4. Ceramics and Semiconductors.
In this chapter we analyze the consequences of a filled valence band. The chemical
bond that is obtained in this case is covalent for elemental solids and ionic or mixed
covalent-ionic for compounds. The nature of the chemical bond controls the crystal
structure of the materials. These bonds are generally stronger than the metallic
bond and give the solid a high melting temperature that prevents casting from a
melt. Ceramics are hard and brittle: they cannot be machined the way metals are.
Ceramic pieces are fabricated by forming a paste, consisting of the ceramic powder
and water, into a near-final shape and solidifying it by firing. Firing causes
sintering of the ceramic particles through diffusion of atoms or molecules. Glass is
an amorphous silicon oxide with the addition of sodium, magnesium or boron.
These additions form positive ions that neutralize the oxygen atoms and allow a
disordered structure. Glass does not have a melting point but increases its viscosity,
upon cooling, to values so high that the glass cannot be deformed at room
temperature. The fabrication of glass objects makes use of its high viscosity.
Cement is a ceramic that hardens by a chemical reaction with water. Hardening
increases with time and reaches its final value in more than a year.
4.1.
A filled valence band and the resulting properties of the solid.
Diamond (i.e. carbon), silicon and germanium, have 4 valence electrons. Their
valence band is completely filled. An energy gap separates the filled valence band from a
higher, empty band of electron orbitals. (See figure 4.1.C).
Electron
Energy
Energy
EnergyGap
Gap
N
A
Metal
2N
B
Metal
2N
2N
C
Insulator
D
Semiconductor
Figure 4.1. Energy bands in metals and ceramics. Gray fields: occupied energy levels.
White fields: unoccupied energy levels. A) Metal with less than 2 valence electrons per
atom. B) Metal with 2 valence electrons per atom and overlapping bands. C) Insulating
ceramic with full valence band separated by large energy from upper, empty band. D)
Semiconductor with full energy band separated by small energy from upper, empty,
conduction band.
In diamond, this energy gap is 8.5 eV, which is too large for thermal excitation of
electrons at any practicable temperature. By virtue of Pauli’s exclusion principle, none of
the electrons can move from one orbital to another in the band since two electrons
already occupy any possible orbital. The consequences are illustrated in figure 4.2. An
applied electric field cannot accelerate the electrons in this material: no current can flow.
The orbitals (shapes) of the four valence electrons dictate the positions of the four atoms
to which any atom is bonded. To change the position of any atom would require novel
orbitals (red arrow in figure 4.2) which do not exist in the valence band; the formation of
an intermediate bonding orbital i would require the excitation of electrons into the higher,
empty, band. This, in diamond, with a gap of 8.5 eV, would require an impossibly large
stress. No plastic deformation is possible and diamond is the hardest material known.
The energy gaps of other ceramics, such as silica (SiO2), alumina (Al2O3), silicon nitride
(Si3N4), are somewhat smaller than that of diamond, but still large enough to prevent
electric conduction or plastic deformation.
E
Empty Band
i
Energy Gap
i
Full Valence Band
Figure 4.2. The properties of a solid with a full valence band. Impossible processes are
marked in dotted arrows. Blue arrow: the kinetic energy of valence electrons can not be
altered either because the final state does not exist (energy gap) or is already occupied.
Yellow arrow: a photon of visible light cannot be absorbed because the absorbing
electron would have to be excited into the energy gap or into an occupied state. Red
orbital energy (i): No orbital for a transition position of atoms can be formed: it cannot
exist in the energy gap or would require an excessive energy to be formed in the empty
band: plastic deformation is not possible.
Now to the optical properties: light has photon energy between 2 and 3 eV. No
electron can be excited by this amount: ceramics are transparent to all light with photon
energy hν smaller than the energy gap.
The only solid elements with full valence band are diamond, silicon, germanium,
sulfur and phosphorus. All other ceramics are compound. The most important are the
oxides silica (SiO2), alumina (Al2O3), titania (rutile TiO2), zirconia (ZrO2); the carbides
such as silicon carbide (SiC), Titanium Carbide (TiC), tungsten carbide (WC); and the
nitrides such as silicon nitride (Si3N4), titanium nitride (TiN) and boron nitride (BN).
The sulfides such as zinc sulfide (ZnS) and cadmium sulfide (CdS) are interesting for
their optical properties.
4.1.1. Semiconductors.
Silicon and germanium are ceramics with a relatively small energy gap between
the filled and the empty energy band as illustrated in figure 4.1.D. This gap is 1.15 eV in
silicon and 0.76 eV in germanium. The III-V compounds, such as GaAs, GaP, etc. (see
the periodic table), have similar band gaps. This allows the promotion of electrons from
the valence band to the empty band, called conduction band, and the conduction of
electrons. These are the semiconductors which form the basis of modern electronics. We
shall see later that the addition of minute amounts (a few parts per million) of chosen
impurities allow the semiconductors to conduct electricity and permits the formation of
electrical rectifiers, transistors, light-emitting diodes (LED), transistors and solar cells.
In ceramics, the energy band or valence electrons is completely filled. It is
separated by a large energy gap from the next higher band of energy levels which
are empty. As a consequence, ceramics are electrical insulators, can be transparent
to visible light, and cannot be deformed plastically but are very hard.
Semiconductors are ceramics that possess a small energy gap, usually between 0.3
and 2.5 eV.
4.2.1. Covalent, ionic and mixed bonds.
In solid diamond, silicon and germanium, all atoms being the same, the sharing of
the electron orbitals between neighboring atoms is symmetrical as shown in figure 4.3A.
All atoms remain neutral. This constitutes the covalent bond.
+
A
B
Figure 4.3. (A) Covalent bond, (B) Mixed, covalent-ionic bond. The atom on the right
has a higher electronegativity than the one on the left. The circles are the atomic orbitals
before bonding: the thicker line indicates the molecular orbital; the positive ion decreases
in size, the negative ion increases.
In compound ceramics, the energy with which atoms bind electrons differs from
one element to the other. The power to attract electrons in a chemical bond, the
electronegativity, has been measured and is shown in figure 4.4. It is smallest at the left
of the periodic table, where the atoms have one electron in addition to completed shells;
they give up this extra electron relatively easily. The electronegativity increases as we go
to the right of the table and is largest for the halogens, which attract electrons more
strongly in order to complete their shell. The shape of the molecular orbital in a
compound is sketched in figure 4.3B. The valence electrons move towards the atom with
higher electronegativity. As a result, this atom carries a negative charge and is enlarged;
the atom with the lower electronegativity carries a positive charge and diminishes in
size;. The result is an ionic bond. In all real compounds, the transfer of charge from the
positive to the negative ion is not a full electron. As sketched in figure 4.3 B, some
electron charge remains on the positive ion and only a fraction of an electronic charge is
transferred to the other atom. The ionicity of the bond, that is, the fraction of an
electronic charge that is transferred from the positive to the negative ion is approximated
by the equation
% ionicity = [1 – exp–0.25(XA-XB)2]
where XA and XB are the electronegativities of the two elements in the bond.
Figure 4.4.
Electronegativities of the elements.
A pure ionic bond, in which the electron is totally transferred from one atom to
the other, does not exist. The most ionic bond is that of cesium fluoride, with an ionicity
of 95%. In this compound, the cesium atom retains only 5% of its original electronic
charge. Sodium chloride has an ionicity of 68 %. At the other extreme, SiC possesses
12% ionicity and GaAs 9.5%. The covalent bond in diamond, silicon and germanium, of
course, has zero ionicity. As a general rule, the farther the columns from which the
components of a compound are taken, the higher the degree of ionicity, the closer the
columns, the smaller the ionicity and the more covalent the bonds.
The degree of ionicity of the chemical bonds has practical implications for the
properties of the solids. In a covalent solid, the shapes of the molecular orbitals govern
the positions of the atoms. This is especially important in the covalently bonded
materials such as diamond, silicon and their compounds SiC, Si3N4, SiO2 because the
orbitals of carbon and silicon are formed from sp3 and sp2 hybrids. The sp3 hybrid is a
new atomic orbital that is formed by the combination of the s and the three p orbitals.
