Ai, liu, zhang, gong, he, and zhang microwave sintering of graphene-nanoplatelet-reinforced al2o3-based composites electricity use in the us

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In this study, we performed a microwave sintering (MWS) of Al 2O 3 ceramic and Al 2O 3-based composites with nominal contents of graphene nanoplatelets (GPLs) of 0.2, 0.4, 0.6, and 0.8 vol%. The GPL dispersion in N-methyl pyrroleketone was optimized to deagglomerate the GPLs without damaging their structure. gas density problems Dense composites were then obtained by MWS at 1500°C for 30 min. The effects of different GPL contents on the phase compositions, microstructures, and mechanical properties of the composites were investigated. The microstructures of the composites became finer with the incorporation of the GPLs. The well-dispersed GPL fillers led to higher sintered densities in the composites. The optimal mechanical properties were achieved with 0.4 vol% GPLs. For this sample, the hardness, fracture toughness, and bending strength were 2000 kgf/mm 2, 6.19 MPa·m 1/2, and 365.10 MPa, respectively. The addition of GPL could improve the microstructure of the Al 2O 3 ceramic and has potential to improve the fracture toughness of the ceramics.

Al 2O 3 ceramics have been widely used as high-speed cutting tools, dental implants, chemical and electrical insulators, resistance parts, and various coatings 1) owing to their excellent properties, such as high strength, hardness, temperature resistance, carrying-capacity corrosion resistance, and good chemical stability. 2- 3) However, the fracture toughness of the pure Al 2O 3 ceramic material is very low. This significantly affects the work reliability and operation safety of the ceramic parts. 4) In order to overcome the limitations of the mechanical properties of Al 2O 3 ceramics, studies have been performed to prepare Al 2O 3-based composite ceramics by adding secondary phases. For example, Puchy et al. 5) used spark plasma sintering (SPS) to prepare Al 2O 3/carbon nanotube (CNT) nanocomposites and reported a fracture toughness of 4.14 MPa·m 1/2 at a CNT concentration of 5%. electricity flow direction Ahmad et al. 6) employed hot-press sintering (HPS) to prepare Al 2O 3-based nanocomposites containing MgO, and reported that the optimal fracture toughness, flexural strength, and hardness were increased by 37%, 22%, and 20%, respectively, compared to those of monolithic alumina. Guo et al. 7) prepared zirconia-toughened alumina (ZTA) ceramics by mixing Al 2O 3 and yttria-tetragonal zirconia polycrystal (Y-TZP), isostatically pressed at 200 MPa and sintered at 1450°C for 2 h in air, whose fracture toughness and strength were up to 7.2 MPa·m 1/2 and 680 MPa, respectively. Li et al. 8) reported a novel approach to improve the hardness of a ZTA composite with a zirconia content of 20 vol%, whose Vickers hardness, flexural strength, and fracture toughness were 17.1 ± 2.5 GPa, 738 ± 88 MPa, and 4.2 ± 0.11 MPa·m 1/2, respectively. Parchoviansky et al. 9) reported the microstructures and mechanical properties of Al 2O 3/SiC nanocomposites prepared by HPS. The results indicated that the flexural strengths of the nanocomposites increased with the volume fraction of the silicon carbide particles; the maximum flexural strength was 655 ± 90 MPa for 20 vol.% SiC.

Figure 2 illustrates the SEM morphologies of the fractured surfaces of the sintered samples with and without GPLs. Fig. 2(a) shows few larger particles in the sintered specimen of pure Al 2O 3; the structure of the tissue is loose with few pores. Figs. 2(b) and 2(c) show SEM micrographs of the sintered samples with 0.2- and 0.4-vol.% GPLs, respectively. According to Figs. 2(b) and 2(c) and average Al 2O 3 particle size statistics in Figs. 2(a)-(c), after the addition of the GPLs, the grains of the sintered samples seem slightly smaller than those of the pure Al 2O 3 ceramic material ( Fig. 2(a)). This could be explained as the GPLs embedded in the grain boundaries (white circles) can stop grain growth and prevent the movement of grain boundaries, referred to as pinning effect of the GPLs. This reveals the good distribution of the GPLs. electricity electricity song Another factor may be that the GPLs exhibit large thermal conductivities, 23) so that the GPLs not only promote a uniform sintering temperature, but also accelerate the cooling. Simultaneously, the temperature distributions in the samples are uniform owing to the coupling between the whole specimen and electromagnetic wave in the MWS. However, when the GPL content reaches 0.6 vol.%, a significant agglomeration of GPLs occurs, as shown in Fig. 2(d). When the GPL content is higher (0.8 vol.%), an abnormal grain growth is observed, as shown in Fig. 2(e), as the too high content of GPLs hinders their uniform distribution in the composites, leading to the agglomerated GPLs. This is not conducive to the densification of the composites, as it reduces the relative density. This implies the formation of pores in the composites, which also provides space for the growth of grains. gas in dogs The aggregation of the GPLs leads to pinning effect reduction, which in turn leads to an increase in grain size.

Figure 3 shows SEM morphologies and energy-dispersive spectra (EDS) of fractured surfaces of the Al 2O 3/0.4-vol.%-GPL composite. Fig. 3(c) shows that Spectrum 1 contains only C, O, and Al elements; the carbon content is relatively high, which is attributed to the GPLs. This could be explained as only graphene was used as a carbon material to prepare the Al 2O 3-based composite ceramics and thus the content of graphene is high in the test area. Fig. 3(b) shows that flaky GPLs exist between adjacent Al 2O 3 grains, which are evenly bent along the Al 2O 3 grain boundary. The anchoring effect emerges when the GPLs are wrapped around the Al 2O 3 grain boundaries and bundled together. This effect can provide a higher energy to resist the removal of GPLs from the Al 2O 3 ceramic matrix, which can enhance the interfacial adhesion. Therefore, the fracture toughness can be improved owing to the enhanced interfacial friction. Therefore, this toughening mechanism is better than the traditional mechanism. 24)

Figure 4 shows the relative densities of the Al 2O 3/ x-vol.%-GPL ( x = 0, 0.2, 0.4, 0.6, 0.8) composite ceramics prepared by the MWS at 1500°C for 30 min. Compared with that of the Al 2O 3 ceramic, the relative densities of the Al 2O 3/GPL composite ceramics are significantly decreased upon the addition of the GPLs. This occurs mainly as the addition of the GPLs hinders the complete densification of the Al 2O 3-based composites during the consolidation process. However, the relative densities of the Al 2O 3/GPL composites increase with the GPL volume content, at low volume contents of the GPLs. The maximum relative density is reached at a GPL content of 0.4 vol.%. electricity 1 unit how many watts The relative densities of the Al 2O 3/GPL composites decrease with further increase in the GPL volume content. This could be explained as the GPLs can be evenly distributed in the composites at low volume contents of the GPLs (≤ 0.4 vol.%) so that fewer pores are formed in the composites in the MWS, which is favorable to promote the sintering. Therefore, the relative densities of the composites increase. However, when the content of GPLs is higher (> 0.4 vol.%, e.g., 0.6 and 0.8 vol.%), the GPLs easily agglomerate. Therefore, more pores easily form, which reduces the relative densities of the composites in the MWS.

The effects of the volume content of GPLs on the hardness values of the Al 2O 3/GPL composites are shown in Fig. 5. The Vickers hardness rapidly decreases, varying with the addition of the GPLs, from 2550 kgf/mm 2 for the pure Al 2O 3 ceramic to 1610 kgf/mm 2 for the Al 2O 3/0.8-vol%-GPL composite. When the content of GPLs is smaller than 0.4 vol.%, the Vickers hardness values of the Al 2O 3/GPL composites are increased; however, an excessive amount of GPLs leads to a reduced Vickers hardness. When the content of GPLs is 0.4 vol.%, the Vickers hardness reaches the maximum value of 2000 kgf/mm 2. gas in chest The Vickers hardness and relative density exhibit the same trend. According to the Hall-Petch relationship, the hardness of a material is inversely proportional with the grain size. As the Al 2O 3 grain growth was inhibited and the grain size distribution was uniform after the addition of the GPL powder (smaller than 0.4 vol.%) (see Figs. 2(a) and 2(b)), the Vickers hardness values of the samples could be improved to some extent. In addition, the low modulus of elasticity of the GPLs could decrease the Vickers hardness of the Al 2O 3/GPL composites. 25)

However, Figs. 6 and 7 show that both bending strength and fracture toughness values of the Al 2O 3/GPL composites initially increase and then decrease with the increase in the content of GPLs. Compared with those of the pure Al 2O 3 ceramic, the fracture toughness and bending strength values of the Al 2O 3/GPL composites are significantly improved upon the addition of the GPLs. When the content of GPLs is 0.4 vol.%, both fracture toughness and bending strength reach their peak values. electricity and circuits physics The maximum fracture toughness and bending strength of the Al 2O 3/GPL composites are 6.19 MPa·m 1/2 and 365.10 MPa, approximately 79% and 12% higher than those of the pure Al 2O 3 ceramic, respectively. Although both fracture toughness and bending strength decrease when the content of GPLs is larger than 0.4 vol.%, they are higher than those of the pure Al 2O 3 ceramic. This phenomenon may be attributed to the pores in the composite, which are believed to be the origins of cracks and weakening of the boundaries of the ceramic matrix. 26) Consequently, the optimal mechanical properties can be obtained with 0.4 vol.% GPLs. For this sample, the relative density, micro-hardness, fracture toughness, and bending strength are 98.8%, 2000 kgf/mm 2, 6.19 MPa·m 1/2, and 365.10 MPa, respectively. The relative density is decreased by approximately 0.4%, the hardness is decreased by approximately 22.5%, while the bending strength and fracture toughness are increased by approximately 12% and 79%, respectively, compared to those of the pure Al 2O 3 ceramic.