@article{articleafbbcec1,
title   = {Radiopacity and microhardness changes and effect of x-ray operating voltage in resin-based materials before and after the expiration date},
author = {Tirapelli, Camila and Panzeri, Heitor and Pardini, Luiz Carlos and Zaniquelli, Osvaldo},
year = {2004},
doi = {10.1590/s1516-14392004000300006},
journal   = {Materials Research}
}