domingo, 30 de mayo de 2010

Applications of Nanocomposite Particles

Applications of Nanocomposite Particles

Several strategies are being pursued for creating nanoparticles containing multipleshell structures (for multifunctionality), with each shell composed of different materials.Template-based approaches are effective in this situation, for which an inner removabletemplate particle (silica, polymer beads) can be used to coat shells of othermaterials (e.g., a metal) via multi-step colloidal or vapor-phase assembly and can laterbe easily removed to create empty shells. These could even be filled with differentmaterials to produce multiple-shell composites [142]. Creating uniform coatings onparticle templates by colloidal self-assembly is based on the concept of self-assembledorganic molecular species. The two ends of the molecules to be joined have specificfunctional groups (e.g.. thiols, amines, carboxylic groups) that can be targeted for specificinteractions with the template and the clusters that are used to make the coatings.Uniform, dense packing of the molecules around the templates leads to close packing

of the clusters that form a porous but space-filled shell around the template (Figure). Noninteracting metal-coated magnetic particles (SiO2/Au, Fe3O4/Au, NiO/Co,etc.) or coated semiconducting particles (PbS/CdS) are examples of such composite particle structures.
These structures can have applications in magnetic recording, as multilayered catalyst materials, or in drug delivery. In such applications of drug delivery systems [143], for which a biocompatible outer layer and a drug-containing inner core are necessary, the multilayered particle approach will be crucial. As an alternative
to colloidal templating, structures such as PbS-coated CdS nanocomposite particles (a few nanometers in diameter) may be synthesized by ion displacement in inverse microemulsions and could be useful in nonlinear optical applications. The observed large refractive nonlinearity in these nanocomposite particles may be attributed to the optical Stark effect and to strong interfacial and internanoparticle interactions.
Asignatura: E.E.S
Saithrhu R. Gonzalez C.

sábado, 29 de mayo de 2010

Elemental and Chemical Composition of Organic/Inorganic Nanostructures

Elemental and Chemical Composition of Organic/Inorganic Nanostructures

Experimental Methods of Composition Study

All traditional methods of elemental and chemical composition study, such as XPS,Auger spectroscopy, Raman spectroscopy, and Furrier Transformed Infrared Spectroscopy(FTIR), were adopted for nanostructural materials [11, 17, 46]. Electronspectroscopy is based upon two schemes of interaction of either high-energy electronsor X-rays with electrons on core atomic levels (see Figure 3.16).In the first case [Figure 3.16(a)], an incident high energy electron beam maycause secondary electron emission from one of the core levels. The vacancy is immediatelyfilled with an electron from the higher level, and results in either the emissionof an electromagnetic wave in the X-ray spectral region (1), or the energy transfer toanother electron and its subsequent emission (2). The second event is called theAuger process. Both processes are exploited for the elemental analysis of thematerial [47].The spectrum of emitted X-rays provides a crude elemental analysis calledenergy dispersion X-ray (EDX) analysis. This technique goes along with standardSEM, and requires an additional X-ray detector. The accuracy of this method is notgreat, but it is sufficient to get a general idea of the material’s elemental compositionand stoicheometry. Another advantage of this technique, combined with electron

beam scanning, is the possibility of mapping the material by plotting twodimensionalimages formed by different elements. Similar information can beobtained with Auger electron spectroscopy (AES).The method of XPS, which is based on the registration of electron spectra emittedfrom core atomic levels as a result of X-ray irradiation gives amore accurate information on the material’s composition. The resolution of thespectra of secondary electrons emitted from the atomic levels is in the range of fractionsof electron volts, which is enough to distinguish between different chemicalforms of the same elements.Another excellent technique for chemical analysis is infrared spectroscopy (IR),which is based upon the registration of characteristic spectra of molecularvibrations in the materials. Such spectra can be measured with either conventionalIR, FTIR spectroscopy, or Raman spectroscopy. The latter is based on the registrationof vibration spectra excited by a powerful laser in the visible spectral range.Raman spectral shift between the main excitation line and coupled vibrationfrequencies is the characteristic parameter for the recognition of molecularvibrations. An extensive database exists for vibration frequencies of differentelements in different materials, which allows researchers to find chemical compositionfairy easily

Examples of Composition Study of Materials Prepared by Chemical Routes

Typical EDS graphs of sol-gel TiO2 films, as shown in Figure 3.17(a), demonstrate
the presence of titanium and oxygen; however, it is difficult to establish the stoichiometry
due to the interference of SiO2 and other oxides existing in the sol-gel film, as
well as in the glass substrates [35]. Glass is a complex compound, containing a
number of different elements (Si, Na, K, Mg, and Ca), as can be judged from EDS
data. TiO2 films deposited by sputtering on silicon wafers yield much more defined
spectra [see Figure 3.17(b)], practically without impurities, other than from silicon
coming from the substrate.
XPS confirmed the formation of the PbS phase in thin films of lead phthalocyanine
(PbPc) after exposure to H2S gas [49]. The XPS peaks of both Pb-4f and S-2p in
Figure 3.18(a) show a complex chemical composition of the material.

Control of Impurities in Chemically Deposited Nanostructures
The problem of the registration and control of low concentrations of impurities (onthe level of parts per million, or parts per billion) in organic materials is of highimportance. Since organic materials have become increasingly popular in nanoelectronicdevice applications, their purity must be comparable to that in inorganicmaterials. Unfortunately, this is not always the case. Even the use of high puritygrade (99.99 or 10−4) chemicals during the formation of nanostructures (e.g., nanoparticles,layered structures, or thin films) does not match the grade of semiconductormaterials in microelectronics, which is typically of 10−6 and higher. That explainswhy materials produced by chemical routes vary in their properties so much. Itrecalls, to a large extent, the situation in semiconductor material technology in1950s and 1960s, when the same types of materials produced in different laboratoriesshowed absolutely different electrical properties. Organic materials technologyis, however, progressing much faster because of great experience in materialsscience, advanced analytical techniques, and technologies of materials processingnow available.The importance of purity control of nanostructured materials has been recognized,and the research in this direction is constantly growing. It includes a number
the resolution limit of Auger and X-photoelectron spectroscopy, and of optical (luminescence)methods [46, 47]. An obvious conclusion is to deploy electrical methodsfor impurity control in nanostructured materials, particularly in nanoparticles. Electricalmethods may also be useful in monitoring the density of surface states, whichplay a significant role in the electrical properties of semiconductor nanoparticles.
Asignatura: E.E.S
Saithrhu R. Gonzalez C.

Inorganic Nanocomposites for Optical Applications

Inorganic Nanocomposites for Optical Applications

Nonlinear optical effects, such as nonlinear optical absorption and second- and thirdorderoptical nonlinearities, can be used to make optical limiters, optical modulators,etc. Although many organic materials have high optical absorption and nonlinearity,their thermal and optical stabilities are poor. Often, it helps to create hybrids or composites(organic/inorganic, inorganic/inorganic) that have acceptable optical propertiesand stability. Quantum-confined nanoparticles have been extensively used in thefabrication of such composites because of their novel optical and electronic properties.The organic-matrix nanocomposites for optical applications are described in chapter 2,which briefly discusses some of the organic materials that contain dispersed nanoparticlesin various hosts.Recent advances in controlling the fabrication and dispersion of semiconductornanoparticles in polymer and ceramic matrices have suggested possible uses forsuch nanocomposites in optical applications. A good example of an optically functionalceramic nanocomposite is GaAs nanocrystals embedded in SiO2 matrix. Theinterest in the novel optical properties of semiconductor nanocrystals [144] has resultedin strategies to package them as nanocomposites. A variety of techniques,such as colloidal synthesis, self-assembly, and electrochemistry, can be used to producethe semiconductor nanoparticles; however, ion coimplantation (Ga+, As+) is anefficient way of creating well-dispersed nanocomposite materials (e.g., GaAs/SiO2)[145]. Typically, the ions are implanted at fluxes of 1016 cm-2 into 100 nm silica filmson Si substrates and annealed at appropriate temperatures to create nanocrystals ofGaAs (several nanometers in size) in the matrix (Figure 1.27).

