Wireless power transfer (WPT) technology has developed rapidly in recent years because of its unique advantages over traditional cable power supply methods [1,2,3,4,5]. It can significantly improve the reliability, convenience, and safety of the electric energy supply and solve the problems of sparks and maintenance difficulties caused by traditional plug-in power transmission modes [6,7,8]. WPT technology has broad application prospects in low-power scenarios such as mobile phones [9,10], wearable devices [11,12], implantable medical care [13,14,15], and smart home products [16,17], as well as high-power fields such as electric vehicles (EVs) [18,19,20,21,22,23], unmanned aerial vehicles (UAVs) [24,25], unmanned underwater vehicles (UUVs) [26,27,28], electric ships [29] and aerospace equipment [30,31]. There is no doubt that WPT technology eliminates the problem of insulation and wire wear due to contact friction and significantly improves the safety and reliability of charging systems.

Generally, the magnetic coupling resonant wireless power transfer (MCR-WPT) technology is considered the most promising WPT method for its major transmission power, high transmission efficiency, and long transmission distance [32,33,34]. The working principle of the MCR-WPT is to generate an alternating current from a high-frequency inverter power supply and pass it into the transmitting coil to generate an alternating high-frequency magnetic field in the space around the transmitting coil. The high-frequency magnetic field passes through the receiving coil and forms the high-frequency induction current. Then, the current through the secondary energy conversion links (rectifier and filter), provides stable electric energy to the load. MCR-WPT technology can be applied to power transmission of W ~ kW level and adapted to a wide range of applications. Since both the primary and secondary sides of the coupling coil use a tuning circuit to make it work in the resonance state of the same frequency, the energy exchange efficiency is very high. In addition, the electric vehicle wireless power transfer (EV-WPT) international standard SAE-J2954 [35] was proposed and adopted, which recommended using the magnetic coupling resonant method for wireless charging of EVs.

As an essential component of the MCR-WPT system for energy conversion and transmission, the magnetic coupler (MC) can be regarded as a non-contact loose coupling transformer composed of coil winding and magnetic core [36]. The coil winding is the key to realizing magnetic coupling, while the magnetic core made of soft magnetic materials is often overlooked. Generally, for a complete MC, including the primary and secondary sides, the coil winding realizes the construction of the space electromagnetic field based on Faraday’s electromagnetic induction law, and the magnetic material realizes the reshaping, restriction, and guidance of the space magnetic path. This is due to the remarkable permeability of magnetic materials compared with air medium, and most of the flux lines generated by coils will pass through the magnetic path with magnetic cores. Therefore, high-performance soft magnetic materials are recommended to be added to the magnetic coupler structure as a magnetic core, which significantly effects the improvement of coupling performance and electromagnetic shielding [37,38,39]. On the one hand, magnetic materials can effectively improve the quality factor and mutual inductance coupling coefficient, which plays a vital role in enhancing the system’s power level and transmission efficiency. On the other hand, magnetic materials can effectively decrease the electromagnetic leakage of magnetic couplers and reduce the electromagnetic radiation to electronic equipment of the system and surrounding environment, which is conducive to the realization of the electromagnetic compatibility (EMC) design.

However, from a negative perspective, WPT systems generally work in the higher frequency region of magnetic materials, bringing additional power loss. In addition, the use of magnetic cores increases the volume, weight, and cost of the system. WPT is a new field of magnetic materials with special needs and concerns different from the traditional application scenarios of magnetic materials (such as transformers and motors). As far as the research progress of magnetic materials for WPT is concerned, it is still in the stage of how to make good use of basic soft magnetic materials. Few magnetic material companies have proposed special magnetic materials specially designed and developed for WPT.

Nevertheless, with the deepening of the research on WPT technology, the contradiction between the requirements of high efficiency, high power density, low cost, lightweight, and the shortcomings of current WPT magnetic materials have gradually been exposed. The existing traditional magnetic materials will be challenging to adapt to the high-transmission performance requirements of the WPT system and may become a bottleneck restricting the further development of WPT technology. In brief, from the perspective of the current development heat of WPT technology in the field of consumer electronics and the promotion of EVs, the research on magnetic materials and magnetic structures of magnetic couplers in WPT systems will become a research hotspot in the future. It is also one of the critical ways to further break through the bottleneck problems mentioned above.

