Crystallization Mechanism of CMAS Glass-Ceramics with Interaction Mecha-nism between Iron and Niobium
With the exploitation and utilization of Bayan Obo mine,a large number of tailings were produced which had not been used effectively.The waste of calcium oxide,silicon oxide,alumina,magnesium oxide and other resources caused by the accumula-tion and landfill tailings is more urgent to solve the high value utilization of rare metal elements such as rare earth,niobium,titanium and so on.The preparation of CaO-MgO-Al2O3-SiO2(CMAS)glass-ceramics with ferric oxide and niobium oxide as nucleating agents was one of the effective means for high value utilization of Bayan Obo tailings.The results showed that the crystal phase composition of glass-ceramics samples can be controlled by changing the doping ratio of ferric oxide and niobium oxide by changing the doping ratio of ferric oxide and niobium oxide.Ferric oxide could reduce the crystallization temperature of glass-ceramics and promoted crystalliza-tion,and diopside phase was the main crystal phase.However,niobium oxide as nucleating agent could promote the niobium-rich crys-tal phase precipitation of pyrochlore and CaNb2O6,but increase the crystallization temperature inhibiting the crystallization of diopside phase in glass-ceramics.The solution ratio of calcium to niobium in Ca2Nb2O7 and CaNb2O6 was 1∶1 and 1∶2,respectively.Therefore,when the amount of niobium was too much,CaNb2O6 precipitated,because it could make more niobium from the glass phase with high-er internal energy to the crystal phase with lower internal energy.It could be found that the glass-ceramics of the target crystal phase composition could be obtained by changing the proportion of Fe-Nb addition.When the niobium content was constant,the crystalliza-tion temperature of the sample decreased obviously with the increase of the iron content,which indicated that iron played a role in pro-moting the crystallization of glass-ceramics.When the amount of iron was constant,the crystallization temperature increased with the increase of the amount of niobium,which meant that the addition of niobium could inhibit the crystallization of glass-ceramics.This was due to the fact that niobium may form[NbO6]structural motifs,which were conducive to the formation of glass and increase the degree of polymerization of the entire silicon oxygen network,thus inhibiting the crystallization of glass-ceramics.The addition of niobi-um oxide caused an obvious Raman shift peak at v=821 cm-1 in this range which belonged to vibration of silicon oxygen network,indi-cating that niobium existed in the form of[NbO6]structural unit and entered into the silicon oxygen network.Nb54 in[NbO6]structural unit would be negatively charged after connecting six O2-,so two[NbO6]structural units would be extremely unstable when connected.However,when[NbO6]structural unit and[SiO4]structural unit were connected into the silicon oxygen network,the external ele-ments of the positively charged network would play a charge compensation role and make the structure stable.However,when[NbO6]structural unit and[SiO4]structural unit were connected into the silicon oxygen network,the external elements of the positively charged network would play a charge compensation role and make the structure stable.Therefore,the addition of niobium increased the polymerization degree of the irregular network in the glass,and more energy was needed to break the glass network for crystalliza-tion.In the glass of CMAS system,niobium could be used as a network component to enter the silicon oxygen network,improve the po-lymerization degree of the silicon oxygen network,and thus inhibit the crystallization of the sample.According to the analysis of crys-tallization mechanism of glass-ceramics nucleation and crystal growth,firstly,the degree of polymerization of glass network increased,which led to the decrease of ion mobility and phase separation in the process of nucleation heat treatment,and finally led to the de-crease of heterogenous nucleation points of diopside.In the process of crystallization heat treatment,the rearrangement energy of atoms would also be affected and increase,which would reduce the growth rate of crystals.The slower precipitation rate of the main crystal phase would precipitate the bytownite phase in the form of surface crystallization.In addition,fluoride ions,which were originally flux-es,also precipitated as fluorophlogopite under the action of the combined mechanism of niobium and iron.This was due to the fact that both the 4d5s orbital of niobium and the 3d4s orbital of iron could provide electrons at the same time,which could increase the density of states near Fermi level and increase the atomic activity.Cordierite phase,which was difficult to precipitate in CMAS glass-ceram-ics,could also be obtained by changing the doping ratio of Fe and Nb.The study on the mechanism of Fe-Nb interaction provided a the-oretical basis for the high value utilization of Bayan Obo tailings and the design of glass-ceramics with unique properties.The strength of Raman peak near v=821 cm-1 caused by[NbO6]connecting two non-bridging oxygen bonds would decrease with the increase of iron,which indicated that the niobium which originally improved the polymerization degree of silicon oxygen network could not enter the silicon oxygen network under the action of iron.The study on the mechanism of Fe-Nb interaction provided a theoretical basis for the high value utilization of Bayan Obo tailings and the design of glass-ceramics with unique properties.
glass-ceramicscombined action mechanism of iron and niobiumRaman spectroscopyglass network
NETL
NSTL
万方数据
