A review of shape memory alloy research, applications and opportunities

Advances in soft robotics, materials science, and stretchable electronics have enabled rapid progress in soft grippers. Here, a critical overview of soft robotic grippers is presented, covering different material sets, physical principles, and device architectures. Soft gripping can be categorized into three technologies, enabling grasping by: a) actuation, b) controlled stiffness, and c) controlled adhesion. A comprehensive review of each type is presented. Compared to rigid grippers, end-effectors fabricated from flexible and soft components can often grasp or manipulate a larger variety of objects. Such grippers are an example of morphological computation, where control complexity is greatly reduced by material softness and mechanical compliance. Advanced materials and soft components, in particular silicone elastomers, shape memory materials, and active polymers and gels, are increasingly investigated for the design of lighter, simpler, and more universal grippers, using the inherent functionality of the materials. Embedding stretchable distributed sensors in or on soft grippers greatly enhances the ways in which the grippers interact with objects. Challenges for soft grippers include miniaturization, robustness, speed, integration of sensing, and control. Improved materials, processing methods, and sensing play an important role in future research.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/adma.201707035

Soft Robotic Grippers.

This review comprises a detailed survey of ongoing methodologies for soft actuators, highlighting approaches suitable for nanometer- to centimeter-scale robotic applications. Soft robots present a special design challenge in that their actuation and sensing mechanisms are often highly integrated with the robot body and overall functionality. When less than a centimeter, they belong to an even more special subcategory of robots or devices, in that they often lack on-board power, sensing, computation, and control. Soft, active materials are particularly well suited for this task, with a wide range of stimulants and a number of impressive examples, demonstrating large deformations, high motion complexities, and varied multifunctionality. Recent research includes both the development of new materials and composites, as well as novel implementations leveraging the unique properties of soft materials.

Soft Actuators for Small-Scale Robotics.

Stimulus-responsive shape memory materials: A review

TiNi thin films have attracted much attention in recent years as intelligent and functional materials because of their unique properties. TiNi thin film based micro-actuators will become the actuator of choice in many aspects in the rapidly growing field of micro-electro-mechanical systems (MEMSs). In this review paper, some critical issues and problems in the development of TiNi thin films are discussed, including preparation and characterization considerations, residual stress and adhesion, frequency improvement, fatigue and stability, modeling of behavior as well as functionally graded or composite thin films. Comparison is made of TiNi SMA micro-actuation with other micro-actuation methods. Different types of TiNi thin film based microdevices, such as microgrippers, microswitches, microvalves and pumps, microsensors, etc. are also described and discussed.

/pdf/tini-based-thin-films-in-mems-applications-a-review-23wqos5t8n.pdf

TiNi-based thin films in MEMS applications: a review

Thin film shape-memory alloys (SMAs) have been recognized as promising and high performance materials in the field of microelectromechanical systems (MEMS) applications. In this investigation, chemical composition, microstructure and phase transformation behaviors of sputter deposited TiNi films were studied. The surface and cross-section morphology of the deposited coating was analyzed using atomic force microscopy (AFM) and scanning electron microscopy (SEM). The results from the differential scanning calorimeter (DSC) showed clearly the martensitic transformation upon heating and cooling. X-Ray diffraction analysis (XRD) also revealed the crystalline structure changing with temperature. By depositing TiNi films on the bulk micromachined Si cantilever structures, micro-beams exhibiting a good shape-memory effect were obtained. Finite element simulation results of the deformation of micro-beam (using the measured NiTi thin film parameters) agree quite well with the measured behavior.

Characterization of TiNi shape-memory alloy thin films for MEMS applications

TiNi films with different Ni/Ti ratios were prepared by co-sputtering of a Ti50Ni50 (at.%) target with a separated Ti target at a temperature of 723 K. The stress values in the deposited films changed significantly with Ti contents and post-annealing temperatures due to the differences in phase transformation behaviors and intrinsic stress. For the film with Ti content of 51.3%, a two-step transformation was observed among martensite, R-phase and austenite, and residual stress was quite low at room temperature. After post-annealing the above film at 923 K, only one-stage transformation was observed. For the films with Ti contents of 47.3 and 53%, residual stress was quite high due to the high intrinsic stress and partial relaxation of stress caused by the R-phase transformation. When the films with Ti contents of 47.3 and 53% were annealed at 923 K, residual stress in films decreased significantly, because post-annealing could probably modify the film structure, reduce the intrinsic stress, increase the transformation temperatures, and cause martensite transformation above room temperature. For all the three types of films annealed at 1023 K for 1 h, the high thermal stress in the thin films could cause non-recoverable deformation during phase transformation, damage the shape memory effect, and result in the peeling-off of film from Si substrate.

Effects of film composition and annealing on residual stress evolution for shape memory TiNi film

SiC whisker toughened Al2O3-(Ti, W)C ceramic matrix composites

Residual stress, relaxation and recovery of stress during martensite transformation of the sputtering-deposited TiNi films were studied in this paper. Results showed that a wide range of residual stress levels (both tensile and compressive) were obtained with the change of Ti contents in the deposited films. After film deposition, the thermal and intrinsic stress in the film could be very large, but due to the differences in phase transformation stress and transformation temperatures, the real stress values under room temperature, and stress evolution with temperature during phase transformation were quite different. For the film with a Ti content of 50.2%, due to the significant relaxation of stress caused by the perfect martensite transformation, the residual stress under room temperature was very low (−20 MPa) and recovery stress during heating was as high as 750 MPa. With both decrease and increase in Ti content in the deposited films, the tensile residual stress increased significantly. For the films with a Ti content of 51.3%, a two-step transformation was observed among martensite, R-phase and austenite. The recovery stress and average increase rate of stress during both heating and cooling were lower than those of film with 50.2% Ti. For the films with a Ti content of 47.3 and 53%, only partial relaxation of stress occurred due to the existence of R-phase transformation at room temperature. Also, because of non-equilibrium contents of Ni and Ti in these films, shape memory effect and the recovery of stress due to the phase transformation was not so significant. High residual stress in the thin films could cause non-recoverable deformation during phase transformation.

Hejun Du

Papers

TiNi-based thin films in MEMS applications: a review

Characterization of TiNi shape-memory alloy thin films for MEMS applications

Effects of film composition and annealing on residual stress evolution for shape memory TiNi film

SiC whisker toughened Al2O3-(Ti, W)C ceramic matrix composites

Relaxation and recovery of stress during martensite transformation for sputtered shape memory TiNi film