Effect of In, Ce and Bi dopings on sintering and dielectric properties of Ba(Zn1/3Nb2/3)O3 ceramics

In, Ce and Bi doped Ba(Zn_1/3Nb_2/3)O₃ (BZN) ceramics were prepared by conventional mixed oxide technique. In doping between 0.2 and 4.0 mol% increased the density of BZN at 1300 °C, Ce doping caused a decrease in density at 1250 °C. Levels of Bi₂O₃ up to 1.0 mol% had negative effect on densification, while high level doping could significantly improve the densification of the specimens. XRD of the samples indicated that In, Ce and Bi doping resulted in single phase formation at all concentrations, except 0.5 mol% Bi. SEM of Bi doped BZN indicated only single phase structure and Ce doping even at 0.2 mol% gave some secondary phases. In and Ce doping increased the dielectric constant from 41 to around 66 at 1 MHz. Bi doping decreased the dielectric constant to about 37 at 0.2 mol%, and then higher doping led to dielectric constant to increase to about 63.     

Effect of the Nb2O5 content on electrical properties of lead-free BaTiO3–Bi0.5Na0.5TiO3 ceramics

(1−x)BaTiO₃–xBi_0.5Na_0.5TiO₃ (BT–BNT) ceramics were prepared by the solid-state reaction method. With an increase of BNT content, both the Curie temperature and the room temperature resistivity increased. At 1 mol% BNT addition, the sample was not semiconducting, due to Bi₂O₃ volatilization resulting from the decomposition of pre-calcined BNT during sintering. Appropriate extra Nb₂O₅ doping in the raw materials could offset Bi₂O₃ volatilization and neutralize the redundant acceptor Na⁺ ions. When the extra Nb₂O₅ content was 0.6 mg, the sample room-temperature resistivity was 6.3×10³ Ω cm, with the Curie point about 135 °C and a high PTC effect of ∼3 orders of magnitude.     

Dielectric and ferroelectric properties of Ba0.8Sr0.2Ti1−5x/4NbxO3 ceramics

Ba_0.8Sr_0.2Ti_1−5x/4Nb_xO₃ ceramics, x = 0, 0.01, 0.05, 0.10, were fabricated by conventional solid-state reaction. With increasing niobium content the ferroelectric phase transition temperature decreases linearly, and the dispersivity of the transition increases. Niobium B-site decreases transition temperature more pronounced than Sr²⁺ at A-site. The heterovalent substitution of Nb⁵⁺ in low content causes local defect dipole, while more substitutions introduce disorder to disturb the long-range dipole correlation. Ba_0.8Sr_0.2Ti_1−0.5/4Nb_0.1O₃ ceramic shows weak ferroelectric loop at room temperature far from its transition temperature, 153 K.     

Synthesis and microstructure of (LaSr)MnO3 and (LaSr)FeO3 ceramics by a reaction-sintering process

(La_xSr_1−x)MnO₃ (LSMO) and (La_xSr_1−x)FeO₃ (LSFO) (x = 0.2–0.4) ceramics prepared by a simple and effective reaction-sintering process were investigated. Without any calcination involved, La₂O₃ and SrCO₃ were mixed with MnO₂ (LSMO) or Fe₂O₃ (LSFO) then pressed and sintered directly. LSMO and LSFO ceramics were obtained after 2 and 4 h sintering at 1350–1400 and 1200–1280 °C, respectively. Grain size decreased as La content increased in LSMO and LSFO ceramics.     

Preparation, characterization and electrical conductivity studies of NdYb1−xGdxZr2O7 (0 ≤ x ≤ 1.0) ceramics

Several compositions of NdYb_1−xGd_xZr₂O₇ (0 ≤ x ≤ 1.0) ceramics were prepared by pressureless-sintering method at 1973 K for 10 h in air. The relative density, microstructure and electrical conductivity of NdYb_1−xGd_xZr₂O₇ ceramics were analyzed by the Archimedes method, X-ray diffraction, scanning electron microscopy and impedance plots measurements. NdYb_1−xGd_xZr₂O₇ (0 ≤ x ≤ 0.3) ceramics have a single phase of defect fluorite-type structure, and NdYb_1−xGd_xZr₂O₇ (0.7 ≤ x ≤ 1.0) ceramics exhibit a single phase of pyrochlore-type structure; however, the NdYb_0.5Gd_0.5Zr₂O₇ composition shows mixed phases of both defect fluorite-type and pyrochlore-type structures. The measured values of the grain conductivity obey the Arrhenius relation. The grain conductivity of each composition in NdYb_1−xGd_xZr₂O₇ ceramics gradually increases with increasing temperature from 673 to 1173 K. NdYb_1−xGd_xZr₂O₇ ceramics are oxide-ion conductor in the oxygen partial pressure range of 1.0 × 10⁻⁴ to 1.0 atm at all test temperature levels. The highest grain conductivity value obtained in this work is 1.79 × 10⁻² S cm⁻¹ at 1173 K for NdYb_0.3Gd_0.7Zr₂O₇ composition.     

