Study On The Pyroelectric Properties Of Tourmaline And Its Artificial Synthesis

Tourmaline is a kind of borosilicate mineral with spontaneous polarization,pyroelectricity,infrared radiations and anion producing characteristics.The pyroelectricity is the earliest and the most important physical property of tourmaline.Due to the complex chemical composition and crystal defects of natural tourmaline,the mechanism of pyroelectricity in tourmaline has not been deeply studied.Therefore,the influence of temperature and crystal structure on the pyroelectric properties of natural Fe-Mg tourmaline is studied preliminarily in this paper.On the basis,the synthesis of tourmaline is explored,which provides an important experimental basis for the further study of the pyroelectric mechanism of tourmaline.Firstly,based on the crystallographic data,the electric dipole moments in tourmaline are calculated at various temperature.The results show that the electric dipole moments of the typical coordination polyhedral firstly increase with temperature rising from 25?to 250?and reach a maximum of 0.73225×10^-29C·m at 250?,then decrease in temperature range of 250?to 300?.In particular,temperature variation can greatly affect the electric dipole moments of the[SiO₄]and[BO₃]polyhedra,indicating that[SiO₄]tetrahedron and[BO₃]polyhedron play an important role in the change of total intrinsic electric dipole moment.Secondly,the pyroelectric coefficients of tourmaline sample are measured experimentally from room temperature to 300?.It can be seen that the relationship between the pyroelectric coefficient and the temperature is linear in the range of 25?to 250?.However,the pyroelectric coefficient is nonlinearly varied with the temperature at the scope of 250?-300?.In addition,the theoretical pyroelectric coefficients have been obtained by using the difference of intrinsic electric dipole moments at varying temperature,finding that it is consistent between the theoretical pyroelectric coefficients and the experimental test.On this basis,the microscopic mechanism of tourmaline's pyroelectric properties has been discussed by analyzing the spontaneous polarization,pyroelectricity and thermal vibration of tourmaline.The results indicate that the polarization forms of tourmaline can be composed of turning-direction polarization of electric dipole and thermionic relaxation polarization.The thermal vibration of[SiO₄]and[BO₃]polyhedra is more violent with the increase of temperature,which caused the emergence of pyroelectric properties in tourmaline.Finally,the synthesis of dravite by solid-state method is investigated,indicating that Na₂O₂,NaOH,KOH,MgO,Mg?OH?₂,B₂O₃ have the highest chemical activity in this study.The volatilization of boron should be prevented,which is the key to the successful synthesis of tourmaline.In view of above phenomena,the dravite NaMg₃Al₆Si₆B₃O₂₇?OH?₄ with good crystalline and simple chemical composition has been firstly synthesized by high pressure solid-state synthesis in this paper.