Prism Axis Research Articles

Production of synchrotron X-ray biprism interference patterns with control of fringe spacing

An X-ray biprism has been constructed from a pair of rectangular-section polished diamond bars meeting edge-to-edge, and rotatable about the prism axis so as to vary the convergence of the refracted beams. Interference patterns have been recorded at wavelengths of 0.08, 0.096 and 0.11 nm with good agreement between calculated and observed fringe spacings. Application of the fringe-observing arrangement employed in this work to the assessment of X-ray source characteristics is proposed.

The birefringence of the optically nonlinear crystal 4-amino benzophenone

A precise method is presented for measuring the refractive index of an anisotropic prism with light polarized orthogonal to the prism axis which is also a dielectric axis. Prisms have been cut from crystals of the optically nonlinear 4-amino benzophenone with exceptionally low defect concentration as identified by synchrotron white light topography. The linear refractive indices of these have been measured and fitted to four-parameter Sellmeier equations to an accuracy of 7×10−6–3×10−3. The values are accurately verified by observation of the external angle of incidence to a (001) plane required to meet the conditions of type-I phase matching with a fundamental wavelength of 1064 nm. The phase matching is of Hobden class six for this wavelength. In common with similar organic molecular materials, the molecular charge transfer axis defines the most polarizable direction in the crystal, with the C=O direction being much less significant.

Studies on sound and carious enamel with the quantitative acoustic microscope.

The scanning acoustic microscope gives strong contrast from small caries lesions in sections of human enamel. The uniqueness of the acoustic microscope lies in its ability to image elastic properties. In addition to revealing the extent and the shape of lesions, the microscope may also be used to measure the elastic properties point by point across an area of interest. Since enamel is anisotropic, measurements of the Rayleigh velocity and attenuation were made as a function of direction on a section of sound enamel. The velocity was greatest parallel to the prism axis, and the attenuation was least in this direction. Measurements of V(z) across a section through a lesion are presented. The variation of attenuation can be interpreted in terms of the development of demineralization, initially along prism boundaries and then along cross-striations. The variation of velocity indicates a substantial reduction of elastic stiffness in the lesion.

Pressureless-sintered ceramics based on the compound Si 2N 2O

Si 2N 2O is a tetrahedral structure of SiN 3O units which has many of the required intrinsic properties of a major phase in high temperature ceramics, similar to the more intensively studied Si 3N 4. The rapid decomposition of Si 2N 2O above 1700°C dictates the use of ternary eutectics to effect liquid phase sintering below this temperature. In this work the eutectic has been used to sinter Si 2N 2O ceramics to nearly the theoretical density without the application of pressure. Sintered microstructures are composed of submicron faceted Si 2N 2O crystals within a semicontinuous matrix of YSiAlON eutectic glass of about 10 vol.%. The crystal morphology is that of elongated pseudo-hexagonal prisms, similar to the β′ phase in Syalon ceramics. The prism axis is [001] with facet planes parallel to (100), (110) and (1 1 0) in the orthorhombic unit cell. A post-sintering heat treatment at 1250–1350°C induces a crystallization of the matrix glass to Y 2Si 2O 7 with a redistribution of aluminium and oxygen by substitution in Si 2N 2O. A brief property survey shows that hardness, fracture toughness and oxidation resistance are comparable with the corresponding values for the related ceramics based on β-Si 3N 4.

An electron microscopy study of exsolved phases in natural black Australian sapphire

Natural Australian sapphire (α-Al 2O 3 with around 1% Fe and up to 0.03% Ti) occurs in many colours, generally with an abundance of exsolved phases. Such ‘silky’ stones, cut en cabochon, exhibit the well-known phenomenon of asterism. For stones with a well-defined ‘star’ needle-like precipitate phases can be seen under an optical microscope. The needle axes are perpendicular to the star axes and generally parallel to the prism a-axes. (For artificial star sapphire the precipitate needle axes are perpendicular to the prism axes, e.g. Phillips et al., 1980.) Many authors have assumed that these precipitate phases in natural sapphire were needles of rutile (TiO 2) only, as is the case for artificial sapphire (Phillips et al., 1980). Our studies on natural Australian black sapphire, however, confirm the existence of a variety of iron/titanium rich phases, as well as rutile. Sample specimens were prepared by illuminating a polished basal plane with a He-Ne laser to obtain the star directions and then cutting slices of the stone parallel to the star axes. The polished sections were then ion-beam thinned and examined in a JEOL 100CX (S)TEM, equipped with an energy dispersive X-ray spectrometer. Precipitate phases were identified by electron diffraction and from their X-ray spectrum.

