Journal of the Ceramic Society of Japan 102 [2] 109-113 (1994) Paper

Preparation and Characterization of -Spodumene Powders

by Sol-Gel Process

Moo Chin Wang

Department of Mechanical Engineerig, National Kaohsiung Institute of Technology, Kaohsiung, 80782, Taiwan, Rep. of China

ゾル-ゲ ル法 による β-スポ ジューメン粉末の調製及び特性評価

王 木琴

国立高雄工商専科学校機械工程科, 中華民国高雄市

[Received March 1, 1993; Accepted November 16, 1993]

A fine -spodumene type amorphous powder has been

prepared by the sol-gel process using LiOCH3, Al(OC2H5)3 and Si(OC2H5)4 as a staring materials. By the rapid hydrolysis of alcohol solution of starting metal alkoxides in the presence of hydrochloric acid as a catalyst and with large excess of water, a gel powder was obtained. For the single-phase gel, the DTA curve closely resembled the amorphous to -spodumene (Li2A234SiO2) reaction. At 800, the crystalline phase comprised the major phase of -spodumene and the minor phase of -eucryptite (Li2A232SiO2). For 1000 and 1200 calcination, the XRD patterns showed pronounced growth of -spodumene. The diffraction lines of -eucryptite disappeared at 1200.

Key-words: -spodumene, Sol-gel, Rapid hydrolysis, Metal alkoxides

1. Introduction

Over the past several years, much interest has

been focused on -spodumene glass-ceramics be

cause they have intrinsic low thermal expansion

coefficient and are practically used as industrial fur

nace material and heat exchangers for gas turbine.

spodumene glass-ceramics commercially available

are prepared mainly by the recrystallization of

solidified melt, but it has been impossible to sinter

pure -spodumene without any sintering aid except the alkoxide-derived precursors.1), 2) Adding the sin

tering aid resulted in a large thermal expansion.

Therefore, the preparation of homogeneous and fine -spodumene powder is considered to be indispensa

ble.

The sol-gel method is a chemical technique including the following stages: (1) mixing of metal alkoxides independently or in combining acetates, oxalates and inorganic salts, (2) addition of distilled water under which hydrolysis and condensation reaction take place.3) Since this technology allows the control of the manufacturing process at micro and macro-scopic scale, the sol-gel method of synthesizing glass or ceramics has many advantages over other methods such as melt processing or solid state reaction sintering.4) 5

In the present study, the raw alkoxides were used for the synthesis of -spodumene powders by sol-gel

process. The -spodumene powders were formed at 800. The main purpose of the present investigation was to examine the crystallization behavior of the -spodumene phase by differential thermal analysis (DTA), X-ray diffraction study (XRD), Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), and electron diffraction (ED).

2. Experimental procedure

Gel powders having the -spodumene composition were prepared from tetraethylorthosilicate (TEOS), Al(OC2H5)3 and LiOCH3 as the starting materials. Gel powders with a stoichiometric -spodumene com

position were prepared by rapid hydrolysis of alcohol solution of starting materials, using formamide as a drying control chemical additive (DCCA). A schematic diagram of the sol-gel process was shown in Fig. 1. TEOS was partially hydrolyzed at 60 for 60h. The catalyst used was hydrochloric acid (HC/TEOS=0.1). The concentration of the alcoholic solution of TEOS was 1M (mol/1) and the molar ratio of water to TEOS was 2.2), 6) The partially hydrolyzed TEOS solution reacted with aluminum alkoxide followed by the reaction with lithium alkoxide solution. The molar ratios of hydrolytic water to Al(OC2H5)3 and LiOCH3 were 3 and 2, respectively. This preparation process resulted in the transparent solution with the same cation ratio as that of the stoichiometric spodumene. This solution was completely hydrolyzed by adding excess water (H2/TEOS=30mol/mol). The monolithis gel obtained was thoroughly dried at 120 for 24h to obtain the gel powders.

The precursors of the Li2-A23-4SiO2 (LAS)

gel, dried at 120 for 24h and heat-treated under various conditions, were analyzed. Differential thermogravimetric analyses (DTA/TGA) were carried out in the air at various heating rate of 5/min, 10/min, 15/min and 20/min, respectively, up to 1000. Crystalline phases precipitated were identified by X-ray diffraction (XRD) using Cu K radiation and Ni filter. The lattice constants of -spodu

1 0 9

1 1 0 Preparation and Characterization of -Spodumene Powders by Sol-Gel Process

mene were calculated from the angular position of the reflections (201), (113), (213), (320), (400) and (402) using the least-square method.7) FT-IR analysis was conducted in order to investigate the formation of chemical bonds. The FT-IR spectra have been recorded in the frequency ranging from 4000 to 400cm-1. The gels have been studied as

powders dispersed in KBr pellets. The powders were observed by transmission electron microscopy (TEM).

Fig. 1. Schematic diagram of the sol-gel process for preparing

spodumene ceramic powders.

3. Results and discussion

Figure 2 showed the DTA/TGA curves for LAS

gel powders at heating rate of 10/min. The endothermic peak in the interval 25 to 400 was due to the release of adsorbed water, and the exothermic

peak at 330 corresponding to the pyrolysis of the organic ligands such as CH3 and C2H5 in the LAS gel.3) The amount of weight loss below 330 was 18%. The exothermic peak at 900 was probably due to the formation of -spodumene phase. At this temperature the weight loss reached to 20%.

The second exothermic peak of the DTA curves with different heating rate, h, ranging from 5/min to 20/min was shown in Fig. 3. As the heating rate increases, the exothermic peak position shifts to a higher temperature. The crystallization kinetic of the gel was evaluated using DTA with different heating rate. Taking into account of the maximum exothermic peak temperature, Tc, Johnson-MehlAvrami equation8), 9) was described as follows;

(1)

where QC was the apparent activation energy for crystallization and R denoted the gas constant.

Fig. 2. DTA and TGA curves for the gel precursor with -spodumene composition at a heating rate of 10/min.

Plotting the n h versus 1/Tc, the straight line was obtained as shown in Fig. 4. The apparent activation energy was calculated from the slope of straight line to 91.2kcal/mol.

Fig. 3. The second exothermic peak of the DTA curves with

different heating rate for LAS gel.

Fig. 4. Plots of n h versus 1/T.

The phase relationships in the LAS gel have been

examined earlier by using XRD.1), 10)-13) Suzuki et

Moo Chin Wang Journal of the Ceramic Society of Japan 102 [2] 1994 1 1 1

a.1 used the partial hydrolysis method of TEOS coupled with the double alkoxide method resulted in the copolymerized precursor solutions with a stoichiometric -spodumene composition. The precursor

powders crystallized to the -spodumene (-eucryptite/-quartz solid solution) above 600. The to transformation of the spodumene occurred around 900.1) Using S(OCH3)4, Al(N3)3 and LiNO3 as the starting materials, the first crystallization phases observed by XRD were Li2SiO3 and -spodumene. The appearances of -spodumene and -quartz were observed at 700 and the crystalline phases of spodumene and Li2Si25 were found at 800.11) Orcel and Hench10) investigated the crystallization of LAS gel doped with Ti2 using drying control chemical additives. The stable crystalline phase was spodumene which appeared between 375 and 400 in this case. Veltri and Scola12) have used Al(OC4H9)3, Si(OC2H5)4 and LiNO3 as the starting materials and hot pressing at 1350 and 6.9MPa for 15min. In this case the crystalline phase observed by XRD was spodumene and mullite. In the alumina sol, silica sol and LiNO3 system, Kobayashi et al.13 found that the phases developed were spodumene and LiAlSi3

Figure 5 illustrated the X-ray diffraction patterns of the LAS gel dried at 120 for 24h and calcined at various temperatures for h. The effect of heat treatment on the progressive development of the major crystalline phase (-spodumene, Li2A234SiO2) and the minor -eucryptite phase (Li2A232SiO2) are as follows: (a) the amorphous state was maintained in the 120 to 400 runs; (b) at 600, the

peak of the (201) reflection of -spodumene was observed at 3.463; (c) at 800, the major reflections

corresponding to the (111), (201), (113), (213) and (400) of a -spodumene type crystal were identified.14) Furthermore, a minor crystalline phase,

-eucryptite (Li2A232i2) , was evolving; (d) above 1000, the (102) and (211) reflections of a spodumene appeared, the diffraction lines of -eucryptite disappered at 1200.

Fig. 5. XRD patterns of the LAS gel dried at 120 for 24h and calcined at various temperatures for lh:(a) dried at 120, (b) calcined at 400, (c) calcined at 600, (d) calcined at 800, (e) calcined at 1000, (f) calcined at 1200.S: -spodumene, E: -eucryptite.

Table 1. XRD and ED Data of -Spodumene from the JCPDS

and LAS Gel During Calcined at Various Temperature for h

In this study, the crystallinity of the -spodumene

phase was improved with increasing temperature. Lattice constants of the powders were calculated on the assumption that the crystal form of the LAS gels calcined above 800 was tetragonal.

