High temperature tensile behavior of zirconia ceramics under DC current

These Flash sintering phenomena, which occurs by applying DC current directly to ceramic powder compacts, has been the subject of many paper of ceramic sintering. This is because the flash event can succeed to lower the sintering temperature/time of several ceramic powders. On the other hand, Conrad and his colleagues examined the effect of electric fields on the high temperature tensile properties of 3Y-TZP and confirmed that the fields can lower the tensile flow stresses of 3Y-TZP enough to attain superplasticity. The enhanced deformation was explained by suppressed grain growth due to the electric bias effect. However, the mechanism/phenomena of the flash event are still unclear. In order to clarify the effect of electric current on high temperature deformation, therefore, the present study was carried out to examine the tensile behavior of polycrystalline zirconia ceramics under the several temperature and electric field/current conditions. By applying the DC electric power higher than a critical value Ec, the flash event similar to that of powder sintering occurs even in dense zirconia ceramics. At around 1000 °C, for example, the Ec value is about 100 - 200 mW/mm3, which is slightly larger than those reported in the powder compacts. For lower than Ec, the applied electric current increases sample temperature depending on the applied value, but does not enhance the rate of deformation. For higher than Ec, on the other hand, the electric current enhances the rate of the deformation to about several times as compared with that of without current conditions. The enhanced deformation cannot be interpreted only by the increment of sample temperatures and is likely to occur by the flash event. After the deformation under the electric current conditions, the tested sample shows slight gray color even under air condition. This suggests that the enhanced deformation would be related to oxygen vacancy formation. In the presentation, we will discuss the detailed current effect obtained at wide range testing conditions

Effects of Initial Punch-Die Clearance in Spark Plasma Sintering Process

In recent years the number of publication on SPS has grown exponentially although the process itself is still not completely understood. The SPS process is governed by a complex bulk and contact multiphysics. The contact resistance between SPS system elements is crucial to understand the overall current and temperature distribution. This work is undertaken in order to improve our fundamental understanding on the contact resistance behaviour in the SPS system. The effect of the initial punch-die clearance is experimentally investigated for the first time during SPS heating. The results are given in terms of temperature-time and voltage-time profiles for various clearance values. It is shown that the initial punch-die clearance is a key SPS parameter to be specified when different SPS experiments or apparatus features have to be compared and offers another means to control the temperature gradient along the radius

Processing of zirconia and calcium aluminate cement mixtures by spark plasma sintering

Spark Plasma sintering (SPS) was applied for the densification of Calcia stabilized ZrO2 based composites obtained frommixtures of pure zirconia (m-ZrO2) and calcium aluminate cement (HAC). Two commercial powders of pure zirconia wereemployed as reactants. One of these powders had a coarse mean particle size (d50 = 8 μm) and the other was a submicrometersized power (d50 = 0.44 μm). Several compositions containing different proportions of HAC (5 to 30 mol. % CaO in ZrO2)were sintered by SPS at temperatures between 1200 and 1400ºC under a pressure of 100 MPa during 10 min. The effect ofprocessing conditions on phase composition, densification, microstructure and Vickers hardness of the obtained compositeswas examined. SPS significantly enhanced the densification in both type of composites (relative density > 93 %) as comparedto those previously produced by conventional sintering. Composites with low CaO content consisted of mixtures of c-ZrO2,(Ca0.15Zr0.85O1.85), unreacted m-ZrO2 and calcium dialuminate (CaAl4O7 or CA2). The highest hardness was determined forcomposites sintered at 1400ºC being related to the maximum relative density (~ 99 %). High densification of compositeswith 30 mol. % CaO composed by similar proportions of CaAl4O7 and c-ZrO2 were obtained even at 1200ºC but led to aslightly lower hardness. In general, the use of the finer m-ZrO2 powder contributed to increase both the c-ZrO2 content anddensification of composite sintered at a relatively lower temperature. For these composites, best hardness (Hv near to 10GPa) resulted when the microstructure consisted of a fine grained ZrO2 matrix surrounding the dispersed CaAl4O7 grainsinstead of large interconnection between grains of both phases existed.Fil: Bruni, Yesica Lorena. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Centro de Tecnología de Recursos Minerales y Cerámica. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Tecnología de Recursos Minerales y Cerámica; ArgentinaFil: Suarez, Gustavo. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Centro de Tecnología de Recursos Minerales y Cerámica. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Tecnología de Recursos Minerales y Cerámica; ArgentinaFil: Sakka, Yoshio. National Institute for Materials Science; JapónFil: Garrido, Liliana Beatriz. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Centro de Tecnología de Recursos Minerales y Cerámica. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Tecnología de Recursos Minerales y Cerámica; ArgentinaFil: Aglietti, Esteban Fausto. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Centro de Tecnología de Recursos Minerales y Cerámica. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Tecnología de Recursos Minerales y Cerámica; Argentin

Processing of zirconia and calcium aluminate cement mixtures by spark plasma sintering

Spark Plasma sintering (SPS) was applied for the densification of Calcia stabilized ZrO2 based composites obtained frommixtures of pure zirconia (m-ZrO2) and calcium aluminate cement (HAC). Two commercial powders of pure zirconia wereemployed as reactants. One of these powders had a coarse mean particle size (d50 = 8 μm) and the other was a submicrometersized power (d50 = 0.44 μm). Several compositions containing different proportions of HAC (5 to 30 mol. % CaO in ZrO2)were sintered by SPS at temperatures between 1200 and 1400ºC under a pressure of 100 MPa during 10 min. The effect ofprocessing conditions on phase composition, densification, microstructure and Vickers hardness of the obtained compositeswas examined. SPS significantly enhanced the densification in both type of composites (relative density > 93 %) as comparedto those previously produced by conventional sintering. Composites with low CaO content consisted of mixtures of c-ZrO2,(Ca0.15Zr0.85O1.85), unreacted m-ZrO2 and calcium dialuminate (CaAl4O7 or CA2). The highest hardness was determined forcomposites sintered at 1400ºC being related to the maximum relative density (~ 99 %). High densification of compositeswith 30 mol. % CaO composed by similar proportions of CaAl4O7 and c-ZrO2 were obtained even at 1200ºC but led to aslightly lower hardness. In general, the use of the finer m-ZrO2 powder contributed to increase both the c-ZrO2 content anddensification of composite sintered at a relatively lower temperature. For these composites, best hardness (Hv near to 10GPa) resulted when the microstructure consisted of a fine grained ZrO2 matrix surrounding the dispersed CaAl4O7 grainsinstead of large interconnection between grains of both phases existed.Fil: Bruni, Yesica Lorena. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Centro de Tecnología de Recursos Minerales y Cerámica. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Tecnología de Recursos Minerales y Cerámica; ArgentinaFil: Suarez, Gustavo. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Centro de Tecnología de Recursos Minerales y Cerámica. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Tecnología de Recursos Minerales y Cerámica; ArgentinaFil: Sakka, Yoshio. National Institute for Materials Science; JapónFil: Garrido, Liliana Beatriz. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Centro de Tecnología de Recursos Minerales y Cerámica. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Tecnología de Recursos Minerales y Cerámica; ArgentinaFil: Aglietti, Esteban Fausto. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Centro de Tecnología de Recursos Minerales y Cerámica. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Tecnología de Recursos Minerales y Cerámica; Argentin

Dense lanthanum silicate oxyapatite ceramics obtained by uniaxial pressing and slip casting

Lanthanum silicate oxyapatite (LSO) is a promising ion conductive ceramic material, which has higher oxygen ion conductivity at intermediate temperatures (600–800 °C) compared to yttria-stabilized zirconia. Its mechanical properties, though important for any of its applications, have been scarcely reported. In this study, we compare apparent densification, open porosity and Vickers hardness of samples conformed by uniaxial pressing and slip casting and fired up to 1600 °C. Colloidal processing was optimized for slip casting in order to get high green densities. At sintering temperatures higher than 1400 ºC, both processing routes yielded comparable densities, although uniaxially pressed samples show slightly better mechanical properties, evidencing that slip cast ones already underwent a grain growth process.Fil: Moreira Toja, Ramiro Julián. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Centro de Tecnología de Recursos Minerales y Cerámica. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Tecnología de Recursos Minerales y Cerámica; ArgentinaFil: Rendtorff Birrer, Nicolás Maximiliano. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Departamento de Química; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata; ArgentinaFil: Aglietti, Esteban Fausto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Departamento de Química; ArgentinaFil: Uchikoshi, Tetsuo. Tsukuba University. National Institute For Materials Science; JapónFil: Sakka, Yoshio. Tsukuba University. National Institute For Materials Science; JapónFil: Suarez, Gustavo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Departamento de Química; Argentin

Influence of the zirconia transformation on the thermal behavior of zircon-zirconia composites

During a heating–cooling cycle, zirconia (ZrO2) undergoes a martensitic transformation from monoclinic to tetragonal structure phases, which presents special hysteresis loop in the dilatometry curve at temperatures between 800 and 1100 °C. Monoclinic zirconia (m-ZrO2) particles reinforced ceramic matrix composites not always present this behavior. In order to elucidate this fact a series of zircon–zirconia (ZrSiO4–ZrO2) ceramic composites have been obtained by slip casting and characterized. The final properties were also correlated with the zirconia content (0–30 vol.%). The influence of the martensitic transformation (m–t) in well-dispersed zirconia grains ceramic composite on the thermal behavior was analyzed. Thermal behavior evaluation was carried out; the correlation between the thermal expansion coefficients with the zirconia content showed a deviation from the mixing rule applied. A hysteresis loop was observed in the reversible dilatometric curve of composites with enough zirconia grains (≥10 vol.%). Over this threshold the zirconia content is correlated with the loop area. The transformation temperatures were evaluated and correlated with the zirconia addition. When detected the m–t temperature transformation is slightly influenced by the zirconia content (due to the previously evaluated decrease in the material stiffness) and similar to the temperature reported in literature. The reverse (cooling) transformation temperature is strongly decreased by the ceramic matrix. The DTA results are consistent with the dilatometric analysis, but this technique showed more reliable results. Particularly the endothermic m–t transformation temperature showed to be easily detected even when the only m-ZrO2 present was the product of the slight thermal dissociation of the zircon during the processing of the pure zircon material.Centro de Tecnología de Recursos Minerales y Cerámic

