Study On The Growth And Properties Of CNGS And CTGS Crystals

Piezoelectric materials mainly consist of piezoelectric crystals, ceramic and polymer, which own their strongpoint and weakness respectively. As for the stabilization, technical flow, yield scale and the tendency to miniaturization, piezoelectric crystals are dominant in the three materials. At present the crystal used widely in piezoelectric engineering isα-quartz, whose yield in a year is up to several kilotons. Quartz possesses high frequency-temperature stability and low cost, which is the main reason for its extensive applications in the past decades. However, the small electromechanical coupling coefficients and large insert loss restrict the usage of quartz in wideband filter.With the rapid development of electronic technology and communication, the requirement for high-frequency and rapidly-propagating devices makes researchers focus on the search for new piezoelectric materials. The aimed materials must be superior to quartz and can be used in wideband filter and oscillation of high frequency-temperature stabilization. Recently researchers show great interest in the following piezoelectric crystals: Li₂B₄O₇ (LBO), GaPO₄ and La₃Ga₅SiO₁₄ (LGS)-family crystals. They have high acoustic Q factor, easily-obtained high electromechanical coupling coefficients and temperature-compensated orientations.Ca₃NbGa₃Si₂O₁₄ (CNGS), Ca₃TaGa₃Si₂O₁₄ (CTGS), Sr₃NbGa₃Si₂O₁₄ (SNGS) and STGS (Sr₃TaGa₃Si₂O₁₄) are new piezoelectric crystals with ordered langasite-structure in recent years. The crystals have zero temperature coefficient orientations, better piezoelectric properties than LGS crystal and much reduced cost due to much reduced gallium content. Relatively low temperature and Ga content lessen the volatilization of Ga₂O₃, which correspondingly improve the crystals' uniformity from ups to downs. Moreover, the crystals have good mechanical strength resulting in less handling difficulty and less thermal expansion anisotropy than LGS crystal.In this paper, many experiments were carried out to grow the Ca₃NbGa₃Si₂O₁₄ (CNGS), Ca₃TaGa₃Si₂O₁₄ (CTGS) and their isostructural compounds by using the czochralski technology. The optimal growth procedure was investigated and crystals with high quality were gained. Their properties were widely studied, especially the piezoelectric parameters and their effects to temperature. Furthermore, the study on the growth and optical properties of the doped CNGS crystal were carried out. New isostructural crystal Ba₃TaGa₃Si₂O₁₄ (BTGS) was also attempted to grow. The main works of the article are as follows:1. We carefully studied the growth habits of the two crystals and correspondingly improved growth techniques gradually. According to the stochiometric ratio, CNGS and CTGS crystals with good quality were obtained by using the czochralski technique. The crystals were transparent and no crack was observed. Smooth faces appeared in some crystals, which is rare among the crystals by Cz technique. Taking cubic taper as the theoretical model, we deduced the relationship between the area of shoulder and time on the basis of the analysis on thermodynamics and kinetics.2. X-Ray powder diffraction analysis confirmed that all grown crystals were single phase and belonged to LGS-type structure. Based on the space group theory, the normal vibration modes of Ca₃NbGa₃Si₂O₁₄ (CNGS) crystal were predicted. Raman spectra of Ca₃NbGa₃Si₂O₁₄ (CNGS) were calculated based on the space group theory (SPT) and measured under the room temperature. Two clusters (Ca₃NbGa₂SiO₁₄ and Ca₃NbGaSi₂O₁₄) were constructed according to the structure of CNGS crystal. The density-functional theory (DFT) calculations were made on the two clusters to model the Raman spectra. An excellent agreement has been achieved between Raman spectra calculated by the SPT theory, DFT theory and those observed in the experiments. The excellent piezoelectric propertied of CNGS are mainly attributed to the large polarizability anisotropy of Ca₃NbGa₂SiO₁₄ and Ca₃NbGaSi₂O₁₄ clusters.An arrangement of parallel steps and a clear height change were observed in (001) facet by atomic force microscopy (AFM). The height change was obvious and appeared periodically. High-resolution X-ray diffraction (HRXRD) was used to check the quality of as-grown CNGS single crystals. CNGS wafer vertical toα-axis namely (2110) facet was oriented and mechanically polished on both sides. The rockingcurve of (2110) diffraction plane was performed on HRXRD D5005 made byBruker-axis. The diffraction angles as well as FWHM values are almost identical of the three measured areas, which show that the crystal lattice has no large aberration close to the growth line. The results indicate that CNGS crystals have good quality and free low-angle boundaries.The extinction ratio measurements showed the high optical quality of CNGS and CTGS crystals. The two crystals had small thermal expansion coefficients and expansion anisotropy, which were beneficial