PhD Thesis call : MECHANICAL BEHAVIOR OF NANOSTRUCTURED SILICA BASED INSULATION PANELS: DISCRETE SIMULATIONS

Thermal renovation is the first lever to reduce greenhouse gas emission, especially in France. Thin panel for insulation from the inside can only be achieved with super-insulation materials (thermal conductivity < 25mW/(m.K). The most mature technology, Vacuum Insulation Panels (VIP), suffers from excessive brittleness that limits its use. The use of highly porous nanostructured silica grains in VIP allows the reduction of thermal conductivity (dual effect of gas nanoconfinement and large tortuosity of the solid skeleton) but induces significant brittleness. Higher densities or addition of fibers can be used but at the cost of lower thermal performance. The thermal/mechanical compromise is presently not optimal and EDF is pushing forward research with various industrial and academic partners to improve this compromise.

The Discrete Element Method (DEM) is particularly well-suited to model damage and fracture behavior of porous materials with a granular character. In DEM, each grain is explicitly described and interacts with its neighbors via appropriate contact laws. The mechanical equilibrium of the system is solved by application of Newton’s second law. The SIMaP laboratory has a strong expertise on this topic.

The objectives of the PhD work are to use DEM simulations to model and understand the mechanical behavior of VIP core material consisting of nanostructured silica grains, binder and fibers. Then, the knowledge and modeling tools developed will be used for the optimization of the material in terms of composition, grain size distribution, grain shape…

The present work is mainly numerical, but a strong collaboration with EDF is anticipated with an ongoing PhD covering the experimental counterpart of this work. This PhD thesis is a unique opportunity to work on a very active and dynamic research field with several ongoing projects on the subject at EDF, SIMaP and MATeB (joint research laboratory between EDF and INSA Lyon)

Anticipated work program

- Generation of realistic numerical microstructures. Fibers can be represented by a string bead approach and non-spherical particles by a multisphere approach. X-ray tomography will be used to characterize the material morphology.

- Implementation (if necessary), calibration and validation of appropriate contact laws. Special attention will be given to the influence of the binder on the contact behavior. Confrontation with experimental data from EDF is anticipated.

- Material optimization

The DEM code dp3D (in-house code) and/or LIGGGHTS (open-source) will be used. New features

will have to be developed and implemented such as new contact laws or multi-sphere approach

for non-spherical grains (some work has already started on this last aspect).
 

Figure: VIP panel showing core and envelope (left) / X-ray tomography 3x3x3 mm (center) / DEM

model of non-spherical grains (right).
 

Jauffres, D., Martin, C. L., Lichtner, A. & Bordia, R. K. Simulation of the toughness of partially sintered

ceramics with realistic microstructures. Acta Mater. 60, 4685 (2012).

Pizette, P., Martin, C. L., Delette, G., Sans, F. & Geneves, T. Green strength of binder-free ceramics. J. Eur.

Ceram. Soc. 33, 975–984 (2013).

Starting date: Sept. 2015

Host Laboratory: SIMaP / Grenoble - France

david.jauffres@simap.grenoble-inp.fr

remy.dendievel@simap.grenoble-inp.fr