China
Africa
Explore chapters and articles related to this topic
+ Graphite-Intercalation Compounds
Published in Ming-Fa Lin, Wen-Dung Hsu, Green Energy Materials Handbook, 2019
Shih-Yang Lin, Wei-Bang Li, Ngoc Thanh Thuy Tran, Wen-Dung Hsu, Hsin-Yi Liu, Ming-Fa Lin
It is well known that graphite is one of the most investigated materials both theoretically and experimentally. Up to now, it has served as the best anode in the Li+-band battery. This layered system is purely composed of the hexagonal-symmetry carbon layers, in which the weak but significant van der Walls interactions would greatly modify the low-lying π-electronic structure and thus dominate the essential physical properties. Monolayer graphene is identified to be a zero-gap semiconductor, while a 3D graphite belongs to a semimetal. The electronic properties strongly depend on the way the graphitic planes are stacked on each other. In general, there are three kinds of stacking configurations: AAA (simple hexagonal), ABAB (Bernal), and ABCABC (rhombohedral). The total free carrier density is predicted to be 3.5 × 1020 e/cm3 in simple hexagonal graphite and ∼1019 in Bernal graphite at room temperature. When various atoms and molecules are further intercalated into the AB-stacked graphite, many graphite-intercalation compounds are formed. When many free conductions (holes) are induced after the intercalation, such systems exhibit the donor-type (acceptor-type) behaviors. Among these compounds, only the stage n lithium intercalation systems display the AAA stacking configuration, as confirmed from the X-ray diffraction patterns. Here, n clearly indicates the number of graphitic sheets between two periodical guest-atom layers, in which n = 1, 2, 3, and 4 (Figure 3.8c–f), being arranged from the highest concentration to the lower one, will be studied thoroughly in terms of the essential properties. It is also noticed that the other alkali intercalation compounds present the MC8 structure in the stage-1 configuration (M = K, Rb; Cs). Obviously, both LiC6 and MC8 should have very different band structures and fundamental properties.
Introduction
Published in Lin ByChiun-Yan, Chen Rong-Bin, Ho Yen-Hung, Lin Ming-Fa, Electronic and Optical Properties of Graphite-Related Systems, 2017
Lin ByChiun-Yan, Chen Rong-Bin, Ho Yen-Hung, Lin Ming-Fa
Carbon atoms can form various condensed-matter systems with unique geometric structures, mainly owing to four active atomic orbitals. Zero- to three-dimensional carbon-related systems cover diamond [1], graphite [2–4], graphene [5], graphene nanoribbons [6], carbon nanotubes [7] and carbon fullerene [8]. All of these have sp2 bonding except for diamond which has sp3. The former might exhibit similar physical properties, for example, π-electronic optical excitations [9,10]. Graphite is one of the most extensively studied materials, theoretically and experimentally. This layered system is very suitable for exploring diverse 3D and 2D phenomena. The interplane attractive forces originate from the weak Van der Waals interactions of the 2pz orbitals. The honeycomb lattice and the stacking configuration are responsible for the unique properties of graphite, for example, its semimetallic behavior due to the hexagonal symmetry and the interlayer atomic interactions. The essential properties are dramatically changed by the intercalation of various atoms and molecules. Graphite intercalation compounds can achieve a conductivity as good as copper [4,11–13]. In general, there exist three kinds of ordered configurations in the layered graphites and compounds, namely AA, AB and ABC stackings. Simple hexagonal (AA), Bernal (AB) and rhombohedral graphites (ABC) exhibit rich and diverse electronic and optical properties in the presence/absence of a uniform magnetic field false(B=B0z^false). To present a systematic review of them, the generalized tight-binding model is developed under magnetic quantization. This model, combined with the Kubo formula, is utilized to investigate the essential properties of layered carbon-related systems. The dimensional crossover from graphene to graphite and the quantum confinement in nanotube and nanoribbon systems are discussed thoroughly. A detailed comparison with other theoretical studies and experimental measurements is also made.
3D Coulomb Excitations of Simple Hexagonal, Bernal, and Rhombohedral Graphites
Published in Chiun-Yan Lin, Jhao-Ying Wu, Chih-Wei Chiu, Ming-Fa Lin, Coulomb Excitations and Decays in Graphene-Related Systems, 2019
Chiun-Yan Lin, Jhao-Ying Wu, Chih-Wei Chiu, Ming-Fa Lin
For three kinds of graphites, there are a lot of theoretical and experimental researches on the electronic properties and Coulomb excitations, clearly illustrating the diversified phenomena by distinct stacking configurations. According to the first-principles calculations [51, 338, 397], the AA-, AB-, and ABC-stacked graphites, respectively, possess the free carrier densities of ~ 3.5 × 1020, ~ 1019, and 3.0 × 1018 e/cm3, being in sharp contrast with one another. As a result of configuration-induced free electrons and holes, such semimetallic systems are predicted/expected to display the unusual low-frequency single-particle excitations and plasmon modes [113,168,234,398-402]. The simple hexagonal graphite exhibits parallel and perpendicular collective oscillations relative to the graphitic planes, with frequency higher than 0.5 eV [31,397,403]. Furthermore, certain important differences between two distinct oscillation modes lie in the frequency, intensity, and critical momentum, as revealed in Bernal graphites [113, 401, 402]. Such plasmon modes are strongly modified by doping effects. The interlayer bondings become weaker in the natural graphite, so that few free carriers only show the lower-frequency plasmons of ωp < 0.2 eV. In addition to the transferred momentum, the collective excitations are very sensitive to the changes in temperature (T) [404,405]. They could survive at larger momenta in the increase of temperature, and the frequencies are enhanced by T. The T-dependent plasmons are the prominent peaks in the energy loss spectra as well as abrupt edge structures of the optical reflectance spectra. Up to now, there is absence of theoretical predictions on the low-frequency Coulomb excitations of the ABC-stacked graphite. This study will provide the full information from random-phase approximation (RPA) calculations, e.g., the lowest plasmon frequency among three systems and the most difficult observations using electron energy loss spectroscopy (EELS) and optical spectroscopies. After the intercalation of atoms or molecules, the donor-type (acceptor-type) graphite intercalation compounds possess much conduction electrons (valence holes), so that their electrical conductivity might be high as copper [44-47]. The Coulomb excitations of free carriers have been investigated by the 2D superlattice model [234,261], being responsible for the threshold edge in the measured optical spectra [231-233] and the ωp ~ 1-eV optical plasmon in the momentum-dependent EELS spectra [37,38,167,168,215,231-233].
Mechanism of catalytic ozonation in expanded graphite aqueous suspension for the degradation of organic acids
Published in Environmental Technology, 2023
Yang Song, Sha Feng, Wen Qin, Jun Ma
Expanded graphite (EG) is a type of modified graphite that is exfoliation of graphite intercalation compound between most of its carbon layers. The graphite intercalation process is conducted by thermal, chemical and electrochemical methods which could be produced at a large scale. The microscopic appearance of EG is characterized by a worm-like or accordion-like with a large number of mesh holes in its structure [15]. EG has been applied in many fields as catalysts such as chemical synthesis, electrochemistry and catalytic oxidation due to its properties including larger surface area, corrosion resistance, oxidation resistance and good thermal conductivity [16]. Xu et al. found that peroxymonosulfate could be activated with EG and EG loaded CoFe2O4 particles enhancing degradation of sulfamethoxazole [17]. However, very few studies focus on EG catalysts for heterogeneous catalytic ozonation of organic pollutants. Since EG can be produced on large scale with relative low toxicity and cost, it may serve as a promising catalyst support for catalytic ozonation. In this study, the novelty mainly focuses on the performance of the new carbon material EG to catalyze ozonation and the exploration of the possible mechanism.
Expandable Graphite in Polyethylene: The Effect of Modification, Particle Size and the Synergistic Effect with Ammonium Polyphosphate on Flame Retardancy, Thermal Stability and Mechanical Properties
Published in Combustion Science and Technology, 2020
Jianing Liu, Xiuyan Pang, Xiuzhu Shi, Jianzhong Xu
In order to prevent combustion and delay fire spread, flame retardants (FRs) are commonly used in polymers since the 1960s (Kemmlein et al., 2003). Although the halogenated flame retardants (HFRs) show excellent efficiency in suppressing ignition and slowing the spread of the flame (Wit, 2002), the use of HFRs is limited due to the release of toxic gases and smoke (Inagaki et al., 1977; Sen et al., 1991). As typical halogen-free FRs (HFFRs), metal hydroxides (Gao et al., 2011; Liu et al., 2010, 2016), layered double hydroxides (Kalali et al., 2018), phosphorus FRs (Nguyen et al., 2012; Seefeldt et al., 2012; Wang et al., 2018), hosphorus-silicon FRs, etc. (Wang et al., 2010) have been extensively used as smoke suppressants and non-toxic additives. Meanwhile, intumescent flame retardant (IFR) has been confirmed to be a kind of highly effective and environmentally friendly FR. Especially, the graphite intercalation compound (GIC), namely expandable graphite (EG), is known as a new generation of IFR which is halogen-free, non-dropping, and has low-smoke. It has multiple functions such as char-forming agent, blowing agent, and smoke suppressant (Ge et al., 2012; Kruger et al., 2017). Due to its excellent performance, EG has been extensively used in the flame retardation of polyurethane (PU) or PU coatings (Lorenzetti et al., 2017; Ming et al., 2017), polyolefin (Shen et al., 2017), acrylonitrile-butadiene-styrene (ABS) (Pang et al., 2016; Zhang et al., 2013), ethylene vinyl acetate (EVA), and so on (Pang et al., 2015).
Structure and thermal properties of expanded graphite/paraffin composite phase change material
Published in Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2019
Hua JianShe, Yuan Chao, Zhao Xu, Zhang Jiao, Du JinXing
Carbon materials have the advantages of high thermal conductivity and light weight. They are most commonly applied to improve the thermal conductivity of PCMs. Commonly used carbon materials include carbon nanotubes, expanded graphite (EG), and graphene. These can increase the thermal conductivity of PCMs (Zhang, Yuan, and Cao et al. 2018). Expandable graphite is a kind of flake graphite intercalation compound and a layered crystal consisting of sheets of carbon atoms tightly bound by covalent bonds to each other in the same plane (Xu et al. 2014). EG is obtained by oxidation, intercalation, washing, drying, and high-temperature expansion using natural graphite flakes. After high-temperature heating, EG exhibits a porous structure, high specific surface area, high surface activity, nonpolarity, and high thermal conductivity. Therefore, it can improve the thermal conductivity of paraffin with its excellent properties (Mochane and Luyt 2015). Li and Wu (2011) prepared paraffin/diatomite with EG and found that EG can greatly reduce the heating time for compositing PCMs, which were heated from 18°C to 53°C. With the decrease of heating time, the heating rates in the solid–liquid phase change processes, which occur from 38°C to 53°C, are increased. When mixed with 8 wt% EG, the heating rates of the PCM in the whole heating process and the phase change process are increased by 2.43 and 2.44 times, respectively. It indicates that the heat conductivity property of PCM is obviously enhanced by mixing with EG. Yu et al. (2016) prepared stearic acid (SA)/EG composite PCMs with different mass ratios and investigated the structural and thermal properties of the PCMs. The experimental results show that EG uniformly adsorbs SA by its porous, networked structure without any chemical interaction and improves the heat transfer efficiency of PCMs.