First Review of Conductive Electrets for Low-Power Electronics

: This is the ﬁrst review of conductive electrets (unpoled carbons and metals), which provide a new avenue for low-power electronics. The electret provides low DC voltage ( µ V) while allowing low DC current ( µ A) to pass through. Ohm’s Law is obeyed. The voltage scales with the inter-electrode distance. Series connection of multiple electret components provides a series voltage that equals the sum of the voltages of the components if there is no bending at the connection between the components. Otherwise, the series voltage is below the sum. Bending within the component also diminishes the voltage because of the polarization continuity decrease. The electret originates from the interaction of a tiny fraction of the carriers with the atoms. This interaction results in the charge in the electret. Dividing the electret charge by the electret voltage V ’ provides the electret-based capacitance C ’, which is higher than the permittivity-based capacitance (conventional) by a large number of orders of magnitude. The C ’ governs the electret energy (1/2 C ’ V ’ 2 ) and electret discharge time constant ( RC ’, where R = resistance), as shown for metals. The discharge time is promoted by a larger inter-electrode distance. The electret discharges occur upon short-circuiting and charge back upon subsequent opencircuiting. The discharge or charge of the electret amounts to the discharge or charge of C ’.


Low-Power Electronics
Low-power electronics for low-power-consumption analog and digital circuits and low-power data storage and processing are receiving increasing attention. There are numerous electronic functionalities, including swing field effect transistor devices, nonvolatile memory, memristors, logic device, electronic skin, sensors, medical monitoring systems, electronic displays, and neuromorphic computing. The growth of low-power electronics is partly fueled by the reduction in power requirements for working electronic and energy systems (including portable and wearable electronics), as enabled by improved efficiency and component size reduction. It is also fueled by the growing demand for the "Internet-of-Things" (IoT).
Because of the lack of conductivity, the electrets involving nonconductors functioning as capacitors, with DC electric current being not able to flow through them continuously, though a DC pulse can flow in the direction from the positive electrode to the negative electrode (Figure 1a). On the other hand, electrets that are conductors function as resistors ( Figure 1b), with continuous DC flow being feasible. With the provision of both DC voltage and DC, the conductive electret serves as a power source, which is enabled by the electret and conductive behavior of a conductor. This concept is new in the field of low-power electronics. This work is directed at providing the first review of the emerging subject of conductive electrets for low-power electronics.  Graphite differs from the abovementioned graphene [2,3] in terms of its large number of carbon layers and its wide availability in various bulk forms. Graphite is widely used for heating elements and electrochemical electrodes [63][64][65][66][67][68][69][70][71][72][73][74][75][76][77][78][79], but its electret behavior is only emerging [57,58]. Because of the electret behavior, graphite is a power-providing Graphite differs from the abovementioned graphene [2,3] in terms of its large number of carbon layers and its wide availability in various bulk forms. Graphite is widely used for heating elements and electrochemical electrodes [63][64][65][66][67][68][69][70][71][72][73][74][75][76][77][78][79], but its electret behavior is only emerging [57,58]. Because of the electret behavior, graphite is a power-providing material in the absence of semiconductors or other two-dimensional materials, which are used in prior work on graphene [2]. Comparison of graphite with various grain sizes indicates that both the electric field in the electret and the electrical resistivity increase slightly as the grain size decreases [58].

