Zircon sintering

The measures of solid state reactivity to be described include experiments on solid-gas, solid-liquid, and solid-solid chemical reaction, solid-solid structural transitions, and hot pressing-sintering in the solid state. These conditions are achieved in catalytic activity measurements of rutile and zinc oxide, in studies of the dissolution of silicon nitride and rutile, the reaction of lead oxide and zirconia to form lead zirconate, the monoclinic to tetragonal transformation in zirconia, the theta-to-alpha transformation in alumina, and the hot pressing of aluminum nitride and aluminum oxide. 

Babilo, P. and Haile, S. M. Enhanced Sintering of Yttrium-Doped Barium Zirconate by Addition of ZnO, Journal of the American Ceram. Soc., 88, 2362 (2005). 

Zirconate pyrochlores have a low isomorphic capacity with respect tetravalent actinides. However the amount of isomorphic substitution can be increased by coupled substitution of Ca, according to the scheme 2REE = An" -i- Ca. Production of single phase ceramic can be attained only for actinide waste streams with rather simple chemical compositions. In the case of a complex waste stream composition, extra phases occur [129]. Finally, zirconate matrices require high temperatures for their synthesis. In order to prepare REE-zirconates from oxide mixtures, sintering at 1500-1600 for about 50 hours is required... 

S. Venkataramani, Calcining and Its Effects on Sintering and Properties of Lead Zirconate Titanate Ceramics, Ph.D. thesis, Pennsylvania State University, 1981. 

Fhiosilicate fusion has been used in the Soviet Union [SI] to produce feed for separation of hafnium from zirconium by fractional crystallization of KjMF . Zircon is ground to pass 200 medi and mixed with potassium fluosilicate and potassium chloride (to act as promoter). The mixture is sintered in a rotary furnace at 650 to 700°C. The following reaction takes place ... 

Nano-sized and homogeneous hematite particles were obtained into sintered cristobalite-zircon matrix after heat treatment at 1000°C/3h and 1100°C/3h. Occluded hematite particles have been spherical shapes with 5-10 nm diameters. Changes of sintering temperatures did not show any effect on hematite morphology but it was important about red shade, inclusion efficiency and thermal-chemical stability of the pigments. 

Khor K A, Yu L G, Li Y. Spark plasma reaction sintering of Zr02 mullite composites from plasma sphe-roidized zircon/ alumina powers. Mater Sci Eng A, 2003, 339 (12) 286-296,... 

Garrido L B, Aglietti E F. Reaction-sintered mullite-zirconia composites by colloidal processing of alumina-zircon-Ce02 mixtures. Mater Sci Eng A, 2004, 369 (12) 250-257. 

A properly chosen support may stabilize a catalyst against sintering. In the case of NijS2 catalysts such supports are commonly used. Supports of this type include refractory oxides which do not react appreciably with hydrogen sulfide, for example alumina, silica, thoria, zirconia, and titania, as well as certain silicates, aluminates, thorates, zirconates, and titanates. Active carbon can be used as a support for NiS or NiaS2 catalysts for reactions which do not involve oxygen. 

Chem. Descrip. Zircon-based cone, in water-based carrier Uses Release coating for sintering ferrous or nonterrous parts, tor use on any graphite, ceramic, or metal surf. 

Pressure sintering techniques have also been used to fabricate transparent lanthanum-doped lead zirconate titanate (PLZT) ceramics in air and in oxygen-gas atmosphere [31]. The microstructure of the sintered body was not uniform it was completely dense near the surface, but it was porous at the center. The thickness of the dense layer increased with sintering time and oxygen-gas pressure in the sintering atmosphere. Vaporization of PbO from the specimen surface and resultant formation of lattice vacancies were attributed to this microstructural evolution. Diffusion of the gas trapped in the pores was also important in determining the thickness of the dense layer. When the PLZT specimen was sintered in air at 1200 °C for 8 h, the thickness of the dense layer was 0.25 mm. Therefore, if the specimen thickness was 0.5 mm, the whole specimen was dense and transparent. When the specimen was sintered in an oxygen-gas atmosphere under the same conditions, the specimen thickness increased markedly. 

Abe Y, Kakegawa K (2002) Fabrication of optically transparent lead lanthanum zirconate titanate ((Pb, La)(Zr, Ti)03) ceramics by a three-stage-atmospheie-sintering technique. J Am Ceram Soc 85 473-475... 

Zhou LJ, Zhao Z, Zimmermann A, Aldinger F, Nygren M (2004) Preparation and properties of lead zirconate stannate titanate sintered by spark plasma sintering. J Am Ceram Soc 87 606-611... 

For classes (a)-(c) and for SiC, the main analysis solution is prepared via a low flux/sample ratio fusion or, in the case of alumina, a sinter at 1200°C. The flux used is a mixture of fusion mixture (NaKCOs) and boric acid a lower flux/sample ratio is used for aluminous materials, and a larger flux/ sample ratio is used for zircons. The melt is dissolved and the silica is dehydrated in a mixture of hydrochloric and sulfuric acids with the assistance of polyethylene oxide. The main silica is then determined gravimetrically. Other constituents, including residual silica, are determined on the filtrate combined with the residue remaining after the gravimetric silica determination. 

A process is operated in the U.S.S.R., on a production scale for the breakdown of zircon by sintering with potassium silicofluoride. The principal reason for the employment of this route is the fact that the product potassium fluorozirconate, is suitable for feeding directly to the Russian fractional crystallization purification process. This removes the 1-5 to 2-5 per cent of hafnium which originates in the zircon mineral. 

A special class of materials that is also ferroelectric are electrooptic ceramics. Materials such as lanthanum-modified lead zirconate titanate (PLZT) produce excellent electrooptic devices. These polycrystalline ceramics exhibit voltage-variable behavior—that is, they can be switched from optically transparent to opaque by the application of voltage. Most of these devices, which are used for shutters, modulators, and displays, are processed by hot pressing to full density. Experiments in many laboratories are being carried out to tape-cast these materials into thin sheets. The main problem encountered to date has been the ability to sinter to full density. The use of nanosized powders has helped in this regard. The ability to tape-cast large sheets could open a wide variety of applications for these materials. 

Aerogel powders of binary aluminate and zirconate oxides were prepared by a sol-gel technique and super-critical drying. The synthesis conditions are described and initial properties. Changes in the surface area and phases present in these powders and several reference materials are followed as they are heated at various temperatures in air cmtaining 10 vol.% water vapor. In addition, a time series was done on the most sintering resistant material (LaAli lOig) in ordtf to estimate its sintering rate. 

---