Performance of Silicone Rubber Composites Filled with Aluminum Nitride and Alumina Tri-Hydrate

materials

Article Performance of Silicone Rubber Composites Filled with Aluminum Nitride and Alumina Tri-Hydrate

Jianjun Zheng 1, Shaojian He 2 , Jiaqi Wang 2, Wenxuan Fang 1, Yang Xue 3,*, Liming Xie 1 and Jun Lin 2,*

1 Inner Mongolia Electric Power Science & Research Institute, Hohhot 010020, China; [email protected] (J.Z.); [email protected] (W.F.); [email protected] (L.X.) 2 School of Renewable Energy, North China Electric Power University, Beijing 102206, China; [email protected] (S.H.); [email protected] (J.W.) 3 State Key Laboratory of Multi-Phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China * Correspondence: [email protected] (Y.X.); [email protected] (J.L.)

Received: 8 May 2020; Accepted: 28 May 2020; Published: 29 May 2020

Abstract: In this study, silicone rubber (SR) composites were prepared with various amounts of aluminum nitride (AlN) and alumina tri-hydrate (ATH), and vinyl tri-methoxysilane (VTMS) was also introduced to prepare SR/ATH/AlN–VTMS composites for comparison. Compared to the SR/ATH composites, the SR/ATH/AlN composites with higher AlN loading exhibited higher breakdown strength and thermal conductivity, which were further improved by the addition of VTMS. Such results were related to the enhanced rubber–ﬁller interfacial interactions from VTMS coupling, as demonstrated by scanning electron microscopy (SEM) analysis and the curing behaviors of the SR composites. Moreover, by replacing ATH with VTMS-coupled AlN, the SR/ATH/AlN–VTMS composites also exhibited lower dielectric loss along with an increased dielectric constant, suggesting the promising application of VTMS-coupled AlN as a ﬁller for the preparation of the SR composites as high-voltage insulators.

Keywords: silicone rubber; aluminum nitride; alumina tri-hydrate; vinyl tri-methoxysilane; dielectric loss; breakdown strength

1. Introduction High-temperature-vulcanized silicone rubber (SR) is widely used to manufacture high voltage insulators due to its relatively light weight and excellent anti-contamination ability [1–4]. It is often filled with suitable additives so that the performance of the SR composites can be improved [5–11]. To date, alumina tri-hydrate (ATH) has been one of the most commonly used fillers in SR composites, which can endow the composites with good flame retardancy and erosion resistance. However, under arc discharging, the evaporation of the crystal water from ATH at over 220 ◦C can increase the roughness of the composite surface, which would be more inclined to be wet-out, and result in dry band arcing [12–14]. Therefore, alternative filler materials including boron nitride (BN), mica, aluminum oxide, etc. have been sought to replace ATH in high voltage SR composite insulators [15–19]. By incorporating 7 wt.% thermally conductive BN particles in the SR composites, Du et al. [20] found an improvement in the thermal dissipation and a decrease in both erosion depth and weight loss, indicating improved electrical erosion resistance. By the addition of mica modified by polydopamine and silane coupling agents, Liao et al. [16] reported SR composites with ~43% higher breakdown strength as compared to that of the ATH-filled SR composites. Aluminum nitride (AlN) is a thermally conductive material with a combination of good electrical insulation ability and low dielectric loss,

Materials 2020, 13, 2489; doi:10.3390/ma13112489 www.mdpi.com/journal/materials Materials 2020, 13, 2489 2 of 10 which is desired for applications in high voltage insulation. However, to the best of our knowledge, the use of AlN in high-voltage SR composites has rarely been reported, even though AlN has been used to prepare various polymer composites [21–25]. Therefore, it would be of great importance to find out whether AlN would be a promising filler to replace ATH for the preparation of high-voltage insulating SR composites. In this work, SR composites with various amount of AlN and ATH were prepared. Since the filler–rubber interfacial compatibility significantly influences the performance of the composites, a commonly used silane coupling agent, vinyl tri-methoxysilane (VTMS), was also introduced, with the intention to strengthen the rubber–filler interfacial interactions. To understand the structure–property relationship in these SR composites, the microstructure, thermal properties, dielectric properties and mechanical performance were investigated.

