1,721,062 research outputs found
Fibre/matrix reactions in plasma sprayed composite monotapes (SiC-Ti6Al4V) at high temperatures
No full textTitanium-based alloys have found increased use in
high temperature applications due to their excellent
strength-to-weight ratio. In many cases, noticeable
increases in specific strength and stiffness can be
achieved through continuous fibre reinforcements.
Of primary interest in many applications are the
SiC-Ti6Al4V composites. One of the major problems faced with this composite system is the
degradation of composite properties by the reaction
of the titanium matrix with silicon carbide fibres at
high temperature. Interfacial reactions can be due to
fabrication methods and to the long term use at high
temperatures. Typical fabrication conditions result in
the formation of a reaction zone consisting of TiC
near the fibre surface, and various amounts of brittle
titanium silicides, including Ti5Si3, TiSi or TiSi2 [1–
8]. To reduce the thickness of the reaction zone it is
possible to fabricate monotapes by inert plasma
spraying [9], because this technology reduces liquid
metal=ceramic fibre contact time (about 100 ms) due
to the high solidification rate of sprayed metal
droplet
Ancient Bones Characterization and Preparation Through Freeze-Drying Process
No full textAncient bones, presumably dating back to 6000 BC and 1500 BC, have been characterized employing different physical analyses (optical and electron microscopy, characteristic X-ray fluorescence, and others) and prepared to be submitted to further investigations by nuclear techniques for dating analysis. Particular attention has been devoted to the freeze-drying process (also known as lyophilization) of the biological samples, which is based on the tissues drying under frozen environment, enabling removal of water and absorbed gases. Such a process presents different advantages, such as the possibility to store the samples at room temperature preserving their chemical properties, and the enhancement of the weight concentration of elements in the matrix. In this work attention is devoted to freeze-drying of cortical and trabecular bones, which loss water slowly with respect to soft tissues. The essential parameters to control during the process are the temperature and the pressure, the maintenance of a low gas pressure and the drying time, allowing the proper ice sublimation and the water degassing from the sample. The aim of the presented work is that to describe the lyophilization of cortical bones, to freeze and subsequently store the sample at low pressure, sublimating the ice on their surface and eliminating the gas under vacuum conditions. Experimental measures were performed to reduce the water content in the range of 1–5 wt.%. Measurements of water desorption, electronic microscopy before and after their treatment process, as well as Raman and FTIR spectroscopy were performed on the treated samples. The described procedure allows to prepare samples at high carbon concentration from which, employing nuclear analysis, it is possible to evaluate the content of stable isotopes and of radiocarbon, which have a crucial relevance for dating of biological samples
Effect of repeated salt fog-dry cycles on the performances reversibility of flax fiber reinforced composites
Full text linkThis paper aims to evaluate the effectiveness of flax fiber reinforced epoxy composites (FFRCs) under realistic environmental conditions, contributing to the development of sustainable composite materials for semi-structural outdoor applications. For this purpose, the reversibility of FFRC performance under consecutive and repeated humid-dry aging cycles was investigated for the first time. In particular, FFRCs were exposed to three cycles (total aging time of 12 weeks), with each cycle (duration 4 weeks) comprising 10 days of wetting (salt-fog spray; 35 °C, 95 % RH, 5 wt% NaCl solution) and 18 days of drying (22 °C, 50 % RH). Three-point bending tests were carried out up to 18 days of drying within each cycle to monitor changes in the mechanical performances of composites. Water absorption capacity, density and void content were also assessed by tracking weight changes throughout the aging campaign. The experimental results highlighted that the humid phase of each cycle causes degradation of FFRC materials, even though they are able to fully recover their flexural strength during each dry phase, indicating reversible aging. On the other hand, a stiffness permanent reduction was observed due to irreversible degradation phenomena
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