Piezoresistance (PZR) is the change in the electrical resistivity of a solid induced by an applied mechanical stress. Its origin in bulk crystalline materials like silicon is principally a change in the electronic structure which leads to a modification of the effective mass of charge carriers. The past few years have seen a rising interest in the PZR properties of semiconductor nanostructures, motivated in part by claims of a giant PZR (GPZR) in silicon nanowires more than two orders of magnitude bigger than the known bulk effect. This review aims to present the controversy surrounding claims and counterclaims of GPZR in silicon nanostructures by summarizing the major works carried out over the past 10 years. The main conclusions to be drawn from the literature are that (i) reproducible evidence for a GPZR in ungated nanowires is limited; (ii) in gated nanowires, GPZR has been reproduced by several authors; (iii) the giant effect is fundamentally different from either the bulk silicon PZR or that resulting from quantum confinement, the evidence pointing to an electrostatic origin; (iv) released nanowires tend to have slightly larger PZR than unreleased nanowires; and (v) insufficient work has been performed on bottom-up grown nanowires to be able to rule out a fundamental difference in their properties when compared with top-down nanowires. On the basis of this, future possible research directions are suggested.

The well-ordered TiO2 nanotube arrays with controlled aspect ratio are fabricated via potentiostatic anodization. The aspect ratio of TiO2 nanotube array can be tuned conveniently by changing the water content in electrolyte and anodization time. The formation of well-ordered TiO2 nanotube array is good for the photogenerated electron transfer. So, the well-ordered TiO2 nanotube array photoelectrodes have been used to fabricate dye-sensitized solar cells (DSSCs). It is found that, with the optimum nanotube length and aspect ratio, DSSCs with TiO2 nanotube array photoelectrodes show better photoelectric conversion efficiency (2.60%) than that with TiO2 nanoparticles on Ti foil photoelectrode. It is elucidated by the interfacial electron transport of DSSCs, which are characterized quantitatively, using the electrochemical impedance spectra. The DSSC with optimal nanotube length and aspect ratio displays the fastest interfacial electron transfer and longer electron lifetime.

Amorphous Ta–O nanotubes (NTs) prepared by anodization in a sulfuric-acid-based solution have been found to contain considerable amounts of extra oxygen and sulfur. Their structural and thermal stability has been studied by combining x-ray diffractometry, transmission electron microscopy, and thermal analysis. The amorphous Ta–O, whose composition was estimated to be Ta2O6.6S0.7, crystallizes into orthorhombic β-Ta2O5 at temperatures around 1073 K by an endothermic reaction, at which excess oxygen and impurity sulfur are released. The amorphous NTs were found to be thermally more stable than stoichiometric amorphous Ta2O5, whose crystallization temperature is around 973 K. Excess oxygen and impurity sulfur, which form chemical bonds with Ta atoms in the amorphous solid, must be the origin of the stability. The crystallization follows the out-diffusion of oxygen and sulfur from the solid at temperatures where the mobility of atoms is high enough, indicating that the crystallization is kinetically arrested.

Two types of carbon nanotube reinforced nickel (CNT/Ni) nanocomposites were processed, both involving spark plasma sintering (SPS) of precursor powders consisting of nickel and carbon nanotubes. The first type involved simple mechanical dry milling of nickel and CNT powders, followed by sintering using SPS, resulting in nanocomposites exhibiting a tensile yield strength of 350 MPa (about two times that of SPS processed monolithic nickel with a strength of 160 MPa) and about 30% elongation to failure. In contrast, the nanocomposites processed by SPS of powders prepared by molecular-level mixing (MLM) exhibited substantially higher tensile yield strength of 690 MPa but limited ductility with an 8% elongation to failure. While the former type of processing involving dry-milling is expected to be lower in cost as well as easy to scale-up, the latter type of processing technique involving MLM leads to a more homogeneous distribution of nanotubes, leading to extraordinarily high strength levels.

Cu/Sn alloy matrix composites reinforced with β-Si3N4 whiskers (β-Si3N4w/Cu/Sn) were prepared by powder metallurgy method with the aim of improving the mechanical property. Although the β-Si3N4 whisker additions played a side effect on the densification of the composites, the mechanical property was significantly improved. The highest bending strength of β-Si3N4w/Cu/Sn reached up to 353 MPa, and the highest hardness of composites was also improved to 97.13 HRF. β-Si3N4 whiskers could induce the improved work of fracture and grain refinement of composites and thus brought about the enhancement of mechanical performance. However, excessively high β-Si3N4 whisker content was not beneficial to the high mechanical property because of the low densification and the severe aggregation of whiskers. The β-Si3N4w/Cu/Sn still had a quite high thermal conductivity. As a result, the β-Si3N4w/Cu/Sn could be a promising material applied in a severe environment.

