- Email: [email protected]
Mechanism of low temperature calcite decomposition
Solid State lonics 32/33 (1989) 413-419 North-Holland, Amsterdam
MECHANISM OF LOW TEMPERATURE CALCITE DECOMPOSITION ~ Giorgio S P I N O L O and U m b e r t o A N S E L M I - T A M B U R I N I Department ql~Physical Chemisto, and ('. S, 7~E./C. N.R. University o f Pa via, ~Tale Tara melli 16. 1 27 l O0 Pa via. ltall,
Received 6 June 1988: accepted for publication 4 July 1988
Wide angle X-scattering (WAXS) and nitrogen adsorption isotherm (NAI) determinations and SEM observations have been used to characterize the microstructure of CaO particles obtained by thermal decomposition of calcite powders at low temperalures (820-870 K), and yew low reaction rates (10 -s tool cm 2 s ~). The combined analysis of WAXS and NAI measurements shows that the microstructure is made of bundles of NaCl-structured and rod-shaped crystallites which are separated from each other by cylindrical pores and completely fill the volume of the corresponding calcite grain. Crystallite and pore sizes do not change during a single decomposition run but significanlly grow when higher working Pco: are used, starting from a low pressure limit which is almost indepenOent of the decomposition temperature. Both size distributions change in a parallel way and the size increases correspond to a marked decrease of shape anisotropy and microstrain content. The conflicting evidence that the microstructure is constant during the decomposition time, but heavily depends on the external conditions, rules out the possibility of describing the primal' reaction step with reference to a single mechanism, either of the shear-transformation type or transportdriven. The work discusses a mechanism that takes into account both aspects at the same time. and shows how the heat- and masstransport processes can affect the microstructure without affecting its time evolution.
I. Introduction The thermal d eco m p o s i ti o n o f calcite is probably the most investigated heterogeneous reaction because o f its technological importance. Moreover, t h e r m o d y n a m i c , structural and transport data on both reagents and reaction products are very well known, so that calcite, and other structurally related divalent carbonates, are a very convenient chemical model to gain a better understanding o f the mechanism o f e n d o t h e r m i c d e c o m p o s i t i o n reactions described by the stoichiometric scheme: A (solid)--. B (solid) + C ( g a s ) . It is well known that, under irreversible conditions, the reaction produces "specially reactive" [ 1 ] oxides whose microstructure is made o f bundles o f rod-shaped crystallites separated from each other by cylindrical pores. A n u m b e r o f studies on topotactic relations or microstructure (see [2,3] and references therein q u o t ed ) are the main argument for classiBased on a thesis submitted by U. Anselmi-Tamburini in partial fulfillment of the requirements for a Ph.D. at University of Pavia.
fying this reaction am o n g the shear transformations, in spite o f the experimental evidence [4] that the external (7", P) variables influence the microstruclure. Indeed, t h e r m o d y n a m i c , kinetic, structural and microstructural investigations on this subject show complex, and somewhat conflicting, results [ 2 , 4 - 7 ] , thus indicating that different reaction steps are relevant to the overall reaction mechanism, and the external variables have different effects on these steps. We may argue that, in some cases, the m ech an i sm is mainly affected by transport properties, i.e. heat transport or transport o f the gaseous reaction product through the porous microstructure o f the solid product. As a further difficulty, the experimental investigation is made more complex by a microsintering effect which is active also when the partial pressure o f the gaseous product is well below the equilibrium value [ 8 ]. This work describes a systematic study o f the microstructure o f calcium oxide produced by decomposing calcite powders at the lowest practical temperatures and under very low reaction fluxes, when we can presume that the features o f the primary reaction steps are less masked by following rearrange-
0 167-2738/89/$ 03.50 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing D iv is i o n )
414
G. .S'Fil~o/o. ~ '. I m c h m 7~mhttrinl / L lcalcilu duc, Unl~,~imm
rearrangements, Isothermal-isobaric decompositions have been investigated at T , = 820 and 870 K and carbon dioxide partial pressures P,, ,: in the range [10 " P~,
2. Experimental Microstructural parameters were measured with wide-angle X-scattering ( W A X S ) and nitrogen adsorption isotherm ( N A I ) d e t e r m i n a t i o n s on CaO samples produced by decomposing reagent grade calcite powders previously ground and sieved ( 5-30 ~m). X-ray scattering line profiles were taken with a Philips PW 1730 diffractometer equipped with Cu radiation tube operated at 45 kV and 22 mA. Soller slits, graphite m o n o c h r o m a t o r on the diffracted beam and scintillation counter. The oxide material was produced m situ under i s o t h e r m a l - i s o b a r i c conditions using an environmental chamber [9] enclosing a small electrical furnace and connected 1o a vacuum system (mechanical pump, diffusion p u m p and liquid nitrogen trap). Step scan data were collected at 0 . 0 2 (20) steps over the 25-85 range with 100 s (peaks) or 10 s (background) step times. Each scan program was looped until 40000 counts/step had been collected for the diffraction lines to be analysed. Processing of the raw scattering data was done [ 10] according to the following steps: background subtraction, smoothing, sampling at equal sin 0 intervals, FFT, Stokes deconvolution of the instrumental aberrations, and W a r r e n - A v e r b a c h strain subtraction using the (l 1 1 ) / ( 2 2 2 ) and ( 2 0 0 ) / ( 4 0 0) couples. The whole set of X-ray scattering lines, ( 1 1 1 ) , ( 2 0 0 ) , ( 2 2 0 ) , ( 3 1 1 ) , ( 2 2 2 ) , ( 4 0 0 ) was also analysed with single line methods [l I ]. Data acquisition and analysis was done on a dcsktop c o m p u t e r with h o m e m a d e software. Each experimental run required 2-10 days for decomposition and two more days for data acquisition. Calcite decomposition was m o n i t o r e d with X-ray patterns taken from time to time on t e m p e r a t u r e and recorded in continuous scan mode: For NAI d e t e r m i n a t i o n s we used a Cahn RG 3885 microbalance connected to a v a c u u m line and to a
high vacuum p u m p i n g system. The calcite samplcs, placed in a Pl crucible, were first d e c o m p o s e d using an external furnace. After complete decomposition (in some cases also after half d e c o m p o s i t i o n ) , NAI data were collected by replacing the furnace with a liquid nitrogen filled Dewar flask. Another flask was then placed around the rate arm of the microbalance to keep its symmetric arrangement. BET surface areas were calculated in the usual way using the low I L . points [/',q, <0.3 I',,], pore size distributions using the high t'~,. data according to the pertinent hysteresis loop type [12]. After WAXS or NAI determinations, part of the samples was transferred under inert atmosphere to a gold sputtering unit for examination in a ( ' a m bridge Steroscan S.E. Microscope.
