MATERIALS CHEMISTRY II CHEM2007 - PowerPoint PPT Presentation

Title: MATERIALS CHEMISTRY II CHEM2007


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MATERIALS CHEMISTRY IICHEM2007

• DR. M. J. MOLOTO
• OFFICE SC204

PRESCRIBED TEXT (1) Shriver and Atkins,
inorganic Chemistry, 4th Ed (2) D.R. Askeland
and P.P. Phule, The Science and Engineering of
Materials 5th Ed, Thompson
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TOPICS TO BE COVERED

• Metal oxides/nitrides/fluorides
• Glasses
• Sulfur compounds
• Pigments
• Chemistry of Si and Al Group 13 with emphasis
  on Al
• Group 14 elements
• Semiconductors
• Extended silicas
• Aluminosilicates
• Zeolites
• Ceramics properties
• Synthesis and processing
• Glasses (ceramics)
• Clays

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Synthesis of Materials
Synthetic reactions generally involves molecular
rearrangements or substitution of one group or
ligand by another. This requires small activation
energies, low temperature (0 150 C) and in
solvents that aid diffusing reacting species.
New materials can be obtained by two main
methods the breaking up of one or
more continuous, or so-called extended lattice,
structures followed by the slow diffusion of ions
and crystallization of new structures, and the
linking of polyhedral building units from
solution and deposition of the newly formed
solids.
Methods of direct synthesis Many complex solids
can be obtained by direct reaction of the
components at high temperatures. Heating metals
in the presence of oxygen gives metal oxides but
complex oxides are obtained by heating a mixture
of oxides with different metals. Examples
ternary oxides, BaTiO3 and quaternary oxides,
YBa2Cu3O7, BaCO3(s) TiO2(s)
BaTiO3(s) CO2(g) ½Y2O3(s) 2BaCO3(s)
3CuO(s) YBa2Cu3O7 2CO2(g)
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3 CsCl(s) 2ScCl3(s) ? Cs3Sc2Cl9(s)
Complex chlorides NaAlO2(s) SiO2(s) ?
NaAlSiO4(s) Aluminosilicates High gas pressure
and inert gases may be used to control the
composition of the product. Tl2O(s)
Ta2O5(s) 2
TlTaO3(s) V2O5(s) V2O3(s) For volatiles
reactants are normally sealed in glass tube,
under vacuum, prior to heating. Sulfur and
thallium(III) oxide are volatile at the reaction
temperature. Ta(s) S2(l)
TaS2(s) Tl2O3(l) 2 BaO(s) 3CaO(s)
4CuO(s) Tl2Ba2Ca3Cu4O12(s)
High pressures such as 100GPa at high
temperatures about 1500 C are used.
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• Solution methods
• Condensation reactions can lead to the frameworks
  of polyhedral species.
• Results in crystallization from solution. The
  following examples are reactions
• that occur in water
• ZrO2(s) 2H3PO4(l) ? Zr(HPO4)2.H2O
  H2O(l)
• 12NaAlO2(s) 12Na2SiO3.9H2O
  Na12Si12Al12O48.nH2O(s) (zeolite
• LTA) 24NaOH(aq)
• These are useful for preparing open structure
  aluminosilicates (zeolites).
• 2La3 Cu2 2La(OH)3.Cu(OH)2(s)
  gel La2CuO4(s)
• 4H2O(g)
• High temperature, direct combination methods and
  solvothermal techniques
• are synthesis methods used in materials
  chemistry. Temperatures used in
• solution methods are lower than in direct methods
  and low temperature may
• also be used to reduce the size of particles of
  materials prepared.

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• Chemical deposition
• Thermal decomposition of volatile inorganic
  compounds can be used to
• deposit electronic materials on substrates. A
  thin layer of film is deposited
• using chemical materials over substrate such as
  silicon. One such technique
• is chemical vapour deposition (CVD), in which
  volatile inorganic molecules
• are decomposed on the substrate. If a
  metal-organic compound used
• Metallo-organic CVD. Metal alkyls may be used to
  deposit metal or reacted
• with gases above the substrate to generate Group
  12/16 (II/VI) semiconductor
• metal chalcogenides
• Zn(CH3)n H2S ? ZnS Group II/VI semiconductor
• Examples of volatile metal-organic compounds used
  include carbonyls,
• dithiocarbamates, acetoacetonates,
  cyclopentadienes. Another approach for
• depositing films is the single-molecular
  precursor in which more than atom
• type is involved.
• Other techniques for depositing films are laser
  ablation and sputtering.