Photoluminescencestudies show an efficient, broad luminescence band in the visible and near-infraredspectral regions due to quantum confinement in GaAs nanocrystals and defect statesin SiO2.Nanoparticles in matrices have interesting photoluminescence properties, due tothe effects of quantum confinement on their optical properties. Indirect band semiconductorssuch as Si and Ge have very poor luminescence efficiency (hence, efficient46 1 Bulk Metal and Ceramics Nanocompositeslight emitters cannot be fabricated directly from them), because the band center transitionis optically forbidden. But, by making the particle sizes smaller than the excitonBohr radius (4.9 nm for Si, 24.3 nm for Ge), the resulting confinement produces anincrease in oscillator strength, shifting the luminescence to higher energies; this allowstailoring novel optical materials, which are otherwise impossible. To take fulladvantage of the particle size effect, the particles must be separated, and the best possibleway of accomplishing this is to disperse the particles in a matrix. Understandingand minimizing the interaction of the host matrix and the particles that create interfacestates are crucial to many of the optical applications. Embedding nanoparticles in differentmatrices and studying the optical properties provides a way to decipher interfaceeffects.

Ge quantum dots have been studied in various hosts, and one way to minimizehost interaction is to embed them in oxygen-free environments, such as AlN; AlN/Gemultilayer structures deposited by pulsed-laser deposition show a blue-shifted photoluminescencepeak [146]. Similarly, indium oxide (InO) nanoparticles dispersed (bysolvent-phase impregnation) within the pores of mesoporous silica (prepared by sol–gel technique) show multiple photoluminescence peaks related to the size and structureof the particles [147]. Amorphous InO particles (<6>

The problemwith porous Si is the structural nonuniformity, lack of reproducibility in emissionand aging, and environmental degradation problems. Incorporating light-emittingSi structures in a matrix that is chemically inert and has a wide band gap suitablefor quantum confinement can overcome this problem. Nanocomposites of Si nanoparticles( 5 nm) embedded in polycrystalline diamond matrix have been preparedFig. 1.27 Transmission electron micrograph, left,GaAs nanocrystals embedded inside SiO2 glassmatrix. Sequential ion implantation followed bythermal annealing was used to form GaAs nanocrystalsin SiO2 films. Right, efficient, broadphotoluminescence band observed in the red andnear infrared spectral regions. The efficient luminescenceis attributed to both quantum confinementstates in GaAs nanocrystals and defects inSiO2.

(Source [145] used with permission)1.9 Inorganic Nanocomposites for Optical Applications 47and studied. The room temperature photoluminescence behavior of such materials inthe 1.6–2.5 eV range shows a strong increase in emission efficiency from the Si nanostructures.

The studies conducted on this nanocomposite reveal that selecting thesize of the embedded Si particles makes it possible to tune the luminescence frequencyin the yellow–green spectral region.Nanocomposites of nanosized metal particles in transparent dielectrics can also beapplied as nonlinear optical materials in photonic devices. These materials are characterizedby large third-order optical nonlinearity (v3) and fast response times, whichare important for device applications such as optical computing, real time holography,and phase conjugators. To incorporate metal nanoparticles into dielectrics, severaltechniques such as ion implantation, sol–gel techniques, and sputtering can be employed.It is even possible to prepare graded layers of metal nanoparticle distributions(with different particle sizes and interparticle separations) by implantation, and thesestructures can be carefully tailored to produce interacting and noninteracting nanoparticlelayers that produce different optical response in the plasma resonance frequencies.

Several of these composite systems have been studied, for example, Au/SiO2, Ag/SiO2, Au/Al2O3, Au/TiO2 with metal concentrations varying from 15%–60%, for nonlinearity and plasma resonance frequency shifts. High values of susceptibilities(v3 6 10-7 esu, compared to low values of 10-12–10-11 esu for glass) havebeen reported for composites containing an optimized fraction of the metal component[148, 149]. Groups II–VI semiconducting nanocrystals (e.g., CdS) prepared inglass hosts have been studied in great detail, because of their large optical nonlinearresponse and small carrier lifetimes. Enhancement of carrier recombination rates isobserved in nanocrystals with large surface-to-volume ratios, and the enhancementresults from increases in the density of surface states and fast surface recombinationand capture due to multiphoton emission. Surface recombination in nanocrystals embeddedin glass matrices exhibits thermally activated nonradiative recombination,which is enhanced at reduced particle size.

The semiconductor/glass interfaces inthe composites may give rise to deep traps responsible for photoactive phenomena.In recent years, nanoparticles have been used to make transparent nanocompositestructures having high refractive indices. Polymers containing inorganic particles in arange of 1–100 nm (nanocomposites) are interesting in this regard. In contrast tocomposites having particles in the micron size range, nanocomposites do not scatterlight and are interesting for optical applications. Preparation of nanocomposites withrefractive indices over the entire range of 3, which is by far the lowest andhighest ever achieved for a polymer composite, has been possible. Transparent polymericmaterials can be coated with surface layers ( 100 nm thick) of UV-absorbingnanocomposites to inhibit degradation of the polymer by UV light [150]. High-refractive-index transparent materials are mainly used for improving the optical couplingefficiencies in photonic devices.

The refractive indices of polymers vary between1.3 and 1.7, those for inorganic semiconductors vary between 2 and 5, and forhigh bandgap semiconductors this value is <3.>

Asignatura: E.E.S

Saithrhu R. Gonzalez C.

Nanocristales utilizados para crear el mejor aislante del mundo

 Nanocristales utilizados para crear el mejor aislante del mundo

Conseguir un alto nivel de protección en aislantes no es un problema planteado sólo para los viajeros del espacio, sino también para las personas en su vida cotidiana. Uno de los mejores aislantes conocidos hasta ahora, el revestimiento de vacío, ayudan a mantener el café o la sopa caliente en el termo que utilizas comúnmente. Sin embargo recientemente, un equipo de ingenieros han descubierto que las capas de cristales fotónicos dentro de un revestimiento de vacío puede incluso prevenir la pérdida de calor desde la radiación infrarroja invisible.El calor normalmente viaja a través de métodos tales como la convección y conducción, los cuales requieren de un material medio, que carece de vacío convenientemente. Pero el calor también puede transferirse a través de la radiación infrarroja, de tal modo que podría traspasar sin dificultad un revestimiento de vacío hasta alcanzar la parte externa de un termo por ejemplo.

Ingenieros de la Universidad de Stanford, comenzaron a preguntarse si podrían crear un mejor aislante natural superando a cualquiera de los existentes, y finalmente después de un largo período de investigaciones, descubrieron que a través de los cristales fotónicos era posible alcanzar la hazaña. Los cristales de este tipo existen en la naturaleza y pueden producirse en el laboratorio sin problemas. Concretamente se componen de diminutas nanoestructuras que afectan a cómo la luz pasa a través de ellos. Pueden incluso configurarse para bloquear ciertos rangos de frecuencias de la luz, incluida la radiación infrarroja.

