rus
Issue #1/2019
B.G.Gribov, K.V.Zinoviev, O.N.Kalashnik, S.G.Dorofeev, N.N.Kononov, N.N.Gerasimenko, D.I.Smirnov, V.N.Sukhanov, L.S.Sukhanova, V.A.Chetverikov, M.A.Shermetova
Multilayer silicon nanostructures as micropower sources of energy
Multilayer silicon nanostructures as micropower sources of energy
Multilayer structures composed of metal / silicon / metal based on Si-nanoparticles have been investigated. Studied is a possibility of such structures to generate electricity due to interaction of nano-silicon and water present in atmospheric air. The prospect of using multilayer Si-nanostructures as an elementary cell of micro-power batteries is shown.
Теги: disproportionation of sio electrophoresis; barrier layer; proton conductivity multilayer structure si-nanoparticles многослойная структура; наночастицы кремния; диспропорционирован
INTRODUCTION
Rapid development of miniature electronic devices fabrication technology makes it necessary to create micro-sources and energy storage systems. Nowadays, thin film micro batteries [1] are being developed as micro-sources of energy, but it is necessary to have large areas of batteries in order to provide a significant amount of accumulated electrical energy, because the energy accumulating structures are, practically, two-dimensional. In order to increase a stored energy density the micro-batteries with matrix of special shape electrodes (so called 3D electrodes) placed close to each other (known as a 3D battery) instead of one flat electrode are being developed. Micro-batteries with porous electrodes capable to leak air and water are in a focus of research because of low weight and cost of such devices [3,4]. Nano- and micro-scale silicon plays an important role in design of porous electrodes because its use in Li-ion batteries allows of increasing their capacity approximately 10 times [5].
In this work we present results of the investigation of multilayer structures based on silicon nano-particles which are capable of generating electrical energy due to interaction of nano-crystal silicon with the atmospheric air water.
FABRICATION OF SAMPLES AND MEASUREMENT METHODOLOGY
Silicon nanoparticles prepared by pyrolysis of silane [6] and disproportionation of silicon oxide [7] were used to form the nanocrystalline silicon (nc-Si) films. Silicon nanoparticles prepared by silane pyrolysis have average diameter about 15 nm, their external shells were partially oxidized as SiOx (where 0 ≤ x ≤ 2). Silicon nanoparticles synthesized due to disproportionation of SiO had an oxide shell and the average core diameter of 2 nm. To prepare the layered structures, a metal electrode (hereinafter the lower electrode) has been manufactured in vacuum by physical deposition of aluminum, titan, indium or bismuth on glass substrates. Afterwards, the nc-Si films of thickness ranging from 50 nm to 1 µm were deposited onto the surface of this electrode using electrophoresis or high-voltage electrical spraying method. Thickness of films was controlled by TalyStep (Taylor-Hobbson) surface profilometer. In some cases an aluminum substrate was used as the lower electrode to deposit the nc-Si films. Samples were tempered in vacuum at 10–5 Torr and the stage temperature of 600 °С. Top electrode was deposited in vacuum onto the layered structure using the same metals as for the lower one. Sometimes, in case of samples with ITO (indium tin oxide) the conductive layer placed on a glass substrate, we used it as the lower electrode. See geometry of the samples in Fig.1.
To measure the electromotive force and volt-ampere characteristics, the stands capable of recording electrical properties of samples within the temperature range 20–150 °С with accuracy of 0.1 °С were used. The results were recorded using high-resistance 24-bits and 4-channels digital X-Y recorders Expert-001 (Econix). Samples were placed in a vacuum chamber with controlled gas media (gas composition, pressure and humidity) to study the influence of the external gas mixture on them.
