The effect of the sintering atmosphere on the densification of B4C ceramics

Densification of boron carbide during sintering may be improved by a two-stage process, namely heating to 2000°C under vacuum and sintering at 2190°C under argon. This sintering regime allows achieving a relative density of the ceramic bodies fabricated from a fine powder higher than 95%. The nitrogen treatment of the boron carbide phase at 1900°C leads to the formation of the BN phase and precipitation of graphite. Vacuum treatment of these samples at 2000°C leads to decomposition of the boron nitride phase. The liberated free boron may again react with graphite to form in situ boron carbide particles. The experimental investigations of the sintering behavior of the boron carbide phase under various atmospheres supported the thermodynamic predictions regarding the phase transformation. No evidence, however, was found for enhanced sintering under a nitrogen atmosphere.     

Densification behavior of gas and water atomized 316L stainless steel powder during selective laser melting

The densification during selective laser melting (SLM) process is an important factor determining the final application of SLM-part. In the present work, the densifications under different processing conditions were investigated and the densification mechanisms were elucidated. It was found that the higher laser power, lower scan speed, narrower hatch spacing and thinner layer thickness could enable a much smoother melting surface and consequently a higher densification.The gas atomized powder possessed better densification than water atomized powder, due to the lower oxygen content and higher packing density of gas atomized powder. A large number of regular-shaped pores can be generated at a wider hatch spacing, even if the scanning track is continuous and wetted very well. The densification mechanisms were addressed and the methods for building dense metal parts were also proposed as follows: inhibiting the balling phenomenon, increasing the overlap ratio of scanning tracks and reducing the micro-cracks.     

Densification study of ITO films during high temperature annealing by GISAXS

Three thermal routes were treated on the sol-gel ITO films, i.e. conventional thermal annealing (CTA), rapid thermal annealing (RTA) and thermal cycle annealing (TCA). The near surface and internal structures of films were characterized by grazing incidence small angle X-ray scattering (GISAXS) technique. It is found that slit-like pores show fractal structures laterally and the near surface is sparser with bigger pores. Ordered pore structure normal to the film appears when films are annealed at high heating rate. The shrinkage of pores is mainly owing to structural relaxation and diffusion during the superheating process. However, the supercooling process has no significant effect on the structures. Furthermore, CTA samples have the greatest porosity and surface roughness due to the prevailing crystallization as well as the coarsening procedure. However small pores inside the films are eliminated at low temperature.     

Factors influencing particle agglomeration during solid-state sintering

Discrete element method(DEM) is used to studythe factors affecting agglomeration in three-dimensionalcopper particle systems during solid-state sintering.A newparameter is proposed to characterize agglomeration.Theeffects of a series of factors are studied,including particlesize,size distribution,inter-particle tangential viscosity,temperature,initial density and initial distribution of particleson agglomeration.We find that the systems with smallerparticles,broader particle size distribution,smaller viscosity,higher sintering temperature and smaller initial densityhave stronger particle agglomeration and different distributions of particles induce different agglomerations.This studyshould be very useful for understanding the phenomenon ofagglomeration and the micro-structural evolution during sintering and guiding sintering routes to avoid detrimental agglomeration.     

Electrostatic lenses

The various types of electrostatic lenses that are used to control beams of ions or electrons are briefly described, together with the computational techniques for evaluating their properties. Particular emphasis is placed on the calculation and minimization of the aberrations of these lenses. In computer-aided designs of lens systems it is convenient to have lens properties available in the form of numerical approximations, and the achievement of this is discussed. Special types of lens, such as the zoom lens, are also discussed.     

Coercivity enhancement of HDDR-processed Nd-Fe-B permanent magnet with the rapid hot-press consolidation process

High coercivity, fully dense anisotropic permanent magnets of submicron grain sizes were produced by rapid hot-press consolidation of hydrogenation-disproportionation-desorption-recombination (HDDR) processed Nd-Fe-Co-B powders. In the hot-press process, the coercivity of the consolidated material showed a sharp minimum prior to full densification. Thereafter, it reached a value 25% higher than that of the initial powder. Scanning electron microscopy and transmission electron microscopy observations revealed that the variation in H_cJ was caused by a redistribution of Nd along the grain boundaries during hot pressing and that the high coercivity was attributable to the formation of thin, continuous Nd-rich phase along the grain boundaries.     

Formation,densification and properties of sol–gel TiO<Subscript>2</Subscript> films prepared from triethanolamine-chelated soluble precursor powders

Amorphous, soluble powders were synthesized with triethanolamine (TEA) as chelating agent as precursors for TiO₂ sol–gel solutions. Dip coating on glass substrates and subsequent annealing yielded thin films with excellent optical properties. Furthermore as-dried films were scraped of substrates, annealed at different temperatures and characterized in order to investigate the structural changes during processing. The observations were systematically compared with previous studies on precursor powders based on acetylacetone. Results indicate that triethanolamine provides both a sufficient hydrolytical stability of the Ti-precursor during coating and an adequate plasticity of the film material throughout thermal densification. These characteristics significantly improve the practical workability of the respective process. Additionally former assumptions regarding the relationship between film properties and intermediate structural features were verified and refined.     

Analysis of instantaneous dynamic states of vibrated granular materials

The behavior of granular materials subjected to continuous vertical vibrations is dependent on a variety of factors, including how energetically the containment vessel is shaken as well as particle properties. Motivation for the investigation reported here is based on phenomenon in which bulk solids attain an increase in density upon relaxation. The results of a detailed, discrete element study designed to examine the dynamic state of a granular material is presented, in which particles are represented as inelastic, frictional spheres. The phase in which the assembly finds itself immediately before vibrations are stopped is quantified by computing depth profiles of the translational energy ratio R in conjunction with profiles of solids fraction ν and granular temperature T. The use of particles that are more frictional tends to hinder or delay thermalization, while particle restitution coefficient plays a role when the flow is collision dominated. The structure before vibrations are applied plays an important role in determining the depth profiles and the phase pattern only at low accelerations. On the other hand, large accelerations can easily dislodge the poured configuration very quickly so that the initial condition is not major factor in the phase pattern.     

Automated image analysis to characterize the melt densification stage of polymer sintering processes

Knowledge of melt densification during sintering is generally acquired by visualization methods, recording bubble formation and dissolution with time. Only manual visualization methods have been reported for polymer studies, which restrict the availability of data to a few discrete moments in time for the overall process. An automated vision system is presented in this paper to provide an improved level of analysis on densification, validated against a manual method. The machine vision technique was applied in the analysis of sintering for various polyethylenes of differing melt flow index and particle size. The automated technique was found to be very accurate and capable of collecting bubble size distributions on a timescale of seconds, which is an improvement in data collection. The method was prone to underestimate bubble numbers (∼20% error), especially those less than 100 μm in size, till the bed temperature rose significantly above the melting temperature of the polymer.     

Preparation of ceramics by hydrothermal hot-pressing

Abstract

The densification mechanism of borosilicate glass, silica gel and amorphous titania powders by hydrothermal hot-pressing is described. The glass powders were densified by a viscous flow mechanism, and fully dedfied compacts were obtained by hydrothermal hot-pressing. On the other hand, porous ceramics were produced from silica gel and amorphous titania. The pore size distribution of these ceramics could be controlled by hydrothermal hot-pressing conditions. In the case of silica gel, it remained amorphous, but amorphous titaaia was crystallized to anatase by hydrothermal hot-pressing. The pore diameter and mechanical strength of the compacts prepared from silica gel increased with reaction time. The densification of the amorphous titania was improved by increasing temperature and pressure.