Preparation,Properties And Polarization Mechanism Of ?Li,Nb? Co-Doped TiO₂ Colossal Permittivity Ceramics

Colossal permittivity dielectric materials have been drawing increasing attention due to their potential application in the miniaturization of capacitors and high energy storage density.Acceptors and donor ions co-doped Ti O₂ ceramics have been studied and excellent colossal permittivity dielectric properties have been obtained.However,the relationship between the different polarization mechanisms and colossal permittivity needs to be further explored in co-doped Ti O₂ ceramics.On the other hand,the extremely low breakdown strength is difficult to meet the practical needs and how to realize high breakdown strength in colossal permittivity dielectric materials is still a problem to be solved.In this thesis,we studied the dielectric properties and the polarization mechanisms of?Li,Nb?co-doped Ti O₂ ceramics and explored the method to improve the breakdown strength of?Li,Nb?co-doped Ti O₂ ceramics.Besides,new co-doping matrix material was tried to explore novel high performance colossal permittivity dielectric materials.?Na,Sm?co-doped dielectric ceramics based on the matrix of Ca Ti Si O₅ titanite were studied to explore the applications at the elevated temperature.The main results are as followed:?1?(Li_1/4Nb_3/4)_xTi_1-xO₂?x=0.25%?10%?ceramics with different concentrations were prepared by solid-state reaction synthesis method.All the ceramics are rutile phase without the secondary phases detected and the grain sizes increase with the co-doping concentration increasing.All the ceramics show colossal permittivity and exhibit excellent frequency stability.However,the dielectric loss rapidly increases for high concentration doping,which demonstrates that high concentration doping deteriorates dielectric properties.Among all co-doped ceramics,(Li_1/4Nb_3/4)_xTi_1-xO₂ ceramics with1%co-doped concentration exhibit excellent dielectric properties with??7600,tg??0.06 at the frequency ranging from 20 Hz to 10⁷ Hz at room temperature.There exist various polarization mechanisms in co-doped Ti O₂ ceramics,like space charge polarization,relaxor-like polarization,polaron hopping polarization and defects dipoles related to electron polarization.Whereas,the colossal permittivity mainly relates to electron polarization originated from defects dipoles.The dielectric properties of(Li_1/4Nb_3/4)_0.01Ti_0.99O₂ ceramics exhibit good temperature stability at different measuring frequencies from 173 K to 423 K,which meet the requirement of X8F capacitors.However,(Li_1/4Nb_3/4)_xTi_1-xO₂ ceramics have a very low breakdown strength?<12 V/mm?for 1%co-doped concentration,which is disadvantage to its practical application.?2?xSi O₂-(Li_1/4Nb_3/4)_0.01TiO₂?x=1%?20%?ceramics were synthesized by solid-state reaction synthesis method and the optimum sintering temperature of ceramics is1250?1300?with the relative density beyond 99.5%.The average grain size of x Si O₂-(Li_1/4Nb_3/4)_0.01Ti O₂ ceramics decreased from 3.18?m at x=1%to 1.97?m at x=9%.The permittivity of x Si O₂-(Li_1/4Nb_3/4)_0.01Ti O₂ ceramics for x=1%shows good temperature stability???3600?over a wide temperature range,which satisfy the requirement of X8F and X9R.Besides,the dielectric properties of the ceramics show good DC bias stability.The results of I-V characteristic curve show that the breakdown strength rapidly increases from?12 V/mm to 230-317 V/mm with the introduction of Si O₂.The improvement of breakdown strength of co-doped Ti O₂ ceramics is vital to realize its practical application.?3?MgTi_1-x(Al_1/2,Nb_1/2)_xO₃?x=1%,3%?and Mg Ti_1-xNb_xO_3+1/2x?x=1%,3%?ceramics were prepared by solid-state reaction and their dielectric properties were measured to explore novel colossal permittivity dielectric material by co-doping method.The dielectric constant is almost unchangeable.Further work needs to be carried out to understand the results.Besides,Ca_1-x(Na_1/2,Sm_1/2)_xTi Si O₅?x=0%-16%?ceramics were synthesized by solid-state reaction.The results show Na⁺ion and Sm³⁺ion co-doping is an effective method to improve the stability of dielectric constant of Ca Ti Si O₅ ceramics at elevated temperature.However,with the co-doping concentration increasing,the dielectric loss increases.Further exploration should focus on how to improve the dielectric properties at elevated temperature.