Thermal conductivity of epoxy composites modified by microspheric molybdenum disulfide

In order to improve the thermal conductivity of epoxy resins (EP) without damaging their dielectric properties, a kind of molybdenum disulfide with microspheres structure (S-MoS2) was used as modifiers to add into EP matrix, which were prepared via the surfactant promoting hydrothermal process, and a new kind of S-MoS2/EP composites was finally obtained. The morphology of the prepared S-MoS2 was observed by X-ray diffraction (XRD) scanning electron microscopy (SEM), while the influence of different S-MoS2 loading on the dielectric properties, thermal conductivity and thermal resistance of S-MoS2/EP composites was also researched. The results suggested that the reasonable content of S-MoS2 can highly improve the thermal conductivity of S-MoS2/EP, which can be attributed to the excellent thermal resistant and uniform dispersion of S-MoS2 in EP matrix.


Introduction
Epoxy resin (EP) is a kind of commonly used materials with high mechanical properties, low creep tendency, excellent resistance to chemical corrosion and thermal deformation [1,2]; thus it has been widely used in electronic and electrical materials. However, the traditional EP exhibits the poor thermal conductivity; it could not transfer heat in time when used as a micro-electronic component, which is easy to cause damage to the material itself. Therefore, it needs to be further modified [3,4]. Modification of epoxy resin can be divided into chemical method and physical method [5,6]. The chemical method is to introduce heteroatoms or functional groups into the EP molecular chain segment to improve its thermal conductivity [7], and the physical method is to add a variety of other thermal conductivity fillers or additives into the EP matrix [8]. Among which, the preparation of filled thermal conductivity EP composites has the advantages of flexible material selection, simple process and obvious improvement of thermal conductivity [9]. Commonly used thermal conductive fillers are boron nitride and aluminum nitride, carbon fiber, steel fiber, etc. [10]. Although these packing add significantly can improve the thermal conductivity of EP resin, they also affect the mechanical properties and other performance of EP to a certain extent [11]. Therefore, how to reduce the additional damage of thermal conductive filler to EP resin in the maximum extent, the selection and modification of thermal conductive fillers should be further studied.
As one of the most important low-dimensional materials, two-dimensional materials have attracted extensive research interest and shown great application prospects [12]. Different from traditional 3D materials, due to quantum confinement effect, the carrier transport behavior of 2D materials will be greatly changed, which has a great impact on the thermoelectric properties of the materials [13]. In twodimensional materials, there are many new strange heat transport phenomena different from traditional bulk materials, such as size effect, ballistic transport and anisotropy, which play a certain role in the thermal conductivity of twodimensional materials, and make them show special thermoelectric properties [14]. In addition, due to the unique lamellar structure, the interface formed in the two-dimensional material facilitates the scattering of phonons, thus allowing heat conduction within the materials [15]. At present, there have been some studies on the thermal conductivity of two-dimensional metal sulfide, but the research on its performance after composite with polymer materials is limited. As one of the twodimensional metal sulfides, molybdenum disulfide (MoS 2 ) has attracted extensive attention because of its unique three-layer structure (Mo atoms sandwiched between two layers of S atoms) [16], and its electronic, optical and mechanical properties are comparable to those of graphene [17]. In addition, MoS 2 also has the characteristics of high electron mobility, valley polarization, strong orbital coupling, strong mechanical strength, etc. [18,19], which can be used as an improver for electronic devices. Furthermore, MoS 2 with apparent closed structure (spherical or tubular) not only has the chemical structure and physical properties of traditional MoS 2 , but also has high chemical stability and thermal resistance, so it can be widely used in catalysts, supercapacitors, lithium ion batteries, solid nano-lubricants and other fields [20].
From what has been discussed above, a new kind of MoS 2 particles with microspheres structure (S-MoS 2 ) was prepared via a hydrothermal process in this article. The surface-active agent sodium dodecyl sulfate was used as promoting additive, the TiO 2 were used as carries, while the sodium molybdate and thiosemicarbazide were used as raw materials to prepare the S-MoS 2 . The S-MoS 2 was then added into EP as modifiers to prepare S-MoS 2 /EP composites. The dielectric properties, thermal conductivity and thermal resistant properties of the S-MoS 2 /EP composites were measured to study the effect of S-MoS 2 on the EP resins. This study is aiming to provide a new method for the development of a high thermal conductive resin that can be used in microelectronic component fields.

