Electrostatic Excitation Research Articles

Quantifying squeeze film damping in four-leaf clover-coupled micro-resonators: A comprehensive study under variable vacuum degrees

The squeeze film effect has a significant impact on the performance of micro-resonators when utilizing the electrostatic excitation and capacitive detection technique, due to the narrow air gaps present. It becomes the most critical damping effect that needs to be addressed. However, when micro-resonators are encapsulated in vacuum or rarefied air, squeeze film damping analysis can become considerably complex due to high-quality factors. In this study, we systematically investigate the squeeze film damping of a widely-used four-leaf clover-coupled micro-resonator under different vacuum degrees. The characteristic frequencies of micro-resonators are determined by applying the principle of mode superposition. Subsequently, the energy transfer theory is utilized to derive the theoretical formula for the quality factor caused by squeeze film damping, for varying levels of vacuum. Finite element analysis is then conducted to simulate the characteristic behavior and squeeze film damping of the resonator, confirming the accuracy of the proposed theory. In the experiment, a novel four-leaf silicon micro-resonator is fabricated using bulk anisotropic wet etching and other micromachining technologies. An experimental platform is established to study the squeeze film damping of the micro-resonator under various vacuum degrees, where the micro-resonator is excited electrostatically and detected capacitively. The frequency response and quality factor of the resonator under different vacuum degrees are measured using a house-made modal test circuit board. The experimental results demonstrate excellent agreement with the theoretical and simulated results, indicating that the proposed method can be a reliable and precise guide for understanding the squeeze film damping effect in advance, once the primary model of a micro-resonator has been designed.

Quantum chemical study of symmetricalnon-fullerene acceptor chromophores for organic photovoltaics

It is well reported that the fused ring acceptor modification can be an effective method for improving the optoelectronic and photovoltaic properties of organic materials. The addition of acceptor groups to a core can enhance its electron-withdrawing ability, resulting in improved charge transport and higher power conversion efficiencies in organic solar cells (OSCs). Thieno[3,2-b]thiophene, and diketopyrrolopyrrole are commonly used groups in OSCs due to their powerful electron-withdrawing capabilities and well-established synthetic protocols. Their incorporation into donor–acceptor systems has been shown to enhance the materials absorption spectra, increase the open-circuit voltage (VOC), and improve device performance. Following integrating several acceptor groups onto the recently synthesized core based on thieno[3,2-b]thiophene, and diketopyrrolopyrrole, we have created six novel electron donors (BDPTM1-BDPTM6) with interesting optoelectronic and photovoltaic properties. It is indeed common practice to use computational methods such as density functional theory (DFT) and time-dependent density functional theory (TD-DFT) to investigate the optoelectronic and photovoltaic properties of newly created molecules. These methods allow researchers to obtain crucial parameters such as the HOMO and LUMO energies, energy gap (Egap), frontier molecular orbital alignment, density of states, molecular electrostatic potential, absorption maxima (λmax), excitation energy (Eex), binding energy and transition density matrix…… Among all the designed compounds, BDPTM3 exhibited the lowest energy bandgap (1.63 eV) with bathochromic shift of 921.610 nm in Dichloro-methane and 852.242 nm in gaseous phase. BDPTM2 displayed the highest dipole moment (µtot) at 7.557 eV, BDPTM5, and BDPTM6 demonstrated consistent linear polarizability〈α〉 at 191.612 × 10−22 esu and 196.2823 × 10−22 esu respectively. Notably, BDPTM6 calculated the highest frst hyperpolarizability βtot at 583.849 × 10–30. Based on the results obtained from the theoretical investigation of the optoelectronic and photovoltaic properties of the developed molecules, it may be possible to recommend them as better candidates for use in organic solar cells (OSCs).

Nonlinear dynamics of a piezoelectrically laminated initially curved microbeam resonator exposed to fringing-field electrostatic actuation

In this paper, the nonlinear dynamics of a piezoelectrically sandwiched initially curved microbeam subjected to fringing-field electrostatic actuation is investigated. The governing motion equation is derived by minimizing the Hamiltonian over the time and discretized to a reduced-order model using the Galerkin technique. The modelling accounts for nonlinearities due to the fringing-field electrostatic force, initial curvature and mid-plane stretching. The electrostatic force is numerically computed using finite element simulation. The nonlinear dynamics of the microbeam in the vicinity of primary resonance is investigated, and the bifurcation types are determined by investigating the location of the Floquet exponents and their configuration with respect to the unit circle on the complex plane. The branches on the frequency–response curves, which originate from the period-doubling bifurcation points, are introduced, and the transition from period-1 to period-2 response is demonstrated by slight sweep of the excitation frequency over the time. The effect of DC and AC electrostatic excitation and the piezoelectric excitation on the response of the system are examined, and their effect on the bifurcation types is determined. The force response curves assuming the AC voltage as the bifurcation parameter are also introduced; it is illustrated that in contrast to in-plane electrostatic excitation, in fringing field-based resonators the resonator is not limited by pull-in instability, which is substantially confining the amplitude of the motion in in-plane resonators.

NONLINEAR VIBRATION RESPONSE OF PIEZOELECTRIC NANOSENSOR: INFLUENCES OF SURFACE/INTERFACE EFFECTS

In the present work, surface/interface energy effects for pull-in instability analysis on dimensionless natural frequency (DNF) and nonlinear dynamics response (NDR) of piezoelectric nanosensor (PENS) subjected to nonlinear electrostatic excitation and harmonic force are studied using Gurtin–Murdoch (GM) surface/interface energy approach. To achieve this purpose, the Hamilton’s approach, assumed mode and Lagrange–Euler’s theories and also arc-length continuation and complex averaging methods are used to investigate influences of surface/interface parameters of PENS such as Lame’s constants, residual stress, piezoelectric constants and mass density for analysis of pull-in instability voltage.

Dynamic response amplification of resonant microelectromechanical structures utilizing multi-mode excitation

This work presents an analytical and experimental study to enhance the dynamic response of the higher-order vibration modes of resonant microstructures. The detection of higher order vibration modes is usually very difficult due to their low amplitude response, which gets buried in noise. Higher input voltages can be used to enhance the response; however, it is highly energy demanding, can result in electronics problems, and may lead to pull-in of the lower participating modes. In this paper, we present a multi-mode excitation (MME) technique to enhance the vibration response of the higher order modes of an electrostatically actuated micro cantilever beam. First, we derive the dynamic equations of motion of the micro resonator accounting for two electrostatic excitation sources. Then, we apply the Galerkin method to analyze the dynamics of the system analytically. We investigate the effect of using various dynamic AC combinations on the involved modes to yield optimal conditions of the amplitude magnification. Then, an experimental investigation is conducted based on a micro resonator made of polyimide. The results showed that, compared to the single mode excitation (SME), the MME can significantly raise the level of the response above noise and amplify the displacement amplitude to yield a higher signal-to-noise ratio. The maximum displacement amplitude can be amplified multiple times as determined by the AC loads. Hence, the improved displacement amplitude with the proposed technique makes it promising for future signal processing technologies and for realizing high-sensitivity sensors.

Nonlinearity modulation in a mode-localized mass sensor based on electrostatically coupled resonators under primary and superharmonic resonances

