Manufacturing of Special Glasses-II

Atiar Rahaman Mollaatiar@cgcri.res.in

03/11/2010 & 08/11/2010 (3-4 PM)

• Glass-ceramics• Sealing and solder glass, Opal

glass, Ophthalmic glass, Photochromic and photosensitive glasses, Bullet-proof glass, Silica glass, Vitreous silica products, Laser glass, Colored glasses

Out-lines…

Glass-ceramicsDefinitions: Glass-ceramics are polycrystalline solids

prepared by the controlled crystallization of glassesDifference between glass-ceramics and

traditional ceramics:In glass-ceramics, the crystalline phases are entirely produced by crystal growth from a homogeneous glass phase and this distinguishes these materials from traditional ceramics where most of the crystalline material is introduced when the ceramic composition is prepared.Difference between glass-ceramics

and glass:Glass-ceramics are distinguished from glasses by the presence of major amounts of crystals since glasses are amorphous or non-crystalline.

Brief History of Glass-ceramicsSynthetic glass-ceramics were serendipitously discovered in 1953. Stanley Donald Stookey, then a young researcher at Corning Glass Works, meant to anneal a piece of a lithium disilicate glass with precipitated silver particles (meant to form a permanent photographic image) in a furnace at 600°C. He accidentally overheated the glass to about 900°C. “Damm it, I’ve ruined a furnace!” Stookey thought. Instead of a melted pool of glass, the astonished Stookey observed a white material that had not changed shape.

Pyroceram

Macor

Zerodur

Ceran

Glass-ceramics

Glass-ceramics by definition has 50 vol. % or more crystalline phase

Why glass-ceramics?• Wide range of thermal expansion: Glass-Ceramics

exhibits a wide range of thermal expansion values from ( -60 x 10-7 to 200 x 10-7) much lower than glass.

• Thermal conductivity: Thermal conductivity of glass-ceramics is generally lower than pure crystalline materials but slightly higher than glass.

• Thermal shock resistance: TSR of glass-ceramics is superior than glass and more refractory than common glasses.

• Mechanical strength: It is mechanically much stronger than glass

• Chemical durability: Glass-ceramics have better chemical durability than glass and can be varied over wide ranges. The composition from Li2O-CeO2-Al2O3 is used for coating chemical reactors.

• Crack arresting property: Glass-ceramics have much mechanical toughness than glass.

• Electrical properties: Most of the glass-ceramics are insulators.

• Optical properties: Transparent and opaque glass-ceramics can be made. The degree of transparency is a function of optical properties of crystalline species, grain size and difference of refractive indices of glass and the crystals.

Thermodynamic driving force for a phase transformation

Decrease in Gibbs free energy

Liquid-> solid

gs - gl = g = -ve

phase transformations

When atoms get together to form a cluster, two effects on the energy of the system should be considered1. The lowering of the volume free energy due to the

ordering in a crystalline arrangement

2. The increase in surface energy

Nucleation A nucleus is a precursor to a crystal

A nucleus is a periodic assemblage of atoms, but it does not have recognizable growth habit planes

A crystal is an assemblage of atoms in a regular, periodic array and has recognizable growth habit planes

Nucleation occurs because the atoms are constantly vibrating and moving around as a result of the thermal energy in the system

There are two barriers to accomplishing nucleation to occur

Kinetic barrier Thermodynamic barrier

ggL

gS

gS < gL

gL < gSLiquid is

stable

TmT

Gibbs free energy as a function of temperature,

gL

gS

g

Solid is stable

Tfreesing

Change in free energy of the system due to formation of the solid ball of radius r :

r

)(3

4 3Ls ggrf

+ve: barrier to nucleation 24 r

)(3

4 3Ls ggr

rr*

f

24 r

grf 3

3

4

24 r

gr 3

3

4

rr*

f

24 rSolid balls of radius r < r* cannot grow as it will lead to increase in the free energy of the system !!!

Solid balls of radii r > r* will grow

r* is known as the CRITICAL RADIUS OF HOMOGENEOUS NUCLEATION

grf 3

3

4 24 r

gr 3

3

4

rr*

f

24 r

0*

rrr

f

gr

2*

*f

2

3

)(3

16*

gf

Critical particle

Formation of critical nucleus by statistical flucctuation

Atoms surrounding the critical particle

Diffuse jump of a surrounding atom to the critical particle makes it a nucleation

The Nucleation Rate

Nt=total number of clusters of atoms per unit volumeN* = number of clusters of critical size per unit volume

By Maxwell-Boltzmann statistics

RT

fNN t

*exp*

RT

fNN t

*exp*

s*= no. of liquid phase atoms facing the critical sized particle

Hd = activation energy for diffusive jump from liquid to the solid phase = atomic vibration frequency

The rate of successful addition of an atom to a critical sized paticle

RT

Hsv dexp*'

Rate of nucleation, I , (m3 s-

1)

'*NI

RT

HfsN dt

*exp*

With decreasing T

1. Driving force increases

2. Atomic mobility decreases

= No. of nucleation events per m3 per sec

= number of critical clusters per unit volume (N*)x

rate of successful addition of an atom to the critical cluster (’)

RT

Hs

RT

fN dt exp*

*exp

T

I

Tm

Growth

Increase in the size of a product particle after it has nucleated

dt

drU

T

U

Overall Transformation Kinetics

),( IUfdT

dX

U

I

dX/dt

TI : Nucleation rate

U : Growth rate

dt

dr

Overall transformation rate (fraction transformed per second)

X=fraction of product phase

T-T-T Diagram

As long as heat-treatments are such that one is to the left of the nose, detectable crystallization is avoided

Isothermal

L+

TStable liquid

UnderCooled liquid

log t

Tm

TTT Diagram for liquid-to-solid transformation

U

I

T

Coarse grained crystals

Fine grained crystals

glass

Critical cooling rate?Critical cooling rate?

If we start cooling from Tm, then glass formation will occur as long as the cooling rate is faster than the rate given by the tangent at the nose.

Kinetic theory for glass formation

(dT/dt) ≥ (dT/dt)c, the critical cooling rate

(dT/dt)c = (Tm-Tn)/tn

Johnson-Mehl-Avrami equation

X = 1- exp (-ktn)

Hruby coefficient

kgl = (Tx-Tg)/(Tm-Tx)

A larger kgl suggests greater thermal stability

Which compounds can form Which compounds can form glass?glass?

At what rate should a given liquid At what rate should a given liquid be cooled to bring it in to a glassy be cooled to bring it in to a glassy state?state?Examples of CCR (0C/s) for glass formation

Cooling rate 106 ºC s-1

Inert gas pressure

Molten alloy

Heater coil

Quartz tube

Rotating cooledmetal drum

Jet of molten metal

Ribbon ofglassy metal

From Principles of Electronic Materials and Devices, Second Edition, S.O. Kasap (© McGraw-Hill, 2002)http://Materials.Usask.Ca

Melt Spinning for metallic glass ribbons

Making of Glass-ceramics…T

time

nucleation

growth

glass

Glass ceramic

Liquid

Very fine crystals

DTA trace showing glass transformation, devitrification, and melting of Li2-Al2O3-6SiO2 glass-ceramic compositions

XRD pattern of glass and glass-ceramics

Microstructures

• Technical applicationsRadomes, Photosensitive and Etched Patterned Materials,Fotoform and Fotoceram Foturan (Gas discharge panels, ink-jet printer, magnetic recording head pads), Machinable, Vitronit, Photoveel, Magnetic Memory Disk Substrates, Liquid Crystal displays

• Consumer ApplicationsPyroceram (crockery), Pyrex (bowls, tea pots, glasses and cups etc.)

• Optical applicationsTelescope mirrors, Solar concentrators, RE-doped oxyfluorides for upconversion and amplification

• Medical and Dental

Cerabone, Ceravital, Bioverit, Dicor, IPS Empress

• Electrical and Electronic applications

Insulators, Electronic packaging

• Architectural Applications

Neoparies, Firelite

Suggested Readings

1. A. K. Varshneya, Fundamentals of Inorganic Glasses, Academic Press, Boston, 1994.

2. J. E. Shelby, Introduction to Glass Science and Technology, The Royal Society of Chemistry, Cambridge, 1997.

3. P. W. McMillan, Glass-Ceramics, Academic Press, 1964.

4. W. Holand and G. H. Beal, Glass-ceramic Technology, The American Ceramic Society, Ohio, 2002.

Sealing glass

Tungsten (CTE 4.0-4.4 x 10-6), used to manufacture high temperature incandescent and discharge lamps (Borosilicate glass)

Molybdenum (CTE 5 x 10-6), alkali borosilicate glasses are used for sealing

Kovar (28% Ni, 18% Co, 54% Fe) (CTE 5 x 10-6), Borosilicate glass with relatively higher content of B2O3 is used

Lead Glass High lead content glasses are used for encapsulation of diodes, resistors and capacitors.

