This article provides a review of materials and devices of wide-bandgap oxide semiconductors based on ZnO, highlighting the nature of the chemical bond. The electronic structures of these materials are very different from those of conventional covalently bonded semiconductors, owing to the ionic nature of the chemical bonds. Therefore, one needs to design and optimize fabrication processes and structures of active devices containing such materials, taking into account the peculiar defect formation mechanisms. A variety of active devices that have clear advantages over the conventional ones have been demonstrated, for example, ultraviolet light-emitting diodes, quantum Hall devices, and transparent and flexible thin-film transistors with high electron mobility, paving the way for future applications. The reasons behind the successes identify future challenges in research on oxide semiconductors.

In this issue we have endeavored to answer the question, “Whither oxide electronics?” This issue provides a framework and perspective on the progress in the field of oxide electronics over the past several decades, as well as the challenges and opportunities in the years to come. Building on the foundations laid by the pioneers in the materials community and spurred by the discovery of high-temperature superconductivity, there has been both tremendous progress in understanding the complex science of oxide electronic materials and the discovery of other fascinating new phenomena, including colossal magnetoresistance, multiferrocity, and two-dimensional electron gases in correlated oxide systems. Thin-film heterostructures provide a pathway to create novel devices and combinations of physical phenomena. Indeed, the ability to synthesize and control oxide heterostructures using sophisticated deposition techniques has become a key enabler of the recent advances in this field. These oxides are beginning to enter mainstream products because of their higher performance, for example, ferroelectric memories and oxides with high dielectric constant for computers that run at higher speed and use less power.

Differential speed rolling (DSR) has been carried out on AZ31 alloys with Mn additions of 0 to 0.6 wt% for investigating the effects of Mn on microstructure, texture, mechanical properties, and formability. The Al–Mn compounds were formed in the sample with a Mn addition of only 0.2 wt% because of its low solid-solubility limit. There were tiny differences among the DSR-processed AZ31 alloys with different Mn contents, while the AZ31 alloy without Mn addition exhibited a more homogeneous microstructure, a weaker basal texture intensity, and a much superior formability together with a larger likelihood of grain growth during annealing. The Mn dissolving in αMg matrix exerted a far stronger influence on the resulting properties compared with those existing in form of the Al–Mn compounds. The Mn solute atoms induced an increase in c/a ratio, which may suppress activity of nonbasal slips and in turn degrade the deformation capability.

The phase equilibria and phase transformation of the body-centered cubic (bcc) phase in the Cu–Ti–Al system were investigated by the diffusion couple method, metallographic examination, differential scanning calorimetry, and x-ray diffraction. The isothermal sections at 700 and 900 °C and vertical sections at 18 at.% Al, 22 at.% Al, and 25 at.% Al in the Cu-rich portion were determined. These results indicate that (i) the Cu2TiAl compound with the L21 Heusler structure has a larger solubility range; (ii) the stable B2 + L21 miscibility gap of the ordered bcc phase exists until the liquid phase, and the tie lines of this miscibility gap are almost parallel with the Cu–Ti side; (iii) the composition and temperature for the eutectic reaction (L ↔ B2 + L21) are about 7 at.% Ti and about 970 °C, respectively, and (iv) the velocity of the eutectoid decomposition [bcc ↔ face-centered cubic (fcc) + D83] of the bcc phase with martensitic morphology in the Cu–Ti–Al alloys is slower than that of the Cu–Al alloys.

Depth penetration per cycle and its retardation exhibited in specimens of 6061 Al were studied. By holding the maximum load reached during loading prior to cyclic oscillating, the creep could be effectively attenuated from the cyclic indentation. Constant depth penetration of sub-nanometers per cycle was found. The interrupt segments with different amplitudes (over- and underloading) or frequencies (frequency increasing and decreasing), but with same mean load, induced immediate retardations of cyclic indentation. As the interrupt segments have more than 10 cycles, the retardation is enhanced with the increase of cycle number. Accumulation of strain hardening due to stress concentration under the indenter tip is proposed to be the mechanism by which interrupt oscillation retards subsequent cyclic indentation.

