Glasses

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29 Νοε 2013 (πριν από 3 χρόνια και 7 μήνες)

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1

2

Ceramic materials divide into three categories:

1.
Glasses



melting & solidification processing

2.
Traditional ceramics



particulate processing

3.
New ceramics



particulate processing

Introduction

3

Glass


Glass

is

one

of

three

basic

types

of

ceramics
.

Glass

is

distinguished

by

its

amorphous

(noncrystalline)

structure
.


Structure

:

Network

formers


Molecules

that

link

up

with

each

other

to

form

long

chains

and

networks
.

Hot

glass

cools,

chains

unable

to

organize

into

a

pattern
.

Solidification

has

short
-
range

order

only
.



Amorphous structure occurs by adding impurities
(Na
+
,Mg
2+
,Ca
2+
, Al
3+
).



Impurities: interfere with formation of crystalline structure

4

Glass



Raw Materials


1.
Glass forming oxides
: usually the dominant constituent



SiO
2
, B
2
O
3
, P
2
O
5
, etc.

2.
Fluxes
: reduce melting temperatures



Na
2
O, PbO, K
2
O, Li
2
O, etc.

3.
Property modifiers
: added to tailor chemical durability,
expansion, viscosity, etc.



CaO, Al
2
O
3
, etc.

4.
Colorants
: oxides with 3d, 4f electron structures; minor
additives (<1 wt%)

5.
Fining agents
: minor additives (<1 wt%) to help promote bubble
removal



As
-
, Sb
-
oxides, KNO
3
, NaNO
3
, NaCl, fluorides, sulfates

5

Glassmaking

1.
The

ingredients

for

glass

are

mixed,

and

along

with

a

proportion

of

cullet

(broken

glass),

are

added

to

a

bath

furnace,

where

they

are

heated

to

about

1500
°
C

and

fused

together
.

2.
Molten

glass

is

fed

as

‘gobs’

to

an

automatic

bottle

or

jar

making

machine
.

3.
A

hot

gob

is

first

made

into

a

parison

or

blank

shape

(by

either

pressing

or

blowing),

which

is

then

blown

to

the

final

bottle

or

jar

shape
.

Surface

coatings

(sc)

may

be

applied

while

hot
.

6

4.
The

bottles

or

jars

pass

into

a

lehr

(an

annealing

oven),

where

they

are

first

reheated

to

soften

the

glass

to

remove

stresses,

and

then

cooled

gradually

to

prevent

stresses

developing
.

5.
The

bottles

or

jars

are

inspected

and

tested

to

meet

quality

standards
.

Bottles

not

passing

the

quality

checks

are

broken

and

returned

to

the

furnace

as

cullet
.

Cullet

reduces

the

amount

of

energy

required

to

melt

the

glass

ingredients
.

6.
Bottles

passing

inspection

and

testing

are

packed

for

dispatch

to

where

they

will

be

filled,

capped,

and

labeled
.

7

Shaping Processes in
Glassmaking


Shaping processes to fabricate these products can be
grouped into three categories:



1.
Discrete

processes

for

piece

ware

(bottles,

jars,

plates,

light

bulbs)

2.
Continuous

processes

for

making

flat

glass

(sheet

and

plate

glass)

and

tubing

(laboratory

ware,

fluorescent

lights)

3.
Fiber
-
making

processes

to

produce

fibers

(for

insulation

and

fiber

optics)

8

Shaping of Piece Ware


Ancient

methods

of

hand
-
working

glass

included

glass

blowing
.



Handicraft

methods

are

still

used

today

for

making

glassware

items

of

high

value

in

small

quantities
.

However,

most

modern

glass

shaping

processes

are

highly

mechanized

technologies

for

producing

discrete

pieces

in

high

quantities
.


Piece

Ware

Shaping

Processes


1
.

Spinning



similar

to

centrifugal

casting

of

metals


2
.

Pressing



for

mass

production

of

flat

products

such

as

dishes,

bake

ware,

and

TV

tube

faceplates


3
.

Press
-
and
-
blow



for

production

of

wide
-
mouth

containers

such

as

jars


4
.

Blow
-
and
-
blow

-

for

production

of

smaller
-
mouth

containers

such

as

beverage

bottles

and

incandescent

light

bulbs


5
.

Casting



for

large

items

such

as

large

astronomical

lenses

that

must

cool

very

slowly

to

avoid

cracking
.

9

Spinning of funnel
-
shaped glass parts such as back sections of cathode
ray tubes for TVs and computer monitors:

(1) gob of glass dropped into mold; and

(2) rotation of mold to cause spreading of molten glass on mold surface

10

Pressing of flat glass pieces:

(1)
glass gob is fed into mold from furnace;

(2)
pressing into shape by plunger; and

(3)
plunger is retracted and finished product is removed (symbols
v
and
F
indicate motion (velocity) and applied force)

11

1.
A

gob

of

hot

glass

drops

into

the

blank

(parison)

mould
.

2.
The

mould

is

sealed

shut

by

a

‘base’

part

and

a

plunger

pushes

the

glass

into

the

mould

(made

from

iron)
.


3.
The

glass

is

shaped

into

a

‘blank’

and

also

pushed

into

the

neck

finish

by

the

plunger
.

This

part

of

a

jar

or

bottle

is

finished

to

its

final

shape

at

this

stage
.

4.
The

blank

shape

(parison)

is

removed,

rotated

180
°
,

and

transferred

to

the

blow

(finishing)

mould
.

5.
This

mould

is

in

two

halves,

made

from

fine
-
grain

cast

iron,

and

is

highly

polished
.

6.
Air

is

blown

into

the

hot

parison

to

expand

it

tightly

against

the

mould

walls
.


7.
The

mould

opens,

the

bottle

is

removed,

annealed

in

the

lehr,

inspected

and

tested,

and

shipped

for

filling
.

12

1.
A

gob

of

hot

glass

drops

into

the

blank

(parison)

mould
.

2.
The

end

is

sealed

and

a

puff

of

air

pushes

glass

into

the

neck

(finish)
.


3.
A

puff

of

air

from

below

pushes

glass

into

the

mould

and

shapes

it

into

a

‘blank’

or

parison,

a

thick
-
walled

bottle

looking

vaguely

like

the

final

bottle

shape
.

4.
The

blank

shape

(parison)

is

removed,

rotated

180
°
,

and

transferred

to

the

blow

(finishing)

mould
.

5.
This

mould

is

in

two

halves,

made

from

fine
-
grain

cast

iron,

and

is

highly

polished
.

6.
Air

is

blown

into

the

hot

parison

to

expand

it

tightly

against

the

mould

walls
.


7.
The

mould

opens,

the

bottle

is

removed,

annealed

in

the

lehr,

inspected

and

tested,

and

shipped

for

filling
.

13

Casting


If

molten

glass

is

sufficiently

fluid,

it

can

be

poured

into

a

mold
.


Relatively

massive

objects,

such

as

astronomical

lenses

and

mirrors,

are

made

by

this

method
.


After

cooling

and

solidifying,

the

piece

must

be

finished

by

lapping

and

polishing
.


Casting

of

glass

is

not

often

used

except

for

special

jobs
.


Smaller

lenses

are

usually

made

by

pressing
.

14

Shaping of Flat and Tubular Glass



Processes for producing flat glass such as sheet and plate glass:

Rolling

of

Flat

Plate

Starting

glass

from

melting

furnace

is

squeezed

through

opposing

rolls

whose

gap

determines

sheet

thickness,

followed

by

grinding

and

polishing

for

parallelism

and

smoothness

15

Float

Process

Molten

glass

flows

onto

the

surface

of

a

molten

tin

bath,

where

it

spreads

evenly

across

the

surface,

achieving

a

uniform

thickness

and

smoothness

-

no

grinding

or

polishing

is

needed
.


16

Danner Process

Molten

glass

flows

around

a

rotating

hollow

mandrel

through

which

air

is

blown

while

the

glass

is

drawn
.

17

Forming of Glass Fibers

Glass

fiber

products

can

be

divided

into

two

categories,

with

different

production

methods

for

each
:

1
.

Fibrous

glass

for

thermal

insulation,

acoustical

insulation,

and

air

filtration,

in

which

the

fibers

are

in

a

random,

wool
-
like

condition
.

Centrifugal

spraying

2
.

Long

continuous

filaments

suitable

for

fiber

reinforced

plastics,

yarns,

fabrics,

and

fiber

optics
.

Drawing

18

Heat Treatment




Annealing

of

Glass

Heating

to

elevated

temperature

and

holding

for

a

time

to

eliminate

stresses

and

temperature

gradients
;

then

slow

cooling

to

suppress

stress

formation,

followed

by

more

rapid

cooling

to

room

temperature
.

Annealing

temperatures

are

around

500
°
C
.



Tempering

of

Glass

Heating

to

a

temperature

somewhat

above

annealing

temperature

into

the

plastic

range,

followed

by

quenching

of

surfaces,

usually

by

air

jets
.

When

the

surfaces

cool,

they

contract

and

harden

while

interior

is

still

plastic
.

As

the

internal

glass

cools,

it

contracts,

putting

the

hard

surfaces

in

compression
.

Tempered

glass

is

more

resistant

to

scratching

and

breaking

due

to

compressive

stresses

on

its

surfaces
.

19

20

Ceramics Particulate Processing



Traditional

ceramics

are

made

from

minerals

occurring

in

nature
.


Products

include

pottery,

porcelain,

bricks,

and

cement


New

ceramics

are

made

from

synthetically

produced

raw

materials
.


Products

include

cutting

tools,

artificial

bones,

nuclear

fuels,

and

substrates

for

electronic

circuits


The

starting

material

for

all

of

these

items

is

powder
.


For

traditional

ceramics
,

the

powders

are

usually

mixed

with

water

to

temporarily

bind

the

particles

together

and

achieve

the

proper

consistency

for

shaping
.


For

new

ceramics
,

substances

other

than

water

are

used

as

binders

during

shaping
.


After

shaping,

the

green

parts

are

fired

(
sintered
)
.


21

Usual

steps

in

traditional

ceramics

processing
:


(
1
)

preparation

of

raw

materials,

(
2
)

shaping,

(
3
)

drying,

and

(
4
)

firing

Part (a) shows the workpart during the sequence, while (b) shows
the condition of the powders

22

Traditional Ceramics



Shaping processes for traditional
ceramics require the starting material
to be a plastic paste.



Main Ingredients of Ceramic Paste


1.
Clay

(hydrous aluminum silicates)
-

usually the main ingredient because
of ideal forming characteristics when
mixed with water.


2.
Water



Adding water to the


clay particles produces a "slip":
-



allows material to shear easily


along weak van der Waals bonds




Hydroplastic
(suitable plasticity


for shaping.

23

Shaping of Ceramics



Slip casting




The clay
-
water mixture is a slurry.


Plastic forming methods




The clay is plastic.


Semi
-
dry pressing




The clay is moist but has low plasticity.


Dry pressing




The clay is basically dry (less than 5% water) and
has no plasticity.

24

Slip Casting



A suspension of ceramic powders in water, called a
slip
, is
poured into a porous
plaster of paris mold

so that water from the
mix is absorbed into the plaster to form a firm layer of clay at the
mold surface.


The slip composition is 25% to 40% water.


Two principal variations:




Drain casting

-

the mold is inverted to drain excess slip after a
semi
-
solid layer has been formed, thus producing a hollow
product.




