Showing papers on "Mechanochemistry published in 1996"

Mechanochemistry as activation method of the VPO catalysts for n-butane partial oxidation

Abstract: The mechanochemical treatment of the V P O catalysts caused a substantial increase of both the catalytic activity in n-butane oxidation and the selectivity towards maleic anhydride. It was accompanied by a relative growth of the contribution of vanadyl plane (001) of VOHPO 4 ·0.5H 2 O which transforms into the topologically similar (100) plane of (VO) 2 P 2 O 7 . The mechanochemical treatment of the V P O catalysts containing bismuth changes the sample morphology, favouring the formation of the vanadyl plane and provoking surface chemical reaction. The data obtained for V P O/BiPO 4 and V P O/Bi 2 O 3 samples demonstrate that mechanochemistry is a new promising method for the introduction of promoting additives into the basic V P O composition.

The role of particle size and polymer molecular weight in the formation and properties of an organo-ceramic composite

Abstract: The role of particle size and polymer molecular weight in the formation and properties of a calcium aluminate cement-poly(vinyl alcohol) composite was investigated. Banbury mixing studies, in combination with poly(vinyl alcohol) solution rheology, showed paste formation to be dependent on the polymer degree of polymerization and concentration. Both polymer molecular weight and cement particle size have an effect on the mechanochemistry, the “window of processibility”, and mechanical properties of the hardened matrix. Increasing the polymer chain length appears to accelerate the mechanically induced crosslinking reactions between the polymer and cement hydration phases, thereby diminishing the processing window. Decreasing the cement mean particle size has a similar effect. Polymer degree of polymerization and cement particle size distribution are shown to be important factors in the mechanical properties of the hardened composite. Scanning electron microscopy (SEM) micrographs show that wide polydispersity in the grain phase is important in attaining a macro-defect-free microstructure.

Structural Degradation of Tobermorite during Vibratory Milling

Abstract: Mechanochemistry of hydrothermally prepared Al-free and Al-substituted 11 nm tobermorite was studied using 29 Si NMR, XRD, and TGA-DTA Tobermorite has a double-chain structure and Al/Si substitution occurs preferentially at bridging tetrahedra By using the vibration mill, both tobermorites were observed to decompose as a function of milling time to form amorphous C-S-H-like phases The decomposition rate was higher for the Al-substituted tobermorite The decomposition process occurs mainly at the bridge portion of the silicate double chain In the Al-substituted tobermorite, the breakage seems to occur preferentially at the points where Al has replaced Si On heating the amorphous C-S-H-like phase, wollastonite is formed However, no decrease was observed at the wollastonite formation temperature

Chemistry of powder production

Abstract: 1 The concept of a powder.- 2 Specification and control of powder particles.- 2.1 Powdered materials for fine ceramics.- 2.2 Magnetic powders.- 2.3 Pigment powders.- References.- 3 Powder fundamentals.- 3.1 Particle size.- 3.2 Surface energy.- 3.3 Surface structure.- 3.3.1 Surface activity.- 3.3.2 Lattice defects.- 3.4 Surface properties.- 3.4.1 Adsorption.- 3.4.2 Wetting.- 3.4.3 Surface improvement.- 3.4.4 Catalytic action.- 3.4.5 Mechanochemistry.- References.- 4 The preparation of powders.- 4.1 The grinding of solids.- 4.1.1 Mechanisms of grinding.- 4.1.2 Grinding energy.- 4.1.3 The theory and practice of grinding.- 4.2 Thermal decomposition of solids.- 4.3 Deposition of solids from the liquid phase.- 4.3.1 Dissolution and deposition.- 4.3.2 Application of solubility products to precipitation processes.- 4.3.3 Co-precipitation and homogeneous precipitation.- 4.3.4 The formation of crystal nuclei.- 4.3.5 Deposition of precipitated particles.- 4.3.6 The rate of formation of nuclei and the induction time.- 4.3.7 The degree of supersaturation control of the size of precipitated particles.- 4.3.8 The growth of crystals.- 4.3.9 Crystal systems and crystal shape.- 4.3.10 The control of crystal shape and shape modification.- 4.3.11 Application of precipitation reactions to industrial process.- 4.3.12 The hydrolysis of metal alkoxides.- 4.3.13 Hydrothermal reactions.- 4.3.14 The rapid deposition of solids from the liquid state.- 4.4 Vapour-phase reactions.- 4.4.1 The thermodynamics of reacting systems.- 4.4.2 The preparation of oxide powders.- 4.4.3 The preparation of non-oxide powders.- 4.5 Sintering and solid-state reactions.- 4.5.1 Solid-state sintering.- 4.5.2 Solid-state reactions.- References.- 5 Determination of powder characteristics.- 5.1 Shape and size of particles.- 5.1.1 Shape and size of a single particle.- 5.1.2 Size distribution.- 5.1.3 Mean diameter.- 5.2 Methods of particle size measurement.- 5.2.1 Sieving method.- 5.2.2 Microscope method.- 5.2.3 Sedimentation method.- 5.2.4 Adsorption method.- 5.2.5 Permeability method.- 5.2.6 X-ray diffraction method.- 5.3 Measurement of surface properties.- 5.3.1 Surface energy.- 5.3.2 Heat of wetting.- 5.3.3 Heat of adsorption.- 5.3.4 Zeta potential.- 5.3.5 Surface activity.- References.

Mechanochemistry in preparation and modification of vanadium catalysts

Abstract: V 2 O 5 , V 2 O 5 /TiO 2 and VPO/Bi systems were subjected to mechanochemical treatment and their structure and texture was examined by XRD, DTA, SEM, XPS and STM. Catalytic activity and selectivity in oxidation of butane and benzene to maleic and o-xylene to phthalic an hydride were determined as function of time of mechanochemical treatment. This treatment caused increase of surface area and exposure of the basal (010) plane of V 2 O 5 in V 2 O 5 and V 2 O 5 /TiO 2 resulting in considerable increase in activity and selectivity. In case of VPO catalyst and its mixture with Bi 2 O 3 and BiPO 4 both changes of morphology and surface chemical transformations were observed leading to an increase of activity and selectivity of n-butane oxidation to maleic anhydride

Metastable, mechanically alloyed and nanocrystalline materials : ISMANAM-95 : proceedings of the International Symposium on Metastable, Mechanically Alloyed and Nanocrystalline Materials, held in Québec, Canada, July 24 - 28, 1995

Abstract: Part 1 Metastable phases: thermodynamics and metastable phases bulk metallic glasses layers, multilayers, solid-state reactions and GMR. Part 2 Nanocrystalline materials: mechanisms, modelling and simulations defects and grain boundaries nanostructures from melt-spun precursors mechanical grinding, ordering, disordering and structural recovery mechanical alloying, mechanosynthesis and structural evolution mechanochemistry and gas(liquid)/solid reactions consolidation and sintering. Part 3 Magnetic materials and properties structural materials and mechanical properties electrochemical and electrical properties hydrogen.