Ferroelectricity of 30 nm BaTiO3 ceramics prepared by high pressure sintering

BaTiO₃ ceramics with a grain size of 30 nm were prepared at 6 GPa, 1,273 K using a three-step high pressure sintering method. The sintered bulk is uniform and the relative density is above 96% of the theoretical value. Similar to normal BaTiO₃, successive phase transitions were observed in 30 nm BaTiO₃ ceramics by X-ray diffraction method with the change of temperature. After annealing in O₂, a broadened ferroelectric transition was obtained at 398 K and the relative dielectric constant is 1,700 at 10 kHz.     

Optimizing design of the microstructure of sol–gel derived BaTiO3 ceramics by artificial neural networks

Modeling and application of artificial neural network (ANN) technique to the formulation design of BaTiO₃-based ceramics were carried out. Based on the homogenous experimental design, the results of BaTiO₃-based ceramics were analyzed using a three-layer back propagation (BP) network model. Then the influence of sintering temperatures, holding time, donor additives (La₂O₃, MnO₂, Ce₂O₃) and sintering aids (Al₂O₃–SiO₂–TiO₂ (AST)) on the average grain size (d _a), the degree of grain uniformity given by the ratio of the maximal grain size to the average grain size (d _max/d _a), and the relative density (D _r) of doped BaTiO₃ ceramics system was investigated. The optimized results and experiment data were expressed and analyzed by intuitive graphics. Based on input data and output data, the sintering behavior of BaTiO₃ nano-powder was explained well. Furthermore, the fine and uniform microstructure of sol–gel derived BaTiO₃ ceramics with d _a?≤?3 μm, d _max/d _a?≤?1.20, and D _r?≥?98% was obtained.     

Low-temperature sintering of Li2O-doped BaTiO3 lead-free piezoelectric ceramics

Low-temperature sintering of BaTiO₃ ceramics using Li₂O as sintering aids was investigated with a special influences of Li₂O content (0–4?mol%) and sintering temperature (1000–1100°C) on crystalline structure and electrical properties. The sinterability of BaTiO₃ ceramics significantly improved by adding Li₂O, whose densification sintering temperature reduced from 1300°C to 1000°C. XRD pattern indicated that BaTiO₃-xLi₂O samples were single phase with a tetragonal symmetry as x?=?0–0.3?mol%, while the samples became an orthorhombic symmetry as x?=?0.5–4?mol%. The densification sintering temperature in which samples showed relative density higher than 90?% decreased with increasing Li₂O content. A maximum d ₃₃ value (200 pC/N) was obtained for the BaTiO₃-0.5?mol%Li₂O sample sintered at 1050°C, which is attributed to a vicinity of the phase transition and the high density. Adding Li₂O not only reduced the sintering temperature but also obtained the acceptable piezoelectric properties, which will make BaTiO₃ become a kind of promising and practical lead-free piezoelectric ceramics.     

Effect of sintering temperature on microstructure and piezoelectric properties of Pb-free BiFeO3–BaTiO3 ceramics in the composition range of large BiFeO3 concentrations

Lead-free (1-x)BiFeO₃–xBaTiO₃ [(1-x)BF-xBT] piezoelectric ceramics in the range of large BF concentrations were prepared by conventional oxide-mixed method at various sintering temperatures. The sintering temperatures have a significant effect on the microstructure of the ceramics, and the composition has a remarkable effect on optimal sintering temperature of the ceramics, which are closely related with piezoelectric properties. The grain size increased with increasing sintering temperature and the optimal sintering temperature increased with increasing BT content. The ceramics with x?=?0.275 sintered at 990 °C exhibit enhanced electrical properties of d ₃₃?=?136pC/N and k _p?=?0.312 due to the polarization rotation mechanisms at MPB and desired microstructure. These results show that the ceramic with x?=?0.275 is a promising lead-free high-temperature piezoelectric material.     

Preparation, microstructure and electrical properties of Li1.4Al0.4Ti1.6(PO4)3 nanoceramics

NASICON-type Li_1.4Al_0.4Ti_1.6(PO₄)₃ solid electrolytes were prepared by various processes, such as crystallization of glasses, spark plasma sintering (SPS) and conventional sintering process from nanosized precursor powders synthesized by a sol–gel route. The experimental results showed that grain size and relative density were the main factors determining the ionic conductivity of the bulk materials. The SPS technique produced ceramics with nearly 100% of the theoretical density. Maximum room temperature conductivities, 1.39?×?10^?3 S cm^?1 and 1.12?×?10^?3 S cm^?1 of grain boundary conductivity and total conductivity, respectively were obtained which were the highest values for Li⁺ inorganic oxide conductors as reported. Crystallization of ceramics from a glass was also certified as a favorable route to fabricate a bulk material with high conductivity.     

Modeling of transport properties of interfacially controlled electroceramics: Application to n-conducting barium titanate

Grain boundary regions in n-conducting barium titanate (BaTiO₃) are re-oxidized during the cooling process after sintering the ceramics in air. The kinetics of this re-oxidation process is determined by rapid transport of oxygen along the grain boundaries and slow (rate-determining) diffusion of cation vacancies from the grain boundaries into the grains until the diffusion process is frozen-in. Based on numerical calculations of frozen-in diffusion profiles of cation vacancies at grain boundary regions for various cooling rates, a modified Schottky-barrier model is introduced in order to calculate the grain boundary resistivity as a function of temperature from the Curie-point up to 900°C. A change of the activation energy at approximately 500°C is predicted owing to an enrichment of holes in the space charge layers at elevated temperatures. The modeling results are compared with experimental data for BaTiO₃-based positive temperature coefficient resistors (PTCRs).     

Fabrication and electrical properties of textured Na1/2Bi1/2TiO3-BaTiO3 ceramics by reactive-templated grain growth

The <001> fiber-textured Na_1/2 Bi_1/2TiO₃-BaTiO₃ (6 mol% BaTiO₃) ceramics were fabricated by reactive-templated grain growth (RTGG), using plate-like Bi₄Ti₃O₁₄ (BiT) particles prepared by a molten salt method as templates. The effects of sintering conditions on texture development and microstructure evolvement were both studied, and the mechanisms of grain orientation and densification were discussed. High Lotgering factor (≥96%) and high density (≥96% theoretic density) textured Na_1/2 Bi_1/2TiO₃-6BaTiO₃ ceramics were prepared by using the max templates concentration supplying 100% Bi in the final product, and sintering at 1200 °C for 10 h. The NBT-6BT obtained exhibited good piezoelectric performance with piezoelectric coefficient d ₃₃ ?=?241pC/N, and electromechanical coupling factor k _p ?=?41.2%, k _t ?=?66.5% at room temperature.     

Grain-size effects on the hardness of nanograin BaTiO3 ceramics

Dense nanocrystalline BaTiO₃ (BT) ceramics with grain size (GS) 50 nm were successfully prepared by spark plasma sintering (SPS) method. The nanoindentation experiment was used to test the hardness of different GS BT ceramics. It was found that the hardness of 50 nm nanograin BT ceramics increased 82% than that of 1.2 μm BT ceramics fabricated by conventional sintering (CS) process. Dislocation pinning resulting from the increase of grain boundary by ultrafine GS are believed to be the dominant factor in raising strength. Restriction on dislocation generation and mobility due to the domain wall motion and sliding imposed by ultrafine GS are expected to be exceptionally important. At the same time, the large stress in nanograin BT ceramics may also play a role in producing high strength. These differences of the hardness behavior between BT ceramics are attributed to the variation of the resultant microstructure, especially the GS of the ceramics.     

Sintering and dielectric properties of Bi1.5ZnNb1.5O7 cubic pyrochlore ceramics via high-energy milling technology

The dense Bi_1.5ZnNb_1.5O₇ cubic pyrochlore ceramics is synthesized by high-energy milling technology from the coarse Bi_1.5ZnNb_1.5O₇ cubic pyrochlore powders prepared by solid state route. The sintering and dielectric properties of the Bi_1.5ZnNb_1.5O₇ cubic pyrochlore ceramics are investigated, which show that the sintering temperature of the prepared ceramics could be effectively lowered to 800°C and the bulk density reach 6.889 g/cm³ approximately 97% of the theoretical density of Bi_1.5ZnNb_1.5O₇ cubic pyrochlore ceramics. The excellent dielectric properties of the ceramics sintered at 850°C has been obtained with the relative permittivity of 160 and the dielectric loss of 10^?4. This route would be a low-cost and mass production for lowering the sintering temperature of the Bi_1.5ZnNb_1.5O₇ cubic pyrochlore ceramics without sintering aids.     

