4.2 ER Materials Based on Nanocomposite and Hybrid Design
4.2.1 MMT Based Nanocomposite ER Materials
Montmorillonite (MMT) is a natural candidate for nanocomposite due to special structure of nano-size layer. Figure 23 shows the crystal structure of MMT. Like microporous zeolite, the cations absorbed in the interlayer of MMT, as charge carries, can move to induce strong interfacial polarization under electric field. This special structure makes MMT attract attention for use as ER materials. Furthermore, MMT is also low cost and its lamellar structure may also improve the stability of anti-sedimentation of the ER suspension.
However, the open lamellar structure and the cations absorbed in the interlayer of MMT are often found to cause an unexpected high current density in pure MMT ER fluid, which results in instability of ER effect. Several approaches have been proposed to attempt to obtain ER active MMT nanocomposite based ER materials. We introduce here two interesting routes including polymer conductor intercalated MMT nanocomposite and nanocrystal coated MMT nanocomposite.
Figure 23 the 2D crystal structure of Na+ MMT
Polyaniline/MMT nanocomposite
Choi et al. [64] firstly introduced a kind of conducting polyaniline (PANI)/MMT nanocomposites with intercalated nanostructure as ER material. This nanocomposite possessed an extended single chain conducting PANI inserted between the layers of MMT due to the confinement in the nanometer size gallery. However, its yield stress was much lower than that of pure PANI and no synergistic effect was found. Lu et al [65] also prepared ER fluid based on polymer conductor-montmorillonite clay nanocomposite by an emulsion intercalation method. Figure 24 are the XRD patterns of pure MMT and PANI/MMT nanocomposites. The shift towards lower angle of low-angle diffraction peak indicated that PANI had well intercalated into MMT layers. And the PANI/MMT nanocomposite possessed significantly small particles diameter about 100-200nm. In order to reduce the conductivity of PANI-MMT particles prepared by the chemical oxidation of aniline in the presence of acidic dopant, the PANI-MMT particles were immersed in NH3aqueous solution (pH=10) for 12 h [66]. This immersed time for controlling conductivity was surprise longer compared with immersed time (only several minutes to several hours) for controlling conductivity of pure PANI. This may be related to the protection function of MMT layer to PANI macromolecular (see the schematic structure in Figure 25). Figure 26 shows the yield stress as function of electric field for PANI/MMT nanocomposite ER fluid at room temperature. It was found that the yield stress of PANI-MMT ER fluid was 7.19 kPa in 3 kV/mm, which is much higher than that of pure polyaniline (PANI), that of pure montmorillonite (MMT) as well as that of the mixture of polyaniline with clay (PANI+MMT). But it also showed high zero-field viscosity. Especially, in the range of 10~100 , the yield stress changed only 6.5 % with the variation of temperature. This good temperature stability revealed the merit of inorganic/organic nanocomposite. Furthermore, they also extended this nanocomposite to poly-N-methaniline/montmorillonite (PNMA/MMT) nanocomposite [67], PoPD/MMT nanocomposite particles by an emulsion intercalation method. Figure 27 shows schematic structure of PNMA–MMT and PoPD/MMT. Besides the similar ER effect was found in this nanocomposite, the effect of guest molecular structure on ER effect was also noted.
Furthermore, Lim et al [68] supplied a kind of ER fluids using both PANI-MMT nanocomposite particles and pure PANI particles as dispersed phase. They noted that there was synergistic effect to enhance shear stress by using this mixture. However, no temperature effect was given in their results.
Figure 24 XRD patterns of pure MMT (a) and PANI/MMT nanocomposites (b).
Na+ Na+ Na+ Na+ NH3+ NH3+
H+ N
HN N
H H+N (NH4)2S2O8
NH3+ Clay
Figure 25 Schematic structure of the PANI/MMT nanocomposites
0 1 2 3
0 1000 2000 3000 4000 5000 6000 7000 8000
Electric field (kV/mm)
Shear stress (Pa)
PANI-MMT PANI PANI+MMT MMT
0 10 20 30 40
Leaking current density (¦ÌA/cm 2)
PANI-MMT
Figure 26 Shear stress (shear rate = 5s-1) of PANI/MMT nanocomposite ER fluids as a function of electric field and corresponding leaking current density.
Na+ Na+ Na+ Na+ NH+
N N
NH+ NH2Me
Clay
+ NH+ 2Me
Me Me
Me Me
NH+ 2Me (NH4)2S2O8
(a)
(b)
Figure 27 Schematic structure of the PNMA/MMT (a), and PoPD/MMT (b) nanocomposites.
Nanocrystallite coated MMT nanocomposite
Since Choi et al. firstly introduced MMT composites and both Lu and Lim made some progress in improving the ER properties, but the yield stress of these kinds of ER fluids were still not as high as that researchers expected. Physically, the polarization, which originates from the local migration of cations absorbed in the interlayer of MMT, is important to ER activity of MMT. However, it should be noted that the modification about MMT ER materials are mainly focused on replacing the natural inorganic cations with polymer. Although this replacing greatly decreases the current density of MMT, the ER activity also becomes weak due to the lack of strong polarization sources from local migration of cations absorbed in the interlayer. Recently, Xiang et al [68] designed a novel kind of TiO2 nanocrystallites-coated montmorillonite (MMT/TiO2) nanocomposite ER material that showed high ER effect as well as good temperature. In the composite, MMT is the bases for its low cost and special structure. High dielectric constant anatase with nanocrystallines is well coated on the surface of MMT flakes (see Figure 28), which is expected to confine the long-range movement of active cations in interlayer so as to decrease its current density and induce strong interfacial polarization in composite particles. The content of TiO2 is demonstrated to have an important influence on the ER effect (see Figure 29). When the content of TiO2 is about 20wt%, the ER effect of MMT/TiO2 ER fluid reaches its maximum, which is about 5 times as high as that of pure MMT ER fluid and 27 times as high as that of pure TiO2 ER fluid. They use interfacial polarization mechanism to explain the ER effect based on dielectric spectra technique.
Furthermore, nanocrystal TiO2 coating is found to overcome congregation of MMT flake and greatly increased anti-sedimentation ability.
Figure 28 SEM photograph of pure MMT (a) and titania nanocrystallite coated MMT(b). The inset in (b) is the TEM image of titania nanocrystallite coated MMT with scale bar of 100nm
-5 0 5 10 15 20 25 30 35 40 0
500 1000 1500 2000 2500 3000 3500
E = 3 kV/mm E = 2 kV/mm E = 1 kV/mm
Yield stress (Pa)
TiO2 content (wt%)
Figure 29 The static yield stress of ER fluids with different TiO2 content as a function of electrical field strength. (the volume fraction = 25vol%, T=20oC)
Figure 30 The scheme for formation and polarization of titania nanocrystallite coated MMT nanocomposite particles suspended in silicone oil