Liquid crystal epoxy resins with high intrinsic thermal conductivities and their composites: A mini-review

Highlights

• Research progresses of intrinsically thermally conductive LCER are reviewed.

• The influencing factors on intrinsic thermal conductivities of LCER are discussed.

• The development trends of intrinsically thermally conductive LCER are pointed out.

Abstract

Owing to the excellent mechanical properties and thermal stability, outstanding electrical insulation and chemical resistance, as well as easy processing and low cost, epoxy resins are widely used in electronics and electrical fields. However, the intrinsic thermal conductivity coefficients (λ) of the traditional epoxy resins are low, which is far from being able to meet the high thermal conduction/dissipation requirements of high-power electrical equipment or electronic components. By designing and changing the structures of molecules and chain links to obtain special physical structures (such as orientation structure, liquid crystal structure and crystalline structure, etc.), the high intrinsic λ of epoxy resins can be achieved. This paper reviews the classification, preparation methods, research progresses and thermal conduction mechanisms of intrinsically thermally conductive liquid crystal epoxy resins. The research progresses and academic achievements of existing intrinsically thermally conductive liquid crystal epoxy resins and their composites are focused on, and the influencing factors on the intrinsic thermal conductivities of liquid crystal epoxy resins are discussed. Finally, the development trends and prospects of intrinsically thermally conductive liquid crystal epoxy resins and their composites are pointed out.

Introduction

Epoxy resins have occupied about 70% of the entire market of thermosetting resins due to their excellent mechanical properties and thermal stability, outstanding electrical insulation and chemical resistance, as well as easy processing and low cost, having been widely used in electronics and electrical fields [[1], [2], [3]]. From tiny electronic components to large-capacity generators and motors, from low- and medium-voltage distribution networks to high-voltage and ultra-high-voltage power transmission systems, epoxy resins are used as electrically insulating materials [[4], [5], [6]]. However, with the development of electrical equipment in the directions of high voltage and large capacity, and the rapid development of electronic information technology, especially the technological innovation of high-frequency, digital, high-power, high-density and high-integration electronic components, it is inevitable to cause rapid accumulation of heat inside epoxy resins and their composites, which will easily accelerate the aging of epoxy resins, affect quality, and seriously threaten the stability and reliability of the performances of equipment, components and high-power systems [[7], [8], [9]]. However, the intrinsic thermal conductivity coefficients (λ) of the traditional epoxy resins are low (∼0.2 W/(m·K)), far from meeting the increasing demand for thermal conduction/dissipation capabilities of electrical equipment or electronic components [[10], [11], [12]].

In order to improve the thermal conductivities of epoxy resins, researchers mostly prepare epoxy-based thermally conductive composites by filling epoxy resins with highly thermally conductive fillers [[13], [14], [15]]. However, the filling of thermally conductive fillers will bring new problems such as interfacial thermal resistance, that is, when heat is transferred, phonon scatters severely, which affects the λ [[16], [17], [18]], causing rapid accumulation of heat and temperature rise inside the composites, which seriously threatens the stability and reliability of the composites [[19], [20], [21]]. At the same time, the introduction of too much thermally conductive fillers will inevitably deteriorate the mechanical properties of epoxy resin-based composites, and bring new challenges to processing, cost and density of materials [[22], [23], [24]]. In response to the above problems, researchers have focused on the design and preparation of novel hetero-structured fillers, surface modification of thermally conductive fillers, optimization of interfaces, and exploration of advanced molding processing methods and processes [[25], [26], [27], [28]]. To a certain extent, these methods ease the disadvantages caused by uneven distribution of thermally conductive fillers and interfacial thermal resistance, and improve the λ of composites. However, to obtain a high λ value, large addition of thermally conductive fillers is still necessary, and the problems such as the sharp increase of the interfacial thermal resistance and the drop in mechanical properties are still extremely prominent, having also become new bottleneck problems for researchers [[29], [30], [31]]. In order to obtain high λ value as well as maintaining mechanical properties of composites at the same time, the amount of fillers should not be too high and the intrinsic thermal conductivity of the matrix should be improved [[32], [33], [34]]. Under this circumstance, how to achieve high intrinsic λ of epoxy resins by designing and changing the structure of molecules and chain links to obtain special physical structures (such as orientation structure, liquid crystal structure and crystalline structure, etc.) has become the technical difficulties and scientific problems in urgent need for solution in the field of thermally conductive materials [[35], [36], [37]].

This paper reviews the classification, preparation methods, research progresses and thermal conduction mechanisms of intrinsically thermally conductive liquid crystal epoxy resins. The research progresses and academic achievements of the existing intrinsically thermally conductive liquid crystal epoxy resins and their composites are focused on, and the influencing factors on intrinsic λ of liquid crystal epoxy resins are discussed. Finally, the development trends and prospects of intrinsically thermally conductive liquid crystal epoxy resins and their composites are pointed out.