Highly Textured BaTiO3 via Templated Grain Growth and Resulting Polarization Reversal Dynamics

Samples of bulk textured polycrystalline BaTiO₃ ceramics were fabricated using a templated grain growth (TGG) approach in order to investigate effects of polycrystallinity and texture related to ferroelectric domain reversal under high‐power drive conditions. Barium titanate platelets were formed via two‐step topochemical conversion of bismuth titanate platelets grown via molten salt synthesis, then aligned via tape casting within a matrix of fine BaTiO₃ powder. The coarse‐grained parts showed a high degree of crystallographic texture after sintering. Combined with ceramics of similar density and polycrystallinity, but random orientation and commercial single‐crystal specimens, this sample set enabled direct isolation of crystallographic texture and polycrystallinity as the primary variables for high‐power polarization reversal studies. These studies have also demonstrated a link between grain size and polarization reversal time that strongly suggests that grain boundaries serve effectively as nucleation sites during the ferroelectric switching process.     

Properties of lanthanum doped BaTiO3 produced from nanopowders

Pure and barium titanate (BT) powders doped with different lanthanum concentration were prepared by the polymeric precursor method (Pechini process) which was carried out as a three-stage process from organometallic complexes. Sintering of pressed powders was performed at 1300 °C for 2, 4 and 8 h. XRD analysis showed cubic BT powders with crystallite sizes between 20 and 25 nm and tetragonal crystal structure of BT ceramics. The influence of sintering time on grain growth was fairly obvious. It was found that lanthanum doping has significant effect on powders particle size and ceramics grain size. The influence of lanthanum concentration on grain size inhibition, improving the dielectric properties of BT ceramics was detected. The relation between sintering time, grain size, structure and electrical properties of the BT ceramics was analyzed.     

Fabrication of textured bismuth titanate by templated grain growth using aqueous tape casting

Textured bismuth titanate (Bi₄Ti₃O₁₂) ceramics were fabricated by templated grain growth (TGG), using plate-like Bi₄Ti₃O₁₂ particles prepared by a molten salt method as the templates. The templates were aligned in the fine-grained matrix by aqueous tape casting with their major surface parallel to the casting plane. Effect of sintering conditions on the grain orientation in the material was investigated. It was found that the degree of grain orientation (Lotgering factor, f) increased with the increase in sintering temperature, soaking time and heating rate. High Lotgering factor (f⩾0.92) can be obtained through careful control of the sintering parameters. The textured Bi₄Ti₃O₁₂ ceramics showed a high anisotropy in its dielectric properties in the directions parallel and perpendicular to the casting plane.     

Preparation of Textured Bismuth Titanate Ceramics Using Spark Plasma Sintering

Textured bismuth titanate ceramics were successfully produced using spark plasma sintering and platelike bismuth titanate particles prepared using a molten-salt method. The microstructure and dielectric properties of the samples were investigated. The results showed that the dielectric property of the textured bismuth titanate ceramics was anisotropic in various directions and that spark plasma sintering was an effective sintering technology to obtain textured, dense bismuth titanate ceramics at a low temperature.     

Colossal Permittivity in Microwave‐Sintered Barium Titanate and Effect of Annealing on Dielectric Properties

Colossal permittivity (ε′ = 301,484 at room temperature and 1 kHz) of barium titanate was induced in ceramics synthesized using the microwave sintering method. Three different sintering processes (conventional, spark plasma, and microwave) were performed to better understand colossal permittivity in sintered barium titanate. The dielectric permittivity measurements revealed that the appearance of colossal permittivity has strong dependence on the sintering temperature and atmosphere, and less on the grain size of the sintered ceramics. However, the as‐sintered barium titanate samples produced by microwave sintering show high dielectric loss (tanδ > 1) consistent with oxygen reduction during the microwave sintering process and consequent accumulation of oxygen vacancies and associated charge carriers at the grain boundary. Since the highly conductive state of as‐sintered ceramics precludes their use in dielectric applications, thermal annealing at different conditions was performed to recover insulating characteristics. Microwave‐sintered barium titanate with post annealing process (950°C for 12 h in air) showed low dielectric loss (tanδ = 0.045) at room temperature and 1 kHz, while still showing a much higher permittivity (ε′ = 36,055) than conventionally sintered barium titanate (ε′ = 3500).     

Effects of Preparation Conditions on the Structural and Optical Properties of Spark Plasma-Sintered PLZT (8/65/35) Ceramics

Transparent lanthanum-doped lead zirconate titanate (PLZT) ceramics with high density were fabricated using spark plasma sintering (SPS), a recently developed hot-pressing method. A wet–dry combination method was used to prepare the fine PLZT powders. The average grain size of the PLZT ceramics was less than 1 μm, because of a relatively low sintering temperature and a very short sintering time. The transmittance of PLZT ceramics increased with an increase of calcination temperature up to 700°C and then it slightly decreased with further increase of calcination temperature. The transmittance strongly depended on the SPS temperature and heat-treatment temperature. The pellet sintered at 900°C for 10 min and heat treated at 800°C for 1 h with a thickness of 0.5 mm showed a transmittance of 31% at a wavelength of 700 nm. The relationships between the transmittance and the microstructure were investigated.     

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.     

Grain growth kinetics in Dy-doped Bi0.5Na0.5TiO3 ceramics

Dysprosium-doped bismuth sodium titanate ceramics were prepared using the conventional mixed-oxide method. The amount of dysprosium used was varied from 0 to 2 at.%. The mixed powders were calcined at 800 °C and checked for phase purity using X-ray diffraction technique. The calcined powders were then cold-pressed into pellets and sintered at 1050 °C for the time ranging from 2 to 48 h. The ceramics were checked for phases and microstructures using an X-ray diffractometer and a scanning electron microscope, respectively. The analysis showed that undoped BNT ceramics sintered at longer time exhibited a significant grain growth with non-uniform grain size distribution and shape. The Dy-doped BNT however showed a much more limited grain growth behavior, resulting in smaller grain size and more equiaxed grain shape. It was also found that all Dy-doped BNT ceramics sintered at 48 h possessed lower porosity than those sintered for shorter time.     

Correlation between Surface Texture and Chemical Composition in Undoped, Hard, and Soft Piezoelectric PZT Ceramics

The electrical and mechanical properties of piezoelectric lead zirconate titanate (PZT) ceramics are strongly influenced by domain-wall motion, which can be tailored via the substitution of ions within the perovskite structure. Different domain mobilities are achieved by lead and/or oxygen vacancies, according to the valency and ionic radius of the dopants. To quantify the surface near domain mobility, hard (silver-doped), soft (lanthanum-doped), and undoped PZT ceramics have been prepared. An applied mechanical stress after sintering causes texturing near the surface, because of the ferroelastic behavior of the 90° domains. The texture is quantified via X-ray diffractometry (XRD) analysis of the tetragonal F _T(002)- and F _T(200)-peak intensities, using least-squares refinement with Gaussian profile functions. The samples are subsequently annealed to remove the surface texture and again characterized via XRD measurements. However, annealed samples still reveal a preferred domain orientation that can be removed only by a second annealing of the pulverized samples. A comparison of the tetragonal additive systems clearly reveals the greatest domain mobility for lanthanum-doped PZT ceramics, whereas the silver-doped and undoped samples have similar ferroelastic behavior. Furthermore, the surface texture of all the compositions is dependent on the applied mechanical stress and cannot be removed completely by heat treatment.     

Pressureless Sintering of Hafnium Carbide–Silicon Carbide Ceramics

HfC‐30 vol%SiC ceramics with a relative density of 99.7% was obtained by pressureless sintering at 2300°C for 0.5 h. The resultant ceramics showed fine microstructure with HfC grain size around 1 μm. The hardness (20.5 ± 0.2 GPa), bending strength (396 ± 56 MPa), and fracture toughness (2.81 ± 0.18 MPa·m^1/2) of HfC‐30 vol%SiC ceramics were at least 20% higher than those of monolithic HfC ceramics. The influences of SiC particle size, volume fraction, and the oxide impurity on the microstructure evolution of HfC‐based ceramics were examined. The results indicate that SiC addition and the oxygen impurity introduced by ball milling play opposite roles in the HfC grain growth during sintering. The oxide impurity introduced by ball milling caused the HfC grain coarsening, whereas SiC particles inhibited the grain growth of HfC significantly.     

