Passive components like resistors and capacitors remain mostly unseen in our world, quietly doing their job of resisting current flow or storing charges faithfully. Until, of course, something goes wrong.
These reliable devices are critical to many of the devices we use every day, and people take capacitor supply concerns very seriously.
The lowly capacitor has been a part of nearly every technological advancement, from the first moon landing to the development of the iPhone—an item that our first astronauts could barely imagine. So, what makes one capacitor better, or at least more appropriate for one job than another? While several factors are at play, one of the most important is the dielectric material—the substance that separates positive and negative charges from each other until just the right moment.
Capacitor Types and Dielectric Properties
The first technological divide for capacitors is whether they are polar or non-polarized.
- Polarized capacitors. As the name implies, polarized capacitors only work in one direction positive or negative-wise, while the direction in which you install non-polarized capacitors makes no difference.
- Non-polarized capacitors. Alternatively, non-polarized capacitors are ideal for use in filter applications where applied voltages switch rapidly from positive to negative on each pole.
Use of Dielectric in Capacitors
A third somewhat distinct option comes in the form of variable and tuning capacitors, where you can change values to set up a circuit or to address other conditions. While somewhat rare today, you can create a tuning capacitor as a scientific demonstration, or for use in equipment that can take advantage of such components. Tuning capacitors use air as the dielectric, which has a relative permittivity (basically a capacitance material performance rating) of 1.0006, negligibly better than the worst possible permittivity, which occurs in a vacuum of 1.0000.
We express dielectric permittivity in units of Farads/meter (F/m). In a vacuum, this measures out to:
8.8542 x 10-12 F/m.
Every other type of dielectric has a higher value than this base number, known as relative permittivity or dielectric constant. The relative permittivity equals the permittivity of a material divided by the permittivity of a vacuum. Therefore, the vacuum has a relative permittivity of 1, while other materials are much higher.
Polarized Capacitors: Chemically Formed Dielectrics
Polarized capacitors are most often electrolytic and feature the following setup:
- Anode: a conductive metal
- Cathode: a conductive liquid
The anode forms an oxidation layer to separate the two, which acts as an extremely thin dielectric— somewhere in the 100-nanometer range. A thin dielectric is ideal for a component’s total capacitance, dependent on the following equation:
C = εA/d
Here C is the total capacitance, ε is the permittivity, A is the separated area between electrodes, and d is the distance between these two areas. So as d approaches 0, the capacitance will approach infinity, at least in theory.
Effect of Dielectric on Capacitors
We can construct electrolytic capacitor anodes out of aluminum, tantalum, or niobium, which result in oxides with relative permittivity of 8.5, 27, and 50, respectively. While this takes care of the ε term in the above equation, the surface area, A, is also enhanced by etching or sintering techniques that produce a rough or porous anode surface. When oxidized to form a dielectric, the resulting microscopic pathways greatly increase the effective area of the capacitor, contributing to higher volumetric efficiency (capacitance per unit volume).
Supercapacitors (also discussed here) use another chemical process, but instead of a layer of oxide, ions in an electrolyte form a Helmholtz double layer that acts as a sort of ad hoc dielectric. This double layer allows for a charge separation of less than a nanometer, boosting total capacitance value per this reduced denominator. Relative permittivity can range into the thousands, giving a total capacitance value that’s often in the single farad range, whereas we typically express electrolytic capacitor values in microfarads (μF), or millionths of a farad.
Conversely, supercapacitors can only withstand voltages in single digit range before they break down, while other types can sustain much higher voltages. Also, energy storage and discharge rates in supercapacitors are much slower than traditional capacitors, though much faster than batteries.
Dielectric Materials in Non-Polarized Capacitors
As opposed to polarized capacitors, we can install non-polarized capacitors in either orientation in a circuit. Non-polarized capacitors are also useful electronics filtering applications. Dielectric materials vary, including:
- Ceramic
- Polymer film
- Paper
- Mica