Grain growth kinetics of textured 0.92Na0.5Bi0.5TiO3—0.08BaTiO3 ceramics by tape casting with Bi2.5Na3.5Nb5O18 templates

Plate-like Bi_2.5Na_3.5Nb₅O₁₈ particles were used as templates to fabricate grain-oriented Na_0.5Bi_0.5TiO₃—BaTiO₃ (NBTBT) ceramics by reactive-templated grain growth. The effects of sintering conditions on the grain orientation and microstructure of the textured NBTBT ceramics were investigated, and the kinetic mechanism of grain growth is discussed. The results show that textured ceramics were successfully obtained with orientation factor more than 0.6. NBTBT specimens are composed of strip-like grains and equiaxed shaped grains. The textured ceramics have a microstructure with strip-like grains aligning in the direction parallel to the casting plane and exhibit an {h00} preferred orientation. The degree of grain orientation increases initially, then decreases with increasing sintering temperature and soaking time. The maximum texture fraction is 0.69 when sintered at 1185 °C for 6 h. The kinetic exponent n and activation energy Q of the two types of grain in textured NBTBT ceramics were calculated. The results show that the grain growth mechanism of oriented grains is controlled not only by grain lattice diffusion, but also by grain boundary diffusion. The grain growth mechanism of matrix grains is mainly controlled by the grain boundary curvature.     

Dielectric and sintering properties of NaNbO3 ceramic prepared by Pechini method

NaNbO₃ powders and ceramics were prepared by Pechini method. The pure phase NaNbO₃ was obtained at temperature as low as 350 °C, and then they obtained fine powders were used to prepared ceramics through conventional sintering process. The effect of sintering temperature on microstructure evolution and dielectric properties of NaNbO₃ ceramics has been determined. Results shown that the microstructure of NaNbO₃ ceramics consisted of stacked plates, which was related the liquid phase. It was important to note that the stacked plate configuration forming the grain has not been emphasized before, and these grains support adequate wetting characteristics for the liquid phase in order to achieve fully dense microstructure. Above 1195 °C, a number of angular grains with flat interfaces started to appear, and joined to each other with the sintering temperature increase. The effect of grains morphology on dielectric properties of the NaNbO₃ ceramics was also detected.     

Influence of B2O3/SiO2 Ratio on the Fabrication of Nd:YAG Ceramics

B₂O₃/SiO₂ are used as composite sintering aids to fabricate Nd:YAG ceramics by solid-state reaction and vacuum sintering method at 1750°C for 5h using Nano-Al₂O₃, Y₂O₃, Nd₂O₃ as starting materials. In this article, we focus on the influence of B₂O₃/SiO₂ ratio on grain size, porosity and relative density. Finally, with the increase of B₂O₃/SiO₂ ratio, the density and shrinkage rate of transparent ceramics increase, the grain size becomes uniform and the porosity reduces, for the reason that B³⁺ begins to vaporize at 1300°C and is reduced to trace levels by 1600°C. The best B₂O₃/SiO₂ ratio is 4: 1.     

High piezoelectric properties of (Ba,Ca)TiO3-0.04LiF ceramics sintered at a low temperature

Lead-free piezoelectric ceramics are strongly needed to replace the lead-based piezoelectric ceramics with increasing environmental concerns. Barium titanate (BaTiO₃) systems are one of the most promising candidates due to excellent electrical properties. However, the sintering temperature for traditionally sintered BaTiO₃ ceramics are about 1300°C, which restricts the applications of BaTiO₃ ceramics. It is necessary to develop high piezoelectric properties of BaTiO₃ based ceramics which are able to sinter at low temperature. The (Ba_0.94Ca_x)Ti_0.94O_δ-0.04LiF (x?=?0.00?~?0.05 mol) ceramics were synthesized by a conventional sintering method at 1050°C. All the samples show high relative densities over 90%. X-Ray Diffraction pattern indicated that the crystallographic structure of the samples (x?=?0.00 and 0.01 mol) are orthorhombic phase and changes to pseudocubic one with increasing Ca content to x?=?0.03 mol. Two-phases with orthorhombic and pseudocubic symmetries coexisted at x?=?0.02 mol, which contributes the excellent properties, in which the piezoelectric constant d ₃₃?=?361 pC/N, the planar electromechanical coupling coefficient k_p?=?41.2%, the Curie temperature Tc?=?70°C, the temperature of phase transition T _O-PC?=?34°C near the room temperature, the relative permittivity ε _r?=?4028 and the remanent polarization P _r?=?9.39 μC/cm².     

