A novel preparation route for ATZ (alumina toughened zirconia) ceramics is presented, using a com... more A novel preparation route for ATZ (alumina toughened zirconia) ceramics is presented, using a commercial ATZ powder (a mixture of 20 wt.% alumina and 80 wt.% zirconia containing 3 mol.% yttria) as a solid filler and an ATZ sol or gel (of the same composition) as a liquid binder for paste extrusion at room temperature. The pastes have a total oxide content of approx. 70 wt.% and during heat treatment the binder composition accommodates to the composition of the filler powder. Extruded samples are characterized before and after heat treatment by determining their shrinkage, bulk density, apparent density and apparent porosity. The optimal firing temperature is determined to be about 1550 °C. Quantitative X-ray phase analysis is used to establish the phase composition (ratio of monoclinic to tetragonal zirconia) and to calculate a (spatially averaged) mean value for the true density of the prepared nanocomposite after firing, which is 5.45 g cm -3 . For optimally sintered specimens the bulk density is approx. 5.06 g cm -3 , i.e. 92.8 % of the theoretical value. The total porosity after sintering is approx. 6.8 % (open 4.7 %, closed 2.1 %).
The sigmoidal average between the upper and lower Hashin-Shtrikman bounds is shown to be the appr... more The sigmoidal average between the upper and lower Hashin-Shtrikman bounds is shown to be the appropriate average relation for isotropic two-phase composites consisting of geometrically equivalent grains. This average ensures that the prediction is close the upper bound for low volume fractions of the low-conductivity phase (corresponding to low-conductivity inclusions in a high-conductivity matrix) and vice versa. In the intermediate concentration range the sigmoidal average reflects the fact that the microstructure is bicontinuous and can undergo a percolation-type transition. It is shown that the sigmoidal average of the Hashin-Shtrikman bounds lies automatically within the three-point bounds (Miller bounds) for a practically important class of microstructures (symmetric-cell materials with spherical cells) and that in the infinite-phase-contrast case (porous media) it is close to the exponential relation, which has been very successful in describing the porosity dependence of properties. Theoretical predictions are compared with experimentally measured values for alumina-zirconia composites in the whole range of volume fractions, from pure alumina to pure zirconia. Even after appropriate correction for porosity, essentially all experimental data are below the sigmoidal average. The fact that some values are even below the lower Hashin-Shtrikman bound is indicative of microcracking and / or grain size (interface) effects.
Porous ceramics have a wide range of applications at all length scales, ranging from fi ltration ... more Porous ceramics have a wide range of applications at all length scales, ranging from fi ltration membranes and catalyst supports to biomaterials (scaffolds for bone ingrowths) and thermally or acoustically insulating bulk materials or coating layers. Organic pore-forming agents (PFAs) of biological origen can be used to control porosity, pore size and pore shape. This work concerns the characterization and testing of several less common pore-forming agents (lycopodium, coffee, fl our and semolina, poppy seed), which are of potential interest from the viewpoint of size, shape or availability. The performance of these new PFAs is compared to that of starch, which has become a rather popular PFA for ceramics during the last decade. The PFAs investigated in this work are in the size range from 5 μm (rice starch) to approximately 1 mm (poppy seed), all with more or less isometric shape. The burnout behavior of PFAs is studied by thermal analysis, i.e. thermogravimetry and differential thermal analysis. For the preparation of porous alumina ceramics from alumina suspensions containing PFAs traditional slip casting (into plaster molds) and starch consolidation casting (using metal molds) are used in this work. The resulting microstructures are investigated using optical microscopy, combined with image analysis, as well as other methods (Archimedes method of double-weighing in water, mercury intrusion porosimetry).
Journal of the American Ceramic Society, Jul 1, 2015
The rheology of suspensions and mechanical properties of green bodies with cordierite composition... more The rheology of suspensions and mechanical properties of green bodies with cordierite composition (raw materials 37 wt % kaolin, 41 wt% talc, 22 wt% alumina, resulting in 46.6 wt % SiO 2 , 38.1 wt% Al 2 O 3 , 13.6 wt% MgO) and two types of starch (corn or potato) are investigated. Rotational viscometry of suspensions with solids loading 50, 60, and 70 wt% without starch showed that all tend to be shear-thinning with a small degree of thixotropy. Suspensions with a total solids loading of 60 wt% with 25 wt% replaced by starch exhibited higher viscosity and thixotropy, but the viscometric behavior is almost identical for the two starch types (apparent viscosities 130-50 mPaÁs). Oscillatory rheometry shows that for suspensions with potato starch the onset temperature for gelatinization is 61°C-63°C, that is, lower than for corn starch (72°C-73°C). Maximum storage moduli and phase shift values after gelatinization are similar for both systems. The mechanical properties of green disks, measured via diametral compression tests, reveal clear differences between materials prepared with corn and potato starch, with the latter showing higher elastic modulus, higher strength, and higher deformation at fracture, obviously because of incompletely gelatinized starch granules in the green bodies prepared with corn starch.
Fil: Lambertini, A.. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientific... more Fil: Lambertini, A.. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Mar del Plata. Instituto de Investigacion En Ciencia y Tecnologia de Materiales (i); Argentina. Universidad Nacional de Mar del Plata. Facultad de Ingenieria; Argentina
DOAJ (DOAJ: Directory of Open Access Journals), Sep 1, 2013
The elastic properties of silica phases are reviewed. Available monocrystal data for crystalline ... more The elastic properties of silica phases are reviewed. Available monocrystal data for crystalline SiO 2 polymorphs (low-quartz, high-quartz, low-cristobalite, high-cristobalite, stishovite) are collected from the literature, and effective elastic constants (Young's moduli, shear moduli, bulk moduli and Poisson ratios) are calculated from these using Voigt-Reuss-Hill averaging. Both experimental data and simulation results are taken into account. A table of room temperature elastic constants for crystalline silica polymorphs and silica glass is given that lists the recommended current "state-of-the-art" values. All data are consistent with the well-known auxetic behavior of cristobalite at room temperature, and high-temperature simulation data published for cristobalite confirm auxetic behavior for all temperatures from room temperature up to more than 1500°C. The calculations of this paper show that also quartz can be auxetic, but only in a very limited temperature range around the low-to-high-quartz transition temperature (420-577°C). Experimental measurements of elastic properties of tridymite and cristobalite, including high-temperature measurements, are identified as a desideratum of future research.
Journal of The European Ceramic Society, Aug 1, 2016
Using transparent yttrium-aluminum garnet (YAG) as an example, the microstructure of dense polycr... more Using transparent yttrium-aluminum garnet (YAG) as an example, the microstructure of dense polycrystalline ceramics is investigated by microscopic image analysis. The mean chord length and Jeffries grain size determined via stereological relations are 9.9-11.3 m and 19.0-21.3 m after 2 h and 16 h of firing at 1735 • C, respectively. Distributions of grain size sections are determined via image analysis and transformed via Saltykov, Cruz-Orive and Woodhead transformations. None of the weighted means or deciles of these distributions corresponds to the mean chord length or Jeffries size exactly, but the distributions of 2D grain size sections are almost identical to the 3D grain size distributions obtained after transformation, which is possible only for Rayleigh distributions. It is shown that the grain size distributions determined here are indeed close to Rayleigh distributions that evidently arise as a result of certain grain growth and ripening processes (Ostwald ripening).
The temperature dependence of Young's modulus of silica refractories exhibiting pore volume fract... more The temperature dependence of Young's modulus of silica refractories exhibiting pore volume fractions in the range 18.6-20.9% is investigated via impulse excitation up to 800-1000 1C and four-point bending up to 500 1C. This temperature dependence exhibits a broad valley approximately below 200 1C, where Young's modulus values decrease to 55-62% of their room temperature magnitude. Upon further heating, Young's modulus increases steeply within a few degrees around 230 1C, followed by an S-shaped increase to maximum values corresponding to 174-282% of the room temperature values. Cooling branches are different from heating branches and do not form closed loops, indicating damage accumulation. Dilatation measurements, polarization microscopy, micromechanical calculations and X-ray phase analysis are used to underpin the conclusion that the observed elastic effects are not caused by phase transitions alone, but are combined effects of phase transitions and microcrack opening and partial closure.
