高温环境中基于声学信号的激光诱导击穿光谱校正方法

Acoustic-Based Spectral Correction Method for Laser-Induced Breakdown Spectroscopy in High Temperature Environment

激光诱导击穿光谱(LIBS)信号的不确定性限制了其定量测量的能力。基于等离子体声学信号的光谱校正方法能够有效降低LIBS信号的不确定性,但仍缺乏在高温环境中的研究。在甲烷/空气预混火焰产生的高温气中,测量了不同激光入射能量下等离子体的吸收能量,并同步采集了等离子体的光谱信号和声学信号,对声学波形的正峰进行了修正,利用修正后正峰的脉冲积分强度(PII)对光谱进行了校正,有效降低了LIBS信号强度的不确定性。研究发现,在激光入射能量为80~280 mJ时,等离子体的吸收能量和激光入射能量间具有显著的线性关系,1 150和1 350 K下,线性决定系数(R^(2))分别为0.997 9和0.998 9,随着激光能量从80 mJ升高至280 mJ, 1 150和1 350 K下等离子体吸收能量的RSD(relative standard deviation)分别从33.17%和34.20%降至6.68%和6.79%。同时,在同一激光入射能量下,由于温度的升高导致了气体密度的下降,1 350 K下等离子体的吸收能量低于1 150 K的吸收能量。等离子体的光谱信号和声学信号源于等离子体内能的再分配过程,因此两者的强度与等离子体的吸收能量更为相关。高温环境中等离子体的吸收能量-光谱强度-声波正峰PII两两间具有较强的线性关系。1 150 K下,等离子体吸收能量与H 656 nm、 N 746 nm、 O 777 nm强度的R^(2)分别为0.996 1、 0.988 9和0.994 8,与修正后正峰PII的R^(2)为0.991 6;1 350 K下,等离子体吸收能量与H 656 nm、 N 746 nm、 O 777 nm强度的R^(2)分别为0.997 5、 0.977 5、 0.988 7,与修正后正峰PII的R^(2)为0.988 0。利用修正后正峰PII对光谱进行校正,当等离子体吸收能量小于100 mJ时,谱线强度波动显著较低。当入射能量为160 mJ时,1 150 K下,等离子体吸收能量为69.24 mJ, H 656 nm、 N 746 nm、 O 777 nm的RSD分别从校正前的16.14%、 21.26%、 17.24%降至校正后的8.75%、 9.15%、 8.50%;1 350 K下,等离子体吸收能量为66.92 mJ, H 656 nm、 N 746 nm、 O 777 nm的RSD分别从校正前的18.22%、 24.85%、 19.13%降至校正后的8.46%、 9.52%、 8.84%。结果表明,基于等离子体声学信号的光谱校正方法能够有效降低LIBS技术在高温环境中的测量不确定性。

The fluctuations of laser-induced breakdown spectroscopy(LIBS)signals to compromise its quantitative measurement.Acoustic-based spectral correction method has been successfully used in reducing the fluctuations,but up to now,not been studied in high-temperature environments.In this work,the plasmas were generated in a high-temperature environment produced by a methane/air premixed flame.The absorbed energies were measured,and the spectral and acoustic signals were collected simultaneously under different laser incident energies.The compression half of acoustic waveforms was corrected by N shape shockwave theory,then the pulse integral intensity(PII)was used to correct the spectra.The uncertainty of LIBS signals was reduced significantly.Strong linear correlations were found between the absorbed energy and laser energy with the laser energy range of 80~280 mJ,under 1150 and 1350 K.The coefficient of determination(R^(2))was 0.9979 and 0.9989,respectively.As the laser energy increased from 80 to 280 mJ,the relative standard deviation(RSD)of absorbed energy decreased from 33.17%to 6.68%under 1150 K and 34.20%to 6.79%under 1350 K.Under fixed laser energy,the absorbed energy of plasma under 1350 K was lower than that under 1150 K due to the sparser gas density.Because the spectral and acoustic signals were transferred from the internal energy of plasma,they are more related to the absorbed energy than laser energy.The linear correlations between absorbed energy and spectral signals and between absorbed energy and acoustic signals were established.Under 1150 K,the plasma absorbed energy was 69.24 mJ,and the R^(2) between absorbed energy and H 656 nm,N 746 nm,and O 777 nm was 0.9961,0.9889,and 0.9948,respectively.Moreover,the R^(2) between absorbed energy and PII was 0.9916.Under 1350 K,the R^(2) between absorbed energy and H 656 nm,N 746 nm,and O777 nm was 0.9975,0.9775,and 0.9887,respectively.Furthermore,The R^(2) between absorbed energy and PII was 0.9880.Subsequently,the spectral signals were corrected by PII,showing that the fluctuations of LIBS signals can be reduced significantly with the absorbed energy less than 100 mJ.When the laser incident energy was 160 mJ,under 1150 K,the absorbed energy was 69.24 mJ,RSD of H 656 nm,N 746 nm,and O 777 nm were reduced from uncorrected 16.14%,21.26%,and 17.24%into corrected 8.75%,9.15%,and 8.50%,respectively.Under 1350 K,the absorbed energy was 66.92 mJ,RSD of H 656 nm,N 746 nm,and O 777 nm were reduced from uncorrected 18.22%,24.85%,and 19.13%into corrected 8.46%,9.52%,and 8.84%,respectively.There sults show that the proposed acoustic-based spectral correction method can effectively reduce the measurement uncertainty of LIBS under high-temperature environment.