As-grown Faces Research Articles

The Mechanical Properties of Minimally Processed RDX

We report for the first time the mechanical properties of RDX crystals in a conventionally processed, sub-millimeter form that have had no additional mechanical processing. Nanoindentation of RDX powders was used to measure the elastic modulus (19.1±1.9 GPa), hardness (0.741±0.098 GPa), and yield point (onset of plastic deformation) on the as-grown faces of seven different RDX crystals, selected to provide random orientations. Properties within each crystal showed narrow distributions, while the range of properties across all crystals is indicative of testing a variety of orientations. The elastic modulus and hardness are within the range of other published reports on bulk and mechanically polished RDX. The distribution in yield point behavior, with the onset of plasticity occurring between 0.1 and 0.7 GPa, indicates that powders of RDX likely contain a significant number of dislocation sources in the as-processed condition, suggesting that deformation sources are prevalent in the energetic component of plastic bonded explosives prior to incorporating into pressed forms.

Microindentation deformation of lithium dihydrogen phosphate single crystals: Microhardness measurement and indentation size effect

The Vickers microhardness HV of the (110) and (111) as-grown faces of lithium dihydrogen phosphate (LDP) crystals was investigated as a function of applied load P. The microhardness HV of the two faces increases with load P i.e. reverse indentation size effect (reverse ISE) and the hardness of the (110) face is somewhat lower than that of the (111) face but this difference is not easily recognized for these planes due to large scatter in the data. The origin of observed ISE was analyzed using different approaches. It was found that: (1) Hays–Kendall's and Begley–Hutchinson's relations do not explain the origin of reverse ISE but Meyer's law describes the reverse ISE satisfactorily and its constants provide a link between ISE and formation of radial cracks with applied indentation load P, (2) reverse ISE is associated with tensile surface stresses, (3) despite its failure to explain reverse ISE, Begley–Hutchinson's relation is reliable to obtain load-independent hardness H0, is 2337 MPa for LDP, and (4) the value of fracture toughness KC of LDP crystals lies between 4.7 and 12 MPa m1/2. The load-independent hardness H0 of LDP is higher by a factor of 1.5 than that reported for undoped KDP and ADP crystals whereas its fracture toughness KC is higher by factor of about 20 than that of undoped KDP crystals.

Specific Surface Free Energy of As-Grown and Polished Faces of Synthetic Quartz

Contact angles of water droplet on as-grown Z, +X, –X, and m faces of synthetic quartz crystals with growth term of 20 and 48 days, and polished Z, +X, –X, Y, and 45&#176 cut faces of synthetic quartz crystal were observed. The average of the contact angles on as-grown Z, +X, and –X faces increased with the growth term, and they were larger than that on polished Z, +X, and –X faces. On the other hand, the average of the contact angles of water on m face decreased with the growth term, and they were smaller than that on polished Y cut face. Growth rate of the faces of synthetic crystals was measured and the order of growth rate was, m < –X < +X < Z. Specific surface free energy (SSFE) was calculated using Neumann’s equation. The SSFE of polished face was in the order of, m < –X < +X < Z, which corresponds to the order of the growth rate. The SSFE was larger for the face with larger growth rate.

Crystal Growth, Crystal Morphology and Surface Micromorphology of Type III KGd(PO3)4and KNd(PO3)4

Type III KGd(PO3)4 (KGP) and KNd(PO3)4 (KNP) inclusion-free single crystals can be grown using the top seeded solution growth−slow cooling method without pulling even though the viscosity of the solution is high (around 16 Pa·s-1 near the crystallization temperature). Poisoning by impurities in the crystals was minimized by using the corresponding self-fluxes as solvent. We determined their main growth process by analyzing the typical micromorphologies that appeared on the surface of the as-grown faces. The crystal morphologies of KGP and KNP were identified using X-ray diffraction analysis. The external habit of the crystals appeared to be more symmetrical than it structurally was because, although KGP and KNP are monoclinic with space group P21, they have an almost cubic cell because their cell parameters are similar and the β angle is very close to 90°. We checked the chemical stability of KGP and KNP in basic and acid media. The distribution and geometry of structural defects were studied by analyzing the etch pits that appeared on the {100}, the {010}, and the {001} forms. The crystals cleaved parallel to {010} and {001}, so the surface micromorphology of both forms was studied. We determined the orientation, the density, and the height of the cleaved planes. We also characterized the local plastic deformation of KGP and KNP by measuring the microhardness on the {100}, {010}, and {001} forms using an indentation method.

Effect of Mn(II) ions on the growth of ammonium oxalate monohydrate crystals from aqueous solutions: I. Growth habit and surface morphology

AbstractThe effect of concentration of Mn(II) ions on the growth habit and the surface micromorphology of different as‐grown faces of ammonium oxalate monohydrate (AO) single crystals grown from aqueous solutions was studied at a constant temperature of 30 °C and predefined supersaturations up to 20%. It was observed that the growth habit and the surface morphology of the crystals strongly depend on the supersaturation used for growth and the impurity concentration in the solution. The experimental results were analysed in terms of connected nets determined from different projections of the structure of AO crystals. Analysis of the observations revealed that: (1) the directions of connected nets corresponding to basic growth units composed of single (NH4)2C2O4 · H2O molecules are in excellent agreement with the low‐index crystallographic directions of the orientations of growth layers, (2) all faces appearing in the growth morphology of AO crystals are F faces, and (3) the {001} face growing from pure aqueous solutions is essentially a kinetically rough face but the presence of Mn(II) impurity leads to their appearance in the morphology due to increase in the strength of bonds of the connected nets composing the surface graph.

Microhardness studies on as-grown faces of NaClO3 and NaBrO3 crystals

Single crystals of NaClO3 and NaBrO3 are grown from their aqueous solutions at a constant temperature of 35°C by slow evaporation by using good quality seed crystals. Systematic microhardness studies are made on as-grown faces of these crystals at various loads. Typical cracks are observed at the corners of the impressions in NaClO3 whereas in addition to the cracks at the corners microcracks also appeared in NaBrO3 crystals around the impressions. The impressions formed in NaBrO3 are not very clear as in NaClO3, a possible mechanism for it is discussed. The work hardening index number(n) for both these crystals is around 1.6 suggesting that these are moderately harder samples. The hardness studies point out that NaBrO3 is harder than NaClO3 (ΔH ≈ 100 kg/mm 2,this could be due to strong inter ionic forces acting between Na-Br in NaBrO3 crystals. Using Gilman’s empirical relation, hardness values are calculated from the values of elastic constants (C 44) and are found to be close to the experimental results.

Computer simulation of growth process in synthetic quartz crystals grown from X-bar, Y-bar and rectangular Z-plate seeds

The growth simulations of synthetic quartz crystals grown on seeds with industrialized sizes and orientations were shown on the procedure by means of the setting of virtual lengths between the seed center and principal crystal faces to be grown. The virtual lengths needed for simulations are calculated on the given sizes and shapes of seeds, X-bar, Y-bar and rectangular Z-plate. The simulations of growth processes are executed as a function of growth time under the model values of growth rates on principal crystal faces, m, R, r, Z, and the so-called S. By the comparison of geometrical sizes of as-grown crystal faces with simulated faces, the growth rates of principal faces are quantitatively evaluated in the commercially produced quartz crystals grown from the rectangular Z-plate seeds. The growth rates of R- and r-faces at the upper and lower sides of grown crystals show somewhat different values which may be due to the direction of convection flow in autoclave.

Growth and Defect Characterisation in Single Crystals of Ferroelectric Ammonium Sulphate

Some observations of growth hillocks, growth spirals and layers on the chemically etched as-grown (100) faces of ammonium sulphate, (NH4)2SO4 crystals obtained at 26°C from aqueous solutions of different pH are reported and discussed for the first time with greater details. Optical and scanning electron microscopy show a good correspondence between etch pits and dislocation behaviour.

