纳米Ni在77 K温度下压缩行为的研究

利用低温力学测试系统研究了电化学沉积纳米Ni在77 K温度下的压缩行为. 室温下纳米Ni 的屈服强度为 2.0 GPa, 77 K温度下的屈服强度为3.0 GPa, 压缩变形量则由室温的10%左右下降到5%. 借助应变速率敏感指数、激活体积、扫描电子显微和高分辨透射电子显微分析, 对纳米Ni的塑性变形机制进行了表征. 研究表明, 在77 K温度下的塑性变形主要是由晶界-位错协调变形主导, 晶界本征位错弓出后无阻碍地在晶粒内无位错区运动, 直至在相对晶界发生类似切割林位错行为. 同时分析了弓出位错的残留位错部分在协调塑性变形时起到的增加应变相容性和减小应力集中的作用. 利用晶界-位错协调机制和残留位错运动与温度及缺陷的相关性揭示了纳米Ni室温和77 K温度压缩性能差异的内在原因. 关键词： 塑性变形 强度 位错     

Compression behavior of NiO in a diamond cell

Abstract

The compressibility of synthetic polycrystalline NiO was studied in a diamond anvil cell at room temperature utilizing two different X-ray sources. A standard film with a conventional X-ray source and the energy dispersive X-ray diffraction (EDXRD) method with synchrotron radiation were used for data acquisition. In the film method, the sample was compressed in a 4:1 methanol to ethanol solution up to 7 GPa with ruby fluorescence as a pressure calibrant. In the energy dispersive method, NiO powder was mixed with gold and compressed in two different conditions: gasketed and ungasketed up to 30 GPa. In the gasketed run, water was used as the pressure transmitting medium while gold was used as pressure calibrant in both runs.

Hydrostatic compression of NiO in both diffraction methods yields a bulk modulus (K _o) of 187 ± 7 GPa assuming K′ = 4. The compression of gasketed NiO of the synchrotron experiment, however, showed an obvious break at pressure exceeding 4 GPa due to the loss of hydrostaticity. NiO in a nonhydrostatic condition behaves with less compressibility than the hydrostatic results with a nominal K _o of 238 ± 10 GPa. The lower compressibility of NiO in synchrotron runs is attributed to the uniaxial loading effect which was more easily detected by the EDXRD geometry. The discrepancy in the bulk modulus can be attributed to the contrast in the shear strength between the sample and pressure medium and the Poisson effect of the sample under uniaxial loading.     

Compression behavior of nanocrystalline anatase TiO2

We present a synchrotron X-ray diffraction study of pressure-induced changes in nanocrystalline anatase (with a crystallite size of 30-40 nm) to 35 GPa. The nanoanatase was observed to a pressure above 20 GPa. Direct transformation to the baddeleyite-TiO₂ polymorph was seen at 18 GPa. A fit of the pressure versus volume data to a Birch-Murnaghan equation yielded the following parameters: zero-pressure volume, V₀=136.15 Å³, bulk modulus, K_T=243(3) GPa, and the pressure derivative of bulk modulus, K′=4 (fixed). The bulk modulus value obtained for the nanocrystalline anatase is about 35% larger than that of the macrocrystalline counterpart.     

Characterization of elastomeric materials by NMR-microscopy

This review reflects a long experience with the application of NMR-imaging methods to elastomeric materials. The experimental techniques, used to obtain parameter selective NMR-images (T₁,T₂, T_1ρ-images), are described in detail and the methods required for the data analysis are explained. A special emphasis is put on the analysis of experimental errors within the framework of NMR-imaging.

In order to make parameter selective images generally useful their information should be correlated to material properties, so that images of the material properties can be obtained. This is demonstrated for the case of crosslink density, which is certainly one of the molecular properties in rubber materials, exhibiting the main influence on mechanical and other material properties.

Sulfur cured and carbon black filled technical rubbers with different degree of crosslink density and oxidative aging were investigated using parameter selective imaging techniques. The image data were analyzed by means of gaussian and multiexponential fitting procedures, revealing spatially resolved NMR relaxation parameters. The further interpretation of these parameters was based on physical models describing molecular motions in crosslinked polymers.     

