S. Kambe’s research while affiliated with Yamagata University and other places

We have developed a dissolved oxygen (DO) method with differential equation (DE) correction. We measured the oxygen content in La-based and Y-based superconductors, and succeeded in measuring the oxygen content simply in one-third of the time required by the iodometric titration method. However, there was a problem with Bi-based superconductors where the measured oxygen content was smaller compared to the iodometric titration method. We hypothesized that not only O2 but also Cl2 gas is generated when dissolving Bi-based superconductors and developed a dissolved oxygen/chlorine (DO/Cl) method with DE correction. This method uses only a dissolved oxygen sensor and a dissolved chlorine sensor to measure the dissolved oxygen and dissolved chlorine content in Bi2Sr2−xLaxCuOy, allowing for the calculation of copper valence and oxygen content. The results from the DO/Cl method with DE correction show that the measured copper valence and oxygen content differ very little from those obtained using the iodometric titration method, with discrepancies within 0.016 and 0.008, respectively. Additionally, this method reduces the measurement time by one-third compared to the iodometric titration method. The results demonstrate that the DO/Cl method with DE correction can effectively measure the copper valence and oxygen content in cuprate superconductors, and using hydrochloric acid as the experimental solution is superior to sulfuric acid and nitric acid.

The copper (Cu) valence greatly influences the transition temperature (Tc) in cuprate superconductors. According to the charge neutralization rule, the Cu valence is calculated from the oxygen content. Previously, we developed the dissolved oxygen (DO) method as a novel oxygen measurement method for La2−xSrxCuO4 and YBa2Cu3Oy (YBCO) using a new DO sensor. The measurement time of the DO method is about one-third that of the standard iodometric method, but the difference in the measured Cu valence for YBCO is 0.039. Herein we report that a more accurate method, which combines the dissolved chlorine method and the corrected DO method, improves this difference to 0.013.

Although Pb²⁺ substitution for Bi³⁺ increases the hole concentration, p, in Pb-substituted Bi-based superconductors, the Pb substitution effect has yet to be studied in detail. Herein the Pb substitution effect on the electronic properties of Bi2201, 2212, and 2223 superconductors is investigated to elucidate the role of Pb on the correlation between p and formal valence, v. Consistent with Mott-Hubbard theory, p increases as v increases in the Pb-free Bi2201 and Pb-free Bi2212 phases. In contrast, p decreases with increasing v for the Pb-substituted Bi2201 phase, which contradicts Mott-Hubbard theory but can be explained by the increase in Pb⁴⁺. For the Pb-substituted Bi2212 phase, p increases with increasing v from 2.0 to 2.2, and then remains constant at p = 0.10–0.20 from v 2.2 to 2.63. When v increases from 2.0 to 2.2, the hole is doped at a Cu site, which is the same behavior as that of the Pb-free Bi2212 phase. As v increases from 2.2 to 2.63, the hole is doped in a Pb site and not a Cu one. For the Pb-substituted Bi2223 phase, p decreases with increasing v, and a mixed-valence state between Pb²⁺ and Pb⁴⁺ may coexist. As the v increases, the Pb valence increases and the Cu valence decreases. These results indicate two factors influence p. One is correlated with the substitution of Sr²⁺ for La³⁺ or Y³⁺ for Ca²⁺, which obeys Mott-Hubbard theory. The other is correlated with the substitution of Pb2+/4+ for Bi³⁺, which does not obey Mott-Hubbard theory. The results strongly suggest that Pb substitution for Bi does not necessarily dope a hole but decreases the hole concentration.

For cuprate superconductors, the Cu valency is a determining factor of the superconducting transition temperature (Tc). The Cu valence (v) of the superconductor YBa2Cu3Oy (YBCO) has been calculated from oxygen content (y) by the charge neutralization rule. The conventional methods for measuring the oxygen content are iodometry and coulometry. Recently, we developed a novel oxygen measuring method for La2-xSrxCuO4 that uses a dissolved oxygen sensor (the DO method). In this study, we examine whether the oxygen content of YBa2Cu3Oy can be determined by the DO method. The y value obtained by the DO method was compared with that obtained by the iodometric method. In addition, we developed a new correction method by applying the diffusion equation. After correction, the errors of v and y fit within 0.026 and 0.039, respectively. We found that DO method is effective for measuring the oxygen content of YBCO after correction.

