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Formation of sp3-bonded carbon upon hydrothermal treatment of SiC
?lJMOND ELSEVIER
Diamond and Related Materials 5 (1996) 151-162
Formation of sp3-bonded carbon upon hydrothermal treatment of SIC Yury G. Gogotsi a**,Per Kofstad a, Masahiro Yoshimura b, Klaus G. Nickel ’ a University of Oslo, Centrefor Materials Research, GaustadallCen 21, N-0371 Oslo, Norway b Research Laboratory of Engineering Materials, Tokyo Institute of Technology, Yokohama 227, Japan ’ Eberhard-Karls-Universitiit Ttibingen, Institutfiir Mineralogie, Petrologie und Geochemie, Wilhelmstr. 56, D-72074 Tiibingen, Germany Received 20 July 1995; accepted in final form 21 September
1995
Abstract The composition and structure of carbon produced by selective leaching of silicon carbide under hydrothermal conditions in the temperature range 300-800 “C and at pressures < 500 MPa were examined by Raman and IR spectroscopy, XRD, SEM, TEM and EDS. The results of this study have demonstrated that various carbon allotropes, including diamond, are formed during hydrothermal treatment of SIC. Their structure varies depending on the experimental conditions and SIC precursor. The formation of sp2 vs. sp3 carbon is discussed. Keywords: Carbon;
Diamond;
Hydrothermal
synthesis;
Silicon carbide -
1. Introduction Hydrothermal
of various in the calculations 2)
Thermodynamic of the + 2Hz0
SiO,
CH4
SiOz
CO,
SIC
4Hz0+
SIC
3Hz0-+SiOz
SIC
2H,O-+SiO,
of silicon [ 11.
(1)
4H,
(2)
CO + 3H,
(3)
2H,
(4)
Finding of of SIC
in this
a very limited information about its structure has been published. The general experimental facts listed below have evolved from our previous studies on ( 1) Carbon
formation
depends on the
of
SIC * Corresponding author. Present address: Ttibingen, Institut ftir Mineralogie, Petrologie Wilhelmstr. 56, D-72074 Tiibingen, Germany.
und Geochemie,
0925-9635/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved SSDI 0925-9635(95)00341-X
whiskers or CVD SIC treated under the same conditions produce different amounts of carbon and these carbon layers give slightly different Raman spectra [ 51. Absorptions due to sp3 CH, are always present on IR spectra of hydrothermal carbon. sp2 CH, is absent under most of the experimental conditions [4]. (3) Oxygen and silicon can be present in hydrothermal carbon obtained from SIC in amounts up to a few atomic percent [4]. (4) Graphitic and amorphous carbon have been found by several methods of analysis [ 2-41. (5) Raman bands can shift from the position typical for graphite to the region of diamond phonon [ 71. (6) Crystalline carbon particles give typical of diamond Auger spectra [ 81. Thus, our previous studies have demonstrated the possibility of the synthesis of graphitic carbon of various degrees of ordering on the surface of Sic. At the same time we noticed the presence of sp3 bonded carbon in the reaction products and described the mechanism of its formation [S-lo]. We have continued the studies with the aim of obtaining further information regarding the formation of carbon during hydrothermal corrosion of Sic. Here
152
Y.G. Gogotsi et aLlDiamond and Related Materials 5 (1996) 151-162
we present the results of a more-detailed study of the composition and structure of hydrothermal carbon along with the analysis of the previously reported data.
2. Experimental details A laboratory scale P-Sic powder’ with a specific surface of 8 m2 g-’ (dsO= 1 pm), Cc-Sic platelets2 (dso= 13.5 pm), single crystals and polycrystalline B-Sic samples3 in the form of - 5 mm thick coupons produced by chemical vapour deposition (CVD) were used for this study. The hydrothermal corrosion of some of these materials was briefly discussed in [ 5,8]. Samples and distilled water were placed into an 30-50 mm long and 3-5 mm in diameter Au capsule. The capsules were closed with Au plugs to prevent loss of material and then heated in a tube-type, cold-seal pressure vessel [ 111. Powders after hydrothermal treatments were leached in a 20% HF solution (to remove Si02) and in hot HC104 (to remove non-diamond carbon). Our previous investigations demonstrated that neither of the analytical methods alone could supply sufficient information about the composition and structure of the reaction products. Some of the existing problems are highlighted in [ 121. Taking into account the variety of existing forms of carbon and severe difficulties with analysis of Sic-Si02-C mixtures, we used a combination of various complementary structural and spectroscopic methods of analysis that allowed us to detect both crystalline and amorphous phases, as well as to study structural ordering of the reaction products. The composition and structure of reaction products were examined using vibrational spectroscopy (Raman and infrared), X-ray and electron diffraction, as well as scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Energy dispersive spectroscopy (EDS) was used to identify the composition of the powders. In case of XRD analysis, monochromatic Cu Kxradiation was used. A Guinier camera was employed for X-ray phase analysis of small amounts of powders. Samples for Fourier-transform infrared (FTIR) spectroscopy were prepared by mixing the powders with KBr and pressing to form a pellet. The volume-filling factor of powder was about 0.005. IR-absorption spectra were measured in the frequency range from 400 to 4000 cm-’ using Bruker spectrometers at the University of Oslo. The Raman analyses were conducted using several
Dilor and Jobin Yvon micro- and macro-Raman spectrometers at the University of Oslo, University of Limoges and Tokyo Institute of Technology. In many cases, the same samples were analysed in several laboratories to obtain reliable and reproducible data. An argon laser at a wavelength of 514.5 nm was used for Raman analysis. The samples were placed on a Si plate for micro-Raman measurements. SEM studies were done using Philips XL30 microscope equipped with an EDS spectrometer particularly sensitive to light elements. After hydrothermal treatments under different conditions several samples were chosen for TEM analysis. The powders were observed in JEM 200CX electron microscope operating at 200 kV. The particle types were identified by EDS and diffraction patterns. TEM samples were prepared by mixing powders with ethanol in an ultrasonic bath, followed by filtration of the particles which were collected on a supporting copper grid covered with a perforated amorphous carbon film. The structure of silica and changes in the structure of SIC after hydrothermal corrosion have also been thoroughly studied. These data will be published elsewhere [ 121. Here we discuss these results only as far as they concern the analysis of carbon.
3. Results 3.1. XRD studies The phase compositions of the reaction products after corrosion in the temperature range 600-800 “C were almost independent on the carbide precursor and pressure. a-cristobalite, a-quartz, traces of tridymite, amorphous silica and carbon were found, but the relative amounts of these depended on the experimental conditions. The identification of carbon by XRD was complicated by the presence of silicon oxides and Sic, which dominated the XRD patterns, and low amplitude of X-ray lines of carbon in comparison with that of silicon compounds. After dissolution of silica in HF, carbon lines are more pronounced. However, after hightemperature (> 700 “C), long-term (> 5 h) experiments the amount of remaining material was often not sufficient for XRD measurements. Nonetheless, we could observe some of the graphite peaks and a very weak peak near 2.06 w which gave a first indication for the presence of diamond. 3.2. IR spectroscopy
1 Synthesized by Dr. E.V. Prilutsky, Institute for Problems Science, Ukrainian Academy of Sciences, Kiev, Ukraine. * C-Axis Technology Ltd., Canada. 3 Raytheon/BOMAS Machine Specialties, USA.
of Materials
The absorptions corresponding to hydrogenated sp3 carbon at about 2850,292O and 2960 cm-’ were present in all cases (Fig. la) and their intensity increased with
Y. G. Gogotsi et aLlDiamond and Related Materials 5 (1996) 151-162
#
I 3150
1
3050
2950
Wavenumber
2000
1800
I
J
2850
2750
(cm-‘)
1600
Wavenum
153
1400
1200
1000
800
ber (cm-‘)
Fig. 1. FTIR spectra of CH, (a) and carbon (b) regions recorded after hydrothermal treatment of Sic platelets for 18 h at 800 “C (HF leached). The expected positions of sp2-CH, groups (a) and sp’-C-C (b) are marked with arrows.
