349 research outputs found
Room temperature aging of Laminate Composites of Alumina/3-mol%-Yttria-Stabilized Tetragonal Zirconia Polycrystals
No full textThe stresses of laminate structures obtained by joining single layers of pure alumina (A), pure yttria-stabilized tetragonal zirconia, 3Y-TZP (Z), and an intimate mixture of alumina and zirconia (AZ) have been determined by fluorescence (in alumina) and Raman (in zirconia) piezospectroscopy. Three symmetrical stacking sequences were examined, namely, A/Z/A, A/AZ/A, and AZ/Z/AZ, with the aim of designing structures where the higher coefficient of thermal expansion (CTE) of zirconia could be used to induce compressive stress in the external layers (and ensuing tensile stress in the central layer). Two experimental sessions, 6 years apart, were conducted on the same samples, also taking care to record the spectra from the same locations; during the time elapsed between the two sessions, the samples were kept at room temperature and humidity. The stress values in alumina obtained during the more recent session were markedly different from those observed in the first session
Giovani e Fuoco
No full textNel primo pomeriggio del 5 agosto 1949, quindici operatori anti-incendio del Servizio Forestale degli Stati Uniti vennero lanciati con il paracadute sulla gola di Mann Gulch, nel Montana. Dovevano contrastare e poi arginare un incendio innescato da un fulmine schiantatosi su un crinale il giorno prima. A terra trovarono un altro operatore che stava già lavorando. Meno di due ore dopo il lancio tredici dei sedici giovani (il più vecchio aveva 32 anni ed il più giovane 17) furono raggiunti ed uccisi da una deflagrazione di fiamme che in meno di 10 minuti spazzò tutta la gola, lunga quasi tre chilometri.
Questo libro, veramente unico nel suo genere, si legge come un appassionante romanzo, ma è una vivida ricostruzione dell’accaduto: vengono descritti gli uomini e presentato il nemico che combattono, il fuoco. Il racconto del fulmineo susseguirsi degli eventi e delle emozioni, minuzioso ma sempre lieve, conduce il lettore dentro l’epica vicenda dei giovani protagonisti, tra le fiamme nella gola di Mann Gulch.
L’autore, Norman Mclean, era originario del Montana e durante gli anni della prima guerra mondiale, quindicenne, era stato a sua volta un operatore anti-incendio. Qualche giorno dopo la tragedia visitò la gola di Mann Gulch e rimase impressionato da quello che vide e dal pensiero di quelle tredici vite sacrificate. Si portò dentro questa macerazione finchè, dopo essere andato in pensione, negli anni settanta, decise di provare a ricostruire l’accaduto. Il risultato è questo libro, cui fino l’autore lavorò fino alla morte che lo colse nel 1990, quando aveva 88 anni. Negli Stati Uniti il libro ebbe grande successo e fu nominato Libro dell’anno 1992 dal Circolo di Critica letteraria. Ora, dopo 30 anni, il libro è stato tradotto per l’Italia
Effects of sample orientation in Raman microspectroscopy of collagen fibers and their impact on the interpretation of the amide III band
No full textIn most tissues, collagen is present in the form of fibrils, large supramolecular assemblies of fundamental
structural units of polypeptide chains. In this contribution we report a clear correlation between the
intensity profile of the Raman spectrum of collagen and the orientation of a collagen fiber, depending on
the laser polarization. More specifically, fiber orientation is shown to affect the relative intensity ratio
between the bands at 1245 and 1268cm−1 (amide III doublet). In view of this finding based on the analysis
of a collagen fiber, the customary use of this parameter as an exclusive marker for collagen secondary structure in tissues has to be reconsidered, taking into account fibrils orientation. Moreover, in Raman microspectroscopy of tissues this effect could be exploited to yield information, which could be relevant for diagnostic purposes or fundamental research, about the orientation of collagen fibrils
Low temperature degradation -aging- of zirconia: a critical review of the relevant aspects in dentistry
No full textThis review present a survey of all experimental data about the low temperature degradation of zirconia (often referred to as “aging”) due to the tetragonal-to-monoclinic transformation, which have been collected at temperatures of interest for dental application (room temperature to about 100°). It is shown that the main factors affecting the aging phenomenon are the stabilizers type and content, the residual stress and the grain size. It is also shown that extrapolating the low temperature degradation rate from accelerated aging tests can lead to unacceptable conclusions about the lifetime of the zirconia-based components. Finally, based on the experimental evidence, a set of engineering guidelines for the use of zirconia in restorative and prosthetic dentistry is proposed
Deformation bands in ceria-stabilized tetragonal zirconia/alumina: II, stress-induced aging at room temperature
No full textA stress-induced aging phenomenon was observed to occur at room temperature in deformation bands introduced into a 8.5 mol% ceria-stabilized tetragonal zirconia/alumina composite by flexural loading. Over a period of 100 days, the concentration of monoclinic zirconia within a deformation band increases and, in addition, the wedge-shaped deformation band grows with time. Accompanying these two changes are an increase in the tensile stress in the remaining tetragonal zirconia within the deformation band and a consequential increase in the overall compressive stress within the band. The average value of the monoclinic concentration within the deformation band was found to increase parabolically with time, suggesting the mechanism responsible for the observed aging is diffusion limited. Away from the deformation bands, no aging was observed to occur suggesting aging is stress dependent
Piezo-Spectroscopic Behavior of the emission bands of alfa-alumina in the 13900-14250 cm-1 spectral range
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