Materials
The EP resin (industrial-grade, the purity of > 95 wt%) was purchased from Nantong Xingchen Synthetic Material Co. Ltd. The isophorone diamine (analytical pure) was purchased from Shanghai McLean Biochemical Technology Co., Ltd. The raw materials to prepare S-MoS 2 are sodium molybdate and ammonium thiourea (analytical pure, the purity was > 98%, purchased from Jingzhou fine chemical co., Ltd). The TiO 2 powder was purchased laboratory-made via hydrothermal method. The other reagents and solvents used in the experiments including: sodium dodecyl sulfate (analytical pure), hydrazine hydrate (analytical pure), chlorhydric acid (analytical pure), ethanol (analytical pure), and acetic acid (analytical pure), which were supplied by Tianjin Fuchen Chemical Reagents Factory without further purification.

Preparation of the S-MoS 2
The S-MoS 2 was prepared by hydrothermal method. 0.484 g sodium molybdenate and 0.364 g thiourea were added into the autoclave with polytetrafluoroethylene substrates, while 20 mL sodium dodecyl sulfate solution (which concentration is 0.016 mol/L) and 1 g TiO 2 are also added into the autoclave. The autoclave was put into a high-pressure steam, 220 °C for 24 h. The autoclave was then cool to room temperature, and the mixture was purred out. The mixture was cleaned by deionized water and anhydrous ethanol for 5 times, and the then dried at 50 °C for 12 h in a vacuum. The products S-MoS 2 were finally obtained after calcined at 500 °C.

Preparation of the S-MoS 2 /EP composites
The S-MoS 2 /EP composites were prepared via a casting method, and the preparation process is shown in Fig. 1. The EP were stirred in a glass beaker with isophorone diamine at 40 °C, and S-MoS 2 (the mass ratio is 0.0 wt%, 1.0 wt%, 2.0 wt%, 3.0 wt%, 4.0 wt%, and 5.0 wt%) was added into the EP, respectively. These mixtures were ultrasonic dispersed for 15 min to obtain the well-dispersed matrix. The matrixes were then stirred under 40 °C until the pre-polymer completely dissolved, and the pre-polymer was poured into a pre-heated mould with a release agent and degassed at 50 °C for about 30 min in a vacuum drying oven. Finally, the mould with pre-polymer was put into a blast drying oven, and the cured process is 80 °C/1 h + 100 °C/1 h + 120 °C/2 h.

The X-ray diffraction (XRD)
The X-ray diffraction (Bruker D8, Germany) was chosen to research the crystal structure of S-MoS 2 at room temperature. The acceleration voltage was 30 kV, current was 20 mA, and scanning speed was 8/min.

Scanning electron microscopy (SEM)
The S-3400 NII scanning electron microscope (HITACHI, Japan, high vacuum mode) was chosen to observe the morphology of S-MoS 2 , and the surface morphology of the fractured surface of the EP and S-MoS 2 /EP samples at room temperature. The acceleration voltage was 5 kV; probe current was 145 μA.

Dielectric properties
The dielectric properties of EP and S-MoS 2 /EP samples were measured by dielectric constant dielectric loss tester (ZJD-A type, China Aviation Times Company).

Thermal conductivity
The thermal conductivity was measured by transient fast hot wire method of thermal conductivity tester (KDRX-II, Xiangtan Xiangyi Instrument Co., Ltd, China)

Thermal resistant properties
The thermogravimetric analysis (TGA) was determined by a TGAQ50 in a nitrogen atmosphere, the heating rate was 20 °C min −1 , the nitrogen flow rate was 150 mL/ min, and the sample volume was 155 mg.

Elemental analysis (EDX)
The elemental analysis (EDX, American EDAX Company, Model Genesis Apollo XP) was chosen to determine the elements of S-MoS 2 ; the acceleration voltage was 30 kV.

Morphology structure of the S-MoS 2
The crystallographic structure of S-MoS 2 was studied by X-ray diffraction (XRD); the results are shown in Fig. 2. As can be observed from  (110), respectively. These results confirmed that the S-MoS 2 has been formed in the system. However, the diffraction peak is not very smooth, and also exists some of messy peaks, indicating that the crystal state of S-MoS 2 is not very uniform. This phenomenon is due to the inhomogeneous structure of the synthesized S-MoS 2 system and the existence of a few impurities in the system.
To further research the morphology of S-MoS 2 , and scanning electron microscopy (SEM) and elemental analysis (EDX) was used and the result is shown as Fig. 3. As can be seen from Fig. 3A, the particle size of the powders is relatively uniform and presents the fluffy state, but there is still a small amount of S-MoS 2 agglomeration, which is caused by the hydroxyl group on the surface of the S-MoS 2 prepared by hydrothermal method. In addition, it can be observed that most of the Fig. 2 The XRD result of S-MoS 2 particles show a relatively obvious spherical closed structure, indicating that the prepared S-MoS 2 is spherical structure. The remaining small amounts of particles with smaller particle size are carrier TiO 2 . In order to further observe the apparent morphology of the prepared S-MoS 2 , Fig. 3B shows the high-magnification SEM of Fig. 3A. As can be seen from Fig. 3B, S-MoS 2 has formed a good closed spherical structure, and its surface is relatively smooth, without load impurities. For Fig. 3C, the elemental analysis (EDX) was applied to analyse the constituent elements of the S-MoS 2 . It can be observed that except for molybdenum and sulfur, no other elements were detected. The peak area calculation shows that the atomic ratio of Mo to S element contained in the S-MoS 2 is 1:1.69, which is close to the atomic ratio of MoS 2 , within the allowable range of error. The above conclusions indicate that the prepared S-MoS 2 has a smooth and relatively uniform spherical structure.