Single CaO accelerated densification and microstructure control of highly transparent YAG ceramic

In this work, a small amount of CaO single dopant was adopted to realize the densification and microstructure control of fine grained YAG ceramic with excellent optical quality, by a simple solid‐state reaction and one‐step vacuum sintering method. Then, highly transparent YAG ceramics (T = 84.4% at 1064 nm) were obtained just after vacuum sintering at 1820°C for 8 hours. The average grain size was only 2.7 μm, when the total amount of CaO was as low as 0.045 wt%. The effect of CaO on the microstructural evolution and optical property of the as‐fabricated YAG ceramics was systematically investigated in detail. It was found that CaO dopant promoted both densification and grain growth of YAG ceramics when the sintering temperature was lower than 1660°C, however, it dramatically inhibited grain growth when the sintering temperature was further increased.     

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.     

Effect of Ce and La substitution on dielectric properties of bismuth titanate ceramics

Effect of Ce and La substitution on the microstructure and dielectric properties of bismuth titanate (BT) ceramics was investigated. Bismuth titanate ceramics (Bi_4−xA_xTi₃O₁₂) (A = Ce or La; x = 0, 0.5, 1) were processed by sintering of pressed pellets, prepared from nanopowder synthesized by the modified sol-gel method. Pure and La modified bismuth titanate ceramics have single Bi₄Ti₃O₁₂ phase of Aurivillius type, whereas a small amount of Bi₂Ti₂O₇ pyrochlore phase appears in Ce modified bismuth titanate ceramics. In the same time addition of La and Ce improved sinterability of BT ceramics. The results of the measurement of dielectric constant and loss tangent at different frequencies (100 Hz-1 MHz) as a function of temperature reveal that Ce modified ceramics has a diffuse phase transition. Temperature T_m, corresponding to the maximum value of the dielectric constant, is shifted to higher temperature and the maximum value of the dielectric constant is decreased with increasing frequency, which indicate that relaxor behavior is caused by Ce substitution.     

Yb doped MgO transparent ceramics generated through the SPS method

Yb doped (0, 0.02, 0.1 and 0.5 at%) MgO transparent ceramics were synthesized through spark plasma sintering (SPS) at the relatively low temperature of 1100 °C for 5–60 min under a pressure of 105 MPa. The effects of dopant concentration and sintering holding time on the densification and microstructure evolution of MgO ceramics were investigated. All ceramics reached a relative density greater than 99.20%. The 0.02% Yb-doped MgO ceramic sintered at 1100 °C for 60 min showed the highest in-line transmittance, of 80% at 1030 nm, a value close to that of MgO single crystals. Yb dopant improved the transmittance, degree of densification and control of grain growth. Herein, the influence of Yb doping on the crystalline phase and microstructure was explored, and the photoluminescence properties of Yb in transparent MgO ceramics were investigated.     

Highly oriented α-Al2O3 transparent ceramics shaped by shear force

Alumina platelets were arranged horizontally in submicron alumina particles by shear force in the flow of slurries during casting. The obtained alumina green bodies with platelets were pressureless-sintered in vacuum, producing ceramics with thoroughly oriented grains and high transmittance. The effects of sintering parameters on the densification, microstructure evolution, and orientation degree of alumina ceramics were investigated and discussed. The results showed that the densification, grain size, orientation degree, and in-line transmittance were increased with increasing sintering temperature. The enhancement of orientation degree was mainly coherent with grain growth. The grain-oriented samples exhibited a much higher in-line transmittance (at 600 nm) of 61 % than that of the grain random sample (29 %). Moreover, the transmission remained a high level in the ultraviolet range (<300 nm).     

Engineering grain size and electrical properties of donor-doped barium titanate ceramics

A semiconducting lanthanum-doped barium titanate ceramic has been fabricated for battery safety applications by simple means from nanoparticles prepared at room temperature by kinetically controlled vapor diffusion catalysis. The material, characterized by electron microscopy, X-ray diffraction and electrical measurements, exhibits a difficult to achieve combination of submicron grain size (∼500 nm) and attractive electrical properties of room temperature resistivity below 100 Ω cm and a 12-fold increase in resistivity through the Curie temperature (positive thermal coefficient of resistivity, PTCR). Systematic investigation of sintering conditions revealed that a short period of heating at 1350 °C under air is necessary to suppress abnormal grain growth, while precise control of the cooling rate is needed to achieve the targeted electrical properties. Cooling must be sufficiently fast to avoid complete back-oxidation, yet slow enough to facilitate oxygen adsorption at the grain boundaries to produce the thin oxide layer apparently responsible for the observed PTCR.     

