Lead-Free Piezoelectric Ceramics: Addressing Industry-Relevant Challenges

Gianni Ferrero

Research output: Book/ReportPh.D. thesis

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Despite the last decade’s numerous advancements, lead-free piezoelectric ceramics are still struggling to replace the lead-based piezoceramics widely applied to convert mechanical to electrical energy, and vice versa. Higher cost of raw materials, delicate and irreproducible synthesis, more complex processing and physical limitations of the lead-free composition are just few of the issues that the piezoceramic industry must overcome. The composition 0.06LiNbO3-0.94K0.5Na0.5NbO3 (LKNN) has an overall good performance and is one of the most versatile lead-free piezoelectrics. The aim of this industrial PhD thesis was therefore to investigate several different aspects limiting the lead-free piezoceramic processing, with LKNN as the primary material.

While the detrimental effect of humidity on LKNN powder processing is well known, the effect of humidity on other stages of the processing is not well understood. We therefore studied sintering of LKNN in varying humidity content. Contrary to the expected poor densification due to excessive grain growth, the samples sintered at high humidity showed increased densities. Lower dielectric losses, significantly increased mechanical quality factors and a slight decrease in piezoelectric performance were also observed. These features indicated a ferroelectric hardening of the material and can be explained by increased evaporation of alkali oxides in humid air, which increases the concentration of oxygen vacancies that pin domain wall motion. This study indicated that humidity also can impart positive characteristics to the sintered LKNN, and deliberately introduced humidity is a potential route to tune the sintering and hardness of LKNN.

Since storage and shelf life are important aspects of industrial piezoceramic manufacturing, we studied how humidity affected sintered LKNN over time. Samples were stored in a glovebox with <0.1 ppm H2O, a desiccator with 20 % relative humidity (RH), a sealed container with 95 % RH, or submerged in water, and characterized at specific time intervals. While the samples in the glovebox maintained a slightly higher performance after six months, storage at 20 % RH proved to be a good and inexpensive solution to keep the piezoelectric properties of the samples almost unaltered. On the contrary, storage in high humidity dramatically reduced the piezoelectric performance already after two of days. Small amounts of a Nb-rich phase were also visible at the grain boundaries after six months of storage in contact with humidity. The performance of the submerged sample decayed slower compared to the samples stored at 95 % RH, due to the slower infiltration of water into the interior of the sample. The dielectric losses and d33 of the samples stored at up to 95 % RH could be restored to their initial values after drying at 150 °C for few hours, while the sample submerged in water was permanently damaged due to electrode delamination. This knowledge on the effect of different humidity conditions can guide both the proper storage and need for encapsulation of the piezoelectric ceramic in devices and can help reduce waste and financial losses in the piezoceramic industry.

To further promote the industrial feasibility of the LKNN system, we investigated the use of an alternative, less pure and inexpensive precursor for copper doping to produce ferroelectrically hard LKNN. The alternative precursor (Cu-alt) could be added to the LKNN powder in a dissolved form, potentially aiding a homogeneous distribution in the LKNN bulk. A 0.5 mol% addition of Cu-alt to LKNN showed an increase of Qm from 38 to 215, a reduction of d33 and dielectric losses from 170 to 90 pC/N and from 0.04 to 0.01, respectively. Higher amounts of Cu dopant (2 mol%) further increased the Qm up to 359 and reduced the dielectric loss down to 0.009. Since copper also acts as a sintering aid in LKNN, it was possible to lower the sintering temperature. This in turn reduced the amount of secondary phases due to the limited evaporation of alkali elements. The comparison of Cu-alt dopant with the classical CuO resulted in practically identical ferro- and piezoelectric properties, confirming the possibility of using Cu-alt as a practical way to reduce manufacturing costs of hard LKNN and, in theory, other piezoelectric ceramics.

As lead-free piezoceramics are not as versatile as the PZT family, having an extensive portfolio of different lead-free materials can be strategically important for the piezoceramic industry. We therefore also studied the processing of 0.67BiFeO3-0.33BaTiO3 (BF-BT). We used mechanochemical activation with and without BT as seed particles for the perovskite BF-BT phase. However, the mechanochemical activation only slightly affected the BT seeds, and rather than promoting the formation of the perovskite BT-BT, they caused a heterogeneous microstructure with remnants of BT-rich inclusions. This in turn resulted in ceramics with a reduced low-field piezoelectric response compared to without the seeds, but at the same time a higher field-induced strain up to 150 °C. Both processing routes were able to form ceramics with relative densities above 96% and without significant amounts of secondary phases, confirming the usefulness of mechanochemical activation and the possibility to significantly change the piezoelectric behaviour of 0.67BF-0.33BT ceramics through process adjustment.
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherDTU Energy
Number of pages207
Publication statusPublished - 2022

Note re. dissertation

This thesis is based on the Industrial PhD project “Lead-free Piezoelectric Ceramics: Addressing Industry-Relevant Challenges” between Meggitt A/S and the Technical University of Denmark, conducted from August 2018 to January 2022. The project was partially funded by Innovation Fund Denmark, case nr. 7038-00234B and partially by Meggitt A/S.


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