Behavior of unsaturated organic molecules in the nanospace of stage-2 cesium–graphite intercalation compounds

Intercalation reaction of benzene (C₆H₆) into CsC₂₄ and the characteristics of resulting ternary graphite intercalation compound (GIC) have been investigated, in comparison with the similar CsC₂₄–ternary GICs with ethylene (C₂H₄) and acrylonitrile (C₂H₃CN), and the behavior of these organic molecules within the interlayer nanospace has been discussed. The stability of these ternary GICs under exposure in air, in both stage structure and electrical conductivity was found to be in the order of C₂H₄–, C₆H₆– and C₂H₃CN–GICs. This result was reasonably explained as follows: the C₂H₄ molecules in the interlayer space oligomerize considerably and form network preventing diffusion of the Cs atoms in the interlayer, while the C₆H₆ molecules form only dimers or trimers and unable to form network, and the C₂H₃CN molecules, in spite of their strong tendency to polymerize, apparently do not form oligomers in the interlayer space, and its ternary GIC tends to degrade easily. By the electrical and galvanomagnetic measurements, it is revealed that the dominant carrier of these ternary GICs is electrons, even when the electrical conductivity decreases considerably after prolonged exposure in air, while the host graphite, Grafoil, shows two-carrier conduction. This fact indicates that the degradation of the ternary GICs occurs mainly at the periphery or surface layer of the crystallite, and at the inside of the crystallite, the ternary stage structure is retained unchanged.     

Optical study of electron back-donation in tetrahydrofuran potassium graphite intercalation compounds

We report the results of optical reflectance studies of charge transfer in the stage-one tetrahydrofuran (THF) graphite intercalation compounds (GICs) K(THF)_xC₂₄ (x = 1 and 2). For both x = 1 and 2, intercalation of THF into KC₂₄ was observed to lower the free-carrier plasma frequency, consistent with a lowering of graphitic carbon π electron concentration from one electron per K atom (e/K) in KC₂₄ to a value of ∼0.51 ± 0.07 (e/K) for the x = 1, 2 K(THF)_xC₂₄ compounds. This amount of electron back-donation to the intercalate layers (∼49%) is significantly larger than reported recently in the K(NH₃)_x GICs, where values of 20% (stage one) and 38% (stage two) are obtained upon NH₃ uptake. In contrast to previous studies in the stage-one K(NH₃)_4.1C₂₄ compound, no optical evidence was found for localized (or solvated) electrons in the intercalate layers of K(THF)_xC₂₄.     

C3H6N6 doping effect of synthetic diamond under high pressure and high temperature

In this paper, large diamond crystals were successfully synthesized under 5.6 GPa at temperature 1513 and 1553 K with melamine (C₃H₆N₆) additive. Fourier transform infrared spectroscopy (FTIR) indicated C₃H₆N₆ is an ideal nitrogen source for large diamond crystal synthesized by temperature gradient method (TGM), but its hydrogen is not absorbed by diamond. With the increase of C₃H₆N₆ content, the colour of diamond changed from yellow to green, and the highest nitrogen content of diamond is 2300 ppm. High synthesis temperature can effectively reduce the formation of defects and play a key role in the formation of C-centers during diamond growth. Surprisingly, diamond is likely to grow into twin crystal when the C₃H₆N₆ content is 0.1 wt%. In the process of diamond growth, carbon atoms of C₃H₆N₆ are likely to serve as carbon source to form plane dislocations, which eventually lead to the formation of twin diamond. Raman spectra show that there are few crystal defects in diamond synthesized with C₃H₆N₆ as dopant.     

Formation of aluminium carbide by cast iron and liquid aluminium interaction

Abstract

Aluminium carbide (Al₄C₃) has been identified in ex service and laboratory test specimens of cast irons exposed to liquid aluminium. In cast iron/aluminium couples, the formation of Al₄C₃ and κ-Fe₃AlC was confirmed for the first time. The growth kinetics of Al₄C₃ were parabolic with an activation energy 145±23 kJ mol^?1 (1023–1223 K), which is consistent with the reported activation energy for growth in graphite/liquid aluminium couples. The aim of this work is to shed light upon the origin and growth of Al₄C₃ in cast iron/liquid aluminium couples in the temperature range of 1023–1223 K.     

Structure and electrical conductivity of graphite fibers prepared by pyrolysis of cyanoacetylene

Highly electroconductive graphite fibers are prepared by the pyrolysis (CVD) of cyanoacetylene on the surface of carbon fibers, followed by heat treatment. Many thin graphite strata, cylindrically layered and scrolled around an axial core of the original carbon fiber, are observed. The lattice constant C₀ of the graphite fiber obtained from cyanoacetylene and heat-treated at 3000 °C is 6.712° (002), which is smaller than that of a graphite fiber from benzene. The magnetoresistance of graphite fibers obtained from cyanoacetylene is as high as 800% (4.2K, 15 kG) (HTT 3300 °C), while that of graphite fibers obtained from benzene is around 150%. Measurements of Raman scattering and X-ray diffraction also show that cyanoacetylene forms more highly graphitized fiber than benzene. Cyanoacetylene is a good starting material for synthesizing graphite. The room temperature conductivity is as high as 1.8 × 10⁴ S/cm for fiber heat treated at 3250 °C. Treatment of the graphite fiber by fuming nitric acid increases the conductivity up to 1.3 sx 10⁵ S/cm in less than 10 min. Upon exposure of the nitrated fiber to air at ambient temperature, the conductivity decreases slightly. After this slight decrease, the conductivity is stable for at least 90 days.     

Deriving bridged polysilsesquioxane xerogels with ≡Si(CH2)3NHP(O)(OC2H5)2 functional groups

The reaction of hydrolytic polycondensation of (C₂H₅O)₃SiC₂H₄Si(OC₂H₅)₃ [or (C₂H₅O)₃SiC₆H₄Si(OC₂H₅)₃] and (C₂H₅O)₃Si(CH₂)₃NHP(O)(OC₂H₅)₂ was used to obtain xerogels containing a phosphine oxide complexing group (0.9–1.4 mmol/g). The presence of this functional group, as well as organic bridges, polysiloxane bonds, noncondensed silanol, and ethoxysilyl groups in the synthesized bridged polysilsesquioxane xerogels, was established using IR spectroscopy and solid-state ¹H MAS NMR spectroscopy. The AFM data indicate that the structure of xerogels is similar to that of other materials of this class (conglomerates of aggregates with the size of 35–40 nm); however, all the obtained samples are practically nonporous. They extract ions of neodymium(III) and dysprosium(III) from aqueous solutions (sorption equilibrium is established in 72 h for neodymium and in 24 h for dysprosium). However, some ligand groups remain inaccessible to lanthanide ions. The whole set of the obtained results indicates that the geometrical sizes of a functional group produce a considerable effect on formation of xerogel porous structure and in the case of using bridged alkoxysilanes as structure-forming agents.     

Exchange bias in Cr/Gd multilayers with TC < TN

We report on the exchange bias phenomenon in Cr/Gd multilayers in which the Curie temperature (T_C) of the ferromagnetic Gd is slightly less than the Néel temperature (T_N) of the antiferromagnetic Cr. The exchange bias field H_E and the coercivity H_C of the samples display unusual temperature dependence behaviors in comparison with those of the traditional antiferromagnet (AFM)/ferromagnet (FM) multilayers with T_C > T_N. It is shown that the temperature dependence of H_E and H_C relates to the thickness of the Cr interlayer. For [Cr(20.4 nm)/Gd(18 nm)]₅ multilayers, the ferromagnetic behavior and exchange bias phenomenon persist to T > T_N; Oscillatory H_E and non-monotonic H_C with the temperature are observed. The anomalous temperature dependence of H_E and H_C results from the spin-density wave spin structure of the Cr layer and the non-collinear coupling between the Cr and Gd layers at the interfaces.     

Structural and magnetic properties of nanocrystalline NiFeCuMo powders produced by wet mechanical alloying

The NiFeCuMo nanocrystalline soft magnetic powders were successfully obtained by wet mechanical alloying route in a planetary ball mill using benzene (C₆H₆) as process control agent (PCA). The milling time used was ranging from 2 up to 20 h. The synthesis conditions and alloy formation have been investigated by X-ray and neutron diffraction as well as their influence on the intrinsic physical properties. Nanometer scale (≈10 nm) crystallites were obtained. A decrease of the samples magnetization has been observed and attributed to the stresses induced during the milling and to the benzene adsorbed on the powders surface. Differential scanning calorimetry investigation shows the presence of an exothermic peak related to the presence of benzene. The adsorbed benzene, internal stresses and crystalline defects removal took place during the heat treatment at 350 °C for 4 h, leading to an improvement of the powders magnetization.     

Role of vapor pressure of 1,4-bis(trimethylsilyl)benzene in developing silicon carbide thin film using a plasma-assisted liquid injection chemical vapor deposition process

The temperature dependence equilibrium vapor pressure (p_e)_T data yielded a straight line when ln(p_e) was plotted against the reciprocal temperature in the range of 312.82-367.12 K, leading to a standard enthalpy of sublimation (Δ_subH°) value of 68.2 ± 0.8 kJ mol^− 1 for 1,4-bis(trimethylsilyl)benzene (TMSB). From the depression of the melting point in the DTA-mode, the standard enthalpy of fusion (Δ_fusH°) was found to be 26.9 ± 2.5 kJ mol^− 1. A thin film of silicon carbide was grown on graphite substrate at 573 K using TMSB or bis(trimethylsilyl)acetylene as precursors. The deposited films were characterized by scanning electron microscopy and energy dispersive X-ray analysis for their composition and morphology.     

Thermal properties of cerium and thorium tellurates

Enthalpy increment measurements on CeTe₂O₆ (s) and ThTe₂O₆ (s) were carried out using a Calvet micro-calorimeter. The enthalpy increment values were least squares analyzed, with the constraint that H⁰(T)−H⁰ (298.15 K) at 298.15 equals 0 and C_p⁰ (298.15 K) is equal to the value estimated by Kellog’s method.The dependence of the enthalpy increment with temperature can be given as:

H⁰(T)−H⁰ (298.15 K)(J mol⁻¹)=189.95 T (K)+15.226×10⁻³ T² (K) +15.414×10⁵/T (K)−63157 (CeTe₂O₆ (s), 391.5–848.0 K)

H⁰(T)−H⁰ (298.15 K)(J mol⁻¹)=191.34 T (K)+14.993×10⁻³ T² (K) +14.668×10⁵/T (K)−63300 (ThTe₂O₆ (s), 391.5–909.3 K)

Molar heat capacity C_p⁰(T), S(T) and Gibb’s free energy functions were evaluated.     

Novel graphite salts and their electrical conductivities

A set of novel first stage graphite salts of general formula C₈⁺MF₆^? has been prepared (M = Os, Ir, As). Single crystal X-ray diffraction studies indicate that these salts are hexagonal with a ≈ 4.9 and $\text{c ≈ 8.1}\text{A}\text{?}$. The unit cell volume indicates that the anions are closely packed in the galleries. Platinum hexafluoride, which is the most powerful oxidizer of the third transition series, forms a first stage compound, which analytical, structural, and magnetic studies establish as C₁₂²⁺PtF₆^2?. In this salt the anions are not close packed, but the electron withdrawal from the graphite planes is greater than for the C₈⁺MF₆^? series. The variation in the electrical conductivity (in the a–b plane), as a function of composition, has been investigated with the OsF₆, IrF₆, PtF₆ and AsF₅ intercalates. For OsF₆ and IrF₆, the conductance per plane of graphite is found to be a maximum at approximately C₂₄MF₆ (second stage); the conductivity being an order of magnitude greater than that of the parent material. Intercalation beyond C₂₄MF₆ leads to a marked decrease in conductivity. C₈MF₆ is comparable in conductivity with the parent graphite. This behavior contrasts with the graphite/AsF₅ system in which a steady increase in conductance per graphite plane with increasing AsF₅ content is observed. For the PtF₆ system, the second as well as the first stage materials are poorly conducting.     

Intercalation of graphite by silicon tetrafluoride and fluorine to yield a second-stage salt C24SiF5

Powdered graphite (either finely broken HOPG or Union Carbide SP1) interacts with 1:1 SiF₄/F₂ mixture (～50 atmospheres) at 40°, to yield a second-stage compound (a₀ = 2.46(1), c₀ = 11.29(1) Å) of composition C～₂₄SiF₅. This product interacts with excess PF₅ with quantitative displacement of SiF₄ to yield a first-stage fluorophosphate (a₀ = 2.46(1); $\text{c}{}_0\text{= 7.65(1)}\text{A}\text{?}\text{)}$ according to the equation: C₂₄SiF₅ + 2PF₅ → C₂₄PF₆. PF₅ + SiF₄. The first-stage fluorophosphate falls to a second-stage salt (a₀ = 2.46(1); $\text{c}{}_0\text{= 10.87}\text{A}\text{?}\text{)}$ under vacuum: $\text{C}{}_{24}\text{PF}{}_6\text{.}\text{PF}{}_5\text{20°}\text{→}\text{C}{}_{24}\text{PF}{}_6\text{+}\text{PF}{}_5$. This is identical to that prepared directly: $\text{24}\text{C}\text{+}\text{PF}{}_5\text{+}\text{1}\text{2}\text{F}{}_2\text{→}\text{C}{}_{24}\text{PF}{}_6$. Extensive reduction of C₂₄PF₆, at ≈20° : 2C₂₄PF₆ + PF₃ → 48C + 3PF₅ to graphite of crystallinity comparable to that of the starting graphite establishes that the initial C₂₄SiF₅ does not involve extensive fluorination of the graphite. The ease of displacement of SiF₄ by superior fluoroacids indicates that C₂₄SiF₅ can be a valuable precursor for other graphite salts. Ready formation of a first-stage fluoroborate, $\text{C}{}_7\text{BF}{}_4\text{, I}{}_{\mathrm{<mtext>c</mtext>}}\text{= 7.70(1)}\text{A}\text{?}$ contrasts with the failure to prepare similarly a first-stage SiF₅^? salt.     

Heat capacities and thermal linear expansion coefficients of graphite intercalation compounds with CuCl2 and CoCl2

The heat capacities within the 350 – 620 K temperature interval and the thermal expansion from 293 to 823 K of C_6.35CuCl₂, C_6.97CoCl₂ and C_16.8CoCl₂ graphite intercalation compounds (GICs), as well as CuCl₂, CoCl₂ and graphite from the Zavalievsk location have been characterized for the first time. For the GIC investigated, certain parameters have been shown to differ considerably from the additive values, which made it possible to evaluate the properties of the particle interaction between the Cu/Co/Cl₂ layers and the carbon atoms in the graphite structure. It has been found that for GICs the heat capacity variation within the above temperature interval monotonically increases, while their thermal expansion temperature functions have a complicated nature characterized by irreversibility, anisotropy and, in some cases, by hysteresis and the dependence on the prehistory of the samples.     

X-ray photoelectron spectroscopy studies of SbF5 intercalated graphite

Two samples of graphite intercalated with SbF₅ have been studied by X-ray photoelectron spectroscopy (x.p.s.). In each case two fluorine (1s) peaks were observed, with binding energies 685.0 eV and 688.0 eV. The peak at 685.0 eV is attributed largely to volatile, quasi-liquid SbF₅ present in the interlayer. The peak at 688 eV is assigned to involatile SbF₆^? ions located at fixed sites in the interlayer, the fluorine atoms undergoing strong polarization interaction with the graphite lattice. Changes observed in carbon (1s) and antimony (3d_{$\text{5}\text{2}$}) spectra are consistent with these assignments.     

Influence of H2S on metal dusting

Presence of H₂S in a carburizing atmosphere causes S-adsorption which retards carbon transfer and deposition and can suppress metal dusting of iron and steels. In the latter process cementite Fe₃C is an intermediate, graphite deposition would initiate its decomposition but graphite nucleation is prevented by adsorbed sulfur. Thus continued Fe₃C growth can be observed in the presence of H₂S. Thermogravimetric studies in CO-H₂-H₂O-H₂S mixtures have been conducted at 500°C at various carbon activities a_C and H₂S/H₂-ratios. With increasing a_C higher H₂S/H₂-ratios are needed to suppress metal dusting, with increasing H₂S/H₂-ratio the kinetics of Fe₃C growth change from diffusion controlled parabolic kinetics to linear carbon transfer controlled kinetics. At very high a_C≥1000 besides Fe₃C also the Hägg carbide Fe₅C2 was observed as an outer layer on the cementite.     

Multi-component molecular conductors with supramolecular assembly based on ET radical cation salts with (NO3)− anion

Synthesis, structure and conducting properties of two new multi-component radical cation ET salts with the (NO₃)⁻ counterion: (ET)₂(NO₃)·C₂H₄(OH)₂ (I) and (ET)₂(NO₃)·0.5[C₂H₄(OH)₂]·H₂O (II) are described. Both salts have layered structures with the distinguishing feature of hydrogen bonds being present between the radical cation layers and anion sheets. It was found that introduction of glycol molecules to compositions of I and II significantly affects the structure of their cation and anion layers and, as a result, their conducting properties. I is a semiconductor, while II demonstrates metallic conductivity down to 4.2 K.     

Effect of ascorbic acid on hydrogen peroxide decomposition into an environmentally friendly mixture for pickling of 316L stainless steel

One of main disadvantages of using environmentally friendly chemical mixtures for pickling, is stabilization of hydrogen peroxide (H₂O₂), due to its decomposition is affected by the presence of metal ions. In previous studies, p-toluenesulfonic acid (C₇H₁₀O₄S) was used as a hydrogen peroxide stabilizer; however, benzene rings in its structure have a certain level of toxicity. In this work, ascorbic acid (C₆H₈O₆) was tested as a stabilizer agent for H₂O₂ in a pickling mixture composed by sulfuric acid (H₂SO₄) and hydrofluoric acid (HF)-H₂O₂, this mixture was tested on 316L stainless steel (SS). Decomposition of H₂O₂ in pickling solution was evaluated for different ferric ions concentration between 0 to 40 g/L and at different temperatures from 25° to 60°C. Pickling rates at 25°C for 316L SS were 26873 mg/dm² and 27799 mg/dm² using ascorbic acid and p-toluenesulfonic acid, respectively. Ascorbic acid has a positive influence on stabilization processes of H₂O₂.     

Preparation and characterisation of diamond-like carbon films prepared by MW ECR/PACVD process deposited on 41Cr–Al–Mo7 nitrided steel

In the present work, Diamond-Like Carbon (DLC) films were deposited by the Microwave Electron Cyclotron Resonance/Plasma-Assisted Chemical Vapour Deposition process on 41Cr–Al–Mo7 nitrided steel using a benzene–argon gas mixture. The quality of DLC films grown on nitrided steel using different deposit conditions such as benzene flow rate, temperature and time was investigated. Microstructural analysis by Raman spectroscopy showed main G-band peaks are localised between 1564 and 1595?cm^?1, while the D-band peaks are centred at 1297?cm^?1. Fourier Transform Infrared Spectroscopy showed the peaks attributed to C–H bending and stretching vibration modes are in the range of 2600–3100?cm^?1. The microstructure, surface morphology and mechanical properties of the films were examined by X-ray diffraction, atomic force microscopy and use of a Vickers indenter (Zwick). The deposition parameters of the coatings have been studied and optimised to improve mechanical properties. The observed increase in adhesion is related to the formation of (Fe,Cr)₃C, Cr₃C₂ and Cr₂₃C₆ interlayer phases. DLC films showed a better wear resistance, higher hardness and low friction coefficient compared to nitrided layers tested for comparison.     

Specific heat and magnetocaloric effect of the Mn5Ge3 ferromagnet

The specific heat C_p(T) of the Mn₅Ge₃ ferromagnet has been studied in a wide temperature range of 2–400 K. The ferromagnetic ordering has been confirmed at T_C = 297.4 K, however the behaviour of C_p(T,H) in small magnetic fields suggests a presence of a weak antiparallel component in the magnetic structure. The combination of the C_p(T,H) and magnetization M(T,H) measurements enabled the determination of not only the isothermal magnetic entropy change ΔS_M but the adiabatic temperature change ΔT_ad as well. It has been also found that the mechanical milling process leads only to a moderate drop of the magnetocaloric effect. The reduction of the grain size by 50% decreases the relative cooling power by 28%.     

Optical spectroscopy of unsolvated and solvated crystalline Alq3

Three novel unsolvated crystalline phases of tris-(8-hydroxyquinoline)-aluminum(III) (Alq₃), namely α-, β- and γ-Alq₃, have been synthesized and their crystalline structure determined from X-ray diffraction data on powders and single crystals. Both α- and β-Alq₃ crystals are triclinic, space group P-1, but differ for the molecular packings. In the γ-Alq₃ phase, Alq₃ molecules lie about a 3-position of the trigonal P-3 space group. A solvated Alq₃(C₆H₅Cl)_1/2 phase (monoclinic, space group P2₁/n) was also isolated. The optical and vibrational properties of these newly isolated crystal phases of Alq₃ have been investigated by absorption, photoluminescence excitation, photoluminescence and Raman spectroscopy. The effect of the different molecular packing on the emission properties has been elucidated, the nature of the photoexcitations clarified, and the vibrational fingerprints of the α- and β-crystalline forms highlighted. The origin of the amorphous nature of the vacuum sublimed thin films is discussed on the basis of the accessibility of many different π–π links between homo- and hetero-chiral Alq₃ molecules.