Preparation and characterization of K0.5Bi0.5TiO3 particles synthesized by a stirring hydrothermal method

Nanoparticles of potassium bismuth titanate K_0.5Bi_0.5TiO₃ (KBT) with an average particle size of 38 nm were prepared using a stirring hydrothermal method. The pure KBT was obtained in 8 h reaction time instead of 24–48 h for conventional hydrothermal method. X-ray diffraction, Raman spectroscopy and TG analysis were used to check the proportion of hydroxyl group existing into the crude and the calcined KBT. A Hydroxyl group was found to affect the crystallite structure parameters and cell volume. When temperature increases from 25 to 1050 °C, the tetragonal structure presents a c/a ratio which decreases from 1.048 to 1.012. TG analysis and Raman vibration at high frequencies show that c/a is affected by hydroxyl group content below 750 °C and by potassium and bismuth vacancies above this temperature. The ceramic KBT showing a 300 nm size presents an improved ε_r=780 and a dielectric loss tan δ=0.062 at room temperature. Electric conductivity σ_ac was also lowered to 10⁻⁶ (Ω m)⁻¹ with an activation energy change at 673 K from 0.35 to 0.605 eV.     

Effects of K0.5Bi0.5TiO3 addition on dielectric properties of BaTiO3 ceramics

(1?x)BaTiO₃–xK_0.5Bi_0.5TiO₃ (abbreviated as BT–KBT, 0.10≦x≦0.15) dielectric ceramics were prepared by a conventional oxide mixing method. The effects of KBT content on the densification, microstructure and dielectric properties of BT ceramics were investigated. The density characterization results show that the addition of KBT significantly lowered the sintering temperature of BT ceramics to about 1280 °C. The XRD results showed that the phase compositions of all samples were pure tetragonal phases. The dielectric constant and dielectric loss firstly increased and then decreased with the increase of KBT. In addition, dielectric constant and dielectric loss versus frequency were characterized in the frequency range from 100 Hz to 2 MHz. It is found that the dielectric constant and the dielectric loss changed with the increase of KBT contents regularly.     

Synthesis of (Ni,Mn,Co)O4 nanopowder with single cubic spinel phase via combustion method

(Ni,Mn,Co)O₄ nanopowders with single cubic phase were successfully synthesized using combustion methods. Particle size of the as-burnt nanopowders after combustion was about 20 nm. Crystallization behavior of the NMC was investigated using various techniques such as X-ray diffraction (XRD), thermogravimetric (TG), Fourier transform infrared (FT-IR) spectroscopy, and transmission electron microscopy (TEM). Calcination at different temperature from 400 °C to 700 °C provides the powders with increased crystallinity and grain size. However, further increasing temperature above 800 °C for calcination, cubic spinel phase of NMC partly transformed to tetragonal spinel phase, which implies that cubic spinel phase of NMC nanopowder synthesized by combustion method becomes unstable above 800 °C.     

High performance lead-free piezoelectric 0.96(K0.48Na0.52)NbO3-0.03[Bi0.5(Na0.7K0.2Li0.1)0.5]ZrO3-0.01(Bi0.5Na0.5)TiO3 single crystals grown by solid state crystal growth and their phase relations

0.96(K_0.48Na_0.52)NbO₃-0.03[Bi_0.5(Na_0.7K_0.2Li_0.1)_0.5]ZrO₃-0.01(Bi_0.5Na_0.5)TiO₃ single crystals were grown for the first time by the solid state crystal growth method, using [001] or [110]-oriented KTaO₃ seed crystals. The grown single crystal shows a dielectric constant of 2720 and polarization-electric field loops of a lossy normal ferroelectric, with P_r = 45 μC/cm² and E_c = 14.9 kV/cm, while the polycrystalline samples with a dielectric constant of 828 were too leaky for P-E measurement due to humidity effects. The single crystal has orthorhombic symmetry at room temperature. Dielectric permittivity peaks at 26 °C and 311 °C, respectively, are attributed to rhombohedral-orthorhombic and tetragonal–cubic phase transitions. Additionally, Raman scattering shows the presence of an orthorhombic-tetragonal phase transition at ∼150 °C, which is not indicated in the permittivity curves but by the loss tangent anomalies. A transition around 700 °C in the high temperature dc conductivity is suggested to be a ferroelastic-paraelastic transition.     

