博士論文概要

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Graduate School of Advanced Science and Engineering Waseda University

博士論文概要

Doctoral Thesis Synopsis

論文題目

Thesis Theme

Influence of Oxygen Partial Pressure and Al Content on the Resistivity and Piezoelectric Properties of Ca 3 TaGa 3-x Al x Si 2 O 14 Single Crystals

申請者 (Applicant Name)

Xiuwei Fu

フシュウエイ

( Department of Nanoscience and Nanoengineering, Research on Nano-structured Crystal Chemistry )

May, 2016

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High temperature (HT) piezoelectric sensors are highly desired for a broad range of applications in industry and science due to their fast response, high sensitivity and compactness. For example, HT piezoelectric pressure sensors are demanded for automotive engines with the purpose of optimizing the combustion efficiency and minimizing the environmental harmful gases like CO2 and NOx. However, conventional piezoelectric materials present different drawbacks for HT applications, like structural transformations, pyroelectricity, instability under oxidizing or reducing atmospheres, and difficult single crystal growth. As alternative, the langasite family is attracting much attention. The compounds of this family are characterized by the absence of pyroelectricity and phase transition up to their melting point (1300-1500°C), and a high piezoelectric d11 value (2~3 times larger than that of α-quartz). Further, the langasite compounds can be grown by the standard Czochralski (Cz) technique, so that the production of large-size crystals is feasible. Among the langasite family, La3Ta0.5Ga5.5-xAlxO14 (LTGA) has been widely investigated for HT applications, and even 4 inch single crystals are grown industrially. However, for HT applications, the less-known Ca3TaGa3Si2O14 (CTGS) is of particular interest recently due to its relatively high resistivity at HT. A high resistivity is fundamentally important for HT sensing applications, in order to reduce the electrical losses and to improve the signal/noise ratio of sensors. The resistivity of oxide crystals is generally affected by the oxygen partial pressure during growth, however, this could not be investigated in the case of langasites since so far all these crystals have been grown using Ir crucibles. Besides the resistivity, a high piezoelectric coefficient d11 is required. CTGS presents a very stable structure, each cation fitting very well to its corresponding cationic site. Therefore, by replacing a single cationic site with another cation of different ionic radius, the lattice will be distorted in a way that an increase of d11 value can be expected. The partial substitution of Ga with Al, which has a smaller ionic radius, i.e. the growth of mixed Ca3TaGa3-xAlxSi2O14 (CTGAS) crystals, has been reported to be difficult due to the appearance of severe cracks and inclusions. As a result, the effect of Al content has not been systematically studied yet. From these viewpoints, the objectives of this thesis are defined as follows: (1) the growth of high quality mixed CTGAS single crystals within the full composition range; (2) the study of the influence of the oxygen partial pressure during growth on the electrical resistivity at HT and (3) the first systematical investigation of the effect of Al content in the resistivity and the piezoelectric coefficient. All properties are analyzed as a function of the temperature in order to elucidate the potential of CTGAS crystal for HT sensor applications.

Chapter 1 overviews the well-known candidates for HT piezoelectric sensor applications in detail. The advantageous characteristics of the langasite family, and the state-of-the-art of these compounds are also analyzed and reviewed. Furthermore, the objectives and approaches of the present work are described.

Chapter 2 describes the experimental procedures. The equipment used for growth, sample preparation and characterization is introduced. In addition, the theoretical background for the determination of dielectric and piezoelectric parameters, based on the resonance method, is also presented in detail.

Chapter 3 is a preliminary study of the LTGA. This crystal serves as a reference, since it is the langasite compound which is nowadays considered for HT pressure sensors. A LTGA single crystal has been grown by the Cz technique under N2+1% O2 atmosphere. High temperature X-ray diffraction measurements indicate the

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absence of phase transitions up to 1300°C. The thermal properties of the crystal, including thermal expansion, specific heat, thermal diffusivity and thermal conductivity, are investigated as a function of temperature. The resistivity of grown crystal at 400°C is 6.7×10⁷ Ω cm.

Chapter 4 studies the growth conditions necessary for the incorporation of Al on the Ga site. It is found that to avoid cracking and the segregation of ternary compounds in the coolest central part, it is very important to achieve a well homogenized melt, as well as to create an extraordinary low temperature gradient at the growth interface. The homogenization is achieved by a melt reheating and a long melt convection prior to growth. The low temperature gradient is obtained by the direct heating of grown crystal with a Pt after-heater. It is found that these conditions need to be improved for higher Al content, indicating the lower stability of the Al substituted crystals. Thanks to these approaches, fully substituted Ca3TaAl3Si2O14 (CTAS) single crystals are successfully grown. A single crystal diffraction measurement allows the determination of the interatomic distances, and by the bond valence sum calculation the instability of CTAS compared with CTGS is verified. While the effective valence of all cations in CTGS is close to the ideal, e.g. 2.97 for Ga, the effective valence of Al in CTAS is found to be as small as 2.64. This result confirms the effective distortion of the lattice with the Al incorporation.

Further, the influence of oxygen partial pressure during growth on the optical and electrical properties of CTAS is investigated. For it, CTAS single crystals have been grown by the Cz method with Ir crucibles, as well as Pt ones for the first time, under various oxygen partial pressures. Highly transparent, colorless CTAS single crystals are obtained for the first time by the growth with Pt crucibles, thus eliminating the defects related with the greenish coloration of crystals grown with Ir crucibles. However, the influence of these defects on dielectric and piezoelectric properties is found to be imperceptible within the experimental error. The electrical resistivity remarkably depends on the oxygen partial pressure during growth, while the activation energy doesn’t change, indicating just a change in the donor concentration. Crystals grown under oxygen-free atmosphere exhibit the highest resistivity, reaching a value of ~7×10¹⁰Ω cm at 400°C, which is three orders of magnitude higher than that of LTGA reference. These results suggest that oxygen vacancies do not play a role in the conductivity of CTAS crystals, since its concentration is expected to diminish with the increase of the oxygen partial pressure.

Chapter 5 describes the growth and properties of CTGS single crystals as a function of growth conditions, analogously to the case of CTAS in previous chapter. For it, CTGS single crystals have been grown by the Cz technique using Ir and Pt crucibles under various oxygen partial pressures. Again, colorless crystals are obtained with Pt-crucibles, indicating that the typical yellowish coloration of CTGS crystals is related with the use of Ir crucibles. In an analogous way to CTAS, only the resistivity is found to depend significantly on the oxygen partial pressure of the growth atmosphere, while its activation energy is approximately constant. The highest resistivity is found for crystals grown under oxygen-less (≤1%) atmosphere, reaching also values over 10¹⁰ Ω cm at 400°C.

Chapter 6 analyzes systematically for the first time the effect of Al content in mixed CTGAS crystals.

Taking into account the optimal growth conditions found in previous chapters for the extreme cases of CTAS and CTGS, namely Pt-crucible and N2+1% O2 atmosphere, mixed CTGAS crystals with a gradually varying Ga3-xAlx

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ratio (x=0~3) have been grown by the Cz technique. All grown crystals are scattering-free and visually colorless, in contrast to the typical coloration observed in crystals grown with Ir-crucibles. Transmittance spectra confirmed the absence of absorption peaks in the ultraviolet-visible range, while the band gap increases linearly with Al content, from 5.0 eV for CTGS to 5.5 eV for CTAS. The chemical composition of grown crystals is found to be coincident with the nominal melt composition, indicating that in this matrix the segregation coefficients of both Ga and Al are about one. This fact is very advantageous from the industrial point of view, since it makes feasible the growth of large size homogeneous CTGAS single crystals of any desired composition.

The lattice constants and density decrease linearly with Al content, in good agreement with the Vegard’s law. The effect of Al substitution is also investigated systematically in terms of electrical resistivity, dielectric and piezoelectric properties as a function of temperature. The activation energy of the electrical resistivity increases systematically with the incorporation of Al from 1.31 for CTGS to 1.44 eV for CTAS. In absolute terms, the resistivity of CTGAS crystals is similar, especially in the higher temperature range. At 400^oC, CTGS has the lowest resistivity with 3.0×10¹⁰ Ω cm, while CTAS doubles it with a value as high as 6.5×10¹⁰ Ω cm, i.e. almost three orders of magnitude higher than that of the LTGA reference. The d11 increases gradually with the Al incorporation, in accordance with the lattice distortion confirmed by the bond valence sum calculation. A theoretical analysis of the piezoelectric measurements indicates that this enhancement is mainly due to an increase in the coupling factor. In conclusion, CTAS exhibits the best performance, with the highest resistivity and the highest d11 among the CTGAS single crystals.

Chapter 7 summarizes the present work and derives conclusions. This thesis proves the successful growth of high quality bulk CTGAS single crystals (x=0~3). The growth instabilities related with the Al incorporation are fully overcome by an appropriate homogenization of the melt and a low temperature gradient.

The composition of crystals agrees with the nominal melt composition, so that homogeneous crystal of any Ga-Al ratio can be grown. Colorless crystals are obtained for the first time by the use of Pt crucibles, indicating that the color-related defects have been eliminated. However, the influence of these defects on electric and piezoelectric properties is found to be negligible. The oxygen partial pressure during growth is the parameter that determines the electrical resistivity of CTGAS crystal. The donor concentration increases continuously with the increase in oxygen partial pressure, suggesting that oxygen vacancies do not play any role in the conductivity of CTGAS crystals. Therefore, the growth under oxygen-less atmosphere is preferable to obtain highly resistive crystals; a resistivity of three orders of magnitude higher than that of reference LTGA is obtained. On the other hand, with the Al incorporation the lattice constants decrease linearly, whereas the band gap increases linearly.

The distortion of the lattice with the Al incorporation is verified by bond valence sum calculations. Furthermore, the activation energy of donors increases continuously with the Al content, as well as the piezoelectric coefficient d11, mainly due to a remarkable enhancement of the coupling factor with the increase in lattice distortion. Consequently, CTAS single crystal shows the best performance, exhibiting the highest resistivity and the highest d11 value at any temperature. Therefore, present results prove the high potential of CTAS for HT sensor applications and, as a next step, the production of prototypes will be undertaken.

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No.1

早稲田大学博士（工学）学位申請研究業績書

(List of research achievements for application of doctorate (Dr. of Engineering), Waseda University)

氏名 FU Xiuwei 印(seal or signature )

（As of May, 2016）種類別

(By Type)

題名、発表・発行掲載誌名、発表・発行年月、連名者（申請者含む）

(theme, journal name, date & year of publication, name of authors inc. yourself) (1) Paper

(2) Presentatio

n

○ 1. Thermal and Piezoelectric Properties of La3Ta0.5Ga5.1Al0.4O14 (LTGA) for High Temperature Sensors

Journal of Alloys and Compounds, 2015, 647, 1086-1090 (June 23, 2015) X.W. Fu, E. G. Víllora, Y. Oshima, K. Shimamura and N. Ohashi

○ 2. Influence of Oxygen Partial Pressure During Growth on Optical and Electrical Properties of Ca3TaAl3Si2O14 Single Crystals

Crystal Growth & Design, 2016, 16, 2151-2156 (February 11, 2016)

X.W. Fu, E. G. Víllora, Y. Matsushita, Y. Kitanaka, Y. Noguchi, M. Miyayama, K.

Shimamura and N. Ohashi

○ 3. Influence of Growth Conditions on the Optical, Electrical Resistivity and Piezoelectric Properties of Ca3TaGa3Si2O14 Single Crystals

Journal of The Ceramic Society of Japan, 2016, 124, 523-527 (May 1, 2016)

○ 4. Temperature Dependence of Electrical Resistivity, Dielectric and Piezoelectric Properties of Ca3TaGa3-xAlxSi2O14 Single Crystals As a Function of Al Content

Journal of Alloys and Compounds, 2016, 687, 797-803 (June 21, 2016)

1. Piezoelectric Langasite Single Crystals for High Temperature Sensors (Verbal Presentation), AY2013 Students Seminar: Kyushu University and Waseda University, Tsukuba, Japan.

(January 15, 2014)

X.W. Fu, E. G. Víllora, Y. Sugahara and K. Shimamura

2. Czochralski Growth and Characterization of Piezoelectric LTGA Single Crystals (Verbal Presentation), The 5^th NIMS/MANA-Waseda University International Symposium, Tsukuba, Japan (March 24, 2014)

3. Growth and Characterization of Piezoelectric Langasite Single Crystals for High Temperature Sensors (Verbal Presentation), AY2014 Students Seminar: Kyushu University and Waseda University, Tsukuba, Japan (January 14, 2015)

4. Influence of Growth Conditions on Optical, Electrical and Piezoelectric Properties of Ca3TaAl3Si2O14 Single Crystals (Verbal Presentation), The 6^th NIMS/MANA-Waseda University International Symposium, Tokyo, Japan (July 29, 2015)

X.W. Fu, E. G. Víllora, Y. Kitanaka, Y. Noguchi, M. Miyayama, K. Shimamura and N.

