Preparation and dielectric properties of BaCu(B2O5)-doped SrTiO3-based ceramics for energy storage

BaCu(B₂O₅) (BCB) was used as sintering aids to lower the sintering temperature of multi-ions doped SrTiO₃ ceramics effectively from 1300 °C to 1075 °C by conventional solid state method. The effect of BCB content on crystalline structures, microstructures and properties of the ceramics was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and dielectric measurements, respectively. The addition of BCB enhanced the breakdown strength (BDS) while did not sacrifice the dielectric constant. The enhancement of BDS should be due to the modification of microstructures, i.e., smaller and more homogeneous grain sizes after BCB addition. The dielectric constant of BCB-doped ceramics maintained a stable value with 1.0 mol% BCB, which was dominated by the combination of two opposite effects caused by the presence of second phases and the incorporation of Cu²⁺ and Ba²⁺, while further increase was owing to the increase of dissolved Ba²⁺ ions when the content of BCB is more than 2.0 mol%. The multi-ions doped SrTiO₃ ceramics with 1.0 mol% BCB addition showed optimal dielectric properties as follows: dielectric constant of 311.37, average breakdown strength of 28.78 kV/mm, discharged energy density of 1.05 J/cm³ and energy efficiency of 98.83%.     

Non-linear current–voltage characteristics related to native defects in SrTiO3 and ZnO bicrystals

SrTiO₃ and ZnO bicrystals with various types of boundaries were fabricated in order to examine their current–voltage characteristics across single grain boundaries. Their grain boundary structures were also investigated by high-resolution transmission electron microscopy. In Nb-doped SrTiO₃, electron transport behaviors depend on the type of boundaries. Random type boundaries exhibit highly non-linear current–voltage characteristics, while low angle boundaries show a slight non-linearity. On the contrary, undoped ZnO does not exhibit non-linear current–voltage characteristics in any type of boundaries including random ones. It is suggested that the differences observed in current–voltage properties between the two systems are mainly due to the difference in the accumulation behavior of acceptor-like native defects at grain boundaries. A clear non-linearity is obtained by means of Co-doping even for the highly coherent Σ1 boundary in a ZnO bicrystal. This is considered to result from the production of acceptor-like native defects by Co-doping.     

Non-linear current–voltage characteristics related to native defects in SrTiO3 and ZnO bicrystals

SrTiO₃ and ZnO bicrystals with various types of boundaries were fabricated in order to examine their current—voltage characteristics across single grain boundaries. Their grain boundary structures were also investigated by high-resolution transmission electron microscopy. In Nb-doped SrTiO₃, electron transport behaviors depend on the type of boundaries. Random type boundaries exhibit highly non-linear current—voltage characteristics, while low angle boundaries show a slight non-linearity. On the contrary, undoped ZnO does not exhibit non-linear current—voltage characteristics in any type of boundaries including random ones. It is suggested that the differences observed in current—voltage properties between the two systems are mainly due to the difference in the accumulation behavior of acceptor-like native defects at grain boundaries. A clear non-linearity is obtained by means of Co-doping even for the highly coherent ∑ 1 boundary in a ZnO bicrystal. This is considered to result from the production of acceptor-like native defects by Co-doping.     

BaTiO3-SrTiO3 layered dielectrics for energy storage

BaTiO₃-SrTiO₃ (BST) thick films (~ 250-390 μm) with layered structures were fabricated by tape-casting and lamination process. Layered composites with various Ba/Sr ratios were obtained by lamination of BaTiO₃ (BT) and SrTiO₃ (ST) tapes in different spatial configurations (2-2). As-prepared BST ceramics showed much improved sinterability over the laminates of pure BT or pure ST tapes. Dielectric properties of materials were measured in the temperature range of 25 °C to 200 °C. The method of utilizing of layered structures offered flexibility to maximize the energy storage capability at specific operating conditions: (temperature and electric field) by tailoring the dielectric properties through varying the spatial configurations of BT and ST films.     

