Synthesis and Characterization of Ti0.33Ta0.33Nb0.33C and Ti0.33Ta0.33Nb0.33CxN1-x Whiskers

Ta_0.33Ti_0.33Nb_0.33C and Ta_0.33Ti_0.33Nb_0.33C _x N_{1− x} whiskers were synthesized via a carbothermal vapor-liquid-solid growth mechanism in the temperature range 900°-1450°C in Ar or N₂. The optimum temperature was 1250°C. Whiskers were obtained in a yield of 70-90 vol%. The whiskers were 0.5–1 µm in diameter and 10–30 µm in length. The starting materials that produced the highest whisker yield were: TiO₂, Ta₂O₅, Nb₂O₅, C, Ni, and NaCl. C was added to reduce the oxides, and Ni to catalyze whisker growth. NaCl was used as a source of Cl for vapor-phase transportation of Ta and Nb oxochlorides and Ti chlorides to the catalyst. The catalyst metal was recycled several times during the synthesis and was transported as NiCl₂( g ) according to thermodynamic calculations. The rate of formation and the chemical composition of the whiskers depended on the synthesis temperature, the choice of catalyst, and the atmosphere. At low temperatures, the whiskers were enriched in Nb and Ta, whereas the Ti content increased with increased synthesis temperature.     

Crystallization of Monoclinic 2TiO2·5Nb2O5

Monoclinic 2TiO₂·5Nb₂O₅ crystallizes at 810° to 835°C from an amorphous material prepared by the simultaneous hydrolysis of titanium and niobium alkoxides. Crystallization isotherms are described by the contracting cube equation 1 − (1 − f)¹¹³= k(t − t₀); the activation energy is 315 kJ·mol⁻¹. Monoclinic 2TiO₂·5Nb₂O₅ transforms to the orthorhombic modification at ∼1200° to 1300°C.     

Effect of V2O5 Doping on the Sintering and Dielectric Properties of M-Phase Li1+x−yNb1−x−3yTix+4yO3 Ceramics

The effect of the addition of V₂O₅ on the structure, sintering and dielectric properties of M -phase (Li_{1+ x − y} Nb_{1− x −3 y} Ti _x +4 _y )O₃ ceramics has been investigated. Homogeneous substitution of V⁵⁺ for Nb⁵⁺ was obtained in LiNb_{0.6(1− x )}V_{0.6 x} Ti_0.5O₃ for x ≤ 0.02. The addition of V₂O₅ led to a large reduction in the sintering temperature and samples with x = 0.02 could be fully densified at 900°C. The substitution of vanadia had a relatively minor adverse effect on the microwave dielectric properties of the M -phase system and the x = 0.02 ceramics had [alt epsilon]_r= 66, Q × f = 3800 at 5.6 GHz, and τ_f= 11 ppm/°C. Preliminary investigations suggest that silver metallization does not diffuse into the V₂O₅-doped M -phase ceramics at 900°C, making these materials potential candidates for low-temperature cofired ceramic (LTCC) applications.     

A- and B Site Cosubstituted Ba6−3xSm8+2xTi18O54Microwave Dielectric Ceramics

Tin (Sn) substitution into the B-site and Nd/Sn cosubstitution into the A- and B-sites were investigated in a Ba _{6−3 x} Sm_{8+2 x} Ti₁₈O₅₄solid solution ( x = 2/3). A small amount of tin substitution for titanium improved the temperature coefficient of resonant frequency (τ_f) but led to a decrease of the relative dielectric constant (ɛ) and the quality factor ( Qf ). The Ba_{6−3 x} Sm_{8+2 x} (Ti_{1− z} Sn_z)₁₈O₅₄-based tungsten-bronze phase became unstable for compositions with a tin content of ≥10 mol%, where BaSm₂O₄and Sm₂(Sn,Ti)₂O₇appeared, and finally, these phases became the major phases. On the other hand, Nd/Sn cosubstitution led to a good combination of high ɛ, high Qf , and near-zero τ_f. Excellent microwave dielectric properties were achieved in Ba_{6−3 x} (Sm_{1− y} Nd _y )_{8+2 x} (Ti_{1− z} Sn _z )₁₈O₅₄ceramics with y = 0.8 and z = 0.05 sintered at 1360°C for 3 h: ɛ= 82, Qf = 10 000 GHz, and calculated τ_f=+17 ppm/°C. The tolerance factor and electronegativity difference exhibited important effects on the microwave dielectric properties, especially the Qf value. A large tolerance factor and high electronegativity difference generally led to a higher Qf value.     

