| As the media of mechanic,heat,electricity,magnetism,light,acoustic and other forms of energy,nowadays,functional crystals play an important role in high-technology era.With the rapid development of opto-electronics industry,communication,aerospace and other fields,researchers have also put forward higher requirements and broader demands on the photoelectric crystals.Laser crystal is the leading force in the development of laser technology,being the core and foundation of lasers.The development direction of laser oriented to high power and short pulse,researchers are paying more and more attention to explore new laser gain crystals.In recent years,the disordered crystals have attracted a great deal of interest.In the structure of disordered crystals,two or more cations with different valences occupy the same crystallographic position in the lattice,which allows the disordered crystals have both large inhomogeneous broadening of the spectra and the excellent thermal and mechanical properties,being one of the most competitive laser matrix materials.Piezoelectric crystals serve as the core components of sensors,being widely used in aerospace,petroleum exploration and chemical industry.However,traditional piezoelectric ceramics,quartz and LiNO3 crystals are difficult to meet the needs of high-temperature piezoelectric conditions.Therefore,the exploration of novel piezoelectric crystal materials applied in the field of high-temperature sensors has became one of the research hotspots in piezoelectric field.Melilite family crystals(ABC3O7)are known as functional crystals,it belongs to both disordered structure and noncentrosymmetry,therefore ABC3O7 crystals has great potential in the laser and high-temperature piezoelectric fields.The ABC3O7 crystals belongs to the tetragonal system with the space group of P421m.A and B are randomly distributed in the octahedron center,resulting in the disorder characteristics of the crystals.Lately,researchers payed attention to the research of laser,piezoelectric,fluorescence properties of ABGa3O7 and Ca2Al2SiO7 crystals and achieved many fruitful results.However,the researchers ignored the study of the same structure aluminate crystals ABAl3O7.Compared with the gallium crystal,aluminum crystal usually has higher thermal conductivity,higher melting melt and larger resistivity.Therefore,this paper selected melilite CaYAl3O7 crystal as the research object.Therefore,based on the growth of CaYAl3O7 crystal,we systematically measured the thermal properties,optical properties,piezoelectric properties and laser properties of the CaYAl3O7 crystal,Nd:CaYAl3O7 crystal and Yb:CaYAl3O7 crystal.The main content and conclusions are listed as follow:?.Crystal Growth,Characterization and First-principle Calculations of CaYAl3O7We explored the growth of CYAM crystal by means of Cz method and acquired a single crystal by using Ir rod as seed.Afterwards,the bulk crystal growth was carried out by the(001)seed,during which the necking part and thermal gradient had been modified and optimized,and finally high-quality bulk crystal was acquired.Dislocations of crystals in different directions were investigated by chemical etching method,the causes of defect formation and anisotropy were analyzed from the perspective of crystal structure.The crystal structure of CYAM was resolved by X-ray single crystal diffraction.CYAM crystal belongs to tetragonal system with the space group of P421m,Al connected with 4 O atoms forming the AlO4,and AlO4 tetrahedrons connected by edge sharing formed X Y plane.Ca and Y occupy the center of octahedron with the ratio of 1:1,possessing the disordered structure.The basic physical properties of CYAM single crystal were measured,including the hardness,density,the specific heat,thermal expansion,thermal diffusivity and thermal conductivity.The results show that the density and thermal properties has obvious anisotropy.The thermal conductivity of a-and c-direction is 1.816 W·m-1·K-1 and 1.372 W·m-1·K-1,respectively,which are higher than that of glass.The thermal conductivity increases with the temperature rising,which is similar to the glass.We concluded this phenomenon is related to the disordered structure.?.Electric-elastic Properties of CaYAl3O7 CrystalThe electro-elastic properties of C YAM crystal at room temperature were measured by resonance method.The dielectric constants of the CYAM crystal ?11 and ?33 are 16.88 and 12.21,respectively.The piezoelectric constants d14 and d36 are 12.6 pC/N and 2.18 pC/N,respectively.The electromechanical coupling coefficients k14 and k36 are 18.06%and 5.45%,respectively.All the results show that the CYAM crystal has good piezoelectric properties.In the temperature range of 25?700?,the resistivity of CYAM crystals was measured.The results demonstrate that CYAM crystal owns considerable resistivity(At 600?,?11=2.86×109 ?