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The boulders formed by cutting the structural surface of the hard rock tunnel may cause instability and disaster during excavation. The Universal Distinct Element Code numerical simulation was carried out based on 64 sets of orthogonal tests for the surrounding rock of the stratified and dislocated block fracture structure. Seven types of collapse modes were obtained for the stratified dislocated crack block structure. Some factors affecting the collapse height were quantitatively analyzed using the range analysis method. Also, six factors influencing the collapse height of a tunnel surrounding rock were ranked. In addition, a collapse height prediction model with six factors was developed for the surrounding rock around the tunnel. In order to analyze the collapse mechanism of the rock mass from the tunnel crack block, six sets of model tests were carried out for a rock mass with block crack stratified dislocation structure. Based on the optical flow method, the progressive evolution process of the rock mass instability induced by tunnel construction was studied, and the characteristics of sequence instability and the influence mechanism of multi-factor coupling of the block group were revealed. The simulation results were verified with the above six groups of laboratory simulation tests, and it was found that the accuracy of the prediction results was higher than 85.21%. It is expected that the results of the present research will provide a scientific basis for predicting, preventing, and controlling the collapse of the surrounding rocks during the construction of tunnels and underground caverns in the block fissure area. [ABSTRACT FROM AUTHOR]

Gas convection transport in porous media plays a pivotal role in various engineering and natural systems, such as oil and gas reservoir behavior and carbon dioxide sequestration. In response, this paper presents a novel peridynamics model for pressure-driven gas convection transport in porous media. By peridynamics, we mean a non-local continuum mechanics theory that accounts for interactions within a finite distance, allowing for the modeling of discontinuities and complex material behavior without relying on classical spatial derivatives. The proposed peridynamics model intends to provide a comprehensive account for simulating gas convection in porous media by incorporating key factors such as the Klinkenberg effect, thermal-flow coupling, and heterogeneous materials. The effectiveness, accuracy, and versatility of the proposed peridynamics approach are demonstrated by numerical results from benchmark examples and complex simulation scenarios. The validity and reliability of this peridynamics model are confirmed under various conditions through convergence studies, sensitivity analyses, and comparisons with finite element method results. Conclusions drawn from the validation studies are that the proposed framework is capable of addressing practical issues such as the prediction of pore pressure in high-temperature concrete and that the proposed methodology is accurate, stable, convergent, and a promising alternative to traditional methods. [ABSTRACT FROM AUTHOR]

Various periodic motions are investigated for a mass-spring-damper oscillator described by second-order discontinuous delayed systems with a parabolic boundary. The G -function related to a delayed variable is first developed to judge the crossing, sliding, and grazing motions at the boundary points. Then, basic and local mappings, together with their motion equations, are classified by current and delayed states of a local flow. It is shown that periodic motions can be predicted analytically in terms of the return mapping and the initial value function provided posteriorly. Finally, numerical simulations are given to illustrate the existence of slowly oscillating periodic solutions without or with sliding portions as well as a crossing periodic solution not slowly oscillating. These results not only enrich and extend switchability theory of discontinuous delayed systems, but also reveal some complicated periodic behaviors which cannot be found in continuous delayed systems and discontinuous systems without time delay. [ABSTRACT FROM AUTHOR]

This study proposes a new fractal permeability model for fractured rocks that comprehensively accounts for the geometric fracture characteristics and the fluid transport mechanism. Then, the permeability changes of fractured rocks are analyzed using discrete fracture networks (DFNs) with or without roughness and different geometry parameters in the DFN modeling and finite element simulation. The results show that the proposed permeability model well agrees with the experimental data, and the established DFN numerical model more realistically reflects the fracture network in fractured rocks. Fluctuation of tortuous fracture lines (rough fractures) increases the fracture intersection probability, consequently increasing the fracture intersection area or connecting adjacent fractures. Moreover, permeability increases with the fractal dimension Df, porosity ϕ, maximum fracture length lmax, and proportionality coefficient β, and it decreases with increasing fractal dimension DTf of fracture tortuosity. When the fracture proportionality coefficient is 0.001 ≤ β ≤ 0.01, different DFNs yield similar simulation results for permeability. However, with increasing fracture network complexity, the predictive model created using conventional DFN (C-DFN) increasingly overestimates the fractured rock permeability. Thus, building a permeability model for a fractured rock using rough DFN (R-DFN) is more effective than that using C-DFN. Our findings are helpful for real permeability predictions via DFN and analytical modeling. [ABSTRACT FROM AUTHOR]

We applied the first-principles method with empirical Hubbard U parameters to study the magnetism of Zn0.75Cr0.25S with the varied doping configurations. Depending on the doping configurations, calculations using generalized gradient approximation (GGA) methods with and without U were compared to study the electronic structure and magnetic properties of Zn0.75Cr0.25S. Simulation with GGA method confirms the presence of ferromagnetic ordering in Zn0.75Cr0.25S as is theoretically concluded by previous literature work, while calculations with GGA+U methods predict either antiferromagnetic or ferromagnetic ordering depending on the doping configurations. The appearance of magnetic ordering is rationalized in terms of the coupling between two adjacent doping Cr atoms that primarily governs the magnetic ordering. Therefore, changing the coupling chains in the Cr-containing semiconductors may provide an alternative route to tune the magnetic properties. [ABSTRACT FROM AUTHOR]

Proton uptake in undoped and Y-doped BaPrO3 has been measured by cold neutron prompt-gamma activation analysis, and compared to the proton uptake in Gd-doped BaCeO3, as determined by the same technique. The conventional proton incorporation model of perovskites in which oxygen ion vacancies, generated by the introduction of the trivalent dopant onto the tetravalent perovskite site, are filled with hydroxyl groups upon exposure of the sample to H2O containing atmospheres, predicts that the proton concentration in such materials should be no greater than the dopant concentration. In contradiction to this model, the proton concentration in BaPr1-xYxO3 after humidification is as much as three times greater than the dopant concentration, and even undoped samples contain a high concentration of protons. Moreover, x-ray photoemission spectra suggest that the Pr oxidation state is lowered upon hydration. In contrast, BaCe0.9Y0.1O3 shows a typical hydrogen concentration, of close to 90% of the yttrium concentration. The results are interpreted in terms of the variable valence of Pr, which can become reduced from the 4+ to the 3+ oxidation state upon exposure to water, and effectively behaves as a self-dopant within the structure. [ABSTRACT FROM AUTHOR]

In this study, lauric acid was introduced in sodium carboxymethyl cellulose via esterification reaction in dimethylacetamide in the presence of polylactic acid (PLA) as the supporting material, affording lauric acid/carboxymethyl cellulose ester (LACCE) phase change materials. The resulting LACCE phase change materials were characterized by FTIR, 1H-nuclear magnetic resonance, and X-ray diffraction analyses. The mechanical testing studies showed that the optimal dosage of LACCE to blend with PLA was 15%. The tubular structure of cellulose and the microstructure of the grafted LACCE were obtained by scanning electron microscopy. After the normalized value, the phase change latent heat of LACCE and LACCE-PLA was 153.06 and 86.40 J/g, respectively, and the phase change temperature was 39.62 and 40.10 °C by differential scanning calorimetry analysis. The thermal gravimetric analysis results revealed that PLA enhanced the thermal stability of LACCE and improved the initial thermal degradation temperature by 30.43%, indicating that the composites had good thermal properties. [ABSTRACT FROM AUTHOR]

This work explores size-induced lattice modification and its relevance to photoluminescence properties of scheelite nanostructures. CaWO4 nanocrystals, a prototype scheelite compound, exhibited a lattice expansion and an increased symmetry of structural units with physical dimension reduction, which is in contradiction to the trend previously reported in bulk CaWO4 at high pressures or high temperatures. Lattice variations in CaWO4 nanocrystals are probably due to the “negative pressures” that originated from strong defect dipole interactions on surfaces. The increased structural symmetry along with surface citric modifications produced a significant enhancement in photoluminescence of CaWO4 nanocrystals, indicating a quantitatively structural control over the electronic properties. [ABSTRACT FROM AUTHOR]

The nature of room-temperature ferromagnetism in diluted magnetic semiconductors, ZnO:M (M is Ni, Co, Fe, etc.), remains in debate, most likely because previous theoretical and experimental studies have excluded the magnetic contribution from secondary phases of transition metal oxide nanocrystals including NiO and CoO. In this work, the authors initiated a study on NiO nanocrystals that demonstrated room-temperature ferromagnetic behavior with relatively large coercive forces, in apparent contradiction to the previous conjectures in the literature. With size reduction, NiO nanocrystals showed an abnormal magnetic crossover, which is closely related to weakened superexchange interactions in their multisublattices. [ABSTRACT FROM AUTHOR]

Lattice variations and bonding characteristics in rutile TiO2 nanocrystals were examined by x-ray diffraction and x-ray photoelectron spectroscopy. With a reduction in the physical dimensions, rutile TiO2 nanocrystals show a linear lattice expansion and an anomalous covalency enhancement in apparent contradiction to the ionicity increase in BaTiO3 and CuO nanocrystals as reported recently by S. Tsunekawa et al. [Phys. Rev. Lett. 2000, 85, 3440] and V. R. Palkar et al. [Phys. Rev. B 1996, 53, 2167]. A surface defect dipole model is proposed to explain these physical phenomena in terms of the strong interactions among the surface dipoles that produce an increased negative pressure. The covalency enhancement is interpreted according to the critical properties of the increased Ti[Single_Bond]O bond lengths in the expanded lattice. [ABSTRACT FROM AUTHOR]

Niobate perovskite oxides of KNb[SUB1-x]Mg[SUBx]O[SUB3-δ] were synthesized directly at high temperature and pressure. X-ray diffraction confirmed the formation of single-phase orthorhombic structures. High-temperature Raman spectroscopy in combination with thermal analysis showed that phase transitions occurred from orthorhombic to tetragonal, to pseudo cubic, and finally to cubic, in sequence. Addition of Mg[SUP2+] at the Nb[SUP5+] sites of perovskite KNbO[SUB3] lattice led to a dramatic enhancement of oxide-ion conduction. This enhancement in oxide-ion conduction was through the suppression of the contribution from p-type holes, which shows that these high-temperature phases can be promising candidates as conductive materials. [ABSTRACT FROM AUTHOR]

Pr0.987Na0.013CoO3 exhibits an anomaly in magnetic susceptibility and transport behavior around 200 K, which is associated with a thermally excited spin transition of Co3+. We show that introduction of Na+ in PrCoO3 induced a variable range hopping (VRH) conduction. At the transition temperature, the density of state near Fermi level increased, while the hopping distance and hopping energy decreased. Extrinsic electrode polarization was also observed at low frequency, leading to a giant dielectric constant. Both conductivity and dielectric relaxation frequency in bulk follow a simple temperature dependence, T-1/4, indicating a close relationship between VRH conduction and dielectric relaxation. [ABSTRACT FROM AUTHOR]

Anticipating chaotic synchronization based parameter estimation of chaotic system is investigated. We propose a method to estimate the unknown parameters of the interested chaotic system even if only a scalar time series is available. Although the Krasovskii-Lyapunov functional method often results in delay-independent stability condition, stability of the synchronization manifold usually relates to time delay. We analyze the stability of anticipating synchronization based parameter estimation numerically. Series of driven systems are used to increase the anticipation time, however, result in longer time to estimate the unknown parameters. These results are also confirmed by numerical simulations., ((c) 2010 American Institute of Physics.)