Norio Miura

Environmental gas sensing

Abstract Solid-electrolyte sensors for detecting air pollutants are urgently needed for the sake of environmental protection. Rapid progress is being marked in the development of sensors for CO 2 , NO 2 , NO, SO 2 , ozone and fluorocarbons. Some of them, such as the CO 2 sensor, have almost reached a stage of practical application, while others are very promising. Most of these sensors have been fabricated with ceramic materials that are solid electrolytes and oxide semiconductors. The current status of research and development for air-pollutant sensing is described briefly.

Grain‐Size Effects in Tungsten Oxide‐Based Sensor for Nitrogen Oxides

The effects of the grain size of WO[sub 3] on WO[sub 3]-based sensor for NO[sub x] were investigated. By hydrolyzing ammonium paratungstate with hot nitric acid in solution and calcining the resulting precipitate in air at prescribed temperatures in the range of 300 to 600 C, the mean crystallite sizes (D) of WO[sub 3] were varied in the range of 16 to 57 nm. The sensitivities of the WO[sub 3] sensor elements to NO[sub 2] (10 ppm) as well as to NO (200 ppm) remained independent of D down to D = 33 nm, below which the sensitivities increased steeply with a decrease in D, reaching at D = 25 nm the sensitivity values three to four times as large as those for D > 33 nm.

New auxiliary sensing materials for solid electrolyte NO2 sensors

Abstract Solid-state electrochemical devices were fabricated by combining an Na + conductor (NASICON) with auxiliary phases of NaNO 2 ue5f8M 2 CO 3 (Mue5fbNa or Li). These devices showed good sensing characteristics to NO 2 in synthetic air. The use of NaNo 2 ue5f8 Li 2 CO 3 (9:1 in molar ratio ) gave quite excellent sensing properties especially to extremely dilute NO 2 . The electromotive force of the device followed the Nernst equation in the wide NO 2 concentration range of 0.005 ppm (5 ppb)–200 ppm at 150°C, its slope suggesting the one-electron reduction of NO 2 . The 90% response times of the sensor to 10 ppm and 5 ppb were ca. 8 s and ca. 3 min, respectively. In addition, the device was found to be utterly insensitive to CO 2 up to 40 vol%.

Improvement of copper oxide–tin oxide sensor for dilute hydrogen sulfide

The influence of the loading and dispersion of CuO on the H2S-sensing properties of CuO–SnO2 sensors has been investigated. The response rates to H2S depend on various factors such as CuO loading, specific surface area (or size) of SnO2 grains, method of loading the SnO2 grains with CuO and the operating temperature. For the CuO–SnO2 sensor prepared by a chemical fixation method, the response rates to dilute H2S (1–10 ppm) increased with decreasing CuO loading, while the sensitivity to H2S decreased monotonically with decreasing amount of CuO dispersed per unit surface area of SnO2. By optimizing these factors, it was possible to obtain a CuO–SnO2 sensor which responded to H2S above 1 ppm at 160 °C with sufficient sensitivity and fairly good response kinetics.

H2S-sensitive thin film fabricated from hydrothermally synthesized SnO2 sol

A porous thin film of SnO2 prepared on an alumina substrate by using hydrothermally synthesized SnO2 sol shows excellent sensitivity to dilute H2S in air together with good response transient characteristics when calcined at 700 °C. The pore size of the film increases uniformly with calcination temperature, and this well developed porous structure appears to be responsible for the excellent H2S sensing characteristics.

Preparation of gold-dispersed vanadium oxide thin films by an alternate spin-coating method for electrochromic applications

Thin films of V2O5 with dispersed gold particles were prepared from ethanol solutions of vanadyl isopropoxide and HAuCl4 by means of an alternate spin-coating method. The Au content was maintained at 12–36 atom% by selection of the appropriate concentration of the alkoxide solution. After calcination at 400 °C, the mean diameter of dispersed Au particles was 7.0 nm, as observed for the film containing 24 atom% Au. Violet (cathodic)↔green (anodic) electrochromism was exhibited by the Au (24 atom%)–V2O5 film. Owing to the plasma resonance of the Au particles, the film had a strong optical absorption band centred at 570 and 610 nm under cathodic and anodic polarization, respectively.

Tc enhancement of excess Sr-doped Bi-2223 oxides by control of oxygen content

Abstract The superconducting properties of excess Sr-doped 2223 phase samples, Bi 1.84 Pb 0.34 Sr 1.91+x Ca 2.03 Cu 3.06 O y ( x =0.1, 0.3, 0.5, 1.0, 1.5), were examined. All the oxides as-prepared showed almost the same Tc of 109∼110 K, After heat-treatment in He at 600 °C for 3h, however, Tc increased up to 114∼115 K for the oxides with x =0.3∼, and decreased down to 102∼106 K for x =0∼0.1. The oxygen contents of the oxides as determined by iodometry increased with increasing x and slightly decreased with the heat-treatment in He. The heat treatment also brough about small increases in lattice constant of c-axis.

Relationship between oxygen content and seebeck coefficient of Bi-based superconducting oxides

Abstract The correlations among Seebeck coefficient, oxygen content and superconducting property were examined for four Bi-based oxides (2223 and 2212 phases). Each oxide underwent reversible sorption and desorption of small amounts of oxygen (ca.3x10−5mol/g) in the temperature range 100–600 °C. In good agreement with such behavior, the Seebeck coefficient (Q) of each oxide was found to change reversibly with changing temperature, suggesting that Q is a reversible function of oxygen content. It was further found that the highest Tc was reached at the oxygen content at which Q was incidentally brought to be around zero at 100 °C for each oxide.