Showing papers in "Journal of Materials Research in 2004"

Measurement of hardness and elastic modulus by instrumented indentation: Advances in understanding and refinements to methodology

Abstract: The method we introduced in 1992 for measuring hardness and elastic modulus by instrumented indentation techniques has widely been adopted and used in the characterization of small-scale mechanical behavior. Since its original development, the method has undergone numerous refinements and changes brought about by improvements to testing equipment and techniques as well as from advances in our understanding of the mechanics of elastic–plastic contact. Here, we review our current understanding of the mechanics governing elastic–plastic indentation as they pertain to load and depth-sensing indentation testing of monolithic materials and provide an update of how we now implement the method to make the most accurate mechanical property measurements. The limitations of the method are also discussed.

Organic solar cells: An overview

Abstract: Organic solar cell research has developed during the past 30 years, but especially in the last decade it has attracted scientific and economic interest triggered by a rapid increase in power conversion efficiencies. This was achieved by the introduction of new materials, improved materials engineering, and more sophisticated device structures. Today, solar power conversion efficiencies in excess of 3% have been accomplished with several device concepts. Though efficiencies of these thin-film organicdevices have not yet reached those of their inorganic counterparts (η ≈ 10–20%); the perspective of cheap production (employing, e.g., roll-to-roll processes) drives the development of organic photovoltaic devices further in a dynamic way. The two competitive production techniques used today are either wet solution processing or dry thermal evaporation of the organic constituents. The field of organic solar cells profited well from the development of light-emitting diodes based on similar technologies, which have entered the market recently. We review here the current status of the field of organic solar cells and discuss different production technologies as well as study the important parameters to improve their performance.

Organic thin film transistors: From theory to real devices

Abstract: The organic thin-film transistor (OTFT) is now a mature device that has developed tremendously during the last twenty years. The aim of this paper is to update previous reviews on that matter that have been published in the past. The operating mode of OTFTs is analyzed in view of recent model development. This mainly concerns the distribution of charges in the conducting channel and problems connected with contact resistance. We also delineate what differentiates n- and p-type semiconductors, and show how this concept differs from what it covers in conventional semiconductors. In the chapter devoted to fabrication techniques, emphasis is placed on solution-based techniques and particularly printing processes. Similarly, soluble materials are given a prominent place in the section dedicated to the performance of devices. Finally, special attention is given to devices at the nanoscale level, which demonstrate a new route toward molecular electronics.

Fe-based bulk metallic glasses with diameter thickness larger than one centimeter

Abstract: Fe–Cr–Mo–(Y,Ln)–C–B bulk metallic glasses (Ln are lanthanides) with maximum diameter thicknesses reaching 12 mm have been obtained by casting. The high glass formability is attained despite a low reduced glass transition temperature of 0.58. The inclusion of Y/Ln is motivated by the idea that elements with large atomic sizes can destabilize the competing crystalline phase, enabling the amorphous phase to be formed. It is found that the role of Y/Ln as a fluxing agent is relatively small in terms of glass formability enhancement. The obtained bulk metallic glasses are non-ferromagnetic and exhibit high elastic moduli of approximately 180–200 GPa and microhardness of approximately 13 GPa.

Growth of aromatic molecules on solid substrates for applications in organic electronics

Abstract: The growth of molecular adlayers on solid substrates is reviewed with aspecial emphasis on molecules of relevance for organic electronics. In particular,we will consider planar molecules with extended π-systems, namely acenes (anthracene, tetracene, pentacene), perylene, coronenes, diindenoperylene, 3,4,9,10-perylene-tetracarboxylicacid-dianhydride, poly-phenylenes, oligothiophenes, and phthalocyanines. Special consideration is given to the importance of the formation of ordered molecular overlayers, which are compared with the structure of the corresponding bulk crystals. Whenever possible, aspects relevant for device fabrication (morphology of deposited films, mobilities of charge carriers) will be addressed.

A room-temperature TiO2-nanotube hydrogen sensor able to self-clean photoactively from environmental contamination

Abstract: Described is a room-temperature hydrogen sensor comprised of a TiO2-nanotube array able to recover substantially from sensor poisoning through ultraviolet (UV) photocatalytic oxidation of the contaminating agent; in this case, various grades of motor oil. The TiO2 nanotubes comprising the sensor are a mixture of both anatase and rutile phases, having nominal dimensions of 22-nm inner diameter, 13.5-nm wall thickness, and 400-nm length, coated with a 10-nm-thick noncontinuous palladium layer. At 24 °C, in response to 1000 ppm of hydrogen, the sensors show a fully reversible change in electrical resistance of approximately 175,000%. Cyclic voltammograms using a 1 N KOH electrolyte under 170 mW/cm2 UV illumination show, for both a clean and an oil-contaminated sensor, anodic current densities of approximately 28 mA/cm2 at 2.5 V. The open circuit oxidation potential shows a shift from 0.5 V to −0.97 V upon UV illumination.

A survey of instrumented indentation studies on metallic glasses

Abstract: The development of instrumented nanoindentation equipment has occurred concurrently with the discovery of many new families of bulk metallic glass during the past decade. While indentation testing has long been used to assess the mechanical properties of metallic glasses, depth-sensing capabilities offer a new approach to study the fundamental physics behind glass deformation. This article is a succinct review of the research to date on the indentation of metallic glasses. In addition to standard hardness measurements, the onset of plasticity in metallic glasses is reviewed as well as the role of shear banding in indentation, structural changes beneath the indenter, and rate-dependent effects measured by nanoindentation. The article concludes with perspectives about the future directions for nanocontact studies on metallic glasses.

Synthesis and Thermal Analyses of TiO2-Derived Nanotubes Prepared by the Hydrothermal Method

Abstract: TiO2-derived nanotubes were prepared by hydrothermal treatment of TiO2 powder in NaOH aqueous solution. High-temperature x-ray diffraction (HT-XRD) andthermogravimetry-differential thermal analysis (TG-DTA) demonstrated the formation of TiO2 (B) phase (a metastable polymorph of titanium dioxide) from the nanotubes under heating at ∼800 °C, which indicates the as-prepared nanotubes should be composed of layered titanate, most probably as H2Ti3O7·nH2O (n < 3). Dehydration behavior and phase transformation confirmed by the HT-XRD study have suggested reliable reaction path and have well-solved the contradictions on the nanotube-formation mechanism among previous studies.

Application of nucleation theory to the rate dependence of incipient plasticity during nanoindentation

Abstract: We propose a nucleation theory-based analysis for incipient plasticity during nanoindentation and predict the statistical distribution of rate-dependent pop-in events for many nominally identical indentations on the same surface. In the framework of stress-assisted, thermally activated defect nucleation, we quantitatively rationalize new nanoindentation measurements on 4H SiC and extract the activation volume of the nucleation events that mark the onset of plastic flow. We also illustrate how this statistical approach can differentiate between unique nucleation events for different indenter tip geometries.

Evaluation of elastic modulus and hardness of thin films by nanoindentation

Abstract: Simple equations are proposed for determining elastic modulus and hardness properties of thin films on substrates from nanoindentation experiments. An empirical formulation relates the modulus E and hardness H of the film/substrate bilayer to corresponding material properties of the constituent materials via a power-law relation. Geometrical dependence of E and H is wholly contained in the power-law exponents, expressed here as sigmoidal functions of indenter penetration relative to film thickness. The formulation may be inverted to enable deconvolution of film properties from data on the film/substrate bilayers. Berkovich nanoindentation data for dense oxide and nitride films on silicon substrates are used to validate the equations and to demonstrate the film property deconvolution. Additional data for less dense nitride films are used to illustrate the extent to which film properties may depend on the method of fabrication.