2. Experimental

2.1. Materials SR (MVQ110, vinyl weight percentage: 0.23%, molecular weight: 680,000 g/mol) was provided by Wynca Chemical Group Co., Ltd. (Xin’an, China). AlN (ZH-AlN010, ~10 µm, 99.0%) was purchased from Hefei Zhonghang Nanometer Technology Development Co., Ltd. (Hefei, China). ATH (2~5 µm, 97.0%) was bought from Zhongke Flame-retardant New Material Co., Ltd. (Hefei, China). VTMS was purchased from Chenguang Chemical Industry Co., Ltd. (Qufu, China). 2,5-bis(tert-butylperoxy)-2,5-dimethyl hexane (DBPMH) was purchased from Meixing Chemical Industry Co., Ltd. (Laiwu, China). All the above-mentioned materials were used as received.

2.2. Sample Preparation Preparation of SR composites: Filler (including ATH and AlN), curing agent and silane coupling agent were mixed with pure SR using a 6 inch two-roll mill to prepare SR compounds, which were then placed in a standard mold and vulcanized using a XLB-D 350 hydraulic hot press (Dongfang Machinery, Co., Ltd., Huzhou, China) at 170 ◦C and 15 MPa for 10 min to obtain SR composites. The recipe for the SR composite preparation was as follows (parts by weight): SR, 100; ﬁller, 150; DBPMH, 0.5; VTMS, 0 or 1.0. The as-prepared SR composites were named SR/ATHx/AlNy and SR/ATHx/AlNy–VTMS, respectively, where x and y refer to the amounts of the parts of ATH and AlN in the composites, respectively.

2.3. Sample Characterization The curing behaviors of the SR composites were recorded by moving a die rheometer (MDR, D&G Measure Instrument Co. Ltd., Shanghai, China) at 170 ◦C by following ASTM D5289-2017. A TG-Q500 analyzer (TA Instruments, New Castle, DE, USA) was used to perform thermogravimetric analysis 1 (TGA) with a heating rate of 20 ◦C min− under a nitrogen atmosphere. A SU8010 scanning electron microscopy (SEM) (Hitachi Co. Ltd., Tokyo, Japan) was utilized to observe the morphologies of the composites after the tensile tests at an accelerating voltage of 5 kV. A 6500B impedance analyzer (Wayne Kerr Electronics, Shenzhen, China) was used to detect the dielectric constant and dielectric loss. An HCDJC-50kV dielectric strength tester (Beijing Huace Testing Instrument Co. Ltd., Beijing, China) was used to measure the breakdown strength with a stepping voltage of 1 kV/s. To obtain the thermal conductivity (κ) of the composites, the specific heat (Cp), density (ρ) and thermal diffusivity (α) were evaluated by differential scanning calorimetry (DSC, NETZSCH, Selb, Germany) (with a heating rate of 10 C min 1 under a nitrogen atmosphere), a density tester (MH-300A, MatsuHaku, Xiamen, China) ◦ · − and laser flash apparatus (LFA 427, NETZSCH, Selb, Germany), respectively, and then calculated by the equation κ = α Cp ρ. An AI-7000S1 electrical tensile tester (Goodtechwill Testing Machines, Co. Ltd., × × Qingdao, China) was used to record the stress–strain behaviors with a tensile speed of 500 mm min 1 · − following ASTM D412-2016. All measurements were performed at 25 ◦C unless mentioned otherwise. MaterialsMaterials2020 2020, 13, ,13 2489, x FOR PEER REVIEW 3 of3 10of 11

