MWNT Distribution in Composites Fabricated Under Different Flows
Image Analysis of MWNT Distribution State in the Composites Fabricated Under Different Flows
Through preliminary tests on MWNT-embedded cement composites, it is found that the flow of the fresh mixture may influence the MWNT distribution. Accordingly, MWNT-embedded cement composites fabricated with different W/C values were prepared and the MWNT distribution state of the composites was evaluated. The constituent materials are listed in Table 1. The mixing ratio of MWNT was 0.6 wt% in order to provide a sufficient amount of MWNT. 0.6 wt% of MWNT exceeded the percolation threshold as reported in the literature (Nam et al. 2015). Accordingly, the distribution of MWNT was expected to be pronouncedly visible. The volumetric fractions (vol%) of MWNT were also provided in the table. In determination procedures of volumetric fractions of MWNT, volume of each constituent material had to be calculated by using true specific gravity of the materials shown in the Sect. 2, then volume ratio of MWNT to total volume of the mixture was obtained. 20 % (by cement weight) SF was added in the mixtures under the consideration that SF can improve the MWNT distribution (Nam et al. 2012). The white cement was substituted for OPC and the incorporation of nylon fiber was omitted, as addressed in Sect. 3.1.1. The specimens were prepared and images of fractured surfaces of the specimens were obtained according to the method described in Sect. 3.1.2.
Figure 3 presents processed images that were obtained from the fabricated specimens. In the processed images, the total area of the MWNT agglomerates appeared to decrease with an increase of the flow (or W/C ratio). Moreover, MWNT clumps, which are excessively entangled-MWNT agglomerates, were generated as the flow increased. Based on these observations, it can be surmised that MWNT agglomerates disentangled as the flow (or W/C ratio) decreased.
In an effort to express the change of the MWNT distribution states in a quantitative manner, the proportion of MWNT agglomerates, indicated by white color, to the total area of the image, was calculated by the MATLAB image processing tool box. This proportion is designated as a q value. The q value refers to the planar occupation ratio of MWNT on a fractured surface of the composite.
Figure 4 shows an increase of the q value as the W/C ratio of the cement composites decreases. The q value doubled due to a 16 % reduction of the W/C ratio. The increase of the q value was attributed to disentanglement of the MWNT agglomerates. The disentanglement of the MWNT agglomerates can be explained by variation of the microstructure of the fresh mixture. Figure 5a shows the microstructure of the fresh mixture of the MWNT-embedded cement composite prepared under a high flow condition. The distance between unhydrated cements increased as the W/C ratio (or SP/C ratio, i.e. SP versus cement ratio) increased (Daimon and Roy 1979). When the interspace among the unhydrated cements increased, highly entangled MWNTs floated in the free water. This is ascribed to hydrophobic characteristics of MWNT. As a result, the images obtained from the W38–M06 and W42–M06 samples showed the highly entangled MWNTs, as presented in the Fig. 3. On the contrary, the microstructure of the fresh mixture of the composites can be changed if the flow of the mixture is decreased. The distance between unhydrated cements in the composites prepared under a low flow condition was narrowed, as shown in Fig. 5b. If the interspace is narrowed, the MWNT agglomerates can be disentangled during the mixing process. This is attributable to the characteristic that the size of the MWNT agglomerates can be restricted by the size of the interspace among the unhydrated cements. As a result, the images obtained from the W26–M06 and W30–M06 specimens in the Fig. 3 showed disentangled MWNT agglomerates.
The planar occupation ratio of MWNT, q, was increased twofold with decrease of W/C ratio from 42 to 26 % as shown in the image analysis result of Fig. 4. However, electrical conductivity of the composites is expected to increase more than hundreds of times as the W/C ratio decreases. This stems from that disconnected CNTs can be electrically connected attributable to the enhancement of MWNT distribution. Accordingly, it can be said that the twofold greater q value can lead to a dramatic increase in the electrical conductivity. This will be dealt in the electrical property characterization section.
