Studies On The Preparation And Properties Of CoTiNb₂O₈-based Microwave Dielectric Ceramics

With the rapid development of microwave technology,high Q dielectric ceramics with medium-permittivity are key materials for resonators in satellite communication and base station.The drawbacks for current middle dielectric materials were limited types?lack structural diversity and poor performance.The dielectric constant??r?of CoTiNb₂O₈ can be reached to 63.5.Meanwhile,this single-phase ceramic maintains excellent quality factor?Q±f?.However,it commonly requires to be sintered at high temperature and its ?f is large for practical applications.But now,there is little relevant research on it.For the first time,low melting point compounds?ionic substitution and composite synthesis were introduced for optimal microwave dielectric performance.A series of novel low-loss dielectric ceramics with medium-permittivity was obtained in this dissertation.XRD?FT-Raman?TEM and TSDC were employed in order to detect the correlations between crystal structure and microwave dielectric property.Of particular interest was the investigation of microwave dielectric loss mechanisms in the view of defects.CoTiNb₂O₈ ceramic was prepared by conventional solid state method.At 1250?/4h,it exhibited excellent microwave dielectric properties with ?r?63.5,Q×f?25,300GHz and ?f?86.06ppm/?.Based on this finding,low melting point oxides such as CuO and B₂O₃ were introduced to lower the sintering temperature.The main issues here were which type of sintering aids existed and how they assisted the sintering process.The CuO addition was proven to reduce the sintering temperature from 1250? to 950?.This phenomenon was attributed to the formation of liquid phase CuNb₂O₆,and the substitution of Cu for Co in CoTiNb₂O₈.The CoTiNb₂O₈ ceramic with 2wt%CuO addition did not react with Ag at 950?,making it a promising candidate for LTCC applications.While in all B₂O₃ doped samples,the ceramics remained tetragonal rutile phase.Due to liquid phase effect,the sintering behavior was enhanced considerably.Even so,the Q×f was still degraded to 18,793GHz.The explanation for this reduction in Q×f that related to the increased oxygen vacancies concentration and newly formed defect dipoles were tentatively proposed.In an attempt to decrease the sintering temperature,CoTiNb₂O₈ ceramic was first synthesized through a developed sol-gel process using Nb₂O5·nH₂O as starting Nb source.Its processing conditions?crystal structure and microwave dielectric property were studied systematically.On the analysis of the fourier transform infrared spectroscopy?FT-IR?,the metal ions were believed to be chelated with citrate acid mainly in two-tooth complexing.When pH was 4,highly reactive CoTiNb₂O₈ nano-powder was achieved by the reaction between TiO2 and intermediate phase CoNb₂O₆ at 800? for 4h.The synthesis of target product was similar to that of solid-state reaction meathod.After tabletting,CoTiNb₂O₈ ceramic could be densified at around 1000?,realizing its low-temperature sintering with metal electrode.Meanwhile,it possessed optimal microwave dielectric properties with an ?r of 64.2,a high Q×f of 16,800GHz and a?f of 66.17ppm/?.This synthetic process was simple and convenient.Introducing Zr and V by ionic substitution into CoTiNb₂O₈ separately were aimed at adjusting the large ?f and reducing the high sintering temperature to meet practical requirements.Combined with Rietveld full spectrum fitting method and TEM,the relationship between crystal structure and microwave dielectric properties was analyzed.With the replacement of Ti by Zr,the system remained tetragonal rutile phase when x<0.2,and for x?0.6,a monoclinic wolframite-type structure was observed,whereas for x=0.4,CoZrNb₂O₈ phase coexisted with rutile phase.The improvement of Q×f was largely dependent on the variation of packing fraction and relative density.Due to the increase of oxygen octahedron distortion,the ?f moved to the negative direction.The incorporation of V at Nb-site assisted the densification behavior of host material.With increasing V content,a range of rutile-structured solid solution was formed for all samples tested.The dielectric properties of CoTi?Nb1-xVx?2O₈ ceramics were also found to have a significant association with the shift of Raman scattering peaks.CoNb₂O₆ was selected as a ?f compensator for CoTiNb₂O₈ in this work.With the introduction of CoNb₂O₆,the sintering behavior of the composite ceramics was improved significantly to some extent.X-ray diffraction patterns revealed that rutile CoTiNb₂O₈ phase always coexisted with columbite CoNb₂O₆ phase in the entire composition range.According to the Lichtenecker mixing rule,the estimated dielectric properties basically agreed well with the measured values.Preliminary analysis on the variation of microwave dielectric property of?1-x?CoTiNb₂O₈-xCoNb₂O₆ composite was also provided.Tunability of ?f with improved Q×f had been done at x=0.8.