- Email: [email protected]
Investigation of microstructure thermal evolution in nanocrystalline Cu
Physica B 406 (2011) 760–765
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Investigation of microstructure thermal evolution in nanocrystalline Cu Kai Zhou, Hui Li, JinBiao Pang, Zhu Wang n Department of Physics, Wuhan University, Wuhan 430072, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 September 2010 Received in revised form 21 November 2010 Accepted 22 November 2010 Available online 8 December 2010
The microstructure of nanocrystalline Cu prepared by compacting nanoparticles (50–60 nm in diameter) under high pressures has been studied by means of positron lifetime spectroscopy and X-ray diffraction. These nanoparticles were produced by two different methods. We found that there are order regions interior to the grains and disorder regions at the grain boundaries with a wide distribution of interatomic distances. The mean grain sizes of the nanocrystalline Cu samples decrease after being annealed at 900 1C and increase during aging at 180 1C, which are observed by X-ray diffraction, revealing that the atoms exchange between the two regions. The positron lifetime results clearly indicate that the vacancy clusters formed in the annealing process are unstable and decomposed at the aging time below 6 hours. In addition, the partially oxidized surfaces of the nanoparticles hinder grain growth when the samples age at 180 1C, and the vacancy clusters inside the disorder regions, which are related to Cu2O, need longer aging time to decompose. The disorder regions remain after the heat treatment in this work, in spite of the grain growth, which will be good for the samples keeping the properties of nanocrystalline material. & 2010 Elsevier B.V. All rights reserved.
Keywords: Nanocrystalline Cu Microstructure Positron annihilation X-ray diffraction
1. Introduction Nanocrystalline materials, with a grain size of typically o100 nm, contain a large number of interfaces, where a large volume fraction of the atoms resides, and exhibit a variety of properties that are different and often considerably improved in comparison with those of conventional coarse-grained materials [1–4]. These include increased strength or hardness, improved ductility or toughness, reduced density, reduced elastic modulus, high electrical resistivity, increased specific heat, high coefficient of thermal expansion, low thermal conductivity, and superior soft magnetic properties. These advantageous properties are connected with the structure of nanocrystalline materials. Therefore, in order to understand the unique properties of nanocrystalline materials, which are a combination of the properties of the crystalline and intercrystalline regions, it is essential to know precisely the structures of the crystalline and intercrystalline regions of nanocrystalline materials. For their future technical use, the thermal stability of the microstructure of these materials is crucial [5]. It is important, from a practical point of view, to study the microstructure evolution of these materials with long time use. The positron annihilation technique can sensitively detect open volume defects in materials, and gives detailed information on defects, such as types and concentrations of defects, and the
n
Corresponding author. E-mail address: [email protected] (Z. Wang).
0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2010.11.073
chemical identity of the elements surrounding the defects. Many positron annihilation measurements have been performed on nanocrystalline materials to investigate defects associated with the extraordinary structure of the grain boundaries in nanocrystalline materials [6–9]. These studies have led to a conclusion that open volume defects with the size of about one monovacancy were present in the nanocrystalline samples together with larger defects whose size is comparable to a vacancy cluster. However, interpretation of the positron lifetime results has to be different because of the obviously different structure of nanocrystalline materials prepared by different techniques such as high pressure compaction of nanometer-sized particles and high pressure torsion. The samples prepared by the former exhibit low dislocation density [6], while dislocations are large in the samples prepared by the latter due to high deformation [5]. Ultrafine grained (mean grain size about 150 nm) copper has been studied by various methods [5,10]. It is found that the grain interiors with low dislocation density are separated by distorted regions with high number of dislocations, and obvious property changes such as heat release, recovery of electrical resistance and relaxation of elastic strains are prior to grain growth at the annealing temperatures. In Ref. [11] an extreme extensibility (elongation exceeds 5000%) without a strain hardening effect was observed when the nanocrystalline copper specimen was rolled at room temperature, originating from a deformation mechanism dominated by grain boundary activities rather than lattice dislocation. A computer simulation of nanocrystalline copper [12] and iron [13] indicates that the volume fraction occupied by the grain boundaries is
K. Zhou et al. / Physica B 406 (2011) 760–765
very large and increases; the density is reduced and the interatomic spacing is expanded, when the grain size is reduced. The microstructure evolution of nanocrystalline Cu during mechanical attrition was investigated by the authors in Ref. [14] using XRD and DSC techniques. They found that even within the milling stage of steady-state grain size, the microstructure of nanocrystalline materials is still changing. Several positron lifetime measurements have been performed on nanocrystalline copper [5–7,15]. A substantial increase in the relative intensity of the component belonging to the microvoids starting from 200 1C in samples prepared by the gas condensation method is observed [15]. The thermal stability of the nanocrystalline samples prepared by the gas condensation method is strongly influenced by the presence of gas impurities and residual porosity [15–17]. In this work, we have studied the microstructure thermal evolution of nanocrystalline Cu using positron lifetime spectroscopy (PLS) and X-ray diffraction (XRD). Special focus is given to the microstructure evolution of nanocrystalline Cu during isothermal aging at 180 1C. Furthermore, the impact of the partial surface oxidation of nanoparticles on the microstructure thermal evolution of nanocrystalline Cu is discussed.
2. Experimental Samples A and B, about 13 mm in diameter and 1 mm thick, were prepared by compacting Cu nanoparticles at room temperature under different conditions (listed in Table 1), and the nanoparticles were produced by the flow-levitation (FL) method [18] and chemical vapor deposition (CVD) [19], respectively. The latter, with purity of 99.9% and mean grain size of about 60 nm, were commercial products. Sample B is prepared for investigating the impact of surface oxidation of nanoparticles on the microstructure thermal evolution of nanocrystalline Cu. In order to reduce the gas impurities and the residual porosity, samples A and B were annealed at 900 1C for 30 min under argon atmosphere. XRD measurements were performed on the Table 1 Characteristics of samples. Samples
Consolidation parameters
Nanoparticle producing method
Mean grain size (nm)
A
625 MPa
FL
55
B
10 MPa
CVD
60
High vacuum In air
761
as-pressed and as-annealed samples to estimate the mean grain size in the samples, respectively, and a BRUKER AXS D8 ADVANCE X-ray diffractometer using CuKa radiations is adopted for the measurements. After aging at 180 1C from 1 to 108 hours (just abbreviated to h in the following) in a vacuum furnace, all samples were cooled slowly to room temperature and subsequently polished manually for positron lifetime and XRD measurements. A digital positron lifetime spectrometer with a time resolution of 220 ps, the structure of which is shown in Fig. 1, was used to perform the lifetime measurements at room temperature. The positron source employed was 22NaCl encapsulated with Ti foil, which is sandwiched between two identical samples. For each spectrum one million total counts were collected, and after source correction and subtraction of background all the spectra were well decomposed into three lifetime components.
3. Results and discussion 3.1. XRD results and discussion The XRD patterns of the as-pressed and as-annealed samples are presented in Fig. 2. It is obvious that Cu nanoparticles in both samples are of the same face-centered-cubic (FCC) structure as the bulk Cu since three characteristic peaks of FCC Cu, corresponding to the (1 1 1), (2 0 0) and (2 2 0) surfaces, appear in Fig. 2. A slight {1 1 1} texture exists in the as-pressed and as-annealed samples according to the relative maximum intensity of each Bragg reflection peak. Compared to the as-pressed samples, the Bragg reflection peak intensities of the as-annealed samples obviously increased, suggesting high structural quality of copper nanoparticles in the samples annealed at 900 1C, while the peaks are significantly broadened, indicating an increase in the fraction volume of the disorder region (grain boundary). We can see in Fig. 2 that samples A and B annealed at 900 1C, to some extent, have been obviously oxidized and the oxides are Cu2O as its three diffraction peaks corresponding to (1 1 1), (2 0 0) and (2 2 0) surfaces appear in the XRD spectra. For sample A, the oxide comes mainly from the sample surface during the annealing, and because of the high vacuum preparing condition the surface oxidation of nanoparticles at the inner region of the sample can be neglected. But for sample B, the oxide comes mainly from two
BaF2 scintillator
PMT stop
PMT start
Samples and 22Na source
Digital oscilloscope
Delay Fig. 1. Schematic diagram of digital positron lifetime spectrometer. Digital oscilloscope—LeCroy WavePro 7100 A (4 inputs, 1 GHz bandwidth, 20 GS/s maximum sampling rate, Windows XP system). Photomultiplier tubes (PMT)—HAMAMATSU H3378-50 photomultiplier tube (R2083Q, rise-time of 1 ns).
Fig. 2. XRD lines of as-pressed and as-annealed samples A and B. Mean grain size is shown in right part of each XRD line.
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aspects—one is the surface oxidation of nanoparticles due to exposure in air or residual air in the samples during the compacting process and the other is the sample surface oxidation during the annealing process. The mean grain size D can be estimated from the XRD lines by using the Debye–Scherrer equation FWðSÞcosðyÞ ¼
Kl D
ð1Þ
where FW(S) is the full width at half maximum (FWHM) of the Bragg reflection peak, y is the peak position, K, the value set at 0.89, is the crystallite shape factor, and wavelength l of CuKa1 radiation is 0.15406 nm. We used only three characteristic peaks of FCC Cu in Fig. 2 for the estimation. The values of D as well as the spacing between two adjacent (1 1 1) surfaces (denoted as d111 ) are 28 and 0.2082 nm for as-annealed sample A, while 33 and 0.20756 nm for as-annealed sample B. What is surprising is the decrease in the mean grain size after annealing at 900 1C in both samples. This could be explained by that the grain size obtained using XRD profile analysis is the coherent domain size, namely, the order region size. After annealing the order regions decrease because the atoms in the edge of these regions are activated thermally and move to the disorder regions (see Fig. 3). In Refs. [5,10] the grain size obtained by XRD is smaller than that obtained by transmission electron microscopy, which can be explained in the same way. After aging for 60 and 108 h, the samples were carefully polished manually to move the oxidized surface away and subsequently measured by XRD to obtain the mean grain size using the method mentioned above. In Fig. 4 we can obviously see the characteristic peaks of Cu2O in sample B aged for 60 and 108 h. This is attributed to the partial surface oxidation of the nanoparticles in sample B. The mean grain sizes for samples A and B aged for 60 h are 70 and 40 nm, respectively. This increase in the mean grain sizes indicates that the volume fraction of the disorder region in the samples decreases and more non-equilibrium atoms are recovered to the equilibrium sites. The faster increase in the mean grain size of sample A compared to that of sample B suggests that the partly oxidized surface of the nanoparticles retard the increase in the mean grain size in the sample. For samples A and B aging for 108 h, the mean grain sizes are 73 and 50 nm, respectively. From the aging time of 60 to 108 h, the mean gain size of sample A grows very slowly compared to that of sample B. Correlated with the positron lifetime results (discussed below), which indicate the vacancy clusters related to the oxide decomposed completely after an aging time of 60 h, we can conclude that the fast grain growth of sample B is the contribution of this decomposition.
3.2. PLS results and discussion In previous studies [20–22], the following four possible states of positron in nanocrystalline were suggested: (1) the free positron state, (2) a trapped state related to a vacancy-like defect such as dislocation jog in the grains or grain boundaries, (3) a trapped state related to a vacancy cluster containing some of the monovacancy at the grain intersection or grain boundaries, and (4) a trapped state in large voids such as missing grains, which gives a long lifetime of the order of nanoseconds. In this work, the spectrum of the well annealed crystalline Cu was well fitted to one lifetime component of t ¼110 ps, which is the bulk lifetime of Cu, coinciding with that of other authors [23]. The spectra of nanocrystalline Cu samples were decomposed into three components t1, t2, and t3, with relative intensities I1, I2, and I3. No contribution of free positrons was found in these positron lifetime spectra because of the short lifetime t1 larger than the bulk lifetime (see Fig. 6(a) and Fig. 7(a)), which indicates that most positrons were efficiently trapped into defects. The longest lifetime t3, the value between 1 and 2 ns, is attributed to a positronium formed at the intergranular region annihilation through the pick-off process. Because the intensity I3 of this lifetime is too small (0.3–0.6%) and
Fig. 4. XRD lines of annealed samples A and B after aging at 180 1C for 60 and 108 h. Mean grain size is shown in right part of each XRD line.
Fig. 3. Snapshot of grain structure and grain boundary in as-pressed (a) and as-annealed (b) samples. Filled circles represent atoms associated with individual grains and open circles represent those constituting grain boundary network.
K. Zhou et al. / Physica B 406 (2011) 760–765
have no significant changes during the aging process, further discussion of t3 is not done. Considering the mean grain sizes of nanocrystal to be smaller than the typical positron mean diffusion length (about 100 nm) in crystals, we reasonably assume that most positrons annihilate in the grain boundaries. As we know that the positron average lifetime tav is a robust parameter that does not depend on the number of components to which a positron lifetime spectrum is decomposed. Therefore, we firstly use this parameter for a description of changes that occur during the aging. From Fig. 5 we can see that the aging behavior of average lifetime tav for annealed samples A and B is similar at the intervals of about 0–10 h and 70–108 h. In the former interval the average lifetime decreases to their minimum values for samples A and B and in the latter interval increases slowly to the maximum values. Furthermore, the average lifetime for both samples may reach their saturation values when the aging time is beyond 108 h, and then the microstructure of the samples would be in a stabilization state. The obvious difference in the aging behavior of tav is at the mediate interval of the aging time, where the average lifetime for sample A slightly reduces but for sample B increases in general. We consider this difference to be due to the partial surface oxidization of nanoparticles in sample B, which impedes the decomposition of vacancy clusters and causes the continuous increase in the size of vacancy clusters. When the vacancy clusters are too large, they become unstable and then might collapse. The details will be discussed below according to the decomposed positron lifetimes and their relatively intensities. The positron lifetimes and intensities for annealed samples A and B are shown in Figs. 6 and 7, respectively, as a function of aging time at 180 1C. The calculated positron lifetimes of V7, V9, V11, and V13 in Cu crystal (where Vn means a vacancy cluster consisting of n monovacancies), obtained from Refs. [5,23] are also shown by horizontal dashed lines in Figs. 6 and 7 in order to evaluate the vacancy size in samples. According to Refs. [5,23] the calculated lifetime of monovacancy in bulk Cu is about 170 ps. For samples A and B, the shorter lifetime t1, with value between 110 and 170 ps, hence, is due to positron annihilation at a vacancy-type defect with the size less than a monovacancy. This type of defect is mostly like to be located at the disorder region. This suggested that the grain boundaries are in an amorphous state with various interatomic spacing, which has been confirmed by XRD [24]. The longer lifetime t2, presented in Figs. 6 and 7, explicitly comes from a positron annihilating at a vacancy cluster, located in the grain boundaries, consisting of V7–V11 for sample A and V9–V13 for sample B. Due to the obviously bigger compacted pressure for
Fig. 5. Average lifetime for annealed samples A and B as function of aging time at 180 1C. Lines are shown to guide the eyes.
763
Fig. 6. (a) Positron lifetimes and (b) intensities of annealed sample A as function of aging time at 180 1C. Calculated positron lifetimes of V7 and V11 in Cu crystal, obtained from Refs. [5,23], are shown in (a) by horizontal dashed lines.
sample A than that for sample B, the longer lifetime t2 for asannealed sample A (intensity about 22%) is significantly smaller than that for as-annealed sample B (intensity about 29%). This indicates that the size and the number of the vacancy cluster in asannealed sample A are smaller than those in as-annealed sample B, and hence the density of as-annealed sample A is larger. We also found that the intensities of the longer lifetime t2 for both asannealed samples are small (below 30%), suggesting that in the grain boundaries the vacancy-like defects are dominant rather than the vacancy clusters or large voids. For sample A (see Fig. 6), the shorter lifetime t1 slowly increases through the whole aging process from 125 to 139 ps, while the corresponding intensity I1 sharply increases at the aging time below 6 h, and then declines to a nearly constant value of 83% at the aging time of 16 h and above. The increase in I1, and of course, the opposite change in intensity I2 (because of I1 + I2 100%), are attributed to the decomposition of the unstable vacancy agglomerations formed during the annealing at 900 1C. One can see in Fig. 6 that intensity I1 is mostly unchanged and lifetime t1 increases slowly when the aging time is above 16 h. This indicates that the atoms around the grains undergo the recovery process, therefore the grain growth remains during this stage. This explanation agrees
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K. Zhou et al. / Physica B 406 (2011) 760–765
lifetime of crystal Cu ( 110 ps), with the intensities of 84% and 73%, respectively. This suggests that the disorder regions still have a large volume fraction. However, the mean grain sizes have increased after a long aging time. One would expect the decrease in the longer lifetime t2 when the vacancy clusters had decomposed in the samples during the aging. But we considered the unstable vacancy clusters formed during annealing to be only a part of the total ones. As seen in Figs. 6(b) and 7(b), we have observed that even if the aging time is up to 108 h, intensity I2 remains about 15% and 25% for samples A and B, respectively. This reveals that some of the vacancy clusters are quite stable at the temperature of 180 1C. It is noteworthy that the copper oxide Cu2O has a negative effect on the decomposition of the vacancy clusters. This is can be seen in Fig. 6(b)—for sample B, the first valley of intensity I2 corresponding to the decomposition of the unstable vacancy agglomerations appears at the aging time of 6 h, whereas the second valley corresponding to the decomposition of the vacancy clusters related to Cu2O appears at a longer aging time of 24 h. 4. Conclusions
Fig. 7. (a) Positron lifetimes and (b) intensities of annealed sample B as function of aging time at 180 1C. Calculated positron lifetimes of V9 and V13 in Cu crystal, obtained from Refs. [5,23], are shown in (a) by horizontal dashed lines.
with the above analysis for the XRD measurement. After aging for 108 h, we observed that the longer lifetime t2, whose value is 318 ps, still has an intensity of 15%, which indicates that the vacancy clusters are difficult to eliminate at the temperature used in the present work. As compared with sample A, intensity I1 corresponding to the shorter lifetime t1 of sample B changes more complexly with aging time (see Fig. 7). However, the shorter lifetime t1 of sample B keeps increasing slowly during the aging process. The aging behavior of the relative intensities is significantly different between samples A and B. As shown in Fig. 7(b), intensity I1 fast increases from 70% to 75% at the aging time below 6 h, and after a slight decline, reaches another maximum at the aging time of 24 h. The first peak of intensity I1 is attributed to the decomposition of the unstable vacancy agglomerations, while the second one is due to the decomposition of the vacancy clusters related to Cu2O, which has been completed at the aging time of 60 h, and both of them were probably formed during the annealing at 900 1C. We also observed that the vacancy clusters in sample B are difficult to eliminate at this temperature, the same as sample A. After aging for 108 h, both samples A and B have a shorter lifetime t1 139 ps, which is obviously higher than the bulk
Positron annihilation spectroscopy and XRD were used for investigating the microstructure thermal evolution in nanocrystalline Cu, prepared by compacting very small crystallites (grain size about 55 nm), which are produced by the FL method and CVD. We observed that the shorter lifetime t1 for both samples in Figs. 6(a) and 7(a) are between the bulk lifetime and the monovacancy lifetime, evidently suggesting that the positrons are effectively trapped into defects. The longer lifetime t2 of as-annealed samples A and B indicates that the vacancy cluster size is larger in the sample compacted by higher pressure (i.e. V7 in sample A and V10 in sample B). The decomposition of the unstable vacancy clusters was observed in both samples at the aging time of about 6 h. Obviously, the vacancy clusters related to the copper oxide need a longer aging time to decompose. The disorder regions still exist in samples A and B after the whole heat treatment. The mean grain sizes decrease after annealing at 900 1C and increase during aging at 180 1C, as observed by XRD. The volume fraction of the order regions increases during the aging. The characteristic peaks of Cu2O is evidently observed in sample B after heat treatment, which is due to the partial oxidation of nanoparticles’ surface. This oxide has a negative effect on grain growth.
Acknowledgements We gratefully acknowledge Professor Z.Q. Chen and China Academy of Engineering Physics for providing sample A. This work was financially supported by the Natural Science Foundation of China (Project no. 10775107). References [1] [2] [3] [4] [5]
[6] [7] [8] [9] [10] [11] [12]
C. Suryanarayana, C.C. Koch, Hyperfine Interact. 130 (2000) 5. M.A. Meyers, A. Mishra, D.J. Benson, Prog. Mater. Sci. 51 (2006) 427. R. Birringer, H. Gleiter, H.P. Klein, P. Marquardt, Phys. Lett. A 102 (1984) 365. H. Gleiter, Prog. Mater. Sci. 33 (1989) 223. ˇ ´zˇek, Ivan Procha´zka, Miroslav Cieslar, Radomı´r Kuzˇel, Jan Kuriplach, Jakub Cı Frantiˇsek Chmelı´k, Ivana Stulı´kova´, Frantiˇsek Becˇva´rˇ, Oksana Melikhova, Rinat K. Islamgaliev, Phys. Rev. B 65 (2002) 094106. ¨ R. Wurschum, M. Scheytt, H.-E. Schaefer, Phys. Status Solidi A 102 (1987) 119. ¨ H.-E. Schaefer, R. Wurschum, R. Birringer, H. Gleiter, Phys. Rev. B 38 (1988) 9545. Nitesh Kumar, D. Sanyal, A. Sundaresan, Chem. Phys. Lett. 477 (2009) 360. P.M.G. Nambissan, C. Upadhyay, H.C. Verma, J. Appl. Phys. 93 (2003) 6320. R.K. Islamgaliev, F. Chmelı´k, R. Kuzˇel, Mater. Sci. Eng. A 237 (1997) 43. L. Lu, M.L. Sui, K. Lu, Science 287 (2000) 1463. Jakob Schiøtz, Francesco D. Di Tolla, Karsten W. Jacobsen, Nature 391 (1998) 561.
