Lourdes Ferre Llin

Charged particle tracking with the Timepix ASIC

A prototype particle tracking telescope was constructed using Timepix and Medipix ASIC hybrid pixel assemblies as the six sensing planes. Each telescope plane consisted of one 1.4 cm2 assembly, providing a 256 ×256 array of 55μm square pixels. The telescope achieved a pointing resolution of 2.4μm at the position of the device under test. During a beam test in 2009 the telescope was used to evaluate in detail the performance of two Timepix hybrid pixel assemblies; a standard planar 300μm thick sensor, and 285μm thick double sided 3D sensor. This paper describes a charge calibration study of the pixel devices, which allows the true charge to be extracted, and reports on measurements of the charge collection characteristics and Landau distributions. The planar sensor achieved a best resolution of 4.0±0.1μm for angled tracks, and resolutions of between 4.4 and 11μm for perpendicular tracks, depending on the applied bias voltage. The double sided 3D sensor, which has significantly less charge sharing, was found to have an optimal resolution of 9.0±0.1μm for angled tracks, and a resolution of 16.0±0.2μm for perpendicular tracks. Based on these studies it is concluded that the Timepix ASIC shows an excellent performance when used as a device for charged particle tracking.

Ge/SiGe superlattices for thermoelectric energy conversion devices

Ge-rich multiple quantum well heterostructures have been investigated as engineered material for efficient thermoelectric generators monolithically integrated on silicon substrates. Thick Ge/SiGe multilayers on Si substrates designed for lateral thermoelectric devices have been grown and characterized in which electrical and thermal conduction occur parallel to the heterostructure interfaces. In this study, an overview of the investigated structures is presented together with results from X-ray scattering and transmission electron microscopy experiments. These analyses confirm the high quality of the material and the uniformity of the structure over the whole deposited thickness. Important parameters in terms of the optimization of the material quality which could affect thermoelectric properties, such as the interfaces roughness and the threading dislocation density, have also been evaluated. Preliminary electrical and Seebeck coefficient measurements indicate the viability of this material for the realization of thermoelectric devices.

Synchrotron tests of 3D Medipix2 and TimePix X-ray detectors

In this article we report on the use micro-focus synchrotron X-ray radiation and pion beams to compare the detection efficiencies and charge sharing properties of novel 3D detectors to that of the current planar technology. Detector substrates are bump-bonded to the Medipx2 and Timepix chips. 55μm square pixel maps of the detection efficiencies have been produced using X-ray and MIP beams. For X-rays, a drop of 3-4% detection efficiency over the pixel area was found due to the central electrode. The corner electrodes show no degradation in efficiency compared to that of the planar device. For MIPs a drop of 0.5% in efficiency due to the central electrode was observed. Evidence of a considerable reduction in charge sharing in the 3D detectors compared to the planar devices is also shown.

Modelling and experimental verification of a Ge/SiGe thermoelectric generator

Thermoelectric generators (TEG) are devices that generate electricity when a temperature gradient is created across it. Therefore these generators can be used to power micro-scaled devices by harvesting the heat that is released to the environment in different systems. This work presents the Finite Element (FE) model and experimental approaches for investigating the performance of a Ge/SiGe-based TEG. The TEG studied in this work was fabricated using a novel p- and n-type nano-fabricated 2-D Ge/SiGe superlattice grown by low-energy plasma-enhanced chemical vapour deposition (LEPECVD). A single p- and n-leg were coupled together using indium as the bonding material. Results for open and short circuit voltages are presented, using both the experimental and FE modeling approaches. An effective Seebeck voltage of 200μV/K and a maximum power density of 60μW/m2 with a temperature difference of 1.15 K were obtained for this device. The FE model was validated using an analytical method in open circuit and both results closely matched. Although the fabricated module is unoptimized at this stage, it is hoped that the FE model will be used to design an optimal and feasible TEG module in the near future, using an improved Ge/SiGe-based material.

Thermoelectrics, Photovoltaics and Thermal Photovoltaics for Powering ICT Devices and Systems

The conversion of heat into electricity through the thermoelectric effect and light into electricity through photovoltaic solar cells both allow useful amounts of power for a range of ICT systems from a few milli-Watts (mW) for autonomous sensors up to kiloWatts (kW) for complete ICT computing or entertainment systems. Photovoltaics at the large scale can also be used to produce MW power stations suitable for the sustainable powering of high-performance computing (HPC) and dataservers for cloud computing. This chapter provides a background to the physics of operation of both types of sustainable energy sources along with the fundamental limits of both technologies. The present performance is presented along with promising research directions to allow for a comparison of the useful power along with the limits for deployment of each approach to power ICT devices and systems. Finally, the developing field of thermal photovoltaics is reviewed, where the overall thermodynamic conversion efficiency of turning light into electricity and useful heat can be increased through the addition of thermoelectrics or heat transfer modules to a photovoltaic cell.

Specially designed solar cells for hybrid photovoltaic-thermal generators

The performance of hybrid photovoltaic-thermal systems can be improved using PV cells that are specially designed to generate both electricity and useful heat with maximum efficiency. Present systems, however, use standard PV cells that are only optimized for electrical performance. In this work, we have developed two cell-level components that will improve the thermal efficiency of PV-T collectors, with minimal loss of electrical efficiency. These are a spectrally-selective low-emissivity coating to reduce radiative thermal losses, and a nanotextured rear reflector to improve absorption of the near-infrared part of the solar spectrum for heat generation.

Preliminary Results using Timepix as a Particle Tracking Detector

A series of tests in CERN’s North Area beam facility have been used to demonstrate the suitability of the Timepix chip, combined with a silicon sensor, as a particle tracking device. Specifically of interest is the potential of a successor to the current chip to be used in the context of an LHCb VELO upgrade. The 55mm square pixels, large active fraction and analogue information make the chip very attractive for forward, high precision tracking systems such as the VELO. In this contribution preliminary results are presented showing the resolution achieved by a Timepix assembly in a 120GeV p beam, over a wide range of incident angles. At the optimum angle the detector was able to provide an unbiased track residual of 5.5mm. The telescope constructed for these measurements contributed a track extrapolation error of 2.5mm. The plans for a future development of this telescope, also based on Timepix assemblies are discussed, with proposals for upgrading its spatial and timing resolution.