3. Results and discussion SEM observation of the samples produced by high vacuum decomposition (see fig. 1 ) shows that the external /latqlu.s of the parent calcite grains is kept almost unmodified, but for the presence of large cracks. The same c o m m e n t could be made for the low resolution photographs of samples obtained with 1 Torr ( - l 0 t Pa) decomposition. Now, high resolution photographs (fig. 2) show a fine microstruclure, typically an orange-peal roughness, which differently affects different laces. Also, the large
Fig. 1. gEM m icrograph of('a() produced b} calcite dccomposb lional - I0
'Paand
870K.
G. Spinolo, U. Ansehni- Tamburini / L T calcite deco.lposilion
Fig. 2. SEM nlicrograph o f ( a O produced bs' calcite decomposilion at ~ 10" Pa and 870 K.
cracks tend to disappear and are replaced by holes with irregular shapes. Air decomposed samples (see fig. 3), show a similar roughness, which can now be seen at lower magnifications due to its larger dimensions. Moreover, the rhombohedral habitus of the calcite grains begins to be masked by rounded edges and by some degree of neck formation between adjacent grains. Finally, for the sake of comparison, fig. 4 is pertinent to CaO particles after two days heating in air at 1600 K, and shows the typical morphology of a large sintering where original habitus and surface roughness have alsmost completely disappeared.
415
Fig. 4. SEM micrograph of CaO produced by calcite decomposilion and heated in air at 1600 K for two da~s.
X-ray scattering measurements show that the decomposition (P, T) conditions affects the microstructure of the CaO product, while its crystallographic nature always corresponds to the regular rock salt structure. Fig. 5 shows an example of the Fourier transform of the X-ray line profiles as obtained after WA microstrain elimination. Tables 1, 2 and 3 summarize the WA and single-line determinations of the size and microstrain parameters. These data show that higher working pressures correspond to remarkably larger crystallite sizes and smaller microstrains. The dependence from decomposition temperature is much smaller, since a 50 K increase of temperature is almost insignificant for high vacuum decomposi-
1.0
0.5
~,
\
0.0 0
Fig, 3. SEM micrograph of CaO produced by calcite decomposition in air and at 870 K.
10 Particle size (nm )
20
Fig. 5. Fourier transfbrm (after Stokes deconvolution and WA strain sublraction) of the (0 0 2)/(0 0 4) couple for CaO produced by calcite decomposition at 10 2 Pa and 870 K.
(i. ,S'pz#lo/o./_' ,.Ins'e/#~ti 778n//)/o't#ll/ L 7"ca~eric de<'Onll)O,~tlitm
416
Table 1 Crystallite mean sizes (nm) from Warren-Averbach analysis for CaO samples produced by calcite decomposition under different conditions. ,
Crystallographic direction
(I 1 11/(2221 (0 0 2)/(0041
Decomposition conditions
7
T P (K) (10 epa)
eq
820 870 820 870
8.5 10.5 12.0
P air (10 ePa) 10.5 19.5 15.0 18.5
i
j."\\
/~ /
I J i
i
1
36.5 4
0,25
37.5
0.35
0.45
0.55
0 65
sin(0)
tions, and only gives a slight increase o f crystallite sizes for ~ 102 Pa d e c o m p o s i t i o n s . C o n c e r n i n g the W A X S data. it is also worth n o t i n g thc a n i s o t r o p y o f m i c r o s t r u c t u r a l p a r a m e t e r s , which is best discussed using the W i l l i a m s o n - H a l l plots (fig. 6), where the q u a n t i t y (2w c o s 0 ) (2w is the full width at h a l f m a x i m u m , F W H M , 0 the d i f f r a c t i o n angle) is plotted against sin O. In these plots, scattered points indicate that the c o h e r e n t d i f f r a c t i o n d o m a i n s have different m e a n sizes along different crystallographic directions. If the d o m a i n shapes are almost isotropic (i.e. roughly s p h e r i c a l ) , the experi m e n t a l data are e x p e c t e d to lie on a single straight line, whose slope is p r o p o r t i o n a l to the m i c r o s t r a i n m e a n s q u a r e d v a l u e w i t h i n a single d o m a i n . Fig. 6 clearly shows, for samples d e c o m p o s e d at 820 K, the progressive change with d e c o m p o s i t i o n pressure o f the crystallites, which are first (Pd = 10 -~ Pa) small, a n i s o l r o p i c and strained, then (t',~= 10 e Pa) larger, isotropic and less strained, and finally ( a i r d e c o m p o s i t i o n ) still larger and unstrained. Fig. 7 shows a typical n i t r o g e n a d s o r p t i o n isot h e r m . T h e whole set o f N A I data are s u m m a r i z e d
Vig. (~. Williamson tlall plot l'or ('a() produced b\ calcilc decomposition al 0;211Km air (crosses). al 10' Pa (diamonds) and at I I Pa ( StlU[II'?S 1. in table 4, where the pore d i m e n s i o n s c o r r e s p o n d to the m a x i m u m o f the size d i s t r i b u t i o n f u n c t i o n s as calculated a s s u m i n g a roughly cylindrical porc shape (de Boer's [12] class A f o r m o f the hysteresis loop). T h e s e results agree well with those o f W A X S deter* m i n a t i o n s : we can see that the d e c o m p o s i t i o n t e m p e r a t u r e does not seem to directly affect in a significant way the m i c r o s t r u c t u r e , whereas higher d e c o m p o s i t i o n pressures p r o d u c e particles with s m o o t h e r surfaces and larger pores. T h e parallel change o f c w s t a l l i t e and pore m o r phologies are shown in fig. 8 for samples d e c o m posed u n d e r respectively similar e x p e r i m e n t a l c o n d i t i o n s . T h e c o r r e l a t i o n o f the m e a n crystallite sizes with B E T surfaces and with m e a n pore sizes is also nicely i n d i c a t e d by figs. 9 and 10 where both kinds o f m i c r o s t r u c t u r a l p a r a m e t e r s are plotted against each o t h e r in the p e r t i n e n t scales. In p a n i c ular, the e m p i r i c a l p r o p o r t i o n a l i t y shown by these
lablc 2 ("rystallile mean sizes ( nm ) from single line analysis for CaO samples produced by calcite decomposition tinder differcnl conditions. Decomposition conditions
('r$ stallographic direction
/ IK)
/'(Pal
(1 I 1)
('~.~001
1"~-20)
(3 I I)
(2_~.'3)
820 820 870 870 870
10 e 1(): 10 " 10 ~ air
8.5 17.(1 12.0 27.0 92.0
13.5 25.0 16.0 31.5 95.5
12.5 20.0 12.5 28.0 96.0
10.0 14.0 1(I.0 21.5 74.(I
11.5 16.0 10,0 21,5 74,(i
40 (}) tL5 90 0.() 21.5 54.0
G. Spinolo, U. ,,tnsehni- Tamburim / L T calcite decomposition Fable 3 Mean squared strains (10 ~') from Warren-Averbach analysis for CaO samples produced by calcite decomposition under different conditions. The results arc for both couples (1 1 I )/(2 2 2) and (0 0 2)/(0 0 4).
417
Table 4 Nitrogen adsorption data for CaO samples produced by calcite decomposition under different conditions. Decomposition conditions
S(BET)
Hysteresis
(m 2 g-~ )
type "~
Mean pore size (nm)
Decomposition conditions T(K)
P(10 ~Pa)
P (10-" Pa)
air
820 870
8.2 7.1
3.4 3.3
0.8
plots suggests that a mlcrosintering step is active during chemical decomposition and agrees well with previous observations [8] that fine particles oP'active" CaO can be microsintered after decomposition by interaction with CO2 gas at partial pressures lower than the equilibrium decomposition value. As a first conclusion, the present SEM, WAXS and NAI investigations clearly point to a nucleation-andgrowth mechanism. However, other measurements of the same type seem to indicate a different inference. For example, fig. 11 shows that the line width does not change significantly during decomposition time. This result, which is well known and is here reported only for the sake of completeness, has been invoked for a long time to support a shear transformation mechanism for decomposition of calcite structured-carbonates [2,3]. Once again, there is a good agreement with NAI measurements. Table 5 compares for a similar sample, BET surface and pore size data obtained at (roughly) half decomposition and on the final decomposition product.
T(K)
P(Pa)
820 820 870 870 870
10 -2 102 10-: 102 air
a}
79.0 54.0 82.5 39.0 12.0
A A/B A A/B
7.0 12.0 5.0 15.0
According to deBoer's classification [ 12 ].
Therefore, the analysis of these seemingly conflicting experimental results rules out the possibility of describing the thermal decomposition of calcite and i
J
]
•i/'V
4.0
Y
i
2.0 0
100
200
Particle size
0.0 ~ - ~ - ' ~ - - - ~ 0,0 0.5
P/Po
1.0
Fig. 7. Nitrogen adsorption isotherm for CaO produced by' calcite decomposition at 10- ~Pa and 820 K.
300
400
0
,
. . . . . . 1 O0
T--.
200
300
, 400
Pore diameter
Fig. 8. Crystalline size (left side) and pore size ( right side) probability distributions for CaO produced by calcite decomposition under different conditions: ~ 10 -2 Pa and 820 K (top line), ~ 10 --~ Pa and 870 K, ~ 102 Pa and 820 K, ~ 102 Pa and 870 K
(bottom line). Ordinates are arbitrary units. Abcissae are in /~ngstr6m.
418
(;. S't/~1~/¢, { '.. In%c/mi- J~ml,lu'lni / i. 7 caAw(, d{'c¢~mp¢)~ltl~}~t 100
.
.
.
1.0
.
.
.
.
.
{~
E
[:
[]
05
50 J
/ / c: 0.0
0.020
0.040
0.060
0.080
0.10
•
()
-
~
20(}
1/(nml)
,
-
.100
(}{lO
Thnc { rain )
Fig. % lc mpirical correlation bct\~een BET surl~ce areas and rccH~rocal cr}stalliR, sizes tor {'a() produced bx calcitc decomposition under dillL'renl conditions.
Fig. I I. T i m e c \ o l u l i o n of F W H M of the ( 2 0 0) X R I ) Imc during d e c o m p o s i t i o n at 820 K and I() ' P a .
other related divalent carbonates with reference to a single mechanism, either of the shear transformation type or transport-driven, but suggests that both mechanisms are parallely active. In our opinion, the overall mechanism is best described by a sequence of two steps. The first step is a very fast martensilic rearrangemcnt which completely transforms a given calcite gram as a whole when the activities of the reaction components at the external surface of the grain satist~ the t h e r m o d y n a m i c requirements for the thermal decomposition, including not only chemical, but also surface and strain contributions to free energy. The product of the first step is a bundle o f C a O cryslallites which keeps the external morphology of the parent grain and has a uniform and constant micro-
structure dependent on the geometrical and elastic parameters of the cwstal structures of reagent and solid product. The second step is the removal of the gaseous reaction product, CO> from the pores of the crystallites produced by the first step. Both rate and nature (surface diffusion, Knudsen effusion or fluid dynamic transport) of the second step heavily depend on the external conditions (mainly on the working pressure) and in turn affect the rate of the first step in an indirect way by modifying the t h e r m o d y n a m i c activity of the gaseous producl at the surface of the adjacent and yet u n d e c o m p o s e d calcite grains. Moreover, this step affects the residence time of the gas within the pores of the already decomposed grain, thus producing a more or less marked microsintering effect on this particular grain. The final result is that kinetics and microstruclure of the reaction depend on the external decomposition conditions, but is not possible to experimentally observe any time evolution of the microstructure.
200
-
-
/-
j~
_/ / /
10(} /
Table 5 Nitrogen adsorption data for ( ' a O samples produced by calcite h a l f a n d dull d e c o m p o s i t i o n at 820 K and 10 ~Pa.
12
16
(
nm
20
Ad,,ancement ofthcreaction
S( BET ) (m 3g I I
Hysteresis type
{nm }
)
Fig. l{}. I m p i r i c a l correlation bctx~een pore and cr',stallitc mean sizes /.~ l~}r('a{) produced b) calcite d e c o m p o s i t i o n under dif~'erent Londdi{}ns.
Mean pore size
0.5 1.0
71.0 79.0
A A
7.0 7.(}
(:. Spino/o, ('. 4 nse/mi- I'amt)urini / L T calcite decomposition
Acknowledgement This work has been supported by the Department of Education of the Italian Government (MPI). Help by dr. A m e d e o M a r i n i in t a k i n g S E M p h o t o g r a p h s is g r a t e f u l l y a c k n o w l e d g e d .
References [ 1] D. Beruto and A.W. Scare.\, Nature 263 (1976) 221. [2] N. Floqucl and J.-C. Niepce, J. Mat. Sci. 13 (1978) 766. [3] G. Bertrand, J. Chem. Phys. 82 (1978) 2536. [ 4 ] E.K. Powcll and A.W. Searcy, J. Am. Ceram. Soc. 66 ( 1982 ) ('42.
4t9
[5] D. Beruto and A.W. Searcy, J. Chem. Soc. Faraday 1 70 (1974) 2145. [6] D. Beruto. L. Barco and A.W. Searcx, J. Am. Ceram. Soc. 66 (1983) 893. [7] G. Spinolo and U. Anselmi-Yamburini, in: High tech ceramics, ed. P. Vincenzini (Elsevier, Amsterdam, 1987) p. 367. [8] D. Beruto. L. Barco and A.W. Searcy, J. Am. Ceram. Soc. 67 (1984) 512. [9] G. Spinolo, V. Massarotti and G. Campari, J. Phys. E-Sci. Instrum. 12 (1979) 1059. [ 10] B.E. Warren. X-ray diffraction (Addison-Wesley, Reading, 1969). [ 11 ] T. DeKeyser, l.J. Langford, E.J. Mittemejer and B.P. Vogels, J. Appl. CD'st. 15 ( 1982 ) 308. [12]J.H. deBoer, The structure and properties of porous materials ( Butterworths, London, 1958).
MECHANISM OF LOW TEMPERATURE CALCITE DECOMPOSITION ~ Giorgio S P I N O L O and U m b e r t o A N S E L M I - T A M B U R I N I Department ql~Physical Chemisto, and ('. S, 7~E./C. N.R. University o f Pa via, ~Tale Tara melli 16. 1 27 l O0 Pa via. ltall,
Received 6 June 1988: accepted for publication 4 July 1988
Wide angle X-scattering (WAXS) and nitrogen adsorption isotherm (NAI) determinations and SEM observations have been used to characterize the microstructure of CaO particles obtained by thermal decomposition of calcite powders at low temperalures (820-870 K), and yew low reaction rates (10 -s tool cm 2 s ~). The combined analysis of WAXS and NAI measurements shows that the microstructure is made of bundles of NaCl-structured and rod-shaped crystallites which are separated from each other by cylindrical pores and completely fill the volume of the corresponding calcite grain. Crystallite and pore sizes do not change during a single decomposition run but significanlly grow when higher working Pco: are used, starting from a low pressure limit which is almost indepenOent of the decomposition temperature. Both size distributions change in a parallel way and the size increases correspond to a marked decrease of shape anisotropy and microstrain content. The conflicting evidence that the microstructure is constant during the decomposition time, but heavily depends on the external conditions, rules out the possibility of describing the primal' reaction step with reference to a single mechanism, either of the shear-transformation type or transportdriven. The work discusses a mechanism that takes into account both aspects at the same time. and shows how the heat- and masstransport processes can affect the microstructure without affecting its time evolution.
I. Introduction The thermal d eco m p o s i ti o n o f calcite is probably the most investigated heterogeneous reaction because o f its technological importance. Moreover, t h e r m o d y n a m i c , structural and transport data on both reagents and reaction products are very well known, so that calcite, and other structurally related divalent carbonates, are a very convenient chemical model to gain a better understanding o f the mechanism o f e n d o t h e r m i c d e c o m p o s i t i o n reactions described by the stoichiometric scheme: A (solid)--. B (solid) + C ( g a s ) . It is well known that, under irreversible conditions, the reaction produces "specially reactive" [ 1 ] oxides whose microstructure is made o f bundles o f rod-shaped crystallites separated from each other by cylindrical pores. A n u m b e r o f studies on topotactic relations or microstructure (see [2,3] and references therein q u o t ed ) are the main argument for classiBased on a thesis submitted by U. Anselmi-Tamburini in partial fulfillment of the requirements for a Ph.D. at University of Pavia.
fying this reaction am o n g the shear transformations, in spite o f the experimental evidence [4] that the external (7", P) variables influence the microstruclure. Indeed, t h e r m o d y n a m i c , kinetic, structural and microstructural investigations on this subject show complex, and somewhat conflicting, results [ 2 , 4 - 7 ] , thus indicating that different reaction steps are relevant to the overall reaction mechanism, and the external variables have different effects on these steps. We may argue that, in some cases, the m ech an i sm is mainly affected by transport properties, i.e. heat transport or transport o f the gaseous reaction product through the porous microstructure o f the solid product. As a further difficulty, the experimental investigation is made more complex by a microsintering effect which is active also when the partial pressure o f the gaseous product is well below the equilibrium value [ 8 ]. This work describes a systematic study o f the microstructure o f calcium oxide produced by decomposing calcite powders at the lowest practical temperatures and under very low reaction fluxes, when we can presume that the features o f the primary reaction steps are less masked by following rearrange-
0 167-2738/89/$ 03.50 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing D iv is i o n )
414
G. .S'Fil~o/o. ~ '. I m c h m 7~mhttrinl / L lcalcilu duc, Unl~,~imm
rearrangements, Isothermal-isobaric decompositions have been investigated at T , = 820 and 870 K and carbon dioxide partial pressures P,, ,: in the range [10 " P~,
2. Experimental Microstructural parameters were measured with wide-angle X-scattering ( W A X S ) and nitrogen adsorption isotherm ( N A I ) d e t e r m i n a t i o n s on CaO samples produced by decomposing reagent grade calcite powders previously ground and sieved ( 5-30 ~m). X-ray scattering line profiles were taken with a Philips PW 1730 diffractometer equipped with Cu radiation tube operated at 45 kV and 22 mA. Soller slits, graphite m o n o c h r o m a t o r on the diffracted beam and scintillation counter. The oxide material was produced m situ under i s o t h e r m a l - i s o b a r i c conditions using an environmental chamber [9] enclosing a small electrical furnace and connected 1o a vacuum system (mechanical pump, diffusion p u m p and liquid nitrogen trap). Step scan data were collected at 0 . 0 2 (20) steps over the 25-85 range with 100 s (peaks) or 10 s (background) step times. Each scan program was looped until 40000 counts/step had been collected for the diffraction lines to be analysed. Processing of the raw scattering data was done [ 10] according to the following steps: background subtraction, smoothing, sampling at equal sin 0 intervals, FFT, Stokes deconvolution of the instrumental aberrations, and W a r r e n - A v e r b a c h strain subtraction using the (l 1 1 ) / ( 2 2 2 ) and ( 2 0 0 ) / ( 4 0 0) couples. The whole set of X-ray scattering lines, ( 1 1 1 ) , ( 2 0 0 ) , ( 2 2 0 ) , ( 3 1 1 ) , ( 2 2 2 ) , ( 4 0 0 ) was also analysed with single line methods [l I ]. Data acquisition and analysis was done on a dcsktop c o m p u t e r with h o m e m a d e software. Each experimental run required 2-10 days for decomposition and two more days for data acquisition. Calcite decomposition was m o n i t o r e d with X-ray patterns taken from time to time on t e m p e r a t u r e and recorded in continuous scan mode: For NAI d e t e r m i n a t i o n s we used a Cahn RG 3885 microbalance connected to a v a c u u m line and to a
high vacuum p u m p i n g system. The calcite samplcs, placed in a Pl crucible, were first d e c o m p o s e d using an external furnace. After complete decomposition (in some cases also after half d e c o m p o s i t i o n ) , NAI data were collected by replacing the furnace with a liquid nitrogen filled Dewar flask. Another flask was then placed around the rate arm of the microbalance to keep its symmetric arrangement. BET surface areas were calculated in the usual way using the low I L . points [/',q, <0.3 I',,], pore size distributions using the high t'~,. data according to the pertinent hysteresis loop type [12]. After WAXS or NAI determinations, part of the samples was transferred under inert atmosphere to a gold sputtering unit for examination in a ( ' a m bridge Steroscan S.E. Microscope.
3. Results and discussion SEM observation of the samples produced by high vacuum decomposition (see fig. 1 ) shows that the external /latqlu.s of the parent calcite grains is kept almost unmodified, but for the presence of large cracks. The same c o m m e n t could be made for the low resolution photographs of samples obtained with 1 Torr ( - l 0 t Pa) decomposition. Now, high resolution photographs (fig. 2) show a fine microstruclure, typically an orange-peal roughness, which differently affects different laces. Also, the large
Fig. 1. gEM m icrograph of('a() produced b} calcite dccomposb lional - I0
'Paand
870K.
G. Spinolo, U. Ansehni- Tamburini / L T calcite deco.lposilion
Fig. 2. SEM nlicrograph o f ( a O produced bs' calcite decomposilion at ~ 10" Pa and 870 K.
cracks tend to disappear and are replaced by holes with irregular shapes. Air decomposed samples (see fig. 3), show a similar roughness, which can now be seen at lower magnifications due to its larger dimensions. Moreover, the rhombohedral habitus of the calcite grains begins to be masked by rounded edges and by some degree of neck formation between adjacent grains. Finally, for the sake of comparison, fig. 4 is pertinent to CaO particles after two days heating in air at 1600 K, and shows the typical morphology of a large sintering where original habitus and surface roughness have alsmost completely disappeared.
415
Fig. 4. SEM micrograph of CaO produced by calcite decomposilion and heated in air at 1600 K for two da~s.
X-ray scattering measurements show that the decomposition (P, T) conditions affects the microstructure of the CaO product, while its crystallographic nature always corresponds to the regular rock salt structure. Fig. 5 shows an example of the Fourier transform of the X-ray line profiles as obtained after WA microstrain elimination. Tables 1, 2 and 3 summarize the WA and single-line determinations of the size and microstrain parameters. These data show that higher working pressures correspond to remarkably larger crystallite sizes and smaller microstrains. The dependence from decomposition temperature is much smaller, since a 50 K increase of temperature is almost insignificant for high vacuum decomposi-
1.0
0.5
~,
\
0.0 0
Fig, 3. SEM micrograph of CaO produced by calcite decomposition in air and at 870 K.
10 Particle size (nm )
20
Fig. 5. Fourier transfbrm (after Stokes deconvolution and WA strain sublraction) of the (0 0 2)/(0 0 4) couple for CaO produced by calcite decomposition at 10 2 Pa and 870 K.
(i. ,S'pz#lo/o./_' ,.Ins'e/#~ti 778n//)/o't#ll/ L 7"ca~eric de<'Onll)O,~tlitm
416
Table 1 Crystallite mean sizes (nm) from Warren-Averbach analysis for CaO samples produced by calcite decomposition under different conditions. ,
Crystallographic direction
(I 1 11/(2221 (0 0 2)/(0041
Decomposition conditions
7
T P (K) (10 epa)
eq
820 870 820 870
8.5 10.5 12.0
P air (10 ePa) 10.5 19.5 15.0 18.5
i
j."\\
/~ /
I J i
i
1
36.5 4
0,25
37.5
0.35
0.45
0.55
0 65
sin(0)
tions, and only gives a slight increase o f crystallite sizes for ~ 102 Pa d e c o m p o s i t i o n s . C o n c e r n i n g the W A X S data. it is also worth n o t i n g thc a n i s o t r o p y o f m i c r o s t r u c t u r a l p a r a m e t e r s , which is best discussed using the W i l l i a m s o n - H a l l plots (fig. 6), where the q u a n t i t y (2w c o s 0 ) (2w is the full width at h a l f m a x i m u m , F W H M , 0 the d i f f r a c t i o n angle) is plotted against sin O. In these plots, scattered points indicate that the c o h e r e n t d i f f r a c t i o n d o m a i n s have different m e a n sizes along different crystallographic directions. If the d o m a i n shapes are almost isotropic (i.e. roughly s p h e r i c a l ) , the experi m e n t a l data are e x p e c t e d to lie on a single straight line, whose slope is p r o p o r t i o n a l to the m i c r o s t r a i n m e a n s q u a r e d v a l u e w i t h i n a single d o m a i n . Fig. 6 clearly shows, for samples d e c o m p o s e d at 820 K, the progressive change with d e c o m p o s i t i o n pressure o f the crystallites, which are first (Pd = 10 -~ Pa) small, a n i s o l r o p i c and strained, then (t',~= 10 e Pa) larger, isotropic and less strained, and finally ( a i r d e c o m p o s i t i o n ) still larger and unstrained. Fig. 7 shows a typical n i t r o g e n a d s o r p t i o n isot h e r m . T h e whole set o f N A I data are s u m m a r i z e d
Vig. (~. Williamson tlall plot l'or ('a() produced b\ calcilc decomposition al 0;211Km air (crosses). al 10' Pa (diamonds) and at I I Pa ( StlU[II'?S 1. in table 4, where the pore d i m e n s i o n s c o r r e s p o n d to the m a x i m u m o f the size d i s t r i b u t i o n f u n c t i o n s as calculated a s s u m i n g a roughly cylindrical porc shape (de Boer's [12] class A f o r m o f the hysteresis loop). T h e s e results agree well with those o f W A X S deter* m i n a t i o n s : we can see that the d e c o m p o s i t i o n t e m p e r a t u r e does not seem to directly affect in a significant way the m i c r o s t r u c t u r e , whereas higher d e c o m p o s i t i o n pressures p r o d u c e particles with s m o o t h e r surfaces and larger pores. T h e parallel change o f c w s t a l l i t e and pore m o r phologies are shown in fig. 8 for samples d e c o m posed u n d e r respectively similar e x p e r i m e n t a l c o n d i t i o n s . T h e c o r r e l a t i o n o f the m e a n crystallite sizes with B E T surfaces and with m e a n pore sizes is also nicely i n d i c a t e d by figs. 9 and 10 where both kinds o f m i c r o s t r u c t u r a l p a r a m e t e r s are plotted against each o t h e r in the p e r t i n e n t scales. In p a n i c ular, the e m p i r i c a l p r o p o r t i o n a l i t y shown by these
lablc 2 ("rystallile mean sizes ( nm ) from single line analysis for CaO samples produced by calcite decomposition tinder differcnl conditions. Decomposition conditions
('r$ stallographic direction
/ IK)
/'(Pal
(1 I 1)
('~.~001
1"~-20)
(3 I I)
(2_~.'3)
820 820 870 870 870
10 e 1(): 10 " 10 ~ air
8.5 17.(1 12.0 27.0 92.0
13.5 25.0 16.0 31.5 95.5
12.5 20.0 12.5 28.0 96.0
10.0 14.0 1(I.0 21.5 74.(I
11.5 16.0 10,0 21,5 74,(i
40 (}) tL5 90 0.() 21.5 54.0
G. Spinolo, U. ,,tnsehni- Tamburim / L T calcite decomposition Fable 3 Mean squared strains (10 ~') from Warren-Averbach analysis for CaO samples produced by calcite decomposition under different conditions. The results arc for both couples (1 1 I )/(2 2 2) and (0 0 2)/(0 0 4).
417
Table 4 Nitrogen adsorption data for CaO samples produced by calcite decomposition under different conditions. Decomposition conditions
S(BET)
Hysteresis
(m 2 g-~ )
type "~
Mean pore size (nm)
Decomposition conditions T(K)
P(10 ~Pa)
P (10-" Pa)
air
820 870
8.2 7.1
3.4 3.3
0.8
plots suggests that a mlcrosintering step is active during chemical decomposition and agrees well with previous observations [8] that fine particles oP'active" CaO can be microsintered after decomposition by interaction with CO2 gas at partial pressures lower than the equilibrium decomposition value. As a first conclusion, the present SEM, WAXS and NAI investigations clearly point to a nucleation-andgrowth mechanism. However, other measurements of the same type seem to indicate a different inference. For example, fig. 11 shows that the line width does not change significantly during decomposition time. This result, which is well known and is here reported only for the sake of completeness, has been invoked for a long time to support a shear transformation mechanism for decomposition of calcite structured-carbonates [2,3]. Once again, there is a good agreement with NAI measurements. Table 5 compares for a similar sample, BET surface and pore size data obtained at (roughly) half decomposition and on the final decomposition product.
T(K)
P(Pa)
820 820 870 870 870
10 -2 102 10-: 102 air
a}
79.0 54.0 82.5 39.0 12.0
A A/B A A/B
7.0 12.0 5.0 15.0
According to deBoer's classification [ 12 ].
Therefore, the analysis of these seemingly conflicting experimental results rules out the possibility of describing the thermal decomposition of calcite and i
J
]
•i/'V
4.0
Y
i
2.0 0
100
200
Particle size
0.0 ~ - ~ - ' ~ - - - ~ 0,0 0.5
P/Po
1.0
Fig. 7. Nitrogen adsorption isotherm for CaO produced by' calcite decomposition at 10- ~Pa and 820 K.
300
400
0
,
. . . . . . 1 O0
T--.
200
300
, 400
Pore diameter
Fig. 8. Crystalline size (left side) and pore size ( right side) probability distributions for CaO produced by calcite decomposition under different conditions: ~ 10 -2 Pa and 820 K (top line), ~ 10 --~ Pa and 870 K, ~ 102 Pa and 820 K, ~ 102 Pa and 870 K
(bottom line). Ordinates are arbitrary units. Abcissae are in /~ngstr6m.
418
(;. S't/~1~/¢, { '.. In%c/mi- J~ml,lu'lni / i. 7 caAw(, d{'c¢~mp¢)~ltl~}~t 100
.
.
.
1.0
.
.
.
.
.
{~
E
[:
[]
05
50 J
/ / c: 0.0
0.020
0.040
0.060
0.080
0.10
•
()
-
~
20(}
1/
,
-
.100
(}{lO
Thnc { rain )
Fig. % lc mpirical correlation bct\~een BET surl~ce areas and rccH~rocal cr}stalliR, sizes tor {'a() produced bx calcitc decomposition under dillL'renl conditions.
Fig. I I. T i m e c \ o l u l i o n of F W H M of the ( 2 0 0) X R I ) Imc during d e c o m p o s i t i o n at 820 K and I() ' P a .
other related divalent carbonates with reference to a single mechanism, either of the shear transformation type or transport-driven, but suggests that both mechanisms are parallely active. In our opinion, the overall mechanism is best described by a sequence of two steps. The first step is a very fast martensilic rearrangemcnt which completely transforms a given calcite gram as a whole when the activities of the reaction components at the external surface of the grain satist~ the t h e r m o d y n a m i c requirements for the thermal decomposition, including not only chemical, but also surface and strain contributions to free energy. The product of the first step is a bundle o f C a O cryslallites which keeps the external morphology of the parent grain and has a uniform and constant micro-
structure dependent on the geometrical and elastic parameters of the cwstal structures of reagent and solid product. The second step is the removal of the gaseous reaction product, CO> from the pores of the crystallites produced by the first step. Both rate and nature (surface diffusion, Knudsen effusion or fluid dynamic transport) of the second step heavily depend on the external conditions (mainly on the working pressure) and in turn affect the rate of the first step in an indirect way by modifying the t h e r m o d y n a m i c activity of the gaseous producl at the surface of the adjacent and yet u n d e c o m p o s e d calcite grains. Moreover, this step affects the residence time of the gas within the pores of the already decomposed grain, thus producing a more or less marked microsintering effect on this particular grain. The final result is that kinetics and microstruclure of the reaction depend on the external decomposition conditions, but is not possible to experimentally observe any time evolution of the microstructure.
200
-
-
/-
j~
_/ / /
10(} /
Table 5 Nitrogen adsorption data for ( ' a O samples produced by calcite h a l f a n d dull d e c o m p o s i t i o n at 820 K and 10 ~Pa.
12
16
(
nm
20
Ad,,ancement ofthcreaction
S( BET ) (m 3g I I
Hysteresis type
{nm }
)
Fig. l{}. I m p i r i c a l correlation bctx~een pore and cr',stallitc mean sizes /.~ l~}r('a{) produced b) calcite d e c o m p o s i t i o n under dif~'erent Londdi{}ns.
Mean pore size
0.5 1.0
71.0 79.0
A A
7.0 7.(}
(:. Spino/o, ('. 4 nse/mi- I'amt)urini / L T calcite decomposition
Acknowledgement This work has been supported by the Department of Education of the Italian Government (MPI). Help by dr. A m e d e o M a r i n i in t a k i n g S E M p h o t o g r a p h s is g r a t e f u l l y a c k n o w l e d g e d .
References [ 1] D. Beruto and A.W. Scare.\, Nature 263 (1976) 221. [2] N. Floqucl and J.-C. Niepce, J. Mat. Sci. 13 (1978) 766. [3] G. Bertrand, J. Chem. Phys. 82 (1978) 2536. [ 4 ] E.K. Powcll and A.W. Searcy, J. Am. Ceram. Soc. 66 ( 1982 ) ('42.
4t9
[5] D. Beruto and A.W. Searcy, J. Chem. Soc. Faraday 1 70 (1974) 2145. [6] D. Beruto. L. Barco and A.W. Searcx, J. Am. Ceram. Soc. 66 (1983) 893. [7] G. Spinolo and U. Anselmi-Yamburini, in: High tech ceramics, ed. P. Vincenzini (Elsevier, Amsterdam, 1987) p. 367. [8] D. Beruto. L. Barco and A.W. Searcy, J. Am. Ceram. Soc. 67 (1984) 512. [9] G. Spinolo, V. Massarotti and G. Campari, J. Phys. E-Sci. Instrum. 12 (1979) 1059. [ 10] B.E. Warren. X-ray diffraction (Addison-Wesley, Reading, 1969). [ 11 ] T. DeKeyser, l.J. Langford, E.J. Mittemejer and B.P. Vogels, J. Appl. CD'st. 15 ( 1982 ) 308. [12]J.H. deBoer, The structure and properties of porous materials ( Butterworths, London, 1958).