Perhaps you should try some spelling variations:
acoustical materials » acoustic materials, acoustic material, classical materials
22optical materials » optical material, optic materials, optics materials
critical materials » chemical materials, artificial materials, frictional materials

You may be able to get more results by adjusting your search query.

• If you are trying to use Boolean operators, they must be ALL CAPS: %((22optical materials) OR (((((acoustical materials) OR (critical materials))) OR (optical materials))))%22.
• The words AND, OR and NOT may confuse the search; try adding quotes: "%((22optical materials) or (((((acoustical materials) OR (critical materials))) or (optical materials))))%22".
• Adding a wildcard symbol may retrieve word variants: %((22optical materials) or (((((acoustical materials) OR (critical materials))) or (optical materials))))%22*.