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.     

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.     

Shrinkage-Free Sintering of Low-Temperature Cofired Ceramics by Loading Dilatometry

Sintering with zero radial shrinkage of low-temperature cofired ceramics (LTCC) was investigated by means of loading dilatometry. The results indicate that constant compressive loads tend to produce either decreased or increased radial shrinkages, and this can vary as a function of density, depending on the magnitude of the applied stresses. Coupling the real-time applied uniaxial load to the radial strain of cylindrical samples afforded a sintering scheme where the radial strain was precisely kept near zero (±0.02%). This approach possibly perfects an existing hot-pressing technique for zero-shrinkage sintering technology of LTCC materials.     

Preparing Low-Loss Low-Temperature Cofired Ceramic Material without Glass Addition

A low-temperature cofired ceramic (LTCC) composition for radio-frequency purposes was accomplished without prior glass preparation. In this process, the formulation was made by mixing the glass-forming oxides (ZnO, SiO₂, and B₂O₃) with the commercial microwave ceramic MgTiO₃–CaTiO₃. The sintering, microstructure, and microwave properties were compared to a formulation with exactly the same composition, but a conventional production route, including glass preparation. The novel preparation route resulted in improved firing properties of the mixture. Also, the densities, porosities, and phases of the samples were almost the same as those of the conventional samples, but the phase fractions were different. Finally, this preparation route produced better dielectric values.     

The Effect of Applied Stress on the Densification of a Low-Temperature Cofired Ceramic-Filled Glass System Under Constrained Sintering

The effect of uniaxial stress on the mechanical response and densification behavior of a low-fire borosilicate glass (BSG)+alumina system during constrained sintering of a multilayer BSG+alumina/alumina laminate has been investigated. Compared with free sintering, the pressure-less constrained sintering of BSG+alumina exhibits poorer densification, and larger porous bulk viscosity at a given temperature. This is caused by the in-plane tensile stress and anisotropic development generated in the transverse directions of the laminate during constrained sintering. The applied uniaxial stress required in the thickness direction to densify BSG+alumina under constrained sintering varies in the range of 50–400 kPa at 700°–800°C. The above results are in agreement with those calculated using the viscous analogy for the constitutive relationships of a porous sintering compact.     

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.     

Use of Titanates to Achieve a Temperature-Stable Low-Temperature Cofired Ceramic Dielectric for Wireless Applications

A low-loss and near-zero temperature coefficient of resonant frequency ( T _f) low-temperature cofired ceramic (LTCC) host dielectric was developed for portable consumer wireless device applications. The low T _f was realized by compensating the Al₂O₃-filled-glass dielectric with admixtures of TiO₂ (negative temperature coefficient of dielectric constant ( T _ɛ)) in the starting formulation. XRD data indicated a portion of the TiO₂ in the starting formulation dissolved into the glass, and extensive formation of crystalline titanium compounds was observed via a nucleation and growth mechanism. The dissolution of TiO₂ in the glass and subsequent formation of titanium compounds was believed to result in the relatively small amount of TiO₂ required to achieve a near-zero T _f in the final sintered structure.     

Microstructural Effects on Microwave Properties of Low-Temperature Cofired Ceramic Striplines: Experiments and Modeling

The effect of microstructure on microwave properties of low-temperature cofired ceramic (LTCC) stripline packages was studied with experiments and simulations. Microstructure was controlled experimentally by modifying organic compositions and heating procedures. Pores existing at the conductor/dielectric interface and inside the conductor line affected the microwave properties, such as the effective dielectric constant (ε_eff), the effective conductivity (σ_eff), and the unloaded quality factor ( Q _U) of the stripline packages. The models for simulation were introduced with a finite-element method using high-frequency simulation software (HFSS). Simulation results showed that the pores existing at the interface made Q _U increase by 7%–8% while [alt epsilon]_eff decrease by more than 10%. Pores inside the conductor line made σ_eff and Q _U decrease. The proposed models and simulation results will contribute to improving future designs and manufacturing of LTCC microwave applications.     

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.     

Near Zero Shrinkage of an Low-Temperature Co-Fired Ceramic Package by Constrained Sintering Using Screen Printed Alumina Paste

Conventional free sintering of low-temperature co-firing ceramic (LTCC) technology has several merits such as sintering temperature below 1000°C that enables co-firing with electrode materials of silver or copper metal and multilayer structure formation. But due to the free sintering process, large shrinkage occurs. To fabricate electronic devices and components with near zero shrinkage within x, y directions constrained sintering (CS) technology is required. In this study a constrained sintering paste (CSP) utilizing alumina powder, which has a higher sintering temperature than LTCC powders, was fabricated for CS technology. The effect of CSP formulated using alumina powder on shrinkage was studied according to variation in paste composition. As a result ceramic package structure with a cavity was fabricated with shrinkage control of 0.028%, which is far smaller than the current CS technology shrinkage of approximately 0.1%.     

Effects of Silver-Paste Formulation on Camber Development during the Cofiring of a Silver-Based, Low-Temperature-Cofired Ceramic Package

The effects of silver-paste formulation on camber development during the cofiring of a two-layered structure of a silver-based, low-temperature-cofired ceramic (LTCC) system have been investigated. The densification kinetics and mechanism of the silver film become more similar to that of LTCC as the amount of LTCC powder in the silver paste increases, which results in a smaller camber during cofiring. By using the densification results of silver film and LTCC, camber development during cofiring can be mathematically described by using viscous analysis, which agrees well with the experimental observations.     

