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Electrical properties of Al–ZrMgMo3O12 with controllable thermal expansion
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CERAMICS INTERNATIONAL
Ceramics International 41 (2015) 2361–2366 www.elsevier.com/locate/ceramint
Electrical properties of Al–ZrMgMo3O12 with controllable thermal expansion Xiao Xiao, Wenjin Zhou, Xiansheng Liu, Mingju Chaon, Yucheng Li, Niu Zhang, Yaming Liu, Yuxiang Li, Dongsheng Feng, Erjun Liang Key Laboratory of Materials Physics of Ministry of Education of China, School of Physical Science and Engineering, Zhengzhou University, Zhengzhou 450052, China Received 17 August 2014; received in revised form 29 September 2014; accepted 8 October 2014 Available online 16 October 2014
Abstract Al–ZrMgMo3O12 composites samples were prepared by the solid state method. The crystal phase and microstructure of the composites were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The XRD patterns show two phases of Al and ZrMgMo3O12. The SEM images show the microstructures of the ceramics containing particles of ZrMgMo3O12 and blocks of Al–ZrMgMo3O12. The ratio of particles to blocks changed on adjusting the mass ratio of Al:ZrMgMo3O12, consequently changing the coefficient of thermal expansion (CTE) and the electrical properties of the composites. When the mass ratio of Al:ZrMgMo3O12 is 2:8 the CTE is 0.77 10 6 K 1 (room temperature to 673 K), which corresponds to a near-zero thermal expansion dielectric material. For Al–ZrMgMo3O12 composite with mass ratio of 6:4, the CTE (8.72 10 6 K 1) is reduced to one-third of that of Al (23.79 10 6 K 1) and the impedance is decreased to 7.68 Ω. The conductivity change of Al–ZrMgMo3O12 composites can be attributed to the conductive percolation phenomenon, resulting from the presence of large mixed blocks of Al–ZrMgMo3O12 among the particles of ZrMgMo3O12. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Negative thermal expansion; Al–ZrMgMo3O12 ceramics; Coefficient of thermal expansion; Conductivity
1. Introduction Aluminum alloys are widely used in aerospace, electronics, construction, transportation, and other fields because of their excellent properties, such as good plasticity, large ductility, low density, simple processibility, fine electrical property and high thermal conductivity. However, thermal mismatches and microcracks could result in poor performance or even device failure when Al is combined with other materials due to high coefficient of thermal expansion (CTE ¼ 23 10 6 K 1) of Al [1]. The combination of Al and ceramics forming cermets could present high hardness and low CTE taking advantage of plasticity of Al and high hardness of ceramics [2,3]. In Refs. [4,5], SiC and Al were combined to obtain a low-CTE n
Corresponding author. Tel.: þ86 371 67767836; fax: þ 86 371 67767758. E-mail addresses: [email protected], [email protected] (M. Chao). http://dx.doi.org/10.1016/j.ceramint.2014.10.048 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
material. The CTE of SiC–Al composite (13.40 10 6 K 1) is lower than that of Al; however, it still does not meet the requirements for the low thermal expansion applications. The combination of negative thermal expansion (NTE) materials and Al is an effective method to obtain materials with controllable or near-zero thermal expansion. However, there are a variety of problems to be solved when NTE materials are combined with Al due to the performance limitations of the reported NTE materials (such as ZrW2O8, ZrV2O7, A2Mo3O12 and A2W3O12 series (A= Y, Sc, etc.). For example ZrW2O8 is metastable at room temperature (RT) and undergoes a reversible order–disorder phase transition at about 440 K and an irreversible cubic–orthorhombic phase transition at pressure 4 0.21 G Pa, which results in the instability of Al–ZrW2O8 composites [2,6–10]. When ZrV2O7 is combined with Al, they react with each other and form AlVO3, AlVO4 and Zr1 xAlxV2O7 with cubic structure [11]; therefore, it is difficult to produce thermostable composites.
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The NTE materials among molybdates (A2Mo3O12) and tungstates (A2W3O12), such as Y2Mo3O12 (CTE ¼ 9.36 10 6 K 1) and Y2W3O12 (CTE ¼ 7.34 10 6 K 1), have a large NTE coefficient in the temperature range of RT– 1200 K [12–15] and could be combined with Al to obtain low thermal expansion composites. However Y2Mo3O12 and Y2W3O12 are strongly hygroscopic, which limits their applications [16–19]. The NTE material ZrMgMo3O12 ( 3.73 10 6 K 1, RT–1000 K) is unhygroscopic and stable, with the following advantages: Low-cost raw materials, easy preparation, and low sintering temperature, which make it possible to be a promising candidate in wide applications [20]. Therefore, in this paper, we prepared Al–ZrMgMo3O12 composites and the adjustable CTE and conductivity were investigated.
2. Experiment details All of the experimental raw materials are commercially available: MoO3 ( Z 99% purity), ZrO2 ( Z99% purity), MgO (Z 99% purity), and metallic Al (Z 99% purity, 200 mesh) powders. ZrMgMo3O12 is prepared using the solid state method. ZrO2, MgO, and MoO3 were weighed and mixed in molar ratio 1:1:3. A powdered form of the mixture (ground for 2 h) was obtained using an agate mortar. The mixed powder was placed into an AY-BF-555-125 box-type furnace. The temperature was then increased from RT to 1073 K at 5 K/min; the mixed powder was heated for 4 h and then cooled naturally to obtain ZrMgMo3O12. Aluminum and ZrMgMo3O12 powders were mixed according to the following mass ratios: 2:8, 4:6, 5:5, 6:4, and 8:2. The mixed powders were ground for 2 h. A 769YP-15A powder tableting machine (200 MPa, 5 min) was used to compress the mixed powders into cylindrical samples (∅10 5 mm), which were then placed into an AY-BF-555-125 box-type furnace for sintering. The temperature was increased from RT to 973 K at 5 K/min; the maximum temperature was maintained for 4 h. The sintered products were cooled naturally to obtain Al–ZrMgMo3O12 composites. The Al–ZrMgMo3O12 composites were analyzed with the Bruker D8 Advance X-ray diffractometer (Cu target, Kα lines, 1.5406 Å wavelength, 10–801 scanning range). A ;field-emission scanning electron microscope (FE-SEM, Model 15 kV JSM6700F, JEOL, Japan) and a SEM with an INCA-ENERGY spectrometer (EDS, ISIS400) were used to observe the microstructure and analyze the chemical composition of the samples. Differential scanning calorimetry (DSC, Ulvac Sinku-Riko DSC Model 1500M/L) was performed in the temperature range from 293 to 873 K at 10 K/min heating/cooling rates. The linear CTE was measured using a dilatometer (LINSEIS DIL L76). Impedance spectroscopy was conducted using an RST5200 electrochemical workstation. The electrode of the samples (discs with a diameter of ∅10 3 mm) was prepared before testing; the disc ends were covered with a silver adhesive. The samples were sintered at 873 K for 1 h after the silver was glued [21].
3. Results and discussion 3.1. Analysis of phase and thermal stability Fig. 1 shows the XRD patterns of Al, ZrMgMo3O12, and Al–ZrMgMo3O12 composites with different mass ratios. Diffraction peaks of only Al and ZrMgMo3O12 are found; this indicates that after mixing Al and ZrMgMo3O12 thoroughly by adding, grinding over time and sintering at 973 K, there are no substitution reactions between Al and ZrMgMo3O12 to form novel phases. That is, a stable composite of Al–ZrMgMo3O12 is obtained. Fig. 2 shows the DSC curves of Al, ZrMgMo3O12, and Al–ZrMgMo3O12 composites with different mass ratios. The curves of ZrMgMo3O12, Al, and the Al–ZrMgMo3O12 composites show no exothermic peaks in the temperature range of RT–1200 K. An endothermic peak is found around 933 K; moreover the height of the peak increases with the increase of Al content, which is attributed to the melting of Al. The results indicate that the Al–ZrMgMo3O12 composite materials with thermal stability could be widely applied in the temperature range of RT–900 K. 3.2. Analysis of microstructure and composition Fig. 3 shows the SEM image of an Al–ZrMgMo3O12 sample (mass ratio of 4:6). It is found that the microstructure of the
Fig. 1. XRD patterns of Al, ZrMgMo3O12 and Al–ZrMgMo3O12 composite with mass ratios of 2:8, 4:6, 5:5, 6:4, and 8:2.
Fig. 2. DSC curves of Al, ZrMgMo3O12 and Al–ZrMgMo3O12 composites.
X. Xiao et al. / Ceramics International 41 (2015) 2361–2366
Fig. 3. Microstructure of Al–ZrMgMo3O12 composites with mass ratio of 4:6.
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sample is mainly composed of particles with the size about 1 μm (white particles). In addition, some blocks with the size of 10–150 μm are dispersed in those particles (dark gray blocks). Fig. 4a is a magnified SEM image corresponding to the rectangular portion in Fig. 3, and Fig. 4b and c corresponds to the EDS spectra of P1 and P2, respectively. It is found that the elemental compositions of the white particles (P1) in Fig. 4b are mainly Zr, Mg, Mo, and O and these particles could be ascribed to the ZrMgMo3O12 based on the XRD analysis (see Fig. 1). Fig. 4c shows that the dark gray blocky phase (P2) contains Al element besides the elements Zr, Mg, Mo, and O. It is reasonable that P2 is a mixture of ZrMgMo3O12 and Al because no other phases were found in the XRD patterns (Fig. 1). Fig. 5a–d shows the microstructure of the Al–ZrMgMo3O12 samples with mass ratios of 2:8, 6:4, 8:2 and 10:1, in order. The microstructure of the sample in Fig. 5a is composed of whitish particles, whereas the microstructure of the sample in Fig. 5b
Fig. 4. (a) Magnified SEM image of Al–ZrMgMo3O12 composites with mass ratio of 4:6; (b, c) EDS spectra corresponding to P1 and P2 in Fig. 3.
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Fig. 5. SEM images of Al–ZrMgMo3O12 composites with different mass ratios of (a) 2:8, (b) 6:4, (c) 8:2 and (d) 10:1.
is composed of numerous dark gray blocks and a small amount of white particles. The interspaces among particles in Fig. 5b are smaller than those in Fig. 5a. With increasing mass ratio from 2:8 to 10:1, the surface morphology changes obviously. From Figs. 3 and 5a–d, it is found that the amount of dark gray blocks in the microstructure of Al–ZrMgMo3O12 composites gradually increases with the increase of Al content. When the ceramic structure of ZrMgMo3O12 and the metal structure of Al are sintered together, in where the sintering temperature at 973 K is higher than the melting point of Al (i.e. 933 K), the Al melts and fills the interspaces between the ZrMgMo3O12 granules. During the cooling stage, the molten Al integrates with ZrMgMo3O12 particles thus forming Al–ZrMgMo3O12 condensed blocks. Because of its NTE characteristics, ZrMgMo3O12 can closely interact with Al when they condense together. This result can be confirmed from the relative density of the samples with different Al mass fractions in Fig. 6. It is evident that the relative density increases with the increase of Al content in the composites. The result can be interpreted as follows: Al continuously fills the interspaces among the ZrMgMo3O12 particles because of the melting of Al during the sintering process of Al and ZrMgMo3O12 composites and, with the increase of Al content, an increasing amount of Al fills the interspaces to form an increasing number of dark gray blocky phase, decreasing the interspaces among the particles of the sample tissue, thereby increasing the relative density of the sample. 3.3. Thermal expansion Fig. 7 shows the relative length change of Al, ZrMgMo3O12, and Al–ZrMgMo3O12 composites with different mass ratios. ZrMgMo3O12, which is not hygroscopic, presents a stable NTE property.
Fig. 6. Relative density of Al–ZrMgMo3O12 samples with different mass percents of Al.
Fig. 7. Relative length change of Al, ZrMgMo3O12 and Al–ZrMgMo3O12 composites with different mass ratios.
Table 1 shows the linear CTEs of the samples calculated based on the linear portion in Fig. 7. It is found that the CTEs of Al–ZrMgMo3O12 composites increase from negative to
X. Xiao et al. / Ceramics International 41 (2015) 2361–2366
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Table 1 CTEs of the samples with different mass ratios. Mass ratio (Al:ZrMgMo3O12)
ZrMgMo3O12
1:15
2:8
4:6
6:4
8:2
Al
CTE (10 6 K 1) Temperature range (K)
3.44 300–673
0.95 300–740
0.77 300–820
3.78 300–830
8.72 300–850
19.46 300–870
23.79 300–850
Fig. 8. Impedance curves versus frequency of ZrMgMo3O12 (a), Al–ZrMgMo3O12 samples with different mass ratios ((b) 2:8, (c) 2.5:7.5, (d) 3:7, (e) 3.5:6.5, (f) 4:6, (g) 5:5, (h) 6:4, (i) 8:2), and Al (j).
Table 2 Impedances of Al–ZrMgMo3O12 composites (mass ratios of 4:6, 5:5, 6:4, 8:2) and Al. Mass ratio (Al:ZrMgMo3O12)
4:6
5:5
6:4
8:2
Al
Impedance (Ω)
960.42
23.04
7.68
1.40
0.66
positive with the increase of the mass ratio Al:ZrMgMo3O12. Thus, by adjusting the mass ratio of Al:ZrMgMo3O12, the desired CTE can be obtained. When the mass ratio of Al: ZrMgMo3O12 is 2:8 the CTE of the composite is 0.77 10 6 K 1, which is close to zero thermal expansion. 3.4. Electrical properties Fig. 8 shows the impedance curves of Al, Al–ZrMgMo3O12, and ZrMgMo3O12 with the change in frequency. For the pure ZrMgMo3O12 phase (curve a) and Al–ZrMgMo3O12 samples with mass ratios of 2:8, 2.5:7.5, and 3:7 (curves b, c, and d, respectively) their impedances decrease as frequency increases, which presents capacitive property. By contrast, for the pure Al phase (curve j) and the Al–ZrMgMo3O12 samples with mass ratios of 4:6, 3.5:6.5, 6:4, and 8:2 (curves e, f, g, h, and i, respectively), the impedance does not change when the frequency increases, indicating a low resistance characteristic. These differences could be attributed to the conductive percolation phenomena discussed in literatures [22,23]. For low contents of Al in Al–ZrMgMo3O12 composites the Al particles with high electrical conductivity irregularly disperse in insulating ZrMgMo3O12, preventing the Al particles from connecting with each other that can form a conductive path; as
a result, the Al–ZrMgMo3O12 composite shows capacitance characteristics. When the Al content reaches a threshold the amount of Al particles increases enough to connect with each other to form a conductive path, and the electrical conductivity of Al–ZrMgMo3O12 composite increases rapidly, leading to nonlinear mutation according to a percolation threshold. The value of percolation threshold could range from 3.5:6.5 to 4:6 (Al:ZrMgMo3O12) in Fig. 8. When the Al content continually increases, the interaction area among Al particles becomes larger; thus, the conductivity of Al–ZrMgMo3O12 composite increases remarkably. Table 2 (Al:ZrMgMo3O12 Z4:6) shows that as Al content increases the impedance of Al–ZrMgMo3O12 composites decreases, indicating that the conductivity of the material increases significantly with the increase of Al content. The magnitude of decrease is from about 40 fold (960.42 Ω (4:6)–23.04 Ω (5:5)) to 3 fold (23.04 Ω (5:5)–7.68 Ω (6:4)). 4. Conclusions (1) Al–ZrMgMo3O12 composites were prepared using the solid state method. The CTE and electrical properties of the composites can be tailored by adjusting the mass ratio of Al:ZrMgMo3O12. (2) The CTE of Al–ZrMgMo3O12 composites with mass ratio of Al:ZrMgMo3O12 2:8 is 0.77 10 6 K 1. The composites belong to a near-zero thermal expansion material with high impedance. (3) When the mass ratio of Al:ZrMgMo3O12 is 6:4 the CTE (8.72 10 6 K 1) of Al–ZrMgMo3O12 is reduced to onethird of Al (23.79 10 6 K 1), and Al–ZrMgMo3O12 exhibits a low resistance. This result could be attributed to the conductive percolation phenomena produced by the mixed block phase structure of Al–ZrMgMo3O12 composite materials.
Acknowledgment The authors acknowledge the financial support given by the key Natural Science Project of Henan Province (Grant no. 142102210073). References [1] P. Sebo, S. Kavecky, P. Stefanik, Wettability of zirconia-coated carbon by aluminium, Theor. Appl. Fract. Mech. 13 (1994) 592–593. [2] Y. Wu, M.L. Wang, Z. Chen, N.H. Ma, H.W. Wang, The effect of phase transformation on the thermal expansion property in Al/ZrW2O8 composites, J. Mater. Sci. 48 (2013) 2928–2933.
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[3] K. Aslantas, I. Ucun, The performance of ceramic and cermet cutting tools for the machining of austempered ductile iron, Int. J. Adv. Manuf. Technol. 41 (2009) 642–650. [4] S. Elomari, R. Boukhili, C.S. Marchi, A. Mortensen, D.J. Lloyd, Thermal expansion responses of pressure infiltrated SiC/Al metal–matrix composites, J. Mater. Sci. 32 (1997) 2131–2140. [5] B.G. Kim, S.L. Dong, S.D. Park, Effects of thermal processing on thermal expansion coefficient of a 50 vol% SiCp/Al composite, Mater. Chem. Phys. 72 (2001) 42–47. [6] T.A. Mary, J.S.O. Evans, T. Vogt, A.W. Sleight, Negative thermal expansion from 0.3 to 1050 K in ZrW2O8, Science 272 (1996) 90–92. [7] J.S.O. Evans, Z. Hu, J.D. Jorgensen, D.N. Argyriou, S. Short, A.W. Sleight, Compressibility, phase transitions, and oxygen migration in zirconium tungstate, ZrW2O8, Science 275 (1997) 61–65. [8] C.A. Perottoni, J.A.H da. Jornada, Pressure-induced amorphization and negative thermal expansion in ZrW2O8, Science 280 (1998) 886–889. [9] E.J. Liang, S.H. Wang, T.A. Wu, M.J. Chao, B. Yuan, W.F. Zhang, Raman spectroscopic study on the structure, phase transition and restoration of zirconium tungstate blocks synthesized with a CO2 laser, J. Raman Spectrosc. 38 (2007) 1186–1192. [10] A. Matsumoto, K. Kobayashi, T. Nishio, K. Ozaki, Fabrication and thermal expansion of Al–ZrW2O8 composites by pulse current sintering process, Mater. Sci. Forum 426–432 (2003) 2279–2284. [11] D.Q. Yun, Q.C. Gu, X.F. Wang, Synthesis and properties of NTE ZrV2O7 and Al/ZrV2O7 composites, Acta. Mater. Compos. Sin. 22 (2005) 25–30. [12] S. Sumithra, A.M. Umarji, Hygroscopicity and bulk thermal expansion in Y2W3O12, Mater. Res. Bull. 40 (2005) 167–176. [13] J.H. Koh, E. Sorge, T.C. Wen, D.K. Shetty, Thermal expansion behaviors of yttrium tungstates in the WO3–Y2O3 system, Ceram. Int. 39 (2013) 8421–8427.
[14] S. Sumithra, A.M. Umarji, Dilatometric studies of Y2W3O12 with added Al2O3, J. Chem. Sci. 115 (2003) 695–701. [15] C. Lind, Two decades of negative thermal expansion research: where do we stand? Materials 5 (2012) 1125–1154. [16] Q.J. Li, B.H. Yuan, W.B. Song, E.J. Liang, B. Yuan, The phase transition, hygroscopicity, and thermal expansion properties of Yb2 xAlxMo3O12, Chin. Phys. B 21 (2012) 046501. [17] J.S.O. Evans, T.A. Mary, A.W. Sleight, Negative thermal expansion in a large molybdate and tungstate family, J. Solid State Chem. 133 (1997) 580–583. [18] B.A. Marinkovic, P.M. Jardim, R.R. Avillez, F. Rizzo, Negative thermal expansion in Y2Mo3O12, Solid State Sci. 7 (2005) 1377–1383. [19] E.J. Liang, Nagative thermal expansion material and their application: a survey of recent patents, Recent Pat. Mater. Sci. 3 (2010) 106–128. [20] W.B. Song, E.J. Liang, X.S. Liu, Z.Y. Li, B.H. Yuan, J.Q. Wang, A negative thermal expansion material of ZrMgMo3O12, Chin. Phys. Lett. 30 (2013) 126502. [21] X.S. Liu, F.X. Cheng, J.Q. Wang, W.B. Song, B.H. Yuan, E.J. Liang, The control of thermal expansion and impedance of Al–Zr2(WO4)(PO4)2 nano-cermets for near-zero-strain Al alloy and fine electrical components, J. Alloys Compd. 553 (2013) 1–7. [22] C. Zhang, M.Q. Chen, C.A. Ma, Research progress of reducing the percolation threshold for conductive composites, Eng. Plast. Appl 37 (2009) 76–79. [23] Y.C. Wang, Z.Y. Fu, The percolation phenomenon in conduction and isolator composite ceramics, J. Wuhan Univ. Technol. 23 (2001) 29–32.
CERAMICS INTERNATIONAL
Ceramics International 41 (2015) 2361–2366 www.elsevier.com/locate/ceramint
Electrical properties of Al–ZrMgMo3O12 with controllable thermal expansion Xiao Xiao, Wenjin Zhou, Xiansheng Liu, Mingju Chaon, Yucheng Li, Niu Zhang, Yaming Liu, Yuxiang Li, Dongsheng Feng, Erjun Liang Key Laboratory of Materials Physics of Ministry of Education of China, School of Physical Science and Engineering, Zhengzhou University, Zhengzhou 450052, China Received 17 August 2014; received in revised form 29 September 2014; accepted 8 October 2014 Available online 16 October 2014
Abstract Al–ZrMgMo3O12 composites samples were prepared by the solid state method. The crystal phase and microstructure of the composites were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The XRD patterns show two phases of Al and ZrMgMo3O12. The SEM images show the microstructures of the ceramics containing particles of ZrMgMo3O12 and blocks of Al–ZrMgMo3O12. The ratio of particles to blocks changed on adjusting the mass ratio of Al:ZrMgMo3O12, consequently changing the coefficient of thermal expansion (CTE) and the electrical properties of the composites. When the mass ratio of Al:ZrMgMo3O12 is 2:8 the CTE is 0.77 10 6 K 1 (room temperature to 673 K), which corresponds to a near-zero thermal expansion dielectric material. For Al–ZrMgMo3O12 composite with mass ratio of 6:4, the CTE (8.72 10 6 K 1) is reduced to one-third of that of Al (23.79 10 6 K 1) and the impedance is decreased to 7.68 Ω. The conductivity change of Al–ZrMgMo3O12 composites can be attributed to the conductive percolation phenomenon, resulting from the presence of large mixed blocks of Al–ZrMgMo3O12 among the particles of ZrMgMo3O12. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Negative thermal expansion; Al–ZrMgMo3O12 ceramics; Coefficient of thermal expansion; Conductivity
1. Introduction Aluminum alloys are widely used in aerospace, electronics, construction, transportation, and other fields because of their excellent properties, such as good plasticity, large ductility, low density, simple processibility, fine electrical property and high thermal conductivity. However, thermal mismatches and microcracks could result in poor performance or even device failure when Al is combined with other materials due to high coefficient of thermal expansion (CTE ¼ 23 10 6 K 1) of Al [1]. The combination of Al and ceramics forming cermets could present high hardness and low CTE taking advantage of plasticity of Al and high hardness of ceramics [2,3]. In Refs. [4,5], SiC and Al were combined to obtain a low-CTE n
Corresponding author. Tel.: þ86 371 67767836; fax: þ 86 371 67767758. E-mail addresses: [email protected], [email protected] (M. Chao). http://dx.doi.org/10.1016/j.ceramint.2014.10.048 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
material. The CTE of SiC–Al composite (13.40 10 6 K 1) is lower than that of Al; however, it still does not meet the requirements for the low thermal expansion applications. The combination of negative thermal expansion (NTE) materials and Al is an effective method to obtain materials with controllable or near-zero thermal expansion. However, there are a variety of problems to be solved when NTE materials are combined with Al due to the performance limitations of the reported NTE materials (such as ZrW2O8, ZrV2O7, A2Mo3O12 and A2W3O12 series (A= Y, Sc, etc.). For example ZrW2O8 is metastable at room temperature (RT) and undergoes a reversible order–disorder phase transition at about 440 K and an irreversible cubic–orthorhombic phase transition at pressure 4 0.21 G Pa, which results in the instability of Al–ZrW2O8 composites [2,6–10]. When ZrV2O7 is combined with Al, they react with each other and form AlVO3, AlVO4 and Zr1 xAlxV2O7 with cubic structure [11]; therefore, it is difficult to produce thermostable composites.
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The NTE materials among molybdates (A2Mo3O12) and tungstates (A2W3O12), such as Y2Mo3O12 (CTE ¼ 9.36 10 6 K 1) and Y2W3O12 (CTE ¼ 7.34 10 6 K 1), have a large NTE coefficient in the temperature range of RT– 1200 K [12–15] and could be combined with Al to obtain low thermal expansion composites. However Y2Mo3O12 and Y2W3O12 are strongly hygroscopic, which limits their applications [16–19]. The NTE material ZrMgMo3O12 ( 3.73 10 6 K 1, RT–1000 K) is unhygroscopic and stable, with the following advantages: Low-cost raw materials, easy preparation, and low sintering temperature, which make it possible to be a promising candidate in wide applications [20]. Therefore, in this paper, we prepared Al–ZrMgMo3O12 composites and the adjustable CTE and conductivity were investigated.
2. Experiment details All of the experimental raw materials are commercially available: MoO3 ( Z 99% purity), ZrO2 ( Z99% purity), MgO (Z 99% purity), and metallic Al (Z 99% purity, 200 mesh) powders. ZrMgMo3O12 is prepared using the solid state method. ZrO2, MgO, and MoO3 were weighed and mixed in molar ratio 1:1:3. A powdered form of the mixture (ground for 2 h) was obtained using an agate mortar. The mixed powder was placed into an AY-BF-555-125 box-type furnace. The temperature was then increased from RT to 1073 K at 5 K/min; the mixed powder was heated for 4 h and then cooled naturally to obtain ZrMgMo3O12. Aluminum and ZrMgMo3O12 powders were mixed according to the following mass ratios: 2:8, 4:6, 5:5, 6:4, and 8:2. The mixed powders were ground for 2 h. A 769YP-15A powder tableting machine (200 MPa, 5 min) was used to compress the mixed powders into cylindrical samples (∅10 5 mm), which were then placed into an AY-BF-555-125 box-type furnace for sintering. The temperature was increased from RT to 973 K at 5 K/min; the maximum temperature was maintained for 4 h. The sintered products were cooled naturally to obtain Al–ZrMgMo3O12 composites. The Al–ZrMgMo3O12 composites were analyzed with the Bruker D8 Advance X-ray diffractometer (Cu target, Kα lines, 1.5406 Å wavelength, 10–801 scanning range). A ;field-emission scanning electron microscope (FE-SEM, Model 15 kV JSM6700F, JEOL, Japan) and a SEM with an INCA-ENERGY spectrometer (EDS, ISIS400) were used to observe the microstructure and analyze the chemical composition of the samples. Differential scanning calorimetry (DSC, Ulvac Sinku-Riko DSC Model 1500M/L) was performed in the temperature range from 293 to 873 K at 10 K/min heating/cooling rates. The linear CTE was measured using a dilatometer (LINSEIS DIL L76). Impedance spectroscopy was conducted using an RST5200 electrochemical workstation. The electrode of the samples (discs with a diameter of ∅10 3 mm) was prepared before testing; the disc ends were covered with a silver adhesive. The samples were sintered at 873 K for 1 h after the silver was glued [21].
3. Results and discussion 3.1. Analysis of phase and thermal stability Fig. 1 shows the XRD patterns of Al, ZrMgMo3O12, and Al–ZrMgMo3O12 composites with different mass ratios. Diffraction peaks of only Al and ZrMgMo3O12 are found; this indicates that after mixing Al and ZrMgMo3O12 thoroughly by adding, grinding over time and sintering at 973 K, there are no substitution reactions between Al and ZrMgMo3O12 to form novel phases. That is, a stable composite of Al–ZrMgMo3O12 is obtained. Fig. 2 shows the DSC curves of Al, ZrMgMo3O12, and Al–ZrMgMo3O12 composites with different mass ratios. The curves of ZrMgMo3O12, Al, and the Al–ZrMgMo3O12 composites show no exothermic peaks in the temperature range of RT–1200 K. An endothermic peak is found around 933 K; moreover the height of the peak increases with the increase of Al content, which is attributed to the melting of Al. The results indicate that the Al–ZrMgMo3O12 composite materials with thermal stability could be widely applied in the temperature range of RT–900 K. 3.2. Analysis of microstructure and composition Fig. 3 shows the SEM image of an Al–ZrMgMo3O12 sample (mass ratio of 4:6). It is found that the microstructure of the
Fig. 1. XRD patterns of Al, ZrMgMo3O12 and Al–ZrMgMo3O12 composite with mass ratios of 2:8, 4:6, 5:5, 6:4, and 8:2.
Fig. 2. DSC curves of Al, ZrMgMo3O12 and Al–ZrMgMo3O12 composites.
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Fig. 3. Microstructure of Al–ZrMgMo3O12 composites with mass ratio of 4:6.
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sample is mainly composed of particles with the size about 1 μm (white particles). In addition, some blocks with the size of 10–150 μm are dispersed in those particles (dark gray blocks). Fig. 4a is a magnified SEM image corresponding to the rectangular portion in Fig. 3, and Fig. 4b and c corresponds to the EDS spectra of P1 and P2, respectively. It is found that the elemental compositions of the white particles (P1) in Fig. 4b are mainly Zr, Mg, Mo, and O and these particles could be ascribed to the ZrMgMo3O12 based on the XRD analysis (see Fig. 1). Fig. 4c shows that the dark gray blocky phase (P2) contains Al element besides the elements Zr, Mg, Mo, and O. It is reasonable that P2 is a mixture of ZrMgMo3O12 and Al because no other phases were found in the XRD patterns (Fig. 1). Fig. 5a–d shows the microstructure of the Al–ZrMgMo3O12 samples with mass ratios of 2:8, 6:4, 8:2 and 10:1, in order. The microstructure of the sample in Fig. 5a is composed of whitish particles, whereas the microstructure of the sample in Fig. 5b
Fig. 4. (a) Magnified SEM image of Al–ZrMgMo3O12 composites with mass ratio of 4:6; (b, c) EDS spectra corresponding to P1 and P2 in Fig. 3.
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Fig. 5. SEM images of Al–ZrMgMo3O12 composites with different mass ratios of (a) 2:8, (b) 6:4, (c) 8:2 and (d) 10:1.
is composed of numerous dark gray blocks and a small amount of white particles. The interspaces among particles in Fig. 5b are smaller than those in Fig. 5a. With increasing mass ratio from 2:8 to 10:1, the surface morphology changes obviously. From Figs. 3 and 5a–d, it is found that the amount of dark gray blocks in the microstructure of Al–ZrMgMo3O12 composites gradually increases with the increase of Al content. When the ceramic structure of ZrMgMo3O12 and the metal structure of Al are sintered together, in where the sintering temperature at 973 K is higher than the melting point of Al (i.e. 933 K), the Al melts and fills the interspaces between the ZrMgMo3O12 granules. During the cooling stage, the molten Al integrates with ZrMgMo3O12 particles thus forming Al–ZrMgMo3O12 condensed blocks. Because of its NTE characteristics, ZrMgMo3O12 can closely interact with Al when they condense together. This result can be confirmed from the relative density of the samples with different Al mass fractions in Fig. 6. It is evident that the relative density increases with the increase of Al content in the composites. The result can be interpreted as follows: Al continuously fills the interspaces among the ZrMgMo3O12 particles because of the melting of Al during the sintering process of Al and ZrMgMo3O12 composites and, with the increase of Al content, an increasing amount of Al fills the interspaces to form an increasing number of dark gray blocky phase, decreasing the interspaces among the particles of the sample tissue, thereby increasing the relative density of the sample. 3.3. Thermal expansion Fig. 7 shows the relative length change of Al, ZrMgMo3O12, and Al–ZrMgMo3O12 composites with different mass ratios. ZrMgMo3O12, which is not hygroscopic, presents a stable NTE property.
