Vanni Lughi

Defect and stress characterization of AlN films by Raman spectroscopy

Raman spectroscopy was used to characterize the residual stress and defect density of AlN thin films reactively sputtered on silicon (100). The authors studied the correlation between the shift of the E2 (high) phonon of AlN at 658cm−1 and the film biaxial stress and obtained a biaxial piezospectroscopic coefficient of 3.7GPa∕cm−1. A correlation was found between the width of the Raman line, the oxygen concentration measured by secondary ion mass spectroscopy, and acoustic losses. This work lays the basis for the nondestructive assessment of two key thin film properties in microelectromechanical systems applications, namely, acoustic attenuation and residual stress.

The effect of wear on the tetragonal-to-monoclinic transformation and the residual stress distribution in zirconia-toughened alumina cutting tools

Abstract Zirconia-toughened alumina cutting tools have been examined after machining an AISI 1040 steel. The Raman spectrum of zirconia and the fluorescence spectrum from alumina have been used to determine the extent of the tetragonal-to-monoclinic (t-m) transformation and the residual stresses in and around the worn-out crater, respectively. Although about 15% vol. m-zirconia is present even in a brand new cutting tool, it was shown that the crater region undergoes the t-m transformation up to 40% vol. In both cases, the transformed zone is concentrated near the surface. The residual stresses in alumina are slightly compressive in the new cutting tool and away from the crater region, but they become tensile inside the crater, where the t-m transformation of zirconia has occurred to a larger extent. A stochastic model has been used to predict the stresses and it has been found to reproduce very well the experimental data. Some evidence of the formation of a spinel reaction product in the crater region has been found, likely (Mg,Fe)AlO 4 .

Economical analysis and innovative solutions for grid connected PV plants

The world total photovoltaic (PV) installed capacity is growing very quickly, due to both government incentives and increased environmental concerns. This situation calls for the realization of large, grid-connected ldquoindustrialrdquo PV plants, aimed at delivering energy to the grid. However, the cost/kWh of PV energy is still quite high. This paper reports some of the most promising research approaches currently in progress on new PV materials and devices, focusing on the reduction of PV generation cost expected from the technological implementation of such research. Because of their large size, industrial PV plants could have different characteristics compared to the small ones that, with few exceptions, have been realised until now. This problem is analyzed with the aim of providing a set of recommendations and guidelines for proper design of industrial PV plants, mainly concerning the choice of the power electronic conditioning systems and the use of simulation-aided design tools.

Towards long lasting zirconia-based composites for dental implants: Transformation induced plasticity and its consequence on ceramic reliability

Zirconia-based composites were developed through an innovative processing route able to tune compositional and microstructural features very precisely. Fully-dense ceria-stabilized zirconia ceramics (84vol% Ce-TZP) containing equiaxed alumina (8vol%Al2O3) and elongated strontium hexa-aluminate (8vol% SrAl12O19) second phases were obtained by conventional sintering. This work deals with the effect of the zirconia stabilization degree (CeO2 in the range 10.0-11.5mol%) on the transformability and mechanical properties of Ce-TZP-Al2O3-SrAl12O19 materials. Vickers hardness, biaxial flexural strength and Single-edge V-notched beam tests revealed a strong influence of ceria content on the mechanical properties. Composites with 11.0mol% CeO2 or above exhibited the classical behaviour of brittle ceramics, with no apparent plasticity and very low strain to failure. On the contrary, composites with 10.5mol% CeO2 or less showed large transformation-induced plasticity and almost no dispersion in strength data. Materials with 10.5mol% of ceria showed the highest values in terms of biaxial bending strength (up to 1.1GPa) and fracture toughness (>10MPa√m). In these ceramics, as zirconia transformation precedes failure, the Weibull modulus was exceptionally high and reached a value of 60, which is in the range typically reported for metals. The results achieved demonstrate the high potential of using these new strong, tough and stable zirconia-based composites in structural biomedical applications. STATEMENT OF SIGNIFICANCE Yttria-stabilized (Y-TZP) zirconia ceramics are increasingly used for developing metal-free restorations and dental implants. Despite their success related to their excellent mechanical resistance, Y-TZP can undergo Low Temperature Degradation which could be responsible for restoration damage or even worst the failure of the implant. Current research is focusing on strategies to improve the LTD resistance of Y-TZP or to develop alternative composites with better stability in vivo. In this work the mechanical characterization of a new type of very-stable zirconia-based composites is presented. These materials are composed of ceria-stabilized zirconia (84vol%Ce-TZP) containing two second phases (α-alumina and strontium hexa-aluminate) and exhibit exceptional strength, toughness and ductility, which may allow the processing of dental implants with a perfect reliability and longer lifetime.

