Effect of CNT on microstructural properties of Zn2SiO4/CNT composite via dry powder processing

This work focused on the influence of carbon nanotubes (CNT) to the microstructural properties of Zn2SiO4/CNT (ZSO/CNT) composite. CNT was synthesized via alcohol catalytic chemical vapor deposition (ACCVD) using cobalt oxide as catalyst and ethanol as carbon source. Zinc silicate (ZSO) glass was prepared from quenching the melted commercial waste glass bottle with zinc oxide powder. ZSO/CNT-x composites with various CNT concentration (0, 1, 2 and 3 wt%) was prepared through introducing CNT into ZSO glass via dry processing technique followed by sintering process in Argon gas (Ar) environment and atmospheric (atm) environment, respectively. FESEM, XRD and EDS were employed to determine the surface morphology, phase composition and elemental distribution of sintered sample. Crystallite trigonal willemite (Zn2SiO4) phase was observed from argon sintered sample and the crystallite size of willemite phase in ZSO/CNT-3/Ar showed the most reduced lattice strain of 22.85% compared to ZSO/CNT-0/Ar. In contrast, semi-crystalline phase exhibited in atmospheric sintered sample resulted in high lattice strain. It is concluded that dry powder processing and inert gas thermal treatment can be an effective technique in fabricating strain-reduced ceramics/CNT composite without alternating the domain phase. Least internal strain in crystal lattice have potential on enhancing the luminescence properties of phosphor material and lattice thermal conductivity of thermoelectric material.


Introduction
Application of carbon nanotubes (CNT) into ceramics matrix, forming CNT/ceramics composite with enhanced thermal [1][2][3], electrical [4][5][6], and optical [7,8] properties drew great demand in this rapid scientific development. Dry powder processing technique was the first and commonly used in fabrication of carbon nanotubes (CNT) with different host matrix such as ceramics and metal in forming composite with enhanced properties. This process was conducted by mixing CNT and the inorganic matrix in milling or manual mixing system. Although dry powder processing faced inhomogeneous CNT dispersion and least interfacial bonding between CNT and inorganic matrix [9], researchers introduced several effective milling process [6,10] and thermal treatment such as spark plasma sintering [11] and low pressure sintering [12], which helped to solve the bottleneck in fabricating desirable CNT composite.
Beside the enhanced properties stated above, CNT plays main role in reinforcing the mechanical properties of domain matrix with very limited amount of CNT needed [13,14]. Mazaheri et al stated the improving in Vickers hardness and Young's modulus was due to the pinning effect of CNT [15]. Pinning effect of CNT was located in the grain boundary, restricting the sliding and lowered the mechanical loss. Furthermore, Bi et al recommended flexural strength of CNT/Al 2 O 3 composite was improved with obtaining small grain size leads to strong interfacial connection between CNT and Al 2 O 3 [16]. However, these studies were based on the synergetic effect of CNT at the grain boundary without noticing the effect of CNT towards the microstructure of the respected domain phase.
The presence of lattice strain in the microstructure of materials played significant role in mechanical performance and the phase information to the domain phase especially for glass-ceramics silica-based material. Willemite (crystallite Zn 2 SiO 4 ) is a type of silica-based glass-ceramics which exhibited low thermal conductivity and green light emission (585 nm) with the excitation of UV. Zn 2 SiO 4 exhibited glass structure at low temperature thermal treatment and initiated crystallization at high temperature [17]. When external (high temperature needed) and internal (low thermal conductivity of glassy Si-based material) factors decelerated the crystallization, crystallite size decreased in the matrix system, leaded to increase in lattice strain and low degree phase transition of glassy to crystalline phase [18,19]. Hence, limiting lattice strain without phase modification in stable lattice dislocations of Zn 2 SiO 4 potentially advancing the thermoelectric behaviour [20,21] and narrow bandwidth emission with high quantum efficiency [22] of the material towards highly functional light emitting diode (LED) application.
In this research, we introduced the cobalt-catalysed CNT as fillers to parent zinc silicate glass powder (ZSO). Sintering process was conducted in controlled argon flow environment in reducing the lattice strain and improving the crystallite size of the crystallite phase. To show comparison with conventional glass-ceramic material synthesis process, ZSO sample with CNT addition were also sintered in atmospheric environment.

