Dielectric analysis of epoxy/amine resins using microwave cavity technique

Dielectric properties (permittivity and dielectric loss factor) of stoichiometric mixtures of DGEBA (diglycidylether of bisphenol A) epoxy (DER 332) and amine (diaminodiphenyl sulfone; DDS) as a function of temperature and extent of cure have been measured at a microwave frequency of 2.45 GHz. Permittivity and dielectric loss factor of this resin increase with increasing temperature and decrease with increasing extent of cure. Dielectric loss factor is more dependent on temperature in the early stages of cure but becomes less dependent on temperature as the cure proceeds. Dielectric loss data can be related to extent of cure in order to monitor the cure process. Online measurements of temperature- and cure-dependent dielectric loss factor show three material structure stages and significant changes in dielectric loss factor during the microwave cure process. Dielectric Joss factor is also found to be the same for both thermal and microwave cured samples.     

Effect of hydrothermal treatment on the performance of RuO2-Ta2O5/Ti electrodes for use in supercapacitors

Hydrothermal treatment (HTT) of RuO₂-Ta₂O₅/Ti electrode, as a method for improving their performance, for use in supercapacitors was investigated.The results show that HTT significantly enhances the stability of the electrodes. The specific capacitance of electrodes, subject to HTT in the temperature range 180-250 °C remains unchanged after 1000 CV cycles between −0.2 and 1.1 V vs. SCE; without HTT a decay to 97% of the initial is observed. The results also show that HTT decreases the activity of the electrodes for O₂ and H₂ evolution and increases the voltage window by 56-135 mV for supercapacitors, but with a specific capacitance decrease of 7-27%. XPS analyses show the existence of more hydroxides after the HTT, which leads to a little increase in the interplanar distance as indicated in the XDR results. Contact angle measurements show the presence of a more hydrophilic surface after HTT.     

Effect of Nanosized TiC0.37N0.63 on Unlubricated Wear Responses of Si3N‐Based Nanocomposites Under Low Hertzian Stress

Si₃N₄‐based nanocomposites containing 0–50 wt% TiC_0.37N_0.63 are directly consolidated at 1700°C by spark plasma sintering, and their reciprocal sliding behavior against a Si₃N₄ counterbody is investigated under a maximum Hertzian stress of 1.27 GPa in unlubricated conditions. The average grain widths of Si₃N₄ and TiC_0.37N_0.63 are about 85 and 90 nm, respectively. The decreasing relative densities of the as‐sintered nanocomposites indicate that the nano‐TiC_0.37N_0.63 may introduce pores and reduce the hardness and fracture resistance of the materials. The brittleness index for sliding contacts in all the samples is 25–31, indicating brittle fracture taking place on the wear surface and inducing cavities. When the mean free paths of nano‐TiC_0.37N_0.63 are slightly greater than grain length of Si₃N₄, the best wear resistance is achieved in Si₃N₄ containing 20/30 wt% TiC_0.37N_0.63 due to the process of surface smoothing by triboproducts. Severe wear response can be observed in Si₃N₄ nanocomposites containing 0, 10, 40, and 50 wt% of TiC_0.37N_0.63. The wear responses are explained by considering the microstructural parameters (like grain characteristics for both phases and mean free path of nano‐TiC_0.37N_0.63) and contact‐induced fracturing behavior, as well as tribochemical reactions.     

Mesoporous nickel electrodes plated with gold for the detection of glucose

Mesoporous nickel electrodes (MNEs) are efficient structural materials for advanced electrochemical sensors due to their extremely high surface areas. However, the poor oxidation resistance of nano-sized nickel in air severely limits their applications. Thus, the purpose of the present study was to improve the oxidation resistance of MNEs by applying relatively stable gold coatings via electroplating. The results revealed that the gold coatings provided excellent oxidation resistance to MNEs, and a loss in amperometric sensitivity for glucose sensing was not observed. The protection time increased with an increase as the thickness of the gold coating (passing charges of 0–25.49 C/cm²), and protection times ranging from a few hours to 2 weeks were obtained. With an alkaline electrolyte (NaOH, 4 M), MNEs exhibited excellent sensitivities. For instance, the sensitivity of the unmodified MNE and the Au coated-MNE was 3238 and 2978 μA mM⁻¹ cm⁻², respectively. Thus, coated and uncoated MNEs displayed remarkably higher linear sensitivities than conventional nickel-based electrodes (394 μA mM⁻¹ cm⁻²) at a wide range of glucose concentrations (0.001–3 M).     

Paired electro-oxidation. I. Production of benzaldehyde

Oxidation of toluene to benzaldehyde by Mn³⁺ and OH free radicals, generated in the anodic and cathodic reactions respectively, was carried out simultaneously in the cathodic and anodic compartments of a cell. The selectivity of benzaldehyde was very high in both the anodic and cathodic reactions. The maximum total current efficiency for benzaldehyde production in the paired electrooxidation was 171%.     

Interlevel dielectric failures in copper/low-k structures

Failure modes for inter-level dielectric layers under accelerated test conditions have been evaluated for a range of dielectric diffusion barriers in copper/low-k structures. The dominant failure mechanism for both constant voltage tests and ramped voltage tests was mechanical cracking at the dielectric barrier/low-k interface. Few occurrences of copper diffusion through the bulk ILD were observed. A simple model for the dominant failure mechanism is proposed which hypothesizes crack formation due to the electrostatic force between interdigitated lines followed by copper extrusion into the cracks. The proposed model is consistent with measurements of interfacial adhesion strengths in Cu/low-k stacks.     

Charge and discharge characteristics of a commercial LiCoO2-based 18650 Li-ion battery

We studied the charge and discharge characteristics of commercial LiCoO₂-based 18650 cells by using various electrochemical methods, including discharging at constant power, ac impedance spectroscopy, and dc-voltage pulse. At 20 °C, these cells deliver 8.7–6.8 Wh of energy when discharged at a power range of 1–12 W between 2.5 and 4.2 V. Ragone plots show that the effect of discharge power on the energy is significantly increased with decreasing of the temperature. For example, energy of the cell is entirely lost when the temperature downs to −10 °C and the discharge rate still remains at 10 W. Impedance analyses indicate that the total cell resistance (R_cell) is mainly contributed by the bulk resistance (R_b, including electric contact resistance and electrolytic ionic conductivity), solid electrolyte interface resistance (R_sei), and charge-transfer resistance (R_ct). Individual contribution of these three resistances to the cell resistance is greatly varied with the temperature. Near room temperature, the R_b occupies up to half of the cell resistance, which means that the rate performance of the cell could be improved by modifying cell design such as employing electrolyte with higher ionic conductivity and enhancing electric contact of the active material particles. At low temperature, the R_ct, which is believed to reflect cell reaction kinetics, dominates the cell resistance. In addition, galvanosatic cycling tests indicate that the charge and discharge processes have nearly same kinetics. The performance discrepancy observed during charging and discharging, especially at low temperatures, can be attributed to these two factors of: (1) substantially higher R_ct at the discharged state than at the charged state; (2) asymmetric voltage limits pre-determined for the charge and discharge processes.     

Adsorption and electrokinetics of glucose reduction

The kinetics of cathodic reduction of glucose were studied by a rotating disk electrode at rotation speeds from 500 to 6000 rpm and zero to 2.0 M concentration of glucose. Sodium sulfite was used as the supporting electrolyte as well as the reducing mediator. Both the experimental results and the theoretical analysis indicated that several reaction steps occurred for the cathodic reduction of glucose. The results also revealed that the adsorption of glucose on a cathode played an important role during the electrolysis. The reaction mechanism was proposed and a mathematical model was developed. The model correlated well with the experimental results. The rate determining step was found to be the cathodic reduction of the sodium cation. The rate current of the electroreduction of glucose can be expressed as

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Effects of ultrasonic oscillations on rheological behavior and mechanical properties of novel propylene‐based plastomers

The effects of ultrasonic oscillations on the die pressure, productivity of extrusion, melt apparent viscosity, melt surface appearance, and die swell of novel propylene‐based plastomers were studied in a specially designed ultrasonic oscillations extrusion system developed in our laboratory. The effects of ultrasonic oscillations on molecular weights, tensile strength, and dynamic mechanical properties of extrudates were also studied. The experimental results showed that the presence of ultrasonic oscillations during extrusion could significantly increase the productivity of plastomers at the same die pressure, and reduce die swell and melt fracture such as sharkskin at a given screw rotation speed. The die pressure and apparent viscosity of plastomers remarkably decreased with increasing ultrasonic intensity. Introduction of ultrasonic oscillations into plastomer melts can improve their processibility. The possible mechanism for ultrasonic improvement of rheological behavior was also proposed in this article. Under certain conditions, ultrasound‐assisted extrusion could slightly decrease the glass transition temperature (T_g) and storage modulus of plastomers due to the minor reduction in molecular weights, but showed no significant impact on yield strength and strength at break. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Annealing effect on the performance of RuO2–Ta2O5/Ti electrodes for use in supercapacitors

The preparation of RuO₂–Ta₂O₅/Ti electrodes, by dip-coating, for use in supercapacitors was investigated. The stability and specific capacitance of the electrodes annealed at various temperatures was examined. The results show that highly stable electrodes with a specific capacitance of 170 F g RuO₂⁻¹ were obtained at approximately 250 °C, while electrodes with a lower capacitance (130 F g RuO₂⁻¹) were obtained at 300 °C. The annealing time needed to obtain a stable RuO₂–Ta₂O₅/Ti electrode at various temperatures correlates well with the Arrhenius’ law: with the activation energy (E) of the annealing reactions for the electrodes being estimated as 73.5 kJ mol⁻¹. SEM images of the electrodes show the coating films to have rough surface morphology with cracks 2–6 μm in width. XRD data indicate that the coating films obtained are composed of crystalline RuO₂ and amorphous tantalum oxide.