Effect of Ba2+ substitution on the structural and electrical properties of SrBi4Ti4O15 ceramic

We studied the effect of Barium doping on the structural and electrical properties of SrBi₄Ti₄O₁₅ (SBBT) ferroelectric ceramic. The samples were synthesized by the conventional solid-state reaction method. X-ray diffraction (XRD) and Raman scattering techniques have been employed to characterize the structural property. Scanning electron microscope images showed plate like grain morphology with random orientation of platelets. Lower Curie temperature and enhanced dielectric constant at the transition temperature were clearly observed with increasing the concentration of Ba²⁺ ion. Detailed studies on the effect of barium on electrical behavior of the SBBT systems have been carried out by the non destructive complex impedance spectroscopy (CIS technique). The Nyquist plots suggest that the grain and grain boundary were responsible in the conduction mechanism of the materials. Ferroelectric measurements revealed that Ba²⁺ doping leads to reduction in the remnant polarization. The piezoelectric coefficients (d₃₃) of the ceramics were enhanced with Ba²⁺ doping.     

Electrical relaxation and conduction mechanisms in iron doped barium strontium titanate

Polycrystalline Ba_0.7Sr_0.3Ti_(1?x)Fe_xO₃ (x = 0.1) (BSTF) ceramics, synthesized via solid-state reaction route were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and complex impedance spectroscopy (CIS). The Rietveld refinement of the XRD data confirmed the presence of tetragonal and cubic phases in the prepared sample. The SEM image revealed that the sample has well distributed grains along with some degree of agglomerations. The electrical behaviour of the BSTF ceramic has been studied by complex impedance spectroscopy (CIS) as a function of frequency (1 Hz to 1 MHz) at different temperatures (RT to 700 K). Two semicircular arcs observed in the Cole-Cole plot confirm the contribution from the grain and grain boundary in overall impedance. Both the electrical as well as ac conduction phenomena take place via correlated barrier hopping (CBH) authenticated by detailed complex modulus analysis and ac fitted conductivity respectively. The values of activation energies calculated from electrical impedance, modulus, and conductivity data clearly reveal that the relaxation and conduction processes in BSTF ceramic are induced by doubly ionized oxygen vacancies.     

AC conductivity,electric modulus analysis and electrical conduction mechanism of RbFeP2O7 ceramic compound

In this work, the diphosphate compound, RbFeP₂O₇, was prepared by the conventional solid-state reaction. The X-ray diffraction pattern revealed that the sample presents a single phase, that crystallizes in the monoclinic structure with a P2₁/C space group. Impedance analysis was performed using the equivalent circuit model, and, it indicated the presence of intra- and inter-granular contribution. Furthermore, the electrical conductivity, the dielectric properties and the relaxation behavior of this material were studied in detail using the impedance spectroscopy technique, in a frequency ranging from 200 Hz to 5 MHz at several temperatures. The temperature dependency of frequency exponent 's' shows that the correlated barrier-hopping model (CBH) is the most responsible mechanism for AC conduction in the investigated compound. In terms of CBH model, the values of maximum barrier s height, the hopping distance and the density of localized states are determined and discussed. A correlation between electrical and structural properties was also discussed.     

Ionic conduction and dielectric properties of yttrium doped LiZr2(PO4)3 obtained by a Pechini-type polymerizable complex route

We report on the ion transport properties of Li_1+xZr_2-xY_x(PO₄)₃ (0.05?≤ x?≤?0.2) NASICON type nanocrystalline compounds prepared through a Pechini-type polymerizable complex method. Structural properties were characterized by means of powder X-ray diffraction, Raman spectroscopy and electron microscopy with selected area electron diffraction. Impedance spectroscopy was utilised to investigate the lithium ion transport properties. Y³⁺ doped LiZr₂(PO₄)₃ compounds showed stabilized rhombohedral structure with enhanced total ionic conductivity at 30?°C from 2.87?×?10^?7 S?cm^?1 to 0.65?×?10^?5 S?cm^?1 for x=0.05 to 0.20 respectively. The activation energies of Li_1+xZr_2-xY_x(PO₄)₃ show a decreasing trend from 0.45?eV to 0.35?eV with increasing x from 0.05 to 0.20. The total conductivity of these compounds is thermally activated, with activation energies and pre-exponential factors following the Meyer-Neldel rule. The tanδ peak position shifts to the high-frequency side with increasing yttrium content. Scaling in AC conductivity spectra shows that the electrical relaxation mechanisms are independent of temperature.     

