Towards Strong Nonapproximability Results in the Lov��sz-Schrijver Hierarchy

Lovász and Schrijver described a generic method of tightening the LP and SDP relaxation for any 0–1 optimization problem. These tightened relaxations were the basis of several celebrated approximation algorithms (such as for max-cut, max-3sat, and sparsest cut).     

Morphology of Single-Component Particles Produced by Spray Pyrolysis

ABSTRACT

This work is a review of the experimental results from the literature for single-component metal and simple metal-oxide particles. Criteria for correlating particle morphology, i.e., whether the particles are solid or hollow, with process parameters and material properties during spray pyrolysis are presented and compared with the data available in the literature. The materials were classified into two categories for which the precursor: (1) melts and (2) does not melt before chemical reaction takes place, and separate criteria were used for each category based on the work of Jayanthi et al. (1993) J. Aerosol Sci. 19:478. In systems where the precursor melts before chemical reaction occurs, e.g., decomposition of nitrates of Mg, Al, Fe, Zn, Pb, Ni, Co, Pd, Mn, Cu, Sr, and Ag, the particle morphology is determined primarily by the densities and formula weights of the reactant and product compounds unless high temperature densification or puffing up of the particles due to gases evolved during the chemical reaction alter the morphology. In systems where the precursor undergoes nucleation to form a solid crust which does not melt before chemical reaction takes place, e.g., Ba(C₂H₃O₂)₂, Al₂(SO₄)₃, Zr(C₂H₃O₂)₂, and Zn(C₂H₃O₂)₂, solubility and density of the precursor as well as the operating temperature are the main factors that affect the product particle morphology. Overall, particle morphologies predicted by the criteria were in agreement with experimental observations reported in the literature.     

Bulk modulus and thermal expansion coefficient of mechano-chemically synthesized Mg2FeH6 from high temperature and high pressure studies

Mg₂FeH₆ was synthesized by ball milling MgH₂ and Fe (2:1 molar ratio) mixture for 72 h followed by heating at 400 °C under H₂ pressure. The hydride formation, its structure and homogeneity were investigated by scanning electron microscopy, X-ray diffraction, transmission electron microscopy and Raman spectroscopy. High pressure in situ synchrotron X-ray diffraction and Vienna ab initio simulation were used to determine bulk modulus of the sample. The bulk modulus of Mg₂FeH₆ was found to be 75.4(4) GPa by optimized experiment and 76.3 GPa by theoretical simulation. From high temperature in situ X-ray diffraction study the volumetric thermal expansion coefficient of Mg₂FeH₆ was found to be α_v = 5.85(3) × 10⁻⁵ + 7.47(7) × 10⁻⁸ (T − T_o)/°C. Decomposition of Mg₂FeH₆ was observed at 425 °C and the decomposition products were Mg, Fe and H₂.     

Use of the nanoindentation technique for studying microstructure/crack interactions in the fatigue of 4340 steel

The objectives of this research are to study the influence of microstructure on the fatigue crack growth behavior in 4340 steel and to explore the application of the nanoindentation technique for determining the plastic deformation zone at a fatigue crack tip. Two heat treatment conditions were chosen for the steel: annealed and quenched plus tempered. The annealed steel consists of coarse pearlite and proeutectoid ferrite, while the quenched and tempered steel consists of fine tempered martensite. Fatigue crack propagation tests were conducted on disklike compact (DCT) specimens. Subsequently, the nanoindentation technique was applied to quantitatively determine the plastic deformation zone at fatigue crack tips. The plastic deformation zone size determined by the nanoindentation test seems larger than the cyclic deformation zone calculated using the fracture mechanics equation, which involves many assumptions. The fatigue crack growth test results show that the annealed steel has a higher resistance to crack growth than the quenched and tempered steel. The fatigue crack in the annealed steel tends to grow along pearlite domain boundaries, or the cementite/ferrite interfaces within a pearlite domain. In contrast, the fatigue crack in the quenched and tempered steel tends to traverse the fine martensite laths. Consequently, the actual crack path in the annealed steel is rougher than in the quenched and tempered steel and more secondary cracks are observed in the annealed steel.     

Heat transfer between a fluidized bed and an immersed vertical U-tube

The design details of a 0.254 m stainless steel cylindrical fluidized bed pilot plant facility, whose fabrication, installation and testing have been recently completed, are described. It primarily consists of a fluidized-bed reactor, fluidizing air-supply system, solids feeder, off-gas cleaning and exhaust system, and cooling water-supply system for heat-transfer tubes provided in the bed and in the freeboard sections. The plant is operated at ambient pressure in the temperature range 300–600 K, both in the batch and continuous modes for solids feed. Bed-pressure drop measurements as a function of fluidizing velocity for two different bed heights reveal that the quality of fluidization is good. Similar experiments have been conducted with the continuous solids feed. The heat-transfer coefficient between the bed and an immersed stainless steel U-tube is measured as a function of fluidizing air velocity at five different temperatures. The effects on bed-to-tube heat-transfer rate of solids feed rate, bed height, air-flow rate, and bed temperature are examined. All of these observed variations are interpreted in terms of the solids mixing and bubble mechanics in the bed.     

Buffer Behavior for Mixed Arrivals and Single Server with Random Interruptions

A queueing model with finite buffer size, mixed input traffic (Poisson and burst Poisson arrivals), synchronous transmission and server interruptions through a Bernoulli sequence of independent random variables is studied. Using average burst length, traffic intensity and input traffic mixture ratio as parameters, the relationships among buffer size, overflow probability and expected message queueing delay are obtained. An integrated digital voice-data system with synchronous time division multiplexing (STDM) for a large number of voice sources and mixed arrival process for data messages is considered as an application for this model. The results of this study are portrayed on graphs and may be used as guidelines in buffer design problems in digital voice-data systems. The queueing model developed is quite general in a sense that it covers pure Poisson and burst Poisson arrival processes and the mixture of the two as well.