Micro texturing of titanium implant surfaces is commonly used to enhance fixation by osseointegration, and devising robust and specific correlations between surface topographic features and implant performance is an area of active current research. In this context, we present a detailed analysis of the topographies of titanium surfaces prepared by grit blasting (GB) and grit blasting followed by acid etching (GB+AE) at two different imaging scales over a full range of statistical parameters. The surfaces were characterized using white light interferometry and atomic force microscopy, and the topographic images were processed to extract the amplitude, spatial, hybrid, and functional parameters of the surface. Although GB+AE surfaces are known to elicit significantly higher bone response than GB surfaces, the topographies differed by less than 20% (over all parameters) when averaged over 242 × 181 μm interferometric images. In contrast, measurements over smaller 25 × 25 μm areas obtained using high-resolution atomic force microscopy indicated that the GB+AE surfaces exhibit a 26% increase in root-mean-square (rms) roughness, a 63% increase in rms slope, a 75% increase in the curvature of the summits, and a 21% increase in surface area over GB surfaces. These results constitute the first identification of rms slope and summit curvatures as important topographic variables that must be considered in ongoing efforts to correlate surface topography with the performance of endosseous titanium implants.

Cyclic stress–strain response of an equal channel angularly pressed Sn-3.8Ag-0.7Cu alloy was investigated to seek a mechanistic understanding of cyclic softening in Sn-rich alloys. The equal channel angular pressing (ECAP) was applied to modify the microstructure of the solder alloy by breaking up the needlelike Ag3Sn intermetallic phase into fine granules and by reducing the large β-Sn dendrites into smaller and equiaxed grains. The extruded alloys were subjected to strain-controlled fatigue test at various strain amplitudes. It was found that the extruded alloy exhibited a sharp decrease of the stress amplitude within the initial few cycles compared with the as-cast alloy. After only a few cycles, the alloy suffered from noticeable surface damage. In situ scanning electron microscopy observations of the cyclic bending specimens revealed an approximately logarithmic relationship between crack density and the number of cycles. A theoretical model of microcrack accumulation was constructed to explain the rapid cyclic softening behavior. The predicted results, based on the model, agreed well with the experimental data and indicated that the rapid softening had resulted from an increased tendency for grain boundary cracking in the ECAPed microstructure due to the increase in the grain boundary area per unit volume and the reduced resistance of Ag3Sn to grain boundary sliding.

We present an off-lattice, on-the-fly kinetic Monte Carlo (KMC) model for simulating stress-assisted diffusion and trapping of hydrogen by crystalline defects in iron. Given an embedded atom (EAM) potential as input, energy barriers for diffusion are ascertained on the fly from the local environments of H atoms. To reduce computational cost, on-the-fly calculations are supplemented with precomputed strain-dependent energy barriers in defect-free parts of the crystal. These precomputed barriers, obtained with high-accuracy density functional theory calculations, are used to ascertain the veracity of the EAM barriers and correct them when necessary. Examples of bulk diffusion in crystals containing a screw dipole and vacancies are presented. Effective diffusivities obtained from KMC simulations are found to be in good agreement with theory. Our model provides an avenue for simulating the interaction of hydrogen with cracks, dislocations, grain boundaries, and other lattice defects, over extended time scales, albeit at atomistic length scales.

Frictional behavior and interfacial adhesion of differently textured pyrolytic carbon layers on Si substrate were investigated by indentation and scratch testing. A large amount of elastic recovery and a low coefficient of friction (μ = 0.05 to 0.09) were observed. Elastic/plastic and frictional behaviors of the coatings are strongly influenced by the microstructure of the pyrolytic carbon films, especially by the texture. The critical load at which the first abrupt increase in the normal displacement occurs was used to characterize interfacial adhesive strength. A pyrolytic carbon film deposited at higher residence time from a gas mixture containing 3% oxygen exhibited higher critical loads than film deposited at lower residence time without oxygen. The results can be understood if one assumes that the gas phase composition during deposition significantly influences the bonding strength at the interface. Failure mechanisms are discussed for both types of films.

Influence of casting temperature on the thermal stability of Cu- and Zr-based metallic glasses (MGs) was analyzed based on the monomer-cluster structural model using the Johnson–Mehl–Avrami (JMA) equation. The result indicates that increasing the casting temperature can enhance the thermal stability of MGs. It is suggested that it be attributed to the decrease in the amount of the local ordering clusters induced by the elevating casting temperature. The prediction is confirmed by continuous heating transformation diagrams constructed for the Cu- and Zr-amorphous samples obtained under different casting temperatures.

