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Abstract: Three SiBCN monoliths: carbon‐lean, ‐moderate, and ‐rich monoliths with the same Si/B/N mole ratio have been prepared by mechanical alloying followed by reactive hot pressing. The role of carbon on phase composition, microstructural evolution, and mechanical properties were investigated. XRD and TEM investigations showed that the as‐sintered monoliths contain several components: Si metal, SiC, t‐BN, and BN(C), strongly correlated with the carbon content. TEM results also revealed that BN(C) enwraps SiC grain forming capsule‐like structure. Raman, NMR, and HRTEM studies suggested that the structural evolution within the BN(C) areas includes the graphitization of amorphous carbon and the lateral growth of BN(C), accompanied by an increase in the defect concentration. The defects form within BN (0002) basal planes resulting from the disordered carbon imbedded, which in turn determines the morphological evolution of BN(C). The ill‐developed microstructures for carbon‐lean monoliths and the excessive carbon in carbon‐rich ones both deteriorate their mechanical properties and densities. [ABSTRACT FROM AUTHOR]

• Amorphous boron-rich Si 2 B 1 - 4 C 2 N monoliths were consolidated by high-pressure low-temperature sintering technique. • Amorphous boron-rich Si 2 B 1-4 C 2 N monoliths oxidize according to a classical parabolic rate law. • The increase in boron content weakens the oxidation resistance of amorphous boron-rich Si 2 B 1-4 C 2 N monoliths. • The formation of a dense and passivating oxide scale is primarily responsible for the high-temperature oxidation resistance. High-temperature oxidation behavior of dense amorphous boron-rich Si 2 B 1-4 C 2 N monoliths in flowing air was studied in detail. The oxidation products are those of amorphous silica and cristobalite that control the diffusion of oxygen inward. Oxygen diffusion leads to a dense and passivating scale, which acts as a barrier to oxygen. Amorphous boron-rich monoliths oxidize through a classical parabolic manner. The amorphous nature, three-layers structure of oxide scale and varying chemical bonds are primarily responsible for the oxidation resistance with respect to the nano monoliths. The increase in boron content is revealed to weaken the oxidation resistance of the amorphous Si 2 B 1-4 C 2 N monoliths. [ABSTRACT FROM AUTHOR]

The high-temperature oxidation behavior of various boron addition SiBCN monoliths has been explored. Boron addition promotes the formation of new B x C phase and growth of SiC gains. At the initial stage of oxidation, surface B x C and BN(C) are prior to SiC generating B 2 O 3 . As time progresses, SiC oxidizes to SiO 2 further reacted with B 2 O 3 forming stable SiO 2 -B 2 O 3 binary melt or borosilicate. Boron-enhanced SiC oxidation occurs immediately with the rapid consumption of BN(C), B x C and loss of B 2 O 3 from borosilicate decomposition, thereby promoting oxidation rate. Substantial SiC and BN(C) in amorphous SiO 2 for transition layer is the main structure feature. [ABSTRACT FROM AUTHOR]

The mixture of 5 wt% LaB 6 and mechanically alloyed Si-B-C-N powders was conventionally ball-milled and then compacted by spark plasma sintering at 1250–2000 °C and 50 MPa. Densification, microstructural evolution and mechanical properties of the as-prepared La/Si-B-C-N monoliths were investigated. At 1360 °C, sintering initiates coupled with rapid densification caused by particle rearrangement and plastic deformation. Grain-boundary sliding and atomic diffusion lead to further densification when sintering continues. Only SiC and La-containing BN(C) phases are detected in La/Si-B-C-N monoliths sintered at temperatures higher than 1900 °C. The monoliths sintered at 2000 °C show a high fracture toughness (4.6 MPa m −1/2 ) attributed mainly to lamellar La-containing BN(C) grains. [ABSTRACT FROM AUTHOR]

Si-B-C-N ceramic monoliths with LaB 6 additive were prepared by mechanical alloying followed by spark plasma sintering. The microstructures and mechanical properties were investigated compared to LaB 6 -free Si-B-C-N counterparts. After sintered at 1900 °C/50 MPa, the La/Si-B-C-N monoliths provide well-developed BN(C) flakes (≤100 nm in thickness) and improved fracture toughness, while the BN(C) ones show no lamellar microstructures in Si-B-C-N systems. The solid-solution of La in BN(C) grains promotes ordered arrangement of BN(C) along the (0002) plane causing in situ growth of BN(C) thus leading to an increase in toughness by 40% (from 5.6 to 7.8 MPa m −1/2 ). [ABSTRACT FROM AUTHOR]

Micro-macroporous hierarchical mordenite (MOR topology) zeolitic monoliths with ultrahigh mechanical stability (crushing strength: 38 MPa) were directly prepared by the one-pot hydrothermal synthesis involving an unconventional acidic hydrolysis route. The obtained monolith exhibited highly crystallized MOR structure with the surface area of 388 m 2 g −1 and pore volume of 0.2 cm 3 g −1 , same as its powder analog. Moreover, it is the first example of binder-free hierarchical zeolites with the high mechanical strength comparable to industrially available binder-shaped counterparts. The micro-shape of primary particles and macro-shape of monoliths could be controlled by varying the elemental synthetic conditions. The synthesis is an efficient, sustainable and environmentally friendly approach because it avoids the employment of sacrificial templates, binders and high-temperature calcination, not only saving energy and time, but also inhibiting waste release. Also importantly, the MOR monolith could be directly used as the shaped catalyst without additional post-mold treatments. Catalytic tests in solvent-free Friedel–Crafts benzylation reaction of benzene with benzyl alcohol showed that the MOR monolith was much more active than its powder and binder-shaped control catalysts. [ABSTRACT FROM AUTHOR]

• The oxidation process for amorphous Si2BC1-4N monoliths was controlled by a classical parabolic or linear manner. • Amorphous phase first undergoes oxidation attack followed by crystallization. • The nucleation and growth of nanocrystals can be promoted by the thermo-oxidative coupling. • The oxidation resistance of dense amorphous Si2BC1-4N monoliths is better than that of the nano monoliths. Dense amorphous Si 2 BC 1-4 N monoliths were subjected to aggressive environments at high-temperatures to explore the oxide scale microstructure and growth kinetics. The effects of high-pressure sintering and crystallization on oxidation resistance were studied in detail. Dense amorphous Si 2 BC 1-4 N monoliths oxidize according to a parabolic rate law or linear equation, depend on the carbon content. Oxidation first attacks at the outermost surfaces of amorphous phases, subsequently proceeds at the interfaces of the crystallized nuclei and neighboring amorphous phases. High-temperature oxidation induces nucleation and thermo-oxidative coupling promotes the nucleation and growth of the nanocrystals. [ABSTRACT FROM AUTHOR]