Electronic Structure of Mn Acceptor Impurity Incorporated SrTiO3 Using Embedded Cluster Method

We have performed atomic-level first principle electronic structure calculations on doped grain boundaries (GB) in SrTiO₃. This was motivated by the electron holography experiments, which were able to quantify the electrostatic potential and the associated space charge distribution across the Mn-doped GB in this material. The embedded cluster Discrete Variational (DV)-X method was used to determine the charge and the densities of states for several idealized models of a single crystal and symmetrical tilt grain boundaries in SrTiO₃. Special attention was given to the role of Mn⁺² and Mn⁺³ acceptors substituting for Ti⁺⁴ resulting in charge segregation across the grain boundaries, which was shown in the electron holography experiments. We have found that Mn replacing Ti prefers to have valence charge around +2 and this picture agrees with the experimental observation of negative grain boundary charges in the GB core.     

Improvement in dielectric properties of Al2O3-doped Li0.30Cr0.02Ni0.68O ceramics

Li_0.30Cr_0.02Ni_0.68O giant dielectric ceramics doped with Al₂O₃ were prepared by solid-state reaction via sol-gel process. The sintered samples were characterized using X-ray powder diffraction and scanning electron microscopy, and dielectric properties were also investigated. All doped samples showed the single phase of cubic rock-salt structure NiO. With increasing Al₂O₃ content, the crystallite size and grain size decreased, possibly due to an occurrence of the secondary phases at grain boundaries which inhibit the grain growth. The sample with 0.2 wt.% Al₂O₃ showed nearly 7 times lower tanδ (2.37) and higher ε_r (7.25 × 10⁶) measured at 1 kHz and room temperature when compared to the pure sample.     

Segregation and Local Structure at Grain Boundaries in SiO2-Doped Tetragonal ZrO2 Polycrystalline Materials

SiO₂ doping in Y₂O₃ stabilized tetragonal ZrO₂ (TZP) materials introduced significant change in mechanical properties around 0.3 wt.% doping level [1]. In order to understand the influence of grain boundary structure and chemistry by doping, high purity undoped and SiO₂-doped 3Y-TZP samples were studied using high-resolution and analytical TEM. Typical grain boundary structures are different for the two types of samples, while amorphous film was not observed at most grain boundaries. A new EDS analysis method was introduced to detect the weak Si and Y signals which overlap with the predominent Zr peaks. It revealed that Si segregation to the grain boundary saturates at 12 at./nm² (or 1.5 monolayer of SiO₂) when the SiO₂ doping level reached and surpassed 0.3 wt.%. It is the segregated atoms which enhanced the grain boundary diffusivity and therefore altered the deformation mechanism.     

Effect of segregative additives on the positive temperature coefficient in resistance characteristics of n-BaTiO3 ceramics

The influence of B₂O₃, and Al₂O₃ as segregative additives in modifying the ρ–T characteristics has been studied in BaTiO₃ ceramics with positive temperature coefficient of resistance (PTCR). Reaction of Al₂O₃ at the grain boundary regions of BaTiO₃ ceramics leads to the segregation of the secondary phase, BaAl₆TiO₁₂ resulting in broad PTCR jump, whereas B₂O₃ addition gives rise to steeper resistivity jump. Microstructure studies by SEM reveal the formation of coherent second phase layer of barium aluminotitanate surrounding the BaTiO₃ grains. The EDX results shows varying Al to Ti ratio in the early stage of phase formation in BaAl₆TiO₁₂ resulting in electrically active layer around the BaTiO₃ grains. The TiO₂-excess melt formation results in lower resistivity for 2–4% Al₂O₃ containing n-BaTiO₃ ceramics whereas at higher alumina contents, BaAl₆TiO₁₂ phase becomes dominant leading to higher resistivity in the sample. Complex impedance analyses support the three-layer regions, corresponding to the contributions from grain interior resistance (R _g), grain boundary resistance (R _gb), and that from secondary phase (R _sec). Electron paramagnetic resonance spectroscopy (EPR) indicated barium vacancies, V _Ba ^/ as the major electron trap centers which are activated across the tetragonal-to-cubic phase transition. A charge trapping mechanism is proposed wherein the segregation of secondary phases bring carrier redistribution among the various acceptor states thereby affecting the electrical conductivity of n-BaTiO₃ ceramics. The presence of Al³⁺–O⁻–Al³⁺ or Ti⁴⁺–O⁻–Al³⁺ type hole centers at the grain boundary layer (GBL) regions results in charge redistribution across the modified phase transition temperature due to symmetry-related vibronic interactions resulting in broad PTCR characteristics extending to higher temperatures.     

Single crystal growth in PMN-PT and PMN-PZT

Single crystal growth of lead-based piezoelectric ceramics Pb(Mg_1/3Nb_2/3)_0.68Ti_0.32O₃ (PMN-32PT) and Pb(Mg_1/3Nb_2/3)_0.42(Ti_0.638Zr_0.362)_0.58O₃ (PMN-37PT-21PZ) ceramics via templated grain growth (TGG) was investigated. (001)- and (111)-oriented BaTiO₃ (BT) single crystals and (001)-oriented SrTiO₃ (ST) single crystals (of approximately 2.5 × 2.5 × 1 mm) were utilized as seeds for the growth experiments. The piezoelectric single crystals were produced in a process that involves at first hot pressing of single crystal in cold isostatically pressed ceramics followed by subsequent sintering of the samples. Growth of (001)-oriented single crystals with BT seeds was observed in both PMN-32PT and PMN-37PT-21PZ matrices. The measured growth lengths were up to 140 and 65 μm, respectively. The grown (001)-oriented single crystals grown were rectangular. The measured growth lengths of the pyramidal-shaped (111) BT single crystals were up to 1 mm, which is much larger than the growth lengths of the (001) single crystals. Experiments on (001) ST-seeded single crystals were not successful. No single crystal growth was observed due to the dissolution of the ST single crystals in the PMN-PZT matrix. The differences were explained by defect-chemical considerations.     

Densification and anisotropic grain growth in Sr2Nb2O7

Sr : Nb stoichiometry and donor-doping with La were found to affect densification behavior and anisotropic grain growth in Sr₂Nb₂O₇ ceramics. La-doping improved the high temperature a.c. resistivity, but inhibits grain growth by grain boundary pinning. The presence of excess Nb was found to promote anisotropic grain growth by forming a liquid at the grain boundaries in both undoped and doped Sr₂Nb₂O₇. Anisotropic grain growth in La-doped Sr₂Nb₂O₇ can be controlled by incorporating large template particles in a Nb-rich matrix. High sintered densities (98% of theoretical) were achieved in both undoped and La-doped samples.     

Synthesis and microstructure of SrTiO<Subscript>3</Subscript> and BaTiO<Subscript>3</Subscript> ceramics by a reaction-sintering process

Strontium titanate and barium titanate ceramics prepared by a reaction-sintering process were investigated. The mixture of raw materials of stoichiometric SrTiO₃ and BaTiO₃ was pressed and sintered into ceramics without any calcination stage involved. A density 4.99 g/cm³ (97.5% of the theoretic value) was found in SrTiO₃ after 6 h sintering at 1,370 °C. Grains less than 1.5 μm were formed at 1,300–1,330 °C and became 2.2–3.3 μm at 1,350–1,370 °C SrTiO₃. A density 5.89 g/cm³ (97.9% of the theoretic value) was found in BaTiO₃ after 6 h sintering at 1,400 °C. Merged grains were observed in BaTiO₃ and were less than 10 μm after sintered at 1,400 °C.     

