A comparative study on combustion synthesis of Ta-B compounds

A comparative study on the preparation of various tantalum borides (including Ta₂B, Ta₃B₂, TaB, Ta₅B₆, Ta₃B₄, and TaB₂) in the Ta-B system was experimentally conducted by self-propagating high-temperature synthesis (SHS) from the elemental powder compacts of their corresponding stoichiometries. Both combustion temperature and reaction front velocity increased and then decreased with increasing boron content in the powder mixture. The fastest flame front with a reaction temperature of 1732 °C and a propagation rate of 11.2 mm/s was observed in the sample of Ta:B = 1:1. The combustion temperature (1205 °C) and flame-front velocity (3.82 mm/s) for the powder compact of Ta:B = 2:1 were the lowest. According to the XRD analysis, single-phase TaB and TaB₂ were produced from the samples of Ta:B = 1:1 and 1:2, respectively. However, multiphase products were synthesized from the samples of other stoichiometries. In the final products from Ta-rich samples of Ta:B = 2:1 and 3:2, two boride phases, Ta₂B and TaB, along with a large amount of residual Ta were detected. The products yielded from boron-rich reactants of Ta:B = 5:6 and 3:4 were composed of TaB, Ta₃B₄, and TaB₂. Based upon the temperature dependence of combustion wave velocity, the activation energies associated with the formation of TaB and TaB₂ by solid state combustion were determined to be 131.1 and 181.4 kJ/mol, respectively.     

基于WO3-MoO3和TiO2/CdS复合电极的光电致变色器件

电致变色广泛应用于智能窗领域,但电致变色材料仍需外部电源驱动,将太阳能电池与电致变色材料结合起来的光电致变色器件可实现无需外部供电的智能变色调控。性能优异的变色阴极和光阳极是当下光电致变色器件的研究热点。通过水热法制备WO₃-MoO₃薄膜,研究其电致变色性能;通过水热法结合连续离子层沉积法制备TiO₂/CdS复合薄膜,研究其光电转换性能。最后将WO₃-MoO₃薄膜和TiO₂/CdS复合薄膜分别作为光电致变色器件的变色阴极、光阳极构建WO₃/MoO₃-TiO₂/CdS光电致变色器件。WO₃/MoO₃-TiO₂/CdS光电致变色器件具有较大的光学调制范围(630nm处为41.99%)、更高的着色效率(35.787%),将其作为智能窗应用在现代建筑、通行工具等领域具有重要应用价值。     

Synthesis and characterization of Al2O3 - ZrO2-based eutectic ceramic powder material dispersion-hardened with ZrB2 and WB particles prepared by SHS

Method of self-propagating high-temperature synthesis (SHS) was used to obtain ceramic powder material based on Al₂O₃-ZrO₂-WB/ZrB₂ system with size of particles ranging from 10 to 200?µm. The routes of chemical reactions taking place in the process of synthesis depending on the source mixture composition have been determined. It was shown that the formation of zirconium boride strengthening phase depends on the content of source components В and W and combustion temperature of mixture. The analysis of microstructure has shown that powder particles feature complex composite structure consisting of oxide eutectics of Al₂O₃-ZrO₂, individual phases of aluminum oxide and zirconium oxide as well as particles of WB and/or ZrB2. The technological properties of produced powders have been investigated.     

Fuel mixture approach for solution combustion synthesis of Ca3Al2O6 powders

Single-phase 3CaO·Al₂O₃ powders were prepared via solution combustion synthesis using a fuel mixture of urea and β-alanine. The concept of using this fuel mixture comes from the individual reactivity of calcium nitrate and aluminum nitrate with respect to urea and β-alanine. It was proved that urea is the optimum fuel for Al(NO₃)₃ whereas β-alanine is the most suitable fuel for Ca(NO₃)₂. X-ray diffraction and thermal analysis investigations revealed that heating at 300 °C the precursor mixture containing the desired metal nitrates, urea and β-alanine triggers a vigorous combustion reaction, which yields single-phase nanocrystalline 3CaO·Al₂O₃ powder (33.3 nm). In this case additional annealing was no longer required. The use of a single fuel failed to ensure the formation of 3CaO·Al₂O₃ directly from the combustion reaction. After annealing at 900 °C for 1 h, the powders obtained by using a single fuel (urea or β-alanine) developed a phase composition comprising of 3CaO·Al₂O₃, 12CaO·7Al₂O₃ and CaO.     

