Carbon nanotube-rich domain effects on bulk electrical properties of nanocomposites

Carbon nanotube (CNT)/epoxy composites are intriguing materials that enable materials scientists and engineers to tailor structural and electrical properties for applications in the automotive and aerospace industries. Recent insights into CNT-rich domain formation and its influence on electrical properties raise questions about which processing variables can be used to tune the overall electrical conductivity. Here, we investigate how mass fraction and curing temperature influence these electrical properties. CNT nanocomposites were fabricated varying the mass fraction of CNT and the epoxy curing temperature. First, scanning lithium ion microscopy coupled with transmission electron microscopy were employed to investigate the morphology of CNT-rich domains that formed more readily at elevated curing temperatures than during room temperature curing. Then, oscillatory shear rheology measurements of the unfilled curing epoxy informed a simple kinetic argument to explain the CNT-rich domain formation. Finally, the electrical conductivity (both alternating and direct current) was characterized with a novel microwave cavity perturbation spectroscopy technique (alternating current conductivity) and a standard four-point probe station (direct current conductivity). The overarching conclusion of the work was that the CNT-rich domains formed a secondary percolated network surrounded by an almost completely unfilled epoxy matrix that allowed for higher conductivities at lower loadings. This work demonstrates that perfect dispersion of the nanoparticulate is, at least in this instance, not necessarily the preferred morphology

Relationship Between Dispersion Metric and Properties of PMMA/SWNT Nanocomposites

Particle spatial dispersion is a crucial characteristic of polymer composite materials and this property is recognized as especially important in nanocomposite materials due to the general tendency of nanoparticles to aggregate under processing conditions. We introduce dispersion metrics along with a specified dispersion scale over which material homogeneity is measured and consider how the dispersion metrics correlate quantitatively with the variation of basic nanocomposite properties. We then address the general problem of quantifying nanoparticle spatial dispersion in model nanocomposites of single wall carbon nanotubes (SWNT) dispersed in poly(methyl methacrylate) (PMMA) at a fixed SWNT concentration of 0.5 % using a \u27coagulation\u27 fabrication method. Two methods are utilized to measure dispersion, UV-Vis spectroscopy and optical confocal microscopy. Quantitative spatial dispersion levels were obtained through image analysis to obtain a \u27relative dispersion index\u27 (RDI) representing the uniformity of the dispersion of SWNTs in the samples and through absorbance. We find that the storage modulus, electrical conductivity, and flammability containing the same amount of SWNTs, the relationships between the quantified dispersion levels and physical properties show about four orders of magnitude variation in storage modulus, almost eight orders of magnitude variation in electric conductivity, and about 70 % reduction in peak mass loss rate at the highest dispersion level used in this study. The observation of such a profound effect of SWNT dispersion indicates the need for objective dispersion metrics for correlating and understanding how the properties of nanocomposites are determined by the concentration, shape and size of the nanotubes

Budżetowanie działalności jednostek gospodarczych Teoria i praktyka. Część V

Z wprowadzenia: "Przekazujemy do rąk Czytelników część monografii dotyczącej budżetowania jednostek gospodarczych. Jej przygotowanie zbiegło się z ukazaniem się manifestu Precz z budżetami Jeremiego Норе’а i Robina Frasera. Autorzy poddają tam ostrej krytyce dotychczasowe praktyki stosowania budżetowania kosztowego. Stąd niniejsza publikacja - między innymi - dlatego różni się istotnie od poprzednich. Od pewnego czasu - także w Polsce - pojawiały się głosy wskazujące na istotne niedoskonałości budżetowania kosztowego, na przykład J. Gierusz [Materiały konferencyjne 2001], G. H. Świderska [Rachunkowośćzarządcza i rachunek kosztów, 2002]. Nie odnosiły one jednak skutku. Nie zauważono też dotąd narastającej listy zarzutów wytaczanych przeciw finansowym jednostkom miary stosowanym w budżetowaniu kosztowym, na przykład G. K. Świderska [jak wyżej], M. Sierpińska, B. Niedbała [Controllingoperacyjny wpnedsiębiorstwie, 2003]. Bez echa pozostało postawione przez autora pytanie: „zmierzch czy rozwój budżetowania?” [„Controlling i rachunkowość zarządcza” 9/2002], gdzie jednoznacznie wskazano, że budżetowanie kosztowe obejmuje jedynie jedną sferę działalności przedsiębiorstwa i tym samym nie może stanowić wystarczającej podstawy do sterowania przedsiębiorstwem. Zwolennicy budżetowania kosztowego nawet zgadzali się ze stawianymi zarzutami, jednak nie reagowali na propozycje zmian w filozofii i metodologii budżetowania."(...

