摘要:

An oxide capacitor consisting of BaTiO3and an oxide is studied as a new type CO2sensor based on capacitance change. Sensitivity to CO2, as well as the optimum operating temperature, was strongly dependent on the particular oxide mixed with BaTiO3. Among the elements investigated in this study, CuO–BaTiO3exhibited the highest sensitivity to CO2. In particular, the CuO–BaTiO3mixed oxide at the equimolar composition is highly sensitive to CO2. The optimum operating temperature and frequency for CuO–BaTiO3are 729 K and 100 Hz, respectively, and the 80% response time to 2% CO2is within 25 s. The equimolar mixture of CuO and BaTiO3can measure the CO2concentration from 100 to 60 000 ppm. Carbonation of oxide seems to play a key role for the detection of CO2on these mixed oxide capacitors. The optimum operating temperature of these mixed oxide capacitors for CO2detection, therefore, correlates with the decomposition temperature of the carbonate corresponding to the oxide mixed with BaTiO3. The capacitance increase of CuO–BaTiO3upon exposure to CO2seems to result from the elevated height of the potential barrier at the grain boundary between CuO and BaTiO3. Carbonation of CuO in the element seems to bring about the elevation in the height of the potential

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07850.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

After a discussion of the thermochemical values of the Si–H–Cl–N system which occur in the literature, CVD phase diagrams are presented which include contours of constant deposition efficiency. The temperature range considered is from 800 to 2600 K. A number of chlorinated silanes as well as silane can be used as a silicon source, while ammonia is used as the nitrogen source. The effects of pressure variation and dilution by nitrogen and hydrogen are also included. Some initial calculations concerning silicon diimide are made. The CVD phase diagrams are used to describe several mechanisms occurring during the formation of silicon nitride from the gas

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07851.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Alumina‐calcia fibers with>50% alumina and 100% alumina fibers cannot be drawn from the melt, or conventionally melt spun, because the viscosities are too low. They can, however, be spun by inviscid melt spinning, an experimental process whereby a molten jet with a viscosity of<1 Pa.s is ejected into propane, a chemically reactive medium. The pyrolytic decomposition of propane stabilizes the molten jet. The consolidated fiber that results has a carbon‐rich skin and usually, i.e., under most process conditions, but not always, a black carbon sheath. This paper identifies the chemistry and morphology of both skin and sheath by depth profile analysis, evaluates potential jet stabilization mechanisms, and concludes that the operative mechanism is rheology‐dominated. Incorporation of particulate carbon in the skin of the jet increases its surface viscosity, prevents breakup into Rayleigh waves and droplets, and facilitates melt spinning of continuous fila

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07852.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Failure mechanisms of an alumina, tested at 1200°C under static and various cyclic loading conditions, were examined. Slow crack growth of a single crack is the dominant mechanism for the failure in specimens under cyclic loading with a short duration of maximum stress at all applied stress levels, as well as at high applied loads for static loading and cyclic loading with a longer hold time at maximum stress. At low stress levels, failure of static loading and cyclic loading with a longer hold time at maximum stress might occur by formation and/or growth of multiple macrocracks. More importantly, for all the given loading conditions. The viscous glassy phase behind the crack tip could have a bridging effect on the crack surfaces. A simplified model for calculating effective stress intensity factor at the crack tip under static and various cyclic loading demonstrated a trend consistent with the stress–life da

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07853.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Ceramic‐air composites were prepared from laminated and sintered tapes that contain ceramic powder in a binder matrix and that have carbon‐containing “fugitive” inks screened onto them. The resulting laminates contained initially more than 50% by volume of organic material. This had to be eliminated (“burned out”) before sintering. The results of thermogravimetric analyses demonstrated the frequent occurrence of exothermic reactions when the burnout was conducted in air. This was prevented by lowering the partial oxygen pressure, but only to a point where reducing conditions were prevented since the latter caused carbonization, incomplete burnout, and increased lead evaporation. This was particularly important when a heavy kiln load consumed a significant fraction of the available oxygen. Weight loss, X‐ray fluorescence, and electrical parameter determinations were then used to optimize the burnout conditions. As a result it became possible to make ceramic–air composites that had consistent electrical behavior and that tolerated high hydros

