Ultrasonic investigation of amorphous carbon in various frequencies at low temperatures from 2.1 to 300 K

Using pulse echo overlap measurement, the elastic behavior of amorphous carbon has been studied at ambient and low temperatures. The smaller ratio B/G of the bulk modulus to shear modulus and smaller Poisson's ratio σ at room temperature indicate that there is an intrinsic stiffening of transverse acoustic phonons in the amorphous carbon. The acoustic velocity and attenuation for longitudinal modes have been measured between 2.1 and 300 K at three frequencies of 7, 21 and 35 MHz, respectively. Their frequency and temperature dependence are observed. The elastic constant C₁₁ increases with decreasing temperature and show enhanced stiffening at low temperatures. In the 130-220 K region, the abnormal change and effect of longitudinal velocity and attenuation with temperature and frequency, and a phase transition associated with structure relaxations are discussed.     

Measurement of internal friction and shear modulus of aluminium-magnesium alloy after neutron irradiation at 78 K

The internal friction and shear modulus of polycrystalline Al + 3 at.% Mg was studied as a function of temperature after fast neutron irradiation, at liquid nitrogen temperature (78 K). The initial increase of shear modulus after irradiation is mainly due to the dislocation pinning of the irradiation induced defects. The further increase of shear modulus during annealing is caused by the bulk-effect. The recovery of shear modulus up to room temperature is similar to that of the residual electrical resistivity after a low temperature neutron irradiation. However, we may point out that in contrast to stage II, in stage III we observe a recovery of shear modulus that is distinctly more pronounced than that of electrical resistivity. The temperature dependence of internal friction, before irradiation, shows a peak at 230 K. After neutron irradiation we observe no peak in the temperature region, from 90 K to 300 K.     

Investigation of the spectra of depolarized light scattering in two viscous liquids

The fine structure of the Rayleigh line wing was studied experimentally in a broad temperature range in the viscous guaiacol liquid. The propagation velocity and the absorption coefficient of the transverse hypersound were determined by the Rayleigh line wing spectra at temperatures from ?27 to ?85°C. The transverse hypersound absorption is shown to pass through its maximum at a temperature of ?34°C. The experimental data for two vitrifying viscous liquids, namely, guaiacol and salol were analyzed. The limiting elastic bulk moduli were found to intersect at the crystallization temperature and the limiting shear modulus to vanish at this temperature. These and other specific features of the guaiacol and salol behavior at the crystallization temperature, revealed in this and previous our studies, point to the possibility of the existence of two phases in a liquid.     

Errata

Gelatin/1-allyl-3-methylimidazolium chloride solutions with different gelatin concentrations were prepared by using the ionic liquid 1-allyl-3-methylimidazolium chloride [AMIM]Cl as a solvent. Rheological properties of the gelatin ionic liquid solutions were investigated by steady shear and oscillatory shear measurements. In the steady shear measurements, all gelatin solutions showed a shear-thinning behavior at low shear rates, which we suggest reflect the characteristics of solvent [AMIM]Cl. In the oscillatory shear measurements, the effects of concentration and temperature on solution viscoelasticity were determined. The results show that the storage modulus G′ of gelatin solutions was essentially independent of gelatin concentration and temperature at all frequencies, while the solution viscosity greatly depended on polymer concentration and temperature. The loss modulus G″ increased with increase in concentration and decreased with rising temperature. This indicated that physical origins of elastic and dissipative behaviors were completely different. The influence of dissolved gelatin on the elasticity of solutions seemed to be minor.     

Phase velocity and attenuation of 9 GHz hypersound in polycrystalline nickel films as a function of quartz-substrate temperature

Using a movable narrow beam of 9 GHz hypersound and wedge-shaped evaporated Ni films, the phase velocity and attenuation of longitudinal as well as of transverse hypersound was determined at a temperature of 2 K by multiple-beam interferometry in these Ni films. They were evaporated onto a quartz substrate at substrate temperatures ranging from 50°C to 500°C. The macroscopic moduliK (bulk modulus) andG (shear modulus) can be obtained from the measured phase velocities. WhereasK appears independent on substrate temperature and agrees within experimental error with theoretical prediction deduced from known elastic constants of the crystallites, the observed shear modulusG possesses at a substrate temperature of 50°C only 2/3 of the predicted value and increases with substrate temperature by reaching the theoretical value at about 300°C.     

