microstructure - properties; ii crystallization of glass

5/21/2018 Microstructure - Properties; II Crystallization of Glass

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Microstructure-Properties: II

Crystallization of Glass27-302

Lecture 4

Fall, 2002

Prof. A. D. Rollett


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Materials Tetrahedron

Microstructure Properties

ProcessingPerformance


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Objective

The objective of this lecture is to providesome background for the experimentinvolving crystallization of glass-ceramic.

The material discussed in this lecture shouldbe familiar to students from the lectures onnucleation and growth.


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References

Phase transformations in metals and alloys, D.A. Porter, &K.E. Easterling, Chapman & Hall.

Physical Ceramics (1997), Y.-T. Chiang, D.P. Birnie III, W.D.Kingery, Wiley, New York,pp430-450.

Materials Principles & Practice, Butterworth Heinemann,Edited by C. Newey & G. Weaver.

Glass-Ceramics (1979), P.W. McMillan, Academic Press, NewYork.

Glass-Ceramic Technology (2002), W. Hland & G. Beall, TheAmerican Ceramic Society, Westerville, OH.

Applications, Production, and Crystallization Behavior of an

Ultra-Low Expansion Glass-Ceramic: Zerodur, presentation byDr. Mark J. Davis, Schott Glass Technologies, Oct. 17th atCMU.


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Growth Rates

Crystallization in glasses is generally a phenomenon to beavoided if at all possible. Crystallization makes glass opaque,for example, and does improve its other properties.

The exception is the case of glass-ceramics.

Most glass-ceramics are valued for a combination of chemical

inertness and thermal shock resistance.

Thermal shock resistance depends on lowCTE. Low CTEmeans that strains developed on cooling from hightemperatures generate small stresses and the breakingstrength is less likely to be exceeded.

Our particular example is opposite: high CTE is needed forcompatibility with metals.


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Glass Ceramics

Other glass ceramic materials are optimized for: High mechanical strength

High temperature capability

Photosensitivity

Low dielectric constant (electronic packaging) Dielectric-breakdown resistance

Biological compatibility

Machinability (through the inclusion of micaceous phases)


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Applications of Glass Ceramics

Radomes - Corning 9606, cordierite glass-ceramic. Requiredproperties: transparency to radar, low dielectric constant, lowCTE, high strength, high abrasion resistance, high thermalshock resistance.

Photosensitive glass-ceramics based on lithium disilicate,

Li2Si2O5, as the crystalline phase that can be selectivelyetched (UV light) to develop very fine features (holes,channels etc.). The parent glass has lithium metasilicate.Example: Foturan, Fotoceram.

Machinable glass-ceramics, e.g. MACOR, based on fluorine-

phlogopite, KMg3AlSi3O10F2), with additions of B2O3and SiO2to form a glass. The fluorine compound is micaceous whichallows easy cleavage over short distances. The material isvery useful as a machinable insulator, used in weldingequipment, medical equipment.

.


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Applications, contd.

Substrates for magnetic recording disks. Spinel-enstatite glass-ceramics allow high modulus, highsoftening point (~1000C), high toughness,insulating substrates.

Cookware based on glass-ceramics with beta-spodumene, LiAlSi2O6-SiO2, e.g. Corning Ware9608. The latter compound has low CTE, is white incolor, and can be easily fabricated.

Low expansion glasses such as Zerodur containingmainly beta-quartz. These are useful for telescopemirrors and ring lasers (low He permeability alsoessential here).


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VLT telescope in Chile (8.2 m mirrors with adaptive optics)

(www.eso.org)

Mirror fabrication

in Mainz, Germany

On the road to Cerro

Paranal, Chile

Dr. Mark J. Davis


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Ring Laser Gyroscope (RLG)

Sagnac Effect

Dr. Mark J. Davis


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RLG examples

Complete navigation

system Dr. Mark J. Davis


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What is Zerodur ?

oxide wt % mol % function

SiO2 55.4 63.8 form beta-quartz ss

Al2O3 25.4 17.2 form beta-quartz ss

P2O5 7.2 3.5 form beta-quartz ss

Li2O 3.7 8.6 form beta-quartz ss

MgO 1.0 1.7 form beta-quartz ss

ZnO 1.6 1.4 form beta-quartz ss

Na2O 0.2 0.2 improve glass melting

K2O 0.6 0.4 improve glass melting

TiO2 2.3 2.0 nucleating agent

ZrO2 1.8 1.0 nucleating agent

As2O3 0.6 0.2 f ining agent Dr. Mark J. Davis


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Annealing Lehr

Up to 1000 liter tank size

Up to 1000 gm/min

flow rate

Zerodur Production

Dr. Mark J. Davis


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Thermal History Used in Production

Ceramization

Melting

glassglass-ceramic

Dr. Mark J. Davis


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Thermal expansion of LAS

high-quartz solid solution crystals

Petzoldt and Pannhorst, 1991

Zerodur

Li2-2(v+w)MgvZnwO.Al2O3.xAlPO4.(y-2x)SiO2

Dr. Mark J. Davis

Residual glass (schematic)


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Control of expansion through ceramization times

Dr. Mark J. Davis


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Exchange 0.1 wt% Li2O for ZnO:

bulk composition effect

Petzoldt and Pannhorst, 1991

Dr. Mark J. Davis


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Microstructure development

The history of glass-ceramics starts with a mistakeby a researcher (Stookey) who left an oven on at toohigh a temperature with a sample of lithium silicateglass containing silver. He expected to find a

puddle of glass once he realized his mistake, butinstead found a piece of white ceramic because hisglass had crystallized with a fine grain size.

This lead to the use of titania as a nucleating agentin alumino-silicate glasses.

Control of crystallization (=devitrification) dependson inclusion of a (well dispersed) nucleating agent.This is akin to grain refinement in solidification(addition of TiB2to aluminum melts).


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Nucleation + Growth

In contrast to metals, where isothermal treatmentsare common, two-steps anneals in glass-ceramicsare the norm.

The typical sequence involves a nucleation stepof a

small volume fraction of, e.g. TiO2, followed by bulkgrowth of other phases.

The nucleation step is carried out at lowertemperatures, presumably to obtain higher driving

forces. Thegrowth step is carried out at higher

temperatures, again presumably to obtain highergrowth rates (more rapid diffusion).


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Heat Treatment of Glass-Ceramics

Typical heat treatments require cooling past the nose of thecrystallization curve, followed by a low temperature treatmentto maximize nucleus density and finally a higher temperaturetreatment to grow the grains.

[Chiang]

N l


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Li2O-Al2O-SiO2

Low temperature nucleationstep included -> fine grain size

Rapid heating to hightemperature (875C)

[McMillan]


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Nucleation Rate - Viscosity

There is a useful relationship between viscosity andnucleation rate for crystallization in glasses.

The nucleation rate, N (orIV),is determined by the critical freeenergy for nucleation and a Boltzmann factor as seenpreviously:

G* = 16g3/3GV2

N = w C0 exp-{G*/kT}where wis an attempt frequency or vibration frequency oforder 1011per second, C0(or NV) is the density of moleculesper unit volume of order 1029per m3.

In glasses one must adjust the attachment frequency basedon the viscosity since this can vary so markedly withtemperature. Using the Stokes-Einstein relation for atomicdiffusivity in a melt:

D = kT/3a0h= wa02

This suggests that we can take wto be inversely proportionalto the viscosity, h.


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Viscosity dependent nucleation rate

Adjusting the attachment frequency to matchexperimental data (larger than theory suggests),

N = 40 C0 kT / 3a02h .exp-{G*/kT}

Given that the viscosity dominates the temperaturedependence of all the terms in this expression, wecan simplify to this:

N = K / h . exp-{G*/kT}

where K is a constant of order 1036m-3sec-1poise foroxide glass formers.

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Viscosity dependent nucleation rate

In the previous development of driving forces, weapproximated the driving force as T Lf/Tm, where Lfis thelatent heat of transformation (melting, e.g.). Hoffmanndeveloped the following improved approximation for the casethat the difference in specific heat between solid and liquid issignificant but constant (with changing temperature):

Gm= HmT.T/Tm2

This can be inserted into the standard expression fornucleation rate:

N K

hexp

16

3kT

Vm

2gSL

3

Hm

2 T T

Tm

2

2

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Comparisons

Calculated andexperimentalTTT curvesagree well forsodiumdisilicate andanorthite.

