A method for preparing direct writing carbon nanotube conductive ink by using microfluidic technology

Abstract

The application discloses a method for preparing direct-writing carbon nanotube conductive ink by using microfluidic technology, which comprises the following steps: (1) pumping carbon nanotube dispersion liquid, polysaccharide derivative solution and natural resin binder into different sample inlets of a microfluidic chip respectively, mixing in a microchannel to obtain mixed liquid A; (2) pumping rheological modifier and crosslinking agent into an auxiliary sample inlet of the microfluidic chip, and continuing to mix mixed liquid A in a mixing channel to obtain mixed liquid B; and (3) performing concentration treatment on mixed liquid B, adjusting solid content and viscosity, and obtaining the product directly. The microfluidic chip is used to mix the components uniformly in the microchannel within several seconds, and the efficiency is improved by hundreds of times. The obtained ink has excellent dispersion stability, resistivity of less than or equal to 10 ohm*cm and direct-writing printing adaptability, realizes continuous production of the carbon nanotube conductive ink, has good batch consistency, and the process is green and environmentally-friendly.

Description

Technical Field

[0001] This invention relates to the fields of conductive ink preparation and microfluidics, specifically to a method for preparing direct-write carbon nanotube conductive ink using microfluidics, which is particularly suitable for preparing flexible electronic devices using direct-write 3D printing technology. Background Technology

[0002] Direct ink writing (DIW), as an emerging additive manufacturing technology, has shown broad application prospects in fields such as flexible electronics, biosensors, and wearable devices. The performance of the conductive ink directly determines the conductivity, flexibility, and reliability of the printed device.

[0003] Carbon nanotubes (CNTs) are ideal conductive fillers for direct-write conductive inks due to their excellent conductivity, high aspect ratio, good mechanical flexibility, and biocompatibility. However, carbon nanotubes suffer from severe aggregation problems in ink systems, caused by their strong π-π stacking interactions and hydrophobic surface properties. Existing technologies typically employ surfactants, polymer dispersants, or ultrasonic treatment to promote the dispersion and stability of carbon nanotubes.

[0004] Currently, the preparation of carbon nanotube conductive inks typically employs a batch process: carbon nanotube powder, along with dispersants, binders, rheology modifiers, and other components, is added to a solvent and then mechanically stirred or ultrasonically treated for several hours to achieve uniform dispersion. This process suffers from the following technical bottlenecks: First, low mixing efficiency, as traditional stirring relies on macroscopic turbulent diffusion, resulting in long mixing times, typically 2-6 hours; second, poor batch consistency, with uneven mass transfer during scale-up production leading to product quality fluctuations, and relative standard deviations often exceeding 10%; third, high energy consumption, as prolonged stirring or ultrasonic treatment consumes significant amounts of energy; and fourth, difficulty in continuous production, as batch processes cannot achieve continuous feeding and discharging, thus limiting production efficiency.

[0005] Microfluidics, by manipulating fluids within micrometer-scale channels, offers advantages such as high mass transfer efficiency, short mixing time, continuous production capability, and ease of scale-up. However, there are currently no reports of applying microfluidics to the formulation of carbon nanotube conductive inks.

[0006] Therefore, developing a carbon nanotube conductive ink with good dispersibility, strong batch consistency, continuous production capability, and suitability for direct-write printing is of great significance. For example, CN10540028B discloses an aqueous UV conductive ink, but it focuses on the UV curing system and does not involve microfluidic-assisted formulation technology, and its dispersion efficiency and batch consistency still need to be improved. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a method for preparing direct-write carbon nanotube conductive ink using microfluidic technology. This method enables rapid and uniform mixing of various components within microchannels, significantly improving formulation efficiency and product consistency, and achieving continuous production.

[0008] The technical solution adopted in this invention is: A method for preparing direct-write carbon nanotube conductive ink using microfluidic technology includes the following steps: (1) Using an existing microfluidic chip, which is equipped with multiple inlet ports and auxiliary inlet ports, carbon nanotube dispersion, polysaccharide derivative solution and natural resin binder are pumped into different inlet ports of the microfluidic chip respectively and mixed in the microchannel to obtain mixture A; (2) Pump the rheology modifier and crosslinking agent into the auxiliary injection port of the microfluidic chip, and continue to mix them with the mixture A in the mixing channel to obtain the mixture B; (3) The mixture B is concentrated and the solid content and viscosity are adjusted to obtain direct-write carbon nanotube conductive ink.

[0009] Preferably, the carbon nanotubes are single-walled carbon nanotubes, multi-walled carbon nanotubes, or mixtures thereof, with a diameter of 1 nm to 50 nm and a length of 1 μm to 30 μm.

[0010] Preferably, the polysaccharide derivative is carboxymethyl chitosan, hydroxyethyl cellulose, sodium alginate, sodium carboxymethyl cellulose, or dextran, and the concentration of the polysaccharide derivative solution is 0.05% to 0.2% (w / v).

[0011] Preferably, the natural resin binder is gum arabic, rosin, peach gum, or gelatin, and the concentration of the natural resin binder is 0.5% to 2% (w / v), which means 0.5 to 2 grams of natural resin binder per 100 ml of water.

[0012] Preferably, the rheology modifier is hydroxypropyl methylcellulose, xanthan gum, guar gum, or fumed silica, and the concentration of the rheology modifier is 0.1% to 0.5% (w / v).

[0013] Preferably, the crosslinking agent is citric acid, glutaraldehyde, or calcium chloride, and the concentration of the crosslinking agent is 0.05% to 0.2% (w / v).

[0014] Preferably, the mixing channel of the microfluidic chip is a serpentine channel, a fishbone channel, or a spiral channel, with a hydraulic diameter of 50μm to 500μm and a channel length of 2cm to 20cm.

[0015] Preferably, in steps (1) and (2), the volumetric flow rate ratio of each fluid is 10:2~5:1~3:1~3:0.5~1 for carbon nanotube dispersion:polysaccharide derivative solution:natural resin binder:rheology modifier:crosslinking agent, the total flow rate is 50μL / min~20mL / min, and the residence time is 1s~120s.

[0016] Preferably, the concentration process in step (3) is performed by rotary evaporation, ultrafiltration or freeze drying, and the solid content of the concentrated ink is 5% to 20%.

[0017] Preferably, based on a total weight of 100%, the weight percentages of each component in the direct-write carbon nanotube conductive ink are as follows: carbon nanotubes 3%–15%, polysaccharide derivatives 0.5%–3%, natural resin binder 2%–10%, rheology modifier 0.5%–5%, crosslinking agent 0.1%–2%, and the balance being water. The viscosity of the ink is 1 Pa·s–50 Pa·s (shear rate 1 s). -1 The yield stress is 30 Pa to 200 Pa.

[0018] Compared with the prior art, the beneficial effects of the present invention are: (1) Significantly improved mixing efficiency: This invention uses the microchannels of a microfluidic chip, which utilizes its extremely high specific surface area and extremely short diffusion distance to enable the components to be uniformly mixed within seconds, while traditional batch mixing requires 2 to 6 hours. The preparation time of this invention is reduced by more than 95%, demonstrating a strong efficiency advantage.

[0019] (2) Significantly improved dispersion stability: This invention achieves rapid and uniform mixing of carbon nanotubes and polysaccharide derivatives through precise fluid control within microchannels, avoiding agglomeration caused by local concentration differences in traditional processes. The resulting ink has an absolute Zeta potential value ≥35mV and shows no sedimentation after standing for 30 days, demonstrating significantly better dispersion stability than traditional stirring methods.

[0020] (3) Excellent conductivity: This invention achieves effective dissociation and uniform dispersion of carbon nanotubes through microfluidic technology, constructing a complete conductive network. The resistivity of the resulting ink is ≤1.0Ω·cm, meeting the conductivity requirements of flexible electronic devices.

[0021] (4) Excellent direct-write printing suitability: This invention constructs a reversible three-dimensional network structure using a rheology modifier, which reduces the ink viscosity during printing (under high shear force) to facilitate extrusion, and rapidly restores viscosity and maintains shape after deposition (under low shear force). The ink viscosity is 1 Pa·s to 50 Pa·s (shear rate 1 s). -1 With a yield stress of 30Pa to 200Pa and controllable shear thinning behavior, it is suitable for high-precision direct-write 3D printing.

[0022] (5) Continuous production and batch consistency: This invention enables continuous production of carbon nanotube conductive ink, and production capacity can be linearly scaled up by increasing parallel chips or enlarging channel size. Product batch consistency is significantly improved (relative standard deviation RSD≤5%), overcoming the bottleneck of difficult scale-up of traditional batch processing processes.

[0023] (6) Mild conditions protect carbon nanotube structure: The present invention uses mild fluid shearing, which avoids damage such as chain breakage and surface defects caused by traditional ultrasonic treatment to carbon nanotubes, and is conducive to maintaining the intrinsic conductivity and mechanical properties of carbon nanotubes.

[0024] (7) Green and environmentally friendly: The present invention adopts an aqueous system, which greatly reduces the use of organic solvents, has low VOC emissions, and the process is green and environmentally friendly. Detailed Implementation

[0025] The present invention will be further described below with reference to specific embodiments and comparative examples, but the scope of protection of the present invention is not limited thereto. Example 1:

[0026] A direct-write carbon nanotube conductive ink prepared using microfluidic technology is described below: (1) Preparation of input fluids: 100 mg of multi-walled carbon nanotubes (MWCNTs, diameter 10-20 nm, length 5-15 μm) were dispersed in 100 mL of deionized water and dispersed with the aid of ultrasonic bath (200 W, 30 min) to obtain a carbon nanotube dispersion (solid content 0.1%); 50 mg of carboxymethyl chitosan was dissolved in 100 mL of deionized water to obtain a polysaccharide derivative solution (concentration 0.05%); 1 g of gum arabic was dissolved in 100 mL of deionized water to obtain a natural resin adhesive solution (concentration 1%); 0.3 g of hydroxypropyl methylcellulose was dissolved in 100 mL of deionized water to obtain a rheology modifier solution (concentration 0.3%); 0.1 g of citric acid was dissolved in 100 mL of deionized water to obtain a crosslinking agent solution (concentration 0.1%). (2) Microfluidic mixing preparation: A PDMS microfluidic chip (serpentine channel, channel width 200 μm, depth 100 μm, length 10 cm) was used. Carbon nanotube dispersion, polysaccharide derivative solution, natural resin binder solution, rheology modifier solution, and crosslinking agent solution were pumped into the five inlets of the chip through a syringe pump at a volume flow rate ratio of 10:5:2:2:1 (500 μL / min, 250 μL / min, 100 μL / min, 100 μL / min, 50 μL / min respectively). The mixture was mixed in the microchannel at room temperature for about 20 seconds, and the outlet mixture was collected. (3) Post-processing: The collected mixture was concentrated to 1 / 10 of its original volume by rotary evaporation at 45°C to obtain direct-write carbon nanotube conductive ink. Example 2:

[0027] A direct-write carbon nanotube conductive ink prepared using microfluidic technology is identical to Example 1 except that the serpentine channel in step (2) is replaced with a fishbone-shaped channel (channel width 200μm, depth 100μm, length 8cm). Example 3:

[0028] A direct-write carbon nanotube conductive ink prepared using microfluidic technology is identical to Example 1 except that the carboxymethyl chitosan in step (1) is replaced with hydroxyethyl cellulose (concentration 0.06%). Example 4:

[0029] A direct-write carbon nanotube conductive ink prepared using microfluidic technology is identical to Example 1 except that the multi-walled carbon nanotubes in step (1) are replaced with single-walled carbon nanotubes (SWCNTs, tube diameter 1-2 nm, tube length 3-10 μm). Example 5:

[0030] A direct-write carbon nanotube conductive ink prepared using microfluidic technology is identical to Example 1 except that the total flow rate in step (2) is increased to 2 mL / min (carbon nanotube dispersion 1000 μL / min, polysaccharide derivative solution 500 μL / min, natural resin binder solution 200 μL / min, rheology modifier solution 200 μL / min, crosslinking agent solution 100 μL / min) and a chip with a channel length of 15 cm is used. Example 6:

[0031] A direct-write carbon nanotube conductive ink prepared using microfluidic technology is identical to Example 1 except that gum arabic in step (1) is replaced with gum arabic (1% concentration). Example 7:

[0032] A direct-write carbon nanotube conductive ink prepared using microfluidic technology is identical to Example 1 except that the hydroxypropyl methylcellulose in step (1) is replaced with xanthan gum (concentration 0.3%).

[0033] Comparative Example 1: Traditional stirring method: A direct-write carbon nanotube conductive ink was prepared by adding 100 mg of multi-walled carbon nanotubes, 50 mg of carboxymethyl chitosan, 1 g of gum arabic, 0.3 g of hydroxypropyl methylcellulose, and 0.1 g of citric acid to 200 mL of deionized water, mechanically stirring (800 rpm) for 4 h, and then rotary evaporating to concentrate to 1 / 10 of the original volume to obtain the conductive ink.

[0034] Comparative Example 2: Traditional ultrasonic method: A direct-write carbon nanotube conductive ink was prepared by ultrasonically treating the mixture of Comparative Example 1 with a probe (200W, 30% amplitude) for 30 minutes, followed by mechanical stirring for 2 hours, and then rotary evaporation to concentrate it to 1 / 10 of the original volume to obtain the conductive ink.

[0035] Comparative Example 3: No polysaccharide derivative added: A direct-write carbon nanotube conductive ink prepared using microfluidic technology, which is completely identical to Example 1 except that the polysaccharide derivative solution in step (1) is omitted (without adding carboxymethyl chitosan).

[0036] Comparative Example 4: Low concentration of polysaccharide derivative: A direct-write carbon nanotube conductive ink prepared using microfluidic technology is exactly the same as Example 1 except that the concentration of carboxymethyl chitosan in step (1) is changed to 0.01% (w / v).

[0037] Comparative Example 5: No rheology modifier added: A direct-write carbon nanotube conductive ink prepared using microfluidic technology, which is completely identical to Example 1 except that the rheology modifier solution in step (1) is omitted (hydroxypropyl methylcellulose is not added).

[0038] Comparative Example 6: No crosslinking agent added: A direct-write carbon nanotube conductive ink prepared using microfluidic technology, which is exactly the same as Example 1 except that the crosslinking agent solution in step (1) is omitted (citric acid is not added).

[0039] Comparative Example 7: Excessive Total Flow Rate: A direct-write carbon nanotube conductive ink prepared using microfluidic technology is completely identical to Example 1 except that the total flow rate in step (2) is increased to 20 mL / min (residence time of about 0.5 seconds).

[0040] Performance testing: The performance test data of the direct-write carbon nanotube conductive inks of Examples 1-7 and Comparative Examples 1-7 are shown below: Table 1. Performance tests of direct-write carbon nanotube conductive inks from Examples 1-7 and Comparative Examples 1-7

[0041] Data Note: Performance testing standardization requirements: Zeta potential testing uses a nanoparticle size and Zeta potential analyzer at a testing temperature of 25℃, with samples diluted 100 times; viscosity testing uses a rotational rheometer with a 40mm diameter flat plate fixture at a testing temperature of 25℃, and a shear rate scan range of 0.1–1000 s. -1 The resistivity test uses the four-probe method, where the ink is directly printed onto the PET substrate to form lines (1 mm wide and 5 cm long), and then tested after drying. The equipment needs to be calibrated with a standard resistance element before testing. The dispersion stability test uses the static observation method, where the ink is placed in a transparent sample bottle and left to stand at room temperature for 30 days to observe the sedimentation at the bottom.

[0042] Key process control points: (1) Microfluidic chip parameter control: channel size tolerance ±10μm; good sealing of the injection port with no leakage; the channel needs to be rinsed with deionized water and dried with nitrogen before use; (2) Microfluidic operating parameters: The flow rate fluctuation of the injection pump shall not exceed ±5%; the residence time shall be controlled by the channel length and total flow rate, with a deviation of no more than ±10%.

[0043] (3) Concentration parameters: Rotary evaporation temperature not exceeding 50℃ to avoid damaging the polysaccharide derivative structure; ultrafiltration centrifugation speed 4000rpm, time 20min, molecular weight cutoff 100kDa.

[0044] Raw material quality control standards: (4) Carbon nanotubes: purity ≥95%; tube diameter 1-50nm; tube length 1-30μm; metal impurity content ≤0.1%.

[0045] (5) Polysaccharide derivatives: degree of substitution ≥0.8; viscosity 50-200 cP (25℃, 1% aqueous solution); pH value 6.0-7.5.

[0046] (6) Natural resin adhesives: acid value ≤20mg KOH / g; ash content ≤0.5%; moisture content ≤5%.

