Dielectric Relaxations in Bi2O3–Nb2O5–NiO Cubic Pyrochlores

Dielectric properties of pyrochlores compositions from Bi₂O₃–Nb₂O₅–NiO system were analyzed. The dielectric properties are dominated with a low-temperature relaxation that is typical for Bi-pyrochlores. A vast pyrochlore homogeneity range that exists in this system allowed to correlate characteristics of the observed relaxations with a compositional variations within the A₂O'- and B₂O₆ pyrochlore sublattice. It was possible to make a distinction between different influences of the two sublattices, which can be satisfactorily described by the existing relaxation model for Bi_3/2ZnNb_3/2O₇. A new relaxor-like room temperature relaxation was found for Bi_1.6Ni_0.57Nb_1.43O_6.55.     

Subsolidus Phase Equilibria in the System Bi2O3-TiO2-Nb2O5

The subsolidus phase equilibria in the system Bi₂O₃-TiO₂-Nb₂O₅ at 1100°C were determined by solid-state reaction techniques and X-ray powder diffraction methods. The system was found to contain 4 ternary compounds, i.e. Bi₃TiNbO₉, Bi₇Ti₄NbO₂₁, a cubic pyrochlore solid solution having a compositional range of 3Bi₂O₃· x TiO₂ (7– x )Nb₂O₅ where x ranges from 2.3 to 6.75, and an unidentified phase, 4Bi₂O₃·11TiO₂·5Nb₂O₅.     

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.     

Pyrochlore Phase Formation in the System Bi2O3–ZnO–Ta2O5

The subsolidus phase diagram of the system Bi₂O₃–ZnO–Ta₂O₅ in the region of the cubic pyrochlore phase has been determined at 1050°C. This phase forms a solid solution area that includes the ideal composition P, Bi₃Zn₂Ta₃O₁₄; possible solid solution mechanisms are proposed, supported by density measurements of Zn-deficient solid solutions. The general formula of the solid solutions is Bi_{3+ y} Zn_{2− x} Ta_{3− y} O_{14− x − y} , based on the creation of Zn²⁺, O²⁻ vacancies in Zn-deficient compositions and a variable Bi/Ta ratio.     

Phase Formation and Crystal-Structure Determination in the Bi2O3–TiO2–TeO2 System Prepared in an Oxygen Atmosphere

Using X-ray diffraction analysis and scanning electron microscopy it was revealed that in an atmosphere of flowing oxygen in the temperature range 700°–800°C, three new compounds are formed in the Bi₂O₃–TiO₂–TeO₂ pseudoternary system. These compounds are Bi₂Ti₃TeO₁₂, Bi₂TiTeO₈, and Bi₆Ti₅TeO₂₂, and all the compounds include Te⁶⁺. All three crystal structures were solved and refined using X-ray powder diffraction data. Based on the results of the phase formation, a solid-state compatibility diagram is proposed.     

Pyrochlore Phases in the System ZnO–Bi2O3–Sb2O5: I. Stoichiometries and Phase Equilibria

Two cubic pyrochlore phases exist in the system ZnO–Bi₂O₃–Sb₂O₅. Neither has the supposed "ideal" stoichiometry, Zn₂Bi₃Sb₃O₁₄. One, P ₁, is a solid solution phase, Zn_{2+ x} Bi_{2.96−( x − y )}Sb_{3.04− y} O_14.04+δ where 0< x <0.13(1), 0< y <0.017(2) and a =10.4285(9)−10.451(1) Å. The other, P ₂, is a line phase, Zn₂Bi_3.08Sb_2.92O_13.92 with a =10.462(2) Å. Subsolidus phase relations at 950°C involving phases P ₁ and P ₂ in the ZnO–Bi₂O₃–Sb₂O₅ phase diagram have been determined.     

Nanopowder Preparation and Dielectric Properties of a Bi2O3–Nb2O5 Binary System Prepared by the High-Energy Ball-Milling Method

The high-energy ball-milling (HEM) method was used to synthesize the compositions of BiNbO₄, Bi₅Nb₃O₁₅, and Bi₃NbO₇ in a Bi₂O₃–Nb₂O₅ binary system. Reagent Bi₂O₃ and Nb₂O₅ were chosen as the starting materials. The X-ray diffraction patterns of the three compositions milled for different times were studied. Only the cubic Bi₃NbO₇ phase, Nb₂O₅, and amorphous matters were observed in powders after being milled for 10 h. After heating at proper temperatures the amorphous matters disappeared and the proleptic phases of BiNbO₄ and Bi₅Nb₃O₁₅ could be obtained. The Scherrer formula was used to calculate the crystal size and the results of nanopowders are between 10 and 20 nm. The scanning electron microscopy photos of Bi₃NbO₇ powders showed drastic aggregation, and the particle size was about 100 nm. The dielectric properties of ceramics sintered from the nanopowders prepared by HEM at 100–1 MHz and the microwave region were measured. Bi₃NbO₇ ceramics showed a good microwave permittivity ɛ_r of about 80 and a Q × f of about 300 at 5 GHz. The triclinic phase of BiNbO₄ ceramics reached its best properties with ɛ_r=24 and Q × f =14 000 GHz at about 8 GHz.     

