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Talk about the application and prospect of nanocrystalline soft magnetic alloy materials in wireless charging

更新时间:2020-05-12 18:03:00点击次数:3725次
Reprinted / Amorphous China Industry Information From / Amorphous China

Nanocrystalline soft magnetic alloy material characteristics and preparation technology

What is a nanocrystal?
The first thing to know is what is amorphous. During the preparation of the metal, if it is cooled with an ultra-fast cooling rate during its solidification process, the atoms are in a disorderly state at this time, and they will be instantly frozen before they can be rearranged. The structure formed at this time is amorphous. Nanocrystals are based on the amorphous state, through special heat treatment, one lets it form crystal nuclei which grow, but the size of the crystal grains is controlled at nanometer level. They do not form a complete crystal, so the structure formed at this time is nanocrystalline.

How is amorphous prepared?
The principle of the amorphous preparation process is very simple, that is, after the mother alloy melts, it is sprayed on a high-speed rotating cooling roller through a nozzle package, and instantaneously cools to form a thin ribbon like a paper strip. It has several characteristics:
High temperature: the temperature of the liquid alloy is basically 1400 ℃ ~ 1500 ℃, instantaneous solidification to near room temperature requires a very high cooling rate, the cooling rate has reached the level of millions of degrees per second.
High speed: even the speed of the spraying belt is very fast, 30m/s, with high precision, the thickness of the ejected strip is 20-30 μm, very thin, such precision control is achieved by the design of the nozzle slit and the gap between the roller and the nozzle.

Preparation method of nanocrystalline alloy
Nanocrystalline soft magnetic alloy is realized by a special heat treatment process for amorphous strip. First, the amorphous strip with a specific composition is put into a heat treatment furnace to generate grains within 100 nanometers through orientation control. In fact, a mixed structure of amorphous and nanocrystals is formed.

Advantages of nanocrystalline alloys
Compared with cobalt-based amorphous alloys and ferrites, nanocrystalline alloys have high saturation magnetic inductance and so can reduce the volume of magnetic devices. High magnetic permeability, small loss, and small coercive force can reduce the energy loss of magnetic devices. Therefore, nanocrystalline alloys are the best soft magnetic materials in high-frequency power electronics applications.

Nanocrystalline alloy characteristics
The current wireless charging Qi standard frequency is between 100-200k. At this frequency, the magnetic permeability of the nanocrystal is very close to that of the cobalt-based amorphous, which is significantly higher than that of the iron-based amorphous and ferrite. The loss is just the opposite, significantly lower than iron-based amorphous and ferrite.
Nanocrystals also have advantages in temperature applications. Not only are nanocrystals wider in application temperature than cobalt-based amorphous and ferrite, but in the range of -40℃-120℃, the stability of nanocrystals is also significantly better than ferrite body.
Nanocrystals also have obvious advantages in the design of magnetic materials. Nanocrystals can be oriented to control permeability and anti-saturation magnetic fields. The magnetic permeability of nanocrystals can be adjusted freely within 1000-30000. The design of the magnetic material requires that the magnetic saturation should not be reached under a specific working current. Once the magnetic saturation is reached, it will stop working. The nanocrystalline adjustable anti-saturation magnetic field can reach 30~350 A/m, making the application range of wireless charging wider.
Comparison between several iron-based nanocrystals and iron-based amorphous, cobalt-based amorphous, and ferrite: saturation magnetic flux density: iron-based nanocrystals are significantly better than cobalt-based except that they are slightly lower than iron-based amorphous and ferrite;
In terms of coercive force, initial permeability, saturation magnetostriction coefficient, Curie temperature, performance change rate, etc., nanocrystals are better than other materials. Therefore, nanocrystals are the best soft magnetic materials.

Nanocrystalline development trends
With the development of electronic products in the direction of high frequency, energy saving, small size, and integration, the application frequency is also constantly increasing, and the strips are updated from generation to generation. From the original traditional tape manufacturing process (current domestic production level) thickness of 22-30μm, to the current development of the third and fourth generation of strip, with advanced tape manufacturing process (international advanced production level) can achieve 14-22μm. And so mastered the thinner strip making technology. The development trend of nanocrystalline strips is towards ultra-thin strips.

Ultra-thin nanocrystalline ribbon characteristics: the thinner the strip, the lower the loss.
The mass production process of magnetic permeable sheets has been changed. Since the mass production of magnetic permeable sheets in 2015, the process has been continuously changed, and the transition from sheet to coil has been gradually improved, which greatly improves production efficiency and meets the growing demand.

Application of Nanocrystalline Soft Magnetic Alloy in Wireless Charging
Wireless charging has become popular in mobile phones, and there are many products in the wearable field. In the future, wireless charging will be popularized in homes, offices, public places, travel tools, and transportation, and electric vehicles will also be popularized in the future.
Wireless power transmission (WPT): The structure of wireless charging for smart phones and smart wearables (small power) is similar to a transformer. It consists of a transmitter and a receiver. The transmitter and the receiver are both made of coils and magnetic materials. The magnetic materials are different. The choices are ferrite, amorphous, nanocrystalline, etc.
 The role of soft magnetic shielding materials in wireless charging
Magnetic shielding: Provides a low-impedance path for magnetic flux, reduces the magnetic field lines radiating outward, reduces the impact on surrounding metal objects, and prevents eddy currents and signal interference.
Permeability reduction: improve the coupling coefficient, improve the magnetoelectric conversion efficiency, use fewer turns to achieve a higher inductive coil, reduce the coil resistance, and reduce the efficiency reduction caused by heating (the more turns, the higher the resistance) .
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