Shape Memory Alloy Thin Films Research Articles

Compositional influence of CuAlMn SMA coated optical fiber towards sensing low temperature

Low temperature sensing with currently available sensors is a challenging task due to the several limitations in usage under harsh environmental conditions. Hence, optical fiber-based sensors could be a better alternative for sensing and monitoring applications due to their immunity at extreme temperatures (both Cryogenic and high temperature), pressure, electromagnetic interference, and power fluctuations. Moreover, the sensing properties are further improved by applying metal coating on optical fiber. In this respect, Shape Memory Alloy (SMA) coating on optical fiber shows enhanced sensitivity due to its unique property of shape memory effect. Particularly, copper-based SMAs are investigated in this regard because of their solid-state actuation characteristics at extreme temperature conditions. Hence, we have coated the optical fiber with CuAlMn SMA by thermal evaporation technique. We have varied the composition of SMA to control the transformation temperature (TT) at low-temperature regions. In the CuAlMn system, TTs are highly dependent on the concentrations of Al and Mn content. We have varied the weight percentages of Al and Mn present in CuAlMn SMA, to achieve the actuation at sub-zero temperature. We have used 2 different wire compositions (Cu-9Al-12.6Mn, and Cu-8.8Al-12.2Mn) for the preparation of the CuAlMn SMA thin films. The film obtained using the Cu-9Al-12.6Mn wires, has shown the lowest transformation temperature and maximum actuation/ displacement (∼10 mm) at low temperature (liquid N2) region. Such actuation of SMA-coated fiber results in significant optical signal attenuation (∼ 8 V peak-to-peak voltage difference under liquid N2 exposure) at the detector end. Thus, we can claim that the CuAlMn-coated optical fiber can be used as a cryogenic sensor due to its significant optical signal attenuation.

High-throughput membrane deflection characterization of shape memory alloy thin films

Nickel–Titanium (Ni–Ti) shape memory alloy thin films are currently gaining significant attention for small-scale systems, but device integration is still challenging due to its high compositional-dependent behavior. Therefore, it is crucial to identify the optimal parameters required to fabricate desirable samples. In this study, combinatorial high-throughput membrane deflection experiments were performed to investigate the mechanical properties of submicron-thick Ni–Ti thin films. The mechanical properties of freestanding Ni–Ti thin films, sputter-deposited with compositions and thicknesses ranging from 45.7 at. % to 52.7 at. % nickel and 270–530 nm were evaluated. With an increase in the Ni content, the crystallinity and existing phases of the films varied and exhibited an overall increasing trend of elastic modulus, transformation stress, percentage of recoverable strain and hysteresis area. Based on the obtained results, the compositional ratio of Ni–Ti films can be tailored accordingly to meet the different requirements of small-scale devices, and to then extend the employment of this high-throughput methodology in exploring advanced metallic alloys and ceramic composites.

Thermal Stability of Cu-Al-Ni Shape Memory Alloy Thin Films Obtained by Nanometer Multilayer Deposition.

Cu-Al-Ni is a high-temperature shape memory alloy (HTSMA) with exceptional thermomechanical properties, making it an ideal active material for engineering new technologies able to operate at temperatures up to 200 °C. Recent studies revealed that these alloys exhibit a robust superelastic behavior at the nanometer scale, making them excellent candidates for developing a new generation of micro-/nano-electromechanical systems (MEMS/NEMS). The very large-scale integration (VLSI) technologies used in microelectronics are based on thin films. In the present work, 1 μm thickness thin films of 84.1Cu-12.4 Al-3.5Ni (wt.%) were obtained by solid-state diffusion from a multilayer system deposited on SiNx (200 nm)/Si substrates by e-beam evaporation. With the aim of evaluating the thermal stability of such HTSMA thin films, heating experiments were performed in situ inside the transmission electron microscope to identify the temperature at which the material was decomposed by precipitation. Their microstructure, compositional analysis, and phase identification were characterized by scanning and transmission electron microscopy equipped with energy dispersive X-ray spectrometers. The nucleation and growth of two stable phases, Cu-Al-rich alpha phase and Ni-Al-rich intermetallic, were identified during in situ heating TEM experiments between 280 and 450 °C. These findings show that the used production method produces an HTSMA with high thermal stability and paves the road for developing high-temperature MEMS/NEMS using shape memory and superelastic technologies.

Fatigue and fracture of shape memory alloys in the nanoscale: An in-situ TEM study

This study is aimed at demonstrating the fatigue and fracture mechanisms in nanoscale NiTi shape memory alloy thin films. During functional fatigue, permanent strains are attributed to residual martensite pinned by Ni4Ti3 precipitates and interfacial dislocations parallel to type II internal twins of the martensite. These residual martensite-austenite interfaces acted as heterogeneous nucleation sites for stress-induced martensite in subsequent cycles thereby explaining the reduction in transformation stress. Finally, structural failure was observed to occur along the dislocation-rich austenite-martensite interface, making it the weakest link in the microstructure thus establishing a direct link between mechanisms of functional fatigue that result in the accumulation of permenant strain and mechanisms of structural fatigue that result in fracture. This work provides a comprehensive understanding of deformation mechanisms in nanoscale shape memory alloys and points to fundamental mechanisms that are applicable to macro scales.

Shape Memory Alloy Thin Film Auxetic Structures

AbstractAuxetic structures provide an interesting approach to solving engineering problems due to their negative Poisson's ratio, which allows for elongation perpendicular to applied stresses, opposite to a conventional structure's necking behavior. Thus, they can function well in applications requiring compacting the device into a small volume during the deployment (e.g., implants inserted with catheters) or stretchability with area coverage (e.g., stretchable electronics). Fabricating them with shape memory alloys (SMAs) expands the possibilities. The high strains experienced by auxetic structures may become reversible compared to ordinary metals due to superelastic or shape memory effect. This work studies four different auxetic microstructures using thin film SMAs that are capable of surviving strains up to 57.4%. Since these structures are fabricated by layer deposition and lithography, other components, such as microelectronics, can be seamlessly integrated into the fabrication process. These auxetic thin films are investigated for their mechanical behavior under tension for their stretchability and stability. Under tension, thin films are known to show wrinkling instabilities. In two of four designs, the large auxetic behavior leads to wrinkling, while the other two display stable, non‐wrinkling behavior. These designs can be candidates for stretchable electronics, wearable medical devices (e.g., biosensors), or implants (e.g., stents).

Photo-electrical Characterization of New CuAlNi/n-Si/Al Schottky Photodiode Fabricated by Coating Thin-Film Smart Material

Micro/nano scale thin-film shape memory alloys (SMAs) have been used in many different miniaturized systems. Using them as thin-film metal components in fabrication of Schottky photodiodes has started a few years ago. In this work, a new SMA-photodiode device with CuAlNi/n-Si/Al structure was produced by coating nano-thick CuAlNi SMA film onto n-Si wafer substrate via thermal evaporation. The photoelectrical I-V, C-V and I-t photodiode signalization tests were performed under dark and varied artifical light power intensities in room conditions. It was observed that the new device exhibited photoconductive, photovoltaic and capacitive behaviors. By using conventional I-V method, the diode parameters such as electrical ideality factor (n), Schottky barrier height (ϕb) and rectification ratio (RR) of the produced photodevice for the condition of dark environment were computed as 12.5, 0.599 eV and 1266, respectively. As good figure of merits, the photodiode’s performance parameters of responsivity (Rph), photosensivity (%PS) and spesific detectivity (D*) maxima values determined for at -5 V reverse voltage bias and under 100 mW/cm2 of light power intensity condition are as 0.030 A/W (or 30 mA/W), 18693 and 1.33×1010 Jones, respectively. The current conduction mechanism analysis revealed that the space charge limited conduction (SCLC) mechanism is the dominant current conduction mechanism. By the drawn reverse squared C-2-V plots, the values of diffusion potential (Vd), donor concentration (ND), Fermi level (EF) and also barrier height (ϕb) were determined for the SMA-photodiode. The results indicated that the new SMA-photodiode device can be useful in optoelectronic communication systems and photosensing applications.

