Ceramization Mechanism of Ceramizable Silicone Rubber Composites with Nano Silica at Low Temperature

materials

Article Ceramization Mechanism of Ceramizable Silicone Rubber Composites with Nano Silica at Low Temperature

Penghu Li 1 , Haiyun Jin 1,*, Shichao Wei 1, Huaidong Liu 1, Naikui Gao 1 and Zhongqi Shi 2 1 State Key Laboratory of Electrical Insulation and Power Equipment, Xi’an Jiaotong University, Xi’an 710049, China; [email protected] (P.L.); [email protected] (S.W.); [email protected] (H.L.); [email protected] (N.G.) 2 State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, China; [email protected] * Correspondence: [email protected]

Received: 22 July 2020; Accepted: 19 August 2020; Published: 21 August 2020

Abstract: Ceramizable composite is a kind of polymer matrix composite that can turn into ceramic material at a high temperature. It can be used for the ceramic insulation of a metal conductor because of its processability. However, poor low-temperature ceramization performance is a problem of ceramizable composites. In this paper, ceramizable composites were prepared by using silicone rubber as a matrix. Ceramic samples were sintered at different temperatures no more than 1000 ◦C, according to thermogravimetric analysis results of the composites. The linear contraction and flexural strength of the ceramics were measured. The microstructure and crystalline phase of ceramics were analyzed using scanning electron microscope (SEM) and X-ray diffraction (XRD). The results show that the composites turned into ceramics at 800 ◦C, and a new crystal and continuous microstructure formed in the samples. The flexural strength of ceramics was 46.76 MPa, which was more than twice that of similar materials reported in other research sintered at 1000 ◦C. The maximum flexural strength was 54.56 MPa, when the sintering temperature was no more than 1000 ◦C. Moreover, glass frit and nano silica played important roles in the formation of the ceramic phase in this research. A proper content of nano silica could increase the strength of the ceramic samples.

Keywords: ceramizable composites; polymer matrix composites; nano silica; liquid-phase sintering; microstructure

1. Introduction A ceramizable silicone rubber composite is a new polymer matrix composite made of silicone rubber, inorganic filler, fluxing agent, reinforcing agent, and so on. The composite has the characteristics of silicone rubber and a good processability at room temperature, and can turn into ceramic with mechanical strength at a high temperature [1–5]. Nowadays, this material is widely used in fire-resistant cables. The composite will turn into ceramic at a high temperature, prevent the spread of flames, and keep cables working normally [6–10]. Because of its good processibility before sintering and lower sintering temperature than traditional ceramics, this material can also be used for the ceramic insulation of a metal conductor such as a bus bar, which is the target material in this paper. Ceramizable composites can be coated on the surface of a copper conductor and sintered to form a ceramic insulating layer, so that the bus bar can work in some specific environments. The application for the ceramic insulation of a metal conductor requires not only a good processability of the composites, but also a lower sintering temperature and a higher strength for the ceramic. Most conductors in power equipment are copper, which melts at 1083 ◦C, so the ceramic should be sintered at a low temperature

Materials 2020, 13, 3708; doi:10.3390/ma13173708 www.mdpi.com/journal/materials Materials 2020, 13, 3708 2 of 12

of no more than 1000 ◦C. In order to get ceramics with better properties, the temperature program must be designed and controlled accurately. There are many achievements in the research on the formulation and processing of ceramizable silicone rubber composites [11–23], but research on the ceramization process and mechanism is not enough. Hanu et al. [1–3] researched the ceramization mechanism of mica/silicone rubber composites, and put forward that ceramization was a process where the edge of mica melted and reacted with silica, which was the residue of silicone rubber, and the liquid phase connected the fillers and improved the residual strength. However, the temperature was too high when the reaction between mica and silica occurred, and no low-melting-point fluxing agent was used in these studies, so the temperature of the ceramization was over 1000 ◦C while the maximum flexural strength was only 8 MPa. Mansouri et al. [4] improved the low-temperature ceramization by adding glass frit into silicone rubber-based composites, and researched the ceramization mechanism further. It was reported that the glass frit was the key to reducing the sintering temperature, because the glass frit could melt to form a liquid phase and connect the mica and silica at a lower temperature. However, the explanation of the mechanism was still based on results at 1000 ◦C, and the mechanism at a lower temperature was not discussed. In recent years, a few researchers studied the ceramization process and mechanism at lower temperatures, and the flexural strength of the ceramics sintered at 1000 ◦C was improved to 20 MPa [24–27]. However, the ceramization performance is still poor, especially at a lower temperature, and the ceramization mechanism should be improved. Researching the ceramization process and mechanism can provide theoretical guidance for the formulation design of ceramizable composites, help to improve ceramization properties and reduce the sintering temperature. In the early research, the strength of ceramics increased obviously only if the sintering temperature reached 1000 ◦C. In this research, ceramizable composites with different contents of nano silica were prepared, and the ceramic samples were sintered at different temperatures of no more than 1000 ◦C. The ceramization process and mechanism below 1000 ◦C were discussed in detail. Low-temperature sintering at 800 ◦C was achieved and the strength of ceramics was improved. The low-temperature ceramization performance was improved considerably. What is more, the effect of nano silica on the ceramization was investigated.

