- Email: [email protected]
Microstructural, crystal structure and electrical characteristics of shock-consolidated Ga2O3 doped ZnO bulk
Powder Technology 208 (2011) 575–581
Contents lists available at ScienceDirect
Powder Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p ow t e c
Microstructural, crystal structure and electrical characteristics of shock-consolidated Ga2O3 doped ZnO bulk Youngkook Kim a,⁎, Ikegami Tomoaki b, Shigeru Itoh a a b
Shock Wave and Condensed Matter Research Center, Kumamoto University Kurokami 2-39-1, Kumamoto City, 860-8555, Japan Graduate School of Science & Technology, Kumamoto University Kurokami 2-39-1, Kumamoto City, 860-8555, Japan
a r t i c l e
i n f o
Article history: Received 14 April 2010 Received in revised form 11 November 2010 Accepted 18 December 2010 Available online 24 December 2010 Keywords: Underwater shock compaction Grain boundary resistance Ga2O3 doped ZnO Nonlinear current–voltage (I–V) characteristic
a b s t r a c t Ga2O3 (5 wt.%) doped zinc oxide (ZnO, 95 wt.%) bulk was fabricated by underwater shock compaction technique. The microstructural, crystal structure and electrical properties of shock-consolidated samples were investigated and compared to a commercially available sintered Ga2O3 (5 wt.%) doped ZnO (95 wt.%). The relative density of shock-consolidated sample was about 97% of the theoretical density, and no grain growth and lattice defects were confirmed. The grain boundary resistance was remarkably higher than that of commercial sintered Ga2O3 doped ZnO and nonlinear current–voltage (I–V) characteristics of shockconsolidated ZnO and Ga2O3 doped ZnO were very lower than that of commercial ZnO varistor. © 2010 Elsevier B.V. All rights reserved.
1. Introduction There is no doubt that ceramics are great potential and promising materials because of their numerous and superior properties such as mechanical [1], electrical [2,3], chemical [4] and optical properties [5,6]. In particular, zinc oxide (ZnO) and doped-ZnO ceramics have been widely used in transparent thin film manufacture as ceramic target materials [7]. In order to obtain ceramics with good properties for various purposes, numerous sintering methods have been extensively studied. For instance, the conventional hot sintering [8,9], spark plasma sintering (SPS) [10,11] and microwave sintering [1,12], and two-step sintering (TSS) [13] are typical methods. However, to obtain higher density ceramics and suppress the grain growth, an explosive compaction technique [14–16] is a useful method because of the very fast consolidation process (microsecond time scale) and high shock pressure (1–100 Gpa). This technique typically uses a high performance explosive to generate a high shock pressure and can easily make high density ceramics or diamond systhesis [17] by means of shock energy generated by explosion of an explosive. Recently, the explosive compaction using an underwater shock wave (called underwater shock compaction) has been reported [16,20,21]. Although this technique is similar to the process of conventional explosive compaction [14,18,19], the main difference is that underwater shock wave is used as a shock loading source
⁎ Corresponding author. Tel.: +81 96 342 3290; fax: +81 96 342 3294. E-mail address: [email protected] (Y. Kim). 0032-5910/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2010.12.020
impacting on the powders. The authours believe that the underwater shock compaction is more useful than the conventional explosive compaction to make ceramics and other hard bulk materials, since the magnitude and homogenous distribution of shock pressure acting on the impacted areas can be easily controlled by the design of the device. In this study, we have tried to make Ga2O3 doped ZnO ceramic bulk, which is widely used in commercial application as a ceramic target material, using the underwater shock compaction technique. Thus, the aim of this study is to introduce the mechanism of underwater shock compaction using numerical calculations and experiments, and discuss microstructural, crystal structure and electrical properties of a shock-consolidated Ga2O3 doped ZnO bulk material. 2. Experimental procedures Fig. 1 shows a schematic illustration of underwater shock compaction device. In the device, a high performance explosive, SEP (Asahi-Kasei Chemicals Corp., Japan) was used to generate a high shock pressure. The explosive, SEP with detonation velocity of 6.97 km/s and density of 1300 kg/m3, was loaded in the explosive container. An explosive lens composed of two types of explosives, SEP and HABW (detonation velocity: 4.75 km/s and density: 2200 kg/m3; Asahi-Kasei Chemicals Corp., Japan) with different detonation velocities and used to generate a planar detonation wave as shown in Fig. 2. A water container was used to create an underwater shock wave and its configuration is a cylindrical type with height of 10 mm and inner diameter of 30 mm. The powders used in this study were copper (Cu) powders (~ 45 μm) and Ga2O3 (~5 μm) doped ZnO (~2 μm) powders
576
Y. Kim et al. / Powder Technology 208 (2011) 575–581
Fig. 1. Schematic illustration of underwater shock compaction device.
with the mixture ratio of 5 wt.%:95 wt.%. The powders were filled in the powder capsule with layer of t1 (22 mm), t2 (8 mm) and t3 (4 mm), respectively and pressed by uniaxial press machine at 50 MPa. The initial densities of pressed Ga2O3 doped ZnO and Cu with layer of t1 and t3 were about 60% and 50%, respectively. The initial densities were measured by the height of the layer of each pressed powder in the powder capsule. The thickness of each filled layer is to prevent spalling effect, which induce a large crack formation in the compacts. Cu powders were used to delay the rapid cooling process of shockconsolidated material and assist the strong surface bonding between powder particles [16]. The configuration dimension of powder capsule was the inner diameter of 30 mm, height of 50 mm and the charging depth of powders was 35 mm. A gas drain hole (1 mm) was made to allow the exhaust of air during compaction. A steel cover plate with thickness of 1 mm was set on the Cu powder with layer of t1 in order to prevent the penetration of impurities (detonation gases or water). All containers were made of mild steel. X-ray diffraction analysis of crystalline phases of Ga2O3 doped ZnO starting powder and shock-consolidated Ga2O3 doped ZnO bulk was carried out by means of X-ray diffractometer (RIGAKU, Rint 2100) at a
voltage of 40 kV, a current of 30 mA with Cu Kα radiation and scanning step size of 0.02°. Microstructures of starting powders and shock-consolidated materials were observed by scanning electron microscopy (SEM, JCM-5700, JEOL, Japan). The relative density and electrical resistance of shock-consolidated Ga2O3 doped ZnO were evaluated by Archimedes method and Nyquist plot method (HIOKI 3532-80 Chemical Impedance Meter, Japan), respectively. A shock pressure of underwater shock wave was tested by a piezofilm stress gauge (PVF2-11-,125-EK, Dyansen, Inc., USA). The piezofilm stress gauge was set below the cover plate. The shock pressure was recored by an ocilloscope measuring device. Fig. 3 shows the measurement system of shock pressure of underwater shock wave. 3. Numerical simulations To understand the generation and propagation process of detonation wave and underwater shock wave, a numerical calculation was performed by means of LS-DYNA (commercial program based on the explicit finite element code). In this calculation, a
Fig. 2. Series photographs of propagation of uniform detonation wave generated by the explosive lens; the explosive (SEP) is charged in PMMA pipe and photographs was taken by high speed video camera (Shimadzu HPV-1, Japan).
Y. Kim et al. / Powder Technology 208 (2011) 575–581
577
Fig. 3. Measurement system of shock pressure impacting on the powders.
Fig. 4. Numerical calculation model of simplified underwater shock compaction device.
Table 1 JWL coefficients for explosive, SEP.
Table 2 Mie–Grüneisen parameters of water and container (steel).
A (GPa)
B (GPa)
R1
R2
ω
364
2.31
4.3
1.00
0.28
simulation model was simplified as a quarter of circle as shown in Fig. 4. The explosive, detonation product gas, water and metal containers were considered as main calculation substance, however powders were modeled as a part of the water in order to simplify the
Water SUS304
ρ0 (kg/m3)
C0 (m/s)
s
Гo
1000 7900
1489 4570
1.79 1.49
1.65 2.17
calculation. For the detonation of an explosive, Jones–Wilkins–Lee (JWL) equation of state was used. Chapman–Jouguet (C–J) detonation process of explosive was solved by C–J volume burn method.
578
Y. Kim et al. / Powder Technology 208 (2011) 575–581
Fig. 5. (a) Propagation processes of detonation wave and underwater shockwave and (b) comparison between results of experiment and numerical calculation for the peak shock pressure of underwater shockwave.
The pressure components generated in water and each container were calculated by Mie–Grüneisen equation of state. In boundary condition, the initial velocity of 1711 m/s was given as particle velocity of explosive. The expression of JWL and Mie–Grüneisen equation of state were described as follows: ω ω ωe PJWL = A 1− expð−R1 V Þ + B 1− exp −R2 V + VR1 VR2 V
ð2Þ
where PJWL is the pressure, V is the ratio of the initial density of explosive to the density of detonation gas products, e is the internal energy and the coefficients A, B, R1, R2, and ω have been determined by the expansion tube test [22]. These parameters are shown in Table 1.
P=
ρ0 C02 η Γ η 1− 0 + Γ0 ρ0 e 2 2 ð1−sηÞ
ð3Þ
where C0 is the sound velocity, η = 1 − ρo/ρ, ρ is density, Г is Grüneisen parameter and s is material constant, and all parameters are listed in Table 2.
4. Results and discussion Fig. 5 shows the detonation wave, underwater shockwave and a measured peak shock pressure of underwater shock wave compared to the result of numerical calculation. As shown in Fig. 5(a), the propagation of detonation wave, generation of underwater shock wave and reflected wave were clearly shown. In particular, the reflected wave was simultaneously generated with the underwater shock wave and converged at the central position of water container. The converging effect of reflected wave can induce a further high shock pressure. Itoh et al. [23] have reported a compaction device using converging effect of reflected wave. The measured peak shock pressure of underwater shockwave was about 6 GPa as shown in Fig. 5 (b). In numerical calculation, there were two peak shock pressures at different time. The first peak was the underwater shock wave and second peak was the converged reflected wave. The result was caused by short distance between the top and bottom of water container. However the time difference between both waves can be reduced by control of inner angle and height of water container. If the time difference is reduced, a further high shock pressure can be obtained because of combination of two waves. In experiment, the reflected wave could not be measured, because the stress gauge was completely destroyed after the shock compaction. Fig. 6 shows a
Y. Kim et al. / Powder Technology 208 (2011) 575–581
579
Fig. 6. (a) Photograph of shock-consolidated Ga2O3 doped ZnO bulk, (b) and (c) are microstructural images of shock-consolidated Ga2O3 doped ZnO and its starting powder, and (d) and (e) are microstructural images of shock-consolidated Cu compacts with layer of t1 and t3, respectively.
photograph of shock-consolidated Ga2O3 doped ZnO bulk and microstructural images of shock-consolidated Ga2O3 doped ZnO and Cu compacts. The shock-consoidated Ga2O3 doped ZnO was cut with an ellipse shape and its size is the diameter of 30 mm, width of 25 mm and thickness of 4 mm. The relative density was about 97% (5.48 g/ cm3) of theoretical density (5.65 g/cm3). We have confirmed the crack-free formation at most of the areas and the color was slightly changed from white to yellow due to a shock energy as shown in Fig. 6 (a). Generally, cracks are easily generated by high velocity of impact, reflected tensile waves and thermal residual stresses [16,24] in explosive compaction. In particular, the thermal residual stresses, which are generated by a thermal gradient of cooling after shockloading, induce large macro-cracks near the wall. In the case of shockconsolidated Ga2O3 doped ZnO, it can be considered that the residual heat generated from the shock-consolidated Cu compacts was
effective to minimize the cracking problem. As shown in Fig. 6(b) and (c), the Ga2O3 particles and ZnO particles are clearly shown. Most of powders seem to be slightly affected by shock energy and joined each other and partially increased compared to the starting powder. Although voids were not observed at most of areas and a high dense microstructure was observed, existence of micro-crack cannot be ignored within the particles or interfacial layers, since voids are a factor leading to cracks during deformation [25]. The high impact behavior of ceramics is quite different from metals, because ceramics are brittle. Therefore, strong surface bonding between powder particles often occurs without interparticle melting. However, in this study, the interparticle melting seems to be partially made, because powder particles were slightly increased. For that result, there is influence of residual heat generated from the shockconsolidated Cu compacts after shock-loading. Unlike the shock-
580
Y. Kim et al. / Powder Technology 208 (2011) 575–581
Fig. 7. X-ray diffraction petterns of (a) shock-consolidated Ga2O3 doped ZnO bulk and (b) its starting powder.
consolidated Ga2O3 doped ZnO, the shock-consolidated Cu compacts were completely melted and some pores were observed as shown in Fig. 6(d) and (e). For that result, the authors believe that the residual heat of Cu compacts was over 1000 °C. Fig. 7 shows the X-ray diffraction analysis of shock-consolidated bulk Ga2O3 doped ZnO and its starting powder. ZnO peaks and very weak Ga2O3 peaks were detected and any chemical reaction was not generated by shock energy. All diffraction peaks are remarkably broadened due to lattice defects and deformation of crystallite size. Fig. 8 shows the Nyquist diagrams of shock-consolidated Ga2O3 doped ZnO bulk and commercially sintered Ga2O3 doped ZnO with the doped ratio of 5 wt.%:95 wt.%. The linear parts indicate the resistivity of electrode. The semicircles are derived from the grain boundary barrier and grain barrier. The diameter of semicircle corresponds to the grain boundary resistivity. We confirmed that the shock-consolidated Ga2O3 doped ZnO bulk has larger grain boundary resistivity (several hundred MΩ) than that of commercial sintered Ga2O3 doped ZnO (several kΩ). The large difference of value between both samples was caused by lattice defects and increase of grain boudaries generated by high shock energy. Fig. 9 shows the current–voltage (I–V) characteristics of shockconsolidated ZnO and Ga2O3 doped ZnO bulks compared to that of a commercial ZnO varistor. It revealed that the shock-consolidated ZnO with high electrical resistance [16] exhibited characteristics of nonconductor due to high electric resistance. Although the shockconsolidated Ga2O3 doped ZnO also had a high electric resistance, it exhibited very low nonlinear I–V characteristic compared to the commercial ZnO varistor. This is consistent with the facts that high nonlinear I–V characteristics depend on the impurity additions doped ZnO [26,27]. Generally, a good varistor must have high resistance values at low voltages and grain boundaries are closely related to the electrical properties of ZnO varistors [2] because, at low voltages, an electron flow of a material not smoothly moves due to potential barriers at the grain boundaries. Moreover, L. Wang et al. have also reported [28] that an ideal varistor has minimal presence of mechanical defects such as voids, porosity and cracks. In underwater shock compaction lattice defects and increase of grain boundaries, which can lead to high electric resistance, are easily formed. We believe that this is a good point to make a varistor.
Fig. 8. Nyquist diagrams; (a) is the shock-consolidated Ga2O3 doped ZnO and (b) is commercial sintered Ga2O3 doped ZnO.
the microstructure and crystalline structure analysis of shockconsolidated Ga2O3 doped ZnO, high dense microstructure and broadened peaks was observed, respectively. The broadened peaks generated from lattice defects are often detected in explosive compaction experiment due to high impact. For the electric characteristics of shock-consolidated Ga2O3 doped ZnO, electric resistance was high and nonlinear I–V characteristics was very poor.
5. Conclusions Ga2O3 doped ZnO ceramic bulk without visible crack was fabricated by underwater shock compaction. Here, we suggest that the residual heat of Cu compacts is effective to minimize cracks. For
Fig. 9. Current density vs electric field relationships for the shock-consolidated ZnO, Ga2O3 doped ZnO and commercial ZnO varistor.
Y. Kim et al. / Powder Technology 208 (2011) 575–581
The results are caused by a lot of grain boundaries, lattice defects and deformation of crystallite. However, we also checked that in the case of shock-consolidated Ga2O3 doped ZnO, nonlinear I–V characteristic was better than shock-consolidated pure ZnO. Therefore, much more studies for electric characteristics of impurity additions are necessary. Acknowledgments The nonlinear I-V characteristic measurements of shock-consolidated samples were performed by Mr. H. Ando of Toshiba Transmission Distribution & Industrial System Company in Japan. This work was supported in part by the Research Funds of Innovative Collaboration Organization of Kumamoto University in Japan. References [1] R.R. Menezes, R.H.G.A. Kiminami, Microwave sintering of alumina–zirconia nanocomposites, J. Mater. Proce. Technol. 203 (2008) 513–517. [2] P. Durán, F. Capel, J. Rartaj, C. Moure, A strategic two-stage low-temperaure thermal processig leading to fully dense and fine-grained doped-ZnO varistors, Adv. Mater. 14 (2002) 137–141. [3] F.M. Filho, A.Z. Somões, A. Ries, L. Perazolli, E. Longo, J.A. Varela, Nonlinear electric behavior of the Cr2O3, ZnO, CoO and Ta2O5-doped SnO2 varistors, Ceram. Inter. 32 (2006) 283–289. [4] H. Tang, K. Prasad, R. Sanjines, F. Levy, TiO2 anatase thin films as gas sensors, Sens. Actuators B 26–27 (1995) 71–75. [5] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Visible-light photocatalysis in nitrogen-doped titanium oxides, Science 293 (2001) 269–271. [6] L.G. Devi, B.N. Murthy, S.G. Kumar, Photocatalytic activity of V+ 5, Mo6+ and Th4+ doped polycrystalline TiO2 for the degradation of chlorpyrifos under UV/solar light, J. Molec. Catal A Chem. 308 (2009) 174–181. [7] N. Sakai, Y. Umeda, F. Mitsugi, T. Ikegami, Characterization of zinc oxide thin films prepared by pulsed laser deposition at room temperature, Surf. Coat. Technol. 202 (2008) 5467–5470. [8] H. Hahn, J. Loga, R.S. Averback, Sintering characteristics of nanocrystalline TiO2, J. Mater. Res. 5 (1990) 609–614. [9] J. Li, Y. Ye, L. Shen, J. Chen, H. Zhou, Densification and grain growth during pressureless sintering of TiO2 nanoceramics, Mater. Sci. Eng. A 390 (2005) 265–270. [10] K. Nishii, M. Horio, Granulation of a hardmetal powder for spark plasma sintering, Powder Technol. 130 (2003) 199–202.
581
[11] P. Angerer, L.G. Yu, K.A. Khor, G. Krumpel, Spark-plasma-sintering (SPS) of nanostructured and submicron titanium oxide powders, Mater. Sci. Eng. A 381 (2004) 16–19. [12] N. Standish, H.K. Worner, D.Y. Obuchowski, Particle size effect in microwave heating of granular materials, Powder Technol. 66 (1991) 225–230. [13] M. Mazaheri, A.M. Zahedi, M. Haghighatzadeh, S.K. Sadrnezhaad, Sintering of titania nanoceramic: densification and grain growth, Ceram. Inter. 35 (2009) 685–691. [14] E.W. LaRocca, J. Pearson, Explosive press for use in impulsive loading studies, Rev. Sci. Instrum. 29 (1958) 848–851. [15] S.S. Shang, K. Hokamoto, M.A. Meyers, Hot dynamic consolidation of hard ceramics, J. Mater. Sci. 27 (1992) 5470–5476. [16] Y. Kim, T. Ueda, K. Hokamoto, S. Itoh, Electric and microstructural characteristics of bulk ZnO fabricated by underwater shock compaction, Ceram. Inter. 35 (2009) 3247–3252. [17] A.B. Sawaoka, M. Takamatsu, T. Akashi, Shock compression synthesis of diamond, Adv. Mater. 6 (1994) 346–354. [18] E. Szewczaka, J. Paszula, A.V. Leonov, H. Matyja, Explosive consolidation of mechanically alloyed Ti–Al alloys, Mater. Sci. Eng. A 226–228 (1997) 115–118. [19] S. Ando, Y. Mine, K. Takashima, S. Itoh, H. Tonda, Explosive compaction of Nd–Fe– B powder, J. Mater. Proce. Technol. 85 (1999) 142–147. [20] A. Chiba, S. Kimura, K. Raghukandan, Y. Morizono, Effect of alumina addition on hydroxyapatite biocomposites fabricated by underwater-shock compaction, Mater. Sci. Eng. A 350 (2003) 179–183. [21] K. Raghukandan, K. Hokamoto, J.S. Lee, A. Chiba, B.C. Pai, An investigation on underwater shock consolidated carbon fiber reinforced Al composites, J. Mater. Process. Technol. 134 (2003) 329–337. [22] S. Itoh, S. Kubota, A. Kira, S. Nagano, M. Fujita, On underwater shock wave generated by underwater explosion of high explosives (I): equation of state of detonating products and underwater shock wave, J. Jpn Explosive Soc. 55 (5) (1994) 202–208. [23] S. Itoh, S. Kubota, S. Nagano, M. Fujita, On generation of ultra-high pressure by converging of underwater shock waves, J. Pressure Vessel Technol. 120 (1998) 51–55. [24] M.A. Meyers, D.J. Benson, E.A. Olevsky, Shock consolidation: microstructurallybased analysis and computational modeling, Acta Mater. 47 (1999) 2089–2108. [25] S.L. Wang, M.A. Meyers, A. Szecket, Warm shock consolidation of IN 718 powder, J. Mater. Sci. 33 (1988) 1786–1804. [26] H.H. Hng, L. Halim, Grain growth in sintered ZnO-1 mol% V2O5 ceramics, Mater. Lett. 57 (2003) 1411–1416. [27] F.M. Filho, A.Z. Simoes, A. Ries, L. Peeazolli, E. Longo, J.A. Varela, Nonlinear electrical behavior of the Cr2O3, ZnO, CoO and Ta2O5-doped SnO5 varistors, Ceram. Inter. 32 (2006) 283–289. [28] L. Wang, G. Tang, Z.K. Xu, Preparation and electrical properties of multilayer ZnO varistors with water-based tape casting, Ceram. Inter. 35 (2009) 487–492.
Contents lists available at ScienceDirect
Powder Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p ow t e c
Microstructural, crystal structure and electrical characteristics of shock-consolidated Ga2O3 doped ZnO bulk Youngkook Kim a,⁎, Ikegami Tomoaki b, Shigeru Itoh a a b
Shock Wave and Condensed Matter Research Center, Kumamoto University Kurokami 2-39-1, Kumamoto City, 860-8555, Japan Graduate School of Science & Technology, Kumamoto University Kurokami 2-39-1, Kumamoto City, 860-8555, Japan
a r t i c l e
i n f o
Article history: Received 14 April 2010 Received in revised form 11 November 2010 Accepted 18 December 2010 Available online 24 December 2010 Keywords: Underwater shock compaction Grain boundary resistance Ga2O3 doped ZnO Nonlinear current–voltage (I–V) characteristic
a b s t r a c t Ga2O3 (5 wt.%) doped zinc oxide (ZnO, 95 wt.%) bulk was fabricated by underwater shock compaction technique. The microstructural, crystal structure and electrical properties of shock-consolidated samples were investigated and compared to a commercially available sintered Ga2O3 (5 wt.%) doped ZnO (95 wt.%). The relative density of shock-consolidated sample was about 97% of the theoretical density, and no grain growth and lattice defects were confirmed. The grain boundary resistance was remarkably higher than that of commercial sintered Ga2O3 doped ZnO and nonlinear current–voltage (I–V) characteristics of shockconsolidated ZnO and Ga2O3 doped ZnO were very lower than that of commercial ZnO varistor. © 2010 Elsevier B.V. All rights reserved.
1. Introduction There is no doubt that ceramics are great potential and promising materials because of their numerous and superior properties such as mechanical [1], electrical [2,3], chemical [4] and optical properties [5,6]. In particular, zinc oxide (ZnO) and doped-ZnO ceramics have been widely used in transparent thin film manufacture as ceramic target materials [7]. In order to obtain ceramics with good properties for various purposes, numerous sintering methods have been extensively studied. For instance, the conventional hot sintering [8,9], spark plasma sintering (SPS) [10,11] and microwave sintering [1,12], and two-step sintering (TSS) [13] are typical methods. However, to obtain higher density ceramics and suppress the grain growth, an explosive compaction technique [14–16] is a useful method because of the very fast consolidation process (microsecond time scale) and high shock pressure (1–100 Gpa). This technique typically uses a high performance explosive to generate a high shock pressure and can easily make high density ceramics or diamond systhesis [17] by means of shock energy generated by explosion of an explosive. Recently, the explosive compaction using an underwater shock wave (called underwater shock compaction) has been reported [16,20,21]. Although this technique is similar to the process of conventional explosive compaction [14,18,19], the main difference is that underwater shock wave is used as a shock loading source
⁎ Corresponding author. Tel.: +81 96 342 3290; fax: +81 96 342 3294. E-mail address: [email protected] (Y. Kim). 0032-5910/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2010.12.020
impacting on the powders. The authours believe that the underwater shock compaction is more useful than the conventional explosive compaction to make ceramics and other hard bulk materials, since the magnitude and homogenous distribution of shock pressure acting on the impacted areas can be easily controlled by the design of the device. In this study, we have tried to make Ga2O3 doped ZnO ceramic bulk, which is widely used in commercial application as a ceramic target material, using the underwater shock compaction technique. Thus, the aim of this study is to introduce the mechanism of underwater shock compaction using numerical calculations and experiments, and discuss microstructural, crystal structure and electrical properties of a shock-consolidated Ga2O3 doped ZnO bulk material. 2. Experimental procedures Fig. 1 shows a schematic illustration of underwater shock compaction device. In the device, a high performance explosive, SEP (Asahi-Kasei Chemicals Corp., Japan) was used to generate a high shock pressure. The explosive, SEP with detonation velocity of 6.97 km/s and density of 1300 kg/m3, was loaded in the explosive container. An explosive lens composed of two types of explosives, SEP and HABW (detonation velocity: 4.75 km/s and density: 2200 kg/m3; Asahi-Kasei Chemicals Corp., Japan) with different detonation velocities and used to generate a planar detonation wave as shown in Fig. 2. A water container was used to create an underwater shock wave and its configuration is a cylindrical type with height of 10 mm and inner diameter of 30 mm. The powders used in this study were copper (Cu) powders (~ 45 μm) and Ga2O3 (~5 μm) doped ZnO (~2 μm) powders
576
Y. Kim et al. / Powder Technology 208 (2011) 575–581
Fig. 1. Schematic illustration of underwater shock compaction device.
