Hot properties

NASA is backing a project to discover new high-temperature piezoelectric materials that could be used to control fuel flow in gas turbines, or act as sensors in jet engines and oil drilling equipment.

Piezoelectric materials (PZs) change their shape when subjected to voltage, and are often used as sensors, accelerometers and actuators. But most known PZ materials will only work below a certain temperature.

US company TRS Technologies and Penn State University hope to find materials suitable for use as PZ actuators to control fuel flow in gas turbine engines, which would ‘dramatically’ increase efficiency, said TRS research and development director Dr Paul Rehrig.

This type of PZ actuator would have a number of aerospace, automotive and marine applications, for example in advanced space propulsion systems, as down-hole sensors in oil drilling, or actuators for morphing aircraft.

‘There are no very high-temperature PZs available,’ said Rehrig. ‘The trend with known materials is that as the temperature rises the PZ performance drops.’

PZs change their crystalline structure and properties at a certain ‘Curie’ temperature, via a phase transition. The key is to find a PZ material with a high Curie temperature. The first avenue the team hope to explore is growth of ‘textured’ ceramic PZs, where grain orientation is controlled on a micron-scale.

‘Most piezoelectric ceramics are polycrystalline materials with randomly orientated grains in the order of a few microns. You can get better properties from a bulk single crystal, but they are expensive and difficult to grow,’ said Rehrig.

‘An intermediate step is a textured ceramic material, which is still polycrystalline but the grains are orientated to get single crystal properties,’ he said. ‘If all the grains are orientated in the same direction, you get a better push when a voltage is applied, increasing the material’s capability.’

The team will also try to grow single crystal ceramic PZs that can only be grown as polycrystallines in conventional furnaces. Single crystals are difficult to produce because it is necessary to grow the material at high temperatures and so much of the crystal is vaporised during growth, said Rehrig.

‘We will also examine different ways of doping the material,’ he added.

‘The composition is also important. The ratio of materials within the PZ can be manipulated to give the highest performance.’

The team first plans to look at materials in the strontium niobate family, such as lanthium. ‘The PZ properties are an order of magnitude lower compared with high-performance materials, but you still get something out. We’re hoping to maximise that.’

The NASA-funded project starts in January. In the UK, Rolls-Royce is leading research in the high-temperature PZ field as part of a separate, e6m (£4m) Europe-wide collaboration on advanced actuation called ADVACT, which was formed earlier this year.