Reinforcement effects of multiwall carbon nanotubes in elastomeric matrices: Comparison with other types of fillers

Mechanical and electrical properties of composites based on butyl rubber and multiwall carbon nanotubes (MWNTs) are investigated. Gradual increases in elastic moduli are observed with the filler content. It was found that the degree of strain affects the electrical resistivity. Finally, the level of reinforcement imparted to a rubbery matrix by carbon nanotubes is compared with that provided by other types of fillers such as carbon black, clay fibers or layered silicates.     

Anisotropic dewetting on stretched elastomeric substrates

We study the instability of a very thin liquid film resting on a uniformly stretched soft elastomeric substrate driven by van der Waals forces. A linear stability analysis shows that the critical fluctuation wavelength in the tensile direction is larger than those in the other directions. The magnitudes of the critical wavelengths are adjustable in the sense that they depend on the principal stretch of the substrate. For example, when the principal stretch of the substrate varies from 1.0 (unstretched) to 3.0, the range of the critical wavelength in the tensile direction increases by 7.0% while that normal to the tensile direction decreases by 8.7%. Therefore, the phenomenon may find potential applications in creating tunable topographically patterned surfaces with nano-to microscale features.     

High-refractive-index measurement with an elastomeric grating coupler

An elastomeric grating coupler fabricated by the replica molding technique is used to measure the modal indices of a silicon-on-insulator (SOI) planar waveguide structure. Because of the van der Waals interaction between the grating mold and the waveguide, the elastomeric stamp makes conformal contact with the waveguide surface, inducing a periodic index perturbation at the contact region. The phase of the incident light is changed to match the guided modes of the waveguide. The modal and bulk indices are obtained by measuring the coupling angles. This technique serves to measure the high refractive index with a precision better than 10(-3) and allows the elastomeric stamp to be removed without damaging the surface of the waveguide.     

High-frequency asymptotic behavior of T-odd optical effects

It is shown that the contribution of low-frequency excitations with characteristic energy ℏω _l to T-odd (nonreciprocal) optical effects, including spatial dispersion effects, at optical frequencies ω≫ω _l can be calculated in the zeroth-order approximation with respect to the parameter ω _l/ω. This greatly simplifies their analysis. Some of these effects are found to be frequency independent in the spectral regions where the refractive index n(ω)≈ const. It is shown that frequency-independent Faraday rotation can be observed in media with zero magnetization, including in media with zero microscopic magnetic moment density. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 7, 510–513 (10 April 1999)     

Low-frequency behavior of optical spatial dispersion effects

A theoretical explanation is given for the frequency independence of the nonreciprocal birefringence of light, which was recently observed in the semiconductors Cd_1?x Mn_xTe, Zn_1?x Mn_xTe, and GaAs in the frequency range below the frequency corresponding to the interband absorption edge. It is shown that the symmetry of the effect becomes higher at such frequencies if the light-induced excitation energy ?ω_n(k) only slightly depends on the photon momentum k. In this case, the nonreciprocal birefringence is completely determined by the second-rank magnetoelectric tensor. It is shown that the nonreciprocal birefringence of light can be observed in magnetic media with a tensor order parameter.     

Kinetic concept of strength in calculations of elastomeric parts

The specific features of the kinetic concept as applied to the prediction of the lifetime of rubber parts are considered. The theoretical and experimental data on the lifetime of rubbers under uniaxial tension, combined stresses, dynamic loading with a dominant tension, and dynamic loading in the presence of a static component are analyzed consistently. Attention is drawn to the fact that standard methods can also be used to determine the strength characteristics of rubbers. It is emphasized that the methods and programs described in this paper are developed for applied purposes.     

The effects of surfaces on dynamic critical behavior

The field theoretic renormalization-group approach for the study of critical behavior near free surfaces is generalized to dynamic properties. Time-dependent Ginzburg-Landau models with nonconserved or conserved order parameters — semi-infinite generalizations of the so-called modelsA andB — are considered. The asymptotic behavior of response and correlation functions is analyzed at the ordinary and special transitions in 4-? dimensions, and dynamic scaling laws for surface quantities are obtained. It is shown that the critical exponents can be expressed entirely in terms of static bulk and surface exponents andz, the dynamic bulk exponent. The critical exponents for the leading frequency, temperature and momentum singularities of the surface two-spin correlation function at the ordinary transition differ appreciably from the corresponding bulk analogues. In addition, the shape function which describes its frequency dependence differs qualitatively from the one of the bulk correlation function.     

