Chemical Compatibility between Silver Electrodes and Low-Firing Binary-Oxide Compounds: Conceptual Study

The chemical compatibility of silver electrodes and low-firing ceramics has been considered, in terms of the existence of a tie line between silver (and/or Ag₂O) and the binary oxide compound in the corresponding ternary phase diagram. The probability of the existence of the tie line is related to the conditions in the subordinated silver-based isothermal binary systems. Greater probabilities have been calculated for the systems with fewer silver-based binary compounds. Based on the concepts that have been developed, several silver-based isothermal binary systems have been investigated to identify the oxides suitable for the development of low-temperature cofired ceramics. The developed concept has been tested by investigating the phase relations in the Bi₂O₃–Nb₂O₅ and Bi₂O₃–V₂O₅ ternary systems with silver. X-ray and microstructural investigations of bismuth niobates and bismuth vanadates reveal that, as a result of the inertness of Bi₂O₃ and the reactivity of Nb₂O₅ and V₂O₅ toward silver, compounds that are rich in niobium or vanadium react with silver to form ternary Nb/V-Bi-Ag oxide compounds, whereas for compounds that are rich in bismuth, tie lines to silver and Ag₂O do exist.     

Structure and Properties of Silver Borate Glasses

Clear glasses form in the system Ag₂O-B₂O₃ up to about 35 mol% (65 wt%) Ag₂O. Infrared absorption, thermal expansion, and density data indicated an analogy to the Na₂O-B₂O₃ system. Pentaborate-triborate group pairs appear to be formed on addition of Ag₂O to B₂O₃ up to 20 mol% Ag₂O and diborate groups from 20 to 33 mol% Ag₂O. This interpretation is supported by the comparison of the infrared absorption spectra of quenched and crystallized glasses. One crystallization product, Ag₂O-4B₂O₃, was identified previously. A new compound starts to appear at 28 mol% Ag₂O. The theory that silver is generally present as a network modifier like sodium was substantiated by the comparison of the molar volume of sodium and silver borate glasses. Above 27 mol% Ag₂O some atomic silver is assumed to be present; below 15 mol%, exploratory studies indicate a two-phase structure within an immiscibility gap. A low-temperature internal friction peak in the glasses up to 28 mol% Ag₂O corresponds to the alkali peak in other glasses; a high temperature peak appearing in the 34 mol% Ag₂O glass is associated with the appearance of nonbridging oxygen in the system.     

Relaxation Dynamics in Superionic Glass Nanocomposites

We have studied the structural and electrical properties of several glass nanocomposite systems. The formation of nanowires, nanoparticles, and nanorods embedded in heat-treated x Ag₂O–(1− x )MoO₃ ( x =0.2, 0.3, and 0.4) glass matrix and the growth of α-AgI nanocrystals and ZnO nanoparticles in the 0.7AgI–0.15Ag₂O–0.15( x ZnO–(1− x )MoO₃) ( x =0.05, 0.20, and 0.30) glass matrix and α-AgI nanocrystals in the x AgI–(1− x )(0.5Ag₂O–0.5MoO₃) ( x =0.70, 0.75, and 0.80) glass matrices have been confirmed from X-ray diffraction and high-resolution transmission electron microscopic studies. We have investigated the electrical conductivity of these glass nanocomposites in a wide frequency and temperature range, and correlated them with their structures. We have analyzed the ac conductivity using a power law model. It has been observed that mobile ion concentration is independent of composition as well as temperature. It was also observed that the conductivity depends on the size of the nanoparticles grown in the host glass matrix. The scaling of the conductivity spectra reveals that the relaxation dynamics of Ag⁺ ions is independent of temperature and is also independent of composition at higher frequencies.     

Conductivity Activation Energy Relations in High-Sodium-Content Borate and Aluminoborate Glasses

Glasses were prepared having compositions x Na₂O + (1 − x )B₂O₃ and x Na₂O + (1 − x )( n B₂O₃·Al₂O₃) where n = 4.0 and 6.7, and their conductivities and activation energies were determined up to 70 mol% Na₂O. Below 50 mol% Na₂O, the conductivity activation energy decreases with Na₂O content, which is attributed to a decreasing cation jump distance. Above 50 mol% Na₂O, however, the activation energy increases with Na₂O content. These measurements are the first to clearly show a conductivity activation energy increasing with alkali oxide content, an effect which is not yet explained by any of the current models of ionic conduction in glass.     

Glass Transition and Infrared Spectra of Low-Alkali, Anhydrous Lithium Phosphate Glasses

Anhydrous glasses in the series x Li₂O + (1 - x )P₂O₅ have been prepared and characterized in the range 0 ≤ x ≤ 0.5. FT-IR spectroscopy and glass transition temperature measurements have been used to explore the structure and a key physical property of the low-alkali phosphate glasses. The structure of v -P₂O₅ is proposed to consist of a 3-D network of trigonally connected tetrahedra decorated with a P=O unit. Contrary to what has long been proposed for these glasses, the addition of alkali degrades the 3-D network through the generation of nonbridging oxygens rather than strengthen the network through the proposed alkali ion bridging. The T _g of v -P₂O₅ is ∼653 K and decreases some 130 K with the addition of 10 mol% Li₂O. T _g then reaches a minimum value at 20 mol% Li₂O and increases with further Li₂O additions. The increase in T _g, even though the fraction of nonbridging oxygens is still increasing, is interpreted in terms of an increasing entanglement of long-chain PO₂ groups in the glass. Such a structural transition from a 3-D network of interconnected PO₄ groups for P₂O₅ to a 1-D chain structure for LiPO₃ is one of the first examples of the importance of intermediate-range order in governing the properties of glass.     

Low-Temperature ac Conductivity of Mixed Mobile Ion Germanate Glasses

The conductivity of alternating current in the x Ag₂O: (1 - x )Rb₂O:4Geo₂ glass series has been measured between 5.0 K and room temperature where x is varied from 0.0 to 1.0. The typical low-temperature conductivity region is observed below 200 K. The temperature and frequency dependence of conductivity in this region can be explained by the thermal excitation of asymmetric double well potential configurations. Further, the classical mixed mobile ion effect is essentially absent, which shows that the effect is associated with the independent movement of the mobile ions.     

Sintering of a Crystallizable CaO-B2O3-SiO2 Glass with Silver

Effects of Ag addition on sintering of a crystallizable CaO-B₂O₃-SiO₂ glass have been investigated at 700°–900°C in different atmospheres. With Ag content increasing in the range of 1–10 vol%, the softening point, the densification, the onset crystallization temperature, and the total amount of crystalline phase formed of the crystallizable glass are reduced when fired in air. A bloating phenomenon is observed when the crystallizable CaO-B₂O₃-SiO₂ glass doped with 1–10 vol% Ag is fired at 700°–900°C for 1–4 h. Fired in N₂ or N₂+ 1% H₂, however, the above phenomena disappear completely. It is thus believed that the diffusion of Ag into the crystallizable glass, which is caused by the oxidation of Ag in air, is the root cause for the above results observed.     

Spectroscopic Properties of Er3+-Doped Na2O–La2O3–Al2O3–SiO2 Glasses

Er³⁺-doped sodium lanthanum aluminosilicate glasses with compositions of (90− x )(0.7SiO₂·0.3Al₂O₃)· x Na₂O·8.2La₂O₃· 0.6Er₂O₃·0.2Yb₂O₃·1Sb₂O₃ (in mol%) ( x = 12, 20, 24, 40, 60 mol%) were prepared and their spectroscopic properties were investigated. Judd–Ofelt analysis was used to calculate spectroscopic properties of all glasses. The Judd–Ofelt intensity parameter Ω _t ( t = 2, 4, 6) decreases with increasing Na₂O. Ω₂ decreases rapidly with increasing Na₂O while Ω₄ and Ω₆ decrease slowly. Both the fluorescent lifetime and the radiative transition rate increase with increasing Na₂O. Fluorescence spectra of the ⁴ I _13/2 to ⁴ I _15/2 transition have been measured and the change with Na₂O content is discussed. It is found that the full width at half-maximum decreases with increasing Na₂O.     

Ag2O-Doped Bi2Sr2Ca1Cu2Ox Superconductors Prepared via Melt-Quenching

Ag₂O-doped superconducting Bi₂Sr₂Ca₁Cu₂O _x ceramics were prepared by a melt-quenching–reheating method. It is found that the Ag₂O-doped, as-cast specimens exhibit superconductivity ( T _c= around 80 K) by heat treatment at temperatures around 800°C even in an evacuated and sealed silica glass tube, while the undoped specimens do not and vaporize by the corresponding heat treatment. Conversion of the Ag₂O-doped, as-cast specimens into superconducting ceramics when heated in an evacuated vessel is explained in terms of the oxygen donor of Ag₂O in the specimen. This finding enables us to fabricate a desired shape of superconducting Bi₂Sr₂Ca₁Cu₂O _x ceramics sealed in metals or glasses. The addition of Ag₂O to Bi₂Sr₂Ca₁Cu₂O _x melt, however, had deleterious influences on the superconducting properties ( T _c and J _c) of the resultant ceramics when obtained by heat treatment in air.     

