Electromagnetic emission under uniaxial compression of ice: II. Analysis of the relationship between an electromagnetic signal and the dynamics of charged dislocation pile-ups

The nature of the electrical activity of mesoscopic defects in I_h ice and the relationship of this activity with the kinetics of plastic deformation and fracture is discussed. The theoretical time dependences of the shear caused by active dislocation pile-ups are compared with the shapes of actual electromagnetic signals generated in different stages of the plastic deformation of I_h ice. Good agreement with the model of free expansion of a conservative dislocation pile-up and the model of nucleation and propagation of a slip band is established.     

Electromagnetic emission under uniaxial compression of ice: I. Identification of nonstationary processes of structural relaxation by electromagnetic signals

Discrete electromagnetic emission caused by a jump of plastic deformation and fracture of single-crystal and polycrystalline ice is revealed and investigated. An album of electromagnetic signals is compiled, which makes it possible, on the basis of the electromagnetic-signal shape, to identify and study the kinetics of mesoscopic events of structural relaxation that are related to the dynamics of dislocation pile-ups and cracks.     

Comparison between flexural and uniaxial compression tests to measure the elastic modulus of silica aerogel

The Young’s moduli of a set of silica aerogels have been measured by two techniques: 3-point bending and uniaxial compression. The data found by the two methods differ strongly. The uniaxial compression test gives generally underestimated values of Young’s modulus, because of geometrical effects. The appropriate gauge lengths were estimated based on the discussion of Euler buckling and nonuniform stress distribution. The measured compressive moduli were analyzed to correct for machine compliance and possible misalignment under compression of the aerogels. Similarly, moduli obtained by 3-point bending depend on the length/thickness ratio of the sample, reaching equilibrium only for ratios above about 10. The corrected compressive moduli were comparable to those measured by 3-point bending on samples of sufficient length.     

MAEAM investigation of the structural stability and theoretical strength of Fe crystals under uniaxial loading

The structural stability and theoretical strength of BCC crystal Fe under uniaxial loading have been investigated with the modified analytic embedded‐atom method (MAEAM). Even if an orthorhombic path is applied, the deformation is spontaneous along the tetragonal path till Milstein modified Born criterion B₂₂‐B₂₃>0 is violated at λ₁=0.9064 in the compressive region. The branched orthogonal path with lower compressive stress σ₁ and energy E is preferred over the conventional tetragonal Bain path. A stress‐free FCC phase with the local maximum energy of ‐4.2186eV appearing either in compressive region (orthorhombic path) at λ₁=0.8923 or in tensile region (tetragonal path) at λ₁=1.2619 is unstable and would slip spontaneously into its near neighbor stress‐free mBCT phase with the local minimum energy of ‐4.2270eV. The initial BCC phase with the lowest energy of ‐4.280eV is the most stable in correspondence with the actual behavior of Fe. Furthermore, the stable region ranges from ‐79.7eV/nm³ to 30.6eV/nm³ in the theoretical strength or from 0.9064 to 1.1788 in the stretch λ₁ correspondingly. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The elastic behaviour of TeO2 glass under uniaxial and hydrostatic pressure

TeO₂ glass of purity exceeding 98.5 mol.% has been made, despite earlier suggestions that some 7.5–10 mol.% of modifier is required to form vitreous telluride networks. It is argued that in view of the high purity of the glass obtained, TeO₂ may well be able to form glass by itself, given an appropriate thermal history in preparation. The hydrostatic and uniaxial pressure dependences of ultrasonic waves propagated in this glass at room temperature have been measured. The results provide the second and third order elastic constants of the glass. The bulk modulus is consistent with a ring diameter averaging about 8 atoms (Te₄O₄ rings) suggesting that the glass is a disordered version of paratellurite; however, if the TeO bending force constant were to be unusually strong, then a larger ring diameter (as in tellurite) would be indicated. Although the anion coordination number is only 4, the pressure derivatives of the second order elastic constants are positive and the third order elastic constants are negative, in marked contrast to the anomalous behaviour of silica-based glasses. These findings suggest that bond bending motions of bridging atoms between the trigonal bypyramidal groups (which are the structural units) do not play an important role in the elastic properties of TeO₂ glass. In consequence the shear and oongitudinal acoustic mode Grüneisen parameters are both positive (γ₁ = +2.14, γ_s = +1.11): the long wavelength acoustic modes stiffen under hydrostatic pressure.     

Corrosion dynamics of electric conductivity of metal films in ice under its structural and phase transformations

The corrosion-induced decrease in the electric conductivity of metal films in ice under its thermally induced structural relaxation and at the water crystallization front have been investigated. Mathematical models of corrosion enhancement are proposed that are based on the suggestion about overcoming the reaction barrier as a result of the release of recombination energy of orientational defects during structural and phase transformations of ice.     