DC Conductivity of the Composites Fabricated Under Different Flows
The image analysis of the MWNT distribution state of the MWNT-embedded cement composites fabricated under different flows (or W/C) studied in Sect. 4.1.1 showed that the MWNT distribution improved as the flow of the composite decreased. In the present section, the DC conductivity of the composites was examined in an effort to understand the change of the MWNT distribution in the composites and also to verify whether it supported the image analysis results. The constituent materials and respective weight content ratios of the composites are given in Table 3. To make specimens with different flows, one group of specimens was fabricated by varying the water content, as described in Sect. 4.1.1, and an additional group of specimens was fabricated by varying the SP content in the present section. The fabrication procedures of the specimens followed descriptions in Sect. 3.2.1 and the measurement method was described in Sect. 3.2.2.
The variations in the DC conductivity of the specimens are plotted in Fig. 6a, b. The change of the DC conductivity of the specimen group prepared with different water content is shown in the Fig. 6a and the results of the specimen group prepared with different SP content are shown in Fig. 6b. An electrical percolation phenomenon can be observed in the electrical conductivity versus W/C ratio plot in the Fig. 6a. This phenomenon indicated that the water content or flow of the mixture is a crucial factor affecting the MWNT distribution of the composites, as found in the image analysis in Sect. 4.1.1. The dramatic increase of electrical conductivity with a decrease of the flow is attributed to disentanglement of the MWNT agglomerates, as shown in the image analysis. In the image analysis, the q value linearly increased with a decrease of the W/C ratio but the electrical conductivity exponentially increased with the decrease of the W/C ratio. This was due to the formation of the percolation network, which induced a remarkable increase of the DC conductivity of some orders. The Fig. 6b, which presents the electrical conductivity versus SP/C ratio plot, also shows the electrical percolation phenomenon. This indicates that the flow plays a decisive role in determining the MWNT distribution, because water content was constant in the specimen group with different SP/C ratio. It is consequently found that the flow of fresh mixture of the composites is an important factor that determines the MWNT distribution state, based on the image analysis and the electrical conductivity evaluation. The improvement of the MWNT distribution with a decrease of the flow is thought to be related to variation of the interspace among unhydrated cement, as discussed in Sect. 4.1.1.
It is worth noting that the electrical conductivity of the M06–W30 was greater than that of the M06–SP04 although the flow of the M06–W30 was higher than that of the M06–SP04. This indicates that control of water content can be a more effective way of enhancing the electrical conductivity of the composites.
Therefore, two categories of specimens were prepared in the subsequent experiments. One category included MWNT-embedded cement composites fabricated under a high flow condition (flow > 250 mm) and another category included composites fabricated under a low flow condition (114 mm < flow < 126 mm) by adjusting the W/C ratio and the SP/C ratio. To understand the MWNT distribution states, the image analysis and DC conductivity measurement for the specimen groups were carried out.
Influence of the Flow on MWNT Distribution in the Composites Incorporating Various MWNT Contents
Image Analysis of the MWNT Distribution State in the Composites
It was found that the flow of fresh mixture was closely related with the MWNT distribution, as observed in Sect. 4.1.2. Accordingly, the MWNT distribution states in MWNT-embedded cement composites with various MWNT contents can be compared by setting the flow of mixtures to be consistently low (flow between 114 and 126 mm). Otherwise, the MWNT distribution is so poor that the reliability of the image analysis data obtained from the composites with various MWNT contents can diminish. To prepare composite mixtures with the low flow values, cement composites with various MWNT contents were fabricated by adjusting the content ratios of water and SP. The water to cement ratio was controlled with reference to the MWNT content in each batch. This was based on a report that MWNT tends to absorb water within its hollow structure (Striolo et al. 2005). The SP content ratio was also adjusted in consideration of the amount of cement and MWNT, but the final SP content was determined by some trials of SP addition in the mixture and flow test. The constituent materials used in the present work are listed in Table 2 but nylon fiber was not used for the image analysis specimens. The mixing ratio of MWNT ranged from 0 to 1.5 %. Figure 7 presents processed images obtained from the fabricated composites. Images observed from LF–M0 showed some particles that were thought to be impurities of the white cement. However, their planar occupation area was so small that it was negligible in a comparison study of the q value.
