Functional Materials Letters

Boron- and nitrogen-based chemical hydrogen storage materials, such as metal borohydrides, ammonia borane, hydrazine borane, metal-nitrogen-hydrogen systems, ammonia, and hydrazine, have been extensively investigated in the past years. A variety of methods have been developed to decrease the reaction temperature and enhance the reaction kinetics of these systems. This feature article is to serve as an up to date account of the recent progress in chemical hydrogen storage with the boron- and nitrogen-based materials.

We have prepared a lithium excess carbon composite material, Li_4+xTi_5-xO_12-δ/C (LTO/C), using various amounts of sucrose as a carbon source by the spray-drying method. The prepared materials were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and elemental analysis. The prepared material had the Li₄Ti₅O₁₂ phase including 3.9–18.4 wt.% carbon. Transmission electron microscopy images and the selected area diffraction (SAD) pattern showed that the prepared materials consisted of a carbon nanonetwork in the LTO/C composite. The charge–discharge cycling tests were carried out using the R2032 coin-type cell under the following conditions; 1.2–3.0 V, 0.1 C–10 C (1 C = 175 mA g^-1), 25°C. Based on the electrochemical results, the electrode performance of the prepared material was improved with increasing amounts of residual carbon, in particular, LTO/C including 6.2 wt.% residual carbon exhibited the best electrode performance of 156 mAh g^-1 at 1 C during 50 cyclings when compared to the other materials.

A facile synthetic approach for CoSb₃/graphene nanocomposite has been developed in this work. By adjusting Co/Sb molar ratio, reaction temperature, and reaction time, we found that nanocrystalline CoSb₃ (5–10 nm) can form at a low temperature of 180°C and a short time of only 1 h via a one-pot solvothermal route. At the same time, graphite oxide can be reduced to graphene with uniformly loaded CoSb₃ nanoparticles. The composites have been characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). The nanocomposite shows improved cycling stability compared to bare CoSb₃.

For the first time, this work indicates that mechanical concerns of inorganic materials are highly demanded when well optimizing their functional applications. We quantitatively give the proper data of hardness on different planes of some representative inorganic functional materials including borides, carbides, nitrides and oxides. This work clearly indicates mechanical importance in studying functionality of inorganic materials, and provides people guidance in the practical applications of these materials.

Hollandite-type MnO₂ and its Co-substituted samples have been synthesized by hydrothermal method from MnSO₄, CoSO₄ and K₂S₂O₈ solutions. The hydrothermal products from MnSO₄ and K₂S₂O₈ solutions consisted of the hollandite-type [2 × 2] tunnel structure at 100°C–150°C for 12 h, which was transformed to pyrolusite-type [1 × 1] tunnel structure by hydrothermal treatment at 150°C for longer times of 24–48 h or at a higher temperature of 180°C. On the other hand, the hydrothermal products from MnSO₄, CoSO₄ and K₂S₂O₈ solutions at 150°C–180°C for 12–48 h consisted of the hollandite-type MnO₂ phase. The Co-substituted samples showed higher initial discharge capacity (180–200 mAh (g-oxide)^-1) than that of non-substituted hollandite-type MnO₂ (130–160 mAh (g-oxide)^-1) at 50 mA g^-1.

The effect of hyperstabilization of martensite implies that the reverse martensitic transformation proceeds in two well separated stages. Namely, a small fraction of martensite (of the order of 10%) retransforms upon heating into the parent phase over a temperature range slightly higher than the nominal reverse transformation temperature, whereas the rest of the martensite retransforms through a re-nucleation of fine lamellae of the parent phase. The renucleation stage of the transformation is well defined and requires strong overheating of the order of 300 K with respect to the nominal transformation. In this letter, the results are discussed of a study of the hyperstabilization effect in different martensitic structures: faulted martensite in Cu–Al–Be system and twinned martensite in ferromagnetic Ni–Fe–Ga crystals by means of differential scanning calorimetry, transmission electron microscopy and internal friction. The conclusion has been drawn that hyperstabilization implies a severe blocking of the motion of interphase boundaries during the reverse transformation, which can be produced either due to a high concentration of highly mobile quenched-in defects ("sweeping" of defects during the reverse transformation) or due to a creation of obstacles by preliminary plastic deformation. The former mechanism requires very intense diffusion of quenched-in defects assisted by dislocations/interfaces, which has been confirmed by internal friction studies. It has been shown that the renucleation stage, which occurs at around 600 K for different alloys, is preceded by a relaxation internal friction peak. A possible role of this relaxation in renucleation of the parent phase is discussed.

Tensile tests were performed in situ in a transmission electron microscope to investigate the twinning mechanism in non-modulated Ni–Mn–Ga martensite. The reorientation of the twin variants occurs via twinning dislocations. Their generation and movement were followed; the glide plane and Burgers vector were verified. Individual twinning dislocations were visualized.

The thermomechanical behavior of shape memory alloys is now well mastered. However, a hindrance to their sustainable use is the lack of knowledge of their fracture behavior. With the aim of filling this partial gap, fracture tests on edge-cracked specimens in NiTi have been made. Particular attention was paid to determine the phase transformation zones in the vicinity of the crack tip. In one hand, experimental kinematic fields are observed using digital image correlation showing strain localization around the crack tip. In the other hand, an analytical prediction, based on a modified equivalent stress criterion and taking into account the asymmetric behavior of shape memory alloys in tension-compression, provides shape and size of transformation outset zones. Experimental results are relatively in agreement with our analytical modeling.

Guaranteed behavior of SMA in damping needs deep study of the alloy behavior according to the requirements. Pseudoelastic window, evolution of properties, self-heating and fatigue are the major concerns. The analysis shows that selected alloys can do the work. The application to portico and to cable oscillations in realistic systems shows an excellent damping effect. Useful simulation requires a good phenomenological model. A cubic model is built with improved results at low deformation.

NiTi wires are successfully employed asthermal actuators: devices able to convert thermal energy into mechanical energy, generating force and displacement. So far, researchers have highlighted the influence of various parameters on shape memory characteristics of the material, such as chemical composition, cold working, heat treatment and cycling. This study is aimed to prove that drawing histories, in particular the number of drawing steps performed to achieve a certain cold working level, are fundamentally important on functional properties of wires. This process parameter involves significant changes in terms of thermal hysteresis width. These alterations are still observable on wires thermo-mechanical cycled, therefore on products ready to be employed for actuators.

Large-sized dye-sensitized solar cells were prepared with TiO₂ cemented and protected Ag grids in the photo and counter electrodes. The addition of high conductive TiO₂ cemented Ag grids can maintain high performance with the enlargement of the cells. The preparation of the compact TiO₂ layer on the Ag grids can prevent the corrosion of the electrolyte, moreover, when it is prepared on the whole area of the photo electrode, it also can play as the blocking layer for further enhancing the performance of cells. The presented method shows a simple and efficient way to prepare high performance large single cells.

The evolution of the ultrafine structure, obtained at thermal treatment below recrystallization, and its effect onto shape memory characteristics in NiTi alloy was under study. It was shown that low temperature annealing (< 0.5 Tmelt) of the hot rolled NiTi leads to the structure refinement accompanied by the growth in accumulated martensite deformation. It was suggested that this is possible due to the increase of the volume fraction of martensite crystals properly oriented in respect to external stress.

Nitinol stents are nowadays widely used for the treatment of occlusions in peripheral arteries. However, the expansion of this indication has also highlighted some complications. In particular, the patient daily activities expose the peripheral arteries to large and cyclic deformations which may cause long-term failure of the device and consequently re-occlusion of the artery. Accordingly, the assessment of the stent fatigue rupture is of primary importance to assure the effectiveness of stenting procedure. However the fatigue behavior characterization of Nitinol for peripheral stent is a quite difficult problem because of the complexity of the in vivo solicitations the stent is subjected to and the strong nonlinearity in the material response. In this paper we approached the problem in two steps: (i) in the first step the study of the stent solicitations under realistic (physiological) conditions was performed through the use of numerical simulations which allowed sophisticated patient-specific models of the stenting procedure; (ii) in the second step, the previous results were used for the design of an experimental campaign and the following execution of the tests for the material mechanical characterization and fatigue life study. The tests were performed on Nitinol specimens derived from the same tubes used for producing a commercial peripheral stent and created following the same procedure employed for the device. As a consequence of the small dimension of the specimens, a preliminary design of the experimental test set-up was also required. The obtained results allowed a sufficiently accurate characterization of the stent material fatigue behavior in the range of interest.

The influence of mechanical and electrical interfacial conditions on the threshold field of ferroelectric nanothin films has been studied by employing Ginzburg–Landau theory and the phase field method. The results obtained show that the threshold field of the ferroelectric thin film increased with increasing compressive misfit strain and extrapolation length. This was because the said factors increased the energy barrier for domain wall motion by affecting the elastic and depolarization fields in the film. These results are useful for the manufacture and application of micro-electromechanical-systems including ferroelectric thin films.

This work proposes the functional characterization of a composite material, suitable for passive suppression of flexural vibration of beams and shells. Two patterned thin sheets of CuZnAl Shape Memory Alloy (SMA) are embedded into a layered beam of glass fiber. The composite combines the density and stiffness of the glass fiber with high damping properties of SMA in martensitic state. Properly shaped patterning of the SMA sheets, for improving adhesion between the SMA and glass fiber, is performed by means of laser technology. The effect of the laser micromachining on transformation temperatures and internal friction properties of the SMA elements are analyzed. Finally, measurements of the structural damping of the layered glass fiber/SMA composite are reported and the flexural vibration suppression, due to the embedded CuZnAl sheets, is shown.

Shape memory alloys (SMA) show several interesting features in their material behavior which are due to martensitic phase transformation. In NiTi, this transformation covers the three crystallographic phases of austenite, martensite, and R-phase. This publication analyzes the influence of the R-phase formation on the overall material behavior by means of a micromechanical model. The model is based on energy minimization with the assumption of a certain energy dissipation when martensite is formed. To simplify the formulation, the dissipation associated with the transformation between austenite and R-phase is neglected, since the geometrical change in unit cell geometry is relatively small in this case. Numerical simulations show that especially the slope reduction in the stress–strain curve, which is known from experiments, can be explained by R-phase formation.

In view to put in light morphology peculiarities of pentacene, the widely used material for organic transistors, a detailed atomic force microscopy (AFM) investigation of the pentacene growth mode in relationship with the film thickness (in the range 6–60 nm) is reported. Local surface energy measurements based on contact AFM spectroscopy were performed to explain the thickness dependent morphology of the pentacene films onto two polymer dielectrics (parylene-C and benzocyclobutene). Our analysis provided an exhaustive description not only of the accessible film surface but also of its inner structural properties. Evidenced by these approaches, a critical thickness of 30 nm was found in relation with a transition from the orthorhombic thin film phase to triclinic bulk phase for pentacene growth on parylene-C and benzocyclobutene. This thickness was previously observed to be corresponding to organic field effect transistors (OFETs) with optimized mobility.

Pure anatase TiO₂ spheres with mesoporous structure were prepared by a simple urea assisted hydrothermal process at low temperature. X-ray Diffractometer, Scanning Electron Microscope, Transmission Electron Microscope, and N₂ adsorption-desorption were used to characterize TiO₂. The dye-sensitized solar cells results show that an overall light conversion efficiency of 6.96% which is 36.7% higher than that of the overall efficiency (5.09%) obtained from the P25 photoanode owing to the superior light scattering effect and the high level of dye loading capacity of microspheres.

Contrast with conventional dielectric tunable materials such as barium strontium titanate (BST), here, we report one new dielectric tunable behavior for Sr_1-xPr_xTiO₃ system at low temperature. Giant dielectric tunability is confirmed in this system. More importantly, the efficient dielectric tunability can be realized just using small bias field. In addition, critical threshold electric field is also confirmed. This phenomenon may be related with the competition interaction of polar state with quantum fluctuations.

A novel vanadium pentoxide (V₂O₅) nanoplatelet has been prepared by an oxalic acid assisted sol–gel method using β-cyclodextrin as an exfoliation agent followed by sintering at a high temperature. Crystal structure and morphology are analyzed using X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. Due to about 10 nm thick, the product shows a large chemical diffusion coefficient of Li⁺ ions over 10^-10 cm² s^-1 at room temperature. In the potential range of 4.0–2.0 V (vs. Li/Li⁺), it can deliver an initial capacity of 291 mAh g^-1 at 0.1 C rate, which equals approximately to the theoretic one. It also shows excellent rate ability and good cycling performance at room temperature. The primary results suggest this two-dimensional nanomaterial could be developed into a promising cathode material for lithium-ion batteries.