羟氨苄青霉素引起大疱性类天疱疮样疹

The aim of this critical review is to provide a broad but digestible overview of mechanochemical synthesis, i.e. reactions conducted by grinding solid reactants together with no or minimal solvent. Although mechanochemistry has historically been a sideline approach to synthesis it may soon move into the mainstream because it is increasingly apparent that it can be practical, and even advantageous, and because of the opportunities it provides for developing more sustainable methods. Concentrating on recent advances, this article covers industrial aspects, inorganic materials, organic synthesis, cocrystallisation, pharmaceutical aspects, metal complexes (including metal–organic frameworks), supramolecular aspects and characterization methods. The historical development, mechanistic aspects, limitations and opportunities are also discussed (314 references).

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Mechanochemistry: opportunities for new and cleaner synthesis

This paper reviews the crystal chemistry, synthesis, densification, microstructure, mechanical properties, and oxidation behavior of zirconium diboride (ZrB2) and hafnium diboride (HfB2) ceramics. The refractory diborides exhibit partial or complete solid solution with other transition metal diborides, which allows compositional tailoring of properties such as thermal expansion coefficient and hardness. Carbothermal reduction is the typical synthesis route, but reactive processes, solution methods, and pre-ceramic polymers can also be used. Typically, diborides are densified by hot pressing, but recently solid state and liquid phase sintering routes have been developed. Fine-grained ZrB2 and HfB2 have strengths of a few hundred MPa, which can increase to over 1 GPa with the addition of SiC. Pure diborides exhibit parabolic oxidation kinetics at temperatures below 1100°C, but B2O3 volatility leads to rapid, linear oxidation kinetics above that temperature. The addition of silica scale formers such as SiC or MoSi2 improves the oxidation behavior above 1100°C. Based on their unique combination of properties, ZrB2 and HfB2 ceramics are candidates for use in the extreme environments associated with hypersonic flight, atmospheric re-entry, and rocket propulsion.

Refractory Diborides of Zirconium and Hafnium

The available literature on the crystal structure of the metastable alumina polymorphs and their associated transitions is critically reviewed and summarized. All the metastable alumina structures have been identified as ordered or partially ordered cation arrays on the interstitial sites of an approximately close-packed oxygen sublattice (either face-centered cubic or hexagonal close packed). The analysis of the symmetry relations between reported alumina polymorphs having an approximately face-centered cubic packing of the oxygen anions allows for an exact interpretation of all the complex domain structures that have been observed experimentally. Possible mechanisms for the phase transitions between the different alumina polymorphs also are discussed.

Metastable alumina polymorphs : Crystal structures and transition sequences

This review article aims to provide an updated and comprehensive description of the development of the Electric Current Activated/assisted Sintering technique (ECAS) for the obtainment of dense materials including nanostructured ones. The use of ECAS for pure sintering purposes, when starting from already synthesized powders promoters, and to obtain the desired material by simultaneously performing synthesis and consolidation in one-step is reviewed. Specifically, more than a thousand papers published on this subject during the past decades are taken into account. The experimental procedures, formation mechanisms, characteristics, and functionality of a wide spectrum of dense materials fabricated by ECAS are presented. The influence of the most important operating parameters (i.e. current intensity, temperature, processing time, etc.) on product characteristics and process dynamics is reviewed for a large family of materials including ceramics, intermetallics, metal–ceramic and ceramic–ceramic composites. In this review, systems where synthesis and densification stages occur simultaneously, i.e. a fully dense product is formed immediately after reaction completion, as well as those ones for which a satisfactory densification degree is reached only by maintaining the application of the electric current once the full reaction conversion is obtained, are identified. In addition, emphasis is given to the obtainment of nanostructured dense materials due to their rapid progress and wide applications. Specifically, the effect of mechanical activation by ball milling of starting powders on ECAS process dynamics and product characteristics (i.e. density and microstructure) is analysed. The emerging theme from the large majority of the reviewed investigations is the comparison of ECAS over conventional methods including pressureless sintering, hot pressing, and others. Theoretical analysis pertaining to such technique is also proposed following the last results obtained on this topic.

Consolidation/synthesis of materials by electric current activated/assisted sintering

When the powder mixture of Zr/B/C=1/1/1 in molar ratio was mechanically activated by grinding in a planetary ball mill and then exposed to air, it self-ignited spontaneously and the self-propagating high-temperature synthesis (SHS) of ZrC and ZrB 2 was simultaneously occurred. In this mechanical activation assisted self-propagating high-temperature synthesis in air (MA-SHS in Air), it was found that high reactivity of disordered carbon formed by mechanical activation of graphite played a decisively important role. On the other hand, the isothermal solid-state reaction of the unground powder mixture of Zr/B/C=1/1/1 in argon formed ZrB 2 at 800 °C and then ZrC at 1000 °C. The reaction mechanism and the morphology of the products in both processes are discussed. The product with fine, homogeneous microstructure obtained in MA-SHS in Air is expected to be a promising candidate as precursor of ZrC–ZrB 2 composites.

Mechanical activation assisted self-propagating high-temperature synthesis of ZrC and ZrB2 in air from Zr/B/C powder mixtures

Mechanochemical phenomena of gibbsite, bayerite and boehmite by grinding

MA-SHS and SPS of ZrB2–ZrC composites

X-ray photoelectron spectra of hydrated aluminas (boehmite, diaspore, bayerite, and gibbsite), transition aluminas (y, δ, η, θ, X, and k –Al2O3) and corundum (α-Al2O3) have been studied for spectral characterization of each compound. The O1s spectra are shifted 0.2–1.2 eV to higher binding energy (Eb) in the order of α-Al2O3 < boehmite, diaspore < bayerite, gibbsite, and this agrees with the order of bulk OH/Al molar ratio in samples. The Eb and FWHM values of O1s spectra of transition aluminas depend on the ratio OH/O, i.e., the amount of OH− ions chemisorbed on them, and tend to decrease toward those of α-Al2O3 with increasing calcination temperature. Therefore, it is considered that an attracting effect of the proton on valence electrons in the hydroxyl oxygen causes the increased binding energy between core electrons and oxygen atomic nuclei. The broad O1s spectra of boehmite and diaspore can be deconvoluted into equal contribution from the two oxygen species in O2− and OH− ions in their structures.

X-ray photoelectron spectroscopic study of hydrated aluminas and aluminas

The effect of solvents, mechanically ground gibbsites and amorphous hydrated alumina gels on the formation of well-crystallized and submicrometer-sized crystals of boehmite was studied. Nearly monodispersed boehmite particles of 0.2–0.3 μm with a parallelogram shape were hydrothermally synthesized from ball-milled gibbsite in water at 200°C for 6 h. The 100 nm-sized crystals of boehmite were prepared from amorphous hydrated alumina gels at 255°C for 24 h.

Takeshi Tsuchida

Papers

Mechanical activation assisted self-propagating high-temperature synthesis of ZrC and ZrB2 in air from Zr/B/C powder mixtures

Mechanochemical phenomena of gibbsite, bayerite and boehmite by grinding

MA-SHS and SPS of ZrB2–ZrC composites

X-ray photoelectron spectroscopic study of hydrated aluminas and aluminas

Hydrothermal synthesis of submicrometer crystals of boehmite