Patent Application
20130277224
1. Field of Invention
The present invention relates to a method for making a wave-absorbing sheet and, more particularly, to a method for making a thin, light, flexible wave-absorbing sheet that absorbs a wide band of frequency of electromagnetic wave at a high rate.
2. Related Prior Art
A microwave-absorbing material (or “wave-absorbing material”) absorbs electromagnetic wave, transforms the electromagnetic energy into heat, and scatters the heat. Thus, the electromagnetic energy is attenuated.
In the military, wave-absorbing materials are used to conceal weapons from radars or protect radars from electromagnetic interference. In everyday life, wave-absorbing materials are used to protect 3C devices from electromagnetic interference that is getting worse as the frequencies of the pulses of CPUs are getting higher, and high-frequency mobile phones and vehicle-navigating devices are getting more popular.
The foregoing electronic devices produce electromagnetic wave that would interfere with other electronic devices and even hurt human bodies. Hence, there is a need for microwave-absorbing materials that fight undesirable electromagnetic wave (or “electromagnetic noise”). A wave-absorbing material is supposed to effectively absorb electromagnetic wave, transforms the electromagnetic energy into heat, and scatters the heat. Thus, the electromagnetic energy is attenuated. Wave-absorbing materials can be used in microwave anechoic rooms, electromagnetic compatibility, and protection from electromagnetic pollution.
Wave-absorbing materials are used for protecting mobile phones, laptop computers and other 3C from EMI. For example, when an RFID tag or reader/writer is attached to a metal surface, the distance for reading is much shorter than expected. Conventionally, the distance of the RFID antenna from the metal surface is increased to avoid the foregoing problem. The increased distance however renders the RFID tab or reader/writer thick.
Wave-absorbing materials are classified into a magnetic-loss type and an electric-loss type. Wave-absorbing materials are made of iron cores, ferrites, silver/nickel and iron/aluminum/silicon. The metal is grounded into powder and mixed with adhesive for use. The power includes solid particles. The problem with the use of metal is expensive and heavy.
The present invention is therefore intended to obviate or at least alleviate the problems encountered in prior art.
It is the primary objective of the present invention to provide a method for making a thin, light, flexible wave-absorbing sheet that absorbs a wide band of frequency of electromagnetic wave at a high rate.
To achieve the foregoing objective, the method includes the steps of providing first emulsified mixture by blending wave-absorbing particles in graphene solution so that the graphene solution is attached to the wave-absorbing particle, providing second emulsified mixture by mixing the first emulsified mixture with resin solution, providing third emulsified mixture by mixing the second emulsified mixture with interface modifier, providing the wave-absorbing particles, resin solution and graphene solution of the third emulsified mixture evenly on two conductive substrates by submerging the conductive substrates in the third emulsified mixture and imposing voltage on the conductive substrates, removing the conductive substrates to provide a wave-absorbing sheet, and washing and drying the wave-absorbing sheet.
In another aspect, the concentration of the wave-absorbing particles in the first emulsified mixture is 0.1% to 5%.
In another aspect, the graphene solution includes graphene dissolved in organic solvent. The concentration of the graphene in the grapheme solvent is 0.1% to 5%.
In another aspect, the organic solvent is methanol, ethanol, isopropyl alcohol or NMP.
In another aspect, each of the wave-absorbing particles includes a core placed in a shell.
In another aspect, the shell is made of metal and the core is made of plastics. The wave-absorbing particles are made by electroplating the metal on the plastics.
In another aspect, the wave-absorbing particles are made with a diameter of 100 nanometers to 50 micrometers. The wave-absorbing particles are spherical, elliptical or hollow spherical.
In another aspect, the core is made of acrylic resin solution, polystyrene, polymethylmethacrylate, poly-epoxy resin or silicon dioxide.
In another aspect, the shell is made of phosphorus ferronickel, NiMoFeP, iron phosphide or nickel phosphide. The shell takes 10 wt % to 80 wt % of each of the wave-absorbing particles.
In another aspect, the shell is made with thickness of 100 to 550 nanometers.
In another aspect, the step of providing the second emulsified mixture includes the step of stirring the second emulsified mixture for 5 minutes to 1 hour.
In another aspect, the step of providing the second emulsified mixture includes the step of providing resin solution as adhesive. The resin solution is acrylic resin solution and epoxy resin solution. The concentration of the resin solution in the second emulsified mixture is 0.1% to 5%.
In another aspect, the interface modifier is silver nitrate, magnesium nitrate, aluminum nitrate, barium nitrate or calcium nitrate. The concentration of the interface modifier is 100 to 400 gram/l.
In another aspect, the step of providing the third emulsified mixture evenly on the conductive substrates includes the step of connecting the conductive substrates to the positive and negative electrodes of a power supply.
In another aspect, the power supply provides operative voltage of 30 to 200 volts. The conductive substrates are made with an area of 1 cm2 to 1 m2. The electrophoresis lasts for 1 minute to 1 hour. The coating is made with thickness of 5 to 500 micrometers.
In another aspect, the conductive substrates are made of copper or aluminum.
In another aspect, the step of removing the conductive substrates includes the step of providing eroding liquid for eroding the conductive substrates for 1 to 24 hours. The concentration of the eroding liquid is higher than 20 vol. %.
In another aspect, the eroding liquid is copper-eroding liquid, iron chloride solution, aluminum-eroding liquid, sodium hydroxide solution or potassium hydroxide solution.
In another aspect, the step of washing and drying the wave-absorbing sheet includes the step of drying the wave-absorbing sheet at 50° C. to 200° C.
Other objectives, advantages and features of the present invention will be apparent from the following description referring to the attached drawings.
The present invention will be described via detailed illustration of the preferred embodiment referring to the drawings wherein:
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The wave-absorbing sheet 5 is made of the wave-absorbing particles 11 and graphene solution 12 so that it is light. The core-shell structure, which consists of the core 111 and the shell 112, enables the wave-absorbing particles 11 to absorb microwave better than conventional wave-absorbing materials. Hence, wave-absorbing sheet 5 exhibits excellent thermal stability and mechanical properties, is light, inexpensive and flexible, and absorbs well.
The wave-absorbing sheet 5 may be used in LF (125 to 134.2 KHz) or HF (13.56 MHz) for example. The wave-absorbing sheet 5 may be attached to an RFID antenna before it is attached to a metal surface. The distance for reading can be as high as 80% of the distance for reading when the RFID antenna is not attached to the metal surface. The wave-absorbing sheet 5 is thin, so that an RFID tag or reader/writer that includes the wave-absorbing sheet 5 is thin. When wave-absorbing materials are made with identical thickness, those with higher magnetic permeability allow more magnetic flux, impose less influence on metal, and allow longer distance for reading. This is why magnetic materials must be added into wave-absorbing materials. The wave-absorbing sheet 5 is better than any other wave-absorbing sheets on the market, and can be used to protect mobile phones, laptop computers, other 3C products, and RFID devices (tag /NFC) against EMI.
The present invention has been described via the detailed illustration of the preferred embodiment. Those skilled in the art can derive variations from the preferred embodiment without departing from the scope of the present invention. Therefore, the preferred embodiment shall not limit the scope of the present invention defined in the claims.
