With the rapid development of smart city, industry 4.0 and Internet of things, there is a large demand for RFID intelligent identification products in the market. RFID (radio frequency identification), also known as radio frequency identification technology, is a non-contact automatic data acquisition technology and one of the core technologies of the Internet of things. Its biggest characteristic is that the information collection speed is fast, does not need the mechanical or the optical contact, completely completes through the wireless communication technology, can simultaneously collect hundreds of thousands of object information in one second, the information collection accuracy is high. At present, this technology has been widely used in almost all fields, such as logistics warehousing, transportation, security anti-counterfeiting, mobile payment and so on.
1. Research status
The United States is an active promoter of the application of RFID sticker, which leads the world in the field of RFID standard establishment, software and hardware technology development and application. European RFID standards follow the EPCglobal standard led by the United States. The research and development of RFID in European and American countries is mainly focused on standard setting, chip manufacturing, reader manufacturing and system integration. China is the world’s largest supplier of RFID stickers, and the total production of RFID stickers accounts for about 60% of the world. Especially for high-frequency RFID stickers, China has basically realized all localization from chips to antennas.
The main bottleneck of popularizing RFID system is the price, size and environmental adaptability of RFID sticker. At present, RFID sticker is manufactured by copper wire winding method and aluminum foil etching method, and ceramic sintering method is basically used. The common problems are as follows:
(1) Environmental pollution;
(2) The label substrate is single and its application field is limited;
(3) The manufacturing efficiency is low;
(4) The label size is large;
(5) High cost;
(6) The manufacturing accuracy is low.
In the past few years, many research institutions at home and abroad believe that screen printing RFID sticker with conductive silver paste is the most effective technology to achieve low cost, miniaturization, high precision, strong adaptability and large-scale production. Although the printing technology in the field of image reproduction is characterized by high speed, high efficiency, fine lines and accurate overprint, due to the poor conductivity of conductive silver paste and the limitation of conductive mechanism, only high silver content conductive silver paste and low wire number screen can be used, which are affected by dozens of factors such as ink viscosity, extensibility, fluidity, scraping pressure, screen stretching and screen wire interference, The printed RFID sticker wire structure deformation, rough boundary, short circuit and open circuit, the actual radiation efficiency and theoretical radiation efficiency are very different. To sum up, on the one hand, the demand of RFID electronic label market is more and more vigorous. On the other hand, RFID electronic tag manufacturing technology still has problems such as low efficiency, high cost, environmental pollution, single substrate, etc. the market needs new manufacturing technology to break through the contradiction between demand and supply.
1.1 trend and demand analysis
With the successful development of graphene materials with high conductivity by Sir Andre Haim and Sir Konstantin Novo Seaan Love, Nobel laureates of Manchester University in the UK, the decline of graphene price and the improvement of product quality greatly stimulate the application research of downstream products, such as various kinds of graphene electronic products such as conductive lines, sensors, medical monitors, etc. Due to the micro topological structure of graphene, it has high conductivity. The conductive mechanism of graphene is different from that of silver particles.
1.2 advantages
There are two advantages in the preparation of filled composite conductive paste by powdering graphene
(1) Strong compatibility. Graphene paste can be printed on almost all substrates such as plastic film, paper, ceramics, cotton cloth, wood, etc.
(2) High cost performance. Compared with the existing conductive silver paste, graphene slurry has better conductivity and higher cost advantage. With the development of graphene production technology and the decrease of cost, graphene conductive paste will gradually occupy the market share. It is estimated that the market scale of graphene in conductive paste field will reach 200 million yuan by 2020.
At present, some foreign RFID system manufacturers have restarted printing RFID electronics with graphene paste
Research on key technologies and industrialization of labels, such as bgtmaterials Limited (BGTM) in the UK.
However, there are no other domestic companies to carry out industrial research and development of this technology.
At present, China’s graphene production capacity is rapidly expanding, and there are many large-scale graphene manufacturers. For example, Ningbo Moxi Technology Co., Ltd., Chongqing mohi Technology Co., Ltd., HONGNA New Material Technology Co., Ltd., Jinan Moxi New Material Technology Co., Ltd., Suzhou gerifeng Nano Technology Co., Ltd., Nanjing Xianfeng Nano Technology Co., Ltd., Changzhou 2D Carbon Technology Co., Ltd., all of which can produce 100 tons of graphene per year.
According to the Research Report on the market prospect and investment opportunities of China’s Internet of things industry in 2017-2022 released by the China Business Industry Research Institute, the scale of China’s RFID market will reach 60 billion yuan in 2018.
Based on this, by analyzing the performance parameters of RFID stickers, optimizing the printability of graphene conductive paste, and applying gravure printing technology, the new RFID sticker manufacturing technology with green environmental protection, mass production, high efficiency, and high quality, low cost and suitable for various substrates has become the inevitable development trend of the industry.
