Papers by Keyword: Magnets

Abstract: The emerging field of soft robots offers the prospective of applying soft actuators as artificial muscles, replacing traditional actuators based on hard materials. Dielectric elastomers (DE), one class of electro-active polymers, represents an attractive technology for the realization of mechatronic actuators, due to their light weight, high energy efficiency and scalability. This work aims at investigating and characterizing a novel design of membrane DE in-plane actuator by magnetic mechanism. A nonlinear dynamic model of the dielectric elastomer actuator (DEA) is established and corresponding material parameters are identified. Natural frequency and response speed of DEAs are studied. It demonstrates that larger stretch and higher response speed can be realized by the proposed DEA.

Abstract: The replacement of NdFeB magnets by ferrite magnets is discussed. For motors, remanence is relevant, implying in a volume three times that of NdFeB, when the relevant index of merit is remanence. However, if the relevant issue is the BH_max (maximum energy product), the volume for replacement should be ten times larger. The high resistivity of ferrites is a big advantage for motors. The temperature of operation is also relevant, because NdFeB magnets loss coercivity even with small increase of temperature. Different applications are discussed, as for instance, motors for electric cars and wind turbines. The choice of the proper volume depends on the evaluation of demagnetizing field in the condition of operation.

Abstract: Compression moulded rare earth magnets are used in many applications, and one of them is in electrical motors, which can operate at temperatures as high as 150 °C. In high temperatures, oxidation and phase transformations may affect magnetic properties. Another issue to be considered is polymer degradation. In this paper cylindrical samples of a commercial ready-to-press raw material (MQPB+, Molycorp-Magnequench) were pressed to densities close to 5.50 g/cm³. These samples were cured (low temperature heat treatment) and thermal aged at 200 °C for 72 hours, under dynamic air. Physical characteristics (weight, dimensions and densities) and magnetic properties were measured at room temperature in a TCH2020 hysteresigraph (Globalmag). Degradation of magnetic properties was observed, and oxidation seems to be the main reason.

Abstract: The curling reversal mode is discussed, with emphasis on the assumptions of the model, many of them unrealistic. The curling mode assumes the Bloch approximation, where a cosine function is approximated by a polynomial of second order. It is argued that if the curling model takes place, this could affect the anisotropy field, and then the anisotropy field would be function of the grain size, but this is not observed. Exchange effects may affect hysteresis curve, as discussed in the SW-CLC model.

Abstract: It is presented a Monte Carlo (MC) study of the dependency of the magnetization of sintered CuMn alloys with the temperature and applied magnetic field using a RKKY function to model the Mn-Mn magnetic interaction. Two kinds of simulations were done: (i) simulations at zero magnetic field, where the energy is obtained as a function of the temperature and (ii) simulations at constant temperature, varying the applied magnetic field. For both kinds of simulations, the variations of the total energy and the magnetization with respect to the concentration x and the range of the interaction (cutoff distance) were calculated. These simulations indicate that the range of the interaction among the magnetic atoms has a significant impact on the thermodynamic properties of the studied alloy.

Abstract: Ceramic magnets as barium ferrite or strontium ferrite have many applications in high technology. One of the reasons is the low cost when compared to competitor materials, as Alnico, MnBi, MnAl or NdFeB. In this study, the advantages and disadvantages of Ba and Sr ferrite magnets are discussed. One clear advantage is that ferrites are already oxides, and do not present the corrosion problems typical of NdFeB and other metallic alloys. As ferrites are oxides, the processing is much easier and cheaper. For example sintering can be done at air, and milling under wet condition. One of the main conclusions is the excellent ratio cost/benefit of ferrites, giving advantage in many applications. Special attention is given for application of ferrites in high efficiency motors.

Abstract: The isotropic Stoner-Wohlfarth model can be applied for the determination of the anisotropy field in powders. Samples of strontium ferrite - SrFe₁₂O₁₉ – were measured in Vibrating Sample Magnetometer (VSM). By means of the fitting of the first quadrant of the hysteresis curve the anisotropy field was estimated as 19-20 kOe for the SrFe₁₂O₁₉ phase. The main advantage of the method is its simplicity, because compacting or sintering of the powder samples is not necessary

Abstract: Due to high magnetic fields causing strong interactions between permanent magnets and other ferromagnetic material, transport and handling of magnetized magnet bodies is challenging. To avoid undesired effects, such as influences on sensitive devices or difficult separation of the single magnets from stack, spacing and shielding of the magnet bodies is required leading to larger package sizes and thus in some cases higher energy demand during transport referred to the transported magnet mass. An optimization of the transport chain can be reached using the software tool presented in this paper. Further magnetizing high coercive rare earth magnets needs strong magnetic fields. To create the necessary field strength, copper coils are used requiring current strengths of several kA. Since the electrical resistance of copper differs from zero, this also means enormous thermal losses. Hence to reduce the losses and to avoid thermal damage of the coil, only short current pulses are applied generated by a pulse magnetizer. However, the efficiency of the process is very poor and lies in the lower per mil range. The presented paper explains the magnetization process in detail with focus on the losses within the magnetization device. Further different material parameters influencing the saturation field strength, such as conductivity, size and diameter to length ratio are presented and possibilities to improve the energy efficiency are shown.

Abstract: In the present study, EBSD was used for the characterization of alloys used for production of SmFeCoCuZr magnets. EBSD is adequate for texture analysis, but may give misleading results for phase identification. EBSD is not suitable for identifying phases with very similar crystalline structure, especially when the phases are crystallographically coherent, due to the superposition of Kikuchi lines. As consequence, for phase identification EBSD should be considered a complementary technique to other methods, as for example x-ray diffraction (XRD).

Abstract: The Brazilian reserves of rare earths were updated. Brazil is among the countries with the largest reserves of rare earths in the world. Rare earths present a series of applications in the high technology industry. Magnet production is one of the applications requesting largest volumes of rare earths. Another relevant application is catalysis and Petrobras needs 1000 tons/year of La₂O₃ for petroleum refining. CBMM mine has a pilot plant able to produce tons of separated rare earth oxides, notably didymium (a mixing of Nd and Pr oxides) used in magnet manufacture. Serra Verde mine in Goiás also produced separated rare earth oxides. The magnet production in Brazil can be important for the high efficiency electrical motors industry and also for the automotive industry.