Investigating rock mass failure precursors using a multi-sensor monitoring system. Preliminary results from a test-site (Acuto, Italy)

In the last few years, several approaches and methods have been proposed to improve early warning systems for managing risks due to rapid slope failures where important infrastructures are the main exposed elements. To this aim, a multi-sensor monitoring system has been installed in an abandoned quarry at Acuto (central Italy) to realise a natural-scale test site for detecting rock-falls from a cliff slope. The installed multi-sensor monitoring system consists of: i) two weather stations; ii) optical cam (Smart Camera) connected to an Artificial Intelligence (AI) system; iii) stress- strain geotechnical system; iv) seismic monitoring device and nano-seismic array for detecting microseismic events on the cliff slope. The main objective of the experiment at this test site is to investigate precursors of rock mass failures by coupling remote and local sensors. The integrated monitoring system is devoted to record strain rates of rock mass joints, capturing their variations as an effect of forcing actions, which are the temperature, the rainfalls and the wind velocity and direction. The preliminary tests demonstrate that the data analysis methods allowed the identification of external destabilizing actions responsible for strain effects on rock joints. More in particular, it was observed that the temperature variations play a significant role for detectable strains of rock mass joints. The preliminary results obtained so far encourage further experiments

Optical fiber sensors for in-situ detection of solid-liquid phase change for n-octadecane

In the past few decades solid-liquid phase change materials (PCMs) have gained an increasingly important role in thermal energy storage applications due to their ability to absorb or release large amounts of energy during melting or solidification. The precise phase change temperature varies with different conditions, such as external pressure, small variations in the PCM composition in the case of multi-component mixtures and/or material purity. In order to achieve better energy efficiency for the energy storage process, it is necessary to be able to accurately detect the solid-liquid phase changes in the bulk of a PCM. Optical fiber sensors allow for direct detection of the phase changes in PCMs while also offering the advantages of a passive nature and small size. The focus of the research presented in this thesis is on the development of a novel approach to detecting the solid-liquid phase changes in selected PCMs using optical fiber sensors. To achieve this goal, initially the correlation between the temperature, changes in the refractive index (RI) and internal pressure acting upon the optical fiber during the phase transitions was studied for the selected PCM, n-octadecane. Based on the results of these studies, several optical fiber sensing structures have been proposed and demonstrated for the detection of phase changes as follows: An optical fiber Fresnel reflection sensor for detection of phase changes. An fiber Fresnel reflection sensor for detection of solid-liquid phase change in n-octadecane is proposed and experimentally demonstrated. The sensor probe consists of a single-mode fiber with a cleaved end immersed in the n-octadecane sample under test. The detection relies on measuring the slope of the output power ratio change which is caused by the RI change during the phase transition. The results of this work suggest that such a simple optical fiber sensor can be used for detection of liquid-solid phase changes in other materials with similar thermo-optic properties to n-octadecane. This sensor realized in-situ detection for a solid liquid phase change, which is a significant advantage compared to the traditional phase change detection methods. A fiber heterostructure based optical fiber sensor for detection of phase changes. A single-mode-no-core-single-mode fiber optical sensor for the detection of solid-liquid and liquid-solid phase changes in n-octadecane is proposed and demonstrated. The transmission-type sensor probe consists of a short section of no-core fiber sandwiched between two sections of a single-mode fiber. The detection relies on measuring the level of the output power ratio which is caused by the large step-like variations in the RI of n-octadecane’s. Importantly, compared to the Fresnel reflection sensor, the proposed fiber heterostructure is resistant to bending and strain disturbances during the measurements. The results of this work suggest that the proposed sensor is potentially capable of detecting liquid-solid phase changes in other materials whose thermo-optic properties are similar to those of n-octadecane. Moreover, this sensor not only has the advantage of achieving in-situ phase change detection, but also has the ability of working in an environment subjected to mechanical disturbance, which makes it has great potential of industry applications. Optical fiber Fabry-Perot sensor based on a singlemode-hollow core-singlemode fiber. An optical fiber Fabry-Perot sensor to monitor the solid-liquid and liquid-solid phase changes in n-octadecane is also proposed and investigated. The sensor probe is fabricated by splicing a short section of a hollow core fiber between two single-mode fibers. By analyzing the changes in the output spectrum of the probe, such as spectral shift of a selected interference dip, the phase change within a material sample in the vicinity of the fiber probe can be accurately detected. The proposed sensor can deal with PCM types whose RI values make it difficult for the other two sensor types to work, and also can be used for detection of the material’s phase state at a particular point of its volume. This work has the potential to better understanding phase change mechanism and its application in energy engineering. Compared to the other sensors developed in the research presented in this thesis, this sensor has the advantage that the application is not limited by the RI of the PCMs

