Several boron-modified polysilazanes of general type {B[C2H4Si(R)NH]3} n (C2H4 = CHCH3 or CH2CH2) were synthesized and their thermal behavior studied. In contrast to the known derivatives with R = alkyl or aryl, we describe ceramic precursors in which the bulky moieties R are substituted with lower weight groups and/or reactive entities. Reactive units enable further cross-linking of the polymeric framework and therefore minimize depolymerization during ceramization. The polymer-to-ceramic conversion of all synthesized polymers was monitored by thermogravimetric analysis. Both low molecular weight substituents and/or cross-linking units increase the ceramic yield from 50% (R = CH3) to 83−88%. High-temperature thermogravimetric analysis in an inert gas atmosphere indicates the ceramics obtained are stable up to ∼2000 °C. XRD studies of the fully amorphous materials point out that, with increasing temperature, formation of α-SiC or α-SiC/β-Si3N4 crystalline phases occurs at 1550−1750 °C, depending on the material's composition. The resistance of these novel materials toward oxidative attack was investigated by TGA in air up to 1700 °C and SEM/EDX, indicating that the materials efficiently self-protect toward oxidation.

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N 4Àx units with x = 0, 1, 2). In addition, a considerable amount of hydrogen is present even at this temperature. The NMR studies have further shown that above 1700°C the amorphous ceramic demixes, along with the formation of crystalline silicon nitride and silicon carbide. Likewise, structural changes for BN domains have been registered that are attributed to the formation of turbostratic BN(C) interface layers. In summary, the present study has demonstrated that the combination of multinuclear solid-state NMR and FT IR spectroscopy is a powerful method to probe the thermolytic preparation of ternary and quaternary ceramic materials.

The synthesis, detailed spectroscopic characterization, polymer-to-ceramic conversion, and high-temperature behavior of a new class of polymeric precursors for Si-B-C-N composites are discussed. The title compoundsCH 2 -CH 2 ; 5a: R 1 , R 2 ) H; 5b: R 1 ) H, R 2 ) CH 3 ; 5c: R 1 , R 2 ) CH 3 ) were designed especially for the preparation of ceramic films and fiber-reinforced ceramic composite matrixes. They are obtained in quantitative yields by the reaction of oligovinylsilazane [(H 2 CdCH)SiH-NH] n (4) with tris(hydridosilylethyl)boranes of general type B(C 2 H 4 SiHR 1 R 2 ) 3 (C 2 H 4 ) CHCH 3 , CH 2 CH 2 ; 3a: R 1 , R 2 ) H; 3b: R 1 ) H, R 2 ) CH 3 ; 3c: R 1 , R 2 ) CH 3 ) in a thermally induced hydrosilylation reaction without catalyst and/or solvent and without the formation of byproducts. Ceramic yields are 83% for 5a, 82% for 5b, and 63% for 5c as shown by thermogravimetric analysis (TGA). High-temperature TGA of the as-obtained amorphous ceramic materials, carried out in an argon atmosphere, reveals a thermal stability toward degradation of the 5b-derived material 6b up to 2000 °C. In contrast, the 6a material, which was obtained from 5a, decomposes around 1850 °C. The least stable is the 6c ceramic, which decomposes at 1450 °C. The microstructure development of 6a-6c was investigated in the temperature range of 1400-2000 °C by X-ray diffraction (XRD), indicating that preferentially crystalline R-silicon carbide is formed at 1700 °C for 6a, 1500 °C for 6b, and 1600 °C for 6c. In addition, there are less intensive reflections observed in the XRD patterns of 6a, caused by the formation of β-silicon nitride.

The preparation of silicon nitride-and carbidebased ceramics by solid-state thermolysis of polysilazanes and polysilylcarbodiimides is described. Results on the ceramization of the preceramic compounds and the architecture of the corresponding amorphous states obtained by spectroscopic means and by X-ray and neutron scattering are reviewed. Fundamental correlations between the composition and structure of the preceramic compounds and the architecture of the amorphous state are revealed. Furthermore, the crystallization behavior of the amorphous precursor-derived Si-C-N ceramics is treated. Moreover, the influence of boron on the thermal stability of the amorphous state is described. The high-temperature behavior of these Si-B-C-N solids can be correlated with their phase composition. Ceramic materials with compositions located close to the three-phase equilibrium SiC BN C exhibit a high temperature stability up to 2000°C. This effect is accompanied by the formation of a metastable solid consisting of Si 3 N 4 and SiC nanocrystals that are embedded in a turbostratic B-C-N matrix phase. Based on thermodynamic considerations, a model for the high-temperature stability effect is proposed.

In atomic layer deposition (ALD), film thickness control by counting the number of deposition sequences is poor in the initial, nonlinear growth region. We studied the growth of TiN films formed by sequentially controlled reaction of TiCl4 and NH3 on thermal SiO2 during the transient, nonlinear period. Using low-energy ion scattering and Rutherford backscattering spectroscopy analysis, we have found that a three-dimensional growth of islands characterizes the ALD TiN growth on SiO2. Growth at different temperatures (350 °C and 400 °C) affects the extent of the transient region and the rapid closure of the film. At 400 °C, a reduced growth inhibition and an earlier start of three-dimensional growth of islands results in film closure at about 100 cycles, corresponding to a TiN thickness of 24±3 Å. At 350 °C the minimum thickness at which the TiN layer becomes continuous is 34±3 Å, deposited with 150 cycles.

Anhydroerythritol (AnEryt) shares some of its ligand properties with furanosides and furanoses. Its bonding to silicon centers of coordination number four, five, and six was studied by X-ray and NMR methods, and compared to silicon bonding of related compounds. Diphenyl(cycloalkylenedioxy)silanes show various degrees of oligomerization depending on the diol component involved. For example, Ph(2)Si(cis-ChxdH(-2)) (1) and Ph(2)Si[(R,R)-trans-ChxdH(-2))] (2; Chxd = cyclohexanediol) are dimeric, Ph(2)Si(AnErytH(-2)) (3) is monomeric, and Ph(2)Si(L-AnThreH(-2)) (4; AnThre = anhydrothreitol) is trimeric both in the solid state and in solution. Ph(2)Si(cis-CptdH(-2)) (5) (Cptd = cyclopentanediol) is monomeric in solution but dimerizes on crystallization. Si(AnErytH(-2))(2) (6) and Si(cis-CptdH(-2))(2) (7) are monomeric spiro compounds in solution but are pentacoordinate dimers in the crystalline state. Pentacoordinate silicate ions are found in A[Si(OH)(AnErytH(-2))(2)] (A = Na, 8 a; Rb, 8 b; Cs, 8 c). Related compounds are formed by substitution of the hydroxo by a phenyl ligand. K[SiPh(AnErytH(-2))(2)]1/2 MeOH (9) is a prototypical example as it shows the two most significant isomers in one crystal structure: the syn/anti and the anti/anti form (syn and anti define the oxolane ring orientation close to, or apart from, the monodentate ligand, respectively). syn/anti Isomerism generally rules the appearance of the NMR spectra of pentacoordinate silicates of furanos(id)e ligands. NMR spectroscopic data are presented for various pentacoordinate bis(diolato)silicates of adenosine, cytidine, methyl-beta-D-ribofuranoside, and ribose. In even more basic solutions, hexacoordinate silicates are enriched. Preliminary X-ray analyses are presented for Cs(2)[Si(CydH(-2))(3)] 21.5 H(2)O (10) and Cs(2)[Si(cis-InsH(-3))] cis-Ins8 H(2)O (11) (Cyd = cytidine, Ins = inositol).

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