Defect induced magnetic properties of CoO nanoparticles produced via mechanical ball milling have been assessed by detailed magnetic measurements. A progressive decrease in the particle size and a concomitant increase in the induced strain have been observed with the milling times. The mechanically milled nanoparticles of CoO exhibit anomalous magnetic properties such as FM hysteresis when compared with the unmilled CoO sample. The presence of weak ferromagnetism, with a highest value of magnetization of 0.532 emu/g at 10 K in the 100 h milled sample, is attributed to the uncompensated surface spins resulting from induced surface defects via mechanical milling. The ZFC coercive force, measured at 10 K, increases with milling time reaching a maximum value of 1066 Oe for the 100 h milled sample. The temperature dependent field-cooled (FC) and zero-field-cooled (ZFC) magnetic measurements indicate a presence of an exchange bias field arising from uncompensated moments generated by mechanical strain and the antiferromagnetic (AFM) core. The exchange bias field measured at 10 K reaches a value 210 Oe for the 50 h milled sample and decreases upon prolonged milling. The exchange bias field vanishes at a temperature approximately 200 K, a temperature much lower than the Neel temperature of CoO (TN approximately 291 K). The observed anomalous magnetic behavior of CoO could be interpreted in terms of the exchanged bias FM-AFM model.
The International Journal of Energy Research (IJER) recognized early on the potential and opportunities of nanomaterials and nanotechnology for energy applications. In the past, several special issues on this theme have been published and separate papers in this category have been regularly published [1,2]. Through improved physical and chemical properties such as increased catalytic activity, large specific surface area, or new functionalities, nanomaterials have shown their applicability for a range of energy technologies such as solar cells, batteries, hydrogen technologies, among others. Nanotechnology is becoming a standard tool to improve the performance or to reduce the cost of novel energy systems. New nano energy devices such as energy harvesting are also emerging.While IJER has launched specific calls for papers in nanotechnology for energy in the past, this time we have chosen extended papers from the 7th International Conference on Surfaces, Coatings & Nanostructured Materials (NANOSMAT 7) which was held in Prague, The Czech Republic, in autumn 2012. A few other paper submissions of interest are also included here. The NANOSMAT conferences have traditionally included several sessions on nanomaterials for energy showing an increasing interest among the science community in this research topic. NANOSMAT conferences have been running since 2005 and due to their great success NANOSMAT-USA and NANOSMAT-Asia have been launched.The papers in this IJER Special Issue on nanotechnology for energy applications deal with solar cells, light-emitting diodes, Li-ion batteries, hydrogen storage, fuel cells, hydrogen production, and energy harvesting. A range of different nanomaterials are also covered such as carbon nanospheres, carbon nanotubes, nanoparticles, nanocomposites, etc. Altogether 14 papers are included in the special issue. A short summary of each paper is presented in the next to introduce the readers of the journal into this special issue.Tsai et al.[3] investigated CuInS 2 thin film solar cells in their paper. They looked on the effects of sulfurization and the Cu/in-ratio on the film quality and morphology, which depend very much on the processing conditions. A solar cell with an efficiency of 6.29% was produced.Pimanpang et al.[4] fabricated dye-sensitized solar cells on plastics and stainless steel. The stainless steel was coated with a film of TiO 2 nanoparticles and the plastic was made conductive through deposition of Pt-nanoparticles. This fully flexible DSSC solar cell showed an efficiency of 2.72%.Su et al.[5] used ZrO 2 nanoparticles to improve the light transmission for a natural pigment-sensitized solar cell in a water-based electrolyte. The nanoparticles improved the light transmission by a factor of eight. An environmentally friendly solar cell with an efficiency of 0.688 % was produced.Ray et al.[6] studied the effect of nitrogen functionalization on the structure of carbon spheres which can potentially be used for energy applications such as solar cells and capacitors. Kasdorf et al. ...
Nanocomposite of Co-SiO2, a soft magnetic material, with Co weight fraction x = 0.3 and 0.7 was prepared via mechanical milling. The magnetic properties of these samples, both zero-field-cooled (ZFC) and field-cooled (FC), have been measured as a function of x, milling time, and temperature. The structural assessment of the composite indicates a presence of only ferromagnetic (FM) hcp-Co phase in the composite. However, reported magnetic properties of these composites appear to be dependent on the presence of antiferromagnetic (AFM) phases of cobalt oxide as well. The observed enhancement in ZFC coercivity and a reduction in saturation magnetization with the milling time are due to an increase in defect density upon milling. The ZFC coercivity for the x = 0.3 samples has been found to be much higher than the x = 0.7 samples for all milling times. The coercivity above 50 K depends on temperature according to the law corresponding to isotropic uniaxial superparamagnetic particles. Below 50 K the presence of an AFM phase Co3O4 (TN approximately 33 K) and increased interparticle interactions bring in a departure from that law. The saturation magnetization is found to be temperature dependent for the x = 0.3 samples and temperature independent for the x = 0.7 samples, which further provides evidence of the presence of higher AFM phase fraction in the composite with a low metal volume fraction. The FC magnetic measurements show a presence of an exchange bias field and an enhanced coercivity which are higher than the ZFC measurements. All magnetic measurements indicate that the overall magnetic properties of the composite are dictated by the presence of a trace amount of cobalt oxides.
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