Four such linear combinations can be formed; these four hybrids extend from the atom in
the four directions shown in figure 4.5. These hybrids are responsible for the crystal
structure of diamond, silicon and germanium, shown in figure 4.6 A. They also
determine the positions of the oxygen atoms in SiO2, illustrated in figure 4.6.B. This
“tetrahedral” placement of the neighbors of silicon atoms is of importance in the structure
of glass, of SiC and Si3N4 and all silicates.
In ionic solids, where a large fraction of the valence electron is transferred to the
more electronegative ion, the ions are attracted by purely coulombic forces between the
electrical charges. The bond is not governed by the shape of the electron orbitals. The
positions of the atoms in the solid are governed by the neutrality of the material (a
negative ion must be surrounded by positive ions and vice versa) and the relative sizes of
the ions.
In practice, one considers as ionic the solids whose structure and chemical
properties are determined by their ionic character. Any solid with ionicity larger than
50% is considered ionic. Similarly, compounds with small ionicity, such as SiC, the
compound semiconductors such as GaAs, GaP, Si3N4, are considered covalent. Covalent
materials are usually harder and more brittle than the ionic. The intermediate compounds
are called mixed or polar covalent bonds. A useful example of a polar covalent bond is
that of water with an ionicity of 40 %. Table 1.1. shows the ionicity of some important
ceramics.
As a rule, the covalent solids are harder than the ionic, the lower the percent
ionicity, the harder the material.
Figure 4.5. The four directions of the sp3 hybrids. These hybrid orbitals are responsible
for the structure of diamond and silicon, as shown in figure 4.6
A
B
Figure 4.6. Left: structure of diamond and silicon. Right: The structural element of
compounds containing silicon: in SiO2, it is the SiO=4 pyramid where the small black
atom is silicon and the larger gray atoms represent oxygen.
Table 1.1 (from Callister)
Material
CaF2
MgO
Al2O3
SiO2
Si3N4
ZnS
SiC
Percent Ionicity
89
73
63
51
30
18
12
The metallic, covalent and ionic bonds, in which electrons are shared between the
atoms, are primary bonds.
Chemical bonds in ceramics are primary bonds: the electrons are shared between
the atoms. In elemental ceramics (C, Si, Ge), the sharing is symmetrical; the bonds
are covalent. In compounds, electron charge is shifted from the less electronegative
to the more electronegative atom and forms ionic bonds. The larger the difference
between the electronegativities, the higher is the ionicity of the bonds. Compounds
of atoms from the far left and the far right of the periodic table (LiF, NaCl) are
ionic, compounds of atoms from the same or neighboring columns (SiC, Si3N4, III-V
compounds) are nearly covalent. Covalent ceramics are harder than ionic solids.
4.3
The structure of Ceramics.
In metals, we have seen that the crystal structure is mainly caused by the non
directional bond: the structure is such that the atoms are packed as close together as
possible, hence the hexagonal close packed and face centered cubic structures. In
covalent ceramics, the structure is determined by the directions of the chemical bonds. In
ionic materials the structure is largely influenced by the sizes of the ions and the electric
charges they carry.
4.3.1. The diamond structure.
We have just seen that the crystal structure of diamond, silicon and germanium is
imposed by the geometry of the sp3 hybrid electron orbitals that form the covalent bond.
As a consequence, the structure of these elements is not the densest possible. When these
solids melt, they contract. (A familiar case of structure dictated by the direction of the
bonds is ice: when ice melts, the water molecules assume random positions that are more
compact. Ice contracts upon melting and water expands when it freezes. This makes ice
float on water and causes the rupture of water pipes in unheated houses in winter).
Figure 4.7.A shows the diamond structure. Figure 4.7.B shows the structure of the III-V
semiconductor compound gallium arsenide. Closer examination reveals that it is in fact
an FCC structure where each FCC site corresponds to a GaAs molecule.
Figure 4.7. (A) The diamond structure. (B) The zincblende structure where one half of
the atom sites are occupied by Ga and the other by As atoms.
4.3.2. The structures of compounds.
The crystal structure of compounds, which are usually ceramic and posses a
mixed covalent-ionic structure, can be very complex.
In ionic crystals, the structure is governed by two principles:
• positive and negative ions must alternate for electrostatic cohesion;
• the ions, which have different sizes, must touch each other so that the
structure does not collapse.
