Optical Materials 34 (2012) 826–831

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Optical Materials

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Behavior of the refractive index of lithium disilicate glass ceramic processedat high pressure and high temperature

Silvio Buchner, Marcelo Barbalho Pereira, Naira Maria Balzaretti ⇑Physics Institute, Universidade Federal do Rio Grande do Sul, CP 15051, Porto Alegre, RS, ZIP 91501-970, Brazil

a r t i c l e i n f o

Article history:Received 8 July 2011Received in revised form 16 October 2011Accepted 21 November 2011Available online 17 December 2011

Keywords:Lithium disilicateRefractive indexHigh pressureHigh temperatureX-ray diffractionGlass ceramic

0925-3467/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.optmat.2011.11.018

⇑ Corresponding author. Tel.: +55 51 3308 6489; faE-mail address: naira@if.ufrgs.br (N.M. Balzaretti).

a b s t r a c t

The aim of this work was to investigate the effect of high pressure and high temperature on the refractiveindex of lithium disilicate glass ceramic with the stoichiometric composition Li2O�2SiO2 (LS2). A firstgroup of monolithic LS2 glass samples were processed at 2.5 GPa, 4 GPa and 7.7 GPa at room temperatureand a second group was submitted to high pressure and, simultaneously, to heat treatments for nucle-ation and growth of the crystalline phases. For comparison, samples submitted to the same heat treat-ments at 1 atm were also investigated. The refractive index of the samples was obtained by spectralellipsometry and the results were clearly dependent on the particular pressure and temperature condi-tions. The crystallization of the samples was investigated by X-ray diffraction. For the samples processedunder high temperature at 1 atm and at 2.5 GPa a fraction of the originally amorphous glass was trans-formed to a monoclinic phase of lithium disilicate. For the samples processed under high temperatureand at 4 GPa, a large fraction of the originally amorphous glass transformed to an orthorhombic phasewhile, at 7.7 GPa, it was observed the formation of lithium metasilicate.

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1. Introduction processed simultaneously under high pressure and high tempera-

Glass–ceramics are polycrystalline materials obtained by crys-tallization of suitable glasses through controlled heat treatmentprocesses. The conventional production of glass–ceramic is a wellknown process and starts with the preparation of a homogeneousglass, followed by the glass shaping and the application of a con-trolled heat treatment to promote nucleation and crystal growth.This process allows the suitable combination of ceramic and glassproperties [1]. The resulting crystalline (fully or partially) materialexhibit better resistance to wear, to oxidation and to chemical deg-radation, and have increased hardness and dimensional stability[2], which is important for engineering applications [1].

Gutzow et al. [3] presented a thorough study of the kinetics ofmelt crystallization under applied static pressures, taking intoaccount the thermodynamic and kinetic consequences of pressureon nucleation and crystal growth rates. They pointed out that onlyat very high loads (in the GPa region) can static pressure exert areal catalytic effect in the crystallization of glasses. According tothem, in most cases, rather than an increase in nucleation rate, ashift of the melt crystallization to higher temperatures should beexpected at higher pressures.

Fuss et al. [4] showed that the crystal growth rate was smallerand the crystallization peak temperature increased when LS2 was

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ture (HPHT). According to them, the smaller growth rate wasrelated to the increase of the glass viscosity due to structuralchanges induced by high pressures.

Buchner et al. [5] investigated the effect of high pressure atroom temperature on the crystallization process and thermalproperties of LS2. They found that the previous densification orthe defects induced by high pressure changed the nucleation andgrowth rates during the subsequent thermal treatment. They alsoinvestigated the effect of HP on the mechanical properties of LS2

[6]. It was shown that the hardness and elastic modulus of thesamples processed at HPHT increased noticeably compared to theglass ceramic obtained through thermal treatment at atmosphericpressure.

