Jan Obrzut

Carbon Nanotubes: Measuring Dispersion and Length

Advanced technological uses of single-walled carbon nanotubes (SWCNTs) rely on the production of single length and chirality populations that are currently only available through liquid-phase post processing. The foundation of all of these processing steps is the attainment of individualized nanotube dispersions in solution. An understanding of the colloidal properties of the dispersed SWCNTs can then be used to design appropriate conditions for separations. In many instances nanotube size, particularly length, is especially active in determining the properties achievable in a given population, and, thus, there is a critical need for measurement technologies for both length distribution and effective separation techniques. In this Progress Report, the current state of the art for measuring dispersion and length populations, including separations, is documented, and examples are used to demonstrate the desirability of addressing these parameters.

Optical measurements of structure and orientation in sheared carbon-nanotube suspensions

We describe an optical metrology for measuring shear-induced structure and orientation in dilute dispersions of multiwalled carbon nanotubes. Small-angle polarized light scattering and optical microscopy are combined in situ to quantify the structural anisotropy of multiwalled carbon nanotubes in semidilute, surfactant-stabilized aqueous suspensions under simple shear flow. Measurements performed as a function of the applied shear rate are used to demonstrate the capabilities and limitations of the experimental technique, which should be suitable for probing the shear response of polymer-nanotube melts and solutions.

Microscale Polymer−Nanotube Composites

Polymer colloids with an interfacial coating of purified single-wall carbon nanotubes (SWCNTs) are synthesized from length- and type-sorted SWCNTs. Aqueous nanotube suspensions sorted through density-gradient ultracentrifugation are used to emulsify spherical polymer colloids of microscale dimensions that are characterized through a combination of optical microscopy, transmission electron microscopy, and impedance spectroscopy. The SWCNT-polymer composite particles exhibit electrical conductivities comparable to or better than those of bulk SWCNT-polymer composites at nanotube loadings of more than 1 order of magnitude lower. The composite particles retain the unique electronic and optical characteristics of the parent SWCNT solution with potential applications as microelectronic and microoptical components.

Dielectric Characterization by Microwave Cavity Perturbation Corrected for Nonuniform Fields

Nonuniform fields decrease the accuracy of dielectric characterization by microwave cavity perturbation. These fields are due to the slot in the cavity through which the sample is inserted and the boundary between the sample and the metallic walls inside of the cavity. To address this problem, we measured the natural frequency and damping ratio of a resonant cavity as a sample is inserted into the rectangular cavity. We found that for a range of cavity filling fractions, a linear regression on the natural frequency and damping ratio versus the effective volume fraction of the sample in the cavity could be used to extract the complex permittivity of the sample. We verified our technique by measuring a known quartz substrate and comparing the results to finite-element simulations. When compared to the conventional technique, we found a significant improvement in the accuracy for our samples and measurement setup. We confirmed our technique on two lossy samples: a neat stoichiometric mixture bisphenol A epoxy resin and one containing a mass fraction of 3.5% multi-walled carbon nanotubes (MWCNTs). At the TE103 mode (7.31 GHz), the permittivity and loss tangent of the epoxy were measured to be εr=2.93±0.11 and tanδ = 0.028±0.002, respectively. The epoxy with a mass fraction of 3.5% MWCNTs had a permittivity of εr=8.01±0.48 and loss tangent of tanδ = 0.137±0.010.

Charge transport in melt-dispersed carbon nanotubes

We investigate the effect of interfacial stabilizer on charge transport in polymer-dispersed carbon nanotubes. Despite mechanical contact, samples with dispersant show poor conductivity, which we attribute to a robust interfacial layer between contacted nanotubes. In comparison, results obtained when nanotubes are mechanically mixed into polymer melts without dispersant show much better conductivity. The difference is striking; at comparable loading, neat melt composites have resistivities five orders of magnitude smaller than those containing interfacial stabilizer. Our results highlight a fundamental issue for the engineering of conducting carbon nanotube composites; dispersion stability will typically be achieved at the expense of conductivity.

Accelerated Stress Test Assessment of Through-Silicon Via Using RF Signals

In this paper, radio frequency signal is demonstrated as an effective probe for assessing the effect of thermal cycling on the reliability of through-silicon vias (TSVs) in stacked dies. It is found that the RF signal integrity in TSV daisy chain, particularly its transmission characteristics, degrades considerably with extended thermal cycling, because of the formation and the growth of voids. Early failures are observed in the reliability analysis of the TSV daisy chain and are attributed to processing-related variability across the wafer. However, the maximum failure rate is found to occur at 500 thermal cycles, which is attributed to the initiation of defects and their subsequent propagation.