[0047] (7) Rheology modifier: moisture ≤5%; pH 5.0-8.0 (1% aqueous solution).

[0048] Product performance acceptance criteria: Electrical conductivity: resistivity ≤10Ω·cm (four-probe method); dispersion stability: no visible sedimentation after 30 days of standing, absolute value of Zeta potential ≥30mV; rheological properties: viscosity 1-50Pa·s (shear rate 1s). -1 Yield stress 30-200Pa; Direct-write printing suitability: Can be continuously extruded for 30 minutes without clogging, with a line width error ≤10%. Safety precautions for production: (1) Carbon nanotube processing: must be carried out under local exhaust ventilation; operators must wear dust masks (N95 level or above) and protective gloves to avoid the spread of carbon nanotube dust.

[0049] (2) Microfluidic operation: When using the high-pressure injection pump, pay attention to the tubing pressure to avoid tubing burst; the chip is made of PDMS material, avoid contact with organic solvents; 3) Waste liquid treatment: Waste liquid containing carbon nanotubes must be collected separately and treated as hazardous waste; waste liquid containing polysaccharide derivatives must be treated biochemically to meet standards before being discharged.

[0050] Precautions for process scale-up: When scaling up microfluidic chips, the hydraulic diameter of the channel should remain constant. Production capacity can be increased by increasing the number of chips connected in parallel. It is recommended to conduct a pilot test with 5-10 chips connected in parallel first. For concentration processing, the rotary evaporator specifications need to be adjusted according to the volume after scale-up. In mass production, each 1L of product constitutes an inspection batch.

Claims

1. A method for preparing a direct-write carbon nanotube conductive ink, characterized in that... Includes the following steps: (1) Using an existing microfluidic chip, which is equipped with multiple inlet ports and auxiliary inlet ports, carbon nanotube dispersion, polysaccharide derivative solution and natural resin binder are pumped into different inlet ports of the microfluidic chip respectively and mixed in the microchannel to obtain mixture A; (2) Pump the rheology modifier and crosslinking agent into the auxiliary injection port of the microfluidic chip, and continue to mix them with the mixture A in the mixing channel to obtain the mixture B; (3) The mixture B is concentrated and the solid content and viscosity are adjusted to obtain direct-write carbon nanotube conductive ink.

2. The preparation method according to claim 1, characterized in that... The carbon nanotubes are single-walled carbon nanotubes, multi-walled carbon nanotubes, or mixtures thereof, with a diameter of 1 nm to 50 nm and a length of 1 μm to 30 μm.

3. The preparation method according to claim 1, characterized in that... The polysaccharide derivative is carboxymethyl chitosan, hydroxyethyl cellulose, sodium alginate, sodium carboxymethyl cellulose, or dextran.

4. The preparation method according to claim 1, characterized in that... The natural resin binder is gum arabic, rosin, peach gum, or gelatin.

5. The preparation method according to claim 1, characterized in that... The rheology modifier is hydroxypropyl methylcellulose, xanthan gum, guar gum, or fumed silica.

6. The preparation method according to claim 1, characterized in that... The crosslinking agent is citric acid, glutaraldehyde, or calcium chloride.

7. The preparation method according to claim 1, characterized in that... The microfluidic chip has a hybrid channel that is serpentine, fishbone, or spiral, with a hydraulic diameter of 50 μm to 500 μm and a length of 2 cm to 20 cm.

8. The preparation method according to claim 1, characterized in that... In steps (1) and (2), the volumetric flow rate ratio of each fluid is 10:2~5:1~3:1~3:0.5~1 for carbon nanotube dispersion:polysaccharide derivative solution:natural resin binder:rheology modifier:crosslinking agent, the total flow rate is 50μL / min~20mL / min, and the residence time is 1s~120s.

9. The preparation method according to claim 1, characterized in that... The concentration process described in step (3) involves rotary evaporation, ultrafiltration, or freeze drying. The solid content of the concentrated ink is 5% to 20%.

10. The preparation method according to claim 1, characterized in that... Based on a total weight of 100%, the weight percentage of each component in the direct-write carbon nanotube conductive ink is as follows: carbon nanotubes 3%–15%, polysaccharide derivatives 0.5%–3%, natural resin binder 2%–10%, rheology modifier 0.5%–5%, crosslinking agent 0.1%–2%, and the balance being water.

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