Anisotropic electrical and magnetic properties in textured Bi5Ti3FeO15 ceramics

Textured Bi₅Ti₃FeO₁₅ (BTFO) ceramics were prepared by molten salt synthesis at 850 °C followed by plasma activated sintering at 800 °C. The structure and the anisotropic electrical and magnetic properties of the ceramics were investigated. The textured BTFO ceramics exhibited [001] preferred orientation with a high degree of texture fraction (Lotgering factor is 0.82), as confirmed from XRD patterns, FE-SEM and TEM micrographs. The anisotropic dielectric relaxation and conduction characteristics along the parallel and perpendicular directions were investigated using the dielectric and impedance spectroscopies. The grain boundary along the parallel direction had a similar value of capacitance to that of grains, whereas the grain boundary along the perpendicular direction dominated the resistive component. The magnetic performance along the perpendicular direction (H_c ∼ 32.5 Oe) was better than that along the parallel direction (H_c ∼ 17.6 Oe). These results could be useful for enhancing the electrical and magnetic properties of BTFO by texturization.     

Fabrication of textured Sr2Na0.9K0.1Nb5O15 ceramics: Anisotropy in structures and electrical properties

Highly textured Sr₂Na_0.9K_0.1Nb₅O₁₅ (SKNN) ceramics were fabricated by reactive templated grain growth method using acicular Sr₂KNb₅O₁₅ (SKN) templates. CuO (1 wt.%) was used as sintering additive. SKN templates were aligned in matrix powders of SrNb₂O₆ and NaNbO₃ via tape casting and sintered at higher temperatures to obtain texture morphology. Through carefully controlling the processing conditions, a texture fraction of 86% was obtained. The structure evolution was explained by liquid-phase-assisted growth mechanism, in which the whole process was divided into 3 steps according to priority: phase formation stage, densification stage, and texture development stage. The textured ceramics exhibited anisotropic properties with the highest electrical properties obtained in c-axis direction: ?_r = 2212, ?_m = 4869, P_r = 15.92 μC cm^?2 and d₃₃ = 82 pC N^?1, showing that reactive templated grain growth method is very effective to improve the physical properties of SKNN ceramics.     

Effect of V2O5 on sintering behaviour,microstructure and dielectric properties of textured Sr0.4Ba0.6Nb2O6 ceramics

The acicular Sr_0.39Ba_0.48K_0.32Nb₂O₆ single crystal particles were first prepared by the reaction of SrCO₃, BaCO₃ and Nb₂O₅ in molten K₂SO₄ at 1300 °C for 3 h. By using these single crystal particles as seeds and V₂O₅ as additives, textured Sr_0.4Ba_0.6Nb₂O₆ (SBN40) ceramics were obtained. The effect of V₂O₅ on sintering behaviour, microstructure and dielectric properties of textured SBN40 ceramics was investigated. The experimental results show that the addition of V₂O₅ can accelerate the densification rate of the material and encourage the texture of SBN40 ceramics, which further improves the anisotropy in dielectric properties between different directions of textured SBN40 ceramics.     

Low-temperature microwave and THz dielectric response in novel microwave ceramics

Low-temperature dielectric properties of BaZn_1/3Nb_2/3O₃-based ceramics, CeO₂-based ceramics and Ruddlesden–Popper Sr_n+1Ti_nO_3n+1 (n = 1–4) ceramics has been studied in microwave, THz and infrared frequency range down to 10 K. Extrinsic dielectric losses originating probably from diffusion of charged defects are observed in two families of compounds by a minimum in the temperature dependence of microwave quality Q. The rise of microwave permittivity and dielectric losses at low temperatures in Sr_n+1Ti_nO_3n+1 (n = 2–4) ceramics was explained by softening of an optical polar mode in SrTiO₃, which is in the Sr_n+1Ti_nO_3n+1 (n = 3, 4) ceramics contained as a second phase.     

Composition design,phase transitions and electrical properties of Sr2+-substituted xPZN–0.1PNN–(0.9−x)PZT piezoelectric ceramics

The influences of PZN content and Sr²⁺ substitution on the structure and electrical properties of Pb(Zn_1/3Nb_2/3)O₃–Pb(Ni_1/3Nb_2/3)O₃–Pb(Zr_0.52Ti_0.48)O₃ (abbreviated as PZN–PNN–PZT) piezoelectric ceramics were studied. All as-prepared PZN–PNN–PZT ceramics presented single phase of perovskite structure, while higher PZN contents favored rhombohedral symmetry and larger grain size. Meanwhile, with the increase in Sr²⁺ content, the phase structure changed from a mix of tetragonal and rhombohedral symmetries to a pure rhombohedral symmetry. Although the ferroelectric Curie temperature (T_C) was decreased with increasing the PZN and Sr²⁺ contents, the piezoelectric constant (d₃₃) exhibited the opposite trend. As a result, optimum comprehensive electrical properties were obtained in the 0.1PZN–0.1PNN–0.8PZT composition with 10 mol% Sr²⁺ substitution: d₃₃~800 pC/N, k_p ~0.65, ɛ_r~4081, T_C~176 °C, P_r~30.92 µC/cm². Thus, the 10 mol% Sr²⁺-substituted 0.1PZN–0.1PNN–0.8PZT ceramic is a promising candidate for high performance applications.     

The anisotropic conductivity of ferroelectric La2Ti2O7 ceramics

Ferroelectric ceramics with perovskite-like layered structure (PLS) have good potential for high temperature piezoelectric applications due to their high Curie point (T_c). The electrical conduction behaviour of these materials is a critical parameter to consider for practical applications. In this study, we prepared textured ceramics of the typical PLS ferroelectric La₂Ti₂O₇ using spark plasma sintering and investigated the electrical properties using impedance spectroscopy and Seebeck measurements. The results reveal that the bulk resistivity along the parallel direction is much higher than that along the perpendicular direction. The activation energy for conduction (E_a) along the parallel direction is 1.45 eV, which is close to half of the optical band gap and much higher than the 0.67 eV along the perpendicular direction. Electrical conduction along both the directions is dominated by p-type hole conduction. No appreciable contribution of oxide ion conduction to the measured conductivity is observed.     

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.     

Enhanced intrinsic permittivity and bulk response in Y2/3Cu3Ti4+xO12 ceramics

Considering the contribution of the mixed valence structure of Ti³⁺ and Ti⁴⁺ to the semiconductivity of grain, compositions with the formula of Y_2/3Cu₃Ti_4+xO₁₂ were designed and prepared. The dielectric bulk responses of Y_2/3Cu₃Ti_4+xO₁₂ ceramics were explored in detail. Changing Ti stoichiometry gives rise to an increase of the intrinsic permittivity. Y_2/3Cu₃Ti_3.925O₁₂ ceramic exhibits a higher intrinsic permittivity of ~120 at 60 MHz than that of pure Y_2/3Cu₃Ti₄O₁₂ ceramics (87 at 60 MHz). Additionally, the activation energies of bulk responses are significantly enhanced by changing Ti stoichiometry, which is closely linked with the increase of Ti³⁺/Ti⁴⁺.     

Dielectric and impedance spectroscopic investigation of the Ba0.3Sr0.7Ti0.873Zr0.097Mn0.03O3 ceramic system

Polycrystalline Ba_0.3Sr_0.7Ti_0.873Zr_0.097Mn_0.03O₃ ceramics were processed via a mixed-oxide solid state sintering route at 1500 °C in air for 6 h. X-ray diffraction and scanning electron microscopy were used for phase and microstructural analyses of the Ba_0.3Sr_0.7Ti_0.873Zr_0.097Mn_0.03O₃ ceramics. The magnitude of relative permittivity (ε_r) was measured as 247 and tan δ as 0.008 at 1 kHz around the room temperature. Complex impedance spectroscopy (CIS) analysis was used to understand the electronic microstructure of the Ba_0.3Sr_0.7Ti_0.873Zr_0.097Mn_0.03O₃ ceramics. The magnitude of total resistance (R_T) was observed to decrease with increasing temperature, which confirmed a negative temperature coefficient of resistance (NTCR) for the Ba_0.3Sr_0.7Ti_0.873Zr_0.097Mn_0.03O₃ ceramics. The Cole–Cole plot showed only one semicircle for the sintered sample, which confirmed the grain boundary contribution to the conduction mechanism. A non-Debye type of relaxation behavior was observed in the Ba_0.3Sr_0.7Ti_0.873Zr_0.097Mn_0.03O₃ sintered sample.     

