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Mechanical properties of Ag-doped Na1.5Co2O4
Journal of Alloys and Compounds 403 (2005) 308–311
Mechanical properties of Ag-doped Na1.5Co2O4 Tosawat Seetawan a,∗ , Vittaya Amornkitbamrung a , Thanusit Burinprakhon a , Santi Maensiri a , Ken Kurosaki b , Hiroaki Muta b , Masayoshi Uno b , Shinsuke Yamanaka b b
a Department of Physics, Khon Kaen University, 123 Mittraparb Road, Muang District, Khon Kaen 40002, Thailand Department of Nuclear Engineering, Graduate School of Engineering, Osaka University, Yamadaka 2-1, Suita, Osaka 565-0871, Japan
Received 24 April 2005; accepted 6 May 2005 Available online 2 August 2005
Abstract The polycrystalline samples of Na1.5 Co2−x Agx O4 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) were synthesized from powder precursors prepared by the polymerized complex (PC) method. Various mechanical properties of the samples, including the elastic modulus, Debye temperature and Vickers hardness, were evaluated. The elastic moduli and Debye temperature increase with increasing Ag content. The relationship between the Vickers hardness and Young’s modulus indicates a metallic characteristic. The microstructure of the non-doped sample shows coarse grains of about 10–20 m. On the other hand, the Ag-doped samples show fine grains with Ag particles around 10 m. © 2005 Elsevier B.V. All rights reserved. Keywords: Sodium cobalt oxide; Mechanical properties; Young’s modulus; Debye temperature; Heat capacity; Thermal barrier coating
1. Introduction Sodium cobalt oxide (Nax Co2 O4 ) was first synthesized in 1974 by Von Jansen and Hoppe [1]. The crystal structure of the ␥-phase of this material is hexagonal, which can be pictured as a layer structure comprising CoO2 conducting layers which are made of edge-shared CoO6 octahedra and inter-layers of Na+ ions alternatively stacked along the caxis. The sodium ions are intercalated in trigonal prismatic or octahedral coordination of oxygen atoms [2]. An important feature in this structure is that sodium ions randomly occupy the available sites by 50%. In this sense, NaCo2 O4 should be written as Nax CoO2 (x = 0.5). Nevertheless, we will call it NaCo2 O4 , because the best thermoelectric properties are realized near the 50% Na occupancy [3]. The ␥-phase Nax Co2 O4 shows a large Seebeck coefficient despite its metallic conductivity. Recently, Terasaki et al. [4] reported the outstanding thermoelectric properties of a single crystal of this oxide, i.e., an unusual Seebeck coefficient of 0.1 mV K−1 accompanied by a low resistivity of 0.2 m cm, at 300 K. To improve the ∗
Corresponding author. Tel.: +81 6 6879 7905; fax: +81 6 6879 7889. E-mail address: [email protected] (T. Seetawan).
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.05.021
thermoelectric efficiency of Nax Co2 O4 , Na or Co is substituted by other metals. For instance, the Seebeck coefficient of 100–150 V K−1 and the dimensionless figure of merit (ZT) of 0.03–0.05 have been obtained for NaCo2−x Tx O4 (T = Mn, Ru, Pb, Cu, Pd, Rh), at 300 K [5–9]. In the present study, the thermoelectric materials of Na1.5 Co2−x Agx O4 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) were prepared from the powder precursors obtained from the polymerized complex (PC) method. The mechanical properties of these materials were measured and calculated. The relationship between the hardness and Young’s modulus of the materials was studied.
2. Experimental procedure Polycrystalline samples of Na1.5 Co2−x Agx O4 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) were synthesized from powder precursors prepared by the PC method [10]. Firstly, citric acid (99.7%) was dissolved in ethylene glycol (99.5%) by heating and stirring at 473 K for 1 h. Secondly, Co(NO3 )2 ·6H2 O (99.95%), NaNO3 (99%) and AgNO3 (99.9%) corresponding to the nominal composition of NaCo2−x Agx O4 were added to this
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309
Table 1 Starting materials for the PC method Sample composition
Citric acid (g)
Ethylene glycol (ml)
NaNO3 (g)
Co(NO3 )2 ·6H2 O (g)
AgNO3 (g)
Non-doped Ag-doped (0.1) Ag-doped (0.2) Ag-doped (0.3) Ag-doped (0.4) Ag-doped (0.5)
42.0280 42.0280 42.0280 42.0280 42.0280 42.0280
503.7241 503.7241 503.7241 503.7241 503.7241 503.7241
7.2242 7.2242 7.2242 7.2242 7.2242 7.2242
29.1030 27.6479 26.1927 24.7376 23.2824 21.8273
0 0.8494 1.6987 2.5481 3.3974 4.2468
Note: The ratio of citric acid:ethylene glycol:metals was 0.2:9:0.05 by molarity.
solution. The actual amounts of the starting materials are given in Table 1. Finally, the mixture was stirred and heated at 573 K for 6 h. The colloidal solution became highly viscous, and this viscous polymeric product was decomposed to a dark-mass precursor at 723 K for 1 h. The mass precursor was ground and calcined at 1073 K for 5 h in order to obtain the powder of the ␥-Nax Co2 O4 phase. The calcined powder was compacted into a pellet of 10 mm diameter and 2 mm thickness under a pressure of 150 MPa and then annealed at 1173 K for 24 h in air. The crystal structure of the samples was analyzed by a powder X-ray diffraction (XRD) at room temperature using Cu K␣ radiation, λ = 0.15406 nm. The microstructures of these samples were determined by scanning electron microscopy (SEM) and energy-dispersive Xray spectroscopy (EDX). For the mechanical property measurements, the density of the samples was calculated from the mass and dimension. The longitudinal and shear sound velocities were measured by an ultrasonic pulse-echo method at room temperature to evaluate the shear modulus, Young’s modulus and Debye temperature. The hardness was measured by a micro-Vickers hardness tester at room temperature. 3. Results and discussion The powder XRD pattern at room temperature of the nondoped sample shows the single ␥-NaCo2 O4 phase. On the other hand, the Ag-doped (x = 0.1, 0.2) samples are composed of the ␥-NaCo2 O4 and Ag2 O phases and the Ag-doped (x = 0.3, 0.4, 0.5) samples are composed of the ␥-NaCo2 O4 , Na2 O2 and Ag2 O phases as shown in Fig. 1. The intensity of the peaks corresponds to the peak data on the JCPDS card, No. 30-1182 (␥-Na0.71 Co0.96 O2 ), No. 15-0068 (Na2 O2 ) and No. 72-2108 (Ag2 O).
Fig. 1. X-ray diffraction patterns of the powder Na1.5 Co2−x Agx O4 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) annealed at 1173 K for 24 h in air.
The hexagonal lattice parameters of the ␥-NaCo2 O4 phase and characteristics of the samples are summarized in Table 2. The lattice parameters are also shown in Fig. 2 as a function of x. As seen in Fig. 2, the lattice parameters a and c are not changed by Ag doping, indicating that the Ag is present as precipitates or inclusions (as shown in Fig. 4(b and c)). The shear modulus, Young’s modulus and Debye temperature were evaluated as shown in Table 2. For isotopic media, the shear modulus (G), Young’s modulus (E), compressibility (β) and Debye temperature (θ D ) can be written in terms of the longitudinal sound velocity VL and shear velocity VS as [11–13]: G = ρVS2 ,
(1)
Table 2 Sample characteristics of Na1.5 Co2−x Agx O4 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) Sample composition
Na1.3 Co2 O4 [9] Na1.5 Co2 O4 Na1.5 Co1.9 Ag0.1 O4 Na1.5 Co1.8 Ag0.2 O4 Na1.5 Co1.8 Ag0.3 O4 Na1.5 Co1.8 Ag0.4 O4 Na1.5 Co1.8 Ag0.5 O4
Lattice parameter of ␥-NaCo2 O4 phase a (nm)
c (nm)
0.2834 0.2839 0.2844 0.2845 0.2837 0.2844 0.2850
1.0899 1.0990 1.0907 1.0891 1.0805 1.0870 1.0817
Bulk density (g/cm3 )
Shear modulus (GPa)
3.88 3.64 3.87 4.03 4.04 4.14 4.25
43.4 19.9 27.0 36.6 32.7 35.2 45.4
Young’s modulus (GPa) 109 49.8 67.4 91.5 81.7 88.0 113
Vickers hardness (GPa)
Debye temperature (K)
0.90 0.17 0.26 0.26 0.14 0.19 0.23
476 359 406 464 440 449 503
T. Seetawan et al. / Journal of Alloys and Compounds 403 (2005) 308–311
310
Fig. 2. Hexagonal lattice parameters of ␥-NaCo2 O4 phase.
