quenching melting quartz

Hi everybody!
I am melting and quenching a quartz phase with the Reaxff potential. I think I have a lot of trouble when I prepare amorphous silica by quenching crystal silica with NPT in Lammps. First, I can well get the amorphous silica when i use a small model and it is mostly equal in X Y Z direction, but the problem come when i use a large model which the dimension in X Y direction is much larger than Z direction, the dimension in X Y direction is almost 5 times the Z direction.The center of model have melt and produce a hole,but the corner is still crystal.I want to know whether the model influence this process? If the size of model in X Y Z direction is not much difference? If the NPT is suitable for this process? Does anybody have any idea about what this problem may be due to?

There are lots of ways to go wrong. Too big a timestep for ReaxFF.
Heating or quenching too fast. Not enough time to equilibrate
the volume in NPT.