2. Materials and Methods

2.1. Materials Ceramizable silicone rubber composites were prepared by methyl vinyl silicone rubber, kilchoanite, low-melting-point glass frit, nano silica, hydroxyl silicone oil and 2,4-dichlorobenzoyl peroxide (DCBP). Silicone rubber had good processibility as the matrix. Kilchoanite (Ca24Si16O56) was the main inorganic ﬁller for ceramization. Low-melting-point glass frit was the ﬂuxing agent, which could melt within the range of 400–500 ◦C to form a liquid phase. Nano silica could improve the mechanical properties of the silicone rubber composites, participate in the ceramization reaction, and improve the strength of ceramics. Hydroxyl silicone oil could soften the silicone rubber to improve the processability of the composites. 2,4-dichlorobenzoyl peroxide (DCBP) was the vulcanizing agent of the silicone rubber. The components of the low-melting-point glass frit were analyzed by XRF and are shown in Table1.

Table 1. The analyzed components of the low-melting-point glass frit.

Components ZnO SiO2 TiO2 P2O5 K2O CaO Others Content (wt.%) 43.58 27.64 10.86 7.56 7.38 2.34 0.64

Table2 presents the formulations of the ceramizable silicone rubber composites. All of the fillers and agents were added into the silicone rubber, in the order from left to right in Table2. S0–S5 were the samples with different contents of nano silica, so as to research the effect of nano silica on the ceramization. S6 was used to research the effect of the sintering temperature on the properties of the Materials 2020, 13, x FOR PEER REVIEW 3 of 11 ceramics. The content of the glass frit in S6 was increased properly based on S3, so that the density and strength of the ceramic samples could be improved [28].

Table 2. The formulations of the ceramizable silicone rubber composites (phr 1).

Hydroxyl Silicone Nano Samples Silicone Kilchoanite Glass Frit DCBP Materials 2020Rubber, 13, 3708 Silica 3 of 12 Oil S0 100 3 0 50 25 1.5 ceramics. The content of the glass frit in S6 was increased properly based on S3, so that the density and S1 strength of the100 ceramic samples3 could be improved 10 [28]. 50 25 1.5 S2 100 3 20 50 25 1.5 S3 100Table 2. The formulations3 of the ceramizable 30 silicone rubber50 composites (phr251). 1.5 S4 Samples Silicone100 Rubber Hydroxyl3 Silicone Oil 40 Nano Silica Kilchoanite50 Glass25 Frit DCBP 1.5 S5 S0100 1003 3 50 050 50 2525 1.5 1.5 S1 100 3 10 50 25 1.5 S6 S2100 1003 3 30 2050 50 2530 1.5 1.5 S3 1001 parts 3 per hundred of 30 rubber. 50 25 1.5 S4 100 3 40 50 25 1.5 S5 100 3 50 50 25 1.5 2.2. PreparationS6 of Ceramizable 100 Composites and 3 Ceramic Samples 30 50 30 1.5 1 parts per hundred of rubber. The materials were mixed by a two-roller internal mixer at 50 °C for a better mixture of fillers and silicone rubber,2.2. Preparation and the of Ceramizablespeed of Compositesrollers was and 30 Ceramic rpm. Samples If the temperature was too high, DCBP would decompose andThe materialsthe crosslink were mixedreaction by a would two-roller take internal place mixer in advance. at 50 ◦C for Then, a better the mixture samples of fillers were and molded and vulcanizedsilicone rubber,in a steel and mold the speed (100 of mm rollers × 100 was 30mm rpm. × 2 If mm) the temperature at 120 °C wasfor 10 too min, high, and DCBP degassed would in an oven at 150decompose °C for 4 and h, so the that crosslink the ceramizab reaction wouldle silicone take place rubber in advance. composites Then, the were samples prepared. were molded and vulcanized in a steel mold (100 mm 100 mm 2 mm) at 120 ◦C for 10 min, and degassed in an To get the ceramic samples, ceramizable× silicone× rubber composites were cut into strip-shaped oven at 150 ◦C for 4 h, so that the ceramizable silicone rubber composites were prepared. samples (80 mmTo get × the 10 ceramic mm × samples,2 mm), ceramizable buried by silicone Al2O3 rubberpowder composites and sintered were cut in into the strip-shaped furnace with the program samplesshown (80in Figure mm 10 1. mm The temperature2 mm), buried prog by Alram2O3 powderwas designed and sintered according in the furnace to the withdecomposition the × × process ofprogram the composites. shown in Figure The1 sintering. The temperature temperature program of was S0–S5 designed was according1000 °C (Step to the decomposition10, X °C in Figure 1). There wereprocess five ofdifferent the composites. sintering The temperature sintering temperature for S6: of600 S0–S5 °C, was700 1000°C, 800◦C (Step °C, 10,900 X °C,◦C inand Figure 10001). °C (Step There were five different sintering temperature for S6: 600 ◦C, 700 ◦C, 800 ◦C, 900 ◦C, and 1000 ◦C 10, X °C in Figure 1). (Step 10, X ◦C in Figure1).

FigureFigure 1. Temperature 1. Temperature program program for for the the sintering of of the the ceramic ceramic samples. samples. 2.3. Characterization of Ceramizable Composites and Ceramic Samples 2.3. Characterization of Ceramizable Composites and Ceramic Samples First of all, thermogravimetric analysis (TGA; TGA/SDTA851, METTLER TOLEDO, Zurich, FirstSwitzerland) of all, thermogravimetric was carried out in nitrogenanalysis with (TGA; a heating TGA/SDTA851, rate of 10 ◦C/min. METTLER A proper temperatureTOLEDO, Zurich, Switzerland)program was for carried the sintering out ofin the nitrogen ceramic sampleswith a was heating designed rate according of 10 to°C/min. the decomposition A proper process temperature of the composites, and the ceramic samples were sintered in the furnace. The linear contraction of the program ceramicfor the samplessintering was of calculated the ceramic by Equation samples (1): was designed according to the decomposition process of the composites, and the ceramic samples were sintered in the furnace. The linear contraction of the ceramic samples was calculated by Equation (1):

