Papers by Keyword: Nanocrystal

Abstract: In this study, susceptibility to SCC of nanostructured Cu-10wt%Zn alloys, produced by equal-channel angular pressing (ECAP) was investigated under the constant stress test in ammonia vapour, which has been well-known typical environment for IGSCC of Cu-Zn alloy. Billets having diameter of 20 mm and length of 100 mm were subjected to ECAP for eight passes at room temperature to obtain structure with grain size of about 100 nm. After ECAP, some of the billets were flush-annealed in 473 K for 60 seconds to decrease excessive unequilibrium dislocations at grain boundaries. Coarse grained specimens without ECAP and one-pass specimens were also tested for comparison. The specimens for SCC were tensioned by a constant load in ammonia vapour inside a glass chamber for 24 hours at room temperature. After the SCC tests, maximum length of cracks was evaluated by SEM. Specimen having UFG structure by 8-passes exhibited cracks in lower applied stress ratio, (=σa/σys) compared with 0- and 1-pass samples, where σa is applied stress and σys is yield stress, respectively. Most importantly, the specimen with annealed at 473K for 60s after ECAP cracked in higher applied stress. It became less sensitive to SCC after flush annealing although mechanical properties were not changed considerably. In our previous studies, we reported that the SCC of UFG copper produced by ECAP, and the sensitivity to SCC becomes lower by flush annealing. Results are discussed in terms of grain boundary state with or without extrinsic grain boundary dislocations

Abstract: Microstructural stability is an important consideration during high temperature deformation and processing of nanomaterials. We will address issues relating to triple junctions in limiting grain growth during creep as well as densification. Although early studies on deformation have considered diffusion creep as a possible rate controlling deformation mechanism in nanocrystals, a critical inspection of available data indicates that there is no strong evidence for conventional diffusion creep in such materials. The possibility of diffusion creep by rapid diffusion along triple junctions will be analyzed, and interface controlled diffusion creep will also be discussed critically. It is shown that the critical grain size for dislocation activity is similar to that for occurrence of conventional diffusion creep.

Abstract: The structure and mechanical properties of amorphous alloy Ni44Fe29Co15Si2B10 after severe plastic deformation (SPD) in Bridgman chamber at the different temperatures (77 and 298 K) have been studied. It is shown that the early stages of the SPD of amorphous alloy cause a noticeable decrease in microhardness HV and significant changes in the physical properties. With increasing the value of SPD the transition from inhomogeneous to homogeneous (or to qualitatively different) mode of plastic flow is observed, which is accompanied by the effects of homogeneous nanocrystallization. The nanoparticle size does not exceed 10 nm. It is established that the thermally activated nanocrystallization processes can occur at very low temperatures (77 K).

Abstract: An evolution of the structure of three glasses of Na2O-Nb2O5-SiO2 system in the course of isothermal heat treatments at 660–700°C and the extinction coefficient of the material were studied. Spinodal phase separation was found to be a primary process followed by precipitation of nano-sized NaNbO3 crystalline phase. It was found that the spectral dependence of the extinction coefficient in the wavelength range λ= 400–800 nm corresponds to light scattering by spinodal structure at the phase separation stage and by independent Rayleigh scatterers (NaNbO3 nanocrystals) at the early stage of crystallization. The extinction coefficient increases at the first half of the crystallization stage and then decreases. At the late stage of crystallization and for the final glass-ceramics, the extinction coefficient α is 10–20 times smaller than that calculated for independently scattering nanocrystals and is characterized by anomalous wavelength dependence (α ∝ λ−6). The model for calculation of extinction coefficient is proposed, in which the interference effects in light scattering by nanocrystals are taken into account. On this basis, the variation of extinction coefficient in the course of crystallization and its wavelength dependence are explained.

Abstract: Repeated cold rolling with intermediate folding (RCR) represents a technique to obtain severe plastic deformation that avoids excessive heating at the internal interfaces and that proceeds without the simultaneous action of a high pressure in the range of several GPa. Aside from the opportunity to obtain amorphous bulk samples, the processing pathway also allows for synthesizing dense, bulk nanocrystalline materials. The sequential combination of different processing routes that drive a material to a different extent -, with different rates - and by different means from thermodynamic equilibrium present new and attractive processing opportunities to obtain bulk nanocrystalline or massive ultrafine grained materials that are widely unexplored. Here, an overview is presented concerning the sequential application of different deformation methods with largely different strain and pressure levels. The basic underlying mechanisms that can lead to ultrafine grained or nanocrystalline microstructures for pure metals or to two-phase nanocomposites or bulk metallic glasses for alloys are discussed and the current state of nanostructure control is highlighted by selected examples.

Abstract: Nanostructured composites inspired by structural biomaterials such as bone and nacre form intriguing design templates for biomimetic materials. Here we use large scale molecular dynamics to study the shock response of nanocomposites with similar nanoscopic structural features as bone, to determine whether bioinspired nanostructures provide an improved shock mitigating performance. The utilization of these nanostructures is motivated by the toughness of bone under tensile load, which is far greater than its constituent phases and greater than most synthetic materials. To facilitate the computational experiments, we develop a modified version of an Embedded Atom Method (EAM) alloy multi-body interatomic potential to model the mechanical and physical properties of dissimilar phases of the biomimetic bone nanostructure. We find that the geometric arrangement and the specific length scales of design elements at nanoscale does not have a significant effect on shock dissipation, in contrast to the case of tensile loading where the nanostructural length scales strongly influence the mechanical properties. We find that interfacial sliding between the composite’s constituents is a major source of plasticity under shock loading. Based on this finding, we conclude that controlling the interfacial strength can be used to design a material with larger shock absorption. These observations provide valuable insight towards improving the design of nanostructures in shock-absorbing applications, and suggest that by tuning the interfacial properties in the nanocomposite may provide a path to design materials with enhanced shock absorbing capability.

Abstract: Real time adsorption behaviors of six proteins with different isoelectric points on hydroxyapatite (HAp) nanocrystal surfaces have been investigated by using HAp sensors for quartz crystal microbalance with dissipation technique (QCM-D). The dissipation (D)–frequency (f) plots clearly showed that the different types of protein adsorption behaviors; the D-f plots of acidic proteins lie on one straight line with a constant slope under all initial protein concentrations, while those of neutral and basic proteins lie on two straight lines with different slopes. The acidic proteins formed a monolayer, while the neutral and basic proteins could cause conformational changes with the adsorbed amount of proteins. The QCM-D technique with novel HAp nanocrystal sensor is useful for the liquid phase changes of proteins on the surface.

Abstract: The adsorption properties of metal ions containing hydroxyapatite (Ca10(PO4)6(OH)2, HAp) were elucidated for the development of protein drug carrier. As-prepared metal ion containing HAp nanocrystal showed plate-like morphology with 10-20nm in length and 5-10nm in width. The metal ion containing HAp microparticles had higher specific surface area than the HAp microparticle. The adsorption amount per unit area of HAp showed higher than those of metal ion containing HAp. The adsorption behavior followed the Langmuir curves for each protein, indicating the monolayer adsorption. The loaded amount of proteins could be one of the most important properties for the application of drug delivery carrier.

Abstract: The properties of germanium implanted into the SiO2 layers in the vicinity of the bonding interface of silicon-on-insulator (SOI) structures are studied. It is shown that no germanium nanocrystals are formed in the buried SiO2 layer of the SOI structure as a result of annealing at the temperature of 1100° C. The implanted Ge atoms segregate at the Si/SiO2 bonding interface. In this case, Ge atoms are found at sites that are coherent with the lattice of the top silicon layer. It is found that the slope of the drain–gate characteristics of the back metal-oxide-semiconductor (MOS) transistors, prepared in the Ge+ ion implanted structures, increases. This effect is attributed to the grown hole mobility due to the contribution of an intermediate germanium layer formed at the Si/SiO2 interface.

Abstract: The behavior of Sb and In atoms embedded into silicon-on-insulator structure (SOI) near the bonding interface was investigated as a function of annealing temperature. Two kinds of the ionimplanted SOI structures were prepared. First kind of the structures contained the buried SiO2 layer implanted with In+ and Sb+ ions near the top Si/SiO2 interface. In second kind, the ion-implanted regions were placed on each side of the bonding interface: Sb+ ions were implanted into Si film; In+ ions were implanted into SiO2 layer. Rutherford backscattering spectrometry (RBS) and crosssectional high-resolution electron microscopy (XTEM) were employed to study the properties of the prepared structures. The formation of InSb nanocrystals was observed within the SiO2 bulk from first kind of the SOI structures as annealing temperature increased to 1100o C. In the case of the double side implanted SOI structures, an increase in annealing temperature to 1100o C was accompanied by the up-hill diffusion of In atoms from the SiO2 bulk toward the bonding interface and by the endotaxial growth of InSb nanocrystals on the top Si/SiO2 interface. It was concluded from the experimental results that Sb atoms were the nucleation centers of InSb phase.