Wartsila 5kW fuel cell prototype

Another smaller example, of a few kilowatts for residential use, is based on a fuel cell generator that also uses natural gas as a fuel. One design uses the more commonly available ceramic cerium gadolinium oxide as the electrolyte as it reduces the processing temperature from 800 ºC to 500 ºC. This allows very thin layers of anode, electrolyte and cathode to be deposited onto a stainless steel substrate, reducing the material cost appreciably.

Batteries are very similar to fuel cells, but whereas the fuel is continuously provided in fuel cells, it has to be stored as chemicals in batteries. Ceramics are also needed in advanced battery systems that often use aggressive chemicals such as the “sodium/sulphur” batteries that operate with molten sodium at 350 ºC. The sodium is kept separate from the sulphur by a sodium aluminate ceramic membrane electrolyte that is a sodium ion conductor. Recent developments make the battery safer by operating at 98 ºC so that the sodium remains solid. It can be used for storage of electricity from solar or wind generators in the home, present models providing 5kW for 4 hours. The energy content of a sodium/sulphur battery is about 4 times that of a lead/acid battery. Lithium batteries have become very popular, some of which use a manganese cobalt oxide cathode.

Commercialisation of fuel cells began in 2007, when some 12,000 units of various sizes were shipped, rising to 18,000 units in 2008. A major development area for fuel cells and batteries is for electric and hybrid automobiles.

Components of a GM fuel cell and a
depiction of a fuel cell powered car

Other current applications of fuel cells include relatively small examples such as those for powering caravan appliances. Fuel cells or batteries are also used as uninterruptible power supplies for telecoms, to power road signs and in fork lift trucks, and ones generating a few watts are used in mobile electronic devices. An interesting “room temperature” application is a proposed battery made of sodium and bromine separated by a sodium aluminate membrane to power a heart pacemaker.

16.24 Temperature Sensitive Resistors 

Resistors that are based on their change in resistance with temperature are called “thermistors”. Thermistors formed from spinel crystal structures were originally used for temperature compensation. They also have the potential to be used for thermal imaging.

Other ceramic resistors developed around 1955, called “positive temperature coefficient resistors”, are based on the abrupt change of resistance from low, below the Curie temperature, to high above it. They are based on ferroelectric ceramics such as barium titanate or barium strontium titanate (BST) that are doped so they become semiconducting. The phenomenon is boundary dependant so not found in single crystal material. Applications include over-temperature protection, level control, self-regulating heaters and current limiters.

16.25 High Thermal Conductivity Ceramics

Beryllia (BeO) has a high thermal conductivity, second only to diamond in non-metals. It can be used to aid the removal of heat as the mounting for power electronics, in rocket and jet engines, in thermal grease, as a moderator and neutron deflector in nuclear reactors and in high performance magnetrons and gas lasers. Silicon carbide and silicon nitride have relatively high thermal conductivities so have uses in high-temperature heat exchangers and can be made into thin films.

LED array with Aluminium Nitride heat sink
- source Fraunhofer Institute

16.26 Pyroelectric (Thermal) Ceramics

Pyroelectricity is based on two effects, firstly the temperature dependence of the dielectric constant of all dielectrics, and secondly the temperature dependence of electrical polarisation of ferroelectric materials - both single crystal and poled polycrystalline ceramics. The latter effect can be very high in ferroelectric ceramics. They are often highly tailored, typically containing as many as six different oxides designed to produce a rapid change in effect for a relatively small change in temperature.