Refine your search:     
Report No.
 - 
Search Results: Records 1-20 displayed on this page of 21

Presentation/Publication Type

Initialising ...

Refine

Journal/Book Title

Initialising ...

Meeting title

Initialising ...

First Author

Initialising ...

Keyword

Initialising ...

Language

Initialising ...

Publication Year

Initialising ...

Held year of conference

Initialising ...

Save select records

Journal Articles

Nanoscale heterogeneity induced by nonmagnetic Zn dopants in the quantum critical metal CeCoIn$$_5$$; $$^{115}$$In NQR/NMR and $$^{59}$$Co NMR study

Sakai, Hironori; Tokunaga, Yo; Kambe, Shinsaku; Zhu, J.-X.*; Ronning, F.*; Thompson, J. D.*; Ramakrishna, S. K.*; Reyes, A. P.*; Suzuki, Kohei*; Oshima, Yoshiki*; et al.

Physical Review B, 104(8), p.085106_1 - 085106_12, 2021/08

Antiferromagnetism in a prototypical quantum critical metal CeCoIn$$_5$$ is known to be induced by slight substitutions of non-magnetic Zn atoms for In. In nominally 7% Zn substituted CeCoIn$$_5$$, an antiferromagnetic (AFM) state coexists with heavy fermion superconductivity. Heterogeneity of the electronic states is investigated in Zn doped CeCoIn$$_5$$ by means of nuclear quadrupole and magnetic resonances (NQR and NMR). Site-dependent NQR relaxation rates $$1/T_1$$ indicate that the AFM state is locally nucleated around Zn substituents in the matrix of a heavy fermion state, and percolates through the bulk at the AFM transition temperature $$T_{rm N}$$. At lower temperatures, an anisotropic superconducting (SC) gap below the SC transition temperature $$T_{rm c}$$, and the SC state permeates through the AFM regions via a SC proximity effect. Applying an external magnetic field induces a spin-flop transition near 5 T, reducing the volume of the AFM regions. Consequently, a short ranged inhomogeneous AFM state survives and coexists with a paramagnetic Fermi liquid state at high fields.

Journal Articles

Effective removal of iodate by coprecipitation with barite; Behavior and mechanism

Tokunaga, Kohei; Takahashi, Yoshio*; Tanaka, Kazuya; Kozai, Naofumi

Chemosphere, 266, p.129104_1 - 129104_10, 2021/03

 Times Cited Count:1 Percentile:65.97(Environmental Sciences)

Radioactive iodine ($$^{129}$$I) is of great concern owing to its high mobility in the environment and long-term radiotoxicity, but there is a lack of effective techniques for removing iodate (IO$$_{3}$$$$^{-}$$) from aqueous solution. The aim of this study is to develop a new technique for removing radioactive iodate from contaminated solution by using barite (BaSO$$_{4}$$). In the present study, we examined the coprecipitation mechanism of iodate by barite at the molecular level for determining optimum conditions for iodate removal. The results showed that iodate was effectively removed from aqueous solution by coprecipitation, even in the presence of competitive anions in solution. Comparing our method with previous studies, iodate removal efficiency by barite was determined to be about two orders of magnitude greater than that by hydrotalcite-like layered double hydroxide at Cl$$^{-}$$ concentration of 10 mmol L$$^{-1}$$. Extended X-ray absorption fine structure (EXAFS) analysis indicated that incorporated iodate was strongly bound in the crystal lattice of barite by substituting the sulfate site in the structure when the iodine concentration was low. The charge compensation problem from the IO$$_{3}$$$$^{-}$$ substitution in SO$$_{4}$$$$^{2-}$$ site is achieved by the substitution of Na$$^{+}$$-IO$$_{3}$$$$^{-}$$ pairs at the nearest Ba$$^{2+}$$ site. Therefore, considering high removal efficiency and strong binding of iodate in barite, coprecipitation with barite is a promising material for removing radioactive iodate from various aqueous solutions contaminated with iodate.

Journal Articles

A Fluorous phosphate for the effective extraction of Ln$$^{III}$$ from nitrate media; Comparison with a conventional organic phosphate

Ueda, Yuki; Kikuchi, Kei*; Tokunaga, Kohei; Sugita, Tsuyoshi; Aoyagi, Noboru; Tanaka, Kazuya; Okamura, Hiroyuki

Solvent Extraction and Ion Exchange, 39(5-6), p.491 - 511, 2021/00

 Times Cited Count:0 Percentile:0(Chemistry, Multidisciplinary)

no abstracts in English

Journal Articles

A New technique for removing strontium from seawater by coprecipitation with barite