Dual-mode dichromatic SrBi4Ti4O15: Er3+ emitting phosphor for anti-counterfeiting application

Fluorescent materials have been widely used for anti-counterfeiting of important documents and currencies, wherein their anti-counterfeit abilities could be improved through multi-mode excitation. Herein, dual-mode-excited double-colour-emitting Er³⁺doped SrBi₄Ti₄O₁₅ up-conversion (UC) phosphors (SBTO: Er³⁺) were synthesised, and their UC spectra included green (2H11/2/4S3/2 → 4I15/2) and red (4F9/2 → 4I15/2) emissions from Er3+ ions under 980 or 1550 nm excitation. However, the green emission colour of phosphors was independent of dopant concentration under 980 nm laser irradiation; whereas the final emission colour was dominated by red emission and significantly affected by contents of Er3+ under 1550 nm excitation. These observations demonstrated potential application in dual-mode double-colour anti-counterfeiting. The possible UC mechanisms and emission characteristics of the phosphors using different 980 and 1550 nm irradiation source were contrastively investigated, and some fluorescent security patterns were also designed to demonstrate the potential applications in anti-counterfeiting and concealing important information.     

Colossal dielectric permittivity and relevant mechanism of Bi2/3Cu3Ti4O12 ceramics

Bi_2/3Cu₃Ti₄O₁₂ (BCTO) ceramics with pure perovskite phase were successfully prepared by traditional solid-state reaction technique. Uniformly distributed and dense grains with the grain size of 2–3 μm were observed by SEM. A giant low-frequency dielectric permittivity of ~3.3×10⁵ was obtained. The analysis of complex impedance revealed that Bi_2/3Cu₃Ti₄O₁₂ ceramics are electrically heterogeneous. There are three kinds of dielectric response detected in Bi_2/3Cu₃Ti₄O₁₂ ceramics, which existed in the low-frequency range, middle-frequency range, and high-frequency range, respectively. Through the study of dielectric spectrum at different temperatures, the relatively low activation energy of 0.30 eV for middle-frequency dielectric response was calculated, which suggested that this Middle-frequency dielectric response can be ascribed to grain boundaries response. In view of the analysis of dielectric spectrum at low temperatures, the activation energy of 0.07 eV for high frequency dielectric response was found. This value illustrated that dielectric response at high frequencies was associated with grains polarization effect. The comparison of dielectric spectra of Bi_2/3Cu₃Ti₄O₁₂ ceramics with different types of electrodes revealed that giant low-frequency dielectric constant was attributed to the electrode polarization effect.     

Dielectric and impedance spectroscopic study of lithium doped potassium tantalum niobium oxide

The (K_0.90Li_0.10) (Nb_0.80Ta_0.20)_0.99Mn_0.01O₃ (KLTN) ceramic has been synthesized by the conventional solid-state reaction route. The Rietveld refinement X-Ray diffraction (XRD) patterns confirmed the single-phase orthorhombic crystal structure with space group Amm2. The frequency dependent electrical properties were examined by the complex dielectric and impedance spectroscopy in the temperature range 30 °C–500 °C where multiple structural phase transition was observed with high dielectric constant, low tangent loss and well saturated electric polarization. The shifting of the ferroelectric phase transition temperature by 30 °C in heating and cooling mode suggests the irreversible motion of the domains and domain walls and significant effects on grain boundaries on structural phase transition temperature. A series of phase transitions from orthorhombic to tetragonal (∼190 °C) and tetragonal to Cubic (∼390 °C in cooling and 420 °C in heating) have been obtained in the wide range of temperature. The different type of analogy such as Modulus formalism, complex impedance spectra, frequency dependent conductivity, the activation energy of charge carriers has been used to understand the microstructure-electrical properties relation.     

Impedance spectroscopic investigation on electrical conduction and relaxation in manganese substituted pyrochlore type semiconducting oxides

The effect of simultaneous substitutions of Mn in both A and B sites of the pyrochlore type semiconducting oxides: (CaCe_1−2xMn_2x)(Sn_1−xMn_xINb)O_7−δ (x=0, 0.1, 0.2, 0.3 and 0.4) on the electrical conduction and relaxation was studied in detail using impedance analysis as a function of frequency over a wide range of temperature. Impedance and modulus analysis clearly explain the relaxation in these materials and its dependence on Mn concentration. Grain boundary dominant electrical characteristics have been observed with progressive Mn substitution. Correlated barrier hopping model was successfully applied for explaining the conduction mechanism in these compounds. Variation of hopping parameters with Mn substitution in these materials indicates strong dependence on the grain and grain boundary contributions. This insight to the conduction mechanism of the system offers in tuning the electrical properties for desired applications such as NTC thermistors.     

