High Temperature Mass Spectrometric Study of Thermodynamic. Properties of the CaO-Si02 System. V. L. Stolyarova, S. I. Shornikov, G. G. Ivanov, and M. M. ...
3710
J. Electrochem. Soc., Vol. 138, No. 12, December 1991 9 The Electrochemical Society, Inc.
REFERENCES 1. J. Zahavi and M. Metzger, This Journal, 119, 1479 (1972). 2. S. Ono and T. Sato, Abstract 38, p. 115, The Electrochemical Society Extended Abstract, Vol. 81-1, Minneapolis, MN, May 10-15, 1981. 3. S. Ono, N. Baba, and T. Sato, in "Abstract of 76th Meeting of J. Metal Finish. Soc. Jpn." pp. 130, 132 (1987). 4. S. Ono and T. Sato, in "Proceedings of Interfinish 80," p. 287, Kyoto (1980). 5. T. Kawaguchi, S. Ono, T. Sato, and N. Masuko, J. Surf. Finish. Soc. Jpn., 41, 690 (1990). 6. D. A. Vermilyea, This Journal, 110, 250 (1963). 7. G. E. Thompson, K. Shimizu, and G. C. Wood, Nature, 286, 471 (1980). 8. R. S. Alwitt and C. K. Dyer, Electrochim. Acta, 23, 355 (1978). 9. C. Crevecoeur and H. J. de Wit, This Journal, 134, 808 (1987). 10. C. K. Dyer and R. S. Alwitt, Electrochim. Acta, 23, 347 (1978). 11. G. E. Thompson, R. C. Furneaux, and G.C. Wood, Corr. Sci., 18, 481 (1978). 12. S. Ono, H. Ichinose, T. Kawaguchi, and N. Masuko,
ibid., 31, 249 (1990). 13. S. Ono, H. Ichinose, T. Kawaguchi, and N. Masuko, J. Right Metal Soc. Jpn., 40, 780 (1990). 14. S. Ono, H. Ichinose, T. Kawaguchi, and N. Masuko, J. Surf. Finish. Soc. Jpn., 41, 1181 (1990). 15. S. Ono, H. Ichinose, and N. Masuko, Corr. Sci., To be published. 16. S. Ono, S. Chiaki, and T. Sato, J. Surf. Finish. Soc. Jpn., 26, 456 (1975) and in "Proceedings of Meeting of International Society of Electrochemistry, p. 1017, Budapest (1978). 17. S. Ono, T. Kawaguchi, H. Ichinose, Y. Ishida, and N. Masuko, J. Surf. Finish. Soc. Jpn., 40, 1361 (1990). 18. D. J. Arrowsmith, E. A. Culpan, and R. J. Smith, in " S y m p o s i u m on Anodizing Aluminum," p. 17, Birmingham, E n g l a n d (1967). 19. N. Sato, Electrochim. Acta, 16, 1683 (1971). 20. J. Yahalom and J. Zahavi, ibid., 15, 1429 (1970). 21. K. Shimizu, G. E. Thompson; and G. C. Wood, Thin Solid Films, 81, 39 (1981). 22. G. E. Thompson, R. C. Furneaux, G. C. Wood, J.A. Richardson, and J. S. Goode, Nature, 272, 433 (1978). 23. R. S. Alwitt, C. K. Dyer, and B. Noble, This Journal, 129, 711 (1982).
