Controlled crystallization of glass–ceramics with two nucleating agents

M A TE RI A L S C H A RAC TE RI ZA T ION 6 0 ( 2 00 9 ) 1 5 2 9–1 5 3 3

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Controlled crystallization of glass–ceramics with two nucleating agents Anmin Hu⁎, Ming Li 1 , Dali Mao 2 State Key Laboratory of the Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiaotong University, 1954 Huashan road, Shanghai, China

AR TIC LE D ATA

ABSTR ACT

Article history:

The optimum nucleating agent had been investigated in Li2O–Al2O3–SiO2 glass–ceramics

Received 26 November 2008

system with 2% ZrO2 and different amounts of TiO2 as nucleating agents. The activation energy

Received in revised form 8 July 2009

(E) of crystallization and the Avrami parameter (n) for the LAS glasses obtained from the DTA

Accepted 1 September 2009

and results show that the most effective addition of TiO2 was about 2.36 wt.%. With the

Keywords:

high transparency and good mechanical properties were obtained, due to the β-quartz solid

Glass–ceramics

solution formed after the crystallization process.

optimum nucleating agents (2% ZrO2 + 2.36% TiO2) addition, LAS glass–ceramics with fine grain,

Crystallization

© 2009 Elsevier Inc. All rights reserved.

Nucleation Properties Thermal shock resistance

1.

Introduction

Glass–ceramics are polycrystalline solids produced by controlled crystallization of glasses. Controlled crystallization usually involves a two-stage heat treatment, namely a nucleation stage and a crystallization stage. In the nucleation stage, small nuclei are formed within the parent glass. After the formation of stable nuclei, crystallization proceeds by growth of a new crystalline phase. It is important to determine nucleation and crystallization parameters, especially, the amount and species of nucleating agents in glass–ceramics [1–3]. These additives act as heterogeneous sites at which the nucleation of desired crystalline phases may take place and therefore by adjusting the type and concentration of the nucleating agent used, it is possible to control over the crystallization process. In many glass–ceramics system, more than one kind of nucleating agents was used to obtain optimum microstructure and properties. In

such a case, how to decide the optimum nucleating agent addition is a very difficult work. Lithium aluminum silicate (LAS) glass–ceramic has been extensively investigated and commercialized, because of its very low thermal expansion coefficient as well as excellent thermal and chemical durability [4–6]. To obtain optical transparency coupled with higher mechanical strength and superior thermal shock resistance, a homogeneous, fine-grained β-quartz solid solution phase was desired. So, it is important to control the nucleation and crystallization processes of LAS glasses, especially the species and the amount of nucleating agents. One of the earliest studies on the nucleation and crystallization of LAS glasses–ceramics was conducted by Doherty [5]. They found that the glasses containing TiO2 as a nucleating agent showed that phase separation occurred on cooling from the melt and subsequent heating caused the formation of a large number of aluminum titanate crystals approximately 5.0 nm in

⁎ Corresponding author. Tel./fax: +86 21 34202748. E-mail addresses: [email protected] (A. Hu), [email protected] (M. Li), [email protected] (D. Mao). 1 Tel.: +86 21 34202542. 2 Tel.: +86 21 34202541. 1044-5803/$ – see front matter © 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2009.09.001

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M A TE RI A L S C H A RAC TE RI ZA T ION 6 0 ( 2 00 9 ) 1 5 2 9– 1 5 3 3

diameter. These crystals act as sites for heterogeneous nucleation and allow crystallization of the remaining glass. Then, Muller [6] found that LAS glasses containing TiO2 and ZrO2 as a mixed nucleating agent had better nucleating effective. Finer grain, higher strength and optical transparency glass–ceramics could be obtained. Schiffner [7,8] found that when TiO2 and ZrO2 served as mixed nucleating agent, ZrO2 was more effective than TiO2. The smaller variation in ZrO2 caused a larger change than did the larger variation in TiO2. So, in LAS glass–ceramic, more ZrO2 is preferred. But Lin [9] had reported that the viscosity of the LAS glass increased sharply as ZrO2 additives increased to 2%. Hsu [10] also found that with more than 2% ZrO2 addition, the melting temperature of LAS glass increased to above 1600 °C, it is difficult to achieve sufficient homogeneous glass. In our previous research, we also found that ZrO2 additives could increase the glass melting temperature greatly [11,12]. It can be concluded that the upper limit for ZrO2 additives employed in LAS glass is around 2%. Thus in this study, LAS glass–ceramics were prepared by adding 2% ZrO2 and different amounts of TiO2 as nucleating agents. The nucleation and crystallization processes were investigated. The mechanical and optical properties such as the flexural strength, elastic modulus and fracture toughness, thermal expansion coefficient, thermal shock resistances and optical transparency were measured.

2.

Experimental Procedures

The initial materials are analytical grade reagents SiO2, Al2O3, Li2CO3, MgO, ZnO, P2O5, TiO2 and ZrO2. Samples T1, T2, T3, T4 and To mean different amounts of TiO2 additives. The detailed compositions of these glasses are given in Table 1. Phosphorus oxide is chosen as the main component to decrease the melt viscosity and elevate optical transparency [1]. P2O5 with an equivalent amount of Al2O3 in the form of AlPO4 quartz-like groupings can enter the structure of β-quartzss crystal, and can result to a more loose crystalline structure which lower the melting temperature [13,14]. The initial bubble free glasses are melted in an electric furnace for 3 h at a temperature of 1580 °C, then poured onto a metal plate, and annealed at a temperature of 600 °C. DTA of annealed glass specimens is done in a Dupont 2100 Thermal Analyzer. After crushing the annealed glasses to about 100–200 μm, non-isothermal experiments are performed by heating 30 mg samples in a Pt crucible with Al2O3 as the reference material in the temperature range between 20 °C and 1200 °C at heating rates of 5–20 °C min− 1. XRD investigations are done with a D-max-RB diffractometer with Cu Ka radiation in the 2θ range from 10° to 70° at 0.02° steps.

SEM is done with a JSM-6301F. The specimens are prepared with standard metallographic techniques followed by chemical etching in an HF solution (5%) for 1.5 min. Etched glass–ceramic samples are coated with a thin layer of gold. For measurement of the mechanical strength, a 3-point bending strength method is employed to the bar type specimens using a MTS Testing Machine at a rate of 0.05 mm/min. Vickers hardness test is applied to measure the hardness of the polished specimens with Vickers micro hardness Testing Machine (HXD-1000), and densities are measured by Archimedes method. The thermal expansion coefficient values are taken in the range of room temperature 600 °C from a TMA measurement (SETRAM-TMA 92, TA Instrument, Germany). Transmission of the glass–ceramics is measured by UV–Vis Recording Spectro-Photometer (UV-3101, Shimadazu Corporation, Japan) with thickness of 4 mm. The thermal shock resistance of specimens is measured by quenching the hot specimens into 20 °C water.

3.

Results and Discussion

DTA curves for the four LAS–2% ZrO2–xTiO2 glass samples at a heating rate of 10 °C/min are shown in Fig. 1. Only one evidence exothermic peak is observed in each curve, the same as in previous investigations [13], which indicate the crystallization of β-quartzss crystals. As TiO2 contents increase from 1% to 4%, the glass crystallization peak temperatures (Tp) decrease from 886 °C to 869 °C. The activation energies of the crystallization processes (E) have been calculated based on Kissinger equation [15–17]: ln

Tp2 α

=

E + Const RTp

ð1Þ

where Tp is the crystallization peak maximum temperature in a DTA curve, α is the heating rate of DTA and R is the gas constant. Values of E derive from the plots of ln(T2p /α) versus 1 /Tp in Fig. 2 are given in Fig. 3 which shows the plot of E versus TiO2 content. As showed in Fig. 3, with increasing TiO2 contents, the activation energy of crystal growth E, corresponds to the energy barrier of transition from glass to crystal, decreases from 1% TiO2 to 2.36%, then increases. This means that with 2.36% TiO2 additives, the

Table 1 – Composition of the glasses (wt.%). Sample Li2O Al2O3 SiO2 MgO ZnO P2O5 ZrO2 TiO2 T1 T2 T3 T4 To

4.0 4.0 4.0 3.9 4.0

25.3 25.0 24.8 24.5 24.94

57.6 57.0 56.3 55.8 56.7

1.0 1.0 1.0 1.0 1.0

1.0 1.0 1.0 1.0 1.0

8.1 8.0 7.9 7.8 8.0

2.0 2.0 2.0 2.0 2.0

1.0 2.0 3.0 4.0 2.36

Fig. 1 – The DTA curves of glass samples, α = 10 °C/min.

M A TE RI A L S C H A RAC TE RI ZA T ION 6 0 ( 2 00 9 ) 1 5 2 9–1 5 3 3

Fig.2 – The plots of ln(T2p / α) versus 1 / Tp for the glasses.

energy barrier of transition from glass to crystal is lowest, the minimum value of E may mean the most effective nucleating agent additives. From the values of activation energy, the Avrami parameters (n) are also calculated by the Augis–Bennett equation [17]: n=

2 2:5 RTp × E ΔT

ð2Þ

where, ΔT is the full width of the exothermic peak at the half maximum intensity. The value of n close to 1 means that surface crystallization, n close to 2 means that two-dimension crystallization, and the value of 3 implies bulk crystallization [10–12]. From calculations, values of n of the four LAS–2% ZrO2– xTiO2 glasses with different amounts of TiO2 are also listed in Fig. 3. When the content of TiO2 is 1%, the value of n is lowest at about 2.2 ± 0.3. When TiO2 content increases to 2.36%, the value of n increases at a maximum value of about 3.4. As TiO2 content increases from 2.36% to 4%, the values of n decrease from about 3.4 to 2.6 ± 0.3. The result indicates that with 2.36% TiO2, the glass systems have maximal trend to bulk crystallization. Combining the value of the activation energy E with the Avrami parameters n, it can be concluded that the optimum nucleating agents in the LAS glass are 2% ZrO2 + 2.36% TiO2. Fig. 4 illustrates the powder XRD pattern of four LAS–2% ZrO2 glasses with different amounts of TiO2 addition crystallized at 840 °C/2 h, respectively. With 1% TiO2, a trace of β-quartzss

Fig. 3 – The plot of E and n versus TiO2 content.

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Fig. 4 – XRD pattern of LAS–2% ZrO2–xTiO2 glasses samples crystallized at 840 °C for 2 h.

appeared in the glass matrix. With 2%–4% TiO2, most of the glass transformed to β-quartzss crystals. Fig. 5(a)–(e) shows the microstructure of four LAS–2% ZrO2– xTiO2 glasses crystallized at 840 °C/2 h, respectively. With additive 1% TiO2, there are small amount of crystals that appear in the glass matrix. These mean that, in the LAS–2% ZrO2– xTiO2 glass sample, 1% TiO2 additives as nucleating agent is deficient. As TiO2 contents increase to 2%, fine equaxial crystals about 100–130 nm appear. As TiO2 contents increase to 2.36%, which is the optimum nucleating agent according to previous analysis, the finest homogenous crystals about 50– 60 nm appear. The glass–ceramics with the finest homogenous crystals may have superiority in optical transparency [5– 7]. With 3% TiO2, the crystals increase again to about 100 nm. With 4% TiO2, the crystals increase to about 150–200 nm. These descriptions prove that TiO2 can improve the bulk crystallization of the LAS–2% ZrO2–xTiO2 glass from 1% to 2.36%, and as TiO2 contents increase to 2.36%, bulk crystallization reaches highest point. When TiO2 content increases to 3%, especially to 4%, there is excessive TiO2 which can increase crystal growth and let the grains coarse. The results are consistent with the tendency of E and n. After being crystallized at 840 °C/2 h, optical transparencies are shown in Fig. 6. With 2.36% TiO2 additives, the LAS–2% ZrO2–xTiO2 glass–ceramics, which have the finest grain size, give the highest optical transparencies. The optical transparency varies in visibility from 11% for λ = 400 nm till approximately 78% for λ = 700 nm. In the near IR (λ = 1600 nm) the optical transparency is 90%. While the glass–ceramics T4 (with 4% TiO2 addition), which have the largest grain size, have the lowest optical transparencies. It had been found that in LAS glass–ceramics, the finer grain means that the higher optical transparency can be obtained [1–6]. The properties of four glass–ceramics crystallized at 840 °C/ 2 h are given in Table 2. Because the matrix is almost glass, the sample T1 with 1% TiO2 has the lowest properties. As TiO2 addition increases to 2%, the properties increase. The maximum values of Vickers hardness, flexural strength, fracture toughness and optical transmission are obtained at the samples with 2.36% TiO2 addition. As TiO2 addition increases to 3% and 4% TiO2, the grain size of glass–ceramics became larger, the properties such as the Vickers hardness, flexural

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Fig. 5 – SEM microstructure of LAS–2% ZrO2–xTiO2 samples crystallized at 840 °C/2 h. (a) T1, (b) T2, (c) T3, (d) T4, (e) To.

strength, and fracture toughness decrease according to Hall– Petch relationship. So, it can be concluded that the optimum nucleating agents are 2.36% TiO2 addition in the LAS–2% ZrO2– xTiO2 glass.

4.

Fig. 6 – Optical transparency of the LAS–2% ZrO2–xTiO2 glass–ceramics with different amounts of TiO2 additives.

Conclusions

The optimum nucleating agent additives had been discussed in LAS glass–ceramics system with mixed nucleating agents (2% ZrO2 and TiO2), the optimum nucleating agents were obtained by fixed 2% ZrO2 with different amounts of TiO2, and the optimum nucleating agent additives were 2% ZrO2 + 2.36% TiO2. With the optimum nucleating agent additives, the glass–ceramics obtained the best mechanical and transmission properties.

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Table 2 – The mechanical properties of LAS–2% ZrO2–xTiO2 glass–ceramics heat-treated at 840 °C/2 h. Sample The Vickers hardness Elastic moduli Flexural strength (GPa) (GPa) (MPa) T1 T2 T3 T4 To

5.8 ± 0.5 6.1 ± 0.5 6.2 ± 0.4 5.8 ± 0.4 6.2 ± 0.5

76 ± 3 90 ± 4 89 ± 4 93 ± 5 92 ± 3

87 ± 2 116 ± 4 113 ± 4 104 ± 5 119 ± 3

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