Nanocrystalline Copper Samples Research Articles

Grain-size-dependent thermal conductivity of nanocrystalline materials

In order to study the influence of grain size and lattice strain on the thermal conductivity of nanocrystalline (NC) materials, both experimental and theoretical studies were carried out on NC copper. The NC copper samples were prepared by hot isostatic pressing of nano-sized powder particles with mean grain size of 30 nm. The thermal behaviors of the samples were measured to be 175.63–233.37 W (m K)−1 by using a laser method at 300 K, which is 45.6 and 60.6 % of the coarse-grained copper, respectively. The average grain size lies in the range of 56–187 nm, and the lattice strain is in the range of −0.21 to −0.45 % (in the direction of 111) and −0.09 to 0.92 % (in the direction of 200). In addition, a modified Kapitza resistance model was developed to study the thermal transport in NC copper. The theoretical calculations based on the presented theoretical model were in good agreement with our experimental results, and it demonstrated that the thermal conductivity of NC materials show obvious size effect. It is also evident that the decrease in the thermal conductivity of NC material can be mainly attributed to the nano-size effect rather than the lattice strain effect.

Deformation characteristics and stress–strain response of nanotwinned copper via molecular dynamics simulation

In this research parallel molecular dynamics (MD) simulations have been performed to study the deformation behavior of nanocrystalline copper samples with embedded nanotwins under approximately uniaxial tensile load. Simulation results reveal that twin boundaries (TBs) act as obstacles to dislocation movements that lead to the strengthening of nanotwinned structures. However, easy glide of dislocations parallel to the TBs contribute primarily to the plastic strain or ductility of these materials. At higher deformation stages, the strengthening effects reach a maximum when abundant dislocations begin crossing the TBs. Due to this highly anisotropic plastic response of the grains, a random polycrystalline sample will show combined properties of ductility and strength. The strengths of the nanotwinned models are found to exhibit an inverse relationship with the twin width and temperature. We also investigate the relation between the deformation behavior in different grains, their orientation with respect to the loading direction, and ultimately the observed response of nanotwinned structures.

THERMAL STABILITY OF NANOCRYSTALLINE COPPER FILMS

The grain growth kinetics of nanocrystalline copper thin film samples was investigated. The grain size of nanocrystalline copper samples was determined from the broadening of X-ray spectra. It was found that the grain size increased linearly with isothermal annealing time within the first 10 minutes, beyond which power-law growth kinetics is applied. The activation energy for grain growth was determined by constructing an Arrhenius plot, which shows an activation energy of about 21 – 30 kJ/mol. The low activation energy is attributed to the second phase particle drag and the porosity drag, which act as the pinning force for grain growth in nanocrystalline copper.

Ultrahigh strength and high ductility of bulk nanocrystalline copper

We have synthesized artifact-free bulk nanocrystalline copper samples with a narrow grain size distribution (mean grain size of 23nm) that exhibited tensile yield strength about 11 times higher than that of conventional coarse-grained copper, while retaining a 14% uniform tensile elongation. In situ dynamic straining transmission electron microscope observations of the nanocrystalline copper are also reported, which showed individual dislocation motion and dislocation pile-ups. This suggests a dislocation-controlled deformation mechanism that allows for the high strain hardening observed. Trapped dislocations are observed in the individual nanograins.

Specific Features of Helium Penetration into Single-Crystal and Nanocrystalline Copper under Deformation in Liquid Helium

This paper reports on experimental data on the penetration of helium atoms into single-crystal and nanocrystalline copper samples subjected to tensile and compressive strains at T=4.2 K, respectively. The dependences of the helium concentration N in the samples on the strain ɛ and the curves of helium extraction in the temperature range 300–1000 K at different strains ɛ are determined. It is found that the dependences N(ɛ) and σ(ɛ) correlate qualitatively with each other for single-crystal copper and do not correlate for nanocrystalline copper. This is associated with the different mechanisms of deformation in these samples. The deformation proceeds through the dislocation mechanism in single-crystal copper and through the jumpwise (twinning, rotational) mechanisms in nanocrystalline copper during local heating in regions of plastic shears. These factors are also responsible for the considerable difference between the curves of helium extraction from samples of both types. The curves of helium extraction exhibit two maxima for single-crystal copper and five maxima for nanocrystalline copper samples. The results obtained are discussed in terms of both the dynamic dislocation pipe diffusion and grain-boundary mechanisms of particle penetration from the surrounding medium into copper through different-type moving defects under applied stresses and due to the gradient of the chemical potential at the metal-surrounding medium interface.