L = (L0 − L1)/L0 (1)

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Materials 2020, 13, x FOR PEER REVIEW L = (L L )/L 4 of(1) 11 0 − 1 0 where L is linear contraction (%), L0 isis original original length length of of the samples before sintering (mm), and L1 isis lengthlength of the samples after sintering (mm). The flexural flexural strength of the ceramic samples was measured with an electronic universal testing machine (CMT4503, MTS Industrial System, Shenzhen, China). The The length length of of the supporting span in three-pointthree-point bending test was 30 mm, and the speed of the applied load was 2 mm/min. mm/min. More details of thethe three-point three-point bending bending test test are are provided provided in in the the Appendix Appendix A.A The. The microstructure microstructure of offracture fracture surface surface for ceramicfor ceramic samples samples S6 S6sintered sintered at atdifferent different temper temperaturesatures was was observed observed with with a a scanning electron microscope (SEM; (SEM; VE-9800S, KEYENCE, KEYENCE, Osaka, Osaka, Japa Japan),n), in in order to investigateinvestigate the microstructuremicrostructure evolution during during ceramization. ceramization. Finally, Finally, the the X-ray X-ray diffr diactionffraction (XRD) (XRD) analysis analysis of the of ceramic the ceramic samples samples was carriedwas carried out on out a on diffractometer a diffractometer (D8 (D8 ADVANCE ADVANCE A2 A25,5, BRUKER, BRUKER, Karlsruhe, Karlsruhe, Germany). Germany). The The ceramic samples were were ground ground into into powder powder and and the the XRD XRD patterns patterns were were recorded recorded over over the the 2θ 2 θrangerange of of 10°–65°. 10–65◦ .

3. Results Results and and Discussion Discussion

3.1. Thermogravimetric Thermogravimetric Analys Analysisis of of Ceramizable Ceramizable Composites Figure 22 showsshows thethe resultsresults ofof thethe thermogravimetricthermogravimetric analysisanalysis forfor thethe ceramizableceramizable compositescomposites sample S6. The The composites composites without without glass glass frit frit and and the the composites composites without without kilchoanite kilchoanite were were prepared prepared based on the formulation of S6, so as toto investigateinvestigate thethe decompositiondecomposition process of of the the sample sample S6. S6. Thermogravimetric analysis of these two samples andand kilchoanite powder were also carried out and are shown in Figure 22.. ThereThere werewere twotwo massmass lossloss stagesstages inin S6S6 (Figure(Figure2 a).2a). TheThefirst first one one was was the the decomposition of of the the silicone silicone rubber rubber matrix. matrix. It wa Its waswithin within the temperature the temperature range rangeof 400–480 of 400–480 °C, which◦C, waswhich almost was almostthe same the temperature same temperature range rangeas the asmeltin the meltingg of the of glass the glassfrit (400–500 frit (400–500 °C). ◦Meanwhile,C). Meanwhile, the firstthe firstdecomposition decomposition process process of the of thesample sample withou withoutt the the glass glass frit frit mainly mainly took took place place within within the temperaturetemperature range range of of 500–560 500–560 °C.◦C. It indicated that the glass frit couldcould reducereduce thethe decompositiondecomposition temperaturetemperature of silicon rubber. Glass Glass frit frit melted melted at at fi firstrst as as the the temperature temperature rose rose up, up, and metal ions such such + 2+ as K + andand Ca Ca2+ inin the the glass glass frit frit had had a acatalytic catalytic effect effect on on th thee decomposition decomposition of of the the silicone silicone rubber, rubber, so that thethe melting melting of of the the glass glass frit frit and and the the decomposition decomposition of of the the silicone silicone rubber rubber took took place place simultaneously. simultaneously. It wasIt was reported reported that that the the melting melting point point of of the the fluxing fluxing agent agent should should be be lower than the decompositiondecomposition temperaturetemperature of the the polymer polymer matrix, matrix, or or else else the the inor inorganicganic fillers fillers would would run run off off during the decomposition decomposition of the the matrix matrix [29]. [29].

Figure 2. Thermogravimetric curves of diﬀerent composites samples: (a) S6 and S6 without glass frit; Figure(b) kilchoanite 2. Thermogravimetric powder and S6 curves without of kilchoanite.different composites samples: (a) S6 and S6 without glass frit; (b) kilchoanite powder and S6 without kilchoanite. Kilchoanite caused the second decomposition process of S6, according to Figure2b. There was no secondKilchoanite stage caused in the the sample second without decomposition kilchoanite, process and of the S6, second according stage to inFigure S6 was 2b. similarThere was to theno seconddecomposition stage in process the sample of kilchoanite. without Moreover,kilchoanite, the and glass the frit second could alsostage accelerate in S6 was the decompositionsimilar to the decompositionof kilchoanite (Figureprocess2 ofa). kilchoanite. It indicated Moreover, that kilchoanite the glass probably frit could reacted also accelerate with the the melted decomposition glass frit of kilchoanite (Figure 2a). It indicated that kilchoanite probably reacted with the melted glass frit during 600–700 °C. The temperature program for the sintering of ceramic samples was designed as shown in Figure 1, according to the decomposition process of the composites. The aims of Step 4 to Step 8 were to make the matrix decompose slowly, to produce enough liquid phase, and to prevent inorganic fillers

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during 600–700 ◦C. The temperature program for the sintering of ceramic samples was designed as Materialsshown 2020 in Figure, 13, x FOR1, according PEER REVIEW to the decomposition process of the composites. The aims of Step5 of 4 to11 Step 8 were to make the matrix decompose slowly, to produce enough liquid phase, and to prevent frominorganic running ﬁllers off. from The runningmatrix of o ﬀS6. Thedecomposed matrix of co S6mpletely decomposed at 500 completely °C. Kilchoanite at 500 in◦C. S6 Kilchoanite decomposed in mainlyS6 decomposed from 600 mainlyto 700 °C. from The 600 residual to 700 mass◦C. The was residual stable from mass 710 was to stable800 °C. from 710 to 800 ◦C.

