Papers by Keyword: Volume Diffusion

Abstract: Within the general context of solid-state sintering process, this work presents a numericalmodelling approach, at the grain scale, of ceramic grain packing consolidation. Typically, the sinteringprocess triggers several matter diffusion routes that are thermally activated: surface, grain boundaryand volume diffusions. Including this physics into a high-performance computing framework wouldpermit to investigate and to track the changes occurring into a granular packing during sintering. Inperforming this kind of simulations, one will face several challenges: the strong topological changesappear during sintering simulation at the grains scale, the evolution of the structure is mainly drivenby the surface tension phenomena through the Laplace's law, and the mechanical properties of thegrains could, possibly, be different. The proposed numerical simulations are carried out within anEulerian Finite Element framework and the Level-Set method is used to cope with changes in themicrostructure. The results obtained with this numerical strategy are compared with success to theusual geometrical models.

Abstract: In order to examine the decarburization behavior in the hot stamping (HS) method, the dependence of the microstructure evolution on the annealing temperature was experimentally studied using a Fe-0.21 mass% C-1.3 mass% Mn-0.2 mass% Si steel. The steel was isothermally annealed in the temperature range of T = 773-1173 K for various times of t = 100-12800 s in an ambient atmosphere. Here, the steel possesses the ferrite (α) + cementite (θ) two-phase microstructure at T = 773-923 K, the α + austenite (γ) two-phase microstructure at T = 1013-1073 K, and the γ single-phase microstructure at T = 1093-1173 K. During annealing at T = 1013-1073 K for t = 1600 s, however, the α layer with a uniform thickness is formed at the surface of the steel due to decarburization and gradually grows into the inside. Such formation of the a layer was not clearly observed at T 973 K and T 1093 K. Thus, the formation of the α layer hardly occurs under the HS annealing conditions. At T = 1033 K, the thickness of the α layer is mostly proportional to the square root of the annealing time. Such a relationship is called the parabolic relationship. Furthermore, the grain size of the α layer monotonically increases with increasing annealing time. Hence, the parabolic relationship guarantees that the growth of the α layer is controlled by volume diffusion.

Abstract: The NbSi2/Nb/-TiAl and NbSi2/Nb functionally graded materials (FGMs) were prepared and their tolerances tested by exposing them to temperatures from 1050 °C to 1250 °C under vacuum and in air. Oxygen resistivity was estimated from metallographic investigations. The FGM lifetime was estimated by using a diffusion equation that considers the disappearance of the NbSi2 and Nb interlayer. These occurred during NbSi2 oxidation and Si diffusion from NbSi2 to Nb and interdiffusion between Nb and -TiAl. The results were validated by diffusion equations.

Abstract: The sintering dynamics, microstructures and microwave dielectric properties of Bi2O3 and MnO2 co-doped [(Pb,Ca) La](Fe,Nb)O3 (PCLFN) ceramics were investigated. Bi2O3 and MnO2 dopants effectively enhanced bulk densities and reduced sintering temperatures by about 100~140°C. Sintering procedure had significant effect on grain size and porosities of ceramics. Investigation of microstructures revealed that the grain growth was controlled by either volume diffusion or second-order interface mechanism in present ceramics. The potential microwave dielectric properties of εr=91.1,Qf=4870GHz and τf=18.5ppm/°C could be obtained when the mass ratio of Bi2O3/MnO2 (k) was 1, the doping content w=1wt% and sintered at 1050°C for 4h.

Abstract: Porous silicon (PS) layers with porosity of 60% on n-type (111) Si substrates were prepared by anodic etching under the white illumination. Metal (Cu, Ag or Au)/PS/Si and metal (Cu, Ag or Au)/Si structures have been fabricated by evaporation of thin metal film onto the PS or Si surface, respectively. The diffusion annealing of structures was carried out in air at 100-250oC. Examination of Cu, Ag and Au concentration distribution in PS layer and monocrystalline Si substrate was performed by successive removal of thin layers from sample and measuring the energy dispersive X-ray fluorescence (EDXRF) intensity of CuKα1, AgKα1 and AuLβ1 peak. The effective diffusion coefficients for investigated metals along PS surfaces decrease in series Cu, Ag and Au and temperature dependences are described as D(Cu)=7.8 exp(-0.62eV/kT), D(Ag)=4.2x10 exp(-0.72eV/kT) and D(Au)=1.2x102 exp(-0.81eV/kT). Diffusion coefficients of Cu, Ag and Au along PS surfaces are larger (by a factor of 104-105) than those into monocrystalline Si. The diffusion mechanism of Cu, Ag and Au along PS surfaces is discussed and data on influence of diffusion of these metals on humidity-sensitive characteristics of metal(Cu, Ag or Au)/PS Schottky type gas sensors is also presented. Diffusion of metals of I group in PS is accompanied by increase of humidity-sensitivity of metal/PS structures by a factor of 1.2-1.4.

Abstract: Gallium grain boundary (GB) diffusion of Ga in polycrystalline magnesium was investigated by radiotracer residual activity technique using 67Ga isotope. The diffusion measurements were carried out under conditions of Harrison’s B-type kinetics in temperature range 639 – 872 K. An approximate evaluation procedure was proposed to calculate both volume diffusion coefficient Dv and GB diffusivity P = s dDb (s is the segregation factor, d the GB width and Db the GB diffusion coefficient). The obtained results showed the following linear Arrhenius relationships: Dv = 1.2 × 10-4 × exp (–134.3 kJ mol-1/RT) m2 s-1 and P = 3.8 × 10-9 × exp (–94.9 kJ mol-1/RT) m3 s-1, where R is universal gas constant and T the temperature. Obtained results were compared with literature data on Mg self-diffusion and with In impurity diffusion in magnesium.