Stress Required for Constrained Sintering of a Ceramic-Filled Glass Composite

The stress required for complete constrained sintering of a low-temperature cofirable ceramic-filled glass composite, i.e., borosilicate glass + alumina, has been investigated under uniaxial constant and cyclic loading. The required uniaxial stress to have complete constrained sintering, i.e., zero strain or strain rate in the perpendicular directions, is in the range of 20–250 kPa, which initially increases with temperature, reaches a maximum at 725°–800°C, and then decreases with increasing temperature. The above measured data exhibit a similar trend to those of required stress and sintering potential calculated using the viscous analogy for the constitutive relationship of a porous sintering compact.     

Effect of Mismatched Sintering Kinetics on Camber in a Low-Temperature Cofired Ceramic Package

Linear shrinkage profiles of an unconstrained gold thick film material and a low -tempearture cofireable glass-ceramic (LTCC) green tape were measured using a noncontact optical technique. A laser beam scans across a sample, at various times during the sintering process. The unconstrained sintering kinetics of the gold film were found to differ significantly from those of the LTCC tape. The densification of the gold film was nearly completed before the LTCC began to densify. The development of camber (warpage) during consintering of a gold/LTCC composite structure was monitored and recorded with a video camera. This camber development is analysed based on the viscous constitutive relations for porous based on the viscous constitutive relations for porous sintering bodies. The mismatched sintering kinetics of the two materials lead to the development of in-plane stresses in the two constituents of the composite structure as one material being constrained from composite structure as one material being constrained of the composite structure as one material being constrained from sintering by the other. The resulting camber during the consintering process is explained by the development of these stresses.     

Sintering of CdO Under Low Applied Stress

A loading dilatometer of novel design was used to study the sintering behavior of CdO powder compacts subjected to stresses between 0 and 0.25 MPa at 973 to 1123 K. Densification and creep occur simultaneously. Although both the volumetric densifcation rate and the uniaxial creep rate show an approximately inverse relation with time, the creep rate increases with stress, whereas the densification rate is independent of stress.     

Stress Required to Densify a Low-Fire NiCuZn Ferrite Under Constrained Sintering

The effect of uniaxial stress on the densification behavior of a low-fire NiCuZn ferrite during constrained sintering of a multilayer structure of a ferrite tape and a pure alumina tape has been investigated. Compared with free sintering, the densification of ferrite becomes significantly reduced and slowed down under pressure-less constrained sintering. To enhance the constrained densification of ferrite in the temperature range required for free sintering, uniaxial stress applied in the thickness direction is needed. The required uniaxial stress to densify ferrite under constrained sintering to reach a relative sintered density of >95% decreases from 1100–1300 kPa at 900°C to 250–450 kPa at 1000°C. Moreover, no significant grain growth is found when the ferrite is densified under pressure-assisted constrained sintering.     

Synthesis and Characterization of Crystallizable Anorthite-Based Glass for a Low-Temperature Cofired Ceramic Application

We report successful identification and preparation of a glass composition in the CaO–Al₂O₃–SiO₂ phase diagram with a judicious choice of fluxes that met all dielectric, electrical, and thermal property requirements for low-temperature cofired ceramic (LTCC) applications. The glass composition sintered at 900°C attains good density (2.45 g/cc) and does not precipitate any crystalline phase. However, when this glass powder is sintered at the same temperature in the presence of 30 vol% cordierite, crystallization of the anorthite phase is observed, which improves the properties of the composite for LTCC application.     

Development, Analysis, and Application of a Glass–Alumina-Based Self-Constrained Sintering Low-Temperature Cofired Ceramic

The effects of an inner constraint layer and alumina particles on the microstructure, strength, and shrinkage of the laminated low-temperature cofired ceramic (LTCC) green sheet were investigated. Alumina particles of several sizes were used in the inner-constraint layer in order to strengthen the LTCC substrate. Smaller alumina particles in the inner-constraint layer produced a substrate with a high bending strength. Sintering shrinkage in the x – y direction of the LTCC is related to the bending strength of the debinded alumina particle layer used for an inner-constraint layer. A larger pore size in the inner-constraint layer was found to increase the distance of the glass penetration from the glass–alumina layer into the inner-constraint layer. The total thickness of the constraint layer changes the shrinkage in the x – y direction and the bending strength.     

