Carbon nanocomposite by solvothermal method

Author’s Accepted Manuscript Preparation of alumina/AlON and AlON/AlN composites from Al 2O3/Carbon nanocomposite by solvothermal method V. Sabaghi, F. Davar, M.H. Taherian www.elsevier.com/locate/ceri

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To appear in: Ceramics International Received date: 3 November 2018 Revised date: 1 December 2018 Accepted date: 10 December 2018 Cite this article as: V. Sabaghi, F. Davar and M.H. Taherian, Preparation of alumina/AlON and AlON/AlN composites from Al 2O3/Carbon nanocomposite by solvothermal method, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.12.080 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Preparation of alumina/AlON and AlON/AlN composites from Al2O3/Carbon nanocomposite by solvothermal method

V. Sabaghi 1 , F. Davar ‫٭‬, 1 , M. H. Taherian 2

1

2

Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Iran

Iran University of Science and Technology, Department of Metallurgy and Material Engineering, Tehran, Iran

‫٭‬

Corresponding author. Tel: +98-31-3391-3289; Fax: +98-31-3391-3233. E-mail address: [email protected]

Abstract Al2O3/Aluminum oxynitride (AlON) and AlON/ Aluminum nitride (AlN) composites were prepared via two-step carbothermal reduction nitridation (CRN) method. A solvothermal method was elected to synthesize Al2O3/Polyacrylonitrile (PAN) nanocomposite as a first-precursor of products. Then, this Al2O3/PAN sample was held under pyrolysis conditions (800 °C for 2 h in an argon atmosphere) for fabricating Al2O3/Carbon nanocomposite. Eventually, with the help of two-step CRN method (first at 1450 °C–1550 °C for 2 h in Ar flow and so 1700 °C-1800 °C for 1 h in N2 flow) Al2O3/AlON and AlON/AlN composites were attained. Results show that a change in the residual carbon content in second-precursor (alumina/carbon nanocomposite) is a decisive factor in controlling the composition of the final product.

Keywords: Carbothermal, Aluminum oxynitride, Aluminum nitride, Solvothermal.

1

1. Introduction The manufacturing of ceramics which that based on Al2O3, AlN, and AlON for various reasons such as chemical resistance and high thermal and mechanical stability as well as the proper optical properties have received a lot of attention [1–7]. Individually, these ceramics have many applications in different fields. For example, AlON ceramics have a high transparency in the UV to mid -IR range [8], and AlN ceramics have a high thermal conductivity [6]. Of course, alumina-based ceramics also have an appropriate thermal resistance or low thermal conductivity [1]. These differences in the structural properties of each pure ceramic precursor led to use the combinations of these precursors to improve the final attributes. Heretofore, the utilization of AlN/Al2O3 composite precursors has been studied to improve oxidation and mechanical properties [1,9]. Al2O3 has an anisotropic crystal form and AlON has an isotropic crystal form, so the polycrystalline ceramic as a product of AlON precursor is optically transparent but alumina-based ceramics is not optically transparent, so the combination of these materials can be reduced the transparency of AlON individually, but increases the thermal diffusively of it, so it is significant to opt the best combination of Al2O3/AlON/AlN composition to receive the intended purpose [5]. In this study, in order to reach the final product, two intermediates were used. The first intermediate or first-precursor was Al2O3/PAN nanocomposite. Polymer selection at this stage was considerable cause of the final product profoundly dependent on the residual carbon content after the pyrolysis of second-precursor. This remaining carbon content is also subordinate on the polymer or resin char yield. The use of polymers for the production of carbon is very common [10–17]. The PAN resin has a suitable functional group for reacting or linking with other metal

2

oxide nanoparticles. The PAN precursor have high char yield that promote carbonitridation process with high efficiency and low amount of polyacrylonitrile [18–20]. The solvothermal approach was selected for preparing of the first precursor (alumina/PAN nanocomposite) since this route significantly enhances the solubility and reactivity of the raw material [21]. This advantage almost unattainable in other wet chemistry methods [21,22]. Furthermore, another principal advantage of this fashion versus other wet chemistry methods is discard annealing step of production, cause the output of the solvothermal method often has a crystalline structure [21,23]. So, in this project Al2O3/PAN nanocomposite as a first-precursor was synthesized by the solvothermal approach. The pyrolysis was performed in order to remove organic compounds of the PAN polymer and remaining the carbon content. The presence of this amorphous thin carbon layer found around the alumina core declined the temperature formation of the product. The answer to this hypothesis is also facilitated by understanding the mechanism of product synthesis. The presence of these thin layers between the alumina cells delays their binding, and as a result, the products do not have the opportunity to grow sufficiently during the operation, and their dimensions are diminished [24]. 2. Experimental

2.1. Preparation of first-precursor (Alumina/PAN nanocomposite) In this stage, at first 1 g of high purity γ-Al2O3 (particle size˂ 50 nm, Merck, Germany) was added to 40 ml of distilled water. The resulting mixture was dispersed for 12 h on the heater stirrer. After this stage, it was subjected to ultrasonic waves for 20 minutes. The aim of this section was to activate the hydroxyl groups of the surface on alumina nanoparticles to produce substantial interactions with polymer functional groups. Simultaneously, in a separate container, 1 g of PAN polymer (MW 862,200 g/mol, Sigma Aldrich, Germany) was appended to the 40 ml 3

of DMF solvent (MW 73,09 g/mol, JEONG Wang, South Korea) and stirred for 1 h. The intention of this section was to form a transparent polymer solution. It is important to note that w/w ratios were considered for this work. Thus, two ratios of 1:1 and 1:2 were elected from alumina to polymer which was appointed S1 and S2 respectively. Then, the contents of the polymer solution were added to the suspension of γ-Al2O3 drop by drop. The ensuing mixture was transferred to an autoclave and placed under a temperature of 160 °C for 8 hours. After the reaction, the flexible dark yellow composite product as first-precursor was dried at 150 °C. Scheme 1

2.2. Preparation of second-precursor (Alumina/C nanocomposite) The Al2O3/PAN nanocomposite flexible film was placed under the pyrolysis treated. This process was carried out in the furnace under the high purity argon atmosphere at 800 °C for 2 hours to burn all heteroatoms (an atom in the ring of a cyclic compound other than a carbon atom) compounds in the structure. The product was black colored that this color was created due to the residual carbon content of the polymer. The temperature rise rate from the ambient temperature to 800 °C was set at 10 L min-1 and argon gas pressure was also considered as 4 Torr. Scheme 2

2.3 preparation of S1 and S2 final product (AlON composite) In the terminal part of this project, Al2O3/C nanocomposite as second-precursor was subjected under the two-step CRN process. This process is divided into two general parts in the 4

first part the black powder is heated under pure nitrogen atmosphere up to 1550 °C, and in the second step, it was heated to 1750 °C. Scheme 2 2.4. Characterization equipment The following devices were used to synthesize and identify completely the precursors and final products. TOPSONICS ultrasonic liquid processor (UP-400 series, Tehran, Iran), Graphite vacuum furnace (NCR-HT 2200, Isfahan, Iran), Fourier transform infrared (680-PLUS, JASCO Company), Field emission scanning electron microscope (TESCAN model, CZECH company), Scanning electron microscope (JEOL 6510 model), X-ray diffraction device (X-Pert-MPD), Elemental analyzer (CHNS-932 model of the LECO Company).