Densification,mechanical and thermal properties of ZrC1 − x ceramics fabricated by two-step reactive hot pressing of ZrC and ZrH2 powders

Densification behavior, mechanical and thermal properties of ZrC_1 ? x ceramics with various C/Zr ratios of 0.6–1.0 have been investigated by two-step reactive hot pressing of ZrC and ZrH₂ powders at 30 MPa and 1500–2100 °C. The two-step reactive hot pressed ZrC_1 ? x ceramic has a higher relative density (> 95.3%) than that (91.9%) of stoichiometric ZrC sintered at 2100 °C. A cubic Zr₂C-type ordered phase forms in the ZrC_1 ? x sample obtained at a ZrC/ZrH₂ molar ratio of 0.6 at a relatively low temperature of 1100 °C. The decrease in C/Zr ratio is beneficial to densification of ZrC_1 ? x ceramic, however, excess grain growth occurs after sintering above densification temperature. The elastic modulus and Vickers hardness decrease with decreasing the C/Zr ratio. With decreasing the C/Zr ratio, both thermal conductivity and specific heat decrease due to the enhanced scattering of conducting phonons and electrons by carbon vacancies.     

Structural parameters and oxygen ion conductivity of Y2O3–ZrO2 and MgO–ZrO2 at high temperature

The oxygen ion conductivity of zirconia-based solid electrolytes doped with 8 mol% Y₂O₃–ZrO₂ (YSZ) and 9 mol% MgO–ZrO₂ (Mg-PSZ) at high temperature was investigated in terms of their thermal behavior and structural changes. At room temperature, YSZ showed a single phase with a fluorite cubic structure, whereas Mg-PSZ had a mixture of cubic, tetragonal and some monoclinic phases. YSZ exhibited higher ionic conductivity than Mg-PSZ at temperatures from 600 °C to 1250 °C because of the existence of the single cubic structure and low activation energy. A considerable increase in the conductivity with increasing temperature was observed in Mg-PSZ, which showed higher ionic conductivity than YSZ within the higher temperature range of 1300–1500 °C. A monoclinic-to-tetragonal phase transformation was found in Mg-PSZ and the lattice parameter of the cubic phase increased at 1200 °C. The phase transformation and the large lattice free volume contributed to the significant enhancement of the ionic conductivity of Mg-PSZ at high temperatures.     

Low-temperature-sintered Bi0.5Na0.5TiO3-based lead-free ferroelectric ceramics with good piezoelectric properties

Li₂CO₃ has been used as a sintering aid for fabricating lead-free ferroelectric ceramic 0.93(Bi_0.5Na_0.5TiO₃)-0.07BaTiO₃. A small amount (0.5 wt%) of it can effectively lower the sintering temperature of the ceramic from 1200 °C to 980 °C. Unlike other low temperature-sintered ferroelectric ceramics, the ceramic retains its good dielectric and piezoelectric properties, giving a high dielectric constant (1570), low dielectric loss (4.8%) and large piezoelectric coefficient (180 pC/N). The “depolarization” temperature is also increased to 100 °C and the thermal stability of piezoelectricity is improved. Our results reveal that oxygen vacancies generated from the diffusion of the sintering aid into the lattices are crucial for realizing the low temperature sintering. Owing to the low sintering temperature and good dielectric and piezoelectric properties, the ceramics, especially of multilayered structure, should have great potential for practical applications.     

Microstructure,ferroelectric and piezoelectric properties of Bi0.5K0.5TiO3-modified BiFeO3–BaTiO3 lead-free ceramics with high Curie temperature

The effects of composition, sintering temperature and dwell time on the microstructure and electrical properties of (0.75 ? x)BiFeO₃–0.25BaTiO₃–xBi_0.5K_0.5TiO₃ + 1 mol% MnO₂ ceramics were studied. The ceramics sintered at 1000 °C for 2 h possess a pure perovskite structure and a morphotropic phase boundary of rhombohedral and pseudocubic phases is formed at x = 0.025. The addition of Bi_0.5K_0.5TiO₃ retards the grain growth and induces two dielectric anomalies at high temperatures (T₁ ～ 450–550 °C and T₂ ～ 700 °C, respectively). After the addition of 2.5 mol% Bi_0.5K_0.5TiO₃, the ferroelectric and piezoelectric properties of the ceramics are improved and very high Curie temperature of 708 °C is obtained. Sintering temperature has an important influence on the microstructure and electrical properties of the ceramics. Critical sintering temperature is 970 °C. For the ceramic with x = 0.025 sintered at/above 970 °C, large grains, good densification, high resistivity and enhanced electrical properties are obtained. The weak dependences of microstructure and electrical properties on dwell time are observed for the ceramic with x = 0.025.     