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METAL OXIDES, NITRIDES AND FLUORIDES
Monoxides of the 3d-metals This include metals
such as Ti, V, Cu, Fe, Ni, etc. which may be
obtained in mixed stoichiometries and hence with
defects
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(a) Defects and Non-stoichiometry The
nonstoichiometry of Fe1-xO arises from the
creation of vacancies on the Fe2 octahedral
sites, with each vacancy charge compensated by
the conversion of two Fe2 ions to two Fe3 ions.
Fe1-xO is metastable at room temperature and It
is thermodynamically unstable with respect to
disproportionation into Fe and Fe3O4 but does not
convert for kinetic reasons. The structure of
Fe1-xO is derived from rock-salt FeO by the
presence of vacancies on the Fe2 octahedral
sites and each vacancy is charge compensated by
the conversion of two adjacent Fe2 ions to two
Fe2 ions. Generally all 3d-metal oxides have
similar defects and clustering of defects except
for NiO. Chromium(II) oxide disproportionates
into Cr and Cr3 species similar to FeO. 3
Cr(II)O ? Cr(III)2O3(s) Cr(0)(s)
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(b) Electronic and Magnetic properties The
3d-metal oxides such as MnO, FeO, CoO and NiO are
semiconductors and TiO and VO are metallic
conductors. 3d-metal oxides - MnO, Fe1-xO,
CoO and NiO have low conductivity that increase
with temperature or have such large band gaps
that become insulators. The electron-hole
migration in these oxides is attributed to the
hopping mechanism. The electron or hole hops
from one localized metal atom site to the other,
and causes the surrounding ions to adjust their
locations and the electron or hole is trapped
temporarily in the potential well produced by
the atomic polarization. The electron reside at
its new site until its thermally activated to
migrate. The electron or hole ends to associate
with local defects, so the activation energy for
charge transport may also include energy of
freeing the hole from the position next to a
defect. Doping of metallic oxides with other
metal substances results in increased
conductivity. In addition to the d-orbital
electrons interactions, the transition metal
oxides have magnetic behaviors that are derived
from the cooperative interaction of the
individual atomic magnetic moments. (See Box
23.2on Page 606 of Shriver and Atkins)
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Higher oxides and Complex oxides
Binary metal oxides that do not have a 11
metaloxygen ratio are higher oxides and those
containing ions with more than one metal are
called complex oxides.
Rhenium trioxide takes the structure ReO6
octahedra sharing all the vertices in three
dimensions (Fig 23.22 page 607). It is a bright
red lustrous solid and its electrical
conductivity is similar to that of copper metal.
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Spinels Spinel MgAl2O4 and oxide spinels have
general formula AB2O4 has a combination of A2
and B3 cations. A spinel structure consists of a
ccp array of O2- ions in which the A cations
occupy one eighth of the tetrahedral holes and
the B cations occupy half of the octahedral
holes. Inverse spinel general formula BABO4
in which the more abundant B cation is
distributed over both coordination spheres. The
occupation factor, ? of a spinel is the fraction
of B atoms in the tetrahedral sites ? 0, for a
normal spinel and ? 0.5 for inverse spinel,
BABO4.
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Structure of spinel
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Perovskite Perovskites CaTiO3 have general
formula ABX3 in which the 12-coordinate hole of
a ReO3 type BX3 structure is occupied by a large
A ion. CaTiO3 exhibits ferroelectric and
piezoelectric properties.
(Perovskite) Structure
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High-temperature superconductors
Superconductors have two features below
critical temperature, TC, perovskites as an
example enter the superconducting state and have
zero electrical resistance, in this state they
also exhibit the Meissner effect (exclusion of
magnetic field). Type I superconductors show
abrupt loss of superconductivity when an applied
magnetic field exceeds a value characteristic of
the material. Type II include high temperature
materials, show gradual loss of superconductivity
above the critical field denoted by HC.
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Collosal magnetoresistance Perovskites with Mn
o the B sites can show very large changes in
resistance on applying magnetic field known as
colossal magnetoresistance. These are manganites
Mn(III) and Mn(IV) complex oxides with the
formulation Ln1-xAxMnO3 (A Ca, Sr, Pb, Ba, Ln
