On the structural and electronic properties of poly(dicarbon monofluoride): solid-state semi-empirical INDO study

Three-dimensional semi-empirical quantum chemical calculations of the structural and electronic properties of the fluorine intercalated graphite compound poly(dicarbon monofluoride)—(C₂F)_n have been performed for several possible stacking sequences of puckered trans-cyclohexane chair layers. Such basic structure consisting from carbon hexagons in chair conformation has been confirmed. Furthermore, based on the geometry optimization, 12 structural sequences have been found to provide a local minima on the potential hypersurface, from which four are considerably more stable and one can assume their statistical distribution in the real poly(dicarbon monofluoride). This is also indicated by comparison with recent Kα XES spectra. In such arrangement the maximal entropy contribution leads to the minimum Gibbs energy of the system. Band structure calculations show that the most stable sequences have insulating properties, which implies that the real poly(carbon monofluoride) behaves as an insulator. The conductive properties of some less stable sequences result from particular interlayer interactions.     

Preparation of poly-(dicarbon monofluoride), (C2F)n from exfoliated graphite

New compound Poly-(dicarbon monofluoride), (C₂F)_n was prepared at low temperatures of 335 to 374°C with faster reaction rate from exfoliated graphite than from natural graphite. The fluorination reaction was not controlled by the diffusion of fluorine into graphite layers, but by the reaction of fluorine with graphite, and the activation energy of the reaction was 35.6 Kcal/mol. The product has the interlayer spacing (d₀₀₁) of 8.8 to 9 Å, half width (β₀₀₁) of 2.7 to 2.9° and F/C ratio of 0.535 to 0.610. The difference in the reaction rates of exfoliated and natural graphites with fluorine was based on the structure of the starting materials.     

A mixed cation nickel diphosphate with a layered intergrowth structure: synthesis, structural and magnetic characterization of Na(NH4)[Ni3(P2O7)2(H2O)2]

A nickel diphosphate with mixed cations, Na(NH₄)[Ni₃(P₂O₇)₂(H₂O)₂] with a layered structure has been synthesized under hydrothermal conditions for the first time and characterized by single crystal X-ray diffraction, IR spectroscope and magnetization measurements. The structure consists of cis- and trans-edge sharing NiO₆ octahedral chains linked via P₂O₇ units to [Ni₃P₄O₁₆]²⁻ layers. The ammonium and sodium cations are alternately located in the interlayer spaces. The mixed cations play an important role in the structural formation of this layered compound, leading to a new layer-stacking variant. The magnetic susceptibility obeys a Curie–Weiss law with μ_eff of 3.32 μ_B, showing the Ni²⁺ character and weak antiferromagnetic interactions.     

Superfine Expanded Graphite with Large Capacity for Lithium Storage

A superfine expanded graphite (s‐EG) fiber material was investigated as an anode material for lithium‐based batteries. The fibers were prepared by decomposition of dicarbon monofluoride‐intercalated graphite. The high resolution transmission electron microscopy (HRTEM) images showed the fiber thickness in range of 2–3 nm with several microns in length. Lithium storage capacity in this material was measured in lithium half cells. High lithium storage capacity of about 1000 mAh · g^–1 at a rate of C/10, corresponding to Li₃C₆ composition was obtained. The material showed fairly good rate capability exhibiting lithium storage capabilities even at 60C. As a effect of ball milling, the s‐EG showed crystallographic ordering in the sample with reduced the lithium storage capacity corresponding to composition of LiC₆. A simple mathematical relation to account for the excess lithium storage capacity in this material is put forward.     

长链烷基季铵盐插层氧化石墨的结构变化

基于改进的Hummers法制备氧化石墨(GO),并以长链烷基季铵盐(CnTAB)对其进行插层处理;通过改变CnTAB的链长、浓度,得到系列CnTAB/GO插层复合物。采用XRD和元素分析对产物的最大底面间距及CnTAB插入量进行表征。结果表明,随着Cn TAB链长的增长、CnTAB浓度的增大,CnTAB/GO插层复合物的最大底面间距逐渐增大。CnTAB通过离子键作用和疏水键作用插入到GO层间,在GO片层上的吸附规律符合修正型(Modified)Langmuir模型,即CnTAB以单分子层吸附在GO片层上。根据CnTAB/GO插层复合物最大底面间距及CnTAB插入量的变化规律分析,得出CnTAB在GO层间的排布模式有单层平躺模式、类双层平躺模式、单层倾斜模式和单层直立模式。     