Influence of processing conditions on the ionic conductivity of holmium zirconate (Ho2Zr2O7)

Nanopowders of holmium zirconate (Ho₂Zr₂O₇) synthesised through carbon neutral sol-gel method were pressed into pellets and individually sintered for 2 h in a single step sintering (SSS) process from 1100 °C to 1500 °C at 100 °C interval and in a two step sintering (TSS) process at (I) −1500 °C for 5 min followed by (II) - 1300 °C for 96 h. Relative density of each of the sintered pellet was determined using the Archimedes’ technique and the theoretical density was calculated from crystal structure data. Grain size was obtained from SEM micrographs using ImageJ. Pellets processed by TSS have been found to be denser (98 %) with less grain growth (1.29 μm) as compared to the pellets processed using SSS process. Ionic conductivity of Ho₂Zr₂O₇ pellets sintered by two different processes was measured using ac impedance spectroscopy technique over the temperature range of 350 °C–750 °C in the frequency range of 100 mHz–100 MHz for both heating and cooling cycles. The temperature dependence of bulk (2.67⨯10⁻³ Scm⁻¹) and grain boundary (2.50⨯10⁻³ Scm⁻¹) conductivities of Ho₂Zr₂O₇ prepared by TSS process are greater than those processed by SSS process suggesting the strong influence of processing conditions and grain size. Results of this study, indicates that the TSS is the preferable route for processing the holmium zirconate as it can be sintered to exceptionally high densities at lower temperature, exhibits less grain growth and enhanced ionic conductivity compared with the samples processed by SSS process. Hence, holmium zirconate can be considered as a promising new oxide ion conducting solid electrolyte for intermediate temperature SOFC applications between 350 °C and 750 °C temperature range.     

Tunable upconversion luminescence in new Ho3+/Yb3+-doped SrBi4Ti4O15 photochromic ceramics for switching application

Upconversion (UC) luminescence modulation is quite important in controlling and processing light for active components of light sources, photoswitches, optical memories, and optical sensing devices. In this work, we reported one kind of novel phosphor, Ho³⁺/Yb³⁺-doped SrBi₄Ti₄O₁₅ ceramics, which displayed both strong UC luminescence and obvious photochromic (PC) reaction. The UC luminescence, PC effect, and the modulation of UC performance based on PC behavior were investigated in detail. By alternating visible light irradiation and thermal stimulus, the UC luminescence could be reversibly regulated. Meanwhile, the modulation was unveiled to tightly rely on the irradiation time and thermal treatment processes. Excellent reproducibility was also achieved. In addition, as an alternative method to thermal treatment, the manipulation of luminescence by electric field was also explored. Finally, the mechanism related to the UC luminescence manipulation was illustrated. The results indicate that these samples could be potentially utilized in optical data storage and anti-counterfeiting security fields.     

AC,DC conduction and dielectric behaviour of solid and liquid phase sintered Al2O3-15 mol% V2O5 pellets

Microstructural and dielectric characterization of porous Al₂O₃-V₂O₅ pellets sintered under both solid (600 °C) and liquid phase (850 °C) conditions are reported. The low temperature solid state sintering (SSS) leaves a very porous network of dense alumina particles surrounded by smaller facetted vanadia, whereas under liquid phase sintering (LPS), the V₂O₅ melts and recrystallizes over the relatively inert Al₂O₃, leading to a connected network structure. The ac conductivity and dielectric parameters of the pellets investigated from room temperature (RT) to 400 °C exhibited both universal dielectric (diffusive) behaviour at low frequencies and nearly constant loss (sub-diffusive) regimes at low temperatures. Activation energies calculated for dc/ac conduction at different frequencies suggests a composite conduction mechanism controlled by the Vanadia phase: at low frequencies, the calculated energies (E_a≈0.5 eV) compare with ionic diffusion of vanadium in V₂O₅, while at high frequencies and low temperatures an additional (E_a≤0.16 eV) polaronic hopping mechanism is seen. This cross-over frequency is significant for the LPS specimen indicating that the vanadia connected network assists conduction by providing a continuous (but disordered) diffusion route. Impedance and modulus spectral analysis show the presence of distributed time constants arising from electrodes, pore/matrix and phase interfaces and GBs.     

