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Zhong L. Wang, et al.

Tribo-Electric Power







R. Ingham  : 'Tribo-electric,' the buzzword of the future?

G. Zhu, et al. : Radial-arrayed rotary electrification for high performance triboelectric generator

Videos

USPA 2014084748 : Triboelectric Nanogenerator for Powering Portable Electronics

WO2013151590 : Triboelectric Generator

US2013049531 : Triboelectric Generator   



http://phys.org/news/2014-03-tribo-electric-buzzword-future.html#jCp
Mar 04, 2014

'Tribo-electric,' the buzzword of the future?

by

Richard Ingham


A hand-held triboelectric generator for harvesting energy from human motions.
Credit: Drs. Guang Zhu and Zhong Lin Wang, Beijing Institute of Nanoenergy and Nanosystems and Georgia Tech

Out at sea, gentle waves provide power for thousands of homes. In cities, dancefloor moves generate electricity for nightclubs. In the countryside, hikers use leg power to recharge their phones.

It is an alluring goal of clean, reliable power free from geo-political risks — and scientists in the United States said Tuesday it lies within reach, thanks to a smart way to harvest energy called tribo-electricity.

Researchers at the Georgia Institute of Technology said they had built a simple prototype device that converts stop-start movement into power.

Waves, walking and dancing — even rainfall, computer keys or urban traffic — could one day be harnessed to drive sensors, mobile gadgets or even electricity plants, they contend.

Zhong Lin Wang, a professor of materials science and engineering, described the invention a "breakthrough."

"Our technology can be used for large-scale energy harvesting, so that the energy we have wasted for centuries will be useful," he told AFP by email.

"Tribo-electric" is a modern term with ancient roots — from the Greek word for "rub."

Its electricity is created from friction between two substances causing a charge of electrons to be transferred from one to the other.

It commonly happens, for instance, when plastic-soled shoes are in contact with a nylon carpet, causing the snap of static discharge when one's hand touches a metal doorknob.

Because tribo-electric is so unpredictable, it has been generally shunned as a power source.

The preferred method has been magnetic induction — a turbine driven by nuclear- or fossil-powered steam or water.

Powering spot lights through harvesting energy from air flow by the triboelectric generator.
Credit: Drs. Guang Zhu and Zhong Lin Wang, Beijing Institute of Nanoenergy and Nanosystems and Georgia Tech

But, in a new study published in the journal Nature Communications, Wang's team said they had overcome key hurdles to converting a haphazardly-generated electrical charge into current.

Their prototype comprises a disc about 10 centimetres (four inches) across, designed to show the potential from a small, portable generator moved by ambient energy.

Inside are two circular sheets of material, one an electron "donor" and the other an electron "receiver," brought together through rotary movement.

If the sheets are separated, one then holds an electrical charge isolated by the gap between them.

Sandwiched between the two discs is a third disc with electrodes, which bridges the gap and helps a small current to flow.

At a top speed of 3,000 revolutions per minute, the device generated 1.5 watts.

This gave it an energy efficiency of 24 percent, three times greater than piezoelectric, the previously best source of mechanical electricity harvesting—and as efficient as magnetic-induction turbines.

It can run on a gentle wind or tap water, or "random jerky motions," including human movement, to provide the rotation, Wang said.

"As long as there is mechanical action, there is power that can be generated."

The prototype used copper for the rotator and gold for the electrodes in lab tests, but these could easily be substituted for low-cost synthetics, he said.

The team is working on ways to scale up tribo-electric energy for harvesting power from the ocean.





http://www.nature.com/ncomms/2014/140304/ncomms4426/full/ncomms4426.html
Nature Communications 5,#3426
doi:10.1038/ncomms4426
4 March 2014

Radial-arrayed rotary electrification for high performance triboelectric generator

Guang Zhu, Jun Chen, Tiejun Zhang, Qingshen Jing & Zhong Lin Wang    

Harvesting mechanical energy is an important route in obtaining cost-effective, clean and sustainable electric energy. Here we report a two-dimensional planar-structured triboelectric generator on the basis of contact electrification. The radial arrays of micro-sized sectors on the contact surfaces enable a high output power of 1.5?W (area power density of 19?mW?cm-2) at an efficiency of 24%. The triboelectric generator can effectively harness various ambient motions, including light wind, tap water flow and normal body movement. Through a power management circuit, a triboelectric-generator-based power-supplying system can provide a constant direct-current source for sustainably driving and charging commercial electronics, immediately demonstrating the feasibility of the triboelectric generator as a practical power source. Given exceptional power density, extremely low cost and unique applicability resulting from distinctive mechanism and structure, the triboelectric generator can be applied not only to self-powered electronics but also possibly to power generation at a large scale.

( Click to enlarge ) --





VIDEOS

http://www.nature.com/ncomms/2014/140304/ncomms4426/extref/ncomms4426-s2.avi
Supplementary Movie 1 (2,884 KB)
Electric potential distribution during rotation in open-circuit condition

http://www.nature.com/ncomms/2014/140304/ncomms4426/extref/ncomms4426-s3.avi
Supplementary Movie 2 (3,609 KB)
Powering spot lights by a rotary triboelectric generator

http://www.nature.com/ncomms/2014/140304/ncomms4426/extref/ncomms4426-s4.avi
Supplementary Movie 3 (4,087 KB)
Reading illumination enabled by a rotary triboelectric generator

http://www.nature.com/ncomms/2014/140304/ncomms4426/extref/ncomms4426-s5.avi
Supplementary Movie 4 (1,177 KB)
Powering a globe light by a rotary triboelectric generator

http://www.nature.com/ncomms/2014/140304/ncomms4426/extref/ncomms4426-s6.avi
Supplementary Movie 5 (1,523 KB)
Powering candelabra lights by a rotary triboelectric generator

http://www.nature.com/ncomms/2014/140304/ncomms4426/extref/ncomms4426-s7.avi
Supplementary Movie 6 (1,864 KB)
Powering LEDs with full brightness by a rotary triboelectric generator

http://www.nature.com/ncomms/2014/140304/ncomms4426/extref/ncomms4426-s8.avi
Supplementary Movie 7 (3,102 KB)
Powering a wireless emitter by a rotary triboelectric generator

http://www.nature.com/ncomms/2014/140304/ncomms4426/extref/ncomms4426-s9.avi
Supplementary Movie 8 (3,836 KB)
Powering a digital clock by a rotary triboelectric generator

http://www.nature.com/ncomms/2014/140304/ncomms4426/extref/ncomms4426-s10.avi
Supplementary Movie 9 (3,185KB)
Charging a cellphone by a rotary triboelectric generator

http://www.nature.com/ncomms/2014/140304/ncomms4426/extref/ncomms4426-s11.avi
Supplementary Movie 10 (2,631 KB)
Harvesting energy from air flow by a rotary triboelectric generator

http://www.nature.com/ncomms/2014/140304/ncomms4426/extref/ncomms4426-s12.avi
Supplementary Movie 11 (2,271 KB)
Harvesting energy from water flow by a rotary triboelectric generator

http://www.nature.com/ncomms/2014/140304/ncomms4426/extref/ncomms4426-s13.avi
Supplementary Movie 12 (2,169 KB)
Harvesting energy from body motion by a rotary triboelectric generator



PATENTS

Triboelectric Nanogenerator for Powering Portable Electronics
US2014084748


Inventor: WANG ZHONG LIN // WANG SIHONG

A triboelectric generator includes a first contact charging member and a second contact charging member. The first contact charging member includes a first contact layer and a conductive electrode layer. The first contact layer includes a material that has a triboelectric series rating indicating a propensity to gain electrons due to a contacting event. The conductive electrode layer is disposed along the back side of the contact layer. The second contact charging member is spaced apart from and disposed oppositely from the first contact charging member. It includes an electrically conductive material layer that has a triboelectric series rating indicating a propensity to lose electrons when contacted by the first contact layer during the contacting event. The electrically conductive material acts as an electrode. A mechanism maintains a space between the first contact charging member and the second contact charging member except when a force is applied thereto.

