Search for sp interference in the emission of prompt neutrons from 235U fission induced by polarized thermal neutrons

It is shown that the softening of the acoustic mode and the ensuing ferroelastic phase transition are due to the linear-quadratic interaction between the symmetric and antisymmetric deformations, which is presently neglected in the literature. An expression is obtained which can be used to predict the phase transition pressure if the initial elastic moduli are known.     

First-order phase transition in potassium dysprosium tungstate induced by the field renormalization and softening of the elastic moduli in the vicinity of a structural Jahn-Teller-type transition

The magnetostriction of KDy(WO₄)₂ single crystals is measured in an external magnetic field at temperatures below the temperature of a structural phase transition of the Jahn-Teller type. A steplike irreversible variation in the elastic strain is observed to occur with an increase in the magnetic field applied along the a or b axis of the monoclinic cell of the crystal. The residual change in the strain is retained after changing the sign of the magnetic field. The return to the initial state characterized by field-induced jumps in the strain is possible only after thermal cycling well above the structural phase transition temperature. The theory of this phenomenon is developed using a phenomenologically derived thermodynamic potential of the elastic sub-system that takes into account the crystal symmetry and the field renormalization of the elastic moduli. The jumplike transitions are interpreted as being due to the magnetic softening of the elastic moduli in the vicinity of the structural phase transition temperature.     

On-line phase transition in La1− x Sr x MnO3 (0.28 ≤ x ≤ 0.36) perovskites through ultrasonic studies

La_{1? x} Sr _x MnO₃ perovskite manganite materials with different compositions (x?=?0.28, 0.31 and 0.36) have been prepared employing solid-state reaction technique. On-line evaluation of ultrasonic velocities and longitudinal attenuation of the above samples has been done over a wide range of temperatures using the transmission method. The observed anomalies in velocities, attenuation and elastic moduli reveal the occurrence of lattice softening and hardening near Curie temperature. The observed dramatic hardening in sound velocities and softening in attenuation are correlated with the phase transition, i.e. ferromagnetic to paramagnetic. The increase in magnitude of maximum velocity with change in Sr content at T _c indicates the existence of linear magnetostriction effect. The elastic moduli study elucidates the observations made from the above-mentioned studies. The variation in the ultrasonic velocities, longitudinal attenuation and its derived parameters help us to understand the competitions between ferromagnetism and paramagnetism.     

用激光空腔锁定技术测量材料的拉,压弹性模量的相对误差

将一激光器的工作频率锁定在Fabry-Perot标准具(简称F-P)的共振峰上,利用F-P标准具光学共振频率随腔长的灵敏响应特性,用光学拍频方法测得F一P腔体材料在微小应力作用下的应变值,就可得到材料的弹性模量.本文就此提出了测量材料拉、压弹性模量相对误差的一种方法,并且实测了石英材料拉、压弹性模量的相对误差     

Complete elastic tensor through the first-order transformation in U2Rh3Si5

The complete elastic tensor of U(2)Rh(3)Si(5) has been determined over the temperature range of 5-300 K, including the dramatic first-order transition to an antiferromagnetic state at 25.5 K. Sharp upward steps in the elastic moduli as the temperature is decreased through the transition reveal the first-order nature of the phase change. In the antiferromagnetic state the temperature dependence of the elastic moduli scales with the square of the ordered moment on the uranium ion, demonstrating strong spin-lattice coupling. The temperature dependence of the moduli well above the transition indicates coupling of the ultrasonic waves to the crystal electric field levels of the uranium ion where the lowest state is a singlet. The elastic constant data suggest that the first-order phase change is magnetically driven by a bootstrap mechanism involving the ground state singlet and a magnetically active crystal electric field level.     

Ductile titanium alloy with low Poisson's ratio

We report a ductile beta-type titanium alloy with body-centered cubic (bcc) crystal structure having a low Poisson's ratio of 0.14. The almost identical ultralow bulk and shear moduli of approximately 24 GPa combined with an ultrahigh strength of approximately 0.9 GPa contribute to easy crystal distortion due to much-weakened chemical bonding of atoms in the crystal, leading to significant elastic softening in tension and elastic hardening in compression. The peculiar elastic and plastic deformation behaviors of the alloy are interpreted as a result of approaching the elastic limit of the bcc crystal under applied stress.     

Elastic properties and spectroscopic studies of lithium lead borate glasses

Elastic properties of xLi₂O — 20PbO — (80-x)B₂O₃ glasses have been measured at a frequency of 10 MHz using X-cut and Y-cut quartz transducers. The trends in the variation of elastic moduli, Poisson’s ratio and Debye temperature have been studied. The elastic moduli namely longitudinal and young’s modulus show strong linear dependence while bulk and shear modulus vary marginally as a function of Li₂O concentration. The Poisson’s ratio is found to be almost constant and Debye temperature increases with the increase of Li₂O concentration. IR, MAS-NMR and glass transition temperature studies have been also carried out. Glass transition temperature is found to increase with increase of Li₂O concentration. IR and MAS-NMR spectra show characteristic features of borate network and systematic change as a function of Li₂O concentration. The variation in the elastic properties and structural features of IR and MAS-NMR indicate that Pb²⁺ ions are likely to occupy network forming positions in this glass system. Paper presented at the 2nd International Conference on Ionic Devices, Anna University, Chennai, India, Nov. 28–30, 2003.     

