On electronic structure of polymer-derived amorphous silicon carbide ceramics

The electronic structure of polymer-derived amorphous silicon carbide ceramics was studied by combining measurements of temperature-dependent conductivity and optical absorption. By comparing the experimental results to theoretical models, electronic structure was constructed for a carbon-rich amorphous silicon carbide, which revealed several unique features, such as deep defect energy level, wide band-tail band, and overlap between the band-tail band and defect level. These unique features were discussed in terms of the microstructure of the material and used to explain the electric behavior

Effect Of Pyrolysis Temperature On The Structure And Conduction Of Polymer-Derived Sic

We studied the electric conductivity and structure of polymer-derived carbon-rich amorphous SiC pyrolyzed at different temperatures. The conductivity of the material increased drastically with pyrolysis temperature followed an Arrhenius relationship with the activation energy of ~3.4 eV. Raman and X-ray photoelectron spectroscopy analysis revealed that the order of free carbon phase increased with pyrolysis temperature, accompanied by sp3→ sp2 transition. The activation energy for such a structure change was 3.1-3.8 eV, which is close to that for the conductivity change. We thus believe that the increase in the conductivity was mainly due to the increase in conductivity of the free carbon phase. © 2014 The American Ceramic Society

Frequency-Dependent Conductive Behavior Of Polymer-Derived Amorphous Silicon Carbonitride

The AC conductive behavior of a polymer-derived amorphous silicon carbonitride ceramic was systemically studied. The conductivity exhibited a frequency-dependent switch: at low frequencies, the conductivity is constant and independent of frequency; while at high frequencies, the conductivity increases with frequency, showing a strong relaxation process. Both the frequency-independent conductivity and the characteristic frequency for the relaxation follow the Arrhenius relation with respect to the annealing temperature and follow a band-tail hopping process with respect to the testing temperature. XPS analysis revealed that a sp3-sp2 transition took place in the free-carbon phase of the material with increasing annealing temperature. The activation energy of the transition is similar to those for the Arrhenius relations. The following conductive mechanisms were proposed to account for the observed behaviors: the frequency-independent conductivity in the low frequency region is dominated by a long-distance transport of charge carriers via matrix-free carbon path, enhanced by an electric-field concentration effect; while the frequency-dependent conductivity in the high frequency region is dominated by a interfacial polarization process governed by charge carrier relaxation within the free-carbon phase

Effects of Short-Term Acidification on the Adsorption of Dissolved Organic Matter by Soil Minerals and Its Mechanism of Action

In order to investigate the impact of soil acidification on the adsorption of dissolved organic matter by soil minerals and understand its mechanism, this study selected commonly found minerals in soils, namely illite, kaolin, and hematite, as the research objects. Glucose and tannic acid were considered as the representative compounds for studying the adsorption of dissolved organic matter in soils. By analyzing the effects of the three minerals on the adsorption characteristics of glucose and tannic acid after a short-term acidification treatment, this study aimed to explore the underlying mechanism. To achieve this, scanning electron microscopy and a specific surface area analyzer were utilized. The results of this study indicate that the adsorption modes of the minerals studied were unaffected by short-term acidification. Chemisorption, as well as surface and mesopore diffusion, were found to dominate the adsorption process. In terms of adsorption behavior, the minerals exhibited multilayer inhomogeneous adsorption with glucose and kaolin, while tannic acid showed monolayer adsorption with illite and hematite. When exposed to the same acidification conditions, the saturated adsorption of glucose and tannic acid was found to be illite ≥ hematite > kaolin. The kinetic adsorption processes exhibited three stages: fast adsorption, slow adsorption, and adsorption equilibrium. Interestingly, as the intensity of the acidification increased, the saturated adsorption capacity generally followed the trend of S3 (test minerals with pH adjusted to 3 value) > S5 (test minerals with pH adjusted to 5 value) > CK (the control group). The acidification-induced solvation led to an increase in the specific surface area and the number of active adsorption sites on the minerals. Additionally, the protonation reaction triggered a change in the surface charge, which in turn affected the hydrogen bonding, ligand exchange, and charge transfer between the minerals and glucose and tannic acids. These interactions ultimately enhanced the adsorption capacity

Structural Evolutions In Polymer-Derived Carbon-Rich Amorphous Silicon Carbide

The detailed structural evolutions in polycarbosilane-derived carbon-rich amorphous SiC were investigated semiquantitatively by combining experimental and analytical methods. It is revealed that the material is comprised of a Si-containing matrix phase and a free-carbon phase. The matrix phase is amorphous, comprised of SiC4 tetrahedra, SiCxOx-4 tetrahedra, and Si-C-C-Si/Si-C-H defects. With increasing pyrolysis temperature, the amorphous matrix becomes more ordered, accompanied by a transition from SiC2O2 to SiCO3. The transition was completed at 1250°C, where the matrix phase started to crystallize by forming a small amount of β-SiC. The free-carbon phase was comprised of carbon nanoclusters and C-dangling bonds. Increasing pyrolysis temperature led to the transition of the free carbon from amorphous carbon to nanocrystalline graphite. The size of the carbon clusters decreased first and then increased, while the C-dangling bond content decreased continuously. The growth of carbon clusters was attributed to Ostwald ripening and described using a two-dimensional grain growth model. The calculated activation energy suggested that the decrease in C-dangling bonds is directly related to the lateral growth of the carbon clusters. (Graph Presented)

Contributions of Basic Chemical Components to the Mechanical Behavior of Wood Fiber Cell Walls as Evaluated by Nanoindentation

Selective chemical extraction was applied to gradually remove classes of chemical components from wood cell walls. Nanoindentation was performed on the control and treated wood cell walls to evaluate the contributions of the chemical components to the cell walls by measuring the elastic modulus, hardness, and creep compliance. Burger’s model was applied to simulate the process of nanoindentation and to gain insight into the response of visco-elastic properties to the chemical components. Wood extractives showed limited effects on the cell-wall mechanics; however, the removal of hemicelluloses and lignin resulted in reductions of 11.7% and 28.4%, respectively, in the elastic modulus and 14.8% and 30.4%, respectively, in the hardness. The extraction of hemicelluloses and lignin reduced the resistance of wood cell walls to creep. Furthermore, the extracted parameters from Burger’s modeling indicated that cellulose exhibited the greatest influence on the mechanical properties of wood cell wall, while hemicelluloses exhibited the greatest contribution to cell-wall viscosity, and lignin contributed extensively to cell-wall elasticity

The robustness of hollow CAPTCHAs

CAPTCHA is now a standard security technology for differentiating between computers and humans, and the most widely deployed schemes are text-based. While many text schemes have been broken, hollow CAPTCHAs have emerged as one of the latest designs, and they have been deployed by major companies such as Yahoo!, Tencent, Sina, China Mobile and Baidu. A main feature of such schemes is to use contour lines to form connected hollow characters with the aim of improving security and usability simultaneously, as it is hard for standard techniques to segment and recognize such connected characters, which are however easy to human eyes. In this paper, we provide the first analysis of hollow CAPTCHAs' robustness. We show that with a simple but novel attack, we can successfully break a whole family of hollow CAPTCHAs, including those deployed by all the major companies. While our attack casts serious doubt on the viability of current designs, we offer lessons and guidelines for designing better hollow CAPTCHAs

Restraining Sodium Volatilization in the Ferric Bauxite Direct Reduction System

Direct reduction is an emerging utilization technology of ferric bauxite. However, it requires much more sodium carbonate than ordinary bauxite does. The volatilization is one of the most significant parts of sodium carbonate consumption, as reported in previous studies. Based on the new direct reduction method for utilization of ferric bauxite, this paper has systematically investigated factors including heating temperature, heating time, and sodium carbonate dosage influencing sodium volatilization. For the purpose of reducing sodium volatilization, the Box–Benhken design was employed, and the possibility of separating iron and sodium after direct reduction was also investigated

Effect Of Acrylic Acid Additive On Electric Conductivity Of Polymer-Derived Amorphous Silicon Carbonitride

The effect of acrylic acid additive on the electric conductivity of amorphous SiCN derived from polymeric precursor was studied. The conductivity showed to follow the Arrhenius dependence on pyrolysis temperature, but with much smaller activation energy, as compared to the unmodified SiCN. Structural analysis using Raman and XPS revealed that the size of the free-carbon clusters within the AC-modified SiCN changed with pyrolysis temperature, but the sp2-to-sp3 ratio remained almost the same. The reason for the effect of AC on the carbon cluster was speculated. The mechanisms governing the conductivity behavior of the AC-modified SiCN were discussed