Input impedance of a coaxial line terminated with a complex gap capacitance - numerical and experimental analysis

A full-wave numerical analysis was performed for a coaxial line terminated with a complex gap capacitance using a finite-element high-frequency structure simulator. The scattering parameters, input impedance, and spatial distribution of the electromagnetic field have been obtained in the frequency range of 100 MHz to 19 GHz for specimens 8 to 320 /spl mu/m thick, with a dielectric constant of up to 80. It was found that the residual inductance of the specimen affects the impedance characteristic of the network. The inductance-capacitance resonance is coupled with the cavity resonance. The specimen inductance is linearly dependent on the specimen thickness. At frequencies near the cavity resonance, the specimen section can be treated as a network of a transmission line with a capacitance, where the fundamental mode propagates along the diameter of the specimen. Results of the numerical analysis were verified experimentally using water as a model material with a high dielectric constant. Our closed-form formula for input impedance of the network is valid in a wider frequency range than the lumped-element method. The results are useful in improving the accuracy of broadband dielectric measurements in the extended frequency range of thin films with high dielectric constant that are of interest to bio- and nanotechnology.

Trade-off between the Mechanical Strength and Microwave Electrical Properties of Functionalized and Irradiated Carbon Nanotube Sheets

Carbon nanotube (CNT) sheets represent a novel implementation of CNTs that enable the tailoring of electrical and mechanical properties for applications in the automotive and aerospace industries. Small molecule functionalization and postprocessing techniques, such as irradiation with high-energy particles, are methods that can enhance the mechanical properties of CNTs. However, the effect that these modifications have on the electrical conduction mechanisms has not been extensively explored. By characterizing the mechanical and electrical properties of multiwalled carbon nanotube (MWCNT) sheets with different functional groups and irradiation doses, we can expand our insights into the extent of the trade-off that exists between mechanical strength and electrical conductivity for commercially available CNT sheets. Such insights allow for the optimization of design pathways for engineering applications that require a balance of material property enhancements.

Noncontact conductivity and dielectric measurement for high throughput roll-to-roll nanomanufacturing.

Advances in roll-to-roll processing of graphene and carbon nanotubes have at last led to the continuous production of high-quality coatings and filaments, ushering in a wave of applications for flexible and wearable electronics, woven fabrics, and wires. These applications often require specific electrical properties, and hence precise control over material micro- and nanostructure. While such control can be achieved, in principle, by closed-loop processing methods, there are relatively few noncontact and nondestructive options for quantifying the electrical properties of materials on a moving web at the speed required in modern nanomanufacturing. Here, we demonstrate a noncontact microwave method for measuring the dielectric constant and conductivity (or geometry for samples of known dielectric properties) of materials in a millisecond. Such measurement times are compatible with current and future industrial needs, enabling real-time materials characterization and in-line control of processing variables without disrupting production.

High Frequency Loss Mechanism in Polymers Filled With Dielectric Modifiers

We analyzed the high frequency dielectric relaxation mechanism in high-k composite materials using film substrates made of low loss organic resin filled with ferroelectric ceramics and with single wall carbon nanotubes (SWNT). We performed broadband permittivity measurements of high-k film substrates at frequencies of 100 Hz to about 10 GHz. In order to analyze the effect of the dielectric thickness, dielectric constant, loss and conductive loss on the impedance characteristics, we used a High Frequency Structure Simulator to perform a full wave numerical analysis of several power planes. Small angle neutron scattering (SANS) was used to probe the dispersion of SWNTs in polymer matrices. It was found that organic-ceramic composites exhibit an intrinsic high frequency relaxation behavior that gives rise to frequency dependent dielectric loss. The highest frequency relaxation process dominates the overall loss characteristic. In the case of polymers modified with SWNTs, we observed that 2 % mass fraction of p-doped semi-conducting SWNTs increases the dielectric constant by 3 orders of magnitude, in apparent violation of the mixing-rule. The hybrid material appears to have preferential coupling within the dispersed phase. The experimental data and numerical simulation indicate that these materials can play a significant role as embedded passive devices with functional characteristics superior to that of discrete components.