Pr or Nb) order ferromagnetically upon cooling
below room temperature (typically 100 and 250 K)
and their resistance occurs near Curie
temperature (TC)
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Glass formation Silicon dioxide readily forms
glass because the three dimensional network of
strong covalent Si-O bonds in the melts does not
readily break and reform upon cooling. A glass is
prepared by cooling a melt more quickly than it
can crystallize. Cooling molten silica gives
vitreous quartz. The lack of strong directional
bonds in metals and simple ionic substances makes
it much more difficult to form glasses from these
materials. Recently techniques with ultrafast
cooling have been developed
Glass composition Low valence metal oxides such
as Na2O and CaO are often added to silica to
soften the Si-O framework and referred to as
modifiers. Bottles and windows are made of
sodalime glass that contains Na2O and CaO as
modifiers. Borosilicate glasses contains B2O3 as
a modifier and have lower thermal expansion
coefficients than sodalime glass and are less
likely to crack when heated.
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Volume change for supercooled liquids, glasses
and crystals
Sol-Gel process
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NITRIDES AND FLUORIDES
Nitrides complex metal nitrides and oxide
nitrides are materials containing the N3- anion.
Examples AlN, GaN and Li3N. Many nitrides are
sensitive to oxygen and water and hence difficult
to synthesize. Li3N is obtained by heating Li in
a stream of N2 gas at 400 C. Sodium azide, NaN3
is used as nitriding agent, 2NaN3(s) 9Sr(s)
6Ge(s)
3Sr3Ge2N2(s) 2Na(g) Ammonolysis of oxides
result in nitrides 3Ta2O5(s) 10NH3(l)
2Ta2N5(s) 15H2O(g) Ca2Ta2O5(s)
2NH3(l) 2CaTaO2N(s)
3H2O(g) The high charge on N3- compared to O2-
results in a degree of increased covalence in
its bonding. Fluorides and other halides
fluorine solid state chemistry parallels much of
oxides because fluorine and oxygen have similar
ionic radii (130 and 140 pm for F and O
respectively).
Page 617 continued
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Layered MS2 and Intercalation
Synthesis and crystal growth d-metal disulfides
can be prepared by direct reaction of elements in
a sealed tube and purified by chemical vapour
transport with iodine TaS2(s) 2 I2(g) ?
TaI4(g) S2(g) effectively prepared at 850
C. Elements on the left of the d-block form
sulfides consisting of sandwich-like layers of
the metal coordinated to six sulfur ions, the
bonding between layers is very weak. See Fig.
23.42 on page 620. Intercalation and insertion
Insertion compounds can formed by from the
d-metal disulfides either by direct reaction or
electrochemically they can also be formed with
molecular guests. TaS2(s) x Na(g) ?
NaxTaS2(s) (0.4 lt x lt 0.7) Insertion by
electrochemical technique called
electrointercalation. This involves current
passed through during synthesis and hence the
amount of alkali metal incorporated (ne- It/F)
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Li(s) ? Li(aq) e- MS2(s)
xLi(aq) xe- ? LixMS2(s)
Alkali metals intercalation compounds of
chalcogenides
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Chevrel phases has a formula such as Mo6X8 or
MxMo6S8 where Se or Te may take the place of S
and the intercalated M atom may have a variety of
metals Mn, Li, Fe, Cd and Pb.
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Structure based on tetrahedral oxoanions are
very stable due to their small size that they
coordinate strongly to the four O atoms in
preference to higher coordination numbers.
Examples are SiO4, AlO4, PO4 although GaO4,
GeO4, AsO4, BO4, BeO4, LiO4, Co(II)O4 and ZnO4
are well known in their structural types.
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Aluminophosphates their structure and physical
properties parallel those of zeolites. The
general formula is AlO4PO4 (unit). Phosphates
and silicates generally calcium
hydrogenphosphates are inorganic materials used
in bone formation. Phospate ions, PO43- a
tetrahedral geometry. Hydroxyapatite,
Ca5(OH)(PO4)3 in which Ca2 ions are coordinated
by PO43- and OH- groups to form a three
dimensional structure and it is the main
constituent of teeth and bones. Other related
minerals octacalcium hydrogenphosphate,
Ca8H2(PO4)6, apatatite, Ca5(OH,F)(PO4)3
Clays, Pillared clays and layered double
hydroxides Sheet-like structures found in many
metal hydroxides and clays can be constructed
from linked metal oxo tetrahedra and octahedra.
Pillaring is stacking and connecting together
two-dimensional materials. Done by chemists in
attempt to increase the pore diameters of
zeolites in order for larger molecules to be
absorbed. Examples of clay hectorite and