3.2.3.2. Ceramization Ceramization Performance Performance at at Different Different Temperatures Temperatures S6S6 was was sintered sintered at at five five different different temperatures (600 (600 °C,◦C, 700 700 °C,◦C, 800 800 °C,◦C, 900 900 °C◦C and and 1000 1000 °C)◦C) in in orderorder to to investigate investigate the the reaction reaction between between kilchoanite kilchoanite and and glass glass frit, frit, and and the the ceramization ceramization process process and and mechanismmechanism of of the the composit composites.es. Figure Figure 3a–e3a–e shows the microstructu microstructurere of the fracture surface for the ceramicceramic samples samples sintered sintered at at different different temperatures. temperatures. Figure Figure 33ff shows the XRD patterns of the ceramic samples.samples. Because Because kilchoanite kilchoanite was was the the only only crystal crystal in in the the composites, composites, all all the the samples samples were were compared compared withwith it it to to judge judge whether whether there there was was a new crystal in the ceramic samples.

Figure 3. SEM images of samples sintered at diﬀerent temperatures: (a) 600 C; (b) 700 C; (c) 800 C; Figure 3. SEM images of samples sintered at different temperatures: (a) 600 °C;◦ (b) 700 °C;◦ (c) 800 °C;◦ (d) 900 C; (e) 1000 C; and (f) XRD patterns of diﬀerent samples. (d) 900 °C;◦ (e) 1000 °C;◦ and (f) XRD patterns of different samples.

The structure of the 600 °C-sintered sample was incompact (Figure 2a). The XRD pattern of 600 °C was almost the same as kilchoanite (Figure 2f), and kilchoanite particles could be observed in Figure 2a. The components did not react to form new crystals. Kilchoanite and silica were only stuck together by the melted glass frit. In the 700 °C-sintered sample, kilchoanite disappeared in both the SEM image (Figure 2b) and XRD pattern (Figure 2f). There was some continuous glassy phase and incompact silica in Figure 2b. Kilchoanite decomposed and reacted with the glass frit to form the

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The structure of the 600 ◦C-sintered sample was incompact (Figure3a). The XRD pattern of 600 ◦C was almost the same as kilchoanite (Figure3f), and kilchoanite particles could be observed in Figure3a. The components did not react to form new crystals. Kilchoanite and silica were only stuck together by Materials 2020, 13, x FOR PEER REVIEW 6 of 11 the melted glass frit. In the 700 ◦C-sintered sample, kilchoanite disappeared in both the SEM image glassy(Figure phase.3b) and The XRD XRD pattern pattern (Figure showed3f). that There there was was some no other continuous crystal, glassy but only phase silica and in incompactthe 700 °C- sinteredsilica in Figuresample.3b. When Kilchoanite the sintering decomposed temperature and reactedreached with800 °C, the there glass was frit a to new form crystal the glassy petedunnite phase. (CaZnSiThe XRD2O pattern6) in the showed XRD thatpattern. there The was existence no other crystal,of Ca butand only Zn silicain petedunnite in the 700 ◦C-sinteredproved that sample. both kilchoaniteWhen the sintering and glass temperature frit participated reached in 800the◦ C,reaction there wasto form a new the crystal crystal. petedunnite A completely (CaZnSi connected2O6) in microstructurethe XRD pattern. had The formed existence in the of Ca sample, and Zn but in petedunnitethere were still proved a lot that of bothpores kilchoanite and silica and(Figure glass 2c). frit Whenparticipated the temperature in the reaction kept to on form rising, the crystal. the silica A completely was invisible connected in both microstructure the SEM images had and formed XRD in patterns.the sample, The but porosity there were of the still samples a lot of poresreduced and and silica the (Figure intensity3c). Whenof petedunnite the temperature in the keptXRD onpatterns rising, increasedthe silica was gradually invisible (Figure in both 2d the–f). SEM It indicated images and that XRD silica patterns. also participated The porosity in of the the reaction samples reducedto form petedunnite.and the intensity of petedunnite in the XRD patterns increased gradually (Figure3d–f). It indicated that silicaFigure also 4 shows participated the linear in the contraction reaction to and form flexural petedunnite. strength of sample S6 sintered at different temperatures.Figure4 shows Both the the linear linear contraction contraction and and flexural flexural strength strength increased of sample rapidly S6 sintered from 600 at to di 800fferent °C, buttemperatures. increased slowly Both thefrom linear 800 to contraction 1000 °C. It and could flexural be explained strength perfectly increased by rapidly the SEM from images 600 toand 800 XRD◦C, patterns.but increased When slowly the sintering from 800 temperature to 1000 ◦C. Itwas could 800 be°C explained, the flexural perfectly strength by was the SEM46.76 images MPa. When and XRD the sinteringpatterns. temperature When the sintering reached temperature 1000 °C, the was flexural 800 ◦ stC,rength the flexural was improved strength wasto 54.56 46.76 MPa. MPa. In Whenthe early the research,sintering the temperature strength of reached the ceramics 1000 ◦C, increased the flexural obviously, strength only was if improved the sintering to 54.56 temperature MPa. In thereached early 1000research, °C, and the the strength flexural of thestrength ceramics was increasedno more than obviously, 20 MPa only [24–27]. if the In sintering this research, temperature 800 °C reachedwas the key1000 point◦C, and of the the ceramization flexural strength performance, was no more which than wa 20s reduced MPa [24 –by27 200]. In °C this compared research, with 800 ◦otherC was research. the key point of the ceramization performance, which was reduced by 200 ◦C compared with other research.