3. Results and discussion 3.1. FT-IR results

3.1.1. FT-IR results of Al2O3 and PAN polymer

The FT-IR spectrum of Al2O3 nanoparticles was shown in figure 1a. A broad absorption band at 400-800 cm-1 related to tetrahedral and octahedral stretching vibrations in the Al-O bond in γ-Al2O3 nanoparticles [24]. The absorption band at 1630 cm-1 and the broad absorption band at 3400-3600 cm-1 related to bending vibration and stretching vibration of H2O and OH groups of the surface of γ-Al2O3, respectively [25]. The absorption band at 2851 cm-1 and 2926 cm-1 related to symmetric and asymmetric C-H bond, respectively [25]. The FT-IR spectrum of PAN was shown in figure 1b. The absorption band at 1365 cm-1 and 1464 cm-1 can be attributed to bending vibration of C-H in the CH2 groups of the carbon chain of PAN polymer [26]. The most 5

important adsorption band in PAN structure can be ascribed to nitrile group of PAN at 2245 cm-1 [27]. The adsorption bands at 2861 cm-1 and 2950 cm-1 in the FT-IR spectrum of PAN can be related to symmetric and asymmetric vibration of the C-H bond in the carbon chain of the polymer [26]. Eventually, the broad absorption band at 3400-3600 cm-1 related to the vibration of the O-H surface group which attributed to H2O adsorbed to surface [26,27]. Figure 1a and Figure 1b.

3.1.2. FT-IR results of first-precursor (S1 and S2 Al2O3/PAN nanocomposites)

When hydroxyl surface groups of γ-Al2O3 were activated by the sonication method, γAl2O3 nanoparticles can be reacted to the nitrile groups of PAN polymer. The result of this process is the formation of hydrogen bonding between hydrogen atoms of hydroxyl surface groups of γ-Al2O3 with the lone pair of nitrogen atoms of nitrile group of PAN chain. Furthermore, this band can be created between π-band in nitrile groups (electron rich part) and OH surface group (electron poor part). These two types of hydrogen interaction help to formation Al2O3/PAN nanocomposite as a first-precursor. As shown in Fig. 2a and Fig. 2b, the combination of absorption bands in the structures of each nanocomposite components is seeable. The appearance of adsorption band in the range 1620-1640 cm-1 could have pertained to the combination of carbon-carbon double bond, the carbon-nitrogen double bond which can occur during the dehydrogenation of PAN molecules. The vibration of the Al-OH bond is also situated in this expanse, which naturally overlaps this number of absorption bands, which stretches the broad band [27][25]. Figure 2a and Figure 2b.

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3.1.3. FT-IR results of second-precursor (S1 and S2 Al2O3/C nanocomposites)

After the first-precursor was fabricated, the alumina/PAN nanocomposite was employed to execute the pyrolysis process interior of the furnace. The pyrolysis comprises dehydrogenation and cyclization steps. During cyclization, single bonds in the polymer chain combine with the functional group (nitrile groups) of other PAN polymer molecules and make benzene cycle structures. Thus, the reduction in adsorption band’s intensity of the 2840-2860 cm-1, 2940-2960 cm-1 is reasonable as well. The absorption band in the 1625-1640 cm-1 range interrelated to multi-agent functional groups. However, with an increment of the number of CC and CN double bonds and the reduction of hydrogen atoms, the intensity of adsorption band has no considerable change. Figure 3a and Figure 3b.

3.1.4. FT-IR results products (S1 (Al2O3/AlON composite) and S2 (AlON/AlN composite) products)

After manufacturing of Al2O3/Carbon nanocomposite as second-precursor, this sample stationed in the furnace for CRN procedure. In abbreviated, the residual carbon adjacent to the alumina nanoparticle acts as a reducing agent. In fact, this agent substitutes nitrogen atoms in the atmosphere with a number of oxygen atoms in the structure of alumina. So, the final output strongly depends on the carbon content. For S1 sample, the carbon content is fewer than the essential amount for reducing the entire alumina particle to AlON, so, the combination of Al2O3 and AlON can be ascertained as the S1 product (Al2O3/AlON composite). The broad bands in

7

Fig. 4a and Fig. 4b at 500-600 cm-1 and 700-800 cm-1 can be ascribed to Al-N and Al-O bond in the Al2O3 and AlON and AlN particles [28,29]. Figure 4a and Figure 4b.

