Unraveling the Mechanism and Practical Implications of the Sol-Gel Synthesis of Spinel LiMn2O4 as a Cathode Material for Li-Ion Batteries: Critical Effects of Cation Distribution at the Matrix Level.

Spinel LiMn2O4 (LMO) is a state-of-the-art cathode material for Li-ion batteries. However, the operating voltage and battery life of spinel LMO needs to be improved for application in various modern technologies. Modifying the composition of the spinel LMO material alters its electronic structure, thereby increasing its operating voltage. Additionally, modifying the microstructure of the spinel LMO by controlling the size and distribution of the particles can improve its electrochemical properties. In this study, we elucidate the sol-gel synthesis mechanisms of two common types of sol-gels (modified and unmodified metal complexes)-chelate gel and organic polymeric gel-and investigate their structural and morphological properties and electrochemical performances. This study highlights that uniform distribution of cations during sol-gel formation is important for the growth of LMO crystals. Furthermore, a homogeneous multicomponent sol-gel, necessary to ensure that no conflicting morphologies and structures would degrade the electrochemical performances, can be obtained when the sol-gel has a polymer-like structure and uniformly bound ions; this can be achieved by using additional multifunctional reagents, namely cross-linkers.

Electrochemical Properties of Multilayered Sn/TiNi Shape-Memory-Alloy Thin-Film Electrodes for High-Performance Anodes in Li-Ion Batteries.

Sn is a promising candidate anode material with a high theoretical capacity (994 mAh/g). However, the drastic structural changes of Sn particles caused by their pulverization and aggregation during charge-discharge cycling reduce their capacity over time. To overcome this, a TiNi shape memory alloy (SMA) was introduced as a buffer matrix. Sn/TiNi SMA multilayer thin films were deposited on Cu foil using a DC magnetron sputtering system. When the TiNi alloy was employed at the bottom of a Sn thin film, it did not adequately buffer the volume changes and internal stress of Sn, and stress absorption was not evident. However, an electrode with an additional top layer of room-temperature-deposition TiNi (TiNi(RT)) lost capacity much more slowly than the Sn or Sn/TiNi electrodes, retaining 50% capacity up to 40 cycles. Moreover, the charge-transfer resistance decreased from 318.1 Ω after one cycle to 246.1 Ω after twenty. The improved cycle performance indicates that the TiNi(RT) and TiNi-alloy thin films overall held the Sn thin film. The structure was changed so that Li and Sn reacted well; the stress-absorption effect was observed in the TiNi SMA thin films.

High Electrochemical Performance Silicon Thin-Film Free-Standing Electrodes Based on Buckypaper for Flexible Lithium-Ion Batteries.

Recently, applications for lithium-ion batteries (LIBs) have expanded to include electric vehicles and electric energy storage systems, extending beyond power sources for portable electronic devices. The power sources of these flexible electronic devices require the creation of thin, light, and flexible power supply devices such as flexile electrolytes/insulators, electrode materials, current collectors, and batteries that play an important role in packaging. Demand will require the progress of modern electrode materials with high capacity, rate capability, cycle stability, electrical conductivity, and mechanical flexibility for the time to come. The integration of high electrical conductivity and flexible buckypaper (oxidized Multi-walled carbon nanotubes (MWCNTs) film) and high theoretical capacity silicon materials are effective for obtaining superior high-energy-density and flexible electrode materials. Therefore, this study focuses on improving the high-capacity, capability-cycling stability of the thin-film Si buckypaper free-standing electrodes for lightweight and flexible energy-supply devices. First, buckypaper (oxidized MWCNTs) was prepared by assembling a free stand-alone electrode, and electrical conductivity tests confirmed that the buckypaper has sufficient electrical conductivity (10-4(S m-1) in LIBs) to operate simultaneously with a current collector. Subsequently, silicon was deposited on the buckypaper via magnetron sputtering. Next, the thin-film Si buckypaper freestanding electrodes were heat-treated at 600 °C in a vacuum, which improved their electrochemical performance significantly. Electrochemical results demonstrated that the electrode capacity can be increased by 27/26 and 95/93 µAh in unheated and heated buckypaper current collectors, respectively. The measured discharge/charge capacities of the USi_HBP electrode were 108/106 µAh after 100 cycles, corresponding to a Coulombic efficiency of 98.1%, whereas the HSi_HBP electrode indicated a discharge/charge capacity of 193/192 µAh at the 100th cycle, corresponding to a capacity retention of 99.5%. In particular, the HSi_HBP electrode can decrease the capacity by less than 1.5% compared with the value of the first cycle after 100 cycles, demonstrating excellent electrochemical stability.

Grain Size and Phase Transformation Behavior of TiNi Shape-Memory-Alloy Thin Film under Different Deposition Conditions.

TiNi shape-memory-alloy thin films can be used as small high-speed actuators or sensors because they exhibit a rapid response rate. In recent years, the transformation temperature of these films, manufactured via a magnetron sputtering method, was found to be lower than that of the bulk alloys owing to the small size of the grain. In this study, deposition conditions (growth rate, film thickness, and substrate temperature) affecting the grain size of thin films were investigated. The grain size of the thin film alloys was found to be most responsive to the substrate temperature.

A Simple Approach for Heat Transfer Enhancement of Carbon Nanofluids in Aqueous Media.

Nanofluids are considered alternative heat transfer fluids because of their excellent thermal and electronic conductivities. Recently, carbon nanomaterials such as carbon nanotubes and graphene have been considered to fabricate enhanced heat transfer nanofluids, but using them to prepare stable nanofluids remains challenging because of their hydrophobicity. Herein, a stable aqueous graphene and carbon nanotube dispersion was prepared using nanostructured cellulose without any additional chemicals. The dispersibility of graphene in cellulose was compared with that in conventional surfactants such as sodium dodecyl benzene sulfonate, sodium dodecyl sulfate, and lauryl betaine. In addition, the optimal mass ratio for the carbon material to cellulose was determined and improvement in the thermal and electrical conductivity of the nanofluid was investigated. The dispersion ability of cellulose was more significant than that of surfactants, and it played a major role in improving the thermal and electrical conductivity. The highest thermal conductivity obtained for the graphene-cellulose nanofluid was 615.23 W/m*K for a mass ratio of 2:1 at 20 °C. The electrical conductivity of the nanofluids increased remarkably with an increase in the cellulose content. Furthermore, the obtained nanofluid improved the heat transfer performance dramatically. It can be assumed that our proposed system can be used to ensure numerous economic and environmental benefits in the domain of heat transfer fluids.

Composition Dependence of the ß Phase Stability and Mechanical Properties of Ti-Nb Thin Films.

Shape memory alloys (SMAs) are commonly used for various applications, e.g., in the aerospace and automotive industries, robotics, and biomedical sciences. Although Ti-Ni SMAs are commercially available, the low biocompatibility of Ni has stimulated research into the development of Ti-Nb based SMAs as potential replacements of Ti-Ni alloys in biomedical applications. Ti-Nb alloys have attracted attention because of their low stiffness and superelasticity. Superelastic thin films can be used in medical applications, including the fabrication of stents for neurovascular blood vessels, which relies on a thin film and on the use of a Ti-Nb alloy coating for less biocompatible alloys. In this study, Ti-Nb thin films were prepared using magnetron sputtering. A Nb content in the range 12.2-35.9 at.% was used in the films, which was determined using energy-dispersive X-ray spectroscopy. X-ray diffraction measurement was used to analyze the crystal structure of the thin films, and their mechanical properties were investigated using nanoindentation.

Phase Stability and Properties of Ti-Nb-Zr Thin Films and Their Dependence on Zr Addition.

Ternary Ti-Nb-Zr alloys were prepared by a magnetron sputtering method with porous structures observed in some of them. In bulk, in order to control the porous structure, a space holder (NH4HCO3) is used in the sintering method. However, in the present work, we show that the porous structure is also dependent on alloy composition. The results from Young's modulus tests confirm that these alloys obey d-electrons alloy theory. However, the Young's modulus of ternary thin films (≈80⁻95 GPa) is lower than that for binary alloys (≈108⁻123 GPa). The depth recovery ratio of ternary Ti-Nb-Zr thin films is also higher than that for binary ß-Ti-(25.9⁻34.2)Nb thin film alloys.

Experimental Study on Characteristics of Grinded Graphene Nanofluids with Surfactants.

In earlier studies, much research has focused on increasing the efficiency of heat exchanger fields. Therefore, in this study, graphene nanofluid was fabricated for use as a heat transfer medium for a heat exchanger. Graphene has excellent electrical conductivity, mechanical properties, and heat transfer properties. It is expected that the heat transfer efficiency will be improved by fabricating the nanofluid. However, graphene is prone to sedimentation, because of its cohesion due to van der Waals binding force. In this experiment, a nanofluid was fabricated with enhanced dispersibility by surfactant and the ball-milling process. The zeta potential, absorbance, and thermal conductivity of the nanofluid were measured. As a result, when using the ratio of 2:1 (graphene:sodium dodecyl sulfate (SDS)), a higher thermal conductivity was obtained than in other conditions.

