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Synthesis and investigation of crystalline modifications of silicon dioxide
293
Reactivity of Solids, 5 (1988) 293-303 Elsevier Science Publishers B.V., Amsterdam
- Printed
in The Netherlands
SYNTHESIS AND INVESTIGATION OF CRYSTALLINE MODIFICATIONS OF SILICON DIOXIDE
M.N. DANCHEVSKAYA, and Yu. D. IVAKIN Chemistry Department, Moscow (U. S. S. R.) (Received
S.N. TORBIN,
G.P. MURAVIEVA,
O.G. OVCHINNIKOVA
M. V. Lomonosov State University of Moscow, 117234,
March 3rd, 1987; accepted
January
14th, 1988)
ABSTRACT The synthesis of a-quartz and other crystalline modifications of silicon dioxide (crystobalite and SiOz-X phase) from amorphous silicon dioxide was carried out via hot steam treatment in the presence of the activating agent, tetra-N-methylammonium hydroxide (TMAH), in an amount of 0.2 wt.-% SiO,. The mass spectrometric study and the electron paramagnetic resonance (EPR) study lead us to the conclusion, that under conditions of hot steam treatment TMAH interacts with surface hydroxides of silicon dioxide and modifies the surface forming amino and methoxy groups. The latter interact with SiO, matrix-based hydroxyls contributing to the formation of interphase contacts. The electron microscopic study and X-ray analysis show, that the structuring of amorphous silicon dioxide proceeds by the mechanism of solid phase crystallization via the consolidation and ordering of SiO, structural elements.
INTRODUCTION
The crystallization of amorphous silicon dioxide, its multiple crystalline modifications each of which is of practical and theoretical importance have been the subject of growing interest. The crystallization of amorphous silicon dioxide is usually carried out under hydrothermal conditions in aqueous solutions of alkali and alkaline-earth metal hydroxides [l-3]. The limitations of the method are obvious: it is impossible to control this process when the hydroxides of alkali metals are used as activators since the selection of metal activators is limited and their action is one-sided. Moreover, the crystalline modifications of silicon dioxide in this case are poorly structured and polluted by silicates and metal carbonates. Therefore it seems that the crystallization via solid-phase transformation of silicon dioxide in water vapour and in the presence of surfactants is more advantageous [4,5]. 0168-7336/88/$03.50
0 1988 Elsevier
Science Publishers
B.V.
294
The introduction of surfactants of varying nature enables the process of silicon dioxide structuring to be controlled. The present article gives the results of the investigation of silicon dioxide crystallization using tetra-N-methylammonium hydroxide as an activating agent.
EXPERIMENTAL
The activator, tetra-N-methylammonium hydroxide (TMAH), was adsorbed from an aqueous solution in an amount of 0.2 wt.-% on the surface of amorphous silicon dioxide of “OSC 8-4” type with a 98 m2/g surface. The hot steam treatment was conducted at 400°C and steam pressure p 200 atm. The specimens subjected to the hot steam treatment of different duration were studied by X-ray, electron microscope, electron paramagnetic resonance (EPR) and mass spectrometry methods. The X-ray phase analysis of the products at various stages of SiO, crystallization was carried out on a diffractometer DRON-2 (Cu-K, radiation). a-quartz added to the specimen in an amount of 20 wt.-% was taken as an internal standard. The thin crystalline structure of newly formed SiO, modifications was studied using Fourier analysis of the line shapes. The zones of Bragg scattering (D) and microdistortions of the crystal lattice ((e*):(t) were determined. The structural morphological peculiarities of the crystalline modifications of silicon dioxide were studied using scanning electron microscopes. The electron microscopes Hitachi-520 and JELL 35 CF were used. A thin layer of gold was deposited on the specimen surface. The magnification level is given on the photographs. The interaction of the activating agent (tetra-N-methylammonium hydroxide (TMAH)) with silicon dioxide during the hot steam treatment and its role in the structuring of the silicon dioxide were studied by EPR and mass spectrometry. The EPR spectra were registered in X-diapason (h 3.2 cm) on the radiospectrometer RE-1306 and RS Varian E-109 within the the specimens temperature range - 196 -+ + 200 o C. Prior to measurements were irradiated with y-rays of 6oCo at - 196 o C. The radiation dose was 5 Mrad. The mass spectrometric studies were conducted on the mass spectrometer MI-1311 with a resolution of 500 and sensitivity to the analysed products of 10’3-10’4 molecules V’ s- ‘. The working vacuum was (l-5) X lo-’ torr. The flows of substances evaporating from silica specimens in vacua were analysed within the temperature range of lOO-1400°C under products conditions of “the open surface”. Thus, the thermal decomposition of the compounds formed in the interaction of the activator TMAH and silicon dioxide during adsorption and hot steam treatment were studied.
295 RESULTS
AND DISCUSSION
The effect of the hot steam treatment with and without TMAH being compared, the decisive role of the activator in the structural transformations of amorphous silicon dioxide becomes obvious. Without the activator even the prolonged steam treatment (72 h) does not initiate transformations of amorphous silica into a crystalline structure. The only effect is the consolidation of structure without further ordering. The structuring stops at the flocculation stage. The specimen is X-ray amorphous. The introduction of 0.01-0.2 wt.-% of TMAH into the system changes the process. The diffractograms of SiO, specimens after the hot steam treatment from 2-72 h in the presence of 0.2% of TMAH are given in Fig. 1. The degree of crystallization grows with time. Both main modifications, crystobalite and SiO,-X phase, are formed simultaneously; small quantities of keatite were also registered. The maximum yield of SiO,-X phase and crystobalite occurred at 12 and 14 h respectively (Fig. 2). Simultaneously lines of a-quartz appeared on the diffractogram (Fig. 1). Further changes in the concentration of crystobalite, SiO,-X phase and a-quartz (Fig. 2) testify to the fact that the final product, a-quartz, is formed from SiO,-X phase and crystobalite. The process of SiO, crystallization and phase transformations initiate monotonous ordering of the system and reduction in the number of defects in the crystal lattice. When the Bragg stattering zone is reached, the transition of SiO,-X phase crystals of D 500 A into a-quartz starts. The transition of crystobalite into a-quartz occurs at lower values of D (160 A). After the transformation of both modifications into a-quartz was completed the size of the a-quartz crystals remained unchanged in spite of the increased duration of the hot steam treatment. The size of blocks (D) was continuously growing as the
Fig. 1. Diffractograms of SiO, crystallization products after hot steam treatment durations: l-2 h; 2-4 h; 3-8; 4-22 h; 5-29 h; 6-72 h.
of different
296
Fig. 2. Kinetics o crystobalite;
of the formation of SO, crystalline SO,-X; 0, a-quartz.
modifications
from the amorphous
state:
A,
microdefects of the crystal lattice diminished (Fig. 4). The metamorphic transformations in the system and peculiarities of crystal structure of SiO, modifications starting with the moment of their initiation were revealed by electron microscopic investigation. The starting amorphous silicon dioxide had a large-globular structure (Fig. 5). After 1 h of the hot steam treatment in the presence of 0.2% of TMAH the structure consolidates causing the appearance of large pores (Fig. 6) and later, floccules (Fig. 7). After 4 h of treatment the initiation of the fibrous structure of SiO,-X phase (Fig. 8, left) and fine-crystalline crystobalite (Fig. 8, right) can be observed. The transition of SiO,-X into a-quartz by the aggregation of similarly oriented SiO,-X crystals occurs after 10 h hot steam treatment. The floccules consist of fragments of unordered crystobalite aggregate. The formation of faces typical of quartz habitus accompanied the transition of the unordered crystobalite into a-quartz. The faces of small and large rhombohedrons are formed more intensively. It may be seen from Fig. 9 that by the time the rhombohedron peaks are well-developed the central part of the particle is still an aggregation of the incompletely coalesced floccules. Even when the crystallization was completed the quartz crystals maintained structural defects caused by incomplete coalescence of floccules in the region of a
D
CT\)
0
L._ 20
/ 40
t 60
_i-80
100 -r(h)
Fig. 3. The size of the Bragg scattering zone (D) for o, crystobalite; a-quartz as a function of the duration of the hot steam treatment (7).
A,
SD-X
and 0,
297
600 400.
2ooOL-M
T
(hl
Fig. 4. The size of the Bragg scattering zone (D) and microdistortion function of the duration of the hot steam treatment.
(clto) of a-quartz
as a
prism. The voids in the crystal structure are regular in shape due to the cutting of primary particles (Fig. 10). The aggregation of quartz crystals (Fig. 10) is typical of the proposed synthesis technique. Apparently, it is caused by the nonuniformal distribution of the activator on the amorphous silicon dioxide surface. The activator contributes to the aggregation of floccules in a single structured system. When the activator is distributed nonuniformly the silicon dioxide particles splits into several independent crystallization zones with random orientation. The number of aggregates was sharply reduced when amorphous silicon dioxide with a uniform adsorption surface was used. The role of the activator TMAH in the structural transformations of silica and its interaction with silicon dioxide during the hot steam treatment were
hexagonal
Fig. 5-10. The photographs of SiO, crystallization products at various stages of the hot steam treatment made by electron microscope: Fig. 5. The starting amorphous silicon dioxide.
298
Fig. 6. Hot steam treatment for 1 h.
by EPR methods. The EPR signals were registered only after silica specimens were irradiated by y-rays of 6oCo at - 196 o C. The EPR spectrum of the y-irradiated amorphous silica with the adsorbed activator TMAH is a three-component signal (Fig. 11) with anisoeropically spread side compostudied
Fig. 7. Hot steam treatment for 3 h.
299
Fig. 9. Hot steam tr~tment
For 24 h.
Fig. 10. Hot steam treatment
for 72 h.
nents. The central component additionally splits into 7 equidistant lines. The observed .EPR spectrum was identified with the aminoalkyl radical (CH,),NCH, [6] adsorbed on the surface of amorphous silicon dioxide. After only 4 h hot steam treatment the EPR spectra changed (Fig. 12)
Fig. 11. The EPR spectrum of the (CH),NCH, radical formed during y-irradiation tetra-N-methylammonium hydroxide adsorbed on the amorphous silica surface.
of
301
Fig. 12. The EPR spectrum of radicals that formed on the SiO, tetra-N-methylammonium hydroxide surface subjected to y-irradiation after hot steam tieatment for 2 h: R, = dH,, R, = =Si-NH-CH, or I$JNCH,.
1
registering the signals of methyl radicals adsorbed on the silica surface and radicals CH,NHSi= and CH,N’SiE based on the SiO, matrix. ’ SiE When the duration of the hot steam treatment was increased, the presence of the activator TMAH or its fragments was no longer detected by EPR. Thus, during the hot steam treatment the adsorbed TMAH molecules and the fit of the fragments of TMAH molecules on the silica surface decomposed and later disappeared forming new siloxane bonds. The mass spectrometric study of the interaction products of the activator with silicon dioxide yielded additional information on the activation process of SiO, crystallization. The products of TMAH surface formations on silicon dioxide evaporating during thermal decomposition in vacua are listed in Table 1. The SiO, specimens with the adsorbed activator TMAH before the hot steam treatment (specimen 1) and after the treatment for 0.5 h (specimen 2) 2 h (specimen 3) and 16 h (specimen 4) were studied. In all cases the transition to the gas phase of the (CH,),NOH molecules heated in vacua was not observed. The flows of methanol and trimethylamine (the products of TMAH dissociative desorption) were detected on specimen 1. The secondary and primary amines formed in fragmentation of the TMAH molecule and interaction with silanol groups on the silica surface were registered on specimens subjected to the hot steam treatment. The mass spectrometric studies revealed that the compound (CH3)qNf-O--Si= is formed on the SiO, surface during the adsorption of the activator TMAH. It decomposes during the hot steam treatment: (CH,),N+OO-Sk and then (CH,),N
El(CH,),N + HO-Sk
+ CH,OSi= + (CH,),N-Si-
+ CH,OH
302 TABLE
1
Products
to be
analysed
T(OC)=200 Specimen
400 1 (before
600
1000
1200
1400
Total
1.5
0.8
0.5
90
4.3
2.1
0.6
660
*
(CH,),NOH (CH,),N
1.8
70
13
CH,OH
1.1
430
190
(CH,),NH
2.5 *
35
l
CH,NH, Specimen
2 (0.5 h of HST) *
(CH,),NOH
t
W,),N CH,OH
14
(CH,),NH
0.5
11 4.1
5
1.3
0.3
-
_
31.6
1.4
0.0
-
_
_
6
0.2
(CH,)NH, Specimen
*
3 (2 h of HST)
* *
(CH,),NOH (CH,),N CH,OH (CH,),NH
0.1
0.7
CH,NH,
.
