Humidity Sensitivity of Chemically Synthesized ZnAl2O4/Al.

Humidity sensitivity is evaluated for chemically synthesized ZnAl2O4/Al devices. We succeeded in synthesizing the ZnAl2O4/Al device by applying chemical techniques only. Hydrothermal treatment for the anodized aluminum (AlOx/Al) gives us the device of the ZnAl2O4/Al structure. All fabrication processes were conducted under 400 °C. The key was focusing on ZnAl2O4 as the sensing material instead of MgAl2O4, which is generally investigated as the humidity sensor. The evaluation of this ZnAl2O4/Al device clarified its effectiveness as a sensor. Both electrical capacitance, Cp, and the resistivity, Rp, measured by an LCR meter, obviously responded to the humidity with good sensitivity and appreciable repeatability. Our synthesis technique is possible in principle to improve on the process for the device with a complex structure providing a large surface area. These characteristics are believed to expand the application study of spinel aluminate devices as the sensor.

Composition induced metal-insulator quantum phase transition in the Heusler type Fe2VAl.

We report the magnetism and transport properties of the Heusler compound Fe2+x V1-x Al at -0.10 ⩽ x ⩽ 0.20 under pressure and a magnetic field. A metal-insulator quantum phase transition occurred at x ≈ -0.05. Application of pressure or a magnetic field facilitated the emergence of finite zero-temperature conductivity σ 0 around the critical point, which scaled approximately according to the power law (P - P c ) (γ) . At x ⩽ -0.05, a localized paramagnetic spin appeared, whereas above the ferromagnetic quantum critical point at x ≈ 0.05, itinerant ferromagnetism was established. At the quantum critical points at x = -0.05 and 0.05, the resistivity and specific heat exhibited singularities characteristic of a Griffiths phase appearing as an inhomogeneous electronic state.

Spectroscopic and crystallographic anomalies of (Co1-xZnx)Al2O4 spinel oxide.

This work investigates the spectroscopic properties of (Co1-xZnx)Al2O4 with a range of x of 0 ≤ x ≤ 1. Spectroscopic and crystallographic evaluations using XRD, Raman, FT-IR and UV-Vis spectroscopy reveal that Zn(2+) substitution systematically changes the lattice constant, which mainly depends on the Co-O bonds, and the related optical characteristics of this material. The x dependence of these properties shows two trends, and the mutation point seems to be at x ≈ 0.5. This implies that the electronic structure of (Co1-xZnx)Al2O4 is not changed monotonically by Zn(2+) substitution. Interestingly, some of the optical phenomena observed in this study become prominent for samples with x ≥ 0.5. That is, we observed sideband peaks near the main peaks in the Raman spectra, and their relative intensities systematically and significantly increased with increasing Zn(2+) substitution. The rates of increase are not constant, and are fast for samples with x ≥ 0.5. The sideband peaks are considered to reflect the unique changes in the local electronic structure of (Co1-xZnx)Al2O4, and they are useful for evaluating the substitution level without the influence of the site change phenomenon. Thus, clarifying them is expected to be important for understanding and controlling the electronic structure of the spinel oxide. On the other hand, investigation of the visible light absorption due to the d-d transition of Co(2+) reveals that the efficiency is also high for samples with high Zn(2+) substitution (x ≥ 0.5). This is also considered to be valuable information for investigation of the optical properties and/or the catalytic function of the spinel oxide. Moreover, the fluorescence of the (Co1-xZnx)Al2O4 samples is also identified as a novel functional property of this material. The intensity of the fluorescence peak also dramatically increases for samples with x ≥ 0.7. The effect of Zn(2+) substitution on the local electronic structure of (Co1-xZnx)Al2O4 has not been clarified yet. However, some of the interesting characteristics reviewed in this study are worth investigating from the viewpoint of materials science and applications.

Reaction temperature variations on the crystallographic state of spinel cobalt aluminate.

In this study, we report a rapid and simple technique for obtaining cobalt aluminate having a spinel structure. The products were prepared from a hydroxide precursor synthesized by coprecipitation of cobalt (Co(2+)) and aluminum (Al(3+)) nitrates with an alkaline solution. The chosen precursor enabled low temperature fabrication of cobalt aluminate with a spinel structure by sintering it for 2 hours at low temperatures (>400 °C). Crystallographic and thermal analyses suggest that the low-temperature-sintered products contain Co(3+) ions stabilized by chemisorbed water and/or hydroxide groups, which was not observed for products sintered at temperatures higher than 1000 °C. The color of the products turned from clear blue (Thenard's blue) to dark green when sintering temperatures were below 1000 °C. Magnetic quantities, Curie constants, and Weiss temperatures show a strong dependence on the sintering temperature. These findings suggest that there are mixed valent states, i.e. Co(2+) and Co(3+), and unique cation distributions at the different crystallographic sites in the spinel structure, especially in the products sintered at lower temperatures.