Nano-sized Microstructure Research Articles

Grain growth in ultrafine grain sized copper during cyclic deformation

The nano-sized grain microstructure of pure copper was achieved by Equal Channel Angular Pressing (ECAP) in which the average grain size of 4 and 8 passed ECAP was about 320 and 300nm. The grain refining increased the tensile strength of ECAPed copper to 420MPa from 210MPa of annealed counterpart. Despite the increase in strength level, the nano-grained copper fabricated by ECAP process did not show the expected level of fatigue strength, such that the fatigue limit was similar to that of coarse grained copper at low applied stress range. In this study, the damage accumulation during fatigue was found to be accelerated in ultrafine grained copper compared to coarse grained counterpart. The damage accumulation mechanism in ultrafine grained copper was discussed based on the micrographic observation.

Small Angle Neutron Scattering Study of Nano Sized Microstructure in Fe–Cr ODS Steels for Gen IV In-Core Applications

The nano-sized microstructures in Fe-Cr oxide dispersion strengthened steel for Gen IV in-core applications were studied using small angle neutron scattering. The oxide dispersion strengthened steel was manufactured through hot isostatic pressing with various chemical compositions and fabrication conditions. Small angle neutron scattering experiments were performed using a 40 m small angle neutron scattering instrument at HANARO. Nano sized microstructures, namely, yttrium oxides and Cr-oxides were quantitatively analyzed by small angle neutron scattering. The yttrium oxides and Cr-oxides were also observed by transmission electron microscopy. The microstructural analysis results from small angle neutron scattering were compared with those obtained by transmission electron microscopy. The effects of the chemical compositions and fabrication conditions on the microstructure were investigated in relation to the quantitative microstructural analysis results obtained by small angle neutron scattering. The volume fraction of Y-oxide increases after fabrication, and this result is considered to be due to the formation of non-stochiometric Y-Ti-oxides.

Influence of Mg doping on the morphology and optical properties of ZnO films for enhanced H2sensing

Highly oriented ZnO and Mg doped ZnO thin films were fabricated on Al2 O3 substrate by sputtering at room temperature. The effect of Mg doping on the structural, optical, and morphological properties of ZnO film was investigated. The intensity of (002) peak in X-ray diffraction measurements revealed the influence of Mg doping on the crystallinity and orientation of ZnO film. Photoluminescence (PL) results show that the Ultraviolet (UV) emission peak was shifted to lower wavelength side for Mg:ZnO film indicating the possibility for quantum confinement. UV-vis-NIR optical absorption revealed an improvement in optical transmittance from 70 to 85%, and corresponding optical band gap from 3.25 to 3.54 eV. Atomic force microscope (AFM) images revealed the nano-size particulate microstructure of the films. The surface topography of Mg doped ZnO film confirmed decreased grain size with large surface roughness and increased surface area, favorable for sensing. Pure ZnO and Mg doped ZnO film were used as active layer and tested for its sensing performance to hydrogen. Compared to undoped ZnO, 22 at.% Mg doped ZnO film showed much higher sensor response to H2 at a concentration as low as 200 ppm and at a lower operating temperature of 180°C. A linear sensor response was observed for H2 concentration in the range of 100-500 ppm.

Highly textured Gd 2Zr 2O 7 films grown on textured Ni-5 at.%W substrates by solution deposition route: Growth, texture evolution, and microstructure dependency

Growth, texture evolution and microstructure dependency of solution derived Gd 2Zr 2O 7 films deposited on textured Ni-5 at.%W substrates have been extensively studied. Influence of processing parameters, in particular annealing temperature and dwell time, as well as thickness effect on film texture and morphology are investigated in details. It is found that a rotated cube-on-cube epitaxy of Gd 2Zr 2O 7< 110>//NiW<100> in-plane texture forms as soon as the (004) out-plane texture appears, implying that epitaxial growth dominates the crystallization processes. Thermal energy plays an important role in minimizing the difference of interfacial energy along two directions in the anisotropic metallic substrate. Growth of Gd 2Zr 2O 7 films displays an ultrafast kinetics under optimized conditions. Independency of sharp epitaxial (004) and polycrystalline (222) orientation is revealed from further synchrotron diffraction studies. Fully covered films with a broad thickness range exhibit a high degree of biaxial orientation, similar surface morphology with crack free and nano-size grains microstructure, seemingly independent of neither heat treatment nor thickness. Particularly, we compared the porosity of the film surface and body according to surface or cross-sectional observation and Rutherford Backscattering Spectrometry analysis, pointing to inhomogeneous structure through film thickness, i.e., dense in the surface layer but porous in the body. This is attributed to trapped gas generated during either decomposition or crystallization in the thicker films. This work not only demonstrates a route for producing textured Gd 2Zr 2O 7 buffer layers with dense structure directly on technical substrates, but also provides some fundamental understandings related to chemical solution derived films grown on metallic substrates.

Contribution of nano-sized lamellar microstructure on recoverable strain of Ti–24Nb–4Zr–7.9Sn titanium alloy

► Nano-sized α lamellar structure was produced by using a short-time annealing technique ( t ≤ 600 s). ► Stress-induced α″ phase transformation was suppressed with limited effect on the β phase stability. ► Stable pseudo-elasticity of about 2.7% was recorded in the full-tensile-range. Intragranular nano-sized α phase with lamellar morphology was induced in the multifunctional Ti–24Nb–4Zr–7.9Sn (wt.%) alloy by short-time thermal treatment. This peculiar α + β dual microstructure resulted in a promising combination of mechanical properties including low modulus (47 GPa), high strength (850 MPa) and reasonable ductility (∼10%). This nano-sized network of lamellar alpha phase has been obtained by short-time ageing at 573 K on solution-treated (ST) samples and at 973 K on cold-rolled (CR) samples, respectively. Suppression of the stress-induced martensitic transformation, keeping the intrinsic modulus at a low level, has been clearly noticed advantageously in both cases. In the treated specimens, a stable strain recovery of about 2.7% was obtained until tensile fracture during cycling loading.

Colour simulation and prediction of complex nano-structured metal oxide films: Test case: Analysis and modeling of electro-coloured anodized aluminium

An optical modeling procedure is developed to predict and model the colour of electro-coloured anodized aluminium that has been modified in pore structure for the generation of interference colours. The relation between the multi-layered, nano-sized oxide microstructure and the colour is experimentally determined and translated into an optical model that is able to predict the colour as a function of microstructural variations. The optical modeling procedure is highly sensitive to changes on the nano-scale (porosity, percentage of metal deposition, presence of barrier regrowth…), and has high potential as a predictive tool to relate the microstructure of a surface to the final colour characteristics of the metal, and as such to the used electrolytic processing conditions.

Polymer‐Derived Ceramics: 40 Years of Research and Innovation in Advanced Ceramics

Preceramic polymers were proposed over 30 years ago as precursors for the fabrication of mainly Si‐based advanced ceramics, generally denoted as polymer‐derived ceramics (PDCs). The polymer to ceramic transformation process enabled significant technological breakthroughs in ceramic science and technology, such as the development of ceramic fibers, coatings, or ceramics stable at ultrahigh temperatures (up to 2000°C) with respect to decomposition, crystallization, phase separation, and creep. In recent years, several important advances have been achieved such as the discovery of a variety of functional properties associated with PDCs. Moreover, novel insights into their structure at the nanoscale level have contributed to the fundamental understanding of the various useful and unique features of PDCs related to their high chemical durability or high creep resistance or semiconducting behavior. From the processing point of view, preceramic polymers have been used as reactive binders to produce technical ceramics, they have been manipulated to allow for the formation of ordered pores in the meso‐range, they have been tested for joining advanced ceramic components, and have been processed into bulk or macroporous components. Consequently, possible fields of applications of PDCs have been extended significantly by the recent research and development activities. Several key engineering fields suitable for application of PDCs include high‐temperature‐resistant materials (energy materials, automotive, aerospace, etc.), hard materials, chemical engineering (catalyst support, food‐ and biotechnology, etc.), or functional materials in electrical engineering as well as in micro/nanoelectronics. The science and technological development of PDCs are highly interdisciplinary, at the forefront of micro‐ and nanoscience and technology, with expertise provided by chemists, physicists, mineralogists, and materials scientists, and engineers. Moreover, several specialized industries have already commercialized components based on PDCs, and the production and availability of the precursors used has dramatically increased over the past few years. In this feature article, we highlight the following scientific issues related to advanced PDCs research: (1) General synthesis procedures to produce silicon‐based preceramic polymers. (2) Special microstructural features of PDCs. (3) Unusual materials properties of PDCs, that are related to their unique nanosized microstructure that makes preceramic polymers of great and topical interest to researchers across a wide spectrum of disciplines. (4) Processing strategies to fabricate ceramic components from preceramic polymers. (5) Discussion and presentation of several examples of possible real‐life applications that take advantage of the special characteristics of preceramic polymers.Note: In the past, a wide range of specialized international symposia have been devoted to PDCs, in particular organized by the American Ceramic Society, the European Materials Society, and the Materials Research Society. Most of the reviews available on PDCs are either not up to date or deal with only a subset of preceramic polymers and ceramics (e.g., silazanes to produce SiCN‐based ceramics). Thus, this review is focused on a large number of novel data and developments, and contains materials from the literature but also from sources that are not widely available.

Net shaped high performance oxide ceramic parts by selective laser melting

An additive manufacturing technique (AM) for ceramics, based on Al2O3–ZrO2 powder by means of Selective Laser Melting (SLM) is presented. Pure ceramic powder is completely melted by a laser beam yielding net-shaped specimens of almost 100% densities without any post-processing. Possible crack formation during the build-up process due to thermal stresses is prevented by a high-temperature preheating of above 1600∘C. Specimens with fine-grained nano-sized microstructures and flexural strengths of above 500 MPa are produced. The new technology allows for rapid freeform manufacture of complex net-shaped ceramics, thus, exploiting the outstanding mechanical and thermal properties for high-end medical and engineering disciplines.

MICROSTRUCTURE AND FUNCTIONAL PROPERTY CHANGES IN THIN Ni–Ti WIRES HEAT TREATED BY ELECTRIC CURRENT — HIGH ENERGY X-RAY AND TEM INVESTIGATIONS

High energy synchrotron X-ray diffraction, transmission electron microscopy and mechanical testing were employed to investigate the evolution of microstructure, texture and functional superelastic properties of 0.1 mm thin as drawn Ni – Ti wires subjected to a nonconventional heat treatment by controlled electric current (FTMT-EC method). As drawn Ni – Ti wires were prestrained in tension and exposed to a sequence of short DC power pulses in the millisecond range. The annealing time in the FTMT-EC processing can be very short but the temperature and force could be very high compared to the conventional heat treatment of SMAs. It is shown that the heavily strained, partially amorphous microstructure of the as drawn Ni – Ti wire transforms under the effect of the DC pulse and tensile stress into a wide range of annealed nanosized microstructures depending on the pulse time. The functional superelastic properties and microstructures of the FTMT-EC treated Ni – Ti wire are comparable to those observed in straight annealed wires.

The cycling performance of LiFePO 4/C cathode materials

Olivine LiFePO 4/C cathode materials were synthesized by carbothermal reduction method at different temperatures. The samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), particle size analysis and charge–discharge test. The results showed that both the crystallization degree and particle size increased remarkably with the synthesis temperature increasing. The LiFePO 4/C synthesized at 750 °C showed a highest initial discharge specific capacity of 145 mA hg −1 at 1C rate, which decreased to 60 mA hg −1 after 500 cycles. However, the sample prepared at 850 °C exhibited a lower initial discharge capacity of 135 mA hg −1 at 1C rate, whose capacity could also reach 105 mA hg −1 after 500 cycles. The material synthesized at lower sintering temperature had lower crystallization degree and plenty of nano-sized microstructure, leading to its higher electrochemical activity and initial discharge capacity. But at the same time, the lower sintering temperature also leads to the instability of the crystal structure and the solution of active materials, which results in the degradation of discharge capacity on long-time cycling. In short, synthesizing the material with perfect crystallization degree and moderate particle size is an effective way to improve the cycling performance of LiFePO 4/C cathode materials.

B, Al, Ga, In의 도핑물질이 투명 전도성 ZnO 박막의 특성에 미치는 영향

The structural, optical and electrical properties of ZnO films doped with 1.5 at% of 3A materials(B, Al, Ga, In) were studied by sol-gel process. The films were found to be c-axis (002) oriented hexagonal structure on glass substrate, when post heated at 500 <TEX>$^{\circ}C$</TEX>. The surface of the films showed a uniform and nano size microstructure and the crystalline size of doped films decreased. The lattice constants of ZnO:B/Al/Ga increased than that of ZnO, while ZnO:In decreased. All the films were highly transparent(above 90 %) in the visible region. The energy gaps of ZnO:B/Al/Ga were increased a little, but that of ZnO:In was not changed. The resistivities of ZnO:Al/Ga/In were less than 0.1 <TEX>$\Omega$</TEX>cm. All the films showed a semiconductor properties in the light or temperature, however ZnO:In was less sensitive to it. A figure of merit of ZnO:In had the highest value of 0.025 <TEX>$\Omega^{-1}$</TEX> in all samples.

Characterization of electrolytic tin dioxide deposition on Pt for lithium ion battery applications

By using cathodic electrodeposition, three structures of Sn, Sn 6O 4(OH) 4 and SnO 2 could be obtained in the 1 mM SnCl 2 aqueous solution for various aeration periods in air. In order to obtain the pure SnO 2 coatings as the anode of thin-film lithium ion battery, the aeration should proceed for 10 days to oxidize tin valence from stannous to stannic in SnCl 2 aqueous solution. The coating films were further annealed and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FTIR), thermogravimetric–differential analysis (TGA), Raman spectroscopy, and cyclic voltammetry (CV). The as-coating SnO 2 film containing 2.4 H 2O in the pristine state could be dehydrated at 300 °C for 2 h, or at 250 °C for 12 h. The crystallinity and surface coarsening of the coating films were promoted with increasing annealing temperature. The dehydrated SnO 2 film used as the anode of a lithium battery showed a reversible capacity as high as 500 mA h/g after 10 cycles at a discharging rate of 0.2 C, which could result from the nano-sized microstructure of coating film.

Electrical and optical properties of Al-doped ZnO thin films by sol–gel process

Al-doped ZnO (AZO) thin films oriented along the (0 0 2) plane have been prepared by the sol–gel process and their electrical and optical properties with post-deposition heating temperature were investigated. The preferred c-axis orientation along the (0 0 2) plane was enhanced with increasing post-deposition heating temperature and the surface of the films showed a uniform and nano-sized microstructure. The electrical resistivity of the films decreased from 73 to 22 Ω cm as the post-deposition heating temperature increased from 500 to 650 °C; however, the film postheated at 700 °C increased greatly to 580 Ω cm. The optical transmittance of the films postheated below 650 °C was over 86%, but it decreased at 700 °C. The electrical and optical properties of the AZO films with post-deposition heating temperature are discussed.

Thermodynamics of nanocrystilline formation in surface layer of Hadfield steel by shot peening

Conventional shot peening tests of Hadfield steel have shown that a nanostructured grains layer can be easily produced. Theoretically, nanostructure formation thermodynamics becomes an important issue to coarse grains transformating into nanocrystalline. In the present paper, thermodynamics of strain induced nanosized microstructure in Hadfield steel after shot peening was discussed based on Pringogine's nonequilibrium thermodynamics and Miedema semi-experimental theory. It was concluded that the difference of Gibbs free energy should be at least increased to 0.1380 kJ/mol in the alloy system by strain hardening, compared to the previous Gibbs energy before the treatment.

A Lead‐Free High‐Curie‐Point Ferroelectric Ceramic, CaBi2Nb2O9.

AbstractFor Abstract see ChemInform Abstract in Full Text.

A Lead‐Free High‐Curie‐Point Ferroelectric Ceramic, CaBi2Nb2O9

Nano-powders of BaTiO3, SrTiO3, Ba0.6Sr0.4TiO3, a mixture of the composition (BaTiO3)0.6(SrTiO3)0.4 with particle sizes in the range of 60 to 80 nm, and Bi4Ti3O12 with an average particle size of 100 nm were consolidated by spark plasma sintering (SPS). The kinetics of reaction, densification and grain growth were studied. An experimental procedure is outlined that allows the determination of a “kinetic window” within which dense nano-sized compacts can be prepared. It is shown that the sintering behaviour of the five powders varies somewhat, but is generally speaking fairly similar. However, the types of grain growth behaviour of these powders are quite different, exemplified by the observation that the kinetic window for the (BaTiO3)0.6(SrTiO3)0.4 mixture is 125 oC, ~75 oC for Bi4Ti3O12, ~25oC for BaTiO3 and SrTiO3, while it is hard to observe an apparent kinetic window for obtaining nano-sized compacts of Ba0.6Sr0.4TiO3. During the densification of the (BaTiO3)0.6(SrTiO3)0.4 mixture the reaction 0.6BaTiO3+0.4SrTiO3 → Ba0.6Sr0.4TiO3 takes place, and this reaction is suggested to have a self-pinning effect on the grain growth, which in turn explains why this powder has a large kinetic window. Notably, SPS offers a unique opportunity to more preciously investigate and monitor the sintering kinetics of nano-powders, and it allows preparation of ceramics with tailored microstructures.The dielectric properties of selected samples of (Ba, Sr)TiO3 ceramics have been studied. The results are correlated with the microstructural features of these samples, e.g. to the grain sizes present in the compacts. The ceramic with nano-sized microstructure exhibits a diffuse transition in permittivity and reduced dielectric losses in the vicinity of the Curie temperature, whereas the more coarse-grained compacts exhibit normal dielectric properties in the ferroelectric region.The morphology evolution, with increasing sintering temperature, of bismuth layer-structured ferroelectric ceramics such as Bi4Ti3O12 (BIT) and CaBi2Nb2O9 (CBNO) was investigated. The subsequent isothermal sintering experiments revealed that the nano-sized particles of the BIT precursor powder grew into elongated plate-like grains within a few minutes, via a dynamic ripening mechanism.A new processing strategy for obtaining highly textured ceramics is described. It is based on a directional dynamic ripening mechanism induced by superplastic deformation. The new strategy makes it possible to produce a textured microstructure within minutes, and it allows production of textured ferroelectric ceramics with tailored morphology and improved physical properties.The ferroelectric, dielectric, and piezoelectric properties of the textured bismuth layer-structured ferroelectric ceramics have been studied, and it was revealed that all textured samples exhibited anisotropic properties and improved performance. The highly textured Bi4Ti3O12 ceramic exhibited ferroelectric properties equal to or better than those of corresponding single crystals, and much better than those previously reported for grain-orientated Bi4Ti3O12 ceramics. Textured CaBi2Nb2O9 ceramics exhibited a very high Curie temperature, d33-values nearly three times larger than those of conventionally sintered materials, and a high thermal depoling temperature indicating that it is a very promising material for high-temperature piezoelectric applications.

