Materials Characterisation Techniques

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Materials Characterisation refers to the use of external techniques to probe into the
internal structure and properties of a material.
Characterisation can take the form of actual materials testing, or analysis, for example
in some form of microscope.
The principles of analytical methods for characterisation of materials for structure and
composition; optical microscopy, scanning electron microscopy, x-ray spectroscopy and
diffraction, atomic absorption, emission spectroscopy, and mass spectrometry.
CLASSIFICATION /DIVISION
Based on the information required, the charcaterisation of materials may be divided into
following groups:
1) Microscopic Analysis
2) Chemical Analysis
3) Thermal Analysis
4) Mechanical Analysis
5) NDT Analysis
Microscopic Analysis Techniques are used simply to magnify the specimen, to
visualise its internal structure, and to gain knowledge as to the distribution of elements
within the specimen and their interactions. Magnification and internal visualisation are
normally done in a type of microscope,such as:
o Optical Microscope
o Scanning Electron Microscope (SEM)
o Transmission Electron Microscope (TEM)
O Scanning Probe Microscope (SPM)
O Scanning Tunneling Microscope (STM)
O Atomic Force Microscope (AFM)
O Field Ion Microscope (FIM)

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Microscopic Characterisation of Materials
Optical Microscopy, one of three general categories of microscopy, entails
examination of materials using visible light to provide a magnified image of the micro-
and macrostructure.
Microscopy (microstructural examination) involves magnifications of approximately
50× or higher while macroscopy (macrostructural examination) is carried out at 50 × or
lower.
In scanning electron microscopy (SEM), the second category, the surface of the
specimen is bombarded with a beam of electrons to provide information for producing an
image.
In Transmission electron microscopy (TEM) consists of passing a beam of electrons
through a very thin specimen and analyzing the transmitted beam for structural
information.
Optical microscopy and, occasionally, SEM are used to characterize structure by
revealing grain boundaries,
phase boundaries,
inclusion distribution, and
evidence of mechanical deformation.
Scanning electron microscopy is also used to characterize
fracture surfaces,
integrated circuits,
corrosion products, and
other rough surfaces, especially when elemental microanalysis of small features is
desired.
Transmission electron microscopy is used to
Examine dislocation arrangements or structures and other small defects in metals and
alloys.
Second-phase particles not observable using optical metallography can frequently be
analyzed using TEM.

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Macro , Micro and Atomic Scale

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LIGHT MICROSCOPY
• Light microscopy in materials analysis generally refers to reflected light
microscopy.
In this method,
• light is directed vertically through the microscope objective and reflected back
through the objective to an eyepiece, view screen, or camera.
• Transmitted light is occasionally used for transparent and translucent materials.
For some low-magnification work (stereo microscopy), external, oblique
illumination can be reflected off the sample into the objective.
• Magnification of the sample image is obtained by light refraction through a
combination of objective lenses and eyepieces.
• The resulting images can be recorded either on traditional films or as digital files
for computer display, analysis, and storage.
What is a Microscope?
• A tool that magnifies and improves resolution of the components of a structure
• Has three components: one or more sources of illumination, a magnifying system,
and one or more detectors
• Light microscopes use a beam of light for illumination and include fluorescence
and confocal microscopes
• Electron microscopes use electrons as a source of illumination and include
transmission and scanning electron microscopes.
Light and Electron Microscopy
Light microscopy
Glass lenses
Source of illumination is
Usually light of visible wavelengths
• Tungsten bulb
• Mercury vapor or Xenon lamp
• Laser
Optical Microscopes: Limitations and Disadvantages
• The maximum magnification obtained with the optical microscope is about 2000X.
• The principal limitation is the wavelength of the visible light, which limits the
resolution of fine detail of the specimen.
Electron microscopy
Electromagnetic lenses
Source of illumination is electrons
•Hairpin tungsten
filament (thermionic emission)
•Pointed tungsten
crystal (cold cathode field emission)
•Lanthanum hexaboride

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INTRODUCTION (SEM)
• Method for hi-resolution imaging of surfaces.
• SEM uses electrons for imaging, much as a OM uses visible light. Hi- magnification >100,000X
and greater depth of field upto 100 times that of OM
• Qualitative & quantitative chemical analysis information is obtained by (EDS)
• Incident electrons cause electrons to be emitted from sample due to elastic & inelastic scattering.
• Hi-energy electrons that are ejected by an elastic collision of an incident electron, with a sample
atom’s nucleus, are referred to as BSE, BSE energy will be comparable to that of the incident
electrons.
• Low-energy electrons resulting from inelastic scattering are called SE. SE can be formed by
collisions with the nucleus where substantial energy loss occurs or by the ejection of loosely
bound electrons from the sample atoms. The energy of SE is typically 50 eV or less.
• Two electron detectors are used for SEM imaging. Scintillator type detectors (Everhart-
Thornley) are used for SE imaging. Detectors for BSE can be scintillator types or solid-state
detector
• SEM column and sample chamber use moderate vacuum to allow electrons to travel freely from
source to the sample and then to the detectors.High-resolution imaging is done with the chamber at
higher vacuum, (1 0-5
to 10-7
Torr.)
PRINCIPLE
• SEM generates a beam of incident electrons in an electron column above the sample chamber.
The electrons are produced by a thermal emission source, such as a heated W-filament, or by a
FEG cathode.
• Energy of the incident electrons can be as low as 100 eV or as high as 30 keV
• The electrons are focused into a small beam by a series of electromagnetic lenses in the column.
• Scanning coils near the end of the column direct and position the focused beam onto the sample
surface.
• Electron beam is scanned in a raster pattern over the surface for imaging.
• Beam can also be focused at single point or scanned along a line for x-ray analysis.
• Beam can be focused to a final probe diameter as small as about 10 Å.
• Incident beam is scanned in a raster pattern across the sample's surface. The emitted electrons
are detected for each position in the scanned area by an electron detector. The intensity of the
emitted electron signal is displayed as brightness on a cathode ray tube (CRT). By synchronizing
the CRT scan to that of the scan of the incident electron beam, the CRT display represents the
morphology of the sample surface area.
FUNCTIONS OF SEM
• To produce the microstructure images with high resolution and high depth of field of the samples.
• To provide the chemical analysis (composition) of micron size area of the structure revealed on
the surfaces.
• Use of Channeling Pattern to evaluate the crystallographic orientation and other information of
micron size region.
• Use of BSE detectors to reveal grain boundaries on unetched samples and domain boundaries in
the ferromagnetic alloys
Use of voltage contrast, electron beam induced currents (EBIC) and Cathodoluminescence for such
purposes as characterization and failure analysis of semiconductor devices.

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SIGNALS IN SEM
Beam Interaction
• Signal detection begins when a beam electron, known as the primary electron enters a specimen.
• When the primary electron enters a specimen it will probably travel some distance into the
specimen before hitting another particle.
• After hitting an electron or a nucleus, etc., the primary electron will continue on in a new
trajectory.
o This is known as scattering.
o It is the scattering events that are most interesting, because it is the components of the scattering events
(not all events involve electrons) that can be detected.
• The result of the primary beam hitting the specimen is the formation of a teardrop shaped reaction
vessel (Figure).
BACKSCATTERED ELECTRONS
• A primary beam electron may be scattered in such a way that it escapes back from the specimen but
does not go through the specimen.
• Backscattered electrons are the original beam electrons and thus, have a high energy level, near that of
the gun voltage.
• Operating in the backscattered imaging mode is useful when relative atomic density information in
conjunction with topographical information is to be displayed.

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SECONDARY ELECTRONS
• Perhaps the most commonly used reaction event is the secondary electron.
• Secondary electrons are generated when a primary electron dislodges a specimen electron from the
specimen surface.
• Secondary electrons can also be generated by other secondary electrons.
• Secondary electrons have a low energy level of only a few electron volts, thus, they can only be detected
when they are dislodged near the surface of the reaction vessel.
• Therefore, secondary electrons cannot escape from deep within the reaction vessel.
• Secondary electrons that are generated but do not escape from the sample are absorbed by the sample.
• Two of the foremost reasons for operating in the secondary electron imaging mode are to obtain
topographical information and high resolution.
• An excellent feature about imaging in the secondary mode is that the contrast and soft shadows of the
image closely resemble that of a specimen illuminated with light.
• Thus, image interpretation is easier because the images appear more familiar.
• Another advantage to viewing specimen in the secondary mode is that the primary beam
electrons may give rise to several secondary electrons by multiple scattering events, potentially
increasing the signal.
• With an increase signal and shallow escape depth of secondary electrons, the spatial resolution
in the secondary mode can be greater than in the other modes.
• Atomic density information also can also be obtained because some materials are better
secondary emitters than others.
• The use of secondary electrons to determine atomic number is not as reliable as with the
backscatter mode.
X-rays
• When electrons are dislodged from specific orbits of an atom in the specimen, X-rays are
emitted.
• Elemental information can be obtained in the X-ray mode, because the X-ray generated has a
wavelength and energy characteristic of the elemental atom from which it originated.
• Problems arise when the X-rays hit other particles, they lose energy, and this changes the
wavelength.
• As the number of hits increases, the x-rays will not have the appropriate energy to be classified
as coming from the originating element and detection of these X-rays will be known as background.
• X-ray spectrometer detectors measure avelength (wavelength Dispersive Spectrometer or WDS)
or energy level (Energy Dispersive Spectrometer or EDS).
• These are the two types of detectors used in X-ray analysis.
Cathode Luminescence
• Some specimen molecule's florescence when exposed to an electron beam.
• In the SEM, this reaction is called cathode luminescence.
• The florescence produces light photons that can be detected.
• A compound or structure labeled with a luminescent molecule can
be detected by using cathode luminescence techniques.
• Few SEM are equipped with capability of detecting photons.

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Transmitted electrons
• If the specimen is thin enough, primary electrons may pass through the specimen.
• These electrons are known as transmitted electrons and they provide some atomic density
information.
• The atomic density information is displayed as a shadow.
• The higher the atomic number the darker the shadow until no electrons pass through the
specimen.
Specimen Current
• When the primary electron undergoes enough scattering such that the energy of the electron is
decrease to a point where the electron is absorbed by the sample, this is known as specimen current.
• In most samples, the induced current is just led to ground.
• If not, the region being bombarded by the beam will build up a negative charge.
• This charge will increase until a critical point is reached and a discharge of electrons occurs,
relieving the pressure of the additional electrons.
• This is known as Charging and can be prevented by properly grounding and sometimes coating
samples with a conductive material.
Transmission electron microscopy
• (TEM) is principally quite similar to the compound light microscope, by sending an electron
beam through a very thin slice of the specimen. The resolution limit (in 2005) is around 0.05
nanometer.
• beam of electrons is transmitted through a specimen, then an image is formed, magnified and
directed to appear either on a fluorescent screen or layer of photographic film or to be detected
by a sensor (e.g. charge-coupled device, CCD camera.
• involves a high voltage electron beam emitted by a cathode, usually a tungsten filament and
focused by electrostatic and electromagnetic lenses.
• electron beam that has been transmitted through a specimen that is in part transparent to
electrons carries information about the inner structure of the specimen in the electron beam
that reaches the imaging system of the microscope.

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Differences between SEM and TEM
TEM SEM
Electron beam passes through thin sample. Electron beam scans over surface of
sample.
Specially prepared thin samples or particulate
material are supported on TEM grids.
Sample can be any thickness and is
mounted on an aluminum stub.
Specimen stage halfway down column. Specimen stage in the chamber at the
bottom of the column.
Image shown on fluorescent screen. Image shown on TV monitor.
Image is a two dimensional projection of the
sample.
Image is of the surface of the sample.

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Resolution of microscopes:
Microscope Resolution Magnification
Optical ± 200 nm ± 1000X
TEM ± 0.2 nm ± 500 000X
SEM ± 2 nm ± 200 000X
The scanning electron microscope:
The transmission electron microscope:

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Chemical analysis of the specimen can also be done in a number of ways:
o Energy-Dispersive X-ray spectroscopy (EDX)
o Wavelength Dispersive X-ray spectroscopy (WDX)
o Electron Energy Loss Spectroscopy (EELS)
o Auger electron spectroscopy
o Mass spectrometry (MS)
o X-ray photoelectron spectroscopy (XPS)

