Alberto Adriano Cavalheiro

Evaluation of the effect of the stoichiometric ratio of Ca/Cu on the electrical and microstructural properties of the CaCu3Ti4O12 polycrystalline system

The structural, microstructural, non-ohmic and dielectric properties of perovskite-type CaCu3Ti4O12 (CCTO) with Ca/Cu stoichiometries of 1/3, 1/1 and 3/1 are discussed. The 1/3 Ca/Cu ratio system presents very high dielectric permittivity (~9000 at 10 kHz) and a low non-ohmic property (α = 9), whereas the 1/1 Ca/Cu ratio system shows the opposite effect, i.e. the dielectric permittivity decreases (2740 at 10 kHz) and the non-ohmic property increases (α = 42), indicating that these properties are not directly correlated. The results of this work reinforce the idea that the greatest contribution to the very high permittivity is caused by the presence of planar defects inside the CCTO grains, generating internal nanometric domains associated with stacking faults, according to the nanoscale barrier layer capacitance model proposed very recently in the literature [1]. The non-ohmic property is related to the presence and distribution of phases such as CaTiO3 (CTO) and CuO, segregated or precipitated at the grain boundary, which generate large numbers of electrically active interfaces.

Síntese, caracterização e estudo das propriedades de um novo complexo mononuclear contendo quercetina e íon Ga(III)

Flavonoids are one of the most important compound groups applied as medicine given their antioxidant properties, but several intrinsic properties can be improved through structural modifications to their molecules. Here, the synthesis and characterization of a new gallium (III) complex with quercetin is described. Electrochemical properties, as well as antioxidant and cytotoxic activities, were investigated and compared to the free flavonoid molecule. The mononuclear complex obtained, [Ga(C15H9O7)3].2H2O.2CH3OH.CH3CH2OH, seems more active as a DPPH radical scavenger given its lower oxidation potential compared to quercetin. The new complex cytotoxic responses have shown to be more effective than those of the free flavonoid and of lapachol used as a control.

The Effect of Iron (III) Co-Insertion in Magnesium-Aluminum Hydrotalcites Obtained by Precipitation Method at pH 11

Hydrotalcite are anionic clay material presenting LDH arrangement and high porosity and specific areas, which make it a good adsorbent for pollutant species in water. Besides that, that material type can be used as catalyst or catalyst support in several industrial processes. The most common compositions is based on metallic mix hydroxide with high content of magnesium, but their adsorptive properties arise from aluminum replacement in layer structure. The present work presents the synthesis of carbonated magnesium-aluminum hydrotalcite through the precipitation method in order to investigate the co-insertion of iron (III) in aluminum site. It was found the iron (III) co-inserted samples obtained at 100 and 200 oC for 4 hours present no substantial harming in relation to the common magnesium-aluminum composition. All of the samples presented high porosity and specific area, becoming an alternative anionic adsorptive.

Effect of Iron and Vanadium on the Phase Transition of Titanium Dioxide Obtained by Polymeric Precursor Method

The titanium dioxide phase formation is dependent on the synthesis method, temperature of calcination and modifiers insertion. By using chemical methods, such as Polymeric Precursor Method, the organic impurities or extrinsic defects caused by doping play an important rule on the formation of precursor structure before the phase crystallization above 500 oC. Some dopants can change the decomposition mechanism of the precursor, which affects the anatase-rutile phase transition. In this work, the Polymeric Precursor Method was used to synthesize titanium dioxide powder samples in order to investigate the effects of iron (III) and vanadium (V) dopants on the phase formation. Through thermal analysis of polymeric precursors and X-ray diffractometry for calcined powder samples it was possible to show the existence of antagonistic effects for both investigated dopants. While the iron doping reduces the anatase phase tetragonality and delays the rutile phase conversion, the vanadium one changes the mechanism of decomposition of polymeric precursor and leads to more amount of rutile phase.

Synthesis of Nickel-Silica Nanocomposite Embedded in Amorphous Carbon through the Polymeric Precursor Method

The direct synthesis methods such as Polymeric Precursor Method can be used as direct route for the synthesis of ceramic-metal composites with high dispersion of metallic nanoparticles. The control of the polymer decomposition in order to avoid the organic matter combustion permits the pyrolysis occurrence, which originates the metallic phase embedded in the ceramic matrix. In this work, a nickel-silica nanocomposite in additional amorphous carbon interphase was successfully obtained through the Polymeric Precursor Method. It was observed by Transmission Electronic Microscopy that metallic nickel nanoparticles nucleate with sizes from 10 to 50 nm inside the mesopore matrix composite. The analysis of Nitrogen Adsorption-Desorption Isotherms for samples pyrolysed at several times showed the existence of two sizes of mesopores, one with diameter of 3.8 nm, associated to the silica-carbon matrix and other above 10 nm, associated to the meso and macropores containing nickel nanoparticles.

