Rapid synthesis of gold nanostructures by a microwave–polyol method with the assistance of CnTAB (n = 10, 12, 14, 16) or C16PC

Gold nanostructures have been synthesized by a microwave(MW)–polyol method with the assistance of such cationic surfactants as alkyltrimethyl ammonium bromide (C_nTAB: n = 10–16, even numbers) or cetylpryridinium chloride (C₁₆PC). Although major products were spherical aggregates for C_nTAB (n = 10, 12, 14), triangular, pentagonal, and hexagonal nanoplates were preferentially synthesized using C₁₆TAB. Spherical spike-ball structures were prepared through C₁₆PC. These results indicated that the morphology of gold nanostructures prepared by the MW–polyol method depends both on the chain length of hydrophobic alkyl group and on the hydrophilic head group.     

Effects of rare earth La on microstructure and properties of Ag–21Cu–25Sn alloy ribbon prepared by melt spinning

Ag–21Cu–25Sn alloy ribbon as a promising intermediate temperature alloy solder (400–600 °C) was prepared by melt spinning technique in this paper. Rare earth La was added into Ag–21Cu–25Sn alloy to refine the microstructures and improve the wettabilities of as-prepared alloy solders. The phase constitutions, microstructures, melting temperatures and wettabilities of selected specimens were respectively tested. The results showed that the dominant phase constitutions of Ag–21Cu–25Sn–xLa alloy ribbons were Ag₃Sn and Cu₃Sn. The grain size of Ag–21Cu–25Sn–xLa alloy decreased with the addition of La increasing. La addition reduced the melting temperatures of Ag–21Cu–25Sn–xLa alloy ribbons, and effectively improved the wettabilities of the alloy ribbons. When the addition of La was 0.5 wt%, the wettability of as-prepared alloy solder achieved the optimal value of 158 cm² g⁻¹ under brazing temperature 600 °C and dwell time 15 min. In addition, raising brazing temperature and prolonging dwell time could improve the wettability of Ag–21Cu–25Sn–xLa alloy ribbon.     

Facile synthesis,characterization and photocatalytic activity of niobium carbide

Niobium carbide (NbC) powders were prepared via a novel route at 550 °C and 8 h, using metallic magnesium powders, niobium pentoxide (Nb₂O₅), and potassium acetate (CH₃COOK) as starting materials. The structure and morphology of the product were characterized by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). The results indicated that as-prepared product was crystallized in pure cubic NbC phase and the size of the sample was estimated to be around 120 nm. The Rietveld refinement of the XRD data gives the cell constant a = 4.4718 Å. According to the Scherrer formula, the real grain size was about 70 nm. The BET surface area of the sample was ca.29.3 m²/g. The grain size distribution of the sample was about 467 nm, which was characterized by N4 PLUS submicron Particle Size Analyzer. The cubic NbC powders exhibited photocatalytic activity in degradation of Rhodamine-B (RhB) under 300 W mercury lamp light irradiation.     

Microwave dielectric properties of BaO–TeO2 binary compounds

A series of BaO–TeO₂ binary ceramic compounds were explored for microwave dielectric applications with ultra-low processing temperatures. During the calcination of mixed BaCO₃ and TeO₂ raw powders, BaTe₄O₉, BaTe₂O₆, BaTeO₃, and Ba₂TeO₅ phases were obtained through the sequential phase formations from Te-rich to Ba-rich phases at temperatures ranging from 500 to 850 °C. Sintering temperatures were as low as only 550 °C for the Te-rich phases. Barium tellurate ceramics exhibited excellent microwave dielectric properties with intermediate dielectric permittivities and high quality factors (Q). The dielectric properties at microwave frequencies were ε_r = 10–21, Q × f = 34,000–55,000 GHz, and TCf = − 51 to − 124 ppm/°C, depending on compositions.     

Microwave-assisted method for preparation of Zn1−xMgxO nanostructures and their activities for photodegradation of methylene blue

Nanostructures of Zn_1−xMg_xO (0 ⩽ x ⩽ 0.2) were prepared in water by one-pot method under microwave irradiation for 5 min. In this method, zinc acetate, magnesium nitrate and sodium hydroxide were used as starting materials without using any additive and post preparation treatment. The nanostructures were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray (EDX), X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectroscopy (DRS), electrochemical impedance spectroscopy (EIS), Fourier transform-infrared (FT-IR), and the Brunauer–Emmett–Teller (BET) techniques. The nanostructures have wurtzite hexagonal crystalline phase and doping of Mg²⁺ ions does not change the phase of ZnO. The SEM and TEM images show that morphology of the samples is changing by doping of Mg²⁺ ions. The EIS data show that by doping the ion, interfacial charge transfer resistance of the nanostructures decreases. Photocatalytic activity of the nanostructures was evaluated by degradation of methylene blue (MB) under UV irradiation. The degradation rate constant on the nanostructures with 0.15 mol fraction of Mg²⁺ ions is about 2-fold greater than for ZnO. Moreover, influence of various operational parameters such as microwave irradiation time, calcination temperature, weight of catalyst, concentration of MB, pH of solution and scavengers of reactive species on the degradation rate constant was investigated and the results were discussed.     

Effect of V2O5 additive to 0.4SrTiO3–0.6La(Mg0.5Ti0.5)O3 ceramics on sintering behavior and microwave dielectric properties

The effect of V₂O₅ addition on the microwave dielectric properties and the microstructures of 0.4SrTiO₃–0.6La(Mg_0.5Ti_0.5)O₃ ceramics sintered for 5 h at different sintering temperature were investigated systematically. It was found that the sintering temperature was effectively lowered about 200 °C by increasing V₂O₅ addition content. The grain sizes, bulk density as well as microwave dielectric properties were greatly dependent on sintering temperature and V₂O₅ content. The 4ST–6LMT ceramics with 0.25% V₂O₅ sintered at 1400 °C for 5 h in air exhibited optimum microwave dielectric properties of ɛ_r = 50.7, Q × f = 15049.6 GHz, T_f = −1.7 ppm/°C.     

Structure and microwave dielectric characteristics of lithium-excess Ca0.6Nd0.8/3TiO3/(Li0.5Nd0.5)TiO3 ceramics

Compositions based on (1−x)Ca_0.6Nd_8/3TiO₃−x(Li_1/2Nd_1/2)TiO₃ + yLi (CNLNTx + yLi, x = 0.30–0.60, y = 0–0.05), suitable for microwave applications have been developed by systematically adding excess lithium in order to tune the microwave dielectric properties and lower sintering temperature. Addition of 0.03 excess-Li simultaneously reduced the sintering temperature and improved the relative density of sintered CNLNTx ceramics. The excess Li addition can compensate the evaporation of Li during sintering process and decrease the secondary phase content. The CNLNTx (x = 0.45) ceramics with 0.03 Li excess sintered at 1190 °C have single phase orthorhombic perovskite structure, together with the optimum combination of microwave dielectric properties of ɛ_r = 129, Q × f = 3600 GHz, τ_f = 38 ppm/°C. Obviously, excess-Li addition can efficiently decrease the sintering temperature and improve the microwave dielectric properties. The high permittivity and relatively low sintering temperatures of lithium-excess Ca_0.6Nd_0.8/3TiO₃/(Li_0.5Nd_0.5)TiO₃ ceramics are ideal for the development of low cost ultra-small dielectric loaded antenna.     

Preparation and properties of sliver and nitrogen co-doped TiO2 photocatalyst

TiO₂ photocatalysts co-doped with different content of Ag and N were prepared by sol–gel method combined with microwave chemical method. The samples were analyzed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), transmission electron microscope (TEM), ultraviolet–visible diffuse reflectance spectrum (UV–vis) and photo-luminescence emission spectrum (PL). The photocatalytic activity was investigated by photocatalytic degradation of methylene blue (MB) under irradiation of fluorescent lamp. The results indicate that Ag and N co-doping can restrain the increase of grain size, broaden the absorption spectrum to visible light region, and inhibit the recombination of the photo-generated electron–hole pairs. Moreover, the photocatalytic activity of Ag–N–TiO₂ in MB degradation is remarkable improved. The degradation rate of the sample with Ag:TiO₂ = 0.05 at%, N:TiO₂ = 18.50 wt% in 5 h is 93.44%, which is much higher than that of Degussa P25 (39.40%).     

Preparation,characterization and photocatalytic activity of Bi2O3–MgO composites

A series of Bi₂O₃–MgO composites were synthesized by solvent-thermal method. It was found that the Bi₂O₃–MgO composites perform much better than TiO₂ (P25), Bi₂O₃ and MgO in the photocatalytic degradation of rhodamine B (RhB) in the presence of HCl and under irradiation of visible light (λ > 400 nm). The effects of Bi/Mg molar ratio, crystallization temperature of Bi₂O₃–MgO and reaction conditions on photocatalytic activity were studied. The best performance was observed over the composite with Bi:Mg molar ratio equal to 2:1 that had been subject to crystallization at 120 °C for 20 h. In addition, the photocatalytic efficiency of the composite can be significantly enhanced by the presence of hydrochloric acid. The prepared samples were characterized by XRD and UV–vis DRS techniques. The relationships between the structure and photocatalytic performance of the as-prepared Bi₂O₃–MgO samples were also investigated.     

