Polyimide properties

Polyimide (PI) is widely used in the fields of microelectronic devices, electronic packaging and aerospace due to its excellent thermal stability, mechanical properties and excellent dielectric properties. However, traditional PI also has the defect of poor thermal conductivity. When used as an electronic package or electronic device, it cannot dissipate heat in time, which seriously affects the performance and service life of the device. Therefore, while maintaining the excellent comprehensive performance of PI itself, improving the thermal conductivity of PI has aroused widespread research interest. At present, filling thermally conductive fillers is one of the main methods to improve the thermal conductivity of polymers. (Picture 1)

Figure 1 Polyimide film and sheet

Heat conduction mechanism of thermally conductive filler

The principle of heat conduction of heat conduction PI: The internal heat conduction carrier of the solid is mainly phonons or electrons (in the dielectric body, heat conduction is achieved through the vibration of the crystal lattice, and the energy of the lattice vibration is quantized. This kind of lattice vibration Quantum is called phonon). Inorganic non-metallic crystals conduct heat through the thermal vibration of neatly arranged crystal grains, which are usually described by the concept of phonons; since amorphous can be regarded as crystals with extremely fine grains, the heat conduction of amorphous can also be analyzed by the concept of phonons, but Its thermal conductivity is much lower than that of crystals; most polymers are saturated systems and have no free electrons. Therefore, adding high thermal conductivity fillers to PI is the main method to improve its thermal conductivity. Thermally conductive fillers are dispersed in PI and contact each other to form a thermally conductive network, so that heat can be quickly transferred along the "thermally conductive network", so as to achieve the purpose of improving the thermal conductivity of PI, as shown in Figure 1. 

Commonly used thermally conductive filler

Commonly used thermally conductive fillers include:

☆Metal (silver, copper, aluminum, etc.)

☆Carbon materials (graphite, carbon nanotubes, carbon fibers, etc.)

☆Inorganic thermal conductive particles (aluminum oxide, aluminum nitride, titanate, silicon carbide, silicon oxide, boron nitride, etc.)

Among them, boron nitride is an ideal filler for preparing materials with high thermal conductivity, low dielectric constant and low dielectric loss due to its high thermal conductivity, low dielectric constant and low loss, excellent oxidation resistance and corrosion resistance.

Characteristics of hexagonal boron nitride material

Boron nitride is a crystal composed of nitrogen atoms and boron atoms. The chemical composition is 43.6% boron and 56.4% nitrogen, with four different variants: hexagonal boron nitride (H-BN), rhombohedral boron nitride (R-BN), cubic boron nitride (C-BN) ) And wurtzite boron nitride (W-BN). Among them, the hexagonal boron nitride material (Figure 2) has:

☆ Higher mechanical strength, high melting point, high thermal conductivity

☆ Very low friction coefficient

☆ Good insulator

☆ Low dielectric constant and loss

☆ Hexagonal boron nitride can withstand the high temperature of 800℃ in the air,

☆ Hexagonal boron nitride can be prepared into a two-dimensional structure similar to graphene, called "white graphene", which has excellent graphene-like properties.

Therefore, hexagonal boron nitride is an excellent thermally conductive PI filling material, and is currently widely used in the field of thermally conductive PI composite materials.

Figure 2 Hexagonal boron nitride powder and polyimide film

Preparation of Hexagonal Boron Nitride/Polyimide Composite

The uniform dispersion of the thermally conductive filler in the polymer matrix is essential for the preparation of polymer matrix composites, especially in the improvement of the performance of the composites. The purpose of various dispersion methods used in the process of preparing polymer matrix composites is to make the fillers well dispersed in the matrix. In current research, PI/BN composites are mostly prepared by liquid phase mixing, and liquid phase mixing mainly includes solution blending and in-situ polymerization.

(1) Solution blending

Solution blending often requires the use of a large amount of solvents. Due to the structure and chemical properties of BN, BN cannot be dissolved in a solvent, and can only be dispersed in a solvent to form a uniform dispersion. Commonly used BN dispersants include water, ethanol, isopropanol, etc.; commonly used PI solvents include chloroform, Dimethyl sulfoxide, dimethyl formamide, dimethyl acetamide, etc. After the BN is uniformly dispersed in the solvent, the uniform mixing of the dispersion of BN and the polymer often requires the assistance of some external forces, such as ultrasonic treatment, mechanical stirring, and magnetic stirring. Sometimes in order to make BN and PI better contact each other, BN is often treated, such as functionalization, surface treatment, etc., and then mixed.

(2) In-situ polymerization

In-situ polymerization is a composite preparation method that emerged with the emergence of nanocomposites. Its main feature is that monomers (or oligomers) are polymerized in the presence of fillers. In-situ polymerization can enhance the interaction between fillers and polymers, and is the most effective method for dispersing fillers in the polymer matrix. Many composite materials are prepared by in-situ polymerization. The composite material prepared by in-situ polymerization technology exhibits better mechanical properties and lower permeation threshold than the composite material prepared by solution mixing or mechanical blending technology. In-situ polymerization also has important applications in PI/BN composites. First, PI monomers (dianhydrides or tetraacids and diamines) and BN are polymerized in situ to prepare PAA (PI precursor)/BN, and then sub- Amine reaction to prepare PI/BN composite material; or first polymerize into PAA, then blend with BN and then perform in-situ polymerization to prepare PI/BN composite material.

The main factors affecting the performance of thermally conductive h-BN/PI composites

The thermal conductiv The thermal conductivity of PI has an impact.

(1) h-BN dosage

When the amount of h-BN is small, h-BN is completely wrapped by PI, and most of the h-BN particles fail to directly contact; at this time, the PI matrix becomes a heat flow barrier between BN particles, suppressing BN phonons Of delivery. As the amount of h-BN increases, BN gradually forms a stable thermal network in the matrix, and the thermal conductivity increases rapidly at this time.

(2) The particle size and geometry of h-BN

When the amount of BN is the same, nanoparticles are more conducive to improving the thermal conductivity of PI than microparticles. The quantum effect of nanoparticles increases the number of grain boundaries, so that the specific heat capacity increases and the covalent bonds become metal bonds. High; At the same time, the small particle size and large number of nanoparticles make their specific surface area larger, and it is easy to form an effective heat conduction network in the matrix, so it is beneficial to improve the thermal conductivity of PI. For micron particles, when the amount of BN filler is the same, large-diameter thermally conductive fillers have a smaller specific surface area and are not easy to be wrapped by adhesives, so they have a greater probability of being connected to each other (more easily to form an effective heat conduction path), which is conducive to the thermal conductivity of the adhesive. Improve. In addition, when the amount of BN is the same, the probability of the thermal network formed by the same fillers of different geometric shapes in the matrix is different, and the thermally conductive fillers with a larger aspect ratio are easier to form a thermal network, which is more conducive to improving the thermal conductivity of the matrix. In short, the choice of particle size should be moderate, not too large or too small.

(3) Surface modification of h-BN

There is a polarity difference between BN and PI matrix interface, resulting in poor compatibility between the two, so BN is easy to aggregate into groups (not easy to disperse) in PI matrix. In addition, the large surface tension of BN makes the surface more difficult to be wetted by the PI matrix, and there are voids and defects between the phase interfaces, which increases the interface thermal resistance. Therefore, modification of the surface of the inorganic filler BN particles can improve its dispersibility, reduce interface defects, enhance interface bonding strength, inhibit the scattering of phonons at the interface and increase the free path of phonon propagation, thereby helping to improve the system's performance Thermal conductivity.

(4) Compound filling of h-BN

At the same time as BN is introduced, other excellent fillers are combined with BN to obtain BN composite fillers, which has become another research hotspot of BN thermally conductive composites. Through the synergistic effect between different fillers and the construction of the thermal network, BN composite fillers can often obtain better comprehensive performance than a single BN filler. Such as the composite of one-dimensional filler and two-dimensional BN, the composite of BN and conductive filler, the composite of one-dimensional BN and two-dimensional filler, etc.