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L0059 - High-temperature thermal characterization
x
G. Etherington, Ceramic Industry, feb 2007
L0058 - The current detour effect observed on materials with random microstucture: experimental evidence from Li3xLa2/3-xTiO3 studied by impedance spectroscopy
Impedance spectroscopy (IS) has been used to study the influence on the low frequency part of the impedance diagrams of the microstructure of a fast ionic conductor, Li3xLa2/3-xTiO3 with x = 0.10 (named hereafter LLTO). This oxide has been synthesised by sol-gel method. After synthesis, the powder of LLTO displays a large distribution of grain size and agglomerates. The grain size distribution and the porosity of the ceramic have been changed by heat-treatment from 600°C to 1200°C in air. The impedance spectra of these ceramics, recorded at different temperatures from room temperature (RT) to 400°C, show a low-frequency depressed arc, which is characteristic of the grain boundary response of the ceramic. Its shape depends strongly on the heat-treatment of the ceramic, and therefore, on its microstructure. It is a simple arc when the pellet is well sintered but becomes very complex for non-sintered ceramics with high resistive grain boundary and pores. The observed "fish" shape indicates the presence of current "detours effect" in the material. This effect means that current detours around blocking grain boundary and/or pores occur to lower the impedance. Consequently, the brick layer model (BLM), which assumes an ideal microstructure, and then no current "detours effect", can not be used to analyse these impedance data.
M. Vijayakumar, O. Bohnke, Journal of the European Ceramic Society 26 (2006) 3221-3231
L0057 - Deformation of steel powder compacts during sintering: Correlation between macroscopic measurement and in situ microtomography analysis
Powder compacts obtained by warm pressing of steel powder containing lubricant were found to swell during a typical delubrication cycle and shrink during sintering. The final dimensional changes of the denser compact were strongly anisotropic. Dilatometry measurements showed that final deformation resulted from microstructure induced by prior die pressing and from various phenomena arising at different stages of the cycle, each of them leading to a specific anisotropic effect. To corroborate this assumption the microstructure evolution of the denser material throughout sintering has been observed using in situ synchrotron X-ray computed microtomography. In addition to visual examination, quantitative analysis of 3D images consisted in searching for a relationship between the morphology and the orientation of pores in 2D sections and measuring local strain with an image correlation technique. From this information a schematic description of the main phenomena responsible for the deformation of steel powder compacts during sintering is proposed.
A. Vagnon, O. Lame, D. Bouvard, M. Di Michiel, D. Bellet, G. Kapelski, Acta Materialia 54 (2006) 513-522
L0056 - Piezo-spectroscopic characterization of alumina-aluminium titanate laminates
A multilayered alumina-aluminium titanate composite was prepared by a colloidal route from aqueous suspensions. The structure of the laminate was symmetric and constituted of two external Al2O3 layers (width=1750 µm), one central Al2O3 layer (width=1200 µm) and two intermediate thin (width=315-330 m) Al2O3-Al2TiO5 layers. Additional monolithic materials with the same compositions as those of the layers were fabricated as reference materials. Young's modulus of the monoliths was determined by three point bending. Dilatometry determinations were performed on green specimens, following the same heating and cooling schedules as those used for sintering the laminate, in order to determine the actual dimensional changes on cooling after sintering. The dimensional changes of the sintered specimens on heating and on cooling were also determined. Microscopic distributions of residual stresses were evaluated by fluorescence piezo-spectroscopy, and they revealed the existence of weak tensile and compressive hydrostatic stresses in the aluminium titanate and alumina layers, respectively. The level and sign of these stresses was in good agreement with those predicted based on analysis of the Young's modulus and the dimensional variations during cooling after sintering of the monoliths with the same compositions as those of the layers. Dimensional variations during cooling after sintering were different from those for sintered materials, which presented hysteresis between heating and cooling. In spite of the presence of compressive residual stresses in the external layers of the laminate, strength values of notched samples of the laminated specimens were lower than those for monoliths of the same composition as the external layers.
