Second phase particles oxides

The most common bulk defect arises from foreign particles being included in the prime material. These second-phase particles, called inclusions, are seldom wanted because they significantly alter the structural properties. An example of an inclusion may be oxide particles in a pure metal or a bit of clay in a glass structure. 

Although the findings described above regarding creep in fine-grained polycrystals have been limited to oxides, it is anticipated that analogous effects are possible for non-oxide ceramics. For example, certain additives and second-phase particles (e.g., Ti, C, and B4C) enhance the creep strength of polycrystalline SiC, while others (e.g., B and Al) degrade it (recall Chapter 3). The mechanisms, however, are not understood. 

The case study demonstrates that even the base NiCr hybrid spray coatings outperformed the alloy 625 coatings in the presence of corrosive salts. The presence of second phase particles, especially Si02, showed improved oxidation and corrosion characteristics. Incorporation of ultrafine and nano sized oxide p>articles is expected to improve the creep properties by pinning the splat boundaries and reduce the oxidation rate. Chromia addition by itself did not help improve the properties significantly. However, it could act as a... 

Adding inert second phase particles results in dispersion hardening. Submicron oxide or carbide particles act as pinning sites that block the motion of dislocations. Intermetallic phases can be made to form, which not only block the motion of dislocations, but also help to stabilize grain boimdaries. The hardening effect of a dispersed phase is not as effective as a coherent precipitated phase, but dispersion hardening can be used in systems that do not... 

Pitting arises when localized/aggressive environments break down the nominally passive and corrosion-resistant film. Pits may form at scratches, mechanical defects, second phase particles, or stochastic local discontinuities in the oxide film. Since by definition pitting refers to local loss passivity, pitting only occurs in the near-neutral pH range for Al. 

Figure 1 shows the powder X-ray diffraction (XRD) pattern of the as-prepared Li(Nio.4Coo.2Mno.4)02 material. All of the peaks could be indexed based on the a-NaFeC>2 structure (R 3 m). The lattice parameters in hexagonal setting obtained by the least square method were a=2.868A and c=14.25A. Since no second-phase diffraction peaks were observed from the surface-coated materials and it is unlikely that the A1 ions were incorporated into the lattice at the low heat-treatment temperature (300°C), it is considered that the particle surface was coated with amorphous aluminum oxide. 

Well-established anode materials are Ni cermets such as Ni/YSZ composites. The presence of the second phase increases the contact area and prevents the catalytically active Ni particles from aggregating. The use of the composite becomes problematic if hydrocarbons are to be directly converted Ni catalyzes cracking, and the resulting carbon deposition deactivates the fuel cells. Therefore either pure H2 has to be used or the fuel has to be externally reformed. A third way is internal conversion of CHV with H20 to synthesis gas. The necessary steam addition, however, reduces the overall efficiency. Another problem of Ni cermets, if they are to be used at lower temperatures, is a potential oxidation of the Ni. Alternatives are Cu/Ce02 cermets in which Cu essentially provides the electronic conductivity and Ce02 the catalytic activity. Note that an efficient current collecting property of the electrode presupposes a metal concentration above the percolation threshold. 

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