Marcus Schwarz

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.

Tri-s-triazine derivatives. Part I. From trichloro-tri-s-triazine to graphitic C3N4 structuresPart II: Alkalicyamelurates M3[C6N7O3], M = Li, Na, K, Rb, Cs, manuscript in preparation.

The first detailed structural characterisation of a functionalised tri-s-triazine derivative, trichloro-tri-s-triazine, is reported, which is a promising starting material for numerous compounds including graphitic C3N4 phases. DFT calculations show that a C3N4 structure based on tri-s-triazine should exist and that it is ∼30 kJ mol−1 more stable than the previously reported C3N4 phase of lowest energy.

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.

NANOTUBES FORMED BY DETONATION OF C/N PRECURSORS

Individual carbon nanotubes, both unfilled and containing metal nanocrystals in a string-of-beads-like arrangement, as shown in the Figure, can be obtained reproducibly by detonative decomposition of a hydrogen-free C/N precursor, it is reported here. This method may challenge traditional techniques for the synthesis of carbon nanotubes, which are energy and hardware intensive.

Orthorhombic In2O3: A Metastable Polymorph of Indium Sesquioxide

The way is open for the physical and chemical characterization and single-crystal growth of the orthorhombic o′-In2O3 polymorph. Orthorhombic In2O3 is synthesized from rhombohedral corundum-type rh-In2O3 under moderately high-pressure and high-temperature conditions (8–9 GPa, 600–1100 °C) followed by recovery to ambient pressure and temperature. The crystal-structure data at ambient conditions confirm unambiguously the Rh2O3(II)-type structure.

Emulsion Processing and Size Control of Polymer‐derived Spherical Si/C/O Ceramic Particles

Abstract Ultrasonic or stirring agitation of silicon‐based pre‐ceramic polymer emulsions in water, followed by crosslinking and consecutive pyrolysis was used to produce fine disperse powders of Si/C/O ceramic particles with spherical morphology. Depending on the shear forces introduced during the formation of the emulsions the diameters of the ceramic spheres can be controlled. Nanosized spheres in the range of 50–600 nm are formed upon sonication of the emulsion with ultrasound, while micrometer‐sized particles in the range of 1–10 and 10–100 μm are obtained when the emulsion is prepared with a high‐performance homogenizer at 20,000 rpm or by magnetic stirring at 1000 rpm, respectively. Dynamic light scattering (DLS), scanning electron microscopy (SEM), simultaneous thermal analysis (TGA/DTA), powder X‐ray diffraction (XRD), transmission electron microscopy (TEM), nitrogen adsorption measurements (BET), infra red spectroscopy (FTIR) and 29Si as well as 13C solid‐state NMR investigations were applied to characterize the products. The novel precursor processing method is a general way to produce fine disperse ceramic powders for advanced ceramics and composites, which have spherical particle morphology, offering an alternative to the sol‐gel and vapor phase routes.

s-Triazine and tri-s-triazine based organic-inorganic hybrid gels prepared from chlorosilanes by exchange reactions

Hybrid polymers [(DeltaO3)4Si3]n and [(DeltaO3)SiMe]n (where Delta = C6N7 or C3N3) have been prepared by a novel sol-gel process based on exchange reactions of MeSiCl3 or SiCl4 with C6N7(OSiMe3)3 and C3N3(OSiMe3)3.

In situ high pressure high temperature experiments in multi-anvil assemblies with bixbyite-type In2 O3 and synthesis of corundum-type and orthorhombic In2 O3 polymorphs

Our in situ high pressure high temperature experiments in multi-anvil assemblies unambiguously evidence the stability of bixbyite-type c-In2O3 at 6 GPa from room temperature to ca. 600°C. At 5.5 GPa and ca. 1100°C, c-In2O3 reacts with free carbon from the amorphous Si‒B‒C‒N capsule being reduced to metallic indium. The material recovered from the ex situ multi-anvil experiment at 6 GPa and 1100°C using the Mo capsule is inhomogeneous, thereby its phase composition depends on the specimen position from the furnace midline that in turn is characterized by the inhomogeneous temperatures. In the midpoint of the furnace, at the highest temperature point, c-In2O3 completely transforms into a corundum-type rh-In2O3 polymorph that is recovered under ambient conditions, as confirmed by X-ray powder and electron diffraction and Raman spectroscopy. Transmission electron microscopic characterization indicates the growth of single crystals of corundum-type rh-In2O3 with an average crystal size of ∼3 μm in the specimen part away from the furnace midline. The automated electron diffraction tomography analysis and X-ray powder-diffraction point out at the possible formation of orthorhombic In2O3 polymorphs.

Formation and properties of rocksalt-type AlN and implications for high pressure phase relations in the system Si–Al–O–N

Pressure-dependent thermodynamic properties of the ambient and high pressure phases of aluminum nitride (w-AlN and rs-AlN) were calculated from first principles in order to determine their phase boundary in the p− T phase diagram. These predictions were checked by static HP/HT experiments, using a multianvil press and an Al/N/H precursor with low decomposition temperature as educt. The experimental data show that at temperatures between 1000 and 2000 K, the boundary line between the two phases is situated between 11 and 12 GPa, which is ∼1.3 GPa lower than the theoretical result and generally lower than previously assumed. The hardness of rs-AlN – measured for the first time – is ∼30 GPa (Knoop indenter at loads of 25–50 g), twice as hard as w-AlN. Shock wave recovery experiments on nano w-AlN allowed testing of the chemical and thermal stability of rs-AlN, and determination of its infrared absorption and 27Al NMR data. The shock wave technique will eventually enable the synthesis of larger amounts of rs-AlN, making it available for technological use. Finally, implications on the high pressure stability of phases in the Si–Al–O–N system are discussed in the light of thermoelastic properties of AlN.

Shock wave synthesis of aluminium nitride with rocksalt structure

The high pressure phase of aluminium nitride with rocksalt structure (rs) is a ceramic with high potential and a challenging material to investigate. The rs-AlN was synthesised and recovered by shock wave experiments using the flyer-plate method with multiple reflections at peak pressures between 15 and 43 GPa. Successful syntheses were carried out using AlN nanopowder with ambient pressure wurtzite structure (w-AlN) as starting material. The high pressure modification could, however, not be obtained when starting from submicron w-AlN. The recovery of rs-AlN is sensitive to the synthesis conditions as these influence the reconversion of rs-AlN to w-AlN.