Mass Transport and Chemical Demixing of Y-Ba-Cu-O Compounds During Sintering in Temperature Gradients

A thin cylindrical powder compact (3-mm diameter by 20-mm) with a nominal composition of YBa₂Cu₃O_x was sintered in a static temperature gradient of 25°C/mm in air for 12 h. The temperature at the hot end of the sample was maintained at 940°±5°C. Considerable mass transport and chemical demixing occurred along the length of the sample. The results are consistent with the formation of a liquid phase at the hot end of the sample, transport of the liquid down the temperature gradient, and the formation of different eutectic compositions along the length of the sample. The implications for processing high-T_c superconductors in the system Y-Ba-Cu-O are discussed.     

Gaseous reduction of an alloy oxide

Ni(Al, Fe)₂O₄ ceramic alloys were reduced by hydrogen gas at a pressure of 1 atm, and at temperatures between 450 and 800° C. The reaction rate was determined from the rate of advance of the porous metal product layer—unreduced oxide interface. A simple analysis was presented permitting assessment of both the interface reaction resistance and the gas transport resistant through the porous product scales. The reaction was under mixed control in all conditions studied. In a range of temperatures and reaction times, preferred grain-boundary attack was observed. The conditions under which this was observed depended strongly on the Al³⁺ content of the ceramic alloy. Al³⁺ also lowered the interface reaction rate and inhibited scale coarsening by formation of dispersed unreduced phases in the product scales.     

Oxidation behaviour of reaction-bonded alumina compacts using an Al88Si12 alloy precursor

The oxidation behaviour of attrition-milled Al₈₈Si₁₂/Al₂O₃ powder mixtures was investigated for the formation of mullite/Al₂O₃ composites by the reaction bonded alumina (RBAO) process. Cylindrical powder compacts were heated at 5°C min^–1 to temperatures between 450 and 1400°C. Oxidation occurred rapidly between ca. 400 and 750°C. Dense, outer reaction layers which formed at the lower temperatures inhibited complete oxidation and led to fracture of the body during continued heating to higher temperatures (above ca. 850°C) While the incorporation of ZrO₂ improved the oxidation of the samples, X-ray analysis indicated that the Si in the alloy reacted with the ZrO₂ to form phases which prevented the formation of mullite at the temperatures used in the experiments.     

Secondary Phases in Hot-Pressed Aluminum-Boron-Carbon–Silicon Carbide

Silicon carbide hot-pressed with aluminum, boron, and carbon as sintering aids (ABC–SiC), was studied by transmission electron microscopy. Both grain-boundary films and inclusions were prevalent in this material. The present study characterized the inclusions located in triple-junctions, grain boundaries, and the interior of the SiC grains, with emphases on phases not scrutinized before. These inclusions were crystalline, in contrast to the amorphous grain-boundary films. Two dominant types of boron-free triple-junction phases containing Al(Si)-O-C-(S) and Al(Si)-O were identified, where sulfur was an unexpected contaminant, and silicon came from SiO₂ or from dissolution of SiC. Boron-containing inclusions with a composition Al-O-B-C were frequently observed inside SiC grains. Although the boron-free aluminum-rich phases wet the grain boundaries completely and are, therefore, effective sintering additives, the boron-containing Al(Si)-O-B-C did not wet the grain boundaries. The structure and chemical composition of these boron-containing intragranular inclusions were determined, and their mechanism of formation is discussed.     

Inclusion Size and Sintering of Composite Powders

Ceramic powders containing dispersed inert particulates often exhibit dramatically lower densification rates than the pure powder. This decrease results from the shrinkage incompatibilities of the matrix and the inclusion during densification. The analyses of this phenomenon all predict that the shrinkage rate is only dependent on the overall volume fraction of the inert second phase, and not on the particle size. Experiments on ZnO containing dispersed SiC particles of different size showed that the retarding effect of the second phase increases significantly with decreasing particle size, at constant SiC volume fraction.     

Reaction Sintering of Zinc Ferrite during Constant Rates of Heating

The reaction sintering of equimolar quantities of zinc oxide and ferric oxide was investigated under conditions of constant rates of heating (1–10°C/min from room temperature to 1350°C) and the data were compared with those for a calcined, single-phase zinc ferrite powder. For the heating rate of 1°C/min, the densifications of the reaction-sintered sample and the calcined sample were approximately the same. However, as the heating rate increased, the density at any temperature increased slightly for the reaction-sintered sample but decreased slightly for the calcined powder. The factors responsible for this slight difference in sintering between the reaction-sintered sample and the calcined sample are discussed. For the constant heating rates used, the reaction was completed prior to any significant densification. Relative densities of >95% were obtained for both the reaction-sintered sample and the calcined sample under identical sintering conditions (1–10°C/min to 1350°C). Reaction sintering in a steep temperature gradient produced a nearly fully dense body prior to complete reaction; a composite microstructure consisting of fine zinc oxide grains in a matrix of zinc ferrite was obtained.     

Synthesis and Biological Evaluation of Pyrrolo[2,1‐f][1,2,4]triazine C‐Nucleosides with a Ribose, 2′‐Deoxyribose,and 2′,3′‐Dideoxyribose Sugar Moiety

The synthesis of hitherto unknown pyrrolo[2,1‐f][1,2,4]triazine C‐nucleosides is described. Structural variations (chlorine, bromine, iodine, and cyano groups) were introduced at position 7 of 4‐aza‐7,9‐dideazaadenine. In addition, pyrrolo[2,1‐f][1,2,4]triazine C‐nucleosides bearing a 2′‐deoxy‐, 2′,3′‐dideoxy‐, and 2′,3′‐dehydrodideoxyribose moiety were also prepared. Among these analogues, the pyrrolo[2,1‐f][1,2,4]triazine C‐ribonucleosides with either a hydrogen atom or cyano group at position 7 of the nucleobase displayed potent cytotoxic activity in a panel of various cancer cell lines.