Materials Science Forum Vols. 727-728

Abstract: In this work a sintering model that assumes infinitesimal displacements rates has been structured using the continuum theory of sintering develop by Olevsky and Skorokhod [1, 2] and the sintering model develop by Sasan Kiani et al. [3, 4]. The model was used to estimate dimensional change and the density distribution using linear displacement rates (LRDs) as the only input. Furthermore, if the sintering potential and dense material shear viscosity are known, the stress distribution can be approximated. The magnitude of two stress norms (i.e. Von Misses stress and the stress tensor first invariant) determined using experimental and calculated LDRs are compared using three different geometries. From FEM simulation results it was inferred that densification can occur in presence of tension stresses if their magnitude is lower than the calculated sintering potential.

Abstract: The microstructures of solid electrolytes play an important role in the level of ionic conductivity of these materials. Thus join the mechanical properties of partially stabilized tetragonal phase and the ionic conductivity displayed by the large area of the grain boundaries of polycrystalline zirconia nanometric is an advantage. The objective of this work is to study the densification of ultrafine particles of yttria stabilized zirconia in the tetragonal phase (Y-TSZ) obtained by chemical routes using the unconventional method of sintering the Two Steps Sintering (TSS). The nanocrystalline powders Y-TSZ (4.5 mol%) were prepared by Pechini method and were characterized by X-ray diffraction and scanning electronic microscopy (SEM). The powders were pressed and sintered per TSS. The density of sintered samples was measured by Archimedes method, the crystalline structure was determinate by X-ray diffraction and the grain size and microstructure were observed by SEM. The TSS prevented the grain growth keeping your submicrometer grain size.

Abstract: The brittle behavior of ceramics limits the use of these materials under conditions of cyclic loading, as is the case of fixed partial dentures. To improve toughness and biocompatibility of ceramics is necessary to employ powders with better purities, adjust the conditions of compaction and sintering, microstructure control and explore mechanisms for increasing the toughness. Yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) is generally used for dental ceramic restorations. The zirconia framework is fabricated using the CAD/CAM system. The ceramic mechanical properties are determined by testing polished sample. Previous work did not analyze the influence of surface defects induced during dentures grinding. Ceramic restorations manufacturing are made with coarse grinding high-speed diamond rotatory cutting instruments. This process induces residual stresses and the high temperature induces surface cracks. Consequently, a lower strength and reliability of the material is observed. In this work the mechanical properties of yttria stabilized zirconia were determined with the use of samples of dental prostheses molded in patients and machined with the use of a dental laboratory CAD/CAM system. The results showed that the mechanical properties of pre-sintered blocks are different from prosthetics machined by CAD/CAM and sintered under the same conditions used in the laboratories of the prosthesis. The defects created during machining reduce the mechanical properties of zirconia.

Abstract: Si₃N₄ based ceramics are widely researched because of their low density, high hardness, toughness and wear resistance. Post-sintering heat treatments can enhance their properties. Thus, the objective of the present paper was the development of a Si₃N₄ based ceramic, suitable for structural applications, by sintering in nitrogen gas pressure, using AlN, Al₂O₃, and Y₂O₃ as additives and post-sintering heat treatment. The green bodies were fabricated by uniaxial pressing at 80 MPa with subsequent isostatic pressing at 300 MPa. The samples were sintered at 1900°C for 1 h under N₂ gas pressure of 0.1 MPa. Post-sintering heat treatment was performed at 1500°C for 48 h under N₂ gas pressure of 1.0 MPa. From the results, it was observed that after post-sintering heat treatment there was a reduction of α-SiAlON phase and increase of β-Si₃N₄ phase, with consequent changing in grain size, decrease of fracture toughness and increase of the Vickers hardness.

Abstract: The effects of adding polymer precursor on the sintering behavior, microstructure and hardness of alumina ceramics have been studied. Polymer sintering aid polymethylhidrogensiloxane were used for alumina based ceramics sintered at 1650º C. The sintered materials were characterized using: (a) helium picnometry to determine apparent density; (b) x-ray diffraction analysis, scanning electron microscopy, transmission electron microscopy and energy dispersive x-ray spectrometry to determine the microstructure and composition; (c) Vickers indentation analysis to determine the hardness and fracture toughness. The addition of polymethylhidrogensiloxane yielded alumina/mullite composites. The preparation of ceramic composites using small amounts of polymer precursors is suited for alumina based ceramics. The processing route being simple, it has the potential to enable the manufacture of complex shaped pieces.

Abstract: In this work, the effect of pressure and direction of compression in the shrinkage and flatness of ZrO₂-(Y₂O₃) dental ceramics were studied. Samples (40x15x15mm) were uniaxial cold pressed at different pressures ranging from 10MPa to 160MPa, and then pre-sintered for further cutting. For comparison, samples were also isostatically cold pressed at 300MPa. The pre-sintered samples were cut longitudinally and transversely to the direction of compression, with final thickness of 1mm. Then the samples were ground and polished, and measured with a micrometer at different positions of the sample. After the measurements, the samples were sintered at 1530C-120 min, and again measures. The results indicate the effect of pressure decline in considerable amounts, up to pressures of about 80MPa. Above this value the differences in shrinkage and warpage are insignificant when compared to isostatically pressed samples.

Abstract: Studies has been reported a significant incidence of chipping of the feldspathic porcelain veneer in zirconia-based restorations. The purpose of this study was to compare the three-point flexural strength (MPa), Weibull parameters, Vickers hardness (VHN) and Vickers indentation fracture toughness (MPa/mm^1/2) in feldspatic porcelains for metal and for zirconia frameworks. Bar specimens were made with the porcelains e.MaxCeram (EM) and VitaVM9 (V9) for zirconia core, and Duceragold (DG) and VitaVMK95 (VK) for metal core (n= 15). Kruskal-Wallis and Dun test were used for statistical analysis. There was no significant difference (p=0.31) among the porcelains in the flexural strength (Median= 73.2; 74.6; 74.5; 74.4). Weibull calculation presented highest reliability for VK (10.8) followed by EM (7.1), V9 (5.7) and DG (5.6). Vickers hardness test showed that EM (536.3), V9 (579.9) and VK (522.1) had no difference and DG (489.6) had the lowest value (p.001). The highest fracture toughness was to VK (1.77), DG (1.58) had an intermediate value while V9 (1.33) and EM (1.18) had the lowest values (p.001). Despite of the suitable flexural strength, reliability and high hardness, the porcelains used to zirconia-based fixed dental prostheses showed lower fracture toughness values.

Abstract: In this work, samples of 15 mol% Sr-doped Al₂O₃ were synthesized by Pechini's method and calcined at 900°C. DRX analysis showed that only corundum phase is present, and no shift on the main peaks was observed, suggesting that the additive is preferentially located on the surface of nanoparticles as a surface excess. SrO show high basic character and is applied as heterogeneous catalyst in transesterification of soybean oil to produce biodiesel. Since SrO is difficult to obtain through Pechinis method, due to high stability of SrCO₃, the use of Al₂O₃ as a catalyst support produced via polymeric precursor was studied, and the reaction of transesterification was successfully made, because the crystallization of strontium carbonate on the corundum surface was avoided using the washing procedure. Also, the catalyst was characterized by DSC, isoelectric point and surface area.

Abstract: Along decades, bioceramics have been used as materials for bone reconstruction, where hydroxyapatite is one of the most used bioceramics. But hydroxyapatite mechanical strength is not so high when compared with another bioceramics. This research aimed to characterize hydroxyapatite-titanium oxide scaffolds with different compositions. Samples were made using a polyurethane sponge with compositions of 70%-30% wt., 60%-40% wt. and 50%-50% wt. of hydroxyapatite-titanium oxide, calcined at 550°C for burning the polymeric sponge and sintered at 1250°C, 1300°C and 1350°C. Samples were characterized by scanning electron microscopy (SEM), x-ray diffraction (XRD) and energy dispersive x-ray spectroscopy (EDS). Results showed that all compositions maintained the polymeric sponge pores structure without any residual traces of the polymeric sponge. Those results are in accordance with technical literature and it is indicated to do in vitro essays to study the scaffolds biocompatibility for using as bone reconstruction materials.

Abstract: ZnAl_1.,9Eu_0.1O₄ nanoparticles were functionalization used of the 3-aminopropyltrimethoxysilane silane and chitosan, in 5:1 proportion of the nanoparticles to chitosan, in order to obtain hybrid materials for the development of luminescent markers. The samples were characterized by XRD, FTIR, TEM, excitation and emission. After functionalized the nanoparticles showed formation of crystalline phase of spinel and chitosan. TEM results have revealed that the samples presented before particle size less than 100 nm and after functionalization was observed that the particle size increases. Emission peak of Eu centered at 614 nm, corresponding to the transition ⁵D₀ ⁷F₂. Nanoparticles before and after functionalization showed the spectral lines characteristics of Eu ion ⁵D₀ ⁷F_j where (J = 0,1,2,3,4). After functionalization the sample presented an increase of luminescence intensity.