Ceramic powders and bulk discs of KBNNO ((K,Ba)(Nb,Ni)O3) and KNBNNO ((K,Na,Ba)(Nb,Ni)O3) have been successfully fabricated using solid-state reaction. The physical properties and microstructure have been characterised. Dielectric, ferroelectric, piezoelectric, pyroelectric and photovoltaic properties of the ceramic samples have been measured. A perovskite-structured photo-ferroelectric composition, (K0.49Na0.49Ba0.02)(Ni0.01Nb0.99)O2.995 has been discovered for the first time ever. Such a composition is able to co-exhibit a narrow band gap and strong ferroelectricity. Conventionally, these two properties were not found to exist simultaneously. The combination of Ni ions and oxygen vacancies has been proved crucial for obtaining narrow band gap and strong ferroelectricity simultaneously. The concentration of the combination of Ni ions and oxygen vacancies should be kept as small as possible, in order to maintain the strong ferroelectricity.
A series of optoelectric and opto-ferroelectric measurements, including the ferroelectric characterisation with illumination, attempt of poling the KNBNNO sample solely by illumination (in absence of an external electric field) and investigation of the influence of polarization on photovoltaic performance have been carried out. A time-dependent, photo-induced domain switching phenomenon has been observed for the first time. Using the KNBNNO, the phenomena of all-optical domain control under illumination, visible-range light-tunable photo-diode/transistor phenomena and opto-electrically tunable photovoltaic properties have also been demonstrated for the first time. As most of the conventional photo-ferroelectrics have wide band gaps, these phenomena were not observed earlier. With the KNBNNO monolithic material, tuning of the electric conductivity independent of ferroelectricity has been achieved, which previously could only be achieved in organic phase-separate blends. Guided by these discoveries, a boost of 5 orders of magnitude in the photovoltaic output power and energy conversion efficiency has been achieved via optical and electrical control of ferroelectric domains in an energy harvesting circuit.
KBNNO and KNBNNO thick-films have been made via laser machining. The thick-films have been attached on stainless steel substrates providing cantilever-structured multi-source energy harvesters. A comprehensive evaluation system has been built, consisting of vibration, heat and light sources as well as data acquisition equipment. The fabricated multi-source energy harvesters have been tested with individual and multiple energy sources. The influence of strain on photovoltaic performance has also been investigated. Neither recognisable effect of strain on photovoltaic output nor degrade between different energy conversion schemes has been observed. Therefore, the KNBNNO ceramics have been proven to be able to simultaneously convert kinetic (e.g. vibration), thermal (e.g. temperature fluctuation) and solar (visible light) energy into electricity. This makes the KNBNNO truly feasible for multi-source energy harvesting applications based on only one piece of material. Such a route is fundamentally new compared to the conventional approaches or even recently developed hybrid energy harvesters.
In terms of the exploitation, the research results obtained in this project has gained many opportunities for both further scientific study and industrial application. Based on the results, two other research proposals which are competitive in higher-level international or national research funding applications have been drafted. The results will also be developed to commercial energy harvesting solutions with industrial partners focusing on smart wearable devices and/or smart infrastructures.
