Hyperfine Interact (2009) 191:55–60 DOI 10.1007/s10751-009-9952-5
Characterization of magnetic nano-fluids via Mössbauer spectroscopy G. Filoti · V. Kuncser · G. Schinteie · P. Palade · I. Morjan · Rodica Alexandrescu · Doina Bica · L. Vekas
Published online: 29 March 2009 © Springer Science + Business Media B.V. 2009
Abstract The laser pyrolysis became a useful tool, providing various ways, in production of nano materials. The iron Mössbauer spectroscopy is one very accurate method in evidencing the physical properties and related processes in the nano scale compounds. The effect of pressure, laser spot area and induced combustion, of gas mixture and laser power on the phase composition and inside particle distribution, grain size as well as the related phenomena were investigated by temperature dependent Mössbauer spectroscopy. A selection of most relevant properties is presented and discussed in details. Keywords Nanopowder and ferrofluids · Laser pyrolysis · Mössbauer spectroscopy · Phase composition and particle size 1 Introduction The magnetic fluids were intensively studied by various methods, including Mössbauer spectroscopy, in the last 30 years, firstly at the micron level and later, due to new processing routes, at nano scale dimensionality. The magnetic particles are mainly iron based oxides, such as magnetite [1] and cobalt ferrite [2] or carbides [3]. The liquids are either various hydrocarbon (non polar) or polar carriers, but more often water [2].
G. Filoti (B) · V. Kuncser · G. Schinteie · P. Palade National Institute of Materials Physics, P.O. Box MG-7, 077125 Bucharest-Magurele, Romania e-mail:
[email protected] I. Morjan · R. Alexandrescu National Institute for Lasers, Plasma and Radiation Physics, P.O. Box MG-36, 077125 Bucharest-Magurele, Romania D. Bica · L. Vekas Centre of Fundamental and Advanced Technical Research, Romanian Academy—Timisoara Division, 300228 Timisoara, Romania
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Among very performing methods to produce nano particles there is the laser pyrolysis of a mixture containing iron pentacarbonyl + air (as oxidizer) and ethylene (as sensitizer). The particle’s properties (dried and desirable stoichiometric single phases) are controlled in several ways: their forecast composition via gas mixtures and pressure, grain phase composition for core and external shell mainly via laser spot area and time cooling, the entity’s dimensionality as function of the laser power. The actual presentation describes, especially using Mössbauer parameters, several cases of samples processed using laser pyrolysis, and the magnetic liquids subsequently processed as well as the observed physical properties for both cases.
2 Experimental The nano-powders were obtained in a flow reactor, by the IR laser irradiation (λ = 10.6 μm) of a gaseous mixture of iron pentacarbonyl and air, with C2 H4 as sensitizer (it is excited by the absorption of the CO2 laser radiation), as described in details elsewhere [4]. First three samples concern the comparison of different pressure: 200 mbar for sample A, 500 mbar (B) and 600 mbar (C) at a laser power of 50 W and similar ratios for gas mixture. Next two samples are dealing with effect of laser spot effects: 1.5 × 4 mm (D) and 1.5 × 10 mm (E) at same 400 mbar pressure and 50 W laser power. The last three samples were produced by varying the laser power between 55 W (sample F), 45 W (G) and 35 W (H) at 300 mbar pressure and flowing rates of 70 sccm (standard cubic centimeter per minute) for air and 145 sccm for ethylene + Fe(CO)5 . The Mössbauer spectra (MS) were acquired using a constant acceleration spectrometer with symmetrical waveform. A 57 Co (∼1.0 GBq) source in Rhodium matrix has been used and the isomer shifts are referred to bcc Fe. A He-bath cryostat was used for temperature dependent Mössbauer measurements. The sample effective thickness was related to a content of 10 mg Fe/cm2 . The fits were performed either using distribution of hyperfine field and quadrupole splitting or individual patterns: sextet, doublet or singlet, depending on physical factors to be emphasized.
3 Results and discussion For the sake of better comparison, most of the Mössbauer data will be presented at 80 K temperature, where the relaxation effects related to super-paramagnetic nano grains behaviour are reduced and also the Brownian movement inside magnetic liquids is frozen. In the Fig. 1, left, are shown MS of samples obtained at different chamber pressure. It is evident that patterns are showing narrower lines,—with increasing pressure, starting with poorly crystallized grain at 200 mbar (sample A) evolving at 400 mbar (B) and further to 600 mbar (C) to well grown grains containing solid solution of maghemite and magnetite, as suggested by the hyperfine distribution, together with a minority paramagnetic phases, scrutinized via quadrupole distribution. For a unitary interpretation the spectra have been fitted with magnetic and quadrupole distribution taking into consideration the spin dynamics as well as the non equivalent surrounding around iron. Both distributions are displayed on right side of Fig. 1. There is a clear effect of increasing pressure on dimensionality
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Fig. 1 Effect of atmosphere pressure of laser pyrolysis evidenced in the Mössbauer spectra at 80 K
Fig. 2 Hyperfine fields of the magnetic components of sample C versus temperature
and grain quality, as reflected by narrower hyperfine field distribution for sample C concomitantly with more definite local co-ordinations for paramagnetic phases, centrally located in MS. The behaviour of both distributions pleads for usefulness of Mössbauer spectroscopy as a powerful tool by putting in evidence valuable information about grain properties, phase formation and inside distribution. The distribution width of the main magnetic peak decreases from a value of 13.2 T for a pressure of 200 mbar (A), to 8.4 T value for 500 mbar (B) and finally to only 4.4 T for 600 mbar (C). Due to high pressure, large grains were induced in the sample C, therefore the spectra were also successfully fitted with individual patterns (two sextets and one doublet). The two magnetic hyperfine field dependences on temperature are
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Fig. 3 a The fitted Mössbauer spectra and the distribution probabilities for sample D. b The fitted Mössbauer spectra and the distribution probabilities for sample E
plotted in Fig. 2 proving the collapse at higher temperature of the inner sextet (H2), belonging to the phase showing a more pronounced super-paramagnetic behaviour as compared with maghemite (H1), and exhibiting a hyperfine field of 48.5 T at room temperature. The next exemplification is related to effect of laser spot area what influence the passing time of particle in the synthesis zone and combustion thermal regime via the D and E samples with a laser spot area 2.5 times larger for latter one. The above Fig. 3a and the following Fig. 3b exhibit their Mössbauer spectra versus temperature. The spectra shapes and their temperature evolution un-doubtfully point to the presence of a large amount of carbide in the sample E, coexisting with the spinel solid solution showing a strong super-paramagnetic relaxation near room temperature. As expected the austenite phase parameters did not modified versus temperature. On the contrary the spinel oxide solid solution appearing in the sample D remains as the major one, even at 270 K. There are no traces of any carbide presence as exhibited by Mössbauer spectra of sample D. It was proved that a mixture of oxides and carbides or presence of only oxides could be tailored via reaction laser spot. In the Fig. 4, left, are displayed the spectra of powders prepared under different laser power. The patterns are showing a mixture, the most external sextet pleading for maghemite and the next inner one standing for spinel solid solution
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Fig. 4 Mössbauer spectra of “as prepared” samples (left) and the pairs via their dispersion in water as magnetic liquid (right)
with magnetite, then an iron carbide (Beff ∼ 20 − 22 T) and finally some austenite (singlet). The line-widths are narrower for higher laser power (sample F). The specific processing of powder to a magnetic liquid has induced a more homogeneous distribution conserving only the large particles in the fluid as proved by Fig. 4, right. This is in agreement with magnetization data showing at room temperature a value per mass several times larger than for “as prepared” powder in the case of sample F. The sample H (only 35 W) showed a value of magnetization three times smaller than sample F proving the effect of higher power on producing larger grain size in sample F. Therefore let’s us to consider that for a particle with axial anisotropy, the E(θ) energy needed to change the magnetic moment direction by an angle θ is: E(θ) = KV sin2 θ , where K is anisotropy constant and V is particle volume. The relaxation regime could be derived from the ratio r = kB T/KV, with kT = thermal energy and KV = magnetic anisotropy. For r < 0.1 there is a collective fluctuation regime, while for r > 0.1 the super-paramagnetic regime appeared. For the first case [5] the effective field dependence is Beff = B0 (1 − kB T/2KV) with B0 the value at very low and there is an equivalent relation for relaxation time τ = temperature τ0 exp KV kB T . The particle size could be determined directly from Mössbauer spectra knowing the blocking temperatures as described in detail elsewhere [5] and, indeed, mean particle size of 10 nm (F), 7 nm (G) and 6 nm (H) were deduced, which are in pretty good agreement with structural determination providing 12 nm (F) and 8 nm (H) data. The laser power of 55 W was found as the most suitable one to obtain materials for sealing via magnetic fluids.
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4 Conclusions Various parameters specific to laser pyrolyses were modified in order to find out their effect on nano-particle properties and subsequently processed magnetic liquids. The increased pressure directly controls the crystalline level of the formed phases and size distributions as proved by Mössbauer spectroscopy. The formation of carbides is influenced by laser spot area and combustion time. Optimized higher laser power gives rise to right particle size to produce magnetic fluids for sealing application. A peculiar approach to relaxation phenomena as seen by Mössbauer spectroscopy was also provided during data discussion. Acknowledgement The partial support of the contract 747/2005 with MATNANTECH and contract 235/2007 with CNCSIS-IDEI, both National Romanian Programs are deeply acknowledged.
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