摘要:

The effect of temperature on the formation of creep substructure in sodium chloride single crystals was studied in the temperature range 573 to 873 K. Microstructural observations conducted on polished specimens deformed under a constant value of normalized stress, σ/G∼ 4 × 10−4, where σ is the applied stress andGis the shear modulus, showed different types of substructural features such as a high density of dislocation etch pits, cells, and subgrains. Detailed measurements of the sizes of the cells and subgrains revealed that the cell size was essentially independent of temperature while the subgrain size increased only by about a factor of 2 for a 300 K increase in temperature. Although the cell and subgrain sizes were not significantly dependent on temperature in comparison to the effect of stress, there was a qualitative change in the substructure morphology with a variation in temperature; i.e., the cells and subgrains were better defined at higher temperatures. The volume fraction of the cell boundaries decreased with increasing temperature, thereby indicating a refinement of the microstructure at the higher temperatures. These observations suggest that while temperature does not change the final dimensions of the substructure significantly, it appears to influence the kinetics of substructure formation by influencing the recovery rate. As a result, the stresses acting on the cell boundaries increase considerably with increasing temperature to values several times the applied

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb08185.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The hydration behavior at 25°C of β‐dicalcium silicate synthesized from hillebrandite (Ca2,(SiO3)(OH)2) at 600°C was studied over a period of 224 d. The hydration rate of the β‐dicalcium silicate having fibrous crystals with specific surface area of 7 m2/g is extremely rapid. For water/solids ratios of 0.5 and 1.0, the hydration reaction is completed in 28 and 14 d, respectively. The hydrate contains almost no Ca(OH)2, and its Ca/Si ratio is close to 2. SEM observations indicate that the hydrate forms an outer shell on the surface of β‐dicalcium silicate and grows inwards. The silicate anion structure is considered to consist of dimers and single‐chain structures from29Si MAS NMR. Variations of physical properties of press‐formed bodies have also

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb08186.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The hydration behavior at 25°C of highly reactive β‐dicalcium silicate synthesized from hillebrandite (Ca2(SiO3)(OH)2) was studied over a period of 7 to 224 d using29Si magic‐angle spinning nuclear magnetic resonance (MAS NMR). The hydration product, C‐S‐H, contains Q2and Q1silicate tetrahedra, the chemical shifts of which are independent of the water/solid (w/s) ratio and curing time. Until the reaction is completed, the amounts of Q1and Q2formed are independent of the w/s ratio, being determined only by the degree of reaction. The ratio Q2/Q1increases as the reaction progresses and as the curing time becomes longer. From the values of Q2/Q1, it appears that the hydrate is a mixture of dimers and short single‐chain polymers. The Ca/Si ratio of the hydrate is high, taking values close to 2.0, but the Ca/Si ratio does not influence the Q2/Q1ratio. It was also found that the NMR peak intensities allow quantitative assessment sim

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb08187.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The strength of bare silica fiber in sodium chloride solutions decreases as the NaCl concentration increases. Static fatigue measurements in deionized water (pH 6) are compared to those in a 2 mM solution of NaCl (pH 5.2), both at 90°C. The pretransition stress corrosion parameter, determined from the power law, is 33 in deionized water and 18 in the NaCl soution. The effect of BaCl on the reaction mechanism in the pretransition region is investigated. The failure rate is proportional to the surface charge of silica raised to the second power

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb08188.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The performance of a stabilized zirconia sensor and electrochemical oxygen pump combination in the isothermal control of oxygen fugacity in a nonflowing atmosphere was studied in a closed furnace system in the temperature range of 800° to 1100°C. Under certain conditions, large differences in oxygen pressure were found between sensors at the same constant temperature in close proximity to each other. This result reflected the existence of a stable steplike oxygen fugacity “front” in the furnace, separating regions which differed by several orders of magnitude in oxygen fugacity. This oxygen pressure profile in the furnace resulted from the steady‐state transport of oxygen from oxygen leakage sources to the pump. The existence, magnitude, and position of the oxygen fugacity front were found to depend on the gas‐phase composition, the relative locations of the leakage sources and the pump, the oxygen fugacity of the reference electrode, and the magnitude of the oxygen flux in the

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb08189.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Experimentally observed deviations from ideal isothermal behavior of solid electrolyte sensors are explained on the basis of gas‐phase diffusion in the furnace atmosphere. The presence of small amounts of CO/CO2or other oxygenbearing gas species is essential to the theoretical explanation of these effects. Solution of the basic transport equations for limiting conditions appropriate to the control of a stagnant atmosphere by a sensor/pump combination indicates that stable fugacity fronts may exist, separating regions of relatively uniform oxygen fugacity which differ from each other by many orders of magnitude. The position of the front depends on the oxygen fugacity difference between the two regions of the furnace and can be moved through the furnace by electrochemically pumping oxygen from one end of the furnace. The relative output of two sensors separated by some finite distance in the direction in the direction of oxygen transport will depend on the position of this front, and as the front passes between the sensors a plateau in the curve of one sensor emf versus the other is predicte

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb08190.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The thermodynamic stability of spinel and rock‐salt structure phases has been investigated for the quasi‐ternary mixed oxide system Co‐Fe‐Mn‐O at 1200°C and at total pressures of the order of 1 atm (∼1.01 × 105Pa) using thermogravimetric and electrical conductivity measurements. The results reveal the stability limits of the spinel and rock‐salt structure phases with varying cationic composition and oxygen partial pressure at 1200°C. The oxygen partial pressure was varied over 12 orders of magnitude, from log10ao2= 0 to log10ao2= ‐12, by using CO/CO2and N2/O2gas mixtures and monitored by an electrochemical, stabilized zirconia‐based EMF cell. The maximum cobalt mole fraction used in the investig

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb08191.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Curium‐doped titanate ceramic containing sodium‐rich high‐level nuclear waste showed a gradual decrease in density up to a dose of 8.5 × 1017α decays ·g−1. After that, the rate of density change increased apparently because of crack formation. Optical microscopy showed cracks>0.1 mm long and>1 μm wide after a dose of 7.9 × 1017α decays ·g−1. Leach tests suggested that the dissolution‐control phases for sodium and cesium changed from freudenbergite and hollandite, respectively, to intergranular phases after significant cracking. Aging also enhanced strontium losses, relative to calcium, indicating that strontium may also be partitioned to the intergranular phases. After the fresh surfaces produced by cracking were exposed to leachant, and the dissolution of soluble intergranular surfaces was complete, the leaching of nonradioactive elements from the samples having a dose of 12.3 × 1017α decays ·g−1was limited by the following dissolution‐control phases: freudenbergite (Na), hollandite (Cs and Ba), perovskite and/or zirconolite (Sr and Ca), and alloys (Mo). The leaching behavior of the nonradioactive indicator elements revealed that chemical durability was reduced by two main factors: (1) increasing the effective surface area by crack formation and (2) decreasing the stability of the actinide‐host phases by α‐recoil damage. In combination these factors increased longer‐term (>7 days) leach rates of sodium and cesium, and strontium and calcium by 1 and 2 orders of magnitude, respectively. In spite of deterioration of the actinide‐host phases, the curium leach rate after a dose of 12.3 × 1017α decays x g−1decreased by 2 orders of magnitude, possibly as a result of preci

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb08192.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The reaction of La1‐xCaxMnO3(x= 0, 0.1, 0.2) with ZrO2‐8 mol% Y2O3(YSZ) has been investigated at temperatures ranging from 1300° to 1425°C in air. Substitution of Ca for La in LaMnO3depresses the reactivity with YSZ. A layer of La2Zr2O7is formed at the La1‐xCaxMnO3/YSZ interface after an induction period, and its formation is accelerated when the La1‐xCaxMnO3phase is porous. The reaction proceeds by unidirectional diffusion of La, Mn, and/or Ca ions, mainly Mn ions, into YSZ. The diffusion coefficients of La and Mn ions in YSZ, which are estimated using a LaMnO3/single‐crystal YSZ couple, are much lower than that of oxygen ion. From the experimental data, a reaction mechanism

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb08193.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The room‐temperature mechanical properties of a SiC‐fiberreinforced reaction‐bonded silicon nitride composite were measured after 100 h treatment in nitrogen and oxygen environments to 1400°C. The composite heat‐treated in nitrogen to 1400°C showed no appreciable loss in properties. In contrast, composites heat‐treated in oxygen from 600° to 1000°C retained ∼65% and 35% of the matrix fracture and ultimate strength, respectively, of the as‐fabricated composites, and those heat‐treated from 1200° to 1400°C retained greater than 90% and 65% of the matrix fracture and ultimate strength, respectively, of the as‐fabricated composites. For all nitrogen and oxygen treatments, the composite displayed strain capability beyond the matrix fracture strength. Oxidation of the fiber surface coating, which caused degradation of bond between the fiber and matrix and reduction in fiber strength, appears to be the dominant mechanism for property degradation of the composites oxidized from 600° to 1000°C. Formation of a protective silica coating at external surfaces of the composites at and above 1200°C reduced oxidation of the fiber coating and hence degrading effects of oxi

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb08194.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY