Reduction in the skin effect for the sintered Si nanopolycrystalline body as an electricity conductor at a high frequency due to its nanostructure was studied. Singular disappearance of electrical resistances near a local high magnetic harmonic frequency of a few MHz was observed. This phenomenon has not been observed for conventional ferromagnetic metals. The measured electrical resistances changed to almost 0 mΩ at room temperature. At the same time, negative resistance of the sintered Si nanopolycrystalline body was observed. It will be applicable to electronic transmittance lines or semiconductors. Numerical calculation was also performed on the electrical resistance with frequency dependency while considering the electric field and magnetic field in the sintered Si nanopolycrystalline body. The experimental and calculated results are compared. The calculation could explain the variation of the relative permittivity of the Si nanopolycrystalline body and the phenomenon for the theoretical disappearance of the resistivity at the MHz frequency. Reduced Si nanoparticles from SiO2 powder were synthesized by laser ablation in liquid. A Si nanopolycrystalline body made of the reduced Si nanoparticles was fabricated. It was found by measuring the magnetization property of the body of the sintered Si nanopolycrystalline body which is ferromagnetic. Dangling bonds (unpaired electrons) have long been known to occur due to defects in Si crystals. Perfect Si without defective crystals has no dangling bonds. However, Si nanoparticles have many dangling bonds. High-density dangling bonds cause the sintered Si nanopolycrystalline body to have ferromagnetism. In this study, the density of the unpaired electrons in the sintered Si nanopolycrystalline body was observed using ESR. It has been clarified that the Si nanopowder and the sintered Si nanopolycrystalline body have numerous dangling bonds. Both densities of the dangling bonds were evaluated.
Reduction of AC resistance owing to the skin effect at high frequency from MHz to GHz in electronic devices or transmittance lines has been attracting attention now due to its ability to realize low-power consumption. We can expect that low-loss electric power lines and low-power-consumption electronic devices with low heat generation can be achieved by developing metals and semiconductors that have the reduced skin effect [
At the same time, a technology has been developed to make electrical circuits at low temperatures of below 470 K using paste or ink consisting of Au [
Moreover, many methods to produce metal or nonmetal nanoparticles exist. Of all of them, the method for producing the metal nanoparticles using laser ablation in liquids with high speed and low cost has especially attracted attention [
Unpaired electrons, i.e., dangling bonds, due to impurities in Si substrates have already been studied by many researchers. E
The goal of our research is to develop low-power-consuming semiconductor devices by applying the prepared Si nanopolycrystalline with a reduced skin effect to semiconductors. We have recently fabricated nanostructured metals by the bottom-up process [
In this study, we investigated if sintered Si nanopaste is a ferromagnetic material. We also report on a significant reduction in the resistance at a specific frequency that was also observed.
We observed the structure of the sintered Si nanopolycrystalline body experimentally. A SEM image of a sintered Si nanopolycrystalline body is shown in Figure
Reflection electron image obtained by SEM (magnification: 5000 times)
Secondary electron image obtained by SEM (magnification: 50000 times)
Secondary electron image obtained by SEM (magnification: 200000 times)
Results of EDX analysis by specific X-rays: (a) part A in Figure
Normal Si has an antimagnetic property. The magnetization property of the reduced Si nanoparticles and the sintered Si nanopolycrystalline body was measured by VSM at a room temperature (293 K) to determine if the sintered Si nanopolycrystalline body has ferromagnetism. The measured magnetization of reduced Si nanopowder and sintered Si is shown as Figure
Magnetic property: (a) magnetization property; (b) Arrott plot.
Magnetic parameter.
