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Abstract
Laser-induced breakdown spectroscopy (LIBS), commonly used for atomic emission analysis, was used to gain some understanding of the process of aging in metalized pyrotechnic substances. In particular, the formation of zirconium dioxide (ZrO2) was investigated through the ZrO molecular band structure. The plasma emission structure that considers the electronic, vibrational and rotational transitions in molecules corresponds to the ZrO α (1,0) bands of the b3ϕ – a3Δ system. The ZrO signal originates from both pure Zr exposed to oxygen, as well as from pre-existing ZrO2, which makes it difficult to discriminate its source. Here, we confirmed that the aging samples showed the same increase in the ZrO signal as the non-aging samples with added ZrO2. A calibration curve for varying ZrO2 concentration with respect to the area under curve (AUC) of the ZrO signal intensity was constructed. The percentage of oxidation in two types of metalized pyrotechnic substances, namely Zr/KClO4 and Zr/Fe2O3, was predicted through the geometrical interpretation of the ZrO molecular emission spectra.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Laser-induced breakdown spectroscopy (LIBS) is an efficient spectroscopic technique for elemental analysis. LIBS has been utilized in various applications where both real-time and stand-off analyses at high detection sensitivity are of great importance. LIBS requires only a small sample mass for identifying the components without the need for chemical preparation. When a pulsed laser is irradiated onto a target sample, during surface ablation, a plasma is generated, and the relevant excited state falls to a natural ground state. During this process, the emitted light is in the form of laser spectra and contains the chemical fingerprint of the ablated sample, which enables its identification from the spectral information [1–4]. The spectrum emitted from the plasma is associated with the size of the energy steps that are required to change the energy level. One photon irradiated onto the sample can exhibit only one energy level state in the form of a specific discrete value, including the effect of degeneracy of the energy associated with a unique wavelength. During LIBS, plenty of photons are emitted in a single laser pulse, and the number of photons corresponding to each energy level corresponds to the signal intensity.
The emission spectral information provided is different for atoms and molecules. In the case of atoms, the emission is due to differences in the electronic energy levels between the orbitals according to the Grotrian diagram, and can be expressed as a peak value [5]. On the other hand, molecules form an interatomic distance by constructing various types of bonds between the elements and have a distinct identity at wavelengths different from the constituent atoms. When a laser beam is focused on the sample, it causes the molecule to transition from a higher energy state, an anti-bonding orbital, to a bonding orbital, which is of lower energy. These molecular emissions can then be classified and analyzed as electronic, vibrational, and rotational energies, based on the Born-Oppenheimer approximation and Franck-Condon principle [6,7]. This series of energy transitions that form the emitted light provide spectroscopic information of the molecules in units of a band structure at different regions depending on the electronic energy levels and electron configuration.
In recent studies, the LIBS technique has evolved into providing a limited molecular spectral signature for various compounds. The signals were obtained by the recombination between the atoms or the dissociated molecule species in a cooling plasma process [8,9]. Bravo et al. [10] observed the CN violet system in organic materials in the plume and noticed that the molecular emission strongly depended on the time evolution and the laser pulse irradiance. The molecular emission has also been investigated for isotopic studies. For example, Mao et al. successfully developed a Laser Ablation Molecular Isotopic Spectrometry (LAMIS) technique and analyzed strontium isotopes for the radiogenic age determination of strontium compounds [11]. Witte et al. studied diatomic carbon spectra by computation and the modeling of each spectrum with temperature, delay time, and laser energy [12]. Kitazima et al. evaluated the isotopic ratio of boron by utilizing the relative band peak heights for several bands of the BO molecular transition [13]. Gaft et al. improved the detection sensitivity for halogens, such as fluorine and chlorine, by combining them with alkali-earth metals and other elements to form molecules [14]. Work has also been carried out on metal oxides. Amir et al. obtained the molecular radiation of AlO to investigate the chemical aging of solid composite propellants and detected that the intensity of its peaks is greater with increased surface oxidation [15]. Alexander et al. demonstrated the possibilities of utilizing LAMIS to predict that CaO emission spectra can be used to infer oxygen isotopic information [16], whereas Mao et al. reported that the emission spectra of SrO molecular radicals have spectrally resolved signatures for the naturally occurring strontium isotopes in ambient air [11]. From observing these metal oxide forms together, the molecular structure can be characterized by the peak values located close to a band origin, which then decreases gradually with increasing wavelength.
