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In this article, we propose a margin-setting model under the assumption of extreme stock price changes. Specifically, extreme stock price changes are caused by the positive feedback effect of lever...

The effects of mechanical boundary conditions, often encountered in thermalstructural engineering, on the thermal shock resistance (TSR) of ultra-high temperature ceramics (UHTCs) are studied by investigating the TSR of a UHTC plate with various types of constraints under the first, second, and third type of thermal boundary conditions. The TSR of UHTCs is strongly dependent on the heat transfer modes and severity of the thermal environments. Constraining the displacement of the lower surface in the thickness direction can significantly decrease the TSR of the UHTC plate, which is subject to the thermal shock at the upper surface. In contrast, the TSR of the UHTC plate with simply supported edges or clamped edges around the lower surface is much better.

In thermal structural engineering, ceramic components can have various shapes and be constrained. It is traditionally known that the thermal shock resistance (TSR) of ceramic components (with conduction occurring in the thickness direction), such as disk, oval plate and rectangular plate, is independent of the in-plane geometric shapes. The present study shows that this is valid for ceramic components that are free, but when the components are constrained, the in-plane geometric shapes can have significant effects on the TSR of ceramics. Three types of components, with the edges of the lower surfaces clamped are studied. Under the second or third type thermal boundary conditions, the TSR of the disk is the best; that of the oval plate decreases as the ellipticity increases; and that of the rectangular plate is approximately equal to that of the oval plate with ellipticity of 1.2. This is because that the asymmetry of the in-plane geometric shapes, as well as the asymmetry of the mechanical boundary condit...

Nomenclature ai−1, ai, ai 1, a 0 i = coefficients in the discrete equation, KW · m−2 · °C−1 B0, B1, B2 = coefficients in Young’s modulus expression Cp = specific heat at constant pressure, J · kg−1 · K−1 E = Young’s modulus, GPa E0 = Young’s modulus at room temperature, GPa fj = term in Eq. (4), where j is 1–5 (f1 and f4 in MPa · m, f2 and f5 in KPa · m , and f3 in Pa · m) h = plate thickness, mm K = coefficient matrix in Eq. (1), KW · m−2 · °C−1 k = thermal conductivity,W · m−1 · °C−1 kFi = thermal conductivity at face Fi, W · m−1 · °C−1 m = ratio of compressive-tensile strength n = number of nodes P = load vector in Eq. (1), MW · m−2 qs = surface heat flux, MW · m −2

The thermal shock resistance of ceramics depends on not only the mechanical and thermal properties of materials, but also the external constraint and thermal condition. So, in order to study the actual situation in its service process, a temperature-dependent thermal shock resistance model for ultra-high temperature ceramics considering the effects of the thermal environment and external constraint was established based on the existing theory. The present work mainly focused on the adjustment of the stress reduction factor according to different thermal shock situations. The influences of external constraint on both critical rupture temperature difference and the second thermal shock resistance parameter in either case of rapid heating or cooling conditions had been studied based on this model. The results show the necessity of adjustment of the stress reduction factor in different thermal shock situations and the limitations of the applicable range of the second thermal shock resistance parameter. Furthermore, the model was validated by the finite element method.

High-resolution X-ray reflectivity has been employed to study the structure, wetting properties, and interfacial roughness of ultra-thin liquid crystal films. The films were prepared at the air–water interface and transferred on to glass substrates by a modified horizontal deposition technique. A 3-layer film was found to partially-wet the substrate in the nematic and isotropic phases and dewet upon cooling to the crystalline phase. The surface roughnesses at the air-film and the film-glass interfaces exhibited a gradual reversible but hysteretic conformal (strongly correlated) to non-conformal transition between the isotropic and smectic-A phases.

High-resolution x-ray diffraction studies have been performed on the polar liquid-crystal binary mixture of octyl- and decyl-oxyphenyl nitrobenxoyloxy benzoate (47.4 mole% DB8ONO2 + 52.6% DB10ONO2). At temperatures below 127 °C this mixture exhibits the phase sequence (with decreasing temperature) smectic-Ad—reentrant nematic Nd—reentrant nematic N1—smectic-A1—tilted smectic antiphase smectic- [Ctilde] -smectic-A2—smectic-C2. The development of lateral fluid antiphase fluctuations from the partial bilayer fluctuations in the smectic-A1 phase have been probed and analyzed as well as the transition from the tilted smectic antiphase to the tilted bilayer smectic-C2 phase that is accompanied by the occurrence of an intermediate smectic-A2 phase.

The structural evolution of films of the liquid crystal mixture mx5112 in the Sm-A and Sm-C* phases has been studied using X-ray scattering techniques. Liquid crystal (LC) films spread on glass substrates with relief grating of 10um period are subjected to competing boundary conditions on opposite sides. On one side, the LC-air interface imposes a homeotropic boundary condition while on the other the glass substrate with grating generally imposes a homogeneous boundary condition. Near room temperature, the smectic layer normal is oriented parallel to the grating lines on the substrate. The measured smectic layer spacing reveals oscillations between a Sm-C* and a Sm-A like structures which depend on the film thickness as well as the distance from LC-air interface and is attributable to the difference in the molecular tilt imposed by a. the air-LC interface; b. the substrate; and c. the gratings' internal walls.

The transient temperature distribution of the ultrahigh-temperature ceramic (UHTC) thermal protection system (TPS) of hypersonic vehicles is calculated using finite volume method. Convective cooling enables a balance of heat increment and loss to be achieved. The temperature in the UHTC plate at the balance is approximately proportional to the surface heat flux and is approximately inversely proportional to the convective heat transfer coefficient. The failure modes of the UHTCs are presented by investigating the thermal stress field of the UHTC TPS under different thermal environments. The UHTCs which act as the thermal protection materials of hypersonic vehicles can fail because of the tensile stress at the lower surface, an area above the middle plane, and the upper surface as well as because of the compressive stress at the upper surface. However, the area between the lower surface and the middle plane and a small area near the upper surface are relatively safe. Neither the compressive stress nor the tensile stress will cause failure of these areas.

A longitudinal ferroelectric smectic phase lacks a reflection or mirror symmetry, which may be achieved with appropriate chemical design of molecules. Using x-ray diffraction, we have examined thre...

Polyphilic compounds were previously reported @F. G. Tournilhac, L. M. Blinov, J. Simon, and S. V. Yablonski, Nature 359, 621 ~1992!# to form a longitudinal ferroelectric liquid crystal phase in the mixture M70 ~70% compound I, 30% compound II!. The ferroelectric nature was confirmed by measurements of acoustically induced piezoelectric response and hysteretic behavior in the smectic-X8 phase. We carefully examined the structure of the smectic-X8 phase with high-resolution x-ray diffraction in an aligned sample and discovered bilayer structure, suggesting that molecules actually adopt antiferroelectric arrangement. @S1063-651X~97!06604-X#

THE key requirement for a material to exhibit nonlinear optical (NLO) activity is the presence of non-centrosymmetric (polar) order, an attribute that is usually restricted to certain crystalline classes and ferroelectric liquid crystals1,2. NLO activity can also be obtained in some amorphous organic materials by applying an intense electric field (corona discharge) above the glass transition temperature, Tg, and subsequently quenching the field-induced polar orientation order3,4. Such materials are attractive for NLO device applications, as they promise lower costs and easier processibility than their crystalline organic and inorganic counterparts5. But field-induced polar order is not stable, and the eventual return to equilibrium (apolar) order results in a deterioration of NLO activity, particularly at temperatures near Tg (refs 6, 7). Here we show that this thermally activated decay of polar order can be circumvented by using a liquid crystal in which both mesogens (molecules that induce a liquid-crystal phase) and NLO-active chromophores are appended to macromolecular siloxane rings. We find that a shear-aligned melt of these composite macromolecules gives rise to a material with a monodomain lamellar superstructure that retains bistable, field-induced polar order above Tg. We attribute the thermal stability of these materials to an energetically favoured polar packing arrangement of the constituent macro-molecules.

High-resolution x-ray scattering measurements of 23.0 and 28.4 mole % mixtures of TBBA in DB6 have led to the observation of a true incommensurate phase in a liquid crystal system. The phase is characterized by two primary quasi-Bragg reflections at incommensurate wave vectors. X-ray scattering measurements of a freely suspended film in the new phase have revealed that it possesses sixfold in-plane symmetry, suggesting it to be an incommensurate crystal- $B$ or - $E$ phase.

