Magnetoresistance of two-phase carbons

The transverse magnetoresistance was studied at liquid nitrogen temperature for carbons with two phases, the turbostratic and the graphitic. Samples investigated were carbons prepared by heat treatment at temperatures between 1320 and 1780°C under pressure of 5 kbar. The starting materials were a coke obtained from polyvinyl chloride carbonized up to 680°C, and an equi-weight mixture of the same coke with a thermal black. Samples obtained from the coke alone by the heat treatment above 1470°C consist of two phases. When the fractionof the graphitic phase ranges 0.5–6%, two new types of field dependence of the transverse magnetoresistance, types 1 and 2, are observed. The field dependence of the magnetoresistance were reproduced by using a model composed of a 2900°C-treated pyrolytic graphite and of a 1900°C-treated extruded petroleum coke carbon, when the resistance ratio between the two components was properly adjusted. This seems to show that the new types of field dependence found in this work are due to the presence of a mixture of graphitic and turbostratic grains in our samples.     

Mercury adsorption by montmorillonite and vermiculite: a combined XRD, TG-MS, and EXAFS study

Synchrotron-based extended X-ray absorption fine structure spectroscopy (EXAFS), X-ray diffraction at room and high temperature, thermal analyses combined with mass spectrometry of evolved gas, and chemical analyses contributed to assess the influence of mercury on montmorillonite and vermiculite layers.The adsorbed Hg amount was higher for montmorillonite (Hg=37.7 meq/100 g) than for vermiculite (Hg=28.0 meq/100 g). The basal spacing for the Hg treated samples was 15.2 (montmorillonite) and 14.5 Å (vermiculite). Thermal and evolved gas spectrometry analyses suggest that Hg was released at T230 °C and at 600 °C for montmorillonite, but at 550, 800 and 860 °C for vermiculite.The effect of temperature on Hg release is also apparent when the basal spacing at 230 °C for montmorillonite (d₀₀₁=10.3 Å) is compared to that for vermiculite (d₀₀₁=11.8 Å).EXAFS analyses provide qualitative evidence that oxygen atoms occupy the first coordination shell of Hg in both clay minerals. The best fit between observed and calculated spectra is obtained when montroydite is assumed as a reference model compound.     

Structure refinement of the as-synthesized and the calcined form of zeolite RUB-3 (RTE)

The structure analysis based on single crystal and powder data revealed that the framework structure of RUB-3 (structure type code RTE) consists of small [4⁴5⁴6²] building units and medium–large [4⁶5⁴6⁶8²] cages with a free volume of ca. 300 ų. Interconnected cages form a one-dimensional channel system with narrow, slightly elliptical pore openings (free diameter 3.6×4.3 Ų). ¹H–¹³C CP MAS NMR spectroscopy proved that the cages are occupied by disordered (±)-exo-2-aminobicyclo[2.2.1]heptane molecules which were used as templates during the synthesis. The unit cell composition of as-synthesized RUB-3 is (C₇H₁₃N)₂·[Si₂₄O₄₈]. The lattice parameters, bond lengths, bond angles and unit cell volumes are nearly identical for the as-synthesized [a=14.039(2) Å, b=13.602(2) Å, c=7.428(1) Å, β=102.22(3)°] and calcined form [a=14.018(1) Å, b=13.612(1) Å, c=7.418(1) Å, β=102.12(1)°] of RUB-3. The RTE structure even keeps its symmetry (C2/m) during the calcination process, indicating that the RTE framework topology is very rigid. All crystals are four-fold twins with twin planes {110}.     

New biintercalation compounds in a potassiumcopper chloride-graphite system

Potassium interaction with stage 1 and stage 2 CuCl₂-graphite intercalation compounds (GICs) at T = 200°C was investigated. Two new phases with I_c = 14.70 Å and 12.55 Å were fixed. The analysis of the intensities of 001 reflections has shown that these two phases seem to be biintercalation compounds with the following sequences of the layers: K-CuCl-K-CuCl, respectively.     

Structural transformations of Muscovite at high temperature by X-ray and neutron diffraction

Structural transformations of Muscovite at temperature up to 1095 °C were determined using powder X-ray and neutron diffraction. Data were collected at room temperature from preliminary heated and quenched samples at 650 °C, 980 °C and 1095 °C. X-ray data were interpreted by either Rietveld method and neutron data, which complete the structural information by a better assignation of oxygen positions. With neutron data atom position was refined by fitting Pair Distribution Functions. It was found to be a progressive but continuous microstructural change, with the formation of an increasingly disorganized structure, but the layered organization of muscovite is maintained up to 1095 °C. Rietveld refinements from X-ray confirm the 6 to 5 coordination of Al atoms above 650 °C. It induces some structural changes as the orientation and mutual position of tetrahedrons in silicate layers. Pair Distribution Function refinements show the weakening of the long range structural organization, above 5 Å. At lower distance, a local order is maintained and the preferential alignments of both alumina unit pairs and silica tetrahedron were observed. This residual structural order of high-temperature muscovite is favorable to the achievement of textured ceramics.     

Static filtration of purified sodium bentonite clay suspensions. Effect of clay content

The effect of clay content on the static filtration properties of purified Na-montmorillonite suspensions was studied at two different pressures. The results of filtering parameters are as expected. When the clay percentage increases, the thickness (e) of the obtained cake increases, leading to the decrease of water loss (WL), and consequently, to the decrease of permeability (k). These decreases of WL and k are more important for the low applied pressure than for the high one.The differences between values of the different experimental parameters (WL and e) obtained at applied pressures of 5.7×10⁵ and 1.5×10⁵ Pa, as a function of clay content in the initial samples, show some singular points. At the clay content corresponding to the sol–gel transition (SGT), already determined by rheological measurements, ΔWL presents a change in concavity and Δe shows a maximum. Localisation of this maximum seems to be a simple way to detect the SGT by performing static filtration experiments of two series' of clay suspensions at two different pressures.According to the water contents obtained at the two pressures, all the cakes were localised in region (2), defined by Norrish (1954) as the “macroscopic” swelling paste of Na-montmorillonite. At low applied pressure, the texture of the cakes is identified by transmission electron microscopy (TEM). Generally, at low magnification, all samples are globally homogeneous. However, they present some regions more dense than others. At high magnification, the number of layers per particle seems to depend on SGT. Indeed, in the cake corresponding to 1% w/w of clay in the initial suspension, isolated layers are most frequent and particles with two to four layers per particle are scarce. In the cake obtained from the gel (3% w/w of clay) the most frequent stacking is about three layers per particle, and isolated layers almost disappear. In the cake obtained from gel, at the SGT (5.5% w/w of clay) which is the less oriented sample, the most frequent stacking is statistically also around three layers per particle but with a greater number of particles, some isolated layers appear again. After the sol–gel transition, in the cake obtained from the 7% sample (which is the most oriented sample), the number of particles increases but the number of layers in each particle decreases (two layers per particle most frequently occurs), and the number of isolated layers again rises. The frequent distances between the aggregates and particles were estimated on the TEM images of the cakes obtained at 1.5×10⁵ Pa. The distances between the particles of the cakes obtained under 5.7×10⁵ Pa was identified by X-ray diffraction (XRD). All these distances measured by the two techniques are compared with a linear extrapolation of Norrish's (1954) experimental data. At SGT, the cakes obtained at the two pressures correspond to aggregates connected with several distance configurations. However, the cakes obtained from the 3% initial clay content always show two distances (19 and 39 Å) at high pressure. At low pressure, one distance is about 300 Å, and the other is about 65 Å. This last distance disappears in the cake obtained from the more diluted suspension (1%) and only the higher distance persists. The 3% initial clay suspension seems to correspond to a “local osmotic transition”, which occurs between aggregated particles (or small aggregates) in the semi-diluted suspensions and isolated particles and/or layers in the more diluted suspensions.     