Synthesis of carbon nanotubes (CNT)
CNT was synthesized from Co 3 O 4 as metal catalyst and ethanol as carbon precursor through alcohol catalytic chemical vapor deposition (ACCVD). Co 3 O 4 powder was weighed and sintered up to 800°C with ramping rate 10°C min −1 in tubular furnace with argon gas flowing (Ar, 50 sccm) in ensuring inert gas environment. At 800°C, vaporized ethanol and hydrogen gas (H 2 , 5 sccm) was introduced into the furnace for 30 min of reaction time. The sample was then annealed to ambient temperature in inert environment. ACCVD was setup as figure 1.

Parent zinc silicate glass (ZSO)
Harvested SLS glass bottle was cleaned, crushed, milled and sieved until the glass powder reached 45 μm. Micron sized SLS powder was mixed with ZnO nanopowder at ratio 1:1 and melted in electrical box furnace at 1400°C for 3 h as figure 2. The melted SLS/ZnO liquid was water-quenched into glass fritz [17]. Glass fritz was milled into 45 μm powder labelled as ZSO glass powder.
2.4. ZSO/CNT composites via dry prowder processing ZSO glass powder was well mixed with CNT with varying concentrations (0, 1, 2 and 3 wt%) as in table 1 and 0.5 g of mixed powder were processed with pelleting process under 5 tonnes pelleting force for 10 min using PVA as binding agent. This pelleting process produced pellet with 3.0 mm thickness. The pellet was labelled as ZSO/CNT-x as x indicated the weight percentage of CNT. The pellets was sintered in tubular furnace with Ar gas flowing at 50 sccm. The heating rate was set at 10°C min −1 and held at 800°C for 2 h. Identical pellet samples were sintered in atmospheric environment with identical sintering parameter. ZSO/CNT-x/Ar and ZSO/CNTx/atm was labelled as sintered sample under argon gas and atmospheric environment, respectively. Figure 3 showed the setup of argon gas sintering and atmospheric sintering.

