Size effects in small crystalline particles at high pressures

Some size effects realized in small particles of alkali metals have been considered in a quasi-chemical approximation. The transition from massive samples to nanoparticles at high pressures is accompanied by an increase in the concentration of vacancies.     

不同粘结剂基TiB_2-C复合阴极中碱金属的渗透迁移行为(英文)

在含K低温电解质熔体中,采用EDS及改进型电解膨胀率测试仪,分别研究电解过程中沥青、呋喃、酚醛、环氧基TiB2-C复合阴极中碱金属(K和Na)的渗透迁移路径。同时,计算并讨论相应的电解膨胀率、碱金属的扩散系数以及复合阴极的腐蚀率。结果表明:无论使用何种粘结剂,电解过程中,碱金属在阴极中表现出相似的渗透迁移路径:碱金属首先渗透进入阴极的孔隙当中,随后渗透进入粘结剂相中,随着电解的不断进行,最后渗透进入复合阴极的骨料相当中。渗透进入粘结剂相和炭质骨料相当中的K和Na均会引起复合阴极的电解膨胀,同时,K比Na有着更强的渗透力。树脂基复合阴极的电解膨胀率、碱金属在其中的扩散系数以及腐蚀率均小于沥青基复合阴极,即,树脂基复合阴极的抗碱金属渗透力强于沥青基复合阴极。而就树脂基TiB2-C复合阴极而言,酚醛基TiB2-C复合阴极的抗渗透力最强,碱金属在其中的渗透速率、扩散系数和相应的腐蚀率分别为4.72mm/h,2.24×10-5 cm2/s和2.31 mm/a.     

Creation of nanospaces by intercalation of alkali metals into graphite in organic solutions

Co-intercalation of alkali metals (Li, Na, K, Rb and Cs) with various organic solvents has been utilized for introducing nanospaces in graphite. The co-intercalation has been conducted by a solution method. Resultant products were studied by X-ray diffraction. For solvents of cyclic ethers, co-intercalation is likely to occur for heavy alkali metals of Rb and Cs. For linear ethers with one oxygen atom, binary graphite intercalation compounds (GICs) were mainly obtained, irrespective of alkali metal species. For linear ethers with two oxygen atoms, light alkali metal, in particular, Li tends to give ternary Li–solvent–GICs. From these results, it is concluded that co-intercalation is mainly influenced by the interaction between alkali metals and solvents and by the size of solvated alkali metals.     

Elastic anisotropy and extreme Poisson’s ratios in single crystals

The relationship between elastic anisotropy and extreme Poisson’s ratio behaviour (either positive or negative) in single-crystalline materials has been investigated using experimentally determined single-crystal elastic constants for a wide range of solid materials. This makes use of a recently proposed elastic anisotropy index that is applicable to all crystal symmetries. For many real materials we find a striking correlation between the value of the elastic anisotropy index and the magnitudes of maximum and minimum Poisson’s ratios this is independent of crystal symmetry. This structure–property relationship provides new examples of auxetics and shows that negative Poisson’s ratios are actually not uncommon among many classes of inorganic (and organic) materials, including elemental metals, alloys, ionic solids, molecular solids and giant covalent networks.     

Polycrystalline elastic moduli of a high-entropy alloy at cryogenic temperatures

CrMnCoFeNi is a FCC high-entropy alloy (HEA) that exhibits strong temperature dependence of strength at low homologous temperatures in sharp contrast to pure FCC metals like Ni that show weak temperature dependence. To understand this behavior, elastic constants were determined as a function of temperature. From 300 K down to 55 K, the shear modulus (G) of the HEA changes by only 8%, increasing from 80 to 86 GPa. This temperature dependence is weaker than that of FCC Ni, whose G increases by 12% (81–91 GPa). Therefore, the uncharacteristic temperature-dependence of the strength of the HEA is not due to the temperature dependence of its shear modulus.     

Structural,electronic, mechanical and thermo-physical properties of TMN (TM = Ti,Zr and Hf) under high pressures: A first-principle study

The ab-initio calculations have been performed to investigate the structural, electronic, elastic and thermo-physical properties of TiN, ZrN and HfN in the stable B1 and high pressure B2 phases. Two different pseudo- and full-potential based approaches have been used in the calculations. The results are in good agreement with the measured data. The elastic constants for the B2 phase of these materials have been calculated for the first time except for ZrN. The observations show that these materials are mechanically stable not only in B1 phase at ambient conditions but also in B2 phase at high pressures. They are brittle in B1 phase while ductile in B2 phase. These materials are anisotropic in both B1 and B2 phases with increased anisotropy in B2 phase. The electronic behaviour of these materials is similar in both B1 and B2 phases except the broadening of the bands in B2 phase. These materials are found to be covalent, ionic and metallic in both the phases concerned. Present observation of electronic nature in CsCl structure of these materials needs validation by future researchers.     

Debye temperature, self-diffusion and elastic constants of intermetallic compounds

The Debye temperature of a material is a suitable parameter to describe phenomena of solid-state physics which are associated with lattice vibrations. It basically depends on the elastic constants. In recent work a simple method was put forward that allows one to derive precise Debye temperatures of crystals with cubic, hexagonal and tetragonal symmetry from the elastic constants. The type of chemical binding does not play any role. It is one aim of the present work to apply this method to various intermetallic compounds, i.e. to critically analyse published Debye temperatures and to calculate hitherto unknown values. It is a further aim to show that the activation energy of self-diffusion is also connected with the elastic constants by a simple law at least for the cubic B2 and L1₂ intermetallics, as it was recently found for face-centred cubic metals. Some consequences for high-temperature plasticity are discussed.     

Über Abhängigkeiten zwischen Polarisationswiderstand und Korrosionsgeschwindigkeit der Metalle

Connections between polarization resistance and corrosion rate of metals The validity of a simplified relation between measured polarization resistance and corrosion rte o some metals has been empirrically evaluated. In the relation K = c. R^k_p (c, k = constants, R_p = polarization resistance, K = corrosion rate) the constants are satisfactiorily equal for steel, Cu Zn, Ag in there active states. However, the proposed alternating current method is suitable principally for technical measurement because the experimental error is ± 15%. One main application might be corrosion monitoring over extended periods of time; in such cases low frequencies (0, 25 — 1 Hz) are useful.     

Anisotropic elastic constants of lotus-type porous copper: measurements and micromechanics modeling

We studied the elastic constants of a lotus-type porous copper, regarding it as a composite material showing hexagonal elastic symmetry with the c-axis along the longitudinal direction of the pores. We used the combination of resonance ultrasound spectroscopy and electromagnetic acoustic resonance methods to determine the elastic constants of the composite. The resulting Young’s modulus E_∥ decreases linearly and c₃₃ does slowly with porosity, while E_⊥ and c₁₁ drop rapidly and then slowly. Micromechanics calculations considering the elastic anisotropy of the copper matrix can reproduce the measured anisotropic elastic constants. This indicates that the elastic properties of various types of porous metals can be predicted and designed with the present approach using micromechanics modeling.     

Elastic constant softening and martensite nucleation in a CuZnAl single crystal

The ultrasonic attenuation A and the elastic constants C₄₄, C_L and C′ have been measured during thermal cycles carried out above the martensite start-temperature M_s (M_s=267 K) in a single crystal of the Cu_67.93Zn_19.03Al_13.04 alloy. The ultrasonic attenuation displays a sharp maximum at a temperature which is about 6 degrees higher than M_s. This temperature difference can be accounted for either in terms of the Clausius–Clapeyron law for the martensite induced by the applied ultrasonic stress-field or in terms of premartensitic effects. Thermal hysteresis occurs for all the elastic constants C₄₄, C_L and C′ and the data are consistent with those previously obtained for the Young’s modulus in a polycrystalline alloy of the same system. The thermal hysteresis is attributed to the formation of stable nuclei of martensite at strong defects.     

Gaseous equilibria in the C-H-O ternary system at 500–2000 K, 0.1–10 atm

The complex gaseous equilibria of approximately 10,000 different compositions in the C-H-O ternary system containing up to 40 gaseous species have been calculated at temperatures of 500, 1000, 1500, and 2000 K for total pressures of 0.1, 1.0, and 10.0 atm. The results are represented by selected isoactivity curves of carbon and isobars of hydrogen and oxygen plotted on C-H-O Gibbs composition triangles. The effects of changing composition, temperature, and pressure upon the chemical equilibria in this system are analyzed. In addition, the role of the C-H-O system in selected industrial applications is briefly discussed, including the choice of gas mixtures for the carburization of metals; the extent of carbon deposition at various temperatures, pressures, and compositions in reforming and gasification processes; the free hydrogen partial pressure in the anode compartment of fuel cells and influence of water addition; the oxygen potential in the products of combustion engines; and chemically buffered gas mixtures in controlled atmospheres.     

Study of effect of covalency in heavy rare-earth monoantimonides upto 31 GPa

In the present paper, we have investigated the high-pressure structural phase transition of rare earth antimonides (HoSb and TmSb). A modified interaction potential model (MIPM) (including the covalency effect) has been developed. Phase transition pressures are associated with a sudden collapse in volume. At compressed volumes, these compounds are found in CsCl phase. The phase transition pressures and associated volume collapses obtained from present potential model show a generally good agreement with available experimental data and others. The elastic constants and bulk modulus are also reported. Our results are in general in good agreement with experimental and theoretical data where available, and provide predictions where they are unavailable.     

Temperature dependence of single-crystal elastic constants of Mo(Si,Al)2

The temperature dependence of single-crystal elastic constants of Mo(Si,Al)₂ with the hexagonal C40 structure has been measured by the rectangular parallelepiped resonance method over the temperature range from room temperature to 1373 K. The silicide has the largest value of Poisson's ratio among transition-metal silicides with the C40 structure. This is interpreted in terms of the reduced directionality in atomic bonding due to the partial substitution of Al for Si atoms. The expression of the elastic constants on the basis of simple interatomic interactions cannot reproduce the elastic anisotropy of crystal with the C40 structure. The disagreement is considered to be a consequence of the occurrence of internal displacements during elastic deformation in crystals with the non-centrosymmetry.     

多晶金属弹性性能的布里渊散射研究

针对不透明样品的多晶金属铝、铜和铁的表面布里渊散射谱进行了测量.通过谱线解读获得了瑞利波和纵波的速度,并进而计算了三种金属的弹性常数,结果与文献中的数值吻合较好,证实了对不透明样品尤其是金属样品谱图解读的正确性.同时发现,对这三种弹性常数依次增大的金属,它们的表面布里渊散射峰强度依次减小,说明弹性性能是影响金属表面晶格振幅的重要因素之一.该方法和结果对于利用布里渊散射技术进行金属表面或薄膜的弹力学性能研究具有一定的参考价值.     

Titanium’s high-temperature elastic constants through the hcp–bcc phase transformation

The five independent elastic constants of hexagonal monocrystal titanium were determined up to the phase-transformation temperature, and the two isotropic elastic constants of polycrystalline titanium were determined beyond, up to 1300 K. Anomalous temperature dependences were observed just below the phase-transformation temperature: C₁₁ and C₆₆ increase with increasing temperature whereas C₃₃ and C₄₄ remarkably decrease, for example. To determine the C_ij, we used the free-vibration resonance frequencies obtained by electromagnetic acoustic resonance. After the phase transformation, the resonance frequencies changed little with the temperature increase, showing that the bcc-phase elastic constants change little with temperature. The polycrystalline elastic constants remained unchanged up to 1300 K after the phase transformation. The anomalous temperature dependence near the transformation is interpreted in terms of the small c/a ratio of the hcp phase and change of the atomic distances to meet the Burgers lattice relationship. Temperature-insensitive elastic constants in the bcc phase suggest the stabilizing of the bcc phase with increasing temperature.     

Changes in the vibrational energies and interatomic spacings upon the formation of vacancies in the volumes and in the cores of crystallite-conjugation regions of polycrystalline transition metals with cubic lattices

The changes in the vibrational energies and the signs of changes in the interatomic spacings upon the formation of vacancies in the bulk of metal and in the cores of the crystallite-conjugation regions (CCR) in polycrystalline transition metals with bcc and fcc lattices have been determined. The vibrational energy increases upon the formation of a vacancy in the bulk of metal because of a positive “relaxation” contribution to the change in the force constant of the atoms surrounding a vacancy. Positive “relaxation” contributions to the changes in the force constants and, correspondingly, an increase in the vibrational energy of the atoms surrounding a vacancy arise also upon the formation of “split” vacancies (S vacancies) in the cores of CCRs of polycrystalline transition metals with a face-centered cubic lattice. The positive “relaxation” contributions to the changes of the force constant of atoms in the region of localization of S vacancies are caused by a decrease in the interatomic spacings upon their formation, just as upon the formation of conventional vacancies in the bulk of metals. The vibrational energy of the nearest environment of the vacancies that are formed in the CCR cores in the polycrystalline d transition metals with a bcc lattice decreases because of a negative “relaxation” contribution to the change in the force constants. The cores of the high-angle CCRs in polycrystalline d transition metals with a bcc lattice are characterized by a negative internal pressure. Therefore, vacancies with positive relaxation volumes ν_BCC > 0 are formed in them, causing an increase in the interatomic distances in the nearest environment of such vacancies.     

Elektrochemische Untersuchungen zur Hochtemperaturkorrosion hochlegierter Werkstoffe in Alkalisulfatschmelzen

Electrochemical investigation into the high temperature corrosion of high alloy materials in alkali sulafte metals The investigations have been carried out in alkali sulfate melts under air at 625 and 800°C with iron, chromium, four ferritic chromium steels, two austenitic chromium nickel steels, one each nickel and cobalt base alloy and three chromium alloys. Clear statements concerning the potential dependence of the corrosion behaviour are feasible only on the basis of mass loss/potential curves. It results from these curves that the behaviour of all the materials studied is characterized by two potential regions with different corrosion behaviour. At relatively negative potentials a protective layer is formed on the material surface and the corrosion rate is low and almost independent from potential. Above a rupture potential (which depends from the system material/melt) porous surface layers are formed. In this potential range the corrosion rate shows a steep increase as potential increases. With increasing chromium contents of the materials the rupture potential is displaced towards more positive potentials, but chromium contents exceeding 20% have no markable influence on the rupture potential. Temperature, too, does not noticeably affect the position of the rupture potential. The interpretation of potentiodynamic accumulated current/potential curves requires great care; the shape of such curves depends from the variation of potential with time. The approximate position of the rupture potential can frequently be recognized at very low potential variations only. At larger potential variations in time the values of the rupture potential appear as a rule to be too negative; this phenomenon may probably attributed to potential dependence growth processes in the passive layer.     

Elastic property of aged duplex stainless steel

We have measured the elastic constants of duplex stainless steel, JIS-SCS14A (CF8M), aged isothermally at 400 °C up to 10,000 h, using resonant ultrasound spectroscopy method. The elastic constant c₁₁ (=λ+2μ) increases monotonically with the aging time, but c₄₄ (=μ) remains virtually unchanged. Using a micromechanics model, we have deduced the elastic constants of α-phase on the assumption that those of γ-phase are unchanged with the aging. The increase of the α-phase elastic constants reflects the spinodal decomposition of α-phase.     

Anisotropic elastic constants and thermal expansivities in monocrystal CrB2, TiB2, and ZrB2

The elastic constants and thermal expansivities in monocrystals of three transition-metal diborides with the AlB₂ structure, CrB₂, TiB₂, and ZrB₂, have been investigated in the temperature ranges from 300 to 1373 K and from 300 to 1073 K. The anisotropic parameters deduced from the measured elastic constants and thermal expansivities indicate that of the three diborides, the anisotropy is the most and least significant in CrB₂ and ZrB₂, respectively. The factors determining the significance in anisotropy in atomic bonding in AlB₂-type diborides are analyzed by an approach similar to the valence-force-field method and are discussed in terms of the deformation of the electronic charge around the metal atoms occurring to fit themselves in the (0 0 0 1) basal plane.     

Elasticity and thermodynamic properties of α-Ta4AlC3 under pressure

First-principles calculations of the crystal structure and the elastic properties of α-Ta₄AlC₃ have been carried out with the plane-wave pseudopotential density functional theory method. The calculated values are in very good agreement with experimental data as well as with some of the existing model calculations. The pressure dependence of the elastic constants c_ij, the aggregate elastic moduli (B, G, E), the Poisson's ratio, and the elastic anisotropy has been investigated. Using the quasi-harmonic Debye model considering the phonon effects, the temperature and pressure dependencies of isothermal bulk modulus, and the thermal expansions, and Grüneisen parameters, as well as Debye temperatures are investigated systematically in the ranges of 0–60 GPa and 0–1500 K as well as compared to available data.