Study On The Synthesis And Modified Mechanism Of Ba/Ca-Nd-Ti Based Microwave Dielectric Ceramics With High Permittivity

The microwave dielectric ceramics with high dielectric constant and low temperature co-fired ceramics?LTCC?materials play a vital role in miniaturization and integration of microwave devices.In this study,the orthorhombic tungsten-bronze-structured Ba3.75Nd9.5Ti18O54?BNT,?r?85?ceramic and orthorhombic perovskite-structured Ca0.61Nd0.26Ti O3?CNT,?_r?107?ceramic were chosen as the study subjects to deeply and systematically investigate their reduction of Ti⁴⁺.Furthermore,the BNT and CNT based LTCC materials were prepared by using appropriate sintering aids based on the liquid phase sintering mechanism.The main research achievements of this study can be concluded as follows:?1?When the substitutions of different mental ions(Cu²⁺,Cr³⁺,Al³⁺,Mn²⁺)for Ti⁴⁺were carried out at B-sites of BNT ceramic,the combination between Ti and free electrons was restrained,and the reduction of Ti⁴⁺was suppressed.As a result,the electrical conductivity decreased and Q×f value of samples increased.The substitution of(Al_0.5Nb_0.5)⁴⁺for Ti⁴⁺in BNT ceramic?BNTAN?not only can promote the Q×f value,but also adjust the?_fvalue to zero.The?_fvalue of+0.3 ppm/?was obtained at x=2 for BNTAN samples.The Raman spectra of BNTAN samples indicated that the blue shifts of Raman peaks were due to the decrease of the unit cell volume,which led to the distortion of oxygen octahedrons and rigidness of vibrational oxygen octahedral framework.Hence,the?_fvalue of BNTAN samples declined as doping content of?Al0.5Nb0.5?⁴⁺increased.The decrease of the unit cell volume also reduced the space of the electron cloud,causing the decline of the molecular polarizability.Then the?_rvalue of samples declined.Baesd on the previous work,the substitution of Sm³⁺for Nd³⁺at A-sites were taken on for the Ba3.75Nd9.5Ti17.5?Cr0.5Nb0.5?0.5O54?BNTCN?ceramic to reduce its?_fvalue by decreasing the tilting angle of oxygen octahedrons.The Q×f value of samples was dependent on the internal strain,and reached the maximum value at x=3,corresponding to the minimum value of internal strain.?2?To obtain the microwave dielectric ceramics with higher?_rvalue and low dielectric loss,the ions-doped(Cr³⁺,Al³⁺,Cu²⁺,Mn²⁺,Sn⁴⁺)CNT ceramics were prepared and analysed in this work.The metal ions Cr³⁺and Al³⁺presented excellent effects on promoting the Q×f value of CNT ceramic due to their good performances on restraining the reduction of Ti⁴⁺compared with Cu²⁺,Mn²⁺and Sn⁴⁺.Based on this finding,the Ca0.61Nd0.26Ti1-x Crx O3?CNTC?samples were investigated by many characterization methods.The results showed that the decline of?_rvalue ascribed to the decline of grain size and molecular polarizability for CNTC samples.On the other hand,the decrease of?_rvalue was also dependent on the decline of the ionicity of Ca-O,Nd-O and B-O bonds.The CNTC samples exhibited maximum Q×f value of 16080 GHz at x=0.01.Moreover,the phase constitutions of the CNT samples substituted by(Cr_0.5Ta_0.5)⁴⁺and(Cr_0.5Nb_0.5)⁴⁺?shorted as CNTCT and CNTCN?were explored.The CNTCT and CNTCN samples presented single phase with orthorhombic perovskite structure in the range of 0?0.1.The Raman spectra of CNTC and CNTCT samples displayed new peaks at about 770 cm^-1,implying the electronic cloud distribution in CNT lattice was influenced by Cr³⁺.The Q×f value of CNTCT and CNTCN samples reached the maximum value at x=0.05 and 0.06,respectively.The substitution of Cr³⁺,(Cr_0.5Ta_0.5)⁴⁺and(Cr_0.5Nb_0.5)⁴⁺for Ti⁴⁺can decrease the?_fvalue of CNT ceramic,which was related to the declined distortion of oxygen octahedrons.?3?The TEM results of Ca0.61Nd0.26Ti1-x Alx O3?CNTA?samples showed that there was superlattice structure in CNTA sample at x=0.05,which may be due to the ordering arrangement of Ti/Al at B-sites.The XPS results indicated the reduction of Ti⁴⁺have been suppressed at x=0.01,and the Q×f value of CNTA sample,consequently,was improved greatly.According to the defect reaction equation,although the acceptor dopant of Al³⁺would introduce the oxygen vacancies in CNT crystal,the oxygen vacancies would combine withAl'_{T i}to form the dipole ofAl'_{T i}-_OV^··,reducing the mobility of oxygen vacancy.As a results,the electrical conductivity and conductance loss of CNTA samples decreased with increasing x value.The Q×f value of CNTA samples increased continuously from 11360 GHz at x=0 to 18130 GHz at x=0.05.For Ca0.61Nd0.26Ti1-x?Al0.5Nb0.5?x O3?x=0?0.12,CNTAN?samples,the 1:1 ordering arrangement of Al/Nb at B-sites was comfirmed by the SAED patterns at x=0.12,which was conducive to the improment of Q×f value.The XPS results showed that the reduction of Ti⁴⁺have been restrained at x=0.04.In summary,the substitutions of Al³⁺and(Al_0.5Nb_0.5)⁴⁺for Ti⁴⁺presented outstanding effects on promotion of Q×f value,and decreased the?_fvalue for CNT ceramic.The Ca0.61Nd0.26Ti0.96?Al0.5Nb0.5?0.04O3 sample exhibited the microwave dielectric properties:?_r=102.4,Q×f=15300 GHz,?_f=+242.5 ppm/?.Based on this results,the?Ca0.61Nd0.26?1-x?Li0.5Nd0.5?x Ti0.96?Al0.5Nb0.5?0.04O3?x=0?0.8,CLNTAN?samples were prepared and investigated to obtain CNT based microwave ceramics with higher?_rvalue.The CLNTAN sample had near zero?_fvalue??_r=129.2,Q×f=2210GHz,?_f=-1.4 ppm/??at x=0.76.The Raman spectra of CLNTAN samples demonstrated the Raman vibration damping increased,increasing the dissipation of Raman phonons.Hence,the internal loss increased and the Q×f value declined.?4?The liquated LB glass?comprised of Li2O-B2O3-Si O2 and Ba O-Zn O-B2O3glasses?formed the liquid phase,wetting the BNTCN ceramic in high temperature.Then the sintering activation energy decreased,which caused the cut down of sintering temperature?1390?to 950??of BNTCN samples.When the doping content of LB glass was 5 wt%,the sample sintered at 950oC had high density and excellent microwave dielectric properties with?_r=73.4,Q×f=5280 GHz,?_f=+7.1 ppm/?.In order to meet LTCC technical requirements,the Ba O-Zn O-Li2O-B2O3-Si O2 glass was chosen as sintering aid to reduce the sintering temperature of Ca0.244Li0.3Nd0.404Ti0.96Al0.02Nb0.02O3ceramic.Subsequently,the sintering temoerature decreased from 1230?to 950?.The sample doped with 4 wt%BZLBS glass sintered at 950?showed the microwave dielectric properties with?_r=104.7,Q×f=2560 GHz,?_f=-2.1 ppm/?.