A
B
Figure 4.8. (A) The NaCl structure (B) the CsCl structure. White atoms are Cl.
These principles are responsible for the two structures of sodium chloride and cesium
chloride, shown in figure 4.8.
The structure of compounds that contain silicon or carbon is governed by the two
principles stated above and the geometry of the sp3 hybrid bonds. Silicon nitride, Si3N4,
for example, possesses 7 atoms per formula. Each Si atom must be surrounded by 4 N
atoms in the sp3 configuration and every N atom touches 3 Si atoms arranged in a plane.
The result is a complex hexagonal structure. All forms of silicon oxide consist of SiO4=
pyramids shown in figure 4.6.B. These pyramids are stacked in such a way that every
oxygen atom belongs to two neighboring pyramids (i.e. is bonded to two silicon atoms).
It is easy to verify that this sharing results in an electrically neutral SiO2. Such a
crystalline structure is sketched in figure 4.9.A.
4.3.3. The structure of glass.
The basic component of glass is silicon oxide and its basic structural element is
the SiO4-4 tetrahedron already examined in figure 4.6B. Besides SiO2, glass contains
elements such as sodium or potassium that form positive ions. These atoms are
structure modifiers. They attach themselves to oxygen atoms at the corners of the
tetrahedra; forming an ionic bond, they transfer an electron to the oxygen and remove the
need for the corner to be shared with another tetrahedron. The result is that the rigid
crystalline structure is no longer necessary; the tetrahedra form an irregular, amorphous
structure as shown in figure 4.9.B. Thus, a silica glass is a disordered arrangement of
ordered SiO4 tetrahedra.
=
+
Figure 4.8. Silica tetrahedron with positive ion attached. This ion provides one electron
to the oxygen and obviates the need to share this oxygen with another tetrahedron.
An amorphous form of pure silica exists as fused quartz. Some of the corners of
silica tetrahedra are not shared, as a consequence the structure is disordered.
In contrast to crystalline solids, amorphous materials do not have an exact
melting temperature. As they cool from the melt, their viscosity increases until it is
so large that no deformation can be obtained in any measurable time. This is shown
in figture 4.10. It is correct to say that a solid glass is a liquid with a very high viscosity.
The fabrication of glassy objects makes use of this important property.
A. Crystalline silica
(quartz, crystobalite)
B. Glass.
C. Fused quartz
Figure 4.9. Structures of crystalline silica, glass and amorphous fused quartz.
4.3.4. The viscosity of glass
The single most important property of glass that influences these processes and
subsequent heat treatments is viscosity (η), which changes by many orders of magnitude
with the temperature as shown in figure 4.10.
Figure 4.10. Variation of the viscosity of various glasses with temperature.
The crystal structure of covalent ceramics is determined by the direction of the
electron orbitals that form the chemical bond. In diamond, silicon and germanium,
these directions are the four space diagonal of a cube and result in the diamond
structure of these materials. The structure of ionic crystals is determined by the
relative sizes of the components and the electrical neutrality which demands that
positive ions touch negative ions and vice versa. In silica and silicates, the short
range structure unit is the SiO=4 tetrahedron. Electrically neutral SiO2 crystals are
formed when every tetrahedron shares an oxygen corner with four other tetrahedra.
When positive ions such as Na, Mg or B are added to the silica, they neutralize some
oxygen atoms and remove the need to connect tetrahedra at their corners: the result
is a disordered array of tetrahedra: this is glass. Since glass does not crystallize, it
does not possess a melting temperature but increases in viscosity as the temperature
decreases and derives its solidity from a very large viscosity.
4.4. Uses and processing of ceramics.
Ceramics have been used earlier than metals. Stones, clay and porcelain are
natural ceramics. Natural materials are compounds of alumina (Al2O3), silica (SiO2) and
water, and contain various mixtures of other oxides. Modern, high performance ceramics
are synthesized; they include alumina (Al2O3), silica (SiO2), other oxides, such as TiO2,
ZrO2, Na2O, Li2O; carbides WC, TiC, SiC, BC; nitrides: Si3N4, TiN, BN and borides
TiB2. Since they are compounds of metals with the lighter elements N, C and O, their
density is usually lower than that of metals. Clay and porcelain have been used for many
centuries for the fabrication of dishes, vases and sanitary equipment. Porcelain is also
used as an electric insulator in furnaces, switches, and in high-voltage overland power
transmission lines. The synthetic ceramics have recently assumed critical importance in
applications where extreme hardness, wear resistance or strength and chemical stability at
high temperatures are required. The low weight of the silicon nitride is also exploited in
turbocharger turbine wheels for their low inertia (Figure 4.11) and in high-speed ball
bearings, for instance in dentist drills. Ceramics, as a rule, have a high melting
temperature. They are used as refractory linings in furnaces and in crucibles for the
melting of metals.