Fabrication of [1 0 0]-oriented bismuth sodium titanate ceramics with small grain size and high density for piezoelectric materials

A layered titanate H_1.07Ti_1.73O₄·nH₂O (HTO) with a plate-like particle morphology was used as a template for the fabrication of [1 0 0]-oriented bismuth sodium titanate (Na_0.5Bi_0.5TiO₃, or BNT) ceramics by a reactive-templated grain growth (RTGG) method. The oriented BNT ceramic with a high degree of orientation (95%), high density (98%), and small grain size (2 μm) was fabricated for the first time by the RTGG method using a HTO-TiO₂-Bi₂O₃-Na₂CO₃ reaction system. The oriented BNT ceramic is formed by a topotactic transformation reaction of plate-like HTO template particles to plate-like BNT mesocrystal particles, and then epitaxial crystal growth of BNT on the BNT mesocrystal particles. The epitaxial crystal growth reaction is affected by TiO₂/HTO mole ratio, chemical component of the starting material, and calcination temperature program. The fabricated oriented BNT ceramic shows a higher d₃₃* value than the non-oriented BNT ceramic, suggesting the promising application to high performance Pb-free piezoelectric materials.     

Improvement of barium titanate properties induced by attrition milling

Barium titanate powder was prepared by soft chemical process from polymeric precursors (modified Pechini process). The synthesized barium titanate (BT) powder was nanosized and the factor of agglomeration (F_agg) pointed the existence of agglomerates. In order to de-agglomerate nanopowder and to enhance BT properties the attrition milling was performed. The milled powder (BTA) possessed smaller particles and the size and number of agglomerates was significantly reduced. To investigate the effect of milling on improvement of ceramics electrical properties, both BT and BTA powders were uniaxially pressed and sintered at 1300 °C for 8 h in air. The temperature dependence of relative permittivity showed three structural phase transitions for ferroelectric barium titanate ceramics. The dielectric constant at Curie temperature was ～6700 for BTA which is much higher than 1340, obtained for non-treated BT. The dielectric losses were below 0.04 in both BT ceramics. At higher temperatures the analysis of impedance measurements showed the presence of both grain interior and grain boundary effects. Much higher grain and grain boundary resistivities were obtained for the BTA ceramics.     

Microstructure development and optical properties of Fe:ZnSe transparent ceramics sintered by spark plasma sintering

Fe:ZnSe transparent ceramics were prepared by spark plasma sintering. Fe:ZnSe powders synthesized via co-precipitation yielded well-dispersed particles with an average particle size of 550 nm. These powders were in the cubic phase Fe:ZnSe, indicating the successful substitution of Fe²⁺ for Zn²⁺. The highest relative density, 99.4%, was obtained by increasing the pressure and sintering time. The effects of sintering temperature, pressure, and time on the microstructure of SPS prepared ceramics were presented by micrographs. With increasing sintering temperature, from 600°C to 900°C, the average grain size increased from < 1 to 10 μm. The intergranular fracture indicated no neck formation in the sintering process. High pressure was essential for the densification process. The average grain size deceased from approximately 10 to 5 μm when the pressure was increased. Increasing the sintering time from 10 to 120 minutes lead to a change in the microstructure, from inter- to transgranular fracture, and eliminated the micropores. The as-prepared Fe:ZnSe ceramics were composed of single-phased cubic ZnSe. The sample sintered at 900°C under a pressure of 90 MPa for 120 minutes yielded a transmittance of approximately 60% at 1.4 μm and 68% at 7.5 μm and had residual micropores as its main scattering source. There was a strong characteristic absorption peak of Fe²⁺ ions at around 3 μm, which was red-shifted compared to Fe:ZnS transparent ceramics. Fe:ZnSe transparent ceramics have a reddish-brown color and it could be a promising mid-infrared laser material.