| Piezoelectric crystal is a type of crystal material with piezoelectric effect,which deforms and generates charges on the surface under the action of external forces.On the contrary,deformation will occur when the external electric field is applied on crystals.According to the feature,piezoelectric crystals can be used for the manufacture of piezoelectric devices,which will be widely used in aerospace,medical,and traffic monitoring and navigation.With the development of science and technology,the demand for high temperature sensor parts in the fields of oil and gas exploration,metal manufacturing,aerospace has gradually increased.The standard for high temperature piezoelectric materials is also increasing.Currently,piezoelectric crystals such as quartz and lithium niobate,which have been commercialized,are difficult to meet the requirements of high-temperature piezoelectric materials because of their poor dielectric and piezoelectric properties under high temperature conditions.In the La3Ga5SiO14 family crystal,the ordered structure CNGS(Ca3NbGa3Si2O14)crystal is more suitable for high temperature piezoelectric field,according to its high mechanical quality factor,high temperature resistivity and low dielectric loss,The doping of the rare earth ions,which will replace other cation sites in the crystal structure,will cause distortion of the structure and change the crystal properties.And different doping rare earth elements will induce different change of crystal properties.However,there are few studies on the change of the structure,basic physical properties and piezoelectric properties of CNGS crystals doped with different rare earth ions.In order to find more crystal materials which can maintain good dielectric and piezoelectric properties under high temperature conditions,Re:CNGS?Re= Yb,Ho?single crystals were grown by the Czochralski method.Their structure,basic thermal properties,optical properties were studied.Dielectric and piezoelectric properties at both room temperature and high temperature were also investigated in this paper.The main work is as follows:?1?Yb:CNGS crystals with doping concentration of 2 at.%and Yb:CNGS,Ho:CNGS crystals with doping concentration of 5 at.%were grown using the Czochralski method.High quality single crystals can be obtained with the[100]-direction CNGS seed crystal.The reasons for the formation of defects in the crystal were analyzed,which provided a theoretical basis for optimizing the process parameters and increasing the crystal quality.?2?The composition and structure of Re:CNGS crystals were studied in this paper.The results of X-ray powder diffraction show that Re:CNGS crystals and CNGS crystal belong to trigonal crystal system,32 point group,and the lattice constants decreased after doping.Buoyancy method was used to measure the density of Re:CNGS crystals,and the results show that the density of Re:CNGS crystals is slightly larger than CNGS.The X-ray photoelectron spectrometer and energy dispersive spectrometer were used to analyze the composition and electronic structure of the crystal.The results show that the bond iconicity of Ca-0 and Nb-O in crystals.?3?The specific heat,thermal expansion coefficient,thermal diffusivity and thermal conductivity of Yb:CNGS crystal were tested.The results show that the Yb:CNGS crystal has large anisotropy in thermal diffusivity and thermal conductivity,and the deformation induced by thermal expansion shows good uniformity.However,the absorption spectrum of Ho:CNGS crystals was analyzed.There are four strong peaks in the range of 300-750 nm.The corresponding peak wavelength and energy level transitions were 418nm?5I8?5G6?,451nm?5I8?SF3?,538nm?5I8?5F4?and 642nm?5I8?5F5?.At the same time,the absorption cross-section values in the 400-700 nm band were calculated.?4?Dielectric constant,piezoelectric constant,elastic compliance coefficient and electromechanical coupling coefficient of Re:CNGS crystals were tested by the resonance method at room temperature.When comparing with the undoped CNGS crystals,the electro-elastic coefficients of crystals increase slightly with the increase of the doping concentration.The temperature dependence of Yb:CNGS crystals from room temperature to 800? was studied.The dielectric constant ?11T/?0,elastic compliance constant s11,piezoelectric constant d11,dielectric loss and electromechanical coupling coefficient k12 of the crystal increase with increasing temperature.The variations are 17.89 to 21.78,9.01 pm2/N to 9.57 pm2/N,3.87 pC/N to 5.10 pC/N,0.00412 to 0.685,and 0.0964 to 0.1118,respectively.Doped crystals exhibit higher electromechanical stability compared with undoped CNGS crystals. |
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