Two kinds of methods for mass production of carbon nanotubes by DC arc discharge are described. One is conventional arc method to prepare multi-walled carbon nanotubes (MWNTs), but newly designed that the cathode graphite rod was automatically fed to get continuous arc discharge. By evaporating in the atmosphere of rarefied pure hydrogen gas, high-quality MWNTs were obtained in the cathode deposit. The second is newly developd arc plasma jet method to produce single-wall carbon nanotubes (SWNTs). High yield of SWNTs was obtained in the cottony carbon soot within the chamber. Prepared MWNTs and SWNTs were characterized by SEM, TEM and Raman spectroscopy. Several breathing modes of MWNTs were obtained at 206-388cm-1.
A carbon nanotube has so extremely high tensile rigidity that engineering applications of it have recently been expected. Although many reports concerned with its physical and chemical properties have been published, there are not enough studies on mechanical properties inevitable for future design. Our recent paper on the molecular dynamics studies shows that the Young's modulus of a single-walled tube is about 0.5TPa. And a cantilever-type tube responds irreversibly to the following lateral load hysteresis due to the topological transition after buckling. It is found that there is geometrical similarity of the buckling shape between the atomic-scaled tube with nanometer-order and the macroscopic titanium tube with millimeter-order. Therefore, the mechanical behaviors may be summarized using the continuum formula of the buckling under bending. In the present paper, the buckling stresses of the cantilever-type tubes are estimated from the relation between the bending moment and the curvature of the deformed tube. The effect of cantilever length on them is further investigated.
We have investigated field emission properties from an individual carbon nanotube protruded-45μm from a tungsten needle tip. The I-V characteristics of the field emitter show no emitter-collector gap dependence at a gap less than 310μm. This experimental result is explained by an analysis under a consideration of the nonuniform electric field near the tip of the nanotube. The tip radius of 2.7nm, which is obtained from the analysis, is consistent with the average tip radius of nanotubes. The field emission device for a flat panel display consisting of the well perpendicularly aligned carbon nanotube array is fabricated by the catalytic chemical vapor deposition using C2H2 as source gas. Patterned emission device is easily obtained by patterning the catalyst. The threshold voltage for the field emission decreases from 400V to 150V by the thermal treatment in air at 700°C for 8min. We also demonstrate that the diode type display consisting of a phosphor coated electrode and the nanotube device emits strong and uniform luminescence.
Reversible photo-induced change has been observed in the polarization dependence of transverse electroabsorption signal in hydrogenated amorphous silicon (a-Si: H), which is indicative of structural change occurred in the whole material. The structural change in a-Si: H saturates in a few minutes upon light exposure, which is a short time in comparison with the saturation time of photoconductivity degradation and more than 10 hours' annealing is necessary for restoring to initial state. Accounting for the results of various investigations, photo-induced structural change may occur through the following processes: (1) The structural disorder increases with light exposure and is observed as a PEA. In particular regions, the local network distortion becomes significant and causes surround network to distort. The strained region enlarges by creating metastable strained bonds. (2) Bond breaking is caused by the collision of such a strained region with another such region. Since the region of intersection will be the most highly strained region. (3) The produced dangling bond pair move through such a bond switching process to reduce strain energy. (4) The PEA ratio saturates when the strained regions cover almost the whole volume. However the density and internal stress will not saturate until they reach a averaged state of distortion distributed throughout the material. Therefore the time to reach the averaged state is considered to depend on the light exposure history. Because the size and distribution of strained region may depend on their consecutive growth time. The breaking process continues with collisions of the strained region in smaller and smaller un-affected parts of the material. On the other hand, in annealing process, the structural restoration begins with the most highly strained regions which is located to near the dangling bond defects.
Dynamics of spontaneous and stimulated emissions in InGaN-based laser-diode (LD) structures have been assessed by employing time-resolved photoluminescence (TRPL) and pump & probe (P & P) spectroscopy at room temperature. The LDs are composed of InxGa1-xN-InyGa1-yN MQWs [(a): x=0.1, y=0.02, (b): x=0.2, y=0.05], whose stimulated emissions correspond to near ultraviolet (390nm) and violet (420nm), respectively. Almost no Stokes shift was observed for the sample (a) (x=10%). However, Stokes shift in the sample (b) (x=20%) was as large as 250meV, and it is probable that the origin of such deep localization is In-rich quantum dots self-formed during the growth. It is likely that large internal quantum efficiency in In-rich InxGa1-xN active layers is as a result of zero-dimensionality because capture-cross-section to non-radiative recombination centers are greatly reduced once excitons are trapped at deep localization centers. P & P spectroscopy has revealed that the optical gain was contributed from the nearly delocalized states [the lowest-quantized MQW levels (LQL)] in the sample (a), while it was from highly localized levels with respect to LQL by 250meV for the sample (b). It was found that the photo-generated carriers rapidly (within a few hundred fs) transferred to LQL, and then relaxed to the localized tail within the time-scale of a few ps, giving rise to the optical gain. Such gain spectra were saturated and other bands appeared in the vicinity of LQL under higher photo-excitation.
Wide band gap semiconductors such as III-V nitride materials and SiC have higher breakdown fields, higher electron saturation velocities and higher thermal conductivity than those of Si and GaAs. These attractive material properties are suitable for high power devices operating at high supply voltages. GaN-based electron devices are especially promising for high frequency operation because modulation-doped AlGaN/GaN heterostructure with high electron mobility is available. Besides, the density of the two dimensional electron gases at the heterointerface is very high owing to the piezoelectric and spontaneous polarization effects. These heterostructures are grown on either sapphire or SiC substrates since no bulk GaN substrate is commercially available widely. For sapphire substrates, the thermal conductivity is relative low, which leads to the reduction of drain currents in the regions of high drain current and high drain voltages due to the self-heating effects. Thus the AlGaN/GaN Heterostructure Field Effect Transistors (HFETs) grown on SiC substrates with high thermal conductivity are more suitable for high power device applications. This paper describes the improvements of process technologies and fabrication of the devices. The fabricated 0.3μm gate HFETs showed a maximum drain current of 740mA/mm, a peak transconductance of 150mS/mm and a breakdown voltage higher than 50V. The cut-off frequency fT and maximum oscillation frequency fMAX were 22GHz and 85GHz, respectively. The fT and fMAX were almost constant with the increase of drain voltage up to 30V. The output power of 1W/mm at 1.9GHz was obtained. And then frequency dispersion was not observed with the high voltage operation. These results indicate that AlGaN/GaN HFETs grown on SiC substrates are quite suitable for high frequency microwave and millimeter-wave devices.
A systematic investigation has been made on generated electric energy from different photovoltaic (PV) solar arrays consisting of three kinds of silicon based solar cells. Clarification of seasonal output power variations with temperature in c-Si and a-Si cells might be important issue for the operations of PV system. It has been shown from the results that output power from a-Si array is 20% larger than that from c-Si in summer. On the other hand, in winter, this scene should be reverted. However output power from c-Si array is only 5% larger than that from a-Si. The analyzed data also show that annually accumulated electric energy generated from a-Si array corresponds to 90% of its nominal efficiency in the year. While in case of c-Si array, this ratio is about 84%.
The distribution of silicic acid gel and boron compounds in wood-mineral composites made using the silicate-boron compound system was examined by electron probe X-ray microanalysis (EPMA). The fire resistance of the composites was also evaluated by applying both oxygen index and percent weight of residual char. The composites were produced by diffusion-penetrating the water glass-boron compound system (double treatment) and the colloidal silica solution system in the sapwood specimens of Sugi (Cryptomeria japonica). For comparison, the composites were also prepared by impregnating boron compounds only into the wood (single treatment). In the single treatment, most of boron compounds in the composites was located in the cell walls as obvious from boron X-ray maps. However, boron compounds themselves were almost all leached out by the leaching procedure, showing no fixation of them in the composites. On the other hand, in the double treatment having the fixation of 5470% after leaching, silicic acid gel was located mostly in the cell lumina and it was very small in quantity in the cell walls as seen from silicon X-ray maps whereas boron compounds existed in the cell walls besides the lumina even after leaching. In the colloidal silica solution system having the high fixation of 86-94%, most of silicic acid gel was deposited in the cell lumina as an insoluble solid body and it was also very little in the cell walls. In addition, the distribution of boric acid was clearly observed not only in the lumina but also in the walls even after leaching. As a result of fire-resisting test, the oxygen index of the composites trended to increase with an increase in weight percent gain (WPG) and there was a high correlation between the two. The percent weight of residual char was highly correlated to WPG independently of leaching and it increased lineally with an increase in WPG. It may be possible that the percent weight of residual char becomes one way for estimating the fire resistance of the composites because of a high correlation with the oxygen index after leaching.
