Intercalation compounds as precursors for oriented catalysts

The oxyhydrolysis of MoCl₅ Ceylon graphite intercalation compounds, in the 400 – 500 °C temperature range, gives MoO₃-graphite oriented catalysts. (010) MoO₃ crystalline faces are parallel to the (001) graphite sheets. Catalytic properties in propylene oxidation are governed by the distribution of the MoO₃ faces and controlled by the temperature of preparation.From the example of the MoO₃-graphite system, it appears that intercalation compounds can be precursors for oriented catalysts with a strong interaction of the graphite support with the active phase, which improves the catalytic properties.     

Intercalation of MoCl5 into graphite-determining factor to control stage structure

The mechanism of formation of MoCl₅–graphite intercalation compounds (MoCl₅–GICs) was proposed on the basis of the experimental results obtained in the present and previous works. The amount of reactive chlorine formed in the reaction system was concluded to be the determining factor for the stage structure in resulting MoCl₅–GICs. Reactive atomic chlorine produced during the decomposition of only 2% MoCl₆, which was formed from MoCl₅ at high temperature, seemed not enough for the formation of pure stage 1 MoCl₅–GIC and therefore only stage 2 structure was obtained. Heating the MoCl₅ in presence of either MoOCl₃ or MoO₃ formed MoOCl₄, which evolved high amount of reactive chlorine which formed the stage 1 MoCl₅–GIC in single phase.     

The oxidative intercalation of selected carbon fibers by bis(fluorosulfuryl)peroxide,S2O6F2

Oxidative intercalation of two selected PAN- and pitch-based carbon fibers by bis(fluorosulfuryl)peroxide, S₂O₆F₂, results in materials with enhanced electrical conductivities. The nature and the extent of intercalation show some differences in both systems. Pitch-based fibers can be intercalated up to a composition of C_7·5SO₃F. PAN-based fibers lose some of the lattice nitrogen atoms during intercalation and a lower degree of intercalation is noted. Conductance enhancements up to 14 and 17 times were observed for intercalated tows of pitch- and PAN-based fibers respectively. Specific resistivity measurements on intercalated filaments showed metallic behaviour of these materials up to 300 K. The observed conductance values are discussed, together with the structural data obtained on them.     

Electronic properties and structure of stage-4 MoCl5 GICs prepared from highly crystallized graphite films

Films of stage-4 MoCl₅ graphite intercalation compound (GIC) were synthesized from laboratory-prepared high-quality graphite films and their electronic properties such as electrical conductivity, Hall coefficient and transverse magnetoresistance were measured at 300, 77 and 1.3 K. The graphite films were prepared from polyimide films of Kapton (25 μm thick) and pyromellitic dianhydride-p-phenylenediamine-3,3′,4,4′-tetraaminobiphenyl (PMDA-PPD-TAB, molar ratio 100/92/4, hereafter called PPT) (45 μm thick) by heating them up to 3200 °C. The stage-4 MoCl₅ GICs were made by heating the graphite films in a glass tube with a mixture of natural graphite flakes and MoCl₅ powder at 450 °C for 7 days. Majority carriers in GICs were holes, the density of which was estimated to be about 5.2 × 10²⁶ m⁻³ at 1.3 K. A Shubnikov-de Haas oscillation with a period of 7.84 × 10⁻³ T⁻¹ and slight modulation of the amplitude was observed in the dependence of the transverse magnetoresistance on magnetic field at 1.3 K for both GIC films. The electronic and structural parameters suggested that the GIC film from a polyimide PPT has less perfect crystallinity than that from a polyimide Kapton, even though the pristine graphite film from PPT has better crystallinity.     

Electrical and mechanical properties of copper chloride-intercalated pitch-based carbon fibers

Using chlorine vapor transport we have intercalated pitch-based fibers, highly oriented pyrolytic graphite (HOPG), and natural single crystals of graphite with CuCl₂. In this paper we report mainly on the electrical properties of pitch-based fibers heat treated to 2500, 2750 and 3000 °C and then intercalated. The electrical resistivity both above and below room temperature, the tensile strength, and Young's modulus are reported. In very high magnetic fields at 4 K the fibers exhibit Shubnikov-de Haas oscillations, which means this powerful technique is now available as a diagnostic tool for intercalated fibers.     

Intercalation of formic acid into carbon fibers and their exfoliation

Intercalation of formic acid in a mesophase-pitch-based fibers was successfully carried out by electrolysis in 50% solution. The formation of intercalation compounds was confirmed by X-ray diffraction (XRD), which showed a broad peak at the interlayer spacing of 0.92 nm. By rapid heating of the intercalated compounds of carbon fibers thus produced up to 1000 °C, exfoliation was occurred instantaneously. Changes of potential was found to depend on electrolysis temperature to prepare intercalation compounds of carbon fibers with formic acid; at the higher temperature potential saturated at the less electrical charge supplied. Suitable synthesis condition of the intercalation compounds was determined to be the current density of 200 A/g at the electrolysis temperature 10 °C. The exfoliation of carbon fibers started at the free end of fibers and proceeds gradually to the fixed end along the fiber axis with increasing charge. This electrolysis behavior suggests that the intercalation of formic acid into the present carbon fibers proceeds from the free end of carbon fibers towards the fixed end with increasing charge.     

Different reactivity of carbon materials for intercalation of iron chloride in its nitromethane solution

Host effect on the intercalation of iron chloride(III) in its nitoromethane (NM) solution was investigated by using host carbons heat-treated at different temperatures, cokes with random orientation and pyrolytic carbons with planar one. Cokes heat-treated below 1900°C were not intercalated even in high concentration solution of FeCl₃ (4.8 M). The reactivity of cokes heat-treated above 2100°C was found to be strongly dependent on the concentration of FeCl₃ in the solution; the less crystallized coke needed the higher concentration of FeCl₃ to be intercalated, and the smaller stage number of resultant intercalation compound from the coke heat-treated at a temperature was obtained in the higher concentration of solution. On the other hand, pyrolytic carbons behaved quite differently; all of them could react even in the 0.6 M solution of FeCl₃, where any cokes did not react, to give the same stage-3 structure independent on the crystallinity. These different reactivities of hosts for the intercalation of FeCl₃ in NM were discussed by comparing with those for other intercalations as reported previously.     

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.     

Interaction between MoO3 and metals Te, Cd, Sb

Eighteen compositions of MoO₃-Te at 800 °C and seven of each of MoO₃-Cd (at 500 °C) and MoO₃-Sb (at 600 °C) were heat treated in vacuum-sealed quartz ampules. The phases of the heat-treated compositions were analyzed using x-ray diffraction (XRD) patterns. The interactions in the three systems are summarized. Three phases in equilibrium are (1) in the MnO₃-Te system at 800 °C, Te, Mo₄O₁₁, TeMo₄O₁₃—(0<x_{MoO 3}<0 889) and MoO₃, Mo₄O₁₁, TeMo₄O₁₃—(0.889<x_{MoO 3}<1); (2) in the MoO₃_Cd system at 500 °C, Cd, MoO₂, CdMoO₄—(0<x_{MoO 3}<0.6667) and MoO₃, MoO₂, CdMoO₄—(0.6667<x_{MoO 3}<1); and (3) in the MoO₃-Sb system at 600 °C, Sb, MoO₂, Sb₄Mo₁₀O₃₁—(0<x_{MoO 3}<0.734) and MoO₃+MoO₂+Sb₄Mo₁₀O₃₁ (0.734<x_{MoO 3}<1). The results lead to construction of ternary phase diagrams: Te-MoO₃-TeMo₄O₁₃, Cd-MoO₃-CdMoO₄, and Sb-MoO₃-Sb₄Mo₁₀O₃₁.     

Interaction between MoO3 and metals Te, Cd, Sb

Eighteen compositions of MoO₃-Te at 800 °C and seven of each of MoO₃-Cd (at 500 °C) and MoO₃-Sb (at 600 °C) were heat treated in vacuum-sealed quartz ampules. The phases of the heat-treated compositions were analyzed using x-ray diffraction (XRD) patterns. The interactions in the three systems are summarized. Three phases in equilibrium are (1) in the MnO₃-Te system at 800 °C, Te, Mo₄O₁₁, TeMo₄O₁₃—(0<x_{MoO 3}<0 889) and MoO₃, Mo₄O₁₁, TeMo₄O₁₃—(0.889<x_{MoO 3}<1); (2) in the MoO₃_Cd system at 500 °C, Cd, MoO₂, CdMoO₄—(0<x_{MoO 3}<0.6667) and MoO₃, MoO₂, CdMoO₄—(0.6667<x_{MoO 3}<1); and (3) in the MoO₃-Sb system at 600 °C, Sb, MoO₂, Sb₄Mo₁₀O₃₁—(0<x_{MoO 3}<0.734) and MoO₃+MoO₂+Sb₄Mo₁₀O₃₁ (0.734<x_{MoO 3}<1). The results lead to construction of ternary phase diagrams: Te-MoO₃-TeMo₄O₁₃, Cd-MoO₃-CdMoO₄, and Sb-MoO₃-Sb₄Mo₁₀O₃₁.     

Agglomeration during reduction of MoO3

Molybdenum powder is manufactured in a two step process starting from MoO₃. The first step reduction of MoO₃ to MoO₂ is carried out in rotary calciners. Agglomeration of powder occurs during this reduction stage resulting in several manufacturing issues. The evolution of agglomeration during the reduction of MoO₃ was investigated in the current study. As-received MoO₃ and MoO₃ milled for 0.5 h were used as the starting powders. The powders were reduced at 550 °C, 650 °C and 750 °C in a hydrogen atmosphere. The starting and reduced powders at various temperatures were analyzed using BET surface area, XRD, and SEM techniques. The surface area of the reduced powders was monitored for quantifying the degree of agglomeration. The surface area was found to be minimum for the samples reduced at 650 °C. SEM observations confirmed the agglomeration of powders during reduction process. XRD analysis showed complete reduction of MoO₃ to MoO₂ at 650 °C and 750 °C. The agglomeration of the powders was either due to melting of eutectic formed between MoO₃ and Mo₄O₁₁ or due to partial melting of MoO₃. The reduction of MoO₃ is recommended to be completed at a low temperature to prevent agglomeration of the oxide powders.     

Characterization of CsC24 prepared from carbon materials with different graphitization degree

Samples of CsC₂₄ were prepared from carbon materials (derived from petroleum cokes) with different graphitization degree and were characterized by X-ray diffraction (XRD), electron spin resonance (ESR) and Raman spectroscopy. The XRD measurement showed that all the host carbon samples heat treated from 1300 to 2800°C being contacted with CsC₈ planar specimen allowed the intercalation of Cs. Correspondingly, the resulting CsC₂₄ samples were able to sorb hydrogen molecules, which is a clear evidence of the formation of the nanospace. The broad ESR signal due to conduction electron observed for carbon samples (heat-treatment temperature, HTT above 2200°C) disappeared after intercalation of Cs because of the spin–orbit interaction caused by the intercalated Cs. The Raman G-band of the CsC₂₄ samples (HTT above 1750°C) shifted from 1584 cm⁻¹ of the host carbon to higher wave number (∼1602 cm⁻¹) in agreement with the reported data for CsC₂₄. In addition, it was confirmed that the D-band signal disappeared for the CsC₂₄ samples (HTT above 2000°C).     

Effect of mechanical activation on aluminothermic reduction of molybdenum trioxide

In the present work, two different methods were used to study the effect of mechanical activation on aluminothermic reduction of MoO₃. In the first method, a stoichiometric amount of Al + MoO₃ and 20% weight percent of aluminum oxide (as the heat absorber) was mixed and mechanically activated by milling process in a planetary ball mill under argon atmosphere. XRD patterns and SEM micrographs of the activated samples indicated that aluminothermic reduction of MoO₃ was done in the milling media after 5 h and increasing the milling time, decreased the remaining non reacted Al and MoO₃. In the second method, as received MoO₃, was mechanically activated in different times in the range of 3 to 24 h. The effect of mechanical activation on molybdenum trioxide grain size was analyzed by Williamson-Hall method, and it was shown that, by increasing the activation time, the grain size and internal strain was reduced and increased, respectively. In the next step, the activated MoO₃ powders were mixed with stoichiometric amount of Al and 20 wt% Al₂O₃ (as the heat absorber) and shaped in the form of cylindrical samples of 10 mm in diameter and height of 15–20 mm by applying 70 MPa uniaxial pressure. Aluminothermic reaction was initiated in the prepared pellets by using conventional microwave oven. The XRD patterns of the combustion products showed that, by mechanical activation of MoO₃, the undesired phases of molybdenum dioxide (MoO₂) and aluminum molybdate Al₂(MoO₄)₃ was decreased. The aluminothermic reduction of activated MoO₃ also was studied by non-isothermal Differential Scanning Calorimetry (DSC) with heating rate of 10 °C min⁻¹ under argon atmosphere. The results showed that by mechanically activating MoO₃ for 24 h, the ignition temperature of reduction reaction decreased from 910 to 780 °C and the practical efficiency of the reaction increased from 82.9% (for the nonactivated MoO₃) to 92.5%. The comparing results between the milling mixture and the milling molybdenum trioxide indicated that the reaction in mixture cannot be completed during milling and a small amount of Al and MoO₃ remained in the activated powder. But for the activated MoO₃, and after igniting in a conventional microwave oven the desired reaction went to completion.     

Oxidation Mechanisms of Copper and Nickel Coated Carbon Fibers

Differential-Thermal Analysis (DTA) and X-ray diffraction analysis were applied to determine the mechanisms of high-temperature oxidation of copper- and nickel-coated carbon fibers. Both kinds of coatings were deposited by electroless plating onto the fiber surface. The as-deposited copper film was crystalline, whereas the nickel coating consisted of an amorphous Ni–P alloy. Coated fibers were heated from room temperature to 900 °C in air at 10 °C min^?1. For the copper coating, the main oxidation product formed at low temperatures was Cu₂O, while at higher temperatures was CuO. The crystallization of Ni–P took place at 280–360 °C with the formation of Ni and Ni₃P. The final compounds were NiO, Ni₂P and Ni₃(PO₄)₂. After complete oxidation of the carbon fibers, copper and nickel-oxidized microtubes were obtained. Besides, while copper reduced the temperature of the fiber oxidation, nickel coatings increased the minimum temperature needed for this reaction.     

连续氮化硅纤维的氧化行为与高温影响研究

连续氮化硅陶瓷纤维是透波／承载一体化陶瓷基复合材料的关键原材料,也是制约复合材料耐高温性能与力学性能的关键因素。本文系统研究了国防科技大学研制的连续氮化硅纤维的抗氧化性能,分析了高温处理后纤维的组成结构与力学性能变化规律。结果表明：1000°C氧化1h后纤维强度高于原始纤维强度,主要是形成的玻璃相能减少和弥补纤维的表面缺陷。随着空气中处理温度提高,氧含量增加,纤维表面形成的SiO₂层逐渐变厚,纤维强度明显降低。纤维在1200°C氧化1h后强度保留率为63%,表明在此温度以下纤维有较好的服役性能。另一方面,氮化硅纤维在1450°C N₂中处理1h的强度保留率为57%,表现出良好的耐高温性能。纤维表面氧化对其在N₂下的耐高温性能具有不利影响,1000°C氧化的纤维在1450°C处理后丧失强度,1500℃处理后形成氮化硅结晶,失重明显增长,纤维内部也开始产生缺陷。     

On the extrusion microstructural evolution of Al–Al3Ni in situ composite

A directionally solidified Al–Al₃Ni in situ composite with fine Al₃Ni eutectic fibers was extruded at 200 and 400°C. Evolution of the microstructures was then investigated by examining the partially and fully extruded portions of the billets. According to the results, extrusion at 200°C will generate numerous tangled dislocations initially. When fiber breakage occurs, subgrains which are formed preferentially near the cracks will grow to eliminate the tangled dislocations and separate the pieces of broken fibers further apart, resulting in a fine subgrain structure with rod-like Al₃Ni particles distributed intergranularly. On the other hand, the billet extruded at 400°C shows formation of large subgrains with many intragranular Al₃Ni fibers in the early stage. Upon further extrusion, the fibers are broken while the large subgrains are refined by forming dislocation bands and changing the bands to sub-boundaries. Consequently, the 400°C-extruded billet shows fine subgrains with intergranular and intragranular rod-like Al₃Ni particles.     

Formation Process of FeCl<Subscript>3</Subscript>-NiCl<Subscript>2</Subscript>-Graphite Intercalation Compounds

The intercalation reaction of NiCl₂ from the mixture of FeCl₃ and NiCl₂ has been studied by using natural graphite at a wide range of residence times from 1 to 24 h. The microstructure changes of reaction products were investigated by XRD, SEM, and EDS, and the formation of ternary FeCl₃-NiCl₂-GICs was confirmed. In the process FeCl₃ was found to be intercalated preferentially at the initial stage of the intercalation, and set up the framework of the stage domain of GICs. Then NiCl₂ intercalated along the channels cut by FeCl₃. The diffusion rate and distribution uniformity of NiCl₂ controlled the rate of the intercalation reaction and the ternary grade of the FeCl₃-NiCl₂-GICs.     

Structural Evolution of Silicon Oxynitride Fiber Reinforced Boron Nitride Matrix Composite at High Temperatures

The structural evolution of a silicon oxynitride fiber reinforced boron nitride matrix (Si-N-O_f/BN) wave-transparent composite at high temperatures was investigated. When heat treated at 1600 °C, the composite retained a favorable bending strength of 55.3 MPa while partially crystallizing to Si₂N₂O and h-BN from the as-received amorphous structure. The Si-N-O fibers still performed as effective reinforcements despite the presence of small pores due to fiber decomposition. Upon heat treatment at 1800 °C, the Si-N-O fibers already lost their reinforcing function and rough hollow microstructure formed within the fibers because of the accelerated decomposition. Further heating to 2000 °C led to the complete decomposition of the reinforcing fibers and only h-BN particles survived. The crystallization and decomposition behaviors of the composite at high temperatures are discussed.     

Oxidation mechanisms in Mo-reinforced Mo5SiB2(T2)–Mo3Si alloys

Results of a systematic investigation of the oxidation in a Mo–Si–B alloy containing the three phases, Mo, Mo₃Si, and Mo₅SiB₂ (T2) are presented. The relative kinetics of B-containing silica-scale formation, permeation of MoO₃ through the viscous scale, the viscosity of the B-containing SiO₂ scale, volatilization of MoO₃ and B₂O₃ from the silica scale, are identified as key parameters that determine the kinetics of the oxidation of the alloy in the temperature range of 500–1300 °C. The oxidation is worst in the intermediate temperature range, 650–750 °C, where MoO₃ begins to volatilize but B₂O₃ does not, resulting in gaseous MoO₃ bubbling through a low viscosity borosilicate scale. In this temperature range, the scale provides insufficient protection suggesting that attempts to improve the oxidation resistance of this system must focus on this temperature range.     

Oxidation features of SiOC/MoSi2 composite at low temperatures

The oxidation behavior of SiOC containing dispersed MoSi₂ particles was studied. An SiOC/MoSi₂ composite was prepared by converting polymethylsiloxane into SiOC via pyrolysis after mixing MoSi₂ particles. The SiOC matrix phase oxidized to SiO₂, and MoSi₂ particles oxidized to SiQ and MoO₃. The composite displayed superior oxidation resistance at 110 0°C, because a thin SiO₂ layer formed on the surface deterred the formation of highly volatile MoO₃. However, accelerated oxidation occurred between 400 and 500°C owing to the concurrent formation of SiO₂ and MoO₃, which led to porous, nonprotective scale formation.