The values of the lattice parameters for LAS gels calcined at various temperatures for h were listed in Table 1. The d-spacings of the main crystalline

phase were identical to those of the tetragonal spodumene and the relative intensity agreed fairly well. This confirmed the fact that the observed crystalline phase can be isostructural with -spodumene phase. Although the values of crystallographic ao and co axes in the -spodumene change at different rates with temperature, the unit cell volume changes varied slightly throughout the series. The unit cell has tetragonal symmetry, with a unit cell volume

(V=a2c) ranging from 520.6 to 525.03.The FT-IR spectra for a series of the dry gels and

the powders calcined at various temperatures for 1h

were shown in Fig. 6. The low-frequency part of the

spectrum under 1100cm-1 is a complicate combina

tion of bands. The characteristic band corresponding

to the Si- stretching vibration was observed about

1025cm-1.15) The absorption peak at 840cm-1 im

plies the vibration of the non-bridging oxygen atoms

1 1 2 Preparation and Characterization of -Spodumene Powders by Sol-Gel Process

at the surface of silica.15) The absorption peak at

800cm-1, which corresponds to the ring structure of Si4 tetrahedra could not be found. These results

may suggest that no serious polymerization of TEOS

would occur.6) The band at 750cm-1 is assigned to

vibrations of A4 tetrahedrons and its appearance

characterizes the change in the coordination number

of Al from 6 to 4.16) The band at 464cm-1 is the

usual silica rocking vibration.17) The absorption

band at 450cm-1 is characteristic for the stretching

of the Si-O-Si bonds,18) and the formation of an ion

ic bond, which is associated with the dehydration

polymerization, and ions or ionic substances is fixed in the three-dimensional network of the SiO4.19)

Fig. 6. FT-IR absorption spectra of the LAS gel dried at 120 for 24h and calcined at various temperatures for h:(a) dried at 120, (b) calcined at 400, (c) calcined at 600, (d) calcined at 800, (e) calcined at 1000, (f) calcined at 1200.

During the calcination, the residual hydroxide ions

or residual organic radicals disappear and the chemi

cal bonds among the elements increase their

regularity.19) After the crystallization of the amor

phous product, two bands are found at 1000 and 740cm-1 characteristic for the -spodumene lattice. The

band of 700cm-1 that the only significant in the spec

trum was the growing of a band with aluminium con

tent, assigned to Al4 localized mode.19) It is conclud

ed by Girin.16) that the spectral indications of LAS

gel crystallization and precipitation of -spodumene in the powders are the appearance of sharp peaks at

750cm-1 and 1000cm-1.

Fig. 7. TEM images and ED pattern of the sample heat treated at various temperatures:(a) calcined at 400 for 1h, (b) ED pattern shows the glassy state, (c) calcined at 1000 for h, (d) ED pattern shows the spodumene phase.

Figure 7 showed the bright and dark field images of TEM and corresponding electron diffraction (ED) pattern of the LAS gels which were calcined at 400 or 1000 for h. In case of the gel calcined at 400 for h, the very fine powders of which primary

particle size was about 300 were observed and the glassy state was maintained up to 400, as shown in Figs. 7 (a) and (b). The ED pattern of -spodumene at 1000 was superimposed, indicating that a lot of particles had been incorporated and crystallized. The fine particles were gradually incorporated into the large particles with increasing temperature. The synthesis was completed in the temperature ranging from 1000 to 1200. No crystalline phase was detected except for the -spodumene with particle size up to 500. Table 1 listed the X-ray index value of, spodumene for JCPDS with those obtained from XRD and ED in this study. The d-spacings listed were closely matched.

4. Conclusions

The results in the present study are summarized as follows:

(1) A fine -spodumene precursor powders were prepared by hydrolyzing a mixture of LiOCH3, Al(OCH5)3 and Si (OC2H5)4 in the presence of hydrochloric acid as a catalyst with excess water.

(2) The major crystalline phase with a tetragonal structure of the -spodumene phase and the minor crystalline phase of a -eucryptite were identified at 800.

(3) The apparent activation energy of -spodumene crystallization in the Li2OAl2O34SiO2 gel

powders was 91.2kcal/mol.(4) For 1000 and 1200 calcination, the XRD

patterns showed pronounced growth of -spodu

Moo Chin Wang Journal of the Ceramic Society of Japan 102 [2] 1994 1 1 3

mene. The diffraction lines of -eucryptite disap

peared at 1200.

Acknowledgments This work was supported by the National Science Council of the Republic of China under Contract No. NSC79-0405-E-151-01 is gratefully acknowledged. The help and service from Dr. M. H. Hon, Dr. H. S. Liu, Mr. M. H. Lin, Ms. L. C. Wang, Mr. J. M. Chen and Mr. S. Y. Yau are appreciated.

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