The influence of mechanochemical treatment of the Bi2O3-ZrO2 system on the structural and dielectric properties of the sintered ceramics

A powder mixture of alpha-Bi2O3 and ZrO2, both monoclinic, in the molar ratio 2:3, was mechanochemically treated in a planetary ball mill in an air atmosphere for up to 20 h, using steel vial and hardened-steel balls as the milling medium. Mechanochemical reaction leads to the gradual formation of an amorphous phase. After 5 h of milling the starting alpha-Bi2O3 and ZrO2 were transformed fully into a non-crystalline phase. After milling for various times the powders were compacted by pressing and isothermal sintering. The pressed and sintered densities depended on the milling time. Depending on the duration of the mechanochemical treatment and sintering temperature, the phases: gamma-Bi-12(ZrxFe1-x)O-20; Bi(ZrxFe1-x)O-3 and Bi-2(ZrxFe1-x)(4)O-9 were obtained by reactive sintering, whereby the Fe originates from vial and ball debris. The dielectric permittivity of the sintered samples significantly depends on the milling time. Samples milled for 10 and 15 h and subsequently sintered at 800degreesC for 24 h exhibit a hysteresis dependence of the dielectric shift (in altering electric fields higher than 10 kV/cm at room temperature), confirming that the synthesized materials possess ferroelectric properties. (C) 2004 Kluwer Academic Publishers

The ball milling induced transformation of alpha-Fe2O3 powder in air and oxygen atmosphere

The mechanochemical treatment of alpha-Fe2O3 powder was done concurrently in air and oxygen atmospheres using a conventional planetary ball mill. The influence of the duration of milling and of the balls-to-powder mass ratio on the transformation of alpha-Fe2O3 was investigated. Under appropriate milling conditions, alpha-Fe2O3, completely transforms to Fe3O4, and for prolonged milling to the Fe-1 _ O-x phase, either in air or oxygen atmosphere. Owing to the higher oxygen pressure, the start of the reaction in oxygen is delayed by similar to 1 h in comparison with the reaction in air. The reverse mechanochemical reaction Fe-1 . O-- GT Fe3O4-- GT , alpha-Fe2O3 takes place under proper oxygen atmosphere. The oxygen partial pressure is the critical parameter responsible for the mechanochemical reactions. The balls-to-powder mass ratio has a considerable influence on the kinetics of mechanochemical reactions. Below the threshold value the reaction does not proceed or proceeds very slowly. Plausibly, three phenomena govern mechanochemical reactions: (i) the generation of highly energetic and localized sites of a short lifetime at the moment of impact; (ii) the adsorption of oxygen at atomically dean surfaces created by particle fracture; and (iii) the change of activities of the constituent phases arising from a very distorted (nanocrystalline) structure. (C) 1999 Elsevier Science S.A. All rights reserved

The ball milling induced transformation of alpha-Fe2O3 powder in air and oxygen atmosphere

The mechanochemical treatment of alpha-Fe2O3 powder was done concurrently in air and oxygen atmospheres using a conventional planetary ball mill. The influence of the duration of milling and of the balls-to-powder mass ratio on the transformation of alpha-Fe2O3 was investigated. Under appropriate milling conditions, alpha-Fe2O3, completely transforms to Fe3O4, and for prolonged milling to the Fe-1 _ O-x phase, either in air or oxygen atmosphere. Owing to the higher oxygen pressure, the start of the reaction in oxygen is delayed by similar to 1 h in comparison with the reaction in air. The reverse mechanochemical reaction Fe-1 . O-- GT Fe3O4-- GT , alpha-Fe2O3 takes place under proper oxygen atmosphere. The oxygen partial pressure is the critical parameter responsible for the mechanochemical reactions. The balls-to-powder mass ratio has a considerable influence on the kinetics of mechanochemical reactions. Below the threshold value the reaction does not proceed or proceeds very slowly. Plausibly, three phenomena govern mechanochemical reactions: (i) the generation of highly energetic and localized sites of a short lifetime at the moment of impact; (ii) the adsorption of oxygen at atomically dean surfaces created by particle fracture; and (iii) the change of activities of the constituent phases arising from a very distorted (nanocrystalline) structure. (C) 1999 Elsevier Science S.A. All rights reserved

Structural and electrical properties of the 2Bi(2)O(3) center dot 3ZrO(2) system

Powder mixtures of alpha-Bi2O3 (bismite) and monoclinic m-ZrO2 (baddeleyite) in the molar ratio 2:3 were mechanochemically and thermally treated with the goal to examine the phases, which may appear during such procedures. The prepared samples were characterized by X-ray powder diffraction, differential scanning calorimetry (DSC), electrical measurements, as well as scanning electron microscopy (SEM) and transmission election microscopy (TEM). The mechanochemical reaction leads to the gradual formation of a nanocrystalline phase, which resembles delta-Bi2O3, a high-temperature Bi2O3 polymorph. Isothermal sintering in air at a temperature of 820 degrees C for 24 h followed by quenching to room temperature yielded a mixture of ZrO2-stabilized beta-Bi2O3 and m-ZrO2 phases, whereas in slowly cooled products, the complete separation of the initial alpha-Bi2O3 and m-ZrO2 constituents was observed. The dielectric permittivity of the sintered samples significantly depended on the temperature. The sintered and quenched samples exhibited a hysteresis dependence of the dielectric shift, showing that the ZrO2-doped beta-Bi2O3 phase possess ferroelectric properties, which were detected for the first time. This fact, together with Rietveld refinement of the beta-Bi2O3/m-ZrO2 mixture based on neutron powder diffraction data showed that ZrO2-doped beta-Bi2O3 has a non-centrosymmetric structure with P $(4) over bar$2(1)c as the true space group. The ZrO2 content in the doped beta-Bi2O3 and the crystal chemical reasons for the stabilization of the beta-Bi2O3 phase by the addition Of m-ZrO2 are discussed. (c) 2008 Elsevier Inc. All rights reserved

A Comparative Study of Ni (II) Removal from Aqueous Solutions on Ca-Alginate Beads and Alginate-Impregnated Hemp Fibers

Ca-alginate particles (CA) and alginate-impregnated hemp fibers (AH) (both, as prepared, dried, and rehydrated) were used as adsorbents for removal of Ni (II) ions from water. Adsorption was examined in the batch system and experimentally obtained data were analyzed by both linear and nonlinear curve fitting using pseudo-first and pseudo-second-order rate, as well as Langmuir, Freundlich, and Sips equations. The concentration of Ni (II) ions was measured by atomic absorption spectrophotometry, while the Fourier transform infrared spectroscopy was used for characterization of adsorbent surface, before and after the adsorption. Although all tested samples showed similar adsorption capacities of 12 mg/g, it was found that rehydration improves adsorption characteristics of AH and increases the removal efficiency above 90 %. Adsorption of Ni (II) ions can be sufficiently described by both kinetic models and Sips isotherm equation, and this relatively fast process presumably occurs through the mechanism of physisorption and ion-exchange. The obtained results proved that Ca-alginate particles and alginate-impregnated hemp fibers have good potential to reduce nickel concentrations in a cost-effective and efficient manner

A comparative study of microstructure, mechanical and fracture properties of Ni3Al-based intermetallics produced by powder metallurgy and standard melting and casting processes

A comparative study of the microstructure, mechanical properties and fractography of Ni3Al macro- and microalloyed intermetallics produced by powder metallurgy (PM) and standard vacuum melting and casting processes has been carried out. Non-porous PM compacts were obtained by vacuum hot pressing of powders produced either by gas atomization or by a rotating electrode process. All materials showed a positive temperature dependence of the compression yield strength. The maximum strength was attained between 600 and 700 degrees C, then the decrease occurs. With increase in temperature the ductility of all materials slightly decreased to a minimum and then abruptly increased. Values of mechanical properties of PM compacts were higher than those of as-cast material. There is a correlation between the fracture morphology and the ductility of Ni3Al, i.e. the higher ductility corresponds to transgranular fracture, while the minimum ductility is a consequence of intergranular fracture