Figure 4.11. Silicon nitride turbine for the turbocharger of an automobile.
Crystalline ceramics cannot be cast because of their high melting temperature,
often, they dissociate chemically before they melt. Because of their high hardness and
lack of ductility, they cannot be shaped by plastic deformation. The starting material in
the fabrication of ceramic objects is a paste composed of small solid particles, water and
often a binder. The paste is easily shaped at room temperature by various methods and
acquires a modest strength after drying. The ceramic piece formed at room temperature
is called a green. Its final solidification and hardness are acquired by firing. We
describe the formation of the green body in section 4.4.1 and the final solidification by
firing in section 4.4.2.
4.4.1
Forming the green body
Green bodies of both traditional and new ceramics are formed by a number of
methods that are enumerated below.
Slip Casting
A slip is first prepared; it consists of a suspension of clay or ceramic powder in
water. This is cast into an absorbent mold as indicated in Fig. 4.12a. After sufficient
water loss, the partially wet solid body is removed and dried further prior to firing. Vases
as well as large and complex parts like Si3N4 turbine rotors have been reproduced this
way.
Pressing
In this case the specially prepared dry powder is placed in a die and pressed under
high uniaxial forces to make a green compact (Fig. 4.12b). Only relatively small parts
can be made this way. An improvement over applying uniaxial compressive loads is to
isostatically compact the powder. Here the part is enclosed in a flexible airtight rubber
bag and immersed in a chamber filled with hydraulic fluid. Hydrostatic (uniform over all
directions) pressure is applied during (cold) compaction, eliminating the complex
pressure distribution throughout the die due to mold wall friction. Electrical insulators,
bushings, magnetic components and spark plug bodies are produced this way for
subsequent sintering.
Hot pressing combining compaction and sintering operations is also practiced to make a
variety of highly dense ceramic components.
Extrusion
The slurry is extruded through a die as shown schematically in Fig.4.12c. Long
tubes, rods and honeycomb structures used to shield thermocouples, or for heat exchanger
applications are made this way. The technique is similar to the extrusion of metals except
that an auger is sufficient to push the soft material through the die.
Tape Forming
This process, shown in Fig.4.12d, is used to make thin ceramic parts like alumina
substrates for integrated circuit chips and special capacitors. A thin layer of slip is laid on
a flat carrier which can be a paper sheet or polymer film. A doctor blade controls the slip
thickness resulting in a tape that can be stamped into small shapes. These can be stacked
or coiled for subsequent sintering.
Injection Molding
In this hybrid process, ceramic powder is mixed with thermosetting (organic)
polymer binder and injected under pressure into a die or mold. The bodies are then
ejected and new ones are replicated. Complex parts with excellent dimensional control
can be made this way.
Figure 4.12. Forming the green (low temperature processes) in ceramic processing, from
Ohring. A) Slip casting, B) Pressing, C) Injection molding; this can also be used for
extrusion when the paste does not enter a mold, D) Tape forming.
4.4.2. Densification
Firing
As everyone knows who has made pottery, the dried green ware is next fired in a
furnace to densify it into a hard body. Firing temperatures vary depending on the
composition of the body as table 4.2. indicates.
As a rough proposition bodies that containing glass-forming silica and alkali
oxides are fired at lower temperatures than bodies richer in alumina and magnesia. (Note
that the colored designs on the surface of ceramic ware require application of an
appropriately compounded glaze and a second firing. The glass that forms fills all the
pores, making the body impervious to water penetration).
