3 Search Results

Multicomponent alloying has displayed extraordinary potential for producing exceptional structural and functional materials. However, the synthesis of single-phase, multi-principal covalent compounds remains a challenge. Here, we present a diffusion-controlled alloying strategy for the realization of covalent multi-principal transition metal carbides (MPTMCs) with a single face-centered cubic phase. The increased interfacial diffusion promoted by the addition of a nonstoichiometric compound leads to rapid formation of the single phase at much lower sintering temperature. Direct atomic-level observations via scanning transmission electron microscopy demonstrate that MPTMCs are composed of a single phase with a random distribution of all cations, which holds the keymore » to the unique combinations of improved fracture toughness, superior Vickers hardness, and extremely lower thermal diffusivity achieved in MPTMCs. The present discovery provides a promising approach toward the design and synthesis of next-generation high-performance materials.« less

Polycrystalline diamond has been successfully synthesized on silicon substrates at atmospheric pressure using a microwave capillary plasma chemical vapor deposition technique. The CH₄/Ar plasma was generated inside of quartz capillary tubes using 2.45 GHz microwave excitation without adding H2 into the deposition gas chemistry. Electronically excited species of CN, C₂, Ar, N₂, CH, H_β and H_α were observed in emission spectra. Raman measurements of deposited material indicate the formation of well-crystallized diamond, as evidenced by the sharp T_2g phonon at 1333 cm^-1 peak relative to the Raman features of graphitic carbon. Furthermore, field emission scanning electron microscopy (SEM) images revealmore » that, depending on the on growth conditions, the carbon microstructures of grown films exhibit “coral” and “cauliflower-like” morphologies or well-facetted diamond crystals with grain sizes ranging from 100 nm to 10 μm.« less

Motivated to explore the formation of novel extended carbon-nitrogen solids via well-defined molecular precursor pathways, we studied the chemical reactivity of highly pure phosphorous tricyanide, P(CN)₃, under conditions of high pressure at room temperature. Raman and infrared (IR) spectroscopic measurements reveal a series of phase transformations below 10 GPa, and several low-frequency vibrational modes are reported for the first time. Synchrotron powder Xray diffraction (PXRD) measurements taken during compression show that molecular P(CN)₃ is highly compressible with a bulk modulus of 10.0±0.3 GPa and polymerizes into an amorphous solid above ~10.0 GPa. Raman and infrared (IR) spectra, together with first-principlesmore » molecular-dynamics simulations, show that the amorphization transition is associated with polymerization of the cyanide groups into CN bonds with predominantly sp² character, similar to known carbon nitrides, resulting in a novel PCN polymeric phase, which is recoverable to ambient pressure.« less