Giant strain in Nb-doped Bi0.5(Na0.82K0.18)0.5TiO3 lead-free electromechanical ceramics

The effect of Nb substitution on the crystal structure, ferroelectric, and electric field induced strain properties of Bi_0.5(Na₈₂K_0.18)_0.5TiO₃ (BNKT) ceramics has been investigated. The coexistence of rhombohedral and tetragonal phases was found in undoped BNKT ceramics, however, Nb doping induced a phase transition to a pseudocubic phase with high electrostriction coefficients. When 3 mol% Nb was substituted on Ti ions, the electric field induced strain was markedly enhanced up to S_max/E_max = 641 pm/V, which is higher than those previously reported on non-textured lead-free electromechanical ceramics.     

Highly enhanced mechanical quality factor in lead-free (K0.5Na0.5)NbO3 piezoelectric ceramics by co-doping with K5.4Cu1.3Ta10O29 and CuO

Effects of Cu doping on the ferroelectric and piezoelectric properties of 0.0038 mol K_5.4Cu_1.3Ta₁₀O₂₉ modified (K_0.5Na_0.5)NbO₃ ceramics have been investigated. On the basis of analyses on crystal structure and polarization hysteresis, it is suggested that Cu ions reveal amphoteric doping behavior in KNN ceramics. At doping levels up to 1 mol%, the Cu ions substitute pentavalent B-site cations, acting as acceptors that generate O-vacancies to resultantly harden the ceramics. At doping levels above 1.5 mol%, however, Cu ions play a role as donors by replacing monovalent A-site cations. A specimen doped with 0.5 mol% CuO shows an extremely high mechanical quality factor of 3053, which is higher than those of any other reports on KNN-based ceramics.     

Domain structure of [(Na0.7K0.2Li0.1)0.5Bi0.5]TiO3 ceramics studied by piezoresponse force microscopy

The domain structure of lead-free ceramics [(Na_0.7K_0.2Li_0.1)_0.5Bi_0.5]TiO₃ was studied by piezoresponse force microscopy (PFM) method. The complicated curved domain structure was observed in the ceramics, and there are some nano domains in the sub-microsized domains, which indicate the relaxor nature of the material. The mechanism for the strong relaxation of the material was discussed in the letter. The reversal behavior of the domain was also studied by PFM method. Only part of the domains reversed after the poling process, and domains of the ceramics reversed back from the center of the domains at first.     

Mechanochemical synthesis and characterization of nanocrystalline 0.4Bi(Zn1/2Ti1/2)O3-0.6PbTiO3 powders

Perovskite 0.4Bi(Zn_1/2Ti_1/2)O₃-0.6PbTiO₃ (BZT-PT) powders were successfully synthesized from precursor oxides using a high-energy planetary ball milling. The phase development of the powders during milling was studied by means of X-ray diffraction and Raman scattering techniques. The microstructure of the powders was characterized using transmission electron microscopy, and the thermal behavior was studied as well. The results reveal that after 15 h of milling the formation of BZT-PT phase can be completed and submicron agglomerates of small crystallite sized ~ 12 nm are present in the powders. However, further prolonging the milling time to 25 h leads to the amorphization of the BZT-PT phase.     

Piezoelectric shear-mode properties of Pb(Yb1/2Nb1/2)O3-PbTiO3 ceramics

A Pb(Yb_1/2Nb_1/2)O₃-PbTiO₃ [PYNT] solid solution was synthesized and characterized for its potential use. The shear-mode dielectric and piezoelectric behaviors of PYNT with a morphotropic phase boundary (MPB) composition were studied as a function of temperature. Dielectric permittivity K₁₁^T and loss were found to be 2310 and 2.5%, respectively, at room temperature. Piezoelectric coefficient d₁₅ and electromechanical coupling factor k₁₅ were calculated to be 710 pC/N and 0.70, respectively, maintaining nearly constant up to 300 °C, resistivity and RC time constant were observed to be 2.4 × 10⁹ Ω cm and 1.07 s, respectively, at 350 °C. These good piezoelectric properties, together with the high Curie temperature (T_c ∼ 370 °C), indicate that PYNT is a promising candidate for high temperature-shear sensor and inkjet actuator applications.     