High Temperature Mass Spectrometric Study of Thermodynamic Properties of the CaO-Si02 System V. L. Stolyarova, S. I. Shornikov, G. G. Ivanov, and M. M. Shultz Institute of Silicate Chemistry of the USSR Academy of Sciences, ul. Odoevskogo 24, korp.2, 199155 Leningrad, USSR ABSTRACT The mass spectrometric K n u d s e n effusion method was used to determine the composition and partial vapor pressures over the CaO-SiO2 system in the concentration range of 0.1-0.5 mole fraction of SiO2 at 1933 K. The new gas molecule CaSiO3 compared with the composition of the individual oxide vapor was found in this system. The formation probability and the relative volatilities in oxide melts can be quantitatively predicted using the partial pressure of oxygen in the systems. The correlation was shown between the thermodynamic properties obtained in the CaO-SiO2 system and its phase diagram. The experimental values of the CaO, SiO2 activities, and the Gibbs integral energies are compared with the resuits calculated by different methods. Materials based on the CaO-SiO2 system play an important role in different fields of materials science, such as ceramics, metallurgy, and geology. Therefore, the high temperature behavior of this system has a great significance for different areas of processing. The thermodynamic properties of the CaO-SiO2 system were studied using electromotive force (EMF) methods (1-5), high temperature calorimetry (6-9), the equilibrium method between slag and gas (10-15), and measurements of the sulfide capacities of slags (16-18). The correlation between the results obtained can be found in (19-21). However, the information about the thermodynamic properties of this system is rather contradicting and, in some cases (1, 4, 11-14, 18), it does not even correspond to the phase diagram. As to the study of the vaporization processes and relative volatilities of the components in the CaO-SiO2 system, these data are limited only by the results obtained in (22, 33) and are absent at concentrations with high CaO molar fractions.
Experimental The samples were prepared by the precursor method (24). The work was done using the MS-1301 mass spectrometer (25). The evaporation of samples was carried out from tungsten effusion cells with a ratio of the effusion and evaporation squares of 1:500, as described earlier (26). The temperature was measured by the optical pyrometer AOP-66 with the vanishing glow lamp filament. Its relative error of temperature definition did not exceed 0.1-0.2%. The procedure of the temperature measuring was the same as described by Semenov et al. (25). The measured gold
vapor pressure at 1680 K was equal to (1.41-+ 0.07) 10-5 atm. The temperature gradient between the upper and lower parts of the cell did not exceed 15 K.
Results It has been shown that the mass spectra of vapor over the CaO-SiO2 system u n d e r investigation contained the following ions at 70 V; SiO § Ca +, O § CaO § SiO~, CaSiO~, Si § CaSi § CaSiO § and CaSiO~. The concentration dependencies of the mass spectra of vapor over the different samples and the appearance potentials determined allowed the authors to conclude that the main components of the gas phase are SiO, Ca, CaO, SiO2, O, and CaSiO~. There are different ways in which the CaS103, " § CaS102 " §, and CaSiO § ions appear in the mass spectra of the vapor. We suppose that these ions may appear from the CaSiO~ gas molecule. The partial pressures of oxygen which are necessary for the determination of the component activities have been calculated using the partial pressures of WO2, WO~, and the equilibrium constant of the gas reaction (28) (27), se~ Tables Ia and Ib. The disagreement between the results obtained by the ion comparison method and the complete isothermal evaporation method was less than 10%. The partial pressures of oxygen which are necessary for the determination of the c o m p o n e n t activities have been calculated using the partial pressures of WO2, WO3, and the equilibrium constant of the gas reaction (28) (WO3) = (WO~) + (O)
