Swirl Formation Research Articles

Numerical simulation of air motion in the swirl chamber of diesel engines. The effect of swirl chamber geometry and the connecting passage.

To clarify the flow in the swirl chamber of a diesel engine, numerical prediction and measurement were performed. The flow model was based on the solution of the finite difference form of the governing differential equations for the transport of mass and momentum, including the concept of the turbulent effective viscosity. To facilitate flow analysis and measurement, swirl chamber flow was assumed to be two-dimensional. Special emphasis was placed on the effects of swirl chamber geometry and the connecting passage. The predicted results were confirmed in experiments using a special air model. Then, flow characteristics in the swirl chamber of a full scale engine were predicted. The results show that swirl chamber geometry and the connecting passage have significant effects on the flow characteristics, such as the tangential velocity and the swirl formation.

Swirl defects in float-zoned silicon crystals

Microdefects in striated (swirls: A and B-defects) and non-striated distribution (D-defects) were observed in dislocation-free crystals with relatively large diameters by x-ray topography following copper decoration. Impurity effect on formation of both the defects was studied by doping with B, C, P and O impurities: Swirl formation is enhanced by doping with by B, C and P, and hindered by O impurity. Whereas, D-defect formation was suppressed by C impurity, and their distribution was striated by O impurity. Comparison of D-defects and swirl defects (interstitial type dislocation-loops) led to the inference that D-defects are clusters of frozen-in vacancies. It was found that the temperature gradient at the interface associated with the growth rate is the critical condition for formation of both the defects rather than the cooling rate. The swirl formation is explained by two actions of the high temperature gradient at the interface: (1) It produces a high stress field to expand their sizes. (2) It decreases concentration of vacancies by out-diffusion to lower the effect by mutual annihilation of interstitials and vacancies.

(Invited) Swirl Defects in Silicon Crystals

The current understanding of microdefects in a striated distribution (swirls) in dislocation-free crystals is reviewed with focusing on their formation mechanism. In float-zoned crystals, their formation is due to clustering of self-interstitials and/or impurity segregation which depend upon both growth conditions and cooling processes. A model for the origin of self-interstitials is proposed which is based on the in-situ X-ray topographic observation that liquid drops are formed inside of crystals by superheating and microdefects are generated by their solidification. A systematic presentation of the swirl formation is attempted in the basis of this model, in contrast with other models of the non-equilibrium incorporation of interstitials at the growth interface and the predominant equilibrium existence of interstitials near the melting point. For Czochralski-grown crystals, the nucleation center of oxygen precipitates is discussed in comparison with the swirl formation of float-zoned crystals.

The swirl formation of defects in Czochralski-grown silicon crystals

Swirl defects in dislocation-free Czochralski silicon were studied by means of X-ray topography cooperated with two annealing processes. After the first annealing in N 2 for 2 h, the Pendellösung fringes of the section topographs were clearly observed as if the crystals were perfect. The second annealing in steam at 1200°C decorated the defects so as to reveal the configuration in the stage of the first annealing. When the temperature of the first annealing was sufficiently high (above ca. 800°C), some of the grown-in defects migrated and aligned along the growth layers. When the temperature was below ca. 800°C, they could not migrate so much that they remained along the crystal pulling direction as observed in as-grown crystal. The configuration of the decorated defects corresponded to the swirl pattern revealed by the Sirtl etching.

Effect of low cooling rates on swirls and striations in dislocation-free silicon crystals

Abstract The occurrence of swirls and the amplitude of dopant striations in dislocation-free silicon is shown to be strongly influenced by the cooling rate during crystal growth. It has been found that the formation of swirls can be prevented if the cooling rate does not exceed 5°C/min. It is argued that at this cooling rate the concentration of thermal defects decreases below the critical value necessary for swirl formation by diffusion to the crystal surface. In addition the amplitude of dopant striations is reduced by solid-state diffusion, which permits the growth of more homogeneous silicon crystals.

Formation and nature of swirl defects in silicon

Point defect agglomerates in dislocation-free silicon crystals, usually called “swirls”, have been investigated by means of high-voltage electron microscopy. It was found that a single swirl defect consists of a dislocation loop or a cluster of dislocation loops. By contrast experiments it could be shown that these loops are formed by agglomeration of self-interstitial atoms. Generally the loops have a/2〈110〉 Burgers vectors, but in specimens with high concentrations of carbon (∼1017 cm−3) and oxygen (∼1016 cm−3) also dislocation loops including a stacking fault were observed. In crystals grown at growth rates higher thanv=4 mm/min no swirls are observed; lower growth rates do not markedly affect the size and shape of the dislocation loops. With decreasing impurity content (particulary of oxygen and carbon) the swirl density decreases, whereas the dislocation loop clusters become larger and more complex. A model is presented which describes the formation of swirls in terms of agglomeration of silicon self-interstitials and impurity atoms.