Thermal and mechanical properties of rare earth aluminate and low-silica aluminosilicate optical glasses

Journal of Non-Crystalline Solids 351 (2005) 650–655 www.elsevier.com/locate/jnoncrysol

Thermal and mechanical properties of rare earth aluminate and low-silica aluminosilicate optical glasses Jacqueline Johnson a

a,*

, Richard Weber b, Marcos Grimsditch

a

Argonne National Laboratory, Energy Technology, 9700 S. Cass Ave., Argonne, IL 60439, USA b Containerless Research Inc., Evanston, IL 60201, USA Received 17 August 2004; received in revised form 21 December 2004

Abstract Aluminate glasses containing 45–71.5 mol% alumina, 10–40 mol% rare earth oxide, and 0–30 mol% silica were synthesized from precursor oxides. The glass transition and crystallization temperatures were determined by differential scanning calorimetry; the structural and mechanical properties were investigated by Raman and Brillouin spectroscopy. The range of the supercooled liquid region varies from 40 °C to 200 °C, providing a useful working range for compositions with 5–30 mol% silica. Raman scattering showed the presence of isolated SiO4 species that strengthen the network-forming structure, enhance glass formation, and stabilize the glass even when they are present at fairly low concentrations. Sound velocities were measured by Brillouin scattering. From these and other values, various elastic moduli were calculated. The moduli increased with both aluminum and rare earth content, as did the hardness of the glasses. YoungÕs modulus was in the range 118–169 GPa, 60–130% larger than that for pure silica glass. Ó 2005 Published by Elsevier B.V. PACS: 61.43.Fs; 62.20.Dc; 65.90.+i

1. Introduction The relative ease with which glasses can be formed into complex shapes, including fibers, makes them well suited to optical devices. In addition to the potential for low-cost manufacturing, glasses have isotropic optical properties and provide a disordered environment for dopant ions that can broaden fluorescence bandwidths to enable broadband amplification or the fabrication of short pulse lasers. The quest for glasses that combine desirable optical properties and environmental stability and that can be readily manufactured has led to development of many oxide glass formulations, in particular, silicate-, phosphate-, and tellurite-based materials [1–6].

*

Corresponding author. Tel.: +1 630 252 5476; fax: +1 630 252 4798. E-mail address: [email protected] (J. Johnson).

0022-3093/$ - see front matter Ó 2005 Published by Elsevier B.V. doi:10.1016/j.jnoncrysol.2005.01.065

Silica-based glasses are widely used in long-haul optical communications devices, notably erbium-doped fiber amplifiers [7], but these glasses are limited to relatively large devices with very low dopant concentrations [1]. At higher dopant concentrations, >500 ppm, ÔclusteringÕ of dopant ions leads to energy transfer processes that decrease device efficiency [2]. Phosphate [3–5], tellurite [6], and aluminosilicate glass materials [1,8] can dissolve relatively large concentrations of dopants compared to pure silica glasses. The high phonon energies of phosphates and silicates limits their use to visible and near infrared applications. Tellurite glasses exhibit excellent optical properties, which include high dopant solubility and low phonon energies. Their poor environmental stability, however, has limited their commercial use. Rare earth-aluminum oxide materials can be formed into bulk single-phase glass [9,10] and glass fibers

J. Johnson et al. / Journal of Non-Crystalline Solids 351 (2005) 650–655

[11,12]. These glasses with up to 20 mol% silicon dioxide can also be formed into rods and plates. All these glasses are strong and stable, and can host large concentrations of dopant ions. Neutron diffraction from Nd-doped rare earth aluminum oxide glasses containing up to 5 mol% Nd2O3 indicates that the dopant ions are homogeneously distributed in the glass and are predominantly in 8-fold coordination with oxygen [13]. Prior work on rare earth aluminosilicate glasses has been on compositions that contain relatively large amounts of silica [14–18] that are easily vitrified. Due to the high SiO2 (30–82 molar%) content, the optical characteristics are dominated by the contributions of silica. The properties of the pure rare earth aluminate and low-silica rare earth aluminosilicate glasses investigated in the present work are dominated by aluminate network formers. In separate work using X-ray and neutron diffraction, it was shown that the materials are highly disordered with distortion of the network forming AlO4 species [19]. In the prior work, Makishima et al. showed that the YoungÕs modulus and shear modulus of glasses containing 30–58 mol% silica were on the order of 100 GPa and 40 GPa respectively. In the present work, glasses containing 0–30 mol% silica were prepared and studied. The new research extends the composition range well beyond prior work and suggests that the mechanical and thermal properties of the glasses change rapidly at the lower silica end of the composition family. In the present work, rare earth-aluminum oxide glasses containing up to 30 mol% silica and up to 20 mol% optically active rare earth element oxides were synthesized, and their physical and optical properties were studied.

2. Experimental procedure Glass formulations were made from mixtures of 5 N purity, 325 mesh metal oxide powders that were blended in a ball mill. Bulk single-phase glass was made in two ways. (i) All the compositions investigated were made into glass using a containerless processing technique that avoided container-derived nucleation of crystals in the undercooled molten oxides. (ii) Samples that contain silica and could be melted at temperatures below 1650 °C were also made by melting blended precursor materials in platinum crucibles [20]. After 2–6 h the melts were removed from the furnace and cast into pyrolytic graphite molds to form rods or plates [20]. The compositions of the fabricated samples are listed in Table 1. For clarity the table is divided into four sections: firstly, those samples that contain only lanthanum oxide, alumina, and silica; secondly, those containing yttria, alumina, and silica; thirdly, those containing both yttrium and lanthanum oxide along with alumina and silica; and lastly, those samples containing optically active components. A Netzsch 404S differential scanning calorimeter (DSC) was used to conduct the thermal characterization. The instrument was calibrated over a broad temperature range using four standards: Sn, Zn, Au, and BaCO3. The system was purged with helium gas, and the heat flow to the sample was compared to a reference and monitored as a function of temperature, while the sample was subjected to a controlled temperature program. The sample was contained in an alumina crucible and heated at a rate of 10 K/min over a temperature range of 600–1300 °C.

Table 1 Glass composition, glass transition temperature (Tg) crystallization onset (Tx) and calculated difference (Tx Glass ID

Al2O3 (%)

71.5A–28.5L 62.5A–37.5L 55A–25L–20S 55A–15L–30S

71.5 62.5 55 55

62.5A–37.5Y 55A–40Y–5S 55A–35Y–10S 55A–30Y–15S 55A–25Y–20S 55A–15Y–30S

62.5 55 55 55 55 55

37.5 40 35 30 25 15

60A–20Y–15L–5S 65A–15Y–10L–10S 50A–10Y–25L–15S 50A–10Y–20L–20S 50A–10Y–10L–30S

60 65 50 50 50

20 15 10 10 10

45A–10Y–15L–20S 45A–15L–20S 50A–13Y–15L–20S 55A–15Y–17L–10S 55A–12Y–17L–10S

45 45 50 55 55

10

Y2O3 (%)

La2O3 (%)

13 15 12

ErO2 (%)

YbO2 (%)

Tg) Tg (°C)

Tx (°C)

Tx

20 30

840 843 845 870

951 927 1009 969

111 84 164 99

5 10 15 20 30

869 870 874 872 874 891

917 914 931 968 985 1004

48 44 57 96 111 113

15 10 25 20 10

5 10 15 20 30

846 866 848 848 870

1010 948 1035 1058 999

164 82 187 210 129

15 15 15 17 17

20 20 20 10 10

861 854 845 839 851

1043 1037 1019 1030 1030

182 183 174 191 179

28.5 37.5 25 15

SiO2 (%)

651

10 20 2 3 6

Tg (°C)

J. Johnson et al. / Journal of Non-Crystalline Solids 351 (2005) 650–655

Raman spectra were obtained in a 90° scattering geometry using power levels of 100 mW at 458, 476, 488, 515 and 568 nm; the spectra were recorded on a Jobin-Yvon T64000 triple spectrometer. Brillouin spectra were also obtained in a 90° scattering geometry using power levels of 100 mW at 532 nm; spectra were recorded on a Sandercok type 5 + 4 pass Tandem Fabry Perot interferometer. The Vickers hardness of several of the glasses was measured with a Beuhler Micromet II microhardness instrument. The instrument was operated with a load of 100 g in which polished samples were held in rigid mounting material.

880

870

860

Tg (oC)

652

850

840

830 5

10

15

3. Results

25

30

35

40

La2O3 (%)

Fig. 1 is a typical DSC curve showing the glass transition temperature (Tg) and multiple crystallization temperatures (Tx). The majority of the samples had only one crystallization peak. Values of Tg and Tx were extracted using the Proteus software, which is integral to the DSC instrument. DSC measurements give information on the Ôease of processingÕ of the glass. Values of Tg and Tx for the different samples are presented in Table 1. As we are interested in the processing of the glasses, the Tx Tg values are also given. Large values of Tx Tg indicate a wide working temperature range before the onset of crystallization. The most significant variation in Tg occurs with the addition of La, which causes Tg to decrease, (see Fig. 2). There is a slight decrease in Tg with the addition of alumina; Tg increases with either SiO2 or Y2O3 content. In general, though, variations in Tg are relatively minor; doping the glasses with an optically active rare earth has little effect on Tg.

Fig. 2. Variation of Tg with La2O3 content. The line is a guide to the eye.

The onset of crystallization occurs over a wide temperature range that is significantly reduced by the addition of Y2O3 (see Fig. 3) or alumina. There is hardly any variation with increasing La2O3 or silica content. This trend is matched by the variation in Tx Tg. Raman spectra were recorded at several different wavelengths to ascertain whether spectral peaks were truly Raman or due to fluorescence of rare earth impurities. If a peak is due to Raman resonance the shift will remain the same whatever the probe laser wavelength. The position of fluorescence peaks changes as the laser wavelength is varied. Fig. 4 shows two Raman spectra of (Al2O3)0.625(La2O3)0.375 at different wavelengths, in which there is a clear shift in peak position and intensity. The majority of the features in these spectra are due to fluorescence. Fig. 5 shows two of the Raman spectra

0.2

1060

0.0 Tg

Tx

1020

-0.2

Tx (oC)

Exo (µV/mg)

20

-0.4

940

-0.6

-0.8 800

980

900

1000

1100

1200

T(oC) Fig. 1. DSC trace of glass containing 65% Al2O3, 15% Y2O3, 10% La2O3 and 10% SiO2, showing the glass transition temperature and three crystallization peaks.

900 5

15

25

35

45

Y2O3 (%) Fig. 3. Variation of Tx with Y2O3 content. The line is a guide to the eye.

J. Johnson et al. / Journal of Non-Crystalline Solids 351 (2005) 650–655

653

L

Intensity (a.u.)

Intensity (a.u.)

L

0

500

1000

1500

T

-2

-1

-1

0

1

1

2

-1

Raman shift (cm )

Frequency (cm )

Fig. 4. (a) Raman spectrum of (Al2O3)0.625(La2O3)0.375 at 488 nm (lower curve) and 515 nm (upper curve).

Intensity (a.u.)

-1

T

Fig. 6. Brillouin spectrum of (Al2O3)0.715(La2O3)0.285, showing the longitudinal (L) and transverse (V) peaks.

shown in Table 2. Both the Brillouin and the Raman data were obtained from a sample set with slightly different compositions from those used in the DSC experiments. Values of Vickers hardness were in the range 850–1000. Our results show that the glass hardness increases with the addition of Y2O3 or alumina, decreases with the addition of silica, and remains virtually unchanged with the addition of La2O3.

SiO2

4. Discussion

0

400

800

1200

1600

Raman shift (cm-1) Fig. 5. (a) Raman spectrum of (Al2O3)0.55(La2O3)0.25+(SiO2)0.2 at 476 nm (upper curve) and 515 nm (lower curve).

of (Al2O3)0.55(La2O3)0.25+(SiO2)0.2; while there is fluorescence in these spectra the most distinct feature, the peak centered around 850 cm 1, which is associated with isolated SiO4 tetrahedra, remains prevalent. The absence of sharp peaks in all of the spectra confirm the amorphous nature of the material. Brillouin scattering was used to measure the transverse and longitudinal sound velocities. Knowing the density and refractive index we can calculate the longitudinal and transverse elastic moduli, C11 and C44.. Other moduli such as C12, bulk (B), YoungÕs (Y), and biaxial moduli and PoissonÕs ratio can, in turn, be calculated. A typical Brillouin spectrum is shown in Fig. 6; L and T denote longitudinal and transverse, respectively. The velocities and elastic moduli for selected samples are

