A. Bellucci

Radiation-assisted Frenkel-Poole transport in single-crystal diamond

The measurement of the density of occupied states as a function of the applied electric field, performed on single-crystal chemical vapour deposition diamond by x-ray modulated photocurrent technique, is reported. Two regimes of non-linear charge transport were observed: a classical Frenkel-Poole (FP) process at high electric fields (>6800 V/cm), and a radiation-assisted transport mechanism at intermediate electric fields (2000 to 6800 V/cm), consisting of a double-step process in which the direct re-emission into the extended band occurs following multiple photo-induced FP-like hopping transitions.

Preliminary characterization of ST2G: Solar thermionic-thermoelectric generator for concentrating systems

An innovative conversion module (CM) for concentrating solar power applications, named ST2G (Solar Thermionic-Thermoelectric Generator), has been developed and fabricated. The new technology is based on a solid-state converter that works at temperatures up to 1000 °C and exploits a double conversion stage: a thermionic stage and a thermoelectric one, connected thermally in series. Potentially, the CM could reach a combined solar energy-to-electrical efficiency larger than 30%, producing also additional thermal energy to be exploited as a co-generation. Different prototypes have been fabricated and the discussion on the technological key-points has been reported, relating them to the physical requirements necessary for an efficient conversion mechanism. The preliminary results obtained at a lab-level are here discussed, indicating low electrical power output, but also how to increase the performance by solving the identified issues.

Buried Boron Doped Layer for CVD Diamond Photo-Thermionic Cathodes

A buried boron (B) doped layer has been fabricated by ion implantation into the bulk structure of a chemical vapor deposition (CVD) diamond film engineered to act as a photo-thermionic cathode for high-temperature solar cells. The boron layer implantation is a fundamental step in order to obtain an efficient diamond-based solar cell. Implantation of boron ions has been performed by fixing the ion dose to a value of 1 × 1015 at/cm2, while varying the ion beam kinetic energy from 40 to 250 keV which localizes the doped layer at different depths (from tens to a few hundreds of nanometers) below the absorbing surface of the cathode. Characterization of the optical and photoelectronic properties of the different implanted layers has been carried out so as to evaluate their effectiveness within the cathode structure. An ion beam kinetic energy of 40 keV is found to significantly increase the quantum efficiency of the CVD diamond plate. This will be useful for the development of the diamond-based photo-thermionic cathode.

Defect engineering of diamond cathodes for high temperature solar cells

A cathode structure for photon-enhanced thermionic emission was designed for high temperature energy conversion in solar concentrating systems. Surface-hydrogenated diamond is one of the few semiconductors to show negative electron affinity and a work function as low as 1.7 eV if nitrogen-doped, that is connected to a significant thermionic emission at moderate temperatures (up to 800 °C). But diamond is transparent to solar radiation, consequently advanced techniques for preparing an efficient sunlight absorbing diamond are discussed.

Optimization of X-ray beam profilers based on CVD diamond detectors

A complete beam profiler, based on a CVD diamond pixel detector, has been developed for real-time X-ray source imaging, showing good results in terms of sensitivity and response speed. A dedicated microcontroller-based read-out electronics has been designed to ensure real-time monitoring for a wide range of beam intensities and dose-rates (0.001÷1 Gy/h). The influence of two different metal-diamond interfaces (Ag and Ti/Au) on the X-ray induced photocurrent characteristics has been also investigated, in order to identify which of the two types of contacts can optimize the beam profiler performance in terms of sensitivity and spatial resolution. For this purpose, a set of measurements (X-ray induced photocurrent dependence on bias voltage and dose-rate, specific sensitivity) was carried out with a standard Coolidge tube equipped with a molybdenum target (Kα line at 17.5 keV, Kβ line at 19.6 keV).

Upgrade of the compact neutron spectrometer for high flux environments

Abstract In this paper new version of the 6 Li-based neutron spectrometer for high flux environments is described. The new spectrometer was built with commercial single crystal Chemical Vapour Deposition diamonds of electronic grade. These crystals feature better charge collection as well as higher radiation hardness. New metal contacts approaching ohmic conditions were deposited on the diamonds suppressing build-up of space charge observed in the previous prototypes. New passive preamplification of the signal at detector side was implemented to improve its resolution. This preamplification is based on the RF transformer not sensitive to high neutron flux. The compact mechanical design allowed to reduce detector size to a tube of 1 cm diameter and 13 cm long. The spectrometer was tested in the thermal column of TRIGA reactor and at the DD neutron generator. The test results indicate an energy resolution of 300 keV (FWHM), reduced to 72 keV (RMS) excluding energy loss, and coincidence timing resolution of 160 ps (FWHM). The measured data are in agreement with Geant4 simulations except for larger energy loss tail presumably related to imperfections of metal contacts and glue expansion.

Thermoelectric analysis of ZnSb thin films prepared by ns-pulsed laser deposition

Zinc antimonide (ZnSb) is a promising thermoelectric material for the temperature range 300– 600 K. ZnSb thin films were prepared by nanosecond Pulsed Laser Deposition (PLD) to evaluate the performance of nanostructured films for thermoelectric conversion by the determination of the Power Factor. A study of the influence of structural, compositional and thermoelectric properties of thin films is reported as a function of different deposition parameters, such as repetition rate, pulse energy, and substrate temperature. The evaluation of a thin film ZnSb compound with excess Sb has been performed to verify the variation of the thermoelectric properties. The obtained results are reported and discussed in the 300–600 K temperature range.

High-resolution wide-range dynamic neutron monochromators

This work proposes the concept of a high-resolution wide-range dynamic neutron monochromator able to operate up to a few eV, thus significantly extending the reliable operating neutron energy range of the state-of-the-art crystal-based devices, and to improve energy resolution in the epithermal range if compared to mechanical choppers. The proposed device is based on a radiation-hard aluminium nitride (AlN)/diamond heterostructure, operating as a super-high-frequency (> 3 GHz) high-speed (> 10000 m/s) acoustic resonator. The resulting surface acoustic wave (SAW) acts as a dynamic grating for the incoming neutrons: as long as neutron speed is lower than (or at least comparable to) SAW speed, diffraction angles are significantly enhanced, thus allowing for neutron beam monochromatization to be effective up to the eV energy range. SAW amplitude can be electrically tuned to increase first-order reflectivity, leading to an enhanced intensity of the monochromatized beam.