Polymeric dispersants that adsorb onto various types of fine particles have been designed through complex formation between cationic polyethyleneimine (PEI) and anionic oleic acid (OA) to improve particle surface compatibility with nonaqueous solvents. Complex formation involves the OA-assisted dissolution of PEI in a nonaqueous solvent (toluene) by ultrasonication. While PEI itself is immiscible in toluene, various molecular weights (MWs) of PEI (M w = 600, 1800, and 10 000) visibly dissolved when, in the presence of more than 5-mol % PEI (based on the ethyleneimine (EI) unit of PEI), a complex formed between the carboxyl group of OA and the amines of PEI in toluene. Si3N4 and one of the typical multicomponent compositions for Si3N4 ceramic fabrication, Si3N4–Y2O3–MgO, were chosen to demonstrate the usage of PEI–​OA complexes for improving nonaqueous slurry stability. While nontreated Si3N4 particles rapidly formed dense slurry aggregates followed by solidification in toluene, PEI associated with 30 mol % OA effectively adsorbed on Si3N4 fine particles and drastically improved the flow properties with ∼1.8 mg/m2 treatment. Surprisingly, the flow properties were maintained even for dried and redispersed slurries. The effect of the MW of PEI–​OA and its additive content on the stability of Si3N4/toluene slurries has been clarified, and their applicability toward Si3N4–Y2O3–MgO multicomponent systems is discussed.

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To collect the signals of magnetic nanoparticles (MNPs) at a distance from a magnetic nanoparticle tomography, a strong ac magnetic field should be generated by applying a high current to the excitation coil. To this end, sinusoidal excitation using a linear amplifier-type ac power source has been applied to the tomography. Although this source can provide a high-quality sinusoidal voltage, its low power efficiency is not suitable for generating the required high current. To overcome this limitation, we use an H-bridge voltage source inverter to achieve high efficiency by generating a square-wave voltage. However, the third harmonic component in the square wave, undermines the acquisition of MNP signals. Thus, we suppress the third harmonic by shifting the switching phase of the inverter transistors. As a result, the third harmonic in the excitation current is reduced to less than one-tenth of that obtained after conventional suppression. We verify the distribution of MNP signals at depths up to 50 mm using the proposed excitation approach. The results demonstrate the effectiveness of the proposed approach based on square-wave inverter excitation for magnetic nanoparticle tomography.

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