SUBMERGED ARC SYNTHESIS OF MAGNETIC NANOPARTICLES

1. SUBMERGED ARC SYNTHESIS OF MAGNETIC NANOPARTICLES N. Parkanskya, G. Leitusb, B. Alterkopa, R.L.Boxmana, Z. Barkayc, Yu. Rosenbergc. aElectrical Discharge and Plasma Laboratory, Tel Aviv University, POB 39040, Tel Aviv 69978, Israel bDepartment of Chemical Research Support, Weizmann Institute of Science, Rehovot 76100, Israel c Wolfson Applied Materials Research Center, Tel Aviv University, POB 39040, Tel Aviv 69978, Israel Experimental Method: Discharge • C and Ni-C powders were produced by a pulsed arc, ignited between C and Ni electrode pairs, respectively, with a gap of 0.35 mm between them, submerged in 200 ml of analytical (99.8%) ethanol. • 0.05J pulses were generated by a Blumlein transmission line circuit. After each pulse discharge, the transmission lines were charged again, and discharges with ~1 µs duration occurred repetitively during free-running operation at a pulse repetition rate of ~1 kHz. • The breakdown voltage in ethanol was 12 kV. • The powder was examined by HRSEM, TEM, EDX and XRD. • Magnetic properties were studied in a SQUID magnetometer. Electrical circuit of Blumlein pulse generator XRD patterns of Ni-C powder. Pulse energies: 0.05 J (bold line), 0.025 J (broken line). 95% of produced particles had a diameter<30 nm. Magnetization curves at low temperatures. Initial susceptibility ini calculated at H < 20 Oe. ini maximum is observed at blocking temperature TB=8 K. HRSEM micrograph of same “big” nano-particles (marked by arrows). HRTEM micrographs of Ni-C nano-particles Structure of Ni-C powder: FCC Ni – C solid solution, lattice parameter up to 3.587 A. C concentration (7.9%) 3 maximum equilibrium solid solubility of C in Ni. The approximate superimposition of the curves at T > 5 K indicates superparamagnetism Temperature dependence of magnetic susceptibility taken at 10 Oe during ZFC (zero field cooled) and FC (field cooled) measurements. Superparamagnetic transition observed at TB = 8-9 K. Transition from single-domain ferromagnetic behavior - blocked state below TB and superparamagnetic state above TB. ESEM SE images of magnetically separated rod (a) and spherical (b) carbon nano-particles captured on glass slides by bio-ferrography. ZFC and FC temperature dependences of DC magnetic susceptibility measured at H = 100 Oe of C nanoparticles. a b Conclusions 1. Nano-powder consisting of C particles and Ni-C alloy particles coated by C, was produced by a pulsed arc between Ni electrodes submerged in ethanol.. 2. C concentration in the Ni alloy was ~3 maximum solid equilibrium solubility. 3. Ni-C powder was superparamagnetic in a wide temperature interval T>TB = 8 K. The material had a very narrow hysteresis loop Hc<8 Oe (i.e. it is a soft magnet) in the superparamagnetic state. 4. Carbon powder comprised of nano- and micro-particles was produced by a pulsed arc between C electrodes submerged in ethanol. 5. Micro-particles were not magnetic, while nano-particles exhibited magnetic behavior. 6. Nano-particles super-paramagnetic, critical temperature > 300 K. Dependence of DC magnetic moment of the raw C powder on magnetic field at T = 20 K and 300 K. The magnetic field was decreased from 10000 Oe to -10000 Oe and then increased back. Inserts: A) Dependences at H > 2000 Oe; B) at low field.