One-step hydrothermal synthesis and enhanced microwave absorption properties of Ni0.5Co0.5Fe2O4/graphene composites in low frequency band

Ni_0.5Co_0.5Fe₂O₄/graphene composites were synthesized successfully via one-step hydrothermal method. The crystal structure, morphology and corresponding elemental distribution, electromagnetic parameters and microwave absorption performances of the as-prepared composites were measured by XRD, SEM, TEM and VNA, respectively. The results indicated that the microwave absorbing performance can be obviously enhanced through the addition of graphene in a suitable range, the magnetic loss plays a dominant contribution for the microwave absorption of composites. The maximum reflection loss of ?30.92?dB at 0.84?GHz with a ?10?dB bandwidth over the frequency range of 0.58–1.19?GHz is obtained when the composite contains 12?wt% graphene and the thickness of sample is 4?mm. This investigation presents a simple method to prepare Ni_0.5Co_0.5Fe₂O₄/graphene composites with excellent microwave absorption performance in the low frequency band of 0.1–3?GHz.     

Electrochemical investigation of LiMn2O4 cathodes in gel electrolyte at various temperatures

A composite lithium battery electrode of LiMn₂O₄ in combination with a gel electrolyte (1 M LiBF₄/24 wt% PMMA/1:1 EC:DEC) has been investigated by galvanostatic cycling experiments and electrochemical impedance spectroscopy (EIS) at various temperatures, i.e. −3<T<56 °C. For analysis of EIS data, a mathematical model taking into account local kinetics and potential distribution in the liquid phase within the porous electrode structure was used. Reasonable values of the double-layer capacitance, the exchange-current density and the solid phase diffusion were found as a function of temperature. The apparent activation energy of the charge-transfer (∼65 kJ mol⁻¹), the solid phase transfer (∼45 kJ mol⁻¹) and of the ionic bulk and effective conductance in the gel phase (∼34 kJ mol⁻¹), respectively, were also determined. The kinetic results related to ambient temperature were compared to those obtained in the corresponding liquid electrolyte. The incorporated PMMA was found to reduce the ionic conductivity of the free electrolyte, and it was concluded that the presence of 24 wt% PMMA does not have a significant influence on the kinetic properties of LiMn₂O₄.     

Fabrication of SiCN-Sc2Si2O7 coatings on C/SiC composites at low temperatures

SiCN-Sc₂Si₂O₇ environmental barrier coatings were fabricated on the surface of C/SiC composites at low temperatures by adding Li₂CO₃ as sintering aids. With this addition, the fabrication temperature could be lowered about 100-200 °C. The shrinkage of the polysilazane-Sc₂Si₂O₇ bars with and without Li₂CO₃ was tested by dilatometer. The results indicate that the shrinkage speed of the polysilazane-Sc₂Si₂O₇ bar with Li₂CO₃ is faster than the one without Li₂CO₃, indicating that the Li₂CO₃ greatly promotes the sintering of polysilazane-Sc₂Si₂O₇. Water-vapor corrosion behavior of the SiCN-Sc₂Si₂O₇ coated C/SiC composites was carried out at 1250 °C. The results reveal that the SiCN-Sc₂Si₂O₇ coatings can effectively protect the C/SiC composites. The corrosion resistance of SiCN-Sc₂Si₂O₇ coatings is not degraded by adding Li₂CO₃.     

Effect of hydrothermal treatment on the performance of RuO2-Ta2O5/Ti electrodes for use in supercapacitors

Hydrothermal treatment (HTT) of RuO₂-Ta₂O₅/Ti electrode, as a method for improving their performance, for use in supercapacitors was investigated.The results show that HTT significantly enhances the stability of the electrodes. The specific capacitance of electrodes, subject to HTT in the temperature range 180-250 °C remains unchanged after 1000 CV cycles between −0.2 and 1.1 V vs. SCE; without HTT a decay to 97% of the initial is observed. The results also show that HTT decreases the activity of the electrodes for O₂ and H₂ evolution and increases the voltage window by 56-135 mV for supercapacitors, but with a specific capacitance decrease of 7-27%. XPS analyses show the existence of more hydroxides after the HTT, which leads to a little increase in the interplanar distance as indicated in the XDR results. Contact angle measurements show the presence of a more hydrophilic surface after HTT.     

