Spark plasma sintering of alumina nanopowders produced by electrical explosion of wires

An et al. SpringerPlus (2015) 4:581 DOI 10.1186/s40064-015-1383-y Open Access RESEARCH Spark plasma sintering of alumina nanopowders produced by electrical explosion of wires Vladimir An1*, Alexey Khasanov1 and Charles de Izarra2 Abstract Alumina nanopowders produced by electrical explosion of wires were sintered using the spark plasma sintering technique. The results of XRD analysis show that the main phase in the compacted nanopowders is α-Al2O3. According to the SEM observations, the sintered alumina nanopowder consists of micron-sized faceted grains and nano-sized necked grains. The increase in sintering temperature resulted in a higher density of the sintered powders: from 78.44 to 98.21 % of theoretical density. Keywords: Alumina nanopowders, Electrical explosion of wires, Spark plasma sintering Background For ceramic technologies, it is important to obtain ultrafine microstructure that ensures improved physical and physic-mechanical properties: hardness, wear resistance, mechanical and optical properties. Over the last decade, spark plasma sintering has become a powerful technique to produce high quality ceramics including nanostructured ceramics (Bordia and Olevsky 2010; Angerer et al. 2006; Suárez et al. 2013; Monnier et al. 2015; Huang and Nayak 2014). Alumina has attracted great interest and is one of the most used materials in various applications. These materials display excellent properties: high strength, good chemical durability and excellent electrical insulating properties. They can be used as translucent ceramics (Wei 2005; Mao et al. 2008), thermal insulation (Xu et al. 2015), catalysts (Nartova et al. 2015; Gündüz and Dogu 2015), biomedical implant (Deville et al. 2003) etc. In this work, spark plasma sintering (SPS) is considered as a promising fabrication way. The SPS technique has some advantages with respect to usual sintering methods: higher heating rates and local temperature gradients, *Correspondence: 1 Institute of High Technology Physics, National Research Tomsk Polytechnic University, 30 Lenin Ave., 634050 Tomsk, Russia Full list of author information is available at the end of the article particular local temperature distributions. The problem to be solved in the study is how the SPS parameters (pressure, temperature) are related to the product characteristics (size, morphology, porosity). One of the interesting methods for fabrication of alumina nanopowders is electrical explosion of wires (Yavorovskii 1996). The objective of this work was to study processes of densification during spark plasma sintering of alumina nanopowders produced by electrical explosion of wires (EEW) in the gaseous mixture of argon and oxygen at a pressure of 1.5 atm. The authors wanted also to explore polymorph transformations during spark plasma sintering of alumina nanopowders. Results and discussion This research work consisted of four main stages: fabrication of alumina nanopowders by electrical explosion of aluminum wires, characterization of as-prepared nanopowders, spark plasma sintering of alumina nanopowders and their characterization. The specific feature of these experiments is the use of alumina nanopowders produced by electrical explosion of wires. The method of electrical explosion of aluminum wires in the mixture of argon and oxygen was used for the preparation of alumina nanopowders. For this work aluminum wires having the diameter of 0.35 mm were employed. The wire (l = 65 mm) was input in the © 2015 An et al. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http:// creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. An et al. SpringerPlus (2015) 4:581 explosion chamber using a special feeding mechanism. The following parameters of electrical explosion were used: working gas pressure—1.5 atm, voltage—24 kV, capacity—2.3 µF, and inductance—0.72 µH. The BET analysis results showed that the specific surface area of as-prepared nanopowder was 20 m2/g. The product of this process is alumina nanopowder. Figure 1 illustrates the alumina nanopowder prepared using the EEW technique in the mixture of argon and oxygen. It is well known that metal nanopowders produced by EEW can reveal certain deficiency of crystalline structure. The shape of prepared particles seems to be rather spherical. However some of these particles can have a faceted surface. The faceted particle surface is explained by structural defects in electroexplosive nanopowders and features of surface aluminum oxide shell. XRD analysis was carried out in order to characterize the phase and crystal structure of the alumina nanopowder samples produced by electrical explosion of wires. The X-ray pattern of the sample (Fig. 2) was analyzed to detect different polymorph alumina structures in the asprepared powder. According to the data of X-ray analysis, the main phases in the products are γ-Al2O3 and δ-Al2O3. According to the intensity of peaks, the dominant phase is γ-Al2O3. The alumina sintering processes reveal a dual nature. On one hand, the SEM micrograph in Fig. 3 shows that the image of alumina sintered at a temperature of 1400 °C and a pressure of 40 MPa. This micrograph shows that the sintered body consists of micron-sized faceted grains and nanosized necked grains. SPS relates to grain-boundary diffusion and migration processes connected to the electric field impact. It can be assumed that the migration processes can be related to the employment of Fig. 1 SEM micrograph of the alumina nanopowder produced by electrical explosion of aluminum wires in the mixture of argon and oxygen Page 2 of 4 Fig. 2 X-ray pattern of the alumina nanopowder produced by electrical explosion of wires Fig. 3 SEM micrograph of the alumina nanopowder sintered by SPS at 1400 °C using a pressure of 40 MPa higher temperatures and pressures. Electric field can also impact significantly on the growth of alumina nanopowder grains. The X-ray analysis shows that the γ-Al2O3 and δ-Al2O3 phases disappeared in the final products of alumina nanopowder sintering. The only phase found in the sintered alumina nanopowder was α-Al2O3 (Fig. 4). This fact is explained by normal polymorph transformations in alumina when heating it up to temperatures higher than 1400 °C. The presence of γ-Al2O3 and δ-Al2O3 phases in electroexplosive alumina nanopowders is related to special fabrication conditions which provide strong nonequilibrium allowing stabilization of low-temperature modifications in the final products. Spark plasma sintering provokes relaxation processes leading to transformation into more stable high-temperature alumina phases. Table 1 shows the densification parameters of alumina nanopowder sintered at different temperatures (...truncated)