Germano S. Iannacchione

The specific heat and effective thermal conductivity of composites containing single-wall and multi-wall carbon nanotubes.

We present a study of the specific heat and effective thermal conductivity in anisotropic and randomly oriented multi-wall carbon nanotube (MWCNT) and randomly oriented single-wall carbon nanotube (SWCNT) composites from 300 to 400 K. Measurements on randomly oriented MWCNTs and SWCNTs were made by depositing a thin film of CNTs within a calorimetric cell. Anisotropic measurements were made on MWCNTs grown inside the highly ordered, densely packed nanochannels of anodic aluminum oxide. The specific heat of randomly oriented MWCNTs and SWCNTs showed similar behavior to the specific heat of bulk graphite powder. However, the specific heat of aligned MWCNTs is smaller and has weaker temperature dependence than that of the bulk above room temperature. The effective thermal conductivity of randomly oriented MWCNTs and SWCNTs is similar to that of powder graphite, exhibiting a maximum value near 364 K indicating the onset of phonon-phonon scattering. The effective thermal conductivity of the anisotropic MWCNTs increased smoothly with increasing temperature and is indicative of the one-dimensional nature of the heat flow.

Characterization of the cylindrical cavities of anopore and nuclepore membranes

The microscopic properties of the inner surface of the cavities of Anopore and Nuclepore membranes are investigated with scanning electron microscopy (SEM), nitrogen adsorption isotherms, and deuterium nuclear magnetic resonance (2H‐NMR). Useful information about the cavity orientation and morphology, and internal surface area is obtained. Analysis of SEM photographs yields estimates of the surface area and the porosity of these membranes which complements the adsorption results. The orientation of liquid crystals permeated in the cavities of Anopore and Nuclepore membranes is probed with the 2H‐NMR technique. It is found that the orientation of the liquid crystal molecules is governed by the confining volume and surface conditions. The 2H‐NMR spectral line shape of the confined liquid crystal also provides information on the substrate morphology and roughness that is consistent with SEM and adsorption experiments.

Nematic anchoring on carbon nanotubes

A dilute suspension of carbon nanotubes (CNTs) in a nematic liquid crystal (LC) does not disturb the LC director. Due to a strong LC-CNT anchoring energy and structural symmetry matching, CNT long axis follows the director field, possessing enhanced dielectric anisotropy of the LC media. This strong anchoring energy stabilizes local pseudonematic domains, resulting in nonzero dielectric anisotropy in the isotropic phase. These anisotropic domains respond to external electric fields and show intrinsic frequency response. The presence of these domains makes the isotropic phase electric field-responsive, giving rise to a large dielectric hysteresis effect.

Smectic ordering in liquid-crystal-aerosil dispersions. I. X-ray scattering.

Comprehensive x-ray scattering studies have characterized the smectic ordering of octylcyanobiphenyl (8CB) confined in the hydrogen-bonded silica gels formed by aerosil dispersions. For all densities of aerosil and all measurement temperatures, the correlations remain short range, demonstrating that the disorder imposed by the gels destroys the nematic (N) to smectic-A (SmA) transition. The smectic correlation function contains two distinct contributions. The first has a form identical to that describing the critical thermal fluctuations in pure 8CB near the N-SmA transition, and this term displays a temperature dependence at high temperatures similar to that of the pure liquid crystal. The second term, which is negligible at high temperatures but dominates at low temperatures, has a shape given by the thermal term squared and describes the static fluctuations due to random fields induced by confinement in the gel. The correlation lengths appearing in the thermal and disorder terms are the same and show a strong variation with gel density at low temperatures. The temperature dependence of the amplitude of the static fluctuations further suggests that nematic susceptibility becomes suppressed with increasing quenched disorder. The results overall are well described by a mapping of the liquid-crystal-aerosil system onto a three-dimensional XY model in a random field with disorder strength varying linearly with the aerosil density.

Dielectric hysteresis, relaxation dynamics, and nonvolatile memory effect in carbon nanotube dispersed liquid crystal

Self-organizing nematic liquid crystals (LCs) impart their orientational order onto dispersed carbon nanotubes (CNTs) and obtain CNT-self-assembly on a macroscopic dimension. The nanotube-long axis, being coupled to the nematic director, enables orientational manipulation via the LC nematic reorientation. Electric-field-induced director rotation of a nematic LC+CNT system is of potential interest due to its possible application as a nanoelectromechanical system. Electric field and temperature dependence of dielectric properties of a LC+CNT composite system have been investigated to understand the principles governing CNT assembly mediated by the LC. In the LC+CNT nematic phase, the dielectric relaxation on removing the applied field follows a single-exponential decay, exhibiting a faster decay response than the pure LC above a threshold field. The observed dielectric behaviors on field cycling in the nematic phase for the composite indicates an electromechanical hysteresis effect of the director field due...

