Metallurgical and composition analysis of melted marks due to electrical failures/Aplydymo medziagos metalurgine ir chemine analize vertinant elektrinius pazeidimus.
Wu, Ying ; Han, Decai
1. Introduction
Uncontrolled fire can devastate our assets and production sources,
and this relates to the societal costs of fire prevention and loss
restoration. The effects of fire on people and the environment become
social issues that depend on the political ideology and economics that
prevail in the state. The steps involved in the investigation of a fire
are site examination and the selection, examination and testing of
samples, collection of background data, and reconstruction and analysis.
Electrical distribution wires with melted ends showing beaded,
drop-shaped are frequently encountered after fires. These melted copper
wires residues can provide fire investigators and forensic scientists
with useful information on the progress of a fire even though they do
not result from these cause.
Optical microscope has been commonly used for visual inspection of
the microstructure, in which properties like porosity and
cross-sectional metallurgical phases of the arc beads are investigated.
Some other means of physical or chemical testing, such as simple visual
observation of the macrostructure, secondary ion mass spectrometry
(SDMS) and dendrite arm spacing (DAS) methods, have been proposed to
analysis the melting mark of copper wires residues found in fire scenes
[1-8]. However, most of the methods only entail subjective, qualitative
criteria for distinguishing between beads that did or did not start a
fire. Due to both further sensitivity of the instrument and the
precision of chemical state determination of X-ray photoelectron
spectroscopy (XPS) [9-16], XPS has been included to deal with insulating
samples.
Copper oxide (i.e., CuO and [Cu.sub.2]O) compounds are interesting
materials because of their use as catalysts, interconnects in
electronics, corrosion of alloys, etc. The identification of the actual
oxidation state of copper in these systems is critical to understand
their chemical behavior and physical properties. Usually, the
attribution of oxidation states of this element is done by considering
both the Cu 2p photoemission peaks and the X-ray induced Auger lines of
copper (Cu LMM). In particular, the use of these two peaks is necessary
if Cu+ has to be differentiated from Cu0. When copper oxides are
dispersed on the surface of another oxide support, the electronic
parameters of the [Cu.sub.2]+ or Cu+ species may undergo significant
changes.
With present work, we carry out a further study on the some
macroscopic observation, composition and microstructures of primary
melted marks (PMM) and secondary melted marks (SMM) by video
microscopes, X-ray photoelectron spectroscopy (XPS) and optical
microscopes. As a result, the given fire cause is the integrative
judgments of these methods results.
2. Experiments
2.1. Materials
PVC-coated wires ([empty set] = 0.75 [mm.sup.2]) were used to form
primary and secondary arc beads. There are many causes, processes and
kinds of arc beads. In this experiment, however, the primary arc beads
that were formed between two PVC-wounded copper wires in a
half-obturator, when brought in contact with each other. The covering of
the wire was burnt by the petrolic burner flames, and the two wires were
short-circuited in the half-obturator to form secondary arc beads. The
heating process used was an attempt to simulate the real fire during the
accident.
2.2. XPS and data processing
XPS spectra were recorded in an ESCALAB 250 spectrometer from VG
supplied with a preparation chamber (base pressure 10-8 Torrs) where all
the evaporation and adsorption experiments were carried out. The spectra
were recorded with the Al K[alpha] (1486.6 eV) radiation in the constant
pass energy mode with a value of 50 eV. In this way, the small
displacements (less than 1 eV) of the peaks in energy position due to
charging effects were compensated. In some cases, when the difference in
BEs between the peaks was very small, their exact position was
determined by fitting a Gaussian curve to them. A maximum error bar of
0.05 eV can be estimated for the measurements.
At least three independent measurements were carried out on the
samples and the accuracy of the measured binding energies was estimated
as [+ or -]0.1 eV. The instrument was calibrated according to the
procedure reported in the literature [17]. Cu, C and O XPS spectra were
detected as described in detail [18, 19]. The Cu 2p3/2 peak cannot be
used to differentiate between Cu metal and cuprous oxide due to the
small chemical shift between the two. The peak centre for both Cu and
[Cu.sub.2]O is located at a binding energy, Eb, of 932.5 eV (within 0.1
eV). These species can be differentiated on the basis of Cu LMM. Cuprous
and cupric oxides can be distinguished in the photoelectron Cu 2p
spectra, since cupric compounds show two strong shake-up peaks located
approximately 10eV above the principal Cu 2p3/2 peak. So, in addition to
the photoelectron lines examined in-depth analytical studies, special
attention was paid to Cu LMM; their presence is due to the relaxation of
the excited states. One of the most useful concepts in XPS is the Auger
parameter ([alpha]). This parameter can be related to the extra-atomic
relaxation energy of photo-holes in the final state of the atoms
undergoing photoemission. It depends on the electronic and dielectric
properties of the environment around the photo excited atom. Within this
context, we have also studied by XPS the changes in binding energy (BE)
of the photoemission peaks and in the [alpha]' parameter of a
cation. Based on these results, we can obtain the composition and the
chemical state in the near-surface regions of PMM and SMM.
A correlation of such behavior with some macroscopic and
metallurgical analysis of the PMM and SMM has been also studied.
2.3. Metallurgical analysis of PMM and SMM
The sample were mounted in resin and polished with emery paper and
diamond paste. Subsequently, metallurgical analysis by visual inspection
of the microstructure from the beads was recorded with a high-power
magnification optical microscope.
3. Macrostructure analysis
In the investigation of fires, arc beads are frequently encountered
because electricity is available in most buildings. Arc beads often
assume a roughly-spherical shape (beaded) and drop-shaped shown as in
Figs. 1, a and b. The shape however does not necessarily have to be
spherical (beaded) or drop-shaped. Figs. 2, a and b give sunk and
pointed shapes of arcs, respectively.
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
Figs. 3 and 4 give the macroscopic views of these experimented
samples of PMM and SMM. While just simple visual observation is not
sufficient for distinguishing two types of arc beads--'cause'
and 'victim' beads. Consequently, X-ray photoelectron
spectroscopy technique has been proposed.
4. Composition analysis
From the survey spectra (not shown), the presence of the elements
copper, carbon and oxygen was detected on all the samples, as described
elsewhere [17, 18]. In addition to the photoelectron lines examined
in-depth analytical studies; special attention was paid to the X-ray
induced Cu LMM; their presence is due to the relaxation of the excited
states. We studied composition and the chemical state in the
near-surface region of PMM and SMM.
4.1. Depth profiling results
The Cu LMM spectra were observed at 569.7[+ or -]0.15 eV in the
near-surface regions of PMM, while at 568.1[+ or -]0.15 eV in the deeper
depth of copper substrate (Fig. 5). On the other hand, Cu LMM spectra
are 568.35[+ or -]0.1 eV both in the near-surface regions of SMM and in
the deeper depth of copper substrate (Fig. 6).
[FIGURE 5 OMITTED]
A depth vs. concentration profile has been made by progressively
etching away portions of the surface and examining a lower layer. The
quantitative results from the depth profiles of PMM and SMM are shown in
Figs. 7 and 8. Carbon is present only as a subsurface contaminant and
almost disappears after the first minute of sputtering in PMM. The
oxygen concentration at the surface of PMM was with gentle increase till
42 nm but gradually decrease in the range of 42-66 nm and then showed
drastic decrease (Fig. 7). On the other hand the concentration of carbon
at the surface of SMM was also observed to be high, and then gradually
diminished concentration of C was observed whereas higher as compared to
PMM in the residual depth. The concentration of O at the surface of SMM
showed gradually decrease throughout the depth profile (Fig. 8).
4.2. Chemical state plot
Combining the kinetic energy of the X-ray induced Auger lines and
the binding energy of the photoelectron lines of the same element in a
two-dimensional plot, the Wagner chemical state plot [19], a powerful
tool for determining the chemical state, is obtained. This plot has
found numerous applications in the investigation of geological materials
[20]. Indeed, if both the binding energy of the photoelectron and the
kinetic energy of the Auger electron are measured, a new parameter known
as the Auger parameter ([alpha]') can be determined. The use of the
Auger parameter concept permits obtaining more information from the
joint analysis of the Auger and photoelectron lines rather than from
either of them separately. Wagner originally defined the Auger parameter
as the difference between the kinetic energy of the most intense Auger
line and the most intense photoelectron line, making reference to the
Fermi level rather than the vacuum level. The definition of the Auger
parameter a' [21] used most frequently is
[alpha]' = [alpha] + hv
that is [alpha]' = [KE.sub.A] + [BE.sub.p]
[FIGURE 6 OMITTED]
[FIGURE 7 OMITTED]
[FIGURE 8 OMITTED]
[FIGURE 9 OMITTED]
[FIGURE 10 OMITTED]
The modified parameter [alpha]' as defined above is then
independent of hv and always positive, and it is the sum of the kinetic
energy of the Auger signal and the binding energy of the photoelectron
line. This sum will be the same, independent of sample charging, and is
found in the chemical state plot as a series of diagonal lines
representing equal Auger parameters. The chemical state plot for PMM
shows that the data points in the depth range of 18-66 nm with an Auger
parameter of 1849.0 [+ or -] 0.15 eV, close to the value of [Cu.sub.2]O.
The data points in the depth range of 66-246 nm of PMM with an Auger
parameter of 1850.8[+ or -]0.2 eV, similar to Cu metal (Table 1).
[FIGURE 11 OMITTED]
[FIGURE 12 OMITTED]
The chemical state plot for SMM shows that the all data points
throughout the depth profile with an Auger parameter of 1849.0 [+ or -]
0.15 eV, close to the value of Cu metal (Table 2).
As a result, the film formed in PMM is constituted mainly of
Cu[O.sub.2]. It thickness is estimated to be 66 nm. While the subsurface
layer of molten product on SMM is not present Cu[O.sub.2].
5. Metallurgical analysis
A typical cross-section of PMM in Fig. 9 possesses a sharp
demarcation between a roughly-spherical bead and the fine cylindrical
portion of the wire, and has little and small voids in melted part,
while that of SMM (Fig. 10) does not show a sharp transition between
molten and virgin material, a roughly cylindrical portion of the wire
with much and large voids in melted part.
Optical microscope has been commonly used for visual inspection of
the microstructure of the fire copper wire debris in fire investigation,
but in some cases, PMM and SMM are difficultly identified just using
this method, even fallibly giving a false result, these experimental
results shown as Figs. 11 and 12. So the results obtained from visual
inspection are prone to human bias as it needs a skillful analyst and
the results may vary with sample preparation and interpretation. In this
situation, however, we can use the presence of [Cu.sub.2]O and its
quantitative results to identify these electric short circuit beads to
be either the cause of a fire (a PMM) or one caused by the flames of the
fire (a SMM), as complementary technique for judgments of fire cause.
6. Conclusion
Based on the macroscopic observation, XPS and metallurgical results
the following conclusions can be drawn:
1. Electrical distribution wires with melted ends show not only
roughly-spherical (beaded), drop-shaped, and pointed shapes, but also
sunk and pointed ones. Just simple visual observation is not sufficient
for distinguishing two types of arc beads--'cause' and
'victim' beads.
2. The molten product on a PMM can be distinguished as three
portions: surface layer with drastic decrease of carbon content;
intermediate layer with gentle change of oxygen content, gradually
diminished carbon peak, and consisted of [Cu.sub.2]O; transition layer
without [Cu.sub.2]O and with rapid decrease of oxygen content. While the
molten product on a SMM can be distinguished as two portions: surface
layer with carbon content decreasing quickly; subsurface layer without
[Cu.sub.2]O and with carbon, oxygen content decreasing gradually. In
addition, the presence of [Cu.sub.2]O and the quantitative results can
be used to identify these electric short circuit beads to be either the
cause of a fire (a PMM) or one caused by the flames of the fire (a SMM),
as complementary technique for judgments of fire cause.
3. A typical microstructure of PMM possesses a sharp demarcation
between a roughly-spherical bead and the fine cylindrical portion of the
wire, and has little and small voids in melted part, while that of SMM
does not show a sharp transition between molten and virgin material, a
roughly cylindrical portion of the wire with much and large voids in
melted part. As a result, there was an obvious interface between the
layered surface product and the substrate for the PMM, while as to SMM
there was no interface.
Acknowledgment
This work was supported by Project 2009ZDYJSYXF027 of Key Research
Program of Ministry of Public Security, PR China.
Received March 23, 2011
Accepted March 08, 2012
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Ying Wu, Key laboratory of fire scene investigation and evidence
identification, Ministry of Public Security, PR China, Shenyang Fire
Research Institution, Ministry of Public Security, 110034, China,
E-mail: yingw78@sina.com
Decai Han, Shenyang Gas Cylinder Safety Technology Co., Ltd., China
http://dx.doi.org/ 10.5755/j01.mech.18.2.1558
Table 1
Binding energy and Auger kinetic energy of copper Cu
2p3/2 and Cu LMM signals of copper compounds with the Auger
parameter for PMM
Depth, Cu 2p3/2 Cu [L.sub.3] [alpha]', Compound
nm BE, eV [M.sub.45] eV
[M.sub.45]
KE, eV
18 932.2 916.9 1849.1 [Cu.sub.2]O
42 932.1 916.75 1848.85 [Cu.sub.2]O
66 932.2 916.8 1849 [Cu.sub.2]O
126 932.5 918.5 1851 Cu
186 932.45 918.3 1850.75 Cu
246 932.45 918.3 1850.75 Cu
Table 2
Binding energy and Auger kinetic energy of copper Cu 2p3/2
and Cu LMM signals of copper compounds with the Auger
parameter for SMM
Depth, Cu 2p3/2 Cu [L.sub.3] [alpha]', Compound
nm BE, eV [M.sub.45] eV
[M.sub.45]
KE, eV
18 932.75 918.25 1851 Cu
36 932.7 918.25 1850.95 Cu
60 932.75 918.2 1850.95 Cu
84 932.7 918.35 1851.05 Cu
144 932.7 918.35 1851.05 Cu
