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doi: 10.1098/rsta.2002.1152, 575-589361 2003 Phil. Trans. R. Soc. Lond. A

Wolfram Höland, Volker Rheinberger and Marcel Schweiger Control of nucleation in glass ceramics

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10.1098/rsta.2002.1152

Control of nucleation in glass ceramicsBy Wolfram H o land, Volker Rheinberger

and Marcel Schweiger

Ivoclar Vivadent AG, Bendererstrasse 2, 9494 Schaan,Principality of Liechtenstein

Published online 29 January 2003

Glass ceramics are advanced materials composed of one or more glass and crystalphases. By developing base glasses with appropriate compositions and by controllingcrystal nucleation and growth in these glasses, glass ceramics with tailor-made prop-erties can be fabricated. The key to developing this type of material is control of thenucleation processes. Both volume and surface nucleation can be exploited. Hetero-geneous volume nucleation has been used to develop glass ceramics showing minimalthermal expansion and high strength. Two nucleation mechanisms can be combinedand the precipitation of two crystal phases can be controlled. That the nucleationprocesses can be controlled by nano- and microscale immiscibility is a special fea-ture, allowing selective nanophase formation or the development of needle-like apatitephases demonstrating a natural morphology. This represents a biomimetic process.The control of nucleation has enabled the development of biomaterials for dentalapplications.

Keywords: glass ceramics; twofold nucleation; biomimetic process; dental materials

1. Introduction

Glass ceramics are modern materials with an inorganic–inorganic microstructure.This dense and void-free microstructure consists of one or more glassy and crys-talline phases. The chemical composition and microstructure of the glass ceramicdetermine its properties and main applications. Glass ceramics are most commonlymade by forming special base glasses, mostly by melting, and then using controlledheat treatment to nucleate and precipitate crystals in the glassy matrix. Holand &Beall (2002) have reviewed the formation of glass ceramics based on alkaline and alka-line earth silicates, aluminosilicates, fluorosilicates, silicophosphates, iron silicates,phosphates, niobates or titanates, with special properties for specific applications.The microstructures that can be formed include nanoscale phases, highly uniformcrystals with an interlocking microstructure, and crystals similar to those in naturalbone.

2. Mechanisms of controlled nucleation

Nucleation is the key factor for controlling crystallization in glass ceramics. Classicaltheory can describe the temperature dependence of nucleation and crystallization inOne contribution of 15 to a Discussion Meeting ‘Nucleation control’.

Phil. Trans. R. Soc. Lond. A (2003) 361, 575–589575

c© 2003 The Royal Society



576 W. Holand and others

glasses. The nucleation rate and crystal growth rate as a function of temperatureare accurately measured experimentally (Stookey 1959; McMillan 1979, Holand &Beall 2002). Nucleation mechanisms have been reviewed by Kingery et al . (1975),Zanotto (1994), James (1982) and Weinberg et al . (1997). Two general nucleationmechanisms have been exploited to develop glass ceramics: volume nucleation andsurface nucleation. In this article, these two mechanisms will be discussed from thepoint of view of practical applications.

(a) Volume nucleation

(i) Homogeneous and heterogeneous nucleation

In the crystallization of glasses, as in other phase transformations, a distinctionmust be made between homogeneous and heterogeneous nucleation. For homogeneousnucleation, the classical theory gives the work of formation ∆G of a spherical nucleusof radius r as

∆G = −43πr3 ∆Gv + 4πr2γ + ∆GE, (2.1)

where ∆Gv is the free-energy change per unit volume associated with the formationof the new phase, γ is the interfacial energy (per unit area) of the new surface of thenucleus, and ∆GE is the elastic distortion energy (often not considered).

To develop glass ceramics, special nucleation agents can be incorporated into thebase glass that acts as a catalyst for nucleation in the glassy matrix. For nucleationon catalytic substrates, the classical picture is of a spherical-cap nucleus making acontact angle θ with the substrate. The critical work of formation for heterogeneousnucleation, ∆G∗

H, is lower for smaller θ according to

∆G∗H = ∆G∗ f(θ), where f(θ) = 1

4(2 + cos θ)(1 − cos θ)2. (2.2)

Based on (2.2), the nucleation barrier is very small if the nucleant substrate iscompletely wetted by the nucleus and the contact angle θ is close to 0◦. Hetero-geneous nucleation is particularly effective if there is epitaxy between the nucleusand substrate. There can be an epitaxial relationship if the lattice geometry of thenucleus and substrate crystals is similar (less than 15% mismatch in lattice param-eter). Further influences on epitaxy in glass ceramics include: the bonding state inthe substrate and nucleus crystals, structure defects, and the degree of coverage ofthe nucleant surface with foreign nuclei.

Weinberg et al . (1997) and Zanotto (1994) have reported in detail on crystallizationkinetics in glasses. The standard theory of this type of phase-transformation kineticswas developed by Johnson and Mehl, Avrami and Kolmogorov (JMAK theory). Thekinetics of the nucleation rate, I, were investigated by James (1982) and Gutzow(1980) as functions of time t and one suggested form is

I = I0 exp(

−τ

t

), (2.3)

where I0 is the steady-state nucleation rate and τ the non-steady-state time lag, atime before the steady-state rate is reached. The steady-state rate can be expressedas

I0 = A exp[∆G∗ + ∆GD

kT

], (2.4)

Phil. Trans. R. Soc. Lond. A (2003)



Control of nucleation in glass ceramics 577

where ∆G∗ is the work of formation of the critical nucleus and is the thermodynamicbarrier, ∆GD represents the kinetic barrier for nucleation, k is Boltzmann’s constant,and A a pre-exponential factor. I0 is inversely proportional to the viscosity, η, of theglass phase. The characterization of the nucleation behaviour is possible by exper-imental determination of the nucleation rate, I, and the nucleus number, N , as afunction of the time of heat treatment under isothermal conditions. Thus, the func-tions I(t) and N(t) can be determined at different temperatures. The time lag τ canalso be characterized. From the nucleation kinetics it may be possible to distinguishthe mechanism.

In many cases of glass-ceramic formation, the crystalline phases are metastable.Temperature–time–transformation (TTT) diagrams are convenient for displaying thecompetition between metastable and stable phases for a given glass-ceramic composi-tion (Uhlmann 1980). TTT diagrams for multi-component systems have been shownby Holand & Beall (2002).

(ii) The role of micro-immiscibility

Particular glasses from the systems SiO2–Al2O3–Li2O, SiO2–Al2O3–MgO andSiO2–Al2O3–K2O tend to phase separate into a SiO2-rich glassy matrix and analkaline or alkaline-earth oxide-rich glassy droplet phase. The relationship betweennucleation and micro-immiscibility was summarized by Uhlmann & Kolbeck (1976).First, by using phase separation in the base glass, volume crystallization can beachieved at an earlier stage or delayed by changing the composition of the matrixphase; surface crystallization or uncontrolled volume crystallization can thus be sup-pressed. Second, phase separation may lead to the formation of a low-viscosity phasedemonstrating homogeneous crystallization, while the matrix crystallizes heteroge-neously, either simultaneously or later. Third phase separation leads to the formationof interfaces that may be preferred sites for crystallization.

Importantly, it has been clearly demonstrated that phase separation can be con-trolled, in turn enabling further control of the nucleation of primary crystals. Withphase separation, material transport delivers the molecular building blocks for thenucleus to the nucleation site (the droplet glass phase). The term ∆GD in (2.4)appears to be reduced to allow direct and rapid formation of the desired primarynucleus in the droplet phase. Beall & Duke (1983) showed that TiO2, ZrO2, P2O5,Ta2O5, WO3, Fe2O3 and fluoride promote special microphase separation in variouschemical systems permitting the development of glass ceramics. These nucleatingagents may accumulate in a specific microphase of the phase-separated base glass.

(b) Surface nucleation

Most glass ceramics exploit volume nucleation, but there are also base glasses inwhich controlled volume nucleation cannot be initiated. In these glasses, controlledcrystallization can be achieved only with surface nucleation. This process, however,is more difficult to control. It is clear that nucleation and crystallization can beaccelerated and controlled by tribochemical activation of the glass surface. The pos-sibilities of controlling the nucleation process were successfully studied by the TC7group of the International Commission on Glass (Pannhorst 2000). Their studies ofglass-ceramic formation from near-stoichiometric cordierite glass showed that seed-ing of the glass initiates surface nucleation in addition to tribochemical reactions.





Seeding of the glass surface was possible with fine powdered glass of the same com-position or heterogeneous particles, e.g. Al2O3. Further investigations of the kineticsof nucleation demonstrated that the elastic-strain term (∆GE) in (2.1) could also beof significance in surface nucleation.

Kokubo (1991) developed bioactive glass ceramics for bone replacement in humanmedicine. He used the mechanism of surface nucleation to precipitate apatite andwollasonite as main crystal phases of the glass ceramic.

3. Application of nucleation control tomulti-component glass ceramics

Base glasses with simple, stoichiometric compositions have been used mainly to makequantitative tests of nucleation theories. For glass ceramics with important propertiesfor technical, consumer, medical or biological applications, however, it is necessaryto develop multi-component base glasses and still to apply the different nucleationmechanisms to convert the base glasses into glass ceramics.

(a) Glass ceramics with low thermal expansion

The development of glass ceramics having a very low or even zero coefficient ofthermal expansion (CTE) in a wide temperature range demonstrates the controlof nucleation by phase separation of the base glass and additional heterogeneousnucleation. Glass ceramics derived from the SiO2–Al2O3–Li2O system were selected.The unusual CTE was achieved by forming crystals of metastable solid solutionsbased on β-quartz and β-spodumene. The key to controlled crystallization of theβ-quartz solid solution is controlled nucleation, by choosing the correct nucleatingagent, the addition level of the agent and the heat treatment of the base glass. Beallet al . (1967) developed an effective nucleation initiation method for the precipitationof very small crystals of the β-quartz solid solution in the SiO2–Al2O3–Li2O–MgO–ZnO system through additions of TiO2, ZrO2, or occasionally Ta2O5. The desiredcrystallites in this case were less than 100 nm in diameter and are an example ofnanophase formation. The combination of the two nucleating agents TiO2 and ZrO2was examined by Beall et al . (1967) and Petzoldt (1967). An effective nucleatingagent ratio of 2 wt% TiO2 and 2 wt% ZrO2 was established (Beall 1992) for thenucleation of highly dispersed ZrTiO4 crystallites at 780 ◦C. During subsequent heattreatment at 980 ◦C, crystallites of β-quartz-solid solution less than 100 nm acrossgrew heterogeneously as the main crystal phase on the primary nuclei of ZrTiO4.Given the special properties of low CTE and thermal conductivity, the glass ceramichas been successfully used as telescope mirror blanks in precision optics and for avariety of household applications.

(b) Glass ceramics with high mechanical strength

The development of glass ceramics with high mechanical strength is also basedon control of volume nucleation by phase separation of the base glass. High-strengthglass ceramics with very good processibility (permitting pressing to shape at less than1000 ◦C) were developed in base glasses of SiO2–Li2O–P2O5–ZrO2 (ZrO2-containingglass ceramics), and SiO2–Li2O (lithium disilicate glass ceramics) (Holand & Beall2002).





1 µm

Figure 1. Microstructure of a ZrO2-containing base glass with fine primary Li3PO4 crystals.Scanning electron micrograph (SEM) of etched sample (2.5% HF, 10 s).

(i) ZrO2-containing glass ceramics

Glass ceramics containing ZrO2 are known in different chemical systems (Holand& Beall 2002). In the SiO2–Li2O–ZrO2–P2O5 system, glass ceramics are found inthe composition range of 42–59 SiO2, 7–15 Li2O, 4–15 P2O5, 15–28 ZrO2 (all compo-sitions given as wt% unless stated otherwise) with additions of K2O, Na2O, Al2O3and F up to 11 wt%. These glass ceramics can be shaped under hot-pressing condi-tions. In a base glass containing 20 wt% ZrO2, nucleation is initiated by phase sep-aration and Li3PO4 primary crystal formation during quenching of the glass melt.Li3PO4 crystals are visible in figure 1 on the edges of the spherical etched areas. Thesubsequent volume crystallization involves precipitation of ZrO2. Two ZrO2 phases(baddeleyite-type and the tetragonal polymorph) are formed as microcrystals mea-suring 200–300 nm at 940 ◦C and grow up to 1–20 µm in length during extendedheat treatments. The growth rate of the ZrO2 microcrystals has its maximum at3.5–5.0 µm h−1 (Holand et al . 1996). The corresponding glass ceramics are producedunder viscous flow conditions; they show bending strengths of 280 MPa and a fracturetoughness KIC of 2.0 MPa m1/2.

(ii) Lithium disilicate glass ceramics

James (1982) and Deubener et al . (1993) investigated non-steady-state nucleationin lithium disilicate glass ceramics. Headley & Loehman (1984) added P2O5 to SiO2–Li2O glasses and discovered heterogeneous nucleation of lithium disilicate by epitax-ial growth on Li3PO4 crystals in a special glass ceramic. Beall (1993) developedchemically durable lithium disilicate glass ceramics in a multi-component system.High-strength glass ceramics were developed by Schweiger et al . (1998), based on apowder-processed multi-component lithium disilicate glass ceramic with composition57–80 SiO2, 0–5 Al2O3, 0.1–6 La2O3, 0–5 MgO, 0–8 ZnO, 0–13 K2O, 11–19 Li2O, 0.5–11 P2O5, 0–6 additives, and colouring substances 0–8. The powdered base glass was





5 µm

Figure 2. Microstructure of a lithium disilicate glass ceramic after hot pressing at 920 ◦C.SEM, etched sample (HF vapour, 10 s).

Table 1. Crystal-phase formation in multi-component (a) Al2O3-free and(b) Al2O3-containing lithium disilicate glass ceramics

(a) (b)︷︸︸︷︷︸︸︷

temp. crystal phase temp. crystal phase

500 ◦C Li2Si2O5 580 ◦C Li2SiO3

Li2SiO3

630 ◦C Li2Si2O5 760 ◦C Li2Si2O5

Li2SiO3, SiO2 Li3PO4

650 ◦C Li2Si2O5

Li3PO4

heat treated at 500–800 ◦C, and a raw glass ceramic was produced in the form of acylindrical ingot. This ingot was transformed into a state of viscosity 105–106 Pa s ina special hot-press apparatus (EP 500, EP 600, Ivoclar Vivadent AG, Liechtenstein),then pressed at 920 ◦C for 5–15 min to form a glass-ceramic body. This lithium dis-ilicate glass ceramic does not require additional heat treatment. In the end productthe main crystal phases are Li2Si2O5 and Li3PO4. Based on intensive studies of thepredominant volume crystallization, nucleation must be initiated by P2O5.

Schweiger et al . (1998) and Holand et al . (2000a) showed that the chemical com-position has an important influence on the nucleation and crystallization of lithiumdisilicate, the main crystal phase. Complex simultaneous and sequential solid-statereactions were studied in an Al2O3-free lithium disilicate glass ceramic of com-position (mol.%) 63.2 SiO2, 29.1 Li2O, 2.9 K2O, 3.3 ZnO, 1.5 P2O5 (Holand et al .2000a). Li3PO4 crystals were formed after the crystallization of Li2SiO3 and Li2Si2O5(table 1a). Therefore, Li3PO4 does not nucleate lithium disilicate crystals, as con-





2 µm

Figure 3. SEM showing primary crystals of leucite formed by heat treatment (12 h at 720 ◦C)of a seeded monolithic glass surface. (Reproduced from Hoeland et al . (1995) with permissionof Elsevier Science.)

cluded previously for glasses of similar composition (Holland et al . 1998). Therefore,the nucleation mechanism may be based on steep compositional gradients. The crys-tals of Li2Si2O5 were precipitated at low temperatures (ca. 520 ◦C) in parallel withLi2SiO3 (table 1a). The growth rate of lithium disilicate increased at 680 ◦C after thedissolution of Li2SiO3 and SiO2 (cristobalite intermediate phase). Thus, the solid-state reaction Li2SiO3 + SiO2 → Li2Si2O5 may have taken place.

Schweiger et al . (2000) and Holand et al . (2000b) found that Al2O3-containing andLa2O3-containing multi-component lithium disilicate glass ceramics show differentnucleation behaviour (table 1b). The crystallization of the Li2Si2O5 as the maincrystal phase clearly occurs via the precursor phase lithium metasilicate, Li2SiO3,as shown by thermal analysis (Holand & Beall 2002) and X-ray diffraction. Theresulting microstructure has a crystallinity of (70±5) vol.% and is shown in figure 2.This glass ceramic is a translucent, high-strength material with a bending strengthof 300–400 MPa; it is used as biomaterial in restorative dentistry.

(c) Surface nucleation and sintering

Glass ceramics derived from the SiO2–Al2O3–K2O system involve surface nucle-ation and crystallization. The main crystal phase is leucite (K Al Si2O6, i.e. K2O ·Al2O3 · 4SiO2). The glass ceramics are characterized by good optical properties,high CTE and good sinterability; they are also used as biomaterials in restorativedentistry.

A typical glass ceramic with leucite as the main crystal phase has a composi-tion in the range 59–63 SiO2, 19–23.5 Al2O3, 10–14 K2O, 3.5–6.5 Na2O, 0–1 B2O3,0–1 CeO2, 0.5–3 CaO, 0–1.5 BaO and 0–0.5 TiO2 (Holand & Beall 2002). Thenucleation mechanism has been studied in a base glass of composition 63.0 SiO2,





2 µm

Figure 4. Dendritic leucite growth in glass granulates shown on an SEM after HF etching.(Reproduced from Hoeland et al . (1995) with permission of Elsevier Science.)

17.7 Al2O3, 11.2 K2O, 4.6 Na2O, 0.6 B2O3, 0.4 CeO2, 1.6 CaO, 0.7 BaO and 0.2 TiO2(Holand et al . 1995). Three experiments were carried out: nucleation with almost flatpolished monolithic samples; nucleation at the flat surface using seeding particles;and nucleation and crystallization of glass powders. When the glass was heat treatedin monolithic form without seeding particles, the nucleation rate was low and thegrowth rate was 5 nm min−1. The leucite crystals demonstrate anisotropic growthin which the crystals grow rectangular to the surface of the glass specimen. Usingglass dust of the same composition, nucleation in the monolithic glass led to two-dimensional crystal growth (figure 3), in which a flat cell-like crystal grew on thesurface of the base glass at 720 ◦C. Subsequently, this crystal grew and transformedinto the predominant crystal-phase leucite. In glass powders there was heteroge-neous surface nucleation of the grains by glass dust. While the leucite crystals grewon seeded areas as well as on unseeded areas (Holand & Beall 2002), seeding didconsiderably increase the nucleation rate and the growth rate was 2 µm min−1.

The population density of the nuclei can be increased and effective surface crystal-lization ensured by finely grinding the base glass. Applying this method to leucite-containing glass ceramics can, on heat treating between 920 ◦C and 1200 ◦C, lead topetal-like, dendritic crystals (figure 4). The glass ceramic is used for dental restora-tion; therefore, it is important that it be translucent, even though the crystals are2–5 µm in diameter. The flexural strength of the final product is 120–140 MPa. Fur-thermore, leucite-glass ceramics must be capable of combining with other productssuch as sintering ceramics, glazes, and glasses with special optical properties in asintering process. With compositions such as 48–66 SiO2, 5–20 Al2O3, 3–15 K2O, 3–20 Me(II)O (CaO, MgO, SrO, BaO), 0.5–5 P2O5, 3–12 Na2O, it is possible to combinetranslucency with opalescence. The microstructure of this opal glass ceramic showsliquid–liquid phase separation and leucite.





(d) Twofold-nucleation mechanisms

Two possibilities for controlling double nucleation were discovered with the aim ofprecipitating two different crystalline phases in a glass ceramic by in situ reactions.First, a mica–apatite glass ceramic was developed in the SiO2–Al2O3–MgO–Na2O–K2O–P2O5–F system (Vogel & Holand 1987). The base glass showed three glassyphases, two droplet phases and a glassy matrix. The small-droplet phase was rich inalkali fluorine, and mica crystals formed in the glassy matrix by a solid-state reaction.The large-droplet phase was rich in CaO, P2O5 and F and showed homogeneousnucleation of apatite. Therefore, during heat treatment of this glass between 750and 1100 ◦C, twofold nucleation was initiated: mica was nucleated heterogeneouslyand apatite homogeneously. Each apatite crystal grew within the droplet phase andgrowth was stopped at the boundary with the glassy matrix. The final distributionof apatite was hexagonal crystals of diameter approximating that of the previousdroplet phase.

A second twofold nucleation mechanism was developed by combining surface andvolume nucleation in the SiO2–Al2O3–CaO–Na2O–K2O–P2O5–F system. The resultwas the formation of a leucite–apatite glass ceramic (Holand et al . 1994). Apatitewas precipitated as needle-like fluorapatite. In recent publications, other needle-likeapatites have been discussed in white opaque-glass ceramics, for example by Moi-sescu et al . (1999) or apatite–mullite needles (Clifford & Hill 1996). A characteristiccomposition of leucite–apatite glass ceramic is 49–58 SiO2, 11–19 Al2O3, 9–23 K2O,1–10 Na2O, 2–12 CaO, 0.5–6 P2O5, 0.2–2.5 F, with additives of up to 6 wt% of CeO2,B2O3, Li2O. A CaO/P2O5 molar ratio of ca. 5.8 and a fluorine content of 0.6 wt%of the base glass composition were preferred. The controlled nucleation and crystal-lization of this base glass was carried out with glass powder of an average grain sizeof 20–40 µm. Nucleation of the glass and the sintering processes of the glass pow-der leading to a monolithic body proceeded almost simultaneously at temperaturesbetween 800 and 1100 ◦C. The twofold nucleation mechanism resulted in the for-mation of the two main crystal phases leucite and apatite. Leucite was produced bysurface nucleation (§ 3 c), while apatite was produced by volume nucleation combinedwith glass immiscibility.

The mechanism of apatite formation in leucite-glass ceramics is different from thatin the mica–apatite glass ceramic. In the leucite–apatite glass ceramic, the stage ofglass immiscibility is rapidly surpassed and the apatite quickly grows past the phaseboundary of the amorphous droplet. The nucleation of apatite is heterogeneouslyinitiated by crystalline precursor phases. The primary crystal phase is NaCaPO4.The sequence of phase formation was studied in special monolithic glasses usinghigh-temperature X-ray diffractometry and nuclear magnetic resonance (Chan et al .2001). Because of the reduced surface nucleation rate of leucite, it is easier to studynucleation of apatite in monolithic glasses than it is in glass powders.

The composition of these bulk samples was 54.8 SiO2, 14.1 Al2O3, 8.4 Na2O,10.6 K2O, 4.9 CaO, 1.0 ZrO2, 0.3 TiO2, 3.9 P2O5, 0.8 CeO2, 0.2 Li2O, 0.3 B2O3 and0.7 F (Holand & Beall 2002). It was very surprising that the primary crystal phaseof NaCaPO4 was formed in the base glass after quenching the melt. This crystalphase was determined by high-temperature X-ray diffractometry up to ca. 610 ◦C.The microstructure (figure 5) shows rounded crystals comparable in shape with phaseseparation of glassy droplet phases. Above 610 ◦C, an additional crystalline phase





1 µm

98.4 nm

128.7 nm

Figure 5. SEM of the etched (2.5% HF, 10 s) surface of primary crystals of NaCaPO4 in anapatite–leucite–glass ceramic after heat treatment for 15 min at 580 ◦C. (Reproduced from Hoe-land et al . (2002) with permission of The American Ceramic Society, copyright 2002. All rightsreserved.)

2 µm

Figure 6. Microstructure of a leucite–apatite glass ceramic for dental restorations. SEM of mate-rial heat treated for 1 h at 850 ◦C and 1 h at 1050 ◦C and then etched (3% HF, 10 s). (Repro-duced from Hoeland et al . (2000c) with permission of Kluwer Academic Publishers.)

precipitated at 640 ◦C and NaCaPO4 crystals were no longer present (table 2). Thesephases nucleate apatite heterogeneously. The first crystals of apatite were identifiedafter 8 h at 700 ◦C. These crystals did not show a needle-like habit. Needle-like flu-orapatite was precipitated at 700 ◦C and an additional heat treatment of 1050 ◦Cfor 2 h. The growth of needle-like apatite was characterized as an Ostwald ripeningprocess.





Table 2. Crystal-phase formation in leucite–apatite glass ceramics

temperature crystal phase

25 ◦Ca NaCaPO4

640 ◦C Na2Ca4(PO4)2SiO4

700 ◦C KAlSi2O6, Na2Ca4(PO4)2SiO4

800 ◦C KAlSi2O6

Na2Ca4(PO4)2SiO4

Ca5(PO4)3F1050 ◦C KAlSi2O6,

needle-like Ca5(PO4)3F

aAfter casting and cooling.

500 nm

Figure 7. SEM of the etched (3% HF, 10 s) surface of fluorapatite glass ceramic, heat treatedfor 4 h at 520 ◦C and 1 h at 700 ◦C, showing the nanoscale microstructure.

In glass powders, the nucleation of leucite and apatite occurred in parallel duringheat treatment for 1 h at 850 ◦C. The leucite crystals were 1–2 µm in diameter.The crystalline content of the glass ceramic was 10–25 vol.% leucite and 5–10 vol.%apatite. The considerably smaller needle-like apatite crystals were located betweenthe leucite crystals (figure 6). The morphology of these apatite crystals is comparablewith that found in natural teeth. The glass ceramic is characterized by good opticalproperties, chemical durability and easy processing by sintering. It is applied as abiomaterial in restorative dentistry.

(e) Nature as the example for a glass-ceramic microstructure

Apatite crystals of needle-like morphology are known to be contained in naturalbone and teeth. The very small crystals in dental microstructures result in very spe-cial optical properties such as translucence and opalescence. Therefore, it was the aim





500 nm

Figure 8. SEM of glasses based on the SiO2–Li2O–K2O–ZnO–CaO–P2O5–F system permit crys-tallization of biomimetic structures: needle-like fluorapatite after 4 h heat treatment at 700 ◦C(etching 3% HF, 10 s).

of a research programme to develop a glass ceramic showing a similar microstructureto that of the natural material. This should provide some direction for the devel-opment of biomimetic microstructures. The main goal was to develop a needle-like,chemically durable apatite in a translucent glass ceramic that is easy to use as adental restorative.

A glass ceramic with needle-like fluorapatite crystals precipitated in a glass matrixwas developed in the SiO2–Li2O–K2O–ZnO–CaO–P2O5–F system (Schweiger et al .2002). A typical composition range was 56–65 SiO2, 1.8–5.3 Li2O, 9–17.5 K2O, 9–16 ZnO, 3.5–10.5 CaO, 2–6 P2O5 and 0.5–1.0 F. The glass of the given overall chem-ical composition with 6.1 CaO, 2.8 P2O5 and 0.8 wt% F was heat treated for 4 hat 520 ◦C and for 1 h at 700 ◦C. The result was nanoscale (30–60 nm) crystals offluorapatite Ca10(PO4)6F2 uniformly and densely precipitated in the volume of thesample (figure 7). It was interesting to note that the microstructure formed by thisheat treatment was not influenced by variation in the content of CaO or P2O5. Higherconcentrations of CaO (8.7 wt%) and P2O5 (5.1 wt%) increased neither the size northe volume percentage of the fluorapatite phase.

