TY - JOUR
AU1 - Kishita,, Keisuke
AU2 - Sakai,, Hisashi
AU3 - Tanaka,, Hiromochi
AU4 - Saka,, Hiroyasu
AU5 - Kuroda,, Kotaro
AU6 - Sakamoto,, Masayuki
AU7 - Watabe,, Akira
AU8 - Kamino,, Takeo
AB - Abstract 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. environmental TEM, gas reaction, high temperature, high resolution, oxidation, reduction Introduction As part of the development efforts conducted on materials in recent years, material control and fabrication have been performed at the nanometer level for a wide variety of materials such as catalysts and fuel cell materials, leading to the discovery of novel properties or methods for the prevention of degradation of the material. In the study of the mechanism and inherent properties of such materials, a transmission electron microscope (TEM) is a powerful tool for the direct observation of the material structure at the nanometer level, as well as for elemental analyses when used in combination with various spectroscopes. In general, a TEM specimen is investigated at room temperature and at a high vacuum (at 10−4 Pa or higher). In analysis of a given material, specimens before and after a change point (for example, before and after degradation) are observed and compared to draw inferences on the mechanism of change in the material. In the development of a material, a critical task is to accurately study the process of change during its fabrication or degradation. There has been a strong demand for the development of in situ analysis techniques for dynamic analysis. In particular, given that in the field of automotive materials, many materials, such as emission treatment catalysts and fuel cells, are fabricated or used under gaseous atmospheres and high temperatures, the development of materials requires the accurate identification of changes in the material under such environmental conditions. Beyond automotive materials, for the study of various material mechanisms, so-called environmental TEMs that permit in situ observation under a variety of temperatures and gaseous atmospheres [1] have been devised and developed. The key point of an environmental TEM is technology that permits the electron gun chamber to be maintained at a high vacuum while the specimen is immersed in a gaseous atmosphere. Environmental TEMs that have been developed to date can be divided into two broad categories based on differences in the mechanism of gas injection. One category is the window type [2–4], and the other is the differential pumping type [5,6]. The former method, the window type, involves confining a gas around the specimen using electron-transparent membranes, such as carbon membranes. As such, while it can attain a relatively higher pressure atmosphere (near atmospheric pressure) than the latter type, and thus does not require extensive modifications to the TEM apparatus, it suffers from the drawback of a reduced image resolution due to the use of membranes. In contrast, the differential pumping type involves maintaining a gaseous environment with an injected gas in the entire specimen chamber by means of vertically arranged orifices and a vacuum pump. As such, while offering the advantage of high image resolution, the method requires extensive modifications to the apparatus and presents a limit beyond which high pressures cannot be attained. In addition, either method is disadvantaged when high temperatures are required. The window type is not amenable to heating to high temperatures due to the heat susceptibility of the O-rings that are used for gas sealing. The differential pumping type uses a furnace for heating that is installed around a 3-mm-diameter specimen-holding grid. However, due to the influence of the thermal expansion and contraction of the specimen-holding grid and the water-cooling mechanism provided around the heating furnace, most of the specimen heating holder is not an adequate tool for performing observation in a stable manner at a high resolution. In contrast to these conventional technologies, a specimen holder has been developed [7] that provides high image stability at high temperatures and is capable of operating under a gaseous atmosphere. This holder has been used to observe the gas reaction behavior involving several materials [8]. The developed technology does not use membranes. Since the gas that is injected is discharged by means of differential pumping in the TEM system, it can be considered a differential pumping-type system. In the present research, a new environmental TEM system and gas-supply unit have been developed using this type of holder as a basic component, and featuring an improved differential pumping capacity for the TEM main unit and mounting a new gas injection mechanism. The technology permits high-resolution in situ observation, as well as repeatable and comparative observation based on its high condition reproducibility. This technology can be used to study the nature of material phenomena leading to the discovery of new knowledge. Methods—development of instruments Figure 1 shows the components of the developed instruments of a specimen holder, the main TEM unit, a heating control system, a gas control system, a video image capture system and an analytical instrument (EDX/EELS). A detailed description of these components is as follows. Fig. 1 Open in new tabDownload slide Total view of the developed in situ TEM system. Fig. 1 Open in new tabDownload slide Total view of the developed in situ TEM system. Specimen holder and heating control system Figure 2 shows an external view of the specimen holder [7] that represents the framework of the newly developed technology, as well as a schematic diagram of the specimen-mounting unit (tip of the holder), respectively. In the figure, the heating element is a spiral-shaped tungsten wire. For an observation specimen, a powder specimen is applied to the wire. The reaction gas is introduced from the gas injection unit (a nozzle) that is provided on the specimen holder (Fig. 3). Fig. 2 Open in new tabDownload slide Gas injection and heating specimen holder. (a) Overview of the specimen holder. (b) Tip of the specimen holder. Fig. 2 Open in new tabDownload slide Gas injection and heating specimen holder. (a) Overview of the specimen holder. (b) Tip of the specimen holder. Fig. 3 Open in new tabDownload slide Schematic illustration of the basic construction of the specimen holder. Fig. 3 Open in new tabDownload slide Schematic illustration of the basic construction of the specimen holder. The heating element has a mechanism wherein a tungsten wire in the order of 25 μm in diameter, developed by Kamino et al. [9,10], is directly heated by a current supplied from a battery. The mechanism provides a high degree of reproducibility of temperature conditions. It permits specifying a temperature by means of a current/temperature calibration line and facilitates a rapid change in the temperature (Fig. 4). The heating element, which has been employed in the near-atomic level observation of solid–solid reactions and solid–liquid reactions [11–16], provides high image stability at temperatures exceeding 1200°C or more. The Lorentz force generated by the lens magnetic field (in the Z direction) and the heating current (in the X direction) of the heater wire prevents the wire from slacking due to thermal expansion during heating (Fig. 5), and it remains stretched in the horizontal direction (Y direction). For these reasons, the specimen position undergoes a minimal drift in the Z direction during observation conducted at high temperatures and during cooling, which permits high-resolution observation without refocusing. Fig. 4 Open in new tabDownload slide Temperature control system. (a) Schematic illustration. (b) Correlation between the heating current and temperature. Fig. 4 Open in new tabDownload slide Temperature control system. (a) Schematic illustration. (b) Correlation between the heating current and temperature. Fig. 5 Open in new tabDownload slide Feature of the heating mechanism of the holder. Fig. 5 Open in new tabDownload slide Feature of the heating mechanism of the holder. The reaction gas is introduced to the area around the specimen by means of a gas injection nozzle with an inner diameter of 0.5 mm and an injection inlet that extends to the tip of the holder. Also, the reaction gas is adjusted by means of a gas injection unit (see the section ‘Gas control system’) provided externally to the holder. Figure 6 shows an example of the Monte Carlo simulation of the gas pressure distribution around the specimen in the specimen chamber when the gas is injected to specimen chamber. In the figure, the horizontal scale represents the horizontal distance from the nozzle outlet (specimen), and the vertical axis, the pressure. The pressure around the specimen is ∼10 Pa when oxygen is injected at the rate of 103 kPa cc min−1. The figure clearly shows that on this holder, a gaseous atmosphere is formed only in the local space in the vicinity of the specimen and that the influence of the atmosphere on the entire specimen chamber can be minimized, which demonstrates the possibility of achieving a higher pressure atmosphere near the specimen without increasing the pressure of other area. Fig. 6 Open in new tabDownload slide Results of the Monte Carlo simulation of the injected gas pressure distribution around the specimen. Fig. 6 Open in new tabDownload slide Results of the Monte Carlo simulation of the injected gas pressure distribution around the specimen. TEM main unit For the TEM on which the holder described in the section ‘Specimen holder and heating control system’ was mounted, the H9500 (LaB6: 300 kV) was used. In this study, the H9500 analytical TEM was equipped with a differential pumping system. As shown in Fig. 7, there are two vacuum stages developed in the upper portion (stages 2 and 3) of the specimen chamber and one vacuum stage in the lower portion (stage 1), using a turbo molecular pump with a pumping speed of 260 L s−1. Fig. 7 Open in new tabDownload slide Schematic illustration of the differential pumping system. Fig. 7 Open in new tabDownload slide Schematic illustration of the differential pumping system. Gas control system The use of the developed holder, when estimated based on gas injection Monte Carlo calculations, requires the gas flow rate to be controlled from several thousands of kPa cc min−1 in order to set the pressure inside the specimen chamber to several Pa. In this system, the amount of gas injection is controlled through the use of a pressure-regulated tank with pressure control at negative pressure values. Figure 8 shows a schematic diagram of the gas injection system. For each gas line containing oxidation, reduction and inert gases, respectively, a pressure-regulated tank is set up anterior to the specimen chamber. The tank pressure can be set to any pressure level from 10−2 to 104 Pa by means of a flow rate control system and a vacuum pump. The tank pressure is regulated by means of a PC control system. Fig. 8 Open in new tabDownload slide Gas control system. Fig. 8 Open in new tabDownload slide Gas control system. The TEM gas conditions are controlled with the pressure equilibrium between the pressure-regulated tanks and the specimen chamber. Figure 9 shows the relationship between the pressure of the pressure-regulated tanks and that of the specimen chamber. The pressure of the specimen chamber is shown as the function of the pressure of the pressure-regulated tank. As can be seen in the figure, the pressure of N2, O2 and H2 gases at the specimen chamber can be precisely reproduced by controlling the pressure-regulated tank, and the pressure can be set in an arbitrary manner. Fig. 9 Open in new tabDownload slide Pressure of the specimen chamber as a function of the pressure of the pressure regulated tank. Fig. 9 Open in new tabDownload slide Pressure of the specimen chamber as a function of the pressure of the pressure regulated tank. The system enables us to change the pressure of the specimen chamber in a few minutes. Further, since the pressure in the pressure-regulated tank can be held at all times, the pressure inside the specimen chamber can be maintained continuously at a constant level in a manner unaffected by the residual pressure in the gas cylinder. In this system, a variety of gases including oxidizing gases (O2, air, etc.), reducing gases (H2, CO, etc.) and inert gases (N2, Ar, and He) can be injected into the specimen chamber. The gas switching valve, shown in Fig. 8, can also be operated from a PC. Figure 10 shows the change in the pressure of the specimen chamber as a function of the time after the start and stop of the O2 and N2 gas injection. The system allows switching of gases in <10 min and this unique feature can be used for the observation of structural changes in the materials under repeated oxidation and reduction conditions, for instance. Fig. 10 Open in new tabDownload slide Change of the pressure of the specimen chamber as a function of time. Fig. 10 Open in new tabDownload slide Change of the pressure of the specimen chamber as a function of time. In this microscope, the gas pressures in the specimen chamber and the electron gun chamber are continuously monitored by means of installed pressure gauges, and the unit contains a system that automatically saves the gas conditions in the PC. The results in Figs. 9 and 10 represent data that are extracted by means of the data-saving system. The use of the saved data facilitates the comparison of the observation results of the change process with the experimental conditions, upon completion of observation. Video image acquisition system For the acquisition of video images, the ERB digital imaging CCD camera made by AMT Corp. (USA) was adopted. The camera is configured so that images can be recorded on DV tape, DVD or hard disk through the extraction of NTSC-based signals. When data are acquired, the date and time of the experiment conducted as well as time values measured from the start of the experiment are recorded on the video images using the VGT-55D made by FOR-A Co. (Japan). Analytical instruments As analytical instruments, the system incorporates an energy dispersion X-ray spectroscopy (EDX) unit (Genesis XM2) made by EDAX Inc., as well as an electron energy loss spectroscopy (EELS) unit (GIF Tridiem) made by Gatan Inc., providing a system capable of performing elemental analyses. Bulk specimen holder and gas injection Whereas the holder described in the section ‘Specimen holder and heating control system’ principally uses a powder specimen, this system can also accept a bulk specimen holder (Fig. 11), which permits the observation of specimens using a metal grid. This holder has been applied to the analysis of a wide variety of materials to date. The holder is equipped with a double-tilt function but a gas injection nozzle is not built in. When the holder is used, gas pressure is controlled by means of a nozzle (not shown in the figure) extending from outside of the column to near the specimen which permits observation of the specimen in the same gaseous state as in the case of the holder depicted in the section ‘Specimen holder and heating control system’. Fig. 11 Open in new tabDownload slide Specimen heating holder developed for the heating of the standard TEM grid. Fig. 11 Open in new tabDownload slide Specimen heating holder developed for the heating of the standard TEM grid. Time-series observation using reaction chambers While the system configuration described in the above subsections permits in situ observation and analysis (specimen temperature: 1200°C or higher, pressure in the specimen chamber of ∼10 Pa) when a sufficiently high pressure is attained in the specimen chamber, the electron scattering by the gas layer diminishes the inherent resolution of the TEM. In view of this problem, there are two chambers for the ex situ experiment The heating method employed, however, is the same as that described in the previous subsections. One reaction chamber uses a specimen holder pre-evacuation chamber; the other reaction chamber is an external reaction chamber. In the former, a gas injection port is provided in the pre-evacuation chamber during the introduction of the specimen holder, which permits heating and gas treatment (equivalent to the atmospheric pressure) in the pre-evacuation chamber (Fig. 12). Since the chamber is a part of the TEM, the specimen can be introduced to the specimen chamber right after the reaction without exposing to air atmosphere. Fig. 12 Open in new tabDownload slide Gas-injected specimen pre-evacuation chamber. Fig. 12 Open in new tabDownload slide Gas-injected specimen pre-evacuation chamber. The latter is an external reaction chamber on which a specimen holder can be set (Fig. 13). Although this unit requires the transport of the specimen holder from the TEM column to the chamber, it does not require the removal of the specimen from the specimen holder for heating and gaseous atmosphere treatment. The external reaction chamber can handle a wide range of gas species and the setting of gas pressures from 10−2 to 105 Pa. Fig. 13 Open in new tabDownload slide Outside ex situ gas injection chamber. Fig. 13 