- Email: [email protected]
β′-sialon preparation from kaolinitic clays
Applied Clay Science, 7 (1993) 407-420 Elsevier Science Publishers B.V., Amsterdam
407
fl'-sialon preparation from kaolinitic clays A.D. Mazzoni, E.F. Aglietti and E. Pereira Centro de Tecnologia de Recursors Minerales y Ceramica (CETMIC). CONICET-CIC-UNLP, Camino Parque Centenario y 506, C.C. 49 (1897), M.B. Gonnet, Buenos Aires, Argentina (Received February 28, 1992; revised and accepted September 29, 1992)
ABSTRACT Mazzoni, A.D., Aglietti, E.F. and Pereira, E., 1993. ff-sialon preparation from kaolinitic clays. Appl. Clay Sci., 7: 407-420. Kaolinitic clays are suitable raw materials for the production offf-sialons and other compounds of the Si-AI-O-N system. These compounds are obtained by carboreduction and simultaneous nitriding of the mineral. Seven kaolins with different SIO2/A1203 ratios and quartz contents were mixed with carbon-black. They were reacted at about 1400°C in a nitrogen atmosphere. Several series of experiments were carried out to establish the influence of various parameters on fl'-sialon formation. Chemical composition and mineralogy of the starting material were decisive in fl'-sialonyield and determined which by-products were formed. From all clays, ff-sialon was the primary reaction product. Fe203 naturally present in the clay or as an additive favours the formation of nitrogenous phases. Remarkable quantities of silicon originating essentially from quartz are incorporated into the fl'-sialons. On the other hand, a SiO(g) loss occurs in these reactions mainly when working at low carbon/ clay ratios and with large N2 volumes.
INTRODUCTION
Sialons are compounds of the Si-AI-O-N system (Jack, 1976; Sorrell, 1983). The phases of this system, which correspond to the formula Si6_zAlzOzNs_zwith 0 < Z~< 4.2, are the fl'-sialons. Their structure is derived from fl-Si3N4 and, therefore, these compounds have physical properties similar to fl-Si3N4. The chemical properties offf-sialons are analogous to alumina (Jack, 1976; Sorrell, 1983 ). These phases are used primarily in the production of ceramic pieces with high thermomechanical properties and as additives to improve properties of other ceramic materials (A1203, ZrO2, SiC, etc. ). Generally fl'-sialons are prepared by starting from a mixture of compounds Correspondence to: A.D. Mazzoni, Centro de Tecnologia de Recursors Minerales y Ceramica (CETMIC). CONICET-CIC-UNLP, Camino Parque Centenario y 506, C.C. 49 (1897), M.B. Gonnet, Buenos Aires, Argentina.
0169-1317/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
408
A.D. MAZZONIET AL.
such as Si3N4, A1203, A1N, SIO2, Si2N20 (Jack, 1976; Sorrell, 1983 ). Nevertheless, there are alternative methods for preparing these phases, e.g., nitriding of SiO2 and A1 mixtures in a N2 atmosphere (Umebayashi et al., 1977 ); nitriding of silica, aluminium and silicon mixtures in a N2 atmosphere (Umebayashi et al., 1980); nitriding of volcanic ash and aluminium in a N2 atmosphere (Umebayashi et al., 1975 ); kaolinite nitriding in an NH3 atmosphere (Wild, 1976 ); reaction of metakaolinite and Si3N4 in a N2 atmosphere (Wild, 1975 ); nitriding of silicon and aluminium mixtures in a N2 atmosphere (Torre and MoceUin, 1976 ), etc. Other alternative methods include carbonitriding reactions (carboreduction and simultaneous nitriding) of oxygen compounds. These reactions are flexible since they permit the formation of different nitrogenous ceramics depending on the starting material, e.g., Si3N4 when starting from quartz or any other source of SiO2 (Zhang and Cannon, 1984); AIN starting from A1203 ( Silverman, 1988 ) and ff -sialons starting from natural alumino-silicates (Lee and Cutler, 1979 ). Carbonitriding reactions that lead to the formation offf-sialons from minerals such as pyrophilite (1), kaolinite (2) and sillimanite (3) can be described as follows: A1203 "4SIO2 "4"9C+ 3N2 --~Si4Al2O2 N6 d- 9CO
( 1)
3 (AlE03 •2SIO2 ) + 15C + 5N2 --*2Si3AI303 N5 + 15CO
(2 )
2 (A1203" SiO2 ) + 6C + 2N2 -~ Si2A14O4 N4 + 6CO
(3)
The production process offf-sialons according to reactions 1-3 is complex and depends on several parameters such as: C/kaolinite ratio (Mukerji and Bandyopadhyay, 1988; Sugahara et al., 1989); SIO2/A1203 ratio (Mukerji and Bandyopadhyay, 1988 ); N2 volume (Van Dijen and Metselaar, 1985; Higgins and Hendry, 1986 ); Fe203 content (Higgins and Hendry, 1986; Mukerji and Bandyopadhyay, 1988; Sugahara et al., 1989 ). The objective here is to study the influence of various parameters including carbon/clay ratio, Fe203 content, N2 flow and SiO2/A1203 ratio of clay on the formation offf-sialons. The goal of the study was to determine if sialons can be produced from clays of the Argentine Republic whose main mineralogic components are kaolinite and quartz. EXPERIMENTAL METHODS AND TECHNIQUES
Raw materials
Six clays from the Argentina and the Fisher kaolin from the USA were used. Their main mineralogic components are kaolinite and quartz. Chemical analyses of these clays are shown in Table 1. The carbon materials used in the
409
#'-SIALONPREPARATIONFROMKAOLINITICCLAYS TABLE l Chemical composition (wt.%) of the clays used in the experiments
Puente (P) Rhotemberger (R) Lote 8 (L) Veronica (Ve) Frentze (Z) Tincar (T) Fisher (F)
SiO2 (%)
A1203 (%)
Fe203 (%)
TiO2 (%)
CaO (%)
MgO (%)
Na20 K20 (%)
L.O.I. (1000°C)
44.2 48.7 60.5 59.8 69.9 69.3 42.5
36.0 33.6 26.3 26.6 19.6 19.4 40.8
2.00 1.20 0.37 1.90 0.92 1.12 0.52
1.00 1.10 0.12 0.30 0.40 0.45 2.10
0.10 0.12 0.40 0.05 0.40 0.60 n.d.
2.10 1.20 0.06 0.15 0.17 0.10 n.d.
0.98 2.10 0.67 1.48 0.77 0.49 n.d.
13.2 12.0 11.6 9.3 7.4 8.4 13.9
Puente: Departamento Independencia, Provincia La Rioja. Rhotemberger: Tandil, Provincia Buenos Aires. Lote 8: San Juli~in, Provincia Santa Cruz. Ver6nica: Sur del Rio, Provincia Chubut. Frente: San Juli~in, Provincia Santa Cruz. Tincar: San Julifi, Provincia Santa Cruz. Fisher: Georgia, USA. TABLE2 Physicochemical properties of selected carbon materials (wt.%)
Carbon-black
Petroleum coke (calcined) Petroleum coke ( non-calcined )
Particle size
Specific surface area BET (m2/g)
Ash (%)
Volatiles (%)
Carbon (%)
- 325 ASTM MESH - 325 ASTM MESH - 325 ASTM MESH
45.8
2
1
97
5.0
0.15
--
99.8
4.0
0.16
13.9
~ 86
experiments are characterized in Table 2. In addition, C13Fe.6H20 (AR) and calcined Fe203 (BDH) were used as Fe203 additives. The nitrogen applied contained less than 5 ppm of O2 and H20.
Experimental methods and equipment Experiments were carried out in a vertical alumina reactor with N2 flowing through a fixed bed supported on a perforated graphite disk. The heating of samples took place in a SiC furnace, with temperature control and thermocouple of Pt-Pt Rh (10%). The reaction products were analyzed by X R D with a Philips PW 1140/00 equipment. Peak areas were used to calculate the amounts of crystalline phases which were expressed in arbitrary units (A.U.). The validity of each of these A.U. is restricted to one individual figure only
410
A.D. MAZZONIET AL.
and the A.U. indicate the evolution of each crystalline phase in a determined series of experiments. They do not indicate absolute quantities and can not be used for a comparison among different plots. Specific surface areas (BET) were determined using a Micromeritics adsorption equipment with a relative error of approximately 5%. Samples were observed by means of a scanning electron microscope, Philips 505. Crystallite sizes were determined by means of Scherrer's equation using quartz as a standard. Samples were prepared by wet mixing of calculated quantities of clay and carbon material. These mixtures were pressed as pellets or extruded. Fe203 was added to the clay using two methods: ( 1 ) precipitation from a FeC13 solution and (2) mixing of the clay with Fe203 aggregates using acetone as a dispersing medium. RESULTS
Carbon content
From the comparison of physicochemical properties of different carbon material (Table 2) it became obvious that carbon-black was best suited for the experiments. Due to its small particle size and large specific surface area, carbon-black is more reactive than the other available carbon materials which ensures intimate contact of carbon with clay. The influence of the C/clay ( w / w ) ratio was studied for two clays (P and R) by changing the C/clay ratio in the experiments and keeping the other
~18 Stoichio.
>. ¢_
14 o°V L.
,~ 100
J I,~ ~
~.~
/
o Quartz • x-Phase AMuHite • (~ -AI 2 03
o
~ 60
o. 208
0.258
0.333 0.377 C/Clay ( W t / W t )
Fig. 1. Influence o f c a r b o n content o n clay " R " (Temp.: 1400 ° C; time: 210 min.; VN2:5 c m / s ) .
,8'-SIALONPREPARATIONFROMKAOLINITICCLAYS
~zoo
411
Stoichio.
¢_
== #
•
==.
100
X - Phase
A Mullite ¢ t~ALON" 0 /3'- Sialon
:>. ¢-
0~ ¢ ' ~ 0.180 0.208
0.265 0.328 C/Clay ( W t / W t )
Fig. 2. Influence of carbon content on clay "P" (Temp.: 1400°C; time 190 min.; VN2 5 cm/s).
reaction conditions constant. In this way, variation in the reaction products could be attributed to changes in the carbon content. Figures 1 and 2 show the evolution of the principal crystalline phases (in A.U. ) as a function of the C/clay ratio. The experimental results of both clays (P and R ) indicate that the fl'-sialon phase increases with increasing carbon. Positive correlations of carbon with t~-AI203 and "A 1ON" are also noted. Mullite, on the other hand, decreases when the C/clay ratio increases. In samples with carbon contents lower than those displayed in Figs. 1 and 2, a great loss of SiO(g) is observed and muUite becomes the dominant crystalline phase. In these samples, cristobalite is present in smaller quantities than where the clay was calcined without carbon. Small quantities of nitrogenous phases are present as well. Consequently, in samples where carbon is added the following reaction is considered to be the most important:
SiO2+C--,SiO(8 ) +CO(g)
(4)
Silica needles (Fig. 3 ) were noticed in one sample and their formation is due to SiO(g). SiO(g) loss is verified in all samples, but it is minor in those
412
A.D. MAZZONI ET AL.
Fig. 3. Silicaneedlesforming due to SiO volatilization ( × 10). with a high carbon content. These results confirm previous work by Yoshimatsu et al. ( 1989 ) and can be described by reaction ( 5 ).
SiO(g) -k C---~SiC + CO(g)
(5)
Cristobalite was not detected in compositions recorded in Figs. 1 and 2. Quartz (Fig. 1 ) mainly reacts with carbon forming SiC, but also reactions (4) and ( 5 ) occur. SiC was not detected in the final products as it intervenes in mullite reduction. The X-phase is a nitrogenous phase of the system S i - A I - O - N with a N2 content lower than fl'-sialon. This is an intermediate phase in clay carbonitriding (reaction 2). With increasing C/clay ratio the amount of X-phase increases, reaches a m a x i m u m at C/clay values of about 0.28 (Fig. 1 ) and 0.18 (Fig. 2 ) for clays R and P respectively and then decreases with the increase of the carbon content. The X-phase is an intermediate in the reaction and has a lower nitrogen content than p'-sialon. Before reaching the final reaction temperature, thermal decomposition of kaolinite produces mullite and silica. The addition of greater quantities of carbon results in a replacement of oxygen by nitrogen, which causes an increase in the amount of nitrogenous phases and a decrease of oxides (mullitesilica). An appreciable decomposition offl'-sialon and formation of larger amounts of reduced phases such as A I N or A1N polytypes did not take place in these experiments. This is, possibly, a consequence of the temperature range used ( 1600-1700 K). Based upon experimental results, excess carbon was utilized to promote a high yield offl'-sialon.
ff-SIALONPREPARATIONFROMKAOLINITICCLAYS
~...~.~Oio. 5
200 o
~1601 ~120
413
~f
'fJ f /
,/~ o=80
j . o ''~
0.4
c
f ¢_
0 ~'-Sialon II IX- AI203
0.3
-
0.2
~
o~ to i
i
~_ 4 o 0 0.0
n 9.56
i , a 0.0 19.12 28.69 38.25 Pressure (NPA)
Fig. 4. Variations of the reaction products as a function of load pressure (Temp.: 1400°C; time 210 rain; VN2:5 cm/s).
Shape and load pressure of samples Several experiments were carried out to study the influence of molding pressure on the preparation of pellets by using clay R with a C/clay ratio of 0.366. Different pelletising pressures were applied when preparing pellets of similar shape and size. These pellets were subjected to equal reaction conditions. A comparison of the reaction products (Fig. 4 ) shows that with increasing pressure the quantity offl'-sialon increases while the a-A1203 phase tends to decrease. Furthermore, a slight tendency of decreasing weight loss with increasing pressure was noticed. In another series of experiments these results were repeated and are attributed to the increasingly intimate contact of the solid reactives with increasing pelletising pressure. The reduction in weight loss, in spite of increasing amounts of the reaction products, could be explained by a diminution of the SiO (g) loss, which, in turn, could also explain the decrease of 0~-A1203. Kinetic measurements performed on pressed pellets of clay P proved that the reaction is not controlled by diffusion mechanisms (Mariano et al., 1983). As much as porosity is created by dehydration of clay during heating, this observation applies even to strongly compacted samples.
Nitrogenflow The N2 flOWvolume (Van Dijen and Metselaar, 1985 ) is an important factor in controlling the CO content during the reaction. With increasing N2 flow
414
A.D. M A Z Z O N I E T AL.
4"
•
• ( ] ( - AI 2U 3 *
"ALON"
o /3'-Sial0n
• •
I
70
II •-
•¢-
I
65
610
sr5
510
415
410
c Cristobal~te • ALN Pofytype
•_
I
35
310
215
210
Sia,o~
115
o2e(cu K(X) Fig. 5. XRD pattern of the reaction product obtained under conditions of great SiO (g) loss. A Mu{Jite
of3-Si C
• x - Phase
, ,ton Silic,de
~a ~
iA
715 70 6~5 610 515 510 415 410 315 310 2J5 2i0 It5 II0 °2e (Cu KCK)
Fig. 6. XRD pattern of the reaction product obtained with deficient elimination of CO (g).
volumes, CO elimination increases, which leads to the repression of nitrogenous product formation (Higgins and Hendry, 1986; Siddiqi et al., 1986). As N2 may transport SiO (g), high N2 flow volumes are accompanied with high silica losses (Yoshimatsu et al., 1989 ). Several tests were performed in order to find the most suitable N2 flow rate. Pressed samples with high carbon contents, through which gas was flowing at a rate ranging from 1.7 to 13.0 cm/s, yielded similar final products. There were, however, slight differences in the amount of aluminium-rich phases such as ot-A1203. The increase in AI phases was caused by higher SiO (g) losses due to increasing N2 flow rates. Samples with a minor carbon content, prepared as extruded (greater porosity than pressed pellets) form a variety of reaction products (Fig. 5). The results imply that CO (g) elimination in the fixed bed reactor is very efficient, which means that the use of low N2 flow volumes are adequate. On the other hand, experiments performed with defficient CO (g) elimination (alumina crucible) lead to the formation of a mixture of mullite, fl-SiC, some X-phase, little iron silicide and no fl' -sialons (Fig. 6 ). Fe203 content
The catalytic effect o f F e 2 0 3 , added to clays as a precipitate or a solid phase, is shown in Figs. 7 and 8 respectively. Both figures show a positive correlation
ff-SIALON PREPARATIONFROMKAOLINITICCLAYS
415
200 • x-Phase A Mullite oOC -AI203 O f~'-Sialo n"
tn c :=
15(
"7,
~ 100 Q.
=
5(
00
1
2 F % 0 3 (%)
3
Fig. 7. Influence of Fe203 content on the reaction products (Fe203 precipitation on kaolin F; Temp.: 1410°C; time: 190 rain.; VN2:5 c m / s ) .
225 ~, • x-Phase ~ c
~ ~ . z . ~ ~
>" o
~5o-
A Mullite • (~- Al203 O ['~'- Sial0n & Ir0n Silicide
"~o
__ °
C3_
-g
g L
(
Fe203 ('/.)
Fig. 8. Influence of Fc203 content on the reaction products (solid Fe203 mixed with kaolin F; Temp.: 1410°C; time 190 min..; VN2:5 c m / s ) .
416
A.D. MAZZONI ET AL.
of Fe203 content, fl'-sialon, a-A1203 and iron silicide as well as a negative correlation of mullite with the forementioned phases. These results confirm previous work by Lee and Cutler ( 1979 ) who showed the catalytic effect of Fe2Oa in connection with weight loss. Weight loss determinations on samples presented in Figs. 7 and 8 show an increase in weight loss with increasing Fe203 content. This is linked with the increasing degree of reaction which can be detected by XRD. Taking into account that carbonitriding reactions of clays may be divided into three stages, the result shown in Figs. 7 and 8 may be interpreted as follows: (a) Thermal decomposition of clay: Depending on its composition and occurring before the reaction temperature at which a mixture of mullite and silica is formed. (b) r-SiC formation: It is caused by silica carboreduction in connection with reactions under (a). (c) Formation of nitrogenous products: At this stage mullite, r-SiC and carbon remain and N2 reacts to form nitrogenous products (fl'-sialon, X-phase, etc. ). Mullite and fl'-sialon variations reflect best the catalytic effect of Fe203. r-SiC is not represented in Figs. 7 and 8 as it is difficult to detect in samples with the low degree of reaction reached here. According to some authors (Higgins and Hendry, 1986 ) the catalytic effect of Fe203 lies in the formation of liquid phases. In these experiments liquid phases were detected during the reaction. Furthermore, it is noticed that Fe203 (more precisely Fe) has a catalytic effect on the formation of r-SiC. The catalytic effect is partly explained by the presence of r-SiC as one of the reactives in the final reaction, stage (c). It is found that Fe203 has a catalytic effect on the complete reaction and that there is not sufficient evidence yet to attribute it to a particular reaction stage.
Clay commodities Clays used for these experiments are mainly composed of kaolinite and quartz (Table 3 ), whose thermal decomposition produces mullite and silica. Under the assumption of a perfect mixture, fl'-sialon is expected to be formed from a single-phase with the composition given in Table 1. Clay materials (Table 1, 3 ) were mixed with carbon-black at a C/clay ratio of 0.36 and pelletised at 38.3 MPa. Reaction experiments were repeated several times with these samples. Results of a representative series of experiments are shown in Table 4. In all cases the main product is fl'-sialon with a silicon content between that of the starting clay (chemical composition) and pure kaolinite. This denotes that there is always an important quantity of silicon introduced into fl'-sialon
fl'-SIALONPREPARATIONFROMKAOLINITICCLAYS
417
TABLE 3 Some physicochemical characteristics o f the clays used in the experiments Clay identification ( - 3 2 5 ASTM MESH (wt.%)
Quartz content (%)
Minor components
Specific surface area BET (m2/g)
Size of kaolinite crystals (A)
P R L Ve
2 9 21 24
12.2 16.5 5.3 20.4
420 310 600 207
Z T F
32 38 --
-Illite -lllite Smectite Smectite ---
19.3 22.5 17.0
231 204 432
TABLE 4 Reaction products: phases and some physicochemical properties (C/clay = 0.36; Temp.: 1420 °C; Time: 260 rain.; VN2:5 c m / s ) Clay
Reaction products Phases*
Specific surface area BET mZ/g
Crystal size (/~) (fl'-sialon)
P
fl'-sialon > "A 1ON" > > ot-A1203
R L Ve Z T F
fl'-sialon > > a - A1203 fl'-sialon > > M > X-phase, C fl'-sialon > > X-phase, SiEN20 fl'-sialon > Si2N20 > > X-phase > M ff-sialon > Si2N20 > X-phase > > M fl'-sialon > > > C
1.0 1.0 7.1 3.5 5.0 2.6 8.3
550 359 343 600 540 338 438
"A I O N " = [A1,8/3 + 4/3,O4_HNH ] C = Cristobalite M = Mullite *Trace components such as iron silicide and TiN are not recorded.
