A thermodynamic framework to model thixotropic materials

Thixotropic materials are widely used in a variety of industrial applications. The constitutive relations to describe these materials are based on one-dimensional experiments in which the material is subjected to a shear motion and there is no unique methodology to obtain proper three-dimensional models. The path towards generalization to a three-dimensional framework is invariably carried out in a ad hoc manner. Here we propose a three-dimensional model that stems from a general thermodynamic framework that has proved to be quite robust in the development of constitutive relations, namely the application of the second law of thermodynamics together with the maximization of the entropy production. This leads to a constitutive equation that has the same form of a generalized Upper Convected Maxwell equation, if we require that changes of microstructure due to the deformation of each Maxwell element that comprises the model are reversible. Changes in microstructure are governed by a potential that is a measure of the difference between the current structure and the equilibrium structure associated with it. The equilibrium structure associated with the current structure is determined by the current value of stress, considered the main break up agent. We assume that the state of equilibrium would be achieved in a Motion With Constant Stress History, starting from the current stress state, until a steady state where the kinematics is not changing.     

Thin-film capacitance models

Models for distributed capacitance in a thin film are derived in the form of a system of local RC diffusion equations coupled by a global elliptic equation. Such models contain the local geometry of the distributed capacitance on which charge pis stored and the exchange of current flux on its interface with the medium. Certain singular limits are characterized, and the resulting degenerate initial-boundary-value problems are shown to be well posed.     

Elongated hexagonal ZnO micro-fence optical resonator

A highly symmetric ZnO micro-fence is prepared to form an optical resonator, which a symmetric array consisting of six elongated hexagonal microcolumns. It is found that the luminescence emission could be enhanced at the edge of ZnO micro-fence and the wavelength dependent light intensity could be influenced by its geometry. The optical characterization along with theoretical calculations and computer simulation analysis suggest that the Fabry-Perot and Cross-whispering gallery modes are generated in ZnO micro-fence. The individual ZnO micro-fence can regarded as an optical resonator structure. Furthermore, it is proposed that the source concentration around the substrate during the growth is a crucial factor for forming such a micro-fence.     

Enhanced mechanical properties of unidirectional basalt fiber/epoxy composites using silane-modified Na+-montmorillonite nanoclay

This study explores the effects of 3-glycidoxypropyltrimethoxysilane (3-GPTS) modified Na-montmorillonite (Na-Mt) nanoclay addition on mechanical response of unidirectional basalt fiber (UD-BF)/epoxy composite laminates under tensile, flexural and compressive loadings. Fourier transform infrared (FT-IR), X-ray diffraction (XRD) and simultaneous thermal analysis (STA) data confirmed the reaction mechanism between the silane compound and Mt. It was demonstrated that addition of 5 wt % 3-GPTS/Mt resulted in 28%, 11% and 35% increase in flexural, tensile and compressive strengths. Scanning electron microscopy (SEM) clarified the improvement in the adhesion between the basalt fibers and matrix in the case of Mt-enhanced epoxy specimens. Also, a theoretical route based on a Euler-Bernoulli beam-based approach was employed to estimate the compressive properties of the composites. The results demonstrated good agreement between theoretical and experimental approaches. Totally, the results of the study show that matrix modification is an effective strategy to improve the mechanical behavior of fibrous composites.     

Tailoring the properties of electrospun PHBV mats: Co-solution blending and selective removal of PEO

Microstructure, surface topography, thermal and mechanical features of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) electrospun non-woven mats were modified, modulated and tailored through blending with different polyethylene oxide (PEO) amounts (20, 30 and 50% wt/wt). The optimal parameters of the soaking protocol for the selective removal of the sacrificial polymer were accurately identified by means of scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), Fourier transform infrared attenuated total reflectance (FTIR-ATR) spectroscopy, simultaneous thermogravimetric and differential analyses (TG-DTA) and differential scanning calorimetry (DSC). The complete PEO removal after soaking in H₂O for 7 days with daily refreshment was confirmed. The resulting samples were only comprised of PHBV fibers characterized by a remarkable decrease of the average size with respect to the respective blends. Their surface topography was corrugated and rough and presented nodules, pits, nanopores, shallow and elongated nanostructured indents/grooves along the fiber axis. A remarkable reduction (>75%) of the tensile modulus (E) of electrospun PHBV mats (15–20 MPa) was obtained, maintaining comparable elongation at break (ε_max) values (20–30%).     

Zr掺杂对MgB2超导微观结构和超导电性的影响

将Mg粉、Zr粉和B粉按比例混合获得Mg1-xZrxB2(x=5%10%和20%),压制成型后,在流通氩气的条件下于800℃烧结2h.利用X射线衍射仪(XRD)、扫描电子显微镜(SEM)分析烧结后样品的显微结构和化学组成,采用差热分析(DTA)观察Zr掺杂对MgB2分解温度的影响,并用物性测试仪(PPMS)测试样品的超...     

Rapid reduction of titanium dioxide nano-particles by reduction with a calcium reductant

Micro-, submicron-, and nano-scale titanium dioxide particles were reduced by reduction with a metallic calcium reductant in calcium chloride molten salt at 1173 K, and the reduction mechanism of the oxides by the calcium reductant was explored. These oxide particles, metallic calcium as a reducing agent, and calcium chloride as a molten salt were placed in a titanium crucible and heated under an argon atmosphere. Titanium dioxide was reduced to metallic titanium through a calcium titanate and lower titanium oxide, and the materials were sintered together to form a micro-porous titanium structure in molten salt at high temperature. The reduction rate of titanium dioxide was observed to increase with decreasing particle size; accordingly, the residual oxygen content in the reduced titanium decreases. The obtained micro-porous titanium appeared dark gray in color because of its low surface reflection. Micro-porous metallic titanium with a low oxygen content (0.42 wt%) and a large surface area (1.794 m² g⁻¹) can be successfully obtained by reduction under optimal conditions.     

Superhydrophobic nickel films fabricated by electro and electroless deposition

In this work, a superhydrophobic nickel surface is fabricated by coupling electro and electroless deposition without chemical modification. SEM study reveals that electrodeposited nickel surface is characterized by nanocone arrays and has a contact angle of about 135°. After adding electroless deposition, as the second step, hemispherically topped nickel nanocone arrays are formed which leads to a high contact angle of 153.6°. That is, nickel surface has successfully transformed from hydrophobic to superhydrophobic. This transition is investigated both from the aspects of chemical composition and surface structure and proves the latter is the dominant factor. The present study inspires us to do more research about the creation of rough surfaces and enriches our comprehension about superhydrophobicity.     

Acid-induced gelation behavior of sonicated casein solutions

Casein gels were made from solutions sonicated by 24 and 130 kHz ultrasounds for 0, 60 and 120 min, followed by acidification with glucono-δ-lactone at 30 °C. The dynamics of gel formation were studied using rheological methods and microstructure of gels was monitored using scanning electron microscopy. Sonication postponed the gelation point to a lower pH value and increased the elasticity of freshly formed gels. It also resulted in gels with a more interconnected structure and smaller non-distinguishable particulates. This structure was especially dominant for the gel made from the solution already sonicated for 120 min.     

Low-phosporous nickel-coated carbon microcoils: Controlling microstructure through an electroless plating process

Carbon microcoils (CMCs) have been coated with a nickel-phosphorus (Ni-P) film using an electroless plating process, with sodium hypophosphite as a reducing agent in an alkaline bath. CMC composites have potential applications as microwave absorption materials. The morphology, elemental composition and phases in the coating layer of the CMCs and Ni-coated CMCs were investigated by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD), respectively. The effects of process parameters such as pH, temperature and coating time of the plating bath on the phosphorus content and deposition rate of the electroless Ni-P coating were studied. The results revealed that a continuous, uniform and low-phosphorous nickel coating was deposited on the surface of the CMCs for 20 min at pH 9.0, plating bath temperature 70 °C. The as-deposited coatings with approximately 4.5 wt.% phosphorus were found to consist of a mix of nano- and microcrystalline phases. The mean particle size of Ni-P nanoparticles on the outer surface of the CMCs was around 11.9 nm. The deposition rate was found to moderately increase with increasing pH, whereas, the phosphorous content of the deposit exhibited a significant decrease. Moreover, the material of the coating underwent a phase transition between an amorphous and a crystalline structure. The thickness of the deposit and the deposition rate may be controlled through careful variation of the coating time and plating bath temperature.