Showing papers by "Philip A. Parilla published in 2010"

Cellulose crystallinity index: measurement techniques and their impact on interpreting cellulase performance

Abstract: Although measurements of crystallinity index (CI) have a long history, it has been found that CI varies significantly depending on the choice of measurement method. In this study, four different techniques incorporating X-ray diffraction and solid-state 13C nuclear magnetic resonance (NMR) were compared using eight different cellulose preparations. We found that the simplest method, which is also the most widely used, and which involves measurement of just two heights in the X-ray diffractogram, produced significantly higher crystallinity values than did the other methods. Data in the literature for the cellulose preparation used (Avicel PH-101) support this observation. We believe that the alternative X-ray diffraction (XRD) and NMR methods presented here, which consider the contributions from amorphous and crystalline cellulose to the entire XRD and NMR spectra, provide a more accurate measure of the crystallinity of cellulose. Although celluloses having a high amorphous content are usually more easily digested by enzymes, it is unclear, based on studies published in the literature, whether CI actually provides a clear indication of the digestibility of a cellulose sample. Cellulose accessibility should be affected by crystallinity, but is also likely to be affected by several other parameters, such as lignin/hemicellulose contents and distribution, porosity, and particle size. Given the methodological dependency of cellulose CI values and the complex nature of cellulase interactions with amorphous and crystalline celluloses, we caution against trying to correlate relatively small changes in CI with changes in cellulose digestibility. In addition, the prediction of cellulase performance based on low levels of cellulose conversion may not include sufficient digestion of the crystalline component to be meaningful.

Fast-Switching Electrochromic Li + -Doped NiO Films by Ultrasonic Spray Deposition

Abstract: A low cost, high throughput deposition method for films of nickel oxide NiO and lithium-doped nickel oxide with improved electrochromic performance is demonstrated. This method is based on ultrasonic spray deposition of aqueous-based precursor solutions in air at atmospheric pressure, which represents a significant cost savings compared to vacuum deposition methods. The resultant materials are characterized by X-ray diffraction, Raman spectroscopy, electron microscopy, and electrochemical measurements. Electrochromic performance is demonstrated with in situ optical transmission measurements during electrochemical characterization. Nickel oxide materials color anodically and are thereby ideally suited to be used as counter electrode for the well-known tungsten oxide WO3 system in “smart” window applications. The coloration of nickel oxide materials is known to be slow when compared to WO3 and thereby limits the overall response time of a NiO/WO3 tandem device. The analysis of potential step response data shows that our lithium-doped nickel oxide material achieves 90% of its total coloration change in 29 s, which is comparable to reported measurements for WO3. These results significantly mitigate a potential bottleneck to the adoption of metal oxide electrochromic windows not only by demonstrating similar performance between NiO and WO3, but by achieving this result via low cost, highly scalable processing methods.

Solution-Phase Synthesis of Heteroatom-Substituted Carbon Scaffolds for Hydrogen Storage

Abstract: This paper reports a bottom-up solution-phase process for the preparation of pristine and heteroatom (boron, phosphorus, or nitrogen)-substituted carbon scaffolds that show good surface areas and enhanced hydrogen adsorption capacities and binding energies. The synthesis method involves heating chlorine-containing small organic molecules with metallic sodium at reflux in high-boiling solvents. For heteroatom incorporation, heteroatomic electrophiles are added to the reaction mixture. Under the reaction conditions, micrometer-sized graphitic sheets assembled by 3−5 nm-sized domains of graphene nanoflakes are formed, and when they are heteroatom-substituted, the heteroatoms are uniformly distributed. The substituted carbon scaffolds enriched with heteroatoms (boron ~7.3%, phosphorus ~8.1%, and nitrogen ~28.1%) had surface areas as high as 900 m^2 g^(−1) and enhanced reversible hydrogen physisorption capacities relative to pristine carbon scaffolds or common carbonaceous materials. In addition, the binding energies of the substituted carbon scaffolds, as measured by adsorption isotherms, were 8.6, 8.3, and 5.6 kJ mol^(−1) for the boron-, phosphorus-, and nitrogen-enriched carbon scaffolds, respectively.

Stoichiometric analysis of compositionally graded combinatorial amorphous thin film oxides using laser-induced breakdown spectroscopy.

Abstract: Laser-induced breakdown spectroscopy (LIBS) is a recently developed locally destructive elemental analysis technique that can be used to analyze solid, liquid, and gaseous samples. In the system explored here, a neodymium-doped yttrium aluminum garnet laser ablates a small amount of the sample and spectral emission from the plume is analyzed using a set of synchronized spectrometers. We explore the use of LIBS to map the stoichiometry of compositionally graded amorphous indium zinc oxide thin-film libraries. After optimization of the experimental parameters (distance between lens and samples, spot size on the samples, etc.), the LIBS system was calibrated against inductively coupled plasma atomic emission spectroscopy which resulted in a very consistent LIBS-based elemental analysis. Various parameters that need to be watched closely in order to produce consistent results are discussed. We also compare LIBS and x-ray fluorescence as techniques for the compositional mapping of libraries.

Metal-insulator-metal point-contact diodes as a rectifier for rectenna

Abstract: The rectenna (rectifying antenna), if extended to operate at visible light, will revolutionize the way sunlight is harvested for electricity. The focus of this work is to develop a suitable rectifier component of the rectenna for rectifying high-frequency radiation. In this work, a point-contact metal-insulator-metal (MIM) diode based on Nb-Nb 2 O 5 and Nb-TiO 2 was fabricated and successfully tested at low frequency. Fabrication of point-contact MIM diodes was greatly simplified by adopting a facile fabrication approach. The ultra-thin (less than 5 nm) insulator layer was deposited via anodic oxidation (of the metal layer), a non-vacuum technique. The second metal is in the form of a bent wire and hence results in a point-contact diode area. An optimization study was carried out to deposit a smooth insulator film with the desired chemical composition. Although researched for more than six decades, suitable MIM devices for high-frequency rectification (with all the desired rectifier behavior) have not yet been developed, mainly because of the many technical difficulties involved. One such difficulty is the lack of optimization of material properties of MIM structures. A systematic study to fill in this lack of knowledge that relates material properties to MIM device performance is reported here. Based on this systematic study, we find a strong correlation between the work function and electron affinity of the metal and insulator, respectively, on MIM device performance.

Doped Bi-Se thin films for photovoltaic applications

Abstract: Cu x Bi y Se z thin films with copper concentration x up to 29 mol. % are synthesized on glass substrates by chemical-bath deposition. Films with thickness in the range of 550–597 nm are fabricated by a multiple-dip process. Deposition parameters for a single-coating run are optimized to avoid re-dissolution of the particles, and to obtain large-area composition homogeneity in the films. Routine film composition Cu:Bi:Se and thickness are analyzed by X-ray fluorescence. To evaluate the doped Bi-Se thin films for potential use in photovoltaic devices, systematic investigations of the films' structural, morphological, optical, and electrical properties are performed. The physical properties of as-deposited and annealed films are analyzed by X-ray diffraction, scanning electron microscopy, UV-Vis-IR photospectroscopy, and Hall effect and Seebeck effect measurements.