고탄소강 펄라이트 조직의 인발 공정 시 전단응력의 해석

This paper presents a study on defects in pearlite lamella structure of high carbon steel by means of finite-element method(FEM) simulation. High-carbon pearlite steel wire is characterized by its nano-sized microstructure feature of alternation ferrite and cementite. The likely fatigue crack is located on interface of the lamella structure where the maximum amplitude of the longitudinal shear stress and transverse shear stress was calculated during cyclic loading. The FEM is proposed for maximum shear stress from loading of lamella structure, and a method is predicted to analyze the likely fatigue crack generation. It is possible to obtain the important basic data which can be guaranteed in the ductility of high carbon steel wire by using FEM simulation.

신선가공조건에 따른 고탄소강 선재 Pearlite 층상구조의 유한요소해석

This paper presents a study on defects in pearlite lamella structure of high carbon steel by means of finite-element method(FEM) simulation. High carbon pearlite steel wire is characterized by its nano-sized microstructure feature of alternation ferrite and cementite. FEM simulation was performed based on a suitable FE model describing the boundary conditions and the exact material behavior. Due to the lamella structure in high carbon pearlite steel wire, material plastic behavior was taken into account on deformation of ferrite and cementite. The effects of many important parameters(reduction in area, semi-die angle, lamella spacing, cementite thickness) on wire drawing process can be predicted by DEFORM-2D. It is possible to obtain the important basic data which can be guaranteed in the ductility of high carbon steel wire by using FEM simulation.

Crystallization of liquids and glasses to quasicrystals

The local structures of metallic glasses and undercooled liquid metal alloys are presumed to contain a high degree of icosahedral short-range order, incompatible with translational periodicity and creating a barrier to the nucleation of crystal phases. The barrier should be lower for the nucleation of icosahedral quasicrystals (i-phase). A brief overview is provided of studies in liquids and glasses that support this conclusion. Recent X-ray diffraction and undercooling results in a TiZrNi alloy that forms an i-phase are presented, correlating an icosahedral local order in the liquid with a lower nucleation barrier for the i-phase. For a growing number of alloys that can be prepared as glasses, a nanosized i-phase microstructure forms upon glass crystallization (devitrification), consistent with the expected low nucleation barrier. However, since glasses that do not form quasicrystals have similar devitrification nanostructures, other crystallization mechanisms must also be operative. These are reviewed. A new model for homogeneous nucleation that couples the interfacial and the long-range diffusion fluxes, is advanced as one way to explain nanostructure formation in these metallic glasses.

Nano-grained Net-Shaped Fabrication of Re Components by EB-PVD

Cost-effective net-shaped forming components have brought considerable interest into the Department of Defense, the National Aeronautics & Space Administration, and the Department of Energy. Electron beam–physical vapor deposition (EB-PVD) offers flexibility in forming net-shaped components with tailored microstructure and chemistry. High-purity rhenium components, including rhenium-coated graphite balls and rhenium plates and tubes, have been successfully manufactured by EB-PVD. EB-PVD rhenium components exhibited submicron and nano-size microstructure with high hardness and strength as compared with chemical vapor deposition (CVD). It is estimated that the cost of rhenium components manufactured by EB-PVD would be less than the current CVD and powder-high-temperature isostatic pressure technologies.