Effect,of,Na2O,Content,on,the,Structure,and,Properties,of,LAS,Glass-ceramics,Prepared,by,Spodumene

ZHOU Zhiqiang, HE Feng＊, SHI Mingjuan, XIE Junlin, WAN Peng, CAO Dahua

(1. School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, China; 2. Foshan Shunde Midea Electro-thermal Appliances Manufacturing Co., Ltd, Foshan 528000, China)

Abstract: The Li2O-Al2O3-SiO2 (LAS) glass-ceramics with low lithium content were prepared from spodumene mineral by melting method. XRD, DSC, and FTIR were used to study the crystallization process and structure of the samples. The results showed that the addition of Na2O promoted the precipitation of β-quartz solid solution and its transformation to β-spodumene solid solution. Mechanical performance tests and FESEM indicated that the larger grain size and inhomogeneous grain dispersion caused by the increase of Na2O content led to lower mechanical properties. In addition, low expansion glass-ceramics were prepared by an appropriate heat treatment according to DSC result, and when Na2O content was in the range of 1.22 wt% to 2.41 wt%,the average coefficient of thermal expansion (CTE) (30-300 ℃) increased from -5.810 ×10-7 to 5.322×10-7℃-1.

Key words: low thermal expansion; Na2O content; LAS glass-ceramics; spodumene

Glass-ceramic is a kind of polycrystalline material obtained by controlling the crystallization of glass[1].It exhibits unique properties owing to combining the characteristics of both glasses and ceramics. LAS glass-ceramic is one of the most concerned and valued glass-ceramics at present. It has aroused the interest of many researchers due to special properties such as adjustable thermal expansion within a certain temperature range, strong thermal shock resistance, high transparency[2], and high temperature resistance. Thus, it can be used in cook top panels, telescope mirrors and high temperature furnace windows,etc.[3].β-quartz solid solution (β-quartz s.s.) andβ-spodumene solid solution(β-spodumene s.s.) are two common phases in low-expansion LAS glass-ceramics, and they have the characteristic of low expansion due to their unique structure[4].Macroscopically zero expansion or even negative expansion properties can be obtained by adjusting the composition and heat treatment to control the type and fraction of the precipitated phases.

Generally, in order to obtain low-expansion LAS glass-ceramics, the content of Li2O is often in a relatively high state. For example, Soareset al[5]successfully synthesized high-density, low-expansion LAS system glass-ceramics with a thermal expansion coefficient of 0.34×10-6℃-1(40-500 ℃) through non-isothermal sintering, and the Li2O content in his research is 8.43 mol% (about 4.8 wt%). Venkateswaranet al[6]prepared LAS system glass-ceramics with a coefficient of thermal expansion of 0.04×10-6℃-1(-60-400 ℃) by melting method, with a Li2O content of 8.7 mol% (about 5.2 wt%). Liet al[7]prepared a high flexural strength LAS system glass-ceramic as the content of Li2O is 4 wt%, however, its CTE is as high as 2.64×10-6℃-1. The study of Hu[8]also shows that the decrease of Li2O content will greatly increase the thermal expansion coefficient of LAS glass-ceramics. Preparing ultra-low expansion or even negative expansion LAS glass-ceramics at low lithium content is conductive to reducing the raw material cost of the industry. However, the melting of glass will become difficult in this condition due to the lack of sufficient flux. At present,there has been a lot of research to lower the melting point of glass. Common fluxes include alkali metals[9],alkaline earth metals[10], fluorides[11],etc. Among these fluxes, Na2O becomes an ideal additive to improve the glass melting situation[12,13]due to its lower price,good fluxing effect and little pollution. However, large amounts of alkali oxides will destroy the network structure of the basic glasses and adversely affect the thermal and mechanical properties of the glass-ceramics.Therefore, it is very important and necessary to explore the effect of Na2O content on the structure and performance of LAS glass-ceramics. Furthermore, if Li2O is introduced by cheap spodumene mineral instead of expensive chemical material Li2CO3, it will also greatly reduce production costs and have more commercial significance. At present, using lithium ore raw materials as the source of lithium content is not common.

