Caractérisations microstructurale et mécanique de mousses de nickel à cellules ouvertes pour batteries de véhicules hybrides

Abstract : Among other applications, nickel foams are used as an electrode element in Ni-MH (Nickel/Metal Hydride) batteries
for hybrid vehicles. The nickel foam production at NiTECH starts with a 10 µm thick coating of nickel onto an open-cell
polyurethane foam. Afterwards, a heat treatment is made with a twofold objective : the polyurethane foam is removed
from the structure by a non-isothermal combustion under air up to 600°C, then an anneal at roughly 1000°C under a
reducing atmosphere is designed so that the nickel foam meets the targets for elongation and strength to fracture. The
major aim of this study is to reduce the costs of production and at the same time to improve mechanical properties of
nickel foams. One of the possible ways is an optimization of the thermal treatment.
For that purpose, in a first part, the thermal degradation of the polyurethane foam was studied by thermogravimetric
analysis. The degradation under air involves three superimposed phenomena. The corresponding activation energies were
calculated by the Kissinger method. Afterwards, the three phenomena were separated to isolate the contribution of each of
them, and the thermal degradation was modeled.
In a second part, the dependence of the mechanical behavior of nickel foams upon grain size was studied. Thanks to
metallurgical characterizations, the grain coarsening phenomenon which occurs during the anneal of foams was analyzed.
The EBSD technique allowed to observe whether the nickel strut grains keep the texture inherent to the electrolytic
deposition step and also, to check the isotropy of the recrystallized nickel. Furthermore, this technique made the
measurements easier by eliminating the annealing twins from the grain distributions. The grain size effects on the
mechanical properties was then studied, namely via the Hall-Petch law. The foam walls being very thin, roughly 10 µm in
thickness, grain growth and mechanical behavior might be different from that of bulk nickel. The results obtained with
foams were compared, on the one hand, with literature data on bulk pure nickel, and on the other hand with nickel foils of
10 and 50 µm in thickness. The Hall-Petch law is observed when the grain size is lower than the foil thickness or the wall
thickness in the case of foams, whereas once the microstructure becomes “bamboo”, the yield strength remains constant.
Finally, a mechanical model in the spirit of this by Gibson and Ashby was presented incorporating the grain size effect on
yield strength and hardening modulus.
Besides, an other application of nickel foam could be found in the fuel cells operating at high temperatures. In order
to investigate the behavior of nickel foams at high temperatures, tensile creep tests were carried out, on the one hand
between 100 and 200°C under air, and on the other hand, in the range from 500 to 700°C under a primary vacuum. The
creep parameters, i.e. the Norton exponent and the activation energy were estimated experimentally and incorporated in
two mechanical models considering creep bending or creep traction of the struts.
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  • HAL Id : tel-00007551, version 1

Citation

Virginie Goussery-Vafiadès. Caractérisations microstructurale et mécanique de mousses de nickel à cellules ouvertes pour batteries de véhicules hybrides. Mécanique [physics.med-ph]. École Nationale Supérieure des Mines de Paris, 2004. Français. ⟨tel-00007551⟩

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