Effect of Oxidation State of Iron on Phase Separation in Sodium Silicate Glasses

The effect of the oxidation state of iron on the phase separation of x Na₂O·(100 – x )SiO₂ glasses, x = 18.56 and 13, containing 0.5 mol% Fe₂O₃ was studied. The oxidation state of iron in the glasses was varied by changing the melting conditions, such as melting temperature and melting atmosphere. The oxidation states of the iron ion were determined using colorimetric and UV–VIS–NIR spectrophotometric methods, and a comparison was made between the results obtained using these two methods. Immiscibility temperatures of the glasses were determined using opalescence and clearing methods. The immiscibility temperature of the sodium silicate binary glasses decreased ∼25°C with the addition of 0.5 mol% Fe₂O₃. The immiscibility temperature of the doped glasses increased slightly with increased concentration of Fe²⁺ ion in the glass. The prediction of immiscibility tendency on the addition of a minor amount of third component was made using models proposed by Tomozawa and Obara and Nakagawa and Izumitani. The Tomozawa and Obara model showed good agreement with measured immiscibility values.     

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.     

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.     

Mixed-Alkali Effect in Low-Alkali Germanate Glasses

Direct-current conductivities of mixed-alkali Na₂O-K₂O-GeO₂ glasses with ∼0.02 mol% total alkali were investigated. No minima in conductivity isotherms were indicated as K₂O is substituted for Na₂O. However, a conductivity-suppressing effect with the-addition of K₂O to Na₂0-GeO₂ glass was demonstrated, from the comparison with the conductivity ofNa₂O-GeO₂ glasses of varied sodium content (0.0079∼0.30 mol% Na₂O.     

Effects of Silver on Barium Titanate as a Function of Stoichiometry

This work examines the effects of Ag on stoichiometric and nonstoichiometric BaTiO₃ in terms of the unit cell dimensions, the polycrystalline microstructure, and the dielectric properties. Stoichiometric BaTiO₃ and compositions with 0.5 mol% TiO₂ excess and 0.5 mol% BaO excess were prepared via solid-state synthesis with varying amounts of Ag up to 0.3 mol%. Experimental results indicate that stoichiometry plays a significant role in the solubility of Ag and its effects on the physical properties. Overall, the solubility of Ag was negligible in stoichiometric BaTiO₃. However, compositions with excess TiO₂ stabilized the solubility of Ag as evidenced from changes in the unit cell dimensions and dielectric properties. Based on these data, it is proposed that Ag occupies the A-site in the perovskite structure with an upper limit of Ag solubility of 0.06 mol% Ag in BaTiO₃ with 0.5 mol% excess TiO₂. Dielectric measurements showed that Ag concentrations approaching 0.3 mol% gave rise to an increase in the space charge effect, especially at temperatures above T _C. In both nonstoichiometric compositions, the presence of a liquid Ag phase during thermal processing was found to affect microstructural development and sintering.     

Phase Formation and Dielectric Characterization of the Bi2O3–TeO2 System Prepared in an Oxygen Atmosphere

Solid-state synthesis of compositions from the Bi₂O₃–TeO₂ system show that, under an oxygen atmosphere, Te⁴⁺ oxidizes to Te⁶⁺ and yields four room-temperature stable compounds: Bi₂Te₂O₈, Bi₂TeO₆, Bi₆Te₂O₁₅, and new a compound with the nominal composition 7Bi₂O₃·2TeO₂. Dense ceramics can be prepared from all these compounds by sintering between 650° and 800°C under an oxygen atmosphere. The permittivity of these compounds varies from ∼30 to ∼54, the Q × f value from 1.100 to 41.000 GHz (∼5 GHz), and the temperature coefficient of resonant frequency from −43 to −144 ppm/K. Bi₆Te₂O₁₅ and 7Bi₂O₃·2TeO₂ do not react with silver, and, therefore, they have the potential to be used for applications in low-temperature cofired ceramic (LTCC) technology.     