The integration of ferroelectrics in nanodevices requires firstly the preparation of high-quality ultrathin films. Chemical solution deposition is considered a rapid and cost-effective technique for preparing high-quality oxide films, but one that has traditionally been regarded as unsuitable, or at least challenging, for fabricating films with good properties and thickness below 100 nm. In the present work we explore the deposition of highly diluted solutions of pure and Ca-modified lead titanates to prepare ultrathin ferroelectric films, the thickness of which is controlled by the concentration of the precursor solution. The results show that we are able to obtain single crystalline phase continuous films down to 18 nm thickness, one of the lowest reported using these methods. Below that thickness, the films start to be discontinuous, which is attributed to a microstructural instability that can be controlled by an adequate tailoring of the processing conditions. The effect of the reduction of thickness on the piezoelectric behavior is studied by piezoresponse force microscopy. The results indicate that films retain a significant piezoelectric activity regardless of their low thickness, which is promising for their eventual integration in nanodevices, for example, as transducer elements in nanoelectromechanical systems.

A combustion synthesis method for the synthesis of α-Si3N4 from a reactant compact composed of Si, NaN3, and NH4X and wrapped up with an igniting agent was investigated. Wrapping the reactant compact with the igniting agent (i.e., a mixture of Ti and C powders) was found necessary for the synthesis of Si3N4. In addition to NH4Cl, which was considered previously to function as a catalytic agent, other ammonium halides (i.e., NH4F, NH4Br, and NH4I) were found to be capable of catalyzing the synthesis reaction with NH4Cl being the most effective. Si3N4 could not be produced when NaN3 was replaced by C3H6N6. NaN3 was thus considered to exert an essential effect on the combustion synthesis reaction other than functioning as a solid-state nitrogen source as considered previously. It was proposed that Na vapor produced by decomposition of NaN3 reduces SiXx (formed by reaction of Si and NH4X), promoting the nitridation reaction to form Si3N4. NaN3 thus plays a role as a reducing agent in the synthesis reaction.

Rapidly solidified nanocrystalline RE–TM–B (RE = Nd, Pr, Dy, TM = Fe, Co) alloys with enhanced hard magnetic properties were synthesized by melt spinning. The composition- and microstructure-dependent elevated temperature magnetic properties were investigated. The temperature coefficients of remanence (α) and coercivity (β) were determined. The effects of Pr substituting Nd, Co substituting Fe, Dy substituting RE, and grain size on the Curie temperature and thermal stability were studied. Co or Dy substitutions were found to have a significant beneficial effect on the thermal stability. Reducing grain size could also improve elevated temperature behavior. Maximum energy product (BH)max > 100 kJ/m3 could be obtained in compositionally optimized nanophase alloys at temperature of 473 K. Extremely low coefficients of α and β were realized in exchange coupled nanocomposite alloys. Bonded nanocomposite magnets with α = −0.052%/K and β = −0.0365%/K for 300–400 K were also successfully fabricated.

In this work, we report a method to synthesize a peculiar composite structure of tubular carbon nanofibers (CNFs) growing on a microsized tin (Sn) whisker. The material used is a commercially available copper clad laminate (CCL). The CCL is composed of a surface copper (Cu) layer and a bottom polymer (phenol-formaldehyde resin) board, in which the polymer board is used as the carbon source. Using lithography and lift-off techniques, the Cu layer was patterned to a stripelike Cu trace. A Sn thin film was then evaporated on the polymer board near the Cu trace. To release the residue stress that resulted from the evaporation; Sn whiskers with diameters of about 2 to 5 μm were formed on the Sn thin film after evaporation. By passing an electric current through the Cu trace, the Cu trace was heated due to Joule heating and served as a heating source for the thermal decomposition of phenol-formaldehyde. After heat treatment, the CNFs grew on the surface of the Sn whiskers with tubular hollow-cored structure. The diameter of the tubular CNFs is about hundreds of nanometers and their length can reach several micrometers. The growth mechanism of the brushlike composite structure is also discussed.

Tungsten oxide comblike nanostructures were synthesized using a two-step thermal evaporation method. The first step involving high reactor pressure and temperature was to synthesize the cores of the comb structures, upon which the teeth of the comb were grown in the second step using low operation pressures and temperatures. The teeth of the comb structure are well aligned and vertical to the side surfaces of the cores. The effects of growth parameters were examined, and the growth mechanism was investigated.