Journal of The European Ceramic Society, Dec 1, 2021
Abstract Pure tin oxide (SnO2) ceramics is well known for its bad sinterability, more precisely f... more Abstract Pure tin oxide (SnO2) ceramics is well known for its bad sinterability, more precisely for the difficulty to densify without additives by conventional pressureless sintering. This is related to the fact that the sintering mechanisms in pure tin oxide ceramics are non-densifying (surface diffusion at low temperature and evaporation-condensation at high temperature). On the other hand, for the same reason, pure tin oxide ceramics is a very unusual model system that can be used to demonstrate the effects of microstructural changes on effective properties without the otherwise dominating effect of changes in porosity. In this paper we show that pure tin oxide ceramics uniaxially pressed at 50 MPa, pre-sintered at 500 °C and re-sintered at 1000, 1200 and 1400 °C exhibit relative Young’s modulus increases of 30, 70 and 120 % while the porosity remains essentially constant at a value of 51.6 ± 0.7 %.
In this work we demonstrate the application of stereology-based image analysis for the characteri... more In this work we demonstrate the application of stereology-based image analysis for the characterization of highly porous cellular ceramics (alumina foams) prepared by biological foaming with yeast and subsequent drying (80-105 °C) and firing (1570 °C). It is shown that the ceramics prepared usually have total porosities in the range 78-84 % and that the porosities made up by large pores (volume fraction of foam bubbles) are usually in the range 58-75 %. Further it is shown that the mean chord length and the Jeffries size, i.e. pore size measures related to the interface density and the mean curvature integral density, respectively, are relatively close to each other (usually 0.8-1.4 and 0.8-1.2 mm) with a ratio close to unity (0.9-1.3) and that the mean surface-to-surface distance of pores gives a realistic picture of the average pore wall thickness (usually 0.46-0.69 mm). Using a special processing variant (excess ethanol addition) it is possible to obtain microstructures with lower porosity (total porosity 68-70 %, foam bubble volume fractions 50-56 %) and smaller pore size (approx. 0.5 mm). Absolute errors are calculated using normalized deviations corresponding to 95 % reliability in the Student distribution and the standard errors for the quantities in question (both observed and estimated). Relative errors are found to be below 12 % when the number of measurements is of order 400-1000.
Using high-temperature impulse excitation the temperature dependence of damping, quantified by th... more Using high-temperature impulse excitation the temperature dependence of damping, quantified by the so-called inverse quality factor, has been measured for silica refractories with porosities of 13 − 17% up to 1200°C. Damping-temperature curves exhibit closed loops between heating and cooling and do not show any indication of damage accumulation after two complete heating-cooling cycles. The temperature dependence of damping exhibits features that are due to an interplay between phase transitions (between the subpolymorphs of tridymite and cristobalite) and microstructure (reversible closure reopening of preexisting microcracks) and are also known from resonant frequency measurements. Additionally, however, the temperature dependence of damping reveals some details that are undetectable by resonant frequency measurements and may tentatively be attributed to the competition between microcrack closure and increased dry friction (damping peak at approximately 325°C) and to a phase transition of the dicalcium silicate phase shannonite (damping increase in the range 700-800°C).
Cristobalite and tridymite are the main SiO 2 phases in silica bricks, a widespread refractory pr... more Cristobalite and tridymite are the main SiO 2 phases in silica bricks, a widespread refractory product. The elastic properties of cristobalite at room temperature have been extensively studied, because it is known for auxetic behavior, i.e. negative Poisson ratios, whereas the elastic properties of tridymite are essentially unknown. Here we show that silica brick materials, consisting almost entirely of tridymite and cristobalite, exhibit remarkable anomalies in the temperature dependence of the Young modulus: in the intermediate temperature range between approximately 50 and 250 °C these materials become very compliant, with stiffness minima of around 60 % of the room temperature values, with a broad transition region at the low-temperature end, a sharp transition at the high-temperature end and a precisely reproducible hysteresis during heating and cooling. Furthermore, it is shown that the Young's moduli at around 800 °C can be more than three times as high as the room temperature values.
