Boron-modified polysilylcarbodi-imides as precursors for Si-B-C-N ceramics: Synthesis, plastic-forming and high-temperature behavior

Abstract

The synthesis, by two different reaction pathways, of boron-modified polysilylcarbodi-imides of general type {B[C 2 H 4 Si(R)NCN] 3 } n (R = singly bonded organic ligand) and the plasticforming and the thermal behavior of these polymers are described. Compounds {B[C 2 H 4 Si(R)NCN] 3 } n [2a, R = H; 2b, R = CH 3 ; 2c, R = (NCN) 0.5 ] can be obtained by treatment of the vinyl-substituted polysilylcarbodi-imides [(H 2 C=CH)(R)SiNCN] n [1a, R = H; 1b, R = CH 3 ; 1c, R = (NCN) 0.5 ] with borane dimethylsulfide BH 3 ÁS(CH 3) 2. The polysilylcarbodi-imides 1a-1c themselves are accessible via the reaction of vinyl-substituted chlorosilanes (H 2 C=CH)-(R)SiCl 2 with cyanamide H 2 N-CN in the presence of pyridine or by a non-oxide sol-gel process of vinylated chlorosilanes and bis(trimethylsilyl)carbodi-imide, (H 3 C) 3 SiN=C=N-Si(CH 3) 3. In the second method for the synthesis of 2a-2c, hydroboration of vinylsubstituted chlorosilanes (H 2 C=CH)(R)SiCl 2 with borane dimethylsulfide, borane trimethylamide or borane triethylamide to yield the tris[(chlorosilyl)ethyl]boranes B[C 2 H 4 Si(R)Cl 2 ] 3 (3a, R = H; 3b, R = CH 3 ; 3c, R = Cl) is followed by treatment of the as-obtained compounds with bis(trimethylsilyl)carbodi-imide, which results in the formation of the hydroborated polysilylcarbodi-imides 2a-2c. The thermogravimetric behavior of the polymers 1a-1c and 2a-2c up to 2300°C is reported. It is shown that boron-modified polysilylcarbodi-imides are suitable precursors for the preparation of dense bulk ceramics. Therefore, the preparation of green bodies of the hydroborated polysilylcarbodiimides 2a-2c by plastic forming (PF) is described. A series of experiments points to the fact that the microstructure of the as-obtained ceramic monoliths obtained by subsequent thermolysis of the plastic-formed green bodies is strongly influenced by the conditions during plastic forming.

Synthesis and pyrolysis of novel polysilazane to SiBCN ceramic

“…1, indicates the formation of various covalent bonds. Asymmetric stretching bands of the N-H and Si-H units are observed at 3393 cm À1 , 1176 cm À1 , and 2156 cm À1 , respectively [17]. A strong and broad vibration band overlapping with C-H group appears at 1403-1408 cm À1 and at 1380 cm À1 .…”

Silicon-Based Polymer-Derived Ceramics: Synthesis Properties and Applications-A Review

“…[61][62][63][64][65][66][67] Another route for the synthesis of Si-B-C-N ceramics is by means of thermolysis of borazine-based oligosilazanes 2,6,[68][69][70][71][72][73] or boron-modified polysilylcarbodiimides. [74][75][76] Thus, a great variety of preceramic organosilicon polymers have been developed that could be used to produce ceramic materials with a wide range of compositions in the system Si-B-C-N-O and additional metallic constituents, such as Ti, Al and Zr. 77,78 In order to understand the materials properties of PDCs at a fundamental level, several modelling and computational studies have been reported, [79][80][81][82][83][84][85] not at least to derive a meaningful and understandable interpretation of the experimental data.…”

“…Polyborosilylcarbodiimides are another important class of precursors for SiBCN ceramics. These polymers can be successfully synthesized by the reaction of tris chlorosilyl-ethyleneboranes BC 2 H 4 SiRCl 2 3 with an excess of bistrimethylsilylcarbodiimide Scheme 9: 74,75,166 Unfortunately, the ceramics obtained from these polymers do not possess satisfactory ultra high-temperature properties, since the thermal degradation already starts at Tࢽ1500c C. Preliminary studies suggested that the degradation temperature strongly depends on the nitrogen content of the samples. 163 In order to reduce the amount of nitrogen, a dehydrocoupling reaction of trishydridosilyl-ethyleneboranes with cyanamide, H 2 N-CN was developed.…”

Al2O3–SiC composites prepared by warm pressing and sintering of an organosilicon polymer-coated alumina powder

“…Here the warm pressing is used as the means of shaping the pre-ceramic polymers by heating them to the temperature where particles of the polymer can be deformed by viscous flow, or above their melting point, and pressing them into a die of desired geometry. [1][2][3][4][5][6] After cooling down to ambient temperature a polymer "green body" is obtained. The method has been successfully applied for shaping of precursors of aluminium nitride, 1 silicon carbonitrides, [4][5][6] and silicon boron carbonitrides.…”

“…The method has been successfully applied for shaping of precursors of aluminium nitride, 1 silicon carbonitrides, [4][5][6] and silicon boron carbonitrides. 2,3,6 In most cases the warm pressed green bodies have much higher density than the bodies prepared by, e.g., cold isostatic pressing (CIP) of polymer powders. This is explained by plastic deformation of polymer particles so that the porosity in green body is partially eliminated by mutual sliding of polymer particles and by viscous flow.…”