Search Results

You are looking at 1 - 10 of 11 items for

  • Author or Editor: A. Prado x
  • All content x
Clear All Modify Search

Thermogravimetric measurements of the equilibrium oxygen partial pressure ( ) as a function of the oxygen contenty were performed on Nd1.85Ce0.15Cu1+δOy samples with controlled Cu contents (1+δ=1.00, 1.01 and 1.02) at 1173 and 1237 K. The data obtained showed a dependence of the oxygen non-stoichiometry on the nominal Cu content. These measurements are discussed in terms of the presence of Cu defects in the T′ structure. The superconducting response of this material in samples with controlled oxygen and Cu contents was also studied. The superconducting response was found to be very sensitive to the presence of small amounts of cationic and anionic defects.

Restricted access

Abstract  

An important example of oxide materials exhibiting oxygen non-stoichiometry are the high temperature superconductors (HTSC). In this family of materials the variation of the oxygen content modifies the defect structure and the carrier concentration, causing phase transitions and structural distortions with the consequent change in their physical properties. In some systems, a continuous variation of the physical properties as a function of the oxygen content is observed, while in others, small changes of the oxygen content strongly modify the superconducting behavior of these materials. In this paper, we review an analysis of the influence of the oxygen content on the physical properties of three HTSC materials: the electron doped Nd–Ce–Cu–O superconductor, the R–Ba–Cu–O (R=Nd, Gd, Pr) 123 compounds and the (Hg, Re)-1201 material.

Restricted access

Abstract  

Commercial humic acid (HA) was anchored onto silica gel (SiAPTS) previously modified with 3-aminopropyltrimethoxysilane (APTS). HA was anchored onto SiAPTS through two routes: adsorption and covalent chemical immobilization onto the surface. The adsorption occurred by adding SiAPTS to HA in an aqueous solution, producing SiHA1, while chemical immobilization was performed by reacting HA suspended in N,N-dimethylformamide with SiAPTS, to yield SiHA2. The infrared spectra confirm HA immobilization using both procedures and the termogravimetric results showed that the anchored compounds have significantly thermal stability increased. While natural HA presents a thermal stability up to 200C, the anchored compound presents a thermal stability near to 750C.

Restricted access
Journal of Thermal Analysis and Calorimetry
Authors: A. Caneiro, L. Mogni, N. Grunbaum, and F. Prado

Abstract

This is the first part of a review devoted to present the physicochemical properties of non-stoichiometric oxides that exhibit simultaneously ionic and electronic conductivity. Oxides mixed conductors are candidates to be used in electrochemical applications such as oxygen separation membranes, Solid Oxide Fuel Cells (SOFC) and Solid Oxide Electrolysis Cell (SOEC). In the present article, divide in two parts, we review the thermodynamic and transport properties of mixed conductors systems. In Part I is presented the layered mixed conductors Sr3FeMO6+δ (M = Fe, Co, Ni) belonging to the n = 2 member of the Ruddlesden–Popper series An+1BnO3n+1, while in Part II we discuss results obtained for the Sr1−xLaxFe0.2Co0.8O3−δ perovskites. These perovskite related intergrowth oxides, Sr3FeMO6+δ, intercalate rock-salt layers in the perovskite structure decreasing the tendency of structural transformation at high temperature. The defect structure of these materials has been determined by thermodynamic measurements of the oxygen chemical potential (μO2) as a function of oxygen content and temperature. The knowledge of the defect structure has shown to be essential to analyze electrical resistivity measurements and neutron powder diffraction data at high temperature to propose mechanisms for the electronic and ionic transport.