2. History, Theory and Applications of WPT

2.1. Brief History of WPT

The turning point of the first realization of electric energy transmission from wire to wireless can be traced back to the last century: Hertz provided conclusive evidence of the existence of electromagnetic waves, and Nicola Tesla successfully lit the phosphorescent lamp with his Tesla coil [40,41,42]. However, embodiments thereof involve an unnecessarily large electric field. Until 1964, the microwave radiation type wireless charging system was first proposed and applied to supplement energy for a special helicopter [43]. Although microwave radiation is very suitable for transmitting information, it will cause a lot of energy loss due to the divergent radiation space when transmitting power, and the power transmission efficiency is very low. Since then, electromagnetic induction radio energy transmission based on Faraday’s electromagnetic induction law has become a new research direction [44,45,46,47,48] and has continued into the 21st century. On this basis, the research institute has made many research achievements in lightweight, high-power capacity and density, and high misalignment tolerance WPT systems [49,50,51,52,53,54,55,56]. It is difficult not to highlight some of these landmark achievements. In 1976, the concept of dynamic radio energy transmission was first introduced [57], and LBNL evaluated its system feasibility. Dynamic wireless power transfer (DWPT) realizes the energy supply during the operation of EVs and the unlimited mileage endurance of EVs under ideal conditions. In 2007, Marin Soljačić [58] from Massachusetts Institute of Technology (MIT) proposed a mid-range MCR-WPT system, which acted as a leading role in academia. In 2009, Korea Advanced Institute of Science and Technology (KAIST) developed the first on-line electric vehicle (OLEV) prototype [59]. In the following years, KAIST rapidly commercialized and updated its OLEV products [60,61,62,63], which was named as one of the top 50 inventions in the world by Time magazine.

2.2. Classification, Principle, and Comparison of WPT

The WPT technology can be realized based on different physical principles. The development of WPT technology is advancing in two major directions, near-field and far-field transmission, based on the distance of power transfer. Then, according to the power accumulation medium and transfer technique, the WPT technology is mainly divided into three types: electromagnetic induction type, electrostatic induction type, and electromagnetic radiation type [64,65,66]. The classification of WPT is shown in Figure 1. Moreover, WPT technology can be further divided into five categories: magnetic coupling resonant WPT (MCR-WPT), electric coupling WPT (EC-WPT), microwave WPT (MW-WPT), ultrasonic wireless power transfer (US-WPT), and optical wireless power transfer (OWPT). The operating frequency range, power range and applicable transmission distance of several WPT types can be visualized and specifically obtained from Figure 2 and Table 1. ,73,74]. The OWPT system is more like an ultralong transmission wire. However, its efficiency is very low, and there are high requirements for directivity and unshielded laser beams.

2.2.3. EC-WPT

The EC-WPT is usually referred to as capacitive power transfer (CPT), which usually uses two pairs of metal plates form an equivalent capacitor to transmit power. The system composition and basic principle of EC-WPT are shown in Figure 5. Due to its unique operating principle, EC-WPT can be used in applications requiring power transfer through metal materials [75]. Unlike the MCR-WPT system, the EC-WPT system transfers energy by the electric field, so it is unnecessary to carry a large number of magnetic cores. Thus, the EC-WPT system has a simple structure with no magnetic hysteresis loss. Furthermore, the main advantage of EC is that almost no electric flux can escape beyond the dielectric material, thus eliminating the problem of electromagnetic field exposure [76]. Due to the limitation of high resonant voltage, EC-WPT is usually designed as a system with high operating frequency and low power level [77,78]. With the continuous progress of technology, some kilowatt power level EC-WPT systems have been proposed, which can realize the wireless transfer of electric energy over a long distance [79,80,81]. In addition, the coupling capacitors are diversified in design for different applications. However, to improve the power level, it is necessary to increase the operating frequency of the system to more than MHz. The existing semiconductor technology limits the ultra-high switching frequency and high-power performance. Furthermore, continuous operation in a high-frequency range easily causes high-voltage stress on electronic devices in the compensation network. Gallium nitride (GaN) switching technology and multiphase modular design will be important solutions to overcome the development bottleneck of high-power EC-WPT systems [82].

