Cl2MeSi–NH–BCl2 and ClMe2Si–NH–BCl2: novel processable single source precursors of amorphous Si/C/B/N ceramics

Abstract

The synthesis and polymerization of Cl 2 MeSi-NH-BCl 2 (MADB) and ClMe 2 Si-NH-BCl 2 (DADB) is reported. MADB and DADB are obtained in two-step reaction sequences from MeSiCl 3 , Me 3 Si-NH-SiMe 3 (HMDS), and BCl 3 (/ MADB) or Me 2 SiCl 2 , HMDS and BCl 3 (/ DADB). The monomeric single source precursors are structurally characterized by means of NMR and IR spectroscopy and mass spectrometry. The molecular structure of DADB is additionally investigated by single crystal X-ray diffraction. It crystallizes orthorhombically in space group Pbca with a ¼ 1334.4 pm, b ¼ 1375.4 pm, c ¼ 1607.8 pm, and Z ¼ 8. In the solid state two molecules are associated via intermolecular B-N coordination, forming planar B 2 N 2 rings with almost identical B-N bond lengths. Polymerization of MADB and DADB is performed by ammonolysis and aminolysis (MeNH 2 ) releasing MADB-NH, DADB-NH, MADB-NMe, and DADB-NMe as colorless solids (MADB-NH) or colorless viscous liquids. The viscosity increases with increasing cross-linking density from DADB-NMe to MADB-NMe to DADB-NH. Accordingly, ceramic yields decrease from 77% (MADB-NH) to 45% (DADB-NH) to below 30% DADB-NMe). As-obtained ceramics are fully amorphous. Their near-range order is investigated using solid state 11 B, 13 C, and 29 Si NMR spectroscopy. All materials display BN 3 coordination at boron whereas the bonding situation at silicon is different. Silicon atoms in DADB-NH have 'mixed' SiN x C y (x + y ¼ 4) environments, whereas the 29 Si NMR spectra of MADB-NMe and DADB-NMe suggest SiN 4 coordination. High temperature TGA performed in a He atmosphere up to 2000 C indicates that DADB-NH ceramics are most stable; the onset of decomposition is above 1900 C. SiC and Si 3 N 4 reflections in the XRD patterns of the heated samples indicate that partial crystallization occurs during the heat treatment at elevated temperature.

Borazine Based Molecular Single Source Precursors for Si/B/N/C Ceramics

“…Single source molecular precursors of the general type R 1 R 2 R 3 Si-X-BR 4 R 5 with X = NH [1, [8][9][10], N(CH 3 ) [11][12][13], CH 2 [14,15], C 2 H 4 [16] and R 1 -R 5 = CH 3 or Cl have proven to perform particularly well with respect to versatility and quality of the final ceramic. As measured by the combination of properties relevant for an application as structural ceramics at high temperature, the Si/B/N/C ceramics show superior performance [6].…”

A novel high yield polyborosilazane precursor for SiBNC ceramic fibers

“…ClMe 2 Si-NH-BCl 2 (DADB) [11]. However, the ceramic yields of all preceramic polymers reported, to our knowledge, were not exceeding 78% after pyrolysis at 900°C.…”

Influence of methyl‐modified silyl groups on the properties of polymers and Si/B/N/C ceramics derived from N‐silylated borazine derivatives

“…Si/B/N/C ceramics derived from molecular single‐source precursors and from polymers have been extensively studied during the last two decades because of their remarkable performance in high‐temperature applications . Analytical investigations regarding the network structure of these materials reveal the formation of sub‐nanometer B/N and Si/N phases, whereas carbon forms aromatic carbonaceous regions mostly bonded to nitrogen but, dependent on the nitrogen content, also incorporated in the coordination sphere of silicon and boron . The homogeneous dispersion of these phases within the ceramic network is essential for thermal stability.…”

“…Smaller regions of such Si/N phases are stabilized by turbostratic BN(C) phases due to their ability to bind free carbon and to act as a diffusion barrier . Embedded carbon enhances significantly the thermal stability of such non‐oxide ceramics and can be introduced into the polymer as methyl groups of silyl entities and of —(NMe)— linkages as well as in the form of bridging units like —(CHR)—, —(CHR—CHR)—, —(CH 2 —CH 2 —CH 2 )—, —(HC═CH)— or —(N═C═N)— …”