Nickel ferrite-dispersed carbon could be synthesized by pressure pyrolysis of divinylbenzene (DVB)-vinylferrocene (VF)-nickelocene (CpzNi) polymer in the presence of water under 125 MPa and at temperatures below 700~ By heat treatment at 550~ with water, nickel ferrite particles could be dispersed finely in the carbon matrix, although a small amount of nickel-iron carbide also began to form above 600 ~ The morphologies of the carbon particles formed were observed to be polyhedral, coalescing spherulitic and spherulitic. When 30 wt % H20, spherulitic carbons a few micrometres in diameter were prepared, in which nickel ferrite particles from 10-30 nm were dispersed in the carbon matrix. The saturation magnetization of carbon composites formed from DVB-3.0 mol% Cp2Ni-6.0 mol% VF and 20 wt% H20 at 550~ was about 30 e.m.u.g-1 and increased with pyrolysis temperature. The coercive force of the carbon composite was 120 Oe and was affected by the amount of added water using pressure pyrolysis. Thermomagnetic measurement shows that the Curie temperature of nickel ferrite-dispersed carbon was about 580 ~
1. Introduction
Pressure pyrolysis is characterized by a high yield of carbon and controllability of the morphology of the carbon products. Hirano et al. [11 synthesized mesophase spherulitic carbon by pressure pyrolysis of anthracene, p-terphenyl and a mixture of these, using a hydrothermal apparatus. Hirano et al. [2] demonstrated that pyrolysis under various pressures of 30 200 MPa could produce many kinds of carbon morphology, including isotropic carbon spherulites. Carbon composite containing fine metal or metal compound particles, such as iron [3, 41, cobalt [51, nickel [6], iron-cobalt alloy [71, iron-carbide [8, 91, magnetite [10], could be synthesized by pressure pyrolysis of divinylbenzene copolymerized with vinylferrocene, phenylethynyl cobaltocene, etc. Both the metal-carbon and carbon-carbon bondings of the starting organometallic polymer influence the carbonization process so that the selection of starting material is one of the most important factors in controlling the morphology and the nature of the resultant carbons.
Metal oxide particles, such as magnetite and nickel ferrite, are quite stable compared to metal particles under oxidative conditions. These magnetic particlesdispersed carbons have many attractive applications, such as electromagnetic wave shields, because ferrite particles as magnetic materials are uniformly dispersed in carbon to produce an electrically conductive material. Moreover their composites are potential materials for catalysts and magnetic toners.