Effects of phase and chemical composition of precursor on structural and morphological properties of (Lu0.95Eu0.05)2O3 nanopowders

Europium-doped lutetium oxide nanopowders have been synthesized by the co-precipitation technique using ammonium hydrogen carbonate as a precipitant. Effects of chemical and phase composition of carbonate precursors on the morphology and sinterability of (Lu_0.95Eu_0.05)₂O₃ nanopowders have been studied. Two types of precursors have been obtained differing by the molar ratio R=NH₄HCO₃/Lu³⁺: a mixture of crystalline Lu_0.95Eu_0.05(OH)(CO₃)·4H₂O and unidentified amorphous phases at R=4–7 and crystalline Lu_0.95Eu_0.05(H₂O)_x(HCO₃)₃·nH₂O precursor at R=8–20. The two-phase precursor consists of spherulite-like aggregates, while the crystalline one is characterized by plate-like morphology. Calcination of Lu_0.95Eu_0.05(H₂O)_x(HCO₃)₃·nH₂O leads to formation of (Lu_0.95Eu_0.05)₂O₃ nanopowders that inherit the precursor morphology, while no morphology succession is observed for (Lu_0.95Eu_0.05)₂O₃ nanopowders obtained by heat treatment of the two-phase precursor. Calcination of the two-phase mixture leads to breakdown of the spherulites and to formation of equiaxed particles with an average diameter of 40 nm with the standard deviation of particle size distribution of about 15%. The obtained low-agglomerated nanopowders were used in vacuum sintering to produce (Lu_0.95Eu_0.05)₂O₃ optical ceramics with in-line transmittance of 41% at 611 nm.     

Synergistic effect in structural and supercapacitor performance of well dispersed CoFe2O4/Co3O4 nano-hetrostructures

Herein, we report a facile homogeneous urea – assisted hydrothermal approach for the design of CoFe₂O₄/Co₃O₄ nano hetrostructure. A variation in Co concentration leads to smartly designed composite material namely CFC-11 and CFC-12 where CFC-12 appreciates the benefits of both CoFe₂O₄ and Co₃O₄ nanoparticles. The physico – chemical properties of as developed materials were investigated by X-ray diffraction (XRD), field emission electron microscopy (FE-SEM), high resolution transmission electron microscopy (HR-TEM), X-ray photoelectron microscopy (XPS) and Raman spectroscopy. The specific surface area and pore size distribution was determined by Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halendo (BJH) respectively. Magnetic measurements via. vibrating sample magnetometer (VSM) demonstrate that saturation magnetization decreases with the incorporation of Co₃O₄ antiferromagnetic nanoparticles. To explore the utility of as designed nano-hetrostructures as supercapacitor electrodes, we employed cyclic voltammetry (CV) and electrochemical impedence spectroscopy (EIS) measurements. A high specific capacitance of 761.1?F?g^?1 at 10?mV?s^?1, admirable cyclic durability of 92.2% and a low resistance value obtained from impedence measurements was observed for CFC-12. The favorable performance demonstrates the synergistic effect of CoFe₂O₄ and Co₃O₄ nanoparticles and thus promise an excellent material for energy storage devices.     

Protective properties of SiO2 with SiO2 and Al2O3 nanoparticles sol-gel coatings deposited on FeCrAl alloys

Five layer SiO₂ coatings containing SiO₂ or Al₂O₃ nanopowders were deposited on FeCrAl alloy support by sol-gel method. Studies of protective properties of the coatings were carried out during high temperature cyclic oxidation. Changes in surface topography, structure and chemical composition of the surface layer of FeCrAl alloy were investigated. It has been shown that the type of nanofillers present in the SiO₂ coating (about 2.5?wt%) affects morphology of Al₂O₃ growing scale and determines the heat resistance of FeCrAl alloy. The lowest relative mass change (approx. 1.3%) after 10 oxidation cycles in air at 900?°C (one cycle = 12?h) was measured for the samples with coatings containing hydrophilic nanosilica (Aerosil 380) as filler. The protective efficiency of the coatings in the process of high-temperature oxidation is from 66% to 85%. The thickness of the formed scale and the value of the parabolic rate constant depend on the type of nanopowder in the coating.     

