Enhanced electrical properties in W/Cu co-doped CaBi2Nb2O9 high-temperature piezoelectric ceramics

CaBi₂Nb₂O₉ (CBN)-based high-temperature piezoelectric ceramics with the formula of CaBi₂Nb_2−x(W_3/4Cu_1/4)_xO₉ were prepared via the traditional solid-state reaction method. Both the bulk microstructure and the electrical performance of the W/Cu co-doped CBN-based ceramics were systematically investigated. The results indicated that the W/Cu incorporation into the Nb-site altered the crystal structure, which enhanced the piezoelectricity and resistivity. The ceramic with the composition CaBi₂Nb_1.96(W_3/4Cu_1/4)_0.04O₉ exhibited good performance with a high d₃₃ (~14 pC/N) and T_C (~939℃). Moreover, the ceramic exhibited a good electrical resistivity (ρ) of 4.91 × 10⁵ Ω·cm and a low dielectric loss (tanδ) of 0.1 at 600℃. Furthermore, the ceramic that was annealed at 900℃ for 2 h presented a d₃₃ value of 13 pC/N, thus indicating good thermal stability of the piezoelectric properties. All these results confirm that the CaBi₂Nb_1.96(W_3/4Cu_1/4)_0.04O₉ ceramic may act as a potential promising candidate for piezoelectric device applications in high-temperature environments.     

Remarkable piezoelectric activity and high electrical resistivity in Cu/Nb co-doped Bi4Ti3O12 high temperature piezoelectric ceramics

Cu/Nb co-doped Aurivillius type Bi₄Ti_3-x(Cu_1/3Nb_2/3)_xO₁₂ (BTCN) ceramics were investigated as a potential candidate for high temperature piezoelectric application. The microstructure, phase structure and resulting piezoelectric properties and conduction behaviors were systematically investigated. A remarkable d₃₃ of 38 pC/N was achieved in the ceramic with a composition of x = 0.015, which may be ascribed to the enhancement of remanent polarization and decrease of coercive field. Moreover, a high DC resistivity of 8.39 × 10⁶ Ω·cm at 500 °C was also obtained in the composition, due to the decrease of the oxygen vacancy concentration induced by the doped Cu/Nb. Furthermore, the ceramic also exhibited stable thermal annealing behaviors and excellent fatigue resistance. All the results demonstrated the great potential of the Cu/Nb co-doped Bi₄Ti₃O₁₂ ceramics for high temperature piezoelectric applications.     

Enhanced piezoelectric properties of Mn-modified Bi5Ti3FeO15 for high-temperature applications

Bi₅Ti₃FeO₁₅ (BTF) has recently attracted considerable interest as a typical multiferroic oxide, wherein ferroelectric and magnetic orders coexist. The ferroelectric order of BTF implies its piezoelectricity, because a ferroelectric must be a piezoelectric. However, no extensive studies have been carried out on the piezoelectric properties of BTF. Considering its high ferroelectric-paraelectric phase transition temperature (T_c ~ 761°C), it is necessary to analyze the piezoelectricity and thermal stabilities of BTF, a promising high-temperature piezoelectric material. In this study, lightly manganese-modified BTF polycrystalline oxides are fabricated by substituting manganese ions into Fe³⁺ sites via the conventional solid-state reaction method. X-ray diffraction and Raman spectroscopy analyses reveal that the resultant manganese-modified BTF has an Aurivillius-type structure with m = 4, and that the substitutions of Fe by Mn lead to a distortion of BO₆. The temperature-dependent dielectric properties and direct-current (DC) resistivity measurements indicate that the Mn ions can significantly reduce the dielectric loss tanδ and increase the DC resistivity. The piezoelectricity of BTF is confirmed by piezoelectric constant d₃₃ measurements; it exhibits a piezoelectric constant d₃₃ of 7 pC/N. Remarkably, BTF with 4 mol% of Mn (BTF-4Mn) exhibits a large d₃₃ of 23 pC/N, three times that of unmodified BTF, whereas the Curie temperature T_c is almost unchanged, ~765°C. The increased piezoelectric performance can be attributed to the crystal lattice distortion, decreased dielectric loss tanδ, and increased DC resistivity. Additionally, BTF-4Mn exhibits good thermal stabilities of the electromechanical coupling characteristics, which demonstrates that manganese-modified BTF oxides are promising materials for the use in high-temperature piezoelectric sensors.     

