Electromechanical coupling behavior of dielectric elastomer transducers

Dielectric elastomer transducers with large deformation, high energy output, light weight and low cost have been drawing great interest from both the research and industry communities, and shown potential for versatile applications in biomimetics, dynamics, robotics and energy harvesting. However, in addition to multiple failure modes such as electrical breakdown, electromechanical instability, loss-of-tension and fatigue, the performance of dielectric elastomer transducers are also strongly influenced by the hyperelastic and viscoelastic properties of the material. Also, the interplay among these material properties and the failure modes is rather difficult to predict. Therefore, in order to provide guidelines for the optimal design of dielectric elastomer transducers, it is essential to first develop accurate and reliable models, and efficient numerical methods to investigate their performance. First, this thesis purposes a boundary-constraint method to eliminate the electromechanical instability of dielectric elastomer actuators under voltage-control loading condition and improve their actuation deformation. Second, based on the finite-deformation viscoelasticity model, the natural frequency tuning process of viscoelastic dielectric elastomer resonators is examined in this work. It is found that the tuned natural frequency is highly affected by the material viscoelasticity. Also, it is concluded that the electrical loading rate only influences the tunable frequency range and the safe operation voltage of the resonator, but not the tuned natural frequency when the applied voltage is within the safe range. Third, with the finite-deformation viscoelasticity model, the energy conversion efficiency of dielectric elastomer generators under equi-biaxial loading is also investigated in this work. Simulation results show that increasing the maximum stretch ratio and the rate of deformation, and choosing a proper bias voltage can lead to an improvement of the energy conversion efficiency. Furthermore, the fatigue life of dielectric elastomer devices under cyclic loading is explored in this work for the first time. Simulation results have demonstrated that the energy conversion efficiency of dielectric elastomer generators is compromised by their fatigue life. To tackle the critical challenges for the development and design of dielectric elastomers transducers, this research develops theoretical models and numerical methods that are able to capture the nonlinear electromechanical coupling, the material properties, the typical failure modes and different operating conditions of dielectric elastomer transducers. With more accurate and reliable modeling methods, this work is expected to provide a comprehensive understanding on the fundamentals and technologies of dielectric elastomer transducers and trigger more innovative and optimal design of such devices

Modeling Viscoelastomers With Nonlinear Viscosity

Consisting of highly mobile and flexible polymer chains, elastomers are known to exhibit viscoelastic behavior. Adopting concepts from the theory of polymer dynamics and finite-deformation viscoelasticity, this work presents a micromacro constitutive model to investigate the viscoelastic behavior of elastomers, in which the material viscosity varies with the macroscopic deformation. The developed model is then applied to study the stress response of elastomers. From the simulation results, it is observed that the developed model exhibits strong capability of capturing the typical response behaviors of elastomers (e.g., strain-softening behavior). A comparison of the stress responses between linear and nonlinear viscosity is also considered in this work. The modeling framework in this paper is expected to provide a general approach and a platform to analyze the viscoelastic behavior of rubber-like materials with nonlinear viscosity

Using Pretreatment of Carbon Monoxide Combined with Chlorine Dioxide and Lactic Acid to Maintain Quality of Vacuum-Packaged Fresh Beef

Due to microbial growth, beef easily gets corrupt in retail conditions, and the color and quality of the meat will be deteriorated. Therefore, hurdle technology, namely, pretreatment of carbon monoxide (CO), chlorine dioxide, and lactic acid, is used for vacuum-packaged beef to decontaminate beef and increase its quality stability. Beef was pretreated with 100% CO (C1), 100% CO and 50 mg/L chlorine dioxide (C2), and 100% CO and 50 mg/L chlorine dioxide and 30 g/L lactic acid (C3). The untreated samples were used as control (CK). During storage, the a⁎ color parameters of C1, C2, and C3 were significantly higher than that of CK, indicating CO pretreatment is a good way to maintain color appearance of beef, and chlorine dioxide and lactic acid did not affect the color-protecting role of CO on beef. C3 showed the strongest antimicrobial activity with the lowest total viable counts, followed by C2, C1, and CK. Samples in C3 also showed the lowest total volatile basic nitrogen, pH, thiobarbituric acid reactive substance, and metmyoglobin during the mid-late storage. Moreover, C3 can keep beef with higher unsaturated fatty acids. In conclusion, CO pretreatment combined with chlorine dioxide and lactic acid displayed efficient antimicrobial and color-stability activity for vacuum-packaged beef. It would be a potential way to use pretreatment of CO combined with chlorine dioxide and lactic acid to maintain the quality of vacuum-packaged beef

The influence of the number of free surfaces on the energy distribution and attenuation law of blasting vibration signals from peripheral holes: field experiment and simulation

Tunnels are commonly excavated using drilling and blasting methods, and the surrounding rock is greatly affected by the vibration of surrounding hole blasting. To study the influence of the number of free surfaces on the energy distribution and attenuation law of surrounding hole blasting vibration signals, on-site experiments and numerical simulation experiments were conducted. The research results indicate that the higher the number of free surfaces, the smaller the peak vibration velocity. The longitudinal Fourier main frequency decreases with the distance from the monitoring point. The more free surface, the greater the centroid frequency and zero crossing frequency. In addition, numerical simulation shows that the degree of rock fragmentation after blasting increases with the increase of the number of free surface of rock

Dynamic variation of the dielectric permittivity of elastomers with mechanical constraints

Elastomers are commonly used as insulators or actuators in important fields such as robotics and electronics. The performance of elastomers is strongly influenced by their dielectric permittivity while determining the dielectric permittivity of elastomers appears to be rather challenging. With image processing techniques and a motor-driven biaxial test setup, the dynamic variation of the dielectric permittivity and dielectric loss of elastomers under dynamic mechanical constraints is first examined in this paper. Also, the mechanical rate-dependence and the cyclic behavior of the dielectric permittivity are investigated. At a given mechanical loading rate, the change in the dielectric permittivity is found to follow a cubic polynomial function. The variations of the stress and the dielectric permittivity indicate the mechanical constraints on the polarization of the material. Our experiment setup allows different mechanical loading paths to be applied to the elastomer when simultaneously measuring the dielectric permittivity and the dielectric loss

Analysis on the energy harvesting cycle of dielectric elastomer generators for performance improvement

With attractive features like high energy density and flexibility, dielectric elastomer generators (DEGs) have been designed to harvest mechanical energy from diverse sources. However, their energy harvesting performance could be limited by the material viscoelasticity and various failure modes. Adopting the finite-deformation viscoelasticity model, this work presents a theoretical framework for analyzing the performance of a DEG with a “triangular” harvesting scheme. Simulation results reveal that choosing an appropriate in-plane stretch ratio for the onset of the discharging process can raise the harvested energy of DEGs. It is also found that the energy conversion efficiency of a DEG can be markedly improved by avoiding loss-of-tension of elastomer during the operation of energy harvesting

Synthesis of New Chiral Benzimidazolylidene–Rh Complexes and Their Application in Asymmetric Addition Reactions of Organoboronic Acids to Aldehydes

A series of novel chiral N-heterocyclic carbene rhodium complexes (NHC–Rh) based on benzimidazole have been prepared, and all of the NHC–Rh complexes were fully characterized by NMR and mass spectrometry. These complexes could be used as catalysts for the asymmetric 1,2-addition of organoboronic acids to aldehydes, affording chiral diarylmethanols with high yields and moderate enantioselectivities