Mica is known as a natural mineral. Mica capacitors are the type of capacitors that use mica as their dielectric material. You will find mica capacitors of known clamped mica capacitors and silver mica capacitors. However, clamped mica capacitors are not in use anymore because of their less effective characteristics. The silver mica capacitors are preferred more now. They are constructed by enclosing mica sheets coated with metal oxides on both sides. And then, to protect this assembly from environmental conditions, it is encased in epoxy. Mica capacitors are utilized when the design requires a stable and reliable capacitor with small values. Mica capacitors are low-loss and can be operated at high frequencies; their values remain set, and no changes occur over time.
Mica Minerals
Mica minerals are electrically, mechanically, and chemically very stable. They are crystalline structures binded which gives them a layered structure. This allows the manufacturing of thin sheets ranging from 0.025-0.125 mm. The most commonly used mica minerals are muscovite and phlogopite mica. Muscovite has better electrical properties, and phlogopite has a more significant temperature resistance. Some countries where mica is delved include South Africa, India, and South America. However, there is high variation needed in composition that can lead to higher manufacturing costs and higher costs of inspection and sorting. Mica minerals do not react with most acids, oil, water, and solvents.
Construction of Mica Capacitors
The construction of a silver mica capacitor is relatively simple. The traditional clamped mica capacitors used thin sheets of mica that would be layered with thin sheets of silver. After that, the layers would be connected, and electrodes were added. However, some holes or tiny air gaps were present in both mica and silver layers due to some imperfections, which restricted the precision of clamped mica capacitors. When this happens, the air gaps will create problems over time due to mechanical stress, leading to changing capacitance.
After WW2, the silver mica capacitors were designed by directly plating the silver on the surface of the mica and then layering them to get the desired capacitance. After the assembly of layers, the electrodes are added, and the assembly is then closed in a casing. The encapsulation material for silver mica is usually epoxy or ceramics. These materials protect the silver mica from environmental conditions like moisture.
The silver mica capacitors have small values of capacitance that commonly range from a few pF up to a few nF. The mica capacitor's most significant capacitance is 1µF, but these are not very common. Their voltage rating ranges from 100 and 1000 volts. However, special capacitors are high voltage and are designed to use RF transmitters that are rated up to 10 kV.
Applications of Mica Capacitors
- Silver mica capacitors are mainly required for applications that need low capacitances, high stability, and a low number of losses. Their primary use is in the Radio Frequency or RF circuits, in which stability is given the most importance.
- Silver mica capacitors are utilized in high frequency turned circuits like oscillators and filters. They are often used in pulsed applications as snubbers.
- Ceramics can replace the mica capacitors in low Radio Frequency applications.
- Silver mica capacitors are indispensable in some applications. Designers of circuits still prefer mica circuits for high-power applications like RF transmitters.
- Silver mica capacitors can also be widely used in High-Voltage applications because of their breakdown voltage.
