Characteristics of Fe-Cr-Al Alloy Nanopowders Prepared by Electrical Wire

Explosion Process under Liquid Media

Jung-Yeul Yun1, Hye-Moon Lee1;*, Si-Young Choi1, SangSun Yang1,Dong-Won Lee1, Young-Jin Kim1 and Byoung-Kee Kim2

1Functional Materials Division, Korea Institute of Materials Science, 531 Changwonadaero, Changwon, Gyeongnam 641-831, Korea2School of Materials Science and Engineering, University of Ulsan, Ulsan 680-749, Korea

Fe-Cr-Al alloy nanopowders were successfully prepared by electrical wire explosion method in ethyl alcohol media. The formation of Fe-Cr-Al alloy nanopowder was monitored by X-ray diffraction. The alloy powders have spherical shape and nanometer size. The alloy powdersprepared by the electrical wire explosion method have main crystal structures of bcc Fe-Cr alloy. The geometric mean diameter and geometricstandard deviation of an alloy powder formed from a wire with diameter of 0.1mmwere 10.2 nm and 1.62, respectively. However, the geometricmean diameter and geometric standard deviation of an alloy powder formed from a wire with diameter of 0.2mm were 15.4 nm and 1.66,respectively. These values indicate that the particle size of the alloy powders prepared by the EWEmethod was increased with wire diameter. Fe,Cr, and Al components are uniformly dispersed inside the powder. [doi:10.2320/matertrans.M2010318]

(Received September 17, 2010; Accepted November 12, 2010; Published December 29, 2010)

Keywords: electrical wire explosion, iron-chromium-aluminum, nano, alloy powder, size distribution

1. Introduction

Metal nanopowders exhibit unique physical and chemicalproperties that differ considerably from those of the bulksolid state.1–4) Therefore, metal nanopowders have beenintensively investigated in various high performance appli-cations, such as catalysts, ferrofluids, and microwavedevices.5–7) Various techniques have been developed toprepare metal nanopowders, such as gas-phase chemicalreaction,8) spray pyrolysis,9) water-heating reaction,10) laserablation,11) flame processing,12) vapor deposition,13) micro-wave plasma synthesis,14) and the sol-gel method.15)

Materials often become the critical limitation in techno-logical advances. Therefore, the need for new materials thathave better characteristics is increasing; alloying is one of thepromising techniques for the development of new materials.The conventional preparation processes of ultra-fine metalpowders have drawbacks in the preparation of alloy powderswith complex composition. The electrical wire explosion(EWE) method is widely used in various applicationsbecause this technique has a number of basic advantagesfor the production of ultra-fine metal and alloy powders.Ultra-fine powders of various pure metals have beenproduced by the EWE method.16–22) Nevertheless, in thecase of alloys and intermetallic alloys, the EWE process hasobvious limitations in that the raw materials should be in theform of thin wires with a diameter of less than 1mm. Becauseof this difficulty, the EWE method has been applied to onlya limited number of alloys.23–25)

Alloy materials with porous structures have been widelystudied in the environmental industry for high temperatureapplications. Ni-Cr alloyed steels protected by a chromiumoxide (Cr2O3) are used in the environmental industry.26,27)

The protective chromium oxide is formed on the metalsurface through a reaction with oxygen present in the

environment. The chromium oxide is a good protector ofthe metal at lower temperatures, but it is less adherent athigher temperatures. At high temperatures, this lack ofadhesion will cause the oxide to spall off, leaving anunprotected surface subject to further oxidation. This processleads to rapid oxidation. Deformation of the material can alsooccur. The Fe-Cr-Al alloys, which are protected by alumi-num oxide (Al2O3), have outstanding stability compared toother iron-based alloys at high temperatures and have beenused in the fabrication of gas burners, industrial heaters, andother high-temperature devices.28,29) However, the character-istics of fine size Fe-Cr-Al alloy powders have not been wellstudied due to the difficulty in preparation of powders withphase purity. Fine size Fe-Cr-Al alloy powders could bewidely used in the environmental industry at high temper-atures.

In this study, Fe-Cr-Al nanopowders were prepared by theEWE method from wires with various diameters. The effectsof the diameter of the wire on the particle size and sizedistribution of Fe-Cr-Al nanopowders were investigated.

2. Experimental

Commercial Fe-Cr-Al wire with a composition of 72.2Fe-22Cr-5.8Al was used as the source material for the EWEmethod. Fe-Cr-Al wires with diameters of 0.1 and 0.2mmwere exploded and subsequently condensed in anhydrousethanol (Sigma-Aldrich Inc., Milwaukee, USA) in orderto prevent oxidation during the EWE process. In generala nanopowder of a smaller particle size could be obtainedby the EWE method, which was performed in the liquidphase.30) The schematic diagram of the EWE systemdesigned for liquid media explosion is shown in Fig. 1. Theformation mechanism of the metal nanopowders in the EWEoperated in liquid media has been well described in theliterature.31) A metal wire was installed between the high-voltage electrodes, which were connected to a capacitor bank*Corresponding author, E-mail: hyelee@kims.re.kr

Materials Transactions, Vol. 52, No. 2 (2011) pp. 250 to 253#2011 The Japan Institute of Metals RAPID PUBLICATION

of 60 mF through a triggered spark gap switch. The wireswere fed by a feeding roller installed on the exploding vesseland were guided by a nozzle into the vessel. A voltage of2.0 kV was applied to the metal wire. The metal wire wasoverheated and evaporated swiftly when a very strong pulsecurrent with current density 2:9� 108 Acm�2 passed throughthe wire, resulting in an explosion. The expansion durationcould be as short as several tens of microseconds.24,32)

The crystal structure analyses of the wire and nanopowdersof Fe-Cr-Al alloys made by the EWEmethod were performedusing an X-ray diffractometer (XRD; D-Max 2200,RIGAKU, JPN). The morphologies and compositions of theFe-Cr-Al alloy nanopowders were observed by transmissionelectron microscopy and energy dispersive spectroscopy(TEM; JEM-2100F, JEOL, JPN) and by scanning electronmicroscopy (SEM; JSM-6700F, JEOL, JPN).

3. Results and Discussion

Figures 2(a) and (b) show the X-ray diffraction patterns ofthe nano-sized Fe-Cr-Al alloy powders formed by the EWE

method under ethyl alcohol media and the Fe-Cr-Al alloywire, respectively. The diffraction patterns of the Fe-Cr-Alalloy wire are the same as those identified in the bcc Fe-Cralloy structure. The nano-sized Fe-Cr-Al alloy powdersformed through the EWE method had the same diffractionpatterns as those of the Fe-Cr-Al alloy wire. However, smallimpurity peaks were observed due to the wire explosion,which was conducted in liquid media. Although the impu-rities were not clearly identified due to the extremely smallpeak intensities, we found that the impurity peaks were inaccordance with some of Fe5C2 and Al2O3 main peaks.

Figure 3 shows the morphologies of the Fe-Cr-Al alloypowders prepared by the EWE method from wires withdiameters of 0.1 and 0.2mm. The powder had a sphericalshape and nanometer size irrespective of the diameters of thewires. Generally, the wires were overheated and evaporatedswiftly when a very strong pulse current with high currentdensity passed through them, resulting in explosions thatproduced a shock wave, forming metal powders. Unlesscomplete evaporation of the metal wire occurs due to notenough of a supply of energy, the wire was split into the

Fig. 1 Schematic diagram of the EWE system designed for liquid media

explosion.Fig. 2 X-ray diffraction patterns of (a) the nano-sized Fe-Cr-Al alloy

powders formed by the EWEmethod under ethyl alcohol media and (b) the

Fe-Cr-Al alloy wire.

Fig. 3 TEM images of the Fe-Cr-Al alloy powders prepared by the EWE method form wires with diameters of (a) 0.1 and (b) 0.2mm.

Characteristics of Fe-Cr-Al Alloy Nanopowders Prepared by Electrical Wire Explosion Process under Liquid Media 251

metal vapors and the overheated metal droplets.16,33) Theoverheated metal droplets formed by incomplete evaporationof the wire would be changed to submicro-, micrometer sizedparticles. On the other hand, if complete evaporation of thewire occurs during the EWE process, the content of thesecoarse particles significantly decreases and the content ofnanometer sized particles due to metal vapor increases. Asshown in Fig. 3, the prepared particles are less than 100 nmin size, indicating that almost complete evaporation occurred

in this Fe-Cr-Al alloy powder preparation using the EWEprocess.

Figure 4 shows the particle size distributions of the Fe-Cr-Al alloy powder prepared with the EWE method from wireswith diameters of 0.1 and 0.2mm. The geometric meandiameter and the geometric standard deviation of the alloypowders formed from the wire with a diameter of 0.1mmwere 10.2 nm and 1.62, respectively. However, the geometricmean diameter and the geometric standard deviation of thealloy powders formed from the wire with a diameter of0.2mm were 15.4 nm and 1.66. These values indicate thatthe particle size of the alloy powders prepared by the EWEmethod increased with wire diameter. In the particlegeneration via the nucleation and growth mechanism, thequantity of metal vapor was proportional to the particle sizeand size distribution.34,35) As for the quantity of alloy vaporgenerated by the EWE method, the wire with a 0.2mmdiameter yielded about four times more than did the wirewith a 0.1mm diameter.32) Consequently, the increase ofmetal vapor due to the increase of wire diameter causedan enlargement of the alloy particle size and of the sizedistribution.

Figure 5 shows the results of dot-mapping of the nano-sized Fe-Cr-Al alloy powders prepared by the EWE methodfrom the wire with diameter of 0.2mm. Fe, Cr, and Alcomponents were shown uniformly dispersed inside thepowder. As shown in Fig. 3, the prepared particles are lessthan 100 nm. The Fe-Cr-Al wire almost completely evapo-

Fig. 4 The particle size distributions of the Fe-Cr-Al alloy powder

prepared with the EWE method from wires with diameters of 0.1 and

0.2mm. (Fraction/dln(Dp): concentration of particles included in specific

particle size range dln(Dp)).

Fig. 5 Dot-mapping images of the nano-sized Fe-Cr-Al alloy powders prepared by the EWE method from the wire with diameter of

0.2mm.

252 J.-Y. Yun et al.

rated to metal vapors by the EWE method despite thedifference of the vapor pressure of each element (Fe, Cr, Al).Metal vapors of each element in the liquid media aresufficiently close to each other and they quickly cooled downwith the same composition of wire due to the large heatcapacity of the liquid media.30)

4. Conclusions

The characteristics of Fe-Cr-Al nanopowders prepared bythe EWE method from wires with various diameters wereinvestigated. The Fe-Cr-Al nanopowders formed from thewire have spherical shapes and a main crystal structure ofbcc of Fe-Cr alloy. The particle size and size distributions ofthe alloy nanopowders prepared by the EWEmethod increasewith the wire diameter; Fe, Cr, and Al components in thealloy nanopowders are uniformly dispersed.

Acknowledgement

This research was supported by a grant from theFundamental R&D Program for Core Technology of Materi-als funded by the Ministry of Knowledge Economy, Republicof Korea.

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