Low-temperature antioxidant conductive copper paste, preparation method and RFID tag

Abstract

The application discloses a low-temperature oxidation-resistant conductive copper paste, a preparation method thereof and an RFID label, and relates to the field of conductive paste. The modified copper powder comprises: copper powder; a cysteine self-assembled molecular layer formed by coupling a mercapto group of cysteine with the surface of the copper powder; and a metal phthalocyanine complex molecular layer connected to the surface of the cysteine self-assembled molecular layer by coordinating a central metal ion of a metal phthalocyanine molecule with an amino group of the cysteine. The conductive copper paste is sintered at 140-170 DEG C. The application can effectively solve the problem of oxidation resistance of the copper powder while ensuring the conductive performance between the copper powders, and obtain a low-temperature solidification conductive copper paste with long-term oxidation resistance and high conductivity.

Description

Technical Field

[0001] This application relates to the field of conductive pastes, and in particular to a low-temperature antioxidant conductive copper paste, its preparation method, and an RFID tag. Background Technology

[0002] Aluminum foil etching technology, currently the mainstream process for electronic tag circuit processing, generates large amounts of waste liquid containing heavy metals and strong alkalis during its wet etching process, causing serious environmental pollution. Furthermore, its process relies on non-degradable polymer substrates such as PET, contradicting the global commitment to environmental sustainability. In contrast, printed electronics technology, based on additive manufacturing principles, employs a direct-write molding process, achieving a clean, pollution-free, and simplified production model. It also allows for direct circuit processing on biodegradable paper-based substrates, and is considered by the industry as a crucial future direction for electronic tag technology.

[0003] In conductive paste systems for printing, silver-based materials have become the most widely used conductive materials due to their excellent conductivity and environmental stability. However, the scarcity of silver has led to persistently high raw material costs, resulting in the price of a single RFID tag made with silver paste being more than 50% higher than that of a traditional aluminum foil-etched tag, severely restricting its large-scale commercial application. Copper-based conductive pastes, with material costs only about 1% of silver, exhibit significant economic advantages and potential as an alternative. However, copper powder is highly susceptible to oxidation in air, forming a poorly conductive oxide layer, which causes a sharp deterioration in the electrical performance of the prepared tags, making it difficult to meet the long-term stability requirements of practical applications.

[0004] In existing technologies, constructing a self-assembled monolayer based on alkyl thiols on the surface of copper powder is one of the widely studied antioxidant methods. However, the molecular layer structure formed by short-chain alkyl thiols is sparse and has many defects, resulting in limited oxygen barrier effect. While long-chain alkyl thiols can form a dense and ordered molecular barrier, their thicker molecular layer and wider HOMO-LUMO band gap make it difficult to completely decompose under low-temperature curing conditions suitable for heat-sensitive substrates such as paper. This incompletely decomposed insulating molecular layer introduces a significant tunneling barrier between adjacent copper powder particles, greatly increasing the interparticle contact resistance and causing a decrease in conductivity. Summary of the Invention

[0005] This application provides a low-temperature antioxidant conductive copper paste, its preparation method, and an RFID tag, which can effectively solve the problem of its antioxidant defects while ensuring the conductivity between copper powders.

[0006] In a first aspect, this application provides a low-temperature oxidation-resistant conductive copper paste, comprising modified copper powder, wherein the modified copper powder includes:

[0007] Copper powder;

[0008] A cysteine ​​self-assembled molecular layer is formed by coupling the thiol groups of cysteine ​​to the surface of the copper powder;

[0009] The metal phthalocyanine complex molecular layer is connected to the surface of the cysteine ​​self-assembled molecular layer through the coordination of the amino group of the cysteine ​​with the central metal ion of the metal phthalocyanine complex.

[0010] The conductive copper paste is sintered at 140–170°C.

[0011] This application proposes a strategy for constructing an antioxidant layer on the surface of copper powder through layer-by-layer self-assembly. First, a short-chain cysteine ​​(mercaptoethylamine) molecular layer is grown on the copper powder surface by utilizing the strong coupling between the copper powder matrix and thiol groups. Then, chemical bonding is achieved through the coordination of the lone pair electrons of the terminal amino group of the cysteine ​​with the central metal ion of a metal (such as copper or zinc) phthalocyanine molecule. The area of ​​the metal phthalocyanine complex molecule (approximately 2.2 nm²) is much larger than that of a thiol molecule (approximately 0.18 nm²), which can act as a "molecular umbrella," effectively blocking copper-oxygen contact caused by the sparseness or defects of the cysteine ​​self-assembled molecular layer. As a short-chain molecule, cysteine ​​has a short tunneling length, and low-temperature sintering under nitrogen (e.g., 140°C) is sufficient to break its CN and CS bonds, resulting in a high decomposition rate and no negative impact on the conductivity of the slurry. The metal phthalocyanine complex itself has a conjugated macrocyclic structure. The metal-π interaction still allows the complex to form a protective layer on the incompletely fused region of the copper particles after cysteine ​​decomposition, preventing the printed pattern from oxidizing in air. Furthermore, because the transplanar thickness of the metal phthalocyanine complex molecule is only 0.30–0.40 nm, it possesses the electron delocalization capability provided by the conjugated structure and the carrier supply characteristics of the metal nucleus (such as copper ions or zinc ions), thus exhibiting excellent interfacial electron transport performance.

[0012] In the specific implementation process, through the aforementioned self-assembly and coordination reactions, this application aims to sequentially construct a cysteine ​​self-assembled molecular layer and a metal phthalocyanine complex molecular layer on the surface of copper powder. It should be noted that, limited by molecular dynamics, the inhomogeneity of the copper powder surface microstructure, and steric hindrance effects during the actual reaction process, the formed cysteine ​​molecular layer is not a perfectly dense film achieving 100% coverage at the microscopic level. Rather, it refers to its ability to achieve high-density and stable adhesion to the copper powder surface through intermolecular forces, forming a continuous protective layer that effectively prevents direct contact between the copper powder and air at the macroscopic level. Similarly, the metal phthalocyanine complex molecular layer subsequently constructed through coordination is intended to effectively supplement and cover any microscopic defects or sparse regions that may exist in the underlying cysteine ​​molecular layer using its macromolecular planar structure, rather than forming an absolutely sealed layer without any gaps. The core of this application lies in the fact that the "molecular layer" is a functional concept. As long as the combination of the cysteine ​​molecular layer and the metal phthalocyanine complex molecular layer can synergistically improve the antioxidant stability of copper powder without significantly impairing its conductivity, it should be considered as achieving the purpose of this invention. This non-100% coverage state is a common characteristic of this type of self-assembled molecular layer technology and can fully meet the requirements for copper powder protection performance in practical applications.

[0013] For example, the sintering temperature of the conductive copper paste is 140°C, 143°C, 145°C, 150°C, 155°C, 160°C, 165°C, 168°C, or 170°C.

[0014] In any of the above technical solutions, the metal phthalocyanine complex includes, but is not limited to, any one or a combination of several of copper phthalocyanine, zinc phthalocyanine, nickel phthalocyanine, cobalt phthalocyanine, iron phthalocyanine, aluminum phthalocyanine, manganese phthalocyanine, lead phthalocyanine, magnesium phthalocyanine, tin phthalocyanine, and titanium phthalocyanine.

[0015] In any of the above technical solutions, the metal phthalocyanine complex is zinc phthalocyanine.

[0016] Zinc phthalocyanine has high axial coordination activity and flexible configuration. This characteristic makes the coordination effect of zinc phthalocyanine molecules stronger than other complexes. It can form a more efficient and stable coordination bond with the lone pair electrons of the nitrogen atom of the terminal amino group of the lower cysteine ​​molecule through the zinc ion at its center. This results in the construction of a "molecular umbrella" protective layer with stronger bonding force and higher coverage on the surface of copper powder, resulting in a stable and dense composite antioxidant layer, thus ensuring long-term antioxidant performance.

[0017] In any of the above technical solutions, the cysteamine self-assembled molecular layer is a cysteamine self-assembled monolayer.

[0018] In any of the above technical solutions, the metal phthalocyanine complex molecular layer is a metal phthalocyanine complex monolayer.

[0019] In any of the above technical solutions, the thickness of the metal phthalocyanine complex molecular layer is 0.35–0.45 nanometers.

[0020] In any of the above technical solutions, the thickness of the cysteine ​​self-assembled molecular layer is 0.3 to 0.4 nanometers.

[0021] The thickness of the aforementioned metal phthalocyanine complex molecular layer is the transplane thickness, which refers to the vertical distance from the plane where the central zinc ion is located to the opposite outer edge of the four pyrrole nitrogen atoms coordinated with it within the core framework plane of the planar macrocyclic molecule.

[0022] It is worth noting that due to the extremely strong interaction between thiol groups and copper (generally greater than 2 eV), cysteamine can easily achieve sufficient coverage on the copper powder surface. In contrast, the interaction between amino groups and copper is relatively weak (generally 0.6–0.8 eV). Therefore, given sufficient growth time, the probability of single-thiol coupling is much higher than that of chelation. This is because molecular layer growth is a dynamic process. During growth, the desorption probability of amino-copper is much higher than that of thiol-copper. As molecular density increases, steric hindrance will prevent chelation formation, driving the system towards single-point coupling between thiol and copper. Finally, since there is virtually no coupling between amino groups, thiol groups, and σ bonds, cross-plane stacking is difficult to achieve. Therefore, after the copper surface is covered with cysteamine, cysteamine is difficult to grow into a multilayer structure. Consequently, the film thickness of the cysteamine molecular layer is close to the height of a single cysteamine molecule (approximately 0.3–0.4 nm). The cross-plane thickness of zinc phthalocyanine molecules, according to AFM data, is approximately 0.35–0.45 nm.

[0023] In any of the above technical solutions, the method for preparing the modified copper powder includes:

[0024] Copper powder with its surface oxide layer removed is mixed with a cysteine ​​solution with a concentration of 0.1-0.3 wt%, and a self-assembly reaction is carried out under an inert atmosphere for no less than 3 hours to form a cysteine ​​self-assembled molecular layer on the surface of the copper powder, thus obtaining cysteine-modified copper powder.

[0025] Cysteine-modified copper powder was mixed with a metal phthalocyanine complex solution with a concentration of 0.7–2.8 wt% and subjected to a coordination reaction. After 30–60 min, a metal phthalocyanine complex molecular layer was obtained, thus preparing modified copper powder.

[0026] In the above technical solution, if the concentration of zinc phthalocyanine is too low or the reaction time is too short, the molecular film will not be fully covered; if the concentration is too high or the reaction time is too long, π-π stacking will occur between phthalocyanine molecules, resulting in an excessively thick film layer and increasing the contact resistance between copper powder particles.

[0027] If the cysteine ​​concentration is too low or the reaction time is insufficient, the surface modification will be incomplete, which will affect the film quality of the subsequent metal phthalocyanine molecules and will not be conducive to improving its antioxidant properties.

