- Email: [email protected]
Characterisation of the free-carbon phase in precursor-derived SiCN ceramics
Journal of Non-Crystalline Solids 293±295 (2001) 261±267
www.elsevier.com/locate/jnoncrysol
Characterisation of the free-carbon phase in precursor-derived SiCN ceramics Stephan Trassl a,1, G unter Motz a, Ernst R ossler b,*, G unter Ziegler a b
a Institute for Materials Research (IMA I), University of Bayreuth, 95440 Bayreuth, Germany Physikalisches Institut, Experimetalphysik II, Universitat of Bayreuth, 95440 Bayreuth, Germany
Abstract Dierent polymeric precursors with varying carbon contents were prepared by ammonolysis of functionalised chlorosilanes. Pyrolysis under inert atmospheres at 1000 °C led to amorphous Si±C±N±(H) ceramics. Further heat treatment caused the transformation into the thermodynamically stable crystalline phase assemblage. The structural changes, especially those of the excess carbon, were studied by characterising the solid intermediates via solid state magic angle spinning (MAS) nuclear magnetic resonance spectroscopy (NMR). In addition, Raman spectroscopy, electron spin resonance spectroscopy (ESR), microwave conductivity measurements and chemical analysis were employed. Combination of all these methods provides a comprehensive picture of the formation and of the behaviour of the free-carbon phase present in the polymer-derived ceramics. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction The pyrolysis of carbon-rich polysilazanes, polycarbosilanes, polysiloxanes as well as polysilylcarbodiimides under inert atmosphere yields at about 1000 °C an amorphous ceramic containing excess carbon [1±9]. When annealing at higher temperatures, the amorphous material partly or completely crystallises to the thermodynamically stable phases, in the case of carbon-rich polysilazanes, to SiC, Si3 N4 and free C. These excess carbon atoms are not bonded to silicon atoms, but they are bonded to adjacent carbon atoms by re-
* Corresponding author. Tel.: +49-921 552 606; fax: +49-921 552 621. E-mail addresses: [email protected] (S. Trassl), [email protected] (E. R ossler). 1 Tel.: +49-921 552608; fax: +49-921 552621.
arranging the four valence electrons into sp2 -hybridisations (trigonal bond). The purpose of this paper is to investigate the formation and the nature of the excess carbon in combination with the structural rearrangement during pyrolysis. We have examined selected selfsynthesised precursors with varying carbon contents and have combined the results obtained by 13 C-solid state magic angle spinning (MAS) nuclear magnetic resonance spectroscopy (NMR), electron spin resonance spectroscopy (ESR), Raman spectroscopy and electric conductivity measurements.
2. Experimental procedure The starting silazanes with dierent amounts of carbon were self-synthesised by the reaction of
0022-3093/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 0 6 7 8 - 0
262
S. Trassl et al. / Journal of Non-Crystalline Solids 293±295 (2001) 261±267
di- and trifunctionalised chlorosilanes with gaseous ammonia as reported elsewhere [10]. The preceramic oligomers named TVS, HVNG and ABSE and studied here are listed in Table 1. The TVS and HVNG precursors were crosslinked via hydrosilylation and polymerisation by using dicumylperoxide (DCP) as a radical initiator and subsequent thermal treatment at 300 °C for 5 h in N2 -atmosphere. For the following experiments powders of such crosslinked precursors, which were heated up to 1600 °C (DT 100 K, annealing time 5 h), were used. Chemical analysis of the pyrolysis residues was carried out by Pascher Microanalytical Lab. (Remagen, Germany) with an error of about 1 wt%. Formation and structural changes of the free-carbon phase which occur during annealing were investigated in particular by 13 C NMR spectroscopy. 13 C NMR spectra of the solid intermediates were obtained on a Bruker AVANCE DSX 400 spectrometer applying the MAS technique. For preparation temperatures up to 700 °C, a signal to noise enhancement using the cross polarisation (CP) method was possible due to the presence of a sucient amount of protons in these samples. However, when studying powders heated above 700 °C, the CP method became inecient since the proton concentration was too low. In this case, the 13 C spectra were recorded with a depth pulse excitation in order to avoid the 13 C signal from the sample holder [11]. In the case of cross polarisation no such signal from the sample holder is expected. In order to get insight about the microstructure and the modi®cation of the respective carbon phase, Raman spectra were acquired on a Bruker FRA 106 with a limit of resolution of 4 cm 1 . Potassium bromide (KBr) pellets of the samples were excited in backscattering geometry using a probe wavelength of 1064 nm. ESR investigations were carried out using a
Bruker ESP 300 Q-band CW spectrometer with the cavity tuned for a klystron frequency of approximately 34 GHz. We estimate the error of the ®tted values of the line width and the intensity at about 10%. For further information, microwave conductivity measurements were performed on powder samples ®lled in quartz tubes. The resonance frequency and the quality factor of the microwave cavity are modi®ed by the introduction of the sample into the cavity. In the quasi-static approximation, i.e., when the high frequency electromagnetic ®eld completely penetrates the sample, the frequency shift and the additional loss are related to the complex dielectric constant from which the conductivity can be derived [12]. The most critical point of the evalution of the microwave conductivity rac is the calculation of the ®lling factor. Therefore, we estimate the error of the values of rac at about 50%. All spectroscopic experiments were carried out at room temperature (RT). 3. Results 3.1.
13
C NMR spectroscopy
Changes in the structure of the solid intermediates during thermal treatment at temperatures exceeding 300 °C were investigated by 29 Si and 13 C solid state NMR spectroscopy and reported in our previous publications [13,14]. Fig. 1 shows these 13 C NMR spectra of the precursor HVNG after dierent pyrolysis temperatures. Heating above 600 °C causes, in addition to the peak of CH2 Si2 sites at about 12 ppm, an appearance of a new signal at 135 ppm which increases in intensity at 700 °C. Coincidentally, a high-®eld shift of the peaks in the 29 Si spectra [13,14] corresponds to an increase of the number of Si±N bonds in contrast
Table 1 Structural units and theoretical formula of the precursors TVS, HVNG and ABSE Precursor
Structural unit
Theoretical formula
TVS HVNG ABSE
ACH@CH2 Si NH1:5 ACH@CH2 Si NH1:5 ±H CH3 Si NH ACH3 NHSi±CH2 ±CH2 ±Si NHCH3
SiN1:5 C2 H4:5 SiN1:25 C1:5 H4:75 SiNC2 H6
S. Trassl et al. / Journal of Non-Crystalline Solids 293±295 (2001) 261±267
263
peak at 28 ppm in the 13 C spectrum and at )48 ppm in the 29 Si spectrum recorded on a HVNG sample heated at 1200 °C [13]. Longer annealing times (48 h) at 1500 °C cause the formation of the thermodynamically stable crystalline phases SiC and Si3 N4 . At 1600 °C, no Si3 N4 but only a SiC signal is detected by 29 Si NMR measurements [13]. 3.2. Raman spectroscopy In recent years, Raman spectroscopy has become one of the most sensitive methods for the characterisation of the dierent modi®cations of carbon [16±18]. In the spectra recorded on HVNG-derived samples pyrolysed up to 900 °C, no bands are observed in the interesting frequency region. Fig. 2 shows the Raman spectra after pyrolysis at temperatures above 900 °C. At 1200 °C, two signals in the 1200±1700 cm 1 region were observed. The intensity of these bands centred at
Fig. 1. 13 C NMR spectra of HVNG-derived powder samples heat treated at temperatures between 300 °C and 1500 °C for 5 and 48 h in N2 -atmosphere. The dashed lines indicate the C Si4 peak and the high-®eld shift of the Csp2 signal.
to the number of Si±C bonds. The signal at 135 ppm in the 13 C NMR spectra indicates a high amount of sp2 -carbon in the form of carbon clusters, the so called free carbon [2,4,15]. Pyrolysis at 1000 °C ®nally yields an amorphous ceramic composed of a free-carbon phase and a homogeneous Si±C±N±(H) phase [13,14]. The 13 C spectrum (see Fig. 1) is characterised by one major peak (width: 40 ppm) with a short spin lattice relaxation time at about 135 ppm corresponding to the sp2 -carbon domains (free carbon). Further heating (1000±1500 °C) yields a small high-®eld shift (135 to 125 ppm) and a narrowing of the peak (40 to 30 ppm) corresponding to the free sp2 -carbon in the 13 C NMR spectrum of the HVNG precursor. Above 1100 °C the formation of CSi4 and SiN4 -environments with a short range order (amorphous SiC and amorphous Si3 N4 ) also takes place, as can be seen in the appearance of a
Fig. 2. Raman spectra of pyrolytic residues of the precursor HVNG annealed at temperatures between 1000 °C and 1700 °C for 5 and 48 h in N2 -atmosphere. The dashed lines indicate the spectral position of the D and the G band.
264
S. Trassl et al. / Journal of Non-Crystalline Solids 293±295 (2001) 261±267
(a)
(b)
Fig. 3. (a) FWHH of the D and G peak as a function of temperature. The dashed lines indicate the FWHH of the D and G peak of glassy carbon (Sigradurâ G). (b) Intensity ratio ID =IG as function of temperature. The dashed line indicate the ID =IG ratio for glassy carbon (Sigradurâ G).
about 1280 and 1600 cm 1 increases with rising temperature and reaches a maximum at 1500 °C. These two Raman bands are the most striking features of disordered graphitic-like carbon and are assigned to the D and G band [16]. Both the line width of the G band (238 to 67 cm 1 ) as well as that of the D band (268 to 130 cm 1 ) reduces with increasing pyrolysis temperature (Fig. 3(a)), whereas the position and the intensity ratio of the two bands ID =IG stay about constant. At temperatures between 1200 and 1500 °C, the intensity ratio ID =IG slightly increases (Fig. 3(b)) and is at 1500 °C at about 2.85. At longer annealing times (1500 °C/48 h), the intensities of both the D and the G band decrease and disappear at 1600 °C (see Fig. 2). However, new Raman bands occur at about 830 and 920 cm 1 corresponding to b-SiC [3]. This indicates the formation of crystalline SiC by a free carbon consuming reaction.
(a)
3.3. ESR spectroscopy The appearance of a free-carbon phase in the ceramic matrix should consequently result in a pronounced ESR signal because of the appearance of unpaired electrons during its formation [19]. Hence, ESR may help in illuminating the role of the excess carbon with respect to the pyrolysis chemistry. Thus, ESR spectra were obtained from samples heated between 300 °C and 1600 °C (DT 100 K). The intensity and the line width of the ESR signals as a function of heat treatment are shown in Fig. 4(a). Already at 300 °C, unpaired electrons are present, leading to a broad ESR line (23 Gauss) with a g-factor of about 2.0023. The intensity of the line strongly increases at temperatures above 700 °C and reaches a maximum at 1200 °C. In contrast to the intensity, the line width decreases (23 to 1.8 Gauss) over the whole tem-
(b)
Fig. 4. (a) Line width and integral intensity, and (b) asymmetry factor of the ESR signals of HVNG-derived powder samples as a function of temperature (inset: ESR spectrum of the HVNG sample heated at 1200 °C).
S. Trassl et al. / Journal of Non-Crystalline Solids 293±295 (2001) 261±267
265
Fig. 5. (a) Microwave conductivity of HVNG-derived samples as a function of temperature. The solid lines indicate the microwave conductivity of glassy carbon (Sigradurâ G), crystalline SiC and crystalline Si3 N4 ; (b) Relation between free-carbon content and conductivity for the dierent precursor-derived materials.
perature range. At 1600 °C, the line width rises again to 5.8 G, which corresponds to the value of a SiC reference sample [14]. 3.4. Microwave conductivity We investigated samples prepared from HVNG-precursor and heated between 300 °C and 1600 °C (DT 100 K). The results are shown in Fig. 5(a). The obtained conductivity values were compared with those from glassy carbon, pure, crystalline SiC and pure, crystalline Si3 N4 . At 400 °C, the pyrolysis residue showed an insulator 1 like conductivity with about 2:6 10 6 X cm . Between 500 °C and 800 °C, the microwave conductivity of these samples increased by about two orders of magnitude. A second increase of the conductivity from about 2:1 10 4 to 1 2 1:0 10 X cm was observed between 1000 °C and 1500 °C. The comparison with samples prepared from ABSE- and TVS-precursor heated at 1500 °C and 1550 °C, respectively, is shown in Fig. 5(b). Both materials show a higher microwave conductivity as the HVNG-derived sample. 4. Discussion 13 C-NMR characterisation of the pyrolysis residues shows the formation of a free-carbon
phase in the temperature range between 500 °C and 800 °C. Additionally, we ®nd an increase of the microwave conductivity and an increase of the number of Si±N bonds in contrast to the number of Si±C bonds. Both can be explained with the segregation of aliphatic hydrocarbons, which reorganise in a ®rst step towards cyclic unsaturated hydrocarbons (the so called free carbon). The increase of the number of Si±N bonds in contrast to the reduced number of Si±C bonds during heat treatment between 300 °C and 1000 °C becomes also evident from the chemical analysis of the precursor-derived materials pyrolysed at 1000 °C, as given in Table 2. Based on the assumption that nitrogen atoms are only bonded to silicon atoms [N Si3=4 ] and C atoms are only present as C Si4 units the number of Si±N bonds and Si±C bonds as well as the amount of excess carbon can be calculated (see also Table 2). The free-carbon content in the dierent samples varies from 24.4 mol% in the HVNG-derived material to 35.6 mol% in the TVS-based ceramic. Pyrolysis at 1000 °C ®nally yields an amorphous ceramic composed of a homogeneous Si±C±N±(H) phase and a freecarbon phase. The short spin lattice relaxation time indicates that paramagnetic centres are coupled to carbon atoms in these free-carbon domains. This is due to the elimination of residual hydrogen, which is still bonded to the periphery of the free-carbon regions, in the temperature range between 800 °C and 1200 °C. For carbonaceous
266
S. Trassl et al. / Journal of Non-Crystalline Solids 293±295 (2001) 261±267
Table 2 Chemical analysis of ceramic residues pyrolysed at 1000 °C for 5 h in N2 -atmosphere Precursor
TVS HVNG ABSE
Empirical formula
SiN1:45 C1:41 H<0:06 O0:03 SiN1:07 C0:96 H<0:05 O0:03 SiN0:89 C1:48 H0:11 O0:19
Ratio of Si±C/Si±N bonds Prec.
