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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. [Copyright &y& Elsevier]

Surface Acoustic Wave (SAW) technology has been widely applied in the fields such as communication and sensing. The performance of SAW devices is significantly influenced by designs of their key component, Interdigital Transducers (IDTs), and thus Coupling of Modes (COM) theory has been used as one of the most employed design tools for SAW devices due to its fast computational speed. Accuracy of this model is primarily dependent upon the COM parameters, but the traditional approach to obtain these parameters is heavily relied on accuracy of the input material properties, which has become a key issue for successful applications of this model. This paper proposed a new strategy to utilize the results obtained from the COM model as a dataset and then employ five different machine learning models for performing regression analysis and accurately extracting the COM parameters. To validate the accuracy of this approach, experimental verifications were performed using a 128°Y-X LiNbO 3 based SAW resonator as an example. The machine learning model with the best predictive performance, i.e., Extreme Gradient Boosting, was used to predict the COM parameters corresponding to experimental results, which were subsequently used in conjunction with the COM model for further calculations and comparisons with the experimental results. Results showed that the calculated results exhibit the same trend of resonance Q-values as the experimental results, demonstrating its effective solution for accurately extracting COM parameters. [Display omitted] • Different machine learning regression algorithms were used to build models, and the most suitable model was determined. • This work presents a novel and effective solution for the accurate extraction of COM parameters. • A Novel Strategy of Machine Learning Assisted Design for Surface Acoustic Wave Resonators. [ABSTRACT FROM AUTHOR]

Coherent phonon manipulation is the mechanical analogy to photonic frequency combs while there is lack of a comprehensive evaluation system on the performance. In this paper, coherent phonon manipulation is demonstrated using a single-mode circular electrostatic resonator. Its manipulation mechanism for controlling phononic frequency comb (PFC) parameters is verified experimentally and also theoretically based on waveform derivation of the differential equations. The spacing of PFC system is regulated solely by the pump frequency and a spacing resolution of 0.1 Hz is achieved at room temperature. The amplitude modulation law is investigated and a complete PFC evolution process with a pump threshold of 2 mV pp is achieved. The sweeping experiment of electrostatic resonators is explained from the perspective of phonons. Additionally, feasibility of manipulating phonons using the electrostatic resonators is verified through nonlinearity modulation and damping loss experiments. This new methodology of phonon manipulation can be extended to other resonant energy carriers, with great potentials for frequency detection and phonon-quantum computing. [Display omitted] • Coherent phonon manipulation is demonstrated using a single-mode electrostatic resonator with intra-modal coupling. • A spacing resolution of 0.1 Hz, and a pump threshold of 2 mV pp are achieved. The PFC has a highly-quality performance under the evaluation system. • Feasibility of manipulating phonons using the resonators is verified through nonlinearity modulation and damping loss experiments at the first time. • This new methodologycan be extended to other resonant energy carriers, with great potentials for phonon-quantum applications. [ABSTRACT FROM AUTHOR]

It has been a critical challenge to develop highly sensitive H 2 S gas sensors, due to its wide-range application in industry and frequent leakage, which endangers people's lives. In this work, a highly sensitive room-temperature SAW H 2 S gas sensor based on Si-Cu nanocomposite was developed. The Cu content in this composite layer plays a key role for H 2 S response of the sensor because CuO, an excellent adsorption site for H 2 S, presents on the surface of Cu. The amorphous Si content results in the highly porous structure enhancing the interaction between CuO and H 2 S. The interaction leads to the formation of CuS and a negative frequency response of the sensor. H 2 O molecules effectively participate the reactions between CuO and H 2 S, and the responses of H 2 S were significantly enhanced in a moister environment. With the relative humidity of 60 %, the sensor can detect 50 ppb H 2 S with a response of − 225 Hz. [Display omitted] • The surface CuO in the composite adsorbs and reacts with H 2 S and then form CuS. • The formation of CuS lead to the negative frequency responses of the SAW sensors. • The sensing performance of the senor toward H 2 S is critically influenced by H 2 O. • The SAW sensor can precisely detect H 2 S gas ranging from 50 ppb to16 ppm. [ABSTRACT FROM AUTHOR]

[Display omitted] • The SAW sensor has a sensitivity of -490 mm2 ng−1Hz toward AMC. • Antireflection silica coatings are susceptible to AMC. • Changes of weight of the silica coatings cause the response. Real time monitoring of contamination on antireflection (AR) silica coatings in high peak power laser systems (HPLs) is critically needed in order to avoid reductions of transmission and laser damage to optical surfaces. Herein we proposed to apply a surface acoustic wave (SAW) sensor to real-time monitor trace amounts of airborne molecular contaminants (AMCs) adsorbed on the AR silica coatings. The silica coating is found to be susceptible to AMCs because of its mesoporous structure, huge surface area and polar nature. The adsorbed AMCs caused the increased mass on the silica coating of the SAW sensor, which resulted in a significant increase of its frequency shift. The fabricated sensor showed a high sensitivity of ∼-490 mm2 ng−1Hz and an excellent linearity vs. the areal density of adsorbed AMCs since the frequency shift of the sensor is linearly related to the change of mass of the silica coating. [ABSTRACT FROM AUTHOR]

