1,721,519 research outputs found
Effects of Oxidation on Surface Stresses and Mechanical Properties of Liquid Phase Pressureless-Sintered SiC-AlN-Gamma2O3 Ceramics
No full textThe understanding of the oxidation mechanism of 50 wt% SiC-50 wt% AlN composites obtained by means of pressureless sintering without the protective powder bed and with Y2O3 as sintering-aid were significantly improved by means of Raman spectroscopy. These analyses put in evidence that amorphous carbon started to be formed at 1300 °C as main effect of active oxidation of SiC. At higher temperature the crystallization process began and it was completed at 1500 °C when only graphite could be recognized. On the basis of these new evidences, oxidation effects on the mechanical properties of SiC-AlN-Y2O3 composites were defined. First of all, heat treatment in air was able to induce a compressive surface stress due to the volume gain associated to the oxidation of the intergranular phase. As a consequence apparent fracture toughness showed a value of 6.6 MPa m1/2 after a heat treatment at 1300 °C, while at higher temperature effects of active oxidation caused a decreasing up to 4.7 MPa m1/2. This toughening mechanism was also used to improve the resistance to thermal shock, which was evaluated by performing quenching tests. Furthermore, passive oxidation induced the healing of superficial flaws by means of the formation of α-cristobalite. This phenomenon was assumed to be responsible for the increasing of the flexural strength
Long term oxidation behaviour of liquid phase pressureless sintered SiC-AlN ceramics obtained without powder bed
No full textThis paper describes the results of the investigation of the oxidation behaviour of 50 wt.% SiC-50 wt.%AlN composites obtained by means of pressureless sintering with Y2O3 as sintering-aid and without using a powder bed to limit the weight loss. Sintered bodies show high density (>98% T.D.) and the microstructure reveals a matrix composed by SiC-AlN solid solution with Y10Al2Si3O18N4 as grain boundary phase. This material exhibits a parabolic oxidation kinetic in the temperature range 1200-1500 °C. Long term oxidation (200 h) induces the formation of a protective oxide scale, composed by mullite, cristobalite and yttrium disilicide, up to 1400 °C. At higher temperature (1500 °C), SiC oxidation behaviour changes from passive to active and the oxide layer consequently shows porosity due to the formation of gaseous species like SiO and CO
U-series and radiocarbon dates on mollusc shells from the uppermost layer of the archaeological site of KHB-1, Ra's al Khabbah, Oman
No full textThe archaeological site of KHB-1, Oman, has been investigated in order to improve the chronological knowledge of the site, which is dated to the beginning of the fourth millennium B.C. on the basis of the archaeological context. Mollusc shells of the genus Anadara, coming from the uppermost layers of the site, have been dated using two different methods, radiocarbon and U-series technique. The results of the two techniques date the last occupational phase of the site from the end of the fourth millennium B.C. to the beginning of the third millennium B.C
Effect of the composition and sintering process on mechanical properties and residual stresses in zirconia-alumina composites
No full textSamples of zirconia-toughened alumina (ZTA) with small amounts of chromia and magnetoplumbite-type crystalline phase (CeMgAl11 O19) have been prepared and processed under different conditions. Mechanical properties like hardness and fracture toughness were examined as a function of different parameters. As an example, fracture toughness was increased by the chromia addition, whereas platelets reinforcement addition suppressed the tetragonal zirconia (t-zirconia) - monoclinic zirconia (m-zirconia) transformation. In addition, transformability of the tetragonal zirconia and the residual stress in the alumina phase were examined by Raman and fluorescence piezo-spectroscopy, respectively. In particular, the extent to which t-zirconia transforms to m-zirconia was determined by Raman spectroscopy after Vickers indentation and the transformability was correlated to the fracture toughness. It was demonstrated that the monoclinic content and the toughness were correlated linearly and experimental results were compared with models already available for zirconia-based materials. On the other hand, residual stresses originated by transformation toughening mechanism were correlated to the transformability of the tetragonal phase
SOREME: An Eu Life+ project for reducing mercury emissions through the sorption on a carbon sorbent produced from used tyres (Life 11 ENV/IT/109)
No full textIn this work we demonstrate the use of an innovative sorbent obtained from a pyrolytic process of wasted tyres as mercury sorbent (named SOREME) at laboratory, semi-industrial and at industrial level. The results showed that SOREME sorbent adsorbs in gas flows more mercury Hg0 (from 2.5 up to 4 times) than commercial sorbent and is able also to adsorb toxic volatile organic compounds (VOC) coming from the combustion. All the tests of mercury release form exhausted SOREME coming from lab, semi-industrial and industrial tests showed that mercury is more tightly bound to SOREME sorbent than to traditional activated carbon (AC)
Portfolios of learning in entrepreneurial internationalisation
No full textBy adopting a process approach to entrepreneurial internationalisation, we longitudinally study eleven case firms, discussing entrepreneurial learning as a portfolio of different sources of learning underpinning the exploration and exploitation of international opportunities. First, we identify the most recurring combinations of learning sources: (i) customer-centred learning with either learning by doing/trial and error or market-specific learning; (ii) imitative learning with relational learning and/or learning by doing; (iii) searching with networking learning and/or customer-centred learning. Second, we uncover instances of opportunity exploitation “looping back” to novel exploration via customer-centred, imitative, and grafting learning. Empirical results are systematised within a set of Propositions to be tested in future research
Properties of liquid phase pressureless sintered SiC-based materials obtained without powder bed
No full textHigh density liquid-phase pressureless sintered silicon carbide (LPSSC) bodies were obtained by means of a process which did not require powder bed to protect sample against weight loss. This process was successfully tested with yttria-alumina and yttria-aluminum nitride systems as sintering aids. In both cases, sintered samples showed a fine microstructure with Y4Al2O9 and Y10Al2Si3O18N4 as grain boundary phase, respectively. Sintering behaviour and mechanical properties were examined and compared to other LPSSC-based materials obtained with different sintering processes (hot pressing, gas pressure sintering, pressureless sintering with powder bed). Microstructural analysis confirmed that a glassy-phase scale protects these materials against oxidation up to 1500°C. In this manner we were able to demonstrate that silicon carbide could successfully be sintered by means of liquid phase mechanism also without powder bed. This fact opens liquid phase sintered silicon carbide to a wide range of industrial application
Mechanical properties of pressureless sintered SiC-AlN composites obtained without sintering bed
No full textHigh density (>98%T.D.) SiC-AlN composites were obtained by means of a pressureless sintering process which did not require the powder bed to protect sample against weight loss. Sintered sample showed a fine microstructure formed by 2H SiC-AlN solid solution with Y10Al2Si 3O18N4 as grain boundary phase. Mechanical properties (hardness, toughness and flexural strength) were determined and compared to the properties of SiC-AlN sample obtained with different sintering processes (hot pressing, gas pressure sintering, pressureless sintering with powder bed). Microstructural analysis confirmed that a mullite scale protects material against oxidation up to 1400°C
Defect-free zirconia-based materials for orthopaedic implants
No full textSamples of zirconia-toughened alumina (ZTA) with small amounts of chromia were prepared and processed under different conditions. Mechanical properties like hardness, fracture toughness and flexural strength were determined as a function of different parameters. As example, fracture toughness was increased by the chromia addition, whereas post-hipped samples showed a lower fracture toughness due to the formation of monoclinic phase. In addition, Weibull modulus determination was used to put in evidence that ZTA sintered components were defect-free and highly reliable
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