Using ferroelectric domain walls for active control of heat flow at the nanoscale

In order to satisfy societal demand for continual improvements in microelectronic device performance, there is an ongoing drive for transistor miniaturisation so that spatial packing densities can be maximised. However, the associated increases in operational power density leads to increased heat generation and rises in on-chip temperature that can prevent reliable device performance. This represents a tremendous technological challenge and there is a clear need to identify and characterise materials with novel thermal properties that will enable superior thermal energy management at the nanoscale. In particular, the ability to actively control heat flow with an external stimulus (e.g. voltage) could have dramatic implications for the thermal management demands and lifetimes of next generation microelectronics. In this regard, oxide ferroelectric materials present an exciting opportunity.

In ferroelectric materials, there exist atomically sharp structural interfaces called 'domain walls' (DWs) that are known to impede heat-flow by disrupting thermal vibrations. What is unique about DWs is their remarkable ability to be created, erased or repositioned inside the material in a fully reversible way by using applied voltages or pressure. This property provides an unprecedented means to actively control heat flow by being able to alter the number of DWs present in the material at a given time and the way in which they are arranged. However, to realise heat flow control using DWs, definitive estimates for the thermal interfacial resistance presented by DWs in different materials must first be determined. Therefore, one of the main goals of this project is to quantify DW thermal resistances through direct thermal conductivity measurements. Ferroelectric material systems having DWs that effectively inhibit heat flow will then be identified. Following this, prototype thermal devices will be fabricated where the relative ease of heat flow through the material will be changed by using applied voltages to reversibly alter the DW pattern. This will also provide the foundation for a longer-term research vision to create a more exotic nanostructured 'thermal mirror' device. In this case, it is envisaged that DWs can be engineered to behave as periodic reflectors of thermal waves in order to maximise the rejection of thermal energy, much like how light is reflected with high efficiency by the multiple layers in a dielectric mirror.

Over the last decade, it has become clear that DWs can be considered as a new type of sheet-like functional material with properties that can be remarkably different than bulk. For example, electrical conduction within DWs can be metallic, or even superconducting, when the bulk is comparatively insulating. Prototype active devices have been fabricated where functionality is derived entirely from deployment of electrically conducting DWs. However, the complementary idea that the narrow DW region may have thermal properties entirely of its own is completely new and unexplored. Within conducting DWs, it is likely that heat flow will be enhanced, due to the availability of extra heat carriers (e.g. mobile electrons), and thermal conductivity measurements will be carried out to confirm this. Conducting DWs will also be explored for conversion of waste heat into electricity since recent predictions indicate that the thermoelectric power can be enhanced by up to 100% within DWs, compared to bulk.

Overall, ferroelectric DWs are exciting candidates for use as the active elements in thermal devices since the DWs may behave functionally to either enhance or restrict heat-flow. However, neither case is currently well characterised nor understood. The innate reconfigurability of these DWs means there is real potential to design and build new types of active thermal devices based on ferroelectric materials that has yet to be capitalised upon.

Knowledge Impact: This is a cutting-edge research programme investigating how reconfigurable structural elements, called domain walls (DWs), can be leveraged to manipulate the flow of heat in nanostructured ferroelectric materials. In recent years, computational modelling capabilities have come of age and started to reveal the potential for DWs to influence nanoscale thermal transport. However, definitive experimental testing of these predictions is lacking and will be directly addressed by the planned research. This will be of high interest to the ferroelectrics community, where there is already substantial activity surrounding the functional properties of DWs in recognition of their potential for use in fundamentally new types of active device. Benchmarking the fundamental thermal properties of DWs will be a short-term impact and demonstration of working prototype devices, where the effective thermal response is tuned according to DW type, orientation, and density, will deliver impact in the medium-term. In the longer-term, attempts to create an artificial phonon band gap using a fundamentally new approach involving DWs is expected to be of tremendous impact to the broader phononics community. Throughout the fellowship, new and existing methods for investigating nanoscale thermal properties will also be developed (e.g. Scanning Thermal Microscopy) and this is expected to be of substantial value to both the functional materials academic community and industry for applications well beyond those described in this proposal.

Economy Impact: The EPSRC strategy document "Materially better: Ensuring the UK is at the forefront of material science" states that there is an "urgent need to deliver a portfolio of research that will underpin and accelerate the development of new, advanced materials" in order to help secure the UK's economic growth and prosperity. While the proposed research is largely fundamental in nature, the potential for new disruptive technologies will be assessed through fabrication of prototype active thermal devices. There is also potential for economic impact though development of new nanoscale thermal characterisation techniques; the PI is supported by an EPSRC grant to develop scanning probe microscopy instrumentation in conjunction with Asylum Research and these close ties could potentially be explored for routes to commercialisation, if necessary. In general, every effort will made to identify and protect any research findings with commercial potential and we will also work closely with QUB's dedicated support team, which specialises in IP protection and mechanisms for commercialisation.

People Impact: The PDRAs and PhD students associated with this activity will benefit from a rigorous programme of training in nanoscale functional materials that includes advanced fabrication methods, scanning probe microscopy techniques and use of low-temperature systems. This will be enabled through access to the combined knowledge base and facilities of the Centre for Nanostructured Media and the industry focused ANSIN advanced materials hub. Following this training, the PDRAs/PhDs will be well positioned to take up research posts in internationally competitive materials laboratories. They will also be attractive candidates to regional high-tech employers such as Seagate, Andor and Intel Ireland.

Society Impact: The scientific insights generated from this programme could lead to fundamentally new ways to actively manipulate heat-flow in device applications and could therefore improve quality of life for end-users and also benefit the environment. This is particularly relevant for the microelectronics industry, where innovative ideas for thermal design of components at the nanoscale have become highly important. There is also the possibility of applications for active temperature regulation at a more blue-skies level e.g., in smart-buildings and in human survival suits.