Enhanced electrical property of graphite/Al2O3 composite fabricated by reductive sintering of gel-casted body using cross-linked epoxy polymer

In the present study, graphite/alumina composites are fabricated via reductive sintering of gel-casted green bodies with structurally controlled cross-linked epoxy polymers for the first time. The cross-linking degrees of polymers are tuned by the amount ratio of epoxy monomer/polyvinyl alcohol cross-linker utilized in gel-casting process. Superior electrical properties with respect to 5-fold enhanced electrical conductivity and 2-fold higher carrier mobility are successfully achieved in graphite/alumina composite fabricated from cross-linked epoxy polymer, whose phenomenon is attributed to the excellent conductive path in ceramic matrix established by highly uniform network with improved graphitization degree.

In this study, nano-carbon/alumina composites are fabricated via reductive sintering of gel-casted green bodies with structurally controlled cross-linked epoxy polymers for the first time. The cross-linking degrees in different polymers were tuned by the amount ratio of epoxy monomer/polyvinyl alcohol (PVA) cross-linker, for perusing an enhanced electrical property of nanocarbon/alumina composite. With detailed characterization on the chemical structure and physical property as flow curve of polymer, relative density, microstructure, distribution, and structure of nano-carbon network, the effects of the cross-linked structure in polymer on the electrical properties with respect to electrical conductivity, carrier mobility, and density of the fabricated nanocarbon/alumina composite were systematically investigated. Against the utilization of toxic and air-sensitive monomer as methacrylamide in previous study, the non-toxic and air ambient available epoxy monomer, and facile structure tuning of polymer, as well as the superior electrical property in fabricated nano-carbon composite presented in current study opens a new strategy for the development of functional ceramic material.

1 Sample preparation
The nano-carbon/alumina composite was fabricated by gel-casting and reductive sintering described as the following process, where bare polymer was also prepared as reference sample. Alumina powder (α-Al 2 O 3 , 99.48%, AL-160SG-4, Showa Denko K.K., Japan; median diameter D 50 = 0.59 μm measured by laser diffraction particle size analyzer) was dispersed in distilled water with dissolved ammonium salt of polycarboxylate (Celuna D-305, Chukyo Yushi Co., Ltd., Japan) and PVA (polymerization degree = 500, Kanto Chemical Co., Inc., Japan) as dispersant and cross-linker for gelation, respectively. The slurry was well-mixed via ball-milled for 24 h at 60 rpm. Then, epoxy resin (poly glycidyl ether, Denacol EX-1610, Nagase ChemteX Corporation, Japan) as gelation agent was added into the well-mixed slurry and ball-milled for another 30 min. The slurry composition is shown in Table 1 excepting PVA. The amount ratio of epoxy/PVA was decided as 100/0, 80/20, and 60/40. In our pre-experiment, loading of PVA over 50% was not able to cast green body because of the high viscosity of slurry. The gelation was initiated by the addition of triethylenetetramine (TETA, Kanto Chemical Co., Inc., Japan) as a gelation initiator. Then degassed slurry was poured into mold and cured for 24 h. Demolded wet green bodies were then dried at 25 ℃ gradually, and relative humidity was altered from 90% to 60% for 5 d. For detailed study in cross-linking reaction between epoxy monomer and PVA, bare polymers without alumina powder loading were also prepared by altered epoxy monomer/ PVA ratios of 100/0, 95/5, 90/10, 85/15, 80/20, 70/30, and 60/40. For reductive sintering process, green bodies were sintered at 1600 ℃ for 2 h in argon atmosphere and the obtained nano-carbon/alumina composites were ready for characterizations.

2 Characterization
The chemical structure of bare polymers (without alumina powder) was analyzed by attenuated total reflection (ATR) method of an infrared Fourier transform (FT-IR, FT/IR-6600, JASCO, Japan). The rheological property of slurry was measured by a rotary rheometer (Kinexus lab+, Malvern, UK). Shear rate of 0.1-100 s −1 was induced on slurry with a coneand-plate geometry (cone angle = 4°, diameter = 40 mm). The relative density and the porosity were measured by the Archimedes method. The microstructure of sintered body was observed by scanning electron microscope (SEM, JSM-6010LA, JEOL). For analyzing element distribution on the surface of sintered body, energy dispersive X-ray spectroscopy (EDS) mapping was conducted. The amount of carbon contents in fabricated nano-carbon/alumina composite was measured by a commercial combustion infrared detection instrument (CS analyzer, LECO-CS884). The measurements were conducted on well-grounded powder. The carbon structure was investigated by Raman spectroscopy (in Via Raman microscope, Renishaw, UK) with a 532 nm laser.
The electrical properties were analyzed on a nanocarbon/alumina composite piece with a size of 10 mm × 5 mm × 3 mm by physical property measurement system (PPMS, Quantum Design, Inc., USA). The electrical conductivity was measured using a 4-probe method with the applied 1 mA current and temperatures of 200, 220, 240, 260, 280, and 300 K. The Hall measurement used a 5-probe method with the fixed current of 1 mA, and the applied magnetic field was set from −1.0 to 1.0 T. The mobility and the carrier density were calculated from the electrical conductivity and Hall resistivity.