Characterization
The surface morphologies of CNT and sintered pellet samples were characterized by a field emission scanning electron microscope (FESEM, JSM-7600F, JEOL, Japan) with an Energy Dispersive x-ray Spectrometer detector (EDS, INCA, Oxford, UK) attached to observe the elemental distributions. X-ray diffractometer (XRD-6000, SHIMADZU, Japan) was utilized to determine the phase composition of samples.    Equation (1) shows Williamson-Hall equation for crystallite size (τ WH ) and lattice strain (ε WH ) calculation. Crystallite size only (τ 0 ) (2) was calculated as the lattice strain was neglected and lattice strain only (ε 0 ) (3) was calculated as the crystallite size to infinite value.
3. Results and discussions 3.1. Phase composition, phase shifting and peak narrowing XRD analysis was employed on the CNT filler, argon sintered sample (ZSO/CNT-x/Ar) and atmospheric sintered sample (ZSO/CNT-x/atm), to determine the phase compositions (refer to figures 4 and 5). Crystalline phase shifting was only observed in ZSO/CNT-x/Ar while semi-crystalline phase and amorphous phase were observed in ZSO/CNT-x/atm without phase shifting. Phase and surface formation of CNT (CNT spectrum in figure 4) were analyzed at the first stage. Metal carbides is the well-known product from the reaction between metal and carbon. In this case shown, there is no formation of cobalt carbides from the phase spectrum. There are 3 sharp peaks found in the XRD spectrum which respect to cubic α-cobalt: 44.34°, 51.60°and 75.95°respect to (111), (002) and (020) planes. During the 30 min reaction time (800°C), H 2 gas acted as the carrier gas for vaporized ethanol and the reducing agent onto the Co 3 O 4 catalyst. The oxidation number of Co 3 O 4 (can be written as CoO·Co 2 O 3 ) reduced from +2 and +3 to neutral. However, some of the reduced cobalt didn't react with the carbon atom from the vaporized ethanol remains cobalt particle without any carbon capsulation contributes to the phase of α-cobalt [25].
From figures 4, 2 h sintering process at 800°C provided ZSO glass sufficient energy in rearranging the silicate, zinc and oxygen ion from glassy structure (water-quenched sample) into crystallite trigonal willemite phase (Zn 2 SiO 4 , space group=R 3). Impurities like magnesium (Mg) and calcium (Ca) found in SLS bottle melted and alloyed with silicate towards minor crystalline phase of tetragonal akermanite (Ca 2 MgSi 2 O 7 , space group=P 421m). The presence of minor ceramics in the composite: Akermanite, due to the impurities within the glass bottles during the industrial manufacturing process to improve the mechanical properties of the bottles [26]. Amorphous carbon peak and crystallite alpha-cobalt (α-Co) was found in CNT and ZSO/CNT-x/Ar. Inert gas such as Ar gas preserved the CNT from decomposing under high temperature or reacted with flowing gas. Broad peak of hexagonal carbon peak at 26°indicated crystallite graphite (002) plane which overlapped with (4 20) plane of hexagonal Zn 2 SiO 4 phase. This phase overlapping phenomenon at same Bragg's angle resulted the increase of intensity at the 2θ position.
In figure 5, semi-crystalline phase existed in atmospheric sintered sample as the CNT concentration increased and amorphous phase for ZSO/CNT-3/atm sample. The possible explanation for this glassy phase formation is the decomposition of CNT under oxygen decomposition at high temperature. Near 20% oxygen contained in atmospheric air invaded the graphite layers of CNT and converted into carbon gaseous, leaving the Co particles reacted with the silicate ion in the pellet. After sintering at 800°C for 2 h, the cobalt-silicate compound remained as glassy state, was believed the crystallite temperature of this ceramics is higher than 800°C [27,28].
Increasing of W-H calculated crystallite size showed relationship with phase shifting to left observed in the XRD spectrum ( figure 6). Phase left-shifting indicates increased in crystallite size based on Scherrer equation. However, phase shifting is not the only parameters in crystallite size determination. In addition, the crystallite size was influenced by the β value (FWHM). FWHM values against plane of ZSO/CNT-x/Ar was plotted as in       hexagonal phase showed the value of τ 0 similar with τ exp and ZSO/CNT-3/Ar showed the crystallite size of 61.402 nm. The lattice strain of ZSO/CNT-3/Ar showed the lowest value which is 1.32×10 −3 . W-H calculated t 0 and e 0 was compared with Scherrer equation calculated τ exp and ε exp . The results showed similar result indicating ZSO/CNT-2/Ar has the smallest crystallite size and highest lattice strain. However, ε exp and e 0 of ZSO/CNT-2/Ar show the strain up to 377% and 12.84% compared to ZSO/CNT-0/Ar respected to the identical phase in the system. This gives the e 0 in W-H calculation having higher accuracy in calculating the homogenous distortion in the system with the consideration of crystallite expands to infinite value while ε exp calculated the average strain between phases standalone without taking the expanding of crystallite into account.
From XRD spectrum of ZSO/CNT-x/atm sample ( figure 5), only crystalline willemite phase was considered in crystallite size determination. ZSO/CNT-0/atm showed τ WH of 0.63×10 −3 ε WH with 109.27 nm τ WH , differed 21.33% and 3.15% in lattice strain and crystallite size, as comparing to ZSO/CNT-0/Ar. The crystallite hexagonal willemite phase was not achieved as the CNT concentration increased but semi-crystallite willemite phase was observed, and the crystallite size and the lattice strain were calculated. The calculated lattice strain (ε exp , ε WH and ε 0 ) increased as the crystallite size (τ exp , τ WH and τ 0 ) of willemite phase was decreased (table 3). Semi-crystallite willemite phase and amorphous state in ZSO/CNT-x/atm with increasing CNT showed great in lattice strain as the silicate ion was partly reacted with the exposed cobalt after the graphite layers was invaded by oxygen radical at high temperature. Glassy cobalt-silicate compound reduced the crystalline phase of zincsilicate towards willemite phase formation.
Reducing lattice strain (lattice distortion and imperfections in crystal matrix) with enhancing the crystallite size was reported in [29,30], showing the relationship of crystallite size and lattice strain. CNT with high thermal conductivity (600-750 W mK −1 ) and high melting point (>3000°C) enhance the thermal energy transport from the external heat source (heating coil of the furnace) to the host matrix without changing the carbon structure leads to enhance the crystallite size [2,31]. Near homogenized distribution of 3 wt% CNT in ZSO/ CNT-3/Ar composite showed the least phase left-shifting and resulted the reduced lattice strain with enlarging crystallite size without changing the domain phase of crystal. Figure 8 shows the morphology of the ZSO/CNT/Ar which is observed by FESEM. Figure 8(e) shows microscopy image of CNT with worm-liked fibrous structure (averaged diameter 23.08±5.45 nm), similar with CNT [32,33] and carbon bulks structure exist in the sample, which is in agreement to XRD results. CNT are crystalline carbon plane with hexagonal structure and rolled into tubular shape while amorphous carbon shows random orientation of carbon particle. The nano sized of CNT and the dominating amorphous carbon are the reasons behind obtaining a broadening carbon peak from the XRD spectrum.

