Papers by Keyword: ZnAl₂O₄

Abstract: Zinc aluminate (ZnAl₂O₄) samples were prepared using nanomilling based solid state reaction method for several potential applications. Effect of milling frequency on structural and dielectric behavior of ZnAl₂O₄ has been explored systemically. Investigation of crystal structure reveals that change in lattice parameter by milling does not alter the cubic lattice of ZnAl₂O₄. This milling frequency at the nanosize resulted in a gradual decrease in the particle size, which can be attributed to the inhomogeneous defects. Grain size in nanometers has been calculated by XRD using Debye-Scherrer formula. Dielectric measurements performed in the range of 20Hz-20MHz confirms the Maxwell –Wagner two layer model which is consistent with the Koop’s theory. High value of ac conductivity indicates that milling blocked the ionic transport. As a result of Nyquist plots, a single semicircle was obtained which indicated the leading role of grain (bulk). The variation in the semicircle radii for different samples is due to the influence of milling frequency.

Abstract: This work reports the photoluminescence chitosan/ZnAl₂O₄ films in mass ratios 1:1, 1:2, 1:3, 1:4 and 1:5, respectively. The films were presented flexible, opaque, with a thickness of 0.04 mm and were characterized by XRD, FTIR, emission and excitation. The results show the presence of characteristic peaks of chitosan and ZnAl₂O₄, and bands related to the presence of chitosan, silanol and siloxane groups and silane agent used in surface modification ZnAl₂O₄, which acts as a binding agent with the chitosan all movie reviews. The excitation and emission spectra showed the presence of broadband processes associated with charge transfer from Al ⁺³ the O^-2 in all the films, and that the highest photoluminescence intensities were observed for the films 1:1 and 1:2, which showed higher concentrations than the concentrations Qs 1:3, 1:4 and 1:5. Thus, Qs acted as a binder, transferring energy to Al ⁺³ the O^{- 2}.

Abstract: The present work intends to prepare ZnAl₂O₄/chitosan films with a mass proportion of 3:1and evaluate the influence of the ZnAl₂O₄ filler on the chitosan films morphology, structure and thermogravimetric properties. The films were characterized by XRD, SEM, TG and FTIR. With the XRD, it has been verified that both the chitosan and the ZnAl₂O₄/chitosan films presented the chitosan peaks. However, the ZnAl₂O₄/chitosan film also presented the ZnAl₂O₄ peaks. Through the SEM, it has been observed that the chitosan film presented a flat surface. On the other hand, the ZnAl₂O₄/chitosan presented a protruding surface. The TGA/DTA curves of the ZnAl₂O₄/chitosan film showed an increase in the thermal stability at temperatures greater than 720oC, comparing with the chitosan film.

Abstract: The aim of this work is to evaluate the esterification and transesterification activity of ZnAl₂O₄ catalysts obtained by different ways of heating during the combustion synthesis using glycine as fuel. Samples were prepared according to the propellants and explosives theory using a vitreous silica crucible as container, and as heating source, plate, muffle furnace and microwave oven. After synthesis, the samples were structural and morphologically characterized by: XRD, nitrogen adsorption and carbon content analysis, and employed in the esterification and transesterification reactions, the percentage of biodiesel formed (FAME) was determined by gas chromatography. The samples obtained on the plate, muffle furnace and microwave showed that even using different heating ways, led to the formation of ZnAl₂O₄ as majority phase, with crystallite sizes of 11, 15 and 10 nm, respectively. The samples present values of surface area ranging from 16 to 77 m²/g, particle size from 17 to 81 nm, and carbon content lower than 11%. The forms used for heating influenced the esterification and transesterification reactions, showing that there was a gap in the production of biodiesel, which is a promising indication that this material has potential to be used as catalysts on the biodiesel production.

Abstract: The variations and developments with the reasons on the mechanical properties of MgO-MgAl₂O₄ and MgO-ZnO-Al₂O₃ composite refractories were examined and thermal parameters affecting the durability of composites at high temperatures were investigated. The density, porosity, strength, modulus of elasticity, fracture toughness, fracture surface energy, critical defect size and mean MgO grain size values of composites were measured/calculated and evaluated. In addition, microstructural changes using XRD measurements and SEM analysis were examined. Thermal stress/shock parameters R and R_st that are used for determining high temperature performance of composites were calculated. The relationships between mechanical properties and structural variations for different compositions and the factors affecting this connection were investigated. With the additions of various amounts of ZnO-Al₂O₃ to MgO, significant improvements were achieved on both mechanical properties and R-R_st parameters of in-situ formed M-S-ZnAl₂O₄ composite refractories, compared to MgO-MgAl₂O₄ materials containing preformed spinel, by factors of up to 3.6 and 2.0, respectively. The important parameters increasing mechanical properties and thermal performance of M-S-ZnAl₂O₄ composites were determined as follows: i) formation of ZnAl₂O₄ phase leading to a high resistance to crack initiation and propagation, ii) propagation of microcracks formed in the structure for a short distance by interlinking to each other, iii) arresting or deviation of microcracks when reaching pores or ZnAl₂O₄ particles, and additionally iv) co-presence of both intergranular and transgranular types of cracks on fracture surfaces, and with the incorporations of ZnO-Al₂O₃, v) increase in density, vi) rise in critical defect size, and vii) a significant reduction in MgO grain size. The optimisation of M-S-ZnAl₂O₄ composite refractories that could be used for obtaining longer service life in industrial applications was performed.

Abstract: The aim of this work is to evaluate the influence of fuel in the synthesis of ZnAl2O4 catalytic supports by combustion reaction. For this, it was used the fuels: urea, carbohidrazide, glycine and aniline. The total amount of reagents was calculated according to the theory of propellants and explosive using urea in the stoichiometric proportion (Φe = 1). The structural and morphological characteristics of the powders were evaluated by XRD, FTIR, TEM, SEM and particle size distribution. The results from XRD showed the formation of the normal cubic spinel structure. The powders presented nanosized particles with narrow agglomerates size distribution. The powders prepared with urea showed better value of surface area and smaller crystallite size.