Ceramics and porcelain are some of the oldest examples of material technology: when clay is baked under suitable conditions, it turns into a hard, strong, and heat resistant substance.

Because they are hard, and remain hard at high temperatures, ceramics are used in modern times as abrasives and cutting tools. And as they are extremely heat resistant, ceramic tiles are used in the lining of metal refineries, and of the nose cones of space shuttles, which bear the brunt of the damage as the shuttles return to earth, generating extreme heat due to friction with the atmosphere.

Chemically, ceramics are oxides, and as such are usually extremely poor conductors of electricity. Thus they are used as insulators in high voltage transmission lines.
But ceramics gained sudden fame a few years ago in the world of physics, when some of them were found to be — to everybody's utter surprise — not insulators at all, but superconductors. In a superconductor, an electrical current can flow without being pushed by a voltage.

Ceramics have other properties that make them useful. In many ceramics, the centres of positive charge and negative charge are separated, exhibiting an electric `dipole moment'. By applying an external electric field, the dipole moment can be turned one way or another, and will stay there when the external field is switched off. Ceramics which can remember the external electric field are called ferroelectric ceramics and can be used for developing computer memory devices. These ferroelectric ceramics are also `piezoelectrics': they deform when an electric voltage is applied across them. A rapidly changing voltage will lead to a rapidly changing deformation, which can then generate an ultrasonic wave in the fluid in which it is immersed, or in the air in which it is exposed. These piezoelectrics are therefore key elements in ultrasonic cleaners and toxic gas sensors. Some other ceramics are transparent, and have good electro-optic properties. They are useful for making optical switches and infrared sensors.

In many of these applications, the ceramics are in the form of thin films, a layer with a thickness under 1/10,000 of a centimetre. A thin kite flies when driven by the lightest wind, and can also flutter rapidly when suitably disturbed. So in the same way, thin films have the decided advantage that they can be driven into motion by very low voltages, and when suitably driven, can respond at high frequencies. For example, when an electric voltage is applied across some ferroelectric ceramics, the refractive index of the ceramics is modified and they can be used to make optical switches. Scientists have demonstrated that with a driving voltage of under 5 volts, these ceramics can perform switching at a rate of over a billion times a second, which would be extremely useful in optical communication.

The challenge in research on ceramic thin films is to find better ways of making these films, to characterize the structure of the thin films, and if possible to relate the properties to the structure, and the structure to the process of making the film.

The Physics Department at The Chinese University of Hong Kong has several research projects which study modern materials of technological importance. The study of ceramic thin films by Dr. H.K. Wong and his students has been going on for a number of years, and won competitive funding from the Research Grants Council in 1990. Some significant results were recently published in the prestigious Applied Physics Letters and the Journal of Applied Physics.

Dr. Wong and his students have set up two simple but reliable facilities to synthesize these thin films.

In the first method, high-power ultraviolet laser light is shone onto a sample of oxide (some ceramic material). The bombarded area of the oxide target is heated up to extreme temperatures in a very short time, and the material evaporates in a plume. The plume, which consists of atoms, molecules and small atomic clusters, impinges upon a carefully polished, cleaned and heated crystal surface. The surface acts as a substrate on which the thin film is formed. Under optimized conditions, the deposited atomic species can move and adjust themselves to form an organized array with a composition nearly identical to that of the oxide target.

The second method is similar, but instead of a laser pulse, it uses argon ions to bombard the target and knock out the atoms. This is known as sputtering.

To reduce defects in thin film samples, the substrate material must be carefully selected so that the structure of its crystal lattice matches that of the material being deposited. By selecting the substrate, one can tailor the crystal orientation of the thin film; this is particularly important as the properties of ceramic thin films are highly directional.

The structure of the thin films at the atomic level is then determined by X-ray diffraction experiments. Dr. Wong and his students built the X-ray facilities in-house, at a fraction of the cost for a factory-made instrument.

The research team has by now studied a number of ceramic thin films. The ceramic Pb(Zr,Ti)O₃ is a ferroelectric, and has been deposited on magnesium oxide and spinel crystals. The material YBa₂Cu₃O_{7_x} is a high temperature superconductor, and has been deposited on sapphire and strontium titanate crystals. These thin film samples can be further processed to make devices. Nonvolatile random access memory (RAM) for computers can be made from Pb(Zr,Ti)O₃ thin films. Superconductors can be used to make superconducting quantum interference devices (SQUIDS), which are incredibly sensitive instruments for detecting magnetic fields, even brain waves. The same preparation techniques can be applied to many ceramics which have other novel properties, and which are objects of study by Dr. Wong and his team.