Synthesis, Characterization, Chemical Reduction and Biological Application of Graphene Oxide

As an atomic layer of sp2-hybridized carbon atoms closely packed in a honeycomb lattice, graphene has been attracting increasing attention since its discovery in 2004 due to its extraordinary physicochemical properties. Graphene oxide (GO), a non-stoichiometric graphene derivative with the carbon plane abundantly decorated with hydroxyl, epoxide and carboxylic groups, can be massively and cost-effectively produced from natural graphite following Hummers method. GO has greater aqueous solubility than pristine graphene due to its oxygen-functionalities. Various solution-based chemical methods can be applied to GO, which has stimulated a new research area called ‘wet chemistry of grahene’. Among them, chemical reduction of GO provides a facile route for large-scale synthesis of graphene. With abundant oxygen-functionalities in its structure, GO can potentially act as a suitable precursor for chemical modifications of graphene through methods used in organic chemistry. Special attention should be paid to that the hydroxyl groups in GO belong to tertiary alcohols, and steric hindrance should be considered when performing chemical modifications. Diethylaminosulfur trifluoride (DAST), a fluorinating reagent, is ineffective in fluorinating GO due to the steric hindrance of tertiary hydroxyls. However, DAST is effective in reducing GO. The capability of DAST for GO reduction is close to hydrazine, but the reduction reaction can be performed at lower temperature for DAST. As a two-dimensional (2D) nanomaterial with good aqueous solubility, biocompatibility and excellent intrinsic mechanical properties, GO is particularly useful in preparing 3D hybrid hydrogel scaffolds for tissue engineering applications.1 yea

Using Polymers to Improve the Performance of Sulfur and Organic Cathodes

Pollution, climate change and the rapid consumption of fossil fuel resources are driving the development and adoption of clean and renewable energy sources, including hydro, wind and solar power, as well as non-fossil fuel powered products such as electric vehicles. Therefore, there is an urgent and growing demand for corresponding high-efficiency, high-density, and low-cost energy storage systems. Among them, Li, Na, K, Mg, Zn, and Al-ion batteries and other types of rechargeable batteries have the characteristics of high efficiency and reversibility, light weight, environmental friendliness, and low cost, and have been widely studied in academia and industries. While extensive research over the past decades has led to the highly successful commercialization of lithium-ion batteries in portable electronics and electric vehicles, there have been many efforts to develop next-generation batteries with higher energy density, longer cycle life, and lower cost, such as batteries based on sulfur and organic materials. This thesis has explored the use of functional polymers to improve the specific capacity, discharge voltage, cycling, and rate performance of sulfur and organic cathodes. Through a combination of characterization techniques including thermal gravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), cyclic voltammetry (CV), chronopotentiometry, electrochemical impedance spectroscopy (EIS), galvanostatic cycling, UV-Vis, and peel test, new insights into the strategies for tackling the dissolution issue of sulfur and organic cathodes, conductivity change of conductive polymers in the battery, capacity fading mechanism, and charge storage mechanism have been gained. These findings may inspire the development of novel cathode materials with higher energy density and lower cost than conventional intercalation cathode materials. Firstly, a multi-functional PEDOT:PSS-Mg2+ binder formed by cross-linking PEDOT:PSS with Mg2+ was developed for the sulfur cathode in Li–S batteries. This new binder has a robust 3-D network structure achieved by the cross-linking of PSS- anions with Mg2+ ions, and a strong binding ability toward lithium polysulfides due to the strong interaction between the oxygen atoms in PEDOT and lithium polysulfides. These functionalities can increase the charge transfer reactions, cushion the drastic volume change during discharge/charge cycling, and trap the soluble lithium polysulfides in the cathode. The Li–S battery with a cathode using this new binder exhibited an initial capacity of 1097 mA h g-1 and capacity retention of 74% over 250 cycles at 0.5C, which are significant improvements compared with the Li–S battery using a conventional PVDF binder. Moreover, the preparation of the cathode slurry and the subsequent cathode fabrication using the PEDOT:PSS-Mg2+ binder uses water present in the PEDOT:PSS dispersion as the only dispersing solvent, which eliminates the use of any organic solvent, making the fabrication of Li–S batteries more environmentally friendly. Therefore, this study demonstrated that the cross-linked PEDOT:PSS-Mg2+ is a very promising new binder for high-performance Li–S batteries. Secondly, an innovative facile in-cell electrochemical polymerization method has been developed to incorporate a conductive polymer PEDOT into the sulfur cathode, which enables an intimate contact between PEDOT and other components in the cathode, leading to enhanced electron transport and effective trapping of soluble polysulfides. As a result, the sulfur cathode with the in-cell formed PEDOT shows substantially improved capacity, cycling stability, and rate performance compared with that using the commercial PEDOT. Furthermore, it has been found that the conductivity of PEDOT changes drastically during the battery cycling process, which affects the battery performance. Finally, the in-situ synthesis of PEDOT has been applied to LiFeO4 cathode, and a notable improvement in the specific capacity has been observed. Thirdly, a series of one-dimensional coordination polymers using 2,5-dihydroxy-1,4-benzoquinone (DHBQ) as the ligand and divalent metal ions (Ni, Co, Mn, Zn, and Cu) as the metal center have been synthesized and their electrochemical properties have been compared. It has been found that the coordination polymers using Ni, Co, Mn, and Zn (M-DHBQ·2H2O) exhibit the redox activities of both metal and ligand in the potential range of 0.5~3 V vs. Li+/Li, while the coordination polymer using Cu (Cu-DHBQ) only exhibits the redox activity of the ligand in the same potential range. In the potential range of 1.3~3 V vs. Li+/Li where only the DHBQ ligand is redox active, Cu-DHBQ exhibits the highest utilization of the quinone groups among the as-synthesized coordination polymers. Moreover, the capacity fading mechanism of Cu-DHBQ cathode is identified as the dissolution of the discharged product or intermediate in the electrolyte by UV-Vis analysis. By using the alginate binder (25 wt% in the cathode), which can strongly bind the electrode film and effectively trap the soluble species, the Cu-DHBQ cathode exhibits a high capacity of 261 mA h g-1 (98.1% of the theoretical capacity) at the current rate of 20 mA g-1, and can maintain a capacity of 194 mA h g-1 after 200 cycles at 100 mA g-1 with a capacity retention of 91.5%. Furthermore, our coordination approach is very versatile and can be extended to other ligand such as 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone (DHBQ-Cl) which has a higher discharge voltage than that of DHBQ. The Cu-DHBQ-Cl cathode shows a fast capacity fading, which might be caused by the collapse of the crystal structure after Li+ insertion. Nevertheless, our approach opens up a new avenue for the application of coordination polymers in energy storage. Finally, the stabilization of organic cathode through acid-base interaction with polymer binders has also been studied. It has been found that the binder approach for improving the cycling stability of organic cathode is only an auxiliary approach, whereas the polymerization approach, which includes the formation of conventional polymers, macrostructures, coordination polymers, covalent organic frameworks (COFs) and metal organic frameworks (MOFs), is be considered as the primary approach. Finally, two diketopyrrolopyrrole (DPP) based conjugated polymers, namely diketopyrrolopyrrole-quaterthiophene copolymer (PDQT) and diketopyrrolopyrrole-bithiophene polymer (PDBT) have been explored as the cathode materials for Li-ion storage. The PDQT cathode shows a p-type charge storge mechanism with a theoretical capacity of 52.4 mA h g-1 and an experimental capacity of 44.4 mA h g-1 (corresponding to a high doping level of 42%), while the PDBT cathode shows a bipolar charge storage mechanism with a theoretical capacity of 93.6 mA h g-1 and an experimental capacity of 17.1 mA h g-1. The experimental average discharge voltages of PDQT and PDBT cathodes are ~3.8 and ~2.95 V, respectively, which are one of the highest among organic cathodes. Further optimization of the testing condition (e.g. nanocomposite formation with porous carbon, better electrolyte solvent which is stable over a broad potential range so that both the p- and n-doping reaction can occur) is needed to increase the experimental capacity of PDBT

Numerical Simulation of Dynamic Response of Fiber Reinforced Ceramic Matrix Composite Beam with Matrix Cracks Using Multiscale Modeling

AbstractA multiscale method for simulating the dynamic response of ceramic matrix composite (CMC) with matrix cracks is developed. At the global level, the finite element method is employed to simulate the dynamic response of a CMC beam. While at the local level, the multiscale mechanical method is used to estimate the stress/strain response of the material. A distributed computing system is developed to speed up the simulation. The simulation of dynamic response of a Nicalon/CAS-II beam being subjected to harmonic loading is performed as a numerical example. The results show that both the stress/strain responses under tension and compressive loading are nonlinear. These conditions result in a different response compared with that of elastic beam, such as: 1) the displacement response is not symmetric about the axis of time; 2) in the condition of small external load, the response at first order natural frequency is limited within a finite range; 3) decreasing the matrix crack space will increase the displacement response of the beam

