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Chemistry of Materials 1989, 1, 554-558
Dimethyldiethoxysilane/Tetraethoxysilane Copolymers: Precursors for the Si-C-0 System Florence Babonneau,* Kevin Thorne, and J. D. Mackenzie Department of Materials Science and Engineering, University of California at Los Angeles, Los Angeles, California 90024 Received April 24, 1989
Polymeric organosilicon gels have been prepared from dimethyldiethoxysilane and tetraethoxysilane. The hydrolysis-condensation reactions were investigated by using '%i NMR and infrared spectroscopies, which showed that a copolymerization between the two precursors occurred. These copolpers have potential for use as precursors for silicon oxycarbides. The pyrolysis process was followed by Si MAS NMR and infrared spectroscopy. It can be divided into two stages: between 300 and 1000 "C, the methyl groups are consumed and the polymer is converted to a silica matrix containing carbon. %SiMAS NMR spectra reveal the presence of Si units such as SiCzOzand SiCOs that clearly show that Si-C bonds remain up to 1000 OC. Above 1300 "C, a carbothermal reduction occurs and crystallinesilicon carbide starts to appear. Introduction The preparation of ceramics via solution routes is currently being widely investigated.' Two main processes are under study: the sol-gel route to mainly oxide ceramics and the polymer route to obtain nonoxide ceramics such as carbides and nitrides. The sol-gel approach can lead to the fabrication of monolithic pieces, fibers, or coatings, while the polymer approach has been generally applied to the production of fibers. Much of the work related to silicon carbide preparation has used the polymer route, following the pioneering work of Yajima? the ceramic precursor in this case is a poly(carbosilane), which is derived by thermolysis of poly(dimethylsilane). The main advantage of this precursor is to allow the production of fibers. Little work has been targeted toward the fabrication of monolithic ceramic bodies. For this purpose, sol-gel methods appear quite suitable. Silicon alkoxides have already been used with an outside source of carbon to produce silicon carbide by carbothermal reduction of ~ i l i c a .Trifunctional ~ alkoxysilanes in which the carbon source is internal also appear to be convenient precursors to prepare silicon ~ a r b i d e . ~ However, it seems that the application of these precursors to the preparation of monolithic ceramics has not been reported. In the present work, dimethyldiethoxysilane (DEDMS) has been investigated as a precursor for Si-C-0 systems. It was already known that hydrolysis of DEDMS does not lead to the formation of gels since this precursor easily forms small rings during the hydrolysis-condensation p r o c e s ~ . Consequently, ~ DEDMS was mixed with tetraethoxysilane (TEOS) to achieve a cross-linked network structure.6 Transparent monolithic gels have been prepared that can be considered as part of a new family of hybrid materials incorporating organic groups into a sol-gel oxide network.'^^ These monolithic pieces retain their shape during the firing process, and silicon oxycarbides can be obtained at around 900 "C. The presence of Si-C bonds in the network has been revealed by ?3i MAS NMR experiments. Oxycarbide glasses are currently under investigation since the substitution of some oxygen atoms by carbon atoms in the glass network should improve their mechanical proper tie^.^ Such improvements have already been found in oxynitride glasses.lOJ1 Pyrolysis of the samples a t 1500 "C can lead to the formation of silicon *To whom correspondence should be sent at Chimie de la Matisre CondensBe, Universitd Paris 6,4 place Jussieu 75005 Paris, France. 0897-4756/89/2801-0554$01.50/0
