The effect of processing conditions and porosity on the electrical properties of Y2O3-doped SrTiO3 internal boundary layer capacitors

Stephen J. Lombardo,Daniel S. Krueger 한양대학교 세라믹연구소 2007 Journal of Ceramic Processing Research Vol.8 No.1

Internal boundary layer capacitors of 0.8 mole% Y2O3-doped SrTiO3 were prepared by a co-milling process and then sintered in an oxidizing atmosphere between 1450-1550 oC for hold periods of 1-15 h. Large enhancements in the dielectric constant as a function of sintering conditions were observed, as compared to pure SrTiO3. The electrical properties of effective dielectric constant and effective conductivity were strongly correlated with each other depending on the frequency, and both properties decreased strongly with increasing porosity. To account for this strong dependence, three microstructural models were developed based on random porosity, porosity intersecting the grain boundary region, and porosity localized in the grain boundary region. For cases when the porosity preferentially reduces the grain boundary area, the models qualitatively predict a very strong dependence for the both the dielectric constant and conductivity on the pore volume fraction. Internal boundary layer capacitors of 0.8 mole% Y2O3-doped SrTiO3 were prepared by a co-milling process and then sintered in an oxidizing atmosphere between 1450-1550 oC for hold periods of 1-15 h. Large enhancements in the dielectric constant as a function of sintering conditions were observed, as compared to pure SrTiO3. The electrical properties of effective dielectric constant and effective conductivity were strongly correlated with each other depending on the frequency, and both properties decreased strongly with increasing porosity. To account for this strong dependence, three microstructural models were developed based on random porosity, porosity intersecting the grain boundary region, and porosity localized in the grain boundary region. For cases when the porosity preferentially reduces the grain boundary area, the models qualitatively predict a very strong dependence for the both the dielectric constant and conductivity on the pore volume fraction.

Effect of processing on the microstructure and deformation of MgO-Al2O3, CaOAl2O3, and SiO2-Al2O3 layered composites

Stephen J. Lombardo,Chang Soo Kim 한양대학교 세라믹연구소 2007 Journal of Ceramic Processing Research Vol.8 No.1

A process is developed whereby thin ceramic substrates layered with a coating are induced to adopt a different shape at elevated temperature without the application of external pressure. To achieve such deformation, thin beams of alumina are coated with magnesia, calcia, or silica; these composite beams are then heated to high temperature where deformation occurs. Scanning electron microscopy and compositional profiling indicate that for MgO and CaO coatings on alumina, a bilayer structure forms. For SiO2 coatings on alumina, silicon is found throughout the substrate. A viscoelastic mechanics model based on strain mismatch in bilayer systems indicates how the curvature depends on the thickness of the two regions. A mechanistic model is presented that describes the strain mismatch in terms of the degree of conversion to a new phase, the amount of differential sintering between layers, and the amount of residual porosity present in the two layers of the substrate. A process is developed whereby thin ceramic substrates layered with a coating are induced to adopt a different shape at elevated temperature without the application of external pressure. To achieve such deformation, thin beams of alumina are coated with magnesia, calcia, or silica; these composite beams are then heated to high temperature where deformation occurs. Scanning electron microscopy and compositional profiling indicate that for MgO and CaO coatings on alumina, a bilayer structure forms. For SiO2 coatings on alumina, silicon is found throughout the substrate. A viscoelastic mechanics model based on strain mismatch in bilayer systems indicates how the curvature depends on the thickness of the two regions. A mechanistic model is presented that describes the strain mismatch in terms of the degree of conversion to a new phase, the amount of differential sintering between layers, and the amount of residual porosity present in the two layers of the substrate.

Development of a heating schedule for rapid thermal debinding of green multilayer ceramic capacitors

Stephen J. Lombardo,Kumar Krishnamurthy 한양대학교 세라믹연구소 2010 Journal of Ceramic Processing Research Vol.11 No.4

A method has been demonstrated to debind rapidly green multilayer ceramic capacitors by a thermal route. The binder consists of a blend of an acrylic polymer and a polyester adipate; the dielectric is a titanate-based material. The thermal decomposition kinetics of the binder, the gas permeability of the open-pore green body, and the failure conditions of the green body were determined from the results of both experiments and modeling. An algorithm based on variational calculus was then used to prescribe a rapid heating schedule for binder removal, without introducing defects into the green components. The cycle developed was 80% shorter than the corresponding cycle used in industry

Methods for Introducing Safety Factors into Minimum Time Heating Cycles for Binder Removal from Green Ceramic Bodies

Stephen J. Lombardo,Jeong Woo Yun 한양대학교 세라믹연구소 2007 Journal of Ceramic Processing Research Vol.8 No.6

Heating schedules have been simulated for the minimum time for binder removal from green ceramic components that contain open porosity. To determine such heating schedules, a number of parameters appearing in the model must be specified, including the kinetic parameters of binder degradation, the permeability of the green body, and the temperature and pressure at which the green body fails. Because of uncertainty in these quantities, safety factors can be applied to these model parameters to obtain a more conservative heating cycle that begins at a lower temperature and is longer in duration than the minimum time heating cycle. Heating schedules have been simulated for the minimum time for binder removal from green ceramic components that contain open porosity. To determine such heating schedules, a number of parameters appearing in the model must be specified, including the kinetic parameters of binder degradation, the permeability of the green body, and the temperature and pressure at which the green body fails. Because of uncertainty in these quantities, safety factors can be applied to these model parameters to obtain a more conservative heating cycle that begins at a lower temperature and is longer in duration than the minimum time heating cycle.

Effects of a combined supercritical extraction/thermal cycle on binder removal cycle time, yield, and residual carbon of multilayer ceramic capacitors

Brandon Abeln,Stephen J. Lombardo 한양대학교 세라믹연구소 2011 Journal of Ceramic Processing Research Vol.12 No.5

A combined cycle, consisting of a supercritical extraction cycle (SCE) followed by a thermal cycle (TC), was used to remove binder from green multilayer ceramic capacitors. The dielectric was barium titanate and the binder consisted of poly(vinyl butyral) plasticized with phthalates. Supercritical extraction of approximately one-third of the binder leads to an increase in the porosity and gas permeability of samples and to a decrease in the adhesion strength between layers. The partial removal of the binder also resulted in a decrease in cycle time and defects following the combined SCE/TC. Samples subjected to the combined SCE/TC also had 25-30% less residual carbon as compared to samples subjected to a thermal cycle alone.

Modeling of green body strength, internal pressure, and stress in porous ceramic bodies during thermal debinding

Rajiv Sachanandani,Stephen J. Lombardo 한양대학교 세라믹연구소 2011 Journal of Ceramic Processing Research Vol.12 No.1

A model is developed to describe the evolution of pressure, stress, and green body strength in porous ceramic bodies during the thermal debinding heating cycle. Pressure in the green body arises from the thermally activated decomposition of the binder, and the pressure increase is mitigated by flow through the porous medium. The stress then arises from the pressure gradient within the green ceramic body. The strength model incorporates the effects of solids and binder loading, and the effect of temperature on the yield behavior of the binder. The stress is then compared to the strength of the green body during the heating cycle in order to determine when failure occurs. The assumptions, the necessary experimental data, and the weaknesses in the model are discussed.

Curvature and bifurcation of MgO-Al₂O₃ bilayer ceramic structures

Chang Soo Kim,Stephen J. Lombardo 한양대학교 세라믹연구소 2008 Journal of Ceramic Processing Research Vol.9 No.2

Ceramic bilayer structures are formed by coating a green alumina substrate with an aqueous magnesia slurry and then heating the composite body. At high temperature, diffusion and reaction to a spinel can occur, and the body deforms. The deformation of the bilayer structure is attributed to the phase change accompanying spinel formation and to differential sintering between layers. As compared to other bilayer structures obtained by deposition methods, relatively thick top layers are obtained and large strain mismatches occur. At small ratios of the thickness of the top to bottom layer for bilayer plates, equal curvature is observed in two directions whereas at larger thickness ratios, a bifurcation in the deformation behavior is observed with unequal curvature in two directions. Ceramic bilayer structures are formed by coating a green alumina substrate with an aqueous magnesia slurry and then heating the composite body. At high temperature, diffusion and reaction to a spinel can occur, and the body deforms. The deformation of the bilayer structure is attributed to the phase change accompanying spinel formation and to differential sintering between layers. As compared to other bilayer structures obtained by deposition methods, relatively thick top layers are obtained and large strain mismatches occur. At small ratios of the thickness of the top to bottom layer for bilayer plates, equal curvature is observed in two directions whereas at larger thickness ratios, a bifurcation in the deformation behavior is observed with unequal curvature in two directions.

Effect of green body size and heating rate on failure during thermal debinding and on the debinding cycle time

Rajiv M. Sachanandani,Stephen J. Lombardo 한양대학교 세라믹연구소 2011 Journal of Ceramic Processing Research Vol.12 No.2

Multilayer ceramic green bodies, prepared with barium titanate as the dielectric and poly(vinyl butyral) and butyl benzyl phthalate as the main components of the binder, have been subjected to rapid heating cycles in order to cause failure to occur. In one approach, the heating rate was held constant and the dimensions of the green bodies were varied, while in the second method, the size of the green body was held constant and the heating rate was varied. In all cases, failure of the green body occurred between 115-140 oC. Models were then used to establish that at failure, the binder loading was high and that the internal pressure in the center of the green body was nearly constant, independent of the method used to cause the samples to fail. The internal pressure was then used as an input into a previously-developed algorithm in order to develop rapid debinding cycles without causing component failure.