Materials Letters 75 (2012) 149–151

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Materials Letters

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Evolution of three-dimensional manganese-based nanoporous structure underthermal processing

Ting Huang a,⁎, Yu Gu a, Changsheng Dong a, Minlin Zhong a,⁎, Lin Li b, Mingxing Ma a

a Key Laboratory for Advanced Materials Processing Technology, Department of Mechanical Engineering, Tsinghua University, Beijing, 100084, PR Chinab Laser Processing Research Centre, School of Mechanical, Aerospace and Civil Engineering, The University of Manchester, M13, 9PL, Manchester, UK

⁎ Corresponding author. Tel.: +86 10 6277 2993; faxE-mail addresses: huangting@mail.tsinghua.edu.cn, z

(T. Huang).

0167-577X/$ – see front matter © 2012 Elsevier B.V. Aldoi:10.1016/j.matlet.2012.02.020

a b s t r a c t

a r t i c l e i n f o

Article history:Received 29 December 2011Accepted 7 February 2012Available online 12 February 2012

Keywords:Porous materialsManganeseThermal propertiesMelting pointStructural evolution

Nanoporous metals (NPMs) hold great promise for various applications. However, lack of understanding ofNPMs' thermal response hinders their potentials in high temperature applications. In this letter, we studythe thermal stability of the three-dimensional manganese-based nanoporous structure (3D-Mn-NPS) recent-ly developed by our group using a novel laser processing–dealloying method. We determine the meltingpoint of the 3D-Mn-NPS to be within the range of 513–593 K, significantly lower than that of the bulkalloy (1373–1473 K). We also establish the structural evolution history of the 3D-Mn-NPS under thermal pro-cessing. The nanomorphology evolves from nanowire skeletons into nanowire–nanoparticle complex andthen into agglomerates of nanoparticles.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Nanoporous metals (NPMs) have attracted intensive interest owingto their potential applications in catalysis [1,2], sensors [3], and actua-tors [4]. NPMs are usually fabricated into thin films through dealloying[5–9]. Very recently, our group has reported for the first time a three-dimensional manganese-based nanoporous structure (3D-Mn-NPS),consisting of a 3D network of manganese-based nanowire skeletonsand nanoporous channels, fabricated through a novel hybrid methodusing laser processing coupled with dealloying [10,11]. This methodhas enabled fabrication of NPMs in a thick fusion layer metallurgicallybonded with substrate rather than a thin film, which is important forstructuralization of NPMs.

However, a critical issue limiting the application of NPMs at ele-vated temperatures is that they are thermodynamically unstableand prone to coarsening [12,13]. Thus, it is crucial to investigate thethermal response of NPMs. Studies have been done to evaluate thethermal stability of other existing NPMs, e.g. nanoporous gold (np-Au) [14–16]. Biener et al. [15] reported that the np-Au ligament sizeincreased from 30 nm to 90 nm in a He atmosphere at 450 K, and400 nm in reactive O3 environment at 650 K. According to Sun et al.[16], the ligament in the np-Au film, whose size doubled when chang-ing the annealing temperature from 373 K to 473 K, collapsed ontothe substrate after annealing at 573 K or higher temperatures. Thesework showed that nanoporous structures lost their initial morpholog-ical characteristics with increasing ligament size during annealing. As

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the 3D-Mn-NPS exhibits a new nanomorphology, it is necessary tostudy its thermal stability to determine its applicability at hightemperatures.

In this letter, we demonstrate the systematic thermal characteri-zation of the 3D-Mn-NPS under different thermal processing condi-tions. Specifically, we aim at determining its melting point, andexamining its structural evolution above melting point. Based on abetter understanding of the relationship between nanomorphologyand temperature, an optimized fabrication protocol can be estab-lished for the 3D-Mn-NPS.

2. Experimental

Cu40Mn60 alloy (40 at.% Cu and 60 at.% Mn) precursor was used tofabricate the 3D-Mn-NPS through laser deposition and dealloying thecopper, the processes of which have been detailed elsewhere [10].The as-fabricated samples of identical size (5 mm×5 mm×3 mm)containing a 2-μm thick 3D-Mn-NPS layer on 1045 steel substratewere thermally processed in a tube furnace (KSL 1400X). First, to de-termine the melting point of the 3D-Mn-NPS, individual sampleswere annealed in the furnace at either 513 K or 593 K for 60 min.Next, to study its structural evolution under thermal processing, weannealed each sample at 593 K for an incremental time length from5 min to 40 min. All the samples were initially at room temperature(298 K), and the heat-up rate was uniformly 20 K/min. The nano-morphologies of the 3D-Mn-NPS layer after heat treatment were an-alyzed using a field emission gun scanning electron microscope (FEG-SEM, LEO-1530). X-ray diffraction (XRD) patterns were recorded byan X-ray diffractometer (D8-Advance, Cu radiation). Auger mappings

Fig. 1. SEM micrographs showing surface morphology of the 3D-Mn-NPS samples under different thermal conditions: (a) as-fabricated, (b) 60-min annealing at 513 K, and (c) 60-min annealing at 593 K.

150 T. Huang et al. / Materials Letters 75 (2012) 149–151

were recorded by an Auger electron spectrometer (AES, PHI-700) toidentify its chemical composition.