Sol-gel synthesis of lead-free (K0.5Bi0.5)0.4Ba0.6TiO3 nanorods

A kind of lead-free ferroelectric nanorods, (K_0.5Bi_0.5)_0.4Ba_0.6TiO₃, has been prepared by the sol-gel process. The phase formation, structure and morphological analyses of (K_0.5Bi_0.5)_0.4Ba_0.6TiO₃ were investigated by XRD, FTIR, Raman and TEM. The results revealed that single-crystalline (K_0.5Bi_0.5)_0.4Ba_0.6TiO₃ nanorods with width around 80 to 120 nm, and length around 200-300 nm were obtained by calcining dried gels at 800 °C for 2 h. Raman analysis of (K_0.5Bi_0.5)_0.4Ba_0.6TiO₃ nanorods indicated that A₁(TO₂) and E(TO) mode incorporated into one broad peak at around 285 cm^− 1, which can be attributed to the cation disorder (Bi, K, Ba) on the 12-fold coordinated A site of ABO₃ structure.     

Enhancement of ferroelectric and piezoelectric characteristics in europium substituted SrBi2Ta2O9 ferroelectric ceramics

Europium substituted samples of compositions Sr_1 − xEu_xBi₂Ta₂O₉ were synthesized by solid-state reaction method. The prepared samples were characterized for their structural and electrical properties. X-ray analysis confirms the formation of the single-phase layered perovskite structure. The microstructural studies reveal that the average grain size increases with increase in Eu content. An increase in remanent polarization and d₃₃ values with increasing concentration of europium has been observed. The maximum 2P_r ~ 14 μC/cm² is observed in the sample with x = 0.20. The observed results have been discussed in terms of contribution from the cation vacancies introduced into the lattice structure due to donor doping.     

High piezoelectric d33 coefficient of lead-free (Ba0.93Ca0.07)(Ti0.95Zr0.05)O3 ceramics sintered at optimal temperature

Lead-free (Ba_0.93Ca_0.07)(Ti_0.95Zr_0.05)O₃ (BCZT) ceramics were prepared using a solid-state reaction technique. The structure and electrical properties were investigated with a special emphasis on the influence of sintering temperature. Crystalline structures and microstructures were analyzed by X-ray diffraction and scanning electron microscope (SEM) at room temperature. The BCZT ceramics sintered at 1450 °C show the highest densification and exhibit excellent piezoelectric properties of high piezoelectric coefficient d₃₃ = 387 pC/N, planar mode electromechanical coupling coefficient k_p = 44.2%, mechanical quality factor Q_m = 140 and Curie temperature T_c = 108 °C.     

Effect of Gd substitution on ferroelectric and magnetic properties of Bi4Ti3O12

Polycrystalline samples Bi_4 − xGd_xTi₃O₁₂ (x = 1, 1.5, 2) were investigated by X-ray diffraction, piezoresponse force microscopy and SQUID-magnetometry techniques. Increasing the gadolinium content was shown to suppress the spontaneous polarization in Bi_4 − xGd_xTi₃O₁₂, resulting in a polar-to-nonpolar phase transition near x = 1.5. In contrast to previous expectations, all these samples were found to be paramagnets. It was thus proven that introducing magnetically-active Gd ions into the lattice of the ferroelectric Aurivillius-type compound should not be considered as an effective way to achieve multiferroic behavior.     

Dielectric properties and heating effect of multiferroic BiFeO3 suspension

Bismuth ferrite (BiFeO₃) ceramic particles of micro-meter size were prepared by a solid-state route. This study revealed an appropriate method to measure the dielectric properties of BiFeO₃ particle suspension using a system designed by ourselves. The heating effect of the BiFeO₃ suspension with applied AC voltage was confirmed. Application of this material in hyperthermia treatment of biological tissues with the goal of tumor therapy may be possible.     

Morphotropic phase boundary in Pb(Sc1/2Nb1/2)O3-BiScO3-PbTiO3 high temperature piezoelectrics

The solid solutions in the 0.36BiScO₃-0.64(1 − x)PbTiO₃-0.64xPb(Sc_1/2Nb_1/2)O₃ system were fabricated using wolframite precursor method. A coexistence of rhombohedral and tetragonal phases is formed in the studied compositions range and a wide morphotropic phase boundary region is confirmed by X-ray diffraction results. After the addition of Pb(Sc_1/2Nb_1/2)O₃, a relaxor behavior is induced and the dielectric maximum temperature shifts to higher temperatures with increasing measuring frequencies. The presence of relaxor can be ascribed to the formation of polar nanoregions. The studied composition exhibits the optimal ferroelectric and piezoelectric properties with d₃₃ of 453 pC/N and K_p of 0.58, T_m of 405 °C for x = 0.10 composition, which is suitable for future high-temperature piezoelectric application.     

