The Effects Of Molar Ratio

Published Date: 02 Nov 2017

CHAPTER 2

Sol- gel methods is one of the wet chemical that is used to produce nano particle powder. This is due the fact that, sol-gel method have great potential in producing important materials in forms of bulk, fibers, sheets, coating films, and particles at relatively low temperature. It is also possible to produce materials of new compositions with high purity, high homogeneity, and to control particle size distributions in nano-scale level. [Dimitriev et al, 2008]. Sol gel uses citric acid to chelate the metal ions while ethylene glycol as a solvent for the process of polymerization to form an intermediate of polyester-type resin. According to Popa and Kakihana et al, this process consists of three major steps which are the complexation of metal ions with citric acid in water, polyesterification with ethylene glycol and decomposition of the organic network to obtain the powder precursor. Thus, searching for the minimum molar ratio of citric acid to metal ions and calcination temperature is important to obtain pure crystalline phase of perovskite with nanometric sizes.

2.2. The Effects of Molar Ratio of Citric Acid / Metal Ions

In sol-gel method, Citric acid was used to chelate metal ions, while ethylene glycol was used as solvent for the process of polymerization between citric acid and ethylene glycol, and an intermediate of polyester-type resin was obtained. Yang et al, 2004 reported, that as the molar ratio of citric acid/metal ions increased up to 4, there is much more cubic phase of LSMO with a perovskite structure was observed. Besides that, the amount of citric acid used as a chelating ligand plays an important role in maintaining the homogeneity of metal ions in the resin on a molecular scale. LSMO reaches homogeneous solution when the molar ratio of citric acid to metal ions were 3-4, which no visible precipitation was observed during the polymerization process. At a lower molar ratio of citric acid to metal ions, a less ligand of citric acid to metal ions causes more segregation of metal ions and resulting the formation of second phase.[Yang et.al]. Besides that, Gaki et al, also reported that the molar ratio citric acid to metal ions is 2 is the most favourable for the synthesis of LaMnO3. However, the optimum citric acid to metal ions ratio for synthesis of other manganites nanoparticle powder is yet to be determined.

Figure 2.1 shows the XRD pattens of the LSM powder samples prepared with different citric acid to metal ions ratio after calcination as reported by Conceição et.al, 2010. The sample prepared with citric acid to metal ions ratio of 1:`1 exhibited a mixture of phases, which were not identified. This shows that it is difficult to maintain the homogeneity of the metal ions in the resin on a molecular scale with ratio of citric acid to metal ions is low. The low concentration of citric acid prevents a good polymer matrix formation and consequently poor crystalline structure obtained after calcination. While the sample prepared with citric acid ratio 2:1 presented a great improvement in the phase formation, which there is no secondary phase formation, however, the LSM is still poorly crystallized. For the XRD pattern of the sample with molar ratio citric acid to metal ions of 3:1, shows the only presence of La0.7Sr0.3MnO3 phase with high crystallinity. Thus, the minimum citric acid to metal ions ratio necessary for a proper formation of the polymer matrix and a good crystallization of perovskite phase after calcination is 3.

Figure 2.1 : XRD patterns of La0.7Sr0.3MnO3 powder prepared with CA:MI ratio of (a) 1, (b) 2, (c) 3, after calcination at 7500C for 10h.

[Obtained from Conceição et al. 2010 Synthesis of La1-x Srx MnO3 powders by polymerizable complex method : Evaluation of structural, morphological and electrical properties]

2.3. Colossal Magnetoresistance (CMR) Effect

The discovery of colossal magnetoresistance (CMR) has led to the producing of perovskite manganites with general formula RE1-x AEx MnO3 (RE= trivalent rare earth cation, AE= Divalent alkaline earth cation) powder. These powder exhibit a high magnetoresistance in a magnetic field of several tesla [Chahara et.al, 1993]. CMR effect and strong correlation between the structure and electronic-magnetic phases can all be attributed to the ratio of the Mn3+ and Mn4+ ions. Partial doping of the trivalent RE ion by divalent alkaline earth cation AE leads to the formation of a mixed valence state of Mn to maintain the charge neutrality of the system. The doping with some divalent cation causes the structure to become distorted due to the differences in the size of the various atoms and lead to the Jahn-Teller effect.