FigureFigure 4. PropertiesProperties of of the the ceramic ceramic samples samples si sinteredntered at at different diﬀerent temperatures: ( (aa)) linear linear contraction; contraction; ((b)) flexural ﬂexural strength.

3.3. Effect of Nano Silica on Ceramization 3.3. Effect of Nano Silica on Ceramization S0–S5, which had different contents of nano silica, were sintered at 1000 C. Figure5 shows the S0–S5, which had different contents of nano silica, were sintered at 1000 °C.◦ Figure 5 shows the XRD patterns of the ceramic samples. The crystal phase of S0, which had no nano silica, consisted XRD patterns of the ceramic samples. The crystal phase of S0, which had no nano silica, consisted of of mainly hardystonite (Ca2ZnSi2O7), some perovskite (Ca4Ti4O12), and zinc phosphate (Zn2P2O7), mainly hardystonite (Ca2ZnSi2O7), some perovskite (Ca4Ti4O12), and zinc phosphate (Zn2P2O7), but no but no petedunnite (CaZnSi2O6). Hardystonite and petedunnite were present in sintered sample S1, petedunnite (CaZnSi2O6). Hardystonite and petedunnite were present in sintered sample S1, which hadwhich nano had silica nano of silica 10 part ofs 10 per parts hundred per hundred of rubber of (phr). rubber Then, (phr). hardystonite Then, hardystonite disappeared disappeared in the XRD in patternsthe XRD of patterns S2–S5, and of S2–S5, the intensity and the of intensity petedunnite of petedunnite in the XRD in patterns the XRD increased patterns gradually increased from gradually S1 to S3.from Moreover, S1 to S3. there Moreover, were silica there and were cristobalite silica and in cristobalite S4 and S5, inwhich S4 and had S5, excess which nano had silica. excess The nano intensity silica. ofThe the intensity cristobalite of the increased cristobalite greatly increased and the greatly intensit andy of the the intensity silica also of theincreased silica also in S5. increased Because in the S5. Because the surface of sintered samples S0 and S1 was damaged, Al2O3 could be observed in the XRD surface of sintered samples S0 and S1 was damaged, Al2O3 could be observed in the XRD patterns of S0 andpatterns S1. of S0 and S1. FigureFigure 66 showsshows thethe linearlinear contractioncontraction andand flexuralflexural strengthstrength ofof sinteredsintered samplessamples S0–S5.S0–S5. BothBoth firstfirst increasedincreased and and then then decreased decreased as th thee content of the nano silica increa increased.sed. The The maximum maximum appeared appeared at at the the point of 30 phr. Therefore, a proper content of the nano silica could improve the strength of the ceramic samples effectively. It should be noted that the content of the inorganic fillers was counted based on the mass of the silicone rubber, which was convenient for the preparation of ceramizable composites, but the silicone rubber decomposed and did not participate in the ceramization at all. It was the proportion between each of the inorganic fillers that determined the ceramization performance of the composites. If the content of kilchoanite increases to 100 phr, then the content of nano silica should increase to 60

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point of 30 phr. Therefore, a proper content of the nano silica could improve the strength of the ceramic samples effectively. It should be noted that the content of the inorganic fillers was counted based on Materials 2020, the13, massx FOR of PEER the silicone REVIEW rubber, which was convenient for the preparation of ceramizable composites, 7 of 11 but the silicone rubber decomposed and did not participate in the ceramization at all. It was the phr. A properproportion proportion between of each kilchoanite of the inorganic and fillers nano that silica determined was 5:3 the ceramizationin weight. performanceOf course, of the the proportion composites. If the content of kilchoanite increases to 100 phr, then the content of nano silica should between theincrease polymer to 60 matrix phr. A properand inorganic proportion fillers of kilchoanite could andalso nano affect silica the was ceramization 5:3 in weight. Ofperformance, course, but it was not withinthe proportionthe scope between of the thediscussion polymer matrix in this and part. inorganic What fillers is more, could alsothe astrengthffect the ceramization of S3 was 42.81 MPa, Materials 2020, 13, x FOR PEER REVIEW 7 of 11 which was lowerperformance, than butthe it S6 was sintered not within at the 1000 scope °C. of theAs discussion mentioned in this before, part. What the is content more, the of strength the glass of frit in S6 S3 was 42.81 MPa, which was lower than the S6 sintered at 1000 C. As mentioned before, the content was increasedphr. properly A proper proportion based on of S3,kilchoanite so that and the nano de nsitysilica was and 5:3 strength ◦in weight. ofOf thecourse, ceramic the proportion samples could be betweenof the glass the frit polymer in S6 was matrix increased and inorganic properly fillers based coul on S3,d also so thataffect the the density ceramization and strength performance, of the ceramic but it improved [28].wassamples not within could bethe improved scope of the [28 discussion]. in this part. What is more, the strength of S3 was 42.81 MPa, which was lower than the S6 sintered at 1000 °C. As mentioned before, the content of the glass frit in S6 was increased properly based on S3, so that the density and strength of the ceramic samples could be improved [28].

Figure 5. XRD patterns of the sintered samples with different contents of nano silica. Figure 5. XRDFigure patterns 5. XRD patterns of the of sintered the sintered samples samples with with diﬀ differenterent contents contents of nano silica.of nano silica.

(a) (b)

Figure 6. PropertiesProperties of of ceramic samples samples with with different diﬀerent contents of nano silica: ( (aa)) linear linear contraction; contraction; (b) flexural ﬂexural strength.

Nano silica silica played played an an important important role role in the in formation the formation of the ofceramic the ceramic phase in phase this research. in thisresearch. In order toIn discuss order to the discuss (effecta) theof the eﬀ ectnano of silica the nano on ceramizatio silica on ceramizationn clearly, the clearly, formula the of formulahardystonite(b) of hardystonite is written as is written as CaO 0.5ZnO SiO and the formula of petedunnite is written as CaO ZnO 2SiO . The mol CaO·0.5ZnO·SiO· 2 and the· formula2 of petedunnite is written as CaO·ZnO·2SiO· 2. The· mol2 ratio of Figure 6.Ca:Zn:Si Properties in hardystonite of ceramic was samples1:0.5:1, while with the differentmol ratio of contents Ca:Zn:Si in of petedunnite nano silica: was ( a1:1:2.) linear There contraction; was (b) flexuralmore strength. Zn and Si in the petedunnite. Ca was from the kilchoanite and Zn was from the glass frit. Si was from the kilchoanite, glass frit, and nano silica (there was no nano silica in S0). The content of kilchoanite or glass frit of samples S0–S5 was the same, but the content of nano silica was different. For sample S0, Nano silicawhich played had no nano an importantsilica, although role the inglass the frit formation and kilchoanite of the could ceramic provide phaseenough inSi, therethis research.was no In order to discuss thepetedunnite, effect of thebut onlynano hardystonite silica on ceramizatioin the sinteredn clearly,sample. Nanothe formula silica was of necessary hardystonite to form is written as CaO·0.5ZnO·SiOpetedunnite.2 and When the enoughformula nano of silica petedunnite was added into is thewritten composites, as moreCaO·ZnO·2SiO glass frit participated2. The inmol ratio of Ca:Zn:Si in hardystonitethe reaction, and was there 1:0.5:1, was no whilehardystonite, the mol but ratioonly petedunnite of Ca:Zn:Si in thein petedunnitesintered sample. was Then, 1:1:2. the There was strength of the ceramic samples increased greatly. However, when the nano silica was in excess, only a more Zn andpart Si ofin nano the petedunnite.silica could take Capart was in the from format theion kilchoanite of petedunnite, and and Zn the was rest fromwould the turn glass into frit. Si was from the kilchoanite,crystalline silica glass and frit, cristobalite. and nano This silica made (therethe strength was of no the nano ceramic silica sample ins S0). obviously The contentdecrease, ofas kilchoanite or glass frit of samples S0–S5 was the same, but the content of nano silica was different. For sample S0, which had no nano silica, although the glass frit and kilchoanite could provide enough Si, there was no petedunnite, but only hardystonite in the sintered sample. Nano silica was necessary to form petedunnite. When enough nano silica was added into the composites, more glass frit participated in the reaction, and there was no hardystonite, but only petedunnite in the sintered sample. Then, the strength of the ceramic samples increased greatly. However, when the nano silica was in excess, only a part of nano silica could take part in the formation of petedunnite, and the rest would turn into crystalline silica and cristobalite. This made the strength of the ceramic samples obviously decrease, as

Materials 2020, 13, 3708 8 of 12 ratio of Ca:Zn:Si in hardystonite was 1:0.5:1, while the mol ratio of Ca:Zn:Si in petedunnite was 1:1:2. There was more Zn and Si in the petedunnite. Ca was from the kilchoanite and Zn was from the glass frit. Si was from the kilchoanite, glass frit, and nano silica (there was no nano silica in S0). The content of kilchoanite or glass frit of samples S0–S5 was the same, but the content of nano silica was different. For sample S0, which had no nano silica, although the glass frit and kilchoanite could provide enough Si, there was no petedunnite, but only hardystonite in the sintered sample. Nano silica was necessary to form petedunnite. When enough nano silica was added into the composites, more glass frit participated in the reaction, and there was no hardystonite, but only petedunnite in the sintered sample. Then, the strength of the ceramic samples increased greatly. However, when the nano silica was in excess, only a part of nano silica could take part in the formation of petedunnite, and the rest would turn into crystalline silica and cristobalite. This made the strength of the ceramic samples obviously decrease, as there were defects in the ceramic phase. On the one hand, nano silica had a large specific surface area and high surface energy, which improved the sintering kinetic force and reaction speed; on the other hand, nano silica could migrate efficiently and make the reaction take place more evenly in samples, so that the sintering could be completed at a lower temperature. In addition, when the temperature reached 700 ◦C, the glass frit had melted and kilchoanite also decomposed, then nano silica acted as the frame of the material. Therefore, when the nano silica was inadequate, the material would lose the support and the liquid phase could run off, so the strength of samples S0–S2 was much lower than that of S3.