Effect of Ta2O5 in (Ca,Si, Ta)-doped TiO2 ceramic varistors

TiO₂ varistors doped with 0.2 mol% Ca, 0.4 mol% Si and different concentrations of Ta were obtained by ceramic sintering processing at 1350 °C. The effect of Ta on the microstructures, nonlinear electrical behavior and dielectric properties of the (Ca, Si, Ta)-doped TiO₂ ceramics were investigated. The ceramics have nonlinear coefficients of α = 3.0–5.0 and ultrahigh relative dielectric constants which is up to 10⁴. Experimental evidence shows that small quantities of Ta₂O₅ improve the nonlinear properties of the samples significantly. It was found that an optimal doping composition of 0.8 mol% Ta₂O₅ leads to a low breakdown voltage of 14.7 V/mm, a high nonlinear constant of 4.8 and an ultrahigh electrical permittivity of 5.0 × 10⁴ and tg δ = 0.66 (measured at 1 kHz), which is consistent with the highest and narrowest grain boundary barriers of the ceramics. In view of these electrical characteristics, the TiO₂–0.8 mol% Ta₂O₅ ceramic is a viable candidate for capacitor–varistor functional devices. The characteristics of the ceramics can be explained by the effect and the maximum of the substitution of Ta⁵⁺ for Ti⁴⁺.     

Dielectric properties of Sm, Nd and Fe doped Bi1.5Zn0.92Nb1.5O6.92 pyrochlores

New pyrochlore ceramics have been produced by doping Sm and Nd into the Bi site and Fe into the Nb site in the Bi_1.5Zn_0.92Nb_1.5O_6.92 (BZN) pyrochlore. Doped pyrochlore ceramics were produced by conventional solid state mixing of oxides at different doping levels using the compositions of Bi_1.5−xSm_xZn_0.92Nb_1.5O_6.92, Bi_1.5−xNd_xZn_0.92Nb_1.5O_6.92 and Bi_1.5Zn_0.92Nb_1.5−xFe_xO_6.92−x. The solubility limit of cations was determined as x = 0.13, 0.18 and 0.15 for Sm, Nd and Fe, respectively. While Sm and Nd increased the dielectric constant (?), Fe doping led a decrease in ?. Dielectric constant of Sm and Nd doped BZN increased to 199 at x = 0.13 (Sm) and to 219 at x = 0.18 (Nd). At low Fe dopings (x = 0.05), the dielectric constant of BZN increased to 242 but decreased to 211 at x = 0.15. The dielectric losses were lower for Sm and Nd dopings than Fe but in all cases it was lower than 0.006. The dielectric constant of Sm, Nd and Fe doped BZN ceramics was nearly independent of frequency within the frequency range between 1 kHz and 2 MHz, but decreased considerably with temperature between 20 and 200 °C. Temperature coefficient of Sm doped BZN (−354 ppm/°C) was lower than Nd and Fe doped BZN ceramics at solubility limits (−538 ppm/°C for Nd and −565 ppm/°C for Fe).     

Synthesis and properties of (BaxPb1−x)(Zn1/3Nb2/3)O3 relaxor ferroelectric ceramics using a reaction-sintering process

(Ba_xPb_1−x)(Zn_1/3Nb_2/3)O₃ (BPZN; x = 0.06–0.1) relaxor ferroelectric ceramics produced using a reaction-sintering process were investigated. Without any calcination involved, the mixture of raw materials was pressed and sintered directly. BPZN ceramics of 100% perovskite phase were obtained. Highly dense BPZN ceramics with a density higher than 98.5% of theoretical density could be obtained. Maximum dielectric constant K_max 13,500 (at 75 °C), 19,600 (at 50 °C) and 14,800 (at 28 °C) at 1 kHz could be obtained in 6BPZN, 8BPZN and 10BPZN, respectively. Dielectric maximum temperature (T_max) in BPZN ceramics via reaction-sintering process is lower than BPZN ceramics prepared via B-site precursor route.     

New binary (1 − x)Ba(Lu1/2Nb1/2)O3-xPbTiO3 solid solution with morphotropic phase boundary

A new ferroelectric solid solution of (1 − x)Ba(Lu_1/2Nb_1/2)O₃-xPbTiO₃ (BLN-PT) (0 ≤ x ≤ 1) has been synthesized by solid state reactions. Its structure and electric properties have been studied by X-ray diffraction and di-/ferro-electric measurements. Based on the investigation, a partial solid state phase diagram of the binary BLN-PT ceramics system has been established, which exhibits a morphotropic phase boundary (MPB) region in the composition range of 0.64 ≤ x ≤ 0.68. The Curie temperature is measured to be around 250 °C in the vicinity of the MPB region, which is much higher than that of PMNT or PZNT system. The dielectric behavior has been discussed based on Curie-Weiss Law and Lorentz-type quadratic relationship. With increasing PT content, a transformation from relaxor to ferroelectric phase has been demonstrated in the solid solution system.     

Fabrication and characterisation of Cr and Co doped Bi1.5Zn0.92Nb1.5O6.92 pyrochlores

Cr and Co doped Bi_1.5Zn_0.92Nb_1.5O_6.92 pyrochlore ceramics were produced by solid state mixing of oxides. Cr and Co were doped into the Nb and Nb-Zn sites considering the compositions of Bi_1.5Zn_0.92Nb_1.5−xCr_xO_6.92−x, (Bi_1.5Zn_0.46)(Zn_0.46−3x/6Nb_1.5−3x/5Cr_x)O_6.92−x/2 for Cr doping and Bi_1.5Zn_0.92Nb_1.5−3x/5Co_xO_6.92, (Bi_1.5Zn_0.46)(Zn_0.46−3x/6Nb_1.5−3x/5Co_x)O_6.92−x/2 for Co doping. The solubility limit of Cr in BZN was higher than that of Co and the solubility limit increased when doping was made both into Nb and Zn sites. The second phases appeared when x > 0.2 for Cr and x > 0.15 for Co doping into the Nb-Zn sites. Simultaneous Cr doping into the Nb- and Zn-sites of BZN pyrochlore gave higher dielectric constant than doping into the Nb-site of pyrochlore. However, Co doping into the Nb- and Zn-sites and only into the Nb-site of BZN gave identical dielectric results in the range of 202-218. The temperature coefficient of dielectric constant decreased with Cr doping and increased with Co doping.     

Structural and electrochemical behaviors of metastable Li2/3[Ni1/3Mn2/3]O2 modified by metal element substitution

Layered metastable lithium manganese oxides, Li_2/3[Ni_1/3−xMn_2/3−yM_x+y]O₂ (x = y = 1/36 for M = Al, Co, and Fe and x = 2/36, y = 0 for M = Mg) were prepared by the ion exchange of Li for Na in P2-Na_2/3[Ni_1/3−xMn_2/3−yM_x+y]O₂ precursors. The Al and Co doping produced the T^#2 structure with the space group Cmca. On the other hand, the Fe and Mg doped samples had the O6 structure with space group R-3m. Electron diffraction revealed the 1:2 type ordering within the Ni_1/3−xMn_2/3−yM_x+yO₂ slab. It was found that the stacking sequence and electrochemical performance of the Li cells containing T^#2-Li_2/3[Ni_1/3Mn_2/3]O₂ were affected by the doping with small amounts of Al, Co, Fe, and Mg. The discharge capacity of the Al doped sample was around 200 mAh g⁻¹ in the voltage range between 2.0 and 4.7 V at the current density of 14.4 mA g⁻¹ along with a good capacity retention. Moreover, for the Al and Co doped and undoped oxides, the irreversible phase transition of the T^#2 into the O2 structure was observed during the initial lithium deintercalation.     

The influence of pentavalent Nb substitution for Zr on electrical property of oxide-ion conductor Gd2Zr2O7

Gd₂(Zr_1−xNb_x)₂O_7+x (0 ≤ x ≤ 0.2) ceramics are prepared via the solid state reaction process at 1973 K for 10 h in air. Gd₂(Zr_1−xNb_x)₂O_7+x (x = 0.1, 0.2) ceramics exhibit an ordered pyrochlore-type structure, whereas Gd₂Zr₂O₇ has a defective fluorite-type structure. The electrical property of Gd₂(Zr_1−xNb_x)₂O_7+x ceramics is investigated by electrochemical impedance spectroscopy over a frequency range of 10 Hz to 8 MHz from 623 to 923 K. The electrical conductivity obeys the Arrhenius equation. The grain conductivity of Gd₂(Zr_1−xNb_x)₂O_7+x ceramics varies with doping different Nb contents, and exhibits a maximum at the Nb content of x = 0.1 in the temperature range of 623-923 K. The conductivity in hydrogen atmosphere is a little bit higher than in air in the temperature range of 723-923 K, which indicates that the doping of Zr⁴⁺ by Nb⁵⁺ can increase the proton-type conduction and reduce the oxide-ionic conduction. The conduction of Gd₂(Zr_1−xNb_x)₂O_7+x is not a pure oxide-ionic conductor.     

