Microstructure,phase transition and electrical properties of LiSbO<Subscript>3</Subscript>-doped (K<Subscript>0.49</Subscript>Na<Subscript>0.51</Subscript>)NbO<Subscript>3</Subscript> lead-free piezoelectric ceramics

Microstructure, phase transition and electrical properties of (1 − x)K_0.49Na_0.51NbO₃ − xLiSbO₃ (x = 0–0.08) lead-free piezoceramics prepared by the conventional solid-state sintering method were investigated with an emphasis on the effects of LiSbO₃ doping amount x. SEM results showed that the ceramic became denser by increasing LiSbO₃ doping amount x. Being indexed by XRD profiles, the ceramics changed from an orthorhombic perovskite structure to a tetragonal one across a composition region of 0.04 ≤ x≤0.05. The sample of LiSbO₃ doping amount x = 0.05 in tetragonal side of the region had the maximum values of piezoelectric constant (d ₃₃ = 256 pC/N) and planar electromechanical coupling coefficient (k _p = 42.7%). Meanwhile, this ceramic sample showed other good properties such as ε _r = 1,463, tgδ = 0.036, Q _m = 48, P _r = 19.8 μC/cm², E _c = 1.9 kV/mm and T _c = 340 °C, which indicated it was a promising lead-free piezoelectric material for ultrasonic transducer applications.     

Phase component and conductivities of Co-doped BaTiO<Subscript>3</Subscript> thermistors

BaTi_1−x Co _x O_3−δ (0.01 ≤ x ≤ 0.4) ceramics were prepared by a wet chemical process polymerized with polyvinyl alcohol. The phases and related electrical properties of the ceramics were investigated. The phase component of the ceramics changes from a tetragonal phase to a hexagonal one with the Co concentration increase. A pure hexagonal phase formed in the ceramic with x = 0.2. The measurement of the temperature dependence of resistances revealed that the ceramic resistivities increase with temperature rising at the temperatures (T) lower than half of the related Debye temperature (Θ_D), and the ceramics show a negative temperature coefficient (NTC) effect at T > Θ_D/2. The material constants B _50/120 of the BaTi_1−x Co _x O_3−δ NTC thermistors were calculated to be 3,187, 2,968 and 2,648 K for x = 0.2, 0.3 and 0.4, respectively. Narrow-band conduction and non-adiabatic hopping models are proposed for the conduction mechanisms at T < Θ_D/2 and T > Θ_D/2, respectively.     

Effects of addition of BiFeO<Subscript>3</Subscript> on phase transition and dielectric properties of BaTiO<Subscript>3</Subscript> ceramics

Samples of xBiFeO₃–(1 − x)BaTiO₃ (x = 0, 0.02, 0.04, 0.06, 0.07 and 0.08) were synthesized by solid state reaction technique and sintered in air in the temperature range 1,220–1,280 °C for 4 h. X-ray diffraction data showed that 2–8 mol% BiFeO₃ can dissolve into the lattice of BaTiO₃ and form single perovskite phase. The crystal structure changes from tetragonal to cubic phase at room temperature when 8 mol% of BiFeO₃ was added into BaTiO₃. Scanning electron microscope images indicated that the ceramics have compact and uniform microstructures, and the grain size of the ceramics decreases with the increase of BiFeO₃ content. Dielectric constants were measured as functions of temperatures (25–200 °C). With rising addition of BiFeO₃, the Curie temperature decreases. For the sample with x = 0.08, the phase transition occurred below room temperature. The boundary between tetragonal and cubic phase of the BiFeO₃–BaTiO₃ system at room temperature locates at a composition between 7 and 8 mol% of BiFeO₃. The diffusivity parameter γ for compositions x = 0.02 and x = 0.07 is 1.21 and 1.29, respectively. The relaxor-like behaviour is enhanced by the BiFeO₃ addition.     

