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Gel synthesis of glass powders in the BaOAl2O3SiO2 system
278
Journal of Non-Crystalline Solids 100 (1988) 278-283 North-Holland, Amsterdam
GEL S Y N T H E S I S OF G L A S S P O W D E R S IN THE BaO-AI203-SiO 2 SYSTEM
William K. TREDWAY United Technologies Research Center, East Hartford, CT 06108, USA
Subhash H. RISBUD Department of Materials Science and Engineering The University of Arizona, Tucson, A Z 85721, USA
Homogeneous glass powders were prepared from multicomponent silicate gels in the BaO-AI203-SiO 2 system. Gels synthesized from TEOS, Al-sec-butoxide, and Ba-acetate were characterized by TGA, DSC, and single-point BET analysis. Amorphous oxide powders of controlled composition were obtained through an appropriate drying and heat treatment schedule. Inclusion-free clear oxynitride glasses were also prepared by co-melting gel synthesized oxide powders with silicon nitride powders.
1. Introduction
The use of sol-gel technology in the preparation of ceramic materials has increased at a phenomenal rate over the past several years. The characteristics imparted to chemically prepared powders and monoliths (e.g., high reactivity, extreme purity, ultrahomogeneity) make them especially suitable for a number of applications where traditional ceramic processes are not adequate, one of which is the preparation of high temperature glasses. Homogeneous glasses in the BaO-A1203-SiO 2 system near the composition of celsian (BaSi2A1208) can be particularly difficult to prepare due to the high temperatures required (> 1600°C). By chemically preparing the glass batch powder via a sol-gel technique, however, it is possible to obtain glasses of good quality at reduced temperatures. This paper describes the chemical preparation of amorphous glass batch powders in the BaO-A1203-SiO 2 system for the purpose of using them in the synthesis of Ba-Si-A1-O-N oxynitride glasses of good quality [1].
2. Sol-gel glasses Gel formation through the use of metal alkoxides relies on the hydrolysis and subsequent 0022-3093/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
condensation of the hydrolyzed alkoxides. The hydrolysis and condensation reactions that occur in single and multi-component silicate gels based on tetraethylorthosilicate (TEOS) as well as the influence of such factors as solution pH, H20/TEOS ratio, and drying conditions on gel structure and quality have all been well documented in the literature [2-5] and hence will not be covered here. Likewise, the reported applications of the sol-gel process in the preparation of glasses and glass-ceramics are too numerous and the reader is referred to an excellent recent review by Zelinski and Uhlmann [4]. The emerging potential of sol-gel processing in glass, ceramic, and coating technologies has been recently reported by Uhlmann and co-workers [11-13]. Perhaps the earliest work on the use of gel mixtures for homogeneous glass preparation is due to studies on phase equilibria in ceramics by Roy and co-workers [6-8]. These studies showed that clear glasses could be formed from the gel-derived powders at temperatures 100°C to 200°C below those required using conventional batch materials in a shorter amount of time. The only drawback to the method was that one or more of the nitrates used in gel preparation often crystallized during drying, thus destroying the homogeneity gained by stirring the mixture in solution. By keeping the pH low and allowing evaporation of water to occur rapidly before gelation, however,
IV.K. Tredway, S.H. Risbud / Gel synthesis of glass powders
the amount of crystallization could be kept to a minimum. Several recent investigations have shown that it is possible to produce glasses in alkaline earth aluminosilicate systems using alkoxides of silicon and aluminum and a soluble salt of the alkaline earth metal. For example Pancrazi et al. [9] studied gel formation in the CaO-A1203-SiO 2 system. In preparing the gel, it was found that calcium acetate could not be used since its relative insolubility in alcohol resulted in precipitation of calcium acetate even when added as a water solution. Consequently, calcium had to be added as a nitrate in an alcohol solution. No detectable crystallization of calcium nitrate occurred during drying of the gel. Holand et al. [10] studied the preparation of gels in the MgO-AI203-SiO 2 system. In this study, magnesium was introduced into the system as a water solution of magnesium acetate. No report of any precipitation or crystallization of magnesium acetate during gelation and desiccation was noted. Bulk gels were converted to clear glass samples at a temperature of 700 o C.
3. Experimental procedure 3.1. Gel synthesis The precursor materials used to prepare the o x i d e b a t c h e s were t e t r a e t h y l o r t h o s i l i c a t e ( T E O S ) *, aluminum-sec-butoxide **, and barium acetate **. All reactions were carried out under constant stirring conditions in a threenecked flask fitted with a reflux condenser. Fig. 1 shows a flow chart of the chemical preparation procedure. In step 1, a dilute solution of TEOS and ethanol (1 : 10 molar ratio) was prepared. Partial hydrolysis of the TEOS was then initiated by adding deionized water (] theoretical) to this solution and raising the temperature to - 60 o C. A few drops of 1.0 N HC1 were added to catalyze the hydrolysis. After several hours, the solution was allowed to cool to room temperature. Step 2 consisted of * F i s h e r Scientific Co., F a i r L a w n , N J . * * Alfa Products, Danvers, MA.
279
60°C, 3 hours J
TEOS/Ethanol/Water (1:10:1 molar ratio) 1.0 N HCI
Reduce to ambient
"[
Add AI-s-butoxide/Isopropanol ( 1: 10 molar ratio)
12 hours at ambient
Pour solution into covered Petri dishes; Gelation soon occurs
A
Stirring for 10-15 minutes
Add Ba-acetate/Water/ Acetic acid
Slow drying for 2 weeks followed by rapid drying
Dried ge~ containing residual organics
I ]
Crushed I I Calcined in air I at 750°C
Homogeneous oxide glass batch; Highly reactive; X-ray amorphous
Fig. 1. Flow diagram of the chemical preparation procedure. slowly adding a dilute solution of Al-sec-butoxide and isopropanol ( 1 : 1 0 molar ratio) to the partially hydrolyzed T E O S / e t h a n o l mixture. This solution was allowed to react for - 12 h to ensure complete reaction. In step 3, the barium acetate was dissolved in deionized water (4.75 times the amount theoretically required to hydrolyze the TEOS). Acetic acid was added to the barium a c e t a t e / w a t e r solution until the p H was lowered to - 3 - 4 , after which the barium acetate/water solution was slowly added to the TEOS/Al-sec-butoxide mixture. Any resulting Ba acetate precipitate was taken back into solution with the addition of more acetic acid. This complete solution was then stirred for 10-15 min or until the solution became slightly translucent, indicating the onset of gelation. At this point the solution was cast into covered Petri dishes. Complete gelation normally occurred within 30 min of casting. The gels were allowed to dry slowly in the covered Petri dishes for 10-14 days. These slow drying conditions were maintained so that most of the alkoxyl groups (C2H5, C4H9) could react and subsequently evaporate as alcohols. The covers were then removed to permit more rapid drying. The dried gel was crushed with a mortar and pestle to break up the larger pieces and then ball milled using ZrO 2 balls in a plastic container to
280
W.K. Tredway, S.H. Risbud / Gel synthesis of glass powders
reduce the gel to a fine powder. The powdered gels were then converted to oxide glass batches by slowly heating them at 5 ° C / m i n in air up to a temperature of - 7 5 0 ° C and calcining them for -12h.
3.2. Characterization of gels The nature and temperatures of the various reactions that occur during heating o f the gels (e.g., volatilization, pyrolysis) were determined using differential scanning calorimetry * (DSC) and thermogravimetric analysis * (TGA). Powders of each sample were subjected to DSC and T G A runs in air at a heating rate of 10 ° C / r a i n from room temperature up to either 600 ° C (DSC) or 750 ° C (TGA). The specific surface area of the gels was determined using single-point BET analysis ** with N 2 as the adsorbate and helium as the inert carrier gas. Powdered samples (-100 mesh) were heated at - 1 5 0 ° C under vacuum for several hours prior to analysis in order to remove any volatiles from the surface. The specific surface area was taken as the average of the two runs which were made for each sample. The amorphous nature of the gels before and after calcining was ascertained using an automated X-ray diffractometer t. Powders of each sample (-100 mesh) were mounted on glass slides and scanned from 20 = 10 ° to 70 ° at a rate of 6 ° 2 0 / m i n using C u K a radiation.
4. Results and discussion
4.1. Gel synthesis Initial attempts at producing good quality gels in the BaO-AI203-SiO 2 system were unsuccessful. The major difficulty encountered was the precipitation of Ba-acetate when it was added as a water solution to the T E O S / A l - s e c - b u t o x i d e / * 1090 Thermal Analysis System, E.I. duPont de Nemours & Co., Wilmington, DE. * * Monosorb Surface Area Analyzer, Quantachrome Corp., Syosset, NY. t Model APD 3520, Phillips Electronic Instruments, Mahwah, NJ.
alcohol mixture. This problem, which is due to the relative insolubility of Ba-acetate in alcohol, was resolved by adding acid to the mixture until Baacetate was taken back into solution. This corresponded to a solution p H of approximately 4-6. The reason for the increased solubility of Baacetate at lower pH values is not entirely clear. A separate experiment showed that Ba-acetate was not soluble in pure ethanol even at low p H values, while it was soluble in an equivolume mixture of ethanol and water with the addition of enough acid to lower the p H to - 4 . 5 . This seemed to imply that water (in which Ba-acetate is soluble) was required in order for Ba-acetate to go into solution in the presence of alcohols. The solubility of Ba-acetate apparently increases at p H values below 6. The first acid used was 1.0 N HCI, which was very effective in clearing up the solution. However, significant amounts of C1- ions were retained in the calcined gels, which was not advantageous since this would in turn lead to the presence of C1- in any glasses made from these gels. A gel in which HC1 was used was found to contain - 1 wt% C1- after calcining in air at 950 o C. Besides failing to remove the CI- from the gels, this high temperature heat treatment also resulted in sintering of the gels and a subsequent loss in reactivity of the gel-derived powders. This was very undesirable, since high reactivity was one of the essential requirements of the gel-derived oxide powders to be used for oxynitride glass synthesis. In an attempt to find an acid which could be easily removed during calcining of the gel while also effectively preventing precipitation, acetic acid (HC2H302) was used. Much larger quantities of acetic acid were required due to its relatively weak acidic nature (compared to HC1), but it was quite effective in increasing the solubility of Ba-acetate in the T E O S / A l - s e c - b u t o x i d e / a l c o h o l mixture. Taking this into consideration along with the ease with which the acetyl groups could be removed via heat treatment, acetic acid was used in the processing of all the gels.
4.2. Thermoanalysis Thermal analysis was used to characterize the reactions that took place during the conversion of
281
W.K. Tredway, S.H. Risbud / Gel synthesis of glass powders
12C
2.0 lO0°C I /%
11c
1.6
/ \
g
o
._~
70
~
-
9
-
60
-0.4 I 100
I
I 200
I
~ J I 500 400 Temperoture
I
/ 500 (°C)
I
i 600
I
I 700
I
800
Fig. 2. TGA trace of the gel along with the derivative curve (dashed) showing the transition temperatures. the "acetic acid" gels to oxide powders. T G A of the gels revealed three principal temperature regimes where weight loss occurred, these regions corresponding to temperatures of - 1 0 0 ° C , 230°C, and - 4 1 5 - 4 3 5 ° C (fig. 2). From the derivative curve, it appears that the high temperature weight loss is actually the result of two separate reactions, while the two lower temperature weight losses seem to be attributable to single reactions. Above - 6 0 0 ° C, all of the significant reactions have taken place and no more weight loss occurs. DSC was also run on the gels to determine the nature of the reactions responsible for the weight loss at each temperature. Endothermic reactions are normally due to a phase change involving an increase in free energy, such as evaporation or volatilization of a hquid to its vapor phase. Exothermic reactions, on the other hand, are the result of reactions involving the evolution of energy, such as crystallization or pyrolysis. From the DSC trace (fig. 3) it was found that the first two reactions were endothermic, while the two reactions occurring at - 415-435 ° C were exothermic. The presence of two separate reactions in the high temperature regime (as suggested by the T G A data) was confirmed in the DSC trace, indicated by the "shoulder" on the largest peak (at 435°C). Taking all of the thermal analysis data into consideration, it was determined that the weight
loss at - 1 0 0 ° C was due to the evaporation of residual water and alcohols in the gel, while the weight loss that took place at - 230-270 ° C was probably the result of the volatilization of some species formed as a result of the reactions taking place during the sol-gel process. Several different types of reactions can take place which involve the acids and alcohols in the system, producing such organic species as ethers and esters. It is likely that some species of this type was formed during the processing of the gel and that this species volatilizes in the temperature range of 230-270 ° C. The exothermic reactions occurring at 415-435 ° C were probably due to the pyrolysis of residual organic groups in the gel (e.g., C2H5, C4H9, C2H302) which had not reacted by the end of the drying step and consequently did not evaporate from the gel prior to calcining. In order to test the thermal analysis results, one sample of gel was heated in air up to 300°C, while another sample was heated in air to a temperature of 500 ° C. The gel heated to 300 ° C was still white in color; the sample subjected to the higher temperature heat treatment, however, had turned gray. This gray color, which was due to the presence of residual carbon in the calcined gel, confirmed that the reactions occurring at 4 1 5 - 4 3 5 ° C were indeed pyrolysis reactions. The white color of the gel heated to 300 ° C agreed well with the idea that volatilization was responsible for the weight loss at lower temperatures.
40
l
l
l
~
l
415°C I
50
U_ 1 0
a 85°C -10
I 0
I 1(30
225 °C I
I ~ I 200 500 Temperature
i
I 400 (°C)
~
I 500
i 600
Fig. 3. DSC curve of the gel with the transition temperatures indicated next to the curve.
W..K. Tredway, S.H. Risbud / Gel synthesis of glass powders
282 I
i
I
200
150
8 loc
5C
I
I
]
200 40D 600 CGIcining Temperoture (°C)
800
Fig. 4. BET surface area of the gel as a function of temperature.
In the development of a heat treatment schedule to convert the gels to oxide powders, several factors were considered: (1) the heating rate had to be slow enough to allow the pyrolysis reactions to take place prior to closure of the gel pores; (2) the soak temperature needed to be high enough to enable the residual carbon to oxidize in a reasonable length of time; (3) the soak temperature could not be so high that the gel would begin to sinter appreciably, since this would result in a loss of reactivity. From the T G A data, it was known that all the weight loss had taken place prior to reaching temperatures of 500-600°C. This established a practical lower limit to the soak temperature. 4. 3. Surface area and chemical analysis In order to determine the upper temperature limit, the specific surface area of the gels was measured as a function of calcining temperature. Fig. 4 shows the variation of BET surface area with calcining temperature for one of the gels used in this investigation. The specific surface area was found to increase on heating up to - 250-300 ° C, followed by a decrease with further heating up to 750 o C. This agreed well with the thermal analysis data. The occurrence of volatilization reactions up
to a temperature of - 270 ° C leads to an increase in surface area due to the creation of more porosity, with further heating causing shrinkage of the pores and a general decrease in surface area. The plateau region around 4 0 0 - 5 0 0 ° C can be attributed to additional porosity created during the pyrolysis reactions. The upper limit to the heat treatment temperature was set at 750 ° C since this retained a fairly high degree of reactivity in the calcined gel (BET surface area of - 5 0 m2/g) while also enabling the residual carbon to oxidize at a reasonable rate. The final heat treatment schedule used to calcine the gels, then, consisted of heating at a rate of - 5 o C / m i n . up to 750 ° C in air and soaking for 12 h. This generally produced oxide powders which were white in color, (i.e., free of excess carbon), X-ray amorphous, and highly reactive. The only problem encountered arose when the gels were dried too rapidly during the drying step of the gel preparation procedure. This rapid drying led to closed porosity in the gels which in turn resulted in carbon being trapped in the gels after pyrolysis. The closed pores severely hindered oxidation of the carbon even at temperatures above 750 ° C, resulting in gray-colored powders containing carbon. Slow drying of the gels was essential in order to maintain open porosity, which thereby enabled the volatiles to escape and the carbon to oxidize. Chemical analysis of the calcined gels showed that very good composition control was afforded by the sol-gel process (table 1). The analyzed and theoretical values are in fairly good agreement for all the constituents. The analyzed A1203 content seems to be somewhat higher than the theoretical value. This is due to the fact that the organometallic precursor for A1203, Al-sec-butoxide, contained solvents in the amount of - 5 % . In an
Table 1 Chemical analysis of a calcined gel Constituent
Analyzed wt%
Theoretical wt%
BaO SiO 2 A1203
42.5 43.7 13.8
42.6 45.0 12.4
W.K. Tredway, S.H. Risbud / Gel synthesis of glass powders
283
Table 2 Batched and analyzed oxynitride glass compositions Constituent
BaO SiO 2 A1203 Si 3N4 at.% N wt% N H (as O H - )
Glass A
Glass A-3
Theo. wt%
Theo. wt%
Anal. wt%
Glass A-6 Theo. wt%
Anal. wt%
42.6 45.0 12.4 -
43.0 41.5 12.5 3.0 2.5 1.2
43.3 40.1 13.6 3.0 2.5 1.17
43.3 38.1 12.6 6.0 5.1 2.4
43.7 37.7 14.0 4.6 3.7 1.76
17 ppm
attempt to correct for this, a slight excess of Al-sec-butoxide was undoubtedly added. The analyzed SiO2 content is also somewhat lower than the theoretical value. It is possible that some of the TEOS could have evaporated during processing. Although refluxing conditions were used throughout the majority of the procedure, the system was open to the atmosphere during transfer of the solution to the Petri dishes.
4.4. Oxynitride glasses from gel batches Gel derived oxide powders (-100 mesh) were mixed with electronic grade Si3N 4 powders (-325 mesh) and melted in Mo crucibles at 1550-1600°C for 30 min as detailed in our previous publications [14-16]. Table 2 shows the analyzed and batched glass compositions. The overall agreement between batched and analyzed values for the oxynitride glass compositions (A-3 and A-6) is quite good. It is specially noteworthy that the amount of N loss ( - 23%) is lower in the present gel derived oxynitride glasses than in those prepared by conventional melting of batches [14]. The microstructure and crystallization behavior of gel derived versus conventional oxynitride glasses has been investigated [17] and will be reported subsequently. Support for this work was provided by the Ceramics and Electronic Materials Program of the Division for Materials Research at NSF under Grant DMR 85-14324. W.K. Tredway also
199 ppm
acknowledges an IBM Pre-doctoral Fellowship Award.
References [1] W.K. Tredway and S.H. Risbud, Mater. Lett. 3 (1985) 435. [2] C.J. Brinker and G.W. Scherer, J. Non-Cryst. Solids 70 (1984) 301. [3] S.P. Mukherjee, J. Non-Cryst. Solids 42 (1980) 477. [4] B.J.J. Zelinski and D.R. Uhlmann, J. Phys. Chem. Solids 45 (1984) 1069. [5] B.E. Yoldas, J. Mater. Sci. 14 (1979) 1843. [6] R. Roy, J. Am. Ceram. Soc. 39 (1956) 145. {7] R. Roy, J. Am. Ceram. Soc. 52 (1969) 344. [8] G.J. McCarthy and R. Roy, J. Am. Ceram. Soc. 54 (1971) 639. [9] F. Pancrazi, J. Phalippou, F. Sorrentino and J. Zarzycki, J. Non-Cryst. Solids 63 (1984) 81. [10] W. Holand, E.R. Plumat and P.H. Duvigneaud, J. NonCryst. Solids 48 (1982) 205. [11] D.R. Uhlmann, B.J.J. Zelinski and G.E. Wnek, in: Better ceramics through chemistry (MRS, Pittsburgh, 1984) p. 59. [12] D.R. Uhlmann, in: Ceramics: today and tomorrow, eds. S. Naka, N. Soga and S. Kume (Ceram. Soc. Japan, Tokyo, 1986) p. 51. [13] D.R. Uhlmann and G.P. Rajendran, Proc. 3rd Conf. on Ultrastructure Processing (Wiley, New York, 1987). [14] W.K. Tredway and S.H. Risbud, J. Amer. Ceram. Soc. 66 (1983) 324. [15] L.M. Bagaasen and S.H. Risbud, J. Amer. Ceram. Soc. 66 (1983) C29. [16] P.E. Jankowski and S.H. Risbud, J. Mater. Sci. 18 (1983) 2087. [17] W.K. Tredway, PhD Thesis, University of Illinois, Urbana-Champaign (1986).
Journal of Non-Crystalline Solids 100 (1988) 278-283 North-Holland, Amsterdam
GEL S Y N T H E S I S OF G L A S S P O W D E R S IN THE BaO-AI203-SiO 2 SYSTEM
William K. TREDWAY United Technologies Research Center, East Hartford, CT 06108, USA
Subhash H. RISBUD Department of Materials Science and Engineering The University of Arizona, Tucson, A Z 85721, USA
Homogeneous glass powders were prepared from multicomponent silicate gels in the BaO-AI203-SiO 2 system. Gels synthesized from TEOS, Al-sec-butoxide, and Ba-acetate were characterized by TGA, DSC, and single-point BET analysis. Amorphous oxide powders of controlled composition were obtained through an appropriate drying and heat treatment schedule. Inclusion-free clear oxynitride glasses were also prepared by co-melting gel synthesized oxide powders with silicon nitride powders.
1. Introduction
The use of sol-gel technology in the preparation of ceramic materials has increased at a phenomenal rate over the past several years. The characteristics imparted to chemically prepared powders and monoliths (e.g., high reactivity, extreme purity, ultrahomogeneity) make them especially suitable for a number of applications where traditional ceramic processes are not adequate, one of which is the preparation of high temperature glasses. Homogeneous glasses in the BaO-A1203-SiO 2 system near the composition of celsian (BaSi2A1208) can be particularly difficult to prepare due to the high temperatures required (> 1600°C). By chemically preparing the glass batch powder via a sol-gel technique, however, it is possible to obtain glasses of good quality at reduced temperatures. This paper describes the chemical preparation of amorphous glass batch powders in the BaO-A1203-SiO 2 system for the purpose of using them in the synthesis of Ba-Si-A1-O-N oxynitride glasses of good quality [1].
2. Sol-gel glasses Gel formation through the use of metal alkoxides relies on the hydrolysis and subsequent 0022-3093/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
condensation of the hydrolyzed alkoxides. The hydrolysis and condensation reactions that occur in single and multi-component silicate gels based on tetraethylorthosilicate (TEOS) as well as the influence of such factors as solution pH, H20/TEOS ratio, and drying conditions on gel structure and quality have all been well documented in the literature [2-5] and hence will not be covered here. Likewise, the reported applications of the sol-gel process in the preparation of glasses and glass-ceramics are too numerous and the reader is referred to an excellent recent review by Zelinski and Uhlmann [4]. The emerging potential of sol-gel processing in glass, ceramic, and coating technologies has been recently reported by Uhlmann and co-workers [11-13]. Perhaps the earliest work on the use of gel mixtures for homogeneous glass preparation is due to studies on phase equilibria in ceramics by Roy and co-workers [6-8]. These studies showed that clear glasses could be formed from the gel-derived powders at temperatures 100°C to 200°C below those required using conventional batch materials in a shorter amount of time. The only drawback to the method was that one or more of the nitrates used in gel preparation often crystallized during drying, thus destroying the homogeneity gained by stirring the mixture in solution. By keeping the pH low and allowing evaporation of water to occur rapidly before gelation, however,
IV.K. Tredway, S.H. Risbud / Gel synthesis of glass powders
the amount of crystallization could be kept to a minimum. Several recent investigations have shown that it is possible to produce glasses in alkaline earth aluminosilicate systems using alkoxides of silicon and aluminum and a soluble salt of the alkaline earth metal. For example Pancrazi et al. [9] studied gel formation in the CaO-A1203-SiO 2 system. In preparing the gel, it was found that calcium acetate could not be used since its relative insolubility in alcohol resulted in precipitation of calcium acetate even when added as a water solution. Consequently, calcium had to be added as a nitrate in an alcohol solution. No detectable crystallization of calcium nitrate occurred during drying of the gel. Holand et al. [10] studied the preparation of gels in the MgO-AI203-SiO 2 system. In this study, magnesium was introduced into the system as a water solution of magnesium acetate. No report of any precipitation or crystallization of magnesium acetate during gelation and desiccation was noted. Bulk gels were converted to clear glass samples at a temperature of 700 o C.
3. Experimental procedure 3.1. Gel synthesis The precursor materials used to prepare the o x i d e b a t c h e s were t e t r a e t h y l o r t h o s i l i c a t e ( T E O S ) *, aluminum-sec-butoxide **, and barium acetate **. All reactions were carried out under constant stirring conditions in a threenecked flask fitted with a reflux condenser. Fig. 1 shows a flow chart of the chemical preparation procedure. In step 1, a dilute solution of TEOS and ethanol (1 : 10 molar ratio) was prepared. Partial hydrolysis of the TEOS was then initiated by adding deionized water (] theoretical) to this solution and raising the temperature to - 60 o C. A few drops of 1.0 N HC1 were added to catalyze the hydrolysis. After several hours, the solution was allowed to cool to room temperature. Step 2 consisted of * F i s h e r Scientific Co., F a i r L a w n , N J . * * Alfa Products, Danvers, MA.
279
60°C, 3 hours J
TEOS/Ethanol/Water (1:10:1 molar ratio) 1.0 N HCI
Reduce to ambient
"[
Add AI-s-butoxide/Isopropanol ( 1: 10 molar ratio)
12 hours at ambient
Pour solution into covered Petri dishes; Gelation soon occurs
A
Stirring for 10-15 minutes
Add Ba-acetate/Water/ Acetic acid
Slow drying for 2 weeks followed by rapid drying
Dried ge~ containing residual organics
I ]
Crushed I I Calcined in air I at 750°C
Homogeneous oxide glass batch; Highly reactive; X-ray amorphous
Fig. 1. Flow diagram of the chemical preparation procedure. slowly adding a dilute solution of Al-sec-butoxide and isopropanol ( 1 : 1 0 molar ratio) to the partially hydrolyzed T E O S / e t h a n o l mixture. This solution was allowed to react for - 12 h to ensure complete reaction. In step 3, the barium acetate was dissolved in deionized water (4.75 times the amount theoretically required to hydrolyze the TEOS). Acetic acid was added to the barium a c e t a t e / w a t e r solution until the p H was lowered to - 3 - 4 , after which the barium acetate/water solution was slowly added to the TEOS/Al-sec-butoxide mixture. Any resulting Ba acetate precipitate was taken back into solution with the addition of more acetic acid. This complete solution was then stirred for 10-15 min or until the solution became slightly translucent, indicating the onset of gelation. At this point the solution was cast into covered Petri dishes. Complete gelation normally occurred within 30 min of casting. The gels were allowed to dry slowly in the covered Petri dishes for 10-14 days. These slow drying conditions were maintained so that most of the alkoxyl groups (C2H5, C4H9) could react and subsequently evaporate as alcohols. The covers were then removed to permit more rapid drying. The dried gel was crushed with a mortar and pestle to break up the larger pieces and then ball milled using ZrO 2 balls in a plastic container to
280
W.K. Tredway, S.H. Risbud / Gel synthesis of glass powders
reduce the gel to a fine powder. The powdered gels were then converted to oxide glass batches by slowly heating them at 5 ° C / m i n in air up to a temperature of - 7 5 0 ° C and calcining them for -12h.
3.2. Characterization of gels The nature and temperatures of the various reactions that occur during heating o f the gels (e.g., volatilization, pyrolysis) were determined using differential scanning calorimetry * (DSC) and thermogravimetric analysis * (TGA). Powders of each sample were subjected to DSC and T G A runs in air at a heating rate of 10 ° C / r a i n from room temperature up to either 600 ° C (DSC) or 750 ° C (TGA). The specific surface area of the gels was determined using single-point BET analysis ** with N 2 as the adsorbate and helium as the inert carrier gas. Powdered samples (-100 mesh) were heated at - 1 5 0 ° C under vacuum for several hours prior to analysis in order to remove any volatiles from the surface. The specific surface area was taken as the average of the two runs which were made for each sample. The amorphous nature of the gels before and after calcining was ascertained using an automated X-ray diffractometer t. Powders of each sample (-100 mesh) were mounted on glass slides and scanned from 20 = 10 ° to 70 ° at a rate of 6 ° 2 0 / m i n using C u K a radiation.
4. Results and discussion
4.1. Gel synthesis Initial attempts at producing good quality gels in the BaO-AI203-SiO 2 system were unsuccessful. The major difficulty encountered was the precipitation of Ba-acetate when it was added as a water solution to the T E O S / A l - s e c - b u t o x i d e / * 1090 Thermal Analysis System, E.I. duPont de Nemours & Co., Wilmington, DE. * * Monosorb Surface Area Analyzer, Quantachrome Corp., Syosset, NY. t Model APD 3520, Phillips Electronic Instruments, Mahwah, NJ.
alcohol mixture. This problem, which is due to the relative insolubility of Ba-acetate in alcohol, was resolved by adding acid to the mixture until Baacetate was taken back into solution. This corresponded to a solution p H of approximately 4-6. The reason for the increased solubility of Baacetate at lower pH values is not entirely clear. A separate experiment showed that Ba-acetate was not soluble in pure ethanol even at low p H values, while it was soluble in an equivolume mixture of ethanol and water with the addition of enough acid to lower the p H to - 4 . 5 . This seemed to imply that water (in which Ba-acetate is soluble) was required in order for Ba-acetate to go into solution in the presence of alcohols. The solubility of Ba-acetate apparently increases at p H values below 6. The first acid used was 1.0 N HCI, which was very effective in clearing up the solution. However, significant amounts of C1- ions were retained in the calcined gels, which was not advantageous since this would in turn lead to the presence of C1- in any glasses made from these gels. A gel in which HC1 was used was found to contain - 1 wt% C1- after calcining in air at 950 o C. Besides failing to remove the CI- from the gels, this high temperature heat treatment also resulted in sintering of the gels and a subsequent loss in reactivity of the gel-derived powders. This was very undesirable, since high reactivity was one of the essential requirements of the gel-derived oxide powders to be used for oxynitride glass synthesis. In an attempt to find an acid which could be easily removed during calcining of the gel while also effectively preventing precipitation, acetic acid (HC2H302) was used. Much larger quantities of acetic acid were required due to its relatively weak acidic nature (compared to HC1), but it was quite effective in increasing the solubility of Ba-acetate in the T E O S / A l - s e c - b u t o x i d e / a l c o h o l mixture. Taking this into consideration along with the ease with which the acetyl groups could be removed via heat treatment, acetic acid was used in the processing of all the gels.
4.2. Thermoanalysis Thermal analysis was used to characterize the reactions that took place during the conversion of
281
W.K. Tredway, S.H. Risbud / Gel synthesis of glass powders
12C
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-
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I
I 200
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I
/ 500 (°C)
I
i 600
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I 700
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Fig. 2. TGA trace of the gel along with the derivative curve (dashed) showing the transition temperatures. the "acetic acid" gels to oxide powders. T G A of the gels revealed three principal temperature regimes where weight loss occurred, these regions corresponding to temperatures of - 1 0 0 ° C , 230°C, and - 4 1 5 - 4 3 5 ° C (fig. 2). From the derivative curve, it appears that the high temperature weight loss is actually the result of two separate reactions, while the two lower temperature weight losses seem to be attributable to single reactions. Above - 6 0 0 ° C, all of the significant reactions have taken place and no more weight loss occurs. DSC was also run on the gels to determine the nature of the reactions responsible for the weight loss at each temperature. Endothermic reactions are normally due to a phase change involving an increase in free energy, such as evaporation or volatilization of a hquid to its vapor phase. Exothermic reactions, on the other hand, are the result of reactions involving the evolution of energy, such as crystallization or pyrolysis. From the DSC trace (fig. 3) it was found that the first two reactions were endothermic, while the two reactions occurring at - 415-435 ° C were exothermic. The presence of two separate reactions in the high temperature regime (as suggested by the T G A data) was confirmed in the DSC trace, indicated by the "shoulder" on the largest peak (at 435°C). Taking all of the thermal analysis data into consideration, it was determined that the weight
loss at - 1 0 0 ° C was due to the evaporation of residual water and alcohols in the gel, while the weight loss that took place at - 230-270 ° C was probably the result of the volatilization of some species formed as a result of the reactions taking place during the sol-gel process. Several different types of reactions can take place which involve the acids and alcohols in the system, producing such organic species as ethers and esters. It is likely that some species of this type was formed during the processing of the gel and that this species volatilizes in the temperature range of 230-270 ° C. The exothermic reactions occurring at 415-435 ° C were probably due to the pyrolysis of residual organic groups in the gel (e.g., C2H5, C4H9, C2H302) which had not reacted by the end of the drying step and consequently did not evaporate from the gel prior to calcining. In order to test the thermal analysis results, one sample of gel was heated in air up to 300°C, while another sample was heated in air to a temperature of 500 ° C. The gel heated to 300 ° C was still white in color; the sample subjected to the higher temperature heat treatment, however, had turned gray. This gray color, which was due to the presence of residual carbon in the calcined gel, confirmed that the reactions occurring at 4 1 5 - 4 3 5 ° C were indeed pyrolysis reactions. The white color of the gel heated to 300 ° C agreed well with the idea that volatilization was responsible for the weight loss at lower temperatures.
40
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Fig. 3. DSC curve of the gel with the transition temperatures indicated next to the curve.
W..K. Tredway, S.H. Risbud / Gel synthesis of glass powders
282 I
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150
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Fig. 4. BET surface area of the gel as a function of temperature.
In the development of a heat treatment schedule to convert the gels to oxide powders, several factors were considered: (1) the heating rate had to be slow enough to allow the pyrolysis reactions to take place prior to closure of the gel pores; (2) the soak temperature needed to be high enough to enable the residual carbon to oxidize in a reasonable length of time; (3) the soak temperature could not be so high that the gel would begin to sinter appreciably, since this would result in a loss of reactivity. From the T G A data, it was known that all the weight loss had taken place prior to reaching temperatures of 500-600°C. This established a practical lower limit to the soak temperature. 4. 3. Surface area and chemical analysis In order to determine the upper temperature limit, the specific surface area of the gels was measured as a function of calcining temperature. Fig. 4 shows the variation of BET surface area with calcining temperature for one of the gels used in this investigation. The specific surface area was found to increase on heating up to - 250-300 ° C, followed by a decrease with further heating up to 750 o C. This agreed well with the thermal analysis data. The occurrence of volatilization reactions up
to a temperature of - 270 ° C leads to an increase in surface area due to the creation of more porosity, with further heating causing shrinkage of the pores and a general decrease in surface area. The plateau region around 4 0 0 - 5 0 0 ° C can be attributed to additional porosity created during the pyrolysis reactions. The upper limit to the heat treatment temperature was set at 750 ° C since this retained a fairly high degree of reactivity in the calcined gel (BET surface area of - 5 0 m2/g) while also enabling the residual carbon to oxidize at a reasonable rate. The final heat treatment schedule used to calcine the gels, then, consisted of heating at a rate of - 5 o C / m i n . up to 750 ° C in air and soaking for 12 h. This generally produced oxide powders which were white in color, (i.e., free of excess carbon), X-ray amorphous, and highly reactive. The only problem encountered arose when the gels were dried too rapidly during the drying step of the gel preparation procedure. This rapid drying led to closed porosity in the gels which in turn resulted in carbon being trapped in the gels after pyrolysis. The closed pores severely hindered oxidation of the carbon even at temperatures above 750 ° C, resulting in gray-colored powders containing carbon. Slow drying of the gels was essential in order to maintain open porosity, which thereby enabled the volatiles to escape and the carbon to oxidize. Chemical analysis of the calcined gels showed that very good composition control was afforded by the sol-gel process (table 1). The analyzed and theoretical values are in fairly good agreement for all the constituents. The analyzed A1203 content seems to be somewhat higher than the theoretical value. This is due to the fact that the organometallic precursor for A1203, Al-sec-butoxide, contained solvents in the amount of - 5 % . In an
Table 1 Chemical analysis of a calcined gel Constituent
Analyzed wt%
Theoretical wt%
BaO SiO 2 A1203
42.5 43.7 13.8
42.6 45.0 12.4
W.K. Tredway, S.H. Risbud / Gel synthesis of glass powders
283
Table 2 Batched and analyzed oxynitride glass compositions Constituent
BaO SiO 2 A1203 Si 3N4 at.% N wt% N H (as O H - )
Glass A
Glass A-3
Theo. wt%
Theo. wt%
Anal. wt%
Glass A-6 Theo. wt%
Anal. wt%
42.6 45.0 12.4 -
43.0 41.5 12.5 3.0 2.5 1.2
43.3 40.1 13.6 3.0 2.5 1.17
43.3 38.1 12.6 6.0 5.1 2.4
43.7 37.7 14.0 4.6 3.7 1.76
17 ppm
attempt to correct for this, a slight excess of Al-sec-butoxide was undoubtedly added. The analyzed SiO2 content is also somewhat lower than the theoretical value. It is possible that some of the TEOS could have evaporated during processing. Although refluxing conditions were used throughout the majority of the procedure, the system was open to the atmosphere during transfer of the solution to the Petri dishes.
4.4. Oxynitride glasses from gel batches Gel derived oxide powders (-100 mesh) were mixed with electronic grade Si3N 4 powders (-325 mesh) and melted in Mo crucibles at 1550-1600°C for 30 min as detailed in our previous publications [14-16]. Table 2 shows the analyzed and batched glass compositions. The overall agreement between batched and analyzed values for the oxynitride glass compositions (A-3 and A-6) is quite good. It is specially noteworthy that the amount of N loss ( - 23%) is lower in the present gel derived oxynitride glasses than in those prepared by conventional melting of batches [14]. The microstructure and crystallization behavior of gel derived versus conventional oxynitride glasses has been investigated [17] and will be reported subsequently. Support for this work was provided by the Ceramics and Electronic Materials Program of the Division for Materials Research at NSF under Grant DMR 85-14324. W.K. Tredway also
199 ppm
acknowledges an IBM Pre-doctoral Fellowship Award.
References [1] W.K. Tredway and S.H. Risbud, Mater. Lett. 3 (1985) 435. [2] C.J. Brinker and G.W. Scherer, J. Non-Cryst. Solids 70 (1984) 301. [3] S.P. Mukherjee, J. Non-Cryst. Solids 42 (1980) 477. [4] B.J.J. Zelinski and D.R. Uhlmann, J. Phys. Chem. Solids 45 (1984) 1069. [5] B.E. Yoldas, J. Mater. Sci. 14 (1979) 1843. [6] R. Roy, J. Am. Ceram. Soc. 39 (1956) 145. {7] R. Roy, J. Am. Ceram. Soc. 52 (1969) 344. [8] G.J. McCarthy and R. Roy, J. Am. Ceram. Soc. 54 (1971) 639. [9] F. Pancrazi, J. Phalippou, F. Sorrentino and J. Zarzycki, J. Non-Cryst. Solids 63 (1984) 81. [10] W. Holand, E.R. Plumat and P.H. Duvigneaud, J. NonCryst. Solids 48 (1982) 205. [11] D.R. Uhlmann, B.J.J. Zelinski and G.E. Wnek, in: Better ceramics through chemistry (MRS, Pittsburgh, 1984) p. 59. [12] D.R. Uhlmann, in: Ceramics: today and tomorrow, eds. S. Naka, N. Soga and S. Kume (Ceram. Soc. Japan, Tokyo, 1986) p. 51. [13] D.R. Uhlmann and G.P. Rajendran, Proc. 3rd Conf. on Ultrastructure Processing (Wiley, New York, 1987). [14] W.K. Tredway and S.H. Risbud, J. Amer. Ceram. Soc. 66 (1983) 324. [15] L.M. Bagaasen and S.H. Risbud, J. Amer. Ceram. Soc. 66 (1983) C29. [16] P.E. Jankowski and S.H. Risbud, J. Mater. Sci. 18 (1983) 2087. [17] W.K. Tredway, PhD Thesis, University of Illinois, Urbana-Champaign (1986).