Nanostructuring and stabilization of metastable rock-salt ZnO: impact of high-pressure media and compression geometry

For complete and reproducible recovery of a high-pressure polymorph of zinc oxide, rock-salt ZnO (rs-ZnO), nanostructured wurtzite ZnO (w-ZnO) is typically used as a precursor for high-pressure synthesis. In the case of polycrystalline w-ZnO, only small amounts (less than 30 vol.%) of disordered/nanosized rs ZnO were occasionally observed in the recovered products. Here we report the conditions for the synthesis of single-phase rs ZnO from microcrystalline (40-50 µm) w ZnO powder at 7.7 GPa and 2000 K. The complete recovery of metastable rs-ZnO is possible only by using a multianvil apparatus for quasi-hydrostatic (triaxial) compression/decompression and pyrolytic boron nitride as a pressure medium. Single-phase rs ZnO was produced as colorless nanocrystalline well-sintered bulks with Vickers hardness up to 7 GPa - the record value for ZnO due to a fortunate combination of Hall-Petch nanostructuring effect and high intergranular purity. This unexpected phenomenon can be related to the mechanism of the direct and reverse phase transitions in ZnO, which requires "uniaxial" tensile deformation. Texture analysis of the recovered samples, as well as previous kinetic studies and ab initio simulation of strain-structure relationships, strongly support this model. Thus, 3-axial decompression is a more efficient tool – neglected until now – for reproducible recovery of high-pressure ZnO-based materials than the nature of the w-ZnO precursor and the presence of isostructural rock-salt phases

In Situ High-Pressure Synthesis of New Outstanding Light-Element Materials under Industrial P-T Range

International audienceHigh-pressure synthesis (which refers to pressure synthesis in the range of 1 to several GPa) adds a promising additional dimension for exploration of compounds that are inaccessible to traditional chemical methods and can lead to new industrially outstanding materials. It is nowadays a vast exciting field of industrial and academic research opening up new frontiers. In this context, an emerging and important methodology for the rapid exploration of composition-pressure-temperature-time space is the in situ method by synchrotron X-ray diffraction. This review introduces the latest advances of high-pressure devices that are adapted to X-ray diffraction in synchrotrons. It focuses particularly on the “large volume” presses (able to compress the volume above several mm3 to pressure higher than several GPa) designed for in situ exploration and that are suitable for discovering and scaling the stable or metastable compounds under “traditional” industrial pressure range (3–8 GPa). We illustrated the power of such methodology by (i) two classical examples of “reference” superhard high-pressure materials, diamond and cubic boron nitride c-BN; and (ii) recent successful in situ high-pressure syntheses of light-element compounds that allowed expanding the domain of possible application high-pressure materials toward solar optoelectronic and infra-red photonics. Finally, in the last section, we summarize some perspectives regarding the current challenges and future directions in which the field of in situ high-pressure synthesis in industrial pressure scale may have great breakthroughs in the next years

Nanostructuring and stabilization of metastable rock-salt ZnO: Impact of high-pressure media and compression geometry

For complete and reproducible recovery of a high-pressure polymorph of zinc oxide, rock-salt ZnO (rs-ZnO), nanostructured wurtzite ZnO (w-ZnO) is typically used as a precursor for high-pressure synthesis. In the case of polycrystalline w-ZnO, only small amounts (less than 30 vol.%) of disordered/nanosized rs ZnO were occasionally observed in the recovered products. Here we report the conditions for the synthesis of single-phase rs ZnO from microcrystalline (40-50 µm) w ZnO powder at 7.7 GPa and 2000 K. The complete recovery of metastable rs-ZnO is possible only by using a multianvil apparatus for quasi-hydrostatic (triaxial) compression/decompression and pyrolytic boron nitride as a pressure medium. Single-phase rs ZnO was produced as colorless nanocrystalline well-sintered bulks with Vickers hardness up to 7 GPa - the record value for ZnO due to a fortunate combination of Hall-Petch nanostructuring effect and high intergranular purity. This unexpected phenomenon can be related to the mechanism of the direct and reverse phase transitions in ZnO, which requires "uniaxial" tensile deformation. Texture analysis of the recovered samples, as well as previous kinetic studies and ab initio simulation of strain-structure relationships, strongly support this model. Thus, 3-axial decompression is a more efficient tool – neglected until now – for reproducible recovery of high-pressure ZnO-based materials than the nature of the w-ZnO precursor and the presence of isostructural rock-salt phases

Thermoelastic equation of state and melting of Mg metal at high pressure and high temperature

International audienceThe p-V-T equation of state of magnesium metal has been measured up to 20 GPa and 1500 K using both multianvil and opposite anvil techniques combined with synchrotron X-ray diffraction. To fit the experimental data, the model of Anderson-Grüneisen has been used with fixed parameter δ T. The 300-K bulk modulus of B 0 = 32.5(1) GPa and its first pressure derivative, B 0 ' = 3.73(2), have been obtained by fitting available data up to 20 GPa to Murnaghan equation of state. Thermal expansion at ambient pressure has been described using second order polynomial with coefficients a = 25(2)×10-6 K-1 and b = 9.4(4)×10-9 K-2. The parameter describing simultaneous pressure and temperature impact on thermal expansion coefficient (and, therefore, volume) is δ T = 1.5(5). The good agreement between fitted and experimental isobars has been achieved to relative volumes of 0.75. The Mg melting observed by X-ray diffraction and in situ electrical resistivity measurements confirms previous results and additionally confirms the p-T estimations in the vicinity of melting

Heat capacities of nanostructured wurtzite and rock salt ZnO: Challenges of ZnO nano-phase diagram

International audienceLow-temperature heat capacities (Cp) of nanostructured rock salt (rs-ZnO) and wurtzite (w-ZnO) polymorphs of zinc oxide were measured in the 2-315 K temperature range. No significant influence of nanostructuring on Cp of w-ZnO has been observed. The measured Cp of rock salt ZnO is lower than that of wurtzite ZnO below 100 K and is higher above this temperature. Using available thermodynamic data, we established that the equilibrium pressure between nanocrystalline w-ZnO and rs-ZnO is close to 4.6 GPa at 300 K (half as much as the onset pressure of direct phase transformation) and slightly changes with temperature up to 1000 K

Ice‐Templating: Integrative Ice Frozen Assembly to Tailor Pore Morphology of Energy Storage and Conversion Devices

International audienceIce‐templating, also known as directional freezing or freeze‐casting, features the tunability of microstructure, the wide applicability of functional nanomaterials, and the fabrication of multiscale well‐controlled biomimetic materials. Recently, integrating ice‐templating with other materials’ processing technologies (such as, spraying, spinning, filtration, and hydrothermal), it has been investigated to tailor pore morphology of scaffolds for emerging applications. Such integration endows materials with various structures (cellular, dendritic, and lamellar) and dimensions (0D, 1D, 2D, and 3D), which opens up a new avenue for improving material properties and developing new materials. Herein, this review probes into the relationship of integrative ice frozen assembly with structure and describes the fundamental principles and synthesis strategies for preparing multi‐scale materials with complex biomimetic structures via ice‐templating. Focusing on ice crystal nucleation and growth, it summarizes the performance of ice‐templating in constructing pore geometries. Additionally, the review analyzes in depth the correlation between microstructure and macromorphology of final scaffolds, highlighting the application of integrative ice frozen assembly in electrochemical energy storage and conversion, and prospects for future research directions for this field

High-pressure synthesis of superhard and ultrahard materials

International audienceA brief overview on high-pressure synthesis of superhard and ultrahard materials is presented in this tutorial paper. Modern high-pressure chemistry represents a vast exciting area of research which can lead to new industrially important materials with exceptional mechanical properties. This field is only just beginning to realize its huge potential, and the image of "terra incognita" is not misused. We focus on three facets of this expanding research field by detailing: (i) the most promising chemical systems to explore (i.e. "where to search"); (ii) the various methodological strategies for exploring these systems (i.e. "how to explore"); (iii) the technological and conceptual tools to study the latter (i.e. "the research tools"). These three aspects that are crucial in this research are illustrated by examples of the recent results on high pressure-high temperature synthesis of novel super-and ultrahard phases (orthorhombic γ-B 28 , diamond-like BC 5 , rhombohedral B13N2 and cubic ternary B-C-N phases). Finally, some perspectives of this research area are briefly reviewed

High-Pressure Melting Curve of Zintl Sodium Silicide Na4Si4 by In Situ Electrical Measurements

International audienceThe inorganic chemistry of the Na–Si system at high pressure is fascinating, with a large number of interesting compounds accessible in the industrial pressure scale, below 10 GPa. In particular, Na4Si4 is stable in this whole pressure range and thus plays an important role in understanding the thermodynamics and kinetics underlying materials synthesis at high pressures and high temperatures. In the present work, the melting curve of the Zintl compound Na4Si4 made of Na+ and Si44– tetrahedral cluster ions is studied at high pressures up to 5 GPa, by using in situ electrical measurements. During melting, the insulating Na4Si4 solid transforms into an ionic conductive liquid that can be probed through the conductance of the whole high-pressure cell, i.e., the system constituted of the sample, the heater, and the high-pressure assembly. Na4Si4 melts congruently in the studied pressure range, and its melting point increases with pressure with a positive slope dTm/dp of 20(4) K/GPa

Mg–C System up to 20 GPa: Its Phase Diagram and Stable Magnesium Carbides

International audienceThe phase diagram of the Mg−C system has been constructed up to 20 GPa and ∼4000 K based on complementary Thermo-Calc simulations and experimental data obtained in both ex situ and in situ experiments using X-ray diffraction with synchrotron radiation. Three high-pressure magnesium carbides, namely, β-Mg 2 C 3 , its high-temperature form γ-Mg 2 C 3 , and antifluorite Mg 2 C, have p−T domains of thermodynamic stability. At the same time, the carbides accessible by ambient-pressure synthesis, α-Mg 2 C 3 and MgC 2 , are either metastable or unstable, depending on the temperature, at least up to 20 GPa. Experimental observations show that at ambient conditions, all carbides are metastable and remain unchanged at least for years