Hi I am trying to model a thin membrane pressure sensor, and was going through the COMSOL MEMS piezoresistive membrane based pressure sensor models (Shell and conventional). I had a couple questions that might seem basic. I am trying to understand the use of some modelling conditions and their role in the membrane physics.

First of all, the SHELL pressure sensor model gives similar results to the conventional 3D one. So when is it that we go for SHELL based solid mech physics?

Secondly, what is the BOX definition for and what does that do?

Thirdly, I see there is no deforming mesh domain specified in the pressure sensor models that are membrane based because in these models we do not model any air. Can we model membrane deflections without air around?

And to follow up to that, I understand COMSOL models membrane deflection assuming linear physics. And the assumption is that the membrane is deflecting linearly. Is there a way to model the membrane non-linear deflection component (due to stretching) as well?

I guess if we have a very thin membrane then there will be some stretching as well.

Thanks all!