The sealing glasses used in electronic engineering are characterized by

Excellent electrical insulation

Low dielectric loss

Gaseous impermeability

High absorption of certain radiations

‘tailor-made’ thermal properties

Vacuum tube

Semiconductor technology

Solder glass

Glass solders are glasses with a very low melting temperature that are used to join glass to other glasses, metals or ceramics with as little thermal impact as possible to the materials to be joined.

Glass soldering is usually done when direct fusion is not technically feasible

Viscosity of glass at soldering temperatures must lie between 104 to 106 P.

Glass solders usually come in powdered form with grain sizes of 60 m or less

Glassy solders crystallizing solders Composite glass solders

Opal glassDefinition: Opal glasses are essentially phase separated glasses, with their opacity resulting from light refraction and internal scattering between the separated phases.

This phase separation may be either liquid-liquid or liquid-crystal

Crystalline opal glasses contain about 3-10 vol. % crystalline phase

The over all opacity of an glass is controlled by three factors

RI differences, Degree of phase separations and size and distribution of the phase separated phases

Ophthalmic glass

Ophthalmic glasses are used to correct the vision and to protect the eyes against undesirable light radiation.

These glasses are extremely homogeneous

The curves of the lens surfaces and the RI of the glass determine the optical power of the ophthalmic lens.

As the power increases, glass spectacles become thick and unpleasant to wear…..lighter, hi-index glasses for thinner lenses with same refractive power is used

For safety, mechanical strength of such lenses are improved by thermal and chemical toughening

Sunglasses have brown or gray or even green tones with less transmission values and good sunglasses restrict IR and UV rays

Photochromic glass When Photochromic ophthalmic lenses are exposed to UV or IR radiation, the transmission of visible light is automatically reduced and after removing the exposure they return to their initial state.

Closely controlled thermal treatment during and after the melting process leads to the formation of silver halide containing phase-separated particles of sizes 5-30 nm in the glass containing small amount of Cu+

AgCl + UV Ag+ + Cl-

Ag+ + Cu+ Ag0 + Cu++

The metallic Ag causes the darkening of the glass

Photomachined middle ear implant

Bullet-proof glassLaminated safety glass of over 60 mm thick which consists of at least four layers joined with viscous plastic layer is called bullet proof glass.

The solid joining of the glasses occurs in a pressurized vessel called an autoclave where under simultaneous heating of the pre-processed ‘sandwich’ the lamination takes place.

When laminated safety glass breaks, the broken pieces of glass stick to the internal tear-resistant plastic layer.

Cashier’s office, bank counters, jewelry store windows or armored transport vehicles can be equipped with bullet proof glass

• Vitreous silica: It is the most important single component

glass. Highly cross linked vitreous silica is viscous and has a thermal expansion coefficient of about 5.5 x 10-7/ºC. It requires very high temp. for melting (>2000ºC).

• Vycor Glass: SiO2 – 96%, Al2O3 – 0.4%, Na2O – 0.2%, B2O3

– rest.

Fusion point is much lower.

Silica Glass

Laser glassThe addition of rare earths, in the glass creates narrow absorption bands. Glasses of this nature are used as laser sources.

Lasers are light sources in which the stimulating energy of a pulsed light (xenon or krypton flash lamp) is transformed into monochromatic, coherent light of high intensity.

e.g. Stimulated neodymium ions in the laser glass emit infrared radiation at the 1.06 m wavelength.

Photon of Resonant energy = E2-E1

MonochromaticCoherentVery intense very focused

Medical:Reattaching of detached retinaPainless removal of tumor as a “Bloodless Knife”Industrial:Fine weldingMicro-machining and micro-drilling

Strategic Sector: Inertial Confinement Fusion (ICF) ResearchWeapon guidanceDetection and tracking of aircraft, missiles, satellitesLaser range finder

APPLICATIONS

Colored glasses

Group III transition element ions and their colours

Ion Colour in a soda lime –silica glass

1 Ti3+ Violet 

2 V3+ Green 

3 Cr3+ Green 

4 Mn3+ Purple 

5 Mn2+ Colourless 

6 Fe3+ Yellow-green 

7 Fe2+ Blue 

8 Co2+ Intense blue 

9 Ni2+ Grey Brown 

10 Cu2+ Blue 

   

 

 

 

APPLICATION OF IR-ABSORBING FILTER

  Laser safety goggles as Nd-dopped laser glass emit light of wavelength is 1.06 m

Movie projection lamps

This glass is also used in operation theater light and in endoscopes to protect body cell from heat radiation where high intense light is required.