The effect of different GNPs addition on the electrical conductivities and percolation thresholds of the SiAlON matrix composites

Journal Pre-proof The effect of different GNPs addition on the electrical conductivities and percolation thresholds of the SiAlON matrix composites Sinem Baskut, Servet Turan

PII:

S0955-2219(19)30802-7

DOI:

https://doi.org/10.1016/j.jeurceramsoc.2019.11.057

Reference:

JECS 12882

To appear in:

Journal of the European Ceramic Society

Received Date:

3 June 2019

Revised Date:

4 November 2019

Accepted Date:

17 November 2019

Please cite this article as: Baskut S, Turan S, The effect of different GNPs addition on the electrical conductivities and percolation thresholds of the SiAlON matrix composites, Journal of the European Ceramic Society (2019), doi: https://doi.org/10.1016/j.jeurceramsoc.2019.11.057

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The effect of different GNPs addition on the electrical conductivities and percolation thresholds of the SiAlON matrix composites Sinem Baskut*, Servet Turan

Eskisehir Technical University, Faculty of Engineering, Department of Materials Science and Engineering, 26480, Eskisehir, Turkey *

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Corresponding author E-mail: [email protected] (Sinem Baskut)

Abstract

Insulating SiAlON ceramics may become electrically conductive with the addition of a conductive phase such as GNPs and can be used more widely. However, the differences in the properties of the used GNPs significantly affect the amount of electrical conductivity that they

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provide to the matrix. In this study, four different GNPs with different properties such as lateral dimension, thickness and aspect ratio were added to SiAlON in the amount of 1.5, 2, 3 and 4

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wt. % and the effects of different properties on the conductivity of composites were investigated. The thinnest GNPs with largest dimension and aspect ratio among the used GNPs

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provided the highest electrical conductivity and lowest percolation thresholds to SiAlON. The decrease in dimension, aspect ratios and the increase in thickness decreased the electrical conductivity of GNPs. Composites exhibited anisotropic behavior with better conductivity and

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percolation threshold values in the in-plane direction than through-plane direction.

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Keywords: Lateral dimension of GNPs, Thickness of GNPs, Electrical Conductivity,

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Percolation Threshold, SiAlON

1. Introduction

In recent years, outstanding electrical conductivity of the graphene nanoplatelets (GNPs) (electrical conductivity of 107 Sm−1 and charge carrier mobility of 2.0x105 cm2V-1s-1) [1, 2] ensured the preference of GNPs as a second phase to improve the electrical properties of many insulating advanced technology ceramic materials used intensively in many industrial fields as structural materials. Silicon-aluminum-oxynitride (SiAlON), which has good mechanical and 1

thermal properties but display quite low electrical conductivity (~ 10-9 Sm-1 [3] is in this group of insulating ceramics. In order to expand the usage of SiAlON, machining it to complex microparts could be easy and fast with electrical discharge machining (EDM), which requires the material to be electrically conductive [4-6]. It has been noticed that the electrical conductivity of many ceramic matrix materials has increased significantly with the contribution of GNPs [3, 7-13]. However, literature survey has shown that different electrical conductivities and percolation threshold values were obtained in different studies related with the same matrix even if GNPs contents were close to each other’s. Yun et al. [9], who added GNPs to the AlN matrix in the range of 0.22 to 10.10 vol. % achieved electrical conductivity of ~10 Sm-1 with an addition of ~2.9 vol. % GNPs and calculated the percolation threshold as 2.5 vol. %. On the

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other hand, ~1 Sm-1 electrical conductivity at ~ 2.9 vol. % GNPs and 1.42 vol. % percolation threshold values were obtained in the study related to AlN matrix containing GNPs between 1.4-11.4 vol. % [8]. Additionally, in two other studies investigating the electrical conductivity and percolation values of GNPs-Al2O3 composites, percolation threshold values were

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calculated as 3±0.405 vol. % [10] and 7.1±1.36 [11], respectively.

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Although the same matrices were used in these studies, it was observed that the used GNPs had different properties such as lateral dimension, thickness (number of layers) and aspect ratios. It

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was reported that the changes in GNPs dimension and thickness, which also affects the homogeneous/inhomogeneous dispersion of GNPs in the matrix, has an effects on the electrical conductivities of the GNPs and composites [14, 15]. Fang et al. [16] measured the electrical

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resistance and mobility of GNPs with different thicknesses such as single-layer (SLG), fewlayer (FLG) and multi-layer (MLG). They have determined that the electrical resistance increased (SLG: 1.4, FLG: 1.9-8.2, MLG: 9.1-147 µ Ω cm) and mobility decreased (SLG: 23.3,

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FLG: 2-8.8, MLG: 0.5-1.8) as the number of layers increased. Moreover, the intrinsic electrical conductivities of the two commercial GNPs with lateral dimensions of 25 μm (SA: 120 m2g-1)