[0028] Preferably, the reaction time of the self-assembly reaction is 3 to 4 hours.

[0029] In any of the above technical solutions, the conductive copper paste comprises 60-70 wt% modified copper powder, 29-39 wt% resin solution, 0.7-0.9 wt% curing agent, and 0.1-0.3 wt% coupling agent.

[0030] In any of the above technical solutions, the particle size of the copper powder is 5 to 10 μm.

[0031] In any of the above technical solutions, the resin is selected from any one or a combination of several of polyurethane resin, phenoxy resin, epoxy resin, and phenolic resin.

[0032] In any of the above technical solutions, the resin solution comprises phenoxy resin and organic solvent in a mass ratio of 1:2 to 3.

[0033] In any of the above technical solutions, the solvent is a composition of divalent ester, terpineol and isophorone in a mass ratio of 2 to 3:1:1.

[0034] In any of the above technical solutions, the divalent ester is selected from any one or a combination of several of diethylene glycol butyl ether acetate, dimethyl succinate, dimethyl glutarate, and dimethyl adipate.

[0035] In any of the above technical solutions, the curing agent is a formic anhydride curing agent and a blocked isocyanate curing agent with a mass ratio of 1.5 to 2.5:1, and the unblocking temperature of the blocked isocyanate curing agent is lower than the sintering temperature of the conductive copper paste (140 to 170°C).

[0036] This application uses a combination of formic acid anhydride curing agent and blocked isocyanate curing agent to achieve simultaneous crosslinking through two curing agents with different reaction mechanisms, thereby optimizing the final resin crosslinked network structure. Under low-temperature flexible conditions (140–170°C), a single curing agent is insufficient to form a sufficiently dense and uniform three-dimensional network within the expected time. The formic acid anhydride curing agent and the blocked isocyanate curing agent are simultaneously activated upon heating. The former reacts with the resin through anhydride ring-opening, while the latter undergoes deblocking, releasing active isocyanate groups that react with hydroxyl groups for crosslinking. These two reactions occur in parallel and can mutually promote each other, enabling the resin system to rapidly construct a continuous phase with higher crosslinking density and a more complete network structure at relatively low temperatures. This optimized resin skeleton not only provides stronger mechanical anchoring for the conductive copper powder, reducing the risk of conductive path breakage due to stress concentration, but also allows the resin volume to shrink during curing. This larger resin volume shrinkage facilitates more efficient contact and adhesion of the metal powder dispersed in the resin, improving the performance of the conductive network. Furthermore, its lower free volume enhances its barrier properties against oxygen and moisture, indirectly contributing to the oxidation resistance of copper powder. Curing can also be achieved using a single type of curing agent system, but the uniformity and density of the cross-linked network may be slightly weakened, potentially negatively impacting the final adhesion, conductivity, and long-term stability of the slurry.

[0037] In any of the above technical solutions, the coupling agent includes, but is not limited to, any one or more of aminosilane coupling agents, epoxysilane coupling agents, and acryloyloxysilane coupling agents.

[0038] Secondly, this application provides a method for preparing low-temperature antioxidant conductive copper paste, which is obtained by mixing the raw materials according to the proportions of any of the low-temperature antioxidant conductive copper pastes described in the first aspect, followed by grinding and screening.

[0039] Thirdly, this application provides an RFID tag, including a substrate, an antenna, and a chip, wherein the antenna is printed from any of the conductive copper pastes described in the first aspect or the conductive copper pastes prepared in the second aspect.

[0040] In any of the above technical solutions, the substrate is a paper-based substrate, including but not limited to coated paper, kraft paper, or corrugated paper.

[0041] In summary, this application has the following beneficial effects:

[0042] This application provides a novel low-temperature antioxidant conductive copper paste and its applications. The core of this invention lies in employing a layer-by-layer self-assembly technology to construct a composite antioxidant structure on the surface of copper powder, consisting of a short-chain cysteine ​​self-assembled molecular layer and a metal phthalocyanine complex molecular layer. This structure cleverly synergizes the advantages of both materials. Cysteine, as a short-chain molecule, ensures efficient decomposition after low-temperature sintering without introducing excessively high contact resistance, laying the foundation for the paste's high conductivity. Meanwhile, the metal phthalocyanine molecules covering it act as a "molecular umbrella" due to their large planar structure, physically blocking oxygen. Furthermore, their extremely thin transplanar thickness and intrinsic electron delocalization properties significantly enhance the antioxidant stability of the copper powder without significantly hindering electron transport. Simultaneously, by optimizing the paste formulation and low-temperature curing process, this conductive copper paste can be adapted to low-cost, biodegradable flexible substrates such as paper. In summary, this application effectively solves the core problems of easy oxidation and poor low-temperature processing adaptability of copper paste while ensuring conductivity, providing a technological foundation for manufacturing high-performance, low-cost, and environmentally friendly RFID tags. Attached Figure Description

[0043] Figure 1 This is a schematic diagram of the structure of the RFID tag in this application;

[0044] Figure 2 This is an atomic force microscopy image (2μm×2μm) of the surface of the modified copper sheet prepared in accordance with Example 1.

[0045] Figure 3 This is an atomic force microscopy image (230nm×230nm) of the surface of the modified copper sheet prepared in accordance with Example 1.

[0046] Figure 4 This is a schematic diagram showing the height difference between the defective and non-defective portions on the surface of the modified copper sheet prepared in accordance with Example 1;

[0047] Figure 5 This is the XPS spectrum of the 2p orbital of copper element after the printed pattern in Example 1 has undergone double 85 processing for 72 hours;

[0048] Figure 6 This is the XPS spectrum of the 2p orbital of zinc element after the printed pattern in Example 1 has been treated with double 85 for 72 hours;

[0049] Figure 7 This is a frequency-reading distance spectrum of copper paste labels after fresh preparation, after being exposed to air for 1 week, and after 72 hours of double 85 treatment.

[0050] Explanation of reference numerals in the attached figures:

[0051] 1. Substrate; 2. Antenna; 3. Chip. Detailed Implementation

[0052] Example 1: An RFID tag is prepared according to the following steps:

[0053] Step 1: Add 100 g of flake copper powder (particle size 5–10 μm) to 300 mL of an aqueous solution composed of citric acid (2.8 wt%) and sodium hypophosphite (0.8 wt%). Under a nitrogen atmosphere, stir at 200 rpm for 50 min to remove surface oxides. After the reaction, centrifuge at 3000 rpm for 5 min and remove the supernatant. Redisperse the resulting copper powder in 200 mL of deionized water, stir and wash at 200 rpm for 5 min, then centrifuge again at 3000 rpm for 5 min and discard the supernatant. Repeat this washing process until the pH of the washing solution reaches 7, and finally collect the copper powder with the surface oxide layer removed.

[0054] Step 2: Prepare a 200 mL cysteamine ethanol solution (0.2 wt%), and add the copper powder with the surface oxide layer removed as described above to this solution. Under a nitrogen atmosphere, stir at 100 rpm for 5 h to allow cysteamine to self-assemble into a cysteamine monolayer on the copper powder surface via thiol groups. After the reaction is complete, centrifuge at 3000 rpm for 5 min and remove the supernatant. Redisperse the precipitated copper powder in 200 mL ethanol, stir and wash at 100 rpm for 3–5 min, and centrifuge again at 3000 rpm for 5 min, discarding the supernatant to remove physically adsorbed and unreacted cysteamine. Repeat this ethanol washing step 1–2 times to finally obtain cysteamine-modified copper powder.

[0055] Step 3: Prepare 200 mL of zinc phthalocyanine solution (solvent: toluene:chloroform = 4:1, mass ratio; zinc phthalocyanine concentration 2.0 wt%), add cysteine-modified copper powder to it, and stir for 45 min to allow the lone pair electrons in the amino group to coordinate with the zinc center of zinc phthalocyanine, forming a zinc phthalocyanine molecular layer. After the reaction is complete, centrifuge at 3000 rpm for 5 min and remove the supernatant. Redisperse the precipitated copper powder in 200 mL of toluene, stir and wash at 200 rpm for 3–5 min, and centrifuge again to remove the supernatant to remove physically adsorbed or uncoordinated zinc phthalocyanine molecules. Spread the obtained copper powder evenly in a ceramic dish (approximately 2 mm thick) and dry in a vacuum oven at 60℃ for 3 h to obtain modified copper powder.

[0056] Step 4: Prepare a solvent by mixing divalent ester, terpineol, and isophorone at a mass ratio of 3:1:1. Mix the solvent and resin at a mass ratio of 2.5:1. Stir at 500 rpm at 60°C until the resin is completely dissolved (no visible particles are observed). Then, place the solution in an ultrasonic bath (power density 20 W / L, frequency 40 kHz) for degassing until no visible bubbles are observed, obtaining a resin solution. Add modified copper powder, resin solution, and KH550 silane coupling agent to a vacuum mixer at a mass ratio of 65:34:0.2 and stir at 500 rpm for 60 min to obtain crude copper paste. The crude copper paste was ground three times in sequence by a three-roll mill (the roller spacing was set to 15 μm, 10 μm and 5 μm respectively), passed through a 300-mesh sieve, and 0.8wt% curing agent (formic anhydride curing agent and 110℃ unsealed blocked isocyanate curing agent were mixed in a 2:1 ratio) was added. The mixture was stirred at 200 rpm for 60 min to obtain low-temperature antioxidant conductive copper paste.

[0057] Step 5: The prepared low-temperature antioxidant conductive paste is screen-printed onto coated paper using a 200-mesh screen to form a paste like... Figure 1 The antenna pattern shown is shown (printing direction is the first direction; the antenna structure is designed according to the matching requirements of the paste conductivity and chip impedance). The printed substrate was cured in a 140℃ nitrogen oven for 30 min, and then flattened using a 6 MPa pressure roller. The NXP-U9 chip was coupled to the antenna using a flip-chip bonding process: conductive adhesive (DELOMONOPOX AC268) was used to connect the chip and antenna contacts, and the RFID tag was hot-pressed at 140℃ for 15 s to complete the RFID tag fabrication.

[0058] Example 2, an RFID tag, differs from Example 1 in that the concentration of the cysteine ​​ethanol solution in step 2 is 0.1 wt%, and the stirring reaction time is 3 h; the concentration of the zinc phthalocyanine solution in step 3 is 2.8 wt%, and the stirring time is 30 min.

[0059] Example 3, an RFID tag, differs from Example 1 in that the concentration of the cysteine ​​ethanol solution in step 2 is 0.3 wt%, and the stirring reaction time is 6 h; the concentration of the zinc phthalocyanine solution in step 3 is 0.7 wt%, and the stirring time is 60 min.

[0060] Example 4, an RFID tag, differs from Example 1 in that, in step 3, an equal amount of copper phthalocyanine is used instead of zinc phthalocyanine, and a copper phthalocyanine solution of the same concentration is prepared.

[0061] Comparative Example 1, an RFID tag, differs from Example 1 in that, in step 4, an equal amount of untreated flake copper powder (particle size 5-10 μm) is used instead of modified copper powder.

[0062] Comparative Example 2, an RFID tag, differs from Example 1 in that, in step 2, a tetradecyl mercaptan ethanol solution of the same concentration is used instead of a cysteine ​​ethanol solution, and step 3 is omitted, proceeding directly to subsequent steps; the specific operation of step 2 is as follows:

[0063] Prepare a 200 mL solution of tetradecyl mercaptan (0.2 wt%) in ethanol, and add the copper powder from step 1 (with the surface oxide layer removed) to this solution. Under a nitrogen atmosphere, stir at 100 rpm for 5 h to allow tetradecyl mercaptan to form a self-assembled monolayer on the copper powder surface via its thiol groups. After the reaction is complete, centrifuge at 3000 rpm for 5 min and remove the supernatant. Redisperse the precipitated copper powder in 200 mL of ethanol, stir and wash at 100 rpm for 3–5 min, and centrifuge again at 3000 rpm for 5 min, discarding the supernatant to remove physically adsorbed and unreacted tetradecyl mercaptan. Repeat this ethanol washing step 1–2 times to finally obtain tetradecyl mercaptan-modified copper powder.

[0064] Comparative Example 3, an RFID tag, differs from Example 1 in that, in step 2, a hexanethiol ethanol solution of the same concentration is used instead of a cysteine ​​ethanol solution, and step 3 is omitted, proceeding directly to subsequent steps; the specific operation of step 2 is as follows:

[0065] Prepare a 200 mL solution of n-hexylthiol (0.2 wt%) in ethanol, and add the copper powder from step 1 (with the surface oxide layer removed) to this solution. Under a nitrogen atmosphere, stir at 100 rpm for 5 h to allow n-hexylthiol to form a self-assembled monolayer on the copper powder surface via thiol groups. After the reaction is complete, centrifuge at 3000 rpm for 5 min and remove the supernatant. Redisperse the precipitated copper powder in 200 mL of ethanol, stir and wash at 100 rpm for 3–5 min, and centrifuge again at 3000 rpm for 5 min, discarding the supernatant to remove physically adsorbed and unreacted n-hexylthiol. Repeat this ethanol washing step 1–2 times to finally obtain n-hexylthiol-modified copper powder.

[0066] Comparative Example 4 is an RFID tag that differs from Example 1 in that step 3 is not performed after step 2, and subsequent steps are performed directly.

[0067] Comparative Example 5, an RFID tag, differs from Example 1 in that, in step 2, copper powder with the surface oxide layer removed is stirred at 100 rpm for 2 h under a nitrogen atmosphere.

[0068] Comparative Example 6, an RFID tag, differs from Example 1 in that, in step 3, cysteamine-modified copper powder is stirred and reacted in zinc phthalocyanine solution for 20 min.

[0069] Comparative Example 7, an RFID tag, differs from Example 1 in that, in step 3, cysteine-modified copper powder is stirred and reacted in zinc phthalocyanine solution for 2 h.

[0070] Comparative Example 8 is an RFID tag that differs from Example 1 in that steps 1 to 4 are omitted, and in step 5, commercially available conductive silver paste (silver content 65 wt%) is used instead of the self-made low-temperature antioxidant conductive copper paste.

[0071] Performance testing: Test 1: Determination of the quality of the modified copper powder anti-oxidation film

[0072] The film-forming effect of the antioxidant film (cysteine ​​self-assembled molecular layer + metal phthalocyanine complex molecular layer) on the surface of copper particles is difficult to characterize directly. In this experiment, an antioxidant film was prepared on a copper sheet (1 cm × 1 cm, replacing sheet copper powder) using the same method as in Example 1. See [link to example]. Figure 2 (Field of view 2 micrometers × 2 micrometers) and Figure 3 (Field of view 230 nm × 230 nm, white arrows indicate structural defects). The atomic force microscope image after growth shows that the film successfully covered more than 93% of the copper sheet surface. See also... Figure 4 The height difference between the uncovered and covered areas is approximately 0.4 nm (i.e., the thickness of the zinc phthalocyanine molecular layer), which is basically consistent with the thickness of a single layer of zinc phthalocyanine. This indicates that zinc phthalocyanine has successfully covered the copper surface with a high coverage and near-single-layer structure, forming an effective "molecular umbrella" protective layer.

[0073] In this experiment, copper sheets were used instead of copper powder primarily because the morphology and surface of copper powder are random, making it impossible to characterize using atomic force microscopy (which requires a large, flat surface). Copper sheets, while chemically similar to copper powder, more easily achieve a large, flat surface, making them more suitable for characterizing molecular films. It is important to emphasize that because copper powder has a much larger specific surface area than copper sheets, it is more reactive. Therefore, if molecular layers can grow at a high quality on copper sheets, it can be inferred that they can also grow at a high quality on sheet-like copper powder.

[0074] Experiment 2: Conductivity Measurement

[0075] Test Method: Printing was performed immediately after the paste preparation was completed. The structure was changed to a 1cm × 1cm square, with coated paper as the substrate. After printing, the paper was dried at 140℃ for 30 minutes and pressed with a 160MPa roller. The sheet resistance of the square printed product was measured using a 4-probe sheet resistance meter (DWR1C, Shanghai Instrumentation). The four probes were placed on the diagonal of the square, and the corresponding sheet resistance values ​​were read. The thickness of the printed film was measured using a film thickness meter (Shenda Weiruipu SW6300-A), and the sheet resistance was converted to conductivity. The conversion formula is: conductivity = 1 / (sheet resistance × film thickness). The test results for each example are shown in Table 1. The sheet resistance and conductivity values ​​are averaged from 10 printed patterns.

[0076] Table 1. Conductivity Test Results

[0077]

[0078] As shown in Table 1, the printed pattern in Comparative Example 1 exhibits the highest sheet resistance. This is because during the drying process (140℃), the copper particles have high surface activity. Even under a nitrogen atmosphere, a small amount of oxygen remains, causing the copper surface to react with oxygen atoms to form nearly insulating copper oxide and cuprous oxide layers. These layers cover the particle surface, significantly increasing the interparticle contact resistance and thus rapidly raising the sheet resistance. Comparative Example 2 also shows a high sheet resistance. Although its surface is modified with a tetradecyl mercaptan protective layer to inhibit copper atom oxidation, this protective layer itself is approximately 2 nm thick, has a large HOMO-LUMO energy difference, exhibits insulating properties, and cannot be completely degraded under the curing conditions of 140℃. The tunneling efficiency between particles decreases exponentially with the thickness of the insulating layer, resulting in a high particle interface resistance and ultimately increasing the sheet resistance of the printed pattern.

[0079] The conductivity of Comparative Example 3 is significantly better than that of Comparative Examples 1 and 2. This is because hexyl thiol can form a molecular protective layer on the copper surface, effectively preventing copper powder oxidation during heating. Simultaneously, the shorter hexane chain allows for partial decomposition at 140°C, enabling direct contact between copper particles. However, due to the lower molecular density of short-chain thiols compared to long-chain thiols, the formed protective layer still contains intermolecular gaps and structural defects. Some copper atoms can oxidize through these defects and contact with air. Therefore, although its performance is superior to Comparative Examples 1 and 2, its conductivity is still significantly lower than the actual embodiment of this work (the difference is nearly three times or more). If used for RF antenna fabrication, it may sacrifice some readout distance. The conductivity of Comparative Example 4 is close to that of Comparative Example 3. This may be because the short-chain cysteine ​​provides some protection for copper particles during curing, but gaps and defects still exist, leading to localized copper exposure and oxidation.

[0080] The conductivity of Examples 1-4 and Comparative Example 6 is higher than that of Comparative Example 4, indicating that the additional protective layer formed by phthalocyanine metal can further block the contact between copper and oxygen and inhibit the formation of oxide layer. However, the conductivity of Comparative Example 6 is about half that of Example 1. This is because its phthalocyanine zinc growth time is shorter, and it failed to form a tightly packed molecular layer, which weakens the protective effect of the phthalocyanine metal "molecular umbrella" structure.

[0081] The conductivity of Comparative Example 5 was significantly lower than that of Examples 1-4 and Comparative Example 6. This is because the growth period of cysteine ​​was too short, resulting in insufficient intermolecular forces and difficulty in forming a dense monolayer. In the non-dense membrane structure, some molecules may chelate with the copper surface through both thiol and amino groups, leading to insufficient sites for metal phthalocyanine complexation and failure to successfully construct a phthalocyanine protective layer. Therefore, the protection of copper powder is incomplete, resulting in lower conductivity.

[0082] The conductivity of Comparative Example 7 was also significantly lower than that of Examples 1-4. This was because the phthalocyanine metal growth time was too long, leading to intermolecular π-π stacking and increasing interfacial contact resistance. However, thanks to the conjugated structure of the porphyrin ring and the carrier contribution of zinc metal, its resistance was much lower than that of copper oxide or alkyl chain insulating layers. Therefore, the conductivity of Comparative Example 7 was still significantly higher than that of Comparative Examples 1 and 2. The conductivity of Comparative Example 8 was about twice that of the examples. This was mainly attributed to the higher intrinsic conductivity of silver itself, the relatively lower welding temperature, and the advantage of not needing to introduce an additional antioxidant layer. However, the cost of silver paste was high, at approximately 7000 yuan / kg, which was more than 10 times that of the materials used in the examples.

[0083] Experiment 3: Slurry stability test

[0084] Test method: The slurry was left to stand in air (temperature 25℃, humidity 50%) for one month. Before use, it was stirred at 500 rpm for 30 minutes, and the conductivity analysis was repeated. Table 2 shows the results of the slurry stability analysis. The slurry state was obtained by visual observation. The conductivity was calculated by averaging the sheet resistance of 10 printed patterns, and the conductivity was calculated according to the formula: conductivity = 1 / (sheet resistance × film thickness). The conductivity value was calculated using the following formula:

[0085] Electrical conductivity decay = (electrical conductivity (freshly prepared slurry) - electrical conductivity (after standing for one month)) / electrical conductivity (freshly prepared slurry);

[0086] The conductivity (for freshly prepared slurry) is shown in Table 1.

[0087] Table 2 Results of slurry stability test in air

[0088]

[0089] Note: Due to the inherent difference of approximately 10% in conductivity between different printed patterns, conductivity attenuation values ​​within the range of ±10% are considered to be without attenuation.