coming from quartz. However, theoretical silicon content is not reached due to SiO (g) loss, and to the formation of products such as Si2N20. The presence of additional phases (Table 4) depends on the clay composition. Clays with low silica contents like P or R (Table 1 ) form aluminium-rich products as 0~-A1203 (R) a n d / o r "A1ON" (P). Clays with higher silica contents form Si2N20 as a by-product, which is most important in the silica-rich clays T and Z. Si2N20 formation could also be explained by a quartz segregation. X-phase which is present in some samples indicates a lesser degree of reduc-
418
A.D.MAZZONIET AL.
Fig. 9. Scanning electron micrographs of some clays and their reaction products. (a) Clay "R" (scale b a r = 10 am). (b) Reaction product from clay "R" (scale b a r = l0 am). (c) Clay " F " (scale bar = 1 am ). (d) Reaction product from clay " F " (scale bar = 10 am ).
tion in the environment of formation than the fl'-sialon. Furthermore, small quantities of iron silicide and titanium nitride were found. The specific surface area (BET) of reaction products is smaller than that of the clay precursor. This observation is in agreement with data of other authors (Van Dijen and Metselaar, 1985 ). Crystalline sizes determined by Scherrer's equation show an increase from the raw clay to the final product. The increase in crystalline size is variable and relatively smaller than expected if compared with the decrease of the specific surface area. From a comparison of SEM photographs of products and clays it is apparent that both have similar morphologies (Fig. 9 ). CONCLUSIONS
(a) Carbonitriding of clay is a suitable method to prepare fl'-sialons. (b) In order to obtain good yields offf-sialon, it is necessary to work with C in excess of the stoichiometric value. (c) The combination of pressed pellets and a N2 flow rate within the range of 1.7 to 13.0 c m / s constitute favorable conditions for the desired reaction. (d) The addition of Fe203 promotes a high yield offl'-sialon. (e) The formation of by-products associated with fl'-sialon is, under the above
fl'-SIALONPREPARATIONFROMKAOLINITICCLAYS
419
conditions, strongly dependent on the clay composition, mainly its SiO2: A1203 ratio. (f) Important silicon quantities originating from quartz are incorporated into fl'-sialons. (g) SiO(g) loss becomes an important factor when working at low C/kaolin ratios and large N 2 volumes. (h) During the reaction process new phases develop whose specific surface areas are much lower than those of the reactives. (i) In most instances the crystal sizes offl'-sialons are larger than those of the kaolinite precursors. (j) Final products and precursor clays have similar morphologies (observed by SEM). (k) The Argentinian clays are suitable for producing fl'-sialons. The nature of the by-products depend on both the chemical composition of the clay and the reaction conditions. REFERENCES Hip,gins, I. and Hendry, A., 1986. Production offl-sialon by carbothermal reduction of kaolinite. Br. Ceram. Trans. J., 85: 161-166. Jack, K.H., 1976. Sialons and related nitrogen ceramics. J. Mater. Sci., 11:1135-1158. Lee, J.G. and Cutler, I.B., 1979. Sinterable simon powder by reaction of clay with carbon and nitrogen. Am. Ceram. Soc. Bull., 58 (91): 869-871. Mariano, W.A., Pinto, A.L., Baldo, J.B. and Casarini, J.R., 1983. Reactividade de materiais cauliniticos para producao de simon. Ceramica, 29:121-126. Mukerji, J. and Bandyopadhyay, S., 1988. Sialons from natural aluminosilicates. Adv. Ceram. Mater., 3(4): 369-373. Siddiqi, S.A., Hip,gins, I. and Hendry, A., 1986. Preparation and densification of nitrogen ceramics from oxides. Non-oxide Tech. Eng. Ceram., (Proc. Int. Congr. ). Elsevier Appl. Sci., London, pp. 119-132. Silverman, L.D., 1988. Carbothermal synthesis of aluminium nitride. Adv. Ceram. Mater., 3 (4): 418-419. Sorrell, C.C., 1983. Silicon nitride and related nitrogen ceramics: II. Phase equilibria and properties of reaction bonded and hot pressed Si-AI-O-N systems. J. Aust. Ceram. Soc., 19 (2): 48-67. Sugahara, Y., Miyamoto, J., Kuroda, K. and Kato, C., 1989. Preparation of nitrides from 1:1 type clay minerals by carbothermal reduction. Appl. Clay Sci., 4:11-26. Torre, J.P. and Mocellin, A., 1976. Some effects of aluminum and oxygen on nitridation of silicon compacts. J. Mater. Sci., 11 (9): 1725-1733. Umebayashi, S. and Kobayashi, K., 1975. Beta silicon nitride solid solution prepared from volcanic ash aluminum powder in nitrogen. J. Am. Ceram. Soc., 58 (9-10): 464. Umebayashi, S. and Kobayashi, K., 1977. Direct preparation of dense silicon-aluminum-oxygen-nitrogen composites from naturally occurring silica and aluminum powder. Am. Ceram. Soc. Bull., 56 (6): 578-579. Umebayashi, S., Kobayashi, K. and Kataoka, R., 1980. Hot pressing of compacts prepared by nitriding powder mixtures of silica, aluminum and silicon Yogyo Kyokai Shi, 88 (8): 46975.
420
A.D. MAZZONI ET AL.
Van Dijen, F.K. and Metselaar, R., 1985. Reaction-rate-limiting steps in carbothermal reduction processes. J. Am. Ceram. Soc., 68 ( l ): 16-19. Wild, S., 1975. "Special Ceramics 6". (British Ceramic Research Association), p. 309. Wild, S., 1976. A novel route for the production offl-sialon powders. J. Mater. Sci., 1 l: 19721974. Yoshimatsu, H., Kawasaki, H., Miura, Y. and Osaka, A., 1989. Carbon-thermal reduction and nitridation of mixtures of SiO2 and A1203"2H20. J. Mater. Sci., 24: 3280-3284. Zhang, S.C. and Cannon, W.R., 1984. Preparation of silicon nitride from silica. J. Am. Ceram. Soc., 67 (10): 691-695.