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and 5 μm (SA: 150 m2g-1) in 6 nm thicknesses were reported as 739.6 and 648.6 Scm-1, respectively [17]. Yu et al [17] compared the electrical conductivities of polymer matrix composites containing two different GNPs having similar morphology but different lateral dimensions. The electrical conductivity of the polymer containing GNPs with an average size of 25 µm was measured ~ 3 times higher than polymer containing GNPs, whose average dimension was less than 1 µm. In addition, it was reported that the lower percolation threshold values were obtained in the GNPs-insulator material composites when GNPs or CNTs with higher lateral dimensions and higher aspect ratios were used [15, 18-23]. However, there are 2

only few studies investigating the effects of different GNP properties on the electrical conductivity performance of GNP-ceramic matrix composites. According to these observations, this work focused on to determine the properties of GNPs that provide the best electrical conductivity performance to SiAlON. For this purpose, four kinds of GNPs with different properties (lateral size, thickness and aspect ratio) were added to SiAlON in the same amounts and the powder mixtures were sintered by using spark plasma sintering (SPS) technique. All measurements were carried out in the directions parallel and perpendicular to the SPS pressing axis, since the uniaxial load applied in the SPS leads to the alignment of GNPs in the matrix

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microstructures.

Materials and Methods

2.1. Production of the Materials

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Four different commercial GNPs, which were called GNP1 (Graph. Chem. Ind. Comp.), GNP2 (Graph. Chem. Ind. Comp.), GNP3 (Graph. Chem. Ind. Comp.) and GNP4 (Graph. Chem. Ind.

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Comp.) were used during the study and their specifications were presented in Table 1. While the average lateral dimensions of GNPs given in Table 1 were measured (Malvern Instruments,

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Hydro 2000) by us, thicknesses and purities were the values specified by the vendors. The thickness values given in Table 1 indicated that the GNP1s and GNP2s are in multi-layer, GNP3s are in few- and multi-layer and GNP4s are in single-layer and few-layer (2-, 3- and 4-

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layers) structures [16, 24]. The aspect ratio given in the Table 1 were calculated by dividing the lateral dimensions by the thickness values. Each GNPs were exfoliated by using tip sonicator (Sonics, 750 Vef) for 3 h under the conditions of successive vibration for 16 s and standby for

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25 s at the 20 kHz frequency and 40 % of amplitude before mixing with SiAlON powders.

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The SiAlON matrices were consist of the commercial α-Si3N4 (UBE Industries Ltd., Japan) with Al2O3 (Alcoa-A16SG), AlN (Tokuyama, H type, d50: 2–2.4 μm, Japan), Sm2O3 (99.9% purity, Stanford Materials Corp., USA), Y2O3 (99.9% purity, H.C. Starck Berlin, Germany), CaCO3 (Reidel-de Haen, Germany) as sintering additives. Each of GNPs were added to SiAlON forming powders in the amount of 1.5, 2, 3 and 4 wt. %. Sodium dodecyl sulfate (SDS, 2.5 wt. % ) was also added to the powder mixture as an anionic surfactant to prevent re-agglomeration of the GNPs.

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Table 1. The properties of the four different GNPs used during the study. GNPs

Average Lateral Size Thickness Aspect Ratios (µm) (nm)

Purity (%)

GNP1

3.28 (1.72-6.17)

5-10

172-1,234

99.9

GNP2

8.82 (4.22-18.28)

50-100

42-357

96-99

GNP3

12.22 (5.40-30.15)

5-8

675-3,770

99.9

GNP4

12.62 (5.40-30.45)

0.8-1.2

6,550-25,000

99.8

The GNPs-SiAlON powders were firstly mixed with a 3 h sonication in the isopropanol

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medium. Then, the mixing was continued in the plenatary ball mill at 120 rpm for 1.5 h. After removal of isopropanol in the evaporator (Heidolph), the 300-mesh sieve were used for each powder mixtures.

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The pure SiAlON and GNPs-SiAlON compositions were sintered by using the SPS furnace (HP 25D, FCT GmbH) at 1800 °C with an uniaxial pressure of 50 MPa for 15 min under vacuum

and a thickness of 6 mm.

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2.2. Characterisation of the Materials

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atmosphere. The heating rate was 100 °C/min. The sintered samples had a diameter of 20 mm

It was expected that the uniaxial load in the SPS furnace resulted in the orientation of GNPs in

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the matrix microstructures. For this reason measurements and investigations were carried out in the through-plane direction (//) which is parallel to the SPS pressing axis and in the in-plane

[8].

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direction (⊥) which is perpendicular to the SPS pressing axis as schematically shown elsewhere

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The SPSed samples were polished down to colloidal silica process by using automatic polisher (STRUERS, TegraPol-25). The morphologic images of the GNP1, GNP2, GNP3 and GNP4 were taken by using secondary electron detector (SE-SEM) whereas the polished surfaces of the GNPs-SiAlON composites examined with backscatter electron detector (BSE-SEM) in the SEM (Zeiss, SUPRA 50 VP).