[0001] This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/704,138, filed Sep. 21, 2012, and U.S. Provisional Patent Application Ser. No. 61/754,992, filed Jan. 22, 2013, the entirety of each of which is hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0003] 1. Field of the Invention

[0004] The present invention relates to electric power generators and, more specifically, to generators that derive power from mechanical contact between surfaces.

[0005] 2. Description of the Related Art

[0006] Wireless, portable and multi-function electronic devices require independent and maintenance-free power sources. The emerging technologies for mechanical energy harvesting are effective and promising approaches for building self-powered systems, because of a great abundance of mechanical energy existing in the environment and the human body. Piezoelectric nanogenerators have been developed to convert tiny-scale mechanical energy into electricity. Another approach to providing power is though triboelectric nanogenerators based on the contact-electrification effect. Triboelectric nanogenerators harvest mechanical energy through a periodic contact and separation of two polymer plates. However, most triboelectric nanogenerators have limited power output.

[0007] Therefore, there is a need for triboelectric nanogenerators with increased power output.

SUMMARY OF THE INVENTION

[0008] The disadvantages of the prior art are overcome by the present invention which, in one aspect, is a triboelectric generator that includes a first contact charging member and a second contact charging member. The first contact charging member includes a first contact layer and a conductive electrode layer. The first contact layer has a contact side and an opposite back side. The first contact layer includes a material that has a triboelectric series rating indicating a propensity to exchange electrons due to a contacting event. The conductive electrode layer is disposed along the back side of the contact layer. The second contact charging member is spaced apart from and disposed oppositely from the first contact charging member. The second contact charging member includes an electrically conductive material layer that has a triboelectric series rating indicating a propensity to exchange electrons when contacted by the first contact layer during the contacting event. The electrically conductive material layer is configured to act as an electrode. A mechanism is configured to maintain a space between the first contact charging member and the second contact charging member except when a force is applied thereto.

[0009] In another aspect, the invention is a triboelectric generating device that includes a first contact charging member and a second contact charging member. The first contact charging member includes a substrate layer having a first side and a second side, a first contact layer and a conductive electrode layer. The substrate layer is configured to maintain a concave shape unless a force is applied thereto. The first contact layer has a contact side and is affixed to the first side of the substrate layer. The first contact layer includes a material that has a triboelectric series rating indicating a propensity to gain electrons due to a contacting event. The conductive electrode layer is affixed to the second side of the substrate layer. The second contact charging member is spaced apart from and disposed oppositely from the first contact charging member. The second contact charging member includes an electrically conductive metal layer that has a triboelectric series rating indicating a propensity to lose electrons when contacted by the first contact layer. The second contact charging member defines a concave shape that is reflective of the concave shape of the substrate layer.

[0010] In yet another aspect, the invention is a triboelectric generating system that includes a first contact charging member, a second contact charging member and at least one spring. The first contact charging member, the first contact charging member includes a first rigid substrate, a conductive electrode layer disposed on the substrate and a first contact layer. The first contact layer is disposed on the conductive electrode layer and includes a material that has a triboelectric series rating indicating a propensity to gain electrons due to a contacting event. The second contact charging member is spaced apart from and disposed oppositely from the first contact charging member, and includes a second rigid substrate and an electrically conductive material layer. The electrically conductive material layer has a triboelectric series rating indicating a propensity to lose electrons when contacted by the first contact layer disposed on the second rigid substrate. The spring is configured to maintain space between the first contact charging member and the second contact charging member except when a force is applied thereto.

[0011] These and other aspects of the invention will become apparent from the following description of the preferred embodiments taken in conjunction with the following drawings. As would be obvious to one skilled in the art, many variations and modifications of the invention may be effected without departing from the spirit and scope of the novel concepts of the disclosure.

BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWINGS

[0012] FIGS. 1A-Eare a series of schematic views showing operation of a first embodiment of a triboelectric generator.

[0013] FIG. 2is a graph relating charge density to separation distance.

[0014] FIG. 3is a schematic view showing a triboelectric generator with patterned surfaces.

[0015] FIG. 4Ais a schematic side view of a second embodiment of a triboelectric generator.

[0016] FIG. 4Bis a schematic side view of the embodiment shown in FIG. 4Awhen compressed by an activating force.

[0017] FIG. 4Cis a schematic plan view of a first contact charging member employed in the embodiment shown in FIG. 4A.

[0018] FIG. 4Dis a schematic plan view of a second contact charging member employed in the embodiment shown in FIG. 4A.



DETAILED DESCRIPTION OF THE INVENTION

[0019] A preferred embodiment of the invention is now described in detail. Referring to the drawings, like numbers indicate like parts throughout the views. Unless otherwise specifically indicated in the disclosure that follows, the drawings are not necessarily drawn to scale. As used in the description herein and throughout the claims, the following terms take the meanings explicitly associated herein, unless the context clearly dictates otherwise: the meaning of "a," "an," and "the" includes plural reference, the meaning of "in" includes "in" and "on."

[0020] U.S. patent application Ser. No. 13/598,132, filed on Aug. 29, 2012 by Wang et al. discloses methods of making triboelectric generators and components thereof and is incorporated herein by reference for the purpose of disclosing such methods.

[0021] As shown in FIG. 1A, one embodiment of a triboelectric generator 100includes a first contact charging member 110that has a first substrate material layer 112, which could include poly(4,4'-oxydiphenylene-pyromellitimide) (which is sold under the trademark "Kapton"), adjacent to a second substrate material layer 113, which can include a material such as SiO 2. The first contact charging member 110also includes a first contact layer 114, which could include, for example, PDMS, PTFE, FEP, PVC, and a Polyimide, or any material that has a triboelectric series rating indicating a propensity to gain electrons due to a contacting event. The first substrate material layer 112and the second substrate material layer 113are applied to each other at an elevated temperature and then cooled so that differential shrinkage during cooling causes the contact charging member to have a concave shape. The first substrate material layer 112can include a polymer sheet having a first thermal expansion coefficient and the second substrate material layer 113can include a ceramic film having a second thermal expansion coefficient that is less than the first thermal expansion coefficient. A second contact charging member 120includes an electrically conductive metal layer (which could include a material such as aluminum, a metal or a conductive polymer) that has a triboelectric series rating indicating a propensity to lose electrons when contacted by the first contact layer 114. The second contact charging member 120defines a concave shape that is reflective of the concave shape of the substrate layer so that when the second contact charging member 120is placed adjacent to the first contact layer 114, the two layers define a space 122therebetween unless a force is applied to the layer to bring them together. A first conductive electrode 116, which is applied to the second substrate material layer 113, and the second contact charging member 120, which acts as an electrode as a result of its conductivity, can be coupled to a load 10to apply current thereto.

[0022] As shown in FIGS. 1B-1E, triboelectric nanogenerator 100generates current by applying a cycled compressive force onto the whole area of the device, so that the bending plates will be periodically pressed to flatten and contact closely with each other, as shown in FIG. 1B. Once released, as shown in FIG. 1C, the two plates will separate apart due to the stored elastic energy and revert instantaneously back to their original arch shape due to resilience. A cycled generation of the potential difference between the two electrodes 112and 120drives the flow of electrons through the load 10. At the original state before the contact of the triboelectric films (as shown in FIG. 1A), there is no charge transferred, thus no electric potential. Upon the pressing of the two films towards each other, they will be brought fully into surface contact (as shown in FIG. 1B) and possibly relative sliding would occur, which results in electrons being transferred from a material in the positive side of the triboelectric series to the one in the negative side in the series. Accordingly, electrons will be injected from the aluminum surface 120to PDMS surface 114, leaving positive charges on aluminum foil 120. The positive triboelectric charges on the conductive aluminum foil 120attract the electrons in the opposite electrode to flow through the external load 10. After cycles of deformation, when the device is press and the surfaces with charges are in close contact with each other, all of the triboelectric charges will stay on the inner surfaces with the same surface density ([sigma]0). Once the pressing force is released, the triboelectric nanogenerator 100will immediately rebound back to its original arch shape due to the elasticity of the film so that a gap 122will form again between the two plates, as shown in FIG. 1C. The electric field generated by the separated surface charges will then give rise to a much higher potential on the aluminum foil side 120than the top electrode 116. Such a potential difference will drive the flow of positive charges from aluminum foil to the top electrode through the external load 10until the potential difference is fully offset by the transferred charges, rendering the top electrode with a surface charge density of ([Delta][sigma]), while the aluminum electrode 120is left with ([sigma]0-[Delta][sigma]), as shown in FIG. 1D. Subsequently, when the triboelectric nanogenerator is pressed again to reach the close contact of the two plates, as shown in FIG. 1E, these redistributed charges will inversely build a positive potential on the top electrode 116, which will drive all of the transferred charges ([Delta][sigma]) to flow back to the inner surface of the aluminum foil 120. Then a cycle is achieved and the device will go back to the equilibrium state depicted in FIG. 1B.

[0023] Both the voltage and current outputs are related to the amount of charges transferred (A[Delta][sigma], A is surface area of the plate), which is determined by the triboelectric charge density ([sigma]0) and the separation distance of the two plates. The results of an analytical calculation 200based on a simplified model of quasi-infinite flat plates, indicates the magnitude of the distance required for the optimum output, as shown in FIG. 2. When the separation distance starts to increase from 0 to 0.7 mm, [Delta][sigma] keeps a very rapid increase from 0 to ~90% of [sigma]0. Then, the slope of this curve starts to decrease. Thus, both an intimate contact and a subsequent separation of nearly 1 mm result in the phenomenal transferring of charges.

[0024] In one experimental embodiment, the electric output measurement was performed on an arch-shaped triboelectric nanogenerator device in a size of 3 cm*2.8 cm, with the triggering frequency of 6 Hz and controlled amplitude. Since the accumulation of the triboelectric charges increases and reaches equilibrium in a certain period of time after multiple cycles, the output will gradually go up in the first stage upon deformation. Then, the open-circuit voltage (VOC) will stabilize at 230 V, measured by an electrometer with infinite input resistance. When the bottom Al is connected to the positive probe of the electrometer, upon the release of the pressing force a positive voltage is generated because of the immediate charge separation. Since the electrons cannot flow in an open-circuit condition, the voltage will hold at a plateau unless there is a quick leakage. The peak value of the short-circuit current (ISC) reaches 94 [mu]A, corresponding to the half cycle of pressing that is in higher straining rate than releasing. The integration of the each current peak gives the total charges transferred in a half cycle of deformation. Moreover, when the connection polarity to the electrometer is switched, both the voltage and current signal will be completely reversed. The gap from the arch-shaped structure is an important factor for the enhanced output, because without the SiO2-film-introduced bending, there will be much smaller and unstable electrical output.

[0025] As shown in FIG. 3, the triboelectric effect can be enhanced by using texture patterned surfaces on the first contact layer 114and the second contact layer 120. For example the surfaces can employ an array of pyramid structures 310or an array of box-like structures 312, or a combination of these structures or one of many other shaped structures formed in the surfaces.

[0026] Fabricating one experimental embodiment began with photolithographic patterning of 4 in. (100) Si wafers with thermally grown SiO 2on top. The patterned wafer with the array of square window openings was firstly etched by buffered-oxide-etching process to transfer the pattern onto the SiO 2layer. Then, the wafer was etched in KOH solution to fabricate the recessed features of pyramid. After cleaned with acetone, isopropanol and ethanol in sequence, the Si molds were treated with trimethylchlorosilane (Sigma Aldrich) by gas phase silanization to enable the easy peel-off of the PDMS film from the Si mold in the following step. In preparing the patterned PDMS film, the elastomer and the cross-linker (Sylgard 184, Dow Corning) were mixed in a 10:1 ratio (w/w), and then casted on the Si mold. After the degassing process under the vacuum, the mixture was spin-coated on the Si mold at 500 rpm for 60 s. After the thermally curing process at 85[deg.] C. for 1 hour, the PDMS inked with pyramid patterns was peeled off from Si mold. The surface without patterns were glued to the inner surface of the bending Kapton substrate.

[0027] The triboelectric nanogenerator can be integrated with a battery, such as a lithium ion battery. In assembling such an embodiment, two different slurries for the two working electrodes were made, which contain 70 wt % active material (LiCoO 2for cathode and graphite for anode), 10 wt % carbon black powders as conductor, 20 wt % polyvinylidene fluoride (PVDF) binder and N-methyl-2-pyrrolidone (Sigma Aldrich) as the solvent. Then the slurries were cast onto the current collectors (Al foil for cathode and Cu foil for anode) respectively with a uniform thickness of 10 [mu]m. The electrodes were baked at 110[deg.] C. under vacuum for 12 hours. Stainless-steel coin cells were used for the battery assembly. The cathode and anode electrodes were stacked with a piece of polyethylene (PE) separator (MTI Corporation) in between. After the system was filled with electrolyte (1M LiPF6 in 1:1:1 ethylene carbonate: dimethyl carbonate: diethyl carbonate, Novolyte Technologies), the coin-cell was finally sealed.

[0028] In another embodiment of a triboelectric nanogenerator 400, as shown in FIGS. 4A-4D, the first contact charging member 410includes a first rigid substrate 412to which a conductive electrode layer 414is applied. The first rigid substrate 412can include a material such as polymethyl methacrylate (PMMA). The conductive electrode layer 414can include, for example, a material such as gold, a metal, ITO, or a conducting polymer. A first contact layer 416is applied to the conductive electrode layer 414. The first contact layer 416is made from a material that has a triboelectric series rating indicating a propensity to gain electrons due to a contacting event, such as, for example, PDMS, PTFE, FEP, PVC, or a polyimide. A second contact charging member 420includes a second rigid substrate 422, which can include can include a material such as polymethyl methacrylate (PMMA). An electrically conductive material layer 423is applied to the second rigid substrate 422. Electrically conductive material layer 423has a triboelectric series rating indicating a propensity to lose electrons when contacted by the first contact layer 416. In one representative embodiment, the electrically conductive material layer 423includes a gold film 424and a plurality of gold nanoparticles 426disposed thereon on. One or more springs 430maintain a space 432between the first contact charging member 410and the second contact charging member 420except when a force is applied thereto. The springs 430may be held in place by receptacles 434defined by the rigid substrates 412and 422.

[0029] Compression of the first contact charging member 410and the second contact charging member 420is shown in FIG. 4B. The use of nanoparticles 426increases the contact are substantially, increasing the triboelectric effect. This embodiment of a triboelectric nanogenerator 400offers a substantially higher power output due nanoparticle 426-based surface modification. In one experimental embodiment, short-circuit current reached a peak value of 2.0 mA, which corresponded to the instantaneous power output of 1.2 W and power density of 322.7 W/m <2>. Average power output and energy conversion efficiency at device level was calculated to be 132 mW and 9.8%, respectively. This embodiment also realized scaling up of nanogenerator power output, making it power not just a single electronic device but hundreds of them simultaneously. Triggered by a commonly available mechanical source such as footfalls, the nanogenerator was capable of instantaneously lighting up 600 commercial LED lamps in real time. This indicates the practicability of harvesting mechanical energy by the nanogenerator on a large scale. The working mechanism demonstrated here can be further applied to potentially harvest large-scale mechanical energy such as from rolling wheels, wind power, and ocean waves.

[0030] In one experimental embodiment, the nanogenerator has a layered structure with two substrates. Polymethyl methacrylate (PMMA) was selected as the material for substrates due to its decent strength, light weight, easy processing, and low cost. On the lower side, a layer of contact electrode is prepared. The contact electrode plays dual roles of electrode and contact surface. It consists of a gold thin film and gold nanoparticles coated on the surface. Alternatively, nanoparticles of non-precious metals can also be used as replacements. They modify the surface both physically and chemically. On the other side, a thin film of gold is laminated between the substrate and a layer of polydimethylsiloxan (PDMS). This electrode is referred to as the "back electrode" for later reference. The two substrates are connected by four springs installed at the corners, leaving a narrow spacing between the contact electrode and the PDMS.

[0031] The electric energy generation process can be explained by the coupling between triboelectric effect and electrostatic effect. At the original position, a separation distance is maintained by springs. When an external impact is applied onto one of the substrates, the gold and PDMS are brought into contact. According to the triboelectric series that ranks materials' tendency to gain or lose electrons, electrons are injected from gold into PDMS, resulting in surface triboelectric charges, which are retained on the PDMS. As the impact is withdrawn, the contacting surfaces move apart due to restoring force from the springs. Once a separation forms, the back electrode possess a higher electric potential than the contact electrode, producing an electric potential difference. Such a potential difference drives electrons through external loads and screens the positive triboelectric charges on the contact electrode. When the nanogenerator reverts back to the original position, positive triboelectric charges on the contact electrode are completely screened, leaving equal amount of inductive charges on the back electrode. Subsequently, mechanical impact once again shortens the separation, producing an electric potential difference with reversed polarity. In consequence, electrons flow in a reversed direction. They keep screening inductive charges on the back electrode until a direct contact is again established. The insulating PDMS allows long-time retention of the triboelectric charges on its surface even through the triboelectric charges on the metal side are periodically screened by inductive charges. In this process, the nanogenerator acts as an electron pump that drives electrons back and forth between the two electrodes.

[0032] The nanogenerator's electric output is strongly related to the contacting force, yielding higher output with larger force. At a force as small as 10 N, the nanogenerator can still produce I scranging from 160 [mu]A to 175 [mu]A. When the force increases to 500 N, the electric output reaches a saturated value, producing a peak I scof 1.2 mA. This result is due to increased contact area with larger force. The two contacting surfaces are neither absolutely flat nor smooth. Surface roughness may be caused by inherent curvature of the substrates, nanoparticle modification, and fabrication defects such as particle contamination from the air. At small contacting force, the surface roughness prevents fully intimate contact between the contact electrode and the PDMS, leaving some areas untouched. With increased force, due to elastic property, the PDMS can deform and fill more vacant space, thus leading to larger contact area. As a result, the electric output increases until all the vacant space is completely filled by the PDMS, reaching a saturated limit.

[0033] As an important figure of merit, energy conversion efficiency of the nanogenerator was calculated. The conversion efficiency is defined as the ratio between the electric energy that is delivered to load by the nanogenerator and the mechanical energy the nanogenerator possesses. FIG. 4 ais a current pulse output produced by human footfall at load resistance of 1 M[Omega]. The time span between t 1and t 2represents a single contact. With an external load of pure resistance, the electric energy delivered by the nanogenerator is equal to the Joule heating energy, which is presented below.

[0000]
E electric =Q=[integral] t1 <t2> I <2> .R.dt=R.[integral] t1 <t2> I <2> .dt=1*10 <6>([Omega]).[integral] 22.7144 <22.7200> I <2> .dt=0.74 mJ (1)

[0000] where Q is the Joule heating energy, I is the instantaneous current, and R is the load resistance. Consequently, the average power output (W average) can be obtained by

[0000] [mathematical formula]

[0034] As soon as the mechanical energy is introduced, it presents in two forms, i.e. elastic energy stored in the springs and kinetic energy carried by a moveable substrate of the nanogenerator. The elastic energy is later released without converting into electric energy, which is calculated by

[0000] [mathematical formula]

[0000] where k is the spring constant (k=1278.88 N/m), x is the displacement of a spring that is equal to the spacing between the two contacting surfaces (x=1 mm), and N is the number of springs (N=4).

[0035] For kinetic energy, at the moment when the two substrates make a contact, it completely transforms to other forms, including electric energy and thermal energy. It can be calculated by the following equation.

[0000] [mathematical formula]

[0000] where m is the mass of the moveable substrate (m=13.45 g, the mass of gold thin film and PDMS layer are negligible), and the v is the velocity of the substrate when a contact is just about to be made (v=0.86 m/s).

[0036] Therefore, the energy conversion efficiency ([eta]) is calculated as

[0000] [mathematical formula]

[0000] It is to be noted that the above result is the overall efficiency at device level. However, at conversion process level, the elastic energy stored in the springs does not participate in energy conversion. Therefore if we solely take into account the kinetic energy that actually partially converts to electric energy, the direct efficiency at conversion process level is

[0000] [mathematical formula]

[0037] The unprecedentedly high power output of the nanogenerator is mainly attributed to three factors. Firstly, the contact electrode plays dual roles of electrode and contacting surface. Compared to previously reported designs in which both of the contacting surfaces are made of polymers, more inductive charges will be generated for the new design. Secondly, the elastic property of PDMS enables conformal contact despite of surface roughness. The PDMS can easily deform in response to small pressure and fill the otherwise vacant space that result from substrate curvature and fabrication defects. The tolerance on surface roughness ensures as much contact area as it can be possibly obtained. Also, the surface modification by gold nanoparticles plays an important role for the output enhancement. It can offer five-fold increase on the current output compared to the device without modification. Physically, the bumpy surface of the nanoparticle provides a larger contact area than a flat surface does. Chemically, the as-synthesized gold nanoparticles are positively charged in nature. The pre-carried positive charges may be able to add up with triboelectric charges upon contact, leading to a largely enhanced surface charge density and thus a substantially higher electric output.

