Lecture - 9 - Microscopy

7/27/2019 Lecture - 9 - Microscopy

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Lecture 9Image Analysis


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Nature Material: Macro, micro, nano

10 centimtres

1X

1 centimtre

10X

100X

1 millimtre

100 microns

1000X

10 microns

10,000X

1 micron

100,000X

1,000,000X

100 nanomtres

10,000,000X

10 nanomtres

100,000,000X

1 nanomtre


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Microstructural Features which concern us

Grain SizeGrain Shapes

Precipitate Size: mostly in the micron regime

Volume fraction and distribution of various phases

Defects such as cracks voids

Atom orientation within crystal

Microstructrue ranging from crystal structure to engine components (e.g Si3N4)


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Characteristics Information : SEM

Crystallographic Information !!How the atoms are arranged in the object; direct relation

between these arrangements and material properties


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Scanning Electron Microscope (SEM) The scanning electron microscope (SEM) is a type of electron microscope that takes images of the surface of

samples by focusing a high energy beam of electrons onto the sample.

The electrons interact with the atoms that make up the sample producing signals that give information aboutthe sample.

Electronic devices are used to capture or detect these signals and either allow them to expose film or, mostcommon today, create an image on a computer screen.

The type of signals made by an SEM can include secondary electrons, characteristic x-rays, and back scatteredelectrons.

In an SEM, these signals come from the beam of electrons striking the surface of the specimen and interactingwith the sample at or near its surface

The SEM has allowed researchers to examine a much bigger variety of specimens The scanning electron microscope has many advantages over traditional microscopes. The SEM has a large

depth of field, which allows more of a specimen to be in focus at one time.

The SEM also has much higher resolution, so closely spaced specimens can be magnified at much higher levels

Because the SEM uses electromagnets rather than lenses, the researcher has much more control in the degreeof magnification.

All of these advantages, as well as the actual strikingly clear images, make the scanning electron microscopeone of the most useful instruments in research today.

SEM micrographs have a very large depth of focus yielding a characteristic three-dimensional appearanceuseful for understanding the surface structure of a sample.

Characteristic x-rays are the second most common imaging mode for an SEM.

These characteristic x-rays are used to tell the chemical composition of the sample.

Back-scattered electrons (BSE) that come from the sample may also be used to form an image.


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Sample Preparation for SEM SEM utilizes vacuum conditions and uses electrons to form an image, special preparations must be done

to the sample.

All water must be removed from the samples because the water would vaporize in the vacuum.

All metals are conductive and require no preparation before being used. All non-metals need to be made conductive by covering the sample with a thin layer of conductive

material. This is done by using a device called a "sputter coater.

The sputter coater uses an electric field and argon gas. The sample is placed in a small chamber that isat a vacuum. Argon gas and an electric field cause an electron to be removed from the argon, makingthe atoms positively charged.

Specimen at high vacuum requires sample fixation and dehydration or freezing.

Minimized charging by coating sample with metal or carbon or lowering the operating kV


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How does SEM work?

What happens when the Electron Beam hits the sample?

SEM Setup

Electron/Specimen InteractionsWhen the electron beam strikes the sample, both photon and electron signals are emitted


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Secondary Electron

Generation

-SEM-SE

-sample electrons ejected by the

primary beam [green line]

-low energy

-surface detail & topography

SEM Imaging Modes

SE

Backscattered Electron

Generation

-SEM-BSE

-primarybeam electrons

-high energy

-composition and topography

[specimen atomic number]

BSE


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Identification of Fracture Mode

SEM micrographs of fracturedsurface of two BaTiO3 samples

D.Sarkar, B.Basu, M.J.Chu and S.J.Cho, R-Curve Behavior of Ti3SiC2, Ceramics International, 33 [5] 789 -793, 2007.


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EDX EDX is an analytical technique used for the elemental analysis or chemical

characterization of a specimen.

it relies on the investigation of a sample through interactions betweenelectromagnetic radiation and matter, analyzing X-rays emitted by the matter inthis particular case.

Each element has a unique atomic structure allowing x-rays to be uniquely

distinguished from each other.

A graph is plotted (x-ray energy vs. count rate). The peaks correspond to characteristic elemental emissions.


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Nanoscale MaterialsNanowires and Nanotubes

Lateral dimension: 1 100 nm Nanowires and nanotubes exhibit novel

physical, electronic andoptical properties due to Two dimensional quantum

confinement

Structural one dimensionality High surface to volume ratio

Potential application in wide range ofnanodevices and systems Nanoscale sensors and actuators

Photovoltaic devices solar cells Transistors, diodes and LASERs

Nanowire Solar Cell: The nanowirescreate a surface that is able to absorb

more sunlight than a flat surface


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Nanofluids

Aluminum Oxide

Particles in Water

Nanofluids are engineered colloids = base fluid (water, organic liquid) + nanoparticles

Nanoparticle size: 1-100 nm

Nanoparticle materials: Al2O3, ZrO2, SiO2, CuO, Fe3O4, Au, Cu, C (diamond, PyC, fullerene) etc.

Previous studies suggest significant enhancement of:

- Thermal conductivity (+40%)

- Single-phase convective

heat transfer (+40%)

- Critical Heat Flux (+100%)


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B i P i i l f TEM


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Basic Principle of TEM

Electron source, electromagnetic lens system, sample holder, and imaging system

Electron Source

The electron source consists of a cathode and an anode. The cathode is

a tungsten filament which emits electrons when being heated. A negative

cap confines the electrons into a loosely focused beam. The beam is

then accelerated towards the specimen by the positive anode. Electrons

at the rim of the beam will fall onto the anode while the others at the

center will pass through the small hole of the anode. The electron source

works like a cathode ray tube.

