Super-slippery carbon nanotubes

The friction between the walls of multi-wall carbon nanotubes is shown to be extremely low in general, with important details related to the specific choice of the walls. This is governed by a simple expression revealing that the phenomenon is a profound consequence of the specific symmetry breaking: super-slippery sliding of the incommensurate walls is a Goldstone mode. Three universal principles of tribology, offering a recipe for lubricant selection are emphasized. Received 8 August 2001     

Determination of calcium and total hardness in natural waters using a potentiometric sensor array

The determination of calcium and total hardness in natural waters is carried out with a potentiometric sensor array which consists of a series of ion-selective electrodes (ISEs) for Ca²⁺, Mg²⁺, NH₄⁺, K⁺, Na⁺, Li⁺, and H⁺. The selectivity of the calcium and magnesium ISEs is not fully achieved as other cation species may interfere with the analysis. The proposed sensor array device can overcome this drawback since it can take advantage of the cross-selectivities of cation species towards each ISE. In this approach, the multivariate data generated by the sensor array results in a richer source of analytical information which allows the quantification of calcium and total hardness in the water samples by means of chemometric methods. Results obtained are in reasonable concordance with those given by the standard method based on complexometry.     

Do the Local Softness and Hardness Indicate the Softest and Hardest Regions of a Molecule?

In this work, we will show that the largest values of the local softness and hardness do not necessarily correspond to the softest and hardest regions of the molecule, respectively. Based on our results, we will argue that it is more useful to interpret the local softness and the local hardness as functions that measure the "local abundance" or "concentration" of the corresponding global properties. This new point of view helps reveal how and when these local reactivity indices are most useful.     

Effect of material stiffness on hardness: A computational study based on model potentials

We investigate the dependence of the hardness of materials on their elastic stiffness. This is possible by constructing a series of model potentials of Morse type; starting with modelling natural Cu, the model potentials exhibit an increased elastic modulus, while keeping all other potential parameters (lattice constant, bond energy) unchanged. Using molecular-dynamics simulation, we perform nanoindentation experiments on these model crystals. We find that the crystal hardness scales with the elastic stiffness. Also the load drop, which is experienced when plasticity sets in, increases in proportion to the elastic stiffness, while the yield point, i.e. the indentation at which plasticity sets in, is independent of the elastic stiffness.     

Significance of stacking fault energy on microstructural evolution in Cu and Cu–Al alloys processed by high-pressure torsion

Disks of pure Cu and several Cu–Al alloys were processed by high-pressure torsion (HPT) at room temperature through different numbers of turns to systematically investigate the influence of the stacking fault energy (SFE) on the evolution of microstructural homogeneity. The results show there is initially an inhomogeneous microhardness distribution but this inhomogneity decreases with increasing numbers of turns and the saturation microhardness increases with increasing Al concentration. Uniform microstructures are more readily achieved in materials with high or low SFE than in materials with medium SFE, because there are different mechanisms governing the microstructural evolution. Specifically, recovery processes are dominant in high or medium SFE materials, whereas twin fragmentation is dominant in materials having low SFE. The limiting minimum grain size (d _min) of metals processed by HPT decreases with decreasing SFE and there is additional evidence suggesting that the dependence of d _min on the SFE decreases when the severity of the external loading conditions is increased.     

The effect of some transition metal oxides on the physical properties of K0.5Na0.5Nb0.95Ta0.05O3 ceramics

Abstract

Both K_0.5Na_0.5Nb_0.95Ta_0.05O₃ (KNNTO) and (K_0.5Na_0.5Nb_0.95Ta_0.05O₃)_0.99-M_0.01, M = Co₃O₄ and Mn₂O₃ (M/KNNTO) Ferromagnetic behaviour was observed for some M/KNNTO compounds. The hardness and compressive strength of all investigated samples are given. Comparisons with similar materials are discussed. Ceramics were synthesised using a solid-state reaction method. X-ray diffraction patterns of all samples revealed that the crystal structure is orthorhombic. Field-emission scanning electron microscopy was performed. Polarisation hysteresis curves indicated a disruption of ferroelectric order with the addition of M into KNNTO ceramics. The dielectric properties of the investigated ceramics have been studied as a function of frequency and temperature.     

Gamma radiation effects on mechanical properties and morphology of a polyurethane derivate from castor oil

In this study, an adhesive of a polyurethane derivate from castor oil was irradiated with gamma radiation from a ⁶⁰Co source, at doses from 0.2 to 25 kGy. This adhesive polyurethane is considered for use in hospital furniture because it does not liberate dangerous solvents. Hardness and elastic modulus were measured by instrumented indentation with a pyramidal Berkovich indenter, using loads from 0.08–40 mN with a nanoindenter XP. The instrumented indentation hardness was 110 MPa for an untreated sample, increasing to 124 MPa after irradiation with 25 kGy, at penetration depths of about 5 μm. The increases in elastic modulus induced by radiation were less pronounced. This polyurethane is naturally cross-linked and the relative modifications in the hardness are attributed to an additional cross-linking process induced by radiation. X-ray diffraction indicates a slight increase in crystallinity. The roughness measured by atomic force microscopy increases after gamma irradiation.     

Hardness and elastic modulus of zircon as a function of heavy-particle irradiation dose: I. In situ α-decay event damage

Abstract

For a natural single crystal of zircon, ZrSiO₄, from Sri Lanka, exhibiting zonation in U and Th contents, the hardness and elastic modulus have been determined as a function of α-decay dose using a mechanical properties microprobe (MPM). The zones vary in thickness from one to hundreds of micrometers, and have uranium and thorium concentrations such that the α-decay dose varies between 2 × 10¹⁵ and 1 × 10¹⁶ α-decay events/mg (0.15 to 0.65 dpa, displacement per atom). The transition from the crystalline to the aperiodic metamict state occurs over this dose range. For a traverse of 75 indent pairs across layers sampling a large portion of the crystalline-to-metamict transition (3.7 × 10¹⁵ to 9.7 × 10¹⁵ α-decay events/mg) both the hardness and elastic modulus decrease linearly with increasing α-decay dose. The radiation-induced softening follows a behavior similar to other radiation-induced changes, that is with the expansion of the unit cell parameters there is a decrease in density, birefringence, hardness and bulk modulus.     

固态二氧化碳电子结构及性能的理论研究

采用基于赝势平面波理论的第一性原理计算方法,研究了9种结构的固态二氧化碳的结构和性质.经过结构的几何优化,得到α石英稳定结构晶格常数与他人的计算值基本一致.通过对平衡态能量的分析,我们得出β方石英(cristobalite)结构是能量最低的结构.这与文献的研究结果一致.对弹性性质的计算结果表明,除了超石英(stishovite)和立方黄铁矿(cubic-pyrite)结构之外,其他的结构都是弹性稳定的.利用基于Mulliken轨道重叠布居数的共价固体本征硬度计算方法,预测了各个结构的本征硬度值.结果表明, 关键词： 第一性原理计算 固态二氧化碳 电子结构 硬度     

Understanding the stability, electronic and molecular structure of some copper(III) complexes containing alkyl and non alkyl ligands: Insights from DFT calculations

DFT calculations have been performed for some Cu(III)-alkyl complexes. Complexes 1-19 were optimized to the square planar (sq) geometry and observed no imaginary frequencies. Although formally copper adopts d⁸ configuration (Cu(III)) in all the complexes, the Natural Population Analysis (NPA) revealed that the copper actually in d¹⁰ (Cu(I)) configuration, Bond order calculation suggested that the Cu(III)-Et_trans bond gets more bond order in the presence of poor π-acidic co-ligand (probe ligand). Relatively smaller bond order was calculated for Cu(III)-Me_cis bond than Cu(III)-Et_trans bond and therefore Cu(III)-Et_trans bond is the strongest bond in all the complexes. Calculated less Chemical hardness (η) of complexes 1-19 suggested that all these complexes are less stable in nature. Energy Decomposition Analysis (EDA) revealed that the Cu(III)-Et_trans bond is relatively more stable than the Cu(III)-Me_cis and Cu(III)-L (L = co-ligand/probe ligand) bonds. And also the Cu(III)-alkyl (Cu(III)-Me_cis and Cu(III)-Et_trans) bond in complexes 1-17 is more of ionic in nature. However, Cu(III)-Et_trans bond is relatively more ionic than Cu(III)-Me_cis bond.