Zirconium (Zr) is a strong transition metal and one of the most abundant metallic fuel elements on earth. In powder form, it is flammable, which leads to its popular use in pyrotechnics. This combustible solid is a highly reactive fuel and has a low electronegativity. When ionized, Zr absorbs oxygen molecules due to oxygen’s high electronic affinity, and a thin oxide film forms immediately on the surface. The oxide layer grows as oxygen permeates into the underlying metal over time. Moreover, oxidation is promoted further by the presence of any humidity or by a solid oxidizer, such as potassium perchlorate (KClO4) or iron oxide (Fe2O3) [17]. Consequently, an aging sample of Zr has an increasing concentration of ZrO2 at the surface, which significantly deteriorates the intended performance of a Zr-based pyrotechnic substance that has been mixed with a solid oxidizer in a common ignition device.
There have been a limited number of reports on the ZrO transitions observed in laser induced plasma emissions. Russo et al. measured the ZrO shift by utilizing 90ZrO and 94ZrO based on the molecular vibronic emission wavelengths [18], and observed an appreciable isotopic shift at the ZrO α(0,1) band of the d3Δ – a3Δ subsystems and the E1Σ+ - X1Σ+ system by using a femtosecond-laser [19]. The band peak values of each ZrO transition were used for the isotopic determination of Zr. Matsumoto et al. obtained the emission spectra of Zr and ZrO in air by using a zirconium metal plate for short (6 ns) and long (100 ns) pulse lasers and found a dramatic enhancement of the atomic and molecular emission in the long-pulse laser relative to a short-pulse one [20].
This present work focuses on the level of oxidation of pyrotechnic substances by analyzing the signal of ZrO in the band form with increasing ZrO2 content. The samples of Zr/KClO4 and Zr/Fe2O3 were prepared as non-aging or aging, where the aging samples comprised of natural and hygrothermal (exposed to both humidity and heat) samples. The properties of the molecules were analyzed by their band form that includes the vibronic information. Thus, the present work advances earlier works [18–20] by utilizing the band structure as opposed to the peak signal of the ZrO α (1,0) bands of the b3ϕ – a3Δ system derived from the spectra of ZrO2 powder. In particular, the ZrO molecular emission detection is a new approach to classify samples that contain Zr and ZrO2. Furthermore, calibration curves for the band area have been constructed to obtain quantitative ZrO2 concentrations in the samples exposed to the aging conditions. Furthermore, X-ray diffraction (XRD), one of the standard elemental analysis techniques, was carried out on the samples to complement and validate the LIBS results in this work.
2. Experimental setup
2.1 LIBS setup
Figure 1 gives a schematic illustration of the experimental setup. The LIBS system which induces laser ablation and generates plasma uses a Q-switched Nd:YAG laser (RT250-Ec, Applied Spectra Inc.) at 1064 nm. The laser pulse is fixed at a 5 ns pulse duration at 30 mJ of energy. First, the laser passes through the beam expander to reduce the divergence of the beam area. Then, a 15 × magnification objective (LMM-15X-P01, Thorlabs) is utilized to focus the laser beam without introducing chromatic aberration. The laser spot size and laser irradiance are 100 μm and 3.82 × 1012 W/m2, respectively. The focal distance can be adjusted by moving the position of the sample on a xyz-stage. To minimize the effect of ablation, the incremental spot locations (at 1 mm apart) are chosen be larger than the single beam spot size. This allows for constant focal length, a thermal effect, and the acquisition of an average spectrum over the entire area. Furthermore, we set up a six-channel charge coupled device (CCD) spectrometer (Applied Spectra Inc.) to detect the plasma in the wavelength range of 198~1050 nm. It can also detect the high resolution spectra (0.1 nm from the ultraviolet (UV) to the visible (VIS) range and 0.12 nm from the visible to the near infrared (NIR) range) from the carefully designed slit size and approximately 13,000 wavelength channels. The laser repetition rate are fixed at 1 Hz.
The generated light is collected from the optic fiber installed at a distance of about 150mm, 45 degrees from the target sample. After that, detecting system operates by receiving the signal during the exposure time corresponding to the gate width(1.05ms) after the gate delay time(1.0μs) from the time of the laser irradiation [21]. At this time, the light entering the spectrometer is decomposed into spectral components by the slit, which are interpreted by digitizing the signal as a function of the wavelength according to the subdivided pixel. The spectrum is composed of these signals from a number of wavelength channels. The collecting position was fixed from the generated plasma, and the experiment proceeded so as not to exceed the first reversal point by the self-absorption effect [22]. To minimize any differences in signal intensity, standard ambient temperature and pressure (SATP) was maintained within a temperature and pressure controlled chamber.
2.2 Sample preparation
Table 1 represents the basic chemical composition for the ZrO2/binder mixture. ZrO2 powder, mixed with a binder namely paraffin, which is inactive during laser ablation, was prepared for confirming the ZrO molecular band structure and for quantitative analysis. Table 2 shows the sample conditions for two types of pyrotechnic substances; zirconium potassium perchlorate (Zr/KClO4 or ZPP) and iron oxide doped zirconium (Zr/Fe2O3). The samples have different zirconium/oxidizer composition ratios. The basic material compositions were Zr (53%), KClO4 (42%), Viton-b (5%) in Zr/KClO4, and Zr (41%), Fe2O3 (49%), SiO2 (10%) in Zr/Fe2O3, respectively. The aging of each sample was investigated by monitoring the increasing concentration of ZrO2, as it is associated with the oxidation process. The non-aging samples mixed with ZrO2 were prepared by varying the concentration of Zr and ZrO2 in the Zr composition of each material to draw the calibration curve. Table 2 lists also the detailed conditions of the actual aging samples.
Table 1. Sample mixture of zirconium dioxide and paraffin binder
Table 2. Two types of zirconium/oxidizer pyrotechnic substances exposed to aging conditions
The mixed powder was pelletized by using 10 tons of pressure for 2.5 min of dwell time and 1.5 min of release time using a Spex model 3635. The process of pelleting is an important step in making the sample surface flat to achieve uniform beam focusing. The paraffin binder consists of hydrogen and carbon (CnH2n + 2), which means it is simple to adjust the concentration and bonding of the powder samples. The amount of the powdered sample and paraffin binder was 3.0 mg and 3.0 g, respectively.
3. Results and discussion
3.1 Construction of a calibration curve for ZrO2/Paraffin mixture
Figure 2 shows the emission spectra of ZrO2 mixed with paraffin binder at various ratios that provide the selective band system. One hundred laser spectra were averaged for each mixture to improve the reliability of the results. Since the paraffin does not react during LIBS, the sequentially increasing ZrO molecular band signal can be obtained directly from the composition change. There are three ZrO band structures, as identified in Fig. 2. They represent the ZrO α (1,0) bands of the b3ϕ – a3Δ system which consist of the b3ϕ4 – a3Δ3, b3ϕ3 – a3Δ2 and b3ϕ2 – a3Δ1 subsystems for the corresponding band origins of approximately 623~634, 634~647, and 647~660 nm, respectively [19]. The emission has a significantly complex band shape due to the combination of electronic, vibrational, and rotational energy levels, which provides a basis for analyzing the signal in the form of a band structure. Similar to the metal oxide form presented in previous papers [11,13,15], it has a gradually decreasing band shape after the peak value near the band origin. Table 3 shows the experimentally measured band origin of the ZrO transitions compared with the theoretical value [23]. The experimental values were measured at the point where new transitions were identified, and may change slightly due to the difference in the internuclear distance between the atoms when electronic transitions occur [24]. As expected, the band signals are greater with an increasing ZrO2 ratio in the sample. Therefore, the result indicates that the magnitude of the entire structure can be determined by the concentration of ZrO2 in the sample.
