We have long discussed the onset mechanism of inelastic deformation of crystalline/amorphous metals based on atomic elastic stiffness or atomic stability. In the present study, we have first applied our “local lattice instability analysis” to silicon with Tersoff interatomic potential. For a comprehensive discussion including the effect of thermal fluctuation and structural inhomogeneity such as surface and grain boundaries, we have performed various tensile simulations against bulk/nanowire of Si single crystal, laminate-bulk/bamboo-nanowire with Σ5 twist grain boundary. Here, we have prepared different 8 set changing the random number for initial Maxwell-Boltzmann velocity distribution for each simulation. Not only the stress-strain response, but also the atomic elastic stiffness at each atom point, Bαij, is evaluated numerically by Δσαi/Δεj(Voigt notation) against local strain perturbation. The change in the average, standard deviation of det Bαij and the number of det Bαij < 0 atoms have brought us many significant insights, especially in e.g. (1) in the case of bulk single crystal under T = 1K, we have found a slight and smooth stress peak before the unstable stress drop, (2) the standard deviation of det Bαij began to increase at the peak of (1) and then the average of det Bαij became negative or reached “global instability” at the stress drop, (3) even in the systems with thermal fluctuation and structural inhomogeneity, the standard deviation of det Bαij decreases at the initial stage of tension, but it increase again when the lower bound of the standard deviation reaches zero well before the unstable stress drop.
ZnO nanorods were grown on various substrates using aqueous solution, aiming at integrating with bio- or organic- molecular structures assembled by bottom-up process. ZnO nanorods with the average diameter of 100 nm and the height of 1.5μm were obtained on an Au/Ti/Si (111) substrate via microwave heating, which is an environmental harmonized system allowing fast heating. Aggregation of ZnO nanorods occurred on a single crystalline ZnMgO film and growth mechanism of ZnO nanorods was discussed. Using ZnO nanorods immobilized with enzymes, amperometric glucose sensors with the detection limit of 0.01mmol/L were fabricated.
The difference of surface treatment method for ZnO single crystals and the epitaxial growth process occurred by the difference in the surface polarity has been studied. In the case of O-polar ZnO single crystal surface, annealing in ceramics box made of ZnO is effective to obtain atomically flat step and terrace surface structure. In the case of Zn-polar ZnO, chemical etching treatment is required to obtain pit free surface. The ZnO single crystal and homo-epitaxial films with atomically flat step and terrace surface structure were obtained for each polar surface. The effect of polarity on the surface treatment process and epitaxial growth process are discussed.
Wide-bandgap semiconductor ZnO potentially exhibits high radiation hardness since large displacement threshold energy of constituent atoms can be expected due to the small lattice constant and large bandgap energy. To study the radiation hardness, the effect of proton irradiation on single-crystalline n-type ZnO films was examined. These films were grown by molecular beam epitaxy, and irradiated by 8MeV protons with fluences of 1.4 × 1015, 2.8 × 1015, 5.6 × 1015 and 1.4 × 1016p/cm2. A rapid increase of electrical resistance by a decrease of carrier density was observed with a threshold fluence of about 1 × 1015p/cm2. This change in electrical properties was associated with a steep deterioration of the near-bandedge emission intensity in cathodoluminescence. These radiation damages were found to recover after a thermal annealing over 600°C. Such high radiation hardness of ZnO exceeds that of GaN, indicating promising application of this material to space- and nuclear-electronics.
Lithium transition-metal oxides have been recognized as key component materials in lithium-ion secondary batteries. In this article we report the fabrication of lithium manganese oxide (LiMn2O4) thin films with spinel structure, which is a promising candidate material as a positive electrode, by a safe, low-cost, and highly-controllable vapor-phase deposition technique, namely a mist chemical vapor deposition (CVD) method. With the use of lithium acetylacetonato (Li(acac)) and manganese acetylacetonato (Mn(acac)3) as the precursors and their water solution as the reaction source being supplied in the form of small particles, that is, mist formed by being applied ultrasonic power, LiMn2O4 thin films with (111)-face spinel structure was deposited on platinum as well as c-face sapphire substrates at 1000°C. Further reduction in the deposition temperature is expected by using modified reactor configuration with fine-channel structure offering sufficient suppression of the reaction sources onto the substrates. This deposition technique may be applicable for a variety of lithium-based oxide thin films possessing promising potentials to be used in lithium-ion batteries.
High quality GaAs layer grown on silicon-on-sapphire (SOS) have been successfully achieved by thermal cyclic annealing (TCA) procedure using the two reactor MOCVD system after Si film deposition. The quality of silicon film measured by X-ray diffraction showed 50% narrowing of full width at half maximum intensity (FWHM) after TCA process. A spherical sapphire substrate was used to investigate substrate Offset angle dependence on crystalline quality of the GaAs layer. By applying TCA for SOS substrate before GaAs layer growth, extra smooth surface whose roughness of less than ±30nm could obtained within a wide area of spherical SOS surface. At the smooth region, clear Franz-Keldysh oscillations (FKOs) were observed by photoreflectance (PR) spectroscopy. It also indicated that the cystalline quality of GaAs layer grown on TCA treated SOS was extremly high.
We performed ESR and magnetization measurements to make clear ferromagnetic properties of GdN thin films. We found two kinds of ferromagnetic phases ; one starts to develop below approximately 50K, and another one below approximately 30K. The Curie temperature estimated from the Arrott plot was 29.0K. The ratio of the 50-K-class-ferromagnetic phase to the 30-K-class-phase becomes high with the increase in the free carrier density, which suggests that the transition of the 50-K-class-ferromagnetic phase can be attributed to be the RKKY interaction. On the other hand, the Curie temperature depends on lattice expansion in the growth direction. This indicates that the 30-K-class-ferromagnetic phase obeys the superexchange interaction. Furthermore, we found that the band gap is dramatically reduced with the ferromagnetic spin ordering. The reduction of the band gap energy depends on magnetization, and the relationship between them was ruled by the Zeeman splitting.
Microfractographic features in fracture surfaces for tensile, fatigue, impact, three point bending specimens of cold forging die steels with Rockwell C scale hardness number of 52 to 68 are presented. The emphasis is placed upon the stretched zone formation ahead of the fatigue crack and the relation between the stretched zone width and fracture toughness. Fatigue crack initiation behavior of plane and notched specimens are also characterized in low cycle up to giga-cycle ranges. A couple of microfractographic analyzed results for failed actual cold forging dies are exemplified. It can be concluded that qualitative and quantitative analysis by use of microfractograpy are possible for cold forging die failures.
In order to investigate the effects of aging structures and humidity on fatigue properties of an extruded bar of age-hardened Al alloy 7075, rotating bending fatigue tests were carried out on T62, T73 and RRA (retrogression and reaging) treated plain specimens in two relative humidity of 25% and 85%. Fatigue strength in low humidity was higher in the order of the RRA, T62 and T73 treated specimens corresponding to the order of static strength. On the other hand, fatigue strength decreased in high humidity in all of the alloys and the decrease was the highest in the RRA treated specimen, which means that the RRA treated alloy is very sensitive to humidity environment. In case of the T73 treated alloy, cracks propagated in shear mode in high humidity and under high stress levels due to the marked texture of the alloy, though tensile mode crack growth is more common in the alloy. Similar crack growth behavior was found in the T62 treated alloy, too. In case of the RRA treated alloy, however, the tendency for a crack to grow in the shear mode was very weak even in high humidity and under high stress levels in spite that the alloy had the same marked texture. The effects of microstructure and humidity on the initiation and propagation of small cracks were marked in all of the alloys.
This paper discusses the corrosion fatigue property of a high-strength cold-rolled eutectoid steel in deionized water to investigate the countermeasures against corrosion fatigue. The effects of temperature, stress cycle frequency and dissolved oxygen concentration on the corrosion fatigue strength in deionized water were investigated. In the case of smooth sample, lower stress cycle frequency, higher temperature and higher dissolved oxygen concentration decreased the fatigue strength in deionized water compared with that in dry air. Note that the corrosion fatigue strength decreased in the deionized water with a dissolved oxygen concentration of even 0.05ppm. The morphology of corrosion pit depended on the specimen orientation relating with a rolled pearlitic microstructure. Moreover, the addition of inhibitor increased the corrosion fatigue strength up to that in dry air because of the surface corrosion protection. It is more effective countermeasures for corrosion protection to add the inhibitor than to decrease the dissolved oxygen. However, even in ion-exchanged water with inhibitor, the fatigue strength of a sample having an artificial crevice was decreased compared with that of sample without the crevice. The corrosion pit formed inside of the crevice caused a decrease in fatigue strength. These results suggest that the corrosion protection effect becomes insufficient inside the crevice because the nitrous ion depleted due to the interruption of diffusion and convection.
We present a new numerical path independent Ê-integral for calculating the stress intensity factors using a known auxiliary solution. The integral is path independent in the similar manner to the well known domain independent M-integral for calculating the stress intensity factors. The Ê-integral is, however, path independent even if the path contains any number of crack-tip and the integral may obtain the stress intensity factors at the onset of crack kinking by the path independent integral ; therefore, the integral path can be far from the crack-tip around which the numerical solution has noticeable error : The domain M-integral may not obtain the stress intensity factors at the onset of crack kinking and may be very difficult to obtain the stress intensity factors when there exist small cracks in the neighborhood of the crack-tip. For illustrative purposes, by using finite element method, numerical examples obtaining the stress intensity factors are presented for an extending straight crack and a kinking crack.
Several silicon nitride ceramics with different compositions and amounts of sintering additive were fabricated by hot-press method. The effect of sintering additive on the sinterability, flexural strength and fractography was examined. The flexural strength of silicon nitrides sintered with alumina as an additive were not high at approximately 400MPa, although the sinterability was good in that bulk densities of 3.05-3.18 × 103kg m-3 were obtained. The sinterability of silicon nitrides sintered with yttria as an additive resulted in low bulk densities of approximately 2.7 × 103kg m-3, with flexural strength ranging from 370 to 650MPa. The sinterability of silicon nitrides that were sintered with both yttria and alumina additives resulted in relatively high bulk densities of approximately 3.21 × 103kg m-3 and extremely high flexural strength in the range of 900 to 1140MPa. The fractography of fracture origin was dependent on the type of sintering additive used; an inclusion size of approximately 30μm for the alumina additive, pores for the yttria additive and small secondary sintering defects for both yttria and alumina as additives were observed at the respective fracture origins.
Patch repair is often applied to the concrete structure deteriorated by the chloride ions. However, the macrocell corrosion may be formed, when there are a lot of chloride ions in the existing concrete. The steel corrosion in concrete progresses while depending the time. Therefore, it is effective to monitor at any time-interval. Based on the above backgrounds in this study, the analyzing method of the corrosion current is developed, to monitor the protection effect after the patch repair. The experiments are investigated in two steps. Firstly, the corrosion currents between measured from cathode to anode using the specimen with the special divided steel bar, and analyzed by this developed method, are compared. Secondly, the analyzed values and the actual corrosion or no-corrosion areas are compared. As results, 1) the analyzed values are equal to the measured values at these investigations ; 2) the analyzed corrosion area is equal to the actual corrosion area. It can be judged that quantitative monitoring method for the protect effect after the patch repair is developed.