Dielectric,thermal and Raman spectroscopy studies of lead-free (Na0.5Bi0.5)1−xSrxTiO3 (x = 0, 0.04 and 0.06) ceramics

ABSTRACT

Lead-free (Na_0.5Bi_0.5)_1?xSr_xTiO₃ (x = 0, 0.04 and 0.06) ceramics with relative densities above 97% were prepared by solid-state synthesis process. Their dielectric, thermal and Raman properties were studied. X-ray diffraction analysis shows perovskite structure with rhombohedral symmetry at room temperature. Sr doping of Na_0.5Bi_0.5TiO₃ (NBT) results in an increase of the dielectric permittivity, diffusing of the permittivity maximum and its shift toward lower temperatures. The temperature of the rhombohedral–tetragonal phase transition indicated by the differential scanning calorimetry (DSC) peak and relaxational dielectric anomaly near the depolarization temperature are also shifted toward lower temperatures. The observed increase and broadening of the permittivity maximum, enhancement of the dielectric relaxation near the depolarization temperature, broadening of the DSC anomaly related to the rhombohedral–tetragonal phase transition and broadening of the Raman bands with increasing Sr content are attributed to the increase of the degree of cationic disorder and evident enhancement of the relaxor-like features in NBT–xST. This enhancement could play a positive role in the improvement of the piezoelectric performance of NBT-based ceramics.     

Dielectric and piezoelectric properties of (Bi1−x−yNdxNa1−y)0.5BayTiO3 lead-free ceramics

New lead-free (Bi_1−x−yNd_xNa_1−y)_0.5Ba_yTiO₃ ceramics were prepared by a conventional ceramic technique and their dielectric and piezoelectric properties were studied. X-ray diffraction studies reveal that Nd³⁺ and Ba²⁺ diffuse into the Bi_0.5Na_0.5TiO₃ lattices to form a new solid solution with a pure perovskite structure, and a morphotropic phase boundary (MPB) of rhombohedral and tetragonal phases is formed at 0.04 < y < 0.10. The partial substitutions of Nd³⁺ and Ba²⁺ decrease effectively the coercive field E_c and increase significantly the remanent polarization P_r. Because of lower E_c, larger P_r and the formation of the MPB, the piezoelectric properties of the ceramics are significantly enhanced at x/y = 0.02/0.06: d₃₃ = 150 pC/N and k_p = 30.5%. The ceramics exhibit relaxor characteristic, which is probably resulted from the cation disordering in the 12-fold coordination sites. The depolarization temperature T_d shows a strong compositional dependence and reaches a minimum value at the MPB. The temperature dependences of the ferroelectric and dielectric properties suggest that the ceramics may contain both the polar and non-polar regions near the depolarization temperature T_d, which cause the polarization hysteresis loop become deformed near/above T_d.     

Raman spectra and anomalies of dielectric properties and thermal expansion of lead-free (1−x)Na0.5Bi0.5TiO3-xSrTiO3 (x = 0, 0.08 and 0.1) ceramics

ABSTRACT

Thermal expansion, Raman and dielectric properties of the lead-free (1?x)Na_0.5Bi_0.5TiO₃-xSrTiO₃ (x = 0, 0.08 and 0.1) ceramic solid solutions, fabricated by the conventional solid-state reaction method, were investigated. The Sr-doping results in an increase of the dielectric permittivity, broadening of the permittivity maximum, enhancement of the relaxation near depolarization temperature, broadening of the Raman bands and shift of all anomalies toward lower temperatures. The observed effects are attributed to an increase of the degree of cationic disorder and enhancement of the relaxor-like features. Anomalies in the thermal expansion strain were observed at the temperatures corresponding to the dielectric anomalies but not related to any phase transitions. These anomalies are supposed to follow changes of the averaged unit cell volume in the temperature interval of tetragonal and rhombohedral phase coexistence.     

Dielectric,ferroelectric and piezoelectric properties of (1−x)[K0.5Na0.5NbO3]−x[LiSbO3] ceramics

Lead-free (1?x)[K_0.5Na_0.5NbO₃]?x[LiSbO₃] (x=0, 0.04, 0.05 and 0.06)/(KNN-LS) ceramics were prepared by conventional solid-state reaction route (CSSR). For dense morphology pure KNN ceramic was sintered at 1120 °C and LS modified KNN ceramics were sintered at 1080 °C for 4 h, respectively. The structural study at room temperature (RT) revealed the transformation of pure orthorhombic to tetragonal structure with the increase in LS content in KNN-LS ceramics. Temperature dependent dielectric study confirmed the increase of diffuse phase transition nature with the increase in LS content in KNN-LS ceramics. The presence of orthorhombic to tetragonal (T_O?T) polymorphic phase transition temperature (PPT) ～43 °C confirmed the presence of two ferroelectric (orthorhombic and tetragonal) phases in 0.95KNN-0.05LS ceramics at RT. 0.95KNN-0.05LS ceramics showed better ferroelectric and piezoelectric properties i.e., remnant polarization (P_r)～18.7 μC/cm², coercive field (E_c)～11.8 kV/cm, piezoelectric coefficient (d₃₃)～215 pC/N, coupling coefficient (k_p)～0.415 and remnant strain ～0.07% were obtained.     

