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VI Chapter

Principles and Applications of X-Ray Microanalysis

Introduction:

The quantitative characterization of X-rays emanating from the specimen forms the basisof X-ray microanalysis. The use of fine and intense electron probe facilitates analysis

with high spatial resolution (at the nm level). Consequently it is possible to study

localized variation in chemical composition. The mechanism of X-ray and Augerelectron production, as already explained, consist of the excitation, relaxation and

emission stages and is schematically illustrated in Fig. 6.1.

Fig. 6.1: Mechanism of X-ray and Auger electron production

The intensity of the X-rays produced varies considerably. The nomenclature for principal

X-ray lines and the values for relative intensities of these lines (K, K, L etc.) areshown in Fig.6.2 and Table 6.1 respectively.

Fig. 6.2: Nomenclature for principal X-ray lines

The emission process

Excitation

Relaxation

Emission

L M

N shell

L shellM shell

K shellK

K

K L

LM

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Table 6.1: Relative intensities of various X-ray lines

K1= 100

K2= 50

K1= 15.30

K2= 1-10K3= 6-15

L1= 100

L2= 50

L1= 50

L2= 20L3= 1-6

L4= 3-5L1 = 1-10

L3 = 0.5-2

M1-2 = 100M= 60

K X-rays are normally used for the study of elements with Z ~ 30, say upto Cu while for

the heavier elements L and M rays are used. L-radiation for the elements with Z


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Fig. 6.4: Electron Excitation of Continuum (Background) Intensity

X-Ray Detectors:

X-rays are characterized by the use of Energy dispersive (EDS) and wave lengthdispersive (WDS) systems. The principle of the energy dispersive detector is illustrated in

Fig. 6.5. The detector uses a lithium-drifted silicon crystal, Si(Li) with surface area in the

range 5 to 200 mm2

and kept at liquid nitrogen temperature and under high vacuum. X-

rays enter through a thin beryllium (0.7m thick) window and create electron-hole pars,the number depending on the energy of the X-rays. The x-ray spectrometer converts the

x-ray photon into an electrical pulse with specific characteristics of amplitude and width.

A multi-channel analyzer measures the pulse and increments a corresponding "energy

slot" in a monitor display. The location of the slot is proportional to the energy of the x-ray photon entering the detector. The display is a histogram of the x-ray energy received

by the detector, with individual "peaks," the heights of which are proportional to the

amount of a particular element in the specimen being analyzed.

There is a trend towards a newer EDS detector, called the Silicon Drift Detector (SDD).The SDD consists of a high-resistivity silicon chip where electrons are driven to a small

collecting anode. The advantage lies in the extremely low capacitance of this anode,thereby utilizing shorter processing times and allowing very high throughput. Benefits of

the SDD include 1) High count rates and processing 2) Better resolution than traditional

SiLi detectors at high count rates 3) Lower dead time (time spent on processing x-ray

event) 4) Faster analytical capabilities and more precise X-ray maps or particle datacollected in seconds and 5) Ability to be stored and operate at relatively high

temperatures, eliminating the need for liquid nitrogen cooling. Since the capacitance of

the SDD chip is independent of the active area of the detector, much larger SDD chips

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can be utilized (40mm sq. or more). This allows for even higher count rate collection.

Further benefits of large area chips include 1) Minimizing SEM beam current allowingfor optimization of imaging under analytical conditions 2) Reduced sample damage and

3) Smaller beam interaction and improved spatial resolution for high speed maps.

In recent years a different type of EDS detector, based upon a microcalorimeter, hasbecome commercially available. This new model allegedly has the simultaneousdetection capabilities of EDS as well as the high spectral resolution of WDS. The EDS

microcalorimeter relies highly on two components: an absorber, and a thermistor. The

former absorbs X-rays emitted from the sample and converts this energy into heat; thelatter measures the subsequent change in temperature due to the influx of heat (in

essence, a thermometer). The EDS microcalorimeter has suffered from a number of

drawbacks; including low count rates, poor collection efficiencies and small detectorareas. The count rate is hampered by its reliance on the tiime constant of the calorimeters

electrical circuit. The collection efficiency is a function of the absorber material and

remains to be optimized. The detector area must be small in order to keep the heat

capacity as small as possible and maximize thermal sensitivity (resolution). Innovativeengineering solutions are necessary for further improvement of spectroscopic

microanalysis.

Fig. 6.5: Principle of the Energy Dispersive Detector

The following are the characteristics of the energy dispersive spectrometer:

Simple, nearly operator independent Large solid angle (0.05-0.3 sr)

Virtually specimen position independent

No moving parts

Parallel detection

Quantification by standardless or standards methods

Poor energy resolution (~130 eV). Superconducting systems (~20 eV)

Poor peak/background ratio (100:1)

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Detection efficiency depends upon x-ray energy

The principle of the wavelength detector is illustrated in Fig. 6.6. It operates using

Fig. 6.6: Principle of Wave Length Dispersion Spectrometer

diffraction principles (Braggs law). X-rays emitted by the sample being analyzed arecollimated by parallel copper blades (called collimator or Soller slits), and irradiate a

known single crystal at a precise angle. The single crystal diffracts the photons (Braggs

law) which are collected by a detector, usually a scintillation counter or a proprtional

counter. The single crystal, the specimen, and the detector are mounted precisely on a

goniometer with the distance from the source of x-rays (the specimen) and the crystalequal to the distance from the crystal to the detector. It is usually operated under vacuum

to reduce the absorption of soft radiation (low-energy photons) by the air and thusincrease the sensitivity for the detection and quantitation of light elements (between

boron and oxygen). Modern systems contain a small number of crystals of known but

differing properties, with automated changing of the crystal depending on the energybeing analysed, enabling elements from the entire periodic table (excepting the very light

elements) to be analyzed.

Details of analysing crsytals used in WDs are given in Table 6.2. It is posible to analysefor elements in the range B to U using a variety of crytals with different interplanar

spacings. The following are the characteristics of the wavelength system:

Excellent energy resolution (~5 eV)

High peak/background ratios (1000:1)

Good detection efficiency for all x-rays

High counting rates

Good light elements capabilities

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Complex mechanical devices, operator intensive

Specimen height dependent focus

Moving components in the AEM

Limited solid angle (


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Probe current 10-11

-10-9

A 10-9

10-6

A

Running costs ~ 500 l liquid nitrogen peryear for cooling the crystal

Argon/methane gas for gasflow proportional counter

Typical output, cps 5000-10000 50,000

Time to collect full spectrum 1 min 30 min.

Artefacts Escape and sum peaks Higher order lines

Field of usefulness Good for quick, readily

interpreted qualitative

analysis

Very good for precise

quantitative analysis and line

scans

The advantages of the EDS are simultaneous collection of entire spectrum from the

sample and hence less time for analysis, ease of fitting to the main instrument. The

disadvantages are the limited resolution and hence overlap of some peaks, e.g. Fe, Cr, Ni

Kand Kand difficulty of detection of light elements due to absorption in the berylliumwindow and outer layers of the detecting crystal. It is possible to analyse the light

elements by replacing the beryllium window with an ultrathin plastic window or an openhole (windowless detector). The nominal FWHM values in modern Si(Li) detectors is ~80 to eV for O K(0.52keV) and 140-160 eV for Mn (~140 to 160 eV). The WDS has

excellent energy resolution and capability to analyse light elements. These facts are

illustrated in Figs. 6.7 by a comparison of the WDS and EDS spectra on a glass sample.

Fig. 6.7: Comparison of EDS and WDS spectra of a glass sample.

The minimum detection limit for the two systems is specified in terms of minimum mass

fraction (MMF) with typical values of 0.001 for EDS and 0.0001 for WDS.

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The two systems can be mounted on a number of electron optical systems to provide

information on chemical composition; the details including typical applications are givenin Table 6.4.

Table 6.4: Instrumentation for X-ray analysis and typical applications

Gun characteristics

The intensity of X-rays depends on the electron energy and probe current while the

spatial resolution for analysis is influenced by the probe size. The relationship between

probe current and probe size is shown in Fig. 6.8. At small probe size values (1 nm) the

FEG is superior to the other two gun sources. For all nanoanalytical work, the FEG ispreferred; note that at large probe size values (1 m) the W filament gun is good enough.It would be useful at this stage to review the characteristics of different types of guns

(although this aspect has already been discussed in II Chapter). The information is

tabulated for various gun sources (Table 6.5).

Quantitative Analysis:

Quantitative analysis involves the measurement of characteristic X-ray intensities fromthe specimen (I) and a standard (Is) (which may be a pure element or a compound)

C = CsI/Is

.where C and Cs are the concentrations of the element in the sample and the standard

respectively

The equation does not take into account a number of phenomena occurring in the sample

before and after the excitation of the X-rays. For this purpose, it is necessary to apply the

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ray photon has energy exceeding the critical excitation energy. Backscattered electrons

are ineffective in exciting X-rays and therefore contribute to the atomic number effect

(Note that increases with Z). Methods of applying these corrections are well

documented in the literature.

The measured intensity of X-rays of a particular species (say K) from the specimendepends on a number of factors, , the ionization cross section, , the radiative partition

function, , the fluorescent yield and a number of constants. The details are given in Fig.

6.19.

Fig. 6.9: Expression for X-ray intensity as a function of properties of the sample

Absorption of X-rays depends on the well known absorption equation,

It= Ioexp(-/)t

where Itand Io are the transmitted and original intensities, (-/) is the mass absorptioncoefficient, is the density and t the thickness. The absorption characteristics of X-rays

in some samples are schematically illustrated in Fig. 6. 10. It is clear that soft X-rays (N,O in the figure) are heavily absorbed even in samples with thickness of ~ 100 nm; the

effect will be more intense when we consider bulk samples where X-rays are generated

from a region of a few m size.

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Fig. 6.10: Absorption of X-rays in various samples

Broadening of the electron probe even can in the thin samples used in transmission

electron microscopy can contribute to errors in analysis This is illustrated in Fig.6.11; the

results of theoretical calculations based on single elastic scattering shown in Table 6.6that beam broadening depends on thickness and atomic number.

