WO2024036611A1 - Gecko-claw-imitating composite thermal interface material and preparation method therefor - Google Patents

Abstract

Disclosed in the present invention are a gecko-claw-imitating composite thermal interface material and a preparation method therefor. The method comprises the following steps: (1) preparing a polymer-based thermal interface material prepolymer on a template having a micron-scale array structure on the surface, and demolding same to obtain a polymer-based thermal interface material prepolymer having a micron-scale array structure on the surface; and (2) implanting carbon nanotubes into the micron-scale array structure of the polymer-based thermal interface material prepolymer obtained in step (1) by means of an electrostatic flocking method, and curing same to obtain a gecko-claw-imitating composite thermal interface material. In the present invention, by means of treating the surface structure of a thermal interface material, a micro-nano two-level gecko-claw-imitating structure is formed, such that an interface contact area is effectively increased, an interface adsorption force is enhanced, and interfacial thermal contact resistance is reduced.

Description

A composite thermal interface material imitating gecko claws and its preparation method Technical field

The invention relates to the technical field of manufacturing non-metallic functional materials for electronic components, and in particular to a composite thermal interface material imitating gecko claws and a preparation method thereof.

Background technique

The interface widely exists inside the device, and due to the presence of roughness, air gaps will inevitably be included when the two surfaces are bonded together. The thermal conductivity of air is extremely poor, resulting in a huge contact thermal resistance and the heat cannot be dissipated. , which will endanger the service life of the device. In order to promote interface thermal conduction, thermal interface materials have emerged. Currently, the most widely used thermal interface material is polymer-based thermal interface material, which accounts for nearly 90% of all thermal interface material products ^[1] . Polymer-based thermal interface materials are composed of polymer matrix and high thermal conductivity fillers (such as metals ^[2] , ceramics ^[3] , carbon-based materials ^[4] , etc.). Polymers have the advantages of softness, cheapness, and good stability, but their thermal conductivity is very low, usually only 0.2W/(m·K). Thermal conductive fillers such as graphene, boron nitride, aluminum powder, etc., although have high thermal conductivity, are relatively hard and are not conducive to bonding with the device interface. However, in order to improve the thermal conductivity of polymer-based thermal interface materials, the filler content has to be increased. This will inevitably affect the mechanical properties of the thermal interface material, and further affect the contact between the thermal interface material itself and the device interface. Thermal interface materials with poor mechanical properties will not only not reduce the interface thermal resistance, but also introduce additional interface thermal resistance. Therefore, it is crucial to develop methods to reduce the interface thermal resistance of thermal interface materials.

The method of reducing interface thermal resistance can be based on both a physical and chemical perspective. Chemical methods use chemical reactions to reduce interface thermal resistance, and the conditions for implementation are relatively harsh. The physical method improves interface contact by optimizing the surface structure, and the use conditions are simpler. Prasher ^[5] studied the effect of surface roughness on interface contact thermal resistance. Several groups of copper substrates with different roughness were used as contact surfaces, and thermal conductive silicone grease and phase change composite materials were used as thermal interface materials to measure the interface heat of different combinations. It is found that as the roughness increases, the contact area between the device and the thermal interface material increases, thereby reducing the contact thermal resistance. The method used is relatively simple and is suitable for device interfaces that have a certain roughness. The roughness of the thermal interface material needs to match the roughness of the device interface to be effective. Regarding the optimization methods of surface structure, the biological world can always bring inspiration. For example, the gecko has become the darling of the bionics community because of its ability to fly over walls and fly over walls ^[6] . Researchers found that gecko feet have dense, high-aspect-ratio, arrays of micron-sized bristles. The bristles are made of flexible material and have numerous nano-sized hairs at their ends. Researchers believe that this micro-nano multi-level structure increases the contact area between the interfaces in a disguised manner, doubling the van der Waals force on the contact surface, thus forming a strong adhesion force ^[6] . Currently, research on imitation gecko claws mainly focuses on improving adhesion properties. Carbon nanotubes are a hollow, highly thermally conductive material with mechanical properties similar to the microstructure of gecko claws, making them an excellent material for imitating gecko claws ^[7][8][9] . Because there is a certain positive correlation between adhesion performance and thermal conductivity, some people have also applied this bionic structure to thermal interface materials. Chen et al. ^[10] proposed a thermal interface material based on carbon nanotube arrays. However, the disadvantage is that the structure he proposed did not completely imitate the gecko claw. The structure of the gecko claw is characterized by a micro-nano two-level structure. Since carbon nanotubes are directly prepared by chemical vapor deposition, they are difficult to transfer to micron-scale arrays.