Some stereological properties of hexagonal prisms

A method is described by which the ratio of base to height in crystals of trigonal symmetry may be determined from metallographic sections of the crystals in a matrix. The method is applied to a WC-25 wt.% Co sample, liquid-phase sintered at 1400°C for 100 hr, during which the WC crystals form triangular prisms. It is shown that the prisms have a base to height ratio of 2.0 ± 0.1 and are slightly truncated along the edges parallel to the prism axis.

Vigezzite, (Ca, Ce) (Nb, Ta,Ti)2O6, a new aeschynite-type mineral from the Alps

SummaryA new mineral species, vigezzite (Ca,Ce) (Nb,Ta,Ti)2O6, was found near Orcesco, Valle Vigezzo, Prov. Novara, Northern Italy. It forms fiat prismatic crystals up to 2–3 mm length of an orange-yellow colour. The crystals are elongated along the a-axis and flattened on (010). Cleavage at right angles to the prism axis is distinct, fracture conchoidal. Vickers hardness VHN = 396 kp/mm2, corresponding to in the Mohs scale. Refraction indices are high: α = 2.14, γ = 2.315 (NaD, 589 µm), birefringence is 0.175. Optic angle 2Vγ is large. Though intensively yellow coloured, the mineral shows no pleochroism. α = c, β = b, γ = a; optic axial plane is parallel to (010).Vigezzite is orthorhombie with space group Pmnb or P21nb; a = 7.559±0.01, b = 11.028±0.01, c = 5.360± 0.004 Å, Z = 4. The six strongest lines of the X-ray powder pattern are 4.821 (9) (011), 3.784 (8) (200), 3.036 (10) (031), 2.974 (10) (211), 1.7129 (6) (242), 1.6018 (7) (431).Electron microprobe analyses lead to the empirical formula (Ca0.82Ce0.24)Σ1.06(Nb0.9Ta0.62Ti0.5)Σ2.02O6. The mineral is not completely homogenous, the ratio Nb/(Nb+Ta) varies from 0.55–0.62. Rynersonite Ca (Ta,Nb)2O6, may represent the Ta-dominant equivalent of vigezzite. Vigezzite occurs together with several other Nb-minerals such as pyrochlore, columbite and fersmite, whose crystallographical and chemical data are reported, in an albitic rock of presumably pegmatitic origin.

Ultrastructural analysis of dissolution of shell of the bivalve Mytilus edulis by the accessory boring organ of the gastropod Urosalpinx cinerea

The shell penetrating mechanism of the muricid gastropod Urosalpinx cinerea follyensis Baker was examined with reference to solubilizing effects of the secretion of the accessory boring organ (ABO) on the ultrastructural organic and mineral components of the shell of the bivalve Mytilus edulis Linne. The fine structure of shell etched by the secretion was contrasted with normal shell and shell solubilized artificially by acids, a chelating agent, and enzymes as an aid in interpreting the pattern etched by the secretion. A synoptic series of scanning electron micrographs of representative regions of the normal shell of M. edulis was prepared to serve as a standard for ultrastructural interpretation of the patterns of dissolution. Intercrystalline conchiolin of the mosaicostracal, prismatic, myostracal, and nacreous strata was dissolved as readily as the periostracum by the ABO secretion. In the prismatic region, maximum depth of dissolution of intercrystalline organic matrix occurred when long axes of prisms were approximately perpendicular to the surface being dissolved. Microscopic solubilization of organic matrix noticeably preceded dissolution of mineral crystals, revealing subsurface prisms and lamellae similar in size and form to the distinctively shaped normal shell units. After solubilization of intercrystalline conchiolin, further dissolution by the ABO secretion revealed a variety of what appeared like internal structures in prisms and lamellae. The form of these subunits varied from that of platelets to nodules in prisms and from laths to nodules in lamellae. These intracrystalline patterns of dissolution probably resulted from preferential etching of (a) soluble intracrystalline conchiolin membranes or other internal aggregates of nacrin, (b) heterogeneously distributed trace and minor elements, or (c) from both. Carriker and Williams (1978) hypothesized that a combination of HCl, chelating agents, and enzymes in a hypertonic mucoid secretion released by the ABO dissolve shell during hole boring. The similarity of patterns of dissolution etched by the ABO secretion and those produced artificially by HCl and EDTA in the present study support the hypothesis that these chemicals, or chemicals similar to them, are constituents of the ABO secretion. Lactic and succinic acids and a chitinase-like enzyme were also suggested by Carriker and Williams as possible agents in shell dissolution. Alteration of the shell surface by experimental application of these, and other, chemical agents was not sufficiently comparable to that etched by the ABO secretion to support the suggestion. Preferential dissolution of shell matrix by the ABO secretion is functionally advantageous to boring gastropods because it increases the surface area of mineral crystals exposed to solubilization and facilitates removal of shell units from the surface of the borehole by the radula.

