Nano-metal ink for flexible sensor and preparation method of nano-metal ink

Abstract

The invention relates to the technical field of nano-metal, and discloses nano-metal ink for a flexible sensor and a preparation method of the nano-metal ink, the ink comprises 60-75 wt% of metal nanoparticles, 8-12 wt% of a thermoplastic elastomer, 15-25 wt% of a high-boiling-point organic solvent with the boiling point of 150-250 DEG C, 2-4 wt% of a dispersing agent and 0.5-2 wt% of a low-temperature sintering accelerant; the method comprises the following steps: S1, synchronously injecting a solution; step S2, performing centrifugal treatment; s3, adding a dispersing agent, a thermoplastic elastomer and a low-temperature sintering accelerant; s4, supplementing the residual high-boiling-point organic solvent to adjust the viscosity of the system to a target range; according to the invention, the low-temperature sintering accelerant opens an oxide layer in situ on the particle surface to form a conductive skeleton which is only locally interconnected, so that the grain boundary brittleness caused by traditional high-temperature sintering is fundamentally weakened, the thermoplastic elastomer is subjected to microphase separation at the grain boundary, and the buffer island absorbs and disperses stress through elastic deformation.

Description

Technical Field

[0001] This invention relates to the field of nanomaterials technology, specifically to a nanomaterial ink for flexible sensors and its preparation method. Background Technology

[0002] With the rapid development of flexible electronics technology, flexible sensors, due to their excellent mechanical flexibility, wearability, and stretchability, are widely used in wearable devices, medical and health monitoring, human-computer interaction, and other fields. Nanoscale metal inks, with their superior conductivity, low-temperature sintering performance, and excellent patterning characteristics, have become one of the important materials for flexible electronics manufacturing. Through inkjet printing, screen printing, and other methods, nanoscale metal inks can directly form conductive patterns on flexible substrates, enabling low-cost, large-area, and patternable flexible sensor manufacturing.

[0003] A search revealed Chinese patent CN111189476B, which discloses a flexible sensor and its fabrication method. The sensor comprises a substrate layer, an electrode layer, and a sensing layer, with the electrode layer located between the substrate layer and the sensing layer. The substrate layer is made of a flexible substrate material. The electrode layer is formed by depositing metal nano-conductive ink onto the surface of the substrate layer using inkjet printing. The sensing layer is formed by applying a sensitive material to the surface of the electrode layer using dispensing or scraping. The fabrication method includes, after fabricating the initial flexible sensor structure consisting of the substrate layer, electrode layer, and sensing layer, curing and sintering the initial flexible sensor structure using a gradient temperature curing sintering method to obtain the flexible sensor.

[0004] A search revealed Chinese patent CN107513310A, which discloses a method for preparing magnetic nano-ink and magnetic flexible circuits or devices. The magnetic nano-ink comprises magnetic nanoparticles, the surface of which is coated with a silver coating layer; the thickness of the silver coating layer is 1-50 nm. By employing the technical solution of this invention, and by coating the surface of the magnetic nanoparticles with a silver layer and controlling the coating thickness to 1-50 nm, an effective reduction in sintering temperature can be achieved. Ink prepared using this technology can achieve low-temperature sintering at 20-100℃ after printing, printing on, or manually coating on a flexible substrate.

[0005] The above-mentioned technical solutions all follow the traditional approach of physical contact between nanoparticles leading to necking driven by a high-temperature external field, and ultimately forming a continuous conductive path in terms of electrode formation mechanism. Although this approach can obtain a conductive layer, high conductivity requires sufficient particle fusion and few grain boundaries, while high flexibility requires the film layer to slip, deflect cracks, or self-heal during mechanical deformation. As a result, the silver layer after sintering exhibits a dense polycrystalline and brittle ceramic structure. When the substrate is subjected to tensile strain, microcracks first appear at the grain boundaries. The cracks rapidly propagate along the grain boundaries and form through gaps, causing the conductive path to break instantly. At the same time, due to the large difference in elastic modulus between the film layer and the flexible substrate, repeated bending at the interface will generate periodic shear stress, further increasing the brittleness of the crack tips. This invention designs a nano-metal ink for flexible sensors and its preparation method to solve the above problems. Summary of the Invention

[0006] The purpose of this invention is to provide a nano-metal ink for flexible sensors and its preparation method, which solves the problem of easy breakage of conductive pathways in the prior art.

