Received: July 20, 2020; Accepted: July 27, 2020; Published: July 30, 2020.
In recent years, many research centers have been intensively researching soft magnetic composites (SMC) based on the use of soft magnetic particles, usually based on iron, with an electrically insulating coating on each particle [1-5]. The main purpose of the low-frequency composite magnetically soft material is the construction with its use of high-efficiency inverter electric motors with inverters, transformers, inductors and other devices for which the working magnetization reversal frequency significantly exceeds the industrial frequency [6,8]. In this regard, the basic properties of a composite material such as magnetic permeability, magnetic induction, magnetization reversal losses and mechanical properties should be no worse than that of traditional laminated metal magnetic materials.
Somaloy powders (Hoganas company) obtained using special iron powders and phosphorus oxide insulation are currently available as commercial composite soft magnetic materials [9-12]. However, some parameters, primarily the loss of magnetization reversal, as well as their high cost, about 6 euros, do not quite suit consumers. The properties of composite magnetic materials depend and are determined by a number of factors. First of all, this is an insulating coating of iron particles - its type, thickness, resistivity, adhesion to iron powder and a number of other properties. Also important is the very basis of the composite magnetic material highly pure iron powder its chemical composition, fractional composition, surface adhesion of particles, and some other factors. Therefore, the aim of the work is to study the properties of low-frequency soft magnetic material by the optimal choice of insulation coating and directly the type of highly pure iron powder.
Materials and Methods
The manufacturing technology of isolated powders of soft magnetic materials and the manufacture of products from them is a multi-stage process, including the following basic operations.
1) The operation of the reaction deposition of insulating coatings from the gas phase at a temperature of 150 - 200° C. When performing this operation, iron powder is suspended in a gaseous medium containing vapor of an applied oxide material together with solvent vapor. Solvent vapors, which were used ethanol, gasoline, acetone, isopropyl alcohol and others, are gradually removed from the reactor. In this work, we studied composite materials based on highly pure iron powders ASC100.29, ABC100.30, and Atomet 1001HP, on the surface of which insulating coatings were applied using various oxide solutions and suspensions. The chemical composition of the investigated iron powders are shown in table 1.
Table 1: Chemical composition, %.
2) The operation of fixing coatings by applying a small amount of a solution of silicone varnish and lubricating an isolated powder by adding a solution of peat wax (peatwax) in an amount of 0.15% of the initial weight of the powder.
3) The operation of manufacturing products by hydrostatic pressing of isolated powders in special molds, on the working surfaces of which grease was applied, under a pressure of 0.7 - 0.8 GPa under normal conditions. Compressed products were heat treated to normalize physical parameters. Samples are annealed at a temperature of 400° C in vacuum or in air.
One of the main advantages of the developed technology, even taking into account that the technology is being finalized, is the possibility of uniformly applying protective and insulating coatings of various compositions in a wide range from nanometers to micrometers. In addition, products made of composite material according to the developed technology retain their dimensions after pressing and subsequent annealing.
To study the magnetic properties, composite magnetic material samples were made in the form of rings with dimensions 24x13x8 mm by powder metallurgy by pressing the prepared isolated powder and then annealed in vacuum at a temperature of 400° C. The density of finished products ranged from 7.6 to 7.75 g/cm3. Magnetic properties were measured on an express magnetometer, where losses and other magnetic parameters were determined from the magnetization reversal curves of the samples.
To study the effect on the magnetic properties of composite materials, primarily the loss of magnetization reversal, we used various solutions and suspensions shown in Table 2.
Table 2: Losses in material based on ASC100.29 with identical by volume of additives at a frequency of 1 kHz and induction of 1.5 Tesla.
?2?5 + ?2?3
It follows from the table that the magnetization reversal losses for all coatings used are practically the same, regardless of the properties of the latter. In this regard, the initial properties of the iron powder itself are decisive in the formation of the properties of composite magnetic materials. Figure 1 shows the results of studies of the dependence of magnetization reversal losses on the magnitude of magnetic induction at a frequency of 1 kHz for composite materials based on iron powders ASC100.29, ABC100.30, and Atomet 1001HP.
Figure 1: Dependence of losses on magnetic induction for composite materials based on powders ASC100.29 -1, 1001HP -2, ABC100.30 -3. at a frequency of 1 kHz and the same thickness of the insulating layer based on boron oxide.
As can be seen from Figure 1, the losses are maximum for composite materials based on ASC100.29 and minimal for materials based on ABC100.30. It can be assumed that the carbon content in the starting iron powders influences the magnetization reversal losses. Figure 2 shows the dependence of the magnetization reversal loss on the carbon content in the initial iron powders.
Figure 2: Dependence of losses at a frequency of 1 kHz and induction of 1.5 T on the carbon content.
The presented results on the dependence of losses on the carbon content can be considered from the following positions. Carbon in the initial powders forms iron carbides, which are centers of inhibition of domain walls during magnetization reversal of the composite material. As a consequence of this, with increasing carbon content, the value of the coercive force grows material and, as a result, losses on magnetization reversal increase. Figure 3 shows the comparative loss curves in the commercial material Somaloy and composite material on industrial iron powder ABC100.30 when titanium oxide is used as an insulating coating. It can be seen from the figure that the losses in the material based on ???100.30 are somewhat lower. In this case, consider the significant difference in the price of materials.
