| Abstract
| - At a particle size of 7 nm, the antiferromagnetic transition in CoO has been suppressed, but not extinguished, with a rounded maximum in the heat capacity and a Néel temperature approximately 20 K lower than that of large single crystals and 60-μm particles.
- CoO nanoparticles ranging in size from 7 to 21 nm were prepared via precipitation andthermal decomposition methods. The particles had water contents ranging from 2.18 to 0.19wt %; water content decreased with increasing particle size. All particles also contained asmall amount of Co3O4 impurity. The surface enthalpy was determined to be 2.82 ± 0.20J·m-2 by acid solution calorimetry at 298 K. Corrections were made for the water and cobaltspinel impurity. Heat capacity measurements using adiabatic (10−320 K) and semi-adiabatic(0.6−40 K) calorimeters were performed on the sample with particle size 7.0 ± 1.0 nm. Thenanoparticle heat capacity had a broad anomaly with a rounded maximum at 265 K, areduction of 23 K from the Néel temperature TN observed as a sharply peaked maximum inthe heat capacity of single-crystal CoO. When corrected for water and Co3O4, the heat capacityof nanophase CoO is greater than that of single-crystal CoO below about 250 K. Above 250K, the much larger magnetic peak in the heat capacity of the single crystal dominates theheat capacity difference. Thus the excess entropy of the nanoparticles, calculated from theheat capacity difference, has a maximum of 2.4 ± 0.3 J·K-1·mol-1 at 245 K but drops to 1.5± 0.3 J·K-1·mol-1 at 298 K. The magnetic and surface contributions to the excess entropycannot be resolved in a definitive manner, but an estimate of the surface entropy, 0.28 ±0.03 mJ·K-1·m-2, is similar to one literature report for MgO, and the excess magnetic entropyis negative, −0.8 ± 0.3 J·K-1·mol-1.
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