LASER INDUSED BREAKDOWN SPECTROSCOPY IN SOME MATERIAL "دراسة الانهيار الطيفي المستحث بالليزر لبعض المواد"

The laser
induced breakdown spectroscopy was studied for the Al alloy samples. The
spectrum of produced plasma collected to study the minor elements in the
aluminum alloy samples. Study the properties of produced plasmas. The samples
irradiated by a Q-switched
Nd:YAG laser at the fundamental wavelength 1064 nm and second harmonic 532 nm.
The energy per pulse (440 and 188 mJ respectively) was measured at the target
surface at pulse duration of 6 ns FWHM.


This thesis consists of five chapters as the
following:


Chapter one:-


In this
chapter we introduce the use of aluminum in
manufacturing and its advantages over other
engineering materials in general. In the end of this chapter the main objective
of this study were mentioned.


Chapter two:-


In chapter
two the LIBS as analytical technique were explained in details. The discovery
of LIBS, the application, the effective parameters, the characteristic of LIBS,
the advantage and the challenge of LIBS were reviewed in details. At the end of
this chapter the analytical technique methods were mentioned.


Chapter three:-


The roles of plasma
spectroscopy in LIBS were studied. At the first, the processes of excitation
and ionization were studied. Secondly, the parameters of plasma decay like
recombination were studied. After that, the emission spectral line was studied.
The equations of plasma temperature and plasma electron density were studied
here. Finally, the equation of calculating Limits of Detection (LOD) and the
precision were mentioned.


Chapter four:-


The
apparatus which used in present study was drawn in this chapter. The system
consists of the laser units, the monochromator, the ICCD camera, the power
meter and the optical lenses. The study talks about these things and the
targets in details.


Chapter five:-


In
the first part of this chapter the self absorption of the emission spectral lines
were tested for the lines of copper, magnesium and manganese at 10, 20, 50 and
100 laser pulses at the target surface for the fundamental’s and second
harmonic wavelength.


After that, the line to noise ratio
(SNR) were studied for copper’s lines, magnesium’s lines, manganese’s lines and
silicon’s line at the same number of pulses, the same laser energies and the
same wavelengths to optimize the LIBS system conditions and obtained the best
SNR to make improvement in the following study.


The
optimized LIBS parameters were presented in table (5-3) for first and second
harmonic laser wavelength. The calibration curves were constructed at these
optimized conditions.


The detection limits were determined
for copper, magnesium, manganese, chromium, iron and silicon for two laser wavelengths.
The best values obtained by second harmonic at 188 mJ for all elements in the
study except those obtained for silicon. The dependence of LOD on laser
wavelength was discussed. After that we analyze commercial samples to show the
precision of that technique in analyzing samples comparing with other techniques.


In
the second part
of this chapter Electron excitation temperatures and electron number density
were determined at the optimum LIBS conditions.


At
the first, the excitation temperature for the produced plasma from the Al alloy
sample which irradiated by fundamentals and second harmonic laser wavelengths
were obtained from, Mn I lines and O I lines. It is in range 1.85 eV and
decreased with time to 0.65 at 10 µs and it is in range 2.4 eV and decreased
with time to 0.65 at 10 µs for fundamental’s and second harmonic respectively.


After
that, the electron density were determined from the stark broadening of the H_α,
O I line, Al II lines and Si I line, at the laser energy 440 mJ by the
fundamental’s wavelength. The electron density was in range 1.15× 10¹⁸
cm^-3 at 0.25 µs to 1× 10¹⁷ cm^-3 at 10 µs.


Finally,
the electron density were calculated from the stark broadening of same lines at
the laser energy 188 mJ by the second harmonic wavelength. The electron density
was in range 9× 10¹⁷ cm^-3 at 0.25 µs to 1× 10¹⁷
cm^-3 at 10 µs.


CONCLUSION:


For the analytical purpose, the optimum
LIBS conditions were investigated previously. Here, we discussed how to avoid
the experimental fluctuations and all types of interferences to observe
calibration curves for the minor elements with good linearity and lower
detection limits than obtained by different authors to improve LIBS technique
results.


Standardization
method has been exploited to avoid any unwanted experimental fluctuations. The
line intensity of trace element plotted as a function of the known
concentration of reference samples. To avoid self-absorption and spectral line
interference, the self absorption was tested and avoids interference between
spectral lines for different species.


The wavelengths of the spectral
lines used throughout the analysis were CuI (324.72, 327.4, 510.55, 515.32, and
521.83 nm), MgI (518.36, 517.26 nm), MnI (478.35, 482.34 nm), CrI (520.84 nm),
FeI (344.04, 375.82 nm) and SiI (288.15 nm). With time-resolved spectroscopy,
the emission signal becomes very low and not all the lines are detected
specially for trace elements. So that, we were enforced to use resonance lines
for Cu. Normally, we have to avoid such lines in the quantitative analysis
since they are subjective of high self-absorption. Since the number density of
the atomic species of Cu in the plasma plume is low due to their low concentration
in the target material, self absorption effect may be neglected within the
experimental uncertainty. These lines are good candidates for a lower limit of
detection (LOD) due to its strength compared with other atomic emission lines.
But at high concentration we enforced to use the no resonant lines 521.83 and 515.32 and 510.55 nm (these
lines corresponding to a transition from higher energy level, with lower
transition probability) linear up to 1.5 %.


The calibration curves of the studied elements were
obtained by drawing the line intensities against their concentrations. The
corresponding calibration curves are given for fundamental and second harmonic
wavelength at different energies for aluminum alloy samples. Each data point
represents the mean value of typically three individual measurements. The given
error bars show the calculated standard deviation for the measured LIBS intensities
for each element content of the aluminum alloy sample. They represent the
variation in our measurements.


Calibration curves of the elements in the aluminum
matrix pass nearly through the origin and have well linear fitting (R²>0.9)
within the experimental uncertainty. In fact, this result gives an expectation
that the proposed LIBS technique has a capability for a good linearity of the
calibration curves attaining a wide range of elemental concentrations.


The LOD was calculated from the equation (5.6). The
obtained LOD values and their relative standard deviation (RSD %), for Cu, Mg,
Mn, Si, Cr and Fe are listed with the previously obtained values in table (5-5)
and table (5-6) for fundamental and second harmonic respectively. It is clear
that we have new LOD values with a precision (RSD 8–25%) with in range of the
published LIBS precision. Silicon has slightly high LOD in aluminum matrix for
fundamental and second harmonic.


Under our new optimized conditions, all the obtained
LOD values are improved by more than (20% to 70%) of the previously observed
values by different authors for all elements.


The attractive remark in our study is that, all
authors studied LIBS using the fundamental wavelength and neglected the visible
wavelength however, it has good stability and it gives us better detection
limits than the fundamental wavelength. So we recommended researchers that,
using of the visible wavelength is more productive to obtain the least results
of LOD with acceptable precisions.


The study presents the OI lines from the
ambient air and the MnI lines from the sample trace elements as useful tool to
diagnose the plasma parameters. The excitation temperatures and electron
density profiles were studied at the optimum LIBS conditions using
spectroscopic technique.


The Boltzmann's plot of OI and MnI lines was
used to determine the excitation temperature first and second harmonic laser
wavelength.Tables (5-9), (5-10) show the parameters of lines used in
these determinations. The excitation
temperature determined from MnI lines in range 1.19 ± 0.05 eV to 0.61±0.03 eV when delay time change from 1.5 to 10 µs and from OI lines in range 2.27±0.1 to
1.05±0.08 eV when delay time change from 0.1 to 2.5 µs for second harmonic at 188 mJ laser pulse
energy on the target. While the excitation temperature determined from MnI
lines in range 1.8 ± 0.12 eV to 0.94±0.06
eV when delay time change from 0.5 to 10 µs and from OI lines in range 1.6±0.08 to 1.04±0.05 eV when delay time
change from 0.1 to 2.5 µs for first harmonic at 440 mJ laser pulse
energy on the target.


The stark broadening parameters of H_α
at 656.27 nm, OI line at 844.64 nm, Al
II lines at 281.65 nm and 466.30 nm and Si I line at 288.15 nm were used to
determine the electron number density of the produced plasma from first
and second laser wavelength. The electron density obtained by different lines
approximately in the same range this due to the produced plasma is homogenous.
The electron density for first harmonic in range (1.02±0.09) ×10¹⁸
to (1.5±0.6) ×10¹⁷ cm^-3 when the delay time varied from
0.1 to 8 µs. while the determined electron density for second harmonic in range (9.1±0.6) ×10¹⁷ to (3.9±0.19) ×10¹⁵
cm^-3when the delay time varied from 0.1 to 8 µs.


These results indicate that the excitation
temperature and electron density at the initial stage is very high and
decreases quickly with time. After the end of laser pulse, the atoms in plasma
are excited and ionized constantly while leaving the target surface and
traveling in direction of laser source; moreover, the impact excitation and
ionization occurring after the end of the laser pulse is the dominant process
in the plasma. It can be seen that the electron number density drops very fast
in the first few microseconds.