Figure 3: Losses at a frequency of 1 kHz Somaloy-1, ???100.30 isolation of TiO2.
A significant reduction in magnetization reversal losses can be achieved due to additional machining of the obtained samples in a vibrating installation. Figure 4 shows the dependences of losses on induction at a frequency of 1 kHz for a composite material based on ASC100.29 with insulation based on titanium oxide after annealing (Fig. 4, curve 1) and after additional processing in a vibrator for 2 and 5 minutes, curves 2 and 3, respectively.
Figure 4: Dependence of losses on magnetic induction at a frequency of 1 kHz for a composite material based on ASC100.29 isolation of titanium oxide after annealing -1 and processing in a vibrator for 2 and 5 minutes, curves 2 and 3, respectively.
The observed phenomenon of a decrease in the magnetization reversal losses is associated with the destruction of the surface layer close to the metallic state and due to an increase in the specific resistance of the material due to the violation of the interparticle contact of particles. At the same time, a violation of interparticle exchange as a result of processing in a vibrator determines a decrease in the magnetic permeability (Fig. 5). Figure 5 shows the curves of changes in magnetic induction as a function of the external magnetic field strength of the sample after annealing curve 1 and, respectively, after processing in a vibrator for 2 and 5 minutes curves 2 and 3. A decrease in magnetic permeability during processing in a vibrator leads to composite material becomes uncompetitive with laminated electrical steels.
Figure 5: Magnetization curves - composite based on ASC100.29 with an insulating coating before machining -1 and also after -2.3.
A similar effect of a decrease in magnetic permeability is observed with an increase in the thickness of the insulating coating (Figure 6,7).
Figure 6: Magnetization reversal losses at a frequency of 1 kHz for a composite material based on iron powder ASC100.29 d <0.1 mm and insulation thickness 1 nm - 1, ASC100.29 d <0.05 mm with insulation thickness 10 nm - 2, ASC100.29 d <0.05mm with an insulation thickness of 20 nm - 3, ASC100.29 d <0.05mm with an insulation thickness of 30 nm - 4.
Figure 7: The magnetization curves of a composite material based on iron powder ASC100.29 with a particle size d> 0.1 mm (insulation layer 1 nm) - 1, ASC100.29 d <0.1 mm (insulation 1 nm) - 2, ASC100.29 d <0.05 mm (insulation 10 nm) - 3, ASC100.29 d <0.05mm (insulation 20 nm) - 4, ASC100.29 d <0.05mm (insulation 30 nm) - 5.
An important and one of the determining criteria in choosing an insulating coating is the adhesion of the coating to iron powder, which ultimately determines the mechanical properties of the composite material and its strength. In order to study the mechanical properties, samples were made and subjected to strength tests (Figure 8).
Figure 8: Samples for testing the mechanical properties of composite materials.
The results of mechanical properties tests are summarized in table3, which gives data on the tensile strength in comparison with steels. It can be seen from the table that the maximum tensile strength is characteristic of a composite material with titanium oxide insulation. The minimum value is typical for a material with boron nitride insulation. In the latter case, the adhesion of the insulation coating to iron particles is minimal.
Table 3: Tensile strength σv for a composite material with various insulating coatings, pressing pressure 7.5 tons/cm2 annealing 400? / 1 hour.
σ? , MPa
The use of composite materials for the construction of electric motors with axial magnetic flux
First of all, the use of composite magnetic materials is of interest for the production of a new generation of electric motors with an axial direction of magnetic flux propagation. The design of such a two-stator electric motor is shown in Figure 9. A prototype electric motor (Figure 10) with a capacity of about 90 kW is currently undergoing debugging and testing.
Figure 9: Axial-directional twin-engine design.
Figure 10: A prototype electric motor with an axial magnetic flux of about 90 kW.
Heaven Storm USA calculated the electric motor with the axial direction of magnetic flux based on our data on composite magnetic material (Figure 11).
Figure 11: Heaven Storm Axial Flux Motor Data.
It follows from the calculated data that the specific power of an electric motor with an axial direction of magnetic flux is 18.3 kW / kg, whereas for the best engines with a radial direction of magnetic flux this value is 4.88 kW / kg. Hoganas AB also intensively conducts research on the development of an engine with an axial direction of magnetic flux.
Discussion of the results of the study
Unlike magnetodielectrics, where each metal particle is completely isolated and conductivity between the particles is excluded, for a composite soft magnetic material adjacent metal particles are connected by conduction channels with the formation of a common conduction band. The isolation of metal particles in this case is local, allowing mutual exchange of conduction electrons. As a result of the mutual flow of electrons, a conduction band is formed in which the population density of the Fermi surface is determined by the degree of overlap of the metal particles. The degree of overlap of particles depending on the thickness of the insulating coating can vary from a zero value characteristic of magnetodielectrics to a maximum value characteristic of a metal. In this case, the population of the Fermi level varies from the minimum for the magnetodielectric Ef = Ef1 to the maximum value for the metal Ef = Ef2 (Figure 12).