[0090] As shown in Table 2, no discoloration or conductivity decrease occurred in the slurry in Examples 1, 2, and Comparative Example 7. This is because the phthalocyanine metal forms a dense protective layer on the surface of the cysteine ​​self-assembled molecular layer, effectively compensating for the insufficient density of the cysteine ​​film. At the same time, the outer surface of the phthalocyanine metal is mainly composed of aromatic rings, which has strong hydrophobicity, further isolating the contact between moisture in the air and the copper powder surface, thereby preventing the oxidation of copper particles.

[0091] Comparative Example 1 turned dark green after being left to stand in air for a month. This is a typical phenomenon of copper paste oxidation in a humid environment (the main components are a mixture of black copper oxide and green basic copper carbonate), indicating that the copper powder in the paste has been severely oxidized. The conductivity of its printed pattern decreased by more than 70%, further confirming the severe degree of oxidation.

[0092] Comparative Example 2 did not show any conductivity decay because the long-chain thiol molecules formed a dense and hydrophobic protective layer on the copper powder surface, effectively preventing copper powder oxidation. However, due to the low intrinsic conductivity of this protective layer, this system is still not suitable for low-temperature sintering processes.

[0093] The appearance and conductivity of the slurry in Comparative Example 3 did not change significantly, indicating that short-chain thiols can effectively protect the surface of copper powder at room temperature, and at the same time, their hydrophobic properties can block moisture and prevent the formation of basic copper carbonate.

[0094] The slurry in Comparative Example 4 was dark in color and showed pale green spots, indicating the formation of copper oxide and basic copper carbonate. This is because the amino groups in cysteine ​​molecules have a certain degree of hydrophilicity, and their water-repellent ability is not as good as that of thiol molecules; in addition, the intermolecular interactions are weak, and the protective layer is less dense, thus resulting in a significant decrease in conductivity.

[0095] In Comparative Example 5, the conductivity decreased significantly due to incomplete coverage by the cysteine ​​protective layer and the presence of numerous exposed copper points on the surface.

[0096] In Comparative Example 6, the phthalocyanine metal failed to form a dense "molecular umbrella" structure, leaving the underlying cysteine ​​layer directly exposed to the environment. Cysteine ​​itself has limited protective capabilities for the copper surface (refer to Comparative Example 4), thus reducing conductivity. However, compared to Comparative Example 4, Comparative Example 6 showed a smaller attenuation, and the slurry did not exhibit significant discoloration, indicating that even with incomplete coverage, the "molecular umbrella" structure can still provide some antioxidant protection.

[0097] Experiment 3: Rapid Aging Test of Printed Materials

[0098] Printing was carried out immediately after the paste preparation was completed. The structure was changed to a 1cm × 1cm square, and the substrate was coated paper. After printing, it was dried at 140℃ for 30 minutes and pressed with a 160MPa roller. The graphic structure was placed in a constant temperature and humidity chamber and placed at 85℃ and 85% humidity for 72 hours to rapidly age the printed material, and the conductivity test was repeated. The conductivity was obtained by averaging the sheet resistance of 10 printed graphics and calculating it according to the formula: conductivity = 1 / (sheet resistance × film thickness). The conductivity attenuation was calculated using the following formula:

[0099] Conductivity decay = (Conductivity (before rapid aging) - Conductivity (after rapid aging)) / Conductivity (before rapid aging).

[0100] Table 3 Results of Rapid Aging Test for Printed Materials

[0101]

[0102] Note: Due to the inherent difference of approximately 10% in conductivity between different printed patterns, conductivity attenuation values ​​within the range of ±10% are considered to be without attenuation.

[0103] As shown in Table 3, after treatment at 85℃ and 85% relative humidity for 72 hours (double 85 test), the sheet resistance of Comparative Example 1 exceeded the upper limit of the equipment range, and the sample was almost in an insulating state (conductivity decayed by more than 99%), with a distinct blackish-purple surface. This phenomenon is attributed to the significant increase in the atomic activity of copper surface in the high temperature and high humidity environment, which reacts with oxygen and moisture in the air to generate products such as copper oxide, cuprous oxide, and basic copper carbonate, resulting in the printed pattern essentially losing its conductivity.

[0104] Comparative Example 2 did not show any conductivity decay, possibly because its long-chain alkyl molecules remained intact and undegraded after drying, effectively blocking moisture and air, thus protecting the printed pattern from oxidation under double 85 conditions. However, due to its intrinsic insulating properties, the conductivity of this system remained at a relatively low level.

[0105] Comparative Example 3 showed a certain degree of decrease in conductivity. This may be because the short-chain thiols used in it underwent partial degradation during the drying process, resulting in some copper particles failing to form effective fusion and only maintaining physical contact. These exposed copper points oxidized in the double 85 environment, causing a decrease in overall conductivity. Similarly, Comparative Example 4 also showed a significant decrease in conductivity due to the degradation of the cysteine ​​self-assembled molecular layer during drying, exposing the copper surface, which then oxidized under harsh conditions.

[0106] In contrast, Examples 1-4 showed only a minor decrease in conductivity (approximately 30%) after the double 85 treatment. Although the cysteine ​​self-assembled molecular layer degraded during drying, the surface metal phthalocyanine molecular layer effectively blocked water and oxygen, preventing copper from being oxidized in high temperature and high humidity environments. Figure 5 XPS spectra of the Cu2p orbitals after double 85 processing are shown: the satellite peaks next to the main peak are related to the oxidation state of copper, and the weaker satellite peak intensity indicates that only a small amount of copper powder is oxidized (hence the slight decrease in conductivity), and most of the region still exists in the form of elemental copper. The position of the Cu2p3 / 2 peak (approximately 932 eV) is consistent with that of metallic copper, further confirming this conclusion (the peak is located at approximately 937 eV in copper oxide). Figure 6 The XPS spectrum of the Zn2p orbital in the processed image of Example 1, Zn 2+ The characteristic peaks of Zn2p1 / 2 and Zn2p3 / 2 indicate that zinc phthalocyanine still plays a protective role on the surface of copper particles. Because the metal phthalocyanine layer is relatively thin (approximately 0.4 nm), see... Figure 4 Furthermore, its metal center and conjugated structure can provide migration channels for charge carriers, thus its obstruction to electron transport is much less than that of the alkyl chain insulating layer, providing protection while maintaining high conductivity.

