Pressure Effects in Sealed Liquid-Crystal Cells

Abstract

Liquid crystal cells are hermetically sealed glass containers completely filled with liquid. Two plane-parallel plates are sealed all around the edges to a frit glass spacer. The cell is then filled with liquid through two holes and sealed off with plugs of fusible metal. This construction gives rise to some internal pressure effects, because the thermal expansion coefficient of the liquid is about 100 times that of the glass. If the cell is filled and sealed off at room temperature, the liquid will exert a pressure at all higher temperatures. At lower temperatures it will be under tension. The pressure will deform the cell, making the walls bow out. This makes the volume enclosed by the cell a little larger and reduces the pressure but does not eliminate it completely. Some pressure or tension will always remain for temperatures different from the filling temperature. Repeated expansion and contraction may lead to loss of hermeticity or other cell failure. In what follows, the magnitude of the effect is calculated and the important factors are analyzed.