Dielectric and piezoelectric properties of Li-substituted lead-free (Bi0.5Na0.5)TiO3–(Bi0.5K0.5)TiO3–BaTiO3 ceramics

Lead-free 0.79(Bi_0.5Na_0.5)TiO₃–0.14[Bi_0.5(K_0.5−xLi_x)]TiO₃–0.07BaTiO₃ (BNBK79 + xLi, x = 0.0, 0.1, 0.2, 0.25, 0.3, and 0.4) ceramics were prepared by conventional solid state reaction process. The crystalline structures and surface morphologies are investigated by X-ray diffraction method and scanning electron microscopy. Dielectric and piezoelectric properties were measured. With increasing of lithium substitution, the Curie temperatures of BNBK79 + xLi ceramics increase, but the maximum value of the dielectric constant decreases. And a relatively large remnant polarization of 17.6 μC/cm² and 157 pC/N of d₃₃ has been obtained when x = 0.3.     

Dielectric, modulus and impedance analysis of lead-free ceramics Ba0.8La0.133Ti1−x Sn x O3 (x=0.15 and 0.2)

Titanate barium (BaTiO₃)-type oxide ceramics Ba_0.8La_0.133Ti_1?x Sn _x O₃ (BLTS) (here x=0.15 and 0.2) have been synthesized by the standard solid-state reaction method. Preliminary room temperature X-ray study confirms the formation of single-phase compounds in a rhombohedral crystal system. The electrical properties of BLTS were studied using the ac impedance spectroscopy technique over a wide range of temperature (120–320 K) in the frequency range of 40 Hz to 10 MHz. The presence of a single arc in the complex modulus spectrum at different temperatures confirms the single-phase character of the BLTS compounds.     

Phase diagram of (1−x%)(0.89Bi0.5Na0.5TiO3–0.06BaTiO3–0.05K0.5Na0.5NbO3)–x%MnO2 lead-free anti-ferroelectric ceramics

A phase diagram for the lead-free ceramics in the (1?x%)(0.89Bi_0.5Na_0.5TiO₃–0.06BaTiO₃–0.05K_0.5Na_0.5NbO₃)–x%MnO₂ (BNBKN-x%Mn) binary system is constructed for the first time based on the ferroelectric and dielectric measurements. The ferroelectric behaviors under different temperatures suggest that the ceramics are basically of relaxor anti-ferroelectric nature near room temperature. The temperature dependent dielectric properties show that when the addition of MnO₂ increases, the relaxor anti-ferroelectric phase can be stabilized to be close to the Curie point, which corresponds to a relaxor anti-ferroelectric to paraelectric phase transition.     

Effect of Li-substitution on piezoelectric and ferroelectric properties of (Bi0.92Na0.92−xLix)0.5Ba0.06Sr0.02TiO3 lead-free ceramics

New lead-free ceramics (Bi_0.92Na_0.92−xLi_x)_0.5Ba_0.06Sr_0.02TiO₃ have been fabricated by a conventional ceramic technique and their electrical properties have been studied. X-ray diffraction studies reveal that Li⁺, Ba²⁺ and Sr²⁺ diffuse into the Bi_0.5Na_0.5TiO₃ lattices to form a new solid solution with a pure perovskite structure. The partial substitution of Li⁺ for Na⁺ increases the remanent polarization P_r of the ceramics. Because of the large P_r and low coercive field E_c, the ceramics with x = 0.075–0.125 exhibit excellent piezoelectric properties: d₃₃ = 189–235 pC/N, k_p = 33.6–36.3% and k_t = 51.6–54.3%. The ceramics exhibit relaxor behaviors after the substitution of Li⁺ for Na⁺. Our results also suggest that polar and non-polar phases may coexist in the ceramics at temperatures above the depolarization temperature T_d.     

