Research Progress Of Carbon Fiber 3D Network Structure Thermal Conductive Composite Materials

 

Abstract:

The methods for preparing high thermal conductivity carbon fiber three-dimensional (3D) network structure composite materials mainly include freeze-drying orientation method, electrophoretic deposition method, electrostatic flocking method, air-laid web-needle felting and 3D printing method. The research progress and characteristics of carbon fiber 3D network structure thermal conductive composite materials prepared by different methods are introduced. Carbon fiber 3D network structure can form a continuous thermal conductive path in the composite material, which has a significant effect on improving the thermal conductivity of the composite material. The future development direction of thermal conductive composite materials is prospected.

 

Background Introduction

 

In recent years, with the rapid development of electronic technology, various electronic components are developing towards miniaturization and integration. The power of electronic components is also getting higher and higher, and a considerable part of the electrical energy is converted into thermal energy.

 

Especially for electronic components in integrated circuits, lighting equipment and high-power equipment, if their heat dissipation is poor, it will lead to heat accumulation and excessive temperature, which will eventually affect the service life of the equipment or damage the equipment. Therefore, people have higher and higher requirements for the thermal conductivity of electronic materials.

 

Due to the special working and processing environment of electronic components, materials are required to have excellent thermal conductivity and low density, excellent mechanical properties, low thermal expansion coefficient, good chemical corrosion resistance, and easy processing.

 

With the continuous development of thermal conductive materials, lightweight high-temperature resistant materials and materials with good thermal conductivity are widely used in high-end fields such as aviation and aerospace.

 

The polymer thermal conductive composite materials prepared by compounding thermal conductive materials with polymer materials can meet the application in various scenarios. Therefore, it is very necessary to summarize the preparation methods of thermal conductive polymer composite materials to provide a reference for the preparation and application of high thermal conductive polymer composite materials.

 

Resin-based composite materials have excellent processing properties and good chemical properties. Compared with traditional metal ceramic materials, they are light and low cost. However, their poor thermal conductivity has greatly limited their application.

 

Under the condition of ensuring the original excellent performance of composite materials, researchers have adopted a variety of methods to improve their thermal conductivity. One of the most commonly used methods to improve its thermal conductivity is the filler filling method. The filler filling method can use various high thermal conductivity fillers to improve the thermal conductivity of composite materials. High thermal conductivity fillers mainly include nano-metal particles (such as Ag), metal oxides (such as Al2O3, MgO), nitrides (such as BN), carbon materials (such as carbon fiber), etc.

 

Although the conventional filler filling method has a simple preparation process, a high filling amount is required to obtain a higher thermal conductivity. A high filling amount will increase the viscosity of the material during mixing and processing, which will bring difficulties to processing, increase the density of the composite material, and damage the mechanical properties of the composite material.

 

Improving the thermal conductivity of the composite material is mainly achieved by forming a percolation structure in the composite material, that is, constructing a thermal conductive path. Carbon fiber has become an excellent thermal conductive filler due to its light weight, excellent thermal conductivity, good fatigue resistance and corrosion resistance.

 

In recent years, the use of carbon fiber as a three-dimensional (3D) network skeleton structure made of thermal conductive fillers to improve the thermal conductivity of composite materials has gradually become a research hotspot.

 

Carbon fiber improves the thermal conductivity of the composite material by forming a thermal conductive network in the composite matrix. The type of composite matrix and the arrangement of carbon fibers in the matrix have a great influence on the thermal conductivity of the composite material. Common matrixes for constructing 3D network structures mainly include epoxy resin, phenolic resin, unsaturated polyester resin, etc.

 

The key to improving the thermal conductivity of materials is to prepare carbon fibers with oriented structures to form 3D network structures.

 

In recent years, several common methods for preparing carbon fiber 3D network structure thermal conductive composite materials mainly include freeze-drying orientation method, electrophoretic deposition method, electrostatic flocking method, air-laid-needle felting method and 3D printing method.

 

These methods can significantly improve the thermal conductivity of composite materials while ensuring a low filler content.

 

01 Freeze-drying Orientation Method

 

The freeze-drying orientation method is a method of controlling the vertical temperature gradient to make the ice grow vertically upward, so that the carbon fibers are arranged vertically along the direction of the ice and maintained in the vertical direction under the action of the adhesive. Through freeze-drying orientation, the carbon fibers can form a parallel oriented structure, and heat can be quickly transferred in the oriented structure, so that a higher thermal conductivity can be obtained.

 

In traditional methods, the characteristics of carbon fibers as one-dimensional thermal conductive materials with high axial thermal conductivity are not fully utilized. The freeze-drying orientation method is an effective method for preparing thermal conductive composite materials developed in recent years. Through freeze drying, a connected carbon fiber 3D network structure is obtained.

 

Ma et al. used freeze-drying to prepare carbon fiber 3D network structure reinforced epoxy resin thermal conductive composite materials, which improved the thermal conductivity of the composite materials.

 

The preparation of carbon fiber 3D network structure thermal conductive composite material is divided into 4 steps:

 

(1) First, carbon fiber is dispersed in a solution containing additives (sodium carboxymethyl cellulose and hydroxyethyl cellulose);

(2) The mixture is placed on the surface of a copper block, and the bottom of the copper block is immersed in liquid nitrogen. The liquid nitrogen freezes the solution in the vertical direction, and the carbon fiber is oriented along the growth direction of the ice crystal;

(3) The sample is placed in a freeze dryer to sublime the ice to obtain a carbon fiber 3D network porous structure;

(4) Finally, the fiber 3D network structure is impregnated into epoxy resin and cured to obtain a carbon fiber 3D network structure reinforced epoxy resin thermal conductive composite material.

 

Figure 1 is a schematic diagram of the preparation of carbon fiber 3D network structure reinforced epoxy resin thermal conductive composite material by freeze-drying orientation method.

 

 

Figure 1 Schematic diagram of carbon fiber 3D network structure reinforced epoxy resin thermal conductive composites prepared by freeze-drying orientation method

 

Table 1 shows the thermal conductivity of several carbon fiber 3D network structure thermal conductive composites prepared by freeze-drying orientation method. The thermal conductivity of pure epoxy resin is 0.19 W/ (m·K). It can be seen from Table 1 that the thermal conductivity of several carbon fiber 3D network structure reinforced thermal conductive composites prepared is 2.84~21.19 W/(m·K), and the thermal conductivity of the thermal conductive composites is significantly improved.

 

 

Table 1 Thermal conductivity W/(m·K) of several carbon fiber 3D network structure thermal conductive composite materials prepared by freeze-drying orientation method

 

The experimental results show that with the increase of carbon fiber 3D network structure content, the thermal conductivity of carbon fiber 3D network structure thermal conductive composite materials increases. As a method widely used in industry to prepare carbon fiber 3D network structure thermal conductive composite materials, freeze-drying orientation method has the advantages of simple operation, low cost and strong orientation. However, the carbon fiber 3D network structure, which is the key to thermal conductivity, still needs to be further optimized to obtain carbon fiber 3D network structure thermal conductive composite materials with higher thermal conductivity.

 

02 Electrophoretic Deposition Method

 

The electrophoretic deposition method is a method of electroplating metal materials onto the surface of carbon fiber to improve the thermal conductivity of the material.

 

There is an interface thermal resistance between carbon fibers, and the electroplated metal can improve the interface performance between fibers, so that a network structure is formed between fibers, thereby forming a high thermal conductivity path, thereby improving the thermal conductivity of the composite material. Electrophoretic deposition is an effective method for coating materials and improving the interface properties of composite materials.

 

Xu et al. coated a layer of copper on the surface of carbon fiber by electroplating to prepare a carbon fiber 3D network structure reinforced epoxy resin thermal conductive composite material.

 

The copper film network formed on the surface of carbon fiber felt forms a continuous heat conduction path. The epoxy resin is impregnated into the copper-plated carbon fiber felt to prepare a thermal conductive composite material with a thermal conductivity of 3.069 W/(m·K), which is significantly improved compared with the thermal conductivity of pure epoxy resin.

 

Compared with traditional spraying such as dip coating and spin coating, electrophoretic deposition can quickly deposit metal materials and effectively control the thickness of metal materials on the surface of carbon fiber. Figure 2 is a schematic diagram of preparing carbon fiber 3D network structure reinforced epoxy resin thermal conductive composite material by electrophoretic deposition.

 

 

Figure 2 Schematic diagram of carbon fiber 3D network structure reinforced epoxy resin thermal conductive composite material prepared by electrophoretic deposition method

 

Table 2 gives the thermal conductivity of several carbon fiber 3D network structure thermal conductive composite materials prepared by electrophoretic deposition method. It can be seen from Table 2 that the thermal conductivity of the thermal conductive composite material is 2.519～6.140 W/(m·K), which is greatly improved compared with the thermal conductivity of pure epoxy resin.

 

Studies have shown that the thermal conductivity of the composite material is related to the thickness of the deposited coating. At first, the thermal conductivity of the composite material increases with the increase of the coating thickness, but when it reaches a certain thickness, the thermal conductivity of the composite material decreases. This may be due to the large amount of copper deposition, which leads to the formation of cracks and air gaps, resulting in a decrease in thermal conductivity.

 

 

 

03 Electrostatic Flocking Method

 

Electrostatic flocking method is a simple method for preparing carbon fiber reinforced composite materials. When the carbon fiber is placed in an electrostatic field, the carbon fiber forms a carbon fiber 3D network structure oriented side by side under the action of the electric field. The carbon fiber is highly oriented in the Z direction, and the heat can be quickly transmitted along the axial direction of the carbon fiber, thereby forming a high thermal conductivity path, and finally improving the thermal conductivity of the composite material.

 

Figure 3 is a schematic diagram of the preparation of carbon fiber 3D network structure reinforced photocurable resin thermal conductive composite material by Uetani et al. through electrostatic flocking method. The carbon fiber contacts the negative electrode plate during the landing process. Under the action of the electric field, the negatively charged carbon fiber is vertically inserted into the adhesive coated on the positive electrode plate to obtain a carbon fiber 3D network structure, which is then impregnated with photocurable resin to obtain a carbon fiber 3D network structure reinforced photocurable resin thermal conductive composite material.

 

 

Figure 3 Schematic diagram of carbon fiber 3D network structure reinforced photocurable resin thermal conductive composite material prepared by electrostatic flocking method

 

Table 3 gives the thermal conductivity of several carbon fiber 3D network structure thermal conductive composite materials prepared by electrostatic flocking method. It can be seen from Table 3 that compared with pure epoxy resin, the thermal conductivity of the thermal conductive composite material is significantly improved, and the thermal conductivity is 1.2～23.3 W/(m·K).

 

 

As an emerging production process, the application of carbon fiber 3D network structure thermal conductive composite materials prepared by electrostatic flocking method is not widely used, but as a simple method that can effectively prepare carbon fiber 3D network structure, the future application prospect is very broad.

 

04 Airflow Netting – Needle Punching Felting Method

 

Airflow netting – needle punching felting method refers to a method of preparing carbon fiber 3D network structure by using airflow net technology, using carbon fiber as raw material to make multi-layer carbon fiber soft pads, using array needles to needle the carbon fiber soft pads, and then compressing, impregnating resin, and carbonizing.

 

During the compression process, a small amount of carbon fiber on the X-Y plane will be transferred to the Z direction, which enhances the bonding force of the carbon fiber between layers. Through sintering, the resin between different layers becomes residual carbon, and the carbon fibers between different layers are connected together to obtain a carbon fiber 3D network structure, thereby forming a high thermal conductivity path, and by impregnating the matrix resin, a carbon fiber 3D network structure thermal conductive composite material is obtained.

 

Wu et al. first used airflow network technology to make carbon fiber felt, and then dissolved phenolic resin in ethanol solution to form a 35% phenolic solution. After impregnation at 175℃ for 3-6 hours, the carbon fiber felt was covered with phenolic resin, and the phenolic resin was carbonized at 2400℃ to obtain a carbon fiber 3D network structure. By impregnating epoxy resin, a carbon fiber 3D network structure reinforced epoxy resin thermal conductive composite material was obtained, and its thermal conductivity reached 6.20 W/(m·K).

 

 

Figure 4 Schematic diagram of preparing carbon fiber 3D network structure reinforced epoxy resin thermal conductive composite material by air-laid and needle-punched felting method

 

 

Table 4 Thermal conductivity of several carbon fiber 3D network structure thermal conductive composite materials prepared by air-laid-needle felting method

 

It can be seen from Table 4 that the thermal conductivity of the thermal conductive composite material is 2.13~6.20 W/(m·K), and the thermal conductivity of the thermal conductive composite material is significantly improved.

 

The air-laid-needle felting method can increase the contact area between the fibers in the carbon fiber 3D network structure. The contact area between the fibers increases with the increase of the compression ratio, so the heat conduction path also increases, which improves the thermal conductivity of the composite material.

 

05 3D Printing Method

 

3D printing method is a method of applying 3D printing technology to mix carbon fiber with a matrix and print the material through a 3D printer. Since the shear force during the printing process causes the carbon fiber to maintain a specific orientation, the carbon fiber will align with the printing pinhole and flow out, and has directionality when leaving the printing pin.

 

Carbon Fiber 3D network structure thermal conductive composite materials with a certain orientation structure can be prepared by 3D printing. When heat is transferred in the orientation direction, it can be quickly transferred along the axial direction of the carbon fiber, so that a carbon fiber 3D network structure thermal conductive composite material with high thermal conductivity can be obtained.

 

Ren et al. dispersed carbon fibers of different contents into the epoxy resin matrix, stirred them to mix them evenly, and after sufficient stirring, evacuated and degassed them in a vacuum drying oven to remove bubbles, heated them, and printed them in a mold using a 3D printer. The mold was placed in an oven for heat preservation, and finally a carbon fiber 3D network structure reinforced epoxy resin thermal conductive composite material was obtained.

 

 

Figure 5 Schematic diagram of 3D printed carbon fiber 3D network structure reinforced epoxy resin thermal conductive composite material

 

Compared with other preparation methods, the biggest advantage of 3D printing is that it can be quickly formed and can be prepared into different shapes according to its own needs. It is easy to process and has low cost.

 

Conclusion

 

The methods for preparing high thermal conductivity carbon fiber 3D network structure composite materials are introduced, including freeze-drying orientation method, electrophoretic deposition method, electrostatic flocking method, air-laid-needle felting method and 3D printing method. The above methods improve the thermal conductivity of composite materials by orienting carbon fibers.

 

However, due to the limitations of the preparation process, the thermal conductivity of composite materials is still difficult to meet people’s current needs for heat dissipation of electronic devices. In order to obtain composite materials with higher thermal conductivity and large-scale industrial production, the arrangement orientation and preparation process of carbon fibers still need to be further improved.

 

Based on the current research foundation, future research can be carried out in the following directions to improve the thermal conductivity of composite materials:

(1) Study new preparation methods to improve the thermal conductivity of composite materials;

(2) Further study the heat transfer mechanism and construct a more scientific heat conduction path;

(3) Improve the thermal conductivity of composite materials by reducing the thermal resistance of composite materials.