Dmitriy Yu. Karpenkov

Polymer-Bonded La(Fe,Mn,Si) 13 H x Plates for Heat Exchangers

Composite materials containing hydrogenated La(Fe,Mn,Si)13 powder and 5 wt.% of polymer binder have very good magnetocaloric properties and can be machined in a heat exchanger. These heat exchangers have to fulfill certain requirements regarding their mechanical and chemical stability. In this paper, we have investigated the magnetocaloric properties of thin composite plates produced by two different methods: 1) wire saw cutting and 2) cold rolling. The flexural strength of the plates was investigated by three-point bending method. Furthermore, the corrosive behavior of the plates in different heat transfer fluids was investigated. Finally, the adiabatic temperature change and the magnetic entropy change of the plates were observed over a seven-month period in order to evaluate their applicability in a real device.

A quantitative criterion for determining the order of magnetic phase transitions using the magnetocaloric effect

The ideal magnetocaloric material would lay at the borderline of a first-order and a second-order phase transition. Hence, it is crucial to unambiguously determine the order of phase transitions for both applied magnetocaloric research as well as the characterization of other phase change materials. Although Ehrenfest provided a conceptually simple definition of the order of a phase transition, the known techniques for its determination based on magnetic measurements either provide erroneous results for specific cases or require extensive data analysis that depends on subjective appreciations of qualitative features of the data. Here we report a quantitative fingerprint of first-order thermomagnetic phase transitions: the exponent n from field dependence of magnetic entropy change presents a maximum of n > 2 only for first-order thermomagnetic phase transitions. This model-independent parameter allows evaluating the order of phase transition without any subjective interpretations, as we show for different types of materials and for the Bean–Rodbell model.Magnetocaloric materials often perform best when their magnetic transitions are at the boundary between first- and second-order behavior. Here the authors propose a simple criterion to determine the order of a transition, which may accelerate future magnetocaloric material searches.

Heat Exchangers From Metal-Bonded La(Fe,Mn,Si) 13 H x Powder

Hydrogenated La(Fe,Mn,Si)13-based alloys have an excellent magnetocaloric properties, but poor mechanical and chemical stability. This hinders their direct machining and implementation in an active magnetic regenerator (AMR). In this paper, we show how machinability and corrosion protection of the particles can be improved by a hot-dip coating. To avoid the loss of hydrogen during the coating process, a low melting temperature eutectic Bi–Sn–In alloy was selected as a metal binder. The coated particles were used to build a packed bed regenerator as an array of fixed particles, avoiding such negative effects as sedimentation, segregation, and channel deformation. Similarity theory, combined with unsteady heat transfer approach was applied in order to calculate the optimal operation frequency and to estimate the maximal cooling power of the magnetocaloric regenerators. Two different geometries of heat exchangers were theoretically compared: stacked flat plate/channel structure and packed bed of equidimensional spherical particles. It is shown that, operating at low frequency, the same amount of magnetocaloric material can expel bigger amount of heat, when formed as packed bed heat exchangers. The metal-bonded packed bed regenerator made from La(Fe,Mn,Si)13Hx powder was tested in a home-build versatile testing device in a magnetic field change of 10 kOe. The maximal achievable temperature span as a function of both parameters—hot end temperature and length of regenerator—was explored. The largest thermal span of 8 K was produced by the regenerator with 40 mm length.

Modification of the field dependence and scaling of the magnetocaloric effect in LaFeSi across the tricritical point

The most straightforward classification of magnetocaloric materials is according to the order of the phase transition that they undergo: either second order (continuous) phase transitions (SOPT), like Gd, or first order magneto-structural phase transitions (FOPT) like the giant magnetocaloric material Gd5Si2Ge2.

Magnetocaloric heat exchangers made from metal-bonded La(Fe,Mn,Si) 13 H x powder

La(Fe,Mn,Si)13Hx-based intermetallic compounds with field-induced first-order magnetic phase transition are one of the most promising materials for application in active magnetic refrigerators (AMR).

