Laser-induced Breakdown Spectroscopy: a New Tool for Process Control
نویسندگان
چکیده
Many industrial activities require adequate and timely evaluation of material composition. Laser-induced breakdown spectroscopy (LIBS) has, in recent years, shown a great potential for rapid qualitative and quantitative chemical analysis of various materials. In this paper, we will illustrate the usefulness of the LIBS technique for elemental analysis at different scales: from depth profilometry of thin coatings and the micron-scale mapping of inclusions in metals, to the continuous in-situ analysis of aqueous liquid effluents and industrial-scale molten metal baths. Introduction: During the lifetime of any material, from its initial mineral state, through numerous processing and manufacturing transformations, to its degradation during use, knowledge of composition is crucial. This is especially true for composite or layered structures. Laser-induced breakdown spectroscopy (LIBS), also known as laser-induced plasma spectroscopy, offers unique capabilities for on-line composition determination. An important advantage of this technique over classical methods stems from the possibility of in-situ analysis of virtually all types of material (solids, liquids, molten materials, and gases) without need for sample preparation. In LIBS, a small volume of the target is intensely heated by the focused beam of a pulsed laser, and thus brought to a transient plasma state where the sample’s components are essentially reduced to individual atoms. In this high-temperature plasma, atoms are ionized, or brought to excited states. Such states decay by emission of radiation, which is observed in the ultraviolet (UV), visible and near-infrared (NIR) regions of the spectrum. An atomic spectrum is obtained by means of a spectrograph, thereby allowing elemental components of the target to be identified and, using a calibration curve, quantified. LIBS measurements are generally carried out in ambient air at atmospheric pressure. For this reason, and also due to its rapidity, non-contact optical nature, and absence of sample preparation, LIBS is particularly suited for at-line or on-line measurements in industrial settings, as well as in the laboratory [1-4]. The capabilities of LIBS for quantitative elemental analysis have been demonstrated in metallurgy, mining, environmental analysis and numerous other fields, including application to specialized materials (e.g. aeronautics and pharmaceuticals). LIBS cannot be considered a non-destructive technique in the strictest sense, since part of the target to be analysed is vaporized and lost. However, the volumes sampled in this manner are very small: 10-10 cm, depending on the material, and the laser wavelength and fluence. Such volumes correspond to masses in the ng to μg range. In the case of a solid sample, the lateral size of the laser-affected zone is typically under 1 mm, and can be made as small as 1 μm. In the vertical dimension, the thickness of material ablated by a laser pulse may, in the case of metals, be only tens of nanometers. When LIBS is applied to the analysis of fluids (e.g. water or molten metal), the issue of destructiveness is irrelevant, since the vaporized mass is negligible and the analysed volume is continuously renewed. The possibility of concentrating laser radiation on a very small area enables the sampling and analysis of solid heterogeneous materials at high spatial resolution. This constitutes a significant advantage of LIBS compared to other techniques of elemental analysis, which often require sample digestion. Admittedly, X-ray fluorescence (XRF) can be considered a strictly non-destructive technique for elemental analysis, but it cannot match the best spatial resolution achievable by LIBS. XRF is also less sensitive than LIBS, often requires sample preparation and, unlike LIBS, cannot be used for light elements (e.g. boron). Other techniques, such as Auger or X-ray photoelectron spectrometry, secondary ion mass spectrometry, or glow discharge methods, all involve sample preparation, are time consuming, sometimes call for ultra-high vacuum conditions, and require sophisticated and expensive instrumentation. For these reasons, they do not meet the industrial requirements for at-site highthroughput compositional mapping of heterogeneous materials. In this paper, the use of LIBS for rapid three-dimensional compositional mapping of solids will be discussed, as well as its on-line application to the real-time analysis of aqueous industrial effluents and molten metals. Depth profilometry and micro-analysis by LIBS: Materials at different stages of transformation from the raw state to finished product often present a heterogeneous elemental composition. In particular, an object’s surface may be protected by one, or more, layers of varying composition. There is a growing need in industry, namely in the context of process development and control, for at-site high-throughput methods that can reveal elemental distributions along one or more spatial dimensions. In the following we provide a few examples that demonstrate how LIBS can meet these demands. The separate analysis of successive laser ablation events at the same position of a solid material enables depth-resolved analysis. In the past, this approach has been applied in our group, and elsewhere, to several types of layered materials (see [5] and references therein). For example, we have used LIBS for the characterization of galvannealed coatings on steel [5,6]. In this case, the zinc coating contains Fe and Al in concentrations of approximately 8-13% and 0.2-0.35%, respectively. Because of widely separated melting and vaporization temperatures, the laser ablation efficiency was different for the coating and the steel substrate: 58 nm/pulse and 17 nm/pulse, respectively. The raw data consisted of Al, Fe and Zn spectral line intensities as a function of pulse number. Calibration strategies were developed to obtain elemental concentrations as a function of depth. The presence of a thin aluminum oxide layer at the surface of the coating was correctly identified, and the global Al content in the coating itself was found to be in good agreement with average concentrations measured using wet chemistry. The Fe profile across the coating was also found to agree with a profile obtained by transmission electron microscopy / energy dispersive X-ray spectrometry, including the region close to the interface with steel where higher Fe concentrations (~25%) were found. The latter work was carried out with a Gaussian laser beam of non-uniform energy distribution. This tended to limit the depth resolution because the central higher-energy portion of the beam entered the substrate, while the beam’s lower-energy periphery was still sampling coating material at a shallower depth [6]. One might instead use a laterally homogenized laser beam to provide a uniform ablation efficiency across the beam [7,8]. This has been shown to significantly improve depth resolution [7]. We have recently developed another approach based on using the same laser to alternately generate beams of different diameter [9,10]. As illustrated in Fig. 1, at first the laser passes through a large diaphragm to produce a large crater on the surface. After a given number of laser pulses, a smaller diaphragm is substituted to produce a narrower beam to thereby generate the analytical plasma inside the crater produced by the larger beam. This eliminates all contributions from the crater walls. Repetition of this sequence serves to generate a depth profile. In Figure 1 we compare the profiles of zinc spectral line intensity for a galvannealed coating on steel using this new approach, with “classical LIBS instrumentation” where the laser beam is only filtered by a large diaphragm. In both cases each data point is based on 100 ablation shots with the large diaphragm, followed by10 measurement shots. It is seen that use of the smaller diaphragm for the measurement shots provided a more accurate profile of the coating thickness. The coating/steel interface is described with more precision: the beginning of the coating/air interface appears in the same place for both profiles, but ends 2 μm sooner using the new approach. The Zn emission quickly falls to zero, which is not the case with conventional instrumentation where the Zn emission persists.
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تاریخ انتشار 2004