Stkk 6014 X-ray Diffraction

7/24/2019 Stkk 6014 X-ray Diffraction

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STKK 6014X-RAY DIFFRACTION

Group Presentation


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WHAT IS X-RAY DIFFRACTION ?


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XRD WORKING CONCEPT

When a monochromatic x-ray beam with wavelength is incident onthe lattice planes in a crystal planes in a crystal at an angle ,

diffraction occurs only when the distance traveled by the rays reflectedfrom successive planes differs by a complete number n ofwavelengths. By varying the angle , the Braggs Law conditions aresatisfied by different d-spacing in polycrystalline materials. Plotting theangular positions and intensities of the resultant diffraction peasproduces a pattern which is characterised of the sample. Where a

mixture of different phases is present, the diffractogram is formed byaddition of the individual patterns.


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WHERE DO X RAYS COME FROM?

!wo principal methods for " raygeneration#

$% &ire beam of electrons at metal target.

' (oni)ation of inner shell electrons results in

formation of an *electron hole.' +elaxation of electrons from upper shells.

!he energy difference /$0 $0 m% isreleased in the form of " rays of specificwavelengths.

' 1ommonly used metals are 1u 23 45$.67$8 9% and :o 23 4 5 0.;$0;< 9%.

' =ery (nefficient. :ost energy dissipated atheat re>uires permanent cooling%.


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?%@ccelerate electrons in a particle

accelerator synchrotron source%.lectrons accelerated at relativisticvelocities in circular orbits. @s velocitiesapproach the speed of light they emitelectromagnetic radiation in the " ray

region.

' !he " rays produced have a range ofwavelengths white radiation orBremsstrahlung).

' +esults in high flux of " rays.' Laue experiment


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BRAGGS LAW

n = 2d sin

T!" B#$%%s &"#" $&$#'"' (!"

N)*"+ P#i," in !.si/s in 010 3)#

(!"i# &)#4 in '"("#5inin% /#.s($+

s(#6/(6#"s *"%innin% &i(! N$C+7 8nS

$n' 'i$5)n'9

A+(!)6%! B#$%%:s +$& &$s 6s"' ()

";+$in (!" in("#3"#"n/" $(("#n )3

X-#$.s s/$(("#"' *. /#.s($+s7

'i33#$/(i)n !$s *""n '"


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B#$%%s L$& $n' Di33#$/(i)n

H)& &$;-#$.s 0 2@ 3#)5 +$n"s &i(!

s$/in% '


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Electron gun

Detector

Sample holder

Known as

goniometer

XRD INSTRMENT


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goniometer


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GONIOMETERTheta : 2-Theta (1:2)


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SAMPLE PREPARATION

' !he single most important determinate of the >uality of "+

data obtained from a powder sample.' !o be able to see all of the diffraction peas, the specimens

powder must present a large number of crystallites in a

random orientation to the incident beam.

' !here is no CstandardD way to prepare a specimen for

powder diffraction, and the most important consideration is

the obEective of the experiment.


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Ftep $ # Fample Preparation :ethods !he sample si)e should be reduced from a fist-si)ed hand

specimen to a powder suitable for "+ analysis.

Ftep ? # Fample :ount :ethods !here are many different ways of mounting specimens for

analysis# &or rapid determination of accurate pea positions, a thin

sample with as much area as possible presented to thebeam is generally best.

&or accurate intensities, you will want a thic sample ofrandomly oriented crystallites.

(mportant note# +un an empty sample holder as a CblanD#@lthough most materials used as sample holders areostensibly amorphous and do not yield a diffraction pattern,in practice this is rarely the case


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Fpecimens and xperimental rrors

Feveral factors that contributed to diffraction errors are as

follows # &lat Fpecimen rror - Gccurs - surface of the specimen is flat, and

does not conform to the curvature of the focusing circle. 1ompositional =ariations between Fample and Fpecimen - related to

grinding, environmental interaction or irradiation effects. Fpecimen isplacement - geometry of the sample mount - causes

positional deviation on the focusing circle. Fpecimen !ransparency - Penetration of the beam into a CthicD

specimen changes the location in which diffraction occurs. Fpecimen !hicness - !hin specimens - produce accurate pea

positionsH thicer specimens - produce more accurate peaintensities.

Particle (nhomogeneity - (nhomogenities can significantly alterdiffraction intensities and peas seen.

Preferred Grientation - Ion-random orientation of crystallites canproduce large variations in intensity and limit the peas seen.


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PPLIC TIONS OF XRD


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' Powder polycrystalline% diffraction - commonly usedfor chemical analysis

' Phase identification searchJmatch%

' (nvestigation of highJlow temperature phases' Folid solutions' eterminations of unit cell parameters of new materials

I'"n(i3i/$(i)n

' crystallinity parts give sharp narrow diffraction peas' amorphous region - a very broad pea halo%' ratio between the intensity of crystallinity and

amorphous part can be used to calculate the amount of

crystallinity in the material.

N)("s

@ polymer can be considered as partly crystalline andpartly amorphous. !he crystalline domain act as areinforcing grid, improves the performance over a widerange of temperature. Kowever, too much crystallinity

causes brittleness.

P)+.5"#

/#.s($++ini(.


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' !he principles of stress analysis by "-ray diffraction isbased on measuring angular lattice strain distributions.

' @ reflection at high ?is chosen and the change in thed-spacing is measured with different orientations of thesample.

' !he stress can be calculated from the strain distributionby using Kooes law.

N)("s +esidual stress is the stress remains in the materialafter the external force that caused the stress have beenremoved. Ftress is defined as force per unit area.Positive values indicate tensile expansion% stress, whilenegative values indicate a compressive state.!he deformation per unit length is called strain.!he residual stress can be introduced by any chemical,mechanical or thermal processH such as machining,plating and welding.

R"si'6$+s(#"ss


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' determination of the preferred orientation of thecrystallites in polycrystalline aggregates.

' C!extureD - synonym to preferred crystallographicorientation in the polycrystalline material, normally singlephase.

' preferred orientation -described in terms of polefigures.' @ polefigure - scanned by measuring the diffractionintensity of a given reflection ? is constant% at a large

number of different angular orientation of the specimen.' polefigures represent sterographic or e>ual areaproEection.

' !he intensity of a given reflection h, , l% is proportionalto the number of h, , l planes in reflecting conditionBraggs law%.

' !he polefigure gives the probability of finding a givencrystal-plane-normal as function of the specimenorientation. (f the crystallites in the sample have arandom orientation, the recorded intensity will beuniform.

T";(6#"$n$+.sis


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APPLICATIONS OF POWDER

DIFFRACTIONDiffraction line arameter Alication!

Pea position '!nit-cell parameter re"inement'Pattern inde#ing

'Space group determination'$nisotropic thermal e#pansion'%acrostress: sin2method'Phase identi"ication

&ntensit' 'Phase aundance'eaction inetics'*r'stal structure anal'sis'iet+eld re"inement'Search,match phase identi"ication'Pre"erred orientation te#ture anal'sis


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Diffraction line arameter Alication!

.idth,readth and shape '&nstrumental resolution "unction

'%icrostructure: line pro"ile anal'sis' %icrostructure (cr'stallite si/e si/edistriution lattice distortion structuremistaes dislocations compositiongradient) cr'stallite growth inetics

'Three dimensional microstructure

0on-amient and d'namicdi""raction

' &n situ di""raction under e#ternal constraints'eaction inetics


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SMMARY

"-ray diffraction provides a powerfultool to study the structure and

composition of the materials which is aey re>uirement for understandingmaterials properties

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Stkk 6014 X-ray Diffraction