Open in new tabDownload slide Outside ex situ gas injection chamber. These units allow the observation of unaffected phenomena by the electron beam in processing under the atmospheric pressure; in some situations, this technique alone can be used to uncover valuable information. Ex situ observation can be conducted in combination with in situ observation to provide a reliable analysis of structural change processes undergone by various gas treatments. Results and discussion This section describes examples of observation, and observation conducted through the effective use of the reproducibility of conditions. Platinum on alumina An observation was conducted during oxygen injection using platinum on alumina. First, the specimen was heated to ∼750°C in a vacuum (∼10−5 Pa) and held in that condition. There was no significant change in the structure, and the structure of the specimen was maintained intact. In this process, it is clear that the specimen can be retained in a stable manner at a high magnification. When oxygen with a measured specimen chamber pressure of 1 Pa was injected into the specimen chamber, as a change that appeared to be attributable to the influence of the gas, it was confirmed that the Pt particles on the alumina migrated and sintered with other Pt particles (particles 1 and 2 in Fig. 14). In this process, even at the high temperatures and under gaseous atmospheres, the image was extremely stable. Fig. 14 Open in new tabDownload slide Observation of platinum particles on Al2O3 (1 Pa O2, 1123 K). Fig. 14 Open in new tabDownload slide Observation of platinum particles on Al2O3 (1 Pa O2, 1123 K). Platinum on carbon Using platinum (Pt) on carbon, observation was conducted during oxygen injection. For carbon, two types of graphite nanofibers were used: a ribbon type in which the graphite layer is oriented parallel to the fiber axis, and a platelet type in which the graphite layer is stacked vertically with respect to the fiber axis. When specimens were heated to ∼300°C in a vacuum (∼10−5 Pa), none of them changed to carbon, and the structure was retained intact (Figs. 15a and 16a). Fig. 15 Open in new tabDownload slide Graphite nanofiber (ribbon-type) with platinum particles (10−3 Pa O2, 573 K). Fig. 15 Open in new tabDownload slide Graphite nanofiber (ribbon-type) with platinum particles (10−3 Pa O2, 573 K). Fig. 16 Open in new tabDownload slide Graphite nanofiber (platelet-type) with platinum particles (10−3 Pa O2, 573 K). Fig. 16 Open in new tabDownload slide Graphite nanofiber (platelet-type) with platinum particles (10−3 Pa O2, 573 K). When oxygen with a measured specimen chamber pressure of 10−3 Pa was introduced, a structural change occurred, apparently due to the combustion of carbon. In conjunction with that carbon change, the Pt particles on the carbon migrated and some particles sintered each other (Figs. 15b and 16b). In the case of the ribbon type, the carbon structure was retained for 30 min or more, and it is clear that the platinum migrated extremely slowly. In contrast, in the case of the platelet type, the platinum migrated extensively, and the carbon structure collapsed in ∼5 min. These phenomena were reproduced in several experiments conducted on different days, clearly demonstrating the high reproducibility of experimental conditions. Detailed analyses are being pursued on an on-going basis, the results of which will be reported separately. The above observation examples represent only some of the trails performed. However, it is clear that the developed system permits extremely stable observation when comparing different specimens under the same conditions. Furthermore, since the developed systems enable us to switch gas conditions, it is thought it will be an effective technology for identifying the reaction processes of various elements, such as repeated oxidation/reduction phenomena. Conclusions This newly developed TEM system permits the observation of changes in the material structure at high temperatures (1200°C or higher) and under gaseous atmospheric (10 Pa or less of the specimen chamber) conditions, as well as elemental analyses. With the newly developed temperature control and gas injection units, the temperature and pressure conditions can be controlled with a high degree of reproducibility, offering the potential for capturing change points during a reaction process. In addition, by mounting a spectrometer, the system can also be deployed to perform analyses. Further more, in combination with an external reaction chamber, the technology can also be used to discover new knowledge. It is intended to apply this technology to various automotive materials, and observe and analyze the mechanism of material reactions at high temperatures and under gaseous atmospheres at the near-atomic level so that the nature of the materials studied can be brought to bear on the development of new materials. It is also hoped that camera technology will make it possible to analyze the mechanism of gas–solid reactions in more details. References 1 Butler E P , Hale K F . , Dynamic Experiments , 1981 Amsterdam North Holland Google Preview WorldCat COPAC 2 Kawasaki T , Ueda K , Tanaka H , Tanji T , Ichihashi M . In-situ observation of gold nano-particle catalysts by high-resolution closed-type environmental-cell transmission electron microscope , Microsc. 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TI - Development of an analytical environmental TEM system and its application
JF - Journal of Electron Microscopy
DO - 10.1093/jmicro/dfp028
DA - 2009-12-01
UR - https://www.deepdyve.com/lp/oxford-university-press/development-of-an-analytical-environmental-tem-system-and-its-uJ7hWEK2Yq
SP - 331
EP - 339
VL - 58
IS - 6
DP - DeepDyve
ER -