Kitamura et al. [7] also pointed out that the permanent densifi-cation after applying HPHT is a phenomenon observed only forglass materials due to their structural freedom. Based on Ramanspectroscopy and radial distribution function, they attributed thisdensification to an increase of the packing density of SiO4 tetrahe-dra due to a decrease of the Si–O–Si bond angle between thetetrahedra.

As far as we know, there is no report in the literature about theeffect of the HPHT treatment on the refractive index of LS2.Therefore, the aim of this work was to study the behavior of therefractive index (real and imaginary parts) of LS2 samples after pro-cessing at HPHT. The refractive index was measured using spectralellipsometry and the crystallization of the samples induced byHPHT was investigated by X-ray diffraction.

Fig. 1. X-ray diffraction patterns for samples processed at (a) 1 atm and 2.5 GPa andthermal treatment TTA and (b) 2.5 GPa and thermal treatment TTB.

Fig. 2. X-ray diffraction patterns for samples processed at 4 GPa and thermatreatments TTA and TTB.

S. Buchner et al. / Optical Materials 34 (2012) 826–831 827

2. Material and methods

2.1. Sample preparation

Lithium disilicate glass of stoichiometric compositionLi2O�2SiO2 (LS2) was prepared using standard reagent grade Li2CO3

(Aldrich Chem. Co., 99+ %) and ground quartz (<99.9% SiO2). The200 g batch was melted in a Pt crucible at 1450 �C during 2 h inan electric furnace. The melt was poured on a steel plate, annealedbelow the glass transition temperature, at 430 �C, during 1 h andcooled down slowly to room temperature.

2.2. High pressure processing

The specific configuration and type of high-pressure chamberused in the experiments were optimized for providing quasi-hydro-static pressure during the experiments [8]. The pressure calibrationwas carried out by the ‘fixed-points’ technique using Bi and Yb,which permitted the calibration of the pressure at the followingpoints: Bi with phase transitions at 2.5 GPa and 7.7 GPa, and Yb witha phase transition at 4.0 GPa. A detailed description of the high-pres-sure calibration method has been provided elsewhere [8].

2.3. Analytical techniques

A Siemens diffractometer Cristalloflex D500 with a copper tube(k = 1.5418 Å) and a graphite monochromator in the secondarybeam was used for the X-ray analysis (h–2h geometry).

The refractive index of the LS2 samples was obtained by spectralellipsometry using an Ellipsometer SOPRA GES-5E with a microspotaccessory to focus the light beam at the samples. The measurementswere made in the wavelength range from 0.25 to 0.85 lm and therefractive index was calculated by the ellipsometer’s analysis soft-ware (WinElli) by the relation described in Ref. [9].

3. Experimental

One group of samples was processed during 5 min at high pres-sure and room temperature (HPRT). In this case, the sample wasplaced inside a lead capsule which acted as a soft pressure trans-mitting medium. A second group was processed simultaneouslyat HPHT. In this case, a hexagonal boron nitride (hBN) capsulewas used, placed inside a graphite cylinder which acted as the hea-ter element. For both groups, the samples were processed at2.5 GPa, 4 GPa and 7.7 GPa, and the thermal treatments investi-gated are listed below:

� TTA: 455 �C during 2 h for nucleation followed by 610 �C during0.5 h for crystal growing;� TTB: 500 �C during 2 h for nucleation followed by 610 �C during

0.5 h for crystal growing, corresponding to a higher nucleationtemperature compared to TTA;� TTC: 455 �C during 2 h followed by 610 �C during 2 h for crystal

growing, corresponding to a longer growing time compared toTTA.

After HPHT processing, the surface of samples was grounded onSiC abrasive paper up to #1200 and polished with CeO2 slurry foroptical measurements.

4. Results and discussion

4.1. X-ray characterization

The crystallization of the group of samples processed at HPHTwas investigated by X-ray diffraction. Fig. 1a shows the X-ray

diffraction pattern for the monolithic LS2 glass ceramic processedat 2.5 GPa simultaneously to the thermal treatment TTA, comparedto the sample heat treated at 1 atm. As can be seen, the results arevery similar. A fraction of the originally amorphous glass trans-formed to a monoclinic phase of LS2 glass ceramic (JCPDS# 010-72-0102 – Li2Si2O5). Fig. 1b shows the pattern for the sample pro-cessed at 2.5 GPa and thermal treatment TTB, corresponding to alarger fraction of the monoclinic phase.

For the sample processed at 4 GPa and thermal treatments TTAand TTB, the crystallization was almost complete (Fig. 2) and theresulting phase was orthorhombic (JCPDS# 010-70-4856 – Li2-

Si2O5). Fuss et al. [4] also observed the orthorhombic phase forLS2 processed at 4.5 GPa at 608 �C.

As shown in Fig. 3, at 7.7 GPa the amorphous fraction is domi-nant in the diffraction pattern after thermal treatment TTA andthe crystalline phase corresponds to the metasilicate Li2SiO3

(JCPDS# 000-29-0829). After thermal treatment TTB, with a higher

l

Fig. 4. X-ray diffraction patterns for samples processed at 1 atm and thermaltreatment TTC.

Fig. 5. (a) Refractive index and (b) extinction coefficient as a function of wavelengthfor samples processed at 2.5 GPa, 4 GPa and 7.7 GPa at room temperature,compared to the pristine glass sample.

Fig. 3. X-ray diffraction patterns for samples processed at 7.7 GPa and thermaltreatments TTA and TTB.

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nucleation temperature, the fraction of crystalline phase increasedand remained metasilicate. This phase was also observed by Fuss etal. [4] for samples processed at 6 GPa and 753 �C.

Fig. 4 shows the results for a sample processed at 1 atm and alonger thermal treatment (TTC) in order to increase the crystallinefraction corresponding to the monoclinic LS2 phase (JCPDS# 010-72-0102).

4.2. Refractive index

Fig. 5a and b shows the behavior of the real part (n) and extinc-tion coefficient (k) of the refractive index, respectively, measuredfor the samples processed at high pressure and room temperaturecompared to the pristine glass sample. The results are very similar,indicating that the previous densification did not affect the opticalresponse of the material to the visible light.

Fig. 6a and b shows the behavior of the real part (n) and extinc-tion coefficient (k) of the refractive index, respectively, for thegroup of samples processed simultaneously at HPHT for both ther-mal treatments TTA and TTB, compared to the sample thermaltreated TTC at ambient pressure.

The behavior of the refractive index of the samples with a largefraction of amorphous phase after HPHT is similar to the resultsshown in Fig. 5. This is the case of the samples processed at TTAand 1 atm, 2.5 GPa and 7.7 GPa. The sample processed at 1 atmand a longer thermal treatment (TTC) showed a large fraction ofcrystalline phase (monoclinic according to Fig. 4) and,

Fig. 6. (a) Refractive index and (b) extinction coefficient as a function of wavelengthfor the samples processed at 1 atm, 2.5 GPa, 4 GPa and 7.7 GPa and thermaltreatments TTA and TTB. Results for a sample processed at 1 atm and TTC are alsoshown for comparison.

Fig. 7. Refractive index (n) measured at 550 nm as a function of the density of thecrystalline phases.

Fig. 8. Images obtained by optical microscopy of the samples processed at: (a) 1 atm and(f) 4 GPa and TTB, (g) 7.7 GPa and TTA, (h) 7.7 GPa and TTB close to the border and (i) c

S. Buchner et al. / Optical Materials 34 (2012) 826–831 829

consequently, it was observed an increase of the refractive index(n), as shown in Fig. 6a. The samples processed at 4.4 GPa, for bothTTA and TTB, showed a very small amount of amorphous phase,according to Fig. 2, and the behavior of the refractive index waspractically the same for both thermal treatments. The increase ofthe refractive index is probably related to the crystallization ofthe orthorhombic phase. This was also the case for the sample pro-cessed at 2.5 GPa and at a higher nucleation temperature (TTB) Thesample processed at 7.7 GPa and at a higher nucleation tempera-ture (TTB) presented the largest values of the refractive index, ascan be shown in Fig. 6a. The X-ray diffraction pattern for this sam-ple corresponds to the metasilicate phase (Fig. 3).

