Considerable attention has been paid to aligned polymer nanofibers because of their appealing mechanical and piezoelectric properties. With modified electrospinning, the aligned poly (vinylidene fluoride) (PVDF) nanofibers can be produced. However, the roles of mechanical stretching and electrical poling during this process require further investigation. Here, we utilize a rotary drum to collect the fibers by both electrospinning and mechanical drawing approaches. The fiber mats were characterized by Scanning Electron Microscope (SEM), Fourier Transform Infrared Spectrometer (FTIR) and X-Ray Diffraction (XRD), and were directly made into sensing films without post electric poling treatment. Their piezoelectricity was measured in our home-made dynamic air pressure sensor. Results show that the β-phase predominates in both electrospun and mechanically-spun nanofibers. However, sensor based on electrospun β-PVDF fiber mat presented a sensitivity of 178 mV/kPa, while only a very weak output signal was observed in the mechanically spun β-PVDF fiber mat. Therefore, mechanical stretching and electrical poling may play key roles in inducing the β-phase and orienting the dipoles in a preferential direction during electrospinning process, respectively.
It is found in our experiments that the depletion layer of anodic bonding is etched faster than the bulk glass (Pyrex 7740) in hydrofluoric acid (HF). Based on this interesting phenomenon, a novel process of a sacrificial layer is proposed in this paper. In order to deeply understand and investigate the rules concerning the influence of bonding parameters on this effect, firstly the width of the depletion layer under different bonding voltages and temperatures and the selection ratio of etching are revealed. To validate the feasibility of the method, a micro-machined accelerometer is designed and fabricated. The test results of resonant frequency and sensitivity of the fabricated accelerometer are 3254.5 Hz and 829.85–844.93 mV/g, respectively. This was further evidence that the depletion layer could be used as a sacrificial layer and the removable structure could be successfully released by fast etching this layer. The important feature of this method is that only one mask is needed in the whole process and therefore it could greatly simplify the fabrication process of the device.
Face masks play a critical role in reducing the transmission risk of COVID-19 and other respiratory diseases. Masks made with nanofibers have drawn increasingly more attention because of their higher filtration efficiency, better comfort, and lower pressure drop. However, the interactions and consequences of the nanofibers and microwater droplets remain unclear. In this work, the evolution of fibers made of polymers with different contact angles, diameters, and mesh sizes under water aerosol exposure is systematically visualized. The images show that capillarity is very strong compared with the elasticity of the nanofiber. The nanofibers coalesce irreversibly during the droplet capture stage as well as the subsequent liquid evaporation stage. The fiber coalescence significantly reduces the effective fiber length for capturing aerosols. The nanofiber mesh that undergoes multiple droplet capture/evaporation cycles exhibits a fiber coalescing fraction of 40%–58%. The hydrophobic and orthogonally woven fibers can reduce the capillary forces and decrease the fiber coalescing fraction. This finding is expected to assist the proper design, fabrication, and use of face masks with nanofibers. It also provides direct visual evidence on the necessity to replace face masks frequently, especially in cold environments.
In this paper, the electrospray technology is used to easily deposit the glass frit into patterns at a micro-scale level. First, far-field electrospray process was carried out with a mixture of glass frit in the presence of ethanol. A uniform, smooth, and dense glass frit film was obtained, verifying that the electrospray technology was feasible. Then, the distance between the nozzle and the substrate was reduced to 2 mm to carry out near-field electrospray. The experimental process was improved by setting the range of the feed rate of the substrate to match both the concentration and the flow rate of the solution. Spray diameter could be less at the voltage of 2 kV, in which the glass frit film was expected to reach the minimum line width. A uniform glass frit film with a line width within the range of 400–500 μm was prepared when the speed of the substrate was 25 mm/s. It indicates that electrospray is an efficient technique for the patterned deposition of glass frit in wafer-level hermetic encapsulation.
A simple and versatile two-step silicon wet etching technique for the control of the width and height of the glass frit bonding layer has been developed to improve bonding strength and reliability in wafer-level microelectromechanical systems (MEMS) packaging processes. The height of the glass frit bonding layer is set by the design of a vertical reference wall which regulates the distance between the silicon wafer and the encapsulation capping substrate. On the other hand, the width of the bonding layer is constrained between two micro grooves which are used to accommodate the spillages of extra glass frit during the bonding process. An optimized thermal bonding process, including the formation of glass liquid, removal of gas bubbles under vacuum and the filling of voids under normal atmospheric condition has been developed to suppress the formation of the bubbles/voids. The stencil printing and pre-sintering processes for the glass frit have been characterized before the thermal bonding process under different magnitudes of bonding pressure. The bonding gap thickness is found to be equal to the height of the reference wall of 10 μm in the prototype design. The bubbles/ voids are found to be suppressed effectively and the bonding strength increases from 10.2 to 19.1 MPa as compared with a conventional thermal annealing process in air. Experimentally, prototype samples are measured to have passed the high hermetic sealing leakage tests of 5 × 10 −8 atm cc s −1 .
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