Ohashi

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No.2

早稲田大学博士（工学）学位申請研究業績書

種類別 By Type

題名、発表・発行掲載誌名、発表・発行年月、連名者（申請者含む）(theme, journal name, date & year of publication, name of authors inc. yourself)

5. Piezoelectric Langasite Ca3TaGa3-xAlxSi2O14 Single Crystals for High Temperature Sensors (Verbal Presentation),第22回材料科学会若手研究者討論会, Tokyo, Japan (August 26, 2015)

X.W. Fu, E. G. Víllora, Y. Kitanaka, Y. Noguchi, M. Miyayama, K. Shimamura and N. Ohashi 6. Piezoelectric Ca3TaAl3Si2O14 (CTAS) Single Crystals for High Temperature Sensors (Verbal

Presentation), JSAP-OSA Joint Symposia 2015 (The 76^th JSAP Autumn Meeting 2015), Nagoya, Japan (September 13-16, 2015)

X.W. Fu, E. G. Víllora, I. Sakaguchi, Y. Kitanaka, Y. Noguchi, M. Miyayama, K. Shimamura and N. Ohashi

7. Piezoelectric Ca3TaAl3Si2O14 Single Crystals for High Temperature Sensors (Verbal Presentation), 第45回結晶成長国内会議 (NCCG-45), Sapporo, Japan (October 20, 2015) X.W. Fu, E. G. Víllora, Y. Kitanaka, Y. Noguchi, M. Miyayama, K. Shimamura and N. Ohashi 8. Influence of Growth Conditions on Optical, Electrical and Piezoelectric Properties of

Ca3TaAl3Si2O14 Single Crystals (Verbal Presentarion), The 9^th International Conference on The Science and Technology for Advanced Ceramics (STAC-9), Tsukuba, Japan (October 19-21, 2015)

X.W. Fu, E. G. Víllora, Y. Kitanaka, Y. Noguchi, M. Miyayama, K. Shimamura and N. Ohashi 9. Ca3TaGa3-xAlxSi2O14 Single Crystals for High Temperature Piezoelectric Sensors (Verbal

Presentation), Japan electronic materials society (JEMS) 第52回講演大会, Tokyo, Japan (November 13, 2015)

X.W. Fu, E. G. Víllora, Y. Kitanaka, Y. Noguchi, M. Miyayama, K. Shimamura and N. Ohashi 10. Piezoelectric Langasite Single Crystals for High Temperature Sensors (Poster Presentation),

The 17^th US-Japan Seminar on Dielectric and Piezoelectric Ceramics, Matsumoto, Japan (November 15-18, 2015)

N. Ichinose, X.W. Fu, E. G. Víllora, Y. Kitanaka, Y. Noguchi, M. Miyayama, K. Shimamura and N. Ohashi

11. Resistivity of Ca3TaAl3Si2O14 As a Function of Oxygen Partial Pressure During Growth (Verbal Presentation), The 32^nd International Japan-Korea Seminar on Ceramics (JK-Ceramics 32), Nagaoka, Japan (November 15-18, 2015)

X.W. Fu, E. G. Víllora, Y. Kitanaka, Y. Noguchi, M. Miyayama, K. Shimamura and N. Ohashi 12. Piezoelectric Langasite Crystals for High Temperature Sensors (Verbal Presentation), The

International Chemical Congress of Pacific Basin Societies, Hawaii, USA (December 15-20, 2015)

K. Shimamura, X.W. Fu, E. G. Víllora, Y. Oshima, Y. Noguchi, M. Miyayama, and N. Ohashi

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No.3

早稲田大学博士（工学）学位申請研究業績書

種類別 By Type

題名、発表・発行掲載誌名、発表・発行年月、連名者（申請者含む）(theme, journal name, date & year of publication, name of authors inc. yourself)

13. Piezoelectric Ca3TaAl3Si2O14 (CTAS) Single Crystals for High Temperature Sensors (Verbal Presentation), JSPS 70^th workshop, Tokyo, Japan (January 29, 2016)

X.W. Fu, E. G. Víllora, Y. Kitanaka, Y. Noguchi, M. Miyayama, K. Shimamura and N. Ohashi 14. Electrical Resistivity, Dielectric and Piezoelectric Properties of Ca3TaGa3-xAlxSi2O14

(CTGAS) Single Crystals As a Function of Al Content (Verbal Presentation), The 63^th JSAP Spring Meeting, 2016, Tokyo, Japan (March 19-22, 2016)

X.W. Fu, E. G. Víllora, Y. Kitanaka, Y. Noguchi, M. Miyayama, K. Shimamura and N.

Ohashi

博 士 論 文 概 要

Graduate School of Advanced Science and Engineering Waseda University