Comparison of the thermal stability between electro-deposited and cold-rolled nanocrystalline copper samples

The objective of this work is to investigate the microstrain effect on grain growth and strain release processes in nanocrystalline (nc) samples. Two kinds of nc Cu samples were studied by using differential scanning calorimetry (DSC), X-ray diffraction (XRD) and transmission electron microscopy (TEM): an electro-deposited nc Cu sample with an average grain size of about 30 nm and one after cold-rolling in which the average grain size remain unchanged but the microstrain is substantially elevated. Measurement results indicated for the electro-deposited nc Cu, grain growth occurs at about 70°C, which was prior to the major strain release process (onset at 150°C). While for the as-rolled nc Cu, the grain growth onset temperature increases up to about 150°C and that of the strain release process drops to about 115°C. These results showed an evident correlation between the grain size stability and the microstrain in the nc materials.

Microhardness of bulk and higher density nanocrystalline copper obtained by hot compaction

Nanocrystalline copper (Cu) samples with relatively higher density (>98.5%) and smaller grain size (20–50 nm), have been prepared by gas-condensation technique with subsequent in-situ hot compaction using a novel consolidation device. It is found that Vickers microhardness of all as-synthesized nanocrystalline copper samples compacted under the different pressure (1.0–3.5 GPa) at 423 K (0.315 T m) are greater than those of coarse grained Cu (50 μm). No significant grain growth is observed. However, the hardness appears to increase with increasing grain size ranging from 20 ± 0.12 nm to 24.8 ± 0.12 nm, in contrast, the normal Hall–Petch relation between 20 ± 0.15 nm and 50 ± 0.15 nm under the same density are indicated. This complicated result is attributed to higher density, smaller grain size and internal microstrains of samples.

Study on microhardness of bulk nanocrystalline copper

Bulk nanocrystalline copper samples weighted 5 gram with average grain size d=25nm were synthesized by inert gas condensation and in situ compacting method. The microhardness study is carried out in this paper considering two influence factors of density and grain size. The specimens with different densities and grain sizes were obtained through compacting the as-evaporated particles under pressures from P=0.5 to 2.5 GPa at room temperature, and annealing the specimen as-compacted under 1 GPa at temperature from T=373K to 773K respectively. The results show that the nanocrystalline copper specimens exhibit higher microhardness than conventional coarse-grained copper, and the microhardness rises with the increase of compacting pressure and reaches a maximum value under P=2.5GPa, which is six times as high as that of conventional copper. The specimen density also plays an important role on the microhardness of bulk nanocrystalline copper except for the grain size.

Fatigue of nanocrystalline copper

Nanocrystalline copper samples have been subjected to pull-pull fatigue, with an upper stress that ranged from 50 to 80% of the yield stress. The minimum stress was 10 MPa. The samples survived several hundred thousand cycles. A moderate increase in grain size was observed after cycling, on the order of 30%. The samples elongated slightly in the course of a prolonged fatigue test. The amount of strain is similar to the room temperature creep strain observed previously in nanocrystalline copper under a constant stress comparable to the maximum cyclic stress. The cyclic deformation appeared to be elastic. However an elastic modulus was measured that is a factor of 2 (after correction for experimental errors) smaller than the modulus for ordinary copper. Parallel micrometer-size extrusions were observed on the surfaces of fatigued specimens. These features extended over several {micro}m and are reminiscent of extrusions associated with PSBs seen in single crystal and coarse-grained fatigued metals.

Calorimetric analysis of the grain growth in nanocrystalline copper samples

Differential calorimetry is used for studying the grain growth in nanocrystalline copper. The samples were prepared by compacting the small copper particles into a tablet-shaped solid in an opposed-targets sputtering system. The grain-growth process starts at temperatures around 400 to 450 K. The total enthalpy change due to grain growth is measured to be about 1200 to 1700 J/mol. The activation energy for grain growth is found to be 1.05 ± 0.05 eV. The average enthalpic part of the grain-boundary energy is found to be about 0.7 J/m 2, implying that the grain-boundary energy in these nanocrystalline copper samples is about the same as that found in conventional polycrystalline copper.

Electrical resistivity of nanocrystalline copper

Nanocrystalline copper samples have been prepared in an opposed-targets sputtering (OTS) system, and the dc resistivity of such samples has been measured and analyzed. The results show that nanocrystalline copper samples have a much higher resistivity and a larger temperature dependence of the resistivity than bulk copper. These features can be understood by grain-boundary-enhanced scattering of electrons. The coefficient of electron reflection by grain boundaries in a nanocrystalline copper sample and its temperature dependence have been deduced and are discussed.