Sandwich Films Research Articles

Flexible and anisotropically conductive film by assembly of silicone rubber and cobalt-coated glass fiber composites

In this study, we prepared electromagnetic cobalt-coated glass fiber (Co@GF) composites via an electroless plating method. Subsequently, a conductive sandwich flexible film consisting of Co@GF composites and liquid silicone rubber (RTV-2) was successfully formed using the tape casting method at room temperature. Based on the perfect coating and excellent electrical conductivity of the Co@GF composites, the resultant RTV-2/Co@GF/RTV-2 sandwich flexible film showed a low volume resistivity of 0.264 Ω·cm and could stretch to 100% (of 4.40 Ω·cm) without obvious fracture. When a magnetic field was applied during the curing process, the electromagnetic Co@GF composites were aligned automatically in the RTV-2 matrix because of their ferromagnetic nature. The as-prepared film exhibited anisotropy in its electrical performance. The volume resistivity parallel to the magnetic field direction is approximately two times lower than that in the perpendicular direction. The maximum difference in the volume resistivity (ρ∥ = 0.768 Ω·cm and ρ⊥ = 1.549 Ω·cm) was obtained at a magnetic field intensity of 800 mT. In addition, a magnetic field intensity of 100 mT helps improve the electrical conductivity of the as-obtained sandwich film. The anisotropic RTV-2/Co@GF/RTV-2 sandwich flexible film is considered a promising flexible electronic sensor, where discrepant inductive sensitivity is required in orthogonal directions.

Multiscale Mixed Elastohydrodynamics in Inclined Fixed Pad Thrust Bearing Involving Surface Roughness and Low Clearance

The film pressures, surface separation profiles and carried loads of the hydrodynamic inclined fixed pad thrust bearing with the sandwich films were calculated by the multiscale approach when the bearing clearance was low and the combined effects of the adsorbed layer, the surface roughness and the surface elastic deformation were incorporated; The sandwich film consists of two physically adsorbed layers respectively on the coupled bearing surfaces and the intermediate continuum fluid film. The results show that the surface roughness intrinsically generates the film pressure ripples, but the rough surface is somewhat flattened and the film pressure ripples are a bit reduced by the surface elastic deformation. For a given sliding speed and a given original bearing clearance, the surface elastic deformation results in the significant reductions of both the maximum film pressure and the carried load of the bearing especially for a strong fluid-bearing surface interaction. In spite of this, the slope of the increase of the bearing load with the magnitude () of the surface roughness is nearly the same for the elastic surfaces with that for the rigid surfaces. The presence of the surface roughness does not qualitatively alter the influence of the surface elastic deformation on the performance of the bearing.

(Invited) Scalable Manufacturing and Application of Piezoelectric Biomaterials

Piezoelectric materials are a group of important functional building blocks that interfacing the human body by coupling biomechanical energy and electricity. So far, many technology innovations have advanced piezoelectric materials and composites toward a broad range of biomedical applications, which possess unique biocompatibility and flexibility. Fundamentally, materials design and engineering draw the boundary where this technology may advance. In this talk, I introduce our most recent development of piezoelectric materials and composites that are particularly designed for implantable nanogenerator applications. First, I present our wafer-scale approach to creating piezoelectric biomaterial thin films based on amino acids. The self-assembled sandwich film and truss-like structures enabled both strong piezoelectricity and largely improved flexibility. Then, new ferroelectric composites are presented as a new material used in 3D printing for directly manufacturing of piezoelectric architectures with tunable piezoelectric and mechanical properties. Toward the end, novel applications of implantable piezoelectric materials are introduced, which enable the closed-loop electrostimulations for many biomedical therapeutics.

Complex conductivity as a tool to investigate the electrical behavior between graphene oxide and reduced graphene in supercapacitors: Correlation between the electrical properties

The storage mechanisms of solid-solid supercapacitors based on graphene oxide (rGO/GO/rGO) were explored by Zhang Q et al. (Zhang et al., 2014) [1], using Density Functional Theory (DFT) and Impedance Spectroscopy (EIS) to elucidate the unusual capacitive behavior of GO sandwich films. Their study revealed alternating charged layers without diffusion zones. Analysis of complex impedance (Z*) data, ranging from 10−3 to 3.106 Hz, was conducted using an equivalent circuit, though this was limited to Nyquist plot data.To further investigate charge storage mechanisms, the current study delved deeper into complex impedance (Z*) and conductivity (σ*). A theoretical approach identified relaxation processes in the imaginary parts of (Z″) and (σ'') across frequencies. Extrapolation up to 3.10^9 Hz and deconvolution confirmed three distinct relaxation processes in Z* and σ*, similar to Cole-Cole relaxation, which describes Maxwell-Wagner-Sillars (MWS) relaxation. This reflects local electrical interactions between active sites on molecular chains and counterions, creating induced dipolar moments or diffusion processes.The identified relaxations at low, medium, and high frequencies correspond well with DFT results, attributed to specific regions: graphene oxide (GO), the (rGO/GO) interface, and reduced graphene (rGO). Key parameters extracted from each relaxation, such as relaxation time, ionic strength, and conductivity values at different frequencies, showed good correlation. This approach, based on deep conductivity (σ*) analysis, effectively highlighted the charge conduction and storage mechanisms in rGO/GO/rGO supercapacitor films.

Study on the film-forming properties of Mo–Na ceramic targets

In order to provide sufficient and stable Na+ for the CIGS flexible thin film solar cells, Mo–Na ceramic targets were prepared by the plasma-assisted cold sintering process (CSPS). And the film-forming properties of the Mo–Na targets were studied. The Mo–Na targets were prepared with MoO3 as precursor. The Mo–Na targets were successfully prepared with uniformity, compactness and high relative density (98.19 %). Then, the Mo–Na monolayer films were prepared by radio frequency (RF) magnetron sputtering. The results show that Mo–Na monolayer films are homogeneous and compact. Significantly, abundant Na+ are stored in the Mo–Na monolayer films. To verify the application of Mo–Na targets in the CIGS cells, the sandwich structure (Mo/Mo–Na/Mo) films and co-sputtered films were studied. The results show that the co-sputtered films have sufficient Na+, and show good photoelectric and mechanical properties. In addition, after heat treatment, Na+ can diffuse to the surface of the sandwich films, which can provide sufficient and stable Na+ for CIGS thin film cells.

All-Organic Piezo-Photocatalytic Film with Highly Efficient Catalysis, Weak-Force Excitation, and Recyclability.

Polyaniline (PANI), a typical organic photocatalyst, has an adjustable structure and good stability, can be easily synthesized on a large scale, and is economical. PANI is doped with ions to regulate its internal structure and improve its photocatalytic performance. However, its photocatalytic performance is limited by the doping concentration and its intrinsic properties, hindering its further application. Herein, PANI films with a piezo-photocatalytic function are fabricated to improve photocatalytic performance and explore their self-powered environmental purification property. PANI/poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) sandwich films, with PVDF-HFP as the interlayer, are prepared by introducing a piezoelectric field into PANI photocatalysts, thereby achieving excellent piezo-photocatalytic performance. The as-fabricated piezo-photocatalyst degrades methyl orange at a rate of 91.2% after 60min under magnetic stirring. Owing to the low Young's modulus of the all-organic catalyst, self-powered purification is realized using the PANI/PVDF-HFP film. Leaf surfaces are functionalized by loading the film in them for removing pollutants under sunlight and water flow. Thus, this study proposes a common strategy, wherein a piezoelectric interlayer is introduced to load the organic photocatalyst for preparing an all-organic piezo-photocatalyst. This piezo-photocatalyst can be easily recycled and responds to weak forces, realizing its application for self-powered environmental purification.

Manipulating epoxy-polycarbonate sandwich films with enhanced dielectric performance via solution-immersion process

Epoxy-polycarbonate sandwiched films with enhanced dielectric performance were prepared in this study. Unlike traditional tedious multiple scratch-coating approach, the sandwich dielectric films were prepared based on a convenient solution-immersion process. Polycarbonate solutions with different concentration were applied to adjust the film composition and compared with epoxy-polycarbonate blending film. We found that rather than deteriorating the breakdown strength as shown in the blending counterpart, sandwiched films prepared by solution immersion shows superior breakdown strength (709.6 MV/m), almost 40 % better than both epoxy and polycarbonate components. Moreover, tan δ of the films prepared from solution-immersion (<0.005) is apparently smaller than blending samples (∼0.008). Both factors lead to admirable ideal dielectric energy storage performance (8.44 J/cm3) of the film, better than many conventional polymers.

