Material s. Presentation 1

8/2/2019 Material s. Presentation 1

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Term paperpresentationon:

SpecialExperimental

Techniques


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Objectives of special experimental

techniques are:

To understand the structure, size and

arrangement of objects which can notbe detect with human`s naked eye

To investigate and search new

findingsTo approve theories etc.


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Special experimental techniquesSpecial experimental techniques

Light microscopy

X-ray diffraction

microscopy

Electro microscopy

TransmissionElectro microscopy

Electron probe x-ray

Electro microscopy

Scanning Electromicroscopy

Scanning transmissionElectro

microscopy

Scanning tunnelingElectro

microscopy

Scanning ForceElectromicroscopy

Energy dispersive

X-ray spectroscopy

Types of special experimental techniques


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INTRODUCTION

Microscopy is the technical field ofusingmicroscopesto view samples andobjects that cannot be seen with the

unaided eye (objects that are not withinthe resolution range of the normal eye).

There are three well-known branchesof microscopy, optical, electron, andscanning probe microscopy.
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OPTICAL MICROSCOPY

Ocular lens (eyepiece) (1)Objective turret (to holdmultiple objective lenses) (2)Objective(3)Focus wheel to move the

stage (4 - coarse adjustment,5 - fine adjustment)Light source, a light or amirror (7)

Diaphragm and condenserlens (8)Stage (to hold the sample)(9)
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With optical microscopy, the light

microscope is used to study the

microstructure.

optical and illumination systems are its

basic elements.


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Normally, careful and meticulous

surface preparations are necessary toreveal the important details of themicrostructure.

The specimen surface must first beground and polished to a smooth and

mirror like finish.

This is accomplished by usingsuccessively finer abrasive papers andpowders.


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The microstructure is revealed by asurface treatment using an appropriatechemical reagent in a procedure termed

etching.


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5.15adepicts the surface structure as it might appearwhen viewed with the microscope.


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Figure 5.15bshows how normally incident light isreflected by three etched surface grains, each

having a different orientation.


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A photomicrograph of a polycrystalline specimenexhibiting these characteristics is shown in Figure


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Also, small grooves form along grain

boundaries as a consequence of etching.

Since atoms along grain boundary

regions are more chemically active, theydissolve at a greater rate than thosewithin the grains.


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These grooves become discernible when viewed under

a microscope because they reflect light at an angle

different from that of the grains themselves.


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Figure 5.16bis a photomicrograph of apolycrystalline specimen in which the grainboundary grooves are clearly visible as dark lines


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Total magnification of a microscope

may be determined by the product ofthe eyepiece number and objective

lens magnification number.

For example: 10X-power eyepiece

used in conjunction with a 50X

objective lens gives a magnification of

500; or X500.


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Advances Material Sciences

MAE612)TAMERATE AWASH


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Presented by Tamerat Awash

X-ray generation & x-ray diffraction


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X-rays are electromagnetic redaction that have high energy & a shortwave length.

Electron beam is focused into the sample being studied.

The higher energy electron also inner-shell electron is ejected(out oforebit) from its orbit through the ionizations processes.

The higher-shell electron falls into this vacancy and mustliberate(loss) some energy (as an X-ray) to do so.

As electron collide with atoms in the target & slow down, acontinuous specterm of x-ray are emitted.

The number and energy of the X-rays emitted from a specimen can

be measured by spectrometers.

X-ray generation


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These quantized x-rays are characteristic of the element


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X-ray diffraction: one of the most useful tools in the study of crystal structure of solids

is x-ray diffraction.

X-ray have wave lengths about equal to the diameters of atoms( 10-10 m).

Diffracted waves from different atoms can interfere with each other and the resultant

intensity distribution is strongly modulated by this interaction.

If the atoms are arranged in a periodic fashion, as in crystals, the diffracted waves will

consist of sharp interference maxima (peaks) with the same. symmetry as in the

distribution of atoms.

Measuring the diffraction pattern therefore allows us to deduce the distribution of atoms

in a material.

X-ray diffraction


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The peaks in a x-ray diffraction pattern are directly related to the atomic distances.

Consider the two parallel planes of atom A-A & B-B which have the same miller indices &

are separate by the inteplaner .

Let us consider an incident x-ray beam interacting with the atoms arranged in a periodic

manner as shown in 2 dimensions in the following illustrations.

n =dsin +dsin

X-ray analysis of crystals


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Application

2dsinq = n l


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Here dis the spacing between diffracting planes, is the incident angle, n is any integer,

and is the wavelength of the beam.

These specific directions appear as spots on the diffraction pattern called reflections.

Thus,X-ray diffraction results from an electromagnetic wave (the X-ray) impinging on a

regular array of scatterers (the repeating arrangement of atoms within the crystal).
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Scanning Electron

Microscopy (SEM)


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The scanning electron microscope (SEM) is a microscope

that uses electrons rather than light to form an image.

It uses a focused beam of high-energy electrons togenerate a variety of signals at the surface of solid

specimens.

