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Page 2 of 11 Qin et al. Microstructures 2023;3:2023035 https://dx.doi.org/10.20517/microstructures.2023.34

INTRODUCTION
Piezoelectric ceramics are extensively employed in electronic devices, such as sensors, actuators, filters,
ultrasonic devices, etc., which are realized through the mutual conversion between mechanical and electrical
energies . Over decades, Pb(Zr Ti )O -based piezoelectric ceramics have been used dominantly in
[1-3]
1-x
3
x
commercial devices. However, the toxicity of lead (Pb) can cause irreversible damage to human health and
the environment, which promotes research hotspots on lead-free materials . Among the lead-free
[4-6]
materials, (K,Na)NbO (KNN)-based and Bi Na TiO (BNT)-based piezoelectric ceramics are considered
3
1/2
1/2
3
to be promising candidates for lead-based piezoelectric ceramics [7-10] . Despite the high piezoelectric
coefficient (d > 500 pC/N), KNN ceramics suffer from certain drawbacks, such as the K and Na
33
volatilization, unstable phase structures near room temperature, and low Curie temperatures (T )
C
~200 °C [1,11-13] . BNT-based ceramics used to exhibit high T , but the volatilization of Bi and Na
C
elements leads to unstable chemical compositions and a high coercive field; consequently, superior
polarization is difficult to achieve [14-17] .
In recent years, BiFeO -xBaTiO (BF-xBT) piezoelectric ceramics have emerged as competitive candidates in
3
3
lead-free materials [2,18-20] . As a kind of multiferroic material, BF has a rhombohedral phase perovskite
structure (ABO ), which has attracted significant attention because of its high T (830 °C) and excellent
3
C
2 [21,22]
spontaneous polarization (P = 90-100 μC/cm ) . Recent studies of BF single crystal [23-25] , polycrystalline
s
thin film [26-29] , and epitaxial thin film [30-34] have also been conducted, which has given researchers additional
ideas for exploring and application. However, the synthesis of pure BF is usually accompanied by the
generation of impurities, where excess Fe O exceeding 5 mol % leads to the formation of pyrochlore
3
2
Bi Fe O and gamma-Fe O . Additionally, the volatilization of Bi and the valence reduction of Fe to Fe 2+
[35]
3+
2
3
4
2
9
at high temperatures cause high leakage currents, resulting in challenges for practical applications [36-38] . BT is
a traditional ferroelectric material exhibiting a tetragonal phase structure at room temperature and
possesses a low T of ~120 °C . It has been found that the sintering of BF-xBT solid solution can effectively
[39]
C
suppress the generation of secondary or impurity phases and reduce the leakage current. Specifically, a
morphotropic phase boundary (MPB) can be constructed when x approaches 0.30 ~ 0.35, which contributes
to enhanced dielectric, piezoelectric, and ferroelectric properties while maintaining a high T C [40-42] .
However, one of the notable disadvantages in BF-xBT-based piezoelectric ceramics is the Bi O 3
2
volatilization during the sintering process, resulting in poor electrical resistivity and piezoelectric
performance. The volatilization of Bi O can be described by the following defect Equation (1) [43]
2
3
The volatilization of Bi O leads to the generation of Bi vacancies ( ) and O vacancies ( ) within the
2
3
ceramics. Various strategies have been attempted to solve the problem. One of the most common strategies
is using nonstoichiometric (excess) Bi to compensate for the loss of Bi during high-temperature sintering, as
listed in Table 1.
In this study, a series of 0.70B F-0.30BT (x = -0.01, 0, 0.01, 0.02, 0.03, 0.04) ceramics were fabricated using
1+x
the conventional solid-state reaction method. The influence of Bi O compensation on the phase structure,
2
3
microstructure, dielectric, ferroelectric, and piezoelectric properties of ceramics are systematically
investigated.