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 .     

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.     

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.     

Preparation of Pb(Zr,Ti)O<Subscript>3</Subscript>–Pb(Mg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)O<Subscript>3</Subscript> piezoelectric ceramics by dry–dry method

Ternary perovskite ceramics of Pb[(Zr_0.5Ti_0.5)_0.8−x (Mg_1/3Nb_2/3)_0.2+x]_0.98Nb_0.02O_3.01 (PZTMN, x = −0.075, −0.05, −0.025, 0, 0.025, 0.05, and 0.075 ), are synthesized via dry–dry method. B-site precursors of PZTMN ([(Zr_0.5Ti_0.5)_0.8−x (Mg_1/3Nb_2/3)_0.2+x ]_0.98Nb_0.02O_2.01, ZTMN) can be synthesized via a two-step solid state reaction method. The first calcination temperature is 1,300 °C, and the second is not higher than 1,360 °C. Incorporation of magnesium and niobium ions promotes the formation of the single phase solid solution with ZrTiO₄ structure. Single phase perovskite PZTMN is formed at 780 °C, much lower than that in conventional process. Dense ceramics can be sintered at about 1,260 °C with dielectric and piezoelectric properties comparable to that of wet–dry method and higher than that of conventional method. It seems that B-site precursor method is cost effective in preparation of ternary piezoelectric ceramics.     

Phase transition and electrical properties of LiNbO<Subscript>3</Subscript>-modified K<Subscript>0.49</Subscript>Na<Subscript>0.51</Subscript>NbO<Subscript>3</Subscript> lead-free piezoceramics

Lead-free (1-x)K_0.49Na_0.51NbO₃-xLiNbO₃ (KNN-LN, x = 0 ~ 0.08) piezoelectric ceramics were prepared by the conventional solid-state sintering method. The effects of LiNbO₃ doping amount x on the phase transition behavior and the electrical properties of KNN-LN ceramics were investigated. By increasing LiNbO₃ doping amount x, the orthorhombic-tetragonal polymorphic phase transition (PPT) temperature (T _o–t) of KNN-LN ceramics shifted downwards, however, the Curie temperature (T _c) slightly moved upwards. The room temperature phase structure thus changed from orthorhombic to tetragonal across the compositions with 0.05 ≤ x ≤ 0.06, named as PPT region. The composition with x = 0.06 in the tetragonal side of PPT region exhibited optimized electrical properties of d ₃₃ = 246pC/N, k _p = 41.6%, ε _r = 679, tgδ = 0.028, and Q _m = 52. In addition to its very high T _c = 467 °C, this ceramic can be an excellent candidate for replacing the lead-based piezoceramics in high temperature applications.     

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.     

Effects of Nd-substitution on microstructures and dielectric characteristics of CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> ceramics

Ca_1−3x/2Nd _x Cu₃Ti₄O₁₂ (x = 0, 0.1, 0.2) ceramics were prepared by a solid state reaction process, and single-phased structures were obtained for all the compositions. The dielectric characteristics of pure and Nd-substituted CaCu₃Ti₄O₁₂ ceramics were investigated together with the microstructures. The mixed-valent structures of Cu⁺/Cu²⁺ and Ti³⁺/Ti⁴⁺ in the present ceramics were confirmed by X-ray photoelectron analysis. The dielectric relaxation in the low temperature range was examined in detail and the variation of dielectric constant and dielectric loss was attributed to the modification mixed-valent structures.     

Investigation of Ba<Subscript>2-x</Subscript>Sr<Subscript>x</Subscript>TiO<Subscript>4</Subscript>: Structural aspects and dielectric properties

Investigation of solid solution of barium-strontium orthotitanates of the type, Ba_2-x Sr _x TiO₄ (0 ≤x≤ 2), show that pure phases exist only for the end members, Ba₂TiO₄ and Sr₂TiO₄, crystallizing in the β-K₂SO₄ and K₂NiF₄ structures, respectively. The intermediate compositions (till≥ 1) lead to a biphasic mixture of two Ba₂TiO₄-type phases (probably through a spinodal decomposition) with decreasing lattice parameters, indicating Sr-substitution in both the phases. Forx > 1, Sr₂TiO₄ along with a secondary phase is obtained. The dielectric constant and dielectric loss were found to decrease with Sr substitution till the nominal composition ofx = 1. However, pure Sr₂TiO₄ shows higher dielectric constant compared to the solid solution composition. Sr₂TiO₄ shows very high temperature stability of the dielectric constant.     

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.     

Microwave dielectric properties of Ba<Subscript>5</Subscript>Nb<Subscript>4</Subscript>O<Subscript>15</Subscript> ceramic by molten salt method

Ba₅Nb₄O₁₅ powders were synthesized by molten-salt method in NaCl–KCl flux at a low temperature of 650–900 °C for 2 h, which is lower than that of the conventional solid-state reaction. This simple process involved mixing of the raw materials and salts in a certain proportion. Subsequent calcination of the mixtures led to Ba₅Nb₄O₁₅ powders at 650–900 °C. XRD and SEM techniques were used to characterize the phase and morphology of the fabricated Ba₅Nb₄O₁₅ powders, respectively. After sintering at 1,300 °C for 2 h, the densified Ba₅Nb₄O₁₅ ceramics with good microwave dielectric properties of ε_r = 39.2, Q × f approximated as 27,200 GHz and τ _f  = 72 ppm/°C have been obtained.     

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.     

Dielectric properties and relaxation of Bi<Subscript>0·5</Subscript>Na<Subscript>0·5</Subscript>TiO<Subscript>3</Subscript>-BaNb<Subscript>2</Subscript>O<Subscript>6</Subscript> lead-free ceramics

A new member of lead-free piezoelectric ceramics of the BNT-based group, (1 − x)Bi_0·5Na_0·5TiO_3−x BaNb₂O₆, was prepared by conventional solid state reaction and its dielectric properties and relaxation was investigated. X-ray diffraction showed that BaNb2O6 diffused into the lattice of Bi_0·5Na_0·5TiO₃ to form a solid solution with perovskite-type structure. A diffuse character was proved by the linear fitting of the modified Curie-Weiss law. The temperature dependence of dielectric constant at different frequencies revealed that the solid solution exhibited relaxor characteristics different from classic relaxor ferroelectrics. The samples with x = 0·002 and 0·006 exhibited obvious relaxor characteristics near the low temperature dielectric abnormal peak, T _f, and the samples with x = 0·010 and 0·014 exhibited obvious relaxor characteristics between room temperature and T _f. The mechanism of relaxor behaviour was also discussed according to the macro-domain to micro-domain transition theory.     

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.     

Low-temperature sintering and microwave dielectric properties of TiO<Subscript>2</Subscript>-based LTCC materials

Temperature stable high-K LTCC material was prepared. The influence of fabrication process on the crystalline phases, microstructures and microwave dielectric properties of TiO₂-Bi₂O₃-CuO ceramics were investigated. The crystalline phases and microstructures of TiO₂-Bi₂O₃-CuO ceramics were investigated by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy. It was found that rutile TiO₂ phase and Bi₂Ti₄O₁₁ phase co-existed in the TiO₂-Bi₂O₃-CuO ceramics. Separate TiO₂ grains and Bi₂Ti₄O₁₁ grains distributed uniformly in the ceramic matrix. The composition 0.92TiO₂-0.08Bi₂Ti₄O₁₁ with 2 wt% CuO addition that was sintered at 900 °C for 2 h showed high dielectric constant (ε_r ~ 81), high quality factor (Q × f ~ 3,500 GHz) and near zero temperature coefficient of resonant frequency (τ_f ~ −5.1 ppm/°C), meanwhile the compatibility test showed that it could co-fire with silver electrode. The processing-microstructure-property interrelationship was also studied.     

Low-firing Li<Subscript>2</Subscript>ZnTi<Subscript>3</Subscript>O<Subscript>8</Subscript> microwave dielectric ceramics with BaCu(B<Subscript>2</Subscript>O<Subscript>5</Subscript>) additive

Phase purity, microstructure, sinterability and microwave dielectric properties of BaCu(B₂O₅)-added Li₂ZnTi₃O₈ ceramics and their cofireability with Ag electrode were investigated. A small amount of BaCu (B₂O₅) can effectively reduce the sintering temperature from 1075°C to 925°C, and it does not induce much degradation of the microwave dielectric properties. Microwave dielectric properties of ε _r = 23·1, Q × f = 22,732 GHz and τ _f = − 17·6 ppm/°C were obtained for Li₂ZnTi₃O₈ ceramic with 1·5 wt% BaCu(B₂O₅) sintered at 925°C for 4 h. The Li₂ZnTi₃O₈ +BCB ceramics can be compatible with Ag electrode, which makes it a promising microwave dielectric material for low-temperature co-fired ceramic technology application.     

Low-field Magnetoresistance,Specific Heat and Magnetocaloric Effect in Sr Substituted Pr<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript>

We investigate the effect of ionic size variation on the electrical and thermodynamic properties in a series of Pr_0.7Ca_0.3−x Sr _x MnO₃ (PCSMO) samples. The increase in Sr content results in an increase of the unit cell volume, as a bigger Sr²⁺ ion replaces the smaller Ca²⁺ ions. Resistivity measurements show that the increase in the Sr content also results in the induction of a metal–insulator transition (T _MI), which increases with increasing Sr content. The activation energy (E _a), calculated from the resistivity data, decreases with increasing Sr content confirming the metallic character. The effect of the magnetic field on resistivity and specific heat has also been studied.     

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.     

Synthesis and properties of AgTi<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>-based NASICON-type phosphates doped with Nb<Superscript>5+</Superscript>, Zr<Superscript>4+</Superscript>, and Ga<Superscript>3+</Superscript>

We have synthesized materials based on a silver titanium phosphate with partial substitution of tri-, tetra-, or pentavalent cations for titanium: Ag_1±x Ti_2−x M _x (PO₄)₃ (M = Nb⁵⁺, Ga³⁺) and AgTi_2−x Zr _x (PO₄)₃. The materials have been characterized by X-ray diffraction and impedance spectroscopy and have been shown to have small thermal expansion coefficients. Their ionic conductivity has been determined. Silver ions in these materials are difficult to replace with protons.     

Effect of secondary heat treatment on the spectroscopic properties of Na<Subscript>2</Subscript>O-Nb<Subscript>2</Subscript>O<Subscript>5</Subscript>-P<Subscript>2</Subscript>O<Subscript>5</Subscript> glasses

We have studied the optical absorption and luminescence spectra of 45Na₂O · xNb₂O₅ · (55 − x)P₂O₅ glasses containing 5, 10, 20, 25, 30, and 35 mol % Nb₂O₅. The results indicate that the absorption band around 26000 cm⁻¹, responsible for the yellow color of the glasses, is due to the [Nb^(5+)--O⁻] center and disappears upon secondary heat treatment. Heat treatment of europium-doped glasses increases the concentration of Eu³⁺ centers in an asymmetric environment, which is accompanied by an increase in luminescence efficiency. The reason for this is that the Eu³⁺ ions are located outside the niobate subsystem of the glass matrix. The europium in the glasses studied acts as a protector ion.