Masato Yoshiya

First principles calculation of chemical shifts in ELNES/NEXAFS of titanium oxides

First principles molecular orbital calculations of three titanium oxides are made in order to quantify the absolute transition energies of ELNES/NEXAFS at the O K and Ti edges and to clarify the origin of their chemical shifts. The absolute transition energies as well as their chemical shifts at two edges are satisfactorily reproduced using clusters composed of 24 to 63 atoms when Slaters transition state method is employed allowing temporary spin-polarization. The O K edge shows a positive shift with the increase of the formal number of d electrons per Ti ion. The shift can be mainly ascribed to the variation of the energy of the -like band, although the energy of the O 1s core-orbital varies slightly. On the other hand, the Ti edge shows negative shift, which is found to be explained by the balance of energies of the Ti 2p and the -like band. The magnitude of the chemical shifts is not significantly altered by the manner of the octahedral linkage.

In situ observation of solidification phenomena in Al–Cu and Fe–Si–Al alloys

Abstract Synchrotron radiation enables the observation of solidification in metallic alloys. In situ observations of solidification for Al–Cu alloys (5, 10 and 15 wt-%Cu) are reported. Nucleation and fragmentation of dendrite arms were often observed in the 15 and 10%Cu alloys when unidirectional solidification was performed from the planar interface. In contrast, nucleation and fragmentation were rarely observed in the 5%Cu alloys. The nucleation ahead of the solidifying front and the fragmentation in the mushy region strongly depended on alloy composition. This paper also presents in situ observation of solidification of Fe–10Si–0·5Al (at-%) alloys. The dendritic growth of δ-Fe was clearly observed using this technique. The development of X-ray imaging techniques enables the solidification of various conventional cast alloys such as Al, Ni and Fe alloys to be observed and will be increasingly used to investigate solidification phenomena.

Theoretical prediction of ELNES/XANES and chemical bondings of AlN polytypes

The first principles calculations of ELNES/XANES of AlN polytypes were carried out by the first-principles OLCAO method using large supercells composed of more than 100 atoms. It can quantitatively reproduce the experimental spectra from wurtzite AlN using a 108-atoms supercell. ELNES from rock-salt and zinc-blend AlN were predicted by using 128 atoms supercells. The spectral features of rock-salt phase are different from other phases, whereas that of zinc-blend phase have numerous similarities with that from wurtzite AlN. Characteristic differences between the wurtzite and zinc-blend phases are predicted to appear at the first peak of Al L(2,3) and K edges. The first peak of zinc-blend AlN is broader than that of wurtzite AlN. The same tendency was found in the case of SiC. In order to elucidate the cause of the broadness at the first peak, partial density of states and chemical bondings were investigated. The theoretical analysis revealed that the broadness of the first peak is related to the covalency of the compounds. This result suggests that the spectral features at the first peak of L(2,3) and K edges contain information about the covalency at the illuminated area.

Atomically ordered solute segregation behaviour in an oxide grain boundary

Grain boundary segregation is a critical issue in materials science because it determines the properties of individual grain boundaries and thus governs the macroscopic properties of materials. Recent progress in electron microscopy has greatly improved our understanding of grain boundary segregation phenomena down to atomistic dimensions, but solute segregation is still extremely challenging to experimentally identify at the atomic scale. Here, we report direct observations of atomic-scale yttrium solute segregation behaviours in an yttria-stabilized-zirconia grain boundary using atomic-resolution energy-dispersive X-ray spectroscopy analysis. We found that yttrium solute atoms preferentially segregate to specific atomic sites at the core of the grain boundary, forming a unique chemically-ordered structure across the grain boundary.