G. de Portu, S. Bueno, L. Micele, C. Baudin, G. Pezzotti, Journal of the European Ceramic Society 26 (2006) 2699-2705
L0055 - Ce4+ modified cordierite ceramics
A dense, low thermal expansion cordierite ceramic was prepared by the co-precipitation method and Ce4+ addition. Ce4+ addition can inhibit m-cordierite crystallization and improve a-cordierite crystallization. Adding 3.2 wt.% Ce4+ enables density and mechanical strength to reach maximum values with a minor increase in thermal expansion coefficient.
Z.M. Shi, X. Bai, X.F. Wang, Ceramics International 32 (2006) 723-726
L0054 - Electrical and thermal properties of La0.7Sr0.3Ga0.6Fe0.4O3 ceramics
Single-phase La0.7Sr0.3Ga0.6Fe0.4O3 (LSGF) ceramic powder was prepared in one step at 1500°C by solid-state reaction in air. The phase is a hexagonal (space group R-3c) at room temperature but becomes cubic at temperatures above 600°C. A high-density (97%) LSGF ceramic shows electrical conductivity of 4.3 S/cm (Ea = 0.32 eV) at 600°C in air. The thermal expansion coefficient of LSGF changes drastically in air due to oxygen loss and phase transition. High-density LSGF ceramic is relatively stable in forming gas (8% H2, 92% Ar). However, the powder decomposes in forming gas above 700°C with formation of LaSrGaO4. LSGF reacts with NiO at 1000°C. The ability of LSGF to split the C-H bond in methane at intermediate temperatures has been also evaluated.
S. Koutcheiko, P. Whitfield, I. Davidson, Ceramics International 32 (2006) 339-344
L0053 - Processing of Al2O3/Y-TZP laminates from water-based cast tapes
Laminated structures have been investigated due to their capability for the reinforcement of ceramics. Crack deflexion and bifurcation, surface strengthening and threshold strength are the mechanisms associated to the fracture of laminated ceramic. In all the cases, a precise control of the thickness and composition of the layers is necessary. In this sense, colloidal processing techniques have proved their adequacy for the fabrication of designed layered structures. This paper deals with the fabrication of layered ceramics by stacking water-based cast tapes at room temperature and using low pressures. In order to control the pressing procedure, the engineering strain-stress curves recorded during the stacking of the tapes were analyzed. Afterwards, the sintering conditions have been optimized by adjusting the green density of the tapes to avoid differential sintering and the associated cracks. Monolithic and layered materials free of cracks have been fabricated using optimized processing conditions.
A.J. Sanchez-Herencia, J. Gurauskis, C. Baudin, Composites: Part B 37 (2006) 499-508
L0052 - Effect of ceria on properties of yttrium-doped strontium titanate ceramics
This work reports the preparation and properties of the ceramic mixtures of the A-site deficient perovskite Sr0.94Y0.04TiO3 (SYT) with CeO2 in a range of ratios with a view to establishing their potential as anode materials for solid oxide fuel cells. Good electrical conductivity that decreased with increasing CeO2 content was observed on reduction in forming gas. The composition with 50 wt.% of CeO2 showed the conductivity of 7.0 S/cm at 900°C in forming gas. The thermal expansion of SYT-CeO2 ceramics in forming gas and in air were investigated in the range 25-900°C at a ramping rate of 3°C/min and thermal expansion coefficients were determined. The addition of ceria was found to have a positive influence on the catalytic behavior of SYT-CeO2 ceramics towards steam methane reforming.
S. Koutcheiko, Y. Yoo, A. Petric, Isobel Davidson, Ceramics International 32 (2006) 67-72
L0051 - Sintering and microstructure of rare earth phosphate ceramics REPO4 with RE = La, Ce or Y
Sintering of rare earth phosphates REPO4 (RE = La, Ce or Y) was studied using dilatometry. The presence of a secondary rare earth metaphosphate phase RE(PO3)3 as sintering aid was investigated. It proved to accelerate the densification but it activated fast grain growth, which was very detrimental to the microstructural design of processed ceramics. A temperature of 1400-1450°C was required to sinter pure LaPO4 and CePO4 ceramics with fine grains. Both compounds behave similar while YPO4 did not densify even at 1500°C. The influence of specific surface area of starting powders, temperature and holding time on the sintering rate and microstructures of dense REPO4 materials is also reported.
