A project under the European Union's Seventh Framework Programme has been completed by the Knowles Capacitors R&D unit based in Norwich, England, together with the UK’s National Physical Laboratory and Euro Support Advanced Materials of the Netherlands. The programme was to develop a lead-free ceramic dielectric, with high energy density and reduced losses, for high temperature applications.
Hybrid and electric vehicles rely on high efficiency power conversion and management, with automotive power electronics representing an emerging £40bn global market. In today’s vehicles, power electronics in energy storage and conversion systems require complex cooling systems, as existing capacitors cannot tolerate very high temperatures. This then adds weight to the vehicle and shortens energy storage/conversion systems lifetime which, to the user translates into low mileage range and increased maintenance costs. Similar challenges are also faced by aerospace, oil and gas and semiconductor sectors, which technologies and reliability relies on high temperature electronics.
The major limiting factors for the use of ceramic capacitors in power electronics are size and temperature stability. Ferroelectric ceramics very often exhibit high energy density but fail to provide stable and efficient performance at high temperatures. The research, therefore, concentrated on finding a new BiFeO3-SrTiO3 ceramic composition that would yield a lead-free multilayer capacitor design. A design that would exhibit what the company claims to be unprecedented low dielectric loss and high thermal stability whilst maintaining high energy density and fast discharge rates up to 200°C – and which ultimately could be used in real applications.
The conclusion of the research resulted in a breakthrough on the development and characterisation of two lead-free compositions based in the BiFeO3-SrTiO3 system and a manuscript entitled Lead-free ceramics with high energy density and reduced losses for high temperature applications has been submitted, and accepted, by Advanced Engineering Materials for publication.
The research team explained the essence of the manuscript: "Our results indicate that our capacitors are characterised by a very high energy density and an impressive low dielectric loss which will reduce capacitor self-heating under a.c. operation conditions. These ferrite-based capacitors are also characterised by very attractive thermal performance, where small variation of capacitance from room temperature up to 200°C, which combined with their remarkable energy storage properties, makes them very attractive for high temperature applications. We believe that our findings in developing a new capacitor with superior performance at high temperatures, yet cost competitive with existing X7R capacitors (125°C maximum temperature and price ~ $0.50), will instigate a new era of high temperature power electronics and pave the way to the stabilisation of Low Carbon markets. These unique properties clearly outperform other state of the art ceramics and potentially create significant new markets for high capacity ceramic capacitors at high frequencies and high temperature operations."
Work now begins at Knowles Capacitors to convert these findings into new product offerings. The research received funding from the European Union's Seventh Framework Programme (FP7/2007-2013) for the Clean Sky Joint Technology Initiative under grant agreement n°641553. The funding was aimed at better exploiting research capacities in Europe and transforming scientific results into new products, processes and services. It had a global budget of €52.52bn.