12 Nano/microstructure - institut de chimie et des materiaux paris-est

DK4027_book.fm Page 322 Monday, May 16, 2005 2:01 PM322 Chemical Processing of Ceramics, Second EditionAlloying the materials obtained from liquid precursors offers a new route tocreate particular micro/nanostructures in order to optimize the properties ofporous materials.E. FUNCTIONALLY GRADED MATERIALSBy selecting the appropriate sol-gel precursors, the dwell temperature requiredto achieve maximum densification can be raised or lowered by about 100°C. 1,36This makes it possible to combine several kinds of impregnated fiber/matrixinterphases in the same composite in order to tailor its physical and/or chemicalproperties for a particular application. Composites have been fabricated thatcombine various kinds of woven fibers (SiC Nicalon ® , Nextel ® , Almax ® ,Saphikon ® ) with various kinds of matrices (LAS, mullite [a pure dielectric witha real microwave permittivity close to 5 at 10 GHz], Nasicon [a solid electrolytewith the structural formula Na 1+x Zr 2 Si x P 3x O 12 (0 x 3) offers the advantage of anelectrical conductivity varying by four orders of magnitude as a function of x],celsian [a corrosion-resistant ceramic], zirconia) 6–10,76,78,79,86,87 in order to tailorthermomechanical and electromagnetic properties simultaneously.The processing can be summarized as follows: The fibers of a two-dimensionalwoven fabric are first impregnated with a liquid alkoxide mixture, whichfills the voids between the fibers, transforms in situ into a gel by reaction withatmospheric moisture, and will be converted into a (glass)-ceramic by pyrolysisduring the hot-pressing of prepreg fabrics covered with the matrix powder precursors.Depending on both the matrix and the fibers, several different “interfaceprecursors” can be used, either alone or in various combinations. Finally, thedoubly impregnated, coated woven fabrics are stacked in a graphite mold andhot-pressed, typically between 950 and 1400°C. By selecting the appropriate solgelprecursors, the dwell temperature required to achieve maximum densificationcan be raised or lowered by about 100°C. This makes it possible to combineseveral kinds of fiber-reinforced layers in the same composites in order to tailor,unidirectionally, the microwave absorption along the direction perpendicular tothe body. Examples given in Figure 12.12 show (1) a composite consisting of azirconia matrix and a mullite matrix, both reinforced by SiC Nicalon NLM 202fibers (4 + 4 layers), and (2) NASICON matrix composite reinforced on one sideby low-permittivity Nextel mullite woven fibers (4 layers) and on the other sideby conducting Nicalon SiC woven fibers (2 layers).V. CHARACTERIZATION OF MULTIPHASEMATERIALSOne of the difficulties in the characterization of multiphase materials is the needto analyze the different phases at various scales: at the scale of each phase, atthe scale of their association (interfaces and intrafaces), and at the scale of theparts. We will address two methods, which can be used very rapidly for this type