Doping level effects in Nb self-doped Bi3TiNbO9 high-temperature piezoceramics with improved electrical properties

Nb self-doped Bi₃Ti_1-xNb_1+xO₉ (x = 0, 0.02, 0.04, 0.06, 0.08, and 0.1) high-temperature piezoelectric ceramics were fabricated through the conventional solid-state sintering method. The effects of different Nb self-doping levels on the microstructure, piezoelectric activities, and electrical conduction behaviors of these Nb self-doped Bi₃Ti_1-xNb_1+xO₉ ceramics were studied in detail. Large doping level effects on piezoelectric activity and resistivity were confirmed, which might be ascribed to the evolution of the crystal structure and the variations of the oxygen vacancy concentration and the grain anisotropy induced by Nb doping. An optimized piezoelectric coefficient (d₃₃) of 11.6 pC/N was achieved at x = 0.04 with a Curie temperature of 906°C. Additionally, an improved DC resistivity of 6.18 × 10⁵ Ω·cm at 600°C was acquired in this ceramic. Furthermore, the ceramic exhibited excellent thermal stability with the d₃₃ value maintaining 95% of its initial value after being annealed at 850°C for 2 hours. These results showed that Nb self-doped Bi₃Ti_1-xNb_1+xO₉ ceramics might have great potentials for high-temperature piezoelectric applications.     

(Bi1/2Na1/2)TiO3–Ba(Cu1/2W1/2)O3 Lead-Free Piezoelectric Ceramics

(Bi_1/2Na_1/2)TiO₃ with 0–6 mol% Ba(Cu_1/2W_1/2)O₃ (BNT-BCW), a new member of the BNT-based group, has been prepared following the conventional mixed oxide route. The compacted bodies were sintered at 1130°C for 2 h to get dense ceramics. The addition of BCW into BNT ceramics facilitated the poling process because of a reduction in leakage current. 0.995BNT·0.005BCW ceramics exhibit a relatively high piezoelectric constant ( d ₃₃= 80 × 10⁻¹² C/N) and a relatively low dielectric loss (tan δ= 1.5%). Increased amount of BCW was found to increase the dielectric constant and loss of BNT-BCW ceramics and to suppress the grain growth. During sintering, some BCW diffuses into the lattice of BNT to form a solid solution and some remains on the grain boundaries.     

Large Piezoelectricity and Ferroelectricity in Mn‐Doped (Bi0.5Na0.5)TiO3‐BaTiO3 Thin Film Prepared by Pulsed Laser Deposition

Mn‐doped (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ (MnBNBT) thin films were prepared on SrRuO₃ (SRO)‐coated (001) SrTiO₃ (STO) single crystal substrates by pulsed laser deposition under different processing conditions. Structural characterization (i.e., XRD and TEM) confirms the epitaxial growth of STO/SRO/MnBNBT heterostructures. Through the judicious control of deposition temperature, the defect level within the films can be finely tuned. The MnBNBT thin film deposited at the optimized temperature exhibits superior ferroelectric and piezoelectric responses with remanent polarization P_r of 33.0 μC/cm² and piezoelectric coefficient d₃₃ of 120.0 ± 20 pm/V.     

Large enhancement of piezoelectric properties in Mn-modified SrBi4Ti4O15 and its thermal stabilities at elevated temperatures

Manganese-modified strontium bismuth titanate (SrBi₄Ti₄O₁₅, SBT) ceramic oxides were synthesized by substituting a small amount of manganese ions into the Ti⁴⁺ sites using conventional solid-state reaction. The resultant Mn-modified SBT (SBT-Mn) exhibits better piezoelectric properties in comparison to unmodified SBT. The piezoelectric properties of Mn-modified SBT is optimized with only 4 mol% Mn substitution. SBT-4Mn exhibits a large piezoelectric constant (d₃₃=30 pC/N), approximately twice the value of unmodified SBT (d₃₃=13 pC/N), while its Curie temperature, T_c, remains almost unchanged at ~530 °C. The temperature-dependent electrical impedance and electromechanical coupling factors k_p and k_t reveal that the SBT-4Mn exhibits thermally stable electromechanical coupling characteristics up to 300 °C but electromechanical coupling factors deteriorate significantly at a higher temperature due to increased conductivity. Our work suggested that Mn-modified SBT ceramics are promising materials for high temperature piezoelectric applications.     

