Microanalysis in Science and Engineering - X-ray Microanalysis

A Workshop for Middle and High School Teachers

sponsored byTennessee Technological UniversityCenter for Manufacturing ResearchDepartments of Chemical, Mechanical, Earth

Sciences and Curriculum and Instructionand The National Science Foundation

FacultyJoseph J. Biernacki (Chemical Engineering)

June 16, 2003

What will we learn

What is X-ray microanalysis? What interactions of electrons and matter are used? How are the electron/matter interactions used to

generate images and compositional information? What linkages can be made between the “technology

fundamentals” and the middle/high school science curriculum?

What is X-ray microanalysis?

X-ray microanalysis is the characterization of X-ray emissions due to the bombardment of matter with electrons.A bit of history:

Mosely (1913) – the frequency of emitted characteristic X-ray radiation is a function of the atomic number of the emitting element (X-ray spectrochemical analysis)Hillier (1947) and Marton (1941) – patented an optical microscopy/X-ray spectrochemical analyzerCastaing and Guinier (1949) – the first elctron microprobeCAMECA (1956) – first commercial electron microprobe (utilized wavelength dispersive spectroscopy technology)

Other forms of microanalysis

When e- interact with matter, a wide range of emissions are produced including:

– Elastic scattering emissions (backscattered electrons)– Inelastic scattering emissions

Secondary electrons Continuum X-rays Characteristic X-rays Auger electrons UV, IR and visible light (photons)

How are characteristic X-rays produces?

There are many forms of interactions, however, the production of characteristic X-rays is among the most widely used for analytical analysis.

N N N

incident e-

scattered e-

ejected e- X-ray

Suggested Curriculum LinksChemistry and Physics: quantum theory,

Bohr atom

Review of electron orbital structure

Electrons are organized into orbitals which are filled in increasing order of energy.

No. of e-

K 1s2 2L 2s2p6 8M 3s2p6d10 18N 4s2p6d10f14 32

N

X-ray

When an electron from a higher energy orbital makes a transition to a lower energy orbital, an X-ray is emitted with a characteristic energy. This energy is unique to the transition and element.

Suggested Curriculum LinksChemistry: electron orbitals,

Aufbau principle

Transitions and nomenclature

Transitions made to a shell are given the name of the shell. Transitions from one energy level above are designated as , from two levels above , etc.

K

L

M

N∞

K K

M M

K e

xcita

tion

Suggested Curriculum LinksChemistry: quantized energy

states

Charcteristic X-rays - examples

Element K L

Mg 1.254Ca 3.691 .341Fe 6.4 .705

Wavelength dispersive spectroscopy (WDS)

WDS utilizes the wave properties of X-rays to discern emitted X-ray energies. X-rays, like all wave energy, can be diffracted.

Some necessary background:

Diffraction of X-rays obeys Braggs law:

n=1, 2, 3, … , =X-ray wavelength, d=the spacing between atomic planes in a crystal, =the diffraction angle

)sin(2 dn

incident X-rays in phasediffracted X-raysout of phase

d

Suggested Curriculum LinksPhysics: light diffraction,

wave theory of matter

How does WDS work?

In WDS the emitted X-rays are diffracted by a crystal. The diffracted X-rays are counted by a detector. The intensity of the diffracted X-rays is recorded as a function of the diffraction angle.

sample

incident e-

X-ray detector

diffraction crystal

emitted X-rays

diffracted X-rays

Energy dispersive spectroscopy(EDS)

EDS uses a solid state X-ray detector (Li-drifted Si crystal). Incident X-rays produce a voltage pulse that is proportional in size to the incoming X-ray energy. In this way, information about all energies (wavelengths) is gathered simultaneously producing the entire X-ray spectrum without “scanning.”

sample

incident e-

solid stateX-ray detector

emitted X-rays

Suggested Curriculum LinksPhysics: semiconductor,

electron-hole pairs

So what can we do with these technologies?

Composition point analysis.

Composition mapping

Summary

EDS and WDS are among the most commonly used forms of SEM microanalysis.

Atoms ionized by incident e- radiation, emit X-rays upon relaxation. These X-rays are unique (characteristic) to the emitting element.

Wavelength dispersive spectroscopy (WDS) utilizes diffraction to discern between X-rays of differing wavelength (energy).

Energy dispersive spectroscopy (EDS) uses a solid state detector to convert X-rays of different energy into voltage pulses proportional to the incident energy.

WDS is more resolved than EDS, but data acquisition can be slower and the instrumentation is more expensive.

Both WDS and EDS can be used to do point analysis, composition mapping and other forms of microchemical imaging.