P. J. Halfpenny

Dislocations in energetic materials

An assessment has been made of the primary dislocation slip systems in the explosives pentaerythritol tetranitrate (PETN) and cyclotrimethylene trinitramine (RDX) using a combination of dislocation etching and microhardness indentation techniques. Hardness measurements were made on all major habit faces as a function of temperature and load. These showed that, within the attainable temperature range (PETN 293 to 353 K, RDX 293 to 373 K) no change in hardness occurred which could be associated with the development of deformation mechanisms additional to those operating at room temperature. The hardness values of both materials were consistent with values obtained in some previous measurements (PETN 17 kg mm−2, RDX 39 kg mm−2). Solvent etching with acetone (5 sec at 274 K) proved to be an excellent method for revealing the emergent ends of growth and mechanically induced dislocations in PETN. Etching of microhardness indentations confirmed that observable slip traces comprised dislocations. These migrated up to 160μm (20g load) from the indentation point on both {110} and {101} faces. The alignments defined a {110} primary slip plane. Parallel experiments with RDX yielded evidence of highly localized slip around the indentation mark (90μm, 50g load). Two alignments of etch pits were noticeable. The better defined of these lay at the intersection of the (010) plane with the habit faces. The second could not be defined absolutely but most probably corresponds to the intersection of either the (011) or (012) plane with the surfaces. Consideration of the Burgers vectors of dislocations which are likely to glide in these planes lead us to speculate that the primary slip systems are, PETN {110} [001], and RDX (010) [001]. Such an assignment would be consistent with the relative hardness of the two materials.

The growth of large single crystals of the organic nonlinear optical material 2-(α-methylbenzylamino)-5-nitropyridine

The crystal growth behaviour of the important organic nonlinear optical material 2-(a-methylbenzylamino)-5-nitropyridine (MBANP) (space group P21) has been investigated and techniques have been developed for the preparation of large (up to 7 cm x 4 cm x 3 cm) high-quality single crystals suitable for detailed optical, dielectric, ultrasonic and piezoelectric studies and for device fabrication. Observation of the crystal growth behaviour and measurement of growth kinetics revealed a strong and anomalous dependence of growth rate on supersaturation. At relative supersaturations less than 0.08, slow growing irregular faces were formed. Once the crystal was completely bounded by such faces, little or no further growth occurred. At supersaturations below 0.24 no growth was observed in either <010> direction. Above this, rapid, but highly imperfect, growth occurred in one <010> direction while the opposite end of the crystal exhibited zero growth rate at all achievable supersaturations. At relative supersaturations greater than 0.4 considerable instability in the growth along [010] was observed. The factors potentially responsible for this behaviour are discussed. A novel technique is described for the preparation of seed crystals elongated parallel to <010>. The use of such seeds allows the growth of large crystals of MBANP at supersaturations less than 0.24, thereby avoiding the highly imperfect and unstable growth at higher supersaturations and yielding crystals of very high quality. The nature, density and distribution of growth induced defects in large crystals of MBANP have been investigated using X-ray diffraction topography. In addition to revealing growth dislocations, growth sector boundaries, inclusions and striations, these studies provide a detailed insight into the growth history of the crystals.

Dislocations in energetic materials: IV. The crystal growth and perfection of cyclotrimethylene trinitramine (RDX)

Abstract Large (typically 10×15×20 mm 3 ) single crystals of cyclotrimethylene trinitramine (RDX) have been prepared from acetone solution by temperature lowering and solvent evaporation techniques. The crystal morphology and perfection are discussed in terms of growth conditions. The basic growth induced defect structure has been revealed by X-ray topography and has been found to comprise dislocations (density 10 to 10 3 lines cm −2 ) and weakly contrasted growth sector boundaries (typical strain 10 −5 ). Many of the dislocations observed had undergone post-growth motion. The extent of this motion was found to be less in crystals grown more slowly at lower temperatures.

X-ray topographic study of flux grown KTP crystals

Abstract The structural quality of crystals of potassium titanyl phosphate (KTiOPO 4 : KTP) prepared by top-seeded solution growth from K 6 P 4 O 13 flux, has been investigated using synchrotron X-ray topography. A very high level of structural perfection was observed, with average dislocation densities less than 10 cm −2 and substantial volumes being dislocation-free. A major factor in this respect is the low level of dislocation formation at the capping region of the crystal. Initial observations relating to the characterisation of dislocations and other defects are discussed together with aspects of the crystal growth history revealed by X-ray topography.

Dislocations in energetic materials Dislocation characterization and post-growth motion in single crystals of cyclotrimethylene trinitramine

Abstract Dislocations in solution-grown crystals of cyclotrimethylene trinitramine (RDX) have been observed and characterized by transmission X-ray topography. The dislocations were found to have the following Burgers vectors: [001], [010], [100], 〈011〉 and 〈110〉. With the exception of one pure edge dislocation type they were all of mixed character. A few of the dislocations were straight and showed characteristics typical of growth induced dislocations. Most, however, exhibited irregular line directions indicative of post-growth motion. In addition, a number of narrow dislocation helices were observed. The observed dislocation configurations are discussed in terms of climb, cross-slip and combined climb/slip mechanisms. Evidence is presented for the existence of an alternative (001) slip plane additional to the (010) plane previously defined by microhardness indentation.

