Browsing by Author "Tsetsgee, Otgontuul"

Filter results by typing the first few letters
Now showing 1 - 1 of 1
  • Thumbnail Image
    ItemOpen Access
    Solid-state NMR studies on precursor derived Si-B-C-N and B-C-N ceramics
    (2009) Tsetsgee, Otgontuul; Klaus, Müller (Prof. Dr.)
    The precursor-derived Si-B-C-N and B-C-N ceramics are of special interest because of their covalent bonding providing mechanical reliability and high-temperature stability. In general, the precursor-derived ceramics posses several advantages, as compared to the ceramics obtained by conventional method. They exhibit a much better homogeneity on the molecular level. Moreover, the precursor route offers a better flexibility for the design of ceramic tools. More recently, it has been demonstrated that Si-B-C-N ceramics exhibit improved thermal stability up to 2200 °C, while the thermal stability of boron free Si-C-N ceramics is limited to 1485 °C due to reaction of Si3N4 with free carbon. The majority of the studies on Si-B-C-N ceramic systems used polysilazanes and polysilylcarbodiimides as precursor polymers, which upon hydroboration and subsequent heat-treatment yield amorphous or crystalline quaternary ceramics. It is anticipated that the thermal stability of Si-B-C-N ceramics primarily relies on their unique structural composition, consisting of nanocrystalline SiC and Si3N4 domains as well as of a turbostratic BNCx phase. In the present contribution, the thermolysis and crystallization behavior of Si-B-C-N and B-C-N ceramic systems are studied by multinuclear (13C, 29Si, 15N and 14N) MAS NMR. Particular emphasis is given to the structural composition of the BNCx phase. For this purpose, for the first time double resonance experiments such as 11B{15N}REDOR, 11B{15N}SEDOR and 11B{14N}REAPDOR and 11B spin echo were applied, which allow the measurement of the dipolar coupling between like and unlike spins and which is directly related to the internuclear distance. From REDOR and SEDOR experiments, the dipolar coupling between boron and nitrogen nuclei were obtained. 11B spin echo experiments provide the homonuclear second moment in the amorphous ceramics. Also, results from 11B{14N}REAPDOR experiments, which are suitable for spin system where two quadrupolar nuclei are involved, are provided. Although this technique gives the same information as the REDOR experiment, it does not require 15N isotopic enrichment. The quaternary Si-B-C-N ceramics were prepared by thermolysis of suitable organometallic precursors, such as boron-modified polysilazanes and polycarbodiimides with the general formula {B[RSi(R')X]3}n (R=C2H4, C3H6, C2H4-SiH(H)C2H4, C2H4-SiH(CH3)C2H4; R'=H, CH3 and X=NH, NCH3, (NCN)0.5). The results of the 13C, 29Si, 15N und 11B NMR studies show that the Si-B-C-N precursor systems at 600 °C are characterized by (i) sp2- and sp3- carbons attributed to the graphite-like carbon and CH4-xSix (x=0, 1, 2 and 3) units, respectively; (ii) an amorphous Si-C-N matrix with the SiC4-xNx (x=0, 1, 2, 3 and 4) structural units; and (iii) BNCx. Furthermore, the NMR results reveal that the microstructure of the amorphous ceramics depend on the functional groups X and R, as they play an important role for crosslinking during pyrolysis. The precursor systems with X=NCN and NCH3 exhibit higher amounts of N-enriched silicon sites (SiCN3 and SiN4 units) as compared to the precursor systems with X=NH. The precursor systems with R=C2H4-SiH(H)C2H4 and C2H4-SiH(CH3)C2H4 have a lower amount of N-enriched silicon sites as they contain the highest amount of silicon and carbon atoms per monomeric unit. Moreover, the Si-B-C-N ceramics containing a higher amount of C-enriched silicon units show a higher amount of CSi4 units as compared to graphite-like carbon. In general, a good match between experimental and theoretical REDOR and REAPDOR curves were achieved for all precursor systems, indicating that the BN3 units have a planar geometry as in h-BN. However, the obtained B-N distances are longer than in pure h-BN (1.44 Å). The most drastic changes occur between 400 and 1400 °C, when the obtained B-N distance of 1.55-1.75 Å decreases continuously with increasing annealing temperature. The obtained B-B distances for the pyrolysis intermediates at 1050 °C were in the range between 2.78 and 3.25 Å and are thus much larger as compared with h-BN. Therefore, a fully developed BN-layer structure does not exist at this temperature. This is confirmed by the 11B and 15N NMR data, which show the presence of the BCN2 and NHB2 units. For example, the shortest B-B distance is observed for the intermediate derived from polysilazane (R'=H and X=H), which does not contain NHB2 structural units. After further increasing the annealing temperature, the B-B distances for the various systems are very similar. However, the temperature dependence of B-B distances is quite pronounced, as expressed by the decrease from 2.86 to 2.53 Å. On the basis of the present results, it can be concluded that intercalated BN and sp2-carbon domains with hexagonal layer structure most likely constitute the (BN)Cx phase above 1400 °C. The graphite-like carbon layers create some internal pressure which in turn is responsible for the observed interatomic distance increase.