Enhanced piezoelectric properties and electrical resistivity in W/Cr co-doped CaBi2Nb2O9 high-temperature piezoelectric ceramics

W/Cr co-doped Aurivillius-type CaBi₂Nb_2-x(W_2/3Cr_1/3)_xO₉ (CBN) (x?=?0.025, 0.050, 0.075, 0.100, and 0.150) piezoelectric ceramics were prepared by the conventional solid-state reaction method. The crystal structure, microstructure, dielectric properties, piezoelectric properties, and electrical conductivity of these ceramics were systematically investigated. After optimum W/Cr modification, the CBN ceramics showed both high d₃₃ and T_C. The ceramic with x?=?0.1 showed a remarkably high d₃₃ value of ~15 pC/N along with a high T_C of ~931?°C. Moreover, the ceramic also showed excellent thermal stability evident from the increase in its planar electromechanical coupling factor k_p from 8.14% at room temperature to 11.04% at 600?°C. After annealing at 900?°C for 2?h, the ceramic showed a d₃₃ value of 14?pC/N. Furthermore, at 600?°C, the ceramic also showed a relatively high resistivity of 4.9?×?10⁵ Ω?cm and a low tanδ of 9%. The results demonstrated the potential of the W/Cr co-doped CBN ceramics for high-temperature applications. We also elucidated the mechanism for the enhanced electrical properties of the ceramics.     

Significantly enhanced dc electrical resistivity and piezoelectric properties of Tb-modified CaBi2Nb2O9 ceramics for high-temperature piezoelectric applications

Bismuth layer–structured ferroelectric calcium bismuth niobate (CaBi₂Nb₂O₉, CBN) is considered to be one of the most potential high-temperature piezoelectric materials due to its high Curie temperature T_c of ∼940°C, but the drawbacks of low electrical resistivity at elevated temperature and low piezoelectric performance limit its applications as key electronic components at high temperature (HT). Herein, we report significantly enhanced dc electrical resistivity and piezoelectric properties of CBN ceramics through rare-earth element Tb ions compositional adjustment. The nominal compositions of Ca_1−xTb_xBi₂Nb₂O₉ (abbreviated as CBN-100xTb) have been fabricated by conventional solid-state reaction method. The composition of CBN-3Tb exhibits a significantly enhanced dc electrical resistivity of 1.97 × 10⁶ Ω cm at 600°C, which is larger by two orders of magnitude compared with unmodified CBN. The donor substitutions of Tb³⁺ ions for Ca²⁺ ions reduce the oxygen vacancy concentrations and increase the band-gap energy, which is responsible for the enhancement of dc electric resistivity. The temperature-dependent dc conduction properties reveal that the conduction is dominated by the thermally activated oxygen vacancies in the low-temperature region (200–350°C) and by the intrinsic conduction in the HT region (350–650°C). The CBN-3Tb also exhibits enhanced piezoelectric properties with a high piezoelectric coefficient d₃₃ of ∼13.2 pC/N and a high T_c of ∼966°C. Moreover, the CBN-3Tb exhibits good thermal stabilities of piezoelectric properties, remaining 97% of its room temperature value after annealing at 900°C. These properties demonstrate the great potentials of Tb-modified CBN for high-temperature piezoelectric applications.     

Effects of cerium on structures and electrical properties of (Nb,Ta) modified Bi4Ti3O12 piezoelectric ceramics

Bi₄Ti₃O₁₂ high-temperature piezoelectric ceramics composed of 0.03 mol (Nb, Ta)⁵⁺ substituting B site and x mol CeO₂ (x = 0–0.05, abbreviated as BCTNT100x) substituting A site were synthesized by the conventional solid-state reaction method. The effects of Ce additive on the structures and electrical properties of resulting Bi₄Ti₃O₁₂-based ceramics were systematically investigated. In-situ temperature-dependent X-ray diffraction (XRD) confirmed that the phase structure of BCTNT100x ceramics change from orthorhombic structure to tetragonal structure as temperature increased. The ceramics at Ce content x = 0.03 illustrated optimal performances with superior piezoelectric constant (d₃₃ = 36.5 pC/N), high Curie temperature (T_C = 649 °C), and large remanent polarization (2P_r = 21.6 μC/cm²). BCTNT3 ceramics also possessed high d₃₃ of 32.5 pC/N at an annealing temperature of 600°C, with electrical resistivity preserved at 10⁶ Ω cm at 500 °C. These results demonstrate that BCTNT100x ceramics can be used as high-temperature piezoelectric devices.     

