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.     

Graphene nanoplatelet/silicon nitride composites with high electrical conductivity

Silicon nitride (Si₃N₄) processed with up to 25 vol.% of graphene nanoplatelets (GNPs) gives conductive composites with the highest electrical conductivity (40 Scm^?1) reported for these ceramics with added conductive particles. During compaction and pressure-assisted densification of the composites in the spark plasma sintering (SPS), a preferred orientation of GNPs occurs. Consequently, the electrical conductivity measured along the direction perpendicular to the SPS pressing axis is more than one order of magnitude higher than the one measured along the parallel direction.Percolation in the composites is observed for 7–9 vol.% of GNPs, depending on the measuring direction, perpendicular or parallel to the pressing axis. Different conduction mechanisms are apparent for the two orthogonal orientations. Charge transport along the direction defined by the graphene ab-plane (perpendicular direction) may be explained by a two dimensional variable range hopping mechanism, whereas conduction in the parallel direction shows a more complex behavior, with a metallic-type transition (dσ/dT < 0) for high GNP contents. A thin amorphous layer was identified at the Si₃N₄/GNPs interface that may affect the conduction for the parallel configuration.     

Raman and dielectric spectroscopic analysis of magnetic phase transition in Y(Fe0.5Cr0.5)O3 multiferroic ceramics

Here, we report the Raman and dielectric spectroscopic studies as a function of temperature of orthorhombically distorted Y(Fe_0.5Cr_0.5)O₃ (YFC) ceramics, measured from 80 to 300 K. The dc-magnetization measurements under field cooled (FC)-zero field cooled (ZFC) protocol indicate a small onset of magnetic ordering at T_N∼270 K. The field dependent magnetization plot recorded at 50 K, 150 K and 200 K show a clear opening in hysteresis loops. The linear dependence of magnetization plot at high field without any saturation of magnetization indicates the coexistence of weak ferromagnetic (WFM) component within the canting antiferromagnetic (CAFM) matrix. Temperature evolution of Raman line-shape parameter of B_2g(4) phonon mode clearly exhibits an anomalous behavior of phonon shift near T_N∼270 K, indicating the spin-phonon coupling in the ceramics. From the temperature dependent dielectric permittivity (ε(T)) study, two dielectric relaxation peaks are detected below 200 K and above 250 K. The appearance of former relaxation peak is responsible for polaronic conduction mechanism, while the later one is associated with magnetic phase transition which might be relevant to the presence of magnetoelectric coupling in YFC ceramics. The observed P-E hysteresis loops at room temperature indicate weak ferroelectric nature of the ceramics.     

Preparation of c-axis textured SiC ceramics by a strong magnetic field of 6 T assisted gel-casting process

C-axis textured SiC ceramics were prepared by a strong magnetic field of 6 T assisted gel-casting and subsequent pressureless sintering. The optimal suspension parameters for gel-casting were determined by analyzing the influences of pH value and dispersant content on the stability and dispersibility of suspensions. The effect of sintering conditions on the texture development and properties of SiC ceramics was discussed. It was found that the increasing sintering temperature or holding time promoted the densification process of SiC ceramics. The c-axis of SiC grain was aligned parallel to the magnetic field by applying a strong magnetic field of 6 T. The degree of texture of SiC ceramics showed a slightly increasing trend with the increase of sintering temperature or holding time. When the samples were sintered at 1950 °C for 4 h or 6 h, the large elongated grains were formed in the samples, leading to the extremely evident anisotropic microstructure on different planes. Textured SiC ceramics exhibited the anisotropic bending strength.     

