Electrical properties of monolithic, amorphous SiCxNyHz-ceramics derived from 1,3,5-trimethyl-1,3,5-trivinyicyclotrisilazane via polymer pyrolysis were investigated from room temperature to 400 degreesC using impedance spectroscopy. Depending on the pyrolysis temperature T-p the d.c. conductivity varies up to 8 orders of magnitude. The temperature dependence of samples pyrolysed at low temperatures (T-p = 700-1200 degreesC) follows a Mott law, whereas samples pyrolysed at high temperature (T-p = 1400 degreesC) show an Arrhenius dependence. Structural changes during pyrolysis were characterized by solid state magic angle spinning nuclear magnetic resonance spectroscopy, Raman spectroscopy and X-ray diffractometry. NMR and especially Raman measurements indicate the formation of sp(2)-carbon atoms, which rearrange towards graphitic-like domains with increasing pyrolysis temperature. This observation can explain the related increase of the d.c.-conductivity. (C) 2002 Elsevier Science Ltd. All rights reserved.

Polyvinylsilazane, as a precursor for Si‐C‐N ceramics, was prepared by ammonolysis of functionalized chlorosilanes. Pyrolysis under inert atmospheres at Tp= 1000°C led to an amorphous Si‐C‐N‐(H) ceramic. Further heat treatment caused the transformation to the thermodynamically stable crystalline phase assemblage. The structural changes, especially those of the excess carbon, were studied by characterizing the solid intermediates via solid‐state magic angle spinning nuclear magnetic resonance spectroscopy. Moreover, Raman spectroscopy, electron spin resonance spectroscopy, and chemical analysis were used. Based on these methods, a comprehensive picture of the formation and behavior of the free‐carbon phase present in polymer‐derived ceramics was obtained.

The excess carbon of various polysilazane precursors with varying carbon contents was investigated using 13C MAS NMR, Raman and ESR spectroscopies, and microwave conductivity measurements. Microstructure characterization was investigated using TEM. The collected data were compared with those from a previous study on the same precursor. This study focused on the distribution and appearance of the free‐carbon phase. Although the spectroscopic techniques showed no differences in the structure of the free‐carbon phase, a clear distinction between the various precursors was found using microwave conductivity and high‐resolution TEM imaging.

Processing of two precursor-derived SiCN ceramic monoliths was performed employing different liquid polymers, which were obtained by ammonolysis of functionalized chlorosilanes. Preparation of monolithic samples was performed by mixing liquid polysilazane with cross-linked SiCN-powder particles, derived from the same precursors by heat treatment at 300 °C, and subsequent annealing upon pyrolysis at temperatures exceeding 1000 °C to initiate crystallization. Characterization of the polymer-derived ceramics was performed after pyrolysis at 1000 °C and, in particular, after annealing at temperatures ranging from 1400 to 1540 °C. Transmission electron microscopy was conducted in order to study the devitrification/thermal stability of the corresponding bulk SiCN glasses. Depending on the functionalities of the SiCN precursor and the processing conditions, different microstructures were obtained. The material prepared from precursor A showed crystallization of large α-Si3N4 grains within the overall homogeneous amorphous bulk material after exposure at 1540 °C for 6 h in nitrogen atmosphere. In contrast, the ceramic monoliths derived from SiCN precursor B remained completely amorphous, with no indication of local nucleation or crystallization. It is thought that devitrification of these polymer-derived glasses is promoted by local rearrangements of the glass network within the amorphous bulk. In addition, the role of the excess free carbon, commonly present in polymer-derived SiCN ceramics,­on the thermal stability is discussed. Copyright © 2001 John Wiley & Sons, Ltd.

Different polymeric precursors with varying carbon contents were prepared by ammonolysis of functionalised chlorosilanes. Pyrolysis under inert atmospheres at 1000 °C led to amorphous Si–C–N–(H) ceramics. Further heat treatment caused the transformation into the thermodynamically stable crystalline phase assemblage. The structural changes, especially those of the excess carbon, were studied by characterising the solid intermediates via solid state magic angle spinning (MAS) nuclear magnetic resonance spectroscopy (NMR). In addition, Raman spectroscopy, electron spin resonance spectroscopy (ESR), microwave conductivity measurements and chemical analysis were employed. Combination of all these methods provides a comprehensive picture of the formation and of the behaviour of the free-carbon phase present in the polymer-derived ceramics.

We used single crystals of [A(+)(C6H5)(4)](2)C60-B- as an ideal model system to determine accurately principal values and principal axes of the g-tensor of C-60 mono radical anion embeded in a crystal field. The g-tensor corresponds to the (static) Jahn-Teller distortion of C-60(-) In contrast to the Jahn-Teller distortion of isolated C-60(-) With D-5d or D-3d symmetry the crystal field stabilizes the D-2h, symmetry. Extending DSC, NMR and ESR to temperatures above 300 K we found the rotational ordering transition of C-60(-) in these salts. Both rotational ordering of C-60 and the transition from static to dynamic Jahn-Teller effect depend on the counterions A(+)(C6H5)(4).

Polymeric precursors with tailored structures were prepared from functionalised chlorosilanes. Pyrolysis under inert atmospheres led to amorphous Si–C–N–(H) ceramics at 1000°C. Further heat treatment caused the transformation into the thermodynamically stable crystalline phase assemblage. The structural changes associated with crosslinking, pyrolysis and crystallisation were studied by characterising the solid intermediates between 300 and 1600°C applying 29Si and 13C solid state nuclear magnetic resonance (NMR) spectroscopy. In addition, Fourier transformed infrared (FTIR) spectroscopy, X-ray diffraction (XRD), thermoanalytical techniques and density measurements were employed. The combination of these methods and the use of polymeric precursors with tailored structures pointed to a correlation of the polymer architecture with the structure of the amorphous ceramic material.

Thermal stability has been recognized as a key aspect with respect to the development and application of polymer derived materials. In this work, polymer precursors, e.g., polysilazanes with defined structure were synthesized starting from functionalized chlorosilanes. Monolithic materials were prepared and heat treated at various temperatures in order to initiate crystallization processes. Besides structural changes within the amorphous structure, surface crystallization is frequently observed in this type of materials. Thermodynamic arguments can be used to rationalize the surface crystallization, the transition from a ternary towards a binary system being an important factor.

Different liquid polymers in the system SiCN with tailored structures were prepared by ammonolysis from functionalized chlorosilanes. Crosslinking to an unmeltable polymer with initiators at low temperatures and subsequent ceramization were studied applying Si-29 solid-state nuclear magnetic resonance (NMR) spectroscopy in combination with Fourier transformed infrared (FTIR) spectroscopy and thermoanalytical techniques.Microstructure development, in particular, the devitrification of the corresponding bulk polymer-derived SiCN glasses was investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Preparation of monolithic samples was performed by mixing liquid polysilazane with SiCN-powder particles, derived from the same precursors by heat treatment at 300 degrees C, and subsequent annealing at temperatures exceeding 1000 degrees C to initiate crystallization. Depending on the functionalities of the SiCN-precursor and the processing conditions, different microstructures were obtained.The material prepared from the HVNG precursor revealed a homogeneous amorphous micro structure with only a small fraction of crystallized spherical inclusions after exposure at 1540 degrees C: for 6 h in nitrogen atmosphere. In contrast, investigating ceramic monoliths derived from another SiCN precursor, a different crystallization sequence was observed. The material derived from the HPS precursor showed crystallization of large alpha-Si3N4 grains within the bulk. As will be discussed in detail, devitrification of these polymer-derived glasses is promoted by local rearrangements and possible phase separations within the amorphous bulk. Moreover, local decomposition and residual porosity can affect the crystallization behavior, which strongly differs depending on the polymer employed.In addition to the crystallization phenomena observed, different oxidation response was monitored for the two SiCN ceramics discussed here. Moreover, fracture strength and hardness data were recorded, which, however, did not substantially differ between the polymer-derived ceramics investigated. (C) 1999 Elsevier Science S.A. All rights reserved.

Thermal stability has been recognized as a key aspect with respect to the development and application of polymer derived materials. In this work, polymer precursors, e.g., polysilazanes with defined structure were synthesized starting from functionalized chlorosilanes. Monolithic materials were prepared and heat treated at various temperatures in order to initiate crystallization processes. Besides structural changes within the amorphous structure, surface crystallization is frequently observed in this type of materials. Thermodynamic arguments can be used to rationalize the surface crystallization, the transition from a ternary towards a binary system being an important factor.