Gaia Early Data Release 3: The celestial reference frame (Gaia-CRF3)

Context. Gaia-CRF3 is the celestial reference frame for positions and proper motions in the third release of data from the Gaia mission, Gaia DR3 (and for the early third release, Gaia EDR3, which contains identical astrometric results). The reference frame is defined by the positions and proper motions at epoch 2016.0 for a specific set of extragalactic sources in the (E)DR3 catalogue. Aims. We describe the construction of Gaia-CRF3 and its properties in terms of the distributions in magnitude, colour, and astrometric quality. Methods. Compact extragalactic sources in Gaia DR3 were identified by positional cross-matching with 17 external catalogues of quasi-stellar objects (QSO) and active galactic nuclei (AGN), followed by astrometric filtering designed to remove stellar contaminants. Selecting a clean sample was favoured over including a higher number of extragalactic sources. For the final sample, the random and systematic errors in the proper motions are analysed, as well as the radio-optical offsets in position for sources in the third realisation of the International Celestial Reference Frame (ICRF3). Results. Gaia-CRF3 comprises about 1.6 million QSO-like sources, of which 1.2 million have five-parameter astrometric solutions in Gaia DR3 and 0.4 million have six-parameter solutions. The sources span the magnitude range G = 13-21 with a peak density at 20.6 mag, at which the typical positional uncertainty is about 1 mas. The proper motions show systematic errors on the level of 12 μas yr-1 on angular scales greater than 15 deg. For the 3142 optical counterparts of ICRF3 sources in the S/X frequency bands, the median offset from the radio positions is about 0.5 mas, but it exceeds 4 mas in either coordinate for 127 sources. We outline the future of Gaia-CRF in the next Gaia data releases. Appendices give further details on the external catalogues used, how to extract information about the Gaia-CRF3 sources, potential (Galactic) confusion sources, and the estimation of the spin and orientation of an astrometric solution

Densification and microstructure evolution of Y-Tetragonal Zirconia Polycrystal powder during direct and hybrid microwave sintering in a single-mode cavity

International audienceThe densification and microstructure changes of 2 mol% yttria-stabilized zirconia nanopowder have been investigated during direct and hybrid microwave sintering. Microwave heating tests were achieved in a resonant single-mode cavity at 2.45 GHz and a cylindrical SiC susceptor was used for hybrid sintering experiments. Constant heating rate runs (25 degrees C/min) were controlled by adjusting the position of a sliding piston at constant forward microwave power. The temperature on the upper surface of the specimen was measured with an infrared camera. The final densities and the microstructures observed by SEM were compared to those of conventionally sintered materials. Homogeneous microstructures have been obtained by hybrid heating whereas direct microwave heating led to rather heterogeneous microstructures due to thermal gradients. Nevertheless, microwave-sintered materials always exhibited higher final densities for a given sintering temperature. This significant enhancement of the densification process was particularly observed in the intermediate sintering stage (1200-1350 degrees C range). Besides, grain growth was found to be mainly influenced by the sintering temperature rather than by the heating mode

Densification and microstructure changes of micron size nickel powder during direct induction sintering

International audienceInduction heating is an attractive technique to sinter metal powders in a short time and with limited energy. A series of direct induction sintering experiments has been performed with a micron size nickel powder in a dedicated set-up with 50 or 150 kHz current frequency and several heating rates, up to 900 degrees C min(-1). With a view to better catching the specific outcome of induction sintering, conventional sintering tests have also been achieved and their results in terms of densification have been depicted by adjusting a Master Sintering Curve model. The main conclusion of this study is that nickel specimens with high density, reasonably low grain size and homogeneous microstructure can be obtained by direct induction sintering with processing times much smaller than typical conventional sintering times. The obtained data also show that powder densification is accelerated during induction sintering, which is supposedly due to the enhancement of diffusion under electric curren

Development of a morphological texture in relation with the crystallographic texture during sintering of strontium hexaferrites

International audienceQuantitative analysis of grain size, grain shape, morphological and crystallographic texture has been performed in strontium hexaferrites materials, sintered from filter-pressed powders oriented or not with a magnetic field. The results provide evidence for anisotropic and abnormal grain growth at high temperature (T ≥ 1250 °C). A morphological texture develops during heating for oriented samples, with plate-like grains preferentially aligned with their larger facets perpendicular to the direction of field application. A quantitative analysis of the morphological texture in 3D has been performed and the relation between morphological and crystallographic texture is identified from the results. The increase in crystallographic texture is related to grain growth

