Study of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">δ</Emphasis></Subscript> Superconductor/Graphene Oxide Composite

The aim of this research was to fabricate YBa₂Cu₃O_7?δ (YBCO) superconductor composite with graphene oxide nanosheets and to study the effect of the graphene oxide nanosheets on YBCO superconductor properties. For this purpose, the samples of pure superconductor and superconductor composite with 0.001, 0.01, and 0.1 wt.% graphene oxide were synthesized. First, graphite oxide was made by Hummer’s chemical method; after that, graphene oxide nanosheets were produced by bath-keeper ultrasonic. Then, different amounts of graphene oxide were added to the process of superconductor fabrication, which was made by solid-state reaction method. The samples were characterized and studied by Meissner effect test, XRD analysis, FESEM imaging, EDX measurement, and ac magnetic susceptibility. The critical current density (J_c) of samples was measured by four probes method. The results showed that by increasing the weight ratio of graphene oxide, J_c and T_c decrease.     

Tailored Crumpling and Unfolding of Spray‐Dried Pristine Graphene and Graphene Oxide Sheets

For the first time, pristine graphene can be controllably crumpled and unfolded. The mechanism for graphene is radically different than that observed for graphene oxide; a multifaced crumpled, dimpled particle morphology is seen for pristine graphene in contrast to the wrinkled, compressed surface of graphene oxide particles, showing that surface chemistry dictates nanosheet interactions during the crumpling process. The process demonstrated here utilizes a spray‐drying technique to produce droplets of aqueous graphene dispersions and induce crumpling through rapid droplet evaporation. For the first time, the gradual dimensional transition of 2D graphene nanosheets to a 3D crumpled morphology in droplets is directly observed; this is imaged by a novel sample collection device inside the spray dryer itself. The degree of folding can be tailored by altering the capillary forces on the dispersed sheets during evaporation. It is also shown that the morphology of redispersed crumpled graphene powder can be controlled by solvent selection. This process is scalable, with the ability to rapidly process graphene dispersions into powders suitable for a variety of engineering applications.     

Graphene/MnO2 hybrid nanosheets as high performance electrode materials for supercapacitors

Graphene/MnO₂ hybrid nanosheets were prepared by incorporating graphene and MnO₂ nanosheets in ethylene glycol. Scanning electron microscopy and transmission electron microscopy analyses confirmed nanosheet morphology of the hybrid materials. Graphene/MnO₂ hybrid nanosheets with different ratios were investigated as electrode materials for supercapacitors by cyclic voltammetry (CV) and galvanostatic charge–discharge in 1 M Na₂SO₄ electrolyte. We found that the graphene/MnO₂ hybrid nanosheets with a weight ratio of 1:4 (graphene:MnO₂) delivered the highest specific capacitance of 320 F g⁻¹. Graphene/MnO₂ hybrid nanosheets also exhibited good capacitance retention on 2000 cycles.     

High-quality production of graphene by liquid-phase exfoliation of expanded graphite

As one of the most promising candidates, graphene exhibits a potential application in post-silicon nanoelectronics. However, it is a key issue to produce high-quality graphene in large scale. Here, a facile method is demonstrated to produce graphene dispersions by exfoliation of expanded graphite in the co-solvent with N,N-dimethylformamide (DMF) and water. We confirm that the optimal ratio of DMF to water for graphene exfoliation is 9:1 (v:v) by means of UV–Vis absorption spectra. This exfoliation results in large flakes ∼2 μm in diameter, which can potentially be improved by adjusting the sonication power. The relatively perfect hexagonal structure of graphene is confirmed by Raman spectroscopy and the as-prepared graphene nanosheet film the as-prepared graphene nanosheet film possesses good electrical conductivity (∼8.3 × 10³ S m⁻¹). DC electrical transport phenomena for the deposited film of graphene nanosheets are well described in terms of conduction models for non-crystal semiconductor. This convenient approach provides an extensive route to prepare high-quality graphene nanosheets.     

