Thermal Expansion Polycrystalline Silicon

Hello,

I was trying to find a similar problem to mine but could not find it.
I am trying to find the stress gradient of a poly crystalline thin film layer deposited on silica and single crystal silicon.
To do so, I built the geometry and added a thermal expansion with a surrounding temperature of 800K and strain reference of 293K, with a fixed constraint at the bottom of the layer.
I am trying to find the steady state condition, so I am using the stationary solver.
The problem is that I can't seem to receive a gradient, the stress is constant along the whole layer, and therefore also the displacement.
I don't know what I'm doing wrong, I feel like it should be a simple simulation.

I would really appreciate your advice, and/or if you've seen or programmed similar simulations in the past, I would really appreciate it if you could share them.

Thanks!
Adam