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SiO2 glasses
Solid State Sciences 12 (2010) 1570e1574
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Formation of nano-crystalline quartz crystals from ZnO/MgO/Al2O3/TiO2/ZrO2/ SiO2 glasses Anja Hunger a, Gunter Carl b, Christian Rüssel a, * a b
Otto-Schott-Institut, Jena University, Fraunhoferstr. 6, 07743 Jena, Germany JSJ Jodeit GmbH, Am Naßtal 10, 07751 Jena-Maua, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 April 2010 Received in revised form 24 June 2010 Accepted 30 June 2010 Available online 8 July 2010
Glasses with the compositions 50.9 SiO2 $ 20.8 Al2O3 $ (20.8 x) MgO$ ZnO $ 3.7 TiO2 $ 3.7 ZrO2 with x ¼ 0, 2.3, 4.6 and 9.3 were annealed at temperatures in the range from 850 to 1100 C. Depending on temperature, high- or low-quartz solid solutions, magnesium aluminosilicate, mullit and spinel precipitated. These glasseceramics exhibit excellent mechanical properties and are potential candidates for applications in micromechanics or as hard disc substrate. The larger the ZnO concentration, the lower is the glass transition temperature. Also microhardnesses and Young’s moduli increased with increasing ZnO concentration. The nucleation temperature was of minor importance. To achieve good mechanical properties, the initially formed high-quartz phase must transform to the corresponding low-quartz phase. This occurs if the quartz phase contains only minor MgO or ZnO concentrations, which can be achieved by increasing the annealing times or temperature. Then MgO, ZnO and Al2O3 occur as separate spinel or gahnite phase. Ó 2010 Elsevier Masson SAS. All rights reserved.
Keywords: Glass Crystallization Quartz
1. Introduction Glasseceramics in the MgO/Al2O3/SiO2 system [1e4] possess excellent mechanical properties. For this system Young’s moduli of up to 140 GPa [1], mechanical strengths of up to 450 MPa, hardnesses of up to 13 GPa have been reported in the literature. The base glass exhibits surface crystallization, however, the addition of components such as TiO2 or ZrO2 leads to volume crystallization and enables the production of fine-grained glasseceramics by controlled crystallization. The glass system requires high melting temperatures and, therefore, has been varied by adding components such as P2O5 [4] or B2O3 [2] which might be suitable to decrease the required melting temperature. After annealing the glasses, depending on composition and annealing temperature various crystalline phases such as spinel, sapphirine low-quartz or high-quartz solid solutions are observed [1,3,5e7]. The low-quartz phase observed at room temperature is formed during cooling the sample from annealing temperatures by the transformation of the initially formed high-quartz phase. This transformation occurs at temperatures in the range from 520 to 580 C [1]. The phase transformation in the pure solid phases takes place at 573 C [8].
* Corresponding author. Tel.: þ49 3641 9 48501; fax: þ 49 3641 9 48502. E-mail address: [email protected] (C. Rüssel). 1293-2558/$ e see front matter Ó 2010 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2010.06.025
This phase transition runs parallel with a volume contraction of 0.8%, which in the studied system occurs below the glass transition temperature. Hence, the stresses formed during the phase transition cannot relax. These stresses are the reason for the high mechanical strength of up to 450 MPa [1] observed in these type of glasseceramics. High-quartz solid solutions, which are still observed at room temperature in this system, and hence have not been transformed to the low-quartz phase typically consist of 90% SiO2, 5% MgO and 5% Al2O3 [1], as already proved by energy dispersive X-ray analysis (EDX) using transmission electron microscopy (TEM) [1]. These highly doped solid solutions are formed at lower temperatures than those with smaller dopant concentrations. Within the past few years, novel fields of application of highstrength glasseceramics, in particular in micromechanics and as hard disc substrate have gained interest [9e12]. The mechanical properties required are primarily high Young’s modulus (>100 GPa) and high hardness (>10 GPa) and besides a good temperature stability to facilitate the subsequent coating processes. In this paper, a study on the system MgO/ZnO/Al2O3/TiO2/ZrO2/SiO2 is presented. The MgO concentration is partly substituted against ZnO, which should allow to decrease the melting temperature. In contrast to other components such as P2O5 or B2O3, hardness, strength and elastic modulus should not decrease in a notable extend.
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2. Experimental Glasses in the system MgO/ZnO/Al2O3/TiO2/ZrO2/SiO2 were melted from reagent grade raw materials 4 MgCO3 . Mg(OH)2 . 5 H2O, ZnO, Al(OH)3, TiO2, ZrO2 and SiO2 (quartz). The batches were heat treated at 800 C for 3 h. Then they were melted in Pt/Rh crucibles at 1550 C in an inductive furnace and kept at this temperature for 4 h. Subsequently, the glasses were cast on a steel mould. Table 1 summarized the investigated glass compositions. Powdered glasses were studied by differential thermal analysis (DTA, Shimadzu DTA 50) and dilatometry (Netzsch 402 ES) using a heating rate of 10 K/s. Glasses and annealed samples were further investigated by X-ray diffraction (Siemens D 5000). The hardness was measured with a microindenter (Duramin-1, Struers) and Vickers pyramid (load: 0.981 N). Young’s moduli were determined by ultrasound measurements (Krautkrämer Branson USD 15) at a frequency of 5 MHz using a sample size of 20 15 2 mm3. The densities of the glasses were around 3.1 g cm3, and those of the crystallized samples around 3.2 g cm3. The sound velocities were in the range from 3600 to 4500 m s1. The mechanical strength was determined by four point bending with a mechanical testing machine Zwick 1445 and a sample geometry of 3*4*45 mm3. The microstructure was studied using transmission electron microscopy (TEM, Hitachi H 8100 II). The samples were annealed at temperatures of 850, 900, 950, 1000, 1050 and 1100 C kept for 3 h. The heating and cooling rates were 5 K$min1. Additionally, samples A and D were annealed in a two-step process first at 750 C for 15 min (denoted as nucleation) and then at 1030 or 1080 C for 3 h (denoted as crystallization). 3. Results Fig. 1 shows DTA-profiles of the glasses AeD. In glass A, the glass transition temperature, Tg, was observed at 784 C and a sharp exothermic peak at around 980 C. The glass transition temperatures and the temperatures attributed to the exothermic peak of all studied samples are summarized in Table 2. The values for Tg all lie in the range from 752 to 784 C and decrease with increasing ZnO concentration. The temperatures of the exothermic peak were in the range from 950 to 974 C and decreased slightly with increasing ZnO concentration. In Table 2 also the thermal expansion coefficients (100e500 C) of the glasses are shown. They were in the range from 3.4 to 4.6$106 K1 and showed a tendency to smaller values with increasing ZnO concentration. Fig. 2 shows XRD-patterns of samples AeD all annealed at a temperature of 1000 C for 3 h. In all samples high-quartz solid solution (JCPDS no-11-0252) was detected. The main peak is observed at around 2q ¼ 26 degrees. Additionally, a peak at around 31 degrees which most probably is attributed to ZrTiO4 (JCPDS no. 34-0415) was seen in all diffraction patterns. Furthermore, small peaks caused by spinel (MgAl2O4; JCPDS no. 21-1152), sapphirine (Mg3.5Al9Si1.5O20 JCPDS no. 21-0549) or gahnite ZnAl2O3 are observed. The intensities of the two peaks at 2q ¼ 31.2 degrees (220) and 2q ¼ 36.7 degrees (311) increase with increasing ZnO concentration, while the peak at 2q ¼ 19 degrees decreases in intensity.
Table 1 Chemical compositions (in mol%). Sample
SiO2
Al2O3
MgO
ZnO
TiO2
ZrO2
A B C D
50.9 50.9 50.9 50.9
20.8 20.8 20.8 20.8
20.8 18.5 16.2 11.6
e 2.3 4.6 9.3
3.7 3.7 3.7 3.7
3.7 3.7 3.7 3.7
Fig. 1. DTA-profiles of samples AeD (heating rate 10 K/min).
In Fig. 3, XRD-patterns of sample D for different annealing temperatures in the range from 850 to 1100 C are shown. While annealing at 850 C led to the crystallization of magnesium aluminosilicate (MgAl2SiO6, JCPDS no. 14-0346), higher temperatures led to the formation of quartz, spinel/gahnite and zirconium titanate. The main peak (100%) attributed to quartz is located at a 2q-value of 26 degrees after annealing at 850 C and is shifted to larger values (up to 26.7 degrees) with increasing annealing temperatures. The intensity of the peaks attributed to spinel/ gahnite as well as that due to zirconium titanate continuously increase with increasing annealing temperature. Fig. 4 presents XRD-patterns of samples B and D, annealed at a temperature of 1050 C for 30 min and 24 h. The formed phases are again spinel/gahnite, zirconium titanate and quartz. The intensities of the peak increase if the samples were annealed for 24 h instead of 30 min. In the XRD-pattern of sample B, the intensity of the peak at 2q ¼ 19 degrees attributed to spinel increases notably. In sample D, the peaks at 2q ¼ 31.2 and 2q ¼ 36.7 degrees increase in intensity. In both samples, the increase in the annealing time from 30 min to 24 h leads to a shift in the 100% peak of quartz from 26 degrees to larger values. In Fig. 5, XRD-patterns of sample D are shown which were annealed in one- or two-step processes. One sample was annealed at 1000 C for 3 h. The other samples were first annealed at temperatures of 800, 820 and 850 C for 2 h and subsequently in a second step at 1000 C for 3 h. These experiments were performed to study the effect of a first annealing step which might induce nucleation. The effect of the first temperature step is not large, however, it led to the occurrence of an additional XRD-peak at 2q ¼ 24.1 degrees. The intensity of this peak is largest at the highest nucleation temperature. Fig. 6 presents the microhardness of samples annealed at various temperatures in the range from 800 to 1100 C. The
Table 2 Glass transition temperature, temperature of the first exothermic peak and linear thermal expansion coefficient. Sample
Tg (onset) in C
Temperature of the exothermic peak in C
Linear thermal expansion coefficient (100e500 C) in 106 K1
A B C D Error
784 780 774 752 5
974 974 960 950 5
4.6 3.9 4.3 3.4 0.1
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Fig. 2. XRD-patterns of samples AeD after annealing at 1000 C for 3 h. -: highquartz solid solution, *: gahnite/spinel, B: ZrTiO4.
Fig. 4. XRD-patterns of samples B and D after annealing at 1050 C for 0.5 and 24 h. (Symbols see Fig. 2).
microhardness of samples annealed at 800 and 850 C were in the range from 7.5 to 8.1 GPa and hence slightly larger than in the corresponding glasses. Further increasing the annealing temperature resulted in a steady increase of the microhardness until after annealing at 1100 C, values in the range from 11.5 to 12.4 GPa were reached. Up to an annealing temperature of 1000 C the microhardness of sample D were largest and decreased with decreasing ZnO concentration. While for samples C and D, the microhardness steadily increased with the temperature, those of samples A and B had a maximum at 1050 C and then decreased again. The effect of the annealing time on the microhardness is shown in Fig. 7 for an annealing temperature of 1050 C. After annealing for 0.5 h, the microhardness were in the range from 10.2 to 10.8 GPa and increased with the annealing time. Annealing for 12 h resulted in microhardness in the range from 12 to 12.6 GPa. Annealing for 24 h within the limits of error resulted in the same values. At the most annealing times, samples with larger ZnO concentrations exhibited larger microhardnesses. In Fig. 8, the Young’s moduli of samples A and D are shown as a function of the crystallization temperature. Young’s moduli of samples A were larger than those of samples D if annealed at the same temperature. The Young’s moduli of the glass samples A and D were 108 and 118 GPa, respectively. Annealing at temperatures
up to 900 C, within the limits of error, resulted in the same Young’s moduli. Further increasing annealing temperatures in the case of sample A led to an increase in the Young’s modulus until after annealing at 1100 C, a value of 152 GPa was reached. In the case of sample D, the Young’s modulus increased up to an annealing temperature of 1050 C where a value of 134 GPa was reached. After annealing at 1100 C, the Young’s modulus redecreased to 115 GPa. Table 3 summarizes microhardness, Young’s moduli and mechanical strengths for the samples A and D before as well as after annealing in a two-step process at 750 and 1030 or 1080 C. The strengths of the glass samples A and D are 96 9 and 117 9 MPa, respectively. Annealing sample A at 1030 C led to a strength of 391 30 MPa, while after annealing at 1080 C a strength of 341 73 MPa was observed. For sample D annealing at 1030 and 1080 C resulted in an increase in strength to 342 23 and 290 20 MPa, respectively. The microhardness and Young’s moduli were approximately the same as after the one step annealing process. Fig. 9 shows a TEM-micrograph recorded from a sample D annealed at 1050 C for 24 h. A fine grained microstructure is observed; the crystals possess sizes in the range from 40 to 80 nm. All annealed samples exhibited bulk crystallization. Quartz, spinel and other phases cannot be distinguished.
Fig. 3. XRD-patterns of sample D after annealing at various temperatures for 3 h. (Symbols see Fig. 2).
Fig. 5. XRD-patterns of sample nucleated at different temperatures for 2 h and subsequently crystallized at 1000 C for 3 h (no nucleation, 800, 820 and 850 C).
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Fig. 8. Young’s modulus as a function of the annealing temperature of samples A (circles) and D (squares).
Fig. 6. Microhardness as a function of the annealing temperature for samples BeD.
4. Discussion The glass transition temperatures of the prepared glasses decreased with increasing equimolar replacement of MgO against ZnO. The cationic field strength [13] is for Mg2þ and Zn2þ 0.51 and 0.59, respectively. Thus, the ZneO bonds are more ionic than the MgeO bonds and a partial replacement of MgO against ZnO should decrease the viscosity and hence facilitate the melting process. As shown in Table 1, the glass transition temperature indeed decreases from 784 to 752 C while increasing the ZnO concentration from 0 to 9.3%. The crystallization temperatures attributed to the exothermic peak in Fig. 1 decreased by 24 K for the glass with 9.3 mol % ZnO. The linear thermal expansion coefficient of 3.4$106 K1 is in agreement with that of borosilicate glasses of the Pyrex type. As already noted in the literature [14e19], both TiO2 and ZrO2 act as nucleating agents, and lead, if both are present, to the precipitation of ZrTiO4 [18e20]. These nanocrystals trigger nucleation of the quartz crystals and lead to volume crystallization which is in contrast to the MgO/Al2O3/SiO2 system without the addition of nucleating agents, such as TiO2 or ZrO2 [21]. Recently [22], it was shown in a Li2O/Al2O3/TiO2/ZrO2/SiO2 glass using a transmission electron microscope combined with high resolution electron energy loss spectroscopy that an alumina enriched layer is formed
around the ZrTiO4 crystals. Hence, the common assumption that the quartz phase grows epitaxially on the seed crystals (here the ZrTiO4 crystal) is highly improbable. Samples annealed at a temperature of 900 C (see Fig. 2) show a main peak at 26 degrees which according to Refs. [1,14,23] is attributed to the high temperature modification of quartz. This phase contains fairly large quantities of MgO (or ZnO) and Al2O3 in equimolar concentrations (>5 mol %) [1,14,23]. With increasing annealing temperature, the peak is shifted to larger values (up to 26.7 degrees) which is then attributed to the low temperature phase of quartz. This phase contains smaller MgO (or ZnO) and Al2O3 concentrations (<1 mol% each [1]). During annealing, first the high temperature phase is formed which subsequently transforms to the low temperature phase during cooling. The occurrence of the peaks at 31.2 and 36.7 degrees is due to spinel or gahnite or respective solid solutions. The intensity of these peaks increases steadily with the annealing temperature. As shown in Fig. 4 the XRD-patterns of samples annealed for 0.5 h at 1000 C show main peaks at around 26.1 degrees. If increasing the annealing time to 24 h, this peak is shifted to larger values and simultaneously the peaks at 31.2 and 36.7 degrees increase in intensity. This means that first the high temperature modification is formed which contains >5 mol% MgO and Al2O3. Due to the occurrence of MgO and Al2O3, the high temperature modification is stabilized and does not transform to the low temperature modification. With increasing annealing temperature, spinel, gahnite or sapphirine are crystallized which leads to a depletion of ZnO and MgO in the residual glassy phase. This is the driving force that the main part of MgO, ZnO and Al2O3 goes out of the quartz phase. The remaining quartz phase is then depleted in MgO, ZnO and Al2O3 and during cooling transfers to the low temperature modification. At larger temperatures within the time scale of the experiment performed, the intermediate existence of the high-quartz phase which during cooling does not transform to
Table 3 Microhardness, Young’s modulus and mechanical strength of samples crystallized in a two-step annealing procedure.
Fig. 7. Microhardness as a function of the annealing time at 1050 C.
Sample
Crystallization Microhardness Young’s Strength Nucleation in GPa modulus in MPa temperature temperature in C in GPa in C
Sample A
e 750 750
e 1030 1080
7.8 0.1 12.3 0.3 12.5 0.3
118 4 138 4 147 4
117 9 391 30 341 73
Sample D e 750 750
e 1030 1080
7.7 0.1 11.6 0.1 12.7 0.2
108 3 128 4 133 4
96 9 342 23 290 20
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By contrast to the microhardness in the Young’s moduli of sample D, a maximum at an annealing temperature of 1050 C is observed, while for sample A, the Young’s modulus increases continuously in the studied temperature range (see Fig. 8). In the entire range of annealing temperatures, the Young’s moduli are larger for sample A than for sample D. 5. Conclusions
Fig. 9. TEM-micrograph of sample D.
the low temperature modification was not observed. The nucleation temperature does not strongly affect the phase formation as shown in Fig. 5. The phase transformation of high-quartz to low-quartz in the pure quartz phase occurs at 573 C. It runs parallel to a volume contraction of 0.7%. This increase in density of the crystalline phase leads to notable tensile stresses in the quartz phase as well as in the glassy phase (in radial direction) whereas in the glassy phase in tangential direction around the inclusion notable compressive stresses occur. As shown in previous papers (see e.g., [1]), the occurrence of the low-quartz phase and the stresses attributed to the phase transition are essential for the high mechanical strength observed in this type of glasseceramics. As shown in Fig. 6, the microhardness increases with increasing annealing temperature up to 1050 C. With increasing ZnO concentration, the microhardness increases for annealing temperatures up to 1000 C. At an annealing temperature of 1050 C, however, sample A (i.e.. that without ZnO) possesses the largest hardness (12.1 GPa). A further increase in the annealing temperature to 1100 C results for sample A in a re-decrease of the hardness to 10.5 GPa. In the case of sample B, also a decrease in hardness is observed if the annealing temperature is increased from 1050 to 1100 C. By contrast, for sample C and D, i.e., those with the largest ZnO concentrations a further increase in the microhardness is observed in this temperature range. According to Ref. [14], in sample A after annealing at 1100 C notable quantities of cristobalite are already formed. As shown in Fig. 3 this is not the case for sample D. In analogy to quartz, cristobalite also occurs in a high and a low temperature modification. The phase transition occurs at 270 C for pure cristobalite and runs parallel to a phase contraction of 2.8%. This high volume contraction is reported to be responsible for a decrease in the mechanical strength [1] as well as the microhardness.
In the studied MgO/ZnO/Al2O3/ZrO2/TiO2/SiO2 system, partial substitution of MgO against ZnO led to a decrease in the glass transition temperature and to an increase in microhardness. Although the Young’s moduli were smaller than in the solely MgO containing system, the maximum value of 134 GPa is sufficient for the most applications. A partial replacement of MgO against ZnO should decrease the viscosity and hence facilitate the melting process. The microhardness of the ZnO containing glasseceramics is even higher. With increasing ZnO concentration, the formation of cristobalite is suppressed and hence the disadvantageous effect of the transformation high-cristobalite to low-cristobalite modification is not observed. To achieve good mechanical properties, the transformation of high-quartz to low-quartz during cooling is essential. This is enabled if the quartz phase contains only minor concentrations of Al2O3 and MgO (or ZnO). It occurs as soon as sufficient quantities of spinel or gahnite are formed. References [1] P. Wange, T. Höche, C. Rüssel, J.D. Schnapp, J. Non-Cryst. Solids 298 (2002) 137. [2] G.H. Beall, Hard, high modulus glasseceramics, US Patent 3873.329, (1975). [3] P. Wange, G. Carl, K. Naumann, J. Vogel, W. Vogel, W. Götz, W. Höland, Silicate Indust. 1 & 2 (1991) 21. [4] A. Katzschmann, P. Wange, Glastech. Ber. Glass Sci. Technol. 68 (1995) 111. [5] V. Maier, G. Müller, cfi Ber. DKG 65 (1988) 208. [6] L.R. Pickney, G.H. Beall, J. Non-Cryst. Solids 219 (1997) 219. [7] W. Höland, P. Wange, G. Carl, W. Vogel, E. Heidenreich, Silikattechnik 35 (1984) 181. [8] J.W. Hinz, Silikate. VEB Verlag für Bauwesen, Berlin, Germany, 1970. [9] X. Zhou, K. Uchida, US 6, 627, 565 B1 (30/09/2003). [10] X. Zhou, K. Azogami, US 6, 774, 072 B2 (10/08/2004). [11] N.Goto US 00000 641 3890B1 “Glass-ceramic substrate for a magnetic information storage medium” (02.07.2002). [12] W. Pannhorst, U. Woelfel, S. Wolf, US 6, 376, 402 B1 (23/04/2002). [13] H. Scholze, Glas, Natur, Struktur und Eigenschaften, Berlin, Heidelberg, New York, 1988. [14] A. Hunger, G. Carl, A. Gebhardt, C. Rüssel, J. Non-Cryst. Solids 354 (2008) 5402. [15] W. Zdaniewski, J. Am. Ceram. Soc. 58 (1975) 16. [16] W. Zdaniewski, J. Mater. Sci. 8 (1973) 192. [17] F.D. Doenitz, K. Koch, W. Vogel, Silikattechnik 33 (1982) 18. [18] A. Ramos, M. Gondais, J. Cryst. Growth. 100 (1990) 471. [19] B. Champagnon, C. Mai, E. Rodeck, J. Non-Cryst. Solids 94 (1987) 210. [20] C. Mai, G. Andrieu, J. Non-Cryst. Solids 108 (1989) 201. [21] P. Amista, M. Cesari, A. Montenero, G. Gnappi, L. Lan, J. Non-Cryst. Solids 192/ 193 (1995) 529. [22] S. Bhattachaya, T. Höche, I. Linschek, I. Avramov, R. Wurth, M. Müller, C. Rüssel, Cryst. Growth Des. 10 (2010) 379. [23] W. Schreyer, J.F. Schairer, J. Petrol. 2 (1961) 419.
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Formation of nano-crystalline quartz crystals from ZnO/MgO/Al2O3/TiO2/ZrO2/ SiO2 glasses Anja Hunger a, Gunter Carl b, Christian Rüssel a, * a b
Otto-Schott-Institut, Jena University, Fraunhoferstr. 6, 07743 Jena, Germany JSJ Jodeit GmbH, Am Naßtal 10, 07751 Jena-Maua, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 April 2010 Received in revised form 24 June 2010 Accepted 30 June 2010 Available online 8 July 2010
Glasses with the compositions 50.9 SiO2 $ 20.8 Al2O3 $ (20.8 x) MgO$ ZnO $ 3.7 TiO2 $ 3.7 ZrO2 with x ¼ 0, 2.3, 4.6 and 9.3 were annealed at temperatures in the range from 850 to 1100 C. Depending on temperature, high- or low-quartz solid solutions, magnesium aluminosilicate, mullit and spinel precipitated. These glasseceramics exhibit excellent mechanical properties and are potential candidates for applications in micromechanics or as hard disc substrate. The larger the ZnO concentration, the lower is the glass transition temperature. Also microhardnesses and Young’s moduli increased with increasing ZnO concentration. The nucleation temperature was of minor importance. To achieve good mechanical properties, the initially formed high-quartz phase must transform to the corresponding low-quartz phase. This occurs if the quartz phase contains only minor MgO or ZnO concentrations, which can be achieved by increasing the annealing times or temperature. Then MgO, ZnO and Al2O3 occur as separate spinel or gahnite phase. Ó 2010 Elsevier Masson SAS. All rights reserved.
Keywords: Glass Crystallization Quartz
1. Introduction Glasseceramics in the MgO/Al2O3/SiO2 system [1e4] possess excellent mechanical properties. For this system Young’s moduli of up to 140 GPa [1], mechanical strengths of up to 450 MPa, hardnesses of up to 13 GPa have been reported in the literature. The base glass exhibits surface crystallization, however, the addition of components such as TiO2 or ZrO2 leads to volume crystallization and enables the production of fine-grained glasseceramics by controlled crystallization. The glass system requires high melting temperatures and, therefore, has been varied by adding components such as P2O5 [4] or B2O3 [2] which might be suitable to decrease the required melting temperature. After annealing the glasses, depending on composition and annealing temperature various crystalline phases such as spinel, sapphirine low-quartz or high-quartz solid solutions are observed [1,3,5e7]. The low-quartz phase observed at room temperature is formed during cooling the sample from annealing temperatures by the transformation of the initially formed high-quartz phase. This transformation occurs at temperatures in the range from 520 to 580 C [1]. The phase transformation in the pure solid phases takes place at 573 C [8].
* Corresponding author. Tel.: þ49 3641 9 48501; fax: þ 49 3641 9 48502. E-mail address: [email protected] (C. Rüssel). 1293-2558/$ e see front matter Ó 2010 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2010.06.025
This phase transition runs parallel with a volume contraction of 0.8%, which in the studied system occurs below the glass transition temperature. Hence, the stresses formed during the phase transition cannot relax. These stresses are the reason for the high mechanical strength of up to 450 MPa [1] observed in these type of glasseceramics. High-quartz solid solutions, which are still observed at room temperature in this system, and hence have not been transformed to the low-quartz phase typically consist of 90% SiO2, 5% MgO and 5% Al2O3 [1], as already proved by energy dispersive X-ray analysis (EDX) using transmission electron microscopy (TEM) [1]. These highly doped solid solutions are formed at lower temperatures than those with smaller dopant concentrations. Within the past few years, novel fields of application of highstrength glasseceramics, in particular in micromechanics and as hard disc substrate have gained interest [9e12]. The mechanical properties required are primarily high Young’s modulus (>100 GPa) and high hardness (>10 GPa) and besides a good temperature stability to facilitate the subsequent coating processes. In this paper, a study on the system MgO/ZnO/Al2O3/TiO2/ZrO2/SiO2 is presented. The MgO concentration is partly substituted against ZnO, which should allow to decrease the melting temperature. In contrast to other components such as P2O5 or B2O3, hardness, strength and elastic modulus should not decrease in a notable extend.
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2. Experimental Glasses in the system MgO/ZnO/Al2O3/TiO2/ZrO2/SiO2 were melted from reagent grade raw materials 4 MgCO3 . Mg(OH)2 . 5 H2O, ZnO, Al(OH)3, TiO2, ZrO2 and SiO2 (quartz). The batches were heat treated at 800 C for 3 h. Then they were melted in Pt/Rh crucibles at 1550 C in an inductive furnace and kept at this temperature for 4 h. Subsequently, the glasses were cast on a steel mould. Table 1 summarized the investigated glass compositions. Powdered glasses were studied by differential thermal analysis (DTA, Shimadzu DTA 50) and dilatometry (Netzsch 402 ES) using a heating rate of 10 K/s. Glasses and annealed samples were further investigated by X-ray diffraction (Siemens D 5000). The hardness was measured with a microindenter (Duramin-1, Struers) and Vickers pyramid (load: 0.981 N). Young’s moduli were determined by ultrasound measurements (Krautkrämer Branson USD 15) at a frequency of 5 MHz using a sample size of 20 15 2 mm3. The densities of the glasses were around 3.1 g cm3, and those of the crystallized samples around 3.2 g cm3. The sound velocities were in the range from 3600 to 4500 m s1. The mechanical strength was determined by four point bending with a mechanical testing machine Zwick 1445 and a sample geometry of 3*4*45 mm3. The microstructure was studied using transmission electron microscopy (TEM, Hitachi H 8100 II). The samples were annealed at temperatures of 850, 900, 950, 1000, 1050 and 1100 C kept for 3 h. The heating and cooling rates were 5 K$min1. Additionally, samples A and D were annealed in a two-step process first at 750 C for 15 min (denoted as nucleation) and then at 1030 or 1080 C for 3 h (denoted as crystallization). 3. Results Fig. 1 shows DTA-profiles of the glasses AeD. In glass A, the glass transition temperature, Tg, was observed at 784 C and a sharp exothermic peak at around 980 C. The glass transition temperatures and the temperatures attributed to the exothermic peak of all studied samples are summarized in Table 2. The values for Tg all lie in the range from 752 to 784 C and decrease with increasing ZnO concentration. The temperatures of the exothermic peak were in the range from 950 to 974 C and decreased slightly with increasing ZnO concentration. In Table 2 also the thermal expansion coefficients (100e500 C) of the glasses are shown. They were in the range from 3.4 to 4.6$106 K1 and showed a tendency to smaller values with increasing ZnO concentration. Fig. 2 shows XRD-patterns of samples AeD all annealed at a temperature of 1000 C for 3 h. In all samples high-quartz solid solution (JCPDS no-11-0252) was detected. The main peak is observed at around 2q ¼ 26 degrees. Additionally, a peak at around 31 degrees which most probably is attributed to ZrTiO4 (JCPDS no. 34-0415) was seen in all diffraction patterns. Furthermore, small peaks caused by spinel (MgAl2O4; JCPDS no. 21-1152), sapphirine (Mg3.5Al9Si1.5O20 JCPDS no. 21-0549) or gahnite ZnAl2O3 are observed. The intensities of the two peaks at 2q ¼ 31.2 degrees (220) and 2q ¼ 36.7 degrees (311) increase with increasing ZnO concentration, while the peak at 2q ¼ 19 degrees decreases in intensity.
Table 1 Chemical compositions (in mol%). Sample
SiO2
Al2O3
MgO
ZnO
TiO2
ZrO2
A B C D
50.9 50.9 50.9 50.9
20.8 20.8 20.8 20.8
20.8 18.5 16.2 11.6
e 2.3 4.6 9.3
3.7 3.7 3.7 3.7
3.7 3.7 3.7 3.7
Fig. 1. DTA-profiles of samples AeD (heating rate 10 K/min).
In Fig. 3, XRD-patterns of sample D for different annealing temperatures in the range from 850 to 1100 C are shown. While annealing at 850 C led to the crystallization of magnesium aluminosilicate (MgAl2SiO6, JCPDS no. 14-0346), higher temperatures led to the formation of quartz, spinel/gahnite and zirconium titanate. The main peak (100%) attributed to quartz is located at a 2q-value of 26 degrees after annealing at 850 C and is shifted to larger values (up to 26.7 degrees) with increasing annealing temperatures. The intensity of the peaks attributed to spinel/ gahnite as well as that due to zirconium titanate continuously increase with increasing annealing temperature. Fig. 4 presents XRD-patterns of samples B and D, annealed at a temperature of 1050 C for 30 min and 24 h. The formed phases are again spinel/gahnite, zirconium titanate and quartz. The intensities of the peak increase if the samples were annealed for 24 h instead of 30 min. In the XRD-pattern of sample B, the intensity of the peak at 2q ¼ 19 degrees attributed to spinel increases notably. In sample D, the peaks at 2q ¼ 31.2 and 2q ¼ 36.7 degrees increase in intensity. In both samples, the increase in the annealing time from 30 min to 24 h leads to a shift in the 100% peak of quartz from 26 degrees to larger values. In Fig. 5, XRD-patterns of sample D are shown which were annealed in one- or two-step processes. One sample was annealed at 1000 C for 3 h. The other samples were first annealed at temperatures of 800, 820 and 850 C for 2 h and subsequently in a second step at 1000 C for 3 h. These experiments were performed to study the effect of a first annealing step which might induce nucleation. The effect of the first temperature step is not large, however, it led to the occurrence of an additional XRD-peak at 2q ¼ 24.1 degrees. The intensity of this peak is largest at the highest nucleation temperature. Fig. 6 presents the microhardness of samples annealed at various temperatures in the range from 800 to 1100 C. The
Table 2 Glass transition temperature, temperature of the first exothermic peak and linear thermal expansion coefficient. Sample
Tg (onset) in C
Temperature of the exothermic peak in C
Linear thermal expansion coefficient (100e500 C) in 106 K1
A B C D Error
784 780 774 752 5
974 974 960 950 5
4.6 3.9 4.3 3.4 0.1
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Fig. 2. XRD-patterns of samples AeD after annealing at 1000 C for 3 h. -: highquartz solid solution, *: gahnite/spinel, B: ZrTiO4.
Fig. 4. XRD-patterns of samples B and D after annealing at 1050 C for 0.5 and 24 h. (Symbols see Fig. 2).
microhardness of samples annealed at 800 and 850 C were in the range from 7.5 to 8.1 GPa and hence slightly larger than in the corresponding glasses. Further increasing the annealing temperature resulted in a steady increase of the microhardness until after annealing at 1100 C, values in the range from 11.5 to 12.4 GPa were reached. Up to an annealing temperature of 1000 C the microhardness of sample D were largest and decreased with decreasing ZnO concentration. While for samples C and D, the microhardness steadily increased with the temperature, those of samples A and B had a maximum at 1050 C and then decreased again. The effect of the annealing time on the microhardness is shown in Fig. 7 for an annealing temperature of 1050 C. After annealing for 0.5 h, the microhardness were in the range from 10.2 to 10.8 GPa and increased with the annealing time. Annealing for 12 h resulted in microhardness in the range from 12 to 12.6 GPa. Annealing for 24 h within the limits of error resulted in the same values. At the most annealing times, samples with larger ZnO concentrations exhibited larger microhardnesses. In Fig. 8, the Young’s moduli of samples A and D are shown as a function of the crystallization temperature. Young’s moduli of samples A were larger than those of samples D if annealed at the same temperature. The Young’s moduli of the glass samples A and D were 108 and 118 GPa, respectively. Annealing at temperatures
up to 900 C, within the limits of error, resulted in the same Young’s moduli. Further increasing annealing temperatures in the case of sample A led to an increase in the Young’s modulus until after annealing at 1100 C, a value of 152 GPa was reached. In the case of sample D, the Young’s modulus increased up to an annealing temperature of 1050 C where a value of 134 GPa was reached. After annealing at 1100 C, the Young’s modulus redecreased to 115 GPa. Table 3 summarizes microhardness, Young’s moduli and mechanical strengths for the samples A and D before as well as after annealing in a two-step process at 750 and 1030 or 1080 C. The strengths of the glass samples A and D are 96 9 and 117 9 MPa, respectively. Annealing sample A at 1030 C led to a strength of 391 30 MPa, while after annealing at 1080 C a strength of 341 73 MPa was observed. For sample D annealing at 1030 and 1080 C resulted in an increase in strength to 342 23 and 290 20 MPa, respectively. The microhardness and Young’s moduli were approximately the same as after the one step annealing process. Fig. 9 shows a TEM-micrograph recorded from a sample D annealed at 1050 C for 24 h. A fine grained microstructure is observed; the crystals possess sizes in the range from 40 to 80 nm. All annealed samples exhibited bulk crystallization. Quartz, spinel and other phases cannot be distinguished.
Fig. 3. XRD-patterns of sample D after annealing at various temperatures for 3 h. (Symbols see Fig. 2).
Fig. 5. XRD-patterns of sample nucleated at different temperatures for 2 h and subsequently crystallized at 1000 C for 3 h (no nucleation, 800, 820 and 850 C).
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Fig. 8. Young’s modulus as a function of the annealing temperature of samples A (circles) and D (squares).
Fig. 6. Microhardness as a function of the annealing temperature for samples BeD.
4. Discussion The glass transition temperatures of the prepared glasses decreased with increasing equimolar replacement of MgO against ZnO. The cationic field strength [13] is for Mg2þ and Zn2þ 0.51 and 0.59, respectively. Thus, the ZneO bonds are more ionic than the MgeO bonds and a partial replacement of MgO against ZnO should decrease the viscosity and hence facilitate the melting process. As shown in Table 1, the glass transition temperature indeed decreases from 784 to 752 C while increasing the ZnO concentration from 0 to 9.3%. The crystallization temperatures attributed to the exothermic peak in Fig. 1 decreased by 24 K for the glass with 9.3 mol % ZnO. The linear thermal expansion coefficient of 3.4$106 K1 is in agreement with that of borosilicate glasses of the Pyrex type. As already noted in the literature [14e19], both TiO2 and ZrO2 act as nucleating agents, and lead, if both are present, to the precipitation of ZrTiO4 [18e20]. These nanocrystals trigger nucleation of the quartz crystals and lead to volume crystallization which is in contrast to the MgO/Al2O3/SiO2 system without the addition of nucleating agents, such as TiO2 or ZrO2 [21]. Recently [22], it was shown in a Li2O/Al2O3/TiO2/ZrO2/SiO2 glass using a transmission electron microscope combined with high resolution electron energy loss spectroscopy that an alumina enriched layer is formed
around the ZrTiO4 crystals. Hence, the common assumption that the quartz phase grows epitaxially on the seed crystals (here the ZrTiO4 crystal) is highly improbable. Samples annealed at a temperature of 900 C (see Fig. 2) show a main peak at 26 degrees which according to Refs. [1,14,23] is attributed to the high temperature modification of quartz. This phase contains fairly large quantities of MgO (or ZnO) and Al2O3 in equimolar concentrations (>5 mol %) [1,14,23]. With increasing annealing temperature, the peak is shifted to larger values (up to 26.7 degrees) which is then attributed to the low temperature phase of quartz. This phase contains smaller MgO (or ZnO) and Al2O3 concentrations (<1 mol% each [1]). During annealing, first the high temperature phase is formed which subsequently transforms to the low temperature phase during cooling. The occurrence of the peaks at 31.2 and 36.7 degrees is due to spinel or gahnite or respective solid solutions. The intensity of these peaks increases steadily with the annealing temperature. As shown in Fig. 4 the XRD-patterns of samples annealed for 0.5 h at 1000 C show main peaks at around 26.1 degrees. If increasing the annealing time to 24 h, this peak is shifted to larger values and simultaneously the peaks at 31.2 and 36.7 degrees increase in intensity. This means that first the high temperature modification is formed which contains >5 mol% MgO and Al2O3. Due to the occurrence of MgO and Al2O3, the high temperature modification is stabilized and does not transform to the low temperature modification. With increasing annealing temperature, spinel, gahnite or sapphirine are crystallized which leads to a depletion of ZnO and MgO in the residual glassy phase. This is the driving force that the main part of MgO, ZnO and Al2O3 goes out of the quartz phase. The remaining quartz phase is then depleted in MgO, ZnO and Al2O3 and during cooling transfers to the low temperature modification. At larger temperatures within the time scale of the experiment performed, the intermediate existence of the high-quartz phase which during cooling does not transform to
Table 3 Microhardness, Young’s modulus and mechanical strength of samples crystallized in a two-step annealing procedure.
Fig. 7. Microhardness as a function of the annealing time at 1050 C.