Effect of ultrasound irradiation on some freezing parameters of ultrasound-assisted immersion freezing of strawberries

In this study, the effect of ultrasound irradiation temperature and ultrasound intensity on the freezing and nucleation in strawberry samples was studied. The application of ultrasound irradiation at different temperatures was able to induce nucleation at lower degree of supercooling compared to the control samples. The achieved degree of supercooling in the ultrasound irradiated strawberries was linearly correlated to the ultrasound irradiation temperature. At the ultrasound irradiation temperature of −1.6 °C, the characteristic freezing time (CFT) was significantly shorter than that in the control sample (p < 0.05). The application of ultrasound at higher intensities was found to effectively shorten the CFT. The degree of supercooling in ultrasound irradiated samples was not linearly correlated to ultrasound intensity. In conclusion, the combination of ultrasound irradiation temperature and intensity can be effective in controlling nucleation and freezing processes of perishable fruits such as strawberry.     

Size distribution-controlled preparation of graphene oxide nanosheets with different C/O ratios

A convenient and efficient preparation method for separation graphene oxide with well-defined size distribution is developed using a centrifugation technique. The graded profile of graphene oxide nanosheets with narrow size distribution is effectively controlled by varying the centrifugation speed. The results show that the oxygen content of graphene oxide is highly dependent on their size distribution. Graphene oxide nanosheet with large size shows a red-shift in UV–vis absorption spectra, compared to graphene oxide with small size. This phenomenon is interpretation by a density functional theory calculation. The present work will provide a simple method to prepare graphene oxide nanosheets with controllable size distribution and C/O ratio, which will be valuable for the functionalization of graphene-based hybrids and the fabrication of graphene nano-devices.     

Superhigh and Robust Ion Selectivity in Membranes Assembled with Monolayer Clay Nanosheets

It is crucial to control the ion transport in membranes for various technological applications such as energy storage and conversion. The emerging functional two-dimensional (2D) nanosheets such as graphene oxide and MXenes show great potential for constructing ordered nanochannels, but the assembled membranes suffer from low ion selectivity and stability. Here a class of robust charge-selective membranes with superhigh cation/anion selectivity, which are assembled with monolayer nanosheets of cationic/anionic clays that inherently have permanent and uniform charges on each layer is reported. The transport number of cations/anions of cationic vermiculite nanosheet membranes (VNMs)/anionic Co-Al layered double hydroxide (CoAl-LDH) nanosheet membranes is over 0.90 in different NaCl concentration gradients, outperforming all the reported ion-selective membranes. Importantly, this excellent ion selectivity can persist at high-concentration salt solutions, under acidic and alkaline conditions, and for a wide range of ions of different sizes and charges. By coupling a pair of cation-selective vermiculite membrane and anion-selective CoAl-LDH membrane, a reverse electrodialysis device which shows an output power density of 0.7 W m⁻² and energy conversion efficiency of 45.5% is constructed. This work provides a new strategy to rationally design high-performance ion-selective membranes by using 2D nanosheets with inherent surface charges for controllable ion-transport applications.     

Reinforcing polyamide 1212 with graphene oxide via a two-step melt compounding process

In this study, graphene oxide (GO) was synthesized from graphite powders via Hummers’ method. Polyamide 1212 (PA1212)/GO composites were prepared via a two-step melt compounding process. First, GO concentrates were prepared via solution coagulation. In this method, a GO solution was mixed with an ethanol-soluble polyamide solution. The resulting product was melt-compounded with a PA1212 matrix. This method enabled GO nanosheets to be well-dispersed in a PA1212 matrix. GO, which functioned as a nucleation template, exhibited heterogeneous nucleation effect in the PA1212 matrix because of its large specific surface area. The mechanical properties of the oriented PA1212/GO composites improved efficiently compared with those of pure PA1212. Crystal orientation degree and crystallinity in the composites increased slightly when GO was added after drawing. The composites’ reinforcing effect was mainly attributed to GO nanosheet alignment. These nanonsheets functioned as the nuclei to reinforce the entire oriented crystals.     

Superheating limits of liquid helium I

It is shown that the maximum superheat temperature ΔT_max in liquid helium I, determined under steady-state peak nucleate boiling conditions, cannot be realistically estimated from the thermodynamic limit of superheat (spinodal), as has been previously claimed. It is suggested that a more meaningful estimate of ΔT_max can be obtained from the kinetic limit of super-heat (homogeneous nucleation temperature). Experimental data on the homogeneous nucleation superheat temperature, ΔT_h, in liquid helium I over an extended temperature range are presented and compared with the ΔT_max measurements of several investigators. It is pointed out that in any quantitative predictions of ΔT_max, a knowledge of ΔT_h alone is not sufficient. In fact, the measured value of ΔT_max can lie either below or even above ΔT_h depending on the bath temperature, conditions of the surface, and the thermophysical properties of both heater and liquid.     

The synthesis of graphene oxide nanostructures for supercapacitors: a simple route

In this work, we report a simple strategy for synthesis of graphene oxide nanostructures with various morphologies including single-, few-layer, and three-dimensional networks. Morphology control is achieved by adding different amounts of Ni²⁺ into a one-step hydrothermal process. The involved growth mechanisms for the morphology control are discussed. A random arrangement of graphene oxide nanosheets is suggested to induce the networks’ formation. Ni²⁺ facilitates the formation of graphene oxide’s preferential face-to-face overlapping structure, and high Ni²⁺ concentrations render adjacent graphene oxide sheets to combine each other tightly to form closely packed, layered structures. Compared with single-, few-layer graphene oxide, the electrode prepared by three-dimensional networks has a mass specific capacitance of 352 F g^?1 at v = 5 mV s^?1, which is much higher than that of recently reported three-dimensional graphene oxide nanostructures (240 F g^?1).     

Preparation of graphene nanosheet/alumina composites by spark plasma sintering

Graphene nanosheet/alumina composite has been prepared by spark plasma sintering. A homogeneous distribution of nanosheets in an alumina matrix could be obtained by the electrostatic attraction between graphite oxide and alumina particles and their subsequent reduction. The introduction of graphene nanosheet leads to refinement of grain size of alumina after hot pressing. The experimental results have shown that the fracture toughness and conductivity of the graphene nanosheet/alumina composite are about 53% and 13 orders of magnitude higher than those of unreinforced alumina material, respectively.     

High Performance of N‐Doped Graphene with Bubble‐like Textures for Supercapacitors

Nitrogen‐doped graphene (NG) with wrinkled and bubble‐like texture is fabricated by a thermal treatment. Especially, a novel sonication‐assisted pretreatment with nitric acid is used to further oxidize graphene oxide and its binding with melamine molecules. There are many bubble‐like nanoflakes with a dimension of about 10 nm appeared on the undulated graphene nanosheets. The bubble‐like texture provides more active sites for effective ion transport and reversible capacitive behavior. The specific surface area of NG (5.03 at% N) can reach up to 438.7 m² g^?1, and the NG electrode demonstrates high specific capacitance (481 F g^?1 at 1 A g^?1, four times higher than reduced graphene oxide electrode (127.5 F g^?1)), superior cycle stability (the capacitance retention of 98.9% in 2 m KOH and 99.2% in 1 m H₂SO₄ after 8000 cycles), and excellent energy density (42.8 Wh kg^?1 at power density of 500 W kg^?1 in 2 m KOH aqueous electrolyte). The results indicate the potential use of NG as graphene‐based electrode material for energy storage devices.     