Structural and electrical properties of La3+-doped Na0.5Bi4.5Ti4O15-Bi4Ti3O12 inter-growth high temperature piezoceramics

New lead-free inter-growth piezoelectric ceramics, Na_0.5Bi_8.5-xLa_xTi₇O₂₇ (NBT-BIT-xLa, 0.00≤x≤1.00), were prepared by the conventional solid-state method. Structural and electrical properties of NBT-BIT-xLa were studied. All the NBT-BIT-xLa samples exhibited a single inter-growth structured phase. XRD and Raman spectroscopy revealed a reduced orthorhombicity, which strongly supports the variation of dielectric and ferroelectric properties. Plate-like grains were found to decrease with the increasing x contents. Impedance spectra analysis indicated that oxygen vacancy defects dominated the contributions to the electrical conductivity. The increased activation energies for dc conductivity evidenced the reduction of oxygen vacancy concentration after La substitution, inducing the enhancement in piezoelectric constant (d₃₃) and remanent polarization (2P_r). The studies of thermal depoling indicated that the optimal d₃₃ of NBT-BIT-0.50La ceramics still remained 22 pC/N at 500 °C, implying that this ceramics could be potentially applied into high temperature devices.     

Deficient or excessive CuO-TiO2 phase influence on dielectric properties of CaCu3Ti4O12 ceramics

The influence of the CuO–TiO₂ phase (CT) on dielectric properties of the CCTO ceramic was investigated. CaCu_XTi_YO₁₂ (CC_XT_YO) powders were prepared based on the coprecipitated method, where 2.70 ≤ x ≤ 3.30 and 3.25 ≤ y ≤ 4.75. XRD patterns confirmed the presence of CCTO and also the secondary phases as CuO, TiO₂, and CaTiO₃ for each sample and aided in its quantification. Scanning Electron Microscopy (SEM) shows secondary phases evolution in the grain boundaries, and its influence on size and morphology of the grains. Impedance spectroscopy measurements showed that the ceramics with lower amount of CuO and TiO₂ phases (CT/deficient ceramics) exhibited the highest ε′ values (2.1 × 10⁴ at 1 kHz for CC_2.9T_3.75O ceramic). Also, CT/deficient ceramics showed lower tanδ values (0.090 at 1 kHz for CC_2.9T_3.75O ceramic) than ceramics prepared with excessive CuO–TiO₂ phase (0.241 at 1 kHz for CC_3.1T_4.25O ceramics). The deficiency of CuO and TiO₂ phases associated with high percentage of CCTO and CaTiO₃ phases resulted in ceramics with the higher ε′ values.     

Electrical Properties of a Bismuth Layer-Structured Ba2Bi4Ti5O18 Single Crystal

Ba₂Bi₄Ti₅O₁₈ single crystals were grown, and their dielectric permittivity, conductivity, and ferroelectricity were investigated along the a -(or b -)axis and the c -axis separately. The dielectric permittivity at 1 MHz along the a -(or b -)axis was 2000 at the Curie temperature (360°C); this value was 8 times greater than that along the c -axis. The dc conductivity was greater along the a -(or b -)axis than that along the c -axis, by one order of magnitude. In regard to the ferroelectricity, the saturated remanent polarization was 120 mC/m² and the saturated coercive field was 3 MV/m along the a -(or b -)axis; values of 8.5 mC/m² and 0.81 MV/m, respectively, were observed along the c -axis. The Ba₂Bi₄Ti₅O₁₈ single crystals had large electrical anisotropies, which were due to the layered structure.     