In this work, relatively cheap spodumene ore was used as the source of lithium, together with other supplementary materials were added to prepare low expansion LAS glass-ceramics. Besides, the influence of Na2O content on the structure, thermal and mechanical properties of glass-ceramics was systematically investigated.

2.1 Preparation of glass-ceramics

The main raw material used in this paper was spodumene mineral from Xinjiang, China, which was tested and analyzed by X-ray fluorescence spectrometer and atomic absorption spectroscopy. The results are shown in Table 1. The others in Table 1 are the loss on ignition of the raw material and small amounts of oxides such as MgO and P2O5. The composition design of the parent glasses is shown in Table 2, wherein Li2O was introduced by Xinjiang spodumene and its content was at a relatively low level, alkali metal oxides and alkaline earth metal oxides were introduced from their corresponding carbonates, and other components were introduced as oxides. Moreover, in order to reduce the influence of other composition variations on the performance of glass-ceramics, when the Na2O in glass-ceramics composition varied from 1.22 wt% to 2.41 wt%,the SiO2which accounted for the largest proportion of the glass-ceramics composition regularly decreased to keep the proportion of other components unchanged.The glass batch was obtained by thoroughly mixing Xinjiang spodumene and other chemical reagents according to Table 2, and melted in an electric furnace at 1 650 ℃ for 3 h. Then, the homogenized molten glass liquid was casted on a preheated steel mold and quickly transferred to an annealing furnace at 550 ℃ for 2 h to eliminate internal stress. After that, the formed glass was cooled to room temperature within the furnace.Finally, the bulk glass sample was cut into suitable size for following heat treatment and tests, and the corresponding glass-ceramics were obtained after sintering at an appropriate heat treatment schedule.

2.2 Structure and performance test

DSC (STA449F3, NETZSCH) was used to investigate the thermodynamic performance from room temperature to 1 000 ℃ in air at a heat rate of 10 ℃/min. And the heat treatment schedule of the LAS glass was determined by the result. The crystalline phases of samples were characterized by using X-ray powder diffractometer (D8 Advance, Brux AXS) with Cu Kα radiation, in the range of 10-70° at a rate of 10 °/min.Infrared spectra of the glass and glass-ceramic samples were carried out by Fourier transform infrared spectrometer (Nicolet 6700, Thermo Electron Scientific Instruments) over the range of 400-1400 cm-1. The glass and glass-ceramics were ground to under 75 μm for the tests of DSC, XRD and FTIR. The microstructure of the glass-ceramics polished and eroded in 5 wt% HF solution for 50-60 s were observed by field emission scanning electron microscope (ULTRA PLUS, Zeiss).The CTE of glass-ceramics were determined by ther-mal dilatometer (DIL 402C, NETZSCH) with the heating rate of 5 ℃/min from room temperature to 700 ℃.The flexural strength was tested by three-point bending method. Universal testing machine (KZY-300-1) was used to perform the test with the experimental span of 25 mm and the loading speed of 9.8 ± 0.1 N/s. A microhardness tester was used to measure the Vickers hardness of samples with a loading time of 10 s and a loading pressure of 1.96 N. The density of the glass-ceramics is obtained by Archimedes drainage method.

