Showing papers on "Microstructure published in 2006"

Three-dimensional reconstruction of a solid-oxide fuel-cell anode

Abstract: The drive towards increased energy efficiency and reduced air pollution has led to accelerated worldwide development of fuel cells. As the performance and cost of fuel cells have improved, the materials comprising them have become increasingly sophisticated, both in composition and microstructure. In particular, state-of-the-art fuel-cell electrodes typically have a complex micro/nano-structure involving interconnected electronically and ionically conducting phases, gas-phase porosity, and catalytically active surfaces. Determining this microstructure is a critical, yet usually missing, link between materials properties/processing and electrode performance. Current methods of microstructural analysis, such as scanning electron microscopy, only provide two-dimensional anecdotes of the microstructure, and thus limited information about how regions are interconnected in three-dimensional space. Here we demonstrate the use of dual-beam focused ion beam-scanning electron microscopy to make a complete three-dimensional reconstruction of a solid-oxide fuel-cell electrode. We use this data to calculate critical microstructural features such as volume fractions and surface areas of specific phases, three-phase boundary length, and the connectivity and tortuosity of specific subphases.

Crystalline WO3 nanoparticles for highly improved electrochromic applications

Abstract: Electrochromic (EC) materials change their optical properties (darken/lighten) in the presence of a small electric potential difference, and are suitable for application in energy-efficient windows, antiglare automobile rear-view mirrors, sunroofs, displays, and hydrogen sensors. [1–4] The operation of conventional EC devices depends on the reversible electrochemical double injection of positive ions (H + ,L i + ,N a + ) and electrons into the host lattice of multivalent transition metal oxide materials, [5–10] with positive-ion insertion required to satisfy charge neutrality. However, diffusion of positive ions into the oxide layer is often slow, taking minutes to complete. Since the chemical diffusion coefficient of protons (DH+ )i s an order of magnitude larger than that of lithium ions (DLi+), EC systems based on proton electrolytes (e.g., aqueous H2SO4) are mandatory for display applications and preferred for other applications. Unfortunately, proton insertion currently results in rapid degradation of EC films. There are two important criteria for selecting an EC material. The first is the time constant of the ion-intercalation reaction, which is limited both by the diffusion coefficient and by the length of the diffusion path. While the former depends on the chemical structure and crystal structure of the metal oxide, the latter is determined by the material’s microstructure. [11] In the case of a nanoparticle, the smallest dimension is represented by the diffusion path length. Thus, designing a nanostructure with a small radius, while maintaining the proper crystal structure, is key to obtaining a material with fast insertion kinetics, enhanced durability, and superior performance. The second important criterion is coloration efficiency (CE), the change in optical density (OD) per unit inserted charge (Q), that is, CE= D(OD)/DQ. [12] A high CE provides

Facile controlled synthesis of MnO2 nanostructures of novel shapes and their application in batteries.

Abstract: In this paper, MnO2 nanomaterials of different crystallographic types and crystal morphologies have been selectively synthesized via a facile hydrothermal route and electrochemically investigated as the cathode active materials of primary and rechargeable batteries. β-MnO2 nano/microstructures, including one-dimensional (1-D) nanowires, nanorods, and nanoneedles, as well as 2-D hexagramlike and dendritelike hierarchical forms, were obtained by simple hydrothermal decomposition of an Mn(NO3)2 solution under controlled reaction conditions. α- and γ-MnO2 nanowires and nanorods were also prepared on the basis of previous literature. The as-synthesized samples were characterized by instrumental analyses such as XRD, SEM, TEM, and HRTEM. Furthermore, the obtained 1-D α- and γ-MnO2 nanostructures were found to exhibit favorable discharge performance in both primary alkaline Zn−MnO2 cells and rechargeable Li−MnO2 cells, showing their potential applications in high-energy batteries.

Volume change during the formation of nanoporous gold by dealloying.

Abstract: We report a macroscopic shrinkage by up to 30 vol % during electrochemical dealloying of Ag-Au. Since the original crystal lattice is maintained during the process, we suggest that the formation of nanoporous gold in our experiments is accompanied by the creation of a large number of lattice defects and by local plastic deformation.

Friction stir welding of carbon steels

Abstract: In order to determine the effect of the carbon content and the transformation on the mechanical properties and microstructures of the FSW carbon steel joints, three types of carbon steels with different carbon contents (IF steel, S12C, S35C) were friction stir welded under various welding conditions. Compared with IF steel, the microstructures and mechanical properties of the carbon steel joints are significantly affected by the welding conditions. The strength of the S12C steel joints increases with the increasing welding speed (decreasing the heat input), while the strength of the S35C steel joints shows a peak near 200 mm/min. This can be explained by the relationship between the peak temperature and the A 1 and A 3 points. When friction stir welding is performed in the ferrite–austenite two-phase region, the microstructure is refined and the highest strength is then achieved.