Microscopic observation and element distribution
From figures 8(a)-(d), the surface morphology of ZSO/CNT-x/Ar were observed. CNT was well dispersed into the glass powder as the non-uniform morphology and size of the glass particle due to the manual crushing and grinding process (particle size distribution refers to table 4). The size of CNT was relatively small (nanosized) to the glass particle (micron-sized) as the CNT capable to fill in the gap between the glass particle. CNT filled in the gap between the glass particle in form of bundle as shown in inset image of figures 8(a)-(d). This showed the agglomeration of the CNT due to the small size of the cobalt catalyst and the CNT. Cobalt particles and nanotubes needed energy to achieve thermodynamically stable which cannot be achieved individually due to small size. During agglomeration in dry powder state, individual nanotubes and particles contribute energy and shared with neighbours to overcome energy insufficient [34,35]. Figure 9 shows the morphology of ZSO/ CNT-x/atm. There is agglomeration of cobalt particles (smaller particles, 66.754±24.807 nm) without worm like structure attached on the semi-crystallite glass grain (bigger particles, 123.37±76.89 nm). Semi-crystallite and glassy structure below crystallite temperature obtained insufficient energy in nucleation but sufficient energy in melting and fusing, comes with greater particle size compare to ZSO/CNT-0/atm (2.95±0.10 μm). The detailed particle size distribution of ZSO/CNT-x/Ar and ZSO/CNT-x/atm were displayed in table 4.
Elemental analysis of the composite was done via EDS detector onto five sample: ZSO/CNT-0/Ar, ZSO/ CNT-3/Ar, ZSO/CNT-0/atm, ZSO/CNT-3/atm and CNT (figures 10(a)-(e)). Atomic ratio of carbon to cobalt is about 6:1, means one mol of cobalt atom capable in producing six mol of carbon nanomaterials. ZSO/CNT-0/Ar without any addition of CNT results 16.15 at% of carbon content referring to table 5. This is believed due to the manufacturing of glass bottle is exposed to atmospheric carbon content as the carbon atom diffuse into the glass and the remaining PVA binding agent. Impurities like 3.71% aluminium is believed to be the mechanical properties enhancer during the manufacturing of glass bottle [26]. The ratio 1:3 of Si: Zn is due to the 60% SiO 2 in SLS glass reacts with mass ratio 1:1 of ZnO. Table 5 showed the carbon content dominating the other elements, having a composition of 61.05% with 3 wt% CNT added. The density of CNT is low [36], while the volume of CNT added is large with 3 wt% per mass of ZnO/SLS powder. The impurified CNT contains not only CNT, but amorphous carbon and different carbon nanomaterials. Table 3. W-H equation calculated crystallite size (τ WH and τ 0 in nm) and lattice strain (ε WH (×10 −3 ) and ε 0 (×10 −3 )) of ZSO/CNT-x/atm and Scherrer equation calculated crystallite size (τ exp in nm) and lattice strain (ε exp (×10 −3 )) as comparison. Change of crystallite size and lattice strain (%) for CNT-added sample was calculated respected to ZSO/CNT-0/atm.

Conclusions
In this study, ZSO/CNT composite was successfully synthesized via dry powder processing technique. Introduction of 3 wt% CNT into ZSO helped in reducing lattice strain of crystallite willemite phase for 22.85% as compared to ZSO without CNT addition. The willemite phase remained unchanged while sintered with higher CNT concentration in Argon gas environment. Atmospheric sintered sample with increasing CNT addition resulted in amorphous structure as the cobalt catalyst reacted with the silicate domain, resulted glassy structure with higher crystallite temperature. We concluded dry powder processing and inert gas sintering   process can be efficient in introducing CNT into glassy zinc silicate and phase-changed into crystallite willemite with limiting lattice strain and preserving the domain phase.