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Title: Study of low-temperature sintered lead-free piezoelectric ceramics for multilayer applications
Authors: Chung, Tat Hang
Degree: M.Phil.
Issue Date: 2017
Abstract: In the present work, two sintering aids, Li₂CO₃ and CuO/TiO₂, have been developed for fabricating lead-free Bi₀.₅Na₀.₅TiO₃ (BNT)-and K₀.₅Na₀.₅NbO₃ (KNN)based piezoelectric ceramics, respectively, at low temperatures. Unlike other commonly used sintering aids, the above mentioned sintering aids are effective not only in lowering the sintering temperature but also in retaining the good piezoelectric properties of the ceramics. The mechanism of low-temperature sintering has also been investigated. Our results reveal that oxygen vacancies as well as a liquid phase are essential for realizing the low-temperature sintering. However, an intermediate phase arisen from the interaction between the sintering aid and the ceramics as proposed by Valant et al. may not be necessary. For studying the feasibility of practical use, the temperature dependence of the piezoelectric properties has also been investigated. All the low-temperature sintered ceramics show good or even improved thermal stability, indicating that they are readily for practical use, even up to 280°C. 0.93(Bi₀.₅Na₀.₅TiO₃)-0.07BaTiO₃ and Bi₀.₄₈₅N₀.₄₈₅Bi₀.₀₂TiO₃ ceramics added with x mol% Li₂CO₃, abbreviated as BNTBT-Li-x and BNBiT-Li-x, respectively, have been prepared by conventional solid-state reaction method. Our results reveal that a small amount (1 mol% and 2 mol%, respectively) of Li₂CO₃ is enough in lowering the sintering temperature of the ceramics to below 1000°C, while their piezoelectric properties are almost not affected. The ceramics are well densified, giving a density close to 98% of their theoretical values. For the BNTBT-Li-1 ceramic, the sintering temperature is reduced by 240°C to 960°C and its piezoelectric coefficient (d₃₃) remains almost unchanged at ~ 172 pC/N. For the BNBiT-Li-2 ceramic, the sintering temperature is as low as 960°C, while its d₃₃ only decreases slightly from 95 to 90 pC/N. A single-phase perovskite structure is observed for both ceramics. As evidenced by the shifting of the diffraction peaks and the calculated tolerate factors, Li+ has diffused into both lattices for replacing the B-site Ti⁴⁺. Owing to the valence difference, oxygen vacancies are generated which increase the lattice-diffusion coefficient and the sintering rate. Together with the liquid phase of Li₂CO₃ formed at 723°C which facilitates the mass diffusion between grains, well densification can then be realized at low temperatures. As the intermediate phase BaCO₃ proposed by Valant et al. for their Ba-containing ceramics does not exist in the BNBiT-Li-x ceramics, reactions between the sintering aids and ceramics may not be necessary for the low-temperature sintering process. Our results also reveal that the sintering aid Li₂CO₃ can improve the thermal stability of both ceramics by extending their "depolarization" temperature up to 100°C.
Owing to the inefficiency of Li₂CO₃ on 0.95K₀.₅Na₀.₅NbO₃-0.06BaTiO₃ (KNNBT) ceramics due to the lack of oxygen vacancies, another sintering aid CT which comprises of Cu and Ti in a molar ratio of 83.3/16.7 has been developed for lowering their sintering temperature. According to the relatively high eutectic temperature of CT (919°C), the present work has then focused on sintering the ceramics at 1000°C, which already allows the use of inexpensive inner electrodes in practical applications. Similarly, a small amount (2-3 mol%) of CT is enough for effectively reducing the sintering temperature as well as promoting the densification and improving the sinterability. For these ceramics, Cu²⁺ as well as Ti⁴⁺ have higher preference of substituting the B-site Nb⁵⁺ or Ti⁴⁺, and thus gernerating oxygen vacancies. Our results have then confirmed that oxygen vacancies as well as a liquid phase are essential for realizing the low-temperature sintering. For the ceramic added with 2.4 mol% CT, it possesses a high density (p = 4.36 g/cm3), good piezoelectric property (d₃₃ = 175 pC/N), as well as good thermal stability up to ~ 290°C. One of the sintering aids, Li₂CO₃, has been applied to fabricate Pr-doped BNTBT multifunctional ceramics with enhanced photoluminescence (PL) and good piezoelectric properties. The ceramics added with 0.25 mol% Pr and x mol% Li₂CO₃, abbreivated as Pr-BNTBT-Li-x, are prepard by conventional solid-state reaction method. Both Pr³⁺ and (part of) Li+ have diffused into lattices, with Pr³⁺ substituting the A-sites and Li+ substituting the B-sites. Typical PL red emissions arisen from Pr³⁺ are observed. Similar to BNTBT and BNBiT, Li₂CO₃ is able to reduce the sintering temperature of the Pr-BNTBT-Li-x ceramics to below 1000°C. Moreover, in agreement with previous studies, Li+ is also effective in enhancing the PL emissions. For the Pr-BNTBT-Li-1 ceramic, its sintering temperature is reduced by 240°C to 960°C, d₃₃ decreases only by ~ 8% to 165 pC/N, while the PL intensity increases by more than 150%. On the basis of the good properties, in particular the low sintering temperature, good piezoelectric properties and high thermal stability, the low-temperature sintered ceramics can readily be used to fabricate multilayered devices using inexpensive inner electrodes for various applications, including high-temperature applications.
Subjects: Piezoelectric ceramics.
Hong Kong Polytechnic University -- Dissertations
Pages: xvi, 143 pages : color illustrations
Appears in Collections:Thesis

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