Since the 1990s, a new type of metallized film, Al Zn composite film, has developed rapidly due to its combination of both advantages: stronger oxidation resistance than pure Zn metallized film, and better attenuation resistance than Al metallized film capacitors. In the Al Zn composite film, the Al content is less than 10%, and its function is to sensitize and substrate, facilitating the evaporation of Zn. Al penetrates into the surface of Zn, forming an Al2O3 protective layer, which plays a protective role.
1. The effect of humid air on Zn-Al film
We have conducted such an experiment where the evaporated Al metallized film was immersed in water for 24 hours without significant changes, indicating that the Al metallized film has good stability in water. Similarly, by directly contacting the evaporated Al Zn metallized film with water, blank spots can appear within seconds. After a few minutes, the film is almost transparent and there is no Al Zn metal layer present, which fully demonstrates the sensitivity of the Al Zn metallized film to water.
We have dissected early failed products multiple times, most of which were caused by oxidation of the metallized film. So we must prevent oxidation and corrosion of the Al Zn film in the production process, and the key is to minimize the humidity of the production environment as much as possible.
2. The existence form of water vapor
To reduce the content of water vapor in the air, it is necessary to explore the forms and patterns of water vapor in the air. The water vapor in the air exists in the form of a gas. In fact, the air around us in our daily environment should be moist air.
The humidity we usually refer to refers to relative humidity. It only represents the relative proportion to saturated water vapor. The definition of saturated water vapor pressure is: when wet air and liquid water are in a saturated state, the pressure of water vapor in the air is the saturated water vapor pressure at this temperature.
The definition of relative humidity is the percentage of the ratio of the actual partial pressure of water vapor in the air to the saturation pressure of water at the same temperature. Among them, Pv is the actual water vapor pressure in the air, and Pw is the saturated water vapor pressure at this temperature. Experiments and data indicate that the saturated water vapor pressure is related to temperature. It is a function related to temperature, and the higher the temperature, the higher the rate of water evaporation, and the higher the water vapor pressure at saturation.
3. Condensation and prevention of water vapor
If the component is moved from a low temperature to a hot and humid air, we can quickly see some small water droplets appearing on its surface. This is because the ambient air temperature is high and the water content is high. When it comes into contact with the surface of low-temperature components, due to the surface temperature of the components being lower than the dew point temperature of the air, the water vapor in the air condenses into small water droplets, which is the phenomenon of condensation.
The definition of dew point temperature is that when the partial pressure of water vapor in the air is greater than the saturated vapor pressure at that temperature, the water vapor in the air may transform into a liquid phase and form dewdrops. This temperature is called dew point temperature. For the actual process of producing capacitors, condensation is absolutely not allowed to occur. Because this will cause fatal damage to the metallized film.
So, how can we prevent it? According to the characteristics of condensation, condensation only occurs when the product temperature is low and the air humidity is greater than the saturated vapor pressure of the product temperature. For example, when a product with a temperature of 25 ℃ is transferred to an environment with an ambient temperature of 30 ℃, condensation will occur when the relative humidity of the environment is greater than the saturated vapor pressure due to the saturation vapor pressures of 2.96kPa and 3.973kPa, respectively.
So, the method we need to avoid condensation is to avoid excessive temperature difference when moving from low temperature to high temperature, and to obtain the maximum temperature difference based on the absolute temperature at high temperature; The second is to reduce the humidity at high temperatures. For the above example, as long as we reduce the relative humidity of the 30 ℃ environment to 50%, condensation can be avoided. Therefore, as long as we adopt scientific and effective methods during the production process, it is not difficult to prevent condensation.
4. Water adsorption and countermeasures
Due to the fact that the oxidation of Zn-Al films takes place after the adsorption of water, it is necessary to explore the conditions under which water adsorption is related. First, let's take a look at the relationship between water vapor pressure and adsorption capacity at a certain temperature. When the temperature is constant, the higher the water vapor pressure, the more water molecules adsorbed on the surface of the Al Zn film per unit time, and the greater the amount of water adsorbed when the adsorption and diffusion equilibrium is reached.
When the vapor pressure of water is constant, the relationship between water adsorption capacity and temperature is: the higher the temperature, the lower the relative humidity, and the less water adsorption capacity.
4.1 Measures to reduce water adsorption capacity
According to the above, as long as the absolute humidity or relative temperature of the air is reduced, the amount of water adsorption can be reduced. The methods adopted include: firstly, dehumidification, which means reducing the moisture content of the air and reducing the vapor pressure of the water. The second is to increase the temperature, which is used to reduce the relative humidity and adsorption capacity by increasing the temperature when the absolute humidity of the air remains constant. The third is to combine the advantages of both heating and dehumidification.
4.2 Method of dehumidification
The method of dehumidification for air conditioners is that when the air conditioner is working in refrigeration and cooling, the hot and humid air passes through the surface of the evaporator and is cooled. The temperature of the humid air is lower than its dew point temperature, so the water in the air condenses into droplets and is discharged, playing a dehumidifying role. Due to its dehumidification effect, it will lower the room temperature. Based on this feature, air conditioning dehumidification is more suitable for environments with people working in hot summers.
The working principle of using a dehumidifier for dehumidification is very similar to that of an air conditioner, but it will increase the room temperature while dehumidifying. So, it is more suitable for colder winters.
The working principle of high-pressure condensation dehumidification is to first compress air and then condense to remove water. Its advantages are high water removal rate, while its disadvantages are high investment and complex process. It is suitable for environments with high drying requirements. When the metallized film is stored in a centralized manner and the cycle is relatively long.
The method of reducing adsorption capacity by heating up is relatively simple, but the process of both heating up and dehumidification is more complex because the working temperature of the dehumidifier must be considered, usually below 32 ℃. So it is necessary to consider separating the compressor from the cold and heat exchanger to ensure that high temperatures do not affect the compressor's operation.
Practice has shown that adopting the above moisture-proof process measures is very effective, but if the production cycle is too long, the metallized film still needs to be oxidized. Therefore, while paying attention to moisture-proof, we must also shorten the production cycle as much as possible to avoid product oxidation. As long as we can take appropriate process measures, we can produce capacitors with good performance. When producing capacitors, it is necessary to prevent oxidation of the metallized film, which can be achieved by:
① Reduce the temperature difference in the production environment and prevent product condensation.
② According to the actual situation, adopt appropriate dehumidification methods to reduce the relative humidity of the production environment.
③ Accelerate product circulation and minimize production cycles as much as possible.