ABSTRACT
We have used the coprecipitation and mechanical-milling methods to fabricate CoFe2O4 nanoparticles with an average crystallite size (d) varying from 81 to â¼12 nm when changing the milling time (t m) up to 180 min. X-ray diffraction and Raman-scattering studies have proved the samples crystalizing in the spinel structure. Both the lattice constant and residual strain tend to increase when t m(d) increases (decreases). The analysis of magnetization data has revealed a change in the coercivity (H c) towards the hard-magnetic properties. Specifically, the maximum H c is about 2.2 kOe when t m = 10 min corresponding to d ≈ 29 nm; beyond this t m(d) value, H c gradually decreases. Meanwhile, the increase of t m always reduces the saturation magnetization (M s) from â¼69 emu g-1 for t m = 0 to 35 emu g-1 for t m = 180 min. The results collected as analyzing X-ray absorption data have indicated a mixed valence state of Fe2+,3+ and Co2+ ions. We think that the migration and redistribution of these cations between the tetrahedral and octahedral sites together with lattice distortions and defects induced by the milling process have impacted the magnetic properties of the CoFe2O4 nanoparticles.
ABSTRACT
OBJECTIVES: Global childhood obesity is of great concern. In 2019, the World Health Organization released global guidelines on movement behaviors for the children under 5 y of age to combat this epidemic. This study aimed to estimate the prevalence of Vietnamese preschoolers meeting the guidelines, and examined whether guideline compliance is associated with adiposity and motor development. METHODS: A cross-sectional study was conducted on 103 healthy preschoolers who were conveniently sampled from preschools in urban and rural areas around Ho Chi Minh city. Time spent in different intensities of physical activity and sedentary behavior was measured using Actigraph GT3X + accelerometers over three consecutive days. Sleep and screen time were obtained via parent questionnaires. Children were classified as meeting or not meeting the global guidelines. Height, weight, and motor skills were measured by the research staff. Regression models were applied to quantify the association between guideline compliance and adiposity and motor development, adjusting for age and sex. RESULTS: While 17.5% of children met all three guidelines, 5.8% met no guidelines. The prevalences of children who met guidelines for physical activity time, sleep duration, and screen time were 50.4%, 81.4%, and 44.7%, respectively. There was no association between guideline compliance and adiposity and motor development. CONCLUSION: This study found a low proportion of children who met the global guidelines, whereas a high proportion of those with overweight and obesity was reported. Health programs should promote more physical activities of various intensities in young children.