Numerical and experimental analysis of the effect of pore formers on the processing and properties of solid oxide fuel cell electrode

International audienceThe fabrication of ceramic components with controlled porosity often requires using pore formers. This paper shows how modeling can help in optimizing pore former quantity and size with respect to sintering behavior and material properties. The case of a mixed ion-electron conductor SOFC cathode is considered. Numerical simulations based on the discrete element method are performed to mimic the sintering of Nd2NiO4 ceramic powders processed with various fractions of pore formers (0-40%) and size ratios (1-5 times the size of ceramic particles). From the results of these simulations, the effect of pore former particle size and amount on sintering densification, conductivity and effective surface area is analyzed. Experiments achieved on a Nd2NiO4 powder with miscellaneous pore formers confirm simulation outcomes as the minor effect of the pore former particle size ratio on the density and the presence of an optimum of oxygen adsorption for 20 vol.%

Constitutive modeling of the behaviour of cermet compacts during reaction sintering

International audienceThis study deals with the identification of a constitutive equation describing the mechanical behaviour of a nickel ferrite based cermet during sintering. This constitutive equation considers the material as a continuum and may enable one to predict the densification behaviour of a powder under different thermal treatments and the impact of compact geometry, external loading on strain and stress generation. A classical viscous equation of the Newtonian type that includes a term describing free sintering densification has been chosen. The method used for the identification of the parameters of this equation is the one proposed Gillia et al., which is based on dilatometry measurement. It includes a stairway thermal cycle for the determination of the free sintering term and intermittent loading for estimating the viscosity. This approach has been successfully applied to nickel ferrite cermet. The model has been found to be adequate to model the densification behaviour up to 1250 degrees C, but experimental and theoretical efforts must be accomplished to describe the behaviour above this temperature, when the material exhibits swelling

Microwave sinter forging of alumina powder

International audienceMicrowave sintering under load is expected to be a promising technique to process ceramics with fine microstructure. This paper presents a new setup allowing sinter forging ceramic compacts in a 2.45 GHz single-mode microwave cavity. This setup has the following features: maximum temperature of 1600 degrees C, heating rate between 1 and 250 degrees C min(-1), maximum stress of 50 MPa applied upon an 8 mm diameter sample. A specific protocol has been defined to calibrate the pyrometer used to measure the sample temperature. Alpha-alumina compacts have been microwave sinter forged under various stresses in the range 0-30 MPa. The results are compared to those obtained in a conventional furnace under 0, 4 and 8 MPa. Final axial and radial shrinkages are identical with the two techniques. The final relative density of the material is not affected by load and is equal to 0.94 and 0.96 in microwave sinter forging and conventional sinter forging, respectively. Two assumptions are proposed to explain this difference: a microwave effect and a temperature discrepancy. Finally the load does not significantly affect grain growth

Deformation and cracking during sintering of bimaterial components processed from ceramic and metal powder mixes. Part II: Numerical simulation.

International audienceIn a companion paper (Part I), the processing of two-layer parts by co-sintering of two powder blends has been investigated. As a complement to this experimental study, a finite element simulation of this operation has been achieved. This simulation was based on constitutive equations identified from specific experiments performed with each blend. The numerical results provided deformation and internal stress information that have been found to be fully consistent with the experimental observation. In particular they confirm that cracking occurs in the course of heating, when one material starts densifying and the other one is still brittle, whereas a large densification mismatch at high temperature is not harmful because the low viscosity of both materials leads to the relaxation of internal stresses

Deformation and cracking during sintering of bimaterial components processed from ceramic and metal powder mixes. Part I: Experimental investigation

International audienceParts composed of two ceramic-metal composite layers have been fabricated by co-sintering of two powder blends. The major constituent was the ceramic powder in one blend and the metal powder in the second one. This paper focuses on the mechanical analysis of the co-sintering process. This process has been observed thanks to an original optical dilatometry set up that provided images of the component in the course of sintering cycle. These images allowed following component shape changes throughout the thermal cycle and also evidenced the formation of cracks at the edges of a part at particular stages of the sintering cycle. These phenomena are interpreted from the mismatch between the densification kinetics of each powder blend sintered alone. In a companion paper the results of a finite element simulation of cosintering are compared to the experimental data displayed in the present paper. (C) 2012 Elsevier Ltd. All rights reserved