Showing papers by "Liangchi Zhang published in 2020"

Array Design of Piezoelectric Micromachined Ultrasonic Transducers With Low-Crosstalk and High-Emission Performance

Abstract: This article presents a resonant cavity-based array design for piezoelectric micromachined ultrasonic transducers (PMUTs). The cavity depth is designed to ensure that its open end achieves a considerably smaller acoustic impedance than the surrounding PMUT cells. The interference acoustic wave generated between every two adjacent PMUT cells at the near surface of the array will take an easy path down to the cavity bottom. As such, the crosstalk effect among different adjacent cells in the array can be largely reduced. An equivalent circuit model of the proposed array is established for its design and optimization. In addition, the solutions for circuit parameters in the electromechanical domain are analytically derived and verified via FEM simulations. Given the low crosstalk effect achieved by the proposed array design, the output sensitivity of the proposed PMUTs can be improved by 259% compared with the traditional PMUTs with a high distribution density of the same size. The cavity-based array design and its model can be used for further advanced PMUT cell structures in other arrays to improve their performance.

A New Polymer-Based Mechanical Metamaterial with Tailorable Large Negative Poisson’s Ratios

Abstract: Mechanical metamaterials have attracted significant attention due to their programmable internal structure and extraordinary mechanical properties However, most of them are still in their prototype stage without direct applications This research developed an easy-to-use mechanical metamaterial with tailorable large negative Poisson's ratios This metamaterial was microstructural, with cylindrical-shell-based units and was manufactured by the 3D-printing technique It was found numerically that the present metamaterial could achieve large negative Poisson's ratios up to -1618 under uniaxial tension and -1657 under uniaxial compression, and the results of the following verification tests agreed with simulation findings Moreover, stress concentration in this new metamaterial is much smaller than that in most of existing re-entrance metamaterials

Equivalent Circuit Models of Cell and Array for Resonant Cavity-Based Piezoelectric Micromachined Ultrasonic Transducer

Abstract: This article presents a design of resonant cavity-based piezoelectric micromachined ultrasonic transducers (PMUTs), including impedance matching tube-integrated (T) and Helmholtz resonant (HR) cavity-integrated PMUTs. In addition, equivalent circuit models for single PMUT cell and PMUT array are developed for structural optimization and complex array design. The model-derived results agree well with the FEM results. On the basis of the proposed models, an optimized design is established to achieve high output pressure and a good array working performance. The working performance of arrays that consist of HR-PMUTs and traditional circular diaphragm PMUTs (C-PMUTs) is compared. Results indicate that the HR-PMUT array has a lower crosstalk effect than the traditional C-PMUT array. Furthermore, the highest ultrasonic output pressure of HR-PMUT array at the resonant frequency can be achieved with an increase of up to 163% compared with that of the C-PMUT array because of the liquid amplification effect. Also, the cavity-based design and its model can be used for further advanced PMUT cell structures in other arrays to improve their performance.

Tensile Properties of Boron Nitride-Carbon Nanosheet-Reinforced Aluminum Nanocomposites Using Molecular Dynamics Simulation

Abstract: The recently discovered hybrid boron nitride-carbon (BN-C) nanostructures have triggered enormous attention in the research on innovative nanocomposite design. Molecular dynamics simulation is conducted in this study to analyze the load-carrying capacity of aluminum nanocomposites reinforced with different types of BN-C nanosheets. By adopting a realistic loading condition in actual composites, the study found that the mechanical performance of the nanocomposite is predominantly affected by the interfacial mechanics between the nanosheet and the inner surface of the matrix. The computed Young’s modulus of Al matrix reinforced by graphene, BN and BN-C nanofiber are 74.61 GPa, 74.65 GPa and 76.48 GPa respectively by using the realistic loading condition. The tensile loading behavior of the nanocomposite is also strongly dependent on the angle of recline of the nanosheet relative to the loading direction. The nanocomposite with a nanofiber reinforcement aligned at 0° to the principal loading axis exhibited maximum tensile resistance compared to that of nanofiber reinforcements aligned at 15° or 30°. The maximum load-carrying capacity under tension decreases with increasing temperature. However, increasing the BN concentration in the reinforcing nanosheet improves the thermal stability of the nanocomposite.

Microstructure-based three-dimensional characterization of chip formation and surface generation in the machining of particulate-reinforced metal matrix composites

Abstract: Particulate-reinforced metal matrix composites (PRMMCs) are difficult to machine due to the inclusion of hard, brittle reinforcing particles. Existing experimental investigations rarely reveal the complex material removal mechanisms (MRMs) involved in the machining of PRMMCs. This paper develops a three-dimensional (3D) microstructure-based model for investigating the MRM and surface integrity of machined PRMMCs. To accurately mimic the actual microstructure of a PRMMC, polyhedrons were randomly distributed inside the matrix to represent irregular SiC particles. Particle fracture and matrix deformation and failure were taken into account. For the model’s capability comparison, a two-dimensional (2D) analysis was also conducted. Relevant cutting experiments showed that the established 3D model accurately predicted the material removal, chip morphology, machined surface finish, and cutting forces. It was found that the matrix-particle-tool interactions led to particle fractures, mainly in the primary shear and secondary deformation zones along the cutting path and beneath the machined surface. Particle fracture and dilodegment greatly influences the quality of a machined surface. It was also found that although a 2D model can reflect certain material removal features, its ability to predict microstructural variation is limited.

Elastic modulus evolution of rocks under heating-cooling cycles

Abstract: Rocks decay significantly during or after heating–cooling cycles, which can in turn lead to hazards such as landslide and stone building collapse. Nevertheless, the deterioration mechanisms are unclear. This paper presents a simple and reliable method to explore the mechanical property evolutions of representative sandstones during heating–cooling cycles. It was found that rock decay takes place in both heating and cooling processes, and dramatic modulus changes occurred near the α − β phase transition temperature of quartz. Our analysis also revealed that the rock decay is mainly attributed to the internal cracking. The underlying mechanism is the heterogeneous thermal deformation of mineral grains and the α – β phase transition of quartz.

Advanced thermal metamaterial design for temperature control at the cloaked region.

Abstract: The present study focuses on maintaining the temperature magnitude around heat-sensitive components (cloaked region) in advanced electronic devices by introducing convective elements using extended surface fins. A finite element analysis confirmed that with the aid of the convection component to thermal cloaking, heat flux can be redirected around the cloaked region as well as control the temperature. The simulation results were verified by experiment under natural convection corresponding to the simulation assumptions. It was found that when the heat source maintains its temperature at 100 °C and the heat sink remains at 0 °C, the temperature within the cloaked region can reduce by up to 15 °C, from ~ 50 °C with conventional cloaking to 35 °C with a well-designed array of surface fins. It is worth noting that experimental results are consistent with the simulation results.

Consistent Computational Modeling of Mechanical Properties of Carbon and Boron Nitride Nanotubes

Abstract: Computational modeling has emerged as a powerful tool in estimating many of the exciting material properties of low-dimensional systems such as nanotubes. There also exists a variation in the reported strength data of nanotubes using different computational techniques. This issue is attributed to the uncertainty in determining the correct thickness of the nanotubes, a fundamental parameter to estimate any mechanics-related properties. The present study establishes a consistent approach in determining the mechanical properties of nanotubes using molecular dynamics (MD) simulation. It was found that the nanotube wall thickness varies with the nanotube radius, which subsequently affects the estimated elastic modulus of the nanotube. There exists a threshold nanotube radius beyond which the elastic modulus remains fairly constant. The results predicted by MD simulation are also consistent with findings from first-principle methods. The findings from this study can be applied for a range of nanomaterials to determine their effective mechanical properties.

Nanoindentation Creep, Elastic Properties, and Shear Strength Correlated with the Structure of Sn-9Zn-0.5nano-Ag Alloy for Advanced Green Electronics

Abstract: This work investigates the influence of an Ag nanoparticle addition on the microstructure, microhardness, creep, temperature-dependent elastic properties, damping capacity, and shear strength of an environmentally friendly eutectic Sn-9Zn (wt.%) material. A microstructure analysis confirmed that adding Ag nanoparticles significantly altered the morphologies of the Zn-rich phase, which includes the size and shape in the presence of fine spherical-shaped AgZn3 intermetallic compound (IMC) particles in the β-Sn matrix. These fine microstructures positively impact on microhardness, creep, damping capacity, and temperature-dependent elastic properties. Furthermore, in the electronic interconnection on an Au/Ni-plated-Cu pad ball grid array (BGA) substrate, adding Ag nanoparticles generates an additional AgZn3 IMC layer at the top surface of the AuZn3 IMC layer. It also significantly improves the oxidation resistance of Sn-Zn material due to the formation of fine AgZn3 IMC particles. Moreover, the interfacial shear strength value of the Sn-Zn material doped with Ag nanoparticles on the Au/Ni-Cu pad BGA substrate increased about 12% as compared to the reference material after five minutes of reaction in the presence of a fine Zn-rich phase and AgZn3 IMC particles, which acted as second phase dispersion strengthening mechanism. Adding Ag nanoparticles also altered the fracture mode to a typical ductile failure with rough dimpled surfaces of the Sn-Zn material.

Effect of ultra-precision fly-cutting on the surface integrity of potassium dihydrogen phosphate crystals

Abstract: The surface integrity of a potassium dihydrogen phosphate (KDP) crystal significantly affects the laser damage threshold of the material. However, the detection of the surface integrity of KDP crystals is difficult due to the material’s special properties including soft, brittle, and sensitive to external environments (e.g., humidity, temperature, and applied stress). This results in conventional characterization methods, such as transmission electron microscopy (TEM) and scanning electron microscope (SEM), which cannot be used to study the mechanisms of surface/subsurface damages of KDP crystals. This paper investigates the ultra-precision fly-cutting effect on the surface integrity of KDP crystals. To explore the fundamentals, nanoindentation was used. The results demonstrated that the elastic-plastic deformation of a KDP crystal occurs more easily on a machined surface than on a cleaved (damage-free) surface. The elastic modulus and hardness of the former surface are lower than that of the latter. Additionally, fly-cutting reduces the anisotropy of the elastic modulus and hardness. To explore the mechanisms behind such variations, a novel method to characterize subsurface damage was proposed by using the grazing incidence X-ray diffraction (GIXD) technique. It was identified that the damages induced by fly-cutting are dislocations and lattice misalignments.

Elastic-plastic-brittle transitions of potassium dihydrogen phosphate crystals: characterization by nanoindentation

Abstract: Potassium dihydrogen phosphate (KDP) crystals are widely used in laser ignition facilities as optical switching and frequency conversion components. These crystals are soft, brittle, and sensitive to external conditions (e.g., humidity, temperature, and applied stress). Hence, conventional characterization methods, such as transmission electron microscopy, cannot be used to study the mechanisms of material deformation. Nevertheless, understanding the mechanism of plastic-brittle transition in KDP crystals is important to prevent the fracture damage during the machining process. This study explores the plastic deformation and brittle fracture mechanisms of KDP crystals through nanoindentation experiments and theoretical calculations. The results show that dislocation nucleation and propagation are the main mechanisms of plastic deformation in KDP crystals, and dislocation pileup leads to brittle fracture during nanoindentation. Nanoindentation experiments using various indenters indicate that the external stress fields influence the plastic deformation of KDP crystals, and plastic deformation and brittle fracture are related to the material’s anisotropy. However, the effect of loading rate on the KDP crystal deformation is practically negligible. The results of this research provide important information on reducing machining-induced damage and further improving the optical performance of KDP crystal components.

Retreatment with immunosuppression for 23 patients with refractory or relapsed severe aplastic anemia

Abstract: Objective: This study aims to evaluate the efficacy and safety of secondary immunosuppressive therapy (IST) in refractory or relapsed severe aplastic anemia. Methods: The hematologic response and safety of 23 patients with refractory or relapsed SAA treated with secondary IST (including ATG/ALG + cyclosporine or HD-CTX) in our hospital were retrospectively analyzed. Results: A total of 23 patients were involved, including 11 males and 12 females, with a median age of 21 (11-62) years. In the refractory group, the interval of IST was 7 (6-12) months. In the relapsed group, on the other hand, the interval between two courses of IST was 39 (14-51) months. At 6 months after IST, the overall response rate was 69.5% (16/23) ; 60% (6/10) of the refractory group vs 77% (10/13) of the relapsed group; 64% (7/11) of the ATG/ALG group vs 75% (9/12) of the HD-CTX group. Among the patients who got the hematologic response, two patients relapsed again, all of them from the relapse group. After the third IST, they got the response again. Conclusion: The second IST is safe and effective for refractory and relapsed SAA patients; the early serologic reaction should be paid attention to when using the same ATG/ALG, and the risk can be reduced by changing the type of ATG/ALG or other IST programs. The third IST can still obtain the treatment response for the second relapse patients.

Mechanisms and optimization for the rapid fabrication method of polymeric microlens arrays.

Abstract: This paper presents a simple and cost-effective rapid method to make defect-free polymeric microlens arrays at room temperature without applying external pressure. This method uses an optically clear and high-transparency Norland Optical Adhesive (NOA) monomer solution. This is realized by using a combination of a mold and an ultraviolet (UV) polymerization technique. NOA can cross-link in a tenth of a second upon UV exposure. The uniformity and surface quality of the manufactured microlens arrays are investigated through atomic force microscopy and optical microscopy techniques. Experimental results show that the microlens arrays manufactured by the polymerization process are of very high quality without any defects. Further, the surface quality of the lenses can be significantly enhanced by increasing the viscosity of the photosensitive monomer solution.