• We manufacture transparent surface acoustic wave ultraviolet photodetectors made on ultra-thin glass. This transmission signal is ∼160 % higher than that the previous polymer based flexible SAW devices. • We apply Graphene quantum dots decorated zinc oxide nanowires (GQDs@ZnO-NWs) to enhance sensing performance of highly flexible SAW sensor. The UV sensitivity of sensors was enhanced by three times and the response time was shorten by four times. • The flexible SAW device maintains good performance under a bending angle of ∼30° for 200 times without visible degradation, showing the excellent flexibility. Graphene quantum dots decorated zinc oxide nanowires (GQDs@ZnO-NWs) were applied to enhance sensing performance of highly flexible and transparent surface acoustic wave (SAW) ultraviolet (UV) photodetectors made on ultra-thin flexible glass. The developed flexible SAW sensors possess better performance than that of the previously developed polymer based flexible SAW devices, due to insignificant acoustic loss of flexible glass substrate. UV sensitivity of the flexible glass based SAW sensors was enhanced by three times, and the response time was shorten by four times after the sensor was coated with the GQDs@ZnO-NWs hybrid nanomaterials. These improvements are mainly attributed to: (1) large specific surface areas of ZnO NWs which can generate a large number of photon-generated carriers; (2) introduction of GQDs which can reduce the carrier recombination rate. The resonant frequency of flexible glass SAW UV photodetectors exhibited a good repeatability and stability in responses to cyclic changes of the UV lights at different wavelengths. They also maintained a good performance under a bending angle of ∼30° for 200 times without apparent degradation, showing the excellent flexibility and stability of the UV photodetector. [ABSTRACT FROM AUTHOR]

Schematic of microfluidic heating within a PDMS microchamber using the AlN/Si thin film SAW device. • Acoustothermal heating characteristics of different thin film (including AlN/Si, ZnO/Si and ZnO/Al) SAW devices have been investigated and compared. • The dynamic heating processes of AlN/Si SAW device for both the sessile droplet and liquid within a PDMS microchamber are characterized. • The liquid heating temperature within a PDMS microchamber can be digitally controlled by the input SAW power with good stability and repeatability. • An improved temperature uniformity using the standing SAW heating is demonstrated. Temperature control within a microreactor is critical for biochemical and biomedical applications. Recently acoustothermal heating using surface acoustic wave (SAW) devices made of bulk LiNbO 3 substrates have been demonstrated. However, these are generally fragile and difficult to be integrated into a single lab-on-a-chip. In this paper, we propose a rapid and controllable acoustothermal microheater using AlN/Si thin film SAWs. The device's acoustothermal heating characteristics have been investigated and are superior to other types of thin film SAW devices (e.g., ZnO/Al and ZnO/Si). The dynamic heating processes of the AlN/Si SAW device for both the sessile droplet and liquid within a polydimethylsiloxane (PDMS) microchamber were characterized. Results show that for the sessile droplet heating, the temperature at a high RF power is unstable due to significant droplet deformation and vibration, whereas for the liquid within the microchamber, the temperature can be precisely controlled by the input power with good stability and repeatability. In addition, an improved temperature uniformity using the standing SAW heating was demonstrated as compared to that of the travelling SAWs. Our work shows that the AlN/Si thin film SAWs have a great potential for applications in microfluidic heating such as accelerating biochemical reactions and DNA amplification. [ABSTRACT FROM AUTHOR]

• The sensor can detect HCl gas with a concentration as low as 1 ppm. • H 2 O on CNCs layers assists the response. • Changes of mass of the CNCs layers cause the response. We report that cellulose nano-crystals (CNCs) can be used as a sensitive and selective layer in surface acoustic wave (SAW) sensors for in-situ HCl gas detection. CNCs were prepared through directly hydrolysis of cotton fiber and were spin-coated on quartz SAW resonators to form the sensitive layer. The CNCs have been identified to have abundant hydroxyl groups on their surfaces, which can act as the perfect adsorption sites for H 2 O, which can further act as the active sites for HCl gas adsorption. The absorption of HCl on the CNCs layer, thus leads to an increase of its mass, causing negative responses of the SAW sensors. Ambient humidity and thickness of CNCs layer are found to have significant influences on the responses of the SAW sensor. With an 80-nm-thick CNCs layer, the sensor shows a response of −2 kHz to 1 ppm HCl at 25 °C and relative humidity of 50 % with an excellent selectivity and recovery characteristics. [ABSTRACT FROM AUTHOR]