Stress Development during Constrained Sintering of Alumina/Glass/Alumina Sandwich Structure

Stress development during the constrained sintering of a sandwich structure of alumina/glass/alumina has been studied. Stress distribution in the glass layer was calculated using a finite element method. The shear stress exhibited a maximum at the edge of the alumina/glass interface. The in-plane tensile stress, formed during constrained sintering, decreased from the interface of the alumina/glass to the middle of the glass in the z -direction, but increased from the edge to the center of glass in the x − y direction. This in-plane tensile stress reduces the driving force of densification, resulting in a larger x − y shrinkage and higher densification in the middle of the glass.     

Fabrication of a Multilayered Low-Temperature Cofired Ceramic Micro-Plasma-Generating Device

Plasma technology is currently being used in innumerable industrial applications. Some of the common uses of this technology include surface cleaning and treatment, sputtering and etching of semiconductor devices, excitation source for chemical analyses, cutting, environmental cleanup, sterilization, and phototherapy. The harsh conditions that these devices must endure require robust refractory materials systems for their fabrication and reliability. Low-temperature cofired ceramic (LTCC) material systems provide a durable and cost-effective platform for the manufacture of such devices, and allow for possible integration into meso-scale microsystems. Our designs are based on RF microstriplines that capacitively couple and ionize small gas discharge sites over the top electrode. In this paper, we have built several iterations of this micro-plasma generating device using LTCC material systems. The impact of electrode ink selection and processing, lamination methods, dielectric layer thickness, and electrode design has been investigated. Several micro-plasma-generating devices were then evaluated for power requirements, output stability, and long-term reliability.     

Effects of Applied Pressure on Densification During Sintering in the Presence of a Liquid Phase

Experimental measurements of the effects of an applied pressure on sintering of powdered materials containing a liquid phase indicate that the applied pressure can be effective by: ( a ) increasing the extent and rate of particle rearrangement, ( b ) increasing the rate of solution at particle contacts, and ( c ) causing plastic flow within the solid particles. Which of these processes predominates depends on the characteristics of each particular system and on the level of applied pressure.     

Constrained Sintering of a Glass Ceramic Composite: I. Asymmetric Laminate

The camber of asymmetric laminates has been experimentally measured and predicted. Two cases are distinguished: (i) sintering of a viscous layer on a viscous substrate and (ii) sintering of a viscous layer on an elastic substrate. In the first case, particular attention is paid to the microstructure of the shrinking layer: a gradient in porosity as well as in pore size has been found along the thickness. Microstructural observations have been rationalized through an asymmetric stress state in the shrinking layer. In the second case, substrate cracking is predicted as function of Young's modulus and thickness ratio of the elastic substrate.     

Sintering of Spherical Glass Powder under a Uniaxial Stress

The sintering of spherical borosilicate glass powder (particle size 5 to 10 μm) under a uniaxial stress was studied at 800°C. The experiments allowed the measurement of the kinetics of densification and creep, the viscosities for creep and bulk deformation, and the sintering stress which was found to increase with density. The data show excellent qualitative agreement with Scherer's theory of viscous sintering. In addition, the quantitative comparison between theory and experiment shows good agreement; the measured viscosity of the bulk glass was ∽1×10⁹ P (∽1×10⁸ Pa·s) compared to ∽3×10⁹ P (∽3 Pa·s) obtained by fitting the data with Scherer's theory.     

Temperature Coefficient of Microwave Resonance Frequency of a Low-Temperature Cofired Ceramic (LTCC) System

A family of low-temperature cofired ceramics (LTCC) based on mixtures of a commercial dielectric, MgTiO₃–CaTiO₃(designated MMT-20) and ZnO, SiO₂, and B₂O₃, has been investigated for microwave applications. The main objective was to optimize the three key properties—relative permittivity (ɛ_r), dissipation factor (DF), and the temperature dependence of the microwave resonance frequency (τ_f)—through adjustment of the composition. A further objective was to estimate the limits on compositional variability while maintaining acceptable properties. The developed microstructures, after firing at 900°C, were studied using X-ray diffractometry and scanning electron microscopy/energy dispersive spectrometry techniques and compared with the dielectric parameters. The optimum composition (wt%) was found to lie in the ranges 45.8–44.9, ZnO; 17.25–17.55, B₂O₃; 6.95–7.05, SiO₂; and 30–30.5, MMT-20, yielding values of ɛ_r= 8.5–9.5, DF < 0.93 × 10⁻³ ppm/K, and τ_f< ±10 ppm/K.     