temperature. Hydrogen bonds preferentially to sp3 sites. Weak absorptions due to sp2 C-H groups were only rarely found. An absorption at 3300 cm-’ that can be assigned to sp’ CH was also observed after hydrothermal treatment of /?-Sic powder [12]. Since the silica band at - 1100 cm-l covers the region from 950 to 1500cm-’ in the hydrothermally treated samples, it was difficult to analyse other bands that could be present in that range. Leaching in HF allowed us to remove the silica bands or drastically decrease their intensity (Fig. lb) and revealed the bonds of sp3 carbon. Absorption at - 1340 cm- ‘, which can be
assigned to sp3-carbon, was not affected by any acid treatments. This is a Raman frequency, rendered IR-active by defects and impurities. It is accompanied by a strong absorption at 1267 cm-’ that can be assigned to SiCH3 bending (observed at 1273 cm-’ [ 131) or to diamond polytypes (predicted at 1280 cm-’ in 15R and at 1257 cm- ’ in 21R [ 141). The latter assumption seems to fit better, because the absorption does not disappear after a treatment in hot HClO,. Absorptions due to CH, groups disappeared after such treatment. This couple of absorptions is typical for hard carbons [ 151. The position of the IR band of sp3 carbon was in the
Y.G. Gogotsi et al./Diamond and Related Materials 5 (1996) 151-162
154
range 1310-1340 cm-’ depending on the carbide precursor and experimental conditions, but it never became as sharp as that in natural or high-quality CVD diamond. 3.3. Raman spectroscopy Raman spectroscopy results are in agreement with XRD and FTIR data and show the presence of Sic, silica and carbon after hydrothermal treatments. In the spectrum of the hydrothermally treated samples, two strong and broad bands appear at - 1590 and - 1350 cm- ’ (Fig. 2). The former one is usually referred to as G mode and assigned to C=C stretching vibrations EZgof graphite [ 16,171. The latter one is usually referred to as D mode of graphite, and known to become active
G
in small graphite crystallites. These bands were present in almost all spectra and confirm the formation of graphitic carbon during hydrothermal corrosion of Sic. Depending on the treatment regimes and Sic precursor, the position of G band of graphite can vary from 1580 to 1610 cm-’ (Fig. 3a). The observed shift of the G band towards the high-frequency edge or its splitting can be caused by a second first order zone boundary phonon at 1620 cm-’ [ 171. The band at frequency 1620 cm-‘, which also corresponds to the maximum density of states of graphite, is sometimes referred to as D’ mode. Thus, our Raman spectra apparently demonstrate sp2bonded carbon in the nanocrystalline or amorphous form. Though some of the spectra of hydrothermal carbon (curve 4 in Fig. 2) are similar to that of disordered graphitic carbon (curve 5 in Fig. 2) it is important to note the down-shift of the D band in comparison with 1350-1355 cm-’ or its splitting to two or more bands (curves 1 and 2 in Fig. 2). The lowest wavenumbers ( 1327- 1330 cm- ‘) were registered after treatments at - 700 “C, but the minimum on the temperature dependencies (Fig. 3b) shifts to lower temperatures (below 500 “C for Tyranno fibres) with increasing time or pressure. In addition to the bands discussed above, we registered several other bands in the range lOOO-1500cm-‘. regions features the Additional appear in 1100-1200 cm-’ (Fig. 4) 1285-1300 cm-’ (Fig. 2) and 1420-1450 cm-’ (Fig. 5). These features can appear as shoulders of the D-peak (Figs. 2 and 5) or well-resolved bands (Fig. 6). The origin of these bands will be discussed later. Bands in the range of diamond phonon at 1330-1340 cm-’ that were not accompanied by G bands have been also observed (Fig. 5). 3.4. Microscopic
observations
SEM + EDS investigations of powders after hydrothermal treatments show the presence of silica and pSIC in all specimens. It was difficult to identify the location of free carbon in SEM pictures of as-treated Sic powders. Carbon particles and films can be better seen on the surface of Sic single crystals (Fig. 7). However, it is not clear whether these particles grow due to direct transformation of Sic to carbon, or formed from the hydrothermal fluid by reaction [20] , 1300
1400 Wavenumber
1500
1600
1700
(cm-l)
Fig. 2. Representative Raman spectra of /?-Sic powder whiskers (2), SIC Tyranno@ fibres (3) and a-Sic single after hydrothermal treatments in comparison with a pyrolytic carbon (5). 1, 5 h at 800 “C under 500 MPa; 700 “C under 100 MPa; 3, 8 h at 700°C under 100 MPa; 750 “C under 10 MPa.
(l), /&Sic crystals (4) commercial 2, 24 h at 4, 15 h at
CO, + CH, -+2C + 2H20
(5)
Etching of hydrothermally treated /?-Sic powders in HF led to the formation of very thin free-standing carbon films that could be easily seen. The selected area diffraction (SAD) patterns from these powders show reflexes corresponding to both graphite and diamond [ 121. However, because of a very small particle size of diamonds (< 50 nm) and numerous graphite and Sic
155
Y.G. Gogotsi et al./Diamond and Related Materials .5 (1996) 151-162
Raman shift (cm-l) 1620
1570
7
200
0
600
400
600
Temperature (“C) Raman shift (cm-l) 1360 [
1350
1340
1330 I 600
I 400
1320 ’ 0
I 800
Temperature (“C) Fig. 3. Dependencies of the positions of G (a) and D (b) Raman bands n , Tyranno fibres treated for 24 h; +, Tyranno fibres treated for 8 h;
on the temperature of hydrothermal treatment under P-Sic powder; 0, /?-Sic whiskers treated for 24 h.
x,
1400
1200 Wavenumber
Fig. 4. Micro-Raman
spectrum
from a single SIC platelet
1600
(Cm- ‘)
treated
for 18 h at 800 “C under
100 MPa pressure.
100 MPa
pressure:
Y. G. Gogotsi et al./Diamond and Related Materials 5 (1996) 151-162
156
1 1200
r
1300 Wavenumber
1500
1400
1600
(cm-‘)
Fig. 5. Two typical Raman spectra from particles on the surface of CVD Sic after corrosion for 20 h at 750 “C under 92 MPa water pressure (2,3) in comparison with the spectra of CVD diamond on a Si substrate [ 181 (4) and nanocrystalline diamond powder [ 191 (1).
particles surrounding them always, we could not obtain single crystal SAD patterns from diamond on these samples. Thin carbon films disappeared after etching in boiling HC104. Graphite was never observed in the powder subjected to wet oxidation treatment. At the same time, we found small (< 0.3 pm) particles on the surface of several larger Sic particles (Fig. 8a). EDS of the crystal covered with particles in Fig. 8a did not demonstrate the presence of any other elements except of Si and
Fig. 6. Raman
spectrum
from the surface
carbon. However, quantitative EDS analysis of the single particles on the surface of Sic was impossible due to their small size. From their morphology, we can suggest that they have rather diamond than graphitic structure. Later, we observed similar particles with TEM (Fig. 8b). At least some of these particles have reflections with d = 2.06 A. Crystalline morphology was generally well developed for crystallites larger than 20 nm. Hexagonal (0001) faces predominated, but face-centered cubic (111) and (100) were also observed (Figs. 7 and 8). The presence of d = 1.92 A reflections in SAD patterns and hexagonal shape of some diamond particles (Fig. 8b) suggest the presence of hexagonal diamond (lonsdaleite or diamond polytypes). Other reflections of lonsdaleite are very close to that of /?- and/or a-Sic. The hexagonally shaped crystals can also arise from the growth of diamond nuclei with two (or more) parallel twin planes. Thus, the exact structure of tetrahedral carbon is still not quite clear. The particle shape and grain size are very similar to that of gas phase diamond powders [ 19, 21). These powders, like hydrothermal carbon produced in our studies, show only a very broad feature at about 1332 cm-’ (curve 1 in Fig. 5) instead of a narrow diamond band.
4. Discussion of reaction products 4.1. Linear and graphitic carbon
It is known [22] that chemical methods of carbyne synthesis are based on decomposition of carboncontaining materials with the formation of linear carbon chains. Carbyne was reported to appear in a C:H films
1400
1500
Wavenumber
(cm-‘)
of an a-SiC single crystal
treated
for 25 h at 750 “C under
100 MPa pressure
Y. G. Gogotsi et al./Diamond and Related Materials 5 (1996) 151-162
157
Si
(a)
(b)
Si
2 Energy
1
2
(keV)
100 nm Fig. 8. SEM image (a) and bright field TEM image (b) of well-faceted crystallites in the P-Sic powder after hydrothermal treatment for 5 h at 740 “C under 100 MPa pressure and etching in HF and HCIO,.
bonding in hydrothermal carbon. Although several reflections on electron diffraction patterns (e.g. d = 2.22-2.24 A) can be assigned to carbyne, we could not find a reflection at 4.47 A. Thus, the presence of carbyne has not been finally confirmed. A more intensive study on TEM observations is necessary to prove or finally exclude the presence of carbyne. Comparisons of the results of various analyses show the formation of graphite and disordered graphitic carbon under hydrothermal conditions. Thus, the fact of the formation of this carbon allotrope has been determined and does not need much further discussion. Fig. 7. EDS spectra of the flat surface (a) of a SIC crystal after hydrothermal treatment for 25 h at 750 “C under 100 MPa and a carbon particle on its surface (b), and SEM images of typical particles (c,d). The intensity ratios correspond to ~50 at.‘+‘& (a) and > 96 at.% C (b).