Phase Relations in the Pyrochlore-Rich Part of the Bi2O3−TiO2−Nd2O3 System

In this study we used solid-state synthesis to determine the phase relations in the pyrochlore-rich part of the Bi₂O₃−TiO₂−Nd₂O₃ system at 1100°C. The samples were analyzed using X-ray powder diffraction and scanning electron microscopy with energy- and wavelength-dispersive spectroscopy. A single-phase pyrochlore ceramic was obtained with the addition of 4.5 mol% of Nd₂O₃. We determined the solubility limits for the three solid solutions: (i) the pyrochlore solid solution Bi_{(1.6–1.08 x )}Nd _x Ti₂O_{(6.4+0.3 x )}, where 0.25< x <0.96; (ii) the solid solution Bi_{4− x} Nd _x Ti₃O₁₂, where 0< x <2.6; and (iii) the Nd_{2− x} Bi _x Ti₂O₇ solid solution, where 0< x <0.35. The determined phase relations in the pyrochlore-rich part are presented in a partial phase diagram of the Bi₂O₃−TiO₂−Nd₂O₃ system in air at 1100°C.     

Crystalline Phases and YAG Laser-Induced Crystallization in Sm2O3–Bi2O3–B2O3 Glasses

The glass formation region, crystalline phases, second harmonic (SH) generation, and Nd:yttrium aluminum garnet (YAG) laser-induced crystallization in the Sm₂O₃–Bi₂O₃–B₂O₃ system were clarified. The crystalline phases of Bi₄B₂O₉, Bi₃B₅O₁₂, BiBO₃, Sm _x Bi_{1− x} BO₃, and SmB₃O₆ were formed through the usual crystallization in an electric furnace. The crystallized glasses consisting of BiBO₃ and Sm _x Bi_{1− x} BO₃ showed SH generations. The formation of the nonlinear optical BiB₃O₆ phase was not confirmed. The formation (writing) region of crystal lines consisting of Sm _x Bi_{1− x} BO₃ by YAG laser irradiation was determined, in which Sm₂O₃ contents were∼10 mol%. The present study demonstrates that Sm₂O₃–Bi₂O₃–B₂O₃ glasses are promising materials for optical functional applications.     

Phase Relations in the System Al2O3–B2O3–Nd2O3

The phase relations at a temperature below "subsolidus" in the system Al₂O₃–B₂O₃–Nd₂O₃ are reported. Specimens were prepared from various compositions of Al₂O₃, B₂O₃, and Nd₂O₃ of purity 99.5%, 99.99%, and 99.9%, respectively, and fired at 1100°C. There are six binary compounds and one ternary compound in this system. The ternary compound, NdAl₃(BO₃)₄ (NAB), has a phase transition at 950°C ± 15°C. The high-temperature form of NAB has a second harmonic generation (SHG) efficiency of KH₂PO₄ (KDP) of the order of magnitude of the form which has been used as a good self-activated laser material, and the low-temperature form of NAB has no SHG efficiency.     

Phase Relations and Dielectric Properties in the Bi2O3–ZnO–Ta2O5 System

Dielectric properties and phase formation of Bi-based pyrochlore ceramics were evaluated for the Bi₂O₃–ZnO–Ta₂O₅ system. The compositional range r Bi₂(Zn_1/3Ta_2/3)₂O₇· (1− r )(Bi_3/2Zn_1/2)(Zn_1/2Ta_3/2)O₇ (0 ≤ r ≤ 1) in Bi₂O₃–ZnO–Ta₂O₅ was investigated to determine the relative solubility of BZT cubic (α-BZT, r = 0) and the pseudo-orthorhombic (β-BZT, r = 1) end members. It was found that extrinsic factors, such as kinetically limited phase formation and bismuth loss, contribute to apparent phase boundaries in addition to thermodynamic stability of each phase. Considering this, the locations of true phase boundaries were r < 0.30 and r ≥ 0.74 for α and β phases, respectively. Dielectric constants between 58 and 80 and low dielectric loss (tan δ < 0.003) were measured for the complete compositional range. The temperature coefficient of capacitance was controlled by composition, which was found to be <30 ppm/°C at the edge of β-phase solid solution. In addition to the excellent dielectric properties these materials can be sintered at low temperatures, which make Bi-based pyrochlores promising candidates for high-frequency electronic applications.     

Investigation on the Microstructure and Dielectric Properties of BaTiO3–Nb2O5–Fe2O3 Materials Doped with La2O3

The influence of La₂O₃ doped on the microstructure and dielectric properties, including the phase structure, temperature dependence of permittivity, and the hysteresis loop of BaTiO₃–Nb₂O₅–Fe₂O₃ (BTNF) materials has been investigated in X-ray diffraction, SEM, and LCR analyzer, respectively. Experiments revealed that incorporation of proper content of La₂O₃ basically soluted in the lattice of BaTiO₃ and can control the grain-growth, reduce the dielectric loss of the BTNF materials. The development of microstructure promoted by the additives can result in the improvement of the dielectric constant. When the doping concentration of La₂O₃ was 3.846 wt%, the relative dielectric constant of the sample sintered at 1280°C only for 2 h could reach 4308, and improve the dielectric-temperature characteristics markedly. As a result, a novel Y5P can be achieved in the BTNF ceramics, which is very promising for practical use in Y5P multilayer ceramic capacitors.     

Phase Diagram of the Bi4Ti3O12–BaTiO3–(Na1/2Bi1/2)TiO3 System

The phases formed in the ternary system (Na_1/2Bi_1/2)TiO₃–Bi₄Ti₃O₁₂–BaTiO₃ (NBT–BTO–BT) were studied at 1150°C in air. A very accurate picture of the ternary phase diagram was obtained examining almost 90 different compositions, exploiting low-angle XRD analyses to study the layer compounds. New compounds with five perovskite blocks ( m =5) were discovered deep in the phase diagram. No compounds with m >5 were found. It was also established that pure perovskite compounds can be obtained only at compositions very close to the NBT–BT line. The relationships between the phases is discussed and it is hypothesized that the number of perovskite blocks in the system is determined by charged sites being created by the progressive substitution of Bi³⁺ in the A site of the perovskite blocks of BTO with the A cations of the perovskite end-member.     

Sintering Characteristics in the BaTiO3–Nb2O5–Co3O4 Ternary System: II, Stability of So-called "Core–Shell" Structure

The sintering characteristics and the reaction of additives with BaTiO₃ (BT) were examined for two materials having Nb-rich composition (Comp.N) and Co-rich composition (Comp.C) to elucidate the relation between the stability of the core–shell microstructure and the Nb/Co ratio in the BT–Nb₂O₅–Co₃O₄ system. TEM observation revealed that the concentration gradient of Nb and Co existed in the shell region although Nb and Co macroscopically distributed homogeneously. X-ray diffraction analysis showed that the shell formation preceded the densification and completed at about 1280°C for both Comp.N and Comp.C as determined from differential scanning calorimetry. A diffusion couple experiment disclosed that Co had a larger diffusivity than Nb and that the diffusion of Co was suppressed when the sample was codoped with a sufficient amount of Nb. On the basis of these experimental results, new mechanisms of the formation and collapse of core–shell structure in the BT–Nb₂O₅–Co₃O₄ system were proposed.     

Electrical Properties of Sr0.7Ba0.3TiO3 Ceramics Doped with Nb2O5, 3Li2O · 2SiO2, and Bi2O3

The electrical properties of Sr_0.5Ba_0.3TiO₃ in the presence of Nb₂O₅ as a donor, 3Li₂O · 2SiO₂ as a sintering agent, and Bi₂O₃ as a dopant have been studied. When the compositions of the ceramics were 1 mol Sr_0.7Ba_0.3TiO₃+ 0.5 mol% Nb₂O₅+ 2 mol% 3Li₂O · 2SiO₂+ 0.2 mol% Bi₂O₃, the ceramics were sintered at 1100°C and exhibited the following characteristics: apparent dielectric constant ɛ, 25000; loss factor tan δ, 2%; insulating resistivity ρ_j, 10¹⁰Ω· cm; variation of dielectric constant with temperature Δɛ/ɛ (−25° to +85°C), +10%, −14%. ɛ and tan δ show only small changes with frequency. The study shows this ceramic can be used in multilayer technology.     

Subsolidus Phase Relationships in the System Fe2O3–Al2O3–TiO2 between 1000° and 1300°C

Subsolidus phase equilibria in the system Fe₂O₃–Al₂O₃–TiO₂ were investigated between 1000° and 1300°C. Quenched samples were examined using powder X-ray diffraction and electron probe microanalytical methods. The main features of the phase relations were: (a) the presence of an M₃O₅ solid solution series between end members Fe₂TiO₅ and Al₂TiO₅, (b) a miscibility gap along the Fe₂O₃–Al₂O₃ binary, (c) an α-M₂O₃( ss ) ternary solid-solution region based on mutual solubility between Fe₂O₃, Al₂O₃, and TiO₂, and (d) an extensive three-phase region characterized by the assemblage M₃O₅+α-M₂O₃( ss ) + Cor( ss ). A comparison of results with previously established phase relations for the Fe₂O₃–Al₂O₃–TiO₂ system shows considerable discrepancy.     

