Xavier Milhet

Untangling the environmental from the dietary: dust does not matter.

Both dust and silica phytoliths have been shown to contribute to reducing tooth volume during chewing. However, the way and the extent to which they individually contribute to tooth wear in natural conditions is unknown. There is still debate as to whether dental microwear represents a dietary or an environmental signal, with far-reaching implications on evolutionary mechanisms that promote dental phenotypes, such as molar hypsodonty in ruminants, molar lengthening in suids or enamel thickening in human ancestors. By combining controlled-food trials simulating natural conditions and dental microwear textural analysis on sheep, we show that the presence of dust on food items does not overwhelm the dietary signal. Our dataset explores variations in dental microwear textures between ewes fed on dust-free and dust-laden grass or browse fodders. Browsing diets with a dust supplement simulating Harmattan windswept environments contain more silica than dust-free grazing diets. Yet browsers given a dust supplement differ from dust-free grazers. Regardless of the presence or the absence of dust, sheep with different diets yield significantly different dental microwear textures. Dust appears a less significant determinant of dental microwear signatures than the intrinsic properties of ingested foods, implying that diet plays a critical role in driving the natural selection of dental innovations.

Very high temperature creep behavior of a single crystal Ni-based superalloy under complex thermal cycling conditions

Very high temperature thermal cycling has been performed on the single crystal superalloy MC2 to evaluate the effect of periodic overheating on creep behavior. The experiments consist of alternately performing 1 min dwell time at 1100°C and 1150°C for every 15 min during creep test at 1050°C/120 MPa. Both thermal cycling and prior γ′-rafting appear to be deleterious to the cyclic creep properties. The observed non-isothermal creep behavior is correlated with γ′-dissolution/coalescence processes, especially during overheatings where γ′ micro-rafts seem to play a significant role.

Mechanical Properties of Sintered Ag as a New Material for Die Bonding: Influence of the Density

Sintered silver powder is a good candidate for die bonding as an alternative to lead alloys. However, little is known about its intrinsic mechanical properties, which are required to model the entire electronic system reliability, especially during ageing. Since interconnections are obtained by sintering, the mechanical properties are expected to be impacted by the residual porosity. However, evaluating the density of the material in its 30-μm-thick form is challenging. This issue was addressed by developing a robust procedure to obtain bulk specimens with the same microstructure as the interconnection. Tensile tests were performed at room temperature to determine the macroscopic characteristics (Young’s modulus, ductility, yield stress, ultimate tensile stress) as a function of the porosity.

Influence of the Porous Microstructure on the Elastic Properties of Sintered Ag Paste as Replacement Material for Die Attachment

Silver pastes are good candidates as alternative materials to lead solder alloys. However, little is known about the relationship between their microstructure and their mechanical properties. This issue is addressed by developing a specific route to obtain standalone sintered bulk specimens representative of the real sintered joints. The relationship between the density and the pore surface fraction is established, allowing the density of the material to be obtained independently from its size and geometry. The elastic constants of both sintered joints and sintered bulk specimens are investigated using dynamic resonant testing. A strong correlation between the elastic constants and the density is established. In contrast to the sintered bulk specimens, for which the Young’s modulus remains constant after annealing, Young’s modulus of the sintered joints evolves significantly towards a stabilized value. This is derived from thermal stresses relaxation within the sintered joint.

On the Dissolution of the γ′ Phase at the Dendritic Scale in a Rhenium-Containing Nickel-Based Single Crystal Superalloy After High Temperature Exposure

MCNG is a nickel base single crystal superalloy containing both rhenium and ruthenium. The γ′ phase dissolution evolutions have been studied after very high temperature exposures for various times up to the thermodynamic equilibrium. Very different γ′ phase dissolution behaviors have been observed across the dendritic structure. In the temperature range investigated, the dissolution of the γ′ phase is more pronounced within the dendrites compared to its dissolution observed within the interdendrites. The physical parameters likely to promote the different dissolution behavior at the dendritic scale such as the diffusion coefficients of the major alloying elements, the γ′ solvus temperature, and the γ′ precipitates morphologies are discussed. Special attention is paid to the very first steps of the dissolution process.

Evolution of the Thermal Conductivity of Sintered Silver Joints with their Porosity Predicted by the Finite Element Analysis of Real 3D Microstructures

Silver paste sintering is a very promising technology for chip bonding in future power electronics modules owing to its high melting temperature and the good electrical and thermal properties among other classic solder alloys. However, in its sintered form, these joints contain nanometric/submicrometric pores that affect their thermal performance. The present study gives insight into the relationship between the material thermal conductivity and the real three-dimensional porous structure using finite element modelling. It is shown that over a certain pore fraction threshold (∼ 13%), the pore morphology has a non-negligible influence on the thermal conductivity. Results are also compared to predictions obtained by analytical models available in the literature.