[0007] To address the aforementioned technical problems, this invention provides the following technical solution: a nano-metallic ink for flexible sensors, wherein the ink, based on a total mass of 100 wt%, comprises: The composition comprises: 60wt%-75wt% of metal nanoparticles, wherein the metal nanoparticles are monodisperse silver particles with an average primary particle size of 20nm-50nm and a particle size distribution index (PDI) < 0.2; 8wt%-12wt% of thermoplastic elastomer, wherein the thermoplastic elastomer is selected from at least one of thermoplastic polyurethane, polyvinylpyrrolidone, or ethyl cellulose with a number average molecular weight of 10000g / mol-100000g / mol and a glass transition temperature (Tg) < -20℃; 15wt%-25wt% of a high-boiling-point organic solvent with a boiling point of 150℃-250℃; 2wt%-4wt% of a dispersant; and 0.5wt%-2wt% of a low-temperature sintering accelerator, wherein the low-temperature sintering accelerator is selected from benzoic acid, which can react with the oxide layer on the metal surface at a temperature below 100℃ to generate a volatile organometallic salt, thereby initiating the formation of necks between particles. Wherein, the viscosity η of the ink at 25°C satisfies: Inkjet printing type: 8mPa·s≤η≤25mPa·s, and surface tension γ=28mN / m-33mN / m; Screen printing type: 5000mPa·s≤η≤15000mPa·s; Furthermore, the electrode is obtained by heat treatment of the ink in two stages: 90℃≤T≤120℃ and t=20min-40min.

[0008] Preferably, the thermoplastic elastomer undergoes microphase separation during the second stage of heat treatment, forming flexible "buffer islands" distributed along grain boundaries, the volume fraction φ of which satisfies: ; Wherein, V_elastomer is the volume of the thermoplastic elastomer in the dry film, V_metal is the volume of the metal nanoparticles in the dry film, and the volume fraction φ ranges from 0.08 to 0.12 to inhibit the rapid propagation of cracks along grain boundaries.

[0009] Preferably, the low-temperature sintering accelerator reacts with the surface oxide layer of the metal nanoparticles to generate a volatile organometallic salt, ensuring that the neck initiation temperature T_neck between particles satisfies the following: ; T_neck is the measured temperature at which the neck begins to form, and T_m is the bulk melting point of the metal nanoparticles, both in K.

[0010] Preferably, the high-boiling-point organic solvent is a combination of diethylene glycol butyl ether, terpineol and dodecyl alcohol, in a mass fraction ratio of 1:1:1.2, used to form a solvent vapor protective layer during the drying process to inhibit the thermal oxidative degradation of the thermoplastic elastomer. The dispersant is a combination of BYK-2155, polyethyleneimine, and castor oil derivatives in a mass fraction ratio of 1:3:3.5, with an amine value ranging from 30 mg·KOH / g to 80 mg·KOH / g, to ensure that the particles remain monodisperse even under high solid content.

[0011] Preferably, the metal nanoparticles are silver-coated copper core-shell structured nanoparticles, wherein the copper core diameter is 15-40 nm, the silver shell thickness is 5-15 nm, and the mass ratio of silver to copper is 8.5:1.

[0012] As can be seen from the above technical solution, the ink components work synergistically through multiple mechanisms: 60wt%-75wt% of monodisperse silver nanoparticles (PDI < 0.2) form a conductive framework, and their narrow particle size distribution (20-50nm) ensures sintering uniformity; 8wt%-12wt% of thermoplastic elastomer (Tg < -20℃) undergoes microphase separation during the second stage of heat treatment at 90℃-120℃, forming flexible buffer islands distributed along the grain boundaries, with the volume fraction φ = V_elastomer / (V_elastomer+V_metal) precisely controlled within the range of 0.08-0.12 (see [reference]). Figure 2The process involves several key steps: 1) Elastic deformation inhibits crack propagation; 2) 0.5wt%-2wt% benzoic acid-based low-temperature sintering accelerators react with the oxide layer on the particle surface to generate volatile organometallic salts, significantly reducing the neck formation temperature to T_neck≤0.28·T_m (T_m is the metal melting point), allowing sintering to be completed at ≤120℃; 3) a high-boiling-point solvent complex (diethylene glycol butyl ether / terpineol / dodecanool = 1:1:1.2) forms a vapor protective layer to inhibit thermal degradation; and 4) a dispersant complex (BYK-2155 / polyethyleneimine / castor oil derivative = 1:3:3.5) maintains monodispersity at high solid content through amine value regulation (30-80 mg·KOH / g), ultimately achieving an electrical conductivity σ≥1.0×10⁻⁶. 4 Flexible electrode with S / m.

[0013] A method for preparing nano-metallic ink for flexible sensors, the method comprising the following steps: Step S1: Under inert gas protection, the metal precursor solution and reducing agent solution are simultaneously injected into the microchannel reactor at a flow rate of 0.5 mL / min-2 mL / min, the reaction temperature is controlled at 60℃-80℃, and the residence time is 2-4 minutes to obtain a metal nanoparticle stock solution with uniform particle size distribution. Step S2: Centrifuge the obtained stock solution, wash it at least three times with a mixed solvent of ethanol and acetone, and then redisperse the washed metal nanoparticles in a high-boiling-point organic solvent to obtain a concentrated dispersion with a solid content of 30wt%-40wt%. Step S3: Under the temperature conditions of 40℃-50℃, add the dispersant, thermoplastic elastomer and low temperature sintering accelerator to the concentrated dispersion in sequence at a stirring speed of 200-400 rpm, and continue stirring for 2-3 hours. Step S4: Add the remaining high-boiling-point organic solvent to adjust the viscosity of the system to the target range, and then perform planetary degassing treatment for 30 minutes under a vacuum degree not higher than -0.08MPa and a rotation speed of 1000-2000rpm. Finally, filter through a 1-5μm PTFE filter to obtain a uniform and stable nano-metal ink.