[0107] Comparative Example 7 exhibits a smaller decrease in conductivity because the multilayered metal phthalocyanine molecules further enhance the water and oxygen barrier properties of the copper surface. However, due to the significant electron transport losses in the multilayered structure, its actual conductivity is still lower than that of Examples 1 and 2, differing by approximately half. Comparative Examples 5 and 6, due to incomplete coverage by the cysteine ​​and zinc phthalocyanine molecular layers and the presence of localized bare copper spots, underwent significant oxidation during the double 85 treatment, resulting in a substantial decrease in conductivity.

[0108] Experiment 4: Copper Paste RFID Performance and Lifetime Tag Test

[0109] The label is designed using CST software based on the conductivity and thickness of the printing paste and impedance matching with the chip, as detailed below. Figure 1 As shown. After the tag of Example 1 was prepared according to step 5, its reading distance was tested using the Voyantic Tagformance UHF test system: the distance between the tag and the transmitting antenna was 0.3 meters, the test environment was 25°C and 50% relative humidity, and the transmitting power was set to 1 watt. The tag response power was measured and converted into reading distance using the built-in algorithm.

[0110] Since the tag's reading distance is highly dependent on the impedance matching between the antenna and the chip, directly comparing the reading distances of different embodiments or comparative examples lacks practical reference value. In initial testing, the tag prepared in this application achieved a reading distance of over 10 meters at a frequency of 915 MHz, comparable to the performance of similar-sized pure silver printed tags and aluminum foil etched tags on the market. After being placed at room temperature (25°C, 50% humidity) for one month, the tag's reading distance showed no significant attenuation, demonstrating good air stability. Furthermore, after undergoing the "double 85" accelerated aging test (85°C, 85% humidity, for 72 hours, approximately equivalent to 2.5-3 years of aging under natural conditions, covering the actual working life of most tags), the tag's actual reading distance was 9.3 meters due to a decrease of approximately 30% in the slurry conductivity. Although the performance is slightly reduced, it still meets the needs of practical RFID applications (see...). Figure 7 ).

[0111] This specific embodiment is merely an explanation of this application and is not intended to limit it. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they fall within the scope of the claims of this application.

Claims

1. A low temperature oxidation resistant conductive copper paste, characterized in that, Contains modified copper powder, wherein the modified copper powder comprises: Copper powder; A cysteine ​​self-assembled molecular layer is formed by coupling the thiol groups of cysteine ​​to the surface of the copper powder; The metal phthalocyanine complex molecular layer is connected to the surface of the cysteine ​​self-assembled molecular layer by coordination between the amino group of the cysteine ​​and the central metal ion of the metal phthalocyanine complex. The conductive copper paste is sintered at 140–170°C.

2. The low temperature oxidation resistant conductive copper paste of claim 1, wherein, The metal phthalocyanine complex is zinc phthalocyanine.

3. The low temperature oxidation resistant conductive copper paste of claim 1, wherein, The thickness of the metal phthalocyanine complex molecular layer is 0.35–0.45 nanometers.

4. The low temperature oxidation resistant conductive copper paste of claim 1, wherein, The thickness of the cysteine ​​self-assembled molecular layer is 0.3–0.4 nanometers.

5. The low temperature oxidation resistant conductive copper paste of claim 1, wherein, The method for preparing the modified copper powder includes: Copper powder with its surface oxide layer removed was mixed with a cysteine ​​solution with a concentration of 0.1-0.3 wt%, and a self-assembly reaction was carried out under an inert atmosphere for no less than 3 hours to form a cysteine ​​self-assembled molecular layer on the surface of the copper powder, thus obtaining cysteine-modified copper powder. Cysteine-modified copper powder was mixed with a metal phthalocyanine complex solution with a concentration of 0.7–2.8 wt% and subjected to a coordination reaction. After 30–60 min, a metal phthalocyanine complex molecular layer was obtained, thus preparing modified copper powder.

6. The low temperature oxidation resistant conductive copper paste of claim 1, wherein, The conductive copper paste comprises 60-70 wt% modified copper powder, 29-39 wt% resin solution, 0.7-0.9 wt% curing agent, and 0.1-0.3 wt% coupling agent.

7. The low temperature, oxidation resistant, conductive copper paste of claim 6, wherein, The resin solution comprises phenoxy resin and organic solvent in a mass ratio of 1:2 to 3.

8. The low temperature oxidation resistant conductive copper paste of claim 6, wherein, The curing agent is a formic anhydride curing agent and a blocked isocyanate curing agent with a mass ratio of 1.5 to 2.5:

1. The unblocking temperature of the blocked isocyanate curing agent is lower than the sintering temperature of the conductive copper paste.

9. An RFID tag, characterized by It includes a substrate, an antenna, and a chip, wherein the antenna is obtained by printing with the low-temperature oxidation-resistant conductive copper paste as described in any one of claims 1 to 8.

Images