Piezoelectric and dielectric properties of 0.98(Na0.5K0.5)NbO3–0.02Ba(ZrxTi(1−x))O3 ceramics

Lead-free 0.98(Na_0.5K_0.5)NbO₃–0.02Ba(Zr_xTi_(1?x))O₃ (0.98NKN–0.02BZT) ceramics with Zr contents were fabricated by a conventional mixed-oxide method. The results indicate that the Zr/Ti ratio significantly influences the structural, piezoelectric, dielectric, and ferroelectric properties of 0.98NKN–0.02BZT ceramics. For the 0.98NKN–0.02BZT (x = 0) ceramics sintered at 1090 °C, the bulk density increased as the Zr contents decreased and showed a maximum value at x = 0. The Curie temperature of the 0.98NKN–0.02BZT ceramics slightly decreased as the Zr contents increased. The dielectric constant, piezoelectric constant, and electromechanical coupling factor of samples were maximized at x = 0, which might be due to the increase in density. A high d₃₃ = 194 pC/N, k_p = 38% were obtained for the 0.98NKN–0.02BZT ceramics sintered at 1090 °C for 4 h.     

Structure and microwave dielectric characteristics of lithium-excess Ca0.6Nd0.8/3TiO3/(Li0.5Nd0.5)TiO3 ceramics

Compositions based on (1−x)Ca_0.6Nd_8/3TiO₃−x(Li_1/2Nd_1/2)TiO₃ + yLi (CNLNTx + yLi, x = 0.30–0.60, y = 0–0.05), suitable for microwave applications have been developed by systematically adding excess lithium in order to tune the microwave dielectric properties and lower sintering temperature. Addition of 0.03 excess-Li simultaneously reduced the sintering temperature and improved the relative density of sintered CNLNTx ceramics. The excess Li addition can compensate the evaporation of Li during sintering process and decrease the secondary phase content. The CNLNTx (x = 0.45) ceramics with 0.03 Li excess sintered at 1190 °C have single phase orthorhombic perovskite structure, together with the optimum combination of microwave dielectric properties of ɛ_r = 129, Q × f = 3600 GHz, τ_f = 38 ppm/°C. Obviously, excess-Li addition can efficiently decrease the sintering temperature and improve the microwave dielectric properties. The high permittivity and relatively low sintering temperatures of lithium-excess Ca_0.6Nd_0.8/3TiO₃/(Li_0.5Nd_0.5)TiO₃ ceramics are ideal for the development of low cost ultra-small dielectric loaded antenna.     

Gd2O3 doped 0.82Bi0.5Na0.5TiO3–0.18Bi0.5K0.5TiO3 lead-free piezoelectric ceramics

Gd₂O₃ (0–0.8 wt.%)-doped 0.82Bi_0.5Na_0.5TiO₃–0.18Bi_0.5K_0.5TiO₃ (BNKT18) lead-free piezoelectric ceramics were synthesized by a conventional solid-state process. The effects of Gd₂O₃ on the microstructure, the dielectric, ferroelectric and piezoelectric properties were investigated. X-ray diffraction (XRD) data shows that Gd₂O₃ in an amount of 0.2–0.8 wt.% can diffuse into the lattice of BNKT18 ceramics and form a pure perovskite phase. Scanning electron microscope (SEM) images indicate that the grain size of BNKT18 ceramics decreases with the increase of Gd₂O₃ content; in addition, all the modified ceramics have a clear grain boundary and a uniformly distributed grain size. At room temperature, the ferroelectric and piezoelectric properties of the BNKT18 ceramics have been improved with the addition of Gd₂O₃, and the BNKT18 ceramics doped with 0.4 wt.% Gd₂O₃ have the highest piezoelectric constant (d₃₃ = 137 pC/N), highest relative dielectric constant (ε_r = 1023) and lower dissipation factor (tan δ = 0.044) at a frequency of 10 kHz. The BNKT18 ceramics doped with 0.2 wt.% Gd₂O₃ have the highest planar coupling factor (k_p = 0.2463).     

Sintering temperature-induced electrical properties of (Ba0.90Ca0.10)(Ti0.85Zr0.15)O3 lead-free ceramics

Effects of the sintering temperature on the microstructure and electrical properties of (Ba_0.90Ca_0.10)(Ti_0.85Zr_0.15)O₃ (BCTZ) lead-free piezoelectric ceramics have been studied, where these ceramics were prepared by the conventional oxide-mixed method at varied sintering temperatures from 1300 °C to 1500 °C. These BCTZ ceramics exhibits a phase transition from a rhombohedral phase to the coexistence of rhombohedral and tetragonal phases with an increase of sintering temperature. With an increase of sintering temperature, their relative density and average grain size gradually increase, and electrical properties are improved greatly. These BCTZ ceramics sintered at ～1440 °C have optimum electrical properties: d₃₃ ～ 442 pC/N and k_p ～ 48.9%, making it a promising material for lead-free piezoelectric ceramics.     

Structure and electrical properties of Er2O3 doped 0.82Bi0.5Na0.5TiO3–0.18Bi0.5K0.5TiO3 lead-free piezoelectric ceramics

Er₂O₃ (0–0.8 wt.%)-doped 0.82Bi_0.5Na_0.5TiO₃–0.18Bi_0.5K_0.5TiO₃ (BNKT18) lead-free piezoelectric ceramics were synthesized by a conventional solid-state reaction method. The effects of Er₂O₃ on the microstructure and electrical properties were investigated. X-ray diffraction (XRD) data shows that Er₂O₃ in an amount of 0.2–0.8 wt.% can diffuse into the lattice of the BNKT18 ceramics and form the pure perovskite phase. Scanning electron microscope (SEM) images indicate that the grain sizes of BNKT18 ceramics decrease with the increase of Er₂O₃ content; in addition, the modified ceramics have the clear grain boundary and a uniformly distributed grain size. At room temperature, the electrical properties of the BNKT18 ceramics have been improved with the addition of Er₂O₃, and the BNKT18 ceramics doped with 0.6 wt.% Er₂O₃ have the highest piezoelectric constant (d₃₃ = 138 pC/N), the highest planar coupling factor (k_p = 0.2382), the highest remnant polarization (P_r = 25.2 μC/cm²), the higher relative dielectric constant (ε_r = 936) and lower dissipation factor (tanδ = 0.047) at a frequency of 10 kHz. Moreover, the T_m and T_d of the samples increase with the addition of Er₂O₃.     

Effect of V2O5 additive to 0.4SrTiO3–0.6La(Mg0.5Ti0.5)O3 ceramics on sintering behavior and microwave dielectric properties

The effect of V₂O₅ addition on the microwave dielectric properties and the microstructures of 0.4SrTiO₃–0.6La(Mg_0.5Ti_0.5)O₃ ceramics sintered for 5 h at different sintering temperature were investigated systematically. It was found that the sintering temperature was effectively lowered about 200 °C by increasing V₂O₅ addition content. The grain sizes, bulk density as well as microwave dielectric properties were greatly dependent on sintering temperature and V₂O₅ content. The 4ST–6LMT ceramics with 0.25% V₂O₅ sintered at 1400 °C for 5 h in air exhibited optimum microwave dielectric properties of ɛ_r = 50.7, Q × f = 15049.6 GHz, T_f = −1.7 ppm/°C.     

Sol–gel-hydrothermal synthesis and sintering of K0.5Bi0.5TiO3 nanowires

The sol–gel-hydrothermal processing of K_0.5Bi_0.5TiO₃ (KBT) nanowires as well as their sintering behavior at 1000–1100 °C were investigated. The morphological analyses indicated that sol–gel-hydrothermal route led to the formation of KBT nanowires with diameters of 4 nm and lengths of 100 nm at low processing temperature of 200 °C with KOH concentration of 6 M. It is believed that the gel precursor and hydrothermal environment play an important role in the formation of the nanowires. The KBT ceramics with a relative density of more than 95% can be successfully fabricated from the high quality KBT nanowires even by a conventional sintering process. The KBT ceramics sintered at 1050 °C showed typical characteristics of relaxor ferroelectrics, and the dielectric properties were better than that prepared by all other methods reported previously.     

Preparation and characterization of the bismuth sodium titanate (Na0.5Bi0.5TiO3) ceramic doped with ZnO

This study investigates effects of the zinc oxide (ZnO) addition and the sintering temperature on the microstructure and the electrical properties (such as dielectric constant and loss tangent) of the lead-free piezoelectric ceramic of bismuth sodium titanate (Na_0.5Bi_0.5TiO₃), NBT, which was prepared using the mixed oxide method. Three kinds of starting powders (such as Bi₂O₃, Na₂CO₃ and TiO₂) were mixed and calcined. This calcined NBT powder and a certain weight percentage of ZnO were mixed and compressed into a green compact of NBT–ZnO. Then, this green compact of NBT–ZnO was sintered to be a disk doped with ZnO, and its characteristics were measured. In this study, the calcining temperature was 800 °C, the sintering temperatures ranged from 1000 to 1150 °C, and the weight percentages of ZnO doping included 0.0, 0.5, 1.0, and 2.0 wt%. At a fixed wt% ZnO, the grain size increases with increase in the sintering temperature. The largest relative density of the NBT disk obtained in this study is 98.3% at the calcining temperature of 800 °C, the sintering temperature of 1050 °C, and 0.5 wt% ZnO addition. Its corresponding dielectric constant and loss tangent are 216.55 and 0.133, respectively.     

Effects of SrTiO3 on dielectric and piezoelectric properties of K0.48Na0.48Li0.04Nb0.96Ta0.04O3-based piezoceramics

In this study, (100 ? x) K_0.48Na_0.48Li_0.04Nb_0.96Ta_0.04O₃ ? xSrTiO₃ (0 ≤ x ≤ 10) ceramics were fabricated via normal sintering of synthesized powder by using solid state reaction. All ceramics revealed pure perovskite structure, indicating formation of solid solution between KNNLT and ST up to 10%. With increasing x, the crystal structure of ceramics changed from orthorhombic to tetragonal and finally pseudocubic symmetry when x = 10. Ceramic containing 1% ST had orthorhombic and tetragonal symmetries, simultaneously. Investigation of the variation of dielectric constant of ceramics versus temperature revealed that for ceramic with x = 1, polymorphic phase transition (PPT) temperature between orthorhombic and tetragonal is less than room temperature. Thus coexistence of two different structures in this ceramic is due to vicinity of its composition to morphotropic phase boundary (MPB). As a result, the maximum piezoelectric constant was measured for this ceramic. Ceramics containing 5 and 7.5% ST tend to appear relaxor ferroelectric behavior which is because of chemical inhomogeneities in both A- and B-sites of the ABO₃ perovskite structure.     

Structure and piezoelectric properties of K0.5Na0.5NbO3–AlFeO3 lead-free ceramics by using AlFeO3 as a sintering aid

The (1 ? x)K_0.5Na_0.5NbO₃–xAlFeO₃ ((1 ? x)KNN–xAF) (x = 0.01–0.08) lead-free piezoelectric ceramics were prepared at low temperature of 1,000 °C by conventional ceramic processing. And AF was used as a sintering aid in order to improve the sintering behavior of KNN. The effect of AF addition on the microstructure, dielectric and piezoelectric properties of the ceramics have been investigated. The results indicate that a small amount of AF can improve the sintering performance and piezoelectric properties of the ceramics effectively. The KNN–AF ceramics for x = 0.03 show the best piezoelectric properties: d ₃₃ = 116 pC/N, k _p  = 32.9 %, Q _m  = 114.8, T _C  = 382 °C, P _r  = 21.8 μC/cm². This also indicates that (1 ? x)KNN–xAF ceramics are a promising lead-free piezoelectric candidate material because of its good properties, low-temperature sintering characteristics and plenty of Al₂O₃ and Fe₂O₃ resources with low cost.     