Preparation and characterization of Li2.9Sc1.9−y Y y Zr0.1(PO4)3 (where y = 0, 0.1) solid electrolyte ceramics

The solid electrolyte Li_2.9Sc_{1.9? y} Y _y Zr_0.1(PO₄)₃ (where y = 0, 0.1) compounds belong to monoclinic symmetry (space group P2₁/n) at room temperature. The Zr 3d, Sc 2p, P 2p, Y 3d, O 1s, and Li 1s core level X-ray photoelectron spectra (XPS) were fitted. The Li ions in ceramics without Y occupy two different positions and in the ceramics with Y they occupy one position in the lattice. The deconvolutions of the Zr 3d, P 2p, Sc 2p, and Y 3d core level XPS are associated with different valence states on the surfaces of the investigated ceramics. Anomalies of enthalpy, change of activation energy of ionic conductivity, anomalies of dielectric permittivity in the temperature range 420–520 K of investigated compounds were found. The phenomena are related to diffuse structure phase transition in the compounds. At temperatures 600 and 900 K, the compounds belong to orthorhombic symmetry (space group Pbcn).     

Synthesis and physicochemical properties of Li2 Me x Zr1 − x O3 − δ (Me = Nb,Ti; x = 0.05, 0.1) solid solutions

Complex lithium metallates Li₂ Me _x Zr_{1 ? x} O_{3 ? δ} (Me = Nb, Ti, x = 0.05, 0.1) with iso-and heterovalent substitutions for Zr⁴⁺ ions in lithium zirconate are synthesized for the first time using a citrate technique. The inclusion of Ti⁴⁺ and Nb⁵⁺ ions in the crystal structure of Li₂ZrO₃ is confirmed by means of X-ray diffraction and NMR. It is shown that in the temperature range of 750–820 K, Li₂Ti_0.1Zr_0.9O₃ solid solution has higher conductivity than phases of undoped lithium zirconate.     

Dielectric,optical and structural properties of Bi4V2−xSrxO11−δ (0.05≤x≤0.20)

Composition Bi₄V_2−xSr_xO_11−δ (0.05≤x≤0.20) is synthesized by melt quench technique followed by heat treatment at 800 °C for 12 h. These compounds are characterised by X-ray diffraction, Fourier transform infrared (FTIR) spectroscopy, UV–visible spectroscopy, impedance spectroscopy and scanning electron microscopy. X-ray diffraction patterns of all the samples show γ-phase stabilization at room temperature except x=0.05 heat treated sample. The optical band gap of all the samples is observed in semiconducting range. The lowest and the highest optical band gap is 2.39 eV and 2.57 eV for x=0.10 heat treated and x=0.20 quenched samples, respectively. The highest value of dielectric constant is obtained ~10⁷ with very low dielectric loss for x=0.15 and 0.20 samples at ~350 °C and below 10 Hz. The grain size increases with dopant concentration leads to increase the dielectric constant.     

Cation Radii and Oxygen Deficiency Effects on the Structural,Magnetic, and Electrical Properties in La 1− x Sr x MnO 3−δ □ δ Manganites ( x = 0.2-0.5 and δ = 0.1)

The series of non-stochiometric strontium substituted lanthanium manganites La 1 m x Sr x MnO 3 m i i ( i = 0.1, x = 0.2-0.5) has been studied and show the effect of oxygen deficiency on the structural transition, magnetic and electrical properties of these compounds. Polycrystalline samples La 1 m x Sr x MnO 3 m i i were synthesised by a new method at atmospheric pressure. In this series of manganites, the Mn 4+ content is systematically decreased due to increase in the non-stoichiometry. X-ray diffraction analysis shows a phase transition from orthorhombic to rhombohedral systems for 0.2 h x h 0.5. The material is ferromagnetic for 0.3 h x h 0.5 and antiferromagnetic for x = 0.2. The Curie temperature T C increases with increasing x . The resistivity as a function of temperature shows that samples with x = 0.3 and ferromagnetic metal between T CF and T P becomes ferromagnetic insulators below T CF where charge-ordering seems to appear.     

Dielectric properties and specific heat of the (1 ? x)PbNi1/3Nb2/3O3-xPbTiO3 ferroelectric ceramics

The temperature dependences of the permittivity and specific heat of a mixed (1 ? x)PbNi_1/3Nb_2/3O₃-xPbTiO₃ system (where x = 0.3, 0.4, 0.5) have been studied over a wide temperature range 290?C700 K. It has been shown that the composition with x = 0.3 undergoes a diffuse phase transition at T _m ?? 315 K. A specific heat anomaly that is characteristic of the phase transition has been revealed at T ?? 315 K in all the compositions under study.     

