1. Understanding the observation of large electrical conductivity in liquid crystal-carbon nanotube composites S. Krishna Prasad and V. Jayalakshmi Centre for Liquid Crystal Research, Bangalore, India Published in Appl. Phys. Lett., 94, 202106 (2009)

2. Abstract We describe the electrical conductivity and dielectric constant measurements in composites of single-walled carbon nanotubes (CNTs) with a nematic liquid crystal, employing electrical and magnetic fields as reorienting fields. Our studies demonstrate that the large magnitude of enhancement in the electrical conductivity reported in voltage-driven reorientation measurements is not due to the inherent property of CNTs, and that dielectric breakdown and local heating effects play an important role. The calculated magnitude of the local heating effect shown to explain the experimental observation including the N-I transition caused by such Joule heating. 2

3. Background CNT: basics The way the graphene sheet is wrapped is represented by a pair of indices (n,m) called the chiral vector. The integers n and m denote the number of unit vectors along two directions in the honeycomb crystal lattice of graphene. If m=0, the nanotubes are called "zigzag". If n=m, the nanotubes are called "armchair". Otherwise, they are called "chiral".vectorscrystal lattice CNT types armchair zigzag chiral 3

4. Mechanical Properties  Extremely strong covalent bonding of atoms.  Represent a very small, high aspect ratio conductive additive for plastics. Electrical Conductivity  Highly conducting, can be either metallic or semiconducting  “Ballistic conduction”  Conductivity has been shown to be a function of the chirality (degree of twist), as well as the diameter  Depending on the chiral vector, nanotubes with a small diameter are either semiconducting or metallic   Thermal Conductivity  One of the best heat conducting materials  Temperature stability estimated to be up to 2800 o C in vacuum and about 750 o C in air.  Ultra-small SWNTs have even been shown to exhibit superconductivity below 20 K.  The thermal conductivity drops significantly to 3000W/m-K when the temperature is increased to 400 K Value > diamond !! 4

5. Motivation Similarities and contrasts of CNT and LC’s CNTLC Anisotropic in nature Diameter ~ nm; length ~  m Diameter < 1nm; length ~ 2-4 nm High Young’s modulus and high tensile strength Elastically soft Excellent electrical conductivity along the tube axis Insulator Excellent heat dissipatorInsulator 5

6. I Dierking, APL., 87,233507 (2005)I. Dierking et al, JAP 97, 044309 (2005) (a)  Note that the enhancement in conductivity is orders of magnitude higher with the use of an electric field as the orienting field in comparison with a magnetic field 6

7. Experimental Material LC: E7, 8CB, ZLI-2857 CNT: SWCNT Mixture: 0.05 wt% SWCNT in LC ITO-coated glass cell thickness: 70  m Method of sample preparation J. Non-Cryst. Solids 353, 4411 (2007)  Normally, the dispersion requires high-power ultra- sonication with a tip sonicator dipped into the mixture Phase separated into CNT-rich dark and CNT-poor light gray volumes  Bottle gently stirred with a magnetic stir bar over a weekend results in uniformly dark material Pure LC 7

8. N S Experimental set up 8

9. Results and Discussion Electrical conductivity investigations using electric field  Large increase in  above V=2V  The small increase in  at low (~0) voltages for the CNT composite over that of the undoped samples is due to ionic impurities of CNT  However, the ratio  composite /  E7 exhibits hyperlinear behaviour of the CNT system ruling out ionic origin for the large increase seen above V th 9

10. Arguments from literature for large increase in  Jang et al [1] and Sandler et al [2] investigated LCE-CNT system. They attributed the large  to a kind of percolation threshold behavior and due to the surface oxidation of CNT. Since no effort was made to re-orient the sample, specifically the CNT, the  observed would be of isotropic origin According to Dierking et al. [3] the large increase is due to reorientation of CNTs along the tube axis Shah et al [4] found an electric field reorientation results in a transformation from N to Iso attributing shorting of the cell by CNTs since the cell gap was smaller than the length of CNT L~60  m 5  m 1.JMC, 13, 679 (2003) 2.Polymer, 40, 5967 (1999) 3.JAP, 97, 044309 (2005) 4.JAP, 103, 064314 (2008) 10

11. Electrical conductivity investigations using magnetic field  Molecular reorientation is seen in both cases  The increase in  is only a factor of 2 higher than for zero field and orders of magnitude smaller than electric field reorientation situation Q: Is reorientation incomplete with H-field? A: Measure dielectric constant as a function of the H-field 11

12. Dielectric investigations  In the magnetic field case, (open symbols) pure and composite behave in a very similar fashion with saturation of  at high H-value  Confirms that low  for composite in H-field is not due to incomplete alignment  In the electric field case (filled symbols) pure E7 behaves normally whereas the composite shows erratic behavior above a certain voltage E7 E7+CNT 12

13. Possible reason for the anomalous behavior of conductivity  Percolation network theory can not be used to explain the discrepancy between magnetic and electric data What could be the reason?????  Large increase in current beyond a certain voltage is characteristic of dielectric breakdown   it is not intrinsic of the medium 13

14. POM observations  At 7.9 V a phase transformation took place to Iso phase  This suggests that local heating is at least of the order of 5 o C Calculation of temperature increase JAP,100,024906,2006  the enhancement in the conductivity is not intrinsic of the system, but an artefact of the applied voltage Conoscopy 14

15. Conclusions The electrical conductivity in LC-CNT composite obtained during voltage-induced reorientation process is not due to the inherent property of CNTs Dielectric breakdown and local heating effects play an important role 15