Ryusuke Nakamura

Hollow oxide formation by oxidation of Al and Cu nanoparticles

The formation of hollow metal oxide nanoparticles through the oxidation process at low temperatures from 295 to 423 K has been studied by transmission electron microscopy for Cu, Al, and Pb. For Cu and Al, hollow oxide nanoparticles are obtained as a result of vacancy aggregation in the oxidation processes, resulting from the rapid outward diffusion of metal ions through the oxide layer during the oxidation process. On the other hand, Pb nanoparticles turn to solid PbO because the diffusivity difference DPb<DO in PbO does not lend itself to the formation of vacancy clusters. The oxide growth behavior of Cu and Al nanoparticles of a larger size at 423 K are summarized as follows: (i) for Al, the rapidly forming oxide layer on its surface stops growing once it reaches a critical thickness of about 1.5 nm, (ii) the growth of Cu2O continues until hollow Cu2O of a certain thickness is formed. This suggests the occurrence of two different diffusion processes in the formation of hollow oxides: the rapid outward ...

Single-phase interdiffusion in the B2 type intermetallic compounds NiAl, CoAl and FeAl

Abstract Interdiffusion coefficients, D , in the B2 type NiAl, CoAl and FeAl phases have been determined by single phase diffusion couples over a wide temperature range from 1073 to 1773 K. The value of D in the NiAl and CoAl phases shows a minimum at about 47 at.% Al deviating slightly from the stoichiometric composition, while the value of D in the FeAl phase has a weak minimum at about 41 at.% Al. The value of the activation energy, Q , for interdiffusion in the NiAl and CoAl phases shows a maximum at about 47 at.% Al, where the value of Q in the CoAl phase is much higher than that in the NiAl phase, while the value of Q in the FeAl phase is nearly constant and much lower than those in the other two phases. It is observed that the larger the lattice constant of the compounds becomes, the lower the activation energy for diffusion in the compounds becomes. Thus, the magnitude of activation energy for diffusion in the B2 type NiAl, CoAl and FeAl is probably related to the lattice constant of the compounds. Using the interdiffusion coefficient and the tracer diffusion coefficients of Ni and Fe with the help of Darkens relation, the diffusion coefficient of Al in the NiAl and FeAl phases has been estimated.

Oxidation behaviour of Ni nanoparticles and formation process of hollow NiO

The oxidation behaviour of Ni nanoparticles at temperatures from 573 to 673 K and the formation process of hollow oxide particles were studied by transmission electron microscopy. In the course of oxidation, a single large void was observed at one site of the interface between inner Ni and outer NiO layer due to vacancy clustering, which occurs during the oxidation process resulting from the rapid outward diffusion of Ni ions through the NiO layer. This suggests that supersaturated vacancies generated at the interface migrate to the site over a long-range distance and aggregate at the site. Ni nanoparticles were fully oxidized to become hollow NiO, in which nano-holes in the form of vacancy clusters were located at the off-centred positions. The de-centring of the voids in hollow NiO is probably due to the large mobility of vacancies inside Ni during oxidation. 1l. (a) 44 567 (1977)

Diffusion mechanisms in B2 NiAl phase studied by experiments on Kirkendall effect and interdiffusion under high pressures

The intrinsic diffusivities of both components and the activation volume for interdiffusion in the B2 type NiAl phase have been measured in the high temperature region from 1473–1773 K. The activation volume for interdiffusion in 40–49 at%Al is found to be almost constant value of 1.0V0 (V0: molar volume of alloys) at 1473–1773 K, suggesting that divacancies contribute to the diffusion. Near 43 at%Al, the diffusion mechanism for Al atoms is most likely the triple defect mechanism with the activation energy of 320 kJmol−1, while that of Ni atoms is probably the anti-structure bridge mechanism with the smaller activation energy of 240 kJmol−1. However, at 50.5 at%Al, Al atoms diffuse by the nearest neighbor mechanism with the smaller activation energy (260 kJmol−1), whereas Ni atoms diffuse probably by the next nearest neighbor jump related to the triple defect with the larger activation energy (360 kJmol−1).

The Kirkendall Effect and Nanoscience: Hollow Nanospheres and Nanotubes

Summary Hollow nanostructures are ranked among the top materials for applications in various modern technological areas including energy storage devices, catalyst, optics and sensors. The last years have witnessed increasing interest in the Kirkendall effect as a versatile route to fabricate hollow nanostructures with different shapes, compositions and functionalities. Although the conversion chemistry of nanostructures from solid to hollow has reached a very advanced maturity, there is still much to be discovered and learned on this effect. Here, the recent progress on the use of the Kirkendall effect to synthesize hollow nanospheres and nanotubes is reviewed with a special emphasis on the fundamental mechanisms occurring during such a conversion process. The discussion includes the oxidation of metal nanostructures (i.e., nanospheres and nanowires), which is an important process involving the Kirkendall effect. For nanospheres, the symmetrical and the asymmetrical mechanisms are both reviewed and compared on the basis of recent reports in the literature. For nanotubes, in addition to a summary of the conversion processes, the unusual effects observed in some particular cases (e.g., formation of segmented or bamboo-like nanotubes) are summarized and discussed. Finally, we conclude with a summary, where the prospective future direction of this research field is discussed.

