Estudo da Sinterabilidade de cerâmicas dentárias de ZrO2 (Y2 O3 )-Biovidro usando dilatometria.

O objetivo deste trabalho é estudar por análise dilatométrica à sinterização por fase líquida da cerâmica ZrO2 utilizando biovidro como aditivo. Pós de ZrO2 estabilizados com Y2 O3 foram misturados com 3%, 5% e 10% em peso de biovidro com a composição baseada no sistema 3CaOP2 O5 -MgO-SiO2 . As amostras foram preparadas por prensagem uniaxial a frio sob 80 MPa e a densidade relativa à verde foi determinada. O comportamento de sinterização nas diferentes temperaturas foi estudado medindo a retração linear das amostras num dilatômetro. As velocidades de aquecimento e resfriamento usados neste estudo foram de 10ºC/min. A temperatura máxima de sinterização foi de 1300ºC com um tempo de retenção isotérmica de 120min. Os resultados de retração e das taxas de retração associadas a temperatura de sinterização ou de tempo foram relacionados com a quantidade de biovidro na amostra. As amostras sinterizadas foram caracterizados por sua densidade relativa e pela análise de Difracção de Raios-X. As micrografias indicam microestruturas semelhantes. Um aumento da quantidade de biovidro leva a aumento da fase monoclínica de ZrO2 . Os resultados da dilatometria indicam uma redução da temperatura quando o máximo de retração ocorre, em função do aumento da quantidade de biovidro. Além disso, o uso da fase líquida reduz a temperatura máxima de sinterização de 1447ºC para 1250ºC – 1280ºC

Atom Skimmers and Atom Lasers Utilizing Them

Atom skimmers are devices that act as low-pass velocity filters for atoms in thermal atomic beams. An atom skimmer operating in conjunction with a suitable thermal atomic-beam source (e.g., an oven in which cesium is heated) can serve as a source of slow atoms for a magneto-optical trap or other apparatus in an atomic-physics experiment. Phenomena that are studied in such apparatuses include Bose-Einstein condensation of atomic gases, spectra of trapped atoms, and collisions of slowly moving atoms. An atom skimmer includes a curved, low-thermal-conduction tube that leads from the outlet of a thermal atomic-beam source to the inlet of a magneto-optical trap or other device in which the selected low-velocity atoms are to be used. Permanent rare-earth magnets are placed around the tube in a yoke of high-magnetic-permeability material to establish a quadrupole or octupole magnetic field leading from the source to the trap. The atoms are attracted to the locus of minimum magnetic-field intensity in the middle of the tube, and the gradient of the magnetic field provides centripetal force that guides the atoms around the curve along the axis of the tube. The threshold velocity for guiding is dictated by the gradient of the magnetic field and the radius of curvature of the tube. Atoms moving at lesser velocities are successfully guided; faster atoms strike the tube wall and are lost from the beam

Steatite/Epoxy Composites for Restoration Works Through a Statistical Mixture Design Methodology

Currently many works of art made of soapstone and recognized as cultural patrimony of humanity are in an advanced stage of degradation. Hence, it is necessary to interrupt this process and recover the deteriorated parts. Composite materials consisted of steatite particles and epoxy polymer are designed and characterised for their application in the repair of sculptures made of soapstone. The material applied in restorations should provide coloration and texture similar to soapstone besides structural requirements. The degree of similarity of the artificial material to the rock is enhanced by the proper selection of the particle size range and the increase of steatite incorporation in the composites. A statistical methodology based on the mixture design is used to optimize the relative amount of three particle size fractions of steatite particles in order to maximise the proportion of the dispersed phase in the composites. The maximum particle packing density (1.50 g/cm³) is obtained for a ternary mixture, composed of 62% of coarse particles (1.18 mm - 0.60 mm), 6% of medium sized particles (0.60 mm - 0.30 mm) and 32% of fine particles (0.30 mm - 0.15 mm). In this manner, the fabrication of composites based on an epoxy polymer matrix with 70wt% of incorporated steatite particles has been possible, increasing the maximum amount by 10% as used in previous works