Impedance spectroscopy and conduction mechanism of magnetoelectric hexaferrite BaFe10.2Sc1.8O19

The dielectric relaxation and conduction properties of hexaferrite BaFe_10.2Sc_1.8O₁₉ (BFSO) ceramics have been investigated by impedance spectroscopy (1 Hz-2 GHz) at various temperatures (253-473 K). The frequency dependence of impedance and modulus spectra of BFSO shows that its dielectric responses are thermally activated. The scaling behaviors of impedance spectra indicate that the distribution of dielectric relaxation times in BFSO is temperature independent. The frequency-dependent conductivity spectra follow the universal-power-law at high temperatures but deviate slightly at low temperatures. An enormous increase in relaxation/conduction activation energies of BFSO above 413 K is also observed in both impedance and conductivity spectra. This indicates that at high temperatures, relaxation/conduction processes may be contributed mainly by the movable oxygen vacancies, whereas at low-temperature electron hopping dominates. The conductivity fitting results further suggest that electron/oxygen vacancy-related small polaron hopping should be the most probable conduction mechanism for BFSO.     

Colossal dielectric response and relaxation behavior in novel system of Zr4+ and Nb5+ co-substituted CaCu3Ti4O12 ceramics

In this work, we developed a novel system of isovalent Zr⁴⁺ and donor Nb⁵⁺ co-doped CaCu₃Ti₄O₁₂ (CCTO) ceramics to enhance dielectric response. The influences of Zr⁴⁺ and Nb⁵⁺ co-substituting on the colossal dielectric response and relaxation behavior of the CCTO ceramics fabricated by a conventional solid-phase synthesis method were investigated methodically. Co-doping of Zr⁴⁺ and Nb⁵⁺ ions leads to a significant reduction in grain size for the CCTO ceramics sintered at 1060 °C for 10 h. XRD and Raman results of the CaCu₃Ti_3.8-xZr_xNb_0.2O₁₂ (CCTZNO) ceramics show a cubic perovskite structure with space group Im-3. The first principle calculation result exhibits a better thermodynamic stability of the CCTO structure co-doped with Zr⁴⁺ and Nb⁵⁺ ions than that of single-doped with Zr⁴⁺ or Nb⁵⁺ ion. Interestingly, the CCTZNO ceramics exhibit greatly improved dielectric constant (~10⁵) at a frequency range of 10²–10⁵ Hz and at a temperature range of 20–210 °C, indicating a giant dielectric response within broader frequency and temperature ranges. The dielectric properties of CCTZNO ceramics were analyzed from the viewpoints of defect-dipole effect and internal barrier layer capacitance (IBLC) model. Accordingly, the immensely enhanced dielectric response is primarily ascribed to the complex defect dipoles associated with oxygen vacancies by co-doping Zr⁴⁺ and Nb⁵⁺ ions into CCTO structure. In addition, the obvious dielectric relaxation behavior has been found in CCTZNO ceramics, and the relaxation process in middle frequency regions is attributed to the grain boundary response confirmed by complex impedance spectroscopy and electric modulus.     

Large enhancement of piezoelectric properties in Mn-modified SrBi4Ti4O15 and its thermal stabilities at elevated temperatures

Manganese-modified strontium bismuth titanate (SrBi₄Ti₄O₁₅, SBT) ceramic oxides were synthesized by substituting a small amount of manganese ions into the Ti⁴⁺ sites using conventional solid-state reaction. The resultant Mn-modified SBT (SBT-Mn) exhibits better piezoelectric properties in comparison to unmodified SBT. The piezoelectric properties of Mn-modified SBT is optimized with only 4 mol% Mn substitution. SBT-4Mn exhibits a large piezoelectric constant (d₃₃=30 pC/N), approximately twice the value of unmodified SBT (d₃₃=13 pC/N), while its Curie temperature, T_c, remains almost unchanged at ~530 °C. The temperature-dependent electrical impedance and electromechanical coupling factors k_p and k_t reveal that the SBT-4Mn exhibits thermally stable electromechanical coupling characteristics up to 300 °C but electromechanical coupling factors deteriorate significantly at a higher temperature due to increased conductivity. Our work suggested that Mn-modified SBT ceramics are promising materials for high temperature piezoelectric applications.     

Photoluminescence,electrical properties and electron band structure of (Ho,Yb)3+ co-doped SrBi4Ti4O15 multifunctional ceramics

SrBi_3.992-xHo_0.008Yb_xTi₄O₁₅ (x = 0.000, 0.002, 0.004, 0.006, abbreviated as SBHT-xYb) multifunctional ceramics were fabricated by solid-phase reaction method. Their electrical, luminescence properties and electronic band structure were investigated. At x = 0.002, samples exhibit enhanced piezoelectric coefficient (d₃₃ = 19 pC/N) and optimal thermal stability. Thermal annealing behavior shows that the d₃₃ of SBHT-0.002 Yb at 460 °C remains 84% of its d₃₃ value at room temperature (RT). Excited by 980 nm laser, the SBHT-xYb samples show two emission bands. Green emission at 548 nm and red emission at 658 nm are considered to be the transfer of two excited states ⁵S₂ and ⁵F₅ to the ground state ⁵I₈ of Ho³⁺ ions, respectively. Furthermore, the electron band structure of SBHT-xYb ceramics was calculated using density functional theory (DFT). It is found that the energy consumed for electron transition first decreases and then increases, while the 4f orbital contributions of (Ho, Yb)³⁺ ions are enhanced by doping with Yb³⁺, elucidating the change in luminescence intensity in terms of microscopic mechanism. All the above results manifest that SBHT-xYb ceramics are good potential alternative for luminescence and piezoelectric properties integrated devices.     

