Günter Motz

Structural characterisation of silicon carbonitride ceramics derived from polymeric precursors

Abstract 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 29 Si and 13 C 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.

Robust Heterogeneous Nickel Catalysts with Tailored Porosity for the Selective Hydrogenolysis of Aryl Ethers

SiC materials with tailored porosity and integrated Ni NPs (Ni@SiC) were synthesized via microphase separation of polycarbosilane‐block‐polyethylene followed by its pyrolysis. Changing the length of organic block allowed the synthesis of micro, meso and hierarchical Ni@SiC materials which were characterized by PXRD, TEM, TGA, and nitrogen physisorption. Selective hydrogenolysis of aryl ethers mimicking the most abundant linkages of lignin was achieved in water avoiding the possible hydrogenation of aromatic rings.

Characterisation of the free-carbon phase in precursor-derived SiCN ceramics

Abstract 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.

The synthesis of pyrroles via acceptorless dehydrogenative condensation of secondary alcohols and 1,2-amino alcohols mediated by a robust and reusable catalyst based on nanometer-sized iridium particles

Pyrroles are important compounds with several applications in medicine and material science. They can be synthesized sustainably from secondary alcohols and amino alcohols. Hydrogen and water are liberated in the course of this reaction. Here, we present that this sustainable catalytic pyrrole synthesis can be mediated efficiently by a novel iridium nanoparticle catalyst. The catalyst synthesis starts from molecular precursors, an N-ligand stabilized Ir complex and a commercially available polysilazane. The generation of nanometer-sized iridium particles was achieved (due to the presence of N atoms in the support). The robust nature of the support allows reuse of the catalyst. The scope of the reaction was verified by the synthesis of 23 pyrrole derivatives (up to 93% isolated yield). Thus, an attractive functional group tolerance (e.g. amines and olefins) could be observed. Commercially available heterogeneous Ir catalysts are inefficient in this pyrrole synthesis and extremely limited in terms of reusability.

Copper-containing SiCN precursor ceramics (Cu@SiCN) as selective hydrocarbon oxidation catalysts using air as an oxidant.

A molecular approach to metal-containing ceramics and their application as selective heterogeneous oxidation catalysts is presented. The aminopyridinato copper complex [Cu(2)(Ap(TMS))(2)] (Ap(TMS)H=(4-methylpyridin-2-yl)trimethylsilanylamine) reacts with poly(organosilazanes) via aminopyridine elimination, as shown for the commercially available ceramic precursor HTT 1800. The reaction was studied by (1)H and (13)C NMR spectroscopy. The liberation of the free, protonated ligand Ap(TMS)H is indicative of the copper polycarbosilazane binding. Crosslinking of the copper-modified poly(organosilazane) and subsequent pyrolysis lead to the copper-containing ceramics. The copper is reduced to copper metal during the pyrolysis step up to 1000 degrees C, as observed by solid-state (65)Cu NMR spectroscopy, SEM images, and energy-dispersive spectroscopy (EDS). Powder diffraction experiments verified the presence of crystalline copper. All Cu@SiCN ceramics show catalytic activity towards the oxidation of cycloalkanes using air as oxidant. The selectivity of the reaction increases with increasing copper content. The catalysts are recyclable. This study proves the feasibility of this molecular approach to metal-containing SiCN precursor ceramics by using silylaminopyridinato complexes. Furthermore, the catalytic results confirm the applicability of this new class of metal-containing ceramics as catalysts.

The generation of palladium silicide nanoalloy particles in a SiCN matrix and their catalytic applications

The synthesis, characterization and catalytic studies of single phase palladium silicide nanoalloy particles supported by a polymer derived, non-oxide SiCN matrix (Pd2Si@SiCN) are reported. Simultaneous chemical modification of a polyorganosilazane as well as its cross-linking was achieved by the use of an aminopyridinato palladium complex at room temperature. Cross-linking takes place with an evolution of hydrogen and increases the ceramic yield by the retention of carbon and nitrogen atoms. Liberation of ligand, as confirmed by 1H NMR spectroscopy, provides indirect evidence of the transfer of palladium to the nitrogen functions producing metal modified polyorganosilazane whose pyrolysis at 1100 °C under nitrogen atmosphere provides Pd2Si@SiCN. Powder X-ray diffraction (PXRD) studies confirmed the presence of the hexagonal Pd2Si phase in the amorphous SiCN matrix. The size of the particles formed seems to depend upon the nature of the solvent used in the cross-linking step. The amount of palladium complex added seems not to affect the size of particles formed but does increase their population density. Pd2Si@SiCN catalysts were found active for the hydrogenation of ketones. The selectivity of the reaction towards the alcoholic product remains very high. The conversion of the reaction however decreases both with increase in alkyl chain length as well as its branching at alpha carbon. The recyclable Pd2Si@SiCN could be a suitable choice for catalytic transformations under a harsh chemical environment and at higher temperatures.

Magnetoceramic nanocrystals from the bulk pyrolysis of novel hyperbranched polyferrocenyl(boro)carbosilanes

We report a novel strategy to synthesize nanostructured magnetoceramics via a direct bulk pyrolysis of hyperbranched polycarbosilanes without any assistance of nanoscaled templates. The hyperbranched polyferrocenyl(boro)carbosilanes (hb-PBCS) were synthesized via a convenient A2+B3 strategy. After the pyrolysis of hb-PBCS under a nitrogen atmosphere at 1100 °C, iron silicide nanocrystal enriched ceramics with a grain size of 20–60 nm were formed. The regular architecture and quite higher ceramic yield of polymeric precursors are superior in retaining iron and silicon that give a possibility to aggregate into regular nanocrystals during the bulk pyrolysis in comparison to their linear counterparts. Furthermore, if the mixed precursor of hb-PBCS with regular architecture and a silylene-diacetylene polymer with high ceramic yield was employed, a hierarchical structure of uniform nanorods with diameters of 20–30 nm and aspect ratios of ∼5 together with microspheres with diameters of 4–8 μm could be obtained under the same pyrolytic conditions. A vibrating sample magnetometer measurement demonstrated that the generated iron silicide nanocrystal enriched ceramics possessed weak ferromagnetism with an ultralow hysteresis loss.

Selective cross-linking of oligosilazanes to tailored meltable polysilazanes for the processing of ceramic SiCN fibres

An interesting alternative for the processing of non-oxide ceramic fibres at lower costs than that for the current commercially available fibre types was developed by modifying different commercially available liquid oligosilazanes (ML33 and HTT1800) into polysilazanes by selective cross-linking via the N–H and Si–H groups with tetra-n-butylammoniumfluoride (TBAF) as a catalyst. Termination of the reaction with calcium borohydride allows the processing of meltable solid polysilazanes (ML33S and HTTS) with tailored chemical and thermal properties, to fulfil the requirements for the melt spinning of mechanically stable and homogeneous polymeric fibres. The chemical and thermal stability of the polysilazanes ML33S and HTTS were investigated by using GPC, DSC and rheological measurements. These techniques indicate the dependency of the molecular weight and glass temperature on the catalytical cross-linking conditions. Polymers with up to ∼10 000 g mol−1 show glass–liquid transition (Tg) between 65 and 81 °C and viscoelasticity, which are essential properties for the melt-spinning process. The thermal stability of ML33S is ensured up to 220 °C. In contrast the thermal stability of HTTS is limited to 170 °C due to the presence of vinyl-groups. The viscoelastic behaviour of the polymer melts, measured by oscillatory rheometry, and the sufficient thermal stability allowed the continuous processing of stable green fibres by melt spinning in the temperature range of 110 to 130 °C. After fast electron beam irradiation curing of the green fibres and pyrolysis of continuous amorphous ceramic SiCN fibres from both ML33S and HTTS polysilazanes were successfully synthesized, while a defined Tg point influences the shape and the smoothness positively.

cBN particle filled SiCN precursor coatings

Abstract A polymer derived ceramic composite coating consisting of the polycarbosilazane ABSE (matrix) and cBN (filler) is developed. The final coating thickness is varied by adjusting the withdrawal speed, the viscosity of the solution and the pyrolysis temperature. The coatings exhibit excellent adhesion to the substrate confirmed by cross-cut and bending tests. The environmental barrier properties of the coatings were investigated with static, isothermal oxidation experiments testing coated and uncoated steel substrates. It was revealed that the weight gain during oxidation was strongly reduced by coating the substrates and hence the coatings offer the possibility to enhance the lifetime and performance of steel substrates under rough conditions.

Micro-/Mesoporous Platinum-SiCN Nanocomposite Catalysts (Pt@SiCN): From Design to Catalytic Applications.

The synthesis, characterization, and catalytic studies of platinum (Pt) nanoparticles (NPs) supported by a polymer-derived SiCN matrix are reported. In the first step and under mild conditions (110 °C), a block copolymer (BCP) based on hydroxyl-group-terminated linear polyethylene (PEOH) and a commercially available polysilazane (PSZ: HTT 1800) were synthesized. Afterwards, the BCP was microphase separated, modified with an aminopyridinato (Ap) ligand-stabilized Pt complex, and cross-linked. The green bodies thus obtained were pyrolyzed at 1000 °C under nitrogen and provided porous Pt@SiCN nanocomposite via decomposition of the PEOH block while Pt nanoparticles grew in situ within the SiCN matrix. Powder X-ray diffraction (PXRD) studies confirmed the presence of the cubic Pt phase in the amorphous SiCN matrix whereas transmission electron microscopy (TEM) measurements revealed homogeneously distributed Pt nanoparticles in the size of 0.9 to 1.9 nm. N2 sorption studies indicated the presence of micro- and mesopores. Pt@SiCN appears to be an active and robust catalyst in the hydrolysis of sodium borohydride under harsh conditions.