Based on the influence of CaO, P2O5 and F, it is obvious that phase separationprovides the basis of the nucleation in the glass. However, primary crystal nucle-ation comparable with that in mica–apatite glass ceramic (homogeneous nucleationof apatite in the droplet phase) or in leucite–apatite glass ceramic (heterogeneousnucleation by a precursor crystal phase) was not detected. It is most likely that glassimmiscibility (glass-in-glass phase separation) and nucleation is so fast in this typeof glass that it is not possible to distinguish the different reaction steps. The result isthe formation of a glass ceramic microstructure with nanoscale fluorapatite crystals,which do not show anisotropic needle-like growth in this temperature range.

The process of nanophase formation was skipped, with a one-step heat treatmentat 700 ◦C. Crystal growth of needle-like fluorapatite was observed by scanning elec-





tron microscopy. These results on monolithic glass ceramics demonstrate that, aftershort thermal treatments, the fluorapatite crystals were smaller and more numerousthan after long heat treatments at 700 ◦C. One requirement for the crystallizationprocess of Ostwald ripening is fulfilled with this result. The fluorapatite crystals grewanisotropically in one direction to form elongated crystals (figure 8). The diameterof the elongated crystals remained constant at ca. 60 nm. The length increased from120 to 300 nm, giving an aspect ratio of up to five. The glass ceramic has good chem-ical durability and translucence. It is used as a highly aesthetic material to veneermetal-free dental restorations of the lithium disilicate type.

4. Conclusion

Fundamental and applied research activities have shown different possibilities forcontrolling nucleation in glasses to tailor properties. Volume nucleation enables thedevelopment of high-strength materials or nanophase products with zero thermalexpansion in a wide temperature range. A twofold nucleation mechanism based onhomogeneous nucleation in phase-separated glasses and on heterogeneous nucleationallows different crystal phases to be combined within a material. Other possibilitiesfor twofold nucleation arise from the combination of surface and volume nucleation,resulting in a leucite–apatite glass ceramic for dental applications. The developmentof glass ceramics with needle-like apatite crystals demonstrates the applicability ofbiomimetic microstructures to a commercial glass ceramic.

References

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Discussion

K. F. Kelton (Department of Physics, Washington University, St. Louis, MO,USA). Are elemental metals still used for nucleants? If not, why not?

W. Holand. Metals such as Au, Ag or Pt were used in the 1960s and 1970s as nucle-ating agents to precipitate crystals in base glasses to form glass ceramics. Today,this type of heterogeneous nucleation is used very rarely. Now we are able to controlnucleation and crystallization of glasses by surface or volume mechanisms. It is veryimportant to combine heterogeneous volume nucleation with phase separation pro-cesses. A special micro-immiscibility is possible with the following nucleating agents:TiO2, ZrO2, P2O5, Ta2O5, WO3, Fe2O3 and F. These agents allow the formationof uniform crystals within the glassy matrix and to control the crystal content ofthe glass ceramic. Therefore, it is possible to produce products with tailor-madeproperties.

B. R. Haywood (Department of Physics, Keele University, UK ). Given that theflux electrolytes are critical, have you considered the potential role of zeolites as‘reagents’ to regulate the crystallization process?

W. Holand. There is the possibility of precipitating zeolite-type crystals in glassceramics. In forming these crystals or any other type of crystals, the viscosity of theglassy matrix changes and the following crystallization is influenced. But there is notnow any special influence of zeolites in controlling crystallization of glasses at hightemperatures.




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