3.4. Ceramization Process and Mechanism Ceramization process could be divided into two stages according to whether petedunnite formed. The first stage was below 800 ◦C. The glass frit melted to form a liquid phase, and accelerated the decomposition of silicone rubber matrix. The liquid phase enfolded the kilchoanite and silica, which finished at about 500 ◦C. During 600–700 ◦C, kilchoanite decomposed and reacted with the melted glass frit to form the glassy phase, and the nano silica acted as the frame of the material instead of kilchoanite. Then, as the temperature rose up, the glassy phases connected with each other to form a stable microstructure. Although the ceramic phase did not form, the sample had a certain mechanical strength because of the glassy phase. The second stage began at a temperature of no more than 800 ◦C. The glassy phase reacted with the silica, petedunnite appeared and the ceramic phase formed. The material had a completely connected microstructure with the ceramic phase and glassy phase bonded together, so that both the contraction and strength increased obviously. Finally, as the temperature rose up to 1000 ◦C, more silica participated in the reaction and the crystal petedunnite grew more perfectly. The density and strength of the samples increased gradually. In conclusion, ceramization mechanism of ceramizable silicone rubber composites is liquid-phase sintering. The ceramic samples had a high strength and low sintering temperature, but the contraction problem was serious [30,31]. There are two keys of ceramization at a low temperature, the proper content of nano silica and low-melting-point glass frit. Glass frit melts at a low temperature to form a liquid phase, which is beneficial to substance transfer, such as flow, diffusion, dissolution, and precipitation. Nano silica plays the role of a frame to support the material, and then takes part in the ceramization reaction, reducing the sintering temperature and improving the strength of the ceramic samples. Figure7 shows the ceramization process and mechanism of ceramizable silicone rubber composites. Small pores formed at 500 ◦C because the silicone rubber decomposed, but the liquid phase could fill up most of the pores. Then, kilchoanite decomposed and reacted with the liquid phase, and the reaction caused contraction as well, so there were more pores of a larger size. Finally, more and more pores were filled up by the formation and growth of petedunnite, but the size of the samples contracted more obviously because of the reaction. Materials 2020, 13, x FOR PEER REVIEW 8 of 11

there were defects in the ceramic phase. On the one hand, nano silica had a large specific surface area and high surface energy, which improved the sintering kinetic force and reaction speed; on the other hand, nano silica could migrate efficiently and make the reaction take place more evenly in samples, so that the sintering could be completed at a lower temperature. In addition, when the temperature reached 700 °C, the glass frit had melted and kilchoanite also decomposed, then nano silica acted as the frame of the material. Therefore, when the nano silica was inadequate, the material would lose the support and the liquid phase could run off, so the strength of samples S0–S2 was much lower than that of S3.

3.4. Ceramization Process and Mechanism Ceramization process could be divided into two stages according to whether petedunnite formed. The first stage was below 800 °C. The glass frit melted to form a liquid phase, and accelerated the decomposition of silicone rubber matrix. The liquid phase enfolded the kilchoanite and silica, which finished at about 500 °C. During 600–700 °C, kilchoanite decomposed and reacted with the melted glass frit to form the glassy phase, and the nano silica acted as the frame of the material instead of kilchoanite. Then, as the temperature rose up, the glassy phases connected with each other to form a stable microstructure. Although the ceramic phase did not form, the sample had a certain mechanical strength because of the glassy phase. The second stage began at a temperature of no more than 800 °C. The glassy phase reacted with the silica, petedunnite appeared and the ceramic phase formed. The material had a completely connected microstructure with the ceramic phase and glassy phase bonded together, so that both the contraction and strength increased obviously. Finally, as the temperature rose up to 1000 °C, more silica participated in the reaction and the crystal petedunnite grew more perfectly. The density and strength of the samples increased gradually. In conclusion, ceramization mechanism of ceramizable silicone rubber composites is liquid-phase sintering. The ceramic samples had a high strength and low sintering temperature, but the contraction problem was serious [30,31]. There are two keys of ceramization at a low temperature, the proper content of nano silica and low-melting-point glass frit. Glass frit melts at a low temperature to form a liquid phase, which is beneficial to substance transfer, such as flow, diffusion, dissolution, and precipitation. Nano silica plays the role of a frame to support the material, and then takes part in the ceramization reaction, reducing the sintering temperature and improving the strength of the ceramic samples. Figure 7 shows the ceramization process and mechanism of ceramizable silicone rubber composites. Small pores formed at 500 °C because the silicone rubber decomposed, but the liquid phase could fill up most of the pores. Then, kilchoanite decomposed and reacted with the liquid phase, and the reaction caused contraction as well, so there were more pores of a larger size. Finally, more and Materialsmore pores2020, 13were, 3708 filled up by the formation and growth of petedunnite, but the size of the samples9 of 12 contracted more obviously because of the reaction.

