A method for preparing a lead-free material with a low coefficient of thermal expansion for flexible electronics

Abstract

This invention discloses a method for preparing a lead-free material with a low coefficient of thermal expansion for flexible electronics, belonging to the field of polymer composite materials technology. The material prepared by this method comprises, along its thickness, layers A, B, and C. Layers A and C use aromatic polyimide as a matrix, with surface-modified hafnium tungstate phosphate nanoparticles dispersed thereon. Layer B uses poly(p-phenylene-2,6-benzobisoxazole) nanofibers as a continuous phase framework, loaded with hexagonal boron nitride nanosheets and Ti3C2T… x MXene nanosheets were obtained by hot pressing three layers together under an argon atmosphere. The resulting material exhibits a coefficient of thermal expansion reduced to 1.2–2.9 ppm / K and an in-plane thermal conductivity of 18.7–25.1 W / (m·K), combining excellent flexibility and mechanical properties. It is suitable for flexible copper-clad laminates, chip packaging, and integrated heat dissipation and shielding substrates for high-power electronic devices.

Description

Technical Field

[0001] This invention belongs to the field of polymer composite materials technology, specifically, it relates to a method for preparing a lead-free material with a low coefficient of thermal expansion for flexible electronics. Background Technology

[0002] With the rapid development of 5G communication, wearable electronics, chip packaging, and high-power modules, the trends of highly integrated, miniaturized, and high-power-density devices have significantly intensified. This has led to increasingly prominent issues such as localized heating, electromagnetic interference, and thermo-mechanical mismatch. Consequently, polymer-based composite materials are required to possess multiple properties simultaneously, including high thermal conductivity, low coefficient of thermal expansion, electromagnetic shielding, flame retardancy, and flexible processing. Currently, resins used for flexible copper-clad laminates, carriers, and integrated heat dissipation and shielding substrates mainly include aromatic polymers such as polyimide (PI) and polybenzoxazole (PBO), which possess excellent heat resistance, dimensional stability, and dielectric properties; however, their bulk thermal conductivity is typically below 0.2 W / m². -1 ·K -1 The coefficient of linear expansion (CTE) is much higher than that of copper foil or silicon chip, making it prone to warping, interface stress concentration and solder joint failure under cyclic thermal shock conditions.

[0003] To improve the thermal conductivity and dimensional stability of polyimide-based films, current techniques commonly employ the introduction of high thermal conductivity inorganic fillers into PI, such as boron nitride (BN), alumina, aluminum nitride, carbon nanotubes, and graphene, to enhance thermal conductivity by forming an in-plane thermal conductivity network. Literature reports that ball milling-shear-induced BN nanosheet orientation can increase the in-plane thermal conductivity of PI / BN composite films to several W·m. -1 ·K -1 Even 7W·m -1 ·K -1 While these measures reduce CTE to some extent, it remains difficult to adjust the CTE to levels close to those of copper or silicon at lower filler contents. Furthermore, high filler content in BN often leads to brittle films and decreased tensile strength. In addition, some patents disclose PI / BN high thermal conductivity insulating films and BN / polymer composite films for roll-to-roll processing for thermal management of electronic devices. However, these materials typically focus on thermal conductivity and dielectric properties, with less emphasis on precise matching of thermal expansion coefficients and electromagnetic shielding functions.

[0004] On the other hand, to achieve flexible electromagnetic shielding, a great deal of work in recent years has focused on MXene (such as Ti3C2T). xComposites of highly conductive fillers such as metal nanowires and carbon materials with polymers are used. Multilayer MXene / PI or PI fiber / MXene composite films are prepared by vacuum filtration-thermal imidization, achieving electromagnetic shielding effectiveness of 30-50 dB or higher in the X-band while maintaining a certain degree of flexibility and environmental stability. However, these MXene / PI shielding films typically form a continuous conductive network in the in-plane direction. While this is beneficial for reflecting and absorbing electromagnetic waves, the overall CTE is still dominated by the positively expanding PI, making it difficult to precisely match the thermal expansion behavior of copper foil and silicon chips. Furthermore, if the conductive layer extends throughout the thickness direction, it may reduce the insulation requirements of the device on the substrate.

[0005] To address the issue of thermal expansion mismatch, inorganic oxide materials with negative thermal expansion (NTE), such as ZrW₂O₈, ZrV₂O₇, and their doped systems, have attracted widespread attention. By blending with a positive thermal expansion matrix, the overall coefficient of thermal expansion can be precisely controlled over a wide temperature range. Related reviews and patents have systematically summarized the structure and applications of the NTE oxide family. In recent years, research has reported the use of novel NTE materials such as Hf₂WO₄(PO₄)₂ to replace Zr-based systems. Nanoparticles were prepared through solid-state sintering and ball milling, and then modified with silane coupling before being filled into PMDA-ODA polyimide. This approach can reduce the CTE of PI films in the 50-400℃ range to near copper levels while maintaining high flexibility and thermal stability, and significantly improve thermal conductivity. However, the aforementioned NTE / PI composite films mainly address the balance between thermal expansion and thermal conductivity, without providing electromagnetic shielding or conductive network designs. A systematic solution for achieving integrated "low CTE-high thermal conductivity-EMI shielding" through multi-scale structures is still lacking.

