NiO particles derived from the thermal annealing of nickel layered hydroxyacetate precursors at different temperatures exhibit clear size variations, with magnetic properties evolving systematically as a function of particle size.

The sintering behavior and the thermal stability of different nanodiamond powders, produced either by detonation or by high-pressure high-temperature (HPHT) synthesis, is studied in situ during spark plasma sintering at 5 GPa. A portable high pressure spark plasma sintering device allowed the transformation of nanodiamonds during the sintering process to be followed by synchrotron radiation X-ray diffraction, in order to determine the optimal sintering conditions and investigate the impact of the sample environment. The powders were also characterized ex situ using advanced bulk and surface techniques to evaluate how the sp2 carbon shell, agglomeration state, and surface chemistry affect sintering while preserving the diamond structure and particle size. A purified and deagglomerated detonated powder (5 nm) and a nanodiamond powder (<30 nm) produced by HPHT synthesis appeared to be the most stable. Sintered nanodiamonds were produced at 5 GPa and temperatures below 2000 °C without using any binders, retaining more than 95% of the diamond phase and limiting grain growth. This study opens up encouraging prospects for the production of binder-free sintered nanodiamonds under pressure and temperature conditions that are less extreme than those currently used.

Nano-structured MgAl2O4 spinel ceramics were fabricated by High-Pressure Spark Plasma Sintering (HP-SPS) up to 5 GPa. High-pressure compaction creates green bodies with fine pores and improved density, enhancing the sintering process. Applying a pressure of 1.5 GPa reduces sintering temperatures by 500 degrees C, resulting in dense, transparent ceramics without grain coarsening. The ceramics show increased hardness due to their nano-scale grain structure, though some optical losses at shorter wavelengths are observed due to stress-induced crystalline defects.

Boron carbide (B4+delta C) possesses a large potential as a structural material owing to its lightness, refractory character, and outstanding mechanical properties. However, its large-scale industrialization is set back by its tendency to amorphize when subjected to an external stress. In the present work, we design a path toward nanostructured boron carbide with greatly enhanced hardness and resistance to amorphization. The reaction pathway consists of triggering an isomorphic transformation of covalent nanocrystals of Na1-x B5-x C1+x (x = 0.18) produced in molten salts. The resulting 10 nm B4.1C nanocrystals exhibit a 4-fold decrease of size compared to previous works. Solid-state 11B and 13C NMR coupled to density functional theory (DFT) reveal that the boron carbide nanocrystals are made of a complex mixture of atomic configurations, which are located at the covalent structural chains between B11C icosahedral building units. These nanocrystals are combined with a spark plasma-sintering-derived method operated at high pressure. This yields full densification while maintaining the particle size. The nanoscaled grains and high density of grain boundaries provide the resulting nanostructured bodies with significantly enhanced hardness and resistance to amorphization, thus delivering a superhard material.

The high-pressure behaviour of Si0.8Ge0.2 alloy is explored using in situ Raman spectroscopy, X-ray dif-fraction techniques and density functional theory (DFT) simulations. High pressure experiments revealed a pressure-induced transition from the stable cubic semiconducting phase (dc-Si0.8Ge0.2) to the tetragonal beta-tin metallic phase (beta-Si0.8Ge0.2) during compression. This sluggish transition is significantly accelerated at moderate temperature (< 300 degrees C). Upon decompression, successive transitions towards metastable phases are observed. A first transition from the metallic beta-Si0.8Ge0.2 toward the rhombohedral r8-Si0.8Ge0.2 phase is observed at 10.3 GPa followed by a partial transition to the body-centered cubic bc8-Si0.8Ge0.2 phase at 2.2 GPa. After releasing the pressure, r8 and bc8 phases coexist at ambient conditions. This transition pathway is similar to that followed by pure silicon and is consistent with the ab initio enthalpy calculations. This phase transition sequence is confirmed by in situ Raman spectroscopy, where signatures of r8 and bc8 phases are observed in the Raman spectra at decompression. An ab initio simulation method is proposed to assign the Raman spectrum of Si0.8Ge0.2 alloy using group theory and projection operators. The exploration of metastable states in these alloys is of major interest both in terms of applications (e.g. optoelectronics) and from a fundamental point of view to better understand the effects of alloying on the physical properties (e.g. vibrational).(c) 2023 Elsevier B.V. All rights reserved.

We investigated the effect of very high pressure on the sintering temperature, phase transition and the grain growth during Spark Plasma Sintering (SPS) of a 15 nm TiO2 nanopowder. Using in situ synchrotron X-ray diffraction during sintering at 1.5 and 3.5 GPa, we followed the evolution of the crystalline phases and the crystallite size as a function of temperature. In comparison, in the laboratory, SPS experiments were performed on two original facilities: A Paris-Edinburgh press and a high-pressure module adapted to standard SPS equip-ment. We studied the effect of the pressure on the sintering in the range 76 MPa to 3.5 GPa. We have shown that highly dense nanostructured ceramics can be prepared under very high pressure at low sintering temperatures. At 1 GPa, we limited the grain growth to an average size of 233 nm by heating at only 560 degrees C, and achieved a relative density of 98 %.

We report the effect of a polyol-mediated annealing on nickel ferrite nanoparticles. By combining X-ray fluorescence spectroscopy, X-ray diffraction, and 57Fe Mössbauer spectrometry, we showed that whereas the as-prepared nanoparticles (NFO) are stoichiometric, the annealed ones (a-NFO) are not, since Ni0-based crystals precipitate. Nickel depletion from the spinel lattice and reduction in the polyol solvent are accompanied with an important cation migration. Indeed, thanks to Mössbauer hyperfine structure analysis, we evidenced that the cation distribution in NFO departs from the thermodynamically stable inverse spinel structure with a concentration of tetrahedrally coordinated Ni2+ of 20 wt-% (A sites). After annealing, and nickel demixing, originated very probably from the A sites of NFO lattice, the spinel phase accommodates with cation and anion vacancies, leading to the (Fe3+ 0.84□0.16)A[Ni2+ 0.80Fe3+ 1.16□0.04]BO4-0.20 formula, meaning that the applied polyol-mediated treatment is not so trivial.

CoO submicrometer-sized pseudo-single crystals were produced in polyol thanks to an oriented aggregation crystal growth driven by the polyol molecules themselves.

Sintering under pressure by means of the spark plasma sintering (SPS) technique is a common route to reduce the sintering temperature and to achieve ceramics with a fine-grained microstructure. In this work, high-density bulk TiO2 was sintered by high pressure SPS. It is shown that by applying high pressure during the SPS process (76 to 400 MPa), densification and phase transition start at lower temperature and are accelerated. Thus, it is possible to dissociate the two densification steps (anatase then rutile) and the transition phase during the sintering cycle. Regardless of the applied pressure, grain growth occurs during the final stage of the sintering process. However, twinning of the grains induced by the phase transition is enhanced under high pressure resulting in a reduction in the crystallite size.

The evolution of the exchange bias at room temperature is studied between a nanopowder and its ceramic counterpart.

Fe-glycolate wires with micrometer-scale lengths can be synthesized by the polyol process. Although the as-produced wires are in the paramagnetic state at room temperature, they are transformed into ferrimagnetic iron oxides and ferromagnetic metallic iron wires by reductive annealing. The shape of the wires is unchanged by reductive annealing, and it is possible to control the magnetic properties of the resulting wire-shaped ferri/ferromagnets by adjusting the annealing conditions. Consequently, the reductive annealing of polyol-derived Fe-glycolate wires is an effective material-processing route for the production of magnetic wires.