Volume defects precipitates

Figure 1.1 Defects in crystalline solids (a) point defects (interstitials) (b) a linear defect (edge dislocation) (c) a planar defect (antiphase boundary) (d) a volume defect (precipitate) (e) unit cell (filled) of a structure containing point defects (vacancies) and (/) unit cell (filled) of a defect-free structure containing ordered vacancies. ...

Three-dimensional (volume) defects—point defect clusters, voids, precipitates. 

Apart from these, there are volume defects that cannot conveniently be described in any other terms. The most important of these consist of regions of an impurity phase—precipitates—in the matrix of a material (Fig. 3.39). Precipitates form in a variety of circumstances. Phases that are stable at high temperatures may not be stable at low temperatures, and decreasing the temperature slowly will frequently lead to the formation of precipitates of a new crystal structure within the matrix of the old. Glasses, for example, are inherently unstable, and a glass may slowly recrystallize. In this case precipitates of crystalline material will appear in the noncrystalline matrix. 

The concept of a defect has undergone considerable evolution over the course of the last century. The simplest notion of a defect is a mistake at normal atom site in a solid. These stmcturally simple defects are called point defects. Not long after the recognition of point defects, the concept of linear defects, dislocations, was invoked to explain the mechanical properties of metals. In later years, it became apparent that planar defects, including surfaces, and volume defects such as rods, tubes, or precipitates, also have important roles to play in influencing the physical and chemical properties of the host matrix. More recently, it has become apparent that interactions between point defects are of considerable importance, and the simple model of isolated point defects is often inadequate with... 

Volume defects include voids and local regions of different phases, such as a precipitate or an amorphous phase. In Si, oxygen precipitation is the most important volume defect. Silicon crystals are grown by either the Czochraski technique or by the float-zone technique. The typical concentration of oxygen in Czochraski Si crystals is about 10-20 ppm (parts per million) or 5 x 1017-1 x 1018 cm-3. The float-zone technique introduces less oxygen in Si than does the Czochraski technique. Most of the oxygen in the as-grown crystal is atomically... 

In summary, we ask again what is special for ceramics— why don t we do this for metals The special feature is the formation of defect associates as a result of the strong attraction between oppositely charged point defects. Of course, defect associates and precipitates are related, and both are clearly volume defects. 

Finally, voids, pores and precipitates are also defects that interrupt the periodicity of a crystal and are known as volume defects . This chapter begins with various point defects followed by lattice defects no extensive coverage of surface or volume defects is included here. 

Finally, volume defects D = 3 can be formed by the inclusion of mother liquor (Figure 2.14). It is interesting to note that the Uquid inclusions can be confined by well-expressed low-indexed (negative) faces. Liquid inclusions, for example, occur under high growth rates, for example, for precipitations. 

Irradiation effects and microstructural changes in Generation IV reactor materials have been discussed in this chapter. The role of irradiation-induced point, hne, and volume defects in performance of steels has been discussed and radiation-induced segregation and precipitation mechanisms have been dehneated. New characterization techniques recently deployed in the nuclear materials field have been introduced and advantages and limitations of each technique have been provided. 

Volume defects consist of inclusions or precipitates of a second phase material or voids. Voids can be formed by vacancy clusters or from the nucleation of bubbles from dissolved gases or from components with high vapor pressures. Such defects can range in size from microscopic to gross. Bear in mind that not all such defects are unwanted. Many are purposely introduced into the final solid to tailor certain electrical, optical, and magnetic properties, or to serve as strengthening mechanisms. These topics are discussed in later chapters. 

Volume defects in the form of a second phase that is purposely added or precipitated from a supersaturated solid solution are often used to improve the mechanical properties. Voids due to the clustering of vacancies or from trapped gases are examples of unwanted 3-D defects. 

The last class of defects considered here are volume defects. These are due to precipitates and domains of materials different from the matrix in which they lie. There is little new to add concerning these as the majority of the problems associated with them are due to interface states at the boundary between one material and another. An interface between different materials, even if perfect structurally, will generally have a contact potential that will produce an electric field and trap one type of carrier. If it has a lower energy gap it may trap both types of carriers. Such a second-phase region is used to advantage in a laser diode, in which the active quantum well traps both types of carriers (see Chapter 3). 

Factors causing low colloid formation (low specific activity) are primarily related to pH, incorrect order of mixing, low heating temperature, heating a large volume, inadequate boiling time, or a defect of kit formulation. Tc eluate used for colloid preparation should be obtained from a generator by daily elution in order to minimize Tc carrier (Ponto et al. 1987). A flocculent precipitate is formed in the presence of cation (1 pg/ml) (Haney et al. 1971 Ponto et al. 1987). 

Whereas film-edge stresses are very localized and tend to create high densities of dislocations in rather small volumes, other sources of stress may afflict the whole volume of the wafer, such as stresses introduced by temperature gradients.Then defects in any location within the wafer may act as dislocation sources, in particular SiOx-precipitates and stacking faults. This may result in warpage or slippage 713/ which renders the whole wafer useless. 

In the bulk phase-separation approach, an organic solution of a polymer dissolved in a water-miscible solvent is injected into the tissue defect. After injection, the solvent diffuses away from the injection site, resulting in precipitation of the water-insoluble polymer. Selection of an appropriate solvent, which must be non-cytotoxic and not harmful to host tissue, is a key factor for success of the bulk phase-separation system. Two solvents that meet these criteria are N-methyl-2-pyrrolidone (NMP) and dimethyl sulfoxide (DMSO). In recent years, improved strategies for removal of the solvent and release of growth factors have been active areas of investigation. However, the requirement of a solvent to induce phase separation of the polymer limits the scale at which this approach can be applied in vivo. Even for relatively biocompatible solvents such as NMP and DMSO, injection of large volumes is anticipated to adversely affect host tissue, as well as the ability to eliminate the solvent from the body. 

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