exchangers. Secondly, a decrease in the pressure drop in these heat exchangers and, as a result, an increase in the coefficient of performance values. The calculated characteristic dimensions of the elements in such heat exchangers are 100–300 µm.

Thus, one of the major research objectives is to improve the mechanical stability of the macro-sized elements of the La(Fe,Mn,Si)13 – based compounds heat exchangers. This will meet the requirements for multi-stimuli cycles realization. There are several approaches to ensure the mechanical integrity of these objects at the required level and at the same time increase their strength. Most of them are based on the introduction of plastic components [28], [29], [30] and/or the development of nondestructive hydrogenation technology [31]. Moreover, it is possible to reduce the negative effect of hydrogen introduction on mechanical characteristics by fabricating textured materials [22].

In this work, a comprehensive study of the performance characteristics of heat exchanger elements based on La(Fe,Si)13 alloys and their hydrides has been carried out. The use in the multi-stimuli cooling cycle implies that magnetic materials must possess to achieve multiple goals. Firstly, magnetic materials must possess efficient heat transfer properties. Secondly, they must have the ability to sustain cyclic external pressure. We determined the optimal value of the secondary phase α-Fe in the La1.12Fe11.6+xSi1.4 (x = 0–4) samples after the calculation of the optimal heat exchangers geometry. It helped us to meet the requirements in yield strength and provides a balance between the caloric and mechanical alloy properties.