Synthesis of Rare-Earth Sulfides (RE=Yb, Eu, and Sm) with Valence-Fluctuated Characters and Their Applications

• Material Type: 博士論文
• Title: Synthesis of Rare-Earth Sulfides (RE=Yb, Eu, and Sm) with Valence-Fluctuated Characters and Their Applications
• Author/Editor: LI, Liang
• Author Heading: LI, Liang
• Publication Date: 2016-09-28
• Publication Date (W3CDTF): 2016-09-28
• Alternative Title: 価数揺動状態がある希土類の硫化物合成とその応用
• Degree grantor/type: 室蘭工業大学
• Date Granted: 2016-09-28
• Date Granted (W3CDTF): 2016-09-28
• Dissertation Number: 甲第390号
• Degree Type: 博士（工学）
• Conferring No. (Dissertation): 甲第390号
• Text Language Code: eng
• Alias of Author: 李, 良
• NDC: 501.4; 436.33
• Target Audience: 一般
• Note (General): application/pdf; In some rare earth compounds, valences of rare earth elements take non-integer values, because valence values of rare earth elements fluctuate spatially and temporally. The fluctuated valences depend strongly on the synthesis temperature. In this study, Yb, Eu, Sm based sulfides with valence-fluctuation characters were to study the synthesis and stability and expected to discover new applications.Ytterbium sulfides were prepared from CS2 sulfurization of Yb2O3 and then heat treatments. Ytterbium sulfides (Yb3S4 and YbS) are expected to be employed as high temperature n-type thermoelectric materials due to their large Seebeck coefficient. The influences of particle size and specific surface area of Yb2O3 powders, sulfurization temperature and time and CS2 gas flow rate on preparation of ytterbium sesquisulfide (Yb2S3) were researched. Small particle size (< 1 μm) and large specific surface area (> 2 m2/g) of Yb2O3 are necessary for fabrication of pure Yb2S3. Single orthorhombic η-Yb2S3 and hexagonal ε-Yb2S3 were synthesized by the sulfurization of fine Yb2O3 powders at 973～1073 K and 1273～1323 K with CS2 gas flow rate of 1.67 mL/s, respectively. Orthorhombic η-Yb2S3 transformed to hexagonal ε-Yb2S3 with increase of temperature. The heat treatments of Yb2S3 were investigated. Upon heat treatment at 1273 K for 3 hr in Ar/CS2 atmosphere, orthorhombic Yb2S3 phase underwent phase transition to hexagonal Yb2S3 phase. Moreover, orthorhombic Yb3S4 was main phase after heat treatment at 1050°C for 8 hr under Ar atmosphere and Yb2S3 disappeared upon prolonged (12 hr) heat treatment. Single Yb3S4 phase could be obtained after treatment at 1273 K for 3 hr, or at 1473 K for 1 hr, under vacuum (～1.2×10-3 Pa). Single-phase YbS with a homogeneity range of YbS1.11-1.15 could be synthesized by treatment at 1773 K for 3 hr. Secondly, europium sulfides were synthesized by CS2 sulfurization of Eu2O3. EuS is a ferromagnetic semiconductor with NaCl type crystal structure. As this temperature is in the proximity of the boiling point of hydrogen, EuS is a potential magnetic refrigeration material. The effects of Eu2O3 character and sulfurization conditions on the preparation of europium sulfides were researched. Single-phase Eu3S4 and EuS can be obtained by CS2 gas sulfurization of spherical Eu2O3 with larger specific surface area and small grain size at 773 K for longer than 0.5 hr and 1073 K for 8 hr, respectively. Moreover, EuS can be fabricated from self-prepared needle Eu2O3 at 1023 K for 8 hr. The higher sulfurization temperature and shorter sulfurization time accelerated the formation of high purity EuS. Specific surface area of synthetics lessened with the rising of sulfurization temperature and time. Synthetic pure Eu3S4 were treated under rich-sulfur atmosphere, inert atmosphere and vacuum, respectively. Single EuS phase was obtained at 973 K under rich-sulfur atmosphere or at 1073 K under inert atmosphere. The stability of Eu3S4 during annealing is weaker than all above mentioned conditions and the transformation finished at 873 K under vacuum of 1.2×10-5 Pa. The transformation of Eu3S4 to EuS was attributed to stability of Eu2+ at high temperature. The synthetic EuS powders were sintered under a uniaxial pressure of 50 MPa in vacuum. The large reversible magnetocaloric effect of polycrystalline EuS was observed, which underwent second-order ferromagnetic to paramagnetic transition at 16.8 K. The maximum of magnetic entropy change is as large as 6.32 J/mol/K and the adiabatic temperature change is 9.1 K under a vary magnetic field change of 5 T. The entropy value for polycrystalline EuS was revised by the combination of the magnetization and heat capacity data. The relative cooling power for polycrystalline EuS reached 69.26 and 125.39 J/mol for ΔH = 3 T and 5 T, respectively. Finally, non-stoichiometric samarium monosulfide (SmSx, 0.55<x<1.2) was synthesized from Sm2S3 and SmH3 at 1273 K for 3 hr under vacuum. Until now, polycrystalline SmS prepared from the direct reaction between samarium and sulfur, has a thermoelectric figure of merit ZT ～0.9 with the optimal composition SmS0.96. The influence of reaction ratio of Sm2S3 to SmH3 on the fabrication of SmS was investigated.The fabrication of SmS required the mole ratio of Sm2S3 to SmH3 above 1. Lattice parameter of synthetic SmS increases firstly and then decreases to saturate following with the addition of SmH3 content. SmS compact was sintered at 1373 K by spark plasma sintering. Density of synthetic SmS is about 99% of theory density. Seebeck coefficient of n-type semiconductor SmSx decreases as temperature rises. The absolute value is distributed between 170-280 μV·K-1. The electrical conductivity of SmSx (0.86<x<1.07) decreases with temperature increasing and shows similar temperature dependence. The surplus Sm which randomly distributed in the SmSx (0.55<x<0.75) matrix, leads to a remarked reduces of electrical resistivity. The optimized power factor for SmS0.6 and SmS0.75 can reach 1500 μW·K-2·m-1 at 600 K.; 価数揺動状態とは、価数が時間的に揺らぐことによって整数値から離れた中途半端な値をとる場合のことである。価数揺動の特徴の一つに、価数が強く温度依存することがある。本研究では、価数揺動状態があるSm, Eu, Ybについて、その用途に見合う単相の硫化物を合成することを目的とする。最初に、Yb2O3のCS2ガス硫化と熱処理によるイッテルビウム硫化物を合成した。斜方晶Yb3S4はp型熱電材料、立方晶YbSも773 K付近においてゼーベック係数が1000μV/Kに達した後、1023～1173 K付近から高温ではn型になる熱電材料として期待されている。硫化実験の結果、粒径が細かく、比表面積が大きなYb2O3の場合、973～1073 Kの低温の硫化ではη-Yb2S3、1273～1373 Kの高温の硫化ではε-Yb2S3 単相が生成した。これに対してさらに大きな比表面積のYb2O3の場合、873Kの硫化でもη-Yb2S3単相の生成が確認された。また、η-Yb2S3を真空中で熱処理するとYb3S4が生成し、とくに長時間または高温で熱処理するとYbSを得ることができた。次に、Eu2O3のCS2ガス硫化によりユーロピウム硫化物を合成した。EuSはNaCl型結晶構造を有する強磁性半導体であり、磁気相転移温度が水素液化温度（20 K）の近くにあるため、水素液化磁気冷凍の液化段に利用できる磁気冷凍材料の候補材料である。Eu2O3には細粒と粗粒を用意し、細粒の場合、硫化時間にかかわらず773 Kの硫化ではEu3S4単相、1073Kの硫化ではEuS単相が生成した。EuSの生成は、Eu2O3からEuS2を経てEu3S4を生成し、次いで一部Eu2O2Sを生成しながらEuSを生成するものと推定した。一方、細粒の場合、硫化条件にかかわらずEu3S4は生成せず、硫化時間が1.8 ksと28.8 ksの場合、低温で生成したEu2O2Sはそれぞれ1273 Kと1073 Kより高温で消滅し、EuS単相となった。すなわち、Eu2O3からEu2O2Sを経てEuSを生成することが確認された。また、EuS焼結体の磁気ゼロ及び磁場中（5T）の比熱を測定したところ、磁気ゼロの場合は16.5 Kに磁気転移に伴う大きなピーク、一方、磁場中は、ピークが磁場により広がりながら高温にシフトするという強磁性－常磁性状態を二次相転移する常磁性体の典型的な変化を示した。また、MPMSを用いた磁化測定から求めた単位体積当たりの磁気エントロピー変化の温度依存性から、16.8 K付近に山型のピークが確認され、単結晶で見られた磁気エントロピー変化の値と一致した。すなわち、EuS焼結体が単結晶と同等の磁気熱量効果を有することが明らかになった。最後に、Sm2S3粉末とSmH3粉末の反応焼結法により非化学量論組成のSmSx, （0.55<x<1.2）焼結体を作製した。これまで、Sm金属と硫黄の直接反応によりSmS0.96が合成され、1000 Kにおいて性能指数（Z）が0.9×10-3 K-1に達することが報告されている。最初に、BN坩堝中で所定の配合比のSm2S3とSmH3の混合粉末を焼成後、パルス通電焼結法により焼結体を作製した。焼結体の電気抵抗（ρ）とSゼーベック係数を測定したところ、ρは、仕込み組成が化学量論組成付近の焼結体では高温ほど減少する半導体的挙動を示した。一方、仕込み組成（SmH3 / Sm2S3）が2～2.5以上となりSmH3が過剰に仕込まれると電気抵抗率は金属的な挙動を示した。また、仕込み組成が化学量論組成付近では温度と共にゼーベック係数が増加する傾向が見られたが、それよりもSmH3を過剰に加えた場合の焼結体ではゼーベック係数の明確な温度依存性が見られなくなった。出力因子は、Sm過剰の焼結体で1000～1500µWK-2m-1と化学量論比付近のものに比べると大きくなった。
• DOI: 10.15118/00009029; https://doi.org/10.15118/00009029
• Persistent ID (NDL): info:ndljp/pid/10224847; https://dl.ndl.go.jp/pid/10224847
• Collection: 障害者向け資料
• Collection (Materials For Handicapped People:1): テキストデータ
• Collection (particular): 国立国会図書館デジタルコレクション > デジタル化資料 > 博士論文; https://dl.ndl.go.jp/collections/A00014
• Acquisition Basis: 博士論文（自動収集）
• Date Accepted (W3CDTF): 2016-12-01T22:39:34+09:00
• Date Created (W3CDTF): 2016-11-19
• Format (IMT): application/pdf
• Access Restrictions: 国立国会図書館内限定公開
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• Availability of remote photoduplication service: 可
• Periodical Title (URI): http://dx.doi.org/10.15118/00009029; http://hdl.handle.net/10258/00009029; https://muroran-it.repo.nii.ac.jp/records/9051
• Data Provider (Database): 国立国会図書館 : 国立国会図書館デジタルコレクション; https://dl.ndl.go.jp

• Summary, etc.: In some rare earth compounds, valences of rare earth elements take non-integer values, because valence values of rare earth elements fluctuate spatial...

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