Dielectric anisotropy and sinterability improvement of Ba4Nd9.33Ti18O54 textured ceramics

The 〈0 0 1〉-textured Ba₄Nd_9.33Ti₁₈O₅₄ (BNdT) ceramics were fabricated by templated grain growth process. Acicular particles prepared using K₂SO₄ molten salt were used as template and aligned by tape casting in the equiaxed BNdT powder. Crystalline phases of sintered specimen exhibited {h k 0} and {0 0 1} development in the plane of parallel and perpendicular to the casting direction, respectively. The formation of a slight amount of secondary phase in the template particles and the compositional deviation inhibited the densification and texture development for BNdT. To improve the sinterability, the composition shifted from BNdT to Ba₂Ti₉O₂₀ (B2T9) rich side slightly in BaONd₂O₃TiO₂ ternary system, which enables to promote a liquid phase during the sintering, was examined as a matrix phase. Resultant ceramics including B2T9-rich matrix phase displayed high density and large dielectric anisotropy. Temperature dependence of dielectric constant showed negative and positive behavior between the direction of parallel and perpendicular to the 〈0 0 1〉-textured BNdT. Near-zero temperature coefficient of resonant frequency (τ_f) in TE_0 1 1 mode was obtained in the textured ceramics with the degree of {0 0 1} orientation of approximately 0.37 on the disk plane. The exceptional temperature behavior of BNdT made it possible to control the τ_f over the wide range by the combination of dielectric anisotropy.     

Effects of Nb-doping on the micro-structure and dielectric properties of (Bi0.5Na0.5)TiO3 ceramics

Bismuth sodium titanate [(Bi_0.5Na_0.5)TiO₃ or BNT] ceramics incorporated with 0, 1, 5, 10, 15 and 20 mol% niobium were prepared by conventional solid state reaction method. The green bodies were sintered at 1050 °C for 2 h to obtain dense ceramics. The effects of substitution of niobium ion for titanium ion in BNT ceramics on micro-structure and dielectric properties were investigated. X-ray diffraction analysis showed the presence of a secondary phase when more than 5 mol% niobium was added. Within the solubility limit, Nb doping caused the grain size of BNTNb to be smaller than the undoped sample. The investigation of the dielectric properties showed that the transition temperature (T_c) was found to shift towards lower temperature as the content of Nb increased. In this research, the donor-type behavior and induced charged defects had significant influence on the electrical properties of Nb-doped BNT ceramics.     

Dielectric and magnetic properties of Nickel ferrite ceramics using crystalline powders derived from DL alanine fuel in sol–gel auto-combustion

Dielectric and magnetic properties of NiFe₂O₄ ceramics prepared with powders using DL-alanine fuel in the sol–gel auto combustion technique are studied. DL-alanine fuel yields crystalline as-burnt powders, and when used for ceramic processing yields varying microstructure at different sintering temperatures. The dielectric properties are influenced by the resulting microstructure and the magnetic properties show slight change in saturation magnetization M_s (~44 – 46 emu/g). The coercive fields, dielectric losses and dispersion are reduced considerably at higher sintering temperatures (1200–1300 °C). The influence of changing microstructure is analyzed through dielectric response, complex impedance analysis and electrical modulus spectroscopy in the frequency range (10⁻²–10⁷ Hz) to understand the interactions from the grain and grain boundary phases. Sintering at 1200 °C, is found to be optimum, yields lower losses & reduced dielectric dispersion, and high resistivity (3.4×10⁸ Ω cm).     

Synthesis,characterization, and dielectric properties of low loss LaBO3 ceramics

The preparation, sintering behaviour, and dielectric properties of low loss LaBO₃ ceramics have been investigated. Single-phase LaBO₃ powder was synthesized by the conventional solid-state ceramics route and dense ceramics (relative density >96%) with uniform microstructure (grain size ～30 μm) were obtained by sintering at 1300 °C in air. The electrical conductivity of LaBO₃ follows the Arrhenius law and the related activation energies for electrical conduction of bulk and grain boundary are 0.62 eV and 0.90 eV, respectively. The LaBO₃ ceramics sintered at 1300 °C exhibit excellent microwave dielectric properties with a relative permittivity, ?_r ～ 11.8, a quality factor, Q × f₀ value ～76,869 GHz (at ～15 GHz), and a negative temperature coefficient of resonant frequency τ_f ～ ?52 ppm/°C.     