Effect of rare earth (Pr2O3, Nd2O3, Sm2O3, Eu2O3, Gd2O3 and Er2O3 ) on the acoustic properties of glass belonging to bismuth-borate system

Bismuth-borate glasses doped with some rare earth ions were studied with respect to the density, molar volume and the elastic moduli, Poisson’s ratio, Debye temperature, microhardness, softening temperature, acoustic impedance, diffusion constant and latent heat of melting. Ultrasonic velocities were measured by the pulse echo overlap technique at a frequency of 10 MHz and at room temperature. From these velocities and density values, various elastic moduli were calculated. The correlation of elastic stiffness, the cross link density, and the fractal bond connectivity of these glasses are discussed. The derived experimental values of shear modulus, bulk modulus, Young’s modulus, and Poisson’s ratio for our glasses are compared with the theoretically calculated values in terms of the bond compression model and Makishima-Mackenize theory.     

Elastic properties of metastable crystalline and amorphous gasb-ge semiconductors synthesized under high pressure

We present the systematic study of the elastic shear G and bulk B moduli in amorphous and crystalline metastable ternary solid solutions (GaSb)_1?x Ge_2x . It is found that the moduli of crystalline phases initially decrease with Ge concentration, falling down to minimum values at 20-30% Ge. The minimal values of elastic moduli for amorphous samples are observed at 50-60% Ge. Elastic softness of crystalline solid solutions is assumed to be related to the increase of chemical disorder and, consequently, of static (non-thermal) geometrical disorder in positions of atoms. An additional topological disorder in amorphous solid solutions leads to additional elastic softening.     

Measurement of the elastic moduli of dense layers of oriented carbon nanotubes by a scanning force microscope

A technique is developed for measuring the modulus of elasticity of a material with a Nanoscan scanning force microscope on the basis of measuring the dependence of probe vibration frequency on the penetration depth of the needle into the specimen. This technique makes it possible to study materials with elastic moduli from 50 to 1000 GPa. The Young moduli of dense films of carbon nanotubes oriented at angles of 45° and 90° to the quartz substrate are measured. From their ratio, the Young modulus in the direction perpendicular to the tubes and the anisotropy of the elastic moduli are determined. A comparison of these values with the corresponding values obtained for a nanotube film deposited on a silicon substrate is carried out. On the basis of this comparison, a conclusion is made concerning the interaction between single-layer nanotubes and between nanotubes in a mixture of single-layer and multilayer ones.     

Temperature effect on the elastic properties of yttrium garnet ferrite Y<Subscript>3</Subscript>Fe<Subscript>5</Subscript>O<Subscript>12</Subscript>

The Young’s moduli along the [100] and [110] crystallographic directions and the shear modulus along the [100] direction in a high-purity yttrium garnet ferrite single crystal are measured in the temperature range from 20 to 600°C. All the independent elastic constants are calculated for this temperature range. The behavior of the elastic moduli and elastic anisotropy factor is analyzed in the vicinity of the critical temperature of the magnetic phase transition.     

Phonon anomalies and martensitic transitions in bcc materials

The general systematics of phonon spectral anomalies are considered for materials with bcc and B2 structures. In a model of central pair-wise interactions, it is shown that anomalies on the TA₂ branch at =1/3 (211), the LA phonon branch at =2/3 (111), and on the TA branch at =1/2 (111) arise simultaneously with the softening of the shear elastic moduli C' and C₄₄. The TA phonon anomaly at =1/3 (110) is not due to softening of the elastic moduli, but rather is due to the long range nature of the pair-wise or multiparticle interaction. A model is proposed for the nucleation of the martensitic phase for B2 compounds, the distinctive feature of which is a consideration of the softening of the shear moduli, which is modulated by lattice relaxation around ordering defects, along with the possibility of additional defect ordering near the transition.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, No. 10, pp. 31–39, October, 1993.     

Elastic moduli of solid helium at megabar pressures

Abstract

Elastic moduli and equation of state of highly compressed solid He are calculated up to the pressure 1.5 Mbar. While calculating the energy of the crystal, pair interatomic interactions and three-body interactions are taken into account. Three-body contribution is significant to a great extent both for the equation ofstate and for the elastic moduli and makes considerable softening of the equation of state. On the other hand, vibrational characteristics, such as the sound velocity and the Debye temperature, do not strongly depend on the inclusion of non-additive interactions.     

Second pressure derivatives of the elastic moduli of fused quartz

The first and second pressure derivatives of the elastic moduli of fused quartz have been determined by ultrasonic measurements of the sound velocity as a function of pressure. The second derivatives are positive in sign and are larger in magnitude than those in crystalline materials. The data are used to calculate the equation of state and the volume dependence of the Grüneisen constant.     

Investigation of interface properties by nanoscale elastic modulus mapping

We present a method for investigating the spatial changes of elastic moduli in a nm-scale vicinity of interfaces. The method is demonstrated on twin walls in PbTiO(3) single crystals. It is revealed that the region near the twin wall is significantly softer than the two domains surrounding it. A comparison with finite element simulations relates this effect to an anelastic relaxation due to point defect accumulation around the twin wall. Local softening around the twin wall can affect the overall elastic modulus in thin films and nanostructured ferroelectric materials, in which the average distance between twin walls is smaller than the thickness of the softer region.     

Ultrasonic,FTIR and thermal investigations of SiO2–Na2O–CaO–P2O5 glasses doped with CeO2

The longitudinal and shear ultrasonic wave velocities for different compositions of SiO₂–Na₂O–CaO–P₂O₅ glasses were measured at room temperature (305 K) using a pulse-echo method at a frequency of 4 MHz. The elastic moduli, Poisson's ratio, microhardness, Debye temperature and other ultrasonic parameters were obtained from experimental data and analyzed using bond compression theory. By calculating the number of network bonds per unit volume, the average stretching constant, and the average ring size, information about the structure of the glass can be deduced. Structural changes after doping with CeO₂ were investigated by FTIR spectroscopy, and by measurements of the thermal expansion coefficient, glass transition and softening temperature to throw more light on the characterization of these glasses.     

Thermal expansion, elastic properties and the Mössbauer effect of stainless Invar

The thermal expansion, the elastic moduli (E, G, B) and the specific heat of three Co-Fe-Cr alloys are reported together with the dependence of these properties. Alloys with fcc structure exhibit the Invar anomaly, i.e., a large spontaneous volume magnetostriction and an elastic softening below T_c. Data for the spontaneous volume magnetostriction and for the ΔE-Effect are given. The Mössbauer effect was measured at 4.2, 77 K and room temperature.     

Alpha-plutonium's polycrystalline elastic moduli over its full temperature range

alpha-plutonium's volume-corrected polycrystal elastic moduli were measured between 18 K and the upper limit of its occurrence, near 400 K. The two independent moduli for a polycrystal-bulk and shear-behave smoothly, indicating no phase transition. Both moduli show the same 50% increase on cooling, an order of magnitude larger than in other metals. The Debye temperature obtained from low-temperature elastic moduli, 207 K, significantly exceeds most previous estimates. The Gruneisen parameter gamma=5.3, obtained from the temperature dependence of the bulk modulus, is intermediate among previous estimates using other approaches, alpha-plutonium's Poisson ratio nu is low: 0.18, nearly temperature independent, and its small decrease on warming opposes usual behavior. The high gamma, large but equal bulk modulus and shear modulus fractional stiffening on cooling, and near-temperature-invariant nu are attributed to a single mechanism: 5-f electron localization-delocalization.     

Softening of surface phonons in the (100) plane of Nb3Sn

The propagation of the surface acoustic mode in the (001) plane is investigated as a function of the temperature dependent elastic moduli of Nb₃Sn in the cubic phase. For the [100] and [110]-directions a softening of the surface mode velocity ν_s is found to follow the softening of the velocity ν_T2 of the bulk shear mode T₂A. It is shown that for the [100]-direction the ratio $\text{(}\text{ν}{}_{\mathrm{s}}\text{ν}{}_{\mathrm{T2}}\text{)}$ tends to ?2 at the structural phase transition. The question is discussed of whether the softening of the surface mode can be observed with Brillouin scattering from opaque surfaces.     

Structural instability and superconductivity in B{a_{1 - x}}{K_x}Bi{o_3}

We report results of the ultrasonic investigation of Ba_1-xK_xBiO₃ superconducting (SC) single crystals for two potassium concentrations and in a wide temperature range including the normal and the SC states. An instability of the crystal lattice that develops above the superconducting phase transition leads to a softening of both the transverse c₄₄ and the longitudinal c₁₁ modes at temperatures between 200 K and 50 K. In the case of Ba_0.65K_0.35BiO₃ a pronounced hysteresis was discovered. Low temperature X-ray powder diffraction analysis does not reveal any change of the cubic structure in the samples within a resolution of our X-ray technique. The softening of the elastic moduli, the hysteresis, the maximum in the attenuation of sound along with the possible short- (or long-) range structural distortion can be explained qualitatively in a simple model by assuming a coupling of the acoustic modes with the anharmonic oscillations of BiO₆ octahedra. Some weak anomalies are discovered in the velocity of the longitudinal sound in the vicinity of the superconducting phase transition. Received 25 June 1999 and Received in final form 14 February 2000