Measurements of the complex shear modulus of polymers under hydrostatic pressure usina the quartz resonator method

Abstract

The quartz resonator method measures the complex shear modulus or compliance of viscoelastic materials in the frequency range from 50 kHz to 140 MHz at temperatures between ?150°C and 300°C and pressures up to 1 GPa. This method can be applied to viscous fluids or polymer melts -even in their glassy or seminystalline regime.

The phase diagram of poly(diethylsiloxane) PDES (a mesophase polymer) was determined for two samples with different molecular weight at pressures up to 400 MPa and temperatures between 20°C and 100°C. Phase transitions are indicated by a sharp bend in the shear compliance although the volume effect of the mesophase-isotropic transition vanishes around 80 MPa.

The pressure dependence of the glass relaxation process (in PVAc), was studied by measuring the change of the complex shear modulus with pressure at constant temperatures between 95°C and 145°C and pressures up to 600 MPa. Additionally to the relaxation process, also the pressure dependence of the real part of the shear modulus in the glassy region can be determined for testing the dislocation concept in the meandermodell by W. Pechhold.     

Non-hydrodynamic modes in viscoelastic behaviour of simple fluids

ABSTRACT

We discuss the role of non-hydrodynamic processes in viscoelastic transition in pure liquids. In particular, using both analytical results and molecular dynamics simulations, we clarify the effect of the shear stress relaxation on the transverse dynamics. We use as an example the Lennard-Jones fluids. We analyse the frequency dependence of the shear modulus and its connection to the non-hydrodynamic shear relaxation mode. The analysis of the relaxation times of the non-hydrodynamic modes in longitudinal and transverse dynamics makes evidence that the emergence of the non-hydrodynamic transverse excitations outside the propagation gap is not directly connected to the onset of the positive sound dispersion.     

Slow nonlinearity kinetics of the viscoelastic properties of oil during shear vibrations

Earlier we discovered the slow evolution of viscoelastic moduli of heavy crude oil. The shear modulus was measured at frequencies of 0.5, 5, and 50 Hz at different temperatures over 72 h. New studies of the dependence of the complex shear modulus on the strain amplitude revealed a logarithmic increase in the nonlinearity parameter as a function of time for this oil sample. It was experimentally established that the complex shear modulus is a linear function of the amplitude of shear perturbations. This is possible in the case of a linear dependence of values of the viscoelastic characteristics on the medium modulus of deformation.     

La0.2Ca0.8MnO3材料电荷有序转变附近弹性模量的研究

通过超声与低频切变模量测量,得到多晶锰氧化物La0.2Ca0.8MnO3材料纵向模量与切变模量随温度变化关系曲线,发现在电荷有序转变温度附近,纵向模量与切变模量都出现最小值.运用合作Jahn-Teller效应理论对实验数据进行了拟合,发现理论与实验曲线符合较好,表明Jahn-Teller效应是发生电荷有序状态转变的主要机制之一.     

The synergetic theory of the glass transition in liquids

The glass transition is treated as a spontaneous emergence of the shear components of strain and stress elastic fields upon cooling a liquid at a rate exceeding the critical value. The stationary elastic strains and stresses and the effective relaxation time are determined within the adiabatic approximation. It is shown that the glass transition process occurs through the mechanism of a first-order kinetic transition with allowance made for the strain dependence of the shear modulus. The critical cooling rate turns out to be proportional to the thermal diffusivity and unrelaxed shear modulus and inversely proportional to the temperature derivative of the relaxed shear modulus and the square of the heat conductivity length of the sample.     

Temperature dependence of the elastic constants of aluminum

The single crystal elastic constants of aluminum have been measured using a piezoelectric composite oscillator from room temperature to just 20 K below the melting point. The elastic moduli differ markedly from previous high temperature results, but match in well with previous cryogenic results. Over the temperature range investigated the isothermal bulk modulus and the two shear moduli have a simple exponential dependence on isobaric volume, and the cryogenic data indicate this dependence may be preserved down to absolute zero. As has been found previously for a wide range of materials, the isothermal bulk modulus and the shear modulus $\text{(c}{}_{11}\text{– c}{}_{12}\text{)}\text{2}$ appear to be continuous functions of volume through the melting expansion, and melting seems to find its origin in the mechanical instability associated with this shear modulus vanishing at the volume of the melt at the freezing point. Grüneisen's parameter divided by the molar volume is very nearly independent of isobaric volume.     

Estimation of the fracture activation parameters of aluminum and α-iron from the lifetime data measured at moderate and low temperatures

The fracture activation energy is determined from the stress dependences of the lifetimes of Al and 6h-Fe measured at moderate and low test temperatures. The shear stresses are analyzed with allowance for the temperature dependence of the shear modulus and the athermal component of stresses.     

Elastic self-healing during shear accommodation in crystalline nanotube ropes

Rigid-tube computations of simple (transverse) shear in crystalline nanotube ropes (CNTRs) reveal that shear modulus and strength increase and decrease with the tube radius, respectively. High modulus to strength ratios suggest that dislocations play a minor role during their plasticity. The computed shear moduli are in agreement with previous studies, although shape change and rolling-based shear may modify low strain and temperature behavior. The instability past the shear strength is due to shear localization via interlayer sliding, wherein stress relief results in significant elastic energy dissipation. Large-tube radius CNTRs accommodate large strains at minimal energetic cost during sliding, due to the increasingly cohesive and short range nature of the intertube potential. Fascinatingly, the crystal aids its recovery, implying that CNTRs may be promising materials for energy absorption and tribology.     

Sound velocity dispersion in a solution with two critical points

The dispersion of the sound velocity in a solution with a region of stratification has been investigated experimentally in the frequency range from 2.6 MHz to 12 GHz. It is found that the dispersion of the sound velocity below the lower critical temperature of stratification reaches 22% and depends linearly to the temperature above the upper critical temperature of separation, the dispersion of the sound velocity is much smaller and has a different linear dependence on the temperature. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 3, 166–168 (10 August 1996)     

The effect of water on some properties of oriented nylon

Samples of 66 nylon were oriented by rolling to produce structures in which the polymer chains are preferentially in the machine direction and the hydrogen bonds are in the transverse direction. Conditioning to 50 or 100% relative humidity (R.H.) produces anisotropic swelling with the transverse direction expanding much less than the other two directions. The dependence on temperature of the tensile modulus in the machine and transverse directions was examined for specimens which were dry, conditioned to 50% R.H., and saturated with water. At high temperatures, the modulus was larger in the transverse direction. At-40°C, it was larger in the machine direction and increased with increasing moisture. The data are interpreted in terms of crystals which have their largest dimension in the hydrogen bond direction.     

Elastic and vibrational properties of cobalt to 120 GPa

Impulsive stimulated light scattering and Raman spectroscopy measurements have been made on hcp cobalt to a static pressure of 120 GPa. We find that at pressures above 60 GPa the shear elastic modulus and the Raman frequency of the E(2g) transverse optical phonon exhibit a departure from a linear dependence on density. We relate this behavior to a collapse of the magnetic moment under pressure that has been predicted theoretically, but until now not observed experimentally.     

Elastic strain response in the modified phase-field-crystal model

To understand and develop new nanostructure materials with specific mechanical properties, a good knowledge of the elastic strain response is mandatory. Here we investigate the linear elasticity response in the modified phase-field-crystal(MPFC) model. The results show that two different propagation modes control the elastic interaction length and time, which determine whether the density waves can propagate or not. By quantitatively calculating the strain field, we find that the strain distribution is indeed extremely uniform in case of elasticity. Further, we present a detailed theoretical analysis for the orientation dependence and temperature dependence of shear modulus. The simulation results show that the shear modulus reveals strong anisotropy and the one-mode analysis provides a good guideline for determining elastic shear constants until the system temperature falls below a certain value.     

Temperature Effect on the Elasticity of Acrylamide-n-Isopropylacrylamide Copolymers

Acrylamide (AAm) – N-isopropylacrylamide (NIPA) copolymers were prepared via free radical crosslinking copolymerization with various weight percentages (wt%) of AAm and NIPA. The temperature dependence of the compressive elastic modulus, G, and toughness, U_T, of the PAAm- NIPA copolymers due to a volume phase transition was found using a compressive testing technique. It was observed that the compressive elastic modulus increased comprehensively when the temperature was increased between 30°C and 60°C. The PAAm- NIPA copolymers presented higher values of the compressive elastic modulus than pure NIPA above the lower critical solution temperature (LCST) (NIPA exhibits a volume phase transition from hydrophilic to hydrophobic in water at 31°C) and their compressive elastic modulus and toughness had a strong temperature dependence.