Novel TEM approaches to imaging of microstructures in carbonates; clues to growth mechanisms in calcite and dolomite

Relating microstructures imaged by transmission electron microscopy (TEM) to specific growth mechanisms requires imaging of both the surface microtopography on crystal faces and its corresponding microstructure in the bulk crystal. Pt-C replicas of as-grown surfaces are ideally suited to this task, as they provide a high-resolution, nearly three-dimensional image of surface topography that can be correlated with microstructures visible in sections at high angle to the as-grown surface. Ultramicrotomy enables the preparation of ultrathin sections more quickly than conventional ion-thinning and can be used to investigate chemical heterogeneities by analytical electron microscopy (AEM). We evaluate the potential of both techniques for the study of microstructures in calcite and dolomite. (1) TEM images were obtained from Pt-C replicas of synthetic calcite. The as-grown (101̅4) face of a Mg/Mndoped crystal growth showed growth hillocks aligned along preferred orientations. In Pb/Mn/Srdoped calcite, sections at high angle to as-grown faces showed a uniform microstructure in {101̅4} sectors. The {011̅2} sectors contained concentric zones that alternated from striated to uniform microstructures, suggesting periodic variations in growth rate and possibly in the growth mechanism. An oscillatory-zoned Mn-doped calcite showed fine-scale banding (30-150 mm) and periodic roughening of (101̅4) surfaces that suggest repetitive transitions between growth mechanisms. (2) In sedimentary dolomites, Pt-C replicas of surfaces cleaved parallel to a (101̅4) face showed a hillocky topography or smooth (101̅4) surfaces modified by non-equivalent facets. Surfaces produced by cleavage at high angle to a (101̅4) face exposed sectors with fine-scale banding crosscut by sharp boundaries, suggestive of closely spaced growth hillocks intercalated with non-equivalent subsectors. Ultrathin sections prepared by microtomy showed some disruption of the crystal structure but, in some dolomite samples, domains with distinctive microstructures similar in scale and shape to those seen in the Pt-C replica were identified. (3) The growth microstructures in {101̅4} sectors of dolomite present striking similarities to the larger scale compositional zoning patterns produced by growth spirals in calcite doped with trace elements. At this stage, Pt-C replicas and ultramicrotomy can distinguish among dolomites that are petrographically very similar.

AFM Study of the Behaviour of Growth Steps on the (100) Faces of KDP Crystals and the Tapering Phenomenon

The experimental results of an investigation of the surface morphology of the as-grown (100) faces of KDP single crystals grown on seeds from aqueous solutions containing different concentrations of Fe3+ and Cr3+ impurities are described and discussed. It was found that the behaviour of steps in terms of their mutual spacing and height does not depend on the presence of an impurity in the growth medium, and that the association of elementary steps to form bunched steps on the growing surface is a statistical process. The bunching of steps and the formation of tapered faces are related phenomena. Step bunching on a face and its tapering are not related with the presence of an impurity in the medium but impurities favour these processes. Our analysis of the evolution of the spacing of elementary steps indicates that the Schwoebel effect is responsible for step bunching and face tapering.

Growth kinetics and surface morphology of crystals grown from solutions: Recent observations and their interpretations

Observations reported since 1980 on the kinetics of growth and surface morphology of crystals obtained from solutions are reviewed and interpreted from the standpoint of crystal growth theories. After a brief introduction to the theories of crystal growth, first the kinetics of growth are surveyed and confronted with the crystal growth theories. Here the origins of deviations in the linear dependences of inclination of growth hillocks and rate of displacement of their growth steps on super saturation are analysed and explained in terms of kink nucleation at steps and solute accumulation at kinks. Thereafter the mechanisms of adsorption of impurities on growing crystal surfaces are briefly presented, and some typical experimental data on the kinetics of crystal growth in the presence of impurities and the tapering of KDP-type crystals are discussed in terms of the proposed mechanisms. The general features of growth hillocks at isolated and composite dislocation sources are then described and discussed in relation to the morphological importance of as-grown faces and growth conditions. The latest results on the bunching of elementary steps and the formation and stability of macrosteps are subsequently presented and discussed in the light of the existing theories of bunch formation. Finally, observations of dislocation hollow cores and decoration of growth steps on the as-grown surfaces of crystals during their extraction from supersaturated solution through an immiscible solvent layer placed above the solution are described.