Processing and properties of monolithic TiB2 based materials

Abstract: Titanium diboride (TiB2) based materials have received wide attention because of their high hardness and elastic modulus, good abrasion resistance, and superior thermal and electrical conductivity. Potential applications include high temperature structural materials, cutting tools, armour, electrodes in metal smelting and wear parts. Despite its useful properties, the application of monolithic TiB2 is limited by poor sinterability, exaggerated grain growth at high temperature and poor oxidation resistance above 1000°C. Pure TiB2 can be densified only at high temperatures (∼2000°C), with an applied pressure generally being necessary during sintering. However, these high sintering temperatures cause abnormal grain growth and microcracks, which are detrimental to the mechanical properties. Various sinter additives are commonly added to obtain dense TiB2 with optimised mechanical properties at lower sintering temperature. The present review surveys the current state of knowledge on development of bulk...

Simultaneously Increasing the Ductility and Strength of Ultra-Fine-Grained Pure Copper

Abstract: Bulk ultra-fine-grained (UFG) materials produced by severe plastic deformation (SPD) usually have high strength but relatively low ductility at ambient temperatures. This low ductility is attributed to insufficient strain hardening due to an inability to accumulate dislocations. For a singlephased UFG material where dislocation slip is the primary deformation mechanism, a long-standing fundamental question concerns the feasibility of developing microstructures that offer high ductility without sacrificing strength. The answer appears to be positive because there are some isolated examples where excellent mechanical behavior has been observed. Nevertheless, the structural features contributing to high strength and good ductility remain undefined, and this lack of understanding has hindered the search for effective procedures to simultaneously improve the strength and ductility of UFG materials. Here, we report a new process in which high ductility is achieved without sacrificing strength by plastically deforming UFG Cu in liquid nitrogen. The enhanced ductility is attributed primarily to the presence of a high density of preexisting deformation twins (PDTs) and also possibly to a large fraction of high-angle grain boundaries (HAGBs) formed during cryogenic processing. We conclude that this procedure provides a new strategy for increasing the ductility of UFG materials without any concurrent loss in strength. Strength and ductility are often mutually exclusive, i.e., materials may be strong or ductile but are rarely both. This also applies to bulk UFG materials. The low ductility of UFG materials has invariably limited their practical application and, accordingly, much attention has been paid to the development of strategies for improving this poor ductility. For singlephase UFG and nanostructured materials, several of the reports documenting high ductility and strength describe experiments on Cu where the stacking-fault energy is relatively low. In some investigations the high ductility was attributed to the development of a bimodal grain size distribution or pre-existing growth twins (PGTs), but in other investigations the reasons for the high ductility were not clearly defined. In practice, however, a bimodal grain size distribution must sacrifice some of the strength gained from nanostructuring. Another challenge is the need to fabricate UFG materials in large bulk form suitable for structural applications. This requirement has been hindered because the evidence suggests that PGTs occur only in electrodeposited thin films of nanostructured Cu, and in nanocrystalline Cu by inert-gas condensation (IGC) followed by compaction. However, the ductility of IGC-prepared nanocrystalline Cu is very low. The objectives of this study were twofold: First, to develop a procedure for increasing the ductility of large bulk UFG Cu without incurring any significant loss in strength. Second, to evaluate the mechanism contributing to high ductility in UFG Cu. A pure Cu (99.99%) bar was initially processed by equalchannel angular pressing (ECAP) to produce a UFG structure (hereafter designated the UFGECAP sample), then cryodrawn (D) to a reduction in area of ca. 95%, followed by cryorolling (R) with a reduction in thickness of ca. 96% (hereafter designated the UFGECAP+D+R sample). Figure 1a shows that the UFGECAP+D+R sample has superior mechanical properties compared to the UFGECAP sample. The UFGECAP Cu sample has a 0.2% yield strength of ca. 410 MPa ( ), which is significantly higher than the value of ca. 40 MPa in coarse-grained (CG) Cu. In addition, necking occurs rapidly after the stress reaches a maximum value, yielding a uniform elongation of only ca. 1.3% and an elongation to failure of only ca. 5.9% in the UFGECAP sample. By contrast, the yield strength is increased to ca. 500 MPa in the UFGECAP+D+R sample, and, more importantly, this sample undergoes strain hardening, giving a uniform elongation of C O M M U N IC A IO N S

ポリマーから誘導されたケイ素系セラミックス ─合成・性質・応用に関する総説─

Abstract: This review presents the synthesis, characterization techniques, processing and potential applications of silicon-based ceramic materials derived from organosilicon polymers. The Si-ceramics are prepared by thermolysis of molecular precursors. The influence of the initial molecular structure of the precursor on the properties of the final ceramic material and its applications is discussed. The thermolytic decomposition of suitable Si-based polymers provides materials which are denoted as polymer-derived ceramics (PDCs). In particular, this procedure is a promising method for the preparation of ternary and multinary silicon-based ceramics in the system SiCNO. There is no other synthetic approach known to produce e.g. SiCO or SiCN based ceramics. In the case of PDCs route, common preceramic polymers are poly(organosilazanes), poly(organosilylcarbodiimides) and poly(organosiloxanes). One basic advantage of the PDC route is that the materials can be easily shaped in form of fibers, layers or bulk composite materials by applying processing techniques established in the plastic industry. The PDCs in general exhibit enhanced thermomechanical properties, i.e., temperature stabilities up to approximately 1500°C. Recent investigations have shown that in some cases the high temperature stability in terms of decomposition and/or crystallization can be increased even up to 2000°C if the preceramic polymer contains some amount of boron. The composition and microstructure of the PDC are a result of the molecular structure of the preceramic polymer. Therefore, the observed differences in the macroscopic properties are also closely related to the variation of composition and solid state structure of these materials.

Ageing effects in a compacted bentonite: a microstructure approach

Abstract: It is suspected that the as-compacted state of heavily compacted clays used as possible engineered barriers for nuclear waste disposal is time dependent, and that further change may occur in the material, even at constant water content and density. This paper presents an investigation of the time-dependent microstructure changes of an MX80 clay bentonite compacted at various dry densities and water contents. Microstructure investigation is based on mercury intrusion pore size distribution measurements and scanning electron microscopy (SEM). Results obtained by other researchers by using X-ray diffraction at low angles are also used. Statically compacted samples of MX80 were kept at constant volume and water content for various periods of time (1, 30 and 90 days) prior to mercury intrusion and SEM micro-structure investigation. A significant change in micro-structure with time was observed, characterised by a decrease in the inter-aggregate porosity and an increase in the very thin porosity not intruded by...

Low-Temperature Sintered Nanoscale Silver as a Novel Semiconductor Device-Metallized Substrate Interconnect Material

Abstract: A nanoscale silver paste containing 30-nm silver particles that can be sintered at 280degC was made for interconnecting semiconductor devices. Sintering of the paste produced a microstructure containing micrometer-size porosity and a relative density of around 80%. Electrical and thermal conductivities of around 2.6times105 (Omegamiddotcm)-1 and 2.4W/K-cm, respectively, were obtained, which are much higher than those of the solder alloys that are currently used for die attachment and/or flip-chip interconnection of power semiconductor devices. The sintered porous silver had an apparent elastic modulus of about 9GPa, which is substantially lower than that of bulk silver, as well as most solder materials. The lower elastic modulus of the porous silver may be beneficial in achieving a more reliable joint between the device and substrate because of increased compliance that can better accommodate stress arising from thermal expansion mismatch

Temperature‐Stable Dielectrics Based on Chemically Inhomogeneous BaTiO3

Abstract: The dielectric properties and chemical homogeneity of BaTiO3 ceramics sintered with additions of the pseudophase “CdBi2Nb2O9” were investigated using SEM, TEM, STEM, and EDX. In materials showing the “X7R” dielectric temperature characteristic, the microstructure exhibits the grain core-grain shell structure. The perovskite material in the shell shows a temperature characteristic determined by mixed crystals of BaTiO3 with the complex perovskites Ba(Bi1/2Nb1/2)O3 and Ba(Cd1/3Nb2/3)O3 having an approximate Curie point of -80°C. The chemical inhomogeneity emerges during a process of reactive liquid-phase sintering. Application of too-high sintering temperatures leads to uniform distributions of the additives via solid-state diffusion and to the loss of the X7R characteristic.

