Miniaturized or microscale generators that can effectively convert weak and random mechanical energy into electricity have significant potential to provide solutions for the power supply problem of distributed devices. However, owing to the common occurrence of friction and wear, all such generators developed so far have failed to simultaneously achieve sufficiently high current density and sufficiently long lifetime, which are crucial for real-world applications. To address this issue, we invent a microscale Schottky superlubric generator (S-SLG), such that the sliding contact between microsized graphite flakes and n-type silicon is in a structural superlubric state (an ultra-low friction and wearless state). The S-SLG not only generates high current (~210 Am−2) and power (~7 Wm−2) densities, but also achieves a long lifetime of at least 5,000 cycles, while maintaining stable high electrical current density (~119 Am−2). No current decay and wear are observed during the experiment, indicating that the actual persistence of the S-SLG is enduring or virtually unlimited. By excluding the mechanism of friction-induced excitation in the S-SLG, we further demonstrate an electronic drift process during relative sliding using a quasi-static semiconductor finite element simulation. Our work may guide and accelerate the future use of S-SLGs in real-world applications.
a state in which the lateral interactions between two incommensurate surfaces are effectively canceled resulting in ultralow sliding friction. The ultralow friction state could also be achieved for tip with radius from 20 to 1000 nm sliding on graphene supported by substrates, e.g., diamond tip with graphene on SiO 2 substrate, [11,12] Si tip with graphene on SiO 2 substrate, [13] and diamond tip with graphene on Cu substrate [14] and the lubrication properties was robust under different normal load. [15] The ultralow friction and high intrinsic strength make graphene an ideal candidate for antiwear coating materials. On the nano and microlevel, Shin et al. [11] carried out microscale scratch tests on exfoliated and epitaxial graphene on a silica substrate. When the indenter was pressed into the monolayer graphene sample at a depth of more than 150 times of its thickness, no damage occurred within the graphene. Using atomic force microscope (AFM) tip with a radius about 100 nm sliding on graphene that mechanically exfoliated to SiO 2 substrates, Qi et al. [12] reported that the monolayer graphene still maintained a low friction coefficient of 0.01 for a prolonged period (4096 cycles) with a normal load up to 9150 nN, and the interior region of graphene shows better wear resistance. Qi et al. [16] further found that the wear resistance of the free edge of graphene could be improved by performing air plasma treatment. With a similar setup, Vasic et al. [17] showed that the SiO 2 substrate gets plastically deformed for lower normal loads, followed by a sudden Adhesion plays an important role in the antiwear property of graphene layer on a substrate. Here the wear property of the inner region of monolayer graphene grown on copper foils via chemical vapor deposition is studied. The adhesive strength is controlled by changing the oxidation of the copper substrate into two oxidation degrees with intact graphene preserved. For graphene layers on copper substrates with either low oxidation degree (LOD) or high oxidation degree (HOD), it is found in both systems wear starts at the wrinkle position under similar normal force. However, with the development of wear, for the LOD substrate the covering graphene layer is worn out gradually, while for the HOD substrate the graphene layer is peeled off rapidly. By measuring the adhesion between graphene and substrates indirectly, together with finite element analysis, it is shown that the underlying mechanism for the different wear phenomena is due to the higher adhesion between graphene and LOD substrates than that between graphene and HOD substrates. This study provides insights on the impacts of adhesion between monolayer material and substrates on the antiwear properties, which can benefit the design of lubrication coatings based on layered materials. Wear
Wear-free sliding between two contacted solid surfaces is the ultimate goal towards extending the lifetime of mechanical devices, especially for inventing new types of MEMS (micro-electromechanical system) where wear is often a major obstacle. Here we report experimental observations of wear-free sliding for a micrometer-sized graphite flake on a diamond-like-carbon (DLC) surface under ambient conditions with speeds up to 2.5 m/s and distance over 100 km. The coefficient of friction (COF) between the microscale graphite flake, a van der Waals (vdW) layered material, and DLC, a non-vdW-layered material, is measured to be of the order of ${10^{ - 3}}$, which belongs to the superlubric regime. Such ultralow COFs are also demonstrated for microscale graphite flake sliding on other six kinds of non-vdW-layered materials with sub-nm roughness. With a synergistic analysis approach, we reveal the underlain mechanism to be the combination of interfacial vdW interaction, atomic-smooth interfaces, and the low normal stiffness of the graphite flake. These features guarantee a persistent full contact of the interface with weak interaction, which contributes to the ultralow COFs. Together with the extremely high in-plane strength of graphene, wear-free sliding is achieved. Our results broaden the scope of superlubricity and promote its wider application in the future.
The structural superlubricity (SSL), a state of near-zero friction between two contacted solid surfaces, has been attracting rapidly increasing research interest since it was realized in microscale graphite in 2012. An obvious question concerns the implications of SSL for micro- and nanoscale devices such as actuators. The simplest actuators are based on the application of a normal load; here we show that this leads to remarkable dynamical phenomena in microscale graphite mesas. Under an increasing normal load, we observe mechanical instabilities leading to dynamical states, the first where the loaded mesa suddenly ejects a thin flake and the second characterized by peculiar oscillations, during which a flake repeatedly pops out of the mesa and retracts back. The measured ejection speeds are extraordinarily high (maximum of 294 m/s), and correspond to ultrahigh accelerations (maximum of 1.1×1010m/s2). These observations are rationalized using a simple model, which takes into account SSL of graphite contacts and sample microstructure and considers a competition between the elastic and interfacial energies that defines the dynamical phase diagram of the system. Analyzing the observed flake ejection and oscillations, we conclude that our system exhibits a high speed in SSL, a low friction coefficient of 3.6×10−6, and a high quality factor of 1.3×107compared with what has been reported in literature. Our experimental discoveries and theoretical findings suggest a route for development of SSL-based devices such as high-frequency oscillators with ultrahigh quality factors and optomechanical switches, where retractable or oscillating mirrors are required.
Resonators and resonator-based oscillators are used in most electronics systems and they are classified as either mechanical or electrical, with fixed or difficult-to-tune resonant frequencies. Here, we propose an electro-superlubric spring, whose restoring force between two contacting sliding solid surfaces in the structural superlubric state is linearly dependent on the sliding displacement from the balanced position. We use theoretical analysis and finite element methods to study the restoring force and stability. The stiffness of this electro-superlubric spring is proportional to the square of the applied electric bias, facilitating continuous tuning from zero to several megahertz or gigahertz for the microscale or nanoscale resonators, respectively. Furthermore, we propose an electro-superlubric oscillator that is easily operated by varying a pair of harmonic voltages. The resonant frequency, resonant amplitude, quality factor, and maximum resonant speed can be continuously tuned via the applied voltage and bias. These results indicate significant potential in the applications of electro-superlubric resonators and oscillators.
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