(Al3+, Nb5+) co–doped CaCu3Ti4O12: An extended approach for acceptor–donor heteroatomic substitutions to achieve high–performance giant–dielectric permittivity

Substitution of (Al³⁺, Nb⁵⁺) co–dopants into TiO₆ octahedral sites of CaCu₃Ti₄O₁₂ ceramics, which were prepared by a solid state reaction method and sintered at 1090 °C for 18 h, can cause a great reduction in a low–frequency loss tangent (tanδ≈0.045–0.058) compared to those of Al³⁺ or Nb⁵⁺ single–doped CaCu₃Ti₄O₁₂. Notably, very high dielectric permittivities of 2.9 ? 4.1 × 10⁴ with good dielectric–temperature stability are achieved. The room–temperature grain boundary resistance (R_gb≈0.37–1.17 × 10⁹ Ω.cm) and related conduction activation energy (E_gb≈0.781–0.817 eV), as well as the non–Ohmic properties of the co–doped ceramics are greatly enhanced compared to single–doped ceramics (R_gb≈10⁴–10⁶ Ω cm and E_gb≈0.353–0.619 eV). The results show the importance of grain boundary properties for controlling the nonlinear–electrical and giant–dielectric properties of CaCu₃Ti₄O₁₂ ceramics, supporting the internal barrier layer capacitor model of Schottky barriers at grain boundaries.     

Influence of TiO2 doping on thermo-optical properties of pyrochlore Sm2(Zr1–xTix)2O7 (0≤x≤0.15) ceramics

Sm₂(Zr_1–xTi_x)₂O₇ (0≤x≤0.15) ceramics have been fabricated by pressureless-sintering method at 1973 K for 10 h in air. The influence of TiO₂ doping on microstructure and thermo-optical properties of Sm₂(Zr_1–xTi_x)₂O₇ ceramics is investigated by X-ray diffraction, scanning electron microscopy and Fourier transform infrared spectroscopy measurements. The partial substitution of Ti⁴⁺ for Zr⁴⁺ results in a significant increase in emissivity at low wavelengths contrasted with undoped Sm₂Zr₂O₇. Sm₂(Zr_0.85Ti_0.15)₂O₇ ceramic exhibits a high emissivity of above 0.70 at 1073 K in a wavelength range of 3–16 µm, where the highest value at this temperature is more than 0.90 especially in the wavelength range of 9–14 µm. FT-IR spectra and optical absorption spectra unveil the mechanisms of enhanced emissivity in Sm₂(Zr_1–xTi_x)₂O₇ (0.05≤x≤0.15) ceramics in the intermediate infrared range, especially at the wavelengths of 3–8 µm, due to Ti⁴⁺ ion substitution for Zr⁴⁺ ion.     

Texturing behaviours of (K0.47Na0.51Li0.02)(Nb0.8Ta0.2)O3 piezoelectric ceramics produced using NaNb1-xTaxO3 templates

Textured (K_0.47Na_0.51Li_0.02)(Nb_0.8Ta_0.2)O₃ (KNLNT20) piezoelectric ceramics were prepared using NaNb_1?xTa_xO₃ templates. The highest degree of grain orientation (97%) and piezoelectric constant (342 pC/N) were obtained upon adding 3 wt% of the NaNb_0.8Ta_0.2O₃ (NNT20) template and sintering at 1150 °C for 1 h. Back-scattered scanning electron micrographs of the textured KNLNT20 samples sintered at 1150 °C for 1 h indicated the presence of templates similar in size to the original ones within the cores of the textured grains. The peak value of the dielectric constant corresponding to the NNT20 core decreased after prolonged holding at 1150 °C, owing to a decrease in the size of the NNT20 core because of the interdiffusion of K, Na, and Li ions between the NNT20 core and KNLNT20 shell. This interdiffusion also decreased the piezoelectric constant, d₃₃ value of the textured KNLNT20 samples by inducing a change in the chemical composition of the shell region.     

