Tetragonal Perovskite Phase Research Articles

Antithermal-Quenching All-Inorganic Perovskite Crystals with Green Ultralong Persistent Luminescence for Advanced Anticounterfeiting Applications.

Long persistent luminescence (PersL) materials have revolutionized many fields of optoelectronics and photonics due to their applications in anticounterfeiting, information encryption, and in vivo bioimaging. Here, we reported a novel PersL crystal prepared by the heterovalent doping of Sb3+ into perovskite tetragonal phase RbCdCl3, comparing with the pristine non-perovskite orthorhombic phase analogue without PersL property. Surprisingly, under the UV light irradiation, the title crystals concurrently exhibit green ultralong PersL (>2400 s), high photoluminescence quantum yield (49.1%), and antithermal quenching in the range from 148 to 328 K. It was revealed by experimental results and theoretical analyses that green ultralong PersL and antithermal quenching of perovskite-phase RbCdCl3/Sb3+ crystals originate from the electron transition between the 5s2 level of the dopant Sb3+ and the electronic defect-induced trap states. Enlightened by the excellent optical properties, the tetragonal perovskite-phase RbCdCl3/Sb3+ PersL materials show promising application prospects in anticounterfeiting and encryption of information.

High thermoelectric performance of n–type SrTiO3 by Dy and Nb co–doping

The effects of Dy/Nb co–doping on the structural and thermoelectric properties of Sr1−xDyxTi1−yNbyO3−δ (0 ≤ x, y ≤ 0.10) are systematically investigated. Single–doped Sr0.95Dy0.05TiO3−δ and SrTi0.95Nb0.05O3−δ consist of tetragonal (I4/mcm space group) and cubic (Pm3¯m space group) perovskite phases. On the other hand, Dy/Nb co–doped SrTiO3, Sr1−xDyxTi1−yNbyO3−δ, has a tetragonal perovskite phase (I4/mcm space group) since the Dy/Nb co–doping induces a distortion of the TiO6 octahedra and lowers the crystal symmetry. The co–doping effectively increases the electrical conductivity and reduces the thermal conductivity, thus noticeably enhancing the dimensionless figure–of–merit (ZT). The highest ZT value (0.24) is achieved for Sr0.95Dy0.05Ti0.95Nb0.05O3−δ at 600 °C. This ZT value is 41 % and 50 % larger than those of single–doped Sr0.95Dy0.05TiO3−δ (0.17) and SrTi0.95Nb0.05O3−δ (0.16), respectively, at 600 °C.

Fabrication and characterization of methylammonium lead iodide-based perovskite solar cells under ambient conditions

<p>This study investigated the fabrication and characterization of CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> based perovskite solar cells (PSCs) using the one-step spin coating technique under ambient conditions, eliminating the need for expensive glovebox and thermal evaporation equipment. The perovskite layer was annealed at 65 °C for 30 seconds and 100 °C for 30 seconds, 1 and 2 minutes. The scanning electron microscope (SEM) images show a smooth and uniform surface coverage for the ETL and CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> layers. SEM results also show an average grain size of 397 nm for CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> and an average particle size of ~17 nm for TiO<sub>2</sub> was confirmed by transmission electron microscopy (TEM). X-ray diffraction (XRD) results confirmed the formation of tetragonal perovskite (CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>) phase with high crystallinity with a crystallite size of 19.99 nm for the samples annealed for 30 seconds at 65 °C and 1 min at 100 °C. FTIR results also confirmed the presence of anatase TiO2 at wavenumber 438 cm<sup>-1</sup> and the formation of the adduct of Pb<sub>2</sub> with dimethyl sulfoxide (DMSO) and MAI is confirmed at 1,015 cm<sup>-1</sup> . From the Tauc plot the bandgap energy of TiO2 and Perovskite layers was determined to be 3.52 eV and 2.06 eV respectively. An open-circuit voltage was 0.9057 V and short circuit current density was 12.2185 mA/cm<sup>2</sup> with a fill factor of 48.05 and power conversion efficiency (PCE) of 5.199%.</p>

Large electrocaloric effect across the non-hysteretic electrostructural phase transition in lead-free (Ba0.88Ca0.12)(Ti0.94Sn0.06)O3 ceramics

Structural, dielectric, and electrocaloric properties were investigated in Pb-free ceramic (Ba0.88Ca0.12)(Ti0.94Sn0.06)O3 (BCST), synthesized using solid-state sintering method. Rietveld refinement of room temperature XRD data revealed the existence of pure perovskite tetragonal (P4mm) phase. Temperature-dependent XRD analyses confirmed a structural transition from tetragonal to cubic phase near 380 K. Temperature-dependent dielectric constant (εr) and specific heat capacity (Cp) confirmed the presence of thermal hysteresis in two phase transitions near 233 ± 10 K and 254 ± 10 K, and a non-hysteretic behavior at the Curie temperature (TC∼373 ± 10 K). A comprehensive Raman study corroborated the existence of phase transitions. Electrocaloric properties were determined using indirect method utilizing the non-hysteretic nature across TC. A large adiabatic temperature change of |∆T| = 0.52 K, isotropic entropy change |∆S| = 0.68 J/kg.K, electrocaloric coefficients, |∆T/∆E |= 0.311 K.mm/kV and |∆S/∆E| = 0.41 J.mm/K.kg.kV were obtained at 380 K for an electric field of 16.7 kV/cm. Obtained value of |∆T/∆E| is relatively good, reported among other BaTiO3-based ceramics.

High energy storage performance induced by the introduction of BiScO3 into (Bi0.5Na0.5)TiO3–BaTiO3 lead-free ferroelectric ceramics

In this work, ternary 0.25(Bi0.5Na0.5)TiO3-(0.75-x)BaTiO3-xBiScO3(abbreviated as 0.25BNT-(0.75-x)BT-xBS, x = 0.19, 0.21, 0.23, 0.25, 0.27) lead-free ferroelectric ceramics were synthesized using the solid-state reaction method. The introduction of BiScO3 into (Bi0.5Na0.5)TiO3–BaTiO3 ceramics had a significant impact on changing the phase structures, and microstructures, as well as the dielectric and ferroelectric properties. All ceramics exhibited a coexistence of tetragonal(P4bm) and rhombohedral(R3c) perovskite phases, while the influence of the BiScO3(BS) content on the average grain size was not significant. Slim polarization-electric field hysteresis loops were recorded for the 0.25BNT-(0.75-x)BT-xBS ceramics, with low remnant polarization (Pr) and large maximum polarization(Pmax). 0.25BNT-0.50BT-0.25BS ceramic had the largest Pmax, which induced a high energy storage density of 2.93 J/cm3 and energy storage efficiency of 91 % under a low electric field of 230 kV/cm 0.25BNT-0.50BT-0.25BS ceramic not only yielded excellent fatigue resistance, but also maintained good energy storage performance at high temperature and high frequency, which are considered of great importance since they can be leveraged for the potential development of high temperature and frequency capacitors. In addition, an excellent pulse performance was measured, with a high discharge energy density of 3.42 J/cm3 and a fast discharging rate t0.9 of 130 ns. This good pulse performance was maintained at high temperatures, suggesting that the proposed material is also highly competitive for applications where high-temperature pulse devices are required.

BaZrO3-BaTiO3-CaTiO3 piezoceramics by a water-based mixed-oxide route: Synergetic action of attrition milling and lyophilization

