Effects Of Cr₂O₃,Fe₂O₃ And CeO₂ Dopants On Nucleation And Crystallization Process In CMAS Based Glass Ceramic

As a typical product of crystallization-controlled phase transition,glass ceramic materials were comprised of crystal phase and residual glass.The precipitation of crystals was induced by additives and nucleating agents.And considering the crystalline process was occurred from glass matrix,structural units,degree of polymerization and selection of nucleating agent would affect the final precipitation of crystals,irreversibly.In this work,we focused on the effects of various dopants or nucleating agents on nucleation and crystallization process in CaO-MgO-Al₂O₃-SiO₂ based glass ceramic.According to experimental results,it concluded that:Cr₂O₃ addition didn't only reduce the crystallization temperature(Tp),but also refine the grain size after "crystallization" treatment,remarkably.In glass matrix,Cr₂O₃ dopant existed in form of MgCr2O₄ or(Mg,Fe)(Al,Fe,Cr)2O₄ spinel phase.After electronic diffraction experiment(both of EBSD and SAED),it proved that orientation relationship between spinel and diopside can be expressed as follow:[(?)12]_spinel//[001]_diopside?(1(?)1)spinel//(200)_diopside[(?)12]_spinel//[101]_diopside?(311)spinel//((?)31)_diopsideBesides,EBSD and SAED results confirmed that habit plane of spinel was {111}.It revealed that lattice plane {111} of spinel crystal served as a nucleation site for subsequent nucleation of diopside.Single Fe₂O₃ addition in CMAS based glass ceramic had remarkable influence on structural unit and DOP of glass matrix.All experimental data verified that Fe₂O₃ additive could reduce the Tg and Tp temperature.Based on Raman profiles,the added Fe3+was existed in forms of[Fe3+O₄-SiO₄]by reacting with isolated Q0 in glass matrix,which can be regarded as a variant of Q2.It was the first experimental evidence to describing the approach of mending the glass network by added Fe₂O₃.Additionally,the Raman results also prove that solubility of Fe₂O₃ additive in this parent glass was about 10wt.%.When Fe₂O₃ adding level was more than this solubility,exceed[FeO₄]distributed outside the glass network,played a role of phase interface.EBSD test acquired from surface and inner diopside cluster confirmed that diopside crystal grew along<001>direction.And all outer plane was(002).Single Cr₂O₃ adding in this parent glass caused a distinct increasing on Tg and Tp temperatures.At the same time,adding Cr₂O₃ also increases the Tg temperatures.To samples with 0?1wt.%Cr₂O₃,the primary phase of glass ceramic after nucleation and crystallization treatments was diopside.Further adding Cr₂O₃ to 2wt.%,diffraction peak of spinel was detectable.It indicated that only sample with more than lwt.%Cr₂O₃ can generate enough spinel phase for XRD observation.However,when Cr₂O₃ and Fe₂O₃ cooped in this glass system,only 5wt.%Fe₂O₃ could achieve the bulk crystallization status.Additionally,further adding Cr₂O₃ in this glass system,reduced Tg and Tp temperatures confirmed that Fe₂O₃ could promote the formation of diopside crystals.Combined with SEM observation,further adding Fe₂O₃ to 15wt.%,the modified surface area restricted the nucleating rate for the abnormal grain growth of spinel crystals,which was attributed to the existence of twin boundary.By comparing the final crystal products in sample with Fe₂O₃ and CeO₂ addition,it indicated[CeO₄]and[CeO₆]were highly similar to[SiO₄]and[CaO₆]in glass matrix.And,it was reasonable to conclude that[CeO₄]and[CeO₆]could replace the[SiO₄]and[CaO₆]in crystal,which resulted in a shift in XRD pattern.Besides,Ce ion gathered surrounding Na+ and Ca2+ to maintain electric neutrality for the valence state differences.Hence,local glass network was repaired for the lack of destructive effect of Na+.Moreover,gathered Na+and Ca2+ modified the glass structural components,made it close to that of feldspar,such as albite and anorthite.However,generated[FeO₄]could react with Q0 to producing([Fe3+O₄SiO₄])in glass matrix.Thus,[Fe3+O₄-SiO₄]could serve as structural component in diopside crystal.