Material of the month: Lead zirconate titanate
Lead zirconate titanate, often referred to as PZT, is a man-made ceramic. It has many uses, most commonly as a piezoelectric ceramic.
Piezo is the Greek word for pressure, therefore piezoelectricity describes pressure electricity. It is an unusual property, characteristic of materials that have complicated crystal structures with a low degree of symmetry, for example sucrose and barium titanate.
The piezoelectric effect was discovered by brothers Pierre and Jacques Curie in 1880. When external forces are applied to a piezoelectric specimen, polarisation is induced and an electric field is established across the specimen. The direction of the field is changed by applying tension instead of compression, or vice versa. This is the direct effect, discovered by the Curies. The inverse effect is the introduction of mechanical strain by applying an electric field, and was deduced theoretically by Gabriel Lippman in 1881 and then confirmed by the Curies.
During World War II, various independent research groups discovered a new class of man-made materials called ferroelectrics, which exhibited higher levels of induced charge per unit of force applied than naturally-occuring materials. This research led to the development of piezoelectric materials with specific properties for particular applications. In 1952, physicists at the Tokyo Institute of Technology developed PZT. Compounds based on PZT are manufactured by combining lead, zirconium and titanate under extremely high pressure and removing the particulates with a filter.
In the context of piezoelectricity, the Curie temperature – Tc – of a material is the temperature above which the material loses its piezoelectric characteristics. The Tc of PZT is more than 200°C, making it a favourable piezoelectric material.
Ultrasonic transducers convert an electrical pulse into a mechanical vibration that is transmitted as ultrasound, and they also perform the reverse operation. They are used in medicine (for example prenatal care) and in industry (for example nondestructive testing). PZT is efficient in reception and transmission and therefore is a popular choice of transducer material.
Ultrasound is also used in medical therapy. Ultrasonic hyperthermia is a cancer treatment where PZT transducers are used to focus heat on tumours. The temperature of the tumour is increased to about 43°C – killing the cells.
Ceramic resonators made from PZT are used in timing circuitry in a vast array of products, including televisions. They are often used in place of quartz as reference clocks in electronic circuitry because they are cheaper and smaller. They are less accurate but frequency accuracy is not critical. These resonators replaced LC circuits in radio receivers because of their higher quality factors and immunity to stray magnetic fields.
PZT is also pyroelectric, which means a voltage difference develops across a piece of the material when it experiences a temperature change. This makes it a suitable material for heat sensors. Despite its very useful properties, alternatives to PZT are being sought because it is more than 60 wt% lead. There is a growing concern regarding the toxicity of lead, driven by the Restriction of Hazardous Substances Directive. Research in the development of environment-friendly piezoelectric materials is ongoing.