Metal and Alloy Bonding - An Experimental Analysis: Charge Density in Metals and Alloys

168 Pages·2012·4.15 MB·English

Metal and Alloy Bonding:
An Experimental Analysis

R. Saravanan M. Prema Rani
•
Metal and Alloy Bonding:
An Experimental Analysis
Charge Density in Metals and Alloys
123
Dr. R.Saravanan M.Prema Rani
Research Centre and PGDepartment Research Centre and PGDepartment
of Physics of Physics
The MaduraCollege The MaduraCollege
Madurai 625011 Madurai 625011
TamilNadu TamilNadu
India India
e-mail: [email protected]; e-mail: [email protected]
[email protected]
ISBN 978-1-4471-2203-6 e-ISBN978-1-4471-2204-3
DOI 10.1007/978-1-4471-2204-3
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Preface
This book has been written based on the experimental results obtained through
several experimental techniques, especially the powder X-ray diffraction method,
on various metals and alloys we encounter frequently. An analysis of the
interactions of electrons in different atoms has been discussed.
Metalshaveusefulpropertiesincludingstrength,ductility,high-meltingpoints,
thermal and electrical conductivity and toughness. The key feature that distin-
guishesmetalsfromnon-metalsistheirbonding.Theexistenceoffreeelectronsin
metals has a number of profound consequences for the properties of metallic
materials. There are a large number of possible combinations of different metals
andeachhasitsownspecificsetofproperties.Thephysicalpropertiesofanalloy,
such as density, reactivity, Young’s modulus and electrical and thermal conduc-
tivity,maynotdiffergreatlyfromthoseofitselements,butengineeringproperties,
such as tensile strength and shear strength, may be substantially different from
those of the constituent materials. Metals and their alloys make today’s manu-
facturing industry, agriculture, construction and communication systems, trans-
portation, defense equipments, etc. possible. Some of the major reasons for the
continuing advancements in alloys are the availability of materials, new manu-
facturing techniques and the ability to test alloys before they are produced. Most
modern alloys are, in fact, preplanned using sophisticated computer simulations,
which help to determine what properties they will display. Semiconductors have
been studied extensively due to their importance in applications. These materials
receivemuchattentionbecausephysicalpropertiessuchasthebandgap,mobility
and lattice parameter can be continuously controlled. Having such continuous
control is of importance in applications such as electronic and optical devices.
Metals and alloys have high-melting temperatures because of the heavy
bonding between the atoms. There are a variety of applications for metals and
alloys. Due to the importance of these materials, a study of their bonding
interactionshasbeencarriedoutinthismonographusingexperimentallyobserved
X-ray diffraction data.
Today’s technological evolution results in developing new and sophisticated
materials of immense use in domestic, technical and industrial applications.
v
vi Preface
Usually, the synthesis of new materials, especially metals and alloys, results in
single-phasematerials,butoftennotinsinglecrystallineform.Hence,acomplete
analysis of the structure, local distribution of atoms and electron distribution in
core, valence and bonding region is necessary using powder diffraction methods,
in addition to single crystal diffraction results, since most of the recent materials
will be initially obtained in powder form. Since one can make efforts to grow
single-crystalsfrompowders,aprioranalysisisrequiredusingpowderstoproceed
for single crystal growth.
Inthiscontext,wehavetakensomesimplemetals(Al,Cu,Fe,Mg,Na,Ni,Te,
Ti,Sn,V,Zn)andalloys(AlFe,CoAl,FeNi,NiAl)andcollectedpowderXRDdata
setsorusedsinglecrystalXRDdatasetsfromtheliterature,tostudythestructurein
termsofthelocalandaveragestructuralpropertiesusingpairdistributionfunction
(hereafter PDF), electron density distribution between atoms using Maximum
Entropy Method (hereafter MEM) and bonding of core and valence electron dis-
tribution using multipole technique. Particularly, the PDF analysis requires data
sets of very high values of Q (=4pSinh/k) which is achievable only through syn-
chrotron studies, but not accessible for common crystallographer/material scien-
tists. The present work gives reasonable results obtained through single crystal
workorthroughhighQdatasets,usingonlypowdersamples.Also,astudyonthe
electronic structure of metals using the most versatile currently available tech-
niques like MEM and multipole method is worthwhile. If the tools available for
analysisyieldhighlypreciseinformation,thenitisappropriatetoapplyittoprecise
datasetsavailableasinthiswork,andthusthemethodologycanalsobetested.In
order to elucidate the distribution of valence electrons and the contraction/expan-
sion of atomic shells, multipole analysisof the electrondensities was alsocarried
out. Recently, multipole analysis of the charge densities and bonding has been
widely used tostudy the electronic structure ofmaterials.
Bondingstudiesincrystallinematerialsareveryimportant,especiallyinmetals,
because of their extensive use. These studies can reveal the qualitative nature of
bonding as well as the numerical values of mid-bond densities which indicate the
strength of the material under study. With the advent of versatile methods like
MEM and multipole method, bonding studies gained impetus because of the
accuracyofthesemethodsandthefactthattheexperimentaldatacanbeusedwith
these methods to accurately determinethe actual bonding between atoms.
The precise study of bonding in materials is always useful and interesting, yet
no study can reveal the real picture as no two sets of experimental data are
identical. This problem is enhanced when the model used for the evaluation of
electrondensitiesisnotentirlysuitable.Fouriersynthesisofelectrondensitiescan
be of use in picturing bonding between two atoms, but it suffers from the major
disadvantages of series termination error and negative electron densities which
prevent the clear understanding of bonding between atoms; the factor intended to
be analysed. The advent of MEM solves many of these problems. MEM electron
densities are always positive and even with limited number of data, one can
determine reliable electron densities resembling true densities. Currently, the
multipole analysis of charge densities has been widely used to study crystalline
Preface vii
materials. This synthesizes the electron density of an atom into core and valence
parts and yields an accurate picture of bonding in a crystalline system.
In this research monograph on metals and alloys, a complete analysis of
bonding has been made on 11 important metals and four alloys. Powder X-ray
diffraction data as well as single crystal data sets have been used for the purpose.
Chargedensityanalysis ofmaterialsprovidesafirmbasisfortheevaluationof
thepropertiesofthematerials.Designingandengineeringofnewcombinationsof
metalsrequiresfirmknowledgeoftheintermolecularfeatures.Recentadvancesin
technology and high-speed computation has put the crystal X-ray diffraction
technique on a firm pedestal as a unique tool for the determination of charge
density distribution in molecular crystals. Methods have been developed to make
experimental probes to unravel the features of charge densities in the intra and
intermolecularregionsincrystalstructures.Inthisreportthestructuraldetailshave
been elucidated from the X-ray diffraction technique through Rietveld technique.
ThechargedensityanalysishasbeencarriedoutwithMEMandmultipolemethod,
and the local and average structure analysis by atomic PDF.
This research work reveals the local and average structural properties of some
technologically important materials, which are not studied along these lines. New
understandingsoftheexistingmaterialshavebeengainedintermsofthelocaland
average structures of the materials. The electron density, bonding, and charge
transferstudiesanalysedinthisworkwillgivefruitfulinformationtoresearchersin
thefieldsofphysics,chemistry,materialsscience,metallurgy,etc.Theseproperties
can be properly utilized for the proper engineering of these technologically
important materials.
Chapter 1 introduces the significance and applications of metals, alloys and
semiconductors studied in this research work. The objectives of this book are
presented. The essential mechanism of ball milling which has evolved to be a
simple and useful method for the formation of nano crystalline materials is
discussed. The current state of art of non-destructive characterisation techniques
such as X-ray diffraction and scanning electron microscope are discussed.
Chapter 2 provides a survey of the current applications of X-ray diffraction
techniquesincrystalstructureanalysis,withfocusontherecentadvancesmadein
the scope and potential for carrying out crystal structure determination directly
from diffraction data. The basic concepts of crystal structure analysis, Rietveld
refinementandtheconceptsusedfortheestimationandanalysisofchargedensity
inacrystalarediscussed.Themorereliable modelsfor charge density estimation
like multipole formalism and MEM are discussed in detail. The local structural
analysis technique and atomic PDF is also discussed.
Chapter3presentstheresultsanddiscussionsofthisresearchwork.Adetailed
account of the results of the materials analysed are presented in the subsections.
Section 3.1 (Sodium and Vanadium Metals) describes about the nature of
bonding and the charge distribution in sodium and vanadium metals are analysed
using the reported X-ray data of these metals. MEM and multipole analysis used
forbondinginthesemetalsareelucidatedandanalysed.Themid-bonddensitiesin
sodiumandvanadiumarefoundtobe0.014and0.723 e/Å3respectively,givingan
viii Preface
indicationofthestrengthofthebondsinthesematerials.Frommultipoleanalysis,
the sodium atom is found to contract more than the vanadium atom.
Section 3.2 (Aluminium, Nickel and Copper) describes the average and local
structures of simple metals Al, Ni and Cu are elucidated for the first time using
MEM,multipoleandPDF.Thebondingbetweenconstituentatomsinalltheabove
systemsisfoundtobewellpronouncedandclearlyseenfromtheelectrondensity
maps. The MEM maps of all the three systems show the spherical core nature of
atoms.Themid-bondelectrondensityprofilesofAl,NiandCurevealthemetallic
bonding nature. The local structure using PDF profile of Ni has been compared
with that of the reported results. The R value in this work using low Q XRD data
forthePDFanalysisofNiisclosetothevaluereportedusinghighQsynchrotron
data. The cell parameters and displacement parameters were also studied and
compared with the reported values.
Section3.3(Magnesium,Titanium,Iron,Zinc,TinandTellurium)describesthe
average and local structures of magnesium, titanium, iron, zinc, tin and tellurium
are analysed using the MEM, and PDF. The structural parameters of the metals
were refined with the well-known Rietveld powder profile fitting methodology.
One-,two-andthree-dimensionalelectrondensitydistributionsofMg,Ti,Fe,Zn,
Sn and Te have been mapped using the MEM electron density values obtained
through refinements. The mid-bond density in Ti is the largest value along [110]
direction among the six metal systems. From PDF analysis the first neighbour
distanceisobservedtodecreaseastheatomicnumberincreasesforallthemetals.
Section3.4(CobaltAluminiumandNickelAluminiumMetalAlloys)describes
thepreciseelectrondensitydistributionandbondinginmetalalloysCoAlandNiAl
ischaracterizedusingMEM andmultipolemethod.Reported X-ray single-crystal
data used for this purpose. Clear evidence of the metal bonding between the con-
stituentatomsinthesetwosystemsisobtained.Themid-bondelectrondensitiesin
thesesystemsarefoundtobe0.358and0.251 e/Å3respectively,forCoAlandNiAl
in the MEM analysis. The two-dimensional maps and one-dimensional electron
density profiles have been constructed and analysed. The thermal vibration of the
individualatomsCo,NiandAlhasalsobeenstudiedandreported.Thecontraction
ofatomsinCoAlandexpansionofNiandcontractionofAlatominNiAlisfound
frommultipoleanalysis,inlinewiththeMEMelectrondensitydistribution.
Section 3.5 (Nickel Chromium (Ni Cr )) describes the alloy Ni Cr was
80 20 80 20
annealed andball milled tostudy the effect of thermal and mechanical treatments
on the local structure and the electron density distribution. The electron density
between the atoms was studied by MEM and the local structure using PDF. The
electron density is found to be high for ball-milled sample along the bonding
direction. The particle sizes of the differently treated samples were realized by
SEMandthroughXRD.Clearevidenceoftheeffectofballmillingisobservedon
the local structure and electron densities.
Section 3.6 (Silver doped in NaCl (Na Ag Cl)) describes the alkali halide
1-x x
Na Ag Cl, with two different compositions (x = 0.03 and 0.10) is studied with
1-x x
regard to the Ag impurities in terms of bonding and electron density distribution.
X-raysinglecrystaldatasetshavebeenusedforthispurpose.Theanalysisfocuses
Preface ix
ontheelectrondensitydistributionandhencetheinteractionbetweentheatomsis
clearlyrevealedbyMEMandmultipoleanalysis.Thebondinginthesesystemsis
studiedusingtwo-dimensionalMEMelectrondensitymapsonthe(100)and(110)
planes and one-dimensional electron density profiles along the [100], [110] and
[111]directions.Themid-bondelectrondensitiesbetweenatomsinthesesystems
are found to be 0.175 and 0.183 e/Å3, respectively, for Na Ag Cl and
0.97 0.03
Na Ag Cl. Multipole analysis of the structure is performed for these two
0.90 0.10
systems, with respect to the expansion/contraction of the ion involved.
Section 3.7 (AluminiumDopedwith Dilute Amounts ofIron Impurities (0.215
and 0.304 wt% Fe)) describes the electronic structure of pure and doped alu-
miniumwithdiluteamountsofironimpurities(0.215and0.304 wt%Fe)hasbeen
analysed using reported X-ray data sets and the MEM. Qualitative as well as
quantitative assessment of the electron density distribution in these samples is
made. The mid-bond characterization leads to a conclusion about the nature of
doping of impurities. An expansion of the size of the host aluminium atom was
observed with Fe impurities.
Chapter 4 presents the conclusion of the results of the reported work.
A complete analysis on the electron density of important metals and alloys is
presented in this book. This book will be highly useful for scientists and
researchersworkingintheareasofmetallurgy,materialsscience,crystallography,
chemistry and physics.