Thermoelectric properties of ytterbium interstitial doped Sr0.7Ba0.3YbxNb2O6-δ ceramics

The thermoelectric properties of interstitial doped Sr_0.7Ba_0.3Yb_xNb₂O_6-δ ceramics were investigated in the temperature range from 323 K to 1073 K. The ytterbium interstitial doping contributes to increasing the power factor (PF) by improving the electrical conductivity. The PF values of Sr_0.7Ba_0.3Yb_0.1Nb₂O_6-δ are comparable to those of the substitution doped sample with twice the doping content. This can be ascribed to the large electronic charge of the dopants relative to the occupied site in the interstitial case. The lattice thermal conductivity of Sr_0.7Ba_0.3Yb_xNb₂O_6-δshows a glass-like behavior due to lattice disorder. The disorder degree deepens as the doping content increases, resulting in plateau values of the lattice thermal conductivity at high temperatures in the Sr_0.7Ba_0.3Yb_0.05Nb₂O_6-δ and Sr_0.7Ba_0.3Yb_0.1Nb₂O_6-δ samples.     

Synthesis of Zn3Nb2O8 ceramics using a simple and effective process

Synthesis of Zn₃Nb₂O₈ ceramics using a simple and effective reaction-sintering process was investigated. The mixture of ZnO and Nb₂O₅ was pressed and sintered directly without any prior calcination. Single-phase Zn₃Nb₂O₈ ceramics could be obtained. Density of these ceramics increased with soaking time and sintering temperature. A maximum density 5.72 g/cm³ (99.7% of the theoretical density) was found for pellets sintered at 1170 °C for 2 h. Pores were not found and grain sizes >20 μm were observed in pellets sintered at 1170 °C. Abnormal grain growth occurred and grains >50 μm could be seen in Zn₃Nb₂O₈ ceramics sintered at 1200 °C for 2 h and 1200 °C for 4 h. Reaction-sintering process is then a simple and effective method to produce Zn₃Nb₂O₈ ceramics for applications in microwave dielectric resonators.     

Effect of holmium substitution on structural and electrical properties of barium zirconate titanate ferroelectric ceramics

The holmium substituted Ba_1−3x/2Ho_xZr_0.025Ti_0.975O₃ (x=0.01, 0.02, 0.025, 0.05) compositions were synthesized by the solid state reaction technique. The synthesized specimens were characterized for their structural and electrical properties using X-ray diffractometer, scanning electron microscopy, impedance analyzer and loop tracer. Phase analysis shows the formation of secondary phase Ho₂Ti₂O₇ for Ho≥2.5 mol% substitution. The microstructural investigation shows that the holmium substitution significantly reduces the grain size. The substitution of holmium increases the Curie temperature for x≤0.02 whereas Curie temperature decreases for x≥0.025. The maximum dielectric constant at transition temperature is observed for x=0.02. The solubility limit is 2 mol% and for x≥0.025 some of the holmium atoms enter B-sites and forms the secondary phase. An increase is observed in the coercive field of the specimens with the increasing holmium content.     

Investigation of ternary system Pb(Sn,Ti)O3-Pb(Mg1/3Nb2/3)O3 with morphotropic phase boundary compositions

(1 − x)Pb(Sn_1−yTi_y)O₃-xPb(Mg_1/3Nb_2/3)O₃ (x = 0.1-0.4, y = 0.45-0.65) ternary system was prepared using two-step columbite precursor method. Phase structure of the synthesized ceramics was studied by using X-ray powder diffraction and the morphotropic phase boundary (MPB) curve of the ternary system was confirmed. The isothermal map of Curie temperature (T_C) in the phase diagram was obtained based on the dielectric-temperature measurements. The coercive field E_C and internal bias field E_i were found to increase with increasing PT content, while decrease with increasing PMN content. The optimum properties were achieved in the MPB composition 0.8Pb(Sn_0.45Ti_0.55)O₃-0.2Pb(Mg_1/3Nb_2/3)O₃, with dielectric permittivity ?_r, piezoelectric coefficient d₃₃, planar electromechanical coupling k_p, mechanical quality factor Q_m and T_C of being on the order of 3040, 530pC/N, 55.5%, 320 and 190 °C, respectively, exhibiting potential usage for high power application.     