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The Raman measurements (WITec, alpha 300) were performed on the each sonicated GNP 1, GNP 2, GNP 3 and GNP 4 powders and also on the 1.5 wt. % GNPs-SiAlON composites in the through-plane direction.

The Lotgering factor (LF) [25] values of the GNPs were calculated by using the through-plane direction XRD peak intensities obtained from the sintered bulk composites and the ground powdered form of the composites. Details were given elsewhere [7].

The electrical resistance values of the samples were measured by using two point method in the

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Precision Impedance Analyzer (Agilent 4294 A) at 50 MHz frequency in the through plane and in-plane directions. All samples were prepared in sizes of 6 mm*6 mm*2 mm (l*w*h) to eliminate the influence of different sample sizes on the measurements and calculations. The surfaces to be measured were coated with conductive Au-Pd film by using sputtering technique. At least ten measurements were carried out and average values were used during the study. The

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electrical conductivities (σ) of the samples were calculated by using the equation given below:

𝐿

(1)

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𝜎 = 𝑅𝑥𝐴

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where L (m) , R (Ω) and A (m2) are the thickness, resistance and surface area of the samples,

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respectively.

3.

Results and Discussions

3.1. Morphological and Microstructural Investigations

The morphologic images of four different GNPs were given in Figure 1. It was observed that morphological images (Fig. 1) and lateral dimensions of GNPs given in Table 1 were compatible with each other. GNP1 (Fig. 1 a) has the smallest lateral dimension with an average 5

of ~ 3.3 μm among the GNPs used in the study and GNP2 (Fig 1 b) followed it with an average lateral dimension of ~ 8.8 µm. When Table 1 and Fig. 1 c, d were evaluated together it was

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obvious that the lateral dimensions of GNP3 and GNP4 were close to each other (d50: ~12 µm).

Figure 1. SEM images of the (a) GNP1, (b) GNP2, (c) GNP3 and (d) GNP4 [36].

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Raman analyses (Fig. 2) were carried out to evaluate the thickness of the GNPs together with the data given in Table 1, since morphological images (Fig. 1) could not provide clear information about the thicknesses. One of the methods used to determine the thickness or layer

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numbers of GNPs in the Raman spectra is dividing the intensity of the 2D band by the intensity of the G band (I2D/IG) [26, 27] and it can be stated that as this ratio approaches to 2, single-layer structure can be obtained [27]. Table 2 shows the average values of the I2D/IG ratios obtained using the Raman results of four different GNP powders and SiAlON composites containing 1.5 wt. % GNPs. Raman analyses were performed at 1.5 wt. % GNP content which is the lowest amount of GNP added to matrix due to avoid the negative effects of the overlapped GNPs. The I2D/IG ratios of GNP1s ranged from 0.67 to 0.86, indicating that GNP1s were in both few-layer 6

and multi-layer structures. While the as-received GNP1s were only in multi-layer structure according to Table 1, the determination of the presence of few-layer GNPs in the Raman results indicated the positive effect of sonication on exfoliation. Considering the ratios obtained from GNP2 (I2D/IG: 0.55-0.96), it can be said that it composed of a mixture of GNPs in multi-layer and few-layer structures. The lower limit I2D/IG ratios obtained from GNP1 (0.67) and GNP2 (0.55) indicated that there were thicker multi-layer structures in the GNP2s than those found in GNP1s. In addition, although the thickness of GNP2 given in Table 1 indicated the multi-layer structure, the ratio value of 0.96 obtained from sonicated GNP2s showed that the sonication was able to exfoliate some GNP2s agglomerates. I2D/IG ratios of GNP3 ranging from 0.77 to

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0.98 have shown that it include few-layer and multi-layer GNPs [27, 28].The I2D/IG ratios of GNP4 ranged from 0.84 to 1.00, showing that it was composed of few-layer (2-, 3-, 4 layers) GNPs. Even if there are single-layer graphene platelets within the GNP4s, they may not be detected due to their agglomeration tendency. While the highest I2D/ID ratio values of GNP3s and GNP4s were close each other, the lowest ratio of GNP3s (0.77) were lower than the ratio

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of GNP4s (0.84). This indicated that both of them contain few layer GNPs with thickness values

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close to each other but GNP3 also include thicker GNPs than GNP4.

The I2D/IG ratios obtained from the GNPs in the sintered composites were lower than the values

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calculated from the powder form (Table 2). This may be explained by the fact that the GNPs were re-agglomerated under the effect of van der Waals and Coulomb attractions during powder

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preparation and/or sintering processes.

When there are defects and/or disorders in the graphene lattices, the D and D' bands are visible in the Raman spectra. The ratio of D band intensity to G band intensity (ID/IG) gives information

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about defects or disorders in the crystal structure of GNPs [26, 27]. Even if the average ID/IG ratios and value ranges of four different GNP powders were close to each other it was

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determined that the ratios of each type of GNPs obtained from sintered composites were higher than those of powdered forms (Table 2). This may be an indication that the sintering process and stages of powder preparation such as sonication and milling caused additional defects/disorders in GNPs lattices.

Table 2. Average I2D/IG and ID/IG values calculated by using at least ten Raman measurements from GNP powders and 1.5 wt. % GNPs containing SiAlON composites in the through-plane (//) direction. The values in brackets are for the measurements range [36]. 7

Average I2D/IG

Average ID/IG

GNP1

0.80 (0.67-0.86)

0.83 (0.81-0.88)

GNP2

0.76 (0.55-0.96)

0.82 (0.70-0.96)

GNP3

0.87 (0.77-0.98)

0.84 (0.73-0.95)

GNP4

0.95 (0.84-1.00)

0.82 (0.59-0.97)

1.5 wt. % GNP1-SiAlON

0.77 (0.62-0.80)

0.85 (0.79-0.89)

1.5 wt. % GNP2-SiAlON

0.72 (0.50-0.90)

0.86 (0.80-0.91)

1.5 wt. % GNP3-SiAlON

0.81 (0.72-0.95)

0.87 (0.79-0.87)

1.5 wt. % GNP4-SiAlON

0.86 (0.76-0.90)

0.88 (0.81-0.93)

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Materials

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Figure 2. Raman spectra obtained from GNP1, GNP2, GNP3, GNP4 powders and SiAlONs containing 1.5 wt. different GNPs in the through-plane (//) direction.

Figure 3 shows in-plane (⊥) direction BSE-SEM images taken from the pure SiAlON (Fig. 3 a, b) and composites containing GNP1 (Fig 3 c, d), GNP2 (Fig. 3 e, f), GNP3 (Fig. 3 g, h) and GNP4 (Fig. 3 i, j) at low and high magnifications. The fact that no porosities were found in the microstructures of the samples showed that they were fully densified. GNP1s representing one of the thickest GNPs in this study were distributed as stacked layers in the SiAlON microstructure (Fig. 3 c, d). The thicker GNP2s have formed isolated regions in SiAlON matrix

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(Fig. 3 e, f). However, the GNP2s that could be separated by sonication showed a more homogeneous distribution and this slightly reduced the negative effects of thicker GNP2s on the dispersion in SiAlON matrix. While the GNP1s have a narrow size distribution, the GNP3s were composed of both large and small GNPs; thereby they exhibited more homogeneous dispersion in the matrix (Fig 3 g, h). Especially their thinness had an important effect on their

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uniform distribution. GNP4s that close to GNP3s in terms of platelet size distribution showed similar dispersion characteristics in SiAlON matrix. However, images (Fig. 2 g-j) have also

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clearly shown that GNP4s were slightly thinner than GNP3s.

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Figure 3. In-plane (⊥) direction BSE-SEM images of the SPSed (a, b) pure SiAlON and SiAlON matrix composites containing 4 wt. % (c, d) GNP1, (e, f) GNP2, (g, h) GNP3 and (i, j) GNP4.

The BSE-SEM images (Fig. 3) taken in the in-plane direction of the each composites showed

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that the basal planes of the GNPs were oriented perpendicular to the SPS pressing axis. These pointed out the anisotropic microstructures and properties agreed with the studies [3, 8, 11, 12,

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29] on GNPs-ceramic matrix composites produced by techniques using uniaxial pressure, such

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as SPS and hot pressing.

In this study, Lotgering Factor (LF) values were calculated to determine the orientation degree of the four different GNPs in the SiAlON matrix. Lotgering factor approaching to 0 and 1 point

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out the random and strong orientation, respectively. Although the GNP peaks obtained from the basal planes were apparent in the through-plane direction XRD pattern, peaks were not visible in the in-plane direction pattern due to GNPs orientation. For this reason, LF values were

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calculated using through-plane direction XRD data. Table 3 shows the LF values of four different GNPs. GNP1 has the lowest LF values (0.712-0.835) among the used GNPs. This

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pointed out that the smaller lateral dimensions of the GNPs make it difficult for the basal planes to align in a certain direction in the matrix microstructure. In this case, it was clear that some of the basal planes of GNP1 were also oriented in other directions from the direction perpendicular to the SPS pressing axis. The fact that the LF values of GNP2, GNP3 and GNP4 were close to each other and higher than the values of GNP1 showed that the large lateral dimension had a positive effect on orientation in particular direction.