Crystallizationdetected by X-

ray diffraction.

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CCT versus TTT

Crucial difference between idealized TTT diagrams thatassume isothermal annealsand realistic quenching is theeffect of continuously decreasing temperature. Re-drawingthe diagrams as CCT diagrams allows the increase in time fora given fraction transformed to be depicted.

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Heterogeneous nucleation

Heterogeneous nucleation is as important inglasses as it is in metals.

In glass, one must be careful to avoid includingphases that can act of nucleation sites for

crystallization. In glass-ceramics, the situation is reversed and one

typically adds nucleating agents deliberately.

Examples are TiO2and ZrO2.

In the glass-ceramic for the Lab, Li3PO4is used topromote nucleation of cristobalite.

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Approximate TTT

Uhlmann and Onorato provide an approximate model for TTTdiagrams in glasses (in the sense of how to avoidcrystallization).

For many cases, the nose of the crystallization curve occurs

at 0.77 Tm. This allows the critical cooling rate to be

calculated based on just one temperature. Also, the nucleation barrier can be approximated by the

following, where T* 0.8Tm:G* 12.6 Sm/R kT* = BkT*

and the constant, B, is of order 50. This allows an formula for

the critical cooling rate to be obtained:dT/dtcrit= ATm2/hexp-(0.212B){1-exp(0.3Hm/RTm)}

0.75where the constantAis of order 40,000 J.m-3K-1and theviscosity, h, is that at the nose of the curve, 0.77Tm.

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Typical glass ceramic compositions

Low expansion glass ceramics result from theparticular compositions used.

Al and Li substitute into beta-quartz (silica): Al3+substitutes for Si4+with Li+providing charge

neutrality as an interstitial ion. Lithia-alumina-silica compositions for Pyroceram

contain beta-spodumene, LiAl[Si2O6], which has aweak positive CTE, ~10-6.C-1.

At higher levels of Al and Li substitution, the CTEcan become negative.

Substitution limit is beta-eucryptite, LiAl[SiO4].

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Glass Crystallization Experiment

The main purpose of the experiment is to:(a) demonstrate in a hands-on experiment thekinetics of phase transformation and the consequentchanges in properties;(b) show you how to measure the change in opticalproperties (transparency) and use themeasurements as a probe of fraction transformed;(c) train you in how to use micro-hardness testing tomeasure to both strength and fracture toughness in

a brittle material;(d) train you how to identify phases using x-raydiffraction and to measure the fraction transformedindependently of the optical measurements.

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Glass Crystallization Expt., contd.

Photometry Measure the ratio of light received by a photometer,

with, I,and without, I0,the specimen.

Measure the specimen thickness, t.

Apply the Beer-Lambert Law: Determine the absorptioncoefficient,(or inverseextinction length).

By making the simplifying assumption that the

volume fraction of crystallized glass is proportionalto the absorption coefficient (see the text of the labmanual) use the measurements to plot a fractiontransformed versus time.

I

I0

exp x

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Micro-Hardness Measure Vickers hardness in the standard manner:

be aware that finding the indent in the glass ismuch less straightforward than with a metal

because of lower contrast! Also be very careful thatyou understand each instrument because we willhave to use both of them. You are expected toobtain hardness values in units of MPa.

Each indent should also produce radial cracksemanating from the tip of each corner of the indent.The longer the cracks, the lower the fracturetoughness.

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X-ray Diffraction For each specimen you will obtain a standard 2q

scan.

The expected results will be a mixture of

amorphous material showing a large broad peak atlow scattering angles with some crystalline phases.

Analysis: measure the area under the amorphouspeak and use this as a measure of the relative

amount of material (un-)transformed. Phases observed in 2001: Lithium Silicate

(Rhombohedral Li2SiO3), plus another unidentifiedphase.

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Summary

The crystallization of glass follows the rules forphase transformation.

The kinetics of crystallization in glasses can berelated to their viscosity because viscous flow

depends on similar atomic mechanisms asdiffusion.

Whereas ordinary glasses need to avoidcrystallization, in glass-ceramics, certain phasesare deliberately nucleated to achieve a fine-grainedcrystalline structure.

The crystalline phases that appear depend on thecomposition: choice of phases is governed by theproperties desired.

microstructure - properties; ii crystallization of glass

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