摘要:

From the viewpoint of industrial and automobile exhaust pollution control, a sensor capable of detecting and measuring the concentration of harmful gases such as carbon monoxide and hydrogen in the ambient is desired. This paper describes the development and use of a titania (TiO2)‐based semiconducting oxide material as a reliable and rugged CO and H2gas sensor in the temperature range of 773 to 1073 K. Significant change in the sensing characteristic of the anatase modification of TiO2was observed when admixed with an insulating second phase, such as alumina or yttria. In the case of TiO2–10 wt% Al2O3, the sensor response was found to be exclusively dependent on the hydrogen concentration alone; the presence of CO or CO2did not affect the sensitivity. On the other hand, the sensor based on TiO2–10 wt% Y2O3showed increased sensitivity to CO and decreased sensitivity to H2, compared to that of the undoped TiO2. Addition of elemental iron, in small concentration, to the two‐phase mixture of titania and yttria seemed to further improve the sensitivity and selectivity of the latte

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04372.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The mechanical properties of a noval laminar composite are investigated. The composite consists of dense alumina sheets bonded between sheets of a uniaxial carbon‐fiber‐reinforced epoxy tape. The behavior of the composite in both flexural and tensile loading is characterized and the results are related to the properties of the constituents. The role of the interlaminar interface in composite behavior is also examined. Implications for the design of laminar composites with complex shapes are briefly discus

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04373.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The cyclic fatigue crack growth behavior in two aluminas with different grain sizes and purities has been investigated using tapered cantilever beam specimens. The curves of load–crack opening displacement revealed a hysteretic loss behavior, in both aluminas, produced by frictional forces generated in the crack wake. From measurements of the specimen complinces it is concluded that the fatigue cracks in these materials were strongly bridged by both “frictional” and “elastic” load‐bearing ligaments. It is shown that the “elastic” contribution to the total crack‐bridging force is far greater than that produced by fristional sliding ligaments. The results support the view that cyclic loads produce degradation of the strength of the bridging ligaments. The effect of grain size and compressive loads on the magnitude of the bridging fo

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04374.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The ZrC‐coated UO2particle is a promising fuel for high‐temperature gas‐cooled reactors. Particle fuels with multiple layers of pyrolytic carbon and ZrC have been irratiation‐tested to a maximum fast‐neutron fluence exceeding 2 × 1025/m2(E<29 fJ). In‐reactor fission‐gas release measured at a burnup of 1.5 at.% was minimal. The failure fraction by postirradiation examination was null for all samples. The ZrC‐coated particles at 4 at.% burnup were postirradiation heated to 2400°C/min without failure until after 6000 s at the maximum temperature. It was found that the ZrC layer could sustain a large strain at such high temperatures. The behavior is in strong contrast with that of SiC of standard Triso coating, which is brittle to very

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04375.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Crack tip shielding is evaluated for observed transformation zones in Ce‐TZP/Al2O3composites, in which the transformation zone sizes were changed significantly by varying the sintering temperature to control the transformation yield stress. The calculated shielding effects are consistent with an observed insensitivity of crack resistance curves to transformation zone size; smaller zones in materials with higher yield stress were associated with larger ratios of wake length to zone width and correspondingly higher normalized shielding stress intensity factors. Shielding due to the dilatational component of the transformation strain accounted for most of the toughening observed in these material

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04376.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The results of two‐step oxidation experiments on chemically‐vapor‐deposited Si3N4and SiC at 1350°C show that a correlation exists between the presence of a Si2N2O interphase and the strong oxidation resistance of Si3N4. During normal oxidation,kpfor SiC was 15 times higher than that for Si3N4, and the oxide scale on Si3N4was found by SEM and TEM to contain a prominent Si2N2O inner layer. However, when oxidized samples are annealed in Ar for 1.5 h at 1500°C and reoxidized at 1350°C as before, three things happen: the oxidationkpincreases over 55‐fold for Si3N4, and 3.5‐fold for SiC; the Si3N4and SiC oxidize with nearly equalkp's; and, most significant, the oxide scale on Si3N4is found to be lacking an inner Si2N2O layer. The implications of this correlation for the competing models of Si3N4oxidation a

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04377.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Aluminofluoride and oxyfluoride melts are essential in the electrolytic smelting of aluminum, as the precursors of fluoride glasses, and as analogue systems for molten silicates. In these systems, the quantification of the abundances of structural species is crucial in the understanding of both thermodynamic and transport properties.27Al NMR spectroscopy at high temperatures can provide important constraints on speciation and coordination number in aluminofluoride liquids, as well as structural and dynamical changes in the crystalline fluorides. We present results on a series of solids and liquids in the NaF–AIF3–Al2O3system. These illustrate the dramatic effects of composition and temperature on liquid structure and suggest that most Al cations are four‐ or six‐coordinated, with a relatively low abundance of five‐coordina

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04378.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Thin films of potassium tantalum niobate (KTN) formed by metalorganic deposition were subjected to one‐ and two‐step sinterings in air and potassium atmosphere. These polycrystalline films were found to be mechanically robust and display electrical properties similar to single‐crystal KTN. This report shows that film grain size increased with sintering temperature and time. Scanning electron micrographs revealed grain sizes ranging from submicrometer to 5 μm in diameter. The maximum relative permittivity, its corresponding rate of change with temperature, and film grain size were measured as a function of sintering temperature an

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04379.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The relationships between the constants of the Herschel‐Bulkley (H‐B) equation describing plastic flow and the Benbow equation for extrusion through a square‐entry die were studied via computer simultions. These relationships are fundamental for characterizing the rheology of ceramic extrudates using the square‐entry rheometer and are difficult to obtain from experiments. Results showed that the apparent shear rate should be used instead of extrudate velocity in the Benbow equation, and the Benbow equation was modified accordingly. The exponent in the H‐B equation was exactly the same as the shear thinning exponent for the extrudate body in the modified Benbow equation. The die‐entry velocity factor was a function not only of the shear rate factor but also of the shear thinning exponent in the H‐B equation. The yield strength of the modified Benbow equation was zero when the yield strength in the H‐B eq

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04380.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The tensile properties of three carbon matrix composites reinforced with SiC (Nicalon) fibers (materials A, B, C) have been measured with and without notches. One of the three materials (material B) had a relatively low strength and exhibited notch brittleness. This material had both a high interface sliding stress and a low fiber bundle strength, caused by particulates in the matrix. These characteristics have been shown to result in a change in failure mechanism that leads to the inferior properties exhibited by material B. The notch properties of the higher‐toughness materials were shown to involve splitting, which alleviates the notch stress concentration and diminishes the notch sensitivit

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04381.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY