Strength improvement of LPS–SiC ceramics by oxidation treatment

In this work, SiC ceramics were liquid phase sintered (LPS), using AlN–Y₂O₃ as additives, and oxidized at 1400 °C in air for up to 120 h. Oxidation was monitored by the weight gain of the samples as function of exposition time and temperature. A parabolic growth of the oxidation layer has been observed and the coefficient of the growth rate has been determined by relating the weight gain and the surface area. The effect of oxidation on strength has been determined by 4-point bending tests. Phase analysis by X-ray diffraction and microstructural observation by scanning electron microscopy indicated the formation of a uniform and dense oxidation layer. The elimination of surface flaws and pores and the generation of compressive stresses in the surface resulted in a strength increase of the oxidized samples.     

Low density polyethylene composites reinforced with Australian King Palm fibers: mechanical and thermal properties

The aim of this work is to obtain and evaluate the mechanical and thermal properties of low Density Polyethylene (LDPE) composites reinforced with fibers from Australian King Palm fibers. Raw fibers were characterized by X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Fourier Transform Infrared (FTIR) Spectroscopy. The chemical composition of the fibers was also evaluated. After characterizing the fibers were mixed into the LDPE, in proportions of 5, 10, 15 and 20% (wt/wt) using a thermokinetic mixer model MH-50H. Furthermore tensile, flexural and impact specimens were prepared for evaluation of mechanical properties. The composites were analyzed through SEM micrographs of fractured surfaces and thermal analysis. The results indicate that the reinforcement decreases the thermal stability of the composites, but caused an increase the mechanical properties of the composites. The composites reinforced with of raw fibers 20% (wt/wt) showed significant increase in the tensile strength, flexural and impact.     

Dose adequacy in measles vaccinations in Tabora region, Tanzania

A study was carried out in Tabora region between July 1991 and September 1991, with the aim of determining the adequacy of measles vaccine given to children in Maternal Child Health clinics. Measles vaccine dose used per child was found to be 0.485 ml, which is about 0.385 ml when wastage due to removal of air bubbles is subtracted. Workers in the field need to be trained on the exact volume of vaccine required to be drawn into the syringe before vaccination, to ensure that a dose measuring exactly 0.5 ml of vaccine is injected to a child.     

Influence of CAD-CAM milling on the flexural strength of Y-TZP dental ceramics

In this work, the influence of milling strategy and cutter wear on the flexural strength of sintered Y-TZP ceramics machined in a CAD-CAM system, was investigated. Pre-sintered Y-TZP blocks were machined with tungsten carbide (WC) milling cutters and divided into three groups: a) samples machined with New Cutting Tools (NCT); b) samples machined with End-of-life cutting tools (ELCT); and c) samples machined and subsequently polished. The samples were machined parallel and perpendicularly to the diameter of pre-sintered Y-TZP blocks aiming to evaluate the effect of the machining orientation on roughness. After cutting, the specimens were sintered at 1530?°C for 2?h and characterized. After sintering, dense Y-TZP samples presenting grain size average of 0.81?±?0.23?μm, hardness of 1205?±?12 HV, K_C of 7.7?±?0.4 MPam^1/2 and flexural strength of 1207?±?199?MPa were obtained. Furthermore, the control roughness (polished surfaces) presented R_a?=?0.058?±?0.011?μm. The surface roughness of the samples was influenced by the machining methodology using NCT mills, with R_a?=?0.386?±?0.149?μm and R_a?=?0.292?±?0.105?μm for samples machined parallel and perpendicularly to the pre-sintered Y-TZP blocks, respectively. The use of ELCT led to a roughness increasing (R_a?=?0.582?±?0.183?μm for samples milled perpendicularly, and R_a?=?0.919?±?0.164?μm for samples cut parallel to the pre-sintered blocks. The flexural strength of the sintered Y-TZP decreases with use of ELCT as consequence of surface roughness increasing, resulting from wear of WC mills, which presented chippings with sizes ranging from 30 to 340?μm in the cutting edge.     

Influence of heat-treatment protocols on mechanical behavior of lithium silicate dental ceramics

In this work, three different commercial lithium silicate (LS) glass-ceramics for computer aided design/computer aided machining systems, CeltraDuo-Dentsply (LS-C), E-MaxCAD-Ivoclar (LS-E), and Suprinity-Vita (LS-S), were comparatively characterized. Following the protocols recommended by the manufacturers, the glass-ceramics were heat-treated under low vacuum and characterized by X-ray diffraction, scanning electron microscopy, hardness, fracture toughness, Young's modulus, and flexural strength. Rietveld refinement indicated that the materials “as-received” present mostly amorphous phase and Li₂SiO₃ as secondary crystalline phase in LS-E and LS-S specimens, while LS-C specimens also present Li₂Si₂O₅ and Li₃PO₄ as crystalline phases. All “as-received” glass-ceramics present hardness, fracture toughness, and Young's modulus of around 647-678 HV, 1.15-1.40 MPa.m^1/2, and 82-92 GPa, respectively. After heat treatment, the LS-C and LS-S specimens presented decreasing of amorphous phase associated to Li₂SiO₃ and Li₂Si₂O₅ grains with low aspect ratio, while LS-E indicates a reduction of amorphous phase and Li₂Si₂O₅ elongated grains. Fracture toughness and Young's modulus increase about 10% due to the crystallization of residual amorphous phase for all materials. Moreover, crystallographic and microstructural characteristics are responsible for the higher flexural strength of LS-E (327 MPa), regarding LS-C and LS-S. However, the glass-ceramics LS-E present lower Weibull modulus (m = 5.4) comparatively to LS-C (m = 9) and LS-S (m = 6).     

Circularly polarised proximity-fed microstrip antenna with polarisation switching ability

The authors propose a design of a circularly polarised proximity-fed microstrip antenna having polarisation switching ability. The proposed antenna has a simple structure, consisting of a truncated-corners square patch in the same plane with a microstrip feed line extended beyond the patch edge, pin diode and a pad connected to a ground by a conducting post. The diode, which is inserted between the end of the feed line and a pad, is used to control feed line termination. By turning the diode on or off, this antenna can radiate either right hand or left hand circular polarisation. Furthermore, we present a technique to improve the input characteristics of the antenna. It involves two diode-controlled tuning stubs connected in shunt with the feed line. Finally, a discussion on how the diode characteristics affect the structure and results of the antenna is given. All analyses are carried out using finite-difference time-domain technique and confirmed by measurements in the 5 GHz band. A good agreement between the simulated and measured results was obtained