Piezoelectric sensor design of graphite-aluminium with dynamic surface interaction method as an environmental technology

Today, graphite is a phenomenal material as an environmentally friendly technology in alternative piezoelectric. Graphite materials with electrical conductivity as electrodes have been widely applied in lithium batteries to support electricity with zero-emissions. Many previous studies used graphene electrodes to harvest electrical energy from raindrops colliding on the electrodes. The voltage generated by the impulse of raindrops on the electrodes is about 129 microvolts. Limitations may arise from rain power generation; namely, the space for system installation is proportional to the amount of power generated. Therefore, electrical power is able to generate using immersing the electrodes into seawater. This type of power generation overcomes the space problem as they can be stacked vertically producing a piezoelectric effect. This study aims to design a piezoelectric sensor of graphite and aluminium reacted to seawater after being subjected to pressure. The experimental results show that the increase in graphite composition greatly affects the voltage results and the composition of seawater will decrease the voltage. Voltage arranged in series from the piezoelectric sensor contributes an increase in voltage about twice the voltage of the unit voltage.


Introduction
Electricity is now a very important need for both household and industrial needs.Limited natural resources and negative effects on the environment have become a global challenge.It is estimated that fossil fuels will run out in 2088 [1].Renewable energy systems have been prioritized to minimize negative effects and reduce consumption of natural resources.In recent years, nanogenerator technology has been developed to produce alternative energy.Among the nanogenerator technologies are electromagneticity, thermoelectricity, triboelectricity, and piezoelectricity [2].
Many studies to develop piezoelectrics have been carried out.Piezoelectric generators are considered as the most promising candidates for energy harvesting because they can generate electricity without interruption through the application of steady pressure [2].Their relatively small size, simple structure, and long-term stability make them favorable for use in powering micro/Nano systems, various small power consumer devices, remote and mobile sensors, and even wearable wireless electronics [3].
One application of piezoelectricity is harvesting energy from rainwater.Lallart and Inman's [4] research succeeded in harvesting energy from falling rainwater with a piezoelectric cantilever bimorph of 10 microWatt/cm3.Research by Guigon et al. obtained energy from raindrops using polyvinylidene difluoride (PVDF) film.With an area of about 30 cm 2 , it is dripped with water with a controlled size.The results show that 1 nJ of energy can be harvested from one drop of water, corresponding to 1 microWatt of instantaneous power (that is, 0.033 microwatt/cm 2 ).
There are two types of piezoelectric materials, natural and synthetic.Natural piezoelectric materials are Quartz (SiO2), Berlinite (AlPO4), Tourmaline and Rossel salt.Meanwhile, Barium Titanate (BaTiO3), Lead Zirconium Titanate (PZT) and Lead Titanate (PBTiO3) are artificial piezoelectric materials made from ceramics.PZT material is the most widely used material, but due to the presence of lead in PZT material which is toxic, it encourages the development of new materials that are safer for humans and the environment.Polyvinylidene Fluoride (PVdF) is a piezoelectric material made of polymer.The material has a high piezoelectric constant value (range of piezoelectric constant values 1 -100 pico Coulomb/Newton).In addition, it is resistant to high pressure.Because of its flexibility so that it is easy to adjust the size and shape, it is safer and lighter because of its low density [5].
To improve the pizoelectric performance, material modifications are continuously carried out, and one of them is by adding graphite material.Chuan Yang et al studied n-butylamine-modified graphite nanoflakes (BMGNFs) were blended in P(VDF-TrFE) copolymers to achieve superior ferroelectric and piezoelectric properties for energy harvesting applications.[2].
Limitations may arise from piezoelectricity used to harvest energy from rainwater.Limitations also arise from similar systems requiring surface area to generate energy.The space for system installation is proportional to the amount of power generated.Therefore, the electrical power that can be generated by immersing the electrodes in seawater was developed in this study.This type of power plant overcomes the problem of space because it can be stacked vertically.However, the electrode materials must be taken into account as they greatly affect the required cost.In this research, piezoelectric made from graphite-aluminum composition.The effect of pressure variations, graphite composition and seawater composition on the graphite-aluminum piezoelectricity is investigated and discussed in this paper.

Piezoelectric
Piezoelectric materials can exhibit switching dipoles when mechanical strain is applied [6].As a result of pressure, negative and positive charges will collect on opposite surfaces which means there is also polarization in the material due to collecting charges in different directions.The collection of charge at different places will produce a potential difference between the two load collection sites.The place of collection of such cargo is none other than on the surface of the material itself.[7] Piezoelectricity is used in a variety of applications such as pressure sensors, data storage, mechanical actuation, ultrasonic waves and energy harvesting.Due to these various applications, a large amount of material has been developed.