Healing performance of Ti2AlC ceramic studied with in situ microcantilever bending

In situ microcantilever bending tests were carried out to evaluate the healing efficiency of pre-notched Ti₂AlC ceramic after annealing at 1200 °C for 1.5 h. Microcantilevers of different orientations were fabricated with focused ion beam method at different locations, i.e. in a singular Ti₂AlC grain, at a grain boundary or at the Ti₂AlC–Al₂O₃ interface after healing. Ti₂AlC microcantilever shows an anisotropic bending strength (ranging between 9.6 GPa and 4.6 GPa depending on the precise crystallographic orientations) that is closely related to the different atomic bonds in the layered structure. After healing, the Ti₂AlC–Al₂O₃ microcantilevers exhibit almost the same strength of about 5.2 GPa, i.e. slightly higher than the cleavage strength (4.6 GPa) of the initial Ti₂AlC microcantilevers. It suggests that the orientation of the matrix grain has no significant influence on the strength of healed microcantilevers. Furthermore, it turns out that the strength of the microcantilever with a healed grain boundary is at least twice the strength of the initial Ti₂AlC cantilever with a grain boundary. It is concluded that the oxidation dominated self-healing mechanism of Ti₂AlC ceramics can result in a perfect recovery of mechanical performance. The paper shows that the in situ microcantilever bending test provides a quantitative method for the evaluation of the strength of self-healing ceramics.     

Investigation on the anti-reduction mechanism of Ti4+ in high dielectric constant system Ca0.9La0.067TiO3 by doping with Al2O3

Ca_0.9La_0.067TiO₃ (abbreviated as CLT) ceramics doped with different amount of Al₂O₃ were prepared via the solid state reaction method. The anti-reduction mechanism of Ti⁴⁺ in CLT ceramics was carefully investigated. X-ray diffraction (XRD) was used to analyze the phase composition and lattice structure. Meanwhile, the Rietveld method was taken to calculate the lattice parameters. X-ray photoelectron spectroscopy (XPS) was employed to study the valence variation of Ti ions in CLT ceramics without and with Al₂O₃. The results showed that Al³⁺ substituted for Ti⁴⁺ to form solid solution and the solid solubility limit of Al³⁺ is near 1.11 mol%. Furthermore, the reduction of Ti⁴⁺ in CLT ceramics was restrained by acceptor doping process and the Q × f values of CLT ceramics were improved significantly. The CLT ceramic doped with 1.11 mol% Al₂O₃ exhibited good microwave dielectric properties: ε_r = 141, Q × f = 6848 GHz, τ_f = 576 ppm/°C.     

Improved dielectric properties and grain boundary response of SrTiO3 doped Y2/3Cu3Ti4O12 ceramics fabricated by Sol-gel process for high-energy-density storage applications

A chemical solution processing method based on sol-gel chemistry (SG) was used to synthesize (1-x)Y_2/3Cu₃Ti₄O₁₂-xSrTiO₃ (x = 0, 0.05, 0.1, 0.15, 0.2, 0.25) ceramics successfully. The 0.85Y_2/3Cu₃Ti₄O₁₂-0.15SrTiO₃ ceramics sintered at 1050 °C for 20 h showed fine-grained microstructure and high dielectric constant (ε′ ≈ 1.7 × 10⁵) at 1 kHz. Furthermore, the 0.85Y_2/3Cu₃Ti₄O₁₂-0.15SrTiO₃ ceramics appeared distinct pseudo-relaxor behavior. Two electrical responses were observed in the combined modulus and impedance plots, indicating the presence of Maxwell-Wagner relaxation. Sr vacancies and additional oxygen vacancies had substantial contribution to the sintering behavior, an increase in grain growth, and relaxation behaviors in grain boundaries. The contributions of semiconducting grains with the nanodomain and insulating grain boundaries (corresponding to high-frequency and low-frequency electrical response, respectively) played important roles in the dielectric properties of (1-x)Y_2/3Cu₃Ti₄O₁₂-xSrTiO₃ ceramics. The occurrence of the polarization mechanism transition from the grain boundary response to the electrode one with temperature change was clearly evidenced in the low frequency range.     

Temperature-stable dielectric ceramics with nominal composition of Ca6Ti2Ta8O30

The dielectric ceramics with nominal composition of Ca₆Ti₂Ta₈O₃₀ were synthesized and characterized. The cubic CaTa₂O₆ major phase combined with minor amount of CaTiO₃ were formed in the present ceramics, and this differed from the similar compositions of Ba₆Ti₂Ta₈O₃₀ and Sr₆Ti₂Ta₈O₃₀, which had the filled tungsten bronze structure. The present ceramics indicated a high dielectric constant of 85–86 with a small temperature coefficient of dielectric constant of +24–29 ppm/°C together with a dielectric loss of 0.0056–0.0060 (at 1 MHz). The dielectric constant was almost a constant, while the dielectric loss and temperature coefficient of dielectric constant were sensitive to the sintering conditions. The present ceramics might be promising in temperature compensated capacitor applications.     