Glass-Forming Ability and Crystallization Tendency Evaluated by the DTA Method in the Na2O–B2O3–Al2O3 System

In order to evaluate the crystallization tendency of glasses, the ratio of the crystallization temperature to the liquidus temperature ( T _c/ T _L) was obtained by DTA measurement for the Na₂O–B₂O₃ and Na₂O–B₂O₃–Al₂O₃ systems. The critical cooling rate for glass formation ( Q *) was also measured. The measurements were performed in the composition range of (100 − x )Na₂O–( x )B₂O₃, ( x = 25–35 and 60–100 mol%), and (100 − y )0.5Na₂O·0.5B₂O₃−( y )Al₂O₃, ( y = 6–34 mol%). The relationship between T _c/ T _L and Q * was discussed. A linear relationship between T _c/ T _L and log Q * for these systems was found. Furthermore, the relationship between T _c/ T _L and Q * was verified by computer simulation based on the crystallization kinetics of glass or supercooled liquid.     

Electrical Conduction Properties of LaP3O9 Glass and Glass–Ceramics

Electrical conduction properties of undoped and 1 mol% Sr-doped LaP₃O₉ glasses and glass–ceramics were investigated over the temperature range of 673–1123 K. Both the materials showed relatively low conductivities in the glassy state. However, the conductivity of the Sr-doped LaP₃O₉ glass significantly increased with a heat treatment above the crystallization temperature, while the conductivity of the undoped LaP₃O₉ glass did not improve even after the heat treatment. It was concluded that crystallization of the Sr-doped LaP₃O₉ glass induced protonic conduction and thus enhanced the conductivity. Electrical conduction properties of the Sr-doped LaP₃O₉ glass–ceramic fundamentally resembled those of the sintered crystalline Sr-doped LaP₃O₉.     

Influence of Pseudo-Alkali Cations on Network Depolymerization in Mixed-Cation Germanate Glasses

Mixed-cation germanate glasses that contained ≥69 mol% GeO₂ and equimolar amounts of Na₂O/K₂O, Ag₂O/Tl₂O, Tl₂O/K₂O, or Ag₂O/K₂O were prepared. Densities, refractive indices, and infrared absorptions were measured and colors noted. In all glasses, the expected shift of the ir absorption to longer wavelengths is enhanced by the presence of the second cation. The mixed-alkali glasses are less voluminous and the mixed-cation glasses more voluminous than predicted. The mixed-cation glasses in some cases also exhibit more intense colors than anticipated and deviations of the refractive index from additivity. These results are interpreted in terms of slight differences in the mode of network depolymerization which appear to be caused by the polarizing nature of Tl⁺ and Ag⁺     

Electrical Conduction in β-Bi2O3 Doped with Sb2O3

Electrical conduction in tetragonal β-Bi₂O₃ doped with Sb₂O₃ was investigated by measuring electrical conductivity, ionic transference number, and Seebeck coefficient. The β-Bi₂O₃ doped with 1 to 10 mol% Sb₂O₃ was stable up to 600°C and showed an oxygen ionic and electronic mixed conduction, where the electron conduction was predominant at low oxygen pressures. The oxygen-ion conductivity showed a maximum at 4 mol% Sb₂O₃, whereas the activation energy for the ionic conduction remained unchanged for 4 to 10 mol% Sb₂O₃-doped specimens. These results were interpreted in terms of the oxygen vacancy concentration and the distortion of the tetragonal structure. The electron conductivity and its oxygen pressure dependence decreased with increasing Sb₂O₃ content. The fact that Sb⁵⁺ is partially reduced by excess electrons in heavily doped β specimens at low oxygen pressures is explained.     

Processing and Properties of Strontium Bismuth Vanadate Niobate Ferroelectric Ceramics

The processing conditions, microstructure, and dielectric properties of strontium bismuth niobate vanadate ceramics, SrBi₂(V _x Nb_{1− x} )₂O₉ (SBVN, with 0 ≤ x ≤ 0.3), were systematically studied. A relative density of >90% was obtained for all the samples using a two-step sintering process. XRD showed that a single phase with the layered perovskite structure of SrBi₂Nb₂O₉ (SBN) was formed with a vanadium content of up to 30 at.%. SEM revealed that the average grain size decreased gradually with an increase in vanadium content. The Curie point was found to gradually increase from ∼418°C for SBN to ∼459°C for SBVN with 30 at.% vanadium. Dielectric constants at room temperature and their respective Curie points were found to peak at a composition with 10–15 at.%; vanadium. Moreover, a high concentration of vanadium (>5 at.%) resulted in a significant increase in tangent loss at low frequencies (<1000 Hz). The relationships between chemical composition, processing condition, microstructure, and dielectric properties of SBVN ferroelectric ceramics are discussed.