Year 2013, Volume 33

Abstract

Highly porous hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂, HA) was prepared through hydrothermal (HT) transformation of aragonitic cuttlefish bones (Seppia Officinalis L. Adriatic Sea) in the temperature range from 140°C to 220°C for 20 minutes to 48 hours. Mechanism of hydrothermal transformation of bones was investigated by DTA/TG analyzer coupled online with FTIR spectrometric gas cell equipment (DTA-TG-EGA-FTIR analysis), X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM). DTA-TG-EGA-FTIR analysis have shown the release of CO₂ at about 400°C, 680°C and 990°C. The first release could be attributed to organics not completely removed from the heat treated bones, and the second release to decomposition of unconverted aragonite, whereas, the third one could be attributed to CO₃ ^2– groups incorporated in the structure of HA. The interconnecting porous morphology of the starting material (aragonite) was maintained during the HT treatment. The formation of dandelion-like HA spheres with diameter from 3 to 8 μm were observed, which further transformed into nanoplates and nanorods with an average diameter of about 200-300 nm and an average length of about 8-10 μm.

Abstract

Felipe Antunes Santos¹, Claudinei dos Santos^1,2, Durval Rodrigues Jr¹, Dolores Ribeiro Lazar³, Dayane Faviero de Castro¹, Daltro Garcia Pinatti¹, Rosa Ana Conte¹
1EEL - USP (Escola de Engenharia de Lorena da Universidade de São Paulo) - Polo Urbo-Industrial, Gleba AI-6, s/n, Mondesir, PC 116, Lorena, SP, Brazil
2 UNIFOA- MeMAT (Centro Universitário de Volta Redonda - Pró-Reitoria de Pesquisa e Extensão) - V. Redonda (RJ), Brazil
3IPEN (Instituto de Pesquisas Energéticas e Nucleares) - Av. Lineu Prestes, 2242, Cidade Universitária, CEP: 05508-000, São Paulo, SP, Brazil

Abstract

Using Mechanical Activation it is possible to obtain small grain size and good homogeneity in a ceramic piece. For ZnO varistor devices Mechanical Activation appears to be a good fabrication technique, since good homogeneity and small grain sizes are advantageous microstructural features. The typical formulation is composed by ZnO, Bi₂O₃, Sb₂O₃, CoO, MnO₂ and Cr₂O₃ as raw materials, and during sintering, several dissolutions and reactions to form pyrochlore and spinel phases occur. When Mechanical Activation is applied to the entire formulation, it is difficult to know what processes are being mechanically activated due to the complexity of the system. The aim of the present work was to clarify how the mechanical activation is taking place in a typical ZnO varistor formulation. The methodology consisted in the formation of all possible combinations of two out of the five oxides above mentioned and to apply mechanical activation on the mixture of each pair of powders. The results showed that systems containing Bi₂O₃ are prone to react during mechanical activation. Also, reduction reactions were observed in MnO₂. In addition, the powder mixture corresponding to the whole formulation was milled in a planetary mill, pressed and sintered, and varistor devices were fabricated. Improvement in the non-linearity coefficient and breakdown voltage was observed.

Abstract

Hetero-epitaxial growth of calcite crystals on calcium dodecyl sulfate (Ca(DS)₂ = CDS) single crystals was studied by alternate soaking method. The calcite (006) oriented single crystals grow on the (001) surface of the CDS. The hetero-epitaxial growth mechanism is discussed by the lattice matching of the a-c planes of calcite and CDS according to the structure data of the CDS single crystal.