Fig. 6.11: Electron beam spread in bulk and thin specimens

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Table 6.6: Theoretical calculation of beam broadening

Complications in analysis:

omplications in analysis arise from a number of effects:

1. X-ray emission characteristic of support grids, goniometer stage, pole pieces,

2. s from the specimen other the one defined by the incident

3. intensities associated with thin specimens

This is illustrated in Fig. 6.12. The extent to which stray sources contribute to the X-ray

Fig. 6.12: Complications in analysis

C

anticontamination device and any other material in the vicinity of the specimen

SYSTEM PEAKS

Peaks due to X-rayelectron beam

Abnormal peak

4. Extra Si peak from detector, sum and escape peaks

spectrum can be evaluated by carrying out a hole count, where the electron probe is

focused on a hole in the sample and X-ray spectrum collected (fig. 6.13).

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Fig. 6.13: Spectral artifacts in AEM: uncollimated radiation : The hole count

Applications:

Fig. 6.13: Illustration of hole count the Ni K peak in the hole is significant

pplications

he ability to carry out high spatial resolution chemical analysis opens up enormous

1. Identification of phasesnear defects such as GB (e.g. precipitate-free zones,

3.

ome examples are given below:

-ray spectra from fired bullets (forming part of a forensic study) are shown in Fig 6.14.

A

Tpossibilities for understanding a number of phenomenon in materials (both organic and

inorganic).

2. Concentration gradientsPFZ), dislocations, coarse precipitate particles; segregation phenomenon

Partition of alloying elements between various phases

S

X

Such studies are important in drawing conclusions about the mode of firing of the pellets.

A metallurgical example from a stainless steel sample containing sigma and chromiumnitride phases and the distribution of the elements Cr, Mo and Fe is illustrated in Figs.

6.15 and 6.16.

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a Pb-Sb pellet fired from Winchester

Fig. 6.14(b): Energy dispersive spectrum from steel pellets fired from Winchester,

is well known that the formation of equilibrium precipitates on grain boundaries during

Fig. 6.14(a) Energy dispersive spectrum from

12 GA

12 GA bird BB

It

the precipitation hardening stages (during quenching of supersaturated solid solution orsubsequent ageing) produces precipitate-free zones (PFZ) near GBs which have an

adverse effect on the resistance to stress corrosion cracking. Examples of analyticalstudies in PFZ in precipitation hardenable aluminium alloys are shown in Figs. 6.17 to

6.19. The width of the PFZ can be measured in the underaged(UA), peakaged(PA) andoveraged (OA) conditions from the microstructures of a 7XXX aluminium alloy

presented in Fig. 6.17. The variation of the elements Zn and Mg in the PFZ and in the

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Fig. 6.15: Composite Decomposition Skin of Sigma Phase and Cr Nitride in 654

SMO Stainless Steel: (left) STEM Image, (right) EDX Mapping Overlap withSignals of Cr (magenta) and Mo (yellow)

Fig. 6.16: Composite Decomposition Skin of Sigma Phase and Cr Nitride in 654

SMO Stainless Steel: (left) STEM Image, (right) EDX Mapping Overlap with

Signals of Fe (green) , Cr (magenta) and Mo (yellow)

surrounding regions can be measured by high resolution X-ray analysis (fig. 6.18). Notethe number of experimental points in the PFZ which ahs a width of < 0.5 m. Themicrostructural and analytical observations can be combined with nanohardness

measurements (fig. 6.19); these studies are useful in understanding the distribution of

alloying elements in various regions during heat treatment.

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Fig. 6.17: PFZ in Al-Zn-Mg alloy aged at 433K in UA, PA and OA conditions

Fig. 6.18: Variation in Zn and Mg concentration across a GB in an Al-Zn-Mg alloy

aged at 433K for 259.2 ks

As the last example we shall consider the X-ray scans on bulk samples in the SEM or

EPMA. As pointed out earlier we can carry out line mapping (variation of a particular

element concentration in one dimension) or area mapping. Such studies are useful inunderstanding a number of metallurgical phenomena (e.g. homogenization of castings).

The X-ray images presented in Fig. 6.20 show how the various elements Si, Mo, Cr and

Co are distributed in the area under observation. A bright region in a given area shows

enrichment of the particular element.

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Fig. 6.19 :Schematic of hardness, solute concentration and precipitate density

variation in Al-Zn-Mg with and without Ag addition

Fig. 6.20

Contamination spots

Contamination spots are formed when high energy electron (typically 200 - 300 keV)probe is focused on the surface of a freshly electropolished thin specimen used for TEM

work. The spots mainly consist of hydrocarbons which diffuse across the surface of the

specimen to the immediate vicinity of electron probe. The amount of contamination is afunction of the time spent at each location. An illustration of this effect is given in Fig.

6.21 for a stainless steel sample observed at 300 kV. The contamination spots tend to

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mask some of the details and therefore need to be cleaned. This is done by reactive gas

plasma processing using argon and oxygen.

UntreatedSpecimen

After 5 minutesof argon

After 5 minutes ofadditional oxygen

Fig. 6.21: Comparison of Results on Electropolished 304 SS

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