Akira Watabe

Development of an analytical environmental TEM system and its application

Many automotive materials, such as catalysts and fuel cell materials, undergo significant changes in structure or properties when subjected to temperature change or the addition of a gas. For this reason, in the development of these materials, it is important to study the behavior of the material under controlled temperatures and gaseous atmospheres. Recently, a new environmental transmission electron microscope (TEM) has been developed for observation with a high resolution at high temperatures and under gaseous atmospheres, thus making it possible to analyze reaction processes in details. Also, the new TEM provides a high degree of reproducibility of observation conditions, thus making it possible to compare and validate observation of various specimens under a given set of conditions. Furthermore, easiness of gas condition and temperature control can provide a powerful tool for the studying of the mechanism of material change, such as oxidation and reduction reactions.

Development of a technique for in situ high temperature TEM observation of catalysts in a highly moisturized air atmosphere.

To clarify the influence of moisture on the structural changes of heated nano materials, in situ high temperature transmission electron microscopy (TEM) has been carried out using a conventional analytical TEM combined with a gas injection-specimen heating holder. Air with high moisture content, above 94% relative humidity (RH), from a humidifier was directly injected onto the heated platinum catalyst dispersed on carbon black (Pt/CB), and the morphological changes of the specimens were observed at high magnification dynamically. The result of the experiment was compared with a result obtained from an experiment using air with a low moisture content, 34% RH. Active movement of the Pt particles, leading agglomeration and grain growth, occurred prior to degradation of the CB support at high moisture content. In contrast, the degradation of the CB support leading agglomeration and grain growth of the Pt particles occurred before the displacement of the Pt particles on the CB supports in a low humidity environment.

In situ TEM observation of solid-gas reactions

Under a gaseous atmosphere at high temperatures, almost all the materials (metal, catalysts, etc.) change their structures and properties. For the research and development of materials, it is of vital importance to clarify mechanisms of solid-gas and liquid-gas reactions. Recently an in situ TEM system combined with an environmental holder, which has a gas injection nozzle close to a specimen-heating element, has been developed. The gas injection nozzle permits gas to flow around the specimens sitting on the heating element made of a fine W filament. The newly developed in situ TEM has a differential pumping system; therefore, the pressure in the specimen chamber is maintained in the range of higher than 1 Pa, while the pressure in the electron gun chamber can be kept in the range of 10-5 Pa. This system was applied to in situ observation of chemical reactions of metals with gases: Observation of oxidation and reduction under a gas pressure ranging from 10-5 Pa to 1 Pa at high temperatures (room temperature to ~1473 K) were successfully carried out on pure metal and rare metal catalysts at near-atomic resolution. This in situ environmental TEM system is promising for clarifying mechanisms of many solid-gas and liquid-gas reactions that take place at high temperatures under a gas atmosphere.

Design of a 300-kV gas environmental transmission electron microscope equipped with a cold field emission gun

A new in situ environmental transmission electron microscope (ETEM) was developed based on a 300 kV TEM with a cold field emission gun (CFEG). Particular caution was taken in the ETEM design to assure uncompromised imaging and analytical performance of the TEM. Because of the improved pumping system between the gun and column, the vacuum of CFEG was largely improved and the probe current was sufficiently stabilized to operate without tip flashing for 2-3 h or longer. A high brightness of 2.5 × 10(9) A/cm(2) sr was measured at 300 kV, verifying the high quality of the CFEG electron beam. A specially designed gas injection-heating holder was used in the in situ TEM study at elevated temperatures with or without gas around the TEM specimen. Using this holder in a 10 Pa gas atmosphere and specimen temperatures up to 1000°C, high-resolution ETEM performance and analysis were achieved.

Environmental Transmission Electron Microscopy Using a Conventional TEM and a Gas Injection-Specimen Heating Holder

In response to the increasing demands from material science, we have developed a gas injection specimen heating holder which allows in-situ observation of gas reacted materials at near atomic resolution at very high temperatures in a conventional TEM. The gas injection nozzle was built in the specimen heating holder and a spirally wound tungsten wire of 25 micrometer in diameter was used as the heating element[1-2]. Specimens sized in the order of micrometers are directly mounted on the heating element and nanometer sized specimens are distributed on a carbon coated heating element. The microscope used in the study was a Hitachi H-9500 300kV high resolution TEM. Spherical aberration co-efficient (Cs) and chromatic aberration co-efficient (Cc) of the microscope are 0.6mm and 1.4mm, respectively. Since the specimen holder was designed not to give any interference to TEM image even at very high temperatures, this performance is thoroughly exploited for high resolution TEM observation of structural changes of specimens during gas reaction. AMT ERB high resolution digital imaging CCD camera system has been attached to the TEM and used for recording TEM images of dynamic behavior of the specimens. The specimen area and the electron gun area of the microscope are constantly evacuated by a 360l/s turbo molecular pump and a 60l/s ion pump, respectively, and the two regions are separated by a 0.3mm diameter fixed aperture located at the middle of the condenser system of the microscope. Thanks to these advanced vacuum technologies, the pressure of the electron gun chamber could be kept in the range of 10Pa while the pressure at the specimen area was maintained at the range of 10Pa. This allows observation of high resolution TEM images using a LaB6 cathode at the accelerating voltage of 300kV. The specimen heating holder with a gas injection nozzle and a carbon film coated heating element are shown in Fig. 1 a and b, respectively. The gas injection specimen heating holder is shown in Fig. 1. Pressures in the specimen chamber and the electron gun of the microscope as a function of the gas flow rate of the specimen heating holder is shown in Fig. 2. Example of dynamic HREM images observation of a growth of cubic-In2O3 at 373K in the air atmosphere of 4.0x10 Pa is shown in Fig.3. The lattice fringes of In2O3 (440) with the distance of 0.179nm are clearly obtained in the video images. References [1] T. Kamino and H.Saka., Microanal. Microstruct. 4 (1993) 127-135 [2] T. Kamino et al., Mater. Trans. JIM 6 (1995) 73-75 Microsc Microanal 12(Supp 2), 2006 Copyright 2006 Microscopy Society of America DOI: 10.1017/S1431927606061940 766 CD