Thermoclastic and cryoclastic action on calcareous building stone: durability to artificial ageing

Short and long-term climate changes are a critical factor of stone decay even in temperate Mediterranean areas, when the combined fluctuations of temperature and moisture lead to thermoclastism and cryoclastism. The effects of weathering are of particular concern in the conservation field, for the assessment of the vulnerability of outdoorexposed stone materials in the historical built heritage. An example of this outline is Apulia, a region of southern Italy characterized by warm, dry summers and mild, rather rainy winters that can bring sub-zero temperatures; here, climate-driven decay can seriously affect the state of conservation of the local calcareous stone used in the monumental heritage, especially the most porous and softest materials. The dynamic behavior of the Apulian calcareous stone in response to thermo-hygrometric stresses was studied here by means of an artificial accelerated ageing test, focusing on a single local stone variety known as “pietra gentile”. It is a fine-grained, soft and porous calcarenite, which is exploited in the Murge area from the outcrops of the “Calcare di Caranna” Fm. (late Campanian?–Maastrichtian). The quarrying activity is concentrated in the territory of Valle d’Itria and mainly of Ostuni, touristically known as “the White Town”, where the stone is widely used for the sacred and civil architecture. The ageing test was programmed according to the typical climatic characteristics of the considered area, following the historical recordings by the official Apulian monitoring institute (Struttura di Monitoraggio Meteoclimatico, Centro Funzionale Regionale); in order to simulate the seasonal climatic changes, the extreme temperatures were considered, while the possible insolation effects for higher temperatures were also taken into account. So, samples of “pietra gentile” were subjected to 100 temperature cycles from 60 to –5 ◦C, in a climatic chamber with a maximum relative humidity of 60%. Before the ageing, the samples were petrographically characterized through optical microscopy on thin section, followed by a geotechnical parameterization with petrophysical measurements (dry density, total porosity, MIP porosity) and indirect ultrasonic and sclerometric tests. Then, after every 20 ageing cycles, the same tests stated above were carried out; in addition, the measurement of residual strains and SEM observations were performed. With this methodology, the gradual modifications in fabric, petrophysical and mechanical properties of the tested stone were analyzed.The results revealed a high durability of the material to the conditions of ageing experimented. The main modifications of the samples concerned fabric, i.e. microcracking due to thermoclastic and cryoclastic action, which occurred only in limited areas for an uneven distribution of internal stresses. For this reason, no evident effects on the macroscopic integrity and physico-mechanical performance were noted, whereas the stone almost preserved the original strength and elasticity. Finally, the data gathered were used to suggest a method for the evaluation of the vulnerability of “pietra gentile” to freeze-thaw microcracking, based on the peculiar porosimetric distribution and the environmental conditions of weathering. New information are globally provided about a stone material that has been largely used in the Apulian monumental heritage, but has received scarce attention from the archaeometric research so far

Expected radiation environment and damage for YBCO tapes in compact fusion reactors

We investigate the neutron damage expected in high-temperature superconducting tapes that will be employed in compact fusion reactors. Monte Carlo simulations yield the expected neutron spectrum and fluence at the magnet position, from which the primary knock-on atom energy distributions can be computed for each atomic species comprising the superconductor. This information is then employed to characterize the displacement cascades, in terms of size and morphology, through molecular dynamics simulations. The expected radiation environment is then compared with the neutron spectrum and fluences achievable at the facilities currently available for experimental investigation in order to highlight similarities and differences that could be relevant to the understanding of the radiation hardness of these materials in real fusion conditions. We find that the different neutron spectra result in different damage regimes, the irradiation temperature influences the number of generated defects, and the interaction of the neutrons with the superconductor results in a local increase in temperature. These observations suggest that further experimental investigations are needed in different regimes and that some neutron shielding will be necessary in compact fusion reactors.Funding Agencies|Italian Ministry of Education, University, and Research through Project PRIN HIBiSCUS [201785KWLE]; Programma Operativo Nazionale (PON) Ricerca e Innovazione 2014-2020; Swedish Research Council [2018-05973]; European Cooperation in Science and Technology, COST Action [CA19108]</p

Fe-doped sol-gel glasses and glass-ceramics for magnetic hyperthermia

This work deals with the synthesis and characterization of novel Fe-containing sol-gel materials obtained by modifying the composition of a binary SiO2-CaO parent glass with the addition of Fe2O3. The effect of different processing conditions (calcination in air vs. argon flowing) on the formation of magnetic crystalline phases was investigated. The produced materials were analyzed from thermal (hot-stage microscopy, differential thermal analysis, and differential thermal calorimetry) and microstructural (X-ray diffraction) viewpoints to assess both the behavior upon heating and the development of crystalline phases. N2 adsorption–desorption measurements allowed determining that these materials have high surface area (40–120 m2/g) and mesoporous texture with mesopore size in the range of 18 to 30 nm. It was assessed that the magnetic properties can actually be tailored by controlling the Fe content and the environmental conditions (oxidant vs. inert atmosphere) during calcination. The glasses and glass-ceramics developed in this work show promise for applications in bone tissue healing which require the use of biocompatible magnetic implants able to elicit therapeutic actions, such as hyperthermia for bone cancer treatment

Facile Chemical Synthesis of Doped ZnO Nanocrystals Exploiting Oleic Acid

Zinc oxide nanocrystals (ZnO-NCs) doped with transition metal elements or rare earth elements can be probed for magnetic resonance imaging to be used as a molecular imaging technique for accurate diagnosis of various diseases. Herein, we use Mn as a candidate of transition metal elements and Gd as a presenter of rare earth elements. We report an easy and fast coprecipitation method exploiting oleic acid to synthesize spherical-shaped, small-sized doped ZnO-NCs. We show the improved colloidal stability of oleate-stabilized doped ZnO-NCs compared to the doped ZnO-NCs synthesized by conventional sol-gel synthesis method, i.e., without a stabilizing agent, especially for the Mn dopant. We also analyze their structural, morphological, optical, and magnetic properties. We are able to characterize the persistence of the crystalline properties (wurtzite structure) of ZnO in the doped structure and exclude the formation of undesired oxides by doping elements. Importantly, we determine the room-temperature ferromagnetism of the doped ZnO-NCs. This oleate-stabilized coprecipitation method can be subjected as a standard procedure to synthesize doped and also co-doped ZnO-NCs with any transition metal elements or rare earth elements. In the future, oleate-stabilized Gd/Mn-doped ZnO-NCs can be exploited as magnetic resonance imaging (MRI) contrast agents and possibly increase the signal intensity on T1-weighted images or reduce the signal intensity on T2-weighted images

Analysis framework for nuclear heating effects on HTS-based conductors in fusion power plants

High-temperature superconducting-slotted cables, including the VIPER cable, are being investigated as possible candidates for winding the toroidal field coils of compact nuclear fusion machines, such as ARC. The higher performance of HTS materials in terms of magnetic fields and currents allows them to reduce their size compared to machines based on Nb alloys and compounds. However, compactness also means a closer interaction between the plasma products and the coils, creating new technological challenges to be faced. Among the others, the thermal effects induced on the superconducting materials by direct particles (i.e., neutrons), secondary particles and gamma-ray heating, will be stronger than for low-field larger machines and must be quantified. In this paper, a detailed thermo-magnetic model is built up using the software COMSOL Multiphysics to simulate the consequences of the D-T plasma operation products on the first turn of a toroidal field coil of the ARC preliminary machine design. The irradiation-induced heat load on the (RE)Ba 2 Cu 3 O 7-δ tapes, computed via Monte Carlo simulations, is used as input for the thermal analysis of the VIPER cable. At the same time, a homogenized T-A formulation is developed for checking the performance reduction in the current/field operating conditions

Study of the thermal distribution for YBCO based Transition Edge Bolometers working above 77 K

ransition Edge Bolometers (TEB) are among the simplest radiation detectors. The straightforward operation mode provides good results only if it is combined with a careful thermal optimization.In a TEB, the strong dependence of the electrical resistivity on the temperature in its transition zone enables the detection of a variation of the local temperature which can reach tens of µK. For this reason, it is essential to study the thermal profile of the superconducting active part of the detector, hence its substrate, to make it as homogeneous as possible.Irradiated YBa 2 Cu 3 O 7-x (YBCO) films can be used for position sensitive detection of infrared radiation. A TEB with a double meander pattern, one of which with a reduced critical temperature due to irradiation with high-energy heavy ions, was designed to work in a portable cryostat at a temperature above the liquid nitrogen (LN 2 ) point.In this work, we present a series of Finite Element Method simulations (using COMSOL Multiphysics ® ) aimed at the optimization of the thermal distribution above the YBCO film. Once the optimal working point for the device is found, various materials for the bolometer hosting are tested to identify the combination that provides the most homogeneous temperature distribution. The optimal configurations are then analyzed in response to a sudden change in the PID current to determine the one which presents the best behavior in a transient situation

Screening magnetic fields by superconductors: A simple model

We introduce a simple approach to evaluate the magnetic field distribution around superconducting samples, based on the London equations; the elementary variable is the vector potential. This procedure has no adjustable parameters, only the sample geometry and the London length determine the solution. This approach was validated by comparing the induction field calculated to the one measured above MgB2 disks of different diameters, at 20K and for applied fields lower than 0.4T. The model can be applied if the flux line penetration inside the sample can be neglected when calculating the induction field distribution outside the superconductor. We conclude by showing on a cup-shape geometry how one can design a magnetic shield satisfying a specific constraint

Modelling and Performance Analysis of MgB2 and Hybrid Magnetic Shields

Superconductors are strategic materials for the fabrication of magnetic shields, and within this class, MgB [Formula: see text] has been proven to be a very promising option. However, a successful approach to produce devices with high shielding ability also requires the availability of suitable simulation tools guiding the optimization process. In this paper, we report on a 3D numerical model based on a vector potential (A)-formulation, exploited to investigate the properties of superconducting (SC) shielding structures with cylindrical symmetry and an aspect ratio of height to diameter approaching one. To this aim, we first explored the viability of this model by solving a benchmark problem and comparing the computation outputs with those obtained with the most used approach based on the H-formulation. This comparison evidenced the full agreement of the computation outcomes as well as the much better performance of the model based on the A-formulation in terms of computation time. Relying on this result, the latter model was exploited to predict the shielding properties of open and single capped MgB [Formula: see text] tubes with and without the superimposition of a ferromagnetic (FM) shield. This investigation highlighted that the addition of the FM shell is very efficient in increasing the shielding factors of the SC screen when the applied magnetic field is tilted with respect to the shield axis. This effect is already significant at low tilt angles and allows compensating the strong decrease in the shielding ability that affects the short tubular SC screens when the external field is applied out of their axis