Fig. 7 shows the results for the refractive index measured at0.55 lm (shown in Fig. 6) for the samples with a large fraction ofcrystalline phase as a function of the density of the correspondingphase. The density of the glass sample was obtained from Ref. [10]

TTA, (b) 1 atm and TTC, (c) 2.5 GPa and TTA, (d) 2.5 GPa and TTB, (e) 4 GPa and TTA,lose to the center of the sample.

Fig. 8 (continued)

830 S. Buchner et al. / Optical Materials 34 (2012) 826–831

and confirmed by the Archimedes method using distilled water.The density of the crystalline phases was obtained from the JCPDSfiles.

Fig. 8 shows typical optical images of the samples after chemi-cal etching with HF (1%) during 2 min to reveal the crystallinegrains. The sample processed at 7.7 GPa and TTB was not homoge-neous: close to the center of the surface, the grains were not coa-lesced while close to the border, the crystallization was complete(Fig. 7h and i, respectively).

The extinction coefficient (k) is related to absorption and scat-tering centers. The contribution of scattering centers for the sam-ples processed under high pressure (amorphous) and polishedwith CeO2 slurry should be relatively small. On the other hand,the samples processed under HPHT contain small grains of crystal-line phases that should act as scattering centers, affecting theextinction coefficient. In fact, the results shown in Fig. 6b changedcompared to the results obtained for the amorphous phases(Fig. 5b).

For the samples processed at 1 atm and TTC and at 4 GPa forboth thermal treatments, the coalescence of the grains at the sur-face of the sample was complete (Fig. 8b, e and f) and, therefore,the contribution of the scattering centers should be small. In fact,the extinction coefficients for these samples were small and similarto the values obtained for the amorphous phase (�0.03). On theother hand, it was not possible to visualize (optical microscopy)the grains for the sample processed at 2.5 GPa and TTA, despitethe results of the XRD demonstrated the existence of monoclinicphase. The samples processed at 2.5 GPa and TTB contained verysmall grains, smaller than 1 lm (Fig. 8d). The sample processedat 1 atm and TTA presented small and dispersed grains,30–50 lm size (Fig. 8a) which could act as scattering centers forthe incident light source. In this case, the extinction coefficientwas, in fact, larger. The situation was similar for the sample pro-cessed at 7.7 GPa and TTA, which contained dispersed grains of

�100 lm and presented a high extinction coefficient. The sampleprocessed at 7.7 GPa and TTB was not homogeneous: the centralregion contained dispersed grains (Fig. 8i) while the border wascrystalline (Fig. 8h).

5. Conclusion

The crystalline phase of LS2 glass ceramic depends on the highpressure and high temperature processing conditions. At ambientpressure and 2.5 GPa, the thermal treatment induces thecrystallization of the monoclinic phase. For 4 GPa the crystalliza-tion to orthorhombic phase was almost complete for both nucle-ation temperatures investigated. For 7.7 GPa and 455 �C, a verylow fraction of the sample crystallized in the metasilicate phase.For 500 �C as the nucleation temperature, the corresponding crys-talline fraction increased. The refractive index of the samples chan-ged significantly comparing the glass sample (average value of 1.4in the visible range) and each one of the crystalline phasesobserved (up to 1.6 in the visible range). An almost linear relation-ship between refractive index and density of the crystalline phasewas observed. The extinction coefficient (k) was affected by thepresence of small and dispersed crystalline grains which acted asscattering centers.

Acknowledgments

The authors would like to thank CNPq for the financial supportand to LaMaV for the fusion of the samples.

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