Francisco Robles-Hernandez

Resonance Raman spectroscopy of G-line and folded phonons in twisted bilayer graphene with large rotation angles

We report the synthesis and systematic Raman study of twisted bilayer graphene (tBLG) with rotation angles from below 10° to nearly 30°. Chemical vapor deposition was used to grow hexagon-shaped tBLG with a rotation angle that can be conveniently determined by relative edge misalignment. Rotation dependent G-line resonances and folded phonons were observed by selecting suitable energies of excitation lasers. The observed phonon frequencies of the tBLG superlattices agree well with our ab initio calculation.

Four-fold Raman enhancement of 2D band in twisted bilayer graphene: evidence for a doubly degenerate Dirac band and quantum interference.

We report the observation of a strong 2D band Raman in twisted bilayer graphene (tBLG) with large rotation angles under 638 nm and 532 nm visible laser excitations. The 2D band Raman intensity increased four-fold as opposed to the two-fold increase observed in single-layer graphene. The same tBLG samples also exhibited rotation-dependent G-line resonances and folded phonons under 364 nm UV laser excitation. We attribute this 2D band Raman enhancement to the constructive interference between two double-resonance Raman pathways, which were enabled by a nearly degenerate Dirac band in the tBLG Moiré superlattices.

A Green Method for Graphite Exfoliation Using a Mechanochemical Route

In the present work, we proposed a method to manufacture exfoliated graphene. This method is low cost and environmentally friendly by mechanochemical means and avoids the use of highly corrosive and dangerous reagents. Our raw material consists of natural graphite and sodium carbonate. The mix is milled followed by a citric acid treatment used as defoliation agents. The mixture consists of an equiatomic mix of graphite and sodium carbonate that is processed in a high-energy ball mill. The milled and raw mixtures are leached with an aqueous acid solution, hot refluxed, washed, and overnight dried. Observed morphological and chemical evidence exhibits a notable reduction of particle size with an important increased level of defoliation that improved the surface area values.

A Green Method for Graphite Exfoliation Using High-Energy Ball Milling

Graphite, an allotropic and stable form of carbon, is a useful material constituted by multiple layers joined with covalent bonds linked together by a weak Van Der Walls interaction. The single units named graphenes have attracted considerable attention, because of their excellent mechanical, chemical, thermal, electrical properties and low thermal expansion coefficient [1]. The strong covalent (sp) bonds in this unique honeycomb structure of graphene and their atomic scale thickness impart them with these unusual properties [2]. Taking advantage of the weakness of these interactions, it is possible to insert ions, atoms or molecules between the layers in order to obtain graphenes from graphite. In the exfoliation process, elimination of the intercalated species leads to a significant expansion up to hundreds of times along the c-axis, forming a highly porous material. Exfoliated graphite (EG) has been synthesized by galvanic, chemical and thermal treatments of the natural graphite. However, the chemical method is widely used because its simplicity and versatility. Usually EG is produced intercalating acid guest species between the stacked graphene layers via liquid-phase by reaction between graphite and 18M H2SO4 in the presence of strong chemical oxidants such as KMnO4, HNO3 or H2O2 [3]. However, reaction leftovers are highly toxic and corrosive materials that required careful manipulation and a special confinement.

A Green Method for Graphite Exfoliation, Effect of Milling Intensity.

Graphite is a natural allotropic form of carbon, being the most stable allotrope under standard conditions; multiple layers or foils named graphitic carbon and graphenes are considered single or double layers of carbon with a honey comb structures that constitute this material. Graphenes have attracted considerable attention of the scientific community, because of their excellent mechanochemical properties, high thermal conductivity and low thermal expansion coefficient [1]. A chemical process to produce single layers graphenes is by inserting ions, atoms or molecules between the layers. During the exfoliation process, elimination of the intercalated species leads to an important material expansion, forming a highly porous material commonly named exfoliated graphite (EG).

Principles of Solidification

This chapter reviews the fundamental aspects of alloy solidification and the various features that are measured or characterized to assess cast structure quality and solidification rate. Also reviewed are the different phases, from primary α-Al dendrites to secondary phase such as primary Si platelet, Fe-bearing phases, and Al2Cu and Mg2Si phases. Finally, the conditions for stable pore nucleation are also reviewed.

Metal Casting Process

This chapter is a fundamental overview of the key metal casting process used in the manufacture of aluminum cast components for the automotive and aerospace industries. In addition, the pros and cons for each of the casting processes are outlined and the feasibility that these processes have toward low-volume production and high-volume production.

Al-Si Alloys, Minor, Major, and Impurity Elements

This chapter gives an introductory description of Al-Si alloys. The intention of this chapter is to provide a brief review and understanding of these alloys including their designation, standards, and the effects of alloying additions to the alloy. The additions to the Al-Si alloy are classified as minor, major, and impurity elements or additions. Minor and major additions are defined based on the fraction or percentage added to the alloy. The impurity element (usually Fe) is present as a byproduct of recycling.

Aluminum Sintering in Air Atmosphere Using High Frequency Induction Heating

For decades, aluminum (Al) has been the most widely used industrial metal (after steel) for its appreciated properties. Unfortunately, the pure metal presents a lower mechanical response; normally to counter this disadvantage some alloy elements are added. However, its high electrical-thermal conductivity and corrosion resistance are compromised. As an alternative, the cold working process can be used to increase mechanical performance, but the ductility is drastically reduced due matrix embrittlement (dislocations saturation). Another hardening route is based on grain refinement (at submicron or nanometric level), where the material properties are positively modified. The mechanical milling (MM) technique involves repeated impacts between the sample and the grinding media causing plastic deformation, fracture and cold welding reaching a highly refined microstructure. After MM, some compaction and heat treatment steps are applied to milled powders to obtain solid samples. However, during the sintering process long term heating promotes a remarkable grain growth, destroying the highly refined state reached after MM.

Microstructural Changes in Aluminum Mechanically Milled Sintered by Conventional Method and Induction

Aluminum (Al) is widely used in the aerospace and automotive industries due its mechanical properties and low density [1]. One way to improve the mechanical performance of pure Al is by cold work and grain size reduction. The last one can be achieved using high-energy ball milling, this process involves different events such as: particle deformations, cold welding and fracture, resulting in a notable crystal size reduction [2]. In order to obtain solid specimens is necessary some additional processing: conventional sintering (CS) requires high temperature and longer processing time; this generates an adverse increment of grain size [3] or high frequency induction sintering (HFIS) wherein some authors have reported high densification levels with not significant changes of grain size [4].