Constrained Sintering Research Articles

Monitoring Constrained Sintering of YSZ Coatings Using Fluorescence Spectroscopy and Micro-Hardness

The constrained sintering behavior of yttria stabilized zirconia coatings containing 1 wt. % alumina produced using electrophoretic deposition has been studied. Coatings have been characterized by scanning electron microscopy, micro-hardness and fluorescence spectroscopy after sintering in air at 1100°C and 1200°C. These coatings were compared with those that additionally contained 1 wt. % soda-lime glass. Micro-hardness and fluorescence spectroscopy are shown to accurately monitor the density of the coatings. Fluorescence spectroscopy showed that the density across the coating is even, as expected from modelling literature. The addition of 1 wt. % of soda-lime glass is shown to improve the sintered density of coatings.

Pore evolution model of ceramic membrane during constrained sintering

Pore size has been found to strongly depend on the sintering program in the preparation of porous ceramic membranes. In this paper, a model was developed to predict the variation in pore size and porosity of membranes during the sintering process. A comparison of shrinkage characteristics was made between the sintering processes of supported membranes and unsupported membranes. For supported membranes, the effect of restriction coming from a rigid substrate on the sintering behavior has been taken into account in the calculation. It is predicted that the pore size increases in supported membranes and decreases in unsupported membranes as the sintering temperature is increased. Calculations also showed that the loss of porosity in the supported membranes was less than that in the unsupported membranes. In order to verify reliability of this model, unsupported and supported membranes were prepared with α-Al2O3 powders at the sintering temperatures ranging from 1125 °C to 1325 °C. The pore size and porosity were measured by gas permeation technique and Archimedes’s method. The experimental results for the unsupported and supported α-Al2O3 membranes showed a good agreement with those calculated from the model. Therefore, this model provides an effective tool in predicting the porosity and the pore size of ceramic membranes at the different sintering temperatures.

Cofiring of Thin Zirconia Films During SOFC Manufacturing

High‐performance solid oxide fuel cells require a thin and gas‐tight electrolyte membrane that must be coated on a porous and relatively rough support. A pretreatment of the delivered submicronmeter electrolyte powder of 8 mol%‐yttria‐stabilized zirconia (8YSZ) yielded a reduced sintering mismatch between the anode substrate made from NiO/8YSZ and the electrolyte coating. Furthermore, it also enhanced the powder packing inside the green film. Constrained sintering usually leads to inadequate film density and an unfavorable pore deformation and orientation. It was demonstrated that these limitations can be resolved by using a coshrinking substrate in a planar cell design. Relative densities of >97% were achieved, which are higher than those for free‐standing layers. Additionally, the camber behavior was investigated in dependence of the temperature program with and without gravity effects, giving an overall suggestion for the cofiring parameters of the electrolyte.

Constrained Sintering of Yttria‐Stabilized Zirconia Electrolytes: The Influence of Two‐Step Sintering Profiles on Microstructure and Gas Permeance

Two‐step sintering was applied to yttria‐stabilized zirconia (YSZ) electrolytes screen‐printed on primed porous prefired substrates. The YSZ layers were sintered at a range of peak and dwell temperatures. Relative density values from 0.79 to 0.97 corresponded to grain sizes ranging from 0.6 to 10 μm, indicating that grain growth was not suppressed when applying these profiles to a constrained system. However, two of the samples sintered using the two‐step technique had a lower gas permeance compared with conventional samples. As the more successful two‐step profiles had lower peak temperatures than the conventional profile, this approach may provide other benefits.

Performance Enhancement of ZnO‐Based Multilayer Varistor by Constrained Sintering

The performance of a ZnO‐based multilayer varistor is affected strongly by the homogeneity of its microstructure. In this study, a homogeneous microstructure of a ZnO‐based multilayer varistor is attained by using constrained sintering, when nonreactive borosilicate glass +90 wt% alumina (Al2O3) was used as the constraining layer laminated on both sides of the multilayer ZnO‐based multilayer varistor (MLV). The mean grain size and the distribution of grain size of a ZnO‐based MLV fired by constrained sintering are both reduced, because an in‐plane tensile stress results from constrained sintering in the x–y plane of the multilayer device, which could modify the densification rate of the ZnO‐based MLV materials. The leakage current and nonlinear coefficient (α) of ZnO‐based MLVs can be greatly improved due to the inhibition of ZnO grain growth when constrained sintering was used instead of free sintering. Note that the energy absorption capabilities in terms of peak current (PC) measurements of ZnO‐based MLVs fired by constrained sintering are remarkably improved due to a homogeneous microstructure.

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.

A further report on finite element analysis of sintering deformation using densification data—Error estimation and constrained sintering

In a previous paper, Kiani et al. [Kiani, S., Pan, J., Yeomans, J. A., Barriere, M. B. and Blanchart, P., Finite element analysis of sintering deformation using densification data instead of a constitutive law. J. Eur. Ceram. Soc., 2007, 27, 2377–2383] proposed an empirical numerical method to calculate the sintering deformation of ceramic powder compacts without knowing the viscosities and sintering potential. The method was validated by free sintering experiments using specimens with non-uniform initial densities. Two new developments are reported in this paper: (a) a method of error estimation is developed which can be used to check if the empirical analysis is valid after the analysis; (b) a range of case studies are presented showing that the empirical solutions provide very good approximations to the solutions obtained using full constitutive laws not only for free sintering but also for highly constrained sintering of single- or multi-layered films.

遠心焼結による厚膜プロセス

Constrained sintering, such sintering as thick films on rigid substrates, inherently involves the problem of macro-defect formation viz. crack and/or de-lamination. The origin of these defects lies in a mismatch of sintering shrinkage between different materials. To overcome the problem of film sintering, we have developed a novel sintering process, namely centrifugal sintering. The sintering is capable of applying the centrifugal acceleration of 100000 G during heating procedure. Due to the external pressure generated by centrifugal acceleration, shrinkage behavior of materials shows anisotropy: densification chiefly progresses along thickness direction of the film, while liner shrinkage along in-plane direction is limited. As a consequence, crack-free homogeneous microstructure is accomplished by the present process.

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 (ΔC/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 (ΔC/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.

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.

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.

Glass Infilteration in Bonding of BaTiO3 and Al2O3 Layers

A novel sintering process is proposed for bonding of BaTiO3 and Al2O3 layers. Common commercial glass was used and infilterated among filler particles. As the kind of commercial glass, the phenomenon of the infilteration is different. Although Sud-1140 glass forms a glass/filler composite, it is not completely infilterated into the filler particles at 900°C. However as the increase of sintering temperature the infilteration of glass was improved. In this study, GA-1 and GA-12 glasses were infilterated the more than Sud-1140 glass. However, they are reacted by BaTiO3 layer. The results of the experiment show that constrained sintering and the co-firing of the different materials were possible for glass infilteration using Sud-1140 glass at 1000°C.

LTCC system with new high- ɛr and high- Q material co-fired with conventional low- ɛr base material for wireless communications

The authors have developed a new LTCC material with characteristics of high dielectric constant ( ɛ r), high quality factor ( Q) and low temperature coefficient of capacitance (TCC). This material can be co-fired with a conventional base LTCC material and buried resistors with low temperature coefficient of resistance (TCR). The base material which consists of Al 2O 3 filler and glass, has low ɛ r of 8.7 at 3 GHz. The newly developed LTCC material, which consists of Ba–(Re)–Ti–O filler, Al 2O 3 filler, and glass, has the following characteristics of ɛ r of 15.1, Q of 900 at 3 GHz, and TCC of −10 ppm/K. The buried resistors consist of RuO 2 and glass. Two different LTCC materials, a resistor material and a silver electrode paste can be co-fired as multi-layer substrates and are regarded as a new LTCC system. Constrained sintering could be applied to this LTCC system and the dimensions of substrates could be controlled with quite high accuracy. This LTCC system is expected to contribute to further miniaturization of RF circuits and the reduction of electrical loss.

Constrained Sintering of the Glass-Alumina Substrate with Inner Alumina Particle Layer

Constrained Sintering of the Glass-Alumina Substrate with Inner Alumina Particle Layer

Simulations of the influence of rearrangement during sintering

A three-dimensional application of the discrete element method to free, constrained and pressure-assisted solid-state sintering of powders is presented. Special emphasis is laid on the generation of realistic random initial configuration as well as on the inertia effects. With this method, internal properties such as coordination number and pair correlation function are directly accessible and compare well with theoretical predictions and experimental data. The simulation scheme also enables one to investigate how grain rearrangements on a mesoscopic scale influence macroscopic properties such as densification rate and bulk and shear viscosity. Finally, crack formation during constrained sintering of low-density films is examined.

Constrained sintering of YSZ/Al 2O 3 composite coatings on metal substrates produced from eletrophoretic deposition

Yttria stabilized zirconia/alumina (YSZ/Al 2O 3) composite coatings were prepared from electrophoretic deposition (EPD), followed by sintering. The constrained sintering of the coatings on metal substrates was characterized with microstructure examination using electron microscopy, mechanical properties examination using nanoindentation, and residual stress measurement using Cr 3+ fluorescence spectroscopy. The microstructure close to the coating/substrate interface is more porous than that near the surface of the EPD coatings due to the deposition process and the constrained sintering of the coatings. The sintering of the YSZ/Al 2O 3 composite coating took up to 200 h at 1250 °C to achieve the highest density due to the constraint of the substrate. When the coating was sintered at 1000 °C after sintering at 1250 °C for less than 100 h, the compressive stress was generated due to thermal mismatch between the coating and metal substrate, leading to further densification at 1000 °C because of the ‘hot pressing’ effect. The relative densities estimated based on the residual stress measurements are close to the densities measured by the Archimedes method, which excludes an open porosity effect. The densities estimated from the hardness and the modulus measurements are lower than those from the residual stress measurement and the Archimedes method, because it takes account of the open porosity.

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.

Mechanics of sintering materials with bimodal pore distribution. II. Mean-square strain rate for the solid phase of a biporous body and instability of the monomodal pore structure during constrained sintering

Sintering of materials with a bimodal porous structure under conditions of external kinematic limitations that make their macroscopic deformation impossible is considered. It is established that in contrast to materials that contain pores of the same size, with sintering of materials that have a bimodal pore distribution under the conditions indicated above there is deformation within the limits of the matrix material. It is demonstrated that a monomodal pore structure under the same conditions is not stable, in the sense that small disturbance of the uniform pore distribution with respect to size may lead to marked stratification during sintering. Tangential stresses that arise in the solid phase as a result of the existence of two types of pores are evaluated.

Accurate Sintering Morphologies FromMonte Carlo Modelling

Through the two-dimensional problems of the sintering of an infinite row and an infinite square arrangement of mono-sized particles, the relevance of the Monte Carlo methodology based on the non-discrete energetical model is pointed out to calculate equilibrium morphologies. In the absence of coarsening, and for the two cases of free and constrained sintering, the calculated stable morphologies are shown to precisely superimpose that obtained from analytical resolution. Therefore, an accurate drawing of surface curvatures and dihedral angles results from that methodology which is one of the required conditions to deal with more complex microstructures and to be able to compare the calculated microstructures with experiments.