Temperature dependence of the yttria-stabilized zirconia Raman spectrum

The Raman spectrum of 5wt% yttria-stabilized tetragonal zirconia from 25to1250°C is reported. All six Raman bands broaden and shift toward lower energy with increasing temperature. The shift associated with thermally induced volume changes is shown to be the main contribution for all bands, but does not explain the unexpected negative value of the thermospectroscopic coefficient found for the mode at 260cm−1 (at room temperature). For all bands, the shift is found to be quasilinear in the range of temperatures considered. As the thermospectroscopic coefficients are large, Raman spectroscopy is well suited as a noncontact, in situ method for monitoring temperature in applications of yttria-stabilized zirconia such as thermal barrier coatings and fuel cells.

Photovoltaics in Italy: Toward grid parity in the residential electricity market

The Italian photovoltaic market, since 2011 the worlds largest, represents a success story having attained grid parity for the residential market of electricity - thus setting the basis for surviving without subsidies. The Levelized Cost Of Energy (LCOE) is calculated for three representative locations in Northern, Central, and Southern Italy, and compared with the residential end-user electricity price. It is shown that grid parity is attained under most conditions for the Italian residential market, thus laying the basis for its survival without subsidies.

Grid parity in the Italian commercial and industrial electricity market

The Italian photovoltaic market is since 2011 the worlds largest and represents a success story having attained grid parity for the commercial and industrial (C&I) market of electricity. In this paper, the Levelized Cost Of Energy (LCOE) is calculated for three representative locations in Northern, Central, and Southern Italy, and compared with the C&I end-user electricity price. The grid parity is shown under certain conditions showing that the photovoltaic market is already ready to survive without the feed-in tariff mechanism.

Sputtered AlN thin films on Si and electrodes for MEMS resonators: relationship between surface quality microstructure and film properties

Aluminum nitride thin films grown by reactive AC magnetron sputtering are characterized using several metrology techniques to examine the correlation between surface quality, microstructure and piezoelectric properties. Atomic force microscopy, X-ray diffraction and electron microscopy are employed to characterize the microstructure. A range of substrate coatings is explored to understand the impact of topography on film crystallinity and piezoelectric performance. A first order approximation model providing the piezoelectric characteristics as a function of the c-axis misorientation in the mosaic-structured wurtzite AlN films is presented. While the model predicts only a small e/sub 33, eff/ and kt change for a misorientation distribution of FWHM of less than 5 degrees, it services as an indication of the impact of AlN crystallinity on film piezoelectric properties.

Beam-supported AlN thin film bulk acoustic resonators

A novel, suspended thin film bulk acoustic wave resonator (SFBAR) has been fabricated from an aluminum nitride film sputtered directly on a (100) silicon substrate. The suspended membrane design uses thin beams to support, as well as electrically connect, the resonator and has been fabricated using both thin film processing and bulk silicon micromachining. The quality factor and the effective electromechanical coupling coefficient were characterized as a function of the number and the length of the support beams. The length of the support beams was found to affect neither the quality factor at resonance nor the effective electromechanical coupling factor. However, longer support beams did facilitate better frequency pair response. Device performance varied with the number of support beams: 70% of the resonators tested showed a higher figure of merit with eight support beams than with four support beams

Fault diagnosis in photovoltaic arrays

This paper proposes a simple automatic technique for fault diagnosis in photovoltaic (PV) arrays, based on the analysis of the an omalies observed in the I-V characteristic. Firstly, the I-V characteristic of the PV array is simulated using Matlab/Simscape tool for different faulty conditions; which is experimentally validated by generating different faults on a PV string installed at the Renewable Energy Laboratory of the University of Jijel (Algeria). Subsequently, we compare the I-V characteristic of the PV string under different faults scenarios, in order to identify the anomalies. Finally, six categories are generated: Normal operation, connection fault, connection fault with shadow effect, partial shadow fault, a group of fault which include shadow effect with faults on bypass diode (open circuit bypass diode, inversed bypass diode, shunted bypass diode), and a group of fault which include: bypass diode fault, cell fault, module fault, and shadow effect with shunted by pass diode fault. The results show that the technique can accurately detect and localize faults occurring in the photovoltaic string.