Electron Magnetic Resonance,Neutron Diffraction and ac Susceptibility Study of CaMn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ru{<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>3</Subscript> (x ≤ 0.40) Manganite System

Electron magnetic resonance (EMR), neutron powder diffraction (NPD) and ac susceptibility techniques were employed for studying the crystallographic structure and magnetic ordering in CaMn_1−xRu_xO₃ (x ≤ 0.40) manganite system. EMR measurements were done on polycrystalline samples at 120 ≤ T ≤ 500 K. High temperature EMR spectra of pristine antiferromagnetic (AFM) CaMnO₃ show a singlet Lorentzian-like line, whose intensity diminishes, zeroing at Neel temperature T _N=120 K. Strong broadening of paramagnetic (PM) lines with increase of Ru-content (Δ H_pp ∼ 1 T for x=0.10) was found. Upon cooling low-doped (x ≤ 0.06) samples remain AFM, whereas higher doped ones (0.10 ≤ x ≤ 0.40) clearly show progressive appearance of ferromagnetic (FM) phases. Thus, EMR evidences that Ru-doping modifies both PM and AFM states and creates an inhomogeneous phase separated FM and AFM ground states at x0.06. Complementary measurements of NPD and ac susceptibility corroborate the complex character of magnetic ordering, revealed by EMR. The changes of the magnetic ordering in CaMn_1−xRu_xO₃ supposed to be solely determined by doping of Mn-sites with Ru.     

Dielectric and piezoelectric properties of MnO<Subscript>2</Subscript>-doped K<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>Nb<Subscript>0.92</Subscript>Sb<Subscript>0.08</Subscript>O<Subscript>3</Subscript> lead-free ceramics

Lead-free MnO₂-doped K_0.5Na_0.5Nb_0.92Sb_0.08O₃ ceramics have been fabricated by a conventional ceramic technique and their dielectric and piezoelectric properties have been studied. Our results show that a small amount of MnO₂ (0.5–1.0 mol%) is enough to improve the densification of the ceramics and decrease the sintering temperature of the ceramics. The co-effects of MnO₂ doping and Sb-substitution lead to significant improvements in the ferroelectric and piezoelectric properties. The K_0.5Na_0.5Nb_0.92Sb_0.08O₃ ceramic with 0.5 mol%MnO₂ doping possesses optimum propeties: d ₃₃ = 187 pC/N, k _P = 47.2%, ε _r = 980, tanδ = 2.71% and T _c = 287 °C. Due to high tetragonal-orthorhombic phase transition temperature (T _O-T ~ 150 °C), the K_0.5Na_0.5Nb_0.92Sb_0.08O₃ ceramic with 0.5 mol%MnO₂ doping exhibits a good thermal stability of piezoelectric properties.     

Synthesis,microstructure and microwave dielectric properties of Ca<Subscript>4-x</Subscript>Mg<Subscript>x</Subscript>La<Subscript>2</Subscript>Ti<Subscript>5</Subscript>O<Subscript>17</Subscript> ceramics

Ca_4-xMg_xLa₂Ti₅O₁₇ ceramics were prepared by a solid state ceramic route for x = 0, 0.5, 1, 2, 3 and 4. The structure and microstructure of the ceramics were investigated using X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy. X-ray diffraction results show that the Ca_4-x Mg _x La₂Ti₅O₁₇ adopts an orthorhombic crystal structure with no secondary phase observed for x from 0 to 0.5. Secondary phase, MgTiO₃ occurs with further increasing doping level (1 ≤ x ≤ 3). When x = 4, mixture phases La_0.66TiO_2.993, MgTiO₃ and a trace of unknown phase coexist. Ca₄La₂Ti₅O₁₇ ceramic exhibits a relative permittivity (ε_r) ~ 65, quality factor (Q × f) ~13,338 GHz (at ~4.75 GHz), and temperature coefficient of resonant frequency (τ _f ) ~ 165 ppm/°C. The sintering temperature was distinctly reduced from 1,580 °C for x = 0 to 1,350 °C for x = 4. With increasing Mg content, ε_r and τ_f obviously decrease, while Q × f value initially decreases and then increases. The ceramic for x = 2 shows ε_r ~ 50, Q × f ~ 9,451 and τ _f  ~ 62.5 ppm/°C. By the complete replacement of Ca with Mg, Mg₄La₂Ti₅O₁₇ ceramic sintered at 1,350 °C for 4 h combines a high dielectric permittivity (ε _r  = 31), high quality factor (Q × f ~ 15,021) and near-zero temperature coefficient of resonant frequency (τ _f  ~ 4.0 ppm/°C). The materials are suitable for microwave applications.     

Influence of Sm substitution on the phase,microstructure and microwave dielectric properties of SrLa<Subscript>4</Subscript>Ti<Subscript>5</Subscript>O<Subscript>17</Subscript>

New dielectric ceramics in the SrLa_4−xSm_xTi₅O₁₇ (0 ≤ x ≤ 4) composition series were prepared through a solid state mixed oxide route to investigate the effect of Sm⁺³ substitution for La⁺³ on the phase, microstructure and microwave dielectric properties. At x = 0–3, all the compositions formed single phase ceramics within the detection limit of in-house X-ray diffraction when sintered in the temperature range 1500–1580 °C. At x = 4, a mixture of Sm₂Ti₂O₇ and SrTiO₃ formed. The maximum Sm⁺³-containing single phase ceramics, SrLaSm₃Ti₅O₁₇, exhibited relative permittivity (ε_r) = 42.6, temperature coefficient of resonant frequency (τ _f ) = −96 ppm/^oC and quality factor (Q _u f _o ) = 7332 GHz. An analysis of results presented here indicates that SrLa_4−xSm_xTi₅O₁₇ ceramics, exhibiting τ _f  ~ 0 and ε_r ~ 53 could be achieved at x ~ 1.4 but at the cost of decrease in Q _u f _o .     

Enhancement of Critical Current Density in YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">y</Emphasis></Subscript> HTS Through Boron Addition

The effect of B₂O₃ addition on the superconducting transition and grain boundary critical current density of the boron free (control) and boron doped HTS ceramics with nominal composition YBa₂Cu₃B _x O_7−y (x=0, 0.025, 0.05, 0.075, 0.1 and 0.15) has been investigated. For the lowest-level boron doping (x=0.025) an increase by nearly 1.5 times was observed in the critical current density J _c compared to the control sample. The small additives of boron in YBa₂Cu₃B _x O_7−y (x=0.025 and 0.05) do not essentially affect the critical temperature T _c =92.5 K of nominally pure Y123. Higher-level boron added compounds revealed a decrease in both T _c and J _c values. The data obtained indicate the possibility of boron dopant being inserted either into interstitial or into substitutional sites of the lattice.     

Temperature dependence of dielectric and piezoelectric properties of (1?x)(BiScO3–0.64PbTiO3)–xLiNbO3 high-temperature relaxor ferroelectric ceramics

(1−x)(0.36BiScO₃–0.64PbTiO₃)–xLiNbO₃ (BSPT64–xLN) high-temperature relaxor ferroelectric ceramics near the morphotropic phase boundary (MPB) composition were investigated. X-ray diffraction showed a change in symmetry from MPB phase to rhombohedral phase with LiNbO₃ content increasing. A change from normal ferroelectric features to relaxor ferroelectric features was observed with LiNbO₃ substitution up to x = 0.06, while high-temperature dielectric relaxation was exhibited at T _max ~230 to 383°C for 0.02 ≤ x ≤ 0.06. The BSPT64–xLN ceramics with x = 0.02 LiNbO₃ exhibited good piezoelectric properties compared with BS–0.64PT ceramics: piezoelectric coefficients d ₃₃ = 505pC/N, planar electromechanical coupling factors k _p = 0.47, and remnant polarization P _r = 40 μC/cm², respectively. The annealing temperature dependence of piezoelectric response for BSPT64–xLN ceramics was measured and it was found that the piezoelectric properties decreased slightly before 210°C for x = 0.02 and 0.04, showing excellent thermal stability, which indicated that BSPT64–xLN relaxor ferroelectric ceramics can be used in the range of high temperature compared to Pb-based relaxor ferroelectrics.     

