Nanograin Boundaries Research Articles

鉄鋼産業における表面科学の役割

Based on the experience of the characterization of steel materials for more than 20 years, the author reviewed the roles and contributions of surface science in the steel industry. Surface analytical techniques have played many important role in steel industry. Surface analytical technique include not only X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and secondary ion mass spectrometry (SIMS) but scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron probe micro analysis (EPMA), x-ray diffraction (XRD), Raman spectroscopy, and so on. Surface analysis applied fields of steel investigation are surface segregation, grain boundary segregation, corrosion, surface treatment, surface passivation, and so on. Recently, the needs of the analysis of buried interface such as the interface between matrix and nano-particle or nano-grain boundary have been increased. So the three dimensional analysis with atomic resolution attracts much attention in the area of steel industry.

Energy filtration of charge carriers in a nanostructured material based on bismuth telluride

The dependences of the electrical conductivity and thermopower on the size of grains in a nanocrystalline material based on Bi2Te3-Sb2Te3 solid solutions of the p type have been investigated theoretically and experimentally. The relaxation time in the case of hole scattering by nanograin boundaries in an isotropic polycrystal has been calculated taking into account the energy dependence of the probability of tunneling of charge carriers and the dependence of the scattering intensity on the nanograin size L n . A decrease in the probability of boundary scattering with an increase in the energy of charge carriers leads to an increase in the thermopower. The dependences of the thermopower and electrical conductivity on the nanograin size, which have been obtained taking into account the boundary scattering and scattering by acoustic phonons, are in good agreement with experimental data. For the material under consideration, the thermopower coefficient increases by 10–20% compared to the initial solid solution at L n = 20–30 nm. This can lead to an increase in the thermoelectric figure of merit by 20–40%, provided that the decrease in the electrical conductivity and the decrease in the lattice thermal conductivity compensate each other. Despite the absence of a complete compensation, it has been possible to increase the thermoelectric figure of merit for the samples under investigation to ZT = 1.10–1.12.

Bismuth-telluride Thermoelectric Nanoparticles Synthesized by Using a Polyol Process

Bismuth-telluride (Bi2Te3) nanoparticles have been synthesized by using BiCl3 and TeCl4 as reactants, ethyleneglycol as the polyol solvent. The microstructures showed that the synthesized bismuth telluride nanoparticles had the 200 nm-sized polycrystalline structures resulting from aggregates of nanocrystallites. The X-ray diraction (XRD) results revealed that the phase of the nanoparticles consisted of mainly Bi2Te3. The sintered Bi2Te3 materials exhibited grain size of 300 nm and significantly decreased the thermal conductivities compared to those of micro-grained bulk materials due to increased phonon-scattering at the nanograin boundaries.

Bulk Nanostructured Polycrystalline p-Bi-Sb-Te Thermoelectrics Obtained by Mechanical Activation Method with Hot Pressing

Bulk nanostructured polycrystals are prospective thermoelectrics. Fabrication and examination of nanostructured polycrystalline p-Bi-Sb-Te are discussed. The influence of fabrication conditions and the composition of the material on sizes of nanograins was investigated experimentally. Basic physical properties of fabricated nanothermoelectrics including nanocomposites were also studied. Theoretical estimation of the influence of phonon scattering on nanograin boundaries in bulk nanostructured thermoelectrics is presented.

The mechanism of the failure of the dispersion-strengthened Cu–Al2O3 nanosystem

The method of “in situ tensile testing in SEM” is suitable for investigations of fracture mechanisms because it enables to observe and document deformation processes directly, thank to which the initiation and development of plastic deformation and fracture can be reliably described. The deformation and fracture mechanisms of Cu–Al2O3 nanomaterials with 5 vol.% of Al2O3 phase has been analyzed using technique of the “in situ tensile testing in SEM.” It has been shown that the deformation process causes break-up of large Al2O3 particles and decohesion of smaller ones. The final fracture path is influenced also by boundaries of nanograins, through which the principal crack propagates towards the sample exterior surface. Based on the experimental observations, a model of damage and/or fracture mechanisms has been proposed.

Isotope Exchange of Gaseous Oxygen with Oxide LaMno<sub>3+δ</sub> Nanograins Boundary

Isotope exchange of oxygen 18О2 with the boundary of nanograins of oxide LaMnO3+ obtained by the method of shock-wave loading was investigated in the temperature range of 400 – 500 °C. It was established that the temperature dependence of the isotope exchange rate is described by the Arrhenius expression, the activation energy and the pre-exponential factor being 1.67 eV and 1.8∙102 cm/s, respectively. Comparison with literature data has shown that for oxide LaMnO3+, a significant difference in activation energies and pre-exponential factors is observed for the isotope exchange rate with a ‘defect-free’ surface and the nanograin boundary. In case of the boundary, these parameters were higher: the activation energy about two times, and the pre-exponential factor, by almost 7 orders of magnitude.

A closer look at the local responses of twin and grain boundaries in Cu to stress at the nanoscale with possible transition from the P–H to the inverse P–H relation

Nanocrystalline copper is considered to be a candidate for electrical contacts for dynamic systems because of its intrinsic conductivity and enhanced fretting resistance. However, the enhanced electron scattering at high-density grain boundaries significantly deteriorates the overall conductivity of nanocrystalline copper. Recent studies suggest that nanosized twin boundaries in copper might be a solution to such a dilemma. To better understand the general mechanical behavior of nanotwin boundaries, we conducted molecular dynamics simulation studies to investigate responses of both nanotwin and nanograin boundaries in copper to stress at the nanoscale, particularly in the critical range of 5–25 nm where the inverse Petch–Hall relation (P–H) may occur in nanocrystalline copper. The obtained results suggest that the twin boundary blocks dislocation movement more effectively and the degree of emitting dislocations under stress is considerably lower than that of grain boundary, leading to superior mechanical behavior. The inverse P–H relation is not applicable to the nanotwinned system. It is also demonstrated that the inverse P–H relation occurring in nanograined materials does not necessarily result from grain boundary sliding.

A microstructure and mechanical property investigation on thermally sprayed nanostructured ceramic coatings before and after a sintering treatment

Coatings have been deposited by air plasma spraying of alumina powders in the form of conventional particles (C), nanostructured agglomerates (N) and sintered–nanostructured agglomerates (S). Sintering alleviated the stresses introduced in the nanopowder by the manufacturing process (high energy ball milling). The coating porosity is a direct consequence of the powder melting degree, which is related to the feedstock porosity. The mechanical performance of the coatings is also closely associated with the powder melting degree. The N coatings present the highest surface roughness due to the lowest melting degree. The slightly higher hardness values of the N and S coatings, as compared to the C coatings, are attributed to the higher percentages of α-Al 2O 3 and the presence of nanostructure. The S coatings exhibit superior adhesion strength, relative fracture toughness and wear resistance, due to sintering consequences (intraparticle cohesion, strain relief, tough splat boundaries), random dispersion of coherent nanozones and stress dissipation at nanograin boundaries.

Zinc Oxide Nanostructures Confined in Porous Silicas

We report on molecular simulations of zinc oxide nanostructures obtained within silica nanopores of diameter D = 1.6 nm and D = 3.2 nm. Both the effects of confinement (by varying the pore size) and degree of pore filling on the structure of the nanomaterial are addressed. Two complementary approaches are adopted: 1) the stability of the three crystalline phases of ZnO (wurtzite, rocksalt, and blende) in the silica nanopores is studied, and 2) ZnO nanostructures are obtained by slowly cooling down a homogeneous liquid phase confined in the silica pores. None of the ideal nanostructures (wurtzite, rocksalt, blende) retains the ideal structure of the initial crystal when confined within the silica pores. Only the structure starting from the ideal wurtzite nanocrystal remains significantly crystalline after relaxation, as revealed by the marked peaks in the pair correlation functions for this system. The morphology and degree of cristallinity of the structures are found to depend on the parameters involved in the synthesis (pore size, filling density). Nanograin boundaries are observed between domains of different crystal structures. Reminiscent features of the bulk behavior, such as faceting of the nanostructures, are also observed when the system size becomes large. We show that the use of nanopores as a template imposes that the confined particles exhibit neutral (basal) surfaces. These predictions provide a guide to experiments on semiconductor nanoparticles.

In situtensile testing in SEM of Al-Al4C3nanomaterials

The deformation and fracture mechanisms of Al-Al4C3 nanomaterials with 4 vol% of Al4C3 phase has been analysed using the technique in situ tensile testing in SEM. It has been shown that the deformation process causes break-up of large Al 4C3 particles and decohesion of smaller ones. The final fracture path is influenced also by boundaries of nanograins, through which the principal crack propagates towards the sample exterior surface. Based on the experimental observations, a model of damage and fracture mechanisms has been proposed.

Nanoscale thermodynamic study on phase transformation in the nanocrystalline Sm2Co17 alloy

The characteristics of phase transformation in nanocrystalline alloys were studied both theoretically and experimentally from the viewpoint of thermodynamics. With a developed thermodynamic model, the dependence of phase stability and phase transformation tendency on the temperature and the nanograin size were calculated for the nanocrystalline Sm(2)Co(17) alloy. It is thermodynamically predicted that the critical grain size for the phase transformation between hexagonal and rhombohedral nanocrystalline Sm(2)Co(17) phases increases with increasing temperature. When the grain size is reduced to below 30 nm, the hexagonal Sm(2)Co(17) phase can stay stable at room temperature, which is a stable phase only at temperatures above 1520 K in the conventional polycrystalline alloys. A series of experiments were performed to investigate the correlation between the phase constitution and the grain structure in the nanocrystalline Sm(2)Co(17) alloy with different grain size levels. The experimental results agree well with the thermodynamic predictions of the grain-size dependence of the room-temperature phase stability. It is proposed that at a given temperature the thermodynamic properties, as well as the phase stability and phase transformation behavior of the nanocrystalline alloys, are modulated by the variation of nanograin size, i.e. the grain size effects on the structure and energy state of the nanograin boundaries.

Phase configuration and stability in the nanocrystalline Sm-Co system

A thermodynamic model that describes the thermal properties of the compound phases in the nanocrystalline （NC） alloy systems has been developed，which provides quantitative predictions on the stabilities and transition features of different phases. By using the NC Sm-Co alloy system as an example，the temperature dependence of the mole Gibbs free energy of the NC alloy phases with different excess volumes in the nano-grain boundary regions were provided. Based on the model calculations，the relative stability of different phases and the phase transition rules in the NC Sm-Co system were analyzed. It was shown by the calculation results that near the room temperature，some of the NC alloy phases，whose mole Gibbs free energy values change from negative to positive due to the decrease of the nano-grain size，will transform into stable NC alloy phases. This is distinctly different from the phase transformation features in the coarse-grained alloy systems，whose thermodynamic properties are only dependent on the temperature. The experimental results on the phase configuration and the phase stability obtained from the prepared NC Sm-Co alloy confirmed the theoretical predictions of the present thermodynamic model.

Nanometric size effect on Ge diffusion in polycrystalline Si

The nanosize effect on Ge diffusion (850≤T≤1000 °C) in polycrystalline Si layers is investigated. The Ge diffusion coefficients in microcrystalline and nanocrystalline Si layers made of 30 μm and 40 nm wide grains, respectively, are measured and compared. In the microcrystalline Si layer, the Ge diffusion coefficient in micrograin boundaries is measured using a conventional analytical solution of Fick’s equations corresponding to the Fisher model. In the nanocrystalline Si layer, the Ge diffusion coefficients in nanograins and in nanograin boundaries are measured via a method based on two-dimensional simulations using the Fisher model geometry. The diffusivities in nanograins and nanograin boundaries are one order of magnitude higher than in micrograins and micrograin boundaries, respectively. However, the nanosize effect appears to be different in grains and grain boundaries; despite that the activation energy for diffusion in 40 nm wide grains is at least 1 eV lower than in Si bulk. The activation energy in nanograin boundaries is about the same as in micrograin boundaries.

Enhanced thermoelectric figure of merit in nanostructured n-type silicon germanium bulk alloy

The dimensionless thermoelectric figure of merit (ZT) of the n-type silicon germanium (SiGe) bulk alloy at high temperature has remained at about one for a few decades. Here we report that by using a nanostructure approach, a peak ZT of about 1.3 at 900 °C in an n-type nanostructured SiGe bulk alloy has been achieved. The enhancement of ZT comes mainly from a significant reduction in the thermal conductivity caused by the enhanced phonon scattering off the increased density of nanograin boundaries. The enhanced ZT will make such materials attractive in many applications such as solar, thermal, and waste heat conversion into electricity.

Structural and magnetic properties of SmCo 7−xMo x alloys

Phase structure and magnetic properties of the as-cast and as-milled/annealed SmCo 7− x Mo x ( x=0, 0.1, 0.2, 0.3, 0.4) alloys have been systematically studied. It is found that all the as-cast series alloys are composed of the CaCu 5-type and Th 2Zn 17-type phases. Saturation magnetization of the samples decreases with the Mo content increasing. Intrinsic coercivities ( i H c) of no more than 0.06 T are observed in these as-cast samples, due to their rather coarse grain microstructures with an average grain size of 50 μm. The as-milled/annealed SmCo 7− x Mo x powders crystallize in the disordered TbCu 7-type (1:7) structure with very fine nanograins, and a minor Co 3Mo phase appears in the samples with x=0.1–0.4. High i H c (⩾0.95 T) are achieved in these samples, with a maximum of 1.26 T located at x=0.2, which can be primarily attributed to strong pinning of the domain wall motion at the nanograin boundaries. The temperature coefficient ( β) of the i H c is about −0.22%/°C in the temperature range of 25–400 °C for the as-milled/annealed samples.

Grain boundary self-diffusion of tracer 18O atoms in nanocrystalline oxide LaMnO3 + δ

Diffusion coefficients of tracer 18O atoms at boundaries of nanograins of LaMnO3 + δ oxide have been measured in the temperature range of 400–500°C. The samples of the nanocrystalline oxide are prepared with the use of the shockwave loading method. The concentration profile of the tracer atoms after diffusion annealing is measured with the use of the nuclear microanalysis method. The activation energies of the grain boundary diffusion amounts to about 2 eV and the boundary width is ∼0.05 nm. The measured coefficients of the grain boundary diffusion at 500°C exceed the corresponding coefficients of the volume diffusion by seven orders of magnitude.

Structural and magnetic properties of SmCo 5.6Ti 0.4 alloy

Three series of SmCo 5.6Ti 0.4 samples were prepared by quenching, melt spinning, and ball milling, respectively. Annealing at different temperatures was carried out for the three series. The influence of the processing routes on the structural and magnetic properties was systematically investigated for this alloy. The as-quenched bulk sample consisted of three phases with a rather coarse grain microstructure. Low intrinsic coercivity ( i H c) of 0.12 T was obtained in this sample. While the as-spun ribbons and as-milled/annealed powders showed the CaCu 5-type phase (1:5) plus Th 2Zn 17-type phase (2:17), and the 1:5 phase plus TbCu 7-type phase (1:7), respectively, with nanograin microstructure. The i H c of as-spun ribbons and as-milled/annealed (700 °C for 2 h) powders was found to be 0.59 and 2.23 T, respectively. Coercivity mechanism of these as-spun ribbons is mainly of nucleation type. In the as-milled/annealed powders, the network of the nanograin boundaries is believed to provide strong pinning sites for the domain wall movement.

Oxygen Isotope Exchange between Gaseous Phase Enriched with <sup>18</sup>O Isotope and Nanocrystal Oxides LaMnO<sub>3+δ</sub> Obtained by Severe Plastic Deformation

The kinetics of the isotope exchange between gaseous oxygen enriched with the 18O isotope and two LaMnO3+δ oxide samples – a nanopowder and a bulk nanocrystal – has been studied. The 18O isotope concentration has been measured by the acceleration nuclear microanalysis method. The coefficients of the volume and the nanograin boundary self-diffusion of oxygen have been evaluated at 500 °C. They are equal to 3.5·10 −20 and 1.5·10 −13 cm2/s, respectively.

Analysis of the strain-rate sensitivity of flow stresses in nanocrystalline FCC and BCC metals

The dependence of the strain-rate sensitivity coefficient of flow stresses S = dlnσ/dln\(\dot \varepsilon \) on temperature, strain rate, and grain size in nanocrystalline (NC) metals is analyzed quantitatively in terms of the dislocation-kinetics approach taking into account the properties of grain boundaries as sources, sinks, and barriers for moving dislocations. The interaction of moving dislocations with a dislocation forest in nanograin boundaries is shown to be responsible for the fact that the values of this coefficient in NC fcc metals (Cu, Ni) are an order of magnitude greater than those in coarse-grained metals and for the strong dependence of the coefficient S on the above factors. This dependence is largely caused by the annihilation of lattice dislocations in grain boundaries controlled by the activation energy of grain boundary diffusion. The values of the coefficient S in NC bcc metals (α-Fe) are an order of magnitude lower than those in coarse-grained samples, because dislocations move in a Peierls relief in nanograins

Anisotropy at nano-grain boundary and effective anisotropy between magnetically soft and hard nano-grains

An expression of anisotropy at grain boundary suitable for different coupling conditions has been given, based on which the effective anisotropy between soft and hard grains 〈 K sh〉 has been calculated in nanoscaled magnetic materials. The results showed that 〈 K sh〉 increases rapidly first, then the increase becomes slow, and 〈 K sh〉 decreases with increasing D s for the same value of D h. 〈 K sh〉 decreases with increasing D s for all given D h, and the decrease rate becomes slow with increasing D h. In order to obtain higher effective anisotropy between magnetically soft and hard grains, D h should not be less than 25 nm, and D s should be about 10 nm.