D. Bregiroux, S. Lucas, E. Champion, F. Audubert, D. Bernache-Assollant, Journal of the European Ceramic Society 26 (2006) 279-287
L0050 - Ba(Zn1/3Nb2/3)O3 sintering temperature lowering for silver co-sintering applications
The complex perovskite oxide Ba(Zn1/3Nb2/3)O3 have been broadly studied due to its attractive dielectric properties which place this material as a good candidate for manufacturing type I capacitors or hyperfrequency resonators. The development of Base Metal Electrodes Multi Layer Ceramic Capacitors (BME-MLCC) require a low sintering temperature to be co-sintered with a low cost metal such as copper or silver. Unfortunately, BZN requires a high temperature (1350°C according to the literature) to reach a satisfactory density (>90% of the theoretical one). The aim of this work is to lower the BZN sintering temperature to allow a co-sintering with copper or silver electrodes. For this goal, different sintering agents (lithium salts and glass phases) have been tested on the nominal compound. It is shown that an addition of 10 molar% B2O3 combined with 5 molar% LiF authorises a sintering temperature lowering near to 350°C. If a slight non-stoichiometry in A site is combined to these sintering agents, it is possible to reach a sintering temperature lower than 950°C without affecting the basic material properties. For each composition obtained, the ceramic is characterised in terms of final density, microstructure and dielectric properties. The silver co-sintering is also performed.
F. Roulland, S. Marinel, Ceramics International 32 (2006) 377-383
L0049 - Titanium effect on phase transformation and sintering behavior of transition alumina
The sintering behavior of doped gamma-alumina powders studied by dilatometry and electron microscopy is analysed with a particular emphasis on the role of titanium on the various densification steps. Compared with other doping elements, such as Mg, Y and Zr, which do not improve alpha-phase densification, titanium enhances this densification step by a fast decrease of internal colony porosity. This internal densification of single crystalline zones is attributed to the lengthening of the elementary bricks in titanium-doped samples.Atransient increase in densification rate is also observed, corresponding to the precipitation when during grain growth, grain boundaries become saturated with titanium. Final densities close to the theoretical values are obtained.
S. Lartigue-Korinek, C. Legros, C. Carry, F. Herbst, Journal of the European Ceramic Society 26 (2006) 2219-2230
L0048 - Effect of type of primary processing on the microstructure, CTE and mechanical properties of magnesium/alumina nanocomposites
Magnesium based composites containing 2.5wt.% of nano-sized Al2O3 (50nm) particulates reinforcement were synthesized using an innovative disintegrated melt deposition technique and blend-press-sinter powder metallurgy technique followed by hot extrusion. Microstructural characterization of the materials revealed uniform distribution of reinforcement, grain refinement and the presence of minimal porosity. Properties characterization revealed that the presence of nano-Al2O3 particulates led to an increase in dimensional stability, hardness, elastic modulus, 0.2% yield strength, UTS and ductility of pure magnesium. Amongst the composites, the ingot metallurgy processed material exhibited superior modulus and ductility while powder metallurgy processed material exhibited superior yield strength and ultimate tensile strength. The results further revealed that the overall combination of tensile properties of these materials remained superior when compared to high strength magnesium alloy AZ91 reinforced with much higher weight percentage of SiC. Fractography studies revealed that the typical brittle fracture of pure magnesium changed to ductile due to the incorporation of nano-Al2O3 particulates. An attempt is made in the present study to correlate the effect of nano-sized Al2O3 particulates as reinforcement and processing type with the microstructural and tensile properties of magnesium.
S.F. Hassan, M. Gupta, Composite Structures 72 (2006) 19-26
L0047 - Variation of the lattice parameter and thermal expansion coefficient of (U,Dy)O2 as a function of DyO1.5 content
Thermal expansions of (U,Dy)O2 solid solutions were investigated between room temperature and 1673K by using a thermo-mechanical analyzer. The lattice parameter of (U,Dy)O2 pellets is lower than that of UO2 and it decreases as Dy content increases. The linear thermal expansion and average thermal expansion coefficients of (U,Dy)O2 are higher than that of UO2. For the temperature range from room temperature to 1673 K, the average thermal expansion coefficient values for UO2 and (U0.8Dy0.2)O2 are 10.97x10^(-6) and 11.37x10^(-6) K-1, respectively.