A novel preparation route for ATZ (alumina toughened zirconia) ceramics is presented, using a com... more A novel preparation route for ATZ (alumina toughened zirconia) ceramics is presented, using a commercial ATZ powder (a mixture of 20 wt.% alumina and 80 wt.% zirconia containing 3 mol.% yttria) as a solid filler and an ATZ sol or gel (of the same composition) as a liquid binder for paste extrusion at room temperature. The pastes have a total oxide content of approx. 70 wt.% and during heat treatment the binder composition accommodates to the composition of the filler powder. Extruded samples are characterized before and after heat treatment by determining their shrinkage, bulk density, apparent density and apparent porosity. The optimal firing temperature is determined to be about 1550 °C. Quantitative X-ray phase analysis is used to establish the phase composition (ratio of monoclinic to tetragonal zirconia) and to calculate a (spatially averaged) mean value for the true density of the prepared nanocomposite after firing, which is 5.45 g cm -3 . For optimally sintered specimens the bulk density is approx. 5.06 g cm -3 , i.e. 92.8 % of the theoretical value. The total porosity after sintering is approx. 6.8 % (open 4.7 %, closed 2.1 %).
The sigmoidal average between the upper and lower Hashin-Shtrikman bounds is shown to be the appr... more The sigmoidal average between the upper and lower Hashin-Shtrikman bounds is shown to be the appropriate average relation for isotropic two-phase composites consisting of geometrically equivalent grains. This average ensures that the prediction is close the upper bound for low volume fractions of the low-conductivity phase (corresponding to low-conductivity inclusions in a high-conductivity matrix) and vice versa. In the intermediate concentration range the sigmoidal average reflects the fact that the microstructure is bicontinuous and can undergo a percolation-type transition. It is shown that the sigmoidal average of the Hashin-Shtrikman bounds lies automatically within the three-point bounds (Miller bounds) for a practically important class of microstructures (symmetric-cell materials with spherical cells) and that in the infinite-phase-contrast case (porous media) it is close to the exponential relation, which has been very successful in describing the porosity dependence of properties. Theoretical predictions are compared with experimentally measured values for alumina-zirconia composites in the whole range of volume fractions, from pure alumina to pure zirconia. Even after appropriate correction for porosity, essentially all experimental data are below the sigmoidal average. The fact that some values are even below the lower Hashin-Shtrikman bound is indicative of microcracking and / or grain size (interface) effects.
Porous ceramics have a wide range of applications at all length scales, ranging from fi ltration ... more Porous ceramics have a wide range of applications at all length scales, ranging from fi ltration membranes and catalyst supports to biomaterials (scaffolds for bone ingrowths) and thermally or acoustically insulating bulk materials or coating layers. Organic pore-forming agents (PFAs) of biological origen can be used to control porosity, pore size and pore shape. This work concerns the characterization and testing of several less common pore-forming agents (lycopodium, coffee, fl our and semolina, poppy seed), which are of potential interest from the viewpoint of size, shape or availability. The performance of these new PFAs is compared to that of starch, which has become a rather popular PFA for ceramics during the last decade. The PFAs investigated in this work are in the size range from 5 μm (rice starch) to approximately 1 mm (poppy seed), all with more or less isometric shape. The burnout behavior of PFAs is studied by thermal analysis, i.e. thermogravimetry and differential thermal analysis. For the preparation of porous alumina ceramics from alumina suspensions containing PFAs traditional slip casting (into plaster molds) and starch consolidation casting (using metal molds) are used in this work. The resulting microstructures are investigated using optical microscopy, combined with image analysis, as well as other methods (Archimedes method of double-weighing in water, mercury intrusion porosimetry).
Journal of the American Ceramic Society, Jul 1, 2015
The rheology of suspensions and mechanical properties of green bodies with cordierite composition... more The rheology of suspensions and mechanical properties of green bodies with cordierite composition (raw materials 37 wt % kaolin, 41 wt% talc, 22 wt% alumina, resulting in 46.6 wt % SiO 2 , 38.1 wt% Al 2 O 3 , 13.6 wt% MgO) and two types of starch (corn or potato) are investigated. Rotational viscometry of suspensions with solids loading 50, 60, and 70 wt% without starch showed that all tend to be shear-thinning with a small degree of thixotropy. Suspensions with a total solids loading of 60 wt% with 25 wt% replaced by starch exhibited higher viscosity and thixotropy, but the viscometric behavior is almost identical for the two starch types (apparent viscosities 130-50 mPaÁs). Oscillatory rheometry shows that for suspensions with potato starch the onset temperature for gelatinization is 61°C-63°C, that is, lower than for corn starch (72°C-73°C). Maximum storage moduli and phase shift values after gelatinization are similar for both systems. The mechanical properties of green disks, measured via diametral compression tests, reveal clear differences between materials prepared with corn and potato starch, with the latter showing higher elastic modulus, higher strength, and higher deformation at fracture, obviously because of incompletely gelatinized starch granules in the green bodies prepared with corn starch.
Fil: Lambertini, A.. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientific... more Fil: Lambertini, A.. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Mar del Plata. Instituto de Investigacion En Ciencia y Tecnologia de Materiales (i); Argentina. Universidad Nacional de Mar del Plata. Facultad de Ingenieria; Argentina
DOAJ (DOAJ: Directory of Open Access Journals), Sep 1, 2013
The elastic properties of silica phases are reviewed. Available monocrystal data for crystalline ... more The elastic properties of silica phases are reviewed. Available monocrystal data for crystalline SiO 2 polymorphs (low-quartz, high-quartz, low-cristobalite, high-cristobalite, stishovite) are collected from the literature, and effective elastic constants (Young's moduli, shear moduli, bulk moduli and Poisson ratios) are calculated from these using Voigt-Reuss-Hill averaging. Both experimental data and simulation results are taken into account. A table of room temperature elastic constants for crystalline silica polymorphs and silica glass is given that lists the recommended current "state-of-the-art" values. All data are consistent with the well-known auxetic behavior of cristobalite at room temperature, and high-temperature simulation data published for cristobalite confirm auxetic behavior for all temperatures from room temperature up to more than 1500°C. The calculations of this paper show that also quartz can be auxetic, but only in a very limited temperature range around the low-to-high-quartz transition temperature (420-577°C). Experimental measurements of elastic properties of tridymite and cristobalite, including high-temperature measurements, are identified as a desideratum of future research.
Journal of The European Ceramic Society, Aug 1, 2016
Using transparent yttrium-aluminum garnet (YAG) as an example, the microstructure of dense polycr... more Using transparent yttrium-aluminum garnet (YAG) as an example, the microstructure of dense polycrystalline ceramics is investigated by microscopic image analysis. The mean chord length and Jeffries grain size determined via stereological relations are 9.9-11.3 m and 19.0-21.3 m after 2 h and 16 h of firing at 1735 • C, respectively. Distributions of grain size sections are determined via image analysis and transformed via Saltykov, Cruz-Orive and Woodhead transformations. None of the weighted means or deciles of these distributions corresponds to the mean chord length or Jeffries size exactly, but the distributions of 2D grain size sections are almost identical to the 3D grain size distributions obtained after transformation, which is possible only for Rayleigh distributions. It is shown that the grain size distributions determined here are indeed close to Rayleigh distributions that evidently arise as a result of certain grain growth and ripening processes (Ostwald ripening).
The temperature dependence of Young's modulus of silica refractories exhibiting pore volume fract... more The temperature dependence of Young's modulus of silica refractories exhibiting pore volume fractions in the range 18.6-20.9% is investigated via impulse excitation up to 800-1000 1C and four-point bending up to 500 1C. This temperature dependence exhibits a broad valley approximately below 200 1C, where Young's modulus values decrease to 55-62% of their room temperature magnitude. Upon further heating, Young's modulus increases steeply within a few degrees around 230 1C, followed by an S-shaped increase to maximum values corresponding to 174-282% of the room temperature values. Cooling branches are different from heating branches and do not form closed loops, indicating damage accumulation. Dilatation measurements, polarization microscopy, micromechanical calculations and X-ray phase analysis are used to underpin the conclusion that the observed elastic effects are not caused by phase transitions alone, but are combined effects of phase transitions and microcrack opening and partial closure.
