Additive fabrication and experimental validation of a lightweight thermoelectric generator.

We develop a thermoelectric generator based on catalytic combustion and operating in the low power range (up to 10 W). Considering the target of small-scale thermoelectric generators, the additive technique was chosen as an enabling technology to customize the different parts of the presented device. The generator consists of a hexagonal shaped combustion chamber coupled to commercial thermoelectric modules, water-cooled at the cold side. Thanks to the components design, heat transfer across each part of the system is properly driven enhancing the thermal management of the system. Moreover, in order to improve the overall efficiency, exhausts outlet is designed to promote heat recovery. The generator is characterized achieving an electrical power output close to 9 W in continuous regime, with an overall efficiency of 3.55%. The compact size, the light weight, the simple design and the reliability in continuous operating conditions are all promising features of the device described. Furthermore, the materials chosen for the device can suggest a way to fabricate cheaper heat exchangers, actually one of the main costs of the device development.

Elastocaloric Properties of Polycrystalline Samples of NiMnGaCu Ferromagnetic Shape Memory Alloy under Compression: Effect of Improvement of Thermoelastic Martensitic Transformation.

Shape memory alloys (SMAs) and ferromagnetic shape memory alloys (FeSMAs) have recently attracted interest for solid state refrigeration applications. Among NiMnGa-based quaternary systems, NiMnGaCu exhibits an interesting giant magnetocaloric effect thanks to the overlapping of the temperatures related to the magnetic transition and the thermoelastic martensitic transformation (TMT); in particular, for compositions with Cu content of approximately 6 at%. In the present work, we investigated the improvement effect of TMT on the total entropy change (ΔS) in the elastocaloric performances of polycrystalline Ni50Mn18.5Cu6.5Ga25 at% alloy samples, just above room temperature. We report an extensive calorimetric and thermomechanical characterization to explore correlations between microstructural properties induced by the selected thermal treatment and elastocaloric response, aiming at providing the basis to develop more efficient materials based on this quaternary system. Both ΔT and ΔS values obtained from mechanical curves at different temperatures and strain recovery tests under fixed load vs. T were considered. Maximum values of ΔS = 55.9 J/KgK and ΔT = 4.5 K were attained with, respectively, a stress of 65 MPa and strain of 4%. The evaluation of the coefficient of performance (COP) was carried out from a cyclic test.

Microstructural and Thermo-Mechanical Characterization of Cast NiTiCu20 Shape Memory Alloy.

Among NiTi-based alloys, one of the most promising and exploited alloys is NiTiCu, since the addition of Cu in substitution of Ni in the binary equiatomic NiTi has a significant influence on the martensitic transformation and the thermomechanical properties of the system. A high content of Cu improves the damping properties at the expense of phase homogeneity and workability. The present study focuses on an alloy with a high copper content, i.e., 20 at.%. For this specific composition, the correlation between the thermal treatments, microstructure, formation of secondary phases, and damping properties are investigated by several analyses. The microscopic observation, together with the compositional analysis, allowed the determination of four different phases in the alloy. Both the calorimetry and dynamic thermo mechanical measurements, which confirmed the high damping ability of the alloy, provided a characterization of the martensitic transition. Finally, the electron backscatter diffraction (EBSD) analysis detected the different crystallographic structures (i.e., cubic austenite, orthorhombic martensite, and cubic (face-centered) NiTi2) and their orientation in the different phases. Therefore, the present work aims to improve the knowledge of the role of secondary phases in the optimization of the NiTiCu20 alloy as a valuable alternative to typical alloys used for damping purposes.

Physical Characterization of Sintered NiMnGa Ferromagnetic Shape Memory Alloy.

The present work focused on the microstructural, thermal, electrical, and damping characterization of NiMnGa samples produced through a powder pressing and a sintering process; the effect of sintering times and of the starting powder size were evaluated. Moreover, an observation of the evolution of martensitic transformation typical of NiMnGa ferromagnetic shape memory alloy was conducted in comparison with the cast material behavior and in correlation with the material densification. The optimum powder size and sintering time for the process, i.e., 50 µm or lower and 72 h, were identified considering the investigated physical properties of the sintered samples in comparison to the cast material. The corresponding sample showed the best compromise between density, thermal and electrical properties, and damping and functional behaviour. In general, the outcomes of this study could be the basis of a useful tool for production processes that include a sintering step as well as being a starting point for the evaluation of an alternative low cost fabrication method of this alloy.

Composition-Nanostructure Steered Performance Predictions in Steel Wires.

Neutron scattering in combination with scanning electron and atomic force microscopy were employed to quantitatively resolve elemental composition, nano- through meso- to metallurgical structures and surface characteristics of two commercial stainless steel orthodontic archwires-G&H and Azdent. The obtained bulk composition confirmed that both samples are made of metastable austenitic stainless steel type AISI 304. The neutron technique's higher detection sensitivity to alloying elements facilitated the quantitative determination of the composition factor (CF), and the pitting resistance equivalent number (PREN) for predicting austenite stability and pitting-corrosion resistance, respectively. Simultaneous neutron diffraction analyses revealed that both samples contained additional martensite phase due to strain-induced martensite transformation. The unexpectedly high martensite content (46.20 vol%) in G&H was caused by combination of lower austenite stability (CF = 17.37, p = .03), excessive cold working and inadequate thermal treatment during material processing. Together, those results assist in revealing alloying recipes and processing history, and relating these with corrosion resistance and mechanical properties. The present methodology has allowed access to unprecedented length-scale (µm to sub-nm) resolution, accessing nano- through meso-scopic properties. It is envisaged that such an approach can be extended to the study and design of other metallic (bio)materials used in medical sciences, dentistry and beyond.

Study of the Performances of a Thermoelectric Generator Based on a Catalytic Meso-Scale H2/C3H8 Fueled Combustor.

In this work the thermoelectric generator (TEG) based on catalytic combustion already developed in our lab has been further investigated and improved. The system made of two thermoelectric (TE) modules coupled with a catalytic combustor has been used in this work to obtain higher overall efficiency by adding hydrogen to the fuel mixture. Since implementation of hydrogen as a fuel has shown low and stable combustion temperature in literature, it is expected to achieve good overall efficiency of TEG. Moreover, hydrogen can be used to improve the system inducing self-ignition. Focus of the present work is the implementation of different mixture proportions, varying the amount of hydrogen, and the investigation of their effects on the overall efficiency. The overall TEG efficiency, has been evaluated by parallel characterization of thermoelectric modules and exhaust gases composition. The system performances have been characterized using different mixtures: the results indicate that addition of H2 to the fuel contribute to increase the chemical and overall TEG efficiency respect to previous work, producing up to 5.92 W of electrical power. Finally, the effects of H2 for on self-ignition conditions have been investigated finding the minimum H2 amount for different gas flow rates.

A new design of a Nitinol ring-like wire for suturing in deep surgical field.

The present work proposes a new suturing procedure based on self-accommodating suture points. Each suture point is made of a commercial NiTi wire hot-shaped in a single loop ring; a standard suture needle is then fixed at one end of the NiTi suture. According to this simple geometry, several NiTi suture stitches have been prepared and tested by tensile test to verify the closing force in comparison to that of commercial sutures. Further experimental tests have also been performed on anatomic samples from animals to verify the handiness of the NiTi suture. Moreover, surface quality of sutures has been carefully investigated via microscopy. Results show that the NiTi suture expresses high stiffness and a good surface quality. In addition, the absence of manual knotting allows for a simple, fast and safe procedure.

Applications of shape memory alloys for neurology and neuromuscular rehabilitation.

Shape memory alloys (SMAs) are a very promising class of metallic materials that display interesting nonlinear properties, such as pseudoelasticity (PE), shape memory effect (SME) and damping capacity, due to high mechanical hysteresis and internal friction. Our group has applied SMA in the field of neuromuscular rehabilitation, designing some new devices based on the mentioned SMA properties: in particular, a new type of orthosis for spastic limb repositioning, which allows residual voluntary movement of the impaired limb and has no predetermined final target position, but follows and supports muscular elongation in a dynamic and compliant way. Considering patients in the sub-acute phase after a neurological lesion, and possibly bedridden, the paper presents a mobiliser for the ankle joint, which is designed exploiting the SME to provide passive exercise to the paretic lower limb. Two different SMA-based applications in the field of neuroscience are then presented, a guide and a limb mobiliser specially designed to be compatible with diagnostic instrumentations that impose rigid constraints in terms of electromagnetic compatibility and noise distortion. Finally, the paper discusses possible uses of these materials in the treatment of movement disorders, such as dystonia or hyperkinesia, where their dynamic characteristics can be advantageous.

Design and thermo-mechanical analysis of a new NiTi shape memory alloy fixing clip.

In this work, a new NiTi shape memory alloy (SMA) bone fixator is proposed. Thanks to the shape memory effect, this device does not need any external tool for the fixation, as the anchorage is obtained only by the self-accommodation of the clip during the parent transformation. Calorimetry and thermo-mechanical tests were used to evaluate the phase transformation temperatures and to estimate the forces generated both during the fixing surgical procedure and after the surgical operation. An application on animal anatomical sample was also performed; an appropriate mechanical tightness as well as a good handiness has been found.

Can passive mobilization provide clinically-relevant brain stimulation? A pilot EEG and NIRS study on healthy subjects.

Lower limb rehabilitation is a fundamental part of post-acute care in neurological disease. Early commencement of active workout is often prevented by paresis, thus physical treatment may be delayed until patients regain some voluntary command of their muscles. Passive mobilization of the affected joints is mostly delivered in order to safeguard tissue properties and shun circulatory problems. The present paper investigates the potential role of early passive motion in stimulating cortical areas of the brain devoted to the control of the lower limb. An electro-mechanical mobilizer for the ankle joint (Toe-Up!) was implemented utilizing specially-designed shape-memory-alloy-based actuators. This device was constructed to be usable by bedridden subjects. Besides, the slowness and gentleness of the imparted motion, make it suitable for patients in a very early stage of their recovery. The mobilizer underwent technical checks to confirm reliability and passed the required safety tests for electric biomedical devices. Four healthy volunteers took part in the pre-clinical phase of the study. The protocol consisted in measuring of brain activity by EEG and NIRS in four different conditions: rest, active dorsiflexion of the ankle, passive mobilization of the ankle, and assisted motion of the same joint. The acquired data were processed to obtain maps of cortical activation, which were then compared. The measurements collected so far show that there is a similar pattern of activity between active and passive/assisted particularly in the contralateral premotor areas. This result, albeit based on very few observations, might suggest that passive motion provides somatosensory afferences that are processed in a similar manner as for voluntary control. Should this evidence be confirmed by further trials on healthy individuals and neurological patients, it could form a basis for a clinical use of early passive exercise in supporting central functional recovery.

Characterization of a cold-rolled 2101 lean duplex stainless steel.

Duplex stainless steels (DSS) may be defined as a category of steels with a two-phase ferritic-austenitic microstructure, which combines good mechanical and corrosion properties. However, these steels can undergo significant microstructural modification as a consequence of either thermo-mechanical treatments (ferrite decomposition, which causes σ- and χ-phase formation and nitride precipitation) or plastic deformation at room temperature [austenite transformation into strain-induced martensite (SIM)]. These secondary phases noticeably affect the properties of DSS, and therefore are of huge industrial interest. In the present work, SIM formation was investigated in a 2101 lean DSS. The material was subjected to cold rolling at various degrees of deformation (from 10 to 80% thickness reduction) and the microstructure developed after plastic deformation was investigated by electron backscattered diffraction, X-ray diffraction measurements, and hardness and magnetic tests. It was observed that SIM formed as a consequence of deformations higher than ~20% and residual austenite was still observed at 80% of thickness reduction. Furthermore, a direct relationship was found between microstructure and magnetic properties.

Intraoral laser welding: ultrastructural and mechanical analysis to compare laboratory laser and dental laser.

The Nd:YAG laser has been used since 1970 in dental laboratories to weld metals on dental prostheses. Recently in several clinical cases, we have suggested that the Nd:YAG laser device commonly utilized in the dental office could be used to repair broken fixed, removable and orthodontic prostheses and to weld metals directly in the mouth. The aim of this work was to evaluate, using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and dynamic mechanical analysis (DMA), the quality of the weld and its mechanical strength, comparing a device normally used in dental laboratory and a device normally used in the dental office for oral surgery, the same as that described for intraoral welding. Metal plates of a Co-Cr-Mo dental alloy and steel orthodontic wires were subjected to four welding procedures: welding without filler metal using the laboratory laser, welding with filler metal using the laboratory laser, welding without filler metal using the office laser, and welding with filler metal using the office laser. The welded materials were then analysed by SEM, EDS and DMA. SEM analysis did not show significant differences between the samples although the plates welded using the office laser without filler metal showed a greater number of fissures than the other samples. EDS microanalysis of the welding zone showed a homogeneous composition of the metals. Mechanical tests showed similar elastic behaviours of the samples, with minimal differences between the samples welded with the two devices. No wire broke even under the maximum force applied by the analyser. This study seems to demonstrate that the welds produced using the office Nd:YAG laser device and the laboratory Nd:YAG laser device, as analysed by SEM, EDS and DMA, showed minimal and nonsignificant differences, although these findings need to be confirmed using a greater number of samples.