Tungsten carbide (WC) particle–reinforced iron matrix surface composites with different content of Cr were fabricated using vacuum evaporative pattern casting technique. It was found that the morphology of carbides changed from continuous net-shape to isolated block-shape patterns. The amount of carbides increase with the increasing Cr content in the matrices. Composites with different Cr content show better abrasive wear resistance than those without Cr. With the increase of Cr content in the matrices, the three-body abrasive wear resistance of the composites increased, while the impact abrasive wear resistance of the composites increased under 1 J impact load, but first increased and then decreased under 3 J impact load. The influences of the addition of Cr in the matrices on the abrasive wear resistance were the synergistic effects of two protecting effects and two supporting effects. The results might provide significant references for the design and practical application of WC particle–reinforced iron matrix surface composites.

Al-induced crystallization (AIC) of amorphous SixGe1–x (a-SixGe1–x) alloys with compositions over the entire range of the isomorphous Si–Ge system has been investigated. The crystallization progress was monitored by dedicated in situ x-ray diffraction analysis while gradually increasing the annealing temperature. Auger sputter-depth profiling was applied to investigate the occurrence of Al-induced layer exchange of the Al and a-SixGe1–x sublayers after complete crystallization. A-SixGe1–x alloys with x < 0.13 and x > 0.41 show largely different AIC behaviors with respect to crystallization rate and possible layer exchange of the Al and a-SixGe1–x sublayers upon crystallization. A thermodynamic model for AIC of a-SixGe1–x alloys is presented, which well explains these experimental observations and thereby demonstrates the dominant role of interface thermodynamics in the AIC process of amorphous semiconductors. The model can be used to predict the AIC behaviors of a-SixGe1–x alloys over the entire composition range at specified annealing temperatures.

A novel series of binuclear transition metal phthalocyanines M2Pc2HnC (M = Mn(II), Fe(II), Co(II), Ni(II), and Cu(II)) were developed for highly efficient electrocatalysts to lithium/thionyl chloride (Li/SOCl2) battery. The capacity of the battery can increase approximately by 40–65% when the binuclear compounds are present in the electrolyte of the battery. To investigate the effect of the binuclear metal phthalocyanines on Li/SOCl2 battery further, this work studied the electrocatalytic reaction of the binuclear compounds to the battery by electrochemical methods (cyclic voltammetry) and other characterization means. The results demonstrate that the order of the electrocatalytic activity of the binuclear compounds with diverse center metal ions is: Fe(II) > Co(II) > Mn(II) > Ni(II) > Cu(II).

Nanoindentation was performed to study the interfacial delamination of SiN/GaAs film/substrate structures, and to determine the adhesion properties of the interface. A sequential dual-indentation approach was developed and the tests were carefully designed to induce interfacial delamination, but avoid the occurrence of the film's through-thickness fracture or buckling. A clamped circular plate model was used to approximate the elastic deflection of the detached film and hence the delamination area. The Griffith energy balance was then used to compute the energy release associated with the delamination. The energy release rate, Gin, calculated was found to be independent on the testing conditions, which agrees with the fundamental assumption of the Griffith energy concept.

The highly ordered mesoporous chromium–alumina catalysts (denoted as OMCA(x)) with different nAl/nCr ratios of 10 and 20 have been successfully synthesized via an evaporation-induced self-assembly (EISA) pathway associated with thermal treatment and were applied to the liquid phase oxidation of cyclohexane with hydrogen peroxide (H2O2) as an oxidant. For comparison studies, a chromium catalyst with the nAl/nCr ratio of 10 supported on an ordered mesoporous alumina support [denoted as Cr/OMA(10)] was also prepared by an incipient wetness impregnation (IWI) method. Although both kinds of catalysts retained a unidimensionally ordered mesoporous structure, textural properties of the catalysts were significantly affected by the preparation method. The characteristic results from XRD, N2 adsorption, TEM, UV-Vis, 27Al MNS NMR, and H2-TPR showed that the OMCA(x) catalysts exhibited much more highly ordered hexagonal mesostructure, narrower pore-size distribution, higher Brunauer-Emmett-Teller surface area and pore volume, and more homogeneous distribution of Cr species in mesoporous alumina framework in the form of tetrahedrally coordinated hexavalent Cr-oxide moieties than those of Cr/OMA(10) synthesized by an IWI method. In addition, in comparison with Cr/OMA(10), OMCA(x) catalysts showed higher catalytic activity and stability for the liquid phase oxidation of cyclohexane.

To prepare hydrogels with ultrarapid response rate and excellent mechanical properties, the poly(N-isopropylacrylamide)/clay nanocomposite hydrogels were synthesized by freezing polymerization technique. The start freezing time, as an important parameter determining the properties of gels, was designed and investigated. The results showed that the properties of gels including mechanical properties, swelling ratio, and swelling/deswelling rate were closely dependent on the freezing polymerization time. Comparably, the gels synthesized with earlier freezing time exhibit a faster swelling rate and an ultrarapid deswelling rate due to the integral interconnecting porous structure, while the swelling ratio, tensile strength and modulus decrease considerably. With the delay of start freezing time, the response rate decreases while the mechanical properties improve. Through the analysis of scanning electron microscope, differential scanning calorimetry, x-ray diffraction, dynamic rheological tests, and mechanical tests, the relevance of gels' structure with the freezing time was explored. It is reasonably presumed that freezing process impacts the effective crosslink of polymer chains by clay significantly, the earlier the freezing started, the more chains with free end existed in gels.