Graphite
Graphite is an excellent conductor owing to the sp 2 hybridization of each of its carbon atom, and its conductivity is three times higher than that of copper.Graphite materials as electrodes have been widely applied in lithium batteries.The composition of the graphite mixture can improve battery performance.This mechanism is related to the electronic conductivity, electrode diffusion, and the ionic nature of the electrode.Now, graphite is a phenomenal material.It is built by many layers of hexagonal carbon atom structure with delocalized electrons making it easy to bond with other elements [8].It has been developed by various applications such as Pencil Graphite Electrodes (PGE) mixing with aniline and glycine, polymeric materials, CuNS, enzymes, lithium batteries (LIBs), solar cells, and sensors.Some researchers started development by modifying the size and constructing composite nanomaterials for the electrodes.There are many different nanocarbons having small designs that can be developed.In addition, graphite has superior electrochemical properties due to its high electrical conductivity and strength stability [9].
Graphite is an original mineral element with a carbon composition (C).The crystal system of graphite is hexagonal, which is a consolidated mass or thin sheets that detach, the opaque structure is generally black.Graphite is the dimorphism of diamonds, but has low hardness (1-2).The mass of graphite is 2.3 g/cm 3 [10].Figure 1 shows the structure of an 8B graphite pencil.Samples were tested using SEM-EDX to determine the morphology and chemical elements.

Aluminium
Those commonly used in energy harvesters include aluminium nitride (AlN).AlN and ZnO have a much weaker piezoelectric effect than the other listed materials.They are usually adopted in thin film configurations in micro scale where figures of merit are different from those of bulk materials [11].Under a harmonic excitation of 4.5 g and 1,011 Hz, the 3 x 3x 1.7-mm 3 AlN unimorph prototype generated a maximum power of 489 mW [12].Aluminium is an abundant metal.Although it is not a type of heavy metal, it is a large amount of element.Aluminium is also a good conductor for electricity because it is light and strong.Aluminium is widely used in many ways.Aluminum alloy contains 99% aluminum and 1% contains manganese, iron, silicon, copper [13].Figure 2 shows the structure of aluminum.Samples were tested using SEM-EDX to determine the morphology and chemical elements.

Covalent bond
Covalent bonds greatly affect the ability of materials used in piezoelectric.Covalent bonding is one of the chemical bonds that occur due to the use of shared electron pairs, which can be seen by the concept of Lewis structure.There are some atoms that are difficult to remove or receive electrons because they require or free up a large amount of energy for the process to take place.The use of shared electron pairs occurs in nonmetallic atoms.Bonds between nonmetallic atoms that occur through the use of shared electron pairs are called covalent bonds.Weakening in the interatomic bond could affect the mechanical properties of the material.Weaker covalent bond is considered to be one of the origins of the elastic softening in piezoelectric material [14].
The piezoelectric materials used are graphite, aluminium, and seawater.Mica plastic is used as a container for placing piezoelectric materials.Double-sided adhesive tape is attached to the mica so that the material can stick.Graphite fine powder is obtained by taking graphite from an 8B pencil.
Aluminium foil with a size of 3x5 cm is affixed to the mica plastic and the graphite is flattened on the surface.Wires installed on both sides (one in graphite and one in aluminum foil).Tissue paper measuring 3x5 cm is installed between the sides.Both sides are closed and glued using plastic tape on the sides.A small hole is made to enter seawater.Measurement of seawater volume using a syringe.Seawater is taken from Jember, East Java, Indonesia with the following characteristics in Table 1

Voltage that occurs in the single piezoelectric
Applying pressure of the same frequency on piezoelectric surfaces causes the appearance of varying voltages.[8] The voltage change that occurs can be seen in figure 6 where the voltage slowly increases to the maximum.At a time of 1 second to 123 seconds, there are fluctuations in voltage.This occurs because the graphs scattered in the aluminium area are uneven.Therefore before data retrieval, it needs Piezoelectric Hand Presure Micro controller Serial Monitor to be sought an emphasis position that produces a maximum and stable voltage that is at 122 seconds to 313 seconds.
Figure 5 shows the voltage that has been generated by a single piezoelectric.The voltage gradually increases to a maximum.From 1 second to 123 seconds, voltage fluctuations occur.Applying pressure with the same frequency on the piezoelectric surface causes a varying voltage.This occurs to the uneven distribution of graphite in the aluminium area.So that before data retrieval, it is necessary to find the position of the emphasis that produces a maximum and stable voltage, which is at a time of 122 seconds to 313 seconds.The voltage that appears is the piezoelectric effect.Pressure on the surface causes polarization of piezoelectric materials or objects.Polarization causes a change in the position of the nucleus of the atom and electrons, resulting in a dipole moment [16].In atomic structures, especially carbon has a permanent dipole moment.Irregular dipole moments if given pressure cause the orientation of the dipole moment to be irregular and in line with the load / pressure [17].This also leads to polarization.The greater the polarization, the greater the piezoelectric effect that it causes.In addition, carbon and aluminium atoms have the same charge (positive).At the symmetrical position at the same charge there is a resisting force due to the emergence of Vander Waals forces between carbon and aluminium that can cause electrical induction [8].