It is generally agreed that an increase in the MWNT content is accompanied with an increase in the total area of MWNT if it is uniformly distributed throughout composites. Such phenomena was in close agreement with the test result provided in Fig. 7 where the total area of MWNT was observed to increase with the MWNT content. In addition, the MWNT clumps were not found even when the MWNT content was increased up to 1.5 %. Observation of the images thus indicated that MWNT was satisfactorily distributed throughout the composites in the LF–M group.
In an effort to express the change of MWNT distribution states in a quantitative manner, the q value was calculated for the LF–M group, as shown in Fig. 8. The q values of the group exhibited a steady increase as a function of the MWNT content in the composites. Accordingly, a dramatic increase in the electrical conductivity is expected as the MWNT content increases.
An image analysis of the MWNT distribution state in the composites fabricated under the high flow condition was not carried out because it was not possible to present reliable q values due to the presence of the MWNT clumps, as shown in the W42–M06 specimen of the Fig. 3.
DC Conductivity of the Composites Incorporating Various MWNT Contents
The change of the DC conductivity of composites fabricated under the two different flow condition, the low and high flow conditions, with various MWNT contents was investigated here. The constituent materials and respective weight content ratios of the composites fabricated under the low flow condition are given in the Table 2. The DC conductivity of the composites in the LF–M group is shown as a function of the MWNT content in Fig. 9. DC conductivity of 1.7 S m−1 was attained for specimen LF–M1.5. This value was greater than the electrical conductivity of 5 mm long carbon fiber-incorporated cement composites and carbon black-filled cement composites (Wen and Chung 2007; Li et al. 2006). The high DC conductivity of the composites in the present section and the image analysis conducted in Sect. 4.2.1 indicated that MWNT was well distributed in the cement matrix.
The electrical percolation phenomenon of the LF–M group was found in the Fig. 9. The percolation threshold, which refers to the critical volume fraction of MWNT inducing remarkable change in the conductive phase, existed in a MWNT content range of 0–0.3 wt% for the LF–M group. Accordingly, the percolation threshold corresponded to 0.196 vol% if it is determined as the mean value of the percolation threshold range.
The percolation phenomena manifested in the MWNT-embedded cement composites fabricated under the low flow condition indicated that MWNT was well distributed throughout the composites and the MWNT distribution was consistent in the specimen group. The results supported the image analysis results obtained in Sect. 4.2.1. Based on the image analysis results and the electrical percolation phenomena, an acceptable MWNT distribution was attained by reducing the flow of the composites. Consequently, it can be concluded that maintaining the low flow of fresh mixtures of the composites is crucial to improve the MWNT distribution.
The change of the DC conductivity of the composites fabricated under the high flow condition with various MWNT contents was also investigated. The constituent materials and respective weight content ratios of the composites are given in Table 4 (Nam et al. 2012). MWNT was incorporated in each composite type at 0, 0.3, 0.6, and 1.0 wt% by weight of cement. To fabricate specimens with high flow, the W/C ratio was fixed to 0.4 in the composites and the SP/C ratio was 0.016.
Figure 10 presents the change of the DC conductivity of the composites as a function of the MWNT content. The electrical percolation phenomenon was not found in Fig. 10. The DC conductivity of all the composites did not exceed 0.001 S m−1, hence all the composites appeared to be under the percolation threshold. The under percolation behavior of the DC conductivity was attributed to a poor distribution of MWNT in the composites. In addition, the DC conductivity of the composites may be dependent on many factors such as the dry condition of the specimens, void ratio, contact resistance between electrodes and the composite specimens, etc. In conclusion, it is not recommended that MWNT-embedded cement composites are fabricated under a high flow condition unless appropriate dispersion methods were carried out.