Atomic rearrangements in amorphous Al2O3 under electron-beam irradiation

The electron-irradiation-induced crystallization of amorphous Al2O3 (a-Al2O3) was investigated by in-situ transmission electron microscopy under the wide electron-energy region of 25–300 keV. The formation of γ-Al2O3 nanocrystallites was induced by irradiating the a-Al2O3 thin film along with the formation of nanovoids in the crystalline grains regardless of the acceleration voltage. The crystallization became more pronounced with decreasing the electron energy, indicating that electronic excitation processes play a dominant role in the formation of γ-Al2O3. Radial distribution analyses suggested that a-Al2O3 transforms to γ-phase via the “excited” (“stimulated”) amorphous state, in which the breaking and rearrangement of unstable short-range Al-O bonds, i.e., fivefold-coordinated Al-O (AlO5) basic units, occur.

Transmission electron microscopy observation of oxide layer growth on Cu nanoparticles and formation process of hollow oxide particles

The growth of a Cu 2 O layer on Cu nanoparticles at 323–373 K was investigated by transmission electron microscopy to elucidate the influence of voids formed at the Cu/Cu 2 O interface on the oxidation rate. The thickness of the Cu 2 O formed on Cu nanoparticles with an initial diameter of 10 to ∼35 nm was measured as a function of oxidation time. During the initial oxidation stage until the oxide film is about 2.5 nm thick, the oxide film on nanoparticles of 10 to ∼35 nm in diameter grows rapidly at an almost consistent rate. After that, however, the growth rate of smaller nanoparticles decreases drastically compared with that of larger ones, suggesting that the voids formed near the Cu/Cu 2 O interface prevent Cu atoms from diffusing outward, because the volume ratio of voids to inner Cu in the case of smaller nanoparticles is considerably higher than that for larger ones at the same oxidation time.

Diffusion of oxygen in amorphous Al2O3, Ta2O5, and Nb2O5

The self-diffusivity of oxygen in amorphous Al2O3 (a-Al2O3), a-Ta2O5, and a-Nb2O5 was investigated along with structural analysis in terms of pair distribution function (PDF). The low activation energy, ∼1.2 eV, for diffusion in the oxides suggests a single atomic jump of oxygen ions mediated via vacancy-like defects. However, the pre-exponential factor for a-Ta2O5 and a-Nb2O5 with lower bond energy was two orders of magnitude larger than that for a-Al2O3 with higher bond energy. PDF analyses revealed that the short-range configuration in a-Ta2O5 and a-Nb2O5 was more broadly distributed than that in a-Al2O3. Due to the larger variety of atomic configurations of a-Ta2O5 and a-Nb2O5, these oxides have a higher activation entropy for diffusion than a-Al2O3. The entropy term for diffusion associated with short-range structures was shown to be a dominant factor for diffusion in amorphous oxides.

Self-diffusion of aluminium in the intermetallic compound Fe-48 at.% Al

The self-diffusion coefficient of Al in the B2-type intermetallic compound Fe-48 at.% Al has been determined using the intrinsic diffusion coefficients of Fe and Al and the self-diffusion coefficient of Fe with the help of the Darken-Manning relation. The self-diffusion coefficient of Al in Fe-48 at.% Al is estimated to be a factor of about 0.6 smaller than that of Fe, and the activation energy for the self-diffusion of Al is obtained to be 280 kJ mol−1 which is a little larger than the value of 262 kJ mol−1 for the self-diffusion of Fe, indicating that the diffusion mechanisms for both components are nearly equal.

Enhancement of nanovoid formation in annealed amorphous Al2O3 including W

The effect of W on the nanovoid formation in annealed amorphous Al2O3 was studied by transmission electron microscopy and molecular dynamics simulations. A comparison of the void formation behavior in electron-beam deposited Al2O3 (without W) and resistance-heating deposited Al2O3 (with 10 at. % W) revealed that W enhances the formation and growth of nanovoids. An analysis of the pair distribution function (PDF) in both types of amorphous Al2O3 showed that the introduction of W into amorphous Al2O3 brings about a significant change in the amorphous structure. Furthermore, it was found by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) that sub-nm sized W clusters exist in as-deposited Al2O3 prepared by resistance-heating and then dissolve in the amorphous matrix with annealing. The combination of PDF analysis and HAADF-STEM observation provides evidence that the enhancement of void formation originates in the heterogeneous short-range atomic configurations induced by t...