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Growth and dislocation studies of β-HMX.

Gallagher HG, Sherwood JN, Vrcelj RM - Chem Cent J (2014)

Bottom Line: Prismatic habit was favoured at low supersaturation, while tabular and columnar crystals were predominant at higher super saturations.The twin plane in β-HMX was identified as a (101) reflection plane.Graphical abstractEtch pits on the twinned (010) face of β-HMX.

View Article: PubMed Central - PubMed

Affiliation: WESTCHEM, Department of Pure and Applied Chemistry, University of Strathclyde, Thomas Graham Building, 295 Cathedral Street, Glasgow, G1 1XL UK.

ABSTRACT

Background: The defect structure of organic materials is important as it plays a major role in their crystal growth properties. It also can play a subcritical role in "hot-spot" detonation processes of energetics and one such energetic is cyclotetramethylene-tetranitramine, in the commonly used beta form (β-HMX).

Results: The as-grown crystals grown by evaporation from acetone show prismatic, tabular and columnar habits, all with {011}, {110}, (010) and (101) faces. Etching on (010) surfaces revealed three different types of etch pits, two of which could be identified with either pure screw or pure edge dislocations, the third is shown to be an artifact of the twinning process that this material undergoes. Examination of the {011} and {110} surfaces show only one type of etch pit on each surface; however their natural asymmetry precludes the easy identification of their Burgers vector or dislocation type. Etching of cleaved {011} surfaces demonstrates that the etch pits can be associated with line dislocations. All dislocations appear randomly on the crystal surfaces and do not form alignments characteristic of mechanical deformation by dislocation slip.

Conclusions: Crystals of β-HMX grown from acetone show good morphological agreement with that predicted by modelling, with three distinct crystal habits observed depending upon the supersaturation of the growth solution. Prismatic habit was favoured at low supersaturation, while tabular and columnar crystals were predominant at higher super saturations. The twin plane in β-HMX was identified as a (101) reflection plane. The low plasticity of β-HMX is shown by the lack of etch pit alignments corresponding to mechanically induced dislocation arrays. On untwinned {010} faces, two types of dislocations exist, pure edge dislocations with b = [010] and pure screw dislocations with b = [010]. On twinned (010) faces, a third dislocation type exists and it is proposed that these pits are associated with pure screw dislocations with b = [010]. Graphical abstractEtch pits on the twinned (010) face of β-HMX.

No MeSH data available.


Related in: MedlinePlus

Etch pits on the (010) face of β-HMX. (a) Optical and (b) interference micrograph, showing type 1 and type 2 etch pits.
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Fig3: Etch pits on the (010) face of β-HMX. (a) Optical and (b) interference micrograph, showing type 1 and type 2 etch pits.

Mentions: Figure 2 shows the general distribution of etch pits on a typical (010) habit face after etching. In detail there are two basic types of pit (Figure 3). Pits of type 1 are smaller in size and appear light in contrast, whereas pits of type 2 are larger and darker. The former type of pit is by far the most predominant. The surface outline of both types of pits are identical, having the shape of a parallelogram with the longer edge aligned along the [100] direction. The measured length to width ratio of almost all the pits had a value of around 2.7:1. In terms of the unit cell, this corresponds to the removal of three unit cells along [100] for every one unit cell in the [001] direction. The three dimensional geometry of the etch pits was revealed using optical and interference microscopy as illustrated in Figure 3. Pits of type 1 exhibit only a few fringes indicating that these are shallow and are bounded by surfaces with high index orientations. In contrast, pits of type 2 show more fringes (thus appear darker) and are therefore much deeper. The small spacing and even distribution of the fringes implies that the sides are steep and planar. The pointed bottom of each pit (type 1 and type 2) coincides with the geometric centre indicating that the associated dislocations emerge normal to the surface. Etch pit densities for different crystals varied in the range 105-108 cm−2 for pits of type 1 and 102-103 cm−2 for pits of type 2.Figure 2


Growth and dislocation studies of β-HMX.

Gallagher HG, Sherwood JN, Vrcelj RM - Chem Cent J (2014)

Etch pits on the (010) face of β-HMX. (a) Optical and (b) interference micrograph, showing type 1 and type 2 etch pits.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4317510&req=5

Fig3: Etch pits on the (010) face of β-HMX. (a) Optical and (b) interference micrograph, showing type 1 and type 2 etch pits.
Mentions: Figure 2 shows the general distribution of etch pits on a typical (010) habit face after etching. In detail there are two basic types of pit (Figure 3). Pits of type 1 are smaller in size and appear light in contrast, whereas pits of type 2 are larger and darker. The former type of pit is by far the most predominant. The surface outline of both types of pits are identical, having the shape of a parallelogram with the longer edge aligned along the [100] direction. The measured length to width ratio of almost all the pits had a value of around 2.7:1. In terms of the unit cell, this corresponds to the removal of three unit cells along [100] for every one unit cell in the [001] direction. The three dimensional geometry of the etch pits was revealed using optical and interference microscopy as illustrated in Figure 3. Pits of type 1 exhibit only a few fringes indicating that these are shallow and are bounded by surfaces with high index orientations. In contrast, pits of type 2 show more fringes (thus appear darker) and are therefore much deeper. The small spacing and even distribution of the fringes implies that the sides are steep and planar. The pointed bottom of each pit (type 1 and type 2) coincides with the geometric centre indicating that the associated dislocations emerge normal to the surface. Etch pit densities for different crystals varied in the range 105-108 cm−2 for pits of type 1 and 102-103 cm−2 for pits of type 2.Figure 2

Bottom Line: Prismatic habit was favoured at low supersaturation, while tabular and columnar crystals were predominant at higher super saturations.The twin plane in β-HMX was identified as a (101) reflection plane.Graphical abstractEtch pits on the twinned (010) face of β-HMX.

View Article: PubMed Central - PubMed

Affiliation: WESTCHEM, Department of Pure and Applied Chemistry, University of Strathclyde, Thomas Graham Building, 295 Cathedral Street, Glasgow, G1 1XL UK.

ABSTRACT

Background: The defect structure of organic materials is important as it plays a major role in their crystal growth properties. It also can play a subcritical role in "hot-spot" detonation processes of energetics and one such energetic is cyclotetramethylene-tetranitramine, in the commonly used beta form (β-HMX).

Results: The as-grown crystals grown by evaporation from acetone show prismatic, tabular and columnar habits, all with {011}, {110}, (010) and (101) faces. Etching on (010) surfaces revealed three different types of etch pits, two of which could be identified with either pure screw or pure edge dislocations, the third is shown to be an artifact of the twinning process that this material undergoes. Examination of the {011} and {110} surfaces show only one type of etch pit on each surface; however their natural asymmetry precludes the easy identification of their Burgers vector or dislocation type. Etching of cleaved {011} surfaces demonstrates that the etch pits can be associated with line dislocations. All dislocations appear randomly on the crystal surfaces and do not form alignments characteristic of mechanical deformation by dislocation slip.

Conclusions: Crystals of β-HMX grown from acetone show good morphological agreement with that predicted by modelling, with three distinct crystal habits observed depending upon the supersaturation of the growth solution. Prismatic habit was favoured at low supersaturation, while tabular and columnar crystals were predominant at higher super saturations. The twin plane in β-HMX was identified as a (101) reflection plane. The low plasticity of β-HMX is shown by the lack of etch pit alignments corresponding to mechanically induced dislocation arrays. On untwinned {010} faces, two types of dislocations exist, pure edge dislocations with b = [010] and pure screw dislocations with b = [010]. On twinned (010) faces, a third dislocation type exists and it is proposed that these pits are associated with pure screw dislocations with b = [010]. Graphical abstractEtch pits on the twinned (010) face of β-HMX.

No MeSH data available.


Related in: MedlinePlus