Second-Phase Particles

The presence of second-phase particles in as-grown crystals named COPs (crystal-originated particles), markedly affecting the optical and electronic bulk quality but also the surface perfection of epiready substrates, is one of the most-studied harmful defect types in compound semiconductor crystals. They are present in II-VIs (e.g. [35, 91]), III-Vs (e.g. [92, 93]) and IV-VIs (e.g. [94]). As is explained 

At faceted interface by capture of melt-solution droplets (or gas bubbles) from the diffusion boundary layer 

Flat or faceted interface Convex-concave interface 

In CdTe, both tellurium and cadmium precipitates have been found (e.g. [91, 99]). This depends on the melt composition from which the crystal was grown (Te- or Cd-rich, respectively). According to the CdTe phase diagram their density can be effectively minimized at growth under near-stoichiometric conditions (as is sketched in Fig. 3.11) [35]. Contrary to this, in melt-grown GaAs crystals only As precipitates were observed. Fornari et al. [95] found a correlation between the As precipitate size and deviation from stoichiometry. Whereas at the stoichiometric 

In contrast to precipitates, inclusions are formed by capture of melt-solution droplets, gas bubbles or foreign particles from the diffusion boundary layer adjacent to the growing interface and enriched by the rejected excess component. 

---

The following mechanisms in corrosion behavior have been affected by implantation and have been reviewed (119) (/) expansion of the passive range of potential, (2) enhancement of resistance to localized breakdown of passive film, (J) formation of amorphous surface alloy to eliminate grain boundaries and stabilize an amorphous passive film, (4) shift open circuit (corrosion) potential into passive range of potential, (5) reduce/eliminate attack at second-phase particles, and (6) inhibit cathodic kinetics. 

Specia.lty Coppers. Additions are made to copper to satisfy specific needs. Tellurium at a nominal 0.5 wt % addition, sulfur at 0.35 wt %, and lead at 1 wt % enhance machinabiHty. These alloys are identified as C145, C147, and C187, respectively. The solubiHty limit for each element is <0.001%, so that the excess is present as second-phase particles which assist in fracture of chips and lubrication during machining. 

The other alternative is to attempt to increase K -. Pure ceramics have a fracture toughness between 0.2 and 2 MPa m. A dispersion of particles of a second phase can increase this a little the advancing crack is pinned by the particles and bows between them, much as a dislocation is pinned by strong second phase particles (Chapter 10). 

POSAP is the only technique available for identifying and locating precipitates, second phases, particles, and interfaces on an atomic scale, and has therefore found considerable application in metallurgical and semiconductor problems. 

The data chain of the collected atoms can be converted to a one-dimensional composition-depth profile. The depth profile shows an average concentration of solute within the aperture, and there is always a possibility that the chemical information from the selected area is a convolution of more than one phase, as indicated diagrammatically in Figure 1.5, which represents the analysis of a FIM specimen containing second phase particles and also an interface across which there is a change of composition. 

The next section describes the processing and microstructural development of particulate composites, and is followed by a section on thermal residual stresses. These stresses are often the most obvious consequence of adding second-phase particles to a matrix and can have a profound effect on properties. Factors determining the toughness, strength and wear resistance of particulate composites are then considered in turn, and the chapter concludes with an assessment of possible future developments in this area. 

The aim of toughening a matrix by adding second-phase particles is essentially to increase its strength. For cracks in homogeneous materials under uniform... 

Another notable example of a reduction in creep rate through the addition of second-phase particles concerns nanocomposites . In alumina-SiC, systems, several investigations have reported significant reductions in creep rate compared with monolithic alumina [48, 49], Figure 4.9 shows the results of Ohji and co-workers [48], At 1200°C the creep rate of an AI203-17vol%SiC nanocomposite was less than that of alumina for a given stress by a factor of 250, and the time to rupture at 50 MPa was increased from 120 h to 1120 h. The SiC inhibits creep primarily because it is difficult to remove or deposit... 

Toughening mechanisms in a/p-sialon composites are similar to those operative in second-phase particle reinforced composites, but, rather than the deliberate addition of a second phase, a/P-sialon composites are fabricated by simultaneous crystallisation of the two solid solutions a- and P-sialon from a eutectic composition liquid. This requires careful design of the starting composition which is usually located within the (a + P)-sialon region of the a-sialon plane as illustrated in Fig. 18.1. 

In terms of fatigue, the materials science community has revealed different length scales of interest as well. Local inclusion/defects, such as pores, second phase particles, inclusions, and constituents, could induce local stress concentrations... 

A statistical framework has been established for describing the spatial dispersion of second phase particles in a continuous matrix. Based on this scheme a computerized image analysis method has been developed for characterizing the morphology of toughened plastics. 

R. Raj, Nucleation of Cavities at Second Phase Particles in Grain Boundaries, Acta Metall., 26, 995-1006 (1978). 

---