Restricted access
Journal of Thermal Analysis and Calorimetry
Authors: A. Caneiro, L. Mogni, N. Grunbaum, and F. Prado

Abstract

One of the most important challenges with solid oxide fuel cells (SOFC) is to find cathode materials with high enough catalytic activity for the dissociation of the molecular oxygen. Oxide mixed conductors with the perovskite structure (ABO3) and high Co content in the B site have been extensively studied to be used as cathode in SOFC. This is the second part of a review of high temperature properties of two mixed conductors systems. The first part was focused on the n = 2 Sr3FeMO6+δ (M = Fe, Co, Ni) Rudlesdden Popper phases, while in this paper we discuss the thermodynamic and transport properties of the perovskite solid solution Sr1−xLaxFe0.2Co0.8O3−δ (0 ≤ x ≤ 0.4) in the temperature range 773 ≤ T ≤ 1173 K. In particular, the interest has been focused on the x = 0 sample, which exhibits large ionic conductivity values (σi ~1 S cm−1), but suffers a structural transformation from cubic to orthorhombic symmetry because the ordering of the oxygen vacancies when the oxygen partial pressure decreases. Measurements of the oxygen chemical potential () as function of oxygen content and temperature, coupled with high temperature X-ray diffraction data, permitted us to broaden the knowledge of the T–δ–p(O2) phase diagram for the x = 0 sample. In addition, we have investigated the effects of the La incorporation on the stability range of the cubic phases of the Sr1−xLaxFe0.2Co0.8O3−δ solid solution.

Restricted access

Thermal diesel-like analysis

Quality control by thermal and chemometric analysis

Journal of Thermal Analysis and Calorimetry
Authors: Alexandre G. S. Prado, Rômulo D. A. Andrade, Jez W. B. Braga, and Paulo A. Z. Suarez

Abstract

Biofuel has been obtained by cracking of soybean (Glycine sp.) oil, which is characterized by acidity index, density, cetane index, copper corrosion, carbon residue, fulgor point, and heat of combustion. In order to evaluate the quality of biofuel as well as detect its adulteration with vegetable oil, partial least squares regression calibration models based on thermogravimetric (TG) analysis were used as a precise and an accurate method. Thirty mixtures of biofuel/diesel/vegetable oil standards were prepared. Twenty of them were used for calibration, and ten for validation. The results have shown that the thermogravimetric analysis, PLS/TG, presented the best performance for the detection of vegetable oil contamination with a root mean square error of prediction (RMSEC% w/w) of 0.23, with a relative error of prediction of 3.6%, corroborating with the success of TG analysis application to determine the quality of biofuels and diesel/biofuel blends, showing that the TG analysis is an excellent tool to control quality of biofuels.

Restricted access

The River Paraná is the second largest river of South America and its flood plain is covered by different kinds of forests and herbaceous vegetation. It is subject to an annual pulse of flooding; floods larger than the normal annual ones at irregular periods of few year and catastrophic extraordinary floods few times in a century. The last catastrophic flood was in 1983, followed by a short lived high flood in 1992. The catastrophic flood destroyed almost completely the herbaceous vegetation. Our hypotheses are, on the one hand, that the plant communities of this area will be restored rapidly, and on the other, that there will be a succession process which will produce a shift of communities so that, those on the higher part of the elevation gradient will encroach the ones at its lower part. We analyse, by means of the floristic composition, the effect of disturbance induced by catastrophic floods on the vegetation stability and dynamic processes, in an internal depression and pond of the riparian plant communities in an island of the River Paraná valley. The results strongly support the first hypothesis.

Restricted access

Abstract

A microcalorimetric method was applied to study microbial soil activity of ornamental flower (Dahlia pinnata) plantations when irrigated with potable water and wastewaters. The samples were irrigated with potable water PW sample (reference) and treated wastewaters from Municipal Wastewater Treatment Station of Asa Norte in Brasilia City (Brazil). Three different water treatments were applied to irrigate soil samples, named TW1, TW2, and TW3 samples. The increase of the microbial soil activity observed in TW1 sample must have occurred because of the high amount of organic waste dissolved in wastewater used for irrigation. This rise indicates that the present treated wastewater can affect natural life cycle. However, only a low alteration in microbial soil activity was observed in the TW2 and TW3 samples, which suggests that these wastewater treatments can be normally used to irrigate soils without bringing environmental consequences, once they offer a great opportunity to upgrade and protect the environment.

Restricted access