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%.     

The mechanism of abnormal grain growth in polycrystalline diamond during high pressure-high temperature sintering

The abnormal grain growth (AGG) in polycrystalline diamond (PCD) during high pressure-high temperature sintering (6 GPa; 1600 °C) was investigated. Some grains grew to a size of several hundreds of micrometers in PCD manufactured with 2-μm diamond powder. However, the AGG distribution was inhomogeneous possibly due to the inhomogeneous pressure distribution. When the initial average particle diameter of diamond powder was 4 μm, no AGG was observed within the experimental range (1 h) due to an increase in the diffusion distance. Electron backscattered diffraction technique was used to show that the abnormally grown grains were single crystals with and without their twins with the {1 1 1} twinning planes. The {1 1 1} faceted planes developed in the abnormally grown crystals, suggesting that AGG in PCD could be explained by the 2D nucleation mechanism.     

Textured (K0.5Na0.5)NbO3 ceramics prepared by screen-printing multilayer grain growth technique

The screen-printing multilayer grain growth (MLGG) technique is successfully applied to alkaline niobate lead-free piezoelectric ceramics. Highly textured (K_0.5Na_0.5)NbO₃ (KNN) ceramics with 〈0 0 1〉 orientation (f = 93%) were fabricated by MLGG technique with plate-like NaNbO₃ templates. The influence of sintering temperature on grain orientation and microstructure was studied. The textured KNN ceramics showed very high piezoelectric constant d₃₃ = 133 pC/N, and high electromechanical coupling factor k_p = 0.54. These properties were superior to those of conventional randomly oriented ceramics, and reach the level of those of textured KNN ceramic prepared by tape-casting technique. Compared with other grain orientation techniques, screen-printing is a simple, inexpensive and effective method to fabricate grain oriented lead-free piezoelectric ceramics.     

Crystal growth and ferroelectric property of Na0.5K0.5NbO3 and Mn-doped Na0.5K0.5NbO3 crystals grown by floating zone method

Na_0.5K_0.5NbO₃ (NKN) and Mn-doped NKN crystals, which are one of the promising candidates of lead-free piezoelectric materials, were grown by using a floating zone (FZ) method. The resulting crystal growth was compared with crystal growth that resulted from using a flux method in a previous study. In the crystal grown by FZ method under where the growth rate was controlled to 3 mm/h, thin layers formed parallel and perpendicular directions to the growth direction. In the crystal grown by FZ method, the crystal structure could not be classified as having the orthorhombic lattice of Amm2, which was observed in the crystal grown using a flux method. It was found that doped Mn was substituted in the perovskite-type lattice of NKN. Pure NKN crystals showed 90° domains that had a zig-zag shape, whereas Mn-doped NKN crystals were aligned to the domain layers in straight lines. It was confirmed that Mn-doped NKN crystal showed a square P–E hysteresis loop.     

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

Textured (Na,K)_0.5Bi_0.5TiO₃ ceramics were fabricated by reactive-templated grain growth in combination with tape casting. The effects of sintering conditions on the grain orientation and the piezoelectric properties of the textured (Na,K)_0.5Bi_0.5TiO₃ ceramics were investigated. The results show that the textured ceramics have microstructure with plated-like grains aligning in the direction parallel to the casting plane. The ceramics exhibit {h 0 0} preferred orientation and the degree of orientation is larger than 0.7. The degree of grain orientation increases with the increasing sintering temperature. The textured ceramics show anisotropy dielectric and piezoelectric properties in the directions of parallel and perpendicular to the casting plane. The ceramics in the perpendicular direction exhibit better dielectric and piezoelectric properties than those of the nontextured ceramics with the same composition. The optimized sintering temperature is 1150 °C where the maximum d₃₃ of 134 pC/N parallel to casting plane, the maximum k₃₁ of 0.31, and the maximum Q_m of 154 in perpendicular direction were obtained.     

Investigations of BaFe0.5Nb0.5O3 nano powders prepared by a low temperature aqueous synthesis and resulting ceramics

A facile method to prepare nanoscaled BaFe_0.5Nb_0.5O₃ via synthesis in boiling NaOH solution is described herein. The nano-crystalline powder has a high specific surface area of 55 m² g⁻¹ and a crystallite size of 15 nm. The as-prepared powder does not show any significant crystallite growth up to 700 °C. The activation energy of the crystallite growth process was calculated as 590 kJ mol⁻¹. Dense ceramics can be obtained either after sintering at 1200 °C for 1 h or after two-step sintering at 1000 °C for 10 h. The average grain sizes of ceramic bodies can be tuned between 0.23 μm and 12 μm. The thermal expansion coefficient was determined as 11.4(3)·10⁻⁶ K⁻¹. The optical band gap varies between 2.90(5) and 2.63(3) eV. Magnetic measurements gave a Néel temperature of 20 K. Depending on the sintering regime, the ceramic samples reach permittivity values between 2800 and 137,000 at RT and 1 kHz.     

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.     

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.     

Low-temperature sintered Bi0.5Na0.5TiO3-SrTiO3 incipient piezoceramics and the co-fired multilayer piezoactuator thereof

Lead-free Bi_0.5Na_0.5TiO₃-SrTiO₃ incipient piezoceramics with Li₂CO₃ and MnO₂ additives were successfully fabricated at low firing temperature for applications in co-fired multilayer piezoactuators. The addition of Li₂CO₃ effectively shifted the sintering temperature from 1230 °C down to 1075 °C, where the ceramics were co-fired with a Ag/Pd (75/25) inner electrode. The prototype actuators were prepared by tape-casting method using ceramics with the composition of 0.74Bi_0.5Na_0.5TiO₃-0.26 SrTiO₃ + 0.15 wt%MnO₂ + 0.45 wt%Li₂CO₃. The total number of active layers was 13, and each ceramic layer had a thickness of 60 μm. The actuator output a large strain up to ∼0.20% at a driving field of 4 kV/mm, due to the field-induced phase transition between the ergodic relaxor and ferroelectric phases. The excellent voltage-displacement performance of the prototype actuator demonstrates the potential for industrial applications.     

Dielectric and piezoelectric properties of lead-free (K0.44Na0.46)NbO3-0.5%MnO2 single crystals grown by the TSSG method

Lead-free (K_0.44Na_0.46)NbO₃-0.5%MnO₂ (KNN-0.5%MnO₂) single crystals with dimensions of Ф30×10 mm were successfully grown by a top seeded solution growth technique (TSSG). The X-ray diffraction pattern has shown that the as-grown crystals have an orthorhombic perovskite structure. The orthorhombic-tetragonal (T_O-T) and tetragonal-cubic phase transition temperature (the Curie temperature T_C) of the single crystal were found at 184 °C and 412 °C, respectively. The KNN-0.5%MnO₂ single crystals exhibited high piezoelectric constants d₃₃ and dielectric permittivity ε_r, being 261 pC/N and 275. Well saturated P-E hysteresis loop with remnant polarization P_r=22.06 µC/cm² and coercive field E_c=17.93 kV/cm was obtained at a maximum electric field of 3 kV/mm. A high strain (0.24%) and electromechanical coupling coefficient K_t (65.9%) were obtained along the (001) orientation. These excellent results indicated that the KNN-0.5%MnO₂ single crystals could be used as high quality lead-free material.     

(Na,K)NbO3–CaCu3Ti4O12 perovskite composites for supercapacitor based piezoelectric devices

The supercapacitor based piezoelectric material composite (Na,K)NbO₃–CaCu₃Ti₄O₁₂ (NKN–CCTO) is investigated for possible application in piezoelectric devices. (1−x)NKN–xCCTO (0.015≤x≤0.06) with different sintering conditions is researched for supercapacitor based piezoelectric applications. The 0.94NKN–0.06CCTO composite sintered at 975 °C shows the highest dielectric permittivity of 796. Clear SEM images of (1−x)NKN–xCCTO reveal that these compositions have high density well-crystallized structures. The composition and sintering temperature dependence of dielectric permittivities and piezoelectric coefficients, plotted in three dimensions, show that the 0.985NKN–0.015CCTO composite sintered at 1025 °C has a moderate dielectric permittivity of 405 and a piezoelectric constant of 98 pC/N.     

Intermediate-temperature conductivity of B-site doped Na0.5Bi0.5TiO3-based lead-free ferroelectric ceramics

Na_0.5Bi_0.5TiO₃ (NBT) based oxide-ion conductor ceramics have great potential applications in intermediate-temperature solid oxide fuel cells (SOFCs) and oxygen sensors. Na_0.5Bi_0.49Ti_1−xMg_xO_3−δ ceramics with x=0, 0.01, 0.02, 0.03, 0.05 and 0.08 were prepared by conventional solid-state reaction. XRD measurement and SEM analysis revealed the formation of pure perovskite structures without secondary phase. MgO doping greatly decreased the sintering temperature and inhibited grain growth. AC impedance spectroscopy measurement was adopted to measure the total conductivity, which was found to increase with MgO doping content ranging from 0 to 3 mol% and subsequently to decrease. High oxygen ionic conductivity σ_t=0.00629 S/cm was achieved for sample doped with 3 mol% MgO at 600 °C in air atmosphere.     