[0014] Preferably, in step S1, the microchannel reactor has a channel diameter of 0.5-2.0 mm, an aspect ratio L / D ≥ 100, and a Reynolds number Re that satisfies the range of 10 < Re < 100, in order to ensure that the reactants are in a laminar flow state.

[0015] Preferably, in step S2, the centrifugal acceleration a satisfies: ; Where N is the centrifugal speed (rpm) and R is the centrifugal radius (cm). The centrifugal acceleration is controlled within the range of 2000-5000g to ensure that the nanoparticles settle sufficiently without hard agglomeration.

[0016] Preferably, after the stirring process in step S3 is completed, the resulting mixture is aged at 25±3℃ for 12-48 hours; the aging process is carried out under inert gas or sealed conditions to prevent solvent evaporation and material oxidation; the viscosity change rate Δη of the ink after aging satisfies: ; η_initial represents the initial viscosity before aging, ensuring the ink's storage stability and consistent printing performance.

[0017] It also includes step S5, quality inspection, which tests the key performance indicators of the filtered ink, including: Step S501: Use a laser particle size analyzer to detect the secondary particle size D50 of the metal nanoparticles, and require D50≤100nm; Step S502, using a rotational viscometer at 25°C for 100 seconds -1 Viscosity detection at shear rates; Step S503: Use a conductivity meter to measure the ink conductivity σ, and require σ ≥ 1.0 × 10⁻⁶. 4 S / m.

[0018] As can be seen from the above technical solution, the ink stability is achieved through continuous flow reaction and precise parameter control in the preparation process: A microchannel reactor (channel diameter 0.5-2.0 mm, L / D ≥ 100) is used to simultaneously inject the precursor and reducing agent under laminar flow conditions (10 < Re < 100), and the reaction is carried out at 60℃-80℃ for 2-4 minutes to obtain uniform nanoparticles (see...). Figure 3 , Figure 4 During the centrifugal phase, the acceleration formula a = 4π is used. 2 N 2 R / 3600 controls the range of 2000-5000g to achieve sufficient sedimentation without hard agglomeration; after adding the components by stirring at 40℃-50℃, it is matured at 25±3℃ for 12-48 hours to ensure that the viscosity change rate |Δη| / η_initial≤5%, ensuring storage stability; planetary degassing (≤-0.08MPa, 1000-2000rpm) and 1-5μmPTFE filtration eliminate bubbles and agglomerates, and the final ink secondary particle size D50≤100nm is verified by conductivity.

[0019] Compared with the prior art, the beneficial effects achieved by the present invention are: 1. In this invention, the low-temperature sintering accelerator opens the oxide layer in situ on the particle surface, forming a "point-neck" conductive framework with only local interconnections; the metal phase can penetrate without sufficient melting and co-existence, retaining fine grains and fundamentally weakening the grain boundary brittleness caused by traditional high-temperature sintering. Thermoplastic elastomers undergo microphase separation at grain boundaries, generating continuously wrapped flexible "buffer islands"; when the film is under tension, the buffer islands absorb and disperse stress through elastic deformation, forcing crack tips to deflect and branch, blocking the rapid penetration path along the grain boundaries.

[0020] 2. In this invention, the elastomer simultaneously reduces the overall modulus of the electrode, forming a gradually changing interface between the elastomer and the flexible substrate. During repeated bending, the interfacial shear stress is continuously released by the gradient layer, avoiding film / substrate peeling or brittle fracture caused by stress concentration. The silver-coated copper core-shell particles constitute a rigid conductive framework, ensuring electronic pathways. The thermoplastic elastic mesh surrounding the framework provides space for slippage and self-healing. The rigid framework and elastic mesh complement each other, achieving both "stable conductive pathways" and "reversible mechanical deformation" simultaneously.

[0021] 3. In this invention, the high-boiling-point mixed solvent forms a vapor curtain during the drying stage, which isolates oxygen molecules from diffusing into the film layer, inhibits high-temperature oxidation of the elastomer, and maintains the flexibility and integrity of the buffer island. Low-temperature heat treatment can be achieved without an additional inert atmosphere. The compound dispersant maintains monodispersity of particles even with high metal solid content through a dual mechanism of steric hindrance and electrostatic repulsion. The ink is uniform and dense during the printing film formation process and has no pore defects after sintering, so that the flexible buffer island and the conductive skeleton are continuously distributed, taking into account both pattern accuracy and electromechanical reliability. Attached Figure Description

[0022] Figure 1 This is a content structure diagram of the nano-metal ink of the present invention; Figure 2 This is a schematic diagram of the composition of the nano-metal ink and the microstructure of the electrode after sintering according to the present invention. Figure 3 This is a flowchart illustrating the preparation process of the nano-metal ink of the present invention. Figure 4 This is a schematic diagram illustrating the working principle of the microchannel reactor of the present invention. Detailed Implementation

[0023] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0024] Example 1; Please see Figures 1-4A nano-metal ink for flexible sensors, comprising, by a total mass of 100wt%,: 60wt%-75wt% of metal nanoparticles, wherein the metal nanoparticles are monodisperse silver particles with an average primary particle size of 20nm-50nm and a particle size distribution index (PDI) < 0.2; 8wt%-12wt% of thermoplastic elastomer, wherein the thermoplastic elastomer is selected from at least one of thermoplastic polyurethane, polyvinylpyrrolidone, or ethyl cellulose with a number average molecular weight of 10000g / mol-100000g / mol and a glass transition temperature (Tg) < -20℃; 15wt%-25wt% of a high-boiling-point organic solvent with a boiling point of 150℃-250℃; 2wt%-4wt% of a dispersant; and 0.5wt%-2wt% of a low-temperature sintering accelerator, wherein the low-temperature sintering accelerator is selected from benzoic acid, which can react with the oxide layer on the metal surface at a temperature below 100℃ to generate volatile organometallic salts, thereby initiating the formation of necks between particles; The viscosity η of the ink at 25℃ satisfies: Inkjet printing type: 8mPa·s≤η≤25mPa·s, and surface tension γ=28mN / m-33mN / m; Screen printing type: 5000mPa·s≤η≤15000mPa·s; Furthermore, the electrode is obtained by heat treatment of ink in two stages: 90℃≤T≤120℃ and time t=20min-40min.

[0025] In the second stage of heat treatment, the thermoplastic elastomer undergoes microphase separation, forming flexible "buffer islands" distributed along the grain boundaries, whose volume fraction φ satisfies: ; Wherein, V_elastomer is the volume of thermoplastic elastomer in the dry film, V_metal is the volume of metal nanoparticles in the dry film, and the volume fraction φ ranges from 0.08 to 0.12, in order to inhibit the rapid propagation of cracks along grain boundaries.

[0026] The low-temperature sintering accelerator reacts with the surface oxide layer of the metal nanoparticles to generate volatile organometallic salts, ensuring that the neck initiation temperature T_neck between particles satisfies the following: ; T_neck is the measured temperature at which the neck begins to form, and T_m is the bulk melting point of the metal nanoparticles; both are in K.

[0027] The high-boiling-point organic solvent is a combination of diethylene glycol butyl ether, terpineol and dodecyl alcohol in a mass fraction ratio of 1:1:1.2. It is used to form a solvent vapor protective layer during the drying process to inhibit the thermal oxidative degradation of thermoplastic elastomers. The dispersant is a combination of BYK-2155, polyethyleneimine, and castor oil derivatives in a mass fraction ratio of 1:3:3.5, with an amine value ranging from 30 mg·KOH / g to 80 mg·KOH / g, to ensure that the particles remain monodisperse even under high solid content.

[0028] The metal nanoparticles are silver-coated copper core-shell nanoparticles, with a copper core diameter of 15-40 nm, a silver shell thickness of 5-15 nm, and a silver to copper mass ratio of 8.5:1.

[0029] The working principle of this invention is as follows: The metal nanoparticles (60wt%-75wt%) in the ink are monodisperse silver particles with an average primary particle size of 20nm-50nm and a PDI < 0.2. Their narrow particle size distribution ensures that the particles can achieve uniform neck formation and dense arrangement during sintering, providing a basis for the continuity of the conductive path. The thermoplastic elastomer (8wt%-12wt%) is a material with a number average molecular weight of 10000g / mol-100000g / mol and a glass transition temperature Tg < -20℃ (such as thermoplastic polyurethane, polyvinylpyrrolidone, or ethyl cellulose). During the second stage of heat treatment (90℃–120℃), microphase separation occurs, forming flexible "buffer islands" distributed along the metal grain boundaries. The volume fraction φ of the buffer island is precisely controlled within the range of 0.08–0.12 using the formula φ=V_elastomer / (V_elastomer+V_metal). During mechanical deformation, it absorbs stress through elastic deformation, effectively inhibiting the rapid propagation of cracks along grain boundaries, thereby significantly improving the flexibility and durability of the electrode.

[0030] The high-boiling-point organic solvent (15wt%-25wt%, boiling point 150℃–250℃) is a compound of diethylene glycol butyl ether, terpineol, and dodecyl alcohol in a mass ratio of 1:1:1.2. During the drying process, it forms a stable protective layer of solvent vapor, slowing down the solvent evaporation rate and inhibiting the degradation of thermoplastic elastomers due to high-temperature oxidation, while also preventing defects in the film layer caused by rapid drying. The dispersant (2wt%-4wt%) is a compound of BYK-2155, polyethyleneimine, and castor oil derivatives in a mass ratio of 1:3:3.5. Its amine value range (30–80 mg·KOH / g) ensures that the nanoparticles remain monodisperse under high solids content through a dual mechanism of steric hindrance and electrostatic repulsion, preventing agglomeration that leads to a decrease in conductivity. Low-temperature sintering accelerators (0.5wt%-2wt%, such as benzoic acid) react with the oxide layer on the metal surface at temperatures below 100℃ to generate volatile organometallic salts, significantly reducing the initiation temperature T_neck of interparticle neck formation, ensuring that T_neck≤0.28·T_m (T_m is the melting point of the metal bulk), thereby achieving particle sintering and conductive network formation under low-temperature conditions of 90℃–120℃.

[0031] The rheological properties of the ink (inkjet printing type: viscosity 8–25 mPa·s, surface tension 28–33 mN / m; screen printing type: viscosity 5000–15000 mPa·s) ensure its applicability and pattern accuracy under different printing processes. Finally, a two-stage heat treatment (90℃–120℃, 20–40 min) yields an ink with high electrical conductivity (conductivity ≥1.0 × 10⁻⁶). 4 (S / m) and an electrode structure with excellent flexibility.

[0032] Example 2; Please see Figures 1-4 A method for preparing nano-metallic ink for flexible sensors, the method comprising the following steps: Step S1: Under inert gas protection, the metal precursor solution and reducing agent solution are simultaneously injected into the microchannel reactor at a flow rate of 0.5 mL / min-2 mL / min, the reaction temperature is controlled at 60℃-80℃, and the residence time is 2-4 minutes to obtain a metal nanoparticle stock solution with uniform particle size distribution. Step S2: Centrifuge the obtained stock solution, wash it at least three times with a mixed solvent of ethanol and acetone, and then redisperse the washed metal nanoparticles in a high-boiling-point organic solvent to obtain a concentrated dispersion with a solid content of 30wt%-40wt%. Step S3: Under the temperature conditions of 40℃-50℃, add the dispersant, thermoplastic elastomer and low temperature sintering accelerator to the concentrated dispersion in sequence at a stirring speed of 200-400 rpm, and continue stirring for 2-3 hours. Step S4: Add the remaining high-boiling-point organic solvent to adjust the viscosity of the system to the target range, and then perform planetary degassing treatment for 30 minutes under a vacuum degree not higher than -0.08MPa and a rotation speed of 1000-2000rpm. Finally, filter through a 1-5μm PTFE filter to obtain a uniform and stable nano-metal ink.

[0033] In step S1, the microchannel reactor has a channel diameter of 0.5-2.0 mm, an aspect ratio L / D ≥ 100, and a Reynolds number Re that satisfies the range of 10 < Re < 100, in order to ensure that the reactants are in a laminar flow state.

[0034] In step S2, the centrifugal acceleration a satisfies: ; Where N is the centrifugal speed (rpm) and R is the centrifugal radius (cm). The centrifugal acceleration is controlled within the range of 2000-5000g to ensure that the nanoparticles settle sufficiently without hard agglomeration.

[0035] After the stirring process in step S3 is completed, the resulting mixture is aged at 25±3℃ for 12-48 hours. The aging process is carried out under inert gas or sealed conditions to prevent solvent evaporation and material oxidation. The viscosity change rate Δη of the ink after aging satisfies: ; η_initial represents the initial viscosity before aging, ensuring the ink's storage stability and consistent printing performance.

[0036] It also includes step S5, quality inspection, which tests the key performance indicators of the filtered ink, including: Step S501: Use a laser particle size analyzer to detect the secondary particle size D50 of the metal nanoparticles, and require D50≤100nm; Step S502, using a rotational viscometer at 25°C for 100 seconds -1 Viscosity detection at shear rates; Step S503: Use a conductivity meter to measure the ink conductivity σ, and require σ ≥ 1.0 × 10⁻⁶. 4 S / m.

[0037] The working principle of this invention is as follows: In step S1, the metal precursor solution and the reducing agent solution are simultaneously injected into the microchannel reactor at a flow rate of 0.5–2 mL / min under inert gas protection. The reaction temperature is controlled at 60℃–80℃, and the residence time is 2–4 minutes. The channel diameter (0.5–2.0 mm) and aspect ratio (L / D ≥ 100) of the microchannel reactor ensure that the reactants are in a laminar flow state (Reynolds number 10 < Re < 100). By enhancing mass and heat transfer, the nucleation and growth of metal nanoparticles are simultaneously controlled, resulting in a stock solution with uniform particle size distribution (PDI < 0.2).

[0038] In step S2, the original solution is centrifuged to achieve particle separation. The centrifugal acceleration 'a' is calculated using the formula a = (4π) / (4π) 2 N 2 The R / 3600 ratio is precisely controlled within the range of 2000–5000 g (N is the centrifugal speed, R is the centrifugal radius). This parameter range ensures sufficient sedimentation of nanoparticles while avoiding hard agglomeration caused by excessive centrifugal force. The washing process uses a mixed solvent of ethanol and acetone to effectively remove residual reaction reagents and byproducts. Subsequently, the particles are redispersed in a portion of a high-boiling-point organic solvent to obtain a concentrated dispersion with a solid content of 30wt%–40wt%, providing a basis for the subsequent addition of components.

[0039] In step S3, the dispersant, thermoplastic elastomer, and low-temperature sintering accelerator are added sequentially at a temperature of 40℃–50℃ and a stirring speed of 200–400 rpm. Stirring is continued for 2–3 hours to allow each component to fully adsorb onto the particle surface and form a stable interfacial layer. Subsequently, a curing treatment is performed (25±3℃, 12–48 hours, inert gas or sealed environment). This achieves system equilibrium through molecular chain relaxation and reduction of interfacial energy. After curing, the viscosity change rate |Δη| / η_initial ≤ 5%, ensuring consistent ink storage stability and printing performance.

[0040] Step S4 involves adjusting the viscosity to the target range by adding the remaining high-boiling-point solvent, and then performing planetary degassing treatment for 30 minutes under a vacuum of ≤-0.08MPa and a rotation speed of 1000–2000rpm to effectively remove entrained air bubbles and prevent line breaks or defects during printing. Finally, the ink is filtered through a 1–5μm PTFE filter to remove potential agglomerates or impurities, resulting in a uniform and stable nano-metallic ink. Quality inspection step (S5) involves laser particle size analysis (D50≤100nm) and rotational viscometer (25℃, 100s). -1 Shear rate) and conductivity test (σ≥1.0×10) 4 S / m) ensures that the product meets the design specifications.

[0041] Example 3; Please see Figures 1-4This paper provides a specific embodiment of a nano-metallic ink for flexible sensors, with a total ink mass of 100 wt%. The ink composition includes: 70 wt% metal nanoparticles, which are monodisperse silver-coated copper core-shell structured particles with an average primary particle size of 35 nm and a particle size distribution index (PDI) of 0.15, wherein the copper core diameter is 25 nm, the silver shell thickness is 10 nm, and the mass ratio of silver to copper is 8.5:1; 10 wt% thermoplastic elastomer, selected as thermoplastic polyurethane with a number average molecular weight of 50,000 g / mol and a glass transition temperature (Tg) of -25 °C, which forms flexible buffer islands distributed along grain boundaries after heat treatment, with a volume fraction φ calculated to be 0.10 to inhibit crack propagation; and 18 wt% high-boiling-point organic solvent, a compound of diethylene glycol butyl ether, terpineol, and dodecyl alcohol in a mass fraction ratio of 1:1:1.2, with a boiling point range of 180 °C to 220 °C, used to form a solvent vapor protective layer during drying to inhibit degradation. A 3wt% dispersant, composed of BYK-2155, polyethyleneimine, and castor oil derivatives in a mass ratio of 1:3:3.5, with an amine value of 60 mg·KOH / g, ensures monodispersity under high solids content. A 1.5wt% low-temperature sintering accelerator, benzoic acid, reacts with the metal surface oxide layer at temperatures below 100℃ to form volatile organometallic salts, ensuring the particle neck initiation temperature T_neck satisfies T_neck ≤ 0.28·T_m (T_m is the bulk melting point of silver 961.8℃, calculated to be 269K). The final ink has a viscosity η of 20 mPa·s and a surface tension γ of 30 mN / m at 25℃, suitable for inkjet printing. Electrodes are obtained through a two-stage heat treatment at 90℃ to 120℃ for 30 minutes. The ink's microstructure is as follows. Figure 2 As shown, the flexible buffer islands are evenly distributed at the metal grain boundaries, effectively improving flexibility.

[0042] The preparation method includes the following steps: Step S1, under the protection of inert nitrogen gas, silver nitrate solution and sodium borohydride reducing agent solution are simultaneously injected into a microchannel reactor at a flow rate of 1.5 mL / min. The reactor channel diameter is 1.0 mm, the aspect ratio L / D is 120, and the Reynolds number Re is controlled at 50 to ensure laminar flow. The reaction temperature is maintained at 70℃, and the residence time is 3 minutes to obtain a metal nanoparticle stock solution with uniform particle size distribution. The reaction process is as follows: Figure 4 As shown.

[0043] Step S2: The stock solution is centrifuged at a speed of 10,000 rpm and a centrifugation radius of 10 cm. The centrifugation acceleration a is calculated to be 4,000 g. The solution is washed three times with a mixed solvent of ethanol and acetone at a volume ratio of 1:1. Then it is redispersed in a portion of a high-boiling-point organic solvent to obtain a concentrated dispersion with a solid content of 35 wt%.

[0044] Step S3: At 45℃, the dispersant, thermoplastic elastomer, and low-temperature sintering accelerator are added sequentially to the concentrated dispersion at a stirring speed of 300 rpm, and stirring is continued for 2.5 hours. Afterwards, a curing treatment is performed at 25℃ for 24 hours under inert gas protection to prevent oxidation. The viscosity change rate Δη / η_initial after curing is 3%, ensuring storage stability. Step S4: The remaining high-boiling-point organic solvent is added to adjust the system viscosity to the target 20 mPa·s. Then, a planetary degassing treatment is performed for 30 minutes under a vacuum of -0.09 MPa and a rotation speed of 1500 rpm. Finally, the mixture is filtered through a 3 μm PTFE filter to obtain a uniform and stable nano-metal ink. The preparation process is as follows: Figure 3 As shown.

[0045] Step S35 includes: Step S501, using a laser particle size analyzer to determine the secondary particle size D50 of the metal nanoparticles to be 85 nm; Step S502, using a rotational viscometer at 25°C for 100 s... -1 The viscosity was measured at a shear rate of 20 mPa·s; in step S503, the ink conductivity σ was measured using a conductivity meter and found to be 1.2 × 10⁻⁶. 4 S / m. All parameters meet the requirements, proving that the ink has high conductivity and excellent flexibility, making it suitable for the manufacture of flexible sensor electrodes.

[0046] Working Principle: The ink components work synergistically through multiple mechanisms. 60wt%-75wt% monodisperse silver nanoparticles form a conductive framework, their narrow particle size distribution ensuring sintering uniformity. 8wt%-12wt% thermoplastic elastomer undergoes microphase separation during the second stage of heat treatment at 90℃-120℃, forming flexible buffer islands distributed along grain boundaries. The volume fraction φ = V_elastomer / (V_elastomer+V_metal) is precisely controlled within the range of 0.08-0.12, inhibiting crack propagation through elastic deformation. 0.5wt%-2wt% benzoic acid-based low-temperature sintering promoters react with the oxide layer on the particle surface to generate volatile organometallic salts, significantly reducing the neck formation temperature to T_neck≤0.28·T_m, allowing sintering to be completed at ≤120℃. A high-boiling-point solvent complex forms a vapor protective layer to inhibit thermal degradation. The dispersant complex maintains monodispersity at high solid content through amine value regulation, ultimately achieving an electrical conductivity σ≥1.0×10⁻⁶. 4 Flexible electrode with S / m.

[0047] Precursor and reducing agent were simultaneously injected into a microchannel reactor under laminar flow conditions, and uniform nanoparticles were obtained by reacting at 60℃-80℃ for 2-4 minutes; during the centrifugation stage, the acceleration formula a=4π was used. 2 N 2R / 3600 controls the range of 2000-5000g to achieve sufficient sedimentation without hard agglomeration; after adding the components by stirring at 40℃-50℃, it is matured at 25±3℃ for 12-48 hours to ensure that the viscosity change rate |Δη| / η_initial≤5%, ensuring storage stability; planetary degassing and 1-5μm PTFE filtration eliminate bubbles and agglomerates, and the final ink secondary particle size D50≤100nm is verified by conductivity.

[0048] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A nano-metallic ink for flexible sensors, characterized in that, The ink, based on a total mass of 100 wt%, comprises: a) 60wt%-75wt% of metal nanoparticles, wherein the metal nanoparticles are monodisperse silver particles with an average primary particle size of 20nm-50nm and a particle size distribution index (PDI) of <0.

2. b) 8wt%-12wt% of thermoplastic elastomer, wherein the thermoplastic elastomer is selected from at least one of thermoplastic polyurethane, polyvinylpyrrolidone or ethyl cellulose with a number average molecular weight of 10000g / mol-100000g / mol and a glass transition temperature Tg < -20℃. c) 15wt%-25wt% of high-boiling-point organic solvents with a boiling point of 150℃-250℃; d) 2wt%-4wt% dispersant; e) 0.5wt%-2wt% of a low-temperature sintering accelerator, wherein the low-temperature sintering accelerator is selected from benzoic acid, which can react with the oxide layer on the metal surface at a temperature below 100°C to generate volatile organometallic salts, causing the formation of interparticle necks. Wherein, the viscosity η of the ink at 25°C satisfies: Inkjet printing type: 8mPa·s≤η≤25mPa·s, and surface tension γ=28mN / m-33mN / m; Screen printing type: 5000mPa·s≤η≤15000mPa·s; Furthermore, the electrode is obtained by heat treatment of the ink in two stages: 90℃≤T≤120℃ and t=20min-40min.

2. The nano-metal ink for flexible sensors according to claim 1, characterized in that, The thermoplastic elastomer undergoes microphase separation during the second stage of heat treatment, forming flexible "buffer islands" distributed along grain boundaries, whose volume fraction φ satisfies: ； Wherein, V_elastomer is the volume of the thermoplastic elastomer in the dry film, V_metal is the volume of the metal nanoparticles in the dry film, and the volume fraction φ ranges from 0.08 to 0.12 to inhibit the rapid propagation of cracks along grain boundaries.

3. The nano-metal ink for flexible sensors according to claim 1, characterized in that, The low-temperature sintering accelerator reacts with the surface oxide layer of the metal nanoparticles to generate a volatile organometallic salt, ensuring that the neck initiation temperature T_neck between particles satisfies the following: ； T_neck is the measured temperature at which the neck begins to form, and T_m is the bulk melting point of the metal nanoparticles, both in K.

4. The nano-metal ink for flexible sensors according to claim 1, characterized in that: The high-boiling-point organic solvent is a combination of diethylene glycol butyl ether, terpineol and dodecyl alcohol, with a mass fraction ratio of 1:1:1.2, used to form a solvent vapor protective layer during the drying process to inhibit the thermal oxidative degradation of thermoplastic elastomers. The dispersant is a combination of BYK-2155, polyethyleneimine, and castor oil derivatives in a mass fraction ratio of 1:3:3.5, with an amine value ranging from 30 mg·KOH / g to 80 mg·KOH / g, to ensure that the particles remain monodisperse even under high solid content.

5. The nano-metal ink for flexible sensors according to claim 1, characterized in that: The metal nanoparticles are silver-coated copper core-shell structured nanoparticles, wherein the copper core has a diameter of 15-40 nm, the silver shell layer has a thickness of 5-15 nm, and the mass ratio of silver to copper is 8.5:

1.

6. A method for preparing a nano-metal ink for flexible sensors, comprising preparing the nano-metal ink for flexible sensors as described in any one of claims 1-5, characterized in that, The preparation method includes the following steps: Step S1: Under inert gas protection, the metal precursor solution and reducing agent solution are simultaneously injected into the microchannel reactor at a flow rate of 0.5 mL / min-2 mL / min, the reaction temperature is controlled at 60℃-80℃, and the residence time is 2-4 minutes to obtain a metal nanoparticle stock solution with uniform particle size distribution. Step S2: Centrifuge the obtained stock solution, wash it at least three times with a mixed solvent of ethanol and acetone, and then redisperse the washed metal nanoparticles in a high-boiling-point organic solvent to obtain a concentrated dispersion with a solid content of 30wt%-40wt%. Step S3: Under the temperature conditions of 40℃-50℃, add the dispersant, thermoplastic elastomer and low temperature sintering accelerator to the concentrated dispersion in sequence at a stirring speed of 200-400 rpm, and continue stirring for 2-3 hours. Step S4: Add the remaining high-boiling-point organic solvent to adjust the viscosity of the system to the target range, and then perform planetary degassing treatment for 30 minutes under a vacuum degree not higher than -0.08MPa and a rotation speed of 1000-2000rpm. Finally, filter through a 1-5μm PTFE filter to obtain a uniform and stable nano-metal ink.

7. The method for preparing a nano-metal ink for flexible sensors according to claim 6, characterized in that: In step S1, the microchannel reactor has a channel diameter of 0.5-2.0 mm, an aspect ratio L / D ≥ 100, and a Reynolds number Re that satisfies the range of 10 < Re < 100, in order to ensure that the reactants are in a laminar flow state.

8. The method for preparing a nano-metal ink for a flexible sensor according to claim 6, characterized in that, In step S2, the centrifugal acceleration a satisfies: ； Where N is the centrifugal speed (rpm) and R is the centrifugal radius (cm). The centrifugal acceleration is controlled within the range of 2000-5000g to ensure that the nanoparticles settle sufficiently without hard agglomeration.

9. A method for preparing a nano-metallic ink for a flexible sensor according to claim 6, characterized in that: After the stirring process in step S3 is completed, the resulting mixture is aged at 25±3℃ for 12-48 hours. The aging process is carried out under inert gas or sealed conditions to prevent solvent evaporation and material oxidation. The viscosity change rate Δη of the ink after aging satisfies: ； η_initial represents the initial viscosity before aging, ensuring the ink's storage stability and consistent printing performance.

10. A method for preparing a nano-metallic ink for a flexible sensor according to claim 6, characterized in that, It also includes step S5, quality inspection, which tests the key performance indicators of the filtered ink, including: Step S501: Use a laser particle size analyzer to detect the secondary particle size D50 of the metal nanoparticles, and require D50≤100nm; Step S502, using a rotational viscometer at 25°C for 100 seconds -1 Viscosity detection at shear rates; Step S503: Use a conductivity meter to measure the ink conductivity σ, and require σ ≥ 1.0 × 10⁻⁶. 4 S / m.

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