[0038] In one experimental embodiment, the following fabrication methods were employed.

[0039] Materials: Hexadecyltrimethylammonium bromide (>=99%) was purchased from Sigma. Sodium tetrachloroaurate dihydrate (99%) and 1,4-Benzenedithiol (99%) were purchased from Aldrich. Hydrazine hydrate solution (78-82%) was purchased from Sigma-Aldrich. Deionized water was obtained using a ultrapure (18.2 M[Omega]-cm) system.

[0040] Synthesis of gold nanoparticles: A solution (50 mL) containing Sodium tetrachloroaurate dihydrate (1 mM) and hexadecyltrimethylammonium bromide (10 mM) was brought to a vigorous boil with stifling in a round-bottom flask fitted with a reflux condenser; Hydrazine hydrate solution (20 [mu]L) was then added rapidly to the solution. The solution was heated under reflux for another 8 min, during which time its color changed from pale yellow to pale red. The solution was cooled to room temperature while stirring continuously. The average size (56 nm) of the synthesized gold nanoparticles was verified through SEM analysis.

[0041] Self-assembly of gold nanoparticles onto Au thin film: Au films were derivatized by immersion in a solution of 1,4-benzenedithiol for 12 h and rinsed with methanol and then water. The derivatized Au films were then immersed in a solution of gold nanoparticles for 12 hours to allow for complete adsorption of a single gold nanoparticle layer. Before the SEM characterization and electrical measurement, non-adsorbed gold nanoparticles were removed by rinsing with water.

[0042] Fabrication of the triboelectric nanogenerator: To fabricate the nanogenerator, two pieces of cast acrylic glass were prepared as substrates with dimensions of 3 inch by 3 inch by 3/32 inch. Four half-thorough holes were drilled at corners as houses for spring installation. 50 nm of gold was deposited on both of the substrates by e-beam evaporator (2 inch by 3 inch). On one of the substrates, fluid PDMS that consisted of base and curing agent in a ratio of 5:1 was spin-coated to form a 10 [mu]m-thick layer. Then it was cured at 100[deg.] C. for 45 minutes. On the other substrate, gold nanoparticles were uniformly distributed on gold surface by self-assembly. Subsequently, four springs (spring constant=7.3 lb/inch) were installed in the houses to connect the two substrates together, leaving a spacing of 1 mm between the gold and the PDMS. The spacing is required to be substantially larger than the polymer thickness to ensure effective generation of inductive charges. Finally, conducting wires were connected to the two metal layers as leads for subsequent electric measurement or for connection to an external load.



TRIBOELECTRIC GENERATOR
WO2013151590


Inventor: WANG ZHONG // LIN LONG

A generator includes a thin first contact charging layer (214) and a thin second contact charging layer (213). The thin first contact charging layer (214) includes a first material that has a first rating on a triboelectric series. The thin first contact charging layer (214) has a first side with a first conductive electrode applied (219) thereto and an opposite second side. The thin second contact charging layer (213) includes a second material that has a second rating on a triboelectric series that is more negative than the first rating. The thin second contact charging layer (213) is disposed adjacent to the first contact charging layer (214) so that the second side of the second contact charging layer (213) is in contact with the second side of the first contact charging layer (214). The second side of at least a selected one of the first contact charging layer (214) and the second contact charging layer (214) includes a molded texture (216).

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to generators and, more specifically, to a system for generating voltage and current using the triboelectric effect.

[0004] 2. Description of the Related Art

[0005] Energy harvesting by converting ambient energy into electricity may offset the reliance of small portable electronics on traditional power supplies, such as batteries. When long-term operation of a large number of electronic devices in dispersed locations is required, energy harvesting has the advantages of outstanding longevity, relatively little maintenance, minimal disposal and contamination. Despite of these benefits, superior performance, miniaturized size and competitive prices are still to be sought after in order for energy harvesting technology becoming prevalent.

[0006] The triboelectric effect is a type of contact electrification in which certain materials become electrically charged after they come into contact with another such as through friction. It is the mechanism though which static electricity is generated.

Triboelectric effect associated electrostatic phenomena are the most common electrical phenomena in our daily life, from walking to driving, but the triboelectric effect has been largely ignored as an energy source for electricity. Some electrostatic microgenerators have been developed and used in research relating to microelectromechanical systems (MEMS), but such designs tend to be based on inorganic materials and the fabrication of such devices requires complex processes. [0007] Therefore, there is a need for a reliable, small and easily manufactured system for harvesting triboelectric energy.

SUMMARY OF THE INVENTION

[0008] The disadvantages of the prior art are overcome by the present invention which, in one aspect, is a generator that includes a thin first contact charging layer and a thin second contact charging layer. The thin first contact charging layer includes a first material that has a first rating on a triboelectric series. The thin first contact charging layer has a first side with a first conductive electrode applied thereto and an opposite second side. The thin second contact charging layer includes a second material that has a second rating on a triboelectric series that is more negative than the first rating. The thin first contact charging layer has a first side with a first conductive electrode applied thereto and an opposite second side. The thin second contact charging layer is disposed adjacent to the first contact charging layer so that the second side of the second contact charging layer is in contact with the second side of the first contact charging layer.

[0009] In another aspect, the invention is a triboelectric generator that includes a first conductive electrode layer, a second conductive electrode layer, a first contact charging layer and a second contact charging layer. The first contact charging layer has a first side and an opposite second side. The first conductive electrode layer is disposed on the first side of the first contact charging layer. The first contact charging layer includes a first material that has a first rating on a triboelectric series. The first contact charging layer has a thickness sufficiently thin so that a positive excess charge on the second side induces an electric field that induces negative charge carriers to form in the first conductive electrode layer. The second contact charging layer has a first side and an opposite second side. The second conductive electrode layer is disposed on the first side of the second contact charging layer. The second contact charging layer includes a second material that his a second rating on the triboelectric series wherein the second rating is more negative than the first rating. The second side of the second contact charging layer is disposed against the second side of the first contact charging layer. The second contact charging layer has a thickness sufficiently thin so that a negative excess charge on the second side induces an electric field that induces positive charge carriers to form in the second conductive electrode layer. Relative movement between contacting portions of the second side of the first contact charging layer and the second side of the second contact charging layer results in excess positive charge on the second side of the first contact charging layer and excess negative charge on the second side of the second contact charging layer.

[0010] In yet another aspect, the invention is a method of generating an electrical current and voltage in which a first contact charging layer is brought in contact with a second contact charging layer. The first contact charging layer has a first side and an opposite second side. A first conductive electrode layer is disposed on the first side of the first contact charging layer. The first contact charging layer includes a first material that has a first rating on a triboelectric series. The first contact charging layer has a thickness sufficiently thin so that a positive excess charge on the second side induces an electric field that induces negative charge carriers to form in the first conductive electrode layer. The second contact charging layer has a first side and an opposite second side. A second conductive electrode layer is disposed on the first side of the second contact charging layer. The second contact charging layer includes a second material that his a second rating on the triboelectric series wherein the second rating is more negative than the first rating. The second side of the second contact charging layer is disposed against the second side of the first contact charging layer. The second contact charging layer has a thickness sufficiently thin so that a negative excess charge on the second side induces an electric field that induces positive charge carriers to form in the second conductive electrode layer. Relative motion between the first contact charging layer and the second contact charging layer is caused. A load is applied between the first conductive electrode layer and the second electrode layer, thereby causing an electrical current to flow through the load.