Structural and Dielectric Properties of Ag(Nb0.8Ta0.2)1−xSbxO3 (x≤0.08) Ceramics

Sb₂O₅ were selected to substitute (Nb_0.8Ta_0.2)₂O₅ and the effects of Sb substitution on the dielectric properties of Ag(Nb_0.8Ta_0.2)O₃ ceramics were studied. The perovskite Ag(Nb_0.8Ta_0.2)_{1− x} Sb _x O₃ ceramics showed no obvious change with x value being no more than 0.08, and the pseudoperovskite unit cell parameters a = c , b and monoclinic angle β decrease with Sb concentration increasing. The dielectric properties of Ag(Nb_0.8Ta_0.2)_{1− x} Sb _x O₃ ceramics were found to be affected greatly by the substitution of Sb for Nb/Ta. The ɛ value of Ag(Nb_0.8Ta_0.2)_{1− x} Sb _x O₃ ceramics sintered at their densified temperature increased from 480 to 825 with x from 0 to 0.08, the tan δ value decreased sharply from 0.0065 to 0.0023 (at 1 MHz) with x increasing from 0 to 0.04, and then kept a stable lower tan δ value ∼0.0024 with x to 0.08. The temperature coefficient of capacitance values continuously decreased from a positive value of 1450 ppm/°C for x =0 to a negative value of −38.52 ppm/°C for x =0.08.     

Electric and Dielectric Properties of Nb-Doped CaCu3Ti4O12 Ceramics

Pure and Nb-substituted CaCu₃Ti_{4− x} Nb _x O_{12+ x /2} (CCTO, x =0, 0.02, 0.1, 0.2, 0.4) ceramics were prepared by a conventional solid-state sintering, and their electric and dielectric properties were investigated using an impedance analyzer. A single-phase CCTO was obtained up to x =0.2 Nb substitution and the lattice parameter increased with Nb substitution concentration. While the grain size decreased with Nb substitution, the resistivity of the grain boundary decreased. The dielectric constant increased with Nb substitution, and the highest value of ∼420 000 was observed in the x =0.2 Nb-substituted specimen at 10 kHz. The obtained electric and dielectric properties in Nb-substituted CCTO were discussed in terms of the internal barrier layer capacitor model, particularly focusing on a ratio of thickness of the grain boundary region to grain size.     

High Q Microwave Dielectric Ceramics in (Ni1−x Znx)Nb2O6 System

(Ni_{1− x} Zn _x )Nb₂O₆, 0≤ x ≤1.0, ceramics with >97% density were prepared by a conventional solid-state reaction, followed by sintering at 1200°–1300°C (depending on the value of x ). The XRD patterns of the sintered samples (0≤ x ≤1.0) revealed single-phase formation with a columbite ( Pbcn ) structure. The unit cell volume slightly increased with increasing Zn content ( x ). All the compositions showed high electrical resistivity (ρ_dc=1.6±0.3 × 10¹¹Ω·cm). The microwave (4–5 GHz) dielectric properties of (Ni_{1− x} Zn _x )Nb₂O₆ ceramics exhibited a significant dependence on the Zn content and to some extent on the morphology of the grains. As x was increased from 0 to 1, the average grain size monotonically increased from 7.6 to 21.2 μm and the microwave dielectric constant (ɛ'_r) increased from 23.6 to 26.1, while the quality factors ( Q _u× f ) increased from 18 900 to 103 730 GHz and the temperature coefficient of resonant frequency (τ_f) increased from −62 to −73 ppm/°C. In the present work, we report the highest observed values of Q _u× f =103 730 GHz, and ɛ'_r=26.1 for the ZnNb₂O₆-sintered ceramics.     