Low energy impact damage characteristics of epoxy coating on the surface of sandwich composites

AbstractWhen composite honeycomb sandwich structures are subjected to low energy impacts, the upper panel or the honeycomb core often suffers large damages, which are invisible, but affects the structural safety. To achieve non‐destructive detection of these damages, an epoxy resin film (ERF) was designed as a functional coating to display the internal failure features on structure surface. The effect of low energy impacts on the damage modes of the ERF was first investigated, and then the correspondence between the ERF damage characteristics and the structural internal failure was established. Simulation and experimental results showed that with the increase of impact energy, cracks on the ERF expanded from the radial radiation with a small extension range to the circumferential pattern with an increased damage area. Meanwhile, localized delamination appeared inside the upper panel, while the degree of flexure and crushing damage to the core continued to increase. Based on the analysis of the crack extension pattern of the ERF and the damage state of the upper panel and honeycomb core, a specific link between the ERF crack characteristics and the internal damages to the sandwich structure was obtained, which might offer a novel non‐destructive testing method for composite impact damages.Highlights Low energy impact damage in honeycomb sandwich structures was a concern. Impact damage response of epoxy resin film on sandwich composites was studied. Crack extension of the film showed a particular change from radial to annular. Specific link between film crack and sandwich structure damage was established. The results would be helpful for damage detection in sandwich composites.

High strength and high damping CFRP composites with co‐curing viscoelastic sandwich film

AbstractThe high temperature co‐curing damping composite was investigated by co‐curing technology to improve the mechanical property and damping performance of the traditional fiber reinforced resin matrix composite. Fluororubber with stable chemical properties and high‐temperature resistance was used as the core layer of novel carbon fiber reinforced bismaleimide resin matrix High‐temperature Co‐curing Damping Sandwich Composites (HCDSC). The curing temperature and curing time of fluororubber were consistent with that of bismaleimide resin by adjusting the formula. The viscoelastic damping properties and thermal stability of zinc methacrylate reinforced fluororubber were investigated in detail. Static mechanical tests, modal tests and interfacial shear tests prove that HCDSC display excellent mechanical and damping properties. The interfacial microscopic analysis reveals that viscoelastic sandwich materials are closely bonded with the composites. The dynamic characteristics of damping sandwich composite plate with fixed support on four sides is analyzed theoretically, and the vibration equilibrium equation of composite plate is derived by combining first‐order shear deformation theory, variational principle, and Hamilton principle. The damping performance of the composite plate are discussed through modal tests to verify the accuracy of theoretical derivation. This research is of great significance to solve the problem of structural dynamics, and provides a new idea for the design of structural and functional composite materials.Highlights A viscoelastic damping material with large damping and high strength was prepared. The dynamic performance of co‐curing sandwich composite structures was studied. The free vibration of composite structures was explored via experiment and theory.

Supercapacitive performance of solution processed free standing TiO2-rGO-TiO2 sandwich film electrodes

The development of flexibility and robustness in functional devices is an important feature for the next generation of devices for long operational periods. It infers applications for the compactness of batteries, bio- sensors, supercapacitors, wearable devices, and photovoltaics as well. Here, we report a two-step low-cost solution-based process to synthesize (a) free-standing films of graphene oxide (GO) and reduced graphene oxide (rGO) and (b) hybrid sandwich structures of titania and rGO by using simple chemical bath deposition. The microstructure and morphological study are carried out using X-ray diffraction, atomic force microscopy, and scanning electron microscopy. Its electrochemical performance for supercapacitor electrodes were evaluated using three electrode cyclic voltammetry and galvanostatic charge-discharge system at different current density. In our experiment, graphene oxide is showing pseudo capacitance like behaviour, rGO and TiO2-rGO-TiO2 sandwich structure shows supercapacitor like behaviour. The value of capacitance for rGO is found 57.61 F/g. The sandwich structure with 0.17 M titania layers showing 121.92 F/g at 1.5 A/g current density with the highest power density observed 416 W/kg. Thus, this hybrid approach is not only having the potential to significantly boost the performance of future energy storage technologies but can also be used to develop cost-effective wearable electronics.

Enhanced dielectric constant and breakdown strength of sandwiched polymer nanocomposite film for excellent energy storage.

Enhanced dielectric constant and high breakdown strength offers immense promise for excellent energy storage performance, which is of critical significance in modern electronics and power systems. However, polymer nanocomposites with traditional routes have to balance between dielectric constant and breakdown strength, hence hindering substantive increases in energy density. Herein, a sandwiched polymer nanocomposite film has been constructed to take full advantage of the individual component layers. BaTiO3 nanoparticles are coated with a fluoropolymer to form core-shell structures and then introduced into a polymer as the top and the bottom layers of a sandwich film for enhancing polarization. Moreover, boron nitride nanosheets (BNNSs) in the middle layer of the sandwich film exert positive effects on the inhibition of current leakage for high breakdown resistance. The breakdown strength increases from 480 MV m-1 of the neat polymer to 580 MV m-1 of the sandwiched film. Additionally, the film exhibits a higher dielectric constant in comparison with the neat polymer. The sandwiched film displays a superior energy density (15.75 J cm-3), which is about 1.9 times that of the neat polymer. This work proposes a feasible route to achieve excellent energy storage of polymer dielectrics by synergistically introducing insulating fillers and additional dipoles in a sandwiched polymer nanocomposite film.

Observing coalescence of aluminum nanoparticles during burning using aluminum/ammonia perchlorate sandwiched films

To observe the agglomeration, an important parasitic process in metal composite combustion, sandwiched structures with laminates of aluminum (Al) and ammonia perchlorate (AP) layers are prepared and characterized by high-speed macro and microscopic imaging systems with pyrometry. The bilayer structure prepared by electrospraying was confirmed by Scanning Electron Microscopy/Energy Dispersive Spectroscopy (SEM/EDS) and the composition was confirmed by thermogravimetric analysis/differential thermal calorimetry (TGA/DSC). We found that the burn rate of the Al/AP sandwich films increases with the decrease of bilayer thickness of Al and AP, owing to the shorter diffusion distance between fuel and oxidizer. More importantly, the complete process of the agglomeration/sintering of Al nanoparticles (NPs) is observed in-situ, which demonstrates the different stages of agglomeration process of Al NPs. Temperature measurement from pyrometry reveals the temperature of agglomerates during combustion, consistent with the agglomerate size and burn rate observation. Simple calculations based on “pocket-theory” support the observed changes in agglomeration size with bilayer thickness.

High transparent conductive Ga-doped ZnO-based multilayer thin films with embedded ultrathin TiN layer deposited in oxygen-containing atmosphere.

To avoid metal layer oxidation during the deposition of transparent conductive oxide (TCO)/metal/TCO multilayer films in an oxygen-containing atmosphere, the ultra-thin (<10 nm) titanium nitride (TiN) layer has been proposed to replace metal embedding in gallium-doped zinc oxide (GZO) film for the development of indium-free transparent electrodes. The effects of TiN thickness on the structure, morphology, electrical, and optical properties of GZO/TiN/GZO multilayer thin films deposited in argon-oxygen mixtures on glass substrates by magnetron sputtering are investigated. The experimental results reveal that multilayers with the 8 nm-thick TiN layer have the optimal performance (figure of merit of 2.75 × 10-1 Ω-1): resistivity of 4.68 × 10-5 Ω cm, and optical transmittance of above 91% in the visible region, which is superior to the sandwich film with the metal embedded layer.

High-quality multi-walled carbon nanotubes toughened glass-ceramic by a two-dimension carbon nanotube film interlayer

The brittleness of bioglass/ceramics limits their applications in bone repair. The toughness of the glass matrix can be enhanced by adding nanoscale reinforcements at random; however, the effect is quite limited. This study proposes a novel strategy to prepare controllable fabricating lamellar precursors with multiwalled carbon nanotubes (MWCNTs) film/glass/MWCNTs film sandwich structure by a high-temperature foaming method. Subsequently, the precursor was sintered by spark plasma to obtain a composite material with strong cross-linking between MWCNTs and the glass matrix. The mechanical properties of the composites were characterized, and mechanisms were analyzed by the finite element method. The results revealed that the glass composite with 1.0 wt% MWCNTs had a significantly enhanced fracture toughness (2.65 MPa m1/2) and flexural strength (145 MPa), which were 2.1 times and 1.8 times higher than those of pure glass, respectively. This increase in toughness is attributed to the strong resistance of the well-combined bent and entangled MWCNTs to the fiber pull-out process. The glass/MWCNTs composite showed a low electrical resistivity of 5.8 Ω cm, which would aid skeletal restoration. Thus, this strategy paves a facile way to prepare the highly toughed bio-glass with multifunctional properties.

(Invited) scalable Manufacturing and Biomedical Applications of Biocompatible Piezoelectric Materials

Piezoelectric materials are a group of important functional building blocks that interfacing the human body by coupling biomechanical energy and electricity. So far, many technology innovations have advanced piezoelectric materials and composites toward a broad range of biomedical applications, which possess unique biocompatibility and flexibility. Fundamentally, materials design and engineering draw the boundary where this technology may advance. In this talk, I introduce our most recent development of piezoelectric materials and composites that are particularly designed for implantable nanogenerator applications. First, I present our wafer-scale approach to creating piezoelectric biomaterial thin films based on γ glycine crystals. The self-assembled sandwich film structure enabled both strong piezoelectricity and largely improved flexibility. Then, new ferroelectric composites are presented as a new material used in 3D printing for directly manufacturing of piezoelectric architectures with tunable piezoelectric and mechanical properties. Toward the end, novel applications of implantable piezoelectric materials are introduced, which enable the closed-loop electrostimulations for many biomedical therapeutics.

Dough‐Like Aramid Nanofiber Putty‐Assisted Design of Free‐Standing Liquid Metal‐Based Films for Ultrahigh Electromagnetic Interference Shielding

Gallium‐based liquid metal (LM) is expected to be an ideal candidate for flexible electromagnetic interference (EMI) shielding materials in wearable electronics due to its excellent electrical conductivity and extraordinary fluidity. However, it is difficult to fabricate LM as a free‐standing material due to its high surface tension and unmanageable fluidity, as well as its poor compatibility with other materials. Herein, a processing strategy is developed to fabricate free‐standing LM‐based films by vacuum filtration with the assistance of dough‐like aramid nanofiber (ANF) putty. The ANF putty improves the compatibility of ANF and LM, and also greatly improves the stability, reliability, and processability of ANF. Rational design of the structure enables the preparation of LM/ANF films with ANF as the nanobridge connecting LM micro/nanodroplets, and ANF‐LM‐ANF sandwich films with ANF as the shells, LM layer as the core. The LM/ANF film (tensile strength: 5.4 MPa) exhibits an ultrahigh EMI SE of up to 105.9 dB (8.0–12.4 GHz) at a thickness of 60 μm. While the ANF‐LM‐ANF sandwich film has higher mechanical strength (33.1 MPa), and also has good EMI shielding properties. The average EMI SE of the 10 μm thick sandwich film exceeds 45 dB in the X‐band.

Tuning magnetoelectric effect in Bi6Fe1.6Co0.2Ni0.2Ti3O18/La0.7Sr0.3MnO3/Bi6Fe1.6Co0.2Ni0.2Ti3O18 sandwich films employing micromagnetic moments and force on dipole

Mutual modulating ferroelectric polarization and magnetization provides the potential for realizing new-type low-energy consumption, high-density memory, and logic processors. Despite extensive investigations, the underlying mechanism remains elusive. In this work, we employed Bi6Fe1.6Co0.2Ni0.2Ti3O18 (BFCNT) as the ferroelectric phase and La0.7Sr0.3MnO3 (LSMO) as the ferromagnetic phase to form environmentally friendly BFCNT/LSMO/BFCNT sandwich films fabricated on flexible FTO substrates using the all-solution chemical-solution deposition route. The ferromagnetism, ferroelectricity, and magnetoelectric coupling effect of the sandwich films were investigated by measuring at macro and micro scales. Then a dynamic micromagnetic moment model and a dipole force model were proposed to study the intermodulation of both ferromagnetism and ferroelectricity. It suggested that the combination of magnetic and electric moments makes the deflection of magnetization (or polarization) and a shift of the electron orbit, causing the movement of domain walls and the flip of magnetic domains, and thus a decrease of the saturation magnetization, offering new insights into the underlying physics of ME coupling in the model system.