G(3VL2 − 4VS2 ) , VL2 − VS2
E= β=
1
ρ VL2 − 43 VS2
θD =
h kB
(2)
,
(3)
9N 4πV (VL−3 + 2VS−3 )
1/3 ,
(4)
where ρ is the sample density, h the Plank constant, kB the Boltzmann constant, N the number of atoms in a unit cell and V is the unit cell volume. The elastic moduli and Debye temperature evaluated from the sound velocities are shown in Fig. 3 as a function of x. Adding of Ag leads to higher elastic moduli and Debye temperature. It seems that these phenomena correspond to the microstructure of the samples.
Fig. 3. Elastic moduli and Debye temperature of Na1.5 Co2−x Agx O4 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5).
Fig. 4. SEM photographs of the surface microstructure: (a) non-doped sample; (b) Ag-doped (x = 0.2) sample; (c) the trace mark of Vickers hardness of Ag-doped (x = 0.2) sample.
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For various materials, the Vickers hardness is also known to be associated with Young’s modulus. For some oxide and carbide ceramics, the hardness was found to be proportional to Young’s modulus, HV /E = 0.05 [14]. We have evaluated the HV /E ratio for pure metals using literature data [15] and obtained the values of 0.006, 0.003 and 0.004 for bcc, fcc and hcp metals, respectively [16]. The relationship between the Vickers hardness and Young’s modulus of the samples shows metallic characteristic as shown in Fig. 6. The HV /E ratio for these samples is around 0.003 and decreases with increasing Ag content. 4. Conclusion
Fig. 5. Vickers hardness as a function of load of Na1.5 Co2−x Agx O4 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5).
The SEM photographs of the microstructure of non-doped and Ag-doped (x = 0.2) samples are shown in Fig. 4(a and b), respectively. The trace mark of the Vickers hardness of Agdoped (x = 0.2) sample is shown in Fig. 4(c). The microstructure of the non-doped sample is composed of significantly coarse grains about 10–20 m in size and it leads to low density. On the other hand, the microstructure of the Agdoped sample is composed of significantly fine grains and Ag particles around 10 m in size that precipitate on the grain boundary and it leads to high density. The hardness of the samples was measured at room temperature, and repeated seven times for a given sample; applied loads were chosen to be 0.245, 0.98 and 2.94 N, used 15 s for the loading time. The relationship between the Vickers hardness and applied load is shown in Fig. 5. The Vickers hardness decreases with increasing applied load, indicating a typical load dependence. The hardness provides the information of the resistance of a material to plastic deformation.
Fig. 6. Relationship between the Vickers hardness, HV , and Young’s modulus, E, of Na1.5 Co2−x Agx O4 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5).