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3.2. Electrical Properties

Table 3 shows the room temperature DC electrical conductivity values of the pure SiAlON and composites containing 1.5, 2, 3 and 4 wt. % GNP1s, GNP2s, GNP3s and GNP4s in the throughplane (//) and in-plane (⊥) directions. The electrical conductivity of the fully insulating SiAlON (~10-9) [3] has dramatically increased ~106-107 times to around 10-2/10-1 with 1.5 wt. % contribution of all GNPs types. However, the lowest electrical conductivities among the composites containing same amounts of GNPs were obtained in the SiAlON containing GNP1s, while the highest electrical conductivity values were measured in the GNP4s-SiAlON

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composites. Figure 4 clearly illustrates the conductivity differences between composites at same GNPs contents. The electrical conductivities of GNP4s-SiAlON composites were from ~1.3 to 12 and ~3 to 15 times higher than GNP1s-SiAlONs in the through-plane and in plane directions, respectively. Even though the SiAlON containing GNP1s and GNP2s have the same and/or close electrical conductivity values in both directions at low GNPs contents (1.5 and 2 wt. %),

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the difference between them became obvious and the electrical conductivities of the composites containing GNP2s composites were found to be higher (~2-8 times) than GNP1s added

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composites when the GNPs content was increased to 3 and 4 wt. %. In addition, the electrical conductivities of composites containing GNP3s were about 2-3 times higher than those of

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composites containing GNP2s. Moreover, the conductivities of GNP3s-SiAlON composites were lower than composites containing GNP4s (Table 3 and Fig. 4). In summary, the performances of the four different GNPs in terms of the electrical conductivity they provides to

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SiAlON can be listed as follows: GNP4>GNP3>GNP2>GNP1.

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Table 3. Lotgering Factor values (//) of the GNPs and room temperature DC electrical conductivity values of the SiAlON matrix composites containing four different GNPs in the through-plane (//) and in-plane (⊥) directions [36].

GNPs (wt. %)

LFs for Electrical Conductivities (σ) (Sm-1) GNPs Through-plane (//) In-plane (⊥) (//)

0

-

10-9

1.5 wt. %

0.712

0.032 ± 0.001

0.037 ± 0.001

1.2

2 wt. %

0.805

0.033 ± 0.002

0.052± 0.005

1.6

3 wt. %

0.823

0.038 ± 0.002

0.072 ± 0.003

1.9

4 wt. %

0.835

0.058 ± 0.003

0.074 ± 0.002

1.4

1.5 wt. %

0.800

0.032 ± 0.0001

0.045 ± 0.0005

1.4

2 wt. %

0.884

0.033 ± 0.0001

0.053 ± 0.001

1.6

3 wt. %

0.946

0.080 ± 0.0001

0.250 ± 0.001

3.1

4 wt. %

0.948

0.200 ± 0.01

1.5 wt. %

0.844

2 wt. %

σ⊥/ σ// -

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GNP1-SiAlON

0.570 ± 0.04

2.9

0.036 ± 0.002

0.089 ± 0.001

2.5

0.899

0.039 ± 0.002

0.150 ± 0.01

3.8

3 wt. %

0.936

0.180± 0.003

0.610 ± 0.02

3.4

4 wt. %

0.950

0.390 ± 0.004

0.800 ± 0.02

2.1

1.5 wt. %

0.787

0.040 ± 0.001

0.120 ± 0.001

3.0

2 wt. %

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GNP2-SiAlON

0.888

0.052 ± 0.002

0.156 ± 0.001

3.0

0.925

0.200 ± 0.01

0.960 ± 0.02

3.0

0.930

0.680 ± 0.03

1.100 ± 0.02

1.6

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GNP4-SiAlON

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3 wt. % 4 wt. %

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GNP3-SiAlON

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Figure 4. Electrical conductivity differences between composites in the (a) through-plane (//) and (b) in-plane (⊥) directions at same GNPs contents.

The fast increase in the measured conductivity value of SiAlON with four different types of 1.5 wt. % GNPs addition and the slower progression of the increase in conductivity as the amount of added GNPs increased can be explained by the percolation theory used to study the behavior of the mixture of conductive and non-conductive components. [30, 31]. The percolation theory

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valid when the concentrations is higher than the percolation value and the DC conductivity of the composites expressed by the equation given below: σdc = σc (PGNPs-Pc)t for PGNPs > Pc

(3)

where σdc and σc are the DC conductivity values of the composites and conducting component, PGNPs is the volume fraction of the GNPs, Pc is the percolation threshold or critical volume fraction and t is the critical exponent.

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The curves obtained by fitting the measured (σdc) and known data (PGNPs) of the composites to the equation 3 are presented in Figure 5. Furthermore, the calculated fitting parameters (σc, Pc and t) of the composites containing GNP1, GNP2, GNP3 and GNP4 in the through-plane (//)

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and in-plane (⊥) directions are given in Table 4.

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Figure 5. The percolation threshold (Pc) values and curves obtained for SiAlON matrices

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containing (a) GNP1 (b) GNP2, (c) GNP3 and GNP4 by fitting the known data (σdc and PGNPs) to the equation 3. Insets are the logarithmic plots of electrical conductivities (σdc) against (PGNPs-

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Pc) [36].

