3 results
Observation of Microstructure of Grain Boundaries of ZnO Varistors using Backscattered-electron
- Masayuki Takada, Yuuki Sato, Shinzo Yoshikado
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- Journal:
- MRS Online Proceedings Library Archive / Volume 1108 / 2008
- Published online by Cambridge University Press:
- 01 February 2011, 1108-A12-04
- Print publication:
- 2008
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The impurities segregated at the grain boundary of ZnO grains, such as Bi2O3, are important factor to obtain the nonlinear voltage-current (V-I ) characteristics of a Bi-added ZnO varistor. The deterioration of V-I characteristics progresses with voltage application. It has been reported that one of the reasons for this deterioration is the movement of oxide ions and interstitial Zn2+ ions across grain boundaries and around the neighborhood of grain boundaries. Thus, the mobility of ions and the pathway of the current formed by voltage application strongly correlate with the structure of grain boundary, such as the crystal orientation of ZnO grains, the phase of Bi2O3 at the grain boundary, and segregated grains. In the present studies, the structure of grain boundary for the ZnO varistor has been evaluated by composition images obtained from energy dispersive X-ray spectrometry (EDS) with scanning electron microscope (SEM) or with transmission electron microscope (TEM). However, the elemental mapping obtained from EDS with SEM is low resolution and the distribution of elements is obscure. Furthermore, the observation of varistor with TEM is difficult because the sample used for TEM needs precise processing. On the other hand, a composition image with high contrast of tone can be easily obtained using back-scattered electron (BSE) detector with SEM, although BSE detector is lack in the quantitative analysis. In this study, to clarify the microscopic distribution of impurities at the grain boundary with simple method, we observed the fractured surface of ZnO varistor using BSE detector with SEM. For the fractured surface, two types of fracture exist; grain boundary fracture and transcrystalline fracture. The microscopic distribution of impurities can be obtained by observing the surface of grain boundary fracture. The characteristics of ZnO varistor deeply correlate with the state of impurities at the grain boundary such as Bi2O3. To clarify the distribution of these impurities, the fractured surface of Bi-Mn-Co-Sb-added ZnO varistor was observed using SEM, EDS, and BSE detector. It was found that the deposit of Bi2O3 as additive had two types of shape on the surface of grain boundary fracture; spot-like and sheet-like, for Bi-Mn-Co-Sb-added ZnO varistor. With dissolving SiO2 in Bi2O3, the surface tension of Bi2O3 decreased and the sheet-like deposit of Bi2O3 increased, while the spot-like deposit of Bi2O3 decreased. Moreover, when the sample was annealed at 700°C, the surface free energy (surface tension) decreased because Zn2+ and Bi3+ were speculated to become the compound such as Bi7.65 Zn0.35 O11.83 and move to the triple point or line.
Radiation of X-rays Using Uniaxially Polarized LiNbO3 Single Crystal
- Shinji Fukao, Yoshikazu Nakanishi, Tadahiro Mizoguchi, Yoshiaki Ito, Toru Nakamura, Shinzo Yoshikado
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- Journal:
- MRS Online Proceedings Library Archive / Volume 1034 / 2007
- Published online by Cambridge University Press:
- 01 February 2011, 1034-K11-11
- Print publication:
- 2007
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It is well known that by changing the temperature for the polarized hemimorphy single crystal, such as LiNbO3 or BaTiO3, the electric field with high intensity is generated and then atmospheric gas atoms or molecules around the crystal are ionized. Using these phenomena, X-rays could be radiated by the bremsstrahlung radiation of electrons in low pressure [1,2]. However, this method has some disadvantages. For example, it is difficult to maintain the intensity of X-rays for a long term. The gas pressure range, where the intensity of X-rays is high, is narrow. The purpose of this study is to increase the intensity of X-rays in a high vacuum. In a low vacuum, positive charges generated by the ionization of gas molecules near the crystal weaken the electric field strength. Consequently, the intensity of X-rays also becomes weak. On the other hand, in a high vacuum, the number of electrons decreases. Thus, thermally emitted electrons are supplied to the X-rays radiation system in high vacuum to increase and stabilize the intensity of X-rays.
The -z plane of the congruent LiNbO3 single crystal polarized in the z-axis direction of a 5 mm thickness and a 10 mm diameter was opposed to the Cu target of a 10 μm thickness placed at a distance of 21 mm from the –z plane in the gas pressure of 10-2-10-4 Pa. The temperature of crystal was changed between from -5 to 75 °C using Peltiert device. The temperature history of the crystal consists of a repetition of a series of the increasing and decreasing processes with the same period. Filament of thorium-added-tungsten as a thermal electrons source was placed at a distance of approximately 20 mm from the crystal side edge. DC current flowing in the filament was adjusted from 0 to 4 A.
In the increasing process of the temperature, the characteristic X-ray of Nb was radiated. This result indicates that the sign of net charge on -z plane of the crystal is positive. Because thermally emitted electrons are supplied to the positively charged –z plane, the electric field strength generated by the crystal is very low. Thus, the intensity of characteristic X-ray of Nb is low. On the other hand, in the decreasing process of the temperature, the characteristic X-ray of Cu was radiated. At the pressure of approximately 10-2 Pa and the filament current of 2.5 A, the intensity of X-rays showed the local maximum. If electrons are supplied more, synthetic electric field strength is weakened by the electric field made by the electron. The intensity of X-rays using thermal electron source was ten or more times higher at the maximum than that without the source and was almost comparable as the case of a low vacuum or more than it. Using thermal electron source, the intensity of X-rays increased with decreasing the pressure down to approximately 10-2 Pa and became constant at lower pressure.
Ozone Gas Generator Using Uniaxially Polarized LiTaO3 Single Crystal
- Nakanishi Yoshikazu, Junko Ide, Jun Kondo, Shinji Fukao, Katsumi Handa, Tatsunori Tochio, Yoshiaki Ito, Akikazu Tanaka, Shinzo Yoshikado
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- Journal:
- MRS Online Proceedings Library Archive / Volume 1034 / 2007
- Published online by Cambridge University Press:
- 01 February 2011, 1034-K10-62
- Print publication:
- 2007
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The phenomenon that a ferroelectrics crystal carries out intrinsic polarization by the temperature change generally is known. The Ozone gas generation was investigated due to a strong electric field of this crystal under atmospheric pressure . When we added a rapid temperature change to the crystal, the charge non–equilibrium occurs around the crystal. Oxygen is influenced due to the non-equilibrium in charge and ozone is considered to be generated. Therefore, we used the high electric field induced due to the polarization of a ferroelectrics crystal in order to produce the ozone under atmospheric pressure.
As a result, we were able to produce ozone of the density of 400ppb in a oxygen gas flow of 1.5 liters per minute using this simple system: The crystals (Yamaju Co. Ltd. and Sumitomo metal mining Co. Ltd.) are used in thickness of 3, 5, 7, 10, 20, and 30 mm with a diameter of 4 inches, respectively. They are poled crystals.
Experiments on the maximum temperature (300 degree), the temperature gradient (100 degree/10 minite), and substrate materials(Cu and Al), the thickness of the crystal, and z face etc. were carried out during the temperature of LiTaO3 single crystal from 20C to about 300C. It is found that the amount of the ozone production increases rapidly, when the maximum temperature of LiTaO3 single crystal is raised and have a relation with the thickness of the crystal. However, the amount of the ozone production doesn't closely related with the X-ray generation that we use the crystal for.
In the present study, when the thickness of the crystal became large, polarization voltage became high, but generated efficiency of ozone was not necessarily proportional to thickness.