13 September 2011
Capacitors continue to evolve
Few passive components exist in the sheer variety of forms that the capacitor has taken. Simple in principle, the design of the capacitor involves a number of tradeoffs that make it impossible to satisfy all possible uses with just a few types of product.
It is possible to regard capacitor design as being the original nanotechnology because of the way in which it relies on achieving a chemically complex construction over very small dimensions – and that research is continuing as manufacturers try to squeeze more energy into a tiny space. Engineers have used an incredible variety of materials to realise new capacitor types, often relying on their detailed molecular structure to improve charge storage, only to discover that commodities prices have made their products uncompetitive.
A simple – and very poor – capacitor consists of two parallel conductive plates, held a certain distance apart by a dielectric insulator with a relative permittivity of er. The capacitance, C, of the device is defined as the plate area divided by their distance apart, multiplied by the relative permittivity and the permittivity of free space, e0. To increase the capacitance of a charge storage device defined in this way means either increasing the area of its plates or the relative permittivity of the dielectric material or reducing the distance between the plates – or some combination of all three.
Capacitor design is all about making the surface area of the plates as large as possible and then packing those plates into a very small volume. A typical construction is a metallic foil separated by a very thin dielectric material, such as a polymer film. The foil and dielectric sandwich is wound into a tight spool, to maximise the energy storage density. But this approach is limited by the availability of very thin dielectric films with suitable breakdown characteristics. Texturing the foil is key to increasing surface area without increasing the volume of the capacitor at the same time.
The energy such a device can store is defined as half of its capacitance multiplied by the applied voltage squared. Increasing energy storage, then, means increasing its capacitance, as discussed, or increasing the voltage applied. But there is a limit to how far you can drive the voltage; beyond a certain field strength, defined by the voltage applied and the thickness of the dielectric material, the dielectric will break down and start to conduct. Trade offs like these control a device's practical ability to store energy, so some will offer very high voltage using film capacitors, whereas others will have high volumetric efficiency in terms of energy storage, but will collapse under a high applied voltage.
King of the capacitor market today is the ceramic capacitor. In 2008, nine out of every ten capacitors shipped were ceramic and most of those were based on the chip like design of the multilayer ceramic capacitor (MLCC) (see fig 1). However, because they have relatively low capacitances and are often scattered around PCBs for filtering and decoupling, they accounted for only 40% of the capacitor market's dollar value.
There are two main classes of dielectric used in ceramic capacitors. Generally based on simple oxides, Class I dielectrics offer poor volumetric efficiency, but have the benefit of offering comparatively stable performance over temperature. Class II materials generally offer much higher dielectric constants, but at the cost of less predictable performance with changes in temperature. For example, barium titanate has become one of the most commonly used dielectrics in Class II capacitors, thanks to its ferroelectricity. However, changes in temperature cause the crystal structure of barium titanate to alter, which causes dramatic shifts in the dielectric constant (see fig 2).
As a result, manufacturers typically use solid solutions of barium titanate with dopants based on strontium, calcium, zirconium, tin and rare earth elements. These dopants broaden the dielectric constant peaks of the core barium titanate.
Alternatives to barium titanate include 'relaxor' dielectrics, which gain their properties from a nanostructure of tiny polar regions surrounded by a non polar matrix. This structure results in permittivity values that are frequency dependent, which is how these materials get the name 'relaxor'. The presence of lead in many candidate materials, however, limits the use of relaxors to specialist applications, such as capacitors for high temperature systems.
One of the biggest changes in the construction of MLCCs has come not so much in the dielectric chemistry and nanostructure, but rather in the electrode materials. MLCCs are typically made by folding a flexible tape of electrode and dielectric into a sandwich that is then sintered at high temperature to remove any solvents and then forms a dense chip capacitor. The trouble with this co-firing process is that it risks the electrode metals reacting with the complex dielectric. Precious metals do not react nearly as readily as base metals, so were favoured in MLCCs for many years.