3. Results and Discussion 3. Results and Discussion

3.1.3.1. Curing Curing Behavior Behavior FigureFigure1 shows 1 shows the the curing curing curves curves of of the the SR SR composites composites with with variousvarious ATHATH andand AlNAlN contents, in inwhich which the the torque reaches a plateauplateau atat aa maximummaximum valuevalue duringduring the the final final stage stage of of the the curing curing process. process. InIn general, general, the the torque torque is is determined determined byby thethe interactions in in the the rubber rubber nanocomposites, nanocomposites, including including the therubber–rubber, rubber–rubber, filler–rubber filler–rubber and and filler–filler filler–filler interactionsinteractions [[26].26]. ForFor thethe SR SR/ATH/AlN/ATH/AlN composites composites (Figure(Figure1a), 1a), the the maximum maximum torque torque decreases decreases with with an an increasing increasing AlN AlN content. content. Such Such a decrease a decrease suggests suggests thatthat the the replacement replacement of of ATH ATH with with AlN AlN could could weaken weaken the the interactions interactions in in the the SR SR composites. composites. On On the the oneone hand, hand, the the same same amount amount of of curing curing agent agent was was added added in in these these composites, composites, which which should should result result in in similarsimilar crosslinking crosslinking density, density, and and thus thus the the rubber–rubber rubber–rubber interactions interactions should should not not change change much much upon upon thethe addition addition of of AlN. AlN. On On the the other other hand, hand, as as compared compared to to the the ATH ATH particles, particles, the the AlN AlN particles particles are are larger larger andand have have smaller smaller surface surface areas, areas, which which should should cause cause fewer fewer filler–rubber filler–rubber and and filler–filler filler–filler interactions interactions whenwhen added added in in the the SR SR composites. composites. As As a result,a result, a lowera lower torque torque value value is is expected expected for for the the SR SR/ATH/AlN/ATH/AlN compositescomposites with with higher higher AlN AlN content. content. In In comparison, comparison, as as shown shown in in Figure Figure1b, 1b, the the SR SR/ATH/AlN–VTMS/ATH/AlN–VTMS compositescomposites exhibit exhibit a highera higher torque torque value value than than the th correspondinge corresponding SR SR/ATH/AlN/ATH/AlN composites composites with with the the samesame AlN AlN content, content, suggesting suggesting that that the the incorporation incorporatio ofn of the the silane silane coupling coupling agent agent VTMS VTMS strengthens strengthens thethe interactions interactions in in the the rubber rubber composites. composites.

FigureFigure 1. 1. CuringCuring curves curves of ofthe the (a) (siliconea) silicone rubber rubber (SR)/alumina (SR)/alumina tri-hydrate tri-hydrate (ATH)/AlN (ATH) and/AlN (b) andSR/ATH/AlN–vinyl (b) SR/ATH/AlN–vinyl tri-methoxysilane tri-methoxysilane (VTMS) (VTMS) composites. composites. During the sample preparation, VTMS was grafted onto the filler particles (ATH and AlN) via the During the sample preparation, VTMS was grafted onto the filler particles (ATH and AlN) via condensation reaction between the hydroxyl groups on the fillers and the alkoxyl groups on VTMS the condensation reaction between the hydroxyl groups on the fillers and the alkoxyl groups on (Figure2, Reaction I). During the vulcanization process, DBPMH decomposed and produced free VTMS (Figure 2, Reaction I). During the vulcanization process, DBPMH decomposed and produced radicals, initiating the radical addition reactions between the vinyl groups on both SR and VTMS free radicals, initiating the radical addition reactions between the vinyl groups on both SR and VTMS (Figure2, Reaction II). Therefore, the VTMS-grafted filler particles were covalently connected to the (Figure 2, Reaction II). Therefore, the VTMS-grafted filler particles were covalently connected to the SR macromolecular chains, leading to the stronger filler–rubber interactions and higher crosslinking SR macromolecular chains, leading to the stronger filler–rubber interactions and higher crosslinking density of the rubber network. density of the rubber network. 3.2. TGA Analysis The TGA curves of the ATH, AlN and SR composites are shown in Figure3. For the ATH powder, a weight loss of ~34 wt.% starting from ~220 ◦C can be observed, which is due to the dehydration of the ATH [27]. As for the AlN powder, no weight loss is found in the TGA curve below 650 ◦C, indicating the excellent thermal stability of AlN. For the SR/ATH150 composite, two steps of weight loss are observed: the first one starting from ~220 ◦C with ~18 wt.% weight loss is mainly ascribed to the dehydration of ATH, and the second one starting from ~360 ◦C with ~36 wt.% weight loss results from the decomposition of the SR matrix. The SR/ATH60/AlN90 composite also exhibits a two-stage thermal degradation behavior, with a smaller weight loss between 220 and 360 ◦C due to the

