A flame-retardant high-thermal-conductivity nitrogen-boron-phosphorus co-doped carbon dot-liquid metal composite epoxy material and a preparation method thereof

Abstract

This invention relates to the field of polymer materials technology, specifically to a flame-retardant, high-thermal-conductivity nitrogen-boron-phosphorus co-doped carbon dot-liquid metal composite epoxy material and its preparation method. This material utilizes the synergistic assembly of nitrogen, boron, and phosphorus co-doped carbon dots with liquid metal to form a composite filler, significantly improving the thermal conductivity, mechanical properties, and flame-retardant properties of epoxy resin. N,B,P-CD is prepared via a hydrothermal method, and its surface Lewis basic sites form a stable coordination with the LM surface oxide layer, achieving uniform dispersion of LM in the composite system. With a total filler content of 70wt%, the optimized composite material achieves a thermal conductivity of 1.90 W·m⁻¹·K⁻¹, 728.82% higher than pure epoxy, with a density of only 1.51 g·cm⁻³, achieving a lightweight and high-thermal-conductivity effect. Mechanical tests show that its shear strength, flexural strength, and fracture toughness are significantly better than the control sample. In terms of flame retardant performance, the material achieves a UL-94V-0 rating and a LOI of up to 26.8%. This invention provides a lightweight, high thermal conductivity, and flame-retardant epoxy composite material design strategy for high-power electronic packaging and thermal management applications.

Description

Technical Field

[0001] This invention relates to the field of polymer materials technology, specifically to a flame-retardant, high thermal conductivity nitrogen-boron-phosphorus co-doped carbon dot-liquid metal composite epoxy material and its preparation method. Background Technology

[0002] With the rapid development of technologies such as 5G communication, high-performance computing, power electronics, and new energy vehicles, the power density and heat load of electronic devices have increased significantly, placing higher demands on the thermal management and safety performance of packaging materials. Epoxy resin (EP) has become the mainstream matrix for electronic packaging and thermal interface materials due to its excellent electrical insulation, adhesion, chemical stability, and processing adaptability. However, EP itself has extremely low thermal conductivity (typically 0.1-0.3 W·m⁻¹·K⁻¹), far lower than that of metal or ceramic materials, which limits the heat conduction efficiency in high-power applications and easily generates local hot spots, leading to shortened device life or even failure. Under combustion conditions, traditional epoxy resin releases a large amount of heat and toxic fumes, increasing the risk of fire. Therefore, improving the thermal conductivity of epoxy composite materials while taking into account flame retardancy and mechanical stability is a core challenge in materials science research (Nano-MicroLetters, 2021, 13(1), 110; Materials Today, 2014, 17(4), 163-174).

[0003] To improve thermal conductivity, researchers have employed filler reinforcement strategies, such as introducing graphene, carbon nanotubes, boron nitride, or metal microparticles to construct thermal conductivity networks. Studies have shown that composite systems using three-dimensional carbon fiber / carbon / metal networks can significantly improve thermal conductivity to the level of 5–10 W·m⁻¹·K⁻¹, demonstrating the crucial role of network structure in thermal conductivity (CompositesScience and Technology, 2021, 205, 108693; Advanced Functional Materials, 2023, 33(36), 2301549). However, these methods typically require high filler loading and complex processing techniques, posing a challenge to achieving a balance between flame retardancy and mechanical properties. Therefore, developing composite material solutions that balance thermal conductivity, flame retardancy, and processability has become a key technological breakthrough (Advanced Materials, 2022, 34(46), 2107905).

[0004] To improve the thermal conductivity of epoxy resins, researchers have employed a composite strategy using high thermal conductivity fillers, including ceramics (Al2O3, BN), carbon-based materials (graphene, carbon nanotubes), and metal microparticles, to construct a continuous thermally conductive network and enhance heat transfer efficiency. However, the type, size, shape, and loading of fillers significantly affect thermal conductivity. While carbon-based materials possess high intrinsic thermal conductivity, they are prone to agglomeration and exhibit poor interfacial compatibility in epoxy systems, hindering the full realization of their thermal conductivity advantages (Composites Part B: Engineering, 2025, 289, 111947; Composites Science and Technology, 2019, 170, 93–100). Researchers have attempted to improve filler dispersibility through three-dimensional network structures and interfacial modifiers, but this method is sensitive to processing conditions and struggles to simultaneously achieve flame retardancy.

[0005] Ceramic fillers exhibit excellent thermal conductivity and insulation properties, but they suffer from high interfacial thermal resistance and discontinuous thermal conduction paths. High filler loading increases system viscosity, reduces processability, and may lead to embrittlement and stress concentration, affecting mechanical stability (Composites Part B: Engineering, 2018, 150, 78-92; Progress in PolymerScience, 2021, 114, 101366). Furthermore, when high thermal conductivity fillers are combined with flame-retardant components, they may interfere with each other, altering the formation rate of carbonization products and leading to a decrease in flame-retardant performance. Therefore, traditional filler composite strategies present a contradiction between improving thermal conductivity, ensuring flame retardancy, and processability (Materials Science and Engineering R: Reports, 2020, 142, 100580). Developing multifunctional filler systems with high interfacial compatibility, flexible thermal conduction pathways, and flame-retardant functions is key to overcoming these bottlenecks.

[0006] To address the limitations of traditional fillers, the synergistic design strategy of multifunctional fillers has become a research hotspot. This strategy achieves synergistic optimization of thermal conductivity, flame retardancy, and mechanical properties through functionalized interface design, heteroatomic doping, and multiphase composites (Progressin Materials Science, 2025, 148, 101362; Matter, 2020, 2(6), 1446-1480). For example, N, B, and P co-doped carbon dots can form strong interfacial coupling in epoxy matrix, while inducing the formation of an expanded carbon layer, enhancing flame retardancy; liquid metal fillers can construct flexible thermal bridges, forming a continuous thermally conductive network, reducing interfacial thermal resistance, and maintaining material processability and mechanical stability (Advanced Materials, 2023, 35(1), 2203391; Advanced Science, 2020, 7(12), 2000192).

[0007] Multiphase synergistic networks fully leverage the complementary effects of different fillers. Carbon-based fillers provide high thermal conductivity pathways, while metal or ceramic fillers strengthen the flame-retardant skeleton, achieving a balance between thermal conductivity and flame retardancy (Small, 2021, 17(52), 2104762). Interface functionalization and network structure optimization not only improve thermal conductivity but also promote carbon layer densification at high temperatures, further enhancing flame retardancy and thermal stability (Composites Part A: Applied Science and Manufacturing, 2022, 163,107216). In summary, the synergistic design of multifunctional fillers provides a theoretical basis for high thermal conductivity epoxy composite materials and provides technical support for the nitrogen, boron, and phosphorus co-doped carbon dots and liquid metal synergistic composite filler scheme proposed in this invention. Summary of the Invention

[0008] To address the shortcomings and deficiencies of the existing technology, this invention provides a flame-retardant, high thermal conductivity nitrogen-boron-phosphorus co-doped carbon dot-liquid metal composite epoxy material and its preparation method.

[0009] The present invention achieves the above objectives by adopting the following technical solution: A flame-retardant, high thermal conductivity nitrogen, boron, and phosphorus co-doped carbon dots-liquid metal composite epoxy material includes an epoxy resin matrix and an LM@CD composite filler dispersed therein. The LM@CD composite filler is assembled by chemical coordination of nitrogen, boron, and phosphorus co-doped carbon dots (N,B,P-CD) and low-melting-point liquid metal (LM).

[0010] The carbon dots have Lewis basic sites on their surface, which can form stable coordination bonds with the oxide layer on the LM surface; the LM is distributed in a core-skin-crown structure in the composite system, forming a continuous thermally conductive network and enhancing interfacial thermal coupling.

[0011] Preferably, the liquid metal is a gallium-indium-tin alloy (Galinstan), which has a melting point below 30°C. It can realize a flexible thermal bridge structure in the epoxy matrix while maintaining uniform dispersion of fillers and processability of the composite material.