[0011] These and other aspects of the invention will become apparent from the following description of the preferred embodiments taken in conjunction with the following drawings. As would be obvious to one skilled in the art, many variations and modifications of the invention may be effected without departing from the spirit and scope of the novel concepts of the disclosure. BRIEF DESCRIPTION OF THE

FIGURES OF THE DRAWINGS

[0012] FIG. 1 is a schematic diagram of one representational embodiment.

[0013] FIG. 2 is a schematic diagram of several generating units stacked and coupled in series.

[0014] FIGS. 3A-3E is a plurality of schematic views showing a pressing-releasing sequence, of one embodiment.

[0015] FIG. 3F is a graph demonstrating voltage relationships during the sequence shown in FIGS. 3A-3E.

[0016] FIG. 3G is a graph demonstrating current relationships during the sequence shown in FIGS. 3A-3E.

[0017] FIGS. 4A-4E is a plurality of schematic views showing a method of making an embodiment with a textured surface.

[0018] FIG. 5 is schematic diagram of an embodiment according to FIGS. 4A-4E.

[0019] FIGS. 6A-6B are schematic views of alternate textures.

[0020] FIG. 7 is schematic diagram of an embodiment employing a surface textured with nano wires.

[0021] FIG. 8 A is a micrograph of a contact charging layer having a surface with a
pyramidal texture.

[0022] FIG. 8B is a micrograph of a contact charging layer having a surface with a
rectangular prism texture.

[0023] FIG. 8C is a micrograph of a contact charging layer having a surface with a row texture.



DETAILED DESCRIPTION OF THE INVENTION


[0024] A preferred embodiment of the invention is now described in detail. Referring to the drawings, like numbers indicate like parts throughout the views. Unless otherwise specifically indicated in the disclosure that follows, the drawings are not necessarily drawn to scale. As used in the description herein and throughout the claims, the following terms take the meanings explicitly associated herein, unless the context clearly dictates otherwise: the meaning of "a," "an," and "the" includes plural reference, the meaning of "in" includes "in" and "on."

[0025] As shown in FIG. 1, a triboelectric energy harvesting system may be embodied as a generator or a sensor unit 100 that includes a first contact charging layer 110 that has a first conductive electrode layer 113 disposed on a first side 112 and that has an opposite second side 114 that has a textured surface. The first contact charging layer 110 includes a material with a relatively less negative triboelectric series rating. Examples of suitable materials can include: polyethylene terephthalate (PET), poly(methyl methacrylate) (PMMA), a conductor, a metal, an alloy and combinations thereof. A first conductive electrode layer 116 is applied to the first side of the first contact charging layer 112.

[0026] A second contact charging layer 120 is disposed against the first contact charging layer 110. The second contact charging layer 120 includes a material that has a rating on the triboelectric series that is more negative than that of the material of the first contact charging layer 110. Examples of such materials can, for example, include poly-oxydiphenylene- pyromellitimide (such as Kapton), polydimethylsiloxane, a conductor, a metal, an alloy and combinations thereof. The second contact charging layer 120 includes a first side 122 and an opposite second side 124 and a second conductive electrode layer 126 is applied to the second side 122. The first side 122 also includes a textured surface. In one embodiment, the textured surfaces may include nanoscale or microscale texture. The electrode layers 116 and 126 can include materials such as gold, silver, aluminum, a metal, indium tin oxide (ITO), and combinations thereof. If ITO is used, the resulting device can be transparent.

[0027] Relative movement between the second sides 114 and 124 of the first contact charging layer 110 and the second contact charging layer 120 can be caused by applying a force to one of the layers. This causes electrons to be transferred from the second contact charging layer 120 to the first contact charging layer 110. This causes the second surface 114 of the first contact charging layer 110 to be negatively charged and the second surface 124 of the second contact charging layer 120 to be positively charged. The charges on the second sides 114 and 124 generate respective electric fields that induce charge accumulation in the electrode layers 116 and 126 and when a load 150 is coupled therebetween, electrons will flow through the load 150.

[0028] A unit 100 can be made as a sandwiched structure with two different polymer sheets stacked alternatively without interlayer binding. In one experimental embodiment, a rectangular (4.5 cmx l .2 cm) Kapton film (125[mu][iota][eta] in thickness, Dupont 500HN) was placed onto another flexible PET substrate (Dura-Lar, 220[mu][iota][eta] in thickness). The two short edges of the device were sealed with ordinary adhesive tape and to ensure an adequate contact between two polymer sheets. Both of the top and bottom surfaces of the structure were covered with a thin layer of Au alloy film (100 nm in thickness) by sputter coating. The metal films play two important roles here: (1) producing equal but opposite sign mobile charges via the electrostatic induction of the tribology generated potential at the interfacial region; and (2) serving as common electrodes for directly connecting the device with an external circuit.

[0029] As shown in FIG. 2, a plurality of units 100 can be stacked and coupled in parallel to generate an increased current, or in series to generate an increased voltage. By stacking two thin polymer films, for example Kapton and polyester (PET), a charge generation, separation and induction process can be achieved through a mechanical deformation of the polymer film. In one experimental embodiment, a power output density of about 10.4 mW/cm <3>was achieved with an output voltage of 3.3 V. This is a simple, low-cost, readily scalable fabrication process of a generator or sensor can convert random mechanical energy found in many environmental sources (e.g., rotating tires, wind, etc.) into electricity using conventional flexible/foldable polymer materials. This technology has a great potential for scaling up to power mobile and personal electronics used in environmental monitoring, personal medical networks, electronic emergency equipment and other self-powered systems.

[0030] The operating principle of the system can be described by the coupling of contact charging and electrostatic induction, as shown in FIG. 3A-3G in an embodiment in which the first contact charging layer comprises a Kapton film and the second contact charging layer comprises PMMA. In FIG. 3A, at the original state, no charge is generated or induced, and no electric potential difference (EPD) exists between the two electrodes. When an externally introduced displacement is applied to the unit 110 in the direction of the arrows, as shown in FIG. 3B, the two contact charging layers are brought into contact with each other. Surface charge transfer then takes place at the contact area due to triboelectric effect. According to the triboelectric series, electrons are injected from PMMA into Kapton, resulting in net negative charges at the Kapton surface and net positive charges at the PMMA surface, respectively. The insulating property of theses polymers allows a long-time retention of triboelectric charges for hours or even days.

[0031] As the displacement decreases, the unit 100 starts to be released and the Kapton film begins to revert back to its original position due to its own resilience. Once the two polymers separate, an EPD is then established between the two electrodes, as shown in FIG. 3C. Defining electric potential of the bottom electrode (UBE) to be zero, electric potential of the top electrode (UTE) can be calculated by:
[deg.]0

where [sigma] is the triboelectric charge density, [epsilon] 0is the vacuum permittivity, and d' the interlayer distance at a given state.

