Chemical control of function beyond the unit cell for new electroceramic materials

Discovery and development of advanced materials requires understanding and control of the relationship between composition, structure and function. In crystalline materials, there is considerable focus on a design process that is informed by a single macroscopic structure defined by the average crystallographic unit cell determined by Bragg diffraction. This is a powerful approach, but it has become increasingly apparent that local chemical and positional deviations from this long-range average view of the structure can have decisive effects even in crystalline systems. Charge stripes in the high temperature superconductors and the role of "panoscopic" order spanning meso- to nano-scopic length scales in thermoelectric performance are just two examples of the limitations of average structure considerations in explaining how an apparently small compositional change can transform functional behaviour. This in turn restricts the utility of such a view of structure in designing new materials with enhanced performance.

This is particularly critical for the many functional materials in which modulation or switching of a ferroic order parameter (i.e., polarization or magnetization) by a stimulus such as an applied field produces the property (e.g., piezoelectricity or magnetoresistance) used in devices (e.g., actuators or data storage). Their properties are optimised by formation of solid solutions e.g., in PbZrO3-PbTiO3 (PZT), responsible for >90% of piezoelectric devices, the Zr/Ti ratio is adjusted to coincide with the boundary between rhombohedral and tetragonal symmetries, at which the piezoelectric charge coefficient maximizes. There is competition between the randomising effect of the local configuration of Zr and Ti cations (which occupy the same position in the average unit cell, but locally exert quite different influences on the displacements producing the polarisation) and the effect of the long-range dipolar and elastic interactions favouring the average polarisation direction. This local structure effect and the finite size correlations it produces exerts decisive control of function that is invisible from the average structure central to traditional design. The properties of the solid solutions are thus not an average of the end members, and simple design rules do not exist.

The project team have recently shown how design based on quantitative local structure analysis can afford materials families with important properties that had not been accessed by classical average structure design approaches. Using nanoscale information from total Bragg scattering studies to control properties, they identified chemistry that would have been disregarded based on the average structure but led to a new lead-free piezoelectric family (Advanced Materials 2015) and then to the first bulk room temperature ferromagnetic ferroelectric multiferroic (Nature 2015): combination of these two long range orders in a single phase has been a longstanding scientific challenge.

This project will develop the control of function by understanding and manipulating symmetry and structure beyond the unit cell length scale. We will build a toolkit that enables this approach by combining solid state materials chemistry, materials science and condensed matter physics to integrate synthesis, crystal chemistry, crystallography, local structure analysis, scanning probe microscopy, magnetism, electroceramic measurement physics, and materials processing. The toolkit exploits the synergies between the skills of the two participating groups.

By designing then preparing new piezoelectric and multiferroic materials, we will demonstrate how this approach can guide synthesis for function, with ramifications for control of properties beyond the exemplar areas studied, for example in heterogeneous catalyst and electrode (fuel cell, battery) materials, contributing to the EPSRC Physical Sciences Grand Challenge of Nanoscale Design of Functional Materials.

The novel approach to materials design will produce new families of functional electroceramics as components of current (piezoceramics) and future (magnetoelectrics and multiferroics) technologies. The multiple length scale understanding will enable the design of materials across a much broader space of functional and structural materials than the initial focus area of electroceramics. The failure to find a lead-free PZT replacement despite extensive worldwide effort emphasises the need for this new approach.

Applications requiring piezoelectric and multiferroic behaviour are diverse and include vibration cancellation, energy harvesting, precision motion and control of machinery, sonar and medical imaging, valve operation, spintronics and data storage in support of developments in low power and portable computing and ICT. The most immediate impact will be in the global piezoelectric devices market which is expected to grow to USD 27 BN by 2020 and represents a significant portion of the total electroceramics market which includes products such as ceramic capacitors (2015 global market USD 10 BN). At the forefront of new piezoelectric devices and materials is our industrial partner Ionix Advanced Technologies, who develop high Curie point materials to address needs in monitoring and measurement at elevated temperatures beyond the capabilities of established materials. Other partners include CeramTec (UK's largest manufacturer of piezo ceramic materials and devices, with interests in multiferroics), Xaar (piezo-driven printing heads), and Thales (sonar devices and multiferroics). The magnetoelectric and multiferroic materials offer longer-term impact by enabling post-CMOS lower energy information storage and processing technologies, which are needed to tackle predicted growth in energy use associated with the Internet of Things, server farms and increasing demand for low power portable electronics.

We will update industrial project partners on materials and technique development by regular discussion, site visits and attendance at project review meetings. The broader industrial community will be engaged via the Knowledge Centre for Materials Chemistry (KCMC), which has a national knowledge transfer role executed through Innovate UK & KTN and supported by a strong industry steering group, ideally placed to identify opportunities for the application of the new materials design methodologies.

The importance of reducing and removing toxic elements such as lead from the supply chain for piezoelectric devices is reinforced by regulation (RoHS, WEEE and REACH). Substitution will benefit raw materials mining through to disposal and recycling, lowering the environmental burden and potential damage to human health. The materials design techniques will help address critical raw materials substitution.

The team have extensive experience in protecting and deploying IP e.g., AB has taken fundamental research to spinout of Ionix Advanced Technologies. The IP and capabilities from the project will be promoted to the partner companies and wider industrial networks through the KCMC KT team and partner University business gateways.

The scientific outputs span the key areas of materials science, solid state chemistry and crystallography, and condensed matter measurement physics. Researchers in these and other fields will benefit from the research results disseminated by high impact publications, conference presentation and networking, where the team have a strong track record.

The project will produce high quality researchers with a multidisciplinary perspective, beyond that of the typical research area of any one group and shaped by engagement with the project partners.

The user engagement strategy will include organisation of events for the relevant technical community e.g., the February 2016 KCMC Future Materials event in London which led to new relationships between the academics and industrial partners for this proposal.