Table 1 Chemical composition of the spodumene/wt%

Table 2 Chemical composition design of the LAS glass-ceramics/wt%

3.1 Thermal analysis

Fig.1 DSC curves of the parent glasses with different Na2O contents

DSC curves of LAS parent glass with different Na2O contents are reflected in Fig.1, while the characteristic temperature points of the DSC curves are showed in Table 3. It can be seen from Fig.1 and Table 3 that the glass transition temperature (Tg) ranges from 530-550 ℃, the nucleation temperature is usually determined to be about 50 ℃ higher than theTgtemperature. The first exothermic peakTC1is between 800-820 ℃, and the second exothermic peakTC2is in the range of 870 to 890 ℃. As a typical glass network modifier, the effect of Na2O in glass is similar to that of Li2O. It can provide more free oxygen to increase the O/Si ratio in the glass network structure, thus destroying the bonding state of the silicate framework and making the network structure have a high degree of polymerization[14]. As a result, the addition of Na2O makes glass structure more relaxed and promotes the precipitation of the crystal in parent glass, which could explain the decreasing trend of the glass transition temperatureTgand second exothermic peakTC2as the content of Na2O increases. However, the first exothermic peakTC1, which demonstrates a slight intensity and broaden feature, displays an opposite trend, indicating that the increase of Na2O content might inhibit the precipitation of the first crystal phase while promoting the precipitation of the second crystal phase. In addition,the first and second crystallization peaks are related toβ-quartz s.s. andβ-spodumene s.s., respectively, according to previous research[15].

A two-step method was used to prepare low expansion glass-ceramics with the main crystalβ-quartz s.s. based on the analysis of DSC curves. The heat treatment was determined as nucleating at 580 ℃ for 2 h and crystallizing at 810 ℃ for 2 h with the heating rate of 5 ℃/min. After that, the samples were slowly cooled to room temperature.

Table 3 DSC characteristic temperature points of the parent glasses with different Na2O content/°C

3.2 Structure analysis

Fig.2 XRD patterns of glasses with different Na2O contents

Fig.3 XRD patterns of glass-ceramics with different Na2O contents

The properties of glass-ceramics depend on the percentages of crystalline and glassy phases formed and the type, composition, shape, and size of the precipitated crystals. Fig.2 and Fig.3 show the XRD patterns of the LAS glasses with different Na2O contents before and after heat treatment, respectively. It can be seen from Fig.2 that all the samples of LAS glasses before heat treatment display a broad diffraction peak,which indicates the amorphous nature of these parent glasses. Characteristic diffraction peaks are observed in Fig.3, demonstrating that a certain fraction of crystal phases have been precipitated inside the LAS glasses after heat treatment. Hexagonalβ-quartz s.s. phases(LixAlxSi1-xO2PDF Number: 40-73) is found as the predominant phase in samples N1 to N5. And the intensity of diffraction peak shows an increasing tendence as the content of Na2O increases. This can be explained from the following two aspects. First, even when the addition of Na2O content reaches the maximum value of 2.41 wt%, the molar ratio of Al2O3/ R2O is 1.08. Researches[16]show that when the molar ratio Al2O3/ R2O ≥ 1(R=Li, Na, K), the increase of Na2O content is conducive to the formation of [AlO4], which is benefit to precipitating the crystals composed of [AlO4] and [SiO4].In addition, Na2O destroys the glass network structure and makes the structure loose, which promotes the precipitation of crystals by promoting the diffusion of group. Furthermore, the enlarged figures of the diffraction peak around 23° corresponding toβ-spodumene s.s.(LiAlSi3O8PDF Number: 35-794) shows that this phase is observed when the content of Na2O is 2.11 wt% and will further increases with increasing the Na2O content.

The result reveals that Na2O promotes the precipitation ofβ-spodumene s.s., which is consistent with the DSC analysis. In addition,β-spodumene s.s. is considered to be formed by recrystallization of aβ-quartz s.s. rather than by direct glass crystallization[5]. Thus,conclusion can be derived that elevated Na2O content contributes to the transformation fromβ-quartz s.s. toβ-spodumene s.s.

