Edwin Kroke

Synthesis of cubic silicon nitride

Silicon nitride (Si3N4) is used in a variety of important technological applications. The high fracture toughness, hardness and wear resistance of Si3N4-based ceramics are exploited in cutting tools and anti-friction bearings; in electronic applications, Si3N4 is used as an insulating, masking and passivating material. Two polymorphs of silicon nitride are known, both of hexagonal structure: α- and β-Si3N4. Here we report the synthesis of a third polymorph of silicon nitride, which has a cubic spinel structure. This new phase, c-Si3N4, is formed at pressures above 15 GPa and temperatures exceeding 2,000 K, yet persists metastably in air at ambient pressure to at least 700 K. First-principles calculations of the properties of this phase suggest that the hardness of c-Si3N4 should be comparable to that of the hardest known oxide (stishovite, a high-pressure phase of SiO2), and significantly greater than the hardness of the two hexagonal polymorphs.

Silazane derived ceramics and related materials

This review highlights the synthesis, processing and properties of non-oxide silicon-based ceramic materials derived from silazanes and polysilazanes. A comprehensive summary of the preparation of precursor compounds containing Si–N–Si units, including commercially available materials, is followed by the discussion of various processing techniques. The fabrication of dense bulk ceramics in the Si/E/C/N systems is reported which involves cross-linking of the polymeric ceramic precursor followed by a polymer-to-ceramic transformation step. The cross-linked precursor can be milled, compacted and pyrolysed to form dense, additive-free, amorphous silicon carbonitride monoliths or polycrystalline composites which withstand oxidation in air at 1600°C. Furthermore, an overview is given on the fabrication of silazane derived powders and coatings involving chemical vapour deposition (CVD) methods utilising volatile precursors. Fibre spinning and fibre properties, as well as other processing techniques like infiltration of preforms, the preparation of porous ceramics and joining are briefly discussed. A state of the art of the mechanical properties of polymer derived amorphous Si/C/N and Si/B/C/N ceramics with respect to hardness as well as high-temperature creep and oxidation resistance is summarised. Finally, some important aspects of industrial applications will be considered. The review is in part based on our own work related to the polysilazane derived ceramics, but will also cover a comprehensive state of the art including the published literature in this field.

Spinel‐Si3N4: Multi‐Anvil Press Synthesis and Structural Refinement

The third known polymorph of silicon nitride, which is cubic and was only recently discovered, has been prepared from two further, different precursors—Si2N2(NH) and a-Si3N4—in a high-pressure, high-temperature synthesis using multi-anvil presses. The synthesis and characterization of the products is described, which included a structural determination by Rietveld refinement of powder X-ray diffraction data. Spinel-type c-Si3N4 is significantly harder than the α and β phases and may possibly find applications as an ultrahard material.

Alkalicyamelurates, M3[C6N7O3].xH2O, M = Li, Na, K, Rb, Cs: UV-luminescent and thermally very stable ionic tri-s-triazine derivatives.

Cyamelurates are salts of cyameluric acid, a derivative of tri-s-triazine (1,3,4,6,7,9-hexaazacyclo[3.3.3]azine or s-heptazine). These compounds are thermally very stable and possess interesting structural and optical properties. Only very few tri-s-triazine derivatives have been reported in the literature. The water-soluble alkali cyamelurates were extensively characterized using NMR, FTIR, Raman, UV, luminescence spectroscopy and elemental analysis. In addition, the single crystal X-ray structure analyses of the four hydrates of lithium, sodium, potassium and rubidium cyamelurates (Li(3)[C(6)N(7)O(3)].6H(2)O; Na(3)[C(6)N(7)O(3)].4.5H(2)O; K(3)[C(6)N(7)O(3)].3H(2)O; Rb(3)[C(6)N(7)O(3)].3H(2)O) are presented. Thermogravimetric analysis shows that the dehydrated salts start to decompose at temperatures above 500 degrees C. The thermal stability does not depend on the cations which is in contrast to the analogous s-triazine salts, i.e. the alkali cyanurates M(3)[C(3)N(3)O(3)]. The photoluminescence spectra indicate a very strong solid state UV-emission with maxima between 280 and 400 nm.

Potassium melonate, K3[C6N7(NCN)3]·5H2O, and its potential use for the synthesis of graphite-like C3N4 materials

Melonates, Mx3/x[C6N7(NCN)3], contain the aromatic and planar tricyanamino-s-heptazine anion. Only one example, the potassium derivative, has been isolated in pure form so far, but it has never been characterised comprehensively. Potassium melonate is obtained from a reaction of the polymer melon, [C6N9H3]n, with KSCN. 13C NMR, FTIR, Raman, UV-vis and luminescence spectroscopy support the proposed molecular structure. The crystal structure of the hydrate K3[C6N7(NCN)3]·5H2O was determined by single crystal X-ray analysis (MoKα: Pbca (no. 161), a = 6.538(2), b = 24.033(6), c = 23.684(6) A and Z = 8). Thermogravimetric analysis shows that the dehydrated salt starts to decompose at temperatures above 500 °C. The [C6N7(NCN)3] unit is an ideal molecular building block for carbon(IV) nitrides, especially s-heptazine based graphite-like C3N4 networks.

Nano-structured metal-containing polymer precursors for high temperature non-oxide ceramics and ceramic fibers—syntheses, pyrolyses and properties

Nano-structured metal-containing ceramic polymer precursors have a potential for progress in the field of polymer-derived ceramics. In this paper the synthesis and characterization of nano-metallopolycarbosilanes (nMPCS) and their transformation into ceramic materials are reported. The formation of metal nano-particles via fast thermolysis of metal- containing compounds in polymer solution or melt previously developed by the authors was applied to preceramic polymers. Tetrabenzyltitanium, tetrabenzylzirconium, bis(cyclopentadienil)dichloride-titanium and zirconium as well as tetrachlorides of these metals and tetrakis(diethylamino)zirconium were used for the introduction of metal nano-particles. The products were characterized by thermo-gravimetric analysis (TGA), differential scanning calorimetry (DSC), differential-thermal analysis (DTA), gel-penetration (GP)-chromatography, X-ray diffraction (XRD), scanning electron microscopy (SEM) and other special analyses. The results of these studies may be useful for the fabrication of non-oxide ceramic fibers, interphase coatings and high temperature ceramic matrix composite (HT-CMC) matrices. Cp2ZrCl2 and Zr[N(C2H5)2]4 provided nano-particles with diameters of 2–4 nm and turned out to be the most suitable compounds for the preparation of ceramic matrices. Besides, they open a new way of oxygen-free curing. Coreless fibers were obtained from nZrPCS with up to 3 mass% of metal. Future investigations will be focused on optimization of the oligocarbosilanes used as starting materials and the development of less reactive polycarbosilane (PCS)–metal-containing compound (MCC) systems.

Synthesis and characterization of alkylene-bridged silsesquicarbodiimide hybrid xerogels

Abstract Hybrid polymers consisting of flexible organic chains within an inorganic silsesquicarbodiimide network of the type [(NCN) 1.5 Si-(CH 2 ) x -Si(NCN) 1.5 ] n (where x =2, 6, and 8) were prepared by mild sol–gel polycondensation reactions of bis(trichlorosilyl)alkanes and bis(trimethylsilyl)carbodiimide. The presence of the NCN groups in xerogel structures was identified by FTIR spectra. The composition and molecular structures were characterized by elemental analysis, solid-state 13 C CP MAS- and 29 Si CP MAS-NMR spectroscopies, and XRD. Scanning as well as transmission electron microscopies were used to examine the morphology of the xerogels. In addition, the pore structure of the materials was examined by the gas adsorption (BET) method and it is found that the surface area decreased with increasing length of the alkylene spacing group.

Synthesis of Monodisperse Spherical Silicon Dicarbodiimide Particles

Spherical particles of silicon dicarbodiimide (SiC2N4, see Figure) are synthesized here via the reaction of tetrachlorosilane (T) and bis (trimethylsilyl)carbodiimide (B) using pyridine as a catalyst and toluene or tetrahydrofuran as solvent. It is shown that the morphology of the thus obtained monodisperse particles strongly depends on the molar ratio of the reagents and the type and amount of solvent used.

High‐Pressure Syntheses of Novel Binary Nitrogen Compounds of Main Group Elements

The application of high-pressure methods in the search for novel materials usually requires additional effort compared to syntheses at ambient pressure. Depending on the desired p/T conditions different methods may be used. Special techniques and experimental apparatus such as shock waves, diamond anvil cells, and multianvil presses, which have been applied mainly by earth scientists and physicists in the past, are increasingly being applied by synthetic chemists and material scientists. A series of fascinating discoveries have been made recently as is demonstrated by three examples of binary nitrogen compounds: 1) Diazenides, compounds with N(2)(2-) ions, were obtained as single-phase products and structurally characterized for the first time. 2) At 11 GPa and 1800 K a phosphorus(V) nitride was prepared, which contains tetragonal PN(5) pyramids as a novel structural motif. 3) Macroscopic amounts of spinel silicon nitride were synthesized by shock-wave techniques, which allows the comprehensive characterization and possibly the implementation of this new hard material.

Synthesis of nanocrystalline aluminum–gallium nitride (Al xGa1− xN; x = 0.1 to 0.5) with oxide precursors via ammonolysis

Oxygen-containing precursor systems for the synthesis of mixed aluminum-gallium nitride (AlxGa1-xN with x = 0.1 to 0.5) through ammonolysis (heat treatment under ammonia) were evaluated. Three different precursor systems were studied: (i) aluminum isopropoxide (aluminum sec-butoxide)/gallium isopropoxide hydrolyzed with excess water and cross-linked with 1,6-hexanediol, (ii) aluminum-gallium hydroxide coprecipitated from aluminum-gallium nitrate solution, and (iii) spray-dried aluminum-gallium nitrate solutions. The specimens were heat-treated between 700 degreesC and 1100 degreesC and were characterized mainly by x-ray diffraction, nuclear magnetic resonance (NMR), and transmission electron microscopy (TEM). NMR was used to follow the conversion of oxygen to nitrogen bonds. TEM in combination with energy-dispersive x-ray spectroscopy was used to determine the solid-solution composition for separated particles. It is possible to synthesize a mixed hexagonal (AI,Ga)N with crystallite sizes in the range of similar to10 nm from all three precursor systems, but all products contained larger GaN crystals ranging from 20 nm (alkoxide-derived) to 200 nm (hydroxide-detived) and a fraction of untransformed Al-O bonds; e.g., (amorphous or gamma-phase) Al2O3.