Tatsuro Harada

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.

Cordless solid-state hydrogen sensor using proton-conductor thick film

Abstract Potentiometric as well as amperometric solid-state gas sensors using a thick film (≈ 10 μm) of proton-conductor have been developed for detecting small amounts of hydrogen in air at room temperature. The sensor elements are composed of the following electrochemical cell: reference (or counter) electrode (Au or Pt) | proton-conductor thick film | sensing electrode (Pt). The thick film is formed on an alumina substrate by applying a paste of antimonic acid-polyvinyl alcohol mixture, mainly by means of a spin-coating method. The electromotive force of the potentiometric sensor varies logarithmically with H 2 concentration, while the short-circuit current of the amperometric sensor is found to be proportional to H 2 concentration. Both give a 90% response time of about 10 s at 30 °C.

Sensing characteristics of ISFET-based hydrogen sensor using proton-conductive thick film

Abstract An FET-based microsensor using a proton conductor for detecting a small concentration of H2 in air at room temperature has been fabricated by attaching an antimonic-acid thick film (≈ 20 μm thick) and a sensing Pt electrode to an ISFET device. Under a constant drain current (ID) at 30°C, the output voltage (VGS) between the sensing electrode and the source is almost linearly correlated with the logarithm of H2 concentration in the range 4−5000 ppm. The slope of the correlation, ≈ − 120 mV/decade, is comparable to that for the non-FET type solid-state potentiometric H2 sensor previously reported. The 90% response time to 5000 ppm H2 is as short as ≈ 5 s. The H2-sensing mechanism of the present microsensor is briefly discussed.

Problem in PFBC boiler (1): Characterization of agglomerate recovered in commercial PFBC boiler

Among various forms of coal ash and bed material purged from the bottom of pressurized fluidized bed combustion boiler, a kind of the fused agglomerate ash grain was analyzed by SEM, EDAX, and XRD. The agglomerate shell was found to consist of black matrix and white particles. The inner and outer surfaces of the shell appeared to consist of different compositions. Inner and outer surfaces showed ratios of Al and Si to Ca which were 3:1 and 1:1, respectively. The white particle was found to be basically CaO, carrying some Al and Si (Al + Si/ Ca = 1:3). The black matrix consisted of Al 2 O 3 /SiO 2 /CaO glassy mineral with melting point around 1350°C. CaCO 3 is calcined to be CaO in the bottom of the combustor where CO 2 partial pressure is low, whereas recarboxylation takes places in the fluidized bed where CO 2 partial pressure is high. The fine particles of Al 2 O 3 , SiO 2 and CaO less than 1 μm gather to form sphere, and then agglomerate glass, which lowers melting point of the mixed oxides. Excess CaO stay as white particles in and on the grain. Combustion of coal grains in the agglomerate releases heat and CO 2 , of which evolution through the melted agglomerates swells the glass to form the spherical shell. Disturbance of fluidization by such agglomerated ash in the bottom inhibits the heat release to raise the temperature beyond its melting point. Such melting accelerates further the formation of agglomerates, which disturb the fluidization through the adhesion in the narrow spaces between tubes to raise the temperature further, finally resulting in the plugging of melt agglomerate to force the shutdown of the operation.