Structural Stability of Transparent Conducting Films Assembled from Length Purified Single-Wall Carbon Nanotubes

Single-wall carbon nanotube (SWCNT) films show significant promise for transparent electronics applications that demand mechanical flexibility, but durability remains an outstanding issue. In this work, thin membranes of length purified single-wall carbon nanotubes (SWCNTs) are uniaxially and isotropically compressed by depositing them on prestrained polymer substrates. Upon release of the strain, the topography, microstructure, and conductivity of the films are characterized using a combination of optical/fluorescence microscopy, light scattering, force microscopy, electron microscopy, and impedance spectroscopy. Above a critical surface mass density, films assembled from nanotubes of well-defined length exhibit a strongly nonlinear mechanical response. The measured strain dependence reveals a dramatic softening that occurs through an alignment of the SWCNTs normal to the direction of prestrain, which at small strains is also apparent as an anisotropic increase in sheet resistance along the same direction. At higher strains, the membrane conductivities increase due to a compression-induced restoration of conductive pathways. Our measurements reveal the fundamental mode of elasto-plastic deformation in these films and suggest how it might be suppressed

Contactless Resonant Cavity Dielectric Spectroscopic Studies of Recycled Office Papers

Current product composition and quality test methods for the paper and pulp industries are rooted in wet-bench chemistry techniques which cannot be used to distinguish between virgin and secondary fibers. We have recently demonstrated the application of an in situ and nondestructive assessment method based on dielectric spectroscopy (DS), which can address this deficiency in the testing of paper. The DS technique, which employs a resonant microwave cavity, could be applicable to quality assurance techniques such as gauge capability studies and real-time statistical process control (SPC), and may have inherent forensic capabilities. In this paper, we show how this DS technique can be used to distinguish between office copier paper products which may contain recycled fibers. We show a reasonable correlation between the dielectric characteristics (e.g., dielectric loss) and the atomistic level chemical changes that result from the paper recycling process

Optical property tuning of single-wall carbon nanotubes by endohedral encapsulation of a wide variety of dielectric molecules

Specific and tunable modification to the optical properties of single-wall carbon nanotubes (SWCNTs) is demonstrated through direct encapsulation into the nanotube interior of guest molecules with widely varying static dielectric constants. Filled through simple ingestion of the guest molecule, each SWCNT population is demonstrated to display a robust modification to absorbance, fluorescence, and Raman spectra. Over 30 distinct compounds, covering static dielectric constants from 1.8 to 109, are inserted in large diameter SWCNTs (d = 1.104−1.524 nm) and more than 10 compounds in small diameter SWCNTs (d = 0.747−1.153 nm), demonstrating that the general effect of filler dielectric on the nanotube optical properties is a monotonic energy reduction (red-shifting) of the optical transitions with increased magnitude of the dielectric constant. Systematic fitting of the twodimensional fluorescence−excitation and Raman spectra additionally enables determination of the critical filling diameter for each molecule and distinguishing of overall trends from specific guest−host interactions. Comparisons to predictions from existing theory are presented, and specific guest molecule/SWCNT chirality combinations that disobey the general trend and theory are identified. A general increase of the fluorescence intensity and line narrowing is observed for low dielectric constants, with long linear alkane filled SWCNTs exhibiting emission intensities approaching those of empty SWCNTs. These results demonstrate an exploitable modulation in the optical properties of SWCNTs and provide a foundation for examining higher-order effects, such as due to nonbulk-like molecule stacking, in host−guest interactions in well-controlled nanopore size materials

Dielectric Characterization of H₂O and CO₂ Uptake by Polyethylenimine Films

The absorption of CO2 by polyethylenimine polymer (PEI) materials is of great interest in connection with proposed carbon capture technologies, and the successful development of this technology requires testing methods quantifying the amount of CO2, H2O, and reaction byproducts under operating conditions. We anticipate that dielectric measurements have the potential for quantifying both the extent of CO2 and H2O absorption within the PEI matrix material as well as insights into subsequent reaction byproducts that can be expected to occur in the presence of moisture. The complexity of the chemistry involved in this reactive binding process clearly points to the need for the use of additional spectroscopic techniques to better resolve the multiple components involved and to validate the model-dependent findings from the dielectric measurements. Here, we employed noncontact resonant microwave cavity instrumentation operating at 7.435 GHz that allows for the precise determination of the complex dielectric permittivity of CO2 films exposed to atmospheres of controlled relative humidity (RH), and N2:CO2 compositions. We find that the addition of CO2 leads to a considerable increase in dielectric loss of the PEI film relative to loss measured in nitrogen (N2) atmosphere across the same RH range. We attribute this effect to a reaction between CO2 and PEI generating a charged dielectrically active species contributing to the dielectric loss in the presence of moisture. Possible reaction mechanisms accounting for these observations are discussed, including the formation of carbamate-ammonium pairs and ammonium cations stabilized by bicarbonate anions that have sufficient local mobility to be dielectrically active in the investigated microwave frequency range. Understanding of these reaction mechanisms and the development of tools to quantify the amount of reactive byproducts are expected to be critical for the design and optimization of carbon capture materials