Repositório Colecção: CF-UM-PQMBCF-UM-PQMBhttps://hdl.handle.net/1822/573872024-03-29T00:40:11Z2024-03-29T00:40:11ZOvercoming the contact problem in quantitative attenuated total reflection spectroscopy analysis of flat samplesVieira, L. G.https://hdl.handle.net/1822/873582023-11-30T20:57:17ZTítulo: Overcoming the contact problem in quantitative attenuated total reflection spectroscopy analysis of flat samples
Autor: Vieira, L. G.
Resumo: A method for measuring the optical functions of a flat sample made of homogeneous and isotropic material, using attenuated total reflection spectroscopy when there is poor contact between the sample and the internal reflection element is presented. The approach consists in treating the spacing between the internal reflection element and the sample as an adjustable parameter, along with the dispersion model parameters, in the simultaneous fitting of s- and p-polarized spectra obtained when the gap distance is unknown. The method is tested with both synthetic and experimental (polystyrene) spectra. The results demonstrate the method’s ability to accurately determine the optical functions even in the presence of a contact problem.
<b>Tipo</b>: articleInfrared optical functions of water retrieved using attenuated total reflection spectroscopyVieira, L. G.https://hdl.handle.net/1822/873552023-11-28T20:57:12ZTítulo: Infrared optical functions of water retrieved using attenuated total reflection spectroscopy
Autor: Vieira, L. G.
Resumo: Comprehensive modeling of non-polarized infrared attenuated total reflection spectrum based on Fresnel equations and wavenumber-dependent dielectric function models of isotropic materials is shown to be a suitable and easy methodology to retrieve optical functions. The scheme is completely general and can be used even for strong dispersion and absorption resonances. Attenuated total reflection spectra in liquid water, measured in the spectral region 100–4400 cm-1 with diamond and germanium as internal reflection elements, were used to illustrate and evaluate the method. The refractive index of water computed from the dispersion analysis is critically compared with literature data.
<b>Tipo</b>: articleDevelopment and production of a CNC machined 420 stainless steel reinforced with Cu by hot pressingCunha, A.Pinto, J.Cerqueira, M. F.Silva, Filipe SamuelTrindade, B.Carvalho, O.https://hdl.handle.net/1822/870322023-10-23T09:11:08Z2023-10-23T09:09:15ZTítulo: Development and production of a CNC machined 420 stainless steel reinforced with Cu by hot pressing
Autor: Cunha, A.; Pinto, J.; Cerqueira, M. F.; Silva, Filipe Samuel; Trindade, B.; Carvalho, O.
Resumo: Multi-material structures make it possible to obtain effective solutions to engineering problems by combining the benefits of different materials to meet the requirements of different working conditions. The aim of this multifunctional 420 stainless steel-copper structure is to create a hybrid solution in which copper acts as heat-transfer enhancer (through cooling channels) while maintaining the required mechanical properties of the steel matrix. This work focuses on a combined engineering process consisting of CNC machining through holes on a 420 stainless steel surface substrate and subsequent filling with copper by hot pressing. The influence of the copper filling on the physical, chemical, microstructural, mechanical, and thermal properties of this multi-material solution was analysed. The machined area (5% of the total surface area) consisted of nine holes with a diameter of approximately 1 mm. The multi-material samples showed high densification, homogeneous microstructures, and a well-defined and sharp interface between the two materials. The microhardness values measured for the 420 stainless steel and copper were 759 and 57 HV, respectively, and the thermal conductivity of the multi-material was ≅ 59% higher than the 420 stainless steel (39.74 and 16.40 W/m K, respectively).
<b>Tipo</b>: article2023-10-23T09:09:15ZDielectric spectroscopy of melt-extruded polypropylene and as-grown carbon nanofiber compositesPaleo, Antonio J.Samir, Z.Aribou, N.Nioua, Y.Martins, Marcos SilvaCerqueira, M. F.Moreira, J. AgostinhoAchour, M. E.https://hdl.handle.net/1822/869992023-10-20T20:02:45ZTítulo: Dielectric spectroscopy of melt-extruded polypropylene and as-grown carbon nanofiber composites
Autor: Paleo, Antonio J.; Samir, Z.; Aribou, N.; Nioua, Y.; Martins, Marcos Silva; Cerqueira, M. F.; Moreira, J. Agostinho; Achour, M. E.
Resumo: In this work, different weight contents of as-grown carbon nanofibers (CNFs), produced by chemical vapor deposition, were melt-extruded with polypropylene (PP) and their morphologic, structure and dielectric properties examined. The morphologic analysis reveals that the CNFs are randomly distributed in the form of agglomerates within the PP matrix, whereas the structural results depicted by Raman analysis suggest that the degree of disorder of the as-received CNFs was not affected in the PP/CNF composites. The AC conductivity of PP/CNF composites at room temperature evidenced an insulator–conductor transition in the vicinity of 2 wt.%, corresponding to a remarkable rise of the dielectric permittivity up to 12 at 400 Hz, with respect to the neat PP ( 2.5). Accordingly, the AC conductivity and dielectric permittivity of PP/CNF 2 wt.% composites were evaluated by using power laws and discussed in the framework of the intercluster polarization model. Finally, the complex impedance and Nyquist plots of the PP/CNF composites are analyzed by using equivalent circuit models, consisting of a constant phase element (CPE). The analysis gathered in here aims at contributing to the better understanding of the enhanced dielectric properties of low-conducting polymer composites filled with carbon nanofibers.
<b>Tipo</b>: articleConfronting Vegard's rule in Ge1-x Sn (x) epilayers: from fundamentals to the effect of defectsMagalhaes, S.Dias, M.Nunes, B.Oliveira, F.Cerqueira, M. F.Alves, E.https://hdl.handle.net/1822/869882023-10-19T10:52:00ZTítulo: Confronting Vegard's rule in Ge1-x Sn (x) epilayers: from fundamentals to the effect of defects
Autor: Magalhaes, S.; Dias, M.; Nunes, B.; Oliveira, F.; Cerqueira, M. F.; Alves, E.
Resumo: Comprehensive and systematic study challenging the application of Vegard's rule to germanium tin solid solutions grown on germanium buffer layers and 100 silicon substrates is presented. The binary's lattice parameters, composition and respective uncertainties are determined through x-ray diffraction via reciprocal space mapping technique employing newly developed software. The tin content is confirmed by Rutherford backscattering spectrometry and energy dispersive x-ray spectroscopy. The statistical agreement between the tin contents derived by the different structural characterization techniques suggests the binary to follow generically the Vegard's rule in the range of low Sn molar fractions (<5%). Phase separation, Sn segregation, point defects, post-growing oxygen impurities, and deteriorated surface morphology are found to be relevant within the similar to 200 nm germanium tin films. Although, complex mechanisms triggering composition/strain heterogeneities are found in the analysed Ge1-x Sn (x) compounds, the deviation from the perfect crystals is suggested to be not enough to distort the in- and out-of-plane lattice parameters away from its empirical linear combination.
<b>Tipo</b>: article