Pyrolysis chemistry of polysilazane precursors to silicon carbonitride Part 2.†—Solid-state NMR of the pyrolytic residues Corine Ge�rardin,*a‡ Francis Taulellea‡ and Djamila Bahloulb aL aboratoire de Chimie de la Matie`re Condense�e, Universite� P. et M. Curie, 75252 Paris, France bL aboratoire de Mate�riaux Ce�ramiques et T raitements de Surface, URA CNRS 320, Faculte� des Sciences, 87060 L imoges, France The chemistry of pyrolytic conversion has been studied for three polysilazanes, (ViSiHNH)n, (ViSiHNMe)n and [(ViSiHNH)0.5–(MeSiHNH)0.5]n, precursors to silicon carbonitride ceramics. 13C and 29Si MAS and CPMAS NMR spectroscopies were used to clarify the processes leading to the formation of the silicon-based mineral network as well as the segregation of a free carbon phase.The assignment of 29Si NMR signals corresponding to SiCnN4-n sites was essential to follow the number of SiMC and SiMN bonds that are cleaved or formed. It was shown that at the organic–mineral transition temperature (ca. 900 °C) the final amount of free carbon as well as the final composition of the silicon-based network were already reached. Above this temperature, redistribution reactions around silicon atoms inside the amorphous silicon-based matrix take place in order to favour nitrogen-rich environments, i.e.crystallized Si3N4 regions. Above 1400 °C, all ceramics contain a similar amorphous silicon carbonitride structure, whose composition is close to SiN0.85C0.35 and which coexists with a crystallizing Si3N4 phase. Finally, the relative amounts of the three possible final carbon states, at 1400 °C, i.e.gas products, carbon incorporated in the silicon-based network and free carbon, could be related to the nature of the carbon-containing substituents in the precursor backbones and to the occurrence of the cross-linking reactions below the mineral transition temperature. Polysilazane polymers whose backbones consist of alternating 29Si NMR spectroscopy can lead to quantitative studies of local Si sites.The fractions of SiCnN4-n environments in the SiMN bonds with carbon-containing substituent groups are widely used as precursors of silicon carbonitride ceramics.1–8 pyrolytic residues can be obtained and those values can be used to estimate a chemical composition of the mineral silicon- The conversion chemistry of these polymeric precursors is still not completely understood.The derived ceramics obtained at based network. The purpose of this paper is to show how NMRcharacteriza- 1400 °C are often formed with three phases: a silicon carbide or nitride phase, an amorphous silicon carbonitride structure tions of the solid residues associated with data obtained previously from chemical analyses, IR spectroscopy, TG, MS and a free carbon phase.The thermomechanical properties of the ceramics depend largely on the proportions of these phases analysis and XRD21 can provide a better description of the major chemical transformations involved in the pyrolytic con- and a goal is to be able to predict the amount and the nature of the different combined structures knowing the backbone version.It will show how local structural details can give an insight into the chemical bonding and also into the chemical structure of the precursor. To achieve this objective, one must elucidate the various chemical transformations of the precer- compositions of the structures present in the multiphased materials at all intermediate temperatures. Special emphasis amic polymer in the course of pyrolysis. The difficulties arise from the characteristics of the intermediate solid residues which will be given to the influence of the nature of the precursor on the final ceramic structure.The polymer to ceramic conversion cannot be completely characterized by conventional solid-state analysis methods such as X-ray diffraction techniques.The chemistry was thus studied for three precursors containing various silazane units, with the same N/Si ratio but different lack of long-range order in the ceramics obtained below 1400 °C severely limits the investigation by XRD. The most C/Si ratios: (ViSiHNH)n (VS), (ViSiHNMe)n (VNMS) and [(ViSiHNH)0.5–(MeSiHNH)0.5]n (VS/MS). common investigation techniques used to characterize the pyrolytic conversion are thermogravimetry (TG), mass spectrometry (MS), IR spectroscopy and chemical analyses.4–12 Experimental These techniques give insights into the global changes in the materials; they answer the questions: what are the gas-phase The synthesis and curing step of the starting oligosilazanes products? When are the gaseous products released and in what have been reported elsewhere.8,10,21,22 The three main func- amounts? But very little information is generally obtained on tionalities present in the polymers are the NMH, SiMH, and the chemical bondings present in the solid inorganic intermedi- Vi (vinyl) groups which can give rise to the typical thermal ates and in the final amorphous ceramics.cross-linking reactions: transamination, hydrosilylation, Few studies centred on the NMR characterizations of the dehydrogenation and vinyl polyaddition.These reactions were solid-phase intermediates have been reported.13–20 Solid-state detailed in other papers.8,21 Note that the VS polymer contains NMR studies of the pyrolytic residues provide a description the three reactive functions per silazane unit; in VNMS, the of local arrangements around atoms. 13C NMR spectroscopy non-reactive NMe group replaces the NH function, and in is very useful for examining the evolutions of C sp3 and C sp2 VS/MS, half of the Vi functions are replaced by Me groups. environments. It can indicate when carbon atoms leave the The precursors were heated under a nitrogen flow at a silicon-based network and segregate as a free carbon phase.pyrolysis temperature in the range 250 to 1450 °C and were kept at that temperature for 1 h.21 The pyrolysis products were analysed by various techniques such as IR, XRD and chemical † Part 1, preceding paper. analyses and the results were detailed in the preceding paper ‡ Present address: Laboratoire de RMN et Chimie du Solide, UMR CNRS 50, Universite� Louis Pasteur, Strasbourg, 67000 France.(Part 1).21 The atomic ratios C/Si and N/Si of the pyrolytic J. Mater. Chem., 1997, 7(1), 117–126 117residues at 1400°C, which were obtained by chemical analyses, are indicated in Table 1. The NMR spectra were recorded on a Bruker MSL 400 spectrometer, operating at 100.62 MHz and 79.5 MHz, for 13C and 29Si nuclei respectively. Magic angle spinning (MAS) NMR spectra were acquired at a rotation frequency of 4 kHz.The 29Si one-pulse experiments required a recycle delay varying from 20 to 60 s with a 25° pulse angle. Some spectra were recorded with a recycle delay of up to 1 h and showed no difference in relative intensities from those of spectra acquired with a recyle delay of 1 min. The 29Si MAS NMR spectra could be quantitatively analysed, they were deconvoluted using the Winfit Bruker software program.Lorentz–Gaussian peaks were used to fit the experimental spectra; the three parameters, position, linewidth and amplitude, could be varied or fixed. The 13C MAS NMR spectra were acquired with a recycle delay of 60 s and a pulse angle of 20°. The 13C repetition times Fig. 1 NMR spectra of the VS/MS residues at different pyrolysis were not optimized in order to gain quantitative 13C MAS temperatures. A, 13C MAS (without cross polarization): (a) precursor, NMR spectra.A contact time of 1.5 ms and a repetition time (b) 250°C, (c) 500 °C, (d) 650 °C; B, 13C CP MAS (with cross polarization): (a) precursor, (b) 250°C, (c) 500°C, (d) 650 °C, (e) 850 °C. of 10 s were used for both 13C and 29Si cross-polarization (CP) MAS NMR spectra.In 29Si and 13C NMR, the compound used as chemical shift reference was tetramethylsilane. For all spectra, between 600 and 1000 scans were accumulated. a decrease of the peak may come from the loss of the methyl group through the evolution of methane; secondly, the loss of resolution may reflect the linking leading to Results progressively less protonated C sites such as CH2Si2 bridges or CHSi3 environments as the temperature increases.In the 13C NMR spectroscopy latter case, carbon insertion into the silicon-based network can The chemical shifts of the signals present in the 13C NMR occur. The 13C NMR resonances corresponding to C sp3 sites spectra lie between d 0 and 200. Signals from d 0 to 50 are are still observed up to 650°C under CPMAS conditions and due to carbon atoms in sp3 configuration, and signals from up to 500 °C under MAS conditions.At temperatures higher d 100 to 200 to carbon atoms in sp2 configuration. More than 850 °C, the number of protons close to carbon atoms is precisely, between d 0 and 25–30 appear signals from too low to obtain a sufficient proton magnetization transfer SiMC(sp3) sites; at d ca. 30 signals from NMC(sp3) sites; and towards 13C nuclei and cross-polarization is inefficient. The at d 135 signals from CNC sites in vinyl groups. The very signal-to-noise ratio of 13C CPMAS NMR spectra at 650 and broad signal centred at d 110 observed in 13C MAS NMR 850°C is rather low, but no significant signal corresponding spectra without cross-polarization (indicated with an asterisk to a still-protonated graphitic-type carbon structure can be on the figures), is due to carbon atoms present in the probe, detected in these samples. and so it does not represent the sample.VS. The 13C, 29Si, 1H, 15N and 14N NMR spectra of the VS VS/MS. Fig. 1A and B show the 13C MAS and CPMAS precursor in solution in C6D6 have been detailed elsewhere.13 NMR spectra of VS/MS pyrolysates.The spectra of the It was shown that 15% of the vinyl groups already transformed precursor show the presence of vinyl groups (signal at d 135) into C sp3 sites during the curing step. Fig. 2A shows the 13C and of SiMCH3 groups (at d ca. 0). The 13C CPMAS spectrum CPMAS NMR spectra of VS residues pyrolysed from 500 to enhances the resonances between d 10 and 30 characteristic of 900°C.At 500°C, no signal from vinyl groups can be observed SiMCHn (n=1 and/or 2) sites; the 1H�13C polarization trans- but a broad signal at d 9 is present, it is due to the C sp3 sites fer is indeed more efficient in these groups compared to the formed from the transformation of vinyl sites. This resonance case of mobile methyl groups.These sites are the result of the does not change much up to 600 °C. Broad and weak signals transformation of vinyl groups by polyaddition or hydrosilyl- appear at low field; they are characteristic of protonated CNC ation leading to SiMCMCMSi or SiMCMSi bridging groups. bonds and are certainly due to polyaromatic compounds which The transformation of vinyl groups is shown to take place are precursors of a free carbon phase.The occurrence of such mainly below 250 °C and it continues up to 500 °C; at this carbonaceous units was discussed in a previous paper.21 Vinyl temperature, all vinyl sites have disappeared. Between 250 and polymerization can lead to ring-type hydrocarbon species; 500 °C, a major rearrangement in the C sp3 environments is then, SiMC cleavage can give rise to six-membered rings observed, the carbon sites newly formed from vinyl groups containing C sp2 atoms.At 900 °C, it can be seen that C sp3 (signal centred at d 30) change considerably. The loss of sites are present in a minor amount compared to the abundant resolution of the peak representing the methyl function C sp2 atoms.The very broad signal centred at d 120 is due to (d ca. 0) may be the result of two different phenomena: first, still protonated C atoms such as CMCHNC. VNMS. Fig. 3A and B show the 13C MAS and CPMAS Table 1 N/Si and C/Si atomic ratios in the precursors (theoretical) and in the ceramics at 1400 °C (determined by chemical analyses) NMR spectra of VNMS pyrolysates. The weak signal at d 135 in the 13C CPMAS NMR spectrum shows that most vinyl global material elements were consumed at 250 °C, they transformed into starting silazane at 1400 °C aliphatic carbon sites whose signal appears at d ca. 10. A large (theoretical) chemical analyses) number of carbon atoms in NMMe groups (at d 30) are still sample N/Si C/Si N/Si C/Si present at this temperature, but most are transformed between 250 and 500 °C; the consumption of NMMe bonds still con- VNMS 1 3 1.19 1.75 tinues up to 650 °C.At 650 °C, a broad signal centred at d 130 VS 1 2 0.98 1.59 appears. It presents a large chemical shift anisotropy; several VS/MS 1 1.5 0.86 1.08 spinning sidebands separated by the spinning speed frequency 118 J. Mater. Chem., 1997, 7(1), 117–126Fig. 2 NMR spectra of the VS residues at different pyrolysis temperatures.A, 13C CP MAS: (a) 500°C, (b) 600 °C, (c) 900°C; B, 29Si MAS: (a) 500 °C, (b) 600 °C, (c) 900 °C, (d) 1200°C, (e) 1300°C, (f ) 1400 °C, (g) 1450°C; C, 29Si CP MAS: (a) 500 °C, (b) 600°C. are observed. These resonances do not appear in spectra without cross-polarization and are due to protonated C sp2 atoms. This observation shows the presence of abundant C atoms in an aromatic carbon structure.These structures may be formed by breaking of SiMC bonds, as explained earlier. From 650 to 850°C, the signals due to C sp3 sites disappear and the resonances due to C sp2 sites become weaker; the main reason for this is the progressive deprotonation of all carbon atoms leading to a less efficient {1H}-13C cross-polarization.By 850 °C, a complete loss of sp3 carbon occurs in the 13C CPMAS NMR spectrum of VNMS, in contrast to the case of VS pyrolytic residues for which a C sp3 signal is still observed at 900 °C. Fig. 3 NMR spectra of the VNMS residues at different pyrolysis temperatures. A, 13C MAS (without cross polarization): (a) 250 °C, 29Si NMR spectroscopy (b) 500°C, (c) 650 °C; B, 13C CP MAS (with cross polarization): (a) 250 °C, (b) 500 °C, (c) 650 °C, (d) 850°C. 29Si NMR spectra were registered under MAS conditions with and without cross-polarization. The comparison between both types of acquisitions was helpful for the assignment of peaks; The method to determine 29Si NMR chemical shifts corresponding to SiCnN4-n first coordination spheres was already it was essentially used to indicate strong 1H–29Si dipolar couplings which arise from direct SiMH bonds. 29Si CPMAS developed elsewhere,23,24 it will not be detailed here. The chemical shifts of SiCnN4-n sites were calculated using the NMR spectra were not acquired for samples pyrolysed above 800 or 900 °C because the hydrogen contents were too low at partial charge model24 and the classical theory of nuclear shielding.It was shown that replacing a carbon atom by a these temperatures. In the spectra analysis, only SiHxCyNz sites will be con- nitrogen atom in the Si environment does not lead to a regular chemical shift variation. The shape of the curve of d(29Si) sidered. The oxygen amount in the materials is indeed small. As was shown in Part 1, the chemical analyses indicated less (SiCnN4-n) vs.n is mainly governed by the nature of the first coordination sphere. The nature of the second coordination than 5 atom% of oxygen. As was also discussed in Part 1, the oxygen contamination occurs during handling and the oxygen sphere mainly shifts that curve to lower or higher field depending on the substituents on the C and N atoms. This is atoms are then present in the amorphous SiMCMN(O) structure.The low oxygen content leads to a distribution of different clearly observed in Fig. 4 which represents the chemical shifts of two series:the first one considers SiMen(NMe2)4-n molecules SiCxNyOz sites (with z=1 or 2) which exist in too small an amount to be identified in the NMR spectra as peaks dis- and the second one presents SiCnN4-n environments with C atoms being CSi4 sites (as in SiC) and N atoms NSi3 sites (as tinguishable from the main SiCnN4-n ones.As a consequence, the oxygenated sites cannot be taken into account in our in Si3N4). In a non-protonated silicon carbonitride structure with only Si, C (as CSi4 sites) and N (as NSi3 ) atoms (which spectr analysis and it would be too hazardous to discuss the oxygen evolution from the NMR results.is expected at high pyrolysis temperatures such as 1400 °C), J. Mater. Chem., 1997, 7(1), 117–126 119Fig. 4 29Si NMR chemical shifts corresponding to SiCnN4-n sites as a function of n for two series: SiMen(NMe2)4-n molecules ($) and SiCnN4-n sites hacing CSi4 and NSi3 environments (%) the chemical shifts of SiCnN4-n sites were calculated to be: d -15 (n=4), -10 (n=3), -19 (n=2), -34 (n=1) and -49 (n=0) with a precision of ±2 ppm.The exact positions of the peaks corresponding to these environments are then experimentally adjusted while fitting the spectra. As will be seen later, the main Si environments present in the samples studied here, from 500 to 1400 °C, are SiC2N2, SiCN3 and SiN4. Fitting the spectra using these three components leads to the exact positions of the individual signals as the temperature increases.It is shown that SiCnN4-n chemical shifts decrease when the temperature increases and this corresponds to the change in the Si second coordination sphere, i.e. the decrease in the number of protons. At temperatures higher than 500 °C, a very small amount of N atoms exist as NHSi2 sites, most of them were already transformed into NSi3 environments.The protonation state of nitrogen atoms changes only slightly from Fig. 6 29Si CP MAS (with cross polarization) NMR spectra of the 500 to 1400 °C, which is why the chemical shift corresponding VS/MS residues at different pyrolysis temperatures: (a) precursor, to SiN4 sites does not change much with temperature (Fig. 5). (b) 250 °C, (c) 500°C, (d) 650°C, (e) 850 °C In contrast, C atoms are still highly protonated at 500 °C. As T increases, carbon deprotonation occurs. In our samples, the non-cross-linked oligomers. All the sites formed by hydro- it is clear that SiCnN4-n 29Si chemical shifts increase with a T silylation and/or Vi polyaddition give the NMR signals shown increase in proportion with n, the number of C atoms in the in Fig. 6. It is emphasized that both types of cross-linking Si sphere (Fig. 5). reactions can lead to the same types of Si first coordination spheres, and so it is not easy to distinguish here which cross- VS/MS. Figs. 6 and 7 show the 29Si CPMAS and MAS linking reaction is predominant. The comparison between 29Si NMR spectra of VS/MS pyrolysates.The assignment of the CPMAS and 29Si MAS NMR spectra shows whether Si first main signals present in the CPMAS spectra is given in Fig. 6. coordination spheres are protonated. It appears that Numerous environments are present in the precursor: SiH(C sp2)N2 and SiH(C sp3)N2 environments are still present SiH2(C sp2)N sites are due to ends of chains, the other Si at 250 °C, with resonances at d ca.-35 and -20 respectively. environments can be explained as shown in Fig. 8. The Si sites Si(C sp3)N3 sites may be present at 500 °C and appear in the indicated with an asterisk represent the initial sites present in same chemical shift range as SiH(C sp3)N2 sites. SiN4 environments appear at 500 °C, at d ca.-45. With increasing temperature to 850°C, Si(C sp3)N3 and Si(C sp3)2N2 become the major Si first coordination spheres, while SiMH bonds are consumed.Between 850°C and 1400 °C, the intensity of the signal due to SiN4 sites increases but never becomes predominant. VS. The NMR study of the VS precursor in solution13 showed the presence of SiH(C sp2)N2, SiH(C sp3)N2 and Si(C sp3)(C sp2)N2 sites in the precursor. Let us add that the nitrogen environments present in the precursor were determined by 15N and 14N NMR spectroscopy; the following distribution was found: 30% NH2Si (at d -356.2), 46% NHSi2 sites (at d -346.8) and 24% NSi3 (at d -330).The chemical shifts were relative to CH3NO2. This result reflected the occurrence of the transamination reaction during the curing step of the starting oligosilazanes.Fig. 2B and C show the 29Si Fig. 5 29Si NMR chemical shifts corresponding to SiCnN4-n sites MAS and CPMAS NMR spectra of VS pyrolysates. At 500°C, (n=0, 1 and 2) in the pyrolytic residues as a function of temperatures. $, SiN4; &, SiN3C; +, SiN2C2 . the main environments are Si(C sp3)2N2, Si(C sp3)N3 and 120 J. Mater. Chem., 1997, 7(1), 117–126SiN4, (signals at d -2, -24 and -46 respectively); there rich sites form.Between 500 and 1200 °C, the intensity of the signal due to SiN4 sites increases and becomes predominant. certainly remain some protonated sites of the type SiH(C sp3)N2 but no SiH(C sp2)N2 since all vinyl groups were Note that among all SiCnN4-n environments, only SiC2N2 sites can be explained by solely cross-linking reactions such as consumed before 500 °C.Between 600 and 900 °C, a marked consumption of Si(C sp3)2N2 sites is observed while nitrogen- hydrosilylation and vinyl polyaddition. One possibility is that SiCN3 environments form from SiHCN2 sites by dehydrocoupling between SiMH and NMH bonds. Another explanation is that SiMC cleavage around SiC2N2 environments can lead to C-deficient Si sites and the formation of new SiMN bonds, thus explaining the occurrence of SiCN3 sites and particularly the increase of SiN4 environments.VNMS. Fig. 9A and B present the 29Si MAS and CPMAS NMR spectra of VNMS pyrolysates. The main component of the spectra at 250 °C, present at d ca. 0, is due to Si(C sp3)2N2 sites, the component at d ca. -15 to -20 corresponds to SiH(C sp3)N2 environments and there remain some initial sites of the type SiH(C sp2)N2 (d ca.-30). The increase of the component at d ca. -20 may correspond to the formation of Si(C sp3)N3 sites, which can be connected to the consumption ofNMMe bonds leading to new NMSi bonds. This proposition is in agreement with 13C NMR spectroscopic observations. The change is abrupt between 500 and 650 °C when a large amount of SiN4 sites appears as a broad signal at d ca.-45. This observation suggests the formation of abundant SiMN Fig. 7 29Si MAS (without cross polarization) NMR spectra of the VS/MS residues at different pyrolysis temperatures: (a) precursor, (b) 250 °C, (c) 500 °C, (d) 650°C, (e) 850°C, (f ) 1400°C Fig. 9 NMR spectra of the VNMS residues at different pyrolysis Fig. 8 Possible firstcoordination spheres of silicon atoms afterhydrosi- temperatures.A, 29Si MAS (without cross polarization): (a) 250 °C, (b) 500 °C, (c) 650°C, (d) 850 °C, (e) 1400 °C, B, 29Si CP MAS (with lylation and/or vinyl polyaddition starting from the initial silicon sites (indicated with a star) cross polarization): (a) 250°C. (b) 500°C, (c) 650°C. J. Mater. Chem., 1997, 7(1), 117–126 121bonds at the expense of SiMH or, more probably SiMC bonds.this temperature range.21 The decrease in C/Si is mainly due to the breaking of NMMe bonds leading to the release of CH4 The change observed in the 13C NMR spectra was also very important in this temperature range. With increasing tempera- and this probably results in the formation of new NMSi bonds at the expense of SiMC and SiMH bonds.This is the first ture, SiN4 sites become more abundant; they are largely predominant at 1400°C. Again, it is observed that SiMN main origin of the formation of nitrogen-rich silicon sites. The second one is the expulsion of C groups from the silicon-based bonds form at the expense of SiMC bonds in the silicon structure observed by 29Si NMR spectroscopy.network to form a carbon phase, which is clearly evidenced through 13C NMR studies from 650 to 850 °C. Vinyl functions first transform into C sp3 groups essentially by polyaddition Discussion reactions. These carbon chains partly lead to the evolution of high molecular mass hydrocarbons as observed by mass spec- This discussion is divided in three parts, the first concerns the formation of the mineral network taking place below the trometry.21 Another large number of SiMC bonds are cleaved leading to free C sp2-type carbonaceous species and more organic–inorganic transition temperature (ca. 850 °C), the second one will be devoted to the evolution of the silicon-based numerous SiMN bonds in the silicon matrix. Carbon segregation thus leads to nitrogen enrichment of the silicon network.matrix at temperatures higher than 900 °C (silicon carbonitride structure and silicon nitride phase) and the third part to the In the case of the VS route, there is a first increase in the amount of SiN4 sites up to 500–650 °C (ca. 30%). This phenom- free carbon phase, its formation and its amount which is determined from the combination of quantitative NMR results enon is associated with a small mass loss and a small C/Si decrease.21 Cross-linking reactions (hydrosilylation, Vi poly- and chemical analyses.merization, dehydrocoupling of SiMH and NMH bonds) have led to SiC2N2 and SiCN3 sites. These sites can transform to Formation of the mineral network N-rich sites (SiN4) through expulsion of C groups as a solid During the first pyrolysis step, the mineral network forms; this free carbon structure.This is what is observed by 13C NMR stage was characterized in detail by MS, TG and IR spec- studies, showing the formation of new C sp2 sites at 600 °C troscopy in the preceding paper.21 It was shown that in the which become very abundant at 900 °C. In this case, the first stage, up to 400 °C, further cross-linking proceeds while a expulsion of carbon from the silicon-based network to give loss of low molecular mass oligomers occurs.In the second free carbon mainly explains the formation of SiN4 sites. stage, from 400 to 800 °C, the mineralization step is charac- In the VS/MS route, the increase in the fraction of SiN4 is terized by a mass loss corresponding to the evolution of much lower up to 900°C compared to the other two precursors.hydrocarbons and hydrogen; it is associated with the breaking This is in accordance with the higher stability with temperature of SiMH, CMH, NMH, NMC and SiMC bonds. Using 29Si of the SiMC bond for Me groups compared to the SiMC bond NMR spectroscopy, it is possible to observe the consequences obtained by hydrosilylation or polyaddition of vinyl species of the cross-linking reactions and the gas evolution on the (stability shown by IR and 13C MAS NMR studies).The local transformations taking place in the solid residues. Silicon formation of SiN4 sites can be mainly related to the departure environments are very numerous below 900 °C (SiHpNnCm of carbon as a gaseous product; the evolution of methane is sites) and it would be hazardous to try to obtain precise values indeed observed up to 900 °C.The formation of free carbon of proportions of Si sites present in the materials because of plays a lesser role in this route, the signal corresponding to the low resolution of the resulting NMR signals. The 29Si MAS new C sp2 environments is indeed very weak in the 13C NMR NMR spectra were completely simulated but only the compo- spectra, in agreement with the results obtained by Raman nent present at high field (d -45) was interpreted quantita- spectroscopy.25 tively.The fraction of SiN4 sites was thus estimated from 500 From 13C and 29Si NMR results, the structural state of the to 900 °C for the three routes, it is the only signal appearing preceramic network can be deduced.The presence of in the chemical shift range d -45 to -50. Fig. 10 shows that SiMCMCMSi bridges formed at low temperature (below at 900 °C, the fraction of SiN4 sites reaches 54% in the VNMS 600°C) renders the matrix more stable towards redistribution route, 33% in the VS route and 23% in the VS/MS route. around Si atoms. Note that redistribution mainly involves In the VNMS route, it is observed that SiN4 sites increase SiMN bonds with SiMN or SiMH ones below 900 °C.Cross- rapidly from 500 to 900°C and become predominant. This can linking involving vinyl groups reduces the segment mobility be related first to the large mass loss observed in TG (ca. 25%) and hinders exchange reactions. It was shown21 that transamin- together with the large decrease in C/Si (from 3 to 1.75) in ation is hindered in the VS and VNMS routes (where vinyl groups lead to a high cross-linking degree) compared to the VS/MS route; the departure of NH3 was reported to be considerable only in the case of VS/MS polymers.At ca. 500°C, considering that all vinyl groups were transformed to C sp3 units, it is observed that VS and VS/MS intermediates contain about the same number of SiMC sp3 bonds per silicon.Note that the 29Si CPMAS NMR spectra at 500 and 600°C of VS and VS/MS are indeed very similar, showing that the proportions of SiMC sp3 and SiMN bonds in the silicon network are about the same in the two routes. But SiMC sp3 bonds, appearing at low temperature by vinyl transformation are not very stable and partly disappear when the temperature is increased to 900 °C. The SiMC cleavage leads to SiN4 sites and enrichment in N atoms of the silicon network.It was observed that the 29Si MAS NMR spectra at 900 °C of VS and VS/MS are different, SiN4 sites being more abundant in VS than in VS/MS. At 900 °C, the silicon network is mainly built of SiMN bonds but it is richer in SiMC bonds in VS/MS than in VS than in VNMS.The combination of 13C and 29Si NMR results was helpful Fig. 10 Fractions of SiN4 sites as a function of the pyrolysis temperature in the three routes. #, VS/MS; ×, VS; &, VNMS to clarify the next point: it is clear that SiMN bonds are 122 J. Mater. Chem., 1997, 7(1), 117–126formed at the expense of SiMC bonds, but do SiMC bonds samples; this is supported by the fact that the fraction of SiC3N sites (which would give a signal at even lower field) is found break to form a solid carbonaceous residue or to lead to the evolution of carbon-containing gaseous products? In each to be zero from 900 to 1400 °C.Moreover, X-ray diffraction peaks corresponding to SiC phases were never observed in route, we showed that it was possible to evaluate which phenomena predominate.those samples pyrolysed under nitrogen, even at high temperature (1450 °C).21 The low oxygen content is neglected in these calculations, and so only SiMC and SiMN bonds are taken Evolution of the silicon-based network from 900 to 1400 °C into account in the silicon matrix. The silicon-based network is now examined in the temperature Fig. 12 shows the evolution of the fractions of SiCnN4-n range 900–1400 °C.It is constituted of all Si atoms and N and sites from 900 to 1400°C. It is observed that a redistribution C atoms directly bonded to Si atoms, and so it excludes the between SiMC and SiMN bonds takes place from 900 to free carbon phase formed by the breaking of SiMC bonds. Up 1400°C. SiN4 and SiC2N2 sites are favoured at the expense of to the organic–inorganic transition temperature, deprotonation SiCN3 sites.A tendency to segregate a silicon nitride phase is in the silicon-based network was considerable.21 From 900 °C, thus observed. The linewidth of the peak due to SiN4 environ- it is assumed that the number of SiMH bonds is negligible. Assuming that at temperatures higher than 900°C, CMH, NMH and NMMe bonds are also negligible, atomic compositions of the silicon matrix can be estimated.C atoms are taken into account as CSi4 sites and N atoms as NSi3 sites. The proportions of Si environments are obtained from deconvolution of the 29Si NMR spectra considering SiCnN4-n firstcoordination spheres; Fig. 11 shows the results obtained at 1400 °C. It is assumed that SiC4 sites are negligible in these Fig. 12 Fractions of silicon sites (SiN4, SiN3C and SiN2C2) in the Fig. 11 29Si MAS NMR spectra of the three samples at 1400 °C: (a) VNMS, (b) VS, (c) VS/MS. Deconvolution into three individual silicon-based networks of (a) VS/MS, (b) VS and (c) VNMS samples pyrolysed from 850 to 1400°C. #, SiN2C2; ×, SiN3C; &, SiN4. peaks corresponding to SiN4, SiN3C and SiN2C2 sites.J. Mater. Chem., 1997, 7(1), 117–126 123Table 3 Estimated proportions of silicon nitride and silicon carbo- ments shows that, at temperatures lower than 1400 °C, the nitride structures in the silicon-based networks at 1400°C silicon nitride structure is still mainly amorphous, which agrees with XRD results.21 With increasing temperature in the VS sample silicon nitride+silicon carbonitride route, the formation of more numerous SiN4 sites is clearly observed in the 29Si NMR spectra at 1400 and 1450 °C VNMS 0.64 SiN4/3+0.36 SiN0.85C0.36 VS 0.44 SiN4/3+0.56 SiN0.88C0.34 (Fig. 2B). This agrees with the XRD results22 which show a VS/MS 0.32 SiN4/3+0.68 SiN0.84C0.37 better crystallization of silicon nitride at 1450 °C. Atomic compositions of the Si networks are calculated using SiCnN4-n fractions.The calculations are performed as follows: if p=SiC2N2 (%), q=SiCN3 (%) and r=SiN4 (%), It is also interesting to compare the composition of the we have: Si(%)=(p+q+r)/S; C(%)=(2p+q)/4S and silicon carbonitride structure of these materials to the composi- N(%)=(2p+3q+4r)/3S with S=(p+q+r)+(2p+q)/4 tion of another silicon carbonitride structure obtained in a +(2p+3q+4r)/3. The deduced C/Si and N/Si atomic ratios material which leads to the major crystallization of SiC and are indicated in Table 2.The compositions are found to be not Si3N4 at high temperature. From carbosilazane precursors quite stable with temperature from 900 to 1400°C, which prepared from thermolysis of (SiMe2)n–(NHSiMeHNH)m means that carbon is not lost from the Si network in this copolymers,27,28 silicon carbonitride ceramics with SiC crystals temperature range.This observation proves that segregation were obtained at 1400 °C.16 Following a similar strategy to of the free carbon phase takes place mainly before the organic– characterize these ceramics,16,23 it was shown that the main Si inorganic transition.After this transition, the main reactions sites present in that silicon network were SiC4, SiC3N and are local rearrangements in the amorphous silicon matrix SiC2N2 from 900 to 1400°C. Again, if we assumed that all which permit the creation of N-rich environments in order to SiC4 sites are only part of the SiC crystalline phase at 1400°C, allow the further silicon nitride crystallization.Exchange it was possible to estimate an atomic composition characteriz- involving SiMC and SiMN bonds is the main reaction. ing the amorphous silicon carbonitride structure that coexists The atomic compositions characterizing the silicon networks with the SiC crystals. The whole silicon phase was found to in the three samples present a similar N/Si ratio, close to unity.have the composition SiC0.71N0.40, and the silicon carbonitride It is seen that C/Si ratios decrease from VS/MS to VS to structure (excluding in this case SiC4 sites) to have the composi- VNMS, suggesting a much higher ability to retain C atoms in tion SiC0.61N0.52, which can be formally written as the silicon phase in VS/MS than in VS than in VNMS. This 0.6SiC+0.4SiN1.33. Note that the atomic composition of the trend is opposite to the initial C/Si values in the precursors.amorphous silicon carbonitride structure is rich in carbon Compared to the atomic compositions in the precursors, atoms in this case and thus seems to be related directly to the ca. 17% of total carbon atoms were retained in the VS/MS nature of the coexisting crystallizing phase, which is thermo- silicon phase compared to 10% in VS and 4% in VNMS.It dynamically expected. Note that carbon atoms initially present is evident that an increase in the total carbon content in the as SiMMe groups in this route lead to a stable incorporation precursor, whatever the polymeric backbone structure, does of carbon in the silicon carbonitride structure and to <5% of not lead to a proportional increase in the carbon content of free carbon.the silicon-based network in the ceramic. Let us now consider the materials pyrolysed at 1400 °C The free carbon phase which present the formation of minor amounts of crystalline silicon nitride, and let us assume that all SiN4 sites segregate It was reported, in Part 1,21 that the global N/Si and C/Si atomic ratios in the pyrolysed materials were almost constant in order to favour Si3N4 crystallization. In this case, it is possible to characterize the silicon carbonitride structure and from 900 to 1400 °C; the corresponding values obtained from chemical analyses at 1400°C are indicated in Table 1.We have calculate an estimated atomic composition of that amorphous structure. The compositions of the silicon carbonitride just shown that these ratios calculated now for the siliconbased network from 29Si NMR results were also constant from structure are then calculated as explained earlier, but only the strictly mixed Si sites (SiC2N2 and SiCN3) are taken 900 to 1400 °C (Table 2).Both results suggest that carbon atoms have left the Si network mainly before the mineral into account.The results are (SiN0.85C0.36) for VNMS, (SiN0.88C0.34) for VS and (SiN0.84C0.37) for VS/MS. Note that transition at 900°C to form a free carbon phase. This combination of quantitative analyses obtained from 29Si MAS NMR the compositions are very similar, close to SiN0.85C0.35 which can be also formally expressed as 0.65SiN1.33+0.35SiC. This studies and chemical analyses is in accordance with the qualitative 13C MAS and CPMAS NMR spectra.It was shown composition is rich in nitrogen atoms, which seems to agree with the fact that the amorphous silicon carbonitride structure that changes in C chemical environments were considerable before 900 °C: a large increase in the fraction of new C sp2 actually coexists with an Si3N4 phase in the process of crystallization.If we compare the relative abundances of the silicon sites was observed from 600 to 900°C at the expense of C sp3 atoms bonded to Si atoms. The new C sp2 sites were attributed nitride phase and the silicon carbonitride structure (Table 3), we observe that the amorphous silicon carbonitride structure to protonated cyclic precursors of a graphite-type structure formed by SiMC bond cleavages; the protons play the role of in the three materials is more abundant in VS/MS than in VS and than in VNMS, which remains in agreement with the poisons in the growth of graphitic carbon cages.These results were supported by TEM data from Delverdier.29 When free order of ability to retain C atoms in the Si matrix. carbon appears, it is present as small aromatic carbon units.The first primary aromatic entities, called basic structural units Table 2 N/Si and C/Si atomic ratios in the silicon-based matrices at (BSU), are embedded in the amorphous silicon carbonitride 900 and 1400 °C, determined by NMR phase. When the temperature increases, the BSUs rearrange into carbon stacks giving rise to more or less complete cages silicon matrix silicon matrix formed from graphite planes.at 900 °C at 1400 °C The abundances of the free carbon phases in the different sample N/Si C/Si N/Si C/Si routes can be estimated from the fractions of C atoms which are no longer bonded to Si atoms. These values can be obtained VNMS 1.11 0.13 1.15 0.13 from the comparison of the C/Si ratios in the global materials VS 1.09 0.19 1.09 0.19 obtained by chemical analyses (CA) and in the Si networks VS/MS 0.97 0.27 1.00 0.25 obtained by NMR studies (Tables 1 and 2).The percentages 124 J. Mater. Chem., 1997, 7(1), 117–126of atoms in the free carbon phase are calculated as follows: mate proportions of phases in the ceramics desired for specific properties. free C(%)=[(C/Si)CA–(C/Si)NMR]/[1+(C/Si)CA+(N/Si)CA]. The results are as follows: 41% (VNMS), 39% (VS) and 28% (VS/MS), which correspond to the following fractions of total Conclusions C atoms: 93% for VNMS, 87% for VS and 76% for VS/MS.It is shown that the abundance of free carbon is inversely The present study shows that solid-state NMR permits the related to the abundance of the amorphous silicon carbonitride elucidation of different aspects of the pyrolytic conversion of structure.This observation is true for pyrolytic residues at polysilazane precursors into silicon carbonitride ceramics. 850 °C and holds up to 1400 °C. The relative abundances of The assignment of the signals corresponding to SiCnN4-n the three different structures present in the materials at 1400 °C (n=0, 1, 2, 3, 4) sites allows us to follow the different reactions are summarized in Table 4.of cleavage or formation of bonds in the silicon-based matrix. The abundance of free carbon can be related directly to the The quantification of these sites leads to a more precise nature and amount of cross-linking reactions occurring below description of the structural evolution with temperature of the 900 °C. The combined results suggest that Vi polyaddition different phases forming the material, namely the silicon-based mainly leads to carbon structures with very little carbon network and the free carbon phase, which at high temperature insertion into the Si network.Polyaddition creates new CMC separate in three distinct structures: the crystallizing silicon bonds and the formation of polymeric carbon chains (CH2)n nitride phase, the amorphous silicon carbonitride structure with n3 gives SiMC sp3 bonds with very low stability.and free graphitic carbon. Hydrosilylation is the main reaction that creates new SiMC From the combination of quantitative NMR results and bonds at low temperature leading to stable carbosilane bridges chemical analyses, the following points are clearly shown.from vinyl groups. Hydrosilylation leads to SiMCMCMSi Below the organic–inorganic transition temperature the carbon chains or SiMCMSi bridges. As the temperature increases, groups leave the silicon network to segregate as a free carbon SiMCMCMSi chains can rearrange into SiMCMSi carbosilane structure. The carbon-bearing functions, i.e. the SiMMe, SiMVi units.Stable carbon incorporation into the silicon network and NMMe groups, are compared by evaluating in the three can thus occur. In the VS route, Vi polyaddition in preference routes the different relative amounts of the three final carbon to hydrosilylation consumes Vi groups, and it is even more states, which are gaseous products, carbon incorporated in the true in VNMS, for which hydrosilylation is certainly hindered silicon matrix and free carbon.Another major point is that by the presence of NMMe groups;21 this leads to high free the chemical composition of the amorphous silicon carbo- carbon contents. Vi groups are not readily evolved as gaseous nitride structure can be estimated; in the present cases, it is products, they lead to high final carbon contents in the close to SiN0.85C0.35.It is shown that that composition is ceramics and that carbon is very slightly incorporated in the essentially related to the nature of the coexisting phase which silicon matrix when polyaddition is preferred to hydrosilyl- is crystallizing (Si3N4 or SiC). Also, the proportions of all the ation. In the VS/MS route, the methyl groups give rise coexisting phases in the ceramics can be deduced.preferentially to either gaseous products such as methane or to stable SiMCH2MSi carbosilane bridges which progressively References deprotonate into CHSi3 and finally transform into CSi4 environments. These processes lead to a low free carbon 1 R. M. Laine, Y. D. Blum, D. Tse and R. Glaser, Inorganic and content but to a reasonable amount of silicon carbonitride Organometallic Polymers; ACS Symp. Ser. 360, ed. M. Zeldin, structure. It is possible to summarize schematically the evol- K. J. Wynne and H. R. Allcock, ACS, Washington, DC, 1988, p. 124. utions of carbon atoms from the precursors to the ceramic 2 D. Seyferth, in Silicon-based Polymer Science. A Comprehensive materials obtained by pyrolysis at 1400 °C for the three routes Resource; Adv.Chem. Ser. 224, ed. J. M. Zeigler and F. W. Fearon, (Table 5). 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