Tin Substitution for Titanium in Ba6−3xNd8+2xTi18O54 Microwave Dielectric Ceramics

Tin (Sn) substitution for titanium (Ti) was investigated in Ba_{6−3 x} Nd_{8+2 x} Ti₁₈O₅₄ ( x =1/2, 2/3, and 3/4) ceramics. A small amount ( z <0.1) of Sn substitution resulted in Ba_{6−3 x} Nd_{8+2 x} (Ti_{1− z} Sn _z )₁₈O₅₄ solid solutions, and some secondary phases were observed with increasing Sn content. A small amount of Sn substitution improved the Q f value significantly, while, due to the formation of secondary phases, the Q f value degraded sharply for larger Sn content. The relative dielectric constant (ɛ_r) decreased with increasing Sn-content, while the temperature coefficient of resonant frequency (τ_f) generally decreased, although an obvious fluctuation was observed for x =3/4.     

Dielectric Properties of the Fluorite-like Bi2O3–Nb2O5 Solid Solution and the Tetragonal Bi3NbO7

Our analysis of the microwave dielectric properties of the δ-Bi₂O₃–Nb₂O₅ solid solution (δ-BN_ss) showed a continuous increase in permittivity and dielectric losses with an increasing concentration of Nb₂O₅. The only discontinuity was found for the temperature coefficient of resonant frequency, which is negative throughout the entire homogeneity range but reaches a minimum value for the sample with 20 mol% Nb₂O₅. At the same composition there is a discontinuity in the grain size of the δ-BN_ss ceramics. For the sample containing 25 mol% Nb₂O₅ two structural modifications were observed. A single-phase tetragonal Bi₃NbO₇, in the literature referred to as a Type-III phase, is formed in a very narrow temperature range from 850° to 880°C. A synthesis performed below or above this temperature range resulted in the formation of the end member of the δ-BN_ss homogeneity range. Compared with the δ-BN_ss the Bi₃NbO₇ ceramics exhibit lower microwave dielectric losses, an increased conductivity, and a positive temperature coefficient of resonant frequency.     

Factors Determining Grain Orientation in Bismuth Sodium Potassium Titanate–Lead Zirconate Titanate Solid Solutions Made by the Reactive Templated Grain Growth Method

Grain-oriented Bi_0.5(Na_0.85K_0.15)_0.5TiO₃-Pb(Zr_{1− x} Ti _x )O₃ (BNKT-PZT) ceramics were prepared via the reactive templated grain growth method, using platelike Bi₄Ti₃O₁₂ particles. Factors that determine the degree of orientation were examined. Prereacted PZT gave a larger degree of orientation than PZT raw materials (PbO, ZrO₂, and TiO₂) in the 75BNKT-25PZT ( x = 0.5) system. Increases in the titanium concentration in the PZT of the 75BNKT-25PZT system and in the BNKT concentration in the y BNKT-(100 − y )PZT ( x = 0.5) system increased the degree of orientation. The direction of material transport between BNKT and PZT was important to obtain ceramics with a large degree of orientation.     

Microwave Dielectric Properties and Far-Infrared Reflectivity Characteristics of the CaTiO3–Li(1/2)−3xSm(1/2)+xTiO3Ceramics

Microwave dielectric properties and far-infrared reflectivity spectra of the 0.3CaTiO₃–0.7Li_{(1/2)−3 x} Sm_{(1/2)+ x} TiO₃ ceramics were investigated as a function of Sm³⁺ substitution (0.0 ≤ x ≤ 0.12). The dielectric constant decreased as the Sm³⁺ substitution increased. The Q × f value increased, up to a solid-solution limit at x = 0.11, because of the change of vibration modes between the A-site cation and the TiO₆ octahedron, and then decreased because of the formation of a secondary phase (Sm₂Ti₂O₇). On the analysis of the far-infrared reflectivity spectra, in the 50–4000 cm⁻¹ range, the change of the dielectric loss and dielectric constant could be explained by the intrinsic factor.     

