Graphitic Ribbons Research Articles

Basal-plane heat transport in three-dimensional graphite ribbons

Basal-plane heat transport in three-dimensional graphite ribbons

Environmental damping and vibrational coupling of confined fluids within isolated carbon nanotubes

Because of their large surface areas, nanotubes and nanowires demonstrate exquisite mechanical coupling to their surroundings, promising advanced sensors and nanomechanical devices. However, this environmental sensitivity has resulted in several ambiguous observations of vibrational coupling across various experiments. Herein, we demonstrate a temperature-dependent Radial Breathing Mode (RBM) frequency in free-standing, electron-diffraction-assigned Double-Walled Carbon Nanotubes (DWNTs) that shows an unexpected and thermally reversible frequency downshift of 10 to 15%, for systems isolated in vacuum. An analysis based on a harmonic oscillator model assigns the distinctive frequency cusp, produced over 93 scans of 3 distinct DWNTs, along with the hyperbolic trajectory, to a reversible increase in damping from graphitic ribbons on the exterior surface. Strain-dependent coupling from self-tensioned, suspended DWNTs maintains the ratio of spring-to-damping frequencies, producing a stable saturation of RBM in the low-tension limit. In contrast, when the interior of DWNTs is subjected to a water-filling process, the RBM thermal trajectory is altered to that of a Langmuir isobar and elliptical trajectories, allowing measurement of the enthalpy of confined fluid phase change. These mechanisms and quantitative theory provide new insights into the environmental coupling of nanomechanical systems and the implications for devices and nanofluidic conduits.

Zigzagging graphitic ribbons: Synthesis, characterization, and acetone vapor sensing

The fast sensing highly volatile and flammable liquids of organic compounds is an urgent scientific goal for developing devices aimed at health and safety monitoring. The high superficial area, high porosity, and electrochemical properties of carbon nanotube/fiber sponges make them promising materials for sensing organic compounds. In this study, we synthesized zigzagging graphitic ribbons (ZGR) sponges employing the aerosol-assisted chemical vapor deposition (AACVD) method at temperatures of 980 °C, 1020 °C, and 1050 °C. We characterized the morphology, composition, thermal stability, and surface chemistry of the ZGRs using various techniques. The ZGRs exhibited a defective, corrugated, and oxygen-functionalized surface. XPS characterizations revealed the presence of ether, carbonyl, carboxyl, aldehyde, and hydroxyl groups on the ZGR surface. TEM characterizations showed graphitic materials with unusual black zones attributed to sp3 carbon hybridization or strained graphite. Sensors based on ZGR sponges were used at room temperature for detecting acetone gas vapor. The electrical resistance response was monitored by varying the concentration of acetone gas vapor. The results revealed that the sensors remained stable over multiple cycles and exhibited fast response times (2–12 s). Interestingly, acetone serves as a selective breath marker for diabetes, suggesting that our sensors could be used for non-invasive diabetes monitoring. Finally, we performed van der Waals density functional theory calculations to elucidate the TEM black zones and the interaction of the acetone molecule with oxygen functional groups hosted in graphitic materials.

Super-Ballistic Width Dependence of Thermal Conductivity in Graphite Nanoribbons and Microribbons.

The super-ballistic temperature dependence of thermal conductivity, facilitated by collective phonons, has been widely studied. It has been claimed to be unambiguous evidence for hydrodynamic phonon transport in solids. Alternatively, hydrodynamic thermal conduction is predicted to be as strongly dependent on the width of the structure as is fluid flow, while its direct demonstration remains an unexplored challenge. In this work, we experimentally measured thermal conductivity in several graphite ribbon structures with different widths, from 300 nm to 1.2 µm, and studied its width dependence in a wide temperature range of 10-300 K. We observed enhanced width dependence of the thermal conductivity in the hydrodynamic window of 75 K compared to that in the ballistic limit, which provides indispensable evidence for phonon hydrodynamic transport from the perspective of peculiar width dependence. This will help to find the missing piece to complete the puzzle of phonon hydrodynamics, and guide future attempts at efficient heat dissipation in advanced electronic devices.

Observation of phonon Poiseuille flow in isotopically purified graphite ribbons

In recent times, the unique collective transport physics of phonon hydrodynamics motivates theoreticians and experimentalists to explore it in micro- and nanoscale and at elevated temperatures. Graphitic materials have been predicted to facilitate hydrodynamic heat transport with their intrinsically strong normal scattering. However, owing to the experimental difficulties and vague theoretical understanding, the observation of phonon Poiseuille flow in graphitic systems remains challenging. In this study, based on a microscale experimental platform and the pertinent occurrence criterion in anisotropic solids, we demonstrate the existence of the phonon Poiseuille flow in a 5.5 μm-wide, suspended and isotopically purified graphite ribbon up to a temperature of 90 K. Our observation is well supported by our theoretical model based on a kinetic theory with fully first-principles inputs. Thus, this study paves the way for deeper insight into phonon hydrodynamics and cutting-edge heat manipulating applications.

Size effect on phonon hydrodynamics in graphite microstructures and nanostructures

The understanding of hydrodynamic heat transport in finite-sized graphitic materials remains elusive due to the lack of an efficient methodology. In this paper, we develop a computational framework enabling an accurate description of heat transport in anisotropic graphite ribbons by a kinetic theory approach with full quantum mechanical first-principles input. A unified analysis of the size scaling of the thermal conductivity in the longitudinal and transverse directions of the system is made within the computational framework complemented with a macroscopic hydrodynamic approach. As a result, we demonstrate a strong end effect on the phonon Knudsen minimum, as a hallmark of the transition from ballistic to hydrodynamic heat transports, along a rectangular graphite ribbon with finite length and width. The phonon Knudsen minimum is found to take place only when the ribbon length is \ensuremath{\sim}5--10 times the upper limit of the width range in the hydrodynamic regime. This paper contributes to a unique methodology with high efficiency and a deeper understanding of the size effect on phonon hydrodynamics, which would open opportunities for its theoretical and experimental investigation in graphitic micro- and nanostructures.

A Comparison of the Abrasive Wear Behaviors of Cast Iron and Cast Steel Materials

The two body abrasive wear of cast iron and cast steel was studied by scratching tests. The tests were carried out by installing a scratching apparatus on the spindle of a CNC machine tool. Cemented carbide inserts were used as tools to scratch the test samples. The abrasive wear behaviors of cast iron and cast steel were investigated by comparing their scratch morphologies and the subsurface microstructures. For cast iron, the crushing of the graphite ribbon and inconsistent deformation at the boundaries of pearlite and graphite are important reasons for the exfoliation of the surface hardening layer and the initiation of surface cracks. The cast steel can undergo sufficient plastic deformation; only when the scratch depth and scratch speed increase to a certain value do cracks occur on the side pile-ups of the groove. Both microploughing and microcutting wear mechanisms exist at the same time, but the dominant wear mechanism changes between the two under different scratch conditions. The pile-up ratio can be used to measure the strength of the microcutting and microploughing actions of cast steel; this parameter increases with increasing scratch depth and decreases with increasing scratch speed and extrusion front rake angle.

Thermal conductivity measurement of an individual millimeter-long expanded graphite ribbon using a variable-length T-type method

Expanded graphite (EG) is a well-known carbon derivative and widely used as the thermally conductive enhancer for thermal management composites. However, the investigation on thermal conductivity measurement of an individual EG particle is still unexploited, which prevents the exploration of the coupling mechanism between EG and matrices and further measures for thermal conductivity enhancement. Herein, using a variable-length T-type method, we measure the thermal conductivity of an individual expanded graphite ribbon (EGR) obtained by mechanically compressing a separate EG particle. The EGR has a micrometer-sized thickness and a millimeter-sized length. By changing the sample length while maintaining the contact junction, we simultaneously obtained the thermal conductivity of the EGR and the thermal contact resistance between the sample and the probe. The longitudinal thermal conductivity of the EGR reaches up to 335.6±27.4 W m−1 K−1 at room temperature and decreases to 254.8±20.8 W m−1 K−1 as the temperature rises from 300 K to 380 K. With a higher thermal conductivity than most graphene paper products and some carbon fibers, this low-cost nanocarbon-based material exhibits a great advantage in the development of thermally conductive composites, and the presented accurate thermal conductivity provides indispensable data for the rational design of composites.

Mechanism studies and fabrication for the incorporation of carbon into Al alloys by the electro-charging assisted process

The incorporation of carbon nanostructures into Al alloys has the potential to further improve the mechanical and electrical properties of these alloys. We report a novel one-step fabrication method of incorporating carbon in aluminum alloys by the application of a high electric current to molten Al metal containing particles of activated carbon at high temperature and under Ar atmosphere. We propose that the mechanism for carbon incorporation is similar to electromigration where the current facilitates ionization of the carbon atoms followed by polymerization of the carbon structures and the formation of graphitic chains and ribbons along preferred directions of the Al lattice. Furthermore, the current induces transformation from amorphous carbon to crystalline graphitic structures that propagate in the metal matrix. The carbon is identified by the C-K edge in electron energy loss spectra, X-ray photoelectron spectroscopy, Raman and Kelvin probe force microscopy. The stiffness is increased in these samples; hardness, on the other hand, decreases for some samples compared to reference Al alloys with no carbon. The electrical conductivity is superior to that of Al-carbon nanotube composites previously reported. The enhanced properties of Al covetics show potential for naval and aerospace applications and power transmission lines.

The carbon substrate in RST Si ribbon technology for solar cells

The carbon substrate used in the RST (silicon Ribbon growth on a Sacrificial carbon Template) process, a silicon ribbon growth technology aimed at the fabrication of silicon solar cells, is a composite made of a compressed expanded natural graphite (CENG) ribbon coated by a protective rough lamellar high-temperature (RLHT) pyrocarbon. It is shown that the reactivity of RLHT pyrocarbon with molten silicon is stimulated by the presence of topological defects of the CENG ribbon. This reactivity generates a contamination of the silicon melt by carbon and its metallic impurities and, in turn, adversely affects the performances of solar cells made from RST silicon films. Reactivity with molten silicon can be drastically reduced by the control of the deposition conditions of pyrocarbon and of the roughness of the CENG ribbon. A new model is proposed to explain the mechanism of the intrinsic reactivity of RLHT pyrocarbon with molten silicon. Based on this model the reactivity problem can virtually be eliminated by the deposition of a SiC coating about 0.3 μm on the RLHT pyrocarbon. Improvements of the carbon ribbon, described in this study, especially its pyrocarbon coating, should lead to flexible silicon solar cells 70–80 μm thick with conversion efficiency above 18%.

Hierarchical Interconnected Expanded Graphitic Ribbons Embedded with Amorphous Carbon: An Advanced Carbon Nanostructure for Superior Lithium and Sodium Storage.

Carbon materials have attracted considerable attention as anodes for lithium-ion and sodium-ion batteries due to their low cost and environmental friendliness. This work reports an advanced carbon nanostructure that takes advantage of the chelation effect of glucose and metal ions, which ensures the uniform dispersion of metal in the precursor. Thus, an effective catalytic conversion from sp3 to sp2 carbon occurs, enabling simultaneously formation of pores with catalyzed graphitic structures. Due to the low carbonization temperature and short carbonization time as well as the different catalytic degree of various metals, a series of expanded graphitic layers from 0.34 to 0.44 nm with defects and amorphous carbon structure are obtained. The structure not only offers accessible graphitic spacings for reversible lithium/sodium ion insertion, but also provides abundant active sites for lithium/sodium ion adsorption in the defects and amorphous structure. Moreover, the hierarchical interconnected porous structure combining graphitic ribbons is beneficial for fast electronic/ionic transport and favorable electrolyte permeation. More importantly, such advanced carbon materials prove their feasibility for balancing the pore structure and degree of graphitization. When serving as the electrode material for lithium-ion and sodium-ion batteries, excellent electrochemical performance along with fast kinetics and long cycle life is achieved.

Synthesis of carbon nano-onions doped with nitrogen using spray pyrolisis

In this work, we have synthesized carbon nano-onions (CNOs) doped with nitrogen and iron carbide core. These nanostructures were synthesized pyrolysing various alcohol-benzylamine reaction mixtures. These CNOs showed a certain degree of functionalization of their surfaces, depending of the solvent, as well as n-type doping due to the inclusion of N atoms in the graphene layers. Ratios of O atoms to C atoms as well as to N atoms of the pyrolyzed solution play an important role in the morphology of the CNOs and on the phase of the iron core. Differences in the morphology of the samples have an important effect on their electrical conductivity as well as in their electrochemical properties. Synthesized samples showing well-defined CNOs, the sintering between them is negligible, have a low conductivity and higher capacitance, while those samples showing the best conductivities and lower capacitances, the CNOs in samples are connected between them by turbostratic graphite ribbons, in similar way to the CNOs synthesized from carbon nanodiamond annealed in argon atmosphere.

Electrosynthesis of Two-Dimensional TiC and C Materials from Ti3SiC2 in Molten Salt

The two-dimensional (2-D) multilayer structured TiC and carbide-derived carbon (CDC) have been prepared by electrochemical etching of ternary layered carbide–Ti3SiC2 in molten CaCl2 at 900°C with 2.5 V and 3.0 V, respectively. Ti3SiC2 powder was pressed to form a pellet which was then served as the anode, and a graphite rod was used as the cathode. The obtained TiC product possesses a specific surface area of 653 m2 g−1 and a total pore volume of 0.56 cm3 g−1 with a narrow pore size distribution (average pore diameter of 2.0 nm). The obtained CDC material shows a layer graphene-like nanosheets structure, and amorphous carbon coexists with graphitic carbon ribbon in the product. The Raman analysis confirms that the ID/IG ratio of the CDC nanosheets is 1.17, indicating the lower degree of graphitization. The layered CDC nanosheets possess a specific surface area of 623 m2 g−1 and a total pore volume of 2.06 cm3 g−1 with an average pore diameter of 20–70 nm. It is suggested that the molten salt electrochemical etching process is a novel method for the production of 2-D multilayer material with tuned pores, and have penitential to be used for a variety of applications.

Electrical properties of in-plane-implanted graphite nanoribbons

We studied the effect of low energy (30 keV) ionic implantation of Ga+ in the direction parallel to the graphene planes (perpendicular to the c-axis) in oriented graphite ribbons with widths around 500 nm. Our experiments have reproducibly shown a reduction of electrical resistance upon implantation consistent with the occurrence of ionic channeling in our devices. Our results allow for new approaches in the modulation of the charge carrier concentration in mesoscopic graphite.

Phonon Hydrodynamic Heat Conduction and Knudsen Minimum in Graphite.

In the hydrodynamic regime, phonons drift with a nonzero collective velocity under a temperature gradient, reminiscent of viscous gas and fluid flow. The study of hydrodynamic phonon transport has spanned over half a century but has been mostly limited to cryogenic temperatures (∼1 K) and more recently to low-dimensional materials. Here, we identify graphite as a three-dimensional material that supports phonon hydrodynamics at significantly higher temperatures (∼100 K) based on first-principles calculations. In particular, by solving the Boltzmann equation for phonon transport in graphite ribbons, we predict that phonon Poiseuille flow and Knudsen minimum can be experimentally observed above liquid nitrogen temperature. Further, we reveal the microscopic origin of these intriguing phenomena in terms of the dependence of the effective boundary scattering rate on momentum-conserving phonon-phonon scattering processes and the collective motion of phonons. The significant hydrodynamic nature of phonon transport in graphite is attributed to its strong intralayer sp2 hybrid bonding and weak van der Waals interlayer interactions. More intriguingly, the reflection symmetry associated with a single graphene layer is broken in graphite, which opens up more momentum-conserving phonon-phonon scattering channels and results in stronger hydrodynamic features in graphite than graphene. As a boundary-sensitive transport regime, phonon hydrodynamics opens up new possibilities for thermal management and energy conversion.

Ultrahigh hardness of carbon steel surface realized by novel solid carburizing with rapid diffusion of carbon nanostructures

In this study, a novel rapid solid carburizing process with a large diffusion depth using nano-diamonds (NDs) was conducted for low carbon steel. Changes of annealed NDs were obtained by Raman spectroscopy and transmission electron microscopy (TEM), and the results suggested that the NDs experience a stripping process before a special solid-reaction with surface iron atoms from steel substrate. Onion-like carbon (OLC) derived from the annealed NDs provided broken graphitic ribbons as carbon sources that accelerated the rate of adsorption and diffusion. Examination of the surface layer at equilibrium using TEM and X-ray photoelectron spectroscopy (XPS) also revealed the special state of carbon, and an ultrafine mixed phase microstructure was obtained by rapid solid-phase transformation. As a result, a surface hardened layer with ultrahigh hardness and a smooth transition region were realized. We believe that these kinds of diamond or graphitic structures with high activity states have an important influence not only on adsorption and diffusion but also on this special solid-phase transformation.