Relaxor behavior and ferroelectric properties of a new Ba4SmFe0.5Nb9.5O30 tungsten bronze ceramic

Unfilled tungsten bronze ceramics with a composition of Ba₄SmFe_0.5Nb_9.5O₃₀ were prepared by the conventional solid-state sintering method. The phase, microstructure, dielectric and ferroelectric properties were studied. Room temperature XRD results indicated that the ceramic occurs in the tetragonal space group P4bm phase with cell parameters of a=b=12.4712(2) Å and c=3.9430(2) Å. The temperature-dependent dielectric properties, XRD data and Raman spectra data indicated that BSFN ceramics exhibit no phase changes from 35 °C to 450 °C. Fitting of a Vogel-Fulcher relationship with an activated energy E_a of 0.11 eV indicates an unambiguous dielectric relaxor state near room temperature. Furthermore, the BSFN ceramics exhibited residual polarization and coercive field of 3.45 µC/cm² and 24.65 kV/cm, respectively.     

Structural,microstructural, ferroelectric and photoluminescent properties of praseodymium modified Ba0.98Ca0.02Zr0.02Ti0.98O3 ceramics

The ‘x’ wt% (x = 0, 0.02, 0.04 and 0.06) Pr₆O₁₁ modified Ba_0.98Ca_0.02Zr_0.02Ti_0.98O₃ (BCZT – x Pr) piezoelectric ceramics have been fabricated by the solid state reaction method with sintering at 1450 °C (x = 0) and 1350 °C (0.02 ≤ x ≤ 0.06) for 2 h. The impact of Pr concentration on the structural, microstructural, photoluminescence and ferroelectric properties has been systematically investigated. The x-ray diffraction (XRD) patterns revealed the co-existence of tetragonal and orthorhombic phases at room temperature upto x = 0.04 Pr concentration. The grain size was found to decrease upto x = 0.04 Pr content. Room temperature Raman spectroscopy results were consistent with the XRD results. The photoluminescence (PL) spectra showed significant emissions consisting of strong blue (489 nm), green (528 nm) and red (649 nm) wavelengths. The emission intensities of PL spectrum were strongly Pr concentration dependent and a maximum value was obtained for 0.04 Pr modified BCZT ceramic. Further, a large remnant polarization (2P_r ~ 13 µC/cm²) and low coercive field (E_C ~ 22 V/cm) were obtained for BCZT – 0.04 Pr ceramic. The crystal structure and microstructure affect the photoluminescence and ferroelectric properties. Such properties of 0.04 Pr modified BCZT ceramic make it the potential candidate for novel integrated and multifunctional devices.     

Compositional range and electrical properties of the morphotropic phase boundary in the Na0.5Bi0.5TiO3–K0.5Bi0.5TiO3 system

We have investigated the Na_0.5Bi_0.5TiO₃–K_0.5Bi_0.5TiO₃ (NBT–KBT) system, with its complex perovskite structure, as a promising material for piezoelectric applications. The NBT–KBT samples were synthesized using a solid-state reaction method and characterized with XRD and SEM. Room-temperature XRD showed a gradual change in the crystal structure from tetragonal in the KBT to rhombohedral in the NBT, with the presence of an intermediate morphotropic region in the samples with a compositional fraction x between 0.17 and 0.25. The fitted perovskite lattice parameters confirmed an increase in the size of the crystal lattice from NBT towards KBT, which coincides with an increase in the ionic radii. Electrical measurements on the samples showed that the maximum values of the dielectric constant, the remanent polarization and the piezoelectric coefficient are reached at the morphotropic phase boundary (MPB) (? = 1140 at 1 MHz; P_r = 40 μC/cm²; d₃₃ = 134 pC/N).     

Ultra-low temperature sintering and microwave dielectric properties of a novel temperature stable Na2Mo2O7-Na0.5Bi0.5MoO4 ceramic

The novel ultra-low temperature sintering (1-x)Na₂Mo₂O₇-xNa_0.5Bi_0.5MoO₄ ceramics have been obtained via solid-state reaction method for passive integration use. The Na₂Mo₂O₇ and Na_0.5Bi_0.5MoO₄ crystal phases are found to be compatible with each other from the results of XRD and SEM-EDS. With the x value changing from 0.36 to 1.00, the ε_r increases from 16.0 to 32.0 and the τ_f value varies from ?58 to 47 ppm/°C. At x = 0.75, the 0.25Na₂Mo₂O₇-0.75Na_0.5Bi_0.5MoO₄ ceramic sintered at an ultra-low sintering temperature of 580 °C can be densified (>96%) and possesses good microwave dielectric properties of an ε_r of 24.0, a Q × f value of 13,000 GHz (at 6.2 GHz), and a τ_f value of 3 ppm/°C. The theoretical ε_r and τ_f of the (1-x)Na₂Mo₂O₇-xNa_0.5Bi_0.5MoO₄ composites were calculated using the mixing law and in accordance with the measured values.     