Results and discussion
The chemical structure of the bare cross-linked epoxy polymers with different epoxy monomer/PVA ratio is analyzed by FT-IR spectroscopy, during which the spectra of epoxy monomer and PVA are also recorded, shown in Fig. 1. The assignments of all appeared peaks are summarized in Table 2. The broad band around 3400 cm −1 corresponding to O-H stretching vibration appears with C-O stretching vibration (peak L) of   [40] secondary alcohol are assigned to the hydroxyl group in side chain of epoxy monomer, PVA, and reacted epoxide group in prepared polymers. Peaks A and C/G attribute to stretching and bending vibrations of CH 3 , suggesting the existence of terminated CH 3 groups in each sample. Peaks B and F initialed from stretching and bending vibrations CH 2 along with peak H of C-H bending vibration indicate the hydrocarbon in all samples. The peak around 1295 cm −1 can be assigned as C-N bond, which is attributed to the trace amount of TETA. Peak N originated from epoxy ring is confirmed in epoxy monomer and disappears after PVA is induced which exemplifies that the polymerization of between epoxy monomer and PVA is progressed by crosslinking reaction. Peak D appears only in cross-linked epoxy polymers, which attributes to CH 2 in ringopened epoxide groups. The peaks J, K, and M are assigned to C-O-C ether group, originated from the main chain of epoxy monomer and cross-linked structure of the polymers, while such C-O-C peak appearing in PVA with C=O (peak E) comes from acetic acid groups in incompletely saponified structure. As a further comparison, the peak of OH stretching vibration is plotted as a function of OH in PVA/epoxide group in monomer and illustrated in Fig. 1(b). The shifting to lower wavenumbers can be ascribed to the increased intermolecular hydrogen bonding resulting from increased amount of PVA in prepared epoxy polymer [39]. In advance, the C-O/C-O-C ratio is also summarized as a function of epoxy monomer/PVA ratio in Fig. 1(c). Such ratio decreases when more PVA is added, which suggests that the cross-linking reaction is preferentially proceeded [40]. In following parts, chemical and physical properties of nano-carbon/ alumina composites fabricated with three selected epoxy monomer/PVA ratios of 100/0, 80/20, and 60/40 are discussed in details. The flow curve of degassed slurries is exhibited in Fig. 2, which demonstrates that each slurry shows pseudoplastic fluid behavior and good castability can be expected. The viscosity of slurry increases with the increase of PVA amount, which shows good agreement with previous report [45]. As the relative density and open porosity of sintered body given in Fig. 2(b), the relative density decreases and open porosity increases as PVA amount increases. Generally, lower density affects to decreasing of electrical conductivity [46]. Nevertheless, electrical conductivity was increased in present study. This phenomenon is considered to be attributed to the pore inducing during gel-casting process with high viscosity slurry involving more PVA. Figure 3 displays the SEM images of sintered bodies and corresponding EDS mapping results. It can be observed that the average grain size in nano-carbon/  alumina composites fabricated from epoxy monomer/ PVA ratios of 100/0, 80/20, and 60/40 decreases gradually. We suggest that the inhibition of grain growth from raw powder is attributed to the residual carbon, calculated from densities of Al 2 O 3 (3.94 g/cm 3 ) and graphite (2.26 g/cm 3 ), that is, 1.15, 1.15, and 1.35 vol% for 100/0, 80/20, and 60/40 cases, respectively. As we previously mentioned, polymer formed in gel-casting transformed to nano-scale carbon coating on individual alumina grains [30]. The generation of highly uniform network in fabricated composites is also confirmed in EDS mapping of carbon. Regards to the result of relative density given in Fig. 2(b), it is also considerable that the decrement of relative density is probably originated from not only the pore induced in gel-casting of high viscosity slurry but also the inhibited sintering behavior caused by large carbon residue.
TEM images and FFT diffraction pattern are depicted in Fig. 4. The pore structure is generated with the increment of PVA amount in gel-casted green body, which is well correlated with open porosity results shown in Fig. 2(b). By focusing the TEM observation of Al 2 O 3 surface, an expended image of Fig. 4(b) is illustrated in Fig. 4(d). Dark area with large contrast belongs to crystalline structure and is observed on Al 2 O 3 surface. The observed interplanar distance of 0.341 nm is identified the same as the spacing of the (002) planes of graphite. Figure 4(c) is illustrated as enlarged Fig. 4(e) for further confirmation of the produced carbon structure between grain boundaries. The FFT pattern in Fig. 4(e) displays 2.92 nm −1 of the distance from central peak to diffraction peak as typical hexagonal crystalline structure of graphene. Figure 4(f) demonstrates the distribution of graphene and graphite in pore-involved composite fabricated in present study. As a result, we think percolation through the graphite/graphene network between Al 2 O 3 grains and pore contributes to the electrical percolation and enhanced electronic conductivity even in 60/40 composite which contains more pores.
Next, the chemical structure of graphite network in fabricated composites is analyzed by Raman spectra, and the results are illustrated in Fig. 5. Three peaks appear at 1350, 1580, and 1620 cm −1 , which can be ascribed to D-band, G-band, and D'-band, respectively, are observed in all samples [47]. D-band is assigned to the edge of crystal and point defects of graphene, while G-band is ascribed to lattice vibration related to C-C (sp 2 ) bond. Furthermore, the defect of graphite-like structure is designated by D'-band [47,48]. The graphitization degree is investigated by R-value, refering to the intensity ratio between D-band and G-band (I D /I G ) [48]. The higher R-value is ascribed to a less graphitized and more disordered carbon structure. To compare epoxy monomer/PVA ratio of 100/0 case with 80/20 and 60/40, R-value is decreased from 0.210 to 0.199 and 0.195, respectively. Thus, it can be concluded  that the graphitization degree is enhanced by the increase of PVA amount. It has been reported that low molecular weight volatile compositions are less generated from covalent bonding of cross-linked structure [49,50]. Therefore, we suggest the highly cross-linked structure in polymer generated by epoxy monomer/PVA ratio of 60/40 contributes to the improved graphitization degree. Meanwhile, it has been mentioned that the hydroxide group of PVA promotes the generation of aliphatic polyene structures under thermal degradation and such structure can form aromatic structure by Diels-Alder reaction and intra-molecular cyclization [51]. Thus, the excess PVA existed in the 60/40 case also plays a possible role in enhanced graphitization degree.
Electrical properties with respect to electrical conductivity, Hall resistivity, carrier mobility, and density of graphite/alumina composites are demonstrated in Fig. 6. As indicated in Fig. 6(a), the result demonstrates that graphite/alumina composite fabricated from epoxy monomer/PVA ratio of 60/40 exhibits the best electrical conductivity, which is attributed to the improved graphitization degree observed in graphite network formed by highly cross-linked polymer. The graphite network coated on alumina grains establishes a continuous conductive path along the grain boundaries, in despite of the different relative densities and the similar carbon content. The graphite generated on alumina particles and percolated around pore is proved . It is worth noting that the electrical conductivity of the fabricated graphite/alumina composite in this study is also significantly higher than those in the other conventional processes [52][53][54][55]. In addition, all the composites exhibit typical semi-conductive character since the electrical conductivity increases linearly with the increase of measuring temperature. The positive Hall resistivity observed in positive magnetic field designated in Fig. 6(b) demonstrates a p-type semiconductive property of the obtained graphite/alumina composite. Our co-author has derived the origin for hole harvested p-type semi-conductive property, that is, the defects in graphite structure act as acceptors of π electrons, and doped aluminum atom in defect center provides excess hole carriers in graphite/alumina composite [32,56]. Regards to the results illustrated in Figs. 6(c) and 6(d), the carrier mobility is increased along the electrical conductivity in graphite/alumina composite fabricated from highly cross-linked polymer structure while the carrier density decreases due to the small amount of defective graphene. Therefore, it can be concluded that the superior electrical property with enhanced electrical conductivity and mobility is successfully achieved by reductive sintering of gelcasted bodies with selective structurally-controlled cross-linked epoxy polymers.

Conclusions
In this work, graphite/alumina composites were fabricated by gel-casting and reductive sintering, where the structure of polymer generated in gelation was controlled by altering the amount ratio of epoxy monomer/PVA. Highly cross-linked polymer structure was obtained with increased PVA amount, which contributes to the generation of graphite network with improved graphitization degree in later sintering process. Although increased slurry viscosity and inhibited sintering behavior lead to a decreased relative density in composite, the highly uniform carbon network coated on alumina grain was confirmed in all samples. In addition, enhanced electrical conductivity and carrier mobility were also successfully achieved in graphite/alumina composite fabricated from cross-linked epoxy polymer. The superior electrical property was attributed to the excellent conductive path established in ceramic matrix by uniform graphite network, with improved graphitization degree induced from polymer with highly cross-linked structure. To view a copy of this licence, visit http://creativecommons. org/licenses/by/4.0/.