Smectic ordering in liquid-crystal-aerosil dispersions. II. Scaling analysis.

Liquid crystals offer many unique opportunities to study various phase transitions with continuous symmetry in the presence of quenched random disorder (QRD). The QRD arises from the presence of porous solids in the form of a random gel network. Experimental and theoretical work supports the view that for fixed (static) inclusions, quasi-long-range smectic order is destroyed for arbitrarily small volume fractions of the solid. However, the presence of porous solids indicates that finite-size effects could play some role in limiting long-range order. In an earlier work, the nematic-smectic-A transition region of octylcyanobiphenyl (8CB) and silica aerosils was investigated calorimetrically. A detailed x-ray study of this system is presented in the preceding paper, which indicates that pseudocritical scaling behavior is observed. In the present paper, the role of finite-size scaling and two-scale universality aspects of the 8CB+aerosil system are presented and the dependence of the QRD strength on the aerosil density is discussed.

Evolution of the isotropic-to-nematic phase transition in octyloxycyanobiphenyl+aerosil dispersions.

High-resolution ac calorimetry has been carried out on dispersions of aerosils in the liquid crystal octyloxycyanobiphenyl (8OCB) as a function of aerosil concentration and temperature spanning the crystal to isotropic phases. The liquid crystal 8OCB is elastically stiffer than the previously well studied octylcyanobiphenyl (8CB)+aerosil system and so general quenched random-disorder effects and liquid crystal specific effects can be distinguished. A double heat capacity feature is observed at the isotropic to nematic phase transition with an aerosil independent overlap of the heat capacity wings far from the transition and having a nonmonotonic variation of the transition temperature. A crossover between low and high aerosil density behavior is observed for 8OCB+aerosil. These features are generally consistent with those on the 8CB+aerosil system. Differences between these two systems in the magnitude of the transition temperature shifts, heat capacity suppression, and crossover aerosil density between the two regimes of behavior indicate a liquid crystal specific effect. The low aerosil density regime is apparently more orientationally disordered than the high aerosil density regime, which is more translationally disordered. An interpretation of these results based on a temperature dependent disorder strength is discussed. Finally, a detailed thermal hysteresis study has found that crystallization of a well homogenized sample perturbs and increases the disorder for low aerosil density samples but does not influence high-density samples.

Dielectric response of multiwalled carbon nanotubes as a function of applied ac-electric fields

The complex dielectric constant (e∗) is reported for multiwalled carbon nanotubes (MWCNTs) up to 105 Hz as a function of ac-electric field amplitudes Erot (in phase and same frequency as the measurement) and Eac (different phase and fixed frequency with respect to the measurement). A slow relaxation process (mode 1) is observed, which shifts to higher frequency with increasing Erot but is independent of Eac. A fast relaxation process (mode 2) is also observed, which is independent of Erot but shifts to higher frequency with increasing Eac (opposite to that of mode 1). An ac-conductivity analysis of MWCNT reveals insights on how Erot and Eac influence the dissipation.

AC calorimetric studies of phase transitions in porous substrates. Superfluid helium and liquid crystals

Abstract In this review, we detail the application of an AC calorimetry technique as it has been employed and optimized for the study of phase transitions, particularly the superfluid-to-normal transition and those undergone by liquid crystals systems while confined to geometries more restrictive than bulk. The theory of operation, the sample preparation, and the electronic equipment involved in such measurements are described in detail. Examples of its use in heat capacity measurements of two-dimensional helium films at sub-Kelvin temperatures and in liquid crystal studies, a discussion on the simultaneously measured phase shift, which contains information regarding the order of the phase transition, is also included.

Thermal properties and glass transition in PMMA + SWCNT composites

The effective thermal and dynamic properties of composites containing polymethyl-metha-acralyte (PMMA) dispersed with single-wall carbon nanotubes (SWCNTs) were studied by modulation/ac calorimetry (ACC) and modulation-differential-scanning (MDSC) calorimetric techniques. The specific heat and effective thermal conductivity were determined by ACC from 300 to 400 K as a function of SWCNT content. A large enhancement of the effective thermal conductivity is observed as the mass fraction (m) of SWCNTs increases from 0.014 to 0.083. These results are in good agreement with a simple geometric model at low SWCNT content but are better described by more sophisticated models above m ~ 0.039. The dynamics of the glass transition were studied by MDSC as a function of temperature scan rate. The hysteresis of the reversible specific heat between heating and cooling scans decreases with decreasing scan rate for pure PMMA but is essentially zero in the composites, indicating that the SWCNT may be quenching slow glass dynamics. In all samples, the effective glass transition temperature, Tg, increases with increasing scan rate (though less so for higher m) but the MDSC determined Tg does not fall inline with that determined by the ACC method. This discrepancy is attributed to the effect of prolonged heat treatment of the composite sample used in the ACC experiments.