Grain size control of lead-free Li0.06(Na0.5K0.5)0.94NbO3 piezoelectric ceramics by Ba and Ti doping

In this study, Ba- and Ti-doped Li_0.06(Na_0.5K_0.5)_0.94NbO₃ [(1 ? x)Li_0.06(Na_0.5K_0.5)_0.94NbO₃–xBaTiO₃ (x = 0–0.07)] ceramics were prepared by using conventional solid state reaction method, and the microstructure and electric properties of these samples were investigated. The grain size distribution of non-doped Li_0.06(Na_0.5K_0.5)_0.94NbO₃ ceramics was relatively wide. The microstructure was composed of grains ranging 1.1–5.0 μm in size. However, with increasing Ba and Ti content, the grain size distribution became narrow and the average grain size decreased from 2.0 to 0.9 μm in size. In particular, the microstructure of x = 0.07 sample was composed of grains ranging 0.5–2.2 μm in size. As a result, the frequency dispersion of dielectric constant for the (1 ? x)Li_0.06(Na_0.5K_0.5)_0.94NbO₃–xBaTiO₃ (x = 0–0.07) ceramics was reduced and the mechanical quality factor Q_m was enhanced with increasing Ba and Ti content.     

Lead-free BaTiO3-Bi0.5Na0.5TiO3-Na0.73Bi0.09NbO3 relaxor ferroelectric ceramics for high energy storage

A series of (1-x)(0.65BaTiO₃-0.35Bi_0.5Na_0.5TiO₃)-xNa_0.73Bi_0.09NbO₃ ((1-x)BBNT-xNBN) (x = 0–0.14) ceramics were designed and fabricated using the conventional solid-state sintering method. The microstructure, dielectric property, relaxor behavior and energy storage property were systematically investigated. X-ray diffraction results reveal a pure perovskite structure and dielectric measurements exhibit a relaxor behavior for the (1-x)BBNT-xNBN ceramics. The slim polarization electric field (P-E) loops were observed in the samples with x ≥ 0.02 and the addition of Na_0.73Bi_0.09NbO₃ (NBN) could decrease the remnant polarization (P_r) of the (1-x)BBNT-xNBN ceramics obviously. The sample with x = 0.08 exhibits the highest energy storage density of 1.70 J/cm³ and the energy storage efficiency of 82% at 172 kV/cm owing to its submicron grain size and high relative density. These results show that the (1-x)BBNT-xNBN ceramics may be promising lead-free materials for high energy storage density capacitors.     

Synthesis of rod-like β-Si3N4 seed crystals with tailored morphology

β-Si₃N₄ seed crystals were synthesized by sintering (α+β)-Si₃N₄ powders with Y₂O₃+MgO additives at 1800 °C. Full α- to β-phase transformation was achievable at 1800 °C for 1 h. The pre-existing β-Si₃N₄ particles acted as nuclei during a sintering process. The length and mean aspect ratio of β-Si₃N₄ seed grains could be tailored by careful control of α/β-Si₃N₄ ratio, which resulted in various nuclei and driving force. The sample A95B5 with 5% β-nuclei shows a bimodal size distribution containing large amount of abnormal elongated β-Si₃N₄ grains with remarkable large diameter. With increasing the β-phase content from 5 wt% to 100 wt%, the average diameter and aspect ratio of the β-Si₃N₄ single crystals decreased from 1.43 µm to 0.92 µm and from 4.36 to 2.79, respectively.     

High-energy storage performance in lead-free (1-x)BaTiO3-xBi(Zn0.5Ti0.5)O3 relaxor ceramics for temperature stability applications

In this paper, we prepared lead-free (1-x)BaTiO₃-xBi(Zn_0.5Ti_0.5)O₃ (x=0.04, 0.08, 0.10, and 0.14) ceramics by a conventional solid-state reaction technique. Pure perovskite structures and dense microstructures were demonstrated for all the compositions. Interestingly, it was found that the sintering temperature tended to decrease with increasing the Bi(Zn_0.5Ti_0.5)O₃ content. It should be stressed that a low sintering temperature of 1050 °C was utilized for the composition of x=0.14. Moreover, the dielectric permittivity-temperature curve became more flat and the relaxor degree became stronger with the augment in Bi(Zn_0.5Ti_0.5)O₃ content. We also conducted a detailed study on the energy storage performance for all the compositions from 25 °C to 180 °C.We found that relatively temperature-stable energy storage performance could be obtained in the compositions with x=0.08, 0.10 and 0.14 regardless of the evolution of dielectric constant during the test temperature range. In particular, due to a higher field of 12 MV m⁻¹, the discharge energy storage densities of x=0.14 could reach 0.81 J cm⁻³, 0.80 J cm⁻³, 0.78 J cm⁻³, 0.72 J cm⁻³, and 0.67 J cm⁻³ with high efficiencies of 94%, 92%, 94%, 88% and 77% at 25 °C, 50 °C, 100 °C, 150 °C, and 180 °C, respectively. All these results demonstrate the (1-x)BaTiO₃-xBi(Zn_0.5Ti_0.5)O₃ ceramics are quite promising for temperature-stable energy storage applications.     

