光谱学与光谱分析, 2020, 40 (9): 2891, 网络出版: 2020-11-26  

低气压下80 ns长脉宽激光诱导击穿铜合金光谱特性研究

Study on Spectral Characteristics of Laser-Induced Breakdown Copper Alloy at 80 ns Long Pulse Width Under Low Air Pressure
作者单位
沈阳理工大学理学院, 辽宁 沈阳 110159
摘要
激光诱导击穿光谱(LIBS)因具有实时快速、 多元素分析、 样品损伤性小等优势, 已成为检测未知物质元素组分以及相应元素含量的重要手段。 近期的一些研究表明, 百纳秒级别激光脉冲由于在确保有效击穿阈值的条件下延长了激光与样品作用时间, 使得其LIBS光谱质量相对于传统10 ns级激光脉冲得到了提高; 适度降低环境气压(至104 Pa量级), LIBS光谱强度和信背比均得到明显提高。 为探究低气压对长脉宽(百纳秒级)激光诱导铜合金等离子体光谱特性的影响, 采用自主研发80 ns脉宽Nd∶YAG激光器(波长1 064 nm, 单脉冲能量20~200 mJ)作为激发光源, 样品为BYG19431的锡青铜(基体元素Cu质量百分数为92.9%, 低含量元素Fe质量百分数为0.007 8%), 通过样品气氛控制系统改变环境气压, 分别研究了低环境压力(1.01×105, 9.6×104, 9.2×104, 8.8×104和8.4×104 Pa)下铜合金基体元素Cu与低含量元素Fe光谱特性。 实验中, 激光脉冲重复频率为1 Hz, 每次打击均为新鲜表面(通过真空腔内的可控旋转平台更换样品位置), 每个能量和气压下分别选取5个脉冲能量较稳定的光谱, 取平均值作为当前实验条件的最终实验结果, 激光脉冲能量的实时监测由透反比1∶1分束镜及能量计完成。 研究发现, 基体元素谱线(Cu Ⅰ 324.75 nm), 常压下低能量(20 mJ, 40 mJ)时均存在较严重的自吸收现象。 在60 mJ时, 虽自吸收效应得到改善, 但谱线背景强度升高, 且激光对样品的损伤加大。 为在低光谱背景, 微样品损伤的条件下实现光谱质量的进一步提升, 实验激光能量为20 mJ。 结果表明, 随着环境气压降低, 基体元素Cu自吸收程度大幅度降低, 样品中低含量Fe元素谱线信背比增加, 等离子温度升高, 谱线展宽变窄。 气压为8.4×104 Pa时, 与常压相比基体元素铜(Cu Ⅰ 324.75 nm)与微量元素铁(Fe Ⅰ 330.82 nm)谱线信背比分别增强5.31和2.43倍; 等离子体温度提升了21.6%; Fe Ⅰ 330.82 nm谱线展宽由0.29 nm降到0.21 nm, 在一定程度提高了LIBS元素谱线的分辨率。
Abstract
Laser induced breakdown spectroscopy (LIBS) technology has become an important method to detect the elemental composition and the content of corresponding elements in unknown substances due to its advantages of real-time, fast, multi-element analysis and less damage to samples. Some studies have shown that the hundred nanosecond laser pulse improves the LIBS spectral quality relative to conventional 10 nanosecond pulses, because of prolonging the action time between laser and sample while maintaining an effective breakdown threshold. By reducing the ambient air pressure (to the order of 104 Pa), both the LIBS spectral intensity and the signal-to-back ratio are significantly improved. In order to investigate the effect of low pressure on the spectral characteristics of copper alloy plasma induced by long pulse width laser (100 nanoseconds), a self-developed 80 ns pulse width Nd∶YAG laser (wavelength 1 064 nm, single pulse energy 20~200 mJ) was used as the excitation source. The sample was tin bronze (base element Cu mass percentage 92.9%, low content Fe is 0.007 8%) numbered BYG19431. The ambient pressure was changed by the sample atmosphere control system, and the spectral characteristics of Cu and Fe in the copper alloy matrix under low ambient pressure (1.01×105, 9.6×104, 9.2×104, 8.8×104 and 8.4×104 Pa) were studied, respectively. In the experiment, the repetition rate of the laser pulse is 1 Hz, and each stroke is a fresh surface (the sample position is replaced by a controllable rotating platform in the vacuum chamber), Five spectra with stable pulse energy are selected for each energy and pressure, and the average value is taken as the final experimental result under the current experimental conditions. The real-time monitoring of the laser pulse energy is performed by a transflective ratio 1∶1 beam splitter and an energy meter. It is found that the matrix element line (Cu Ⅰ 324.75 nm) has a relatively high self-absorption phenomenon at low energy (20 mJ, 40 mJ) under normal pressure. At 60 mJ, although the self-absorption effect is improved, the background intensity of the line is increased and the damage of the laser to the sample is increased. In order to further improve the spectral quality under the condition of low-spectrum background and micro-sample damage, the experimental laser energy is 20 mJ. The results show that the self-absorption effect of matrix element Cu is greatly reduced, the ratio of low-content Fe element in the sample increases, the plasma temperature increases, and the spectral line width narrows with the decrease of ambient pressure. When the gas pressure is 8.4×104 Pa, the signal-to-back ratio of matrix element copper (Cu Ⅰ 324.75 nm) and trace element iron (Fe Ⅰ 330.82 nm) increases by 5.31 and 2.43 times, the plasma temperature increases by 21.6%, and the line width of Fe Ⅰ 330.82 nm decreases from 0.29 to 0.21 nm compared with normal pressure, which improves the resolution of the LIBS element line to a certain extent.

袁备, 宁日波, 李倩, 韩艳丽, 徐送宁. 低气压下80 ns长脉宽激光诱导击穿铜合金光谱特性研究[J]. 光谱学与光谱分析, 2020, 40(9): 2891. YUAN Bei, NING Ri-bo, LI Qian, HAN Yan-li, XU Song-ning. Study on Spectral Characteristics of Laser-Induced Breakdown Copper Alloy at 80 ns Long Pulse Width Under Low Air Pressure[J]. Spectroscopy and Spectral Analysis, 2020, 40(9): 2891.

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