Firing proceeds in two stages. At the lower temperatures (<450 C), water is
expelled and the binder is burned off. As the temperature is increased, a liquid glass is
formed when sintering aids are added to the powder. Above 1000C, sintering occurs that
fuses the ceramic crystals together.
Table 4.2. Firing temperatures of ceramics.
CERAMICS
1. Porcelain enamels (on cast irons and
steels)
2. Clay products (bricks, sewer pipes,
earthenware, pottery)
3. Whitewares (porcelain, china,
sanitary ware)
4. Refractories (alumina, silica and
magnesia brick, silicon carbide)
5. Synthetic high-performance ceramics
(aluminas, nitrides carbides,
titanates)
APPROXIMATEFIRING TEMPERATURE
650 – 950 °C
1000 – 1300 °C
1000 – 1300 °C
1300 – 170 °C
1300 – 1800 °C
Sintering
The final stage of solidification consists of sintering in which the grains of
ceramic fuse together; this is shown in figures 4.13 and 4.14. Two natural phenomena
are crucial to sintering, these are the diffusion of molecules at high temperature and the
decrease of surface energy. Diffusion is sufficiently fast for sintering at temperatures
much lower than the melting point; it can be effective at temperatures as low as half the
melting temperature in degrees Kelvin. Diffusion is the random movement of molecules
made possible by their thermal vibration energy. Surface diffusion is much more rapid
than diffusion through the bulk because the diffusing molecules need not displace their
neighbors in order to change place. Consider two grains that are in contact, as shown in
figure 4.13. Their total surface area is that of two spheres. At elevated temperatures,
molecules at the surface of the grains move in random directions. This is shown as A.
The molecules that diffuse to the contact area fill the crevice between the two
grains and reduce the surface area, and therefore the total surface energy. This lowering
of energy favors the type B diffusion. The result is that the two grains are fused together.
Sintering of the grains is rarely complete: pores are left in the junction of three grains.
This porosity weakens the material and may be undesirable. For this reason, sintered
ceramics are marketed with an indication of density that is a measure of porosity. In this
manner, a 95% theoretical density means that the volume of the material includes 5%
porosity. Two methods are used to minimize porosity where mechanical strength is
important, one is Hot Isostatic Pressing (HIP) in which the ceramic is encapsulated in a
glass skin and subjected to high pressure gas during sintering: the particles are thus
pressed together; the other is liquid phase sintering, in which an substance that liquefies
at the sintering temperature is added to the powder and penetrates the pores.
A
B
B
Figure 4.13. Sintering of two particles. (A): random surface diffusion of molecules. (B)
diffusion into the crevice between the grains reduces the total surface area and therefore
the total energy of the two grains.
Figure 4.14. Partially sintered particles. As sintering proceeds further, the contact areas
grow, the particles move closer together and porosity decreases. The material shrinks in
the process.
Since the particles fuse together, the material shrinks during sintering. Allowance
for this shrinking must be made in the design of ceramic pieces.
After sintering, ceramics can only be machined by abrasion. Abrasion is the
“scratching” of the material by small grains of a material that is still harder: ceramics are
machined by diamond powder bonded to a polishing pad or a metal slicing wheel. The
machining of ceramics is slow and expensive.
4.4.3
Fabrication of glass objects.
The magnitude of the viscosity of glass is critical in all stages of its manufacturing
and forming processes. Viscosity determines virtually all of the melting, forming,
annealing, sealing and high temperature heat treatments of glass. Typical viscosity ranges
for the following operations are as follows:
OPERATION
1. Glass melting and fining (bubble
elimination)
2. Pressing
3. Gathering or gobbing for forming
4. Drawing
5. Blowing
5. Removal from molds
6. Annealing
7. Use
VISCOSITY (Pa-s)
5 to 50
50 to 700
50 to 1300
2000 to 10000
1000 to 3000
103 to 106
1012 to 1015
10 13 to 1015
The temperatures at which these operations are carried out differ with glass
composition and these are depicted in Fig.4.15 for a number of commercial silica based
glasses. Each of the latter has a specific viscosity-temperature dependence and individual
set of strain, annealing, softening and working point temperatures. These are defined in
the following ways:
Strain Point : The temperature at which internal stresses are reduced significantly in a
matter of hours. At this temperature the viscosity is taken as 1013.5 Pa-s.