Materials 2020, 13, 2489 4 of 10 lower ATH loading in the composite than that in the SR/ATH150 composite. By comparison, for the SR/AlN150 composite, there is only one weight loss of ~40 wt.% between 360 and 500 ◦C, which results from the degradation of the SR, since the weight percentage of SR in the composite is right at 40 wt.%. Therefore, the thermal stability of the SR composites could be enhanced through the ﬁller replacement of ATH by AlN. It is also found that the TGA curves of the SR/ATH/AlN–VTMS composites are very close to those of the corresponding SR/ATH/AlN composites, suggesting that the addition of VTMS does notMaterials change 2020, 13 the, x FOR stability PEER REVIEW of the SR composites. 4 of 11

Materials 2020, 13, x FOR PEER REVIEW 5 of 11 FigureFigure 2. 2.Formation Formation ofof thethe SR/BN/ATH–VTMS SR/BN/ATH–VTMS composite. composite.

3.2. TGA Analysis The TGA curves of the ATH, AlN and SR composites are shown in Figure 3. For the ATH powder, a weight loss of ~34 wt.% starting from ~220 °C can be observed, which is due to the dehydration of the ATH [27]. As for the AlN powder, no weight loss is found in the TGA curve below 650 °C, indicating the excellent thermal stability of AlN. For the SR/ATH150 composite, two steps of weight loss are observed: the first one starting from ~220 °C with ~18 wt.% weight loss is mainly ascribed to the dehydration of ATH, and the second one starting from ~360 °C with ~36 wt.% weight loss results from the decomposition of the SR matrix. The SR/ATH60/AlN90 composite also exhibits a two-stage thermal degradation behavior, with a smaller weight loss between 220 and 360 °C due to the lower ATH loading in the composite than that in the SR/ATH150 composite. By comparison, for the SR/AlN150 composite, there is only one weight loss of ~40 wt.% between 360 and 500 °C, which results from the degradation of the SR, since the weight percentage of SR in the composite is right at 40 wt.%. Therefore, the thermal stability of the SR composites could be enhanced through the filler replacement of ATH by AlN. It is also found that the TGA curves of the SR/ATH/AlN–VTMS composites are very close to those of the corresponding SR/ATH/AlN composites, suggesting that the addition of VTMS does not change the stability of the SR composites.

Figure 3.FigureThermogravimetric 3. Thermogravimetric analysis analysis (TGA) (TGA) curves of ofthe the AlN, AlN, ATHATH and SR and composites. SR composites.

3.3. SEM Observation3.3. SEM Observation The tensileThe fracturetensile fracture surface surface morphologies morphologies ofof the the SR SR composites composites with withvarious various ATH and ATH AlN and AlN contents werecontents observed were observed by SEM. by SEM. Both Both ATH ATH and and AlN AlN ar aree spherical spherical particles, particles, so the sodifference the di betweenfference between them is the size of the particles. For the SR/ATH/AlN composites shown in Figure 4a–f, there exist them is thesome size filler of the particle particles. agglomerations For the exposed SR/ATH on/AlN the te compositesnsile fracture surfaces. shown inDu Figuree to the 4largera–f, thereparticle exist some filler particlesize agglomerationsof AlN than that of exposed ATH, the on size the of tensile the agglomerations fracture surfaces. increases Due with tothe the AlN larger content. particle In size of AlN than thataddition, of ATH, somethe voids size are of also the found agglomerations in the composites, increases demonstrating with the relatively AlN content. weak interfacial In addition, some voids are alsoadhesion found between in the the composites, fillers and rubber demonstrating matrix. Meanwhile, relatively for the weak SR/ATH/AlN–VTMS interfacial adhesion composites between the shown in Figure 4g–l, there exist fewer agglomerations and voids, demonstrating that the fillers andincorporation rubber matrix. of VTMS Meanwhile, reduces the for filler the agglomerat SR/ATHions/AlN–VTMS and improves composites the interfacial shown interactions in Figure 4g–l, between the filler particles and SR matrix.

Figure 4. SEM images of the (a) SR/ATH150, (b) SR/ATH120/AlN30, (c) SR/ATH90/AlN60, (d) SR/ATH60/AlN90, (e) SR/ATH30/AlN120, (f) SR/AlN150, (g) SR/ATH150–VTMS, (h)

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Figure 3. Thermogravimetric analysis (TGA) curves of the AlN, ATH and SR composites.

3.3. SEM Observation The tensile fracture surface morphologies of the SR composites with various ATH and AlN contents were observed by SEM. Both ATH and AlN are spherical particles, so the difference between them is the size of the particles. For the SR/ATH/AlN composites shown in Figure 4a–f, there exist some filler particle agglomerations exposed on the tensile fracture surfaces. Due to the larger particle size of AlN than that of ATH, the size of the agglomerations increases with the AlN content. In Materials 2020, 13, 2489 5 of 10 addition, some voids are also found in the composites, demonstrating relatively weak interfacial adhesion between the fillers and rubber matrix. Meanwhile, for the SR/ATH/AlN–VTMS composites shownthere exist in fewerFigure agglomerations 4g–l, there exist and voids,fewer demonstratingagglomerations that and the voids, incorporation demonstrating of VTMS that reduces the incorporationthe ﬁller agglomerations of VTMS reduces and improves the filler the agglomerat interfacialions interactions and improves between the interfacial the ﬁller particles interactions and betweenSR matrix. the filler particles and SR matrix.

Figure 4. 4. SEMSEM images images of the of the(a) SR/ATH150, (a) SR/ATH150, (b) SR/ATH120/AlN30, (b) SR/ATH120/AlN30, (c) SR/ATH90/AlN60, (c) SR/ATH90/AlN60, (d) (SR/ATH60/AlN90,d) SR/ATH60/AlN90, (e) SR/ATH30/AlN120, (e) SR/ATH30/AlN120, (f) SR/AlN150, (f) SR/AlN150, (g) SR/ATH150–VTMS, (g) SR/ATH150–VTMS, (h) (h) SR/ATH120/AlN30–VTMS, (i) SR/ATH90/AlN60–VTMS, (j) SR/ATH60/AlN90–VTMS, (k) SR/ATH30/AlN120–VTMS and (l) SR/AlN150–VTMS composites.

3.4. Dielectric Properties The dielectric properties of the SR composites with various AlN contents as a function of frequency are shown in Figure5. Since the polarity of AlN is weaker than that of ATH, the replacement of ATH by AlN could reduce the interfacial polarization for the SR composite. As a result, both the dielectric constant and dielectric loss of the SR composites decrease with an increasing AlN content. As for the SR/ATH/AlN–VTMS composites, it is found that they exhibit a higher dielectric constant and lower dielectric loss than the corresponding SR/ATH/AlN composites containing the same amount of ATH/AlN. VTMS in situ-modiﬁed ATH and AlN are more compatible with the SR matrix, so both the interfacial polarization and space charge accumulation in the SR/ATH/AlN–VTMS composites should be reduced as compared to those in the SR/ATH/AlN composites. As a result, a lower dielectric loss is observed for the SR/ATH/AlN–VTMS composites. Meanwhile, the increase in the dielectric constant of the SR/ATH/AlN–VTMS composites could rationalized as follows: the introduction of VTMS not only strengthens the ﬁller–rubber interactions but also raises the composite crosslinking density, which results in lower free volume. Such a decrease in the free volume of the SR composites should lead to an increase in the dielectric constant, which has also been reported previously [28–30].