Combustion synthesis of (Ti1−xNbx)2AlC solid solutions from elemental and Nb2O5/Al4C3-containing powder compacts

Preparation of the (Ti_1−xNb_x)₂AlC solid solution (formed from the M_n+1AX_n or MAX carbides, where n = 1, 2, or 3, M is an early transition metal, A is an A-group element, and X is C) with x = 0.2-0.8 was investigated by self-propagating high-temperature synthesis (SHS). Nearly single-phase (Ti,Nb)₂AlC was produced through direct combustion of constituent elements. Due to the decrease of reaction exothermicity, the combustion temperature and reaction front velocity decreased with increasing Nb content of (Ti_1−xNb_x)₂AlC formed from the elemental powder compacts. In addition, the samples composed of Ti, Al, Nb₂O₅, and Al₄C₃ were adopted for the in situ formation of Al₂O₃-added (Ti,Nb)₂AlC. The SHS process of the Nb₂O₅/Al₄C₃-containing sample involved aluminothermic reduction of Nb₂O₅, which not only enhanced the reaction exothermicity but also facilitated the evolution of (Ti,Nb)₂AlC. Based upon the XRD analysis, two intermediates, TiC and Nb₂Al, were detected in the (Ti,Nb)₂AlC/Al₂O₃ composite and their amounts were reduced by increasing the extent of thermite reduction involved in the SHS process. The laminated microstructure characteristic of the MAX carbide was observed for both monolithic and Al₂O₃-added (Ti,Nb)₂AlC solid solutions synthesized in this study.     

Gas sensing properties of multiple networked GaN/WO3 core-shell nanowire sensors

GaN nanowires and GaN-core/WO₃-shell nanowires were synthesized by the thermal evaporation of GaN powders followed by the sputter-deposition of WO₃ and their gas sensing properties were examined. The multiple networked pristine GaN nanowire sensors showed responses of approximately 125%, 140%, 146%, 159%, and 183% to 1, 2, 3, 4, and 5 ppm NO₂ gases, respectively. These responses are comparable to those obtained previously using metal oxide semiconductor one-dimensional nanostructure sensors. The responses of the nanowires to 1, 2, 3, 4, and 5 ppm NO₂ gases were improved 1.3, 1.4, 1.6, 1.7 and 1.8 fold, respectively, further through the encapsulation of GaN nanowires with a WO₃ thin film. The improvement in the response of GaN nanowires to NO₂ gas by encapsulation is attributed to the modulation of electron transport at GaN–WO₃ heterojunction. The electron transport in the core-shell nanowires is modulated by the heterojunction with an adjustable energy barrier height, resulting in an enhanced sensing property of the core-shell nanostructures.     

Fabrication and characterization of WO3 flocky microspheres induced by ethanol

The novel tungsten trioxide flocky microspheres induced by ethanol were successfully obtained via a simple and convenient hydrothermal route. The as-prepared products were characterized by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, BET surface area measurement and UV-Vis diffuse reflectance spectroscopy. WO₃ powder prepared with volume ratio (v_ethanol/v_water) of 40% revealed a complete flocky microsphere structure with particle size of 3.0-4.0 μm and exhibited good photochromic property. The possible formation mechanism of the flocky microspheres was proposed and the improved photochromic properties were also investigated.     

Efficient photocatalytic activity of water oxidation over WO3/BiVO4 composite under visible light irradiation

A coupled WO₃/BiVO₄ thin film has been deposited on FTO substrate by a spin coating method from precursor solutions. The composite films were characterized by AFM, SEM, XPS and XRD techniques. The incident photon to current efficiency (IPCE) of BiVO₄ electrode was increased by 10 times when a WO₃ film was layered between the BiVO₄ layer and the FTO substrate. The enhanced performance of WO₃/BiVO₄ composite film electrode is mainly ascribed to the effective electron-hole separations at the semiconductor heterojunction. A schematic mechanism of charge transfer was proposed to explain the photocurrent enhancement for the WO₃/BiVO₄ electrode surface.     

Giant Persistent Photoconductivity of the WO3 Nanowires in Vacuum Condition

A giant persistent photoconductivity (PPC) phenomenon has been observed in vacuum condition based on a single WO₃ nanowire and presents some interesting results in the experiments. With the decay time lasting for 1 × 10⁴ s, no obvious current change can be found in vacuum, and a decreasing current can be only observed in air condition. When the WO₃ nanowires were coated with 200 nm SiO₂ layer, the photoresponse almost disappeared. And the high bias and high electric field effect could not reduce the current in vacuum condition. These results show that the photoconductivity of WO₃ nanowires is mainly related to the oxygen adsorption and desorption, and the semiconductor photoconductivity properties are very weak. The giant PPC effect in vacuum condition was caused by the absence of oxygen molecular. And the thermal effect combining with oxygen re-adsorption can reduce the intensity of PPC.     

The influence of mechanochemical activation on combustion synthesis of Si3N4

This study addresses itself in the performance of Si₃N₄ combustion synthesis, occurred in the presence of Si₃N₄ and NH₄Cl powders in N₂ atmosphere of 6 MPa. Mechanochemical activation of Si powder, achieved via high-energy attrition milling up to 24 h, increases the intensity and the efficiency of the reactions between Si and N₂ as well as combustion temperature. Benign processing conditions, anticipated with lower mechanochemical activation of Si powder, low N₂ pressures, and low combustion temperatures, favor formation of α-Si₃N₄.     