Sample
Crystallization Microhardness Young’s Strength Nucleation in GPa modulus in MPa temperature temperature in C in GPa in C
Sample A
e 750 750
e 1030 1080
7.8 0.1 12.3 0.3 12.5 0.3
118 4 138 4 147 4
117 9 391 30 341 73
Sample D e 750 750
e 1030 1080
7.7 0.1 11.6 0.1 12.7 0.2
108 3 128 4 133 4
96 9 342 23 290 20
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By contrast to the microhardness in the Young’s moduli of sample D, a maximum at an annealing temperature of 1050 C is observed, while for sample A, the Young’s modulus increases continuously in the studied temperature range (see Fig. 8). In the entire range of annealing temperatures, the Young’s moduli are larger for sample A than for sample D. 5. Conclusions
Fig. 9. TEM-micrograph of sample D.
the low temperature modification was not observed. The nucleation temperature does not strongly affect the phase formation as shown in Fig. 5. The phase transformation of high-quartz to low-quartz in the pure quartz phase occurs at 573 C. It runs parallel to a volume contraction of 0.7%. This increase in density of the crystalline phase leads to notable tensile stresses in the quartz phase as well as in the glassy phase (in radial direction) whereas in the glassy phase in tangential direction around the inclusion notable compressive stresses occur. As shown in previous papers (see e.g., [1]), the occurrence of the low-quartz phase and the stresses attributed to the phase transition are essential for the high mechanical strength observed in this type of glasseceramics. As shown in Fig. 6, the microhardness increases with increasing annealing temperature up to 1050 C. With increasing ZnO concentration, the microhardness increases for annealing temperatures up to 1000 C. At an annealing temperature of 1050 C, however, sample A (i.e.. that without ZnO) possesses the largest hardness (12.1 GPa). A further increase in the annealing temperature to 1100 C results for sample A in a re-decrease of the hardness to 10.5 GPa. In the case of sample B, also a decrease in hardness is observed if the annealing temperature is increased from 1050 to 1100 C. By contrast, for sample C and D, i.e., those with the largest ZnO concentrations a further increase in the microhardness is observed in this temperature range. According to Ref. [14], in sample A after annealing at 1100 C notable quantities of cristobalite are already formed. As shown in Fig. 3 this is not the case for sample D. In analogy to quartz, cristobalite also occurs in a high and a low temperature modification. The phase transition occurs at 270 C for pure cristobalite and runs parallel to a phase contraction of 2.8%. This high volume contraction is reported to be responsible for a decrease in the mechanical strength [1] as well as the microhardness.
In the studied MgO/ZnO/Al2O3/ZrO2/TiO2/SiO2 system, partial substitution of MgO against ZnO led to a decrease in the glass transition temperature and to an increase in microhardness. Although the Young’s moduli were smaller than in the solely MgO containing system, the maximum value of 134 GPa is sufficient for the most applications. A partial replacement of MgO against ZnO should decrease the viscosity and hence facilitate the melting process. The microhardness of the ZnO containing glasseceramics is even higher. With increasing ZnO concentration, the formation of cristobalite is suppressed and hence the disadvantageous effect of the transformation high-cristobalite to low-cristobalite modification is not observed. To achieve good mechanical properties, the transformation of high-quartz to low-quartz during cooling is essential. This is enabled if the quartz phase contains only minor concentrations of Al2O3 and MgO (or ZnO). It occurs as soon as sufficient quantities of spinel or gahnite are formed. References [1] P. Wange, T. Höche, C. Rüssel, J.D. Schnapp, J. Non-Cryst. Solids 298 (2002) 137. [2] G.H. Beall, Hard, high modulus glasseceramics, US Patent 3873.329, (1975). [3] P. Wange, G. Carl, K. Naumann, J. Vogel, W. Vogel, W. Götz, W. Höland, Silicate Indust. 1 & 2 (1991) 21. [4] A. Katzschmann, P. Wange, Glastech. Ber. Glass Sci. Technol. 68 (1995) 111. [5] V. Maier, G. Müller, cfi Ber. DKG 65 (1988) 208. [6] L.R. Pickney, G.H. Beall, J. Non-Cryst. Solids 219 (1997) 219. [7] W. Höland, P. Wange, G. Carl, W. Vogel, E. Heidenreich, Silikattechnik 35 (1984) 181. [8] J.W. Hinz, Silikate. VEB Verlag für Bauwesen, Berlin, Germany, 1970. [9] X. Zhou, K. Uchida, US 6, 627, 565 B1 (30/09/2003). [10] X. Zhou, K. Azogami, US 6, 774, 072 B2 (10/08/2004). [11] N.Goto US 00000 641 3890B1 “Glass-ceramic substrate for a magnetic information storage medium” (02.07.2002). [12] W. Pannhorst, U. Woelfel, S. Wolf, US 6, 376, 402 B1 (23/04/2002). [13] H. Scholze, Glas, Natur, Struktur und Eigenschaften, Berlin, Heidelberg, New York, 1988. [14] A. Hunger, G. Carl, A. Gebhardt, C. Rüssel, J. Non-Cryst. Solids 354 (2008) 5402. [15] W. Zdaniewski, J. Am. Ceram. Soc. 58 (1975) 16. [16] W. Zdaniewski, J. Mater. Sci. 8 (1973) 192. [17] F.D. Doenitz, K. Koch, W. Vogel, Silikattechnik 33 (1982) 18. [18] A. Ramos, M. Gondais, J. Cryst. Growth. 100 (1990) 471. [19] B. Champagnon, C. Mai, E. Rodeck, J. Non-Cryst. Solids 94 (1987) 210. [20] C. Mai, G. Andrieu, J. Non-Cryst. Solids 108 (1989) 201. [21] P. Amista, M. Cesari, A. Montenero, G. Gnappi, L. Lan, J. Non-Cryst. Solids 192/ 193 (1995) 529. [22] S. Bhattachaya, T. Höche, I. Linschek, I. Avramov, R. Wurth, M. Müller, C. Rüssel, Cryst. Growth Des. 10 (2010) 379. [23] W. Schreyer, J.F. Schairer, J. Petrol. 2 (1961) 419.