[1]
d. Electrochem. Soc., Vol. 138, No. 12, December 1991 9 The Electrochemical Society, Inc. Table la. The partial pressuresof components(atm) over the CaO-SiOz system at 1993 K obtained by the complete evaporation method (1) and the ion comparisonmethod (2). SiO2 mole fraction
1
2
1
2
0.50 0.49 0.47 0.44 0.41 0.40 0.39 0.38 0.36 0.33 0.25
2.2 • 0.2 3.5 • 0.3 5.1 • 0.3 9.6 • 0.4 17.6 • 1.0 27.6 • 1.0 36.3 • 2.0 37.1 • 3.0 49.6 • 4.0 50.1 -+ 4.0 49.9 • 4.0
2.5 • 0.3 3.9 • 0.4 5.1 • 0.4 10.3 • 0.5 22.9 _+ 1.5 29.4 • 2.0 38.7 • 3.0 41.6 • 4.0 48.7 • 5.0 49.2 • 5.0 50.9 • 5.0
5.9 • 0.4 8.8 § 0.5 11.9 • 0.5 20.8 _+ 1.0 23.0 - 2.0 41.2 • 3.0 41.3 • 3.0 49.6 • 4.0 61.7 • 4.0 75.8 • 5.0 78.8 • 5.0
6.0 • 0.5 8.9 • 0.6 11.9 • 0.8 18.2 • 1.5 26.9 _+ 3.0 35.6 • 4.0 35.7 _+ 4.0 48.6 • 5.0 60.5 • 5.0 73.4 • 6.0 79.5 • 6.0
Pca" lOv
Pcao" 108
Table lb. The partial pressuresof components(atm) over the CaO-SiOz system at 1933 K obtained by the complete evaporation method (1) and the ion comparison method (2). psio. 106
Psio2 " 10 ~
SiO~ mole fraction
1
2
2
0.50 0.49 0.44 0.41 0.40 0.39 0.38 0.33 0.25
91.4 • 5.0 42.2 • 3.0 45.7 • 3.0 23.8 • 2.0 17.2 • 1.0 15.9 -+ 1.0 13.9 • 1.0 1.9 • 1.0 2.7 -+ 1.0
100 • 10 46.0 • 5.0 43.4 _+ 4.0 26.2 • 3.0 18.9 +_ 2.0 17.5 • 2.0 15.3 • 2.0 5.0 • 2.0 0.8 _+ 1.0
15.9
Po" 101~ Calculated from equilibrium (1) 10.0 10.0 10.0 10.0 6.8 10.0 7.4 6.7 6.8
10.1 (7.7) (5.3) (2.0)
3711
Table Ila. The comparison of the CaO activities in the CaO-SiO~ system at 1933 K obtained using Eq. [2], [5], [7]. SiO2 mole fraction
Eq. [2]
0.50 0.49 0.44 0.41 0.40 0.39 0.38 0.33 0.25
0.07 -+ 0.02 0.11 • 0.03 0.26 • 0.04 0.29 • 0.04 0.52 • 0.04 0.52 • 0.04 0.63 • 0.04 0.96 • 0.03 1.00 -+ 0.02
ac~o obtained using Eq. [5] 0.08 0.10 0.20 0.33 0.38 0.49 0.67 0.95 1.00
Table lib. The comparison of the Si02 activities in the CaO-Si02 system at 1933 K obtained using Eq. [3], [6], [8]. SiO2 mole fraction 0.50 0.49 0.44 0.41 0.40 0.39 0.38 0.33 0.25
Eq. [3] 0.20 • 0.09 • 0.10 • 0.05 • 0.03 • 0.04 • 0.023 • 0.003 • 0.004 •
aslo2 obtained using Eq. [6]*
0.01 0.02 0.02 0.01 0.01 0.01 0.005 0.002 0.001
0.20 0.13 0.05 0.03 0.03 0.02 0.02 0.006 0.005
in asio2 = f X c a o d In (Isio+/Ica+)
k 9Ic~o+T - ~cao~c~o+ k . I~ao+T 9 ~CaO~/CaO+
Icao+ - - -
I~o+
[2a]
w h e r e Pc~o, P ~ o = t h e CaO partial v a p o r p r e s s u r e o v e r t h e CaO-SiO2 s y s t e m a n d t h e i n d i v i d u a l o x i d e ; Ic~o+, I~ao§ = t h e i o n c u r r e n t v a l u e s o f CaO § in t h e m a s s s p e c t r a o f v a p o r o v e r t h e CaO-SiO2 s y s t e m a n d i n d i v i d u a l oxide; k = t h e c o n s t a n t o f t h e i n s t r u m e n t sensitivity; ~cao = t h e ionization c r o s s s e c t i o n o f t h e CaO gas m o l e c u l e ; "]cao+ = t h e s e n s i t i v i t y c o e f f i c i e n t o f t h e s e c o n d e l e c t r o n m u l t i p l i e r for t h e CaO § ion asio2
=
Psioz/P~io2 = P s i o P o k l / P ~ i o P ~ ) k l = P s i o ( P w o 3 / P w o 2 ) k s / P soi o ( P w o J P w o 2 ) ok 5 [2b]
if we do a similar p r o c e d u r e we obtain asio2 = Isio+(Iwo~/Iwo~)/I~io+(Iwo~/Iwo~) ~
[3]
k, = t h e e q u i l i b r i u m c o n s t a n t o f t h e r e a c t i o n (SiO2)g~ = (SiO)g~ + Og~
[4]
k5 = t h e e q u i l i b r i u m c o n s t a n t of t h e r e a c t i o n [1], Is~o+, I~o+ = t h e ion c u r r e n t v a l u e s o f CaO + i n t h e m a s s s p e c t r a o f v a p o r o v e r t h e CaO-SiO2 s y s t e m a n d i n d i v i d u a l o x i d e SiO2; (Iwo~/Iwo~), (Iwos/Iwo~) ~ = t h e ratio o f WO~, WO~ ion c u r r e n t s in t h e m a s s s p e c t r a o f v a p o r o v e r t h e C a O - S i Q s y s t e m a n d i n d i v i d u a l silica for t h e e v a p o r a t i o n p r o c e s s f r o m t h e t u n g s t e n cell. It s h o u l d b e m e n t i o n e d t h a t in o u r c a s e w e h a d a n o p p o r t u n i t y to c h e c k t h e c a o a n d SiOz activities o b t a i n e d acc o r d i n g to Eq. [2] a n d [3] to t h o s e c a l c u l a t e d u s i n g t h e Gibbs-Duhem equations in ac~o = - f ( 1 - Xc~o)/Xc~od In asio2
[5]
i n asio2 = - f ( 1 - Xsio2)/Xsiozd In acao
[6]
or u s i n g t h e B e l t o n - F r u e h a n e q u a t i o n s (29) In acao
=
- f X s i o 2 d In (Isio+/Ic~+)
0.07 0.12 0.19 0.33 0.45 0.52 0.55 0.75 1.00
Eq. [8]* 0.20 0.11 0.07 0.03 0.02 0.016 0.015 0.008 0.004
* Evaluated values.
T h e CaO a n d SiO2 activities at t h e c o n s t a n t t e m p e r a t u r e w e r e o b t a i n e d f r o m t h e m e a s u r e d ion c u r r e n t s o f CaO +, SiO § WO~, a n d WO~ b y t h e f o l l o w i n g acao.T = Pcao/P~ao =
Eq. [7]
[7]
[8]
w h e r e Xcao, Xsio2 = t h e CaO a n d SiO2 m o l e fractions. T h e i n t e g r a t i o n limits w e r e t a k e n f r o m t h e p u r e c o m p o n e n t s u p to c o r r e s p o n d i n g c o m p o s i t i o n s . T a b l e s IIa a n d l i b i l l u s t r a t e t h e c o r r e l a t i o n s b e t w e e n t h e s e values. T h e r e l a t i v e u n c e r t a i n t i e s o f t h e d e t e r m i n a t i o n o f t h e CaO a n d SiO2 activities a c c o r d i n g to Eq. [5]-[8] w e r e n o t m o r e t h a n 15%. F i g u r e 1 s h o w s t h e e v a p o r a t i o n i s o t h e r m s c o r r e s p o n d i n g to t h e c o m p o u n d s CaO-SIO2, 2CaO 9 SIO2, a n d 3CaO 9 SiO~. T h e r e s u l t s o f b o t h m e t h o d s are in g o o d a g r e e m e n t . S u c h c o r r e l a t i o n a l l o w s us to c o n clude that the values of ion currents obtained from the c o m p l e t e e v a p o r a t i o n curve, s e e Fig. 1, are in g o o d agreem e n t w i t h t h e r e s u l t s o b t a i n e d f r o m t h e ion c o m p a r i s o n m e t h o d w h e n w e s t u d y t h e ratio o f t h e c o m p o s i t i o n s similar to t h o s e c a l c u l a t e d f r o m t h e e v a p o r a t i o n curve. A c c o r d ingly, w e s u g g e s t t h a t t h e i n f l u e n c e o f t h e CaO-rich layer w i t h t h e l o w e r v i s c o s i t y c o m p a r e d to t h e o n e o f t h e initial c o m p o s i t i o n s is n o t so i m p o r t a n t . W h e n t h e t e m p e r a t u r e w a s r a i s e d u p to 2150 K w e o b s e r v e d an i n c r e a s e o f t h e CaO, SiO, Ca partial v a p o r p r e s s u r e s . T h e c o n g r u e n t c h a r a c t e r o f v a p o r i z a t i o n p r o c e s s e s in t h e r a n g e f r o m 0.01 to 0.37 -+ 0.02 m o l e f r a c t i o n s o f SiO2 w a s s h o w n on the basis of analysis of the d e p e n d e n c i e s of the CaO, Ca, a n d SiO e v a p o r a t i o n i s o t h e r m s , Fig. 1. O w i n g to this observation, we concluded that the liquidus point on the phase diagram may be corrected compared with the earlier m e n t i o n e d 0.41 m o l e f r a c t i o n o f SiO2 (30).
Discussion T h e e x p e r i m e n t a l v a l u e s o f t h e CaO, SiO2 activities a n d t h e G i b b s i n t e g r a l e n e r g i e s are i n g o o d a g r e e m e n t w i t h t h e r e s u l t s o b t a i n e d w i t h t h e aid o f e x c h a n g e r e a c t i o n s (18), t h e E M F m e t h o d s (3), a n d t h o s e c a l c u l a t e d b y t h e n u m b e r o f t h e m o d e l a p p r o a c h e s in Ref. (31-35). B u k h t o y a r o v e t al., (31) o b t a i n e d t h e s e v a l u e s u s i n g t h e M o n t e Carlo m e t h o d . T h e y u s e d t h e m o d e l in w h i c h t h e Ca a t o m s w e r e s i t u a t e d i n t h e k n o t s o f t h e lattice w h e r e t h e Si a t o m s w e r e p r e v i o u s l y p l a c e d . It s h o u l d b e m e n t i o n e d that, b y t h i s s u b s t i t u t i o n , t h e c o o r d i n a t i o n n u m b e r o f t h e lattice e q u a l e d to 4 d i d n o t c h a n g e . T h e m a t r i x w i t h t h e 504 tet r a h e d r a l p o s i t i o n s w a s c h o s e n , c o m b i n e d in Torr. T h e n u m b e r o f s t e p s w a s 2 - l0 s a n d this v a l u e b e t w e e n t h e n e i g h b o r c o n f i g u r a t i o n s in t h e M a r k o v c h a i n w a s a b o u t
J. Electrochem. Soc., Vol. 138, No. 12, D e c e m b e r 1991 9 The Electrochemical Society, Inc.
3712
,23, l i I
5 1
3SiO 2
a) p.40
o .40
1.5
0.6
0.0 0 c"
(a)
!
too
300
1.5 -
b)
r
p-5 o 40
(7
0,2 O.S0.0 1oo
0
200
J
0,'5 0 .'5 8i0 2, m0le fracti0rL
~00
c)
0
,q
I
t.5
b-5
aca.o
C
0.5 0.0 0
I
I
80
160
t:ime, rain Fig. 1. Time dependences of the ion currents of Ca+(FI), CaO+(E>), SiO+(O), and CaSiO+(O) in the mass spectra of vapor over the CaOSi02 system on the complete evaporation time of a given composition containing CaSi03 (a), Co2Si04 (b), Ca3SiOs (c) at 1933 K. Numbers near the 1 a curve indicate the compositions calculated according to the complete evaporation method during the vaporization of calcium silicate: 1-0.50; 2-0.42; 3-0.39; 4-0.37; 5-0.33 mole fractions Si02.
104. The statistical relative error of calculations using the Monte Carlo m e t h o d did not exceed 5%. Those t h e r m o d y n a m i c data show negative deviations from the ideality, see Fig. 2 and 3. In this work the Gibbs integral energies of the CaO-SiO2 system were evaluated using the generalized lattice theory of associated solutions both in the B203-SIO2 and B20~GeO2 systems (32). In this method, the different sizes of the calcium oxide and silica in the melt were taken into account. In the m o d el of the melt, the interactions between Ca and O, and Si and O, were considered. The Ca-O and Si-O interaction energies due to the type of the atom in the second coordination sphere were the same as those found by Bukhtoyarov et aL (31). The structural elements of oxides were placed in the knots of a quasi-crystalline lattice. The lattice parameters were chosen in such a way that the n u m b e r of knots occupied by CaO and SiO2 corresponded to the ratio of their molar volumes. The structural elements corresponding to the oxide formula were chosen in the continuous lattice. Atoms entering in to contacts with their neighbors, i.e., the contact sites, were identified in such an element. In this case the coordination numbers with regard to the oxygen were four for silicon and six for calcium. It was suggested that before the melt formation the Ca and Si atoms were even connected with some of the oxygen atoms. During the melt formation the n u m b e r of contact sites was assumed to be five for calcium, two for silicon, and one for oxygen, according to the coordination requirements. The adjustable parameters were not used in the model. The disagreement between the experimental and evaluated values can be explained to some extent by choosing the Ca-O interaction energies corresponding to four coordinated calcium by oxygen (31). Nevertheless, the discrepancies between the experimental results and our calculated results, see Fig. 3, are about 40%, which m ay be considered rather satisfactory c o r n -
0.6
(b)
0.2. I
0.5 Si02,
!
O.5 mole fraction.
Fig. 2. Dependences of Si02 (a) and Ca0 (b) activities vs. concentration of Si02 (mole fraction) in the CaO-Si02 system at 1933 K: D-the EMF method (3); --, O-the measurements of the sulfide capacities of slags (18); +, - - --the values calculated by Monte-Carlo method (31); O-our experimental results. pared with the data obtained in Ref. (34, 35), where the adjustable parameters were used. The lack of adjustable parameters in the model suggested allows us to suppose that such an approach is more adequate than that used in Ref. (34, 35). The use of bond energies corresponding to the six-coordinated calcium that are 30% lower than the values applied here will probably allow us to obtain a rather good agreement between the experimental and calculated results. Table III illustrates the data available in the literature (36-39) on the composition of the vapor phase and the Gibbs energy in the binary oxide systems containing silica and calcium oxide. The following consideration shows the correlation between different types of evaporation processes; such as the association or dissociation and the oxygen vapor pressure as a measure of a relative volatility of oxide systems. This is the procedure that we used for the calculating of oxygen vapor pressure in the CaO-SiO2, MgO-SiO2, A120~-SiO2, and CaO-A1203 systems. We suggest that the evaporation process in the similar systems may be presented in a similar way; such as [SiO2] -+ (SiO2) -+ (SiO) + 1/2 O2
[9]
[CaO] --> (CaO) --~ (Ca) + 1/2 02
[10]
S o c . , Vol. 138, No. 12, December 1991 9 The Electrochemical Society, Inc.
J. E l e c t r o c h e m .
Si02, mote fraction
Table III. The vapor species in the binary systems at 1823-1933 K.
Systems
Gaseous phase components
Reference
CaO-SiO2 CaO-A1203 FeO-SiO2 MnO-SiO2 A1203-SiO2 MgO-SiO~
SiO, Ca, CaO, CaSiO2, SiO2, O A1, A10, AI20, O, Ca, CaO Fe, SiO, O, FeO Mn, SiO, O SiO, O Mg, SiO, O
This work (36) (37) (39) (37) (38)
3713
O.2 I
0.6 I
I
I
I
-5
[MgO] --* (MgO) --> (Mg) + 1/2 02
[11]
[Al~.O3]--> (A1203)--->(A1202) + 1/2 O2
[12]
5
where square brackets a n d parentheses correspond to the condense and gas phases. For example, the Gibbs energy in the CaO-SiO2 system may be written as
AG = XcaoRT in acao + Xsio2RT In asio2
[13]
AG = x l R T In [(Psio - Po2)tkiPsio2] o.5 o
/
,
/
/
6
-8
+ x2RT In [(Pca 9Po2)/k2Pc~o] 0.5 o [14] where kl and k2 are the constants of equilibriums [9] and [10]; acao and as~o2 = the CaO and SiO2 activities; Xc~o and Xsio2 = the CaO and SiO2 molar fractions; P~to2 and P~o = the partial vapor pressures of the individual oxides and Pc~, Ps~o, and Po2 = the partial vapor pressures of the components in the CaO-SiO2 system. Using Eq. [9] and [10] we obtained In Po~ = 2/3(AG/RT + Xsio~in k2P~io2 + Xc~o In klP~ao - In 2) [15] The oxygen vapor press~tres calculated in a similar way depending on the concentration of SiO2 or CaO for the FeO-SiO2, MnO-SiO2, A1203-SiO2, MgO-SiO2, and CaOAl~O3 systems at 1933 K are given in Fig. 4. The data on the individual oxides were taken from (28, 40-43). These dependencies illustrate the different relative volatilities of the systems m e n t i o n e d and the tendency for association in the vapor phase of the CaO-SiO2 and CaO-A1203 systems.
Lg P02, arm Fig. 4. Comparison of the dependences of 02 vapor pressures on the composition in the binary systems at 1933 K: 1--the CaO-SiO2 system (our experimental results); 2 the MgO-SiOz system; 3--the FeO-SiO2 system; 4--the CaO-AI203 system; at 1973 K: 5--the AI203-SIO2 system; 6 ~ t h e MgO-SiO2 system.
Conclusions Significant negative deviations from the ideality were observed in the CaO-SiO2 system at 1933 K by high tern-
SiO 2, m o l e 0,3 I
fraction,
Manuscript submitted Oct. 15, 1990; revised manuscript received May 30, 1991. This was Paper 522 presented at the Seattle, WA, Meeting of the Society, Oct. 14-19, 1990.
0.5 I
+
-4
I
-t6
"o
perature mass spectrometry. The possibility of calculation of the thermodynamic properties of the MeO-SiO2 systems using the generalized lattice theory of associated solutions was shown on the basis of the CaO-SiO2 system.
+/
+ aG, kcal/mole
Fig. 3. The Gibbs integral energies vs. concentration of SiO2 (mole fraction) in the CaO-SiOz system at 1933 K: [ ] , - - - - t h e measurements of the sulfide capacities of slags (18); 9 values calculated by Monte-Carlo method (31); --in (34); . . . . in (35); +--the values calculated using the generalized lattice theory of associated solutions in this study; O---our experimental results.
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Chemical Etching of (100)GaAs in Volcanic Mineral Water Sadao Adachi Department of Electronic Engineering, Faculty of Engineering, Gunma University, Kiryu-shi, Gunma 376, Japan ABSTRACT We have successfully demonstrated that a solution of volcanic mineral water [Kusatsu Spa water (KSW)]:H202:H20 etches (100)GaAs wafers. The KSW contains a considerable a m o u n t of positive ions (K + , Na + , Ca 2 + , etc.) and negative ions (CI-, SO~-, and HSO4). The etchant system provides shiny flat surfaces on the etched bottoms. The system has reproducible etching rates and does not erode photoresist masks. Etching rate vs. temperature data for (100)GaAs in an etchant consisting of 5KSW: 1H202:1H20 show that different rate limiting processes operate above and below 15~
Chemical etching of semiconductors plays an essential role in electron and optoelectronic device technology. Important factors determining the choice of an etchant are, generally, the etching rates for the materials in question, the degree of surface quality and undercutting, the solution chemical aggressiveness toward photoresist masks, and the desired etching profile for the relevant purpose. There have been m a n y reports on the etching characteristics of GaAs (1). The chemicals used in such studies were usually of reagent grade and were, therefore, industrial products. There are m a n y hot springs in Japan. The Kusatsu Spa (Kusatsu-cho, Agatsuma-gun, Gunma, Japan) is one of the most famous hot springs which m a n y people visit to take baths. The purpose of this paper is to demonstrate that volcanic.mineral water [Kusatsu Spa water (KSW)]:H202:H20 mixtures can successfully etch GaAs wafers without providing any undesirable roughness or etch pits.
Experimental The GaAs single crystals used were undoped wafers of (100) surface orientation with an uncertainty of 1~ or less. These wafers were mirror-like finished, degreased, and
Chemical Analysis Result for Volcanic Mineral Water I n Fig. 1 we show a location of the Kusatsu Spa and in Table I chemical analysis results for KSW based on the official method of analysis: Near the Kusatsu Spa there is an active volcano n a m e d Mt. Shirane. The KSW originates from its volcanic activity. It is a colorless and transparent liquid [pH - 1.7 (see Table I)]. Note that the KSW contains considerable quantities of positive ions, such as K § Na § Ca 2+, Mg 2+, and A13§ The HC1 and tI2SO4 are the main acid constituent of the KSW. The chemical analysis results during the past few decades also suggest that the concentration of some cations (K +, Na § Fe 2§ and Fe 3§ changes relatively significantly over time b u t the anion concentrations (CI-, SO~, and HSO~) do not (2).
.I ~'
JAPAN
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the I~citic
., Ocean
l Fig. 1. A location of the Kusatsu Spa, Japan.