2.2.4. MCR-WPT

MCR-WPT is now the most widely accepted and applied WPT method. In the MC-WPT system, the receiving coil picks up the flux lines of the magnetic field generated by the high-frequency current in the transmitting coil and converts it into a DC current for charging. The components and fundamentals of MCR-WPT are shown in Figure 6. The magnetic coupler can be regarded as a loosely coupled transformer with a long distance between the primary and secondary windings [83]. The loosely coupled transformer commonly has a large airgap between the double side windings, which leads to a lower coupling coefficient and higher electromagnetic leakage. Through the utilization of the magnetic core, the quality factor of coupling coils can be optimized, and then the effective power transfer can be realized [84]. With the continuous development of MCR-WPT technology, the power level of the existing research has reached hundreds of kilowatts, and the transmission efficiency is very considerable. In addition, the multi-module parallel connection method can even realize megawatts of wireless power transfer [85]. The distance diameter ratio (DDR) is one of the key research directions of WPT systems. DDR is the ratio of the distance between the primary and secondary coils to the coil diameter. It is one of the parameters characterizing the transmission capacity of the magnetic coupler. In some long-distance MCR-WPT systems, the DDR can reach one or even higher. The misalignment tolerance of the magnetic coupler is another important characterization parameter. Through the optimization of magnetic coupler and compensation topology of MCR-WPT, the high misalignment tolerance characteristics can be realized. Such optimization can improve the flexibility and freedom of the MCR-WPT, so that the magnetic coupler can still achieve an efficient state when misalignment. The vertical and horizontal allowable misalignment tolerance can reach a level comparable to half of the coil diameter. The main advantages and potential shortcomings of the five types of WPT techniques are detailed in Table 2.

2.2.1. US-WPT and MW-WPT

The US-WPT system transmits energy through space ultrasound radiation, and its principle is shown in Figure 3. The power amplifier converts the DC signal into ultrasounds, and the transmitting antenna then emits the ultrasound beam. The receiving antenna is used to receive the ultrasounds and eventually rectify them into a DC power supply that can be used by the device [67,68,69]. The ultrasonic transmission was originally designed to transmit information, so its power capacity is often low, usually not exceeding 10 W. The transmission principle of MW-WPT is similar to that of US-WPT. Still, the frequency range of its band and the energy capacity are significantly different, while the MW-WPT can even realize the wireless transmission of MW-level energy. MW-WPT and US-WPT transmissions are radiant, so a large portion of the power is dissipated by radiation during transmission and cannot be captured by the receiving side, resulting in extremely low transmission efficiency [70]. With the improved antenna technology, directional antennas can already achieve directional MW-WPT [71]. However, directional MW-WPT is very sensitive to the transmission medium, does not allow any obstacles in its transmission path, and requires real-time tracking and positioning of microwaves, which is difficult to implement.

