Comparison of ductile material erosion models combining cutting and deformation components of wear

ABSTRACT

Herein we compare seven models of erosion of ductile materials, considering various combinations including cutting and deformation components of wear. The models were fit to three independent sets of experimental results to verify the quality of fitting based upon the coefficient of determination. The results were discussed especially in terms of the friction component, the adjustment of which could lead to better fitting.     

Thermoresistive and thermoelectric properties of coplanar cellulose-MWCNTs buckypaper

Thermoresistive sensors are based on the change in electrical resistance with temperature variation, are easily read, and have a simple design but require external power for their operation. Thermoelectric devices (TDs) based on the Seebeck effect directly convert heat into electrical power without any moving parts, generating voltages from the temperature difference established between the ends of a solid-state material. In recent years, several thermoresistors and TDs have been manufactured with conductive films based on carbon nanotubes (CNTs), i.e., with buckypaper (BP), because they provide lightweight, flexible, and sensitive devices. Nevertheless, the electrical resistance and thermoelectric properties of CNTs are affected when they are randomly assembled to form a BP. Then, this study investigated the thermoresistive and thermoelectric properties of a coplanar BP with an active area of 1.0 cm². Morphological characterization was performed by scanning electron microscopy and showed bundles of multiwalled CNTs agglomerated on the surface but also impregnated into cellulose fibers. BP-based thermoresistive sensor had a maximum sensitivity of ??10.05% at 322 K. Moreover, the thermoelectric configuration presented a maximum thermovoltage and thermoelectric power of ??1.2 mV and ??0.09 mV/K, respectively. These results suggest that this coplanar BP can be easily applied in thermal sensors and thermoelectric device concepts.

     

Water Content in Self-Etching Primers Affects Their Aggressiveness and Strength of Bonding to Ground Enamel

In this study the influence of water concentration in self-etching (SE) primers on their aggressiveness and bond strength to ground enamel was investigated. Five experimental primers with 0, 5, 10, 20, or 40 wt% of water were formulated, using Clearfil SE Bond® (CSEB) as a commercial reference. Primers were applied to bovine incisors following application of bonding resin and composite restoration. Bond strength was evaluated through shear testing and data were statistically analyzed (5%). The etching aggressiveness was evaluated by SEM. Water concentration significantly affected bond strength and etching aggressiveness. The highest bond strength value (MPa) was found for the primer containing 20% of water (24.7), followed by CSEB (23.8). Groups containing 5% (20.5) and 10% (20.2) of water showed an intermediate performance, while the lowest values were observed for the 0% (17.3) and 40% (16.7) groups. SEM analysis showed increasing aggressiveness for increased water content.     

Transport model of controlled molecular rectifier showing unusual negative differential resistance effect

We investigate theoretically the charge accumulated Q in a three-terminal molecular device in the presence of an external electric field. Our approach is based on ab initio Hartree-Fock and density functional theory methodology contained in Gaussian package. Our main finding is a negative differential resistance (NDR) in the charge Q as a function of an external electric field. To explain this NDR effect we apply a phenomenological capacitive model based on a quite general system composed of many localized levels (that can be LUMOs of a molecule) coupled to source and drain. The capacitance accounts for charging effects that can result in Coulomb blockade (CB) in the transport. We show that this CB effect gives rise to a NDR for a suitable set of phenomenological parameters, like tunneling rates and charging energies. The NDR profile obtained in both ab initio and phenomenological methodologies are in close agreement.     

Antioxidant properties of rice grains with light brown,red and black pericarp colors and the effect of processing

The present work evaluated the concentration of total soluble phenolic compounds (TSPCs) and antioxidant activity (AOA) of rice grains with light brown, red and black pericarp colors and the processing effect on the concentration of TSPCs in the grain. Brown rice grains of ten ecotypes and six cultivars with red pericarp, one with a black pericarp and one with a light brown pericarp color, were evaluated. The concentration of TSPCs and AOA of rice grains with different processing (brown, polished, parboiled brown and parboiled polished) was determined, and the concentration of TSPCs was also determined in raw and cooked grains. Significant differences were observed in the concentrations of TSPCs and AOAs among genotypes with higher values obtained for grains with red and black pericarp colors, and a positive and significant correlation between these parameters was found. Parboiling reduced the TSPCs concentration in the grains due to the loss of part of them in the processing water, thermal decomposition and, possibly, interaction with other components. This reduction is related to the lower AOA in these grains. In a similar way, cooking also reduced the concentration of TSPCs, especially in brown and polished grains.     

Modeling Grain Boundary Motion and Dynamic Recrystallization in Pure Metals

The current study proposes a new approach of modeling discontinuous dynamic recrystallization in pure copper and cobalt based on the inverse analysis of experimental data. This approach comprises two steps: First, the mobility of grain boundaries is determined by a mean-field model in the steady state regime, then in a second step the information collected (mobility, nucleation frequency) is used to determine the mechanical behavior and the grain size change. The nucleation criterion is reformulated in a more general expression, and a new expression of the nucleation frequency with a single empirical parameter is proposed. The model predicts the stress–strain curves and the evolution of mean grain size, and is in good agreement with experimental data for both copper and cobalt. The modeling procedure requires a minimum of initial material parameters and could be especially attractive in the case of complex metals and alloys for which these parameters are unknown.     

Channel allocation algorithms for WLANs using distributed optimization

The performance of a wireless local area network (WLAN) depends on the channel assignments among interfering access points (APs). Due to the limited number of non-overlapping channels, severe interference scenarios may arise if no appropriated spectrum planning is employed. In our study we focus on WLANs scenarios where APs may belong to different administrative domains, which is actually a very common situation in dense urban deployments. In such cases the use of centralized algorithms is not feasible and the already proposed distributed methods does not guarantee optimal channel assignment. In this paper, therefore, we formalize the channel allocation as a distributed constraint optimization problem (DCOP) and propose a new distributed channel assignment algorithm named DCAA-O, which can find the optimal solution to the channel assignment problem for a group of APs. A suboptimal strategy denoted DCAA-S is also investigated, which aims at reducing the number of control messages to be exchanged between APs, while still achieving a suboptimal solution which is very close to the optimal one. The simulation results show that the proposed algorithms are able to outperform the best known techniques both in terms of solution quality and number of exchanged messages.     

A sociologically inspired heuristic for optimization algorithms: A case study on ant systems

This paper discusses how social network theory can provide optimization algorithms with social heuristics. The foundations of this approach were used in the SAnt-Q (Social Ant-Q) algorithm, which combines theory from different fields to build social structures for state-space search, in terms of the ways that interactions between states occur and reinforcements are generated. Social measures are therefore used as a heuristic to guide exploration and approximation processes. Trial and error optimization techniques are based on reinforcements and are often used to improve behavior and coordination between individuals in a multi-agent system, although without guarantees of convergence in the short term. Experiments show that identifying different social behavior within the social structure that incorporates patterns of occurrence between states explored helps to improve ant coordination and optimization process within Ant-Q and SAnt-Q, giving better results that are statistically significant.