Besides that, the granular perovskite system a grain surface of the perovskite would have low transition temperature and magnetization due to the magnetically disordered states in the surface of the grains. By decrease the grain size, causes the magnetic order states increases and thus, double exchange (DE) mechanism weakens. This consequently leads to suppression of paramagnetic to ferromagnetic transition temperature as well as the metal to insulator transition.

In the journal reported by Kameli et al 2006, the sample of La0.8 Sr0.2 MnO3 with various grain size reported that in large grain sample (sample SI) a metal-insulator (MI) transition behavior was observed at a temperature 305K which is very close to curie temperature Tc. By decreasing the grain size, the resistivity increases and the MI transition temperature decreases. By further decreasing the grain size, the low temperature peak height increases as shown in figure 2.2. In a granular perovskite system, the perovskite can be assumed as a two-phase system, a body and a surface. The body phase have the same properties as the bulk compound (magnetic and transport propertied) and the surface phase would have low transition temperature and magnetization. This because of the magnetically disordered states is in the surface and not the body phase. According to Kameli et al. 2006, The mangnetically disordered states at grain surface is due to the high degree of oxygen vacancies, breaking of Mn-O-Mn paths, deviation of stoichiometric compositions, termination of crystal structure and dislocation. From figure 2.2, the peaks at high temperature shows the intrinsic metal-insulator transition temperature but the resistivity peak at temperature well below Tc, does not mean a metal-insulator-like transition for the granular system and indicates the interfacial tunneling due to the difference in magnetic order between surface and body.

Figure 2.2 : The temperature dependence of the resistivity for La0.8 Sr0.2 MnO3.

[Obtained from Kameli et al. 2006 Influence of grain size on magnetic and transport properties of polycrystalline La0.8 Sr0.2 MnO3 manganites]

Figure 2.3(a), shows the ρ-T curve at zero field and an applied field H=5KG. The MR in polycrystalline manganites shows two clearly different behaviors. Intrinsic CMR response is shown around MI transition temperature but superimposed to this, extrinsic MR appears, as a consequence of the granular morphology of the samples. While in figure 2.3(b), the intrinsic CMR around MI transition temperature can be tuned by means of particles size variations. The intrinsic CMR peak intensity decreases by decreasing grain size.

Figure 2.3: (a) Temperature dependence of resistivity for sample S48 at zero field and an applied field H=5kG. (b) The temperature dependence of magnetoresistance (MR) at an applied field of 5kOe for all of the sample.

[Obtained from Kameli et al. 2006 Influence of grain size on magnetic and transport properties of polycrystalline La0.8 Sr0.2 MnO3 manganites]

Besides that, according to Panneer et al 2011, the field dependent magnetoresistance of the samples at 5K of bulk sample and nanoparticle (MH25) sample as shown in figure 2.4, indicate the sharp drop of MR(H). This is due to the spin polarized tunneling of charge carriers between ferromagnetic grains contributes to this MR drop at low field regime, which also known as low field magnetoresistance (LFMR). The magnitude of magnetoresistance of bulk sample is always smaller than the nanoparticle sample because of the disordered grain boundary spins or magnetic spins clusters of the nano-sized grains which contribute to magnetoresistance. Response of these disordered spins to the magnetic field is slow due to interactions among spins and this provides a large scope of tuning magnetoresistance. As the increase of measurement temperature, the LFMR strongly decreases at 100k and 200k in all samples. At 300K, bulk sample shows nearly 1% of LFMR while for the rest of the samples indicate no significant LFMR. The decrease of magnetoresistance at higher temperature indicates the decrease of ferromagnetic double exchange correlation among the grain boundary Mn3+-O2- -Mn4+ spins networks. According to the core-shell spin structure of ferromagnetic grains, some of the grain boundary (shell) spins become paramagnetic or disordered with the decrease of grain size.

Figure 2.4 : MR(H) for bulk and nano-grained samples at 5K (a), 100K (b), 200K (c), and 300K (d).

[Obtained from Panneer et al, Grain size dependent magnetization, electrical resistivity and magnetoresistance in mechanically milled La0.67 Sr0.33 MnO3]