Bayesian estimation of the Gaussian mixture GARCH model

Bayesian inference and prediction for a generalized autoregressive conditional heteroskedastic (GARCH) model where the innovations are assumed to follow a mixture of two Gaussian distributions is performed. The mixture GARCH model can capture the patterns usually exhibited by many financial time series such as volatility clustering, large kurtosis and extreme observations. A Griddy–Gibbs sampler implementation is proposed for parameter estimation and volatility prediction. Bayesian prediction of the Value at Risk is also addressed providing point estimates and predictive intervals. The method is illustrated using the Swiss Market Index

Using network science to analyze football passing networks: dynamics, space, time and the multilayer nature of the game

From the diversity of applications of Network Science, in this Opinion Paper we are concerned about its potential to analyze one of the most extended group sports: Football (soccer in U.S. terminology). As we will see, Network Science allows addressing different aspects of the team organization and performance not captured by classical analyses based on the performance of individual players. The reason behind relies on the complex nature of the game, which, paraphrasing the foundational paradigm of complexity sciences "can not be analyzed by looking at its components (i.e., players) individually but, on the contrary, considering the system as a whole" or, in the classical words of after-match interviews "it's not just me, it's the team".Comment: 7 pages, 1 figur

Influence of boron content on the fracture toughness and fatigue crack propagation kinetics of bainitic steels

The relatively good combination of high strength and ductility makes bainitic steels a candidate to replace many other steels in industrial applications. However, in service, ductility and strength are not up to standard requirements. In many industrial components, toughness and fatigue performance are also very relevant. In the present study, bainitic steels with varying content of boron were fabricated, with the aim of analyzing the fracture toughness and changes in the fatigue life. The results show that a relatively small change in the boron content can cause a notable variation in the fracture toughness of bainitic steels. The maximum value obtained in fracture toughness was for the steel with the highest boron content. It was observed that the amount of interlath martensite constituents decreases in steels with the addition of boron, leading to the promotion of the presence of void coalescence and a remarkable rise in the toughness of bainitic steels. An increase on the fatigue life of the bainitic steels with an increase in the boron content was also observed, through analysis by means of Paris’ law. A comprehensive micrographic study was carried out in order to examine the mechanics of fatigue crack growth in the bainitic steels, revealing small longitudinal cracks in bainitic steels that lack boron. These cracks tend to disappear in bainitic steels that contain boron. To elucidate this behavior, micrographs of the surfaces generated by the crack growth process were taken, showing that several nano-cracks appeared between the bainite laths. It is finally argued that this high-energy consumption process of nano-crack nucleation and growth is the reason for the improved toughness and fatigue life observed in bainitic steels.Peer ReviewedPostprint (author's final draft

Al/Fe-, Al/Cu- and Al/(Fe-Cu)-pillared clays: Structural features at low Atomic Active Metal Ratios (AMR)

A set of Al/Fe-, Al/Cu- and Al/(Fe-Cu)-pillared clays were prepared from a Colombian bentonite in sodium or natural (Ca/Mg) form as starting materials. The effect of the loading of the second metal(s) (Fe and/or Cu) related to Al (AMR) in the intercalating solutions at low range of values ≤ 10%, on the main physicochemical properties was investigated. The insertion of the active metals rose almost linearly with AMR for both binary, Al/Fe- and Al/Cu- mixed metal systems. For the three-metal Al/(Fe-Cu)- mixed system, the stabilization of copper behaved randomly as a function of AMR, while the stabilization of Fe was almost not affected by the presence of Cu. On the basis of the ionic radii of the metals involved, a possible competition of the two active metals for the octahedral sites available in the Al13 polycationic structure is proposed for the three-metal system (ionic radii values: Al3+ = 0.68 Å; Fe3+ = 0.67 Å; Cu2+ = 0.73 Å), but also as a good explanation for the efficiency of stabilization for iron around 15 times higher than copper in the binary metal systems. A clear dependence of the compensation of the cationic exchange capacity by polycations with the AMR value was found only for the system Al/Fe-, which was explained in terms of the opposite changes in the final pH obtained in the intercalating solutions, induced by the active metals in the Keggin-like polycations. The change in the basal spacings obtained in the XRD patterns from oriented glasses as a function of AMR was interpreted in terms of the opposite effect expected on the molecular size of the polycations as higher amounts of Fe or Cu would isomorphically substitute Al into the Al13 framework, allowing to propose that both, iron and copper are able to carry out such a substitution, although in different extent. The hydrogen consumption in the H2-TPR analysis of the materials was mainly distributed between two broad peaks: one from about 250ºC to 580ºC, and a second one from about 750ºC to 950ºC. Despite the several metal oxide species that are possible to form, the first signal in samples modified with the Al/Fe- system may correspond with the sequential overlapped reduction of Fe2O3 in up to three steps: (1) Fe2O3 → Fe3O4; (2) Fe3O4 → FeO; and (3) FeO → Fe, and to two sequential reduction effects in the range of 210-260 ºC for Cu2+→ Cu+ and Cu+→ Cu0 at higher temperatures of ca. 400 ºC to 700 ºC. In the Al/Fe- system two shoulders were observed at around 600ºC and 850ºC, whose intensity grow as the active metal loading increases, seem to be related to two kinds of metal sites repeatedly claimed in the literature as responsible for the high catalytic activity displayed by these materials in Fenton-like reactions; Fe “decorating” Al pillars and true mixed Al/Fe pillars, respectively. The textural analysis of the natural modified samples suggest that at AMR below 5.0 %, a discrete maximum exist for the systems Al/Fe- and Al/Cu- over which the sorption capacity starts to be significantly affected. Simultaneous thermal analysis (DSC/TGA) demonstrated that the addition of a second metal in the intercalating system enhanced more the fraction of physisorbed water and decreased the maximum temperature for such a thermal event than intercalation with simple Al- polycations. SEM micrographs showed significant changes in morphology of the particles as AMR increased from 2.0 to 5.0%; as the amount of iron and/or copper added to the system increased, a greater amount of tiny particles deposited on the clay surface was seen, may be corresponding with external oxide aggregates. Likewise, the ratio Fe/Al obtained from the semi-quantitative surface chemical EDX analysis of the samples modified with the system Al/Fe- indicated that the AMR increasing also leads to higher fraction of the iron inserted represented in intercalated, possibly less aggregated mixed species in the materials. Acknowledgements: Financial support from MEC and FEDER funds (Ref. MAT2007-66439-C02)

Modulating the water behavior, microstructure, and viscoelasticity of plasma-derived hydrogels by adding silica nanoparticles with tailored chemical and colloidal properties

The viscoelastic properties of hydrogels depend on the tridimensional polymeric structure and the behavior of the liquid confined in their pores. The objective here is to modulate these characteristics in plasma-derived hydrogels by the addition of glycidoxypropyl-silica nanoparticles. These nanoparticles exhibited a hydrodynamic average size between 105.4 − 151.0 nm and surface coverage with (3-Glycidoxypropyl) trimethoxysilane of 0–96 %. The reinforced hydrogels are porous networks with spherical nanoparticles homogeneously distributed into their walls. The silanol groups of silica increase four-fold humidity retention compared with the native hydrogel. This correlates with bound water &gt; 45 % on these reinforced hydrogels, in contrast with 75 % of free water on the native one (calculated from DSC in frozen hydrogels). The humidity stability can be also achieved in the hydrogel prepared with nanoparticles exhibiting 96 % organic coverage. Furthermore, this organic content promotes the microstructure chemical crosslinking, resulting in 3.9 and 1.6 higher Young's modulus compared with native and silica-reinforced hydrogels, respectively. The presence of glycidoxypropyl-silica nanoparticles in reinforced hydrogels modulated its viscoelasticity behavior, decreasing stress relaxation, which was explained using the generalized Maxwell-Wiechert model. In conclusion, novel organic-inorganic hybrid hydrogels based on plasma-derived ones and glycidoxypropyl-silica nanoparticles were developed. These nanoparticles are versatile and allow the production of hydrogels with improved viscoelastic behavior that also exhibits high water retention and morphological stability.</p