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: www.rsc.org/materials Journal of Materials Chemistry
Ver: http://nanocompositescrf.blogspot.com/

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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

Fuente: http://www.solociencia.com/quimica/08052303.htm

Ver: http://nanocompositescrf.blogspot.com/

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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: www.rsc.org/materials Journal of Materials Chemistry
Ver: http://nanocompositescrf.blogspot.com/

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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.

Introduction
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: www.rsc.org/materials Journal of Materials Chemistry
Ver: http://nanocompositescrf.blogspot.com/

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Sol-gel

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.

Uniformity
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.

Polymerization
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
(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.

Applications
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

Fuente: http://en.wikipedia.org/wiki/Sol-gel

Ver: http://nanocompositescrf.blogspot.com/

Publicado por Sthefany Raga

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Los materiales nanoestructurados

Richard W. Siegel es uno de los pioneros mundiales en la investigación, fabricación y promoción de los materiales nanoestructurados. Físico de profesión, posee un doctorado en metalurgia que le ha permitido profundizar en la investigación de nuevos procesos prácticos de fabricación de materiales. En el año 1985 comenzó su experimentación en el campo de la nanoestructuración dentro de las instalaciones del Laboratorio Nacional de Argonne. Debido al éxito que obtuvo en su trabajo, decidió explotar comercialmente sus descubrimientos con la creación de una empresa que llamó Nanophase Technologies Corporation, la cual es actualmente líder mundial en el campo de la industrialización y comercialización de los materiales nanoestructurados.

¿Qué diferencia existe entre un material común y uno nanoestructurado? Si comparamos dos pedazos de materiales con un volumen idéntico, por ejemplo, dos cubos sólidos de cobre de un centímetro cúbico, la diferencia estriba en que en el interior del pedazo de material común, sus moléculas están organizadas en granos con poblaciones típicas de miles de millones de átomos, cuya dimensión granular oscila entre micrómetros y milímetros de diámetro. En el pedazo del material nanoestructurado, los granos moleculares tienen un tamaño máximo de 100 nanómetros de diámetro y tienen poblaciones granulares menores a decenas de miles de átomos. Dicho de otra forma, los granos nanoestructurados son entre mil y cien veces más pequeños que los de un material común, y además, dentro del mismo volumen poseen el 0.001 por ciento de átomos.

Lo anterior significa un ahorro increíble de materia dentro de cada pedazo de material nanoestructurado y, como consecuencia, una ligereza en peso que puede llegar a ser mil veces mayor que lo normal. Esta distinción física permite también obtener prioridades y características nuevas, singulares y asombrosas que nunca antes han sido vistas en los materiales comunes.

La explicación del éxito obtenido por Siegel (5) radica en el descubrimiento de un proceso práctico y económico para crear materiales nanoestructurados en cantidades industriales, al cual ha llamado y patentado como Síntesis Física de Vapor (6).

El proceso expone un material común a temperaturas superiores a su punto de fundición, propiciando una evaporación superficial de átomos -dentro de una atmósfera constituida por un gas especial- que son capturados en forma de cristales mediante un colector enfriado a bajas temperaturas. Los cristales restantes son retirados del tubo colector y prensados para moldear cualquier tipo de objeto. Lo más importante de este proceso es que mediante el control del ritmo de evaporación, la determinación del tipo correcto de gas y el manejo adecuado de su presión atmosférica, se puede modificar la resistencia a la fractura, la plasticidad, la elasticidad, el color, la transparencia, la resistencia a la corrosión, la reacción química, el comportamiento eléctrico y magnético, y la resistencia térmica y acústica de cualquier material nanoestructurado.

¿Qué tipos de materiales comunes son susceptibles de ser nanoestructurados mediante este proceso? En realidad, todo tipo de sólido conocido puede ser aprovechado para crear estos nuevos materiales. Los cuatro grupos de sólidos presentes en la naturaleza, llamados metales, cerámicas, semiconductores y polímeros, están siendo tratados según este proceso. Debemos recordar que los cuatro tipos mencionados son los materiales constructivos básicos utilizados en la arquitectura moderna.

¿Qué resultados importantes se han obtenido? Existen actualmente metales cuya resistencia es cinco veces mayor que la de sus contrapartes naturales. Se encontraron cerámicas que nunca se fracturan, sólo de deforman. Hay materiales que cambian de color dependiendo del espectro de luz que se aplique a su superficie, y que se vuelven en algunos casos totalmente transparentes. Se han construido semiconductores 300 veces más eficientes que los utilizados en la electrónica convencional. Existen cerámicas que resisten altas temperaturas y atmósferas sumamente corrosivas.