[0032] Here, a forward connection is defined for measurement as a configuration with positive end of the electrometer 150 connected to the bottom electrode (BE). (All electric measurements herein are based on the forward connection unless otherwise stated.)

Therefore, as the unit 100 is being released, V oc(as shown in FIG. 3F) keeps increasing until reaching the maximum value when the Kapton film fully reverts to the original position, as shown in FIG. 3D. Theoretically, such a voltage would remain constant provided that the input impedance of the electrometer is infinite. If renewed pressing is immediately followed, as shown in FIG 3E, the EPD starts diminishing as the two polymer layers get closer to each other. As a result, V ocdrops from the maximum value to zero when a full contact is made again between the two polymers, as shown in FIG. 3F.

[0033] As shown in FIG. 3G, if the two electrodes are shorted, any established EPD shown in Equation (1) as the two polymers separate drives electrons from the top electrode (TE) to the bottom electrode (BE), resulting in a nearly instantaneous positive current during the releasing process. The net effect is that inducted charges accumulate with positive sign on the TE and negative sign on the BE. The inducted charge density ([sigma].) when the generator is fully released can be ex ressed as below:

where s rk, and [epsilon] [psi]are the relative permittivity of Kapton and PMMA, respectively, di and [upsilon][3/4] are the thickness of the Kapton film and the PMMA layer.

[0034] Once the unit 100 is pressed again, reduction of the interlay er distance makes the TE possess a higher electric potential than the BE. As a consequence, electrons are driven from the BE back to the TE, reducing the amount of inducted charges. This process corresponds to the nearly instantaneous negative current shown in FIG. 3G. When the two polymers are in contact again, as shown in FIG. 3B, all inducted charges are neutralized.

[0035] In one experimental embodiment, as triggered by a vibration source with controlled frequency and amplitude, the unit 100 produced an open-circuit voltage and a short-circuit current as predicted in the above analytical model. Electric output with opposite sign was obtained by switching the polarity for electric measurement. The peak value of the V ocand I scwere up to 110 V and 6 uA, respectively. Substituting the experimentally determined V ocinto Equation (1), a theoretical triboelectric charge density was obtained according to the following:
=97.39 [mu][sigma][iota] (3)
d.

Then based on Equation (2), the maximum inducted charge density ([sigma] max) was theoretically calculated to be: [sigma] [Mu]= <G>/ <d ri[pound]rp>, =73.72 [mu][theta] m <2>(4)
d ls v+ d 3[pound] rks rp+ d 2[pound] rk

Therefore, electrons are pumped back and forth between the two electrodes as a result of contact charging and electrostatic induction. For one cycle of contacting-sliding-separating the integration of current over time for releasing has the same value as that for pressing, indicating that equal amount of electrons flow in the opposite direction. The current peak corresponding to releasing has a smaller magnitude but lasts longer than that for pressing. Such an observation can be explained by the fact that pressing is caused by the external vibration source while it is the resilience of the Kapton film that leads to releasing. Therefore, it is likely that releasing corresponds to a slower process and thus a smaller but wider current signal. Having the maximum inducted charge (Q'), the corresponding charge density was obtained as:
<r~ = = <87>- <23>^ <C/ m2>(6)

where S is the electrode area. The experimental result in Equation (6) is only slightly larger than the theoretically calculated one in Equation (4), indicating that the model is fairly valid for explaining the working principle.

[0036] External load matching for the generator was studied in the experimental embodiment. With an increase in the load resistance, the maximum current decreases due to ohmic loss, while the maximum voltage across the load has an opposite trend. Accordingly, the electric power exhibited an instantaneous peak value of 110 [mu]W, in correspondence to a power density of 31.2 mW/cm <3>. The measurement results reveal that the generator is particularly efficient provided that the load has a resistance on the order of mega ohms.

[0037] One embodiment can be employed in a sensor system, such as a self-powered touch screen. To make the device transparent and improve the power generation density, three approaches were used in an experimental embodiment: (1) using a transparent PDMS film as one of the contact charging layers; (2) using transparent ITO for the electrode layers, resulting in a flexible and transparent structure; and (3) fabricating various PDMS pattern arrays to enhance the friction effect, resulting in a high-output generator unit.

[0038] Such an embodiment can be made of two sheets of polymers that have distinctly different triboelectric characteristics, with one easy to gain electrons and the other one easy to lose electrons. By stacking the two sheets together with flexibility of relative sliding, two insulating polymeric materials are touched and rubbed with each other when deformed by an external mechanical deformation. Thus, electrostatic charges with opposite signs are generated and distributed on the two surfaces of the polymer films due to the presence of the nanometer scale roughness, and an interface dipole layer is formed, which is called a triboelectric potential layer. Such a dipole layer induces an inner potential layer between the planar metal electrodes. The induced charges will not be quickly conducted away or neutralized owing to the insulative nature of the polymer films. To minimize the energy created by the triboelectric potential, electrostatically induced free-charges will flow across the external load between the two electrodes coated on the top and bottom polymer sheets, respectively, to reach equilibrium. Once the structure is released and the tribologic force is removed, the two polymer films recover their original shapes, and the tribologically generated positive and negative charges may neutralize, and the electrostatic induced charges across the two electrodes recombine.

[0039] As shown in FIGS. 4A-4E, patterns can be fabricated on the polymer surfaces to increase the triboelectric power output. To make patterned polydimethylsiloxane (PDMS) films, Si wafer molds 210 are fabricated by traditional photolithography methods, followed by a dry or wet etching process to fabricate different recessed features 212 onto the surface, as shown in FIG. 4A. Examples of such features include pyramids 216, rectangular prisms 226 (as shown in FIG. 6A) and rows 236 (as shown in FIG. 6B). The surface of the molds is initially treated with trimethylchlorosilane to prevent the PDMS film from sticking to the recessed features 212. As shown in FIG. 4B, liquid PDMS elastomer and a cross-linker are mixed, degassed and uniformly spin-coated on the surface of the mold 210. As shown in FIG. 4C, after curing thermally, a uniform PDMS layer 214 is peeled off, including inverse 216 of the original pattern features 212 on the surface of the mold 210. As shown in FIG. 4D, the PDMS film was fixed on the insulation surface of a clean indium tin oxide (ITO)- coated 219 polyethylene terephthalate (PET) substrate 217 by a thin PDMS bonding layer, and then the entire structure was covered with another ITO-coated 215 PET film 213 to form a sandwich-structured device. A schematic view of the resulting structure is shown in FIG. 5.

[0040] One advantage of this technique is that hundreds of replicas of patterned PDMS films can be produced from one single mold. Silicon-based molds can be replaced by metal molds (e.g., Ni or Al) due to their excellent mechanical properties and longevity. The entire preparation process of the device is simple and low-cost, making it possible to be scaled-up for large-scale production and practical applications.

[0041] As shown in FIGS. 8A-8C, micrographs of the resulting PDMS pattern arrays show that the above-described fabrication method yields regular and uniform microstructures across the whole area of a 4-inch wafer mold. The shape and lateral dimensions of the polymer structure are well controlled by the initial patterns on the surface of the wafer mold. As shown in FIGS. 8A-8C, the size of single PDMS features is limited to about 10 [mu][iota][eta].