Synthesis and study on phase diagram of 1-10 mol% SnO2-doped Bi2O3

Bi₂O₃ materials doped with various SnO₂ concentrations were prepared by colloidal process and solid state reactions to achieve high density and uniform microstructure. Thermal behavior, and crystalline phases of the SnO₂-doped Bi₂O₃ (BSO) samples were investigated by differential thermal analyses, X-ray diffraction, and scanning electron microscopy. A new phase diagram of Bi₂O₃-(1-10 mol%) SnO₂ system is proposed in this study. The results show that 1 mol% SnO₂ doped concentration is totally dissolved in Bi₂O₃ without the existence of any impurity phases as compared to higher doping SnO₂ concentration.     

Compositional dependence of phase structure and electrical properties in (K0.50Na0.50)0.97Bi0.01(Nb1−xZrx)O3 lead-free ceramics

(K_0.50Na_0.50)_0.97Bi_0.01(Nb_1-xZr_x)O₃ (KNBNZ) lead-free ceramics were prepared by the conventional solid-state sintering process. Their phase structure is dependent on the Zr content in the investigated range, and the ceramics endure a phase transition from pseudocubic to orthorhombic with increasing Zr content. Improved piezoelectric properties have been observed when the poling temperature is located at ~100 °C because of the coexistence of orthorhombic and tetragonal phases. Their dielectric and piezoelectric properties were enhanced by doping Zr, the ceramic with x=0.02 showing optimal electrical properties, i.e., d₃₃~161 pC/N, k_p~0.41, Q_m~81, T_c~370 °C, and T_o−t~130 °C. These results show that the KNBNZ ceramic is a promising lead-free piezoelectric material.     

Combustion synthesis of (Ti1−xNbx)2AlC solid solutions from elemental and Nb2O5/Al4C3-containing powder compacts

Preparation of the (Ti_1−xNb_x)₂AlC solid solution (formed from the M_n+1AX_n or MAX carbides, where n = 1, 2, or 3, M is an early transition metal, A is an A-group element, and X is C) with x = 0.2-0.8 was investigated by self-propagating high-temperature synthesis (SHS). Nearly single-phase (Ti,Nb)₂AlC was produced through direct combustion of constituent elements. Due to the decrease of reaction exothermicity, the combustion temperature and reaction front velocity decreased with increasing Nb content of (Ti_1−xNb_x)₂AlC formed from the elemental powder compacts. In addition, the samples composed of Ti, Al, Nb₂O₅, and Al₄C₃ were adopted for the in situ formation of Al₂O₃-added (Ti,Nb)₂AlC. The SHS process of the Nb₂O₅/Al₄C₃-containing sample involved aluminothermic reduction of Nb₂O₅, which not only enhanced the reaction exothermicity but also facilitated the evolution of (Ti,Nb)₂AlC. Based upon the XRD analysis, two intermediates, TiC and Nb₂Al, were detected in the (Ti,Nb)₂AlC/Al₂O₃ composite and their amounts were reduced by increasing the extent of thermite reduction involved in the SHS process. The laminated microstructure characteristic of the MAX carbide was observed for both monolithic and Al₂O₃-added (Ti,Nb)₂AlC solid solutions synthesized in this study.     

Synthesis and tunable thermal expansion properties of Sc2−xYxW3O12 solid solutions

A new series of rare earth solid solutions Sc_2−xY_xW₃O₁₂ was successfully synthesized by the conventional solid-state method. Effects of doping ion yttrium on the crystal structure, morphology and thermal expansion property of as-prepared Sc_2−xY_xW₃O₁₂ ceramics were investigated by X-ray diffraction (XRD), thermogravimetric analysis (TG), field emission scanning electron microscope (FE-SEM) and thermal mechanical analyzer (TMA). Results indicate that the obtained Sc_2−xY_xW₃O₁₂ samples with Y doping of 0≤x≤0.5 are in the form of orthorhombic Sc₂W₃O₁₂-structure and show negative thermal expansion (NTE) from room temperature to 600 °C; while as-synthesized materials with Y doping of 1.5≤x≤2 take hygroscopic Y₂W₃O₁₂·nH₂O-structure at room temperature and exhibit NTE only after losing water molecules. It is suggested that the obvious difference in crystal structure leads to different thermal expansion behaviors in Sc_2−xY_xW₃O₁₂. Thus it is proposed that thermal expansion properties of Sc_2−xY_xW₃O₁₂ can be adjusted by the employment of Y dopant; the obtained Sc_1.5Y_0.5W₃O₁₂ ceramic shows almost zero thermal expansion and its average linear thermal expansion coefficient is −0.00683×10⁻⁶ °C⁻¹ in the 25–250 °C range.