3.1. Density, porosity, microstructure and total amount of carbon of sintered samples:
Table 1 shows the notations and density of sintered samples. The LECO results indicating the amount of residual carbon inside sintered bodies are showed in Table 1. The theoretical density of each sample was calculated from LECO results, assuming the theoretical densities of alumina and graphite were 3.99 and 2.25 g/cm3 correspondingly. The relative density and open porosity were calculated from the results of Archimedes method and calculated theoretical density. Closed porosity is calculated from results of pycnometer. Relative density, open porosity and closed porosity are also showed in Table 1. The typical sintering behavior was pointed out clearly. Density of sintered bodies tends to be increased with increasing of sintering temperature or applied pressure.
Table 1
Residual carbon content, density and porosity of sintered C-Al2O3 samples:
Sam-ple
|
Conditions
|
Residual carbon (wt%/vol%)
|
Theoretical density (g/cm3)
|
Relative density
(%)
|
Open porosity
(%)
|
Closed porosity
(%)
|
A-1
|
PVA10
1400°C - 20 MPa
|
0.80 / 1.41
|
3.965
|
89.1
|
9.7
|
0.14
|
A-2
|
PVA10
1400°C - 80 MPa
|
0.53 / 0.94
|
3.974
|
98.5
|
0
|
1.64
|
A-3
|
PVA10
1500°C - 20 MPa
|
0.52 / 0.92
|
3.974
|
97.2
|
0.1
|
2.69
|
A-4
|
PVA10
1500°C - 80 MPa
|
0.45 / 0.80
|
3.976
|
98.1
|
0
|
1.43
|
B-1
|
PVA10.PCA1
1400°C - 20MPa
|
1.05 / 1.85
|
3.958
|
87.6
|
10.2
|
0.35
|
B-2
|
PVA10.PCA1
1400°C - 20MPa Aggre. powder
|
1.08 / 1.90
|
3.957
|
84.7
|
12.3
|
0.57
|
Despite of the same sintering conditions, relative density of sample B-1 was lower than that of sample A-1. From TOC results, the absorption of organic carbon on the surfaces of Al2O3 powder was enhanced by the addition of PCA, from 0.6 wt% in the A-series powders to 5 wt% in the B-series powders. With the higher amount of organic carbon absorbed on the Al2O3 surfaces, the residual carbon within sintered body of sample B-1 was also higher, which suppressed more the densification during sintering. In comparison with sample B-1, sample B-2 had even lower relative density, despite of the comparative amount of remaining carbon contents (1.05 and 1.08 wt% correspondingly). It was attributed to the aggregation of the pre-sintered powder. In order to preserve the polymer network in powder, sample B-2 was sintered from as-aggregated powder after drying. As a consequence, the powder was not packed perfectly in the die and generated high porosity from the original aggregates.
The microstructure of all sintered samples is shown in Fig. 3. A slight grain growth was only observed in sample A-4, which was sintered at high temperature and high pressure. All of the other sintered samples showed comparative grain size. It was obvious that the influence of carbon impurity strongly suppressed the grain growth of Al2O3 samples. The increasing in sintering temperature from 1400°C to 1500°C (samples A-1 and A-3) or the increasing in uniaxial pressure from 20 MPa to 80 MPa (samples A-1 and A-2) distributed only to the densification and significantly increased the relative density of sintered samples. From SEM images, samples B-1 and B-2 showed a clearly lower relative density in comparison to other samples.
3.2. Raman results:
Figure 4 shows the Raman spectra investigated on the cross-sectional surfaces of sintered bodies and Fig. 5 demonstrates the ratios ID/IG and IG’/IG of Raman peaks. All Raman spectra were normalized by the intensity of G peak. The three obvious peaks of graphite structure are assigned for D band (1350 cm−1), G band (1590 cm−1) and G’ band (2695 cm−1) [15, 19, 20]. In comparison among samples of A series, the graphite structure was generated more with increasing of sintering temperature or applied pressure during PECS process. The shoulder peak next to G-band at about 1625 cm-1, indicating the structure of graphite-like carbon, had slightly lower intensity when the sintering temperature increased from 1400°C to 1500°C. This shoulder peak was much weaker when the applied pressure in PECS increase from 20 MPa to 80 MPa. From Raman results, the graphitization of carbon was obviously promoted at higher sintering temperature and higher pressure. The amount of layers in graphite structure, which is indicated by intensity of G’-band, also increased at higher temperature and pressure. In comparison with the graphitization in pressureless sintering methods reported in previous works [14, 15], PECS has advantages to the formation of graphite structure. The graphite structure in C/Al2O3 composite sintered at 1500°C and 80 MPa is similar to the pressureless-sintered sample at 1600°C [15]. By the influences of applied pressure in PECS, the ratio of graphite structure tends to be promoted. As discussed above, the graphitization was promoted with higher applied pressure. However, the total amount of carbon remained inside sintered samples was lower than other studies with pressureless sintering. Even with the original concentration of PVA at 10 wt% in comparison with Al2O3 powder, the remained carbon content in sintered bodies was in the range of 0.5-1 wt%, which was only about 10% of the original carbon content in PVA. The amount of remaining carbon inside sintered samples reduced with increments of sintering temperature or pressure. The results seem to be obvious because the oxidization and vaporization of carbon contents may be enhanced by higher temperature or pressure.
3.3. Electrical conductivity and semi-conductive properties:
Electrical conductivity of C/Al2O3 sintered bodies with different sintering conditions is showed in Fig. 6. In all cases, the electrical conductivity of C/Al2O3 composites increased as the temperature increased, indicating that C/Al2O3 composites fabricated by PECS in our study exhibited semi-conductive properties. The highest electrical conductivity in A-series samples at ambient temperature is nearly 0.9 S/cm, which value belongs to the sample sintered at 1400°C and 20 MPa. Despite of the smallest content of graphite as well as fewest layers of graphite structure among sintered samples, sample A-1, sintered at 1400°C and 20 MPa, had the highest electrical conductivity. One of the possible reasons of its highest electrical conductivity is its highest residual carbon amount as well as its lowest relative density. Menchavez et al. reported that electrical conductivity of C/Al2O3 composites increased while the bulk density of sintered sample decreased [13]. Reason of this relationship was reported as the larger amount of carbon filler adding to C/Al2O3 composites, which resulted to both the promotion of electrical conductivity and the reduction of bulk density due to the suppression of sintering ability. In this study, the remaining amount of carbon in sample A-1 was the most among A-series samples. Although the overall graphitization in other A-series samples were increased by higher temperature or/and higher applied pressure, their electrical conductivity was still much lower than sample A-1. Our conjecture is that the increase of sintering temperature and pressure not only promoted the graphitization degree but also pushed more carbon substances out of the bulk bodies. Consequently, the amount of residual carbon contents in bulk samples reduced with increasing of sintering temperature or pressure. More importantly, at high temperature and pressure, the densification as well as the removal of carbon substances were dramatically accelerated, which possibly eliminated many conductive paths of graphite at the grain boundaries. As a result, a good graphite structure with many stacking layers might be formed inside bulk samples but not connected well each other along grain boundaries in the ceramic matrix. Therefore, although the Raman results indicated the higher graphitization degree with more stacks of graphite layers in samples A-2 or A-4, their conductivity was much lower than that of sample A-1.
Comparing among three sample A-1, B-1 and B-2, the electrical conductivity of B-series samples was 3-4 times higher than sample A-1, even though they were sintered at the same sintering condition. The results of electrical conductivity of B-series samples illustrated that the structure of polymer network in pre-sintered powder strongly affected to the graphitization process as well as the graphite structure in sintered bodies and their electrical conductivity. With the addition of PCA as a dispersant, the carbon fillers in the ball-milled slurries were expected to be dispersed more homogeneously, which led to a homogeneous distribution of carbon contents in the sintered bodies. Especially, by packing the aggregated powder directly into the graphite die of PECS, sample B-2 obtained the highest electrical conductivity. This result is consistent with the results on the graphite structure of sample B-2 in comparison to sample A-1 or B-1. The high ratio of graphite content as well as large amount of stacking layers of graphite led to the high electrical conductivity of sample B-2.
The Hall resistivity measurements were conducted with samples A-1, B-1 and B-2. Hall effect could not be observed clearly with the other 3 sample due to the low electrical conductivity and discontinuous graphite structure. The dependence of Hall resistivity of samples A-1, B-1 and B-2 on intensity of magnetic field at different temperature is showed in Fig. 7. While the Hall resistivity of samples B-1 and B-2 is linearly related to the intensity of magnetic field, the points in the graphs belongs to samples A-1 at all temperature are scattered too much. The reason for the scattered values might be the heterogeneous distribution of graphite structure in the sintered bodies. Because the pre-sintered powders were simply mixed with PVA by wet ball milling, the distribution of PVA in the synthesized powders might not be homogeneously optimized. Otherwise, the actual distribution of temperature and pressure during PECS, which cannot be investigated directly, may cause the heterogeneous distribution of graphite structure in sintered bodies. For the B-series samples, the existence of PCA as a dispersant made Al2O3 powder and PVA dispersed well in the slurry, leading to the better polymer network as well as homogeneous distribution of graphite in the sintered bodies.
The results of Hall resistivity also showed that sample A-1 contains positive-charged carriers while samples B-1 and B-2 have negative-charged ones. The modification between n-type and p-type in graphene/Al2O3 composites has been reported by Fan et al. [21]. Although the graphene/Al2O3 composites intrinsically contain negative-charged carriers, the positive charges are assumably introduced by the doping from the Al2O3 matrix or by the point defects in graphite structure which can be considered as the electron acceptors [15, 16, 21]. In the other words, C/Al2O3 composites may contain both intrinsic negative-charged electrons within the graphite structure and conceivably positive-charged holes at the contacting layers between graphite and Al2O3 grains. At the low concentration of graphite, the amount of positive charges possibly overwhelms and produce p-type semi-conductive properties. Otherwise, at high enough concentration of graphite, the amount of electron within graphite structure overcomes and the composites perform as n-type. In this study, by the additional of PCA, the surfaces of Al2O3 particles could adsorbed more organic carbon substances, that was confirmed by TOC results above. Hence, the residual amount of graphite in sintered bodies of B-1 and B-2 was higher than that in sample A-1. This is apparently same phenomena described by Fan et al. [21], so that A-1 samples showed p-type semi-conductive properties with lower concentration of graphite while B-1 and B-2 samples performed as n-type semi-conductive composites with higher concentration of graphite.
Figure 8 shows the carrier density and mobility of three samples A-1, B-1 and B-2. In all three samples, the relationship between Hall coefficient and the measuring temperature was not evident. The carrier density and the carrier mobility of sample A-1 were in the ranges of 0.8-3.8 × 1024 m−3 and 0.14-0.65 × 10−3 m2/Vs correspondingly. In disregard of the sign (+/-) of the results, the carrier mobility of B-1 and B-2 samples were about 10 times higher than that of sample A-1. The improvement in carrier mobility in B-series samples may attributed to the distribution of carbon fillers in pre-sintered powder as well as the homogeneous graphite structure in the sintered bodies.
3.4. XPS results:
The interaction of carbon contents and Al2O3 surface in sintered samples was inspected by XPS results. Fig. 9a, 9b and 9c show the XPS spectra of C1s of 3 sintered samples A-1, B-1 and B-2 correspondingly. The peak fitting for all samples pointed out 5 peaks at around 282.2, 283.5, 284.3, 285.8 and 288.1 eV, distributing to C-Al, C-O-Al, C=C, C-C and C=O bindings [16, 22–26]. The differences in carrier types of the 3 samples are possibly explained by the differences in C-Al and C-O-Al bindings. Fig. 10 shows the ratio of each peak area over the total peak area in the 3 samples. The graph in Fig. 10 illustrated the decrease of C-Al bindings from sample A-1 to B-1 and B-2. In contrast, amount of C-O-Al bindings increased in the same order of 3 sample. The amount of C-Al bindings in sample A-1 from XPS results proved that the positive-charged carriers derived from the doping effect of Al atoms in graphite structure. In samples B-1 and B-2, with the addition of PCA, the absorption of carbon fillers on surface of Al2O3 powder seem to be stronger. Therefore, more C-O-Al bindings were created from the connections between carbon fillers and Al2O3 surface. The C-O-Al bindings tent to remain after sintering process and prevented the doping of Al atoms to graphite structure, that could be confirmed by the reduction of C-Al peak’s area in samples B-1 and B-2. Moreover, residual amount of carbon in samples B-1 and B-2 is higher, leading to the higher stacking of graphene layers and giving B-series samples more negative-charged carriers than positive-charged ones. Besides the results in C-O-Al and C-Al bindings, the higher peaks of C=O bindings at around 288 eV in samples B-1 and B-2 also indicated that the polymer structures formed in B-series samples were more stable than in sample A-1. Hence, after sintering, both C=O and C-O-Al bindings remained more in B-series samples.