Development of miniature all-solid-state potentiometric sensing system

A procedure for the development of a pen-like, multi-electrode potentiometric sensing platform is described. The platform comprises a seven-in-one electrode incorporating all-solid-state ion-selective and reference electrodes based on the conductive polymer (poly(3,4-ethylenedioxythiophene) (PEDOT)) as an intermediate layer between the contacts and ion-selective membranes. The ion-selective electrodes are based on traditional, ionophore-based membranes, while the reference electrode is based on a polymer membrane doped with the lipophilic salt tetrabutyl ammonium tetrabutyl borate (TBA-TBB). The electrodes, controlled with a multichannel detector system, were used for simultaneous determination of the concentration of Pb2+ and pH in environmental water samples. The results obtained using pH-selective electrodes were compared with data obtained using a conventional pH meter and the average percent difference was 0.3%. Furthermore, the sensing system was successfully used for lead-speciation analysis in environmental water samples

Topological engineering of interfacial optical Tamm states for highly-sensitive near-singular-phase optical detection

We developed planar multilayered photonic-plasmonic structures, which support topologically protected optical states on the interface between metal and dielectric materials, known as optical Tamm states. Coupling of incident light to the Tamm states can result in perfect absorption within one of several narrow frequency bands, which is accompanied by a singular behavior of the phase of electromagnetic field. In the case of near-perfect absorptance, very fast local variation of the phase can still be engineered. In this work, we theoretically and experimentally demonstrate how these drastic phase changes can improve sensitivity of optical sensors. A planar Tamm absorber was fabricated and used to demonstrate remote near-singular-phase temperature sensing with an over an order of magnitude improvement in sensor sensitivity and over two orders of magnitude improvement in the figure of merit over the standard approach of measuring shifts of resonant features in the reflectance spectra of the same absorber. Our experimentally demonstrated phase-to-amplitude detection sensitivity improvement nearly doubles that of state-of-the-art nano-patterned plasmonic singular-phase detectors, with further improvements possible via more precise fabrication. Tamm perfect absorbers form the basis for robust planar sensing platforms with tunable spectral characteristics, which do not rely on low-throughput nano-patterning techniques.Comment: 31 pages; 6 main text figures and 10 supplementary figure

A new LED-LED portable CO2 gas sensor based on an interchangeable membrane system for industrial applications

A new system for CO2 measurement (0-100%) by based on a paired emitter-detector diode arrangement as a colorimetric detection system is described. Two different configurations were tested: configuration 1 (an opposite side configuration) where a secondary inner-filter effect accounts for CO2 sensitivity. This configuration involves the absorption of the phosphorescence emitted from a CO2-insensitive luminophore by an acid-base indicator and configuration 2 wherein the membrane containing the luminophore is removed, simplifying the sensing membrane that now only contains the acid-base indicator. In addition, two different instrumental configurations have been studied, using a paired emitter-detector diode system, consisting of two LEDs wherein one is used as the light source (emitter) and the other is used in reverse bias mode as the light detector. The first configuration uses a green LED as emitter and a red LED as detector, whereas in the second case two identical red LEDs are used as emitter and detector. The system was characterised in terms of sensitivity, dynamic response, reproducibility, stability and temperature influence. We found that configuration 2 presented a better CO2 response in terms of sensitivity