Evaporation of Silver during Cofiring with Barium Titanate

Silver and Ag-Pd alloy are cofired with BaTiO₃-based dielectrics during the manufacturing of capacitors. The diffusion of silver into BaTiO₃ during sintering is very slow. However, the vapor pressure of silver is high at temperatures greater than the melting point of silver (960°C). The effect of the evaporation of silver during the sintering with BaTiO₃ at 1200°, 1250°, and 1290°C is investigated in the present study. The silver vapor can transport through the pore channel of the dielectrics to a distance of a few hundred micrometers. The melting point of Ag-Pd alloy is higher than that of silver; therefore, the transportation of silver vapor from Ag-Pd alloy is hardly observed at temperatures >1200°C.     

Behavior of Silver and Palladium Mixtures during Heating

The behavior of mixtures of silver and palladium during heating in both air and an inert atmosphere was studied using X-ray diffractometry (XRD), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), dilatometry, and scanning electron microscopy (SEM). In situ high-temperature XRD studies on a commercial 20% palladium material with submicrometer-sized particles indicated that an intermetallic phase, most likely Ag₃Pd, formed in air between 300° and 400°C, the same temperature range where a 13% linear expansion was measured by dilatometry. The DSC data indicated an exothermic peak at 340°C, a temperature where the TGA results indicated that the material had picked up only 0.2% oxygen, compared with the maximum of 1.4% at 525°C. No PdO was detected by XRD at 400°C, which suggests that oxygen was being incorporated in the intermetallic. Microstructural examination using SEM indicated that larger particles, with internal pores, had formed after heating in air to 375°C. When the material was heated in argon for 1 h at 400°C, no intermetallic phase or alloy formed, and minimal expansion occurred. When mixtures of larger silver particles (5–30 μm) with palladium particles (1–3 μm) were heated in air, the maximum amount of expansion that occurred increased from 0% for pure palladium up to a maximum of 18% at 75% silver. This result supports the conclusion that expansion is a result of formation of this new phase, in the presence of oxygen, not of the oxidation of the palladium.     

The Wulff Shape of Alumina: IV. Ti4+-Doped Alumina

Controlled-geometry cavities, initially ≈20 μm × 20 μm × 0.5 μm, were introduced into     -plane titanium-doped (≈210 ppma;≈500 wt. ppm) sapphire substrates using photolithographic methods, and subsequently internalized by diffusion bonding. The samples were annealed in air for prolonged periods at 1600° and 1800°C to convert the titanium to the 4+ state and to allow the pore shapes to adjust. Pores with an equivalent spherical radius of ≈3.6 μm reached a quasi-equilibrium shape within 160 h at 1600°C and within 48 h at 1800°C. The Wulff shape was determined using optical microscopy, scanning electron microscopy, and atomic force microscopy. The Wulff shape of Ti⁴⁺-doped alumina includes well-defined c(0001),     , and     facets and smoothly curved sections. The     and a     facets, which are components of the Wulff shape of undoped sapphire, are not discernable. In contrast to undoped alumina, for which the r-plane has the lowest energy, the c-plane is the lowest energy plane in Ti⁴⁺-doped alumina. The surface energy sequence of the stable c, r, and p surfaces differs from that in undoped alumina. The Wulff shape varies with temperature. Samples equilibrated at 1800°C were re-annealed at 1600°C. Pore shape changes were reversible, indicating that the observed pore shapes were close to the equilibrium (Wulff) shape.     

Extrinsic OH− Absorption in Transparent Polycrystalline Lanthana-Doped Yttria

Transparent lanthana-doped yttria fabricated by transient solid second-phase sintering under wet hydrogen typically has a broad absorption band with a peak at 3.08 μm. The absorption band shift observed in samples treated in wet deuterium indicated that the 3.08-μm absorption was due to OH⁻ ions. The diffusion rates of hydrogen defects in lanthana-doped yttria were determined in the temperature range from 1000° to 1400°C. The changes in the concentrations of OH⁻ ions upon anneals were determined by measuring infrared absorbance at 3.08 μm. The diffusion coefficient is 1.3 × 10⁻⁷, 9.9 × 10⁻⁷, and 4.1 × 10⁻⁶ cm²/s at 1000°, 1200°, and 1400°C, respectively, with an activation energy of 140 kJ/mol. Annealing in a controlled oxygen partial-pressure environment can remove the OH⁻ absorption band and bring the total absorption in the 3- to 5-μm range closer to the intrinsic values.     

