Metamaterials, as artificially engineered structures with unconventional mechanical and acoustic properties, have recently emerged as a transformative platform for enhancing the capabilities of triboelectric nanogenerator (TENG) systems. Since the invention of TENG devices, extensive efforts have been devoted to improving charge density, output stability, and overall performance. Conventional performance optimization strategies mainly rely on device-level improvements such as surface chemistry modification, microstructuring, and nanopatterning. However, limited emphasis has been given to system-level development of smart self-powered intelligent systems. The integration of metamaterials into TENG devices opens a new era by enabling frequency-selective localization, mechanical impedance matching, and controllable deformation pathways. These engineered mechanical structures not only improve energy harvesting efficiency but also introduce new functionalities into the system. This review systematically summarizes recent advances in metamaterial-integrated TENG systems across four major application domains: (i) energy harvesting, (ii) acoustic telecommunication and acoustic-to-electric conversion, (iii) self-powered sensing, and (iv) vibration suppression and monitoring. Overall, the integration of metamaterials into TENG systems will pave the way for next-generation sustainable, intelligent, self-powered devices with diverse functionalities.

Al-Mo thin films were fabricated using combinatorial sputtering system to realize highly sensitive surface acoustic wave (SAW) devices. The Al-Mo sample library was grown with various chemical compositions and electrical resistivities, which provided important information for selecting the most suitable materials for SAW devices. As the SAWs generated from piezoelectric materials are significantly affected by the resistivity and density of the interdigital transducer (IDT) electrodes, three types of Al-Mo thin films with different Al contents were fabricated. The thickness of the Al-Mo thin film used in the SAW-IDT electrode was fixed at 150 nm. As the Al content of the Al-Mo thin film decreased from 81.2 to 30.3 at%, the resistivity decreased slightly from 5.43±0.15 to 4.87±0.1×10^-5 Ω-cm, whereas the calculated density increased significantly from 4.1 to 7.9 g/㎤. The SAW device composed of Al-Mo IDT electrodes resonated at 143 MHz without frequency shifts; however, the selectivity of the resonant frequency and insertion loss deteriorated as the Al content decreased. This suggest that the resonant characteristics of the SAW devices fabricated with Al-Mo thin films were more strongly influenced by the material density rather than the electrical properties of the IDT electrodes.

This paper investigates the soundness of porcelain insulators associated with the acoustic emission (AE) technique. The AE technique is a popular non-destructive method that measures and analyzes the burst energy that occurs mainly when a crack occurs in a high-frequency region. Typical AE methods require continuous monitoring with frequent sensor calibration. However, in this study, the AE technique excites a porcelain insulator using only an impact hammer, and it applies a high-pass filter to the signal frequency range measured only in the AE sensor by comparing the AE and the acceleration sensors. Next, the extracted time-domain signal is analyzed for the damage assessment. In normal signals, the duration is about 2ms, the area of the envelope is about 1,000, and the number of counts is about 20. In the damage signal, the duration exceeds 5ms, the area of the envelope is about 2,000, and the number of counts exceeds 40. In addition, various characteristics in the time and frequency domain for normal and damage cases are analyzed using the short-time Fourier transform (STFT). Based on the results of the STFT analysis, the maximum energy of a normal specimen is less than 0.02, while in the case of the damage specimen, it exceeds 0.02. The extracted high-frequency components can present dynamic behavior of crack regions and eigenmodes of the isolated insulator parts, but the presence, size, and distribution of cracks can be predicted indirectly. In this regard, the characteristics of the surface crack region were derived in this study.

This study investigated the various physical and electrical effects of silicon direct bonding. Direct bonding means the joining of two wafers together without an intermediate layer. If the surfaces are flat, and made clean and smooth using HF treatment to remove the native oxide layer, they can stick together when brought into contact and form a weak bond depending on the physical forces at room temperature. An IR camera and acoustic systems were used to analyze the voids and bonding conditions in an interface layer during bonding experiments. The I-V and C-V characteristics are also reported herein. The capacitance values for a range of frequencies were measured using a LCR meter. Direct wafer bonding of silicon is a simple method to fuse two wafers together; however, it is difficult to achieve perfect bonding of the two wafers. The direct bonding technology can be used for MEMS and other applications in three-dimensional integrated circuits and special devices.

The power conversion efficiency of organic polymer solar cells was enhanced by introducing a ferroelectric polymer layer at the interface between active layer and metal electrode. The power conversion efficiency was increased by 50% through the enhancement of the open circuit voltage. To investigate the role of the ferroelectric layer on the dissociation process of the excitons, non-radiative portion of the exciton decay was directly measured by using photoacoustic technique. The results show that the ferroelectric nature of the buffer layer does not play any roles on the dissociation process of the excitons, which indicates the efficiency enhancement is not due to the ferroelectricity of the buffer layer.

P3HT:PCBM bulk heterojunction solar cells added with ferroelectric polymer were fabricated and characterized. By incorporating P3HT:PCBM solar cell with P(VDF-TrFE) ferroelectric additive, the power conversion efficiency was increased up to nearly 50%. Photoacoustic analysis on this phenomena was carried out for the first time. Through this study, we find that the ferroelectricity of the polymer additive plays the key role in the enhancement of the power conversion efficiency of the organic solar cell by suppressing the non-radiative recombination of charge transfer exciton more effectively.

This paper dealt with the propagation characteristics of acoustic signal in insulation oil for the purpose of improving the reliability of AE (acoustic emission) method used for condition monitoring of oil-immersed transformers. A discharge source was placed in insulation oil and AE sensors (fc :140 kHz) were attached on the oil tank to study the changes of velocity and propagation path with the depth and distance. The average velocity was 1,436 m/s and the velocity decreased with the increase of depth from the oil surface to 430 mm. The propagation paths were classified into three sections by the shortest reflection path of the detected signal. The minimum distinguishable distance in each section was 70 mm. It was also verified that PD (partial discharge) with a magnitude over than 500 pC can be detected by the AE sensors.

This paper dealt with the frequency component analysis of acoustic signals produced by corona and series-arc discharges as a diagnostic technique for closed-switchboards. Corona and series-arc discharge were simulated by a needle-plane electrode and an arc generator specified in UL1699, respectively. Acoustic signal was detected by a wideband acoustic sensor with a frequency bandwidth of 4 Hz∼100 kHz (-3 dB). We analyzed frequency spectrums of the acoustic signals detected in various discharge conditions. The results showed that acoustic signals mainly exist in ranges from 30 kHz to 60 kHz. From the experimental results, an acoustic detection system which consists of a constant current power supply (CCP), a low noise amplifier (LNA) and a band pass filter was designed and fabricated. The CCP separates the signal component from the DC source of acoustic sensor, and the LNA has a gain of 40 dB in ranges of 280 Hz∼320 kHz. The high and the low cut-off frequency are 30 kHz and 60 kHz, respectively. We could detect corona and series-arc discharges without any interference by the acoustic detection system, and the best frequency is considered in ranges of 30 kHz∼60 kHz.

The osseointegration of dental implant is influenced by many factors such as surface geometry, loading and the amount of bone. Thus, stability of the dental implant should be checked periodically. In order to test the stability of dental implant by using resonance frequency analysis, we designed a structure of transducers and fabricated a piezoelectric devices. Using finite element analysis, the thickness and length of piezoelectric device and transducers were tailorized and the optimized frequency of 10 kHz was obtained. The resonance frequency from simulation analysis and evaluation was estimated to be similar as 10 kHz. The osseointegration was further enhanced with increasing frequency from the evaluation result of the finite element analysis.