Industry News

Atmosphere balance of ferrite magnets

2018-07-21
Especially at high temperatures, the atmosphere around the ferrite magnet blank is different, and the metal ions will change in price and composition will change, causing electrical, magnetic, and mechanical properties. Therefore, it is necessary to discuss the influence of the surrounding atmosphere on the manufacture of ferrite magnets. This section first discusses the theory of the atmosphere balance of ferrite magnets, and then analyzes the oxidation, reduction and volatilization of ferrite magnets and their control methods through some specific ferrite magnets.
Balance atmosphere of ferrite magnet
The physicochemical problems of ferrite magnets have been an extremely important issue in the manufacture of ferrite magnets. The various properties of ferrite magnets are related to the manufacturing process, especially the sintering process. For example, a nickel-zinc ferrite magnet, when sintered at a high temperature, such as a lack of oxygen in the surrounding atmosphere, produces a material having a low electrical resistivity and a large loss due to volatilization of certain components. During the sintering process of the barium ferrite magnet, if oxygen is deficient, the barium ferrite magnet will be reduced, causing its magnetic and mechanical properties to deteriorate, even if it is broken. In another example, if the MnZn ferrite magnet is oxidized during cooling, decomposition occurs and the performance is degraded.
In order to prevent oxidation, reduction and volatilization of the ferrite magnet blank during sintering, it is necessary to understand the chemical equilibrium at high temperature and at what speed to return from equilibrium to equilibrium, that is, how to control the equilibrium atmosphere. .
(1) The concept of a balanced atmosphere
In order to establish the concept of equilibrium air pressure, the balance between water and water vapor is first explained.
As shown, in a closed container with a piston, a small vessel enclosing water is placed. After the air is removed (the position of the piston is fixed), the lid of the water container is broken, and the water in the vessel evaporates, so that the pressure of the water vapor in the sealed container increases. After a certain period of time, the gas pressure of the water vapor in the container reaches a certain fixed value, and it is necessary to leave a little bit of water at any time. If the temperature of the system is known, the water vapor pressure in the vessel can be determined, for example, the steam pressure of water at 20 ° C is 2.32 kPa. Thus, the pressure of the steam of the water in the temperature state in which the water is retained is referred to as the pressure of the equilibrium steam.
In the case of maintaining a balanced state, the piston of the lifting container, as the volume of the container is increased, the pressure of the water vapor in the container is lowered. As a result, a part of the water in the vessel evaporates, so that the pressure of the water vapor in the vessel increases again. When the temperature reaches a certain level, the pressure of the water vapor in the vessel returns to the original equilibrium vapor pressure. If the piston is pressed down, the pressure of the water vapor in the container increases as the volume of the container becomes smaller. When the gas pressure of the water vapor in the vessel is greater than the equilibrium vapor pressure at that temperature, a portion of the water vapor condenses into water, so that the gas pressure of the water vapor in the vessel returns to the equilibrium vapor pressure.
In summary, when the gas pressure of the water vapor in the vessel deviates from the equilibrium vapor pressure at that temperature, evaporation of water or condensation of water vapor occurs. Under certain conditions, water evaporation or water vapor condensation may be as long as the gas pressure of the water vapor in the system is compared with the pressure of the equilibrium steam at that temperature. When the pressure of the water vapor in the system is lower than the pressure of the equilibrium steam at the temperature, the water will evaporate; on the contrary, when the pressure of the water vapor in the system is higher than the pressure of the equilibrium steam at the temperature, the water vapor is Condensation.
It is well known that a large amount of oxygen is contained in both metal oxide and ferrite magnets. For an oxide or ferrite, there is also a defined equilibrium oxygen pressure in a greenhouse. The equilibrium oxygen pressure of metal oxides and ferrite magnets can also be considered similar to the equilibrium vapor pressure of water and water vapor. The equilibrium oxygen partial pressure or equilibrium atmosphere of a ferrite magnet means that the ferrite magnet sample does not increase or decrease in oxygen content under such equilibrium oxygen pressure. In fact, the ferrite magnet sample exchanges oxygen with its surrounding atmosphere during the sintering process. Only when the oxygen pressure is maintained, the number of oxygen ions entering the ferrite magnet sample from the surrounding atmosphere and the number of oxygen ions running from the ferrite magnet sample to the surrounding atmosphere are equal. The atmosphere capable of maintaining the oxygen content of the ferrite magnet sample is called the equilibrium atmosphere of the ferrite magnet; the oxygen partial pressure in the equilibrium state is called the oxygen decomposition pressure of the ferrite magnet sample. Thus, in the equilibrium state, the oxygen partial pressure in the surrounding atmosphere is equal to the decomposition pressure of oxygen in the ferrite sample.
Obviously, at a certain temperature, if the decomposition pressure of oxygen in the ferrite magnet sample is larger than the oxygen partial pressure in the atmosphere around the sample, the ferrite magnet sample will release oxygen, that is, the ferrite magnet sample. The number of oxygen atoms entering the surrounding atmosphere is much higher than the number of oxygen atoms entering the ferrite magnet sample in the surrounding atmosphere. On the contrary, if the decomposition pressure of oxygen in the ferrite magnet sample is smaller than the oxygen partial pressure in the atmosphere around the sample, the ferrite magnet sample will absorb oxygen, that is, the sample of the ferrite magnet enters the surrounding atmosphere. Is the number of oxygen atoms less than the number of oxygen atoms in the ferrite magnet sample entering the surrounding atmosphere?
(2) The equilibrium oxygen partial pressure of ferrite magnet changes with temperature
The equilibrium surrounding atmosphere of a ferrite magnet refers to a single gas or a mixed gas that is balanced with a ferrite magnet at any temperature. Balancing the surrounding atmosphere is a function of temperature. In the equilibrium atmosphere of the ferrite magnet, it is important to balance the partial pressure of oxygen. Under certain conditions, such as air typically at 101. 325 kPa, the partial pressure of oxygen accounts for 21%, or 21.28 kPa. When considering the system, such as the sample and its surrounding atmosphere, the oxygen activity is enhanced during the temperature rise, whether it is the decomposition pressure of oxygen in the ferrite sample or the partial pressure of oxygen in the atmosphere around the sample. Both increase. Since the concentration of oxygen in the ferrite magnet sample is larger than that in the surrounding atmosphere, the temperature rises, and the oxygen decomposition pressure in the ferrite magnet sample grows faster than the oxygen partial pressure in the surrounding atmosphere, so that The oxygen decomposition pressure in the ferrite magnet sample is greater than the oxygen partial pressure in the atmosphere around the sample, and the result is that the ferrite magnet sample is oxygenated; conversely, when the system temperature is lowered, the oxygen in the ferrite magnet sample is The decomposition pressure is smaller than the oxygen partial pressure in the surrounding atmosphere, so that the ferrite magnet sample is oxygen-absorbing.
For heating in air, the exchange of oxygen between the ferrite sample and its surrounding atmosphere is generally negligible at temperatures below 500 °C. When the temperature is between 500 and 1 000 ° C, this oxygen exchange is slow but noticeable. When the temperature is higher than 1 000 ° C, the exchange of oxygen between the ferrite magnet sample and its surrounding atmosphere becomes very rapid and remarkable.
No matter what material, on the side of high oxygen partial pressure, the material absorbs oxygen, 4M02←→2 M2O3 +O2 reaction proceeds to the left; on the side of low oxygen partial pressure, the material emits oxygen, 4M02←→2M2O3 +O2 should be to the right On the curve, the equilibrium is established, and the reactants and the product coexist. For this reason, the oxygen partial pressure is determined, that is, the temperature at which the respective reactions are balanced is determined. Under certain oxygen partial pressure conditions, as the temperature of the system rises, it is beneficial for metal ions to exist in a low-cost form; conversely, when the system temperature is lowered, it is beneficial for metal ions to exist in the form of valence.
Usually in the ferrite sintering process, when the oxidation and reduction of the system are not serious, although some metal ions may change or vacate, they will be dissolved in the original phase, so they can still be maintained. Simplex. If the O2 is insufficient in the atmosphere, the sample will be partially reduced. If the degree of reduction exceeds the solid solubility, the other phase will be desolvated. This is because in the composition of ferrite, some metals (such as iron, manganese, cobalt, copper, etc.) ions will change in price, such as iron ions have trivalent and divalent. The conditions for the price change are the temperature and the partial pressure of oxygen in the surrounding atmosphere. If oxygen is deficient in the atmosphere around the ferrite, the ferrite magnet will release oxygen, and with the deflation, the variable valence metal ions in the ferrite sample will change from high to low, that is, reduction occurs. In the process of manufacturing a ferrite magnet, if a reduction occurs, the ferrite magnet may degrade due to the presence of metal ions that do not meet the required price.