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07854.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Changes in the surface area of anatase TiO2were investigated at 690 K as a function of the gas composition in mixtures of hydrogen chloride, water vapor, and oxygen. The chlorine content on the catalyst support was not dependent on the annealing time, which indicates that the adsorption equilibrium is reached very quickly. The kinetics of surface area were obtained as a function of partial pressure of HCI (from 0.1 to 40 kPa), water vapor (from 0.05 to 10 kPa), and oxygen (from 0.4 to 20 kPa). A mathematical expression for the rate of surface area loss was obtained which includes the partial pressure of water and the chlorine content at the surface of anatase.

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07855.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

From the experimental rate for surface area reduction of anatase TiO2, expressed versus the partial pressure in H2O and the surface content of chloride ion, a kinetic model is proposed in which two parallel mechanisms are involved. Two ways of material transport appear to contribute to the surface area loss: in the vapor phase and at the surface of the oxide. The related rate‐determining steps are attributed to the formation of the volatile compound Ti(OH)2Cl2in the first case, and to the diffusion of hydroxyl ions in the second one. At high partial pressure of water, the rate is enhanced by surface interactions between water molecules and chlorine‐containing spec

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07856.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

A study is made of the factors that contribute to the energy of mica‐mica, silica‐silica, and mica‐silica interfaces in the presence of moist atmospheres. Energies are measured using brittle fracture and contact adhesion techniques. Both “virgin” and “healed” interfaces are investigated, with special attention on the latter. The fracture and adhesion data overlap, reflecting a common underlying separation process by “sharp‐crack propagation.” The study identifies several contributors to the interface adhesion energies. At virgin mica‐mica and silica‐silica interfaces the energy is determined by primary ionic‐covalent attraction, and by the screening of this attraction by condensed moisture from the atmosphere. At healed interfaces the energy depends on both environmental interaction and “lattice” coherence. At retracted cracks in mica‐mica most of the virgin ionic attraction is retained. On misorienting separated cleavage halves prior to recontact the interaction energy drops substantially: in “dry” atmosphers (relative humidity50%) capillary forces dominate. The dissimilar mica‐silica system exhibits the same dominance by capillary forces in wet atmospheres. However, in dry atmospheres the adhesion energy becomes inordinately high, from spontaneous transfer of electronic charge from one surface to the other. The implications of these observations concerning mechanic

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07857.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Reaction‐bonded silicon nitride (RBSN) with little or no residual silicon was formed from laser‐synthesized silicon powders and N2, at low temperatures and in short times (<10 min at 1250°C,<1 h at 1150°C). The nitriding kinetics were studied with thermogravimetric analysis (TGA). Samples formed in the TGA furnace between 1150° and 1350°C were completely converted to silicon nitride near their centers, with residual silicon near the external surfaces. These unusual composition profiles were not observed in samples that were formed in a higher‐purity atmosphere. Various experimental observations were compared to analyses of heat and mass transport, and it was determined that these composition in the nitriding gas. These analyses also indicated that heat transfer, N2diffusion, and Si evaporation and transport all affect the nitriding kinetics under certain c

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07858.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The hydrolysis of uranium oxycarbide (UCO) was studied by injecting water vapor into a fuel element containing a known number of high‐temperature, gas‐cooled reactor (HTGR) coated particles with exposed fuel kernels. The experiments were conducted in the High Flux Isotope Reactor (HFIR) at temperatures between 700° and 1000°C, a system pressure of 200 kPa, and partial pressures of water vapor between 21 and 199 Pa. The general sequential response of the exposed fuel kernels to water vapor addition consisted of (1) a rapid release of stored fission gas with a concomitant increase in the steady‐state release and (2) a period of constant steady‐state release. Upon cessation of the addition of water vapor, a decline in the release to prehydrolysis values generally occurred. The release of stored fission gas was dependent on the square of the partial pressure of water vapor. The ratio of the constant steady‐state release to the prehydrolysis value was independent of the partial pressure of water vapor. The time constant for the decline in the release was the same in all of the hydro

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07859.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY