Papers by Keyword: Mechanical Activation (MA)

Abstract: Alumina is utilized in many areas of modern industry because of its unique mechanical, electrical and optical properties. Various methods are been employed for produce alumina for different end uses. The preparation of fine and sintering-reactive alumina powders is probably one of the most important steps for production alumina ceramics with controlled microstructure. In this work, it was studied the production of alumina powders by “Pechini” method associated to highenergy milling. For this, it was prepared the resin by Pechini method, using aluminum nitrate nonahydrate. This resin was calcined at 500oC. Then, the calcined powders were submitted a high energy milling for different times. The powders mechanically activated were characterized by x ray diffraction, FT-IR and scanning electronic microscopic. After milling, the powders were calcined at 900oC. The results showed that the alumina phase transitions and powders characteristics were modified when the step of activation mechanical was introduced.

Abstract: The use of mechanical activation (the elemental powder mixture is milled for a short time at given frequency and impact energy) as a precursor to self-propagating high-temperature synthesis (SHS) results in the formation of nanostructured porous materials. The mechanical activation step was found necessary (i) to modify the thermal parameters of the combustion front (i.e. combustion front velocity, thermal heating rate…) in the cases of Mo-Si, Fe-Al, Ni-Si (ii) to initiate a combustion front in the case of systems having a low exothermicity. Nevertheless, the control of the mechanically activated mixture characteristics and, the understanding of the mechanical activation role on the SHS parameters are essential to produce end-products with expected microstructure.

Abstract: Mechanical activation (MA) is used extensively as a relatively no expensive method for the modification of physico-chemical properties of dispersed systems in technologies for obtaining powders and ceramics. Different processes that occur during MA of powders lead to the formation of specific structures that promote and accelerate solid-state reactions, as well as densification during sintering. Changes of particle size and structure during MA of the ceramic parent material are the sources of the morphological and structural metastability of the starting powders and they can affect the sintering process, positively or negatively. Many properties of final polycrystalline ceramics strongly depend on a green body microstructure and on conditions under which the green body is sintered. From the other side green body microstructure depend on a powders characteristics such as morphology, particle and pore size distributions. Regarding above mentioned activation and sintering must therefore be carried out under strictly controlled conditions in order to avoid influences that might cause a deterioration of the final properties of the ceramic materials. The present study is focused on the processes of sintering that occurred in mechanically activated single and multiphase oxide powders.

Abstract: Bismuth zinc niobate belongs to the pyrochlore-based microwave dielectrics, with high ε and low dielectric losses. This research reports the results obtained on the bismuth zinc niobate pyrochlore (Bi1.5ZnNb1.5O7 - α-BZN) synthesis by a mechanochemically assisted method. The mechanochemical activation allows reducing the particle size of the initial products, leading to an increase in the specific surface, improving in most cases its reactivity. The effects of changing the milling parameters (ball size, speed, and length of the mechanical treatment) are discussed. The obtained powders were characterized by X-ray diffraction, thermal analysis, optical spectroscopy and scanning electron microscopy. The activated powder was annealed at temperatures ranging from 500 to 800oC. A well crystallized cubic pyrochlore phase can be obtained at 600 oC, which represents a significant decrease in temperature, if compared with the conventional ceramic synthesis method. The mechanically activated powders exhibit a particle size of around 160 nm, which increases with further thermal treatment.

Abstract: Superior properties of nanostructured Al2O3 based materials, such as higher hardness and fracture toughness, have been evidenced. In order to optimize their manufacturing, the mechanical activation of the starting powders (Al2O3-TiO2 and Al2O3-ZrO2) was studied. In the present work, Al2O3 powders blended with 13wt% and 44wt% of titania or 20wt% and 80wt% of yttria partially stabilized zirconia have been high-energy ball-milled using a planetary mill, P4 (Fritsch) with steel vials and balls. The effect of the milling time and operating parameters, such as shock energy and friction to total energy ratio, on the powder structural and microstructural evolutions has been determined by SEM, XRD and BET. The transformation of the metastable anatase TiO2 phase into the high pressure TiO2 II phase and rutile phase was evidenced, simultaneously to the decrease of the alumina crystallite size, in the Al2O3-TiO2 system. In the Al2O3-ZrO2 system, the transformation of the monoclinic phase and the decrease of the alumina and tetragonal zirconia crystallite size have been observed.

Abstract: Starting powder mixtures of ZnO and TiO2, at the molar ratio that is in accordance with the stoichiometry of zinc titanate Zn2TiO4, were mechanically activated using a planetary ball mill in different time intervals from 0 to 90 minutes. X-ray diffraction analysis, scanning electron microscopy and non-isothermal dilatometric measurements were performed in order to investigate zinc titanate formation. Processes occurring during mechanical activation led to the formation of a specific structure of obtained powders that promoted and accelerated solid-state reactions and densification during sintering. The main conclusion based on analysis is that mechanical activation enables better compaction of activated powders, i.e. possibility of achieving higher densities of green bodies without binders, but first of all that Zn2TiO4 ceramics can be obtained by mechanical activation after a certain time with appropriate thermal treatment, i.e. heating rate and sintering time, at temperatures lower than those when non-activated mixtures were used.

Abstract: To study the effects of doping the layered perovskite SrBi2Ta2O9 with an appropriate amount of tungsten, both undoped and W-doped SrBi2Ta2O9 (SBT) of nanocrystallinity were synthesized by mechanical activation of constituent oxides of strontium oxide, bismuth oxide, tantalum oxide and tungsten oxide at room temperature. A nanocrystalline single perovskite phase was observed in SrBi2(Ta1-xWx)2O9 with up to x=0.1. SrBi2(Ta0.9W0.1)2O9 shows a higher Curie temperature than that of undoped SBT, although it exhibits a very similar layered perovskite structure. The ferroelectric and impedance behaviors of SrBi2 (Ta0.9W0.1)2O9 were studied over a range of test temperatures and frequencies, in order to understand the effects of W-doping in SBT, given that W exhibits a similar ionic size to Ta but with a variable valence.

Abstract: Nanocrystalline calcium bismuth titanate (CaBi4Ti4O15), which exhibits a layer structure, has been successfully synthesized by mechanical activation of constituent oxides of CaO, Bi2O3 and TiO2 in a nitrogen atmosphere at room temperature. The phase-forming calcination at elevated temperatures that is always required is skipped. CaBi4Ti4O15 derived from mechanical activation consists of nanocrystallites, which occur as aggregates of ~50 nm in sizes. It demonstrates an improved sinterability and was sintered to a density of 93.4% theoretical density at 1175oC for 2 hours. Ferroelectric properties of sintered CaBi4Ti4O15 derived from mechanical activation have been studied. A peak dielectric constant of 1049 at the Curie temperature of 774oC was measured at 1MHz for CaBi4Ti4O15 sintered at 1175oC.