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Lattice metamaterials have been attracting wide research interests due to their excellent mechanical properties. Most of meta-properties have been implemented by proper geometric designs of microstructures. In this study, we examine another way to obtain outstanding properties, which has been relatively less explored. That is, we aim to adjust the loading bearing capability of lattices by periodically introducing prestress into particular lattice segments. Based on existing related works, we focus on the following two problems deserving further investigations. First, results have been provided based on a single cell with/without taking into account the interactions between each two of neighboring individual cells. Second, it is interesting to search for the optimal distribution of prestress in lattices subjected to a specific load. For the former, we propose a set of constraint equations for implementing periodic boundary conditions (PBC) on a periodic unit cell and confirm its correctness. The significance of PBC related to rotational degrees of freedom is emphasized. We then use the proposed method to calculate the initial damage surface of four kinds of prestressed lattice unit cells under PBC. For the latter, we build a new optimization algorithm with the help of the so-called Symbiotic-Organisms-Search technique (SOS), to calculate the optimal prestress setting corresponding to the requested properties. As an example, the optimal prestress setting is found to almost double the critical load to failure of the lattice in a special direction. This work may be helpful to design lattice metamaterials with programmable strengths.

By extending the long-wave assumption generally employed in most continua, we have proposed a strain gradient (SG) continuum model to simulate one-dimensional (1D) mass-spring chains with local/nonlocal interactions. By accounting for the periodicity of wave motion, we establish a wavelength-dependent Taylor expansion formulation of displacement. The present continuum is reduced to the existing Mindlin-type SG counterparts at the long-wave limit and remains accurate in the medium- and short-wave range. Based on the proposed model, the following fundamental topics are analyzed: (1) The selection of unit cell strongly influences the performance of the continuum. The model is found to work at its best when the periodic unit cell is composed of one single complete mass point and all half springs directly linked to it. (2) Nonlocal interactions increase local extremums in the dispersion curve, and we propose an explicit criterion to judge the workable range of the long-wave approximation, which becomes narrower with increasing the nonlocal interaction span. (3) A higher-order SG continuum does NOT necessarily enhance the ability of predicting short-wave dispersion properties unless the Taylor expansion method proposed here is adopted. The fourth-order equation of motion, hereafter called EOM, of the present SG continuum has been found to adequately capture the dispersion relation throughout the first Brillouin zone. This work sheds light on short-wave homogenizations of periodic structured medium such as mechanical metamaterials.

Here, we synthesize a functional MoO3 photoanode with self-photorechargeability through aerosol-assisted chemical vapour deposition. The role of platinum co-catalyst in Pt-mediated MoO3 photoanode (Pt/MoO3) was investigated to improve and maintain its charge density to enable photoelectrochemical water oxidation under non-irradiated condition. Raman spectroscopy and X-ray diffraction analyses showed that the orthorhombic phase of MoO3 was formed with significant Raman diffraction peaks and crystal planes were grown in the (0k0) crystallographic orientation for better charge storage capacity in MoO3 photoanodes, respectively. Furthermore, the chronoamperometry measurements of the Pt/MoO3 photoanode revealed that both current and charge densities were improved by 29 times and 94 times, respectively, than the bare MoO3 photoanode. This is attributed to the presence of Pt co-catalyst, which acts as an effective electron sink in inhibiting the recombination of photogenerated charge carriers. A density functional theory simulation was then used to validate the experimental findings and fundamental mechanisms of self-photorechargeability in Pt/MoO3 photoanodes. It was found that the distinct characteristic of slow decay in charge density of Pt/MoO3 photoanode is due to the formation of alkali cation/metal layer, where the intercalated photogenerated electrons require a higher energy to overcome the energetic barrier of the alkali cation/metal layer.

Menthol has been employed recently as important temporary consolidant for conservation of cultural heritages. In this paper, a phase field solidification model is proposed and numerical simulations are conducted to investigate the solidification mechanism of menthol. Simulations provide not only the phase transition of menthol during solidification in consistent with experiment, but also how solidification reaction and evolution of heat are mutually interacted. Solidification leads to liberation of latent heat, contributes to the high temperature in un-solidified core and in turn retards solidification. In addition, size effects on solidification quality and mechanical strength are investigated. During solidification, tensile stresses affect solidification and lead to micro defects which are described using relaxation strain in this manuscript, result in size dependent volumetric contraction as well as residual stress after solidification. Operating solidification in smaller size geometries would help lower down the internal stresses and relaxation strain, resulting in denser products of higher mechanical strength. Therefore, multistep casting procedure is suggested rather than one step casting in real consolidations using menthol.

The extended Eshelby method and Ginzburg-Landau theory have been employed to investigate the effect of grain boundary on the microstructure behavior and electromechanical property of BaTiO3 ceramics with no prior constraint on the grains’ shape and material parameters. The results obtained show that, the ferroelastic property of grain boundary plays an important role on the domain structure, ferroelectric and electromechanical properties of ferroelectric ceramics by affecting the elastic and electrostatic field of the material. The macro-properties of ferroelectric ceramics could be effectively functionalized by controlling the average grain size and ferroelastic field of the structure.

Superwettable materials are destined for a wide range of multifunctional applications. An ingenious, cost-effective yet simple method is proposed to control the superwettability of graphene through a thermal curing process at 300 °C under atmospheric pressure. By functionalizing graphene nanoplatelets (GNPs) with oxygenated groups, superhydrophilic surface with a water/air contact angle of 0° manifesting the ultrafast water permeation property is synthesized. Oppositely, superhydrophobic surface with antiadhesive property to water is obtained by functionalizing GNPs with siloxane groups under the same conditions. The superhydrophobicity is justified with a static contact angle of 163° ± 2.8° with a low contact angle hysteresis of 1.53° ± 0.86°. The chemical structures of the superwettable graphene are elucidated using various spectroscopy techniques. Theoretically, by using molecular dynamics simulations, we have further examined and testified the tunable superwettability phenomena of graphene. This study provides important insights into a facile approach in synthesizing advanced materials with superwettability for multifunctional high-performance applications.

Conventional photoanode using a singular semiconductor material is not technically viable for photoelectrochemical (PEC) water oxidation owing to the properties relating to its wide band gap, sluggish charge mobility, as well as poor separation and rapid recombination of photogenerated charge carriers. The main aim of this study was to fabricate an n-n heterojunction photoanode of V2O5/BiVO4 via a facile electrodeposition synthesis method in order to overcome the technical bottlenecks encountered in conventional singular photoanode structures. Additionally, the synergistic effect of band potentials matching and conductivity difference between BiVO4 and V2O5 were studied using LSV, IMPS, EIS, HR-TEM, XRD, XPS, Raman and ultraviolet-visible spectroscopies. This was followed by the performance evaluation of the light-induced water splitting using a standard three-electrode assembly PEC cell under 1.5 AM solar simulator. Results showed that the V2O5/BiVO4 heterojunction photoanode achieved a significantly improved photocurrent density of 1.53 mA/cm2 at 1.5 V vs Ag/AgCl, which was a 6.9-fold and a 7.3-fold improvement over the individual pristine BiVO4 (0.22 mA/cm2) and V2O5 (0.21 mA/cm2), respectively. The improvement was attributed to the lower charge resistances at the FTO/semiconductor, semiconductor/FTO and semiconductor/electrolyte interfaces as well as the fast transit time ( τ ) of 6.4 ms for photo-injected electrons in the V2O5/BiVO4 heterojunction photoanode. Finally, the experimental results were used to reconstruct a theoretical band diagram in validating the heterojunction alignment between V2O5 and BiVO4 as well as in elucidating the photogenerated charge carriers transfer mechanism in the V2O5/BiVO4 heterojunction photoanode.

Laser has become a well-accepted technique for surgical procedures. To fit the physically irradiated area to the clinically planned target area, a multifibre laser system can be used. This paper theoretically investigated the temperature distribution in a three-dimensional biological tissue when irradiated by multifibre lasers with the aids of the dual-phase-lag (DPL) bio-heat conduction model. First, four laser beams located on the corners of a square are used as the heat sources. The method of separation of variables is adopted to obtain the analytical expression of temperature. The characteristics of DPL bio-heat transfer model and the difference with Pennes model were checked graphically. It is found that the effect of multiple laser beams on temperature response differs from those of single laser beam distinctly. A second heating phenomenon occurs and the contour of a square with curved corners are obtained. Then the laser beams are located to form a triangle and a trapezoid and the corresponding temperature responses are studied. Different irradiated zone can be obtained and accomplished by changing the spot size, the arrangement layout and the interval distance of the laser beams.

The flow dynamics of natural convection during paraffin wax melting is numerically investigated using double-population multi-relaxation-time lattice Boltzmann modeling. Flow dynamics manipulation is achieved using inserts of varying tilt angles θ and positions x*, while the use of insulation material isolates the effects of thermal conductivity enhancement. It is found that improvement in melting duration can reach up to 13%, particularly at low θ and high x* due to restructuring of natural convective currents. Natural convective currents show high potency to enhance melting due to a boost in thermal energy transport and solid-liquid interface progression, especially when the liquid melt phase grows significantly in size. More importantly, the mesoscopic nature of LBM permits natural convective multicellular vortex formation and phase change to naturally manifest themselves, as convective currents rotate in the clockwise direction in multiple degrees of vorticity when melting enhancement occurs. Moreover, the melting process can be expressed in four stages via time-dependent Nusselt number Nu variation. Flow dynamics analysis reveals high natural convective activity, albeit low Nu value even after the four stages of melting process complete early. Overall, the present study may provide new insights into the flow dynamic effects that are underrated knowledge in natural-convective melting, and subsequently would impact the development of available crude oil wax mitigation techniques via thermal management.

This paper develops a single-crystal plasticity model with Johnson–Cook-type hardening laws to examine the strain rate and temperature sensitivities of magnesium (Mg). Slip, twinning and their interactions are deemed the dominant plastic mechanisms. The twinning-induced lattice reorientation is implemented by taking the initial grain after reorientation as a “new” grain with an updated orientation. Distinct strain rate and temperature dependences are considered for different slip and twinning modes. Non-basal slip is believed strain rate and temperature dependent, while compression twinning (CT) is assumed to exhibit temperature sensitivity in a certain temperature range. To validate the proposed model, plane-strain compression tests of Mg crystals under different strain rates and temperatures are simulated. The experimental data available in the literature are compared with the predicted results, and the tendencies of stress–strain curves are analyzed based on the corresponding evolution of slip and twinning. It is found that twinning-induced lattice reorientation significantly influences the mechanical behavior of Mg, especially when tension twinning (TT) dominates the plastic deformation. High temperatures and low strain rates enhance the activity of non-basal slip, and CT becomes easily activated as the temperature is increased beyond $$150\,^{\circ }\hbox {C}$$ , which coincides well with experimental observations. The strain rate sensitivity rising with temperature is also predicted by the model.

A new attempt is made to simulate progressive failure processes in heterogeneous brittle materials such as concrete, ceramics, rocks etc., by considering the time-dependence of stress redistributions induced by local breakages. Two mechanisms of stress redistribution are incorporated into the proposed model in order to account for the influence of each local breakage on the remaining specimen: (1) one is the immediate release of internal forces in the breaking element, which is assumed to happen within an infinitesimal time when compared with the characteristic time of external loadings. The release of such internal forces is hence suddenly applied to the remaining specimen, which is considered to take time to deform correspondingly due to material viscosity. This deformation delay is implemented by introducing a viscous force (VF) field prevailing in the entire specimen. (2) The other is the gradual release of previously stored VF fields, whose characteristic time is assumed to be material-dependent. Here the release of VF is approximated as stepwise for simplicity. The proposed model is found to be capable of overcoming the unreasonably-low-ductility problem encountered in many existing lattice models when it comes to the uniaxial tensile test. Furthermore, the force–displacement response obviously depends on the ratio of the VF releasing time to the characteristic time of external loading, showing trends agreeing with experimental observations. Compared with results without viscosity, the failure pattern is more scattering, and the force–displacement curve has a higher peak load and a more ductile post-peak tail.

Recent in-situ experiments show that stress-driven migration of the phase interface during two-phase lithiation in nanosized particles exhibits self-limiting and size-dependent behaviors wherein the mechanism remains unclear. In the reaction-limited regime, we develop a mechano-kinetic coupling model with a nonlinear kinetic law to study the size effect of such phase boundary movement accounting for possible sources of stresses by chemical lithiation, concurrent plasticity, surface/interface elasticity, and elastic softening of the lithiated phase. We show that both hydrostatic and non-hydrostatic stresses contribute to the driving force for the phase interface movement and result in the size-dependent slowing down behavior of the phase interface. The obtained results reveal why the interface movement slows down more dramatically in the smaller particle, and there are similar lithiation time scales in nanoparticles of different sizes observed in experiments.

Ferroelectrics are widely used in the manufacture of transducers, actuators, and memory devices, due to their attractive electromechanical properties. However, the reliability and failure of devices is greatly dependent on their brittleness. In view of the fact that both the polarization distribution and elastic field are at nanoscale and vary greatly in the vicinity of the crack tip, flexoelectricity is expected to strongly affect the domain configuration. In this work, Ginzburg-Landau (TDGL) theory and the phase field method (PFM) are employed to analyze the influence of flexoelectric effect on the domain switching process in the vicinity of the crack tip of ferroelectric materials. The results obtained show that, the domain configuration would become asymmetric with increasing flexoelectric coefficients, and the flexoelectric effect has a larger influence on the polarization field than on the elastic field in the vicinity of the crack tip of ferroelectric materials.

A new category of structural metals with high strength and good ductility is coarse-grained metals strengthened by nanotwinned (NT) regions. This unique combination makes them particularly suitable for applications in bulletproof targets. The extent of this improvement, however, depends on the volume fraction of the NT regions and multiple other microstructural features. Here, a numerical model based on the strain gradient plasticity and the Johnson–Cook failure criterion is developed to study the effects of these attributes. The ballistic performance is quantified by two indices: the limit velocity that measures its ability to resist failure and the limit displacement that measures its ability to resist deformation. The results obtained indicate that, in general, a relatively small twin spacing and a moderate volume fraction of NT regions can achieve both excellent limit velocity and limit displacement. Moreover, array-arranged NT regions are more effective than staggered NT regions in enhancing the limit velocity, but the influence of the array group tends to depend more on the volume fraction of NT regions than the latter one. Mechanism analyses based on the three stages of the impact process and two categories of low-speed impact are performed. The simulated results could provide significant insights into the NT structures for a superior class of nanotwin-strengthened metals for ballistic protection.

Coarse-grained (CG) metals strengthened by nanotwinned (NT) regions possess high strength and good ductility. As such, they are very suitable for applications in bullet-proof targets. Here, a numerical model based on the conventional theory of strain gradient plasticity and the Johnson–Cook failure criterion is employed to study the influences of volume fraction of NT regions on their ballistic performance. The results show that in general a relatively small twin spacing (4–10 nm) and a moderate volume fraction (7%–20%) will lead to excellent limit velocity and that the influences of volume fraction on limit displacement change with the category of impact processes.

Self-assembled heteroepitaxial offers tremendous opportunity to tailor optical and charge transport properties in noble metal–semiconductor interface. Here, we incorporated gold nanoparticles (AuNPs) onto the {001} facets of semiconductor strontium titanate, SrTiO3 (STO), by means of heteroepitaxial approach to investigate the band gap tuning and its effect of photoresponse. We demonstrate that the Fermi energy level of the system can be tuned by controlling the AuNPs size. X-ray photoelectron spectroscopy (XPS) shows that the energy difference between Sr3d and Au4f core levels measured in the AuNPs/STO (100) heterojunction increases from 47.90 to 49.26 eV with decreasing AuNPs size from 65 to 16 nm, respectively. Hence, the Fermi energy level was shifted toward the conductive band of STO (100), and the system charge transfer efficiency was improved. It was also found that smaller AuNPs sizes exhibited a higher photoactivity as the result of the band gap narrowing effect. Photoactivity was improved by bro...

Samarium (5%Sm) and manganese (0.5%Mn) co-doped BiFeO 3 (B5SF0.5MO) polycrystalline multiferroic ceramics were prepared by solid-state-reaction to study the morphology, local polarization switching and piezoresponse, using atomic force microscopy (AFM) and piezoresponse force microscopy (PFM). Room temperature x-ray diffraction (XRD) and micro-Raman spectra reveal that samarium and manganese co-doping retains the parental rhombohedral R 3 c structure of BiFeO 3 (BFO). The out-of-plane (OP) PFM phase images show domains with oppositely oriented polarizations which are distinguished as domains with downward and upward polarizations with respect to the cantilever direction. Polarization switching occurs in the poled sample by locally poling the sample at negative and positive biases. Following the domain switching mechanism in BFO, it was found that 109° domain switching occurred besides the growth of 180° domains upon application of an electric field. The clockwise phase hysteresis implies the Debye model of piezoelectric relaxation effect due to defects that are both elastic and electric dipoles. The negative self-polarization in these samples is supposed to be the result of built-in internal bias field generated from excess electrons and charge defects like oxygen vacancies. The Saturated localized in-field hysteresis phase and amplitude loops from 180° domains suggest the existence of well-defined polarization along the field direction and suggests that Sm and Mn co-doped BFO can be a potential material for nanoscale piezoelectric applications.

Topological defect nucleation and boundary branching in crystal growth on a curved surface are two typical elastic instabilities driven by curvature induced stress, and have usually been discussed separately in the past. In this work they are simultaneously considered during crystal growth on a sphere. Phase diagrams with respect to sphere radius, size, edge energy and stiffness of the crystal for the equilibrium crystal morphologies are achieved by theoretical analysis and validated by Brownian dynamics simulations. The simulation results further demonstrate the detail of morphological evolution governed by these two different stress relaxation modes. Topological defect nucleation and boundary branching not only compete with each other but also coexist in a range of combinations of factors. Clarification of the interaction mechanism provides a better understanding of various curved crystal morphologies for their potential applications.

Within the framework of micropolar theory, we have studied the continuum homogenization procedure of a typical kind of planar chiral structures, in which each two neighboring joints interact through a zigzag beam. Both the triangular and square lattices are taken as the demonstrating examples. Usage of zigzag beams makes the lattices bending-dominant, and thus calls for micropolar theory to predict the constitutive behaviors. According to the equivalence of microstructure- and continuum-level strain energies, the micropolar-type elastic stiffness constants are expressed in terms of the microstructural geometric and material parameters. By considering uniaxial tension, Young's modulus and Poisson's ratio are determined in an analytical form. Particularly, for square lattices, there arises a strong coupling between stretch and shear, meaning that the lattice under uniaxial tension is subject to a shearing deformation along the direction perpendicular to the tension, and vice versa. The theoretical results are validated by comparing with finite element simulations and experiments done on 3D printed specimens. We have also analyzed the influences of microstructural geometric parameters on the homogenized Young's modulus and Poisson's ratio. Furthermore, in this study the micropolar characteristic lengths are given explicitly in terms of microstructural parameters, providing the convenience and guidance of tuning the related size effects.

The Fleck-Hutchinson theory on strain gradient plasticity (SGP), proposed in Adv. Appl Mech 33 (1997) 295, has recently been reformulated by adopting the strategy of decomposing the second order strain presented by Lam et al. in J Mech Pays Solids 51 (2003) 1477. The newly built SGP satisfies the non negativity of plastic dissipation, which is still an outstanding issue in other SGP theories. Furthermore, it explicitly shows how elastic strain gradients and corresponding elastic characteristic length scales come into play in general elastic-plastic loading histories. In this study, the relation between elastic length scales and plastic length scales is investigated by taking wire torsion as an example. It is concluded that the size effects arising when two sets of length scales are of the same order are essentially elastic instead of plastic.

In the present study, thermoelastic behavior of a semi-infinite rod, which is subjected to a time exponentially decaying laser pulse, is formulated. The rod is free at the left end and the laser pulse moves along the axial direction from the left end. The non-Fourier effect of the heat conduction equation is considered and the Laplace transformation method is employed in solving the governing equations. The temperature, displacement, strain, and stress in the rod are derived and the distributions of the parameters at different positions are analyzed. Also the influence of the laser speed is investigated.

The effects of mechanical stresses on the voltage hysteresis of a lithium ion battery during charge–discharge cycles are theoretically investigated. A diffusion–reaction-stress coupling model has been established. It is found that a compressive stress in the electrode surface layer would impede lithium intercalation. Therefore, a higher overpotential is needed to overcome the intercalation barrier induced by stresses. The stress difference between charge and discharge made contribution to the voltage hysteresis which depends on charge rate, electrode particle radius, as well as a combined parameter that reflects the influence of material properties including elastic modulus, partial molar volume, capacity and diffusivity. Calculations show that the stress-induced overpotentials are several orders of magnitude higher in silicon electrodes compared to those in graphite and LiMn2O4 electrodes. Finally, a relaxation simulation shows that the stress variation from a thermodynamically non-equilibrium state to an equilibrium state leads to the relaxation of the electrode potential under an open-circuit operation. This serves as a proof that battery performance is affected by stresses.

For micron-scale elasto-plastic deformations, a theory of strain gradient elasto-plasticity (SGEP) has been established to explain some phenomena that have recently attracted intensive investigations, which include size effects around the elastic limit and in plastic hardening, the anomalous Bauschinger effect, plastic softening, and the unconventional load–unload hysteresis loops. Based on the fact that for most materials the elastic length scale is far smaller than the plastic length scale, our dimensional analysis reveals that the elastic strain gradient is generally dominant over the plastic strain gradient, which indicates the significant role of elasticity. Therefore, a more rigorous consideration of the elastic deformation is implemented within the proposed SGEP framework, featured by a new Taylor plasticity-based yield function and the elasto-plastic decompositions of both strain and strain gradient. The proposed yield function emphasizes the reversibility of elastic deformation-related dislocations, which has been frequently observed experimentally. This proposed SGEP not only does satisfy the principle of nonnegative plastic dissipation for material hardening, but also allows for the existence of material softening which has been observed in recent experiments. The wire torsion and foil bending problems are used as examples, to validate the SGEP’s capability in interpreting the above-mentioned phenomena by comparison with experimental data in the literature.

We conduct experiments to study the effects of incorporation of carbon nanotubes (CNTs) coatings on the thermal performance of micro heat pipe (MHP) arrays. The microchannels of MHP are fully coated with CNTs which are functionalized through a thermal curing process. The cured CNTs coating manifests a superhydrophilic characteristic and fast water permeation property. The rapid water permeation through CNTs nanostructure enhances the evaporation at the evaporator section and the fluid circulation synergically in the MHP. For evaporation, the superhydrophilic highly permeable porous CNTs nanostructures increase the nucleation sites and promote film-wise evaporation which is more efficient than the bulk evaporation. For circulation of working fluid, an intricately interconnected CNTs networks facilitate the fluid transport with enhanced capillary pressure. The effective thermal conductivity, which denotes the overall performance of a micro heat pipe manifests a maximum enhancement of 202%; and the evaporator heat transfer coefficient which represents the evaporation strength is enhanced up to 61%. Computationally, molecular dynamics simulations are performed to investigate the fast water permeation property of CNTs nanostructure which leads to the anomalous thermal performance enhancement. This study provides interesting insight into the viability of incorporating CNTs nanostructures into an MHP for microscale cooling applications.

Two dimensionalization of dielectric oxides will lead to significant variations of the dielectric properties in comparing with their bulk counterparts. To clarify the longstanding controversy how indeed dielectric properties would change is a critical prerequisite and highly demanded by the correlated scientific community and electronic industry. Here, taking titania as an example, a theoretical study by ab-initio calculations is performed for predicting the dielectric property evolution in combining with the structure transformations. Highly enhanced dielectric constant anisotropy is found that out of plane reduces significantly differed from those enlargement predictions, but electronic screening in plane enhances. While for the breakdown strength, which is always ignored in theoretical investigation, it also demonstrates that permittivity and breakdown strength can be enhanced at the expense of each other during two dimensionalization. The anisotropy between permittivity and breakdown strength can cause a much higher dielectric energy density. Such findings are very helpful for designing future atomic scale electronic and optical devices such as high voltage capacitor, excellent gate dielectrics for high frequency field effect transistor (FET) with two-dimensional oxide dielectrics.

There is a growing research interest in exploring the self-photorechargeability of photoanodes, which enables photoelectrochemical (PEC) water oxidation even under non-irradiated conditions. The main aim of this study was to develop a facile synthesis of molybdenum trioxide (MoO3) photoanode displaying self-photorechargeability using an aerosol-assisted chemical vapour deposition (AA-CVD) method. A systematic optimisation of the key synthesis parameters of AA-CVD method, namely: (1) ultrasonication time of precursor solution, and (2) annealing temperature was carried out in order to understand the best trade-off between photocurrent density (illuminated conditions) and charge density (non-illuminated conditions). Field emission-scanning electron microscopy images showed that the MoO3 photoanodes synthesized via AA-CVD method exhibited a 3D plate-like crystalline structure that gave a large voltammogram area, indicating that the MoO3 photoanodes possessed high charge storage capacity for photogenerated electrons. PEC measurements showed that the optimised MoO3 photoanode obtained during an ultrasonication time of 25 min and at the annealing temperature of 500 °C achieved a photocurrent density of 1.47 μA/cm2 at 1.0 V vs Pt electrode. A significantly prolonged on-off illumination cycle (i.e. 1000 s) showed a significant storage capacity of photogenerated electrons within the 3D plate-like MoO3 crystalline structure was discharged during the non-irradiated conditions, and a charge density of 0.35 mC/cm2.

The rate of water transport through graphene nanocapillaries is profoundly enhanced compared to that in microscale capillaries due to the prevalence of exceptionally high capillary pressures and large slip lengths. As an inaugural study, we integrate graphene nanocapillaries into a micro heat pipe (MHP) for enhanced light-emitting diode (LED) cooling. With the use of graphene nanocapillaries, the ultrafast water transport synergically enhances the water circulation and evaporation process in the microfluidic device. The graphene-coated MHP achieves more than 45% enhancement in the overall performance compared to the uncoated counterpart. In turn, the experiments demonstrate a drastic reduction of LED's operating temperature (more than 25 °C) which translates into a significantly prolonged lifespan of LED. The molecular dynamics simulations reveal that the oxygenated functional groups attached on graphene further increase the capillary pressure (~1000 bar) and effective velocity (~20 m/s) of the nanoconfined water, compared to those (~500 bar and ~10 m/s) in a pristine graphene nanochannel. The ultrafast water transport in graphene nanocapillary is justified. This study provides a holistic analysis and important insight into the phenomenon of ultrafast water transport in graphene nanocapillaries that exhibits an enormous potential in thermal energy management applications for LED cooling.

Ethanol-based nanofluids have attracted much attention due to the enhancement in heat transfer and their potential applications in nanofluid-type fuels and thermal storage. Most research has been conducted on ethanol-based nanofluids containing various nanoparticles in low mass fraction; however, to-date such studies based on high weight fraction of nanoparticles are limited due to the poor stability problem. In addition, very little existing work has considered the inevitable water content in ethanol for the change of thermal conductivity. In this paper, the highly stable and well-dispersed TiO2-ethanol nanofluids of high weight fraction of up to 3 wt% can be fabricated by stirred bead milling, which enables the studies of thermal conductivity of TiO2-ethanol nanofluids over a wide range of operating temperatures. Our results provide evidence that the enhanced thermal conductivity is mainly contributed by the percolation network of nanoparticles at low temperatures, while it is in combination with both Brownian motion and local percolation of nanoparticle clustering at high temperatures.

Photocatalytic CO2 reduction over the UV-Vis-NIR broad spectrum was realized for the first time. The presence of surface oxygen vacancy defects on Bi2WO6 resulted in significant photocatalytic enhancement over the pristine counterpart under UV and visible light irradiation. Meanwhile, the photocatalytic responsiveness of Bi2WO6-OV was successfully extended to the NIR region.

Phase separation in an electrode of a lithium ion battery, which is a phenomenon where an active electrode material is separated into Li-rich and Li-poor phases, exists widely in many active materials and has significant impacts on the diffusion of lithium ions and diffusion-induced stresses. A phase field model is developed to study the phase separation. Firstly, the influences of various energies, such as the free energy of uniform Li-ion concentration, gradient energy and elastic energy, on phase separation are discussed. Secondly, the impacts of charge operation, e.g. galvanostatic and potentiostatic, on Li-ion diffusion and diffusion-induced stresses in a planar phase separating electrode are investigated. Calculations are also made for single phase electrodes based on Fick’s law for comparison. The obtained simulation results show that the Li-ion diffusion in a phase separating electrode depends significantly on the phase separating profile and movement of phase boundary, but it is not sensitive to charge operation. The diffusion-induced stresses also separate into high and low stress regions. Finally, based on the diffusion process and diffusion-induced stress, it is suggested that phase separation should be avoided for the sake of fast charging and mechanical reliability.

A theoretical model is proposed to illustrate the effect of detwinning of the secondary twin lamellae on the emission of lattice dislocations from a semi-infinite crack tip in a hierarchically nanotwinned metal. The results obtained show that the potential of the crack tip to emit dislocations can be optimized and greatly enhanced by tuning the secondary twin spacing, which leads to strong crack blunting. Moreover, the hierarchical structure is able to produce significant toughening at very thin secondary twin lamellae, as observed in molecular dynamics simulations. As a result, the proposed model suggests a novel toughening mechanism in ultrafine/nano-grained metals.

A theoretical model has been developed, which reveals the underlying correlation between the strong extra strain hardening achieved in the nano-grained layers of the grain size gradient structure and the non-uniform deformation of the lateral surface in surface nano-crystallized materials, based on some existing experimental observations and the concept of geometrically necessary dislocations. The proposed model led to the establishment of a simple physical law that can be expressed as H n ¼A n , where H n and A n are two dimensionless parameters. The former represents the extra strain hardening, while the latter characterizes the non-uniform deformation of the lateral surface. The values of these two parameters can be measured through experiments.

Indirect nanosecond-pulsed laser ablation of Au film of 10 nm–600 nm has been performed to investigate the ablated surface morphology of Au film as compared to direct pulsed laser ablation. This work shows that both the direct and indirect ablations give similar trend of ablation threshold fluence for film thickness equal or less than 170 nm. In this region, the ablation mechanism is driven by melting and vaporization. However, for film thickness greater than 170 nm, indirect ablation exhibits superior film quality with 3.0–3.6 factors less applied laser fluence compared to direct ablation and thus has no heat-affected zone and no recast material. As a result, the ablation mechanism for indirect ablation is driven by photomechanical process. This work highlighted the significant role of irradiation orientation (direct and indirect) as well as the correlation of heat penetration depth with different Au film thickness in influencing the ablation mechanism of Au film on glass substrate.