Annealing Point : The temperature at which the viscosity is about 10120 Pa-s and the
internal stresses are reduced to acceptable commercial limits in a matter of minutes.
Softening Point : At this temperature glass will rapidly deform under its own weight.
For soda-lime glass this occurs at a viscosity of 107Pa-s.
Working Point : At this temperature glass is soft enough for most of the common hot
working processes. The working point corresponds to a viscosity of 103 Pa-s.
Figure 4.16 illustrates the various methods that are in use in the fabrication of
glass objects. These are:
a. Pressing
b. Blowing
c. Casting
d. Rolling and Float Molding
a. Pressing
An example of pressing involves the forming of glassware by compressing it in a mold
with a plunger. The process (Fig.4.16a) resembles closed die forging of metals and
compression molding of polymers. Parts with diameters of 5 cm, depths of 2 cm and
weights up to 15 kg are pressed using pressures of 0.69 MPa (100 psi).
b. Blowing
Hand blowing of glass is practiced today much as it was in antiquity. Glass is gathered on
the end of an iron blowpipe and with lung power or the use of compressed air the glass
blower shapes the “gathers”. Artistic glassware is hand blown while continually rotating
the glass. The largest pieces reach weights of 15 kg, lengths of almost 2m and diameters
of 1m. Examples of machine blown ware include glass containers and light bulb
envelopes. Gobs of glass are delivered to the blank mold where they are preformed either
by blowing or pressing. After rotating the blank it is then blown in the blow mold to
finish the operation (Fig.4.16b).
c. Casting
In this process glass is poured into or on molds, tables or rolls. The largest piece ever cast
was the Mount Palomar reflector, a cylinder measuring 5.08 m (200 in) diameter and
0.457 m thick. Other products routinely cast are radiation- shielding window blocks and
television and cathode ray tubes. The latter are centrifugally cast, and as the molten gobs
are spun the glass flows up to create a uniform wall thickness (Fig.4.16c).
d. Rolling and Float molding
Both of these methods produce plate glass. In continuous processes, shown in
figure 4.17, raw materials are fed in at one end of very large horizontal furnaces (with
capacities of 1000 tons) where successive melting and refining of glass occurs. At the
other end the glass is fed into a pair of cooled rollers and the emerging ribbon of solid
glass is then conveyed on rollers through an annealing lehr. Plate glass nearly 4 m wide
and 1 cm thick can be rolled at rates of over 6 m/min. The surfaces of rolled plate glass
must be further ground and polished.
Figure 4.16 From Ohring
Figure 4.17. Continuous rolling of plate glass.
The Pilkington float molding process is now widely used to make plate glass. In this
process soft glass is floated on the flat surface of a molten tin alloy that controls the lower
glass surface temperature. The upper surface of the glass develops by gravity. As the
temperature is reduced the glass stiffens and is conveyed to be annealed. This process
produces a distortion-free plate glass of comparable flatness to that produced by rolling
glass. A drawback of the process is incorporation of Sn in the glass.
All one has to do is look through plate glass windows of the last century to
appreciate how far we have come in glass plate manufacture.
4.5
CEMENT AND CONCRETE
Concrete and reinforced concrete are two of the most important civil engineering
materials. In tonnage consumed they far exceed that of steel, wood and polymers
combined. They are the essential ingredient in some of the largest structures built in this
century such as high rise buildings and airport runways.
4.5.1 CEMENT
Concrete is composed of cement, aggregates (sand, gravel, crushed rock) and
water. Cement, a generic term for concrete binder, is the key ingredient. The most
important binder for concrete is Portland cement which is produced from an initial
mixture of 75% limestone (CaCO3), 25% clay, assorted aluminosilicates and iron and
alkali oxides. This mixture is ground and fed into a rotary kiln (a large cylindrical
rotating furnace of about 6 m diameter and 180 m length!) together with powdered coal.
Progressive reactions at temperatures extending to1800K break down clays, decompose
the limestone to yield quicklime or CaO, and then fuse them to produce clinker (pellets)
5-10 mm in size. After cooling, the latter is mixed with 3-5 wt % gypsum (CaSO4) and
ground to the powder that is Portland cement.