Polymer-bonded La(Fe, Mn, Si) 13 H x heat exchangers with optimized magnetocaloric properties

Three main issues need to be solved before the magnetic refrigerators will turn from prototypes to the mass production: (1) the price of the magnetic system, made from permanent magnets and providing the magnetic field change of 1-2 T, needs to be significantly reduced; (2) the switching of heat exchange fluids should be drastically accelerated and the hydraulic valves should be able to commutate the hot and cold circuits at frequencies of 20-50 Hz; (3) The time of heat transfer from the magnetocaloric material to the liquid should be reduced down to 50-20 ms and on this time scale the heat exchange should be 80-90 % of the theoretical efficiency. This last issue requires: magnetocaloric materials with outstanding MCE, excellent mechanical integrity and high thermal conductivity shaped into a porous structure with fine straight channels. This work aims to fulfill these requirements.

The influence of severe plastic deformation on magnetic properties of Ni 48 Fe 48 Zr 4 , Fe 1.5 Co 0.5 BTa 0.3 and Co 80 Zr 16 B 4 alloys

Summary form only given. Since the development of Nd-Fe-B magnets, rare-earth magnets have been the essential components in many fields of technology because of their ability to provide a strong magnetic flux. Nevertheless, the crises of rare-earths during last few years had opened the discussion of rare-earths free permanent magnets once again. The important properties of the permanent magnets include their coercivity, remanence and energy product. There are essentially two ways how to achieve the large values of these properties necessary for todays applications . First, the microstructure of the material can be optimized (in our case with the help of high pressure torsion) to prevent rotation of ferromagnetic domains . The second factor is the intrinsic spin-orbit coupling of electrons that forces the spins to align along a particular crystallographic direction, giving rise to the magnetocrystalline anisotropy energy of the material . Because the strength of the spin-orbit coupling increases as the fourth power of an elements atomic number, maximizing the magnetocrystalline anisotropy energy can be accomplished by utilizing heavier elements. Severe plastic deformation has a great effect on magnetic properties of 4-f elements. For instance, in gadolinium a significant increase of the magnetocrystalline anisotropy (up to 2 orders of magnitude) has been observed . Thus, it is the question - is it possible to improve coercivity in 3-d based alloys with the help of severe plastic deformation or not? In our work we have chosen the objects of the investigation based of the following reasons: 1) The meteoritic tetrataenite phase of FeNi has the outstanding magnetic properties as a rare-earth free permanent magnet, but the synthesis of this phase is extremely difficult . Stabilization of the tetrataenite phase could be possible with addition of some extra elements. After preparation of rapidly quenched precursors of Fe48Ni48X4 (X=Ta, Zr, W, Mo, Re) the sample Fe48Ni48Zr4 shows the highest coercivity and it was the reason to select it as the first object of the investigation. 2) (Fe,Co)2B alloys have easy-axis magnetic anisotropy and they are promising materials as the rare-earth free permanent magnets. After preparation of rapidly quenched precursors (Fe,Co)2B alloys with small addition of Ti, Cr, Zr, Nb and Re the highest coercivity has been observed for the composition Fe1.5Co0.5BTa0.3. That was the second compound for current research. 3) In the literature, there are a few works where the magnetic properties of rapidly quenched Co80Zr16B4 were investigated and an enhancement of the coercivity (up to several kOe) has been reported after a heat treatment. This compound has been selected as the third one . Based on that, the aim of this work is to investigate the influence of high pressure torsion (HPT) on magnetic properties of Ni48Fe48Zr4, Fe1.5Co0.5BTa0.3 and Co80Zr16B4 alloys . HPT (Bridgemens anvils) was performed under 5GPa pressure with 5 complete turns . Such a high plastic deformation was found to dramatically affect microstructure of the samples by reduction of the grain size down to the nanometer scale . The field dependencies of magnetization for the concerned samples are shown . There are two different curves for each sample measured after HPT and after the heat treatment which consists of heating the sample up to 1000K with 2K/sec followed by cooling the sample in the furnace down to room temperature . No significant change was observed for Ni48Fe48Zr4 and Fe1.5Co0.5BTa0.3 alloys before and after the heat treatment, but for the Co80Zr16B4 sample an increase of the coercivity up to 2.25 kOe has been found . The origin of the enhanced coercivity is suggested to be due to the refined grain structure obtained during the HPT process . Thus, it is demonstrated that HPT affects magnetic properties of 3d compounds and in some cases it is possible to enhance the coercivity of these materials . The value of coercivity Hc=2.25 kOe obtained for the HPT-treated Co80Zr16B4 alloy is comparable, where this compound was prepared in the form of ribbons by the melt spinning technique .