16.30 High Frequency Ceramics

Ceramics uses at microwave frequencies include resonators to stabilise oscillators and narrow bandwidth filters. The simplest versions are metalised patterns printed directly on substrates that resonate at particular frequencies. Such resonators on alumina substrates are not very selective and much higher dielectric constant (k) is needed to increase the selectivity (Q) and reduce the size of the resonator components. Using a substrate with a k of 38 reduces the size by a factor of 2 compared with alumina. The characteristics required of the ceramic are high dielectric constant, low dielectric loss and low temperature dependence; however, it is very difficult to find a material to meet all these requirements. Some complex compounds have been developed such as a high-order version of barium titanate with a k of 40 and Q of 2,000 at 20 GHz, but it is somewhat temperature sensitive. There are several alternatives such as barium tin magnesium tantalate with a k of 25 and Q of 10,000 at 20 GHz or neodymium barium bismuth titanate with a k of 90 (making devices smaller) but only practical to a few GHz. 

Rather than have the substrate made of high k material, a separate ceramic resonator can be attached to a conventional substrate. There are two forms of such discrete piezoelectric resonator that are based on standing-waves; firstly coaxial, that are hollow cylinders covering frequencies from 400 MHz to 4 GHz (typically 6mm outer diameter and 15mm long at 950 MHz); and secondly disks covering frequencies from 1 GHz to 100 GHz (typically with a diameter 2.5 times their thickness, with a diameter of 50 mm at 1 GHz and 2.3 mm at 21 GHz).

Microwave (0.3 to 30 GHz) and millimetre wave (30 to 300 GHz) bandpass filters are made using ceramic resonators that have a Q of as high as 1,500 at 94 GHz. Applications include filters for 12 GHz satellite TV amplifiers as well as 400 MHz to 2 GHz filters for mobile phones and digital radios.

Another important application of alumina is in klystrons and magnetrons, which are the means of generating high frequency power. The ceramic has to have very good insulation to withstand the high voltages, but also needs to be transparent at microwave frequencies, low loss, high thermal conductivity, high strength and able to be joined to metal. Examples are a 1.3 GHz klystron for a radar transmitter with a peak power of 30 MW, and a more down to earth one for microwave ovens, having 1.2 kW power at 2.45 GHz. For higher power applications, beryllia, with its very high thermal conductivity, can be used.

16.31 Wearable Electronics and Optics

Ceramic materials are also being applied to clothing, so sensors, electronics and optics can become wearable. There are already aerospace applications of doped PZT formed into fibres that are incorporated into carbon fibre/polymer composites as piezoelectric sensors. The brittleness of the ceramic is overcome by embedding it as very fine fibres in the polymer. Health monitoring applications, for example detecting vibrations such as heartbeat and breathing rate, can utilise special shirts that might also activate drug delivery. Scarves or belts could also be used to send information to a remote receiver. In particular, uniforms for occupations such as firemen could provide information to them on their safety and security and to others on their health and environmental conditions. Clothing incorporating woven optical fibres and micro devices is also being developed.

16.32 Nanotechnology

Nanotechnology was mentioned earlier regarding nanotubes, but it is a huge area receiving worldwide attention and billions of pounds of research and development investment at present. The usable characteristics of minute nano particles are still being revealed, and they show great potential both in powder form and for devices. For example, ceramic nanofibres are being used to permanently lock away radioactive ions in waste water. Nanomaterials as powders are used extensively in cosmetics, coatings and sunscreens typically using titanium dioxide or zinc oxide.

The overall global nanotechnology market in 2010 represented $15.7 billion and is expected to reach $27 billion by 2015. Nanomaterials make up the lion’s share of around 70%, with nanodevices growing from $35m to $234m over this timescale.