Hexagonal Solids Research Articles

Hard coatings with high temperature adaptive lubrication and contact thermal management: review

Progress in the design and exploration of hard coatings with high temperature adaptive behavior in tribological contacts is reviewed. When coupled with most recent surface engineering strategies for high temperature contact thermal management, this progress opens a huge opportunity for adaptive coating applications on machine parts, where oils and coolants are commonly used. The adaptive mechanisms discussed here include metal diffusion and formation of lubricant phases at worn surfaces, thermally- and mechanically-induced phase transitions in hexagonal solids, contact surface tribo-chemical evolutions to form phases with low melting point, formation of easy to shear solid oxides, and others. All of these adaptive mechanisms are combined in nanocomposite coatings with synergistic self-adaptation of surface structure and chemistry to lubricate from ambient temperatures to 1000°C and provide surface chemical and structural reversibility during temperature cycling to maintain low friction coefficients. The review also highlights emerging surface adaptive concepts, where advances with ab initio modeling of intrinsically layered solids point to new compositions for thermally stable, easy to shear ceramic coatings, load- and temperature-adaptive surfaces with arrays of compliant carbon and boron nitride nanotubes as well as low friction two-dimensional structures. Approaches for self-regulation of coating thermal conductivity, heat flow, and thermal spike mitigations are discussed in the context of surface structure evolution and phase transitions. Future progress is linked to the development of in situ exploration techniques, capable of identifying adaptive surface chemistry and structural evolutions in broad temperature regimes. When combined with predictive modeling, such approaches drastically accelerate adaptive coating developments. The review identifies opportunities, strategies, and challenges for designs and applications of hard coatings with high temperature adaptive lubrication and contact thermal management.

Velocity-Stress Equations for Waves in Solids of Hexagonal Symmetry Solved by the Space-Time CESE Method

This paper reports an extension of the space-time conservation element and solution element (CESE) method to simulate stress waves in elastic solids of hexagonal symmetry. The governing equations include the equation of motion and the constitutive equation of elasticity. With velocity and stress components as the unknowns, the governing equations are a set of 9, first-order, hyperbolic partial differential equations. To assess numerical accuracy of the results, the characteristic form of the equations is derived. Moreover, without using the assumed plane wave solution, the one-dimensional equations are shown to be equivalent to the Christoffel equations. The CESE method is employed to solve an integral form of the governing equations. Space-time flux conservation over conservation elements (CEs) is imposed. The integration is aided by the prescribed discretization of the unknowns in each solution element (SE), which in general does not coincide with a CE. To demonstrate this approach, numerical results in the present paper include one-dimensional expansion waves in a suddenly stopped rod, two-dimensional wave expansion from a point in a plane, and waves interacting with interfaces separating hexagonal solids with different orientations. All results show salient features of wave propagation in hexagonal solids and the results compared well with the available analytical solutions.

Disorder in Layered Hydroxides: DIFFaX Simulation of the X-ray Powder Diffraction Patterns of Nickel Hydroxide

AbstractLayered metal hydroxides exhibit non-uniform broadening of lines in their X-ray powder diffraction (XRPD) patterns, which cannot always be explained on the basis of crystallite size effects. In the case of hexagonal solids such as nickel hydroxide, DIFFaX simulations of the XRPD patterns show that: (1) stacking faults and turbostratic disorder at low (<30%) incidence selectively broaden the h0l reflections; (2) turbostratic disorder at high (>40%) incidence causes asymmetric broadening of the hk0 reflections and a complete extinction of the hkl reflections while leaving 00l unchanged; (3) interstratification selectively broadens the non-hk0 reflections; and (4) cation vacancies reduce the relative intensity of the 100 reflection. In contrast, a reduction in the thickness of the crystallites along the stacking direction of the layers selectively broadens the 00l reflections while a reduction in the disc diameter causes the progressive broadening and extinction of the hk0 reflections. Comparison with experimental data shows that several kinds of disorders have to be invoked to account for the observed broadening. DIFFaX simulations enable the quantification of the different kinds of disorder.

Orientational phasetransition between hexagonal solids in planar systems of hardcyclic pentamers and heptamers

Two-dimensional, hard-body systems of cyclic multimers(pentamers and heptamers) are studied by Monte Carlosimulations. Elastic properties (the elastic constants and thePoisson ratio) are computed for hexagonal solid phases withmolecular rotation by using an algorithm based on thestrain-fluctuation method. In both systems a minimum of thePoisson ratio is observed at the same density at which aqualitative change of the orientational singlet distributionfunction occurs. This result confirms that, in each of thestudied systems, a phase transition occurs between twoelastically isotropic solid phases exhibiting orientationaldisorder: a higher-density phase with (strongly) hinderedrotation and a lower-density phase with (almost) free rotation.The study also suggests that it is worth paying moreattention to the dependence of the Poisson ratio on thethermodynamic variables (volume, pressure or temperature) insolids because this quantity can play the role of a sensitiveindicator of (at least some) structural phase transitions.

Ground States and Vortex Orderings in Fields Parallel to Layers of a Layered Superconductor

The ground states and the ordering behaviors on cooling in the liquid regime of vortices of a layered superconductor in fields parallel to the layers are theoretically examined using the lowest Landau level approximation for the Lawrence-Doniach model and neglecting randomness. The field-induced change of ground states is largely determined by the magnitude of Abrikosov factor β A and the sign of shear energy in an assumed pinned hexagonal lattice. We find that, in a narrow field range below the stability region of the pinned rhombic solid with no vacant interlayer spacings, a waving structure suggested in a recent numerical simulation is indeed the true ground state, and that a thermal fluctuation correction to the mean field energy is necessary to stabilize other pinned hexagonal solids realizable in lower fields. It is shown by examining the ordering behaviors of vortices in the liquid regime with significant layering effect that the positional correlation in any situation with negligible magnetic scre...

Structure and phase transitions of D 2 + Ar mixtures adsorbed on graphite

We report elastic neutron scattering experiments which explore the effects of size mismatch and relative composition on the freezing of binary two-dimensional mixtures. We coadsorb on graphite at low temperature two different species, 36Ar and D 2. Due to its large quantum motion, deuterium is highly compressible (more than 50%) when adsorbed as a solid monolayer on graphite. It can therefore span a large range of size mismatch with respect to Ar. Results obtained at 20 K for various partial coverages of Ar and D 2 show that three phases coexist on the surface, two Ar-rich hexagonal solids and a D 2-rich phase which itself can be disordered or a hexagonal solid, depending on the D 2 partial coverage.

A General Solution Technique for Piezothermoelasticity of Hexagonal Solids of Class 6mm in Cartesian Coordinates

AbstractPiezoelastic materials have attracted considerable recent attention because of their potential use in “intelligent” structural systems. In this paper, we propose a general solution technique for three‐dimensional problems of piezothermoelasticity for hexagonal solids of class 6mm in cartesian coordinates. Potential functions are introduced to represent the coupled elastic and electric fields. Simple uncoupled governing equations for the potential functions are derived from he stress equations of is analyzed. Numerical calculations for the stresses and the electric potential are carried out for a cadmium selenide body exhibiting class 6mm symmetry. Stress distributions are compared with those obtained for the similar thermoelastic problem without the piezo‐effect.

Transport Reactions of Some Vanadium(III) Halides. Mixed Halide Formation

The reactions of VCl/sub 2/, VCl/sub 3/, and VCl/sub 3/ -VBr/sub 3/ mixtures with Br vapor at 350 to 450 un. Concent 85% C led to vaporization of the halides and deposition of mixed haiides of vanadium(III) at lower temperatures. A study of solid solution formation in the system VCl/sub 3/--VBr/ sub 3/ showed that the two components were miscible in the solid state, and that the mixed halide VCl/sub 2/Br shouid be regarded as such a solid solution. The pure compounds VCl/sub 3/ and VBr/sub 3/, the mixed halides VCl/sub 2/Br and VBr/ sub 2/I, and solutions of all compositions in the system VCl/sub 3/ -VBr/sub 3/ were hexagonal solids with the BiI/sub 3/ layer structure; lattice constants for all of the compounds are given. Vaporization of VBr/sub 2/ in iodine vapor at 350 to 400 un. Concent 85% C resulted in the transport and deposition of VBr/sub 2/I, but the analogous reaction between VCl/sub 2/ and I yielded only a deposit of VCl/sub 2/. Vanadium(IV) mixed halides containing I were postulated to account for the vaporization at these temperatures. (auth)