with the mixture ratio of 5 wt.%:95 wt.%. The powders were filled in the powder capsule with layer of t1 (22 mm), t2 (8 mm) and t3 (4 mm), respectively and pressed by uniaxial press machine at 50 MPa. The initial densities of pressed Ga2O3 doped ZnO and Cu with layer of t1 and t3 were about 60% and 50%, respectively. The initial densities were measured by the height of the layer of each pressed powder in the powder capsule. The thickness of each filled layer is to prevent spalling effect, which induce a large crack formation in the compacts. Cu powders were used to delay the rapid cooling process of shockconsolidated material and assist the strong surface bonding between powder particles [16]. The configuration dimension of powder capsule was the inner diameter of 30 mm, height of 50 mm and the charging depth of powders was 35 mm. A gas drain hole (1 mm) was made to allow the exhaust of air during compaction. A steel cover plate with thickness of 1 mm was set on the Cu powder with layer of t1 in order to prevent the penetration of impurities (detonation gases or water). All containers were made of mild steel. X-ray diffraction analysis of crystalline phases of Ga2O3 doped ZnO starting powder and shock-consolidated Ga2O3 doped ZnO bulk was carried out by means of X-ray diffractometer (RIGAKU, Rint 2100) at a
voltage of 40 kV, a current of 30 mA with Cu Kα radiation and scanning step size of 0.02°. Microstructures of starting powders and shock-consolidated materials were observed by scanning electron microscopy (SEM, JCM-5700, JEOL, Japan). The relative density and electrical resistance of shock-consolidated Ga2O3 doped ZnO were evaluated by Archimedes method and Nyquist plot method (HIOKI 3532-80 Chemical Impedance Meter, Japan), respectively. A shock pressure of underwater shock wave was tested by a piezofilm stress gauge (PVF2-11-,125-EK, Dyansen, Inc., USA). The piezofilm stress gauge was set below the cover plate. The shock pressure was recored by an ocilloscope measuring device. Fig. 3 shows the measurement system of shock pressure of underwater shock wave. 3. Numerical simulations To understand the generation and propagation process of detonation wave and underwater shock wave, a numerical calculation was performed by means of LS-DYNA (commercial program based on the explicit finite element code). In this calculation, a
Fig. 2. Series photographs of propagation of uniform detonation wave generated by the explosive lens; the explosive (SEP) is charged in PMMA pipe and photographs was taken by high speed video camera (Shimadzu HPV-1, Japan).
Y. Kim et al. / Powder Technology 208 (2011) 575–581
577
Fig. 3. Measurement system of shock pressure impacting on the powders.
Fig. 4. Numerical calculation model of simplified underwater shock compaction device.
Table 1 JWL coefficients for explosive, SEP.
Table 2 Mie–Grüneisen parameters of water and container (steel).
A (GPa)
B (GPa)
R1
R2
ω
364
2.31
4.3
1.00
0.28
simulation model was simplified as a quarter of circle as shown in Fig. 4. The explosive, detonation product gas, water and metal containers were considered as main calculation substance, however powders were modeled as a part of the water in order to simplify the
Water SUS304
ρ0 (kg/m3)
C0 (m/s)
s
Гo
1000 7900
1489 4570
1.79 1.49
1.65 2.17
calculation. For the detonation of an explosive, Jones–Wilkins–Lee (JWL) equation of state was used. Chapman–Jouguet (C–J) detonation process of explosive was solved by C–J volume burn method.
578
Y. Kim et al. / Powder Technology 208 (2011) 575–581
Fig. 5. (a) Propagation processes of detonation wave and underwater shockwave and (b) comparison between results of experiment and numerical calculation for the peak shock pressure of underwater shockwave.
The pressure components generated in water and each container were calculated by Mie–Grüneisen equation of state. In boundary condition, the initial velocity of 1711 m/s was given as particle velocity of explosive. The expression of JWL and Mie–Grüneisen equation of state were described as follows: ω ω ωe PJWL = A 1− expð−R1 V Þ + B 1− exp −R2 V + VR1 VR2 V
ð2Þ
where PJWL is the pressure, V is the ratio of the initial density of explosive to the density of detonation gas products, e is the internal energy and the coefficients A, B, R1, R2, and ω have been determined by the expansion tube test [22]. These parameters are shown in Table 1.
P=
ρ0 C02 η Γ η 1− 0 + Γ0 ρ0 e 2 2 ð1−sηÞ
ð3Þ
where C0 is the sound velocity, η = 1 − ρo/ρ, ρ is density, Г is Grüneisen parameter and s is material constant, and all parameters are listed in Table 2.
4. Results and discussion Fig. 5 shows the detonation wave, underwater shockwave and a measured peak shock pressure of underwater shock wave compared to the result of numerical calculation. As shown in Fig. 5(a), the propagation of detonation wave, generation of underwater shock wave and reflected wave were clearly shown. In particular, the reflected wave was simultaneously generated with the underwater shock wave and converged at the central position of water container. The converging effect of reflected wave can induce a further high shock pressure. Itoh et al. [23] have reported a compaction device using converging effect of reflected wave. The measured peak shock pressure of underwater shockwave was about 6 GPa as shown in Fig. 5 (b). In numerical calculation, there were two peak shock pressures at different time. The first peak was the underwater shock wave and second peak was the converged reflected wave. The result was caused by short distance between the top and bottom of water container. However the time difference between both waves can be reduced by control of inner angle and height of water container. If the time difference is reduced, a further high shock pressure can be obtained because of combination of two waves. In experiment, the reflected wave could not be measured, because the stress gauge was completely destroyed after the shock compaction. Fig. 6 shows a
Y. Kim et al. / Powder Technology 208 (2011) 575–581
579
Fig. 6. (a) Photograph of shock-consolidated Ga2O3 doped ZnO bulk, (b) and (c) are microstructural images of shock-consolidated Ga2O3 doped ZnO and its starting powder, and (d) and (e) are microstructural images of shock-consolidated Cu compacts with layer of t1 and t3, respectively.
photograph of shock-consolidated Ga2O3 doped ZnO bulk and microstructural images of shock-consolidated Ga2O3 doped ZnO and Cu compacts. The shock-consoidated Ga2O3 doped ZnO was cut with an ellipse shape and its size is the diameter of 30 mm, width of 25 mm and thickness of 4 mm. The relative density was about 97% (5.48 g/ cm3) of theoretical density (5.65 g/cm3). We have confirmed the crack-free formation at most of the areas and the color was slightly changed from white to yellow due to a shock energy as shown in Fig. 6 (a). Generally, cracks are easily generated by high velocity of impact, reflected tensile waves and thermal residual stresses [16,24] in explosive compaction. In particular, the thermal residual stresses, which are generated by a thermal gradient of cooling after shockloading, induce large macro-cracks near the wall. In the case of shockconsolidated Ga2O3 doped ZnO, it can be considered that the residual heat generated from the shock-consolidated Cu compacts was
effective to minimize the cracking problem. As shown in Fig. 6(b) and (c), the Ga2O3 particles and ZnO particles are clearly shown. Most of powders seem to be slightly affected by shock energy and joined each other and partially increased compared to the starting powder. Although voids were not observed at most of areas and a high dense microstructure was observed, existence of micro-crack cannot be ignored within the particles or interfacial layers, since voids are a factor leading to cracks during deformation [25]. The high impact behavior of ceramics is quite different from metals, because ceramics are brittle. Therefore, strong surface bonding between powder particles often occurs without interparticle melting. However, in this study, the interparticle melting seems to be partially made, because powder particles were slightly increased. For that result, there is influence of residual heat generated from the shockconsolidated Cu compacts after shock-loading. Unlike the shock-
580
Y. Kim et al. / Powder Technology 208 (2011) 575–581
Fig. 7. X-ray diffraction petterns of (a) shock-consolidated Ga2O3 doped ZnO bulk and (b) its starting powder.
consolidated Ga2O3 doped ZnO, the shock-consolidated Cu compacts were completely melted and some pores were observed as shown in Fig. 6(d) and (e). For that result, the authors believe that the residual heat of Cu compacts was over 1000 °C. Fig. 7 shows the X-ray diffraction analysis of shock-consolidated bulk Ga2O3 doped ZnO and its starting powder. ZnO peaks and very weak Ga2O3 peaks were detected and any chemical reaction was not generated by shock energy. All diffraction peaks are remarkably broadened due to lattice defects and deformation of crystallite size. Fig. 8 shows the Nyquist diagrams of shock-consolidated Ga2O3 doped ZnO bulk and commercially sintered Ga2O3 doped ZnO with the doped ratio of 5 wt.%:95 wt.%. The linear parts indicate the resistivity of electrode. The semicircles are derived from the grain boundary barrier and grain barrier. The diameter of semicircle corresponds to the grain boundary resistivity. We confirmed that the shock-consolidated Ga2O3 doped ZnO bulk has larger grain boundary resistivity (several hundred MΩ) than that of commercial sintered Ga2O3 doped ZnO (several kΩ). The large difference of value between both samples was caused by lattice defects and increase of grain boudaries generated by high shock energy. Fig. 9 shows the current–voltage (I–V) characteristics of shockconsolidated ZnO and Ga2O3 doped ZnO bulks compared to that of a commercial ZnO varistor. It revealed that the shock-consolidated ZnO with high electrical resistance [16] exhibited characteristics of nonconductor due to high electric resistance. Although the shockconsolidated Ga2O3 doped ZnO also had a high electric resistance, it exhibited very low nonlinear I–V characteristic compared to the commercial ZnO varistor. This is consistent with the facts that high nonlinear I–V characteristics depend on the impurity additions doped ZnO [26,27]. Generally, a good varistor must have high resistance values at low voltages and grain boundaries are closely related to the electrical properties of ZnO varistors [2] because, at low voltages, an electron flow of a material not smoothly moves due to potential barriers at the grain boundaries. Moreover, L. Wang et al. have also reported [28] that an ideal varistor has minimal presence of mechanical defects such as voids, porosity and cracks. In underwater shock compaction lattice defects and increase of grain boundaries, which can lead to high electric resistance, are easily formed. We believe that this is a good point to make a varistor.
Fig. 8. Nyquist diagrams; (a) is the shock-consolidated Ga2O3 doped ZnO and (b) is commercial sintered Ga2O3 doped ZnO.
the microstructure and crystalline structure analysis of shockconsolidated Ga2O3 doped ZnO, high dense microstructure and broadened peaks was observed, respectively. The broadened peaks generated from lattice defects are often detected in explosive compaction experiment due to high impact. For the electric characteristics of shock-consolidated Ga2O3 doped ZnO, electric resistance was high and nonlinear I–V characteristics was very poor.
5. Conclusions Ga2O3 doped ZnO ceramic bulk without visible crack was fabricated by underwater shock compaction. Here, we suggest that the residual heat of Cu compacts is effective to minimize cracks. For
Fig. 9. Current density vs electric field relationships for the shock-consolidated ZnO, Ga2O3 doped ZnO and commercial ZnO varistor.
Y. Kim et al. / Powder Technology 208 (2011) 575–581
The results are caused by a lot of grain boundaries, lattice defects and deformation of crystallite. However, we also checked that in the case of shock-consolidated Ga2O3 doped ZnO, nonlinear I–V characteristic was better than shock-consolidated pure ZnO. Therefore, much more studies for electric characteristics of impurity additions are necessary. Acknowledgments The nonlinear I-V characteristic measurements of shock-consolidated samples were performed by Mr. H. Ando of Toshiba Transmission Distribution & Industrial System Company in Japan. This work was supported in part by the Research Funds of Innovative Collaboration Organization of Kumamoto University in Japan. References [1] R.R. Menezes, R.H.G.A. Kiminami, Microwave sintering of alumina–zirconia nanocomposites, J. Mater. Proce. Technol. 203 (2008) 513–517. [2] P. Durán, F. Capel, J. Rartaj, C. Moure, A strategic two-stage low-temperaure thermal processig leading to fully dense and fine-grained doped-ZnO varistors, Adv. Mater. 14 (2002) 137–141. [3] F.M. Filho, A.Z. Somões, A. Ries, L. Perazolli, E. Longo, J.A. Varela, Nonlinear electric behavior of the Cr2O3, ZnO, CoO and Ta2O5-doped SnO2 varistors, Ceram. Inter. 32 (2006) 283–289. [4] H. Tang, K. Prasad, R. Sanjines, F. Levy, TiO2 anatase thin films as gas sensors, Sens. Actuators B 26–27 (1995) 71–75. [5] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Visible-light photocatalysis in nitrogen-doped titanium oxides, Science 293 (2001) 269–271. [6] L.G. Devi, B.N. Murthy, S.G. Kumar, Photocatalytic activity of V+ 5, Mo6+ and Th4+ doped polycrystalline TiO2 for the degradation of chlorpyrifos under UV/solar light, J. Molec. Catal A Chem. 308 (2009) 174–181. [7] N. Sakai, Y. Umeda, F. Mitsugi, T. Ikegami, Characterization of zinc oxide thin films prepared by pulsed laser deposition at room temperature, Surf. Coat. Technol. 202 (2008) 5467–5470. [8] H. Hahn, J. Loga, R.S. Averback, Sintering characteristics of nanocrystalline TiO2, J. Mater. Res. 5 (1990) 609–614. [9] J. Li, Y. Ye, L. Shen, J. Chen, H. Zhou, Densification and grain growth during pressureless sintering of TiO2 nanoceramics, Mater. Sci. Eng. A 390 (2005) 265–270. [10] K. Nishii, M. Horio, Granulation of a hardmetal powder for spark plasma sintering, Powder Technol. 130 (2003) 199–202.
581
[11] P. Angerer, L.G. Yu, K.A. Khor, G. Krumpel, Spark-plasma-sintering (SPS) of nanostructured and submicron titanium oxide powders, Mater. Sci. Eng. A 381 (2004) 16–19. [12] N. Standish, H.K. Worner, D.Y. Obuchowski, Particle size effect in microwave heating of granular materials, Powder Technol. 66 (1991) 225–230. [13] M. Mazaheri, A.M. Zahedi, M. Haghighatzadeh, S.K. Sadrnezhaad, Sintering of titania nanoceramic: densification and grain growth, Ceram. Inter. 35 (2009) 685–691. [14] E.W. LaRocca, J. Pearson, Explosive press for use in impulsive loading studies, Rev. Sci. Instrum. 29 (1958) 848–851. [15] S.S. Shang, K. Hokamoto, M.A. Meyers, Hot dynamic consolidation of hard ceramics, J. Mater. Sci. 27 (1992) 5470–5476. [16] Y. Kim, T. Ueda, K. Hokamoto, S. Itoh, Electric and microstructural characteristics of bulk ZnO fabricated by underwater shock compaction, Ceram. Inter. 35 (2009) 3247–3252. [17] A.B. Sawaoka, M. Takamatsu, T. Akashi, Shock compression synthesis of diamond, Adv. Mater. 6 (1994) 346–354. [18] E. Szewczaka, J. Paszula, A.V. Leonov, H. Matyja, Explosive consolidation of mechanically alloyed Ti–Al alloys, Mater. Sci. Eng. A 226–228 (1997) 115–118. [19] S. Ando, Y. Mine, K. Takashima, S. Itoh, H. Tonda, Explosive compaction of Nd–Fe– B powder, J. Mater. Proce. Technol. 85 (1999) 142–147. [20] A. Chiba, S. Kimura, K. Raghukandan, Y. Morizono, Effect of alumina addition on hydroxyapatite biocomposites fabricated by underwater-shock compaction, Mater. Sci. Eng. A 350 (2003) 179–183. [21] K. Raghukandan, K. Hokamoto, J.S. Lee, A. Chiba, B.C. Pai, An investigation on underwater shock consolidated carbon fiber reinforced Al composites, J. Mater. Process. Technol. 134 (2003) 329–337. [22] S. Itoh, S. Kubota, A. Kira, S. Nagano, M. Fujita, On underwater shock wave generated by underwater explosion of high explosives (I): equation of state of detonating products and underwater shock wave, J. Jpn Explosive Soc. 55 (5) (1994) 202–208. [23] S. Itoh, S. Kubota, S. Nagano, M. Fujita, On generation of ultra-high pressure by converging of underwater shock waves, J. Pressure Vessel Technol. 120 (1998) 51–55. [24] M.A. Meyers, D.J. Benson, E.A. Olevsky, Shock consolidation: microstructurallybased analysis and computational modeling, Acta Mater. 47 (1999) 2089–2108. [25] S.L. Wang, M.A. Meyers, A. Szecket, Warm shock consolidation of IN 718 powder, J. Mater. Sci. 33 (1988) 1786–1804. [26] H.H. Hng, L. Halim, Grain growth in sintered ZnO-1 mol% V2O5 ceramics, Mater. Lett. 57 (2003) 1411–1416. [27] F.M. Filho, A.Z. Simoes, A. Ries, L. Peeazolli, E. Longo, J.A. Varela, Nonlinear electrical behavior of the Cr2O3, ZnO, CoO and Ta2O5-doped SnO5 varistors, Ceram. Inter. 32 (2006) 283–289. [28] L. Wang, G. Tang, Z.K. Xu, Preparation and electrical properties of multilayer ZnO varistors with water-based tape casting, Ceram. Inter. 35 (2009) 487–492.