A CdSO4-like 3-D framework constructed from monosodium substituted Keggin arsenotungstates and copper(II)-ethylenediamine complexes

A 6⁵·8 CdSO₄-like 3-D framework [Cu(en)₂]₃[α-AsW₁₁NaO₃₉]·2H₂O (1) constructed by monosodium-substituted Keggin arsenotungstates and copper(II)-ethylenediamine complexes (en = ethylenediamine) has been synthesized by reaction of Na₈[α-HAsW₉O₃₄]·11H₂O, CuCl₂·2H₂O and ethylenediamine under hydrothermal conditions and structurally characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis and single-crystal X-ray diffraction. Crystal data for 1: C₁₂H₅₂AsCu₃N₁₂NaO₄₁W₁₁, monoclinic, Cc, a = 20.409(9) Å, b = 16.737(8) Å, c = 16.561(7) Å, β = 104.607(7)°, V = 5474(4) Å³, T = 296(2) K; Z = 4, μ = 24.860 mm⁻¹, GOOF = 1.086, R₁ = 0.0284, wR₂ = 0.0759. To the best of our knowledge, 1 represents the first 6⁵·8 CdSO₄-like 3-D monovacant Keggin arsenotungstate derivative in polyoxometalate chemistry.     

Interstitial iron(II)-chloride compounds with graphite—I.: Formation by reduction of iron(III)-compounds with Fe(CO)5

A graphite intercalation compound of FeCl₂ with a composition of 1 Fe:4.7 C, closely approaching the theoretically limiting stoichiometry FeCl₂C_4.22, is obtained by the reduction of the intercalation compound FeCl₃C_7.1 with Fe(CO)₅ under 150 atm of CO at 150°C. The FeCl₂-compound is characterized by its X-ray powder pattern and Mössbauer spectra. The distance between carbon layers increases from 9.40 Å in FeCl₃C_7.1 to 9.56 in FeCl₂C_4.73 by this reduction reaction. Carbon monoxide reduces FeCl₃C_7.1 at 150°C in a sluggish reaction which occurs under removal of iron chlorides from the intercalation compound in contrast to the reduction by Fe(CO)₅, which increases the Fe:C ratio.     

Preparation, structure, and electrical conductivity of vanadium fluoridegraphite intercalation compound

Graphite intercalation compounds (GIC) of vanadium fluoride have been prepared in a fluorine atmosphere. The GICs prepared from highly oriented pyrolytic graphite (HOPG) were stage 1–8 compounds with composition, C_8.4–79.5VF_5.8–6.0. The apparent size of intercalated VF₆⁻(d_i) decreased from 5.33 Å to 4.15 Å along the c axis with increasing x in C_xVF₆. Various intercalated structures of VF₆⁻ between the carbon layers have been proposed for this change in d_i values. The compounds with small d_i around 4.2 Å show high stability in the air, which is due to the nestling of the VF₆⁻ anion between the carbon layers. Electron diffraction measurements have indicated that a well-nestled stage 2 GIC has high regularity in orientation of intercalated VF₆⁻ anions, which make two large unit cells of the hexagonal system ( ) with the different vectors by ±14° from that of graphite lattice. The ¹⁹F-NMR spectra and X-ray diffraction data in the low temperature region suggest a reversible phase transition. The highest electrical conductivity was 1.97 × 10⁵Scm⁻¹, which is 12 times that of pristine HOPG.     

Heat treatment of mesocarbon microbeads under high pressure

Mesophase spherules separated from pitches and asphalt by quinoline (mesocarbon microbeads) were heat-treated under a pressure of 5 kbar at 1300–2000°C for 1 hr. The as-separated microbeads gave a bulk density of 1.8–1.9 g/cm³ after heat treatments at high temperatures. The pre-heated microbeads gave high bulk density above 2.0 g/cm³ after high temperature heat treatment. The mesocarbon microbeads were found to have low graphitizability under pressure. The as-separated microbeads showed a heterogeneous process of graphitization, but the maximum amount of the graphitic component was only 60% even after 2000°C-treatment under 5 kbar. On the pre-heated microbeads, almost no graphitization was observed. The spherical shape of the as-separated microbeads was lost even after the heat treatments at low temperatures under 5 kbar. However, the pre-heated microbeads showed the tendency to keep the original spherical shape even after heat treatment at 1900°C.     

Varied forms of palygorskite and sepiolite from different geologic systems

Both palygorskite and sepiolite occur as sheets in tension fracture zones, fault zones, and caves in rock strata, as well as forming strata in alluvial terraces. The sheets of fibrous clays were commonly named mountain leather or mountain cork. The fibrous clays occur in geologic host rock systems of Paleozoic and Mesozoic ages as well as Quaternary age. The analyzed palygorskites represent a continuous series with Al₂O₃ contents ranging from 16.3 wt.% to 4.8 wt.% and (110) XRD peaks from 10.30 Å to 10.57 Å. When less than about 8 wt.% Al₂O₃ is present secondary NIR Mg related OH bands may be observed along with the palygorskite spectra. The analyzed sepiolites have (110) XRD d-values ranging from 12.03 Å to 12.26 Å. The latter is an Fe-sepiolite (xylotile). DTA analyses show that the palygorskite exotherm temperatures form a linear function with the Al₂O₃ contents, from 1045 °C at 16.3 wt.% to 880 °C at 4.8 wt.% Al₂O₃. The fibrous clays can readily adsorb considerable amounts of metals, such as Pb, As, and Cd. The amounts held can be used for determining whether the clays are classified as hazardous materials, on the basis of U.S. EPA and California State standards.     

An oxide chain constructed from octamolybdate, a Keggin anion and a copper (II) – Tetrapyridylpyrazine binuclear unit

The hydrothermal reaction of MoO₃, CuSO₄·5H₂O, tetra-4-pyridylpyrazine (tpyprz), and diphenyldiarsonic acid in water at 140 °C for 48 h yielded the one-dimensional material [{Cu₂(tpyprz)(H₂O)₃}₂{Mo₈O₂₆}{Mo₁₂O₃₆(AsO₄)}]·8H₂O (1·8H₂O). The chain structure of 1·8H₂O is constructed from alternating β-octamolybdate clusters and molybdoarsonate Keggin clusters linked through {Cu₂(tpyprz)(H₂O)₃}⁴⁺ binuclear subunits. The Cu(II) sites are inequivalent with one copper bonding to two pyridyl donors and a pyrazine nitrogen of one terminus of the tpyprz ligand, two aqua ligands and an oxo-group of the cluster while the second copper site coordinates to the nitrogen donors of the other terminus of the tpyprz ligand, an aqua group, a terminal oxo-group of the cluster and a terminal oxo-group of the mixed valence {Mo₁₁^VIMo^VO₃₆(AsO₄)}⁴⁻ cluster. Crystal data: C₄₈H₆₀AsCu₄Mo₂₀N₁₂O₈₀: FW = 4332.96 Triclinic , a = 12.5087(5) Å, b = 13.6681(6) Å, c = 14.8980(6) Å, α = 92.196(1)°, β = 97.046(1)°, γ = 97.751(1)°, Z = 1, D_calc = 2.877 g cm⁻³; structure solution and refinement based on 12,333 reflections (Mo K_α, λ = 0.71073 Å) converged at R₁ = 0.0540 and wR₂ = 0.1115.     

Hydrothermal reaction between CSH and kaolinite for the formation of tobermorite at 180°C

11 Å tobermorites were made from C---S---H (Ca/Si = 1.14) and kaolinite with Ca/(Si+Al) = 0.8 and Al/(Si+Al)= 0.15 at 180°C. The C---S---H was prepared from colloidal silica and lime at 130°C and 180°C for 2 h. Reaction gives in succession C---S---H, poorly crystalline Al-substituted tobermorite, and highly crystalline Al-substituted tobermorite. The addition of kaolinite markedly accelerates the formation of tobermorite within 4 h, more effectively with C---S---H prepared at 130°C than with that prepared at 180°C. X-ray fluorescence diffractometry shows that the Al coordination number is a mixture of 4 and 6 in the initial products and shifts to 4 with an increase in processing time. This agrees with the results for the degree of reaction of the kaolinite.     

Effect of metal additives on the physico-chemical characteristics of activated carbon exemplified by benzene and acetic acid adsorption

The effects of treatment with inorganic salts exemplified by Mg (NO₃)₂ or Ba (NO₃)₂ on the physical-chemical characteristics of activated carbon were studied. Results indicate that the pore volume remains relatively unchanged upon salt treatment when the pore diameter ranges between 7 Å and 20 Å. Significant variation in pore volume was observed when pore diameter is less than 7 Å; generally, the pore volume of AC is greater than AC–Ba and AC–Mg. Major oxygen functional groups such as hydroxyl group for AC, carboxyl group for AC–Ba, and hydroxyl and carbonyl groups for AC–Mg were created. Regenerated at a temperature of 400°C, creates more pore volume especially when treated with Mg(NO₃)₂ with an increase of 13.0%. The regenerated AC–Mg exhibits the greatest acetic acid adsorption capacity among all activated carbons studies.     

Surface analysis of the electropolishing layer on Si(111) in ammonium fluoride solution

The influence of potential and pH on the thickness and composition of the anodic films formed on n-Si(111) in dilute ammonium fluoride solutions is investigated by photoelectron spectroscopy. A combined electrochemistry/surface analysis system with very low contamination levels is used for the investigation of the oxidic layers. XPS data show that the film thicknesses increase with decreasing pH reaching 25 Å at pH 3. A potential dependence measured at lower coverage at a pH of 4.9 reveals changes in thickness and composition by increasing the potential from flatband to +5 V. Fluorine is incorporated into these films in substantial amounts which otherwise consist predominantly of silicon oxide. Silicon fluoride and silicon oxyfluoride are found. We define a current transmissivity, T_c, of the layers which shows strong variations with potential and pH. For a pH of 3, T_c is about 5×10² larger than for pH 4.9. The potential dependence shows a variation of T_c by a factor of three and an increase for thicknesses exceeding 5 Å. These observations are explained by a structural transition from a high density pseudomorphic interfacial layer to a more open structure at larger thicknesses (up to 25 Å). The analysis of the UP-spectra shows only little changes of the electron affinity χ with coverage. We obtain χ4.15±0.1 eV. A distinct band bending is not detected. From HeII spectra, we determine the valence band offset ΔE_v between Si and the oxidic layer to be 5.2 eV.     

Structure of a methylamine sorption complex of fully dehydrated Cd-exchanged zeolite X, Cd46(CH3NH2)16[Si100Al92O384]-FAU

The structure of a methylamine sorption complex of fully dehydrated, fully Cd²⁺-exchanged zeolite X, Cd₄₆(CH₃NH₂)₁₆[Si₁₀₀Al₉₂O₃₈₄]-FAU (a = 24.863(4) Å), has been determined by single-crystal X-ray diffraction techniques in the cubic space group at 21(1) °C. An aqueous exchange solution 0.05 M in Cd²⁺ was allowed to flow past the crystal for 5 days. The crystal was then dehydrated at 480 °C and 2 × 10⁻⁶ Torr for 2 days (colorless), and exposed to 160 Torr of methylamine gas at 21(1) °C for 2 h (yellow). Diffraction data were then gathered in this atmosphere and were refined using all data to the final error indices (based upon the 524 reflections for which F_o > 4σ(F_o)) of R₁ = 0.069 and wR₂ = 0.200. In this structure, Cd²⁺ ions occupy three crystallographic sites. The octahedral sites I at the centers of the hexagonal prisms are filled with 16 Cd²⁺ ions per unit cell (Cd–O = 2.369(8) Å). The remaining 30 Cd²⁺ ions are located at two non-equivalent sites II with occupancies of 14 and 16. The 16 methylamine molecules per unit cell lie in the supercage where each interacts with one of the latter 16 site-II Cd²⁺ ions: N–Cd = 2.11(8) Å. The imprecisely determined N–C bond length, 1.49(22) Å, agrees with that in gaseous methylamine, 1.474 Å. The positions of the hydrogen atoms were calculated. It appears that one of the amino hydrogen atoms hydrogen bonds to a 6-ring oxygen, and that the other forms a bifurcated hydrogen bond to this and another 6-ring oxygen. The methyl group is not involved in hydrogen bonding.     

Reactivity of Ni/SiO2·MgO toward carbon dioxide reforming of methane under steady state and periodic operations

The carbon dioxide reforming of methane over commercial Ni/SiO₂·MgO catalyst under periodic and steady state operations was investigated at a temperature range of 650–750 °C. Under steady state operation, methane conversions tended to be constant with reaction time but increased with increasing reaction temperature. It was then observed that at low temperature (650 °C) under the periodic operation, methane conversion was also constant at approximately 48% throughout reaction time, but for the operation at a higher temperature, i.e. 750 °C, higher methane conversion (about 67%) was initially achieved but decreased dramatically with reaction time (to 27% in 240 min). The reason for the catalyst deactivation particularly for the periodic operation was further investigated by TPO, BET and XRD. It is suggested that at different operating temperatures, various types of coke occurred on the surface of catalyst and affected the catalytic activity.     

The influence of oxidation upon strength and structure of a needle-coke and a coal extract coke

Calcined commercial needle-cokes and coal extract coke are characterised by optical microscopy of polished surfaces. The needle-coke has an optical texture of coarse-grained mosaic, flow domain and a acicular flow domain; the coal extract coke has an optical texture of medium- and coarse-grained mosaics. The cokes are oxidised in air to 1–25% wt. loss. The microstrengths of original and oxidised cokes are measured. For the needle-cokes, a 1% wt. loss significantly reduces the microstrength values whereas for the coal extract coke the microstrength begins to decrease only after 15% wt. loss. SEM examination of original and oxidised surfaces indicates that oxidation of the needle-coke proceeds by the development of microfissures within the flow domain and by pitting of surfaces of the basal planes of the acicular flow domains. The surfaces of the coal extract coke were uniformly pitted. The decline of microstrength values of needle-cokes is associated with internal oxidation of the coke particles.     

Diffraction evidence for distorted graphite-like ribbons in an activated carbon of very large surface area

A carbon prepared from petroleum coke which possesses a very large active surface area (3500 m²/gm) has been examined with X-ray diffraction techniques. Simultaneous analysis of the low and high angle scattering suggests that the material is composed of distorted ribbons of graphite-like layers. On the average, each ribbon contains only one or a very few graphite-like layers, and these layers are distorted with an average radius of curvature of ~ 25A. This very open structure results in a large surface area and gives rise to extensive small angle scattering.     

The hydrothermal synthesis and structure of a one-dimensional Fe(II)molybdophosphate

A new one-dimensional Fe(II)molybdophosphate of the formula, [(C₁₀H₈N₂)H₂]₂[Fe₄^(II)Mo₁₂^(V)(HPO₄)₆(PO₄)₂(H₂O)₈(OH)₆O₂₄]·8H₂O, (1), has been synthesized by employing hydrothermal methods and characterized by single crystal X-ray diffraction. The structure consists of a network of MO₆ (M = Fe, Mo) octahedra and (H)PO₄ tetrahedra linked through their vertices. The connectivity between the polyhedral units gives rise to one-dimensional chains with eight-membered apertures. The hydrogen bonded interactions between the chains form pseudo two-dimensional layers. Extensive hydrogen bonding also exists between the amine molecule, 4,4^′-bipyridine, water molecule and framework oxygen atoms. Crystal data for 1: monoclinic, space group = P2₁ (no. 4), a=12.549(3) (Å), b=23.496(5) (Å), c=14.551(3) (Å), β=114.87(3)°, , and Z=2.