[0012] Preferably, the nitrogen, boron, and phosphorus co-doped carbon dots (N,B,P-CD) are synthesized by a high-pressure hydrothermal method. The nitrogen source in the synthesis raw materials includes one or more of the following: aniline, p-toluidine, o-toluidine, m-toluidine, p-aminophenol, N-methyl-o-phenylenediamine, tyramine, tryptamine, o-phenylenediamine, m-phenylenediamine, p-phenylenediamine, 2,4-diaminotoluene, 2,6-diaminotoluene, 4,4'-diaminodiphenylmethane, m-phenylenediamine, and 1,8-naphthylenediamine.

[0013] Preferably, the boron source includes one or more of the following: borax, ammonium pentaborate, boric acid, phenylboronic acid, borate ester, methylboronic acid, and isobutylboronic acid.

[0014] Preferably, the phosphorus source includes phytic acid, sodium phytate, trisodium phosphate, sodium pyrophosphate, phenylphosphonic acid, 2-aminoethylphosphonic acid, sodium glycerophosphate, ammonium polyphosphate, or one or a mixture thereof.

[0015] Preferably, the LM in LM@CD has a "core-skin-crown" distribution structure, and the total filler content of the LM@CD composite filler is 60-80 wt%, constructing a continuous thermally conductive network in the composite material; under the condition of 70 wt% total filler, the thermal conductivity of the material can reach ≥1.8 W·m⁻¹·K⁻¹, and the density ≤1.6 g·cm⁻³, ensuring lightweight and high thermal conductivity performance.

[0016] Preferably, the LM@CD composite filler also has flame retardant function, enabling the composite material to achieve a flame retardant rating of UL-94 V-0 and a limiting oxygen index (LOI) ≥26%, forming a dense expanded char layer under combustion conditions to suppress the release of smoke and toxic gases.

[0017] The present invention also provides a method for preparing the flame-retardant, high thermal conductivity nitrogen-boron-phosphorus co-doped carbon dot-liquid metal composite epoxy material, comprising the following steps: Step S1: Preparation of N,B,P co-doped carbon dots (N,B,P-CD). o-Phenylenediamine, borax, and phytic acid were dissolved in deionized water at a specific molar ratio. After thorough stirring to form a homogeneous solution, the solution was transferred to a high-pressure reactor for hydrothermal reaction at 180°C for 12 hours. After the reaction, the reaction solution was cooled to room temperature. Large particulate impurities were removed using a filter membrane, and then purified by dialysis to obtain highly dispersed carbon dot powder with a surface rich in nitrogen, boron, and phosphorus functional groups. This ensured that the surface possessed Lewis basic sites to form stable coordination bonds with liquid metals.

[0018] Step S2: Preparation of LM@CD composite filler. The liquid metal (e.g., Ga / In / Sn alloy) and the N,B,P-CD powder obtained in step S1 are added to a grinding device in a designed ratio. The mixture is thoroughly mixed through solid-phase grinding, ensuring the liquid metal uniformly coats the carbon particles, forming a "core-skin-crown" structure of the LM@CD composite filler. Temperature and time are controlled during the grinding process to prevent liquid metal aggregation or excessive flow, ensuring uniform dispersion and particle stability of the composite filler.

[0019] Step S3: Prepare the epoxy composite adhesive. Add the LM@CD composite filler obtained in step S2 to the epoxy resin matrix, along with the curing agent. Disperse the filler thoroughly using high-shear stirring. Solvent co-precipitation can be used to ensure uniform filler distribution in the composite system, forming a stable composite preform. By controlling the stirring speed, time, and solvent volume, the filler dispersion and matrix viscosity can be optimized, ensuring the preform is suitable for subsequent molding and thermosetting.

[0020] The beneficial effects of this invention are: In this nitrogen, boron, and phosphorus co-doped carbon dot-liquid metal composite epoxy material, LM and N, B, P-CD synergistically assemble to form a "core-skin-crown" structure, allowing LM to be uniformly dispersed in the epoxy matrix, reducing interfacial thermal resistance and aggregation risk. This significantly improves the thermal conductivity of the material with high filler content while maintaining lightweight characteristics. Carbon dots induce matrix phosphate formation to form an expanded carbon layer, and LM promotes the crosslinking of metal phosphates / oxides to form a dense graphitized carbon layer, improving flame retardant performance (UL-94 V-0, LOI 26.8%). The release of smoke and harmful gases is significantly reduced, and the mechanical properties are significantly better than the control material containing only carbon dots. Attached Figure Description

[0021] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments of the present invention or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 The present invention refers to N, B, P-CD powder containing liquid metal droplets (liquid metal is shown in the illustration).

[0023] Figure 2 The black LM@CD hybrid filler prepared by grinding in this invention is shown.

[0024] Figure 3 This is a TEM image of N,B,P-CD in this invention.

[0025] Figure 4The image shows the particle size distribution histogram and Gaussian fitting curve of the N,B,P-CD of this invention.

[0026] Figure 5 Microscopic morphology of coke residue after cone calorimeter testing: SEM images of EP, EPC, EPLC-10 and EPLC-70 from left to right. Detailed Implementation

[0027] 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.

[0028] It should be noted that, in specific embodiments of the present invention, terms such as "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, the use of phrases such as "comprising one" to define an element does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element. Those skilled in the art will understand the specific meaning of the above terms in the present invention through the specific circumstances.

[0029] Further analysis: The raw materials used in the experiments of the embodiments and comparative examples of the present invention are as follows, but are not limited to the following raw materials. The present invention only uses the following raw materials as specific examples to further illustrate the effect of the flame-retardant, high thermal conductivity nitrogen-boron-phosphorus co-doped carbon dot-liquid metal composite epoxy material described in the present invention.

[0030] The raw materials used in the experiments of the embodiments and comparative examples of the present invention are as follows, but are not limited to the following raw materials. The present invention only uses the following raw materials as specific examples to further illustrate the effect of the flame-retardant, high thermal conductivity nitrogen-boron-phosphorus co-doped carbon dot-liquid metal composite epoxy material described in the present invention.

[0031] Raw materials: NPEL 128 epoxy resin was purchased from Nan Ya Electronic Materials (Kunshan) Co., Ltd.; REH-1037 modified amine epoxy curing agent was purchased from Ruichi (Guangdong) Technology Co., Ltd.; phytic acid was purchased from Sinopharm Chemical Reagent Co., Ltd.; sodium tetraborate decahydrate (borax), o-phenylenediamine (OPD), N,N-dimethylformamide (DMF) and hexane were purchased from Adams (Shanghai Titanium Industry Technology Co., Ltd.); liquid metal alloy (Galinstan, 99.99%, containing 62.5wt% gallium, 21.5wt% indium and 16wt% tin) was purchased from Hunan Zhongcai Shengte New Material Technology Co., Ltd.; deionized water was used for carbon dot synthesis.

[0032] N,B,P-CD was synthesized directly via a hydrothermal method. 1.87 g of o-phenylenediamine and 3.3 g of borax were added to a polytetrafluoroethylene container and completely dissolved in 80 mL of deionized water preheated to 100 °C. Then, 5.44 g of 70% phytic acid was added, and the mixture was sonicated for 30 minutes to ensure thorough mixing. The sonicated mixture was then transferred to an autoclave, which was heated in an oven at 180 °C for 12 hours. The supernatant was filtered through a 0.22 μm polyethersulfone (PES) membrane to remove impurities. Furthermore, the liquid fraction in the deionized water was dialyzed for 24 hours using a dialysis bag with a molecular weight cutoff of 1000 Da to obtain a solution containing carbon dots. The carbon dot solution was freeze-dried and ground to obtain CD powder.

[0033] Preparation of LM@CD. Based on the set mass ratio of N,B,P-CD to LM (LM / CD = 0.1-0.7), accurately weigh liquid metal and N,B,P-CD particles and place them in an agate mortar. Grind the mixture under solid-state conditions until the color is uniform, indicating that sufficient mechanochemical reaction and interfacial interaction have occurred between the liquid metal and N,B,P-CD. During grinding, the temperature is maintained at 25°C-30°C, and the time ranges from 30 minutes to 1 hour, depending on the amount of filler being ground. The relationship between grinding amount and time can be roughly summarized as 30 minutes of grinding for every 20g of composite filler.

[0034] Preparation of epoxy composites. 5 g of NPEL 128 epoxy resin was dissolved in 26.83 g of N,N-dimethylformamide (DMF), followed by the addition of 6.5 g of REH-1037 curing agent and 0.02 g of DMP-30 accelerator. The mixture was magnetically stirred at room temperature for 24 hours to obtain a homogeneous polymer solution. This solution was then mixed with LM@CD and premixed by grinding in an agate mortar for 10 minutes. The mixture was then subjected to shear-induced turbulent mixing at 2000 rpm for 2 minutes using a high-speed disperser to obtain a relatively viscous black suspension. The composite material was extracted from the suspension by hexane co-precipitation. The resulting precipitate was dried in a vacuum oven at 80°C for 48 hours. Finally, the material was processed into EP / LM@CD composites of different shapes and thicknesses by hot pressing at 170°C, named EPLC-X, where X represents the LM / CD weight ratio. The preparation process of the EP / N,B,P-CD composite was consistent with the above description, and it was named EPC.

[0035] Performance evaluation: Thermal conductivity was measured using a Hot Disk TPS 2500S instrument with the transient planar source method at 25°C according to ISO 22007-2 standard. UL-94 horizontal / vertical flammability tests were conducted on a VOUCH 5402 system, and the limiting oxygen index (LOI) was measured using a fully automated oxygen index meter (VOUCH 5801A).

[0036] The present invention includes Examples 1-5 and Comparative Example 1, and their formulations and performance data are shown in Table 1.

[0037]

[0038] This invention designed a series of epoxy composite material samples, namely Comparative Example 1 sample EPC (containing only carbon dots) and composite materials Examples 1-5 (EPLC-10, EPLC-25, EPLC-40, EPLC-55, EPLC-70) with different LM / CD ratios, all with a total filler content of 70 wt%. By adjusting the ratio of liquid metal to N, B, P-CD, the thermal conductivity, flame retardant properties, and mechanical properties of the composite materials were optimized.

[0039] Thermal conductivity analysis: Experimental results show that the thermal conductivity of the composite material initially increases and then decreases with increasing LM / CD ratio. EPLC-25 exhibits the highest thermal conductivity, reaching 1.90 W·m⁻¹·K⁻¹, which is nearly 3.7 times that of EPC (0.52 W·m⁻¹·K⁻¹). This indicates that at a moderate LM / CD ratio, the liquid metal and carbon dots form a continuous thermal bridge network, effectively reducing interfacial thermal resistance and enhancing heat transfer efficiency. However, when the LM / CD ratio further increases to 0.79–1.00 (EPLC-55, EPLC-70), the thermal conductivity decreases to 1.25 and 0.55 W·m⁻¹·K⁻¹, respectively. This is mainly because excessive LM causes liquid metal agglomeration, disrupting the continuous thermal bridge structure and discontinuous heat transfer paths.

[0040] Flame retardant performance analysis: The LOI value and UL-94 rating show a similar trend to the LM / CD ratio. EPLC-25 has an LOI of 26.6%, maintaining a V-0 rating, indicating that it can form a dense, expanded char layer during combustion and suppress smoke release. When the LM / CD is too low (EPLC-10) or too high (EPLC-70), the LOI is 26.8% and 23.7%, respectively, and the UL-94 ratings are V-0 and V-1, respectively. It is evident that adding an appropriate amount of LM can enhance the structural density of the flame-retardant char layer, while excessive LM leads to char layer cracking and uneven metal oxide deposition, thereby reducing flame retardant performance.

[0041] Mechanical property analysis: Shear strength, flexural strength, and fracture toughness KIC show a peak-like distribution with varying LM / CD ratio. EPLC-25 exhibits shear strength, flexural strength, and KIC of 14.5 MPa, 98 MPa, and 2.2 MPa, respectively, all 1.7-2 times that of EPC. This indicates that a moderate LM / CD ratio can form a stable interfacial bond within the matrix, with the liquid metal filling the voids between carbon dots, thus improving the load transfer capacity and toughness of the composite material. However, when the LM / CD ratio is too high (EPLC-70), the shear strength decreases to 8.0 MPa, the flexural strength decreases to 74 MPa, and the toughness drops to 1.0 MPa·m1 / 2, indicating that excessive LM aggregation disrupts the matrix continuity, and interfacial stress concentration leads to brittle fracture.

[0042] Comparative analysis of the embodiments: EPC (control) contains only carbon dots, and its thermal conductivity, flame retardancy and mechanical properties are all lower than those of the best embodiment (EPLC-25). This shows that the addition of LM not only constructs an efficient thermal conduction path, but also improves the interfacial coupling of carbon dots to epoxy groups, forming a synergistic enhancement effect, which reflects the innovation and inventiveness of the present invention.

[0043] In summary, this invention achieves synergistic optimization of three performance aspects by adjusting the LM / CD ratio: ① In terms of thermal conductivity, the continuous thermal bridge formed by the liquid metal and carbon dots significantly improves heat transfer efficiency; ② In terms of flame retardancy, the LM modulates the carbon layer density and oxide crosslinking, forming a stable flame-retardant protective layer; ③ In terms of mechanical properties, the synergistic filling of LM / CD enhances interfacial bonding, improving shear strength, flexural strength, and toughness. These results fully verify the multifunctional reinforcing effect of LM@CD composite filler in epoxy-based composite materials and provide a reliable technical basis for high-power electronic packaging and thermal management applications.

[0044] 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, various 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 within the scope of protection of the present invention.

Claims

1. A flame-retardant, high thermal conductivity nitrogen-boron-phosphorus co-doped carbon dot-liquid metal composite epoxy material, comprising an epoxy resin matrix and LM@CD composite filler dispersed therein, characterized in that: The LM@CD composite filler is assembled from nitrogen, boron, and phosphorus co-doped carbon dots and low-melting-point liquid metal through chemical coordination. The surface of the nitrogen, boron, and phosphorus co-doped carbon dots has Lewis basic sites, which can form a stable coordination bond with the oxide layer on the surface of the low-melting-point liquid metal. The low-melting-point liquid metal is distributed in a core-skin-crown structure in the composite system, forming a continuous thermally conductive network and enhancing interfacial thermal coupling.

2. The flame-retardant high-thermal-conductivity nitrogen-boron-phosphorus co-doped carbon dot-liquid metal composite epoxy material of claim 1, wherein: The liquid metal is a gallium-indium-tin alloy with a melting point below 30°C, which can realize a flexible thermal bridge structure in the epoxy matrix while maintaining uniform dispersion of fillers and processability of the composite material.

3. The flame-retardant high thermal conductive nitrogen-boron-phosphorus co-doped carbon dots-liquid metal composite epoxy material according to any one of claims 1, wherein: The nitrogen, boron, and phosphorus co-doped carbon dots are synthesized by a high-pressure hydrothermal method. The nitrogen source in the synthesis raw materials includes one or more of the following: aniline, p-toluidine, o-toluidine, m-toluidine, p-aminophenol, N-methyl-o-phenylenediamine, tyramine, tryptamine, o-phenylenediamine, m-phenylenediamine, p-phenylenediamine, 2,4-diaminotoluene, 2,6-diaminotoluene, 4,4'-diaminodiphenylmethane, m-phenylenediamine, and 1,8-naphthylenediamine.

4. The flame-retardant high thermal conductive nitrogen-boron-phosphorus co-doped carbon dots-liquid metal composite epoxy material according to any one of claims 1, wherein: The boron source includes one or more of the following: borax, ammonium pentaborate, boric acid, phenylboronic acid, borate esters, methylboronic acid, and isobutylboronic acid.

5. The flame-retardant high thermal conductive nitrogen-boron-phosphorus co-doped carbon dots-liquid metal composite epoxy material according to any one of claims 1, wherein: The phosphorus source includes phytic acid, sodium phytate, trisodium phosphate, sodium pyrophosphate, phenylphosphonic acid, 2-aminoethylphosphonic acid, sodium glycerophosphate, ammonium polyphosphate, or one or a mixture thereof.

6. The flame-retardant high thermal conductivity nitrogen-boron-phosphorus co-doped carbon dots-liquid metal composite epoxy material of claim 1, wherein: Low-melting-point liquid metal exhibits a "core-skin-crown" distribution structure in LM@CD. The total filler content of the LM@CD composite filler is 60-80 wt%, constructing a continuous thermally conductive network in the composite material. Under the condition of 70 wt% total filler, the thermal conductivity of the material can reach ≥1.8 W·m⁻¹·K⁻¹, and the density ≤1.6 g·cm⁻³, ensuring lightweight and high thermal conductivity performance.

7. The flame-retardant, high thermal conductivity nitrogen-boron-phosphorus co-doped carbon dot-liquid metal composite epoxy material as described in claim 1, characterized in that: The LM@CD composite filler also has flame retardant properties, enabling the composite material to achieve a flame retardant rating of UL-94 V-0 and a limiting oxygen index of ≥26%. Under combustion conditions, it forms a dense, expanded char layer, which inhibits the release of smoke and toxic gases.

8. A method for preparing a flame-retardant, high thermal conductivity nitrogen-boron-phosphorus co-doped carbon dot-liquid metal composite epoxy material as described in any one of claims 1-7, characterized in that, Includes the following steps: Step S1: Prepare nitrogen, boron, and phosphorus co-doped carbon dots. Dissolve o-phenylenediamine, borax, and phytic acid in deionized water at a certain molar ratio. After stirring thoroughly to form a homogeneous solution, transfer it to a high-pressure reactor for hydrothermal reaction. React at 180°C for 12 hours. After the reaction is completed, wait for the reaction solution to cool to room temperature. Use a filter membrane to remove large particulate impurities, and then purify by dialysis to obtain highly dispersed carbon dot powder with nitrogen, boron, and phosphorus functional groups on the surface. Ensure that its surface has Lewis basic sites so as to form stable coordination bonds with liquid metal. Step S2: Prepare LM@CD composite filler. Add the liquid metal and the nitrogen, boron and phosphorus co-doped carbon dot powder obtained in step S1 into a grinding device according to the design ratio. Mix them thoroughly through solid-phase grinding so that the liquid metal is uniformly coated on the carbon dot particles to form an LM@CD composite filler with a "core-skin-crown" structure. Control the temperature and time during the grinding process to prevent the liquid metal from agglomerating or flowing excessively, and ensure that the composite filler is uniformly dispersed and the particles are stable. Step S3: Prepare epoxy composite adhesive solution. Add the LM@CD composite filler obtained in step S2 to the epoxy resin matrix, and add the curing agent at the same time. Use high shear stirring to fully disperse the filler. Solvent co-precipitation method can be used to make the filler uniformly distributed in the composite system and form a stable composite preform. By controlling the stirring speed, time and solvent amount, the filler dispersion and matrix viscosity can be optimized to ensure that the preform is suitable for subsequent molding and thermosetting.

9. The preparation method of a flame-retardant, high thermal conductivity nitrogen-boron-phosphorus co-doped carbon dot-liquid metal composite epoxy material as described in claim 8, characterized in that, In step S1, the molar ratio of o-phenylenediamine, borax, and phytic acid is 2-4:1-2:

1.

10. The preparation method of a flame-retardant, high thermal conductivity nitrogen-boron-phosphorus co-doped carbon dot-liquid metal composite epoxy material as described in claim 8, characterized in that, In step S2, the mass ratio of liquid metal to nitrogen, boron, and phosphorus co-doped carbon dots is LM / CD = 0.1-0.7.

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