4714
Ind. Eng. Chem. Res. 2000, 39, 4714-4719
Core-Shell Materials Elaboration in Supercritical Mixture CO2/ 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 SmCo5. In both cases the copper source is bis(hexafluoroacetylacetonate)copper(II), which is thermally decomposed in the supercritical mixture CO2/ethanol. This process allows one to obtain new “core-shell structures”, Ni/Cu and SmCo5/Cu. New interesting properties are expected for these structures, more particularly in the magnetic recording field. Introduction Today’s chemical vapor decomposition (CVD) processes 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 particle 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 structure. 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 informationstorage capacity.7,8 Because the elementary particles need to be physically small and magnetically independent, 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 study11 that, under the conditions T ) 473 K, P ) 19 MPa, and CO2/ethanol in * To whom correspondence should be addressed. E-mail:
[email protected]. † 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.
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 described by Thaler et al.12 The second example concerns a deposit of a pure copper layer on SmCo5 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 cm3). 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 suspension. 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 CO2. A known amount of the metallic precursor Cu(hfa)2 is placed inside the reactor, together with the Ni or SmCo5 particles. Ethanol is added as the cosolvent in molar proportion CO2/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 CO2 (Bocquet et al.13). The hot-air oven is heated at a temperature TA ) 403 K, being also the temperature of the cell’s bottom TA. At the same time, the precursor is solubilized in the supercritical mixture CO2/ethanol (see point 2 in Figure
10.1021/ie000155f CCC: $19.00 © 2000 American Chemical Society Published on Web 11/03/2000
Ind. Eng. Chem. Res., Vol. 39, No. 12, 2000 4715
Figure 1. Apparatus for the coating process: (1) CO2; (2) cooler; (3) CO2 pump; (4) ethanol pump; (5) hot-air oven; (6) pressure sensor; (7) temperature regulator; (8) contact breaker; (9) solvent trap; (10) reactor fitted with an external heater.
Figure 3. Magnetic hysteresis loop: saturation magnetization, Ms; remanent magnetization, Mr; coercivity, Bc. Figure 2. Schematic representation of the experimental pathway during a classical coating process in the CO2/ethanol mixture in molar proportion 80/20. The critical coordinates of the mixture (point C) are Tc ) 365 K and Pc ) 14.5 MPa.20
2). By means of the resistive heater, the temperature of the cell’s top is maintained constant at TB (with TB > TA and Tdecomposition (Tdec)). The temperature TB was fixed at 473 K because UV-visible studies14 revealed that Tdec of Cu(hfa)2 in CO2/ethanol in molar proportion 80/20 was equal to 468 K. The increase of temperature from ambient to TB leads to an increase of the pressure to the working pressure P, which is then maintained constant at 19 ( 0.5 MPa (see point 3 in Figure 2). In the reactor, the convective movement created by the temperature gradient (TB - TA) causes the particles to move. In the volume where T > Tdec, the precursor decomposition occurs and releases atomic copper, which coats the in-movement particles. A strong point of this process is that after the decomposition stage the organic part of the precursor is still soluble in the supercritical fluid and is easily removed when the vessel is vented and easily trapped at the outlet of the installation. After the return to ambient conditions, the coated particles are directly obtained free of solvent and organic contamination (see point 4 in Figure 2). Both Ni particles (average diameter ) 3 µm) and bis(hexafluoroacetylacetonate)copper(II) {CF3COCHdC(O)CF3]2Cu}, were provided by Sigma-Aldrich and used without further purification. The intermetallic SmCo5 was prepared by melting of stoichiometric amounts of the constituent elements under a purified argon atmosphere in an induction levitation furnace. The purity of the starting materials was as follows: Sm, 99.9%; Co,
99.99%. SmCo5 particles were obtained by grinding using a Fritsch P5 high-energy instrument. The grinding process was described previously15 (average diameter of the SmCo5 particles ) 10 µm). Our samples were characterized by conventional X-ray powder diffraction using Cu KR radiation. The particle size and morphology were investigated by scanning electron microscopy (SEM) using a JEOL 840 microscope. The granulometer used was Malvern/Mastersizer 2000s. Information on the chemical composition of deposited surface layers was obtained by Auger spectroscopy, using a VG310F apparatus. Magnetization measurements were carried out using a SQUID (superconducting quantum interference device) magnetometer. To familiarize the reader with commonly measured magnetic parameters, Figure 3 schematically illustrates a hysteresis loop (magnetization vs field). The application of a sufficiently large magnetic field causes spins within a material to align with the field. The maximum value of the magnetization achieved in this state is called the saturation magnetization, Ms. As the magnitude of the magnetic field decreases, spins cease to be aligned with the field and the total magnetization decreases. In ferromagnets, a residual magnetic moment remains at zero field. The value of the magnetization at zero field is called the remanent magnetization, Mr. The ratio of the remanent magnetization to the saturation magnetization, Mr/Ms, is called the remanence ratio and varies from 0 to 1. The coercive field Bc is the magnitude of the field that must be applied in the negative direction to bring the magnetization of the sample back to zero.
4716
Ind. Eng. Chem. Res., Vol. 39, No. 12, 2000
Figure 4. X-ray diffraction patterns of nickel particles: (i) before (JCPDS file no. 4-0850), and (ii) after the supercritical treatment, with TB ) 473 K.
Results and Discussion Ni Particles Coating. (a) Supercritical Treatment. Because our process is performed at a pressure and temperature as high as 20 MPa and 473 K, respectively, it must be checked that the supercritical treatment does not modify the initial properties of Ni particles. Thus, a known amount of nickel particles (without precursor) was placed in the reactor for 2 h. X-ray diffraction patterns (Figure 4) and SEM images (Figure 5a,b) of the untreated and treated particles do not reveal any significant changes in both the nature and the morphology of these particles. We have also verified that the magnetic properties of the nickel particles are not affected by the supercritical treatment (Figure 6a,b). It can be concluded from these result in the fact that no reaction occurs between the supercritical mixture CO2/ethanol and the Ni particles. (b) Coating Process. A total of 0.6 g of Cu(hfa)2 was added to 0.1 g of nickel particles in the mixture of CO2/ ethanol. The treatment lasted 30 min. In Figure 7, the X-ray diffraction pattern reveals the presence of both metallic copper and Cu2O. The SEM image (Figure 5c) exhibits a significant modification of the initial particles morphology. The nickel particles present a rounded shape with a rough surface. The coated particles are spherical in shape, but their surface is now smooth. We made an additional experiment in which the mass of the nickel particles was modified (m ) 0.02 g). The precursor mass was still 0.6 g, with the experimental conditions being identical. The X-ray diffraction pattern still reveals the presence of both Cu and Cu2O (Figure 7). In comparison with the first experiment, the Cu2O proportion has been increased. It could be inferred from these results that our process allows one to deposit a Cu2O/Cu layer on Ni particles. The thickness of this layer depends on the initial ratio between the amount of the precursor and Ni particles (studies of granulometry gave values from 0.2 to 1 µm). Further experiments were made in order to explain the Cu2O formation. First, spectroscopic Auger measurements were performed to verify that oxygen was not present on the initial Ni particles (Figure 8a). Second, the coating process was made at a higher temperature, TB ) 573 K (the other working conditions were unmodified). Cu2O formation was not observed according to the X-ray diffraction pattern (Figure 9).
Figure 5. SEM pictures of Ni particles: (a) initial; (b) after the supercritical treatment, with TB ) 473 K; (c) after coating, with TB ) 473 K.
Ind. Eng. Chem. Res., Vol. 39, No. 12, 2000 4717
Figure 7. X-ray diffraction patterns of coated particles at different weight ratios of precursor/nickel, with TB ) 473 K: (i) precursor/ nickel ) 6; (ii) precursor/nickel ) 30. Asterisks indicate diffraction lines corresponding to metallic copper (JCPDS file nο. 4-0836), and b indicate diffraction lines corresponding to Cu2O (JCPDS file nο. 5-0667).
Figure 6. Hysteresis loops at 15 K of Ni particles: (a) initial; (b) after the supercritical treatment, with TB ) 473 K; (c) after coating, with TB ) 473 K.
SEM observation (Figure 10) shows a significant structural modification of the copper layer in contrast with Figure 5c, which corresponds to TB ) 473 K. In fact, after the thermal treatment at 573 K, the copper layer is composed of submicroparticles (>500 nm); however, at 473 K, the copper layer is composed of nanoparticles (
