*Article* **Effects of Oxide Additives on the Phase Structures and Electrical Properties of SrBi4Ti4O<sup>15</sup> High-Temperature Piezoelectric Ceramics**

**Shaozhao Wang <sup>1</sup> , Huajiang Zhou <sup>1</sup> , Daowen Wu <sup>1</sup> , Lang Li <sup>2</sup> and Yu Chen 1,\***

> <sup>1</sup> School of Mechanical Engineering, Chengdu University, Chengdu 610106, China; wangsz0908@163.com (S.W.); zhouhj1996@163.com (H.Z.); wdw132812976202021@163.com (D.W.)

<sup>2</sup> Key Laboratory of Deep Earth Science and Engineering, Ministry of Education, Sichuan University, Chengdu 610065, China; lilang@scu.edu.cn

**\*** Correspondence: chenyuer20023@163.com; Tel./Fax: +86-28-84616133

**Abstract:** In this work, SrBi4Ti4O<sup>15</sup> (SBT) high-temperature piezoelectric ceramics with the addition of different oxides (Gd2O<sup>3</sup> , CeO<sup>2</sup> , MnO<sup>2</sup> and Cr2O<sup>3</sup> ) were fabricated by a conventional solid-state reaction route. The effects of oxide additives on the phase structures and electrical properties of the SBT ceramics were investigated. Firstly, X-ray diffraction analysis revealed that all these oxidesmodified SBT ceramics prepared presented a single SrBi4Ti4O<sup>15</sup> phase with orthorhombic symmetry and space group of *Bb*21*m*, the change in cell parameters indicated that these oxide additives had diffused into the crystalline lattice of SBT and formed solid solutions with it. The SBT ceramics with the addition of MnO<sup>2</sup> achieved a high relative density of up to 97%. The temperature dependence of dielectric constant showed that the addition of Gd2O<sup>3</sup> could increase the *T*<sup>C</sup> of SBT. At a low frequency of 100 Hz, those dielectric loss peaks appearing around 500 ◦C were attributed to the space-charge relaxation as an extrinsic dielectric response. The synergetic doping of CeO<sup>2</sup> and Cr2O<sup>3</sup> could reduce the space-charge-induced dielectric relaxation of SBT. The piezoelectricity measurement and electro-mechanical resonance analysis found that Cr2O<sup>3</sup> can significantly enhance both *d*<sup>33</sup> and *k*<sup>p</sup> of SBT, and produce a higher phase-angle maximum at resonance. Such an enhanced piezoelectricity was attributed to the further increased orthorhombic distortion after Ti4+ at B-site was substituted by Cr3+. Among these compositions, Sr0.92Gd0.053Bi4Ti4O<sup>15</sup> + 0.2 wt% Cr2O<sup>3</sup> (SGBT-Cr) presented the best electrical properties including *T*<sup>C</sup> = 555 ◦C, *tan δ =* 0.4%, *k*<sup>p</sup> = 6.35% and *d*<sup>33</sup> = 28 pC/N, as well as a good thermally-stable piezoelectricity that the value of *d*<sup>33</sup> was decreased by only 3.6% after being annealed at 500 ◦C for 4 h. Such advantages provided this material with potential applications in the high-stability piezoelectric sensors operated below 500 ◦C.

**Keywords:** SrBi4Ti4O15; high-temperature piezoceramics; oxide additives; curie temperature; piezoelectric coefficient; ion substitution