Solid casting

-

to produce solid products, adequate time is
allowed for entire body to become firm.

25

Sequence of steps in drain casting, a form of slip casting:

(1)
slip is poured into mold cavity,

(2)
water is absorbed into plaster mold to form a firm layer,

(3)
excess slip is poured out, and

(4)
part is removed from mold and trimmed

Drain casting

26

27

Solid casting

Sequence of steps in solid casting:

(1)
slip is poured into mold cavity,

(2)
water is absorbed into plaster
mold to form a product,

(3)
part is removed from mold and
trimmed

28

Plastic Forming



The

starting

mixture

must

have

a

plastic

consistency,

with

15
%

to

25
%

water
.


Plastic

Forming

Methods
:





Hand

modeling

(manual

method)





Jiggering

(mechanized

method)





Pressing






Isostatic

pressing





Extrusion

29

Hand Modeling



Creation of the ceramic product by manipulating the mass of
plastic clay into the desired geometry.


Hand molding

-

similar only a mold or form is used to define
portions of the part geometry.


Hand throwing

on a potter's wheel is another refinement of
handcraft methods.




Potter's wheel

= a round table that rotates on a vertical spindle,
powered either by motor or foot
-
operated treadle.




Products of circular cross
-
section can be formed by throwing
and shaping the clay, sometimes using a mold to provide the
internal shape

30

31

Jiggering



Similar to potter's wheel methods, but hand throwing is replaced
by mechanized techniques.

Sequence in jiggering:

(1) wet clay slug is placed on a convex mold;

(2) batting; and

(3) a jigger tool imparts the final product shape

32

33

Pressing



Forming process in which a plastic clay slug is pressed between
upper and lower molds contained in metal rings.



Molds are made of porous material such as gypsum, so when a
vacuum is drawn on the backs of the mold halves, moisture is
removed from the clay.


The mold sections are then opened, using positive air pressure to
prevent sticking of the part in the mold.


Advantages: higher production rate than jiggering and not limited
to radially symmetric parts.

34

Isostatic pressing



A powdered material can be compacted by loading it into a
flexible, air
-
tight container and placing it into a closed vessel filled
with a fluid to which pressure (> 150 MPa) is applied.


Wet bag molds are independent of the press construction. Wet
-
bag served as a carrying case and controls the shape of pressed
item.

35

Extrusion



Compression of clay through a die orifice to produce long
sections of uniform cross
-
section, which are then cut to required
piece length.


Equipment utilizes a screw
-
type action to assist in mixing the clay
and pushing it through die opening.


Products
: hollow bricks, shaped tiles,


drain pipes, tubes, and insulators


Also used to make the starting clay slugs for


other ceramics processing methods such as


jiggering and plastic pressing.

36

37

Semi
-
dry
Pressing



Uses high pressure to overcome the clay’s low plasticity and force
it into a die cavity.

Semi
-
dry pressing:

(1) depositing moist powder into die cavity,

(2) pressing, and (3) opening the die sections and ejection

38

Dry Pressing



Process sequence is similar to semi
-
dry pressing
-

the main
distinction is that the water content of the starting mix is typically
below 5%.


Dies must be made of hardened tool steel or cemented carbide to
reduce wear since dry clay is very abrasive.


No drying shrinkage occurs, so drying time is eliminated and
good dimensional accuracy is achieved in the final product.


Typical products: bathroom tile, electrical insulators, refractory
brick, and other simple geometries.

39

Clay Volume vs. Water Content



Water plays an important role in most of the traditional ceramics
shaping processes. It must be removed from the clay piece before
firing.


Shrinkage is a problem during drying because water contributes
volume to the piece, and the volume is reduced when it is
removed.


Drying

: The drying process occurs in two stages:


Stage 1

-

drying rate is rapid and constant as water evaporates
from the surface into the surrounding air and water from the
interior migrates by capillary action to the surface to replace it.




Stage 2

-

the moisture content has been reduced to where the

ceramic grains are in contact.

40

Firing of Traditional Ceramics



Heat treatment process that
sinters

the ceramic material.


Performed in a furnace called a
kiln
.


Bonds are developed between the ceramic grains, and this is
accompanied by densification and reduction of porosity.


Therefore, additional shrinkage occurs in the polycrystalline
material in addition to that which has already occurred in drying.


In the firing of traditional ceramics,


a glassy phase forms among the


crystals which acts as a binder.


41

Shrinkage during Sintering



Sintering shrinkage depends on
initial green density.


Shrinkage from green body to final
form may be as much as 30%(linear)
: problems in predicting final
product size and shape.


Differential shrinkage is a serious
problem : hence use of lubricants
etc to get good flow in compaction
to green body


uniform green
density.


If there is a difference in density
across a component distortion
occurs.

42

Refractories




Refractory ceramics should withstand to high temperatures
without melting or decomposing, and remain unreactive and inert
when exposed to severe environments.


Porosity is important factor to produce a suitable refractory
material.



-

less thermal expansion/contraction upon thermal cycling



-

Resistance to thermal shock



-

Increased insulation



-

Lighter


Disadvantages:


Worse resistance to Chemical attack







Weaker load bearing capability

43

Processing of New Ceramics



The manufacturing sequence for the new ceramics can be
summarized in the following steps:


1. Preparation of starting materials


2. Shaping


3. Sintering


4. Finishing



Preparation of Starting Materials :
Strength requirements are
usually much greater for new ceramics than for traditional
ceramics. Therefore, the starting powders must be smaller and
more uniform in size and composition, since the strength of the
resulting ceramic product is inversely related to grain size.


Powder preparation includes mechanical and chemical methods.

44

Shaping of New Ceramics



Many of the shaping processes for new ceramics are borrowed from
powder metallurgy and traditional ceramics.


The processes described here are not normally associated with the
forming of traditional ceramics.


Hot Pressing :
Similar to
dry pressing
except it is carried out at
elevated temperatures so sintering of the product is accomplished
simultaneously with pressing. This eliminates the need for a
separate firing step.


Powder Injection Molding (PIM)
: Ceramic particles are mixed with a
thermoplastic polymer, then heated and injected into a mold cavity.
The polymer acts as a carrier and provides flow characteristics for
molding. Upon cooling which hardens the polymer, the mold is
opened and the part is removed. The plastic binder is removed and
the remaining ceramic part is sintered.

45

Engineering Ceramics

46

Heat engine applications




Ceramic materials are used in automobile internal combustion
engines.


The advantage over the metal alloys:


-

The

ability

to

withstand

higher

operating

temperatures,

thereby

increasing

fuel

efficiency
;


-

excellent

wear

and

corrosion

resistance
;


-

the

ability

to

operate

without

a

cooling

system
;


-

low

densities

(lower

engine

weight)
.


47

Electrical/electronic applications

Materials
Applications
Alumina
(
Al
2
O
3
)
Beryllium oxide
(
BeO
)
Boron nitride
(
BN
)
Silicon carbide
(
SiC
)
Aluminum nitride
(
AlN
)
Barium titanate
(
BaTiO
3
)
Capacitors, sensors, etc
.
Zirconia
(
ZrO
2
)
Sensors
Insulators, substrates,
structural components,
etc
.
48

Bioceramics

49



Ceramic
-
matrix composites
(CMCs)



Strength and fracture toughness of ceramics have been improved
significantly by the development of ceramic
-
matrix composites.


CMCs are believed to be toughened by three main mechanism, all of
which result from the reinforcing fibers interfering with crack
propagation in the ceramic as follows :


1.

Crack deflection

2.

Crack bridging



3. Fiber pullout

50


1.

Crack deflection


Upon encountering the reinforcement, the crack is deflected,
making its propagating path more meandering, thus higher
stresses are required to propagate the crack.