800
HST)
0.1
0.1
0.0
0.0
0.0
0.3
0.0
0.0
0.0
1.1
traces Specimen
4 (16 h of HST)
*
(CH,),NOH (CH,),N
l
CH,OH
0
0.1
0.0
0.0
0.0
0.0
0.0
0.1
(CH,),NH
0.0 _
0.4
-
-
-
_
_
0.4
_
0.2
0.1
0.1
0.1
0.0
0.5
(CH,)NH, l
Not detected.
Some surface hydroxyl groups are substituted for amino and methoxy groups. New siloxane bonds are formed in the interaction of functional amino and silanol groups, thus stimulating the interphase contacts. The investigation proved that the activating effect of TMAH on the structuring of amorphous silicon dioxide consists in the formation of functional groups based on the SiO, matrix and in enhancing the charge interactions between the structural elements of the silicon-oxygen frame of SiO,. The activator determines the direction and rate of synthesis and the ordering of crystals. Thus the crystallization proceeds via consolidation and ordering of SiO, structural elements. Under the conditions of the hot steam treatment the mobility of structural elements is caused by the increased capacity of siloxane bonds for splitting and locking. In our opinion the mobile elements of the SiO, structure are large enough to include dozens of links of the siloxane chain with linear, branched or cyclic structures.
303 CONCLUSIONS
1. The synthesis of a-quartz and other crystalline modifications of silicon dioxide (crystobalite and SiO,-X phase) from amorphous silicon dioxide was carried out via hot steam treatment in the presence of the activating agent, tetra-IV-methylammonium hydroxide (TMAH). 2. Under conditions of hot steam treatment TMAH interacts with surface hydroxides of silicon dioxide and modifies the surface forming amino and methoxy groups. The latter interact with SiO, matrix-based hydroxyls contributing to the formation of interphase contacts. 3. The structuring of amorphous silicon dioxide proceeds by the mechanism of solid-phase crystallization via the consolidation and ordering of SiO, structural elements.
REFERENCES 1 S. Kitahara, T. Mutuie and H. Muraishi, Proceedings of the first International Symposium on Hydrothermal reactions, Japan, 1982, pp. 480-495. 2 B.M. Mitsyuk, L.J. Gorogotskaya, and A.J. Rastrenenko, Dokl. Akad. Nauk SSSR, 209 (1973) 926. 3 H. Harder and W. Flehmig, Geochim. Cosmochim. Acta, 34 (1970) 295. 4 V.B. Lazarev, G.P. Panasyuk, M.N. Danchevskaya and G.P. Budova, Advances in Inorganic Chemistry, Moscow, 1983. p. 196. 5 Patentschrift BRD DE 3317327 C2, 24.7.86. 6 P. Wardman and D.R. Smith, Can. J. Chem.. 49 (1971) 1880.
Reactivity of Solids, 5 (1988) 293-303 Elsevier Science Publishers B.V., Amsterdam
- Printed
in The Netherlands
SYNTHESIS AND INVESTIGATION OF CRYSTALLINE MODIFICATIONS OF SILICON DIOXIDE
M.N. DANCHEVSKAYA, and Yu. D. IVAKIN Chemistry Department, Moscow (U. S. S. R.) (Received
S.N. TORBIN,
G.P. MURAVIEVA,
O.G. OVCHINNIKOVA
M. V. Lomonosov State University of Moscow, 117234,
March 3rd, 1987; accepted
January
14th, 1988)
ABSTRACT The synthesis of a-quartz and other crystalline modifications of silicon dioxide (crystobalite and SiOz-X phase) from amorphous silicon dioxide was carried out via hot steam treatment in the presence of the activating agent, tetra-N-methylammonium hydroxide (TMAH), in an amount of 0.2 wt.-% SiO,. The mass spectrometric study and the electron paramagnetic resonance (EPR) study lead us to the conclusion, that under conditions of hot steam treatment TMAH interacts with surface hydroxides of silicon dioxide and modifies the surface forming amino and methoxy groups. The latter interact with SiO, matrix-based hydroxyls contributing to the formation of interphase contacts. The electron microscopic study and X-ray analysis show, that the structuring of amorphous silicon dioxide proceeds by the mechanism of solid phase crystallization via the consolidation and ordering of SiO, structural elements.