S-H. Kim, H-S. Kim, Y-W Lee, D-S. Sohn, D-S. Suhr, Journal of Alloys and Compounds 407 (2006) 263-267
L0046 - Elaboration and characterisation of apatite based mineral supports for microfiltration and ultrafiltration membranes
The development and the characterisation of new supports for microfiltration and ultrafiltration membranes from apatite applied to filtration are presented. The choice of this material is based primarily on its low cost (considering its abundance in the Tunisian ores). The support, with tubular configuration, was prepared from natural apatite: apatite powders were crushed for 30 min and sieved to 200 mm. The resulting powders, mixed with organic additives and water, could be extruded to elaborate a porous structure. The firing temperature of the support is 1160°C. The morphologies of the surface and the cross-section observed on scanning electron microscope (SEM) are homogeneous and do not present any macro defects (cracks, etc.). The mean pore diameter, measured by mercury porosimetry, is 6 mm and the pore volume is 48%.
S. Masmoudi, A. Larbot, H. El Feki, R. Ben Amar, Ceramics International 33 (2007) 337-344
L0045 - Electrical and microstructural characterization of spinel phases as potential coatings for SOFC metallic interconnects
Several spinel samples, i.e., MnxCr3-xO4 (0.5<x<2.5), NiCr2O4 and CoCr2O4, were synthesized and studied in terms of phase analysis, density, electrical resistivity and thermal expansion behaviour. The spinel samples were generally single phase; exceptions included MnCr2O4 and Mn0.5Cr2.5O4 with significant amounts of Cr2O3 and NiCr2O4 with trace amounts of NiO. Porosity, in general, decreased with increasing sintering temperature, except for Mn0.5Cr2.5O4, which showed increasing porosity with increasing sintering temperature. NiCr2O4, CoCr2O4 and MnCr2O4, all had similar thermal expansion behaviour, with thermal expansion coefficients (TEC) ranging from 7.2-7.6x10^(-6) /°C. The TEC difference between the spinels and ferritic stainless steel was larger than the difference between the steel and chromia, which had a TEC of 9.6x10^(-6) /°C. The spinels and chromia exhibited semiconductor-type behaviour, with electrical resistivities decreasing with increasing temperature. Only Mn2CrO4 and NiCr2O4 had resistivities lower than Cr2O3 over the entire temperature range of testing (20-900°C). For MnxCr(3-x)O4, resistivity decreased with increasing Mn content.
W. Qu, L. Jian, J.M. Hill, D.G. Ivey, Journal of Power Sources 153 (2006) 114-124
L0044 - Enhancing strength and ductility of Mg/SiC composites using recrystallization heat treatment
In the present study, magnesium composites with sub-micron size silicon carbide (SiC) particulate reinforcements were successfully synthesized using an innovative Disintegrated Melt Deposition technique followed by hot extrusion. The extruded materials were characterized for their microstructural, thermal and mechanical properties. Systematic investigations on the microstructure of the extruded samples revealed a fairly uniform distribution of SiC particulates with limited clustering. Good SiC/Mg interfacial integrity and limited porosity was observed for all the samples. The results of the properties characterization revealed that SiC in sub-micron length scale are more effective in lowering CTE, and enhancing hardness, 0.2% yield strength, and ultimate tensile strength when compared to SiC particulates in micron length scale. Subsequently, isothermal heat treatment at 150°C for 5 h was carried out for the composite samples. The results of tensile testing revealed that the heat treated samples showed an improvement of 0.2% YS, UTS and ductility. Particular emphasis is placed in this study to investigate the effect of heat treatment at the recrystallization temperature on the tensile properties of magnesium.
S. Ugandhar, M. Gupta, S.K. Sinha, Composite Structures 72 (2006) 266-272
L0043 - Stress relaxation of bulk and ribbon glassy Pd40Cu30Ni10P20
Detailed measurements of linear heating and isothermal stress relaxation of bulk and ribbon glassy Pd40Cu30Ni10P20 below the glass transition temperature have been performed. Using the activation energy spectrum reconstructed from these measurements, the temperature dependence of the shear viscosity has been calculated. It has been found that this dependence agrees with that derived directly from the creep measurements. This provides strong support for understanding the stress relaxation as a result of the stress-oriented irreversible structural relaxation with distributed activation energies.
