The rapid development of smart cities, Industry 4.0, and the Internet of Things has generated a large demand for RFID intelligent identification products in the market. RFID, also known as wireless radio frequency identification technology, is a non-contact automatic data collection technology and one of the core technologies of the Internet of Things. Its biggest feature is its fast information collection speed, which does not require mechanical or optical contact and is completely completed through wireless communication technology. It can simultaneously collect hundreds or thousands of object information within 1 second, with high accuracy in information collection. RFID Sensor Tags, which integrate sensing capabilities, further enhance this technology by enabling not only identification but also real-time monitoring of environmental conditions such as temperature, humidity, and motion.** At present, this technology has been widely applied in almost all fields such as logistics warehousing, transportation, security and anti-counterfeiting, mobile payments, etc.
1. Current research status at home and abroad
The United States is an active promoter of RFID electronic tag applications, leading the world in the establishment of RFID standards, software and hardware technology development, and application fields. The European RFID standard follows the EPCglobal standard led by the United States. The research and development of RFID in European and American countries mainly focuses on standard setting, chip manufacturing, reader manufacturing, and system integration. China is the world’s largest supplier of RFID electronic tags, with a total production volume of about 60% of the world’s RFID electronic tags. Especially for high-frequency RFID electronic tags, China has basically achieved full localization from chips to antennas. The main bottlenecks in promoting the application of RFID systems are the price, size, and environmental adaptability of tags.
At present, copper wire winding method and aluminum foil etching method are commonly used in China to manufacture RFID electronic tags, while ceramic sintering method is basically used abroad. The common problems are:
- Pollution of the environment;
- The label substrate is single and the application field is limited;
- Low manufacturing efficiency;
- Large label size;
- High cost;
- Low manufacturing accuracy.
In the past few years, many research institutions at home and abroad have believed that using conductive silver paste screen printing to manufacture RFID electronic tags is the most effective technology for achieving low-cost, miniaturized, high-precision, adaptable, and large-scale production of tags. Although printing technology in the field of image replication is fast, efficient, has fine lines, and accurate overprinting, due to the poor conductivity of conductive silver paste and the limitations of the conductive mechanism, only high silver content conductive silver paste and low wire mesh screens can be used. Due to dozens of factors such as ink viscosity, extensibility, fluidity, scraping pressure, screen stretching, and wire interference, the printed RFID electronic tag wire structure is deformed, the boundary is rough, short circuits are broken, and the actual radiation efficiency differs greatly from the theoretical radiation efficiency.
Looking at domestic and foreign research, there are almost no scientifically effective control methods for the above-mentioned problems. Therefore, there are currently almost no successful cases of using this process to achieve the industrialization of RFID electronic tags. In summary, on the one hand, the demand for RFID electronic tags in the market is becoming increasingly strong. On the other hand, RFID electronic tag manufacturing technology still faces problems such as low efficiency, high cost, environmental pollution, and single substrate. The market urgently needs new manufacturing technologies to break through the contradiction between demand and supply.
Micro RFID Transponder
1.1 Trend and demand analysis
With the Nobel Prize winner Andre? Sir Heim and Constantine? Sir Novoselov successfully developed graphene materials with high conductivity. The decrease in graphene prices and the improvement in product quality have greatly stimulated the application research of downstream products, such as various conductive circuits, sensors, medical monitors, and other graphene electronic products. Due to the micro topological structure of graphene material, it has high conductivity, and its conductivity mechanism is different from that of metal silver particles.
1.2 Advantages
Processing graphene powder into a filling type composite conductive paste has two major advantages:
- Strong compatibility. Graphene paste can be printed on almost all substrates such as plastic film, paper, ceramics, cotton cloth, wood, etc;
- High cost-effectiveness. Compared with existing conductive silver pastes, graphene pastes have better conductivity and greater cost advantages.
With the continuous maturity of graphene production technology and the continuous reduction of costs, graphene conductive paste will gradually occupy market share. It is expected that the market size of graphene applications in the field of conductive paste will reach 200 million yuan by 2020. At present, some foreign manufacturers developing RFID systems have restarted the key technology and industrialization research of printing RFID electronic tags with graphene paste.
2. Project research methods
2.1 Preparation, Modulation, and Dispersion Control of Graphene Conductive Slurry
Due to the distinct appearance and conductive properties of graphene and conductive silver powder, the formulation and manufacturing process of conductive pastes based on graphene are also completely different. The hydrophobicity of graphene makes it easy for graphene nanosheets to agglomerate through strong van der Waals forces. The use of effective solvents can prevent the agglomeration of graphene, thereby making it a stable graphene dispersion. The ideal solvents are mainly N-methylpyrrolidone (NMP) and dimethylformamide (DMF). This project plans to use DMF/NMP as the solvent and add stabilizers (such as ethyl cellulose) to the graphene slurry formula to disperse graphene, in order to solve the problem of easy agglomeration and difficult dispersion of graphene powder. Use a laser particle size analyzer to test the particle size and dispersion of the slurry, ensuring the dispersion of graphene slurry particles.
Passive RFID Transponder
Adding UV initiators, photosensitive resins, and other components to graphene conductive paste, optimizing their mixing ratios, enables graphene conductive paste to quickly cure under UV light, reducing the drying temperature of RFID electronic tags after printing, shortening the drying time, and improving production efficiency. RFID electronic tags can be printed on various substrates such as paper, plastic films, and silk fabrics. By changing the connecting materials and additives of the slurry, the viscosity, viscosity, flowability, surface tension, dryness, thixotropy, rheology, particle size and other printing adaptability of graphene conductive slurry can be modulated. The viscosity cup or Ubbelohde viscometer, slurry viscosity meter, and surface tension can be used to meet the requirements of gravure printing.
2.2 RFID electronic tag antenna design based on graphene conductive paste printing
The main parameters that affect the electrical performance of RFID electronic tag antennas include antenna shape, size structure, material properties, operating frequency, bandwidth, polarization direction, directionality, gain, lobe width, impedance, sensitivity, quality factors, and application environment. The design of RFID electronic tags needs to balance the above parameters. Input design parameters such as antenna line width, line spacing, bending size, feed gap, feed loop size, electromagnetic signal receiving and feedback material conductivity, dielectric constant, etc. into simulation software HFSS or ADS for computer simulation. The computer simulation obtains the return loss and energy distribution of UHFRFID electronic tags, thereby determining the electrical performance characterization parameters of RFID electronic tags and establishing a data model. Study the influence of graphene conductive paste formula and printing process parameters on antenna performance. Study the influence of ink film thickness on skin effect and antenna performance. By studying the impedance characteristics of graphene ink layers at different chemical potentials, especially the impact of high reactance characteristics in the ultra-high frequency band on the antenna gain of RFID electronic tags.
2.3 Control of gravure printing parameters based on graphene conductive paste
Based on the printing suitability of graphene conductive paste and the structural parameters of RFID electronic tags designed by computer simulation, determine the appropriate number of screen lines, mesh depth, and mesh shape for the gravure printing drum, calculate the ink amount, and adjust the thickness of the paste film layer. For different substrates such as paper, plastic films (such as PET, PI, CPP, etc.), silk fabrics, etc., adjust a series of printing process parameters such as the tension of the roll during gravure printing, the pressure of the printing rubber embossing roller, the thixotropic viscosity of the slurry, the contact angle of the scraper, the printing speed, and the positioning overprint to obtain the optimal printing process solution.
2.4 Optimization of Ink Film Drying Temperature and Drum Compression Pressure
Due to the drying and solidification of the printed conductive ink layer to a certain extent, and then rolling it with a roller, its surface morphology can be changed, the density of the ink layer can be increased, and the conductivity can be greatly improved. However, the ink film that is pressed can easily cause contour enlargement and wire deformation. Adjust the temperature and time of the graphene conductive ink layer in the UV curing channel, test the ability to improve conductivity and the degree of deformation of the ink layer contour, and optimize process parameters such as ink film curing temperature, time, and pressure.
2.5 Analyze and test the performance of RFID electronic tags
Take printed samples, attach chips, and test their electrical performance parameters such as operating frequency, bandwidth, polarization direction, directionality, gain, lobe width, impedance, quality factor bandwidth, directionality, gain, return loss, quality factor, sensitivity, etc. on network analyzers such as Tagform. Test their reading distance in different application environments for verification and correction
3. summary
Through the above research methods and technical routes, we aim to obtain RFID electronic tags with high conductivity that meet the needs of the Internet of Things industry, and achieve large-scale industrial manufacturing of RFID electronic tags. The manufacturing process is completely waste free, ensuring green environmental protection. The technological iteration of graphene RFID electronic tags will undoubtedly become the next trend in the Internet of Things industry.