Dielectric properties of the materials
The dielectric constant of S-MoS 2 /EP composites with different content of S-MoS 2 . As can be seen from Fig. 4, with the increase of S-MoS 2 content, the dielectric constants of S-MoS 2 /EP composites exhibit the increasing trend. This is because as a metal sulfide, the dielectric constant of MoS 2 itself is higher than that of pure EP matrix, meanwhile, the addition of S-MoS 2 can affect the transport ability of the inner sub-chain segments of cyclic oxygen lipid, and thus the polarization is easier to establish, leading to the increase of dielectric constant. In addition, it can also be observed that all of these S-MoS 2 /EP composites with different S-MoS 2 loading do not show obvious frequency dependence, indicating that that the polarization of the interface between S-MoS 2 and EP matrix can always keep up with the change rate of the applied electric field, which can be attributed to the unique spherical structure of S-MoS 2 and its good dispersion inside the EP matrix. Furthermore, although the dielectric constant of S-MoS 2 /EP composites have been improved to a certain extent, they still remain in a relative low range, which is owning to the independent state of S-MoS 2 with each other in the resin matrix, thus they do not form a leaky conductive path to become conductive materials. The relatively low dielectric constant of S-MoS 2 /EP composites can greatly meet the needs of their use in high frequency insulating electronic material.
By observing the curves of the dielectric loss of S-MoS 2 /EP composites changing with frequency (Fig. 5), it can be concluded that the dielectric loss of S-MoS 2 /EP composites has been greatly improved after the addition of S-MoS 2 . This is because Fig. 4 The S-MoS 2 /EP with different content of S-MoS 2 varies by frequency Fig. 5 The dielectric loss of S-MoS 2 /EP with different content of S-MoS 2 varies with frequency the interface polarization of S-MoS 2 inside the EP resin increases their dielectric loss. In addition, the dielectric loss of both EP and S-MoS 2 /EP composites increases with the increase of frequency, which is mainly because with the increase of frequency, the movement of polar molecules inside the S-MoS 2 /EP composites gradually accelerates, and the heat generated by the friction between molecular chains increases. The interfacial polarization and internal molecular polarization also increase, which leads to the increase of dielectric loss of S-MoS 2 /EP composites.

Thermal conductivities of the materials
As shown in Fig. 6, the influence of S-MoS 2 content on the thermal conductivity of S-MoS 2 /EP composites is studied. As can be seen from the figure, the thermal conductivity of S-MoS 2 /EP composites increases with the increase of the content of S-MoS 2 . When the content of S-MoS 2 is 3.0 wt%, the thermal conductivity reaches the maximum of 0.3061 W/mk, which is 80.05% higher than that of pure EP resin (0.1700 W/mk). These results suggest that the thermal conductivity of EP can be significantly improved by adding S-MoS 2 appropriately. The enhancement of thermal conductivity of S-MoS 2 /EP composites can be attributed to the excellent thermal conductivity of S-MoS 2 itself. However, when the content of S-MoS 2 continues to increase, the thermal conductivity of the S-MoS 2 /EP composites does not further increase, and even slightly decreases. This is because when the content of S-MoS 2 is appropriate, it can be uniformly dispersed inside the EP resin, and its spherical surface can also provide a larger surface area for heat transfer; thus the thermal conductivity of the composite material increases. Whereas when the content of S-MoS 2 is too large, they are easy to agglomerate in the EP matrix and form a few voids, so their performance of thermal conductivity is greatly affected.
In order to further investigate the reasons for the improvement of S-MoS 2 on thermal conductivity of S-MoS 2 /EP composites, the fracture morphology of EP resin and S-MoS 2 /EP composites filled with different content S-MoS 2 was observed by SEM. As can be seen from Fig. 7, the fracture surface of EP is relatively smooth and there is no particulate matter, indicating that its heat conduction mainly depends Fig. 6 The thermal conductivities of S-MoS 2 /EP with different content of S-MoS 2 on the crosslinking structure of epoxy resin. But for the S-MoS 2 /EP composite, their fracture surfaces are very rough, with obvious bulges and some granular materials, which proves the existence of S-MoS 2 in the EP matrix. In addition, it can also be observed that when the content of S-MoS 2 is less than 4.0 wt% (as shown in Fig. 7B-D), more small bumps and obvious granular solids can be seen on the surface of the material, indicating that S-MoS 2 can be evenly dispersed in the material, which is very beneficial to the thermal conductivity of the material. The relatively uniform distribution of these S-MoS 2 provides a good path for heat conduction (the thermal conductivity mechanism of EP and S-MoS 2 /EP composites is shown in Fig. 8). At that time, when the content of S-MoS 2 continued to increase, large particulate matter began to appear on the surface (Fig. 7E), and when its content increased to 5.0 wt%, a few pores could be seen (Fig. 7F). This indicates that excessive content of S-MoS 2 will cause partial agglomeration of the fillers, which is of no significance to the improvement of their thermal conductivity. These conclusions correspond to the above results for thermal conductivity.

Thermal resistant of the materials
The thermogravimetric analysis (TGA) results of pure EP resin and S-MoS 2 /EP composite resin with different content of S-MoS 2 content are shown in Fig. 9. It can be seen from the figure that the initial decomposition temperature of the S-MoS 2 /EP composite with different content of S-MoS 2 (1.0 wt, 2.0 wt%, 3.0 wt%, 4.0 wt% and 5.0 wt%) is 302 °C, 349 °C, 302 °C, 312 °C and 335 °C, respectively, which are not different from that of pure EP resin (325 °C), indicating that the addition of S-MoS 2 can not affect the curing process of EP resin. At 800 °C, the carbon residual rate of the S-MoS 2 /EP composites with different content of S-MoS 2 is 5.52%, 12.15%, 20.53%, 5.85% and 5.85%. This phenomenon indicates that when S-MoS 2 is added too much or too little, it has little effect on the thermal properties of the EP, but when the amount of S-MoS 2 added is reasonable (2.0 wt%, 3.0 wt%), the amount of carbon residue is increased to a large extent. Especially when the content is 3.0 wt%, it is 271% higher than that of pure EP resin (5.85%). This high carbon residual rate indicates that the composite has higher thermal stability after the reasonable addition of S-MoS 2 . This phenomenon can be attributed to the high heat resistance of S-MoS 2 itself and the spherical structure of S-MoS 2 which allows heat to transfer well within the EP resin. High heat resistance is also very beneficial to the improvement of its thermal conductivity, which can effectively reduce the damage of external heat to the material when it is subjected to heat.
The reciprocal TG results (DTG) of the above pure EP resin and the S-MoS 2 / EP composite containing 3.0 wt% S-MoS 2 were calculated respectively, and the results are shown in Fig. 10. As can be seen from the figure, the trends of the two curves are similar, and there is only one obvious thermal decomposition peak in both curves. These results indicate that the addition of S-MoS 2 does not significantly change the thermal decomposition mechanism of EP resin, and there is Fig. 9 The TGA curve of the EP and S-MoS 2 /EP composites with 3.0 wt% S-MoS 2 only one main thermal decomposition reaction. In addition, the thermal decomposition temperature of EP is 383 °C, while that of the S-MoS 2 /EP composite is 318 °C, indicating that S-MoS 2 can reduce the thermal decomposition temperature while improving the thermal resistance of the S-MoS 2 /EP composite.

Conclusion
A kind of molybdenum disulfide with microspheres structure (S-MoS 2 ) was prepared via surfactant promoting hydrothermal process, and the S-MoS 2 was then used as modifiers to add into EP matrix to prepare a new kind of S-MoS 2 /EP composites. The results show that the dielectric constant of the S-MoS 2 /EP composite increases and the dielectric loss can be kept at a low level after adding appropriate S-MoS 2 . In addition, when the amount of S-MoS 2 is 3.0 wt%, the thermal conductivity reaches the maximum of 0.3061 W/mk, which is 80.05% higher than that of pure EP resin (0.1700 W/mk), and their thermal resistant is also increased. The enhancement of thermal conductivity on S-MoS 2 /EP composites can be attributed to the high thermal conductivity of S-MoS 2 , as well as its spherical structure to facilitate heat transfer, and the high heat resistance of S-MoS 2 /EP composites is also very beneficial to the improvement of its thermal conductivity.