Formation Of Flakes Research Articles

Deposition mechanism of graphene flakes directly from graphite particles in the kinetic spray process studied using molecular dynamics simulation

Deposition of graphene flakes in a kinetic spray process is a relatively new topic of research. The present work provides insight into the formation of graphene flakes from bulk graphite particles using molecular dynamics (MD) simulations due to the advantage of the short MD simulation timescales. The deposition of nano-scale graphite particles onto a copper (Cu) substrate was studied as a function of various particle sizes (4 nm, 6 nm, 8 nm and 10 nm) and impact velocities. For each particle, there is a critical impact velocity in which graphene flakes (single or few-layer) were separated from bulk graphite particles, and the separated graphene flakes were attached to a Cu substrate to form deposited layers. The critical impact velocity required for layer separation decreased gradually with increasing particle size. The deposition behavior was observed over the whole simulation period for each particle with different impact velocities, where the deposition mechanism at the critical impact velocity involves interlayer separation of graphite particles due to the high impact velocity. Particles deformed plastically during deposition, and Cu substrates deformed elastically/plastically for the deposition of various graphene flake structures, which are strongly dependent on impact velocities. The surface temperature of the Cu substrate increased at the impact zone due to the kinetic energy of the impacted graphite particles. Moreover, the number of deposited layers increased with increasing impact velocities beyond the critical impact velocity for the separation of graphene layers. Apart from these, there is a critical impact velocity for each particle, where excess impaction causes disordering of atoms/etching of the Cu substrate at the impact zone, and the disorder of atoms increased violently beyond this critical impact velocity. The simulation results also revealed a deposition window for graphene flake deposition in the kinetic spray process, where the impact velocity of nano-scale graphite particles plays a crucial role in successful graphene flake deposition.

Near Room Temperature Light-Activated WS2-Decorated rGO as NO2 Gas Sensor.

The NO2 response in the range of 200 ppb to 1 ppm of a chemoresistive WS2-decorated rGO sensor has been investigated at operating temperatures of 25 °C and 50 °C in dry and humid air (40% RH) under dark and Purple Blue (PB) light conditions (λ = 430 nm). Few-layers WS2, exfoliated by ball milling and sonication technique, with average dimensions of 200 nm, have been mixed with rGO flakes (average dimension 700 nm) to yield WS2-decorated rGO, deposited on Si3N4 substrates, provided with platinum (30 μm gap distance) finger-type electrodes. TEM analysis showed the formation of homogeneous and well-dispersed WS2 flakes distributed over a thin, continuous and uniform underlying layer of interconnected rGO flakes. XPS and STEM revealed a partial oxidation of WS2 flakes leading to the formation of 18% amorphous WO3 over the WS2 flakes. PB-light irradiation and mild heating of the sensor at 50 °C substantially enhanced the baseline recovery yielding improved adsorption/desorption rates, with detection limit of 400 ppb NO2 and reproducible gas responses. Cross sensitivity tests with humid air interfering vapor highlighted a negligible influence of water vapor on the NO2 response. A charge carrier mechanism between WS2 and rGO is proposed and discussed to explain the overall NO2 and H2O response of the WS2–rGO hybrids.

DETERMINATION OF THE STRUCTURE OF CARBON PARTICLES FORMED WHEN FORMING A POLYMER FILM IN A PLASMA CHEMICAL REACTOR

Abstract—We study the formation mechanism of carbon nanoparticles with simultaneous formation of a polystyrene film in a AC barrier corona discharge at atmospheric pressure. The importance of the research stems from the need to control the allotropic form of carbon nanoparticles, which affects the physical and technical characteristics of polymer films obtained by this method. It is shown that nucleation of polycrystalline onion-like carbon nanoparticle agglomerates is the basis for graphene flake formation in the corona sheath. Graphene flakes form from these nucleation sites in gas discharge streamers owing to the destruction of monomer molecules remaining in the agglomerates of nucleation sites. It was revealed that the allotropic form of such particles is determined not only by the energy—in this case the barrier corona discharge—but also by the ratio of the duration of its exposure to the characteristic destruction and formation times of covalent bonds participating in the particle process.

Particle Method Simulation for Formation and Flow of Cold Flakes in High-Pressure Die Casting

In high-pressure die casting (HPDC) processing of aluminum alloys, solidified layers generated in the sleeve of a die casting machine that flow into the mold cavity are known as “cold flakes.” The prediction and control of them are a crucially important issue for HPDC. This study developed a method to simulate their formation and flow using smoothed particle hydrodynamics. First, a solidified layer was modeled as a set of solid particles with behaviors defined by mechanical constitutive equations. Second, this study proposed an algorithm for ascertaining the phase of particles and for calculating liquid–solid particle interaction. Numerical results demonstrated that the method can predict the formations of solidified layers in the sleeve, their peeling and folding during the plunger movements, their inflow into the runner and the mold cavity, and flow disturbances caused by solidified layers trapped at the gate.

Carboxymethyl cellulose aided fabrication of flaky structured mesoporous β-Co(OH)2/C nanocomposite for supercapacitors

This paper reports ecofriendly synthesis of mesoporous cobalt hydroxide using carboxymethyl cellulose as carbon source. Different analytical investigations on this material indicate the formation of mesoporous flakes. This kind of morphology is highly favorable for energy storage applications. The electrochemical performance of the electrode material is evaluated for supercapacitor, in an alkaline electrolyte; it shows high specific capacitance and good rate capacity. In addition, it retains 93% of initial capacitance after 3000 repeated charge–discharge cycles. This attractive nanocomposite shows very low value of charge transfer resistance (1 Ω). We design an asymmetric supercapacitor device using β-Co (OH)2/C and activated carbon as electrode. This device exhibits a specific capacitance of 102 F g−1 at 1 A g−1. The outcomes of these studies suggest its usefulness in supercapacitor devices.

Rapid formation of brushite flakes on pyrolytic carbon modified carbon fibers by pulsed electrodeposition

Brushite flakes are rapidly synthesized on pyrolytic carbon modified carbon fibers (PCF) by pulsed electrodeposition (PED) method. The maximum length of the brushite flakes could reach 63.5 μm in 10 s and 423.7 μm in 20 min. The formation mechanism of the brushite flakes is proposed, identifying the influence of fiber-shaped substrate, surface oxygen-containing functional group and PED preparation method. Moreover, good biomineralization ability of the brushite flakes is confirmed by in-vitro test in simulated body fluid. The brushite flakes prepared by this method has the potential applications as bioactive coatings, biological reinforcements, biological fillers and scaffolds for tissue engineering.

Effects of Different Parameters on Porosity Defects Between the Horizontal and Vertical Shot Sleeve Processes

The effects of different casting parameters on porosity defects between the horizontal and vertical shot sleeve processes are quantitatively studied based on experiments and simulations. The castings made using the two processes are characterized in terms of porosity defects by X-ray radiographic inspection and blistering formation by T4 heat treatment process. Experimental results indicate that much fewer porosities and blisters are formed in the castings made using the vertical process than those made using the horizontal one. Meanwhile, numerical simulations have been performed to quantitatively investigate the flow dynamics behavior of molten metal in the horizontal and vertical shot sleeves. The simulation results demonstrate that compared with the conventional horizontal process, the vertical shot sleeve process is more efficient in elimination of waves and reduction of air entrapment and oxide formation. Moreover, the vertical process needs a lower pouring temperature which can decrease hydrogen solubility in the molten metal; it has a smaller temperature gradient of molten melt which can reduce the formation of cold flakes and pre-solidified materials. Therefore, the vertical shot sleeve process is more competitive in improving the internal integrity of die castings.

Insight into ball milling for size reduction and nanoparticles production of H-Y zeolite

High energy ball milling (BM) of micro H-Y zeolites was studied at a wide range of speeds, milling duration, and ball sizes. SEM analysis reveals that BM produces three types of species that can be distinguished namely: nanoparticles, flakes/slabs, and un-ground micro-sized particles. BM at high speeds reduces the particle size of zeolite to the nanometer range in a very short duration. However, BM for longer time led to the formation of zeolite flakes. The particle size reduction was achieved at the cost of loss in zeolite crystallinity. Among several combinations of BM processing parameters, BM at 1000 rpm for short duration were found to be the optimum parameters to reduce the zeolite into nanoparticles while retaining considerable crystallinity. Post BM centrifugation was found to be essential in obtaining a homogeneous particle size distribution. The produced nanoparticles had an average diameter of 50–100 nm, surface area of 465 m2/g, and a crystallinity index of 20%.

Sunlight activated anodic freestanding ZrO2 nanotube arrays for Cr(VI) photoreduction

Visible-light-active freestanding zirconia (ZrO2) nanotube (FSZNT) arrays were fabricated by a facile electrochemical anodization method in fluoride containing ethylene glycol electrolyte added to it was 1 vol% of potassium carbonate (K2CO3) at 60 V for 1 h. Poor adhesion at the metal∣oxide interface was induced by K2CO3 leading to the formation of FSZNT flakes. The effect of the crystal structures of the FSZNTs e.g., amorphous, amorphous/tetragonal, and tetragonal/monoclinic was investigated towards the photocatalytic reduction of 10 ppm hexavalent chromium, Cr(VI) at pH 2 under sunlight. The results demonstrate the amorphous FSZNTs exhibited the highest Cr(VI) removal efficiency than the crystalline FSZNTs (95% versus 33% after 5 h). The high photocatalytic activity of the amorphous FSZNTs can be attributed to enhanced Cr(VI) adsorption, high visible light absorption, and better charge carrier separation. The low photocatalytic activity of the crystalline FSZNTs annealed at 500 °C was mainly attributed to poor Cr(VI) adsorption, low visible light absorption, and less photoactive monoclinic-ZrO2.

Obtaining high crystalline ball milled H-Y zeolite particles with carbon nanostructures as a damping material

Abstract Nanosized zeolites have attracted interest for many applications including catalysis due to their higher surface area and hence more accessible active sites compared to the microsize zeolite particles. Ball milling is considered as a fast and an efficient technique to reduce the particle size down to nanometer range. However, it is usually accompanied by the formation of large undesired flakes and slabs with a loss of crystallinity and surface area. This study attempts to utilize carbon nanostructures (CNS) as a damping material to overcome the reported drawbacks. The idea of using CNS as a damping material is novel as this is the first study which reports the usage of CNS as damping material that prevents the zeolite micro particles from severe loss in crystallinity compared to the same process without CNS addition. Utilization of CNS gives two-fold advantages pertaining to its flexibility and superior mechanical properties and secondly the ease by which it can be removed off through oxidation after ball milling. Different parameters such as the ball milling solvent and post treatment conditions were varied and studied to observe the changes in final ball milled particle properties, such as morphology, crystallinity index, surface area and particle size distribution. Optimum ball milling and post treatment conditions registered a final zeolite surface area of 1522 m2/g compared to the initial 850 m2/g for micron sized particles. Moreover, x-ray diffraction and high resolution transmission electron microscopy (HR-TEM) images revealed highly crystalline zeolite nano particles with a crystallinity index of 38% which is more than 30% higher than previously reported study of ball milled zeolite particles without any damping material.

Spin-Crossover in an Exfoliated 2D Coordination Polymer and Its Implementation in Thermochromic Films

Development of novel 2D materials with singular and thrilling properties has aroused large interest due to the novel unexpected applications that can be derived from there. In this sense, coordination polymers (CPs) have appeared as matching candidates thanks to their rational chemical design and the added-value properties given by the presence of metal ions. This is the case of switchable spin-crossover systems that have been proposed as excellent candidates for data storage or sensing, among others. Here we report the delamination of crystals of the 2D spin-crossover (SCO) {[Fe(L1)2](ClO4)2}∝ (1) CP by liquid-phase exfoliation (LPE) in water. The application of this top-down technique results in the formation of flakes with controlled thicknesses, down to 1–2 nm thick (mostly mono- and bilayer), that retain the chemical composition and SCO interconversion of the bulk material. Moreover, these flakes can be handled as stable colloidal dispersions for many days. This allows for a controlled transfer to so...

Study on Flake Formation Behavior and Its Influence Factors in Cr5 Steel.

A flake is a crack that is induced by trapped hydrogen within steel. To study its formation mechanism, previous studies mostly focused on the formation process and magnitude of hydrogen pressure in hydrogen traps such as cavities and cracks. However, according to recent studies, the hydrogen leads to the decline of the mechanical properties of steel, which is known as hydrogen embrittlement, is another reason for flake formation. In addition, the phenomenon of stress induced hydrogen uphill diffusion should not be neglected. All of the three behaviors are at work simultaneously. In order to further explore the formation mechanism of flakes in steel, the process of flake initiation and growth were studied with the following three coupling factors: trap hydrogen pressure, hydrogen embrittlement, and stress induced hydrogen re-distribution. The analysis model was established using the finite element method, and a crack whose radius is 0.5 mm was set in its center. The cohesive method and Bilinear Traction Separate Law (BTSL) were used to address the coupling effect. The results show that trap hydrogen pressure is the main driving force for flake formation. After the high hydrogen pressure was generated around the trap, a stress field formed. In addition, the trap is the center of stress concentration. Then, hydrogen is concentrated in a distribution around this trap, and most of the steel mechanical properties are reduced. The trap size is a key factor for defining the critical hydrogen content for flake formation and propagation. However, when the trap size exceeds the specified value, the critical hydrogen content does not change any more. As for the crack whose radius is 0.5 mm, the critical hydrogen content of Cr5VMo steel is 2.2 ppm, which is much closer to the maximum safe hydrogen concentration of 2.0 ppm used in China. The work presented in this article increases our understanding of flake formation and propagation mechanisms in steel.

