摘要:

The concepts of fractal geometry are applied to the analysis of 0.4‐μm alumina constrained to agglomerate in two dimensions. Particles were trapped at the bottom surface of a drop of a dilute suspension, and the agglomeration process was directly observed, using an inverted optical microscope. Photographs were digitized and analyzed, using three distinct approaches. The results indicate that the agglomerates are fractal, having a dimension of approximately 1.5, which agrees well with the predictions of the diffusion‐limited cluster–cluster aggregation

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07228.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The plasma‐enhanced chemical vapor deposition (PECVD) of boron nitride films was studied using polymeric cyanoborane, (CNBH2)n, a material previously examined by thermally activated CVD. The PECVD procedure yields boron nitride coatings containing ≅20 wt% paracyanogen as a contaminant. This impurity can be removed by heat treatment under vacuum or in an ammonia atmosphere. The boron nitride coatings are hexagonal and appear to be boron deficient. The PECVD process takes place at 300°C, measured at the backside of the substrate, as compared with 600°C in the thermally activated CVD pr

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07229.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Substitution of Y3+in CaS results in the cubic rock‐salt‐type solid‐solution Ca1−xY2x/3□x/3S, withx0.37 and where □ denotes cation vacancies that form for charge compensation. The substitution range determined from polycrystalline samples prepared at 1025°C corresponds to a solubility limit of 16 mol% Y2S3in CaS. Evolution of the unit cell edge withxindicates that the CaS lattice expands about the cation vacancies. Single‐crystal X‐ray diffraction studies of samples grown from a eutectic CaCl2–KCl flux revealed no indication of superstructure formation, indicating that Ca2+, Y3+, and vacancies are disordered across the cation sites. Thermogravimetric analysis curves for Ca0.7Y0.2S, CaS, and the ternary compound CaY2S4indicate oxidative stability up to 430°, 410°, and

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07230.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

A novel and rapid method to prepare 123 superconductor powder based on microwave processing is reported. A mixture of the nitrates of Y, Ba, and Cu in 123 stoichiometry when subjected to microwaves of frequency 2.45 GHz for 240 s resulted in an appreciable quantity of tetragonal 123 phase. Annealing for a period of 4 h at 940°C in flowing oxygen formed a superconductor having aTcof 91 K. Sintered pellets of the powder formed by microwave processing have shown transportJcof 105 A/cm2

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07231.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Compositions of La1‐xBaxCuO3, where x ranges from 0.0 to 0.5, were fired in air, oxygen‐enriched air, and oxygen. Studies show that BaO cannot make a solid solution with LaCuO3without changing the basic structure. The resulting phases, in all attempts, were the binary compounds La2CuO4, La1‐xBaxCuO3‐δ(x= 0.2 to 0.5), or their mixtures. All samples showed metallic conductivity. Extra oxygen in the reaction atmosphere appeared to encourage the formation of the LaCuO3‐based phases of La1‐xBaxCuO3‐δ(x= 0.2 or 0.5). We provide a defect‐chemical and thermodynamical explanation for

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07232.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Resistively heated filaments are used to manufacture ceramic fibers. When an ac power source is used for heating, the temperature of the filament oscillates. These oscillations influence the deposition rate on the surface of the filament. An analysis of this problem is presented, and it is also shown that these oscillatory effects diminish as the filament gauge increases. The analysis can help to decide whether an ac or a dc power source should be used.

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07233.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Barium titanate (BaTiO3) thin films have been deposited on single‐crystal magnesia (MgO) substrates by pulsed‐laser ablation. Temperature dependence of capacitance measurements show a peak of 110°C, indicative that a ferroelectric phase transition has occurred at this temperature. This value is lower than that determined for single‐crystal BaTiO3, but consistent with that found for BaTiO3thin films produced by other methods. It has been demonstrated that the microstructure of the films can be varied by changes in the deposition parameters. Optimizing these variations permits the formation of thin films with controlled microstructures and prop

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07234.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

During an investigation of the role of alumina particles in ceria‐stabilized tetragonal zirconia polycrystals (Ce‐TZP)1we observed that a group of samples (based on a Ce‐TZP matrix (8.5 mol% Ceria) containing 15 vol% dispersed alumina particles), exhibited an extended plastic deformation after the first elastic response. This deformation could be retained upon a prompt unloading from a fourpoint bend jig. These deformed (bent) specimens recovered their original straight shape when heated above 200°C, exhibiting features like the shape memory effect (SME). A careful examination of the samples revealed the presence of wedgelike transformation zones intruding the bulk of the specimen inwards from the tensioned side.In an attempt to increase the degree of bending, other samples of the same composition and microstructure have been placed under a static load and maintained under that condition for ≅15 min. The penetration depth of the transformation zone reached the opposite face of the samples and the monoclinic‐phase concentration rose about 50% on both sides. Through this inwards movement, these samples, that were initially bent, showed an elongation of about 0.5% and became straightened. These samples returned to their original size after heating at ≅250°C. To get a deeper insight, samples have been examined by X‐ray diffraction (XRD), differential thermal analysis (DTA), and thermod

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07235.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The eutectic composition between Y4Al2O9and Y2O3was determined using electron probe microanalysis (EPMA) on directionally solidified specimens with hypo‐ and hypereutectic compositions. The microstructures of the specimens as a function of composition differ considerably with small deviation from the eutectic composition (70.5 mol% Y2O3and 29.5 mol% Al2O3). Based on the current results and other published data, the pseudobinary system between Al2O3and Y2O3is revise

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07236.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The thermal evolution of amorphous TiO2powders, consisting of spherical particles and prepared by hydrolysis of a titanium ethoxide aerosol, was studied by using Raman spectroscopy. On calcination at 350°C, the solid crystallized, giving anatase as a major phase. A small amount of rutile was also detected and attributed to small seeds localized at the particle outlayer. The nucleation of rutile at so low a temperature was ascribed to the presence of organic impurities in the powders. The transformation of anatase into rutile was clearly observed after heating at 660°

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07237.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY