Synthesis of FeCu Nanopowder by Levitational Gas Condensation Process
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- Sivaprahasam, D., Sriramamurthy, A.M., Vijayakumar, M. et al. Metall and Materi Trans B (2010) 41: 841. doi:10.1007/s11663-010-9370-8
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Condensation from the vapor state is an important technique for the preparation of nanopowders. Levitational gas condensation is one such technique that has a unique ability of attaining steady state. Here, we present the results of applying this technique to an iron-copper alloy (96Fe-4Cu). A qualitative model of the process is proposed to understand the process and the characteristics of resultant powder. A phase diagram of the alloy system in the liquid–vapor region was calculated to help understand the course of condensation, especially partitioning and coring during processing. The phase diagram could not explain coring in view of the simultaneous occurrence of solidification and the fast homogenization through diffusion in the nanoparticles; however, it could predict the very low levels of copper observed in the levitated drop. The enrichment of copper observed near the surface of the powder was considered to be a manifestation of the lower surface energy of copper compared with that of iron. Heat transfer calculations indicated that most condensed particles can undergo solidification even when they are still in the proximity of the levitated drop. It helped us to predict the temperature and the cooling rate of the powder particles as they move away from the levitated drop. The particles formed by the process seem to be single domain, single crystals that are magnetic in nature. They, thus, can agglomerate by forming a chain-like structure, which manifests as a three-dimensional network enclosing a large unoccupied space, as noticed in scanning electron microscopy and transmission electron microscopy studies. This also explains the observed low packing density of the nanopowders.
During the last two decades, more attention has been paid to nanomaterials because of their unusual properties[1–5] when compared with conventional polycrystalline materials. In the area of powder metallurgy, nanostructured,[6,7] nanograined, and nanosize particulate[9–11] materials are being paid great attention. Both chemical and physical techniques exist for the preparation of nanopowders. In the preparation by physical methods, vapor condensation is an important technique that can produce strain-free particles of very high purity. These powders are characterized by very clean reactive surfaces. Consequently, handling such powders requires great care, and in general, nanopowders are consolidated in situ into bulk samples.[12,13] In most of these techniques based on vapor condensation, a fixed quantity of materials is melted and evaporated in an inert gas atmosphere and condensed on walls cooled by circulating very cold fluids like liquid nitrogen. Such techniques are well suited for producing elemental powders. However, for alloy powders, the composition of the melt will be changing continuously during the process as a result of the preferential evaporation of relatively more volatile components. There is no continuous addition of alloy to the evaporating liquid pool that could result in a steady-state process. In levitated drop gas condensation (LGC), material of a chosen composition is fed continuously into the levitated liquid pool. This leads to a steady-state process, producing powder of the same composition as the alloy being fed. Furthermore, because the molten alloy in this process is not in contact with any other material, the powder produced is completely free from any contamination, and the composition is expected to be uniform throughout the process period, unlike other physical vapor processes. Unfortunately, little literature[14–17] has been published on this technique, and whatever work was done covers only elemental and oxide nanopowders. This article covers the application of this technique to prepare Fe-Cu nanopowder and its characterization. A model has been proposed to aid in understanding the results qualitatively. We have chosen Cu as an alloying element because the literature suggests that it catalyzes the reduction of iron oxide that forms during passivation—a necessary step before the removal of the powder from the apparatus. We also have studied elemental iron to serve as reference.
FeCu Nanopowder Synthesis
Experimental Parameter Measured During the Synthesizing of FeCu Nanopowder in LGC
Rate of wire feeding (g/hr)
Argon flow rate (m3/hr)
Inductor coil power (KW)
Temperature of the molten drop (K (°C))
Argon gas velocity in QT (m/s)
The particle size of the FeCu nanopowder was characterized by (1) scanning electron microscopy (SEM) equipped with a high-resolution field emission gun (FEG-SEM-Hitachi S4300 SE/N, Hitachi High Technology America, Pleasanton, CA), (2) TEM and (3) the Brunauer, Emmett, and Teller (BET) method in a Micromeritics ASAP 2020 system (Micromeritics Instrument Corporation, Norcross, GA) using a 373 K (100 °C) degassing temperature. The particle size distribution was measured (on a sample of about 800 particles) on 100 KX SEM micrographs using an image analyzer (Image Pro Plus; Media Cybernetics, Silver Spring, MD) software. The crystallite sizes in the nanopowder, phases such as the passivating oxides over the nanopowder and the amount of various phases were characterized by X-ray diffraction (XRD) in a Bruker’s diffractometer (AXS Model Number D8 Advance System, Bruker AXS GmbH, Karlsruhe, Germany) using Cu Kα radiation (40 kV and 40 mA). The XRD scan was carried out over the 2θ from 20 deg to 120 deg at 0.005 deg/s−1 scan rate. The crystallite size of the nanopowder was determined by the Williamson–Hall method, and the amount of oxide was quantified based on the peak height method. The structure, morphology, passivating oxide layer thickness, and chemical composition of individual nanopowder were investigated by TEM. The samples for TEM were prepared by mixing FeCu nanopowder with gatan two component epoxy glue and casting the mixture on to a thin sheet over aluminum foil followed by curing at 353 K (80 °C) on a hot plate. The glue sheet then was thinned down to 100 μm by mechanical polishing followed by cutting a disk of 3-mm diameter. The disk again was polished down to 40 μm mechanically, which was polished further into a dimple to about 10 μm. This was thinned further by ion beam milling with an argon ion source until perforation. TEM investigations were carried out at an excitation voltage of 200 kV using an FEI TECNAI G2 (FEI Worldwide, North America Nanoport, Hillsboro, OR) transmission electron microscope equipped with energy-dispersive X-ray spectrometry (EDS) for compositional analysis. Electron diffraction of a few individual FeCu nanoparticles was carried out to investigate the crystal structure. Composition analysis of several individual nanoparticles was carried out at random locations. The nanopowder also was examined in TEM by dispersing the as-prepared powder in ethanol and spreading one or two drops on a copper TEM sample grid to study the structure of agglomerates. X-ray photoelectron spectroscopy (XPS) (Omicron system, Omicron Nanotechnology GmbH, Taunusstein, Germany) equipped with an Ar+ sputtering gun was used to investigate the composition of the nanopowder surface and the depth profiling of the Cu concentration. For this purpose, the nanopowder was compacted into a thin disc (10-mm diameter and 0.5-mm thickness) with a smooth surface and then analyzed using monochromatized Al Kα source. The depth profiling was carried out by alternating ion (Ar+) sputtering and XPS analysis, and the Ar+ beam was rastered over an area of 3 × 3 mm2 and was run at a voltage of 3 KV. The surface composition was determined from the characteristic Fe and Cu peaks after different durations of sputtering.
Physical Characteristics and Structure FeCu Nanopowders
Physical Characteristic of the FeCu and Fe Nanopowders
BET surface area, m2/g
Mean particle size, nm
Crystallite size, nm
Packing density, g/cm3
Fe 2p and Cu 2p Binding Energy Peaks of Various Standard Samples Along with FeCu Nanopowder
Calculation of the composition of various constituents present, based on the XRD peak heights after background subtraction, showed that the overall oxide content on FeCu nanopowders is 7.1 pct. The lattice parameter “a” measured from Fe d and d planes of FeCu nanopowders was 2.868A deg and 2.869A deg, respectively. These values are higher than the ones for Fe nanopowders (d = 2.864A deg and d = 2.866A deg) synthesized from same raw material under identical conditions. Because the atomic radius of Cu (1.278A deg) is higher than that of Fe (1.241A deg), it is expected that the lattice parameter of Fe will increase with Cu dissolution. This is evident in a study by Kneller on an Fe-Cu solid solution prepared by the simultaneous vapor deposition of Fe and Cu on a cold substrate; this study reports a significant increase in the aFe with a Cu concentration particularly below 15 wt pct. In this present study, the aFeCu of the FeCu nanopowder is comparable with the theoretically calculated aFeCu based on the atomic radii of Fe and Cu.
Composition of the Liquid FeCu Drop Levitated for Different Times
Cu (wt pct)
4 ± 0.05
2.76 ± 0.05
2.52 ± 0.05
0.2 ± 0.002
The state of the nucleated nanoparticles within the vapor-bearing zone mainly depends on their temperature. An effort has been made to estimate the temperature of the condensed droplets/particles based on heat transfer considerations. The details of the calculation are described in Appendix A. The nanodroplets or particles of 18-nm radii (mode of particle size distribution) was taken for this calculation. For the purpose of comparison, a calculation also was done for smaller and larger particles. It turned out that the lowest estimated temperature of the nanodroplet or particle separated by 10 micrometers from the levitated drop under equilibrium conditions (i.e., dT/dt = 0) is around 1470 K (1197 °C), and at 0.9 mm—the end vapor-bearing zone—it is 1293 K (1020 °C). This suggests that most of the condensed particles in the vapor-bearing zone of 0.9 mm around the levitated drop are in solid state. The time required for the gas or particles to travel around the levitated drop and leave the latter is around 3 ms. During this period, the particles can grow further by the intake of vapor until they exit from the proximity of the levitated drop. The particles also can grow by coalescence with other particles that impinge on them at this temperature. Because the droplets and particles are extremely small, they will be in a state of Brownian motion and will collide and coalesce with each other while being carried by the flowing argon toward the exit side. The rate of cooling estimated through heat transfer calculations (Appendix A) is very high, of the order of 105 K/sec, when droplets are very close to levitated drop, and it falls as the distance from it increases.
Particle size distribution shows that it follows log-normal distribution. However, unlike other vapor-condensation processes, the levitated drop technique shows a much broader distribution, and particularly, particles with larger diameters are significant. This could be a result of a much longer time available for the growth of the droplet nucleated at the entry side of the levitated drop, as stated earlier. Mixing helium with argon or using helium gas alone narrows down the particle size distribution, as reported by Jigatch et al. in Al powder; however, this is suitable only for low-melting-temperature metals and alloys. In the present study on the FeCu system, mixing of even 5 vol pct of He with Ar reduced the levitated drop temperature drastically and resulted in an insignificant nanopowder yield.
An interesting issue of fundamental importance is the formation of nanoparticle aggregates in a chain-like configuration. Such behavior was observed in all types of material, including metals and oxides.[28–30] Tang et al. in their article on the one-dimensional assembly of nanoparticles indicated several driving forces that can lead to template-free self-assembly forming chain-like agglomeration. Out of these, magnetic dipole moments and crystallographically oriented agglomeration are relevant to LGC. As stated earlier, the individual particles produced are single crystals. Because their size is smaller than the magnetic domain size of bulk iron, they are single domain, single crystal magnetic particles. Therefore, they can assemble along their magnetic axis. Because the magnetic vectors of the individual particles are oriented along certain specific crystallographic direction, the chains formed in the magnetic materials also may be referred to as crystallographically oriented agglomerates. The merit of magnetic dipole driven agglomeration is that each of the particles is associated with a magnetic field, which can exert a physical force on other similar particles that are present in, or enter, its magnetic field. When the particles come together, they align their magnetic vector before impingement. An attempt to prove the alignment of magnetic axis in the chain using TEM were unsuccessful because of beam drift. Because of the existence of physical forces in the agglomerate formation of magnetic particles, the growth rate of agglomerates is expected to be much faster than, crystallographically oriented growth alone. Because only the magnetic vector between adjacent particles is aligned, the grain boundary or interface has 1 degree of freedom (i.e., rotation about the magnetic vector), which remains unfixed. Therefore, they are twist boundaries, which are likely to have lower energy than general grain boundaries. This situation should result in a large dihedral angle between adjacent particles. The development of a thick neck between adjacent particles, as shown in Figure 7, supports this view.
We believe that the agglomeration will occur in the blank zone. Heat transfer calculation (Appendix A) performed for a particle with a 36-nm diameter indicates that the particles cool to Curie temperature just 2 mm below the levitated drop in the blank zone. Because the particles forming the chain are generally around 30- to 40-nm diameter, they must cool to Curie temperature much sooner before reaching the end of the blank zone.
A nanopowder of 96Fe:4Cu was made by the levitational gas condensation method. The resultant powder was studied for physical, microstructural, and chemical characteristics. An attempt was made to understand the characteristics through a liquid–vapor phase diagram calculated using the available thermodynamic data, process modeling through heat transfer analysis, and magnetic nature of the powder.
The composition of the levitated drop giving out a vapor of 4-pct Cu in steady state matched well with the prediction of the calculated phase diagram. Though a high level of Cu at the surface of the powder is to be expected from the phase diagram, it takes a negligibly short time for complete homogenization because of the small size and prevalent high temperature. It was argued that the low surface energy of Cu is the major driving force that could stabilize Cu at the surface of the powder.
Modeling of the process involving heat transfer analysis, which indicated that most droplets can undergo solidification while they are still in the proximity of the levitated drop. During the period of residency close to the levitated drop, they can grow by intake of vapor. Once the particles start moving away from the levitated drop, they cool rapidly. Although agglomeration can initiate above the curie temperature, it proceeds rapidly below the curie temperature under the influence of the magnetic field of the particles. The chain-like structure formed, thus, occasionally can develop branching, leading to the formation of a reticulated three-dimensional sponge like structure that is responsible for the observed very low packing density of the resultant powder.
The authors acknowledge Dr. Sandeep Bysakh for his help with TEM investigation and Dr. Sankara Subramanian, DMRL, Hyderabad, for his help with developing a code for the numerical solution of the heat transfer problem.