Saturated magnetization (emu/g) | Corrective force (Oe) | |
---|---|---|
Si nanopowder | 0.011 | 200 |
Si nanocrystalline | 0.08 | 200 |
Fe powder [ |
210 (mean size: 150 nm) | 280 |
Fe oxide [ |
74 (mean size: 30 nm) | 700 |
Ni-doped ZnO [ |
4 | 90 |
The measured maximum magnetization of the reduced Si nanoparticles was 0.011 emu/g, and the measured maximum magnetization of the sintered Si nanopolycrystalline body was 0.075 emu/g. The magnetization of the sintered Si nanopolycrystalline body is 7 times larger than that of the reduced Si nanoparticles. The maximum magnetization of the conventional Fe bulk was 218 emu/g. The evaluated coercive force of the sintered Si nanopolycrystalline body was 200 Oe. From the measured magnetization property, the sintered Si nanopolycrystalline body includes a ferromagnetic phase because of the residual magnetization and coercive force as shown in Figures
Densities of the dangling bond in the Si nanopowder and the Si nanopolycrystalline were measured by electron spin resonance (ESR) and then compared. Results of ESR analysis for the Si nanopowder and the sintered Si nanopolycrystalline are shown in Figures
Results of ESR analysis for Si nanopowder
Results of ESR analysis for sintered Si nanopolycrystalline
Result on evaluated density of dangling bond.
Amount of dangling bond (×1013 spin/g) | ||
---|---|---|
Si nanopowder | 2.0000 | 8.3 |
Si nanopolycrystalline | 2.0025 | 16.0 |
Measured resistance of the sintered Si as a function of frequency is shown in Figure
Measured resistivity as function of frequency when current at 5 MHz was 10 mA: (a) increasing frequency; (b) decreasing frequency.
The resistance of the sintered Si nanopolycrystalline body increased with increasing frequency, and the resistance returned to 60 mΩ at 4 MHz. After that, the resistance decreased continuously, as shown in Figure
The measured resistance of the sintered Si depending on the current is shown in Figure
Results on measured resistance as function of frequency. Currents at 5 MHz were (a) 0.17, (b) 10, and (c) 20 mA. (d) Resistances with dependence on current.
The measured resistances of the Si nanopolycrystalline body depending on the current are shown in Figure
The resistance at high frequency was calculated numerically while considering the permittivity of the Si nanopolycrystalline and the electric field and magnetic field. From the results, the resistance clearly varies in accordance with the relative permittivity of 1 as a function of the frequency as shown by the dashed lines in Figures
Thus, the relative permittivity was changed so as to fit the measured resistance. The evaluated relative permittivities as function of frequency are shown in Figure
Evaluated relative permittivity as a function of frequency. Currents at 5 MHz were 0.17, 10, and 20 mA.
For Figure
For Figure
In the case in Figure
When the magnetic resonance occurred, the real part of the permeability changed to negative at 3 MHz, as show in Figure
Spatial distribution of permeability when magnetic resonance occurred and real part of permeability changed to negative at 3 MHz.
However, at the same time, the sintered Si nanopolycrystalline body had a relative permeability of −1. The numerically calculated result shows that the current does not flow linearly in micro scale of the sintered Si and generation of the eddy current is prevented due to the structure.
The resistance of a sintered Ag nanopolycrystalline body with nonferromagnetism did not become zero at the magnetic-resonance frequencies of a few MHz. Thus, this phenomenon is thought to be intrinsic for a metal nanopolycrystalline body with ferromagnetism.
Common metal has effective permeability below 1, such as 0.5, at the MHz level, and resistance becomes low due to the generated
It is considered that the resistances of almost all ferromagnetic metal nanopolycrystalline bodies change to zero at frequencies of a few MHz with no connection to the relative permeability.
However, resistance from 3 to 20 mA changed to close to zero by some cause. The resistance also changed to negative around 5 MHz when the current was 20 mA, as shown in Figure
In these experiments, we did not observe the reduction of the skin effect by which the current distribution in the Si is close to uniform due to the nanostructure. We believe that sintered Si nanopaste with a Si nanopolycrystalline body will be applicable to magnetic materials.
Figure
Calculated magnetic field intensity: (a) thickness direction; (b) length direction.
It was found that when the effective value of the current flowing in the Si nanopolycrystalline was 0.17, 10, and 20 mA, the calculated magnetic field intensity was 1, 80, and 160 A/m, respectively, in the direction perpendicular to the thickness direction.