Grain Growth of Microtubes During Sintering in Semiconducting Ba3Ce3−xBixTi5Nb5O30 (x=0.5, 1.0, 2.0, and 3.0) Ceramics

Well grown microtubes of Ba₃Ce_{3− x} Bi _x Ti₅Nb₅O₃₀ ( x =0.5, 1.0, 2.0, and 3.0) have been observed in the sintered ceramics. The microtubes have octagonal shapes at the cylindrical ends, hollow cores with diameters of 2–3 μm, lengths over a few tens of micrometers and wall thickness of 0.5–1.0 μm. In morphology, the tubes are very straight and are apparently welded to each other on the sides. The tubular growth as well as increase of wall thickness is observed to be more with greater Bi content, but when Ce is fully substituted by Bi, the tubes have grown into solid rods without hollowness. The growth spirals at the ends of tubes as seen by scanning electron microscope indicate that growth takes place in the longitudinal direction of the tubes. Observation of growth spirals on grains of a sintered ceramic is unusual. In fact, such features are usually seen in liquid phase growth and, therefore, we propose here grain growth in partially fluid conditions. The phase identification by X-ray diffraction shows that the compound has a tetragonal tungsten bronze type structure. The resistivity measurements above room temperature indicate that these compounds are semiconductors and both resistivity and activation energy increase with Bi content.     

Effects of La on Dielectric and Piezoelectric Properties of Pb1−xLa2x/3(Nb0.95Ti0.0625)2 O6 Ceramics

Ceramics with the chemical compositions of Pb_{1− x} La_{2 x /3}(Nb_0.95Ti_0.0625)₂O₆ (0≤ x ≤0.060) (PLTN) were prepared by the conventional solid-state reaction method. X-ray diffraction analysis indicated that Ti and La doping not only decreased the rhombohedral–tetragonal phase transformation temperature, but also stabilized the orthorhombic phase of PLTN ceramics. All ceramics sintered at 1190°–1250°C had shown the pure orthorhombic ferroelectric phase. La doping suppresses grain growth and inhibits the formation of pores and cracks, resulting in an increase in relative density up to 97%. The amount of La doping to PLTN ceramics obviously affect ceramics' piezoelectric constant ( d ₃₃) and dielectric loss (tanδ). The sample with x =0.015 possesses high Curie temperature ( T _c=560°C), low dielectric loss (tanδ=0.0054), and excellent piezoelectric constant ( d ₃₃=92 pC/N), presenting a high potential to be used in high-temperature applications as piezoelectric transducers.     

Bismuth/Samarium Cosubstituted Ba6−3xNd8+2xTi18O54 Microwave Dielectric Ceramics

Modification of the microwave dielectric properties in Ba_{6−3 x} Nd_{8+2 x} Ti₁₈O₅₄ ( x = 0.5) solid solutions by Bi/Sm cosubstitution for Nd was investigated. A large increase in the dielectric constant and near-zero temperature coefficient combined with high Qf values were obtained in modified Ba_{6−3 x} Nd_{8+2 x} Ti₁₈O₅₄ solid solutions where an enlarged solid solution limit of Bi in Ba_{6−3 x} Nd_{8+2 x} Ti₁₈O₅₄ was observed. Excellent microwave dielectric characteristics (ɛ= 105, Qf = 4110 GHz, and very low τ_f) were achieved in the composition Ba_{6−3 x} (Nd_0.7Bi_0.18Sm_0.12)_{8+2 x} Ti₁₈O₅₄.     

High-Dielectric-Constant and Low-Loss Microwave Dielectric in the (1−x)Nd(Zn1/2Ti1/2)O3–xSrTiO3 System with a Zero Temperature Coefficient of Resonant Frequency

High-dielectric-constant and low-loss ceramics in the (1− x )Nd(Zn_1/2Ti_1/2)O₃– x SrTiO₃ system have been prepared by the conventional mixed-oxide route and their microwave dielectric properties have been investigated. A two-phase system was confirmed by the X-ray diffraction patterns, the energy-dispersive X-ray spectrometer analysis, and the measured lattice parameters. Addition of SrTiO₃, having a much smaller grain size in comparison with that of Nd(Zn_1/2Ti_1/2)O₃, could effectively hold back abnormal grain growth in the Nd(Zn_1/2Ti_1/2)O₃ matrix. Evaporation of Zn at high temperatures caused an increase in the dielectric loss of the system. The temperature coefficient of resonant frequency increases with increasing SrTiO₃ content and tunes through zero at x =0.52. Specimens with x =0.52 possessed an excellent combination of microwave dielectric properties: ɛ_r∼54.2, Q × f ∼84 000 GHz, and τ_f∼0 ppm/°C. It is proposed as a suitable candidate material for today's 3G passive components and small-sized GPS patch antennas.     

Structure-Property Phase Diagram of BaZrxTi1−xO3 System

In the course of searching environmental friendly lead-free relaxor ferroelectrics a complete phase diagram of barium zirconate titanate, Ba(Zr _x Ti_{1− x} )O₃ system with compositions 0.00≤ x ≤1.00 has been developed based on their dielectric behavior. It has been shown that BaZr _x Ti_{1− x} O₃ system depending on the composition, successively depicts the properties extending from simple dielectric (pure BaZrO₃) to polar cluster dielectric, relaxor ferroelectric, second order like diffuse phase transition, ferroelectric with pinched phase transitions and then to a proper ferroelectric (pure BaTiO₃). A comprehensive structure–property correlation of BaZr _x Ti_{1− x} O₃ ceramics has been studied to understand the various ferroelectric phenomena in the whole phase diagram.     

Effect of Nb2O5/ZnO Addition on Microwave Properties of (Zr0.8Sn0.2)TiO4 Ceramics

The effects of Nb₂O₅ and ZnO addition on the dielectric properties, especially the quality factor, of (Zr_0.8Sn_0.2)TiO₄ (ZST) ceramics were investigated in terms of the sintered density acquired by the zinc. For ZST ceramics with 2 mol% added ZnO, the relative density of the samples decreased with >0.5 mol% addition of Nb₂O₅. On the other hand, for samples with 6 mol% added ZnO, the relative density remained >97%, even when the amount of Nb₂O₅ was increased to 2.0 mol%. When >0.5 mol% Nb₂O₅ was added, both the quality factor and the dielectric constant exhibited similar trends with sintered density. The ZST ceramics with 6 mol% added ZnO, especially, still manifested a quality factor >40 000 and a dielectric constant of 37, even when the amount of Nb₂O₅ was increased, values that are not explainable by the previously suggested electronic defect model.     

Microstructure, Density, and Dielectric Properties of Lead-Free (K0.44Na0.52Li0.04)(Nb0.96−xTaxSb0.04)O3 Piezoelectric Ceramics

Lead-free (K_0.44Na_0.52Li_0.04) (Nb_0.96−xTa_xSb_0.04)O₃ piezoelectric ceramics were prepared by the conventional solid-state sintering method. The grain growth of the ceramics was inhibited and the relative density was improved with Ta substituting for Nb. Increasing x led to different variations of dielectric properties before and after poling, and prevented the occurrence of orthorhombic–tetragonal phase transition (at T _{o − t} ). All the ceramics show an intermediate relaxor-like behavior between normal and ideal relaxor ferroelectrics. Significantly enhanced dielectric and piezoelectric properties were obtained in the ceramics with x =0.20. The ceramics are very promising lead-free materials for electromechanical device applications.     

Electrical Characterization of Sr0.97Ti1−xFexO3−δ by Complex Impedance Spectroscopy: I, Materials with Low Iron Contents

Impedance spectroscopy has been used to characterize Sr_0.97Ti_{1− x} Fe _x O_3−δ materials. This technique was suitable to evaluate the grain and grain-boundary conductivities of samples with ≤5% iron. The role of grain boundaries was changed by sintering in different conditions, and this was interpreted by accounting for microstructural differences. The corresponding relaxation times were also used to confirm the interpretation of complex impedance spectra. The values of grain-boundary conductivity and the corresponding values of activation energy obtained by this method were relatively close to predictions obtained on assuming a p → n inversion at the grain-boundary core, as proposed by Waser and coauthors.1–3 The temperature dependence of relaxation times confirmed this finding.