Photoluminescence properties of praseodymium doped cerium oxide nanocrystals

The structure and photoluminescence (PL) properties of CeO₂ nanocrystals synthesized by the microwave-assisted hydrothermal (MAH) method with different praseodymium (Pr³⁺) ions contents were performed. X-ray diffraction (XRD), transmission electron microscopy (TEM), diffuse reflectance ultraviolet-visible (UV-vis), Fourier transform Raman (FT-Raman) spectroscopies and PL measurements at room temperature were employed. XRD patterns indicated that the nanocrystals are free of secondary phases and crystallize in the cubic structure while FT-Raman revealed a typical scattering mode of fluorite type. The UV-vis spectra suggested the presence of intermediate energy levels in the band gap of these nanocrystals. The most intense PL emission was obtained for CeO₂ nanocrystals doped with 1.6% of Pr³⁺ ions and smaller particle size.     

Properties of NiFe2O4 ceramics from powders obtained by auto-combustion synthesis with different fuels

Nickel ferrite (NiFe₂O₄) powders derived by auto-combustion synthesis using three different fuels (citric acid, glycine and dl-alanine) have been characterized. The sintering behavior of ceramics using these powders has been compared. Oxygen balance (OB) setting for the chemical reaction is found to regulate the combustion reaction rate. A rapid reaction rate and a high flame temperature are achieved with dl alanine fuel yielding single phase NiFe₂O₄ powder in the as-burnt stage, whereas powders derived with citric acid and glycine fuels show poor crystallinity and necessitate post-annealing. The powder particles are largely agglomerated with a non-uniform distribution in shape and size, and the average particle size is estimated in the range ~ 54–71 nm. Powders derived from dl-alanine fuel show better phase purity, smaller crystallite size, larger surface area and superior sintering behavior. Additional Raman modes discerned for dl-alanine derived powder support a 1:1 ordering of Ni²⁺ and Fe³⁺ at the octahedral sites relating to microscopic tetragonal P4₁22 symmetry expected theoretically for the formation of NiFe₂O₄ with inverse spinel structure. Microstructure of sintered ceramics depends on the precursor powders that are used and sintering at 1200 °C is found to be optimum. Citric acid and glycine derived powders yield high saturation magnetization (M_s~47–49 emu/g), but poor dielectric properties, whereas dl-alanine derived powders yield ceramics with high resistivity (~3.4×10⁸ Ω cm), low dielectric loss (tan δ~0.003 at 1 MHz) and high magnetization (46 emu/g). Dielectric dispersion and impedance analysis show good correlation with the changes in the ceramic microstructure.     

Low-temperature synthesis of SrWO4 nano-particles by a molten salt method

SrWO₄ nano-particles with a scheelite structure were successfully prepared by a molten salt method at 270 °C. The structure, morphology and luminescent property of the resultant powders were characterized by X-ray diffraction (XRD), transmission electron microcopy (TEM), and photoluminescence (PL), respectively. The resultant samples are a pure phase; the size, morphology and properties of SrWO₄ nano-particles were affected by the calcining time and weight ratio of the salt to the SrWO₄ precursor has little influence on it. PL spectra results also show that the optical properties of the SrWO₄ nano-particles strongly relied on their crystallinity.     

Evolution of corrosion on microstructure of ceramic coating produced by plasma electrolytic oxidation in molten salt

Low corrosion resistance in an Al alloy is usually overcome with the help of an Al oxide coating. Plasma electrolytic oxidation (PEO) is a highly promising environment-friendly method to achieve a ceramic surface. Traditionally, PEO is carried out in an aqueous electrolyte; however, in this study, PEO was conducted in a molten salt. This approach conserved more energy and led the formation of a pure oxide coating, as confirmed by subsequent phase analysis. The obtained ceramic coating contained two sublayers: a porous outer sublayer and a dense inner sublayer. A study of corrosion evolution was performed on oxide-coated alloys immersed in a NaCl solution for different durations. The corrosion behavior characterized by electrochemical impedance spectroscopy (EIS) was related to the changes in the surface morphology changes examined by electron microscopy. The appearance of pits on the oxide surface was attributed to the adsorption and incorporation theory, previously described for Al alloys. This study revealed that the progress of the corrosion attack by chloride ions affects both sublayers; the thickness of the outer sublayer decreased, and the inner sublayer became more compact, resulting in high resistance properties.     

Synthesis and characterization of MgAl2O4 micro-rods by a molten salt method

Large amounts of MgAl₂O₄ micro-rods were successfully synthesized using the molten-salt technology. The effect of KCl contents on the formation of MgAl₂O₄ micro-rods was investigated. The structure and morphology of MgAl₂O₄ were investigated by means of powder X-ray diffraction, field emission scanning electron microscopy, and transmission electron microscopy, respectively. The experimental results showed that the contents of KCl significantly influenced the formation of MgAl₂O₄ micro-rods. MgAl₂O₄ micro-rods could be prepared at 1150 °C with a weight ratio of 100:1 between the salt and the starting materials. The formation of MgAl₂O₄ micro-rods could be suggested to be due to the inhomogeneous nucleation and orientated growth perpendicularly to the surfaces of Al₂O₃ grains. An impedance-type humidity sensor was finally fabricated based on the as-prepared MgAl₂O₄ micro-rods. According to tests of the humidity performance, MgAl₂O₄ micro-rods might be suitable for high-performance humidity sensors.     

Synthesis of calcium copper titanate ceramics via the molten salts method

CaCu₃Ti₄O₁₂ has a giant dielectric constant of up to 10⁴ at room temperature and has great potential for various technological applications. In this work, CaCu₃Ti₄O₁₂ ceramic powder was synthesized by heating a stoichiometric amount of CaCO₃, CuO and TiO₂ in molten NaCl–KCl and Na₂SO₄–K₂SO₄, respectively. The synthesis temperature was decreased from 1000 °C (required by conventional solid-state reactions) to 750 °C for NaCl–KCl or to 850 °C for Na₂SO₄–K₂SO₄. The flux type has a larger influence on the phase compositions and morphology of the resultant powders than the synthesis temperature does. The dielectric constant of the resulting ceramics is more than 10⁴ over the wide frequency range from 100 Hz to 100 kHz. The dielectric loss tangent of the resulting ceramics is lower than 0.2 in the frequency range from 100 Hz to 100 kHz. The dielectric behavior of both samples is similar to the results obtained for CaCu₃Ti₄O₁₂ ceramics that were synthesized by the sol–gel method.     

Low temperature synthesis of nano-crystalline gadolinium titanate by molten salt route

Nano-crystalline Gadolinium Titanate (Gd₂Ti₂O₇) powder was successfully synthesized by “Single Step Molten Salt Technique”. LiCl–KCl eutectic mixture was used as a molten medium for the reaction. The duration of the synthesis was 10 h. Stoichiometric proportion of the reactants were mixed in an (LiCl–KCl) eutectic medium and treated at 750 °C in an electrical resistance furnace. Single phase Gadolinium Titanate compound was obtained by the thermal process. The synthesized powders were characterized using XRD, FT-IR, UV, EDAX and XPS analyses. The morphology of the powder was examined using SEM and TEM techniques. From the above studies, it has been concluded that pure crystalline Gadolinium Titanate powders can be synthesized via low temperature molten salt process.     

