High-temperature Route Research Articles

High-Entropy Perovskite Fluorides: A New Platform for Oxygen Evolution Catalysis.

High-entropy perovskite fluorides (HEPFs) have great potential in electrocatalysis that has not been realized because of the limitation of a high-temperature synthetic route and the limited understanding of high-entropy materials. The use of HEPFs in effective oxygen evolution catalysis and a feasible synthesis route for HEPFs in a boiled solution by combining a hydrothermal method with mechanochemistry are first reported here. These HEPFs consisting of cost-effective elements dramatically gave excellent catalytic activity for the oxygen evolution reaction in an alkaline media.

Synthesis and characterization of Ruddlesden-Popper system (Ba1−xSrx)2SnO4

The samples of (Ba1−xSrx)2SnO4 with x = 0, 0.10, 0.20, 0.40, 0.80, 1.00 were synthesized by a high-temperature solid state ceramic route. Lattice parameters obtained from the Rietveld profile analysis; decreased with increasing Sr. The decreasing nature of lattice parameters indicated the complete solubility of Sr at Ba-site. Further, incorporation of Sr at Ba-site has been studied by Fourier transform infrared (FTIR) spectroscopy. The dielectric constant and dissipation factor increases with increasing Sr. Contribution of grain and grain boundary were resolved by complex modulus spectroscopy analysis. AC conductivity spectra of the entire sample follow Universal Jonscher's power law. The similar value of activation energy obtained from relaxation frequency and dc conductivity implies that the origin of conduction and relaxation mechanisms remains the same throughout the samples. The modulus and conductivity spectra of solid solutions nicely follow the time-temperature superposition (TTSP) principle. Further, conductivity spectra of solid solutions were studied by Ghosh and Summerfield scaling.

Strong Narrow Red Emission in a Perturbed Fergusonite System: Y3Mg2Nb3O14:Eu3+ for White LED Applications

A series of intense red-emission phosphors, Y3Mg2Nb3O14:Eu3+, with higher color purity were prepared via the conventional high-temperature ceramic route. The influence of Mg2+ ion incorporation in the highly symmetrical YNbO4 lattice on the luminescence properties was studied using powder x-ray diffraction, scanning electron microscopy with energy dispersive spectrometry, UV–visible absorption spectroscopy, and photoluminescence measurements. The developed phosphors crystallize into a monoclinic fergusonite-type structure with a C2/c space group even with the addition of Mg into the system. These phosphors display intense, sharp, red emission with a maximum at 613 nm under near-UV excitation. The photoluminescence studies suggest that Mg addition perturbs the A site network forming cation pairs of Eu3+-Y3+, Eu3+-Mg2+, and Eu3+-Eu3+ connected with O2− ions. The preferential occupation of cations in the A site network by the incorporation of Mg2+ ions in the YNbO4 host lattice induces Eu3+-Eu3+ separation and results in intense, sharp, red emission due to increased radiative probabilities. Moreover, the color purity of the Y3−xMg2Nb3O14:xEu3+ phosphor could achieve 92.4% that is attributed to the narrow full width at half maximum (FWHM ∼ 4 nm) of the red emission peaking at 613 nm. More importantly, the luminescence intensity of the developed phosphor is 9.4 times greater than that of the Philip’s red phosphor under near-UV excitation. Because of the intriguing red luminescence, phosphors exhibit Commission Internationale de l’Elcairage color coordinates (0.63, 0.35) close to the National Television System Committee standards, low correlated color temperature values for warm white lighting, and greater color purity (92.4%) for improving the color rendering index, which makes them a potential candidate for use in the fabrication of white light-emitting diodes.

Nickel - p-block metal mixed chalcogenides based on AuCu3-type fragments: iodine-assisted synthesis as a way of obtaining new structures.

Two new mixed nickel-gallium chalcogenides, Ni9.39Ga2S2 and Ni5.80GaTe2, and a new mixed nickel-indium telluride, Ni5.78InTe2, have been synthesized by a high-temperature ampoule route with the addition of iodine, and characterized from single-crystal or powder diffraction data. They belong to the relatively uncommon Ni7-xMQ2/Ni10-xM2Q2 type of structures (M = Ge, Sn, Sb, In), and are built from p-block metal-centered nickel cuboctahedra, alternating along the c axis with defective Cu2Sb-type nickel-chalcogen ones. Both tellurium-containing compounds show a small degree of orthorhombic distortion with respect to the idealized tetragonal structure, only detectable in the powder diffraction data. No phase transition to the tetragonal structure was detected for Ni5.80GaTe2 by the in situ powder diffraction measurements from room temperature to 550 °C. DFT calculations show close relationships of electronic structures of these ternary compounds to their parent intermetallics, Ni3M (M = Ga, In). Metallic conductivity and paramagnetic properties are predicted for all three with the latter confirmed by magnetic measurements. The bonding patterns, investigated via the ELF topological analysis, show multi-centered nickel - p-block metal bonds in the AuCu3-type fragments and pairwise covalent interactions in the nickel-chalcogen fragments. Both Ni7-xMTe2 compounds showed no structural or compositional changes upon high-temperature mid-pressure hydrogenation.

Synthesis and characterization of amine stabilized CdSe quantum dots

Herein we portrait CdSe quantum dots synthesized by an organometallic high temperature route using low toxic and cost effective oleylamine as the ligand molecule. The morphological and structural properties of the nanostructures were investigated using X-ray diffraction, high resolution transmission electron microscopy, selected area electron diffraction, energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. The optical properties were studied using UV–visible absorption spectroscopy, photoluminescence spectroscopy and cyclic voltammetry. The nanostructures grown were highly luminescent, spherical, monodisperse and highly crystalline. Further investigations incorporating the quantum dots in a solar cell is progressing.

Structural, thermal, electrical and magnetic features of a new lead-free electronic material: (SbLi)1/2(Fe2/3W1/3)O3

Low electric and magnetic moment features are the main drawbacks that restrict the practical utilities of bismuth ferrite. Successfully, we have substituted Li+ and W6+ in Sb3+ and Fe3+ sites to eliminate further confinements of bismuth ferrite (referred as BFO/BiFeO3). A new lead-free perovskite compound of (SbLi)1/2(Fe2/3W1/3)O3 (symbolized as SLFWO) has been produced through ceramic processing (high temperature) route. The calcination temperature of said compound is confirmed from the thermogravimetric analysis. The structural, microstructural, magnetic and electrical (dielectric constant/loss, impedance, modulus, conductivity) features of the said material have been studied over an extensive range of frequency (100 Hz-1 MHz) and temperature (RT-400 °C). The finding of reflection indices and crystal parameters approve the development of polycrystalline nature with orthorhombic symmetry of the compound. The impedance analysis confirms the existence of non-Debye type relaxation trend. The frequency-temperature dependence of ac-conductivity studies proves the nature and conduction mechanism in the compound. At specific peak frequency, the modulus spectrum movements of the charge carrier vary from wide array to narrow array. The peak frequencies increase with temperature signifying the thermally activated relaxation process. The value of leakage current rises with the temperature that established the NTCR trend of the compound. The observed values of remanent magnetization, saturation magnetization and coercivity of SLFWO are 0.5647 emu/g, 0.04953 emu/g, and 104.242 Oe respectively.

Enhanced properties of LiFePO4/C cathode materials co-doped with V and F ions via high-temperature ball milling route

V and F ions co-doped LiFePO4/C (LFP-VF) cathode material is prepared via high-temperature ball milling route. Effects of V and F ions co-doped on the structure, morphology and electrochemical property of LiFePO4 are investigated in this work, and some analysis of X-ray diffraction, scanning electron microscope, transmission electron microscope, charge-discharge tests, electrochemical impedance spectroscopy and cyclic voltammetry are employed. The results confirm that prepared LFP-VF exhibits excellent electrochemical property. The specific capacities are 165.7 (0.1 C), 154.9 (1.0 C) and 124.9 (10 C) mAhg−1. Moreover, over 500 cycles at 1.0 C-rate, the discharge capacity is 148.2 mAhg−1 with a capacity retention rate of 95.7%. Experimental results indicate that the electrochemical performance of LiFePO4 can be greatly enhanced by co-doping with V and F ions.

