Research On The Cooperative Regulation Mechanism Of Transparency And Electrical Properties Of KNN-based Ferroelectric Ceramics

Transparent ferroelectric ceramics not only exhibit superior piezoelectric and ferroelectric properties,but also own high transparency and excellent electro-optical coefficient;therefore,they can be used to develop a series of devices,such as optical storage,light modulation,light shutter,image processing,etc.In this circumstance,lead-free transparent piezoelectric ceramics play a crucial role in the future development of computer technology,optical communication technology and national defense.KNN-based piezoelectric ceramics with grain piezoelectric properties are one of the most promising candidates.To date,the main problem that needs to be resolved is to improve the optical transmittance.If the grain size is reduced to submicron or nanometer scale,the transmittance can be significantly improved;nevertheless,the piezoelectric properties would be significantly deteriorated or even lost as the result.Therefore,the contradiction between transmittance and piezoelectric properties becomes a key bottleneck restricting the development of KNN-based transparent ferroelectric ceramics.In this thesis,KNN-based ceramics with both decent optical and good electrical properties were realized via rational chemical composition design.The transmittance-microstructure correlation was discussed by studying the pore scattering and birefringence loss in various KNN-based ceramics.On this basis,by modulating the microstructure(such as grain size,pore size)and phase structure,we proposed an effective strategy to resolve the above-mentioned contradiction in KNN-based transparent ferroelectric ceramics,and eventually realized the coordinate regulation of high transparency and decent piezoelectricity.The main conclusions were summarized as follows:(1)The phase structural characters of pure KNN ceramic and the regulation of Bi doping on its transmittance were systematically studied,based on both experimental and first-principles calculations.Pure KNN ceramics retain orthorhombic phase at room temperature,but it can easily transit to rhombohedral or tetragonal phase,due to the low energy difference between them.As Bi is introduced to Nb site in KNN,impurity level generates in the band gap,leading to a reduced excitation energy as confirmed by the gradually reduced optical band gap Eg in experiments.Although KNN-xBi ceramics have high transparency due to the sub-micron grain(<120 nm),according dielectric properties are inferior.(2)The influence of rare earth(Eu3+and Sm3+)doping on the microstructure and transmittance of KNN ceramics was studied;based on pore equivalent scattering model and Rayleigh scattering theory,the correlation between ceramic transmittance and porosity was revealed.KNN-xEB ceramics and KNNB-xSm ceramics both form submicron-sized grains and nanometer-sized pores.To study pore scattering,we first estimated the pore size(dpore)and porosity volume fraction(p)via experimental microstructures,and then calculated the theoretical optical transmittance of KNNB-xSm using Rayleigh's theory.The calculated results were well verified by experiments.According to the calculations,we also revealed that the influence of pores on the transmittance is almost negligible when the pore size is very small(<10 nm)or porosity volume fraction(<0.01%)is very low.At last,we experimentally verified that the dielectric and ferroelectric properties of KNN-0.03Bi ceramic can be effectively improved via pore modulation by Eu3+and Sm3+doping.(3)By chemical composition design(SAT/SAN modulation in KNN),we studied the effects of phase structure and micro structure evolution on transmittance and electrical properties.The KNN-0.16SAT ceramic with an isotropic phase structure shows the highest transmittance close to 70%at 2000 nm;however,its electrical properties of the ceramic are strongly degraded and almost lost due to cubic phase structure.The transparency mechanism was ascribed to the vanishing of birefringence in isotropic cubic phase.Next,we prepared the anisotropic KNN-xSAN ceramics by introducing SAN(x?0.08)into KNN.Heavily SAN doping was found to significantly reduce Pr and d33 despite that it also significantly limited the grain growth.Consequently,superior transmittance and electrical properties:T?55%(780 nm),d33=105 pC/N was realized in a moderately doped 0.96KNN-0.04SAN composition.(4)A general strategy of realizing coordinate regulation of high transparency and piezoelectricity in KNN-based ceramics is proposed based on micro structure(pore size,grain size)and phase structure design.For the KNN-xSIN ceramics(0.02 ?x ?0.07),all ceramics studied have anisotropic phase structure at RT and submicron grain size.We achieved comprehensive superior optical and electrical performance in anisotropic and submicron sized KNN-xSIN ceramics(0.02 ?x ?0.07)thanks to the absence of impurity phase and trace pores.We also studied the structure evolution of perovskite structured KNN-xSSN ceramics with SSN content increasing.The phase structure changes gradually from orthogonal to pseudo-cubic phase,while the grain size decreases from 1.74 ?m to 0.26 ?m.As the result,the transmittance gradually increases and piezoelectric property slightly gets deteriorated.Eventually,at x=0.05,we realized a comprehensively high transmittance(73%at 780 nm)accompanied by a superior piezoelectric constant(d33=101 pC/N),as the synergetic result of moderate submicron grains(?0.34?m),orthorhombic phase and high mass density.