Scientific Origin
The basic science is different from that of all previously reported power sources. Vibrations are not a mechanism for the energy generation of conductive electrets, although they are a mechanism in the case of nonconductive electrets. The nonconductive electrets behave as capacitors, whereas the conductive electrets behave as polarizable conductors ( Figure 1). The mechanism for energy generation is described below for conductive electrets.
The permittivity is the key material property that describes the dielectric behavior. For a conductor, the permittivity originates from the interaction of a tiny fraction of the carriers with the atoms. As shown for graphite, the permittivity increases with the inverse of the grain size linearly [58]. This implies that the carrier-atom interaction occurs at the grain boundaries. The relative permittivity reaches 10 6 [80,81]. This is consistent with the "Giant polarizability" predicted theoretically for conductors by Jonscher (1922Jonscher ( -2005 in 1999 [82]. An electret discharges by short-circuiting, and charges back upon subsequent open circuiting ( Figure 2). The fact that discharge occurs without the need for a prior charge is expected from the fact that the electret does not require poling. In fact, poling enhances the electret negligibly [83]. In other words, the dipole in the electret is inherent (thermodynamically stable). The greater the inter-electrode distance, the longer the discharge time [58]. The discharge/charge of an electret involves that of capacitance. This capacitance is not the well-known capacitance that relates to permittivity but is a novel type of capacitance known as the electret-based capacitance [84]. In contrast to the permittivity-based capacitance, the electret-based capacitance is defined as the magnitude of the electret charge (determined by integrating the curve of discharge current concerning the time for the entire discharge period ( Figure 2) divided by the electret voltage V'. The electret-based capacitance C' is higher than the permittivity-based capacitance C by numerous orders of magnitude [84]. For example, for Sn-4Ag lead-free solder, C' is 620 F, whereas C is only 175 pF [84]. The time constant of the discharge/charge approximately equals RC' (R = resistance) and the energy of the discharge/charge approximately equals 1 2 C'V' 2 , as shown for metals [84]. material in the absence of semiconductors or other two-dimensional materials, which are used in prior work on graphene [2]. Comparison of graphite with various grain sizes indicates that both the electric field in the electret and the electrical resistivity increase slightly as the grain size decreases [58].

Scientific Origin
The basic science is different from that of all previously reported power sources. Vibrations are not a mechanism for the energy generation of conductive electrets, although they are a mechanism in the case of nonconductive electrets. The nonconductive electrets behave as capacitors, whereas the conductive electrets behave as polarizable conductors ( Figure 1). The mechanism for energy generation is described below for conductive electrets.
The permittivity is the key material property that describes the dielectric behavior. For a conductor, the permittivity originates from the interaction of a tiny fraction of the carriers with the atoms. As shown for graphite, the permittivity increases with the inverse of the grain size linearly [58]. This implies that the carrier-atom interaction occurs at the grain boundaries. The relative permittivity reaches 10 6 [80,81]. This is consistent with the "Giant polarizability" predicted theoretically for conductors by Jonscher (1922Jonscher ( -2005 in 1999 [82]. An electret discharges by short-circuiting, and charges back upon subsequent open circuiting ( Figure 2). The fact that discharge occurs without the need for a prior charge is expected from the fact that the electret does not require poling. In fact, poling enhances the electret negligibly [83]. In other words, the dipole in the electret is inherent (thermodynamically stable). The greater the inter-electrode distance, the longer the discharge time [58]. The discharge/charge of an electret involves that of capacitance. This capacitance is not the well-known capacitance that relates to permittivity but is a novel type of capacitance known as the electret-based capacitance [84]. In contrast to the permittivity-based capacitance, the electret-based capacitance is defined as the magnitude of the electret charge (determined by integrating the curve of discharge current concerning the time for the entire discharge period ( Figure 2) divided by the electret voltage V'. The electret-based capacitance C' is higher than the permittivity-based capacitance C by numerous orders of magnitude [84]. For example, for Sn-4Ag lead-free solder, C' is 620 F, whereas C is only 175 pF [84]. The time constant of the discharge/charge approximately equals RC' (R = resistance) and the energy of the discharge/charge approximately equals ½ C'V' 2 , as shown for metals [84].  The fraction of carriers that interact with the atoms is much greater for the electret (DC) than the permittivity (AC polarization) [58]. For graphite (grain size 1 µm), the fraction of carriers that take part in the permittivity reaches 1.2 × 10 −9 , while the fraction of carriers that take part in the electret discharge reaches 6.4 × 10 −3 [58].
A conductor functions as a resistor with low-resistance (mΩ) (usual function), a capacitor with low-capacitance (pF, permittivity-based), and a power source that provides low voltage (µV, electret-based) [85]. The extent of a function compared to the other functions depends on the dimensions. For a given value of the cross-sectional area A, increasing the length (the inter-electrode distance l) increases the voltage V' while reducing the capacitance C (such that the capacitors are in series). For a given value of l, increasing A promotes C while reducing the resistance [85].

Electret Power Source Behavior
Flexible graphite is made by compacting graphite particles in the form of exfoliated graphite without a binder and is commercially available in the form of large sheets ( Figure 3) [86][87][88][89][90][91][92][93][94]. Exfoliated graphite is expanded intercalated graphite, such that it has a cellular structure (Figure 4). The expansion is typically conducted by rapid heating of the intercalated graphite. Because of the cellular structure, multiple pieces of exfoliated graphite, with each piece made from a graphite flake, join by mechanical interlocking without the need for a binder. The fraction of carriers that interact with the atoms is much greater for the electret (DC) than the permittivity (AC polarization) [58]. For graphite (grain size 1 μm), the fraction of carriers that take part in the permittivity reaches 1.2 × 10 −9 , while the fraction of carriers that take part in the electret discharge reaches 6.4 × 10 −3 [58].
A conductor functions as a resistor with low-resistance (mΩ) (usual function), a capacitor with low-capacitance (pF, permittivity-based), and a power source that provides low voltage (μV, electret-based) [85]. The extent of a function compared to the other functions depends on the dimensions. For a given value of the cross-sectional area A, increasing the length (the inter-electrode distance l) increases the voltage V' while reducing the capacitance C (such that the capacitors are in series). For a given value of l, increasing A promotes C while reducing the resistance [85].

Electret Power Source Behavior
Flexible graphite is made by compacting graphite particles in the form of exfoliated graphite without a binder and is commercially available in the form of large sheets ( Figure  3) [86][87][88][89][90][91][92][93][94]. Exfoliated graphite is expanded intercalated graphite, such that it has a cellular structure ( Figure 4). The expansion is typically conducted by rapid heating of the intercalated graphite. Because of the cellular structure, multiple pieces of exfoliated graphite, with each piece made from a graphite flake, join by mechanical interlocking without the need for a binder.  The greater the inter-electrode distance, the longer the discharge time [58], as mentioned in Section 3. Presumably due to the microstructure, flexible graphite in the form of large sheets (hence making large inter-electrode distances feasible) exhibits a discharge time that is so long that the electret essentially does not discharge upon short-circuiting. Thus, this material is particularly suitable for use as a power source and its electret characteristics [95] are described below.
The electret voltage (Table 1) can be increased by a series connection of multiple pieces of the electret. The voltage is doubled when two essentially identical specimens are connected in series without bending the electrical connection between the two specimens, but the series voltage is roughly reduced by 50% when the electrical connection is bent [95]. This is indicated by comparing the case of an unbent connection (a silver-paint butt joint) ( Figure 5) and the case of a bent connection (in the form of aluminum foil attached to the specimen by using silver paint) ( Figure 6). The former gives a voltage that corresponds to the sum of the voltages of the two specimens that are disconnected from one another, but the latter gives a voltage that is less than the sum (Table 2). The greater the inter-electrode distance, the longer the discharge time [58], as mentioned in Section 3. Presumably due to the microstructure, flexible graphite in the form of large sheets (hence making large inter-electrode distances feasible) exhibits a discharge time that is so long that the electret essentially does not discharge upon short-circuiting. Thus, this material is particularly suitable for use as a power source and its electret characteristics [95] are described below.
The electret voltage (Table 1) can be increased by a series connection of multiple pieces of the electret. The voltage is doubled when two essentially identical specimens are connected in series without bending the electrical connection between the two specimens, but the series voltage is roughly reduced by 50% when the electrical connection is bent [95]. This is indicated by comparing the case of an unbent connection (a silver-paint butt joint) ( Figure 5) and the case of a bent connection (in the form of aluminum foil attached to the specimen by using silver paint) ( Figure 6). The former gives a voltage that corresponds to the sum of the voltages of the two specimens that are disconnected from one another, but the latter gives a voltage that is less than the sum (Table 2). Table 1. Electrical characteristics of electret in the form of flexible graphite (specimens labeled S1 and S2). The S1 + S2 column gives the sum of the values for S1 and S2 when these two specimens are not connected electrically. The electric field is obtained by dividing the voltage by the interelectrode distance [95].

S1
S2 S1 + S2 (Sum)  The cellular structure is shown. Table 1. Electrical characteristics of electret in the form of flexible graphite (specimens labeled S1 and S2). The S1 + S2 column gives the sum of the values for S1 and S2 when these two specimens are not connected electrically. The electric field is obtained by dividing the voltage by the inter-electrode distance [95]. S1 S2 S1 + S2 (Sum)  Table 2. Effect of the bending of the series connection of two electrets (S1 + S2) in the form of flexible graphite. The electret voltage is compared for two cases, i.e., the case of no bending of the electrical connection and the case of bending of the connection. The two specimens (S1 + S2) positioned side by side are joined in a butt fashion at their edges. The inter-electrode distance corresponds to the distance between the proximate edges of the two voltage electrodes, with one electrode positioned close to the left end of the left specimen (S1) and the other electrode positioned close to the right end of the right specimen (S2) [95]. * For the case without connection bending, the butt joint is obtained by applying silver paint along the full length of the joint ( Figure 5, for S1 + S2). † For the case with connection bending, the butt joint is obtained by applying a layer of double-sided adhesive tape along the full length of the joint. The two specimens (S1 + S2) are electrically connected via aluminum foil leads, with a lead protruding from each specimen. The protruded leads are connected by applying silver paint on the leads (Figure 6, for S1 + S2). J. Low Power Electron. Appl. 2023, 13, x FOR PEER REVIEW 6 of 14 (a) (b) Figure 5. Configurations for the testing of electrets in the form of flexible graphite. Two specimens (labeled S1 and S2, Table 1) are electrically connected in series. The connection involves silver paint applied to the proximate edges of the two specimens so that the connection between the two specimens is not bent. (a) Schematic illustration (dimensions in mm). (b) Photograph with the 8-inch ruler (with divisions in inches and centimeters) having main divisions in cm in its lower scale [95]. Configurations for the testing of electrets in the form of flexible graphite. Two specimens (labeled S1 and S2, Table 1) are electrically connected in series. The connection involves silver paint applied to the proximate edges of the two specimens so that the connection between the two specimens is not bent.  Figure 6. Configurations for the testing of electrets in the form of flexible graphite. Two specimens (labeled S1 and S2, Table 1) are electrically connected in series, such that they are in contact along the full length of their proximate edges, except for double-sided adhesive tape between the proximate edges. The electrical connection is achieved by using two aluminum foil electrodes that have been applied to the specimens using silver paint, which is applied near the proximate edges of the two specimens. The foils protrude from the specimens and are connected by using silver paint. As a result of the protruded part of the foil, there is much bending (curling) in the connection. (a) Schematic illustration (dimensions in mm). (b) Photograph with the 8-inch ruler (with divisions in inches and centimeters) having main divisions in cm in its lower scale [95]. Configurations for the testing of electrets in the form of flexible graphite. Two specimens (labeled S1 and S2, Table 1) are electrically connected in series, such that they are in contact along the full length of their proximate edges, except for double-sided adhesive tape between the proximate edges. The electrical connection is achieved by using two aluminum foil electrodes that have been applied to the specimens using silver paint, which is applied near the proximate edges of the two specimens. The foils protrude from the specimens and are connected by using silver paint. As a result of the protruded part of the foil, there is much bending (curling) in the connection. (a) Schematic illustration (dimensions in mm). (b) Photograph with the 8-inch ruler (with divisions in inches and centimeters) having main divisions in cm in its lower scale [95].
Due to the electret voltage, current passes through a series of load resistances in accordance with Ohm's Law (Table 3, Figure 7) This supports the possibility of using the electret as a power source [95]. Table 3. Comparison of the measured and calculated values of the voltage across a load resistor. A flexible graphite specimen (a single piece, S1, Table 1) is in series with the load resistor [95].  The specimen is the one labeled S1 (Table 1) [95].

Resistor
The electret material is preferably not bent, as the bending would decrease the voltage because of the reduction in the degree of polarization continuity [95]. Polarization continuity is an emerging concept in the field of the dielectric properties of materials. This is because nonconductive materials that dominate the study of dielectric behavior are investigated in thin dimensions along the capacitance direction and the thin dimensions do not allow variations along the thickness direction for the purpose of studying the polarization continuity [96][97][98][99].

Conclusions
This is the first review of the emerging subject of conductive electrets (metals and carbons without poling), which provide a new avenue for low-power electronics. Electrets The specimen is the one labeled S1 (Table 1) [95].
The electret material is preferably not bent, as the bending would decrease the voltage because of the reduction in the degree of polarization continuity [95]. Polarization continuity is an emerging concept in the field of the dielectric properties of materials. This is because nonconductive materials that dominate the study of dielectric behavior are investigated in thin dimensions along the capacitance direction and the thin dimensions do not allow variations along the thickness direction for the purpose of studying the polarization continuity [96][97][98][99].

Conclusions
This is the first review of the emerging subject of conductive electrets (metals and carbons without poling), which provide a new avenue for low-power electronics. Electrets provide multiple electrical functions, namely, serving as power sources and providing capacitance and resistance, with the functions being tunable via the dimensions (both length and cross-sectional area) of the electret material.
Electrets provide a low-DC current voltage (µV) while allowing a low DC (µA) to pass through. Ohm's Law is obeyed. The voltage scales with the inter-electrode distance. Series connection of multiple electret components is effective if there is no bending at the electrical connection between the components, giving a series voltage equal to the sum of the voltages of the components. In the case of bending at the electrical connection, the series voltage is below the sum. Bending within a component also diminishes the voltage because of the decrease in the degree of polarization continuity.
The science is fundamentally different from that of all previously reported power sources. The electret originates from the interaction of a tiny fraction of the carriers with the atoms. This interaction results in the charge in the electret. Dividing the electret charge by the electret voltage V' gives the electret-based capacitance C', which is higher than the permittivity-based capacitance (conventional) by orders of magnitude. Both V' and permittivity depend on the microstructure, with the values increasing with decreasing grain size. The C' governs the electret energy (1/2 C'V' 2 ) and electret discharge time constant (RC', where R = resistance), as shown for metals. The discharge time increases with increasing inter-electrode distance. Electret discharge occurs upon short-circuiting and charges back upon subsequent open-circuiting. The discharge/charge of the electret involves that of C'.
Low-power power sources in the form of conductive electrets constitute a new field of science. This review focuses on emerging new science. The attraction of this new technology is that poling is not required and the electret materials (whether carbons or metals) are widely available. Moreover, because of the absence of any chemical reaction and the established wide utilization of the materials in structures (e.g., steel structures), there is no environmental issue. In fact, existing structures (as opposed to new structures) may be used as power sources. The electret voltage scales with the dimensions, so large structures are preferred. Because existing large structures are abundant in almost any city, large structures such as the steel rebars in a building may be used as power sources. This would involve applying electrodes to each piece (e.g., each rebar) of the structure. By staggering the discharge start time for different pieces of electret material in the same structure, continuous energy generation by the structure may be feasible. Thus, no energy storage is needed, in contrast to this need for wind and solar forms of energy. Both the common availability of electret materials and the absence of the need for energy storage are advantageous from the cost viewpoint. However, the very low power is a disadvantage, though it is expected to increase with the dimensions and with microstructural refinement, which enhances the abundance of sites for the carrier-atom interaction that is required for the polarization. As the emerging field progresses, the power is expected to increase. Data Availability Statement: Please refer to the original cited papers for the data availability, since this is a review paper.

Conflicts of Interest:
The authors declare no conflict of interest.