Papers by Keyword: Transmission Electron Microscopy (TEM)

Abstract: Pt-based nanoparticles (NPs) have numerous applications, such as, as catalyst, in car exhaust systems, gas sensors, biosensors and cancer therapy. One of the Pt based NPs which has been successfully produced is core-shell Ag@Pt NPs. Numerous methods for the synthesis of this material have been reported. This paper reports a fully new approach of chemical mediated synthesis for core-shell Ag@Pt NPs. Characterization process for the synthesized Ag@Pt NPs, carried out by the UV-vis Spectroscopy, Transmission Electron Microscopy (TEM), High Resolution Transmission Electron Microscopy (HRTEM) showed that the core AgNPs have approximate sizes of 18 nm in diameter are shelled with Pt and the sizes of core-shell Ag@Pt NPs were estimated to be around 29 nm in diameter.

Abstract: The simple-perovskite system Bi_1-xLa_xFeO₃ (BLFO) is one of mixed-oxide systems having the multiferroic BiFeO₃ as an end material, which shows the ferroelectric and antiferromagnetic orders in its ground state. Because of the paraelectric nature of LaFeO₃ as another end material, the ferroelectric-to-paraelectric state change can be expected to occur in the mixed-oxide system BLFO. The interesting feature of BLFO is that there are both the PbZrO₃-type and incommensurately modulated (IM) states in the intermediate La-content range between the ferroelectric-R3c and paraelectric-Pnma states. Although the detailed features of these two states have not been understood sufficiently, in this study, we focus on the IM state around x = 0.20, and have investigated the crystallographic features of prepared BLFO samples with 0.15 ≤ x ≤ 0.35, mainly by transmission electron microscopy. It was found that six kinds of superlattice reflections were, for instance, present in the reciprocal lattice of the state at 300 K for x = 0.20, in addition to fundamental reflections due to the cubic simple-perovskite structure. To understand the appearance of these superlattice reflections, we regarded the state as a modulated-structure state in this study. Concretely, the modulated structure was assumed to be characterized by the appearance of both the incommensurate wave with q_I = <1/(4+δ) 1/(4+δ) 0>_c and the commensurate wave with q_II = <1/4 0 0>_c in the normal-Imma structure. In addition, the appearance of the two modulation waves could also produce the superlattice reflections at the <1/4 1/4 1/4>_c-, <1/2 0 0>_c-, and <1/2 1/2 0>_c-type positions in the reciprocal lattice. From the comparison with the experimental data obtained in this study, our modulated-structure model seems to be appropriate for the IM state in the vicinity of the PbZrO₃-type/IM state boundary.

Abstract: The highly-correlated electron system Sr_1-xSm_xMnO₃ (SSMO) with the simple-perovskite structure has been found to exhibit fascinating electronic states accompanying antiferromagnetic and ferromagnetic orderings. It was, in particular, reported that the electronic state for 0.46 ≤ x ≤ 0.54 was characterized by the coexistence state consisting of the A-type antiferromagnetic and ferromagnetic states. However, the features of the coexistence state in this Sm-content range have not been understood yet. We have thus investigated the crystallographic features of prepared SSMO samples with 0.46 ≤ x ≤ 0.55, mainly by transmission electron microscopy. As a result, all prepared SSMO samples were first confirmed to have the orthorhombic-Pnma structure at 300 K. When the temperature was lowered from 300 K, in the case of x=0.47, the disordered-Pnma state was found to be transformed into an orbital-modulated (OM) state accompanying an incommensurate modulation. The notable feature of the OM state is that the state becomes unstable with increasing Sm contents at 100 K. In other words, the OM state was never changed into the CE-type state with the orbital and charge modulations. In addition, no orbital-ordered state for the A-type antiferromagnetic ordering was also found for 0.46 ≤ x ≤ 0.55.

Abstract: One of the main challenges during continuous casting of microalloyed steel is to avoid the formation of transverse surface cracks on the steel slabs. These cracks occur due to severe mechanical and thermal stresses in the strand during the straightening operation. The reason for this phenomenon is a ductility loss of austenite in a typical temperature range of 700°C - 1100°C. One of the main mechanisms reducing the ductility is the precipitation of carbides and nitrides. In this work, we correlate ductility loss and precipitation state accompanying two model cooling strategies in a microalloyed steel grade. Continuously cooled samples show a minimum of ductility at temperatures around 750°C. With increasing temperature, deformability recovers again to reach full ductility again at 950°C. In contrast, samples treated with a fast intermediate cooling and reheating show constant low ductility in this entire temperature range. A transmission electron microscopy (TEM) investigation shows nanometer-sized NbC precipitates in the low ductility states. In contrast, in the samples with high ductility, larger NbC precipitates with lower number densities are observed. The experimental results show a good accordance with corresponding precipitation kinetics simulations carried out with the MatCalc software package.

Abstract: The main aim of this study is to analyze the effect of the severe plastic deformation (SPD) on the mechanical properties and defect structure of metastable beta Ti alloys. Experiments were performed on two different β-Ti alloys: Ti-15Mo and Ti-6.8Mo-4.5Fe-1.5Al which were subjected to severe plastic deformation (SPD) by high pressure torsion (HPT). The increase of hardness with increasing equivalent strain was determined by microhardness mapping. Dislocation density was studied by advanced techniques of positron annihilation spectroscopy (PAS). Microhardness and dislocation density increases with increasing equivalent strain inserted by severe plastic deformation.

