Conventional PZT piezoceramics – used in everything from spark generators and parking sensors to medical ultrasonics – can’t function at high temperatures, limiting their industrial applications. They’re also lead-based materials, which face legal restrictions due to their toxicity to people and the environment. In light of these issues, there is considerable interest in developing lead-free, high temperature piezoelectric materials. BiFeO3-BaTiO3 (BF-BT) ceramics are promising candidates, and researchers at the University of Manchester have been seeking to understand the mechanisms of how we can process and modify the composition of these materials to improve their properties. In work recently published in the Journal of Materiomics, they investigated the unusual technique of adding strontium as an acceptor dopant. Although the strontium disrupted the microstructure – as expected – the team found that, in small amounts, it still improved the ferroelectric and piezoelectric properties. This work demonstrates that acceptor dopants can be used to tune piezoelectric properties, and that strontium-doped BF-BT ceramics are potential candidates for high temperature lead-free piezoelectric materials.
Increasing demand for applications in piezoelectric sensors, accelerometers and transducers has created considerable interest in high temperature piezoceramics. Conventional lead-based ceramics have two important limitations. Firstly, their low Curie temperature (TC) means they can only be used at temperatures below 200°C. Secondly, they have strong toxic effects on the environment and human health, and their use is increasingly restricted.
Materials based on bismuth ferrite (BF) combine strong ferro/antiferromagnetism and high spontaneous polarization (>100 mC/cm2) with a high ferroelectric Curie temperature of around 825 °C.
Senior author Dr David Hall, from the University of Manchester, said:
For many years now, we’ve been working with Ionix Advanced Technologies, a spin-out company from the University of Leeds. Ionix uses piezoelectric compositions to generate ultrasound for high temperature applications. It’s very similar to the way ultrasound is used for medical imaging, but in this case, it’s used for inspecting structural components like cooling systems that can operate up to nearly 400 °C. There are similar applications in high temperature processing systems operating at around 580 °C. At the moment, there isn’t a lead-free material that can match that performance.
We’ve been working on a lead-free BiFeO3–BaTiO3 (BF-BT) solid solution. In principle, if you make a piezoelectric device from this compound it would operate up to very high temperatures, and then it could have applications in the real world as a piezoelectric electromechanical transducer material.
A solid solution is a mixture of two or more crystalline solids coexisting in the same crystal lattice. Many ceramics are solid solutions, produced by reaction of the constituent oxides, followed by powder pressing and high temperature sintering to form the polycrystalline ceramic. Instead of forming an ideal polycrystalline solid with a uniform composition within each grain (micro-crystal), BF-BT can have a heterogeneous core-shell type microstructure with each grain comprising a BF-rich core surrounded by a BT-rich shell. This micro-chemical heterogeneity can be a problem.
Postdoc Yizhe Li explained:
For this work, we were investigating what happens when you add strontium as an acceptor dopant for the bismuth – what effect that has on the structure and ferroelectric/piezoelectric properties. We used in situ synchrotron X-ray diffraction and digital image correlation (DIC) macroscopic strain measurements to measure the electric field-induced strain. So you're applying an electric field that generates strain, and that's essentially the converse piezoelectric effect. Ideally, the material shows a homogeneous grain structure, with each grain containing a herringbone-type pattern of polar ferroelectric domains. When core-shell features are present, the relatively inactive core regions induce a degradation of the piezoelectric properties. Diamond Light Source is a very important tool for us to understand the mechanisms that govern the functional properties of this material in detail.
Adding dopants is a trick that’s used in traditional piezoelectric ceramics. Adding donor or acceptor dopants can modify the properties of the material. Normal practice would be to add a little strontium (Sr2+) as a substitute for some of the barium (Ba2+), because they have the same charge. However, in these experiments the team added the strontium as a substitute for some of the bismuth (Bi3+), making it an acceptor dopant. They expected crystalline defects, principally oxygen vacancies, to form.
What we expected to find was that the acceptor dopant, the strontium, would produce oxygen vacancies for charge compensation, because we're not incorporating enough oxide ions with the dopant cations. And we expected that would adversely affect the ferroelectric behaviour. What we found was that there’s limited solubility of the strontium when substituted for bismuth. With 0.3% strontium, the material showed minor amounts of core-shell features. However, as that increases up to 1% strontium, the microstructure is a lot more heterogeneous and secondary phases form due to the partial solubility of the dopant. However, adding up to 0.3% strontium produces what we call a ‘softening’ effect, making it easier to switch the orientation of the ferroelectric domains. So although there was more heterogeneity in the microstructure, we achieved better piezoelectric properties.
This work demonstrates that it is feasible to use acceptor dopants to tune piezoelectric properties - instead of the more common donor or multivalent dopants. The lightly Sr-doped 0.7BiFeO3-0.3BaTiO3 ceramics exhibited a high Curie temperature (around 500 °C ) and enhanced ferroelectric and piezoelectric properties, making them potential candidates for high temperature lead-free piezoelectric materials in the future.
To find out more about the I15 beamline or discuss potential applications, please contact Principal Beamline Scientist Annette Kleppe: annette.kleppe@diamond.ac.uk.
Yang Z et al. Acceptor doping and actuation mechanisms in Sr-doped BiFeO3-BaTiO3 ceramics. Journal of Materiomics 10.1 (2024): 57-69. DOI:10.1016/j.jmat.2023.04.007.
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