Mechanism of Silver Nanoparticles Deposition by Electrolysis and Electroless Methods on a Graphite Substrate

This study shows a silver electrodeposition model (EDM) on a graphite ‎substrate. The electrolyte was a 0.01 M solution of pure silver and chromium nitrate using an ‎electrolyzing cell. EDC with current density up to 20 mA/cm2 and 15 mV and pulse current were studied. Results revealed that silver deposited at a ‎rate of 0.515 mg/cm2/min with 12 mA /cm2 that decreases to 0.21 and 0.16 mg/cm2.min ‎with the decrease of current density to 6 and 5 mA/cm2 respectively. The model postulates that ‎silver ions (a) were first hydrated before diffusing (b) from the solution bulk to ‎the cathode vicinity, the next step (c) involved the chemical adsorption of these ions on certain ‎accessible sites of the graphite substrate (anode), the discharged entities (d) adhere to the graphite ‎surface by Van der Vales force. Silver ions are deposited because the ‎discharge potential of silver is low (0.38 mV) as compared to other metal ions like chromium (0.82 mV). Pulse ‎current controls silver deposition due to flexibility in controlling steps (a) - (c) of the ‎deposition mechanisms.

Challenges toward applying UHTC-based composite coating on graphite substrate by spark plasma sintering

In this study, the UHTC-based composite layers where applied on the graphite substrates using SPS method to protect them against ablation. The protective layers had some defects and problems such as crack, fracture, separation, melting, and weak adhesion to the substrate. Several factors such as the thickness of composite layer, the number of protective layers, the SPS conditions (temperature, applied pressure, soaking time and mold), the chemical composition of the layers, the type of the substrate and the mismatch between the thermal expansion coefficients of the substrate and the applied layer(s) affected the quality and connection of the protective layer to the graphite substrate. The amount of additive materials influenced the melting phenomenon in the composite layer; for example, further MoSi2 in the layer led to more melting. The mismatch between the thermal expansion coefficients of the graphite substrate and the composite layer caused stresses during the cooling step, which resulted in cracks in the applied layer. Hence, proximity in the thermal expansion coefficients seems to be necessary for the formation of an acceptable adhesion between the layer and the substrate.

Gold Carbide: A Predicted Nanotube Candidate from First Principle

In the present work, density functional theory (DFT) calculations were applied to confirm that the gold carbide previously experimentally synthesized was AuC film. A crucial finding is that these kinds of AuC films are self-folded on the graphite substrate, leading to the formation of a semi-nanotube structure, which significantly diminishes the error between the experimental and simulated lattice constant. The unique characteristic, the spontaneous archlike reconstruction, makes AuC a possible candidate for self-assembled nanotubes. The band structure indicated, in the designed AuC nanotube, a narrow gap semiconductor with a bandgap of 0.14 eV. Both AIMD (at 300 and 450 K) results and phonon spectra showed a rather high stability for the AuC nanotube because a strong chemical bond formed between the Au–5d and C–2p states. The AuC nanotube could become a novel functional material.

In Situ Observation of the Carbothermic Reduction and Foaming of Slags in Silicomanganese Production

The carbothermic reduction of slag in silicomanganese production is accompanied by the release of carbon monoxide. This gas can accumulate as bubbles within the slag, leading to foaming and, potentially, disturbances to furnace operation. This study investigated the reduction in the slag together with its foaming using a sessile drop furnace. Five silicomanganese slags produced from industrial raw materials (Assmang ore, Comilog ore, high-carbon FeMn slag with quartz, and FeS additions) were reduced by a graphite substrate at isothermal conditions (i.e., 1540–1660 °C) under CO atmosphere. The reduction reaction was tracked by photographing the slag droplet, and the cyclic expansion and burst of the droplet were used to estimate the gas evolution. The reacted samples were analyzed by wavelength-dispersive X-ray spectroscopy (WDS) to determine MnO and SiO2 reduction. While no foaming was observed using Comilog ore, extensive retention of CO in the slag phase was observed when using Assmang ore or Assmang with high-carbon FeMn slag. The beginning of foaming was attributed to an increase in the reaction rate; the absence of foaming when using Comilog can be attributed to the acidity of the charge. Addition of sulfur to the Comilog-based charge did not influence the reduction.

Nanoscale lattice dynamics in hexagonal boron nitride moiré superlattices

Twisted two-dimensional van der Waals (vdW) heterostructures have unlocked a new means for manipulating the properties of quantum materials. The resulting mesoscopic moiré superlattices are accessible to a wide variety of scanning probes. To date, spatially-resolved techniques have prioritized electronic structure visualization, with lattice response experiments only in their infancy. Here, we therefore investigate lattice dynamics in twisted layers of hexagonal boron nitride (hBN), formed by a minute twist angle between two hBN monolayers assembled on a graphite substrate. Nano-infrared (nano-IR) spectroscopy reveals systematic variations of the in-plane optical phonon frequencies amongst the triangular domains and domain walls in the hBN moiré superlattices. Our first-principles calculations unveil a local and stacking-dependent interaction with the underlying graphite, prompting symmetry-breaking between the otherwise identical neighboring moiré domains of twisted hBN.

Development and characterization of Silicon Carbide Coating on Graphite Substrate

The development of materials with unique and improved properties using low cost processes is essential to increase performance and reduce cost of the solid rocket motors. Specifically advancements are needed for boost phase nozzle. As these motors operate at very high pressure and temperatures, the nozzle must survive high thermal stresses with minimal erosion to maintain performance. Currently three material choices are being exploited; which are refractory metals, graphite and carbon-carbon composites. Of these three materials graphite is the most attractive choice because of its low cost, light weight, and easy forming. However graphite is prone to erosion, both chemical and mechanical, which may affect the ballistic conditions and mechanical properties of the nozzle. To minimize this erosion Pyrolytic Graphite (PG) coating inside the nozzle is used. However PG coating is prone to cracking and spallation along with very cumbersome deposition process. Another possible methodology to avoid this erosion is to convert the inside surface of the rocket nozzle to Silicon Carbide (SiC), which is very erosion resistant and have much better thermal stability compared to graphite and even PG. Due to its functionally gradient nature such a layer will be very adherent and resistant to spallation. Despite its very good adhesion due to its functionally gradient nature, this layer due to its porous nature exhibit poor oxidation performance compared to a dense SiC layer. The current research is focused on synthesizing, characterizing and oxidation testing of a bi-layer; a functionally gradient inner layer and dense outer layer, SiC coating on graphite.

Heterogeneous Crystal Nucleation from the Melt in Polyethylene Oxide Droplets on Graphite: Kinetics and Microscopic Structure

It is well known that the crystallization of liquids often initiates at interfaces to foreign solid surfaces. In this study, using polarized light optical microscopy, atomic force microscopy (AFM), and wide-angle X-ray scattering (WAXS), we investigate the effect of substrate–material interactions on nucleation in an ensemble of polyethylene oxide (PEO) droplets on graphite and on amorphous polystyrene (PS). The optical microscopy measurements during cooling with a constant rate explicitly evidenced that the graphite substrate enhances the nucleation kinetics, as crystallization occurred at approximately an 11 °C higher temperature than on PS due to changes in the interactions at the solid interface. This observation allowed us to conclude that graphite induces heterogeneous nucleation in PEO. By employing the classical nucleation theory for analysis of the data with reference to the amorphous PS substrate, the obtained results indicated that the crystal nuclei with contact angles in the range of 100–117° were formed at the graphite interface. Furthermore, we show that heterogeneous nucleation led to a preferred orientation of PEO crystals on graphite, whereas PEO crystals on PS had isotropic orientation. The difference in crystal orientations on the two substrates was also confirmed with AFM, which showed only edge-on lamellae in PEO droplets on graphite compared to unoriented lamellae on PS.

Influence of Interfacial Oxidation on Friction in Structural Superlubricity

We present the results of friction experiments performed by manipulation of oxidized platinum nanoislands on highly oriented pyrolytic graphite (HOPG) substrates through atomic force microscopy (AFM). The oxidation of the platinum nanoislands, performed via mild plasma exposure, is confirmed through X-ray photoelectron spectroscopy (XPS) and high-resolution energy-dispersive X-ray spectroscopy (EDX), the latter of which reveals partial oxidation on the sliding surfaces of the nanoislands. Oxidized platinum nanoislands are found to exhibit higher friction than non-oxidized islands, with a ~ 70% increase in mean shear stress over the investigated contact size regime. An increase in chemical interaction forces between the oxidized platinum and the graphite substrate is proposed to explain the increase in friction forces. Our results reveal that alteration of interfacial chemistry through oxidation leads to a noticeable modulation of friction forces, but not a total breakdown of the superlubric state (as evidenced by the signature observation of decreasing shear stress with increasing contact size), providing further feasibility for the design of superlubric mechanical systems to be operated under ambient conditions. Graphic Abstract