Although bismuth vanadate (BiVO4) has shown excellent photoelectrochemical (PEC) properties and is a good candidate of photoanode materials, the solar-driven PEC water splitting performance is still remained below its full potential due to the fast recombination and sluggish charge mobility of photogenerated charge carriers. Previously, we have communicated a proven Type II staggered vanadium pentoxide (V2O5)/BiVO4 heterojunction photoanode that could improve the photocurrent density. This study aimed to examine the effect of introducing an rGO thin film as an efficient electron conductive interlayer in a proven V2O5/BiVO4 heterojunction photoanode, and subsequently tuning the rGO film thickness in achieving the optimum PEC performance. The resultant ternary photoanode structure of V2O5/rGO/BiVO4 was characterised by using field emission-scanning electron microscopy (FE-SEM), high resolution-transmission electron microscopy (HR-TEM), UV–vis spectroscopy, X-ray diffractometer (XRD), Raman spectroscopy and photoluminescence (PL) measurements. Results showed that the interlayer rGO thin film arising from the sequential drop cast and electrochemical reduction of 320 μ L ultrasonicated GO solution resulted in the optimal photocurrent density of 2.1 mA/cm2 at 1.5 V vs. Ag/AgCl. Furthermore, the chemical physics surrounding the photogenerated charge carrier transfer for heterojunction V2O5/BiVO4 was validated for the structure with and without the rGO interlayer. In particular, the electrochemical impedance spectroscopy (EIS) was used to measure multiple resistances at the FTO/semiconductor, semiconductor/semiconductor and semiconductor/electrolyte interfaces. Additionally, the charge transfer (Kt) and recombination (Kr) rate constants for the heterojunction V2O5/BiVO4 with the rGO interlayer were quantified using intensity modulated photocurrent spectroscopy (IMPS). Finally, the PEC H2 evolution rate from the ternary V2O5/rGO/BiVO4 photoanode was measured to be 32.7 μ mol/hr, which was about 3-fold higher than the bare V2O5/BiVO4 heterojunction photoanode.

Herein, we report the rational design of a novel hierarchical V2O5/BiVO4 heterojunction photoanode structure with rGO interlayer that functionalises as photogenerated electron collector, and dual electrocatalyst thin films of FeOOH and NiOOH as photogenerated hole extractors for solar-driven PEC water splitting. Results showed that the novel hierarchical FTO/V2O5/rGO/BiVO4/FeOOH/NiOOH photoanode exhibited an unprecedented and stable photocurrent density of 3.06 mA/cm2 at 1.5 V vs Ag/AgCl, and an apparent cathodic onset potential shift down to 0.2 V under AM 1.5 G simulated solar light illumination. The significant enhancement in PEC performance is ascribed to band potentials matching between V2O5 and BiVO4 in forming a Type II staggered heterojunction alignment, and further coupling with rGO interlayer and dual-electrocatalyst thin films as photogenerated electron collector and photogenerated hole extractors, respectively. Three different configurations of the novel hierarchical FTO/V2O5/rGO/BiVO4 photoanodes without electrocatalyst, with mono- and dual-electrocatalyst thin films were systematically examined. It was proven though EIS and IMPS measurements that the dual-electrocatalyst configuration photoanode exhibited the shortest transit time (τ) of 31.8 ms for the diffusion of photogenerated electrons to the counter electrode, and the lowest charge transfer resistance across the interface of electrode/electrolyte as estimated using the Randles-Ershel model. We believe that the proof-of-principle work described here not only provides an in-depth understanding on the roles of electrocatalyst thin films but also provides a design guide over the incorporation of electrocatalyst materials for further improving the photogenerated charge carrier dynamics in photoanodes used in solar-driven PEC water splitting.

A phase-field model has been developed to study the influence of epitaxial strains and domain structures on the electrocaloric effects (ECEs) in PbTiO3 (PTO) thin films. The simulated domain-switching dynamics obtained from the proposed model was tallied with the existing experimental results. In the case of single-domain PTO thin film, the ECE gradually increased with the increasing epitaxial strain, whereas, in the case of poly-domain PTO thin film, the influence of epitaxial strain on ECE can be divided into three regions of stable domain structures, i.e., c-domains, a/c-domains, and a-domains regions. It is worth noting that the maximum ECE occurred under a tensile epitaxial strain in the a/c-domains region, which was the consequence of the competition between the enhancement of ECE and the reduction of c-domains volume fraction, both caused by the increasing tensile epitaxial strain. Although the change of temperature arising from the ECE may not be large enough to be viable for practical applications, it does illustrate the important influence of domain structures on the ECEs of thin films, which has not gained much attention of most researchers whose focus is still on single-domain thin films.