Structural properties of (Bi0.5Na0.5)1−xBaxTiO3 lead-free piezoelectric ceramics

A systematic XRD investigation of poled and unpoled ceramics of the system (1 ? x) Bi_0.5Na_0.5TiO₃–x BaTiO₃ (0 ≤ x ≤ 0.2) (BNBT) was performed. The variation of the lattice parameters confirms the existence of a morphotropic phase boundary at 0.06 ≤ x ≤ 0.08; however, significant differences in unit cell parameters between poled and unpoled states appear. Lattice distortions of the rhombohedral and tetragonal phases are significantly increased in poled samples. Dramatic changes in peak intensities of the pseudo-cubic (2 0 0) reflections between poled and unpoled samples reveal a strong enhancement of the tetragonal volume fraction in the poled state. Temperature-dependent XRD studies confirm a transition into a cubic high-temperature phase. This transition is rather smooth in the unpoled state. In poled samples, the tetragonal distortion and the tetragonal volume fraction display a different temperature variation and tetragonal regions seem to persist into the cubic phase field.     

Low-temperature sintering,structure transition and dielectrical properties of Ba0.85Ca0.15Ti0.9Zr0.1O3 with Na0.5Bi0.5TiO3 addition

Na_0.5Bi_0.5TiO₃-modified Ba_0.85Ca_0.15Ti_0.9Zr_0.1O₃ ceramics were prepared by solid state route and their structural and dielectric properties were investigated. The sintering temperature of BCTZ ceramics has been significantly decreased from 1460 °C to 1280 °C with NBT addition. All samples showed a pure perovskite structure and a stable solid solution has been formed between BCTZ and NBT. Some tetragonal phase gradually transformed to rhombohedral or cubic phase with the addition of NBT. Dielectric peak gradually becomes broader, revealing that the diffuser behavior was enhanced. The prominent superimposed loss peaks related to thermally activated relaxation process. The values of activation energy of the relaxation process are 1.034, 1.285, 1.308 and 1.353 eV, which could be associated with the migration of oxygen vacancies.     

Temperature–stable dielectrics based on Cu–doped Bi2Mg2/3Nb4/3O7 pyrochlore ceramics for LTCC

Temperature–stable dielectrics based on Cu–doped Bi₂Mg_2/3Nb_4/3O₇ pyrochlore ceramics were prepared by conventional solid–state reaction. Microstructure analysis indicates that all of the specimen maintain the cubic pyrochlore phase, a fluorite–like phase of Bi₃NbO₇ and a Bi₅Nb₃O₁₅ formed for Cu doping. The dielectric constant is dominated by densification of samples and secondary phases, while the dielectric loss is related by the secondary phases, grain boundaries, and leakage current characteristics. The (1-x)BMN - xCuO(x = 0.1 mol%) ceramic sintered at 925 °C shows excellent dielectric properties with dielectric constant of ~184.06, dielectric loss of ~0.0017 and near zero τ_ε (?20 ppm/°C) is obtained at sintering temperature of 925 °C, which could be a promising candidate for LTCC.     

Structural,magnetic and magnetocaloric properties of nanostructured Pr0.5Sr0.5MnO3 manganite synthesized by mechanical alloying

Nanostructure Pr_0.5Sr_0.5MnO₃ compounds were elaborated by mechanical milling in a planetary high energy ball mill at various milling times followed by high temperature sintering under air at 1400 °C for 20 h. The phase structure, the morphology, the magnetic and magnetocaloric properties of the powders were characterized by X-ray diffractometry, scanning electron microscopy and a vibrating sample magnetometer. Rietveld refinement of the X-ray diffraction patterns shows that all specimens crystallize in the tetragonal system with I4/mcm space group. Increasing the milling time up to 16 h, the average crystallites size decreases to the nanoscale (∼36 nm). During the intermediate stage of milling, significant changes occur in the morphology of the powder particles, due to the severe plastic deformation. Significant refinement in particle size was found evident at the final stage of milling (∼2 µm). From magnetic measurements, it was found that all samples present two magnetic transitions as a function of temperature. The Curie temperature T_C decreases with increasing milling time. Moreover, it was revealed that the antiferromagnetic domains fractions highly dependent on crystallites sizes. A large magnetocaloric effect and an important value of the relative cooling power around Neel temperature was observed in all samples. These characteristics may be related to the first-ordered nature of this transition. Moreover, the magnetic entropy change and the relative cooling power were increased with decreasing crystallites sizes.     