The High‐Strength, Flexible Organic Solvent Driven Reversible “Deformation‐Recovery‐Reverse Deformation” Responsive g‐PLA/PPC/PVA Film for Contactless Actuation

AbstractThe non‐contact deformation response driven by organic solvent has potential applications in the fields of intelligent devices, soft robots, sensing detection, and biomedical micro‐devices. The modified polyactic acid (g‐PLA)/polypropyl carbonate (PPC)/polyvinyl alcohol (PVA) sandwich with deformation response film is prepared by the asymmetric swelling response mechanism. Specially, the flexible sandwich film achieves the non‐contact driving response of “deformation‐recovery‐reverse deformation”, showing excellent deformation performance. The curvature of sandwich film reaches 13.79 cm−1 within 15 s, and the film recovers to its initial state within 3 s, then the curvature of the reverse coil reaches 18.18 cm−1 within 15 s and finally achieves a stable state. This special phenomenon is caused by the different adsorption swelling and desorption rates of volatile organic solvents in g‐PLA, PPC, and PVA layers. Additionally, the flexible film has high strength, reversible circulation, and stable actuation performance. The application scenarios of the bionic flower actuator, artificial octopus, intelligent protective cover, and transportable objects are successfully implemented. This work will significantly provide substantial guidance for the development of intelligent driving technology in the future.

Facile Separation of Cu2+ from Water by Novel Sandwich NaY Zeolite Adsorptive Membrane

Polyethersulfone-sulfonated polyethersulfone (PES-SPES)/NaY zeolite/nylon sandwich structure membranes were prepared and used to adsorb Cu2+ from water. The adsorption kinetics, adsorption isotherm, dynamic adsorption experiment, and reusability were discussed. The experimental data showed that the Langmuir isotherm model, Dubinin–Radushkevich (D-R) isotherm model, and the pseudo-first-order kinetic model can well represent the adsorption of Cu2+ on the membrane, indicating an ion exchange mechanism, with the maximum adsorption capacity of 111.25 mg·g−1. Repeatability experiments show that the sandwich film still has good adsorption performance after five times of adsorption and desorption. The as-prepared membrane showed considerable separation performance in removing Cu2+ from aspirin solution, providing a feasible method to remove heavy metals from drugs.

Combined effects of surface roughness and physically adsorbed layer in hydrodynamic journal bearing with multiscale approach

AbstractNumerical calculations were made for the film pressure and carried load of the hydrodynamic journal bearing with very large eccentricity ratios, that is, very low clearances when the combined effects of surface roughness and physically adsorbed layer were considered. The shaft was rotating with perfectly smooth surface, and the sleeve was stationary with the nanoscale sinusoidal surface roughness. The fluid piezo‐viscous effect was also incorporated. Owing to the coexistence of the nanoscale adsorbed layer and the macroscopic intermediate continuum fluid film, the multiscale approach was used to simultaneously solve this sandwich film lubrication problem. It was found that owing to low bearing clearances the effect of the adsorbed layer normally very significantly increases the lubricant film pressure, depending on the fluid‐bearing surface interaction; While at the same time both the surface roughness effect and the fluid piezo‐viscous effect are particularly significant compared to those classically calculated, owing to the local more severe film squeezing and the resulting local higher pressures. By considering the effect of the adsorbed layer, the surface roughness more significantly increases the load‐carrying capacity of the journal bearing for very large eccentricity ratios, especially for a strong fluid‐bearing surface interaction. It is suggested that in modeling the hydrodynamic journal bearing for large eccentricity ratios, the combined effects of the surface roughness, physically adsorbed layer and fluid piezo‐viscous property should be considered.