3.2. Morphology investigations and particle size of second-precursor FESEM was exploited to determine the morphology of Al2O3/C nanocomposite as the second precursor. Figures 5a and 5b, show the FE-SEM images for Al2O3/C (S1) nanocomposite and Al2O3/C (S2) nanocomposite, respectively. According to these images, polymer content in the S2 sample is twice than the S1 sample in first-precursor. During the pyrolysis operation, the PAN polymer begins to dehydrogenation and cyclization process to construct an amorphous carbon structure. The carbon nanolayer remaining from the polymer has an amorphous structure which covers the alumina nucleus. As can be seen in these images, the resulting nanocomposite dimensions are less than ~ 40 nanometers. In some places, agglomerates with a large number of interconnected cores are visible. These agglomerates can be created for several reasons because the degree of agglomeration of the primary nanoparticles and the growth of these nanoparticles may be the main reasons for these aggregations. By increasing the polymer content and, as a result, the carbon retained by the pyrolysis, the thickness of the coating nanolayers increments, Ultimately, agglomerates with a higher number of alumina nucleus is created.

Figure 5a and figure 5b. 3.3. CHNS Analysis An elemental analysis was exploited to ascertain the exact amount of carbon and nitrogen content remaining in S1 and S2 Al2O3/C nanocomposite samples. Table 1 summarizes these

8

results. The combustion process at CHNS equipment occurs at temperatures up to 1000 ° C. Under these conditions, carbon is converted to carbon dioxide and nitrogen into nitrogen gas or nitrogen oxide. Eventually, each of these gases identified individually and the quantities of each of the atoms are accurately reported. According to Table 1, with increasing polymer content from S1 sample than the S2 sample, the amount of carbon and nitrogen remaining increases, which seems reasonable. Of course, the presence of nitrogen atoms in this compound after pyrolysis indicates that the nitrogen atoms are captured among the remaining carbon structures of the polymer. It should be noted that in the structure of the final products, nitrogen is present so that it cannot be considered as an impurity, but it can even be considered as a contributing factor in achieving the AlON product by CRN approach. Table 1. 3.4. XRD pattern of products Figures 6a and 6b show the pattern of X-ray diffraction for S1 and S2 specimens. Figure 6a shows the final product which obtained from the ratio of 1 to 1 alumina to the PAN polymer. The residual carbon content in second-precursors acts as a reducing agent and substitutes some of the nitrogen atoms in the environment with oxygen atoms in the structure of alumina . Consequently, in the boundary of alumina and carbon, AlON phase commences forming [30]. Therefore, it can be concluded that because the carbon content as a reducing agent in S 1 sample was not sufficient to convert all of the alumina nuclei to AlON, a composite of two phases was generated together. For the S2 sample, the amount of polymer has duplicated, and the content of carbon has also reached from 14.7% to 25% according to the CHNS elemental analysis. Thus, the capacity for reducing agent has increased and in fact, in some places, this amount of carbon has been so high that all of the oxygen replaces with the nitrogen atom and produces the 9

aluminum nitride phase. Another remarkable thing in this section is the conversion of gammaalumina to alpha-alumina, which occurs at temperatures above 1200 °C [30]. Schemes 3 and 4 illustrate the mechanism of formation of products.

Scheme 3 and scheme 4. 3.5. EDS Analysis Energy dispersive x-ray spectrometer was used up to an approximate number of elements in the products. As demonstrated in figure 7a and 7b and relevant table by rising of carbon content as a reducing agent, the value of nitrogen was increased and the extent of oxygen was decreased. Reduced aluminum content can also be attributed to the release of alumina gas species (Al2O and Al) during high-temperature processes [31,32, 33]. Fig 7a and 7b.