Fig.4 FTIR spectra of the glasses with different Na2O contents

FTIR is usually used to investigate the information of glass structure with composition changing.The FTIR results of the LAS parent glasses containing different Na2O contents are displayed in Fig.4. It can be seen from the chart that the absorption peak bands of the LAS glasses with different Na2O contents seem almost the same, and three main absorption bands are observed from 1 400-400 cm-1in all the samples. According to the literature, the vibration peak in the range of 1 300-800 cm-1is attributed to the Si-O-Si(Al)asymmetric stretching vibration in the glass network structure, and the vibration peak in the range of 850-600 cm-1is attributed to Si-O-Si(Al) symmetrical stretching vibration. The vibration peak approach 460-400 cm-1is attributed to the bending vibration of O-Si(Al)-O[17]. Two broad absorption peaks are formed in the two wavenumber ranges of 1 250-850 cm-1and 830-600 cm-1, respectively, which could be explained by the overlap of absorption peaks, which is generated by wavenumbers shift of specific absorption peak due to the different coordination states of Si-O-Si(Al). At the same time, the infrared absorption band of the parent glass has a large full width at half maximum, which is considered as a characteristic of the typical glass network structure, so it can be inferred that no obvious devitrification occurs in the parent glasses. In addition,the absorption bands of the LAS glass get broader as the increase of Na2O content. This may be ascribed to the depolymerization of Na2O, which destroys the network structure of the glass and generates more asymmetric configurations.

Fig.5 FTIR spectra of glass-ceramics with different Na2O contents

Fig.5. demonstrates FTIR spectra of the LAS glass-ceramics with different Na2O contents. The obtained FTIR spectra of the LAS glass-ceramic exhibits some characteristics comparing with that of parent glass. First, the vibration of the in-plane bending vibration of O-Si-O appears to shift from the high wavenumber of the parent glasses to the low wavenumber of the glass-ceramics (450-430 cm-1), which is ascribed to the distortion of [SiO4] inβ-quartz s.s.[18]. Secondly, an obvious vibration peak appeared near 560 cm-1, which belongs to the in-plane bending vibration of Si-O-Al in the glass network structure[19]. Moreover, the absorption band in the range of 700-800 cm-1changes from two broad vibration peaks (780 and 730 cm-1) to a sharper vibration peak at 761 cm-1, and the absorption band in the range of 1 000-1 100 cm-1, and a broad absorption band around 1 055 cm-1splits into two sharp vibration peaks (1 078 and 1 020 cm-1). The splitting of the broad absorption band and the appearance of new vibration peaks are all attributed to the precipitation of crystalline phases in glasses after heat treatment[20].The regular arrangement of [SiO4] and [AlO4] in the glass-ceramics and strengthened interaction between the molecules cause the band splitting. At the same time, the highly symmetrical structure of crystals has the characteristics of low distribution of bond length and the angles between bonds, which makes the vibration band stronger and sharper.

Fig.6 FESEM of the glass-ceramics with different Na2O contents

Fig.6 shows the FESEM images of LAS glass-ceramics with different Na2O contents. It can be seen from the figure that there are numerous granular crystals inside the N1-N5 samples. The average size of the crystals is between 30 and 100 nm, which leads to a high transparency in glass-ceramics. However, there are still obvious differences in the grain size and distribution inside the LAS glass-ceramic samples with different Na2O contents. When the Na2O content is less than 1.52 wt% (N1-N2), highly fine and densely dispersed granular crystals can be seen from the pictures.The crystal grains precipitated inside the samples are fine and uniform, with an average size at about 30-40 nm, and smaller crystals fill the voids or microcracks inside the glass, making the inside of the glass-ceramics denser. With the further increase of Na2O content(N3-N4), the size of the crystals inside the samples gradually increases, and the average size is between 40-70 nm. The granular crystals are in contact with each other and closely stacked into larger-sized clusters, and the defects and voids inside the glass-ceramics gradually expand with the growing of the crystals filling in these sites before, which reduces the density inside the glass-ceramics. These differences are since the addition of Na2O content makes the diffusion of groups in the samples easier, which is beneficial to the growth of crystals. The size of the crystal particles inside the N5 glass-ceramic sample is the largest and the average size is between 70-90 nm. However, some smaller crystals around 30 nm are also found in N5 sample. This phenomenon is explained that crystalline phase transformation occurs in this sample and the larger crystals in size areβ-spodumene s.s. while the smaller crystals in size areβ-quartz s.s., which is consistent with the XRD analysis.