The adsorption geometry and molecular self-assembly of graphene for 1,3,5-triphenylbenzene on Cu(111)

Electronic structures and adsorption geometry of 1,3,5-triphenylbenzene (TPB) on the Cu(111) surface, and followed by formation of graphene nano flakes (GNFs) upon annealing the sample were investigated by using angle resolved ultraviolet photoelectron spectroscopy (ARUPS) and density functional theory (DFT) calculations. The emission features d, e and f originated from TPB molecule at a coverage of 1.3 monolayer (ML) in UPS spectra are located at 4.65 eV, 6.76 eV and 8.57 eV below Fermi level, and the decrease of the work function upon increasing of TPB coverage on Cu(111) is attributed to the interfacial dipole formed between the adsorbate and the substrate. Formation of GNFs upon annealing of TPB overlayer on Cu(111) is approved by appearance of new features g, h, i, j and k originating from GNFs, and located at 1.09, 4.97, 6.64, 9.23 and 11.23 eV below Fermi level. The ARUPS measurements and DFT calculations indicate that both TPB molecules and GNFs planes are nearly parallel to the Cu(111) substrate in the monolayer regime. Most importantly, the results show that, with decreasing of the coverage of TPB as precursor, the formation of GNFs, or graphitization temperature can be decreased down to 350 °C, indicating the crucial influence of intermolecular interaction on the dehydrogenation process.

Suppressing Aluminum Carbide in Welding Aluminum Silicon Carbide Composite

Weldability of aluminum-based metal matrix composite A359/SiC/10p using gas tungsten arc welding and R356.0 filler material is investigated. The welding current, welding speed, and preheat temperature affect the weld quality significantly. Finite element analysis is successfully applied to map the weldment temperature during preheating and welding. During mechanical testing of welded specimens, a crack propagates in the parent composite or in the weld, but not in the stronger zone between the weld and the parent material. The weld region contains reasonably uniform distribution of SiC particles due to high viscosity of the molten weld and its fast cooling rate. Proper control of heat input and addition of silicon-rich filler material hinder the interface reaction between aluminum matrix and the reinforcing SiC particles, and successfully suppresses the formation of harmful aluminum carbide flakes in the weld. The average tensile and flexural strengths of optimally welded specimens approach those of the parent composite while its ductility exceeds that of the parent material.

Effect of friction on oxidative graphite intercalation and high-quality graphene formation

Oxidative wet-chemical delamination of graphene from graphite is expected to become a scalable production method. However, the formation process of the intermediate stage-1 graphite sulfate by sulfuric acid intercalation and its subsequent oxidation are poorly understood and lattice defect formation must be avoided. Here, we demonstrate film formation of micrometer-sized graphene flakes with lattice defects down to 0.02% and visualize the carbon lattice by transmission electron microscopy at atomic resolution. Interestingly, we find that only well-ordered, highly crystalline graphite delaminates into oxo-functionalized graphene, whereas other graphite grades do not form a proper stage-1 intercalate and revert back to graphite upon hydrolysis. Ab initio molecular dynamics simulations show that ideal stacking and electronic oxidation of the graphite layers significantly reduce the friction of the moving sulfuric acid molecules, thereby facilitating intercalation. Furthermore, the evaluation of the stability of oxo-species in graphite sulfate supports an oxidation mechanism that obviates intercalation of the oxidant.

Flake Formation in Phase Layers of a Cutting Ceramic

Flake formation (cracking) is observed in inserts of untreated cutting ceramic and also during diffusional strengthening of the inserts’ surface layer by nitrogen on heating in a muffle furnace to 450–500°C, with the creation of γ-phase (a protective layer), the removal of carbon from the atomic cells, and the formation of titanium nitride. Heat-treatment conditions for inserts of cutting ceramic are established such that the flake content at their surface is reduced. Treatment to prevent flake formation is proposed: heating in a sealed muffle furnace with a nitrogen atmosphere to 250–280°C at a pressure of 0.25 MPa. In this process, the titanium carbide at the surface of the inserts is converted to titanium nitride as a result of carbon diffusion, with the formation of a protective γ-phase layer. By removing carbon from the surface layer of the inserts and ensuring the diffusion of nitrogen into the cutting ceramic, with the formation of titanium nitride in the γphase, the durability and performance of the inserts are improved, along with their mechanical properties.

