Perhaps you should try some spelling variations:
22optical materials » optical material, acoustical materials, optic materials
critical materials » artificial materials, frictional materials, crystal materials
chemical materials » medical materials, chemical potentials, biomedical 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 (((((critical materials) OR (chemical materials))) OR (optical materials))))%22.
• The words AND, OR and NOT may confuse the search; try adding quotes: "%((22optical materials) or (((((critical materials) OR (chemical materials))) or (optical materials))))%22".
• Adding a wildcard symbol may retrieve word variants: %((22optical materials) or (((((critical materials) OR (chemical materials))) or (optical materials))))%22*.