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[13] Da Chen, Comput. Mater. Sci. 3 (1995) 327. [14] Y.H. Zhao, K. Lu, K. Zhang, Phys. Rev. B 66 (2002) 085404. [15] M. Eldrup, P.G. Sanders, J.R. Weertman, Mater. Sci. Forum 255-257 (1997) 436. [16] S. Okuda, F. Tang, Nanostruct. Mater. 6 (1995) 585. [17] R. Birringer, Mater. Sci. Eng. A 117 (1989) 33. [18] C.-M. Li, H. Lei, Y.-J. Tang, J.-S. Luo, W. Liu, Z.-M. Chen, Nanotechnology 15 (2004) 1866.
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[19] Mark T. Swihart, Curr. Opinion Colloid Interface Sci. 8 (2003) 127. [20] S. Chakrabarti, S. Chaudhuri, Phys. Rev. B 71 (2005) 064105. [21] D. Segers, S. Van Petegem, J.F. Liiffler, H. Van Swygenhoved, W. Wagner, C. Dauwe, Nanostruct. Mater. 12 (1999) 1059. ¨ [22] H.-E. Schaefer, R. Wurschum, Phys. Lett. A 119 (1987) 370. [23] Hideaki Ohkubo, Z. Tang, Y. Nagai, M. Hasegawa, T. Tawara, M. Kiritani, Mater. Sci. Eng. A 350 (2003) 95. [24] X. Zhu, R. Birringer, U. Herr, H. Gleiter, Phys. Rev. B 35 (1987) 9085.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Investigation of microstructure thermal evolution in nanocrystalline Cu Kai Zhou, Hui Li, JinBiao Pang, Zhu Wang n Department of Physics, Wuhan University, Wuhan 430072, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 September 2010 Received in revised form 21 November 2010 Accepted 22 November 2010 Available online 8 December 2010
The microstructure of nanocrystalline Cu prepared by compacting nanoparticles (50–60 nm in diameter) under high pressures has been studied by means of positron lifetime spectroscopy and X-ray diffraction. These nanoparticles were produced by two different methods. We found that there are order regions interior to the grains and disorder regions at the grain boundaries with a wide distribution of interatomic distances. The mean grain sizes of the nanocrystalline Cu samples decrease after being annealed at 900 1C and increase during aging at 180 1C, which are observed by X-ray diffraction, revealing that the atoms exchange between the two regions. The positron lifetime results clearly indicate that the vacancy clusters formed in the annealing process are unstable and decomposed at the aging time below 6 hours. In addition, the partially oxidized surfaces of the nanoparticles hinder grain growth when the samples age at 180 1C, and the vacancy clusters inside the disorder regions, which are related to Cu2O, need longer aging time to decompose. The disorder regions remain after the heat treatment in this work, in spite of the grain growth, which will be good for the samples keeping the properties of nanocrystalline material. & 2010 Elsevier B.V. All rights reserved.
Keywords: Nanocrystalline Cu Microstructure Positron annihilation X-ray diffraction
1. Introduction Nanocrystalline materials, with a grain size of typically o100 nm, contain a large number of interfaces, where a large volume fraction of the atoms resides, and exhibit a variety of properties that are different and often considerably improved in comparison with those of conventional coarse-grained materials [1–4]. These include increased strength or hardness, improved ductility or toughness, reduced density, reduced elastic modulus, high electrical resistivity, increased specific heat, high coefficient of thermal expansion, low thermal conductivity, and superior soft magnetic properties. These advantageous properties are connected with the structure of nanocrystalline materials. Therefore, in order to understand the unique properties of nanocrystalline materials, which are a combination of the properties of the crystalline and intercrystalline regions, it is essential to know precisely the structures of the crystalline and intercrystalline regions of nanocrystalline materials. For their future technical use, the thermal stability of the microstructure of these materials is crucial [5]. It is important, from a practical point of view, to study the microstructure evolution of these materials with long time use. The positron annihilation technique can sensitively detect open volume defects in materials, and gives detailed information on defects, such as types and concentrations of defects, and the
n
Corresponding author. E-mail address: [email protected] (Z. Wang).
