1. Preparation of ferrite powder
At present, in large-scale production, the preparation of ferrite powder is still mainly based on the oxide method. The main points of the process are as follows:
(1) Raw materials
The raw materials require high purity, and chemical reagents are generally used. Impurities often form non-magnetic substances with iron, which seriously reduces the magnetic permeability of ferrite. Alkali metal ions such as K+ and Na+ not only affect magnetic permeability, but also increase high-frequency loss due to higher electrical conductivity, which is the most harmful. SiO2 is a ubiquitous impurity. It combines with Fe2O3 to synthesize iron silicate, and at the same time releases oxygen, which increases the porosity of the product and reduces the permeability.
(2) Pre-sintering and sintering
In order to reduce the firing shrinkage rate of the product, make the reaction complete, and improve the quality factor and magnetic permeability of the ferrite. The ball-milled mixture is often pre-fired. One method is low-temperature burn-in at around 500°C; the other method is high-temperature burn-in around 1000°C. Sometimes it is even necessary to use two pre-burning, that is, after low-temperature pre-burning, after grinding, and then high-temperature pre-burning.
Sintering is one of the keys in the ferrite production process, and the sintering temperature is generally controlled at 1000~1400°C. Since iron ions in ferrite are usually trivalent, in order to avoid reduction of Fe3+, an oxidizing atmosphere must be ensured. The firing equipment mostly uses silicon-carbon rod electric kilns. Generally, soft magnetic materials with high magnetic permeability must be slowly cooled to eliminate their internal stress. The moment magnetic material as a memory element must be cooled quickly to meet certain magnetic requirements.
Also worth mentioning is the production method of anisotropic ferrite ceramics. The anisotropic ferrite grains produced by this method are arranged in a certain direction, and its magnetic energy product (BH)max is 3 to 4 times larger than that of general anisotropic hard ferrite. There are two production methods: one It is molded under magnetic field conditions, dried and fired. The second is to use raw materials with larger particles in a flat or strip shape. Because they are easy to align in the molding process, hot pressing sintering is used after molding. For example: α-Fe2O3 is a hexagonal sheet, γ-MnOOH is a thin strip, and ZnO is a flat shape. They can be used as raw materials for the production of spinel manganese-zinc ferrite heads.
2. Preparation of ferrite film
(1) Preparation of ferrite single crystal film
In the early 1970s, it was discovered that garnet ferrite single crystal films can produce cylindrical magnetic domains (magnetic bubbles). Because magnetic bubbles are very promising as high-density, low-power, and high-reliability memories, the research and preparation of single crystal thin films have led to the development.
The thickness of the film is usually ≤1μm. Such a thin film must be attached to certain substrates (substrates are also called substrates) before they can be used in practical applications. The substrate material is a non-magnetic material. In order to make the film firmly adhere to the substrate without breaking, in addition to the delicate process, it is very important to select the appropriate substrate material. In principle, the requirement for the substrate is that the thermal expansion coefficient of the film and the substrate are similar.
The techniques for growing ferrite single crystal thin films mainly include sputtering method, chemical vapor deposition method (CVD), and liquid phase epitaxy (Liquid Phase Epitaxial Growth Method). At present, the liquid phase epitaxy method is used more frequently, with better quality and simple equipment. The basic method of liquid phase epitaxy is to melt the ferrite component to be epitaxial on the substrate at a high temperature in a flux, and then reduce the temperature to make the solution supersaturated. At this time, the substrate is immersed, and then the temperature is rapidly reduced , To promote the epitaxial growth of the film on the substrate. Due to the different immersion methods, it can be divided into two types: pouring method and dipping method. At present, the impregnation method is dominant.
(2) Preparation of ferrite polycrystalline film
Ferrite polycrystalline film is an inevitable requirement for the development of semiconductor integrated circuits. Up to now, there have been arc plasma spraying methods, radio frequency sputtering methods, gas phase methods, spray thermal decomposition methods, coating methods or chemical adhesion post-sintering methods, and metal vacuum evaporation high-temperature oxidation methods. From the current point of view, the arc plasma spraying method and the radio frequency sputtering method can produce uniform and dense films, which are suitable for mass production.
The principle of the arc plasma spraying method is to use the arc discharge between the electrodes to generate a current of hundreds of amperes to form a plasma flame, the temperature of which can be as high as 2000 ℃, and the ferrite powder (particle size ≤ 40 μm) conveyed by the carrier gas is melted , Sprayed on the ceramic substrate to obtain a thin film.