A Quantum Scattering Interferometer

The collision of two ultra-cold atoms results in a quantum-mechanical superposition of two outcomes: each atom continues without scattering and each atom scatters as a spherically outgoing wave with an s-wave phase shift. The magnitude of the s-wave phase shift depends very sensitively on the interaction between the atoms. Quantum scattering and the underlying phase shifts are vitally important in many areas of contemporary atomic physics, including Bose-Einstein condensates, degenerate Fermi gases, frequency shifts in atomic clocks, and magnetically-tuned Feshbach resonances. Precise measurements of quantum scattering phase shifts have not been possible until now because, in scattering experiments, the number of scattered atoms depends on the s-wave phase shifts as well as the atomic density, which cannot be measured precisely. Here we demonstrate a fundamentally new type of scattering experiment that interferometrically detects the quantum scattering phase shifts of individual atoms. By performing an atomic clock measurement using only the scattered part of each atom, we directly and precisely measure the difference of the s-wave phase shifts for the two clock states in a density independent manner. Our method will give the most direct and precise measurements of ultracold atom-atom interactions and will place stringent limits on the time variations of fundamental constants.Comment: Corrected formatting and typo

Sinterização de Pós Nanoparticulados de ZrO2 (Y2 O3) - Efeito da pressão de compactação na densificação

Neste trabalho, o estudo da sinterização de pós nanoparticulados de ZrO2 (Y2 O3 ) por dilatometria foi realizado visando identificar o efeito da variação de pressão de compactação nosresultados finais de densificação de materiais sinterizados. Pós nanoparticulados foram compactados em diferentes pressões de compactação, utilizando prensagem uniaxial. Os compactos foram submetidos à dilatometria, em temperaturas finais de 1250 a 1400º C, com patamares de sinterização variando de 0 a 8h. Corpos de prova sinterizados foram caracterizados por Difração de Raios-X e densidade relativa, utilizando Método de Arquimedes. Os resultados foram comparados com pós microparticulados de mesma composição química visando comparar o efeito do tamanho de partículas nos parâ- metros de densificação. As amostras foram ainda submetidas à caracterização microestrutural visando identificar o tamanho médio dos grãos em cada condição de sinterização utilizada, em ambos os materiais

Si3N4 ceramics sintered with Y2O3/SiO2 and R2O3(ss)/SiO2: a comparative study of the processing and properties

A comparative study was made of Si3N4 sintered with two types of additives, namely, Y2O3/SiO2 and R2O3(ss)/SiO2, R2O3(ss) being a rare earth metal oxide in solid solution. The processing conditions for both types of Si3N4 were 14 vol% of additives and a sintering temperature of 1800 ºC for 30, 60 and 240 min. To compare the efficiency of the additives in the material's sintering process, the density, flexural strength, fracture toughness and hardness were measured and the phase composition and microstructure determined. The results indicated that R2O3(ss)/SiO2 as a sintering aid improved the material's high temperature strength and slowed down grain growth when compared with the Y2O3/SiO2 additive

Fracture toughness measurements of LPS-SiC: a comparison of the indentation technique and the SEVNB method

Many methods are currently used to measure the fracture toughness of ceramic materials. Methods based on crack-length measurements of cracks introduced into the sample surface by the Vicker's indentor have the advantage that they are easy to use, but are very unreliable due to subcritical crack growth and the difficulty in determining the exact length of the cracks. Furthermore, depending on the crack shape there are many equations to calculate K Ic. Other methods like the Chevron Notch or Single Edge Pre-cracked Beam (SEPB) are often difficult to execute or expensive. The simple and inexpensive Single-Edge-V-Notched Beam (SEVNB) on the other hand gives reliable values of fracture toughness of ceramic materials. In this method a saw cut is tapered to a sharp V-notch using a razor blade sprinkled with diamond paste. Thus, it is possible to introduce a sharp crack with a notch width of less than 20 micrometers, necessary to conduct valid tests. In this investigation, fracture toughness measurements on LPS-SiC materials carried out by the indentation technique and the SEVNB method have been compared