Preparation,phase structure and microwave dielectric properties of CoLi2/3Ti4/3O4 ceramic

A new low loss microwave dielectric ceramic with composition of CoLi_2/3Ti_4/3O₄ was prepared by a conventional solid-state reaction method. The compound has a cubic spinel structure [Fd-3m (227)] similar to MgFe₂O₄ with lattice parameters of a = 8.3939 Å, V = 591.42 Å³, Z = 8 and ρ = 4.30 g/cm³. This ceramic has a low sintering temperature (～1050 °C) and good microwave dielectric properties with relative permittivity of 21.4, Q × f value of 35,000 GHz and τ_f value of ?22 ppm/°C. Furthermore, the addition of BaCu(B₂O₅) (BCB) can effectively lower the sintering temperature from 1050 °C to 900 °C and does not induce much degradation of the microwave dielectric properties. Compatibility with Ag electrode indicates that the BCB added CoLi_2/3Ti_4/3O₄ ceramics are good candidates for LTCC applications.     

Microwave-assisted synthesis of BaWO4 nanoparticles and its photoluminescence properties

Single-crystal BaWO₄ nanoparticles have been successfully synthesized under microwave irradiation. The results show that nearly monodisperse BaWO₄ nanoparticles have been successfully prepared without using surfactants. The products were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), and photoluminescence (PL). The XRD results indicated that the BaWO₄ nanoparticles obtained had a tetragonal unit cell (a = 0.5612, c = 1.2706 nm). The TEM images show that the as-prepared BaWO₄ have good narrow particle-sized distributions containing a number of nanoparticles with uniform sizes. The products show a strong photoluminescence peak at 432–436 nm with the excitation at 365 nm.     

Characterization,sintering and dielectric properties of nanocrystalline barium titanate synthesized through a modified combustion process

Nanocrystalline barium titanate has been synthesized through a modified combustion process in a single step for the first time. The as-prepared barium titanate powder is cubic perovskite with lattice constant a = 4.018 Å. The phase purity of the nanopowder was examined using thermo gravimetric analysis, differential thermal analysis and Fourier transform infrared spectroscopy. Transmission electron microscopic investigations have shown that the particle size of the as-prepared powder is in the range 20–40 nm. The agglomerate size distribution of the as-prepared powder was studied using atomic force microscopy. The nanoparticles of barium titanate were sintered to 97% of the theoretical density at a temperature of 1350 °C for 3 h. The microstructure of the sintered surface was examined using scanning electron microscopy. The dielectric constant and loss factor of the sintered pellets at 1 MHz measured at room temperature were 1223 and 3.5 × 10^? 3 respectively.     

Double-layer microwave absorber based on nanocrystalline Zn0.5Ni0.5Fe2O4/α-Fe microfibers

The microwave absorption properties of the nanocrystalline NiZn ferrite (Zn_0.5Ni_0.5Fe₂O₄) and iron (α-Fe) microfibers with single-layer and double-layer structures were investigated in the frequency range of 2–18 GHz. The double-layer absorbers have much better microwave absorption properties than the single-layer absorbers, and the microwave absorption properties of the double-layer structure are influenced by the coupling interactions between the absorbing layer and matching layer. With the absorbing layer thickness 0.7 mm of α-Fe microfibers–wax composite and the matching layer thickness 1.5 mm of Zn_0.5Ni_0.5Fe₂O₄ microfibers–wax composite, the minimum reflection loss (RL) reaches about −71 dB at 16.2 GHz and the absorption band width is about 9.2 GHz ranging from 8.8 to 18 GHz with the RL value exceeding −10 dB. While, when the absorbing layer is the Zn_0.5Ni_0.5Fe₂O₄ microfibers–wax composite with thickness 1.8 mm and the matching layer is the α-Fe microfibers–wax composite with thickness 0.2 mm, the RL value achieves the minimum about −73 dB at 13.8 GHz and the absorption band width is about 10.2 GHz ranging from 7.8 to 18 GHz with the RL value exceeding −10 dB, which covers the whole X-band (8.2–12.4 GHz) and K_u-band (12.4–18 GHz).     

Microwave dielectric ceramics in (Ca1− xBax)(Zn1 / 3Nb2 / 3)O3 system

(Ca_1−xBa_x)(Zn_1 / 3Nb_2 / 3)O₃ (x = 0–1.0) microwave dielectric ceramics were prepared and investigated. The Ba(Zn_1 / 3Nb_2 / 3)O₃-based solid solution was observed for x = 0.9, and the compositions with x = 0.1–0.7 resulted in the mixture of two phases. Dielectric constant ε_r and temperature coefficient of resonant frequency τ_f of the present ceramics varied anomalously and reached their maximum at x = 0.7–0.9, and these phenomena were originated from the partial substitution of small Ca²⁺ ions for larger Ba²⁺ at A-site. On the other hand, a good combination of microwave dielectric properties (ε_r = 36, Qf = 16,170 GHz, τ_f = − 12 ppm/°C) were obtained at x = 0.1, while the decreased Qf value was observed in other compositions.     

