Study On Physical Properties Of α-phase Silicon Nitride Under High Pressure

Silicon nitride materials are receiving increasing attention from experimental and theoretical research due to their properties in basic science and technology applications.Silicon nitride has good chemical stability,good compression resistance,good corrosion resistance,high hardness and good mechanical properties.Silicon nitride semiconductors are widely used in the ceramic industry,machinery industry,nuclear industry,aerospace industry,automobile engines,solar cells,etc.The field has a wide range of applications.In recent decades,with the development of large-scale integrated circuits,silicon nitride semiconductors have great application potential in the optoelectronic industry.The silicon nitride material has two kinds of homogenous heterogeneous bodies under normal temperature and normal pressure conditions,namely α-SbN4 and β-Si3N4,both of which are hexagonal crystal systems.It is generally believed that the α-Si3N4 is in a metastable state,and the β-Si3N4 is a low-temperature phase of silicon nitride.Pressure,temperature and chemical composition(P-T-X)are the basic thermodynamic conditions that determine the structure and state of existence of a substance.As one of the three important independent thermodynamic parameters,pressure plays an important role in material preparation and research.Zerr et al[Nature 340,(1999)400]for the first time synthesized a third isomorphous body of Si3N4 by high-pressure experiments,y-Si3N4,whose structure is a cubic spinel structure.In theoretical research,the microstructure determines the macroscopic properties of the material.Therefore,the study of the basic structure of the sample is the basis for studying its macroscopic properties under extreme conditions.The first part of this paper introduces the research background and significance of silicon nitride materials.The second part introduces the experimental characterization methods and theoretical calculation methods of α-Si3N4,as well as the CASTEP module and Gibbs(GIBBS)Software used in theoretical calculations;the third part introduces the experimental results of α-Si3N4,including the spectrum generated by the X-Ray diffractometer and the Rietveld refinement analysis of the data to determine the crystal structure by means of GSAS software,and the sample is displayed by means of scanning electron microscopy(SEM).Particle morphology,Fourier transform infrared(FT-IR)spectroscopy provides spectroscopic evidence for the phase transition of a phase silicon nitride samples under high pressure(22 GPa).During the experiment,the pressure is provided by Diamond Anvil Cell(DAC);The four parts introduce the theoretical calculation results of α-Si3N4,the mechanical,electrical and thermodynamic properties at 0 to 30 GPa;the fifth chapter is the summary and prospect of this work.The main research results are as follows:X-ray diffraction(XRD)method and structural refinement were used to characterize the sample of α-Si3N4.Rietveld analyses of XRD data confirmed that the sample belongs to hexagonal crystal(space group of P31c,No:159).The refined structural parameters are in agreement with the available theoretical and synchrotron radiation experimental results.Then,the infrared(IR)spectra of α-Si3N4 under 22 GPa were investigated with the diamond anvil cell(DAC).According to the IR absorption spectra at 296.3 K and 2 hPa(Observation range from 400 cm-1 to 12500 cm-1),we observed 23 IR active peaks.Normalized FT-IR absorption spectra under high pressure and room temperature(296.3 K)show that α-Si3N4 undergoes a phase transition at 16.25 GPa and changes from hexagonal to cubic phase,namely,γ-Si3N4,which is very close to the result of our theoretical calculations,i.e.the phase transition pressure is 15.66 GPa.In addition,the electrical,mechanical and thermodynamic properties of α-Si3N4 in the range of 0 to 30 GPa at 0 K were studied by first-principles calculations.Electronic structure calculations show that α-Si3N4 is an indirect semiconductor with a wide band gap of 4.611 eV.Under high pressure,the degree of freedom of movement of electrons is greatly reduced,and the band gap is gradually increased,reflecting the tendency of α-Si3N4 to become an insulator from a semiconductor.According to the mechanical stability criterion of hexagonal system,it is predicted by theory that α-Si3N4 is mechanically stable in the range of 0 to 30 GPa at 0 K.In addition,the calculated B/G ratio indicates that the α-Si3N4 is brittle at 0 GPa to 10 GPa at 0 K,and tends to be ductile in the range of 10 GPa to 30 GPa.At the same time,the quasi-harmonic Debye model is adopted.The thermodynamic properties of 0a-S13N4 under high temperature(0 GPa-30 GPa)are studied.