Molten salt synthesis of titanate pyrochlore waste-forms

Actinide chlorides, such as those arising from pyrochemical reprocessing operations can be problematic to immobilise, as the high chloride content often makes their solubilities in melts very low, and even in small quantities can seriously affect the properties of the waste-form. Rather than attempt to immobilise the chlorine, one potential approach is to utilise the chloride salt as a reaction medium from which the An(III) cations can be extracted and immobilised. To this end, lanthanide titanate pyrochlores have been prepared by molten salt synthesis in CaCl₂:MgCl₂, CaCl₂:NaCl and MgCl₂:NaCl eutectics. Single-phase pyrochlore is found to be formed at temperatures as low as 650 °C in the CaCl₂:NaCl system, whereas in the CaCl₂:MgCl₂ and MgCl₂:NaCl eutectics reaction with Mg produces a magnesium titanate secondary phase. Compositions of Yb₂Ti₂O₇ doped with Sm³⁺ as an actinide surrogate have been synthesised, and cold-pressing and sintering at 1500 °C yields fully dense pellets.     

Graphitic mesoporous carbons synthesised through mesostructured silica templates

In this paper the fabrication and characterization of graphitizable and graphitized porous carbons with a well-developed mesoporosity is described. The synthetic route used to prepare the graphitizable carbons was: (a) the infiltration of the porosity of mesoporous silica with a solution containing the carbon precursor (i.e. poly-vinyl chloride, PVC), (b) the carbonisation of the silica–PVC composite and (c) the removal of the silica skeletal. Carbons obtained in this way have a certain graphitic order and a good electrical conductivity (0.3 S cm⁻¹), which is two orders larger than that of a non-graphitizable carbon. In addition, these materials have a high BET surface area (>900 m² g⁻¹), a large pore volume (>1 cm³ g⁻¹) and a bimodal porosity made up of mesopores. The pore structure of these carbons can be tailored as a function of the type of silica selected as template. Thus, whereas a graphitizable carbon with a well-ordered porosity is obtained from SBA-15 silica, a carbon with a wormhole pore structure results when MSU-1 silica is used as template. The heat treatment of a graphitizable carbon at a high temperature (2300 °C) allows it to be converted into a graphitized porous carbon with a relatively high BET surface area (260 m² g⁻¹) and a porosity made up of mesopores in the 2–15 nm range.     

Selective formation of carbon nanotubes over Co-modified beta zeolite by CCVD

Multiwall carbon nanotubes containing carbon-zeolite composite materials have been obtained by ethylene catalytic chemical vapor deposition (CCVD) on Co-modified beta zeolite powder. High carbon selectivity to nanotubes was achieved. The combined physical characterisation through several techniques, such as TEM, HRTEM, SEM, XRD, N₂ adsorption-desorption, allows to determine the geometric characteristics of nanotubes and their dependence of catalyst and synthesis conditions.     

Synthesis of carbon nanotubes filled with Fe3C nanowires by CVD with titanate modified palygorskite as catalyst

Multi-walled carbon nanotubes (MWCNTs) have been successfully synthesized with titanate-modified palygorskite as catalyst and acetylene as carbon source by chemical vapor deposition (CVD) at high temperature. Transmission electron microscopy (TEM) studies showed that there were a lot of carbon nanotubes partially filled with elongated foreign material in their inner cavities. X-ray energy dispersive spectrum (EDS) analyses and selected area electron diffraction (SAED) investigations on the encapsulated material revealed that it was single crystalline iron carbide (Fe₃C) derived from ferric precursors in the mineral. The yield of carbon nanotubes was influenced by preparation temperature based on thermal gravimetric analyses (TGA). The relative quantity of Fe₃C nanowires was influenced by the temperature and the local structure of nanotubes upon TEM observations. A growth mechanism is also proposed in the paper.     

Preparation of carbon-encapsulated iron carbide nanoparticles by an explosion method

Fe₇C₃ nanocrystals encapsulated in carbon nanoparticles, with sizes ranging from 10 to 40 nm, were synthesized via the explosion of a hybrid xerogel containing oxidized pitch and iron nitrate. Explosion of the hybrid xerogel was induced by heat treatment. Transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) were employed to determine the structure of the nanoparticles in the explosion product. The results show that the Fe₇C₃ nanocrystals are nested inside amorphous carbon shells that protect them from oxidation by air.     

High energy-storage properties of [(Bi1/2Na1/2)0.94 Ba0.06] La(1−x) ZrxTiO3 lead-free anti-ferroelectric ceramics

Energy-storage properties of [(Bi_1/2Na_1/2)_0.94Ba_0.06]La_(1−x)Zr_xTiO₃ (BNT-BLZT, x=0, 0.02, 0.04, and 0.06) lead-free anti-ferroelectric ceramics fabricated via the conventional sintering technique were first investigated. Calculation from the X-ray diffraction results reveals that BNT-BLZT ceramic possesses a single perovskite structure phase. In addition, the P–E hysteresis loops measured at room temperature show that the BNT-BLZT (x=0.02) ceramics obtain the maximum P value of 37.5 μC/cm² and the largest energy-storage density W_max is 1.58 J/cm³. The temperature dependence of dielectric permittivity ε_r and dielectric loss tanδ illustrate that the addition of Zr can improve the piezoelectric properties of BT-BLZT ceramics. These properties indicate that BNT-BLZT ceramics might be a promising lead-free anti-ferroelectric material for energy storage application.     

Synthesis,characterization and influence of fuel to oxidizer ratio on the properties of spinel ferrite (MFe2O4, M = Co and Ni) prepared by solution combustion method

The effect of fuel to oxidizer ratio on the crystallite size and the physical properties of cobalt and nickel ferrite samples synthesized by solution combustion method using glycine as fuel are reported. Powder X-ray diffraction studies indicate that pure nanocrystalline ferrite phase is formed when the fuel to oxidizer ratio is less than one. The crystallite size can also be controlled by controlling the fuel to oxidizer ratio. The Mössbauer and Raman studies on the sample prepared at F/O 0.6 indicate that high quality samples having inverse or mixed spinel structure can be prepared by solution combustion method. Room temperature magnetization curves of the samples prepared under fuel lean condition have a systematic trend of coercivity with increasing crystallite size.     

Effects of annealing temperature on some structural and optical properties of ZnO nanoparticles prepared by a modified sol-gel combustion method

Plate-shaped zinc oxide nanoparticles (ZnO-NPs) were successfully synthesized by a modified sol-gel combustion method. Zinc acetate, pure water and isopropanol were used as the starting materials. Acetic acid, diethanolamine and nitric acid were used as the polymerization agent, complexing agent and fuel, respectively. The precursors were formed by mixing aqueous solutions of zinc acetate, acetic acid and diethanolamine. Nitric acid was used to dry the produced gel. The resulting xerogel was annealed at 600 °C, 650 °C and 750 °C for 1 h. The synthesized ZnO-NPs were characterized by X-ray diffraction analysis (XRD), thermogravimetric analysis (TGA) and high-magnification transmission electron microscopy (TEM). The XRD results revealed that the samples produced were crystalline with a hexagonal wurtzite phase. The TEM results showed single-crystal ZnO-NPs with nearly hexagonal plate shapes. The optical properties of the ZnO-NPs were studied by UV-visible and Fourier-transform infrared spectroscopy (FTIR). The UV-vis absorption spectra of the ZnO-NPs indicated absorption peaks in the UV region, which were attributed to the band gap of the ZnO-NPs. The results of the FTIR and UV-vis studies showed that the optical properties of the ZnO-NPs depended on the annealing temperature.     

Synthesis, characterization and optical properties of Zn1−xTixO nanoparticles prepared via a high-energy ball milling technique

Zn_1−xTi_xO (x = 0, 0.01, 0.03 and 0.05) nanoparticles were prepared by high-energy ball milling at 400 rpm. The milled powders were characterized by X-ray diffractometer (XRD) and the results exhibited that Ti-doped ZnO nanoparticles consisted of single phase with hexagonal structure when the mixtures of ZnO and TiO₂ powders were milled for 20 h. The crystallite size reduced as a function of the doping content and milling time from 1 to 10 h then increased after milling for 20 h and when the annealing temperature increased. The strain changed inversely to the crystallite size. A wider band-gap was obtained by increasing the doping content and annealing temperature because of a reduction in defect concentration. Both ZnO- and Ti-doped ZnO nanoparticles caused damage to S. aureus, E. coli, P. mirabilis, S. typhi and P. aeruginosa.     