Los ingenieros desarrollaron el primer prototipo de aislante mediante la agrupación de 10 capas de cristales fotónicos en una pequeña estructura con un grosor de 100 micras. Para hacernos una idea de su tamaño, tengamos en cuenta que 1.000 micras constituyen tan solo un milímetro. Las pruebas con el nuevo aislante puso de manifiesto que la transferencia de calor no se basa en el espesor de la capa, sino tan sólo en la rapidez con la que la luz pueda viajar a través del material en concreto.

El equipo de investigación espera que los cristales fotónicos puedan ahora encontrar un empleo más allá de las comunicaciones y aplicaciones informáticas, como por ejemplo en los sistemas de energía solar térmica que tratan de capturar el calor del sol para su utilización como fuente de energía, ya que los cristales fotónicos podrían permitir que la luz visible al pasar atrape el calor en el interior.
Asignatura: E.E.S
Saithrhu R. Gonzalez C.

Estimaciones de nanopartículas en el medio ambiente


Estimaciones de nanopartículas en el medio ambiente

Sin saber qué cantidad de producto químico industrial se produce, es casi imposible para los científicos determinar si realmente plantean una amenaza para el medio ambiente o la salud humana. Por ello, un grupo de ingenieros civiles en la Universidad de Duke han descubierto una nueva forma de calcular la cantidad de nanopartículas que se genera (como el dióxido de titanio), por lo que se establecen las bases para futuros estudios en la evaluación de los posibles riesgos.

Esta información es especialmente valiosa si los productos químicos se encuentran en forma de nano-partículas, las cuales poseen unas propiedades únicas debido a su minúsculo tamaño. Las nanopartículas son atractivas para una amplia gama de productos, sin embargo poco se conoce sobre sus consecuencias en el medio ambiente. Uno de los más utilizados es la forma de nanopartículas del dióxido de titanio, que puede encontrarse en diversos productos desde cremas, pasta de dientes a pinturas o papel, incluso se emplea en el tratamiento del agua.

"El mayor problema al que nos enfrentamos al tratar de determinar los riesgos de las nanopartículas de dióxido de titanio es que nadie sabe realmente cuánto hay de este en el ambiente", decía Christine Robichaud, estudiante graduado en Ingeniería Civil y Ambiental de la Universidad de Duke. Los resultados de su análisis fueron publicados en la edición americana de la revista Environmental Science and Technology.
Robichaud consideró especialmente difícil tratar de reunir todos esos datos, ya que las empresas que utilizan en sus procesos de fabricación el dióxido de titanio no están dispuestos a revelar esa información. Así que utilizó un nuevo enfoque desarrollado por los colaboradores Lynne Zucker y Michael Darby de la Universidad de California en Los Angeles para estimar la tasa de innovación en la industria de la biotecnología. "Hemos combinado la ciencia y la ingeniería del conocimiento con las empresas y los modelos económicos para llegar a lo que creemos que es la máxima cantidad de partículas de dióxido de titanio", comentaban los investigadores.

Sobre la base de sus cálculos, el equipo de ingenieros encontró que la producción de nanopartículas de dióxido de titanio fue insignificante en 2002, aumentando en alrededor del 2,5 por ciento de la cantidad total de dióxido de titanio producido hoy en día. Para 2015, la producción de nanopartículas se estima en alrededor del 10 por ciento del total, a medida que más empresas evolucionen a tecnologías más novedosas. En el marco del escenario más agresivo, prácticamente todo el dióxido de titanio en los EE.UU. sumaría alrededor de 2,5 millones de toneladas métricas en forma de nanopartículas para el año 2025, según la conclusión de los ingenieros.

"Saber la cantidad de este material es importante para conocer la probabilidad de daños y consecuencias hasta ahora desconocidas que pueden causar al medio ambiente y a los seres humanos", decía Mark Wiesner, profesor de ingeniería civil en la Universidad de Duke y miembro superior del equipo de investigación. También dirige el Centro de fondos federales para las implicaciones ambientales de la nanotecnología (CEINT).
Ahora que los investigadores tienen una mejor idea de cómo gran parte de estos nanomateriales podrían producirse en los próximos años, van a centrarse en tipos específicos de productos. "Queremos tener una mejor idea de cuando en el proceso de estas nanopartículas podrían ser liberadas en el aire, el agua o el suelo", decía Robichaud. "Podría ser durante la extracción minera, durante la producción de las nanopartículas, la fabricación del producto específico utilizando las nanopartículas, el uso del producto, o en su eliminación definitiva".
Asignatura: E.E.S
Saithrhu R. Gonzalez C.

Aplicación de nuevos nanomateriales en plásticos

Aplicación de nuevos nanomateriales en plásticos

Los ingenieros de la Universidad Estatal de Michigan, en Estados Unidos, han desarrollado un nuevo nanomaterial que hace al plástico más rígido, ligero y fuerte, resultando una solución más económica de cara al ahorro en los costos de mantenimiento y fabricación de aviones y automóviles, así como de equipamiento médico y deportivo más duradero.

El material XGnP (NanoPlaquetas Exfoliadas de Grafito) será el instrumento para el desarrollo de nuevas y más amplias aplicaciones en la industria aeroespacial, de automoción y la industria del envasado, según comentaba Lawrence Drzal, profesor de Ingeniería Química y Ciencia de los Materiales en la Universidad Estatal de Michigan.

El profesor Drzal dirigió al grupo de investigación que ha desarrollado este producto, quién considera desde un punto de vista práctico, que este material de bajo costo tiene un conjunto único de características físicas, químicas y atributos morfológicos, dando como resultado un material a nanoescala, que es eléctricamente y térmicamente conductor, reduciendo la inflamabilidad y las propiedades de barrera.
El grafito de nanopartículas están siendo fabricados en la actualidad por XG Sciences Inc, empresa situada en Michigan, que tiene acceso a los derechos sobre la propiedad intelectual en la Universidad, por lo que cuenta con una licencia exclusiva para la fabricación del producto.

El XGnP puede ser utilizado como un aditivo para plásticos o de sí mismo pudiendo hacer un cambio transformacional en el desempeño de muchos dispositivos de electrónica avanzada. La clave para este nuevo material es la capacidad de una forma rápida y poco costosa en su proceso para separar las capas de grafito (grafeno) en pilas de menos de 10 nanómetros de grosor, pero con dimensiones en su parte lateral de 500 nm a decenas de micras, junto con la capacidad para adaptar la superficie de partículas químicas para hacerla compatible con el agua, resina o plástico.

Funcionalidades del XGnP:

◦Podría ser usado para hacer más ligeras las piezas de aeronaves y automóviles, turbinas de viento más fuertes, implantes médicos y equipamiento deportivo.◦Es un buen conductor eléctrico, muy atractivo para las baterías de iones de litio, pudiendo ser utilizado para hacer recubrimientos transparentes conductores para células solares y pantallas.◦Puede hacer los tanques de gasolina ligeros, reducir las fugas, y mejorar el precintado de plásticos para mantener los alimentos frescos durante semanas.Como nos comentaba el profesor Drzal; "Ahora que sabemos cómo hacer este material y la forma de modificarlo para que pueda ser utilizado en plásticos, nuestra atención está dirigida a las aplicaciones del alto rendimiento en las que podemos realmente hacer algunos cambios sustanciales en el camino de la electrónica. Las pilas de combustible, baterías y células solares podrían realizarse como resultado del uso de este material".
"Como ingeniero de investigación tenemos en cuenta la comprensión no sólo de los fundamentos de cómo funcionan las cosas, sino también de la elaboración de soluciones para resolver los importantes problemas a los que se enfrenta el mundo en el que vivimos", decía Drzal.
Asignatura: E.E.S
Saithrhu R. gonzalez C.