Enhanced thermal conductivity of nanofluids containing graphene nanoplatelets prepared by ultrasound irradiation

Stable ethylene glycol (EG)-based nanofluids containing graphene nanoplatelets (GnPs) were prepared by intensive ultrasonication without any surfactant. The structural properties of the commercially produced GnPs were confirmed using the nitrogen gas adsorption method, Fourier transform infrared spectroscopy, X-ray diffraction method, Raman spectroscopy, and high-resolution transmission electron microscopy. After ultrasound irradiation, the GnP aggregates were broken into thinner and smaller-sized nanosheets, which is beneficial for a stable dispersion. The ultrasonic-treated GnPs showed a constant value of thermal conductivity enhancement, k/k _o (= 1.127 ± 0.002) at 2 vol% in the temperature range of 10–90 °C. From the analyses of the thermal conductivities of the GnP nanofluids as functions of GnP concentration and temperature, it was concluded that the thermal conductivity increased as the GnP concentration and the temperature increased. Furthermore, the experimentally measured thermal conductivities of the EG-based GnP nanofluids were much higher than the theoretically calculated values based on the Hamilton–Crosser correlation, which is due to higher specific surface area and two-dimensional structures of the GnPs.     

Size and temperature consideration in the liquid layer growth from nanovoids and the melting model construction

A new model for the solid melting point T_m(D) from nanovoids is proposed through considering the liquid layer growth behavior. This model, which does not have any adjustable parameter, introduces the classical thermodynamic treatment, i.e., the liquid nucleation and growth theory, for nanoparticle melting. With increased void diameter D, T_m(D) approaches to T_m0. Moreover, T_m(D) > T_m0 for a small void (T_m0 is the bulk melting point). In other words, the solid can be significantly superheated especially when D decreases, even if the difference of interface energy is larger than zero. This finding can be expected from the negatively curved surface of the void. The model predictions are consistent with the molecular dynamic (MD) simulation results for argon solids. Moreover, the growth of liquid layer from void surface relies on both size and temperature, which directly determine liquid layer thickness, and only when liquid layer thickness reaches to a critical value, can void become instable.     

Converting graphene oxide monolayers into boron carbonitride nanosheets by substitutional doping

To realize graphene-based electronics, bandgap opening of graphene has become one of the most important issues that urgently need to be addressed. Recent theoretical and experimental studies show that intentional doping of graphene with boron and nitrogen atoms is a promising route to open the bandgap, and the doped graphene might exhibit properties complementary to those of graphene and hexagonal boron nitride (h-BN), largely extending the applications of these materials in the areas of electronics and optics. This work demonstrates the conversion of graphene oxide nanosheets into boron carbonitride (BCN) nanosheets by reacting them with B(2) O(3) and ammonia at 900 to 1100 °C, by which the boron and nitrogen atoms are incorporated into the graphene lattice in randomly distributed BN nanodomains. The content of BN in BN-doped graphene nanosheets can be tuned by changing the reaction temperature, which in turn affects the optical bandgap of these nanosheets. Electrical measurements show that the BN-doped graphene nanosheet exhibits an ambipolar semiconductor behavior and the electrical bandgap is estimated to be ≈25.8 meV. This study provides a novel and simple route to synthesize BN-doped graphene nanosheets that may be useful for various optoelectronic applications.     

CuS nanoplatelets arrays grown on graphene nanosheets as advanced electrode materials for supercapacitor applications

CuS nanoplatelets arrays grown on graphene nanosheets are successfully synthesized via a facile low-temperature solvothermal reaction with graphene oxide (GO), CH₃CSNH₂ and Cu(CH₃COO)₂·H₂O as the reactants. CH₃CSNH₂ plays an important role in being the reducing agent for GO and the sulfur source of CuS. Supercapacitive performance of the graphene/CuS nanocomposite as active electrode materials has been evaluated by cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy measurements. The results indicate that graphene/CuS electrode delivers a high capacitance of 497.8 F g^–1 at a current density of 0.2 A g^–1, which outperforms bare CuS electrode. This excellent performance is ascribed to the short diffusion path and large surface area of the unique hierarchical nanostructure with nanoflakes building blocks for bulk accessibility of faradaic reaction.     