to the crystals' application under high temperature. The density and hardness of the two crystals are larger than quartz, but smaller than LGS crystal.The optical properties of CNGS and CTGS were systemically investigated. The transmittance spectra and optical property were recorded on a TU-1901 spectrophotometer in the range of 250-850 nm. The ultraviolet absorption edge is about 260 nm. We can find that optical transmittance is very high in c-direction. While the transmittance inα-direction is lower than c-direction. A comparatively wide absorption peak exists between 400 and 600 nm centered at 477 nm. The specific rotationρalong the c-direction of CTGS sample was determined by a TU-1901 spectrophotometer via measuring the optical transmission in dependence on the wavelength between crossed polarizers. The maximum specific rotationρof CTGS crystal was found to be 180°/mm at the wavelength of 301nm. High specific rotation can make the CTGS crystals useful as electric-optical switches and optical insulating instruments. Using V-shape prism method, the refractive indices n_o and n_e were measured in the visible region and the two crystals were found to be positive uniaxial crystals as quartz. The powder double-frequency effects were observed in CNGS and CTGS crystals for 1. 064μm laser. And the phase matching angles were calculated. The results show the two crystals may have a certain application in nonlinear optical areas.3. With reasonable measure steps and adequate crystal samples, the relative dielectric constants, the piezoelectric strain constants and elastic compliance constants of CNGS single crystals have been determined by the LCR bridge and resonant-antiresonant method. Compared with the results of other researchers, our results are exacter. Using the same procedure, we firstly measured the piezoelectric parameters, which were very close to those of CNGS. The piezoelectric coefficients of CNGS and CTGS crystals are greatly larger than those of quartz. Compared with the disordered LGS crystal, CNGS and CTGS have smaller dielectric coefficients and larger piezoelectric coefficients. So the two new-typed crystals have greater application potential.A more strongly piezoelectric effect can be obtained by using the cuts of the piezoelectric crystal with the maximum coefficients. The values of the elements of piezoelectric coefficient matrixes change with the different cutting orientation. For the cuts x-y doubly rotated, they are continuous functions of two rotation angles. The absolute value of every matrix element has a maximum. Some of the maximum values are greater than that given in the original coordinate system. The formulas of the elements of the piezoelectric coefficient matrix for the cuts of the crystal CNGS that doubly rotated were obtained. The maximum piezoelectric effects can be applied on piezoelectric devices and films under the circumstance that the temperature effects need not be considered.The temperature effect of elastic constants of CTGS was investigated. Two Y-cut minus 23.8°plates of CNGS and CTGS crystals were prepared and their frequency-temperature curves were described. The stabilizations to temperature of the two plates are good and the plates demonstrate large piezoelectric effects, which show a promising application in piezoelectric oscillation devices. After the succedent process and encapsulation on the CNGS Y-cut -23.8°plate, we successfully obtained the oscillation based on CNGS crystal.4. The propagating velocities of bulk acoustic waves in new-type piezoelectric crystals CNGS and CTGS were calculated along the x, y, and z axes. Under the conditions considering the piezoelectric effect and without considering the piezoelectric effect, the curves of slowness of CNGS in yz, xy and xz planes were plotted. Using the measured elastic constants, we calculated the corresponding angles of pure mode direction. The results can provide a certain theoretical guidance for design and application of the CNGS crystal in BAW and SAW piezoelelctric device.5. Single crystal of Co²⁺-doped Ca₃NbGa₃Si₂O₁₄(CNGS) has been grown along the a-axis using the Czochralski technique. The structure of the crystal has been studied by X-ray powder diffraction (XRPD) method. The differential scanning calorimetric experiment shows that the Co²⁺-doped CNGS crystal melted at 1339.3℃. The transmittance and fluorescence spectra of Co²⁺-doped CNGS crystal were measured at room temperature. When the 622nm light was irradiated on the sample, infrared light at 1252nm was observed using a Continues Xe-flash lamp.Single Ba₃TaGa₃Si₂O₁₄ (BTGS) crystals were grown by the Czochralski technique. The results of single X-ray analysis indicated that each cation occupied its respective crystallographic site in an ordered way. The differential scanning calorimetric (DSC) experiment showed that BTGS crystal melted at 1373.9℃. Using first-principles calculations based on density-functional theory, the elastic constants of BTGS were calculated.