Although the above technical solution reduces the contact thermal resistance to a certain extent, it also has other problems:

Making some simple micron-level structures on the surface of thermal interface materials can usually only reduce the interface thermal resistance when it matches the roughness of the device interface itself, which has certain limitations;

The imitation gecko claw structure can effectively enhance the interaction of the contact surface, but it is rarely applied to thermal interface materials. The main reason is that the current research on imitation gecko claws mainly focuses on adhesive materials, and some problems need to be overcome when transferring to thermally conductive materials. Thermal interface materials are usually composite materials composed of highly thermally conductive fillers and matrices. The materials are uneven and it is more difficult to prepare micron double-layer structures.

Since carbon nanotubes are prepared by chemical vapor deposition, they cannot grow directly on the surface of the thermal interface material. Even if the micron-scale array structure is first etched on the surface of the thermal interface material, it is difficult to transfer the carbon nanotube array to the micron-scale structure.

references:

[1] Yang Bin, Sun Rong. Current status and research progress of thermal interface materials industry [J]. Chinese Basic Science, 2020(2):56-62.

[2]Batmunkh M,Tanshen M R,Nine M J,et al.Thermal Conductivity of TiO ₂ Nanoparticles Based Aqueous Nanofluids with an Addition of a Modified Silver Particle[J].Industrial&Engineering Chemistry Research,2014,53(20):8445–8451 .

[3]Choi S, Kim J.Thermal Conductivity of Epoxy Composites with a Binary-Particle System of Aluminum Oxide and Aluminum Nitride Fillers[J].Composites Part B:Engineering,2013,51:140-147.

[4]Balandin A A, Ghosh S, Bao W, et al. Superior Thermal Conductivity of Single-Layer Graphene[J]. Nano Letters, 2008, 8(3):902-907.

[5]Prasher R S.Surface chemistry and characteristics based model for the thermal contact resistance of fluidic interstitial thermal interface materials.Journal of Heat Transfer, 2001,123(5):969-975.

[6]Autumn K,GravishN.Gecko Adhesion:Evolutionary Nanotechnology[J].Philosophical Transactions,2008,366(1870):1575-1590..

[7]Qu L, Dai L, Stone M, et al.Carbon Nanotube Arrays with Strong Shear Binding-on and Easy Normal Lifting-off[J].Science.2008,322(5899):238-242.

[8]Cui Y, Ju Y, Xu B, et al. Mimicking a Gecko Foot With Strong Adhesive Strength Based on a Spinnable Vertically Aligned Carbon Nanotube Array[J].RSC Advances.2013,4(18):9056-9060.

[9] Li Yang. Preparation and performance study of carbon nanotube biomimetic adhesion arrays [D]. Nanjing, University of Aeronautics and Astronautics, 2016.

[10]Chen L, Ju B, Feng Z, et al. Vertically Aligned Carbon Nanotube Arrays as Thermal Interface Material for Vibrational Structure of Piezoelectric Transformer[J]. Smart Materials and Structures, 2018.

Contents of the invention

In view of the above technical problems, the present invention provides a composite thermal interface material imitating a gecko claw and a preparation method thereof, which reduces the thermal contact resistance between the thermal interface material itself and the device interface.

In order to achieve the above objects, the technical solutions adopted by the present invention are:

On the one hand, the present invention provides a method for preparing a composite thermal interface material imitating gecko claws, which includes the following steps:

(1) Prepare a polymer-based thermal interface material prepolymer on a template with a micron-scale array structure on the surface, and obtain a polymer-based thermal interface material prepolymer with a micron-scale array structure on the surface after demoulding;

(2) Implant carbon nanotubes into the micron-scale array structure of the polymer-based thermal interface material prepolymer obtained in step (1) through electrostatic flocking. After solidification, the composite thermal interface imitating gecko claws is obtained. Material.

As a preferred embodiment, the density of the carbon nanotubes implanted is 30 to 100 mg/cm ² .

As a preferred embodiment, in step (1), the micron-scale array structure includes a plurality of micron-scale columnar holes; the hole depth H of the columnar holes is 20 microns to 40 microns, and the hole diameter D of the columnar holes is 20 to 30 microns, and the center distance L between adjacent holes of the plurality of micron-sized columnar holes is 40 to 90 microns.

In some specific embodiments, in step (1), the template with a micron-scale array structure on its surface is a silicon substrate,

Preferably, the micron-scale array structure is etched by photolithography technology;

Preferably, it also includes pre-treatment of coating the template with a hydrophobic film;

Preferably, the hydrophobic film is a polytetrafluoroethylene film, and the thickness is preferably 100 to 200 nanometers;

Preferably, the polytetrafluoroethylene film is plated on the surface of the template using plasma enhanced chemical vapor deposition (PECVD).

As a preferred embodiment, in step (1), the components of the polymer-based thermal interface material prepolymer include a polymer matrix and fillers;

In the technical solution of the present invention, the polymer matrix is not particularly limited. Specific examples include epoxy resin, polybutadiene, polyurethane, acrylic resin, polydimethylsiloxane, etc., and is preferably a flexible polymer. Polymers such as polydimethylsiloxane (PDMS);

In some specific embodiments, the polymer matrix includes a monomer and a cross-linking agent; the monomer is preferably a vinyl-terminated silicone oil, and the cross-linking agent is preferably a side chain hydrogen-containing silicone oil; the vinyl-terminated silicone oil The mass ratio of the base silicone oil and the side chain hydrogen-containing silicone oil is preferably 1: (0.4~0.8);

In the technical solution of the present invention, the type of filler is not particularly limited. Specific examples include aluminum powder, alumina powder, boron nitride, silver powder, graphene, carbon fiber, etc., preferably aluminum powder; There is no special restriction on the amount. Aluminum powder is used as a filler, and its mass fraction accounts for 80wt% to 92wt% of the polymer-based thermal interface material prepolymer. Carbon fiber or graphene is used as a filler, and its mass fraction accounts for 80wt% to 92wt% of the polymer-based thermal interface material prepolymer. 20wt%~40wt% of the base thermal interface material prepolymer.

Preferably, the filler is proportioned according to a mass ratio of particle size 1 to 6 microns: particle size 10 to 20 microns (2/8 to 3/7): 1;

Specifically, in step (1), the specific steps for preparing the polymer-based thermal interface material prepolymer are: after mixing each component of the polymer-based thermal interface material prepolymer evenly, apply it on the surface with micron On the template of the hierarchical array structure, pre-cure at 100°C to 150°C for 10 to 20 minutes, and then demould; in the technical solution of the present invention, the surface of the polymer-based thermal interface material prepolymer obtained through the above steps has a micron-scale array structure and viscosity.

As a preferred embodiment, in step (2), the outer diameter of the carbon nanotube is 5 to 15 nanometers, and the length is 0.5 to 2 microns;

Preferably, the carbon nanotubes undergo hydroxylation treatment.

As a preferred embodiment, in step (2), the voltage of the electrostatic flocking is 20-100kV;

Preferably, the flocking time of the electrostatic flocking is 20 to 60 seconds.

As a preferred embodiment, in step (2), the curing temperature is 100-150°C;

Preferably, the curing time is 1.5 to 2.5 hours.

In another aspect, the present invention provides a gecko claw-like composite thermal interface material obtained by the above preparation method.

The above technical solution has the following advantages or beneficial effects:

The invention provides a composite thermal interface material imitating a gecko claw and a preparation method thereof. By processing the surface structure of the thermal interface material, the contact thermal resistance of the interface between the thermal interface material and the device is reduced. The invention first uses a template with a micron-scale array structure on the surface to pour a micron-scale array on the surface of the polymer-based thermal interface material prepolymer, and then uses electrostatic flocking technology to implant the carbon nanotube array onto the micron-scale array structure. Forming a micro-nano two-level imitation gecko claw structure, this structure can effectively increase the interface contact area, enhance the interface adsorption force, thereby reducing the interface contact thermal resistance. The present invention uses electrostatic flocking to introduce carbon nanotube arrays. Compared with directly growing carbon nanotubes using chemical vapor deposition, this method is easier to realize the transfer of carbon nanotubes and has a simple preparation process.

Description of drawings

Figure 1 is a schematic diagram of the surface bionic structure of a composite thermal interface material imitating a gecko claw in Embodiment 1 of the present invention.

Figure 2 is a silicon wafer template in Embodiment 1 of the present invention.

Figure 3 is a structural diagram of a micron-scale array in Embodiment 1 of the present invention.

Figure 4 is a surface profile curve of the micron-scale array structure in Embodiment 1 of the present invention.

Figure 5 is a scanning electron microscope picture of the carbon nanotubes in Example 1 of the present invention.

Figure 6 is a graph showing the adhesion test results of the composite thermal interface material imitating gecko claws in Example 1 of the present invention.

Detailed ways

The following embodiments are only some of the embodiments of the present invention, rather than all of the embodiments. Therefore, the detailed description of the embodiments of the invention provided below is not intended to limit the scope of the claimed invention but rather to represent selected embodiments of the invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without any creative work fall within the protection scope of the present invention.

In the present invention, unless otherwise specified, all equipment and raw materials can be purchased from the market or are commonly used in the industry. The methods in the following examples are all conventional methods in the art unless otherwise specified.

In the following examples:

Carbon nanotubes (CNT) were purchased from Aladdin, product number C419574, brand name short multi-walled carbon nanotubes;

Vinyl-terminated silicone oil was purchased from Ambia Specialty Silicones, with the brand name VS285CV;

The side chain hydrogen-containing silicone oil was purchased from Ambia Specialty Silicones, with the brand name XL1B.

Example 1:

The preparation process of the gecko claw-like composite thermal interface material in this embodiment is as follows:

(1) The silicon wafer template is etched through the photolithography process to make the surface have a micron-scale array structure. The specific parameters of the micron-scale array structure are: hole depth H 40 microns, hole diameter D 20 microns, and adjacent hole center distance L is 60 microns (observed by confocal microscope, see Figure 2), and then the surface is coated with a layer of polytetrafluoroethylene film with a thickness of 100 nanometers;

(2) The matrix of the polymer-based thermal interface material prepolymer is obtained by mixing vinyl-terminated silicone oil (monomer) and side chain hydrogen-containing silicone oil (cross-linking agent) with a mass ratio of 1:0.4, and the filler is aluminum powder. The mass fraction accounts for 90wt% of the polymer-based thermal interface material prepolymer; the aluminum powder is proportioned according to a mass ratio of 3:7 with a particle size of 1 to 6 microns: a particle size of 10 to 20 microns; the preparation process: combine the matrix and After the aluminum powder is stirred and mixed evenly, it is coated on the silicon wafer template obtained in step (1), pre-cured at 120°C for 10 minutes, and then demoulded; the surface of the obtained polymer-based thermal interface material prepolymer has a micron-scale array , and is sticky;

(3) Hydroxylate the carbon nanotube powder. The process is to immerse the carbon nanotube powder in a mixed solution of 30% mass fraction hydrogen peroxide solution and 20% mass fraction ammonia water for 12 hours. The hydrogen peroxide solution and ammonia water are The volume ratio is 1:1, then wash and dry with alcohol;

Take 1g of hydroxylated carbon nanotube powder (shown in Figure 5, outer diameter is 15 nanometers, length is 0.5-2 microns), spread it in a 5cm*5cm plane, and place it at the bottom of the electrostatic flocking equipment; follow the steps (2) The obtained polymer-based thermal interface material prepolymer is placed at a distance of 20cm directly above the carbon nanotube powder to flock the micron-scale array structure on its surface. The size of the polymer-based thermal interface material prepolymer is 3cm. *3cm, the thickness is 500mm; the electrostatic flocking voltage is 50kV, the flocking time is 30 seconds, the thickness of the carbon nanotube implant is about 200 nanometers, and the implantation density is 100mg/cm ² ; after the flocking is completed, put it Put it into the oven for secondary curing, the curing temperature is 120°C, and the curing time is 2 hours; the schematic diagram of the bionic structure on the surface of the gecko claw-like composite thermal interface material obtained in this example is shown in Figure 1.

Effect test:

(1) Morphological characterization

A Keyence confocal microscope was used to observe the micron-scale array structure on the surface of the silicon wafer template and the thermal interface material prepolymer.

Figure 2 shows the surface morphology of the silicon wafer template after etching in step (1). It can be seen that the surface of the silicon wafer has uniform holes;

Figure 3 shows the actual picture after the polymer-based thermal interface material prepolymer is demolded from the silicon wafer template in step (2), which shows that the demoulding effect is good, and it can be seen that the micron-scale array structure is evenly distributed in The surface of the thermal interface material prepolymer; its surface profile was observed with a confocal microscope. As shown in Figure 4, the height of the micron cylinder is basically the same, 40 microns; the distance between adjacent wave peaks is the same, also 40 microns.

(2) Adhesion test

Test method: Use welding strength tester DAGE4000 to test the adhesion force.

Test sample: Sample ① The polymer-based thermal interface material prepolymer with a micron-scale array on the surface obtained in step (2) and implanted with carbon nanotubes according to the relevant operations in step (3) (without secondary curing) ;Sample ② Common thermal interface material prepolymer obtained with the same components on a common silicon wafer substrate;

The test sample preparation is shown in the illustration in the upper left corner of Figure 6: Four small samples are prepared for each group of samples. Among them, sample ① and sample ② are placed on the surface of the large silicon wafer (2.56cm*2.56cm). 1cm*1cm area, and then cover it with a small silicon wafer of 1cm*1cm, and treat it at 120°C for 2 hours to completely solidify (after sample ① is cured, the micron-scale array structure and carbon nanotubes form a bionic structure); among them, The surface of the small silicon wafer in contact with the sample is a rough surface, and the surface of the sample ① with the bionic structure faces the small silicon wafer.

During the test, the large silicon wafer is used as a tray, and the probe is pushed outward from the bottom of the small silicon wafer. The small silicon wafer and the large silicon wafer are bonded with a thermal interface material. The thrust the small silicon wafer receives when it is pushed is the adhesive force; Figure 6 shows the stress on the probe during the entire process. The maximum stress is the bonding force of the thermal interface material. Test results show that the bonding force of thermal interface materials with bionic structures on their surfaces becomes stronger.

(3) Interface thermal resistance based on photothermal method test

Test method: Laser photothermal method, which is a non-contact thermal measurement method that can directly measure the contact thermal resistance of different interfaces in multi-layer samples. Based on the photothermal effect, the surface of the sample to be tested is periodically modulated laser heating, causing Measuring the periodic fluctuations of the sample surface temperature, and using a multi-layer physical model to fit the phase or amplitude of the test signal to determine the contact thermal resistance between the interfaces is one of the most important means of studying the interface thermal resistance.

In this example, the contact thermal conductivity of two groups of samples was tested using the laser photothermal method:

① Contact thermal resistance between the gecko claw-like composite thermal interface material obtained in step (3) and the rough silicon wafer surface with a thickness of 100 microns;

②Contact thermal resistance between an ordinary thermal interface material with the same components as the above polymer-based thermal interface material without surface treatment and without composite carbon nanotubes and a rough silicon wafer surface with a thickness of 100 microns;

Laser photothermal method test process: First, a layer of chromium with a thickness of about 100 nanometers is coated on the surface of the smooth side of the silicon wafer as a light absorption layer. During the test process, the laser is emitted to the chromium layer, and the heat is transferred from the chromium to the silicon wafer. , and then transmitted to the thermal interface material, thereby obtaining the contact thermal resistance between the silicon wafer and the thermal interface material, and the contact thermal resistance between the silicon wafer and the chromium layer.

Test results: Each group of samples is tested at four different points and the average value is taken. The above test results are shown in Table 1. The interface contact thermal resistance between silicon wafer and chromium is basically the same, which is in line with expectations. However, the contact thermal resistance between the thermal interface material with the bionic structure on the surface and the rough silicon wafer surface is significantly lower than the contact thermal resistance between the ordinary thermal interface material and the silicon surface. It shows that this solution can effectively reduce the interface contact thermal resistance.

Table 1

The above are only the preferred embodiments of the present invention. It should be pointed out that those of ordinary skill in the art can make several improvements and modifications without departing from the principles of the present invention. These improvements and modifications can also be made. should be regarded as the protection scope of the present invention.

Claims (19)

1. A method for preparing a composite thermal interface material imitating gecko claws, which is characterized by including the following steps:

   (1) Prepare a polymer-based thermal interface material prepolymer on a template with a micron-scale array structure on the surface, and obtain a polymer-based thermal interface material prepolymer with a micron-scale array structure on the surface after demoulding;

   (2) Implant carbon nanotubes into the micron-scale array structure of the polymer-based thermal interface material prepolymer obtained in step (1) through electrostatic flocking. After solidification, the composite thermal interface imitating gecko claws is obtained. Material.
2. The preparation method according to claim 1, characterized in that the density of the carbon nanotubes implanted is 30 to 100 mg/cm ² .
3. The preparation method according to claim 1, characterized in that, in step (1), the micron-scale array structure includes a plurality of micron-scale holes; the hole depth H of the holes is 20 microns to 40 microns, and the holes The hole diameter D is 20-30 microns, and the distance L between the centers of adjacent holes of the plurality of micron-sized holes is 40-90 microns.
4. The preparation method according to claim 1, characterized in that in step (1), the micron-scale array structure is etched by photolithography technology.
5. The preparation method according to claim 1, characterized in that step (1) further includes pre-treatment of plating the template with a hydrophobic film.
6. The preparation method according to claim 5, characterized in that the hydrophobic film is a polytetrafluoroethylene film.
7. The preparation method according to claim 6, wherein the thickness of the hydrophobic film is 100 to 200 nanometers.
8. The preparation method according to claim 6, wherein the polytetrafluoroethylene film is plated on the surface of the template using a plasma enhanced chemical vapor deposition method.
9. The preparation method according to claim 1, characterized in that in step (1), the components of the polymer-based thermal interface material prepolymer include a polymer matrix and a filler.
10. The preparation method according to claim 9, characterized in that the polymer matrix includes monomers and cross-linking agents.
11. The preparation method according to claim 10, characterized in that the monomer is vinyl-terminated silicone oil, and the cross-linking agent is side chain hydrogen-containing silicone oil.
12. The preparation method according to claim 11, characterized in that the mass ratio of the terminal vinyl silicone oil and the side chain hydrogen-containing silicone oil is 1: (0.4-0.8).
13. The preparation method according to claim 1, characterized in that, in step (2), the outer diameter of the carbon nanotube is 5 to 15 nanometers, and the length is 0.5 to 2 microns.
14. The preparation method according to claim 1, characterized in that, in step (2), the carbon nanotubes undergo hydroxylation treatment.
15. The preparation method according to claim 1, characterized in that in step (2), the voltage of the electrostatic flocking is 20-100kV.
16. The preparation method according to claim 15, characterized in that in step (2), the flocking time of the electrostatic flocking is 20 to 60 seconds.
17. The preparation method according to claim 1, characterized in that in step (2), the curing temperature is 100-150°C.
18. The preparation method according to claim 17, characterized in that in step (2), the curing time is 1.5 to 2.5 hours.
19. A composite thermal interface material imitating a gecko claw obtained by the preparation method according to any one of claims 1 to 18.

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