Tokunaga, Kohei; Kozai, Naofumi; Takahashi, Yoshio*

Journal of Hazardous Materials, 359, p.307 - 315, 2018/07

 Times Cited Count:11 Percentile:59.27(Engineering, Environmental)

In the present study, we looked at the removal efficiency of Sr by using barite (BaSO$$_{4}$$) under various experimental conditions to develop techniques for the direct removal of Sr from seawater. The uptake of Sr by barite was found to be dependent on pH, saturation state, and [Ba$$^{2+}$$]/[SO$$_{4}$$$$^{2-}$$] ratio in initial aqueous solution, showing that most of the aqueous Sr can be removed from the aqueous solution by adjusting these parameters. However, the effects of ionic strength and competitive ions were negligible, suggesting the effectiveness of its application to removal of Sr from seawater. Batch experiments were also conducted in a seawater system, and a rather high removal efficiency of Sr from seawater (more than 90%) was achieved. Considering its high removal and retention efficiency of Sr in seawater systems, barite is a reliable material for the removal of Sr from seawater.

Journal Articles

Effective removal of selenite and selenate ions from aqueous solution by barite

Tokunaga, Kohei*; Takahashi, Yoshio*

Environmental Science & Technology, 51(16), p.9194 - 9201, 2017/08

 Times Cited Count:27 Percentile:81.48(Engineering, Environmental)

In the present study, we explore a new application of barite (BaSO$$_{4}$$) as a sequestering phase for selenite (Se(IV)) and selenate (Se(VI)) ions from aqueous solutions due to the low solubility and high stability of barite. The uptake of Se(IV) and Se(VI) during coprecipitation with barite was investigated through batch experiments to understand the factors controlling effective removal of Se(IV) and Se(VI) from polluted water to barite. The uptake of Se(IV) by barite is dependent on pH, coexistent calcium ion, and sulfate concentration in the initial solution, possibly due to their effects on the chemical affinity and structural similarity. On the other hand, the uptake of Se(VI) by barite was strongly dependent on sulfate concentration in the initial solution, which is only related to the structural similarity. This study provides a good estimate of its ability to effectively remove Se(IV) and Se(VI) from aqueous solutions (more than 80%) under optimized experimental parameters.

Journal Articles

Ligand exchange adsorption and coordination structure of Pd on $$delta$$-MnO$$_{2}$$ in NaCl solution

Tanaka, Kazuya; Tanaka, Masato*; Watanabe, Naoko*; Tokunaga, Kohei*; Takahashi, Yoshio*

Chemical Geology, 460, p.130 - 137, 2017/06

 Times Cited Count:5 Percentile:28.38(Geochemistry & Geophysics)

Pd is highly accumulated in ferromanganese nodules and crusts relative to its concentration in seawater but the mechanism by which Pd(II) is incorporated remains poorly understood. We investigated the local coordination structure of Pd(II) adsorbed on $$delta$$-MnO$$_{2}$$, using X-ray absorption fine structure spectroscopy. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analyses indicated that Pd was adsorbed on $$delta$$-MnO$$_{2}$$ through ligand exchange, from Cl coordination to O coordination. Furthermore, curve fitting of EXAFS spectra demonstrated the formation of two different inner-sphere complexes, bidentate-mononuclear and bidentate-binuclear complexes, and this finding was supported by density functional theory calculations. The formation of inner-sphere complexes is reasonable given the relatively large distribution coefficients obtained from adsorption experiments.

Oral presentation

NQR/NMR study on Zn-doped CeCoIn$$_5$$

Sakai, Hironori; Hattori, Taisuke; Tokunaga, Yo; Kambe, Shinsaku; Ronning, F.*; Zhu, J.-X.*; Suzuki, Kohei*; Oshima, Yoshiki*; Yokoyama, Makoto*

no journal, , 

Dilute substitutions of non-magnetic dopants Zn can induce a long-ranged antiferromagnetic ordering to a heavy fermion superconductor CeCoIn$$_5$$. We have microscopically investigated the Zn-doped CeCoIn$$_5$$ by means of the nuclear quadrupole/magnetic resonance (NQR/NMR) techniques.

Oral presentation

NQR/NMR study of Zn-doped CeCoIn$$_5$$

Sakai, Hironori; Hattori, Taisuke; Higa, Nonoka; Tokunaga, Yo; Kambe, Shinsaku; Ronning, F.*; Zhu, J.-X.*; Suzuki, Kohei*; Oshima, Yoshiki*; Yokoyama, Makoto*

no journal, , 

We have used Nuclear Quadrupole Resonance (NQR) to probe microscopically the response of prototypical quantum critical metal CeCoIn$$_5$$ to substitutions of small amounts of Cd or Zn for In. Such non-magnetic Cd or Zn substitutions induce a long range antiferromagnetism. In the case of Cd substitutions, approximately half of the Cd substituents induce local Ce moments in their close proximity, as observed by site-dependent longitudinal nuclear spin relaxation rates $$1/T_1$$. In the paramagnetic state, the values of $$1/T_1$$ at the main peak position are similar to those in the pure CeCoIn$$_5$$, while those are enhanced at the skirt position of spectrum. We will discuss about the heterogeneous electronic state realized in such a Zn substituted system and discuss about the coexistence of superconductivity with the antiferromagnetism, as well.