Surface Modification of Graphene Nanoparticles by Acid Treatment and Grinding Process.

Surface modification is necessary to decrease graphene's (GN) stacking process and increase its advantageous properties. In this study, the effects of acid treatment and grinding processes on the structural integrity of GN have been studied. Morphological and structural characteristics of modified GN were investigated by field emission scanning electron microscopy, transmission electron microscopy, gas Pycnometer, particle size analyzer, X-ray diffractometer, UV-Vis spectroscopy and thermal conductivity measurement system which expose some strong evidences of the effects of purification and grinding process on GN nanoparticles in order to get GN based better nanofluid dispersed in water which gives 1.66% and 3.38% enhancement of thermal conductivity at 20 °C and at 40 °C respectively compared to that of DW in this experiment.

Effect of Substrate Roughness on Adhesion and Structural Properties of Ti-Ni Shape Memory Alloy Thin Film.

Ti-Ni shape memory alloy (SMA) thin films are very attractive material for industrial and medical applications such as micro-actuator, micro-sensors, and stents for blood vessels. An important property besides shape memory effect in the application of SMA thin films is the adhesion between the film and the substrate. When using thin films as micro-actuators or micro-sensors in MEMS, the film must be strongly adhered to the substrate. On the other hand, when using SMA thin films in medical devices such as stents, the deposited alloy thin film must be easily separable from the substrate for efficient processing. In this study, we investigated the effect of substrate roughness on the adhesion of Ti-Ni SMA thin films, as well as the structural properties and phase-transformation behavior of the fabricated films. Ti-Ni SMA thin films were deposited onto etched glass substrates with magnetron sputtering. Radio frequency plasma was used for etching the substrate. The adhesion properties were investigated through progressive scratch test. Structural properties of the films were determined via Feld emission scanning electron microscopy, X-ray diffraction measurements (XRD) and Energy-dispersive X-ray spectroscopy analysis. Phase transformation behaviors were observed with differential scanning calorimetry and low temperature-XRD. Ti-Ni SMA thin film deposited onto rough substrate provides higher adhesive strength than smooth substrate. However the roughness of the substrate has no influence on the growth and crystallization of the Ti-Ni SMA thin films.

Forced Convective Heat Transfer of Aqueous Al2O3 Nanofluid Through Shell and Tube Heat Exchanger.

This study presents the forced convective heat transfer of a nanofluid consisting of distilled water and different weight concentrations (1 wt% and 2 wt%) of Al2O3 nanoparticles flowing in a vertical shell and tube heat exchanger under counter flow and laminar flow regime with certain constant heat flaxes (at 20 °C, 30 °C, 40 °C and 50 °C). The Al2O3 nanoparticles of about 50 nm diameter are used in the present study. Stability of aqueous Al2O3 nanofluids, TEM, thermal conductivity, temperature differences, heat transfer rate, T-Q diagrams, LMTD and convective heat transfer coefficient are investigated experimentally. Experimental results emphasize the substantial enhancement of heat transfer due to the Al2O3 nanoparticles presence in the nanofluid. Heat transfer rate for distilled water and aqueous nanofluids are calculated after getting an efficient setup which shows 19.25% and 35.82% enhancement of heat transfer rate of 1 wt% and 2 wt% aqueous Al2O3 nanofluids as compared to that of distilled water. Finally, the analysis shows that though there are 27.33% and 59.08% enhancement of 1 wt% Al2O3 and 2 wt% Al2O3 respectively as compared to that of distilled water at 30 °C, convective heat transfer coefficient decreases with increasing heat flux of heated fluid in this experimental setup.

Synthesis and Characterization of the Graphene-Fe3O4 Hybrid Composite.

Graphene and iron oxide composites have attracted huge attention in the fields of nanoelectronics and nanodevices due to their superior magnetic and electric characteristics. However, their synthesis methods are composed of many steps and use toxic chemical reactants. Accordingly, in this study, a GN-Fe3O4 NP hybrid composite was prepared using an eco-friendly and facile method. Its morphological and structural characteristics were then investigated by scanning electron microscopy, transmission electron microscopy, X-ray diffractometer and UV-visible spectroscopy. The results indicated that the GN structures as well as Fe3O4 NPs were significantly associated with the composite of GN-Fe3O4 NPs.

Microstructural and Electrochemical Properties of LiCoO2 Thin Films Prepared by Metal-Induced Crystallization.

LiCoO2 thin films were fabricated using the metal-induced crystallization (MIC) method. The effect of MIC on the microstructural and electrochemical properties of the films was investigated. The crystal structures and surface morphologies of the deposited films were investigated by X-ray diffraction (XRD), Raman spectroscopy, and field emission electron microscopy (FE-SEM). Charge-discharge tests were carried out in order to examine the electrochemical properties of the films. The LiCoO2 thin film fabricated using MIC exhibited better microstructural and electrochemical properties at a lower annealing temperature.

Compositionally graded Ti-Ni alloys prepared by diffusion bonding.

A Ti-Ni alloy compositionally graded along the thickness direction in order to obtain a shape change over a wide temperature range, which is beneficial to the actuator for precise position control, was prepared by spark plasma sintering (SPS) after stacking Ti-Ni alloy ribbons in the sequence of Ti-51Ni, Ti-50Ni, Ti-49Ni and Ti-48Ni (at%) followed by annealing. Then, the microstructure and martensitic transformation behavior were investigated by using FE-SEM, DSC and thermal cycling tests under a constant load. The inter-ribbon defects observed after SPS due to insufficient diffusional bonding between the ribbons were eliminated by post-SPS annealing at 1023 K for 36 ks. The compositionally graded sample showed compositional variation of 1.5 at% Ti along the thickness direction (- 120 µm) and a martensitic transformation temperature window as large as 91 K on cooling and 79 K on heating. A recoverable elongation of 0.9% was obtained under a stress of 80 MPa and the deformation rate, which is defined as the ratio of the recoverable elongation to the temperature range where the elongation occurred was 0.015%/K in the compositionally graded sample.

Electrochemical properties of Si film electrodes grown on current collectors with CuO nanostructures for thin-film microbatteries.

Si film electrodes were deposited onto Cu foil current collectors fabricated with well-formed CuO nanostructures. The structural and electrochemical properties of the Cu foils oxidized for 1, 3, and 6 h and of the Si film electrodes were investigated using field-emission scanning electron microscopy, X-ray diffraction (XRD), and charge/discharge tests. The morphologies and XRD profiles suggested that the oxidized Cu foils consisted of a top CuO layer and a bottom Cu2O layer. The surface roughness of the Cu foils decreased with increasing oxidation time since the flower-like CuO nanostructures weakly adhered to the surface were easily detached by ultrasonic cleaning. The cycle performance of the Si film electrode with the rougher CuO layer rapidly deteriorated, whereas the flat Cu2O layer showing a relatively high electric conductivity induced the formation of a dense Si film and improved the electrochemical performance of the Si film electrode.

Protection effect of ZrO2 coating layer on LiCoO2 thin film fabricated by DC magnetron sputtering.

Bare and ZrO2-coated LiCoO2 thin films were fabricated by direct current magnetron sputtering method on STS304 substrates. Deposited both films have a well-crystallized structure with (003) preferred orientation after annealing at 600 degrees C. The ZrO2-coated LiCoO2 thin film provide significantly improved cycling stability compared to bare LiCoO2 thin film at high cut-off potential (3.0-4.5 V). The improvement in electrochemical stability is attributed to the structural stability by ZrO2 coating layer.

Influences of Ti film thickness on electrochemical properties of Si/Ti/Cu film electrodes.

Si and Si/Ti films were fabricated on a Cu current collector (substrate) using the DC sputtering system. The Ti film as a buffer layer was inserted between the Si film and the Cu current collector. Their structural and electrochemical properties were investigated with various Ti film thicknesses of 20-90 nm. The Si and Ti films deposited on a polycrystalline Cu substrate were amorphous. The Si/Ti/Cu film electrode exhibited better electrochemical properties than the Si/Cu electrode in terms of capacity, charge-discharge efficiency, and cycleability. In the Si/Ti/Cu electrode, the film electrode with a 55 nm Ti film thickness showed the best electrochemical properties: 367 microA h/cm2 initial capacity, 91% efficiency, and 50% capacity retention after 100 cycles. These good electrochemical properties are attributed to the enhanced adhesion between the Si and Ti films. Additionally, the modified surface morphology of Si film with a cluster structure could withstand the lateral volume change during the charge-discharge process.

Patterned Si thin film electrodes for enhancing structural stability.

A patterned film (electrode) with lozenge-shaped Si tiles could be successfully fabricated by masking with an expanded metal foil during film deposition. Its electrochemical properties and structural stability during the charge-discharge process were examined and compared with those of a continuous (conventional) film electrode. The patterned electrode exhibited a remarkably improved cycleability (75% capacity retention after 120 cycles) and an enhanced structural stability compared to the continuous electrode. The good electrochemical performance of the patterned electrode was attributed to the space between Si tiles that acted as a buffer against the volume change of the Si electrode.