In situ observation of nucleation, fragmentation and microstructure evolution in Sn–Bi and Al–Cu alloys

Abstract This paper presents recent progress of in situ observation for the microstructure evolution during solidification. Nucleation and fragmentation of dendrite arms are important issues for controlling microstructure during solidification. However, there are few studies on in situ observation of nucleation and fragmentation in metallic alloys. Time resolved X-ray imaging technique has been developed to observe solidification of metallic alloy systems in situ. Fragmentation of dendrite arms often occurred at the root after growth velocity was reduced for the Sn–13 at.-%Bi alloys and the Al–15 mass%Cu alloys. In the Al–15 mass%Cu alloys, both of nucleation and fragmentation contribute to formation of grain structure. The result suggested that fragmentation should be considered for controlling grain structure.

Electron-energy-loss near edge structures of six-fold-coordinated Zn in MgO

Electron-energy-loss near edge structures (ELNES) at the Zn-L(2,3) edge and the O-K edge have been measured for 10 mol% ZnO-doped MgO, and were compared with spectra from reference materials. In order to interpret the spectra, first principles molecular orbital calculations were made using model clusters composed of 125 and 153 atoms. Photoabsorption cross sections (PACS) were computed at the Slaters transition state in which a half-filled core hole was included in the self-consistent calculations. The difference in the coordination numbers of Zn was found well distinguishable by the Zn-L(2,3)-edge ELNES. The experimental spectra in the first 25 eV were well reproduced by the theoretical PACS. In this energy region, the Zn-L(2,3)-edge ELNES from four-fold coordinated Zn showed four sets of peaks, whereas the six-fold coordinated Zn exhibits three sets of peaks. The origin of these peaks can be explained by the point symmetry within the first coordination unit. A small shift toward the lower energy side was observed in the O-K edge ELNES of the ZnO-doped MgO as compared with pure MgO. This can be ascribed to the lower energy of the Zn-4s orbital as compared with the Mg-3s orbital, which is the common mechanism to the difference in the band gap between MgO and ZnO.

Energetical role of modeled intergranular glassy film in Si3N4-SiO2 ceramics

Abstract We have performed theoretical calculations to clarify the energetical role of an intergranular glassy film (IGF) by a static energy minimization technique using pair-potentials. A model structure of the IGF composed of silicon oxynitride (SON) is employed, which is consistent with the experimental electron energy-loss near-edge structure (ELNES). Two sets of calculations, i.e., for 0°- and 180°-twist boundaries, are done for Si 3 N 4 /SON/Si 3 N 4 and Si 3 N 4 /Si 3 N 4 interfaces. It is found that the presence of SON at the 180°-twist grain boundary is energetically favorable. The SON phase plays an important role in relieving the strain energy at the grain boundary. The dependence of the interface energy on composition of the SON phase is also discussed.

Evaluation of thermal conductivity of zirconia coating layers deposited by EB-PVD

