Fabrication of Graded Nickel–Alumina Composites with a Thermal-Behavior-Matching Process

Composites of nickel and Al₂O₃ with compositionally graded microstructures were fabricated from powders through an empirically determined thermal-behavior-matching process that was designed to minimize processing-induced stresses. Compositions ranged from pure Al₂O₃ to pure nickel. Specimen geometries included round disks 25 mm in diameter and 5–25 mm thick, as well as rectangular bars 25 mm × 25 mm in cross section and 75 mm long. Several different gradients were produced, including samples with single interlayers. Compacts were formed by cold uniaxial pressing in a die, followed by consolidation through sintering at 1 atm or hot isostatic pressing. Several different particle sizes of nickel and Al₂O₃ comprised the composite interlayers. The compaction behavior, sintering start temperature, sintering rate, and total linear shrinkage of each composition were evaluated. Careful data analysis, coupled with sintering theory, led to a layer configuration with matched green density and sintering behavior. Thermomechanically matched layers allowed large, crack-free, graded composites to be produced.     

Surface Exchange of Oxygen in La1−x Sr x FeO3−δ (x = 0, 0.1)

The electronic charge carrier concentration in La_1?x Sr _x FeO_3?δ was shown to depend on the partial pressure of O₂ (pO ₂). Chemical diffusion coefficient and surface exchange coefficient, k _chem, were determined by conductivity relaxation in O₂/N₂ and CO/CO₂ mixtures. k _chem was proportional to pO ₂ ^1.06 in O₂/N₂, while in CO/CO₂ k _chem was controlled by a reaction mechanism involving both CO and CO₂.     

Carbon dioxide reforming of methane over lanthanum-modified catalysts in a fluidized-bed reactor

Nickel on lanthanum-modified aluminas were prepared and tested as catalysts for the CO₂ + CH₄ 2CO + 2H₂ reaction in a fluidized-bed reactor. Attrition tests show that lanthanum increases the strength of the carrier significantly. TPR and XRD results indicate that nickel enters into positions in the Al-O spinel blocks of the subsurface LaAl₁₂O₁₉ phase. This results in lower reducibility and thus lower activity for the reforming reaction as compared to nickel on unmodified alumina. While the unmodified catalyst deactivated rapidly under the chosen test conditions, the modified catalysts seem to be stable over several hours. However, the stability is sensitive to the pretreatment conditions of the catalysts.     

Role of Oxygen in Microstructure Development at Solid-State Diffusion-Bonded Cu/α-Al2O3 Interfaces

Microstructure development at solid-state diffusion-bonded Cu/α-Al₂O₃ interfaces has been studied using optical and electron microscopy. High-purity Cu foil was bonded between basal-oriented α-Al₂O₃ single-crystal plates at 1040°C for 24 h in a vacuum of ∼1.3 × 10⁻⁴ Pa (1 × 10⁻⁶ torr). Optical microscopy of as-bonded specimens revealed a large Cu grain size, fine pores, and long needles of Cu₂O at the interface. Bulk specimens were annealed at 1000°C for various times under controlled oxygen partial pressures in CO/CO₂ mixtures. Consistent with a thermochemical analysis, CuAlO₂ could be formed at the interfaces. The CuAlO₂ was acicular and discontinuous, but occurred in a uniform distribution over the bulk specimen interfaces.     

Uncertainties in heat-pump coefficient of performance (COP) and exergy efficiency based on standardized testing

The uncertainties allowed in testing following international (ISO) and European (CEN) standards for testing of residential heat pumps (HPs) were investigated and quantified. For air/air HPs, the relative uncertainties of COP and exergy efficiency allowed by the CEN standard were found to be from 7% to 9%. For the ISO standard the figures were from 5% to 10%. For air/water HPs, the CEN standard allowed up to 9% relative uncertainty in the COP and 7% in the exergy efficiency. In addition to the uncertainties described in the standards, it was also shown how to estimate the uncertainty of COP due to uncertainties in the levels of temperature where the heat transfer occurs. For air/air HPs at standard rating conditions, this contributed 3% to the relative uncertainty of the COP and the exergy efficiency. Exemplary calculations based on published test data showed that very few HPs had performance clearly above or below those with the median results. The exergy efficiency of the air/air HPs, with some exceptions, showed a decrease with increasing outdoor temperature. A similar, but weaker tendency was observed for air/water HPs. The exergy efficiencies for air/water HPs were typically found in the range from 0.3 to 0.4.     

Analysis of moiré data for near-interface cracks

The analysis of moiré data obtained in bimaterials with near-interface cracks is examined. To extract stress intensity factors, a collocation-type method is developed where Westergaard crack-tip expansions are used for displacements in the cracked portion of the bimaterial, expansions from the method of fundamental solutions are used for displacements in the uncracked portion of the bimaterial, and continuity conditions at the interface are used to couple the two expansions. Proof-of-principle numerical experiments performed on synthetic data from a boundary element analysis of a cracked bimaterial successfully demonstrated the analysis method. Mixed-mode stress intensity factors were then determined from actual moiré data obtained in a copper–tungsten specimen.     

The energy market in Norway and Sweden: congestion management

This paper gives an expanded discussion of congestion management in the deregulated electric power system in Scandinavia (presently Norway and Sweden). Previous paper provided an overview of the Nordic power system and deregulated structure and described spot and futures market operation for the uncongested case     

够了，让我们实践吧！

众多的软件开发过程到底是为我们解决的问题多,还是造成的问题更多?它们是否真的能够为改进团队的工作方式提供深刻的见解和所需的指导?文中对当代软件开发过程所面临的问题做了深刻的分析,并指出为什么我们现在拥有如此多的过程。