Thermoelectric power generation is a promising green energy system recovering waste heat. CoSb3 skutterudite compound is a candidate material for thermoelectric power generation. For improving thermoelectric performance of the material, it is necessary to lower the thermal conductivity. In this work, Al2O3 nanoparticles were added and dispersed by mechanical grinding (MG) into n-type CoSb3 compound prepared by pulse discharge sintering (PDS) method. Over long MG treatment brought high electrical resistivity, lowering thermoelectric performance of the Al2O3 nanoparticles added CoSb3 compound. Seebeck coefficient almost did not change regardless of additional amount of Al2O3 nanoparticles. Increasing additional amount of Al2O3 nanoparticles brought increasing electrical resistivity and decreasing thermal conductivity. It is found that there is an optimal additional amount of Al2O3 nanoparticles that leads to improvement of thermoelectric performance of the CoSb3 compound. The maximum thermoelectric performance (ZT=0.58) was achieved at 684 K in the CoSb3 compound including Al2O3 nanoparticles of 0.1 mass%.
The thermal characteristic and the structure of rapidly solidified Al-Cr-Si ribbons by single roll melt spinning were examined and ability of sintering by pulsed current sintering was investigated. A single phase of quasicrystal is produced in Al74Cr20Si6, and a mixed phase of Al and quasicrystal was produced in Al77Cr13Si10. The mixed phase of Al and quasicrystal was produced in Al90CrxSi(10-x) (x=3,6,8,10), also. The quasicrystal in Al90CrxSi(10-x) x=3, 6, 8) ternary system was different thermal stability from the quasicrystal in Al90Cr10 binary system because of with a different quasicrystalline structure. The transform temperature of the quasicrystal to a stable phase in Al-Cr-Si ternary system was 100 degree higher than Al-Cr binary alloy. The porosity of a sintered material decreased with increasing Al phase in the rapidly solidified powder from the result of sintered Al74Cr20Si6, Al77Cr13Si10 and Al90Cr6Si4, and Al90Cr6Si4 was the highest density. The effect of sintering temperature and applied pressure on sintering density was investigated in Al90Cr6Si4. The sintering density at 513K in 500MPa was above 99%. Then, the sintering density at 613K in 250MPa was 99% or more, and the applied pressure could be decreased in half.
The effect of phase formation from unstable to stable phase on densification process of pressure sintering was studied using mechanically alloyed powders. Powder mixture of Ti and Si having a composition of Ti-37.5mol%Si was milled for 360ks and 3600ks. The 3600ks milled powder showed homogeneous microstructure and amorphous-like XRD profile, although the 360ks milled showed (Ti+Si) lamellar microstructure. Milled powders were vacuum hot pressed at a heating rate of 20K/min up to 1273K with applying constant pressure from 10 to 200MPa, then kept for 10.8ks. It was observed that the density of the compact suddenly increased at the temperature range between 870K and 900K for 3600ks milled powder and about 820K and 910K for 360ks milled powder. This temperature showed good agreement with the exothermic reaction of DTA run, which corresponded to the phase formation from amorphous to Ti5Si3 and from elemental phases to Ti-silicides. It was assumed that the reasons of this extraordinary densification behavior were temperature increase, volume change, rearrangement and acceleration of plastic deformation during phase formation. From experimental results to examine these factors, with the exception of plastic deformation, other factors did not show any effect in increasing density. It is concluded that the phase formation from unstable (amorphous) to stable (Ti5Si3) phase accelerates plastic deformation of MA powder, and therefore it leads to increase the density. After the formation of stable phase, densification was stopped for a while then restarted at 1050K (3600ks milled powder) and (360ks milled powder). These temperatures seem to be a softening temperature of MA powders.
Modified 9Cr-1Mo steel has been applied to the parts of boiler in thermal power plants due to its excellent properties at high temperatures. In recent years, this steel also has been applied to the parts of turbine in ultra super critical power plants. Our company has been manufactured forged products by using large ingots of Mod.9Cr-1Mo steel that exceeded 20t for ten years or more, and it has been recognized that internal defects were liable to occur in Mod.9Cr-1Mo steel ingot. Especially the generation of the casting defects becomes more serious problem with the size of ingot, therefore measures were taken to improve the quality of forgings about both of ingot making and forging procedure. As a result of investigation of the defects in large Mod.9Cr-1Mo steel ingots, we concluded that defects formed in forgings were caused by casting defects and eutectic Nb (C,N). And the defect was proved to be influenced considerably by solidification rate and the cooling rate by means of solidification analysis that was performed because complete sticking of defect surface was difficult and the improvement of the steel ingot was necessary. Afterwards, large ingots were produced based on the analysis and the effectiveness of the analysis was confirmed by UT and the investigation of the microstructure. As mentioned above, we were able to raise quality and the stability of the product by improving the quality of the steel ingots.