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Glass, MICA, Air and Vacuum Capacitors

  • Posted by doEEEt Media Group
  • On December 16, 2022
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This article briefly introduces niche types of electrostatic capacitors – Glass, MICA, Air, and Vacuum Capacitors.

Introduction

Aluminum or tantalum plate capacitors with an oxide dielectric material are widely used for applications requiring high capacitance.

These electrolytic capacitors have high instability, and poor tolerances, and their performance deteriorates when they are subjected to high leakage current or high temperatures. Plastic film and paper capacitors are usually used in circuits that demand capacitors with good aging performance, high stability, and high current-carrying capability. For high-frequency applications, ceramic capacitors are a good option. However, these capacitors have poor aging performance.

Aluminum capacitors or tantalum plate capacitors

Mica and glass capacitors are suitable for applications that demand extremely high stability, reliability, and tolerance to high voltages and temperatures. However, these capacitors are typically expensive and are not commonly used in lower-cost devices.

Glass Capacitors

The one in all respects most environmentally resistant capacitor we know is the glass capacitor. Except for common environments, the capacitor also will stand high nuclear radiation doses and strong neutron radiation. It is specified in US MIL-C-11272 and –23269. The capacitance range is limited to a tenth of nanofarads compared to other technologies. The capacitor is, just like the MLCC, manufactured in a stacked form. The dielectric and encapsulation comprise glass that constitutes a homogeneous and highly resistive product in all environments.

Figure 1. glass capacitors structure

Figure 1. glass capacitors structure

The electrodes most often consist of aluminum foils folded over the edges of the end surfaces and welded to the connecting leads. A relatively thick dielectric – at least 50 µm – and a thick encapsulation make the capacitor bulky with the aluminum foil. On the other hand, the Al foils permit high surge currents. The dielectric absorption of older production was approximately 0.5%, while testing of recent manufacture accounts for 0.01 %. The most common failure usually is defective weld joints between Al foils and leads. High pulse currents burn the weak joints, thus causing an open circuit. The failure rate, in general, increases with the proportions λ120° : λ25° ≈ 50 : 1.

Figure 2. leaded glass capacitors

Figure 2. leaded glass capacitors

Summary of glass capacitors

  • very high stability, no corrosion or degradation, no microfractures, delaminations …
  • the hermetically axial case is practically immune to severe environmental effects such as shock, vibration, radiation resistance, moisture, salt spray, and solder heat.
  • zero aging rate, high Q factor, large RF Current Capability, no piezoelectric noise, low dielectric absorption, ±5ppm temperature coefficient retraceability, high breakdown voltage, zero voltage coefficient, up to -75°C – +200°C
  • pF to hundreds / th. pF, several hundred V

MICA Capacitors

Previously MICA capacitors have, due to their stability and their good HF characteristics, been dominating for filter purposes. Today there are plastic film capacitors that in many respects are more favorable. What still makes the mica capacitor interesting is its stability, the small tolerances, and the temperature range (-55/+125 and up to 200°C). Mica is a mineral that, for capacitor purposes, is mined dominantly in India. Mica has a high voltage strength and low losses. The capacitors are manufactured in a stacked form, either with metal foil electrodes (stacked mica) or with electrodes consisting of a screen-printed silver oxide paste that under reduction to silver, is burnt to the mica surface, so-called silvered mica.

The stacked mica can, among other things, stand higher currents and are more used in power electronics. We only mention their occurrence and will not deal with the subject further. The silvered mica capacitors that are most common normally can stand currents only in the mA range, but there exist special designs for stronger currents. Silvered mica is also produced as chips. Some examples show that the capability to stand power sometimes is better than that of ceramic Class 1 chips.

A special variant of mica capacitors is the mica paper type. Small mica scales are treated approximately like paper pulp and are transferred to sheets and either wound with aluminum foils or stacked with electrodes. The result will be a kind of mica capacitor with relatively high losses (similar to polyester and paper) and, in other respects, equivalent to conventional mica. Also, this type belongs more to power electronics and will not be treated further in this context. The connection to the silvered mica electrodes is made at the edge of the end surfaces by special foil sheets, usually silver.

The silvered mica packet is then pressed together, the silver foil sheets are folded and soldered to the connecting leads after that the packet is supplied with fixing metal clamps. The stack is vacuum impregnated, usually with epoxy. Because the silver electrodes are extremely apt to migration in moisture the capacitor has to be encapsulated. Either it is molded in plastic or dipped in some kind of phenolic lacquer. The more layers, the better the vapor barrier. For professional applications, there should be at least four layers of lacquer. The pressure clamps lead at temperature changes to traces of capacitance hysteresis. When we return to the starting temperature a certain ∆C appears due to the TCE of the clamps causing an incomplete mechanical pressing together to the initial pressure.

Figure 3. leaded MICA Capacitor

Figure 3. leaded MICA Capacitor

Measurement frequencies:

  • C ≤ 1000 pF Measurement frequency 1 MHz.
  • C > 1000 pF Measurement frequency 1 kHz.

MICA capacitors summary

  • Mica minerals are very stable electrically, chemically, and mechanically. Because of its specific crystalline structure binding, it has a typical layered structure. This makes it possible to manufacture thin sheets in the order of 0.025-0.125 mm
  • with high thermal stability performance at temperatures of 175°C+
  • high power applications requiring low inductance at high frequencies, temperatures up to +200°C, and voltages up to 1,000Vdc.
  • axial led with voltage ratings up to 5KVdc, capacitance ratings as high as 3.0µF, and a temperature rating of +175°C.
  • Hermetic metal or glass bead seal in radial and axial mounting options; the rated voltage range is 63Vdc – 250Vdc, and the operating temperature range -55°C – +200°C. These capacitors are designed for extreme temperature applications such as oil/gas exploration and high-temperature engines.

Air/Vacuum capacitors

Very specific groups of capacitors are air and vacuum capacitors. The relative permittivity dielectric constant is close to 1 as per its definition. Thus capacitors are large in size. Nevertheless, its losses are low under a high power load. These capacitors are used specifically for RF applications from VHF to microwave frequencies in power broadcasting as trimmer capacitors etc.

Figure 4. Vacuum variable capacitors

Figure 4. Vacuum variable capacitors

Air/Vacuum capacitors summary

  • Air trimmer capacitors
  • Specifically for RF applications, VHF through microwave
  • High Q and temperature stability
  • Vacuum – high power RF – broadcasting, networks, tenth of kV, hundreds pF

Comparison

Table 1. GLASS / MICA CAPACITORS CHARACTERISTICS

Table 1. GLASS / MICA CAPACITORS CHARACTERISTICS

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