secondary ion mass spectrometry of automotive coating systems.

by:ZHENHUA     2020-09-02
This is the last article in a series focusing on surface analysis techniques.
The previous article checked x-
The energy spectrum of Ruiguang electron and the electron energy spectrum of Russia.
These auxiliary methods are valuable tools to solve the problem of surface composition of solid materials in applications such as automotive paint systems. [
Slightly] Illustrations
Secondary ion mass spectrometry (SIMS)
It is a special form of surface analysis, which is mainly used to determine the elements and chemical structure of solid materials. (1),(2)
It is based on mass spectrometry, a family of technologies that are classified according to the atomic/molecular weight of elements, molecules, and molecular fragments.
This technique uses a source to excite the substances of interest into the gas phase as part of the ionization species.
Ions then go through a mass filter and classify them according to their massto-charge ratio.
The resulting mass spectrometry gives a representation of the elements and/or molecular composition of the material of interest. (1-3)
In general, a list of molecular fragments in the spectrum can be pieced together to obtain a detailed understanding of the chemical structure of the material. (1),(3),(4)
In SIMS theory, the excitation source is composed of a vacuum chamber and an anion gun.
Place the sample in a vacuum, keep the surface clean and allow the ion to be transported easily through the instrument.
An ion gun produces a stream of ions called primary ions that illuminate the sun.
When the main ions reach the surface of the sample, they pass energy through elastic collisions to atoms in the solid, a process called \"collision cascade \". \"(5)
The schematic diagram of this process is shown in Figure 1.
Sufficient energy is transferred in the collision cascade to break the chemical bond and replace the atom from the position in the solid lattice.
A small fraction of these atoms and molecular fragments get enough energy to escape the solid into the gas phase.
Some of the ejected particles are charged (ionized).
These particles, called secondary ions, can produce SIMS spectra of solids by mass spectrometry. [
Figure 1 slightly]
Many different ion sources and mass spectrometer can be used to perform SIMS experiments, which gives analysts great flexibility in designing how experiments are performed.
Experimental device-
Up also affects the type of data that can be generated.
There are many kinds of SIMS instruments, which can be divided into two categories: Dynamic SIMS and static SIMS. (1,6)
Historically, the first SIMS instrument developed was a dynamic SIMS system designed to take advantage of the methodological sensitivity that could approach the partper-billion (PPB)levels. (7)
This part is achieved by using high-throughput primary ions.
During the analysis, a crater is eroded into a solid, producing enough secondary ions to achieve high sensitivity.
These instruments have developed into complex systems designed to measure ultra-low concentration, depth distribution (depth profiles)
And lateral distribution (maps)of elements. (8)
One of the main uses of DynamicSIMS is to determine the distribution of semi-doping
Conduction device.
This information is essential when customizing the electrical energy of the device, in part because of
Conductor performance. (3)
However, the high primary flux used in dynamic SIMS destroys most of the chemical bonds in solids, resulting in minimal structural and chemical information.
Therefore, the utility of dynamic SIMS is limited for the Study of coating systems that are usually composed of organic materials.
At the end of 1960, when benninghovenven introduced the concept of static SIMS, the restriction was corrected. (9),(10)
If the total amount of primary ions used in the experiment remains below a certain threshold, statistically, each primary ion interacts with solids in areas where the previous primary ion bombardment was not damaged.
Since secondary ions have experienced only one collision cascade reaction, they contain a higher proportion of molecular fragments that contain structural information about the material in the solid.
This threshold is a static limit ([Less than]5 x [10. sup. -12]ions/[cm. sup. 2])
There is also the technology of static Sims.
Since the flux of secondary ions produced in static samples is very low, an efficient mass spectrometer is required to maintain sufficient sensitivity.
The most useful spectrometer type for static SIMS experiments is time-of-flight (ToF)
Mass Spectrometry. (1)
No signal-
Waste the slit in This spectrometer;
Instead, it relies on the separation of ions of different mass through the flight time difference of the instrument.
Therefore, a high proportion of secondary ions produced were detected in this type of instrument.
As shown in Figure 2, a schematic diagram of the time of flight spectrometer.
Note that the ion source produces an ion pulse, not a continuous stream.
The arrival time of this primary ion pulse starts to be timed, as this is the moment in which secondary ions are produced.
Secondary ions accelerate to constant kinetic energy using a grid at a high potential.
Then they went through the long flying tube and hit the detector where the clock stopped.
Determine the arrival time of secondary ions in the detector by a simple kinetic energy equation :[
Figure 2:KE = 1/2m[V. sup. 2]
KE is the kinetic energy received by the ion from the accelerated grid, m is the mass of the ion, and V is the final speed of the ion. (11)
If we replace the length of the flight tube (d)
This equation can be rearranged in the time of flight of the speed: flight time = dl [square root of[2KE/m]]
Therefore, the flight time is inversely proportional to the square root of the ion mass.
Light Ions take a very short time to reach the detector, and heavy ions take a longer time.
Given the number of ions of the mass, resulting in the strength under that mass.
The resulting spectrum consists of a functional diagram of the ion intensity with mass.
The rich set of peaks in the obtained spectrum represents the composition of the solid and the way the elements are combined.
Ability to fly time
SIMS measures molecular fragments with high sensitivity and is ideal for studying coating systems. (12)
Table 1 lists some key features of flight timeSIMSexperiment. (1)
This instrument can detect positive ions and ions.
This increases the sensitivity of this technique because some materials produce a large amount of positive ions, while others produce more negative ions.
Collisions only spread very little single layers in solids, so this technique is very surface sensitive.
Working in a static state results in minimal surface damage, but SIMS are very sensitive to the detection limits within the 1ppm range.
The main beam can be focused on a smaller diameter with a spatial resolution of 150 nm.
This makes it possible to generate small local regions and/or elements and molecular fragment graphs with high spatial details.
In addition, the modern instrument has a movable stage to check the relatively large area (up to 50 m[m. sup. 2]).
A huge advantage of flight time
SIMS instruments are that sample analysis during the analysis process is often not a serious problem even when processing non-samples
Conductive samples such as paint and polymer.
There are many reasons for this.
First of all, since the primary ion source is pulsed, the duty cycle of the primary ion is very low.
Typically, the primary ion beam \"turns on\" in each pulse for only nanoseconds, and then the electronic device \"waits\" milliseconds in all the ions on the detector and is counted.
Secondly, during this dead time waiting for the ion to reach the detector, the electronic flood gun can be used to guide the electronic pulse to the surface of the sample, which balances the positive primary ion and greatly reduces any sampling.
Ions composed of different elements usually have the same mass.
The comparison of the low and high quality resolution spectra contained in Figure 3 illustrates this.
In this example, aluminum ([Al. sup. [Plus]])
Fragments of hydrocarbons [C. sub. 2][H. sub. 3. sup. +]are present.
Low quality resolution spectrum of M/[DELTA]ssimko@ford. com.
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