The signals that derive from electron-sample interaction

reveal information about the sample including externalmorphology (texture), chemical composition, and

crystalline structure and orientation of materials making up

the sample.


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The SEM is also capable of performing analyses of

selected point locations on the sample; this approachis especially useful in qualitatively or semi-

quantitatively determining chemical compositions

using EDS, crystalline structure, and crystal

orientations using diffracted backscattered electrons(EBSD).

The design and function of the SEM is very similar to

the EPMA and considerable overlap in capabilities

exists between the two instruments.


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Figure 1. A typical SEM instrument


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Figure 2. Cross section of generic SEM

F d t l P i i l f SEM


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Accelerated electrons in an SEM carry significant amounts of

kinetic energy, and this energy is dissipated as a variety of

signals produced by electron-sample interactions

Fundamental Principles of SEM

Figure 3. Electron sample interaction


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These signals include secondary electrons (that produce

SEM images), backscattered electrons (BSE), diffracted

backscattered electrons (EBSD) that are used to determinecrystal structures and orientations of minerals, photons

(characteristic X-rays that are used for elemental analysis),

visible light (cathodo-luminescence-CL), and heat.

Secondary electrons and backscattered electrons arecommonly used for imaging samples: secondary electrons

are most valuable for showing morphology and topography

on samples and backscattered electrons are most valuable

for illustrating contrasts in composition in multiphasesamples.


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SEMs always have at least one detector (usually asecondary electron detector), and most haveadditional detectors.

The SEM is routinely used to generate high-resolution images of shapes of objects (SEI) and toshow spatial variations in chemical compositions:

acquiring elemental maps or spot chemical analysesusing EDS,discrimination of phases based on mean atomicnumber using BSE, and compositional maps based

on differences in trace element "activators"(typically transition metal and Rare Earthelements) using CL.


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SEMs equipped with diffracted backscattered electron

detectors (EBSD) can be used to examine micro fabric andcrystallographic orientation in many materials.

Backscattered electron images (BSE) can be used for rapid

discrimination of phases in multiphase samples


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Figure 4a.Silica spheres image formed by SEM


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Figure 4b. Ag powder image formed by SEM


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Transmission electron microscopy

(TEM) Transmission electron microscopy(TEM) is a

microscopytechnique whereby a beam ofelectrons istransmitted through an ultra thin specimen,interacting with the specimen as itpasses through.

Details of internal micro-structural features are accessibleto observation

An image is formed from the interaction of the electronstransmitted through the specimen; the image is magnifiedand focused onto an imaging device, such as a fluorescent

screen, on a layer ofphotographic film, or to be detected bya sensor such as a CCD camera. Magnifications approaching1,000,000 times are possible

with transmission electron microscopy
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WorkingPrinciple


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Scanning

Transmission ElectronMicroscopy


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Scanning Transmission Electron

MicroscopyAscanning transmission electron microscope

(STEM) is a type oftransmission electron microscope(TEM). As with any transmission illumination scheme,

the electrons pass through a sufficiently thinspecimen.

However, STEM is distinguished from conventionaltransmission electron microscopes (CTEM) by

focusing the electron beam into a narrow spotwhichis scanned over the sample in a raster.
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The Scanning Transmission Electron Microscope(STEM) produces images of the internalmicrostructure of thin specimens using a high-energy scanning electron beam, in the same way aScanning Electron Microscope (SEM) produces imagesof bulk surfaces.

STEM is also used to describe the group ofcrystallographic and compositional analysis methodsknown collectivelyas Analytical Electron Microscopy(AEM)


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SCANNING TUNNELLING MICROSCOPE (STM)

AND

ATOMIC FORCE MICROSCOPE (AFM)

Presented by: Tadele Belay


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SCANNING TUNNELLING MICROSCOPE (STM)

Developed in 1981 ByGerd Binnig and Heinrich Rohrer (at IBM Zrich), the Nobel Prize

winners in Physics in 1986

A powerful instrument for imaging surfaces at the atomic level

Expected to possess: 0.1 nm lateral resolution and

0.01 nm depth resolution


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The scanning tunnelling microscope (STM) is widely used to obtainatomically resolved images of metal and other conducting surfaces.

This is very useful for characterizing:

oSurface roughness,o Observing surface defects, ando Determining the size and conformation of aggregates of atoms

and molecules on a surface

Increasingly STM is used to manipulate atoms and molecules on a surface.


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Components of STM


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Operation of STM

It works by measuring the tunneling current between a conducting tip

and conducting sample when a DC bias voltage is applied.

Resolution in z (perpendicular to surface) is ~0.01 nm and in x/y is ~0.1nm,resulting in atomic-resolution images.


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Operation of STM

Consider two metals, one is a tip and the other is a surface,separated by a finite distance implies that a uniform potential barrier.

Electron wave function can tunnel through the barrier and leads to

finite current.

As the two metals move apart, the probability of electrons tunnelingdecreases exponentially and as they move closer the tunnelingincreases.


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Scan directiontip

Lower currentConstant height

Higher current

Atomic surface

The air

gap(barrier) isseveralnm

I (nA)

x (nm)

The currentprofileduplicates theatomic surface

Operation of STM


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it is possible to image individual nickel atoms.

Application areas:

Application areas:


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it is also possible to manipulate individual iron atoms

on a copper surface

Application areas:

it is also possible to have some fun
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Iron on copperCarbon monoxide on platinum

it is also possible to have some fun

Xenon on nickel

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The atomic force microscope (AFM) is widely used to obtain atomically

resolved images of non-metal and other non-conducting surfaces

An AFM works by scanning a ceramic tip over a surface. The tip ispositioned at the end of a cantilever arm shaped like a diving board.

The tip is repelled by or attracted to the surface and the cantilever armdeflected.

The deflection is measured by a laser that reflects at an oblique anglefrom the very end of the cantilever.
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Principle of operation

Measure the forces between the sharp tip and sample surface.
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How are forces measured?Hookes law: F= -ks

Laser Beam Deflection Method
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Fabrication of Cantilever

Made from SiN or Si As soft as possible to achieve high sensitivity

Spring constant of the cantilever is less than equivalentspring constant between atoms of sample in order to

avoid dragging the atoms out of its atomic site.
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Fabrication of Tip

Made from SiN or Si As sharp as possible

The radius of curvature of the tip does not influence

the height of a feature but the lateral resolution

The machine looks:-
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Materials Science

Polymer Science

Data Storage : Help in design hard disk drive at nanoscale level

Semi conductor

Surface topography ofZnO

film

Image of polymer blend DVD

Applications
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With an atomic force microscope it is possible to image the carbon

atoms of a carbon tube.
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Electron Probe X-RayMicroanalysis (EPMA )

Electron Probe X Ray
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Electron Probe X-Ray

Microanalysis (EPMA )

EPMA is non-destructive analytical technique thatuses a narrow electron beam( usually with a diameterless than 1 millimeter) focused on a solid specimen to

excite an x-ray spectrum that provides qualitativeand quantitative information characteristic of theelements in the sample.

Electron probe microanalyzers are designedfrom the ground up for the analysis of x-rays whichare emitted from the specimen whenprobed(explored) with an electron beam.

F d l P i i l f El
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Fundamental Principles of Electron

probe micro-analyzer (EPMA)An electron microprobe operates under the

principle that if a solid material is bombarded byan (accelerated and focused) electron beam, the

incident electron beam (has sufficient energy to)liberate both matterand energy from the sample.

These electron-sample interactions mainly liberateheat, but they also yield both derivative electronsandx-rays.

X ray Generation
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Electron beam is focused into the sample being studied.An inner-shell electron is ejected from its orbit.

A higher-shell electron falls into this vacancy and must liberate someenergy (as an X-ray) to do so.The number and energy of the X-rays emitted from a specimen canbe measured by spectrometers.These quantized x-rays are characteristic of the element.

X-ray Generation
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Electron probe micro-analyzer (EPMA)

Instrumentation - How Does It Work?
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A li ti f EPMA


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Applications of EPMA Quantitative EPMA analysis is used for chemical

analysis of geological materials at small scales. In most cases, EPMA is chosen in cases where

individual phases need to be analyzed (e.g., igneous andmetamorphic minerals), orwhere the material is of small

size or valuable for other reasons (e.g., experimental runproduct, sedimentary cement, volcanic glass, matrix of a meteorite,

archeological artifacts such as ceramic glazes and tools).

EPMA is also widely used for analysis of synthetic

materials such as optical wafers, thin films, microcircuits, semi-conductors, and superconducting ceramics.

Strengt s of EPMA
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g

An electron probe is essentially the same instrument as anSEM, but differs in that it is equipped with a range of

crystal spectrometers that enable quantitative chemicalanalysis at high sensitivity.

An electron probe is the primary tool for chemical analysisof solid materials at small spatial scales (as small as 1-2micron diameter); hence, the user can analyze even minute

single phases (e.g., minerals) in a material (e.g., rock) with"spot" analyses. Spot chemical analyses can be obtained, which allows the

user to detect even small compositional variations withintextural context or within chemically zoned materials.

Electron probes commonlyhave an array of imagingdetectors that allow the investigator to generate images ofthe surface and internal compositional structures that help

with analyses. Flexible in varying parameter or another to circumvent instrumental

weaknesses.

Limitations of EPMA
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Although electron probes have the ability to analyze

for almost all elements, they are unable to detect thelightest elements (H, He and Li); as a result, forexample, the "water" in hydrous minerals cannot beanalyzed.

Some elements generate x-rays with overlapping peakpositions(by both energy and wavelength) that mustbe separated.

Due to the low X-ray intensity, images usually take anumber of hours to acquire.

Probe analysis also cannot distinguish between thedifferent valence states of Fe, so the ferric/ferrous ratiocannot be determined and must be evaluated by othertechniques.
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Thank you!

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