Pseudoacids. III: formation and structures of new cyclic oxocarboxylic acids

Crystal and molecular structures for aryl- and benzyl-substituted 3-hydroxy-3,4-dihydroisobenzopyran-1-one are described. For the 4,4-dimethyl derivative, two monoclinic modifications are produced on sublimation: (1) P2₁/c, a = 23.668(6) Å b = 11.749(4) Å c = 7.215(2) Å = 96.70(2); V = 1992.6(10) ų; (2) P2₁/c, a = 11.509(4) Å, b = 12.429(4) Å, c = 6.933(2) Å, = 91.12(3), V = 991.6(6) Å. In both, hydroxy groups are pseudoaxial, and molecules form dimeric complementary but noncooperative hydrogen bonds. In the 6-methoxy derivative (3), P2₁/c; a = 14.185(7) Å b = 4.105(4) Å c = 15.598(13) Å = 98.59(6); V = 898.1(10) ų, the hydroxy group is pseudoequatorial, and the intermolecular hydrogen bonding forms infinite chains. In the 4,4-dimethyl-7-nitro derivative (4), C2/c, a = 19.365(4), b = 8.425(2), c = 13.464(4), = 92.62(2), V = 2194.2(7) ų, the hydroxy group is pseudoaxial, and the intermolecular hydrogen bonding forms chains. Compared to the unmethylated compounds, the interbond angles within the ring at the dimethylated carbon are contracted.     

Propagation of elastic waves near acoustic axes in crystals under the action of extreme external forces: II. Centrosymmetric crystal in an electric field; III. Crystal under mechanical stress

The problem of degeneracy of elastic waves propagating in the vicinity of acoustic axes in centrosymmetric crystals in an external electric field and in crystals with arbitrary symmetry under uniaxial mechanical stress is considered. For the case of acoustic axes coinciding in the absence of external forces with the symmetry axes, equations describing the directions of external forces at which the difference in the velocities of initially degenerate transverse waves or angles of splitting and shift of the initial degeneracies reach maximum values are obtained. By the example of acoustic axes in LiNbO₃, La₃Ga₅SiO₁₄, Bi₁₂SiO₂₀, and SrTiO₃ crystals, extreme directions of uniaxial mechanical stress and the corresponding changes in the characteristics of transverse waves are calculated. The behavior of acoustic axes under uniaxial compression is analyzed.     

Optical properties of a nematic liquid crystal under the binary effect of periodic shear and compression

The optical response (clearing) of a liquid crystal cell, containing a layer of a nematic crystal with homeotropic alignment, under the binary effect of periodic shear and compression has been theoretically described. The effect was calculated taking into account the presence of orientational waves arising at displacement of one of the boundary plates in its plane, and the mobility of this plate along the normal upon statement of boundary conditions. The influence on the effect of the alignment energy of molecules at the layer boundaries is analyzed. The calculation results are compared with the experimental data.     

Degenerate Reflections in Acoustics of Solids. III: Cubic Crystals

Crystallography Reports - Particular cases of reflection of bulk acoustic waves from a free surface of cubic crystal, when both the plane of incidence and the crystal surface are symmetry planes,...     

Blue emission of ZrO2:Tm nanocrystals with different crystal structure under UV excitation

In this paper, thulium doped ZrO₂ nanocrystals with different phases were prepared by Pechini sol-gel complex method. The effects of host structure, phases and concentration on luminescence properties of Tm³⁺ ions were investigated. With the increase of the concentration of Tm³⁺ ions, the phosphors phase transformed from monoclinic to tetragonal structure and all the phosphors emitted blue light. However, because the environment symmetry around Tm³⁺ in m-ZrO₂ (C_2h) is worse than that of t-ZrO₂ (D_4h), there were still obvious differences in the emission and excitation. The reason was investigated with the help of energy level diagram. In addition, the quenching concentration of Tm³⁺ in ZrO₂ host is 1 m/m%.     

水热反应温度对碱性条件下TiO2纳米晶生长动力学及其光电性能的影响

以钛酸丁酯为原料,在碱性条件下水热合成了锐钛矿相TiO2纳米晶.采用X射线衍射仪、比表面积测试仪、扫描电子显微镜等方法对纳米晶粒的晶体结构、比表面积以及光阳极的表面形貌分别进行表征,研究水热反应温度对TiO2纳米晶生长动力学过程的影响规律.结果表明:随着温度的升高,锐钛矿相纳米晶的平均晶粒尺寸先增大后减小.光电转换性能测试表明,电池的光电转换性能随着水热反应温度的升高而增大,在220℃时获得最大光电转换效率4.05;( 1sun,100 mW/cm2).并分析了水热反应温度对碱性条件下TiO2纳米晶粒的生长动力学过程和电池光电转换性能的影响机制.     

Melting and Crystallization Processes of EBBA III. Microscopic Observation

Melting and crystallization processes of EBBA (N-p-ethoxybenzylidene-p'-butyl aniline) have been studied by the method of microscopic observation. A solid sample obtained by rapid cooling (cooling rate: 40 K/min) from nematic to 5°C (sample A) shows granular texture. A band like crystal grows in sample A at room temperature. A solid sample obtained by quenching (cooling rate: 13 K/s) from nematic to about - 100°C (sample B) shows cloudy texture. In the heating process sample B transforms to a more stable crystalline form which shows mosaic texture. A needle crystal grows slowly in a supercooled nematic state, then smaller needle crystals grow rapidly around the needle crystal. The formation and transformation processes of solid modifications are explained on the basis of the idea of instability of them, and schematic phase diagram of those solid forms is presented.

Keywords: EBBA, liquid crystals, nematic, solids, crystal growth, microscopic observation     

Glow curves and the emission of BaFCl:Eu2+

Thermoluminescence and the emission of γ-irradiated BaFCl:Eu²⁺ has been studied. A well defined glow peak at 400 K and a shoulder at 480 K are obtained in addition to those obtained as in undoped BaFCl. Thermoluminescence emission has shown a band at about 390 ∼ 400 nm. Additional glow peaks have been attributed to the luminescence centers as caused by europium impurity.     

ThePoranthera corymbosa alkaloids. III. Molecular structure and absolute configuration of poranthericine hydrobromide

Crystals of the hydrobromide of poranthericine, C₁₅H₂₇NO, an alkaloid isolated from the plantPoranthera corymbosa Brogn. (family Euphorbiaceae) were found to belong to the orthorhombic space groupP2₁2₁2₁, with unit cell dimensionsa = 12·761,b = 15·144 andc = 8·030 Å, andZ = 4. The structure was solved by the heavy-atom method and refined with 1276 reflections to a residual of 0·091 The molecule was found to consist of three saturated six-membered rings fused on to a common central nitrogen atom, with methyl, ethyl and hydroxyl groups attached to the periphery. All rings had thechair conformation and were fusedtrans to one another. The absolute configuration was determined using the anomalous scattering power of the bromine atom.     

Thermal expansion of microcracks and grain boundaries in crystals (II). Thermal expansion and compression under pressure of intermicrocrystallite gaps in liquids

The changes of melting points under pressure (melting curves) in terms of the microcrystallite model of liquids has been studied. These curves allowed the estimation of temperature dependencies of the scale factor n (or effective temperature) for microcrystallite surfaces for a representative group of liquids above the melting points. Further, by using the results of the author's previous works, general temperature relationship of the scale factor n for microcracks and gaps over a wide range of bulk temperature has been plotted. This relationship is characterized by the sharp jump at the melting point and is asymmetrical with respect to the melting temperature line. Solid-liquid transitions from the standpoint of the microcrystallite model have been discussed.     

Thermal Studies on the Phase Transitions of p-n-Dodecyloxy and p-n-Tetradecyloxy Benzoic Acids.: (Part III)

Thermal properties of p-n-dodecyloxy and p-n-tetradecyloxy benzoic acids have been investigated using a differential scanning calorimeter. Heat capacities of thermally stable and metastable crystals of these acids were estimated and the effects of the paraffinic chain attached to the benzoic acid on the formation of a metastable crystalline state are discussed. It is suggested that the disorder of hydrogen bond configuration in associating carboxylic acids in pairs brings about the formation of a metastable crystal state for the alkoxy benzoic acids in general. Under the same conditions the mobile paraffinic chains control the modification of the hydrogen bond during the course of crystallization.     

Dynamics in glass-forming mixtures: Comparison of behavior of polymeric and non-polymeric components

Recent experiments have shown that the segmental dynamics of dilute polymer chains in glass-forming matrices are substantially different than the dynamics of the host. In contrast, we show here that the dynamics of dilute low molecular weight species (tetracene and rubrene) are very similar to the dynamics of the host matrix. This is consistent with the view that self-concentration is a dominant factor in determining the dynamics of polymer chains in glass-forming mixtures; as a polymeric effect, self-concentration should be negligible for low molecular weight species. In addition, we show that self-concentration may play an important role in understanding segmental dynamics in concentrated polymer solutions.