A novel nickel/carbon catalyst for CH4 and H2 production from organic compounds dissolved in wastewater by catalytic hydrothermal gasification

A novel Ni/carbon catalyst recently developed by the authors was used to gasify organic compounds dissolved in the wastewater with TOC concentration from 0.2 to 2%. The process removes the organic compounds by gasifying them into high calorific gases like methane and hydrogen. The investigations were focused on the efficiency of the Ni/carbon catalyst in terms of carbon conversion, conversion of big organic molecules, and catalyst deactivation due to sintering. The preliminary results showed that up to 99% carbon conversion can be achieved at 360 °C, and 20 MPa. A conversion mechanism was suggested which consists of: first, decomposition of big molecules to small molecules on the metal surface, steam gasification of small molecules to produce CO and H₂ followed by CO methanation and CO shift reaction to produce CH₄ and CO₂. The catalyst was found to be highly active and stable and no sintering was observed even after 100 h of reaction time.     

Influence of CaCl2 on the hydrothermal modification of Mg(OH)2

The influence of CaCl₂ on the hydrothermal modification of Mg(OH)₂ was investigated in this paper. The experimental results indicated that dispersive Mg(OH)₂ hexagonal plates with the regular shape, the enlarged size and the perfect crystallinity were formed by treating the irregular Mg(OH)₂ agglomerates at elevated temperatures in dilute CaCl₂ solutions. Thermodynamic analysis showed that the presence of CaCl₂ at the elevated temperature promoted the formation of MgOH⁺, which was favorable for the formation of Mg(OH)₆⁴⁻ and the growth of Mg(OH)₂ crystals.     

Uniform dispersion of Ni nano particles in a carbon based catalyst for increasing catalytic activity for CH4 and H2 production by hydrothermal gasification

A carbon based nickel (Ni) catalyst was prepared by ion exchange method in which a large amount of Ni is dispersed as almost uniform nano particles. The method ion exchanges the ion exchangeable sites in an ion exchange resin with Ni followed by carbonization at 500 °C. Two different catalysts were prepared by ion exchanging Ni from an aqueous solution containing same amount of Ni ion concentration but at different pH of the solution. The amount of Ni ion exchanged in both cases was about 15%. The metal load after carbonization was about 47% and 46% in the two catalysts, respectively. The XRD pattern and TEM images of the catalysts showed that Ni particles in NiWB500 (pH = 8.8) were bigger in size in comparison to the Ni particles in NiLG500 (pH = 9.4). The BET surface area was 178 and 183 m²/g, respectively. The catalytic hydrothermal gasification (CHTG) experiments at 350 °C, 20 MPa, and 50 h⁻¹ LHSV for 50 h with organic water containing 0.2% and 2% TOC concentration showed that conversion was almost 100% with NiLG500 (pH = 9.4) catalyst and 100% and 96% with NiWB500 (pH = 8.8) catalyst, respectively. The XRD and TEM patterns of the two catalysts after 50 h gasification run showed higher sintering in NiLG500 (pH = 9.4) in which Ni particles were smaller in size.     

A non-sintering fabrication method for porous Si3N4 ceramics via sol hydrothermal process

A non-sintering fabrication method for porous Si₃N₄ ceramics with high porosity and high mechanical strength was proposed. Strength of the porous ceramics can be obtained by silica sol mass transfer process in hydrothermal conditions rather than a traditionally controlled high temperature sintering process. Under hydrothermal circumstances, silica sol is continuously transferred to the necks of Si₄N₃ powder compact, depositing there and thus consolidating the ceramic skeleton. The key of the method to obtain homogeneous microstructure and mechanical strength is how to keep the silica sol from gelatin during hydrothermal procedure. The stabilization of silica sol and its affecting factors were studied. The results indicated that ultrasonic treatment makes alkali-catalyzed silica sol remain stable even in 200?℃ hydrothermal condition, which insures consecutive silica transportation. The effect of hydrothermal time on open porosity/mechanical strength of the porous Si₄N₃ ceramics were also thoroughly investigated. The porous Si₄N₃ ceramics with open porosity above 42% and flexural strength of 45?MPa were obtained without any high temperature sintering process. This method can be widely employed for the preparation of other porous ceramics as well.     

