G. Breglio

Current distribution effects in organic sexithiophene field effect transistors investigated by lock-in thermography: Mobility evaluation issues

In this Letter, a lock-in thermography technique has been used to investigate the actual current distribution profile in the active channel region of organic field effect transistors. The high accuracy of the setup shows an evidence of nonuniformity in the current flow over the device area. The physical origin of this experimental occurrence is tentatively ascribed to a not uniform contact resistance distribution along the channel width or to inhomogeneities in the interface traps distribution. The subsequent implications on the carrier mobility evaluation are discussed too.

Radiation hard polyimide-coated FBG optical sensors for relative humidity monitoring in the CMS experiment at CERN

This work investigates the performance and the radiation hardness capability of optical thermo-hygrometers based on Fibre Bragg Gratings (FBG) for humidity monitoring in the Compact Muon Solenoid (CMS), one of the four experiments running at CERN in Geneva. A thorough campaign of characterization was performed on 80 specially produced Polyimide-coated RH FBG sensors and 80 commercial temperature FBG sensors. Sensitivity, repeatability and accuracy were studied on the whole batch, putting in evidence the limits of the sensors, but also showing that they can be used in very dry conditions. In order to extract the humidity measurements from the sensor readings, commercial temperature FBG sensors were characterized in the range of interest. Irradiation campaigns with ionizing radiation (γ-rays from a Co60 source) at incremental absorbed doses (up to 210 kGy for the T sensors and up to 90 kGy for the RH sensors) were performed on sample of T and RH-Sensors. The results show that the sensitivity of the sensors is unchanged up to the level attained of the absorbed dose, while the natural wavelength peak of each sensor exhibits a radiation-induced shift (signal offset). The saturation properties of this shift are discussed.

Thermal feedback blocks for fast and reliable electrothermal circuit simulation of power circuits at module level

Thermal Feedback Blocks are a viable approach to perform thermal and electrothermal simulations of electronics systems with very fast-switching inputs, for which the coupling of a finite-element method thermal solver with a physics-based or a circuit simulator cannot be used. In this work, we describe a RC-based thermal network improving the conventional Foster and Cauer solutions, which is used to perform (i) an accurate and effective dynamic thermal analysis of an IGBT power module, and (ii) a fast electrothermal analysis of the aforementioned module used as a half-bridge DC-AC power converter in both normal operating mode and during a load short-circuit event.

A Comparative Study of Radiation-Tolerant Fiber Optic Sensors for Relative Humidity Monitoring in High-Radiation Environments at CERN

In this paper, we report a comparative study of fiber optic sensors for applications of relative humidity (RH) monitoring in high-radiation environments. In particular, we present investigations carried out since 2011 by our multidisciplinary research group, in collaboration with the European Organization for Nuclear Research (CERN) in Geneva. Our research has been first focused on the development of polyimide-coated fiber Bragg grating (FBG) sensors, and recently, it has been extended to nanoscale metal oxide-coated long-period gratings (LPGs). Experimental tests in the [0-70] %RH range at different temperatures, before and after γ-ionizing radiation exposures, have been carried out to assess the sensors performances in conditions required in experiments running at CERN. The advantages and disadvantages of the two proposed technologies are discussed in this paper in light of their possible application in high-energy physics environments. In particular, reported results suggest that LPG-based sensors can be preferred in some applications (particularly in presence of very low humidity levels) mainly because they are able to provide very high RH sensitivity (up to 1.4 nm/% RH), which is up to three orders of magnitude higher than that exhibited by FBG-based hygrometers. On the other side, compared with FBGs, LPGs are more difficult to multiplex due to limitations in terms of available bandwidth.

A fiber optic Bragg grating seismic sensor

Here we present a fiber-optic seismic waves sensor based on in-fiber Bragg gratings. Fiber-Bragg-grating (FBG) sensors have been demonstrated to have very high sensitivity to dynamical strain and very extended dynamical response. The seismic sensing system is based on the integration of three FBGs dynamical strain sensors in a mechanical structure acting as an inverse pendulum. Polar symmetry of the mechanical system and 120deg placement of the FBG sensors could enable directional capability of the seismic sensor. Design, manufacturing, and preliminary dynamical testing of the seismic sensor are discussed.

All-fiber optoelectronic sensor with Bragg gratings for in-situ cure monitoring

Real-time, in situ monitoring for quality control of the polymer cure process is of high interest, since thermoset polymer-matrix composite are widely used in large industrial areas: aeronautical, aerospace, automotive and civil due to their low cost/low weight features. However, their final properties are strongly dependence on the processing parameters, such as temperature and pressure sequence. The key-point for advanced composite materials is the possibility to have distributed and simultaneous monitoring of chemoreological and physical properties during the cure process. To this aim, we have developed and tested an optoelectronic fiber optic sensor based on the Fresnel principle able to monitor the variations of the refractive index due to the cure process of an epoxy based resin. Experimental results have been obtained on sensor capability to monitor the cure kinetics by assuming the refractive index as reaction co-ordinate. The integration with in-fiber Bragg grating in order to measure the local temperature has been discussed and tested.

Physics of current limited failures during avalanche for 600V Fast Recovery Diodes

Fast Recovery Epitaxial Diodes (FREDs) in the 600V voltage range are widely used for the design of Switch Mode Power Supply or snubberless DC-DC converters, where the devices can operate close to their intrinsic limitation in terms of maximum sustainable energy during avalanche operation. Experimental characterization of 600V rated FREDs in avalanche conditions has highlighted a failure due to a current level limitation. The mechanism cannot be attributed to the achievement of an avalanche energy threshold, therefore a current dependent mechanism is involved in the failure of the device. In this work this failure mechanism is addressed and the relevance of the termination design is highlighted.

Model-Order Reduction Procedure for Fast Dynamic Electrothermal Simulation of Power Converters

Thermal Feedback Blocks represent an efficient means to perform thermal and electrothermal analyses of power converters, for which the adoption of strategies relying on 3-D numerical tools is too onerous or even impossible. In this work, we describe a RC-based thermal network extracted with a model-order reduction procedure improving the conventional Foster and Cauer solutions, which is used for the study of a DC/DC boost converter. The accuracy of the proposed approach is verified by comparing: (i) the output of purely-thermal simulations with those obtained through a commercial software based on the finite-element method, and (ii) electrothermal simulation results with measurements.

Fiber optic cryogenic sensors for superconducting magnets and superconducting power transmission lines at CERN

The design, fabrication and tests of a new generation of superconducting magnets for the upgrade of the LHC require the support of an adequate, robust and reliable sensing technology. The use of Fiber Optic Sensors is becoming particularly challenging for applications in extreme harsh environments such as ultra-low temperatures, high electromagnetic fields and strong mechanical stresses offering perspectives for the development of technological innovations in several applied disciplines.

Radiation hard fiber optic thermo-hygrometers for relative humidity detection in the CMS experiment at CERN

This work investigates the performances and the radiation hardness capability of optical thermo-hygrometers based on Fiber Bragg Gratings (FBG) technology for humidity monitoring in the Compact Muon Solenoid experiment (CMS) at CERN, in Geneva. Extensive characterizations in terms of sensitivity, repeatability and accuracy on 80 specially produced polyimide-coated FBG sensors and 80 commercial temperature FBG sensors are presented. Progressive irradiation campaigns with γ- ionizing radiations were also performed. Results showed that the sensors sensitivity is unchanged after each radiation exposure; while the wavelength peak exhibits a radiation-induced shift. The saturation properties of this shift are discussed.