Portland cement is composed of the following identifiable compounds;
Calcium oxide (lime) CaO usually denoted by
Silicon oxide (silica) SiO2
Aluminum oxide (alumina) Al2O3
C
S
A
Some three quarters by weight is composed of
tricalcium silicate (C3S) (i.e. 3CaO.SiO2 or Ca3SiO5)
dicalcium silicate (C2S) (i.e. 2CaO.SiO2).
tricalcium aluminate (C3A) (i.e. 3 CaO.Al2O3).
Different proportions of these and other ingredients are blended to produce cements that
either set slowly or more rapidly, liberate less or more heat of hydration, or are intended
to resist degradation by water containing sulfates etc.
Water is added to the cement which solidifies through chemical reactions which are
complex and not entirely understood. Cement does not harden by drying but the water is
incorporated in the solid. Cement does not shrink when hardening.
Several important hydration reactions occur at different rates and can be written as:
C3A + 6H→ C3AH6 + heat
2C3S + 6H → C3S2H3 + 3CH + heat
2C2S + 4H → C3S2H3 + CH + heat
(hours)
( days )
(months)
The first reaction causes the cement to set, while the second and third reactions cause
hardening. Note that all these reactions are exothermic: the cement heats up as it sets.
In constructions where a very large volume of concrete is needed, such as in water dams,
one must avoid high temperatures that could result from rapid exothermic reactions. In
this case, one uses a cement with a larger proportion of dicalcium silicate C2S which
takes months to complete its reaction. Fast hardening cements, used for small
constructions, contain a larger proportion of C3S which completes its reaction in a matter
of days. One often observes water spray on top of constructions sites; the purpose of this
spray is to cool the concrete.
Wet cement is only workable as long as it sets, and this period establishes the pot
life of the mix. The typical time dependence of heat evolution and hardening of cement is
shown in Fig.4.18. After a large initial burst of heat there are much smaller secondary
peaks that coincide with the start of solidification. During the setting period cement has
no strength, but with solidification, strength rises. The slow setting of cement is used in
building an attractive stone wall. When you build a wall with cement between the stones,
let the cement set for a few hours. (The calcium aluminate hardens). It is then possible
to remove any excess cement with a brush and water and obtain a neat wall (See figure
4.19). It is advisable to build the wall in the morning and clean it in the evening. If one
waits overnight, the cement is too hard and the excess can no longer be removed. Several
days are then necessary for sufficient solidity.
Figure 4.18. The setting and hardening of Portland Cement. Dotted line and left scale:
evolution of heat during the reaction. Solid line and right scale: increase in the
compressive strength of cement with time.
Figure 4.19. Brushing of cement after a few hours of setting. Left: stone wall with
cement as applied. Right: stone wall with excess cement removed by simple brushing
and rinsing with water.
The fabrication of ceramic pieces proceeds in two major steps: forming of a green
(at room temperature) and firing.
In the preparation of a green, one prepares a paste or slurry consisting of the
ceramic powders, water and, often, a binder. The green is formed by any method
appropriate for a past:
Slip casting (where a fluid slurry is poured into a porous mold; the latter extracts
the water from a thin layer which solidifies, the remainder of the slurry is poured
out)
Pressing (where the paste is pressed into a mold)
Extrusion (where the paste is continuously driven through a dye
Injection molding (where the paste is driven into a mold)
Tape forming (where the paste in formed into a tape).
The green is allowed to dry and acquires a modest strength. In that condition, it can
be machined.
Firing consists of heating the ceramic to high temperatures. In this process, the
green is dried, the binder is burned off, and the ceramic grains are fused together by
surface diffusion. This process leaves the material porous. High temperature
pressing is needed to remove porosity.
The fabrication of glass uses the increasing viscosity as the temperature decreases.
Glass objects can be pressed into a mold, or blown into a mold. Plate glass is
formed by rolling or flowing the still liquid glass on liquid tin where it cools and
solidifies.
Cement hardens by a chemical reaction of the cement powder with water. This
reaction takes many months to complete. It does not shrink. Cement is a
combination of calcium oxide, silica, alumina. Larger amounts of calcium oxide
produce a faster hardening cement. Thick sections require a slow-setting cement
(with less calcium oxide) to avoid overheating and cracking.