Descubren nueva forma de estudiar nanoestructuras

Descubren nueva forma de estudiar nanoestructuras

El efecto Josephson se refiere al trabajo que Brian Josephson publicó en 1962 con relación al flujo de una corriente eléctrica entre superconductores. En este trabajo, por el que compartió un Premio Nobel en 1973, Josephson predijo que cuando se mantuviera una diferencia de voltaje constante entre dos superconductores débilmente unidos, separados por una delgada barrera aislante (un arreglo ahora conocido como Unión de Josephson), fluiría una corriente eléctrica alterna a través de la unión. La frecuencia de las oscilaciones de la corriente está directamente relacionada con el voltaje aplicado.

Estas predicciones fueron totalmente confirmadas por un inmenso número de experimentos, y el voltio estándar se define ahora en términos de la frecuencia de la corriente alterna de Josephson. El efecto Josephson tiene numerosas aplicaciones en la física, la computación y las tecnologías de los sensores. Puede ser utilizado para la detección con sensibilidad extraordinariamente alta de la radiación electromagnética, los campos magnéticos muy débiles y en los bits de la computación cuántica en superconductores.
El físico experimental Alexei Marchenkov y el teórico Uzi Landman del Tecnológico de Georgia han descubierto ahora que el efecto Josephson puede emplearse para detectar el movimiento mecánico de los átomos colocados en la unión de Josephson.

La perspectiva de poder explorar y quizás utilizar fenómenos de la escala atómica usando este efecto, resulta muy prometedora.
Recomienda esta página El efecto Josephson puede emplearse para detectar el movimiento mecánico de los átomos colocados en la unión de Josephson.
Marchenkov y Landman planean continuar explorando los efectos oscilatorios en las uniones de enlaces débiles, empleando la información obtenida a través de estos estudios para determinar las características oscilatorias, los arreglos atómicos y los mecanismos de transporte en nanoestructuras metálicas, orgánicas y biomoleculares.
Uno de sus objetivos es el desarrollo de dispositivos y metodologías para sensores que se aprovechen de las peculiaridades desveladas con esta nueva investigación.
Asignatura: E.E.S
Saithrhu R. Gonzalez C.

Nanocompuesto polimérico

Nanocompuesto polimérico
Un nanocompuesto polimérico es un material caracterizado por la dispersión homogénea de partículas de relleno de dimensiones nanométricas en el interior de una matriz polimérica. Como relleno se usan por lo general nanopartículas de silicato y metálicas.

En los polímeros compuestos de tipo convencional (es decir, a los cuales se ha agregado un componente inorgánico no nanoestructurado, por ejemplo los plásticos reforzados con fibra de vidrio) hay una separación neta a nivel macroscópico entre las fases orgánica e inorgánica, lo que representa una limitación al mejoramiento de los materiales poliméricos; la ventaja de los nanocompuestos polímero/filosilicato es que permiten superar dicho límite, mejorando las características mecánicas y térmicas y la permeabilidad del mismo polímero, con el agregado de cantidades mínimas (del orden del 5%) de silicatos.
Es importante subrayar que tales mejoras no van en detrimento del color, de la procesabilidad ni de la densidad aparente.

Este tipo de materiales están teniendo amplia aplicación, sobre todo en el campo de los envases para alimentos, por su propiedad de barrera a la penetración de los gases, de hasta cinco a quince veces mayor que la del polímero puro y de polímeros cargados que a menudo contienen hasta un 20 - 30% de material silíceo (mica, talco o carbonato de calcio). Por otra parte, los nanocompuestos de silicato/polímero presentan también un poder de retardo de llama mejorado; los ensayos muestran que el pico de velocidad de la emisión de calor, que es una medida de la inflamabilidad del material, en el caso de un nanocompuesto llega a ser del 60 al 80% más bajo que el de un polímero puro. Al mismo tiempo, las propiedades mecánicas exhiben mejoras significativas, como mayor tenacidad y resistencia a la abrasión.
Asignatura: E.E.S
Saithrhu R. Gonzalez C.

Nuevo proceso de fabricación de un nanocompuesto mejora los condensadores

Nuevo proceso de fabricación de un nanocompuesto mejora los condensadores

Debido a sus elevadas propiedades dieléctricas, el titanato de bario ha sido objeto de interés por mucho tiempo para su empleo en los condensadores, pero hasta hace muy poco los científicos habían sido incapaces de producir una buena dispersión del material dentro de una matriz de polímero.
Usando ácidos diseñados especialmente para encapsular y modificar la superficie de las nanopartículas, investigadores del Centro para la Fotónica y la Electrónica Orgánicas, del Instituto Tecnológico de Georgia, pudieron superar el problema de la dispersión de partículas que obstaculizaba la creación de un nanocompuesto uniforme.
Los nuevos materiales nanocompuestos han sido probados a frecuencias de hasta un megahercio, y, según los investigadores, es plausible que funcionen a frecuencias aún más altas. Aunque los nuevos materiales podrían tener aplicaciones comerciales sin otras mejoras, su contribución más importante puede ser la de haber demostrado la eficacia de la nueva técnica de encapsulado con la que los han hecho. Dicha técnica podría tener amplias aplicaciones en otros materiales nanocompuestos.
Debido a su propiedad de almacenar y descargar rápidamente la energía eléctrica, los condensadores se utilizan en una amplia variedad de productos cotidianos, como los ordenadores y los teléfonos móviles. Y, debido a la creciente demanda para energizar vehículos y otros nuevos equipos con electricidad, también tienen aplicaciones importantes en esos campos.

Aunque los nuevos materiales ya pueden ofrecer suficiente ventajas para justificar su comercialización, los investigadores creen que existen posibilidades de mejorar su funcionamiento. También quieren variar algunos aspectos de la producción para hacer muestras más grandes con las que experimentar (hasta ahora, han sido producidas en películas de cinco por ocho centímetros aproximadamente), y ponerlas a disposición de otros investigadores que quieran desarrollar aplicaciones adicionales.
Los investigadores están trabajando ahora en el empleo de estos nuevos nanocompuestos en transistores de películas delgadas orgánicas, en los cuales se utilizan técnicas basadas en disoluciones para fabricar componentes electrónicos baratos.

Más allá de los condensadores, hay muchas áreas donde el nuevo material puede ser útil. Entre las aplicaciones potenciales, figuran ciertos transistores, pantallas y otros aparatos electrónicos. Con este nuevo material resulta posible lograr una capa altamente dieléctrica que puede incorporarse en esos dispositivos.
Asignatura: E.E.S
Saithrhu R. Gonzalez C.

Estructuras hibridas que combinan la fuerza de nanotubos de carbono y nanocables

Estructuras hibridas que combinan la fuerza de nanotubos de carbono y nanocables

La impresionante conductividad de los nanotubos de carbono les convierte en materiales prometedores para una amplia variedad de aplicaciones electrónicas, pero encontrar técnicas para fijar los nanotubos individuales a los contactos de metal ha demostrado ser un desafío. El nuevo método empleado por los investigadores del Instituto Politécnico Rensselaer permite precisamente esto, ofreciendo una solución práctica al problema del empleo de los nanotubos de carbono como dispositivos de interconexión y en los chips de ordenador.

Como los diseñadores de chips buscan continuamente incrementar la potencia de computación, su objetivo pasa por la disminución de las dimensiones de los componentes de los chips hasta la escala nanométrica. Los nanotubos de carbono y los nanocables, que empezaron a estar disponibles en los años noventa, son candidatos prometedores para actuar como conexiones en esta escala porque ambos poseen interesantes propiedades.