407
fl'-sialon preparation from kaolinitic clays A.D. Mazzoni, E.F. Aglietti and E. Pereira Centro de Tecnologia de Recursors Minerales y Ceramica (CETMIC). CONICET-CIC-UNLP, Camino Parque Centenario y 506, C.C. 49 (1897), M.B. Gonnet, Buenos Aires, Argentina (Received February 28, 1992; revised and accepted September 29, 1992)
ABSTRACT Mazzoni, A.D., Aglietti, E.F. and Pereira, E., 1993. ff-sialon preparation from kaolinitic clays. Appl. Clay Sci., 7: 407-420. Kaolinitic clays are suitable raw materials for the production offf-sialons and other compounds of the Si-AI-O-N system. These compounds are obtained by carboreduction and simultaneous nitriding of the mineral. Seven kaolins with different SIO2/A1203 ratios and quartz contents were mixed with carbon-black. They were reacted at about 1400°C in a nitrogen atmosphere. Several series of experiments were carried out to establish the influence of various parameters on fl'-sialon formation. Chemical composition and mineralogy of the starting material were decisive in fl'-sialonyield and determined which by-products were formed. From all clays, ff-sialon was the primary reaction product. Fe203 naturally present in the clay or as an additive favours the formation of nitrogenous phases. Remarkable quantities of silicon originating essentially from quartz are incorporated into the fl'-sialons. On the other hand, a SiO(g) loss occurs in these reactions mainly when working at low carbon/ clay ratios and with large N2 volumes.
INTRODUCTION
Sialons are compounds of the Si-AI-O-N system (Jack, 1976; Sorrell, 1983). The phases of this system, which correspond to the formula Si6_zAlzOzNs_zwith 0 < Z~< 4.2, are the fl'-sialons. Their structure is derived from fl-Si3N4 and, therefore, these compounds have physical properties similar to fl-Si3N4. The chemical properties offf-sialons are analogous to alumina (Jack, 1976; Sorrell, 1983 ). These phases are used primarily in the production of ceramic pieces with high thermomechanical properties and as additives to improve properties of other ceramic materials (A1203, ZrO2, SiC, etc. ). Generally fl'-sialons are prepared by starting from a mixture of compounds Correspondence to: A.D. Mazzoni, Centro de Tecnologia de Recursors Minerales y Ceramica (CETMIC). CONICET-CIC-UNLP, Camino Parque Centenario y 506, C.C. 49 (1897), M.B. Gonnet, Buenos Aires, Argentina.
0169-1317/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
408
A.D. MAZZONIET AL.
such as Si3N4, A1203, A1N, SIO2, Si2N20 (Jack, 1976; Sorrell, 1983 ). Nevertheless, there are alternative methods for preparing these phases, e.g., nitriding of SiO2 and A1 mixtures in a N2 atmosphere (Umebayashi et al., 1977 ); nitriding of silica, aluminium and silicon mixtures in a N2 atmosphere (Umebayashi et al., 1980); nitriding of volcanic ash and aluminium in a N2 atmosphere (Umebayashi et al., 1975 ); kaolinite nitriding in an NH3 atmosphere (Wild, 1976 ); reaction of metakaolinite and Si3N4 in a N2 atmosphere (Wild, 1975 ); nitriding of silicon and aluminium mixtures in a N2 atmosphere (Torre and MoceUin, 1976 ), etc. Other alternative methods include carbonitriding reactions (carboreduction and simultaneous nitriding) of oxygen compounds. These reactions are flexible since they permit the formation of different nitrogenous ceramics depending on the starting material, e.g., Si3N4 when starting from quartz or any other source of SiO2 (Zhang and Cannon, 1984); AIN starting from A1203 ( Silverman, 1988 ) and ff -sialons starting from natural alumino-silicates (Lee and Cutler, 1979 ). Carbonitriding reactions that lead to the formation offf-sialons from minerals such as pyrophilite (1), kaolinite (2) and sillimanite (3) can be described as follows: A1203 "4SIO2 "4"9C+ 3N2 --~Si4Al2O2 N6 d- 9CO
( 1)
3 (AlE03 •2SIO2 ) + 15C + 5N2 --*2Si3AI303 N5 + 15CO
(2 )
2 (A1203" SiO2 ) + 6C + 2N2 -~ Si2A14O4 N4 + 6CO
(3)
The production process offf-sialons according to reactions 1-3 is complex and depends on several parameters such as: C/kaolinite ratio (Mukerji and Bandyopadhyay, 1988; Sugahara et al., 1989); SIO2/A1203 ratio (Mukerji and Bandyopadhyay, 1988 ); N2 volume (Van Dijen and Metselaar, 1985; Higgins and Hendry, 1986 ); Fe203 content (Higgins and Hendry, 1986; Mukerji and Bandyopadhyay, 1988; Sugahara et al., 1989 ). The objective here is to study the influence of various parameters including carbon/clay ratio, Fe203 content, N2 flow and SiO2/A1203 ratio of clay on the formation offf-sialons. The goal of the study was to determine if sialons can be produced from clays of the Argentine Republic whose main mineralogic components are kaolinite and quartz. EXPERIMENTAL METHODS AND TECHNIQUES
Raw materials
Six clays from the Argentina and the Fisher kaolin from the USA were used. Their main mineralogic components are kaolinite and quartz. Chemical analyses of these clays are shown in Table 1. The carbon materials used in the
409
#'-SIALONPREPARATIONFROMKAOLINITICCLAYS TABLE l Chemical composition (wt.%) of the clays used in the experiments
Puente (P) Rhotemberger (R) Lote 8 (L) Veronica (Ve) Frentze (Z) Tincar (T) Fisher (F)
SiO2 (%)
A1203 (%)
Fe203 (%)
TiO2 (%)
CaO (%)
MgO (%)
Na20 K20 (%)
L.O.I. (1000°C)
44.2 48.7 60.5 59.8 69.9 69.3 42.5
36.0 33.6 26.3 26.6 19.6 19.4 40.8
2.00 1.20 0.37 1.90 0.92 1.12 0.52
1.00 1.10 0.12 0.30 0.40 0.45 2.10
0.10 0.12 0.40 0.05 0.40 0.60 n.d.
2.10 1.20 0.06 0.15 0.17 0.10 n.d.
0.98 2.10 0.67 1.48 0.77 0.49 n.d.
13.2 12.0 11.6 9.3 7.4 8.4 13.9
Puente: Departamento Independencia, Provincia La Rioja. Rhotemberger: Tandil, Provincia Buenos Aires. Lote 8: San Juli~in, Provincia Santa Cruz. Ver6nica: Sur del Rio, Provincia Chubut. Frente: San Juli~in, Provincia Santa Cruz. Tincar: San Julifi, Provincia Santa Cruz. Fisher: Georgia, USA. TABLE2 Physicochemical properties of selected carbon materials (wt.%)
Carbon-black
Petroleum coke (calcined) Petroleum coke ( non-calcined )
Particle size
Specific surface area BET (m2/g)
Ash (%)
Volatiles (%)
Carbon (%)
- 325 ASTM MESH - 325 ASTM MESH - 325 ASTM MESH
45.8
2
1
97
5.0
0.15
--
99.8
4.0
0.16
13.9
~ 86
experiments are characterized in Table 2. In addition, C13Fe.6H20 (AR) and calcined Fe203 (BDH) were used as Fe203 additives. The nitrogen applied contained less than 5 ppm of O2 and H20.
Experimental methods and equipment Experiments were carried out in a vertical alumina reactor with N2 flowing through a fixed bed supported on a perforated graphite disk. The heating of samples took place in a SiC furnace, with temperature control and thermocouple of Pt-Pt Rh (10%). The reaction products were analyzed by X R D with a Philips PW 1140/00 equipment. Peak areas were used to calculate the amounts of crystalline phases which were expressed in arbitrary units (A.U.). The validity of each of these A.U. is restricted to one individual figure only
410
A.D. MAZZONIET AL.
and the A.U. indicate the evolution of each crystalline phase in a determined series of experiments. They do not indicate absolute quantities and can not be used for a comparison among different plots. Specific surface areas (BET) were determined using a Micromeritics adsorption equipment with a relative error of approximately 5%. Samples were observed by means of a scanning electron microscope, Philips 505. Crystallite sizes were determined by means of Scherrer's equation using quartz as a standard. Samples were prepared by wet mixing of calculated quantities of clay and carbon material. These mixtures were pressed as pellets or extruded. Fe203 was added to the clay using two methods: ( 1 ) precipitation from a FeC13 solution and (2) mixing of the clay with Fe203 aggregates using acetone as a dispersing medium. RESULTS
Carbon content
From the comparison of physicochemical properties of different carbon material (Table 2) it became obvious that carbon-black was best suited for the experiments. Due to its small particle size and large specific surface area, carbon-black is more reactive than the other available carbon materials which ensures intimate contact of carbon with clay. The influence of the C/clay ( w / w ) ratio was studied for two clays (P and R) by changing the C/clay ratio in the experiments and keeping the other
~18 Stoichio.
>. ¢_
14 o°V L.
,~ 100
J I,~ ~
~.~
/
o Quartz • x-Phase AMuHite • (~ -AI 2 03
o
~ 60
o. 208
0.258
0.333 0.377 C/Clay ( W t / W t )
Fig. 1. Influence o f c a r b o n content o n clay " R " (Temp.: 1400 ° C; time: 210 min.; VN2:5 c m / s ) .
,8'-SIALONPREPARATIONFROMKAOLINITICCLAYS
~zoo
411
Stoichio.
¢_
== #
•
==.
100
X - Phase
A Mullite ¢ t~ALON" 0 /3'- Sialon
:>. ¢-
0~ ¢ ' ~ 0.180 0.208
0.265 0.328 C/Clay ( W t / W t )
Fig. 2. Influence of carbon content on clay "P" (Temp.: 1400°C; time 190 min.; VN2 5 cm/s).
reaction conditions constant. In this way, variation in the reaction products could be attributed to changes in the carbon content. Figures 1 and 2 show the evolution of the principal crystalline phases (in A.U. ) as a function of the C/clay ratio. The experimental results of both clays (P and R ) indicate that the fl'-sialon phase increases with increasing carbon. Positive correlations of carbon with t~-AI203 and "A 1ON" are also noted. Mullite, on the other hand, decreases when the C/clay ratio increases. In samples with carbon contents lower than those displayed in Figs. 1 and 2, a great loss of SiO(g) is observed and muUite becomes the dominant crystalline phase. In these samples, cristobalite is present in smaller quantities than where the clay was calcined without carbon. Small quantities of nitrogenous phases are present as well. Consequently, in samples where carbon is added the following reaction is considered to be the most important:
SiO2+C--,SiO(8 ) +CO(g)
(4)
Silica needles (Fig. 3 ) were noticed in one sample and their formation is due to SiO(g). SiO(g) loss is verified in all samples, but it is minor in those
412
A.D. MAZZONI ET AL.
Fig. 3. Silicaneedlesforming due to SiO volatilization ( × 10). with a high carbon content. These results confirm previous work by Yoshimatsu et al. ( 1989 ) and can be described by reaction ( 5 ).
SiO(g) -k C---~SiC + CO(g)
(5)
Cristobalite was not detected in compositions recorded in Figs. 1 and 2. Quartz (Fig. 1 ) mainly reacts with carbon forming SiC, but also reactions (4) and ( 5 ) occur. SiC was not detected in the final products as it intervenes in mullite reduction. The X-phase is a nitrogenous phase of the system S i - A I - O - N with a N2 content lower than fl'-sialon. This is an intermediate phase in clay carbonitriding (reaction 2). With increasing C/clay ratio the amount of X-phase increases, reaches a m a x i m u m at C/clay values of about 0.28 (Fig. 1 ) and 0.18 (Fig. 2 ) for clays R and P respectively and then decreases with the increase of the carbon content. The X-phase is an intermediate in the reaction and has a lower nitrogen content than p'-sialon. Before reaching the final reaction temperature, thermal decomposition of kaolinite produces mullite and silica. The addition of greater quantities of carbon results in a replacement of oxygen by nitrogen, which causes an increase in the amount of nitrogenous phases and a decrease of oxides (mullitesilica). An appreciable decomposition offl'-sialon and formation of larger amounts of reduced phases such as A I N or A1N polytypes did not take place in these experiments. This is, possibly, a consequence of the temperature range used ( 1600-1700 K). Based upon experimental results, excess carbon was utilized to promote a high yield offl'-sialon.
ff-SIALONPREPARATIONFROMKAOLINITICCLAYS
~...~.~Oio. 5
200 o
~1601 ~120
413
~f
'fJ f /
,/~ o=80
j . o ''~
0.4
c
f ¢_
0 ~'-Sialon II IX- AI203
0.3
-
0.2
~
o~ to i
i
~_ 4 o 0 0.0
n 9.56
i , a 0.0 19.12 28.69 38.25 Pressure (NPA)
Fig. 4. Variations of the reaction products as a function of load pressure (Temp.: 1400°C; time 210 rain; VN2:5 cm/s).
Shape and load pressure of samples Several experiments were carried out to study the influence of molding pressure on the preparation of pellets by using clay R with a C/clay ratio of 0.366. Different pelletising pressures were applied when preparing pellets of similar shape and size. These pellets were subjected to equal reaction conditions. A comparison of the reaction products (Fig. 4 ) shows that with increasing pressure the quantity offl'-sialon increases while the a-A1203 phase tends to decrease. Furthermore, a slight tendency of decreasing weight loss with increasing pressure was noticed. In another series of experiments these results were repeated and are attributed to the increasingly intimate contact of the solid reactives with increasing pelletising pressure. The reduction in weight loss, in spite of increasing amounts of the reaction products, could be explained by a diminution of the SiO (g) loss, which, in turn, could also explain the decrease of 0~-A1203. Kinetic measurements performed on pressed pellets of clay P proved that the reaction is not controlled by diffusion mechanisms (Mariano et al., 1983). As much as porosity is created by dehydration of clay during heating, this observation applies even to strongly compacted samples.
Nitrogenflow The N2 flOWvolume (Van Dijen and Metselaar, 1985 ) is an important factor in controlling the CO content during the reaction. With increasing N2 flow
414
A.D. M A Z Z O N I E T AL.
4"
•
• ( ] ( - AI 2U 3 *
"ALON"
o /3'-Sial0n
• •
I
70
II •-
•¢-
I
65
610
sr5
510
415
410
c Cristobal~te • ALN Pofytype
•_
I
35
310
215
210
Sia,o~
115
o2e(cu K(X) Fig. 5. XRD pattern of the reaction product obtained under conditions of great SiO (g) loss. A Mu{Jite
of3-Si C
• x - Phase
, ,ton Silic,de
~a ~
iA
715 70 6~5 610 515 510 415 410 315 310 2J5 2i0 It5 II0 °2e (Cu KCK)
Fig. 6. XRD pattern of the reaction product obtained with deficient elimination of CO (g).
volumes, CO elimination increases, which leads to the repression of nitrogenous product formation (Higgins and Hendry, 1986; Siddiqi et al., 1986). As N2 may transport SiO (g), high N2 flow volumes are accompanied with high silica losses (Yoshimatsu et al., 1989 ). Several tests were performed in order to find the most suitable N2 flow rate. Pressed samples with high carbon contents, through which gas was flowing at a rate ranging from 1.7 to 13.0 cm/s, yielded similar final products. There were, however, slight differences in the amount of aluminium-rich phases such as ot-A1203. The increase in AI phases was caused by higher SiO (g) losses due to increasing N2 flow rates. Samples with a minor carbon content, prepared as extruded (greater porosity than pressed pellets) form a variety of reaction products (Fig. 5). The results imply that CO (g) elimination in the fixed bed reactor is very efficient, which means that the use of low N2 flow volumes are adequate. On the other hand, experiments performed with defficient CO (g) elimination (alumina crucible) lead to the formation of a mixture of mullite, fl-SiC, some X-phase, little iron silicide and no fl' -sialons (Fig. 6 ). Fe203 content
The catalytic effect o f F e 2 0 3 , added to clays as a precipitate or a solid phase, is shown in Figs. 7 and 8 respectively. Both figures show a positive correlation
ff-SIALON PREPARATIONFROMKAOLINITICCLAYS
415
200 • x-Phase A Mullite oOC -AI203 O f~'-Sialo n"
tn c :=
15(
"7,
~ 100 Q.
=
5(
00
1
2 F % 0 3 (%)
3
Fig. 7. Influence of Fe203 content on the reaction products (Fe203 precipitation on kaolin F; Temp.: 1410°C; time: 190 rain.; VN2:5 c m / s ) .
225 ~, • x-Phase ~ c
~ ~ . z . ~ ~
>" o
~5o-
A Mullite • (~- Al203 O ['~'- Sial0n & Ir0n Silicide
"~o
__ °
C3_
-g
g L
(
Fe203 ('/.)
Fig. 8. Influence of Fc203 content on the reaction products (solid Fe203 mixed with kaolin F; Temp.: 1410°C; time 190 min..; VN2:5 c m / s ) .
416
A.D. MAZZONI ET AL.
of Fe203 content, fl'-sialon, a-A1203 and iron silicide as well as a negative correlation of mullite with the forementioned phases. These results confirm previous work by Lee and Cutler ( 1979 ) who showed the catalytic effect of Fe2Oa in connection with weight loss. Weight loss determinations on samples presented in Figs. 7 and 8 show an increase in weight loss with increasing Fe203 content. This is linked with the increasing degree of reaction which can be detected by XRD. Taking into account that carbonitriding reactions of clays may be divided into three stages, the result shown in Figs. 7 and 8 may be interpreted as follows: (a) Thermal decomposition of clay: Depending on its composition and occurring before the reaction temperature at which a mixture of mullite and silica is formed. (b) r-SiC formation: It is caused by silica carboreduction in connection with reactions under (a). (c) Formation of nitrogenous products: At this stage mullite, r-SiC and carbon remain and N2 reacts to form nitrogenous products (fl'-sialon, X-phase, etc. ). Mullite and fl'-sialon variations reflect best the catalytic effect of Fe203. r-SiC is not represented in Figs. 7 and 8 as it is difficult to detect in samples with the low degree of reaction reached here. According to some authors (Higgins and Hendry, 1986 ) the catalytic effect of Fe203 lies in the formation of liquid phases. In these experiments liquid phases were detected during the reaction. Furthermore, it is noticed that Fe203 (more precisely Fe) has a catalytic effect on the formation of r-SiC. The catalytic effect is partly explained by the presence of r-SiC as one of the reactives in the final reaction, stage (c). It is found that Fe203 has a catalytic effect on the complete reaction and that there is not sufficient evidence yet to attribute it to a particular reaction stage.
Clay commodities Clays used for these experiments are mainly composed of kaolinite and quartz (Table 3 ), whose thermal decomposition produces mullite and silica. Under the assumption of a perfect mixture, fl'-sialon is expected to be formed from a single-phase with the composition given in Table 1. Clay materials (Table 1, 3 ) were mixed with carbon-black at a C/clay ratio of 0.36 and pelletised at 38.3 MPa. Reaction experiments were repeated several times with these samples. Results of a representative series of experiments are shown in Table 4. In all cases the main product is fl'-sialon with a silicon content between that of the starting clay (chemical composition) and pure kaolinite. This denotes that there is always an important quantity of silicon introduced into fl'-sialon
fl'-SIALONPREPARATIONFROMKAOLINITICCLAYS
417
TABLE 3 Some physicochemical characteristics o f the clays used in the experiments Clay identification ( - 3 2 5 ASTM MESH (wt.%)
Quartz content (%)
Minor components
Specific surface area BET (m2/g)
Size of kaolinite crystals (A)
P R L Ve
2 9 21 24
12.2 16.5 5.3 20.4
420 310 600 207
Z T F
32 38 --
-Illite -lllite Smectite Smectite ---
19.3 22.5 17.0
231 204 432
TABLE 4 Reaction products: phases and some physicochemical properties (C/clay = 0.36; Temp.: 1420 °C; Time: 260 rain.; VN2:5 c m / s ) Clay
Reaction products Phases*
Specific surface area BET mZ/g
Crystal size (/~) (fl'-sialon)
P
fl'-sialon > "A 1ON" > > ot-A1203
R L Ve Z T F
fl'-sialon > > a - A1203 fl'-sialon > > M > X-phase, C fl'-sialon > > X-phase, SiEN20 fl'-sialon > Si2N20 > > X-phase > M ff-sialon > Si2N20 > X-phase > > M fl'-sialon > > > C
1.0 1.0 7.1 3.5 5.0 2.6 8.3
550 359 343 600 540 338 438
"A I O N " = [A1,8/3 + 4/3,O4_HNH ] C = Cristobalite M = Mullite *Trace components such as iron silicide and TiN are not recorded.
coming from quartz. However, theoretical silicon content is not reached due to SiO (g) loss, and to the formation of products such as Si2N20. The presence of additional phases (Table 4) depends on the clay composition. Clays with low silica contents like P or R (Table 1 ) form aluminium-rich products as 0~-A1203 (R) a n d / o r "A1ON" (P). Clays with higher silica contents form Si2N20 as a by-product, which is most important in the silica-rich clays T and Z. Si2N20 formation could also be explained by a quartz segregation. X-phase which is present in some samples indicates a lesser degree of reduc-
418
A.D.MAZZONIET AL.
Fig. 9. Scanning electron micrographs of some clays and their reaction products. (a) Clay "R" (scale b a r = 10 am). (b) Reaction product from clay "R" (scale b a r = l0 am). (c) Clay " F " (scale bar = 1 am ). (d) Reaction product from clay " F " (scale bar = 10 am ).
tion in the environment of formation than the fl'-sialon. Furthermore, small quantities of iron silicide and titanium nitride were found. The specific surface area (BET) of reaction products is smaller than that of the clay precursor. This observation is in agreement with data of other authors (Van Dijen and Metselaar, 1985 ). Crystalline sizes determined by Scherrer's equation show an increase from the raw clay to the final product. The increase in crystalline size is variable and relatively smaller than expected if compared with the decrease of the specific surface area. From a comparison of SEM photographs of products and clays it is apparent that both have similar morphologies (Fig. 9 ). CONCLUSIONS
(a) Carbonitriding of clay is a suitable method to prepare fl'-sialons. (b) In order to obtain good yields offf-sialon, it is necessary to work with C in excess of the stoichiometric value. (c) The combination of pressed pellets and a N2 flow rate within the range of 1.7 to 13.0 c m / s constitute favorable conditions for the desired reaction. (d) The addition of Fe203 promotes a high yield offl'-sialon. (e) The formation of by-products associated with fl'-sialon is, under the above
fl'-SIALONPREPARATIONFROMKAOLINITICCLAYS
419
conditions, strongly dependent on the clay composition, mainly its SiO2: A1203 ratio. (f) Important silicon quantities originating from quartz are incorporated into fl'-sialons. (g) SiO(g) loss becomes an important factor when working at low C/kaolin ratios and large N 2 volumes. (h) During the reaction process new phases develop whose specific surface areas are much lower than those of the reactives. (i) In most instances the crystal sizes offl'-sialons are larger than those of the kaolinite precursors. (j) Final products and precursor clays have similar morphologies (observed by SEM). (k) The Argentinian clays are suitable for producing fl'-sialons. The nature of the by-products depend on both the chemical composition of the clay and the reaction conditions. REFERENCES Hip,gins, I. and Hendry, A., 1986. Production offl-sialon by carbothermal reduction of kaolinite. Br. Ceram. Trans. J., 85: 161-166. Jack, K.H., 1976. Sialons and related nitrogen ceramics. J. Mater. Sci., 11:1135-1158. Lee, J.G. and Cutler, I.B., 1979. Sinterable simon powder by reaction of clay with carbon and nitrogen. Am. Ceram. Soc. Bull., 58 (91): 869-871. Mariano, W.A., Pinto, A.L., Baldo, J.B. and Casarini, J.R., 1983. Reactividade de materiais cauliniticos para producao de simon. Ceramica, 29:121-126. Mukerji, J. and Bandyopadhyay, S., 1988. Sialons from natural aluminosilicates. Adv. Ceram. Mater., 3(4): 369-373. Siddiqi, S.A., Hip,gins, I. and Hendry, A., 1986. Preparation and densification of nitrogen ceramics from oxides. Non-oxide Tech. Eng. Ceram., (Proc. Int. Congr. ). Elsevier Appl. Sci., London, pp. 119-132. Silverman, L.D., 1988. Carbothermal synthesis of aluminium nitride. Adv. Ceram. Mater., 3 (4): 418-419. Sorrell, C.C., 1983. Silicon nitride and related nitrogen ceramics: II. Phase equilibria and properties of reaction bonded and hot pressed Si-AI-O-N systems. J. Aust. Ceram. Soc., 19 (2): 48-67. Sugahara, Y., Miyamoto, J., Kuroda, K. and Kato, C., 1989. Preparation of nitrides from 1:1 type clay minerals by carbothermal reduction. Appl. Clay Sci., 4:11-26. Torre, J.P. and Mocellin, A., 1976. Some effects of aluminum and oxygen on nitridation of silicon compacts. J. Mater. Sci., 11 (9): 1725-1733. Umebayashi, S. and Kobayashi, K., 1975. Beta silicon nitride solid solution prepared from volcanic ash aluminum powder in nitrogen. J. Am. Ceram. Soc., 58 (9-10): 464. Umebayashi, S. and Kobayashi, K., 1977. Direct preparation of dense silicon-aluminum-oxygen-nitrogen composites from naturally occurring silica and aluminum powder. Am. Ceram. Soc. Bull., 56 (6): 578-579. Umebayashi, S., Kobayashi, K. and Kataoka, R., 1980. Hot pressing of compacts prepared by nitriding powder mixtures of silica, aluminum and silicon Yogyo Kyokai Shi, 88 (8): 46975.
420
A.D. MAZZONI ET AL.
Van Dijen, F.K. and Metselaar, R., 1985. Reaction-rate-limiting steps in carbothermal reduction processes. J. Am. Ceram. Soc., 68 ( l ): 16-19. Wild, S., 1975. "Special Ceramics 6". (British Ceramic Research Association), p. 309. Wild, S., 1976. A novel route for the production offl-sialon powders. J. Mater. Sci., 1 l: 19721974. Yoshimatsu, H., Kawasaki, H., Miura, Y. and Osaka, A., 1989. Carbon-thermal reduction and nitridation of mixtures of SiO2 and A1203"2H20. J. Mater. Sci., 24: 3280-3284. Zhang, S.C. and Cannon, W.R., 1984. Preparation of silicon nitride from silica. J. Am. Ceram. Soc., 67 (10): 691-695.