Emergence of skyrmion lattices and bimerons in chiral magnetic thin films with nonmagnetic impurities

Skyrmions are topologically protected field structures with particlelike characteristics that play important roles in several areas of science. Recently, skyrmions have been directly observed in chiral magnets. Here, we investigate the effects of pointlike nonmagnetic impurities on the distinct initial states (random or helical ones) and on the formation of the skyrmion crystal in a discrete lattice. Using Monte Carlo techniques, we have found that even a small percentage of spin vacancies present in the chiral magnetic thin film considerably affects the skyrmion order. The main effects of impurities are somewhat similar to thermal effects. The presence of these spin vacancies also induces the formation of bimerons in both the helical and skyrmion states. We also investigate how adjacent impurities forming a hole affect the skyrmion crystal

Uso da técnica fotoacústica para caracterizar grãos de cafés quanto a qualidade e conservação - uma metodologia.

Entrada padronizada MELO, W. L. de B

Fouling in heat exchangers: a study of the mechanisms of formation of kaolin deposits

Fouling caused by a water-kaolin suspension in an annular heat exchanger was studied. Deposition seemed to be controlled by ma!!s transfer for lower Reynolds numbers and by adhesion for higher Re. The data was satisfactorily described by the generalized model of Pinheiro. The relative cohesion of the deposits was measured using a concentric cylinders apparatus, which also helped in confirming the existence of a loose and a hard layer in the kaolin deposits

Particulate fouling: controlling processes and deposit structure

Fouling caused by a water-kaolin suspension in an annular heat exchanger was studied. Deposition seemed to be controlled by mass transfer for lower Reynolds numbers and by adhesion for higher Re. The data was satisfactorily described by the generalized model of Pinheiro. The relative cohesion of the deposits was measured using a rotating cylinder apparatus, which also helped in confirming the existence of a loose and a hard layer in the kaolin deposits.(undefined

Fouling by aqueous suspensions of magnetite particles: the effect of pH and ionic strength

Particles adhesion Is one of the fundamental steps in heat exchanger fouling. Surface Interactions are affected by a con siderable number of factors related to the nature of the surfaces Involved. and to the composition and dynamics of the fouling fluid. Although quantitative predictions of U1e rate of adhesion are still not feasible !n practical situations. the evaluation of the potential energy of Interaction between two surfaces "in each vicinity" can be a valuable tool for the qualitative assessment of the fouling tendencies. For that purpose experimental determination of the parameters characterizing the physico-chemical properties of the Interacting surfaces are required. The present paper reports studies on particulate fouling caused by aqueous suspensions of magnetite on copper surfaces. A series of experiments were carried out at different pH values in the range of 7.5 - 9.5. For each pH two distinct types of experiments were performed: In one case the pH was controlled wlth NaOH and In the other the pH controlling medium was Na2co3. Each run lasted 20 days and at the end the thi ckness of the deposits was measured by means of a micrometer coupled to an electrical circuit. The zeta potentials and surface tensions of the solids were measured and used !n the Interpretation of the results. which was carried out In terms of the DLVO Theory

The use of hot wire techniques in fouling tests: a word of awareness

Hot metal wires have been used in fouling tests, mainly when corrosion or chemical reaction is present. Most authors who have reported data obtained with hot wire techniques make no refence about the phenomenon related to the increase In surface area due to the accumulation of fouling layers. Yet this can cause problems of reverse insulation (critical radius), affecting the conclusions taken directly from the readouts. The aim of this paper is to discuss this problem and show through a few selected tases the Importance of this aspect in fouling resistance determinations using hot wire techniques

Efeitos de superfície na deposição de partículas de magnetite e de caulino sobre superfícies de cobre

A deposição de partículas em meta aquoso é regida pela ação combinada de diferentes interações superficiais e forças hidrodinâmicas. De entre o elevado número de factores que podem afectar as interações superficiais seleccionaram-se para estudo o efeito do pH e da força iónica do meio. Realizaram-se ensaios de deposição, sobre superfícies de cobre, com suspensões aquosas de magnetite e de caulino para valores de pH compreendidos entre 7 e 11. Para cada valor de pH foram feitos dois tipos distintos de ensaios: num caso o controlo do meio foi feito com NaOH, no outro com Na2CO3. Efectuaram-se ainda, outros ensaios em que, para um dado valor do pH, se alterou a força iónica do meio por adição de sais neutros (NaNO3 e NaC1). Para auxiliar a interpretação dos resultados obtidos nos ensaios anteriormente referidos, determinaram-se os potenciais zeta das partículas e do cobre em função do pH e calcularam-se as energias livres de adesão partícula-liquido-superfície. Os resultados obtidos têm sido analisados com base na teoria DLVO, com vista à compreensão dos mecanismos que lhe estão subjacentes. Embora não fosse previsível, pelo menos no caso do caulino, a melhor justificação para os resultados obtidos é conseguida admitindo que a interação entre as duplas camadas eléctricas de partículas e superfície se processa a potencial constante

Simbiose micorrízica em plantas forrageiras.

Esse projeto teve como objetivos: verificar a condição micorrízica em plantas forrageiras nas condições do semi-árido, avaliar a eficiência de fungos micorrízicos arbusculares em promover o crescimento vegetal e o teor nutricional de plantas de Cajanus cajan var. Petrolina, passível de utilização na alimentação animal, e conhecer o potencial dessas forragens em incrementar a propagação desses fungos no solo