Anita Müller

Correlation of boron content and high temperature stability in Si-B-C-N ceramics II

Abstract This study treats the preparation of polyborosilazanes which are obtained by hydroboration of oligovinylsilazane [–(H 2 CCH)SiHNH–] n with different amounts of H 3 B . SMe 2 . Pyrolysis of these precursors yields amorphous ceramic materials, which only differ in their relative boron content whereas their Si/C/N ratio is very similar. High temperature thermogravimetric analysis reveals that with increasing boron content the materials thermal degradation is shifted toward higher temperatures. Furthermore, the observed mass loss due to nitrogen evaporation during the materials decomposition is noticeably smaller in boron-rich materials than thermodynamically calculated. X-ray diffraction (XRD) experiments support the presence of crystalline silicon nitride in heat treated samples with a minimum boron content of 5 at.%. The decomposition temperature of this phase strongly depends on the amount of boron and can exceed 2000°C. However, high temperature stable materials are not only characterized by a definite B/N ratio, but also by the ability to develop appropriate microstructures.

Self-diffusion studies on 15N in amorphous Si3BC4.3N2 ceramics with ion implantation and secondary ion mass spectrometry

Nitrogen self-diffusion studies in amorphous precursor-derived Si3BC4.3N2 ceramics were carried out, using ion implanted stable 15N isotopes as tracers and secondary ion mass spectrometry for depth profiling. The analysis of the diffusion profiles in the range of 1500–1700 °C did not show the typical Gaussian broadening of the implantation profiles. Instead, we observed the occurrence of a high concentration region where the width of the implantation profile is nearly unchanged due to implantation damage, and a low concentration region where diffusion occurs. The experimentally determined diffusivities obey an Arrhenius behavior with an activation enthalpy of about H=7 eV and a pre-exponential factor D0 in the order of 5 m2/s which indicates a diffusion mechanism via vacancy-like defects.

Dehydrocoupling of tris(hydridosilylethyl)boranes and cyanamide: a novel access to boron-containing polysilylcarbodiimides

Abstract The synthesis by a novel reaction pathway and the polymer-to-ceramic conversion of boron-containing polysilylcarbodiimides are reported. The title compounds are accessible by reaction of tris(hydridosilylethyl)boranes, B[C 2 H 4 Si(R)H 2 ] 3 (R=H, CH 3 ; C 2 H 4 =CH 2 CH 2 , CHCH 3 ) with different amounts of cyanamide, H 2 NCN, in tetrahydrofuran solution. Polymerization occurs by dehydrocoupling without the formation of solid by-products and without the use of catalysts. Overall compositions range from C 6.5 H 20 BNSi 3 to C 9 H 15 BN 6 Si 3 in the case of the polymers obtained from B(C 2 H 4 SiH 3 ) 3 and from C 9.5 H 26 BNSi 3 to C 12 H 21 BN 6 Si 3 in the case of the polymers obtained from B[C 2 H 4 Si(CH 3 )H 2 ] 3 , depending on the B[C 2 H 4 Si(R)H 2 ] 3 : H 2 NCN ratio used. Molecular structure and chemical composition were investigated by spectroscopic methods (solid state MAS-NMR, FT-IR) and elemental analysis. Thermolysis in an Ar atmosphere delivers SiBCN ceramics in 65–85% yield, depending on the elemental composition and the molecular structure, i.e. the cross-linking of the polymers.

Lithiierte Phosphanimin‐Komplexe. Die Kristallstrukturen von [LiCH(Me)PEt2NSiMe3]4 und des Cuprats [Li{Me3SiNPMe2CH2–Cu–CH(SiMe2OLi)PMe2NSiMe3}]2

[LiCH(Me)PEt2NSiMe3]4 (1) wird als farblose sauerstoff- und feuchtigkeitsempfindliche Kristalle bei der Reaktion des silylierten Phosphanimins Me3SiNPEt3 mit nButyllithium in nHexan bei 0 °C erhalten. 1 kristallisiert in der tetragonalen Raumgruppe I41/acd mit acht Formeleinheiten pro Elementarzelle. Gitterkonstanten bei –80 °C: a = b = 1505,2(1); c = 4747,4(6) pm, R1 = 0,0278. In 1 besetzen die Lithium-Atome die Ecken eines Tetraeders, auf dessen Flachen die C-Atome der carbanionischen CH(Me)-Gruppen liegen. Die N-Atome der Phosphanimin-Reste koordinieren die Lithium-Atome durch „innere Solvatation”. Das Cuprat [Li{Me3SiNPMe2CH2–Cu–CH(SiMe2OLi)PMe2NSiMe3}]2 (4) entsteht aus dem bekannten [LiCH2PMe2NSiMe3]4 und Kupfer(I)iodid unter Mitwirkung des Dichtungsmittels (-OSiMe2-)n in Diethylether als farblose sauerstoff- und feuchtigkeitsempfindliche Kristalle. 4 kristallisiert in der triklinen Raumgruppe P 1 mit einer Formeleinheit pro Elementarzelle. Gitterkonstanten bei –50 °C: a = 1025,4(2); b = 1145,5(2); c = 1261,0(2) pm; α = 65,19(1)°; β = 79,55(1)°; γ = 77,94(1)°; R1 = 0,039. 4 bildet ein zentrosymmetrisches dimeres Molekul mit einem zentralen Li2O2Vierring, dessen O-Atome uber -SiMe2-Brucken mit den Cuprat-Fragmenten > CH–CuCH2- verbunden sind. Lithiated Phosphoraneimine Complexes. Crystal Structures of [LiCH(Me)PEt2NSiMe3]4 and of Cuprate [Li{Me3SiNPMe2CH2–Cu–CH(SiMe2OLi)PMe2NSiMe3}]2 [LiCH(Me)PEt2NSiMe3]4 (1) has been obtained as colorless, oxygen and moisture sensitive crystals from the reaction of the silylated phosphoraneimine Me3SiNPEt3 with nbutyllithium in nhexane at 0 °C. 1 crystallizes in the tetragonal space group I41/acd with eight formula units per unit cell. Lattice dimensions at –80 °C: a = b = 1505.2(1), c = 4747.4(6) pm, R1 = 0.0278. 1 forms a Li4 tetrahedron, the faces of which are capped with the carbon atoms of the carbanionic -CH(Me)- groups. The nitrogen atoms occupy the corners of the Li4 tetrahedron by means of “inner solvation”. The cuprate [Li{Me3SiNPMe2CH2–Cu–CH(SiMe2OLi)PMe2NSiMe3}]2 (4) has been obtained from the known [LiCH2PMe2NSiMe3]4 and copper(I) iodide in the presence of silicon grease (-OSiMe2-)n in diethylether, forming colorless oxygen and moisture sensitive crystals. 4 crystallizes in the triclinic space group P 1 with one formula unit per unit cell. Lattice dimensions at –50 °C: a = 1025.4(2), b = 1145.5(2), c = 1261.0(2) pm, α = 65.19(1)°, β = 79.55(1)°, γ = 77.94(1)°, R1 = 0.039. 4 forms a centrosymmetric dimeric molecule with a central Li2O2 four-membered ring, the oxygen atoms of which are connected by -SiMe2- bridges with the cuprate fragment > CH–Cu–CH2-.

Nanostructured SiC/BN/C ceramics derived from mixtures of B3N3H6 and [HSi(Me)C≡C]n

Abstract Three borazine-modified polycarbosilanes were synthesized by reaction of poly[1,2-ethynediyl-(methylsilylene)], [HSi(Me)C≡C]n, and borazine, B3N3H6 using different stoichiometries. The polymeric precursors were transformed into inorganic Si–B–C–N materials by solid state thermolysis at 1400°C each in 88% yield. High temperature thermogravimetric analysis in an argon atmosphere showed no substantial decomposition reactions below 1800°C. Crystallization behavior up to 2000°C in a nitrogen atmosphere was studied by post-thermolysis heat treatment of as-obtained ceramics at various temperatures and subsequent examination of samples at room temperature using powder X-ray diffraction. Ceramics annealed at 1800°C were additionally characterized by transmission electron microscopy.

Thermal evolution of free volumes and of crystallization in amorphous Si–B–C–N ceramics

Abstract We report on the crystallization behavior of initially amorphous precursor-derived Si3B1C4.3N2 ceramics, making use of macroscopic and atomic scale investigation techniques. As derived from our kinetic studies upon heating, the following temperature sequence of solid state processes is observed with approximately the same kinetic time constants: after an initial densification in the amorphous state (1673K), crystallization occurs (1873K) with subsequent strain relaxation and disappearance of nanovoids (2073K). This behavior indicates an increasing activation enthalpy of the atomic mechanisms giving rise to these processes when the annealing temperature is increased. The nanovoids may be located in the turbostratic BNCx layers as derived from positron annihilation studies. For grain growth at high temperatures an activation energy of HG=3.9eV is derived.