Fe [Co¯Fe¯], if one considers the moment of the cobalt ion being reduced due to the stabilization of the lowest Kramers doublet. [16] The slow rise of the virgin magnetization is consistent with a pinning-type magnet with no reversible region. [2] However, the S-type increase with field of the initial (virgin) magnetization after ZFC is not that expected for a random distribution of super-paramagnets, and this anomaly may be a consequence of the presence of a mean dipolar field acting on the particles, which depends on the direction and magnitude of the magnetization of surrounding particles. [18] We, therefore, think that the large coercive field reflects the ªintrinsic anisotropy of the particles enhanced by the inter-particle dipolar fieldsº. To test this hypothesis, we examine a sample (7.5 % CoFe 2 O 4 ±SiO 2 , annealed at 800 C) consisting of 3.2 nm particles with varying separation between particles. These particles are super-paramagnetic at room temperature and the ZFC±FC magnetization measurements in 5 Oe indicate a blocking temperature of 55 K (see supplementary material S2, available from the author). The super-paramagnetism is exemplified by the absence of remanance magnetization and coercivity above 55 K, while a wide hysteresis loop with a remanance magnetization of 50 % of that at saturation is observed and the coercivity is 13.5 kOe at 2 K (see supplementary material S3, available from the author). We also note that the remanance in this case is as expected for the model of Stoner for a random oriented polycrystalline magnetic sample. [19] Further work is in progress on samples prepared at different dilution of spinel and different annealing temperatures between 800 C and 1100 C in order to elucidate the effective dipolar field as a function of size and inter-particle distances. We can only note presently that the size of the particle, its blocking temperature and coercive field decrease on lowering both the concentration and sintering temperature. The lack of evidence for the presence of c-Fe 2 O 3 in the composite does not rule out the model of Skomski and Coey for the enhancement of magnetic hardness in composites consisting of hard and soft magnetic materials. This remains to be verified. [20] We have observed an unusually high magnetic hardness, characterized by a 20 kOe coercive field at 2 K, for CoFe 2 O 4 particles of a size of 12 nm in amorphous SiO 2 prepared by the sol±gel method and annealed at 1000 C. The coercivity reflects the intrinsic anisotropy enhanced by dipolar fields. Its high value for smaller particles tends to confirm this hypothesis. ExperimentalThe sample was prepared by the sol±gel method as follows: Co(NO 3 ) 2´6 H 2 O (2.18 g) and Fe(NO 3 ) 3´9 H 2 O (6.06 g) were dissolved in 4.5 g of water acidified with nitric acid (0.03 M) as a catalyst. To this solution tetraethoxysilane (TEOS, 10.6 g), dissolved in 7 g of methanol, and formamide (2.25 g), as a modifier, were added. After gelation (approximately 2 h at 40 C) and ageing (24 h), the sample was dried at 40 C f...

A simple formula for the thermal conductivity enhancement in carbon nanotube composites is presented by incorporating the interface thermal resistance with an effective medium approach. This model well describes the thermal conductivity enhancement observed recently in nanotube suspensions. In particular, this simple formula predicts that a large interface thermal resistance across the nanotube-matrix interface causes a significant degradation in the thermal conductivity enhancement, even for the case with ultrahigh intrinsic thermal conductivity and aspect ratio of the carbon nanotubes embedded.

Fe 3 O 4 magnetic nanoparticles (MNPs) were synthesized by a co-precipitation method using sodium citrate and oleic acid as modifiers. Phase composition and microstructure analysis indicate that the sodium citrate and oleic acid have been successfully grafted onto the surface of Fe 3 O 4 MNPs. The magnetic behaviors reveal that the modification can decrease the saturation magnetization of Fe 3 O 4 MNPs due to the surface effect. Fe 3 O 4 MNPs modified by sodiumcitrate and oleic acid show excellent dispersion capability, which should be ascribed to the great reduction of high surface energy and dipolar attraction of the nanoparticles.

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Many electroactive polymer (EAP) actuators use diverse configurations of carbon nanotubes (CNTs) as pliable electrodes to realize discontinuous, agile movements, for CNTs are conductive and flexible. However, the reported CNT-based EAP actuators could only accomplish simple, monotonous actions. Few actuators were extended to complex devices because efficiently preparing a large-area CNT electrode was difficult, and complex electrode design has not been carried out. In this work, we successfully prepared large-area CNT paper (buckypaper, BP) through an efficient approach. The BP is highly anisotropic, strong, and suitable as flexible electrodes. By means of artful graphic design and processing on BP, we fabricated various functional BP electrodes and developed a series of BP-polymer electrothermal actuators (ETAs). The prepared ETAs can realize various controllable movements, such as large-stain bending (>180°), helical curling (∼ 630°), or even bionic actuations (imitating human-hand actions). These functional and interesting movements benefit from flexible electrode design and the anisotropy of BP material. Owing to the advantages of low driving voltage (20-200 V), electrolyte-free and long service life (over 10000 times), we think the ETAs will have great potential applications in the actuator field.