However, the use of metals such as platinum and palladium in automotive catalyst converters brought capacitor makers into conflict with car makers for sources of these metals. Car makers had the benefit of being able to switch much more readily between metals when the price of one increased too far than capacitor manufacturers, who would be forced to swallow often sudden cost changes or risk becoming uncompetitive. By the end of the internet bubble at the end of the 1990s, the price of palladium had risen tenfold in less than ten years, before plummeting again.
These dramatic price swings convinced MLCC makers to push harder on alternatives to precious metals, such as nickel. Doping with rare earths has helped to limit the reaction between the nickel and oxygen in the dielectric layers and has boosted overall device lifetime, particularly at high temperatures.
The volumetric efficiency of MLCCs has gradually increased, with customers in response choosing to move to smaller case sizes. Whereas the 2012 case size dominated in the mid 1990s, manufacturers have shifted to the 1005, with smaller packages such as the 0603 and 0402 gaining in popularity. And the almost microscopic 01005 is beginning to see usage in volume.
One factor limiting the size of MLCCs lies in the porosity of any ceramic, which keeps the breakdown voltage relatively low. So manufacturers are now looking at the use of glassy dielectrics to try to improve volumetric efficiency and to increase the breakdown voltage.
The other main capacitor in general use is the electrolytic, split between the aluminium and tantalum families. The electrolytic technology dates back to the end of the 19th Century, when Charles Pollak received a patent for an aluminium electrolytic capacitor based on a borax electrolyte. Pollak discovered that a very high capacitance formed between a very thin aluminium oxide layer deposited on the electrode and an electrolyte solution. The downside was that the electrolyte would eat away at the electrode when power was removed. Borax turned out to work as an electrolyte without destroying the electrode.
The reliability of electrolytics has improved dramatically over the years, but the components still need some care and attention. For example, the oxide layer can degrade if the capacitor is not used for long periods, although it is possible to rejuvenate the oxide with a sustained voltage. The chemistry of the electrolytic means the capacitor can only be used in one polarity. Reversing the polarity destroys the oxide layer – the resulting gas expelled at high temperature will often rupture the package.
Aluminium electrolytic capacitor technology is now seen as mature, although some improvements are being made to their chemistry. Tantalum based devices, meanwhile, have seen more extensive changes in the past decade. These typically offer higher capacitance in a smaller space and tighter tolerances than their aluminium cousins, although they are not available with capacitances as high as the largest aluminium products. But tantalums are not without their problems; not least being the core metal's scarcity.
During the internet boom of the late 1990s, tantalum prices soared because of the capacitors' use in computers and mobile phones. Capacitor makers attempted to secure supplies in the face of increasing demand and tied themselves into contracts that quickly became punitive after spot market prices tumbled in the wake of the 2001 bust. Tantalum prices have risen and fallen with other commodities since then, seeing a smaller but significant peak in 2009.
Although similar problems have afflicted ceramics because of the use of precious metals in electrodes, many manufacturers have decided that the tantalum capacitor is too volatile in terms of price to be a good choice where alternatives exist. As a result, designers have shifted to using high capacity MLCCs, squeezing tantalums into a smaller niche where aluminium products cannot fit.
Despite their better space efficiency, tantalums have traditionally suffered a big drawback compared with aluminium products: relatively high effective series resistance. This is due to the poor conductivity of the manganese oxide used in the cathode. A move towards polymer cathodes has helped reduce this resistance, although aluminium chemistries have joined this trend: to improve overall efficiency and to avoid the shortcircuit failure mode of electrolytics.
Polymer cathode materials help with one other problem: the tantalum capacitor's explosive nature. The manganese oxide that forms the cathode in a standard tantalum capacitor can release oxygen as it heats and that allows tantalum to burn – sometimes with catastrophic results. Conductive polymers do not release oxygen as readily as manganese oxide, although they will shortcircuit, and sometimes more readily than the metal oxide versions. This is because there is a limited self healing mechanism with manganese oxide that protects against local shortcircuits, at least up to a point.
