Twin Plane Re-entrant Edge Mechanism Research Articles

Growth kinetics of primary Si particles in hypereutectic Al-Si alloys under the influence of P inoculation: Experiments and modelling

A quantitative study on the growth kinetics of primary Si particles (PSPs), especially under the effect of P inoculation, during isothermal melt solidification of hypereutectic Al-Si-Cu alloys has been realized by using a unique in-situ micro-focus X-radiography method. The morphology evolution has been captured by in-situ observation and the growth velocity has been measured. It is found that P inoculation reduces the branching ability and the growth velocity of PSPs. As a result, the morphologies were changed from star-like, interconnecting multi-plate shapes into more compact, block-shaped and individual plate-shaped by P addition. A post-situ crystallographic study of PSPs by electron backscatter diffraction (EBSD) shows that the growth of plate-shaped PSPs in both alloys is through the twin-plane reentrant-edge mechanism, while block-shaped particles in the P inoculated alloy do not show any preferential growth. A simple growth model has been developed to simulate the growth kinetics of plate-shaped Si particles. A comparison between the simulated and measured growth velocity indicates that growth of plate-like particles is close to the diffusion-controlled lengthening kinetics at high undercooling while the effect of P addition in reducing growth velocity of PSP plates is due to the lower nucleation undercooling and stronger solute impingement effect caused by nucleation of high density PSPs.

Crystallization kinetics in Si-1 at%Sn during rapid solidification in undercooled melt

In order to elucidate the cause of the morphological transition of crystals growing in an undercooled melt of semiconducting materials, we carried out the containerless solidification of undoped Si and Si-1at%Sn using a CO2 laser-equipped electromagnetic levitator (EML). The crystallization of these materials was successfully achieved under controlled undercooling. The relation between the shape of growing crystals and the degree of undercooling in Si-1at%Sn was similar to that in undoped Si; that is, plate-like needle crystals were observed at low undercooling, whereas at medium and high undercooling the shape of growing crystals changed to massive dendrites. The grain-size of as-solidified samples of Si-1at%Sn was remarkably small compared with that of undoped Si. The surface morphologies of samples solidified by dropping the melt onto a chill plate of mirror-polished silicon consisted of typical twin-related <110> dendrites. On the other hand, samples that were dropped from the undercooled state consisted of twin-free <100> dendrites. The nucleation rate of two-dimensional nuclei calculated on the basis of two mechanisms, which are the twin-plane re-entrant edge mechanism and the twin-free mechanism, suggested that the morphological transition to twin-free <100> dendrites from twin-related <110> dendrites occurs when the degree of undercooling becomes larger than the critical value. These results indicate that the cause of the morphological transition of Si growing in the undercooled melt is not the roughening transition of the crystal–melt interface but the transition of the nucleation kinetics to the twin-free mechanism from the twin-related mechanism.

Synthesis and structural property of Si nanosheets connected to Si nanowires using MnCl2/Si powder source

Si nanosheets connected to Si nanowires were synthesized using a MnCl2/Si powder source with an Au catalyst. The synthesis method has benefits in terms of avoiding conventionally used air-sensitive SiH4 or SiCl4. The existence of the Si nanosheets connected to the Si<111> nanowires, like sprouts or leaves with petioles, was observed, and the surface of the nanosheets was Si{111}. The nanosheets were grown in the growth direction of <211> perpendicular to that of the Si nanowires. It was evident from these structural features of the nanosheets that the nanosheets were formed by the twin-plane reentrant-edge mechanism. The feature of the observed lattice fringes, which do not appear for Si bulk crystals, of the Si(111) nanosheets obtained by high resolution transmission electron microscopy was clearly explained due to the extra diffraction spots that arose by the reciprocal lattice streaking effect.

The roles of Eu during the growth of eutectic Si in Al-Si alloys.

Controlling the growth of eutectic Si and thereby modifying the eutectic Si from flake-like to fibrous is a key factor in improving the properties of Al-Si alloys. To date, it is generally accepted that the impurity-induced twinning (IIT) mechanism and the twin plane re-entrant edge (TPRE) mechanism as well as poisoning of the TPRE mechanism are valid under certain conditions. However, IIT, TPRE or poisoning of the TPRE mechanism cannot be used to interpret all observations. Here, we report an atomic-scale experimental and theoretical investigation on the roles of Eu during the growth of eutectic Si in Al-Si alloys. Both experimental and theoretical investigations reveal three different roles: (i) the adsorption at the intersection of Si facets, inducing IIT mechanism, (ii) the adsorption at the twin plane re-entrant edge, inducing TPRE mechanism or poisoning of the TPRE mechanism, and (iii) the segregation ahead of the growing Si twins, inducing a solute entrainment within eutectic Si. This investigation not only demonstrates a direct experimental support to the well-accepted poisoning of the TPRE and IIT mechanisms, but also provides a full picture about the roles of Eu atoms during the growth of eutectic Si, including the solute entrainment within eutectic Si.

Revisiting the Twin Plane Re-entrant Edge Growth Mechanism at an Atomic Scale by Electron Microscopy

We conducted an extensive electron microscopy study on surface and defect structures of boron suboxide/suboxycarbide platelets by examining them under various imaging conditions, e.g., side-view and top-view perspectives. It was determined that a twin plane re-entrant edge mechanism was responsible for the growth process at an atomic scale. Moreover, this thorough investigation provided an opportunity to resolve several critical issues regarding this otherwise well-known growth mechanism for metallic nanostructures. In this study, the platelets contained multiple {001} twin lamellae parallel to basal planes and their side faces were mainly enclosed by {101} facets. Vertical growth was heterogeneously nucleated of {001}-type growth twins that were confined at the corners. In lateral growth, nucleation sites were greatly extended to twin re-entrant edges around all side faces. No secondary growth twins were introduced during lateral growth because {101}-type side faces were not twin planes. This work clearly establishes that surface structures of twinned platelets determine nucleation behaviors on basal/side faces and thus control the final morphology, which is relevant to the shape control of metal nanoplates.

Modification of Eutectic Si in Al-Si Based Alloys

The paper provides a new insight into the modification of eutectic Si in Al-Si based alloys. To date, impurity-induced twinning mechanism and twin plane re-entrant edge mechanism are the well-accepted theories. However, neither IIT nor TPRE can be used to interpret all modification observations. Therefore, a re-consideration of modification mechanisms is still required. In this contribution, recent advances on the understanding the modification of eutectic Si are reviewed. Two different cases are highlighted. In the case of Sr, Na and Eu addition, eutectic Si was modified from a faceted to a fibrous morphology, which involves the formation of multiple Si twinning. In the case of Yb and Ca addition, eutectic Si was refined to a smaller size, but still maintained a plate-like morphology. The possible modification mechanism was thus discussed in terms of (i) adsorption of atoms at twin re-entrant edge, and (ii) segregation across {111}Si growth planes. Furthermore, solute entrainment of modifying elements (M) was introduced to interpret the formation of Al2Si2M phase or M-rich clusters within Si crystals.

Growth Characteristics of CVD Beta‐Silicon Carbide

The films of were prepared from the methyltrichlorosilane (MTS) by low pressure chemical vapor deposition onto the graphite substrates. The results revealed that the growth rate of increased with the MTS flux; besides, the growth rate increased to a maximum and then decreased with increasing deposition temperature. The preferred orientation of the films was examined by x‐ray diffraction, and was found to exhibit a (220) texture in the films of high growth rate and/or long deposition time. The structural morphology was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Observed features indicated the twin plane reentrant edge (TPRE) mechanism as responsible for the growth. The growth features of particulate crystals (e.g., dendrites, whiskers, needles) were identified, and also could be interpreted in terms of the TPRE mechanism.

Crystallography of the Al-Al 3 Fe eutectic

Crystallographic characteristics and limits of stability of the Al–Al 2Fe abnormal eutectic microstructure have been observed. When the G R ratio is low, Al 3Fe grows as frequently branched, narrow platelets in a dendritic aluminium matrix. When G R > 1000°C sec/mm 2 branching is largely eliminated and [010]Al 3Fe ∥ [100]Al ∥ growth direction. The Al 3Fe platelets grow in the [010] direction by a twin plane re-entrant edge mechanism in which grooves between faceting {111} planes are formed between multiple (001) twins. (001) twins were observed at 50–200 Å intervals for all freezing conditions, and (100) twins were also observed at higher values of G R .

Effect of growth parameters on habit and morphology of electrodeposited lead dendrites

Changes in habit and morphology of lead dendrites grown by electrodeposition from aqueous solutions were studied as a function of various growth parameters. Changes resulting from current density and concentration of the electrolyte are given in chart form. The following changes in morphology were observed in order of increasing current density, (a) polyhedral growth, (b) dendrites growing in the 〈110〉 directions, (c) acicular dendrites growing in the 〈110〉 directions, (d) dendrites growing in the 〈211〉 directions, (e) dendrites growing in the 〈100〉 directions, and (f) spongy growth. Not all of these morphologies were found, however, at each concentration. Dendrites were faceted in all cases by {111} planes. (a), (e), and (f) did not contain twin planes. Dendrites in (b) and (d) contained twin planes parallel to the main faces, while those in (c) had twin planes radiating from a common line. Evidence suggests that the dendrites containing twin planes grow by the twin plane re-entrant edge mechanism.

The dendritic growth of lead from gels

Lead dendrites were grown from gels, and their habit, morphology, and internal structure were studied. Growth was found to proceed in three stages. The first stage showed mainly single crystal dendrites growing in the 〈100〉 direction while the second and third stages gave dendrites growing in the 〈211〉 direction. All dendrites were faceted by {111} planes. The dendrites from the second and third stages were shown to grow by the twin plane re-entrant edge mechanism

The growth of semiconductor crystals from solution using the twin-plane reentrant-edge mechanism

Experimental conditions are described for growing crystals of diamond lattice and zinc blende semiconductors from metal solutions using the twin plane reentrant edge mechanism. In addition to the expected < 211 > type twinned platelets, some unusual < 110 > extended forms with intersecting twin configurations were obtained. Etch techniques were used to study microresistivity striations and to determine the growth history of the platelets. The relative number of nucleation events at different sites and the possible effect of polarity on the growth of III-V compound platelets are discussed. Electrical measurements are reported on some of the platelets.

The growth of semiconductor crystals from solution using the twin plane reentrant-edge mechanism

The growth of semiconductor crystals from solution using the twin plane reentrant-edge mechanism