...funding research, sharing discoveries.
Vincenzo M Sglavo
professor of glasses and ceramics, university of trento - dimti
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2011 Feb; 196(4)
Flash-sintering of Co 2 MnO 4 spinel for solid oxide fuel cell applications
We show that cobalt manganese oxide (Co 2 MnO 4 ) spinel can be sintered (without the application of external pressure) in a few seconds at about 325 °C by applying a DC electrical field of 12.5 V cm −1 , by a process known as flash-sintering. A tran... expand abstractsition from normal to flash-sintering occurs when the field is ≥7.5 V cm −1 . The flash sintering phenomenon has also been observed in yttria-stabilized zirconia (3YSZ). Together, the results for 3YSZ and Co 2 MnO 4 point towards the generality of the process, since 3YSZ is an ionic conductor while the spinel is a predominantly electronic conductor. The Co 2 MnO 4 spinels are used to protect metals, such as stainless steels, in solid oxide fuel cells. The low temperatures employed in flash sintering can obviate interfacial interdiffusion with the metal substrate; in nominal sintering these interfacial reactions can produce deleterious interfacial phases. collapse abstract
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International Journal of Applied Ceramic Technology 2010 Oct; 7(6)
Sintering and Deformation of Solid Oxide Fuel Cells Produced by Sequential Tape Casting
No abstract is available for this article.
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2010 Sep; 58(17)
Vertical sintering to measure the uniaxial viscosity of thin ceramic layers
A novel method is proposed for the determination of the uniaxial viscosity of porous ceramic layers upon sintering. This approach is based on the application of a continuous but very low tensile stress to the densifying powder compact whose deformati... expand abstracton is continuously monitored by an optical system. The viscosity of the system can be determined as a function of temperature and density from the sintering rate differences measured between loaded and unloaded samples. The uniaxial viscosity of porous Y 2 O 3 doped ZrO 2 (YSZ) and NiO–YSZ composites was measured using the proposed approach. The results were used to predict the curvature evolution of bilayers used in solid oxide fuel cell applications, obtaining a fairly good agreement between the model and the data recorded experimentally. collapse abstract
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Journal of the American Ceramic Society 2009 Oct; 92(11)
AluminaSilicon Carbide Laminated Composites by Spark Plasma Sintering
Ceramic laminates composed of alumina/silicon carbide composite layers were produced by spark plasma sintering (SPS). Monolithic composite disks containing up to 30 vol% of silicon carbide were fabricated by stacking together and cosintering by SPS g... expand abstractreen layers prepared by tape casting water-based suspensions. An engineered laminate with a specific layer combination that is able to promote the stable growth of surface defects before final failure was also designed and produced. Fully dense materials with an optimum adhesion between the constituting layers and a homogeneous distribution of the two phases were obtained after SPS. Monolithic composites showed an increasing strength with SiC load, and biaxial strength values as high as 700 MPa were observed for a SiC content of 30 vol%. The engineered laminate showed a peculiar crack propagation that is responsible for the high strength value of about 600 MPa and for the evident insensitivity to surface defects. collapse abstract
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Journal of the American Ceramic Society 2008 Mar; 91(4)
Nondestructive Measurement of the Residual Stress Profile in Ceramic Laminates
Millimeter-thick symmetric ceramic laminates, designed to possess a specific through-thickness residual stress profile, were produced by tape casting from blends of alumina, zirconia, and mullite powders. The residual stress profile was checked nonde... expand abstractstructively by X-ray energy-dispersive diffraction using synchrotron white-beam radiation. Measurement of the average stress on very small volumes (ca. 10 μm along the specimen thickness) provided results in good agreement with the design data. Moreover, the possibility of independently measuring each crystalline phase composing the laminate allowed inferences to be made on stress partition and grain–grain coupling in the laminas. collapse abstract
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Journal of Thermal Spray Technology 2007 Jun; 3(1)
Indentation method for fracture resistance determination of metalceramic interfaces in thick TBCs
The indentation technique has been used to measure the adhesion of plasma- sprayed ceramic coatings on metals intended for thick thermal barrier coating ( TTBC) applications. This approach provides the adhesion value as the critical strain energy rel... expand abstractease rate,Gc, of the interface, which also takes into account any residual stresses. The theoretical background of the method is outlined, and specific examples are reported with respect to the effect of substrate temperature on the metal/ceramic adhesion of thick TBCs. Keywords adhesion - fracture toughness - indentation methods - interface properties - substrate temperature - thermal barrier - coatings - yttria-stabilized zirconia - ttbcs - vickers indentation collapse abstract
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2007 May; 353(16-17)
Strengthening of soda-lime-silica glass by surface treatment with sol–gel silica
In this study, the failure resistance of soda-lime-silica glass was increased by surface treatment with sol–gel silica. Samples annealed and ion-exchanged in KNO 3 for 24 h at 450 °C were considered. Sol–gel silica coating was carried out by dipping ... expand abstractthe glass samples into a sol suspension prepared by hydrolysis of Si(OEt) 4 in ethanol/water solution. The deposited layer was consolidated in air for 24 h and subjected to mild thermal treatment at 300 °C for 1 h. The surface treatment increased the fracture resistance of annealed glass of about 35 MPa; conversely, ion-exchanged specimens showed an average increase of about 90 MPa. The strengthening effect induced by the surface treatment was attributed to the reduction of the effective crack length generated by the silica coating. The different strength increase between annealed and ion-exchanged samples is discussed in terms of fracture toughness which, for ion-exchanged glass, is not constant, due to the presence of the surface residual stresses and thus the reduction of the crack length due to the silica coating determines a higher strength increase than for annealed glass. collapse abstract
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2007 May; 74(9)
In situ observation of crack propagation in ESP (engineered stress profile) glass
Important features of the ESP (engineered stress profile) glasses are the crack arrest and multiple cracking phenomena that occur even in an unstable stress field. In this work a detailed “in situ” observation of crack observation and analysis was pe... expand abstractrformed with the aim to examine crack propagation in detail and relate it to the residual stress field produced by ion exchange and to the final mechanical performances of the material. The results showed that the peculiar residual stress field with a maximum below the surface is responsible for the formation of a multitude of stable cracks on the tensile surface of the glass that evolved into through-thickness flaws. The propagation within the material is limited by the increasing compressive residual stress, which also leads to kinking of the cracks in a direction parallel to the surface. The observed fracture phenomena are also responsible for a shielding effect that makes the measured failure resistance of ESP glass larger than predicted by simplistic single crack models. collapse abstract
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2006 Oct; 177(19-25)
Tape casting fabrication and co-sintering of solid oxide “half cells” with a cathode–electrolyte porous interface
Solid oxide fuel cells in the planar anode-supported configuration were prepared by water-based tape casting and screen printing. In order to obtain a large extent of three phase boundaries (TPBs) at the electrolyte/cathode interface, a porous Yttria... expand abstract-Stabilized Zirconia with Yttria content equal to 10 mol% (10YSZ) interlayer was inserted in the green laminate. A composite cathode was then obtained by impregnating the porous electrolyte with a (La 0.8 Sr 0.2 )MnO 3 (LSM20) gel precursor and a LSM20 current collector was finally screen printed on the cathode layer. Electrochemical performances of the as-prepared cells were investigated in terms of open circuit voltage measurements, polarization and impedance spectroscopy analyses in the temperature range between 700 and 900 °C. For cells impregnated before the current collector sintering it was observed that the shrinkage of the LSM20 phase leads to a change in cathode/electrolyte morphology when samples are treated to 1150 °C. The electrochemical behaviour of cells impregnated before and after sintering at 1150 °C were investigated and it was observed that samples with the 10YSZ interlayer post-impregnated after the heat treatment at 1150 °C achieve higher power densities than pre-impregnated cells. collapse abstract
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Journal of the American Ceramic Society 2005 Oct; 88(11)
Processing and Thermal Shock Resistance of a PolymerDerived MoSi2SiCO Ceramic Composite
In this paper, we report a study on the thermal shock resistance (TSR) of MoSi2/SiCO ceramic composites obtained through controlled pyrolysis of a gel-derived precursor. MoSi2-filled gel is prepared by casting a sol obtained from MoSi2 powder dispers... expand abstracted in methyltriethoxysilane. The pyrolysis product can be described as a porous ceramic composite formed by a SiCO matrix with a dispersion of MoSi2 particles. Mechanical characterization is performed on bar samples by four-point bending. The TSR is investigated either by evaluating the R parameter (associated with strength, elastic modulus, and thermal expansion coefficient), or with the conventional water quenching technique. In both cases, the results suggest that the studied ceramic material displays a good TSR, which makes it a candidate for high-temperature application. collapse abstract
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Journal of the American Ceramic Society 2005 Sep; 88(10)
Tailored Residual Stresses in High Reliability AluminaMullite Ceramic Laminates
A design and processing approach to fabricate ceramic laminates with high mechanical reliability, i.e., high failure resistance, limited strength scatter, and increased damage tolerance is presented in this paper. Different ceramic layers are stacked... expand abstract together to develop a specific residual stress profile after sintering. By changing the composition of the laminae and the composite architecture it is possible to produce a material with predefined failure stress which can be evaluated from the fracture toughness curve correlated to the residual stresses. In addition, by tailoring the fracture toughness curve, surface defects can be forced to grow in a stable way before reaching the critical condition, thus obtaining a unique-value strength ceramic material. Laminates composed of alumina/mullite composite layers are designed and created in this work by the implementation of the proposed approach. The material obtained shows a “constant” strength of 456 MPa (standard deviation <7%) even when large surface damage is produced by Vickers indentation. collapse abstract
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2005 Jul; 37(8)
Procedure for residual stress profile determination by curvature measurements
A procedure is presented for the measurement of residual stress profile in regular geometry bodies such as plates and disks. The experimental work is based on the sole measurement of the sample curvature deriving from progressively etching one of its... expand abstract surfaces. A relationship is obtained to correlate curvature data as function of etching depth to the original residual stress profile. The sensitivity of the method and the influence of possible experimental errors are analyzed. The technique is then applied to strengthened glass samples produced by both thermal tempering and ion exchange and obtained results are compared to data obtained by available alternative technique to demonstrate the validity of the proposed approach. collapse abstract
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Journal of Materials Science 2005 Apr; 28(23)
Fabrication and characterization of polymer-derived Si
Amorphous Si-Zr-N-O powders, obtained by nitridation in an NH3 flow of zirconium modified polycarbosilane, have been sintered to full density by hot pressing at 1500C. The resulting ceramic shows an extremely fine-grained microstructure composed of S... expand abstracti2N2O and ZrO2 crystallites 20–30 nm in diameter. Thermal stability measured in air appears excellent up to 1300C for 48 h. Mechanical characterization pointed out good values of flexural strength (330 MPa), fracture toughness (4.1 MPam0.5) and Weibull modulus. collapse abstract
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Journal of Sol-Gel Science and Technology 2004 Dec; 2(s 1-3)
Aging effect on the mechanical properties of hybrid gels
Sol-gel derived unsupported films and thin rods have been obtained from co-hydrolysis of triethoxysilane and methyldiethoxysilane. The materials are flexible, dense and transparent. Films and rods have been aged for different periods of time in air a... expand abstractt room temperature. The elastic modulus has been measured by means of tensile or flexural tests. Measurements showed an increase of elastic modulus with aging time and showed different values for films and rods. The observed evolution of mechanical properties has been related to a corresponding structural modification as highlighted mainly by MAS-NMR studies. Analyses pointed out the crucial role of condensation processes and showed that the stiffness increase arises from the formation of relatively few bonds which link and constrain pre-existing mobile network regions.Keywords hybrid materials - elastic modulus - solid state NMR - aging - tensile strength collapse abstract
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Journal of Materials Science 2004 Oct; 34(3)
Indentation fatigure testing of soda-lime silicate glass
Dynamic, cyclic and static fatigue testing was performed on soda lime silicate glass using indentation strength measurements. Using the conventional analysis, the cyclic and static fatigue data were inconsistent with the remaining data for the case w... expand abstracthen the indentations were annealed prior to testing. Using an analysis that included the measured variations in the fracture mechanics geometric parameters, all three data sets were consistent. Using a numerical analysis, the lifetime for materials in the static and cyclic fatigue tests was shown to be sensitive to variations in the stress intensity factor at short crack lengths. It is therefore very important to understand any crack size dependent of the fracture mechanics parameters in this crack size region for accurate lifetime predictions. collapse abstract
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2004 Sep; 344(1-2)
Soda–borosilicate glass: normal or anomalous behavior under Vickers indentation?
