Qualitative Analysis Of Metal Materials Based On Photoacoustic Effect And LIBS

Nanosecond and femtosecond lasers focus on the metal surface and can form metal plasmas.The metal plasma expands outward and produces acoustic waves due to the photoacoustic effect;while the metal plasma emits atomic spectroscopy,that is,laser-induced breakdown spectroscopy(LIBS).Both the photoacoustic effect and LIBS technology can be used for material qualitative analysis.Therefore,the laser-generated plasma can be used to study these two methods for qualitative analysis of different materials.Based on the same laser,two kinds of spectral means can complement each other to improve the accuracy of material identification.In this dissertation,we use nanosecond and femtosecond lasers,to carry out photoelectric effect and LIBS experiments for material qualitative analysis.Firstly,the mechanism of photoacoustic effect in solid is summarized,including the thermos-elastic mechanism and the plasma mechanism.The temperature field of the thermos-elastic mechanism is numerically simulated by MATLAB.The simulation results show that the femtosecond laser and nanosecond laser have the thermal influence depth in the order of nanometer and micrometer,respectively.Secondly,the experimental study on the photoacoustic effect of four kinds of metals was carried out.The relationship between the acoustic wave and the laser power and the detection distance is studied.It is found that the acoustic intensity is inversely proportional to the laser power,which is inversely proportional to the square of the detection distance.Then,the relationship between the plasma and the laser power is studied.It is found that the plasma sonic intensity increases slowly with the increase of the laser energy to the rapid rise and then rises slowly and finally becomes saturated for the nanosecond laser experiment.For the femtosecond laser experiment,the plasma sonic intensity increases linearly with the increase of laser power.Then,the difference of the acoustic waves for different materials in the frequency domain is studied.It is found that the laser power does not affect the main frequency of the acoustic wave.The main frequency of the acoustic wave is related to the ionization energy of the metals.The smaller the ionization energy is,the higher the frequency of the acoustic wave is.Finally,for the nanosecond and femtosecond laser experiments,we found that the femtosecond laser produced sound waves with lower frequencies,and sound waves exist less time.Lastly,the LIBS qualitative analysis of the four kinds of metals is carried out.For the nanosecond LIBS,it is necessary to obtain the time delay to obtain the atomic emission spectrum with higher signal to noise ratio,and for the femtosecond LIBS,no delay detection can be realized.Secondly,by comparing the atomic emission spectra of different metals from 300 nm to 600 nm,it is found that different metal species show different atomic emission spectra.The type of laser does not affect the characteristic spectral wavelengths which can be compared with the reference sample LIBS Spectra,determining the type of reference sample.In this dissertation,the difference between the frequency domain of the plasma acoustic waves and the difference between the atomic emission spectrum for four different metals are used to distinguish and classify the metals.These two orthogonal experimental methods can be used as a rapid identification of metal and alloy samples.