Deposition and Sintering of Particle Films on a Rigid Substrate

Experimental techniques for the deposition and sintering of ceramic particle films on a rigid substrate have been investigated. Three representative systems (SiO₂, ZnO-Bi₂O₃, and Al₂O₃) were chosen to determine the validity of the conceptual processing model. Crack-free, sintered films 1 to 50 μm thick were produced using single or multiple deposition/sintering cycles.     

Viscous Sintering on a Rigid Substrate

Two analyses are presented for the sintering kinetics of a porous glass layer on a rigid substrate. The first treatment uses a continuum model, with constitutive equations and the free strain rate derived from an appropriate microstructural model. Predictions are obtained for the sintering kinetics and the magnitude of the tensile stress in the layer. During sintering, shrinkage is not permitted in the plane of the substrate, but the resulting microstructural anisotropy is ignored by the model. A second treatment represents the sintering layer by tubes whose axes are normal to the substrate. The densification kinetics of this model are in reasonable agreement with the results of the continuum model. Therefore, the effects of microstructural anisotropy (pore orientation) are likely to be small, and either model can be used     

Constrained Sintering of Alumina Stripes on Rigid Substrates: Effect of Substrate Roughness and Coating

Micromolding in capillaries has been used to fabricate alumina stripes on smooth (R_a = 0.5 nm) and rough substrates (R_a = 900 nm). Different lateral (10–500 μm) and vertical stripe dimensions (8–27 μm) were used to study the influence of substrate roughness (smooth and rough sapphire) and substrate material (platinum coated and plain sapphire) on sintering behavior. Alumina stripes experience edge delamination during sintering on a rigid substrate independent of substrate roughness. However, enhanced substrate roughness reduced delamination length by half and lowered lateral strains by up to 0.10. Grooves in the rough substrate were found to be responsible for this feature as they act as crack propagation barriers and generate local density minima. Accompanying discrete element simulations revealed a localized triaxial stress state at the grooves of a sinusoidal‐shaped substrate interface as the main cause of the density minima. Platinum interlayers also resulted in reduced delamination by 40% in some stripe geometries while density was enhanced by 4% and lateral strains doubled in some geometries. Creep of the metal layer during sintering is thought to be the reason for this seemingly contradictory behavior.     

Crack Growth and Damage in Constrained Sintering Films

The constrained sintering of films on substrates leads to a reduction in densification rate and may lead to processing flaws. This paper reports on a study of damage and cracking in sintering films, with particular emphasis on the growth of preexisting cracks. Experiments have been conducted with glass and polycrystalline Al₂O₃ films on various substrates. The effect of important variables (viz., film thickness, crack length, and friction with the substrate) on crack growth is reported. The experiments with glass films show that cracking occurs above a critical film thickness which is in good quantitative agreement with a recent analysis for this problem. In the case of Al₂O₃ films, we observe a diffuse damage zone ahead of cracks. Crack growth occurs by the coalescence of microcracks with each other and with the main crack. Some possible reasons for this difference between the glass and Al₂O₃ films are presented. As a model for diffuse damage, the stability of a sintering film under spatial variations in constitutive parameters is analyzed. It is shown that the film is unstable to small perturbations only in the early stages of densification, and that for viscous sintering the films are usually kinetically stable.     

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.     

Measurement of the Sintering Pressure in Ceramic Films

An experimental technique is described to estimate the intrinsic sintering pressure in ceramic films. It involves measuring the time-dependent axial strain of the film during isothermal sintering under a varying tensile load. The applied stress when the axial strain rate goes to zero gives the measurement of the sintering pressure. Such experiments with titania tapes fabricated by tape casting showed the intrinsic sintering pressure to lie in the range of 200 to 130 kPa for densities in the range of 85% to 95% of the theoretical density. The sintering pressure decreased with increasing density. These results are in agreement with measurements of sintering pressure in alumina and titania obtained from sinter-forging experiments.     

Pressureless Sintering of a Ceramic Matrix with Multiple Rigid Inclusions: Finite Element Model

Interaction among the dispersed second phase of rigid inclusions in a ceramic matrix undergoing densification is investigated by a viscoelastic finite element method with a two-dimensional multiple-inclusion model. The interaction is negligible when the volume fraction of inclusions is low (<20%) and/or when the inclusions are uniformly distributed. The interaction becomes significant when the volume fraction of inclusions is high or the inclusions are agglomerated. The densification rates in the region between the inclusions are enhanced to some degree when the inclusions are closely packed. This enhancement will, however, cease when the inclusions get much closer, eventually coming in contact with each other. In general, the agglomeration or close packing of inclusions results in retardation in the overall densification of the sintering matrix.     

Image Analysis and Modeling of the Orientation of Pores in a Constrained Film on a Rigid Substrate

The nature of porosity in functional materials is often a critical parameter in determining their functionality, for example, in structure materials and fuel cell electrodes. Here, we study the development of the anisotropy of porous yttria‐stabilized zirconia (YSZ), focusing particularly upon the contribution of pore orientation to this anisotropy. Simulation from when the ink is deposited on the surface of a rigid substrate shows that platelike pores are found tend to align along the transverse direction of the substrate. Cross‐sectional image analysis of the pores from the attendant pores of YSZ particles or pore‐forming agent (PFA) matches with the simulated modeling when materials transport is insignificant in determining the shape of pores. The anisotropy created in the densification stage is separated from that in the green body by analyzing the image of porous structures formed with spherical glassy carbon which is unable to contribute to anisotropy during green‐state processing.     

Constrained Sintering of Alumina Thin Films: Comparison Between Experiment and Modeling

Alumina thin films deposited by dip coating on alumina substrates were sintered between 1150° and 1350°C. A new measuring system using a rocking arm as a mechanical amplifier allows in situ measurement of the shrinkage of the film. Comparison of experimental densification behavior with the predictions of the isotropic continuum mechanics model (using values of constitutive parameters determined by sinter forging) highlights the inadequacy of the isotropic models. These results, together with other published evidence, provide justification to consider anisotropic models.     

Anisotropic Microstructural Development During the Constrained Sintering of Dip-Coated Alumina Thin Films

Alumina thin films were manufactured from aqueous slurries by dip coating. Film thickness as function of substrate withdrawal velocity could be correctly modeled by Landau and Levich's theory. Samples were sintered on a rigid substrate at 1350°C for different isothermal times to achieve relative densities from 84% to 97%. Microstructural analysis of polished cross sections revealed a continuous development of pore alignment, as expected by theoretical considerations. With increasing density pores become more anisometric and orientate along the thickness direction. A further preferential orientation of pores was found in the normal plane, apparently due to the coating process. Constraining conditions had less influence on the size and shape of the grains; they tend to become more equiaxed in the constrained plane, presumably due to the biaxial tensile stress state.     

Improvement in the Temperature Stability of a BaTiO3-Based Multilayer Ceramic Capacitor by Constrained Sintering

In this study, a self-constrained BaTiO₃ material system, composed of a low-fire BaTiO₃-based X7R (Δ C / C ±15% within −55° to 125°C) MLCC dielectric and a high-fire, BaTiO₃-based X7R MLCC dielectrics eliminated sintering aid that are laminated on both sides of the BaTiO₃-based X7R MLCC, has been developed. The temperature dependence of capacitance of the BaTiO₃-based X7R MLCC is significantly improved to satisfy X8R requirements over the entire temperature range studied (Δ C / C ±15% within –55° to 150°C) using the self-constrained sintering. The curie temperature of BaTiO₃-based X7R MLCC increases on increasing the thickness of the constraining layers. Compared with the specimen fired by free sintering, a substantial reduction in grain size, leading to a decrease in dielectric constant, was observed in the specimen fired by constrained sintering with the thickness of the constraining layer being 170 μm. The increase in Curie temperature and decrease in grain size of the specimen fired by constrained sintering can be explained in terms of the presence of in-plane tensile stress. The in-plane tensile stress that introduces a friction force between the multilayer matrix and the nonshrinkage constraining layers to suppress the in-plane shrinkage and to inhibit the rate of grain growth was formed during constrained sintering.     

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%.     

烧结升温速率对低温共烧陶瓷基板性能的影响

烧结是低温共烧陶瓷(LTCC)基板工艺中关键工序之一,对LTCC基板的各项性能指标具有重要的影响。本文以国产MG60生瓷带为研究对象,研究了不同烧结升温速率对LTCC基板介电性能、翘曲度、膜层附着力、抗折强度等性能指标的影响,分析了基板性能变化的原因。结果表明,当升温速率为8 ℃/min时,基板介电常数为5.788,介电损耗为8.21×10^-4,基本无翘曲,烧结致密,附着力强,抗折强度达到175 MPa。     

Flaw Generation During Constrained Sintering of Metal-Ceramic and Metal–Glass Multilayer Films

Sintering of symmetrical multilayer films has been studied theoretically and experimentally. Experiments were conducted on ceramic/metal/ceramic and glass/metal/glass films. The sintering rate of free films was compared to the sintering rate of multilayers. The origin of processing flaws was examined. The following results were obtained: (1) Differential sintering rates of the components in the multilayer give rise to in-plane tensile and compressive stresses. The film that is stressed in tension in the early stage of sintering is most susceptible to fracture. Experiments with ceramic/metal/ceramic multilayer are in agreement with this prediction. (2) A theoretical prediction that the glass/metal/glass multilayer will not develop defects because of a high value of the shear relaxation factor in glass is confirmed by experiments. (3) The likelihood of developing a tensile stress in the multilayer depends only on geometry, the green density, and the ratio of the intrinsic sintering pressures. (4) The in-plane shrinkage of the multilayer depends on the difference in the free-sintering rates and the shear relaxation factors, and is reasonably well predicted by the analysis. (5) We have evidence that the metal layer deforms plastically when it is placed in tension by differential sintering.     

Attaining a Homogeneous Microstructure of a ZnO-Based Multilayer Varistor by Constrained Sintering

The performance of a ZnO-based multilayer varistor (MLV) is affected strongly by the homogeneity of its microstructure. In this study, a homogeneous microstructure of ZnO-based MLV was attained by using constrained sintering when nonreactive borosilicate glass +90 wt% alumina (Al₂O₃) was used as a constraining layer laminated on both sides of the multilayer ZnO-based MLV. The mean grain size and the distribution of grain size of ZnO-based MLVs are both reduced because by constrained sintering, an in-plane tensile stress results from constrained sintering in the x – y plane of a multilayer device, which could modify the densification rate of the dielectric materials.     

Sintering and Crystallization Behavior of Machinable Fluorphlogopite–Gehlenite Glass–Ceramic

Easy machining is a characteristic of fluorphlogopite glass–ceramics, that sets them apart from other glass–ceramic systems. However, these materials have low hardness and mechanical strength, which limit their performance as a structural ceramic. In this paper, we attempted to overcome this problem by incremental addition of calcium oxide to a base glass composition. In this way, various parameters of the resulting glasses like their sintering and crystallization behavior were investigated, using X-ray diffractometry, differential thermal analysis, and scanning electron microscopy. According to our results, while small amounts of CaO improved the crystallization of fluorphlogopite and induced a decrease in the sinterability of glasses, higher amounts of CaO had the opposite effect, that is, it led to a reduction in the dimensions of mica crystalline particles and caused gehlenite to precipitate.     

Constrained Sintering of Silver Circuit Paste

Densification kinetics and stress development during constrained sintering of a silver film on a rigid silicon substrate have been studied. Compared with free sintering, the sintering of constrained silver film exhibits a much lower densification and slower densification kinetics. The densification-controlled mechanism changes from fast grain-boundary diffusion kinetics for free sintering to slow lattice diffusion kinetics for constrained sintering. The in-plane tensile stress developed during constrained sintering of silver film, measured using a noncontact laser-scanning optical system, increases rapidly to a maximum level of 1.0–1.5 MPa initially, gradually decreases, and then becomes constant at 0.8–1.0 MPa. The maximum stress observed increases with increasing sintering temperature as a result of the faster densification rate. It is believed that the retardation of densification kinetics of constrained silver film is caused by a change in densification mechanism and the existence of in-plane tensile stress.     

Constrained Sintering of Low-Temperature Co-Fired Ceramics

This paper discusses the effect of uniaxial compressive stress and pressureless constraint on the microstructure, density, and shrinkage anisotropy during the sintering of two commercial low-temperature co-fired ceramic (LTCC) systems, i.e., Heraeus CT2000 (CT) and DuPont 951Tape (DU). Under uniaxial compression, the ratio of axial to transverse shrinkage of DU is significantly higher than that of CT. A simple linear viscous theory was used to estimate the change in the strain rates produced by the external stress and the stress required to produce zero shrinkage. The theory was found to overestimate the measured stress-induced strain rates. The uniaxial compressive stress required for zero overall shrinkage was estimated to be ∼60 kPa for DU and 80 kPa for CT. The estimate for the DU materials was in good agreement with the experimental data, but there was significant deviation for the CT material. Higher viscosity and higher constraining stresses led to lower densities in pressure-less constrained CT specimens compared with DU.     

Energy Conversion during Microwave Sintering of a Multiphase Ceramic Surrounded by a Susceptor

A quasi-analytic model has been developed to examine energy conversion during the microwave sintering of a ceramic that is surrounded by a susceptor. Low-loss ceramics, such as ZrO₂, couple poorly with microwave radiation at low temperatures; however, because the dielectric loss usually increases rapidly as temperature increases, coupling improves dramatically at high temperatures. To improve heat transfer at low temperatures, susceptors are used. Three processes of energy flow are considered: microwave absorption due to dielectric losses, blackbody radiation, and heat convection. As expected, the susceptor (SiC) heats rapidly, relative to the ceramic (ZrO₂), at low temperatures; however, the ceramic attains higher temperatures after a prolonged period of microwave exposure. Below a critical temperature (800°C), the primary heat-transfer mechanism to the ZrO₂ is blackbody radiation from the susceptor. Above this temperature, microwave radiation is the main source that contributes to the temperature increase of the ceramic. The results of the simulation are in reasonable agreement with recent experimental data.     

Sintering of a Ceramic at Very Low Temperatures

Surface area, linear shrinkage, and compressive strength measurements show that, with the help of neutron irradiation, sintering of high-purity alumina can be initiated at temperatures lower than 150°C; such radiation sintering might be beneficial to many other ceramics.