Rapid coating of Ti6Al4V at room temperature with a calcium phosphate solution similar to 10× simulated body fluid

Abstract: In this paper, we report the utilization of high ionic strength (>1100 mM) calcium phosphate solutions in depositing 20–65-μm-thick, bonelike apatitic calcium phosphate on Ti6Al4V within 2–6 h, at room temperature. The super-strength solution used here multiplied the concentrations of calcium and phosphate ions in human plasma or simulated body fluid (SBF) by a factor of ten. The interesting features of the technique are given in the following. First, the solutions did not contain any buffering agents, such as Tris or Hepes. Second, during the process, homogeneous formation of calcium phosphate nano-clusters took place. However, their presence did not adversely affect the coating process. Third, other than simple surface treatments to begin with, no other additional intermediate steps were necessary. The only step needed after the preparation of the solution from reagents is the addition of proper amounts of NaHCO3 to raise the pH to 6.5 prior to the coating procedure. Fourth, there is no CO2 bubbling required, and hence, this is a robust process. Fifth, such a procedure led to a significant enhancement of coating rate enabling the formation in as little as 2–6 h. Coating proceeded with a linear rate. Sixth, the adhesion strength (12 ± 2 MPa) of the present coatings was comparable to coatings produced by soaking in 1.5× SBF solutions over a prolonged period of time, typically two to three weeks. Finally, the carbonate content (8 wt%) and Ca/P molar ratio (1.57) qualify the coating as bonelike.

Size effects of nanoindentation creep

Abstract: The size effects on indentation creep were studied on single-crystal Ni3Al, polycrystalline pure Al, and fused quartz samples at room temperature. The stress exponents were measured by monitoring the displacement during constant indentation loads after correction for thermal drift effects. The stress exponents were found to exhibit a very strong size effect. In the two metals Al and Ni3Al, the stress exponent for very small indents is very small, and for Al, this even approaches unity, suggesting that linear diffusional flow may be the controlling mechanism. The stress exponents in these two metals rise rapidly to over 100 as the indent size gets larger, indicating a rapid change of the dominating mechanism to climb-controlled to eventually glide-controlled events. In fused quartz, the stress exponent also exhibits a sharply rising trend as the indent size increases. The stress exponent is also close to unity at the smallest indents studied, and it rises rapidly to a few tens as the indent size gets larger.

From polymer transistors toward printed electronics

Abstract: Printed organic circuits have the potential to revolutionize the spread of electronic applications. This will be enabled by inexpensive and fast fabrication with printing techniques using soluble organic materials. Two main challenges have to be mastered on the way towards printed electronics. First, the development of stable transistors and an adapted chip design for organic materials, and second, the development of a reliable fabrication process. We present our results on high performance polymer transistors, mainly based on poly-3alkylthiophene (P3AT) as semiconducting material. Fast circuits up to 200 kHz and stable circuits with operation lifetimes of more than 1000 h under ambient conditions without any encapsulation are shown. We also report on a fully printed, all organic ring oscillator.

Manufacturing and commercialization issues in organic electronics

Abstract: Techniques for manufacturing organic electronic devices [organic light-emitting diodes (OLEDs), photovoltaic cells, transistors, and solid-state memory] are reviewed and analyzed with respect to cost and market fitness in comparison to competitive approaches based on silicon electronics. The conclusions are (i) OLED displays will be successful using infrastructure largely borrowed from liquid crystal displays, because they provide fundamental customer value not dependent on lower cost; (ii) OLEDs for general lighting and organic–inorganic hybrid photovoltaic cells currently confront substantial barriers in cost and efficiency, but solutions appear feasible and would lead to very large volume businesses; (iii) organic crossbar memories are promising, but require innovations in driver architecture and interconnection; and (iv) organic transistors have not yet found a viable major market, but have great promise for highly customized, small-volume product runs using digital patterning techniques.

The microstructure of Sn in near-eutectic Sn-Ag-Cu alloy solder joints and its role in thermomechanical fatigue

Abstract: During the solidification of solder joints composed of near-eutectic Sn–Ag–Cu alloys, the Sn phase grows rapidly with a dendritic growth morphology, characterized by copious branching. Notwithstanding the complicated Sn growth topology, the Sn phase demonstrates single crystallographic orientations over large regions. Typical solder ball grid array joints, 900 μm in diameter, are composed of 1 to perhaps 12 different Sn crystallographic domains (Sn grains). When such solder joints are submitted to cyclic thermomechanical strains, the solder joint fatigue process is characterized by the recrystallization of the Sn phase in the higher deformation regions with the production of a much smaller grain size. Grain boundary sliding and diffusion in these recrystallized regions then leads to extensive grain boundary damage and results in fatigue crack initiation and growth along the recrystallized Sn grain boundaries.