EXPERIMENTAL RESULTS AND THEIR DISCUSSION
It was observed that all samples exhibit a certain difference of potentials between the lower and upper electrodes. The value of this difference at 20 °С stays within 10 mV – 1.5 V, depending on electrode types and deposition methods. The maximum difference of potentials (up to 1.5 V) in Al/nc-Si/Al structures is achieved after the samples were tempered in vacuum. The lower electrode always has a positive potential relatively to the upper one. Electromotive force temperature dynamic was the same for all samples and was increasing as the sample temperature became higher (See Fig.2). Obviously, the electromotive force value increases, practically, linearly with temperature growth in the region 30–90 °С. It is important that the observed electromotive force appears spontaneously, without any influence of external electrical sources on the metal/film – nc-Si/metal structure. Fig.3 indicates the difference of potentials time dependence on Al/film – nc-Si/Al layered structure cell electrodes previously shorted electrically for 20 hours and connected to the load of 50 MOhm. This figure presents the temperature changes of a sample in the range 1 °С as a result of daily temperature fluctuations in the laboratory with taking into account the thermostat temperature shift. Fig.3 indicates rapid changes of voltage immediately after switching of the layered structure to the load and achievement of the maximum (47 mV) in 1.5 hour. Then this maximum of the voltage decreases monotonically according to the temperature changes, however, if compare its change with changes in the temperature it becomes evident that the temperature changes in 1.5 hour depend on daily temperature fluctuations in the laboratory. In other words, after connecting to the load Al/film nc-Si/Al structure gives the specific capacity to the external circuit in 1.5 hour and stays stable and equal to 9 · 10–4 mWt/gr. The structure remained stable for several days without visible changes.
To determine the reasons of electromotive force appearing, the structures were placed in a vacuum chamber with controllable pressure, temperature and gas composition. Fig.4 shows the difference in potentials dependence at 1 MOhm load on a type of gas in the chamber. Wet oxygen, laboratory air, dry and moist argon were used in measurements. Moreover, some quantity of laboratory air was present in the chamber due to the ballast volume leak valve operation. Temperature of the sample in all measurements was 24 °С. Whenever the gas mixture in the chamber contains oxygen and water, Al/film nc-Si/Al structure has a slightly varying voltage on the load in the wide range of these components proportions and sharply decreases without them (see Fig.4).
Figure 5 presents the volt-ampere characteristics of Al/film nc-Si/Al structure measured in laboratory air and vacuum at pressure Р~10–2 Torr. The volt-ampere characteristics are non-symmetrical in the laboratory air media and the dependence of current on voltage is exponential with high accuracy at the forward biased current. This fact indicates that barrier layer of, primarily, unidirectional conductivity from the upper electrode to the lower one appears during preparation of the sample. The reasons leading to appearance of such barrier are not clear, but its presence helps to separate the electrical charges in Al/film nc-Si/Al structure more effectively. The volt-ampere characteristics in the laboratory air media exhibit a hysteresis effect which is similar to cases with quasi-capacity. The specific capacity value is calculated by the formula (here – rate of potential variation on the sample contacts, m – weight of the nc-Si film) at voltage biase U = 0 is equal to 3 F/gr. It means that when Al/film nc-Si/Al structure is in the laboratory air media it has the super capacitor properties. The volt-ampere characteristics of this structure in vacuum have no hysteresis effect.
As the electromotive force in measured structures arises in air (oxygen) and water media only, it can be explained as follows. In all cases the upper electrode of metal/ film nc-Si /metal structure was deposited onto the nanocrystalline silicon irregular surface. So, the upper electrode surface should be irregular according to the film’s relief too. Moreover, upper electrode may have micro-holes capable to leak water and oxygen (as a part of air) and may be considered as a microporous membrane. It was shown [8, 9] that nano-sized silicon reacts with water and pushes out hydrogen up to 14% of the silicon weight. When size of nano-silicon particles is 10 nm and less, the hydrogen generation rate is especially high. We suggest that water (moisture from the air) passing through the upper electrode reacts with nano-silicon in accordance with the following reaction:
Si + 4H2O – 4e– → H4SiO4 + 4H+.
The upper electrode and nano-sized silicon particles are charged negatively. Protons diffuse inside the structure as the nano-silicon films used in such structures have strongly pronounced proton conductivity [10].