The Use of Long Chain Diol for Obtaining Nickel Embedded in Silica-Carbon Matrix through the Polymeric Precursor Method

Silica-carbon composites have large surface area and porosity and combines low density with refractoriness and high corrosion resistance, which make it a good choice for application as mesoporous matrix for composite catalysts. The insertion of nanoparticulate active phase can originate new properties for that composite type, such as catalysts for industrial methane hydro-reforming or other applications. However, there is a strong dependence among the composite morphology, active phase dispersion and porosity, originating a wide variation in the final properties. In this work, the Precursor Polymeric Method was modified to insert a long chain diol as polymerizing agent in order to insert large amount of carbon phase in the composite. The samples were obtained through the pyrolysis of the polymeric precursor prepared with tetraethylene glycol. Composite samples with high dispersion of nickel nanoparticles were obtained even after 1 hour of pyrolysis, but the porosity showed to be dependent of time of pyrolysis, mainly for mesopores with 3.8 nm in diameter.

Influence of Polyester Chain Type in the Morphology of Silica-Carbon Composites Obtained by the Polymeric Precursor Method

Particulate composite materials containing metallic phase in ceramic matrices may be obtained by direct routes with the advantage of avoiding the partial collapse of the composite structure when a subsequent step for metal insertion is carried out. The non-metallic silica-carbon matrix combines high values of pore volume and surface area with chemical refractivity and may be applied as molecular sieves, adsorbents, filters and catalyst support. The Polymeric Precursor Method is a versatile method to obtain this composite type as the metal precursor can be reduced to metallic phase by pyrolysis of organic matter. In this work, it was used three different diol chain sizes obtaining silica-carbon composites through the pyrolysis of polyester precursor at 600°C for 3 hours in closed tubular oven. It was observed a direct dependence between the amorphous carbon phase amount and the polyester chain size. All of the composite samples presented dual distribution for mesopore size, situated at 3.8 and 11 nm in diameter. However, the pore volume and surface area significantly lowers for larger polyester chain sizes.

Morphological Characterization of Nickel-Silica Nanocomposite in Residual Carbon Obtained by Direct Polymeric Route

Nickel-silica nanocomposites can be obtained by direct chemical route, such as the Polymeric Precursor Method. That methodology type permits to obtain material powders with significant specific area and porosity suitable for reactive gases or fluids permeation, which are required characteristics for application in heterogeneous catalysis process. The composite material obtained from pyrolysis of polymeric precursor has its porosity strongly dependent from precursor constitution, which affects the decomposition kinetic. In this study, it was obtained a polyester precursor based on triethylene glycol, which has been submitted at pyrolysis at 600 oC for several times in nitrogen atmosphere. The nickel-silica nanocomposite obtained through that methodology presents a residual amorphous carbon phase playing an important rule on the mechanism of pore formation. Nickel nanoparticles nucleate with sizes close to 10 nm in diameter and are highly dispersed in a hybrid amorphous carbon-silica matrix. The composite pore volume, calculated through the JBH method, presents a continuous increasing as a function of pyrolysis time, reaching more than 0.15 cc/g after 7 hours from initial decomposition process.

Morphological Analysis of the Amorphous Carbon Embedded Nickel-Silica Composite Submitted at Gradual Oxidation

Amorphous embedded nickel-silica composites can be directly obtained by Polymeric Precursor Method when the polymeric precursor is pyrolysed in controlled atmosphere. The metallic particles dispersion into the matrix composite is very important to material applications, but the resistance against the oxidation under extreme conditions is also required. In this work, nickel-silica nanocomposite sample was synthesized for containing amorphous carbon as matrix interphase through the direct route based on the Polymeric Precursor Method. The composite sample was obtained by pyrolysis of polymeric precursor at 600 oC for 1 hour under nitrogen flow and then submitted to oxidation in oxygen flow at 400 oC up to 7 hours. It was observed by Transmission Electron Microscopy the metallic nickel nanoparticles present high dispersion into matrix composite. However, in spite of the border of the composite particles are easily oxidized and originate nickel oxide phase, as proven by X-Ray Diffraction, the amorphous carbon phase acts as protective phase for the majority of the composite sample. By Nitrogen Adsorption-Desorption Isotherms at 77 K it was demonstrated the carbon phase also preserves significantly the mesoporous nature of the composite under oxidation process.