The mechanical properties of Na1.5 Co2−x Agx O4 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) were measured at room temperature. The shear modulus and Young’s modulus increase with increasing Ag content. The Young’s moduli for non-doped (x = 0) and Ag-doped (x = 0.1–0.5) samples are around 50 and 88 GPa, respectively. The value of HV /E equals 0.003 for all samples, which corresponds to the metallic characteristics. Acknowledgments This work had financial support from the Rajabhat Sakon Nakhon University, the National Research Council of Thailand and Faculty of Science, Khon Kaen University, Thailand. References [1] M. Von Jansen, R. Hoppe, Z. Anorg. Allg. Chem. 408 (1974) 104. [2] H. Nagatsugawa, K. Nagasawa, J. Solid State Chem. 177 (2004) 1137. [3] M. Ohtaki, Y. Nojiri, E. Maeda, Proceedings of the 19th International Conference on Thermoelectrics (ICT2000), Babrow, Wales, 2000, p. 190. [4] I. Terasaki, Y. Sasago, K. Uchinokura, Phys. Rev. B 56 (1997) R12 685. [5] S. Li, R. Funahashi, I. Matsubara, S. Sodeoka, Mater. Res. Bull. 35 (2003) 2371. [6] A. Mrotzek, E. M¨uller, J. Plewa, H. Altenburg, 22nd International Conference on Thermoelectrics, 2003, p. 219. [7] I. Terasaki, Proceedings of the 19th International Conference on Thermoelectrics (ICT2000), Cardiff, UK, 2000, p. 20. [8] I. Terasaki, I. Tsukada, Y. Iguchi, Phys. Rev. B 65 (2002) 195106. [9] K. Kurosaki, H. Muta, M. Uno, S. Yamanaka, J. Alloys Compd. 315 (2001) 234. [10] M. Ito, T. Nagira, D. Furumato, S. Katsuyama, H. Nagai, Scripta Mater. 48 (2003) 403. [11] K. Yamada, S. Yamanaka, M. Katsura, Tech. Rep. Osaka Univ. 47 (1997) 181. [12] M. Fukuhara, I. Yamauchi, J. Mater. Sci. 28 (1993) 4681. [13] H. Inaba, T. Yamamoto, Netsu Sokutei 10 (1983) 132. [14] K. Tanaka, H. Koguchi, T. Mura, Int. J. Eng. Sci. 27 (1989) 11. [15] S. Yamamoto, T. Tanabe, Atarashii Zairyoukagaku, Showadou Kyoto (1990). [16] S. Yamanaka, K. Yamada, T. Tsuzaki, T. Iguchi, M. Katsura, Y. Hoshino, W. Saiki, J. Alloys Compd. 271 (1998) 549.
Mechanical properties of Ag-doped Na1.5Co2O4 Tosawat Seetawan a,∗ , Vittaya Amornkitbamrung a , Thanusit Burinprakhon a , Santi Maensiri a , Ken Kurosaki b , Hiroaki Muta b , Masayoshi Uno b , Shinsuke Yamanaka b b
a Department of Physics, Khon Kaen University, 123 Mittraparb Road, Muang District, Khon Kaen 40002, Thailand Department of Nuclear Engineering, Graduate School of Engineering, Osaka University, Yamadaka 2-1, Suita, Osaka 565-0871, Japan
Received 24 April 2005; accepted 6 May 2005 Available online 2 August 2005
Abstract The polycrystalline samples of Na1.5 Co2−x Agx O4 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) were synthesized from powder precursors prepared by the polymerized complex (PC) method. Various mechanical properties of the samples, including the elastic modulus, Debye temperature and Vickers hardness, were evaluated. The elastic moduli and Debye temperature increase with increasing Ag content. The relationship between the Vickers hardness and Young’s modulus indicates a metallic characteristic. The microstructure of the non-doped sample shows coarse grains of about 10–20 m. On the other hand, the Ag-doped samples show fine grains with Ag particles around 10 m. © 2005 Elsevier B.V. All rights reserved. Keywords: Sodium cobalt oxide; Mechanical properties; Young’s modulus; Debye temperature; Heat capacity; Thermal barrier coating
1. Introduction Sodium cobalt oxide (Nax Co2 O4 ) was first synthesized in 1974 by Von Jansen and Hoppe [1]. The crystal structure of the ␥-phase of this material is hexagonal, which can be pictured as a layer structure comprising CoO2 conducting layers which are made of edge-shared CoO6 octahedra and inter-layers of Na+ ions alternatively stacked along the caxis. The sodium ions are intercalated in trigonal prismatic or octahedral coordination of oxygen atoms [2]. An important feature in this structure is that sodium ions randomly occupy the available sites by 50%. In this sense, NaCo2 O4 should be written as Nax CoO2 (x = 0.5). Nevertheless, we will call it NaCo2 O4 , because the best thermoelectric properties are realized near the 50% Na occupancy [3]. The ␥-phase Nax Co2 O4 shows a large Seebeck coefficient despite its metallic conductivity. Recently, Terasaki et al. [4] reported the outstanding thermoelectric properties of a single crystal of this oxide, i.e., an unusual Seebeck coefficient of 0.1 mV K−1 accompanied by a low resistivity of 0.2 m cm, at 300 K. To improve the ∗
Corresponding author. Tel.: +81 6 6879 7905; fax: +81 6 6879 7889. E-mail address: [email protected] (T. Seetawan).
