Paramagnetic centers in nanocrystalline TiC/C N.Guskos1,2,* , T. Bodziony1 , M. Maryniak1 , J.Typek2 , and A. Biedunkiewicz3 1
Solid State Physics, Department of Physics, University of Athens, Panepistimiopolis, 15 784
Zografos, Athens, Greece. 2
Institute of Physics, Szczecin University of Technology, Al.Piastow 17, 70-310 Szczecin, Poland.
3
Institute of Material Engineering, Szczecin University of Technology, Al.Piastow 17, 70-310
Szczecin, Poland.
Electron paramagnetic resonance is applied to study defect centers in nanocrystalline titanium carbide dispersed in carbon matrix (TiC x /C) synthesized by the nonhydrolytic sol- gel process. The presence of Ti3+ paramagnetic centers is identified below 120 K along with a minor contribution from localized defect spins coupled with the conduction electron system in the carbon matrix. The temperature dependence of the resonance intensity of the latter signal indicates weak antifferomagnetic interactions. The presence of paramagnetic centers connected with trivalent titanium is suggested to be the result of chemical disorder, which can be further related to the anomalous behavior of conductivity, hardness, and corrosion resistance of nanocrystalline TiC x /C.
PACS: 33.35.+r, 73.63.Bd, 75.20. g
*
Corresponding author: e- mail:
[email protected]
1
Titanium carbides (TiC x ) belo ng to the class of refractory metal compounds characterized by electrical and thermal conductivities comparable to those of pure transition metals in combination with the extreme hardness and very high melting points typically observed in covalent crystals [1]. These unique properties have long attracted interest from both scientific and technological perspective and made these materials particularly important in several engineering applications for hardness and corrosion resistant coatings [2,3]. Many theoretical studies have been conducted on the electronic structure, elastic properties and lattice stability of transition metal carbides, especially of TiC, in order to understand the interplay of ioniclike and metallic physical properties [4-6]. TiC x compounds can vary widely in both stoichiometry and vacancy content. Carbon vacancies are the dominating defects in the crystal lattice of TiC x , while the concentration of titanium vacancies is much lower. The presence of defects at high concentrations has a profound effect on many macroscopic properties of TiC x , causing changes in hardness, thermal expansion coefficient, and resistivity to oxidation [2,3]. Moreover, many physical characteristics of TiC x display specific dependence on the concentration of carbon vacancies. For example, the lattice constant, melting temperature, hardness, and the activation energy of carbon diffusion increase significantly with the concentration of carbon vacancies up to x~0.78 – 0.73, whereas their value remains constant or even start to decrease upon further increase of the number of vacancies. Interesting conducting properties of titanium carbide, in particular a semiconductor – to – metal transition has been recently identified in nanocrystalline TiC [7] that was related to disorder connected with the existence of trivalent titanium ions [8]. Moreover, titanium carbide dispersed in a carbon matrix displayed a rather peculiar thermal behavior of its electron paramagnetic resonance (EPR) spectra arising from conduction electrons [9,10]. On the other hand, previous studies have shown rather broad EPR spectra in TiC compounds indicative of high electrical conductivity and it was suggested that the electronic behavior is
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not dissimilar to that of bulk metals, in which the electrons exhibit fairly short spin- lattice relaxation times, giving rise to excessively broad lines and render the observation of EPR resonances inaccessible to ordinary spectrometers [11]. In this work, EPR spectroscopy is applied to study nanocrystalline titanium carbide dispersed in carbon matrix (TiC x /C) synthesised by a new method, the nonhydrolytic sol-gel process [12]. The presence of Ti3+ paramagnetic centers is identified along with a minor contribution from localized defect spins coupled with the conductio n electron system. This study could provide additional data for a better understanding of the oxidation phenomena in titanium carbide such as the anomalous behavior of conductivity, hardness, and corrosion resistance. Figure 1 presents the SEM picture of the titanium carbide dispersed in carbon. Carbon cages of sub micron sizes are seen, containing TiC nanoparticles with an average size in the 40-50 nm range. The content of free carbon in the sample is 3-5% wt. EPR measurement were performed on powder samples sealed in quartz tubes using a Bruker E 500 X-band spectrometer (?=9.48 GHz) with 100 kHz field modulation and an Oxford flow cryostat for temperature dependent measurements (3.5-300 K). Figure 2 shows the EPR spectra of the nanocrystalline TiC/C sample at different temperatures. At 3.7 K two EPR lines are recorded: a very narrow centered at Hr = 3373 Oe (geff = 2.0030(3)) with peakto-peak linewidth ?Hpp = 3.27(3) Oe (referred further as CE line) and the other at Hr = 3447(2) Oe (geff = 1.96(1)), with ? Hpp =68 Oe (designated as Ti line). Figure 3 shows the temperature dependence of the g- factor, peak-to-peak linewidth and intensity of the CE line. The g- factor of the narrow line remains very close to the free electron g-value (ge=2.0023), while a similar weak temperature variation is observed for the linewidth. This behavior resembles the temperature dependence of the g factor of the conduction electron spin resonance within the band model of quasi-two-dimensional doped graphite coupled with that of localized
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paramagnetic defects [13,14]. The temperature dependence of the resonant intensity (I) can be adequately fit to the sum of a dominant Curie-Weiss term ICW =C/(T-T) with T=-3.7(5) K and a temperature independent contribution I0 of small magnitude. This variation indicates a rather small Pauli- like contribution I0 from the conduction electrons and a major fraction of localized defects in the graphite matrix with weak antiferromagnetic interactions (T