Influence of composition on the unipolar electric fatigue of Ba(Zr0.2Ti0.8)O3‐(Ba0.7Ca0.3)TiO3 lead‐free piezoceramics

The lead‐free (1?x)Ba(Zr_0.2Ti_0.8)O₃‐x(Ba_0.7Ca_0.3)TiO₃ system is considered as promising candidate for the replacement of lead‐based piezoceramics in actuation applications, during which electric fatigue is a major concern. This issue was addressed in this work, where the unipolar fatigue resistance of three (1?x)Ba(Zr_0.2Ti_0.8)O₃‐x(Ba_0.7Ca_0.3)TiO₃ compositions with different crystallographic structures (rhombohedral, orthorhombic, and tetragonal) was evaluated. Strain asymmetry and development of an internal bias field were observed in all compositions. The decrease in the remanent polarization and the large signal piezoelectric coefficient after 10⁷ unipolar cycles was found to lie between 6%‐12% and 2%‐13%, respectively. The most pronounced fatigue was observed for the orthorhombic composition, which has the largest extrinsic contribution to strain. On the other hand, the best fatigue resistance was observed for the tetragonal composition, which has a predominantly intrinsic strain response. The correlation of fatigue resistance with strain mechanism was corroborated with determination of the Rayleigh parameters and changes in the domain morphology after cycling as confirmed by piezoresponse force microscopy.     

Enhanced pyroelectric and piezoelectric responses in W/Mn-codoped Bi4Ti3O12 Aurivillius ceramics

The pyroelectric and piezoelectric properties of 4 at% Mn-doped Bi₄Ti_2.9W_0.1O₁₂ (BiTW-Mn) Aurivillius ceramic were investigated and compared to Bi₄Ti_2.9W_0.1O₁₂ (BiTW) counterpart, which were fabricated using a conventional solid state reaction method. High resistivities of 4.9?×?10¹² and 2.5?×?10¹¹ Ω?cm at 100?°C were obtained in the W-doped and W/Mn-codoped BiT ceramics, respectively. They showed similar activation energies and ionic-p-type mixed conduction mechanisms. Higher pyroelectric coefficients of 57.1?μC/m²K and piezoelectric coefficients of 21 pC/N, as well as much lower dielectric loss of 0.003 were achieved in W/Mn-codoped ceramics. These property changes were mainly induced by $\text{M}{\text{n}}_{\text{Ti}}^{}?{\text{V}}_{\text{o}}^{}$ defect dipoles. The effect of acceptor doping was evidenced by an internal bias field, shown by a horizontal offset in the polarization-field behavior. The improved properties together with high thermal stability indicate that BiTW-Mn may be a promising candidate for pyroelectric and piezoelectric devices at elevated temperatures.     

Dielectric and magnetic studies of 0.5Bi2/3Cu3Ti4O12 - 0.5Bi3LaTi3O12 nano-composite ceramic synthesized by semi-wet route

A nano-composite electro ceramic with the chemical composition of 0.5Bi_2/3Cu₃Ti₄O₁₂ - 0.5Bi₃LaTi₃O₁₂ was synthesized by a semi-wet route using high purity metal nitrate and solid TiO₂ in a stoichiometric ratio. X-ray diffraction (XRD) analysis showed the presence of Bi₃LaTi₃O₁₂ (BLTO) and Bi_2/3Cu₃Ti₄O₁₂ (BCTO) phases in the composites sintered at 900 °C for 8 h. Transmission electron microscope (TEM) analysis of the composite shows the presence of nanoparticles in the range of 55±3 nm. Atomic force microscopy (AFM) study also substantiates the presence of nanoparticles in the composite. Scanning electron microscope (SEM) images show that the surface morphology consists of plates like and spherical grains. The study of PE hysteresis loop revealed no saturation polarization which suggested lossy capacitor behavior of the composite. Magnetic behavior of the composite shows the weak ferromagnetic nature in M-T and M-H curve. The High observed value of dielectric constant (ε’=13.94×10³) of the composite may be due to the presence of space charge polarization.     

Piezoelectricity,thermal stability,and fatigue resistance in Nb and Ta-doped Bi4Ti3O12 high-temperature piezoceramics

The effect of Nb/Ta donor doping on the piezoelectricity, thermal stability, and fatigue resistance of bismuth titanate Bi₄Ti₃O₁₂ (BIT) ceramics was investigated in relation to their structural and oxygen vacancy-related electrical properties. As the Nb/Ta doping amount increased, the activation energy of oxygen vacancy conduction increased, indicating a reduction in the concentration of oxygen vacancies. The improved electrical insulating properties of the Nb/Ta-doped Bi₄Ti₃O₁₂ ceramics (BTNT) with fewer oxygen vacancies, contributed to their effective poling and strong piezoelectricity. Outstanding piezoelectric performance with high piezoelectric constant (39 pC/N) and Curie temperature (690 °C) could be achieved in the 0.005 mol Nb/Ta-doped BTNT ceramic with high density and anisotropic grain growth. The BTNT ceramics exhibited superior thermal aging stability and fatigue resistance compared to the BIT ceramic, suggesting that the reduction of oxygen vacancy defects plays a decisive role in enhancing elevated-temperature-induced and electric-field-induced degradation stabilities.     