Rapid synthesis of nanocrystalline Co3O4 by a microwave-assisted combustion method

A facile and rapid microwave-assisted combustion method was developed to synthesize the nanocrystalline Co₃O₄. The study suggested that application of microwave heating to produce the homogeneous porous Co₃O₄ was achieved in a few minutes. The structure and morphology of the as-prepared nanocrystalline Co₃O₄ were investigated by means of X-ray diffraction (XRD), Fourier transform infrared spectra (FTIR), Raman spectra, UV-vis absorbance spectra and scanning electron microscopy (SEM). The XRD, FTIR and Raman spectra confirmed the formation of spinel structural Co₃O₄, and the SEM results indicated the porous surface characteristic of the products. Magnetic measurement was carried out using a vibrating sample magnetometer (VSM). The field dependence of the magnetization at room temperature showed a tiny hysteresis loop with a coercivity of 56.7 Oe.     

Characterization of electrodeposited WO3 films and its application to electrochemical wastewater treatment

WO₃ films have been prepared on to IrO₂-coated Ti substrate by cathodic deposition, and as-deposited and annealed films have been characterized using XRD, TEM, Raman and FT-IR spectroscopy. The as-deposited film consists of nanocrystalline, orthorhombic WO₃·H₂O and this phase transforms to amorphous WO₃ by annealing at 250 °C and to monoclinic WO₃ by annealing at and above 350 °C. The as-deposited and annealed films have been used as anodes for electrochemical decomposition of phenol in aqueous solutions with and without chloride ions. The monoclinic WO₃ anodes prepared by annealing at 350 and 400 °C show relatively high electrochemical activity in the chloride-containing solution. In addition, the anodes possess high chemical and physical stabilities: very low dissolution rate of WO₃ during the electrolysis and good adhesion to the substrate. Thus, WO₃ anodes may be promising materials for anodic oxidation of bio-refractory organics in wastewater, although further improvement of electrochemical activity is needed for more effective decrease in total organic carbons in wastewater.     

Ultra high temperature high-entropy borides: Effect of graphite addition on oxides removal and densification behaviour

The introduction of 0.5–1.0 wt.% graphite to the powders prepared by Self-propagating High-temperature Synthesis (SHS) is found to be highly beneficial for the removal of oxide impurities (from 2.7-8.8 wt.% to 0.2–0.5 wt.%) during spark plasma sintering (1950°C/20 min, 20 MPa) of (Hf_0.2Mo_0.2Ta_0.2Nb_0.2Ti_0.2)B₂ and (Hf_0.2Mo_0.2Ta_0.2Zr_0.2Ti_0.2)B₂ ceramics. Concurrently, the consolidation level achieved is enhanced from about 92.5% and 88%, respectively, to values exceeding 97%. While a further increase of graphite slightly improves samples densification, final products become progressively richer of the unreacted carbon.It is assumed that graphite plays a double role during SPS, e.g. not only as a reactant during the carbothermal reduction of oxides contaminant, but also as lubricating agent for the powder particles. The latter phenomenon is likely the main responsible for the densification improvement when 3 wt.% or larger amounts of additive are used. Another positive effect is the crystallite size refinement of the high-entropy phases with the progressive abatement of oxides, to confirm that their presence promotes grain coarsening during the sintering process.     

Enhanced photoelectrocatalytic activity of FTO/WO3/BiVO4 electrode modified with gold nanoparticles for water oxidation under visible light irradiation

Gold nanoparticles were successfully deposited on FTO/WO₃/BiVO₄ electrode surface by means of electrolysis of AuCl₄⁻ ions. The composite films were characterized by SEM, XPS and XRD techniques. An increase in photocurrent and a negative shift of onset potential for water oxidation were observed upon modification of the electrode surface with the Au particles. The electrochemical impedance spectroscopy was used to confirm the acceleration of charge transfer process by Au deposition at the electrode surface. The photocurrent action spectrum did not correlate with the plasmonic absorbance of Au nanoparticles at 560 nm, suggesting that the Au nanoparticles increased charge separation without undergoing a plasmon resonance effect under visible light irradiation.     

