Preparation of nanotubes and nanofibers from silicon carbide precursor polymers by using polymer blend and spinning techniques

Many potential applications have been proposed for silicon carbide (SiC) nanostructures, including high-strength composites, nanosensors and nanodevices. Limitations in processing are an important barrier that has to be overcome in order to develop these applications. The aim of this study is to explore the possibility of fabricating SiC nanotubes and to improve the elaboration process of SiC nanofibers prepared by using polymer blend and spinning techniques. In the general introduction, the structure of SiC and the recent uses of polycarbosilane (PCS) as a SiC precursor for nanofibers are discussed. A review of the synthesizing methods for SiC nanotubes and nanofibers and their possible applications are presented. The present work proposes the use of polymer blend technique in order to prepare the SiC nanotubes and nanofibers. Due to the versatility of the polymer blend industrially, it could be used to mass-production of nanostructures. The polymer blend technique presented is based on the use of a precursor polymer with a ceramic residue and a thermally decomposable polymer without carbon residue after heating. Chapter 1 presents an attempt to prepare nanotubes from core/shell particles by using polymer blend and wet-spinning techniques. Polystyrene (PS) was used as a core decomposable polymer and polydimethylsilylvinylacetylene (PDMSVA) as the shell silicon carbide precursor polymer. The core/shell particles were prepared by emulsion polymerization. They were subjected to wet-spinning, stretched mechanically six times their original length, stabilized by UV and also in air, and finally heat-treated in an inert atmosphere. Nanotubes were prepared although the silicon carbide precursor polymer (PDMSVA) was insufficiently stabilized. Preparation of nanotubes from poly(methyl methacrylate) / polycarbosilane (PMMA/PCS) and poly(methyl methacrylate) / polyacrylonitrile / polycarbosilane (PMMA/PAN/PCS) core/shell particles by the aforementioned techniques is presented in Chapter 2. The core/shell particles were prepared by the spray drying method. They were subjected to wet-spinning, stretched, stabilized and heat-treated. Few nanotubes were obtained from the PMMA/PAN/PCS heat-treated sample because the stretching technique was not efficient in order to elongate all the core/shell particles. Recently, SiC fibers have found many applications in composites at higher temperatures in oxidizing environments. One of the most important factors related to the chemical composition of these fibers is the oxygen content because it decreases the mechanical properties of the fibers used for such applications. In this study, how to improve the preparation process in order to obtain SiC nanofibers with low oxygen content was sought. Chapter 3 refers to preparation and study of the oxidation behavior of nanofibers derived from polycarbosilane by using polymer blend and melt-spinning techniques. Polycarbosilane (PCS) and novolac-type phenol-formaldehyde resin (PF) were dissolved in tetrahydrofuran (THF) and after removal of THF, the polymer blend was melt-spun. The polymer blend fibers were soaked in an acid solution in order to cure the phenolic matrix. The stabilized polymer blend fibers were heated at 1000 oC under a nitrogen atmosphere and kept in nitric acid solution to remove the matrix carbon. The oxidation behavior was characterized. A part of the nanofibers was further heated at 1500 oC. Nanofibers of several 100 nm in diameter were prepared. A large amount of oxygen was introduced into the nanofibers during the oxidation process. After heating at 1500 oC, the nanofibers changed from an amorphous phase to β-SiC. In Chapter 4, the microstructural changes of nanofibers were examined. The preparation procedure was the same as that of Chapter 3, but the nanofibers were heated to two different temperatures, 1000 oC and 1200 oC. The nanofibers heated at 1200 oC were straight, longer and had a smoother surface compared with the nanofibers heated at 1000 oC. The nanofibers heated at 1200 oC had higher resistance to oxidation by nitric acid treatment than the nanofibers heated at 1000 oC, revealing the importance of the heat treatment temperature. Finally, the general conclusions are presented.学位記番号：工博甲31

Implementation of the Z-scan technique using NI cRIO 9074 system

This paper presents the implementation of Z-scan technique using NI cRIO 9074 system to characterize different types of nanofluids and films. This technique was implemented in the Universidad Autonoma del Estado de Mexico. For this experiment a chassis NI cRIO9074 of National Instruments, a linear translation stage NRT150E and a stepper motor controller BSC203 were used, both of Thorlabs. Three steps were followed for this implementation first, the connection between NI cRIO9074 and BSC203 controller. Second, the program on LabVIEW was developed and finally, all optic part of the z-scan technique was implemented. This implementation can be used to characterize relatively thin (< 5mm) optical materials. The system testing was done with gold nanoparticles. The results showed the nonlinear optical properties of these samples

Conservación de productos hortofrutícolas mediante el uso de nanopartículas de quitosano y agentes naturales

Actualmente se buscan nuevas alternativas amigables con el ambiente que permitan la conservación de productos agrícolas expuestos al deterioro durante la cosecha, transporte y distribución en el mercado para su venta con la subsecuente reducción de sus atributos de calidad, fisiológicos y propiedades sensoriales, aunados a la manifestación de microorganismos patógenos. En este sentido, se presenta el estado del arte del uso de recubrimientos nanoestructurados de nanopartículas de quitosano con agentes naturales, sus beneficios y un enfoque académico, industrial y gubernamental del uso de la nanoagrotecnología, así como las perspectivas de esta técnica para la conservación de frutas y hortalizas como una solución viable

Conservación de productos hortofrutícolas mediante el uso de nanopartículas de quitosano y agentes naturales

Nowadays, new environmentally friendly alternatives that will allow the conservation of agricultural products exposed to deterioration during harvesting, transportation and distribution in the market for sales with the subsequent reduction of their quality, physiological and sensory properties, together with the manifestation of pathogenic microorganisms are sought. In this science divulgation paper, the state of the art of the use chitosan nanoparticles with natural agents nanostructured coatings is presented, along with its benefits. Also, an academic, industrial, and governmental approach to the use of nano-agrotechnology, as well as the perspectives of this technique for fruit and vegetable preservation as a viable solution.Actualmente se buscan nuevas alternativas, amigables con el ambiente, que permitan la conservación de productos agrícolas que se exponen al deterioro durante la cosecha, transporte y distribución en el mercado para su venta, con la subsecuente reducción de sus atributos de calidad, fisiológicos y propiedades sensoriales, aunados a la manifestación de microorganismos patógenos. En este artículo de divulgación se presenta el estado del arte del uso de recubrimientos nanoestructurados de nanopartículas de quitosano con agentes naturales, sus beneficios y un enfoque académico, industrial y gubernamental del uso de la nano-agrotecnología, así como las perspectivas de esta técnica para la conservación de frutas y hortalizas como una solución viable

Biodegradability Assessment of Prickly Pear Waste&ndash;Polymer Fibers under Soil Composting

Nowadays, solving the problems associated with environmental pollution is of special interest. Therefore, in this work, the morphology and thermal and mechanical properties of extruded fibers based on polylactic acid (PLA) and poly(butylene adipate-co-terephthalate) (PBAT) added to prickly pear flour (PPF) under composting for 3 and 6 months were evaluated. The highest weight loss percentage (92 &plusmn; 7%) was obtained after 6-month degradation of the PLA/PBAT/PPF/CO/AA blend, in which PPF, canola oil (CO), and adipic acid (AA) were added. Optical and scanning electron microscopy (SEM) revealed structural changes in the fibers as composting time increased. The main changes in the absorption bands observed by Fourier transform infrared spectroscopy (FTIR) were related to the decrease in -C=O (1740 cm&minus;1) and -C-O (1100 cm&minus;1) groups and at 1269 cm&minus;1, associated with hemicellulose in the blends with PPF. Differential scanning calorimetry (DSC) showed an increase in the cold crystallization and melting point with degradation time, being more evident in the fibers with PPF, as well as a decrease in the mechanical properties, especially Young&rsquo;s modulus. The obtained results suggest that PPF residues could promote the biodegradability of PLA/PBAT-based fiber composites

Study of the thermal diffusivity of nanofluids containing SiO2 decorated with Au nanoparticles by thermal lens spectroscopy

In this work, silicon dioxide (SiO2) spheres and gold nanoparticles (Au) were prepared. The SiO2 was used as a platform to deposit the gold nanoparticles. The SiO2 structures were synthesized employing the Stöber method. Monodisperse spherical particles with mean size of 293 nm were observed by transmission electron microscopy (TEM). The gold nanoparticles were attached to the dielectric platform trough in situ reduction. The UV–Vis spectrum of SiO2–Au showed an absorption band in the visible region associated with the presence of the gold nanoparticles. The TEM micrographs confirmed decorated SiO2 spheres with the metallic nanoparticles of 5 nm in size. Nanofluid concentrations of 0.1–0.6 mg/ml of SiO2 decorated with Au nanoparticles dispersed in water were prepared. The functional groups of SiO2-functionalized spheres were followed by FTIR. The formation of gold nanoparticles was evidenced by UV–Vis spectroscopy. The crystalline structure of SiO2 spheres and SiO2 decorated with Au nanoparticles was determined by XRD. The thermal diffusivity as a function of concentration using the mode-mismatched thermal lens (TL) spectroscopy was studied. The results of TL spectroscopy showed an increase in the thermal diffusivity with an increase of SiO2 decorated with Au nanoparticle concentration in the nanofluid

Use of Natural Products on the Control of <i>Aspergillus flavus</i> and Production of Aflatoxins In Vitro and on Tomato Fruit

Aspergillus flavus affects fresh and dry fruit and vegetable products, and its toxic metabolites, namely aflatoxins, cause serious damage in humans. The objective of this research study was to evaluate the effect of commercial natural products as well as edible and nanostructured chitosan coatings on the development of A. flavus and on the production of aflatoxins in vitro and in tomato. Treatments were as follows: chitosan 1%, chitosan coating, chitosan nanostructured coating, Citrocover 1% (citrus seed extract), Resinadher 0.5% (pine resin extract), mancozeb 2%, and water. The variables were as follows: halo inhibition, spore production, and aflatoxins content. In fruit, the following were evaluated: disease incidence, mycelial growth, and aflatoxin production. An ANOVA (Tukey: p 5 spores mL−1, respectively), while chitosan inhibited the production of aflatoxins. With Resinadher and Citrocover, tomato fruit had the lowest incidence, mycelial growth, and aflatoxin production with corresponding values of 0%, 0.0 cm2, and 0.95 ppb, respectively, and 7%, 0.2 cm2, and 1.77 ppb, respectively. The use of Citrocover and Resinadher could be a viable alternative to decrease the development of A. flavus in tomato fruit

Effect of Nanostructured Chitosan/Propolis Coatings on the Quality and Antioxidant Capacity of Strawberries During Storage

Strawberries have a thin epidermis and a high respiration rate. The use of edible coatings containing chitosan nanoparticles (CSNPs) and propolis (P) has been effective in preserving the shelf life and antioxidant capacity of various fruit and vegetable products. The present research evaluated the effect of coatings with CSNPs and P on the quality, antioxidant compounds, and antioxidant capacity of strawberries. The specific coatings that were evaluated were chitosan (CS), CS+CSNPs33%, CS + CSNPs + P10%, CS + CSNPs + P20%, CS + CSNPs + P30%, and a control with no coating. The variables were weight loss, firmness, total soluble solids (TSS), color, phenols, total flavonoids, antioxidant capacity, and sensory characteristics. An ANOVA and a Tukey test (p &le; 0.05) were used to analyze the data. Strawberries covered with CS + CSNPs + P10% showed the lowest weight loss (9.77%), while those covered with CS + CSNPs + P20% had the greatest firmness (4.96 N). CS + CSNPs + P coatings at 10%, 20%, and 30% concentrations maintained the antioxidant compounds and antioxidant capacity in the evaluated fruit (28.49 mg GAE g&minus;1, 554.61 &mu;g quercetin g&minus;1, and 92.48% DPPH, respectively). The application of nanostructured coatings did not modify the sensory characteristics of the fruit. Coatings with CSNPs and/or P could therefore be a viable alternative for preserving the quality and antioxidant capacity of strawberries