Engineering grain size and electrical properties of donor-doped barium titanate ceramics

A semiconducting lanthanum-doped barium titanate ceramic has been fabricated for battery safety applications by simple means from nanoparticles prepared at room temperature by kinetically controlled vapor diffusion catalysis. The material, characterized by electron microscopy, X-ray diffraction and electrical measurements, exhibits a difficult to achieve combination of submicron grain size (∼500 nm) and attractive electrical properties of room temperature resistivity below 100 Ω cm and a 12-fold increase in resistivity through the Curie temperature (positive thermal coefficient of resistivity, PTCR). Systematic investigation of sintering conditions revealed that a short period of heating at 1350 °C under air is necessary to suppress abnormal grain growth, while precise control of the cooling rate is needed to achieve the targeted electrical properties. Cooling must be sufficiently fast to avoid complete back-oxidation, yet slow enough to facilitate oxygen adsorption at the grain boundaries to produce the thin oxide layer apparently responsible for the observed PTCR.     

Effect of Ce and La substitution on dielectric properties of bismuth titanate ceramics

Effect of Ce and La substitution on the microstructure and dielectric properties of bismuth titanate (BT) ceramics was investigated. Bismuth titanate ceramics (Bi_4−xA_xTi₃O₁₂) (A = Ce or La; x = 0, 0.5, 1) were processed by sintering of pressed pellets, prepared from nanopowder synthesized by the modified sol-gel method. Pure and La modified bismuth titanate ceramics have single Bi₄Ti₃O₁₂ phase of Aurivillius type, whereas a small amount of Bi₂Ti₂O₇ pyrochlore phase appears in Ce modified bismuth titanate ceramics. In the same time addition of La and Ce improved sinterability of BT ceramics. The results of the measurement of dielectric constant and loss tangent at different frequencies (100 Hz-1 MHz) as a function of temperature reveal that Ce modified ceramics has a diffuse phase transition. Temperature T_m, corresponding to the maximum value of the dielectric constant, is shifted to higher temperature and the maximum value of the dielectric constant is decreased with increasing frequency, which indicate that relaxor behavior is caused by Ce substitution.     

Synthesis of mesoporous strontium titanate by molten salt assisted template-free method

Mesoporous SrTiO₃ was synthesized via the NaOH/KOH molten salt method. The detailed formation mechanism for the mesoporous SrTiO₃ should be obtained to adjust the microstructure, which is an import factor affecting the photocatalytic activity. The detailed mechanism is explored by studying the intermediate phase. It is found that the NaOH/KOH molten salt method for mesoporous SrTiO₃ synthesis, using four kinds of TiO₂ with different morphologies as precursors, follows the precursor dissolution-precipitation growth mechanism. The mesoporous SrTiO₃ structure originates from the reservation of space for NaOH/KOH. The crystallinity, number of pores, and size of particles are affected by the cleaning temperature, which can thus influence the photocatalytic activity.     

Synthesis of bismuth oxide nanoparticles by the polyacrylamide gel route

A polyacrylamide gel route has been adopted for the preparation of bismuth oxide nanoparticles. Thermal behaviour of the polyacrylamide gel has been studied by means of thermogravimetry (TGA). The formation of monoclinic Bi₂O₃ nanocrystallites is confirmed by X-ray diffraction (XRD). Transmission electron microscopy (TEM) investigation revealed the average particle size of Bi₂O₃ nanoparticles to range from 30 nm to 50 nm.