Improvement of electrical conductivity and leakage current in co-precipitation derived Nd-doping BiFeO3 ceramics

BiFeO₃ and Bi_0.925Nd_0.075FeO₃ ceramics were prepared by co-precipitation method. The crystal structure and electrical properties of the samples were characterized by X-ray diffraction (XRD), impedance spectra and leakage current measurement. XRD result implied that the impurity phases are weakened by suitably doping Nd. The impedance spectra of BiFeO₃ sample indicate that low grain boundary resistance and non Debye-type relaxation below the Néel temperature. Complex impedance spectra suggested that the doped samples are closer to Debye-type, and the grain boundary resistance increases which lead to low leakage current density.     

Improved current limiters based on mixed-phase BaTiO3 ceramic semiconductors

Very stable and highly reproducible current-limiting characteristics have been observed for polycrystalline ceramics prepared from sintering mixtures of coarse-grained, donor-doped BaTiO₃ (tetragonal) as the major phase and ultrafine, undoped cubic perovskite such as BaSnO₃, BaZrO₃, SrTiO₃ or BaTiO₃ (cubic) as the minor phase. The initial linear current-voltage (I-V) relationship becomes current-limiting with increase in applied potential and the consequent onset of thermal equilibrium. The strong current maximum of theI-V curve of donor-doped BaTiO₃ can be eliminated when the ceramics are constituted of mixed phases. The voltage drop at the insulating grain boundaries minimizes the temperature gradient between the interior and the surface, and subdues the thermal runaway. The magnitude of the limiting current, and hence the power-handling capacity, can be varied with the controlled addition of grain boundary layer modifiers and by optimizing the processing parameters. The dielectric constant versus temperature or voltage variation in power dissipation with ambient temperature and resistivity-temperature relations point to the necessity of the mixed phase character for the current-limiting property.     

Doped barium titanate nanoparticles

We have synthesized nickel (Ni) and iron (Fe) ion doped BaTiO₃ nanoparticles through a chemical route using polyvinyl alcohol (PVA). The concentration of dopant varies from 0 to 2 mole% in the specimens. The results from X-ray diffractograms and transmission electron micrographs show that the particle diameters in the specimen lie in the range 24–40 nm. It is seen that the dielectric permittivity in doped specimens is enhanced by an order of magnitude compared to undoped barium titanate ceramics. The dielectric permittivity shows maxima at 0.3 mole% doping of Fe ion and 0.6 mole% of Ni ion. The unusual dielectric behaviour of the specimens is explained in terms of the change in crystalline structure of the specimens.     

The effect of Sb and Nb on the electrical conductivity of tin dioxide based ceramics

The electrical conductivity of Mn doped SnO₂ systems prepared by an organic route (Pechini’s method) has been investigated as a function of antimony and niobium concentration. The conductivity increases with the increase of both concentration ions, however, in a different manner. While the conductivity of niobium doped ceramics increases with the power of 1.6 for the entire range of concentrations studied (0.01–0.7 mol%), the conductivity of antimony doped ceramics increases with the power of 1.9 in the range 0.01–0.05 mol% of Sb; 3.7 in the range 0.05–0.30 mol% and 1.8 in the range 0.30–0.70 mol%. This behavior is attributed to the existence of two stable oxidation states for antimony: Sb³⁺ and Sb⁵⁺, while for niobium there is only one: Nb⁵⁺. The power of 3.7 for Sb would be related to the segregation of this ion on the grain boundary accompanied by an additional contribution coming from the substitution of Sn²⁺ by Sb³⁺ on the grain surface.     

Temperature dependence of fractal dimension of grain boundary region in SnO2 based ceramics

Fractal dimensions of grain boundary region in doped SnO₂ ceramics were determined based on previously derived fractal model. This model considers fractal dimension as a measure of homogeneity of distribution of charge carriers. Application of the derived fractal model enables calculation of fractal dimension using results of impedance spectroscopy. The model was verified by experimentally determined temperature dependence of the fractal dimension of SnO₂ ceramics. Obtained results confirm that the non-Debye response of the grain boundary region is connected with distribution of defects and consequently with a homogeneity of a distribution of the charge carriers. Also, it was found that C−T ⁻¹ function has maximum at temperature at which the change in dominant type of defects takes place. This effect could be considered as a third-order transition.     

R-curve and subcritical crack growth in lead zirconate titanate ceramics

R-curves and subcritical crack growth curves (V–K_I) were determined for undoped, K doped (PKZT) and Nb doped (PNZT) lead zirconate titanate (PZT) ceramics and the results are discussed including the effect of doping, grain size and the polarisation state. A pronounced crack growth resistance was observed in the soft PNZT ceramic, which is attributed to ferroelastic domain switching. Subcritical crack growth in the studied PZT materials is governed by both environmentally stress-induced corrosion at the crack tip and the crack shielding due to domain switching. Increasing domain switching capacity by structural modification of the material (i.e. by donor doping or by increasing the grain size) or by poling sifts the V–K_I curve to higher values of the stress intensity factor.     

Influence of rare-earth addition on microstructure and dielectric behavior of Ba0.6Sr0.4TiO3 ceramics

Ba_0.6Sr_0.4TiO₃ (BST) ceramics with 0.5 mol% various trivalent rare-earth additions prepared by a solid-state route are investigated. A strong correlation is observed between the microstructure, dielectric properties and rare-earth element dopant. The results display that comparing with the lattice constants of undoped and doped rare-earth BST, the structure transforms from cubic to tetragonal structure. In addition, the dopant improves the tetragonal distortion with the ionic radius of rare earth decreasing, and then deteriorates it with further decreasing. Large ions rare-earth additions effectively suppress the grain growth of BST. It is found that the temperature-permittivity characteristics for the BSTR (R, namely, rare earth) system could be controlled using various rare-earth elements. Especially, such as Sm, Eu, Gd dopants are effective to satisfy the tunable microwave devices application due to the decrease of permittivity and the improvement of dissipation factors of BST ceramic with the accompanying high-tunability.     

The abnormal grain growth and dielectric properties of (Nb,Ba) doped TiO2 ceramics

The effect of consolidation pressure and crystallite size of powders crystal phases of TiO₂ on sintered microstructure of TiO₂ ceramics doped with 0.25 mol % Nb and 1.0 mol % Ba were investigated. Also, the development sequence of abnormal grain growth of (niobium, barium) doped TiO₂ ceramics was proposed. The second phases of as-sintered surface were determined. The dielectric properties of Ag-electroded samples were correlated with the resistivity of the bulk (Nb, Ba) doped TiO₂ ceramics. Abnormal grain growth lowered the resistivity of bulk material of (Nb, Ba) doped TiO₂ ceramics, and moved the relaxation frequency of fan δ to high frequency region over 10⁵ Hz. Controlling the sintered microstructures can obtain reasonably good dielectric properties.     

Dielectric properties and relaxation behavior of Sm substituted SrTiO3 ceramics

Sr_1–1.5xSm_xTiO₃ (x = 0–0.025) ceramics were fabricated by the conventional solid-state reaction method. Experimental results show that all ceramics are pure cubic perovskite structure, and the lattice parameters and average grain sizes of the ceramics decrease with the Sm concentration increasing. Compared with the pure SrTiO₃ ceramics, the relative dielectric constant could be enhanced to 3,681 (at 1 kHz and room temperature)with dielectric loss less than 0.02 when x = 0.02. The good stability of ε _r under 0–20 kV/cm is beneficial to applying in high voltage conditions. Two sets of relaxation peaks of Sm doped SrTiO₃ ceramics are observed which are all related to the oxygen vacancies. The medium-temperature relaxation is ascribed to the coupling of oxygen vacancies with strontium vacancies and the origin of high-temperature relaxation is the motion of oxygen vacancies.