Colossal dielectric permittivity and relevant mechanism of Bi2/3Cu3Ti4O12 ceramics

Bi_2/3Cu₃Ti₄O₁₂ (BCTO) ceramics with pure perovskite phase were successfully prepared by traditional solid-state reaction technique. Uniformly distributed and dense grains with the grain size of 2–3 μm were observed by SEM. A giant low-frequency dielectric permittivity of ~3.3×10⁵ was obtained. The analysis of complex impedance revealed that Bi_2/3Cu₃Ti₄O₁₂ ceramics are electrically heterogeneous. There are three kinds of dielectric response detected in Bi_2/3Cu₃Ti₄O₁₂ ceramics, which existed in the low-frequency range, middle-frequency range, and high-frequency range, respectively. Through the study of dielectric spectrum at different temperatures, the relatively low activation energy of 0.30 eV for middle-frequency dielectric response was calculated, which suggested that this Middle-frequency dielectric response can be ascribed to grain boundaries response. In view of the analysis of dielectric spectrum at low temperatures, the activation energy of 0.07 eV for high frequency dielectric response was found. This value illustrated that dielectric response at high frequencies was associated with grains polarization effect. The comparison of dielectric spectra of Bi_2/3Cu₃Ti₄O₁₂ ceramics with different types of electrodes revealed that giant low-frequency dielectric constant was attributed to the electrode polarization effect.     

Textured CaBi4Ti4O15 ceramics with large piezoelectricity,excellent thermal stability and high resistivity

The bismuth layer-structured ferroelectrics (BLSF) are promising high-temperature piezoelectric materials, in which large piezoelectricity, good thermal stability and high electrical resistivity are desired. Here highly textured CaBi₄Ti₄O₁₅ BLSF ceramics with orientation factor of 82% have been fabricated by spark plasma sintering technique. The piezoelectric coefficient d₃₃ is significantly enhanced by 250%, from 7.2 pC/N for the texture-less sample to 25.3 pC/N for the textured one, accompanied by a high Curie temperature T_C= 788 °C. The variation of d₃₃ is below 5% in the temperature range of 25–500 °C, showing excellent thermal stability. The textured sample exhibits high electrical resistivity ρ = 2.1 × 10¹¹ Ω·cm, an order of magnitude larger than that of the texture-less sample. At the temperature as high as 500 °C, the textured sample still maintains excellent electrical properties of d₃₃ = 24.2 pC/N, tanδ = 9.9% and ρ = 2.7 × 10⁶ Ω·cm, suggesting that the textured CaBi₄Ti₄O₁₅ ceramics could be a potential candidate for high-temperature piezoelectric sensor or detector applications.     

Microwave dielectric properties and thermally stimulated depolarization of Al-doped Ba4(Sm,Nd)9.33Ti18O54 ceramics

Ba₄(Sm_0.15Nd_0.85)_9.33Ti_18-zAl_3z/4O₅₄ (BSNT-zAl, 0.0 ≤ z ≤ 2.5) ceramics were prepared via a solid-state reaction, and the effects of Al doping on the microwave dielectric properties and defect behavior of the title compound were studied. X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM) photographs suggested that Al ions successfully entered the lattice to form tungsten-bronze-like solid solutions. With a small amount of Al substitution, the relative dielectric constant (ε_r), and the temperature coefficient of resonant frequency (τ_f) values decreased, whereas the quality factor (Q × f) substantially increased by approximately 50%. The defect-related extrinsic dielectric loss was clarified via the thermally stimulated depolarization current (TSDC) technique. With Al doping, the TSDC relaxation of across-grain-boundary oxygen vacancies () vanished, whereas that of defect dipoles () appeared at relatively low temperatures. Therefore, in the BSNT-zAl ceramics, oxygen vacancies were more inclined to interconnect with to form defect dipoles. This could reduce the activity of and account for the notable improvement in the Q × f values. In particular, the excellent characteristics of ε_r = 67.33, Q × f = 16 530 GHz, and τ_f = +0.87 ppm/°C were achieved in the specimens with z = 1.5 sintered at 1350°C for 4 hours.