CONCLUSIONS
Multilayer metal/silicon/metal structures based on nano-sized silicon particles have been prepared and studied. It was discovered that these structures are capable of generating electricity because of interaction between nano-silicon and air water. Electromotive force value depends on the sample temperature and outer media, e.g. air moisture and oxygen. Specific power may be produced on an external load under the influence of the electromotive force and may reach up to 2 · 10–3 to 5 · 10–2 mWt/gr within the temperature range 20–115°С. In future these structures may be used as an elementary micro-power battery cell – power sources for the needs of microelectronics and in other industries. ■
Rapid development of miniature electronic devices fabrication technology makes it necessary to create micro-sources and energy storage systems. Nowadays, thin film micro batteries [1] are being developed as micro-sources of energy, but it is necessary to have large areas of batteries in order to provide a significant amount of accumulated electrical energy, because the energy accumulating structures are, practically, two-dimensional. In order to increase a stored energy density the micro-batteries with matrix of special shape electrodes (so called 3D electrodes) placed close to each other (known as a 3D battery) instead of one flat electrode are being developed. Micro-batteries with porous electrodes capable to leak air and water are in a focus of research because of low weight and cost of such devices [3,4]. Nano- and micro-scale silicon plays an important role in design of porous electrodes because its use in Li-ion batteries allows of increasing their capacity approximately 10 times [5].
In this work we present results of the investigation of multilayer structures based on silicon nano-particles which are capable of generating electrical energy due to interaction of nano-crystal silicon with the atmospheric air water.
FABRICATION OF SAMPLES AND MEASUREMENT METHODOLOGY
Silicon nanoparticles prepared by pyrolysis of silane [6] and disproportionation of silicon oxide [7] were used to form the nanocrystalline silicon (nc-Si) films. Silicon nanoparticles prepared by silane pyrolysis have average diameter about 15 nm, their external shells were partially oxidized as SiOx (where 0 ≤ x ≤ 2). Silicon nanoparticles synthesized due to disproportionation of SiO had an oxide shell and the average core diameter of 2 nm. To prepare the layered structures, a metal electrode (hereinafter the lower electrode) has been manufactured in vacuum by physical deposition of aluminum, titan, indium or bismuth on glass substrates. Afterwards, the nc-Si films of thickness ranging from 50 nm to 1 µm were deposited onto the surface of this electrode using electrophoresis or high-voltage electrical spraying method. Thickness of films was controlled by TalyStep (Taylor-Hobbson) surface profilometer. In some cases an aluminum substrate was used as the lower electrode to deposit the nc-Si films. Samples were tempered in vacuum at 10–5 Torr and the stage temperature of 600 °С. Top electrode was deposited in vacuum onto the layered structure using the same metals as for the lower one. Sometimes, in case of samples with ITO (indium tin oxide) the conductive layer placed on a glass substrate, we used it as the lower electrode. See geometry of the samples in Fig.1.
To measure the electromotive force and volt-ampere characteristics, the stands capable of recording electrical properties of samples within the temperature range 20–150 °С with accuracy of 0.1 °С were used. The results were recorded using high-resistance 24-bits and 4-channels digital X-Y recorders Expert-001 (Econix). Samples were placed in a vacuum chamber with controlled gas media (gas composition, pressure and humidity) to study the influence of the external gas mixture on them.