2.2.2. OWPT

Optical Wireless Power Transfer (OWPT) is one of the WPT technologies specifically researched for long-distance directional devices. The components and principles of the OWPT system are shown in Figure 4. The most significant advantage of OWPT is that it can realize long-distance and high-power directional energy transmission, and its transmission distance can even be greater than 1 km. At this stage, the leading service objects of OWPT include UAVs, satellites, and other remote power facilities. The system composition and fundamental principle of OWPT are shown in Figure 4. The electricity in the power grid is converted into light via laser diodes. Then, the laser beam is shaped by optical elements and directed to a remote photovoltaic receiver through the beam director. The particular photovoltaic receiver can match the laser wavelength and beam intensity and convert the laser back to electric energy to charge the battery load [72 In addition, companies such as Qualcomm Halo [97], WiTricity [98,99], Momentum Dynamics [100,101], Bombardier [102], ZTE New Energy, Zone Charge, and INVIS Power [103] are sparing no effort to promote the research of wireless power transfer technology and its application. The main electrical parameters of typical WPT products of the typical companies are shown in Table 4. _s of Mn-Zn soft ferrite can theoretically reach 600 mT [130]. However, the products prepared by the prior art are lower than 500 mT no matter at room temperature 25 °C or high temperature. For the 4H45 material of the FDK Company, the B_s is 450 mT at 100 °C, and the B_s of 4H47 material is about 470 mT. The power consumptions of the two materials at 100 °C are 450 mW/cm³ and 650 mW/cm³, respectively [126]. Therefore, to this day, improving the saturated magnetic flux density B_s of materials is still the research hotspot of Mn-Zn ferrite [131,132,133,134]. Compared with Mn-Zn ferrites, Ni-Zn ferrite materials are not very popular. Ni-Zn ferrites usually have low B_s and μ_i, but their resistivity is high, so it is more suitable for high-frequency applications. NiCuZn ferrite can be prepared by adding CuO to the main formula [135,136,137]. Because of its high resistivity, high-frequency characteristics, and low sintering temperature, it can be used to prepare laminated chip inductors [138]. The FDK Company proposed L47H [126] type Ni-Zn ferrite material for high-voltage power conversion, and the University of Electronic Science and Technology of China developed a similar HN120B type material [139]. As excellent high-frequency magnetic materials, MnZn ferrite and NiZn ferrite have been widely used in the field of wireless charging and mentioned in the two authoritative standards of Qi and SAE-J2954 [35,121]. Among them, the Qi standard proposed by the Wireless Power Consortium (WPC) is mainly for mobile consumer electronic devices such as mobile phones with the specified operating frequency range of 110~205 kHz. According to the Qi standard, application scenarios can be divided into fixed position type, single-coil free position type and multi-coil free position type. As the power level and current level of the products corresponding to the Qi standard are not high, the requirements for the temperature characteristics and saturated magnetic flux density of the required soft ferrite products are not strict. Table 7 shows the requirements for magnetic materials and corresponding recommended soft ferrite materials under the three application scenarios of the Qi standard [140,141,142,143,144]. 146]. The METGLAS series of Fe-based, Co-based, and FeNi-based amorphous alloy ribbons were produced based on the planar flow casting technology. Since then, soft magnetic amorphous alloys have entered the era of industrialization and commercialization. Among them, Fe-based amorphous alloys have occupied the mainstream of the amorphous alloy industry with their high magnetic properties, low cost, and good amorphous formation ability [147]. The METGLAS 2605SA1(1K101) series FeSiB Fe-based soft magnetic amorphous alloys have been used in large quantities for manufacturing transformer cores and are now gradually used for wireless charging fields. Nanocrystalline soft magnetic materials have been developed in three systems, Finemet-type, Nanoperm-type Fe-based nanocrystalline alloys, and Hitperm-type FeCo-based nanocrystalline alloys [148]. Their structures and magnetic properties are shown in Table 8. The anisotropy of the average magnetic crystal of nanocrystalline alloys is weak, so the magnetostriction coefficient can be reduced close to zero by adjusting the composition and process [149,150,151]. The permeability and saturation flux density of the alloy can be effectively increased because of the exchange coupling between the amorphous matrix and the nanocrystalline grains [152]. Yoshizawa [153] developed the first five-membered nanocrystalline alloy with a typical composition of Fe_73.5Si_13.5B₉Nb₃Cu₁ by adding small amounts of Cu and Nb to FeSiB alloy and registered it as FINEMET alloy. The saturation magnetic induction strength of this material is 1.1–1.5 T, which is slightly lower than that of commonly used amorphous alloys (1.56 T) and silicon steel. It had relatively small losses in the medium and high frequencies and a Curie temperature T_C of 570–600 °C. In the 1990s, Finemet-type nanocrystalline alloys produced by Hitachi Metals in Japan and Vacuum Metallurgical Corporation (VAC) in Germany had matured. Hitachi Metals formed nine FT-1 products, and VAC formed three Vitroperm products [154].

2.3. The State-of-the-Art of MCR-WPT

The structure of the MCR-WPT system is shown in Figure 7, including the inverter, the compensation network, the magnetic coupler, the rectifier, DC/DC module, and the load [86]. The significant difference between the magnetic coupling resonance type and the traditional inductive power transfer is that the MCR-WPT system adds a resonance compensation network to eliminate the reactive power component in the circuit, which means that the magnetic coupling resonant type can obtain higher active power transmission under the same degree of coil coupling. In other words, when in the case of a longer distance (weak coupling state), it still can have a higher active power output. In recent years, many institutes have conducted in-depth research on WPT technology, mainly focusing on system modeling and control, magnetic coupler and compensation topology design, anti-misalignment capability, electromagnetic leakage, and shielding. The representative institutions include Auckland University [87], Korea Advanced Institute of Science and Technology (KAIST) [63,88,89,90], Korea Railroad Research Institute [85], Oak Ridge National Laboratory (ORNL) [91], University of Michigan [92], Saitama University [93], Harbin Institute of Technology (HIT) [94,95] and Chongqing University [96]. Table 3 shows some typical research results and related technical parameters of universities and research institutions. At present, hundreds of kilowatts power level and more than 90% transmission efficiency have been achieved in the existing research reports. However, compared with the conventional wired charging method, there are still problems of lower power density. At this stage, high efficiency and high-power density are the important development directions of the MCR-WPT technology. With the continuous progress of power electronics technology, the new generation of SiC and GaN switching power electronics further reduces the switching losses of the inverters and rectifiers. Improving the Litz wire preparation process reduces the proximity effect and skin effect of the coil so that the copper loss of the magnetic coupler is further optimized. Therefore, improving the electromagnetic properties of magnetic materials has become one of the most effective solutions to break the bottleneck problem of increasing the power density of the WPT system. The development of soft magnetic materials with high permeability, high saturation magnetic induction, and low power consumption has gradually become the key to improving the efficiency of the WPT system.