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TWO TYPES OF DETECTION
1) The resulting x-ray spectrum can be displayed according to energy:
• Energy Dispersive X-ray Spectroscopy – EDS requires energy-sensitive detector, usually
Si(Li) semiconductor
2) wavelength
• Wavelength Dispersive X-ray Spectroscopy – WDS requires x-ray monochromator and a
propotional counter
Advantages of WDX analysis over EDX analysis:
• much better energy resolution, preventing many peak overlap errors frequently encountered in EDX
analysis;
• lower background noise allowing a more accurate quantitative analysis.
Disadvantages:
• higher time consumption;
• greater sample damage and chamber contamination because of the high beam currents required;
• high cost.
Thermal Analysis Techniques
• Thermal analysis is a group of techniques in which changes of physical or chemical properties of
the sample are monitored against time or temperature, while the temperature of the sample is
programmed. The temperature programmer involve heating or cooling at a fixed rate, holding the
temperature constant (isothermal), or any sequence of these.

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Where is Thermal Analysis used?
• Thermal Analysis techniques are used in virtually every area of modern science and technology.
• The basic information that these techniques provide, such as crystallinity,specific heat and
expansion, are relied on heavily for the research and development of new products.
• Thermal Analysis techniques also find increasing use in the area of quality control and assurance,
where demanding requirements must be met in an increasingly competitive world.

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DILATOMETER
• A dilatometer is a scientific instrument that measures volume changes caused by a physical or
chemical process.
• Dilatometers have been used in the fabrication of metallic alloys, compressed and sintered
refractory compounds, glasses, ceramic products, composite materials, and plastics.
Thermo-mechanical Analyser (TMA)
• Thermo-mechanical analysis (TMA) is used to determine the deformation of a sample (changes
in length or thickness) as a function of temperature. The measuring range may extend from -150
°C to +600 °C.
Differential Scanning Calorimetry (DSC)
• Differential scanning calorimetry (DSC) is a technique used to study what happens to polymers
when they're heated. By studying the thermal transitions occurring within the material
information is acquired on the nature of the polymer.
• In this technique the heat flow rate to the sample is monitored.
Applications include (DSC):
_ Melting temperature
_ Glass transition
Temperature of polymers
_ Crystallinity of polymers
_ Polymorphism of polymers
_ Purity
_ Heat capacity
_ Curing reactions
_ Reaction kinetics
_ Decomposition
Thermogravimetry Analysis (TGA)
• Thermogravimetric analysis involves heating a sample in an inert or oxidising atmosphere and
measuring the weight. The weight change over specific temperature ranges provides indications
of the composition of the sample and thermal stability.
• Measurements of changes in sample mass with temperature are made using a thermobalance.
Applications include (TGA):
_ Asesssment of moisture and volatiles
_ Asesssment of composition
_ Thermal stability & Oxidative stability
_ Decomposition kinetics
WHAT CAN THERMAL ANALYTICAL TECHNIQUES MEASURE?
•Glass transitions
•Melting and boiling points
•Crystallisation time and
temperature
•Percent crystallinity

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•Heats of fusion and reactions
•Specific heat capacity
•Oxidative/thermal stability
•Rate and degree of cure
•Reaction kinetics
•Purity
Non-Destructive Characterisation of Materials
• Conventional test methods, such as overlap shear test, compression shear and cyclic fatigue all
result in the destruction of the joint.
• A number of non-destructive test (NDT) methods are available, but their use is currently limited
to a few industries.
• NDT methods include:
Visual Inspection, Tap Test, Eddy Current
X-ray Radiography, Ultrasonic Testing
Acoustic Emission
Chemical Composition by NDT
• The XRF is widely used for the qualitative and
quantitative elemental analysis of metal, ceramic, glass, polymer, composites, rubber, paint, jewellery,
fuel, food, and forensic samples.
• XRF is a non-destructive, multielemental, fast and cost-effective technique with detection
limit from a 100% to few parts per million (ppm) in the range of Sodium to Uranium of the
periodic table.
Fundamentals of X-Ray Fluorescence Spectroscopy
• XRF is based on the principle that individual atoms, when excited by an external energy source,
emit X-ray photons of a characteristic energy or wavelength.
• By counting the number of photons of each energy emitted from a sample, the elements present
may be identified and quantitated.

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