Growth habit and surface morphology of L-arginine phosphate monohydrate single crystals

The results of a study of the growth habit and the surface topography of L-arginine phosphate monohydrate (LAP) single crystals as a function of supersaturation are described and discussed. Apart from a change in the growth habit with supersaturation, it was observed that most of the as-grown faces of LAP exhibit isolated growth hillocks and macrohillocks and parallel bunched layers and that the formation of bunched layers is pronounced on faces showing macrohillocks. Observations of bunching of growth layers emitted by macrohillocks on the {100} faces revealed that, for the onset of bunching close to a macrospiral, there is a characteristic threshold distance whose value depends on the interstep distance and supersaturation, but is independent of step height. The theoretical habit of LAP deduced from PBC analysis showed that all faces exhibiting growth hillocks and macrohillocks are F faces. Analysis of the results on bunch formation revealed that growth of LAP takes place by the direct integration of growth entities at the growth steps, that the bunching is facilitated by an increasing value of the activation energy for their integration, and that the observed dependencies of threshold distance on interstep distance, supersaturation and step height are qualitatively in agreement with van der Eerden and Müller-Krumbhaar's theory of bunch formation.

Micromorphology study of mercuric iodide crystals by atomic force microscopy

Atomic force microscopy was used to study the surfaces of mercuric iodide crystals at ambient. Various surface conditions were examined, including as-grown (001), (110), and (013) faces, and cleaved, chemically etched, vacuum etched, and aged (001) surfaces. As-grown (001) faces show a terraced structure and are smoother than as-grown (110) faces. Freshly cleaved surfaces exhibit a terraced structure, with riser heights corresponding to multiples of the c-lattice constant. All surfaces become progressively pitted and rougher by vacuum treatment or chemical etching. Defects identified as pits or hillocks with dimensions of the order of 100 nanometers were also observed, and surface roughness values were obtained for quantitative characterization of the various treatments.

Study of surface morphology of vapour-grown HgI2 platelets

Fresh and old as-grown (001) surfaces of a-HgI, platelets were studied by optical microscopy. The presence of growth hillocks and the high value of the surface entropy factor are proofs of the spiral growth of platelets. In air surfaces became gradually dull. Consequently, the final micromorphology of HgI, is the result of both growth (primary) and post-growth (secondary) processes. For the revelation of defects a new, reliable etchant was used. 1. Introduction Tetragonal a-HgI, crystals are useful material for detection of X-ray and nuclear radiation. Different techniques of vapour and solution growth are used to obtain mercuric iodide bulk crystals or platelets (e.g. SCHIEBER et al.; WILKE, BOHM). Platelets seem to be especially useful for detection of low energy gamma radiation because no sawing and slicing is needed in fabrication of devices, and electrical contacts are deposited on as-grown or polished surfaces. The present paper describes the results of an investigation of the micromorphology of as-grown (001) faces of crystalline platelets by optical microscopy. The aim of the study is to obtain information on the crystallization and post-crystallization processes and on the distribution of defects. 2. Experimental Red, a-HgI, platelets were obtained from vapour in the presence of hydrocarbons, organic polymers or noble gases (LASKOWSKI et al. (a); PRZYLUSKI, LASKOWSKI). The best quality platelets of 1 cmz area and thickness up to 0.5 mm were grown at total pressure of 2- 10 Torr. The temperatures of the source and crystallization zones were about 150- 180 C and 118 - 127 C, respectively. Details of crystallization procedure Mere described previously (LASKOWSKI et al. (a)). The faces of the platelets were determined by Laue technique as {OOl) planes. The crystals were mercury-rich and the total concentration of 23 elemental impurities was less than 100 ppm (LASKOWSKI et al. (b)). As-grown and etched (001) surfaces of platelets were examined under an optical microscope in reflection. Inclusions were observed in transmission. For the revelation and examination of the distribution of defects in platelets a new etchant consisting of acetone, HAuCI, aq. (0.5 - 1% wt.) and MnCI, aq. (satd) was used. Dislocations are revealed after etching for a few seconds. 3. Results and discussion

Growth of improved quality 3C-SiC films on 6H-SiC substrates

Previously reported chemical vapor deposition of 3C-SiC on 6H-SiC has resulted in films with a high density of double positioning boundaries (DPBs). We have found that growth on as-grown faces of 6H-SiC crystals can yield films that are largely free of DPBs. The (111) 3C-SiC films, up to 12 μm thick, were evaluated by optical and electron microscopy and low-temperature photoluminescence (LTPL). The LTPL spectra of the films were similar to those of high quality Lely-grown 3C-SiC.

Growth and characterisation of triammonium citrate

Single crystals of triammonium citrate are grown from aqueous solution and characterized using X-ray and etching studies. Using single crystal X-ray diffraction, the crystal is found to be orthorhombic with lattice parametersa = 6·223 Å,b = 15·048 Å andc = 11·056 Å. Surface studies on the as-grown faces reveal stratigraphic patterns. Of the several etchants tried, glacial acetic acid appears to be the best etchant for revealing dislocation sites on the as-grown faces.

Effect of solvent metals upon the morphology of synthetic diamonds

Diamond crystals up to 3 mm in diameter were grown from various metal solvents, and the effect of solvent metals upon the morphology was analyzed by the observation of the surface microtopography of as-grown faces and internal morphology. From a Ni solution only {111} and {100} faces develop, whereas {110} and {113} are associated when solvents of Ni-based alloys, Co and Fe, are used. In any metal solvents, {111} and {100} appear as smooth faces on which growth layers thinner than 30 Å develop, whereas {110} and {113} are hoppered. The internal morphology of the respective growth sectors indicates that {110} and {113} are transientfaces, whereas {111} and {100} are persistent faces. The order of morphological importance is thus expressed as {111}>{100}å {110} ≈ {113}. The observed characteristics of the {100} faces are interpreted in terms of a surface reconstruction model, and the appearance of the {110} and {113} faces is explained in terms of the adsorption of molecules of metallic nitride bonding with two carbon atoms at third neighbour sites on the diamond surface.

Crystal growth of synthetic zircon round natural seeds

A new method of hydrothermal growth of zircon on natural and synthetic seeds is proposed. The reaction mixture is made of Zr(SO 4) 2·4H 2O+SiO 2 gel. The mixture is raised to a temperature above 750°C under a pressure between (0.5 and 1.0)×10 8 Pa. Synthetic crystals are obtained from seeds of about 100 to 200 μm, which can reach 700 μm in size. The overgrowths obtained show the form {101} well developed, and sometimes the faces {001} not much developed disregarding of the forms already present on the original seed. The as-grown faces obtained are generally of a very good quality. The interface between the seed and the overgrowth is well defined.

Microindentation analysis of di-ammonium hydrogen citrate single crystals

Microindentation analysis of di-ammonium hydrogen citrate single crystals is reported here for the first time. The single crystals grown by a slow evaporation method are indented on the as-grown (0 0 1) and cleavage (1 1 0) faces using a diamond pyramid indenter, and the hardness variation with load is studied. The toughness is determined by measuring the crack length. The values of hardness and toughness are used to evaluate the brittleness of these crystal faces. The crack system formed around the indentations is analysed by etching and surface removal techniques, and it is shown that the threshold crack is radial. As the indentation load is increased, lateral cracks initiate and take the shape of median cracks with further increase in load.

Growth and characterization of tin disulphide crystals grown by physical vapour transport method

Tin disulphide single crystals have been grown by the physical vapour transport method. Optical measurements gave a direct band gap of 2.40 eV. Both circular and hexagonal growth spirals have been observed on the as-grown faces of these crystals. Spirals having a hollow core with a change of curvature showing the influence of the stress field on the shape of the growth spirals have also been observed. Other interesting observations — such as inner spirals starting from the centre of the hollow core — have been presented here.