2.

Crack bridging



Fibers can bridge the crack and help keep the material together,
thus increasing the stress level needed to cause further cracking.

3.
Fiber pullout


The friction caused by fibers being pulled out of the cracking
matrix absorbs energy, and thus higher stresses must be applied to
produce further cracking. Therefore, a good interfacial bond is
required between the fibers and the matrix for higher strengths.

51



Ceramic
-
matrix composites



Transformation toughening

Stress promotes transformation from one phase to another with a higher
volume, causing a compressive stress.



Partially stabilized zirconia particles (tetragonal phase made to be stable
at ambient conditions instead of the expected and higher volume
monoclinic phase). An approaching crack can be pinched shut.

52

Cement and concrete




Cement
is a mixture of compounds made by burning limestone and
clay together at very high temperatures (1400 to 1600
o
C).



Portland cement

: it is consumed in the largest amount.



The major constituents of Portland cement

are tricalcium silicate
(3CaO
-
SiO
2
), and dicalcium silicate (2CaO
-
SiO
2
).


Concrete

is a composite material consisting of aggregate particles
(sand and stone) bound together in a solid body by a cement.


Water

is the key ingredient, which when mixed with cement, forms a
paste that binds the aggregate together. The water causes the
hardening through a process called hydration.



Tricalcium silicate + Water

Calcium silicate hydrate + Calcium hydroxide + heat



2 (3CaO
-
SiO
2
) + 7 H
2
O → 3 CaO
-
2SiO
2
-
4H
2
O + 3 Ca(OH)
2

+ 173.6kJ


The hydration will continue as long as water is present and there are
still unhydrated compounds in the cement paste.

53

Cement and concrete



The reaction of water with the cement in concrete is extremely
important to its properties and reactions may continue for many
years. When concrete dries, it actually stops getting stronger.
Concrete with too little water may be dry but is not fully reacted.



Advantages of concrete



-

Economic benefits : long life and low maintenance requirements


-

Resistant to wind, water, rodents, and insects


-

Concrete has the ability to be molded or cast into almost any
desired shape on the work
-
site.


-

Non
-
combustible material


Disadvantages of concrete



-

Relatively low tensile strength


-

Low strength
-
to
-
weight ratio


-

Susceptible to cracking

54



Reinforced concrete




Reinforced concrete
is concrete containing steel reinforcements in
the form of rods, wire, wire mesh, etc.



In the reinforced concrete, the tension forces are transferred from
the concrete to the steel reinforcement through bonding.


55



Pretensioned
concrete




The tensile strength of reinforced concrete can be further improved
by introducing compressive stresses into the concrete by
pretensioning

or
posttensioning

using steel reinforcements called
tendon
.


Pretensioned (prestressed) concrete


Posttensioned concrete


56



Pretensioning

:


1. Steel tendons are stretched between external tendon anchorage
and adjustable jack for applying tension.


2. Concrete is poured over the tendons.


3. When the concrete reaches the required strength, the jack
pressure is released.


Posttensioning
:


1. Hollow conduits containing steel tendons are placed in concrete.


2. When concrete is sufficiently strong, tendon is anchored at one
end and jacking tension is applied at the other end.


3. When jacking pressure is sufficiently high, a fitting is replaced the
jack.


4.The space in conduits is filled by cement grout by forcing.

57

Colorant
Color
Iron oxides
greens, browns
Manganese oxides
deep amber, amethyst, decolorizer
Cobalt oxide
deep blue
Gold chloride
ruby red
Selenium compounds
reds
Carbon oxides
amber
/
brown
Mix of mangnese, cobalt, iron
black
Antimony oxides
white
Uranium oxides
yellow green
(
glows
!)
Sulfur compounds
amber
/
brown
Copper compounds
light blue, red
Tin compounds
white
Lead with antimony
yellow