IoT System for Real-Time Posture Asymmetry Detection.

The rise of the Internet of Things (IoT) has enabled the development of measurement systems dedicated to preventing health issues and monitoring conditions in smart homes and workplaces. IoT systems can support monitoring people doing computer-based work and avoid the insurgence of common musculoskeletal disorders related to the persistence of incorrect sitting postures during work hours. This work proposes a low-cost IoT measurement system for monitoring the sitting posture symmetry and generating a visual alert to warn the worker when an asymmetric position is detected. The system employs four force sensing resistors (FSR) embedded in a cushion and a microcontroller-based read-out circuit for monitoring the pressure exerted on the chair seat. Java-based software performs the real-time monitoring of the sensors' measurements and implements an uncertainty-driven asymmetry detection algorithm. The shifts from a symmetric to an asymmetric posture and vice versa generate and close a pop-up warning message, respectively. In this way, the user is promptly notified when an asymmetric posture is detected and invited to adjust the sitting position. Every position shift is recorded in a web database for further analysis of the sitting behavior.

Influence of carbon nanoparticles/epoxy matrix interaction on mechanical, electrical and transport properties of structural advanced materials.

The focus of this study is to design new nano-modified epoxy formulations using carbon nanofillers, such as carbon nanotubes, carbon nanofibers and graphene-based nanoparticles (CpEG), that reduce the moisture content and provide additional functional performance. The chemical structure of epoxy mixture, using a non-stoichiometric amount of hardener, exhibits unique properties in regard to the water sorption for which the equilibrium concentration of water (C eq) is reduced up to a maximum of 30%. This result, which is very relevant for several industrial applications (aeronautical, shipbuilding industries, wind turbine blades, etc), is due to a strong reduction of the polar groups and/or sites responsible to bond water molecules. All nanofillers are responsible of a second phase at lower glass transition temperature (Tg). Compared with other carbon nanofillers, functionalized graphene-based nanoparticles exhibit the best performance in the multifunctionality. The lowest moisture content, the high performance in the mechanical properties, the low electrical percolation threshold (EPT) have been all ascribed to particular arrangements of the functionalized graphene sheets embedded in the polymeric matrix. Exfoliation degree and edge carboxylated groups are responsible of self-assembled architectures which entrap part of the resin fraction hindering the interaction of water molecules with the polar sites of the resin, also favouring the EPT paths and the attractive/covalent interactions with the matrix.

The role of carbon nanofiber defects on the electrical and mechanical properties of CNF-based resins.

Heat treatment of carbon nanofibers has proven to be an effective method in removing defects from carbon nanofibers, causing a strong increase in their structural perfection and thermal stability. It affects the bonding states of carbon atoms in the nanofiber structure and causes a significant transformation in the hybridization state of the bonded carbon atoms.Nanofilled resins made of heat-treated CNF show significant increases in their electrical conductivity even at low concentrations. This confirms that enhancement in the perfection of the fiber structure with consequent change in the morphological features plays a prominent role in affecting the electrical properties. Indeed heat-treated CNFs display a stiff structure and a smooth surface which tends to lower the thickness of the unavoidable insulating epoxy layer formed around the CNF which, in turn, plays a fundamental role in the electrical transport properties along the conducting clusters. This might be very beneficial in terms of electrical conductivity but might have negligible effect on the mechanical properties.

Thermal and Electrical Characterization of Polyester Resins Suitable for Electric Motor Insulation.

This paper undertakes the thermal and electrical characterization of three commercial unsaturated polyester imide resins (UPIR) to identify which among them could better perform the insulation function of electric motors (high-power induction motors fed by pulse-wide modulation (PWM) inverters). The process foreseen for the motor insulation using these resins is Vacuum Pressure Impregnation (VPI). The resin formulations were specially selected because they are one-component systems; hence, before the VPI process, they do not require mixing steps with external hardeners to activate the curing process. Furthermore, they are characterized by low viscosity and a thermal class higher than 180 °C and are Volatile Organic Compound (VOC)-free. Thermal investigations using Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) techniques prove their excellent thermal resistance up to 320 °C. Moreover, impedance spectroscopy in the frequency range of 100 Hz-1 MHz was analyzed to compare the electromagnetic performance of the considered formulations. They manifest an electrical conductivity starting from 10-10 S/m, a relative permittivity around 3, and a loss tangent value lower than 0.02, which appears almost stable in the analyzed frequency range. These values confirm their usefulness as impregnating resins in secondary insulation material applications.

Role of MWCNTs Loading in Designing Self-Sensing and Self-Heating Structural Elements.

This work proposes nanocomposites with carbon nanotubes characterized by self-sensing and self-heating properties. Recently, a growing interest in these two properties has been found in many industrial sectors, especially in the aerospace and automotive fields. While the self-sensing function allows diagnosing the presence of micro-damage in the material thanks to the detection of residual resistance, the self-heating function is exploited to properly tune the heating performance in terms of the heating rate and final temperature values. An electrical percolation value of around 0.5% by weight of carbon nanotubes was found by electrical characterization. The AC conductivity of the nanocomposites, in the range of 100 Hz to 1 MHz, evidences that beyond a CNTs amount of 0.5% wt/wt, they are characterized by a purely resistive behavior. The self-sensing analysis displayed a gauge factor value of 4.1. The solid thermal stability up to 300 °C makes the material suitable as a heating element at high temperatures. SEM investigations and temperature maps evidence a good dispersion of the conductive filler in the epoxy matrix and, consequently, good isotropy in heat distribution. As regards the trend of electrical resistance by varying the temperature, the electro-thermal investigation has shown the presence of both Positive Temperature Coefficient (PTC) and Negative Temperature Coefficient (NTC) behaviors with a predominance of NTC as soon as the temperature becomes closer to the glass transition temperature of the epoxy resin.

The Performance of Graphene-Enhanced THz Grating: Impact of the Gold Layer Imperfectness.

We report the performance of a graphene-enhanced THz grating fabricated by depositing a gold layer on the femtosecond micromachined SiO2 substrate. The morphology of the gold plated patterned substrate was studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM), while the quality of the chemical vapor deposition (CVD) graphene was evaluated by Raman spectroscopy. The electromagnetic (EM) response of the metasurface comprising the graphene sheet and the gold plated substrate was studied by THz time domain spectroscopy in the 100 GHz-1 THz frequency range. We employed the finite elements method (FEM) to model the metasurface EM response by adjusting the ac conductivity of the gold layer covering the patterned SiO2 substrate to reproduce the measured transmission/reflection spectra. The results of the numerical simulation reveal the impact of the imperfectness of the gold layer on the performance of the THz metasurface. The experimental results are well described in terms of the Drude-Smith model of metal conductivity that takes into account the anisotropic scattering of the carriers in thin metal films.

Self-Sensing Nanocomposites for Structural Applications: Choice Criteria.

Epoxy resins containing multi-wall carbon nanotubes (MWCNTs) have proven to be suitable for manufacturing promising self-sensing materials to be applied in the automotive and aeronautic sectors. Different parameters concerning morphological and mechanical properties of the hosting matrices have been analyzed to choose the most suitable system for targeted applications. Two different epoxy precursors, the tetrafunctional tetraglycidyl methylene dianiline (TGMDA) and the bifunctional bisphenol A diglycidyl ether (DGEBA) have been considered. Both precursors have been hardened using the same hardener in stoichiometric conditions. The different functionality of the precursor strongly affects the crosslinking density and, as a direct consequence, the electrical and mechanical behavior. The properties exhibited by the two different formulations can be taken into account in order to make the most appropriate choice with respect to the sensing performance. For practical applications, the choice of one formulation rather than another can be performed on the basis of costs, sensitivity, processing conditions, and most of all, mechanical requirements and in-service conditions of the final product. The performed characterization shows that the nanocomposite based on the TGMDA precursor manifests better performance in applications where high values in the glass transition temperature and storage modulus are required.

Numerical Evaluation of the Effect of Geometric Tolerances on the High-Frequency Performance of Graphene Field-Effect Transistors.

The interest in graphene-based electronics is due to graphene's great carrier mobility, atomic thickness, resistance to radiation, and tolerance to extreme temperatures. These characteristics enable the development of extremely miniaturized high-performing electronic devices for next-generation radiofrequency (RF) communication systems. The main building block of graphene-based electronics is the graphene-field effect transistor (GFET). An important issue hindering the diffusion of GFET-based circuits on a commercial level is the repeatability of the fabrication process, which affects the uncertainty of both the device geometry and the graphene quality. Concerning the GFET geometrical parameters, it is well known that the channel length is the main factor that determines the high-frequency limitations of a field-effect transistor, and is therefore the parameter that should be better controlled during the fabrication. Nevertheless, other parameters are affected by a fabrication-related tolerance; to understand to which extent an increase of the accuracy of the GFET layout patterning process steps can improve the performance uniformity, their impact on the GFET performance variability should be considered and compared to that of the channel length. In this work, we assess the impact of the fabrication-related tolerances of GFET-base amplifier geometrical parameters on the RF performance, in terms of the amplifier transit frequency and maximum oscillation frequency, by using a design-of-experiments approach.

Cure behavior and physical properties of epoxy resin-filled with multiwalled carbon nanotubes.

Different experimental procedures were investigated for incorporating Multi walled Carbon nanotubes (MWCNT) into epoxy diglycidil-ether bisphenol-A (DGEBA), cured with 4,4' diamine-dibenzyl-sulfone (DDS): (i) mechanical mixing for either 60 or 120 minutes; (ii) high energy ball milling for 30 minutes; (iii) ultrasonication for 20 minutes. The mechanical properties of the obtained samples were monitored and used in order to guide the selection of the most promising composite system. The best results were obtained by using, as method of incorporation of carbon nanotubes in the resin, sonication for 20 minutes. Moreover it was found that the presence of an accelerating agent, BF3, commonly used for the cure reaction, is ineffective in the presence of carbon nanotubes, besides leading to composites with a lower glass transition. Afterwards resins with increasing MWCNT concentration were prepared and the thermal properties analyzed, showing a slight improvement in either the glass transition or the degradation temperature. The electrical conductivity of the selected composite system is characterised by a percolation threshold (lower than 0.1%) comparable to that found for similar systems, but beyond threshold it assumes greater values. The conductivity is characterised by only a slight increase with temperature in the range 30-90 degrees C. The positive temperature coefficient can be related to a decrease of the small gaps separating the CNTs clusters. A simple power law describes the relation between the composite conductivity and CNT concentration near the percolation threshold. A dependence of all the parameters of the power law on the temperature has also been evidenced, and it should be properly considered and indicated when using such a model in describing the behaviour of composites near the percolation threshold.

Damage Monitoring of Structural Resins Loaded with Carbon Fillers: Experimental and Theoretical Study.

In the present study, nanocomposite materials for structural applications with self-sensing properties are proposed. In particular, suitable processing of epoxy resins filled with carbon nanotubes and expanded graphite characterized by very different aspect ratio leads to nanocomposite systems with high glass transition temperatures and remarkable values of the gauge factor. In particular, this notable property ranges between four, for composites filled with one-dimensional nanofiller, and 39 for composites with two-dimensional (2D) graphite derivatives. The greater sensitivity of the 2D system against permanent deformations is interpreted on the basis of an empirical mathematical model and morphological descriptions. The larger inter-contact area among the graphite layers determines a larger contact resistance change than that occurring among carbon nanotubes. The proposed systems turn out to be very advantageous in strain-sensor applications where damage detection is a key requirement to guarantee the reliability of the structures and the safety of the end-users.

Electrical Current Map and Bulk Conductivity of Carbon Fiber-Reinforced Nanocomposites.

A suitably modified resin film infusion (RFI) process was used for manufacturing carbon fiber-reinforced composites (CFRCs) impregnated with a resin containing nanocages of glycidyl polyhedral oligomeric silsesquioxane (GPOSS) for enhancing flame resistance and multi-wall carbon nanotubes (MWCNTs) to contrast the electrical insulating properties of the epoxy resin. The effects of the different numbers (7, 14 and 24) of the plies on the equivalent direct current (DC) and alternating current (AC) electrical conductivity were evaluated. All the manufactured panels manifest very high values in electrical conductivity. Besides, for the first time, CFRC strings were analyzed by tunneling atomic force microscopy (TUNA) technique. The electrical current maps highlight electrically conductive three-dimensional networks incorporated in the resin through the plies of the panels. The highest equivalent bulk conductivity is shown by the seven-ply panel characterized by the parallel (σ//0°) in-plane conductivity of 16.19 kS/m. Electrical tests also evidence that the presence of GPOSS preserves the AC electrical stability of the panels.

Nanocarbon/Poly(Lactic) Acid for 3D Printing: Effect of Fillers Content on Electromagnetic and Thermal Properties.

Electromagnetic and thermal properties of a non-conventional polymer nanocomposite based on thermoplastic Polylactic acid (PLA, Ingeo™) filled, in different weight percentage, with multi-walled carbon nanotubes (MWCNTs), graphene nanoplatelets (GNPs), as well as a mixture of both fillers (MWCNTs/GNPs), are analyzed. The combination of notable electrical, thermal, and electromagnetic (EM) properties of the carbon fillers, in concentrations above the percolation threshold, together with the good processability of the PLA matrix gives rise to innovative filaments for 3D printing. In particular, the shielding efficiency (SE) in the frequency range 26-37 GHz of samples increases from 0.20 dB of unfilled PLA up to 13.4 dB for composites containing MWCNTs and GNPs, corresponding to 4% and 95% of SE, respectively. The thermal conductivity of the PLA loaded with 12 wt % of GNPs is 263% higher than that of the unfilled polymer, whereas an improvement of about 99% and 190% is detected for the PLA matrix loaded with MWCNTs and both fillers, respectively. The EM and thermal characterization is combined with a morphological investigation allowing us to correlate the dispersion states of the fillers within the polymer matrix with the observed EM and thermal properties. The EM and thermal characteristics exhibited by the nanocomposites make them suitable for packaging applications of electronic devices with electromagnetic interference (EMI) shielding and thermal dissipation features.

Morphological, Rheological and Electromagnetic Properties of Nanocarbon/Poly(lactic) Acid for 3D Printing: Solution Blending vs. Melt Mixing.

The limitation of poor mechanical stability and difficulties in printing electrically conductive components can be overcome owing to the recent introduction of nanotechnology into the field of additive manufacturing (AM) and the consequent development of nonconventional polymer nanocomposites suitable for 3D printing. In the present work, different weight percentages (up to 6 wt % in total) of carbon-based nanostructures-multiwalled carbon nanotubes (MWCNTs), graphene nanoplatelets (GNPs), and a combination of both fillers (MWCNTs/GNPs)-were incorporated into poly(lactic) acid (PLA, Ingeo™) in an attempt to overcome several limitations of conventional 3D manufacturing based on insulating materials. Solution blending and melt mixing were the two fabrication methods adopted for preparation of the samples under test. A comparison of the morphological, rheological, and electrical properties of the resulting nanocomposites was carried out. Moreover, for the same weight concentrations, the influence of physical and geometrical features (i.e., functionalization and aspect ratio) of the embedded fillers was also investigated. Rheological methods were applied to control the quality of fillers dispersion in PLA matrix. The rheological percolation threshold was considered as reference in order to evaluate the internal structure of nanodispersions. TEM visualization, combined with rheological characterizations, was used for efficient control of the nanofiller dispersion. DC characterization revealed that lower electrical percolation thresholds and higher values of electrical conductivity were achieved using fillers with a larger aspect ratio and melt mixing, respectively. Moreover, given the possibility of obtaining complex and appropriate shapes for electromagnetic compatibility (EC) applications, electromagnetic (EM) response of the nanocomposites at the highest filler concentration was investigated in GHz and THz regions. It was found that the electromagnetic shielding efficiency (EMI) of nanocomposites strongly depended on the aspect ratio of the nanofillers, whereas the type of processing technique did not have a significant effect. Therefore, a careful choice of methods and materials must be made to address the final application for which these materials and further 3D printed architectures are designed.

Influence of uncertain electrical properties on the conditions for the onset of electroporation in an eukaryotic cell.

A detailed numerical investigation concerning the influence on the electrical response of an eukaryotic cell model due to the variations of the conductivity and permittivity of the plasma and nuclear membranes is carried out by means of a vertex analysis. The dynamics of three cell performances, the voltages across the external and the nuclear membrane, and the pores density are analyzed by adopting a novel FEM-based model coupled with the nonlinear equation describing the electroporation (EP) phenomenon. The variations of the electrical and morphological performances, when the cell is stressed by a nonideal step input, are studied in three interesting regimes, i.e., at the nominal threshold for the onset of EP, and when the applied stress is over or under the EP threshold. The performed numerical analysis puts in evidence that, around the nominal EP threshold, uncertainties may lead to an indeterminate state of the plasma membrane, whereas the other two regimes remain well definite.