Synthetic opals

Fig. 7. Electron microscope view of Gilson synthetic opal each sphere is j p.m in diameter.

Discredited Synthetics. There are several materials that have in the past been considered to be synthetics, but were found on closet examination not to deserve such a designation, being merely imitations. Examples include imitation coral, lapis la2uli, and turquoise, all made by ceramic processes. This same point has been raised (17) with respect to synthetic opal, which does contain some substances not present in natural opal and somewhat less water. However, the composition of natural opal is quite variable and is usually intermixed with significant amounts of rock-derived materials hence the synthetic designation is usually retained. 

Gilson A process for making synthetic opals, invented in France in 1974 by P. Gilson, Sr. 

Ordered macroporous materials (OMMs) are a new family of porous materials that can be synthesized by using colloidal microspheies as the template. - The most unique characteristics of OMMs are their uniformly sized macropores arranged at micrometer length scale in three dimensions. Colloidal microspheres (latex polymer or silica) can self assemble into ordered arrays (synthetic opals) with a three-dimensional crystalline structure. The interstices in the colloidal crystals are infiltrated with a precursor material such as metal alkoxide. Upon removal of the template, a skeleton of the infiltrated material with a three-dimensionally ordered macroporous structure (inverse opals) is obtained. Because of the 30 periodicity of the materials, these structures have been extensively studied for photonic applications. In this paper, the synthesis and characterization of highly ordered macroporous materials with various compositions and functionalities (silica, organosilica, titana, titanosilicate, alumina) are presented. The application potential of OMMS in adsorption/separation is analyzed and discussed. 

Yoshino, K., Satoh, S., Shimoda, Y. et al.. Tunable optical properties of conducting polymers infiltrated in synthetic opal as photonic crystal, Synthet. Met., 121, 1459, 2001. 

FIGURE 2.20 Schematic illustrating preparation of an inverse CEP opal using a synthetic opal template. (With permission from G. G. Wallace and P. C. Innis, J. Nanosci. Nanotech. 2, 441 (2002). 2002, American Scientific Publishers.)... 

Systems with a solid dispersion medium are represented by rocks, minerals, a variety of construction materials. Most such systems are of the S,/S2 types. Various synthetic and natural porous materials (with closed porosity), such as pumice and solid foams (e.g. styrofoam, bread), belong to the G/S type. The systems of L/S type include natural and synthetic opals and... 

Three-dimensional opal-VO based photonic crystals were prepared by the chemical bath deposition technique. The x-ray diffraction and Raman spectroscopy confirm the crystalline perfection of VO2 impregnated into synthetic opal pores. It is shown from the optical reflectivity measurements that the photonic bandgap of the opal-V02 based photonic crystals composite is governed by the phase transition in VO2. The shift of Bragg diffraction spectra under the pulsed (10 ns) illumination of YAG.Nd laser has been also observed in the opal-VO2 composites. 

A good example of a three-dimensional photonic crystal is synthetic opal [2], having a fee lattice composed of close-packed monodisperse amorphous Si02 spheres with a diameter varying within 100-1000 mn. The PBG position may be tuned in a wide spectral range (UV-near IR) by varying the size of Si02 spheres. 

To conclude, we have synthesized VO2 with a perfect crystal stmcture in opal pores using the chemical bath deposition technique. The parameters of the semiconductor-metal phase transition in the prepared material indicate the presence of a small amount of oxygen defects. We have achieved a controllable and reproducible variation of the PEG properties of the opal-V02 composite and inverted VO2 composite during heating and cooling. This is due to the change in the dielectric constant of VO2 at the phase transition. We demonstrated dynamical tuning of the PEG position in synthetic opals filled with VO2 imder laser pulses. 

Other templates have also been used. Polypyrrole nanowires have been produced by growing the polymer in porous alumina. Highly porous conducting polymers have also been produced using the inverse opal method [38], in which the polymer is deposited around a matrix of tightly packed spheres, which form the synthetic opal. When the spheres are removed, a highly porous film is left with... 

Sol-gel growth techniques. The most common example is the production of synthetic opal. The first step consists to produce a suspension of monodisperse silica nanospheres in ethanol obtained by the direct hydrolysis of tetraethyl ester of orthosilicic acid, SifOC H l with ethanol using ammonia as a catalyst according to the following reaction ... 