A study on the growth and structure of titania nanotubes

Abstract: Titania nanotubes synthesized by a soft chemical process are described, having diameters of 8 nm to 10 nm and lengths ranging from approximately 0.1 μm to 1 μm. X-ray diffraction studies show the structure of the as-prepared nanotubes is the same as that of the starting anatase TiO2 nanoparticles. Energy-dispersive x-ray analysis and electron energy loss spectroscopy studies further indicate that the as-prepared nanotubes are composed of titania. Studies using transmission electron microscopy verified that the nanotubes are formed during alkali treatment, with subsequent acidic treatments having no effect on nanotube structure and shape.

Raman spectroscopy studies on the thermal stability of TiN, crN, TiAlN coatings and nanolayered TiN/CrN, TiAlN/CrN multilayer coatings

Abstract: About 1.5-um-thick single-layer TiN, CrN, TiAlN coatings and nanolayered TiN/CrN, TiAlN/CrN multilayer coatings were deposited on silicon (111) substrates using a13; reactive direct current magnetron sputtering process. Structural characterization of the coatings was done using x-ray diffraction (XRD) and micro-Raman spectroscopy. All13; the coatings exhibited NaCl B1 structure in the XRD data. Raman spectroscopy data of as-deposited coatings exhibited two broad bands centered at 230x2013;250 and 540x2013;630 cmx2212;1.13; These bands have been assigned to acoustical and optical phonon modes, respectively. Thermal stability of the coatings was studied by heating the coatings in air in a13; resistive furnace for 30 min in the temperature range 400x2013;900 xB0;C. Structural changes as a result of heating were characterized using Raman spectroscopy and XRD. Raman13; data showed that TiN, CrN, TiN/CrN, TiAlN, and TiAlN/CrN coatings started to oxidize at 500, 600, 750, 800, and 900 xB0;C, respectively. To isolate the oxidation-induced spectral changes as a result of heating of the coatings in air, samples were also annealed in vacuum at 800 xB0;C under similar conditions. The Raman data of vacuum-annealed coatings showed no phase transformation, and intensity of the optical phonon mode increased and shifted to lower frequencies. The origin of these spectral changes is discussed in terms of defect structure of the coatings. Our results indicate that the thermal stability of nanolayered multilayer coatings is superior to the13; single-layer coatings.

Elastic properties, hardness, and indentation fracture toughness of intermetallics relevant to electronic packaging

Abstract: Many intermetallics, such as Ag3Sn, AuSn4, Cu3Sn, Cu6Sn5 (η and η`), Ni3Sn4, and γ–Cu5Zn8 are present in modern solder interconnects as a result of solder chemistry and/or due to the interfacial reaction between solder and metallization scheme. Coarse-grained, single-phase intermetallics are produced by conventional casting followed by annealing for long time. Ambient temperature isotropic elastic moduli (bulk, Young’s, shear, and Poisson’s ratio) and selected plastic properties (hardness and indentation fracture toughness) of these intermetallics are presented. The isotropic elastic moduli of these intermetallics are determined by the pulse-echo technique. The measured bulk, Young’s and shear moduli lie in the range of 6.3 to 11.4 × 1010 N/m2, 7.1 to 12.3 × 1010 N/m2 and 2.7 to 4.5 × 1010 N/m2, respectively. The hardness and fracture toughness are determined by an indentation method. The loads used for indentation experiments were: 100–10,000 g for Ag3Sn and γ–Cu5Zn8, 10–50 g for AuSn4, 200–1000 g for Cu3Sn, 50–100 g for Cu6Sn5, and 100–200 g for Ni3Sn4. The measured Vickers hardness lies in the range of 50 to 470 Kg/mm2, and the measured indentation fracture toughness lies in the range of 2.5 to 5.7 MPa m1/2. Due to coarse grain size of the specimens, the indentation cracks were contained within one grain. In Cu3Sn, Cu6Sn5 (η and η`) and Ni3Sn4 intermetallics, the indentation cracks were found to be nearly straight and run along the indent diagonal. However, the cracks in AuSn4 showed significant zig-zag and branching phenomena, and they seemed to propagate along particular cleavage plane(s). The presence of slip bands are also observed in AuSn4, Ag3Sn, Cu3Sn, γ-Cu5Zn8, and Ni3Sn4. In the case of Ag3Sn and γ–Cu5Zn8, indentation cracks cannot be induced by applying loads up to 10 kg. Rather, extensive plastic deformation occurs resulting in the formation of a large number of shear/kink bands, and possibly twins, that spread across several grains. At a load of 5000 g or higher, Ag3Sn exhibits grain boundary decohesion near the indents. Among the intermetallics studied, Ag3Sn is shown to be the most ductile.