Potential of the Pb,PbSO4/H2SO4 electrode at temperatures up to 240°C

Standard lead—lead sulphate electrode potential was determined over the temperature range 20–240°C from emf measurements of the Pb, PbSO₄H₂SO₄ (0.05M)K₂SO₄KClHCl(0.1M)/AgCl, Ag and Pb, PbSO₄H₂SO₄(m)K₂SO₄H₂SO₄(0.05M)PbSO₄, Pb cells where m = 0.005, 0.01, 0.1 and 0.5 M. To this effect lead—lead sulphate electrode potential was calculated using the temperature relationship of the standard silver—silver chloride electrode potential and activity coefficients of hydrochloric acid determined by Greeley et al. at temperatures up to 260°C. Diffusion potentials occurring at the phase boundaries in the cells under investigation were calculated using the Henderson's equation. Values of the standard lead—lead sulphate electrode potential were determined by extrapolation of the E°′ function to the zero ionic strength which was calculated using the second sulphuric acid dissociation constant determined by Lietzke et al. at temperatures up to 300°C. The standard electrode potential was described in the temperature range 20–240°C by the following relationship: E°_{Pb, PbSO4/SO^2?4}(V) = 0.040-0.00126T. A change in entropy ΔS° of the electrode reaction Pb + SO^2?₄ = PbSO₄ + 2e^? is constant in this temperature range and is ?243 JK^?1 mol^?1 (?1018 cal K^?1 mol^?1).     

Synthesis and physical characteristics of ZnAl2O4 nanocrystalline and ZnAl2O4/Eu core-shell structure via hydrothermal route

Nanocrystalline zinc aluminate (ZnAl₂O₄) particles with a spinel structure were prepared by hydrolyzing a mixture of aluminum chloride hexahydrate and zinc chloride in deionized water. It was found that pH value and reaction temperature play critical roles in the formation of nano-sized ZnAl₂O₄. Depending on pH values in the precursor solution, ZnAl layered double hydroxide (ZnAl-LDH), ZnO, boehmite or gibbsite could be formed. At pH 7 and T>120 °C, the nanocrystalline ZnAl₂O₄ particles with average particle size of ∼5 nm are easily synthesized through ZnAl layered double hydroxide (ZnAl-LDH). After surface treatment with R-OH by using the cationic surfactant CTAB, the ZnAl₂O₄/Eu core-shell structure can be developed. The ZnAl₂O₄/Eu core-shell structure can show both emissions from ⁵D₀ to ⁷F₂ sensitivity energy level and ⁵D₂ to ⁷F₀ depth energy level.     

Improved formaldehyde sensor of Zn2SnO4/SnO2 microcubes by compositional evolution and Y2O3 decoration

Zn₂SnO₄/SnO₂(ZTO/SnO₂) and Y doped Zn₂SnO₄/SnO₂(ZTO/SnO₂) microcubes were synthesized by a hydrothermal route at 130?°C and subsequently used for obtaining gas sensors. To evaluate the structure, morphology, chemical state and optical bandgap, our sensors were characterized by XRD, SEM, XPS and UV–vis analysis. Compared with sensors based on ZTO/SnO₂ microcubes, the Y doped ZTO/SnO₂ microcubes had an optimum sensing performance to 100?ppm formaldehyde (HCHO), for instance lower working temperature (210?°C) and better response (46.07). In addition, the enhanced sensing mechanism of Y doped ZTO/SnO₂ microcubes was discussed in detail.     

One-step synthesis of fine SrTiO3 particles using SrSO4 ore under alkaline hydrothermal conditions

The employment of mineral SrSO₄ crystals and powders for preparing SrTiO₃ compound was investigated, with coexistence of Ti(OH)₄·4.5H₂O gel under hydrothermal conditions, at various temperatures (150–250 °C) for different reaction intervals (0.08–96 h) in KOH solutions with different concentrations. The complete dissolution of the SrSO₄ crystal occurred at 250 °C for 96 h in a 5 M KOH solution, resulting in the synthesis of SrTiO₃ particles with two different shapes (peanut-like and cubic). In contrast, very fine SrTiO₃ pseudospherical particles were crystallized when SrSO₄ powders were employed as precursor. Variations on the SrTiO₃ particle shape and size were found to be caused by the differences in the dissolution rate of the SrSO₄ phase in the alkaline KOH solution. The crystallization of SrTiO₃ particles was achieved by a bulk dissolution–precipitation mechanism of the raw precursors, and this mechanism was further accelerated by increasing the reaction temperature and concentration of the alkaline media. Kinetic data depicted that the activation energy required for the formation of SrTiO₃ powders from the complete consumption of a SrSO₄ single crystal plate under hydrothermal conditions, is 27.9 kJ mol⁻¹. In contrast, when SrSO₄ powders were employed (28–38 μm), the formation of SrTiO₃ powder proceeded very fast even for a short reaction interval of 3 h at 250 °C in a 5 M KOH solution.     