Por ejemplo, los nanotubos de carbono muestran una resistencia mecánica asombrosa y son excelentes conductores de la electricidad, con la capacidad de producir interconexiones muchas veces más rápidas que las actuales basadas en el cobre. Los nanocables de oro también tienen propiedades ópticas y eléctricas muy interesantes y son compatibles con las aplicaciones biológicas.
Con el fin de aprovecharse de lleno de estos materiales, los investigadores prueban la idea de combinarlos para obtener una nueva generación de nanomateriales híbridos. Esta estrategia es un buen método para unir las fuerzas de ambos materiales.

Los nanocables de metal en esta técnica son fabricados usando una plantilla de alúmina que puede diseñarse para tener el tamaño de sus poros en el rango nanométrico. Los cables de cobre o de oro se depositan dentro de esos poros, y todo el conjunto es puesto en un horno donde está presente un compuesto rico en carbono. Cuando el horno se calienta a altas temperaturas, los átomos de carbono se autocolocan a lo largo de la pared de la plantilla y los nanotubos de carbono crecen directamente sobre los cables de cobre.
Es una técnica muy fácil, y podría aplicarse a muchos otros materiales. El aspecto más interesante es que permite manipular y controlar las uniones entre los nanotubos y los nanocables sobre longitudes de varios cientos de micras. Las plantillas de alúmina ya se fabrican en serie para su utilización en la industria de los filtros, y la técnica puede adaptarse fácilmente para otros usos.
Asignatura: E.E.S
Saithrhu R. Gonzalez C.

sábado, 22 de mayo de 2010

Hybrid materials for micro-optics

Wafer-scale UV-embossing can be applied to substrates other than glass, for example Si and semiconductor III–V based wafers with prefabricated devices. In these cases, it is often advantageous to use the same hybrid materials in a combined lithographic and embossing mode to produce free-standing micro-optical elements, for example the lenslet on VCSEL elements for fiber coupling (CSEM in collaboration with Avalon Photonics Ltd., CH-Zurich). Fig. 16 shows SEM images of processed microoptical components on VCSEL wafers: (a) diffractive lenses, (b) an array of refractive lenses.

All the Pyramid Optics' collimator arrays shown in Fig. 17 are constructed with a silicon V-groove fiber array mounted with a microlens array. The microlens arrays are replicated in a ORMOCER1 thin film on a BK7 glass substrate.

The new collimator arrays are offered for several wavelength regions: 630–690 nm; 780–850 nm and 1250–1650 nm. Other wavelengths are possible on request. Several of the above used hybrids for optical applications are now licensed by Fraunhofer-ISC to the company Micro Resist Technology GmbH and they are producing them in large scale and marketing them worldwide under the names: ''Ormocore'', ''Ormoclad'' and ''Ormocomp''. As a result, besides the given industries, lots of companies from Japan and South Korea as well as from Europe (Germany, Switzerland, Sweden, and Finland) have integrated these materials mostly in micro-optical products.

Nanohybrids are also used for interference optical coatings. Dielectric mirrors or reflectors can be prepared using interference quarterwave stacks of colloidal-based low-refractive index material and a hybrid dense material as the high refractive index layer. In the literature, a possible high index layer used for laser optical thin films139 is a nanohybrid material prepared from mixing a nanosized-zirconia suspension with a transparent polymer alcoholic solution. Using a hybrid system with a polymeric binder in the high index oxide sol helps to decrease the total layer number required for the same reflection value. The selected polymer must be soluble in the suspending medium, preventing the sol from flocculation, and also needs to be transparent at the wavelength of interest. That is the reason why polyvinylpyrrolidone (PVP) was chosen as possessing the best combination of properties for binder use. Considering the structure of the monomer unit, it is seen to have an amphiphilic character.

Indeed, PVP contains a highly polar amide group conferring hydrophilic and polar-attracting properties, and also apolar methylene (CH2) and methine (CH) groups in the backbone and the ring conferring hydrophobic properties. When added to a colloidal suspension, the amphiphilic character of PVP helps to maintain colloidal stability and to reduce flocculation through a steric stabilization mechanism. As the purpose was to get the maximum refractive index value, the oxide/polymer ratio of the hybrid system needed to be optimized. It was demonstrated that the index value was directly dependent on the PVP/oxide ratio and the optimum ratio was determined in a separate experiment in which the hybrid refractive index variation was plotted regarding the PVP/oxide ratio (see Fig. 18).

The PVP polymer is supposed to ''smother'' the oxide nanoparticles leading to a dense hybrid structure with hydrogenbonding between the amide carbonyl groups of PVP and surface hydroxyl groups of oxide particles (Fig. 19). A similar dense hybrid structure was described by Toki et al.177 for silica nanosized particles.

Because PVP was soluble in alcoholic solvents, multilayer deposition required a UV-curing step to avoid redissolution of the previous deposited polymeric layer. PVP is a photosensitive polymer that could be UV-cured using short wavelength irradiation. In the UV-curing of PVP, FT-IR measurements show that the increase of the hydroxyl-band is correlated to the decrease of the ketone and amide bands. This UV-induced hydroxylation of PVP explains the modification of its solubility because, as for polyvinyl alcohol (PVA), crystallinity of hydroxylated-PVP (OH-PVP) has changed.

Using such nano-hybrid materials, 99.5% reflection coatings have been prepared on a 42 6 46 6 9 cm deformable mirror (BK-7 substrate) for near infrared use (1053 nm wavelength) exhibiting high optical uniformity and low optical losses (as low as 0.35%) (Fig. 20)

Asignatura: CRF
Fuente: Journal of Materials Chemistry

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Materiales híbridos para mejorar y abaratar a los ordenadores

(NC&T) Este nuevo material permitiría una integración perfecta de las funciones lógicas y la memoria, y se espera que permita el diseño de dispositivos que operen a velocidades mucho mayores y que usen bastante menos energía que los dispositivos electrónicos actuales.
El objetivo primario del equipo de investigación es explorar nuevos métodos para integrar el magnetismo y los materiales magnéticos con materiales electrónicos emergentes como los semiconductores orgánicos.La investigación puede conducir a dispositivos mucho más compactos y energéticamente eficientes. Se estima que los costos de elaboración de estos materiales híbridos serán mucho menores que los de los chips semiconductores tradicionales, resultando en dispositivos cuya producción debería ser más barata.En este nuevo enfoque de diseño, el acoplamiento entre componentes magnéticos y no magnéticos se haría por medio de un campo magnético o el flujo del espín del electrón. El espín, una manifestación de la mecánica cuántica que puede describirse como apuntando hacia "arriba" o hacia "abajo", es la propiedad fundamental de un electrón y es responsable de la mayoría de los fenómenos magnéticos.

La espintrónica es una gran promesa para enriquecer o incluso reemplazar a la electrónica tradicional. Mientras los circuitos electrónicos hacen circular a los electrones gracias a su carga, los circuitos de la espintrónica funcionarían basándose en el espín. Gracias a ello, operaciones típicas de la circuitería clásica, como la conmutación (el mecanismo que produce los ceros y los unos del código binario) podrían ser realizadas más deprisa y usando menos energía.
Asignatura: CRF

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Hybrid materials for microelectronics

Organically modified resins retain important roles in electrical component coatings such as resistors and molding compounds, as well as spin-on dielectrics in microelectronic interlayer and multilayer dielectric and planarization applications.

Simply methyl or hydride substitution of alkoxysilanes allowed the development of commercial products such as:
Glass-Rock1 by Owens–Illinois, for sealing cathode-ray tubes; Techneglas1 by NEC, for dielectric applications, Accu-Spin1 by Honeywell, for spin-on glass applications, etc. However, although numerous commercial hybrid-based products in electronics merit mention, only several representative examples will be detailed.

To demonstrate the feasibility of ORMOCER1s for use as an MCM L/D material, a substrate for a Pentium2 multi-chip module (MCM) with a BGA interface was realized. The substrate of that demonstrator consists of a metal interconnection separated by ORMOCER1 dielectrics on top of a FR-4 laminate including power and ground planes.

Two sputtered aluminium layers were used for the signal interconnection. The signal layers were separated from each other and from the power plane by 6 mm thick spin-coated ORMOCER1 layers in which via-holes were defined by direct photo-patterning. The FR-4 laminate was provided with micro-vias and through hole vias connecting to a 1.27 mm pitch, ball grid array underneath.

In Fig. 14(a), from right to left, the different steps of sequentially built-up (SBU) processing are given: Cu-metallised FR-
4-substrate (ground plane), first ORMOCER1 layer with power plane, second ORMOCER1 layer with first signal plane, third ORMOCER1 layer with second signal plane, and finally a Pentium2 chip set was also mounted on the substrate. This example shows the possibilities offered by low temperature curing of hybrid materials to achieve compact low cost devices by integration of the MCM substrate with the package.

As these hybrids show beside good dielectric and processing properties also high optical transmission, opto-electrical (o/e) and opto-electronic applications were done. The concept enabled by ORMOCER1 technology aims at very low cost and comprises high density electrical interconnects and optical waveguides integrated in three layers of ORMOCER1s.

An optoelectronic-MCM-L/D demonstrator is depicted (an electro-optical board, laminate with a hybrid in SBU-technology on top as well as a 5-channel optical transmitter and a 5-channel optical receiver). The thin film layers have been put on top of an FR-4 laminate with microvias. The laminate is furnished with a ball grid array (BGA) underneath, eliminating the need of any extra package.

The advantage of tuning the hybrid's flexibility and adhesion properties allows their use on flexible substrates, even as optical waveguides. Fig. 15 shows ORMOCER1 waveguides in which the hybrid coating is deposited on a flexible foil.

The chemistry of these hybrid polymers is based on the 1 : 1 polycondensation (alkoxolation) of diphenyl silanediol and silicon trialkoxides with methacryl and/or epoxy respectively cyclohexylepoxy functionalities. The ''water-free'' alkoxolation reaction causes condensates nearly free of residual SiOH, essential for low optical loss in the near infrared around 1550 nm. The high aryl content is chosen to get also a low-loss window in the NIR around 1310 nm. This easy chemistry therefore offers a perfect fit for implementation as an optical waveguide material in systems like transmitters and receivers in medium and long distance telecommunication, thermooptical switches and couplers etc., as the laser-sources as well as the glass-fibres work in these low-loss windows. Having a low permittivity e around 3.5 that composition could also be in parallel used as dielectrics. Because of the methacryl group such systems are photo-definable i.e. they can be used in photolithography, projection lithography as well as all kinds of UV-laser technology.

Dielectric ORMOCER1-hybrid polymers offer perfect media for high resolution photolithography because: (i) the precursor oligomers of the siloxane part have a small size around 1–5 nm, and (ii) the chemical cross-linking during mask-aligning (UV-polymerisation based patterning) is strongly sterically hindered by the oligomers which cause a fast break-down of radical polymerisation avoiding parasitic reactions into the mask-shaded areas. In general, their chemistry is based on the polycondensation of phenylfunctionalized silanols with tetraalkoxysilane as well as some percentage of zirconium alkoxides. As reactive cross-linking species such as silanes with methacryl groups (UV-polymerisation) and epoxy groups (thermal postcuring) can be used. Recent publications give information about the use of ORMOCER1 hybrids for 2-photon-polymerisation within femto-laser technology. It allows by careful choice of photoinitiators very high resolution for e.g. build-up of photonic crystals. Here, an additional benefit of hybrids: the mechanical stability allows the build-up of very fine structures ,100 nm.

Asignatura: CRF
Fuente: Journal of Materials Chemistry

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Applications of hybrid organic–inorganic nanocomposites

Organic–inorganic hybrid materials do not represent only a creative alternative to design new materials and compounds for academic research, but their improved or unusual features allow the development of innovative industrial applications. Nowadays, most of the hybrid materials that have already entered the market are synthesised and processed by using conventional soft chemistry based routes developed in the eighties. These processes are based on: a) the copolymerisation of functional organosilanes, macromonomers, and metal alkoxides, b) the encapsulation of organic components within sol–gel derived silica or metallic oxides, c) the organic functionalisation of nanofillers, nanoclays or other compounds with lamellar structures, etc. The chemical strategies (self-assembly, nanobuilding block approaches, hybrid MOF (Metal Organic Frameworks), integrative synthesis, coupled processes, bio-inspired strategies, etc.) offered nowadays by academic research allow, through an intelligent tuned coding, the development of a new vectorial chemistry, able to direct the assembling of a large variety of structurally well defined nano-objects into complex hybrid architectures hierarchically organized in terms of structure and functions. Looking to the future, there is no doubt that these new generations of hybrid materials, born from the very fruitful activities in this research field, will open a land of promising applications in many areas: optics, electronics, ionics, mechanics, energy, environment, biology, medicine for example as membranes and separation devices, functional smart coatings, fuel and solar cells, catalysts, sensors, etc.

For the past five hundred million years nature has produced materials with remarkable properties and features such as the beautifully carved structures found in radiolaria or diatoms. Another of nature's remarkable features is its ability to combine at the nanoscale (bio) organic and inorganic components allowing the construction of smart natural materials that found a compromise between different properties or functions (mechanics, density, permeability, colour, hydrophobia, etc.). Such a high level of integration associates several aspects: miniaturisation whose object is to accommodate a maximum of elementary functions in a small volume, hybridisation between inorganic and organic components optimizing complementary possibilities, functions and hierarchy.

Current examples of natural organic–inorganic composites are crustacean carapaces or mollusc shells and bone or teeth tissues in vertebrates. As far as man-made materials are concerned, the possibility to combine properties of organic and inorganic components for materials design and processing is a very old challenge that likely started since ages (Egyptian inks, green bodies of china ceramics, prehistoric frescos, etc.).

However, the so-called hybrid organic–inorganic materials are not simply physical mixtures. They can be broadly defined as nanocomposites with organic and inorganic components, intimately mixed. Indeed, hybrids are either homogeneous systems derived from monomers and miscible organic and inorganic components, or heterogeneous systems (nanocomposites) where at least one of the components' domains has a dimension ranging from some A°to several nanometers. It is obvious that properties of these materials are not only the sum of the individual contributions of both phases, but the role of the inner interfaces could be predominant. The nature of the interface has been used to grossly divide these materials into two distinct classes. In class I, organic and inorganic components are embedded and only weak bonds (hydrogen, van der Waals or ionic bonds) give the cohesion to the whole structure. In class II materials, the two phases are linked together through strong chemical bonds (covalent or iono-covalent bonds).

Maya blue is a beautiful example of a remarkable quite old man-made class I hybrid material whose conception was the fruit of an ancient serendipitous discovery. Ancient Maya fresco paintings are characterized by bright blue colors that had been miraculously preserved (Fig. 2). That particular Maya blue pigment had withstood more than twelve centuries of a harsh jungle environment looking almost as fresh as when it was used in the 8th century. Maya blue is indeed a robust pigment, not only resisting biodegradation, but showing also unprecedented stability when exposed to acids, alkalis and organic solvents.

Maya blue is a hybrid organic–inorganic material with molecules of the natural blue indigo encapsulated within the channels of a clay mineral known as palygorskite. It is a manmade material that combines the color of the organic pigment and the resistance of the inorganic host, a synergic material, with properties and performance well beyond those of a simple mixture of its components.

Considering the industrial era, successful commercial hybrid organic–inorganic polymers have been part of manufacturing technology since the 1950s.14 Paints are a good link between Mayas and modern applications of hybrids. Indeed, some of the oldest and most famous organic–inorganic industrial representatives are certainly coming from the paint industries, where inorganic nano-pigments are suspended in organic mixtures (solvents, surfactants, etc.). While the name of ''hybrid'' materials was not evoked at that time, the wide increase of work on organic–inorganic structures was pursued with the development of the polymer industry. The concept of ''hybrid organic–inorganic'' nanocomposites exploded in the eighties with the expansion of soft inorganic chemistry processes. Indeed the mild synthetic conditions offered by the sol–gel process (metallo-organic precursors, organic solvents, low processing temperatures, processing versatility of the colloidal state) allow the mixing of inorganic and organic components at the nanometric scale.3–11 Since then, the study of so-called functional hybrid nanocomposites became a mushrooming field of investigation yielding innovative advanced materials with high added value. These materials being at the interface of organic and inorganic realms are highly versatile offering a wide range of possibilities to elaborate tailor-made materials in terms of processing and chemical and physical properties. Today, this potential is becoming real and many hybrid materials are entering niche markets that should expand in the future because new and stricter requirements are now being set up to achieve greater harmony between the environment and human activities. New materials and systems produced by man must in future aim at higher levels of sophistication and miniaturisation, be recyclable and respect the environment, be reliable and consume less energy. Without any doubt, hybrid materials will soon generate smart membranes, new catalysts and sensors, new generation of photovoltaic and fuel cells, smart microelectronic, micro-optical and photonic components and systems, or intelligent therapeutic vectors that combine targeting, imaging, therapy and controlled release properties.

This review summarizes the general chemical pathways to prepare hybrid materials and presents the most striking examples of applications of functional hybrids, selecting them among the existing prototypes or commercially available hybrid materials.
Asignatura: CRF
Fuente: Journal of Materials Chemistry

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The sol-gel process, also known as chemical solution deposition, is a wet-chemical technique widely used in the fields of materials science and ceramic engineering. Such methods are used primarily for the fabrication of materials (typically a metal oxide) starting from a chemical solution which acts as the precursor for an integrated network (or gel) of either discrete particles or network polymers. Typical precursors are metal alkoxides and metal chlorides, which undergo various forms of hydrolysis and polycondensation reactions. The formation of a metal oxide involves connecting the metal centers with oxo (M-O-M) or hydroxo (M-OH-M) bridges, therefore generating metal-oxo or metal-hydroxo polymers in solution. Thus, the sol evolves towards the formation of a gel-like diphasic system containing both a liquid phase and solid phase whose morphologies range from discrete particles to continuous polymer networks.
In the case of the colloid, the volume fraction of particles (or particle density) may be so low that a significant amount of fluid may need to be removed initially for the gel-like properties to be recognized. This can be accomplished in any number of ways. The simplest method is to allow time for sedimentation to occur, and then pour off the remaining liquid. Centrifugation can also be used to accelerate the process of phase separation.

SEM micrograph of amorphous colloidal silica particles (average particle diameter 600 nm) precipitated in basic solution from TEOS using ammonium hydroxide as a morphological catalyst.

Removal of the remaining liquid (solvent) phase requires a drying process, which is typically accompanied by a significant amount of shrinkage and densification. The rate at which the solvent can be removed is ultimately determined by the distribution of porosity in the gel. The ultimate microstructure of the final component will clearly be strongly influenced by changes imposed upon the structural template during this phase of processing. Afterwards, a thermal treatment, or firing process, is often necessary in order to favor further polycondensation and enhance mechanical properties and structural stability via final sintering, densification and grain growth. One of the distinct advantages of using this methodology as opposed to the more traditional processing techniques is that densification is often achieved at a much lower temperature.

SEM micrograph of surface of colloidal solid. Structure and morphology consists of ordered domains with both interdomain and intradomain lattice defects.(Amorphous colloidal silica particles of average particle diameter 600 nm).
The precursor sol can be either deposited on a substrate to form a film (e.g., by dip coating or spin coating), cast into a suitable container with the desired shape (e.g., to obtain monolithic ceramics, glasses, fibers, membranes, aerogels), or used to synthesize powders (e.g., microspheres, nanospheres). The sol-gel approach is a cheap and low-temperature technique that allows for the fine control of the product's chemical composition. Even small quantities of dopants, such as organic dyes and rare earth elements, can be introduced in the sol and end up uniformly dispersed in the final product. It can be used in ceramics processing and manufacturing as an investment casting material, or as a means of producing very thin films of metal oxides for various purposes. Sol-gel derived materials have diverse applications in optics, electronics, energy, space, (bio)sensors, medicine (e.g., controlled drug release), reactive material and separation (e.g., chromatography) technology.

Highlighted image of surface of colloidal solid. Emphasis on microstructural defects to illustrate the defect/domain morphology typical of a simple one-component colloidal solid.

The interest in sol-gel processing can be traced back in the mid-1880s with the observation that the hydrolysis of tetraethyl orthosilicate (TEOS) under acidic conditions led to the formation of SiO2 in the form of fibers and monoliths. Sol-gel research grew to be so important that in the 1990s more than 35,000 papers were published worldwide on the process.

In the processing of fine ceramics, the irregular particle sizes and shapes in a typical powder often lead to non-uniform packing morphologies that result in packing density variations in the powder compact. Uncontrolled agglomeration of powders due to attractive van der Waals forces can also give rise to microstructural inhomogeneities.
Differential stresses that develop as a result of non-uniform drying shrinkage are directly related to the rate at which the solvent can be removed, and thus highly dependent upon the distribution of porosity. Such stresses have been associated with a plastic-to-brittle transition in consolidated bodies, and can yield to crack propagation in the unfired body if not relieved.

In addition, any fluctuations in packing density in the compact as it is prepared for the kiln are often amplified during the sintering process, yielding inhomogeneous densification. Some pores and other structural defects associated with density variations have been shown to play a detrimental role in the sintering process by growing and thus limiting end-point densities. Differential stresses arising from inhomogeneous densification have also been shown to result in the propagation of internal cracks, thus becoming the strength-controlling flaws.
It would therefore appear desirable to process a material in such a way that it is physically uniform with regard to the distribution of components and porosity, rather than using particle size distributions which will maximize the green density. The containment of a uniformly dispersed assembly of strongly interacting particles in suspension requires total control over particle-particle interactions. Monodisperse colloids provide this potential.

Monodisperse powders of colloidal silica, for example, may therefore be stabilized sufficiently to ensure a high degree of order in the colloidal crystal or polycrystalline colloidal solid which results from aggregation. The degree of order appears to be limited by the time and space allowed for longer-range correlations to be established. Such defective polycrystalline colloidal structures would appear to be the basic elements of submicrometre colloidal materials science, and, therefore, provide the first step in developing a more rigorous understanding of the mechanisms involved in microstructural evolution in inorganic systems such as polycrystalline ceramics.

Particles and polymers
The sol-gel process is a wet-chemical technique used for the fabrication of both glassy and ceramic materials. In this process, the sol (or solution) evolves gradually towards the formation of a gel-like network containing both a liquid phase and a solid phase. Typical precursors are metal alkoxides and metal chlorides, which undergo hydrolysis and polycondensation reactions to form a colloid. The basic structure or morphology of the solid phase can range anywhere from discrete colloidal particles to continuous chain-like polymer networks.

The term colloid is used primarily to describe a broad range of solid-liquid (and/or liquid-liquid) mixtures, all of which contain distinct solid (and/or liquid) particles which are dispersed to various degrees in a liquid medium. The term is specific to the size of the individual particles, which are larger than atomic dimensions but small enough to exhibit Brownian motion. If the particles are large enough, then their dynamic behavior in any given period of time in suspension would be governed by forces of gravity and sedimentation. But if they are small enough to be colloids, then their irregular motion in suspension can be attributed to the collective bombardment of a myriad of thermally agitated molecules in the liquid suspending medium, as described originally by Albert Einstein in his dissertation. Einstein concluded that this erratic behavior could adequately be described using the theory of Brownian motion, with sedimentation being a possible long term result. This critical size range (or particle diameter) typically ranges from tens of angstroms (10−10 m) to a few micrometres (10−6 m).
Under certain chemical conditions (typically in base-catalyzed sols), the particles may grow to sufficient size to become colloids, which are affected both by sedimentation and forces of gravity. Stabilized suspensions of such sub-micrometre spherical particles may eventually result in their self-assembly—yielding highly ordered microstructures reminiscent of the prototype colloidal crystal: precious opal.

Under certain chemical conditions (typically in acid-catalyzed sols), the interparticle forces have sufficient strength to cause considerable aggregation and/or flocculation prior to their growth. The formation of a more open continuous network of low density polymers exhibits certain advantages with regard to physical properties in the formation of high performance glass and glass/ceramic components in 2 and 3 dimensions.

In either case (discrete particles or continuous polymer network) the sol evolves then towards the formation of an inorganic network containing a liquid phase (gel). Formation of a metal oxide involves connecting the metal centers with oxo (M-O-M) or hydroxo (M-OH-M) bridges, therefore generating metal-oxo or metal-hydroxo polymers in solution.

In both cases (discrete particles or continuous polymer network), the drying process serves to remove the liquid phase from the gel, yielding a micro-porous amorphous glass or micro-crystalline ceramic. Subsequent thermal treatment (firing) may be performed in order to favor further polycondensation and enhance mechanical properties.

With the viscosity of a sol adjusted into a proper range, both optical quality glass fiber and refractory ceramic fiber can be drawn which are used for fiber optic sensors and thermal insulation, respectively. In addition, uniform ceramic powders of a wide range of chemical composition can be formed by precipitation.

Metal alkoxides are members of the family of organometallic compounds, which are organic compounds which have one or more metal atoms in the molecule. Metal alkoxides (R-O-M) are like alcohols (R-OH) with a metal atom, M, replacing the hydrogen H in the hydroxyl group. They constitute the class of chemical precursors most widely used in sol-gel synthesis.

The most common mineral in the earth's crust is silicon dioxide (or silica), SiO2. There are at least seven different crystalline forms of silica, including quartz. The basic building block of all of these crystalline forms of silica is the SiO4 tetrahedron. Since each tetrahedron shares 2 of its edges with other SiO4 tetrahedra, the overall ratio of oxygen to silicon is 2:1 instead of 4:1 (thus SiO2). The intricate and highly specific geometry of this network of tetrahedra takes years to form under incredible terrestrial pressures at great depth. That is why SiO2 is such a good glass former. Crystallization in a reasonable amount of time under the most ideal laboratory conditions is highly unlikely. Thus, amorphous silica is the major component of nearly all window glass.

A well studied alkoxide is silicon tetraethoxide, or tetraethyl orthosilicate (TEOS). The chemical formula for TEOS is given by: Si(OC2H5)4, or Si(OR)4 where the alkyl group R = C2H5. Alkoxides are ideal chemical precursors for sol-gel synthesis because they react readily with water. The reaction is called hydrolysis, because a hydroxyl ion becomes attached to the silicon atom as follows:
Si(OR)4 + H2O → HO-Si(OR)3 + R-OH

Depending on the amount of water and catalyst present, hydrolysis may proceed to completion, so that all of the OR groups are replaced by OH groups, as follows:
Si(OR)4 + 4 H2O → Si(OH)4 + 4 R-OH

Any intermediate species [(OR)2–Si-(OH)2] or [(OR)3–Si-(OH)] would be considered the result of partial hydrolysis. In addition, two partially hydrolyzed molecules can link together in a condensation reaction to form a siloxane [Si–O–Si] bond:
(OR)3–Si-OH + HO–Si-(OR)3 → [(OR)3Si–O–Si(OR)3] + H-O-H
(OR)3–Si-OR + HO–Si-(OR)3 → [(OR)3Si–O–Si(OR)3] + R-OH
Thus, polymerization is associated with the formation of a 1, 2, or 3- dimensional network of siloxane [Si–O–Si] bonds accompanied by the production of H-O-H and R-O-H species.

By definition, condensation liberates a small molecule, such as water or alcohol. This type of reaction can continue to build larger and larger silicon-containing molecules by the process of polymerization. Thus, a polymer is a huge molecule (or macromolecule) formed from hundreds or thousands of units called monomers. The number of bonds that a monomer can form is called its functionality. Polymerization of silicon alkoxide, for instance, can lead to complex branching of the polymer, because a fully hydrolyzed monomer Si(OH)4 is tetrafunctional (can branch or bond in 4 different directions). Alternatively, under certain conditions (e.g., low water concentration) fewer than 4 of the OR or OH groups (ligands) will be capable of condensation, so relatively little branching will occur. The mechanisms of hydrolysis and condensation, and the factors that bias the structure toward linear or branched structures are the most critical issues of sol-gel science and technology.
The applications for sol gel-derived products are numerous. For example, scientists have used it to produce the world's lightest materials and also some of its toughest ceramics. One of the largest application areas is thin films, which can be produced on a piece of substrate by spin coating or dip coating. Other methods include spraying, electrophoresis, inkjet printing or roll coating. Optical coatings, protective and decorative coatings, and electro-optic components can be applied to glass, metal and other types of substrates with these methods.

Cast into a mold, and with further drying and heat-treatment, dense ceramic or glass articles with novel properties can be formed that cannot be created by any other method. Macroscopic optical elements and active optical components as well as large area hot mirrors, cold mirrors, lenses and beam splitters all with optimal geometry can be made quickly and at low cost via the sol-gel route.

One of the more important applications of sol-gel processing is to carry out zeolite synthesis. Other elements (metals, metal oxides) can be easily incorporated into the final product and the silicate sol formed by this method is very stable.

Another application in research is to entrap biomolecules for sensory (biosensors) or catalytic purposes, by physically or chemically preventing them from leaching out and, in the case of protein or chemically-linked small molecules, by shielding them from the external environment yet allowing small molecules to be monitored. The major disadvantages are that the change in local environment may alter the functionality of the protein or small molecule entrapped and that the synthesis step may damage the protein. To circumvent this, various strategies have been explored, such as monomers with protein friendly leaving groups (e.g. glycerol) and the inclusion of polymers which stabilize protein (e.g. PEG).

Other products fabricated with this process include various ceramic membranes for microfiltration, ultrafiltration, nanofiltration, pervaporation and reverse osmosis.
If the liquid in a wet gel is removed under a supercritical condition, a highly porous and extremely low density material called aerogel is obtained. Drying the gel by means of low temperature treatments (25-100 °C), it is possible to obtain porous solid matrices called xerogels.

Finally of historical note, a sol-gel process was developed in the 1950s for the production of radioactive powders of UO2 and ThO2 for nuclear fuels, without generation of large quantities of dust.
Asignatura: CRF
Publicado por Sthefany Raga

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