A general model of a mode-localized mass sensor incorporating two weakly coupled clamped-clamped microbeams under electrostatic excitation is presented, and a reduced-order model considering quadratic and cubic nonlinearities is established. The multiple time scales method is used to solve the dynamic characteristics of the coupled resonators under primary resonance, simultaneous superharmonic and primary excitations, and one-third superharmonic resonance respectively, and to analyze the contribution of each harmonic excitation term. It is shown that the sensor can display softening, hardening, and linear behaviors by tuning the overall nonlinear coefficient in three different excitation scenarios. Furthermore, the conditions for restoring linear behavior with the highest possible amplitude without any hysteresis under different excitations are obtained. Finally, the mass sensitivities represented by the relative shift of amplitude ratio are calculated for all the resulting dynamic behaviors. The results show that the sensitivity is highest, for the hardening behavior in the in-phase mode and for the softening behavior in the out-of-phase mode. Interestingly, the sensitivities of the linear behavior obtained by nonlinearity modulation are the same for the two vibration modes, which is improve the output stability. Consequently, the sensor resolution can be significantly enhanced below the pull-in instability, while avoiding noise mixing.

On the nonlinear dynamics of a piezoresistive based mass switch based on catastrophic bifurcation

This research investigates the feasibility of mass sensing in piezoresistive MEMS devices based on catastrophic bifurcation and sensitivity enhancement due to the orientation adjustment of the device with respect to the crystallographic orientation of the silicon wafer. The model studied is a cantilever microbeam at the end of which an electrostatically actuated tip mass is attached. The piezoresistive layers are bonded to the vicinity of the clamped end of the cantilever and the device is set to operate in the resonance regime by means of harmonic electrostatic excitation. The nonlinearities due to curvature, shortening and electrostatic excitation have been considered in the modelling process. It is shown that once the mass is deposited on the tip mass, the system undergoes a cyclic fold bifurcation in the frequency domain, which yields a sudden jump in the output voltage of the piezoresistive layers; this bifurcation is attributed to the nonlinearities governing the dynamics of the response. The partial differential equations of the motion are derived and discretized to give a finite degree of freedom model based on the Galerkin method, and the limit cycles are captured in the frequency domain by using the shooting method. The effect of the orientation of the device with respect to the crystallographic coordinates of the silicon and the effect of the orientation of the piezoresistive layers with respect to the microbeam length on the sensitivity of the device is also investigated. Thanks to the nonlinearity and the orientation adjustment of the device and piezoresistive layers, a twofold sensitivity enhancement due to the added mass was achieved. This achievement is due to the combined amplification of the sensitivity in the vicinity of the bifurcation point, which is attributed to the nonlinearity and maximizing the sensitivity by orientation adjustment of the anisotropic piezoresistive coefficients.

Analysis and Compensation of Bias Drift of Force-to-Rebalanced Micro-Hemispherical Resonator Gyroscope Caused by Assembly Eccentricity Error

Aiming at the excitation coupling error caused by assembly eccentricity error in micro-hemispherical resonator gyroscope ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula> HRG), this paper proposes a method to identify excitation coupling coefficients by using the relationship between input angular velocity and excitation forces during mode reverses. Firstly, this paper studies the influence mechanism of eccentricity error on displacement detection and electrostatic excitation under force-to-rebalanced (FTR) mode. Furthermore, theoretical derivation and simulation results show that the coupling error of excitation force will lead to the mutual coupling between the driving and the sensing loops, which reduces the stability of the driving force and introduces additional zero-bias drift into the sensing mode. According to the proposed identification method of the coupling coefficient of excitation force, the coupling coefficients are calibrated, and the principle of direct compensation in the FPGA system is given. The experimental results show that in the FTR mode, the driving force amplitudes at different input angular rates and the zero-bias drift at room temperature before and after error compensation are compared, which verifies the effectiveness of the proposed error calibration and compensation method. At room temperature, under the FTR mode, compared with the uncompensated test results, the average value of the measured bias stability ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1\sigma $ </tex-math></inline-formula> ) of the three groups is reduced from 74.667 °/h to 9.670°/h, which is improved by nearly 7.72 times. When at 40°C, the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula> HRG’s bias stability ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1\sigma $ </tex-math></inline-formula> ) is reduced from uncompensated 177.151°/h to compensated 9.256°/h. [2022-0174]

Achieving High‐Performance Triboelectric Nanogenerator by DC Pump Strategy

AbstractTriboelectric nanogenerator (TENG) has been demonstrated as a promising solution for powering widely distributed electronics in the new era of Internet of Things (IoTs), however, high‐performance TENG always relies on tribo‐materials with high triboelectric property. Herein, a simple self‐charge excitation technique that charge injection is directly realized by self‐generated electrostatic breakdown charge instead of voltage‐multiplying circuit is proposed to break through the limitation of triboelectrification. By using the designed electrostatic breakdown charge excitation TENG (EBE‐TENG), the output performance of poor tribo‐material based TENG such as polyethylene (PE) can be enhanced by 5.7 times, which even approximates that of high tribo‐material based TENG such as fluorinated ethylene propylene (FEP). Moreover, electrostatic breakdown charge excitation technique also exhibits a universal applicable ability for other different triboelectric materials. This work not only greatly simplifies the charge excitation system, but also broadens the availability of materials for achieving high‐performance TENG.

Self-Sensing WS2 Nanotube Torsional Resonators.

We demonstrate self-sensing tungsten disulfide nanotube (WS2 NT) torsional resonators. These resonators exhibit all-electrical self-sensing operation with electrostatic excitation and piezoresistive motion detection. We show that the torsional motion of the WS2 NT resonators results in a change of the nanotube electrical resistance, with the most significant change around their mechanical resonance, where the amplitude of torsional vibrations is maximal. Atomic force microscopy analysis revealed the torsional and bending stiffness of the WS2 NTs, which we used for modeling the behavior of the WS2 NT devices. In addition, the solution of the electrostatic boundary value problem shows how the spatial potential and electrostatic field lines around the device impact its capacitance. The results uncover the coupling between the electrical and mechanical behaviors of WS2 and emphasize their potential to operate as key components in functional devices, such as nanosensors and radio frequency devices.

Freestanding Laser-Induced Graphene Ultrasensitive Resonative Viral Sensors.

Early and reliable detection of an infectious viral disease is critical to accurately monitor outbreaks and to provide individuals and health care professionals the opportunity to treat patients at the early stages of a disease. The accuracy of such information is essential to define appropriate actions to protect the population and to reduce the likelihood of a possible pandemic. Here, we show the fabrication of freestanding laser-induced graphene (FLIG) flakes that are highly sensitive sensors for high-fidelity viral detection. As a case study, we show the detection of SARS-CoV-2 spike proteins. FLIG flakes are nonembedded porous graphene foams ca. 30 μm thick that are generated using laser irradiation of polyimide and can be fabricated in seconds at a low cost. Larger pieces of FLIG were cut forming a cantilever, used as suspended resonators, and characterized for their electromechanics behavior. Thermomechanical analysis showed FLIG stiffness comparable to other porous materials such as boron nitride foam, and electrostatic excitation showed amplification of the vibrations at frequencies in the range of several kilo-hertz. We developed a protocol for aqueous biological sensing by characterizing the wetting dynamic response of the sensor in buffer solution and in water, and devices functionalized with COVID-19 antibodies specifically detected SARS-CoV-2 spike protein binding, while not detecting other viruses such as MS2. The FLIG sensors showed a clear mass-dependent frequency response shift of ∼1 Hz/pg, and low nanomolar concentrations could be detected. Ultimately, the sensors demonstrated an outstanding limit of detection of 2.63 pg, which is equivalent to as few as ∼5000 SARS-CoV-2 viruses. Thus, the FLIG platform technology can be utilized to develop portable and highly accurate sensors, including biological applications where the fast and reliable protein or infectious particle detection is critical.