Electromagnetic lens system

After leaving the electron source, the electron beam is tightly focused using electromagnetic lens and metal

apertures. The system only allows electrons within a small energy range to pass through, so the electrons in the

electron beam will have a well-defined energy.

1.Magnetic Lens: Circularelectro-magnets capable of generating a precise circular magnetic field. The field acts

like an optical lens to focus the electrons.

2.Aperture: A thin disk with a small (2-100 micrometers) circular through-hole. It is used to restrict the electronbeam and filter out unwanted electrons before hitting the specimen.

Sample Holder


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Sample Holder

The sample holder is a platform equipped with a mechanical arm for holding the specimen and controlling its

position.

Imaging system

The imaging system consists of another electromagnetic lens system and a screen. The electromagnetic lenssystem contains two lens systems, one for refocusing the electrons after they pass through the specimen, and the

other for enlarging the image and projecting it onto the screen. The screen has a phosphorescent plate which

glows when being hit by electrons. Image forms in a way similar to photography.

Working principle

TEM works like a slide projector. A projector shines a beam of light which transmits through the slide. The patterns

painted on the slide only allow certain parts of the light beam to pass through. Thus the transmitted beam

replicates the patterns on the slide, forming an enlarged image of the slide when falling on the screen.

TEMs work the same way except that they shine a beam of electrons (like the light in a slide projector) through the

specimen (like the slide). However, in TEM, the transmission of electron beam is highly dependent on the

properties of material being examined. Such properties include density, composition, etc. For example, porous

material will allow more electrons to pass through while dense material will allow less. As a result, a specimen with

a non-uniform density can be examined by this technique. Whatever part is transmitted is projected onto aphosphor screen for the user to see.

The following movie will help you to understand more about the operation of a TEM:

Imaging Mode of TEM: Bright Field Mode

Dark Field Mode

Diffraction Mode


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In a TEM, the specimen you want tolook at must be of such a low

density that it allows electrons to

travel through the tissue

There are different ways to prepare

your material for that purpose. You

can cut very thin slices of your

specimen from a piece of tissue

either by fixing it in plastic or

working with it as frozen material.

Another way to prepare your

specimen is to isolate it and study a

solution of for example viruses or

molecules in the TEM.

Preparation of Specimen


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TEM Sample Prep for Materials


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How to Measure SAED (Selected Area Electron Diffraction)

Au (111) ring [2.35 d-spacing]

With 200KV, (111) ring should distance Rfrom the transmitted beam

Rdhkl=lL

R is measuredd inter atomic distancel is the electron wavelengthL is the camera length(lL is the camera constant)

Think of TEM as a diffraction camera

Transmitted Beam

Diffracted Beam

L

R


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Electron DiffractionFour conditions in Back Focal Plane (BFP) of the

objective lens: No sample No reflections (only transmitted beam)

Amorphous Transmitted beam + random scattering

Polycrystal Transmitted beam + rings

Single crystal Transmitted beam + spots

Metal particles Polymer mix

Electron Diffraction

Bright field TEM of FeNbO4 and (1 wt %) Pt

impregnated FeNbO4. (Inset) SAED pattern

of FeNbO4

Particle Size : 30-60nm


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TiO2 Nanoscale Rods

TEM images of Cu on ZnO (a model methanol catalyst)


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(b) bright field image

TEM Characterization of TiO2 Nanoparticles

The structure of all as-grownsamples is anatase.

The particle sizes from TEM rangebetween 15 and 25 nm.

(a) dark field image (c) diffraction patterns

(d) lattice image

20nm

(d) Lattice Image

Cr stallite Si e is Different than Particle Si e


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Crystallite Size is Different than Particle Size

A particle may be made up of several different crystallites

Crystallite size often matches grain size, but there are exceptions

Grain boundaries are interfaces where

crystals of different orientations meet.

A grain boundary is a single-phase

interface, with crystals on each side of

the boundary being identical except in

orientation. The term "crystallite

boundary" is sometimes, though

rarely, used. Grain boundary areas

contain those atoms that have been

perturbed from their original lattice

sites, dislocations, and impurities that

have migrated to the lower energygrain boundary.

Crystallite is the average size of the particle whereas the

particle size denotes the individual size of the particle

Crystallite size is usually measured

from X-ray diffraction patterns and

grain size by other experimental

techniques like transmission electron

microscopy.

Most materials are polycrystalline;

they are made of a large number ofsingle crystals crystallites.

S i P b Mi


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Scanning Probe Microscopy

atomic force microscope

sharp probe moves over surface of specimen atconstant distance

up and down movement of probe as it maintainsconstant distance is detected and used to create image

scanning tunneling microscope

steady current (tunneling current) maintained betweenmicroscope probe and specimen

up and down movement of probe as it maintains currentis detected and used to create image of surface ofspecimen


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5 6

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Transmission Electron Microscopy Study of

TiO2 Phase Transformation

As-deposited 700 oC 800 oC

TEM diffraction patterns

for annealed and as-deposited 12-nm sample.


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Dark Field Imaging

If the transmitted beamis excluded from theimage formation process

off-axis imaging

tilted beam imaging


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Bright Field Imaging

If the main portion of the near-forwardscattered beam is used to form the image

transmitted beam 000 beam

zero-order beam


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Lecture - 9 - Microscopy

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