1000 °C
Siliconcarbonitride corresponding stoichiometry
1/3 1/2.5 2/2
0/4 0.8/3.2 1.3/2.7
SiN1:33 1:41C 0:12N SiN1:07 C0:20 0:76C SiN0:89 C0:33 1:15C
materials, the evolution of hydrogen is related to an increased number of free radicals, which are observed in the ESR spectra. Therefore the cyclic unsaturated hydrocarbons had changed towards disordered or nanocrystalline graphitic carbon as shown by the appearance of the D and the G band in the Raman spectra. During the following heat treatment, at 1200 °C < T < 1500 °C, we ®nd a high-®eld shift and a narrowing of the Csp2 peak in the 13 C spectra, a narrowing of the Raman bands, changes in the ESR signal and an increase of the microwave conductivity possibly because of an increased degree of order, the growth and rearrangement of free-carbon regions leading to a carbon network with nanocrystalline graphitic-like clusters. The domain size of the graphite is calculated to be 16 in the direction of the graphite plane, using the A equation found by Tuinstra and Koenig [17]. The spectral parameters of the D and G band for the 1500 °C sample are similar to those of glassy carbon (ID =IG 2:93; G bandwidth 53 cm 1 ). Above 1100 °C, the ESR line shapes show a slight asymmetry (Fig. 4(b)). The occurrence of this asymmetric line shape, known as a Dysonian line shape [20], indicates the presence of conductive metallic like components within the amorphous ceramic matrix prepared at temperatures above 1100 °C. The conductive material in the HVNGderived amorphous ceramic is most likely graphitic-like carbon, which is also detected by Raman spectroscopy. The increased ordering causes a higher mobility of the electrons yielding an increased conductivity, which is by an order of magnitude larger as the conductivity of a measured pure SiC sample. The conductivity is related to the free-carbon content in the dierent samples 1 and increases from 1:0 10 2 X cm for the HVNG-derived material (free-carbon content 24.4
Free carbon (mol%) 35:6 1:0 24:4 1:0 31:0 1:0
mol%) to 2:6 10 1 X cm 1 for the TVS-based ceramic (free-carbon content 35.6 mol%). Further heat treatment causes the decrease of the free-carbon content in the ceramic material due to its reaction with silicon carbonitride and silicon nitride to yield silicon carbide and nitrogen. 5. Conclusion Pyrolysis of the carbon-rich polysilazanes HVNG, ABSE and TVS under inert atmospheres carried out at about 1000 °C resulted in amorphous ceramics consisting of a Si±C±N±(H) phase and excess carbon in a trigonally bonded phase. Our results obtained from the dierent experimental methods (NMR spectroscopy, Raman spectroscopy, ESR spectroscopy, microwave conductivity measurements) are consistent and suggest the formation of free carbon due to the cracking of aliphatic hydrocarbons and its reorganisation towards aromatic hydrocarbons at about 600 °C. In the temperature range between 800 °C and 1200 °C, the residual hydrogen bonded to the periphery of the free carbon is eliminated. During the following heat treatment, at 1200 °C < T < 1500 °C, the changes in Si±C±N network structure, density and bonding are minimal, but the microwave conductivity changes essentially at these temperatures possibly because of the growth and rearrangement of free-carbon regions leading to a carbon network with nanocrystalline graphitic-like clusters. The comparison between the dierent precursor types, HVNG (free-carbon content at 1000 °C: 24.4 mol%), ABSE (free-carbon content at 1000 °C: 31.0 mol%) and TVS (free-carbon content at 1000 °C: 35.6 mol%), showed no signi®cant differences in the 13 C NMR spectra of the samples
S. Trassl et al. / Journal of Non-Crystalline Solids 293±295 (2001) 261±267
heated at 1000 °C, in the Raman spectra and in the ESR spectra. This fact indicates a similar structure at the atomic scale of the free-carbon phase of the dierent precursor-derived materials. However, there is a distinction in the microwave conductivity, which is related to the free-carbon content. References [1] M. Peuckert, T. Vaahs, M. Br uck, Adv. Mater. 2 (1990) 398. [2] C. Gerardin, F. Taulelle, J. Livage, J. Chem. Phys. 89 (1992) 461. [3] Y. Sasaki, Y. Nishina, M. Sato, K. Okamura, J. Mater. Sci. 22 (1987) 443. [4] R.M. Laine, F. Babonneau, K.Y. Blowhowiak, R.A. Kennish, J.A. Rahn, G.J. Exarhos, K. Waldner, J. Am. Ceram. Soc. 78 (1995) 137. [5] M. Monthioux, O. Delverdier, J. Eur. Ceram. Soc. 16 (1996) 721. [6] D. Bahloul, M. Pereira, P. Goursat, J. Am. Ceram. Soc. 76 (1993) 1163.
267
[7] G.T. Burns, R.B. Taylor, Y. Xu, A. Zangvil, G.A. Zank, Chem. Mater. 4 (1992) 1313. [8] G.D. Soraru, G. D'Andrea, R. Campostrini, F. Babonneau, G. Mariotto, J. Am. Ceram. Soc. 78 (1995) 379. [9] J. D urr, S. Schempp, P. Lamparter, J. Bill, S. Steeb, F. Aldinger, Solid State Ionics 101±103 (1997) 1041. [10] J. L ucke, J. Hacker, D. Suttor, G. Ziegler, Appl. Organomet. Chem. 11 (1997) 184. [11] M.R. Bendall, R.E. Gordon, J. Magn. Res. 53 (1983) 365. [12] A. Schilder, H. Klos, I. Rystau, W. Sch utz, B. Gotschy, Phys. Rev. Lett. 73 (1994) 1299. [13] S. Trassl, D. Suttor, G. Motz, E. R ossler, G. Ziegler, J. Eur. Ceram. Soc. 20 (2000) 215. [14] S. Trassl, G. Motz, E. R ossler, G. Ziegler, J. Am. Ceram. Soc., (2001) accepted. [15] J. Seitz, J. Bill, N. Egger, F. Aldinger, J. Eur. Ceram. Soc. 16 (1996) 885. [16] Y. Wang, D.C. Alsmeyer, R.L. McCreery, Chem. Mater. 2 (1990) 557. [17] F. Tuinstra, J.L. Koenig, J. Chem. Phys. 53 (1970) 1126. [18] M.I. Nathan, J.E. Smith Jr., K.N. Tu, J. Appl. Phys. 45 (1974) 2370. [19] L.S. Singer, I.C. Lewis, Appl. Spec. 36 (1982) 52. [20] G. Feher, A.F. Kip, Phys. Rev. 98 (1955) 337.
www.elsevier.com/locate/jnoncrysol
Characterisation of the free-carbon phase in precursor-derived SiCN ceramics Stephan Trassl a,1, G unter Motz a, Ernst R ossler b,*, G unter Ziegler a b
a Institute for Materials Research (IMA I), University of Bayreuth, 95440 Bayreuth, Germany Physikalisches Institut, Experimetalphysik II, Universitat of Bayreuth, 95440 Bayreuth, Germany
Abstract Dierent polymeric precursors with varying carbon contents were prepared by ammonolysis of functionalised chlorosilanes. Pyrolysis under inert atmospheres at 1000 °C led to amorphous Si±C±N±(H) ceramics. Further heat treatment caused the transformation into the thermodynamically stable crystalline phase assemblage. The structural changes, especially those of the excess carbon, were studied by characterising the solid intermediates via solid state magic angle spinning (MAS) nuclear magnetic resonance spectroscopy (NMR). In addition, Raman spectroscopy, electron spin resonance spectroscopy (ESR), microwave conductivity measurements and chemical analysis were employed. Combination of all these methods provides a comprehensive picture of the formation and of the behaviour of the free-carbon phase present in the polymer-derived ceramics. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction The pyrolysis of carbon-rich polysilazanes, polycarbosilanes, polysiloxanes as well as polysilylcarbodiimides under inert atmosphere yields at about 1000 °C an amorphous ceramic containing excess carbon [1±9]. When annealing at higher temperatures, the amorphous material partly or completely crystallises to the thermodynamically stable phases, in the case of carbon-rich polysilazanes, to SiC, Si3 N4 and free C. These excess carbon atoms are not bonded to silicon atoms, but they are bonded to adjacent carbon atoms by re-
* Corresponding author. Tel.: +49-921 552 606; fax: +49-921 552 621. E-mail addresses: [email protected] (S. Trassl), [email protected] (E. R ossler). 1 Tel.: +49-921 552608; fax: +49-921 552621.
arranging the four valence electrons into sp2 -hybridisations (trigonal bond). The purpose of this paper is to investigate the formation and the nature of the excess carbon in combination with the structural rearrangement during pyrolysis. We have examined selected selfsynthesised precursors with varying carbon contents and have combined the results obtained by 13 C-solid state magic angle spinning (MAS) nuclear magnetic resonance spectroscopy (NMR), electron spin resonance spectroscopy (ESR), Raman spectroscopy and electric conductivity measurements.
2. Experimental procedure The starting silazanes with dierent amounts of carbon were self-synthesised by the reaction of
0022-3093/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 0 6 7 8 - 0
262
S. Trassl et al. / Journal of Non-Crystalline Solids 293±295 (2001) 261±267
di- and trifunctionalised chlorosilanes with gaseous ammonia as reported elsewhere [10]. The preceramic oligomers named TVS, HVNG and ABSE and studied here are listed in Table 1. The TVS and HVNG precursors were crosslinked via hydrosilylation and polymerisation by using dicumylperoxide (DCP) as a radical initiator and subsequent thermal treatment at 300 °C for 5 h in N2 -atmosphere. For the following experiments powders of such crosslinked precursors, which were heated up to 1600 °C (DT 100 K, annealing time 5 h), were used. Chemical analysis of the pyrolysis residues was carried out by Pascher Microanalytical Lab. (Remagen, Germany) with an error of about 1 wt%. Formation and structural changes of the free-carbon phase which occur during annealing were investigated in particular by 13 C NMR spectroscopy. 13 C NMR spectra of the solid intermediates were obtained on a Bruker AVANCE DSX 400 spectrometer applying the MAS technique. For preparation temperatures up to 700 °C, a signal to noise enhancement using the cross polarisation (CP) method was possible due to the presence of a sucient amount of protons in these samples. However, when studying powders heated above 700 °C, the CP method became inecient since the proton concentration was too low. In this case, the 13 C spectra were recorded with a depth pulse excitation in order to avoid the 13 C signal from the sample holder [11]. In the case of cross polarisation no such signal from the sample holder is expected. In order to get insight about the microstructure and the modi®cation of the respective carbon phase, Raman spectra were acquired on a Bruker FRA 106 with a limit of resolution of 4 cm 1 . Potassium bromide (KBr) pellets of the samples were excited in backscattering geometry using a probe wavelength of 1064 nm. ESR investigations were carried out using a
Bruker ESP 300 Q-band CW spectrometer with the cavity tuned for a klystron frequency of approximately 34 GHz. We estimate the error of the ®tted values of the line width and the intensity at about 10%. For further information, microwave conductivity measurements were performed on powder samples ®lled in quartz tubes. The resonance frequency and the quality factor of the microwave cavity are modi®ed by the introduction of the sample into the cavity. In the quasi-static approximation, i.e., when the high frequency electromagnetic ®eld completely penetrates the sample, the frequency shift and the additional loss are related to the complex dielectric constant from which the conductivity can be derived [12]. The most critical point of the evalution of the microwave conductivity rac is the calculation of the ®lling factor. Therefore, we estimate the error of the values of rac at about 50%. All spectroscopic experiments were carried out at room temperature (RT). 3. Results 3.1.
13
C NMR spectroscopy
Changes in the structure of the solid intermediates during thermal treatment at temperatures exceeding 300 °C were investigated by 29 Si and 13 C solid state NMR spectroscopy and reported in our previous publications [13,14]. Fig. 1 shows these 13 C NMR spectra of the precursor HVNG after dierent pyrolysis temperatures. Heating above 600 °C causes, in addition to the peak of CH2 Si2 sites at about 12 ppm, an appearance of a new signal at 135 ppm which increases in intensity at 700 °C. Coincidentally, a high-®eld shift of the peaks in the 29 Si spectra [13,14] corresponds to an increase of the number of Si±N bonds in contrast
Table 1 Structural units and theoretical formula of the precursors TVS, HVNG and ABSE Precursor
Structural unit
Theoretical formula
TVS HVNG ABSE
ACH@CH2 Si NH1:5 ACH@CH2 Si NH1:5 ±H CH3 Si NH ACH3 NHSi±CH2 ±CH2 ±Si NHCH3
SiN1:5 C2 H4:5 SiN1:25 C1:5 H4:75 SiNC2 H6
S. Trassl et al. / Journal of Non-Crystalline Solids 293±295 (2001) 261±267
263
peak at 28 ppm in the 13 C spectrum and at )48 ppm in the 29 Si spectrum recorded on a HVNG sample heated at 1200 °C [13]. Longer annealing times (48 h) at 1500 °C cause the formation of the thermodynamically stable crystalline phases SiC and Si3 N4 . At 1600 °C, no Si3 N4 but only a SiC signal is detected by 29 Si NMR measurements [13]. 3.2. Raman spectroscopy In recent years, Raman spectroscopy has become one of the most sensitive methods for the characterisation of the dierent modi®cations of carbon [16±18]. In the spectra recorded on HVNG-derived samples pyrolysed up to 900 °C, no bands are observed in the interesting frequency region. Fig. 2 shows the Raman spectra after pyrolysis at temperatures above 900 °C. At 1200 °C, two signals in the 1200±1700 cm 1 region were observed. The intensity of these bands centred at
Fig. 1. 13 C NMR spectra of HVNG-derived powder samples heat treated at temperatures between 300 °C and 1500 °C for 5 and 48 h in N2 -atmosphere. The dashed lines indicate the C Si4 peak and the high-®eld shift of the Csp2 signal.
to the number of Si±C bonds. The signal at 135 ppm in the 13 C NMR spectra indicates a high amount of sp2 -carbon in the form of carbon clusters, the so called free carbon [2,4,15]. Pyrolysis at 1000 °C ®nally yields an amorphous ceramic composed of a free-carbon phase and a homogeneous Si±C±N±(H) phase [13,14]. The 13 C spectrum (see Fig. 1) is characterised by one major peak (width: 40 ppm) with a short spin lattice relaxation time at about 135 ppm corresponding to the sp2 -carbon domains (free carbon). Further heating (1000±1500 °C) yields a small high-®eld shift (135 to 125 ppm) and a narrowing of the peak (40 to 30 ppm) corresponding to the free sp2 -carbon in the 13 C NMR spectrum of the HVNG precursor. Above 1100 °C the formation of CSi4 and SiN4 -environments with a short range order (amorphous SiC and amorphous Si3 N4 ) also takes place, as can be seen in the appearance of a
Fig. 2. Raman spectra of pyrolytic residues of the precursor HVNG annealed at temperatures between 1000 °C and 1700 °C for 5 and 48 h in N2 -atmosphere. The dashed lines indicate the spectral position of the D and the G band.
264
S. Trassl et al. / Journal of Non-Crystalline Solids 293±295 (2001) 261±267
(a)
(b)
Fig. 3. (a) FWHH of the D and G peak as a function of temperature. The dashed lines indicate the FWHH of the D and G peak of glassy carbon (Sigradurâ G). (b) Intensity ratio ID =IG as function of temperature. The dashed line indicate the ID =IG ratio for glassy carbon (Sigradurâ G).
about 1280 and 1600 cm 1 increases with rising temperature and reaches a maximum at 1500 °C. These two Raman bands are the most striking features of disordered graphitic-like carbon and are assigned to the D and G band [16]. Both the line width of the G band (238 to 67 cm 1 ) as well as that of the D band (268 to 130 cm 1 ) reduces with increasing pyrolysis temperature (Fig. 3(a)), whereas the position and the intensity ratio of the two bands ID =IG stay about constant. At temperatures between 1200 and 1500 °C, the intensity ratio ID =IG slightly increases (Fig. 3(b)) and is at 1500 °C at about 2.85. At longer annealing times (1500 °C/48 h), the intensities of both the D and the G band decrease and disappear at 1600 °C (see Fig. 2). However, new Raman bands occur at about 830 and 920 cm 1 corresponding to b-SiC [3]. This indicates the formation of crystalline SiC by a free carbon consuming reaction.
(a)
3.3. ESR spectroscopy The appearance of a free-carbon phase in the ceramic matrix should consequently result in a pronounced ESR signal because of the appearance of unpaired electrons during its formation [19]. Hence, ESR may help in illuminating the role of the excess carbon with respect to the pyrolysis chemistry. Thus, ESR spectra were obtained from samples heated between 300 °C and 1600 °C (DT 100 K). The intensity and the line width of the ESR signals as a function of heat treatment are shown in Fig. 4(a). Already at 300 °C, unpaired electrons are present, leading to a broad ESR line (23 Gauss) with a g-factor of about 2.0023. The intensity of the line strongly increases at temperatures above 700 °C and reaches a maximum at 1200 °C. In contrast to the intensity, the line width decreases (23 to 1.8 Gauss) over the whole tem-
(b)
Fig. 4. (a) Line width and integral intensity, and (b) asymmetry factor of the ESR signals of HVNG-derived powder samples as a function of temperature (inset: ESR spectrum of the HVNG sample heated at 1200 °C).
S. Trassl et al. / Journal of Non-Crystalline Solids 293±295 (2001) 261±267
265
Fig. 5. (a) Microwave conductivity of HVNG-derived samples as a function of temperature. The solid lines indicate the microwave conductivity of glassy carbon (Sigradurâ G), crystalline SiC and crystalline Si3 N4 ; (b) Relation between free-carbon content and conductivity for the dierent precursor-derived materials.
perature range. At 1600 °C, the line width rises again to 5.8 G, which corresponds to the value of a SiC reference sample [14]. 3.4. Microwave conductivity We investigated samples prepared from HVNG-precursor and heated between 300 °C and 1600 °C (DT 100 K). The results are shown in Fig. 5(a). The obtained conductivity values were compared with those from glassy carbon, pure, crystalline SiC and pure, crystalline Si3 N4 . At 400 °C, the pyrolysis residue showed an insulator 1 like conductivity with about 2:6 10 6 X cm . Between 500 °C and 800 °C, the microwave conductivity of these samples increased by about two orders of magnitude. A second increase of the conductivity from about 2:1 10 4 to 1 2 1:0 10 X cm was observed between 1000 °C and 1500 °C. The comparison with samples prepared from ABSE- and TVS-precursor heated at 1500 °C and 1550 °C, respectively, is shown in Fig. 5(b). Both materials show a higher microwave conductivity as the HVNG-derived sample. 4. Discussion 13 C-NMR characterisation of the pyrolysis residues shows the formation of a free-carbon
phase in the temperature range between 500 °C and 800 °C. Additionally, we ®nd an increase of the microwave conductivity and an increase of the number of Si±N bonds in contrast to the number of Si±C bonds. Both can be explained with the segregation of aliphatic hydrocarbons, which reorganise in a ®rst step towards cyclic unsaturated hydrocarbons (the so called free carbon). The increase of the number of Si±N bonds in contrast to the reduced number of Si±C bonds during heat treatment between 300 °C and 1000 °C becomes also evident from the chemical analysis of the precursor-derived materials pyrolysed at 1000 °C, as given in Table 2. Based on the assumption that nitrogen atoms are only bonded to silicon atoms [N Si3=4 ] and C atoms are only present as C Si4 units the number of Si±N bonds and Si±C bonds as well as the amount of excess carbon can be calculated (see also Table 2). The free-carbon content in the dierent samples varies from 24.4 mol% in the HVNG-derived material to 35.6 mol% in the TVS-based ceramic. Pyrolysis at 1000 °C ®nally yields an amorphous ceramic composed of a homogeneous Si±C±N±(H) phase and a freecarbon phase. The short spin lattice relaxation time indicates that paramagnetic centres are coupled to carbon atoms in these free-carbon domains. This is due to the elimination of residual hydrogen, which is still bonded to the periphery of the free-carbon regions, in the temperature range between 800 °C and 1200 °C. For carbonaceous
266
S. Trassl et al. / Journal of Non-Crystalline Solids 293±295 (2001) 261±267
Table 2 Chemical analysis of ceramic residues pyrolysed at 1000 °C for 5 h in N2 -atmosphere Precursor
TVS HVNG ABSE
Empirical formula
SiN1:45 C1:41 H<0:06 O0:03 SiN1:07 C0:96 H<0:05 O0:03 SiN0:89 C1:48 H0:11 O0:19
Ratio of Si±C/Si±N bonds Prec.
1000 °C
Siliconcarbonitride corresponding stoichiometry
1/3 1/2.5 2/2
0/4 0.8/3.2 1.3/2.7
SiN1:33 1:41C 0:12N SiN1:07 C0:20 0:76C SiN0:89 C0:33 1:15C
materials, the evolution of hydrogen is related to an increased number of free radicals, which are observed in the ESR spectra. Therefore the cyclic unsaturated hydrocarbons had changed towards disordered or nanocrystalline graphitic carbon as shown by the appearance of the D and the G band in the Raman spectra. During the following heat treatment, at 1200 °C < T < 1500 °C, we ®nd a high-®eld shift and a narrowing of the Csp2 peak in the 13 C spectra, a narrowing of the Raman bands, changes in the ESR signal and an increase of the microwave conductivity possibly because of an increased degree of order, the growth and rearrangement of free-carbon regions leading to a carbon network with nanocrystalline graphitic-like clusters. The domain size of the graphite is calculated to be 16 in the direction of the graphite plane, using the A equation found by Tuinstra and Koenig [17]. The spectral parameters of the D and G band for the 1500 °C sample are similar to those of glassy carbon (ID =IG 2:93; G bandwidth 53 cm 1 ). Above 1100 °C, the ESR line shapes show a slight asymmetry (Fig. 4(b)). The occurrence of this asymmetric line shape, known as a Dysonian line shape [20], indicates the presence of conductive metallic like components within the amorphous ceramic matrix prepared at temperatures above 1100 °C. The conductive material in the HVNGderived amorphous ceramic is most likely graphitic-like carbon, which is also detected by Raman spectroscopy. The increased ordering causes a higher mobility of the electrons yielding an increased conductivity, which is by an order of magnitude larger as the conductivity of a measured pure SiC sample. The conductivity is related to the free-carbon content in the dierent samples 1 and increases from 1:0 10 2 X cm for the HVNG-derived material (free-carbon content 24.4
Free carbon (mol%) 35:6 1:0 24:4 1:0 31:0 1:0
mol%) to 2:6 10 1 X cm 1 for the TVS-based ceramic (free-carbon content 35.6 mol%). Further heat treatment causes the decrease of the free-carbon content in the ceramic material due to its reaction with silicon carbonitride and silicon nitride to yield silicon carbide and nitrogen. 5. Conclusion Pyrolysis of the carbon-rich polysilazanes HVNG, ABSE and TVS under inert atmospheres carried out at about 1000 °C resulted in amorphous ceramics consisting of a Si±C±N±(H) phase and excess carbon in a trigonally bonded phase. Our results obtained from the dierent experimental methods (NMR spectroscopy, Raman spectroscopy, ESR spectroscopy, microwave conductivity measurements) are consistent and suggest the formation of free carbon due to the cracking of aliphatic hydrocarbons and its reorganisation towards aromatic hydrocarbons at about 600 °C. In the temperature range between 800 °C and 1200 °C, the residual hydrogen bonded to the periphery of the free carbon is eliminated. During the following heat treatment, at 1200 °C < T < 1500 °C, the changes in Si±C±N network structure, density and bonding are minimal, but the microwave conductivity changes essentially at these temperatures possibly because of the growth and rearrangement of free-carbon regions leading to a carbon network with nanocrystalline graphitic-like clusters. The comparison between the dierent precursor types, HVNG (free-carbon content at 1000 °C: 24.4 mol%), ABSE (free-carbon content at 1000 °C: 31.0 mol%) and TVS (free-carbon content at 1000 °C: 35.6 mol%), showed no signi®cant differences in the 13 C NMR spectra of the samples
S. Trassl et al. / Journal of Non-Crystalline Solids 293±295 (2001) 261±267
heated at 1000 °C, in the Raman spectra and in the ESR spectra. This fact indicates a similar structure at the atomic scale of the free-carbon phase of the dierent precursor-derived materials. However, there is a distinction in the microwave conductivity, which is related to the free-carbon content. References [1] M. Peuckert, T. Vaahs, M. Br uck, Adv. Mater. 2 (1990) 398. [2] C. Gerardin, F. Taulelle, J. Livage, J. Chem. Phys. 89 (1992) 461. [3] Y. Sasaki, Y. Nishina, M. Sato, K. Okamura, J. Mater. Sci. 22 (1987) 443. [4] R.M. Laine, F. Babonneau, K.Y. Blowhowiak, R.A. Kennish, J.A. Rahn, G.J. Exarhos, K. Waldner, J. Am. Ceram. Soc. 78 (1995) 137. [5] M. Monthioux, O. Delverdier, J. Eur. Ceram. Soc. 16 (1996) 721. [6] D. Bahloul, M. Pereira, P. Goursat, J. Am. Ceram. Soc. 76 (1993) 1163.
267
[7] G.T. Burns, R.B. Taylor, Y. Xu, A. Zangvil, G.A. Zank, Chem. Mater. 4 (1992) 1313. [8] G.D. Soraru, G. D'Andrea, R. Campostrini, F. Babonneau, G. Mariotto, J. Am. Ceram. Soc. 78 (1995) 379. [9] J. D urr, S. Schempp, P. Lamparter, J. Bill, S. Steeb, F. Aldinger, Solid State Ionics 101±103 (1997) 1041. [10] J. L ucke, J. Hacker, D. Suttor, G. Ziegler, Appl. Organomet. Chem. 11 (1997) 184. [11] M.R. Bendall, R.E. Gordon, J. Magn. Res. 53 (1983) 365. [12] A. Schilder, H. Klos, I. Rystau, W. Sch utz, B. Gotschy, Phys. Rev. Lett. 73 (1994) 1299. [13] S. Trassl, D. Suttor, G. Motz, E. R ossler, G. Ziegler, J. Eur. Ceram. Soc. 20 (2000) 215. [14] S. Trassl, G. Motz, E. R ossler, G. Ziegler, J. Am. Ceram. Soc., (2001) accepted. [15] J. Seitz, J. Bill, N. Egger, F. Aldinger, J. Eur. Ceram. Soc. 16 (1996) 885. [16] Y. Wang, D.C. Alsmeyer, R.L. McCreery, Chem. Mater. 2 (1990) 557. [17] F. Tuinstra, J.L. Koenig, J. Chem. Phys. 53 (1970) 1126. [18] M.I. Nathan, J.E. Smith Jr., K.N. Tu, J. Appl. Phys. 45 (1974) 2370. [19] L.S. Singer, I.C. Lewis, Appl. Spec. 36 (1982) 52. [20] G. Feher, A.F. Kip, Phys. Rev. 98 (1955) 337.