Citrate Gel Route Research Articles

Ultrasonic velocity studies in the vicinity ofTC of bismuth doped manganites

Materials with the compositional formulaLa0.67−xBixSr0.33MnO3 (where x = 0, 0.1, 0.2, 0.3) prepared by a citrate gel route were used with a view toinvestigating the elastic behaviour of manganites in the vicinity of theirTC. The structural characterization of the materials clearly indicates that all the samples havea rhombohedral structure with space group. The magnetic (TC)as well as the metal–insulator (TP) transition temperatures determined by AC susceptibility and resistivity measurements,respectively, are found to decrease continuously with increasing bismuth concentration.Finally, the ultrasonic longitudinal velocities of all the samples are found to exhibitconsiderable softening in the vicinity of their magnetic transition temperatures, and aneffort has been made to explain the observed behaviour by mean field theory and theJahn–Teller phenomenon.

Structural, optical and magnetic properties of nanocrystalline Zn 0.9Co 0.1O-based diluted magnetic semiconductors

With a view to understand the influence of nano size on various properties of cobalt-doped ZnO-based diluted magnetic semiconductors, a series of materials were prepared by the citrate gel route. The phase and morphology studies have been carried out by X-ray diffraction and transmission electron microscopy, respectively. All the samples of the present investigation are found to have hexagonal wurtzite structure and crystallite sizes are found to vary from 25 nm to 65 nm. From the optical absorption measurements it has been observed that upon doping with cobalt, the energy band gap is found to shift towards lower energy side (red shift) while it shifts towards higher energy side (blue shift) when the crystallite size is increased continuously. It has been observed from the XPS results that oxidation state of Cobalt is +2 and that the difference in binding energies of Co 2p 3/2 and Co 2p 1/2 is found to increase continuously with increasing crystallite size. Finally, all the samples are found to exhibit room temperature ferromagnetism and the specific magnetization decreases with increasing crystallite size.

Electrical transport properties of Nd0.67−xEuxSr0.33MnO3 (0 ≤ x ≤ 0.67) manganites

A systematic investigation of electrical transport properties viz., electrical conductivity and thermopower of Eu-doped Neodymium-based colossal magnetoresistive manganites with compositional formula, Nd0.67−xEuxSr0.33MnO3 (x = 0–0.67) has been undertaken. These materials were prepared by citrate gel route and characterized by X-ray diffraction, scanning electron microscopy, AC susceptibility, and electrical resistivity measurements. With a view to understand the complex conduction mechanism of these materials, electrical resistivity and thermoelectric power (TEP) data have been analyzed using various theoretical models. It has been concluded that the ferromagnetic metallic part of the conduction mechanism is explained by grain/domain boundary, electron–electron, and magnon scattering mechanisms, while that of high temperature paramagnetic insulating region might be due to small polaron hopping mechanism. The sign change of charge carriers observed in TEP measurements is attributed to the oxygen deficiency of the samples.

Microwave Response of La<SUB>0.7</SUB>Sr<SUB>0.3</SUB>MnO<SUB>3</SUB> Nanoparticles for Heating Applications

La07Sr03MnO3 (LSMO) nanoparticles having average particle size of the order of 20 nm are synthesized by citrate-gel route and evaluated for their microwave response. The structural and magnetization studies establish phase pure nature of nanoparticles with the intended stoichiometry (Curie temperature ∼360 K). These nanoparticles exhibit a temperature increase from room temperature to 45 � C in 120 s when subjected to microwave exposure of 2.45 GHz. Considering hyperthermia as one of the possible application domain of such materials, the cytotoxicity studies on human skin carcinoma and human fibrosarcoma cell lines was done. These studies reveal the percentage cell viability with the exposure to these nanoparticles with a dose of 10 g/ml to be about 85% and 78% respectively. To study the effect of nanoparticles on normal cells, results were compared with primary mice fibroblast cells which show the corresponding cell viability of 94%.

Influence of cation mismatch on electrical, magnetic and magnetoresistance properties of Pr0.67A0.33MnO3manganites

AbstractWith a view to investigate the influence of A‐site cation mismatch on electrical, magnetic and magnetoresistance properties of Pr0.67A0.33MnO3, (A = Ca, Sr, Pb and Ba) manganites, a systematic investigation has been undertaken by preparing the materials using citrate gel route. The samples were characterized by X‐ray diffraction (XRD) and scanning electron microscopy. The XRD data were analyzed by Rietveld refinement method, while the crystallite sizes were estimated using the peak broadening technique. The unusual behavior of large magnetoresistance at low temperatures and at high magnetic fields has also been investigated. Finally, the high temperature electrical resistivity data were analyzed using variable range hopping and small polaron conduction mechanisms. A systematic investigation of the influence of average ionic radius of the A‐site (〈rA〉) and cation mismatch (σ2) on electrical and magnetic properties, viz., transition temperatures, density of states at Fermi level, polaron activation energies etc., has been undertaken. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Structural, magnetic and magneto-transport studies on the (La2/3Sm1/3)0.67Ba0.33−xSrxMnO3 (x = 0, 0.1, 0.2 and 0.33) manganites

The effect of substituting Sr for Ba on the magneto-transport and magnetic properties of the (La2/3Sm1/3)0.67Ba0.33MnO3 system has been investigated. The samples of (La2/3Sm1/3)0.67Ba0.33−xSrxMnO3 (x = 0.0, 0.1, 0.2 and 0.33) are synthesized by the citrate gel route and crystallize in an orthorhombic structure. The unit cell volume decreases while the metal–insulator transition temperature (TMI) increases with increasing Sr content. The atomic disorder, which is represented by the size variance, increases with Ba content. The small anomaly in ρ(T) due to the atomic disorder which leads to the different magnetic ordering followed by a broad metal–insulator transition is observed in Ba-rich systems. The peak at T* (small anomaly) is presumed to be due to spin-dependent interfacial tunnelling that causes the difference in magnetic order between the surface and the core. The suppression in the resistivity values with increasing Sr content could be due to the more symmetric MnO6 octahedra network. The paramagnetic phase coexists with the magnetically ordered phase between T* (∼TC) and TMI. The suppression of the metastable magnetic phase leads to the high magnetoresistance in Ba-rich samples. The transport data has been analysed using a small polaron correlated model. The magnetic behaviour may be understood by the Ising model. The overall magnetic behaviour could be accounted for by the competing antiferromagnetic and ferromagnetic interactions.

Thermopower studies of Pr0.67D0.33MnO3 manganite system

With a view to explaining the structural and electrical transport behaviour of some rare earth manganites, having the general formula Pr0.67D0.33MnO3 (D = Ca, Sr, Pb and Ba), a systematic investigation of the electrical resistivity, thermopower and magnetic properties has been undertaken. These materials were prepared using the citrate–gel route by sintering at 900 °C. All the materials were characterized by x-ray diffraction, scanning electron microscopy, etc measurements. The x-ray data were analysed using the Rietveld method and the variation of various parameters involved in the process as explained. The average crystallite size of the materials has been estimated using the peak broadening method, while TC and TP values were determined from ac magnetic susceptibility and electrical resistivity measurements, respectively. Finally, the magnetoresistance (MR) measurements were also carried out over a magnetic field of 1–7 T and the data clearly exhibit the extrinsic MR in ferromagnetic metallic region.The influence of average A-site cation radius ⟨rA⟩ and cation mismatch parameter σ2 on thermoelectric power (TEP), MR, electrical and magnetic transitions has been analysed. The TEP data at low temperatures (T < TP) have been analysed by considering the magnon concepts, while the high temperature (T > TP) data were explained by the Mott's small polaron conduction mechanism. The effects of ⟨rA⟩ and σ2 on polaron activation and hopping energies were also studied systematically.

Enhancement of magnetoresistance in La2/3Ca1/3MnO3/xCuMn2O4 nanocomposites

Nanocomposites of La2/3Ca1/3MnO3(LCMO)/xCuMn2O4 (0 ⩽ x ⩽ 40%) have been prepared by a citrate gel route and characterized for microstructural and magnetotransport properties. Results show that fabrication with CuMn2O4 has an important effect on the electrical transport behaviour of the composites. With the increment of CuMn2O4 content x, the metal–insulator transition temperature TMI for the composites shifts downwards and the resistivity increases. The susceptibility analysis indicates that the composites with x = 4, 20, 30 and 40% have a similar paramagnetic–ferromagnetic transition temperature TC ∼ 240 K, which is lower than TC of pure LCMO (∼262 K). The high temperature (T > TMI) semi-conducting part of the resistivity (ρ) data follows a small polaron hopping conduction mechanism, and the metallic behaviour of the samples (T < TMI) fits the model in terms of electron–magnon scattering of the carriers. Furthermore, a significant enhancement both in low-field magnetoresistance (LFMR) and in high-field magnetoresistance (MR) is observed for the composites at a wide temperature range below TMI. The LFMR measured at 0.3 T reaches the maximum for the x = 40% sample when T = 10 K with the value of ∼23%, which is much larger than that of the pure LCMO (∼6.9%). We argue that such an enhancement in MR is attributed to the enhanced spin-polarized tunnelling, which is manipulated by the spin disorder at the LCMO surfaces caused by CuMn2O4 addition.

Lattice effect on the magnetic and magneto-transport properties of (La 1/3Sm 2/3) 0.67Ba 0.33− xSr xMnO 3 ( x = 0.0, 0.1, 0.2 and 0.33) compounds

The effect of substituting Sr for Ba on the magneto-transport and magnetic properties of (La 1/3Sm 2/3) 0.67Ba 0.33MnO 3 system, has been investigated. The samples, (La 1/3Sm 2/3) 0.67Ba 0.33− x Sr x MnO 3 ( x = 0.0, 0.1, 0.2 and 0.33), synthesized by citrate gel route, crystallize in an orthorhombic structure (space group Pnma, no. 62). The unit cell volume decreases while the metal-insulator transition temperature ( T MI) increases with increasing Sr content. The localization of charge carriers occurs at low temperatures and becomes more pronounced with decreasing Sr content which leads to an enhancement of resistivity. This could be understood by the variation of Mn O Mn bond-distance and angle. Reappearance of semiconducting behavior (d ρ/d T < 0) is observed only in samples with x = 0 and x = 0.1 below certain temperature ( T < T MI). These samples exhibit thermal irreversibility behavior for a field-cooled (FC) and zero-field-cooled (ZFC) magnetization data in a magnetic field of 100 Oe. This is ascribed to the competition between the superexchange and double exchange interactions. The change in physical properties has been correlated to chemical parameters such as ionic radii, tolerance factor, electronegativity and variation in Mn O Mn angle.

Anomalous variation of magnetoresistance in Nd 0.67− yEu ySr 0.33MnO 3 manganites

A systematic investigation of Eu-doped Nd-based colossal magnetoresistive manganites with compositional formula Nd 0.67− y Eu y Sr 0.33MnO 3( y=0–0.67) has been undertaken to understand their structural, magnetic as well as electrical behavior. These materials were prepared by the citrate gel route, and were later characterized by X-ray diffraction (XRD), AC susceptibility, electrical resistivity etc; measurements. A detailed structural characterization of the XRD data has also been undertaken, using Rietveld refinement method. From a systematic analysis of electrical resistivity versus temperature data, it has been found that the last two samples of the series ( y = 0.57 and 0.67) are found to exhibit charge order (CO) phenomenona, and that the CO state melts completely in the case of former sample ( y = 0.57 ) while it melts partially only in the case of later one ( y = 0.67 ) , even in a magnetic field of 7 T. The observed behavior has been explained qualitatively. Some of the samples of the series are found to exhibit unusually large magnetoresistance over a wide temperature range in the low temperature region, and their behavior has been explained on the basis of phase segregation and an inter-grain spin polarized tunneling effect.

Ba-Zr codoped sodium bismuth titanate by novel alkoxyless wet chemical route: processing and electromechanical behavior

Sodium bismuth titanate (NBT) based ceramics have been shown to be a promising lead-free alternative to commercial piezoelectrics and high capacitance materials. This article reports a novel aqueous citrate gel route for the synthesis of (Na0.5Bi0.5)xBa1-xTiyZr1-yO3 powders. Instead of alkoxides, usually used in citrate gel route, aqueous citrate solutions of titanium and bismuth were prepared from their respective inorganic salts. These solutions were used to prepare powder of (Na0.5Bi0.5)0.88Ba0.12Ti0.96Zr0.04O3. The process produced an amorphous xerogel that was readily crystallized to a pseudocubic perovskite phase when calcined at temperatures as low as 500 °C. The decomposition and oxidation of the xerogel was studied using TG-DTA, which showed complete removal of volatile components by 580 °C. The formation of crystalline phases was studied using XRD. SEM was used to evaluate the sintering behavior of the powder. The powder exhibits significant sintering at temperatures 800–900 °C. The polarization behavior of the sintered material was similar to one prepared by conventional solid state route. A maximum polarization of ∼24 μC/cm2 and peak bipolar strain ∼0.095% at 44 kV/cm was achieved in the sintered material.

Low temperature synthesis and magneto- electrical studies in Nd1−xPbxMnO3 system

The system Nd1 − xPbxMnO3 with x ≤ 0.50 has been synthesized by citrate-gel route. All these samples have shown single phase structure with cubic symmetry. Magnetic and electrical behavior has been studied in the temperature range 4–300 K. The temperature dependent resistivity data shows that metal-semiconductor transitions occurs at temperature TP∼ 166, 174 and 182 K for x = 0.30, 0.40 and 0.50 respectively. Under the field of 1 T, this transition shifts to higher temperature. These samples have shown maximum MR upto 40% under the field of 1 T at temperature lower than TP. These results are supported by magnetic measurements over the observed temperature range. Resistivity data above TP has been found to fit well with Tn where n = −1/4 for x = 0.30 and n = −1/2 for x = 0.50 which is attributed to the variable range hopping of small polarons.

Electronic phase separation in (La1∕3Sm2∕3)2∕3A1∕3MnO3 (A=Ca, Sr and Ba) compounds

Electrical and magnetotransport properties of (La1∕3Sm2∕3)2∕3A1∕3MnO3 (A=Ca, Sr, and Ba) compounds, synthesized by the citrate gel route, have been investigated. These compounds crystallize in an orthorhombic structure (space group Pnma). Semiconducting-like behavior is observed only in Ca-substituted sample while Ba and Sr-substituted samples show metal-insulator type transition. The high and weakly temperature dependent magnetoresistance at low temperature, thermal irreversibility in field-cooled and zero-field-cooled magnetization, and nonsaturating magnetization behavior are expected in systems with electronic phase separation. The cationic size disorder by varying alkaline earth ions causes the electronic phase separation.

Synthesis of bismuth titanate by citrate method

A simple citrate gel route was adopted for the preparation of bismuth titanate (Bi 4Ti 3O 12) powders. Stoichiometric quantities of BiCl 3 and TiOCl 2 were mixed with required amount of citric acid, and the mixture was heated on a water bath. This leads to formation of gel, which was decomposed at 673 K. Formation of bismuth titanate was observed on calcining powders at 973 K by X-ray diffraction studies. The average particle size is found to be 20 nm by microscopy. The room temperature dielectric constant is found to be 175 at 1 kHz. The hysteresis loop parameters were also obtained by home built Sawer–Tower circuit.

Magneto-transport studies on (Pr1/3Sm2/3)2/3A1/3MnO3 (A = Ca,Sr and Ba) compounds

The magnetic and transport properties of(Pr1/3Sm2/3)2/3A1/3MnO3 (A = Ca,Sr and Ba) compounds, prepared by the citrate-gel route, have been investigated. Thesecompounds are found to crystallize in the orthorhombic structure. Charge orderingtransport behaviour is indicated only in the Ca-substituted compound. The Sr- andBa-substituted compounds show a metal–insulator transition and semiconducting-likebehaviour, respectively. The magnetoresistance is highest in the Ba-substituted compound.All three samples show irreversibility in magnetization as a function of temperature inzero-field cooled (ZFC) and field cooled (FC) plots. Non-saturating magnetization,even at 5 K and 4 T field, is observed in Ca- as well Ba-substituted compounds.

Magnetic and electrical transport properties ofLa0.67Ca0.33MnO3 (LCMO):xZnO composites

We have synthesized La0.67Ca0.33MnO3 (LCMO):xZnO () composites through a citrate gel route and have characterized them for magnetic andmagnetotransport properties. In lower concentrations (), ZnO mostly goes into the perovskite lattice substituting Mn in LCMOand segregates less in the grain boundary region, but at higher concentration(x>0.13) it segregates mostly at the grain boundaries of LCMO and influences the transport propertiessignificantly. A model is proposed which describes the overall resistivity of the system as aparallel combination of a low resistive intragrain conducting path and a high resistiveintergrain insulating path. Using this approach, the grain and grain boundary contributionsto the overall resistivity are separated for all the composites. The field dependent resistivityshows that all the composites have higher values of MR at the transition temperatures(TMI) compared to thatin pure LCMO (x = 0). The highest value of MR is obtained forx = 0.10 and is 76.6% at80 kOe field near TMI.

Influence of synthesis route on the properties of doped lanthanum cobaltite and its performance as an electrochemical reactor for the partial oxidation of natural gas.

La(0.6)Sr(0.4)Co(0.2)Fe(0.8)O(3-delta)(LSCF) perovskite powders have been synthesised by solid-state reaction, co-precipitation, drip pyrolysis and citrate gel routes, and characterised using XRF, XRD, SEM and BET. Co-precipitation using oxalic acid or aqueous ammonia as precipitant failed to achieve the correct chemical composition. Perovskite structures were achieved in all other cases. Surface areas ranging from 0.6 to 17.4 m(2) g(-1) were obtained, which was reflected in the different microstructures observed. The citrate gel product exhibited a convoluted network morphology resulting in its large surface area. Thin-walled (approximately 200 microm) tubular membranes have been manufactured from the LSCF powders using viscous plastic processing. The tubes have been characterised using a custom-built gas analysis rig with on-line mass spectrometry. Porosity levels of the membranes were found to be very low (<0.1%). The spontaneous oxygen flux across the tubular membranes was determined as a function of temperature. Oxygen permeation rates ranged from 0.1 to 0.3 micromol cm(-2) s(-1) at 1273 K. The catalytic behaviour of the LSCF tubes towards methane oxidation has been studied using temperature programmed reaction and conventional catalytic measurements. The tubes favoured combustion reactions, with smaller amounts of partial oxidation and oxidative coupling products observed. Powder coatings have been incorporated to establish the effect of increasing surface area.

Influence of synthesis route on the powder properties of a perovskite-type oxide

La0.6Sr0.4Co0.2Fe0.8O3δ (LSCF) powders intended for membrane reactors for the catalytic partial oxidation of natural gas have been synthesised by a range of conventional and novel techniques. The powders were manufactured by solid state reaction, co-precipitation, drip pyrolysis and citrate gel routes, with subsequent heat treatment of the precursors creating the perovskite-type oxides. The LSCF powders have been characterised using XRF, XRD, SEM, and BET and compared against a commercially available powder of identical composition.

Preparation and microwave dielectric properties of (Pb0.45Ca0.55)(Fe1/2Nb1/2)O3 ceramics by citrate-gel processing route

A synthetic procedure has been developed for the (Pb0.45Ca0.55)(Fe1/2Nb1/2)O3 (PCFN) system in terms of liquid-mixed citrate precursor. The stability of the citrate gels against phase separation is investigated as a function of the pH value and the citrate/metal ratio in the initial solution. The elaboration process is described in detail. A single-phase PCFN powder with a particle size of 30–50 nm can be synthesized at the low temperature of 850 °C using the citrate-gel processing route, which is 200 °C lower than that for the conventional process. Sintering properties and microwave dielectric properties of specimens derived from the citrate-gel process were also studied in this paper. A dense compound with a bulk density up to 6.20 g cm-3 could be obtained when the specimens are sintered at 1000 °C/4 h. The dense single-phase PCFN ceramics were found to have microwave dielectric properties of ɛγ=86.2, Q · f=3450 GHz and τf=4 ppm°C-1.

Synthesis of Calcium Silicate by a Chelate Gel Route with Aqueous Solution of Citric Acid.

Recently, novel gel techniques such as the metal-chelate gel method, in situ polymerized complex method and polymer precursor method have been utilized to prepare many kinds of ceramics. These techniques offer the potential advantage of achieving compositionally homogeneous and fine powders with a narrow size distribution. Since the hydration activity of calcium silicates is affected by phase changes and the surface area, it is of vital importance to take into account both polymorphic forms and particle size. In this study, a metal-chelate gel route based on gelation of the aqueous solution of citric acid has been successfully applied to the synthesis of calcium silicates (Ca2SiO4 and Ca3SiO5) for the first time. In addition, their phase transformations and particle size are discussed in comparison to the conventional solid-state reaction route. The novel citrate gel route and the conventional solid-state reaction route were found to produce β-Ca2SiO4 (high-temperature phase) and γ-Ca2SiO4 (low-temperature phase), respectively. This result can be explained in terms of the particle size effect and the energy barrier. The nucleation and propagation of microcracks were responsible for overcoming a comparatively high-energy barrier, ΔG*β→γ. The particle size effect governs both the statistic of martensitic nucleation and the propagation of the β→γ transformation. In contrast to Ca2SiO4, triclinic Ca3SiO5 (low-temperature phase) was obtained by both the citrate gel route and the conventional solid-state reaction route. The nucleation and propagation are not responsible for the transformation, thus the energy barrier of the monoclinic (M) to the triclinic (T) transformation, ΔG*M-T, are considered to be small.