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Ships in 15 business days. Link Either by signing into your account or linking your membership details before your order is placed. Description Table of Contents Product Details Click on the cover image above to read some pages of this book! Industry Reviews "Based on a March symposium held during the th ACS meeting, this collection of 16 papers presents research on the effects and possible benefits of reduced gravity for polymerization. Preface p.
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Eyewitness Periodic Table Eyewitness. 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. 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:. Depending on the amount of water and catalyst present, hydrolysis may proceed to completion to silica:. 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.
The Basics: Polymer Definition and Properties
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. 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.
This reaction is favored in both basic and acidic conditions. Sonication is an efficient tool for the synthesis of polymers. Furthermore, multi-phase systems are very efficient dispersed and emulsified , so that very fine mixtures are provided. This means that ultrasound increases the rate of polymerisation over conventional stirring and results in higher molecular weights with lower polydispersities.
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Ormosils organically modified silicate are obtained when silane is added to gel-derived silica during sol—gel process. The product is a molecular-scale composite with improved mechanical properties. Sono-Ormosils are characterized by a higher density than classic gels as well as an improved thermal stability. An explanation therefore might be the increased degree of polymerization. For single cation systems like SiO 2 and TiO 2 , hydrolysis and condensation processes naturally give rise to homogenous compositions.
For systems involving multiple cations, such as strontium titanate , SrTiO 3 and other perovskite systems, the concept of steric immobilisation becomes relevant. To avoid the formation of multiple phases of binary oxides as the result of differing hydrolysis and condensation rates, the entrapment of cations in a polymer network is an effective approach, generally termed the Pechini Process.
Subsequently a polymer network is formed to immobilize the chelated cations in a gel or resin. This is most often achieved by poly-esterification using ethylene glycol. The resulting polymer is then combusted under oxidising conditions to remove organic content and yield a product oxide with homogeneously dispersed cations. 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 flocculation of powders due to attractive van der Waals forces can also give rise to microstructural heterogeneities. 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 heterogeneous 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 heterogeneous 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 structures would appear to be the basic elements of nanoscale 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 sintered ceramic nanomaterials.
The applications for sol gel-derived products are numerous. One of the largest application areas is thin films, which can be produced on a piece of substrate by spin coating or dip-coating.
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. Other coating methods include spraying, electrophoresis , inkjet   printing, or roll coating.
With the viscosity of a sol adjusted into a proper range, both optical and refractory ceramic fibers can be drawn which are used for fiber optic sensors and thermal insulation, respectively. Thus, many ceramic materials, both glassy and crystalline, have found use in various forms from bulk solid-state components to high surface area forms such as thin films, coatings and fibers. Ultra-fine and uniform ceramic powders can be formed by precipitation. Robertson, Mostafa Yourdkhani, Polette J.
Ivanoff, Elyas Goli, Evan M. Lloyd, Leon M. Dean, Nancy R. Sottos, Philippe H. Geubelle, Jeffrey S.
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Moore, Scott R. Rapid energy-efficient manufacturing of polymers and composites via frontal polymerization.
Nature , ; : DOI: ScienceDaily, 9 May University of Illinois at Urbana-Champaign. New polymer manufacturing process saves 10 orders of magnitude of energy.