An alternative that has appeared in recent years is the niobium capacitor, chosen because niobium has a higher ignition temperature than tantalum and because it is up to 100 times more abundant than tantalum, which should keep its price under closer control.
Niobium has been used in ceramic capacitors, but problems with the way in which its oxide forms delayed the introduction of solid electrolytic designs. Chemical treatments have made it possible to build stable devices.
Chemistry has taken the capacitor into the realm dominated by batteries, using a combination of reversible chemical changes and electrostatics to boost energy storage. Supercapacitors, also known as ultracapacitors or electrochemical double layer capacitors, have a double layer construction of two non reactive porous carbon electrodes immersed in an organic electrolyte (see fig 3). When a voltage is applied to the plates, the potential on the positively charged plate attracts the negatively charged ions, and vice versa. This creates two separate layers of capacitive storage, one at each plate.
These high capacity devices benefit from massive plate areas, thanks to the use of a porous carbon based electrode material whose structure gives it a surface area on the order of 1000m²/g. Supercapacitors also benefit from the distance between the plate and the stored charge – the charge separation distance – being less than 1nm, controlled by the size of the ions in the electrolyte. The combination of large surface area and very small charge separation gives the supercapacitor its increased capacitance relative to conventional parts.
Although supercapacitors are electrochemical devices, relying on the polarisation of an electrolyte, there's no chemical reaction involved in the energy storage mechanism. This means it is highly reversible, enabling supercapacitors to be charged and discharged hundreds of thousands of times, as well as quickly. Supercapacitors also have low equivalent series resistances, which allows them to give up their energy very quickly to create very large currents. Supercapacitors are also less likely to be affected by temperature than chemical batteries, since they're not reliant on a chemical reaction that could be slowed by low temperatures.
The parts have been used in applications that need to be charged in seconds and then discharged over minutes, for example in power tools and toys. They are also used in uninterruptible power systems, where the ultracapacitor provides the power for short outages, or as a bridge to a generator set or other continuous back-up power supply.
This combination of characteristics makes supercapacitors an attractive supplement to, or perhaps future replacement for, chemical batteries. Supercapacitors can be used to take over the short term heavy current delivery necessary to drive a starter motor to turn over a car engine or to provide fast load smoothing in applications with very irregular current demands.
Supercapacitors are getting a lot of attention in vehicle applications. An MIT team points out that the devices have a greater power rating than a conventional lead-acid battery, superior charging and discharging characteristics, and can be fully discharged without being damaged.
There is a downside: the discharge profile of a supercapacitor has a significant drawback when it comes to safety. Vehicle makers are concerned about what could happen if an electric vehicle breaks down and the driver decides to open up the bonnet and have a poke around to see what is wrong. A spanner in the wrong place could lead to a very sudden and fatal discharge through the driver – something that is less likely to happen with a conventional battery pack. So, if they are used, supercapacitors will need much more protection to prevent accidental discharge.
Self discharge is also a problem; it is much higher than with a battery. An array of supercapacitors could not be used to hold the bulk of an electric vehicle's energy because it would need to be topped up regularly when parked for long periods.
At the other end of the power scale, developers are investigating using supercapacitors to provide peak currents in handheld devices, such as in mobile phones and wireless modems during transmission or polling. Supercapacitors are likely to be a good match for the kind of 'bursty' power demands of such intermittent high current demands. Their ability to reduce the peak power demands on the chemical battery are also likely to extend its lifetime, a critical factor in consumer electronics devices, although there is a clear tradeoff between the energy lost through self discharge versus the problems caused by continual battery charging.
Improvements in materials science and nanotechnology are not just likely to lead to better forms of individual capacitor families, but also point to a possible crossover between different technologies. For example, improvements in polymer and glass composite design could lead to capacitors that have the high volume production advantages of a ceramic construction, but with the properties associated with an electrolytic. Despite their humble appearance, there is still plenty of evolution to come in the world of the capacitor.