Effect of Temperature on Ostwald Ripening of Silver in Glass

The effect of temperature and volume fraction on the growth of silver particles in a lead borosilicate glass has been studied. The particle growth obeyed the Ostwald ripening theory and was controlled by diffusion. An anomaly of growth rate coefficient was observed around 700°C and explained in terms of the dissolution behavior of silver. Also, the solubility of silver in glass was evaluated from the normalized rate coefficient K using volume fraction of silver particles and viscosity of the glass.     

Dielectric Behavior and Cofiring with Silver of Monoclinic BiSbO4 Ceramic

Pure monoclinic phase of BiSbO₄ ceramic was synthesized using the solid-state reaction method. BiSbO₄ ceramics can be well sintered between 960° and 1080°C. The dielectric constant of BiSbO₄ ceramic was about 20.1 and dielectric loss was about 0.055–2.5 × 10⁻⁴ at 1 MHz. With the measured frequencies increasing from 100 to 900 kHz, the temperature coefficients of permittivity decreased from +224.1 to +95.9 ppm/°C. At the microwave range, the best microwave dielectric properties were obtained in the ceramic sintered at 1080°C/2 h with a permittivity of 19.3, a Q _f value of about 70 000 GHz, and a temperature coefficient of resonant frequency of −62 ppm/°C. Cofiring between BiSbO₄ ceramic and 20 wt% Ag was also investigated, and it was found that after cofiring at 900°C for 5 h only very little unknown phase containing a little amount of Ag was formed and most Ag was scattered in grain boundaries.     

Preparation of Small-Grained and Large-Grained Ceramics from Nb-Doped BaTiO3

Barium titanate powders with average grain sizes of 0.07 and 0.65 μm were coated with an organic Nb ester, using a liquid-solid mixing process. Small-grained (1 to 3 μm) ceramic dielectrics from both powders and large-grained (>50 μm) dielectrics from the 0.65 μm powder were fired using 0 to 2 at.% Nb. Microstructures, densities, and electrical resistivities were investigated.     

Preparation and Dielectric Properties of Low-Temperature-Sinterable (Zr0.8Sn0.2)TiO4 Powder

Low-temperature-sinterable (Zr_0.8Sn_0.2)TiO₄ powders were prepared using 3 mol% Zn(NO₃)₂ additive and then compared with powders prepared using 3 mol% ZnO additive and no additives. Sintering at 1200°C for 2 h produced a dielectric ceramic with ρ= 98.6% of theoretical density (TD), ɛ_r= 38.4, Q × f (GHz) = 42000, and τ _f =−1 ppm/°C. Sintering at 1250°C resulted in an excellent dielectric with ρ= 99% of TD, epsilon_r= 40.9, Q × f (GHz) = 49000, and τ _f =−2 ppm/°C. Scanning electron microscopy showed a microstructure with grains measuring 0.5 to 1 μm, and transmission electron microscopy revealed secondary phase in the grain boundaries.     

Mechanism of Preventing Crystallization in Low-Firing Glass/Ceramic Composite Substrates

The formation of a crystal phase in the glass matrix of low-firing glass/ceramic composite substrates limits the efficiency of the ceramic substrate when it is used in circuit boards. In the present study, adding approximately 30 vol % or more of alumina to a borosilicate-glass/ceramic composite system as a ceramic filler caused the diffusion of aluminum ions from the alumina filler into the glass matrix and prevented the formation of a cristobalite crystal phase. The diffusion distance between the aluminum ions was ∼30 μm when the system was fired at 900°C for 10 min. Raman spectroscopic analysis proved that some of the aluminum ions had diffused into the glass matrix during firing, working as a network former in the glass matrix. Raman spectra near 460 and 1100 cm⁻¹ indicate the change of network structure in the borosilicate glass. These phenomena indicate that crystallization of the borosilicate glass was prevented in the alumina-filled borosilicate system.     

Effect of Additives on Ostwald Ripening of Silver in Glass

The effect of additives on the growth of silver particles in a lead borosilicate glass was studied. Growth of silver particles was expressed by the Ostwald ripening theory and controlled by diffusion. Addition of palladium retarded the growth of silver particles. This is attributed to the reduced activity of silver caused by the formation of silver–palladium solid solutions. Addition of ruthenium dioxide promoted the growth, whereas addition of tin dioxide had almost no effect on the growth rate. A "solubility parameter" was introduced to evaluate the temperature dependence of solubility of silver in glass, and the effect of additives has been discussed. Association of diffusion species has been proposed as a possible factor responsible for the temperature dependence of the solubility parameter.     