Synthesis of porous Mg-doped CeO2 powders via self-propagating high-temperature synthesis route

The aim of this study was synthesis of Mg-doped porous cerium oxide powder by self-propagating high-temperature synthesis reaction of Mg-xCeO2-O2 system. Results indicated that doping Mg in the CeO2 crystal structure increased its lattice parameter slightly, and shifted the Ce-O band in the FTIR spectra from 490 to 574 cm−1. Moreover, the EDS results revealed the distribution of Mg in the CeO2 microstructure. There were a high volume fraction of interconnected macro-pores in the microstructure of cerium oxide after the synthesis process. Increase in the x-value from 0.05 to 0.25 mol decreased the mean size and volume fraction of pores from 2.4 to 1 µm and 50 to 30 vol%, respectively. In addition, the BET surface area of porous CeO2 varied between 0.3 and 1.57 m2/g. Finally, it was inferred that the SHS technique can be introduced as a rapid and novel method for synthesis of Mg-doped porous CeO2 powders.

Tuning of emission from copper-doped ZnSe quantum dots

In this paper, different concentrations of copper-doped zinc selenide quantum dots (Cu:ZnSe QDs) were synthesized and compared with differently sized (2.0–3.7 nm) Cu:ZnSe QDs by keeping the concentration of Cu fixed at 5%. In both cases a high-temperature wet chemical route was employed for synthesis. As the concentration of Cu increased from 2.5% to 12.5% in the solution, a clear redshift was observed in the absorption spectrum, which is an indication that Cu ions have been incorporated into the host lattice. In order to identify the state in which the Cu ions are present, electron paramagnetic resonance (EPR) studies were carried out on the samples. The EPR spectra reveal a resonance signal which is characteristic of Cu2+ ions. With an increase in concentration of Cu ions it is found that the component increases. Photoluminescence emission and excitation studies revealed both band-edge and Cu-related emission, further strengthening the incorporation of Cu ions in the host lattice.

Electrochemical CH4 oxidation into acids and ketones on ZrO2:NiCo2O4 quasi-solid solution nanowire catalyst

Efficient utilization of abundant methane (CH4) from natural gas remains a major catalysis challenge. One recommended strategy is to convert methane into liquid fuels, such as alcohols, ketones and acids, but most processes usually proceed through high-temperature routes. Here, we employ a ZrO2:NiCo2O4 quasi-solid solution catalyst as the electrochemical anode for partial methane oxidation. Without high temperature, noble metal catalysts and expensive oxidants, this quasi-solid solution anode produces propionic acid, acetic acid and acetone. After the analysis of the products, we demonstrate that the intermediate products from partial methane oxidation, 1-propanol, acetaldehyde and 2-propanol, are further oxidized into propionic acid, acetic acid and acetone, respectively. Long-term stable production via methane oxidation is a new strategy for the electrochemical conversion of organic chemicals.

Optically Stimulated Nanodosimeters with High Storage Capacity.

In this work we report on the thermoluminescence (TL) and optically stimulated luminescence (OSL) properties of β-Na(Gd,Lu)F4:Tb3+ nanophosphors prepared via a standard high-temperature coprecipitation route. Irradiating this phosphor with X-rays not only produces radioluminescence but also leads to a bright green afterglow that is detectable up to hours after excitation has stopped. The storage capacity of the phosphor was found to be (2.83 ± 0.05) × 1016 photons/gram, which is extraordinarily high for nano-sized particles and comparable to the benchmark bulk phosphor SrAl2O4:Eu2+,Dy3+. By combining TL with OSL, we show that the relatively shallow traps, which dominate the TL glow curves and are responsible for the bright afterglow, can also be emptied optically using 808 or 980 nm infrared light while the deeper traps can only be emptied thermally. This OSL at therapeutically relevant radiation doses is of high interest to the medical dosimetry community, and is demonstrated here in uniform, solution-processable nanocrystals.

Investigation on structural and electrical properties of Co and W modified BaTiO3

The influence of co-substitution of Co and W on structural and electrical properties of BaTiO3 (i.e., Ba(Co1/3Ti1/3W1/3)O3) was investigated. This perovskite ceramic was synthesized using a conventional ceramic technology (i.e., high-temperature solid state reaction route, calcination temperature = 1100 °C and sintering temperature = 1200 °C). The compound crystallizes in the orthorhombic phase. The Nyquist plots exhibit the activation of grain boundary effect at 300 °C and the Non-Debye behavior in the sample. The relaxation distribution parameter for the sample at different temperatures is calculated from the depression angle of the semicircular arcs in the Nyquist plot. The activation energy of 0.69 eV estimated for the charge carriers in the grain is almost in good agreement with that of 0.66 eV calculated from the relaxation process of grains. The sample possesses a dielectric constant of 1140 and loss tangent of 0.27 at room temperature which can be exploited for some applications including memory devices, etc. The existence of room temperature hysteresis loop confirms the ferroelectric nature of the sample. On the basis of variation of the exponent in the Jonscher's power law with temperature, correlated barrier hopping is favored as the dominant conduction mechanism in the sample.