By adding further amounts of reactant, the particles of silica grow up to the diameter de-sired-that is to about 300 nm size. Secondly, the raw opal precursor is precipitated either by spontaneous sedimentation or by centrifugation. Finally, the opal precursor is then dried in order to remove liquid from its pores. Afterwards, the dried opal precursor is sintered by thermal treatment at 400-800°C in a furnace. For the production of synthetic opal of gem quality, it is then necessary to complete the process by filling pores in the opal substance with a silica gel. 

This synthesis was announced in 1967 (396). Since then synthetic opals have become available from Gilson of Switzerland (397). These have the appearance of natural opal and many of the same properties, for example hydrated, density 2.113, refractive index 1.45, singly refractive, amorphous, hardness 6.15 mohs (398). The synthetic black opal is said to be indistinguishable from the natural ones by visual examination. Some difference in fluorescent properties was reported. The method of manufacture is a trade secret of the manufacturing company, Gilem S. A. 

More recently another material known as Slocum Stone has been developed by John Slocum (399, 400). Specimens exhibiting brilliant interference colors of all shades have been made with a wider range of background color than in any natural opals. All types of opals can be simulated. However, in spite of the similarity the material is not claimed to be a synthetic opal because it has a unique, characteristic appearance of its own and also because it does not have the composition of opal. It is nonhydrated, nonporous, harder, and with higher density (2.4-2.5) and higher refractive index (1.51). The colors appear to originate from interference Films dispersed throughout the matrix. The method of manufacture has not been revealed. 

If all particles in a gel are of very uniform size and they are packed in a uniform manner, sintering will only occur suddenly as a given temperature is reached and the pores are all eliminated at the same time. Her has observed such a phenomenon in uniformly close-packed silica spheres 200 nm in diameter (see section in Chapter 4 on synthetic opal). This white opaque gel shrinks quickly as a certain temperature is reached (1000-1200 C, depending on Na content) and is converted to clear nonporous SiOa glass far below its normal softening point. 

A further advantage of PMMA relies on its availability. Uniformly sized PMMA spheres are prepared by polymerization of methyl methacrylate (MMA) in water. The product of the polymerization then takes the form of a colloidal suspension of solid particles that are so small that they tend not to settle. By centrifugation, the PMMA particles are forced to settle and pack into a solid, often called a colloidal crystal. In such colloidal crystals, the PMMA spheres are arranged in a close-packed fashion in the same manner as the silica spheres that make up natural opal [178]. Therefore, these materials can be referred to as synthetic opals. Several textbooks cover selective aspects of the physicochemical properties of PMMA [181,182]. 

In a wide context, the synthesis of inherently conducting polymers, PTh, PANI and PPy, and their properties and applications (capacitance, sensors, artificial muscles, biomolecular interactions, cell growth) related to the attainment of nanodimension, have been reviewed and a section is dedicated to physical templates (pore sized membranes, synthetic opals) that induce the doped polymer fibrillar morphology [164]. 

Yoshino, K Lee, S.B., Tatsuhara, S., Kawagishi, Y, Ozaki, M and Zakhidov, A.A. (1998) Observation of inhibited spontaneous emission and stimulated emission of rhodamine 6G in polymer replica of synthetic opal. Appl. Phys. Lett., 73 (24), 3506-3508. 

F. Garcia-Santamaria, V. Salgueiriiio-Maceira, C. L6pez, and L. M. Liz-Matzin, Synthetic opals based on silica-coated gold nanoparticles, Langmuir 18,4519-4522 (2002). 

K. Yoshino, Y. Shimoda, Y. Kawagishi, K. Nakayama, M. Ozaki, Temperature tuning of the stop band in transmission spectra of liquid crystal infiltration synthetic opal as tunable photonic crystal, Appl. Phys. Lett., 75, 93 (1999). 

Escobedo, C.A.C., Munoz-Saldana, J., Vigueras, D.J., and Beltran, F.J.E. (2006) Preparation of size controlled nanometric spheres of colloidal silica for synthetic opal manufacture. Mater. Scl Forum, 509, 187-192. 

Bueno, L.A., Bertholdo, R., Barros Filho, D.A., Messaddeq, Y., and Ribeiro, S.J.L. (2002) Rare earth doped synthetic opals and inverse opals. Proc. SPIE. doi 10.1117/12.456543. 

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