Synthesis of nanoporous structures in metallic materials under laser action

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Abstract

We defined conditions of the laser-aided formation of nanoporous structures with nanopores ranging in size from 40 to 50 nm using laser pulses of 10.6 μm wavelength at a pulse-repetition rate of up to (4–5)×103 Hz for a model metallic material (a two-component alloy “brass of 62%”). It has been established that the exposure to a uniform laser light at depths of up to 25–30 μm results in the formation of nanopores with a relatively uniform distribution across the surface. The resulting pattern contains both solitary pores and ramified porous channels. The nanopores are uniformly distributed within a subgrain, being fairly stable in size and shape. The nanopore size and shape feature larger non-uniformity on the subgrain boundary. The resulting metallic structures show promise for use as catalysts and ultrafiltration membranes.

Highlights

► We defined conditions of the laser-aided formation of nanoporous structures. ► We used laser pulses of 10.6 μm at a pulse-repetition rate of up to (4-5) 103 Hz. ► In alloy “brass of 62%” nanopores ranging in size from 40–50 nm are formed. ► The resulting structures contain both solitary pores and ramified porous channels. ► Such structures show promise for use as catalysts and ultrafiltration membranes.

Introduction

Grouped in a separate class of nanomaterials, nanoporous materials possess a range of unique physical properties associated with the presence of a great number of pores and (or) channels of nano-scale cross-section size. Today, current products made of non-metal nanoporous materials, including polymers, glass, ceramics, graphite have gained wide acceptance [1], [2]. As distinct from the above-listed materials, metals boast a range of improved physio-chemical and technological properties, such as mechanical strength, heat resistance, high thermal, and electro conductivity, featuring long service life and enhanced chemical stability.

The nanoporous surface morphology of metal materials can be synthesized by electrochemical de-alloying [3], [4] of a single-phase solid solution of Au–Ag. The selective anodic etching is used for fabricating nanoporous nickel films from Ni–Cu alloys [5]. Methods of selective dissolution of electrodeposited metallic materials show promise for the production of three-dimensional components of microelectromechanical systems (MEMS) [6], [7]. The local selective dissolution is used to form a nanostructured surface morphology in lithography and for synthesis of optoelectronic devices. To these ends, electrodeposited polymolecular layers of NiCu/Cu, NiFeCu/Cu [8], and CoNiFeCu/Cu [9] are subjected to the selective etching to generate periodic nanostructured surfaces. Note that the above-listed electrochemical methods, also employed in Refs. [10], [11], [12], [13], are mostly suited for synthesizing materials with closed pores and a well-developed surface morphology. At the same time, generating through pores by means of the electrochemical methods presents a considerable challenge.

Metallic structures that contain gas-and-water permeable nanopores are synthesized by electrochemical preferential dissolution of a selected phase from monocrystalline heat-resistant Ni-based alloys, but the manufacture costs are rather high. By way of illustration, synthesis of a nanoporous material with ramified, several-hundred-nanometers wide channels produced from a Ni-based alloy by selective phase dissolution was reported in Refs. [14], [15], [16]. Here, the first stage of synthesizing a unified disperse structure composed of a base and a selective phase is followed by phase separation via preferential dissolution. A new technique for applying coatings onto the resulting nanoporous surface with the aim of producing biocompatible or environmentally friendly functional meso- and nano-structures was dealt with in Ref. [17]. The coatings were deposited onto the surface of the material containing channel-like pores of width ∼200 nm by pulsed laser deposition (PLD).

The state-of-the-art methods and technologies for synthesizing metallic porous materials have essential restrictions in terms of pore size stability, resulting in either relatively expensive products or inferior mechanical properties when exposed to blow, bending, and other deformations, which have prevented them from being widely used. Thus, developing new methods for synthesizing nanoporous metallic structures have been of significant scientific and practical interest.

In Ref. [18], a method for fabricating nano-scale porous structures in crystalline materials was proposed, in which vacuum heating was preceded by the preliminary exposure to high-intensity energy flows with a deliberately varied spatial power distribution. Because high cooling rates following the laser heating result in a structure with high inhomogeneity, enhanced dispersivity, and other crystalline lattice defects, preliminary exposure to laser light enables producing fine-dispersed structure material, thus allowing the pore density to be increased and their spatial distribution made more uniform. The use of laser pulses at pule-repetition rate of up to (4–5)×103 Hz with the aim of activating diffusion processes in the material under processing allows the functional capabilities of this method to be essentially increased.

In this work, we experimentally study the distinctive features of synthesizing nanoporous structures in a Cu–Zn alloy when its surface is exposed to laser light.

Section snippets

Material under study and experimental setup

We have experimentally studied distinctive features of synthesizing Cu–Zn alloy structures when its surface is exposed to high-intensity energy flows. As a model, we used a two-component Cu–Zn alloy “brass of 62%” with a 60.5–63.5% content of copper (Cu), which is peculiar for a significant concentration of the higher-density vapor component (Zn). When conducting experimental studies, an important advantage of the said model material is that the reduction of Zn concentration in the surface

Studying the synthesized material structure

We have identified the modes of laser energy deposition that enable the metallic material structure to be changed at a nanoscale level. The exposure to periodic laser pulses with the pulse-repetition rate of (4–5)×103 Hz—provided that the material is being heated below the melting point—makes it possible to produce a steady stress on the sample surface. The maximal incident power density was chosen from the condition q<4.88λ(T)Tnл/Df, where λ(T) is the material heat conductivity; Tnл is the

Conclusion

We have defined conditions of the laser-aided formation of nanoporous structures with nanopores ranging in size from 40 to 50 nm using laser pulses of (4–5)×103 Hz rate for a model metallic material—a two-component alloy “brass of 62%”. Laser treatment modes have been identified making it possible to change the metallic material structure at a nanoscale level, with the material being heated below the melting point. Microstructural studies of the samples have revealed the formation of single pores

Acknowledgements

The work was financially supported by the RF Ministry on Education and Science as part of the program “Scientific and Educational Manpower of Innovation Russia” in 2009–2013, by the Russian–American Program “Basic Research and Higher Education” (“BRHE”, CRDF Grant PG08-014-1) and RF Presidential grant for support of leading scientific schools NSh-7414.2010.9.

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