EXPERIMENTAL RESULTS AND THEIR DISCUSSION
It was observed that all samples exhibit a certain difference of potentials between the lower and upper electrodes. The value of this difference at 20 °С stays within 10 mV – 1.5 V, depending on electrode types and deposition methods. The maximum difference of potentials (up to 1.5 V) in Al/nc-Si/Al structures is achieved after the samples were tempered in vacuum. The lower electrode always has a positive potential relatively to the upper one. Electromotive force temperature dynamic was the same for all samples and was increasing as the sample temperature became higher (See Fig.2). Obviously, the electromotive force value increases, practically, linearly with temperature growth in the region 30–90 °С. It is important that the observed electromotive force appears spontaneously, without any influence of external electrical sources on the metal/film – nc-Si/metal structure. Fig.3 indicates the difference of potentials time dependence on Al/film – nc-Si/Al layered structure cell electrodes previously shorted electrically for 20 hours and connected to the load of 50 MOhm. This figure presents the temperature changes of a sample in the range 1 °С as a result of daily temperature fluctuations in the laboratory with taking into account the thermostat temperature shift. Fig.3 indicates rapid changes of voltage immediately after switching of the layered structure to the load and achievement of the maximum (47 mV) in 1.5 hour. Then this maximum of the voltage decreases monotonically according to the temperature changes, however, if compare its change with changes in the temperature it becomes evident that the temperature changes in 1.5 hour depend on daily temperature fluctuations in the laboratory. In other words, after connecting to the load Al/film nc-Si/Al structure gives the specific capacity to the external circuit in 1.5 hour and stays stable and equal to 9 · 10–4 mWt/gr. The structure remained stable for several days without visible changes.
To determine the reasons of electromotive force appearing, the structures were placed in a vacuum chamber with controllable pressure, temperature and gas composition. Fig.4 shows the difference in potentials dependence at 1 MOhm load on a type of gas in the chamber. Wet oxygen, laboratory air, dry and moist argon were used in measurements. Moreover, some quantity of laboratory air was present in the chamber due to the ballast volume leak valve operation. Temperature of the sample in all measurements was 24 °С. Whenever the gas mixture in the chamber contains oxygen and water, Al/film nc-Si/Al structure has a slightly varying voltage on the load in the wide range of these components proportions and sharply decreases without them (see Fig.4).
Figure 5 presents the volt-ampere characteristics of Al/film nc-Si/Al structure measured in laboratory air and vacuum at pressure Р~10–2 Torr. The volt-ampere characteristics are non-symmetrical in the laboratory air media and the dependence of current on voltage is exponential with high accuracy at the forward biased current. This fact indicates that barrier layer of, primarily, unidirectional conductivity from the upper electrode to the lower one appears during preparation of the sample. The reasons leading to appearance of such barrier are not clear, but its presence helps to separate the electrical charges in Al/film nc-Si/Al structure more effectively. The volt-ampere characteristics in the laboratory air media exhibit a hysteresis effect which is similar to cases with quasi-capacity. The specific capacity value is calculated by the formula (here – rate of potential variation on the sample contacts, m – weight of the nc-Si film) at voltage biase U = 0 is equal to 3 F/gr. It means that when Al/film nc-Si/Al structure is in the laboratory air media it has the super capacitor properties. The volt-ampere characteristics of this structure in vacuum have no hysteresis effect.
As the electromotive force in measured structures arises in air (oxygen) and water media only, it can be explained as follows. In all cases the upper electrode of metal/ film nc-Si /metal structure was deposited onto the nanocrystalline silicon irregular surface. So, the upper electrode surface should be irregular according to the film’s relief too. Moreover, upper electrode may have micro-holes capable to leak water and oxygen (as a part of air) and may be considered as a microporous membrane. It was shown [8, 9] that nano-sized silicon reacts with water and pushes out hydrogen up to 14% of the silicon weight. When size of nano-silicon particles is 10 nm and less, the hydrogen generation rate is especially high. We suggest that water (moisture from the air) passing through the upper electrode reacts with nano-silicon in accordance with the following reaction:
Si + 4H2O – 4e– → H4SiO4 + 4H+.
The upper electrode and nano-sized silicon particles are charged negatively. Protons diffuse inside the structure as the nano-silicon films used in such structures have strongly pronounced proton conductivity [10].
CONCLUSIONS
Multilayer metal/silicon/metal structures based on nano-sized silicon particles have been prepared and studied. It was discovered that these structures are capable of generating electricity because of interaction between nano-silicon and air water. Electromotive force value depends on the sample temperature and outer media, e.g. air moisture and oxygen. Specific power may be produced on an external load under the influence of the electromotive force and may reach up to 2 · 10–3 to 5 · 10–2 mWt/gr within the temperature range 20–115°С. In future these structures may be used as an elementary micro-power battery cell – power sources for the needs of microelectronics and in other industries. ■
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