Yakushima-sugi woods have high potential as structural timber owing to having their high mechanical properties. However, the factors influencing the mechanical properties have not been investigated. In this study, the wood properties of 25-50 years old Yakushima-sugi trees grown at four locations in Yakushima, two locations in Tanegashima and one location in Togo district (Kagoshima) are determined. The dynamic modulus of elasticity, the latewood percentage, and the basic density of Yakushima-sugi woods were higher than those of common sugi-woods. The woods with higher latewood percentages had bands of compression-wood like tracheids in the latewood. In addition, Yakushima-sugi woods had longer tracheids than common sugi-woods. Both the basic density and the S2 microfibril angle affect the dynamic modulus of elasticity of Yakushima-sugi woods. There were differences in how these two factors affected the dynamic modulus of elasticity of Yakushima-sugi woods among sites. Higher basic density and/or lower S2 microfibril angle were the cause of the higher mechanical properties of Yakushima-sugi woods.
Swelling behaviors in the cross sections of various woods were measured using an optical microscope and a digital image correlation method (DIC). The interactions between ‘earlywood and latewood’ and ‘longitudinal cells and rays’ were observed to make clear the mechanism of anisotropy in swelling and shrinkage. It was suggested from the observations with akamatsu (Pinus densiflora) and western hemlock (Tsuga heterophylla) that the interaction between earlywood and latewood was the major factor of swelling anisotropy. Difference of tangential swelling between earlywood and latewood was large at the edge, but became smaller with the distance from the edge. Large shear strains were observed at annual ring boundaries where the density varies discontinuously. Distribution of swelling in radial and tangential direction of keyaki (Zelkova serrata) indicated that radial swlling was restricted by rays. Observation of swelling in red meranti (Shorea sp.) showed that diameter of pores decreased greatly in radial direction with the increase of water absorption. According to the results of 8 species (5 softwoods and 3 hardwoods), it was concluded that hardwoods had smaller swelling in radial direction than softwoods because of the restriction by rays and decrease of diameter of pores in radial direction.
The acoustic emission (AE) behavior on dowel-bearing tests with a drift pin was investigated. The comparison of AE behavior in elastic region and that in plastic region was carried out. The drift pin diameter and the end distance from center of drift pin were changed to compare their effect on AE behavior. From AE behavior side, the safety of embedding stress from ASTM-D5764 and EN383 was examined. The results were as follows. 1) AE was observed in plastic region, while few AE was found in elastic region. It is estimated that minute fracture in embedding wood causes the AE generation. 2) On the specimen with short end distance, AE was observed actively. On the contrary, on the specimen with long end distance, AE was observed slowly. 3) As regards AE amplitude, large number of ratio over 70dB was seen in plastic region. The ratio was increased approximately with increasing AE count rate. Very small number of the ratio was observed in the region showing less than 100 (event/sec) AE count rate. 4) Both 5% offset stress value from ASTM-D5764 and embedding strength from EN383 did not show significant difference. However through the examination of the AE behavior, it was indicated that both values might be accompanied with the minute fracture.
To compare and investigate the air permeability in heartwood, white zone wood and sapwood in green logs of Cryptomeria japonica, each of the points in the log specimen set in vacuum chamber was connected to mercury manometer, and the changes of pressures in those were measured in the stages of vacuuming. The several experiments were carried out at the different log conditions, i.e. the conditions of opening or sealing of transverse section, radial section and log surface were changed, and the air permeability of longitudinal, tangential and radial directions were investigated with respect to each of the points. The results obtained were as follows. 1) The air permeability in white zone wood was remarkably high, in comparison with those in heartwood and sapwood. 2) In heartwood, the air permeation was scarcely detected for all of three directions whether the moisture content was high or low. 3) In white zone wood, the air permeability was high for both of longitudinal and tangential directions, and that of longitudinal direction was higher than that of tangential direction. 4) In sapwood, the air permeability of tangential direction was obviously higher than that of radial direction. 5) The air permeability in each of the points were graded on the basis of opening or not of radial section for slitting by saw, i.e. white zone wood (there is opening of radial section for slitting)≥white zone wood (there is not opening of radial section)>sapwood (opening)>>sapwood (not opening)>>heartwood (both).
Mechanical properties of particleboards such as surface hardness, modulus of elasticity (MOE), bending strength (MOR) can be improved by high densification of its face layers. One of the effective ways for this purpose is introducing a steam pre-treatment technique into preparations of wood furnish for face layers, because steaming wood at high temperature increases wood compressibility. Also steam treated and then compressed particles show much less irreversible swelling than ordinary particles. This excellent dimensional stability would contribute to reductions of panel thickness swellings. Three-layered particleboards with a density ranging 0.5-0.8g/cm3, a face to core ratio of 2/3, and a thickness of 12mm were manufactured from Sugi (Cryptomeria japonica D. Don) face strands (75mm×9.0mm×0.67mm) and core particles (14.9mm×2.9mm×0.97mm). The face strands were steam pre-treated in an autoclave by introducing high pressure steam at 210°C for 10 minutes. Phenol-formaldehyde resin was used as a binder. The manufactured panels showed the following characteristics, when compared to control panels with non-treated face strands: 1) the maximum density through the thickness increased by 10-15 percent, 2) surface hardness increased by 30 percent, 3) thickness swellings were greatly improved, this being pronounced at higher density panels, 4) however, the reduction of linear expansion was not obvious because this layer structure showed excellent resistance against for in-plane swelling, 5) in spite of face layer's high densification, MOE and proportional limit stress in bending were almost the same as the controls, 6) MOR and withdrawal resistance of wood screw decreased at most by 30 percent due to the reduction of wood cohesion itself caused by steaming.
Gusset truss systems using metal plate connectors have been developed and used in residential constructions in Japan. Several softwoods, such as SPF (Picea spp., Pinus spp., and Abies spp.) and white Spruce (Picea abies Karst), have been used as structural members of the systems. However, Sugi (Cryptomeria japonica D. Don), the most important species in Japan has not been used so far. To examine the feasibility of Sugi, tensile strength tests were made on Sugi timber joints with two kinds of metal plate connectors (GN80 and GNQ). Furthermore, full size bending tests of roof trusses composed of Sugi and GN80 were conducted to discuss the availability of current structural design method. Sugi joints showed lower maximum strength and toughness than SPF and White Spruce. However, it was found that Sugi had enough performance required for structural members. Coefficient of correlation between the density of wood and tensile strength of the joint was positive in any kinds of specimens, but most of them were less than 0.3. The current design method for plate size and truss members can be applied to the Sugi gusset trusses, though its estimation for deformation is rather conservative. Since the mechanical properties of Sugi vary with its cultivar, provenance, or other factors, such as the amounts of small knots, more experimental data should be accumulated to expand the practical applications.
Fracture tests of gusset welded joint with various conditions, tensile and compressive pre-strain, static and dynamic loading, tension and bending loading, were performed and fracture characteristics were discussed. Ductile fracture strength was estimated using tensile strength of material at tested temperature. Brittle fracture strength was estimated by fracture mechanics using CTOD criteria, CTOD design curve of WES2805 was applied to estimate strain at weld toe of gusset joint. Intrinsic crack model was used to determine effective crack size parameter of gusset joint. Effect of plastic pre-strain and strain rate on fracture toughness was estimated using equivalent static temperature, considering plastic pre-strain and strain rate to temperature shift. Test results of all conditions are well expressed by above estimation method.
Many bridges were severely damaged by the Hyogo-ken Nanbu Earthquake. Taking this lesson into account, the fundamental design concept was changed so that bridge might be prevented from falling down by the equivalent earthquakes. Many researchers have studied the shock absorber for bridge restrainer system. In this study, the static compress test was performed for the various type of the laminated fiber reinforced rubber, which will be expected to be of use as a shock absorber in bridge restrainer system. The following results are obtained: (1) The optimum shape to be used as a shock absorber is the cubic, (2) An empirical equation for initial yield load of laminated fiber reinforced rubber is proposed for the various cube and cylindrical specimens.
It is discussed the effects of a high strength additive on reduction of environmental burden from the viewpoint of LCA in this paper. The high strength additive is mainly consisted of anhydrite and silica powder, the CO2 primary unit of one is calculated from the power consumption at its production, which amounts to be one order smaller figure compared to that of the cement. This implies to be an effective re-utilizing of industrial wastes and the material itself is considered to be of low environmental burden. It is found that placing of high strength concrete is more effective in reducing environmental burden than repetitive placing of ordinary strength concrete, when highly durable concrete is required. It is revealed that production of high strength concrete by using high strength additive which produce ettringite, can reduce the unit weight of cement for the required design strength, and can reduce environmental burden. It is also found that the compressive strength can be achieved approximately equivalent to that with autoclave curing by using blastfurnace slag cement and ettringite-formed high strength additive and with only steam curing, and thus that high strength concrete products are possibly produced in this manner. In addition, the method is revealed to be able to reduce the CO2 primary unit significantly compared to the production with autoclaving, and effective in reducing environmental burden by calculating the CO2 primary unit required for the production of concrete products.