Processing and electrical properties of (1 − x)[(1 − y)(Pb(Mg1/3Nb2/3)O3)–y(Pb(Yb1/2Nb1/2)O3)]–xPbTiO3 ceramics

Ferroelectric ceramics in the vicinity of morphotropic phase boundary (MPB) with compositions represented as (1 ? x)[(1 ? y)(Pb(Mg_1/3Nb_2/3)O₃)–y(Pb(Yb_1/2Nb_1/2)O₃)]–xPbTiO₃ were prepared by solid state reaction. The addition of PYbN to PMN–PT decreased the sintering temperature from 1200 °C (y = 0.25) to 1000 °C (y = 0.75). The PT content, where the MPB was observed, increased with the PYbN addition. A remanent polarization value of 28.5 µC/cm² and a coercive field value of 11 kV/cm were measured from 0.62[0.25PMN–0.75PYbN]–0.38PT ceramics, which were close to the ones measured from PMN–0.32PT ceramics. In addition, the Curie temperature was found to increase with PYbN additions.     

Microstructure, dielectric and piezoelectric properties of La-modified Bi0.5(Na0.84K0.16)0.5TiO3 lead-free ceramics

Lead-free ceramics (Bi_1?xLa_x)_0.5(Na_0.84K_0.16)_0.5TiO₃ were prepared by a conventional ceramic technique and the effects of La doping and sintering temperature on the microstructure, ferroelectric and piezoelectric properties of the ceramics were studied. All the ceramics possess a pure perovskite structure and La³⁺ diffuses into the Bi_0.5(Na_0.84K_0.16)_0.5TiO₃ lattices to form a solid solution with a rhombohedral symmetry. The addition of La leads to the significant change in the grain morphology and size for the (Bi_1?xLa_x)_0.5(Na_0.84K_0.16)_0.5TiO₃ and a number of rod grains with the length of 10–50 μm and the diameter of 1–2 μm are observed in the ceramic with x = 0.04 sintered at 1,140 °C for 2 h. However, as sintering temperature increases to 1,160 °C, the rod grains disappears and the uniform and rectangular grains are observed in the ceramics with x = 0.04. As x increases from 0 to 0.06, the coercive field E _c of the ceramics decreases from 4.33 to 2.81 kV/mm and the remanent polarization P _r of the ceramics retains the high values of 25.9–27.7 μm/cm². The depolarization temperature T _d decreases from 154 to 50 °C with x increasing from 0 to 0.10. All the ceramics exhibit the diffusive phase transition at high temperature (280–320 °C). The ceramic with x = 0.04 sintered at 1,150 °C for 2 h exhibit the optimum piezoelectric properties, giving d ₃₃ = 165 pC/N and k _p = 32.9 %. The optimum sintering temperature is 1,150 °C at which the improved piezoelectric properties (d ₃₃ = 165 pC/N and k _p = 32.9 %) are obtained. At the high La³⁺ level (x = 0.10 and 0.12), the ceramics exhibit weak ferroelectricity (P _r = 13.0–21.2 μm/cm²) and thus possess poor piezoelectricity (d ₃₃ = 17–27 pC/N).     

Effects of Ge<Superscript>4+</Superscript> acceptor dopant on sintering and electrical properties of (K<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>)NbO<Subscript>3</Subscript> lead-free piezoceramics

Lead-free (K_0.5Na_0.5)(Nb_1-xGe _x )O₃ (KNN-xGe, where x = 0-0.01) piezoelectric ceramics were prepared by conventional ceramic processing. The effects of Ge⁴⁺ cation doping on the phase compositions, microstructure and electrical properties of KNN ceramics were studied. SEM images show that Ge⁴⁺ cation doping improved the sintering and promoted the grain growth of the KNN ceramics. Dielectric and ferroelectric measurements proved that Ge⁴⁺ cations substituted Nb⁵⁺ ions as acceptors, and the Curie temperature (T_C) shows an almost linear decrease with increasing the Ge⁴⁺ content. Combining this result with microstructure observations and electrical measurements, it is concluded that the optimal sintering temperature for KNN-xGe ceramics was 1020°C. Ge⁴⁺ doping less than 0.4 mol.%can improve the compositional homogeneity and piezoelectric properties of KNN ceramics. The KNN-xGe ceramics with x = 0.2% exhibited the best piezoelectric properties: piezoelectric constant d₃₃ = 120 pC/N, planar electromechanical coupling coefficient k_p = 34.7%, mechanical quality factor Q_m = 130, and tanδ = 3.6%.     

Microstructures and microwave dielectric properties of Ba4LiNb3O12–BaWO4 composite ceramics

Composite ceramics of (1 ? x)Ba₄LiNb₃O₁₂–xBaWO₄ (x = 0.36–0.69) had been synthesized by co-firing the mixtures of Ba₄LiNb₃O₁₂ and BaWO₄ powders. The structures and microwave dielectric properties of the ceramics were studied by XRD, SEM, and TEM. These ceramics consisted of hexagonal Ba₄LiNb₃O₁₂ and tetragonal BaWO₄. The two phases co-existed well in the ceramics, and there was not obvious reaction at interfacial areas among grains. These ceramics had low sintering temperatures and excellent microwave dielectric properties, especially the small temperature coefficients of resonant frequency (τ_f) and high quality factor (Q × f) values. For composition at x = 0.69, the ceramic sintered at 1070 °C had Q × f value of 75,500 GHz and τ_f value of +8.7 ppm/°C.     

Effect of ZnO on the microstructure and electrical properties of (K<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>)NbO<Subscript>3</Subscript> lead-free piezoelectric ceramics

K_0.5Na_0.5NbO₃–x ZnO (KNN–xZn) lead-free ceramics have been prepared using the conventional sintering technique and the effects of ZnO addition on the phase structure and piezoelectric properties of the ceramics have been studied. Our results reveal that a small amount of ZnO can improve the density of the ceramics effectively. Because of the high density and ZnO doping effects, the piezoelectric and dielectric properties of the ceramics are improved considerably. The good piezoelectric and dielectric properties of d ₃₃ = 114 pC/N, k _p = 0.36, ε _r = 395, and Q _m = 68 were obtained for the KNN ceramics doped with 1 mol% ZnO. Therefore, the KNN-1.0 mol%Zn ceramics is a good candidate for lead-free piezoelectric application.     

Application feasibility of Pb(Zr,Ti)O3 ceramics fabricated from sol–gel derived powders using titanium and zirconium alkoxides

It is believed that what may be termed the ‘Nanoscaled Century’ will lead to a new industrial revolution, particularly in terms of sol–gel methods of assembly for nanostructure devices. A propyl alcohol (1-Pro) based sol–gel chemical has been developed to replace 2-methoxyethanol (MOE), 1,1,1-tris(hydroxymethyl)ethane (THOME) for the fabrication of PbZr_0.53Ti_0.47O₃ (PZT) piezoelectric ceramics. This chemical is prepared from sol–gel derived powders that are near to the morphotropic phase boundary (MPB). The pyrochlore phase was still apparent when calcining at 900 °C with a shorter calcining time, such as 30 min. However, it disappeared for longer calcining times, for example 3 h or more. From the results of the analysis, PZT ceramics calcinations at 900 °C for 4 h, and sintering at 1100 °C for 2 h could reach a pyrochlore-free crystal phase with relative density of approximately 7.9 g/cm³—close to 98% of the theoretical value. The P–E hysteresis loop, measured by the Sawyer–Tower circuit, revealed that the remanent polarization (P_r) and coercive field (E_c) were 8.54 μC/cm² and 15.6 kV/cm, respectively. The vibration modes of the PZT ceramics were between 150 and 1.5 MHz. Morevoer, under such processing conditions the PZT piezoceramics had uniform grain size distribution less than 1 μm and zero temperature coefficient of resonant frequency (TCF). In summary, the PZT ceramics derived from the sol–gel method were confirmed to possess excellent piezoelectric properties. Furthermore, the processing temperatures were scaled down by 100–200 °C, compared to conventional oxide reaction. Finally, from an energy-saving viewpoint, this experiment can potentially make a very positive contribution.     

Effects of GeO2 addition on physical and electrical properties of BaFe0.5Nb0.5O3 ceramic

This paper presents the effect of GeO₂ glass former on the physical and electrical properties of BaFe_0.5Nb_0.5O₃ (BFN) perovskite ceramics. The BFN powder was prepared by a conventional mixed-oxide method and the GeO₂ contents, ranging from 1 to 5 wt.%, were subsequently added to the calcined BFN powder. The mixtures were pressed and sintered to form dense ceramics. We showed that, with the addition of GeO₂, the maximum density was achieved at lower sintering temperature, approximately 200–225 °C lower than those required by the pure BFN ceramic. However, the densities of these GeO₂ doped BFN ceramics were slightly lower than those of pure BFN due to the occurrence of pores. We also found that the addition of GeO₂ reduces the dielectric loss at room temperature from 4.29 to 0.39–0.79 but the dielectric constant at room temperature decreased with the increased GeO₂ concentrations. With small amount of added GeO₂, ferroelectric property of BFN ceramics was also obtained, as confirmed by their hysteresis loops.     

Low-temperature sintering and microwave dielectric properties of CaTi1−x(Fe0.5Nb0.5)xO3 ceramics with B2O3 addition

CaTi_1−x(Fe_0.5Nb_0.5)_xO₃ (0 ≤ x ≤ 1) dielectrics were synthesized via the solid state reaction route and structure analysis was performed together with the dielectric characterization. The substitution of Ti⁴⁺ by Fe³⁺/Nb⁵⁺ and developed phase were studied by X-ray diffraction. The dielectric constant and temperature coefficient of resonant frequency decrease rapidly with an increase of x. The influence of 1–5 wt.% B₂O₃ as a sintering additive investigated at CaTi_0.5(Fe_0.5Nb_0.5)_0.5O₃ solid solutions. The dielectric properties were found to strongly depend on the sintering conditions and contents of B₂O₃ additions. ɛ_r = 52.3, Q × f_o = 2930 GHz and T_f = 13 ppm/°C were obtained for CaTi_0.5(Fe_0.5Nb_0.5)_0.5O₃ specimen 3 wt.% B₂O₃ sintered at 900 °C for 2 h.     

On the synthesis and dielectric studies of (1 − x)Bi(Mg1 / 2Zr1 / 2)O3 − xPbTiO3 piezoelectric ceramic system

Scandium free piezoelectric ceramics of the composition (1 − x)Bi(Mg_1 / 2Zr_1 / 2)O₃ − xPbTiO₃ (BMZ − xPT) were fabricated by the solid state reaction method. Dielectric and structural properties were measured and phase diagram was constructed from the temperature dependent dielectric and impedance data. The morphotropic phase boundary (MPB) was found to be located in the range 0.55 < x < 0.60 with paraelectric–ferroelectric phase transition temperature, T_C (∼ 280 °C). The ceramics near the MPB showed high room temperature dielectric constant (∼ 1387). The room temperature values of the remanent polarization (P_r) and coercive filed (E_C), were ∼ 29 μC/cm² and ∼ 23 kV/cm, respectively.     

Enhanced densification and microwave dielectric properties of Mg2TiO4 ceramics added with CeO2 nanoparticles

We report the study of the effects of processing parameters and additive concentration on the structure, microstructure and microwave dielectric properties of MTO–CeO₂ (x wt.%) ceramics with x = 0, 0.5, 1.0 and 1.5 prepared by solid-state reaction method by adding CeO₂ nanoparticles as a sintering aid. The pure Mg₂TiO₄ ceramics were not densifiable below 1450 °C. However, when CeO₂ nanoparticles were added to MTO, the densification achieved at 1300 °C along with the increase in average grain size with the uniform microstructure and improved microwave dielectric properties. This is mainly driven by the large surface energy of CeO₂ nanoparticles and their defect energy during the sintering process. While the addition of CeO₂ nanoparticles in MTO ceramics does not change the dielectric constant (?_r), the unloaded quality factor (Q_u) was altered significantly. MTO–CeO₂ (1.5 wt.%) ceramics sintered at 1300 °C exhibit superior microwave dielectric properties (?_r ～ 14.6, Q × f₀ ～ 167 THz), as compared to the pure Mg₂TiO₄ ceramics. The observed results are correlated to the enhancement in density and the development of uniform microstructure with the enhanced grain size.