X-ray topographic investigations of solid state reactions. I. Changes in surface and bulk substructure during incipient thermal decomposition in sodium chlorate monocrystals

On heating, sodium chlorate monocrystals undergo a limited degree of thermal decomposition in the solid state (0.1-1 % mass loss) between 450 K and the melting point, ca. 535 K. The degree to which this occurs depends on the perfection of the sample, the more perfect crystals suffering lower mass loss. Examination of thermally treated pristine surfaces with a microscope reveals distributions of decomposition sites characteristic of the reaction initiating where emergent dislocations intersect the crystal surface. There is little correlation, however, between dislocation etch-pits and decomposition centres. X-ray topographic studies of changes in bulk substructure during thermal treatment confirm that ‘grown in’ dislocations play little or no part in the nucleation of the decomposition process in the bulk. There is some evidence, however, that they may contribute to the surface reaction. In the bulk, the potential nuclei are strained regions in the lattice. From these, and possibly as a consequence of decomposition, dislocation loops are punched out into the surrounding lattice along <100> and <110> directions. Similar defects are emitted from localized regions at the surface and it is possible that these give rise to the surface decomposition patterns. Impurity doped crystals show considerably enhanced decomposition.

Dislocations in energetic materials: I. the crystal growth and perfection of pentaerythritol tetranitrate (PETN)

Abstract Crystals of the secondary explosive material pentaerythritol tetranitrate (PETN) were grown from ethyl acetate and from acetone solutions by temperature lowering and solvent evaporation techniques. The growth-induced defect structure revealed by X-ray topography was found to consist of growth bands, growth sector boundaries, dislocations and solvent inclusions. All of the crystals grown exhibited only the forms {110} and {101} with the former dominating. Additional growth sectors were revealed by topography and were found to correspond to {111} facets which had grown out in the early stages of growth. The crystal perfection was found to increase with more accurate control of growth conditions. In general the {110} growth sectors exhibited a higher defect density than the {101} sectors. This can be explained on the basis of the relative growth rates of the habit faces and the nature of the growth dislocations present.

The crystal growth behaviour of the organic non-linear optical material 2-(α-methylbenzylamino)-5-nitropyridine (MBANP)

Observation of the growth behaviour and measurement of the growth kinetics of 2-(α-methylbenzylamino)-5-nitropyridine (MBANP) has revealed a strong and anomalous dependence of growth rate on supersaturation. At relative supersaturations less than 0.08, slow growing irregular faces were formed. At supersaturations below 0.24 no growth was observed in either 〈010〉 direction. Above this, rapid, but highly imperfect, growth occurred in one 〈010〉 direction while the opposite end of the crystal exhibited zero growth rate at all achievable supersaturations. The factors potentially responsible for this behaviour are discussed. On the basis of these observations, techniques have been developed for the preparation of large (up to 7 x 4 x 3 cm 3 ) high quality single crystals suitable for detailed optical studies and for device fabrication.

Crystal growth and characterisation of the electro-optic material 3-(2,2-dicyanoethenyl)-1-phenyl-4,5-dihydro-1H-pyrazole

The growth of 3-(2,2-dicyanoethenyl)-1-phenyl-4,5-dihydro-1H-pyrazole crystals from toluene and ethyl acetate solutions is described. Kinetic data indicate the development of a polar morphology due to anisotropic solvent adsorption. A double-solvent technique has been used to grow crystals up to 1 cm2 in cross-section and their perfection was examined by X-ray section topography.

X-ray topographic studies of organic and non-linear optical materials

SummaryThe flexible and non-destructive nature of X-ray topography is ideally suited to the study of large single crystals for both fundamental research and technological applications as well as the optimisation of crystal growth processes. Three examples are discussed, illustrating the application of X-ray topographic methods to non-linear optical (NLO) crystals. Synchrotron radiation section topography has been applied to the examination of large (up to 2 cm thick) organic crystals. Examples taken from the important NLO materials urea and 2-(α-methyl benzylamino)-5-nitropyridine (MBANP) demonstrate the advantages of this totally non-destructive assessment of crystal quality and illustrate the valuable insight into crystal growth history which can be obtained. X-ray topography has been used to examine growth defects and the quality of crystals of m-nitroaniline (mNA) grown by the Bridgeman method. These studies allow evaluation of growth parameters together with their influence on defect density and show that in the case of mNA, remarkably low defect densities can be achieved under optimum growth conditions. Double-crystal reflection topography, with synchrotron radiation has been used to image defects intersecting the {011} faces of the inorganic NLO material potassium titanyl phosphate (KTP). X-ray images have been combined with optical microscopy and interferometry to provide valuable information on the crystal growth process.