Table 3. Experimentally and theoretically measured band origins of the ZrO transitions for ZrO2 samples
Meanwhile, an AUC method was applied for quantifying the area of the ZrO band signal by integrating the relevant band region in Fig. 2 [25–27]. In this study, we set the spectrum of the binder being inactive during LIBS as the background line for the AUC result in the corresponding region. Figure 3 shows the resulting calibration curve of the ZrO α (1,0) bands of the b3ϕ – a3Δ system as the ZrO2 concentration increases. OriginPro software (OriginLab, OriginPro 8.5.1, USA) was used to perform data processing. Also, the denoise method is applied to present more accurate results. The relation between ZrO2 concentration (x) and the corresponding area under the band structure (y) is expressed by the equation (y = 289 x - 31) with a correlation coefficient, R2 = 0.97. Because R2 indicates the linearity of the calibration curve, the accuracy of the quantification of the signals was confirmed. The points on the graph represent the respective mean and standard deviation of the calculated area, irradiated for 100 shots. As shown, quantification of the molecular signal is possible, in particular from the LIBS analysis of the ZrO2 sample and so the method was then extended to analyze two Zr-based pyrotechnic substances.
Fig. 3 Calibration curve for the ZrO molecular band AUC (area under curve) according to ZrO2 concentration.
3.2 LIBS signals of Zr/KClO4 and Zr/Fe2O3 with varying Zr:ZrO2 concentrations
The molecular signal detection approach was applied to the analysis of oxidizer-doped zirconium substances that are associated with the reliability assessment of aging pyrotechnics. The ZrO molecular band signal process follows steps that include, i) the recombination of Zr and O in the plasma, ii) the emission of molecular signals from ZrO fragments in the sample produced by intermolecular bonds, and iii) the reaction within the plasma of Zr with O surrounded by air [14,28]. Moreover, the emission of the ZrO band appears not only from ZrO2 but also from Zr alone, as expected from iii) above. However, we considered that the degree of influence on the generation of the ZrO band signal would be different for Zr and ZrO2 under the same experimental conditions. For this reason, the present experiment was devised with respect to the changes in both Zr and ZrO2 concentrations to analyze their effect on the generation of the ZrO signal.
Figure 4 shows the LIBS spectra of the two types of pyrotechnic substances, each containing solid oxidizer (KClO4 and Fe2O3) doped Zr. Each laser spectrum was averaged from 100 points of the sample LIBS data. The three molecular bands, belonging to the b3ϕ – a3Δ system, were successfully observed with a distinct band structure. Furthermore, the magnitude of Zr/Fe2O3 is smaller compared to Zr/KClO4 due to its low Zr composition. As mentioned in step iii) above, a certain amount of intensity is expected even at a high concentration of Zr due to the recombination of Zr and oxygen in the air during the cooling process. These results indicate that the ZrO band signals are generated in both Zr and ZrO2.
Figure 5 shows the increasing signal intensity with aging duration. Table 4 summarizes the AUC result associated with the ZrO band in the spectra. The AUC values of the aging samples follow closely the increasing trend of the non-aging samples with added ZrO2. This is clear confirmation of the oxidation of Zr within the aging samples, both natural and hygrothermal, to form more ZrO2.
Fig. 5 The emission spectra of aging samples of (a) Zr/KClO4, hygrothermally aged and (b) Zr/Fe2O3, naturally aged.
Table 4. AUC results of the spectra
Figure 6 shows a comparison of a non-aging sample vs. an aging sample for both pyrotechnic substances with respect to the calibration line. The non-aging AUC results from Table 4 were used for the construction of the calibration curve. In Fig. 6(a), the x-axis indicates the concentration of ZrO2 with respect to the total Zr-related content (Zr + ZrO2) within the non-aging Zr/KClO4. The linear fit follows the equation y = 270 x + 6439 (R2 = 0.93). In Fig. 6(b) for Zr/Fe2O3, the regression equation of calibration curve was y = 58 x + 4183 (R2 = 0.91) for the AUC results. The red circles with error bars represent the mean value and standard deviation of the applied AUC method to both aging samples. Both aging samples, one being hygrothermal and the other being natural, follow the behavior of the standard sample with increasing ZrO2 concentration, indicating that the oxidation of Zr is strongly related to the aging of the Zr-based pyrotechnic substances.
Fig. 6 AUC results of the aging samples (red circles) in comparison to the calibration curve of the non-aging samples (black squares) at various concentrations. (a) Zr/KClO4, (b) Zr/Fe2O3.
Table 5 summarizes the predicted ZrO2 concentrations in the aging pyrotechnic samples. By comparing the average values of the band region with respect to a regression line, the concentrations of ZrO2 within each aging substance were 23.1%, 33.2%, 38.3% and 41.4% in the case of Zr/KClO4, and 2.05%, 8.7%, 13.2% and 17.5% in the case of Zr/Fe2O3. By utilizing the area under the curve of the ZrO band, we have illustrated a method for predicting the concentration of the oxidation of Zr in aging pyrotechnic samples.
Table 5. Prediction of ZrO2 concentration ( wt. %) in aging pyrotechnic substances: [ - ] indicates wt. % with respect to the whole substance
3.3. Validation for LIBS results via XRD method
X-ray diffraction (XRD) is used for component analysis aimed at determining the chemical structure of the target sample by observing the scattered intensity of an X-ray beam. The crystal structures were recorded by using an high-resolution x-ray diffractometer (Rigaku SmartLab) with Cu K-α radiation (A = 1.541 Å) operated at 9 kW. An image detector, namely Hypix-3000 was mounted on the top of the scanner and performed with a step of 0.02° in the 2-theta range from 15 to 90° (scan rate 0.5°/min).
Figure 7 represents the XRD patterns for the two types of pyrotechnic substances. It shows a noticeable intensive diffraction patterns at 2θ = 24.2° and 28.2°, which correspond to ZrO2 crystal phase (m-ZrO2) according to JCPDS 37-1484. In addition, the difference in intensities of diffraction patterns at 25.4° (JCPDS 17-0385) was also observed in oxygen-deficient zirconium oxide (ZrO0.35), which represents the unique characteristics of ZrO2 crystal [29]. In both samples, the signals of the corresponding ZrO2 diffraction peaks were clearly increased with the aging duration. This complemental spectroscopic method demonstrates the same tendency and thus validate the obtained LIBS results.
4. Conclusion
This paper presents how LIBS as a standard atomic spectroscopic method can exploit the ZrO molecular band signals to estimate the premature onset of oxidation of Zr-containing substances in pyrotechnic igniters. Aging samples of Zr/KClO4 and Zr/Fe2O3 were analyzed by the calibration curve constructed on the basis of the AUC results obtained from non-aging samples with the addition of ZrO2. Furthermore, an XRD analysis was conducted to verify the validity of the LIBS experiment. The present method of predicting the percentage of oxidation can be developed further to assist in estimating the life expectancy and safety of pyrotechnic devices that use metalized fuels such as zirconium as the exothermic source of ignition.
Funding
Korea National Research Foundation under the National Space Laboratory Program 2014 (NRF-2014M1A3A3A02034903) through the IAAT at Seoul National University; The Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2016R1D1A1A02937421); The Hanwha Yeosu Grant 2018.
Acknowledgments
This work was financially supported by a grant from the Korea National Research Foundation under the National Space Laboratory Program 2014 (NRF- 2014M1A3A3A02034903) through the IAAT at Seoul National University. This research was also supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF- 2016R1D1A1A02937421). Additional support was provided by the Hanwha Yeosu Grant 2018.
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