This manuscript shows an innovative water-based processing route to fabricate lead-free piezoceramics. It faces the challenge of substituting conventionally used organic solvents by innocuous water. The procedure involves two methods of widespread use in industry: attrition ball-milling and freezing-lyophilization. Powders with nominal composition (Ba0.92Ca0.08)(Ti0.95Zr0.05)O3 were effectively obtained. A narrow monomodal size distribution (600 nm) proved high reactivity that allowed solid-state synthesis at ultra-low temperature (700 °C-2 h). Optimal two-step sintering method (900 °C-3 h,1280 °C-6 h) allowed well-sintered homogeneous ceramics with pure tetragonal perovskite phase. They showed high piezo-sensitivity (d33 =320 pC/N, d31 =−110 pC/N, kp =31.45% and Np =2750 kHz·mm, ε′33T=3670, tanδ=0.036 at 1 kHz), comparable to that found for routes in organic media. Therefore, attrition ball milling in water and lyophilization work in synergy to successfully activate the precursors, influencing on the structural, microstructural and electrical properties of the final BCZT ceramics. This is an unprecedented development to produce piezoelectric ceramics using a sustainable, environmentally benign water-based process, with a positive impact in noteworthy industrial applications.

Heat treatments for thermal sprayed lead-free piezoelectric ceramic coatings

[0.94(Bi0.5Na0.5)TiO3-0.06BaTiO3]0.98-(LiNbO3)0.02 (BNBTLN) piezoelectric ceramic coating was fabricated by thermal spray process on stainless steel substrates with thermal barrier coating (TBC) as an intermediary layer. The morphology, structure, and electrical properties of the thermal-sprayed BNBTLN coatings after furnace heat treatment (F-HT) and plasma torch heat treatment (P-HT) were studied, respectively. The BNBTLN coatings after F-HT and P-HT both had the coexisting rhombohedral and tetragonal perovskite phases and exhibited excellent electrical properties, with an effective piezoelectric coefficient d33 of 68 p.m./V and 40 p.m./V, respectively. These results reveal that high quality piezoelectric coatings can be achieved by the P-HT method, which is promising for scaling up the fabrication of piezoelectric ceramic coatings.

Fabrication of 0.7BaTiO3-0.3Ni0.7Zn0.3Fe2O4 Multiferroic Composite Ceramics Prepared by the Solid-State Combustion Technique

Lead-free multiferroic composite ceramics of 0.7BaTiO3-0.3Ni0.7Zn0.3Fe2O4 (BT-NZF) were prepared by the solid-state combustion technique. The BT and NZF powders were calcined in the range of 1100 to 1200 °C for 4 h and 900 to 1100 °C for 2 h, respectively. Afterward, BT-NZF composite ceramics were sintered between 1100 and 1250 °C for 4 h. The effect of the firing temperature on the phase formation, microstructure, electrical, and magnetic properties of BT-NZF composite ceramics was investigated. X-ray diffraction of BT and NFZ powders showed the highest purity phase were prepared the calcination temperatures of 1100 and 1050 °C, respectively. The BT-NZF composite ceramics exhibited coexisting tetragonal perovskite and cubic spinel phases. The maximum density of 5.85 g/cm3 was obtained when sintering the composite ceramics at 1125 °C. The dielectric constant at room temperature increased with increased sintering temperature. The hysteresis loops showed leakage current in all samples. Ferromagnetic properties showed the highest Ms ∼ 24.56 emu/g measured under a magnetic field of 10,000 Oe from the sample sintered at 1150 °C.

Large electric field-induced strain and excellent photoluminescence properties of Pr-modified 0.94Bi0.5Na0.5TiO3-0.06BaTiO3 lead-free ferroelectric ceramics

The 0.94Bi0.5-xPrxNa0.5TiO3-0.06BaTiO3 (BNT-6BT-xPr) lead-free ceramics with doping content of x = 0, 0.01, 0.025, 0.04, 0.05, 0.075, and 0.1 were synthesized by the conventional solid-state reaction method. After doping with the Pr element, a composition-induced phase transition took place, which from the rhombohedral perovskite (R3c) and tetragonal perovskite (P4bm) phases coexisting into the pseudo-cubic phase. At the same time, the introduction of the Pr element had a beneficial effect on grain growth, and the average size of the crystal particle increased from 299 to 742 nm. The Curie temperature Tm showed no obvious change with the increasing Pr content, but the temperature TF-R of the ferroelectric-relaxor phase transition was gradually shifted toward lower temperatures. In addition, the depolarization temperature Td gradually decreased from 125 to 53 °C. In the BNT-6BT-0.025Pr ceramics, giant electric field-induced strain (Smax) and normalized piezoelectric coefficient (d33*) were obtained (Smax = 0.58%, d33* = 723 pm/V). Furthermore, a significant photoluminescence emission intensity was achieved, exhibiting a bright red color. Due to significant enhancement of the piezoelectric and the photoluminescence properties, the BNT-6BT-0.025Pr ceramics exhibited strong competitiveness in actuators and multifunctional devices, which may broaden their application fields.