To develop these glasses for use as optical materials, an understanding of their thermal stability is necessary. The greater the value of Tx Tg, the stronger is the inhibition to nucleation and crystallization processes. In turn, the greater the inhibition to these processes, the more stable the glass. This translates into a wider working-temperature range in which the samples can be processed. In the glasses studied here, Tx Tg was reduced significantly by the addition of Y2O3 and Al2O3 and slightly by the addition of La2O3. In general, therefore, large working-temperature ranges require high SiO2 content. The Raman measurements suggest that the SiO4 species are mainly isolated. In spite of the apparent lack of silica network, even small additions of silica significantly increase the supercooled liquid range in the glasses and decrease the elastic modulus. The Brillouin results indicate that both the longitudinal and transverse velocities increase with increasing yttria (see Fig. 7) or alumina content. The large moduli of these glasses, compared to those of pure silica and most glasses used in the optical industry, reflect their

654

J. Johnson et al. / Journal of Non-Crystalline Solids 351 (2005) 650–655

Table 2 Measured values of the transverse (VT) and longitudinal (VL) sound velocities and density for the glasses Velocity (km s 1)

Glass ID

1

71.5A–28.5Y 76A–24Y 62.5A–37.5L 71.5A–28.5L 55A–40Y–5S 55A–35Y–10S 55A–30Y–15S 55A–25Y–20S 55A–25L–20S 50A–10Y–20L–20S SiO2

n

Density (g cm 3)

Moduli (GPa) C11

C44

C12

B

Y

Biaxial

m

1.74 1.74 1.80 1.77 1.80 1.79 1.76 1.74 1.75 1.76 1.46

3.87 3.81 4.56 4.23 4.35 4.13 3.84 3.49 4.28 4.23 2.20

158 206 166 224 223 199 194 177 162 169 79

45 38 47 65 65 59 57 51 46 48 32

67 129 71 93 92 82 80 74 70 72 15

97 155 103 136 135 121 118 108 100 105 36

118 106 123 169 169 151 147 133 121 126 74

168 173 176 239 239 214 208 189 172 180 88

0.30 0.39 0.30 0.29 0.29 0.29 0.29 0.29 0.30 0.30 0.16

1

nVT (km s )

nVL (km s )

5.95 5.51 5.79 6.96 6.98 6.74 6.77 6.68 5.77 5.95 5.56

11.11 12.79 10.85 12.87 12.88 12.43 12.52 12.38 10.78 11.13 8.74

Calculated values of the moduli of elasticity (C11, C44, and C12); bulk moduli (B); YoungÕs moduli (Y); biaxial moduli and PoissonÕs ratio, m, are given. Amorphous silica is shown for comparison. Refractive indices of 55A–Y–10S, –15S and –20S were measured, silica obtained from the literature, and others estimated.

Overall, the work has determined important properties that support the idea that aluminate glasses are strong materials that have a useful working-temperature range and could be exploited in making glass products.

Elastic Moduli (GPa)

250

200

150

Acknowledgments 100

50

0 5

15

25

35

45

Y2O3 (%) Fig. 7. Variation of longitudinal, C11 (d), and transverse, C44 (j), elastic modulus as a function of Y2O3 content. The lines are to guide the eye.

The work was supported under a Department of Energy (DOE) Cooperative R&D Agreement, project number C0300101, FWP 73C01. Glass development research was supported under a National Science Foundation Phase II SBIR, contract number DMI-0216324. MG was supported by DOE, Office of Basic Energy Sciences, Materials Sciences, under contract W-31-109-ENG-38. We thank Ms. Kirsten Hiera and Mr. Richard Scheunemann for measuring the density, Vickers hardness, and index of refraction of the glasses.

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5. Conclusions DSC results have shown the range of compositions over which the rare earth aluminate glass is stable and easy to process. Raman scattering has given insight into the glass structure and composition that leads to greater stability. The glasses have large elastic moduli, indicating that they are stiff compared to most silicate and phosphate glass materials.

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