Synthesis of cementite-dispersed carbons by pressure pyrolysis of organoiron copolymers

Cementite-dispersed carbons were synthesized by pressure pyrolysis of divinylbenzene-vinylferrocene and styrene-vinylferrocene copolymer at temperatures below 600° C and the pressure of 125 MPa. The pyrolysis process of both copolymers was analysed by infrared spectra and magnetization of the pyrolysed substances. The absorption band of iron-carbon bond of divinylbenzene-vinylferrocene copolymer decreased on increasing its pyrolysis temperature from 300 to 450° C and finally disappeared at 500° C. The carbonization of divinylbenzene-vinylferrocene proceeded more rapidly than styrene-vinylferrocene at temperatures between 450 and 500° C. Styrene-vinylferrocene was heat-treated at 250° C for 2 h under 100 MPa affording a paramagnetic product, whereas the paramagnetic character of divinylbenzene-vinylferrocene was revealed after heat-treatment at 380° C. The saturation magnetization of cementite-dispersed carbon synthesized from both kinds of copolymers was comparable when the pressure pyrolysis was carried out at temperatures between 520 to 600° C at 125 MPa. The saturation magnetization of cementite-dispersed carbon formed at 550° C under 125 MPa was correlated linearly with the iron content in carbon. Threedimensional cross-linked divinylbenzene-vinylferrocene copolymer gave the highly dispersed cementite particles less than 50 nm with the coercive force of 950 Oe. On the other hand, the larger particle size of cementite up to 120 nm and the lower coercive force about 400 Oe were obtained in carbon matrix prepared by the pressure pyrolysis of styrene-vinylferrocene copolymer.     

Effects of crystallinity of starting carbons on diamond formation in presence of nickel under high pressure and high temperature condition

The processes of graphitization and diamond formation of several carbons in the presence of nickel were investigated under 8 GPa at temperatures up to 1800° C. Diamond was formed easily from graphitized pitch coke which had a well-developed graphitic structure and in less amount from glassy carbon preheated at about 3000° C which was partly graphitized. On the other hand, pitch coke and glassy carbon, preheated at about 2000° C and not graphitized, did not transform to diamond but remained graphitized even in the diamond stable region. Diamond from graphitized pitch coke and glassy carbon preheated at about 3000° C grew to form by direct bonding.     

Effect of high-pressure pre-treatment of starting carbon on diamond formation

Four starting carbons differing in crystallinity and grain size were pre-treated with or without nickel at 3 GPa and 1800° C or at 6 GPa and 1700° C. Diamond synthesis from carbons pre-treated and then further treated in vacuum was carried out at 8 GPa and 1700° C. Pre-treated carbons with or without nickel, which were fully or partly graphitized, changed a little or did not convert to diamond at 8 GPa and 1700° C. Diamond did form from the pre-treated carbons after treatment in a vacuum at 1000° C. Diamond formation, even from the graphitized carbons, was found to be inhibited mainly by gases adsorbed on the treated carbon during the pre-treatment under high pressure.     

Synthesis and properties of cobalt-dispersed carbons by pressure pyrolysis of organocobalt polymers

Cobalt-dispersed carbons were synthesized by pressure pyrolysis of cobaltocene-divinylbenzene and phenylallylcobaltocene-divi nylbenzene at 125 MPa below 700° C. The carbons resulting from cobaltocene-divinylbenzene contained uniformly dispersed fine particles, < 20 nm diameter, of metallic cobalt of lower crystallinity, which were composed of ferromagnetic and superparamagnetic particles. Metallic cobalt particles of cubic and hexagonal structures with higher crystallinity were formed during pyrolysis of phenylallylcobaltocene-divinyibenzene. Cobaltucence-divinylbenzene and phanylallylcobaltocene-divinylbenzene changed their magnetic properties from diamagnetism to paramagnetism at 250 and 200° C, respectively. The infrared absorption band of the cyclopentadienyl ring at 995 cm^–1 disappeared at 350° C in cobaltocene divinylbenzene and at 300° C in phenylallylcobaltocene-divinylbenzene. Superparamagnetic particles from cobaltocene-divinylbenzene aggregated and crystallized to produce larger particles of diameter 30 to 100 nm, which increased the magnetization during thermomagnetic measurement. The saturation magnetization of cobalt-dispersed carbons from phenylallylcobaltocene-divinylbenzene was higher by about 10% than that from cobaltocene-divinylbenzene. The coercive forces of cobalt-dispersed carbon from phenylallylcobaltocene-divinylbenzen and cqbaltocene-divinylbenzene were 350 and 250 Oe (2.79 × 10⁴ and 1.99 × 10⁴ Am^–1), respectively.     

Effects of starting carbon and solvent-catalyst on the reaction sintering of diamond

Polycrystalline diamond sintered compact was prepared under high pressure and temperature conditions (7 GPa, 1700° C, 10 to 15 min) from purified natural graphite (NG) or graphitized pitch coke (GPC) using iron, cobalt or nickel as the catalytic active metal and titanium or zirconium as the solvent metal. The effects of the combinations of starting carbon and solvent-catalyst on the transformation behaviour and morphology of the converted diamond were investigated in relation to the starting powder compositions. Diamond crystals converted from NG tended to have euhedral habits, when twin crystals were occasionally found due to a relatively rapid conversion and growth rate of diamond. On the other hand, a skeletal structure of diamond was easily formed by a mild conversion from GPC in the 15 to 30 vol% ( 40 to 60 wt%) nickel solvent-catalyst. The degree of catalytic action in the 8A group of 3d transition metals for the GPC system was in the order: Ni Co > Fe. The grain growth of the converted diamond was depressed by the addition of 4A transition metals (titanium or zirconium) which results in the fine-grained and homogeneous sintered microstructure.     

Influence of size and distribution of W phase on strength and ductility of high strength Mg-5.1Zn-3.2Y-0.4Zr-0.4Ca alloy processed by indirect extrusion

A high strength Mg-5.1Zn-3.2Y-0.4Zr-0.4Ca(wt%) alloy containing W phase(Mg_3Y_2Zn_3) prepared by permanent mold direct-chill casting is indirectly extruded at 350?C and 400?C, respectively. The extruded alloys show bimodal grain structure consisting of fine dynamic recrystallized(DRXed) grains and unrecrystallized coarse regions containing fine W phase and β2' precipitates. The fragmented W phase particles induced by extrusion stimulate nucleation of DRXed grains, leading to the formation of fine DRXed grains, which are mainly distributed near the W particle bands along the extrusion direction. The alloy extruded at 350?C exhibits yield strength of 373 MPa, ultimate tensile strength of 403 MPa and elongation to failure of 5.1%. While the alloy extruded at 400?C shows lower yield strength of 332 MPa,ultimate tensile strength of 352 MPa and higher elongation to failure of 12%. The mechanical properties of the as-extruded alloys vary with the distribution and size of W phase. A higher fraction of DRXed grains is obtained due to the homogeneous distribution of micron-scale broken W phase particles in the alloy extruded at 400?C, which can lead to higher ductility. In addition, the nano-scale dynamic W phase precipitates distributed in the un DRXed regions are refined at lower extrusion temperature. The smaller size of nano-scale W phase precipitates leads to a higher fraction of un DRXed regions which contributes to higher strength of the alloy extruded at 350?C.     

Effects of added c-BN seed crystals on the reaction sintering of c-BN accompanied by a conversion from h-BN to c-BN

Reaction sintering behaviour of c-BN which is accompanied by a conversion from h-BN to c-BN was investigated under high pressure (7 GPa) and temperature (1700°C) conditions for 30 to 60min. A high conversion yield of c-BN in the sintered compact was attained by adding fine-grained c-BN seed crystals (particle size 0.5 to 8m) to h-BN powder in the presence of 1 wt% NH₄NO₃ as a catalyst. An induced transformation from h-BN to c-BN occurs over a large surface area of c-BN seed crystals, which results in the formation of direct interparticle bonding between c-BN grains in the sintered compact. A fully dense sintered compact of c-BN (bulk density 98% theoretical) was obtained from the specimen of 70wt% h-BN with 30wt% added c-BN crystals having a particle size of 0.5m. This c-BN compact had an average microhardness of 5100 kg mm^–2 and a specific dielectric constant of 10.0 at a frequency of 1 MHz.     

Synthesis and properties of carbons dispersed with Fe-Co alloy by pressure pyrolysis of organoiron-organocobalt copolymer

Carbons dispersed with Fe-Co alloy were synthesized by the pressure pyrolysis of vinylferro cene-phenylethynylcobaltocene-divinylbenzene copolymer at temperatures below 700° C and at 125 M Pa. As-prepared carbon synthesized at 550° C contained finely dispersed metallic particles of less than 10nm diameter with low crystallinity, which crystallized to form Fe-Co alloy particles with a higher crystallinity by subsequent heat treatment at 800° C. Larger particles of the alloy of more than 50nm diameter were dispersed in the carbon matrix synthesized at 700° C. Thermomagnetization measurement of the as-prepared carbon synthesized from divinylbenzene-2.1 mol% vinylferrocene-4.8mol% phenylethynylcobaltocene copolymer at 550° C and 125 M Pa confirmed that iron formed an alloy with cobalt in the carbon matrix. Fine, superparamagnetic metallic particles in the as-prepared carbon aggregated and crystallized by the heat treatment during the thermomagnetic measurement to increase the magnetization of the alloy-dispersed carbon. The saturation magnetization and the coercive force of alloy-dispersed carbon increased from 128 to 187e.m.u.g^–1 and from a few to 50 Oe, respectively, on increasing the pyrolysis temperature of the starting copolymer from 550 to 700° C. The saturation magnetization of alloy-dispersed carbon from divinylbenzene containing iron and cobalt with a ratio of 52 was higher than that from divinylbenzene including those with a ratio of 25. The carbon with finely dispersed Fe-Co alloy showed a high saturation magnetization of 213 e.m.u.g^–1 and a coercive force of 230 Oe, and the magnetization persisted above 800° C.     

Anodic oxidation of coal slurries in flow-through packed bed electrodes

Experimental studies were made to determine the characteristics of anode systems depolarized by coal slurries. The anode was a packed bed of platinum-plated titanium pellets, through which the slurries were recirculated. This flow system increased collisions between coal particles and the anode surfaces so that the anodic oxidation of the coal was enhanced. At the same time, soluble organic compounds and Fe²⁺ ions dissolved from the coal were oxidized at the anode, and Fe³⁺ was reduced by thermochemical reactions with reducing compounds in the coal; this reduction was important in maintaining the current density in long-term electrolysis.     

Acceleration of Metastable to Alpha Transformation of Al2O3 Scale on Fe?CAl Alloy by Pure-Metal Coatings at 900???C

The effect of pure-metal coatings on the transformation of a pre-formed ??-Al₂O₃ scale on an Fe?C50Al alloy was investigated. The elements Fe, Cr, Ti, and Ni were deposited on the pre-formed Al₂O₃ having a thickness of about 50?nm. Ti, Cr or Fe coatings on pre-formed ??-Al₂O₃ were found to accelerate the transformation to ??-Al₂O₃, while no apparent effect was observed from the Ni coating. Since the corundum crystal structure of Ti₂O₃, Cr₂O₃ and Fe₂O₃ was indentified on Ti, Cr or Fe coated samples and the transformation to ??-Al₂O₃ started at the outer part of the ??-Al₂O₃ scale, these corundum oxides were considered to act as heterogeneous nucleation sites for ??-Al₂O₃, therefore promoting the transformation from ??-Al₂O₃ to ??-Al₂O₃. The growth rate of ??-Al₂O₃ grains and the rate of advance of the ??-Al₂O₃/??-Al₂O₃ interface were found to be faster with the Fe coating.