montmorillonite have layer structures constructed
from vertex- and edge-sharing octahedra, MO6, and
tetrahedra, TO4.
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Sheet-like structure of the clay hectorite,
tetrahedra and octahedra centred on Al, Si or Mg
and separated by cations Cs or K.
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Pillaring of clay by ion exchange of a simple
monoatomic interlayer cation with a large
polynuclear hydroxometallate followed by
dehydration and cross-linking of the layers to
form cavities.
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Colours inorganic pigments Adoption of the
tetrahedral site by Co(II) in the spinel, AlCo2O4
result in the deep blue colour. Many inorganic
solids are used as pigments in colouring inks,
plastics, glasses and glazes. Intense colours can
arise from the d-d transitions, charge transfer
or intervalence charge transfer.
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White pigments TiO2 is used as a universal
white pigment. Other examples ZnO, ZnS, Pb(CO3),
lithophene (mixture of ZnO and BaSO4). TiO2
either in its rutile or anatase forms (see Fig.
23.64 page 633) is produced fro Ti ores (ilmenite
FeTiO3) by sulfate process. TiO2 dominates the
white pigment market and its found in paints,
coatings, printing ink to provide brightness in
the coloured pigments, plastics, fibres, paper
and white cement.
Black, absorbing and specialist pigments Carbon
black is the most important black pigment. It is
obtained by partial combustion or pyrolysis (i.e
heating in the absence of air) of hydrocarbons.
Copper(II) chromite CuCr2O4 with spinel structure
is also used as black pigment. Unlike carbon
black it can also absorb outside the visible
region including the infrared.
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SEMICONDUCTOR CHEMISTRY
Semiconductor typical band gaps lies in the few
electron volts in the 100 200 kJ.mol-1 . The
valence and conduction bands separation is in the
desired range 0.2 4 eV. Further influence of
the band gap is particle size in semiconductors.
Group 14 semiconductor crystalline and
amorphous silicon are cheap semi conducting
materials and widely used in electronic devices.
Si (pure form) has band gap of 1.1 eV Ge has
smaller band gap of 0.66 eV C in diamond form
has band gap of 5.47 eV. Doping of Si with Group
13 and 15 elements are extrinsic semiconductors.
Conductivity of pure Si is around 10-2 S.cm-1 at
room temperature. Amorphous Si is obtained by
chemical vapour deposition or by heavy ion
bombardment of crystalline solid. It is used in
silicon solar cells pocket calculators but
production costs diminish, are likely to find
much wider applications as renewable energy
sources.
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Systems isoelectronic to silicon Groups 13/15
and 12/16 elements can have enhanced properties
based on changes in the electronic structure and
electron motion. Group 13/15 semiconductor
GaAs, GaP, InP, AlAs and GaN. GaAs has better
response to electrical signals than Si as
important for number of tasks such as amplifying
the high frequency (1-10 GHz) signals of
satellite TVs, and can also be used with signals
up to 100 GHz. Group 12/16 semiconductors
ZnS (3.64 eV for cubic and 3.74 eV for hexagonal
phase), CdS (2.41 eV), CdTe (1.475 eV).
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Fullerides Solid C60 are considered as close
packed array of fullerenes interacting only
weakly through van der Waals forces holes in
arrays of C60 molecules may be filled by simple
and solvated cations and small inorganic
molecules. General formula, MxC60, M alkali
metals K, Rb, Cs sometimes Na or Li. Examples
K3C60, which is superconducting on cooling to 18
K and Rb3C60 at 29 K, CsRb2C60 at 33 K. Cs3C60
does not form fcc structure and is not
superconducting
Molecular magnets Molecular solid containing
individual molecules, clusters, or linked chains
of molecules can show three dimensional magnetic
effects such as ferromagnetism. Examples of
ferromagnetic materials, decamethylferrocene
tetracyanoethenide (TCNE), Fe(-Cp)2(C2(CN)4),
Mn analogue, MnCu(2-hydroxy-1,3-propylenebisoxamat
o).3H2O. The incorporation of several d-metal
ions into a single complex provides molecule that
acts as a tiny magnet termed single molecule
magnets (SMM).
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Inorganic liquid crystals inorganic metal
complexes with disc- or rod-like geometries can
show liquid crystalline properties. Liquid
crystalline or mesogenic are compounds with
properties that lie between those of solids and
liquids and include both. They are widely used in
displays. Rod-like calamitic and disc-like
discotic.