Transparent nanocrystalline MgO by rapid and low-temperature spark plasma sintering

Abstract: We investigated superfast densification of nanocrystalline MgO powders by spark plasma sintering (SPS) between 700 °C and 825 °C under applied pressures of 100 and 150 MPa. Fully-dense transparent nanocrystalline MgO with a 52-nm average graun size was fabricated at 800 °C and 150 MPa for 5 min. In-line transmissions of 40% and 60% were measured compared to MgO single crystal, for the yellow and red wavelengths, respectively. Densification occurs by particles sliding over each other; the nanometric graun size and pores lead to the optical transparency. The light brownish color of the nanocrystalline MgO is due to the oxygen vacancy color centers, originating from the reducing atmosphere of the SPS process.

Atomic layer deposition of noble metals: Exploration of the low limit of the deposition temperature

Abstract: The low limit of the deposition temperature for atomic layer deposition (ALD) of noble metals has been studied. Two approaches were taken; using pure oxygen instead of air and using a noble metal starting surface instead of Al2O3. Platinum thin films were obtained by ALD from MeCpPtMe3 and pure oxygen at deposition temperature as low as 200 °C, which is significantly lower than the low-temperature limit of 300 °C previously reported for the platinum ALD process in which air was use da s the oxygen source. The platinum films grown in this study had smooth surfaces, adhered well to the substrate, and had low impurity contents. ALD of ruthenium, on the other hand, took place at lower deposition temperatures on an iridium seed layer than on an Al2O3 layer. On iridium surface, ruthenium films were obtained from RuCp2 and oxygen at 225 °C and from Ru(thd)3 and oxygen at 250 °C, whereas no films were obtained on Al2O3 at temperatures lower than 275 and 325 °C, respectively. The crystal orientation of the ruthenium films was found to depend on the precursor. ALD of palladium from a palladium -ketoiminate precursor and oxygen at 250 and 275 °C was also studied. However, the film-growth rate did not saturate to a constant level when the precursor pulse times were increased.

New lead-free relaxors based on the K0.5Na0.5NbO3–SrTiO3 solid solution

Abstract: New lead-free relaxors have been produced from the K0.5Na0.5NbO3–SrTiO3 (KNN-STO) system. The solid solubility within the studied range of compositions (1 -x) K0.5Na0.5NbO3–xSrTiO3 was observed for x up to 0.33. A pseudo-cubic perovskite structure was determined for x = 0.15 to 0.25. The high density and the uniform distribution of fine grains and pores were confirmed by the translucency of these ceramics. The 0.85KNN-0.15STO composition reaches the dielectric permittivity of above 3000 at room temperature. Dielectric spectroscopy measurements revealed that, as with lead-based complex perovskites, the cationic distribution disorder is reflected in relaxorlike properties, thus suggesting possible applications based on this environmentally friendly lead-free ceramic system.

Elastic contact to a coated half-space - Effective elastic modulus and real penetration

Abstract: A new approach to the contact to coated elastic materials is presented. A relatively simple numerical algorithm based on an exact integral formulation of the elastic contact of an axisymmetric indenter to a coated substrate is detailed. It provides contact force and penetration as a function of the contact radius. Computations were carried out for substrate to layer moduli ratios ranging from 10−2 to 102 and various indenter shapes. Computed equivalent moduli showed good agreement with the Gao model for mismatch ratios ranging from 0.5 to 2. Beyond this range, substantial effects of inhomogeneous strain distribution are evidenced. An empirical function is proposed to fit the equivalent modulus. More importantly, if the indenter is not flat-ended, the simple relation between contact radius and penetration valid for homogeneous substrates breaks down. If neglected, this phenomenon leads to significant errors in the evaluation of the contact radius in depth-sensing indentation on coated substrates with large elastic modulus mismatch.

