**1. Introduction**

Piezoelectric ceramics, which are a kind of synthetic polycrystalline ferroelectric material, have been used as sensing materials for many electrical devices such as ultrasonic transducers, vibration sensors and multi-layer actuators [1–5]. In recent years, the concern for the environmental pollution and people's health highlights the main differences between the commercial lead-based piezoelectric ceramics (lead zirconate titanate-PZT) and the developed lead-free piezoelectric ceramics (such as calcium barium titanate-BTO, potassium sodium niobate-KNN, etc.) [6–8]. Bismuth-layered structure ferroelectrics (BLSFs, also called Aurivillius phase), with large spontaneous polarization and fatigue-free properties, are promising candidates for ferroelectric random access memories (FRAMs) [9]. The chemical formula of BLSFs can be described as (Bi2O2) 2+ (A*m*−1BmO3*m*+1) <sup>2</sup>−, where *m* delegates to the number of octahedral layers in the perovskite layer between the bismuth

**Citation:** Wang, S.; Zhou, H.; Wu, D.; Li, L.; Chen, Y. Effects of Oxide Additives on the Phase Structures and Electrical Properties of SrBi4Ti4O<sup>15</sup> High-Temperature Piezoelectric Ceramics. *Materials* **2021**, *14*, 6227. https://doi.org/10.3390/ma14206227

Academic Editor: Andres Sotelo

Received: 31 August 2021 Accepted: 8 October 2021 Published: 19 October 2021

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oxide layers and values normally from 1 to 5. Such as Bi2WO<sup>6</sup> (*m* = 1, *T*<sup>C</sup> = 950 ◦C) [10], Bi3TiNbO<sup>9</sup> (*m* = 2, *T*<sup>C</sup> = 904 ◦C) [11], Bi4Ti3O<sup>12</sup> (*m* = 3, *T*<sup>C</sup> = 675 ◦C) [12], SrBi4Ti4O<sup>15</sup> (*m* = 4, *T*<sup>C</sup> = 520 ◦C) [13], Sr2Bi4Ti5O<sup>18</sup> (*m* = 5, *T*<sup>C</sup> = 267 ◦C) [14]. Because of high Curie temperature, BLSFs have received more and more attention due to the urgent need of high-temperature sensor, actuator, and transducer applications in recent years [15].

Among the Aurivillius family, SrBi4Ti4O<sup>15</sup> (SBT) captures an orthorhombic symmetry with space group *A*21*am* at room temperature, including four perovskite-like TiO<sup>6</sup> octahedron units stacked in between (Bi2O2) 2+ layers [16]. However, some disadvantages associated with SBT such as difficulty in polarization, high leakage current, volatilization of the bismuth during sintering, and a low density and undesirable properties caused by the random arrangement of plate-like crystal grains [17], such as a tenuous piezoelectric activity (*d*33~10 pC/N) [18]. Over the last few decades, a large number of investigations focused on the modification of the electrical properties of SBT piezoceramics through the ionic substitution at A-site [19–21] or B-site [22–24]. Cao et al. [25] found a large enhancement of piezoelectric properties (*d*<sup>33</sup> = 30 pC/N) in Mn-modified (B-site) SrBi4Ti4O<sup>15</sup> as well as good thermal stability at elevated temperatures, while its *T*<sup>C</sup> remains almost unchanged at ~530 ◦C. However, most reports on enhancing the ferroelectric and piezoelectric properties of SBT ceramics concentrate on A-site rather than B-site [26]. For example, the Curie temperature increased by doping SBT with Na<sup>+</sup> and Pr3+ at A-site [27]. The dielectric constant and loss decreased, whereas the Curie temperature increased when Na<sup>+</sup> and Nd3+ were substituted to A-site in SBT [17]. A-site cerium-modified SrBi4Ti4O<sup>15</sup> ceramics showed a high stability of dielectric properties [28]. On the other hand, some oxide compounds like Cr2O3, MnO<sup>2</sup> and U3O<sup>8</sup> as additives have been also proved to play a notable effect on the physical and electrical properties of BLSF ceramics [29].

Among these modified SBT ceramics, the compositions with both high Curie temperature and high piezoelectric property at the same time were rarely reported, and most of the as-reported works focused on the modification of the SBT ceramics with one kind of element/oxide. In this work, two kinds of oxides (Gd2O3, CeO2, MnO<sup>2</sup> and Cr2O3) were co-doped into the SBT ceramics, and after these the oxide-modified SBT piezoceramics were fabricated via a traditional solid-state reaction process; the effect of these oxides on the phase structure and electrical properties of SBT ceramics have been investigated in detail.