Borosilicate glasses, together with silica glass, are commonly considered as ‘anomalous’ glasses. This definition regards the occurrence of Hertzian-like cone cracks during Vickers indentation. Such a particular behavior can be related to anelastic d... expand abstractensification processes occurring in the highly stressed region beneath the indenter. Conversely, glasses defined as ‘normal’ show mainly a median/radial crack system and plastic deformation is accounted for shear faulting. Quite surprisingly, in this work both median and conical crack systems were observed in soda–borosilicate glasses under Vickers indentation. Three glasses, with different composition mainly related to the network-modifier content, were considered. The surface and in-depth crack morphology was analyzed as a function of chemical composition, test environment and indenter geometry and the influence of the experimental conditions on the actual glass behavior was discussed. collapse abstract
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2004 Sep; 344(1-2)
Processing of glasses with engineered stress profiles
The introduction of a compressive residual stress with a maximum below the glass surface can promote the stable growth of surface flaws and result in high strength value with limited scatter. Such approach has been reduced to practice in recent years... expand abstract using a double ion-exchange process. Obtained glasses have been named as engineered stress profile (ESP) glasses to emphasize the engineering design approach. In the present work, an alternative production technique, based on physical processes to obtain the specific residual stress profile in soda-lime silicate glass is presented. The proposed technique consists of a conventional physical tempering followed by a fast heating treatment necessary to relax the compressive stress on the surface only. Such relaxation is produced by rapid treatment in a fused tin bath. ESP glasses produced in this manner are compared in terms of residual stress profile, mechanical strength and failure behaviour. collapse abstract
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2003 Jun; 23(8)
Comparison of the sandwiched beam (SB) and opposite roller loading (ORL) techniques for the pre-cracking of brittle materials
The “sandwiched beam” (SB) and “opposite roller loading” (ORL) methodologies suitable to introduce sharp through-thickness cracks in brittle materials are critically reviewed and compared in this work. In both cases a sharp crack is obtained in a not... expand abstractched specimen by means of a suitable loading. In the SB technique the specimen is placed between two support bars and bent in a 3- or 4-point configuration. The ORL procedure is based on the symmetrical loading by four rollers which induces a local tensile stress. Results show that both techniques are successfully usable on brittle materials: in both cases suitable specimens are obtained for fracture toughness measurements. The crack length can be reasonably controlled and varies in a wide range. The SB procedure typically provides cracks with α 0.5, while shorter cracks are obtained by the ORL technique. Fracture toughness is measured on specimens prepared using the two techniques. The obtained values result in good agreement with literature data. collapse abstract
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2003 Jun; 116(1-2)
Assessment of high power HEV lead-acid battery advancements by comparative benchmarking with a European test procedure
The technical and practical suitability of lead-acid batteries for applications in vehicles with electrical drivetrains (battery-powered or hybrid electric) has been experimentally investigated in a variety of testing programmes. Under the direction ... expand abstractand funding support of the Commission of the European Community, since early 1990s, the R&D Organisation EUCAR, a collaborative partnership of most European car manufacturers, has been conducting battery technological assessment projects, through bench tests carried out by different independent laboratories throughout Europe, using agreed test procedures. In this framework, ENEA acted as independent testing institute and tested, among others, three high power lead-acid batteries of various technologies (flat plate electrodes and spiral wound) for EV and HEV applications. In addition, different battery sizes and operating conditions have been tested at ENEA in a separate collaboration with ALTRA-IRISBUS. This paper intends to trace technological and performance improvements of high power lead-acid battery technology through the analysis of experimental data during parameter and life cycle tests, including the effects of battery sizes, charge/discharge profiles and testing procedures, with special emphasis on the reduction of the internal resistance and the variation of peak power and cycle life. collapse abstract
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Journal of the American Ceramic Society 2002 Sep; 85(10)
Influence of Composition on Fatigue Behavior and Threshold Stress Intensity Factor of Borosilicate Glasses
Fatigue behavior of borosilicate glasses was studied using the analysis of subcritical propagation of Vickers indentation cracks. Glasses containing various amounts of glass-network modifiers, mainly soda, were considered. Cone and median/radial crac... expand abstractk systems were observed, depending on glass composition, indenter geometry, and test environment. Indentation tests were performed in water, holding the maximum load for durations ranging from 15 s to 6 d. The analysis of the crack length as a function of dwell time allowed evaluation of the subcritical growth parameters and fatigue limit at crack arrest. The influence of composition on fatigue parameters and fatigue limit was discussed in terms of fourfold-coordinated boron atoms fraction compared with the content of glass-network modifier ions. collapse abstract
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Journal of the American Ceramic Society 2001 Jul; 84(8)
FlawInsensitive IonExchanged Glass: I, Theoretical Aspects
Stable crack growth can be produced on residually stressed surfaces, provided that the residual-stress profile is carefully chosen. Once identified, these conditions will lead to strengthening and an insensitivity of strength to the size of the initi... expand abstractal surface flaw. Theoretical calculations are performed that involve choosing idealized forms of either the required toughness curve or the residual-stress profile. The results of this analysis allow the range of stable growth and the strengthening to be determined. Thus, in the material-design process, if the range of surface flaws in the stress-free surface are known, then the required stress profile or toughness curve for optimum performance can be selected. collapse abstract
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Journal of the American Ceramic Society 2001 Jul; 84(8)
FlawInsensitive IonExchanged Glass: II, Production and Mechanical Performance
A two-step ion-exchange process that introduces strengthening and multiple cracking into glasses has been developed. The experimental parameters in the second step must be carefully controlled to avoid spontaneous cracking, because the compressive st... expand abstractress is reduced in the surface region. Control of the near-surface stress gradient is an important role of the second ion-exchange step. Double ion exchange leads to strength values as high as those that are obtained after the first ion-exchange step but remarkably produces multiple cracking on the specimen surface on loading. The multiple cracking, which acts as a failure warning, is initiated at 300 MPa, and the crack spacing decreases as the applied stress increases. The multiple cracking is shown to be a result of the crack stability that is induced by the designed residual-stress profiles. This behavior has been confirmed, using the measured residual stresses to calculate the apparent-toughness curves for surface cracks. The duration of the first ion-exchange step has been shown to be important in determining the degree of strengthening; however, the duration of the second step controls the extent of the crack stability. collapse abstract
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2001 Apr; 21(5)
Fatigue limit in fused silica
The threshold stress intensity factor for sub-critical crack growth in fused silica was measured using the interrupted static fatigue (ISF) test and from observations of indentation crack growth under load. Good agreement was obtained between the two... expand abstract techniques with values of 0.31–0.34 MPa√m and 0.30±0.02 MPa√m, respectively. The relative advantages of the two experimental approaches are discussed. collapse abstract
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2000 Feb; 20(3)
Bauxite ‘red mud’ in the ceramic industry. Part 2: production of clay-based ceramics
Some potential uses of red mud as a raw component in clay mixtures for ceramic bodies production are presented. The influence of increasing amounts of red mud on the forming procedure, sintering and final properties was analyzed. Samples were produce... expand abstractd by uniaxial pressing and slip casting. Two different clays are used as basic materials, the former being currently employed for the production of bricks by extrusion, the second — almost pure Kaolin — for high quality ceramic manufacturing. In both cases the addition of red mud led to more deflocculated solid–water systems and an increase of the critical moisture content. Mixtures prepared with the first clay and red mud loads up to 50% were fired at 850°C. The red mud content did not influence the sample porosity while determining a strength decrease attributed to the inertness of red mud at the working temperature. Samples produced using the second clay and red mud (0 – 20%) were fired at 950 and 1050°C. The addition of red mud determined increases of density and flexural strength which can be accounted for by the formation of a larger amount of glassy phase at higher red mud contents. The results of this work indicate excellent perspectives for using ‘red mud’ as raw material in mixtures with clay for the production of ceramic bodies. collapse abstract
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2000 Feb; 20(3)
Bauxite ‘red mud’ in the ceramic industry. Part 1: thermal behaviour
Samples of red mud, by-products of alumina production from bauxite, are studied in the 120–1400°C interval. An extensive characterization was performed by thermal and X-ray diffraction analyses. The identification of gaseous species released upon hea... expand abstractting was carried out by coupling the thermal analizer with a gas-chromatographic/mass spectrometer. Density evolution was also determined as a function of the heat treatment. Results indicate primary H 2 O release from aluminium hydroxides, followed by carbonate decomposition with CO 2 evolution below 900°C. Alkaline oxides, mainly CaO and Na 2 O, lead to the formation of Ca 3 Al 2 O 6 and NaAlSiO 4 between 900 and 1100°C. At the highest temperatures, reduction of Fe 3+ to Fe 2+ , involving O 2 release, promotes the formation of Fe 2 TiO 4 , with the disappearance of the rutile-TiO 2 phase. The various solid state reactions, ascertained at different stages of the heating process, and possible mass balances are discussed with reference to the state diagrams of principal red mud components. collapse abstract
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