Nanoindentation of bone : Comparison of specimens tested in liquid and embedded in polymethylmethacrylate

Abstract: Elastic modulus of bone was investigated by nanoindentation using common methods of sample preparation, data collection, and analysis, and compared to dynamic mechanical analysis (DMA: three-point bending) for the same samples. Nanoindentation (Berkovich, 5 μm and 21 μm radii spherical indenters) and DMA were performed on eight wet and dehydrated (100% ethanol), machined equine cortical bone beams. Samples were embedded in polymethylmethacrylate (PMMA) and mechanical tests repeated. Indentation direction was transverse to the bone long axis while DMA tested longitudinally, giving approximately 12% greater modulus in DMA. For wet samples, nanoindentation with spherical indenters revealed a low modulus surface layer. Estimates of the volume of material contributing to elastic modulus measurement showed that the surface layer influences the measured modulus at low loads. Consistent results were obtained for embedded tissue regardless of indenter geometry, provided appropriate methods and analysis were used. Modulus increased for nanoindentation (21 μm radius indenter) from 11.7 GPa ± 1.7 to 15.0 GPa ± 2.2 to 19.4 GPa ± 2.1, for wet, dehydrated in ethanol, and embedded conditions, respectively. The large increases in elastic modulus caused by replacing water with ethanol and ethanol with PMMA demonstrate that the role of water in fine pore space and its interaction with collagen strongly influence the mechanical behavior of the tissue.

Fabrication and structural characterization of self-supporting electrolyte membranes for a micro solid-oxide fuel cell

Abstract: Micromachined fuel cells are among a class of microscale devices being explored for portable power generation In this paper, we report processing and geometric design criteria for the fabrication of free-standing electrolyte membranes for microscale solid-oxide fuel cells Submicron, dense, nanocrystalline yttria-stabilized zirconia (YSZ) and gadolinium-doped ceria (GDC) films were deposited onto silicon nitride membranes using electron-beam evaporation and sputter deposition Selective silicon nitride removal leads to free-standing, square, electrolyte membranes with side dimensions as large as 1025 μm for YSZ and 525 μm for GDC, with high processing yields for YSZ Residual stresses are tensile (+85 to +235 MPa) and compressive (–865 to -155 MPa) in as-deposited evaporated and sputtered films, respectively Tensile evaporated films faul via brittle fracture during annealing at temperatures below 773 K; thermal limitations are dependent on the film thickness to membrane size aspect ratio Sputtered films with compressive residual stresses show superior mechanical and thermal stability than evaporated films Sputtered 1025-μm membranes survive annealing at 773 K, which leads to the generation of tensile stresses and brittle fracture at elevated temperatures (923 K)

Analysis of the spherical indentation cycle for elastic-perfectly plastic solids

Abstract: A finite element analysis of frictionless indentation of an elastic-plastic half-space by a rigid sphere is presented and the deformation behavior during loading and unloading is examined in terms of the interference and elastic—plastic material properties. The analysis yields dimensionless constitutive relationships for the normal load, contact area, and mean contact pressure during loading for a wide range of material properties and interference ranging from the inception of yielding to the initiation of fully plastic deformation. The boundaries between elastic, elastic-plastic, and fully plastic deformation regimes are determined in terms of the interference, mean contact pressure, and reduced elastic modulus-to-yield strength ratio. Relationships for the hardness and associated interference versus elastic-plastic material properties and truncated contact radius are introduced, and the shape of the plastic zone and maximum equivalent plastic strain are interpreted in light of finite element results. The unloading response is examined to evaluate the validity of basic assumptions in traditional indentation approaches used to measure the hardness and reduced elastic modulus of materials. It is shown that knowledge of the deformation behavior under both loading and unloading conditions is essential for accurate determination of the true hardness and reduced elastic modulus. An iterative approach for determining the reduced elastic modulus, yield strength, and hardness from indentation experiments and finite element solutions is proposed as an alternative to the traditional method.