3. Results and discussion

The nanomorphologies of the 3D-Mn-NPS before and after annealingat 513 K and 593 K are shown in Fig. 1. The as-fabricated structure con-sisted of nanopores and nanowires averaging 60 nm thick (Fig. 1a).Annealing at 513 K resulted in a similar structure as in the un-heat-treated sample (Fig. 1b). However, annealing at 593 K destroyed thenanowire structure into agglomerates of nanoparticles (Fig. 1c), indicat-ing melting of nanowires. As we found 60-minute annealing wassufficient for the nanostructure to reach a steady state, we may confirmthat the melting point of the as-fabricated 3D-Mn-NPS is between513 K and 593 K.

The composition at the surface of the as-fabricated 3D-Mn-NPS sam-ple was 5 at.% Cu and 95 at.% Mn (referred to as Cu5Mn95) [10]. It isreported that the melting point of bulk Cu5Mn95 is 1373–1473 K [17],whereas the 3D-Mn-NPS Cu5Mn95 completely melted below 593 K, im-plying that the structural evolution history of the 3D-Mn-NPS Cu5Mn95should be distinct from that of its bulk counterpart.

Hence, we next examined the evolution history of the 3D-Mn-NPS at593 K. By observing the nanomorphology in each sample annealed at593 K for a different time from 5min to 40 min, we obtained a series oftransient nanomorphologies during structural evolution, from which acomplete evolution history of the 3D-Mn-NPS was constructed (Fig. 2).

Fig. 2. SEM micrographs showing surface structural evolution at 593 K: (a)

With 5-minute annealing at 593 K, the first 3D-Mn-NPS samplestarted to melt on the very surface, generating small droplets, whichaggregated in the mean time by surface tension. The aggregatingdroplets then solidified into nanoparticles after immediate cooling(Fig. 2a, marked by arrow). In the second sample finished with 10-minute annealing, the entire 3D-Mn-NPS layer had achieved meltingpoint. The droplets on the surface continued to grow slowly, whilesmall droplets started to form inside the nanostructure. Upon solidifi-cation, larger nanoparticles formed on the surface and smaller onesformed inside (Fig. 2b, marked by arrow). For the third sample with 15-minute annealing, the droplets on the surface remained approximatelythe same size, but increased significantly in quantity, forming a 3Dnanostructure coupled with a great amount of autogenous nanoparticlesafter cooling (Fig. 2c). For the samples annealed for 20 min and 30 min,forming of new droplets gradually stopped. Existing droplets grew yetlarger and aggregated with neighboring droplets, transforming theoriginal 3D-Mn-NPS into a 2D structure (Figs. 2d and e). Finally, after40-minute annealing, the nanowire-nanoporous structure in the lastsample had completely disappeared (Fig. 2f).

Next, we examined the crystallographic and compositional featuresof the 3D-Mn-NPS before and after thermal processing. Fig. 3a showsthe XRD results bearing crystallographic information. For both the sam-ples without annealing and with 60-minute annealing at 513 K, asingle-phase solid solution Cu–Mn alloy with face-centered cube struc-ture (denoted as (Cu, γMn)) was identified, indexed with the main dif-fraction peaks of (111), (200), and (220). Whereas in the sampleannealed at 593 K for 60 min, low peaks of CuO and MnO were

5 min, (b) 10 min, (c) 15 min, (d) 20 min, (e) 30 min, and (f) 40 min.

Fig. 3. (a) XRD patterns of the nanostructure in the samples: without annealing, andannealed at 513 K and 593 K, and (b) Auger electron spectroscopy of nanoparticlesafter 5-min annealing at 593 K.

151T. Huang et al. / Materials Letters 75 (2012) 149–151

identified alongwith themain peaks of the (Cu, γMn) phase, indicatingthat though the main phase remained (Cu, γMn), oxide phases hademerged. We further analyzed the composition of the nanoparticlesgenerated after 5 min into annealing at 593 K (Fig. 2a). Oxygen peakwas observed (Fig. 3b), which again proved that the (Cu, γMn) phasewas partially oxidized to CuO and MnO at 593 K.

4. Conclusions

Thermal characterization of the 3D-Mn-NPS has been carried out.The melting point of the 3D-Mn-NPS has been found to be between513 K and 593 K. The initial single-phase (Cu, γMn) nanowire structureremains stable below513 K. At 593 K, however, the 3D-Mn-NPS starts tomelt and displays a very different structural evolution history from thatof the alloy in bulk form. First, melted droplets gradually emerge and de-velop amidst the nanowires. As the droplets develop, they grow first in

quantity and then in size. In the end, the nanowire structure in the initial3D-Mn-NPS is entirely destroyed, replaced by agglomerates of nanopar-ticles. Also, a small portion of the (Cu, γMn) phase is oxidized to formMnO and CuO. These findings will facilitate the design for 3D-Mn-NPSfabrication and application. It is also noteworthy that the nanowire–nanoparticle complexmorphology created during the incomplete trans-formation from nanowire to nanoparticle in the 3D-Mn-NPS has neverbeen reported before, which may exhibit desirable material propertiesas it possesses the dual nature of nanowire and nanoparticle, andhence worth further investigation.

Acknowledgments

The authors would like to acknowledge the support from the Nat-ural Science Foundation of China (grant number 90923021), Indepen-dent Scientific Research Plan of Tsinghua University (2010THZ0) andKey Program for Science and Technology Project of Beijing MunicipalEducation Commission (KZ201110005001).

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