长链烷基季铵盐插层氧化石墨的结构变化

基于改进的Hummers法制备氧化石墨(GO),并以长链烷基季铵盐(C_nTAB)对其进行插层处理;通过改变C_nTAB的链长、浓度,得到系列C_nTAB/GO插层复合物。采用XRD和元素分析对产物的最大底面间距及C_nTAB插入量进行表征。结果表明,随着C_nTAB链长的增长、C_nTAB浓度的增大,C_nTAB/GO插层复合物的最大底面间距逐渐增大。C_nTAB通过离子键作用和疏水键作用插入到GO层间,在GO片层上的吸附规律符合修正型(Modified)Langmuir模型,即C_nTAB以单分子层吸附在GO片层上。根据C_nTAB/GO插层复合物最大底面间距及C_nTAB插入量的变化规律分析,得出C_nTAB在GO层间的排布模式有单层平躺模式、类双层平躺模式、单层倾斜模式和单层直立模式。     

X-ray spectroscopic study of graphite fluoride (C2F) n intercalated with benzene

Samples of intercalated graphite fluoride of the C₂F·zR type (R is C₆H₆) before and after heating to 150 °C in a spectrometer vacuum chamber were studied by X-ray fluorescence spectroscopy. The C-Kα differential spectra of the samples mainly characterizes the electron state of carbon atoms in the benzene molecule inside the C₂F matrix. The differential spectrum is distinct from the spectrum of solid benzene by additional maxima, which indicate the interaction between the benzene molecules and the graphite fluoride matrix. Comparative analysis of the spectrum of the heated sample and those of graphite and graphite fluoride (CF) _n suggests that the layers of the C₂F matrix contain considerable regions of both completely fluorinated and graphite-like regions. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 705–708, April, 2000.     

Uncompensated magnetization in the layered molecular antiferromagnet {N(n-C5H11)4[MnFe(ox)3]}∞

Studies on the magnetic properties of the molecular antiferromagnetic material {N(n-C₅H₁₁)₄[Mn^IIFe^III(ox)₃]}_∞, carried out by various physical techniques (AC/DC magnetic susceptibility, magnetization, heat capacity measurements and Mössbauer spectroscopy) at low temperatures, have been presented. Different experimental observations complement each other and provide a clue for the observation of an uncompensated magnetization below the Néel temperature and short-range correlations persisting high above T_N. It is understood that the honeycomb layered structure of the compound contains non-equivalent magnetic sub-lattices, (Mn^II–ox–Fe^III_A–...) and (Mn^II–ox–Fe^III_B–...), where different responses of the Fe^III_A and Fe^III_B spin sites towards an external magnetic field might be responsible for the observation of the uncompensated magnetization in this compound at T < T_N. The present magnetic system is an S = 5/2 2-D Heisenberg antiferromagnet system with the intralayer exchange parameter J/k_B = −3.29 K. A very weak interlayer exchange interaction was anticipated from the spin wave modeling of the magnetic heat capacity for T < 0.5T_N. The positive sign of the coupling between the layers has been concluded from the Mössbauer spectrum in the applied magnetic field. Frustration in the magnetic interactions gives rise to the uncompensated magnetic moment in this compound at low temperatures.     

Lithium intercalation into CrO<Subscript>3</Subscript>-GIC and its derivatives

Graphite intercalation compound with chromium trioxide (CrO₃-GIC) was used as a precursor for new graphite composites. By heating CrO₃-GIC at 800 °C in air a composite material, expanded graphite/Cr₂O₃, was obtained, whereas further heating of this composite at 1000 °C in the stream of hydrogen/argon mixture led to the reduction of Cr₂O₃ to chromium carbide (Cr₃C₂). A new material thus obtained was expanded graphite/Cr₃C₂ composite. The chromium species having catalytic properties gave rise to the improvement of the crystal structure of expanded graphite. All materials were tested as anode materials for lithium-ion cell. It was found that graphitic material treated in H₂/Ag at 1000 °C has greater reversible capacity in comparison to the original graphite and the shape of the charge/discharge curve shows very well-defined stage structure which supports the crystallographic data.Presented at the 3rd International Meeting Advanced Batteries and Accumulators, June 16th–June 20th, Brno, Czech Republic     

2,4,6‐Trifluorobenzonitrile

The title compound, C₇H₂F₃N, contains three crystallographically independent molecules in the crystal structure; two of these molecules have symmetry m and the third has symmetry mm. Each independent molecule forms a planar or approximately planar layer with its own kind. There are three different types of interlayer contacts, two of which are similar to each other, while the third is distinctly different. The packing within the layers is similar to that found in 2,5‐ and 3,6‐difluorobenzonitrile, with weak C—H...N interactions holding the molecules in the layers. The remarkable feature of this structure is the presence of more than one type of interlayer interaction.     

Acceptor-type graphite intercalation compounds and new carbon materials based on them

The problems of synthesis and study of the physicochemical properties of graphite intercalation compounds (GIC) formed upon insertion of various molecules into the interplanar space of graphite are considered. Binary and ternary intercalation compounds with protonic acids (HNO₃, CH₃COOH, H₃PO₄, H₂SO₄, etc.) are described. The results of systematic research into graphite intercalation by potentiometry, calorimetry, powder X-ray diffraction, conductivity measurements, DTA, chemical analysis, and other methods are given. These results underlie elucidation of the characteristic and peculiar features of acid insertion into graphite. The physicochemical properties and practical applications of GIC and low-density carbon materials are analyzed. __________ Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1699–1716, August, 2005.     

Original close-packed structure and magnetic properties of the Pb4Mn9O20 manganite

The crystal structure of the Pb₄Mn₉O₂₀ compound (previously known as “Pb_0.43MnO_2.18”) was solved from powder X-ray diffraction, electron diffraction, and high resolution electron microscopy data (S.G. Pnma, a=13.8888(2) Å, b=11.2665(2) Å, c=9.9867(1) Å, R_I=0.016, R_P=0.047). The structure is based on a 6H (cch)₂ close packing of pure oxygen “h”-type (O₁₆) layers alternating with mixed “c”-type (Pb₄O₁₂) layers. The Mn atoms occupy octahedral interstices formed by the oxygen atoms of the close-packed layers. The MnO₆ octahedra share edges within the layers, whereas the octahedra in neighboring layers are linked through corner sharing. The relationship with the closely related Pb₃Mn₇O₁₅ structure is discussed. Magnetization measurements reveal a peculiar magnetic behavior with a phase transition at 52 K, a small net magnetization below the transition temperature, and a tendency towards spin freezing.     

Facile synthesis of exfoliated and highly conductive poly(arylene disulfide)/graphite nanocomposites

Exfoliated graphite has been synthesized by first synthesizing H₂SO₄ intercalated compound in a H₂O₂‐H₂SO₄ mixture, followed by exfoliation under microwave irradiation. Poly(arylene disulfide)/graphite nanocomposites were then fabricated by absorbing cyclic(arylene disulfide) oligomers into the pores of exfoliated graphite. Subsequently, the nanocomposite precursor was subjected to heat treatment to carry out the in situ ring‐opening polymerization of the oligomers via free radical mechanism. The as‐prepared nanocomposite exhibited a exfoliated nanostructure as evidenced by transmission electron microscopy (TEM) observation. The nanocomposite with a very small amount of graphite, 5 wt%, possesses a highly electrical conductivity of 4 S/cm, therefore, many applications can be found as conductive materials. Copyright © 2004 John Wiley & Sons, Ltd.     

Electrochemical wettability control on conductive TiO2 nanotube surfaces modified with a ferrocene redox system

This work reports on an electrochemical system which allows the control of surface wettability properties by voltage induced changes in contact angle (Θ) of ΔΘ  50°. For this we used conductive TiO₂ nanotubular layers that were modified with ferrocene coupled to the TiO₂ surface via triethoxysilane. To enhance the hydrophobic character of the nanotubular TiO₂ surface, also mixed organic monolayers namely perfluorotriethoxysilane, were explored. Formation of the ferrocene and mixed organic monolayer was confirmed by X-ray-photoelectron-spectroscopy (XPS). Contact angle combined with electrochemical measurements show that ferrocene in these monolayers can successfully be switched from Fe²⁺ to Fe³⁺ and that this change in the redox state considerably alters the wetting properties. Using a conductive nanotube substrate allows us to amplify this change by a factor of more than 10, and thus this surface can be used to trigger significant wetting alterations.     

Suppression of lithium deposition at sub-zero temperatures on graphite by surface modification

Lithium deposition on graphite anodes is considered as a main reason for failures and safety for lithium ion batteries (LIB). Different amounts of carbon coating on the surface of natural graphite are used in this work to suppress the amount of lithium deposited at − 10 °C. Pulse polarization experiments reveal relative polarization of graphite anodes at various temperatures and show that lithium deposition is accelerated at lowered temperatures. Electrochemical experiments, along with photographs, scanning electron microscopy (SEM) images and ex-situ X-ray diffraction (XRD) data suggest that carbon coating not only suppresses the lithium deposition but also enhances the formation of LiC₆ at − 10 °C. The homogeneous potential profile on the graphite surface attained by the carbon coating explains such an improved low temperature performance, as it allows efficient Solid Electrolyte Interface (SEI) film formation, which is a prerequisite for safety LIB.     

Spherical filler-promoting thermally conductive pathway in graphite-containing polymer composites for high heat radiation

Graphite is an efficient and affordable filler for polymer composites, allowing the control of thermal conductivity. In comparison to other thermally conductive fillers, graphite is lightweight and flexible but affords anisotropic thermal conductivity. Herein, the control of thermal conductivity of graphite-containing polymer composite sheet using spherical polymer particles as additional fillers is described. The thermal conductivity in the through-plane direction (λ_t) of the composite sheet is enhanced by varying the composition ratio of the two fillers (flaky graphite and spherical particles), and optimizing the forming temperature and pressure. Graphite-containing (25 wt%) polymer composite sheet formed by compression at 150 °C and 10 MPa exhibits λ _t value of 0.66 W/m K. Upon mixing of polystyrene microspheres, λ _t is successfully increased. The maximum value of thermal conductivity for a composite sheet with 35 wt% of graphite and 50 wt% of spherical particles is 7.51 W/m K, at 180 °C and 10 MPa. The graphite-containing polymer matrix forms a sequentially connected network-like structure in the composite sheet. Excess polymer microspheres lead to the formation of void structures inside the composite sheet, reducing the thermal conductivity. Thermo-camera observations proved that the composite sheets with higher λ _t value showed comparably high heat radiations. © 2020 Wiley Periodicals, Inc. J. Polym. Sci. 2020 , 58, 607–615     

Microstructure and ferroelectric properties of r.f. magnetron sputtering derived PZT thin films deposited on interlayer (PbO/TiO2)

Lead zirconate titanate (PZT) thin films were deposited on Pt/Ti/SiO₂/Si and interlayer/Pt/Ti/SiO₂/Si substrate by radio frequency (r.f.) magnetron sputtering with a Pb_1.1Zr_0.53Ti_0.47O₃ target. The crystallization of the PZT thin films was formed only by substrate temperature. When interlayer (PbO/TiO₂) was inserted between the PZT thin film and the Pt electrode, the grain growth and processing temperature of the PZT thin films were considerably improved. Compared to PZT/Pt structure, the dielectric constant and polarization properties of the PZT/interlayer/Pt structure were fairly improved. In particular, PZT/interlayer/Pt at the substrate temperature of 400 °C showed prevalent ferroelectric properties (_r=475.97, tanδ=0.0591, P_r=23 μC/cm²). As a result of an X-ray photoelectron spectroscopy (XPS) depth-profile analysis, it was found that PZT/interlayer/Pt deposited only by substrate temperature without the post-annealing process via r.f. magnetron sputtering method remained independent of each other regardless of substrate temperatures.     

Si-O-C骨架支撑型高循环性能锂离子电池硅基负极材料

通过经济有效的方法制备得到一种具有长循环寿命的高效稳定性硅/硅氧碳/无定形碳的复合负极材料结构. 在这种结构中,以具有稳定化学性能的硅氧碳结构作为骨架,来支撑和隔离硅纳米颗粒结构. 材料中包含的无定形碳组分可提高硅/硅氧碳结构的电导性能. 这种复合负极结构在0.3C电流充放电情况下,不仅能发挥出637.3 mAh·g^-1的比容量,而且在经过100 周的充放电循环后,其容量保持率也达到86%. 这种新型硅基负极材料的设计为其他功能材料的设计提供了一种潜在可能的方法.     

Is the brittle–ductile transition preceding melting related to 5- and 7-membered ring defects in graphite?

The brittle–ductile transition (BDT) is a very general phenomenon in materials science. The temperature T _BDT in numerous materials correlates somewhat with the higher melting temperature T _m. We have earlier proposed the disclination (or rotation-dislocation) as a universal ingredient in the BDT. The present study is exclusively concerned with graphite, where T _m?≈?4000?K. Our conclusion is that the energetics of 5- and 7-membered rings play a crucial role in determining these temperatures. However, experiment is to be invoked, should T _BDT eventually be measured for graphite, so as to decide between various mechanisms, all of which here depend on 5- and 7-membered rings. One mechanism involves interlayer van der Waals coupling, whence a pressure experiment may hold the key.     

X-ray photoelectron study of fluorinated graphite intercalation compounds

Intercalated compounds of fluorinated graphite C₂F·yR, where R is benzene, hexafluorobenzene, acetone, or germanium tetrachloride, are studied by X-ray photoelectron spectroscopy. The binding energies of the C1s and F1s inner levels indicate that the C-F chemical bond in fluorinated graphite differs dramatically from the covalent bond in graphite monofluoride. The binding energies of the inner levels in atoms of the graphite fluoride matrix and GeCl₄ are analyzed and it is concluded that there is no chemical binding between the host matrix and the guest molecule. Translated fromZhurnal Struktumoi Khimii, Vol. 39, No. 6, pp. 1127–1133, November–December, 1998.