Preparation And Dielectric Properties Of Li-doped KNbO₃ Ceramics And Films

Although both the electro-optical and nonlinear optical properties of bulk KNbO₃ single crystals compare rather favorably with other optoelectronic materials,the widespread application of KNbO₃ crystals for optical devices is limited because of the difficult?and cost of growing large single crystals.Compared with bulk single crystal,the ceramic form is generally more superior in the degrees of applicative freedom and productivity. However,the synthesis of high-density KNbO₃ ceramics has been difficult by ordinary pressure sintering,because the high volatile activity of potassium oxide (K20) limits stoichiometric control.This problem prevents KNbO₃ ceramics from fully densifving and deliquescence also appears at times when exposed to humidity by the existence of unstable secondary phases.So,until now,there are little reports about the sintering of pure KNbO₃ ceramics. Much effort has been made to modifying the sinter character of KNbO₃ and enhances the possibility as a lead-free candidate in the ceramic form. Doping is the most effective method to prepare KNbO₃ ceramics with high density.Now, increasing attention has been paid to LiNbO₃ as a lead-free ceramics with spontaneous polarization and high Curie temperature.So,in this paper, Li was chosen as the additive.The Li-doped KNbO₃ ceramics has been prepared by ordinary pressure sintering using Nb₂O₅,K₂CO₃,Li₂CO₃ as raw materials.The sintering characters and the dielectric properties of the samples have been systemic studied by methods ofXRD,Raman and SEM et al.In the last few years,increasing attention has been paid to the preparation of KNbO₃ thin films as the tremendous development of integration. As the assumption of electro-optic integration,the preparation of KNbO₃ flms on Si crystalline substrate which is the basis of semiconductor system has great sense.The KNbO₃ films prepared on Si substrates have wide potential application at the fields of optical waveguide,optical modulation and acoustic surface wave.So,the KNbO₃ films have been prepared on Si substrates by pulsed laser deposition using the Li doped KNbO₃ ceramics as target.The influence of processing parameters on the composition and structure of KNbO3 films has been systemically studied by methods of XRD, Raman,EPMA and FESEM et al.As the experimental results shows,the fine KN powders can be synthesized at 800℃for 2h. The volatile activity of potassium has been effectively suppressed by sintering in the powders with the same content as that of the samples,which benefits for stoichiometric control.The doped Li has introduced deformation of KNbO₃ crystal lattice, improvement of the sintering ability and decrease of sintering temperatures.To determine the inherent potential of sintered KNbO₃ ceramics,the amounts of additives must be lowered to as small as possible.In this study, the single KNbO₃ phase has been obtained by controlling the doped Li content less than 7mo1ï¼…. The samples with the optimum Li content 5mo1ï¼…sintered at 950℃for 3h have well-developed grains and density 4.34g/cm³ã€‚The dilelectric constant decreases and dielectric loss increases as the Li content increases and sintering temperature decreases.The values of dielectric constant and dielectric loss of samples doped Li 5moiï¼…sitered at 950℃for 3h respectively are 259,0.21.The The Curie temperature shifts to a higher value as the Li content of samples and sintering temperature increase,which benefits for the application in the areas of ultrasonic transducer.When KN thin films were prepared by PLD,the Si substrates more suit for the growth of crystalline KNbO₃ films than SiO₂ substrates.In our experimental conditions,the optimum condition is 650℃of substrate temperature,40ram of substrate-target distance,7.SmJ/pulse of laser energy,20Pa of oxygen partial pressure.The molar ratio of K/Nb of the films deposited under these conditions can reach to 0.98 using KNbO₃ ceramics with K/Nb 0.96 as target.K and Nb species have a spatial segregation effect in the spread process of the plume.The atomic mass of K is 39,much lighter than that of Nb (92).The K species is more likely to spread at a large oblique angle from the axial direction of the plume,which can explain the serious Kdeficiency in the films deposited on the substrates located in the plume axial direction.A considerable number of K back scatter to the target surface, which induces K-rich regions on the target surface that is not ablated. The composition of KNbO₃ films can be controlled by adjusting the target-substrate distance and the oblique angle of substrate from the plume axial direction. The almost stoichiometric KNbO₃ thin films (molar ratio of KfNb was 0.98) were successfully prepared on Si (100) substrates by PLD using the KNbO₃ ceramics target with K/Nb molar ratio of 0.96,when the substrates were set at an oblique angle of 3～12°from the plume axis and the target-substrate distance was kept at 40mm. The complete stoichiometric KNbO₃ films were obtained when the oblique angle increases up to 14 at the oxygen partial pressure of 20Pa.