Microwave dielectric properties of low loss microwave dielectric ceramics: A0.5Ti0.5NbO4 (A = Zn,Co)

The microwave dielectric properties of low-loss A_0.5Ti_0.5NbO₄ (A = Zn, Co) ceramics prepared by the solid-state route had been investigated. The influence of various sintering conditions on microwave dielectric properties and the structure for A_0.5Ti_0.5NbO₄ (A = Zn, Co) ceramics were discussed systematically. The Zn_0.5Ti_0.5NbO₄ ceramic (hereafter referred to as ZTN) showed the excellent dielectric properties, with ɛ_r = 37.4, Q × f = 194,000 (GHz), and τ_f = −58 ppm/°C and Co_0.5Ti_0.5NbO₄ ceramic (hereafter referred to as CTN) had ɛ_r = 64, Q × f = 65,300 (GHz), and τ_f = 223.2 ppm/°C as sintered individually at 1100 and 1120 °C for 6 h. The dielectric constant was dependent on the ionic polarizability. The Q × f and τ_f are related to the packing fraction and oxygen bond valence of the compounds. Considering the extremely low dielectric loss, A_0.5Ti_0.5NbO₄ (A = Zn and Co) ceramics could be good candidates for microwave or millimeter wave device application.     

Catalytic performance of LaCo0.5M0.5O3 (M = Mn,Cr, Fe,Ni, Cu) perovskite-type oxides and LaCo0.5Mn0.5O3 supported on cordierite for CO oxidation

Catalytic combustion of CO over perovskite-type oxides LaCo_0.5M_0.5O₃ (M = Mn, Cr, Fe, Ni, Cu) and LaCo_0.5Mn_0.5O₃ supported on cordierite were investigated. The catalysts were synthesized by impregnation method with citrate and characterized by XRD, SEM and TPR. The LaCo_0.5Mn_0.5O₃ catalyst showed much higher activity in CO oxidation compared with LaCo_0.5M_0.5O₃ (M = Cr, Fe, Ni, Cu) due to different kinds of valence state and lattice oxygen content. When LaCo_0.5Mn_0.5O₃ was supported on cordierite, the activity was improved significantly. However, calcining temperature and the presence of water vapor affected the catalytic activity due to sintering and competition of H₂O with CO for adsorption, respectively.     

A composition design rule for crystal growth of centimeter scale by normal sintering process in modified potassium sodium niobate ceramics

It has been reported that single crystals could be grown by normal sintering process, without the addition of a seed, in (K,Na)NbO₃ (KNN)-based ceramics. The growth of huge grains (approximately 5–30 mm) is due to the donor effect on abnormal grain growth (AGG) in KNN-based ceramics. In this study, a composition design rule is suggested to obtain the large single crystal without the seed addition in KNN-based ceramics. In addition, it is also identified by the microstructure observation that the huge grains can be due to the donor effect on the abnormal grain growth which is found in perovskite materials. The donor ratio must be designed to be located at the range between 0.7% and 0.9% compared to that of Na⁺ ions, in order to obtain the large single crystals in KNN-based ceramics. This range of the donor ratio is narrower than that in BaTiO₃ (or SrTiO₃) ceramics.     

Effects of sintering atmosphere on grain morphology of liquid-phase-sintered SiC with Al2O3 additions

The effects of sintering atmospheres of Ar and N₂ on grain morphology were investigated for pressureless liquid-phase-sintered (LPS) SiC with Al₂O₃ additions. When increasing the sintering temperature, the SiC grain size and its aspect ratio increased in both sintering atmospheres. With a 2 mass% Al₂O₃ addition, no distinct difference was observed between the grain morphology of SiC sintered in the Ar atmosphere and that sintered in the N₂ atmosphere. With a 15 mass% Al₂O₃ addition, sintering in a N₂ atmosphere led to retarded grain growth and this resulted in a fine homogeneous microstructure, whereas sintering in an Ar atmosphere enhanced the grain growth compared with that in 2 mass% Al₂O₃. The effects of atmosphere on the grain morphology depend on the amount of Al₂O₃ addition, and this also affects the grain growth process of solution-reprecipitation. The mechanical properties of the SiC are also considered.