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.     

低温烧结绝缘材料的研究

低温烧结绝缘材料是当前电子陶瓷行业迫切需要解决的问题之一，本研究采用Ａｌ２Ｏ３与Ｚｎ－Ｖ－Ｂ系易熔玻璃按一定比例在６００℃左右进行烧结，并获得具有高绝缘性（室温ρｖ〉１０＾１２Ω．ｃｍ，５００℃下，ρｖ〉１０＾８Ω．ｃｍ），高抗电强度（Ｅｊ〉１５ｋＶ／ｍｍ），热稳定性好的材料，并探讨了低温烧结的机理方法和途径。     

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.     

Low-Temperature Sintering of Iron Oxides

To study agglomeration in iron oxides during low-temperature sintering, submicron magnetite (Fe₃O₄) powders were processed in air and in vacuum at 773, 793, and 843 K. Specific surface area measurements were used to monitor sintering progress. Oxidation of the magnetite altered the sintering kinetics, which were dominated by surface diffusion in this temperature range.     

A Sintering Kinetics Model for Ceramic Dual‐Phase Composite

An analytical model is developed for the sintering kinetics of ceramic dual‐phase composites of the cathodes in solid oxide fuel cells (SOCFs). The model simulates the isothermal and pressureless sintering processes and formulates volumetric three‐phase boundary (TPB) length and porosity as a function of sintering time, surface/interface energies, grain‐boundary diffusivities, particle sizes, and dual‐phase composition. Lanthanum strontium manganite (LSM)–yttria‐stabilized zirconia (YSZ) composite is used as an example to develop and validate the model. LSM–YSZ composites are sintered at 1100°C for various sintering time, and the TPB length and porosity are estimated from SEM images by using stereological analysis to validate the model. Parametric studies are performed at various conditions, illustrating novel insights into the sintering kinetics. This analytical model is generic and applicable to the sintering kinetics of ceramic dual‐phase composites for use in solid‐state electrochemical devices, such as SOCFs, electrolyzers, and gas separation membranes. This analytical model can also be easily extended to the sintering processes of other ceramic dual‐phase and triple‐phase composites.     

Low-Temperature Sintering of Uranium Dioxide

Studies of the fabrication of uranium oxide fuel pellets by the low-temperature sintering of nonstoichiometric oxide are described. Completer reversion to stoichiometric UO₂ in the sintered pellets was attained by two methods: (1) Sintering in a nitrogen atmosphere which contained a small amount of hydrogen and (2) sintering in pure nitrogen and then exposing the pellets to hydrogen at the sintering temperature. Large variations in sinterability were found in commercially procured ceramic-grade UO₂ powder. In studying these variations, it was discovered that fluorine was a powerful inhibitor of low-temperature sintering. This impurity could be readily removed by oxidation in air at 500°C. The data strongly indicated that the primary mechanism responsible for the removel of residual fluorine was pyrohydrolysis. It was found that a preliminary oxidation-reduction cycle activated the less-sinterable oxides so that in every case densities of at least 95% of theoretical were obtained by sintering at 1200°C.     

Low-Temperature Sintering of Aluminum Oxide

A fine-sized (∼0.1 μm), agglomerate-free Al₂O₃ dispersion was used to prepare homogeneous green bodies with ∼69% relative density and ∼10-nm median pore radius. Samples could be sintered at 1150°C to a relative density >99.5% and an average grain size of 0.25 μm.     

Effect of Densification Mismatch on Camber Development during Cofiring of Nickel-Based Multilayer Ceramic Capacitors

The development of camber during the cofiring of a two-layered structure of Ni-electrode/BaTiO₃-dielectric as a function of temperature has been investigated. At a given thickness of Ni electrode, less camber and camber rate with increasing thickness of BaTiO₃ dielectric have been observed. This phenomenon is attributed to the densification mismatch between the Ni electrode and the BaTiO₃ dielectric during cofiring. The mathematical analysis of camber development, based on a viscous model, shows significant agreement with experimental observations.     

Effect of Green Density on the Thermomechanical Properties of a Ceramic During Sintering

The thermomechanical properties of a commercial barium titanate were experimentally or theoretically determined for samples with green densities ranging from 45% to 55%. For stresses less than 300 kPa, sample deformation was determined to be linear viscous for all three stages of sintering. The shrinkage rates at a given temperature can differ by up to ∼25% as the green density changes from 45% to 55%, and the maximum shrinkage rate increased with decreasing green density. The increase in shrinkage rate with lower green density samples persisted through the final sintering stage. The viscosity was determined by cyclic loading dilatometry to range from 5 to 6 GPa·s in the initial stage of sintering, to 2 GPa·s in the intermediate stage, and to increase to 10–20 GPa·s for all specimens in the final stage of sintering. Differences in the final-stage viscosity were attributed to grain size differences. Relaxation times for the sintering body were estimated to be less than 1 s, indicating that viscous behavior is dominant throughout the sintering process.