2 project research methods
2.1 preparation, modulation and dispersion control of graphene conductive paste
Due to the different appearance and conductive properties of graphene and conductive silver powder, the formulation and process of conductive paste based on graphene are also different. The hydrophobicity of graphene can make graphene Nano meter
2.1 preparation, modulation and dispersion control of graphene conductive paste
Due to the different appearance and conductive properties of graphene and conductive silver powder, the formulation and process of conductive paste based on graphene are also different. The hydrophobicity of graphene makes it easy to agglomerate through strong van der Waals force. The effective solvent can prevent the aggregation of graphene and make it a stable graphene dispersion. The ideal solvents are N-methylpyrrolidone (NMP) and dimethylformamide (DMF). In this project, we plan to use DMF / NMP as solvent to disperse graphene by adding stabilizer (such as ethyl cellulose) in the graphene slurry formula, so as to solve the problem that graphene powder is easy to agglomerate and difficult to disperse. The particle size and dispersion of graphene slurry were measured by laser particle size analyzer to ensure the dispersion of graphene slurry particles.
Adding UV initiator, photosensitive resin and other components into graphene conductive paste, and optimizing its mixing ratio, the graphene conductive paste can be rapidly solidified under ultraviolet light, reduce the drying temperature of printed RFID electronic tag, shorten the drying time, improve the production efficiency, and can print RFID electronic tags on various substrates such as paper, plastic film, silk and so on. The viscosity, viscosity, fluidity, surface tension, dryness, thixotropic, rheological property and particle size of graphene conductive paste are modulated by changing the binder and additives of the slurry. The viscosity cup or Ubbelohde viscometer, slurry viscosity meter and surface tension are used to meet the requirements of gravure printing.
2.2 design of RFID sticker antenna based on graphene conductive paste printing
The main parameters that affect the electrical performance of RFID sticker antenna are antenna shape, size structure, material characteristics, operating frequency, bandwidth, polarization direction, directivity, gain, lobe width, impedance, sensitivity, quality factors and application environment, etc. these parameters need to be balanced in the design of RFID sticker.
In the simulation software HFSS or ads, input antenna line width, line spacing, bending size, feed gap, feed ring size, electromagnetic signal receiving and feedback material conductivity, dielectric constant and other design parameters for computer simulation, as shown in Figure 9, the computer simulation obtains the RFID sticker return loss and energy distribution, so as to determine RFID sticker electrical performance characterization parameters, establish the data model. The effects of graphene conductive paste formula and printing process parameters on antenna performance were studied. The effect of ink film thickness on skin effect and antenna performance is studied. The impedance characteristics of graphene ink layer at different chemical potentials, especially the influence of high reactance in UHF band on the gain of RFID sticker antenna are studied.
2.3 control of gravure printing parameters based on graphene conductive paste
According to the printability of graphene conductive paste and the structural parameters of RFID sticker designed by computer simulation, the appropriate screen line number, mesh depth and mesh hole shape of gravure printing cylinder are determined, and the ink amount is calculated and the thickness of slurry film is adjusted. Aiming at different substrates such as paper, plastic film (such as pet, PI, CPP, etc.), silk and so on, a series of printing process parameters, such as the tension of winding and unwinding, the pressure of printing rubber imprint roller, the thixotropic viscosity of slurry, the contact angle of scraper, printing speed and positioning overprint, are adjusted to obtain the optimal printing process scheme.
2.4 optimize ink film drying temperature and roller pressing pressure
Because the conductive ink layer is dried and solidified to a certain extent, and then calendared by roller, the surface morphology can be changed, the density of ink layer can be increased, and the conductivity can be greatly improved. However, the pressed ink film is easy to cause the contour amplification and wire deformation. Adjust the temperature and time of graphene conductive ink layer in UV curing channel, test the conductivity improvement ability and ink layer contour deformation degree, optimize the ink film curing temperature, time and grinding pressure and other process parameters.
2.5 analyze and test the performance of RFID sticker
Take the printed sample sheet, paste the chip, and test its electrical performance parameters such as working frequency, bandwidth, polarization direction, directivity, gain, lobe width, impedance, quality factor bandwidth, directivity, gain, return loss, quality factor, sensitivity and other electrical performance parameters on the network analyzer such as tag format, and test its reading distance in different application environments, verify and correct.
Conclusion
Through the above research methods and technical routes, we hope to obtain RFID electronic tags with high conductivity and meet the needs of the Internet of things industry, realize the large-scale industrial manufacturing of RFID electronic tags, completely free of waste in the manufacturing process, and ensure green environmental protection. The technical iteration of graphene RFID sticker will become the next outlet in the Internet of things industry.
