Application and Prospect Of Nanocrystalline Soft Magnetic Alloy Materials In Wireless Charging

What is nanocrystalline?

The first thing to know is what is amorphous. In the process of metal preparation, if the metal is cooled with an ultra fast cooling rate during its solidification, at this time, the atoms will be frozen in a disordered state before rearrangement, and the structure formed at this time is amorphous. On the basis of amorphous state, nanocrystalline is made to form crystal nucleus and grow up through special heat treatment. However, it is necessary to control the grain size at the nanometer level and not to form complete crystal. At this time, the structure formed is nanocrystalline.

How is the amorphous prepared?

The principle of amorphous preparation is very simple, that is to melt the master alloy, spray it on a high-speed rotating cooling roller through the nozzle package, and instantly cool it to form a thin strip like paper. It has several features:

At high temperature, the temperature of liquid alloy is about 1400 ℃ ~ 1500 ℃, and it needs a very high cooling rate, which reaches the level of millions of degrees per second.

High speed, that is, the speed of the spray strip is also very fast, 30m / s, high precision, the thickness of the spray strip is 20-30 μ m, very thin, such precision control is achieved through the design of the slit under the nozzle package and the distance between the roller nozzles.

nanocrystalline material

Preparation of nanocrystalline alloy

Nanocrystalline soft magnetic alloy is made of amorphous strip by special heat treatment process. Firstly, the amorphous strip with specific composition is put into the heat treatment furnace to generate grains within 100 nm by directional control. In fact, the mixture structure of amorphous and nanocrystalline is formed.

Advantages of nanocrystalline alloy

Compared with cobalt based amorphous and ferrite, nanocrystalline has higher saturation magnetic induction and can reduce the volume of magnetic devices. Nanocrystalline alloy is the best soft magnetic material in the application of high frequency power electronics because of its high permeability, low loss and low coercive force.

Properties of nanocrystalline alloy

At present, the frequency of wireless charging Qi standard is between 100-200k. At this frequency, the permeability of nanocrystalline is very close to that of cobalt based amorphous, which is significantly higher than that of iron-based amorphous and ferrite, while the loss is just the opposite, which is significantly lower than that of iron-based amorphous and ferrite.

Nanocrystalline also have advantages in temperature application. Nanocrystals not only have wider application temperature than cobalt based amorphous and ferrite, but also have better stability than ferrite in the range of – 40 ℃ – 120 ℃.

Nanocrystalline also have obvious advantages in the design of magnetic materials. Nanocrystalline can directionally control the permeability and anti saturation magnetic field. The permeability of nanocrystalline can be adjusted in the range of 1000-30000. The design of magnetic materials requires that the magnetic saturation should not be reached under the specific working current. Once the magnetic saturation is reached, the work will be stopped. The adjustable anti saturation magnetic field of nanocrystalline can reach 30 ~ 350A / m, which makes the application range of wireless charging wider.

Comparison among several kinds of Fe based nanocrystalline, Fe based amorphous, Co based amorphous and Ferrite: saturation flux density: Fe based nanocrystalline are obviously superior to Co based amorphous and ferrite except that they are slightly lower than Fe based amorphous.

Nanocrystalline is superior to other materials in coercive force, initial permeability, saturation magnetostriction coefficient, Curie temperature, performance change rate, etc. Therefore, nanocrystalline is the best soft magnetic material.

The development trend of nanocrystalline

With the development of electronic products in the direction of high frequency, energy saving, miniaturization and integration, the application frequency is also constantly increasing, and the strip is updated from generation to generation. From the original traditional belt making process (the existing production level in China) with a thickness of 22-30 μ m to the current strip development to the third and fourth generation, the advanced belt making process (the international advanced production level) can achieve 14-22 μ M. Moreover, we have mastered the thinner belt making technology. The development trend of nanocrystalline strip is ultrathin strip.

Characteristics of ultra-thin nanocrystalline strip: the thinner the strip, the lower the loss.

Since the mass production of the magnetic disc in 2015, the process has been constantly changing, from sheet to coil, greatly improving the production efficiency and meeting 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 field of wearing. In the future, wireless charging will be popular in homes, offices, public places, travel tools and transportation, and electric vehicles will be popular in the future.

Wireless energy transfer (WPT): the structure of wireless charging for smart phones and smart wear (small power) is similar to that of transformers, which is composed of transmitting end and receiving end. Both transmitting end and receiving end are composed of coils and magnetic materials. Magnetic materials have different choices, including ferrite, amorphous, nanocrystalline, etc.

The function of soft magnetic shielding material in wireless charging

Magnetic isolation and shielding: provide a low impedance path for magnetic flux, reduce the magnetic field lines radiated out, reduce the impact on surrounding metal objects, and prevent eddy current and signal interference.

Conduct magnetism and reduce resistance: improve coupling coefficient, improve magnetoelectric conversion efficiency, use less turns to realize higher inductance coil, reduce coil resistance, reduce efficiency reduction caused by heating (the more turns, the higher resistance).

Types of commonly used soft magnetic shielding materials

Comparison of charging efficiency of nanocrystalline magnetic sheet:

Simulation of the real scene, under the same conditions for comparative testing, using different thickness of nanocrystalline magnetic guide and different permeability, different thickness of ferrite charge efficiency comparison. With the increase of the thickness, the charging efficiency is constantly improving, but the thicker the nanocrystalline is, the better it is. When it reaches 0.1mm, it is basically saturated. Therefore, when designing the wireless charging module, the nanocrystalline magnetic vane does not need to be too thick, which will increase the material cost. The law of ferrite is similar to that of nanocrystalline. The higher the permeability, the higher the charging efficiency, the thicker the thickness, and the higher the charging efficiency. But under the same charging efficiency, the thickness of nanocrystalline magnetic sheet is only half of that of ferrite.

The development of wireless charging for smart phones

In 2012, Nokia launched lumia 920, a wireless charging mobile phone, which uses hard ferrite as its magnetic material. In 2013, hike 868, a mobile phone sold overseas, designed the integration of wireless charging and NFC, equipped with WPC ferrite (hard) and NFC ferrite (soft) magnetic materials. In 2015, the wireless charging of mobile phones took a milestone change. Samsung launched the first wireless charging flagship mobile phone Galaxy S6, which is not only compatible with two wireless charging standards, WPC and PMA, but also equipped with two payment standards, NFC and MST. Besides ferrite, the matching soft magnetic shielding material uses amorphous magnetic plates for the first time, which makes the mobile phone not only light and delicate, but also greatly improves Wireless charging efficiency. By 2016, Samsung has made improvements, replacing all magnetic materials with more advanced nanocrystalline magnetic guides, leading the innovation of wireless charging technology, and always in the leading position. From the development process of these years, we can see that the function of NFC and MST near-field communication is added to the pure wireless charging. The magnetic materials gradually transition from ferrite to nanocrystalline.

Application case

The application of nanocrystalline in wireless charging starts from S7. A material realizes all functions, replacing the combination of amorphous and ferrite. It is generally believed that ferrite is the best soft magnetic material for NFC, while nanocrystalline is not suitable, because at high frequency, the loss of nanocrystalline is far greater than that of ferrite, but Samsung just made a breakthrough. The successful application of S7 proves that nanocrystalline can be used for NFC. Subsequently, many models of products such as S8 / N8 / A7 / J5 / J7 expand the application of nanocrystalline from WPC to NFC and MST.

At the transmitter, Antai technology, a representative enterprise of China nanocrystalline production, has also made some attempts. It has made several wireless charger products with nanocrystalline magnetic sheet, which have multi station, multi-function and other characteristics. There is no problem in performance. Now the only problem is that the cost of magnetic sheet is higher than that of ferrite.

Mobile wireless charging development trend:

Functions: WPC → WPC + NFC → WPC / airfule + NFC

Wireless charging – Wireless charging + – random charging

Power: 5W → 7.5W → 10W → 15W

Slow charging – general charging – fast charging – Flash charging

Development trend of magnetic guide:

Receiving end: absorbing material → ferrite → amorphous + ferrite → nanocrystalline

Nanocrystalline magnetic sheet:

Thin ultra thin: 0.14 → 0.12 → 0.11 → 0.10

High magnetic conductivity, low loss – high Q

Application and Popularization:

N small power: mobile phone, smart wear, etc.

N medium power: computers, kitchen appliances, etc.

High power: electric vehicles, roads and other infrastructure

In the future, it will be a wireless world, changing life, changing the world.