Determination of the activation energy for crystal growth by differential thermal analysis

A method for determining the activation energy for crystal growth was calculated on the basis of the heat balance in the differential thermal analysis (DTA) measurements and the mechanism of nucleation and growth. The theoretical analysis showed that the term ln[C _pd(δT)/dt+KδT] should be a linear function of l/T, whereC _p is the heat capacity of sample and sample holder,K is the heat transfer coefficient,δT is the temperature difference between the sample and reference substance andt is the time. The energy term,E _D, obtained by multiplying the slope of the resulting straight line byR is indicative of the activation energy for crystal growth. It was shown thatE _D should be three times the activation energy for crystal growth when bulk nucleation is dominant, and equal to that for crystal growth only when surface nucleation predominates. The result of the analysis was tested by comparing the experimentally determinedE _D's with the activation energy for viscous flow, which was known to represent that for crystal growth. TheE _D for Li₂O·2SiO₂ glass with dominant bulk nucleation, approached three times the activation energy for viscous flow, as the heating rate in DTA decreased. TheE _D for 33.3Li₂O·66.7SiO₂·3TiO₂ glass with dominant surface nucleation approached the activation energy for viscous flow as the heating rate increased, suggesting the validity of the analysis.     

Morphology Tunable Hybrid Carbon Nanosheets with Solvatochromism

The tunable photoluminescence of carbon‐based nanomaterials has received much attention for a wide range of applications. Herein, a unique, broad‐solvatochromic hybrid carbon nanosheet (CNS) synthesized through the hydrothermal carbonization of molecular precursors exploiting graphene oxide as a template is reported, resulting in the formation of clusters of carbon nanorings on the surface of graphene‐oxide nanosheets. Under UV and visible‐light excitation, the hybrid CNS exhibits tunable emission spanning the wide range of colors in a series of solvents with different polarities. This interesting spectroscopic behavior is found to originate from hydrogen‐bonding interactions between CNS and solvents, which eventually induce the morphological transition of CNS from 2D sheets to 3D crumpled morphologies, affecting the lifetimes of emissive states. This novel soft carbon nanostructure may open up a new possibility in tailoring the photophysical properties of carbon nanomaterials.     

Surface modification of a flexible polyimide film using a reactive fluorinated polymer nanosheet

This paper describes an effective approach to surface modification of a flexible polyimide film using a reactive fluorinated polymer nanosheet. N-(1H,1H-pentadecafluorooctyl)methacrylamide copolymers containing carboxyl group as a reactive moiety form stable monolayer on the water surface and highly ordered reactive polymer nanosheets can be fabricated by the Langmuir-Blodgett technique. This reactive fluorinated polymer nanosheet was utilized to modify the surface properties of polyimide film through its immobilization using thermal treatment. The modification process was studied by X-ray photoelectron spectroscopy (XPS) and modified PI surface was characterized by contact angle measurements and atomic force microscopy (AFM).     

Preparation of graphene nanosheets/SnO2 composites by pre-reduction followed by in-situ reduction and their electrochemical performances

The Graphene nanosheets/SnO₂ composites were synthesized using stannous chloride to restore the semi-reduction graphene oxide (SRGO) under a simple hydrothermal reduction procedure. First graphene oxide was pre-reduced by glucose for a certain time to get SRGO, which keeps the good water-solubility of graphite oxide (GO) and has a good conductivity like graphene nanosheets. The higher electrostatic attraction between SRGO and Sn²⁺ makes SnO₂ nanoparticles tightly anchor on the graphene sheets in the hydrothermal reduction process. The formation mechanism of the composite was investigated by SEM, TEM, XRD, AFM and Raman. Moreover, the electrochemical behaviors of the Graphene nanosheets/SnO₂ nanocomposites were studied by cyclic voltammogram, electrical impedance spectroscopy (EIS) and chronopotentiometry. Results showed that the Graphene nanosheets/SnO₂ composites have excellent supercapacitor performances: the specific capacitance reached 368 F g⁻¹ at a current density of 5 mA cm⁻², and the energy density was much improved to 184 Wh kg⁻¹ with a power density of 16 kW kg⁻¹, and capacity retention was more than 95% after cycling 500 cycles with a constant current density of 50 mA cm⁻². The experimental results and the thorough analysis described in this work not only provide a potential electrode material for supercapacitors but also give us a new way to solve the reunification of the graphene sheets.