Electron beam-physical vapor deposition (EB-PVD) is a widely used technique for depositing thermal barrier coatings (TBCs) on metal substrates for high temperature applications, such as gas turbines, in order to improve thermal efficiency [1]. Characterization of the thermal conductivity of the coating layers is therefore very important for developing superior thermal barrier coatings, but because of the irregular nature of the coated specimens it is difficult to derive the thermal conductivity of the coating layer from measurements of the thermal conductivity of the combined coating and substrate. Two steps are therefore involved in determining the thermal conductivity of a thin coating film: (i) separation of the coating film from the combined coating and substrate specimen, and (ii) measurement of the thermal conductivity of the film. With regards to the first step, it is known that coating layers deposited by EB-PVD have a porous structure so that they are easily damaged because of their poor strength [2, 3]. In other words, in practice it is not easy to physically separate the coating film from the coated substrate by machining or some other method without damaging it. Regarding the second step, even if the coating can be successfully separated from the substrate, it is not a simple matter to measure directly the thermal conductivity of the coating. The laser flash method is generally used to accurately measure the thermal diffusivity and specific heat capacity of materials, from which the thermal conductivity can be calculated. The technique was developed by Parker et al. [4], and is usually carried out assuming the specimen to be uniformly dense and opaque. However, coated layers deposited by EB-PVD have a columnar non-uniform structure, making it difficult to measure the thermal conductivity directly. The aim of the present work is therefore to derive a practical method for determining the thermal conductivity of coating layers based on theoretical calculations, and comparing the values obtained with direct experimental measurements. We have therefore adopted the response function method as a means of determining the thermal conductivity of the coating layers. It has been reported that the response function method is a powerful method to analyze one-dimensional heat diffusion across multi-layer materials [5]. We also present the experimental results from thermal conductivity measurements of coated substrates as well as coating layers detached from their substrates as a function of substrate thickness. In this work, ZrO2-4 mol% Y2O3 coatings have been applied by EB-PVD to zirconia substrates with the same composition as the coating material to minimize interface effects on thermal conductivity. Disc-type zirconia substrates were prepared by pressureless sintering at 1600◦C. The sintered substrates were machined to 10.0 mm diameter and 0.1–3 mm thickness. The substrates were first preheated at 900–1000 ◦C in a heating chamber using a graphite heating element. An electron beam evaporation process was used to deposit the film in a coating chamber under a vacuum level of 10−4 Pa using a 45 kW electron gun at a rate of 4 μm/min and substrate rotation speed of 5 rpm. The average coating thickness was about 300 μm. The density of each specimen was determined by measuring its mass on an electronic balance and its volume with a micrometer. All thermal diffusivity and specific heat capacity measurements were carried out three times for each specimen at room temperature by the laser flash method. The microstructure of the coated specimens was observed by SEM. A typical microstructure of a specimen coated on a zirconia substrate is shown in Fig. 1. The crosssectional surface of the coated specimen clearly reveals the columnar microstructure, with all columnar grains oriented in the same direction, i.e., perpendicular to the substrate. This columnar structure is very similar to those reported for metal substrates coated by EB-PVD [3]. In other words, the distinctive columnar microstructure can be obtained regardless of whether the substrate being used is metal or ceramic. The correlation between temperature rise at the rear surface of a specimen and time is shown in Fig. 2 when the front surface of the specimen is uniformly heated using a laser pulse. For bulk materials, the thermal diffusivity (α) is described by the following equation:

Direct observation of intergranular cracks in sintered silicon nitride

Crack propagation along grain boundaries in sintered silicon nitride (Si3N4) was investigated by in-situ straining experiments at room temperature in a transmission electron microscope, using a high-precision microindenter. Using this in-situ technique, cracks introduced were introduced in situ and observed propagating along grain boundaries. High-resolution electron microscopy observation revealed that the propagation of the intergranular crack takes place at an interface between the Si3N4 grains and the intergranular glassy film (IGF). This suggests that the Si3N4/IGF interface has a relatively high excess energy. The result was compared with a theoretical calculation using a molecular dynamics simulation.

In-situ observation of peritectic solidification in Sn-Cd and Fe-C alloys

Time-resolved absorption imaging using synchrotron radiation X-rays allows us to observe solidification of metallic alloys of interest. This paper presents peritectic solidification in Sn-Cd alloy and Fe-C alloys. In unidirectional solidification of Sn-Cd alloy, the formation of a banded structure, in which two phases were alternatively piled up in the growth direction, was clearly observed. Sequence of nucleation or fluctuation of triple junction (primary phase / secondary phase / liquid) resulted in the banded structure. Ease of nucleation for both phases contributed to the banded structure formation. In carbon steel (Fe-0.45mass%C), the transformation from δ phase to γ phase was observed. At lower cooling rates, γ phase was produced in semisolid state of δ phase and liquid, indicating the peritectic reaction occurred during solidification. In contrast, δ phase transformed into γ phase when solidification nearly completed at temperatures 100K below the liquidus temperature. Namely, the transformation seemed to be massive. The observation showed that two different transformation modes operated in Fe-C alloy.