3. Magnetic Materials and Their Applications in WPT

3.1. Brief History of Soft Magnetic Materials

In 1831, through an experiment, Michael Faraday found that when a part of the conductor of a closed circuit cut the magnetic induction line in the magnetic field, the current would be generated in the conductor. Faraday’s law of induction was proposed [104]. Iron is chosen as the magnetic core because of its highest saturation magnetization among all elements. In addition, it also has the characteristics of high permeability and low coercivity. Since then, soft magnetic materials have been developing continuously. Later, the researchers found that the annealing process of iron can improve its mechanical properties and reduce its coercivity through stress relief, making it more suitable for induction applications. In 1900, British metallurgist Robert Hudfield invented non-oriented silicon steel by adding 3% silicon to iron, which improved both the resistivity and saturation magnetization [105]. In 1933, American metallurgist Norman Goss invented grain-oriented silicon steel by promoting grain growth along the direction of low anisotropy crystallization, thus further improving the saturation magnetization. Even today, due to the high saturation magnetization and relatively low cost of silicon steel, it still occupies the main share of the global soft magnet market. The most common applications of silicon steel are large transformers (oriented silicon steel) and motors (isotropic non-oriented silicon steel). However, low resistivity (~0.5 mΩ·m) makes silicon steel lose more at high frequency [106]. Recently, electrical steel manufacturers have developed a method to increase the silicon content in steel to 6.5% by using a chemical vapor deposition process [107]. This method can increase the resistivity of silicon steel material to 82 μΩ·cm but still cannot meet the current high-efficiency requirements of high-frequency power electronic equipment and high-speed motors. In the 1910s, Gustav Elmen of Bell Laboratories carried out experiments on nickel-iron and discovered the nickel-rich (78%) permalloy composition [108]. A significant advantage of permalloy is its high relative permeability (up to 100,000). Nickel-iron is still used in some special induction applications today, but it is not common in power electronics and motors because of their high eddy current loss. Adding nickel can reduce soft magnetic materials’ saturated magnetic flux density. Moly permalloy powder (MPP) can be produced by adding a small amount of molybdenum (2%) to permalloy [109]. MPP is used to fabricate the powder cores with the lowest loss [110], and it is still the best choice for high-frequency inductor cores within the frequency range of 450 kHz. Then in the late 1940s, the soft magnetic ferrites were invented by J. L. Snoek [111]. These materials have high resistivity, which can effectively suppress eddy current loss. In addition, the preparation process of ferrite is often simple so that the ferrite core can be produced at a very low cost. Soft ferrite has been developed rapidly in recent years due to its high resistivity and economic performance and has been widely used in electromagnetic induction and high-frequency equipment. Today, the market share of ferrite in the world’s soft magnetic materials is only second to that of silicon steel sheets [112]. Manganese zinc ferrite is also the most commonly used soft magnetic material in the WPT system at present. However, the saturation flux density of ferrite is relatively low (almost a quarter of that of silicon steel sheets), which limits the energy density of sensing elements containing ferrite cores. Therefore, increasing the maximum saturation flux density of soft ferrite has always been the development direction of the ferrite process. In 1967, Duwez and Lin reported the first amorphous soft magnetic alloy in the form of small disc-shaped samples [113]. They used a rapid solidification technique called splat cooling for Fe-P-C systems. Then interest in Fe- and Co-based amorphous alloys surged by the mid of 1970s. Amorphous alloys obtained some applications because of their excellent coercivity and saturation magnetic density compared with ferrite. In 1988, Hitachi researchers added Nb and Cu additives and added an annealing step in the production of amorphous alloys to produce small and closely distributed the iron or cobalt-based nanocrystals (about 10 nm in diameter) in the matrix of amorphous materials, which marked the invention of nanocrystalline alloys [114]. Amorphous and nanocrystalline alloys have low power loss and competitive saturation flux density. Although the cost is higher than that of silicon steel, due to the low power loss, these advanced alloys can reduce the total lifetime cost of power electronics and motors. In the early 1990s, powder cores (also known as soft magnetic composites or SMCs) were proposed [115]. These materials combine magnetic particles, anywhere between ~1 to 500 mm in diameter, and either coat or mix them with an insulating material before consolidating with high pressures. Moreover, the heat process can also be applied either during or after densification to improve magnetic properties. Magnetic particles are usually iron powder but can also be composed of alloys. Powder magnetic core can be quickly processed into a more complex magnetic core shape, improving its applicability in special equipment and significantly reducing manufacturing costs. Their isotropy, low cost, and the ability to make complex mesh parts make SMC quite successful in rotating electrical machines [116,117]. Although the magnetic permeability of the powder core is usually low, its stability at high frequencies is impressive (such as the MPP mentioned earlier). SMC-based magnetic cores are attractive in high-frequency inductor design. The desired overall core permeability of SMC core can be achieved by adjusting the powder size, addition of insulation material and phosphoric acid, and pressure during the preparation process to reduce the air gap loss and ease the inductor design [118]. Figure 8 shows the brief history and development trend of soft magnetic materials [119,120]. From the perspective of power electronics applications, the saturated magnetic flux density, resistivity, and cost of soft magnetic materials are important concerns in their development. Ferrite material is a competitive core material at high frequency, so it has been in a leading position in the field of WPT technology. In recent years, with the application and popularization of advanced alloys such as amorphous and nanocrystalline alloys, researchers have gradually applied them to various WPT scenarios and obtained some applied results. In the following section, the application of soft magnetic materials and their magnetic structures in WPT technology will be discussed in detail.