A Quadrilateral Element-based Method for Calculation of Multi-scale Temperature Field

AbstractIn the analysis of functionally graded materials (FGMs), the uncoupled approach is used broadly, which is based on homogenized material property and ignores the effect of local micro-structural interaction. The higher-order theory for FGMs (HOTFGM) is a coupled approach that explicitly takes the effect of micro-structural gradation and the local interaction of the spatially variable inclusion phase into account. Based on the HOTFGM, this article presents a quadrilateral element-based method for the calculation of multi-scale temperature field (QTF). In this method, the discrete cells are quadrilateral including rectangular while the surface-averaged quantities are the primary variables which replace the coefficients employed in the temperature function. In contrast with the HOTFGM, this method improves the efficiency, eliminates the restriction of being rectangular cells and expands the solution scale. The presented results illustrate the efficiency of the QTF and its advantages in analyzing FGMs

A Review on Ceramic Matrix Composites and Environmental Barrier Coatings for Aero-Engine: Material Development and Failure Analysis

Ceramic matrix composites with environmental barrier coatings (CMC/EBCs) are the most promising material solution for hot section components of aero-engines. It is necessary to access relevant information and knowledge of the physical properties of various CMC and EBCs, the characteristics of defects and damages, and relevant failure mechanisms. Then, effective failure prediction models can be established. Individually assessing the failure of CMC and EBCs is not a simple task. Models considering the synergetic effect of coating properties and substrate fibrous architecture are more reasonable and more challenging. This paper offers a review and a detailed description of the materials features, failure mechanism, and failure modeling for both CMC substrate and EBC coatings. The various methods for failure analyses and their pros and cons are discussed. General remarks on technical development for failure modeling are summarized subsequently

A Review on Ceramic Matrix Composites and Environmental Barrier Coatings for Aero-Engine: Material Development and Failure Analysis

Ceramic matrix composites with environmental barrier coatings (CMC/EBCs) are the most promising material solution for hot section components of aero-engines. It is necessary to access relevant information and knowledge of the physical properties of various CMC and EBCs, the characteristics of defects and damages, and relevant failure mechanisms. Then, effective failure prediction models can be established. Individually assessing the failure of CMC and EBCs is not a simple task. Models considering the synergetic effect of coating properties and substrate fibrous architecture are more reasonable and more challenging. This paper offers a review and a detailed description of the materials features, failure mechanism, and failure modeling for both CMC substrate and EBC coatings. The various methods for failure analyses and their pros and cons are discussed. General remarks on technical development for failure modeling are summarized subsequently

Multiscale acoustic emission of C/SiC mini-composites and damage identification using pattern recognition

In this paper, multiscale acoustic emission (AE) signal analysis was applied to acoustic emission data processing to classify the AE signals produced during the tensile process of C/SiC mini-composites. An established unsupervised clustering algorithm was provided to classify an unknown set of AE data into reasonable classes. In order to correctly match the obtained classes of the AE signals with the damage mode of the sample, three scales of materials were involved. Single fiber tensile test and fiber bundle tensile test were firstly performed to achieve the characteristics of AE signal of fiber fracture. Parameter analysis and waveform analysis were added to extract the different features of each class of signals in the In-situ tensile test of C/SiC mini-composite. The change of strain field on the sample surface analyzed by DIC (Digital Image Correlation) revealed the corresponding relationship between matrix cracking and AE signals. Microscopic examinationwas used to correlate the clusters to the damage mode. By analyzing the evolution process of signal activation for each class against the load, it also provided a reliable basis for the correlation between the obtained classes of the AE signals and the damage mechanism of the material

Thermal Stress Analysis of Environmental Barrier Coatings Considering Interfacial Roughness

A numerical analysis of the effect of roughness interface on the thermal stress in the environmental barrier coatings for ceramic matrix composites was performed. Based on the concept of representative volume elements, a micromechanical finite element model of the coated composites was established. The rough interfaces between the coating layers were described using sine curves. The cooling process after preparation and the typical service conditions for the CMCs component were simulated, respectively. The results show that the rough interface has little effect on the temperature distribution along the depth direction for the studied T/EBC coatings for SiC/SiC composites. The stress concentration occurs at the rough EBC/BC interface, which is prone to cause delamination cracking. Under typical service conditions, the high temperature can eliminate part of the thermal residual stress. Meanwhile, the thermal gradient will cause large thermal stress in the TBC layer and the stress will result in surface cracks. The stress concentrations appear at the peaks and valleys of rough interfaces. The variation range of thermal stress increases with the roughness amplitude and decreases with the wavelength