Precursors shrinkage

Specialty Aluminas. Process control (qv) teclmiques permit production of calcined specialty aluminas ha nng controlled median particle sizes differentiated by about 0.5 ]lm. Tliis broad selection enables closer shrinkage control of high tech ceramic parts. Production of pure 99.99% -AI2O2 powder from alkoxide precursors (see Alkoxides, metal), apparently in spherical form, offers the potential of satisfying the most advanced appUcations for calcined aluminas requiring tolerances of 0.1% shrinkage. 

Characterization of the samples by TGA and CHN analysis shows that the template was effectively removed (C < 0.2 wt%). Small-angle X-ray scattering data of the calcined solid shows a reduction in the unit cell due to thermal shrinkage, while the values for the Fenton samples coincide with the starting precursor. Our approach therefore completely preserves the unit cell corresponding to the diameter of the micelles contained in the mesophase. 

The ethylene glycol-containing silica precursor has been combined, as mentioned above, with most commercially important polysaccharides and two proteins listed in Table 3.1. In spite of the wide variety of their nature, structure and properties, the jellification processes on addition of THEOS to solutions of all of these biopolymers (Scheme 3.2) had a common feature, that is the formation of monolithic nanocomposite materials, proceeding without phase separation and precipitation. The syner-esis mentioned in a number of cases in Table 3.1 was not more than 10 vol.%. It is worthwhile to compare it with common sol-gel processes. For example, the volume shrinkage of gels fabricated with the help of TEOS and diglyceryl silane was 70 and 53 %, respectively [138,141]. 

IR spectra measurements as well as variation of the film thickness, shrinkage, and refractive index demonstrated substantial differences in the mechanisms of thermal decomposition of films prepared from the exclusively metal alkoxide precursor and from the metal alkoxides modified by 2-ethylhexanoic acid. These differences affect the evolution of film microstructure and thus determine the different dielectric properties of the obtained films. The dielectric permittivity of the films prepared from metal alkoxide solutions was relatively low (about 100) and showed weak dependence ofthe bias field. This fact may be explained by the early formation of metal-oxide network (mostly in the... 

But, in the general case, inhomogeneities are induced by fast chemical reactions and are very difficult to control. For this reason, precursors of controlled architectures have been developed for different purposes, such as to decrease the viscosity in high-solids systems, to lower shrinkage, to improve material properties and, for some electronic and nonlinear optical applications, to build nanosized ordered regions in the networks. 

A rich variety of materials (including precursors) have been incorporated into the template-directed synthesis.33 169 170 Curable liquid prepolymers are particularly attractive because they have been widely used in replica molding with resolutions better than a few nanometers. By judicially selecting a monomer, the volume shrinkage involved in... 

There are two further requirements for a suitable matrix precursor. Firstly, the carbonization shrinkage of the matrix should not damage the carbon fiber of the skeleton. Secondly, the porosity formed by carbonization of the precursor should be open... 

Table I. Carbonization Shrinkage and Translation of Fiber Strength In Unidirectional C/C Composites Fabricated with Polyphenyleneacetylene (HA 43) as Matrix Precursor (20)...

Figure 23. Cross-sectional shrinkage as a function of carbon yield in the first carbonization. Unidirectional composites fabricated with PAN-based fibers and various matrix precursors.

---