Topography of etched rhombohedral faces of quartz crystals: evidence for orientation effects

A study has been made of the rate at which the rhombohedral faces of a natural quartz crystal are etched in a concentrated ammonium bifluoride solution. The thickness of the disturbed surface layer created by an initial mechanical lapping is estimated from rate data. The etching rate as well as this thickness are found to be sensitive to natural face orientation. The changes in the surface texture of the rhombohedral faces with repeated chemical etchings are investigated. The variation in the roughness parameters with the average depth of etch shows both directional and orientation effects. The chemical attack results in the formation of stable etch figures characteristic to the orientation of the surface on which they lie and which enlarge with repeated etchings. finally, schematic etch figures are proposed for the differently oriented rhombohedral faces.     

Calculation of the stress fields in a polysynthetic twin located near the crystal surface

Without using the thin twin approximation, assuming a continuous distribution of dislocations on the boundaries, a procedure to calculate the stress fields in a polysynthetic twin located near the crystal surface has been developed. It is shown that the stresses in the polysynthetic twin are localized at the boundaries and apices of the twins that enter into the composition of the former twin. Examples of calculations of the cleavage stresses of the polysynthetic twin having rectilinear and curvilinear boundaries with uniform and nonuniform distribution of dislocations at them are given. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 82, No. 1, pp. 184–190, January–February, 2009.     

Plasticity bands near the tips of edge cracks in a body with circular hole

Within the framework of the model of plasticity bands, we consider a two-dimensional elastoplastic problem of fracture mechanics for a body with one circular hole and two collinear cracks whose ends lie on the contour of the hole under the conditions of plane stressed state and plane deformation. It is assumed that plastic strains near each tip of the edge cracks are concentrated along three (for the plane stressed state) or two (under the conditions of plane deformation) plasticity bands. Their lengths and orientations are determined as a result of the solution of the problem. Numerical results are obtained for the cases of a biaxial tensile load acting at infinity and constant pressure applied to the contour of the hole and edge cracks (under the conditions of plane deformation). Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 34, No. 2, pp. 79–86, March–April, 1998.     

Impact fatigue of an alumina ceramic

A new type of pendulum impact apparatus is described for determining single and repeated impact strength of ceramics, It has been demonstrated that specimens of a Sintox alumina ceramic exhibit fatigue behaviour, having a high stress plateau followed by progressively increasing endurance with decrease in applied impact energy. A fatigue limit, at least from the engineering view point, has been drawn at high endurances (10⁵ impacts). The influence of environment (static fatigue) and/or plastic deformation to explain the fatigue behaviour is suggested.     

The effects of C ion implantation on the near surface microstructure and properties of alpha alumina

Biomedical grade (>99.5% purity) alpha-alumina has been implanted with carbon ions at fluences ranging from 5 × 10¹⁶ to 5 × 10¹⁷ C ions/cm² at an ion energy of 75 keV. The surfaces of the alumina have been examined in cross-section using transmission electron microscopy (TEM) and the data correlated with both nanohardness measurements and computer based simulations (Transport and Range of Ions in Matter, TRIM). TEM examination of the implanted surface has demonstrated the formation of a sub-surface amorphous layer as well as other microstructural modifications that are characteristic of ion damage. The nanohardness of the near-surface alumina was determined as a function of depth and was found to be strongly dependent on the fluence used.     

Crystalline orientation of alumina ceramics prepared by electrophoretic deposition under a high magnetic field

Electrophoretic deposition (EPD) of alumina in a superconducting magnet was performed at various magnetic field strengths. A stable colloidal suspension of alumina appropriate for magnetic alignment was prepared in an ethanol medium by using a phosphate ester (PE) as a dispersant. The amount of PE appropriate for the stability of the alumina suspension was investigated by measuring the pH, zeta-potential and the relative density of the green compacts. The consolidation of alumina powder was performed by EPD under a magnetic field of 0–12 T. The degree of crystalline orientation of the sintered bodies was evaluated by X-ray diffraction (XRD) as a function of the applied magnetic field during deposition and the sintering temperature.     