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.05.021
thermoelectric efficiency of Nax Co2 O4 , Na or Co is substituted by other metals. For instance, the Seebeck coefficient of 100–150 V K−1 and the dimensionless figure of merit (ZT) of 0.03–0.05 have been obtained for NaCo2−x Tx O4 (T = Mn, Ru, Pb, Cu, Pd, Rh), at 300 K [5–9]. In the present study, the thermoelectric materials of Na1.5 Co2−x Agx O4 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) were prepared from the powder precursors obtained from the polymerized complex (PC) method. The mechanical properties of these materials were measured and calculated. The relationship between the hardness and Young’s modulus of the materials was studied.
2. Experimental procedure Polycrystalline samples of Na1.5 Co2−x Agx O4 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) were synthesized from powder precursors prepared by the PC method [10]. Firstly, citric acid (99.7%) was dissolved in ethylene glycol (99.5%) by heating and stirring at 473 K for 1 h. Secondly, Co(NO3 )2 ·6H2 O (99.95%), NaNO3 (99%) and AgNO3 (99.9%) corresponding to the nominal composition of NaCo2−x Agx O4 were added to this
T. Seetawan et al. / Journal of Alloys and Compounds 403 (2005) 308–311
309
Table 1 Starting materials for the PC method Sample composition
Citric acid (g)
Ethylene glycol (ml)
NaNO3 (g)
Co(NO3 )2 ·6H2 O (g)
AgNO3 (g)
Non-doped Ag-doped (0.1) Ag-doped (0.2) Ag-doped (0.3) Ag-doped (0.4) Ag-doped (0.5)
42.0280 42.0280 42.0280 42.0280 42.0280 42.0280
503.7241 503.7241 503.7241 503.7241 503.7241 503.7241
7.2242 7.2242 7.2242 7.2242 7.2242 7.2242
29.1030 27.6479 26.1927 24.7376 23.2824 21.8273
0 0.8494 1.6987 2.5481 3.3974 4.2468
Note: The ratio of citric acid:ethylene glycol:metals was 0.2:9:0.05 by molarity.
solution. The actual amounts of the starting materials are given in Table 1. Finally, the mixture was stirred and heated at 573 K for 6 h. The colloidal solution became highly viscous, and this viscous polymeric product was decomposed to a dark-mass precursor at 723 K for 1 h. The mass precursor was ground and calcined at 1073 K for 5 h in order to obtain the powder of the ␥-Nax Co2 O4 phase. The calcined powder was compacted into a pellet of 10 mm diameter and 2 mm thickness under a pressure of 150 MPa and then annealed at 1173 K for 24 h in air. The crystal structure of the samples was analyzed by a powder X-ray diffraction (XRD) at room temperature using Cu K␣ radiation, λ = 0.15406 nm. The microstructures of these samples were determined by scanning electron microscopy (SEM) and energy-dispersive Xray spectroscopy (EDX). For the mechanical property measurements, the density of the samples was calculated from the mass and dimension. The longitudinal and shear sound velocities were measured by an ultrasonic pulse-echo method at room temperature to evaluate the shear modulus, Young’s modulus and Debye temperature. The hardness was measured by a micro-Vickers hardness tester at room temperature. 3. Results and discussion The powder XRD pattern at room temperature of the nondoped sample shows the single ␥-NaCo2 O4 phase. On the other hand, the Ag-doped (x = 0.1, 0.2) samples are composed of the ␥-NaCo2 O4 and Ag2 O phases and the Ag-doped (x = 0.3, 0.4, 0.5) samples are composed of the ␥-NaCo2 O4 , Na2 O2 and Ag2 O phases as shown in Fig. 1. The intensity of the peaks corresponds to the peak data on the JCPDS card, No. 30-1182 (␥-Na0.71 Co0.96 O2 ), No. 15-0068 (Na2 O2 ) and No. 72-2108 (Ag2 O).