Journal of The European Ceramic Society, Dec 1, 2021
Abstract Pure tin oxide (SnO2) ceramics is well known for its bad sinterability, more precisely f... more Abstract Pure tin oxide (SnO2) ceramics is well known for its bad sinterability, more precisely for the difficulty to densify without additives by conventional pressureless sintering. This is related to the fact that the sintering mechanisms in pure tin oxide ceramics are non-densifying (surface diffusion at low temperature and evaporation-condensation at high temperature). On the other hand, for the same reason, pure tin oxide ceramics is a very unusual model system that can be used to demonstrate the effects of microstructural changes on effective properties without the otherwise dominating effect of changes in porosity. In this paper we show that pure tin oxide ceramics uniaxially pressed at 50 MPa, pre-sintered at 500 °C and re-sintered at 1000, 1200 and 1400 °C exhibit relative Young’s modulus increases of 30, 70 and 120 % while the porosity remains essentially constant at a value of 51.6 ± 0.7 %.
In this work we demonstrate the application of stereology-based image analysis for the characteri... more In this work we demonstrate the application of stereology-based image analysis for the characterization of highly porous cellular ceramics (alumina foams) prepared by biological foaming with yeast and subsequent drying (80-105 °C) and firing (1570 °C). It is shown that the ceramics prepared usually have total porosities in the range 78-84 % and that the porosities made up by large pores (volume fraction of foam bubbles) are usually in the range 58-75 %. Further it is shown that the mean chord length and the Jeffries size, i.e. pore size measures related to the interface density and the mean curvature integral density, respectively, are relatively close to each other (usually 0.8-1.4 and 0.8-1.2 mm) with a ratio close to unity (0.9-1.3) and that the mean surface-to-surface distance of pores gives a realistic picture of the average pore wall thickness (usually 0.46-0.69 mm). Using a special processing variant (excess ethanol addition) it is possible to obtain microstructures with lower porosity (total porosity 68-70 %, foam bubble volume fractions 50-56 %) and smaller pore size (approx. 0.5 mm). Absolute errors are calculated using normalized deviations corresponding to 95 % reliability in the Student distribution and the standard errors for the quantities in question (both observed and estimated). Relative errors are found to be below 12 % when the number of measurements is of order 400-1000.
Using high-temperature impulse excitation the temperature dependence of damping, quantified by th... more Using high-temperature impulse excitation the temperature dependence of damping, quantified by the so-called inverse quality factor, has been measured for silica refractories with porosities of 13 − 17% up to 1200°C. Damping-temperature curves exhibit closed loops between heating and cooling and do not show any indication of damage accumulation after two complete heating-cooling cycles. The temperature dependence of damping exhibits features that are due to an interplay between phase transitions (between the subpolymorphs of tridymite and cristobalite) and microstructure (reversible closure reopening of preexisting microcracks) and are also known from resonant frequency measurements. Additionally, however, the temperature dependence of damping reveals some details that are undetectable by resonant frequency measurements and may tentatively be attributed to the competition between microcrack closure and increased dry friction (damping peak at approximately 325°C) and to a phase transition of the dicalcium silicate phase shannonite (damping increase in the range 700-800°C).
Cristobalite and tridymite are the main SiO 2 phases in silica bricks, a widespread refractory pr... more Cristobalite and tridymite are the main SiO 2 phases in silica bricks, a widespread refractory product. The elastic properties of cristobalite at room temperature have been extensively studied, because it is known for auxetic behavior, i.e. negative Poisson ratios, whereas the elastic properties of tridymite are essentially unknown. Here we show that silica brick materials, consisting almost entirely of tridymite and cristobalite, exhibit remarkable anomalies in the temperature dependence of the Young modulus: in the intermediate temperature range between approximately 50 and 250 °C these materials become very compliant, with stiffness minima of around 60 % of the room temperature values, with a broad transition region at the low-temperature end, a sharp transition at the high-temperature end and a precisely reproducible hysteresis during heating and cooling. Furthermore, it is shown that the Young's moduli at around 800 °C can be more than three times as high as the room temperature values.
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