Dislocation grain boundary interactions in martensitic steel observed through in situ nanoindentation in a transmission electron microscope

Abstract: Dislocation-interface interactions in Fe-0.4 wt% C tempered martensitic steel were studied through in situ nanoindentation in a transmission electron microscope (TEM). Two types of boundaries were imaged in the dislocated martensitic structure: a low-angle (probable) lath boundary and a coherent, high-angle (probable) block boundary. In the case of a low-angle grain boundary, the dislocations induced by the indenter piled up against the boundary. As the indenter penetrated further, a critical stress appeared to have been reached, and a high density of dislocations was suddenly emitted on the far side of the grain boundary into the adjacent grain. In the case of the high-angle grain boundary, the numerous dislocations that were produced by the indentation were simply absorbed into the boundary, with no indication of pileup or the transmission of strain. This surprising observation is interpreted on the basis of the crystallography of the block boundary.

Spark plasma sintering and characterization of bulk nanostructured fully stabilized zirconia: Part II. Characterization studies

Abstract: Dense fully stabilized cubic zirconia, sintered by the spark plasma sintering (SPS) method, was characterized through hardness, fracture toughness, and electrical impedance measurements. The effect of sintering temperature on hardness and fracture toughness was evaluated. Samples sintered at 1200 °C for 5 min, which had crystallite size of <100 nm, exhibited the highest hardness. Impedance measurements showed an increase in bulk contribution relative to grain boundaries as sintering temperature is increased. Calculation of the activation energy for conduction gave a value, 1.13 eV, in agreement with previously published results.

Raman spectroscopy of CaTiO3-based perovskite solid solutions

Abstract: Perovskite-structured solid solutions intended for use as microwave dielectric resonators were studied by Raman spectroscopy. Two distinct categories were investigated: (i) simple perovskite–simple perovskite solid solutions, that is, CaTiO3–SrTiO3 (CTST), CaTiO3–CaZrO3 (CTCZ), CaTiO3–NdAlO3 (CTNA), and CaTiO3–LaGaO3 (CTLG); and (ii) simple perovskite–complex perovskite solid solutions, such as CaTiO3–SrMg1/3Nb2/3O3 (CTSMN). In the latter category, the influence of A-site ion radius was also addressed by examining 0.5CaTiO3– 0.5LaMg1/2Ti1/2O3 (0.5CT–0.5LMT), 0.5SrTiO3 (ST)–0.5LMT, and 0.5BaTiO3 (BT)–0.5LMT. Raman data from the end members and solid solutions are compared, paying particular attention to F2g and A1g mode bands, often associated with ordering of B-site species.

Photoelectrochemical properties of titania nanotubes

Abstract: N-type nanocrystalline titania is a promising material for use in semiconductor photoelectrochemical cells and, potentially, the solar generation of hydrogen. In this study, we examined the photochemical properties of titania nanotube arrays made by anodization of a starting Ti foil in a fluoride ion containing electrolyte. The absorption properties of the titania nanotube samples were investigated using diffuse reflectance ultraviolet (UV)-visible (vis) spectroscopy, with a broadening of the absorption spectra seen as a function of material phase, nanotube diameter, and Pd sensitization. The magnitude of the anodic photocurrent obtained from the polycrystalline nanotube samples, measured under band gap UV illumination, appeared to be significantly higher than that reported for any other form of nanocrystalline titania. A maximum photoconversion efficiency (UV light exposure at 365 nm, intensity 146 mW/cm2) of 4.8% was obtained for 22 nm diameter nanotubes annealed at 500 °C and coated with a discontinuous palladium layer of 10 nm average effective thickness.

Grain Refinement at the Nanoscale Via Mechanical Twinning and Dislocation Interaction in a Nickel-Based Alloy

Abstract: A nanostructured surface layer was formed on an Inconel 600 plate by subjecting it to surface mechanical attrition treatment at room temperature. Transmission electron microscopy and high-resolution transmission electron microscopy of the treated surface layer were carried out to reveal the underlying grain refinement mechanism. Experimental observations showed that the strain-induced nanocrystallization in the current sample occurred via formation of mechanical microtwins and subsequent interaction of the microtwins with dislocations in the surface layer. The development of high-density dislocation arrays inside the twin-matrix lamellae provides precursors for grain boundaries that subdivide the nanometer-thick lamellae into equiaxed, nanometer-sized grains with random orientations.