Capacitor manufacturers have used mica as a dielectric for many decades, and while mica capacitors are far less common than aluminum, ceramic or plastic film devices, their impressive stability and reliable performance make them suitable still for various applications including high frequency and high voltage.
Properties of mica
Mica is a silicate mineral with a dielectric constant ranging between 5 and 7 and high electrical, chemical, and mechanical stability. This natural mineral has a distinct layered structure that allows manufactures to cut it into thin and flat sheets, typically thinner than 0.025mm. In addition, mica is highly inert and stable and does not react with water and most acids, alkalis, and solvents. The most common types of mica that are used in capacitor construction are muscovite (potassium aluminium silicate) and phlogopite (potassium aluminum silicate) mica.
Muscovite mica has a high dielectric strength and is thermally stable for temperatures up to 500°C. On the other hand, phlogopite mica is thermally stable for temperatures up to 900°C, but it has less desirable electrical characteristics. In addition, as compared to muscovite mica, phlogopite mica is softer.
Although mica is a commonly available mineral, the cost of producing mica for use in capacitor manufacturing is relatively high because of variations in its composition. In most cases, this naturally occurring mineral contains sodium, iron, lithium, and ferric oxide. Careful inspection and classification is required to remove impurities from mica.
Construction of mica capacitors
Early stacked mica capacitors were constructed using mica and copper or aluminum. Thin sheets of mica separated by thin sheets of aluminum or copper were stacked together to form a mica capacitor of desired capacitance. Sheets of mica acted as the dielectric material while sheets of metal acted as electrodes. These capacitors had poor performance, and they are no longer manufactured.
The performance of mica-metal capacitors improved when aluminum and copper were replaced by silver. In these clamped mica capacitors, thin sheets of mica separated by thin sheets of silver were layered to form an assembly. These mica-silver layers were clamped before connections were attached.
The precision of clamped mica capacitors was low due to the air gaps that formed between the two materials. These air gaps were caused by physical imperfections in the two materials. For these capacitors, it was common for capacitance to change over time due to air gaps. Manufacturers enclosed mica-metal assemblies in plastic cases to protect components from moisture and mechanical damage.
Sliver mica capacitors are constructed using layers of silver-coated mica. Several layers of silver-plated mica are assembled together to form a component. In silver mica capacitors, mica acts as the dielectric while silver acts as the electrodes. After adding connections to the assemblies, epoxy resins or ceramics are used to encapsulate the assembled component. This protects assemblies from moisture and air humidity. Silver mica capacitors have high accuracy, quality factor (Q), and stability.
Compared to clamped mica capacitors, silver mica capacitors offer higher stability, reliability, and greater value per volume. The performance characteristics of these components are better mainly because there are no gaps between mica and plates. The airtight enclosures of modern mica capacitors eliminate possibility of corrosion or oxidation of connections or plates. In addition, the conducting surfaces of modern silver mica capacitors are thinner.
Characteristics and applications of mica capacitors
Mica capacitors have been used in electronic circuits for many years. These passive components are suitable for applications that demand high operating stability, reliability, and capability to withstand high temperatures. Their small temperature coefficient and low loss make them a suitable option for a broad spectrum of applications.
The performance characteristics of mica capacitors are greatly dependent on the purity of mica used to construct them. These characteristics can be significantly affected if the dielectric material has foreign pores and air bubbles. Silver mica capacitors have a voltage rating of between 100 and 1000 v, but mica capacitors for special applications can have higher voltage ratings. Moreover, these capacitors have relatively low capacitance values, typically less than 1µF.
Silver mica capacitors have tight tolerances, typically approximately ±1% and retain their values for long periods of time. In addition, these capacitors are hermetically sealed to protect them from harmful environmental conditions.
Mica capacitors offer high stability levels, and changes in operating voltage, frequency, or temperatures have minimal effects on their capacitance. This means that the capacitance of a silver mica capacitor is stable over a broad range of conditions. In addition, these capacitors can withstand high temperatures and voltages.
Mica capacitors exhibit low inductive and resistive losses. As a result, these passive components have a high quality factor (Q). The low-loss property of mica capacitors makes them a suitable option for high-frequency applications. Moreover, mica capacitors are capable of withstanding high voltages, and they usually have high voltage ratings.
Although silver mica capacitors have impressive characteristics, they are relatively expensive compared to most dielectric types. These passive components are unsuitable for applications where cost is a key consideration. Ceramic capacitors are commonly used in such applications that demand low-cost components. Furthermore, mica capacitors are only available in low capacitance values.
Silver mica capacitors have an impressive temperature coefficient that ranges between 35 and 75 ppm/°C. The average temperature coefficient of a typical silver mica capacitor is approximately 50 ppm/°C.
Although silver mica capacitors have performance characteristics that make them suitable for a wide range of applications, there is a decline in the use of these components. This decline is mainly due to their physical size and the high cost of manufacturing them. Improvements in other capacitor technologies have also contributed to this decline.
The performance characteristics of mica capacitors make them a suitable option for applications that demand high stability and low-loss components. They are suitable for power RF circuits, high-frequency tuned circuits, coupling circuits, and pulsed applications. In RF oscillators, the low-loss property of these components allows high stability and low phase noise to be achieved.
The low temperature coefficient of silver mica capacitors enables oscillators to achieve high stability and low drift. Filters require components with tight tolerance levels and low loss. The characteristics of silver mica capacitors allow realization of high performance and high Q filters.
Mica capacitors have been used in the electronics industry for many years, but their use has been declining. These passive components offer very high operating reliability, extremely low induction, and long-term stability. In addition, the loss factor of these components is nearly frequency-independent, and they can be used in applications with elevated operating temperatures. Silver mica capacitors are commonly used in resonance circuits, coupling circuits, power conversion circuits, radio and TV amplifiers, cable TV amplifiers, time constant circuits, and high-power RF broadcast transmitters.