Figure 12: The position of the Fermi level in magnetodielectrics Ef = Ef1 and metals Ef = Ef2.
The theory of such composite materials has not been developed to date; a process of a set of experimental data is underway, which allows us to further develop theoretical foundations of such materials. However, it follows from general considerations that, based on the theory of direct exchange interaction, the electron density on the Fermi surface for both the metallic state and composite materials should be close. In this case, the magnetic properties of the metallic ferromagnet and the composite material should be identical. This condition can be fulfilled in the case of composite magnitic materials if the insulation of the grains is translucent and has the smallest possible thickness. As the present studies have shown, the calculated thickness of the insulating layer should be equal to or less than one nanometer. In this case, the chemical composition of the insulating coating has practically no special effect on the magnetic parameters of the composite material. As for the mechanical properties and strength of the composite material, in this case, the decisive role is played by the adhesion of the insulating layer to the metal. Studies have shown that maximum strength is achieved when using an insulating layer of based on titanium oxide, the minimum strength is typical for insulation based on hexagonal boron nitride.
Studies have shown that further progress in improving the magnetic properties of composite materials and, in the first place, achieving losses equal to or less than losses in metal ferromagnets is associated with an improvement in the properties of iron powder itself. As follows from the studies, the defectiveness of the grains of the iron powder, apparently determined by the content of carbon, forming iron carbides, which are centers of inhibition of domain walls. As a result, the high value of the coercive force of the order of Hc = 70-80 A / m determines the main hysteresis losses in the composite material. The main task of the next stage of the work is to reduce the coercive force of the composite material. To this end, a study was conducted to improve the properties of iron powder ASC100.29 by passing iron powder through the temperature zone 900C for 48 hours. As a result of zone cleaning of iron powder, carbides are destroyed with the formation of ferrites and free carbon. As shown by studies of the properties of a composite material on a powder after zone cleaning, the number of defects resulting from the decomposition of nitrides increases, and the coercive force increases after annealing.
1) It was found that the optimal thickness of the insulating layer for low-frequency composite materials should be less than one nanometer. Moreover, the magnetic properties of the composite material are practically independent of the chemical composition of the insulation coating. 2) Mechanical properties - the strength of the products depends on the adhesion of the insulation coating and is maximum for insulation based on titanium oxide σb = 510 MPa and minimum for insulation based on hexagonal boron nitride σb = 270 MPa. 3) For composite materials based on highly pure iron powders ASC100.29, ABC100.30, and Atomet 1001HP, the minimum magnetization reversal losses are characteristic of the material based on ABC100.30, which is associated with the minimum carbon content in these powders. 4) Further progress in improving the properties of low-frequency composite materials is associated with an improvement in the properties of directly iron powders - a decrease in carbon content compared to its content in ABC100.30.
LP Lefebvre, S Pelletier, C Gelinas (1997) Effect of electrical resistivity on core losses in soft magnetic iron powder materials. Journal of Magnetism and Magnetic Materials 176: pp. 93-96.
Bingyang Meng, Jiexin Hou et al. (2019) Low-loss and high-induction Fe-based soft magnetic composites coated with magnetic insulating layers. Journal of Magnetism and Magnetic Materials 492: 165651.
Jingxin Li, Jing Yu et al. (2018) The preparation and magnetic performance of the iron-based soft magnetic composites with the Fe@Fe3O4 powder of in situ surface oxidation. Journal of Magnetism and Magnetic Materials 454: pp. 103-109.
Govor GA, Larin AO (2007) The Magnetic Properties of a Magnetically Soft Composite Material for Use in the Low-Frequency Range. Inorganic Materials: Applied Research 10: pp.387-390.
Govor GA, Mihnevich VV (2007) Composite soft magnetic materials based on iron powders. Inorganic materials 43: pp.805-807.
Grande MA, Ferraris L, Francici F, Poskovic E (2018) New SMC Material for Small Electric Machine. IEEE Transaction on Industry Applications 54: pp.195-203.
Xinran Y, Yongjian L, Qingxin Y, Changgeng Z, Yang L, et al. (2019) Reseach of Harmonic Effects on Core Losses in SMC Materials. IEEE Transaction on Magnetic 55: pp.1-5.
Sustarsic B, Sirc A, Milyavec D (2004) SMC materials in the design of small electric motors for domestic application. Euro PM 2004, Proc. conf. PM Functional Materials 4: p.629.
Skorman B, Chzhou E, Jansson P, RF Patent 2389099, 2010.
Jansson P (1998) Advance in soft magnetic composites, Symp. on Soft Magnetic Materials 98, Barcelona, no. 7.
Govor GA, Mityk VI, Tamonov AV (2012) [A method of manufacturing a soft magnetic composite material]. Patent RF ?2465669. Publ. 27.10.2012.
Sychov VV (1980) Complex thermodynamic systems. Moscow, Nauka Publ., 208 p.