Study On Transparent Glass Ceramics With High Dielectric Constant

Glass-ceramics are a kind of new materials in which crystalline phases coexist with glass phase through proper heat treatment process.They combine many excellent properties of glass and ceramics,while having stronger toughness than glass and higher transparency than ordinary ceramics.A new type of glass material,i.e.transparent glass-ceramics with high dielectric constant,can be prepared with use of strict heat treatment for some basic glasses with specific composition,which results in controlled precipitation of large number of high polarizability microcrystalline phases in the glass matrix.Because of their excellent properties such as transparent to visible light,stable dielectric constant,high mechanical strength,wear resistance and corrosion resistance,this kind of materials have broad application prospects in optoelectronic sensing,optical communication,fingerprint identification,capacitive sensing,etc.However,it is difficult for existing glass-ceramics to meet the requirements of both high dielectric constant and high transparency,and the theoretical and experimental researches on the temperature stability of dielectric constant,dielectric loss,hardness and other properties of such materials have not been fully carried out,while the actual commercial applications put forward higher requirements for the comprehensive properties of the materials.Therefore,it is particularly important to improve the comprehensive properties of existing high dielectric constant or high transparent glass-ceramics.In this paper,perovskite glass-ceramics of Nb₂O₅-Na₂O-SiO₂?NNS?system,tetragonal tungsten bronze glass-ceramics of SrO-BaO-Nb₂O₅?SBN?system and transparent glass-ceramics of MgO-Al₂O₃-SiO₂?MAS?system were studied respectively.Differential thermal analysis?DSC?,scanning electron microscopy?SEM?and X-ray diffraction?XRD?were used to study the effects of glass composition and heat treatment on controlled precipitation of microcrystalline phase and microcrystalline grain morphology of glass-ceramics.The potential relationship between the structure of glass-ceramics and physical properties such as ultraviolet-visible transmittance,dielectric constant,dielectric loss and hardness was explored.Glass-ceramics with high dielectric constant and transparency were prepared by optimized glass composition and designed heat treatment process.The main research includes:?1?For the Nb₂O₅-Na₂O-SiO₂?NNS?glass-ceramic system,transparent glass-ceramics with a thickness of less than 5 mm were prepared when the content of Nb₂O₅ was higher?65.4%?.The size of the precipitated NaNbO₃ ferroelectric microcrystals ranges from 40 to 60 nm,the dielectric constant of the sample reaches a maximum of 136.9?@1kHz,25??and have good temperature stability.The change rate of dielectric constant in the temperature range of25-290?is about-0.0302/??@1000kHz?,and the transmittance of 1 mm thick sample in the visible band achieves 70-75%.The glass composition and heat treatment process were further optimized to improve the crystallization performance of the base glass.When the glass composition is optimized as 33.7Nb₂O₅-15.72Na₂O-34.28SiO₂-8B₂O₃-8ZrO₂-0.3Sb₂O₃?wt%?,the basic glass has good forming performance,and the size of glass-ceramics is not limited by the forming method,for which the best heat treatment regime is 630?/12h+720?/2h,resulting in an increase of dielectric constant of the glass-ceramics by 31.4%compared with the base glass to 14.22?@100kHz?and improvement of the transmittance of the glass-ceramics at 600 nm up to 85%.?2?With regard to the Na₂O-BaO-Nb₂O₅-SiO₂?NBNS?glass-ceramic system,by optimizing the BaO/Na₂O ratio,the size of precipitated tetragonal black bronze structure Ba₂NaNb₅O₁₅5 microcrystals is only 16-30nm and the resulting transmittance?87%?@600nm??close to the base glass was obtained.The dielectric constant of glass-ceramics?14.0?@100kHz??is 29.4%higher than that of base glass,the dielectric loss(0.006?@100kHz?is about one order of magnitude lower than that of ordinary glass,and its change rate is only about 2.6%in a wide frequency range?1-100kHz?,showing high frequency stability.?3?For the MgO-Al₂O₃-SiO₂?MAS?glass-ceramic system,use of ZrO₂?3wt%?+TiO₂?5wt%?composite nucleating agent effectively promoted phase separation and crystallization of the base glass.By introducing part of ZnO to replace MgO,two kinds of spinel crystals,MgAl₂O₄ and ZnAl₂O₄,were formed in the glass-ceramics,and the number of precipitated crystals were higher.When the content of ZnO reaches 4wt%,the dielectric constant?10.22@1kHz?of the prepared glass-ceramics is 62.7%higher than that of the base glass,meanwhile the sample maintained a high transmittance of 85%?@600 nm?.Under 820?/6h+970?/2.5h heat treatment conditions,the vickers hardness of produced sample is 905.7kgf/mm² and is 64.6%higher than that of base glass.Finally,a conductivity measurement device for high temperature glass melts was designed and built independently,and the relationship between the conductivity,viscosity and temperature of the glass was studied.The viscosity of the MAS glass melt at 1450?is measured to be 3.6 log dPaS and the conductivity is0.0015S/cm.These results can provide key technical parameters for the design of the glass electric melting furnace for MAS systems.