Influences of the Glass Phase on Densification, Microstructure, and Properties of Low-Temperature Co-Fired Ceramics

Multilayer ceramic devices based on low-temperature co-fired ceramics (LTCC) materials provide a very promising technology. Most LTCC tapes available today contain considerable fractions of glass powders to lower the sintering temperature. However, the glassy phases offer more possibilities to set a proper sintering behavior, on the one hand, and to tailor the desired properties of the final LTCC substrate, on the other. The exploitation of demixing and subsequent crystallizing glass compositions was shown on an example of a low-permittivity (4.4)—low-loss (1.5 × 10⁻³) LTCC with a high quartz content. In another LTCC material, undesired demixing could be restricted and the crystal phase anorthite could be triggered by partial dissolution of alumina in the liquid phase during sintering. To estimate the effect of silver diffusion in the latter material, the surroundings of a pure silver via were studied. A silver-contaminated range of 50 μm was detected. Using model glasses containing silver oxide, a strong influence of dissolved silver on viscosity and crystallization behavior of the liquid phase was demonstrated. The dielectric properties of the sintered substrates were not degraded.     

Effect of Co-Precipitation on the Low-Temperature Sintering of Biphasic Calcium Phosphate

Three types of biphasic calcium phosphate (BCP) powders were prepared and their sintering behavior was investigated. The specific surface area and HA/TCP ratio were similar in all three specimens. Most of the densification in the co-precipitated s-BCP occurred before the β- to α-TCP phase transformation, and a maximum density of ∼95% was obtained at 1100°C. The mixture of separately precipitated and calcined hydroxyapatite (HA) and tricalcium phosphate (TCP) (m-BCP) showed a poor sintering behavior, and the apparent density was below 70% at temperatures up to 1200°C. In the commercial HA and TCP mixture (c-BCP), the low temperature sintering was poor, but densification continued without the phase transformation due to the presence of MgO, achieving almost full densification at 1200°C.     

Effect of Green Density on Densification and Creep During Sintering

The effect of green density on both the densification rate and the creep rate was measured simultaneously during sintering by loading dilatometry. The experiments were performed on zinc oxide powder compacts with five different green densities covering a range of 0.39 to 0.73 of theoretical. The samples were heated at a constant rate of 4°C/min up to 1100°C in air. The densification rate at any temperature increases significantly with decreasing green density. The data for the densification rate and creep rate as a function of density show two quite distinct regimes of behavior; the rates were strongly dependent on density below 0.80, while above this value they were weakly dependent on density. The ratio of the densification rate to the creep rate was almost independent of temperature but increased almost linearly with increasing green density. The representation of the data in terms of models for sintering and creep is discussed.     

玻璃应力对玻璃强度的影响

介绍了玻璃的应力与玻璃强度之间的关系,并讨论了几种不同类型的应力对玻璃强度的影响。     

Influence of Ag on the Composition and Electromagnetic Properties of Low-Temperature Cofired Hexaferrites

We have studied the compatibility of Ag and (Co,Cu)Z-hexaferrite with the composition Ba₃Co_1.4Cu_0.6Fe₂₄O₄₁ (Z). In order to do this, the (Co,Cu)Z-hexaferrite was cofired with various amounts of Ag at 900°C. The degradation of the (Co,Cu)Z-hexaferrite to Y- and U-hexaferrite occurred during the cofiring and was confirmed by X-ray diffraction as well as by thermomagnetic measurements. The permeability and permittivity of the cofired ceramics were measured in the range 1 MHz to 10 GHz. For the (Co,Cu)Z-hexaferrite, both these properties were significantly influenced by the Ag when the amount of Ag exceeded a minimum of 4.5 wt%. The influence of the Ag and the degradation of the (Co,Cu)Z-hexaferrite on the permeability were evaluated using the Bruggeman effective media theory.     

Effect of Sintering Atmosphere on the Densification and Electrical Properties of Alumina

The effect of sintering atmosphere on the final density and electrical properties of alumina compacts has been investigated using two different oxygen pressures: air and CO/CO₂. Measuring of electrical behavior has been considered a tool for determining the mechanism responsible for densification. Finally, the importance of a reducing atmoshphere on the electrical behavior of polycrystalline alumina is pointed out.