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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.


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Interference micrograph of etch pits on the (010) face of growth twinned β-HMX.
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Fig5: Interference micrograph of etch pits on the (010) face of growth twinned β-HMX.

Mentions: Typical etch pit patterns on growth twinned {010} facets are shown in Figure 4. Three types of pit geometry are in evidence. Pits of type 1 and type 2 are associated with the untwinned regions of the crystal and are described above. Type 3 pits only occur along the surface trace of the composition plane of the twin. They have a kite-shaped surface outline which shows mirror symmetry associated with twinning about the (101) plane. The edges of the pits are parallel to the [100] and [001] directions. Optical and interference microscopy (Figure 5) indicates that the pits are deep with high index planar sides and the apex lying in the twin plane. This fact, together with pit symmetry, suggests that the dislocation line must lie in the twin boundary. Figure 6 shows how the basic pit geometry can be reconstructed from a type 1 or type 2 pit modified by reflection in the (101) mirror plane of the twin. The additional etching of the tail section that is observed probably arises from the tendency for dissolution to take place at the re-entrant surfaces, caused by the close proximity of the twin, in order to achieve a minimum energy configuration. The dimensions of the pit edges parallel to [100] and [001] directions estimated from many observations expressed as a ratio is 1:1.1. This is in excellent agreement with the relative dimensions of the pits calculated from the model presented in Figure 6. The ratio corresponds to the ratio of the unit cell parameters along the a- and c-axes. It can be concluded that the apex of the pit lies at the centre of the figure and indicates that the dislocation line is normal to the surface. The etch pits are irregularly spaced along the twin plane with typically 102-103 cm−1.Figure 4


Growth and dislocation studies of β-HMX.

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

Interference micrograph of etch pits on the (010) face of growth twinned β-HMX.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Fig5: Interference micrograph of etch pits on the (010) face of growth twinned β-HMX.
Mentions: Typical etch pit patterns on growth twinned {010} facets are shown in Figure 4. Three types of pit geometry are in evidence. Pits of type 1 and type 2 are associated with the untwinned regions of the crystal and are described above. Type 3 pits only occur along the surface trace of the composition plane of the twin. They have a kite-shaped surface outline which shows mirror symmetry associated with twinning about the (101) plane. The edges of the pits are parallel to the [100] and [001] directions. Optical and interference microscopy (Figure 5) indicates that the pits are deep with high index planar sides and the apex lying in the twin plane. This fact, together with pit symmetry, suggests that the dislocation line must lie in the twin boundary. Figure 6 shows how the basic pit geometry can be reconstructed from a type 1 or type 2 pit modified by reflection in the (101) mirror plane of the twin. The additional etching of the tail section that is observed probably arises from the tendency for dissolution to take place at the re-entrant surfaces, caused by the close proximity of the twin, in order to achieve a minimum energy configuration. The dimensions of the pit edges parallel to [100] and [001] directions estimated from many observations expressed as a ratio is 1:1.1. This is in excellent agreement with the relative dimensions of the pits calculated from the model presented in Figure 6. The ratio corresponds to the ratio of the unit cell parameters along the a- and c-axes. It can be concluded that the apex of the pit lies at the centre of the figure and indicates that the dislocation line is normal to the surface. The etch pits are irregularly spaced along the twin plane with typically 102-103 cm−1.Figure 4

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