High thermally Sm3+-activated SrBi2Ta2O9 orange-red phosphor: Preparation, characterization, and optical properties

A new orange-red SrBi2Ta2O9:Sm3+ phosphor is prepared via high-temperature solid-state route in air atmosphere. The phase purity of phosphor is characterized using X-ray diffraction (XRD). The phosphor shows the intense excitation peak at 403 nm and emits bright orange-red light at 564, 600, 647, and 701 nm in photoluminescence (PL) spectrum. Among them, the strongest peak is located at 600 nm due to 4G5/2 – 6H7/2 transition of Sm3+. The critical transfer distance of Sm3+ is calculated as 26.31 Å. The optimal doping concentration is determined to be about 2 mol%. The temperature dependent emission spectra exhibit excellent thermal stability. The Commission International de L'Eclairage (CIE) coordinates of the SrBi2Ta2O9:xSm3+ (0.005 ≤ x ≤ 0.30) are located in the reddish orange region. These results indicate that the orange-red emitting SrBi2Ta2O9:Sm3+ phosphor can be used as potential substitute red phosphors for white light emitting diodes (w-LEDs).

Preparation of LiFePO4/C via high-temperature ball-milling route and Box-Behnken design-based optimization

LiFePO4/C cathode material was prepared via high-temperature ball-milling route with ultrasonic dispersion as mixing process using eutectic molten salt (0.76 LiOH·H2O-0.24 Li2CO3) as lithium source. Box-Behnken design was used to study the combined effects of ultrasonic time, ball-milling temperature, and ball-milling time on the discharge capacity to obtain the optimum predicted conditions. The optimum conditions were as follows: ultrasonic time was 63 min, ball-milling temperature was 638 °C, and ball-milling time was 7 h. LiFePO4/C prepared from the optimized experimental conditions exhibited a well electrochemical performance; its discharge capacity was 161.3 mAh g−1 at a 0.1 C-rate which was in consistence with the predicted discharge capacity of 160.2 mAh g−1. Moreover, its capacity retention rate achieved 93.6% at a 10 C-rate over 100 cycles.

Mechanosynthesis of the Whole Y1-xBixMn1-xFexO3 Perovskite System: Structural Characterization and Study of Phase Transitions.

Perovskite BiFeO3 and YMnO3 are both multiferroic materials with distinctive magnetoelectric coupling phenomena. Owing to this, the Y1−xBix Mn1−xFexO3 solid solution seems to be a promising system, though poorly studied. This is due to the metastable nature of the orthorhombic perovskite phase of YMnO3 at ambient pressure, and to the complexity of obtaining pure rhombohedral phases for BiFeO3-rich compositions. In this work, nanocrystalline powders across the whole perovskite system were prepared for the first time by mechanosynthesis in a high-energy planetary mill, avoiding high pressure and temperature routes. Thermal decomposition temperatures were determined, and structural characterization was carried out by X-ray powder diffraction and Raman spectroscopy on thermally treated samples of enhanced crystallinity. Two polymorphic phases with orthorhombic Pnma and rhombohedral R3c h symmetries, and their coexistence over a wide compositional range were found. A gradual evolution of the lattice parameters with the composition was revealed for both phases, which suggests the existence of two continuous solid solutions. Following bibliographic data for BiFeO3, first order ferroic phase transitions were located by differential thermal analysis in compositions with x ≥ 0.9. Furthermore, an orthorhombic-rhombohedral structural evolution across the ferroelectric transition was characterized with temperature-dependent X-ray diffraction.

Structural, Morphological and Electrical Impedance Spectroscopy of Bi2MnCdO6 Double Perovskite Electronic Material

In this communication, the systematic studies of structural (basic crystal data, microstructure), and electrical (dielectric constant and loss, impedance, modulus, conductivity, etc.) properties of the double perovskite material Bi2MnCdO6 have been reported. The compound has been synthesized by means of a high temperature route (a solid state reaction method). From the X-ray diffraction structural investigation, a stable phase of the orthorhombic crystal system has been recognized. The microstructure obtained from the scanning electron microscope shows uniform, dense and compact grain distribution. Detailed investigation of dielectric as well as electrical parameters of the fabricated compound through a broad frequency range (i.e. 1 kHz–1 MHz) and temperature (i.e., 25 °C to 300 °C) have provided many interesting data and results in order to understand its ferroelectric and electrical relaxation mechanism. It has been experimentally affirm that this material (Bi2MnCdO6) has more advantages like high relative dielectric constant and low tangent loss over Bi2MnCoO6 material.

Thermally stable double-perovskite Ca3TeO6:Eu3+ red-emitting phosphors with high color purity

Abstract A series of novel Eu3+-activated tellurate Ca(3-2x)EuxNaxTeO6 (x = 0.01–0.25) red-emitting phosphors were synthesized through a high-temperature solid-state route at 1220 °C. The phosphors were well crystallized in the space group P21/c (No. 14). Their photoluminescence emission and excitation spectra were investigated. The results indicate that the Ca3TeO6:Eu3+ phosphor can be effectively excited under 394 nm irradiation, which is suitable for UV LED chip. Upon 394 nm excitation, the Ca3TeO6:Eu3+ phosphor shows five peaks, corresponding to the 5D0–7FJ (J = 0, 1, 2, 3 and 4) transitions of Eu3+. The optimal doping concentration of Eu3+ in Ca3TeO6:xEu3+ was determined at 20 mol%. By temperature dependent emission spectra, the quenching temperature for Ca3TeO6:0.20Eu3+ was found to be over 500 K. The CIE chromaticity coordinates of Ca3TeO6:0.20Eu3+ are very close to the National Television System Committee (NTSC) standard red.

Crystalline Carbon Nitride Semiconductors for Photocatalytic Water Splitting.

Conjugated carbon nitride (CN) is an emerging and promising semiconductor photocatalyst for water photolysis owing to its unique properties. However, the traditional thermally induced polymerization of N-containing precursors typically produces melon-based CN solids with amorphous or semi-crystalline structures with only moderate photocatalytic performance. Many strategies have been developed to prepare crystalline CNs (CCNs), such as high-temperature and high-pressure routes, ionothermal synthesis, and microwave-assisted synthesis. In this Minireview, we summarize the progress that has been made in the synthesis of CCNs and their application in photocatalytic water splitting reactions. Three kinds of CCNs are mainly discussed according to their polymeric subunits. Challenges associated with CCNs and their future development are also included.

A novel UV pumped yellow-emitting phosphor Ba2YAlO5:Dy3+ for white light-emitting diodes

A series of novel Dy3+ activated aluminate Ba2Y1-xDyxAlO5 (x = 0.01–0.30) red-emitting phosphors were synthesized through a high-temperature solid-state route at 1380°C. The X-ray diffraction patterns revealed that the samples were well crystallized in the space group P21/c (No. 14). The spectrum analysis revealed that under UV light excitation, Ba2YAlO5:Dy3+ phosphor exhibited blue and yellow peaks corresponding to 4F9/2–6H15/2 transition and 4F9/2–6H13/2 transition, respectively. The optimum dopant concentration of Dy3+ ions is around 2 mol% and the critical transfer distance of Dy3+ is calculated as 29 Å. The concentration quenching mechanism between Dy3+ has been investigated. Results indicate that Ba2YAlO5:Dy3+ offers the excellent optical properties as a potential yellow-emitting phosphor candidate for n-UV LEDs.

Structural and Optical Characterization of CuInS2-In2Se3 Green Nanostructures Synthesized by Hot Injection Method

Structural and optical properties of core-shell CuInS2-In2Se3 (CIS-InSe) quantum dots (QDs) synthesized by organometallic high temperature route, privileged with remarkable size and shape control is investigated. The elemental composition of the samples is confirmed using EDAX and XPS. The x-ray diffractograms show diffraction peaks pertaining to the tetragonal structure. The well resolved lattice planes of the TEM images ratify the high crystallinity of the QDs. The optical characterization of the samples is done using UV-Vis absorption spectroscopy. The absorption spectrum, extending to the near infrared region, shows that the nanostructures grown are suitable for photovoltaic applications.