Structural features of various kinds of carbon fibers as determined by small-angle X-ray scattering

The structural features of polyacrylonitrile and pitch-based carbon fibers were analyzed from a comprehensive point of view by X-ray measurements and related techniques. The results indicated that the undulating graphite ribbon with embedded microvoid was the main structural unit for graphitic fibers. The void’s parameters for these fibers could be obtained directly by small-angle X-ray scattering following the classic method deduced based on the typical two-phase system (i.e., Porod’s law, Guinier’s law and Debye’s law). The non-graphitic fibers, however, were composed of two-dimensional turbostratic crystallites in the aggregation of microfibril and thus had a quasi two-phase structure (microfibril, interfibrillar amorphous structure and microvoid embedded within the microfibril). The extended Debye or Beaucage model in this case should be applied in order to obtain the structural parameters. It also revealed that the quasi two-phase system would complete its transformation to two-phase system during high-temperature graphitization. Therefore, the degree of graphitization was speculated to be the essential indicator distinguishing graphitic fibers from non-graphitic ones and would be helpful in understanding the transformation of structural features during the graphitization of carbon fibers.

Complex Metal–Organic Frameworks from Symmetrically Backfolded Dendrimers

AbstractWe study symmetrically backfolded building blocks (Sierpinski dendrimers) that generate metal‐organic frameworks featuring graphitic ribbons, free‐standing carboxylic functions and intricate topologies in clear correspondence with molecular structures. Unlike the starburst shape of traditional dendrimers, the Sierpinski dendrimer features side branches bent away (e. g., by 120°) from the outward direction of the main branch. This paper reports a systematic attempt to correlate the backfolded shape with the connectivity (topology) of the resultant network. Our discovery is surprisingly simple: as building blocks for extended nets, the backfolded Sierpinski dendrimer behaves as a collection of subsidiary building units of the conventional starburst shape (e. g., the regular tritopic unit); and the resultant networks can be deconstructed into subnets corresponding to the star‐shaped subunits of the molecule. These seemingly exotic and highly branched molecules are not only compatible with the purpose of network construction, but also present fundamentally new features that open new horizons for molecular design.

Reuseable Monolithic Nanoporous Graphite-Supported Nanocatalysts (Fe, Au, Pt, Pd, Ni, and Rh) from Pyrolysis and Galvanic Transmetalation of Ferrocene-Based Polyamide Aerogels

Polyamide aerogels with ferrocene as a monomer repeat unit were prepared in one step from ferrocene dicarboxylic acid and tris(4-isocyanatophenyl)methane. Pyrolysis at ≥800 °C yielded nanoporous carbons doped throughout with crystallites of α-Fe (about 50 nm in diameter), which in turn were shrouded in graphitic ribbons (<30 graphene layers thick). Transmetalation was carried out with aqueous solutions of Au, Pt, Pd, Rh, and Ni salts, via a path akin to galvanic corrosion, whereas graphitic ribbons separated anodes (α-Fe particles) from cathodes (defects along the ribbons). The new metallic phases formed clusters of smaller crystallites (10–20 nm in diameter) on the graphitic ribbons, leaving behind empty cage-like formations previously occupied by the Fe(0) nanoparticles. All metal-doped carbons were monolithic and over 85% porous. Catalytic activity was demonstrated with the oxidation of benzyl alcohol to benzaldehyde catalyzed with carbon-supported Au or Pt, the reduction of nitrobenzene by hydrazine t...

Nitrogen doped graphitic carbon ribbons from cellulose as non noble metal catalyst for oxygen reduction reaction

Nitrogen doped graphitic carbon ribbons are prepared by simultaneously graphitization and nitrogen doping of nano crystalline cellulose at high temperature. Ferrous salt works as catalyst for graphitization, without which amorphous carbon is resulted. The graphitic carbon ribbons and amorphous carbon are compared in detail in morphology, crystalline structure, defects, nitrogen groups and ORR activity by means of SEM, TEM, Raman, XRD, XPS and electrochemical characterizations. It is found the nitrogen doped graphitic carbon ribbons has comparable ORR activity with Pt/C catalyst in alkaline medium, which is much better than nitrogen doped amorphous carbon. The improved ORR activity is ascribed to the nitrogen group introduced by NH3 treatment of graphitic carbon. The as prepared material can work as efficient non noble metal catalyst for the oxygen reduction reaction.