Diffusion controlled precipitation of austenitic bi-crystals possessing twin related orientation in the ferrite of a duplex stainless steel

The ferritic matrix in the Fe-22Cr-5Ni-3Mo-0.03C ferritic-austenitic duplex stainless steel can undergo a variety of decomposition processes when aged in the temperature range 650–550°C. One of these processes is the formation of austenite in the shape of a spearhead. Unlike the one found at high temperature, this austenite is characterized by a habit plane, which is similar to the midrib of the martensite. This feature suggests that a diffusionless process as referred to in the literature controls its formation. In the present investigation, transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) results indicate that the proposed precipitation mechanism must be reviewed. Based on the crystallographic results in conjunction with the clear-cut difference in chemical composition between the matrix and the austenitic second phase, the equilibrium shape is explained and the ambiguity of precipitation mechanism has been brought to light. It has been suggested that the latter one goes through the following steps: (i) enrichment of the (110)-ferritic planes with -forming elements by diffusion, (ii) double shear to transform (110)-ferritic planes into (111)-austenitic planes, i.e., to change from BCC () to FCC (), (iii) twinning of the FCC structure to reduce local strains and formation of bi-crystals and (iv) growth of the twinned towards the ferritic matrix by volume diffusion.     

Characterization of asymmetric rhombohedral twin in epitaxial α-Cr2O3 thin films by X-ray and electron diffraction

Epitaxial chromium oxide (α-Cr₂O₃) films grown by atomic layer deposition at 375 °C from CrO₂Cl₂ and CH₃OH on (1 1¯ 0 2) oriented α-Al₂O₃ have been studied by reflection high-energy electron diffraction (RHEED), X-ray diffraction (XRD) and X-ray reflection (XRR). The thickness of the films ranged from 10 to 310 nm, and the average growth rate was 0.1 nm per deposition cycle. According to the XRD analysis, the orientation relationship in thinner films was (1 1¯ 0 2)[1 1 0]Cr₂O₃ || (1 1¯ 0 2)[1 1 0]Al₂O₃. Confirmed by the RHEED and XRD analyses, (1¯ 1 0 2) became the preferred growth plane at the thicknesses above 40 nm. This change has been interpreted as the appearance of an asymmetric rhombohedral twin with the orientation relationship between the layers (1¯ 1 0 2)[1 1 0]top || (1 1¯ 0 2)[1 1 0]bottom and (1¯ 1 0 2)[1 1¯ 1]top || (1 1¯ 0 2)[1¯ 1 1]bottom. The match of the anion and cation sublattices of both layers was characterized in terms of the structural model of the twin interface.     

Polyol process for the synthesis of LiFePO4 rhombohedral particles

As a new approach, LiFePO₄ nanoparticles were directly synthesized from precursors iron(III) nitrate and lithium dihydrogen phosphate by a polyol process without post heat treatment in one step. The obtained powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and combined thermogravimetry and differential scanning calorimetry and mass spectroscopy TG/DSC/MS. The X-ray diffraction showed the orthorhombic crystal structure of LiFePO₄ without any impurity phases. The synthesized LiFePO₄ has rhombohedral morphology with high aspect ratio with a thickness of less than 100 nm. TG/DSC/MS revealed a weight loss of only 10.9 wt.% when heating up to 1000 °C. Electrodes prepared from the LiFePO₄ particles were electrochemically characterized by cycling at 0.1C current rate and temperatures in half cell measurements against lithium foil between 2 and 4.2 V in an EC/DMC electrolyte with 1 M LiPF₆ as conductive salt. A reversible specific capacity of 146 mAh/g was achieved by applying carbon coating on the rhombohedral particles.     

Effect of the grain boundary on the evolution of deformation in a bicrystal

The role of grain boundary constraint in strain localization and concomitant constitutive response was examined by performing a series of uniaxial compression tests on a tantalum bicrystal. Tantalum single crystals were diffusion bonded to form a (011) 90^∘ twist boundary that was compressed along the common [011] direction. The plastic deformation resulted in the creation of deformation bands away from the highly constraining grain boundary, resembling those bands known from single crystal plastic deformation. Near the grain boundary, such deformation band formation could not be detected. Instead a distinctive pattern of crystal lattice rotation was observed that filled a rather large volume (several millimeters in size) around the bicrystal grain boundary. The internal deformation band structure as well as the crystal lattice rotation pattern near the bicrystal grain boundary were characterized and found to give greater rates of work hardening in the neighborhood of the grain boundary.     

Effects of scratching on losses in 3-percent Si-Fe single crystals with orientation near

The effects of scratching perpendicular to the [001] direction on losses were studied as a function of the tilt angle of the [001] out of the crystal surface β both with and without tensile stress in 3-percent Si-Fe single crystals. The reduction of the total losses by scratching becomes larger with decreasing β. The total losses of scratched samples further decrease with the application of tensile stress parallel to the [001] direction. Observations of domain structure showed that scratching causes a decrease in 180° main domain wall spacings and also the occurrence of reverse subdomains in the vicinity of the scratch. It was found that the total losses of scratched specimens are lower than those of unscratched samples for equal 180° main domain wall spacing observed in the demagnetized state. This may be because the losses of scratched samples are influenced by the dynamic behavior of reverse subdomains in addition to the function of 180° domain wall displacements, The variation of dc hysteresis loss by scratching is very small. This may be caused by the effect of the new domain configuration at the scratch line, which weakens the domain wall pinning due to free poles or internal stress caused by scratching.     

Formation of lamellar M23C6 on and near twin boundaries in austenitic stainless steels

Thin foil electron microscopy studies were made on the precipitation of lamellar M₂₃C₆ during aging at 973 K and 1073 K in water-quenched specimens of two austenitic stainless steels. After the precipitation on incoherent twin boundaries M₂₃C₆ formed on coherent twin boundaries and in the regions adjacent to incoherent twin boundaries. These precipitates showed lamellar morphology and were aligned in a specific manner with respect to the twin boundaries. Such lamellar precipitates were absent in the specimens which were isothermally treated at 1073 K after being transferred from the solution treatment temperature. The lamellar morphology of M₂₃C₆ is suggested to be developed by the influence of residual specific stress field around twin boundaries resulted from quenching.     

The permeability of alumina over an extended temperature range

A semiempirical mathematical model has been developed to predict the permeability of porous alumina over a wide temperature range at atmospherie pressure. Although the thermal expansion of the alumina is of the order of 1 % over the range of temperatures considered, the apparent permeability of the sample to gas flow varies by over 400%. This behavior is due to the dependence of the mean free path of a gas on temperature and to the corresponding slip conditions that occur in the pores of the solid. The model developed correlates the apparent permeability data with temperature, true permeability, gas viscosity, and gas molecular weight, variables suggested by kinetie theory on mean free path and slip. Apparent permeability was found to be a very strong function of temperature. It exhibited both a direct thermal dependence and an indirect dependence, manifest through thermally driven variations in the gas viscosity. A mathematical model from the literature was used for gas viscosity. The inverse correlation with gas molecular weight, suggested by kinetie theory, is demonstrated. The model covers the temperature range between 250 and 1600 K. Small molelcule gases including air, nitrogen, argon, and helium were used in the development.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.     

Fracture of alumina: an experimental and numerical study

A numerical analysis of the fracture behaviour of alumina has been performed based on experimental crack growth studies. Single-edge notch bend and short-double cantilever beam specimens of diverse grain size alumina were tested under quasi-static growth conditions. Constitutive equations for the alumina were obtained from experimental results and used to carry out finite element analyses. The agreement between numerical and experimental results is very promising, e.g. the influence of grain size on fracture behaviour can be predicted accurately. The underlying toughening mechanisms are discussed.     

In-situ studies of the deformation and dynamic recrystallization of rhombohedral camphor

Polycrystalline specimens of rhombohedral camphor have been deformed in compression in the temperature range 283 to 343 K and the deformation processes and the development of microstructure followed by transmission polarized light microscopy. Processes of slip, kinking and twinning have been observed, and in particular, the mechanisms of dynamic recrystallization and its effect on the development of zones of heterogeneous deformation have been investigated. The points of similarity between the behaviour of camphor and metals and minerals of low symmetry are discussed.     

Removal of mercury from an alumina refinery aqueous stream

Digestion condensate is formed as a by-product of the alumina refinery digestion process. The solution exhibits a high pH and is chemically reducing, containing many volatile species such as water, volatile organics, ammonia, and mercury. Because digestion condensate is chemically unique, an innovative approach was required to investigate mercury removal. The mercury capacity and adsorption kinetics were investigated using a number of materials including gold, silver and sulphur impregnated silica and a silver impregnated carbon. The results were compared to commercial sorbents, including extruded and powdered virgin activated carbons and a sulphur impregnated mineral. Nano-gold supported on silica (88% removal under batch conditions and 95% removal under flow conditions) and powdered activated carbon (91% under batch conditions and 98% removal under flow conditions) were the most effective materials investigated. The silver and sulphur impregnated materials were unstable in digestion condensate under the test conditions used.