[23]. Therefore, we could expect its formation upon preferential leaching of SIC under hydrothermal conditions. FTIR spectra may give evidence of some sp’
4.2. Tetrahedral carbon A down-shift of the D band was observed after hydrothermal treatments at 450-750 “C. It can indicate the presence of sp3 carbon bonds as well as disorder. in /?-Sic powder The D band shifts from -1350cm-l treated at 600 “C to below 1340 cm-’ after treatment for 8 h at 730 “C (Fig. 3b). An even stronger downward
Y.G. Gogotsi et al./Diamond and Related Materials 5 (1996) 151-162
158
shift to -1330 cm-’ was observed for SIC fibres and whiskers treated at 700 “C. This position of the peak is characteristic for diamond. A single narrow peak is observed at - 1332 cm-’ in diamond. This band may be broadened by defects, and/or small crystalline size, and shifted by as much as 8 cm-’ (either higher or lower) depending upon preparation conditions and substrate [24]. Data plotted in Fig. 3b, along with TEM and electron diffraction results suggest that with increasing temperature of hydrothermal treatment carbon may change its form from amorphous material to a mixture of nanocrystalline graphite and disordered ultra-fine diamond. This can be explained by the increasing etching rate of graphitic carbon by H,O and Hz with temperature. An increase in the frequency of D band at 800 “C is due to enhanced graphitisation of carbon. Increasing duration of the treatment at that temperature leads to a further shift of the D band to the high-frequency edge (Fig. 3b). According to our experiments, synthetic highpressure diamonds4 are stable and do not interact with water below 750 “C. A similar crystallisation of carbon leading to the formation of both graphite and diamond was observed upon laser heating of a-C:H films [ 251. We must note that the assignment of the D band at - 1330 cm-’ to sp3-bonded carbon is not universally accepted, and it has been argued that this peak can arise from a highly disordered and strained graphitic phase [26]. However, the G band does not shift downward along with the D band (Fig. 3), as would be expected in the case of stress-induced shift. Also, extremely high stresses are necessary for such shift of a Raman band. The analysis of the literature (e.g. Ref. [ 171) and our experiments with various carbons available to us show that the D band of graphitic materials is always above 1343 cm-’ (at 514.5 nm laser wavelength). At the same time, D bands were observed at 1338 cm-’ for a-C:H [25], at 1336 cm-’ for DLC obtained from a polymer precursor [27], at 1332 cm-’ for nanosize multiplephase diamond powder (Fig. 5) and nanocrystalline CVD diamond films [28] and at 1320 cm-’ for Sic-C from polymer precursors [ 133. These were attributed both to the presence of sp3 carbon and to bond-angle bending disorder. However, the assignment of the broad (FWHM =70 cm-l) band at 1336 cm-’ accompanied by a G band at 1580 cm-’ to diamond bonding [27] caused much criticism in the scientific community (e.g. Ref. [26]). Indeed the explanations of the shift and/or broadening of the diamond line by the overlap of D band of graphite [27] and many polytypes of diamond [ 193 may seem to be speculative until sufficient proofs are presented. However, a computer-assisted analysis of the bands centred at 1330 cm-’ (curves l-3 in Fig. 2) shows that, unlike the D band of pyrolytic carbon (curve 5 in Fig. 2), those can be described by neither a Lorentzian 4 General
Electric,
USA.
nor a Gaussian shape. Contributions of at least three bands centred at -1295 cm-’ (its origin will be discussed later), -1330cm-’ (diamond band) and - 1360 cm- ’ (typical position for a D band of graphitic carbon) are necessary to get the best agreement between the measured and computer simulated spectra. Using micro-focus technique, we observed occurrence of the discussed band at 1336-1339 cm-‘, and it was not accompanied by a G band (curves 2 and 3 in Fig. 5). Therefore, it cannot be related to graphitic carbon, which always shows a band at N 1580 cm-‘. The micron-size regions of the surface giving the spectra shown in Fig. 5 were bright and transparent in optical microscope, but non-distinguishable from the surrounding material in SEM. Thus, these were nanocrystalline particles. The peaks at 1336-1340 cm-’ observed on Raman spectra of CVD Sic (Fig. 5) may give an evidence of the formation of diamond under hydrothermal conditions. The shift of the peak from 1332 cm-’ can be due to stresses when diamond grows on the SIC substrate (stress of +2.1 GPa is necessary to shift the band to 1337 cm-’ [ 171). It is only slightly broader than that of a median-quality CVD diamond on Si and even more narrow compared with that of nanocrystalline diamond powder (Fig. 5). For other samples (Fig. 6), we observed three single bands in this range positioned at 1303, 1336 and 1378 cm-l, overlapping of which will produce broad bands with shoulders shown in Figs. 2 and 5. The above results allow us to explain the observed (Fig. 3b) downward shift of D band by its overlapping with the bands of cubic diamond and diamond polytypes. Broad bands in the frequency range 1100-1200 cm-’ (Fig. 4) can be assigned to amorphous diamond. In the amorphous state the Raman spectrum must reflect the vibrational density of states of material. The maximum of the density of states is expected at 1140 cm-’ for cubic diamond (TO mode) and at 1175 cm-’ for hexagonal diamond [ 291. A broad band centred at - 1170-l 180 cm-’ was reported for spherulitic diamond [ 301 and bands at 1130-l 150 were observed in various nanocrystalline or amorphous diamond hlms [ 24,291. Diamond films with crystallites < 0.1 urn exhibit Raman bands in the range 1loo-1260 cm-’ at laser wavelength 1064 nm [31]. Thus, the spectrum in Fig. 4 can be assigned to amorphous diamond. A very strong and broad band of sp3 carbon on FTIR spectrum of the same sample (Fig. lb) supports this assumption. After identification of all phases of SiOZ, Sic, graphitic carbon and cubic diamond, and excluding carbynes from the discussion, we still have several bands on Raman spectra in the range 1250-1520cm-’ that cannot be assigned to any of the above compounds. We have done a literature search looking for similar features on the spectra of related materials: various forms of carbon such as a-C, a-C:H, DLC, CVD diamond, a-SiC:H,
Y. G. Gogotsi et al.lDiamond and Related Materials 5 (1996) 151-162
a -Si:H, etc. These features cannot be accounted for by any of the known SiO,, Sic or SiH, structures, though more complex phases in the Si-C-H-O system cannot be completely excluded from the discussion. On the other hand, the features with similar wavenumbers have been observed in various hard carbons and diamond materials obtained under non-equilibrium conditions: (1) A peak at - 1300 cm-’ was observed in a diamond crystal obtained by CVD in an oxygen-containing environment [32], in DLC films with the estimated content of sp3 bonding of 76% obtained by laser ablation [ 331 and in a-C:H films [34]. In the latter case, it was accompanied by a band at 1250 cm- ‘, which is close to the band at 1240 cm-’ registered for hydrothermally treated Sic powder (curve 1 in Fig. 2). The presence of the band at 1300 cm-’ did not depend on the substrate (MgO or Si [ 331) and thus, this band cannot be due to Si-containing compounds. Moreover, the calculated positions of Raman bands of 15R (1280 and 1295 cm-‘) and 21 R (1306 and 13 10 cm- ‘) diamond polytypes are in good agreement with the bands that were observed for hydrothermally treated P-Sic powder (1285 and 1294 cm-‘) and single crystals (1303 cm-‘). (2) The Raman bands at around 1420-1440 cm-’ [30] and a luminescence band at 1445 cm-l [35] were observed in the spectra of diamond films excited at 514.5 nm line. The latter band was assigned to the presence of twins in diamond. (3) A peak at - 1520 cm- ’ was observed in various diamond and DLC films and attributed to “bridged graphite” [ 161. On the basis of these points we can conclude that these peaks are not unusual for CVD diamond and related materials and, at least some of them, can be attributed to diamond polytypes and/or disordered diamond containing Si or 0. The latter statement is supported by the presence of forbidden diamond reflections in SAD patterns [ 121. The appearance of forbidden reflections was attributed to the formation of a diamond superstructure formed by the ordering of inclusions incorporated in the crystallites during growth [ 361. This suggests a growth of the layers with Si, 0 or other defects. Some of the observed Raman bands are close to those of fullerenes. For example, in the frequency range of interest C,,, has Raman bands at about 1248,1318,1426, 1573 and 1632 cm-’ and the strongest one at 1469 cm-’ [37]. However, we cannot explain how fullerens can be formed under conditions of our experiments and fullerens were found neither by XRD nor by electron diffraction studies. Using the above data and our results for other Sic materials, we can summarise all facts supporting the presence of amorphous or crystalline tetrahedral carbon on the surface of SIC hydrothermally treated below 500 MPa by the following. (1) IR band at 1310-1340 cm-‘.
159
(2) Shift of the D-band on the Raman spectra to < 1340 cm-’ or its splitting to two bands positioned or its occurrence at at - 1330 and - 1350cm-‘, 1336-1340cm-’ without being accompanied by a G-band. (3) Broad Raman bands at 1100-1200 cm-’ that can be assigned to amorphous diamond. (4) Several Raman bands with frequencies typical for diamond polytypes. (5) Diamond reflections on the electron diffraction patterns. (6) Well-shaped non-graphitic carbon particles (analysis of such particles on the surface of CVD Sic produced typical for diamond Auger spectra [lo]). (7) XRD peak at d = 2.06 A. Thus we have enough supporting data to confirm the formation of tetrahedral forms of carbon upon hydrothermal leaching of SIC. However, since we did not observe a narrow Raman peak at 1332 cm-’ and did not obtain enough material for high quality XRD studies on carbon, we cannot claim the formation of wellordered cubic diamond. Formation of amorphous diamond, disordered diamond-like carbon and tetrahedral carbons with different stacking sequences seems to be highly probable. The presence of additional Raman bands at - 1150, 1245, 1285, 1300, 1440 and 1520 cm-’ supports the hypothesis of the formation of carbons with the atomic structure different from that of either graphite or cubic diamond. The absence of a sharp Raman band at 1332 cm-’ can be explained by very small size and content of diamond crystallites [ 3 11. Our TEM measurements show that the size of crystallites, arising in the bulk of carbon films, is usually < 50 nm. At such small particle sizes the wavevector conservation rule for Raman spectroscopy of single crystals of diamond breaks down [ 313. The phonon-confinement effect should play a substantial role leading to the shift and to the strong broadening of a fundamental diamond mode [38]. It is quite possible that, keeping its integral intensity constant, this line would have a too low amplitude to be extracted from the graphite background even in the case of comparable contents of diamond and graphite phases in the sample. Particularly, it has been reported that the Raman cross-section of graphite is much higher than that of crystalline diamond [ 391 and hence sp2-bonded regions dominate the Raman spectra. Our TEM observation showed only a small percentage of diamond in the hydrothermal carbon and we could not expect to see a sharp diamond peak at 1332 cm-’ in our spectra. The Raman response of hexagonal diamond is smaller than that of cubic diamond [ 141. Even 30-40 urn thick hexagonal diamond films exhibit only a very weak and broad peak at 1333 cm-‘, accompanied by D band at 1360 cm-’ [40]. An additional factor is the expected presence of silicon
Y. G. Gogotsi et al./Diamond and Related Materials 5 (1996) 151-162
160
in hydrothermal carbon. According to [41], the addition of 2 at.% Si to CVD diamond substantially decreases the intensity and increases width of the band at 1332 cm-‘. It also decreases the intensity of X-ray peaks of diamond [ 351. What are the critical features that allow the formation of sp3-bonded carbon, and not simple amorphous or graphitic carbon network? We suggest the following. The presence of an appropriate substrate. SIC is widely used as a substrate for diamond deposition. The formation of P-Sic buffer layer, with a partial matching between the diamond lattice (Fd3m group) and the pSic lattice (F43m group), has been suggested to be necessary for diamond nucleation on the surface of silicon [24]. This means that cubic /?-Sic due to its structure can serve as a good substrate for hydrothermal synthesis of diamond. The formation of hydrogen during hydrothermal treatment ofSiC. It was found that atomic hydrogen can etch
graphite with a much higher rate than diamond [42]. The atomic hydrogen is known to be important in diamond growth and also plays an important role in its nucleation. Tetrahedral bonding of carbon in Sic. We believe that carbon can maintain the cubic structure of the carbide precursor due to stabilisation of dangling bonds by hydrogen, if silica is dissolved in the hydrothermal fluid. Transformation of tetrahedral carbon in SIC to diamond may be energetically more advantageous than a graphite to diamond transformation. Preferential
oxidation
of sp2-bonded
carbon
by water.
It was shown that water addition to CH4-H, precursor gas reduces the substrate temperature necessary for CVD diamond formation and increases its growth rate [43]. Reaction (4) can lead to the formation of diamond nuclei on the surface of Sic, provided certain conditions are established and maintained during the process. The mechanism of the formation of carbon nuclei is shown schematically in Fig. 9. The fact that diamond particles grow preferentially on the surface of SIC (Figs. 7 and 8) is in agreement with the hypothesis, that diamond nucleates due to reaction (4). However, for further growth, transport through the hydrothermal fluid is necessary. The presence of carbon deposits not only on the SIC surface, but also on other parts of the system (gold capsule, test tube) confirms the transport of carbon through the hydrothermal fluid. The substantial solubility of carbon in C-H-0-(Si) fluids and the possibility of graphite deposition from these fluids have been known for years [20]. It was proposed that the deposition of graphite from metastable fluid containing CO, and CH, occurs according to the reaction (5) taking place in an H,O-dominated fluid. Thus, carbon can be formed by deposition from a C-H-0-Si fluid formed due to reactions (l-4). This mechanism of carbon deposition can
H-O
H-O
H
S&-C--Si-C-Si-C
’ I
I
Si-C-S&C
I/II
I I
Si
3-j
I
C
I
Fig. 9. Schematic two-dimensional sketch of the reaction mechanism, showing that the process includes the following stages: (i) interaction of Sic surface with an H,O molecule; (ii) stabilisation of dangling bonds by hydrogen; (iii) release of hydrosilica and hydrogen with the formation of sp3 C-C bonds.
contribute to the growth of diamonds after the formation of nuclei due to differential etching of SIC by (4). Diamond is metastable with respect to graphite in the pressure/temperature range used in our experiments. Thus, thermodynamic considerations and more-detailed analyses of the reaction mechanisms are necessary to understand why diamond formation is possible. To show the mechanism of diamond formation, we can rewrite the reaction (4) as follows: SIC + 2H,0+Si02 .
+ C~si,+ 4H ’
4H + Ctsi, +Ccdiamond)+ 2H2
(6)
(7)
where C~si,is a carbon atom that has at least one bond with a Si atom. Assuming these reactions to be thermodynamically coupled, we can suggest that the chemical energy of reaction (4) is partly used to form diamond. For the further growth of a diamond nucleus by (5) thermodynamic and kinetic explanations that were given for low-pressure gas-phase synthesis of diamond can be used (see Ref. [24] for a review). We must also note that the chemistry in the hydrothermal medium may contrast with that associated with ambient or low-pressure conditions. This must be taken into consideration when explaining the mechanism of diamond formation and growth. The above. discussion shows that formation of DLC or even diamond upon hydrothermal leaching of Sic can be explained on the basis of the existing theories of low-pressure growth of diamond. An extremely small grain size of the hydrothermal diamonds can be again understood by comparison with the vapour phase synthesis. Since graphitic carbon is usually formed along with sp3-carbon, it retards the growth rate of diamond as observed upon CVD synthesis [24].
Y. G. Gogotsi et al.lDiamondand
Hydrothermal origin of some natural diamond crystals has been considered in the literature and very good reviews of the hydrothermal systems pertinent to the diamond synthesis have been published [44,45-j. Diamond formation under hydrothermal conditions was predicted at the lowest possible temperatures, but highest feasible pressures. Recently, hydrothermal synthesis of diamond from hydrocarbons on diamond substrates has been reported [46-481. However, hydrothermal synthesis of diamond from carbides has never been considered. It may open a new route to synthesising diamond at relatively low pressure. The formation of tetrahedrally-bonded carbon on the surface of SIC under hydrothermal conditions is not surprising in view of the facts that carbon can be synthesised from Sic at high pressures [49]. Moreover, natural occurrence of SIC coexisting with diamond and quartz was found [SO]. It indicates diamond-sic paragenesis. The phase composition of that system (cr-Sic, P-Sic, quartz and diamond) corresponds exactly to the phase composition of the reaction products in our system. Crystallisation of quartz instead of coesite indicates that the growth of these natural diamonds occurred at a maximum pressure of ~2000 MPa at -=z1000 “C. Thus, we probably came close to a natural process, but with less favourable for diamond growth parameters, as the yield and size of our crystals are very small. From the experience of CVD synthesis we know that the growth of diamond is dependent on preventing or destroying graphite nuclei. Following [45], we compared the Bachmann diagram [Sl] with the Rumble diagram [ 201 and added the results of our thermodynamic analysis [2] (Fig. 10). Although there can be not a continuous connection from the low to high pressure regimes, under certain conditions of hydrothermal corrosion of SIC we can have equilibrium H,/CO/CH4 ratios [2] corresponding to the region of diamond formation. We approach these regimes at >600 “C and intermediate H,O/SiC ratios. At low (e.g. 2/l) water/carbide ratio we approach the sufficient H&H, ratio only above 800 “C. At very high ratios (e.g. 10/l) we shall not get any carbon deposition. On the basis of this analysis, we see possibility to get stable diamond growth on the surface of SIC under hydrothermal conditions. Hydrothermal methods are widely used for producing large single crystals of various materials. If one understands how to control the process of hydrothermal diamond synthesis, one may be able to synthesise large crystals that cannot be obtained by CVD or highpressure synthesis. Finally, we wish to note that seeding with diamond crystals led to an increasing yield and size of hydrothermal diamonds. These results will be reported in details elsewhere [52]. However, in that case different mechanisms of diamond formation, including dissolutionprecipitation processes, may be involved.
Related Materials
5 11996) 151-162
161
0 H2O
Fig. 10. Comparison of C-H-0 ranges for CVD diamond formation from the gas phase in the presence of atomic hydrogen at 800 “C and < 1 atm [ 51 J and the equilibrium compositions of liquids and graphite at 400 MPa and 600 “C [20]. The black triangle corresponds to the suggested region of diamond growth under hydrothermal conditions.
5. Conclusions The results of this study have demonstrated that the structure of carbon formed after hydrothermal treatment of P-Sic varies depending on the experimental conditions. Electron diffraction and XRD gave a pattern consistent with the idea that carbon was present as graphite, diamond and amorphous phase. Reflections at 2.06 A that can be assigned to diamond have been found for several samples. IR spectroscopy confirms the presence of H-bonded and free sp3 carbon. Raman spectroscopy suggests the presence of disordered diamond and other non-graphitic carbon phases. Some crystals were big enough to be observed in SEM. While yields of diamond in our experiments were always very small, amorphous carbon and graphite were formed concurrently in larger quantities. By varying experimental conditions it ought to be possible to grow diamond with larger crystal sizes. For considering this method for diamond synthesis, it is necessary to identify exactly the conditions under which diamond will be nucleated and the formation of graphite and/or other non-diamond phases will be inhibited. We believe that the present results offer exiting prospects for future synthesis of diamond under hydrothermal conditions.
Acknowledgements
We appreciate the assistance of Ms. A. Horn of the University of Oslo with FTIR analysis, Professor
162
Y. G. Gogotsi et al./Diamond and Related Materials 5 (1996) 151-162
P. Klaeboe of the University of Oslo and Dr. T. MerleMejean of the University of Limoges with Raman spectroscopy and Dr. G.E. Khomenko of the Institute for Materials Science, Kiev with TEM. This research was supported in part by the NATO Linkage Grant 940706. Y.G.G. was supported by Alexander von Humboldt and NATO/NFR Fellowships.
~231A.G. Fitzgerald
and A.E. Henderson,
Materials Sci. Forum,
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c51 Y.G. Gogotsi, M. Yoshimura, M. Kakihana, Y. Kanno and M. Shibuya, in G.L. Messing et al. (eds.) Ceramic Processing Science and Technology, Ceramic Transactions, Vol. 51, Am. Ceram. Sot., 1995, pp. 243-241. C61Y.G. Gogotsi, Method for Producing Carbon Coatings on Silicon Carbide, Ukrainian Patent, Application 94041927 of 13.04.1994, Issued 16.02.1995. c71 Y.G. Gogotsi, M. Kakihana, Y. Kanno and M. Yoshimura, Formation of Diamond-Like Carbon upon Hydrothermal Leaching of Sic, Unpublished letter, 1994. CSI Y.G. Gogotsi, in R. Telle and P. Quirmbach (eds), Korrosion und VerschleiJ uon keramischen Werkstoffen, DKG, 1994, pp. 114-122. c91 Y.G. Gogotsi and N.S. Jacobson, in Extended Abstracts, Fall Meeting Electrochem.
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Diamond and Related Materials 5 (1996) 151-162
Formation of sp3-bonded carbon upon hydrothermal treatment of SIC Yury G. Gogotsi a**,Per Kofstad a, Masahiro Yoshimura b, Klaus G. Nickel ’ a University of Oslo, Centrefor Materials Research, GaustadallCen 21, N-0371 Oslo, Norway b Research Laboratory of Engineering Materials, Tokyo Institute of Technology, Yokohama 227, Japan ’ Eberhard-Karls-Universitiit Ttibingen, Institutfiir Mineralogie, Petrologie und Geochemie, Wilhelmstr. 56, D-72074 Tiibingen, Germany Received 20 July 1995; accepted in final form 21 September
1995
Abstract The composition and structure of carbon produced by selective leaching of silicon carbide under hydrothermal conditions in the temperature range 300-800 “C and at pressures < 500 MPa were examined by Raman and IR spectroscopy, XRD, SEM, TEM and EDS. The results of this study have demonstrated that various carbon allotropes, including diamond, are formed during hydrothermal treatment of SIC. Their structure varies depending on the experimental conditions and SIC precursor. The formation of sp2 vs. sp3 carbon is discussed. Keywords: Carbon;
Diamond;
Hydrothermal
synthesis;
Silicon carbide -
1. Introduction Hydrothermal
of various in the calculations 2)
Thermodynamic of the + 2Hz0
SiO,
CH4
SiOz
CO,
SIC
4Hz0+
SIC
3Hz0-+SiOz
SIC
2H,O-+SiO,
of silicon [ 11.
(1)
4H,
(2)
CO + 3H,
(3)
2H,
(4)
Finding of of SIC
in this
a very limited information about its structure has been published. The general experimental facts listed below have evolved from our previous studies on ( 1) Carbon
formation
depends on the
of
SIC * Corresponding author. Present address: Ttibingen, Institut ftir Mineralogie, Petrologie Wilhelmstr. 56, D-72074 Tiibingen, Germany.
und Geochemie,
0925-9635/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved SSDI 0925-9635(95)00341-X
whiskers or CVD SIC treated under the same conditions produce different amounts of carbon and these carbon layers give slightly different Raman spectra [ 51. Absorptions due to sp3 CH, are always present on IR spectra of hydrothermal carbon. sp2 CH, is absent under most of the experimental conditions [4]. (3) Oxygen and silicon can be present in hydrothermal carbon obtained from SIC in amounts up to a few atomic percent [4]. (4) Graphitic and amorphous carbon have been found by several methods of analysis [ 2-41. (5) Raman bands can shift from the position typical for graphite to the region of diamond phonon [ 71. (6) Crystalline carbon particles give typical of diamond Auger spectra [ 81. Thus, our previous studies have demonstrated the possibility of the synthesis of graphitic carbon of various degrees of ordering on the surface of Sic. At the same time we noticed the presence of sp3 bonded carbon in the reaction products and described the mechanism of its formation [S-lo]. We have continued the studies with the aim of obtaining further information regarding the formation of carbon during hydrothermal corrosion of Sic. Here
152
Y.G. Gogotsi et aLlDiamond and Related Materials 5 (1996) 151-162
we present the results of a more-detailed study of the composition and structure of hydrothermal carbon along with the analysis of the previously reported data.
2. Experimental details A laboratory scale P-Sic powder’ with a specific surface of 8 m2 g-’ (dsO= 1 pm), Cc-Sic platelets2 (dso= 13.5 pm), single crystals and polycrystalline B-Sic samples3 in the form of - 5 mm thick coupons produced by chemical vapour deposition (CVD) were used for this study. The hydrothermal corrosion of some of these materials was briefly discussed in [ 5,8]. Samples and distilled water were placed into an 30-50 mm long and 3-5 mm in diameter Au capsule. The capsules were closed with Au plugs to prevent loss of material and then heated in a tube-type, cold-seal pressure vessel [ 111. Powders after hydrothermal treatments were leached in a 20% HF solution (to remove Si02) and in hot HC104 (to remove non-diamond carbon). Our previous investigations demonstrated that neither of the analytical methods alone could supply sufficient information about the composition and structure of the reaction products. Some of the existing problems are highlighted in [ 121. Taking into account the variety of existing forms of carbon and severe difficulties with analysis of Sic-Si02-C mixtures, we used a combination of various complementary structural and spectroscopic methods of analysis that allowed us to detect both crystalline and amorphous phases, as well as to study structural ordering of the reaction products. The composition and structure of reaction products were examined using vibrational spectroscopy (Raman and infrared), X-ray and electron diffraction, as well as scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Energy dispersive spectroscopy (EDS) was used to identify the composition of the powders. In case of XRD analysis, monochromatic Cu Kxradiation was used. A Guinier camera was employed for X-ray phase analysis of small amounts of powders. Samples for Fourier-transform infrared (FTIR) spectroscopy were prepared by mixing the powders with KBr and pressing to form a pellet. The volume-filling factor of powder was about 0.005. IR-absorption spectra were measured in the frequency range from 400 to 4000 cm-’ using Bruker spectrometers at the University of Oslo. The Raman analyses were conducted using several
Dilor and Jobin Yvon micro- and macro-Raman spectrometers at the University of Oslo, University of Limoges and Tokyo Institute of Technology. In many cases, the same samples were analysed in several laboratories to obtain reliable and reproducible data. An argon laser at a wavelength of 514.5 nm was used for Raman analysis. The samples were placed on a Si plate for micro-Raman measurements. SEM studies were done using Philips XL30 microscope equipped with an EDS spectrometer particularly sensitive to light elements. After hydrothermal treatments under different conditions several samples were chosen for TEM analysis. The powders were observed in JEM 200CX electron microscope operating at 200 kV. The particle types were identified by EDS and diffraction patterns. TEM samples were prepared by mixing powders with ethanol in an ultrasonic bath, followed by filtration of the particles which were collected on a supporting copper grid covered with a perforated amorphous carbon film. The structure of silica and changes in the structure of SIC after hydrothermal corrosion have also been thoroughly studied. These data will be published elsewhere [ 121. Here we discuss these results only as far as they concern the analysis of carbon.
3. Results 3.1. XRD studies The phase compositions of the reaction products after corrosion in the temperature range 600-800 “C were almost independent on the carbide precursor and pressure. a-cristobalite, a-quartz, traces of tridymite, amorphous silica and carbon were found, but the relative amounts of these depended on the experimental conditions. The identification of carbon by XRD was complicated by the presence of silicon oxides and Sic, which dominated the XRD patterns, and low amplitude of X-ray lines of carbon in comparison with that of silicon compounds. After dissolution of silica in HF, carbon lines are more pronounced. However, after hightemperature (> 700 “C), long-term (> 5 h) experiments the amount of remaining material was often not sufficient for XRD measurements. Nonetheless, we could observe some of the graphite peaks and a very weak peak near 2.06 w which gave a first indication for the presence of diamond. 3.2. IR spectroscopy
1 Synthesized by Dr. E.V. Prilutsky, Institute for Problems Science, Ukrainian Academy of Sciences, Kiev, Ukraine. * C-Axis Technology Ltd., Canada. 3 Raytheon/BOMAS Machine Specialties, USA.
of Materials
The absorptions corresponding to hydrogenated sp3 carbon at about 2850,292O and 2960 cm-’ were present in all cases (Fig. la) and their intensity increased with
Y. G. Gogotsi et aLlDiamond and Related Materials 5 (1996) 151-162
#
I 3150
1
3050
2950
Wavenumber
2000
1800
I
J
2850
2750
(cm-‘)
1600
Wavenum
153
1400
1200
1000
800
ber (cm-‘)
Fig. 1. FTIR spectra of CH, (a) and carbon (b) regions recorded after hydrothermal treatment of Sic platelets for 18 h at 800 “C (HF leached). The expected positions of sp2-CH, groups (a) and sp’-C-C (b) are marked with arrows.
temperature. Hydrogen bonds preferentially to sp3 sites. Weak absorptions due to sp2 C-H groups were only rarely found. An absorption at 3300 cm-’ that can be assigned to sp’ CH was also observed after hydrothermal treatment of /?-Sic powder [12]. Since the silica band at - 1100 cm-l covers the region from 950 to 1500cm-’ in the hydrothermally treated samples, it was difficult to analyse other bands that could be present in that range. Leaching in HF allowed us to remove the silica bands or drastically decrease their intensity (Fig. lb) and revealed the bonds of sp3 carbon. Absorption at - 1340 cm- ‘, which can be
assigned to sp3-carbon, was not affected by any acid treatments. This is a Raman frequency, rendered IR-active by defects and impurities. It is accompanied by a strong absorption at 1267 cm-’ that can be assigned to SiCH3 bending (observed at 1273 cm-’ [ 131) or to diamond polytypes (predicted at 1280 cm-’ in 15R and at 1257 cm- ’ in 21R [ 141). The latter assumption seems to fit better, because the absorption does not disappear after a treatment in hot HClO,. Absorptions due to CH, groups disappeared after such treatment. This couple of absorptions is typical for hard carbons [ 151. The position of the IR band of sp3 carbon was in the
Y.G. Gogotsi et al./Diamond and Related Materials 5 (1996) 151-162
154
range 1310-1340 cm-’ depending on the carbide precursor and experimental conditions, but it never became as sharp as that in natural or high-quality CVD diamond. 3.3. Raman spectroscopy Raman spectroscopy results are in agreement with XRD and FTIR data and show the presence of Sic, silica and carbon after hydrothermal treatments. In the spectrum of the hydrothermally treated samples, two strong and broad bands appear at - 1590 and - 1350 cm- ’ (Fig. 2). The former one is usually referred to as G mode and assigned to C=C stretching vibrations EZgof graphite [ 16,171. The latter one is usually referred to as D mode of graphite, and known to become active
G
in small graphite crystallites. These bands were present in almost all spectra and confirm the formation of graphitic carbon during hydrothermal corrosion of Sic. Depending on the treatment regimes and Sic precursor, the position of G band of graphite can vary from 1580 to 1610 cm-’ (Fig. 3a). The observed shift of the G band towards the high-frequency edge or its splitting can be caused by a second first order zone boundary phonon at 1620 cm-’ [ 171. The band at frequency 1620 cm-‘, which also corresponds to the maximum density of states of graphite, is sometimes referred to as D’ mode. Thus, our Raman spectra apparently demonstrate sp2bonded carbon in the nanocrystalline or amorphous form. Though some of the spectra of hydrothermal carbon (curve 4 in Fig. 2) are similar to that of disordered graphitic carbon (curve 5 in Fig. 2) it is important to note the down-shift of the D band in comparison with 1350-1355 cm-’ or its splitting to two or more bands (curves 1 and 2 in Fig. 2). The lowest wavenumbers ( 1327- 1330 cm- ‘) were registered after treatments at - 700 “C, but the minimum on the temperature dependencies (Fig. 3b) shifts to lower temperatures (below 500 “C for Tyranno fibres) with increasing time or pressure. In addition to the bands discussed above, we registered several other bands in the range lOOO-1500cm-‘. regions features the Additional appear in 1100-1200 cm-’ (Fig. 4) 1285-1300 cm-’ (Fig. 2) and 1420-1450 cm-’ (Fig. 5). These features can appear as shoulders of the D-peak (Figs. 2 and 5) or well-resolved bands (Fig. 6). The origin of these bands will be discussed later. Bands in the range of diamond phonon at 1330-1340 cm-’ that were not accompanied by G bands have been also observed (Fig. 5). 3.4. Microscopic
observations
SEM + EDS investigations of powders after hydrothermal treatments show the presence of silica and pSIC in all specimens. It was difficult to identify the location of free carbon in SEM pictures of as-treated Sic powders. Carbon particles and films can be better seen on the surface of Sic single crystals (Fig. 7). However, it is not clear whether these particles grow due to direct transformation of Sic to carbon, or formed from the hydrothermal fluid by reaction [20] , 1300
1400 Wavenumber
1500
1600
1700
(cm-l)
Fig. 2. Representative Raman spectra of /?-Sic powder whiskers (2), SIC Tyranno@ fibres (3) and a-Sic single after hydrothermal treatments in comparison with a pyrolytic carbon (5). 1, 5 h at 800 “C under 500 MPa; 700 “C under 100 MPa; 3, 8 h at 700°C under 100 MPa; 750 “C under 10 MPa.
(l), /&Sic crystals (4) commercial 2, 24 h at 4, 15 h at
CO, + CH, -+2C + 2H20
(5)
Etching of hydrothermally treated /?-Sic powders in HF led to the formation of very thin free-standing carbon films that could be easily seen. The selected area diffraction (SAD) patterns from these powders show reflexes corresponding to both graphite and diamond [ 121. However, because of a very small particle size of diamonds (< 50 nm) and numerous graphite and Sic
155
Y.G. Gogotsi et al./Diamond and Related Materials .5 (1996) 151-162
Raman shift (cm-l) 1620
1570
7
200
0
600
400
600
Temperature (“C) Raman shift (cm-l) 1360 [
1350
1340
1330 I 600
I 400
1320 ’ 0
I 800
Temperature (“C) Fig. 3. Dependencies of the positions of G (a) and D (b) Raman bands n , Tyranno fibres treated for 24 h; +, Tyranno fibres treated for 8 h;
on the temperature of hydrothermal treatment under P-Sic powder; 0, /?-Sic whiskers treated for 24 h.
x,
1400
1200 Wavenumber
Fig. 4. Micro-Raman
spectrum
from a single SIC platelet
1600
(Cm- ‘)
treated
for 18 h at 800 “C under
100 MPa pressure.
100 MPa
pressure:
Y. G. Gogotsi et al./Diamond and Related Materials 5 (1996) 151-162
156
1 1200
r
1300 Wavenumber
1500
1400
1600
(cm-‘)
Fig. 5. Two typical Raman spectra from particles on the surface of CVD Sic after corrosion for 20 h at 750 “C under 92 MPa water pressure (2,3) in comparison with the spectra of CVD diamond on a Si substrate [ 181 (4) and nanocrystalline diamond powder [ 191 (1).
particles surrounding them always, we could not obtain single crystal SAD patterns from diamond on these samples. Thin carbon films disappeared after etching in boiling HC104. Graphite was never observed in the powder subjected to wet oxidation treatment. At the same time, we found small (< 0.3 pm) particles on the surface of several larger Sic particles (Fig. 8a). EDS of the crystal covered with particles in Fig. 8a did not demonstrate the presence of any other elements except of Si and
Fig. 6. Raman
spectrum
from the surface
carbon. However, quantitative EDS analysis of the single particles on the surface of Sic was impossible due to their small size. From their morphology, we can suggest that they have rather diamond than graphitic structure. Later, we observed similar particles with TEM (Fig. 8b). At least some of these particles have reflections with d = 2.06 A. Crystalline morphology was generally well developed for crystallites larger than 20 nm. Hexagonal (0001) faces predominated, but face-centered cubic (111) and (100) were also observed (Figs. 7 and 8). The presence of d = 1.92 A reflections in SAD patterns and hexagonal shape of some diamond particles (Fig. 8b) suggest the presence of hexagonal diamond (lonsdaleite or diamond polytypes). Other reflections of lonsdaleite are very close to that of /?- and/or a-Sic. The hexagonally shaped crystals can also arise from the growth of diamond nuclei with two (or more) parallel twin planes. Thus, the exact structure of tetrahedral carbon is still not quite clear. The particle shape and grain size are very similar to that of gas phase diamond powders [ 19, 21). These powders, like hydrothermal carbon produced in our studies, show only a very broad feature at about 1332 cm-’ (curve 1 in Fig. 5) instead of a narrow diamond band.
4. Discussion of reaction products 4.1. Linear and graphitic carbon
It is known [22] that chemical methods of carbyne synthesis are based on decomposition of carboncontaining materials with the formation of linear carbon chains. Carbyne was reported to appear in a C:H films
1400
1500
Wavenumber
(cm-‘)
of an a-SiC single crystal
treated
for 25 h at 750 “C under
100 MPa pressure
Y. G. Gogotsi et al./Diamond and Related Materials 5 (1996) 151-162
157
Si
(a)
(b)
Si
2 Energy
1
2
(keV)
100 nm Fig. 8. SEM image (a) and bright field TEM image (b) of well-faceted crystallites in the P-Sic powder after hydrothermal treatment for 5 h at 740 “C under 100 MPa pressure and etching in HF and HCIO,.
bonding in hydrothermal carbon. Although several reflections on electron diffraction patterns (e.g. d = 2.22-2.24 A) can be assigned to carbyne, we could not find a reflection at 4.47 A. Thus, the presence of carbyne has not been finally confirmed. A more intensive study on TEM observations is necessary to prove or finally exclude the presence of carbyne. Comparisons of the results of various analyses show the formation of graphite and disordered graphitic carbon under hydrothermal conditions. Thus, the fact of the formation of this carbon allotrope has been determined and does not need much further discussion. Fig. 7. EDS spectra of the flat surface (a) of a SIC crystal after hydrothermal treatment for 25 h at 750 “C under 100 MPa and a carbon particle on its surface (b), and SEM images of typical particles (c,d). The intensity ratios correspond to ~50 at.‘+‘& (a) and > 96 at.% C (b).
[23]. Therefore, we could expect its formation upon preferential leaching of SIC under hydrothermal conditions. FTIR spectra may give evidence of some sp’
4.2. Tetrahedral carbon A down-shift of the D band was observed after hydrothermal treatments at 450-750 “C. It can indicate the presence of sp3 carbon bonds as well as disorder. in /?-Sic powder The D band shifts from -1350cm-l treated at 600 “C to below 1340 cm-’ after treatment for 8 h at 730 “C (Fig. 3b). An even stronger downward
Y.G. Gogotsi et al./Diamond and Related Materials 5 (1996) 151-162
158
shift to -1330 cm-’ was observed for SIC fibres and whiskers treated at 700 “C. This position of the peak is characteristic for diamond. A single narrow peak is observed at - 1332 cm-’ in diamond. This band may be broadened by defects, and/or small crystalline size, and shifted by as much as 8 cm-’ (either higher or lower) depending upon preparation conditions and substrate [24]. Data plotted in Fig. 3b, along with TEM and electron diffraction results suggest that with increasing temperature of hydrothermal treatment carbon may change its form from amorphous material to a mixture of nanocrystalline graphite and disordered ultra-fine diamond. This can be explained by the increasing etching rate of graphitic carbon by H,O and Hz with temperature. An increase in the frequency of D band at 800 “C is due to enhanced graphitisation of carbon. Increasing duration of the treatment at that temperature leads to a further shift of the D band to the high-frequency edge (Fig. 3b). According to our experiments, synthetic highpressure diamonds4 are stable and do not interact with water below 750 “C. A similar crystallisation of carbon leading to the formation of both graphite and diamond was observed upon laser heating of a-C:H films [ 251. We must note that the assignment of the D band at - 1330 cm-’ to sp3-bonded carbon is not universally accepted, and it has been argued that this peak can arise from a highly disordered and strained graphitic phase [26]. However, the G band does not shift downward along with the D band (Fig. 3), as would be expected in the case of stress-induced shift. Also, extremely high stresses are necessary for such shift of a Raman band. The analysis of the literature (e.g. Ref. [ 171) and our experiments with various carbons available to us show that the D band of graphitic materials is always above 1343 cm-’ (at 514.5 nm laser wavelength). At the same time, D bands were observed at 1338 cm-’ for a-C:H [25], at 1336 cm-’ for DLC obtained from a polymer precursor [27], at 1332 cm-’ for nanosize multiplephase diamond powder (Fig. 5) and nanocrystalline CVD diamond films [28] and at 1320 cm-’ for Sic-C from polymer precursors [ 133. These were attributed both to the presence of sp3 carbon and to bond-angle bending disorder. However, the assignment of the broad (FWHM =70 cm-l) band at 1336 cm-’ accompanied by a G band at 1580 cm-’ to diamond bonding [27] caused much criticism in the scientific community (e.g. Ref. [26]). Indeed the explanations of the shift and/or broadening of the diamond line by the overlap of D band of graphite [27] and many polytypes of diamond [ 193 may seem to be speculative until sufficient proofs are presented. However, a computer-assisted analysis of the bands centred at 1330 cm-’ (curves l-3 in Fig. 2) shows that, unlike the D band of pyrolytic carbon (curve 5 in Fig. 2), those can be described by neither a Lorentzian 4 General
Electric,
USA.
nor a Gaussian shape. Contributions of at least three bands centred at -1295 cm-’ (its origin will be discussed later), -1330cm-’ (diamond band) and - 1360 cm- ’ (typical position for a D band of graphitic carbon) are necessary to get the best agreement between the measured and computer simulated spectra. Using micro-focus technique, we observed occurrence of the discussed band at 1336-1339 cm-‘, and it was not accompanied by a G band (curves 2 and 3 in Fig. 5). Therefore, it cannot be related to graphitic carbon, which always shows a band at N 1580 cm-‘. The micron-size regions of the surface giving the spectra shown in Fig. 5 were bright and transparent in optical microscope, but non-distinguishable from the surrounding material in SEM. Thus, these were nanocrystalline particles. The peaks at 1336-1340 cm-’ observed on Raman spectra of CVD Sic (Fig. 5) may give an evidence of the formation of diamond under hydrothermal conditions. The shift of the peak from 1332 cm-’ can be due to stresses when diamond grows on the SIC substrate (stress of +2.1 GPa is necessary to shift the band to 1337 cm-’ [ 171). It is only slightly broader than that of a median-quality CVD diamond on Si and even more narrow compared with that of nanocrystalline diamond powder (Fig. 5). For other samples (Fig. 6), we observed three single bands in this range positioned at 1303, 1336 and 1378 cm-l, overlapping of which will produce broad bands with shoulders shown in Figs. 2 and 5. The above results allow us to explain the observed (Fig. 3b) downward shift of D band by its overlapping with the bands of cubic diamond and diamond polytypes. Broad bands in the frequency range 1100-1200 cm-’ (Fig. 4) can be assigned to amorphous diamond. In the amorphous state the Raman spectrum must reflect the vibrational density of states of material. The maximum of the density of states is expected at 1140 cm-’ for cubic diamond (TO mode) and at 1175 cm-’ for hexagonal diamond [ 291. A broad band centred at - 1170-l 180 cm-’ was reported for spherulitic diamond [ 301 and bands at 1130-l 150 were observed in various nanocrystalline or amorphous diamond hlms [ 24,291. Diamond films with crystallites < 0.1 urn exhibit Raman bands in the range 1loo-1260 cm-’ at laser wavelength 1064 nm [31]. Thus, the spectrum in Fig. 4 can be assigned to amorphous diamond. A very strong and broad band of sp3 carbon on FTIR spectrum of the same sample (Fig. lb) supports this assumption. After identification of all phases of SiOZ, Sic, graphitic carbon and cubic diamond, and excluding carbynes from the discussion, we still have several bands on Raman spectra in the range 1250-1520cm-’ that cannot be assigned to any of the above compounds. We have done a literature search looking for similar features on the spectra of related materials: various forms of carbon such as a-C, a-C:H, DLC, CVD diamond, a-SiC:H,
Y. G. Gogotsi et al.lDiamond and Related Materials 5 (1996) 151-162
a -Si:H, etc. These features cannot be accounted for by any of the known SiO,, Sic or SiH, structures, though more complex phases in the Si-C-H-O system cannot be completely excluded from the discussion. On the other hand, the features with similar wavenumbers have been observed in various hard carbons and diamond materials obtained under non-equilibrium conditions: (1) A peak at - 1300 cm-’ was observed in a diamond crystal obtained by CVD in an oxygen-containing environment [32], in DLC films with the estimated content of sp3 bonding of 76% obtained by laser ablation [ 331 and in a-C:H films [34]. In the latter case, it was accompanied by a band at 1250 cm- ‘, which is close to the band at 1240 cm-’ registered for hydrothermally treated Sic powder (curve 1 in Fig. 2). The presence of the band at 1300 cm-’ did not depend on the substrate (MgO or Si [ 331) and thus, this band cannot be due to Si-containing compounds. Moreover, the calculated positions of Raman bands of 15R (1280 and 1295 cm-‘) and 21 R (1306 and 13 10 cm- ‘) diamond polytypes are in good agreement with the bands that were observed for hydrothermally treated P-Sic powder (1285 and 1294 cm-‘) and single crystals (1303 cm-‘). (2) The Raman bands at around 1420-1440 cm-’ [30] and a luminescence band at 1445 cm-l [35] were observed in the spectra of diamond films excited at 514.5 nm line. The latter band was assigned to the presence of twins in diamond. (3) A peak at - 1520 cm- ’ was observed in various diamond and DLC films and attributed to “bridged graphite” [ 161. On the basis of these points we can conclude that these peaks are not unusual for CVD diamond and related materials and, at least some of them, can be attributed to diamond polytypes and/or disordered diamond containing Si or 0. The latter statement is supported by the presence of forbidden diamond reflections in SAD patterns [ 121. The appearance of forbidden reflections was attributed to the formation of a diamond superstructure formed by the ordering of inclusions incorporated in the crystallites during growth [ 361. This suggests a growth of the layers with Si, 0 or other defects. Some of the observed Raman bands are close to those of fullerenes. For example, in the frequency range of interest C,,, has Raman bands at about 1248,1318,1426, 1573 and 1632 cm-’ and the strongest one at 1469 cm-’ [37]. However, we cannot explain how fullerens can be formed under conditions of our experiments and fullerens were found neither by XRD nor by electron diffraction studies. Using the above data and our results for other Sic materials, we can summarise all facts supporting the presence of amorphous or crystalline tetrahedral carbon on the surface of SIC hydrothermally treated below 500 MPa by the following. (1) IR band at 1310-1340 cm-‘.
159
(2) Shift of the D-band on the Raman spectra to < 1340 cm-’ or its splitting to two bands positioned or its occurrence at at - 1330 and - 1350cm-‘, 1336-1340cm-’ without being accompanied by a G-band. (3) Broad Raman bands at 1100-1200 cm-’ that can be assigned to amorphous diamond. (4) Several Raman bands with frequencies typical for diamond polytypes. (5) Diamond reflections on the electron diffraction patterns. (6) Well-shaped non-graphitic carbon particles (analysis of such particles on the surface of CVD Sic produced typical for diamond Auger spectra [lo]). (7) XRD peak at d = 2.06 A. Thus we have enough supporting data to confirm the formation of tetrahedral forms of carbon upon hydrothermal leaching of SIC. However, since we did not observe a narrow Raman peak at 1332 cm-’ and did not obtain enough material for high quality XRD studies on carbon, we cannot claim the formation of wellordered cubic diamond. Formation of amorphous diamond, disordered diamond-like carbon and tetrahedral carbons with different stacking sequences seems to be highly probable. The presence of additional Raman bands at - 1150, 1245, 1285, 1300, 1440 and 1520 cm-’ supports the hypothesis of the formation of carbons with the atomic structure different from that of either graphite or cubic diamond. The absence of a sharp Raman band at 1332 cm-’ can be explained by very small size and content of diamond crystallites [ 3 11. Our TEM measurements show that the size of crystallites, arising in the bulk of carbon films, is usually < 50 nm. At such small particle sizes the wavevector conservation rule for Raman spectroscopy of single crystals of diamond breaks down [ 313. The phonon-confinement effect should play a substantial role leading to the shift and to the strong broadening of a fundamental diamond mode [38]. It is quite possible that, keeping its integral intensity constant, this line would have a too low amplitude to be extracted from the graphite background even in the case of comparable contents of diamond and graphite phases in the sample. Particularly, it has been reported that the Raman cross-section of graphite is much higher than that of crystalline diamond [ 391 and hence sp2-bonded regions dominate the Raman spectra. Our TEM observation showed only a small percentage of diamond in the hydrothermal carbon and we could not expect to see a sharp diamond peak at 1332 cm-’ in our spectra. The Raman response of hexagonal diamond is smaller than that of cubic diamond [ 141. Even 30-40 urn thick hexagonal diamond films exhibit only a very weak and broad peak at 1333 cm-‘, accompanied by D band at 1360 cm-’ [40]. An additional factor is the expected presence of silicon
Y. G. Gogotsi et al./Diamond and Related Materials 5 (1996) 151-162
160
in hydrothermal carbon. According to [41], the addition of 2 at.% Si to CVD diamond substantially decreases the intensity and increases width of the band at 1332 cm-‘. It also decreases the intensity of X-ray peaks of diamond [ 351. What are the critical features that allow the formation of sp3-bonded carbon, and not simple amorphous or graphitic carbon network? We suggest the following. The presence of an appropriate substrate. SIC is widely used as a substrate for diamond deposition. The formation of P-Sic buffer layer, with a partial matching between the diamond lattice (Fd3m group) and the pSic lattice (F43m group), has been suggested to be necessary for diamond nucleation on the surface of silicon [24]. This means that cubic /?-Sic due to its structure can serve as a good substrate for hydrothermal synthesis of diamond. The formation of hydrogen during hydrothermal treatment ofSiC. It was found that atomic hydrogen can etch
graphite with a much higher rate than diamond [42]. The atomic hydrogen is known to be important in diamond growth and also plays an important role in its nucleation. Tetrahedral bonding of carbon in Sic. We believe that carbon can maintain the cubic structure of the carbide precursor due to stabilisation of dangling bonds by hydrogen, if silica is dissolved in the hydrothermal fluid. Transformation of tetrahedral carbon in SIC to diamond may be energetically more advantageous than a graphite to diamond transformation. Preferential
oxidation
of sp2-bonded
carbon
by water.
It was shown that water addition to CH4-H, precursor gas reduces the substrate temperature necessary for CVD diamond formation and increases its growth rate [43]. Reaction (4) can lead to the formation of diamond nuclei on the surface of Sic, provided certain conditions are established and maintained during the process. The mechanism of the formation of carbon nuclei is shown schematically in Fig. 9. The fact that diamond particles grow preferentially on the surface of SIC (Figs. 7 and 8) is in agreement with the hypothesis, that diamond nucleates due to reaction (4). However, for further growth, transport through the hydrothermal fluid is necessary. The presence of carbon deposits not only on the SIC surface, but also on other parts of the system (gold capsule, test tube) confirms the transport of carbon through the hydrothermal fluid. The substantial solubility of carbon in C-H-0-(Si) fluids and the possibility of graphite deposition from these fluids have been known for years [20]. It was proposed that the deposition of graphite from metastable fluid containing CO, and CH, occurs according to the reaction (5) taking place in an H,O-dominated fluid. Thus, carbon can be formed by deposition from a C-H-0-Si fluid formed due to reactions (l-4). This mechanism of carbon deposition can
H-O
H-O
H
S&-C--Si-C-Si-C
’ I
I
Si-C-S&C
I/II
I I
Si
3-j
I
C
I
Fig. 9. Schematic two-dimensional sketch of the reaction mechanism, showing that the process includes the following stages: (i) interaction of Sic surface with an H,O molecule; (ii) stabilisation of dangling bonds by hydrogen; (iii) release of hydrosilica and hydrogen with the formation of sp3 C-C bonds.
contribute to the growth of diamonds after the formation of nuclei due to differential etching of SIC by (4). Diamond is metastable with respect to graphite in the pressure/temperature range used in our experiments. Thus, thermodynamic considerations and more-detailed analyses of the reaction mechanisms are necessary to understand why diamond formation is possible. To show the mechanism of diamond formation, we can rewrite the reaction (4) as follows: SIC + 2H,0+Si02 .
+ C~si,+ 4H ’
4H + Ctsi, +Ccdiamond)+ 2H2
(6)
(7)
where C~si,is a carbon atom that has at least one bond with a Si atom. Assuming these reactions to be thermodynamically coupled, we can suggest that the chemical energy of reaction (4) is partly used to form diamond. For the further growth of a diamond nucleus by (5) thermodynamic and kinetic explanations that were given for low-pressure gas-phase synthesis of diamond can be used (see Ref. [24] for a review). We must also note that the chemistry in the hydrothermal medium may contrast with that associated with ambient or low-pressure conditions. This must be taken into consideration when explaining the mechanism of diamond formation and growth. The above. discussion shows that formation of DLC or even diamond upon hydrothermal leaching of Sic can be explained on the basis of the existing theories of low-pressure growth of diamond. An extremely small grain size of the hydrothermal diamonds can be again understood by comparison with the vapour phase synthesis. Since graphitic carbon is usually formed along with sp3-carbon, it retards the growth rate of diamond as observed upon CVD synthesis [24].
Y. G. Gogotsi et al.lDiamondand
Hydrothermal origin of some natural diamond crystals has been considered in the literature and very good reviews of the hydrothermal systems pertinent to the diamond synthesis have been published [44,45-j. Diamond formation under hydrothermal conditions was predicted at the lowest possible temperatures, but highest feasible pressures. Recently, hydrothermal synthesis of diamond from hydrocarbons on diamond substrates has been reported [46-481. However, hydrothermal synthesis of diamond from carbides has never been considered. It may open a new route to synthesising diamond at relatively low pressure. The formation of tetrahedrally-bonded carbon on the surface of SIC under hydrothermal conditions is not surprising in view of the facts that carbon can be synthesised from Sic at high pressures [49]. Moreover, natural occurrence of SIC coexisting with diamond and quartz was found [SO]. It indicates diamond-sic paragenesis. The phase composition of that system (cr-Sic, P-Sic, quartz and diamond) corresponds exactly to the phase composition of the reaction products in our system. Crystallisation of quartz instead of coesite indicates that the growth of these natural diamonds occurred at a maximum pressure of ~2000 MPa at -=z1000 “C. Thus, we probably came close to a natural process, but with less favourable for diamond growth parameters, as the yield and size of our crystals are very small. From the experience of CVD synthesis we know that the growth of diamond is dependent on preventing or destroying graphite nuclei. Following [45], we compared the Bachmann diagram [Sl] with the Rumble diagram [ 201 and added the results of our thermodynamic analysis [2] (Fig. 10). Although there can be not a continuous connection from the low to high pressure regimes, under certain conditions of hydrothermal corrosion of SIC we can have equilibrium H,/CO/CH4 ratios [2] corresponding to the region of diamond formation. We approach these regimes at >600 “C and intermediate H,O/SiC ratios. At low (e.g. 2/l) water/carbide ratio we approach the sufficient H&H, ratio only above 800 “C. At very high ratios (e.g. 10/l) we shall not get any carbon deposition. On the basis of this analysis, we see possibility to get stable diamond growth on the surface of SIC under hydrothermal conditions. Hydrothermal methods are widely used for producing large single crystals of various materials. If one understands how to control the process of hydrothermal diamond synthesis, one may be able to synthesise large crystals that cannot be obtained by CVD or highpressure synthesis. Finally, we wish to note that seeding with diamond crystals led to an increasing yield and size of hydrothermal diamonds. These results will be reported in details elsewhere [52]. However, in that case different mechanisms of diamond formation, including dissolutionprecipitation processes, may be involved.
Related Materials
5 11996) 151-162
161
0 H2O
Fig. 10. Comparison of C-H-0 ranges for CVD diamond formation from the gas phase in the presence of atomic hydrogen at 800 “C and < 1 atm [ 51 J and the equilibrium compositions of liquids and graphite at 400 MPa and 600 “C [20]. The black triangle corresponds to the suggested region of diamond growth under hydrothermal conditions.
5. Conclusions The results of this study have demonstrated that the structure of carbon formed after hydrothermal treatment of P-Sic varies depending on the experimental conditions. Electron diffraction and XRD gave a pattern consistent with the idea that carbon was present as graphite, diamond and amorphous phase. Reflections at 2.06 A that can be assigned to diamond have been found for several samples. IR spectroscopy confirms the presence of H-bonded and free sp3 carbon. Raman spectroscopy suggests the presence of disordered diamond and other non-graphitic carbon phases. Some crystals were big enough to be observed in SEM. While yields of diamond in our experiments were always very small, amorphous carbon and graphite were formed concurrently in larger quantities. By varying experimental conditions it ought to be possible to grow diamond with larger crystal sizes. For considering this method for diamond synthesis, it is necessary to identify exactly the conditions under which diamond will be nucleated and the formation of graphite and/or other non-diamond phases will be inhibited. We believe that the present results offer exiting prospects for future synthesis of diamond under hydrothermal conditions.
Acknowledgements
We appreciate the assistance of Ms. A. Horn of the University of Oslo with FTIR analysis, Professor
162
Y. G. Gogotsi et al./Diamond and Related Materials 5 (1996) 151-162
P. Klaeboe of the University of Oslo and Dr. T. MerleMejean of the University of Limoges with Raman spectroscopy and Dr. G.E. Khomenko of the Institute for Materials Science, Kiev with TEM. This research was supported in part by the NATO Linkage Grant 940706. Y.G.G. was supported by Alexander von Humboldt and NATO/NFR Fellowships.
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