Improving the Efficiency of Natural Raw Water Pretreatment at Thermal Power Stations

In the treatment of make-up water for thermal power stations (TPS) and heat networks, raw water from surface water bodies is used. It contains organic and mineral pollutants in the form of particulates or colloids. Coagulation and flocculation are reagent methods for removing these pollutants from water. Chemicals are used to assist in the formation of large structured flakes that are removed easily from water. The Kuibyshev water reservoir was selected as the object of investigation. Basic physical and chemical properties of the raw water are presented. The application of various coagulating agents, their mixtures in different proportions, and flocculating agents for clarifying the Volga water was examined. The required dose of a coagulant or flocculant was determined based on test coagulation of the treated water. Aluminum sulfate and iron (III) chloride were used a coagulant, and Praestol 2500 (nonionic) as a flocculant. A method of enhancement of coagulation and flocculation by injecting air into the treated water is examined. The results of experimental investigation of the effect of water treatment method on water quality indices, such as alkalinity, pH, iron content, suspended material content, and permanganate value, are presented. It is demonstrated that joint use of ironand aluminum containing coagulation agents brings the coagulation conditions closer to the optimum ones. Aeration does not affect the coagulation process. The methods for supplying air to a clarifier are proposed for practical implementation.

Green synthesis of micron-sized silver flakes and their application in conductive ink

A facile, green method for fabrication of conductive ink composed of Ag flakes was developed for use in flexible printed electronics. The Ag flakes were prepared using AgNO3, nontoxic l-ascorbic acid (Vc) and polyvinylpyrrolidone (PVP) serving as a metal salt, reducing agent and capping agent, respectively. The prepared Ag flakes were characterized by XRD, SEM, and TGA. The combination of PVP and FeCl3 was found to be critical for the formation of the Ag flakes, and reaction activity was affected by temperature. Samples obtained at 140 °C were composed of Ag flakes with an average size of 0.94 ± 0.3 μm. A new fabrication method for producing conductive patterns was designed using a syringe with varying dispersion areas, allowing the ink (Ag flakes and organic solvent) to be applied directly onto a flexible photo paper. The conductivity and flexibility of the pattern were experimentally tested under varying reaction temperatures and bending cycles. Increasing contact area and packing density between Ag flakes resulted in an improved conductivity of the bending pattern. After more than 5000 bending cycles, the patterns were sintered at 160 °C and the resistivity increased from 4.6 ± 0.6 to 22.4 ± 0.8 μΩ cm, acceptable values for practical applications. Sample conductive lines drawn by the Ag flake ink exhibited excellent conductive performance and mechanical integrity, demonstrating a promising method for the formation of flexible microelectrodes or electronic devices.

Scalable MoS2/graphene hetero-structures grown epitaxially on sapphire substrates for phototransistor applications

Bi-layer graphene is grown directly on sapphire substrates by using ethane as the precursor without the assistance of a metal catalyst. A growth model of graphene flake formation in the furnace, followed by a complete film growth is also proposed. Using the graphene/sapphire sample as the new substrate, scalable MoS2 films with good layer number controllability can be grown directly on the substrate. After fabricating the MoS2/graphene hetero-structures into bottom-gate photo-transistors, a Dirac point shift is observed for the device under the light irradiation condition, which is attributed to the extraction of photo-excited electrons in the MoS2 layer to the graphene channel. The photo-voltaic response observed for the photo-transistors may provide a potential application of the 2D material hetero-structure in thin-film solar cells.

Study on the Mechanism of Nano-Flake Defect during Tungsten Contact Chemical Mechanical Polishing

Tungsten chemical mechanical polishing (CMP) has been used for metal interconnect formation in semiconductor manufacturing for a long time. Its application continues to the advanced device manufacturing with transistor scaling. However, the defect concerns of the tungsten CMP process have become critical to manufacturing such as metal flake defects that can cause electrical shorts between contacts. Although there are many published studies on tungsten CMP fundamentals, process applications and CMP-induced defects, the mechanism of thin metal flake formation in nanoscale (nano-flake defect) has not been clearly known to industry. In this study, nano-flake defects are identified after tungsten contact CMP, and mechanism of its formation is explored. Significant CMP dishing at wide contact area removes all tungsten and open barrier metal. Friction force by 3-body interaction among wafer, abrasive particle and pad asperity breaks the adhesion of barrier material from intermetallic dielectric material. And it allows re-deposition of barrier metal to the wafer surface. Insufficient CMP in-situ cleaning results in residual nano-flake defects at the end of CMP process. Analysis of device macros, thickness of flake, cross sectional analysis by transmission electron microscope (TEM) and topographical analysis support this suggested mechanism.