Low-temperature synthesis of Bi2Fe4O9 nanoparticles via a hydrothermal method

Bi₂Fe₄O₉ (BFO) nanoparticles were successfully synthesized by a hydrothermal method at a temperature as low as 100 °C. The as-prepared powders, characterized by X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), transmission electron microscope (TEM) and physical property measurement system (PPMS), exhibited a pure BFO phase about 100 nm size with uniform sheet-like shape and exhibited an AF order at room temperature. It was found that high alkali concentration and alkali ion Na⁺ played a key role in the formation of BFO nanoparticles at a low temperature of 100 °C.     

Improvement of the high-rate discharge capability of phosphate-doped spinel LiMn2O4 by a hydrothermal method

Micrometer-scale pristine and phosphate-doped spinel LiMn₂O₄ materials with homogeneous size distribution were synthesized by a one-step hydrothermal method. The composition, structure and morphology of the as-prepared samples were characterized using inductively coupled plasma atomic emission spectroscopy (ICP-AES), chemical analysis, X-ray diffraction (XRD) and scanning electron microscopy (SEM). The effect of phosphate doping on the structural and electrochemical properties of spinel LiMn₂O₄ was investigated by Fourier transform infrared (FT-IR) spectroscopy, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The phosphate-doped LiMn₂O₄ cathode (with a molar ratio of PO₄³⁻:LiMn₂O₄ = 1.5%) exhibits good high-rate discharge capability with 94 mAh/g at a current density of 2960 mA/g. The analyses demonstrate that compared with the pristine LiMn₂O₄ sample, the phosphate-doped samples have a relatively large Li-ion diffusion coefficient and smaller charge-transfer resistance due to the increase of the unit cell volume of spinel LiMn₂O₄ caused by the doping of phosphate.     

Modified hydrothermal synthesis of SnOy-SiO2 for lithium-ion battery anodes

A modified hydrothermal method was developed to synthesize tin oxide doped with highly dispersed silicon oxide. The microstructure, morphology and electrochemical performance of the mixtures were analyzed by X-ray diffraction (XRD), infra-red (IR), scanning electron microscopy (SEM) and electrochemical methods. The average size of the particles is about 30 nm. Silicon oxide as matrix should be able to support the anode changes accompanied by the formation of lithium-tin alloys, thus an improvement of the cycleability of the Li-ion battery would be expected. The electrochemical results showed that addition of silicon oxide reduces the irreversible capacity during the first discharge/charge cycle. The material delivers a charge capacity of more than 750 mAh g⁻¹. The capacity loss per-cycle is about 0.9% after cycling 20 times. The electrochemical performance indicates that silicon oxide is an appropriate matrix and these composites are good anode candidates for application in lithium-ion batteries.     

The effect of the hydrothermal treatment with aqueous NaOH solution on the photocatalytic and photoelectrochemical properties of visible light-responsive TiO2 thin films

The effect of the hydrothermal treatment with aqueous NaOH solution on the photoelectrochemical and photocatalytic properties of visible light-responsive TiO₂ thin films prepared on Ti foil substrate (Vis-TiO₂/Ti) by a radio-frequency magnetron sputtering (RF-MS) deposition method has been investigated. The hydrothermally treated Vis-TiO₂/Ti electrodes exhibited a significant increase in their photocurrent under UV and visible light irradiation as compared to untreated Vis-TiO₂/Ti electrode. SEM investigations revealed that the surface morphology of Vis-TiO₂/Ti are drastically changed from the assembly of the TiO₂ crystallites to the stacking of nanowires with diameters of 30–50 nm with increasing hydrothermal treatment time (3–24 h), accompanying the increase in their surface area. The separate evolution of H₂ and O₂ from water under solar light irradiation was successfully achieved using the Vis-TiO₂/Ti/Pt which is hydrothermally treated for 5 h, while the H₂ evolution ratio was 15 μmol h⁻¹ in the early initial stage, corresponding to a solar energy conversion efficiency of 0.23%.     