Solenoids and plectonemes in stretched and twisted elastomeric filaments

We study the behavior of a naturally straight highly extensible elastic filament subjected to large extensional and twisting strains. We find that two different phases can coexist for a range of parameter values: the plectoneme and the solenoid. A simple theory based on a neo-Hookean model for the material of the filament and accounting for the slender geometry suffices to explain these observations, and leads to a phase diagram that is consistent with observations. Extension and relaxation experiments on these phases show the presence of large hysteresis loops and sawtooth-like force-displacement curves which are different for the plectoneme and the solenoid.     

Compression behavior and spectroscopic properties of insensitive explosive 1,3,5-triamino-2,4,6-trinitrobenzene from dispersion-corrected density functional theory

Using dispersion corrected density functional theory, we systematically examined the pressure effect on crystal structure, cell volume, and band gap of 1,3,5-triamino-2,4,6-trinitrobenzene(TATB) to understand its extraordinary chemical stability. Analysis of the Mulliken population and the electron density of states implied a possible charge transfer in TATB with increasing pressure. Raman and infrared spectra of TATB under hydrostatic pressure up to 30 GPa were simulated.The observed strong coupling between NH_2 groups and NO_2 groups with increasing pressure, which is considered to have a tendency of energy transfer with these vibrational modes, was analyzed. The pressure-induced frequency shift of selected vibrational modes indicated minor changes of molecular conformation mainly by the rotation of NH_2 groups. Compression behavior and spectroscopic property studies are expected to shed light on the physical and chemical properties of TATB on an atomistic scale.     

Size effects and stochastic behavior of nanoindentation pop in

A statistical model for pop in initiated at preexisting dislocations during nanoindentation is developed to explain size-dependent pop-in stresses. To verify theoretical predictions of this model, experiments were performed on single-crystal Mo, utilizing indenter radii that vary by over 3 orders of magnitude. The stress where plastic deformation begins ranges from the theoretical strength in small volumes, to 1 order of magnitude lower in larger volumes. An intermediate regime shows wide variability in the stress to initiate plastic deformation. Our theory accurately reproduces the experimental cumulative probability distributions, and predicts a scaling behavior that matches experimental behavior.     

Compression behavior of the delafossite-type metallic oxide PdCoO2 below 10 GPa

The compression behavior of delafossite-type metallic oxide PdCoO₂ below 10 GPa has been investigated by in situ high pressure X-ray diffraction measurement using synchrotron radiation. It is found that the delafossite-type structure of PdCoO₂ is stable below 10 GPa. It should be noted that compression behavior of PdCoO₂ is anisotropic. Pressure dependence of the lattice parameters indicates that the a-axis is more compressible than the c-axis. The lattice parameter ratio c/a in the hexagonal unit increases with increasing pressure. The calculated zero-pressure bulk modulus is 224 GPa. It is found that the above characteristic compression behaviors of PdCoO₂ are the same as those of the delafossite CuFeO₂. The compressibilities of the a-axis of both PdCoO₂ and CuFeO₂ are highly different although those of the c-axis are almost the same.     

Synthesis of nanoparticles and its effect on properties of elastomeric nanocomposites

Calcium carbonate (CaCO₃) nanoparticles (9, 15, and 21 nm) were synthesized by solution spray of CaCl₂ and NH₄HCO₃ with sodium lauryl sulfate (SLS) as a stabilizing agent, and their effect was studied on polybutadiene rubber (PBR) with variations in wt% loading (4, 8, and 12%). The results of PBR nanocomposites were compared with commercial CaCO₃ (40 μm) and fly ash (75 μm) filled PBR microcomposites. Properties such as tensile strength, young modulus, elongation at break, glass transition temperature, decomposition temperature, and abrasion resistances were determined. Profound effect in properties was observed, because nanometric size of CaCO₃ particles synthesized using solution spray technique. Maximum improvement in mechanical and flame retarding properties was observed at 8 wt% of filler loading. This increment in properties was more pronounced in 9-nm size CaCO₃. The results were not appreciable above 8 wt% of nanofillers because of agglomeration of nanoparticles. In addition, an attempt was made to consider modeling Young’s modulus of PBR–nano CaCO₃ which was predicted by modified Halpin–Tsai equation. It was observed that the predication by the Guth equation and modified Halpin–Tsai equation agreed very well with experimental, whereas the Halpin–Tsai equation can only applied to predict the modulus of rubber nanocomposites in the range of low addition of nanofiller, which agrees the Nielsen equation.