Intensity of sulfonitric treatment on multiwall carbon nanotubes

We present a study of sulfonitric treatment and its effect on MWCNTs at different temperatures (90, 110, 130 and 150ºC) using DRX, XPS, FTIR, Raman spectroscopy, TEM and zeta potential. It was found that oxidation starts with the C-C and C-H bonds generating different oxidized groups from alcohol to carboxylic acid, following a sequential oxidation. Given that heterocoagulation needs a maximum zeta potential gap between the ceramic and the MWCNT surface and it significantly exist a risk of manipulate acids at high temperature it is recommended to use acid treatment of CNT at 110 ºC for generating ceramic composites by heterocoagulation.Facultad de Ciencias ExactasCentro de Tecnología de Recursos Minerales y Cerámic

Hybrid hydrogels containing vertically aligned carbon nanotubes with anisotropic electrical conductivity for muscle myofiber fabrication

Biological scaffolds with tunable electrical and mechanical properties are of great interest in many different fields, such as regenerative medicine, biorobotics, and biosensing. In this study, dielectrophoresis (DEP) was used to vertically align carbon nanotubes (CNTs) within methacrylated gelatin (GelMA) hydrogels in a robust, simple, and rapid manner. GelMA-aligned CNT hydrogels showed anisotropic electrical conductivity and superior mechanical properties compared with pristine GelMA hydrogels and GelMA hydrogels containing randomly distributed CNTs. Skeletal muscle cells grown on vertically aligned CNTs in GelMA hydrogels yielded a higher number of functional myofibers than cells that were cultured on hydrogels with randomly distributed CNTs and horizontally aligned CNTs, as confirmed by the expression of myogenic genes and proteins. In addition, the myogenic gene and protein expression increased more profoundly after applying electrical stimulation along the direction of the aligned CNTs due to the anisotropic conductivity of the hybrid GelMA-vertically aligned CNT hydrogels. We believe that platform could attract great attention in other biomedical applications, such as biosensing, bioelectronics, and creating functional biomedical devices

Heterocoagulation and SPS sintering of sulfonitric-treated CNT and 8YZ nanopowders

Reinforcement of ceramic composites using carbon nanotubes (CNT) has been extensively studied for materials such as Al2O3, Si3N4 and tetragonal ZrO2. Knowledge concerning CNT composites based on a matrix of cubic zirconia (8YZ) is in short supply, however. This paper presents a study on the addition of 1 wt% CNT to an 8YZ matrix. CNT was functionalized by sulfonitric treatment at three different temperatures: 50, 90 and 130°C. To obtain strong bond between the CNT and the 8YZ particles, the composites were produced by electrostatic heterocoagulation followed by consolidation by spark plasma sintering (SPS). Dense 8YZCNT composites were successfully processed by the proposed route. A study of the influence of the surface treatment temperature of CNT on the final properties of ceramics is also presented. CNTs are dispersed uniformly and individually within the 8YZ matrix in 8YZ-CNT 90 and 8YZ-CNT130 composites. 8YZ-CNT50 displayed a less uniform CNT distribution and the largest grain size, suggesting that the lowest temperature acid pretreatment is less effective for the subsequent heterocoagulation mixture. The reinforcement of ceramic materials by the addition of 1 wt% CNT was confirmed by an evaluation of fracture toughness.Centro de Tecnología de Recursos Minerales y Cerámic

Near-infrared photoluminescence from molecular crystals containing tellurium

We report the observation of near-infrared photoluminescence from Te4(Ga2Cl7)2 and Te4(Al2Cl7)2 molecular crystals containing Te42+ polycations. The experimental and theoretical results clearly revealed that Te42+ polycation is one smart near-infrared emitter with characteristic emission peaks at 1252 and 1258 nm for Te4(Ga2Cl7)2 and Te4(Al2Cl7)2 crystals, respectively, resulting from the intrinsic electronic transitions of Te42+. Furthermore, it was also found that the emissions strongly depend on the excitation wavelengths for both Te4(Ga2Cl7)2 and Te4(Al2Cl7)2 samples, most possibly owing to the co-existence of other Te-related optically active centers. This research not only enriches the species of luminescent charged p-block element polyhedra and deepens the understanding of Te-related photophysical behaviors, but also may stimulate efforts for designing novel material systems using such active centers. It is also greatly expected that these sub-nanometer optically active species could exist in other systems such as glasses, polymers, and bulk optical crystals, and the stabilization of these centers in widely used hosts will pave the way for their practical applications