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Powder metallurgy deals with heating a compressed powder to a temperature less than its melting point (sintering). A wide range of economically significant products, such as the many metal parts of modern engineering systems, are presently developed by this method.
The particle size distribution (PSD) affects vital characteristics of the resulting metallurgical compositions, producing accurate measurements of PSD essential for the production of materials produced by powder metallurgy. This is specifically true for nanopowders that comprise of complex and/or broad PSDs that cannot be readily measured using standard electron microscopy or dynamic light scattering techniques.
This article briefly explores the history of powder metallurgy and then examines the significance of powder quality to the production of nanomaterials. Using a case study from the Vecchio lab at the University of California, San Diego, this article will highlight the need for accurate particle sizing in the production of nanoparticles by spark erosion. Data from MANTA Instruments’ ViewSizer® 3000 (Figure 1) specify that particle quality and process control can largely depend on capacitance charge and that the choice of liquid dielectric has a major impact on the resulting size distribution.
Figure 1. The ViewSizer® 3000.
The manufacture of materials from metal powders (powder metallurgy) has ancient origins. The early Incans and Egyptians made use of this technology in order to produce small metal objects. By 300 A.D., much bigger objects were being manufactured, particularly in India, but it was not until the early 19th century that powder metallurgy experienced a major advancement.
It was soon realized that the production of materials from powders was not limited by the distribution of solid and liquid phases that limit alloys made by simple melting. Refinements in the blending of powders with additives resulted in a diversification of vital industrial metal products in the early 20th century, such as the tungsten filament for incandescent lamps, remarkably strong tungsten carbide and porous (self-lubricating) bearings. By the latter half of the 20th century, powder metallurgy had become strongly established as the means of economically producing parts, mainly in the aerospace and automotive industries.
The early 21st century has seen extraordinary developments in the application of powder metallurgy based on nanomaterials. Products made up of nanoscale particles can have unique magnetic and electrical properties, as well as greater strength with minimal loss in ductility. An example refers to the development of orthopedic weight-bearing implants.
A significant aspect of powder metallurgy refers to the production of the powders themselves since the qualities of the starting powder frequently dictate electrical, optical and mechanical properties of the final material. Metal powders have been historically produced by mechanical attrition, such as ball milling, or by physical methods, such as gas condensation or atomization.
Increasingly refined techniques were introduced, such as chemical vapor deposition and plasma processes, toward the end of the 20th century. A current focus on the production of nanoscale powders by electrical arc discharge highlights the significance of PSD and stresses on the need for accurate particle sizing.
Advanced Powder Metallurgy Case Study
The Nanoengineering Materials Research Center (NEMRC) is a University of California San Diego-based research group under the leadership of Dr. Kenneth Vecchio. Research interests at NEMRC include the processing and function of high-performance nanostructured steels, bulk amorphous alloys, entropy-stabilized carbides, ceramics and various other composites. One focus at NEMRC is on comprehending and controlling the microstructural properties and development of these materials for fabrication of progressive structural applications.
Compression of powders via high-temperature hot press or spark plasma sintering, among other techniques, is employed for synthesizing bulk samples. Innovative powder production techniques, such as spark erosion, are generally used for producing unique nanopowders that sinter to bulk materials with improved mechanical properties.
Spark erosion, developed by Ami Berkowitz et al., is considered to be a greatly versatile technique and has been used for producing powders ranging in size from a few nm to >100 µm and has been applied to materials including semi-conductors, metals and ceramics.[1-5] The process involves the breakdown of bulk samples by electric discharge in a dielectric fluid that is capable of producing a high-temperature arc, as shown in Figure 2a. Erosion through an electric discharge is also the basis of a popular machining technique known as electric discharge machining (EDM).
Figure 2. (a) Schematic diagram of the spark erosion process; (b) SEM and (c) TEM of spark-eroded powders.
During the erosion process, the arc touches temperatures as high as ~104 K and pressures as high as 280 MPa, which melts and then vaporizes the sample material.[1-3,6,7] Both the molten droplets and the small droplets of condensed vapor are then quenched in situ.[3,8] These two separate mechanisms of particle formation result in a bimodal powder distribution: the quenched molten droplets form bigger particles and the condensed vapors form nanoscale particles (Figure 2b,c).
The spark erosion rate and the resulting PSD are largely dependent on both the choice of dielectric fluid and the energy parameters. Higher energy sparks (>100 mJ) have been demonstrated to produce ~1-5% by mass of nanostructured powder, whereas lower energy sparks (~9 mJ) can produce up to >60% nanostructured particles. Much of the earlier research in spark erosion has focused on the minimization of the occurrence of the larger particle mode in order to produce nanostructured powders.
However, a bimodal grain distribution can be desirable when fabricating bulk metallic structures. The sintering of mixtures of powders with bimodal grain size distributions results in final samples which are capable of showing a drastic increase in strength because of Hall-Petch strengthening in the finer-grained regions, but can still maintain ductility due to the work-hardening ability of the coarser-grained regions.
The Importance of Particle Size Distribution Measurements
There is immense interest in understanding the effect of capacitance charge and dielectric fluid on PSD. For instance, by monitoring the voltage discharges happening during the sparking process it has been discovered that a 100 µF nitrogen (N2) dielectric spark has the same energy as a 120µF ethanol dielectric spark. Understanding these effects enables the process to be better tuned in order to produce perfect PSDs for high density, enhanced ductility materials and high strength.
In spite of spark erosion’s potential to produce nanopowders, it has been challenging to exactly quantify the distribution of multiple particle sizes developed by this technique. Systems made up of heterogeneous and asymmetric particles present distinctive analytical challenges. Earlier work with SEM, TEM and STEM has highlighted the extensive range of particle sizes possible, but an inability to efficiently count sufficiently huge numbers of particles rapidly prevents these techniques from offering a comprehensive representation of the PSD.
Results developed using dynamic light scattering have been mostly questionable and unreliable because of the polydispersity of the size distribution. Thus, the multispectral particle analysis technique of MANTA Instruments’ ViewSizer® 3000 was employed in order to better understand and quantify PSD of the nanopowders.
The ViewSizer® 3000 characterizes nanoparticles by examining their Brownian motion and analyzes bigger micron-sized particles by tracking their settling motion (driven by gravity). The system leverages novel illumination and detection techniques that allow video recording of scattered light from a wide range of sizes of separate particles simultaneously. Figure 3 shows a schematic diagram of the ViewSizer® 3000 optical system.
Figure 3. Schematic diagram of the ViewSizer ® 3000 optical system.
A main advancement of this system refers to its potential to address the extremely large dynamic range of scattered light intensity from differently sized nanoparticles coexisting in polydisperse colloids. In other light scattering methods, the extremely intense scattered light from even only a few larger particles overwhelms standard detection systems and obscures the analysis of smaller particles in the sample. MANTA Instruments’ ViewSizer®3000 overcomes these boundaries and is capable of quantifying an extensive range of particles sizes from 10 nm to 15 µm simultaneously.
The only inputs required for the experiments using the ViewSizer® 3000 are temperature, which was controlled at 22 °C in this case and liquid viscosity; the instrument is capable of automatically imputing the value for water. The instrument recorded 25 seven-second-long videos of particle motion for each test. The sample was stirred between each video to guarantee that a fresh aliquot of sample is used for each video.
Powders were produced from spark erosion performed on 316L ingots that produced a 316L nanopowder. All powders were cleansed and purified in the same manner.
In total, eight samples were prepared. Spark erosion capacitance settings of 60 µF, 80 µF, 100 µF and 120 µF were employed for nanopowders produced in both ethanol and N2 liquid dielectrics. Powders were suspended in Xzero Type1 reference water in order to limit contaminants.
Nanoparticles were the key focus of this analysis and hence, after preparation of the suspensions, bigger particles were allowed to settle before sampling from the upper portion of the suspension to guarantee that the majority of the particles in the test suspension would be under 1 µm.
For both ethanol and N2liquid dielectrics, decreasing capacitance clearly resulted in a reduction in the average particle size as well as a “smoothing” of the distribution. Lowering the capacitance also brought about a lower variability in the right shoulder of the distribution, as depicted in Figure 4. Similarly, a narrowing of the particle size distributions with lowered capacitance proposes that the sparking mechanism became more refined as the sparks shrank in size and energy. Thus, particle quality and process control were determined to be heavily dependent on the chosen capacitance.
Additionally, size distributions produced from same energy but varied dielectric conditions show the dielectric has a major impact on the size distribution produced. The average particle size produced in liquid N2 was 80 nm bigger and the size distribution considerably broader than that in ethanol (Figure 5). This effect is attributed to a blend of the quenching and dielectric breakdown properties of either liquid.
The optimization of high-performance nanomaterials produced by powder metallurgy needs nanopowders produced with particular particle sizes. Spark erosion produces nano-powders with broad PSDs that cannot be quantified in a reliable manner using standard sizing techniques. This has indeed hindered optimization of the spark erosion process to refine the production of nanopowders.
Figure 4. Size distributions of spark erosion particles produced using liquid nitrogen dielectric and under varying capacitance levels.
Figure 5. Results of particle size distributions extracted from same-energy sparks using different dielectric liquids.
The ViewSizer® 3000 from MANTA Instruments readily characterizes nanoparticles of a wide range of sizes simultaneously, enhancing the understanding of the effect of dielectric fluid and capacitance charge on the PSD produced. Data from the ViewSizer®3000 specify that i) the liquid dielectric had a major impact on the PSD produced, with larger average particle sizes produced in liquid N2 than in ethanol, and ii) for a given liquid dielectric, decreased capacitance leads to a reduction in the average particle size. These results highlight the significance of accurate measurements of PSD in the production of nanopowders for powder metallurgy and demonstrate that the ViewSizer® 3000 is perfect for routine use in Quality Control laboratories or R&D.
 Walter, J.L., 1987. Fine Powders by Spark Erosion.JOM Journal of the Minerals, Metals and Materials Society, 39(8), pp.60-60.
 Nguyen, P.K., Lee, K.H., Moon, J., Kim, S.I., Ahn, K.A., Chen, L.H., Lee, S.M., Chen, R.K., Jin, S. and Berkowitz, A.E., 2012. Spark erosion: a high production rate method for producing Bi0.5Sb1.5Te3 nanoparticles with enhanced thermoelectric performance. Nanotechnology, 23(41), p.415604.
 Berkowitz, A.E., Hansen, M.F., Parker, F.T., Vecchio, K.S., Spada, F.E., Lavernia, E.J. and Rodriguez, R., 2003. Amorphous soft magnetic particles produced by spark erosion. Journal of Magnetism and Magnetic Materials, 254, pp.1-6.
 Berkowitz, A.E., Harper, H., Smith, D.J., Hu, H., Jiang, Q., Solomon, V.C. and Radousky, H.B., 2004. Hollow metallic microspheres produced by spark erosion. Applied physics letters, 85(6), pp.940-942.
 Hsu, M.S., Meyers, M.A. and Berkowitz, A., 1995. Synthesis of nanocrystalline titanium carbide by spark erosion.Scriptametallurgicaetmaterialia, 32(6), pp.805-808.
 Albinski, K., Musiol, K., Miernikiewicz, A., Labuz, S. and Malota, M., 1996. The temperature of a plasma used in electrical discharge machining. Plasma Sources Science and Technology, 5(4), p.736.
 H. Tsuchiya, T. Inoue, Y. Mori, in: J.R. Crookall (Ed.), Proceedings of the Seventh International Conference on Electromachining, North-Holland, Amsterdam, 1983, p. 107.
 R.W. Siegel, G.E. Fougere,.G.C. Hadjipanayis, R.W. Siegel (Eds.), Nanophase Materials, Kluwer Academic Publishers, Dordrecht, 1994, p. 233.
 International Organization for Standardization, 2016. Particle size analysis – Particle tracking analysis (PTA) method, ISO 19430, Geneva, Switzerland.
This information has been sourced, reviewed and adapted from materials provided by MANTA Instruments.
For more information on this source, please visit MANTA Instruments.