The usefulness of surface wave devices is the relative simplicity of designing interdigital metallic fingers that can generate and receive complex signals, which are used, for example, as resonators in the remote operation of garage doors. They can also be used in radar processing (such as pulse compression to improve signal-to-noise ratio), and for frequency filters such as intermediate frequency (IF) filters in television sets. These devices are practical at low power levels up to 1 GHz.

Diagram of surface acoustic wave device

Surface wave devices: three from the 1960’s
including the first TV IF filter in a Heathkit TV;
two ceramic microwave circuits and a modern
liquid density sensor from Durham University

16.17 Transparent Ceramics

Translucency is achieved in a ceramic when the refractive indices of the final constituents are very nearly the same, and transparency when they are the same, as with amorphous glass. So porcelain that is made up of mullite crystals, glass and a few air voids is translucent because the refractive indices are close. Earthenware has a lot of air pores making it opaque. However, if these air pores are eliminated in an otherwise opaque ceramic, it can become translucent, and an example is the almost transparent Lucalox ™ that is a pure alumina ceramic without pores.   

Ceramics are used as windows across the electromagnetic spectrum, from microwave to infrared, visible, ultraviolet and X-ray, often also as a physical barrier. The most common, of course, is normal window glass that transmits in the visible but also in the near infrared, which can be a disadvantage by causing overheating in buildings. It also forms a barrier against the weather and noise. Similarly, coloured glass restricts transmission to a narrower range of the visible spectrum.

Spinel laser window and Murata lumicera ™
ceramic camera lens

Transparent single crystal ferroelectrics such as barium titanate and gadolinium molybdinate have been used as electro-optical materials for some time. However, until the discovery in the1960’s that lanthanum enhanced the transparency in PZT forming polycrystalline PLZT, no polycrystalline ceramic was known that was both transparent to visible light and highly sensitive to an electric field. Subsequently, advanced polycrystalline ceramics have been developed that are fully dense (no pores) and have been tailored to have no light scattering at grain boundaries, and they can achieve exceptional transparency.

Transparent ceramics need purer raw materials than most electroceramics. The precipitation method is used to achieve very pure small particle sizes of around 0.03 to 0.1 microns. These are dried, calcined at the lower temperature of about 500 ºC for one hour, and ball milled using alumina or zirconia balls. The resultant powder can be used for cold pressing, extrusion, slip casting, tape casting, screen-printing or injection moulding. The densification of the powder into a pore free, fully dense ceramic body is extremely critical to achieving a high quality product. Firing can be by sintering or hot pressing. Sintering is cheaper and can achieve 96% of theoretical maximum density and even higher with additives and control of the atmosphere. As an example the preparation of transparent PZT requires calcining at 900 ºC and sintering for 5 hours at 1250 ºC.

Hot pressing is a more reliable method of firing these ceramics consistently. Some transparent electro-optic materials, such as PLZT, are usually made by hot pressing where an appropriate quantity of the prepared material is placed in a silicon carbide mould and heated at 1,200 to 1,250 ºC for 5 to 18 hours at 2,200 to 5,800 psi (15 to 40MPa). This method can produce highly transparent, fully dense optical quality disks up to 6” (15 cm) diameter and 1” (2.5 cm) thick.