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MATERIALS
CHEMISTRY II
CHEM2007
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
TOPICS TO BE COVERED
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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
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)
1000 °C
BaTiO3(s) + CO2(g)
½Y2O3(s) + 2BaCO3(s) + 3CuO(s) 930°C/air and 450°C/O2
+ 2CO2(g)
YBa2Cu3O7
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.
N2/600°C 2 TlTaO (s)
Tl2O(s) + Ta2O5(s)
3
V2O5(s)
H2/1000 °C V O (s)
2 3
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) 500 °C TaS2(s)
Tl2O3(l) + 2 BaO(s) + 3CaO(s) + 4CuO(s) 860 °C
Tl2Ba2Ca3Cu4O12(s)
High pressures such as 100GPa at high temperatures about 1500 °C are used.
• 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 90 °C Na12[Si12Al12O48].nH2O(s) (zeolite
LTA) + 24NaOH(aq)
These are useful for preparing open structure aluminosilicates (zeolites).
2La3+ + Cu2+ OH-(aq) 2La(OH)3.Cu(OH)2(s) – gel 600 °C 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.
• 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.
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
Compound Structure
Composition, x
Electrical
character
CaOx
Rock-salt
1
Insulator
TiOx
Rock-salt
0.65 – 1.25
Metallic
VOx
Rock-salt
0.79 – 1.29
Metallic
MnOx
Rock-salt
1 – 1.15
Semiconductor
FeOx
Rock-salt
1.04 - 1.17
Semiconductor
CoOx
Rock-salt
1 - 1.01
Semiconductor
NiOx
Rock-salt
1 - 1.001
Semiconductor
CuOx
PtS (linked CuO4
square planes)
1
Semiconductor
ZnOx
Wurtzite
Slight Zn excess
Wide band gap ntype semiconductor
(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)
(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 it’s 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)
Higher oxides and Complex oxides
Binary metal oxides that do not have a 1:1 metal:oxygen 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.
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 B[AB]O4 in which the more abundant B cation is distributed over both
coordination spheres. The occupation factor, λ of a spinel is the fraction of B
atoms in the tetrahedral sites λ = 0, for a normal spinel and λ = 0.5 for inverse
spinel, B[AB]O4.
A
B
Al3+
Cr3+
Mn3+
Fe3+
Co3+
d0
d3
d4
d5
d6
Mg2+
d0
0
0
0
0.45
Mn+
d5
0
0
Fe2+
d6
0
0
Co2+
d7
0
0
Ni2+
d8
0.38
0
0.1
0.5
0.5
0.5
0
Cu2+ Zn2+
d9
d10
0
0
0
0
0.5 0
0
Structure of spinel
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
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.
Element TC/K
Compound
TC/K
Zn
Cd
Hg
Pb
Nb
Nb3Ge
Nb3Sn
LiTi2O4
K0.4Na0.6BiO3
YBa2Cu3O10
Tl2Ba3Ca3Cu4O12
MgB2
K3C60
23.2
18.0
13
29.8
93
134
40
39
0.88
0.56
4.15
7.19
9.50
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 Ln1xAxMnO3 (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)
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.
Volume change for
supercooled liquids,
glasses and crystals
Sol-Gel process
Sol
Volume
Liquid
Hydrolysis
Supercooled
liquid
Gel
Dry
Glass
Porous
Ceramic
Crystal T
g
Temperature
Tf
Heat
Heat
Dense
Ceramic
or Glass
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) 750°C, sealed Nb tube 3Sr3Ge2N2(s) + 2Na(g)
Ammonolysis of oxides result in nitrides;
3Ta2O5(s) + 10NH3(l) 700 °C 2Ta2N5(s) + 15H2O(g)
Ca2Ta2O5(s) + 2NH3(l)
800 °C
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
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 < x < 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)
CP
e-
e-
Li
R
MS2
+
LixMS2
Li+
Li(s) → Li+(aq) + e- ;
MS2(s) + xLi+(aq) + xe- → LixMS2(s)
Alkali metals intercalation compounds of chalcogenides
Compound
Δ/pm
Compound
Δ/pm
K1.0ZrS2
160
K0.4MoS2
214
Na1.0TaS2
K1.0TiS2
Na0.6MoS2
117
192
135
Rb0.3MoS2
Cs0.3MoS2
245
366
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.
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.
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 edgesharing octahedra, MO6, and tetrahedra, TO4.
Cs/K
SiO4
(Al, Mg)O6
SiO4
SiO4
(Al, Mg)O6
SiO4
Sheet-like structure of the clay hectorite, tetrahedra and octahedra centred on
Al, Si or Mg and separated by cations Cs+ or K+.
Pillaring of clay by ion exchange of a simple monoatomic interlayer cation with
a large polynuclear hydroxometallate followed by dehydration and crosslinking of the layers to form cavities.
Heat
Dehydration
Void
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.
Material
Colour
AlCo2O4
CaCuSi4O10
PbCrO4
Blue
Blue
Yellow-orange
Zn2SiO4 (Mn2+ dopant)
Y2O3 (Eu3+ dopant)
CaMg(SiO3)2 (Ti dopant)
Green
Red
Blue
CaSiO3 (Mn dopant)
Ca5(PO4)3F,Cl (Mn dopant)
Yellow-orange
orange
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 it’s 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.
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.
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).
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-propylenebisoxamato).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).
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.