Table 4. The parameters obtained by fitting the GNPs content (PGNPs) and electrical

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conductivity values of composites (σdc) to equation 3 [36]

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σc

Pc (wt. %)

t

Adj R2

Materials

(⊥)

(//)

(⊥)

(//)

(⊥)

(//)

(⊥)

GNP1-SiAlON

0.030

0.050

1.120

1.115

0.410

0.350

0.887

0.934

GNP2-SiAlON

0.020

0.040

0.580

0.560

1.980

1.850

0.893

0.932

GNP3-SiAlON

0.030

0.090

0.575

0.535

2.000

1.740

0.932

0.967

GNP4-SiAlON

0.040

0.100

0.555

0.475

2.000

2.000

0.916

0.913

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(//)

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The percolation threshold values of the SiAlON matrices containing GNP1, GNP2, GNP3 and GNP4 were 1.120, 0.580, 0.575 and 0.555 wt. % in the through-plane direction and 1.115, 0.560, 0.535 and 0.475 wt. % in the in-plane direction, respectively (Table 4). The percolation threshold values obtained in the both directions were summarized as follows: GNP4
Further, in-plane direction electrical conductivity values were 1.2-1.6, 1.4-3.1, 1.6-3.0, 2.1-3.8 times higher than the through-plane conductivities of SiAlON containing GNP1, GNP2, GNP3 and GNP4, respectively (Table 3 and Fig. 4). These results are in agreement with the study [12]

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where the σ⊥/σ// ratios of GNPs-yttria tetragonal zirconia composites produced by using SPS were 3-6 times higher in the in-plane direction than through-plane direction. In addition to these, the percolation threshold values were exhibited anisotrophic behavior by the fact that the values of in-plane and through-plane directions of composites containing same kind of GNPs were different. The in-plane direction percolation threshold values were found to be lower than the

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through-plane direction values (Fig 5 and Table 4). The percolation threshold values of ceramics/polymer matrix composites in which GNPs [3, 11, 15] aligned were determined lower

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in the in-plane direction than the through-plane direction similar to this study. Higher electrical conductivity and lower percolation values in the in-plane direction than through-plane direction

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can be attributed with orientation of GNPs in the SiAlON microstructure and also their high electrical conductivity along the basal planes (in the in-plane direction) and poor conductivity in the direction perpendicular to the basal planes (through-plane direction) [32]. Moreover, in

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our study, the ratio of σ⊥/σ// of GNP1-SiAlON composites were much lower in comparison to others because GNP1s with lower LF values were more randomly oriented in SiAlON microstructure than other GNPs. Lower differences between conductivities in the in-plane and

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through-plane directions can be attributed to the rotation of some of the GNP1s in different

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directions from the in-plane direction and they are twisted.

The main reason of the differences in the electrical conductivity and percolation threshold values of composites containing four different GNPs is that the GNPs used have different properties such as lateral dimension, thickness and aspect ratio. Electrical conductivity is generated by the transfer of electrons from one GNP to the other GNP in the non-conductive SiAlON matrix. The mechanism by which the electrons are transferred along the formed network between neighboring GNPs without any physical contact is the tunneling effect [18, 33]. However, the insulating ceramic matrix surrounding the GNPs creates a resistance against 18

transmission of the free electrons. Moreover, this tunneling resistance is affected by the physical properties of GNPs and the gaps between GNPs [34]. As shown in Table 3 and Fig. 4, conductivity values have changed in directly proportional to the lateral dimensions of GNPs. The electrical conductivity of composites containing GNP4s, which has the largest lateral dimension among the used GNPs, had the highest values (σ//:0.680, σ⊥:1.100) whereas the smallest GNP1s added composites exhibited lowest conductivities (σ//:0.058, σ⊥:0.074) at same (4 wt. %) GNPs contents. In addition, when the conductivities of SiAlONs containing GNP1s and GNP2s representing thick GNPs during the study were compared, it was observed that the conductivity performance of GNP2 with ~ 8.8 µm lateral size was almost one order of

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magnitude higher than GNP1 with 3.3 µm size. The tunneling resistance displayed a decreasing tendency when the lateral dimension of GNPs increased and this is in agreement with the literature [14, 18, 34]. It was also obvious that the effects of GNPs lateral dimensions on the electrical conductivities of composites were more pronounced in the in-plane direction than in

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the through-plane direction. Jun et al. [14] measured the electrical conductivity values of the polymer composites containing GNPs with different lateral dimensions. They reported that the in-plane direction electrical conductivity value (3.2 Sm-1) achieved with the addition of 0.4 wt.

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% GNPs with a large lateral dimension (~100 µm) was two orders of magnitude higher than the conductivity value (1.4x10-2 Sm-1) obtained by adding the 1.9 wt. % small GNPs (2-15 µm).

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Additionally, the tunneling resistance of CNTs with 2 nm diameter (1012 Ω) was two orders of magnitude higher than those of CNTs with 10 nm diameter (1010 Ω) among the CNTs coated

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with 1 nm thick polymer [34].

Percolation occurs when complete conductive network formed between the GNPs for the

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transmission of free electrons. The percolation threshold values of the GNPs-SiAlON composites decreased as the lateral dimensions of the used GNPs increased (Fig. 5 and Table 4) and it is a good agreement with polymer or ceramic matrix containing GNPs or CNTs with

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different dimensions [15, 18, 22]. In one of these studies [15], the percolation thresholds of the polypropylene matrix containing GNPs with 5 µm lateral dimension was calculated as 5.05 vol. % and the percolation threshold value decreased to 3.66 vol. % when the GNPs, which have 25 µm lateral dimension and same thickness were added to the same matrix. These observations point out that an effective conduction path for electron conduction can be achieved more easily with large GNPs in smaller quantities in comparison to small-sized GNPs. Larger GNPs or

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CNTs have more junctions than the smaller ones, which provides an increasing number of conductive paths and reducing the percolation resistance [23].

Another important parameter that has an impact on tunneling resistance is the length of the gap formed by the matrix between GNPs [3, 34]. As the length of gap between the GNPs in the overall microstructure increases, the tunneling resistance also increases. This indicated the importance of the dispersion states of GNPs in the microstructure of matrix material. While homogeneous dispersion of GNPs affect the conductivity positively as they prevent the formation of large gaps between them, the inhomogeneous dispersion caused by GNP-rich and

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GNP-poor regions in the matrix negatively affects the conductivity by formation of large gaps in the matrix. It was found that at the low GNP quantities (1.5 and 2 wt. %), the electrical conductivities of SiAlON containing GNP2s were close to those of containing GNP1s in the same amount, even though GNP2s had a larger lateral size in comparison to GNP1s. The fact that GNP1s have small, thick and narrow size distribution and thick GNP2’s formed GNP-rich

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and GNP-poor regions in the matrix made it difficult to construct conductive networks especially at low GNP contents (Fig. 2 d, f). Moreover, high thickness may caused the reduction

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in the number of dispersed GNPs per unit volume compared to thin GNPs in the matrix microstructure. Yu et al. [34] who investigate the variation in tunneling resistances depending

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on the length of the gaps between CNTs in a CNTs-polymer matrix composite, calculated the tunneling resistance as ~1010 Ω at a distance of 1 nm and they reported that the resistance value

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increased to ~1019 Ω when the length increased to 1.8 nm, as consistent with our results.

Since the GNP3s and GNP4s have a wide platelet size distribution and they were composed of few-layer GNPs, they facilitated the electron transfer because of more homogeneously

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dispersing in the SiAlON microstructure than GNP1s and GNP2s. Furthermore, despite they have almost the same lateral dimension (~ 12 µm), GNP4s were composed of thinner GNPs in

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comparison to GNP3 and this led to the formation of more homogeneous dispersion of GNP4 and shorter gaps between them than GNP3s in the SiAlON microstructure. Therefore, higher electrical conductivity values were achieved in the GNP4-SiAlON compared to GNP3-SiAlON (Table 3 and Fig. 4.). These results are in agreement with a study [35] in which investigating the effects of different thicknesses of GNPs (but same lateral dimension) on the electrical conductivity of the polymer matrix composite. In that study, the electrical conductivity of the polymer containing 1-/2-layer GNPs was found as 10-9 Scm-1, while the conductivity of polymer containing few-layer GNPs was measured as 10-12 Scm-1, at the same GNPs amounts. These 20

findings also showed that the gaps lengths created by the insulator matrix between the GNPs had greater effects on the resistance of the composite in comparison to lateral dimensions. In a study [3], investigating the effects of different exfoliation techniques on the properties of GNPs and GNP-SiAlON matrix composites also supported this observation. Although one of the GNPs used were very small (~3 µm), they were more uniformly dispersed with smaller gaps in the matrix due to their thin structure (less than 3-layer) than the other GNPs, which were larger (~12 µm) but multi-layered. Thus, due to the low tunneling resistance, small GNPs provided lower percolation thresholds and ~ 2-3 times higher electrical conductivity than larger GNPs to

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SiAlON.

Another parameter that affects the composite resistance is the aspect ratio, which varies depending on the dimension and thickness of GNPs. In the same volume fractions, GNPs with high aspect ratio make the formation of percolation network faster in comparison to GNPs with low aspect ratio, resulting in higher electrical conductivity and lower percolation threshold

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values [15, 18, 20 - 23]. In our study, when the electrical conductivities of composites containing 1.5 wt. % GNP4s and GNP1s were compared, it was determined that the

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conductivity of the composites containing GNP4s having the highest aspect ratio (6,550-25,000) were one order of magnitude higher than those containing GNP1s, which is one of those with

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low aspect ratios (172-1,234). In addition, percolation threshold values were increased as the aspect ratio of the GNP decreased. According to the Table 1, while the as-received GNP2s had lowest aspect ratio values, the presence of exfoliated GNP2s as a result of sonication led to the

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presence of GNP2s with high aspect ratio in sintered compositions and positively affected the electrical conductivity. Furthermore, Hicks et al. [21], who studies the effects of changes in the aspect ratio values on tunneling resistance by using tunneling-percolation modeling and Monte

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Carlo simulation, have determined that tunneling resistance decreased by six orders of magnitude when the aspect ratio value increased from 1 to ~30 at the thin film with 0.67 vol.

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% GNPs content.

In addition to tunneling effect, electron conduction occurs through direct contact between the conductive components [33]. The thickness of the GNPs also significantly influences the overall conductivity of GNPs-ceramic composites by effecting the scale of the contact resistance of GNPs that occurs when they are in direct contact with each other. As shown in Table 3 and Fig. 4, the use of thick GNPs (GNP1 and GNP2) improved the electrical conductivity of SiAlON less than thinner GNPs (GNP3 and GNP4). Nirmalraj et al. [24] 21

measured the electrical resistance of the ~100 GNPs with different thicknesses (1-, 2-, 3- and multi-layer) in the graphene film by using conductance imaging atomic force microscopy (CIAFM). They found an interplatelets resistance jump of 550 Ω at the intersection between 1layer and 2-layer GNPs while the resistance jump was determined as 6500 Ω at the junction of multi-layer and tri-layer GNPs. These results showed that the contact resistance highly dependent on the thickness of the GNPs and high interplatelets resistance values were obtained at the junctions of thick GNPs while low resistances occur between thin/individual GNPs.

One of the reasons why the electrical conductivities of composites containing thick GNPs

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(GNP1 and GNP2) were lower than those containing thin GNPs (GNP4) was the intrinsic conductivities of GNPs. The intrinsic conductivity of GNPs, which is one of the mechanisms provide electrical conduction in the GNPs-insulating matrix significantly influences from the number of layers forming GNPs. While the free electrons are transmit in one direction with high mobility along the surface channels in the single-layer GNP, their movement slows down

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due to three dimensional scattering at the interlayer channels as the number of layers increases [16, 17]. In this case, as the thickness of the GNP increases, the electrical resistances increase

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due to the increase of the interlayer channels. Fang et al [16] who developed the numerical simulation for making the correlating between thicknesses of GNPs and their electrical

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conductivity behavior, reported that the decrease in the electrical conductivity of few-/multilayer GNPs was caused by the inhibition of charge carriers movement, not by the reduction in

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the amount of charge carrier.

After achieving the percolation, the number of conductive paths and the number of charge carriers increased as the amount of GNPs addition increased, thereby the electrical conductivity

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of the composites continued to increase. As the GNPs content increased, the tunneling

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resistance decreased by reduction in the SiAlON matrix thickness between the GNPs.

Conclusion

In this study, the effects of the addition of four different types of GNPs with different properties such as lateral dimension, thickness and aspect ratio to the SiAlON matrix were investigated. In-plane direction electrical conductivities were higher than the through-plane direction. However, in-plane direction percolation threshold values were lower than those in the through22

plane direction. The electrical conductivity of the composites containing the GNP4s, which have highest lateral dimension and aspect ratio among the used GNPs were found to be one order of magnitude higher than those containing GNP1s with the smallest lateral dimension and low aspect ratio. In addition, the percolation threshold values of composites containing GNP4s were lower than that containing GNP1s. Considering the electrical conductivity of SiAlONs containing GNP1s and GNP2s, which represent thicker GNPs in the study, it was observed that the composites containing GNP2s with a larger lateral dimension (~8.8 µm) had an electrical conductivity 2-8 times higher than those containing small GNP1s (~3.3 µm). Additionally, the percolation threshold values of GNP2s-SiAlON composites were lower than GNP1s-SiAlON

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composites. Despite close lateral dimension values, the SiAlONs containing the thicker GNP3 had lower electrical conductivity and higher percolation threshold values than that containing the thin GNP4s. All these results showed that in order to provide fast and high amount of electrical conductivity to the insulating matrix, the used GNPs should have a high lateral

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dimension and high aspect ratio but low thickness properties.

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Declaration Of Interest Statement The authors whose names are listed immediately below certify that they have no affiliations with or involvement in any organization or entity with any financial interest or non-financial interest in

Acknowledgements

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the subject matter or materials discussed in this manuscript.

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The Eskisehir Technical University Scientific Research Projects under the project numbers of 1606F570 and 19ADP019 supported this work. Authors would like to thank the MDA Advanced Ceramics Ltd. for providing all the raw materials for the production of SiAlON

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matrices and also to Mr. Mert Gul, Prof. Dr. Aydın Dogan and Mr. Emre Uraz for the provision

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of equipment to measure electrical conductivities and calculations of percolation thresholds.

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