Design, characterization, fabrication, and performance evaluation of ferroelectric dielectric resonator antenna for high-speed wireless communication applications

The research comprehensively analyzes the structural, electrical, and electrodynamic characteristics of Ba0.7Sr0.3TiO3 nanoparticles synthesized using a sol-gel approach. The formation of a tetragonal perovskite phase and the nano-crystalline nature have been confirmed using X-ray diffraction (XRD) analysis. Fourier transform infrared (FTIR) spectroscopy has been employed to identify characteristic absorption bands correlated with the crystal lattice vibrations of Ba0.7Sr0.3TiO3. The sample’s morphology has been examined using scanning electron microscopy (SEM), revealing a relatively dense and homogeneous composition composed of small spherical particles. The electrical properties of Ba0.7Sr0.3TiO3 nanoparticles have also been investigated using a ferroelectric measurement technique. The soft electric hysteresis loop with a small coercive field value makes the sample suitable for photovoltaic applications. Finally, a compact dielectric resonator antenna (DRA) based on Ba0.7Sr0.3TiO3 nanoparticles has been designed and characterized. The designed DRA demonstrates good impedance matching, broadside radiation pattern, and high antenna gain and efficiency at the frequency band of 5.6–6.15 GHz, making it suitable for sub-6 GHz (5 G) applications.

Dielectric, ferroelectric, and piezoelectric properties of rare earth Sm-doped 0.94 Bi0.5Na0.5TiO3-0.06 BaTiO3 lead-free ceramics

Bi0.47-xSmxNa0.47Ba0.06TiO3 (BNT-6BT-xSm) ferroelectric ceramics with different doping contents of samarium (Sm) were prepared by solid-state reaction method. All ceramics lie in the coexistence region of the rhombohedral perovskite phase (R3c) and tetragonal perovskite phase (P4bm). When the Sm content increases, the grain growth is limited, and the average grain size is reduced from 1.69 µm (at the composition of x = 0.005) to 0.86 µm (at the composition of x = 0.05). When Sm is introduced into Bi0.5Na0.5TiO3-BaTiO3 binary system, the composition-induced local structure transition occurs, namely the transition from order to disorder. And after the introduction of Sm3+ ions, the depolarization temperature (Td) increases considerably. With the increase of the Sm content, the remnant polarization (Pr) decreases from 52 μC/cm2 to 8 μC/cm2, and the coercive field (Ec) gradually decreases from 37 kV/cm to 14 kV/cm, respectively. Under an electric field of 80 kV/cm, the maximum electric field-induced strain and corresponding normalized strain (d33*) reached 0.35% and 318 pm/V (at the composition of x = 0.03), respectively. The maximum piezoelectric coefficient (d33) is 158 pC/N (at the composition of x = 0.015), resulting from the structure transition between ferroelectric phase and anti-ferroelectric phase. Therefore, BNT-6BT-0.03Sm lead-free ferroelectric ceramic is a promising candidate of lead-based ferroelectric ceramics for piezoelectric actuators.

Preparation, electrical and thermal properties of new anode material based on Ca-doped perovskite CeAlO3

Tetragonal perovskite phase Ce0.9Ca0.1AlO2.95 + x was obtained for the first time. Such phase, containing cerium in the oxidation state of 3+, can be promising anode materials for a solid oxide fuel cells (SOFCs). Ce0.9Ca0.1AlO2.95 + х (space group I4/mcm) was synthesized by the solid-phase method at 1400°С in a nitrogen flow with using ammonium oxalate (NH4)2C2O4 to create a reducing atmosphere. Thermogravimetry results showed that Ce0.9Ca0.1AlO2.95 + x was stable to oxidation up to 500°С in air and up to 700°С in argon (partial pressure of oxygen рО2 = 10−4 bar). The thermal expansion coefficient measured by dilatometry was equal to 11.16·10−6 К−1. The temperature dependences of the electrical conductivity (for undoped phase CeAlO3σ ≈ 1·10−3 S/cm and for doped Ce0.9Ca0.1AlO2.95 + xσ ≈ 3·10−2 S/cm at 500°С in air) were obtained by the electrochemical impedance spectroscopy measurements). The electrical conductivity of these samples at the temperatures range of 350–500°С was almost independent of the partial pressure of oxygen рО2 from 10−18 to 0.21 bar, however, there was a slight negative slope at T > 500 °C (рО2). The total ionic transport numbers measured by the EMF method were close to 1·10−3, which indicated the dominance of electronic conductivity. The measurement of the sign of the thermal-EMF showed that positive charge carriers (holes) were dominant charge carriers.

Solid State Processing of BCZT Piezoceramics Using Ultra Low Synthesis and Sintering Temperatures.

Lead-free (Ba0.92Ca0.08) (Ti0.95 Zr0.05) O3 (BCZT) ceramics were prepared by a solid-state route (SSR) using ultra-low synthesis (700 °C/30 min and 700 °C/2 h) and sintering temperatures (from 1150 °C to 1280 °C), due to prior activation and homogenization by attrition milling of the starting high purity raw materials for 6 h before the synthesis and of the calcined powders for 3 h before the sintering. The comparison of the thermal analysis of the mixture of the starting raw materials and the same mixture after 6 h attrition milling allowed to evidence the mechanisms of activation, resulting in a significant decrease of the perovskite formation temperature (from 854 °C down to 582 °C). The secondary phases that limit the functional properties of the ceramic and their evolution with the sintering conditions were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), which allowed the design of a two-step sintering method to eliminate them. A pure tetragonal BCZT perovskite phase (P4mm, c/a = 1.004) and homogeneous ceramic microstructure was obtained for synthesis at 700 °C for 2 h and sintering with the use of a two-step sintering treatment (900 °C for 3 h and 1280 °C for 6 h). The best electromechanical properties achieved were d33 = 455 pC/N, kp = 35%, Qm = 155.

Reducing the Cobalt Content in SrCo0.95Ti0.05O3-δ-Based Perovskites to Produce Cleaner Cathodes for IT-SOFCs

Recent studies have demonstrated the possibility of stabilizing the cubic or tetragonal perovskite phases in the SrCoO3-δ system at room temperature by partially replacing Co atoms with small amounts of highly charged cations like Ti4+ in SrCo0.95Ti0.05O3-δ. This material gives extraordinary performances as a cathode for solid oxide fuel cells (SOFCs) at intermediate temperatures (800–850 °C). In this work, we have tried to still improve this cathode in a cleaner and low-priced material by reducing its Co content, since Co is a high-cost and toxic element, additionally reducing its thermal expansion coefficient (TEC), to make it more mechanically compatible with the rest of the components of a SOFC. SrCo0.50Fe0.45Ti0.05O3-δ perovskite has been obtained by the citrate method, introducing 45% of Fe in the Co and Ti positions. Its crystalline structure has been studied by X-ray diffraction (XRD) and neutron powder diffraction (NPD) and has been correlated with mechanical and electrical properties. A chemical compatibility study between SrCo0.50Fe0.45Ti0.05O3-δ and the electrolyte La0.8Sr0.2Ga0.83Mg0.17O3-δ (LSGM) revealed that both materials do not interact at high temperatures. TEC measurements show a linear expansion over the entire temperature range. Finally, the perovskite showed good electrical conductivity and an encouraging power density value of 667 mW/cm2 at 850 °C, which indicate that this material is a promising cathode for SOFCs at intermediate temperatures.

