One question I have in mind for quite some time:
Ion exchange in glasses leads to higher strength, because literally speaking “bigger” ions are incorporated into the glass network and build up compressive stresses.
Has there ever been the idea of incorporating ions to induce a different coefficient of thermal expansion (CTE) which then, when cooling the glass, will lead to compressive stresses, because of different CTE between inner and outer of the glass?
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Generally ion-exchange is used to generate residual compression in a glass surface by exchanging large ions for small ions (let us call this the ionic size effect). Your suggestion to add to this residual compression by designing an appropriate CTE mismatch which will give additional residual compression upon cooling is interesting (Let us call it the CTE effect). In my opinion, it will most likely not work because glasses containing larger size alakli ions generally have higher CTE than those containing smaller size ions (for the same mole percent alkali - see for example the glass properties handbook by Bansal and Doremus). A higher CTE on the surface will lead to residual tension in the surface.
ReplyDeleteyes, I agree. You need a lower CTE in the surface than in the bulk. So one needs to seperate the K and Na and Ca and Mg ions which are already in the glass (I wrongly wrote "ion-exchange"...). maybe by an electric field.. ok, this now is quite theoretical...
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