Amorphous ceramics

Products Polymer Semiceramics Ceramics (amorphous) Ceramics (P-sic)... 

Cylindrically converging shock waves on powders were used to make mixtures of diamonds and hBN. BN fiber reinforced Zr02 was described . A nanostructured composite of magnetic particles of FOj N in a nonmagnetic matrix of BN is made via an inorganic geP. Fabrication of BN-B4 composites was reported . Consolidation of novel sintered composites formed from high pressure crystallization of amorphous ceramics was also described. The literature discusses other ceramics reinforced with BN fibers, as welF . ... 

Furthermore, the lack of long-range order in amorphous ceramics results in more phonon scattering than in crystalline solids and consequently leads to lower values of Ath-... 

It is the authors opinion that one has to concede that structures of amorphous ceramics are also possible which are not related to structures of corresponding supercooled liquids ... 

The arrangement of the atoms or ions in the material also needs to be considered. Crystalline ceramics have a very regular atomic arrangement whereas in noncrystalline or amorphous ceramics (e.g., oxide glasses) there is no long-range order, although locally we may identify similar polyhedra. Such materials often behave differently relative to their crystalline counterparts. Not only perfect lattices and ideal structures have to be considered but also the presence of structural defects that are unavoidable in all materials, even the amorphous ones. Examples of such defects include impurity atoms and dislocations. 

As far as the final heat treatment step is concerned, it is well known that the maximum pyrolysis temperature governs the densification of the amorphous ceramic phase and its crystallization process [44]. The densification of polycarbosi-lane-derived amorphous silicon carbide was studied [17,45] in the temperature... 

Recent studies have shown that incorporation of boron element into sihcon-based ceramics increases their thermal stability and retard crystallization [202-204]. For example, the materials of the binary system Si—N start to crystallize at 7 =1,000 °C forming a-Si3N4, while metastable solid solutions of the ternary and quaternary systems Si—C—N and Si—B—C—N withstand crystallization up to 1,450 and 1,700 °C, respectively [205]. In order to form an amorphous uniform phase in the final multinary ceramics, the ceramic elements are preferably distributed homogeneously in the preceramic polymers. The general consensus in the ceramics community is that the quaternary system Si—B—C—N as well as the ternary systems Si—B—N and Si—B—C would be particularly suitable for producing amorphous ceramics that resist the microstructural changes even at top loads. 

Pyrolysis of poly(organoborosilazane) (entry 6) under argon at 1,050 °C gives an amorphous ceramics, which resist crystallization up to 1,700 °C and thermally degradation up to 2,200 °C [210]. It should be noticed that the ratio of ceramic elements (B Si N) in the ceramic chars is about the same as that in the polymer precursors, illustrating the importance to control the ratio of ceramic elements in the preceramic polymers. Ceramic fibers can be obtained from this type of polymer for high temperature application... 

Controlled hydroboration of [—MeSi(Vi)—NH—] with BH3 leads to precursors with different content of boron (entry 10), which are then converted to ceramics at 1,400 °C [218]. The thermal stability of the obtained amorphous ceramics is strongly dependent on the boron content. The boron-free ceramics (sample D), which is obtained from the pyrolysis of the parent polymer [—MeSi(Vi)—NH—] , decomposes at about 1,500 °C. The decomposition temperatures of the boron-modified ceramics E and F are raised to 1,650 °C and 1,900 °C, respectively, showing that boron not only retards the crystallization of SiC and Si3N4 but also protects the thermodynamically not stable Si3N4 against decomposition at elevated temperature. 

Ceramics tend to possess ionic and covalent bonding, and Si02, for example, can exist in either form—the crystalline form occurs when Si02 melt is slowly cooled from above the mp (1723°C), while the amorphous form is obtained if rapid cooling is applied. The type of bonding formed affects the properties (e.g. amorphous ceramics are poorer conductors of heat due to lack of an ordered lattice). 

Important amorphous ceramics are the glass-Uke materials. A schematic stress-strain curve is indicated in Fig. 1.1b and shows a lower overall stress—strain curve. However, it would be difficult to enumerate aU the many types of glass with a wide variety of compositions, ranging from the most common window glass to the various metallic glasses. In general however, internal and external factors influence the performance of ceramic materials even to the point at which ductility can be induced. Here are some of these factors, especially those that have critical effects on ceramic (and glass) behavior ... 

Point Defects in Amorphous Ceramics and Their Strengthening (Effect)... 

The most representative amorphous ceramics are the various glasses, among them the well-known silica glass. There are various silicate ceramics, such as oxide and halide glasses. Amorphous ceramics can be obtained by several means ... 

The properties are related to the atomic and molecular structure of the materials. For example, we could compare iron, glass, salt, and polyethylene plastics. They represent a metallic crystalline solid, a vitrified amorphous ceramic, a nonmetallic crystalline solid, and an amorphous high polymer. Figures 1-1, 1-2, 1-3, and 1-4 show the structures in schematic form for each of these materials. 

The results were consistent with a diffusion mechanism that was mediated by vacancy-like defects in the amorphous ceramics. 

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