O.P. Bobrov, K. Csach, V.A. Khonik, K. Kitagawa, S.N. Laptev, M.Yu. Yazvitsky, Scripta Materialia 54 (2006) 369-373
L0042 - Cofiring behavior and interfacial structure of NiCuZn ferrite/PMN ferroelectrics composites for multilayer LC filters
The cofiring behavior, interfacial structure and cofiring migration between NiCuZn ferrite and lead magnesium niobate (PMN)-based relaxor ferroelectric materials were investigated via thermomechanical analyzer (TMA), X-ray diffractometer (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). Mismatched sintering shrinkage between NiCuZn ferrite and PMN was modified by adding an appropriate amount of sintering aids, Bi2O3, into NiCuZn ferrite. Pyrochlore phase appeared in the mixture of NiCuZn ferrite and PMN, which is detrimental to the final electric properties of LC filters. EDS results indicated that the interdiffusion at the heterogeneous interfaces in the composites, such as Fe, Pb, Zn, existed which can strengthen combinations between ferrite layers and ferroelectrics layers.
C. Miao, J. Zhou, X. Cui, X. Wang, Z. Yue, L. Li, Materials Science and Engineering B 127 (2006) 1-5
L0041 - Surface resistance and sensitivity to hygrometry of various ceramic compositions for multilayer capacitors
Developing new dielectric compositions for multilayer ceramic capacitors, the authors of this work have observed an influence of hygrometry on the insulation resistance behavior of some of them. The swiftness of the phenomenon let us suppose a surface mechanism. Prompted by these observations, we have investigated the sensitivity of various conventional ceramic compositions to hygrometry by measuring their surface resistance versus the moisture rate in air. The studied materials are formulations for type I and type II ceramic capacitors. In order to understand some breakdown phenomena in ceramic multilayer capacitors, we have also investigated the influence of the screen printed electrodes composition.
J. Bernard, D. Houivet, J-M. Haussonne, Journal of the European Ceramic Society 25 (2005) 2977-2979
L0040 - Low temperature sintering of MgTiO3 with bismuth oxide based additions
MgTiO3 is a classical material for type I MLCC. However, its densification temperature is too high to allowco-sintering with copper electrodes. Investigations into low temperature sintering of MgTiO3 show the complexity of this problem particularly when co-sintering with copper electrodes and thus a reducing atmosphere are involved in the process. Though feasibility of MgTiO3/copper MLCC seem to be possible [Bernard, J., Houivet, D., El Fallah, J. and Haussonne, J. M., MgTiO3 for base metal multilayer ceramic capacitors. J. Eur. Ceram. Soc., 2004, 24, 1877-1881] studies have pointed out several remaining problems such as sensibility to hygrometry [Bernard, J., Houivet, D., El Fallah, J. and Haussonne, J. M., Effect of hygrometry on dielectric materials. J. Eur. Ceram. Soc., 2004, 24, 1509-1511]. Investigations into low temperature sintering of MgTiO3 are thus still necessary to find a formulation which is not detrimental to dielectric behaviour. Sintering aid capabilities and/or good dielectric properties of bismuth oxide based additives make them good candidate materials to lower MgTiO3 sintering temperature without altering its final properties. In this study bismuth titanate is either formed in situ by reaction-sintering of Bi2O3-B2O3 additives with MgTiO3 or obtained by pre-calcination of Bi2O3 and TiO2 powders and then added to MgTiO3 prior to sintering. Bismuth titanate appears as a good sintering aid that allows sintering of MgTiO3 under 1000°C. It is however highly sensitive to the nature of the sintering atmosphere, oxidizing or reducing. Dielectric properties of sintered specimens are compatible with type I capacitors applications.
F. Belnou, J. Bernard, D.d Houivet, J-M. Haussonne, Journal of the European Ceramic Society 25 (2005) 2785-2789
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