3.6. Morphology investigations and particle size of products Figure 8a and 8b display the SEM images of S1 and S2 samples, respectively. For S1 sample cause less carbon monolayer thickness, the penetration of alumina nucleus toward each other is faster and thus the first nuclei of the product are fabricating ultimately, the primary nuclei, has an opportunity to grow and form symmetric structures. But, for S2 samples, the content of carbon and the resulting thickness is higher than S1 sample thus the process of gas penetration into the boundary between alumina and carbon is slower so the producing of the first nuclei of the product occurs late in such a situation. Thus, the product does not have the opportunity to grow equally in all directions and so irregular structures (Fig 8b) are formed. The 10

characteristics of particle size charts for both samples show that products have same average size, and therefore, it can be said that carbon content has no direct effect on the size of the product, but their crystalline structure depends on the carbon content.

Figure 8a,8b,9a and 9b 4. Conclusion In summary, Al2O3/AlON and AlON/AlN composites were synthesized through two-step carbothermal reduction nitridation process. For this intent, at first Al2O3/polyacrilonitrile nanocomposites with different ratios of alumina to PAN (1:1 and 1:2) as a first-precursor were prepared. First-precursors were pyrolyzed to construct Al2O3/carbon nanocomposite as secondprecursor. The amount of carbon nanolayers which that created at this stage depends directly on the quantity of polymer used in the first step. This nanocomposite was subjected to a CRN process to form final products. The results showed that the carbon content as a reducing agent plays a significant role so that the stoichiometry of the final product is fully affected by this parameter. The remarkable thing to note in this study was that the higher thickness nanolayer of the carbon causes penetration of the gases in the common boundary between the alumina nucleus and the reducing agent is harder and longer, and thus, during the process, there isn’t a sufficient growth time of the formation of symmetric spherical particles. Therefore, the lower thickness of the carbon nanolayers, in addition, to prevent the combination of alumina core during the CRN process, it also reduces the temperature of the products.

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Figures and Table captions. Figure 1a. FTIR spectrum of Al2O3 nanoparticle. Figure 1b. FTIR spectrum of polyacrylonitrile polymer. Figure 2a. FTIR spectrum of Al2O3/PAN S1 nanocomposite. Figure 2b. FTIR spectrum of Al2O3/PAN S2 nanocomposite. Figure 3a. FTIR spectrum of Al2O3/C S1 nanocomposite. Figure 3b. FTIR spectrum of Al2O3/C S2 nanocomposite. Figure 4a. FTIR spectrum of Al2O3/AlON S1 composite. Figure 4b. FTIR spectrum of AlON/AlN S2 composite. Figure 5a. Field emission scanning electron microscopy of Al2O3/C S1 sample. Figure 5b. Field emission scanning electron microscopy of Al2O3/C S2 sample. Figure 6a. Diffraction pattern of S1 product. Figure 6b. Diffraction pattern of S2 product Figure 7a. X-ray spectrum through energy dispersive x-ray spectrometer of S1 product. Figure 7b. X-ray spectrum through energy dispersive x-ray spectrometer of S2 product. Figure 8a. Scanning electron microscopy of S1 product. Figure 8b. Scanning electron microscopy of S2 product.

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Figure 9a. Particle size distribution of S1 product. Figure 9b. Particle size distribution of S2 product.

Table 1. CHNS data from Al2O3/C S1 nanocomposite and Al2O3/C S2 nanocoposite.

17

Scheme 1

18

19

Scheme 2

Scheme 3

20

Scheme 4

21

22

Figure 1a

Figure 1b

Figure 2a

23

Figure 2b

24

Figure 3a

Figure 3b

25

Figure 4a

Figure 4b

26

Figure 5a 27

Figure 5b

28

Figure 6a

29

Figure 6b

30

Figure 7a

Figure 7b

31

Figure 8a

Figure 8b 32

Figure 9a

33

Figure 9b

Table 1.

sample Al2O3/C S1 nanocomposite

Al2O3/C S2 nanocomposite

Carbon

Nitrogen

unit Weight percent

14.7

4.6

25

5.5

Weight percent

34