Structure of the molluscan bivalve hinge ligament, a unique calcified elastic tissue

The microstructure of a calcified molluscan bivalve hinge ligament has been studied. Similar to bivalve shell and vertebrate dental enamel, the ligament is composed of oriented crystal bundles or prisms. Electron micrographs of Spisula solidissima ligament show two types of prisms. The lateral surfaces of the ligament are composed of long cylindrical prisms oriented perpendicular to the growing margin of the ligament. The prism crystals are oriented parallel to the prism long axis. In shape and orientation these prisms are similar to those of dental enamel. The prisms observed in the central region of the ligament are cylindrical but constricted at regular intervals, giving the appearance of three-dimensional standing waves. The crystal spacings within these prisms vary regularly along the prism axis. This structure probably responds to compression like an elliptical spring and accounts for the elastic properties of the ligament.

Refractive Indices, Densities, Polarizabilities and Molecular Order in Nematic p-Ethoxy Benzylidene-p-n- Butylaniline

Refractive index measurements of EBBA (p-Ethoxy-Benzylidine, p-n-Butylaniline) have been made in the crystalline, liquid crystalline and liquid phases using prism technique. The principal refractive indices of the crystalline phase have been obtained by making measurements on two different specimens of crystals grown in the prisms with small angles, with molecular axis parallel and perpendicular to the prism axis respectively. Orientational order parameters have been evaluated making use of the density data of liquid crystalline phase extrapolated from the density value at one temperature in the liquid crystalline phase and the thermal expansion coefficient and the densities of the crystal and liquid measured by floatation and specific gravity bottle techniques respectively. The estimated values of the S-factors are in very good agreement with those obtained by wide line NMR studies on deuterated EBBA molecule.

An Optical Technique for Measuring the Velocity of a Fluid

The technique described is based on a refraction phenomenon. Objects observed through a rotating transparent parallel-faced prism have images which move relative to the object. When the objects are small tracer particles suspended in a moving fluid, the velocity of the fluid can be measured by varying the rotational speed of the prism until the component of the image velocity normal to the prism axis is cancelled. The point in space at which the measurement is made is fixed by the intersection of the line of sight of the viewing telescope with a thin transverse beam of light used for illuminating the tracer particles.

Alignment of Rotational Prisms

An analytical expression is derived for the alignment of optical elements, such as the Dove or Pechan prism, that are used to rotate optical images. Improper alignment is characterized by a nutation of the system boresight as the prism is rotated. An analytical description is developed for this nutational boresight error in terms of specific misalignments between the prism, mechanical, and optical axes. This error is shown to be similar in form to that given by a mathematical expression commonly known as thelimacon of Pascal. Unique nutation patterns are noted by which one can distinguish and measure specific misalignments. Proper alignment is seen to involve both lateral and angular adjustments. Appropriate adjustment axes are chosen, and suitable alignment procedures are developed.

Theory of Dislocation Cells: The Dislocation Quadrupole

The prototype of all dislocation cells composed of dislocations with two mutually perpendicular Burgers vectors is the terminated quadrupole. It consists of two similar, mutually perpendicular, rectangular glide dislocation loops which interpenetrate such that their edge dislocation segments are parallel to each other and that each of the two pairs of like-handed screw dislocation segments form a cross of fourfold symmetry. The stress components of the terminated quadrupole have been computed numerically and have been presented in the form of contour maps in, or parallel to, the midplane of the quadrupoles as well as in a cut parallel to the prism axis of the quadrupole. It is found that the intensity of the stresses falls with distance from the cell as 1/r5 for medium short or squat quadrupoles, and as 1/r4 for very elongated quadrupoles.

The nature of the Hunter-Schreger bands in enamel

It has been shown that in polishing longitudinal sections of enamel adjacent Hunter-Schreger bands show a different resistance to abrasion. By preparing specimens of enamel with two surfaces at right angles it could be seen that this effect was not due to differences in hardness between successive bands but was a directional effect. Polishing along the prism axis was easier than polishing at right angles to the prism axis. A similar effect was produced by etching surfaces. A polished longitudinal section of enamel in consequence varies in thickness depending on the direction of the prisms. This variation in thickness of sections accounts for the appearance of the Hunter-Schreger bands on microradiographs.

Structure of mature human dental enamel as observed by electron microscopy

The ultrastructure of mature human dental enamel was studied by direct electron microscopy of ultrathin sections cut with a diamond knife and of replicas made from lightly etched cut surfaces. The enamel prisms were found to resemble keyholes when cut in cross-section, with the prisms measuring about 5 μ in dia. across the heads of the keyholes and about 9 μ in height over the extreme head to tail dimensions. The prisms were found to be oriented so that the heads of the prism cross-sections were directed cuspally and the tails were directed cervically. The crystallite orientation within any single prism was characterized by being coaxial with the prism in the extreme head region of the prism, but tipped more and more away from the prism axis in moving towards the tail of the prism. This orientation of crystallites in prisms was observed regardless of the plane of sectioning used to obtain a sample for observation. There was no evidence for an “interprismatic substance” between the prisms. Three-dimensional plastic models were prepared based on an idealized representation of enamel structure and sectioned for comparison with the electron micrographs. All structures observed in the micrographs could be simulated by sectioning the models in the proper planes.

TiAs2-type Phases

THE structure of TiAs2, which is of a new type, has recently been determined by Wenglowski et al.1. The orthorhombic unit cell contains eight formula units which comprise two kinds of titanium and four kinds of arsenic atoms. The co-ordination of TiI and TiII is very similar. Both have six As neighbours forming a trigonal prism and three more neighbours lying at the corners of a triangle perpendicular to the prism axis. The corners of this triangle are occupied by 3As in the case of TiI and by 2As + 1TiII in the case of TiII. AsI is located at the centre of a tetrahedron formed by 1TiII + 2TiII + 1AsI. AsII and AsIII lie at the centre of a distorted tetrahedron of 3TiI + 1TiII. Finally, AsIV is placed at the centre of a trigonal prism of 2TiI + 4TiII, a co-ordination which is also found in α-TiAs (TiP type) and β-TiAs (NiAs type). This coordination immediately reveals that TiAs2 must be metallic though its composition would suggest the possibility of a semi-conductor of ionic formula Ti4+(As2)4−. However, only one of the four different arsenic atoms forms pairs, but half the cations are connected as well. Assuming single bonds between TiII–TiII and AsI–AsI, we are left with 44 anion valence electrons per unit cell for the cationanion bonds which cannot be balanced by the 28 available cation electrons and thus induce metallic conductivity. My resistivity measurements in fact confirm these conclusions.

The arrangement of crystallites in enamel prisms

A series of X-ray diffraction patterns has been recorded from sections of permanent and deciduous human enamel. The sections were cut in three different planes so that an idea of the spatial arrangement of the crystallites could be found. From any point in the enamel the pattern consisted of arcs of reflection, the variation in crystallite orientation being indicated by the limits of the arcs, and the mean direction of orientation being shown by a line joining the midpoints of the two 002 reflections. In cuspal enamel of longitudinal sections and in enamel cut in the transverse oblique plane, the mean direction of the crystallites was parallel with the direction of the prisms. In enamel on the sides of the crown (lateral enamel) in longitudinal sections, the mean direction of orientation was displaced away from the prism axes towards the cervical margin. No true “double” orientation of crystallites within the prism complex was demonstrated. This was in agreement with a new interpretation of the optical behaviour of enamel between the crossed polars of a polarizing microscope. There appeared to be a gradual change in the orientation of crystallites from one side of a prism to the other. Such a pattern would account for the features of enamel structure observed here and would agree well with the appearance of enamel as seen with the electron microscope. Occasionally, diffraction patterns with two sets of arcs were recorded but they were the result of including two differing layers of enamel in the X-ray beam. Specimens producing such an artifact have been used to demonstrate the cause of the asymmetric, distorted reflection patterns which are frequently obtained when working with calcified tissues.