[0006] In the integration of electromagnetic shielding and thermal management, PBO fibers, due to their high modulus, flame retardancy, and high intrinsic thermal conductivity, have been used to construct nanopaper-based composite materials. Reports have already documented the use of PBO nanofibers, BN nanosheets, and Ti3C2T... x The ordered assembly of MXene gel microparticles was used to prepare a PBO / BN / MXene nanocomposite paper with a brick-and-mortar-like structure and a sandwich structure. This material can achieve 20 W·m³ at a thickness of approximately 13 μm. -1 ·K -1 The above-mentioned in-plane thermal conductivity, EMI shielding effectiveness of approximately 30 dB or more, and close to 1 ppm·K -1 Extremely low CTE. This type of ordered assembly nanopaper fully demonstrates the advantages of hierarchical structure design in terms of thermal conductivity and electromagnetic shielding. However, since it is a free paper material prepared in an acid system, it still needs to be combined with an engineered matrix layer to meet the comprehensive requirements of flexible copper clad laminates, packaging substrates, etc. for mechanical strength, insulation and processability.

[0007] In summary, existing technologies suffer from the following problems: While adding BN or other high thermal conductivity fillers to PI can improve thermal conductivity and reduce CTE to some extent, high filler content can lead to increased brittleness and processing difficulties, and there is a general lack of electromagnetic shielding and interlayer insulation structure design. When using MXene / PI or MXene / fiber composite films to achieve flexible EMI shielding, the conductive layers are mostly simple stacked or uniformly distributed structures, resulting in a relatively high overall CTE, making it difficult to simultaneously meet the comprehensive requirements of low CTE, high thermal conductivity, and thickness-direction electrical insulation. NTE oxide / PI composite materials can effectively adjust the coefficient of thermal expansion, but they usually only focus on the microstructure control within the PI single matrix layer and are not organically combined with high thermal conductivity-electromagnetic shielding nanopaper or multilayer structures, making it difficult to simultaneously achieve multiple properties such as low CTE, high thermal conductivity, electromagnetic shielding, and flame retardancy in the same film. PBO / BN / MXene ordered nanopaper exhibits excellent EMI shielding and thermal conductivity-low CTE characteristics, but as an independent paper material, its interface matching with engineered resin matrices such as PI, lamination structure design, and roll-to-roll process compatibility still need to be addressed. Therefore, a new polyimide-based multilayer nanocomposite film structure is needed, which simultaneously introduces negative thermal expansion inorganic nanoparticles, ordered thermally and electrically conductive nanopaper, and an electrically insulating interlayer design in terms of material composition and interlayer structure. This would allow for the achievement of low thermal expansion coefficient, high in-plane thermal conductivity, and stable electromagnetic shielding performance that match copper foil or silicon chips, while also meeting the comprehensive requirements of flame retardancy and dielectric properties, under the premise of flexible processing. Summary of the Invention

[0008] To address the technical problems of existing flexible electronic materials, such as high coefficient of thermal expansion, severe thermal mismatch with copper foil or silicon chips, low thermal conductivity, insufficient electromagnetic shielding effectiveness, increased brittleness due to high filler content, poor interfacial bonding in multilayer structures, and difficulty in simultaneously achieving low thermal expansion, high thermal conductivity, electromagnetic shielding, and good flexibility and insulation, this invention provides a method for preparing a lead-free material with a low coefficient of thermal expansion for flexible electronics.

[0009] This invention employs the following technical solution: a method for preparing a lead-free material with a low coefficient of thermal expansion for flexible electronics, comprising the following steps: the lead-free material comprises at least the following along its thickness direction: layer A: a first negative thermal expansion polyimide-based composite layer; layer B: a nano-paper layer located in the middle; layer C: a second negative thermal expansion polyimide-based composite layer, wherein: the resin matrix of layers A and C is aromatic polyimide, and surface-modified hafnium tungstate phosphate nanoparticles are dispersed in layers A and C; layer B comprises, by mass, 20-60 parts of poly(p-phenylene-2,6-benzobisoxazole) nanofibers, 20-70 parts of hexagonal boron nitride nanosheets, and 20-70 parts of Ti3C2T xMXene nanosheets, with poly(p-phenylene-2,6-benzobisoxazole) nanofibers as the continuous phase framework, loaded with hexagonal boron nitride nanosheets and Ti3C2T x MXene nanosheets are obtained by stacking layers A, B, and C in sequence, hot-pressing them at 1-2.5 MPa pressure and 220-300℃ for 60-90 min under an argon atmosphere, and then cooling them.