Structural and thermoelectric properties of hydrothermal-processed Ag-doped Fe2O3

Fe_2-xAg_xO₃ (0?≤?x?≤?0.04) nanopowders with various Ag contents were synthesized at different hydrothermal reaction temperatures (150?°C and 180?°C). Their structural properties were fully investigated through an X-ray diffraction, a Fourier transform infrared spectroscopy, and an X-ray photoelectron spectroscopy. The hydrothermal reaction temperature, time, and Ag content remarkably affected the morphological characteristics and crystal structure of the synthesized powders. The Fe_2-xAg_xO₃ (0?≤?x?≤?0.04) powders synthesized at 150?°C for 6?h and the Fe_2-xAg_xO₃ (0.02?≤?x?≤?0.04) powders synthesized at 180?°C for 12?h formed the orthorhombic α-FeOOH phase with a rod-like morphology, whereas the Fe_2-xAg_xO₃ (0?≤?x?≤?0.01) powders synthesized at 180?°C for 12?h formed the rhombohedral α-Fe₂O₃ phase with a spherical-like morphology. The Fe_1.98Ag_0.02O₃ fabricated by utilizing Fe_1.98Ag_0.02O₃ powders synthesized at 180?°C showed the largest power factor (0.64?×10^?5 Wm^?1 K^?2) and dimensionless figure-of-merit (0.0036) at 800?°C.     

Characterization and tribological investigation of sol-gel Al2O3 and doped Al2O3 films

Thin films of Al₂O₃ and doped Al₂O₃ were prepared on a glass substrate by dip coating process from specially formulated ethanol sols. The morphologies of the unworn and worn surfaces of the films were observed with atomic force microscope (AFM) and scanning electron microscope (SEM). The chemical compositions of the obtained films were characterized by means of X-ray photoelectron spectroscopy (XPS). The tribological properties of obtained thin films sliding against Si₃N₄ ball were evaluated and compared with glass slide on a one-way reciprocating friction tester. XPS results confirm that the target films were obtained successfully. The doped elements distribute in the film evenly and exist in different kinds of forms, such as oxide and silicate. AFM results show that the addition of the doped elements changes the structure of the Al₂O₃ films, i.e., a rougher and smoother surface is obtained. The wear mechanisms of the films are discussed based on SEM observation of the worn surface morphologies. As the results, the doped films exhibit better tribological properties due to the improved toughness. Sever brittle fracture is avoided in the doped films. The wear of glass is characteristic of brittle fracture and severe abrasion. The wear of Al₂O₃ is characteristic of brittle fracture and delamination. And the wear of doped Al₂O₃ is characteristic of micro-fracture, deformation and slight abrasive wear. The introduction of ZnO is recommended to improve the tribological property of Al₂O₃ film.     

Yb2O3 and Y2O3 co-doped zirconia ceramics

A suspension stabilizer-coating technique was employed to prepare x mol% Yb₂O₃ (x = 1.0, 2.0, 3.0 and 4.0) and 1.0 mol% Y₂O₃ co-doped ZrO₂ powder. A systematic study was conducted on the sintering behaviour, phase assemblage, microstructural development and mechanical properties of Yb₂O₃ and Y₂O₃ co-doped zirconia ceramics. Fully dense ZrO₂ ceramics were obtained by means of pressureless sintering in air for 1 h at 1450 °C. The phase composition of the ceramics could be controlled by tuning the Yb₂O₃ content and the sintering parameters. Polycrystalline tetragonal ZrO₂ (TZP) and fully stabilised cubic ZrO₂ (FSZ) were achieved in the 1.0 mol% Y₂O₃ stabilised ceramic, co-doped with 1.0 mol% Yb₂O₃ and 4.0 mol% Yb₂O₃, respectively. The amount of stabilizer needed to form cubic ZrO₂ phase in the Yb₂O₃ and Y₂O₃ co-doped ZrO₂ ceramics was lower than that of single phase Y₂O₃-doped materials. The indentation fracture toughness could be tailored up to 8.5 MPa m^1/2 in combination with a hardness of 12 GPa by sintering a 1.0 mol% Yb₂O₃ and 1.0 mol% Y₂O₃ ceramic at 1450 °C for 1 h.     

Citric based sol-gel synthesis and photoluminescence properties of un-doped and Sm doped Ca3Y2Si3O12 phosphors

The photoluminescent properties of un-doped and Sm³⁺ doped Ca₃Y₂Si₃O₁₂ phosphors were prepared by the citrate sol-gel method. Un-doped sample has shown a strong blue emission, which has its maximum intensity at 389 nm. Among all the observed emission transitions ⁴G_5/2→⁶H_J (J = 5/2, 7/2 & 9/2) of Sm³⁺, the reddish-orange (RO) emission transition ⁴G_5/2→⁶H_7/2 is more prominent, which matches well with the emission wavelength of near UV (n-UV) LED. The reasons for the observance of such prominent visible color emissions from these phosphors have been substantiated appropriately. Besides, structural details of these samples have also been analyzed from the measured XRD, TEM, TG-DTA and FT-IR profiles.     

Improving sintering behavior of MWCNT/BaTiO3 ceramic nanocomposite with Bi2O3-B2O3 addition

The present research systematically investigated the novel low-temperature fabrication of a multi-walled carbon nanotube (MWCNT)/barium titanate nanocomposite using a two-step mixing technique. The synthesis was conducted using different amounts of MWCNT (0.25%, 0.5%, 1%, 2%, 4%, and 8% wt) with different compositions of (Bi₂O₃ + B₂O₃) as a sintering aid. Scanning and transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, three-point bending strength, Vickers hardness indentation and Archimedean technique were used to characterize the as-synthesized specimens. It was found that the appropriate content of sintering aid (Bi₂O₃+B₂O₃) strongly decreased the sintering temperature from 1200 °C to 950 °C. The results also revealed that any sintering aid with the optimum composition that included 30% (mol) Bi₂O₃ was optimal for a sintering aid content of 6% (wt). Consequently, the highest values of the flexural strength and fracture toughness were achieved by applying the optimal amounts of MWCNT (1% wt) and sintering aid (6% wt).     

Sol-gel synthesis of substoichiometric cobalt ferrite (CoFe2O4) spinels: Influence of additives on their stoichiometry and magnetic properties

Spinel cobalt ferrites (CoFe₂O₄) with varying levels of substoichiometry were prepared via sol-gel synthesis with different combinations of citric acid, dextrose and PVP (polyvinylpyrrolidone). The gels, prepared from the metal nitrates, were dried at 110?°C, further treated at 850?°C and finally subjected to thorough structural and magnetic characterization in order to correlate the composition of the gel to the structural and magnetic properties displayed by the ferrites. The materials synthesized in the present work have shown to be rather iron-depleted, reaching over 50% deficiency of the metal, which leaves the spinel lattice and forms Fe₂O₃ instead. The fuel/oxidant ratio and the thermal behavior of the xerogels exert a direct influence on the compositional variation of the prepared spinels, which could in turn be correlated to the magnetic properties displayed by the particles. A maximum in coercivity of 2154.4?Oe was achieved without the application of any additive to the nitrate precursors. On the other hand, the magnetic remanence displayed by the ferrites shows a linear relationship to the iron content in their chemical formula. The results open up the possibility of fine-tuning the structural and magnetic properties displayed by the spinel product via careful control of the composition of the reaction medium.     

Glass transition and crystallization of ZnO-B2O3-SiO2 glass doped with Y2O3

The effect of Y₂O₃ on the glass transition kinetics, crystallization kinetics, phase separation and crystallization behavior of 60ZnO–30B₂O₃–10SiO₂ glass has been investigated by non-isothermal differential thermal analysis, scanning electron microscopy (SEM) and X-ray powder diffraction (XRD). The glass transition activation energies E_g calculated by using both Kissinger and Moynihan model decrease from 668?kJ/mol to 573?kJ/mol for Kissinger model, and 682?kJ/mol to 587?kJ/mol for Moynihan model with the increase of yttrium oxide doping content from 0 to 6?mol%. And the glass crystallization kinetics parameters, crystallization activation energy E_c and Avrami exponent n stands for crystal growth, are also obtained on the basis of several well developed equations. Increase of about 58?kJ/mol in Ec values obtained by different theoretical equations is caused by addition of 6?mol% yttrium oxide into 60ZnO–30B₂O₃–10SiO₂ glass, and the Avrami exponent (n close to 2) suggests that crystal growth in 60ZnO–30B₂O₃–10SiO₂ glass doped with or without yttrium is mainly one-dimensional growth of crystals. The results on the phase separation and crystallization behavior occurred at 893?K and 993?K respectively for base and doped glass, are well consistent with the glass transition and crystallization kinetics results. Hence, addition of yttrium oxide into 60ZnO–30B₂O₃–10SiO₂ glass decrease the glass transition activation energy while increase the crystallization activation energy of glass, thereby the stability of glass structure is improved. Phase separation phenomenon and crystallization behavior occurred at glass surface provide some useful information for preparing glass ceramics with micro- or nano-crystals in surface.     

Effect of sol-gel and solid-state synthesis techniques on structural,morphological and thermoelectric performance of Ca3Co4O9

Two methods of obtaining calcium cobalt oxide (Ca₃Co₄O₉) thermoelectric materials were studied: (I) solid state synthesis (SS) followed by high-energy ball-milling (HEBM) and (II) modified sol-gel method (SG). The obtained powders were heated at 900?°C for 12?h. They were subsequently compacted using spark plasma sintering (SPS). The crystal structure and morphology were characterized by X-ray diffraction and scanning electron microscopy. The calculated average crystallite sizes, before and after SPS, alter from 43 to 61?nm and from 38 to 41?nm, for the SS and SG powders, respectively. XRD studies of the obtained powders revealed presence of Co₃O₄ secondary phases, which turned into metallic cobalt after SPS. Thermoelectric measurements of sintered samples were performed in the range, 50–500?°C. The maximum Seebeck coefficient is 120?µV/K and 110?µV/K, for the SS and SG synthesized samples, respectively. The highest ZT was found to be 0.02 and 0.04 at 500?°C for the SS and SG samples, respectively.     

Phase stability and electric conductivity of Er2O3-Nb2O5 co-doped Bi2O3 electrolyte

Two oxides, Er₂O₃ and Nb₂O₃, are used to stabilize delta-phase Bi₂O₃ used as electrolyte of solid oxide fuel cell. Optimization of dopant ratio and total doping concentration (TDC) is determined by X-ray diffraction, and successfully reduce the TDC (Er + Nb) to 10-15 mol.%. Conductivities of different compositions are measured by two-probe method. The results show that highest conductivity appears at the minimum doping concentrations. Phase stability of ENSB samples with Er/Nb ratio of 2/1 and TDC of 10-20 mol.% at 650 °C up to 300 h is analyzed showing two newly formed (alpha- and gamma-) phases in the samples. Degradation of conductivity at 650 °C is studied in detail by DTA and TEM. The abnormity of lattice contraction of delta-phase is discussed.     

A novel approach to synthesis of nanosize MgAl2O4 spinel powder through sol–gel citrate technique and subsequent heat treatment

This paper summarizes a study on the influence of heat-treatment atmosphere in synthesis of nanosize MgAl₂O₄ spinel powder employing sol–gel citrate route. Based on measurement of BET specific surface area, crystallite size, simultaneous thermal analysis, and microscopic observation, it was clarified that the argon atmosphere used for heat treatment of precursor reduced the particle size of synthesized ceramic powder significantly. Comparing with heat treatment in air atmosphere, it was elucidated that the carbonaceous material remaining from citrates has an important role in hindering the particle sintering. Also the slower heat generation in argon atmosphere creates a chance to achieve a nanocrystalline MgAl₂O₄ spinel powder with crystallite size in the range of 5–7 nm. The present novel approach shows that the argon treatment confronts the problem of sudden heat generation in sol–gel citrate method to synthesize nanosize MgAl₂O₄ spinel particles in the range of 20–100 nm.     

Citrate gel process and thermoelectric properties of Ge-doped In2O3 bulk ceramics

Indium oxide ceramics doped with different doping levels of germanium (from 0 to 10 at.%) were prepared with nano-sized powders obtained by citrate gel process. Ge for In substitution in the In₂O₃ bixbyite structure below the solubility limit (about 0.5-1 atom%) leads to a large decrease in the electrical resistivity and the absolute value of the Seebeck coefficient. X-ray diffraction and scanning electron microscopy show the presence after sintering of well dispersed secondary phases In₂Ge₂O₇ when the Ge solubility in the In₂O₃ matrix is passed. The thermal properties measurements confirm that the decrease in the lattice thermal conductivity observed in these composite materials can be attributed to the presence of insulating In₂Ge₂O₇ phases. Furthermore, preliminary microwave sintering experiments have been tested in order to keep the nanoscale after sintering. Particular nano-microstructures were obtained due to minimal grain growth induced by this rapid sintering process.     

The nucleation and crystallization of MgO-B2O3-SiO2 glass

The nucleation and crystallization of MgO-B₂O₃-SiO₂ (MBS) glass were studied by means of a non-isothermal, thermal analysis technique, X-ray diffraction and scanning electron microscopy. The temperature range of the nucleation and the temperature of the maximum nucleation rate for MBS glass were determined from the dependences of the inverse temperature at the DSC peak (1/T_p) and the maximum intensity of the exothermic DSC crystallization peak ((δT)_p) on the nucleation temperature (T_n). For MBS glass the nucleation occurred at 600-750 °C, with the maximum nucleation rate at 700 °C, whereas the nucleation and crystal growth processes overlapped at 700 °C < T ≤ 750 °C. The analyses of the non-isothermal data for the bulk MBS glass using the most common models (Ozawa, Kissinger, modified Kissinger, Ozawa-Chen, etc.) revealed that the crystallization of Mg₂B₂O₅ was three-dimensional bulk with a diffusion-controlled crystal growth rate, that n = m = 1.5 and that the activation energy for the crystallization was 410-440 kJ/mol.     

Synthesis and magnetic properties of CoAlFe2O4 nanoparticles

Nanosized particles of CoAl_xFe_2-xO₄, where 0?≤?x?≤?2, were synthesized by the sol–gel combustion method and the magnetic properties of these compounds were investigated. According to X-ray diffractograms, the samples are single phase and the crystallite size is between 7 and 25?nm. The room temperature saturation magnetization of the samples was estimated from the cation distribution and ferromagnetic resonance spectra were used to determine the magnetocrystalline anisotropy. The results show that the saturation magnetization and the magnetocrystalline anisotropy vary over a wide range, from maxima of M_s =?0.42?MA/m and K?=?0.39?kJ/m³ for x?=?1.0 to minima of almost zero for x?≈?1.4, a result that could be useful for practical applications of these materials.     

Microwave dielectric properties and structure of ZnO-Nb2O5-TiO2 ceramics

Ceramic samples based on ZnO-Nb₂O₅-TiO₂ compositions have been prepared using solid state ceramic route. The work was carried out over a wide range of initial ZnNb₂O₆ and Zn_0.17Nb_0.33Ti_0.5O₂ compounds concentration. The crystal structure and microstructure developments were studied using X-ray diffraction (XRD) and scanning electron microscopy (SEM). It was shown that the phase compositions of the samples present itself a columbite type and mixture of two phases—solid solutions of columbite and rutile types.The sintering behavior, permittivity, its temperature coefficients and quality factor had been characterized for ceramic samples in depending on compositions. The permittivity of the samples in this system is within the limits from 24 to 80, τ_? from 150 to −560 ppm/°C. For the samples with τ_? ∼ 0, ?_r ∼ 43.8 and Q·f = 35000 GHz at f = 9 GHz. The comparatively low sintering temperature (≤1080 °C) and high dielectric properties in microwave range make these ceramics promising for application in electronics.     

Green synthesis of Dy2Ce2O7 ceramic nanostructures using juice of Punica granatum and their efficient application as photocatalytic degradation of organic contaminants under visible light

An effective and simple route is developed for production of Dy₂Ce₂O₇ nanostructures by applying juice of Punica granatum as fuel and metal nitrates. Juice of Punica granatum has for the first time been exploited as a new fuel for simple synthesis of various structures of Dy₂Ce₂O₇ in terms of shape, photocatalytic efficiency and particle size at different time and temperature for production. TEM, Raman, DRS, XRD, FESEM and EDS are exploited for study and characterization the obtained nano-sized structures of Dy₂Ce₂O₇. Photocatalytic efficiencies have been evaluated with destruction of erythrosine and methylene blue pollutants under visible illumination over the various structures of Dy₂Ce₂O₇. Results clearly demonstrated that the nano-sized structures of Dy₂Ce₂O₇ obtained by application of juice of Punica granatum as new fuel at 450 °C for 4 h revealed excellent photocatalytic efficiency for the destruction of erythrosine and methylene blue pollutants.     

Selective catalytic reduction of NO by hydrocarbons on Ga2O3/Al2O3 catalysts

Catalytic properties of supported gallium oxides have been examined for the selective reduction of NO by CH₄ in excess oxygen. The activity was greatly affected by the support; Ga₂O₃/Al₂O₃ (Al₂O₃ supported Ga₂O₃) and Ga₂O₃–Al₂O₃ mixed oxide exhibited high activity and selectivity as comparable to Ga-ZSM-5, while unsupported Ga₂O₃ and the other supported Ga₂O₃ were ineffective. For Ga₂O₃/Al₂O₃, the activity changed with Ga₂O₃ content, and was highest at about 30 wt% Ga₂O₃, which corresponds to a theoretical monolayer coverage. Gallium oxide highly dispersed on Al₂O₃ is considered to be responsible for the high activity and selectivity. The reaction characteristics of Ga₂O₃/Al₂O₃ were studied and compared with Ga-ZSM-5 and Co-ZSM-5. Ga₂O₃/Al₂O₃ exhibited the highest activity and selectivity at high temperature. In addition, Ga₂O₃/Al₂O₃ showed higher tolerance against water than Ga-ZSM-5. C₃H₈ and C₃H₆ were also evaluated as reducing agents, and Ga₂O₃/Al₂O₃ showed higher activity than Ga-ZSM-5 above 723 K achieving almost complete reduction of NO to N₂.     

A modified B2O3 containing Li2O-Al2O3-SiO2 glass with ZrO2 as nucleating agent - Crystallization and microstructure studied by XRD and (S)TEM-EDX

Glass-ceramics based on lithium-alumo-silicate glasses are commercially important for a wide range of applications, due to their special properties, like a vanishing thermal expansion. In order to tailor these properties, the composition of the glass and the temperature/time schedule are crucial factors. For the industrial production of most lithium-alumo-silicate glasses, high melting temperatures are required due to the high viscosities of the respective melt compositions. In this study, a simplified lithium-alumo-silicate glass composition with ZrO₂ as nucleating agent, on the basis of the commercially available Robax^® composition, is studied. Adding boron oxide leads to lower viscosities of the glass melt and notably lower melting temperatures may be supplied. The resulting glass is investigated using X-ray diffraction and transmission electron microscopy. During the crystallization process, phases such as ZrO₂ and β-quartz types are formed. The microstructure of the glass ceramics is notably coarser than that of glass-ceramics which are obtained from lithium-alumo-silicate glasses of standard compositions. EDX-analyses indicate a considerable enrichment of chemical elements in comparatively small areas of the microstructure. Especially boron oxide is found to be enriched in the residual glass of the investigated glass-ceramics.