Microstructure and electrical properties of the rare-earth doped 0.94Na0.5Bi0.5TiO3-0.06BaTiO3 piezoelectric ceramics

Phase structure, microstructure, piezoelectric and dielectric properties of the 0.4 wt% Ce doped 0.94Bi_0.5Na_0.5TiO₃-0.06BaTiO₃ (Ce-BNT6BT) ceramics sintered at different temperatures have been investigated. The powder X-ray diffraction patterns showed that all of the Ce-BNT6BT ceramics exhibited a single perovskite structure with the co-existence of the rhombohedral and tetragonal phase. The morphologies of inside and outside of the bulk indicated that the different sintering temperatures did not cause the second phase on the inside of bulk. However, the TiO₂ existed on the outside of the bulk due to the Bi₂O₃ and Na₂O volatilizing at higher temperature. The ceramics sintered at 1,200 °C showed a relatively large remnant polarization (P _r) of about 34.2 μC/cm², and a coercive field (E _c) of about 22.6 kV/cm at room temperature. The permittivity ? _r of the ceramics increased with the increasing of sintering temperature in antiferroelectric region, the depolarization temperature (T _d) increased below 1,160 °C then decreased at higher sintering temperature. The resistivity (ρ) of the Ce-BNT6BT ceramics increased linearly as the sintering temperature increased below 1,180 °C, but reduced as the sintering temperature increased further. A maximum value of the ρ was 3.125?×?10¹⁰ ohm m for the Ce-BNT6BT ceramics sintered at 1,180 °C at room temperature.     

Improved piezoelectric properties of Ag doped 0.94(K0.5–βNa0.5–δ)NbO3–0.06Li1–γNbO3 ceramics by templated grain growth method

In this work, Ag doped 0.94(K_0.5–βNa_0.5–δ)NbO₃–0.06Li_1–γNbO₃ lead-free piezoelectric ceramics have been prepared by templated grain growth (TGG) using cubic (K,Na)NbO₃ single crystal seeds as templates. Specimens were prepared by the conventional mixed method and sintered in controlled atmosphere. TGG method was well known, because that this method can improve piezoelectric properties by increasing the grain size during the sintering process. Sintering caused densification and grain growth of ceramics by the expense of matrix particles. Densification prior to grain growth was found to be necessary to obtain highly textured ceramics. Crystalline properties were analyzed by the XRD method. The effects of TGG on phase structure, microstructure, piezoelectric and dielectric properties of Ag doped 0.94(K_0.5–βNa_0.5–δ)NbO₃–0.06Li_1–γNbO₃ ceramics were investigated.     

Fabrication of amorphous bulk and multi-phase ceramics by melting method in the HfO2-Al2O3-Gd2O3-Eu2O3system

Ceramics have generally been fabricated from powders by shape forming & sintering methods except for glasses and glass ceramics. Glasses and glass ceramics can be fabricated by melting methods. The melting method has not only higher productivity but also higher shape forming ability than powder processes via forming & sintering methods. Thus we have reinvestigated melting methods in binary and ternary oxides systems to fabricate amorphous bulk ceramics and bulk nano composites. We have successfully fabricated amorphous phases by simple melt solidification methods in ternary eutectic melts in the HfO₂-Al₂O₃-Gd₂O₃system. The present study demonstrates the formation of the amorphous phases in quaternary systems HfO₂-Al₂O₃-Gd₂O₃-Eu₂O₃. Furthermore, we have also succeeded to fabricate nano-structured bulk ceramics, which consisted of constituent oxide grains with 20–100 nm in size, by post annealing of the amorphous phase.     

Influence of sintering temperatures on the electrical property of bismuth sodium titanate based piezoelectric ceramics

A systematic investigation of cerium and stannum doped 0.94(Bi_0.5Na_0.5)TiO₃−0.06BaTiO₃ (Sn&Ce-BNT6BT) based lead-free piezoelectric ceramics is undertaken to understand the influence of sintering temperature on electrical properties. The X-ray diffraction patterns showed that all of the Sn&Ce-BNT6BT ceramics exhibited a single perovskite structure with the co-existence of the rhombohedral and tetragonal phase. The smaller grain size of Sn&Ce-BNT6BT ceramics was obtained at lower sintering temperature, and more cubical grains of Sn&Ce-BNT6BT ceramics were obtained at higher sintering temperature. The temperature dependence of dielectric permittivity of the compositions exhibited strong dispersion with the increasing temperature, and the dielectric loss tangent increased dramatically while the temperature over 225^∘C. The depolarization temperature T _d of Sn&Ce-BNT6BT ceramics sintered at 1160^∘C was 92.6^∘C. The remnant polarizations P _r for Sn&Ce-BNT6BT ceramics sintered at 1120 and 1200^∘C were found to be 28.8 and 33.4 μC/cm² at room temperature, respectively.     

Microstructure and dielectric properties of ferroelectric barium strontium titanate ceramics prepared by hydrothermal method

Ba _x Sr_1-x TiO₃, nanoparticles with different Ba compositions were synthesized by a hydrothermal method. The mechanism of hydrothermal reactions was discussed based on DTA/TG, XRD and TEM characterizations. The result showed that perovskite structure was developed through the mutual diffusion between the intermediate phases and TiO₂ phase. The grain size of the Ba_0.77Sr_0.23TiO₃ (BST77) powders was about 20–40 nm. BST ceramics were made from the hydrothermal-derived BST powders and the dielectric properties of the BST ceramics were measured. Due to the small grain size and active surface energy of the BST powders prepared by hydrothermal method, the BST ceramics showed low sintering temperature. It was found that the BST77 ceramics sintered at 1280 °C showed dielectric constant peak dispersion which was believed to be caused by dimension domino effect.     

Application of spark plasma sintering for growing dense Pb(Mg1/3Nb2/3)O3–35 mol% PbTiO3 single crystal by solid-state crystal growth

This study examined the effect of spark plasma sintering (SPS) on the densification behavior and resulting dielectric and piezoelectric properties of Pb(Mg_1/3Nb_2/3)O₃–35 mol% PbTiO₃ ceramics with a 5 mol% excess of PbO. Through normal sintering at 1200^∘C, the density of the specimen reached only 92% of the theoretical density (TD). However, with the SPS treatment, the density of the PMN-PT ceramics increased to more than 99% of the TD at 900^∘C, and maintained over 98% of the TD during subsequent heat-treatment at 1200^∘C for 10 h. The increased density of the Pb(Mg_1/3Nb_2/3)O₃–35 mol% PbTiO₃ ceramics resulted in an improvement in the dielectric and piezoelectric properties. The SPS treatment was also successfully applied to the densification of a PMN-PT single crystal grown on a BaTiO₃ seed crystal using a solid-state crystal growth (SSCG) process.     

The effect of Bi4Ti3O12 particles addition in lead-free Bi0.5(Na0.75K0.25)0.5TiO3 ceramics

We studied the effect of Bi₄Ti₃O₁₂ (BiT) platelet addition in Bi_0.5(Na_0.75K_0.25)_0.5TiO₃ (BNKT) ceramics by preparing two kinds of BNKT ceramics. One type of BNKT ceramic was fabricated by a conventional solid state reaction method (normal sample), while the other by addition of 15 wt% BiT platelets to BNKT powders (BiT-added sample). In the case of BiT-added BNKT ceramics, plate like grains were formed by the reaction of BiT platelets with Na₂CO₃, K₂CO₃, and TiO₂ during the sintering process. The grain size of BiT-added BNKT ceramics was 10 times larger than that of normal BNKT ceramic. The piezoelectric strain and d₃₃ values of BiT-added BNKT ceramics were 0.135% and 225 pm/V, respectively. These values were 35% higher than those of normal BNKT ceramics. The piezoelectric properties of BiT-added BNKT ceramics were enhanced by the higher domain activity due to a decrease in domain density at larger grain sizes.