Enhanced thermal properties of polyamide 6, 6 composite/aluminum hybrid via injection joining strategy

Abstract

Polymer- metal hybrid with high thermal conduction and lightweight are of great importance for reducing material and energy consumptions. Firstly, the synergistic improvement in the thermal and mechanical properties of polyamide 6, 6 (PA 66) were investigated. By adding hexagonal boron nitride (hBN) and short carbon fiber (SCF) to PA 66, the thermal conductivity coefficient of the composite was significantly increased from 0.4 to 1.21 W/(m·K), with an excellent tensile strength about 100.9 MPa. Then, a new injection joining strategy was presented to fabricate the polymer composite-metal hybrid using in heat transfer and weight reduction fields. Thanks to the three-dimensional conductive networks embedded within the metal and polymer composites, the molded PA 66 composites-Aluminum alloy (Al) hybrid structure with good interface bonding strength exhibits outstanding thermal diffusivity, reaching to 2.88 m²/s which is about 9-folds compared to that of the neat PA 66. The surface temperature of the fabricated samples over time in heating and cooling process was also recorded by an infrared camera. As an environment friendly, scalable methodology, it provides much more convenience than those of the existing commercial technologies for production of the polymer-metal hybrid which can be used in fields with requirement of heat transfer, lightweight and complex structure.

Introduction

In the past few years, with the continuous development of micro-mechanical systems, the integration of electronic devices with high performance increases rapidly. The electro heat issue that seriously affects the working efficiency and lifetime of electronic devices becomes a major concern in the fields of electronic communication, aerospace, automobile, biomedicine, etc. [1,2]. To solve the demand for high-efficiency heat dissipation caused by heat concentration in a small space, it is urgent to design compact and efficient micro-structure radiators [3]. The materials employed for heat dissipation include conventional metals [[4], [5], [6], [7]], inorganic materials [8,9], polymers [10,11] or modified composite materials. Considering the characteristics of the processing and materials currently used, metals with high thermal conductivity have the disadvantages of relatively large mass density, poor corrosion resistance and difficult to process complex parts, which causes severe environmental issues and substantial economic losses [[12], [13], [14]]. Polymer is believed to be potential substitute due to its good anti-corrosion ability, easy in molding, oxidation resistance and low electrical conductivity. The thermal transfer properties can be improved by processing complex micro-structures on the polymer part surface. However, further application of polymers in heat exchange field has been significantly restrained because of its low thermal conductivity, which was considered to be only about 0.2–0.4 W/(m·K). Large numbers of researches have carried out to improve the thermal conductivity of the polymer and make it applicable for heat exchange fields.

Fillers such as carbon-based material [[15], [16], [17], [18]], ceramic material and metal particles including silver, copper, aluminum are mixed with polymer matrix so as to enhance the thermal conductivity [12,19,20]. It was concluded that the thermal conductivity of polymer composites increased with the filler content, and the improvement is 150–250% with the filler content ranging from 10% to 30%. The performance of these polymer/filler composites rely on the intrinsic properties of the filler including their chemical and physical composition, dimension and distribution in the matrix. Among the fillers, boron nitride (BN) is regarded as an attractive candidate because of its remarkably high thermal conductivity, superior chemical stability, great electrical insulation properties, and relatively low cost. For instance, Naguib et al. [21,22] used hexagonal boron nitride (hBN) to improve the thermal conductivity of polyphenylene sulfide and liquid crystal polymer, the thermal conductivities increased from 0.22, 0.20 W/m.K, to 1.83 and 2.94 W/m.K, respectively. Yang et al. [23] fabricated polyethylene (PE)/hBN composite sheet, it was reported that when the volume content of hBN attained 5.97%, the thermal conductivity was enhanced to about 4 times comparing to that of the neat PE. Hybrid fillers or structures are also investigated for the purpose of improving the thermal and mechanical performance of polymers. Yoo [24] investigated the effect of graphite and carbon fibers on PA 6 material, it was reported that the fillers distributed uniformly in the PA 6 matrix leading to high matrix reinforcement, and the thermal conductivity increased by up to 25-fold, comparing with neat PA 6. Yang [15] presented a carbon nanotube (CNT) hybrid structure and the through-thickness thermal properties of polymer was found to be significantly improved by 31% with long CNT resulting from the percolating network of CNT and the densification effect.

Recently, polymer-metal hybrids which possess characteristics of lightweight and low cost are developed [[25], [26], [27], [28], [29]]. As metals having excellent thermal conductivities, the polymer section in the hybrids can be conveniently constructed with microstructure for heat dissipating unit, the radiation emission rate also can be remarkably improved [30,31]. Therefore, it is also considered that the polymer-metal hybrid is a promising structure for the application of heat exchange filed. To date, there are many methods to produce these structures, such as the adhesion method [32,33], laser or friction assisted welding [26,34], ultrasonic method [35,36] and so on. However, the adhesives used in adhesion method need long curing time. The laser or friction assisted welding can easily result in formation of large number of bubbles in the welding interface, affecting the bonding strength of the dissimilar material. Besides, as a result of the restrained equipment power, the thickness of work piece is currently limited to less than 3 mm by ultrasonic method. Therefore, the injection technology was proposed to mold polymer - metal hybrids directly in our previous researches [[37], [38], [39], [40]]. The presented method has the advantages of high processing efficiency, low cost, light weight and space saving, which makes it quite potential to be applied in the fabrication of the hybrids.

Herein, the flexible injection technology was used to fabricate polymer-metal hybrid structure with high thermal conduction and dissipation for heat exchange applications. The effects of hBN content on the thermal and mechanical performance of the PA 66 composite were investigated. As a result of the three-dimensional conductive networks embedded within the metal and polymer composites, the molded PA 66 composite - Al hybrid structure exhibits excellent thermal conductive performance with a thermal diffusivity of up to 2.88 m²/s, which is about 9-fold compared to the neat PA 66. As far as we know, the presented methodology provides much more convenience than those of the existing commercial technologies for producing heat exchange parts with lightweight and complex structure requirements.