Scanning Electron Microscope (SEM)
High Temperature Materials KGP003
Div. of Process Metallurgy Spring 2006
SCANNING ELECTRON MICROSCOPY (SEM): In the scanning electron microscope (SEM) a very fine 'probe' of electrons with energies up to
40 keV is focused at the surface of the specimen in the microscope and scanned across it in a
'raster' or pattern of parallel lines. A number of phenomena occur at the surface under electron
impact: most important for scanning microscopy are the emission of secondary electrons with
energies of a few tens eV and re-emission or reflection of the high-energy backscattered
electrons from the primary beam. The intensity of emission of both secondary and
backscattered electrons is very sensitive to the angle at which the electron beam strikes the
surface, i.e. to topographical features on the specimen. The emitted electron current is
collected and amplified; variations in the resulting signal strength as electron probe is scanned
across the specimen are used to vary the brightness of the trace of a cathode ray tube being
scanned in synchronism with the probe. There is thus a direct positional correspondence
between the electron beam scanning across the specimen and the fluorescent image on the
cathode ray tube.
The magnification produced by scanning microscope is the ratio between the dimensions of
the final image display and the field scanned on the specimen. Usually magnification range of
SEM is between 10 to 200 000 X. and the resolution (resolving power) is between 4 to 10 nm
(40 - 100 Angstroms) .
There are many different types of SEM designed for specific purposes ranging from routine
morphological studies, to high-speed compositional analyses or to the study of environment -
sensitive materials. Our laboratory has a combination SEM and EDS, which in combination,
provide a powerful analytical approach for many research or quality control applications.
Here is example of using SEM in the material research.
The left image shows a Nimonic 115 heat and corrosion resistant wrought nickel alloy that
has not been exposed to excessive overheating. There is no evidence of significant growth of
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gamma prime precipitates. By using the backscattered electrons, two phases differing only
slightly in composition, can be distinguished.
The right image shows the effect of overheating. The material was exposed to a temperature
of 1200 C. The grain boundary has been effected, two phases have disappeared and the
structure of the edges has changed. The dark lobes correspond to an aluminum rich phase. A
typical triangular precipitate at the end of the "crack" can also be seen. It is composed of
titanium with a few percent of nitrogen.
ENERGY DISPERSIVE SPECTROSCOPY (EDS): Chemical analysis (microanalysis) in the scanning electron microscope (SEM) is performed
by measuring the energy or wavelength and intensity distribution of X-ray signal generated by
a focused electron beam on the specimen. With the attachment of the energy dispersive
spectrometer (EDS) or wavelength dispersive spectrometers (WDS), the precise elemental
composition of materials can be obtained with high spatial resolution. When we work with
bulk specimens in the SEM very precise accurate chemical analyses (relative error 1-2%) can
be obtained from larger areas of the solid (0.5-3 micrometer diameter) using an EDS or WDS.
Bellow is an example of EDS spectrum collected in the SEM with EDS. The spectrum shows
presence of Al, Si, Ca, Mn and Fe in the steel slag phase.
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THE PRINCIPLES BEHIND SEM:
Electron gun
Condenser lenses
Aperture
Detector
SEM sampleVacuum pump
Vacuum pump
Scan coils
Objective lens
Electron Beam In a SEM the electron beam energy and current are variables. The voltage is variable from
about 1 - 60keV and the current from 1e-7 to 1e-12 A. These are rough guidelines - the exact
ranges available depend on the instrument type and manufacturer.
Condenser lenses These magnetic lenses focus an image of the electron gun onto the aperture, in order to
spatially filter the electron beam.
Aperture Controls the spread of electrons in the column by de-selecting off-axis electrons. Vacuum pumps The specimen chamber and the column must both be evacuated, in order to prevent scattering
of the beam electrons by air molecules.
Electron gun Several different types of electron gun are used in SEMs, the most primitive being a simple
tungsten "hairpin" filament. The filament is heated by passing a current through it, and
electrons are then emitted via the process of thermionic emission. The effective size of this
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electron source (and hence the final electron spot size) can be reduced by instead using a
single crystal of lanthanum hexaboride (LaB6). Still finer resolution can be achieved using an
almost atomically sharp single crystal tungsten tip, which allows electrons to be emitted by a
field emission process. Scan coils These coils deflect the beam in order to scan the electron spot in a raster pattern across the
sample. Objective lens This magnetic lens focuses the electron beam onto the sample.
Detector At each beam position, some quantity or characteristic of the sample must be measured, so
that an image can be built up. The most commonly measured quantity is the rate of secondary
electrons emitted from the sample, and this is detected by using a scintillator in conjunction
with a photomultiplier.
SEM sample Conducting samples provide a path to earth for the beam electrons, and therefore require no
special preparation. Insulating materials, however, require a thin coating of a conductor (often
carbon or gold) in order to prevent charging.
DIFFERENT SEM MODES: Primary electron imaging When an electron from the beam encounters a nucleus in the sample, the resultant Coulombic
attraction results in the deflection of the electron's path, known as Rutherford elastic
scattering. A few of these electrons will be completely backscattered, re-emerging from the
incident surface of the sample. Since the scattering angle is strongly dependent on the atomic
number of the nucleus involved, the primary electrons arriving at a given detector position
can be used to yield images containing both topological and compositional information. Secondary electron imaging The high-energy incident electrons can also interact with the loosely bound conduction band
electrons in the sample. The amount of energy given to these secondary electrons as a result
of the interactions is small, and so they have a very limited range in the sample (a few nm).
Because of this, only those secondary electrons that are produced within a very short distance
of the surface are able to escape from the sample. This means that this detection mode boasts
high-resolution topographical images, making this, the most widely used of the SEM modes.
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Energy-Dispersive analysis of X-rays (EDX) Another possible way in which a beam electron can interact with an atom is by the ionization
of an inner shell electron. The resultant vacancy is filled by an outer electron, which can
release it's energy either via an Auger electron or (more importantly here) by emitting an X-
ray. This produces characteristic lines in the X-ray spectrum corresponding to the electronic
transitions involved. Since these lines are specific to a given element, the composition of the
material can be deduced. This can be used to provide quantitative information about the
elements present at a given point on the sample, or alternatively it is possible to map the
abundance of a particular element as a function of position.
Linescan (qualitative) The electron probe traverses along the line on the surface of stationary sample. Line scanning
produces graphs of elemental distributions across boundaries and through phases in sample.
This method is very illustrative for showing concentration gradients e.g. at grain or phase
boundaries.
Linescan (quantitative) The specimen is traversed along a predetermined line at steady rate with the help of the stage
stepping motors. After every step the specimen stops and quantitative element analysis is
performed. The number and the length of steps on the line are controllable.
Element map Along with a video image the distribution of predetermined elements is obtained for the
observed image area. The video and element images can easily be superimposed into a
composite image. This is excellent eg. for observing adjoining phases of different
composition, microchemical composition of inclusions in steel, and distribution of impurities
on the sample surface.
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EXAMPLES:
SEM-X-ray mapping
This Micrograph shows a secondary electron image of the cadmium telluride surface of
a polycrystalline CdS/CdTe cell.
Iron oxide Cancer Cells
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X-RAY DIFFRACTION (XRD)
High Temperature Materials
KGP003 Div. of Process Metallurgy
Spring 2006
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Background: X-rays are electromagnetic radiation of wavelength about 1 Å (1x10-9 m), which is about the
same size as an atom. They occur in that portion of the electromagnetic spectrum between
gamma rays and the ultraviolet. The discovery of X-rays in 1895 enabled scientists to probe
crystalline structure at the atomic level. X-ray diffraction has been in use in two main areas,
for the fingerprint characterization of crystalline materials and the determination of their
structure. Each crystalline solid has its unique characteristic X-ray pattern that may be used as
a "fingerprint" for its identification. Once the material has been identified, X-ray
crystallography may be used to determine its structure, i.e. how the atoms pack together in the
crystalline state and what the interatomic distance and angle are etc. X-ray diffraction is one
of the most important characterization tools used in solid-state chemistry and materials
science. The XRD technique requires only a small amount of material and is non-destructive
so that further analysis can be completed on the same material afterwards.
The workings of a modern x-ray diffractometer are quite complex but the general layout and
geometry is given in Figure 1. The basic components of an XRD diffractometer are the x-ray
source, the sample specimen and an x-ray detector.
Fig. 1 Arrangement and geometry of an x-ray diffractometer The X-ray radiation most commonly used is that emitted by copper, whose characteristic
wavelength for K radiation is =1.5418Å. When the incident beam strikes a powder sample,
diffraction occurs in every possible orientation of 2θ. The diffracted beam may be detected by
using a moveable detector such as a Geiger counter, which is connected to a chart recorder. In
normal use, the counter is set to scan over a range of 2θ values at a constant angular velocity.
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Routinely, a 2θ range of 5 to 70 degrees is sufficient to cover the most useful part of the
powder pattern. The scanning speed of the counter is usually 1-2 degrees/min and therefore,
about 30 minutes to 1 hour is needed to obtain a trace.
Theory: A crystal system can be thought of as a structure built from unit cells that, when stacked, fill
three-dimensional space. There are seven discrete unit cell shapes in three-dimensional space.
These seven unit cell shapes are known as the seven crystal systems: triclinic, monoclinic,
orthorhombic, tetragonal, hexagonal, rhombohedral, cubic. Each point in a unit cell is an
atom within the crystal structure. Hence, the physical aspects of crystal structure on an
atomic level are possible to investigate using XRD.
The concept of XRD is based on Bragg’s law, λ=2dsinθ. Where λ is wavelength, d is the
interplanar distance between atoms in the crystal and θ is the incident and reflection angle of
x-rays.
Fig. 2 Reflection of x-rays from two planes of atoms in a solid Figure 2 illustrates the path difference between two x-rays reflecting from two planes of
atoms in a crystal:
2λ = 2dsinθ
For constructive interference (destructive interference cancels the reflection) between these x-
rays, the path difference must be an integral number of wavelengths:
This leads to the Bragg equation:
nλ= 2dsinθ
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Figure 3 shows the x-ray diffraction pattern from a single crystal of layered clay. Strong
intensities can be seen for a number of values of n, i.e. different atomic planes; from each of
these lines we can calculate the value of d, the interplanar spacing between the atoms in the
crystal.
Fig. 3 X-ray diffraction pattern from layered structure vermiculite clay EXAMPLE: Unit Cell Size from Diffraction Data
The diffraction pattern of copper metal was measured with x-ray radiation of wavelength of
1.54 Å. The first order Bragg diffraction peak was found at an angle 2θ of 69.32 degrees.
Calculate the spacing between the diffracting planes in the copper metal.
The Bragg equation is:
nλ = 2dsinθ
We can rearrange this equation for the unknown spacing d:
d = nλ/2sinθ
θ is 34.66 degrees, n =1, and wavelength = 1.54 Å, and therefore
d= 1 x 1.54/(2 x 0.5687) = 1.354 ÅPractical Application: What criteria should a powder sample have to be suitable for XRD?
• Suitable grain size
The size should be between 70 μm (high symmetry) of 50 μm (lower symmetry) to
about 0.5 μm. Overly small grain sizes give peak broadening. Overly large grain
sizes give preferred orientation and “spotty” peaks. Use a mill, but not for too long or
you will create amorphous material! (noncrystalline).
• Randomly oriented crystals
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The sample loading into the specimen holder is important. Do not work with the sample
more than necessary, otherwise preferred orientation may occur and alter the relative
intensity of the resulting peaks.
• Flat, smooth and aligned
Theoretically a curved sample would be optimal (better focusing) but most specimen
holders have a flat design for ease of loading. The sample must be aligned at the correct
height, otherwise a height error will shift the 2θ scale.
• Suitable sample area
The length of the x-ray beam is about 12 mm on the sample and the width is controlled by
slits. We often use variable slits so that the width is 20 mm independent of θ. If the
sample size is too small, the intensity will be lowered.
• Suitable sample thickness
The x-rays penetrate a few micrometers into the sample surface, therefore, the sample
should be thicker than this.
• Homogenous
If a standard powder is added to the sample, mix them thoroughly in order to maintain a
relative homogenous sample.
• Avoid using amorphous sample holders
If you still want to use for instance a glass slide as sample holder, take in to account that
the glass can add amorphous pattern to your sample original highly crystalline pattern.
How to identify an unknown sample with XRD?
• Identify the three most intense reflections and determine their d-spacing
• Search for corresponding d-values in a reference (Hanawalt Search Manual, computer
software such as EVA), start with the most intense peak and work in descending order
of peak intensity
• After finding the d-values which correspond the best with d-values from the sample
compare the intensities of the sample reflections with the reference reflections
• Compare d and intensity values for the entire diffractogram, only when full agreement
with reference values has been reached is the identification of unknown sample
complete.
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THERMAL ANALYSIS
-4
-2
0
2
4
DSC /(mW/mg)
60.0
65.0
70.0
75.0
80.0
85.0
90.0
95.0
100.0
TG /%
50 100 150 200 250 300Temperature /°C
-0.5
0
0.5
1.0
1.5
Ion Current *10-8 /A
Sample: NaHCO3
(theor. 36.90%)
0.25 ml CO2 pulse
1 ml CO2 pulse
amu 44 (CO2)
0.436 mg 1.745 mg-> 1.34 mg evolved(theor. 1.30 mg)
2 NaHCO3 -> Na2CO3 + H2O + CO2 157.7 °C
452.04 J/g
-36.80 %
↓ exo
High Temperature Materials KGP003
Div. of Process Metallurgy Spring 2006
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Background:
Thermal analysis includes a group of methods by which the physical and chemical properties
of a substance, a mixture and/or reaction mixtures are determined as a function of
temperature or time, while the sample is subjected to a controlled temperature program. The
program may involve heating or cooling (dynamic), or holding the temperature constant
(isothermal), or any combination of these.
TGA stands for Thermo-Gravimetric Analysis. TGA can be used for the determination of
decomposition weight loss, combustion analysis, temperature stability, and moisture content
and reaction mechanism.
TGA monitors weight versus temperature. It can detect changes in weight of 1 ug. This is
accomplished with an extremely sensitive balance hanging inside a furnace. A thermocouple
mounted just a few millimeters from the sample pan ensures accurate temperature of the
sample. There are several events that can be calculated for any run.
Thermal Analysis methods: Differential Thermal Analysis (DTA) Differential Scanning Calorimetry (DSC) Thermogravimetry (TG) Simultaneous Thermal Analysis (STA) Thermomechanical Analysis (TMA) Dilatometry (DIL) Dynamic Mechanical Analysis (DMA) Combined (Hyphenated) Techniques (TA - MS, TA - FT-IR, PTA) Thermal Conductivity Testing (TCT) Thermal Diffusivity Measurement (LFA) Refractories Testing (RUL, CIC, MOR)
THERMOGRAVIMETRY (TG):
This is a technique by which the mass of the sample is monitored as a function of
temperature or time, while the sample is subjected to a controlled temperature program.
Dm = mass change dm/dt = rate of mass change/decomposition DTG = derivative thermogravimetry DTG Peak = characteristic decomposition temperatures ® identification Tonset = thermal stability Composition = moisture content, solvent content, additives, polymer content,
filler content , dehydration , decarboxylation, oxidation, decomposition.
The curve of thermogravimetry analysis is build around a furnace where the sample is
mechanically connected to an analytical balance. The first thermobalance was developed by
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K. Honda in 1915. I t be came widespread from 1950s when the derivated thermogravimetric
(DTG) was solved.
Balance, furnace and control/data handling system are the three essential parts of a modern
TG instrument. There are three main possibilities to place the sample relative to the balance:
a- suspended, b- horizontal, c- top-loading see figure 2.
Figure1 – Schematic of Thermogravimetry system
A B C
Figure 2 – Position of the sample in thermobalance
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DIFFERENTIAL THERMAL ANALYSIS (DTA):
DTA is a technique measuring the difference in temperature between a sample and a
reference (a thermally inert material) as a function of time or the temperature, when they
undergo a temperature scanning in a controlled atmosphere. The DTA method enables any
transformation to be detected for all the categories of materials.
Applications: Melting and crystallisation behavior, Heat of melting and crystallisation, Heat
of reaction, Reaction kinetics, Glass transitions, Oxidative stability, Thermal stability.
DIFFERENTIAL SCANNING CALORIMETRY (DSC):
DSC is a technique which determines the variation in heat flow released or captured by a
sample when it undergoes temperature scanning in a controlled atmosphere. When heating or
cooling, any transformation taking place in a material is accompanied by an exchange of
heat; DSC enables the temperature of this transformation to be determined and the heat from
it to be quantified. Calibration is necessary.
1. Power compensated DSC
2. Heat flux DSC
3. Applications, see DTA
SIMULTANEOUS THERMAL ANALYSIS (STA):
This technique combines thermogravimetry with differential thermal analysis or differential
scanning calorimetry in one run.
- Possible to consider the real sample mass at a given temperature in Cp determination.
- No temperature differences between signals of TG and DTA/DSC measurement.
Simultaneous Thermal Analysis (STA) is used for the following applications:
Characteristic temperatures
Identification
Glass transitions
Melting and crystallization behavior
Heat of melting and crystallization
Polymorphism
Solid-liquid ratio
Specific heat capacity
Reaction behavior
Heat of reaction
Reaction kinetics
Oxidative stability
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COMBINED (HYPHENATED) TECHNIQUES:
With these thermoanalytical methods, for example TG-DTA-MS (Thermogravimetric-
Differential Thermal-Mass Spectrometric analysis), the gases evolved from the sample
during a thermal analysis experiment are detected/analyzed.
APPARATUS (STA 409C)
The STA 409C instrument consists of the following main parts (see figure 3):
Registration control cabinet, Measuring part, Computer system with printer, Thermostat,
Evacuating system (vacuum pump)
20 °
10
control cabinet
TA-controller measuring part Computer sustem
printer
thermostatvacuum pump
Figure 3 – Assembly diagram of STA 409 C/7/E measuring equipment
The schematics of STA 409C and STA with QMS are shown in figures 4 and 5 respectively.
The balance and furnace are sequentially evacuated and purged with inert gas. Two programs
are used with this instrument. One for setting operating conditions, running the STA and data
acquisition, another for evaluating the data obtained. In order to avoid buoyancy effect, a
TG/DTA correction must be run with empty crucibles and the data obtained will be used as
reference when running with a crucible containing sample.
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Gas outlet
Furnace
Sample carrier
radiation shield
protective tube
vacuum
reactive gasprotective gas
inductive displacement transducerelectromagnetic compensation system
vacuum tight casing
DSC and TG carrier
thermostatic control
evacuation system
Figure 4 - Scheme of STA-409 C (B).
Figure 5 – Schematic of Skimmer
THE QUADRAPOLE MASS SPECTROMETER: The quadrapole mass filter was originally proposed by W. Paul Its basic design is illustrated
in figure 6. In a high-frequency, quadrupole electric field, which in the ideal case is generated
by four hyperbolic rod electrodes a distance of 2ro apart at the tips, it is possible to separate
ions according to their mass/charge ratio (m/e). The hyperbolic surfaces are approximated
with sufficient accuracy by cylindrical rods of circular cross-section. The voltage between
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these electrodes is composed of high-frequency alternating component Vcoswt and a
superposed direct voltage U.
Figure 6 – Structure of quadrapole analyser
Classification of TG curve
Figure 7 shows different types of TG curves can be obtained from TA 409C equipment. Their
classification is as following:
Type A. It shows no mass change over temperature range selected. DSC can be used to investigate if non - mass changing processes have occurred.
Type B. Large initial mass loss followed by mass plateau evaporation of volatile compounds during polymerization drying and deposition give rise to such a curve.
Type C. Single stage decomposition. Type D. Multi-stage decomposition, where the reaction steps are easily resolved. Type E. The individual reaction steps are not well resolved, DTG curve is preferred to
characterize this type of curve. Type F. Presence of an interacting atmosphere, a mass increase is observed, surface
oxidation reactions. Type G. Mass increase followed by decomposition, such as surface oxidation followed by
decomposition of the reaction products.
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A
B
C
D
E
F
G
MA
SS IN
CR
EA
SE
TEMPERATURE 1°C
Figure 7 – Behavior of different TG curves.
Behavior of TG/DTA curves
TG DTA/DSC Glass Transition
Melting
Crystalisation
Evaporation
Sublimation
Decomposition
Reduction
Oxidation MA
SS IN
CR
EA
SE
dH/d
t, E
ND
O
Figure 8 – Comparison of TG and DTA curves
EXAMPLE: Decomposition of Calcium oxalate ( CaC2O4.H2O)
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The behavior of calcium oxalate (CaC2O4.H2O) in inert atmosphere is shown in figures 9 and
10. Figure 9 illustrates a thermogram obtained from TG + QMS analysis. It shows the weight
loss % and the coinciding evolution of gas with different composition during heat treatment of
CaC2O4.H2O. The following mass units can be seen: 18 (H2O), 28 (CO), 44 (CO2). Figure 10
is a recorded thermogram obtained from TG analysis by increasing the temperature of pure
calcium oxalate at a rate 5°C/min. The clearly defined horizontal regions correspond to
temperature ranges in which the indicated calcium compounds are stable. The decomposition
of calcium oxalate in inert atmosphere is as follows:
Temperature °C Compounds Residue Volatiles
100 - 226 CaC2O4.H2O CaC2O4 H2O
346 - 420 CaC2O4 CaCO3 CO
660 - 840 CaCO3 CaO CO2
Figure 9 – TG+QMS of the Calcium oxalate treated in helium atmosphere.
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CaC2O4.H2O
CaC2O4
CaCO3
CaO
CO
H2O
CO2
100 °
226 °
420 ° 660 °
840 ° 980 °
346 °
Wei
ght,
g
Temperature, °C
Figure 10 – Decomposition of CaC2O4.H2O in an inert atmosphere
Figure 11 shows the derivative of the thermogram shown in figure 10. This derivative curve
may reveal information that is not identified in ordinary thermogram. For example, the three
peaks at 140 °C, 180 °C and 205 °C, are related to three hydrates which lose moisture at
different temperatures. At 450 °C, CO results during weight loss and at 780 °C, CO2 results
during weight loss.
140 °
dm/d
t
Temperature, °C
180 ° 205 °
450 °
780 ° 1030 °
Figure 11 – DTG of CaC2O4.H2O in an inert atmosphere (4).
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