Hydrothermal synthesis of flower-like Zn2SnO4 composites and their performance as anode materials for lithium-ion batteries

Flower-like Zn₂SnO₄ composites had been prepared through a green hydrothermal synthesis. The structural, morphological and electrochemical properties were investigated by means of XRD, BET, SEM, TEM, and electrochemical measurement. The results show that the as-prepared sample is in high purity phase and of good crystallinity; meanwhile it has a particular 3-D structure and large surface area. Electrochemical measurement suggests that flower-like Zn₂SnO₄ composites exhibit better cycling properties and lower initial irreversible capacities than the solid Zn₂SnO₄ cubes. The first discharge and charge capacities of the material are 1750 mA h g⁻¹ and 880 mA h g⁻¹ respectively. A higher reversible capacity of 501 mA h g⁻¹ was obtained after 50 cycles at a current density of 300 mA g⁻¹. The higher reversible capacity and good stability can be related to the special nanostructural features of the material. Such Zn₂SnO₄ structures synthesized by the simple and cheap method are expected to have potential application in energy storage.     

Catalytic hydrothermal conversion of cellulose over SnO2 and ZnO nanoparticle catalysts

The hydrothermal conversion of cellulose in the presence of nanometal oxide particles (SnO₂ and ZnO) was investigated in this study. Both catalysts were synthesized hydrothermally and characterized using TEM, FESEM-EDX, X-ray diffraction spectroscopy and BET (Brunauer, Emmett and Teller) methods. In order to reveal the effect of nano-scale catalysts, experiments were conducted using bulk (non-nano) metal oxide and pure cellulose without any catalyst. The hydrothermal conversion experiments were carried out in a micro autoclave at 300, 400, 500, 600 °C and 1 h reaction time. The compositions of the gaseous products and the aqueous phase were determined with various analytical techniques (GC, ion chromatography, HPLC, UV-vis). Contribution of carbon containing products to the carbon mass balance was also represented. The results indicated both nano and bulk ZnO and SnO₂ to have an effect on the water-gas shift reaction at varying temperatures. The water-gas shift reaction (WGS) proceeded fast at 300 °C in the presence of ZnO, while the rate of WGS was lower at 300 °C in the presence of SnO₂. Nano ZnO led to improved hydrogen yield, while ethane and propane were formed as a result of side reactions in the presence of nano and bulk SnO₂.     

Effect of potassium hydrogen phthalate (C8H5KO4) on the one-step electrodeposition of single-phase CuInS2 thin films from acidic solution

The effect of potassium hydrogen phthalate (C₈H₅KO₄) as a special additive on the one-step electrodeposition of single-phase CuInS₂ thin films from acidic solution (pH 2.5) was investigated in detail. The XRD, SEM and UV-vis-NIR characterization confirms that the addition of an adequate concentration of C₈H₅KO₄ (23 mM) to the electrolytic bath containing 12.5 mM Cu²⁺, 10 mM In³⁺, 40 mM S₂O₃²⁻ and 100 mM LiCl can contribute greatly to the controllable growth of pure chalcopyrite CuInS₂ films with uniform surfaces and an ideal band gap of approximately 1.54 eV. Complexation studies of C₈H₅KO₄ with Cu²⁺ and In³⁺ in electrolytic solutions indicated that C₈H₅KO₄ can complex Cu²⁺ more strongly than In³⁺ and move the electrode potentials of Cu²⁺ and In³⁺ near each other as determined by polarization analysis. Furthermore, the potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) analysis performed in a series of solution systems revealed a three-step reaction mechanism for CuInS₂ deposition and considerable adsorption of C₈H₅O₄⁻ and Cu(C₈H₅O₄)⁺ to the cathode surface. This deposition shows that the synergetic effects of complexation and adsorption originated from the additive on the Cu²⁺ electro-reduction, thus promoting the co-deposition of copper, indium and sulfur in the form of single-phase CuInS₂.     

Resistance switching in electrodeposited polycrystalline Fe3O4 films

Resistance random access memory (RRAM) is an emerging nonvolatile memory that offers advantages of scalability, fast switching, and low voltages. Magnetite, Fe₃O₄, has been shown to exhibit resistance switching in nanoscale architectures such as superlattices. Here, we show that electrodeposited polycrystalline films of Fe₃O₄ exhibit multistate resistance switching. Experiments suggest that the insulator-to-metal transition may be facilitated by the presence of a thin nano-crystalline layer which is critical for resistance switching to occur at lower bias. We also show that the switching behavior can be tuned through the applied deposition potential. The multiple resistance states accessible in these simple architectures open up new possibilities for multi-bit data storage and retrieval.