Figure
Calculated electric field: (a) 0.17 mA, normalized by
For Figure
For Figure
Reduction in the skin effect for the sintered Si nanopolycrystalline for semiconductor material at a high frequency due to its nanostructure has been studied. Singular disappearance of electrical resistances near a local high magnetic harmonic frequency of a few MHz has been observed in the experiments. Negative resistance of the sintered Si nanopolycrystalline has also been observed. Numerical calculation has also been performed on the electrical resistance with frequency dependency while considering the electric field and magnetic field in the sintered Si nanopolycrystalline. The experimental and calculated results are compared. The calculation could theoretically explain the phenomenon of vanishing resistivity at the MHz frequency. It was found by measuring the magnetization property that the sintered Si nanopolycrystalline has ferromagnetism. The density of the unpaired electrons in the sintered Si nanopolycrystalline was observed using ESR. It has been recognized that the sintered Si nanopolycrystalline has numerous dangling bonds.
The laser ablation method in liquid is described in the paragraph below. When laser pulses are irradiated onto metal oxides in liquid, the metal oxides melt and resolve, and the melted oxides are set outside the metal nanoparticles [
A sintered Si nanopaste (Si nanopolycrystalline body) was made using the reduced Si nanoparticles. The dried Si nanopowders were mixed with 5 mg of Ag nanopastes (NAG-10 Daiken Chemical); the viscosity of the paste was high. The size of the sintered Si nanopolycrystalline body was determined to be 4 × 10 × 0.3 mm. The current was conducted in the longitudinal dimension, and the resistivity at a high frequency was measured. The Si paste was sintered using an electrical hot plate (CHP-170AN, ASONE) at 473 K (1 min) and 533 K (4 min), enabling us to obtain sintered Si pastes.
The magnetization properties of the reduced Si nanoparticles and the sintered Si nanopolycrystalline body were measured using a vibrating sample magnetometer (VSM) (BHV-30T, Riken Denshi, Japan) at room temperature (20°C). The sintered Si was observed by a scanning electron microscope (SEM) (S-4700 with low resolution and SU8240 with high resolution, Hitachi High-Technologies, Japan), and the existence of Ag, Si, and O atoms was analyzed by energy-dispersive X-ray spectrometry (EDX) (EMAX7000, Horiba, Japan).
We show the measurement condition for ESR analysis in Table
Measurement condition.
Microwave frequency | 9.3–9.4 GHz |
Intensity of microwave | 12 mW |
Range of maneuvering magnetic field | 20 mT |
Modulated magnetic field frequency | 100 kHz |
Modulated magnetic field amplitude | 1.0mT |
Sampling time | 81.92 ms |
Accumulation count | 3 times |
Measurement temperature | 10 K (liquid He cooling) |
Standard substance | 1-Diphenyl-2-picrylhydrazyl (DPPH) |
Index | Mn marker (Mn2+) |
FT-IR, S-FA200 type, JEOL was used for ESR analysis. The weight of the used sintered Si nanopolycrystalline body is 170 mg. The size was 20 mm × 2.5 mm × 1 mm.
The resistance and inductance of the sintered Si pastes were measured using an LCR meter (3532-50 LCR, high tester, Hioki, Japan). The inductance and resistance from 42 Hz to 5 MHz were measured. The inductors and electric power transmitters were assumed to have components of inductance and resistance. The phase angles of cascaded resistances and inductors in the stick-type sintered Si pastes were also measured.
We resolved the integration Maxwell-Faraday equation numerically and calculated the electric field generated by the current-generated magnetic field inside the metal. We observed spatial distribution of the electric field in the thickness and longitudinal directions. The electric field was added to the original electric field applied from the sine wave signal source. The resistance was calculated with the recalculated electric field. We considered only the real part of the relative permeability. The special mesh was set to be 40 in both length and thickness directions. It was assumed that the permeability is uniform in the profile.
The current-generated magnetic field intensities in the directions of the length and thickness are given as
No data were used to support this study.
The authors declare that they have no competing interests.
T. S., Y. I., and M. I. contributed to the experimental design, data analyses and interpretation of the findings, and preparation of the manuscript. All authors approved the final version of the manuscript.