0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2010.11.073
chemical identity of the elements surrounding the defects. Many positron annihilation measurements have been performed on nanocrystalline materials to investigate defects associated with the extraordinary structure of the grain boundaries in nanocrystalline materials [6–9]. These studies have led to a conclusion that open volume defects with the size of about one monovacancy were present in the nanocrystalline samples together with larger defects whose size is comparable to a vacancy cluster. However, interpretation of the positron lifetime results has to be different because of the obviously different structure of nanocrystalline materials prepared by different techniques such as high pressure compaction of nanometer-sized particles and high pressure torsion. The samples prepared by the former exhibit low dislocation density [6], while dislocations are large in the samples prepared by the latter due to high deformation [5]. Ultrafine grained (mean grain size about 150 nm) copper has been studied by various methods [5,10]. It is found that the grain interiors with low dislocation density are separated by distorted regions with high number of dislocations, and obvious property changes such as heat release, recovery of electrical resistance and relaxation of elastic strains are prior to grain growth at the annealing temperatures. In Ref. [11] an extreme extensibility (elongation exceeds 5000%) without a strain hardening effect was observed when the nanocrystalline copper specimen was rolled at room temperature, originating from a deformation mechanism dominated by grain boundary activities rather than lattice dislocation. A computer simulation of nanocrystalline copper [12] and iron [13] indicates that the volume fraction occupied by the grain boundaries is
K. Zhou et al. / Physica B 406 (2011) 760–765
very large and increases; the density is reduced and the interatomic spacing is expanded, when the grain size is reduced. The microstructure evolution of nanocrystalline Cu during mechanical attrition was investigated by the authors in Ref. [14] using XRD and DSC techniques. They found that even within the milling stage of steady-state grain size, the microstructure of nanocrystalline materials is still changing. Several positron lifetime measurements have been performed on nanocrystalline copper [5–7,15]. A substantial increase in the relative intensity of the component belonging to the microvoids starting from 200 1C in samples prepared by the gas condensation method is observed [15]. The thermal stability of the nanocrystalline samples prepared by the gas condensation method is strongly influenced by the presence of gas impurities and residual porosity [15–17]. In this work, we have studied the microstructure thermal evolution of nanocrystalline Cu using positron lifetime spectroscopy (PLS) and X-ray diffraction (XRD). Special focus is given to the microstructure evolution of nanocrystalline Cu during isothermal aging at 180 1C. Furthermore, the impact of the partial surface oxidation of nanoparticles on the microstructure thermal evolution of nanocrystalline Cu is discussed.
2. Experimental Samples A and B, about 13 mm in diameter and 1 mm thick, were prepared by compacting Cu nanoparticles at room temperature under different conditions (listed in Table 1), and the nanoparticles were produced by the flow-levitation (FL) method [18] and chemical vapor deposition (CVD) [19], respectively. The latter, with purity of 99.9% and mean grain size of about 60 nm, were commercial products. Sample B is prepared for investigating the impact of surface oxidation of nanoparticles on the microstructure thermal evolution of nanocrystalline Cu. In order to reduce the gas impurities and the residual porosity, samples A and B were annealed at 900 1C for 30 min under argon atmosphere. XRD measurements were performed on the Table 1 Characteristics of samples. Samples
Consolidation parameters
Nanoparticle producing method
Mean grain size (nm)
A
625 MPa
FL
55
B
10 MPa
CVD
60
High vacuum In air
761
as-pressed and as-annealed samples to estimate the mean grain size in the samples, respectively, and a BRUKER AXS D8 ADVANCE X-ray diffractometer using CuKa radiations is adopted for the measurements. After aging at 180 1C from 1 to 108 hours (just abbreviated to h in the following) in a vacuum furnace, all samples were cooled slowly to room temperature and subsequently polished manually for positron lifetime and XRD measurements. A digital positron lifetime spectrometer with a time resolution of 220 ps, the structure of which is shown in Fig. 1, was used to perform the lifetime measurements at room temperature. The positron source employed was 22NaCl encapsulated with Ti foil, which is sandwiched between two identical samples. For each spectrum one million total counts were collected, and after source correction and subtraction of background all the spectra were well decomposed into three lifetime components.
3. Results and discussion 3.1. XRD results and discussion The XRD patterns of the as-pressed and as-annealed samples are presented in Fig. 2. It is obvious that Cu nanoparticles in both samples are of the same face-centered-cubic (FCC) structure as the bulk Cu since three characteristic peaks of FCC Cu, corresponding to the (1 1 1), (2 0 0) and (2 2 0) surfaces, appear in Fig. 2. A slight {1 1 1} texture exists in the as-pressed and as-annealed samples according to the relative maximum intensity of each Bragg reflection peak. Compared to the as-pressed samples, the Bragg reflection peak intensities of the as-annealed samples obviously increased, suggesting high structural quality of copper nanoparticles in the samples annealed at 900 1C, while the peaks are significantly broadened, indicating an increase in the fraction volume of the disorder region (grain boundary). We can see in Fig. 2 that samples A and B annealed at 900 1C, to some extent, have been obviously oxidized and the oxides are Cu2O as its three diffraction peaks corresponding to (1 1 1), (2 0 0) and (2 2 0) surfaces appear in the XRD spectra. For sample A, the oxide comes mainly from the sample surface during the annealing, and because of the high vacuum preparing condition the surface oxidation of nanoparticles at the inner region of the sample can be neglected. But for sample B, the oxide comes mainly from two
BaF2 scintillator
PMT stop
PMT start
Samples and 22Na source
Digital oscilloscope
Delay Fig. 1. Schematic diagram of digital positron lifetime spectrometer. Digital oscilloscope—LeCroy WavePro 7100 A (4 inputs, 1 GHz bandwidth, 20 GS/s maximum sampling rate, Windows XP system). Photomultiplier tubes (PMT)—HAMAMATSU H3378-50 photomultiplier tube (R2083Q, rise-time of 1 ns).
Fig. 2. XRD lines of as-pressed and as-annealed samples A and B. Mean grain size is shown in right part of each XRD line.
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aspects—one is the surface oxidation of nanoparticles due to exposure in air or residual air in the samples during the compacting process and the other is the sample surface oxidation during the annealing process. The mean grain size D can be estimated from the XRD lines by using the Debye–Scherrer equation FWðSÞcosðyÞ ¼
Kl D
ð1Þ
where FW(S) is the full width at half maximum (FWHM) of the Bragg reflection peak, y is the peak position, K, the value set at 0.89, is the crystallite shape factor, and wavelength l of CuKa1 radiation is 0.15406 nm. We used only three characteristic peaks of FCC Cu in Fig. 2 for the estimation. The values of D as well as the spacing between two adjacent (1 1 1) surfaces (denoted as d111 ) are 28 and 0.2082 nm for as-annealed sample A, while 33 and 0.20756 nm for as-annealed sample B. What is surprising is the decrease in the mean grain size after annealing at 900 1C in both samples. This could be explained by that the grain size obtained using XRD profile analysis is the coherent domain size, namely, the order region size. After annealing the order regions decrease because the atoms in the edge of these regions are activated thermally and move to the disorder regions (see Fig. 3). In Refs. [5,10] the grain size obtained by XRD is smaller than that obtained by transmission electron microscopy, which can be explained in the same way. After aging for 60 and 108 h, the samples were carefully polished manually to move the oxidized surface away and subsequently measured by XRD to obtain the mean grain size using the method mentioned above. In Fig. 4 we can obviously see the characteristic peaks of Cu2O in sample B aged for 60 and 108 h. This is attributed to the partial surface oxidation of the nanoparticles in sample B. The mean grain sizes for samples A and B aged for 60 h are 70 and 40 nm, respectively. This increase in the mean grain sizes indicates that the volume fraction of the disorder region in the samples decreases and more non-equilibrium atoms are recovered to the equilibrium sites. The faster increase in the mean grain size of sample A compared to that of sample B suggests that the partly oxidized surface of the nanoparticles retard the increase in the mean grain size in the sample. For samples A and B aging for 108 h, the mean grain sizes are 73 and 50 nm, respectively. From the aging time of 60 to 108 h, the mean gain size of sample A grows very slowly compared to that of sample B. Correlated with the positron lifetime results (discussed below), which indicate the vacancy clusters related to the oxide decomposed completely after an aging time of 60 h, we can conclude that the fast grain growth of sample B is the contribution of this decomposition.
3.2. PLS results and discussion In previous studies [20–22], the following four possible states of positron in nanocrystalline were suggested: (1) the free positron state, (2) a trapped state related to a vacancy-like defect such as dislocation jog in the grains or grain boundaries, (3) a trapped state related to a vacancy cluster containing some of the monovacancy at the grain intersection or grain boundaries, and (4) a trapped state in large voids such as missing grains, which gives a long lifetime of the order of nanoseconds. In this work, the spectrum of the well annealed crystalline Cu was well fitted to one lifetime component of t ¼110 ps, which is the bulk lifetime of Cu, coinciding with that of other authors [23]. The spectra of nanocrystalline Cu samples were decomposed into three components t1, t2, and t3, with relative intensities I1, I2, and I3. No contribution of free positrons was found in these positron lifetime spectra because of the short lifetime t1 larger than the bulk lifetime (see Fig. 6(a) and Fig. 7(a)), which indicates that most positrons were efficiently trapped into defects. The longest lifetime t3, the value between 1 and 2 ns, is attributed to a positronium formed at the intergranular region annihilation through the pick-off process. Because the intensity I3 of this lifetime is too small (0.3–0.6%) and
Fig. 4. XRD lines of annealed samples A and B after aging at 180 1C for 60 and 108 h. Mean grain size is shown in right part of each XRD line.
Fig. 3. Snapshot of grain structure and grain boundary in as-pressed (a) and as-annealed (b) samples. Filled circles represent atoms associated with individual grains and open circles represent those constituting grain boundary network.
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have no significant changes during the aging process, further discussion of t3 is not done. Considering the mean grain sizes of nanocrystal to be smaller than the typical positron mean diffusion length (about 100 nm) in crystals, we reasonably assume that most positrons annihilate in the grain boundaries. As we know that the positron average lifetime tav is a robust parameter that does not depend on the number of components to which a positron lifetime spectrum is decomposed. Therefore, we firstly use this parameter for a description of changes that occur during the aging. From Fig. 5 we can see that the aging behavior of average lifetime tav for annealed samples A and B is similar at the intervals of about 0–10 h and 70–108 h. In the former interval the average lifetime decreases to their minimum values for samples A and B and in the latter interval increases slowly to the maximum values. Furthermore, the average lifetime for both samples may reach their saturation values when the aging time is beyond 108 h, and then the microstructure of the samples would be in a stabilization state. The obvious difference in the aging behavior of tav is at the mediate interval of the aging time, where the average lifetime for sample A slightly reduces but for sample B increases in general. We consider this difference to be due to the partial surface oxidization of nanoparticles in sample B, which impedes the decomposition of vacancy clusters and causes the continuous increase in the size of vacancy clusters. When the vacancy clusters are too large, they become unstable and then might collapse. The details will be discussed below according to the decomposed positron lifetimes and their relatively intensities. The positron lifetimes and intensities for annealed samples A and B are shown in Figs. 6 and 7, respectively, as a function of aging time at 180 1C. The calculated positron lifetimes of V7, V9, V11, and V13 in Cu crystal (where Vn means a vacancy cluster consisting of n monovacancies), obtained from Refs. [5,23] are also shown by horizontal dashed lines in Figs. 6 and 7 in order to evaluate the vacancy size in samples. According to Refs. [5,23] the calculated lifetime of monovacancy in bulk Cu is about 170 ps. For samples A and B, the shorter lifetime t1, with value between 110 and 170 ps, hence, is due to positron annihilation at a vacancy-type defect with the size less than a monovacancy. This type of defect is mostly like to be located at the disorder region. This suggested that the grain boundaries are in an amorphous state with various interatomic spacing, which has been confirmed by XRD [24]. The longer lifetime t2, presented in Figs. 6 and 7, explicitly comes from a positron annihilating at a vacancy cluster, located in the grain boundaries, consisting of V7–V11 for sample A and V9–V13 for sample B. Due to the obviously bigger compacted pressure for
Fig. 5. Average lifetime for annealed samples A and B as function of aging time at 180 1C. Lines are shown to guide the eyes.
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Fig. 6. (a) Positron lifetimes and (b) intensities of annealed sample A as function of aging time at 180 1C. Calculated positron lifetimes of V7 and V11 in Cu crystal, obtained from Refs. [5,23], are shown in (a) by horizontal dashed lines.
sample A than that for sample B, the longer lifetime t2 for asannealed sample A (intensity about 22%) is significantly smaller than that for as-annealed sample B (intensity about 29%). This indicates that the size and the number of the vacancy cluster in asannealed sample A are smaller than those in as-annealed sample B, and hence the density of as-annealed sample A is larger. We also found that the intensities of the longer lifetime t2 for both asannealed samples are small (below 30%), suggesting that in the grain boundaries the vacancy-like defects are dominant rather than the vacancy clusters or large voids. For sample A (see Fig. 6), the shorter lifetime t1 slowly increases through the whole aging process from 125 to 139 ps, while the corresponding intensity I1 sharply increases at the aging time below 6 h, and then declines to a nearly constant value of 83% at the aging time of 16 h and above. The increase in I1, and of course, the opposite change in intensity I2 (because of I1 + I2 100%), are attributed to the decomposition of the unstable vacancy agglomerations formed during the annealing at 900 1C. One can see in Fig. 6 that intensity I1 is mostly unchanged and lifetime t1 increases slowly when the aging time is above 16 h. This indicates that the atoms around the grains undergo the recovery process, therefore the grain growth remains during this stage. This explanation agrees
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lifetime of crystal Cu ( 110 ps), with the intensities of 84% and 73%, respectively. This suggests that the disorder regions still have a large volume fraction. However, the mean grain sizes have increased after a long aging time. One would expect the decrease in the longer lifetime t2 when the vacancy clusters had decomposed in the samples during the aging. But we considered the unstable vacancy clusters formed during annealing to be only a part of the total ones. As seen in Figs. 6(b) and 7(b), we have observed that even if the aging time is up to 108 h, intensity I2 remains about 15% and 25% for samples A and B, respectively. This reveals that some of the vacancy clusters are quite stable at the temperature of 180 1C. It is noteworthy that the copper oxide Cu2O has a negative effect on the decomposition of the vacancy clusters. This is can be seen in Fig. 6(b)—for sample B, the first valley of intensity I2 corresponding to the decomposition of the unstable vacancy agglomerations appears at the aging time of 6 h, whereas the second valley corresponding to the decomposition of the vacancy clusters related to Cu2O appears at a longer aging time of 24 h. 4. Conclusions
Fig. 7. (a) Positron lifetimes and (b) intensities of annealed sample B as function of aging time at 180 1C. Calculated positron lifetimes of V9 and V13 in Cu crystal, obtained from Refs. [5,23], are shown in (a) by horizontal dashed lines.
with the above analysis for the XRD measurement. After aging for 108 h, we observed that the longer lifetime t2, whose value is 318 ps, still has an intensity of 15%, which indicates that the vacancy clusters are difficult to eliminate at the temperature used in the present work. As compared with sample A, intensity I1 corresponding to the shorter lifetime t1 of sample B changes more complexly with aging time (see Fig. 7). However, the shorter lifetime t1 of sample B keeps increasing slowly during the aging process. The aging behavior of the relative intensities is significantly different between samples A and B. As shown in Fig. 7(b), intensity I1 fast increases from 70% to 75% at the aging time below 6 h, and after a slight decline, reaches another maximum at the aging time of 24 h. The first peak of intensity I1 is attributed to the decomposition of the unstable vacancy agglomerations, while the second one is due to the decomposition of the vacancy clusters related to Cu2O, which has been completed at the aging time of 60 h, and both of them were probably formed during the annealing at 900 1C. We also observed that the vacancy clusters in sample B are difficult to eliminate at this temperature, the same as sample A. After aging for 108 h, both samples A and B have a shorter lifetime t1 139 ps, which is obviously higher than the bulk
Positron annihilation spectroscopy and XRD were used for investigating the microstructure thermal evolution in nanocrystalline Cu, prepared by compacting very small crystallites (grain size about 55 nm), which are produced by the FL method and CVD. We observed that the shorter lifetime t1 for both samples in Figs. 6(a) and 7(a) are between the bulk lifetime and the monovacancy lifetime, evidently suggesting that the positrons are effectively trapped into defects. The longer lifetime t2 of as-annealed samples A and B indicates that the vacancy cluster size is larger in the sample compacted by higher pressure (i.e. V7 in sample A and V10 in sample B). The decomposition of the unstable vacancy clusters was observed in both samples at the aging time of about 6 h. Obviously, the vacancy clusters related to the copper oxide need a longer aging time to decompose. The disorder regions still exist in samples A and B after the whole heat treatment. The mean grain sizes decrease after annealing at 900 1C and increase during aging at 180 1C, as observed by XRD. The volume fraction of the order regions increases during the aging. The characteristic peaks of Cu2O is evidently observed in sample B after heat treatment, which is due to the partial oxidation of nanoparticles’ surface. This oxide has a negative effect on grain growth.
Acknowledgements We gratefully acknowledge Professor Z.Q. Chen and China Academy of Engineering Physics for providing sample A. This work was financially supported by the Natural Science Foundation of China (Project no. 10775107). References [1] [2] [3] [4] [5]
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