Low-cost flame synthesized La2/3Cu3Ti4O12 electro-ceramic and extensive investigation on electrical,impedance, modulus,and optical properties

An environmentally-friendly synthesis route and low-cost starting materials are more appropriate for the production of ceramic materials at the industry level. With this concern we prepared the La_2/3Cu₃Ti₄O₁₂(LCTO), which is isostructural of CaCu₃Ti₄O₁₂ (CCTO), using the low-cost TiO₂ instead of a high-cost of titanium source (titanium isopropoxide or titanium chloride) using a low-cost wet-chemical route. Although, there are lots of synthetic methods reported for LCTO fabrication in terms of duration, cheap reagents, energy consumption, feasibility, etc. The present method is far better than the others. The prepared ceramic samples were sintered at 1050 ^°C/12 h and studied their structural, morphology and impedance, and modulus studies for further confirmation. The prepared LCTO ceramic shows the pure phase with the cubic type of morphology. The homogenous distribution of all the elements was observed through dispersive X-ray analysis. X-ray photoelectron spectroscopy studies revels that La is in +3 oxidation state, Cu is in a +2 oxidation state, and Ti is multiple (+3 and + 4) oxidation state. The LCTO ceramic displayed the very high dielectric constant (∼3852) and dielectric loss (0.322), at 1 kHz and at room temperature. Calculated the activation energy using the impedance and modulus data and it shows the superior to that of CCTO ceramic synthesized by the same method. The prepared samples exhibited Debye-type relaxation, which is evoked from the impedance and modulus studies. The calculated optical energy bandgap of LCTO (2.06 eV) is found to be lesser than that of the well-known structure of perovskites (BaTiO₃ (3.28 eV), PbTiO₃ (3.18 eV), LiNbO₃ (3.78 eV) and BiFeO₃ (2.67 eV) as well as structure of spinel CoCr₂O₄ (3.10 eV) and LuFe₂O₄ (2.18 eV)) materials.     

The effect of second phase La0.67TiO2.87 on the phase structure and impedance spectroscopy of La2Ti2(1 + x)O7 piezoelectric ceramics

The Ti excess La₂Ti_{2 (1+x)} O₇ (x = 0, 0.005, 0.01, 0.02, 0.05, 0.1) piezoelectric ceramics have been prepared by sol-gel technology and solid state synthesis method. Through refinement analysis, the phase structure of the ceramics varies with Ti content. Most monoclinic phase (∼93%) and a handful of orthogonal phase (∼7%) coexist in La₂Ti_{2 (1+0)} O₇ ceramics. Pure monoclinic phase La₂Ti₂O₇ with space group P2₁ appears in La₂Ti_{2 (1+0.005)} O₇ and La₂Ti_{2 (1+0.01)} O₇ ceramics. Monoclinic phase La₂Ti₂ O₇ and a certain proportion of tetragonal phase La_0.67TiO_2.87 coexist in La₂Ti_{2 (1+0.02)} O₇, La₂Ti_{2 (1+0.05)} O₇ and Ti_{2 (1+0.1)} O₇ ceramics. With the excess of Ti content, the monoclinic phase ratio and distortion angles in a-b projection plane of the ceramics increase first and then decrease, which is consistent with the variation tendency of piezoelectric constant d₃₃. The excellent piezoelectric constant for Ti_{2 (1+0.01)} O₇ ceramics is 2.8 pC/N.Impedance analysis shows that the conductive mechanisms of all samples include both grain and grain boundary conductivity at temperature range T ≥ 500 °C. The formation of tetragonal phase La_0.67TiO_2.87 derives from Ti excess in pure monoclinic phase La₂Ti₂O₇. The existence of tetragonal phase La_0.67TiO_2.87 can obviously increase the capacitance of ceramics at x ≥ 0.05. All prepared piezoelectric La₂Ti_{2 (1+x)} O₇ ceramics have highly frequency stability and are candidates for ultrahigh temperature piezoelectric application.