Effect Of CeO₂ Addition On Microstructure And Properties Of Diopside Glass Ceramics

Glass-ceramics with many kinds and different functions combine various physical and chemical properties,such as high mechanical strength,high toughness,high wear resistance,high insulation,high light transmission,strong acid and alkali corrosion resistance and low thermal expansion coefficient.It has been widely and importantly applied in all areas of the national economy and in the daily lives of the people as well as in national defense military technology.The raw materials for preparing glass-ceramics are rich and varied.Among them,the research and application of structural microcrystalline glass prepared by using various industrial wastes such as tailings and slags as raw materials,have received more and more attention in recent years,because of its extremely important economic and practical significance in efficient recycling and utilization of mineral resources,ecological security and environmental protection.The results reported previously for the diopside-type glass-ceramics prepared by using the rare earth contained tailings of Bayan Obo mineral showed that the glass-ceramics with a small amount of rare earth ions exhibited unusually high mechanical strength,super wear resistance and excellent resistance to acid and alkali corrosion.However,the influences of rare earth ions on the phase composition,phase structure,crystallization process,microstructure,and mechanical properties of diopside-type glass-ceramics remains a highly controversial subject so far.Clarifying the influence mechanism of rare earth ions is not only an important academic issue,but also of important theoretical significance to guide the design of the functions,compositions,and processes of related glass-ceramics.Accordingly,this dissertation is designed to focus firstly on the revealing of the influence of rare earth ions Ce⁴⁺and different preparation processes on the intrinsic nature of the phase formation and mechanical properties and related mechanism,majored in the single-phased CaMgSi₂O₆ diopside crystals with[Si₂O₆]^4-single-chain structure.On this basis,CaO-Al₂O₃-MgO-SiO₂?CAMS?-baseddiopside-typeglass-ceramics,with composition optimized to be similar to that of the conventional tailings and slags,were synthesized with chemically pure raw materials,the effects of different rare earth oxides and ZrO₂ nucleating agents on the crystallization kinetics,microstructure,mechanical properties,and corrosion resistance of CAMS-based glass-ceramics were investigated systematically.The main research contents and results of this dissertation are concluded as follows:1.The single-phase crystalline samples of Ca_1-xCe_xMgSi₂O₆?x=0,0.02,0.04,0.06,0.08,and 0.1?diopside were fabricated for the first time by direct melt-cooling process.Through crystal structure analysis,it is found that the lattice constant and unit cell volume of Ce-containing samples generally decrease,and when x exceeds0.04,it tends to be stable,which means that it is difficult for Ce⁴⁺to substitute Ca²⁺sites or occupy the interstitial sites of the diopside crystal extensively.For the single-chain diopside phase with a Si/O ratio of 1:3,the crystalline and effective ionic radius of Ce⁴⁺are much smaller than the corresponding radii of Ca²⁺sites of VIII coordination,and much larger than those of Mg²⁺or Al³⁺sites of VI coordination.A large amount substitution with Ce4+will destroy the thermodynamic stability of the diopside phase structure.This result is consistent with the measurement results of the mechanical parameters.The Vickers hardness?⁶700 MPa?with Ce containing in the range of 0.02-0.04 is slightly higher than that of other component samples?⁶500MPa?.It can be seen that a small amount of Ce⁴⁺ions entering the crystal lattice does not significantly affect the crystal structure and mechanical properties of the single-chain diopside phase.However,the grain sizes calculated using the Scherrer formula indicate that the addition of Ce⁴⁺contributes significantly to the refinement of the diopside phase grains.Currently,it is difficult to prepare large single crystal samples that can be used for conventional mechanical property testing.Therefore,for the two effects of Ce,that is,refining grains and replacing Ca to modify the intrinsic mechanical properties of the crystal,it is not possible to determine which effect is more important for the influence of mechanical properties such as Vickers hardness.2.A single-phase diopside-type CaMgSi₂O₆ crystalline sample can be prepared also by utilizing glass crystallization processes.Microstructural analysis showed that the crystallization was first started on the outer surface of the sample,and the preferred orientation of the grains was mixed one of[001]and[010],while in the vicinity of the interface between the glass layer and the precipitated crystal layer,the preferred orientation of the grains is dominantly in the[010]direction.After adding CeO₂ with 2 wt%,the preferred orientation of the grains became[001].The addition of Ce⁴⁺increases the crystallization and melting temperatures,crystallization activation energy E_a and pre-exponential factor V_a of the single-phase diopside glass-ceramic.In addition,it is also found that the glass-ceramic sample with more complete crystallinity and more compact morphology can be obtained if it is prepared by sintering the glass powders pulverized and compacted at a high temperature.More interestingly,its Vickers hardness is much higher than that obtained by direct cooling or glass crystallization.In particular,2wt%Ce⁴⁺can increase the Vickers hardness of the sample by 9.0%to 8460 MPa,originating that Ce⁴⁺is favorable for the regular arrangement of diopside crystallites.The results above show that the microstructure has a more significant impact on the mechanical properties of diopside glass-ceramics,and it could have certain referring significance for the preparation of high-hardness glass-ceramics.3.The CaO-Al₂O₃-MgO-SiO₂?CAMS?-based diopside glass-ceramics with optimized composition have higher hardness,stronger crystallization ability and more excellent corrosion resistance performance than the single-phase diopside CaMgSi₂O₆glass-ceramics.The presence of additives such as CeO₂,ZrO₂,Al₂O₃,and ZnO does not affect the crystal structure type of the precipitated crystal,but influences the mechanical properties and corrosion resistance of the glass-ceramic by modifying the kinetic process and microstructure of the crystallization behavior.Both CeO₂ and ZrO₂ are found to be beneficial to the whole crystallization.Relatively,nano-scale ZrO₂ agents can significantly reduce the activation energy,so that crystallization can be carried out at a lower temperature,and the effect on whole crystallization is more obvious.Similar to the role in single-phase CaMgSi₂O₆ glass-ceramics,Ce⁴⁺ions are also mainly present in the glass phase to affect the mechanical properties of the glass-ceramics resulting from the variations of the density,viscosity,and crystal phase content of the glass-ceramic.The Vickers hardness of the crystallized glass without CeO₂ is higher than that of not-crystallized one,but there is inconsiderable difference was found between the glass state and the crystallized state for samples with CeO₂ added.It is indicated that the rare earth ions improves the stability and viscosity of the CAMS base glass itself,whereas inhibits the crystallization,so that the two kinds of effects on mechanical properties of glass ceramics cancel each other out.With the increase of CeO₂ content,the diopside grain size increases,and the c-axis of the precipitated diopside crystal grains changes from perpendicular to parallel to the interface between the devitrified layer and the glass matrix.Moreover,nanocrystals with[010]orientation were found existing in the vicinity of the interface in the glass ceramic with CeO₂.