carbide. At this temperature, the material can be described as a mixture of silicon carbide, silica, and carbon. The relative amount of these three components is greatly influenced by the conditions of the heat treatment. Thus, it appears quite important to study the transformation of the precursor in the pyrolyzed products to obtain better control of the process. This paper presents a structural investigation of (1)the hydrolysis-condensation process of the precursors and (2) the pyrolysis of the gels, mainly using infrared and 29SiNMR spectroscopies. Experimental Section
Most of this study was done on the system corresponding to
a DEDMS:TEOS:EtOH:H,O ratio of 1:1:4:6. In this case, 50.4 g of DEDMS (Alfa) and 67.3 g of TEOS (Alfa) are stirred together for 15 min. A mixture of water (34.9 g) and ethanol (59.5 g) is
added, and the solution is stirred overnight to obtain a homogeneous clear solution. The solution was aged in an opened Petri dish, and the gelation occured after several weeks. In this general procedure, no catalyst was used. Monolithic transparent pieces of gel were dried at 80 "C and then heat treated. The heat treatments were usually done under nitrogen flow. It will be specified in the text when argon was used. The chemical analysis was performed on the dried gel by Galbraith Laboratories for Si, C, and H. The oxygen was estimated by difference. The result gave SiC0.8301,43H2,56. For the experiment done in acidic medium, the water was acidified with HCl (HC1/HzO= 2 X lo4) before mixing with DEDMS and TEOS. A series of samples was prepared according to the same general procedure for infrared in(1) Ultrastructure Processing of Advanced Ceramics; Mackenzie, J. D., Ulrich, D. R., Eds.; Wiley: New York, 1988. (2)Yajima, S.;Hasegawa, Y.; Hayashi, J . Iimura, M. J. Mater. Sci. 1978,13, 2569. Hasegawa, Y.; Iimura, M.; Yijima, S. Ibid. 1980,15,720. Hasegawa, Y.; Okamura, K. Ibid. 1983, 18,3633. (3)Wei, G.C.; Kennedy, C. R.; Harris, L. A. Am. Ceram. SOC.Bull. 1984, 63, 1054. (4) White, D.A.; Oleff, S. M.; Boyer, R. D.; Budinger, P. A.; Fox, J. R. Adu. Ceram. Mater. 1987,2,45. White, D. A.; Oleff, S. M.; Fox, J. R.
Ibid. 1987, 2, 53. (5) Sakka, S.; Tanaka, Y.; Kokubo, T. J. Non-Cryst. Solids 1986,82, 24. (6)Chen, K. C.;Thorne, K. J.; Chemseddine, A.; Babonneau, F.; Mackenzie, J. D.Mater. Res. Soc. Symp. Proc. 1988,121,571. (7) Huang, H. H.; Orler, B.; Wilkes, G. L. Macromolecules 1987,20, 1322. (8) Schmidt, H.; Seiferling, B. Mater. Res. Soc. Symp. Proc. 1986, 73, 739. (9)Homeny, J.; Nelson, G. G.; Risbud, S. H. J.Am. Ceram. SOC.1988, 71,386. (10)Loehman, R. E.J. Non-Cryst. Solids 1983,56, 123. (11) Coon, D. N., Rapp, J. G.; Bradt, R. C.; Pantano, C. G. J. NonCryst. Solids 1983, 56, 161.
0 1989 American Chemical Society
Chemistry of Materials, Vol. 1, No.5, 1989 555
DimethyldiethoxysilanelTetraethoxysilane Copolymers vestigation. The different DEDMS:TEOSH20EtOHratios were 1:2124,1:1:124,21:124, and 2.5:1:124. %Siliquid NMR spectra were recorded on an AM 360 Bruker spectrometer at a frequency of 71.5 MHz. A pulse width of 10 ps was applied with a relaxation delay of 6 s. The number of scans varies from 16 to 100. %Si MAS NMR spectra were obtained on an MSL 400 Bruker spectrometer at a frequency of 79.5 MHz. A pulse width of 3 ws and a relaxation delay of 100 s were used. The number of scans was between 500 and 1000. TMS was used as a reference for all the NMR experiments. Infrared spectra were recorded in transmission mode on a Perkin-Elmer 1330 spectrometer in the 4000-200-cm-' range. The KBr pellet technique was used. Thermogravimetric analysis was performed using Perkin-Elmer TGS2 equipment. X-ray diffraction patterns were recorded on a Phillips diffractometer.
Results and Discussion A goal of this study was to determine whether the hydrolysis-condensation process of a solution of DEDMS/ TEOS leads to the formation of copolymers between the two structural units, D units ((CH3)2Si(Oo.s)2) from DEDMS and Q units (Si(00.s)4)from TEOS. The hydrolysis-condensationprocess of a solution of DEDMS has been investigated by =Si NMR to get NMR reference data concerning chains of D units. The spectrum of pure DEDMS exhibits one peak at -4.9 ppm. After 8 days, the spectrum of the hydrolyzed solution (H20/Si = 6) presents two peaks, an intense one at -12.5 ppm and a less intense peak a t -3.1 ppm (Figure la). After 2 months, the spectrum recorded on the same solution still shows these two peaks (Figure lb) with an observed decrease in the intensity of the peak at -3.1 ppm and the formation of a new peak at -21.3 ppm. If DEDMS is hydrolyzed with less water, H20/Si = 2, the spectrum after 6 days shows the two peaks at -12.1 and -21.3 ppm in roughly a 1:2 ratio. The signal at -3.1 ppm has disappeared. The assignment of these different peaks follows earlier reports.'"14 During the hydrolysis process, two kinds of reactions may occur: hydrolysis: (CH3)2Si(OEt)2+ H 2 0
-
(CH,),Si(OEt)(OH) + EtOH (1) (CH,),Si(OEt)(OH) + H 2 0 (CH3)2Si(OH)2 + EtOH (2) condensation: (CH,),(OH)Si-OH + HO-Si(OH)(CH,), (CH3)2(OH)Si-O-Si(OH)(CH,)2 + H 2 0 (3)
V
20
o
-20
-40
-60
-80
-100
-120
(PpNI)
Figure 1. %SiNMR spectra of a hydrolyzed solution of DEDMS (DEDMS:EtOH:H20= 1:46): (a) after 8 days; (b)after 2 months,
v, TMS.
-
-
(CH3)2(00.s)Si-OH+ HO-Si(OH)(CH,),
-
(CH3)2(Oo.s)Si-O-Si(OH)(CH3)2 + H 2 0 (4)
Published data on organosilicon polymers12 show that the D units in MoHD,MoH oligomers (MoH: (CH3)2Si(OH)(OO.d)exhibit chemical shifts ranging from -20 to -22 ppm, while MoH units range from -11.1 (n = 0) to -12.3 ppm (n > 5). Thus, the two peaks at -12.1 and -21.3 ppm can be respectively assigned to MoHand to D units. The assignment of the peak at -3.1 ppm is more uncertain. It presents a shift of 1.8 ppm downfield as compared to pure DEDMS. Considering that the hydrolysis of one ethoxy group of TEOS lead to a 2 ppm downfield shift of the resonance peak,15it can be reasonably proposed that the (12) Harris, R. K.; Robins, M. L. Polymer 1978,19, 1123. (13) Engelhardt, G.; Jancke, H. Polym. Bull. 1981,5, 577. (14) Williams, E. A.; Cargioli, J. D.; Larochelle, R. W. J. Organomet. Chem. 1987,108, 345. (15) Pouxviel, J. C.; Boilot, J. P.; Beloeil, J. C.; Lallemand, J. Y. J. Non-Cryst. Solids 1987,89, 345.
20
0
-20
-40
-60
-80
-100
-120
PPM)
Figure 2. %SiNMR spectra of a hydrolyzed solution of DEDMS and TEOS (DEDMS:TEOS:H20:EtOH= 1:1:6:4): (a) after 16 h; (b) after 10 days; V, TMS.
peak at -3.1 ppm indicates the existence of hydrolyzed species such as (CH,),Si(OEt)(OH) or (CH3)2Si(OH)2. The hydrolysis-condensation process seems to follow reactions 1-4. The chemical shift of the peak due to (CH3),Si(OH)(O0.,)units corresponds to short MoHD,MoH chains (n < 5). However, the presence of small rings must also be considered. TEOS was mixed with DEDMS to achieve a three-dimensional extended gel-network structure. Many studies have been carried out on the hydrolysis of TEOS ming =Si NMR.1"17 Si units are usually referred as Qj units, where j represents the number of briding oxygens surrounding the silicon atoms. The chemical shifts exhibit the following ranges: -72 to -82 ppm for Qo units, -82 to -89 ppm for (16) Turner, C. W.; Franklin, K. J. J. Non-Cryst. Solids 1987,91,402. (17) Lin, C. C.; Basil, J. D. Mater. Res. SOC. Symp. Proc. 1986, 73,585.
556 Chemistry of Materials, Vol. 1, No. 5, 1989
Babonneau et al.
"
0
-80
-40
-120
(PPM)
Figure 3. 29SiNMR spectrum of a solution of DEDMS/TEOS hydrolyzed under acidic conditions (DEDMS:TEOS:EtOH:HzO = 1:1:4:6; HC1/H20 = 2 X lo4); V, TMS.
Q1 units, -92 to -96 ppm for Q2 units, -100 to -104 ppm for Q3 units, and around -110 ppm for Q4 units. The hydrolysis of a solution of DEDMS/TEOS in ethanol (DEDMS:TEOS:EtOH:H20 = 1:1:4:6) was followed by NMR (Figure 2). Before hydrolysis, the spectrum of a mixture of DEDMS/TEOS in a 1:l ratio showed two peaks at -4.9 and -82.0 ppm corresponding to the two precursors, DEDMS and TEOS, respectively. This solution was hydrolyzed, and after 16 h, two peaks at -3.1 and -12.0 ppm were present in the D unit region, due to hydrolyzed species, (CH3)2Si(OH)2, and condensed species, (CH3)2Si(OH)(00.5), respectively. No peak corresponding to pure DEDMS was present. Two peaks also appeared in the Q unit region at -82.0 and -72.4 ppm, corresponding respectively to Si(OEt), and Si(OH)& At this stage, the hydrolysis of TEOS had just started, according to the relative intensity of the two peaks. After 10 days, the D unit region still exhibited a peak at -12.0 ppm due to (CH3)2Si(OH)(0)o.s species. The (CH3)2Si(OH)2species had all been consumed. Several peaks at -16, -19, and -21 ppm appeared. In the hydrolysis study of a solution of DEDMS, no peaks between -16 and -20 ppm were observed. The assignment of these peaks will be discussed later. In the Q unit region, peaks were visible in the Q1 region, at -81.7 and -83.9 ppm, in the Q2 region at -90.8, -91.7, and -93.3 ppm, and in the Q3 region at around -102 ppm. Peaks correspondingto Q ,. units could appear around -110 ppm, but they would be hidden by the broad band due to the presence of silica in the probe and in the tube. The peaks at -81.7 and -83.9 ppm can be assigned to (OH),(OEt)&3i(O),, species with x = 3 and n = 2, respectively. The peak at -81.7 ppm could also be due to TEOS, but this assumption is less probable since no peaks due to monomeric hydrolyzed species are present in the NMR spectrum. The peaks at -91.7 and -93.3 ppm could correspond to Q2 units present in linear species such as (OH)2Si(Oo.5)2 and (OEt)(OH)Si(O,,),. The peak at -90.8 ppm could be due to cyclic species. The peaks due to Q3 units such as (OH)Si(Oo.s)3and (OEt)Si(O,,), species near -102 ppm are not easily visible because of the presence of a broad band due to silica. To increase the hydrolysis-condensation rates, the hydrolysis of a DEDMS/TEOS solution was performed in acidic conditions (DEDMS:TEOS:EtOH:H20 = 1:1:4:6; HCl/H20 = 2 X lo4). The %i NMR spectrum has been recorded just before gelation and should represent the final stage of evolution of the solution in the liquid state (Figure 3). It exhibits two broad peaks corresponding to Q2 and Q3 units, at -91.6 and -101.1 ppm. The most interesting features of this spectrum are the peaks at -15.8, -17.6,
2Ooo
lsoo
1200
800
400
WAVENUMBERS (cm.1)
Figure 4. Infrared spectra of a hydrolyzed solution of DEDMS (a) and dried gels prepared with various x = DEDMS'IEOS ratios (b) x = 2.5, (c) x = 2, (d) x = 1, (e) x = 0.5, (0 x = 0. -19.1, and -21.5 ppm. They are the only peaks left in the D unit region. Are these peaks due to chains or rings of D units or to copolymers between D and Q units? As previously mentioned, no peaks were present between -15 and -20 ppm in the spectrum of the hydrolyzed solution of DEDMS. Much work has been published on %i NMR studies of oligomeric and polymeric siloxanes. According to these results, chains of D units exhibit characteristic chemical shifts ranging from -20 to -22 ppm.12-14 As far as the rings are concerned, the chemical shift range is from -20 to -23 ppm, except for the ring with 3 D units (6 = -9.2 ppm).'* Thus three of the four peaks in this region, i.e., those at -15.8, -17.6, and -19.1 ppm, should be assigned to copolymers between D and Q units. It is known that a change in the nature of the first and even of the second neighboring atom of a given Si atom can lead to a shift of several ppm of the resonance peak.lg These peaks can, for example, correspond to the presence of QDD, QDQ, or DDD sequences. A definite assignment of the peaks is difficult to achieve. However, it seems clear that during the condensation process of TEOS and DEDMS, copolymers between D and Q units have been formed. To confirm this copolymerization, different gels were prepared with various DEDMS/TEOS ratios. Infrared spectra of the dried gels are presented in Figure 4. The spectrum of a hydrolyzed solution of DEDMS is shown in Figure 4a, and that of a silica gel prepared from TEOS is shown in Figure 4f. The spectrum of hydrolyzed DEDMS ) , 1095 and 1015 ( v s i a ) , presents bands at 1260 ( v ~ i x ~ , at at 860 and 800 (vSi-(CH9)J, and at 390 cm-' (6si-o-s:). The infrared spectrum of sihca gel exhibits bands at 1080 ( v s ~ ) , (18) Engelhardt, G.; Jancke, H.; Magi, M.; Pehk, T.;Lippmaa, E. J. .~ Organomet. Chem. 1971,28, 293. (19) Harris, R. K.; Kimber, B. J. J. Organomet. Chem. 1974, 70, 43.
DimethyldiethoxysilaneITetraethoxysilane Copolymers
Chemistry of Materials, Vol. 1, No.5, 1989 557
700°C
80 400
200
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TEMPERATURE ("C)
Figure 5. TGA under argon flow of a dried gel (DEDMS:TEOS = 1:l). Rate: 20 OC/min.
950 (vSiaH), and 800 and 460 cm-' (8simi). In the infrared spectra of the dried gels prepared from DEDMS and TEOS, bands due to D and Q units are present. Two bands are of particular interest. The band at 860 cm-' (vSi(CHs)n),present in the hydrolyzed solution of DEDMS, shifts to 845 cm-' and increases in intensity in the mixed gels. According to the literature,2oblocks of D units show a relatively weak band at 860 cm-'. But in many copolymers containing D units (not in a block), this band shifts to 845 cm-' and becomes stronger. It seems clear that copolymers between D and Q units are present in the gels. Furthermore, the band corresponding to the angular deformation 8Si+si, shifts from 400 to 440 cm-' as the DEDMS/TEOS ratio varies from 2.5 to 0.5. Blocks of D units show a band at 390 cm-' while blocks of Q units exhibit a band at 460 cm-l. In the mixed gels, only one band appears between those two values. This band was never resolved into two components. It can be assumed to be due to copolymers between D and Q units. The dried gel prepared from a 1:l DEDMS/TEOS ratio has been characterized by '%i MAS NMR (Figure 6). Two peaks are present at -19 and -110 ppm characteristic of D and Q units. The peak due to D units does not exhibit any discernible structure. The peak due to Q units at -110 ppm is characteristic of Q4units. A shoulder is present near -100 ppm and could be due to Q3 units. The ratio between the two peaks is roughly 1:l and corresponds to the initial ratio in solution. The theoretical chemical formula of the gel should be SiC01.5H3and is close to the experimental formula (SiC0.8301.43H2.56). TGA was performed on the dried gel from room temperature to 1000 "C under flowing argon (Figure 5). Two weight losses clearly appear, from 300 to 450 "C (5%) and from 450 to 600 "C (7%). The total weight loss represents 14.5%. To examine the chemical reactions occurring in the fired gels, the pyrolysis process was followed by infrared6 and '?3i MAS NMR spectroscopies up to 1500 "C. The infrared spectrum of the gel that was heat treated at 300 "C is essentially the same as the spectrum of the dried gel. At 500 "C, the intensities of the bands corresponding to Si-CH3 groups at 1275,845, and at 800 cm-' have decreased. At 700 "C, few CH3groups are still bound to silicon. At 1550 "C, the spectrum shows a band characteristic of Sic near 900 cm-', together with a band due to Si02 at around 1100 cm-l. (20) Lamer, P. J. Silicon Compounds;Petrach Systems Inc.: 1984;
p 11.
100
-100
0
-200
( PPM )
Figure 6. %i MAS NMR spectra of a dried gel (DEDMSTEOS = 1:l)pyrolyzed at various temperatures for 12 h under flowing argon.
0
400
800
1200
1600
TEMPERATURE ("C) Figure 7. Evolution of the percentage of the different Si units with firing temperature: 8 , D units; E, T units; S i c 4 units.
+, Q units, 0,
29SiMAS NMR spectra were recorded for gels fired at various temperatures from 300 to 1500 "C (Figure 6). A t 300 "C, the spectrum is similar to that of the dried gel with two peaks at -19 and -110 ppm due to D and Q units. A t 500 "C, the peak due to the D units has decreased, while a new peak at -64 ppm is present. This peak can be assigned to T units,21where Si atoms are surrounded by one carbon atom and three oxygen atoms. At 700 "C, the peak due to the D units has disappeared and the peak due to T units has started to decrease. A t 1000 "C, Si atoms are mainly surrounded by four oxygen atoms. At 1300 "C, a small peak around -10 ppm appears, characteristic of Si atoms surrounded by four carbon atoms. This peak is stronger at 1500 "C and could indicate the formation of silicon ~ a r b i d e . ~The ~ - ~different ~ peaks have been inte(21) Engelhardt, G.; Jancke, H.; Lippmaa, E.; Samoson, A. J. O g a nomet. Chem. 1981,210, 295. ( 2 2 ) Hartman, J. S.; Richardson, M. F.; Sheriff, B. L.; Winsborrow,B. G. J. Am. Chem. SOC.1987,109,6059. (23)Finlay, G. R.; Hartman, J. S.; Richardson, M. F.; Williams, B. L. J. Chem. SOC.,Chem. Commun.1985, 159.
558
Chemistry of Materials 1989,1, 558-563
grated and the variation of the area during the pyrolysis process is represented in Figure 7. According to TGA, infrared and MAS NMR, the gel is thermally stable up to 300 "C. The first stages of the pyrolysis process from 300 to 1000 "C correspond to the consumption of the CH, groups and the conversion of the D units into T and then Q units. The sample can be described as a matrix of silica containing carbon, with at least some carbon atoms bound to silicon as revealed by NMR. At temperatures beyond 1300 "C, the carbothermal reduction of silica starts and silicon carbide is formed. As already noted in the l i t e r a t ~ r ethe , ~ heating rate should be quite low during the pyrolysis process to prevent significant weight losses at high temperatures. A gel was pyrolyzed for 1 h at 1500 "C under an argon flow (purity: 99.999%) by using a heating rate of 2 "C/min. The starting sample was a monolithic piece of dried gel, while the pyrolyzed material appeared as a powder. An X-ray diffraction pattern obtained from this material exhibited sharp peaks corresponding to /3-SiC.26A small amount of a-Sic was also evidenced. No peaks due to Si02 were present. If the same starting material is pyrolyzed by using argcn gas with a lower purity (99.9%), the monolithic piece retains its shape but X-ray diffraction measurements reveal the presence of a mixture of crystalline Sic and Si02 in the sample. It seems that silica has to be present in the material to obtain monolithic pyrolyzed pieces. The starting gel has an oxide-based network, and it seems that this network has to be retained to keep a monolithic piece.
Conclusion The preparation of new precursors for the Si-C-0 system using the sol-gel route has been investigated. The application of the hydrolysis-condensation process to a mixture of tetraethoxysilane and dimethyldiethoxysilane leads to the formation of monolithic pieces. A structural investigation performed by 29SiNMR has evidenced the formation of copolymers between the two precursors. Solid-state NMR also confirmed the presence of two kinds of units in the dried gels, D and Q units. '%i MAS NMR proved to be a powerful technique for following the evolution of the local environment of the Si units during the pyrolysis process. A cleavage of the S i 4 bonds and their conversion into Si-0 bonds start above 300 "C. The presence of Si-C bonds in the silica network remains up to 900 "C. The material can be considered as a silicon oxycarbide glass. At 1000 "C, the transformation of Si-C bonds into Si-0 bonds is completed, and the material can be described as a mixture of silica and carbon. Above 1300 "C, Si-C bonds are formed again by carbothermal reduction of the silica matrix, and at 1500 "C, crystalline silicon carbide is obtained. These copolymers between a tetraalkoxysilane and a dialkyldialkoxysilane appear to be attractive precursors for the preparation of silicon oxycarbide glasses and silicon carbide at higher temperatures. The ratio C/O can be modified according to the respective nature of the alkyl and alkoxy groups. Furthermore, monolithic pieces can be prepared that retain their shape as long as a silicon oxide or oxycarbide network is present.
(24)Inkrott, K.; Wharry, S. M.; O'Donnell, D. J. Mater. Res. SOC. Symp. h o c . 1986,73, 165. (25)Powder Diffraction File; JCPDS 29 1129,1979.
Acknowledgment. We greatly acknowledge Jocelyne Maquet for recording the MAS NMR spectra. Registry No. DEDMS, 7862-6; TEOS, 7810-4; Sic4,409-21-2.
Solution/Gelation of Arsenic Trisulfide in Amine Solvents Theresa A. Guiton and Carlo G. Pantano* Department of Materials Science and Engineering, Pennsylvania State University, University Park, Pennsylvania 16802 Received June 8, 1989
Arsenicsulfur solutions were synthesized by dissolving amorphous arsenic trisulfide (a-As#d in anhydrous amine solvents. Ethylenediamineprovided the highest solubility for As2S3and led to optimum gelation behavior. Thus, it was selected as the model system for this study. The emphasis of the study was the identification of the active polymerizing species in the As&-ethylenediamine solutions. The molecular transformations that occurred during the sol-to-gel and gel-to-glass transitions of fibers prepared from the As2S3-ethylenediaminesolutions were also examined and correlated to the proposed solution species. Introduction Many of the non-oxide amorphous chalcogenides are readily synthesized in bulk by melt quenching.' These glasses can then be used to fabricate bulk optics or to draw fiber-optic wave guides. In the case of thin films, vapor deposition has widely been applied.2 However, the ultimate effect of such a high-temperaturepreparation method upon the structure, and thus properties, of the thin films cannot be ignored.,-" For example, mass spectroscopic (1) Hartouni. E.: Hulderman, F.: Guiton. T. SPZE h o c . 1984,505. 131-140. (2)Keneman, S.A. Thin Solid Films 1974,21,281-285. (3) DeNeufville, J. P.; Moss,S. C.; Ovshinsky, S. R. J. Non-Cryst. S o l t d ~1973/1974,13,191-223. (4)Lu, C . S.;Donohue, J. J. Am. Chem. SOC.1944,66,818.
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and Raman analyses have identified and As4S45-7 species in the vapor phase of a - w , . Furthermore, reporta indicate the presence of these and other discrete molecular species in the as-deposited Due to such inho(5) Leadbetter, A. J.; Apling, A. J.; Daniel, M. F. J. Non-Cryst. Solids 1976,21,47. (6)Apling, A. J.; Leadbetter, A. J.; Wright, A. C. J.Non-Cryst. Solids 1977,23,369. (7) Solin, S. A.; Papatheodorou, G. N. Phys. Reu. 1977, B15, 2084-2090. (8)Takahasi, T.;Harada, Y. Solid State Commun. 1980,35, 191. (9)Zallen, R. The Physics of Amorphous Solids; Wiley: New York, 1983;pp 94-96. (10)Wright, A. C.;Sinclair, R. N.; Leadbetter, A. J. J. Non-Cryst. Solids 1985,71,295-302. (11)Onari, S.;Asai, K.; Arai, T. J. Non-Cryst. Solids 1985, 76, 243-251.
0 1989 American Chemical Society