La empresa Nanophase Technologies Corporation fabrica y comercializa una línea de producción que abarca actualmente materiales abrasivos, catalizadores, cosméticos, magnéticos, pigmentos y recubrimientos, componentes electrónicos y cerámicas estructurales.(7) Este último conjunto de productos permite la fabricación de partes estructurales mediante el proceso de moldeo en malla que, en un futuro inmediato, será utilizado principalmente por la industria automotriz y aeroespacial en la construcción de estructuras, motores y laminados.

CITAS
(5) Richard W. Siegel, Creating Nanophase Materials, Scientific American, diciembre de 1996, pp. 74-79.
(6) Llamada PVS (Physical Vapor Synthesis), la cual no es, actualmente, el único medio de obtención de materiales nanoestructurados.

(7) Todos sus productos tienen el nombre comercial NanoTek ®. Por ejemplo, el óxido de aluminio nanoestructurado es llamado NanoTek ® Aluminum Oxide. Todos están patentados y registrados.

Asignatura: CRF

Fuente: http://nanotecnologiayarquitectura.blogspot.com/2008/07/los-materiales-nanoestructurados.html

Ver: http://nanocompositescrf.blogspot.com/

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Sol-gel: Un método de temperatura ambiente para formar vidrios.

La mayoría de las personas creen que se necesitan altas temperaturas para obtener vidrios de alta calidad. La técnica sol-gel permite formar a temperatura ambiente vidrios de calidad excepcional.

Qué es el vidrio

Es una sustancia amorfa fabricada sobre todo a partir de sílice (SiO2) que puede contener con boratos o fosfatos. También se encuentra en la naturaleza, por ejemplo en la obsidiana, un material volcánico, El vidrio es una sustancia amorfa porque no es ni un sólido ni un líquido, sino que se halla en un estado intermedio en el que las unidades moleculares, aunque están dispuestas de forma desordenada, tienen suficiente cohesión para presentar rigidez mecánica.

El vidrio se enfría hasta solidificarse sin que se produzca cristalización; el calentamiento puede devolverle su forma líquida. Suele ser transparente, pero también puede ser traslúcido u opaco. Su color varía según los ingredientes empleados en su fabricación.

Cómo se forma el vidrio a altas temperaturas.

El vidrio se fabrica a partir de una mezcla compleja de varias sustancias, como sílice, fundentes, como los álcalis, y estabilizantes. El horno debe alcanzar una temperatura de 1.500 °C y después enfriarlo en hasta la temperatura de 1.200 a 800 °C.

Al vidrio así obtenido se le da forma por laminación. Estas altas temperaturas fueron una limitante para hacer dispositivos electrónicos con los vidrios, hasta que recientemente se desarrollo la técnica sol-gel.

Qué es la técnica sol-gel.

Consiste en la reacción química, la hidrólisis, de los precursores moleculares y la policondensación a una forma vidriosa. En otras palabras, el proceso sol-gel es una reacción química, donde un liquido se va condensando hasta formar un sólido.

El proceso se parece mucho a la formación de la gelatina, la cual primero es un líquido y después de esperar un tiempo se hace sólido.Por medio de esta técnica se pueden obtener vidrios a temperatura ambiente (alrededor de los 20 grados centígrados), en condiciones de presión normal y en menos de 30 minutos se crea un vidrio rígido. La técnica permite la incorporación de sustancias orgánicas e inorgánicas durante la formación de la red vidriosa, pues está es altamente porosa. Veamos ahora las aplicaciones del sol-gel.

Aplicaciones tecnológicas del sol-gel

La técnica sol-gel permite obtener materiales gruesos, también conocidos como en bulto; recubrimientos (también llamados películas delgadas), fibras ópticas, entre otros formatos. Cada uno de ellos tiene características que tecnológicamente se pueden explotar para el bien de la sociedad.Se puede hacer una mezcla de material inorgánico (como el vidrio) y un material orgánico (como un polímero). De tal modo que se pueden conseguir materiales poliméricos con propiedades prediseñadas y protegidos por un vidrio. Los materiales híbridos presentan mejor respuesta para alcanzar una forma específica, son más estables ante esfuerzos mecánicos y de cambios de temperatura. Veamos un ejemplo puntual:Láseres entonables sólidos.

Moléculas como la Rodamina 6G, al ser iluminada con luz verde (e.g. 532 nm) presenta el fenómeno de fluorescencia, es decir, reemitirá esa luz en otros colores (otras longitudes de onda), en su fase liquida dicha molécula orgánica se puede emplear para hacer láseres que varían su color por medio de un selector o entonador apropiado. Sin embargo, esta molécula se degrada rápidamente, es cancerigena y no se puede condesar para obtener un sólido útil para un láser. Afortunadamente, la molécula de Rodamina 6G se puede incorporar al sol-gel, mientras es un líquido; se incorpora fácilmente y queda encerrada cuando el sol-gel solidifica. De tal modo que el líquido sol se puede colocar en moldes o manipular para obtener un material sólido para un láser eficiente.

Conclusiones

Nuestra sociedad se ha beneficiado grandemente por el desarrollo de materiales vítreos. Existen muchas clases de vidrios y sus modos de preparación son variados. La técnica sol-gel nos permite obtener vidrios de calidad excepcional en condiciones muy cómodas: a temperatura ambiente, a presión atmosférica y en menos de 30 minutos.

Al incorporar al vidrio una molécula orgánica, ópticamente activa (e.g. rodamina) se puede obtener la base de un desarrollo tecnológico de vanguardia, como un láser de estado sólido, al cual se le puede seleccionar el color de emisión. Se pueden hacer otras combinaciones vidrio-polímero para hacer diferentes dispositivos tecnológicos.

El estudio del sol-gel representa una ventana de oportunidades científicas y tecnológicas muy innovadoras que se deben aprovechar por el bien de nuestra comunidad.

Asignatura: CRF
Fuente: http://vicente1064.blogspot.com/2007/04/sol-gel-un-mtodo-de-temperatura.html
Ver: http://nanocompositescrf.blogspot.com/

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Nanoestructuras híbridas detectan magnetismo en la nanoescala

Un desafío clave de la investigación en nanotecnología está en conocer cómo se comportan los diferentes materiales con longitudes de sólo "una milmillonésima de un metro". Cuando se reduce a esos diminutos tamaños, muchos materiales de la vida cotidiana muestran nuevas propiedades interesantes y potencialmente beneficiosas. Ahora un grupo de ingenieros de Rensselaer han descubierto nuevas nanoestructuras híbridas que podrían allanar el camino hacia nuevos dispositivos de almacenamiento de datos, así como mejoras en sistemas de suministros farmacológicos.El comportamiento magnético es un tipo de fenómeno que pueden cambiar significativamente dependiendo del tamaño del material. Sin embargo, el gran reto de la observación de las propiedades magnéticas de materiales a nanoescala ha impedido un estudio más profundo del tema.

Los investigadores del Instituto Politécnico Rensselaer han desarrollado y demostrado un nuevo método para detectar el comportamiento magnético de los nanomateriales. Para ello crearon un nuevo proceso en la creación de una única multi-capa de nanotubos de carbono que se encuentra incrustado con nanoestructuras de cobalto. Las agrupaciones de cobalto tienen medidas que oscilan entre 1 y 10 nanómetros.

Después de una serie de experimentos, el equipo de ingenieros ha llegado a la conclusión de que la conductividad eléctrica de los nanotubos de carbono es lo suficientemente sensible para detectar y ser afectado por trazas de actividad magnética, como los presentes en el cobalto incorporado en las nanoestructuras. Con ello se demostró en primera instancia la detección de campos magnéticos de estos pequeños imanes individuales mediante un nanotubo de carbono.
Los resultados completos del estudio se detallan en un artículo titulado; "Detección de la actividad magnética a nanoescala a través de un único nanotubo de carbono", recientemente publicado por la revista "Nano Letters".

Dado que los grupos de cobalto en este sistema están incrustados en el interior del nanotubo más que en la superficie, esto no causa la dispersión de electrones y, por tanto, no parecen afectar al atractivo de las propiedades conductoras de acogida de nanotubos de carbono. Desde un punto de vista genérico, estas nanoestructuras híbridas pertenecen a una nueva clase de materiales magnéticos.

Estas nuevas nanoestructuras híbridas abren nuevas vías de investigación únicas en física aplicada, allanando así el camino para una mayor funcionalidad en nanotubos de carbono utilizando la electrónica magnética con un gran grado de libertad, pudiendo dar lugar a importantes aplicaciones espintrónicas.

Las posibles aplicaciones de este material con las nuevas generaciones de sensores de conductividad a nanoescala, dará lugar a nuevos avances en dispositivos de almacenamiento digital, además de un mayor desarrollo de la espintrónica, que ofrecerá nuevas aplicaciones en campos médicos.

Asignatura: CRF
Fuente: http://www.fierasdelaingenieria.com/nanoestructuras-hibridas-detectan-magnetismo-en-la-nanoescala/
Ver: http://nanocompositescrf.blogspot.com/

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Estudian las propiedades magnéticas de nanoestructuras

Un grupo de investigadores de la Universidad de Valladolid se ha adentrado en el análisis de diferentes características magnéticas de elementos de tamaño microscópico.

Si tomamos un elemento material y lo reducimos progresivamente llega un momento en que sus propiedades físicas, como la eléctrica o la magnética, varían de forma drástica. El punto donde todos los materiales ven modificadas sus prestaciones se sitúa en la escala nanométrica, o lo que es lo mismo, al nivel de átomos y moléculas (lo que equivale la millonésima parte de un milímetro). En el caso de los materiales magnéticos, como el hierro o el acero, sus propiedades varían de forma notable a esa escala, lo que es objeto de investigación del Grupo de Propiedades Nanométricas de la Materia de la Universidad de Valladolid, ubicado en el Departamento de Física Teórica, Atómica y Óptica de la Facultad de Ciencias.

Entre las aplicaciones de estos estudios se encuentra el avance en la electrónica tradicional, pero también en la denominada espintrónica. Frente a la electrónica convencional, que solo explota la carga del electrón, la espintrónica aprovecha también el espín. El electrón, al girar sobre su propio eje, genera un campo magnético que se deonima espín (término procedente del inglés spin, que significa giro).

Según Carlos Balbás, coordinador del grupo, el efecto físico que ha originado el desarrollo de la espintrónica es la Magnetorresistencia Gigante, descubierta en los años 80 y que le ha valido hace dos años a sus descubridores, Peter Grünberg y Albert Fert, el Premio Nobel de Física. Se trata de un efecto nuevo que se produce en determinadas multicapas magnéticas nanoestructuradas, en las que, aplicando un campo magnético, se puede modificar su resistencia eléctrica. La magnetorresistencia gigante se caracteriza por su intensidad "y por ponerse de manifiesto sólo en materiales nanométricos.

Andrés Vega y Amador García son los investigadores del grupo que centran su labor en estas propiedades. En concreto, explica Balbás, han contribuido estudiando otro tipo de sistema interfase entre dos metales que tienen un comportamiento de spin diferente dependiendo de la estructura y el tamaño. El equipo de físicos realiza simulaciones de propiedades de materiales a nivel nanométrico y, en este caso, analizan "cómo se acoplan los espines dependiendo del tamaño y del campo magnético que se aplique". Estimar la manera más rápida puede aprovecharse en la espintrónica.

Además de la línea de investigación sobre nanomagnetismo, el Grupo de Propiedades Nanométricas de la Materia trabaja en campos como los líquidos metálicos, los materiales nanoensamblados o la catálisis. Estas líneas tienen potencial aplicación en la electrónica molecular, las aleaciones de metales líquidos en metalurgia y en reactores de fusión, los nuevos catalizadores nanométricos (por ejemplo para la eliminación del óxido de nitrógeno en la combustión de hidrocarburos en automóviles) o en materiales optoelectrónicos para tecnología del medio ambiente. El equipo está compuesto por siete investigadores fijos y personal fluctuante.

Sensor de campo magnético. Foto: Grupo de Propiedades Nanométricas de la Materia.

Asignatura: CRF
Fuente: http://www.plataformasinc.es/index.php/Noticias/Estudian-las-propiedades-magneticas-de-nanoestructuras
Ver: http://nanocompositescrf.blogspot.com/

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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.

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.

El efecto Josephson puede emplearse para detectar el movimiento mecánico de los átomos colocados en la unión de Josephson. (Foto: Georgia Tech)

Asignatura: CRF

Fuente: http://www.solociencia.com/ingenieria/07090301.htm

Ver: http://nanocompositescrf.blogspot.com/

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