Effect of ZnO on the microstructure and electrical properties of (K<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>)NbO<Subscript>3</Subscript> lead-free piezoelectric ceramics

K_0.5Na_0.5NbO₃–x ZnO (KNN–xZn) lead-free ceramics have been prepared using the conventional sintering technique and the effects of ZnO addition on the phase structure and piezoelectric properties of the ceramics have been studied. Our results reveal that a small amount of ZnO can improve the density of the ceramics effectively. Because of the high density and ZnO doping effects, the piezoelectric and dielectric properties of the ceramics are improved considerably. The good piezoelectric and dielectric properties of d ₃₃ = 114 pC/N, k _p = 0.36, ε _r = 395, and Q _m = 68 were obtained for the KNN ceramics doped with 1 mol% ZnO. Therefore, the KNN-1.0 mol%Zn ceramics is a good candidate for lead-free piezoelectric application.     

Microstructure,electrical properties of CeO<Subscript>2</Subscript>-doped (K<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>)NbO<Subscript>3</Subscript> lead-free piezoelectric ceramics

CeO₂-doped K_0.5Na_0.5NbO₃ lead-free piezoelectric ceramics have been fabricated by a conventional ceramic fabrication technique. The ceramics retain the orthorhombic perovskite structure at low doping levels (<1 mol.%). Our results also demonstrate that the Ce-doping can suppress the grain growth, promote the densification, decrease the ferroelectric–paraelectric phase transition temperature (T _C), and improve the dielectric and piezoelectric properties. For the ceramic doped with 0.75 mol.% CeO₂, the dielectric and piezoelectric properties become optimum: piezoelectric coefficient d ₃₃ = 130 pC/N, planar electromechanical coupling coefficient k _p = 0.38, relative permittivity ε_r = 820, and loss tangent tanδ = 3%.     

Properties of neodymium-doped Ca<Subscript>0.15</Subscript>Sr<Subscript>1.85</Subscript>Bi<Subscript>4</Subscript>Ti<Subscript>5</Subscript>O<Subscript>18</Subscript> ferroelectric ceramics

Bismuth-layered compound Ca_0.15Sr_1.85Bi_4−xNd_xTi₅O₁₈ (CSBNT, x = 0–0.25) ferroelectric ceramics samples were prepared by solid-state reaction method. The effects of Nd³⁺ doping on their ferroelectric and dielectric properties were investigated. The remnant polarization Pr of CSBNT ceramics increases at beginning then decreases with increasing of Nd³⁺ doping level, and a maximum Pr value of 9.6 μC/cm² at x = 0.05 was detected with a coercive field Ec = 80.2 kV/cm. Nd³⁺ dopant not only decreases the Curie temperature linearly, but also the dielectric constant (ε_r) and dielectric loss tangent (tan δ). The magnitudes of ε_r and tan δ at the frequency of 100 kHz are estimated to be 164 and 0.0083 at room temperature, respectively.     

Synthesis and sintering of nano-sized BaSnO<Subscript>3</Subscript> powders containing BaGeO<Subscript>3</Subscript>

The formation of solid solutions of the type [Ba(HOC₂H₄OH)₄][Sn_1−x Ge _x (OC₂H₄O)₃] as BaSn_1−x /Ge _x O₃ precursor and the phase evolution during its thermal decomposition are described in this paper. The 1,2-ethanediolato complexes can be decomposed to nano-sized BaSn_1−x /Ge _x O₃ preceramic powders. Samples with x = 0.05 consist of only a Ba(Sn,Ge)O₃ phase, whereas powders with x = 0.15 and 0.25 show diffraction patterns of both the Ba(Sn,Ge)O₃ and BaGeO₃ phase. The sintering behaviour was investigated on powders with a BaGeO₃ content of 5 and 15 mol%. These powders show a specific surface area of 15.4–15.9 m²/g and were obtained from calcination above 800 °C. The addition of BaGeO₃ reduced the sintering temperature of the ceramics drastically. BaSn_0.95Ge_0.05O₃ ceramics with a relative density of at least 90% can be obtained by sintering at 1150 °C for 1 h. The ceramic bodies reveal a fine microstructure with cubical-shaped grains between 0.25 and 0.6 μm. For dense ceramics, the sintering temperature could be reduced down to 1090 °C, when the soaking time was extended up to 10 h.     

Dielectric and piezoelectric properties of YMnO<Subscript>3</Subscript> modified Bi<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript> lead-free piezoelectric ceramics

Perovskite lead-free piezoelectric ceramics Bi_0.5Na_0.5TiO₃, modified with yttrium and manganese to form a new compound, (1 − x) Bi_0.5Na_0.5TiO₃–xYMnO₃ (BNT-YM100x) with x = 0–1.2 mol%, was synthesized by a conventional solid-state reaction method. The effect of YMnO₃ on crystal structure, dielectric and piezoelectric properties was investigated. X-ray diffraction analysis shows that the materials have a single phase perovskite structure with rhombohedral symmetry. Addition of small amount of YMnO₃ improves piezoelectric properties and the optimal piezoelectric properties of d ₃₃ = 115 pC/N, k _p = 0.207 and Q _m = 260 were obtained at 0.9% YMnO₃ addition. The loss tangent tanδ is approximately constant while Curie temperature decreases with increasing YMnO₃ concentration.     

Effect of Oxygen Concentration on Superconducting Properties of Rubidium Tungsten Bronzes Rb<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>WO<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>

Superconducting transition temperature (T _c) as a function of oxygen concentration for hexagonal rubidium tungsten bronzes Rb _x WO _y with 2.80 ≤ y ≤ 3.07 and x = 0.19, 0.23, and 0.27 has been systematically investigated. Three regions corresponding to T _c < 2 K (denoted as superconductivity suppressed region), T _c∼ 3 K (superconductivity uniform region) and T _c > 3 K (superconductivity enhanced region) were identified in T _c–y phase diagram for Rb_0.19WO _y and Rb_0.23WO _y . No superconductivity enhanced region was observed for Rb_0.27WO _y . The superconductivity suppressed region shifts toward higher oxygen content as rubidium concentration increases. The local ordering of the intercalated rubidium atoms might be responsible for the intriguing T _c–y phase diagram of Rb _x WO _y .     

Crystal chemistry and domain structure of rare-earth doped BiFeO<Subscript>3</Subscript> ceramics

Bi_(1−x)RE _x FeO₃ (BREF100x, RE = La, Nd, Sm, Gd) has been investigated with a view to establish a broad overview of their crystal chemistry and domain structure. For x ≤ 0.1, the perovskite phase in all compositions could be indexed according to the rhombohedral, R3c cell of BiFeO₃. For Nd and Sm doped compositions with 0.1 < x ≤ 0.2 and x = 0.15, respectively, a new antipolar phase was stabilised similar in structure to PbZrO₃. The orthoferrite, Pnma structure was present for x > 0.1, x > 0.15, and x > 0.2 in Gd, Sm, and Nd doped BiFeO₃, respectively. For x > 0.2, La doped compositions became pseudocubic at room temperatures but high angle XRD peaks were broad and asymmetric. These compositions have been indexed as the orthoferrite structure. It was concluded therefore that the orthoferrite phase appeared at lower values of x as the RE ferrite, end member tolerance factor decreased. However, the compositional window over which the PbZrO₃-like phase was stable increased with increasing end member tolerance factor but was not found as single phase in La doped compositions at room temperature. On heating, the PbZrO₃-like phase in BNF20 transformed to the orthoferrite, Pnma structure. T _C for all compositions decreased with decreasing A-site, average ionic polarizabilty and tolerance factor. For compositions with R3c symmetry, superstructure and orientational, and translational (antiphase) domains were observed in a manner typical of an antiphase-tilted, ferroelectric perovskite. For the new PbZrO₃-like phase orientational domains were observed.     

