Mixed nitrogen source effect in the hydrothermal synthesis of cubic BN

Mixed nitrogen source effect was applied in the synthesis of boron nitride (BN) by hydrothermal method. The experimental results revealed that, compared to those obtained by using only two kinds of nitrogen sources, when three kinds of nitrogen sources were used during the synthesis of BN, it was more easier to obtain BN samples with cBN as the dominant phase. Furthermore, if this effect was applied together with the two-step reaction method, nearly pure cBN can be obtained. The results of X-ray diffraction (XRD), Fourier transformation infrared spectroscopy (FTIR), transmission electron microscopy (TEM), selective area electron diffraction (SAED), high-resolution transmission electron microscope (HRTEM) and X-ray photoelectron spectroscopy (XPS) measurements proved that the major phases in the samples are cBN.     

Synthesis of cubic niobium nitride by reactive diffusion under nitrogen pressure

The synthesis of niobium nitride by reactive diffusion in a furnace at 1395-1475 °C and under nitrogen pressure in the range 2-25 MPa was investigated. In experiments, we used compacted Nb powder with a mean particle size of 43 μm. Phase transformations in the product as studied by electron probe microanalysis (EPMA) were found to proceed in the following order: Nb → α-Nb(N) → β-Nb₂N_1±x → γ-Nb₄N_3±x → δ-NbN_1±x. The size of niobium particles which could react with nitrogen to yield cubic niobium nitride was estimated (SEM analysis) from the dependence of the thickness Δ of the δ-NbN_1±x outer layer formed on the surface of Nb particles on the dwell time t_dw at 1460-1473 °C. It was shown that Δ grew nearly proportional to t_dw. At t_dw = 30 min and P(N₂) = 2 MPa, Δ was found to attain a value of about 15.5 μm. Prolonged heating (t_dw ≈ 60 min) was found to result in decomposition of the single-phase cubic niobium nitride into a two-phase (multiphase) product. This was confirmed by XRD data and magnetic measurements which showed the occurrence of two different critical temperatures T_c in the same sample. The maximum critical temperature T_c was found to attain a value of 15.6 K.     

Synthesis of cubic boron nitride from rhombohedral form under high static pressure

The cubic, zincblende-type boron nitride (z-BN) has been synthesized from the rhombohedral form (r-BN) under high static pressures greater than 6 GPa without any planned addition of catalysts. The process of forming z-BN has been delineated from isobaric and isothermal series of data. At 6GPa, r-BN begins conversion to the graphite-type form (g-BN) upon heating to 600 °C. This conversion terminates at 1200 °C forming single-phase g-BN, which in turn transforms into z-BN at temperatures higher than 1300 °C. The appearance of z-BN occurs at lower temperatures when the pressure is raised to 7 or 8 GPa. At pressures beyond 10 GPa the wurtzite-type form (w-BN) is observed between 400 and 1200 °C, whereas z-BN is formed above 1000 °C. The boundary of pressure-temperature conditions for synthesizing z-BN from r-BN runs through 6GPa and 1300 °C, and is located near to the lowest bound hitherto known for non-catalytic z-BN synthesis from g-BN.     

Enhancing form factor and light collection of multiplex imaging systems by using a cubic phase mask

The bulky form factor of traditional optical sensors limits their utility for certain applications. Flat multiplex imaging-sensor architectures face the light-gathering challenges inherent with small collection apertures. We examine a wavefront-coding approach wherein a cubic phase mask is used to increase the aperture sizes of multiplex imaging systems while maintaining the distance from the lens to the detector array. The proposed approach exploits the ability of cubic-phase-mask systems to operate over a large range of misfocus values. An exact expression for the optical transfer function of cubic-phase-mask systems is presented, and its misfocus-dependent spatial-filtering properties are described. Criteria for form-factor enhancement are assessed and trade-offs encountered in the design process are evaluated.     

Comparative analysis of performance of cubic boron nitride and microcrystalline alumina tools in profile grinding of form cutters

A comparative analysis of performance of cubic boron nitride, microcrystalline alumina, and white fused alumina wheels has been carried out in profile grinding of long-length HSS profile broach under production conditions. In a particular case, where grinding is accompanied by impact loading and the wheel has to be frequently dressed to provide precise profiling, the vitrified cubic boron nitride wheel has demonstrated an essentially better performance in terms of removal rate, minimum time for dressing cycles, and overall labor input in shaping a working profile of the broach.     

Physical model of the transition from a hexagonal to a cubic structure during growth of boron nitride under nitrogen and argon ion irradiation

A model based on original experimental data is proposed to describe the transition of the structure from hexagonal to cubic during the growth of boron nitride layers. It is postulated that boron and nitrogen ions entering the growing layer form additional atomic planes parallel to the (ab) planes of the hexagonal structure or complete existing dislocations in the growing crystal, causing the boron nitride to undergo a transition to a cubic structure. Pis’ma Zh. Tekh. Fiz. 23, 66–70 (July 12, 1997)     

Flux synthesis of cubic potassium antimonate

Although the compound KSbO₃ has been reported to occur in a high temperature cubic polymorph, only the rhombohedral ilmenite phase was obtained in this study for pure material at ambient pressure and temperatures up to about 1350°C in open Pt crucibles or sealed Pt tubes. With the addition of about 2 mole percent SiO₂, B₂O₃ or other oxides of small cations a primitive cubic phase was found with $\text{a}\text{?9.578}\text{A}$. Small single crystals of a body centered cubic phase (a=9.605A) were easily synthesized by a flux evaporation technique at about 1000°C in an open Pt crucible from a composition of about 5 mole % Sb₂O₄:95 mole % KF. The exact composition and structure of this phase (presumably K_1?xSbO_3?xF_x, 0<x≤0.5) is now under investigation.     

High-pressure sintering of cubic boron nitride

Cubic boron nitride (cBN) powder was compacted at high pressure and high temperature using a solid solution of titanium carbide and titanium nitride (TiC _x N_1–x ) as binding material in the presence of a small amount of aluminium. Different compositions of (TiC _x N_1–x ), 0<x<1 were used as binders. The weight percentage of cBN, TiC _x N_1–x and aluminium were optimized and found to be critical; any marked deviation from these optimized values deteriorated the quality of compacts. Various high-pressure sintering parameters such as pressure, temperature and sintering time, etc, were optimized for this binder. The compacts were also characterized using X-ray diffraction, scanning electron microscopy and microhardness measurements. The microstructural and X-ray diffraction observations indicated no marked changes in the compacts as the value ofx in TiC _x N_1–x was varied, but the microhardness was found to depend on the value ofx.     

Dissolution slowness surfaces of cubic crystals

The first part of this paper offers a tensorial method that gives analytical equations for the dissolution slowness surface of cubic crystals. Conditions for the proposed equations are outlined. The non-centrosymmetric class 23 is treated as an example. The evolution of the shape of the slowness surface with the higher rank of the dissolution tensors is studied, using in particular a three-dimensional graphical representation of the slowness surface. The conditions for obtaining slowness surfaces with a complex shape and an increasing number of extrema are discussed.     

Convex forms of the cubic model

In the paper a generalised model is proposed which encompasses a number of known models of the mechanical material behaviour: the normal stress hypothesis, the maximum strain hypothesis, rotationally symmetric potentials (v. MISES, BELTRAMI, cone, paraboloid- and hyperboloid potentials) and the model of SAYIR. The model gives rise to some new solutions which extend the classic hypotheses. The model is build up of the three invariants of the stress tensor and has five free parameters. Parameters are restricted to yield a convex form. Some convex forms of the model, which contains the stresses to the power three or two, are constructed in the paper.     

Dissolution slowness surfaces of cubic crystals

Procedures are developed to derive the etching shapes encountered in conventional crystal etching and in localized crystal dissolution. These procedures are used to predict the shapes produced on class 23 crystals. Problems of practical importance in photolithography techniques, such as the extent of the underetch, the bordering angle and the etched shape of star-like structures, are studied. In some cases the results are compared with those obtained from geometrical constructions based on Wulff's procedure.     

Errors in cubic spline interpolation

Summary This paper deals with the problem of finding error bounds for cubic spline interpolation of functions of the classC ⁴[a, b], andC ⁵[a, b], by examining a relationship between cubic spline interpolation and piecewise cubic Hermitian interpolation.The method also gives an indication of what happens, in the case of almost uniform meshes, especially if the approximated function is in the classC ⁵[a, b].Comparison is made with recent work carried out by K. E. Atkinson [3], in dealing with natural cubic spline interpolation.     

Quadratic and cubic transition elements

Quadratic and cubic quadrilateral isoparametric elements are investigated for use as transition elements for use a crack tip elements. Computations for a single element are explicitly carried out using a symbolic algebraic manipulation program and in addition to the desired square root singularity, additional singularities are revealed. Cubic transitional elements were used for a double-edge crack problem and only marginal improvement is found for a well-laid out grid.     

Functionalized cubic mesostructured silica films

Cubic Pm3n mesoporous silica films have been prepared on silicon wafers and quartz crystal microbalance (QCM) devices covered with gold electrodes by a spin-coating process from preformed silica/CTABr/ethanol/water assemblies under acidic conditions. Post-synthesis functionalization of mesoporous films with amino- and thiol-containing organosilanes is performed in order to modify the mesoporous surface for further confinement of nanoscale structures. The type of mesophase structure and the functionalization process was followed with surface sensitive techniques such as grazing incidence diffraction (GID), reflection-absorption FT-IR spectroscopy and gravimetric measurements by applying QCMs technique. Nitrogen sorption data using QCM devices were obtained for the calcined and functionalized mesoporous films.     

Permeability of dendrite network of cubic alloys

Abstract

The permeability for liquid in interdendritic space was measured using a unidirectionally frozen borneol–paraffin organic binary alloy the columnar dendrite structure of which is the same as that of bcc and fcc metallic alloys. For flow parallel to and normal to columnar dendrites, the permeability for volume fractions of liquid from 0·21 to 0·22 is in fairly good agreement with the value calculated from the empirical formulae obtained previously for volume fractions of liquid > 0·27. Thus, the formulae are applicable over a wide range of liquid volume fractions. The permeability is approximately proportional to the cube of the volume fraction of liquid. When the volume fraction of liquid is < 0·20, the permeability begins to decrease steeply with decreasing volume fraction of liquid. The permeability obtained in the present work is confirmed to be applicable to bcc and fcc metallic alloy systems.

MST/1028     

Construction of irreducible polynomials using cubic transformation

The transformationf(x)f(x ³–3x) is studied. We show that for allF _q of characteristic >3 andn1, there existsf(x)F _q [x] of degreen which generates an infinite sequence of irreducible polynomials of degree 3 ⁱ n by the iteration of this transformation.     

Solubility of Mn stabilized cubic zirconia nanostructures

Mn-stabilized cubic zirconia nanostructures (Zr_1?xMn_xO₂, x = 0.10, x = 0.15 and x = 0.20) have been prepared using chemical precipitation method for different doping concentrations. The prepared powders have been characterized by X-ray diffractometer for structural analysis, vibrating sample magnetometer for analyzing magnetic properties, scanning electron microcopy–elemental compositional analysis using X-ray for morphological and elemental compositional analysis. Room temperature ferromagnetism has been found at around x = 0.15. For high doping effect of Mn impurity at host lattice, secondary phase has been detected at around x = 0.20 when exceeding the solubility limit of cubic zirconia. Due to ferromagnetic behavior, Mn doped ZrO₂ nanostructures can be considered as good candidates for spintronics applications within their solubility limit.