A nonlinear Hermitian transfinite element method for transient behavior analysis of hollow functionally graded cylinders with temperature-dependent materials under thermo-mechanical loads

In the present paper, an algorithm for nonlinear transient behavior analysis of thick functionally graded cylindrical vessels or pipes with temperature-dependent material properties under thermo-mechanical loads is presented. In contrast to researches presented so far, a Hermitian transfinite element method is proposed to improve the accuracy and to prevent artificial interference or cohesion formation at the mutual boundaries of the elements. Time variations of the temperatures, displacements, and stresses are obtained through a numerical Laplace inversion. Another novelty of the present research is using the transfinite element method to solve nonlinear problems. A sensitivity analysis includes investigating effects of the volume fraction index, dimensions, and temperature-dependency of the material properties is performed. Results confirm the efficiency of the present algorithm and reveal the significant effects of the temperature-dependency of the material properties and the elastic wave reflections and interferences on the responses. In comparison to other techniques, the present technique may be used to obtain relatively accurate and stable results in a less computational time.     

Magnetothermoelastic interactions in hollow structures of functionally graded material subjected to mechanical loads

This paper considers the magnetothermoelastic problem of functionally graded material (FGM) hollow structures subjected to mechanical loads. Exact solutions for stresses and perturbations of the magnetic field vector in FGM hollow cylinders and FGM hollow spheres are determined using the infinitesimal theory of magnetothermoelasticity. The material stiffness, thermal expansion coefficient and magnetic permeability are assumed to obey the same simple power-law variation through the structures’ wall thickness. The aim of this research is to understand the effect of composition on magneto- thermoelastic stresses and to design optimum FGM hollow cylinders and hollow spheres.     

Development of new analytical solutions for elastic thermal stress components in a hollow cylinder under sinusoidal transient thermal loading

For the analysis of high-cycle thermal fatigue due to striping (such as has been observed due to turbulence at mixing tees of class 1–2–3 piping of nuclear power reactors) it can be necessary to consider the time-dependent temperature gradient within the pipe wall thickness rather than just at the surface. To address this, a set of analytical solutions with several new features has been developed for the temperature field and the associated elastic thermal stress distributions for a hollow circular cylinder subjected to sinusoidal transient thermal loading at the inner surface. The approach uses a finite Hankel transform and some properties of Bessel functions. The analytical predictions have been successfully benchmarked by comparison with results from finite element analysis, and also with some results of independent studies.     

A review on recent advances in hollow spheres for hydrogen storage

Recent advancements in synthesizing materials potential for hydrogen storage have greatly forced the hydrogen storage technology ahead in recent years. Hollow spheres, with unparalleled characteristics like low density and high specific surface area, have emerged as one of the most promising alternatives for hydrogen storage applications. In the present review, the main synthesis approaches of hollow spheres including spray drying, Kirkendall, template-free and, template-assisted methods are surveyed and concisely described. In addition, different types of hollow spheres such as hollow carbons, hollow glasses and other less-common types like Boron nitrides and metal hollow spheres have been tackled with special focus on adsorption/desorption capacities as well as the kinetic of hydrogen storage/release. In addition to the recent progresses, some perspective and outlook on the advancement of hollow spheres and challenges in terms of synthesis methods and hydrogen storage performance were presented.     

Electrocatalytic performance of Ba-doped TiO2 hollow spheres in water electrolysis

Barium doped titania hollow sphere catalysts of various compositions were prepared and their electrocatalytic performance was evaluated in acidic media. Titania hollow sphere particles were prepared using poly (styrene–methacrylic acid) latex as template material, and BaCl₂ was used precursor material during barium doping over spheres. The morphology and structure of Ba/TiO₂ hollow spheres were characterized by BET, XRD, TGA and TEM analysis. XRD results had confirmed the presence of rutile TiO₂ and BaTiO₃ phases in the catalysts. The catalysts have shown almost similar surface area values (around 97 m²/g) irrespective of their compositions. On the other hand, surface area values were diminished remarkably with rise in the calcination temperatures. In TEM images, hollow sphere formation with uniform layer of barium over the spheres could clearly be observed. Diameter of Ba-doped TiO₂ hollow sphere was found 0.4–0.5 μm irrespective of the variation in calcination temperatures. In cyclic voltammograms, both the hydrogen and oxygen evolution peaks were present for all the hollow sphere samples; although the peak positions had changed to some extent for few samples. Anodic polarization curves have shown 60 mA cm⁻² current density for 20 wt% Ba/TiO₂ hollow sphere sample. Tafel slope is revealed as 122 mV for the same electrocatalyst. Barium doped titania hollow spheres have also shown long time electrocatalytic stability in acid solution.     

Enhanced electrochemical performance of nickel hydroxide electrode with monolayer hollow spheres composed of nanoflakes

Nickel hydroxide electrodes with hollow spheres were fabricated using a PS (polystyrene) sphere template and electrochemical deposition. The nickel hydroxide grew perpendicular to the electrode substrate during anodic deposition and around the PS spheres during cathodic deposition. After the removal of the PS template, hollow spheres or open hollow spheres were formed via cathodic deposition. The nickel hydroxide electrode with hollow spheres and nanoflakes showed greatly enhanced electrochemical performance in alkaline solution compared with the bare nickel hydroxide electrode. The opening of the hollow spheres facilitated easy electrolyte transport to the reaction sites and led to a further increase in the specific capacitance of the nickel hydroxide electrode. The specific capacitance of the electrode with the open hollow spheres reached 800 F g⁻¹, which was much higher than that of the bare electrode (224 F g⁻¹) and the hollow-sphere electrode (342 F g⁻¹) at a discharge current density of 10 A g⁻¹.     

Effect of Initial Imperfections on Thermal Buckling of Functionally Graded Plates

ABSTRACT

Thermal buckling analysis of rectangular functionally graded plates with initial geometrical imperfections is presented in this article. The equilibrium, stability, and compatibility equations of an imperfect functionally graded plate are derived using the first-order shear deformation plate theory. It is assumed that the nonhomogeneous mechanical properties of the plate, graded through the thickness, are described by a power function of the thickness variable. The plate is assumed to be under three types of thermal loading, namely: uniform temperature rise, nonlinear temperature rise through the thickness, and axial temperature rise. Resulting equations are employed to obtain the closed-form solutions for the critical buckling temperature change of an imperfect functionally graded plate. The influence of transverse shear on thermal buckling load is discussed.     

Nanostructured photoelectrode consisting of TiO2 hollow spheres for non-volatile electrolyte-based dye-sensitized solar cells

A photoelectrode consisting of titania hollow spheres for dye-sensitized solar cells (DSSCs) is prepared by a paste method and the effect of the nanostructure on the performance of DSSCs with non-volatile electrolytes is investigated. The structure of the hollow sphere (HS) electrode with a large pore size and a high porosity allows highly viscous non-volatile electrolytes to penetrate the electrode thoroughly. Furthermore, its outstanding light-harvesting efficiency and long electron diffusion length make the efficiency of the DSSCs with the HS electrode comparable with those of a conventional nanocrystalline electrode, in spite of the smaller amount of the adsorbed dye, when oligomer electrolytes are used. The results show that the structure of a photoelectrode highly improves the performance of the device and the HS electrode is an effective structure for the use of non-volatile electrolytes in DSSCs.     

Thermal Buckling of Axially Functionally Graded Thin Cylindrical Shell

In the present research, thermal buckling of shell made of functionally graded material (FGM) under thermal loads is investigated. The material properties of functionally graded materials (FGMs) are assumed to be graded in the axial direction according to a simple power law distribution in terms of the volume fractions of the constituents. In the previous articles that published, these properties are assumed to be graded in the thickness direction. Nonlinear kinematic (strain-displacement) relations are considered based on the first order shear deformation shell theory. By substituting kinematic and stress-strain relations of functionally graded shell in the total potential energy equation and employing Euler equations, the equilibrium equations are obtained. Applying Euler equations to the second variation of total potential energy equation leads to the stability equations. Then, buckling analysis of functionally graded shell under three types of thermal loads is carried out resulting into closed-form solutions.     

Preparation and properties of proton exchange membranes based-on Nafion and phosphonic acid-functionalized hollow silica spheres

Vinyl functionalized hollow silica spheres (HSSs) were prepared via a template method and surface modification thereafter. Poly(vinylbenzyl phosphonic acid) (PVBPA) grafted HSSs (HPSSs) were prepared via emulsion polymerization of diisopropyl p-vinylbenzyl phosphonate (DIPVBP) on the surface of HSSs, and hydrolysis thereafter. The chemical structure and morphology of HPSSs were characterized by FTIR and TEM. A series of proton exchange membranes based-on Nafion^®212 and HPSSs were prepared via solution casting. The water uptake, swelling ratio, mechanical properties, thermal behavior, proton conductivity, and chemical oxidative stability of the composite membranes were investigated. The addition of HPSSs in Nafion^® membranes can improve the water retentivity of the composite membranes. The composite membranes with HPSSs exhibit higher water uptake and proton conductivity than that of the recast Nafion^® membranes. The water uptake and the proton conductivity of the composite membranes increase with increasing HPSSs loading. With the higher water retentivity, the membranes exhibit high proton conductivity at high temperature (1.6 × 10⁻¹ S cm⁻¹ at 125 °C).     

Transient stress analysis of sandwich plate and shell panels with functionally graded material core under thermal shock

A finite element formulation for stress analysis of functionally graded material (FGM) sandwich plates and shell panels under thermal shock is presented in this work. A higher-order layerwise theory in conjunction with Sanders’ approximation for shells is used to develop the finite element formulation for transient stress analysis of FGM sandwich panels. The top and the bottom surfaces of FGM sandwich panels are made of pure ceramic and metal, respectively, and core of the sandwich is assumed to be made of FGM. The temperature profile in the thickness direction of the panels is considered to be varying as per the Fourier’s law of heat conduction equation for unsteady state. The heat conduction equations are solved using the central difference method in conjunction with the Crank–Nicolson approach. Transient thermal displacements of the sandwich panels are obtained using Newmark average acceleration method and the transient thermal stresses are obtained using stress–strain relations, subsequently. Results obtained from the present layerwise finite element formulations are first validated with available solutions in literature. Parametric studies are taken up to study the effects of volume fraction index, temperature dependency of material properties, core thickness, panel configuration, geometric and thermal boundary conditions on transient thermal stresses of FGM sandwich plates and shells.     

CoO-loaded graphitable carbon hollow spheres as anode materials for lithium-ion battery

A new type of CoO nanoparticles encapsulated by graphitable hollow carbon sphere (GHCS) composite material was synthesized. The core–shell structure CoO/GHCS composite shows the improved cyclability as an anodic material in Li-ion battery. The core–shell composite containing 50 wt% CoO exhibits a reversible capacity of 584 mAh g⁻¹ at a constant current density of 100 mA g⁻¹ between 0 and 3.0 V (vs. Li⁺/Li), and remains a capacity retention of 95% after 50th cycle. The improvement could be attributed to that the GHCS with a good electronic conductivity and high surface severs as dispersing medium to prevent CoO nanoparticles from aggregating, and provide the enough space to buffer the volume change during the Li-ion insertion and extraction reactions in CoO nanoparticles.     

Enhanced room temperature hydrogen storage capacity of hollow nitrogen-containing carbon spheres

Novel hollow nitrogen-containing carbon spheres have been prepared by a simple CO₂ activation method at 1223 K using the precursor of ordered mesoporous carbon nitrides which were replicated from SBA-15 template. Rich narrow-distributed micropores have been found in the sphere shell. The structural transformation from solid to hollow sphere is correlated with the surface curvature stress and mass transportation. This material exhibits a high hydrogen storage capacity of 2.21 wt.% at room-temperature under 8 MPa.     

Thermal Buckling of Functionally Graded Plates Resting On Elastic Foundations Using the Trigonometric Theory

In this article, thermal buckling analysis of functionally graded material (FGM) plates resting on two-parameter Pasternak's foundations is investigated. Equilibrium and stability equations of FGM plates are derived based on the trigonometric shear deformation plate theory and includes the plate foundation interaction and thermal effects. The material properties vary according to a power law form through the thickness coordinate. The governing equations are solved analytically for a plate with simply supported boundary conditions and subjected to uniform temperature rise and gradient through the thickness. Resulting equations are employed to obtain the closed-form solution for the critical buckling load for each loading case. The influences of the plate aspect ratio, side-to-thickness ratio, gradient index, and elastic foundation stiffnesses on the buckling temperature difference are discussed.     

Transient thermal stress analysis of a functionally graded thick hollow cylinder with temperature-dependent material properties

This article deals with the study of temperature distribution and thermal stresses of a functionally graded thick hollow cylinder with temperature dependent material properties. All the material properties except Poisson’s ratio are assumed to be dependent on temperature and spatial coordinate z. The two-dimensional transient heat conduction equation is solved under convective heat transfer condition with varying point heat source. The influence of inhomogeneity parameters on the thermal and mechanical behavior is examined. Numerical computations are performed for ceramic-metal-based functionally graded material, in which alumina is selected as ceramic and nickel as metal.     

Analysis of Al A359/SiCp Functionally Graded Cylinder Subjected to Internal Pressure and Temperature Gradient with Elastic-Plastic Deformation

In this article, an analytical elastic-plastic solution for thick-walled cylinders made of Functionally Graded Materials (FGMs) subjected to internal pressure and thermal loading is presented. Based on the experimental results, a mathematical model to predict the yielding through the thickness of FG AlA359/SiCp cylinder is developed. It is shown that under the temperature gradient loading, there is a point in the cylinder where the circumferential stress changes from compressive to tensile. The position of this point depends on the geometry and material properties of the FG cylinder and is independent of the temperature gradient.     

Transient Thermal Stress Analysis of Orthotropic Functionally Graded Materials with a Crack

The transient thermal fracture problem of a crack (perpendicular to the gradient direction) in a graded orthotropic strip is investigated. Most of the materials properties are assumed to vary as an exponential function of thickness direction. The transient two-dimensional temperature problem is analyzed by the methods of Laplace and Fourier transformations. A system of singular integral equations are obtained and solved numerically. Numerical results are figured out to show the variation of the temperature on the crack faces and extended line and stress intensity factors for different material parameters with dimensionless time.     

Investigation of Aerothermoelastic Behaviors of Functionally Graded Panels in Supersonic Flows

Considering the potentials of Functionally Graded Panels (FGPs) in aerospace field, it is necessary to study the aerothermoelastic behaviors of FGPs in supersonic flows. In this study, Piston Theory Aerodynamics (PTA) and Eckert reference enthalpy method are used to model aerodynamic force and heating, respectively. The 2-D heat conduction equation is solved and the impact of elevated temperature on the mechanical properties of FGPs is considered to build an aerothermoelastic two-way coupling model of FGPs, and Finite Element Method (FEM) is used to approach the solution. As the results, it is found that there exist three different regions in the bifurcation diagram, namely, thermal buckling region, critical region and flutter region. Due to the inhomogeneous distribution of thermal expansion coefficient, the panel buckles up first and then buckles down via vibration, as thermal buckling happens. Also, irregular vibrations are observed in the critical region of bifurcation diagram. In the flutter region, the dynamic behavior of FGPs is discontinuous and very sensitive to initial conditions. With the impact of aerothermoelastic two-way coupling, different FGPs behaviors lead to the differences in temperature distribution. In particular, the final buckling position and vibration center move to lower positions, and lower temperature region near leading edge is left in the FGPs, because of thermal moment. Also, regular vibrations, rather than irregular vibrations, are easy to extract more principal and regular POD (Proper Orthogonal Decomposition) modes. The results presented could be applied to the analysis and design of Functionally Graded Panels in supersonic flows.     

Thermal behavior of functionally graded solid sphere with nonuniform heat generation

This article deals with the thermoelastic analysis of the functionally graded solid sphere due to nonuniform heat source inside the body under the constant surface temperature. The sphere material is considered to be graded along the radial direction where an exponentially varying distribution is assumed. Also, the material assumed with constant Poisson’s ratio. The implicit finite difference scheme is used to determine the transient temperature, radial displacement, and stress field within the sphere. The results are illustrated numerically and graphically for functionally graded solid sphere consists of metal and ceramic.     

Nanoconfinement of borohydrides in CuS hollow nanospheres: A new strategy compared to carbon nanotubes

Herein, the nanoconfinement of LiBH₄ and NaBH₄ in a carbon (carbon nanotubes, MBH₄@CNT) and an inorganic support (CuS hollow nanospheres, MBH₄@CuS) is compared. Both led to drastic improvements in hydrogen storage properties, with hydrogen desorption occurring from room temperature, and the reversibility greatly enhanced. However, successive hydrogen desorption and absorption cycles for MBH₄@CNT led to a decrease in hydrogen storage capacity, most likely due to partial oxidation from oxygen-containing groups on the surface of the carbon nanotubes. In contrast, little to no decrease in capacity was observed for MBH₄@CuS, indicating that similar materials may be a more viable alternative for future nanoconfinement research.