Fabrication of Free-Standing Titania-Based Gas Sensors by the Oxidation of Metallic Titanium Foils

A simple method for fabricating TiO₂-based sensors of CO( g ) is demonstrated: the oxidation of Ti-bearing foils. Metallic foils (35 μm thick) were converted into free-standing, porous rutile foils (60 μm thick) by exposure to O₂( g ) at 800°—965°C. The oxidized foils contained thin (0.5—1 μm thick), regularly spaced oxide layers oriented parallel to the external surface. The exposure of such porous foils to increasing concentrations of CO( g ) resulted in a monotonic increase in the steady-state electrical resistance. Rutile foils sensitive to 50 ppm changes in CO( g ) content with response times of a few minutes were produced. The effects of oxidation conditions and copper doping on sensing performance are discussed.     

Characterization of Silver Interdiffusion into (Zn,Mg)TiO3+x:Bi:Sb Multilayer Ceramic Capacitor

In this study, (Zn,Mg)TiO_{3+ x} +Bi₂O₃+Sb₂O₅ multilayer ceramic capacitors (MLCCs) with different proportions of a silver (Ag)–palladium (Pd) mixture acting as the inner electrode were sintered at 925°C for 2 h to evaluate the effect of the inner electrode on reliability. The main results reveal that the lifetime is inversely proportional to the Ag content in the Ag/Pd inner electrode. Ag diffusion into the (Zn,Mg)TiO_{3+ x} +Bi₂O₃+Sb₂O₅ MLCC during cofiring at 925°C for 2 h and Ag migration at 140°C against 200 V are both responsible for the short lifetime of the (Zn,Mg)TiO_{3+ x} +Bi₂O₃+Sb₂O₅ MLCC, particularly the latter factor. A Bi,Sb-rich secondary phase was present at the triple junction and a small amount of Ag was detected from the second phase for a (Zn,Mg)TiO_{3+ x} +Bi₂O₃+Sb₂O₅ MLCC with a high Ag content in the inner electrode of Ag/Pd=99/01. However, this was not the case with a low Ag content in the inner electrode of Ag/Pd=90/10. This means that the Bi,Sb-rich second phase plays an important role in determining the degradation of insulation resistance due to the excessive formation of oxygen vacancies from the reaction of Ag⁺ for Bi³⁺ or Sb⁵⁺ substitution to lower the activation energy of the (Zn,Mg)TiO₃+Bi₂O₃+Sb₂O₅ dielectrics and to enhance markedly the effect of Ag migration.     

Dielectric Behavior of Na0.5Bi0.5TiO3-Based Composites Incorporating Silver Particles

The dielectric properties of Na_0.5Bi_0.5TiO₃ (NBT) -based composites incorporating silver particles prepared by sintering at a low temperature of ∼900°C are reported. The dielectric constant increases with the amount of metal silver particles in the measured frequency range (150 Hz to 1 MHz), and could be enhanced up to ∼20 times higher than that of pure NBT ceramics, which was ascribed to the effective electric fields developed between the dispersed particles in the matrix and the percolation effect. Further investigation revealed that the dielectric constant of the composites has weak frequency and temperature dependence (−50°C to +50°C).     

Creep of CaO-Stabilized ZrO2

The compressive creep of 18 mol% CaO-stabilized ZrO₂ was studied at 1200° to 1400°C and 500 to 4000 psi. The specimens were polycrystalline with grain diameters from 7 to 29 μm. The activation energy for creep is 94 kcal/mol, and the creep rates are linearly proportional to the stress and to the inverse of the grain size. These results lead to the conclusion that creep in 18 mol% CaO-stabilized ZrO₂ may be controlled by cation diffusion associated with grain-boundary sliding.     

Preparation of a Dense/Porous BiLayered Ceramic by Applying an Electric Field During Freeze Casting

Dense/porous bilayered ceramics were fabricated by introducing an electric field into the freeze-casting process. A unidirectional solidification was performed by placing an aqueous ceramic on refrigerating equipment, at a constant cooling rate of 6°C/min. When the bottom temperature of the slurry was reduced to 0°C, an electric field along the vertical direction was applied until the freezing process was completed. After drying and sintering of the green part, an as-prepared ceramic with dense/porous bilayered architectures was obtained. It was shown that the thickness of the dense layer in the sample could be controlled by tuning the electric field intensity from 51 to 155 μm as the electric field increased from 15 to 90 V. On the other hand, all samples exhibited almost an identical lamellae thickness and interlayer distance in the porous region, as large as ∼14 and ∼24 μm, respectively.     

High-Temperature Structural Phase Transition in Ca0.7Ti0.7La0.3Al0.3O3: Investigation by Synchrotron X-Ray Diffraction

Mixed-oxide prepared Ca_0.7Ti_0.7La_0.3Al_0.3O₃ (CTLA) ceramics (≈96% dense), grain size 6–7 μm, with dielectric properties (at 4 GHz) of ɛ_r≈46, Q × f ≈38 000 GHz, and τ_f+13 ppm/°C, were studied at 25°–1300°C using synchrotron X-ray powder diffraction. At room temperature, CTLA exhibits a distorted orthorhombic structure, with two tilt systems: a =5.40383 (4) Å, b =5.41106 (6) Å, and c =7.64114 (7) Å with space group Pbnm . At 1050°±25°C, there is a transition from orthorhombic ( Pbnm ) to tetragonal ( I 4/ mcm ), with a simpler tilt arrangement. The lattice parameters at 1100°C were: a =5.44285 (4) Å and c =7.68913 (8) Å.     

Sintering and Properties of Monazite-Type CePO4

Monazite-type CePO₄ powder (average grain size 0.3 μm) was dry-pressed to disks or bars. The green compacts began to sinter above 950°C. Relative density ≧ 99% and apparent porosity <1% were achieved when the specimens were sintered at 1500°C for 1 h in air. The linear thermal expansion coefficient and thermal conductivity of the CePO₄ ceramics were 9 × 10⁻⁶/°C to 11 × 10⁻⁶/°C (200° to 1300°C) and 1.81 W/(m · K) (500°C), respectively. Bending strength of the ceramics (average grain size 4 μm) was 174 ± 28 MPa (room temperature). The CePO₄ ceramics were cracked or decomposed by acidic or alkaline aqueous solutions at high temperatures.     

Electrical Breakdown Strength of Alumina at High Temperatures

The dc electrical strength of sapphire and poly crystalline alumina was studied up to 1400°C. The electrical strength was essentially identical for both materials. It was > 10⁶ V/cm at room temperature and decreased gradually with temperature up to 900°C (2.6×10⁵ V/ cm), then dropped rapidly to 2×10⁴ V/cm at 1400°C for a sample thickness of ∼ 100 μm. The electrical strength decreased with the sample thickness. It was inversely proportional to the thickness for samples thicker than ∼ 600 μm at 1200°C. The breakdown behavior was explained on the basis of a thermal breakdown model.     

Microwave Dielectric Properties and Low-Temperature Cofiring of BaTe4O9 with Aluminum Metal Electrode

Polycrystalline BaTe₄O₉ ceramic compound was investigated as a promising microwave dielectric compound for low-temperature cofired ceramics (LTCC) applications. The binary phase BaTe₄O₉ was synthesized and subsequently densified over the temperature range of only 500°–550°C, which allows for low-temperature cofiring with aluminum metal. The dielectric properties of BaTe₄O₉ ceramics sintered at 550°C for 2 h were determined in the microwave region of 12–14 GHz. The dielectric constant and Q × f product obtained were 17.5 and 54 700 GHz at 12 GHz, respectively. The temperature coefficient of resonance frequency showed a negative value of −90 ppm/°C. In terms of its evaluation for LTCC, the BaTe₄O₉ composition was found to be chemically compatible and successfully cofired with highly conductive aluminum electrode, while maintaining good electrical performance.     

Crack-Healing Behavior of Al2O3 Toughened by SiC Whiskers

Al₂O₃ reinforced by SiC whiskers (Al₂O₃/SiC-W) was hot-pressed to investigate the crack-healing behavior. Semielliptical surface cracks of 100 μm in surface length were introduced using a Vickers indenter. The specimens containing precracks were crack-healed at temperatures between 1000° and 1300°C for 1 h in air, and their strengths were measured by three-point bending tests at room temperature and elevated temperatures between 400° and 1300°C. The results show that Al₂O₃/SiC-W possesses considerable crack-healing ability. The surface cracks with length of 2 c = 100 μm could be healed by crack-healing at 1200° or 1300°C for 1 h in air. Fracture toughness of the material was also determined. As expected, the SiC whiskers made their Al₂O₃ tougher.