Improved osteointegration response using high strength perovskite BaTiO3 coatings prepared by chemical bath deposition

A wide range of bioactive materials have been investigated for tissue engineering and regeneration. Barium titanate is a promising smart material to be used as scaffold for bone tissue engineering. Barium titanate coatings are prepared in the present study using chemical bath deposition technique. Coatings are prepared at room temperature with the variation in solution molarity from 0.1 to 1.2 M. Perovskite tetragonal phase is observed after annealing the samples at 300 °C using 1.0–1.2 M solutions. Normal-anomalous dielectric response is observed for annealed coatings. Maximum transmission of ∼55% and ∼82% is observed under as-prepared and annealed coatings, respectivly. Variation in direct band gap, i.e. 3.45–3.64 eV, is observed with varying molarity. High hardness of the coatings (∼1180 HV) is observed at 1.2M with fracture toughness of ∼22 MPam−1/2. Biodegradation studies show smaller values of weight loss even after immersion in simulated body fluid (SBF) after 26 weeks. Barium titanate coatings also show high antioxidant activity. BaTiO3's antibacterial reaction is evaluated against microorganisms such as Escherichia coli (E. coli) and Staphylococcus aureus. Antibacterial activity shows highest zone of inhibition (∼31 mm) against Staphylococcus aureus bacteria. Quantitative real-time PCR is used to assess the gene expression profile in cultivated cells. Thus, coatings produced without the use of hazardous solvents/reagents utilizing CBD technique are a potential material for biomedical applications.

A+1Nb5+O3 (A = Li, Na, K) Perovskites with Different Fine Structures for Oxidative Coupling of Methane: Tracing the Crystalline Phase Effect on the Surface Active Sites

Targeting to unravel the catalytic sites of some specific perovskites for OCM reaction, and elucidate the influences of crystal phase changes on the active sites, three A+1Nb5+O3 (A = Li, Na, K) complex oxides were synthesized in the pure phase. The reaction performance is ranked as NaNbO3 > KNbO3 > LiNbO3 for the catalysts. It is revealed that LiNbO3 possesses a hexagonal ilmenite-type phase, and NaNbO3 and KNbO3 possess orthorhombic and tetragonal perovskite phases, respectively. Due to the fine structure difference, NaNbO3 and KNbO3 have higher oxygen mobility than LiNbO3, as testified by the electric conductivity measurements. Moreover, the Nb–O bond strength of NaNbO3 and KNbO3 is much weaker than that of LiNbO3, which promotes also the oxygen mobility and phase transformation of the two samples, especially in the high temperature OCM process. As a result, more oxygen vacancies/defects are generated on NaNbO3 and KNbO3, which facilitates the efficient formation of OCM active and selective surface O2– species and basic sites. Subsequently, the OCM performance of NaNbO3 and KNbO3 can be evidently improved. Combining the above advantages, NaNbO3 exhibits the optimal OCM performance in all the samples.

Complex dielectric function and opto-electronic characterization using VEELS for the lead-free BCZT electro-ceramic perovskite

The current work presents the complex dielectric function and the opto-electronic properties of lead-free Ba0.8Ca0.2Ti0.9Zr0.1O3 (BCZT) electro-ceramic, derived from valence electron energy loss spectroscopy, in transmission electron microscopy (VEELS–TEM).A single tetragonal perovskite phase, with P4mm space group, was determined by Rietveld refinement of the x-ray diffraction pattern. The VEELS–TEM experiment scanned the energy interval from 0–50 eV. The spectroscopic analysis started with the chemical identification of the atoms that conforms the BCZT solid-solution. Bulk and surface plasmons were located at 27.2 eV and 12.9 eV, respectively in the energy loss function. Complex dielectric function was obtained using Kramers–Kronig analysis from the Gatan Microscopy Suite software. Dielectric constant was calculated from the real part of the complex dielectric function, while the inter-band transitions were identified in the joint density of states function. The refraction index n and the extinction coefficient k, as a function of energy, were obtained from the complex dielectric function. The bandgap energy was determined using a polynomial fit in the optical absorption coefficient plot with an Eg = 3.2 eV.