Formation of chromium borides by combustion synthesis involving borothermic and aluminothermic reduction of Cr2O3

Chromium borides of various phases were fabricated through combustion synthesis in the mode of self-propagating high-temperature synthesis (SHS) by adopting the powder compacts of Cr₂O₃ + xB (with x = 4–9) and Cr₂O₃ + 2Al + yB (with y = 1–8). Because aluminothermic reduction of Cr₂O₃ is more exothermic than borothermic reduction, the reaction temperature and combustion front velocity of the Al-added samples are much higher than those of the Cr₂O₃–B samples. In agreement with the composition dependence of reaction exothermicity, the fastest combustion wave was observed in the compact with x = 6 for the Cr₂O₃–B sample and y = 4 for the Cr₂O₃–Al–B sample. According to the XRD analysis, Cr₅B₃, CrB, and CrB₂ were produced in the monolithic form respectively from the Cr₂O₃–B samples of x = 4, 5, and 9, or in the composite form from samples of other stoichiometries. On the other hand, five different borides were identified in Al₂O₃-added products. Among them, Cr₂B, CrB, and CrB₂ were yielded as the sole boride compound from the Al-added samples of y = 1.2, 3, and 7 or 8, respectively. Cr₅B₃ and Cr₃B₄ were produced along with CrB as the secondary phase. Based upon experimental evidence, it was found that an excess amount of boron in the reactant mixture was required to facilitate the formation of chromium borides.     

Metal-organic framework Cu3 (BTC)2(H2O)3 catalyzed Aldol synthesis of pyrimidine-chalcone hybrids

A typical metal organic framework, [Cu₃ (BTC)₂(H₂O)₃, BTC = 1,3,5-benzene tricarboxylate] has been used for the synthesis of pyrimidine-chalcones. We have explored a green synthesis of pyrimidine chalcones under Cu₃(BTC)₂ catalysis by Aldol condensation. Easy isolation of product, excellent yield, and recyclable catalyst makes this reaction eco-friendly. The technology was demonstrated to be applicable to the synthesis of a host of chemical hybrids.     

Electrochemical characterization of amorphous LiFe(WO4)2 thin films as positive electrodes for rechargeable lithium batteries

Amorphous LiFe(WO₄)₂ thin films have been fabricated by radio-frequency (R.F.) sputtering deposition at room temperature. The as-deposited and electrochemically cycled thin films are, respectively, characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, selected area electron diffraction, and X-ray photoelectron spectra techniques. An initial discharge capacity of 198 mAh/g in Li/LiFe(WO₄)₂ cells is obtained, and the electrochemical behavior is mostly preserved in the following cycling. These results identified the electrochemical reactivity of two redox couples, Fe³⁺/Fe²⁺ and W⁶⁺/W^x+ (x = 4 or 5). The kinetic parameters and chemical diffusion coefficients of Li intercalation/deintercalation are estimated by cyclic voltammetry and alternate-current (AC) impedance measurements. All-solid-state thin film lithium batteries with Li/LiPON/LiFe(WO₄)₂ layers are fabricated and show high capacity of 104 μAh/cm² μm in the first discharge. As-deposited LiFe(WO₄)₂ thin film is expected to be a promising positive electrode material for future rechargeable thin film batteries due to its large volumetric rate capacity, low-temperature fabrication and good electrode/electrolyte interface.     

Hydrothermal synthesis of Li(Ni1/3Co1/3Mn1/3)O2 for lithium rechargeable batteries

Ultrafine powders of Li(Ni_1/3Co_1/3Mn_1/3)O₂ cathode materials for lithium-ion secondary batteries were prepared under mild hydrothermal conditions. The influence of the molar ratio of Li/(Ni + Co + Mn) was studied. The products were investigated by XRD, TEM and EDS. The final products were found to be well crystallized Li(Ni_1/3Co_1/3Mn_1/3)O₂ with an average particle size of about 10 nm.     

Effect of CeO2 on the thermoelectric properties of WO3-based ceramics

The thermoelectric properties of tungsten trioxide (WO₃) ceramics doped with cerium dioxide (CeO₂) were investigated. The results demonstrated that the addition of CeO₂ to WO₃ could promote the grain growth and the densification. The magnitude of the electrical conductivity (σ) and the absolute value of the Seebeck coefficient (|s|) depended strongly on the CeO₂ content. The sample doped with 2.0 mol% CeO₂ yielded higher σ and |s|, resulting in a significant increase in the power factor (σs²). In addition, the power factor value of all samples increased abruptly at high temperatures, which revealed that WO₃-based ceramics could have greater thermoelectric properties at high temperatures.     

Preparation and characterization of NaBiO3 nanopowders by different methods for photocatalytic reduction of CO2

In this work we prepared NaBiO₃ nanopowders via three synthesis methods (sol-precipitation, dehydration and hydrothermal methods). To evaluate and compare the physical properties of the prepared materials X-ray diffraction analysis, BET measurements, UV–vis spectroscopy and TEM were applied. The results showed changes to the NaBiO₃ crystallinity, the specific surface area and the particle shape and size, depending on the method of synthesis. To determine the photocatalytic efficiency of the prepared materials, we evaluated the photocatalytic reduction of CO₂.