A Unique Mechanochemical Redox Reaction Yielding Nanostructured Double Perovskite Sr2FeMoO6 With an Extraordinarily High Degree of Anti-Site Disorder.

Strontium ferromolybdate, Sr2FeMoO6, is an important member of the family of double perovskites with the possible technological applications in the field of spintronics and solid oxide fuel cells. Its preparation via a multi-step ceramic route or various wet chemistry-based routes is notoriously difficult. The present work demonstrates that Sr2FeMoO6 can be mechanosynthesized at ambient temperature in air directly from its precursors (SrO, α-Fe, MoO3) in the form of nanostructured powders, without the need for solvents and/or calcination under controlled oxygen fugacity. The mechanically induced evolution of the Sr2FeMoO6 phase and the far-from-equilibrium structural state of the reaction product are systematically monitored with XRD and a variety of spectroscopic techniques including Raman spectroscopy, 57Fe Mössbauer spectroscopy, and X-ray photoelectron spectroscopy. The unique extensive oxidation of iron species (Fe0 → Fe3+) with simultaneous reduction of Mo cations (Mo6+ → Mo5+), occuring during the mechanosynthesis of Sr2FeMoO6, is attributed to the mechanically triggered formation of tiny metallic iron nanoparticles in superparamagnetic state with a large reaction surface and a high oxidation affinity, whose steady presence in the reaction mixture of the milled educts initiates/promotes the swift redox reaction. High-resolution transmission electron microscopy observations reveal that the mechanosynthesized Sr2FeMoO6, even after its moderate thermal treatment at 923 K for 30 min in air, exhibits the nanostructured nature with the average particle size of 21(4) nm. At the short-range scale, the nanostructure of the as-prepared Sr2FeMoO6 is characterized by both, the strongly distorted geometry of the constituent FeO6 octahedra and the extraordinarily high degree of anti-site disorder. The degree of anti-site disorder ASD = 0.5, derived independently from the present experimental XRD, Mössbauer, and SQUID magnetization data, corresponds to the completely random distribution of Fe3+ and Mo5+ cations over the sites of octahedral coordination provided by the double perovskite structure. Moreover, the fully anti-site disordered Sr2FeMoO6 nanoparticles exhibit superparamagnetism with the blocking temperature T B = 240 K and the deteriorated effective magnetic moment µ = 0.055 µ B per formula unit.

Emergence of ferromagnetism due to charge transfer in compressed ilmenite powder using super-high-energy ball milling.

Ilmenite, FeTiO3, is a common mineral in nature, existing as an accessory phase in the most basic igneous and metamorphic rocks, for example, it is derived from the upper mantle. Therefore, an understanding of the high-pressure physics of FeTiO3 is of fundamental importance in the study of rock magnetization. Here, we provide experimental evidence of lattice compression of FeTiO3 powder using super-high-energy ball milling, enabling the very high collision energy of 420 times gravitational acceleration. A sample obtained as an ilmenite- hematite 0.5FeTiO3·0.5Fe2O3 solid solution showed a decrease in molar volume of approximately 1.8%. Consequently, the oxidation state in FeTiO3 powder was changed into almost Fe3+Ti3+, corresponding to 87% Fe3+ of the total Fe for FeTiO3, resulting in the emergence of ferromagnetism. This new ferromagnetic behaviour is of crucial importance in the study of rock magnetization which is used to interpret historical fluctuations in geomagnetism. In addition, the super-high-energy ball mill can be used to control a range of charge and spin states in transition metal oxides with high pressure.

Chemical and thermal properties of VO2 mechanochemically derived from V2O5 by co-milling with paraffin wax.

A novel mechanochemical reduction process of V2O5 to VO2 was established by milling with paraffin wax (PW, average molecular weight 254-646), serving as a reductant. The reduction progressed with increasing milling time and mass ratio V2O5 : PW (MRVP). The mechanochemically derived VO2 became phase pure after milling for 3 h with an MRVP of 30 : 1 and exhibited a reversible polymorphic transformation between tetragonal and monoclinic phases at around 53-60 °C and 67-79 °C during heating and cooling, respectively. The latent heat was above 20 J g-1 in both processes, being superior to those of commercial VO2. Doping of starting V2O5 with Cr, Mo or W at 1 at% in the form of oxide did not increase the latent heat. This is another difference from the conventionally prepared doped VO2. These anomalous heat storage properties of mechanochemically derived VO2 were discussed mainly on the basis of X-ray photoelectron spectroscopy V2p3/2 peaks combined with ion etching. The observed relatively high heat storage capacity of undoped VO2 is primarily ascribed to the abundance of V4+ ionic states introduced during milling with PW, which were stabilized with simultaneously introduced structural degradation throughout the entire particles. The possible role of a remaining small amount of PW was also discussed.

Solid-state reduction of silica nanoparticles via oxygen abstraction from SiO4 units by polyolefins under mechanical stressing.

Metal oxides with an oxidation number lower than the highest often exhibit attractive functional properties. However, conventional chemical or thermal reduction of the stable oxides is often laborious and cannot be stopped at an appropriate level of reduction. Therefore, we here try to explore non-conventional reduction processes in a solid-state without external heating. Unique features of reduction processes of SiO2 toward suboxides, SiO x (1 ≤ x < 2), were made possible by milling fumed silica nanoparticles with polyolefins (POL), i.e., polypropylene (PP) or polyethylene (PE) and a fluorine-containing one, polyvinylidene difluoride (PVDF). We mainly examined the electronic and coordination states of Si by Si2p XPS spectra and 29Si MAS NMR, respectively. They significantly differ from a similar commercial product obtained via a thermal route. Judging from the chemical shift of 29Si MAS NMR as a criterion of the degree of reduction of SiO2, the function of POL as a reductant is in the order PP ≈ PE > PVDF. Since the present solid-state reaction is free from the formation of unstable gaseous SiO as an intermediate, the products are free from the Si component in a Q0 state close to that of metallic Si. From these results we conclude that the present silicon suboxides obtained by co-milling silica with POL are closer to those defined as a random bonding model of SiO, than a random mixture model, the former being unachievable by a thermal process. The main mechanism of the present solid-state reduction is the oxygen abstraction from the SiO4 units by the polarized POL, with its simultaneous oxidative decomposition up to the state of carbon. The reaction process is simple and scalable so that it may offer a new affordable fabrication method of silicon suboxides.

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).

Preparation of core-shell poly(L-lactic) acid-nanocrystalline apatite hollow microspheres for bone repairing applications.

In this paper, hybrid inorganic-organic core-shell hollow microspheres, made of poly(L-lactic acid) (PLLA) and biomimetic nano apatites (HA), were prepared from biodegradable and biocompatible substances, suitable for bone tissue applications. Preparation is started from Pickering emulsification, i.e., solid particle-stabilized emulsions in the absence of any molecular surfactant, where solid particles adsorbed to an oil-water interface. Stable oil-in-water emulsions were produced using biomimetic 20 nm sized HA nanocrystals as particulate emulsifier and a dichloromethane (CH(2)Cl(2)) solution of PLLA as oil phase. Hybrid hollow PLLA microspheres at three different HA nanocrystals surface coverage, ranging from 10 to 50 µm, were produced. The resulting materials were completely characterized with spectroscopic, calorimetric and microscopic techniques and the cytocompatibility was established by indirect contact tests with both fibroblasts and osteoblasts and direct contact with these latter. They displayed a high level of cytocompatibility and thus represent promising materials for drug delivery systems, cell carriers and scaffolds for regeneration of bone useful in the treatment of orthopaedic, maxillofacial and dental fields.

Preparation of injectable auto-forming alginate gel containing simvastatin with amorphous calcium phosphate as a controlled release medium and their therapeutic effect in osteoporosis model rat.

Highly soluble amorphous calcium phosphate powder (ACP) was added to the alginate gel as a buffering agent, in an attempt to enable widely controlled release while avoiding an acidification of the gel-environment. Therapeutic effects of the ACP-containing alginate gel which slowly releases a drug, simvastatin, on osteoporosis model rats were examined. A model drug, simvastatin, incorporated in the alginate gel with ACP up to 0.5%, was continuously released for a long time under the acidic condition. The release rate was controlled by the amount of ACP, serving as a buffer to suppress acidity. When the alginate solution intramuscularly injected in the rat, a soft gel was formed in the injected site. Simvastatin released from the gel containing 0.5% of ACP showed high therapeutic effect on osteoporosis rat. Thus, the present injectable long-sustained release system is expected to be a novel drug delivery controlling device.

Effects of alkali pretreatment of silk fibroin on microstructure and properties of hydroxyapatite-silk fibroin nanocomposite.

Nanocomposites comprising hydroxyapatite (HAp) and silk fibroin (SF) were synthesized from Ca(OH)2 suspension co-dispersed with SF fine particles and H3PO4 solution via a wet-mechanochemical route. The SF particles were modified with an alkali solution to increase contact points between HAp phase and SF matrix. HAp crystallites grow more preferentially along c-axis on alkali pretreated SF substrates. The composites exhibit porous microstructure with 70% of open porosity and about 70% of the interpores ranging from 40 to 115 microm in diameter. The peak shifts in amide II band of SF indicate that the chemical interactions between HAp crystals and SF matrix are intensified by the alkali pretreatment of SF. The stronger inorganic-organic interactions promote the formation of three-dimensional network extending throughout the composites, bringing about an increase of 63% in the Vickers hardness to the composite.

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.

Prediction of apparent equilibrium solubility of indomethacin compounded with silica by 13C solid state NMR.

The apparent equilibrium solubility (AES) of indomethacin increased by co-grinding with silica. Change in the long- and short range disorder of indomethacin by co-grinding was examined by X-ray powder diffraction and 13C solid state NMR, respectively, to elucidate the increased AES. Since the increase in AES was particularly marked after complete disappearance of X-ray diffraction peaks, we attributed the enhanced AES primarily to the short range disorder on the molecular basis. This was confirmed by a high correlation between the standardized full width at half maximum (SFWHM) of the specific peaks observed by 13C solid state NMR and log (AES). The correlation enables the prediction of AES as well.

Stabilization of amorphous indomethacin by co-grinding in a ternary mixture.

Mechanochemical amorphization of indomethacin (IM) was substantially enhanced by grinding with SiO(2), talc and a Mg(OH)(2)-SiO(2) mixture. The rates of the mechanochemical amorphization were in the order of Mg(OH)(2)-SiO(2) mixture>talc>SiO(2). Amorphous state stability of IM compounded with the carrier was examined by crystallization behavior under the condition of 30 degrees C and 11% relative humidity. Superiority of the binary mixture as a carrier was explained in terms of the mechanically induced strong acidic sites of the carrier.