Oral presentation

Development of removal method for radioactive elements by coprecipitation with barite (BaSO$$_{4}$$)

Tokunaga, Kohei; Kozai, Naofumi; Takahashi, Yoshio*; Ohara, Yoshiyuki

no journal, , 

In the present study, we explore a new application of barite (BaSO$$_{4}$$)) as a sequestering phase for selenite (Se(IV)) and selenate (Se(VI)) ions from aqueous solutions. Barite is a common phase in many geological environments, and it can be used to remove toxic and/or radioactive elements from aqueous solutions, but it has not been widely used in environmental studies. This study describes the mechanisms of Se distribution between barite and water, thus providing a good estimate of its ability to effectively remove Se(IV) and Se(VI) from aqueous solutions (more than 80%) using the optimized experimental parameters examined here.

Oral presentation

A New technique for removing radioactive nuclides from aqueous solution by coprecipitation with barite

Tokunaga, Kohei; Takahashi, Yoshio*; Kozai, Naofumi

no journal, , 

In the present study, we explore a new application of barite (BaSO$$_{4}$$) as a sequestering phase for radioactive nuclides from aqueous solutions. Barite is a common phase in many geological environments, and it can be used to remove toxic and/or radioactive elements from aqueous solutions, but it has not been widely used in environmental studies. This study describes the mechanisms of SeO$$_{3}$$$$^{2-}$$, SeO$$_{4}$$$$^{2-}$$, IO$$_{3}$$$$^{-}$$, and Sr$$_{2}$$$$^{+}$$ distribution between barite and water, thus providing a good estimate of its ability to effectively remove these ions from aqueous solutions (more than 80%) using the optimized experimental parameters examined here.

Oral presentation

A New technique for removing radioactive nuclides from aqueous solution by coprecipitation with barite

Tokunaga, Kohei; Takahashi, Yoshio*; Kozai, Naofumi

no journal, , 

In the present study, we explore a new application of barite (BaSO$$_{4}$$) as a sequestering phase for radioactive nuclides from aqueous solutions. Barite is a common phase in many geological environments, and it can be used to remove toxic and/or radioactive elements from aqueous solutions, but it has not been widely used in environmental studies. This study describes the mechanisms of SeO$$_{3}$$$$^{2-}$$, SeO$$_{4}$$$$^{2-}$$, IO$$_{3}$$$$^{-}$$, and Sr$$_{2}$$$$^{+}$$ distribution between barite and water, thus providing a good estimate of its ability to effectively remove these ions from aqueous solutions (more than 80%) using the optimized experimental parameters examined here.

Oral presentation

A New technique for removing oxyanions from aqueous solution by coprecipitated with barite

Tokunaga, Kohei; Takahashi, Yoshio*; Kozai, Naofumi

no journal, , 

In the present study, we explore a new application of barite as a sequestering phase for radioactive oxyanions, such as selenium-79, technetium-99, and iodine-129, from aqueous solutions. Barite is a common phase in many geological environments, and it can be used to remove toxic and/or radioactive elements from aqueous solutions, but it has not been widely used in environmental studies. This study mainly describes the mechanisms of selenite and selenate distribution between barite and water, thus providing a good estimate of its ability to effectively remove these ions from aqueous solutions (more than 80%) using the optimized experimental parameters examined here.

Oral presentation

Effective removal of oxyanions from aqueous solution by coprecipitation with barite

Tokunaga, Kohei; Kozai, Naofumi; Takahashi, Yoshio*

no journal, , 

no abstracts in English

Oral presentation

A New technique for removing selenite and selenite from aqueous solution by coprecipitation with barite

Tokunaga, Kohei; Kozai, Naofumi; Takahashi, Yoshio*

no journal, , 

no abstracts in English

Oral presentation

A New technique for removing radioactive strontium by Ca-substituted barite

Tokunaga, Kohei; Kozai, Naofumi; Takahashi, Yoshio*

no journal, , 

no abstracts in English

Oral presentation

A Fluorous phosphate for the extraction of Zr(IV) from HNO$$_{3}$$ solution; Comparison with tri-$$n$$-butyl phosphate

Ueda, Yuki; Kikuchi, Kei; Sugita, Tsuyoshi; Tokunaga, Kohei; Motokawa, Ryuhei; Naganawa, Hirochika

no journal, , 

The extraction and separation of Zr(IV) as a major fission product from aqueous HNO$$_{3}$$ solutions have been investigated extensively because the presence of Zr in the TBP-based PUREX process for reprocessing spent nuclear fuel is problematic. Moreover, the formation of liquid-liquid (water, oil) interfacial cruds or a third phase during Zr extraction by TBP from HNO$$_{3}$$ aqueous solution occurs with a high metal loading in the organic phase. In this study, we develop a fluoroalkylated phosphate (TFP) for Zr(IV) extraction to increase the extractability and prevent third-phase formation. We investigated the extraction performance and mechanism of TFP extraction of Zr(IV) from HNO$$_{3}$$ solutions into perfluorohexane and compared them with the conventional organic extraction system using TBP in n-octane. The Zr(IV) extraction strength of TFP in the fluorinated diluent perfluorohexane was much higher than that of TBP in n-octane.

Oral presentation

NMR study of Zn- and Ni-substitution effect in heavy fermion superconductor CeCoIn$$_5$$

Sakai, Hironori; Tokunaga, Yo; Haga, Yoshinori; Kambe, Shinsaku; Zhu, J.-X.*; Ronning, F.*; Ramakrishna, S. K.*; Reyes, A. P.*; Kotegawa, Hisashi*; To, Hideki*; et al.

no journal, , 

In heavy fermion superconductor CeCoIn$$_5$$, slight Zn substitutions for In atoms induce antiferromagnetism, while slight Ni substitutions for Co atoms suppress superconductivity. The both substituted CeCoIn$$_5$$ exhibit non- fermi-liquid behaviors near the phase boundaries which have been detected by magnetization and specific heat measurements. We have performed NMR experiments in these systems to investigate the microscopic effects by substituents. We will discuss about the spin fluctuations near the boundaries by NMR relaxation rates.

Oral presentation

Heterogeneous electronic state by Zn-substitutions in heavy fermion superconductor CeCoIn$$_5$$

Sakai, Hironori; Tokunaga, Yo; Haga, Yoshinori; Kambe, Shinsaku; Zhu, J.-X.*; Ronning, F.*; Ramakrishna, S. K.*; Reyes, A. P.*; Suzuki, Kohei*; Oshima, Yoshiki*; et al.

no journal, , 

Substitutions of In sites by Zn atoms in heavy fermion superconductor CeCoIn$$_5$$ is reported to induce an antiferromagnetic order by Yokoyama et al. in Ibaraki University. Near the critical region, non-Fermi-liquid behavior has been observed in magnetization and specific heat. We have performed NMR measurements to reveal the spin fluctuations near the critical region.

Oral presentation

NMR study of Ni-substitution effect in heavy fermion superconductor CeCoIn$$_5$$

Sakai, Hironori; Tokunaga, Yo; Haga, Yoshinori; Kambe, Shinsaku; Zhu, J.-X.*; Ronning, F.*; Kotegawa, Hisashi*; To, Hideki*; Suzuki, Kohei*; Oshima, Yoshiki*; et al.

no journal, , 

The Ni-substitutions of Co in the heavy fermion superconductor CeCoIn$$_5$$ are reported to suppress the superconductivity by Yokoyama et al. in Ibaraki Univ. To microscopically investigate the Ni doping effect, we have performed NMR study in the alloy system of Ce(Co, Ni)In$$_5$$. We would like to discuss the spin fluctuations and the electronic state near the critical composition for the superconductivity in this system.

Oral presentation

Microscopic study of Ni and Zn substitution effects in heavy fermion superconductor CeCoIn$$_5$$

Sakai, Hironori; Tokunaga, Yo; Haga, Yoshinori; Kambe, Shinsaku; Ronning, F.*; Zhu, J.-X.*; Thompson, J. D.*; Kotegawa, Hisashi*; To, Hideki*; Suzuki, Kohei*; et al.

no journal, , 

Heavy fermion superconductor CeCoIn$$_5$$ is considered as a quantum critical antiferromagnetic material. When the In elements are slightly substituted by Zn elements, a long-range antiferromagnetism is induced. On the other hand, slight Sn substitutions make the system step away from the quantum critical point. In the case of Zn substitutions, heterogeneity of antiferromagnetic area in the matrix of heavy fermion state has been microscopically confirmed by NQR measurements. In the case of Ni substitutions for the Co elements, the NQR result appears to be the same as in Sn substituted system.

21 (Records 1-20 displayed on this page)