摘要:

Traditionally, new materials are developed in response to the requirements of an emerging device technology. In the optical fiber case, development of glass as a transmission medium came as the culmination of a century‐long quest for improved broadband telecommunications, a search that established the superiority of light signals transmitted through glass over electronic signals transmitted by wir

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb04256.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

A batch coprecipitation of 1‐2‐3 superconductor precursor powder was accomplished by adding an aqueous solution of mixed nitrates rapidly to a mixture of sodium or potassium hydroxide and carbonate. The blue hydrogel, an intimate mixture of Y(OH)3, BaCO3, and Cu(OH)2, was filtered and washed by decantation, using water sparged with CO2to control the pH. After a second filtration, drying and calcination, the product was a homogeneous powder, with an average particle size of about 1 μm, free of alkali, and with excellent superconducting properties. The process is reproducible, inexpensive, and easy to scale to higher production rates. Important process parameters are the following: the time and temperature of precipitation; alkali ratios; volume, temperature, and pH of the wash water; and time of calcina

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb04257.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

Pure BaTiO3exhibits a paraelectric‐to‐ferroelectric phase transition at 130°C. When stoichiometric BaTiO3is combined with 10 mol% ZrO2, the relative permittivity (ε) changes to a broad, relatively insignificant temperature dependence, and the Curie point,Tc, is not sharply defined. However, the transition sharpens atT= 95°C when these samples are sintered for a longer period of 60 h. SEM, EDAX analysis coupled with TEM observation gives three types of core‐shell structures of different microstructural characteristics which are related to the diffuse phase transition. Chemical inhomogeneity, due to Zr4+distribution in the core‐shell structure, is proposed to account for the diffuse phase transitio

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb04258.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

Superconducting coupling nature at grain boundaries in Bi2Sr2CaCu2Oxglass‐ceramics consisting mainly of the low‐Tcphase was first examined by measuring superconducting properties and temperature or ac field dependence of ac complex susceptibility. It was found from the ac loss peaks that superconducting coupling at grain boundaries was basically characterized by three types of weak links. The weak‐link behaviors at grain boundaries depended strongly on cooling conditions after annealing and annealing time and temperature. Particularly, it was found that the weak links at grain boundaries were improved by prolonged annealing at 840°C. The furnace‐cooled glass‐ceramics obtained by annealing at 820° or 840°C for about 200 h exhibited a critical transport current density (77 K, zero magnetic field) of abo

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb04259.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

Near‐eutectic composition Y2O3─SiO2melts were formed as bulk samples or as an intergranular phase in Si3N4. Upon cooling to room temperature the bulk material partially crystallized to γ‐Y2‐Si2O7whereas the intergranular phase was glass. On heat‐treating at 1500°C the bulk material transformed to γ‐Y2Si2O7whereas the intergranular glass crystallized first to γ‐Y2Si2O7and then to β‐Y2Si2O7. Possible reasons for the different be

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb04260.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

The oxidation of chemically vapor‐deposited SiC in wet O2(water vapor partial pressure = 0.01 MPa, total pressure = 0.1 MPa) was examined using a thermogravimetric technique in the temperature range of 1823 to 1923 K. The oxidation kinetics follow a linear‐parabolic relationship over the entire temperature range. The activation energies of linear and parabolic rate constants were 428 and 397 kJ · mol−1, respectively. The results suggested that the rate‐controlling step is a chemical reaction at an SiC/SiO2interface in the linear oxidation regime, and the rate‐controlling step is an oxygen diffusion process through the oxide film (cristobalite) in the parabolic oxidat

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb04261.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

The effects of Si and ZrO2dopants on the crystallization and phase transformation process in Li2O · Al2O3· 6SiO2glasses were investigated using differential thermal analysis, X‐ray powder diffractometry (XRD), and high‐resolution transmission electron microscopy (TEM) interactively. Phase separation was observed in the studied glasses prior to substantial crystallization. Elemental Si (1 mol%) significantly aided in glass devitrification. Dropletlike phase‐separated regions in the as‐quenched or heat‐treated glass devitrified at ∼760°C, which in turn provided sites for the heterogeneous nucleation and growth of β‐quartz(ss) (solid solution), which transformed to β‐spodumene(ss) at higher temperature. Low‐temperature surface crystallization in these glasses occurred as low as 760°C. ZrO2has limited solubility in this glass system. Small ZrO2crystallites (·5 nm) in the as‐quenched glass acted as sites for the heterogeneous nucleation and subsequent growth of large (<5 μm) β‐quartz(ss) crystals in glasses containing 1.0 mol% or more ZrO2. The transformation from β‐quartz(ss) to β‐spodumene(ss) was increasingly inhibited with ZrO2additions. The nucleating efficiency of Si was significantly greater t

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb04262.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

Monazite‐type CePO4powder (average grain size 0.3 μm) was dry‐pressed to disks or bars. The green compacts began to sinter above 950°C. Relative density ≧ 99% and apparent porosity<1% were achieved when the specimens were sintered at 1500°C for 1 h in air. The linear thermal expansion coefficient and thermal conductivity of the CePO4ceramics were 9 × 10−6/°C to 11 × 10−6/°C (200° to 1300°C) and 1.81 W/(m · K) (500°C), respectively. Bending strength of the ceramics (average grain size 4 μm) was 174 ± 28 MPa (room temperature). The CePO4ceramics were cracked or decomposed by acidic or alkaline aqueous solution

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb04263.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

Subcritical crack growth under mixed‐mode loading was studied in soda‐lime glass. Pure mode I, combined mode I and mode II, and pure mode II loadings were achieved in precracked disk specimens by loading in diametral compression at selected angles with respect to the symmetric radial crack. Crack growth was monitored by measuring the resistance changes in a microcircuit grid consisting of parallel, electrically conducting grid lines deposited on the surface of the disk specimens by photolithography. Subcritical crack growth rates in pure mode I, pure mode II, and combined mode I and mode II loading could be described by an exponential relationship between crack growth rate and an effective crack driving force derived from a mode I‐mode II fracture toughness envelope. The effective crack driving force was based on an empirical representation of the non‐coplanar strain energy release rate. Stress intensities for kinked cracks were assessed using the method of caustics and an initial decrease and a subsequent increase in the subcritical crack growth rates of kinked cracks were shown to correlate with the variations of the mode I and the mode II stress inte

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb04264.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

Oblique impact damage to gas‐pressure‐sintered silicon nitride is investigated by examining changes in the stress field beneath the impact site of the silicon nitride. Varying the impact angle changes the morphologies of the initiated cracks from Hertzian cone crack to surface ring crack as the impact angle decreases. Moreover, impact at greater angles (90° and 60°) creates Hertizian cone cracks that drastically decrease the strength of the target materials; impact at smaller angles (45° and 30°), on the other hand, creates surface ring cracks that cause only moderate strength degr

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb04265.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY