Mamoru Senna

Hallmarks of mechanochemistry: from nanoparticles to technology

The aim of this review article on recent developments of mechanochemistry (nowadays established as a part of chemistry) is to provide a comprehensive overview of advances achieved in the field of atomistic processes, phase transformations, simple and multicomponent nanosystems and peculiarities of mechanochemical reactions. Industrial aspects with successful penetration into fields like materials engineering, heterogeneous catalysis and extractive metallurgy are also reviewed. The hallmarks of mechanochemistry include influencing reactivity of solids by the presence of solid-state defects, interphases and relaxation phenomena, enabling processes to take place under non-equilibrium conditions, creating a well-crystallized core of nanoparticles with disordered near-surface shell regions and performing simple dry time-convenient one-step syntheses. Underlying these hallmarks are technological consequences like preparing new nanomaterials with the desired properties or producing these materials in a reproducible way with high yield and under simple and easy operating conditions. The last but not least hallmark is enabling work under environmentally friendly and essentially waste-free conditions (822 references).

Relationship between optical properties and crystallinity of nanometer Y2O3:Eu phosphor

Y2O3-doped Eu3+ (Y2O3:Eu) nanometer particles (NPs) were synthesized via a chemical route. The particle size estimated by x-ray diffractometry and transmission electron microscopy was about 61 nm. Two photoluminescence peaks were observed at 582 and 587 nm, being attributed to the transition of 5D0→7F1a of Eu3+ in the S6 and C2 symmetry sites of Y2O3, respectively. The intensity ratio of the photoluminescence peaks at 582 nm to at 587 nm was larger for NPs than for micrometer particles (MPs). The excitation peak of NPs due to the charge-transfer band shifted toward the high-energy side as compared with that of MPs. According to x-ray diffractometry, the lattice distortion and the lattice constant were larger for NPs than for MPs, showing the restructure at the near surface and the increase in ionicity of the Eu–O bond with decreasing particle size. This explains the blueshift in the excitation peak.

Stability of amorphous indomethacin compounded with silica

The stability of indomethacin (IM) compounded with SiO(2) either by co-grinding or by melt-quenching was examined by recrystallization kinetics under the conditions 30 degrees C and 11% relative humidity. A decrease of the recrystallization rate with and without an appreciable induction period was observed in both compounds. Higher stability of amorphous IM compounded with SiO(2) was attained by prolonged co-grinding than by melt-quenching. This was explained by the stronger chemical interaction at the interface between IM and SiO(2) by co-grinding, as revealed by (29)Si and (13)C solid state NMR. Incomplete co-grinding with the rest of the crystalline state, however, made the amorphous state appreciably unstable, since the remaining crystallites serve as seeds for recrystallization.

Optical properties and characteristics of ZnS nano-particles with homogeneous Mn distribution

Unique properties of nano-particles of Mn-doped ZnS prepared by a new chemical process are studied. Homogeneously Mn-doped ZnS nano-particles, as demonstrated from electron spin resonance (ESR), were accomplished by precipitation in a methanolic media with the aid of methacrylic acid at room temperature. Photoluminescence (PL) intensity of ZnS nano-particles increased with decreasing particle size. Quantum size effects were confirmed from the blue shift in the optical band gap of ZnS nano-particles. The enhanced PL intensity was also explained by the nanosize effect.

Comparison between polyvinylpyrrolidone and silica nanoparticles as carriers for indomethacin in a solid state dispersion.

States of interaction between indomethacin (IM) and polyvinylpyrrolidone (PVP) in an amorphous solid dispersion prepared by co-grinding were compared with those between IM and silica nanoparticles. Changes in the carbon chemical states of the solid dispersions were evaluated based on the chemical shift in the 13C-CP/MAS-NMR. Hydrogen bonds between the amide carbonyl of PVP particles and the carboxyl groups of IM molecules were formed by co-grinding. Despite the wide difference in carrier properties, the apparent equilibrium solubility (AES) of IM in the ground IM-PVP mixture was predicted by solid state NMR on the basis of the relationship previously established for IM with SiO(2). This indicates that AES is affected solely by the state of IM, irrespective of the carrier species, and despite carrier-dependent chemical interactions.

Magnetic properties of ultrafine magnetite particles and their slurries prepared via in-situ precipitation

Abstract Ultrafine magnetite particles of 4–10 nm were prepared in aqueous solutions of polyvinylalcohol (PVA) or PVA with partially exchanged carboxyl groups. The mode of agglomeration and dispersion of the particles were different depending on the properties of polymers in the solvent. The dispersion of magnetite particles prepared in PVA 1 wt.% aqueous solution at pH 13.8, was particularly stable. In contrast, those precipitated in an aqueous solution of PVA with 0.1 mol.% exchanged carboxyl groups agglomerated in the form of a chain-like cluster. The magnetic properties were sensitive to the state of agglomeration of the magnetite particles with adsorbed polymers. An increase of the coercive force after ball-milling was observed, due to the formation of the network structure.

Cyclic Properties of Si-Cu/Carbon Nanocomposite Anodes for Li-Ion Secondary Batteries

Si/carbon nanocomposites with different Si distribution were prepared to the anode of Li-ion batteries for better cyclic properties. Si particles were milled by a high-energy multi-ring-type mill to decrease the crystallite size to 15 nm. Nanocomposite particles with controlled inhomogeneous spatial Si distribution were obtained by co-grinding with Cu powder. Heterogeneity of the Si distribution exhibited better cyclability than those with homogeneous Si distribution prepared from well-dispersed Si nanoparticles. We attribute the better cyclability to two factors, reduction of mechanical stress developed by electrochemical volume change of Si and networking of Si particles for maintaining conductive paths in the electrode.

Crystallization of titania and magnesium titanate from mechanically activated Mg(OH)2 and TiO2 gel mixture

Abstract By preliminary grinding of TiO 2 gel and Mg(OH) 2 mixture with a laboratory vibration mill, formation of magnesium titanate (MgTiO 3 ) on subsequent heating predominantly took place instead of crystallization of anatase and MgO. This can be explained by the incipient interaction between TiO 2 and Mg(OH) 2 coupled with mechanochemical dehydration during the mechanical activation. The rate of mechanochemical complexation with TiO 2 was smaller than with SiO 2 , presumably due to the weaker acidity of TiO 2 than SiO 2 .

Change in the structure of niobium pentoxide due to mechanical and thermal treatments

Abstract The structure and properties of amorphous niobium pentoxide obtained from its H-phase by prolonged grinding were studied by X-ray diffractometry, IR and Raman spectroscopy, and thermal analysis. The short range order at distances less than 0.5 nm was always preserved as detected from the radial distribution function, even after prolonged grinding. The amorphized Nb 2 O 5 showed no peak corresponding to the stretching of collinear NbONb bonding around 850 cm −1 in both Raman and IR spectra. The mechanically amorphized niobium pentoxide has a structure similar to the TT-phase and not to the starting H-phase. The amorphous state obtained by grinding is an intermediate in the course of the transformation from H- to TT-phase, either with the insufficient shift of Nb or the incomplete construction of the bipyramidal hexagonal polyhedra necessary to form a stable TT-phase. The structural details for these processes are also discussed.

Mechanochemical dehydration and amorphization of hydroxides of Ca, Mg and Al on grinding with and without SiO2

Abstract Mechanochemical dehydration and amorphization of Ca(OH) 2 , Mg(OH) 2 and Al(OH) 3 (gibbsite) were substantially enhanced by grinding with SiO 2 . The rates of those mechanochemical reactions were in the order of Al(OH) 3 > Ca(OH) 2 > Mg(OH) 2 . These phenomena are explained by such a model that the decomposition of hydroxide and the incipient formation of metal silicates are stimulated by each other. Apart from Mg(OH) 2 and Ca(OH) 2 , gibbsite was amorphized and dehydrated easily by grinding without SiO 2 , as well, because of intrinsic structural vacancy.