Temperature/Composition Phase Diagram of the System Bi2O3-PbO

The Bi₂O₃-PbO phase diagram was determined using differential thermal analysis and both room- and high-temperature X-ray powder diffraction. The phase diagram contains a single eutectic at 73 mol% PbO and 635°C. A body-centered cubic solid solution exists above ∼600°C within a composition range of 30 to 65 mol% PbO. The compounds α-Bi₂O₃, σ5-Bi₂O₃, and γ-PbO (litharge) have wide solubility ranges. Four compounds, 6Bi₂O₃·PbO, 3Bi₂O₃·2PbO, 4Bi₂O₃,5PbO, and Bi₂O₃·3PbO, are formed in this system and the previously unreported X-ray diffraction patterns of the latter three compounds are reported. Diffraction patterns for some of these mixed oxides have been observed in ZnO-based varistors grown using Bi₂O₃ and PbO as sintering aids.     

Microwave Dielectric Properties of 5Li2O–0.583Nb2O5–3.248TiO2 Ceramics with V2O5

The effects of V₂O₅ addition on the sintering behavior, microstructure, and the microwave dielectric properties of 5Li₂O–0.583Nb₂O₅–3.248TiO₂ (LNT) ceramics have been investigated. With addition of low-level doping of V₂O₅ (≤2 wt%), the sintering temperature of the LNT ceramics could be lowered down to around 920°C due to the liquid phase effect. A secondary phase was observed at the level of 2 wt% V₂O₅ addition. The addition of V₂O₅ does not induce much degradation in the microwave dielectric properties but lowers the τ_f value to near zero. Typically, the excellent microwave dielectric properties of ɛ_r=21.5, Q × f =32 938 GHz, and τ_f=6.1 ppm/°C could be obtained for the 1 wt% V₂O₅-doped sample sintered at 920°C, which is promising for application of the multilayer microwave devices using Ag as an internal electrode.     

Phase Relationships in the Si3N4–SiO2–Lu2O3 System

Phase relationships in the Si₃N₄–SiO₂–Lu₂O₃ system were investigated at 1850°C in 1 MPa N₂. Only J-phase, Lu₄Si₂O₇N₂ (monoclinic, space group P 2₁/ c , a = 0.74235(8) nm, b = 1.02649(10) nm, c = 1.06595(12) nm, and β= 109.793(6)°) exists as a lutetium silicon oxynitride phase in the Si₃N₄–SiO₂–Lu₂O₃ system. The Si₃N₄/Lu₂O₃ ratio is 1, corresponding to the M-phase composition, resulted in a mixture of Lu–J-phase, β-Si₃N₄, and a new phase of Lu₃Si₅ON₉, having orthorhombic symmetry, space group Pbcm (No. 57), with a = 0.49361(5) nm, b = 1.60622(16) nm, and c = 1.05143(11) nm. The new phase is best represented in the new Si₃N₄–LuN–Lu₂O₃ system. The phase diagram suggests that Lu₄Si₂O₇N₂ is an excellent grain-boundary phase of silicon nitride ceramics for high-temperature applications.     

Single-Calcination Synthesis of Pyrochlore-Free 0.9Pb(Mg1/3Nb2/3)O3–0.1PbTiO3 and Pb(Mg1/3Nb2/3)O3 Ceramics Using a Coating Method

A coating approach for synthesizing 0.9Pb(Mg_1/3Nb_2/3)O₃–0.1PbTiO₃ (0.9PMN–0.1PT) and PMN using a single calcination step was demonstrated. The pyrochlore phase was prevented by coating Mg(OH)₂ on Nb₂O₅ particles. Coating of Mg(OH)₂ on Nb₂O₅ was done by precipitating Mg(OH)₂ in an aqueous Nb₂O₅ suspension at pH 10. The coating was confirmed using optical micrographs and zeta-potential measurements. A single calcination treatment of the Mg(OH)₂-coated Nb₂O₅ particles mixed with appropriate amounts of PbO and PbTiO₃ powders at 900°C for 2 h produced pyrochlore-free perovskite 0.9PMN–0.1PT and PMN powders. The elimination of the pyrochlore phase was attributed to the separation of PbO and Nb₂O₅ by the Mg(OH)₂ coating. The Mg(OH)₂ coating on the Nb₂O₅ improved the mixing of Mg(OH)₂ and Nb₂O₅ and decreased the temperature for complete columbite conversion to ∼850°C. The pyrochlore-free perovskite 0.9PMN–0.1PT powders were sintered to 97% density at 1150°C. The sintered 0.9PMN–0.1PT ceramics exhibited a dielectric constant maximum of ∼24 660 at 45°C at a frequency of 1 kHz.