[0010] Preferably, the aromatic polyimide is prepared as follows: by mass, under nitrogen protection, 400-600 parts of a polar aprotic solvent are used as the reaction medium, and 40-60 parts of pyromellitic dianhydride (CAS No.: 89-32-7) and 40-60 parts of 4,4'-diaminodiphenyl ether (CAS No.: 101-80-4) are mixed and polycondensed, followed by a stepwise temperature program to obtain the polyimide.

[0011] Preferably, the polar aprotic solvent is N,N-dimethylacetamide or N-methylpyrrolidone; the parameters of the polycondensation reaction are as follows: mechanical stirring at 50-70℃ and 60-120 rpm for 18-24 hours; the parameters of the stepped temperature program are as follows: 80℃ for 1-2 hours, 150℃ for 1-2 hours, 250℃ for 1-2 hours, and 300℃ for 1-2 hours.

[0012] Preferably, the hafnium tungstate nanoparticles are prepared by the following steps: hafnium oxide (CAS No.: 12055-23-1), tungsten trioxide (CAS No.: 1314-35-8), and ammonium dihydrogen phosphate (CAS No.: 7722-76-1) are added to a ball mill jar at a molar ratio of Hf:W:P = (2-6):(1-5):(2-4), and 80-150 parts of anhydrous ethanol (CAS No.: 64-17-5) are added. The mixture is then wet-milled at 250-450 rpm for 10-20 h to obtain a slurry, wherein the particle size of the slurry is 3 μm. 00-500nm; the slurry is dried at 90-120℃ for 8-18h to obtain precursor dry powder; under argon atmosphere protection, the precursor dry powder is heated to 1200-1400℃ at 4-8℃ / min, held for 2-2.5h and then cooled to room temperature in the furnace to obtain block reaction product; the block reaction product is mixed with 20-40 parts of γ-aminopropyltriethoxysilane (CAS No.: 919-30-2) and 10-30 parts of N,N-dimethylformamide (CAS No.: 68-12-2), and then dry ball milled for 60-96h to obtain the product.

[0013] Preferably, the surface modification method of the hafnium tungstate nanoparticles is as follows: 100 parts by mass of hafnium tungstate nanoparticles are dispersed in 800-1200 parts of anhydrous ethanol and ultrasonically dispersed for 20-50 min; 10-20 parts of deionized water are added to adjust the pH of the system to 4.5-5.5, and the mixture is magnetically stirred for 0.8-2 h, then stirred at 50-70℃ for 3-6 h, centrifuged at 2000-4000 rpm for 10-20 min, and finally dried at 110-125℃ for 6-10 h to obtain the final product.

[0014] Preferably, the mass fraction of surface-modified hafnium tungstate nanoparticles in layer A is 10-20%; the mass fraction of surface-modified hafnium tungstate nanoparticles in layer C is 10-20%.

[0015] Preferably, the poly(p-phenylene-2,6-benzobisoxazole) nanofibers in layer B are prepared by the following method: 1 part by weight of poly(p-phenylene-2,6-benzobisoxazole) fiber (CAS No.: 60857-81-0) is added to a mixed acid system composed of 45-55 parts by weight of methanesulfonic acid (CAS No.: 75-75-2) and 45-55 parts by weight of trifluoroacetic acid (CAS No.: 76-05-1), and the mixture is magnetically stirred at 25-32°C for 36-72 hours to obtain a sol with a solid content of 0.6-1.5% by weight. The sol is then diluted to a solid content of 0.3-0.8% by weight to obtain the final product.

[0016] Preferably, the hexagonal boron nitride nanosheets in layer B are obtained through the following steps: bulk hexagonal boron nitride powder is pre-calcined in air at 820-880℃ for 2-3 hours to remove surface impurities; 1 part of the pre-calcined powder is added to 60-150 parts by weight of isopropanol (CAS No.: 67-63-0); the mixture is continuously sonicated at 300-550W ultrasonic power for 15-25 hours; then centrifuged at 1500-4000rpm for 15-25 minutes, and the supernatant is collected; the Ti3C2T nanosheets in layer B are obtained through the following steps. x MXene nanosheets were prepared by the following method: a 7-8.5 mol / L mixed acid etching solution was prepared by mixing lithium fluoride (CAS No.: 7789-24-4) and hydrochloric acid (CAS No.: 7647-01-0). Ti3AlC2MAX phase powder (CAS No.: 196506-01-1) was slowly added at a LiF:Ti3AlC2 mass ratio of 1.5-2.5:1. The mixture was stirred at 32-38℃ for 18-30 h. The etching product was washed with deionized water and centrifuged until the pH of the supernatant was 6-7. The supernatant was then ultrasonically exfoliated in water or ethanol for 10-50 min to obtain the final product.

[0017] Preferably, the preparation method of layer B is as follows: the prepared poly(p-phenylene-2,6-benzobisoxazole) nanofibers are mixed with hexagonal boron nitride nanosheets, and then Ti3C2T is added.x MXene nanosheets are homogenized under high pressure of 5-10 MPa for 10-30 s, and then hot-pressed at 15-25 MPa and 110-130 °C for 30-50 min to obtain nanopaper layers.

[0018] Preferably, the thickness ratio of layer A, layer B and layer C after hot pressing is (0.5-1.5):1:(0.5-1.5).

[0019] Compared to existing technologies, this invention offers the following advantages: the coefficient of thermal expansion is significantly reduced to 1.2-2.9 ppm / K, achieving near-perfect matching with copper foil (≈17 ppm / K) and silicon chips (≈2.6 ppm / K). The mechanism lies in the intrinsic negative thermal expansion characteristics of hafnium tungstate phosphate nanoparticles (Hf2WO4(PO4)2 type) modified with γ-aminopropyltriethoxysilane on the surface of layers A and C. The transverse vibrations (rocking / bending phonon modes) of the MOM (Hf-OW, Hf-OP) bridging oxygen in its crystal structure increase with increasing temperature, leading to a shortening of the second nearest neighbor distance in the lattice and generating a negative Grüneisen parameter (α≈-8--5×10⁻⁸). -6 K -1 The silane coupling agent forms chemical bonds between Si-O-Si and PI imide groups on the particle surface, significantly improving interfacial compatibility and stress transfer efficiency. This allows NTE particles to effectively "anchor" polyimide molecular chains, offsetting their positive thermal expansion. Compared to existing single-layer NTE / PI composites, the symmetrical sandwich structure of this invention (A / C layer thickness ratio 0.5-1.5:1:0.5-1.5) balances thermal stress on both sides, avoiding warping caused by unilateral expansion mismatch, and achieving ultra-low CTE across the entire temperature range (-50-200℃). The in-plane thermal conductivity is as high as 18.7-25.1 W / m·K: the B layer uses poly(p-phenylene-2,6-benzobisoxazole) (PBO) nanofibers as a continuous phase framework, and hexagonal boron nitride (h-BN) nanosheets are induced by high-pressure homogenization and hot-pressing, while Ti3C2T... x MXene nanosheets are highly oriented, forming a "brick-and-mortar" biomimetic network. h-BN's sp... 2In-plane phonon transport (κ≈300-600 W / m·K) and the synergistic electron-phonon contribution of MXene together construct a percolation channel with low interfacial thermal resistance. The high modulus (>200 GPa) of PBO fibers further suppresses phonon scattering. In existing technologies, randomly dispersed BN / PI systems typically have thermal conductivity <10 W / m·K due to filler agglomeration and interfacial thermal resistance. The ordered multi-scale assembly of this invention significantly reduces Kapitza thermal resistance, achieving efficient in-plane heat transport. X-band electromagnetic shielding effectiveness reaches 41.2-52.6 dB, while maintaining excellent electrical insulation in the thickness direction: the high electrical conductivity (>5000 S / cm) of MXene nanosheets forms a dense conductive network in the B layer, synergistically attenuating electromagnetic waves through surface reflection and internal multiple reflections / absorption; the insulating PI matrix in the A / C layers blocks the conductive path in the thickness direction, avoiding the risk of short circuits. The flame retardancy of PBO fibers and the dielectric synergy of h-BN further enhance absorption loss. Compared to existing simple MXene / PI stacked or uniformly dispersed systems, this invention's "insulation-conductivity-insulation" sandwich structure achieves both high shielding and high insulation, solving the dielectric breakdown problem caused by the penetration of the conductive layer in traditional systems. It exhibits a tensile strength of 115.6-142.3 MPa and a minimum bending radius of 0.9-1.6 mm, combining excellent flexibility and interfacial bonding. Three-layer hot pressing (1-2.5 MPa, 220-300℃) enables interdiffusion of PI molecular chains and PBO fibers, forming a metallurgical-grade interfacial bond; silane modification further provides covalent bridging. The high modulus of the PBO backbone and the lubricating effect of BN synergistically enhance resistance to crack propagation. Existing high-filler BN / PI systems are prone to stress concentration and embrittlement due to agglomeration; this invention achieves a triple balance of strength, flexibility, and low CTE through a low-filler (only 10-20% in the A / C layers) + ordered nanopaper design. The process is green, lead-free, and suitable for roll-to-roll mass production: the entire process uses argon-protected hot pressing to avoid oxidation; the raw material ratio is precisely controlled within the scope of the claims to ensure sufficient reaction (solid-state sintering at 1200-1400℃ completes the crystal phase transformation, and mixed acid dissolution at 25-32℃ avoids PBO degradation). Compared with existing technologies with high filler content, solvent residue, or complex multi-step processes, this invention significantly reduces costs and environmental impact.

[0020] In summary, this invention, through a multi-scale collaborative design of "precise control of negative thermal expansion particles + ordered nano-paper thermal conductivity / shielding network + symmetrical sandwich interface engineering", completely breaks through the bottleneck of the prior art that it is difficult to achieve "low CTE-high thermal conductivity-high shielding-good flexibility", and provides an ideal flexible substrate material for 5G, wearable electronics and high-power chip packaging. Attached Figure Description

[0021] Figure 1 This is the infrared spectrum of the aromatic polyimide prepared in Example 1.

[0022] Figure 2This is a transmission electron microscope (TEM) image of the hafnium tungstate nanoparticles prepared in Example 1.

[0023] Figure 3 This is a sample image of the lead-free material with a low coefficient of thermal expansion for flexible electronics prepared in Example 3. Detailed Implementation

[0024] The present invention will now be described in detail through specific embodiments. However, these illustrative embodiments are for purposes and uses only to illustrate the invention and do not constitute any limitation on the actual scope of protection of the invention, nor are they intended to restrict the scope of protection of the invention to these embodiments. For parameter ranges not mentioned, intermediate values ​​are selected. Also, for mass ratios not explicitly stated or mentioned, the mass ratio after addition generally refers to the mass ratio. Furthermore, in the present invention, the unit of mass is grams (g).

[0025] Example 1

[0026] A method for preparing lead-free materials with low thermal expansion coefficient for flexible electronics includes the following steps: (1) Preparation of aromatic polyimide: Under nitrogen protection, using 500g N,N-dimethylacetamide as the reaction medium, 50g pyromellitic dianhydride and 50g 4,4'-diaminodiphenyl ether are added, and mechanically stirred at 60℃ and 100rpm (stirrer model: IKA RW 20digital) for 21h to carry out polycondensation reaction. Then, the temperature is increased in a stepwise program: 80℃ for 1.5h, 150℃ for 1.5h, 250℃ for 1.5h, and 300℃ for 1.5h to obtain a polyimide solution (solid content of about 18%). Figure 1 As shown, the characteristic doublet of the imide carbonyl group (C=O) is ~1778 cm⁻¹. -1 (Strong and sharp): corresponds to the asymmetric stretching vibration of the carbonyl group (C=O) on the imide ring; ~1720 cm⁻¹ -1 (Extremely strong): Corresponds to the symmetric stretching vibration of the carbonyl group (C=O) on the imide ring; this is the most significant recognition feature of polyimide. Analysis conclusion: The presence of these two peaks directly proves that the imide ring has been successfully closed. Stretching vibration of the CN bond in imide: ~1377cm -1 (Strong): Corresponds to the stretching vibration of the CN bond in the imide ring. Analysis conclusion: This further confirms the formation of the imide ring skeleton. Stretching vibration of the aromatic ether bond (COC): ~1240 cm⁻¹ -1 (Strong): Corresponds to the COC asymmetric stretching vibration in the diphenyl ether structure derived from the diamine monomer (ODA) in the molecular structure. Analysis conclusion: Confirms that the ODA structural unit is retained in the molecular chain. Benzene ring skeletal vibration: ~1500 cm⁻¹ -1 : Corresponds to the C=C skeletal stretching vibration of the aromatic nucleus (benzene ring). Bending vibration of the imide ring: ~725 cm⁻¹ -1: Corresponds to the out-of-plane bending vibration of the carbonyl group of the imide ring. Imidization degree analysis: 3000-3500 cm⁻¹ -1 Region: This region in the graph is relatively flat, without any significantly broad hydroxyl or amino absorption peaks. Significance: In polyamic acid (PAA) precursors, this region usually contains significant hydroxyl and amino peaks. The flatness of this region in the graph indicates that after high-temperature treatment at 300℃ for 1.5 hours, the dehydration and ring-closing reaction proceeded very thoroughly, and PAA has been highly converted to PI. (2) Preparation of hafnium tungstate nanoparticles: 83.2 g of hafnium oxide, 69.3 g of tungsten trioxide, and 34.5 g of ammonium dihydrogen phosphate were added to a planetary ball mill jar (ball mill model: QM-3SP4) at a molar ratio of Hf:W:P = 4:3:3. 120 g of anhydrous ethanol was added, and the mixture was wet-milled at 350 rpm for 15 h to obtain a slurry (slurry particle size 400 nm). The slurry was dried at 105 °C for 12 h to obtain a precursor powder. Under an argon atmosphere, the precursor powder was heated to 1300 °C at 6 °C / min, held at that temperature for 2.2 h, and then cooled to room temperature with the furnace to obtain a blocky reaction product. The blocky reaction product was mixed with 30 g of γ-aminopropyltriethoxysilane and 20 g of N,N-dimethylformamide, and then dry-milled for 78 h to obtain hafnium tungstate nanoparticles. The transmission electron microscope image is shown below. Figure 2 As shown. (3) Surface modification of hafnium tungstate nanoparticles: 100g hafnium tungstate nanoparticles were dispersed in 1000g anhydrous ethanol and ultrasonically dispersed for 35min (ultrasonic instrument model: KQ-500DE); 15g deionized water was added to adjust the pH of the system to 5.0, and magnetic stirring (stirrer model: HJ-6) was performed for 1.2h. Then, stirring was continued at 60℃ for 4.5h, centrifuged at 3000rpm for 15min, and finally dried at 118℃ for 8h to obtain surface-modified hafnium tungstate nanoparticles. (4) Preparation of B-layer nanopaper: Preparation of poly(p-phenylene-2,6-benzobisoxazole) nanofibers: 1g of poly(p-phenylene-2,6-benzobisoxazole) fibers were added to a mixed acid system consisting of 50g of methanesulfonic acid and 50g of trifluoroacetic acid. The mixture was magnetically stirred at 28℃ for 54h to obtain a sol with a solid content of 1.0% by mass. The sol was diluted to a solid content of 0.5% by mass to obtain the final product. Preparation of hexagonal boron nitride nanosheets: Bulk hexagonal boron nitride powder was pre-calcined in air at 850℃ for 2.5h to remove surface impurities. One part of the pre-calcined powder was added to 100 parts by mass of isopropanol and ultrasonically sonicated continuously at 400W for 20h. Then, the mixture was centrifuged at 2500rpm for 20min, and the supernatant was collected to obtain the final product. Ti3C2T xPreparation of MXene nanosheets: An 8 mol / L mixed acid etching solution was prepared by reacting lithium fluoride with hydrochloric acid. Ti3AlC2MAX phase powder was slowly added at a LiF:Ti3AlC2 mass ratio of 2:1. The mixture was stirred at 35°C for 24 h. The etching product was washed with deionized water and centrifuged until the supernatant pH reached 6.5. The supernatant was then ultrasonically exfoliated in water for 30 min to obtain the final product. Then, 40 g of the prepared poly(p-phenylene-2,6-benzobisoxazole) nanofibers were mixed with 45 g of hexagonal boron nitride nanosheets, followed by the addition of 45 g of Ti3C2T… x MXene nanosheets were homogenized under high pressure of 7.5 MPa for 20 s, and then hot-pressed at 20 MPa and 120 °C for 40 min to obtain nanopaper layer (B layer, total mass 130 g). (5) Preparation of A and C layer composite materials: Take two portions of the polyimide solution (100 g dry polyimide after solid content conversion) obtained in step (1), add 15 g of surface-modified hafnium tungstate nanoparticles (mass fraction 15%) obtained in step (3), mechanically stir to disperse evenly, cast into a film and dry to obtain A layer and C layer pre-film (total mass 115 g each). (6) Assembly and hot pressing of multilayer materials: Stack A layer, B layer and C layer in sequence (thickness ratio 1:1:1), hot press at 1.75 MPa pressure and 260 °C for 75 min under argon atmosphere (hot press model: Carver 4122), and after cooling, obtain a lead-free material with low thermal expansion coefficient for flexible electronics, such as Figure 3 As shown.

[0027] The specific parameters for Examples 2-12 and Comparative Examples 1-12 are listed in the following tables. The tables are designed according to the progression of the steps, with each table reflecting different parameter values ​​for the examples / comparative examples, covering all endpoint and intermediate values. Only the parameters that have changed from those in Example 1 are listed in the tables; the descriptions of the remaining parameters are the same as in Example 1.

[0028] Table 1: Preparation parameters of aromatic polyimide in step (1)

[0029]

[0030] Table 2: Preparation parameters of hafnium tungstate nanoparticles in step (2) (Hf:W:P molar ratio, ethanol, ball milling, drying, sintering)

[0031]

[0032] Table 3: Surface modification parameters in step (3) and particle mass fraction of A / C layer in step (5)

[0033]

[0034] Table 4: Step (4) B-layer nanopaper preparation parameters (PBO nanofibers, hexagonal boron nitride nanosheets, Ti3C2T) x MXene)

[0035]

[0036] Table 5: Step (4) B-layer nanopaper preparation parameters 2 (pre-firing, ultrasonication, etching, homogenization, hot pressing) and Step (6) hot pressing parameters

[0037]

[0038] Table 6: Comparative parameters for steps (2), (3), (4), and (6) (adjusted based on Example 1)

[0039]

[0040] To verify the performance of the lead-free material with low thermal expansion coefficient for flexible electronics described in this invention, multi-dimensional tests were conducted on the products prepared in Examples 1-12 and Comparative Examples 1-12. The tests included thermal expansion coefficient, in-plane thermal conductivity, electromagnetic shielding effectiveness (X-band), tensile strength, and bending flexibility. The test methods are as follows: Thermal expansion coefficient test: Measured using a thermomechanical analyzer (model: TA Q400) within the range of -50~200℃ at a heating rate of 5℃ / min, according to ASTM E831 standard, and the average linear expansion coefficient (ppm / K) was calculated; In-plane thermal conductivity test: Measured at room temperature using a laser flash thermal conductivity meter (model: Netzsch LFA 467) according to ASTM E1461 standard, with a sample diameter of 12.7mm and a thickness of 0.2mm; Electromagnetic shielding effectiveness test: Measured total shielding effectiveness (SE, dB) in the 8.2-12.4GHz frequency band using a vector network analyzer (model: Agilent N5230A) according to GB / T 30142-2013 standard; Tensile strength test: Measured using an electronic universal testing machine (model: Instron) according to GB / T 1040.3-2006 standard. 5967) Test at a speed of 5 mm / min, sample size 15 mm × 150 mm; Bending flexibility test: measure the minimum bending radius (mm) using a bending tester according to GB / T 13542.2-2009 standard.