Structural chemistry,electrical resistivity,magnetization and specific heat in La1.85Sr0.15Cu1−xZn x O4

Samples from the oxide series of solid solutions La_1.85Sr_0.15Cu₁–xZn _x O₄, quenched from 1100°C, have been characterized by X-ray powder analysis. We report on the structural chemistry and on measurements of the electrical resistivity, magnetization, ac susceptibility and specific heat of these samples. Zinc was found to statistically replace the copper atoms in the body centered tetragonal K₂NiF₄-type of structure up to values ofx smaller than 0.15. At higher zinc-concentrations (0.15<x<0.45) the crystal structure transformes into the orthorhombically distorted K₂NiF₄-type, known for the low-temperature form of Sr-and Zn-free La₂CuO₄. Limited solid solutions are encountered when Cu is substituted by Cd and Hg. Whilst the Zn-free sample La_1.85Sr_0.15CuO₄ was superconducting with a sharp onset at 35 K, substitution of Cu by Zn by as little as 6 mole percent was sufficient to suppress superconductivity belowT _c<4.5 K. Isoelectronic substitution of Cu by (Ni_0.5Zn_0.5) was found to cause an even larger degradation ofT _c than equivalent amounts of Zn. At the superconducting transition the discontinuity of the specific heat C _p is observed in agreement with recent publications, but is found to decrease with Zn substitution. A linear term inC _p(T) is resolved at low temperatures which significantly rises upon Cu/Zn substitution.     

Composition-related structural,thermal and mechanical properties of Ba1−xSrxTiO3 ceramics (0 ≤ x ≤ 0.4)

The Ba_1?xSr_xTiO₃ (BST) ceramics were prepared by conventional ceramic method. The crystalline structure and morphology were studied by X-ray diffraction and scanning electron microscopy, respectively. Experimental results show that increase of sintering temperature leads to an uncontrolled precipitating of the phase with a lower content of Ti. The dielectric constant and specific heat as a function of composition and temperature were investigated. The increasing concentration of Sr ions leads to a shift of the Curie point below room temperature. To determine the elastic constants (the Young's modulus E, the shear modulus G and the Poisson's ratio v) of BST, a method of measurement of the longitudinal (ν_L) and transverse (ν_T) ultrasonic wave velocities for this type of material was developed. The structural, dielectric and mechanical properties of BST ceramics were discussed in terms of microstructure and chemical composition     

Impedance spectroscopy and piezoresponse force microscopy analysis of lead-free (1 − x) K0.5Na0.5NbO3 − xLiNbO3 ceramics

(1 ? x) K_0.5Na_0.5NbO₃ ? xLiNbO₃ (where x = 0.0, 5.0, 5.5, 6.0, and 6.5 wt.%) (KNLN) perovskite structured ferroelectric ceramics were prepared by the solid-state reaction method. X-ray diffraction patterns indicate that single phase was formed for pure KNN while a small amount of second phase (K₆Li₄Nb₁₀O₃₀, ～3%) was present in LN doped KNN ceramics. Phase analysis indicated the change in the crystal structure from orthorhombic to tetragonal with increase in LN content. The electrical behavior of the ceramics was studied by impedance spectroscopy technique in the high temperature range. Impedance analysis was performed using an equivalent circuit model. The impedance response in pure KNN and KNLN ceramics could be deconvoluted into two contributions, associated with the bulk (grains) and the grain boundaries. Activation energies for conductivity were found to be strongly frequency dependent. The activation energy obtained from dielectric relaxation data was attributed to oxygen vacancies. From PFM we found that the composition with 6.5 wt.% LN displays stronger piezocontrast as compared to pure KNN implying an evidence of a pronounced piezoelectric coefficient.     

Influence of Ba addition on the dielectric and optic properties of (1 – x) Na0.5Bi0.5TiO3–xBaTiO3 (x = 0, 0.025, 0.0325 and 0.05) single crystals

High-quality, large-size lead-free (1 – x)Na_0.5Bi_0.5TiO₃–xBaTiO₃ ((1 – x)NBT–xBT) single crystals (x = 0, 0.025, 0.0325 and 0.05) were grown using the Czochralski method. Dielectric and transmitted light intensity properties were measured for these crystals. The broad anomalies exhibited in the temperature dependence of the transmitted light corresponded to structural and dielectric anomalies and were related to the temperature dependence of polar regions and the appearance of a long-range ferroelectric state. We explain our results based on local electromechanical fields, by inhomogeneity of the ion distribution and the mismatch in ion size. We suggested that the NBT–BT system can be a promising lead-free piezoelectric material for ultrasonic delay-line applications, broadband transducers and sensors.