- Nano Express
- Open Access
Influence of Oxidation on Electrical Properties of Compacted Cu Nanopowders
© The Author(s). 2016
- Received: 13 October 2016
- Accepted: 21 October 2016
- Published: 28 October 2016
The phase composition and electrical transport properties of Cu powder obtained by electric spark dispersion and the pellets manufactured from this powder were studied by X-ray phase analysis and electric resistance measurements. The compacted powders were annealed in pure Ar atmosphere. It was shown that electrical resistance of the compacted Cu specimens essentially depends on the annealing temperature. In particular, the electrical resistance of the pellet after annealing at 873 K decreases on heating at low temperatures (semiconducting mechanism). As the temperature is increased, semiconducting behavior of resistivity is altered for metallic one. This change of conductivity type is ascribed to formation of metallic oxide and modification of its content during annealing.
- Electrical spark dispersion
- Highly dispersed Cu nanopowders
- X-ray diffraction analysis
- Electric resistance
The Cu-based Heusler alloys with restricted solubility are prospective materials for spintronic applications [1–3]. Nanopowders of the Cu-13.1 % Mn-12.6 % Al alloy were obtained by electrical spark dispersion (ESD) in different media including ethanol, gas, and water . Electric conductivity of compacted powders (pellets) produced in ethanol and gas and annealed at 1073 K was found to be characteristic of semiconductors increasing with temperature in contrast to as-cast alloys, which demonstrate metallic-type conductivity.
Detailed analysis of all the results obtained led to conclusion that the type of conductivity is conditioned by formation of copper and manganese oxides in the process of preparation. The effect of copper oxide on electric conductivity is suggested to employ for controllable oxidization of pure copper to govern its conductivity. Thus, the aim of this study can be designated as determination of effect of phase content on electric conductivity of compacted copper powders prepared by successive procedures of electric spark dispersion, compacting, and annealing in argon atmosphere.
The subjects of this investigation were highly dispersed powder (nanopowder) and compacted specimens of Cu. They were prepared from high purity Cu (99.99 %). Highly dispersed powder was produced by electrical spark dispersion in distilled water; specimens were made by pressing powder . After ESD, powder particles were collected from operating fluid. First, they have been dried in air at room temperature and then at temperature of 200 °C for 1 h in drying oven to remove moisture. Next, powder has been compacted at room temperature, and finally, some of compacted specimens were annealed at 873 and 1073 K for 30 min in argon environment. X-ray diffraction measurements were carried out using diffractometer DRON −3.0 with cobalt anode. Diffraction patterns were analyzed by comparison of lines with maximum intensity corresponding to different crystalline phases. The size of coherently scattering domains (CSD) was calculated using the Scherrer equation .
Phase content and sizes of CSD (D) for as-received powder and pellets
Phase content, %
Powder, after ESD
Pellet, after compacting
Pellet, after annealing at 873 K for 30 min
Pellet, after annealing at 1073 K for 30 min
The variation of electric resistance for the compacted powders can be explained from results of X-ray phase analysis (Fig. 2, Table 1). It is evident that semiconducting character of conductivity is caused by the formation of copper oxide during ESD. Its content reduces due to decomposition as temperature of annealing is increased. These results are consistent with those  where semiconducting behavior associated with presence of copper and manganese oxides was reported for compacted specimens of highly dispersed Cu-13.1 % Mn-12.6 % Al powder. It should be noted that in case of disordered intermetallic alloys, semiconducting behavior can be attributed to strong atomic scattering, which results in quantum interference effects such as a weak localization or hopping conductivity [7–10]. Nevertheless, analysis of all collected experimental results allowed authors to conclude that the Cu-13.1 % Mn-12.6 % Al-compacted powders comprised of ordered and disordered phases demonstrated semiconducting behavior owing to appearance of oxide layer on the surface of particles rather than by other reasons . Here, the compacted Cu powder specimens contain only copper and copper oxides without any disordered phase constituents. This and the other experimental data unambiguously confirm correlation between degree of oxidization of highly dispersed Cu particles and conduction mechanism.
As it follows from the X-ray diffraction data, thickness of oxide layers on the surface of Cu particles forming pellets is reduced with decreasing the content of Cu oxide in specimen and hence in particles. That is why, conductivity type depends on temperature of annealing. This contention is also supported by resistivity behavior at low-temperature characteristic of semiconductors in spite of only 2 % of Cu2O in the specimen annealed at 873 K (Fig. 3b). If nanoparticles of Cu2O had been distributed in the pellet regardless of Cu nanoparticles, resistivity of Cu2O phase would never have effect on total resistivity being bridged by several orders lower resistivity of Cu. In fact, the measurements of R (Fig. 3b) show that below 120 K, semiconducting copper oxide phase dominates temperature behavior of resistivity and even at higher temperatures up to 180 K, there are no signs of prevalence of metallic type of conductivity associated with Cu phase. It is possible only if Cu2O phase develops mainly on the surface of highly dispersed particles of Cu.
Without going into details of temperature dependence of resistivity over the whole temperature interval of the measurements, one can restrict the consideration to temperatures T S = 100 K and T M = 273 K, where respective semiconducting and metallic behavior of resistivity are well defined.
This result suggests that at T S , resistivity decreases while at T M , it increases with the temperature in accordance with the experimental data (Fig. 2b).
Electric conductivity of compacted Cu powders prepared by ESD method and annealed in Ar environment varies with the annealing temperature as oxidation level of powder particles is changed. Appearance of semiconducting Cu2O phase modifies metallic-type behavior of electric conductivity causing its growth with the temperature.
This work was financially supported by 15/12-H and 15/13-H projects within the framework of target N.A.S. of Ukraine program of the fundamental studies “Fundamental problems of nanostructured systems, nanomaterials, nanotechnologies.”
VN organized the whole work and wrote a draft of the manuscript; AP performed the XRD analysis and contributed to the manuscript drafting; VK supervised the work and revised the manuscript; SK finalized the manuscript, conducted resistivity measurements, and estimated resistivity behavior of sample at different temperatures; TK performed the SEM observations. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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