Core-Shell Materials Elaboration in Supercritical Mixture CO 2 / Ethanol V. Pessey, R. Garriga, † F. Weill, B. Chevalier, J. Etourneau, and F. Cansell* Institut de Chimie de la Matie ` re Condense ´ e de Bordeaux (ICMCB), CNRS-UPR 9048, Universite ´ Bordeaux I, Avenue Albert Schweitzer, 33608 Pessac Cedex, France Supercritical fluids exhibit a range of unusual properties that can be exploited for developing new processes. In this paper a new way for particle coating is presented. It consists of depositing on a core particle a thin layer of a material based on copper in a supercritical medium. Two examples are described: a deposit of Cu on nickel particles and a deposit of Cu on particles of permanent magnet SmCo 5 . In both cases the copper source is bis(hexafluoroacetylacetonate)- copper(II), which is thermally decomposed in the supercritical mixture CO 2 /ethanol. This process allows one to obtain new “core-shell structures”, Ni/Cu and SmCo 5 /Cu. New interesting properties are expected for these structures, more particularly in the magnetic recording field. Introduction Today’s chemical vapor decomposition (CVD) pro- cesses and their derivatives are well-known to deposit a thin metallic layer on plane surfaces. 1-3 The nature and quality of these layers are now under control. Concerning particles coating, numerous papers present results about the encapsulation by a polymer. In such processes, the polymer is dissolved in a supercritical fluid containing the solid particles to be coated. A rapid expansion of the solution causes the precipitation of the polymer on the solid particles. 4,5 However, there are few examples quoted in the literature concerning the par- ticle coating with metal or oxide. 6 The development of processes which allow one to deposit a metallic layer is very interesting because new properties can be obtained in such a core-shell struc- ture. For instance, the deposit of a thin metallic layer permits one to protect the particles against external corrosion and may favor the use of an ultrafine powder in metallurgical and ceramic processing or in electronic applications. In the magnetic recording field, there has been an ongoing effort to improve the information- storage capacity. 7,8 Because the elementary particles need to be physically small and magnetically indepen- dent, the powder coating process may be a solution to obtain isolated magnetic particles. Supercritical fluids have been used for film deposition applications. 9 This study presents a new process of fine particles coating via thermal decomposition of a metallic precursor dissolved in a supercritical fluid. 10 Precursor solubility in supercritical fluids is higher than that in gases. Thus, in comparison with the CVD process, the mass transport is enhanced in the supercritical media. Moreover, the tunable properties of supercritical fluids with P and T allow a better control of the process. We have shown in a previous study 11 that, under the conditions T ) 473 K, P ) 19 MPa, and CO 2 /ethanol in molar proportion 80/20, the decomposition of bis- (hexafluoroacetylacetonate)copper(II) [Cu(hfa) 2 ] directly leads to pure copper particles which are spherical in shape. Both their size and their size distribution depend principally on the initial precursor concentration. The aims of this work are, first, to demonstrate the capability of this new process in coating micron-sized particles with complex shape and, second, to compare the magnetic properties of the uncoated and of the coated materials. Two examples will be described. The first one concerns a deposit of a copper layer on Ni particles. Some properties of the Ni/Cu system will be studied because new properties are expected as de- scribed by Thaler et al. 12 The second example concerns a deposit of a pure copper layer on SmCo 5 particles. Experimental Details The experimental setup is shown in Figure 1. The high-pressure reactor (10) is made of 316 stainless steel (diameter of the cylinder ) 1.8 cm, volume ) 20 cm 3 ). An external heating resistor allows one to reach the precursor decomposition temperature. This resistor is placed at the top of the cell in order to generate an important natural convective movement. It permits one to avoid the sedimentation of the particles in suspen- sion. A poly(tetrafluoroethylene) film (PTFE; thickness ) 0.25 mm) is rolled up inside the reactor to avoid contamination of the cell’s wall. The system itself is placed in a hot-air oven. Two thermocouples measure the temperature, one inside the hot-air oven and the other inside the reactor. The high-pressure generator is a fluid metering pump (CM 3200 P/F) which permits one to introduce the initial quantity of CO 2 . A known amount of the metallic precursor Cu(hfa) 2 is placed inside the reactor, together with the Ni or SmCo 5 particles. Ethanol is added as the cosolvent in molar proportion CO 2 /ethanol 80/20, because both the solubility of the precursor is enhanced and the precursor decomposition temperature is about 30 Κ lower than that in pure CO 2 (Bocquet et al. 13 ). The hot-air oven is heated at a temperature T A ) 403 K, being also the temperature of the cell’s bottom T A . At the same time, the precursor is solubilized in the supercritical mixture CO 2 /ethanol (see point 2 in Figure * To whom correspondence should be addressed. E-mail: cansell@chimsol.icmcb.u-bordeaux.fr. † Present address: Departamento de Quimica Organica y Quimica Fisica (Area Quimica Fisica), Facultad de Ciencias, Universidad de Zaragoza, Zaragoza, Spain. E-mail: rosa@ posta.unizar.es. 4714 Ind. Eng. Chem. Res. 2000, 39, 4714-4719 10.1021/ie000155f CCC: $19.00 © 2000 American Chemical Society Published on Web 11/03/2000