3.5. Breakdown Strength The breakdown strength of the SR/ATH/AlN composites with various ATH and AlN contents is shown in Figure6. It is found that the breakdown strength increases with an increasing AlN content for either the SR/ATH/AlN or SR/ATH/AlN–VTMS composites. The breakdown strength of the SR/AlN150 1 1 composite reaches 26.2 kV mm− , ~18% higher than that of the SR/ATH150 composite (22.2 kV mm− ). This should be due to the better insulating property of AlN and the lower interfacial polarization for the AlN-ﬁlled composites. With the introduction of VTMS, all the SR/ATH/AlN–VTMS composites show higher breakdown strength than the corresponding SR/ATH/AlN composites containing the same amount of AlN. Such a result could be related to the fact that there are fewer structural defects in Materials 2020, 13, x FOR PEER REVIEW 6 of 11

SR/ATH120/AlN30–VTMS, (i) SR/ATH90/AlN60–VTMS, (j) SR/ATH60/AlN90–VTMS, (k) SR/ATH30/AlN120–VTMS and (l) SR/AlN150–VTMS composites.

3.4. Dielectric Properties The dielectric properties of the SR composites with various AlN contents as a function of frequency are shown in Figure 5. Since the polarity of AlN is weaker than that of ATH, the replacement of ATH by AlN could reduce the interfacial polarization for the SR composite. As a result, both the dielectric constant and dielectric loss of the SR composites decrease with an increasing AlN content. As for the SR/ATH/AlN–VTMS composites, it is found that they exhibit a higher dielectric constant and lower dielectric loss than the corresponding SR/ATH/AlN composites containing the same amount of ATH/AlN. VTMS in situ-modified ATH and AlN are more compatible with the SR matrix, so both the interfacial polarization and space charge accumulation in the SR/ATH/AlN–VTMS composites should be reduced as compared to those in the SR/ATH/AlN composites. As a result, a lower dielectric loss is observed for the SR/ATH/AlN–VTMS composites. Meanwhile, the increase in the dielectric constant of the SR/ATH/AlN–VTMS composites could rationalized as follows: the introduction of VTMS not only strengthens the filler–rubber interactions Materials 2020, 13, 2489 6 of 10 but also raises the composite crosslinking density, which results in lower free volume. Such a decrease in the free volume of the SR composites should lead to an increase in the dielectric constant, whichthe SR /hasATH also/AlN–VTMS been reported composites previously because [28–30]. of the strengthened rubber–ﬁller interactions from VTMS incorporation, as demonstrated in the above-mentioned SEM results.

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kV mm−1). This should be due to the better insulating property of AlN and the lower interfacial polarization for the AlN-filled composites. With the introduction of VTMS, all the SR/ATH/AlN– VTMS composites show higher breakdown strength than the corresponding SR/ATH/AlN composites containing the same amount of AlN. Such a result could be related to the fact that there are fewer structural defects in the SR/ATH/AlN–VTMS composites because of the strengthened rubber–filler interactions from VTMS incorporation, as demonstrated in the above-mentioned SEM results. Figure 5. (a) Dielectric constant and (b) dielectric loss of SR composites. Figure 5. (a) Dielectric constant and (b) dielectric loss of SR composites.

Figure 6. BreakdownBreakdown strength strength of SR composites. 3.6. Thermal Conductivity 3.6. Thermal Conductivity The thermal conductivity of both the SR/ATH/AlN and SR/ATH/AlN–VTMS composites gradually increasesThe withthermal an increasingconductivity AlN of content both (Figurethe SR7/ATH/AlN), mainly because and SR/ATH/AlN–VTMS AlN has higher intrinsic composites thermal gradually increases with an1 increasing1 AlN content (Figure1 1 7), mainly because AlN has higher conductivity (~150 W m− K− ) than ATH (~30 W m− K− ). After all of the ATH particles are −1 −1 −1 −1 intrinsicreplaced bythermal AlN, theconductivity SR/AlN150 (~150 and theW SRm /AlN150–VTMS K ) than ATH composites (~30 W m exhibit K ). thermalAfter all conductivities of the ATH particles are replaced by AlN, the SR/AlN150 and the SR/AlN150–VTMS composites exhibit thermal conductivities of 0.275 W m−1 K−1 and 0.307 W m−1 K−1, respectively, 37.5% and 30.1% higher than those for the SR/ATH150 (0.200 W m−1 K−1) and SR/ATH150–VTMS (0.236 W m−1 K−1) composites, respectively. As expected, the introduction of VTMS further enhances the thermal transport properties of the composites, so the SR/ATH/AlN–VTMS composites always exhibit higher thermal conductivity than the corresponding SR/ATH/AlN composites with the same AlN content. As illustrated previously, the addition of VTMS facilitated the strengthening of the filler–rubber interfacial interactions in the SR/ATH/AlN–VTMS composites as compared to those in the SR/ATH/AlN composites, resulting in lower interfacial heat resistance [31]. As a result, in addition to the improved insulation properties, the incorporation of AlN particles along with VTMS into the SR composites could also significantly enhance the heat transfer to effectively dissipate the heat generated within the composites.