Numerical simulations of spherical indentation of superelastic-plastic thin films

Depth sensing indentation is a useful tool to characterize the mechanical behavior of shape memory alloys (SMAs), which are often used in small volumes, such as thin films. However, it is difficult to interpret these test results since SMAs exhibit various phenomena such as superelasticity (SE), shape memory effect and plasticity. Hence, in this work, a finite element investigation of spherical indentation of a SMA thin film bonded to a stiff elastic substrate as well as a bulk SMA specimen is performed pertaining to two fixed temperatures, which are below and above the austenite finish temperature (Af). A SMA constitutive model which can represent the combined effects of SE and plasticity is employed. The simulations are aimed at revealing the roles of above two phenomena as well as influence of the substrate on the indentation response. While the load-depth (P−h) curve for the thin film rises well above that of the bulk SE sample as h increases, the inclusion of plasticity reduces this deviation. The Oliver–Pharr modulus is close to the Young’s modulus pertaining to the parent austenite phase at small h and tends to that of martensite at large h for bulk samples, whereas it reaches a minimum and thereafter increases corresponding to the thin film specimens. The mean contact pressure versus indentation strain curves for both specimens agree quite well which enables interpretation based on expanding cavity models. The results are rationalized from the evolution of transformed martensite and plastic zone underneath the indenter.

Electrochemical Properties of Multilayered Sn/TiNi Shape-Memory-Alloy Thin-Film Electrodes for High-Performance Anodes in Li-Ion Batteries.

Sn is a promising candidate anode material with a high theoretical capacity (994 mAh/g). However, the drastic structural changes of Sn particles caused by their pulverization and aggregation during charge–discharge cycling reduce their capacity over time. To overcome this, a TiNi shape memory alloy (SMA) was introduced as a buffer matrix. Sn/TiNi SMA multilayer thin films were deposited on Cu foil using a DC magnetron sputtering system. When the TiNi alloy was employed at the bottom of a Sn thin film, it did not adequately buffer the volume changes and internal stress of Sn, and stress absorption was not evident. However, an electrode with an additional top layer of room-temperature-deposition TiNi (TiNi(RT)) lost capacity much more slowly than the Sn or Sn/TiNi electrodes, retaining 50% capacity up to 40 cycles. Moreover, the charge-transfer resistance decreased from 318.1 Ω after one cycle to 246.1 Ω after twenty. The improved cycle performance indicates that the TiNi(RT) and TiNi-alloy thin films overall held the Sn thin film. The structure was changed so that Li and Sn reacted well; the stress-absorption effect was observed in the TiNi SMA thin films.

Nano Scaled Checkerboards: A Long Range Ordering in NiCoMnAl Magnetic Shape Memory Alloy Thin Films with Martensitic Intercalations

Magnetic shape memory Heusler alloys, such as NiCoMnAl, are considered as promising candidates for magnetocaloric cooling applications. Grown in thin film systems of adjacent layers with austenite and martensite crystal structures of almost equal thicknesses, a long-range ordering phenomenon in the shape of a 3D checkerboard pattern occurs in NiCoMnAl samples. The crystallographic origin of the pattern is proven by transmission electron microscopy (TEM) techniques. The darker fields of the arrangement consist of martensite nuclei superposed with austenite, while the purely austenite regions appear bright in TEM cross sections. The nucleation process is presumably triggered by inhomogeneous local elastic stray fields of primary martensitic nuclei in the austenite matrix and limited by the thicknesses of the martensite and austenite thin films.

Technique of TiNi-based shape memory alloy thin film coating on optical fibers

<sec>Intelligent, integrated and cost-effective micro-electro-mechanical system (MEMS) and micro sensors can be developed with TiNi-based memory alloy thin film and optical fibers. Such devices can work in harsh environment, like in deep sea, in space with flammable or explosive objects, or with strong electromagnetic interference; and examples of their possible applications include gas concentration detection in underground mines, dynamic detection of production parameters in oil or gas mining, etc. As TiNi-based memory alloy thin film possesses good biocompatibility, such devices can also be used in intracranial/endocardial pressure test, surgical resection, early cancer assessment, etc. The successful development of the above MEMS and micro sensors involve optical fibers coated with memory alloy films. However, unlike the common planar substrates, optical fiber is of a cylinder with a small diameter, and how to grow good-quality memory alloy film on its surface remains to be explored.</sec><sec>In this work, the silica fibers are coated with TiNi memory alloy films by magnetron sputtering. How to choose the proper operating parameters in the sputtering process, and also the effects of subsequent annealing treatment on the films, are discussed in detail. Uniform thin films are grown on the 125-μm-diameter cylindrical surfaces of optical fibers with our built coating mask device specially designed for fibers. The experiments show that when target-substrate distance, background vacuum degree, Ar gas flow and sputtering time are fixed in the sputtering process, the sputtering power can be optimized, while a higher sputtering pressure results in lower film deposition rate but better surface roughness. The thin film is well crystallized under annealing, and the major martensite B19′ phase and minor austenite B2 phase coexist in the Ti<sub>49.09</sub>Ni<sub>50.91</sub> film. In the experiments, with the optimal operating parameters (sputtering power of 150 W and sputtering pressure of 0.23 Pa), TiNi memory alloy film about 852.2 nm in thickness is grown on the fiber at a deposition rate of 0.118 nm/s, and surface root mean square roughness of the unannealed film is 15.1 nm. Annealing at temperatures of 500, 550 and 600 ℃ are respectively tried, and such a thermal treatment evidently refines the crystalline grains inside the film. Surface root mean square roughness of the film annealed at 600 ℃ is reduced to 6.32 nm.</sec><sec>This work indicates that a glass fiber can be coated with high-quality TiNi-based memory alloy film, and it thus forms a part of the bases of further development of relevant MEMS and micro sensors.</sec>

Microstructure evolution and strain recovery characteristic of proton-irradiated Ti-rich NiTi thin films

The effect of 120 ​keV proton irradiation with different fluences on the phase constitution, microstructure, and shape recovery characteristics of Ni-51.4 ​at.%Ti shape memory alloy thin films was systematically investigated. The results showed that the proton irradiation induced the formation of a multiple-layer structure at the surface of NiTi thin film. Meanwhile, the hybrid defects such as vacancies, dislocations, and nano-scale precipitates were observed in the irradiation layer. The transition temperature and shape recovery strain decreased gradually with the increasing irradiation fluence, which can be attributed to the formation of the multilayer structure and the generation of irradiation-induced defects.

NONLINEAR DYNAMIC CHARACTERISTICS AND OPTIMAL DESIGN OF VENTILATOR PIPELINE POLYMER COMPOSITE DAMPER UNDER RANDOM EXCITATION

For severe patients with new coronavirus pneumonia, the use and stability of the ventilator are important. In this paper, we proposed a new type of shock-absorbing structure that can be used to improve the stability of the ventilator’s gas pipeline, thereby reducing the risk of the gas pipeline falling due to external interference. The film shape memory alloy (SMA) compared with bulk SMA is more suitable for gas pipeline shock absorption due to its large volume ratio and extremely high energy density. In order to optimize the performance of actuator and enhance the utilization rate of SMA, the nonlinear characteristics of Ti-Ni thin film actuator are researched and the effect of SMA film position on the system are investigated. The van der Pol term is introduced to fit the hysteresis loop of shape memory materials. Thus, the nonlinear dynamic response of SMA thin film actuator under stochastic excitation is studied. The numerical and experiment results show that the position of SMA can change the system’s stability, the proposed structure can reduce the system’s vibration to the maximum extent. The results indicate that the new proposed polymer/SMA composite film micro-actuator exhibits high energy dissipation ability, and the utilization rate of smart material is enhanced. Therefore, the new damping structure proposed in this paper is of great significance to the damping of gas pipelines.

A Critical Review on Nickel–Titanium Thin-Film Shape Memory Alloy Fabricated by Magnetron Sputtering and Influence of Process Parameters

A Critical Review on Nickel–Titanium Thin-Film Shape Memory Alloy Fabricated by Magnetron Sputtering and Influence of Process Parameters

Nano-indentation and nano-scratching of pure nickel and NiTi shape memory alloy thin films: an atomic-scale simulation

Issues with contact and friction in thin-film devices are inevitable in the field of micro-engineering, especially in micro-electro-mechanical systems (MEMS). In this study, the nano-indentation and nano-scratching behaviours of pure nickel and NiTi shape memory alloy (SMA) nano-films, two common materials used in MEMS, were comparatively investigated using large-scale molecular dynamics simulations. For the nickel nano-film structure, the load–displacement (P–h) curve demonstrated distinct elasto-plastic characteristics, and visible permanent deformation remained after unloading. In contrast, for the NiTi SMA, the P–h curve formed a hysteresis loop, and no evident deformation remained on the surface after unloading. It is noteworthy that during scratching, despite a lower applied normal pressure on the nickel nano-film than on the NiTi SMA, the friction and friction coefficient of the former were much higher. These differences in performance are closely related to the super-elasticity induced by the martensitic transformation within the NiTi SMA.

Effect of annealing temperature on microstructure and mechanical response of sputter deposited Ti-Zr-Mo high temperature shape memory alloy thin films

The shape memory thin films are most suitable candidates as actuators in MEMS applications than other smart materials due to its ability to produce larger recovery strains at low operation voltages. Recently, Ti based shape memory alloys are gaining attention due to its ability to produce stable shape memory properties at high temperatures. In current study, Ti-Zr-Mo high-temperature shape memory thin films were fabricated using multi-target DC/RF magnetron sputtering system on Si (100) substrate. Deposition time and target power were varied to obtain the desired composition. Post-annealing was carried out at 500 °C, 600 °C and 700 °C for 15 min each to crystallize the thin films that were amorphous in the homogenized condition. The annealing temperature was varied to observe the structural and morphological changes. The total thickness of the film was found to be around 300 nm measured from the cross-sectional micrographs. The results suggest that flatness of surface topography is attributed to higher ad-atom mobility with increasing the annealing temperature. The martensite phase (α’) was observed after annealing at 500 °C and the formation of silicides was observed with annealing at higher temperatures. The transformation temperature of 220 °C was obtained from electrical resistivity measurement. The enhanced mechanical properties with maintaining structural stability was observed in Ti-Zr-Mo thin film annealed at 500 °C for 15 min.

Origami-inspired thin-film shape memory alloy devices

We describe the design and fabrication of miniaturized origami structures based on thin-film shape memory alloys. These devices are attractive for medical implants, as they overcome the opposing requirements of crimping the implant for insertion into an artery while keeping sensitive parts of the implant nearly stress-free. The designs are based on a group theory approach in which compatibility at a few creases implies the foldability of the whole structure. Importantly, this approach is versatile and thus provides a pathway for patient-specific treatment of brain aneurysms of differing shapes and sizes. The wafer-based monolithic fabrication method demonstrated here, which comprises thin-film deposition, lithography, and etching using sacrificial layers, is a prerequisite for any integrated self-folding mechanism or sensors and will revolutionize the availability of miniaturized implants, allowing for new and safer medical treatments.

The Influence of Martensitic Intercalations in Magnetic Shape Memory NiCoMnAl Multilayered Films.

Hysteresis and transformation behavior were studied in epitaxial NiCoMnAl magnetic shape memory alloy thin films with varying number martensitic intercalations (MIs) placed in between. MIs consists of a different NiCoMnAl composition with a martensitic transformation occurring at much higher temperature than the host composition. With increasing number of intercalations, we find a decrease in hysteresis width from 17 K to 10 K. For a large difference in the layers thicknesses this is accompanied by a larger amount of residual austenite. If the thicknesses become comparable, strain coupling between them dominates the transformation process, which manifests in a shift of the hysteresis to higher temperatures, splitting of the hysteresis in sub hysteresis and a decrease in residual austenite to almost 0%. A long-range ordering of martensite and austenite regions in the shape of a 3D checker board pattern is formed at almost equal thicknesses.