Smaller features down to 5 [mu][iota][eta] can also be produced with consistent quality. As shown in FIG. 8A, the pyramid features 400 have a near perfect geometric structure and a sharp tip, which can be beneficial for increasing the friction area and the efficiency in the power generation process. A micrograph of a rectangular prismatic embodiment 402 is shown in FIG. 8B and an embodiment employing elongated rows 404 is shown in FIG. 8C.

[0042] It should also be noted that the resulting PDMS film is stretchable and transparent, as illustrated in Figure 2D. Given that the electrode layers include transparent ITO, these embodiments are especially applicable to touch sensors, such as touch screen displays.

[0043] As shown in FIG. 7, one embodiment includes a triboelectric generator 300 wherein a side of one of the contact charging layers 310 includes a texture formed by a plurality of elongated nanowires 312 extending outwardly therefrom. This embodiment can result in a substantial charge density during use.

[0044] Dry etching is applied on the Kapton surface 310 to create vertically aligned polymer nanowires 312. Use of these nanowires 312 results in increased surface friction as they are brought in contact with the opposite polymer layer 328. A spacer structure 326 improves electric output.

[0045] In one experimental embodiment, the fabrication process starts with a square glass sheet 320, on which is deposited with a thin layer of aluminum 322 as the bottom electrode using electron beam evaporator. Then a thin layer of PMMA 324 is spun-coated, followed by adding a spacer layer 326 at the edges, leaving a square cavity 328 at the center. One side of a Kapton film 310 is deposited with a layer of aluminum as the top electrode 311, while the other side was dry-etched to create vertically aligned polymer nanowires 312. Then the Kapton layer 310 was anchored on the spacer 326 with the top electrode 311 facing up. The spacer 326 can be made of an insulating polymer with double-sided adhesive, keeping the Kapton film 310 at a fixed distance away from the PMMA layer 324 underneath.

[0046] The above described embodiments, while including the preferred embodiment and the best mode of the invention known to the inventor at the time of filing, are given as illustrative examples only. It will be readily appreciated that many deviations may be made from the specific embodiments disclosed in this specification without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is to be determined by the claims below rather than being limited to the specifically described embodiments above.



Triboelectric Generator    
US2013049531


Inventor: WANG ZHONG L // FAN FENGRU

A generator includes a thin first contact charging layer and a thin second contact charging layer. The thin first contact charging layer includes a first material that has a first rating on a triboelectric series. The thin first contact charging layer has a first side with a first conductive electrode applied thereto and an opposite second side. The thin second contact charging layer includes a second material that has a second rating on a triboelectric series that is more negative than the first rating. The thin first contact charging layer has a first side with a first conductive electrode applied thereto and an opposite second side. The thin second contact charging layer is disposed adjacent to the first contact charging layer so that the second side of the second contact charging layer is in contact with the second side of the first contact charging layer.

BACKGROUND OF THE INVENTION

[0003] 1. Field of the Invention

[0004] The present invention relates to generators and, more specifically, to a system for generating voltage and current using the triboelectric effect.

[0005] 2. Description of the Related Art

[0006] Energy harvesting by converting ambient energy into electricity may offset the reliance of small portable electronics on traditional power supplies, such as batteries. When long-term operation of a large number of electronic devices in dispersed locations is required, energy harvesting has the advantages of outstanding longevity, relatively little maintenance, minimal disposal and contamination. Despite of these benefits, superior performance, miniaturized size and competitive prices are still to be sought after in order for energy harvesting technology becoming prevalent.

[0007] The triboelectric effect is a type of contact electrification in which certain materials become electrically charged after they come into contact with another such as through friction. It is the mechanism though which static electricity is generated. Triboelectric effect associated electrostatic phenomena are the most common electrical phenomena in our daily life, from walking to driving, but the triboelectric effect has been largely ignored as an energy source for electricity. Some electrostatic microgenerators have been developed and used in research relating to microelectromechanical systems (MEMS), but such designs tend to be based on inorganic materials and the fabrication of such devices requires complex processes.

[0008] Therefore, there is a need for a reliable, small and easily manufactured system for harvesting triboelectric energy.

SUMMARY OF THE INVENTION

[0009] The disadvantages of the prior art are overcome by the present invention which, in one aspect, is a generator that includes a thin first contact charging layer and a thin second contact charging layer. The thin first contact charging layer includes a first material that has a first rating on a triboelectric series. The thin first contact charging layer has a first side with a first conductive electrode applied thereto and an opposite second side. The thin second contact charging layer includes a second material that has a second rating on a triboelectric series that is more negative than the first rating. The thin first contact charging layer has a first side with a first conductive electrode applied thereto and an opposite second side. The thin second contact charging layer is disposed adjacent to the first contact charging layer so that the second side of the second contact charging layer is in contact with the second side of the first contact charging layer.

[0010] In another aspect, the invention is a triboelectric generator that includes a first conductive electrode layer, a second conductive electrode layer, a first contact charging layer and a second contact charging layer. The first contact charging layer has a first side and an opposite second side. The first conductive electrode layer is disposed on the first side of the first contact charging layer. The first contact charging layer includes a first material that has a first rating on a triboelectric series. The first contact charging layer has a thickness sufficiently thin so that a positive excess charge on the second side induces an electric field that induces negative charge carriers to form in the first conductive electrode layer. The second contact charging layer has a first side and an opposite second side. The second conductive electrode layer is disposed on the first side of the second contact charging layer. The second contact charging layer includes a second material that his a second rating on the triboelectric series wherein the second rating is more negative than the first rating. The second side of the second contact charging layer is disposed against the second side of the first contact charging layer. The second contact charging layer has a thickness sufficiently thin so that a negative excess charge on the second side induces an electric field that induces positive charge carriers to form in the second conductive electrode layer. Relative movement between contacting portions of the second side of the first contact charging layer and the second side of the second contact charging layer results in excess positive charge on the second side of the first contact charging layer and excess negative charge on the second side of the second contact charging layer.

[0011] In yet another aspect, the invention is a method of generating an electrical current and voltage in which a first contact charging layer is brought in contact with a second contact charging layer. The first contact charging layer has a first side and an opposite second side. A first conductive electrode layer is disposed on the first side of the first contact charging layer. The first contact charging layer includes a first material that has a first rating on a triboelectric series. The first contact charging layer has a thickness sufficiently thin so that a positive excess charge on the second side induces an electric field that induces negative charge carriers to form in the first conductive electrode layer. The second contact charging layer has a first side and an opposite second side. A second conductive electrode layer is disposed on the first side of the second contact charging layer. The second contact charging layer includes a second material that his a second rating on the triboelectric series wherein the second rating is more negative than the first rating. The second side of the second contact charging layer is disposed against the second side of the first contact charging layer. The second contact charging layer has a thickness sufficiently thin so that a negative excess charge on the second side induces an electric field that induces positive charge carriers to form in the second conductive electrode layer. Relative motion between the first contact charging layer and the second contact charging layer is caused. A load is applied between the first conductive electrode layer and the second electrode layer, thereby causing an electrical current to flow through the load.

[0012] These and other aspects of the invention will become apparent from the following description of the preferred embodiments taken in conjunction with the following drawings. As would be obvious to one skilled in the art, many variations and modifications of the invention may be effected without departing from the spirit and scope of the novel concepts of the disclosure..




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