An Electrostatic Comb Excitation Resonant Pressure Sensor for High Pressure Applications

In order to meet growing attentions and demands in the high-accuracy and high-pressure applications of deep-sea sciences, petroleum exploration, special industrial manufacturing, a silicon resonant high-pressure sensor with the measurement range of 50 MPa was developed based on double resonators of dual electrostatic comb excitation and piezoresistive detection in this paper. When operating, the frequency shifts of the resonators suspended in a sensitive diaphragm were produced due to the deformation of the diaphragm by applied pressure. The sensor structures were investigated by adopting the finite element analysis to extend the pressure-sensitive range with focusing on sensitivities, strength and the amplitude of the output signal and were fabricated by bulk-micromachining processes. Besides, by adopting dual synthetic comb-driven-force in the resonator, the direct current biased voltage and the negative stiffness effect of the sensor were decreased. The developed sensor here exhibited a high Q-factor value of ~46000, a differential pressure sensitivity of ~66 Hz/MPa (~930 ppm/MPa) with a linear coefficient of 0.99999, a low differential temperature sensitivity of ~0.1 Hz/°C (~1.4 ppm/°C). Benefiting from aforementioned characteristics, fitting errors of the sensor were better than 0.01% full scale (gas pressure of 0.11 to 7 MPa) under temperature range (−30 to 80°C) by using a polynomial algorithm and measurement errors of the sensor were better than 0.018% full scale (gas pressure of 0.11 to 7 MPa) under temperature range (−10 to 80°C).

Fused Quartz Dual-Shell Resonator Gyroscope

This paper introduces a 3-D fused quartz dual-shell microstructure, discussing its design, simulation, fabrication, and instrumentation as a Microelectromechanical System (MEMS) gyroscope. The dual-shell was realized from plastic deformation of fused quartz using triple-stack wafer bonding and high-temperature micro-glassblowing processes. The fabrication process was developed and dual-shell prototypes with different design parameters were fabricated. A Finite Element (FE) model was presented to simulate the deformation of viscous fused quartz during the micro-glassblowing process. The model was utilized to predict the final 3-D geometry from the design process parameters. A modeling framework based on FE simulations was presented for designing the resonators as well as predicting the resonator’s response to transient excitation, such as shock and vibrations. Fused quartz dual-shell resonator prototype with Q-factors as high as 1.83 million and amplitude ringdown time of 120 seconds was demonstrated. An electrode substrate was designed and integrated with the dual-shell resonator for electrostatic excitation and detection of the vibration modes, as well as 3-D packaging and vacuum encapsulation of the sensing element. An open-loop angular rate gyro operation with electrostatic excitation and capacitive detection was demonstrated on the dual-shell resonator. The Allan deviation of zero rate output bias revealed Angle Random Walk (ARW) of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.058 \deg /\sqrt {hr}$ </tex-math></inline-formula> and in-run bias instability of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.4 \deg /hr$ </tex-math></inline-formula> without temperature control on the dual-shell resonator gyroscope. The proposed structure can be instrumented to operate as a resonator, a gyroscope, or other vibratory sensors, and is anticipated to have advantages for precision operation in a harsh environment.

In the existence of a transverse dc electric field, the kinetic theory of current‐driven electrostatic ion cyclotron waves excitation in a magnetized dusty plasma

AbstractThe theoretical modelling for the electrostatic ion cyclotron (EIC) waves in the existence of a transverse direct current (dc) electric field in a collisionless magnetized dusty plasma has been investigated using the kinetic theory model. It is examined that after incorporation of dc electric field with the dust grains can alter the dispersion properties of EIC waves in a magnetized dusty plasma. Our model developed accounts for the critical drift velocity, electron‐to‐ion temperature ratio, magnetic field, the number density of dust grains, relative density of negatively charged dust grains, and finite gyroradius parameter. Our investigation found that normalized growth rate and frequency increase with the relative density ratio and electric field. Moreover, the critical drift velocity for the excitation of the mode with negatively charged dust grains and electron‐to‐ion temperature ratio has been studied, and it was found that critical drift velocity decreases with the increase in negatively charged dust grains. The impact of dust grains on the low‐frequency waves has also been studied, and it was found that an increase in dust grain number density results in the reduction of growth rate. Some of our theoretical findings are consistent with the experimental observations.

A three million Q factor tuning fork resonator based on a vibration isolation structure

The quality factor (Q factor) is one of the most important parameters for a resonant sensor since it determines the performance of the resonant system. The mass distribution imbalance caused by the fabrication error has a great impact on the anchor loss, and the Q factor will, thus, decrease drastically. In this work, a vibration isolation structure of the tuning fork is proposed to eliminate the influence of weight imbalance so that an ultrahigh Q factor can be obtained. With this designed vibration isolation structure, we prove that the anchor loss can be reduced significantly via numerical simulations, and we experimentally demonstrate that Qanchor increases by at least 78.6%. Moreover, the surface loss of the fused quartz tuning fork is also reduced after chemical etching, and we use interdigital electrodes for electrostatic excitation to avoid loss caused by the metal film employed in the conventional excitation method. Finally, the Q factor increases to 3 × 106, which is one of the highest test values known at present, and it is 127% larger than that of the tuning fork that uses a coated metal film for electrostatic excitation.

Impact of outgassing and its corrective methods to improve the vacuum stability in the Coriolis Vibratory Gyroscope

Coriolis Vibratory Gyroscope (CVG) is an inertial angular rate measurement sensor. CVG sensor comprises of metal coated hemispherical quartz vibrating structure as the rotation sensing element. It is forced to vibrate at one of its resonant modes by electrostatic excitation. Because of the limited flexibility of the quartz sensing element, the amplitude is limited to the submicron level. An ultra-high vacuum environment is required for the sensing element to sustain vibration for a long time. The criticality in the sensor development is to maintaining an ultra-high vacuum environment for the sensing element. Sensor suffers a problem of vacuum instability during the operation due to the outgassing from its components. This paper presents a novel approach in the identification of various outgassing sources that exist in the sensor, mitigation plan to minimize outgassing rate by selection of suitable raw material, suitable fabrication process of the components, and the surface characterization etc. The research work also presents the cost-effective experimental methodology to measure the outgassing rate from the sensor components to assess the CVG vacuum life, the requirement of the getter to maintain the ultra-high vacuum level throughout the CVG operation.

Electrostatic Ion-cyclotron Wave Excitation by a Gyrating Ion Beam in a Magnetized Plasma Containing Heavy Positive Ions

In the present paper, we have developed a theoretical model of cylindrical magnetized plasma column consisting of electrons, ions &amp; heavy positive ion species. A gyrating ion beam of potassium having radius ~1.5 cm and energy 10eV is launched through one end of the cylinder parallel to the static magnetic field and the electrostatic ion cyclotron wave is assumed to propagate nearly perpendicular to the magnetic field. The presence of heavy positive ions in the collisionless magnetized plasma decreases the critical drift for the excitation of EIC waves and hence the injected beam drives the electrostatic ion cyclotron (EIC) waves to instability through Cerenkov interaction. Therefore, the plasma is observed to contains two excited EIC wave modes; a (light positive) ion mode and a (heavy positive) ion mode. The frequencies and growth rates for both the positive unstable modes are worked out using fluid theory for the typical existing experimental parameters which are found to increase with the increase in ion beam energy and beam density. It is also observed that the unstable wave frequency and growth rate for both the modes increases with the increase in relative ion concentration of and heavy positive ions respectively. Magnetic field is also one of the important factors which influence the plasma instabilities appreciably. As the magnetic field increases, the frequency of both the ion modes increases linearly.

Ponderomotive force driven mechanism for electrostatic wave excitation and energy absorption of electromagnetic waves in overdense magnetized plasma

The excitation of electrostatic waves in plasma by laser electromagnetic (EM) pulse is important as it provides a scheme by which the power from the laser EM field can be transferred into the plasma medium. The paper presents a fundamentally new ponderomotive pressure-driven mechanism of excitation of electrostatic waves in an overdense magnetized plasma by a finite laser pulse. Particle-in-cell simulations using the EPOCH-4.17.10 framework have been utilized for the study of a finite laser pulse interacting with a magnetized overdense plasma medium. The external magnetic field is chosen to be aligned parallel to the laser propagation direction. In this geometry, the EM wave propagation inside the plasma is identified as whistler or R and L waves. The group velocity of these waves being different, a clear spatial separation of the R and L pulses are visible. In addition, excitation of electrostatic perturbation associated with the EM pulses propagating inside the plasma is also observed. These electrostatic perturbations are important as they couple laser energy to the plasma medium. The excitation of electrostatic oscillations are understood here by a fundamentally new mechanism of charge separation created by the difference between the ponderomotive force (of the EM pulse) felt by the two plasma species, viz., the electrons and the ions in a magnetized plasma.

A versatile sample fabrication method for ultrafast electron diffraction

Integral to the exploration of nonequilibrium phenomena in solid-state systems is the study of lattice motion after photoexcitation by a femtosecond laser pulse. For the past two decades, ultrafast electron diffraction (UED) has played a critical role in this regard. Despite remarkable progress in instrumental development, this technique is still bottlenecked by a demanding sample preparation process, where ultrathin single crystals of large lateral size are typically required. In this work, we describe an efficient, versatile method that yields high-quality, laterally extended (≥ 100 µm), and thin (≤ 50 nm) single crystals on amorphous films of Si3N4 windows. It applies to most exfoliable materials, including those reactive in ambient conditions, and promises clean, flat surfaces. Besides the natural extension to fabricating van der Waals heterostructures, our method can also be applied to future-generation UED that enables additional control of sample parameters, such as electrostatic gating and excitation by a locally enhanced terahertz field. Our work significantly expands the type of samples for UED studies and also finds application in other time-resolved techniques such as attosecond extreme-ultraviolet absorption spectroscopy. This method hence provides further opportunities to explore photoinduced transitions and to discover novel states of matter out of equilibrium.

Piezoelectric response of energetic composites under an electrostatic excitation

Several high-explosive (HE) crystals are known to be piezoelectric. However, no systematic study has been carried out on how this effect can be utilized. In this paper, we report the results of an analysis on the response of composites consisting of HE crystals and a polymeric binder under electrostatic excitation. The HE crystals considered are 1,3,5-trinitroperhydro-1,3,5-triazine, octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine, pentaerythritol tetranitrate, and ammonium perchlorate. To explore avenues for enhancing the piezoelectric effect, the binder of the composites is taken to be piezoelectric polyvinylidene fluoride. The focus is on the distributions of induced electric field vector and mechanical stress in the microstructures. The effects of crystal–binder volume fraction, HE crystal size, and dielectric constants of the HE crystals are investigated. To further explore the effect, microparticles of lead zirconate titanate piezoelectric ceramic are introduced to some microstructures. For the HE crystals considered here, a coupled electromechanical analysis shows that the microstructural heterogeneities can enhance the local electric fields to as high as 1.34 times the applied E-field, causing the dielectric breakdown field strength of the overall composite to be much lower than the breakdown strengths of the constituents in the microstructure. In addition, the induced stress levels just prior to dielectric breakdown are well below the yield strengths of the respective constituents. As such, controlled dielectric breakdown, rather than mechanical damage, should primarily be used to facilitate hotspot formation, ignition, and chemical reaction. The likelihood of local dielectric breakdown within the HE crystals is systematically quantified as a function of applied electric field, microstructural attributes, and constituent behavior. To gauge the effect of the direct piezoelectric effect, one material case is also subjected to mechanical excitation in the form of compression. Under an applied external stress, the results show that the direct piezoelectric effect can lead to local yielding and thereby serve as a hotspot generation mechanism. On the other hand, the induced E-field is weak and unlikely to serve as a practical or efficient means of effecting hotspots within an energetic material. The analysis points out that simultaneous application of electrostatic excitation and mechanical excitation can also be considered.