Fig. 1. X-ray diffraction patterns of the powder Na1.5 Co2−x Agx O4 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) annealed at 1173 K for 24 h in air.
The hexagonal lattice parameters of the ␥-NaCo2 O4 phase and characteristics of the samples are summarized in Table 2. The lattice parameters are also shown in Fig. 2 as a function of x. As seen in Fig. 2, the lattice parameters a and c are not changed by Ag doping, indicating that the Ag is present as precipitates or inclusions (as shown in Fig. 4(b and c)). The shear modulus, Young’s modulus and Debye temperature were evaluated as shown in Table 2. For isotopic media, the shear modulus (G), Young’s modulus (E), compressibility (β) and Debye temperature (θ D ) can be written in terms of the longitudinal sound velocity VL and shear velocity VS as [11–13]: G = ρVS2 ,
(1)
Table 2 Sample characteristics of Na1.5 Co2−x Agx O4 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) Sample composition
Na1.3 Co2 O4 [9] Na1.5 Co2 O4 Na1.5 Co1.9 Ag0.1 O4 Na1.5 Co1.8 Ag0.2 O4 Na1.5 Co1.8 Ag0.3 O4 Na1.5 Co1.8 Ag0.4 O4 Na1.5 Co1.8 Ag0.5 O4
Lattice parameter of ␥-NaCo2 O4 phase a (nm)
c (nm)
0.2834 0.2839 0.2844 0.2845 0.2837 0.2844 0.2850
1.0899 1.0990 1.0907 1.0891 1.0805 1.0870 1.0817
Bulk density (g/cm3 )
Shear modulus (GPa)
3.88 3.64 3.87 4.03 4.04 4.14 4.25
43.4 19.9 27.0 36.6 32.7 35.2 45.4
Young’s modulus (GPa) 109 49.8 67.4 91.5 81.7 88.0 113
Vickers hardness (GPa)
Debye temperature (K)
0.90 0.17 0.26 0.26 0.14 0.19 0.23
476 359 406 464 440 449 503
T. Seetawan et al. / Journal of Alloys and Compounds 403 (2005) 308–311
310
Fig. 2. Hexagonal lattice parameters of ␥-NaCo2 O4 phase.
G(3VL2 − 4VS2 ) , VL2 − VS2
E= β=
1
ρ VL2 − 43 VS2
θD =
h kB
(2)
,
(3)
9N 4πV (VL−3 + 2VS−3 )
1/3 ,
(4)
where ρ is the sample density, h the Plank constant, kB the Boltzmann constant, N the number of atoms in a unit cell and V is the unit cell volume. The elastic moduli and Debye temperature evaluated from the sound velocities are shown in Fig. 3 as a function of x. Adding of Ag leads to higher elastic moduli and Debye temperature. It seems that these phenomena correspond to the microstructure of the samples.
Fig. 3. Elastic moduli and Debye temperature of Na1.5 Co2−x Agx O4 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5).
Fig. 4. SEM photographs of the surface microstructure: (a) non-doped sample; (b) Ag-doped (x = 0.2) sample; (c) the trace mark of Vickers hardness of Ag-doped (x = 0.2) sample.
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For various materials, the Vickers hardness is also known to be associated with Young’s modulus. For some oxide and carbide ceramics, the hardness was found to be proportional to Young’s modulus, HV /E = 0.05 [14]. We have evaluated the HV /E ratio for pure metals using literature data [15] and obtained the values of 0.006, 0.003 and 0.004 for bcc, fcc and hcp metals, respectively [16]. The relationship between the Vickers hardness and Young’s modulus of the samples shows metallic characteristic as shown in Fig. 6. The HV /E ratio for these samples is around 0.003 and decreases with increasing Ag content. 4. Conclusion
Fig. 5. Vickers hardness as a function of load of Na1.5 Co2−x Agx O4 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5).
The SEM photographs of the microstructure of non-doped and Ag-doped (x = 0.2) samples are shown in Fig. 4(a and b), respectively. The trace mark of the Vickers hardness of Agdoped (x = 0.2) sample is shown in Fig. 4(c). The microstructure of the non-doped sample is composed of significantly coarse grains about 10–20 m in size and it leads to low density. On the other hand, the microstructure of the Agdoped sample is composed of significantly fine grains and Ag particles around 10 m in size that precipitate on the grain boundary and it leads to high density. The hardness of the samples was measured at room temperature, and repeated seven times for a given sample; applied loads were chosen to be 0.245, 0.98 and 2.94 N, used 15 s for the loading time. The relationship between the Vickers hardness and applied load is shown in Fig. 5. The Vickers hardness decreases with increasing applied load, indicating a typical load dependence. The hardness provides the information of the resistance of a material to plastic deformation.
Fig. 6. Relationship between the Vickers hardness, HV , and Young’s modulus, E, of Na1.5 Co2−x Agx O4 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5).
The mechanical properties of Na1.5 Co2−x Agx O4 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) were measured at room temperature. The shear modulus and Young’s modulus increase with increasing Ag content. The Young’s moduli for non-doped (x = 0) and Ag-doped (x = 0.1–0.5) samples are around 50 and 88 GPa, respectively. The value of HV /E equals 0.003 for all samples, which corresponds to the metallic characteristics. Acknowledgments This work had financial support from the Rajabhat Sakon Nakhon University, the National Research Council of Thailand and Faculty of Science, Khon Kaen University, Thailand. References [1] M. Von Jansen, R. Hoppe, Z. Anorg. Allg. Chem. 408 (1974) 104. [2] H. Nagatsugawa, K. Nagasawa, J. Solid State Chem. 177 (2004) 1137. [3] M. Ohtaki, Y. Nojiri, E. Maeda, Proceedings of the 19th International Conference on Thermoelectrics (ICT2000), Babrow, Wales, 2000, p. 190. [4] I. Terasaki, Y. Sasago, K. Uchinokura, Phys. Rev. B 56 (1997) R12 685. [5] S. Li, R. Funahashi, I. Matsubara, S. Sodeoka, Mater. Res. Bull. 35 (2003) 2371. [6] A. Mrotzek, E. M¨uller, J. Plewa, H. Altenburg, 22nd International Conference on Thermoelectrics, 2003, p. 219. [7] I. Terasaki, Proceedings of the 19th International Conference on Thermoelectrics (ICT2000), Cardiff, UK, 2000, p. 20. [8] I. Terasaki, I. Tsukada, Y. Iguchi, Phys. Rev. B 65 (2002) 195106. [9] K. Kurosaki, H. Muta, M. Uno, S. Yamanaka, J. Alloys Compd. 315 (2001) 234. [10] M. Ito, T. Nagira, D. Furumato, S. Katsuyama, H. Nagai, Scripta Mater. 48 (2003) 403. [11] K. Yamada, S. Yamanaka, M. Katsura, Tech. Rep. Osaka Univ. 47 (1997) 181. [12] M. Fukuhara, I. Yamauchi, J. Mater. Sci. 28 (1993) 4681. [13] H. Inaba, T. Yamamoto, Netsu Sokutei 10 (1983) 132. [14] K. Tanaka, H. Koguchi, T. Mura, Int. J. Eng. Sci. 27 (1989) 11. [15] S. Yamamoto, T. Tanabe, Atarashii Zairyoukagaku, Showadou Kyoto (1990). [16] S. Yamanaka, K. Yamada, T. Tsuzaki, T. Iguchi, M. Katsura, Y. Hoshino, W. Saiki, J. Alloys Compd. 271 (1998) 549.