Development of texture in Bi0.5Na0.5TiO3 prepared by reactive-templated grain growth process

The effect of sodium concentration on texture development in Bi_0.5Na_0.5TiO₃ [BNT] bulk ceramics was examined. The 〈1 0 0〉-textured specimens were prepared by the reactive-template grain growth process using platelike Bi₄Ti₃O₁₂ particles. Texture did not extensively develop in stoichiometric and Na-deficient BNT, but excess Na promoted extensive texture development together with densification. The role of excess Na was discussed based on the formation of a liquid phase.     

Pulsed electric current,in situ synthesis and sintering of textured TiB2 ceramics

Textured TiB₂ monolithic ceramics were prepared in a one step in situ synthesis and densification process during pulsed electric current sintering (PECS) using TiH₂ and amorphous B as raw materials. The Lotgering orientation factors of the (0 0 l) and (h k 0) planes were measured to be 0.66 and 0.68, respectively. The crystallographically textured ceramics also display a clear microstructural anisotropy. The addition of SiC as a secondary phase was found to deteriorate the orientation in the TiB₂ ceramic due to grain growth inhibition.     

Li ion conduction of perovskite Li0.375Sr0.4375Ti0.25Ta0.75O3 and related compounds

In this work, perovskite-structured Li_0.375Sr_0.4375M_0.25N_0.75O₃ (M=Ti, Sn, N=Nb, Ta) solid electrolytes were synthesized by conventional solid state reaction method. Phase compositions, fractured morphologies and conductivities of these compounds were investigated by X-ray diffraction, scanning electron microscope and AC-impedance spectroscopy, respectively. X-ray diffraction analysis confirms that all of Li_0.375Sr_0.4375M_0.25N_0.75O₃ (M=Ti, Sn, N=Nb, Ta) ceramics present perovskite structure. Pure Li_0.375Sr_0.4375Ti_0.25Ta_0.75O₃ and Li_0.375Sr_0.4375Sn_0.25Ta_0.75O₃ perovskite ceramics were obtained. But impurities were detected in Li_0.375Sr_0.4375Ti_0.25Nb_0.75O₃ and Li_0.375Sr_0.4375Sn_0.25Nb_0.75O₃. Among all investigated compounds, Li_0.375Sr_0.4375Ti_0.25Ta_0.75O₃ shows the highest total ionic conductivity of 2.60 × 10^?4 S cm^?1 at room temperature and the lowest activation energy of 0.347 eV. Conductivities of Li_0.375Sr_0.4375Sn_0.25Ta_0.75O₃ and Li_0.375Sr_0.4375Sn_0.25Nb_0.75O₃ were 4.4 × 10^?5 S cm^?1 and 1.82 × 10^?6 S cm^?1, respectively. Their conductivities were much lower than Li_0.375Sr_0.4375Ti_0.25Ta_0.75O₃ and Li_0.375Sr_0.4375Ti_0.25Nb_0.75O₃.     

Enhancement of giant dielectric response in Ga-doped CaCu3Ti4O12 ceramics

The influences of Ga³⁺ doping ions on the microstructure, dielectric and electrical properties of CaCu₃Ti₄O₁₂ ceramics were investigated systematically. Addition of Ga³⁺ ions can cause a great increase in the mean grain size of CaCu₃Ti₄O₁₂ ceramics. This is ascribed to the ability of Ga³⁺ doping to enhance grain boundary mobility. Doping CaCu₃Ti₄O₁₂ with 0.25 mol% of Ga³⁺ caused a large increase in its dielectric constant from 5439 to 31,331. The loss tangent decreased from 0.153 to 0.044. The giant dielectric response and dielectric relaxation behavior can be well described by the internal barrier layer capacitor model based on Maxwell?Wagner polarization at grain boundaries. The nonlinear coefficient, breakdown field, and electrostatic potential barrier at grain boundaries decreased with increasing Ga³⁺ content. Our results demonstrated the importance of ceramic microstructure and electrical responses of grain and grain boundaries in controlling the giant dielectric response and dielectric relaxation behavior of CaCu₃Ti₄O₁₂ ceramics.