Usage Of Complex Chemical Bond Theory In The Microwave Characteristic Regulations Of A_0.5B_0.5CO₄ Systems

As essential key materials applied in electronic communication field,microwave dielectric ceramics have huge potential in practical application.It is always crucial and difficult to improve the dielectric properties of microwave dielectric ceramics at microwave frequency.The“experience”is always regarded as the guidance in this field,however,it shows referential value to a certain extent.This guidance can not be used to control experiment fundamentally,which leads to an urgent requirement of an effective theory.It is widely acknowledged that the uniqueness of dielectric ceramics lies in the fact that the microwave characteristics vary with the crystal structure.It is the key to realize the fundamental regulation of dielectric properties with a thorough understanding of the influence of crystal structure on dielectric properties.On the condition of achieving great sintering characteristics of material systems,it is necessary to investigate the intrinsic properties of microwave dielectric ceramics caused by crystal structures.A complex dielectric theory of crystal(shorten as P-V-L chemical bond theory)describes the chemical bond traits,including bond ionicity,bond covalency and bond susceptibility of binary crystals,and correlates the crystal structure and dielectric properties.The P-V-L complex chemical bond theory then can be applied into multi-systems after the binary bonding formula theory of polycrystalline was proposed,and provide guidance for dielectric properties of microwave ceramics.Therefore,from the perspective of chemical bond traits,to improve the"blind zone"in the field of microwave dielectric ceramics and put forward the relationship between crystal structure and dielectric properties,correlation model was established between the chemical bond characteristics and microwave dielectric properties by using the P-V-L complex chemical bond theory,crystal structural refinement and lattice vibration spectroscopy.The excellence of the theory in the analysis of crystal structure,evaluation and prediction of microwave dielectric properties is systematically elaborated.Profiting from the model,an effective method to guide the doping modification of materials is proposed.Based on that,the AO-BO2-C2O5 systems were used as the verification objects,and the model was used to guide the doping modification experiments in Wolframite,Ixiolite,Trirutile and Rutile phases from low dielectric to high dielectric systems,respectively.The consistence between theoretical and experimental microwave characteristics verifies correctness and effectiveness of the model.To expand the application prospects of microwave dielectric ceramics in Low-temperature Co-fired Ceramics(LTCC)technology,the low temperature sintering mechanism of ZnO-TiO2-Nb2O5 based materials was profoundly analyzed via image sintering,activation energy and high temperature solubility of ceramics,and the ZnO-TiO2-Nb2O5 based LTCC ceramics with competitive microwave dielectric properties were prepared at low temperature.The main contents of this dissertation are as follows:(1)In(Zn1/3Nb2/3)x Ti1-xO2(x=0.45,0.75,1)systems with the same element and different substitution amount,there are microwave dielectric ceramic systems with three typical crystal structural types,including low?r,medium?r and high?r.The P-V-L complex chemical bond theory was used to calculate the three kinds of material systems,followed by that,the impact origins to dielectric polarization and lattice stability were summarized,and modification methods were also proposed to regulate the microwave characteristics,which is supported by the experimental results and research reports.For instance,columbite ZnNb2O6 ceramics were synthesized when x=1,it is found that Nb-O bonds are crucial for crystal structure after comparing the bond ionicity,bond susceptibility and lattice energy,that is to say,Nb-O bonds play dominant roles in the regulation of intrinsic dielectric properties of Zn Nb₂O₆ crystal.This conclusion is well testified by using far-infrared reflectance spectroscopy and complex permittivity function.Similarly,ixiolite Zn₀.5Ti₀.5NbO₄ and rutile Zn₀.15Nb₀.3Ti₀.55O₂ structures were prepared,and the contributions of chemical bond traits to dielectric polarization and lattice stability at different cation sites were evaluated via P-V-L complex chemical bond theory calculation.Ionic doping experiments provide great testifications.(2)Three kinds of microwave dielectric ceramics with different crystal structures were found in(A²⁺_1/3C⁵⁺_2/3)_0.75B⁴⁺_0.25O₂ systems with different elements but the same substitution amount,such as rutile A₀.5Ti₀.5NbO₄ ceramic with high?r,trirutile A₀.5Ti₀.5TaO₄ ceramic with medium?r,and wolframite A₀.5Zr₀.5NbO₄ ceramics with low?r.Correspondingly,improving strategies were suggested based on the estimations of bond features calculating through P-V-L complex chemical bond theory.For instance,the bond ionicity,bond susceptibility and lattice energy of A-O bonds are the lowest,reflecting their negligible effects on microwave characteristics.On the contrary,Nb-O and Ti-O bonds show obvious regulatory effects.Therefore,Nb and Ti sites were substituted by equivalent Ta⁵⁺and Sn⁴⁺cations,and their adjusting impacts were compared through far-infrared reflectance spectroscopy and intrinsic dielectric properties analysis,and the P-V-L complex chemical bond theory conclusions were also verified.In trirutile A₀.5Ti₀.5TaO₄ structure,M₂ cation site(occupied by Ti/Ta₂ in Ni0.5Ti₀.5TaO₄)predominates in lattice stability based on P-V-L complex chemical bond theory calculation,thus,Ti site was replaced by Sn element,achieving the effects of reducing the dielectric loss and increasing the Q×f value.While in wolframite A₀.5Zr₀.5NbO₄(A=Mn,Zn,Mg,Co)systems,it is calculated that Nb-O bonds own the largest bond ionicity,bond susceptibility and lattice energy proportion.Firstly,when the A site element directly varies from Mn to Co,the sintering ability of ceramics shows large differences,extrinsic loss containing the growth of grain and grain boundary loss should not be ignored.Based on the consideration of avoiding these effects,research reports show that the Nb site doped with Sb⁵⁺cation presents the greatest effect on reducing the dielectric loss,which is consistent with the P-V-L complex chemical bond theory conclusion.(3)The low temperature sintering behavior of two different ZnO-TiO₂-Nb₂O₅ based microwave dielectric ceramics were studied by using Li₂O-B₂O₃-SiO₂ and ZnO-B₂O₃-SiO₂ glass frits.The low-temperature sintering kinetics mechanism of Zn O-Ti O₂-Nb₂O₅based LTCC materials was analyzed by characterization of wettability,solubility,sintering activation energy and dynamic shrinkage.Benefitted from the formation of liquid phase or eutectic mixture of glass at high temperature,the activation energy of sintering is greatly reduced due to the infiltration and dissolution of ceramics in the high temperature glass phase,which makes the low temperature sintering process much easier to be carried out.By adjusting the content of glass additives and sintering process parameters,excellent microwave dielectric properties were obtained at 900?:?r=36.7,Q×f=20000 GHz,?f=7 ppm/?.The application prospects of ZnO-TiO₂-Nb₂O₅ based microwave dielectric ceramics in the field of LTCC is broaden.