INTRODUCTION
The crystallization of amorphous silicon dioxide, its multiple crystalline modifications each of which is of practical and theoretical importance have been the subject of growing interest. The crystallization of amorphous silicon dioxide is usually carried out under hydrothermal conditions in aqueous solutions of alkali and alkaline-earth metal hydroxides [l-3]. The limitations of the method are obvious: it is impossible to control this process when the hydroxides of alkali metals are used as activators since the selection of metal activators is limited and their action is one-sided. Moreover, the crystalline modifications of silicon dioxide in this case are poorly structured and polluted by silicates and metal carbonates. Therefore it seems that the crystallization via solid-phase transformation of silicon dioxide in water vapour and in the presence of surfactants is more advantageous [4,5]. 0168-7336/88/$03.50
0 1988 Elsevier
Science Publishers
B.V.
294
The introduction of surfactants of varying nature enables the process of silicon dioxide structuring to be controlled. The present article gives the results of the investigation of silicon dioxide crystallization using tetra-N-methylammonium hydroxide as an activating agent.
EXPERIMENTAL
The activator, tetra-N-methylammonium hydroxide (TMAH), was adsorbed from an aqueous solution in an amount of 0.2 wt.-% on the surface of amorphous silicon dioxide of “OSC 8-4” type with a 98 m2/g surface. The hot steam treatment was conducted at 400°C and steam pressure p 200 atm. The specimens subjected to the hot steam treatment of different duration were studied by X-ray, electron microscope, electron paramagnetic resonance (EPR) and mass spectrometry methods. The X-ray phase analysis of the products at various stages of SiO, crystallization was carried out on a diffractometer DRON-2 (Cu-K, radiation). a-quartz added to the specimen in an amount of 20 wt.-% was taken as an internal standard. The thin crystalline structure of newly formed SiO, modifications was studied using Fourier analysis of the line shapes. The zones of Bragg scattering (D) and microdistortions of the crystal lattice ((e*):(t) were determined. The structural morphological peculiarities of the crystalline modifications of silicon dioxide were studied using scanning electron microscopes. The electron microscopes Hitachi-520 and JELL 35 CF were used. A thin layer of gold was deposited on the specimen surface. The magnification level is given on the photographs. The interaction of the activating agent (tetra-N-methylammonium hydroxide (TMAH)) with silicon dioxide during the hot steam treatment and its role in the structuring of the silicon dioxide were studied by EPR and mass spectrometry. The EPR spectra were registered in X-diapason (h 3.2 cm) on the radiospectrometer RE-1306 and RS Varian E-109 within the the specimens temperature range - 196 -+ + 200 o C. Prior to measurements were irradiated with y-rays of 6oCo at - 196 o C. The radiation dose was 5 Mrad. The mass spectrometric studies were conducted on the mass spectrometer MI-1311 with a resolution of 500 and sensitivity to the analysed products of 10’3-10’4 molecules V’ s- ‘. The working vacuum was (l-5) X lo-’ torr. The flows of substances evaporating from silica specimens in vacua were analysed within the temperature range of lOO-1400°C under products conditions of “the open surface”. Thus, the thermal decomposition of the compounds formed in the interaction of the activator TMAH and silicon dioxide during adsorption and hot steam treatment were studied.
295 RESULTS
AND DISCUSSION
The effect of the hot steam treatment with and without TMAH being compared, the decisive role of the activator in the structural transformations of amorphous silicon dioxide becomes obvious. Without the activator even the prolonged steam treatment (72 h) does not initiate transformations of amorphous silica into a crystalline structure. The only effect is the consolidation of structure without further ordering. The structuring stops at the flocculation stage. The specimen is X-ray amorphous. The introduction of 0.01-0.2 wt.-% of TMAH into the system changes the process. The diffractograms of SiO, specimens after the hot steam treatment from 2-72 h in the presence of 0.2% of TMAH are given in Fig. 1. The degree of crystallization grows with time. Both main modifications, crystobalite and SiO,-X phase, are formed simultaneously; small quantities of keatite were also registered. The maximum yield of SiO,-X phase and crystobalite occurred at 12 and 14 h respectively (Fig. 2). Simultaneously lines of a-quartz appeared on the diffractogram (Fig. 1). Further changes in the concentration of crystobalite, SiO,-X phase and a-quartz (Fig. 2) testify to the fact that the final product, a-quartz, is formed from SiO,-X phase and crystobalite. The process of SiO, crystallization and phase transformations initiate monotonous ordering of the system and reduction in the number of defects in the crystal lattice. When the Bragg stattering zone is reached, the transition of SiO,-X phase crystals of D 500 A into a-quartz starts. The transition of crystobalite into a-quartz occurs at lower values of D (160 A). After the transformation of both modifications into a-quartz was completed the size of the a-quartz crystals remained unchanged in spite of the increased duration of the hot steam treatment. The size of blocks (D) was continuously growing as the
Fig. 1. Diffractograms of SiO, crystallization products after hot steam treatment durations: l-2 h; 2-4 h; 3-8; 4-22 h; 5-29 h; 6-72 h.