Fig. 6. Relative density of Al–ZrMgMo3O12 samples with different mass percents of Al.
Fig. 7. Relative length change of Al, ZrMgMo3O12 and Al–ZrMgMo3O12 composites with different mass ratios.
Table 1 shows the linear CTEs of the samples calculated based on the linear portion in Fig. 7. It is found that the CTEs of Al–ZrMgMo3O12 composites increase from negative to
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Table 1 CTEs of the samples with different mass ratios. Mass ratio (Al:ZrMgMo3O12)
ZrMgMo3O12
1:15
2:8
4:6
6:4
8:2
Al
CTE (10 6 K 1) Temperature range (K)
3.44 300–673
0.95 300–740
0.77 300–820
3.78 300–830
8.72 300–850
19.46 300–870
23.79 300–850
Fig. 8. Impedance curves versus frequency of ZrMgMo3O12 (a), Al–ZrMgMo3O12 samples with different mass ratios ((b) 2:8, (c) 2.5:7.5, (d) 3:7, (e) 3.5:6.5, (f) 4:6, (g) 5:5, (h) 6:4, (i) 8:2), and Al (j).
Table 2 Impedances of Al–ZrMgMo3O12 composites (mass ratios of 4:6, 5:5, 6:4, 8:2) and Al. Mass ratio (Al:ZrMgMo3O12)
4:6
5:5
6:4
8:2
Al
Impedance (Ω)
960.42
23.04
7.68
1.40
0.66
positive with the increase of the mass ratio Al:ZrMgMo3O12. Thus, by adjusting the mass ratio of Al:ZrMgMo3O12, the desired CTE can be obtained. When the mass ratio of Al: ZrMgMo3O12 is 2:8 the CTE of the composite is 0.77 10 6 K 1, which is close to zero thermal expansion. 3.4. Electrical properties Fig. 8 shows the impedance curves of Al, Al–ZrMgMo3O12, and ZrMgMo3O12 with the change in frequency. For the pure ZrMgMo3O12 phase (curve a) and Al–ZrMgMo3O12 samples with mass ratios of 2:8, 2.5:7.5, and 3:7 (curves b, c, and d, respectively) their impedances decrease as frequency increases, which presents capacitive property. By contrast, for the pure Al phase (curve j) and the Al–ZrMgMo3O12 samples with mass ratios of 4:6, 3.5:6.5, 6:4, and 8:2 (curves e, f, g, h, and i, respectively), the impedance does not change when the frequency increases, indicating a low resistance characteristic. These differences could be attributed to the conductive percolation phenomena discussed in literatures [22,23]. For low contents of Al in Al–ZrMgMo3O12 composites the Al particles with high electrical conductivity irregularly disperse in insulating ZrMgMo3O12, preventing the Al particles from connecting with each other that can form a conductive path; as
a result, the Al–ZrMgMo3O12 composite shows capacitance characteristics. When the Al content reaches a threshold the amount of Al particles increases enough to connect with each other to form a conductive path, and the electrical conductivity of Al–ZrMgMo3O12 composite increases rapidly, leading to nonlinear mutation according to a percolation threshold. The value of percolation threshold could range from 3.5:6.5 to 4:6 (Al:ZrMgMo3O12) in Fig. 8. When the Al content continually increases, the interaction area among Al particles becomes larger; thus, the conductivity of Al–ZrMgMo3O12 composite increases remarkably. Table 2 (Al:ZrMgMo3O12 Z4:6) shows that as Al content increases the impedance of Al–ZrMgMo3O12 composites decreases, indicating that the conductivity of the material increases significantly with the increase of Al content. The magnitude of decrease is from about 40 fold (960.42 Ω (4:6)–23.04 Ω (5:5)) to 3 fold (23.04 Ω (5:5)–7.68 Ω (6:4)). 4. Conclusions (1) Al–ZrMgMo3O12 composites were prepared using the solid state method. The CTE and electrical properties of the composites can be tailored by adjusting the mass ratio of Al:ZrMgMo3O12. (2) The CTE of Al–ZrMgMo3O12 composites with mass ratio of Al:ZrMgMo3O12 2:8 is 0.77 10 6 K 1. The composites belong to a near-zero thermal expansion material with high impedance. (3) When the mass ratio of Al:ZrMgMo3O12 is 6:4 the CTE (8.72 10 6 K 1) of Al–ZrMgMo3O12 is reduced to onethird of Al (23.79 10 6 K 1), and Al–ZrMgMo3O12 exhibits a low resistance. This result could be attributed to the conductive percolation phenomena produced by the mixed block phase structure of Al–ZrMgMo3O12 composite materials.
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