Hendrik Lorentz | EPFL Graph Search

Hendrik Antoon Lorentz (ˈlɒrənts; 18 July 1853 – 4 February 1928) was a Dutch physicist who shared the 1902 Nobel Prize in Physics with Pieter Zeeman for the discovery and theoretical explanation of the Zeeman effect. He derived the Lorentz transformation of the special theory of relativity, as well as the Lorentz force, which describes the combined electric and magnetic forces acting on a charged particle in an electromagnetic field. Lorentz was also responsible for the Lorentz oscillator model, a classical model used to describe the anomalous dispersion observed in dielectric materials when the driving frequency of the electric field was near the resonant frequency, resulting in abnormal refractive indices. According to the biography published by the Nobel Foundation, "It may well be said that Lorentz was regarded by all theoretical physicists as the world's leading spirit, who completed what was left unfinished by his predecessors and prepared the ground for the fruitful recep

Switchable Selective Absorber Coatings for Overheating Protection of Solar Thermal Collectors

Antonio Paone

Overheating is a common problem both with the use of active and passive solar energy in solar thermal energy systems and in highly glazed buildings. In solar thermal collectors, the elevated temperatures occurring during stagnation result in reduced lifetime of the collector materials. Highly glazed building facades provide high solar gains in winter, but imply in most cases high energy needs for air conditioning in summer. The aim of this thesis is to propose ”smart” thermochromic coatings as a solution to such problems. A durable inorganic thermochromic material is vanadium dioxide. At 68°C, VO2 undergoes a reversible crystal structural phase transition accompanied by a strong change in optical properties. By doping the material with tungsten, it is possible to lower the transition temperature, making it suitable as a window coating. Controlled doping of VO2 based films was possible using a co-sputtering installation. This doping lowered the transition temperature from 67.9 ± 0.5°C to 49.5 ± 0.5°C. In order to simulate the optical behavior of multilayered solar coatings, precise knowledge of the optical properties of the material is required. In this study the complex dielectric functions of VO2 and VO2 : W were determined by spectroscopic UV-VIS-NIR-MIR ellipsometry above and below the transition temperature. For this purpose, a point-by-point analysis of the ellipsometric angles Ψ/∆ was performed. A validation of our results was obtained comparing the optical constants determined by point-by-point fitting with those determined by the Lorentz - Drude and the empirical Lorentz - Cauchy dispersion formula. The optical constants of VO2 show a considerable change in the near/middle infrared range. The maximum k (extinction coefficient) change of a factor 7.4 between the semiconducting state and the metallic state occurs at 13490 nm. Reflectance and absorptance were measured by spectrophotometry in the near infrared range up to 20 μm in order to be compared with the computer simulations based on the determined optical properties of the material. A solar absorptance of 0.96 below the transition temperature was reported for a VO2 based absorber. Structural and electrical analyses were performed as well. The thermal emittance of new nanocomposite materials based on VO2 was also investigated applying the Bruggeman effective medium approximation. A thermal emittance switch from 0.08 to 0.32 was simulated for a 350 nm thick VO2 : W film mixed with a 40% volume fraction of SiO2. The glycols used in solar thermal collectors start to degrade above 170°C, the use of this coating as solar absorber lowers the stagnation temperature below this critical point. As alternative solution to VO2, gas chromic MgyTi1−y switchable mirrors were studied by XRD and ellipsometry. In this thesis, the characterization of the optical properties of VO2 and VO2 : W is reported. This characterization shows that these coatings are efficient absorbers of thermochromic solar thermal panels.

EPFL2013

WC/a-C nanocomposite thin films: Optical and electrical properties

Rosendo Sanjines

WC/amorphous carbon (a-C) thin films were deposited by dual magnetron sputtering from individual WC and graphite targets. The influence of film composition and microstructure on the optical and electrical properties was investigated. As evidenced by x-ray photoelectron spectroscopy and grazing angle x-ray diffraction measurements, the WC/a-C films are composite materials made of hexagonal W2C and/or cubic beta-WC1-X nanocrystallites embedded in (a-C) matrix. The optical properties were studied by spectroscopic ellipsometry and the electrical resistivity was measured by the van der Pauw method between 20 and 300 K. Both the optical and the electrical properties of the WC/a-C films are correlated with the chemical composition and microstructure evolution caused by a-C addition. The optical properties of W2C/a-C and beta-WC1-x/a-C films with a-C content

2009

Temperature-dependent multiangle FTIR NIR-MIR ellipsometry of thermochromic VO2 and V1-xWxO2 films

Antonio Paone, Rosendo Sanjines, Andreas Schueler

VO2 and W-doped VO2 thin films were prepared by Radio Frequency (RF) reactive magnetron sputtering using V-metal target. The amount of W doping was quantified by Rutherford Backscattering Spectrometry (RBS). The optical constants of VO2 and V1-xWxO2 films were inferred above and below the transition temperature by temperature-dependent multiangle ellipsometry in the Near InfraRed (NIR) spectral range and by temperature-dependent multiangle Fourier Transform InfraRed ellipsometry (FTIR) in the Near InfraRed Middle InfraRed (NIR MIR) spectral range up to 20,000 nm. The effect of the doping concentration on their optical constants was studied. A validation of the results was obtained comparing the optical constants determined by point-by-point fitting with those determined by the Lorentz Drude model and the empirical Lorentz Cauchy dispersion formula. (C) 2015 Elsevier Ltd. All rights reserved.

Pergamon-Elsevier Science Ltd2015

Switchable Selective Absorber Coatings for Overheating Protection of Solar Thermal Collectors

Antonio Paone

EPFL2013