[0041] Table 7: Performance Test Results 1 (Examples 1-6)

[0042]

[0043] Table 8: Performance Test Results II (Examples 7-12)

[0044]

[0045] Table 9: Performance Test Results 3 (Comparative Examples 1-6)

[0046]

[0047] Table 10: Performance Test Results 4 (Comparative Examples 7-12)

[0048]

[0049] The test results show that the products in the examples exhibit excellent overall performance, with coefficients of thermal expansion ranging from 1.2 to 2.9 ppm / K, in-plane thermal conductivity from 18.7 to 25.1 W / m·K, electromagnetic shielding effectiveness from 41.2 to 52.6 dB, tensile strength from 115.6 to 142.3 MPa, and minimum bending radius from 0.9 to 1.6 mm. In contrast, the comparative examples show a significant decrease in performance due to the absence of key components or parameter deviations (e.g., the coefficient of thermal expansion increases to 6.5 to 15.9 ppm / K, and the electromagnetic shielding effectiveness decreases to 19.7 to 39.2 dB). This demonstrates the superiority of the preparation method of this invention. In Example 1, surface-modified hafnium tungstate phosphate nanoparticles form stable chemical bonds on the particle surface through a silane coupling agent, which improves the interfacial compatibility with the polyimide matrix and fully utilizes the negative thermal expansion effect to counteract the positive expansion of the polyimide, thereby significantly reducing the overall coefficient of thermal expansion. In the B layer, poly(p-phenylene-2,6-benzobisoxazole) nanofibers serve as a continuous phase framework, along with hexagonal boron nitride and Ti3C2T... x MXene collaboratively constructs an ordered thermally and electrically conductive network, achieving a simultaneous improvement in high thermal conductivity and electromagnetic shielding effectiveness. Simultaneously, multi-layer hot pressing ensures interlayer metallurgical bonding, avoiding the interface delamination problem common in single-layer structures. Comparative Example 1 lacks surface modification, leading to particle agglomeration, reduced negative thermal expansion contribution, and a significant increase in the coefficient of thermal expansion; Comparative Example 2 lacks Ti3C2T... x MXene causes discontinuity in the conductive network, significantly reducing electromagnetic shielding effectiveness; in Comparative Example 4, the sintering temperature was too low, resulting in incomplete particle crystallization, poor dispersibility, and simultaneous reduction in thermal conductivity and mechanical properties; in Comparative Example 11, the unbalanced thickness ratio led to an excessively thick A / C layer, mismatch between the negative thermal expansion layer and the conductive layer, and deterioration of overall performance.

[0050] The above description, in conjunction with specific embodiments, provides a further detailed explanation of the present invention. It should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, several simple deductions or substitutions can be made without departing from the concept of the present invention, and all such deductions or substitutions should be considered to fall within the scope of protection defined by the claims submitted herein.

Claims

1. A method for preparing a lead-free material with a low coefficient of thermal expansion for flexible electronics, characterized in that, Includes the following steps: The lead-free material comprises at least the following layers along its thickness: Layer A: a first negative thermal expansion polyimide-based composite layer; Layer B: a nano-paper layer located in the middle; Layer C: The second negative thermal expansion polyimide-based composite layer, wherein: the resin matrix of layers A and C is aromatic polyimide, and surface-modified hafnium tungstate phosphate nanoparticles are dispersed in layers A and C; layer B, by mass, includes 20-60 parts of poly(p-phenylene-2,6-benzobisoxazole) nanofibers, 20-70 parts of hexagonal boron nitride nanosheets, and 20-70 parts of Ti3C2T x MXene nanosheets, with poly(p-phenylene-2,6-benzobisoxazole) nanofibers as the continuous phase framework, loaded with hexagonal boron nitride nanosheets and Ti3C2T x MXene nanosheets are obtained by stacking layers A, B, and C in sequence, hot-pressing them at 1-2.5 MPa pressure and 220-300℃ for 60-90 min under an argon atmosphere, and then cooling them.

2. The method for preparing a lead-free material with a low coefficient of thermal expansion for flexible electronics according to claim 1, characterized in that, The preparation method of the aromatic polyimide is as follows: by mass, under nitrogen protection, 400-600 parts of polar aprotic solvent are used as the reaction medium, 40-60 parts of pyromellitic dianhydride and 40-60 parts of 4,4'-diaminodiphenyl ether are mixed and polycondensed, and then the mixture is obtained by stepwise temperature increase.

3. The method for preparing a lead-free material with a low coefficient of thermal expansion for flexible electronics according to claim 2, characterized in that, The polar aprotic solvent is N,N-dimethylacetamide or N-methylpyrrolidone; the parameters of the polycondensation reaction are as follows: mechanical stirring at 50-70℃ and 60-120rpm for 18-24h; the parameters of the stepped temperature program are as follows: 80℃ for 1-2h, 150℃ for 1-2h, 250℃ for 1-2h, and 300℃ for 1-2h.

4. The method for preparing a lead-free material with a low coefficient of thermal expansion for flexible electronics according to claim 3, characterized in that, The hafnium tungstate nanoparticles are prepared by the following steps: hafnium oxide, tungsten trioxide, and ammonium dihydrogen phosphate are added to a ball mill jar in a molar ratio of Hf:W:P = (2-6):(1-5):(2-4), and 80-150 parts of anhydrous ethanol are added. The mixture is wet-milled at 250-450 rpm for 10-20 h to obtain a slurry with a particle size of 300-500 nm. The slurry is dried at 90-120℃ for 8-18 h to obtain a precursor powder. Under an argon atmosphere, the precursor powder is heated to 1200-1400℃ at 4-8℃ / min, held at that temperature for 2-2.5 h, and then cooled to room temperature in the furnace to obtain a blocky reaction product. The blocky reaction product is mixed with 20-40 parts of γ-aminopropyltriethoxysilane and 10-30 parts of N,N-dimethylformamide, and then dry-milled for 60-96 h to obtain the final product.

5. The method for preparing a lead-free material with a low coefficient of thermal expansion for flexible electronics according to claim 4, characterized in that, The surface modification method of the hafnium tungstate nanoparticles is as follows: 100 parts by mass of hafnium tungstate nanoparticles are dispersed in 800-1200 parts of anhydrous ethanol and ultrasonically dispersed for 20-50 min; 10-20 parts of deionized water are added to adjust the pH of the system to 4.5-5.5, and the mixture is magnetically stirred for 0.8-2 h. Then, stirring is continued at 50-70℃ for 3-6 h, centrifuged at 2000-4000 rpm for 10-20 min, and finally dried at 110-125℃ for 6-10 h to obtain the final product.

6. The method for preparing a lead-free material with a low coefficient of thermal expansion for flexible electronics according to claim 1, characterized in that, The surface-modified hafnium tungstate nanoparticles in layer A have a mass fraction of 10-20%; the surface-modified hafnium tungstate nanoparticles in layer C have a mass fraction of 10-20%.

7. The method for preparing a lead-free material with a low coefficient of thermal expansion for flexible electronics according to claim 1, characterized in that, The B-layer poly(p-phenylene-2,6-benzobisoxazole) nanofibers were prepared by the following method: 1 part by weight of poly(p-phenylene-2,6-benzobisoxazole) fiber was added to a mixed acid system consisting of 45-55 parts by weight of methanesulfonic acid and 45-55 parts by weight of trifluoroacetic acid. The mixture was magnetically stirred at 25-32°C for 36-72 hours to obtain a sol with a solid content of 0.6-1.5% by weight. The sol was then diluted to a solid content of 0.3-0.8% by weight to obtain the final product.

8. The method for preparing a lead-free material with a low coefficient of thermal expansion for flexible electronics according to claim 7, characterized in that, The hexagonal boron nitride nanosheets in layer B are obtained through the following steps: Bulk hexagonal boron nitride powder is pre-calcined in air at 820-880℃ for 2-3 hours to remove surface impurities. One part of the pre-calcined powder is added to 60-150 parts by weight of isopropanol, and the mixture is continuously sonicated at 300-550W for 15-25 hours. Subsequently, the mixture is centrifuged at 1500-4000 rpm for 15-25 minutes, and the supernatant is collected. The Ti3C2T nanosheets in layer B are obtained through the following steps: x MXene nanosheets were prepared by the following method: a 7-8.5 mol / L mixed acid etching solution was prepared by mixing lithium fluoride and hydrochloric acid. Ti3AlC2MAX phase powder was slowly added at a LiF:Ti3AlC2 mass ratio of 1.5-2.5:

1. The mixture was stirred at 32-38℃ for 18-30 h. The etching product was washed with deionized water and centrifuged until the pH of the supernatant was 6-7. The supernatant was then ultrasonically exfoliated in water or ethanol for 10-50 min to obtain the final product.

9. The method for preparing a lead-free material with a low coefficient of thermal expansion for flexible electronics according to claim 8, characterized in that, The preparation method of layer B is as follows: the prepared poly(p-phenylene-2,6-benzobisoxazole) nanofibers are mixed with hexagonal boron nitride nanosheets, and then Ti3C2T is added. x MXene nanosheets are homogenized under high pressure of 5-10 MPa for 10-30 s, and then hot-pressed at 15-25 MPa and 110-130 °C for 30-50 min to obtain nanopaper layers.

10. The method for preparing a lead-free material with a low coefficient of thermal expansion for flexible electronics according to claim 1, characterized in that, The thickness ratio of layer A, layer B, and layer C after hot pressing is ( 0.5-1.5）：1：（0.5-1.5）。

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