Hydrothermal synthesis of single-crystal BaTiO3 dendrites

In this paper, we report a simple hydrothermal method without any surfactants, for the first time, to synthesize single-crystal BaTiO₃ dendrites. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron diffraction (ED), and high-resolution transmission electron microscopy (HRTEM). The KOH concentration was found to be vital to the final formation of BaTiO₃ dendrites. An obvious morphology evolution from sphere-like shape to dendrite was observed when KOH concentration was decreased from 1 M to 0.1 M. It is rational to expect that dendritic structures of other perovskite oxides may also be synthesized by this simple method.     

Synthesis of KNbO3 nanostructures by a microwave assisted hydrothermal method

One-dimensional nanostructures of KNbO₃ have attracted a great interest in the scientific community, mainly because of their promising application as nanoelectromechanical systems (NEMS). However, the synthesis of KNbO₃ structures becomes complex due to the natural tendency to form non-stoichiometric potassium niobates. In this context, we report on the crystallization of one-dimensional KNbO₃ nanostructures through the reaction between Nb₂O₅ and KOH under microwave-assisted hydrothermal synthesis (M-H). The use of this synthesis method made possible a very fast synthesis of singlecrystalline powders. Based on SEM, TEM and XRD characterizations, the influence of the synthesis time and the reactants concentration in the structure and morphology of the resultant KNbO₃ was established. The conditions that favor the crystallization of nanofingers were determined to be small amounts of Nb₂O₅ and short reaction times.     

Preparation and electrical properties of nanoporous BaTiO3

Nanoporous barium titanate with high specific surface area was prepared from co-gel precursors through solvothermal method followed by supercritical drying. The samples were accumulated by BaTiO₃ nanoparticles with excellent crystallinity. The BaTiO₃ obtained at 60 °C exhibited a high BET surface area of 117 m²/g. The porosity reduced with the increasing solvothermal temperature. Raman spectra indicated that the solvothermal-synthesized BaTiO₃ was composed by both cubic phase and tetragonal phase. The relations between dielectric properties and the porosity of the samples were also investigated. The introduction of pores reduced the dielectric constant obviously. The dielectric constant of the obtained sample increased with the decrease of the porosity.     

KxNa1 − xNbO3 powder synthesized by molten-salt process

K_xNa_1 − xNbO₃ ceramic powders have been successfully synthesized in different salts (NaCl, KCl, NaCl-KCl). Our results reveal that K_xNa_1 − xNbO₃ powders with single-phase perovskite structure can be formed at a low temperature such as 750 °C. The type of salts has significant effects on the morphology and chemical composition of the powders. As Na⁺ has a higher diffusing rate and occupies the A-site in the perovskite structure more easily as compared to K⁺, the powder contains only a small amount of K⁺ (x ∼ 0.10) when it is synthesized according to formula K_0.5Na_0.5NbO₃ and in a flux containing the same molar content of Na⁺ and K⁺. By using a NaCl or KCl salt, the K⁺ concentration x can be adjusted to almost 0 and 0.77, respectively.     

Structural and chemical characterization of BaTiO3 nanorods

An electron-microscopy investigation was performed on BaTiO₃ nanorods that were processed by sol-gel electrophoretic deposition (EPD) into anodic aluminium oxide (AAO) membranes. The BaTiO₃ nanorods grown within the template membranes had diameters ranging from 150 to 200 nm, with an average length of 10-50 μm. By using various electron-microscopy techniques we showed that the processed BaTiO₃ nanorods were homogeneous in their chemical composition. The BaTiO₃ nanorods were always polycrystalline and were composed of well-crystallized, defect-free, pseudo-cubic BaTiO₃ grains, ranging from 10 to 30 nm. No intergranular phases were observed between the BaTiO₃ grains. A low-temperature hexagonal polymorph that is coherently intergrown with the BaTiO₃ perovskite matrix was also observed as a minor phase. When annealing the AAO templates containing the BaTiO₃ sol in an oxygen atmosphere the presence of the hexagonal polymorph was diminished.     