Ferroelectric phase transition and electrical properties of high-TC PMN-PH-PT ceramics prepared by partial oxalate route

The high-Curie temperature (T_C) 0.15Pb(Mg_1/3Nb_2/3)O₃-0.38PbHfO₃-0.47PbTiO₃ (PMN-PH-PT) piezoelectric ceramics were prepared by the partial oxalate route via the B-site oxide mixing method. The obtained uniform nm-sized PMN-PH-PT precursor powders provide high calcining and sintering activity for synthesizing ceramics, based on which the synthesis conditions were tailored as calcining at 775 °C and sintering at 1245 °C. The partial oxalate route synthesized PMN-PH-PT ceramics are far superior to the counterparts synthesized by the columbite precursor method and exhibit excellent thermal stability of the piezoelectric properties under T_C (～292 °C), ensuring the potential application in transducers under elevated environmental temperatures. The temperature dependent Raman spectroscopy not only proves the occurrence of the ferroelectric to paraelectric phase transition around T_C, but also confirms the successive phase symmetry transitions, which correlate with the polar nanoregions (PNRs) and/or the coexistence of multiple ferroelectric phases, revealing the origin of the enhanced electrical properties in the PMN-PH-PT ceramics.     

Compositional inhomogeneity and segregation in (K0.5Na0.5)NbO3 ceramics

In this report, the effects of the calcination temperature of (K_0.5Na_0.5)NbO₃ (KNN) powder on the sintering and piezoelectric properties of KNN ceramics have been investigated. KNN powders are synthesized via the solid-state approach. Scanning electron microscopy and X-ray diffraction characterizations indicate that the incomplete reaction at 700 °C and 750 °C calcination results in the compositional inhomogeneity of the K-rich and Na-rich phases while the orthorhombic single phase is obtained after calcination at 900 °C. During the sintering, the presence of the liquid K-rich phase due to the lower melting point has a significant impact on the densification, the abnormal grain growth and the deteriorated piezoelectric properties. From the standpoint of piezoelectric properties, the optimal calcination temperature obtained for KNN ceramics calcined at this temperature is determined to be 800 °C, with piezoelectric constant d₃₃=128.3 pC/N, planar electromechanical coupling coefficient k_p=32.2%, mechanical quality factor Q_m=88, and dielectric loss tan δ=2.1%.     

Low-temperature sintering of Ti1−xCux/3Nb2x/3O2 (x = 0.23) microwave dielectric ceramics with CuO and B2O3 addition

The influence of CuO and B₂O₃ addition on the sintering behavior, microstructure and microwave dielectric properties of Ti_1?xCu_x/3Nb_2x/3O₂ (TCN, x = 0.23) ceramic have been investigated. It was found that the addition of CuO and B₂O₃ successfully reduced the sintering temperature of TCN ceramics from 950 to 875 °C. X-ray diffraction studies showed that addition of CuO-B₂O₃ has no effect on the phase composition. The TCN ceramics with 0.5 wt% CuO-B₂O₃ addition showed a high dielectric constant of 95.63, τ_f value of + 329 ppm/°C and a good Q × f value of 8700 GHz after sintered at 875 °C for 5 h, cofirable with silver electrode.     

High-temperature dielectric and relaxation behavior of Yb-doped Bi0.5Na0.5TiO3 ceramics

Microstructure, phase transition and dielectric properties of Yb-doped Bi_0.5Na_0.5TiO₃ (BNT) ceramics were investigated. It is found that ytterbium promotes the grain growth and densification of the ceramics while Ti-rich impurity appears due to the compensation of Ti-vacancy. The dielectric operational temperature range of the ceramics with a±15% tolerance was greatly broaden until 500 °C by ytterbium doping. Meanwhile, the diffuseness of the diffuse phase transition increases with the increase of doping Yb. BNT ceramics with 3 mol% Yb doping shows a near-plateau dielectric behavior in a broad temperature range from 147 to 528 °C and a low dielectric loss (<0.025) from 154 to 356 °C, indicating that it is a promising material for applications in high-temperature capacitor.