Materials 2020, 13, x FOR PEER REVIEW 9 of 11

4. Conclusions In this research, ceramizable silicone rubber composites used for the ceramic insulation of a metal conductor, such as a bus bar, were prepared. Low-temperature sintering at 800 °C was achieved by adding Figurelow-melting-point 7. Ceramization processglass andfrit mechanismand nano of ceramizable silica. The silicone low-temperature rubberrubber composites.composites. ceramization performance4. Conclusions of the composites was improved considerably. When the sintering temperature was 800 °C, the flexural strength was 46.76 MPa. When the sintering temperature reached 1000 °C, the flexural In this research, ceramizable silicone rubber composites used for the ceramic insulation of a metal strength improved to 54.56 MPa. The ceramization process was divided into two stages. The first conductor, such as a bus bar, were prepared. Low-temperature sintering at 800 ◦C was achieved by stageadding was the low-melting-point formation of the glass glassy frit and phase nano silica.and th Thee second low-temperature stage was ceramization the formation performance of the ceramic of phase.the There composites were wastwo improved keys to ceramization considerably. at When low thetemperature, sintering temperature the proper was content 800 ◦ C,of thenano flexural silica and low-melting-pointstrength was 46.76 glass MPa. frit. The When contraction the sintering of the temperature ceramic samples reached was 1000 not◦C, low the flexuralenough, strength because the proportionimproved of to silicone 54.56 MPa. rubber The ceramizationmatrix was processabout 47 was wt divided.% in S6. into If two the stages. proportion The first of stage the wasmatrix the was reduced,formation the contraction of the glassy phaseof the and ceramic the second samples stage would was the decrease formation and of the the ceramic flexural phase. strength There werecould be improvedtwo keys further. to ceramization at low temperature, the proper content of nano silica and low-melting-point glass frit. The contraction of the ceramic samples was not low enough, because the proportion of silicone Authorrubber Contributions: matrix was aboutInvestigation 47 wt.% and in S6. experiment, If the proportion P.L.; theoretica of the matrixl guidance, was reduced, H.J.; data the contractionanalysis, S.W. of and H.L.;the writing—original ceramic samples draf wouldt preparation, decrease and P.L.; the flexuralwriting—review strength couldand editing, be improved H.J. further.and P.L.; experimental guidance, N.G. and Z.S. All authors have read and agreed to the published version of the manuscript. Author Contributions: Investigation and experiment, P.L.; theoretical guidance, H.J.; data analysis, S.W. and H.L.; Funding:writing—original This research draft received preparation, no external P.L.; writing—review funding. and editing, H.J. and P.L.; experimental guidance, N.G. and Z.S. All authors have read and agreed to the published version of the manuscript. Acknowledgments:Funding: This research Hongchuan received Tang, no external Guowen funding. Kuang and Qian Li provided a lot of help with the experiment in this research. Acknowledgments: Hongchuan Tang, Guowen Kuang and Qian Li provided a lot of help with the experiment in Conflictsthis research. of Interest: The authors declare no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest. Appendix A. Three-Point Bending Test Appendix A. Three-Point Bending Test In theIn thethree-point three-point bending bending test, test, each each result result ofof flexuralflexural strength strength is is the the average average of fiveof five samples. samples. FigureFigure A1 A1shows shows the the geometry geometry of of the the ceramic ceramic samplessamples in in the the three-point three-point bending bending test. test. The The samples samples beforebefore sintering sintering had had uniform uniform geometry geometry parameters of of 80 80 mm mm 10× 10 mm mm2 ×mm. 2 mm. However, However, the samplesthe samples × × contractedcontracted during during sintering sintering and and th thee contraction contraction was didifferentfferent according according to theto the sintering sintering temperature temperature and andthe thecontent content of of nano nano silica,silica, so so the the average average geometry geometry parameters parameters for each kindfor ofeach sample kind are of provided sample are providedin Table in A1Table. The A1. lengths The lengths of samples of samples were measured were measured using a ruler using with a ruler an accuracy with an of accuracy 1 mm. It wasof 1 mm. It wasused used to calculateto calculate the linear the linear contraction contract andion did and not did affect not the affect flexural the strength. flexural strength.

FigureFigure A1. A1. GeometryGeometry ofof thethe ceramic ceramic samples. samples.

Table A1. The average geometry parameters of the ceramic samples.

Samples Length L1 (mm) Width b (mm) Thickness h (mm) S0 78 8.386 2.330 S1 66 8.430 2.315 S2 64 8.435 2.315 S3 62 7.860 1.575 S4 65 8.280 1.460 S5 69 8.710 1.605 S6 (600 °C) 73 9.250 2.140 S6 (700 °C) 68 8.750 1.725 S6 (800 °C) 63 7.912 1.513 S6 (900 °C) 62 7.944 1.577 S6 (1000 °C) 61 7.780 1.424

Materials 2020, 13, 3708 10 of 12

Table A1. The average geometry parameters of the ceramic samples.

Samples Length L1 (mm) Width b (mm) Thickness h (mm) S0 78 8.386 2.330 S1 66 8.430 2.315 S2 64 8.435 2.315 S3 62 7.860 1.575 S4 65 8.280 1.460 S5 69 8.710 1.605 S6 (600 ◦C) 73 9.250 2.140 S6 (700 ◦C) 68 8.750 1.725 S6 (800 ◦C) 63 7.912 1.513 S6 (900 ◦C) 62 7.944 1.577 Materials 2020, 13, xS6 FOR (1000 PEER◦C) REVIEW 61 7.780 1.424 10 of 11

FigureFigure A2 A2shows shows the the test test device device of the three-pointthree-point bending bending test. test. The The length length of supporting of supporting span (l )span (l) waswas 30 30 mm, mm, and and the speedspeed of of the the applied applied load load was was 2 mm 2 mm/min./min. R was R 5was mm 5and mmr wasand 0.5r was mm. 0.5 mm.

FigureFigure A2. A2. TestTest device device of three-point bending bending test. test. The ﬂexural strength of the ceramic samples is calculated by Equation (A1). The flexural strength of the ceramic samples is calculated by Equation (A1). σ = 3Pl/(2bh2) (A1) σ = 3Pl/(2bh2) (A1) wherewhere σ is σtheis theflexural ﬂexural strength strength (MPa), (MPa), PP isis the the fracture loadload (N), (N),l isl is the the length length of supportingof supporting span span (mm),(mm), b is btheis thewidth width of ofthe the sample sample (mm), (mm), and and h is thethe thicknessthickness of of the the sample sample (mm). (mm).

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