Ceramics for Sustainable Energy Technologies with a Focus on Polymer-Derived Ceramics - Novel Combustion Concepts for Sustainable Energy Development - page 498

Al 2 O 3 , and SiO 2 . The Si-based precursor family is the most investigated; however,
research has also been done on aluminum-, titanium-, zirconium-, and boron-based
systems (Greil 1995 ).
Preceramic polymers have certain polymeric characteristics, leading to many
advantages with respect to processing that are unavailable to processing of ceramics
using traditional techniques (vapor phase or powder processing). For example, by
altering the main chain chemistry or that of the side groups, coupled with the
introduction of a reactive atmosphere during pyrolysis, the

final ceramic compo-
sition, porosity, pore structure, and grain morphology can be tailored (Colombo
et al. 2010 ). Precursors can be made in liquid form, which allows ease of processing
by slurry methods. This also allows for control of the viscosity of the precursor
which can be useful in processes such as coating. As liquids, precursors are easily
puri ed; this leads to a reduction in the impurity content of the ceramic product.

Another signi cant advantage of preceramic polymers is that they have rela-

tively low pyrolysis temperatures compared with traditional ceramic processing
methods, often forming dense, amorphous ceramics at temperatures as low as 600 -

800 C (Torrey and Bordia 2007 ). While traditional ceramic processing methods

such as powder compaction can be used, there are also many polymer processing
techniques that can be employed when forming polymer-derived ceramics. These
include

°

fiber spinning, pressure casting, dip and spin coating, painting, spraying,
printing, injection molding, UV curing, inkjet printing and biotemplating (Yajima
et al. 1975 ; Torrey and Bordia 2008a , b ; Walter et al. 1996 ; Schef

er et al. 2005b ).
The main disadvantage of preceramic polymer technology is the large density
change that occurs when the polymer precursor, typically a density of 1 g cm − 3 ,is
converted to the ceramic product, often >2 g cm − 3 . Densi

fl

cation leads to volu-

metric shrinkage on the order of 50 75 %, which makes production of net-shape

-

components dif cult. This is often accompanied by pore formation during cross-

linking and pyrolysis due to the off-gassing of organic side groups. This disad-
vantage has been largely solved by the use of passive or active filler materials and

by careful heat treatment to minimize the shrinkage on pyrolysis. As described by
Greil ( 1995 ), these preceramic polymers can be

filled with either inactive or active

fillers such as Si, Al, Ti or inter-
metallics react either with the residual carbon during pyrolysis or with the gases
used during processing. Typically,

fillers. In a process known as AFCOP, active

fillers are used such that the speci c volume of

the product phase is greater than that of the filler. This expansion partially com-

pensates the shrinkage of the polymer leading to signi

cantly lower, and in some

cases, zero net shrinkage. The final phases in these materials depend on the

pyrolysis products of the precursors and the reacted fillers.

Due to the advantages mentioned above, the polymer precursor approach is now
used to make ceramics in many forms including fibers, coatings, porous ceramics

and ceramic foams, high-temperature MEMS devices, and bulk composites, to
name a few. Arguably, the most commercially successful application of preceramic
polymers to date has been their use in the fabrication of bers and nanorods (Motz

et al. 2000 ; Schef fl er et al. 2004 ). The process for mass production of SiC bers