Enhanced ferroelectricity in Ti-doped multiferroic BiFeO3 thin films

Abstract: Ti4+ ion-doped BiFeO3 thin films were prepared by sol-gel spin-coating technique on (111)Pt∕Ti∕SiO2∕Si substrates. X-ray diffraction and scanning electron microscope revealed the single phase, and good surface and cross-section morphologies of the films, respectively. Leakage current density measurement indicated that the quality of the BiFeO3 films was improved by Ti4+ doping. By introducing a small amount of Ti ions into the sol-gel solution-processed BiFeO3 films, large enhancement in both remnant and saturation polarizations of the doped-BiFeO3 films in comparisons with the undoped BiFeO3 films was observed, due to the reduced leakage current, stabilization of the ferroelectric distortion by Ti4+, and more homogenous microstructure.

The cold spray process and its potential for industrial applications

Abstract: Cold spraying has attracted serious attention since unique coating properties can be obtained by the process that are not achievable by conventional thermal spraying This uniqueness is due to the fact that coating deposition takes place without exposing the spray or subtrate material to high temperatures and, in particular, without melting the sprayed particles Thus, oxidation and other undesired reactions can be avoided Spryy particles adhere to the substrate only because of their high kinetic energy on impact For successful bonding, powder particles have to exceed a critical velocity on impact, which is dependent on the properties of the particular spray material This requires new concepts for the description of coating formation but also indicates applications beyond the market for typical thermal spray coatings The present contribution summarizes the current “state of the art” in cold spraying and demonstrates concepts for process optimization

A comparative study of reactive direct current magnetron sputtered CrAlN and CrN coatings

Abstract: Approximately 1.5 μm thick CrN and CrAlN coatings were deposited on silicon and mild steel substrates by reactive direct current (DC) magnetron sputtering. The structural and mechanical properties of the coatings were characterized using X-ray diffraction (XRD) and nanoindentation techniques, respectively. The bonding structure of the coatings was characterized by X-ray photoelectron spectroscopy (XPS). The surface morphology of the coatings was studied using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The XRD data showed that the CrN and CrAlN coatings exhibited B1 NaCl structure. Nanoindentation measurements showed that as-deposited CrN and CrAlN coatings exhibited a hardness of 18 and 33 GPa, respectively. Results of the surface analysis of the as-deposited coatings using SEM and AFM showed a more compact and dense microstructure for CrAlN coatings. The thermal stability of the coatings was studied by heating the coatings in air from 400 to 900 °C. The structural changes as a result of heating were studied using micro-Raman spectroscopy. The Raman data revealed that CrN coatings got oxidized at 600 °C, whereas in the case of CrAlN coatings, no detectable oxides were formed even at 800 °C. After annealing up to 700 °C, the CrN coatings displayed a hardness of only about 7.5 GPa as compared to CrAlN coatings, which exhibited hardness as high as 22.5 GPa. The potentiodynamic polarization measurements in 3.5% NaCl solution indicated that the CrAlN coatings exhibited superior corrosion resistance as compared to CrN coatings.

Atomic pillar-based nanoprecipitates strengthen AlMgSi alloys.

Abstract: Atomic-resolution electron microscopy reveals that pillarlike silicon double columns exist in the hardening nanoprecipitates of AlMgSi alloys, which vary in structure and composition. Upon annealing, the Si2 pillars provide the skeleton for the nanoparticles to evolve in composition, structure, and morphology. We show that they begin as tiny nuclei with a composition close to Mg2Si2Al7 and a minimal mismatch with the aluminum matrix. They subsequently undergo a one-dimensional growth in association with compositional change, becoming elongated particles. During the evolution toward the final Mg5Si6 particles, the compositional change is accompanied by a characteristic structural change. Our study explains the nanoscopic reasons that the alloys make excellent automotive materials.

Effect of nanosilica on characterization of Portland cement composite

Abstract: Both the filling effect and the pozzolanic reaction make siliceous materials as one of major ingredients of high-performance Portland cement-based composites. Hence, the introduction of nanosilica with finer particle size and larger silicon dioxide to the composite becomes a great deal of interest in recent years. In this study, a liquid-form of nanosilica particle with a spherical diameter of about 20 nm was incorporated into the Portland cement paste at five different dosages and analyzed at four different ages to identify the nanosizing effects on the microstructures and material properties of composite cement paste. Experimental results show that the Portland cement composite with 0.60% of added nanosilica by weight of cement has an optimum compressive strength, in which the increase of compressive strength is about 43.8%. Moreover, the corresponding nanosilica paste of one portion of water mixed with nanosilica of 1.08 wt.% of water has the maximum absolute value of zeta potential of 41.3 mV. Properties through the analyses of NMR, BET and MIP also indicate that the microstructure of Portland cement composite with nanosilica evidently has a more solid, dense and stable bonding framework.