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.     

Physical and electrical properties of Nb doped Bi0.5Na0.5[Zr0.59Ti0.41]O3

This research studied the effect of Nb doping on Bi_0.5Na_0.5[Ti_0.41Zr_0.59]O₃ (when Nb concentration = 0.00, 0.01, 0.03, 0.05, 0.07 and 0.09 mol fraction). Nb doped BNTZ ceramics were fabricated using a conventional mixed-oxide method. All samples were calcined at a temperature of 700 °C for 2 h and sintered at a temperature of 900 °C for 2 h. X-ray diffraction patterns suggested that the compounds possessed rhombohedral perovskite structure. SEM micrographs indicated that average grain size decreased as the amount of Nb additives increased. The electrical resistivity showed a decreasing trend with increasing Nb concentration due to excess charge present in the sample. The dielectric constant and dielectric loss of samples showed no particular trend when Nb was added but the optimum was observed when 0.05–0.07 Nb mol fraction was present in BNTZ ceramics. In this study, both microstructure and donor-type effects played an important role in determining electrical resistivity and dielectric properties of these ceramics.     

Microstructure and electrical properties of Mn/Y codoped Ba0.67Sr0.33TiO3 ceramics

Pure and Mn/Y codoped Ba_0.67Sr_0.33TiO₃ (BST) ceramics were fabricated via the citrate–nitrate combustion technique, and the microstructure and electrical properties of BST ceramics were mainly investigated. The Mn/Y codoping concentration has a strong influence on the microstructure and electrical properties of BST ceramics. All BST ceramics possess a pure polycrystalline structure. The density, dielectric loss, leakage current, and ferroelectric properties are improved by codoping 0.5 mol% Mn and 1.0 mol% Y to BST. The relative density of 0.5 mol% Mn/1.0 mol% Y-codoped BST (BST0510) ceramics reaches 97.5% of the theoretical value. BST0510 ceramics have the lowest dielectric loss (tanδ < 0.0073 at 1 kHz) among all BST ceramics. BST0510 ceramics also demonstrate a low leakage current density (1.23 × 10^?7 A/cm²) at an applied field of 10 kV/cm, and excellent ferroelectric properties with a remanent polarization of 2P_r = 15.327 μC/cm² and a coercive field of 2E_c = 3.456 kV/cm. Therefore, the Mn and Y with optimum content help improve the electrical properties of BST materials.     

Energy storage density and tunable dielectric properties of BaTi0.85Sn0.15O3/MgO composite ceramics prepared by SPS

(100-x) wt.% BaTi_0.85Sn_0.15O₃–x wt.% MgO (BTS/MgO) composite ceramics were prepared by spark plasma sintering (SPS) technology. Phase constitution, microstructure, dielectric and electrical energy storage properties of BTS/MgO composite ceramics were investigated. The samples prepared by SPS had smaller grain size and presented layer-plate substructure. Dielectric permittivity and dielectric loss of BTS/MgO composite ceramics decreased significantly with the content of MgO increasing, and dielectric tunability maintained a relatively high value (>45%). Meanwhile, the dielectric breakdown strength was improved when addition of MgO in BTS matrix, which resulted in a significant improvement of energy storage density. The high dielectric breakdown strength of 190 kV/cm, energy storage density of 0.5107 J/cm³ and energy storage efficiency of 92.11% were obtained in 90 wt.% BaTi_0.85Sn_0.15O₃–10 wt.% MgO composite ceramics. Therefore, BTS/MgO composites with good tunable dielectric properties and electrical energy storage properties could be exploited for energy storage and phase shifter device applications.     

Microstructure and electrical properties of (Ba0.85Ca0.15)1−xLix(Ti0.90Zr0.10)1−xNbxO3 ceramics with a low dielectric loss and a low sintering temperature

A low sintering temperature is demonstrated for (Ba_0.85Ca_0.15)_1?xLi_x(Ti_0.90Zr_0.10)_1?xNb_xO₃ (BCLTZN-x) piezoelectric ceramics, where BCLTZN-x lead-free piezoelectric ceramics were prepared by the normal sintering. Effects of Li and Nb on the microstructure and electrical properties of these ceramics were investigated. The sintering temperature of BCLTZN-x ceramics was decreased greatly by introducing Li and Nb, and the grain size of these ceramics decreases with increasing x. These ceramics with a small amount of Li and Nb maintain good piezoelectric properties, together with a low sintering temperature and a lower dielectric loss. These ceramics with x = 0.01 demonstrate optimum electrical properties: d₃₃ ～ 353 pC/N, k_p ～ 41.1%, T_c ～ 86 °C, ?_r ～ 4236, and tan δ ～ 0.75%.     

Microwave dielectric properties of nanocrystalline TiO2 prepared using spark plasma sintering

TiO₂ bulk ceramics were fabricated by using both spark plasma sintering (SPS) and the conventional sintering method (CSM). Starting materials were ultra fine rutile powders (<50 nm) prepared via the sol–gel process. CSM achieved the relative sintering density of 99.2% at 1300 °C. The grain size of 1300 °C sintered specimen was 6.5 μm. However, the sintering temperature of SPS for the density of 99.1% was as low as 760 °C, where the grain size was only 300 nm. In order to re-oxidize the Ti³⁺ ions due to the reducing atmosphere of the SPS process and the high temperature of the CSM process, the prepared TiO₂ specimens were annealed in an oxygen atmosphere. The dielectric constant (ɛ_r) and quality factor (Q × f) of SPS-TiO₂ re-oxidized specimens in a microwave regime were 112.6 and 26,000, respectively. These properties were comparable to those of 1300 °C sintered CSM specimens (ɛ_r ∼ 101.3, Q × f ∼ 41,600). These microwave dielectric properties of nanocrystalline TiO₂ specimens prepared using SPS were discussed in terms of grain size variation and Ti⁴⁺ reduction.     

Effect of grain size on the energy storage properties of (Ba0.4Sr0.6)TiO3 paraelectric ceramics

(Ba_0.4Sr_0.6)TiO₃ (BST) ceramics with various grain sizes (0.5–5.6 μm) were prepared by conventional solid state reaction methods. The effect of grain size on the energy storage properties of BST ceramics (T_c ≈ −65 °C) was investigated. With decreasing grain sizes, a clear tendency toward the diffuse phase transition was observed and the dielectric nonlinearity was reduced gradually, which can be explained by the Devonshire's phenomenological theory (from the viewpoint of intrinsic polarization). Based on the multi-polarization mechanism model, the relationship between the polarization behavior of polar nano-regions (the extrinsic nonlinear polarization mechanisms) and grain size was studied. The variation of the grain boundary density was thought to play an important role on the improvement of dielectric breakdown strength, account for the enhanced energy density, which was confirmed by the complex impedance spectroscopy analysis based on a double-layered dielectric model.     

Preparation and magnetoelectric properties of NiFe2O4–PZT composites obtained in-situ by gel-combustion method

Magnetoelectric composites of xNiFe₂O₄–(1 ? x)Pb(Zr,Ti)O₃ with x = 2, 5, 10, 20, 30% were prepared by citrate–nitrate combustion using PZT-based template powders. In order to ensure a better connectivity of dissimilar phases, we have used chemical methods for preparation in situ composites, followed by adequate sintering procedure. The structural, microstructural and functional properties of di-phase magnetoelectric composites of NiFe₂O₄–PZT are reported. The XRD analysis is demonstrating the synthesis of pure ferrite phase directly on the ferroelectric templates. An excellent mixing was obtained in the composite powders, as proved by a detailed SEM analysis.The magnetic and dielectric behaviors of the ceramic composites vary with the ratio of the two phases. The dielectric behavior is greatly influenced by the magnetic phase. The magnetoelectric (ME) coefficient was measured as a function of applied DC magnetic field. The maximum ME coefficient (dE/dH) varies from 0.0011 mV/(cm Oe) to 0.5 mV/(cm Oe) with increasing of NF addition.     

Effect of CuO addition on magnetic and electrical properties of Sr2Bi4Ti5O18 lead-free ferroelectric ceramics

The effect of CuO addition on magnetic and electrical properties of Sr₂Bi₄Ti₅O₁₈ (SBT) lead-free bismuth layered structure ferroelectric ceramics have been studied and reported. Interestingly, the prepared samples exhibit multiferroic behavior with the coexistence of magnetic and ferroelectric phase transition temperature. Magnetic phase transition with Neel׳s temperature (T_N) of 657 K is observed at 0.75 mol% of CuO added SBT ceramics, which is higher than the well known multiferroic BiFeO₃ (643 K) and the ferroelectric phase transition with Curie temperature (T_C) of 587 K is observed at 1 mol% of CuO added SBT ceramics, which is relatively higher than the reported pure SBT ceramics (558 K). Further, the electrical properties such as dielectric, ferroelectric, piezoelectric, leakage current density characteristics and optical properties were investigated as a function of x (x=0, 0.25, 0.5, 0.75 and 1 mol%). Presence of strong magnetic super-exchange interactions in CuO and the creation of oxygen vacancies play a vital role in the enhancement of magnetic and electrical properties of CuO doped SBT ceramics. Moreover, the present results indicate that, small amount (0.25 mol%) of CuO addition in SBT ceramics enhances the electrical properties significantly and vice versa, large amount (0.75 mol%) of CuO addition enhances the magnetic properties. Thus, the presence of magneto-electric coupling effect was observed in CuO doped Sr₂Bi₄Ti₅O₁₈ ferroelectric ceramics.     

Processing and mechanical response of highly textured Al2O3

Dense, high quality textured alumina was fabricated by templated grain with only 0.14 wt% (SiO₂ + CaO) and 1–15 wt% tabular alumina templates From stereological measurements, texture fraction was 95% and unaffected by template loading or dopant concentration. Due to the excellent template alignment during casting, the full-width at half the maximum (FWHM) of the rocking curve was exceptionally low at 4.6°. Samples with the largest templated grains (1% templated alumina) have much lower strength (300–320 MPa) than those with smaller templated grains (400–500 MPa). Predominantly transgranular crack paths were observed in fracture surfaces both parallel and perpendicular to the oriented grains. The fracture toughness was anisotropic with slightly higher toughness perpendicular to the basal surface than parallel the direction grain basal surface. Surprisingly, the lowest density sample (93%) tested, which started with 15 wt% templates had the highest strength of 511 MPa ± 0.45 and highest fracture toughness of 4.58 ± 0.44 MPa m^1/2 when measured perpendicular to the basal surface of the grains.     

Anisotropy on SrTiO3 templated textured PMN–PT monolithic ceramics

In this work, templated grain growth (TGG) and reactive templated grain growth (RTGG) texture techniques combined with uniaxial hot pressing were used for the first time to produce high dense monolithic textured 0.6PMN–0.4PT ceramics. Microstructural analysis of the textured ceramics showed that both TGG and RTGG texture methods are efficient to promote anisotropic grain nucleation around the SrTiO₃ single crystal templates. The structural data remarkably revealed that although there was no previous reaction of the powder to form the lead magnesium niobate–lead titanate (PMN–PT) compound in the RTGG samples and the ceramic phase formation was 100% perovskite indicating that RTGG texture technique is more attractive than TGG in the case of SrTiO₃ templated PMN–PT. Rather, dielectric characterizations performed in the RTGG samples parallel and perpendicular to the template axis revealed a high anisotropy in the electrical permittivity for RTGG samples (1.48) that were comparable to an estimated value for 0.62PMN–0.38PT single crystals. Piezoelectric characterization of RTGG samples resulted in strain levels up to 0.34% and the highest d₃₃ coefficient was 1100 pC/N, which showed significant increase compared to random ceramic.