Abstract: Cast iron is an iron alloy mainly composed of carbon and silicon, the amount of carbon is more than 2.1 mass%. Cast irons, gray cast iron and ductile cast iron, have been used as industrial parts and automobile parts widely because they have a good wear resistance and an excellent machinability. Graphite formation mechanism have been proposed, but, it is not established clearly yet. In this study, the microstructure of flake graphite was investigated to reveal the graphite formation mechanisms using FC250 alloy. Transmission electron microscopy (TEM) samples were prepared using focused ion beam (FIB). In the case of a cross section of flake graphite taken perpendicular to its elongated direction using TEM, internal microstructure of flake graphite was observed layered structure. In the case of a cross section of flake graphite taken parallel to its elongated direction, clear microstructure was not observed. Selected area electron diffraction (SAED) from flake graphite showed <0001> direction of graphite are mostly parallel to their thickness.

Abstract: In the highly-correlated electron system Sr_1-xNd_xMnO₃ (SNMO) having the simple perovskite structure, there are interesting electronic states, which are related to degrees of charge, orbital, and spin freedoms for e_g electrons in Mn ions. Among these states, in the case of SNMO, the C-, A-and CE-type antiferromagnetic states were reported for 0.20 ≤ x ≤ 0.38, for 0.38 ≤ x ≤ 0.48, and for 0.48 ≤ x ≤ 0.52, respectively. The points to note here are that these antiferromagnetic states are directly associated with corresponding orbital orderings, and that the CE-type state also accompanies charge ordering. Because of these features, we were interested in the (A → CE) state change in SNMO. The crystallographic features of prepared SNMO samples with 0.46 ≤ x ≤ 0.50 have thus been investigated mainly by means of a transmission electron microscope equipped with a low-temperature holder. As a result, the state around 100 K for x = 0.48 was first understood to be identified as the Imma state, which includes a large number of orbital-modulated (OM) regions with an average size of about 10 nm. The feature of such regions is that the orbital modulation has an incommensurate periodicity and a charge modulation is absent in them. On the other hand, the CE-type state having the commensurate orbital and charge modulations was also confirmed to be present for x = 0.50. In addition to these two states, the state around 100 K for x = 0.46 was found to be characterized by the coexistence state consisting of the C-type orbital-ordered state and the Imma states including OM regions. In other words, the presence of the A-type orbital-ordered state could not be confirmed in the temperature range between 300 K and about 100 K for 0.46 ≤ x ≤ 0.50 in this study.

Abstract: In Bi_1-xSm_xFeO₃ (BSFO) having the multiferroic BiFeO₃ as an end material, when the Sm content increases from x = 0, it has been reported that the ferroelectric-R3c state is changed into the paraelectric-Pnma state around x = 0.14. The R3c/Pnma state boundary around x = 0.14 can be regarded as a temperature-independent morphotropic-phase boundary (MPB). The notable feature in BSFO is that, in addition to these two states, the antiferroelectric PbZrO₃-type state was also found in the vicinity of the MPB. Although the PbZrO₃-type state appears as a modulated structure, its detailed features have not been understood yet. We have thus examined the crystallographic features of prepared BSFO samples around x = 0.14, mainly by transmission electron microscopy. The PbZrO₃-type state was confirmed to be present in samples with x = 0.15 on the basis of x-ray powder diffraction profiles measured from prepared samples at 300 K. On the other hand, the observation made by transmission electron microscopy indicated that the state for x = 0.15 is characterized by a coexistence state consisting of the ferroelectric-R3c and antiferroelectric PbZrO₃-type states. In particular, the crystal structure of the PbZrO₃-type state could be identified as a modulated structure with two transverse modulation waves, whose wave vectors are given by q₁ = [1/2 0 0]_o and q₂ = [0 1/2 0]_o in the orthorhombic-Pnma notation. In addition, eigenvectors of these two transverse waves were also determined to be parallel to the same [001]_o direction.

Abstract: We studied three series of Z-phase strengthened steels using scanning electron microscopy, transmission electron microscopy, and atom probe tomography to reveal the detailed microstructure of these steels. In particular, the phase transformation from M(C,N) to Z-phase (CrMN) was studied. Carbon content in the steels is the governing factor in this transformation. The impact toughness of some test alloys was rather low. This is attributed to the formation of a continuous W-rich film along prior austenite grain boundaries. Cu and C addition to the test alloys changed Laves phase morphology to discrete precipitates and improved toughness dramatically. BN particles were found in some steels. Formation of BN is directly linked to the B concentration in the steels.

Abstract: The effects of Temperature-Strain-Time parameters at the Thermomechanical Processing (TMP) of austenitic, duplex and pearlitic structural steels on the mesostructure formation has been studied based on the laboratory, industrial experiments and TEM analysis. The fragmented dislocation substructure observed in steels with a different carbon, nitrogen, titanium, niobium content as well as recrystallization gave evidence that TMP effects the work-hardening and softening behaviour. The problem of mesostructure appearing in various steels and alloys due to various modes of TMP used hot and hot-warm deformation is discussed. The role of plastic strain in the formation of mesostructure and the relation between the changes in the crystal structure due to TMP and the mechanical properties of the steels are considered.