We report results of x-ray reflectivity studies of thin conducting films, organic alignment layers, and liquid crystal films on flat glass substrates. An understanding of these films is important for liquid crystal science and technology. Specular reflectivities of conducting layers of indium tin oxide, organic overlayers of lecithin, 12-8 (poly)diacetylene polymerized with linearly polarized uv light, and spin-coated diheptylazoxybenzene films were measured. The evolution of surface roughness of soda-lime glass substrates as a function of chemical etching time was also studied. Using theoretical models based on the Fourier transform of electron density gradients and the matrix formalism of x-ray optics, we quantitatively determined film thickness, layer spacing, electron density, and rms surface roughness and height-height correlations.

The delicate interplay between the effects of mechanical rubbing and subsequent photo-induced chemical reactions on polyimide films has been studied for aligning liquid crystals. Exposure to linearly polarized ultraviolet (LPUV) light was found to profoundly alter the direction and the degree of molecular orientations obtained by rubbing. A simple model is presented to describe the observed changes in the director orientation. The results show that LPUV exposure can be very effectively used to control and fine-tune liquid crystal alignment.

The mechanism responsible for liquid crystal (LC) alignment on mechanically buffed or UV exposed polymer films is poorly understood. A comprehensive study of LC alignment on variously prepared substrates unequivocally shows that the anisotropy in the surface roughness of the substrate completely determines the direction of LC alignment. In all the cases studied, including those where an anchoring transition occurs with temperature, the LC director (re)aligns in the directions of low roughness.

High-resolution x-ray diffraction studies of a binary mixture of the n=8 and n=10 homologs of alkoxyphenyl nitrobenzoyloxy benzoate ${\mathrm{DB}}_{\mathit{n}}$${\mathrm{ONO}}_{2}$ (52.6 mole % ${\mathrm{DB}}_{10}$${\mathrm{ONO}}_{2}$) have been performed at the onset of the tilted smectic antiphase (Sm-C\ifmmode \tilde{}\else \~{}\fi{}) upon cooling from the smectic-${\mathit{A}}_{1}$ phase. Fluctuations of the two competing smectic orders are found to be related to monolayer and partial bilayer ordering. With decreasing temperature, the period of the antiphase in-plane modulation increases while the smectic layer normal tilts at an angle of \ifmmode\pm\else\textpm\fi{}7\ifmmode^\circ\else\textdegree\fi{} with respect to the director. This tilt partially recovers upon the transition to the smectic-${\mathit{A}}_{2}$ phase. \textcopyright{} 1996 The American Physical Society.

We present the results of an X-ray scattering study on partially confined smectic-A (layered) LC films. This partial confinement is achieved by placing the LC material inside the open grooves of a glass grating. Samples prepared in this manner are confined in the direction perpendicular to the gratings. Samples in which the LC is contained entirely inside the grooves develop an induced molecular tilt, which results in a compression of the smectic layers. The molecular tilt varies as the amount of the LC material increases, eventually forming a thin overlayer film above the gratings. As the thickness varies, a second region develops in the films. The layer spacing in this region is close to the bulk layer spacing. This structural evolution is coupled to a variation in the nematic-to-smectic-A phase transition temperature of the samples from the bulk. In addition, the nature of the phase transition is driven first order. The effects of partial confinement on a sm-C* LC film is discussed briefly.

By using the techniques of normal Bragg scattering and glancing incidence scattering, we have measured both the out-of-plane and in-plane structures of GaixInxAs epilay-ers grown on GaAs (001) face by MOVPE. The data presented in this paper were collected from two samples: a thick epilayer of 40, 000A Ga0.84In0.16As, and a thin epilayer of 500Å Ga0.84In0.19As, which are lattice-mismatched with the substrate by about 1.14% and 1.36% respectively according to the Vegard's law. The measured average lattice constants of the thick epilayer agree with the expectation both out-of-plane and in-plane and show nearly complete structural relaxation. An uniaxial in-plane modulation in the thick epilayer has been observed. The out-of-plane and in-plane data of the thin epilayer demonstrate a tetragonal distortion of the lattice, and the Poisson's ratio is calculated to be 0.36. The averaged in-plane lattice constant is still 0.18% larger than that of the substrate, which results in a complex structure at the epitaxial interface. An in-plane misalignment along one (110) axis in the thin epilayer has been observed.