Effect of the composition (% weight) of graphite
In this experiment, the piezoelectric was made with 3 variations of graphite composition (% by weight) respectively 13% (0.46 grams), 40% (1.38 grams) and 80% (2.76 grams).Seawater is given with a constant volume of 0.6 ml.The voltage that occurs can be seen in Figure 6.The voltage measurement is repeated 3 (three) times.The time for each repetition is 30 seconds.6 it can be seen that the addition of graphite composition (% by weight) causes an increase in the piezoelectric voltage.After the composition is reduced of 40% (1.38 grams) there was a voltage fluctuation until the seventh second.When pressing the piezoelectric surface, the position of graphite collects at the bottom as shown in Figure 7.This causes the maximum voltage generated to be unstable.So it is necessary to adjust the position of the graphite in the middle area which is connected to the positive cable.This phenomenon occurs due to changes in the carbon structure.With the addition of the composition, the structure of the carbon atom will change and its density will increase.At a fixed piezoelectric volume the addition of graphite increases the density in the carbon atoms.So the addition of graphite will increase the polarization of positively charged carbon atoms.The repulsion increases with the same charge on the carbon atoms.In addition, there is a chemical reaction between carbon, aluminum and seawater.The reaction is polar and nonpolar covalent [18].Sea water consists of the dominant elements SO42-, K, Ca, while the lighter elements consist of Na, Cl and Mg elements.Composition 13% ( 0,46 g) and seawater will disrupt the double bonds on the carbon atoms.The C-C and C-O bonds are broken by H2O because the O atom will release electrons on the C atom so that the electronegativity and affinity are getting stronger, causing electron delocalization.These electrons will react with seawater elements.Then the carbon element will bind to Cl to form a polar covalent bond.This will increase the number of electrons that jump (move).This causes an increase in voltage.

Effect of sea water volume on piezoelectric
In this treatment, the piezoelectric was made on the same graphite composition, 13% (0.47 grams) but the volume of seawater added was different 0.2 ml, 0.6 ml and 1 ml, respectively.The voltage measurement was carried out 3 (three) times.The time for each repetition is 30 seconds.The change in piezoelectric voltage due to the addition of seawater volume is shown in Figure 8.With 0.2 ml seawater, the voltage is in the range of 0.51-0.56volts.At a volume of 0.6 ml, the voltage reaches the range of 0.51 -0.54 volts.The voltage difference that occurs is very small 0.04 volts.Meanwhile, at a volume of 1 ml of seawater the voltage reaches 0.38 volts.This result is in line with previous research which states that the addition of seawater reduces the impedance of the graphite material [1].
The amount of impedance is directly proportional to the resistance that arises.This is in accordance with Pouillet's law, where the resistance of the material is inversely proportional to the area (A) where the greater the area, the less resistance.The reaction between graphite, aluminum (Al) and seawater causes the movement of electrons and the formation of new compounds.When the volume of seawater rises, the larger the area of the graphite reacts.So that the impedance of the graphite material is reduced.Each element, graphite, water and aluminum has a different impedance.The conductor material has a small impedance.

Effect of composition (% by weight) of graphite on piezoelectrics arranged in series
Piezoelectric arranged in series to see the magnitude of the voltage that occurs.Two piezoelectrics arranged in series were then subjected to constant pressure for 30 seconds with three repetitions.At 80% graphite composition (2.76 grams) the maximum voltage is 1.4 volts, the minimum voltage is 1.15 volts.The average piezoelectric voltage for the three treatments was 1.27 volts.In figure 9, the voltage change in the piezoelectric series arrangement increased by 77% compared to the single piezoelectric.At 40% graphite composition (1.38 grams) the maximum voltage is 1.21 volts, the minimum voltage is 1.11 volts.The average piezoelectric voltage for the three treatments was 1.13 volts.In Figure 10, the voltage change in the piezoelectric series arrangement increased by an average of 89% compared to the single piezoelectric.While the composition of 13% graphite (0.46 grams) the maximum voltage reaches 0.95 volts, the minimum voltage is 0.78 volts.The increase in voltage in the graphite composition by 13% caused the smallest increase, with an average increase of only 20% (Figure 11).

Conclusion
From the results of the analysis and discussion, the addition of pressure on the surface will increase the voltage that occurs in the piezoelectric.The pressure on the surface is directly proportional to the output stress.This is in accordance with the characteristics of the piezoelectric material as a sensor.Besides, the addition of graphite (% by weight) causes an increase in stress as a result of polarization of carbon atoms, chemical reactions between atoms in graphite, aluminium atoms (Al) and seawater.The maximum voltage obtained is 0.76 volts with a composition of 80% graphite (2.76 grams) and the smallest voltage is 0.51 volts with a composition of 13% (0.46 grams).The more water there is, the lower the voltage generated.The maximum voltage is 0.56 volts at 0.2 ml seawater volume, while the minimum voltage is 0.38 volts at 1 ml seawater volume.Piezoelectric arranged in series will provide a voltage increase of about twice the unit voltage.Therefore, this is in accordance with ohm's law.

Figure 7 .
Figure 7. Graphite position on piezoelectric sensors indicated by a white rectangular.
Positively charged carbon atoms will react with negatively charged Cl and OH.Mg, Ca, Cu and Na atoms form new bonds.The reaction of carbon 0

Figure 8 .
Figure 8. Piezoelectric voltage with difference in sea water volume.
Piezoelectric experiment steps are carried out with the following steps: