Electric storage in de-alloyed Si-Al alloy ribbons

During the past three decades, the demand for the storage of electrical energy has mushroomed both for portable applications and for static applications [1–6]. For the Japanese nation which contemplates a nuclear-power–free future after the tremendous disaster of the earthquake and tsunami, especially, the development of renewable-energy source including electrical storage is an urgent problem to be solved.

A capacitor, a device for storing electric charge, consists of a pair of electric conductors separated by a dielectric (insulator). The electric charge cannot be stored in metals (or alloys) without such a dielectric open space. Recently, we have found that the (Ni_0.36Nb_0.24Zr_0.40)₉₀H₁₀ glassy alloy, showing a Coulomb dot oscillation at room temperature [7] and a semi-true circular Nyquist diagram with total capacitance of 17.8 μF [8], can be regarded as a dc/ac converting device having a large number of 0.23 nm capacitors with femtofarad capacitance among its distorted icosahedral Zr₅Ni₅Nb₃ clusters (dots of ca. 0.55 nm in size [9]). In this material, hydrogen atoms flow into spaces outside the clusters, enlarging the spaces and resulting in the construction of zigzag tunnels of 0.23 nm average width due to the high-pressure effect of hydrogen during electrolysis [10, 11]. However, its discharge experiment put severe constraints on the practical prompt discharging, because the electric charge behaves as a polarized glutinous liquid that is absorbed between pairs of metallic clusters [12]. This is known as electroadsorption [13]. In practice, we found that the low resistance of a Ni-Nb-Zr-H glassy alloy in the input circuit prevented its prompt discharging to an output circuit with larger resistance. Rapid, powerful, and energy-rich storage effects in such materials are necessary for future electronic devices and electric power applications such as hybrid electric vehicles and backup power supplies.

In this study, Si-Al alloys were chosen as starting alloys for the formation of nanometer-sized porous structures with high resistivities of 1 Ωcm to 1 kΩcm using a de-alloying method, with the aim to obtain rapid charge/discharge characteristics. In case of higher resistivity of over 1 kΩcm, the specimen loses suitable electric conductivity and fails in setting the simultaneously homogeneous distribution of electric charges. If the de-alloyed Si-Al specimen is organized as a distributed constant equivalent parallel circuit of femtofarad Si capacitors, as well as in the case for Ni-Nb-Zr-H glassy alloys [12], we will be able to obtain an ultra-super capacitor by their parallel connection. Especially, the electron charging/discharging rate in the specimen will be extremely large compared with those of conventional electric double-layer capacitors (EDLC) based on ion or radical diffusion between electrodes [3,5,6]. Furthermore, the development of electric capacitors with ac current charging/discharging ability will fall into disuse of power-transmission lines. The storage of electric charge in such metals and alloys has been overlooked in the literature, as far as we know.

Si-Al alloy ingots (compositions given in nominal atomic percentage) were prepared by arc-melting mixtures of Si (99.999% purity) and Al (99.95% purity) in an argon atmosphere purified by Ti-gettering. Ribbon samples of 20–50 μm in thickness and 1 mm in width were prepared from these ingots by rapid solidification of the melt on a single copper roller at a tangential velocity of 31.4 m/s. De-alloying of the samples was carried out for 259.2 ks in 1N HCl solution at room temperature.

The specimen density was measured using Archimedes' principle by weighing specimens in distilled water and air. The sample structure was examined by X-ray diffraction (XRD) in reflection mode with monochromatic Cu Kα radiation. Surfaces of the de-alloyed specimens were examined by scanning electron microscopy (SEM, Hitachi S3800).

Electrical resistances of the Si-Al alloys before and after de-alloying were measured at room temperature using a four-terminal measuring method with electrode distances of 2–21 mm. Capacitances for specimens with lower electric resistivity were calculated as a function of frequency between 1 mHz and 1 MHz from electric charge/discharge pulse curves of 10 V applied at 100 ms–15 ns intervals, using a mixed-signal oscilloscope (MSO 5104, Tektronix) and 30 MHz multifunction generator (WF1973, NF Co.) on the basis of a simple exponential transient analysis (fig. 3(a)). Two gold wires each having a diameter of 100 μm were contacted to the specimen with copper tapes. The four-terminal pair configuration on the LCR meter (EM2372, NF Co.) measured the complex impedance at frequencies between 1 mHz and 100 kHz under a constant voltage of 1 V at room temperature, to remove undesirable influences such as noise derived from electromagnetism in the environment, interference of the test signals, or unwanted residual factors in the connection method cidental to ordinary termination methods. The signal path connection between the specimen and the LCR meter was made as short as possible. Before each run, we touched the round wire to completely eliminate any offset charge. All electronic measurements were carried out in an Al shield box, to prevent electromagnetism of the environment from influencing the results.

To select alloys with charging/discharging abilities suitable for electric storage capacitors, we measured the electrical resistivity at room temperature of the Si-Al alloys before and after acid leaching. The results are shown in fig. 1, along with reference data for Si-V [14], -Ce [15], and -Nb [16] alloys. The observed resistivity change of the Si-Al alloys is fairly consistent with those reported for Si-V, -Ce, and -Nb alloys. The resistivity of the de-alloyed samples decreased from 10⁹ to 10⁶ μΩcm values 10^–2–10^–5 lower than that of Si (2 × 10¹¹ μΩcm), as the Al content increased. Figures 2(a) and (b) show XRD spectra of representative Si-20 at.% Al specimens before and after de-alloying, respectively. Before de-alloying, the sample consists of a mixture of crystalline Si and Al, and a very small amount of Al₂O₃, while after de-alloying the sample exhibited crystalline Si, also with low Al₂O₃ content. The densities of Si-20 at.% Al specimens before and after de-alloying were 2.31 and 2.24, respectively.

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We then measured voltage transient phenomena as a function of time for charging and discharging in input and output circuits at 0.1 Hz, using de-alloyed Si-20 at.% Al, the results of which are shown in figs. 3(b) and (c). Transient analysis of figs. 3(b) and (c) gave capacitance values of 0.86 and 1.1 μF for charging and discharging, respectively. Since both values are almost the same, the capacitance value for discharging hereafter serves conveniently as capacitance one. Capacitance as a function of frequency for this sample is presented logarithmically in fig. 4, along with those of de-alloyed Si-30 and -40 at.% Al specimens. Although the discharging capacitance of the de-alloyed Si-20 at.% Al alloy was somewhat larger than those of the de-alloyed Si-30 at.% and -40 at.% Al, all capacitances decreased parabolically from around 0.1 mF (0.55 F/cm³) to around 1.2 pF (53 nF/cm³) with increasing frequency up to 10 kHz at 100 ms–25 ns intervals, before becoming saturated in the frequency region from 100 kHz to 1 MHz. This behaviour implies ac current charging/discharging, with the observed decrease in capacitance derived from dielectric dispersion at higher frequencies. To confirm the frequency dependence of capacitance, we measured the capacitance from 1 mHz to 100 kHz, using the LCR meter based on parallel series circuit. The result presented in fig. 4 is not necessarily in accordance with the data by the exponential transient analysis. We hereafter use the Si-20 at.% Al sample as the representative specimen.

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Since it is considered that de-alloyed Si-Al is organized as a complex circuit of a m-rank parallel and n-row series combination of femtofarad Si capacitors, as is the case for Ni-Nb-Zr-H glassy alloys [9], we measured the electrode distance dependence on discharging capacitance for the de-alloyed specimen. As can be seen from fig. 5(a), the capacitance C for all frequencies increased linearly with the electrode distance d.

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At 0.1 Hz, C = 8.372d + 956.8 (fig. 5(b)), suggesting the existence of parallel circuits in the alloy. However, if it had a perfect parallel combination, the capacitance would be expressed by C = 58.1d. Actually, the alloy would be composed of distributed constant equivalent circuits of series C with 1.7% parallel C (fig. 5(c)). This is the reason why the capacitance curves obtained by the exponential transient analysis are not in accordance with that obtained by the LCR meter on the basis of the parallel series circuit in fig. 4.

Figure 5(d) presents the surface microstructure of the de-alloyed specimen prepared in the HCl solution, showing Si grains with narrow canyons which store electric charge. Although the investigation of the three-dimensional cluster configuration is indispensable to the construction of a giant capacitance, it was very difficult to determine the actual canyon structure because of its brittleness. Further precise observation is called for. Since large electric storage could be obtained by the parallel combination network of huge numbers of atomic-size spaces, further improvements are also needed such as supercooling through higher roller speeds [11] and hydrogen penetration [8–12] for construction of surface structures with a higher density of narrow canyons.

We lastly measured the I-V characteristics of the de-alloyed specimen at room temperature to investigate its structure, the results of which are presented in fig. 6(a) and show a nonlinear electronic transport behaviour. It should be noted that Wang et al. [17] reported that the thin end of diameter-graded Ag nanowires with diameters from about 8 to 32 nm exhibited similar semiconducting properties due to Coulomb blockade at room temperature, while Ag nanowires with a uniform diameter of 30 nm showed linear I-V characteristics and normal metallic electronic transport behaviour. Hence, by analogy, we infer that the de-alloyed specimen had an electronic transport structure similar to that of a metal-semiconductor junction (i.e., Schottky junction) [18]. This is attributable to the residue of Al in the Si skeleton, also elucidating why the observed resistivity of the specimen was lower than that of Si, as shown in fig. 1. When we note the fractal structure of an EDLC, which is composed of a distributed constant equivalent circuit of active carbon (R) and electrolyte (C) (fig. 6(b)) [19, 20], we can assume that the Si skeleton and Al backbone are to an electron in this study what active carbon is to the electrolyte solution in EDLC. However, the transport rate of the electrons is extremely fast compared with that of alkaline ions in EDLC. As a result, the Al backbone of the de-alloying specimens contributed to prompt charging/discharging in the range 100 ms–25 ns, gaining an advantage over conventional batteries such as EDLC. The RC structure of the specimen used in this study is schematically presented in fig. 6(c).

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In summary, with the aim to obtain rapid charge/discharge abilities suitable for electric storage capacitors, we chose Si-Al alloys as starting alloys for the formation of nanometer-sized porous structures with high resistivity, using a de-alloying method. The resistivity of the de-alloyed samples decreased from 10⁹ to 10⁶ μΩcm, values 10^–2–10^–5 lower than that of Si (2 × 10¹¹ μΩcm), as the Al content increased. After de-alloying the sample exhibited crystalline Si, also with low Al₂O₃ content. The de-alloyed Si-20 at.% Al specimen showed rapid charge/discharge abilities at 100 ms–25 ns intervals. Capacitances of the specimen decreased parabolically from around 0.1 mF (0.55 F/cm³) to around 1.2 pF (53 nF/cm³) with increasing frequency up to 10 kHz, before becoming saturated in the frequency region from 100 kHz to 1 MHz. From the electrode distance dependence on the discharging capacitance for the de-alloyed specimen, the alloy would be composed of distributed constant equivalent circuits of series C with 1.7% parallel C. Since theI-V characteristics showed Schottky junction, we found that the Si skeleton and Al backbone are to an electron in this study what active carbon is to the electrolyte solution in EDLC.

We gratefully acknowledge support for this work by a Grant-In-Aid for Science Research in a priority Area "Research and Development Project on Advanced Metallic Glasses, Inorganic Materials and Joining Technology" from the Ministry of Education, Science, Sports and Culture, Japan.