of different
296
Fig. 2. Kinetics o crystobalite;
of the formation of SO, crystalline SO,-X; 0, a-quartz.
modifications
from the amorphous
state:
A,
microdefects of the crystal lattice diminished (Fig. 4). The metamorphic transformations in the system and peculiarities of crystal structure of SiO, modifications starting with the moment of their initiation were revealed by electron microscopic investigation. The starting amorphous silicon dioxide had a large-globular structure (Fig. 5). After 1 h of the hot steam treatment in the presence of 0.2% of TMAH the structure consolidates causing the appearance of large pores (Fig. 6) and later, floccules (Fig. 7). After 4 h of treatment the initiation of the fibrous structure of SiO,-X phase (Fig. 8, left) and fine-crystalline crystobalite (Fig. 8, right) can be observed. The transition of SiO,-X into a-quartz by the aggregation of similarly oriented SiO,-X crystals occurs after 10 h hot steam treatment. The floccules consist of fragments of unordered crystobalite aggregate. The formation of faces typical of quartz habitus accompanied the transition of the unordered crystobalite into a-quartz. The faces of small and large rhombohedrons are formed more intensively. It may be seen from Fig. 9 that by the time the rhombohedron peaks are well-developed the central part of the particle is still an aggregation of the incompletely coalesced floccules. Even when the crystallization was completed the quartz crystals maintained structural defects caused by incomplete coalescence of floccules in the region of a
D
CT\)
0
L._ 20
/ 40
t 60
_i-80
100 -r(h)
Fig. 3. The size of the Bragg scattering zone (D) for o, crystobalite; a-quartz as a function of the duration of the hot steam treatment (7).
A,
SD-X
and 0,
297
600 400.
2ooOL-M
T
(hl
Fig. 4. The size of the Bragg scattering zone (D) and microdistortion function of the duration of the hot steam treatment.
(clto) of a-quartz
as a
prism. The voids in the crystal structure are regular in shape due to the cutting of primary particles (Fig. 10). The aggregation of quartz crystals (Fig. 10) is typical of the proposed synthesis technique. Apparently, it is caused by the nonuniformal distribution of the activator on the amorphous silicon dioxide surface. The activator contributes to the aggregation of floccules in a single structured system. When the activator is distributed nonuniformly the silicon dioxide particles splits into several independent crystallization zones with random orientation. The number of aggregates was sharply reduced when amorphous silicon dioxide with a uniform adsorption surface was used. The role of the activator TMAH in the structural transformations of silica and its interaction with silicon dioxide during the hot steam treatment were
hexagonal
Fig. 5-10. The photographs of SiO, crystallization products at various stages of the hot steam treatment made by electron microscope: Fig. 5. The starting amorphous silicon dioxide.
298
Fig. 6. Hot steam treatment for 1 h.
by EPR methods. The EPR signals were registered only after silica specimens were irradiated by y-rays of 6oCo at - 196 o C. The EPR spectrum of the y-irradiated amorphous silica with the adsorbed activator TMAH is a three-component signal (Fig. 11) with anisoeropically spread side compostudied
Fig. 7. Hot steam treatment for 3 h.
299
Fig. 9. Hot steam tr~tment
For 24 h.
Fig. 10. Hot steam treatment
for 72 h.
nents. The central component additionally splits into 7 equidistant lines. The observed .EPR spectrum was identified with the aminoalkyl radical (CH,),NCH, [6] adsorbed on the surface of amorphous silicon dioxide. After only 4 h hot steam treatment the EPR spectra changed (Fig. 12)
Fig. 11. The EPR spectrum of the (CH),NCH, radical formed during y-irradiation tetra-N-methylammonium hydroxide adsorbed on the amorphous silica surface.
of
301
Fig. 12. The EPR spectrum of radicals that formed on the SiO, tetra-N-methylammonium hydroxide surface subjected to y-irradiation after hot steam tieatment for 2 h: R, = dH,, R, = =Si-NH-CH, or I$JNCH,.
1
registering the signals of methyl radicals adsorbed on the silica surface and radicals CH,NHSi= and CH,N’SiE based on the SiO, matrix. ’ SiE When the duration of the hot steam treatment was increased, the presence of the activator TMAH or its fragments was no longer detected by EPR. Thus, during the hot steam treatment the adsorbed TMAH molecules and the fit of the fragments of TMAH molecules on the silica surface decomposed and later disappeared forming new siloxane bonds. The mass spectrometric study of the interaction products of the activator with silicon dioxide yielded additional information on the activation process of SiO, crystallization. The products of TMAH surface formations on silicon dioxide evaporating during thermal decomposition in vacua are listed in Table 1. The SiO, specimens with the adsorbed activator TMAH before the hot steam treatment (specimen 1) and after the treatment for 0.5 h (specimen 2) 2 h (specimen 3) and 16 h (specimen 4) were studied. In all cases the transition to the gas phase of the (CH,),NOH molecules heated in vacua was not observed. The flows of methanol and trimethylamine (the products of TMAH dissociative desorption) were detected on specimen 1. The secondary and primary amines formed in fragmentation of the TMAH molecule and interaction with silanol groups on the silica surface were registered on specimens subjected to the hot steam treatment. The mass spectrometric studies revealed that the compound (CH3)qNf-O--Si= is formed on the SiO, surface during the adsorption of the activator TMAH. It decomposes during the hot steam treatment: (CH,),N+OO-Sk and then (CH,),N
El(CH,),N + HO-Sk
+ CH,OSi= + (CH,),N-Si-
+ CH,OH
302 TABLE
1
Products
to be
analysed
T(OC)=200 Specimen
400 1 (before
600
1000
1200
1400
Total
1.5
0.8
0.5
90
4.3
2.1
0.6
660
*
(CH,),NOH (CH,),N
1.8
70
13
CH,OH
1.1
430
190
(CH,),NH
2.5 *
35
l
CH,NH, Specimen
2 (0.5 h of HST) *
(CH,),NOH
t
W,),N CH,OH
14
(CH,),NH
0.5
11 4.1
5
1.3
0.3
-
_
31.6
1.4
0.0
-
_
_
6
0.2
(CH,)NH, Specimen
*
3 (2 h of HST)
* *
(CH,),NOH (CH,),N CH,OH (CH,),NH
0.1
0.7
CH,NH,
.
800
HST)
0.1
0.1
0.0
0.0
0.0
0.3
0.0
0.0
0.0
1.1
traces Specimen
4 (16 h of HST)
*
(CH,),NOH (CH,),N
l
CH,OH
0
0.1
0.0
0.0
0.0
0.0
0.0
0.1
(CH,),NH
0.0 _
0.4
-
-
-
_
_
0.4
_
0.2
0.1
0.1
0.1
0.0
0.5
(CH,)NH, l
Not detected.
Some surface hydroxyl groups are substituted for amino and methoxy groups. New siloxane bonds are formed in the interaction of functional amino and silanol groups, thus stimulating the interphase contacts. The investigation proved that the activating effect of TMAH on the structuring of amorphous silicon dioxide consists in the formation of functional groups based on the SiO, matrix and in enhancing the charge interactions between the structural elements of the silicon-oxygen frame of SiO,. The activator determines the direction and rate of synthesis and the ordering of crystals. Thus the crystallization proceeds via consolidation and ordering of SiO, structural elements. Under the conditions of the hot steam treatment the mobility of structural elements is caused by the increased capacity of siloxane bonds for splitting and locking. In our opinion the mobile elements of the SiO, structure are large enough to include dozens of links of the siloxane chain with linear, branched or cyclic structures.
303 CONCLUSIONS
1. The synthesis of a-quartz and other crystalline modifications of silicon dioxide (crystobalite and SiO,-X phase) from amorphous silicon dioxide was carried out via hot steam treatment in the presence of the activating agent, tetra-IV-methylammonium hydroxide (TMAH). 2. Under conditions of hot steam treatment TMAH interacts with surface hydroxides of silicon dioxide and modifies the surface forming amino and methoxy groups. The latter interact with SiO, matrix-based hydroxyls contributing to the formation of interphase contacts. 3. The structuring of amorphous silicon dioxide proceeds by the mechanism of solid-phase crystallization via the consolidation and ordering of SiO, structural elements.
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