Analysis Of Crystal Behavior Of Alkali Borosilicate Glass With Transition Metal Oxide

TiO₂ plays the key role in the fabrication of glass,glass-ceramics.It is introduced as nucleation agent in the commercial Li₂O-Al₂O₃-SiO₂ and MgO-Al₂O₃-SiO₂glass-ceramics with a lower content no more than 10wt%.In this work,we investigated the crystallization behavior of borosillicate glasses with a large concentration range of TiO₂,and the effects of Ti O₂ and Nb₂O₅ in these glasses was studied.The main results are as follows:?1?The glasses of SiO₂-B₂O₃-Li₂O-K₂O-TiO₂-Sb₂O₃ system have a strong phase-separation tendency,which favors the crystallization by lowering the interface energy of nucleation.The rutile crystals were precipitated by proper heat treatment in the glasses with the substitution of Li₂O for K₂O from 0 to 9 wt%.Increase of Li₂O content leads to an increase of phase-separation tendency accompanying the decrease of T_g temperature.Similarly,with the replacement of Li₂O by Na₂O,the phase separation and crystallization trend are greatly reduced due to the lower field strength of the latter.?2?Differential scanning calorimetry?DSC?was used to determine the nucleation rate of the glass of Ti O₂-Li₂O-?SiO₂+B₂O₃?system.We found that,in the DSC curves,peak areas phase-separation and crystallization increase with the increase of heating rate,this is due to the fact that the critical nucleation radius increases with increasing temperature,thus,the nuclei formed at low temperature can not exist at higher temperature.This explains why the hither heating rate,e.g.,25K/min or 30K/min,corresponds to larger peak areas of crystallization compared to that with a lower heating rate of 5K/min or 10K/min.We found that the nucleation rate first increases and reaches the maximum at 853 K,then it decreases.?3?For Li₂O-B₂O₃-SiO₂?LiB?,Nb₂O₅-Li₂O-SiO₂?LiBNb?,TiO₂-Li₂O-B₂O₃-SiO₂?LiBTi?glass,because the Li BNb glass is a multiple rings layered structure mainly composed of[BO₄]tetrahedron,[BO₃]triangle,[NbO₆]octahedron,the structure hinders the diffusion of Li⁺and makes LiBNb samples surface crystallization after heat treatment.In the DSC test,the LiBTi sample has a split peak before appeared crystallization peak.This is due to the occurrence of boron abnormal phenomenon in LiBTi sample,Ti was mainly present in the form of[TiO₄]tetrahedron in the LiBTi sample,[TiO₄]is immiscible with[BO₃]and facilitates the separation of the sample.The competition between[TiO₆]octahedron and[NbO₆]octahedral to get free oxygen will let B-O bond and Si-O bond weakened,so the glass transition temperature of LiBTi and Li BNb was significantly lower than that of LiB in DSC test.In the DSC test,the LiBTi sample has a split peak before appeared crystallization peak,this is due to the occurrence of boron abnormal phenomenon in LiBTi sample,Ti was mainly present in the form of[TiO₄]tetrahedron in the LiBTi sample,[TiO₄]and[BO₃]had the structure of immiscible and facilitates the separation of the sample.The competition between[TiO₆]octahedron and[NbO₆]octahedral to get free oxygen will let B-O bond was strong and the Si-O bond was weakened so the glass transition temperature of LiBTi and Li BNb was significantly lower than that of LiB in DSC test.