To elucidate the effect of Pb substitution on the modulation structure of the Bi2212 phase, we prepared Pb-free Bi2Sr1.8(Ca1-xYx)Cu2Oy and Pb-substituted Bi1.6Pb0.4Sr1.8(Ca1-xYx)Cu2Oy. For both the Pb-free and Pb-substituted Bi-2212 phases, the modulation wave number σ increased with increasing oxygen content y. This tendency is explained by the excess oxygen that is inserted into the BiO layer and leads to the formation of the modulation structure. For the Pb-free Bi2212 phase, σ increased with increasing O-Cu-O length l. This tendency is explained by the lattice mismatch that exists between the O-Bi-O and O-Cu-O layers. In contrast, for the Pb-substituted Bi2212 phase, σ changed only slightly with increasing l, revealing that the relaxation mechanism for lattice mismatch in the Pb-substituted Bi2212 phase is different from that of the Pb-free Bi2212 phase. A possible mechanism for relaxation of lattice mismatch is the change in Pb valence. As the Pb valence increases from 2+ to 4+, the ionic radius of Pb decreases, suggesting that l decreases and the mismatch between the O-Cu-O and Bi(Pb)O lengths decreases.

Higher superconducting transition temperature Tc is achieved in longer Cu-Cu bond distance samples. In this study, indium-based cuprate compounds with long Cu-Cu bond distance were prepared via solid-state reaction. The parent compound InBa2CuO4.44 is not electrically conducting due to the lack of oxygen ions in the CuO2 layer. In order to make it conducting, the oxygen content should be increased. For this purpose, polycrystalline InBa2-xLaxCuOy samples were prepared. In case of x = 1.0, single-phase compounds were prepared. Unfortunately, it is not superconductor but semiconductor.

It is known that Cu valence of cuprate superconductors govern transition temperature (Tc). For La2−xSrxCuOy, when x = 0.15, that is, v is 2.15, the highest Tc around 40 K is achieved. The v value is determined both by strontium and oxygen content. So far, coulometry and iodometry have been used for determining oxygen content. However, chemical analysis usually requires much experience. The purpose of this research is to develop a novel measuring method of oxygen content in cuprate superconductors with a newly developed dissolved oxygen (DO) sensor and to discuss correction method. Signal received from the DO sensor is sent to PC automatically without any external power supply. Oxygen content obtained with DO sensor was compared with iodometry. It was found that this method is reliable enough to determine oxygen content in La2−xSrxCuOy, and is called dissolved oxygen (DO) method.

It is known that Cu valence of cuprate superconductors govern transition temperature (Tc). For La2-xSrxCuOy, when x = 0.15, that is, Cu valence is 2.15, the highest Tc around 40K is achieved. The Cu valence is determined both by strontium and oxygen content. So far, coulometry and iodometry have been used for determining oxygen content. However, these chemical analyses usually require experience. The purpose of this research is to develop a novel measuring method of oxygen content in cuprate superconductors with a developed dissolved oxygen sensor. Signal received from the sensor is sent to PC automatically without any external power supply. Oxygen content obtained with the sensor was newly compared with iodometry. It was found that this method is reliable enough to determine oxygen content in La2-xSrxCuOy, and is called dissolved oxygen method.

Polycrystalline Gd1−xCaxBaSrCu3O7−δ samples (0 ≤ x ≤ 0.1) were prepared via solid-state reaction. The superconducting critical temperature Tc and lattice parameter decrease with doping content. Temperature dependence of the specific heat of all samples was measured with the thermal relaxation technique using a Physical Property Measurement System from about 2 K to 150 K. The calculated Debye temperature ΘD at 10 K is found to be inversely proportional to Tc. Subsequently, the electron–phonon coupling constant was estimated based on standard BCS theory (λBCS) in the weak coupling limit and two-dimensional Van Hove scenario (λVH). The calculated values of λVH in this study (around 0.04) are close to the experimental data as reported by other researchers on Bi-based and YBCO samples. Hence, the two-dimensional Van Hove scenario seems to be a viable candidate for the mechanism of superconductivity if electron–phonon coupling plays a role in superconductivity of these materials.

Avrami’s equation was found to be the best kinetics theory to describe the high-Tc Bi-2223 crystal formation so far. There are three parameters (exponential index n, activation energy E and parameter k) in the equation. A series of research works have been done on the determination of parameters n and E, but few on another parameter k are to be found. The purpose of the approach was to determine and study the role of k in the formation of Bi-2223 crystals. The analysis in the research showed that the parameter k could describe the exhausting speed of reactants, and determine reaction probability and mean lifetime of reactants. The growth mechanism of the 2223 polycrystals was found to show some differences from the prediction of the Avrami’s theory model. At the initial duration, the 2223 crystal growth was much lower than the theoretical prediction; especially there was a mature duration to be found at the last growth duration, at which the 2223 crystal almost did not increase, but its crystal grains obviously became larger. This was a typical morphological transformation. The morphological transformation was the first time to be reported for the growth of the 2223 phase.