Universidade Federal de Santa Catarina

About: Universidade Federal de Santa Catarina is a education organization based out in Florianópolis, Brazil. It is known for research contribution in the topics: Population & Context (language use). The organization has 28408 authors who have published 55433 publications receiving 714461 citations.

Effects of metakaolin on autogenous shrinkage of cement pastes

Abstract: The effects of partial replacements (5%, 10%, 15% and 20%) of Portland cement by high-purity metakaolin (MK) on the autogenous shrinkage of pastes (water/binder ratios 0.3 and 0.5) were investigated. In order to distinguish the effect of heterogeneous nucleation and pozzolanic activity from the dilution effect, some mixes were prepared using a coarse powder (Qref) instead of MK (10% and 20% for both w/b ratios). The hydration of cement–MK pastes was followed qualitatively by differential thermal analysis (DTA). DTA showed that C–S–H and gehlenite (C2ASH8) were the main compounds produced by MK pozzolanic reaction. Results showed that the long-term autogenous shrinkage of cement–MK pastes, for both w/b ratios, decreased as the cement replacement level by MK increased. No expansion was observed at early ages, contrary to the findings of other authors. With the elimination of the dilution effect, it was shown, at early ages, that the increase of autogenous shrinkage of the cement–MK pastes relative to cement–Qref mixes was due to heterogeneous nucleation. At later ages, autogenous shrinkage became lower for cement–MK pastes than for cement–Qref pastes, surely because the pozzolanic activity of MK became predominant. This behavior i.e. reduction of autogenous shrinkage, is one more benefit confirming the interest of using MK in concrete.

Two-dimensional pressure-induced electronic topological transition in Bi 2 Te 3

Abstract: A structural peculiarity of the electronic topological transition (ETT) occurring within the pressure stability range of the low-pressure rhombohedral phase I has been evidenced in Bi${}_{2}$Te${}_{3}$. On both sides of the ETT the structure remains unchanged. Nevertheless, precise investigation of x-ray diffraction patterns allows us to conclude that this ETT obeys the lamellar character of this compound but in a counterintuitive way. Indeed, the signature of this ETT can be detected only in the layers' plane in the pressure variation of the lattice parameter $a$ with a 25% increase of the lattice modulus and a 68% decrease of its pressure derivative. On the contrary, no singularity occurs perpendicularly to the layers of the Bi${}_{2}$Te${}_{3}$ structure.

Cloud-point extraction for the determination of Cd, Pb and Pd in blood by electrothermal atomic absorption spectrometry, using Ir or Ru as permanent modifiers

Abstract: The phase-separation phenomenon of nonionic surfactants occurring in aqueous solution was used to perform the extraction of Cd, Pb and Pd from digested blood samples. After complexation with O,O-diethyldithiophosphate (DDTP) in hydrochloric acid medium, the analytes are quantitatively extracted to the phase rich in the nonionic surfactant octylphenoxypolyethoxyethanol (Triton X-114) after centrifugation. 0.1 mol L−1 HNO3 in methanol was added to the surfactant-rich phase prior to its analysis by electrothermal atomic absorption spectrometry (ETAAS). The adopted concentrations for DDTP, Triton X-114 and hydrochloric acid were all optimized, and the performance of 500 µg of either iridium or ruthenium as a permanent modifier was evaluated for the determination of Cd and Pb. No modifier was required for the Pd determinations. Detection limits (3σB) of 0.02 µg L−1, 0.08 µg L−1 and 0.014 µg L−1, along with enrichment factors of 71, 34 and 100 for Cd, Pb and Pd, respectively, were achieved. The proposed procedure was applied to the analysis of two certified blood reference samples (lyophilized bovine blood and reconstituted human blood) after microwave-assisted digestion in a mixture of H2O2 and HNO3. Comparison with certified values and recovery tests were performed to validate the methodology; good accuracy was obtained within the 95% confidence interval in both cases.