3.3 Properties analysis

Fig.7 Thermal expansion curves of glass-ceramics with different Na2O contents

The CTE of N series glass-ceramics were measured by dilatometer. Fig.7 shows the thermal expansion curves of LAS system glass-ceramics with different Na2O contents. Table 4 demonstrates the CTE values calculated from Fig.7 for N series glass-ceramics in different temperature ranges. The relationship between CTE of N series glass-ceramics and Na2O contents is shown in Fig.8.

Table 4 CTE of glass-ceramics with different Na2O contents(×10-7℃-1)

Fig. 8 CTE of glass-ceramics with different Na2O contents

Fig.9 Flexural strengths and microhardness of glass-ceramics with different Na2O contents

Fig.10 Density of glass-ceramics with different Na2O content

The result reveals that with the increase of Na2O content (N1-N5), the CTE of N series glass-ceramics show an obvious upward trend, behaving as the variation from initial negative expansion to positive expansion in the given temperature ranges, which indicates that the addition of Na2O promotes the CTE of LAS glass-ceramics. On the one hand, Na+is prone to form sodium-rich residual glass phase instead of entering into the structure of crystalline phases during the crystallization due to the large radius. The enrichment of the Na2O content in the glass phase makes the network structure more relaxed, thereby increasing the CTE of glass phase, and eventually affecting the CTE of glass-ceramics. On the other hand, it can be known from the results of XRD and DSC that Na2O promotes the precipitation ofβ-spodumene s.s., which exhibits greater CTE due to the lack of unique structure ofβ-quartz s.s., thus, the transformation of crystal phases can also increase the CTE of glass-ceramics. Moreover,as the results shown by FESEM, the addition of Na2O promotes the growth of crystal grains, which leads to crack propagation and looser structure of samples, and ultimately increases the CTE of glass-ceramics.

Fig.9 and Fig.10 display the flexural strength,microhardness, and density of glass-ceramics with different Na2O contents, respectively. It can be seen from the figures that flexural strength, microhardness and density of N series glass-ceramics show a decrease trend with the increase of Na2O content. This trend can be explained from the perspective of grain size,just as many studies have pointed out, uniform and fine grains are beneficial to improve the mechanical properties of glass-ceramics, when the Na2O content in glass-ceramics is maintained at a low level, the crystals precipitated inside the samples are fine, uniform and dispersed, the smaller crystals can be filled in the voids or micro-cracks to get a denser structure. However, the crystals are easier to stack into clusters and grow larger with the increase the Na2O content, making the microcracks inside the glass-ceramics gradually expand,thus, leading a decrease trend of flexural strength and density. Moreover, the crystal phase transition in the glass-ceramics will precipitate the crystal grains of different sizes, which may cause the growth stress and decrease the mechanical properties of glass ceramics. In addition, Na2O will destroy the network structure of the glass and make the structure loose, thereby reducing the density and microhardness of the LAS glass-ceramics.

The Na2O content influences the crystal grain size and dispersion inside glass-ceramics by promoting crystal growth and crystal phase transformation, thereby affecting the thermal expansion coefficient, flexural strength, and microhardness of LAS glass-ceramics.When the Na2O content is less than 1.52 wt%, the LAS glass-ceramics containing fine and uniformly dispersed crystal grains display negative expansion and good mechanical properties. When the Na2O content is more than 1.52 wt%, LAS glass-ceramics show higher thermal expansion coefficient and poor mechanical properties due to further looser sodium-rich glass phase and crystal growth. When the Na2O content increases to 2.41 wt%, the crystal phase transition fromβ-quartz s.s.toβ-spodumene s.s. can be proved by XRD, resulting in an obvious difference in grain size. And the highest thermal expansion coefficient and poorest mechanical properties in samples containing 2.41 wt% Na2O content are mainly attributed to the precipitation of crystal phase with higher CTE and crack propagation caused by the phase transition.