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

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DK4027_book.fm Page 316 Monday, May 16, 2005 2:01 PM316 Chemical Processing of Ceramics, Second EditionFIGURE 12.7 (a) Scanning electron micrograph (SEM) of a mullite matrix CMC reinforcedwith submicronic zirconia (fiber diameter 12 µm). (b) TEM of a cobalt dispersionin an aluminosilicate matrix synthesized by the infiltration-H 2 firing route. 87particles are dispersed together in an anhydrous solvent (e.g., propanol) beforethe addition of alkoxides. After mixing, pH-controlled water is poured in undervigorous mechanical stirring, which has to be maintained until a paste of thickconsistency is obtained. The paste is then dried to give a flour-like powder. Thepowder is compacted and sintered. Gel embedding promotes a homogeneousload/pressure transfer and hence suppresses the densification hindrance inducedby the platelet addition. For example, zirconia gel embedding of the mullitepowder increases the microhardness by 30% and the toughness by 40%. Thecombination of zirconia gel embedding and platelets addition increases the K ICvalue to about 5 MPa.m 1/2 . 37B. METAL- AND CERAMIC-CERAMIC NANOCOMPOSITESMetal-ceramic microcomposites (cermets) have been prepared for a long time,especially by dispersion of nickel particles in a silica matrix. Metal nanocomposites(iron, nickel, tin, and copper particles in a silica or alumina matrix) havebeen prepared by different groups. 60–64 More complex materials with nanodispersionof pure or alloyed iron, cobalt, and nickel in aluminosilicate matriceshave also been prepared. 65,66 The main methods can be classified as follows: (1)the mixing of the (sub)micronic metal powder within a liquid matrix precursor;(2) the mixing of the alkoxides with an aqueous solution of metal ions andcontrolled H 2 reduction on firing; and (3) the preparation of a porous host matriximpregnated by a concentrated solution of transition metal nitrates and controlledreduction. Homogenous dispersions are achieved at the submicronic scale(Figure 12.7b). About 5% of matter in volume (15% in weight) can be incorporatedin one cycle of impregnation/firing. The metal content can be increased bysuccessive impregnation-firing cycles (up to ~50% in volume). Combined infiltrationswith different metal and alloy precursors can tailor the magnetic andelectromagnetic properties, in particular the microwave absorption between 0.1and 10 GHz. 66 These new materials can find applications at higher temperatures,as their magnetic properties are maintained up to 550°C.
DK4027_book.fm Page 317 Monday, May 16, 2005 2:01 PMNano/microstructure and Multiphase Materials 317The preparation of ceramic (oxide-oxide, carbide-carbide, etc.) nanocompositesis also rather old and many authors have reported some exceptional properties.49,50 This field of study developed rapidly. It is clear that more work is neededin order to further understand the mechanics of nanocomposites.C. ONE-DIMENSIONAL AND TWO-DIMENSIONALFIBER-REINFORCED COMPOSITESOne of the main problems in the preparation of ceramic matrix composites (CMCs)is achieving a low open porosity in the matrix in order to protect the fiber from theenvironment and to optimize the mechanical strength and toughness. Fibrous fractureof CMCs is mandatory to get a stable and reliable strength value to be used for partsdesign. In the case of weavable fibers forming a yarn, the interfiber voids are a fewmicrons or less in size, which makes infiltration of liquid or gaseous precursorsmandatory. 12,23,67 In many cases, interyarn voids (1 to 100 mm 3 ) of textile preformsare accessible only through interfiber voids close to 1 to 5 µm. Chemical vaporinfiltration (CVI), a method based on the infiltration of gaseous precursors, is moreexpensive because of the duration of the synthesis cycle and the need for specifictools for each cycle. 68,69 Furthermore, gaseous precursors are not well suited for themulticomponent oxide compositions required for functional composites. The presenceof long-fiber geometrically invariant reinforcement inhibits the coherent shrinkage ofthe matrix. In the case of one-dimensional or two-dimensional (textile) reinforcements,this phenomenon can be solved by hot-pressing: the in-plane shrinkage canbe counterbalanced by the thickness reduction if a viscous behavior is achieved duringthe hot-pressing. 67 A flowchart for this process is summarized in Figure 12.8.AlkoxidesM(OR) nPrepreg routeFabricsUltrafine powderImpregnation+in situ gelationDepositHot-pressingStackingFIGURE 12.8 Schematic of the sol-gel process for the fabrication of woven fabricsreinforced CMCs. 87
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