carbon lamellar nanocomposites

0022~3697(95)00390-8

Pergamon

J. Phys. Chem Solids Vol57, No 6-8, pp. 1019~1029,1996 Copyright 0 1996 Elwvier Science Ltd Printed in Great Britain. All rights reserved 0022-3697/96 $15.00 + 0.00

ELABORATION AND STRUCTURE OF SILICATE/CARBON LAMELLAR NANOCOMPOSITES F. BkGUIN,

J. N. ROUZAUD,

CRMD, CNRS-University,

I. BEN MAIMOUN

and A. SERON

IB Rue de la Ferollerie, 45071 Orleans Cedex 02, France

(Received 28 May 1995; accepted 3 1 May 1995) Abstract-The interlamellar cations of Na-montmorillonite and Li-taeniolite were exchanged for aromatic ammonium cations (acriflavine, aminoacridine, safranine, indoine) to lead to silicate-organic cation complexes. The subsequent thermal treatment of the complexes, under neutral atmosphere, led to the lamellar silicate-carbon nanocomposites. X-ray diffraction measurements on the silicate-organic cation complexes demonstrated that the organic cations are perpendicular to the silicate sheets, but slightly tilted. The nanocomposites were formed when carbonization of the interlamellar organic cation started, i.e. at 400°C. Transmission electron microscopy observations strongly suggested that, at 40&5OO”C, basic structural units (BSUs), stackings of two or three aromatic carbon layers of nanometric dimensions, are formed, and are almost randomly distributed in the interlayer space of the nanocomposite. Upon increasing the heat treatment temperature, the interlayer distance &,r in the composite decreases, and due to the lattice constraints, the BSUs tend to become more and more parallel to the silicate sheets. Above lOOo”C,the highly dispersed carbon starts to react with the silicate host, and through carbo-reduction a ceramic material can be formed.

Keywords: A. nanostructures, A. inorganic compounds, B. chemical synthesis, C. electron microscopy, C. X-ray diffraction.

INTRODUCTION

During the last decades, composite materials were extensively developed at the industrial scale for their remarkable physical properties. They are usually obtained by a ‘mechanical’ process to favor a mixture of two or more components which are of the micrometric order. More recently, nanocomposite terminology was introduced for a geometrical arrangement of solid phases among which, at least one, has nanometric dimensions. Although this term was coined only recently, nanocomposites are pervasive throughout the biological system (coal, wood, bones, shells, . . .). Special attention must be paid to the synthetic lamellar nanocomposites which appear the most interesting. They associate an inorganic lamellar substrate (silicate, aluminosilicate, .) with an interlamellar organic or inorganic entity which could play the role of the pillar between the layers. As an example, silicate-carbon nanocomposites are obtained by the pyrolysis of silicate-organic molecule complexes. The organic component can be intercalated in the interlamellar space either by exchanging the compensating cation with an organic cation (acriflavine hydrochloride, aliphatic amine hydrochloride) [l-4] or by substituting the hydration water by a polar organic

molecule (acrylonitrile, furfuryl alcohol,. . .) [S-8]. In this last case, the molecules are in weak dipolar interaction with the substrate and, during the pyrolysis, an important part of them is vaporized. Therefore, cationic exchange appears a better process for obtaining nanocomposites with important carbon content, owing to the strong electrostatic attraction of the charged layers on the intercalated organic cations, which must allow the maintenance of the cations in the interlamellar space during the pyrolysis. The most remarkable property of these new materials is that they keep their lamellar structure up to 1000°C [ 1,9], above which carbo-reduction of the silicate matrix by the interlamellar carbon occurs [3,5, lo]. Though no nanotextural study of the intercalated carbon has been published yet, most of the authors assume that carbon is a monoatomic graphene layer between the silicate layers [l, 71. Therefore, the organization (structure and nanotexture) of the intercalated carbon has to be elucidated. Depending on the texture of the nanocomposites, very different characteristics can be expected, for instance, anisotropic electrical conductivity or adsorptive properties inside the interlamellar space. Moreover, due to variable lattice constrains, changes of the properties may be anticipated, especially as a function of pyrolysis temperature. In this paper, we present the synthesis of 1019

F. BfiGUIN ef a/.

1020

various precursors, using two kinds of lamellar hosts and various aromatic cations. The conditions for obtaining the nanocomposites were determined, and the influence of in-situ carbonization temperature on their nanotexture was particularly considered. EXPERIMENTAL We used two host lamellar substrates which differed in cationic exchange capacity (CEC), acido-basic properties and thermal stability: Na-montmorillonite

Nao.ssSi8[AJ~Mg)0.65Feo.3~1020(OH)4, n&O Wming, CEC = 87 meq/lOO g) and Li-taeniolite Li(Mg2Li)Si4010F4, nH20 (Topy Co. Ltd, CEC = 260meq/ 1OOg); these two silicates are stable up to 700 and 9OO”C,respectively. The aromatic ammonium cations (Aldrich) used are represented in Fig. 1; they were selected for their different number of positive charges and for their different number of aromatic rings. The cationic exchange was realized at 80°C on 100ml of a 3 g aqueous suspension of the silicate by introducing the aromatic cation either in excess (1.5 x CEC) with respect to montmorillonite or taeniolite, or in a substoichiometric amount in the particular case of taeniolite (0.8 x CEC). The mixture was stirred for 4 days for montmorillonite and 1 day for taeniolite. After eliminating the solution by filtration, the solid was rinsed several times with distilled water, until the solution became colourless, and adsorbed water was evaporated by freeze-drying. To obtain the nanocomposite, the silicate/organic cation complex was pyrolyzed under nitrogen flow (200ml/min) at a ramp of 20O”C/h and at final

temperatures ranging from 400 to 1100°C (plateau 3 h). In order to analyze the nanotexture of the carbonlayer in the composite, we selectively dissolved the mineral part in an acidic medium. Such a demineralization has actually been used for separating the organic matter from the sediments [13] and assumed that any important modification of the carbon organization was avoided. The nanocomposite (1 g) was stirred at room temperature in a 40% HF solution for 2 days. In the particular case of taeniolite nanocomposites, an additional treatment was necessary: boiling in 12 M HCl in order to remove some fluorides unsoluble in the HF medium. After dissolution and filtration, the carbon residue was dried at 100°C under vacuum. In some cases, the chars were submitted to a graphitization treatment, for 15min in a furnace at 2800°C under argon atmosphere, to obtain carbons. The FTIR spectra were realized on KBr pellets (0.5% dispersion of the solid) with a Nicolet 710 spectrometer at a resolution of 4 cm-‘. The variations of the mean interlayer distance, provoked either by the intercalation of the organic cation or by the thermal treatments of the silicate-organic cation complexes, were determined by X-ray diffraction in the reflection mode with a Siemens D 500 diffractometer working at 1.5418A. The elemental analyses on the solids were performed at ‘Centre d’Analyses’, CNRS, Vernaison, France. The amount of Na or Li presented in the filtrate after the exchange was measured by atomic absorption spectrometry (Perkin Elmer 3300), using standards (Aldrich) in the range of O-l ppm for sodium and O-3 ppm for lithium.

NH,,HCl

Hcl**2NJ3qQNH2 o-i+?

cl’

CH,

Acriflavine

hvdrochloride

Aminoacridine

hvdrochloride

0\’ Safkanine

Indoine blue

Fig. 1.Structure of the aromatic ammonium cations

Si/C lamellar nanocomposites The Transmission Electron Microscopy observations of the nanocomposites were performed on a Philips CM 20, whereas the chars and carbons were studied on a Philips EM 400. The different modes used on carbon were: (1) bright field (BF) to image the particles morphology; (2) selected area electron diffraction (SAED) to obtain local structure information such as graphitization degree; (3) h/cl lattice fringe (hkl LF) to image the profile of the hkf planes (especially the 002 lattice fringe mode to visualize the profile of the aromatic layers); (4) 002 dark field mode (002 DF) to image the polyaromatic basic structural units (BSUs) placed at the Bragg angle, i.e. practically parallel to the incident beam, and of a given orientation.

RESULTS AND DISCUSSION (a) Formation complexes

of the silicate-organic

cation

Bringing the charge and the number of carbon atoms of the organic cation into play will allow one to obtain composites with various carbon contents. The cationic exchange was brought out by titrating the amount of alkali metal in the filtrate after the reaction. In the case of montmorillonite, the values obtained by atomic absorption on the solution (Table 1(a)) are in good agreement with those deduced from the elemental analysis of Na in the solid after the exchange (Table l(b)). In most cases the exchange rate is higher than 90%, indicating almost complete reaction. With taeniolite, in the same conditions (1.5 x CEC), the amount of Li+ exchanged, which was determined by atomic absorption on the filtrate, is close to only 80% (Table 2). Such a value, lower than with montmorillonite, could be due to the higher crystallinity of taeniolite which prevents easy diffusion of the cations from the edge planes to the center of the particles. Another more reasonable explanation is that, due to the large molecular volume of the organic cations and to the large value of CEC of taeniolite (260 meq/lOO g), the Li+ cations cannot be substituted

Table 1. Exchange (%) of the sodium cation of montmorillonite with the organic cations: (a) values determined on the filtrate; (b) elemental analyses of Na in the montmorillonite/organic cation complex Exchange (%) Cation Safranine Indoine blue Acriflavine Aminoacridine

(a) solution

(b) solid

95 92 93 92

93 94 93 87

1021

Table 2. Exchange (%) of the lithium cation of taeniolite with the organic cations, determined from the analysis of Li on the residual solution, as a function of the initial amount of organic cation (0.8 CEC or 1.5 CEC) Exchange (%) _ Cation Safranine Acriflavine Aminoacridine Indoine blue

0.8 CEC

1.5 CEC

36.5 36.5 37

76. I

80.6

completely by ammonium cations, except if a double layer of these cations would be formed. In order to have almost the same concentration of the organic cations in the aqueous solution used for the exchange, whatever the lamellar substrate, their concentration was lowered in the case of taeniolite to 0.8 x CEC. In such conditions, the exchange was less than 40% of the total amount of lithium initially present in taeniolite (Table 2). A very simple calculation, which accounts for the respective CEC of taeniolite and montmorillonite and for the percentages of exchange obtained, respectively, at 1.5 x CEC (case of montmorillonite) and 0.8 x CEC (case of taeniolite), shows that the amount of organic cations intercalated is almost the same in the two cases. Therefore, for what will follow, we decided to select to work on the silicate-organic cation complexes prepared with 1.5 x CEC and 0 > 8 x CEC respectively, for montmorillonite and taeniolite. Infrared spectra were recorded, in order to bring out the organic cation in the solid phase. The spectrum realized on the taeniolite/safranine complex exhibits both the bands belonging to safranine and to the taeniolite substrate (Fig. 2(a)). The main bands observed are at 3452, 3372 and 3260 (N-H stretching in NH*), 3174 (N-H stretching, dimer), 1642, 1612 (C = C stretching, C = N stretching, N-H deformation), 1532 and 1490 (C = C stretching), 1095 (Si-0 stretching), 998 (SiO asymmetric stretching), 468 cm-’ (Mg-0, Mg-F stretching or Si-0-Si angular deformation). The X-ray diffraction patterns showed an important shift of the 001 lines towards the small angles, as compared to the pristine host, demonstrating the intercalation of the organic cation (Fig. 3). The values of the 001 distance (&l) are given in Table 3, respectively, for the complexes of montmorillonite and taeniolite. The thickness e of the intercalated layer, given in Table 3, is obtained by substracting the thickness of the silicate sheet (9.4A for taeniolite, 9.6 A for montmorillonite) from do,,,. The values of e strongly depend on the organic cation. Since, all these cations have almost the same thickness (~4 A), this is

F. BBGUIN et al.

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b a

Fig. 2. X-ray diffraction patterns at 1S418 A of the taeniolite-aromatic ion complexes: (a) lithium-taeniolite; (b) taenioliteacriflavine; (c) taeniolite-aminoacridine; (d) taeniolite-safranine; (e) taeniolite-indoine.

the hypothesis of a double layer of cations lying parallel to the silicate sheet, as proposed by Van Damme [2]. As already suggested for aliphatic amines [4], it could be proposed that the mean plane of the cations is almost perpendicular to the silicate sheets. If one compares the dimensions of the cations (Table 4) to e, it is likely that, in most cases, the cations would be with the direction of their width almost perpendicular against

to the silicate sheets. The values of e are always larger with the complexes of taeniolite, which must be attributed to the higher CEC of this material, imposing a higher cationic occupation than in montmorilionite. This difference is particularly striking in the case of the taeniolite-indoine complex: the indoine cation is constrained to be with its length almost perpendicular to the silicate sheets.

Table 3. &, distance, thickness e of the intercalated layer and tilting angle of the organic cation in the complexes with taeniolite and montmorillonite Montmorillonite complexes

Taeniolite complexes Cation Safranine Indoine blue Acriflavine Aminoacridine

0 &I (4

e 64

a0

21.2 29.6 17.5 16.4

11.8 20.2 8.1 7

25.3 57 48.4 40.6

0 &I (A) 20.1 18.1 15.3 15.7

eA 10.5 8.5 5.7 6.1

a” 17 14.5 36.8 35.6

Fig. 3. X-ray diffraction patterns at 1.5418A of the montmorillonite-safranine nanocomposites: (a) montmorillonite; (b) montmorillonite-safranine complex at 20°C; nanccomposites at: (c) 400°C; (d) 500°C; (e) 700°C; ( f) 900°C; (g) 1000°C.

1023

B/C lamellar nanocomposites

Ii-‘,\ 6

(

,

,

14

10

z.JYy, , .--y-,d I8

26.

22

30

34

a 38

70

Fig. 4. X-ray diffraction patterns at 1.5418A of the taeniolite-indoine nanocomposites: (a) taeniolite; (b) taeniolite-indoine complex at 20%; nanocomposites: (c) 400°C; (d) 500°C; (e) 700°C; ( f) 900°C; (g) 1000°C.

In fact, the proportion of intercalated cations is adjusted by the relation which exists between the specific surface area and the cationic exchange capacity of the host lattice: the surface occupation of the cation on the silicate sheet cannot be higher than the surface of the sheet corresponding to one charge. The surface corresponding to one negative charge of the silicate layer, S_, can be determined from the CEC and the half-specific surface area (400 m2/g for montmorillonite, 3 10 m2/g for taeniolite). The calculated values of S_ are 76w2 for montmorillonite and 19 A2 for taeniolite. In the case of montmorillonite, it is interesting to compare S_ with the surface occupied by the cations when they are lying parallel to the silicate sheets (Si) or when their length is parallel to the c axis (S2) (Table 4). Since S- is smaller than Si and higher than S2, the aromatic cations must be standing up, but slightly tilted. For reasons of symmetry, it is better to consider that the width 1of the cation is perpendicular to the c axis, and the tilting angle cycan be calculated from the following equation:

Taking into account the dimensions of the cations given in Table 4 and the experimental values of e, the tilting angles Q were deduced from the previous equation (Table 3). Since the cationic exchanges were never complete (Tables 1 and 2), some interlayer spaces may be unfilled, and the values of &i only express an average interlayer distance. This is particularly the case for the montmorillonite-indoine complex, for which &i is very low. In fact, in some trials with this specimen, we could observe another 001 line at 24.96A which would lead to a more reasonable value of o. It is likely that, especially with the big cations, some of them may concentrate on the edge planes, preventing further diffusion in the interlayer space. (b) Formation and characteristics of the nanocomposites The silicate-organic cation complexes were heat treated under nitrogen flow at temperatures ranging d

(A)

e=Lcoscr+tsincu where L and t are, respectively, the length and the thickness of the cation. Table 4. Dimensions and surfaces of the various organic cations Sl (AZ)

s2 (A21

Cation

I(‘%

L (4

tile

Lllc

Safranine

11.6 11.6 8.58 8.03

13.55 21.87 13.17 11.52

157 254 113 93

47 47 35 33

Indoine blue Acriflavine Aminoacridine KS5?:6,8-H

10t,,.‘...‘...‘,..;.. 0 200 400

;,..! 600

SW

1000

1200 T(‘Ci

Fig. 5. Thermal evolution of doOrin the nanocomposites formed from the taeniolite-organic cation complexes.

F. BkGUIN ef

1024

from 400 to 1000°C in order to form the nanocomposites. For example, the X-ray diffractograms of the nanocomposites obtained from montmorillonitesafranine and taeniolite-iodoine complexes are, respectively, presented in Figs 4 and 5. The most striking fact is that a lamellar structure is still observed, even at the highest temperatures at which the non-intercalated host lattice was destroyed, as indicated by the presence of 001 lines. Moreover, since the h/c0 lines still appear at the same position as in the pristine host, it can be concluded that the structure of the silicate layers is almost unchanged. As montmorillonite and taeniolite are, respectively, destroyed at 700 and 9OO”C, it is obvious that the increase of thermal stability is due to the intercalated

al.

aromatic structures which prevent sintering of adjacent silicate sheets [9]. Above lOOO”C, the nanocomposites were no longer stable, because the reduction of the silicate by the intercalated carbon started to be very important, and a @-SiAlON was formed [3, 5, lo]. It should also noticed that the u&, distance continously decreases as the heat treatment temperature increases (Fig. 6). However, one can see that the decrease is more marked at c. 400-6OO”C, when carbonization of the organic cation proceeds, i.e. when most of the functional groups are released. Whatever the starting aromatic cation, the G&,,values observed at 900°C are very close to each other, in a range of 13- 17 A. Taking into account the thickness of the layer in taeniolite (9.4& the thickness of the carbon layer in the nanocomposite can be deduced; it ranges from about 3.5 to 7.5A. Most of these values are very far from 3.35 A, which would be obtained for an ideal monoatomic graphene layer. Therefore, all the models assuming such a layer in the nanocomposite must be reconsidered [ 1, 71. The nanocomposites were also analyzed by transmission i.r. spectrometry in order to examine the thermal decomposition of the cations (Fig. 2). At c. 400-5OO”C, there is an important decrease of the lines corresponding to the N-H stretching, in the range 3000-3600cm-‘, and two broad bands at 1628 and 1473 cm-’ (aromatic C-C stretching) are indicative of the condensation of the aromatic rings. Above 5OO”C, aromatization is almost over, as shown by the impossibility of obtaining good i.r. spectra. These data are in good agreement with the strong decrease of dool observed by XRD. The thermogravimetric analysis demonstrates that there is no evolution of interlayer water at low temperature, which proves a complete exchange of the sodium cations. Indeed, the polarizing power of the organic cation is so low that it cannot be solvated by water molecules. The most important weight loss, mainly attributed to ammonia, starts at 4OO”C, as

cm-1 Fig. 6. Infrared spectra of the nanocomposites formed from the taeniolite-indoine complex at various temperatures: (a) 20°C; (b) 300°C; (c) 400°C: (d) 500°C; (e) 700%; (f) 900°C.

Fig. 7. Evolution of the carbon percentage in the nanocomposites as a function of their formation temperature.

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Si/C lamellar nanocomposites by the decrease of the N-H band in the i.r. spectra. An other important weight loss appears above lOoo”C, when carbo-reduction of the silicate by the interlamellar carbon proceeds [3,5, lo]. The carbon content of the nanocomposites is only slightly varying with the temperature (Fig. 7). Due to the important aromaticity of the cations, only cases such as hydrogen or ammonia contribute to the vapor. In the case of aminoacridine, the carbon content decrease is due to the rather important vapor pressure of this molecule; a part of these molecules is vaporized instead of being carbonized. To confirm the lamellar structure, TEM was performed on the as-received nanocomposites in the 001 LF mode in order to image the protile of the silicate layers (Plate I). The micrograph shows an increase of the mean interlayer distance as compared to the pristine host, and local variations of the spacings, due to the existence of local defects such as layer distorsions and dislocations (arrows on the micrograph). Unfortunately, the carbon sheets could not be imaged directly, probably due to the poor organization, coupled with the weak atomic number of carbon. Therefore, we decided to analyze the chars obtained after the dissolution of the matrix. X-ray diffraction on the chars, coming from nanocomposites formed above 4OO”C,showed a broad 002 line, demonstrating the existence of nanometric basic structural units (BSUs). Moreover, SEM observations on the chars proved the improvement of a lamellar morphology when carbonization temperature increased. In order to bring out the possible subsequent nanotextural evolution, the 002 TEM mode was carried out. All the 002DF images of the chars coming from nanocomposites formed below 500°C showed almost similar densities of brightened BSUs in two orthogonal directions of aperture (micrographs 1 and 2, Plate II); the BSUs are therefore randomly oriented. As the already suggested

pyrolysis temperature increases from about 500 to 9OO”C,the 002 DF images, obtained for various orientations of BSUs in the Bragg angle, exhibit higher and higher contrast, due to an increasing difference in density of brightened BSUs (micrographs 3 and 4, Plate II). This proves the development of a preferential orientation of the BSUs, and it is in good agreement with the observation of a more and more lamellar nanotexture in the BF mode. The chars issued from the nanocomposites heated above 900°C give quasiperfect lamellae in which the BSUs are parallel to the lamella plane; such lamellae are lying on the supporting film, and they always appear dark except on the folded parts in which BSUs are at the Bragg angle (micrographs 5 and 6, Plate II). TEM brought out drastic nanotextural modifications of the chars coming from nanocomposites elaborated at different temperatures. Such increasing order cannot be attributed to the demineralization, which in fact would rather destroy the established order. The observation of various nanotextures for carbon after the demineralization is consequently strongly in favor of the existence of hierarchized BSUs in the nanocomposite. As already suggested by SEM observations, a lamellar nanotexture is developed during in-situ carbonization. The improvement

of the lamellar

nanotexture

was

a posteriori confirmed

by the study of the carbons resulting from the graphitization of the chars at 2800°C. The chars issued from low temperature nanocomposites remained porous (micrograph 1, Plate III) and are nongraphitizable as seen by the absence of hkl reflections in SEAD patterns (inset, micrograph 1, Plate III) and confirmed by XRD. On the other hand, the high temperature chars led to polycrystalline graphite lamellae, as proved by SAED patterns (inset, micrograph 3, Plate III) showing hkl reflections, and especially the 112 ones (arrow). The

Plate I. As-received taeniolite-indoine nanocomposite formed at 1000°C: 001 lattice fringe image showing the profile of the silicate layers. The arrows indicate local distorsions of the lattice.

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Plate II. Chars coming from the demineralization of nanocomposites formed in the range 500-900°C. 002 dark field images, visualizing two normal orientations of the BSU. Micrographs 1 and 2: char from a 500°C nanocomposite. The BSUs are strongly misoriented, as shown by similar density of lighted BSU from micrograph 1 to 2. Micrographs 3 and 4: char from a 700°C nanocomposite. The BSU acquired a statistical preferential planar orientation, as shown by the strong difference of lighted BSU from micrograph 3 to 4. Micrographs 5 and 6: char from a 900°C nanocomposite. These micrographs are typical of a lamellar microtexture. The parts lying flat are always lighted out, whereas only the folded parts can be bright.

structure is also confirmed by the perfectly planar aromatic layers (micrograph 2, Plate III). Such carbons are made of large crystallites, up to one micrometer, as demonstrated by the 1lODF mode (micrograph 3, Plate III). The size of the crystallites depends on the nature of the host lattice; for a given cation, they are larger with carbons coming from taeniolite based nanocomposites. graphite

Our observations are in complete disagreement with previous hypotheses assuming the development of single graphene layers between the silicate sheets. On the other hand, they strongly suggest the formation of BSUs at 40&5OO”C, which progressively tend to become parallel to the silicate sheets when the pyrolysis temperature of the nanocomposite increases (Fig. 8).

Si/C lamellar

nanocomposites

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Plate III. Carbons resulting from heat-treatment of the chars at 2800°C. Micrograph 1: 2800°C heat-treatment of the char resulting of the safranine powder. The 002 lattice fringe image shows the still porous nature of carbon. Inset: SAED pattern showing the still turbostratic order by the absence of kkl reflections. Micrographs 2 and 3: polycrystalline graphite formed after 2800°C heat-treatment of the char coming from a 1000°C taeniolite-safranine nanocomposite. Micrograph 2: 002 lattice fringe image showing perfectly planar aromatic layers. Micrograph 3: 11 dark field image showing moirC fringes due to superimposed crystallites; inset: SAED pattern showing the kkl reflections (the arrow indicates the 11.2 reflection) characteristic of the triperiodic order developed in such carbons.

1028

F. BEGUlN er al.

T=20”C

T =500 “C

T = 9oooc

Fig. 8. Model showing the various steps in the formation of

the nanocomposite: (20°C) structure of the silicate-organic cation complex formed by cationic exchange; (500%) the BSU arc formed, but they are misoriented; (900°C) the BSU are tending to be parallel to the silicate sheets; note that they are not independent.

CONCLUSION Cationic exchange with the interlamellar cations of lamellar silicates leads to interesting precursors for the formation of silicate-carbon lamellar nanocomposites with a rather high carbon content. Initially, i.e. at room temperature, the aromatic cations are almost perpendicular to the silicate sheets (1D nanotexture). When the temperature increases, carbonization starts to proceed at 400°C with the formation of basic structural units, parallel stackings of two or three aromatic structures. Up to about 5OO”C, the BSUs are randomly orientated in the interlameliar space of the nanocomposite, but they are probably almost perpendicular to the silicate layers (1D order). In the range of 500-9OO”C, due to the lattice constraints, the BSUs are progressively tilting, tending to acquire a preferential planar orientation, in parallel to the silicate sheets (2D order). Consequently, the d,-,aidistance decreases and, at 9OO”C,the interlamellar carbon has an almost perfect lamellar (2D) nanotexture.

However, due to the volume contraction provoked by the carbonization of the organic cations, one cannot expect a regular occupation of all the interlamellar spaces in the nanocomposite. Therefore, it is better to consider that the lamellar nanocomposite is made of a random sequence of empty and free intervals separated by pleats of the silicate sheets, as it is the case of graphite intercalation compounds. This assumption is quire reasonable because, even if the thick silicate layers are considered as more rigid than graphene layers for instance, such pleats are often imaged in the TEM observations of the precursor lamellar silicates and even on the nanocomposite itself. The interlamellar carbon plays an important role, preventing sintering of the adjacent silicate layers. Therefore, the lamellar structure of the nanocomposite is still observed up to 1000°C. Above this temperature, the highly divided carbon reacts with the silicate host, and carbo-reduction leads to ceramic materials, such as P’-SiAlON. The 1DP2D nanotextural transition occuring in the nanocomposite is responsible for the subsequent graphitization of the chars, even if the precursor molecules themselves, when heat-treated outside the silicate structure, would give nongraphitizable chars. This is a good example in which the graphitizability is the consequence of increasing anisotropic constraints developed at the nanometric scale, between silicate sheets. In nature, similar phenomena occured when shear strains were applied during tectonics, at a kilometric scale, leading to lamellar graphitizable highrank coals (anthracites). Due to the particular nanotexture of the interlamellar carbon, these nanocomposites should have very specific properties, such as the anisotropy of electrical conductivity or selective adsorption, depending on their heat-treatment temperature. In particular, the partial oxidation of carbon should lead to porous bidimensional materials. These new solids could find applications as molecular sieves or supports for the catalysis.

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11. Oya A., Saito M. and Otani S., Appl. Clay. u Sci. 3,291 (1988). 12. Oya A., Sato A., Hanakoa H. and Otani S., J. Am. Ceram. Sot. 73,689 (1990). 13. Durand B. and Nicaise G., in Kerogen (Edited by B. Durand), p. 36., Paris (1980).