The effect of growth temperature on the nanostructure and dielectric response of BaTiO3 ferroelectric films

BaTiO₃ ferroelectric films were grown on Si/SiO₂/Ti/Pt/Au/Pt templates at different temperatures in the range 560-680 °C by pulsed laser deposition. Cross section scanning electron microscopy images and atomic force microscopy surface morphology analysis reveal films with columnar structure and in-plane grain size distribution, in the range 10-60 nm, depending on growth temperature. Low-field dielectric measurements were performed as functions of temperature in the range 40-500 K and external dc field up to 400 kV/cm. The apparent permittivity of ferroelectric films grown at 680 °C shows Curie-Weiss behavior above 400 K with Curie temperature and Curie-Weiss constant 240 K and 1 · 10⁵ K, respectively. The films grown at lower temperatures reveal a decrease of Curie temperature down to − 80 K, reduced values of apparent permittivity and loss tangent, and broadening of maximum of temperature dependence of apparent permittivity. The film grown at 590 °C demonstrates state of the art combination of temperature stability (temperature coefficient of apparent permittivity 300 ppm/K in the range 50-350 K), high tunability of apparent permittivity (up to 60% at room temperature), and relatively low loss tangent (less than 0.05 in the frequency range up to 10 GHz). The change in apparent permittivity and its temperature dependence, with variation of growth temperature are analyzed using two different composite models. The first model assumes the film to be a composite with vertical inclusions of low permittivity dielectric material associated with grain boundaries. This model may explain the observed decrease of permittivity with decreasing growth temperature, but not the shift of Curie temperature. The second model assumes a layered type of composite with low permittivity material associated with the film/electrode interfaces, and allows explanation of the Curie temperature shift.     

Preparation of polycrystalline BaTi2O5 ferroelectric ceramics

Polycrystalline BaTi₂O₅ (BT₂) was prepared by solid state sintering in air using BaCO₃ and TiO₂ as starting materials. A rod-like BT₂ sintered bodies in a single phase were obtained above 1025 °C by adding 5 wt.% B₂O₃. The densification of BT₂ sintered bodies decreased with increasing sintering temperature. The maximum permittivity of BT₂ sintered body was 640 at the Curie temperature (T_c) of 461 °C and the frequency of 100 kHz. The addition of B₂O₃ was effective to obtain BT₂ in a single phase at low temperatures with elongated microstructure and to improve the permittivity, indicating the formation of preferred orientation at low temperature was related with liquid-phase sintering mechanism.     

Properties of TiO2 prepared by acid treatment of BaTiO3

TiO₃ powders were prepared by acid treatment of BaTiO₃ and their properties were investigated. The BaTiO₃ powder was subjected to HNO₃ in concentrations ranging from 10⁻³ to 8 M at 90 °C for 0.5-6 h. Dissolution of BaTiO₃ and precipitation of TiO₂ occurred at acid concentrations of 2-5 M. BaTiO₃ dissolves completely to form a clear solution at reaction times of 0.5-1 h, but a rutile precipitate is formed after 2 h of acid treatment. By contrast, anatase is precipitated by adjusting the pH of the clear solution to 2-3 using NaOH or NH₄OH solution. The rutile crystals were small and rod-shaped, consisting of many small coherent domains connected by grain boundaries with small inclination angles and edge dislocations, giving them a high specific surface area (S_BET). With increasing HNO₃ concentration, the S_BET value increased from 100 to 170 m²/g while the crystallite size decreased from 25 to 11 nm. The anatase crystals obtained here were very small equi-axial particles with a smaller crystallite size than the rutile and S_BET values of about 270 m²/g (higher than the rutile samples). The photocatalytic activity of these TiO₂ was determined from the decomposition rate of Methylene Blue under ultraviolet irradiation. Higher decomposition rates were obtained with larger crystallite sizes resulting from heat treatment. The maximum decomposition rates were obtained in samples heated at 500-600 °C. The photocatalytic activity of the TiO₂ was found to depend more strongly on the sample crystallite size than on S_BET.     

Hydrothermal synthesis of ultrafine A5Nb4O15 (A = Ba, Sr) powders

A₅Nb₄O₁₅ (A = Ba, Sr) powders with hexagonal perovskite structures have been firstly synthesized by a two-step hydrothermal process. These oxides were crystallized directly from niobium oxide in the presence of potassium basic solutions at 240 °C for 48 h. The products were characterized by powder X-ray diffraction and scanning electron microscopy. It was found that the pretreatment of the niobium oxide through the hydrothermal technique firstly was the critical factor to prepare these niobates. Moreover, the alkaline concentration also played an important role in the formation of these niobates. On basis of the experimental data, a possible reaction process for the formation of A₅Nb₄O₁₅ (A = Ba, Sr) was proposed.