Electrical properties of La-Mn-codoped BaTiO3-(Bi0.5Na0.5)TiO3 lead-free PTCR ceramics

La-Mn-codoped 0.97BaTiO3-0.03(Bi0.5Na0.5)TiO3 ceramics (abbreviated as xLa-yMn-BBNT3, x = 0.001, 0.002, 0.003, 0.004; y = 0, 0.0005, 0.001, 0.0015, 0.002) for application of lead-free positive temperature coefficient of resistivity (PTCR) thermistors were prepared using a wet chemical route. The analysis of XRD patterns shows that all prepared powders and ceramics are composed of pure tetragonal perovskite phase. The x = 0.002 La-doped ceramic sintered in air has a Curie temperature of Tc about 140 °C, room-temperature resistivity (ρ25) of ~103 Ω cm and resistivity jump (log(ρmax/ρmin)) of ~3 orders in magnitude. It is also found that the resistivity jump of prepared ceramic reaches up to 4.4 orders in magnitude when y = 0.0015 Mn was doped. Influence of grain and grain-boundary effect on PTCR effect was analyzed by complex impedance spectra measured at different temperatures, and the results reveal that the grain boundary effect plays key role in the PTCR effect.

Improved ferroelectric properties and softening effect in BLTF ceramics

A comparative study is reported on the ferroelectric ceramics with compostional formula Ba0·98La0·02Ti0·98Fe0·02O3 (BLTF1) and Ba0·97La0·02Ti0·98Fe0·02O3 (BLTF2; valency compensated). The samples were prepared by solid state route and sintered in air at 1325 °C for 4 h. The X-ray diffraction analysis confirmed the formation of tetragonal perovskite phase. The prepared samples were characterized for their dielectric and ferroelectric properties at different temperatures at 1, 5, 10 and 100 kHz. Enhancement in ferroelectric properties (Pr = 11.25 μC/cm2, Ec = 9.05 kV/cm) was observed in BLTF2 ceramics as compared to BLTF1 ceramics (Pr = 6.53 μC/cm2, Ec = 9.35 kV/cm) along with the observation of electrocaloric effect. The value of piezoelectric coefficient ‘d33’ was found to be 140 pC/N and 163 pC/N for BLTF1 and BLTF2 samples respectively. The present investigations show that the valency compensation method can improve the ferroelectric properties of the ferroelectric ceramics thereby making them more suitable for different applications.

Preparation and Functional Properties of BaTiO3-BaGeO3 Ceramics.

The phase formation and functional properties (dielectric, ferroelectric switching, and tunability) in x BaGeO3-( 1- x )BaTiO3 ceramics with compositions x = 0 , 0.01, 0.018, 0.1, 0.68, and 1, produced by solid-state reaction, are presented. For small Ge additions x ≤ 0.10 for which the perovskite tetragonal phase is predominant, the low-field dielectric properties are quite similar to ones of BaTiO3 ceramics, with all the structural phase transitions in the same temperature range and a small shift of the Curie temperature to higher values when increasing Ge addition. The eutectic composition x = 0.68 is a composite containing mostly nominal amounts of hexagonal α -BaGeO3 phase and tetragonal BaTiO3, and shows permittivity below 100 and a lossy linear dielectric character, with zero tunability and lack of switching, similar as the BaGeO3 composition ( x = 1 ). The role of Ge in increasing the density by liquid-phase sintering is beneficial for improving the electrical properties. In this sense, the composition x = 0.10 is optimum in the present ceramic series: it shows the highest relative density of 99% and large grains (tens of micrometers), excellent switching properties with the highest polarization and a rectangular switching loop, the highest permittivity above room temperature and a good ε(E) tunability of 60%-70%, comparable with the best values reported in other BaTiO3 solid solutions.