Electroceramics can be defined as ceramic materials which are able to perform an electronic function for a particular application. This rather wide umbrella-term includes materials for a wide range of applications, of varying complexity, from relatively simple insulating materials, to complex ferroelectrics. The materials for such applications are usually prepared from specifically formulated compositions (typically not found in nature), and processed under strictly controlled conditions. Some of the materials and applications are described below:
Dielectric (Insulating) materials
Most ceramics, and indeed electroceramics, are electrically insulating in nature; they resist the flow of current and are able to separate charge as a result. This allows ceramics to be used:
- as simple insulating materials, e.g. Al2O3
- in capacitors (e.g. BaTiO3 ), where temperature stability/reproducibility are important
- as printed circuit board substrates (e.g. BeO, Al2O3 ), where thermal conductivity is a major factor
- as microwave resonant cavities (e.g. ZrTiO4 ), facilitating both the reduction in size and increase in available lines for satellite and mobile phone communication systems.
Ceramic materials can exhibit changes in properties with a number of external factors. This principle can be used to facilitate detection of environmental changes, including both temperature and atmosphere.
Ceramic gas sensors based on zirconia* are able to detect a number of the gases such as CO2 , SO2 , and NOx that are of current environmental concern. By monitoring oxygen levels, they have also been used for the regulation of air/fuel ratio in engines.
Ceramic temperature sensors can detect changes in temperature by changes in their resistance. Such changes may be as a result of a continuous intrinsic change, such as increased ionic conductivity with increasing temperature, or a change in crystallographic structure. Positive temperature coefficient (PTC) resistors are based on BaTiO3 materials. Applications include temperature sensors for electric motor protection, liquid level and air flow, self regulating heaters in cars and appliances, and current-switching devices in refrigerants and air conditioning systems.
*Zirconia-based materials were found to exhibit high oxygen ion conductivity at elevated temperatures by Nernst in 1900. The zirconia is 'stabilised' at elevated temperatures by the addition of around 8mol% yttria. The yttrium ions substitute directly for the zirconium ions. This produces an electronic imbalance that is compensated for by the production of 'vacancies' in the lattice. It is the presence of these vacancies that results in the oxygen ion conductivity in this material.
Typical ferroelectric ceramics include lead zirconate titanate (PZT) and barium titanate. They contain electrically polarised domains in many ways analogous to the magnetic domains present in ferromagnetic materials such as iron. In much the same way as magnets can be poled by applying a strong magnetic field, applying an electric field can align ferroelectric domains. High proportions of these domains then remain aligned when the electric field is removed.
If the material is operated along the linear part of its hysteresis loop (see diagram below), then the piezoelectric effect is observed. This can be either dimensional change resulting from the application of an electric field, or the generation of potential difference through the application of compressive/tensile stress. Such devices are used as spark igniters, positional sensors and actuators, accelerometers, transformers, and sonar/ultrasonic imaging equipment.
If the material is operated past this linear region, with the application of an electric field to the point switching the direction of polarisation, then the device can be used as a kind of memory, remaining in this polarisation state until switched again.
In addition to the ferroelectric response to electric field and mechanical loading, modified compositions can also respond to polarised light (electro-optic) and heat (pyroelectric) for a variety of switching and imaging applications.