Capacitors were the original electrical energy storage devices. They are now among the world’s most widely used electronic components, with a vital role in the drive systems of today’s electric and hybrid vehicles. As capacitor technology continues to evolve, it could play an even bigger part in the mobility solutions of tomorrow.
Jonathan Ward
The first electric battery wasn’t a battery at all. In the mid-18th century, scientists and inventors around the world were fascinated by the mysterious phenomenon of electricity. The first practical way to store electrical energy was known as the Leyden jar, named after the hometown of Dutch scientist Pieter van Musschenbroek, one of several pioneers to independently create the device.
The ideas that led to the Leyden jar were wrong: its inventors thought that electricity could be stored in liquid form, dissolved in water or alcohol. The jars were designed to facilitate that process, using a glass container with a conductive metal rod inserted through its lid.
It was Benjamin Franklin, scientist and one of the Founding Fathers of the United States, who later discovered that Leyden jars worked just as well with nothing in them. He used empty jars wrapped and lined with metal foil, eventually replacing the jars with flat glass plates. In a letter of 1749, Franklin described an arrangement of multiple plates as “… what we call’d an Electrical Battery.”
We now know that the Leyden jar and its descendants were not batteries but rather early examples of the capacitor, a device that has become ubiquitous in modern electrical and electronic systems. Batteries store and release charge through chemical reactions. Capacitors work by catching and releasing electrons. They consist of two conductive plates, held close together but separated by a barrier of insulating material. When a positive charge is applied to one of the plates, it attracts electrons to the other. Unable to cross the barrier, those electrons simply hang around until conditions change and they are free to flow back into the circuit.
A modern battery can store many times more energy than a capacitor of equivalent size and weight, but capacitors have other properties that batteries can’t match. Batteries work best when they work slowly. Their chemical reactions take time, and charging or discharging a battery too quickly can damage or destroy it. A capacitor, by contrast, can cope with extremely rapid charge and discharge cycles with no ill-effects.
That ability to absorb and release energy quickly is what makes capacitors so useful in power electronics applications, including the drive systems of electric vehicles. Large capacitors perform multiple functions in modern electric vehicles. They smooth the flow of energy through the circuits that convert direct current from the battery to alternating current at the motor, for example.
And they can act as a buffer at times of high demand, absorbing excess energy during regenerative braking or discharging to boost acceleration. Making capacitors that perform well in electric mobility applications is tricky. The storage capacity of a device is closely related to the area of its conductive plates, but engineers need compact, robust components they can easily integrate into their designs. One way to do that is to make the plates and insulating layer from thin, flexible materials that can be rolled up into a small package.
Today’s component makers are taking that approach to extremes. Ultra-thin film capacitors, which are common in automotive applications, use oriented polypropylene (OPP) or polyester (PET) films as their insulators. “In automotive applications, we see film thicknesses down to 1.2 micrometers,” says Michael Mücke, Head of R&D and Product Management for Bühler Leybold Optics’ Flexible segment, the part of the business that makes the machines that apply coatings of conductive material onto the insulator material.
Manufacturing a thin film capacitor involves multiple process steps. Films receive a thin aluminum coating to form the plates, with thicker layers added in select areas to create connection points. Prior to coating, a printing system masks parts of the film with a layer of oil, so the conductive coatings only go where the component design requires them.
Precision vacuum coating using physical vapor deposition, including thermal evaporation, is a core Bühler expertise. However, applying coatings that are 50 times thinner than a human hair to thin polymeric films creates its own special challenges.
“You need to ensure that you don’t damage the film during handling,” says Mücke. “Even allowing it to wrinkle would negatively impact the coating process.” The Leybold Optics CAP vacuum coating systems incorporate multiple innovations to achieve the best possible quality and process reliability.
They include sophisticated control systems that precisely manage the tension and position of the film as it travels through the coating chamber. “We develop our own software for the control system,” says Mücke. “We want to continually adapt the speed of the coating and winding systems to achieve a consistent result, while minimizing oscillations that could disturb the film.”
Temperature management is critical, too. Within the coating chamber, supporting process drums are cooled to between -15ºC and -20ºC to ensure the hot aluminum does not melt the film. An optical verification system inspects the entire coated surface as it leaves the chamber, backed up by cameras that take high resolution images of select areas.
The rolls of film can be as long as 60 kilometers, yet resource efficiency and quality criteria have to be maintained throughout the process. “Our aim is to minimize scrap and achieve 100 percent quality over a full roll of film,” says Mücke.
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