The photo response properties of shape memory alloy thin film based photodiode

In this work, the CuAlMn shape memory alloy (SMA) composed as Cu-12.28Al-2.84Mn in wt.% (=72.49Cu-24.70Al-2.81Mn in at.%) was used as a thin Schottky contact layer on an n-Si wafer to produce a Schottky type Al/CuAlMn/n-Si/Al photodiode. To do this, in the first stage of the experimental route, the CuAlMn alloy was prepared in succession by casting in arc melter, homogenizing in β-phase region and lastly by quenching in iced-brine water. Then, the shape memory effect characteristics were revealed by the widely-used thermal (DSC and DTA) and structural (EDS and XRD) characterization tests that been carried out on the alloy. Then, the alloy was thermally evaporated and deposited on the n-Si wafer as a thin film nano layer to make the Al/CuAlMn/n-Si/Al photodiode. The current-voltage (I-V) analysis of the diode taken under dark and different light intensities revealed the rectifying behavior of the diode. The work function value of CuAlMn alloy film was determined as 4.747±0.17 eV. The reverse bias current values of the diode under light were found to be greater than the value observed under dark. The results showed that the diode has a considerable photoconducting feature. The characteristic electrical diode parameters of the junction were computed by using the traditional I-V method. By taking into consideration the literature works related to SMA layered photodiodes, some of the photodiode parameters such as responsivity, photosensitivity, and detectivity, not been reported so far, were determined for the first time. The SMA/n-Si device showed an excellent photosensitivity (~105) performance, and also good merits of responsivity, and detectivity. Which implied that the produced device can be used in various optoelectronic applications. The transient photocurrent (TPC) measurement was performed under the various light intensity conditions and the related characteristic photodiode parameters were determined. Additionally, the capacitance-voltage (C-V) measurement of the SMA-layered photodiode was also carried out at room temperature and different frequencies. The capacitance of the device was observed to be quite sensitive to frequency. Furthermore, a solution suggestion for SMA film cracks and related fractures was also discussed.

Shape Memory Alloys and Polymers for MEMS/NEMS Applications: Review on Recent Findings and Challenges in Design, Preparation, and Characterization

Rapid progress in material science and nanotechnology has led to the development of the shape memory alloys (SMA) and the shape memory polymers (SMP) based functional multilayered structures that, due to their capability to achieve the properties not feasible by most natural materials, have attracted a significant attention from the scientific community. These shape memory materials can sustain large deformations, which can be recovered once the appropriate value of an external stimulus is applied. Moreover, the SMAs and SMPs can be reprogrammed to meet several desired functional properties. As a result, SMAs and SMPs multilayered structures benefit from the unprecedented physical and material properties such as the shape memory effect, superelasticity, large displacement actuation, changeable mechanical properties, and the high energy density. They hold promises in the design of advanced functional micro- and nano-electro-mechanical systems (MEMS/NEMS). In this review, we discuss the recent understanding and progress in the fields of the SMAs and SMPs. Particular attention will be given to the existing challenges, critical issues, limitations, and achievements in the preparation and characterization of the SMPs and NiTi-based SMAs thin films, and their heterostructures for MEMS/NEMS applications including both experimental and computational approaches. Examples of the recent MEMS/NEMS devices utilizing the unique properties of SMAs and SMPs such as micropumps, microsensors or tunable metamaterial resonators are highlighted. In addition, we also introduce the prospective future research directions in the fields of SMAs and SMPs for the nanotechnology applications.

Study on technique of TiNi-based shape memory alloy thin film coating on optical fibers

Study on technique of TiNi-based shape memory alloy thin film coating on optical fibers