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1 1 1 1 of 0.275 W m− K− and 0.307 W m− K− , respectively, 37.5% and 30.1% higher than those for the 1 1 1 1 SR/ATH150 (0.200 W m− K− ) and SR/ATH150–VTMS (0.236 W m− K− ) composites, respectively. As expected, the introduction of VTMS further enhances the thermal transport properties of the composites, so the SR/ATH/AlN–VTMS composites always exhibit higher thermal conductivity than the corresponding SR/ATH/AlN composites with the same AlN content. As illustrated previously, the addition of VTMS facilitated the strengthening of the filler–rubber interfacial interactions in the SR/ATH/AlN–VTMS composites as compared to those in the SR/ATH/AlN composites, resulting in lower interfacial heat resistance [31]. As a result, in addition to the improved insulation properties, the incorporation of AlN particles along with VTMS into the SR composites could also significantly enhanceMaterials 2020 the, 13 heat, x FOR transfer PEER REVIEW to effectively dissipate the heat generated within the composites. 8 of 11

Figure 7. Thermal conductivity of SR composites. Figure 7. Thermal conductivity of SR composites. 3.7. Mechanical Properties 3.7. Mechanical Properties The tensile strength, elongation at break and Shore A hardness of the SR composites as a function of theThe AlN tensile content strength, are summarized elongation in at Figurebreak 8and. As Shor demonstratede A hardness from of the the SR curing composites behaviors as a offunction these SRof the composites, AlN content the filler–fillerare summarized and filler–rubber in Figure 8. interactionsAs demonstrated decrease from with the an curing increasing behaviors AlN content,of these leadingSR composites, to smaller the valuesfiller–filler of Shore and filler–rubber A hardness interactions when the loading decrease of with AlN an is higher.increasing Besides, AlN content, due to theleading larger to particlesmaller sizevalues and of smaller Shore A surface hardness area when of AlN, the theloading rubber of chainsAlN is arehigher. less Besides, restrained due by to AlN the thanlarger ATH. particle Therefore, size and thesmaller composites surface witharea of higher AlN, the AlN rubber content chains exhibit are lowerless restrained tensile strength by AlN than and elongationATH. Therefore, at break. the After composites the incorporation with higher of VTMS, AlN the content SR/ATH exhibit/AlN–VTMS lower composites tensile strength are found and to exhibitelongation higher at break. values After for tensile the incorporation strength, elongation of VTMS, at breakthe SR/ATH/AlN–VTMS and Shore A hardness composites as compared are found to the SRto /exhibitATH/AlN higher composites values for with tensile the same strength, AlN content,elongation which at break is mainly and Shore ascribed A hardness to the increase as compared in both rubber–fillerto the SR/ATH/AlN interfacial composites interactions with and the composite same AlN crosslinking content, which density is mainly facilitated ascribed by VTMS. to the According increase toin JBboth/T 10945-2010, rubber–filler the interfacial SR materials interactions for composite and composite insulators shouldcrosslinking achieve density a tensile facilitated strength by of VTMS. 3 MPa andAccording an elongation to JB/T at10945-2010, break of 100%. the SR Therefore, materials the fo tensiler composite strength insulators of the AlN-filled should achieve SR composites a tensile needsstrength to beof 3 improved MPa and furtheran elongation to satisfy at break applications of 100% in. Therefore, the high-voltage the tensile insulation strength field. of the AlN-filled SR composites needs to be improved further to satisfy applications in the high-voltage insulation field.

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Figure 8. Tensile strength, elongation at break andand Shore AA hardnesshardness ofof thethe SRSR composites.composites. 4. Conclusions 4. Conclusions In this work, SR composites with various AlN and ATH content were used to prepare SR/ATH/AlN composites,In this andwork, VTMS SR was composites also introduced with various to prepare AlN SR /ATHand /AlN–VTMSATH content composites were used for comparison.to prepare UponSR/ATH/AlN replacing composites, ATH with AlN and as theVTMS filler, was the SRalso composites introduced demonstrated to prepare improvements SR/ATH/AlN–VTMS in thermal composites for comparison. Upon replacing ATH with AlN as the filler, the SR composites stability, breakdown strength and thermal conductivity, while the mechanical properties deteriorated. demonstrated improvements in thermal stability, breakdown strength and thermal conductivity, In addition, higher AlN loading resulted in a reduced dielectric constant and dielectric loss for the while the mechanical properties deteriorated. In addition, higher AlN loading resulted in a reduced SR composites. With the introduction of VTMS, the SR composites exhibited an enhancement of dielectric constant and dielectric loss for the SR composites. With the introduction of VTMS, the SR rubber–filler interfacial interactions, as demonstrated by the curing behaviors and SEM morphology composites exhibited an enhancement of rubber–filler interfacial interactions, as demonstrated by the analysis. This led to an improvement of the performance of the SR composites, including higher curing behaviors and SEM morphology analysis. This led to an improvement of the performance of tensile strength and Shore A hardness. Furthermore, the dielectric constants of the SR composites the SR composites, including higher tensile strength and Shore A hardness. Furthermore, the were increased along with a reduction in the dielectric loss after VTMS was added, which could be dielectric constants of the SR composites were increased along with a reduction in the dielectric loss related to the lower free volume and lower space-charge accumulation at the filler–matrix interface. after VTMS was added, which could be related to the lower free volume and lower space-charge The breakdown strength and thermal conductivity of SR composites were also further improved by accumulation at the filler–matrix interface. The breakdown strength and thermal conductivity of SR the addition of VTMS. Therefore, our work has demonstrated that AlN coupled with VTMS could composites were also further improved by the addition of VTMS. Therefore, our work has be very promising for the preparation of the SR composites to be applied in the field of high voltage demonstrated that AlN coupled with VTMS could be very promising for the preparation of the SR insulation as long as we overcome the issue of the decrease in mechanical performance. composites to be applied in the field of high voltage insulation as long as we overcome the issue of Authorthe decrease Contributions: in mechanicalData curation, performance. J.Z., S.H., J.W., W.F.and L.X.; Funding acquisition, S.H. and J.L.; Investigation, J.Z. and S.H.; Methodology, J.Z.; Resources, Y.X. and J.L.; Supervision, Y.X. and J.L.; Writing–original draft, J.Z. and S.H.; Writing–review & editing, Y.X. and J.L. All authors have read and agreed to the published version of Author Contributions: Data curation, J.Z., S.H., J.W., W.F. and L.X.; Funding acquisition, S.H. and J.L.; the manuscript. Investigation, J.Z. and S.H.; Methodology, J.Z.; Resources, Y.X. and J.L.; Supervision, Y.X. and J.L.; Writing – Funding: This research was financially supported by the National Natural Science Foundation of China, China original draft, J.Z. and S.H.; Writing – review & editing, Y.X. and J.L. All authors have read and agreed to the (grant No. 51973057), the Science and technology project of Inner Mongolia Electric Power Corporation, China (grantpublished No. 2019-37),version of and the the manuscript. Fundamental Research Funds for the Central Universities, China (grant No. 2016YQ08). ConflictsFunding: ofThis Interest: researchThe was authors financially declare supported no conflicts by ofthe interest. 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