Mechanochemistry of Hexagonal Boron Nitride: 1. Destruction and Amorphization During Mechanical Treatment

The regularities of the mechanical activation of hexagonal boron nitride are analyzed using the X-ray diffraction, IR spectroscopy, transmission electron microscopy, dynamic light scattering, and adsorption methods. At the initial state of mechanical activation, the main process is material destruction. At this stage, the specific surface area increases to 400 m2/g and crystallographically oriented nanosized needles are formed. At the same time, boron nitride crystal structure is disordered with an increase in interplanar distance d(002). The disordering is assumed to be due to a shift along planes (001). At a specific dose of supplied mechanical energy above 6–8 kJ/g, the disordering processes dominate and the material is amorphized. At this stage, the specific surface area of samples decreases

Destruction, Amorphization and Reactivity of Nano-BN Under Ball Milling

The processes of mechanical activation of a hexagonal boron nitride (h-BN) and its reactivity upon interaction with hydrogen and water were investigated using X-ray, TEM, Microdiffraction, Dynamic Light Scattering, FTIR-spectroscopy, adsorption (BET). Initial h-BN samples were monocrystalline plates 70–80 nm thick. Mechanical treatment of h-BN is accompanied by plate splitting and formation of crystallographically oriented “rods.” The rod thickness gradually diminishes to less than 5 nm. Specific surface area of the rods (400 m2/g), is found to be equal to the outer geometrical surface of rods. As nanocrystallites form “c” parameter of h-BN increases. When nanocrystallites are less than several nanometers in size, mechanical treatment results in BN amorphization; in this case specific surface of the system begins to decrease. Splitting of BN plates in the atmosphere of hydrogen is accompanied by the material hydrogenation and formation of BH and NH bonds. The amount of adsorbed hydrogen corresponds to monolayer filling. The amorphous part of activated BN interacts with water even at room temperature

Landscape science: a Russian geographical tradition

The Russian geographical tradition of landscape science (landshaftovedenie) is analyzed with particular reference to its initiator, Lev Semenovich Berg (1876-1950). The differences between prevailing Russian and Western concepts of landscape in geography are discussed, and their common origins in German geographical thought in the late nineteenth and early twentieth centuries are delineated. It is argued that the principal differences are accounted for by a number of factors, of which Russia's own distinctive tradition in environmental science deriving from the work of V. V. Dokuchaev (1846-1903), the activities of certain key individuals (such as Berg and C. O. Sauer), and the very different social and political circumstances in different parts of the world appear to be the most significant. At the same time it is noted that neither in Russia nor in the West have geographers succeeded in specifying an agreed and unproblematic understanding of landscape, or more broadly in promoting a common geographical conception of human-environment relationships. In light of such uncertainties, the latter part of the article argues for closer international links between the variant landscape traditions in geography as an important contribution to the quest for sustainability

Kinetic, chemical and mechanical factors affecting mechanical alloying of Ni-bcc transition metal mixtures

The influence of enthalpy of mixing and elemental mechanical characteristics on the kinetics of mechanical alloying (MA) of 80 at.% Ni-20 at.% bcc transition metals (Fe, Cr, W, Nb, and Ta) has been studied. The features and structures of milled powders were characterized by X-ray diffraction, optical microscopy, and particle size analysis. Powder caking of the grinding media - in varying amounts depending on the materials alloyed and the alloying time - occurs when these elemental combinations are mechanically alloyed. Differences, if any, between the structures of \u27free\u27 and \u27caked\u27 powders were also determined. As expected, for systems with low enthalpies of mixing (Ni/Fe, Ni/Cr, Ni/W) crystalline solid solutions form during MA. Moreover, the compositions of these solid solutions are the same in both free and caked powders. Also as anticipated, for systems with high enthalpies of mixing (Ni/Nb, Ni/Ta), extended MA produces amorphous phases that form from precursor solid solutions. In distinction to systems with low enthalpies of mixing, the structures of the free and caked powders differ for systems with high mixing enthalpies. Caked powders were inhomogeneous, consisting of powder in varying degrees of solid solution and, if the milling time was sufficiently long, also some amorphous powder. However, the free powder was almost entirely noncrystalline. With extended milling, the fraction of free powder increases suggesting that the formation of the amorphous phase takes place on the surface layer of the coated grinding media. Subsequent to its formation, the amorphous phase is abraded from the coated surfaces. The kinetics of solid solution and/or amorphous phase formation is discussed in terms of the differing mechanical characteristics of the bcc transition elements, as well as the system thermodynamics. © 2000 Elsevier Science S.A

Mechanochemistry of Hexagonal Boron Nitride: 1. Destruction and Amorphization During Mechanical Treatment

The regularities of the mechanical activation of hexagonal boron nitride are analyzed using the X-ray diffraction, IR spectroscopy, transmission electron microscopy, dynamic light scattering, and adsorption methods. At the initial state of mechanical activation, the main process is material destruction. At this stage, the specific surface area increases to 400 m2/g and crystallographically oriented nanosized needles are formed. At the same time, boron nitride crystal structure is disordered with an increase in interplanar distance d(002). The disordering is assumed to be due to a shift along planes (001). At a specific dose of supplied mechanical energy above 6–8 kJ/g, the disordering processes dominate and the material is amorphized. At this stage, the specific surface area of samples decreases