Synthesis and Superconductivity in Ba-doped (Ru,Cu)(Sr,Nd)<Subscript>2</Subscript>(Nd,Ce)<Subscript>2</Subscript>Cu<Subscript>2</Subscript>O<Subscript><Emphasis Type="Italic">z</Emphasis></Subscript> Compounds

We prepared various (Ru_0.5Cu_0.5)(Sr_1.67−x Ba _x Nd_0.33)(Nd_1.34Ce_0.66)Cu₂O _z samples with varying Ba contents by x=0, 0.1, 0.2, 0.33, 0.5 and 0.835. X-ray diffraction revealed that the phase purity of the samples increases as the Ba-doping content increases and single- phase compounds could be obtained for samples with 0.1≤x≤0.33. Contrary to the Ba-free sample, superconductivity could be induced by partially substituting Ba for Sr and superconductivity with onset T _c of about 15 K is observed for the sample with x=0.2. The superconducting behavior of the Ba-doped samples is discussed in conjunction with thermoelectric power measurements.     

Wetting of grain boundaries in Al by the solid Al<Subscript>3</Subscript>Mg<Subscript>2</Subscript> phase

The microstructure of binary Al_100−x –Mg _x (x = 10, 15, 18 and 25 wt%) alloys after long anneals (600–4000 h) was studied between 210 and 440 °C. The transition from incomplete to complete wetting of Al/Al grain boundaries (GBs) by the second solid phase Al₃Mg₂ has been observed. The portion of completely wetted GBs increases with increasing temperature beginning from T _wsmin = 220 °C. Above T _wsmax = 410 °C all Al/Al GBs are completely wetted by the Al₃Mg₂ phase.     

Construction of new morphotropic phase boundary in 0.94(K0.4?xNa0.6BaxNb1?xZrx)O3–0.06LiSbO3 lead-free piezoelectric ceramics

0.94(K_0.4−x Na_0.6Ba _x Nb_1−x Zr _x )O₃–0.06LiSbO₃ ceramics were prepared by conventional technique, and the effect of BaZrO₃ on the phase transitions, dielectric, ferroelectric, and piezoelectric properties of the ceramics were investigated. The phase transitions for the ceramics were determined by the temperature dependence of dielectric properties and X-ray diffraction patterns. BaZrO₃ changes the symmetry of the ceramics from tetragonal dominant phase with x = 0–0.06 to rhombohedral phase with x = 0.07–0.09. The phase transition near room temperature for the composition with x ~ 0.06 is different from previously reported phase transition between orthorhombic and tetragonal phases. It is suggested that a new morphotropic phase boundary (MPB) is constructed with both rhombohedral–orthorhombic and orthorhombic–tetragonal phase transitions near room temperature, and the enhanced piezoelectric properties (d ₃₃ = 344 pC/N and k _P = 32.4% with x = 0.06) are obtained. The results indicate that the construction of new MPB is of significance for further development of KNN-based ceramics.     

Relaxor behavior and ferroelectric properties of Na<Subscript>0.5</Subscript>Bi<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript>-K<Subscript>0.5</Subscript>Bi<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript>-KNbO<Subscript>3</Subscript> lead-free ceramics

Lead-free ferroelectric ceramics of (1−x) [0.88Na_0.5Bi_0.5TiO₃-0.12K_0.5Bi_0.5TiO₃]-x KNbO₃(x = 0, 0.02, 0.04, and 0.06) were prepared by the conventional ceramic fabrication technique. The crystal structure, dielectric properties and P-E hysteresis loops were investigated. XRD data showed that all compositions could form pure perovskite structure. Temperature dependence of dielectric constant ε _r and dissipation factor tanδ measurement between room temperature and 500^∘C revealed that the compounds experience phase transitions that from ferroelectric to anti-ferroelectric and anti-ferroelectric to paraelectric in the range of x = 0–0.04. The frequency dependent dielectric constant showed these compounds were relaxor ferroelectric. At low frequency and high temperature, dielectric constant and dissipation factor increased sharply attributed to the superparaelectric clusters after the KNbO₃ doped.