arXiv subjects

C. Rodrigues

Publications and source records attributed to C. Rodrigues.

5 recordsLinked to original sources

arXiv2018

An Electromagnet-Based Magnetically-Activated Thermal Switch Without Moving Parts

With the ever increasing power dissipation in electrical devices, new thermal management solutions are in high demand to maintain an optimal operating temperature and efficient performance. In particular, recently developed magnetically-activated thermal switches (MATSs) provide an alternative to existing devices, using the magnetic and thermal properties of superparamagnetic nanofluids to dissipate heat in a controlled manner. However, the presence of moving parts is a major drawback in those systems that must still be addressed. Herein, we present a compact and automatized MATS composed by an encapsulated superparamagnetic nanofluid and an electromagnet allowing to activate the MATS without any moving part. We investigate the effect of different temperature gradients (40, 26 and 10 C) and powers applied to the coil (6.5, 15, 25 and 39 W) on the performance of this novel MATS. The results show that the highest (44.4$) and fastest (0.6 C/s) temperature variation occur for the highest studied temperature gradient. On the other hand, with increasing power, there is also an increase in the efficiency of the heat exchange process between the two surfaces. These results remove one of the main barriers preventing the actual application of magnetic thermal switches and opens new venues for the design of efficient thermal management devices.

physics.app-ph↗

arXiv2018

Influence of Thickness and Contact Area on the Performance of PDMS-Based Triboelectric Nanogenerators

Triboelectric nanogenerators (TENGs) are an emerging mechanical energy harvesting technology that was recently demonstrated. Due to their flexibility, they can be fabricated in various configurations and consequently have a large number of applications. Here, we present a study on the influence of the thickness of the triboelectric layer and of the contact surface area between two triboelectrical materials on the electric signals generated by a TENG. Using the PDMS-Nylon tribo-pair, and varying the thickness of the PDMS layer, we demonstrate that the generated voltage decreases with increasing thickness. However, the maximum generated current presents an inverse behaviour, increasing with increasing PDMS thickness. The maximum output power initially increases with increasing PDMS thickness up to 32 um, followed by a sharp decrease. Using the same tribo-pair (but now with a constant PDMS thickness), we verified that increasing the contact surface area between the two tribo-materials increases the electrical signals generated from the triboelectric effect.

cond-mat.mes-hall↗

arXiv2001

29-Si NMR and Hidden Order in URu2Si2

We present new 29-Si NMR spectra in URu2Si2 for varying temperature T, and external field H. On lowering T, the systematics of the low-field lineshape and width reveal an extra component (lambda) to the linewidth below T_N ~ 17 K not observed previously. We find that lambda is magnetic-field independent and dominates the low-field lineshape for all orientations of H with respect to the tetragonal c axis. The behavior of lambda indicates a direct relationship between the 29-Si spin and the transition at T_N, but it is inconsistent with a coupling of the nuclei to static antiferromagnetic order/disorder of the U-spin magnetization. This leads us to conjecture that lambda is due to a coupling of 29-Si to the system's hidden-order parameter. A possible coupling mechanism involving charge degrees of freedom and indirect nuclear spin/spin interactions is proposed. We also propose further experiments to test for the existence of this coupling mechanism.

cond-mat.str-el↗

arXiv1999

Spectropolarimetry of the Luminous Narrow-Line Seyfert Galaxies IRAS 20181-2244 and IRAS 13224-3809

We observed the narrow-line Seyfert 1 galaxies IRAS 20181-2244 and IRAS 13324-3809 with a new spectropolarimeter on the RC spectrograph at the CTIO 4m telescope. Previously it had been suggested that IRAS 20181-2244 was a Type 2 QSO and thus might contain an obscured broad-line region which could be detected by the presence of broad Balmer lines in the polarized flux. We found the object to be polarized at about 2%, and constant with wavelength, (unlike most narrow-line Seyfert 1s), but with no evidence of broad Balmer lines in polarized flux. The spectropolarimetry indicates that the scattering material is inside the BLR. IRAS 13224-3809, notable for its high variability in X-ray and UV wavelengths, has a low polarization consistent with a Galactic interstellar origin.

astro-ph↗

arXiv1997

Multiwavelength Monitoring of the BL Lacertae Object PKS 2155-304 in May 1994. I. The Ground-Based Campaign

Optical, near-infrared, and radio observations of the BL Lac object PKS2155-304 were obtained simultaneously with a continuous UV/EUV/X-ray monitoring campaign in 1994 May. Further optical observations were gathered throughout most of 1994. The radio, millimeter, and near-infrared data show no strong correlations with the higher energies. The optical light curves exhibit flickering of 0.2-0.3 mag on timescales of 1-2 days, superimposed on longer timescale variations. Rapid variations of ~0.01 mag/min, which, if real, are the fastest seen to date for any BL Lac object. Small (0.2-0.3 mag) increases in the V and R bands occur simultaneously with a flare seen at higher energies. All optical wavebands (UBVRI) track each other well over the period of observation with no detectable delay. For most of the period the average colors remain relatively constant, although there is a tendency for the colors (in particular B-V) to vary more when the source fades. In polarized light, PKS 2155-304 showed strong color dependence and the highest optical polarization (U = 14.3%) ever observed for this source. The polarization variations trace the flares seen in the ultraviolet flux.

astro-ph↗