·cm),which is 2?3 orders of magnitude higher than that of SrGdGa3O7 and equal to the resistivity of Ca2Al2SiO7 crystal,which is a research hot spot in the high temperature piezoelectric field.Afterwards,the relationship between resistivity and the structure of CYAM crystal was explained.We studied the temperature stability of the electro-elastic properties of CYAM crystal in the range of 25?500?.The results showed that it has good temperature stability.Besides,no phase transition takes place before 1630 ?.Therefore,CYAM crystal has great potential for high temperature piezoelectric materials.Based on the coordinates of each atom and the corresponding bond length,the valence of Ca2+,Y3+,Al3+ and O2-were calculated,respectively.The dipole moments of YO8?CaO8 and AlO4 were calculated by the dipole moment formula.The relationship between the piezoelectric properties and the dipole moments of crystals were also analyzed.?.Crystal Growth,Characterizations and Laser Performance of Nd:CaYAl3O7 CrystalThe Nd:CaYAl3O7 crystal has been successfully grown for the first time.The segregation coefficient of Nd3+ is 0.67,which is much larger than the segregation coefficient of Nd3+ in YAG(?0.2)and GGG(?0.4)crystals.Large segregation coefficient is beneficial to increase the homogeneity of Nd3+ in crystals.The thermal properties of Nd:CYAM crystals were systematically measured.The Nd:CYAM crystal's thermal conductivity of the a-and c-direction at room temperature was 1.83 and 1.16 W·m-1·K-1,respectively,meaning the doping of Nd3+ has less influence on the structure;The thermal expansion coefficient of the a and c direction is 6.21×10-6K-1 and 4.45×10-6K-1?respectively.The small anisotropy of thermal expansion coefficient is beneficial for crystal growth.The absorption spectra,emission spectra and fluorescence lifetime of Nd:CYAM crystal were systematically characterized.In the absorption spectra,the FWHM of 808 nm is as wide as 31 nm,which is much wider than Nd:GGG(8.6 nm),Nd:YAG(0.9?2.1 nm)and other melilite crystals(8?18 nm).It has proved that Nd:CYAM crystal has higher disorder degree,thus the inhomogeneous broadening effect of spectrum is more obvious.The center of main peak appeared at 1063 nm,and the FWHM is as wide as 27 nm,which is equal to Nd:glass.The life time of Nd:CYAM crystal has been measured to be 261 ?s.All above properties demonstrate that Nd:CYAM has great potential for tunable lasers.? Crystal Growth,Characterizations and Laser Performance of Yb:CaYAl3O7 CrystalYb:CYAM crystal was successfully grown for the first time,the doping concentration of Yb3+ is about 3.Sat.%.The effective segregation coefficient of Yb3+ was 0.66,which was slightly higher than that of Yb3+ in the Yb:CaGdAlO4 crystal(0.6).The thermal properties of Yb:CYAM crystal were systematically characterized,the thermal conductivity of crystals in the a-and c-direction at room temperature was 1.65 W·m-1·K-1 and 1.48 W·m-1·K-1,respectively.The thermal conductivity gradually increased with the temperature raising.The thermal expansion coefficient of the a and c direction of is about 5.83×10-6 K-1 and 5.30×10-6K-1,respectively.The polarization absorption spectra,polarization emission spectra and fluorescence lifetime(low temperature,room temperature)of the Yb:CYAM crystal were carefully characterized.At room temperature,the strongest absorption peaks of the Yb:CYAM crystals was located at 982 nm and 978 nm,respectively,the FWHM of main peak is 133 nm and 116 nm.The strongest peak of the fluorescence spectrum appears at 1008 nm with the FWHM being about 27nm.The fluorescence lifetime of 298 K and80 K is 1.22 ms and 977.3 ?s,respectively,which is much longer than that of Yb:CaGdAlO4 crystal.The long lifetime of the upper energy level is favorable for energy storage,so this crystal has great potential for Q-switched lasers.The continuous laser performance of the Yb:CYAM crystal was studied.The maximum laser output power of 505 mW was achieved at 1026.5 nm using c-cut Yb:CYAM crystal,corresponding to a slope efficiency of 12.5%.The results show that Yb:CYAM crystal have broad absorption and emission spectra,which will promote its potential applications in the field of ultrafast and tunable lasers... |
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