3.2. Mn-Zn and Ni-Zn Soft Ferrites

Soft ferrite materials are the most widely used magnetic material in WPT systems at present and are also typical magnetic core materials recommended in SAE-J2954 and Qi standards [35,121,122]. They have remarkable performance in the field of consumer electronics and EVs wireless charging. Among them, Mn-Zn ferrites have high saturated magnetic flux density, permeability, and low resistivity compared with Ni-Zn ferrite. In addition, the performance of Mn-Zn ferrites is generally better than Ni-Zn below 2 MHz. The application of Mn-Zn ferrite materials accounts for about 80% of all ferrite materials. According to different application conditions and performance indicators, Mn-Zn ferrite materials can be divided into two categories. One is high permeability ferrite (generally greater than 15,000), which is usually used in low-frequency broadband transformers and inductance components of communication equipment. The other is high-frequency low-loss ferrite, which is often called power ferrite. Under high frequency and magnetic flux density, the loss of power ferrites does not change much with the increase of temperature in a particular range [123,124]. Power ferrite materials can be used for power conversion and transmission due to the high magnetic flux density B_s, high initial permeability μ_i, and low power loss P_c characteristics. Mn-Zn power ferrite materials were first used in household appliances, switching power supplies, and adaptive transformers. With the rise of WPT technology, Mn-Zn power ferrites have gradually become the most widely used core material. At the end of the 20th century, material companies developed a set of landmark Mn-Zn power ferrite, which solved the problem of large power consumption and the rapid decline of magnetic properties at 100~500 kHz high-frequency ranges. The maximum saturated magnetic flux density of this high-performance power ferrite is about 500 mT, and the loss at high temperature and frequency (100 kHz, 200 mT, 100 °C) is about 400~500 kW/m³. The most well-known products include PC40 from TDK [125], 6H20 from FDK [126], and N72 from SIEMENS [127]. In the past decade, low P_c ferrite materials have made remarkable progress. The PC45, PC46, and PC47 series Mn-Zn ferrites with low power loss near a temperature range are produced by TDK. The power consumption valley of PC45 is 60~80 °C, PC46 is 40~50 °C, and PC44 and PC47 are around 100 °C. Then, the PC90 and PC95 series Mn-Zn ferrite materials with wide temperatures and low power loss were developed. PC95′s P_c was lower than 350 mW/cm³ in the range of 25~120 °C, while PC90′s P_c was about 320 mW/cm³ at 100 °C. In addition, the B_s of PC90 are 450 mT and 540 mT at 100 °C and 25 °C, respectively. The PC90 and PC95 series Mn-Zn ferrites have wide temperatures and low power loss with excellent comprehensive performance [128]. The performance parameters of TDK PC series Mn-Zn ferrites are shown in Table 5. It is worth mentioning that the Netherlands’ Ferroxcube Company developed 3C series power ferrite materials [129]. 3C92 has high saturation magnetic induction, 3C95 has excellent temperature stability, and 3C94 has a low cost. 3C series Mn-Zn ferrite has comparable performance to TDK products, and the performance parameters of 3C ferrite materials are shown in Table 6. In fact, the saturated magnetic flux density B SAE-J2954 standard is an international standard for the wireless charging of electric vehicles proposed by the Society of Automotive Engineers International (SAE International). SAE-J2954 imposed design constraints on three power levels (WPT1-3.7 kW, WPT2-7 kW, and WPT3-11 kW) [35]. The power capacity of EVs is much larger than mobile phones, so the magnetic field intensity generated by the magnetic coupler is significant [20,21]. In order to avoid the magnetic saturation failure of the magnetic core, the soft ferrite must have a high saturation flux density. In addition, the EV-WPT system will produce an obvious temperature rise during continuous operation, so there is an increased requirement for the temperature characteristics of soft ferrite. The soft ferrite recommended in the SAE-J2954 standard are the N96 and PC95 materials of the TDK Company, which are the most widely used high-performance Mn-Zn ferrite materials in the WPT field at the moment.

3.3. Amorphous and Nanocrystalline Alloys

Amorphous and nanocrystalline soft magnetic materials mainly include Fe, Ni, Co, Fe-Ni, and Fe-Co-based materials. Amorphous materials are produced by ultra-rapid cooling solidification technology with a cooling rate of approximately 106 °C/s. The thin ribbons of 15–30 μm thick alloys are formed from the liquid state of the metal in a single process. The atoms of amorphous alloys do not crystallize in an orderly arrangement under the effect of rapid cooling. There are no grains and grain boundaries of crystalline alloys, so they show a long-range disorderly arrangement and exhibit isotropic characteristics. This amorphous state has excellent soft magnetic properties, including high permeability, low coercivity, low magnetic loss, and high saturation magnetic induction strength, while the material is strong and wear resistant. Since amorphous alloys are in a thermodynamically nonequilibrium sub-stable state, under appropriate heat treatment process conditions, amorphous alloys crystallize to obtain precipitated crystalline phases with grain sizes below 20 nm, leading to the preparation of nanocrystalline materials or nanocrystalline/amorphous composites [145]. With their excellent magnetic properties, amorphous and nanocrystalline alloys can replace silicon steel, permalloy, and ferrites. They are widely used in many electromagnetic fields such as distribution transformers, sensors, and electromagnetic shielding. With the rise of near-field communication and wireless charging, the application of amorphous and nanocrystalline materials in the field of electromagnetic shielding and WPT is gradually gaining attention. In the 1970s, researchers developed Fe-Ni-P-B, Fe-Ni-P-B-M, Fe-B, Fe-B-C, Fe-Si-B, Fe-Si-B-M series Fe-based amorphous alloys and Co-based soft magnetic amorphous alloys based on the alloy melt-spin quenching technology [ Due to their attractive properties [155], amorphous and nanocrystalline soft magnetic materials have become a hot research topic since their invention [156]. The maximum B_s and T_c characteristics of the material can be significantly enhanced by adjusting the typical element content of amorphous and nanocrystalline materials [157,158,159,160,161,162,163,164,165,166]. Suzuki [167] developed a FeZrB amorphous nanocrystalline duplex alloy system with high Fe content and registered it as the NANOPERM alloy. It is a FeMB-based alloy, where M is Zr, Nb, or Hf in the three-element alloy, and M is further increased by Cu, P, V, and Co in the four or five-element alloys. The B_s of the alloy increases to 1.5–1.7 T, which is higher than that of the FINEMET alloy and almost equal to that of the iron-based amorphous alloy, and its T_c reaches 770 °C. However, it has a high temperature during melting and rapid cooling for ribbon production. Thus, the easy oxidation of Zr, Nb, and Hf elements in the alloy and the requirement for preparation and production under vacuum or gas protection make it difficult for low-cost industrial applications [168]. Willard [169] added Co to create a new variety of Fe-Co-based nanoalloys. The typical composition is a Hitperm-type five-element alloy with Fe₄₄Co₄₄Zr₂B₄C₁, which precipitates α-FeCo nanocrystalline phase with a grain size of 10–15 nm. However, the cost of this alloy is high due to a large amount of use of the Cobalt. Then, Ogawa [170] invented HB1 iron-based amorphous alloy with B_s reaching about 1.64 T. Makino [171] studied Fe-Si-B-P-Cu nanocrystalline, and its B_s can reach about 1.9 T without the addition of Cobalt precious metal, which dramatically reduces the cost. The earliest typical application of amorphous and nanocrystalline materials is in motor stators [172,173,174,175,176,177]. Hitachi used laminated amorphous cores in an 11 kW motor prototype with a system efficiency of IE5 efficiency class [178]. In addition, high-efficiency nanocrystalline motor stator cores have been proposed and validated to effectively improve the iron loss of permanent magnet synchronous motors [179,180]. Another typical application of amorphous and nanocrystalline materials is the cores of high-frequency transformers [181,182]. Amorphous iron core distribution transformers have been widely adopted in Asia to reduce grid losses. By the end of 2010, the total capacity of these transformers reached 70 million kVA in China and 35 million kVA in India [183]. Nanocrystalline materials have gained the attention of scholars in the field of WPT in recent years. They have started to apply the matured Fe-based nanocrystalline materials to various wireless charging products, including but not limited to cell phones and electric vehicles [184]. The Samsung Galaxy S6 mobile phone uses ferrite and amorphous soft magnetic sheet as the magnetic core to reduce the phone’s weight and improve its wireless charging efficiency. Starting from the Galaxy S7 series phones, Samsung replaced the previous core material with nanocrystalline. The nanocrystalline core can be compatible with both the NFC function and wireless charging function of cell phones and has achieved good application results, which marked the gradual transition from ferrite to nanocrystalline as the soft magnetic material for mobile phone WPT. In addition to Samsung, more and more brands such as Apple and Huawei have adopted similar nanocrystalline soft magnetic sheet solutions as the magnetic cores for the wireless charging systems of electronic products. 在EV-WPT领域，剑桥大学的Long [185，186，187，188]提议使用日立FT-3M型铁基纳米晶作为SAE-J2954标准中规定的三个EV-WPT功率水平的接收器侧磁芯。其中，在11kW WPT3系统中，与使用TDK的N87 Mn-Zn铁氧体相比，系统效率提高了2%，耦合系数提高了13%。在WPT1和WPT2系统中，纳米晶磁芯的效果略低于铁氧体，但可以接受。实验结果表明，在大功率EV-WPT应用场景中，铁基纳米晶材料发挥了高饱和磁密度和低磁滞损耗的优势，能够有效减小车端尺寸和重量，具有很大的应用前景。Xiong[189]还提出在WPT2和WPT3 EV-WPT系统中使用Fe基纳米晶作为磁芯。设计铁芯厚度为2 mm的磁耦合器的实验效率达到97.4%。最高核心温度约为80.9 °C，漏磁完全符合国际非电离辐射防护委员会提出的ICNIRP 2010指南[190]。3M系统研究所开发了基于Fe基纳米晶芯的7.7 kW EV无线充电实验平台[191]。他们的实验结果表明，使用Fe基纳米晶磁芯可以实现传统铁氧体平台65%的重量，84%的垂直空间体积优化和16%的电磁泄漏。纳米晶优异的一致性和柔韧性使系统运行更加稳定可靠，不易因运输而损坏。Jiang [192] 比较了由不同软磁材料的环形磁芯制成的共模电感器的电磁特性，这些共模电感器可用作 EV-WPT 系统的典型 LCC-LCC 或 LCC-S 拓扑中的补偿电感。实验分析了Kool Mμ、X通量、高通量、MPP和Fe基纳米晶磁芯共模电感器的磁滞损耗、热分布和频率特性。结果表明，纳米晶和MPP电感的电磁温度特性相当，纳米晶材料在工作频率高于450 kHz时的磁热分布和电磁性能方面更具优势。 此外，柔性纳米晶带允许磁芯产生良好的变形力，使其适用于某些具有特殊外壳的WPT器件中的应用。来自HIT的Bie [193]提出了一种防错位无人机无线充电平台，该平台在无人机支架的接收器侧缠绕了一个螺线管线圈。该样机使用纳米晶带作为磁芯，并连接到支架上，实现了保形设计和良好的传输效率。来自HIT的Wang[194，195]和Cai [196]提出了将Fe基纳米晶带应用于UUV WPT系统，与铁氧体磁芯原型相比，磁耦合器的尺寸和重量优化了约40%。此外，该系统被设计为轻巧和保形。[194]中所示的UUV WPT磁耦合器及其Fe基纳米晶磁芯如图9所示。相信在早期的将来，纳米晶磁芯将越来越多地应用于航空航天、水下和其他具有特定领域的无线充电设备。同时，全球纳米晶材料的年产量已超过100万公斤，并且还在继续增加[156]。研究人员还在不断努力提高非晶态和纳米晶合金的性能，特别是寻求增加最大饱和磁通密度[197]，以适应更多高功率、集成的WPT应用。