The thermal stability of high-energy ball-milled nanostructured Cu

The thermal stability of nanostructured (NS) Cu prepared by high-energy ball milling was investigated. The as-prepared samples were isothermal annealed for 1 h in the temperature range of 200–1000 °C. Effects of annealing on NS Cu samples were studied by means of Vickers hardness test, differential scanning calorimetry (DSC) and stress relaxation test. The exceptional high microhardness of as-prepared Cu sample of 1.7 GPa was not detected to decrease after annealing at 500 °C for 1 h with corresponding small value of activation volumes V^* of 22.6b³ and high value of strain rate sensitivity m of 0.0176. A prominent decrease of microhardness was detected after higher temperature annealing with a rapidly increase of activation volume and decrease of strain rate sensitivity. The present investigation demonstrates that the thermal stability of NS Cu prepared by high-energy ball milling is determined by not only the grain size but also the microstructure of grain boundaries, and during annealing process, the strain release process occurred prior to the grain growth process, therefore, the NS Cu has a relatively high thermal stability.     

Enhanced densification and microwave dielectric properties of Mg2TiO4 ceramics added with CeO2 nanoparticles

We report the study of the effects of processing parameters and additive concentration on the structure, microstructure and microwave dielectric properties of MTO–CeO₂ (x wt.%) ceramics with x = 0, 0.5, 1.0 and 1.5 prepared by solid-state reaction method by adding CeO₂ nanoparticles as a sintering aid. The pure Mg₂TiO₄ ceramics were not densifiable below 1450 °C. However, when CeO₂ nanoparticles were added to MTO, the densification achieved at 1300 °C along with the increase in average grain size with the uniform microstructure and improved microwave dielectric properties. This is mainly driven by the large surface energy of CeO₂ nanoparticles and their defect energy during the sintering process. While the addition of CeO₂ nanoparticles in MTO ceramics does not change the dielectric constant (?_r), the unloaded quality factor (Q_u) was altered significantly. MTO–CeO₂ (1.5 wt.%) ceramics sintered at 1300 °C exhibit superior microwave dielectric properties (?_r ～ 14.6, Q × f₀ ～ 167 THz), as compared to the pure Mg₂TiO₄ ceramics. The observed results are correlated to the enhancement in density and the development of uniform microstructure with the enhanced grain size.     

Cryogenic low noise 2.2–3 GHz amplifier

A simple design of one stage, low power cryogenic amplifier at 2.2–3 GHz range is presented. The design was numerically simulated by freely available microwave library SuperMix. The amplifier constructed according to the numerically optimized design was measured in cryogen-free refrigerator. The measured noise temperature T_N ≈ 6 K and gain G ≈ 15.5 dB are in good agreement with numerical simulations.     

Novel Ag@TiO2 nanocomposite synthesized by electrochemically active biofilm for nonenzymatic hydrogen peroxide sensor

A novel nonenzymatic sensor for H₂O₂ was developed based on an Ag@TiO₂ nanocomposite synthesized using a simple and cost effective approach with an electrochemically active biofilm. The optical, structural, morphological and electrochemical properties of the as-prepared Ag@TiO₂ nanocomposite were examined by UV–vis spectroscopy, X-ray diffraction, transmission electron microscopy and cyclic voltammetry (CV). The Ag@TiO₂ nanocomposite was fabricated on a glassy carbon electrode (GCE) and their electrochemical performance was analyzed by CV, differential pulse voltammetry and electrochemical impedance spectroscopy. The Ag@TiO₂ nanocomposite modified GCE (Ag@TiO₂/GCE) displayed excellent performance towards H₂O₂ sensing at ? 0.73 V in the linear response range from 0.83 μM to 43.3 μM, within a detection limit and sensitivity of 0.83 μM and ~ 65.2328 ± 0.01 μAμM^? 1 cm^? 2, respectively. In addition, Ag@TiO₂/GCE exhibited good operational reproducibility and long term stability.     

Crystal structure and electrical properties of CaxWO3 (0.01 ≤ x ≤ 0.15) prepared by hybrid microwave synthesis

Calcium tungsten bronzes Ca_xWO₃ (0.01 ≤ x ≤ 0.15) were synthesized by hybrid microwave method from mixtures of CaO, WO₃ and tungsten powder. Single-phased samples can be obtained by microwave heating within 40 min. With the increase of calcium content, the crystal structure of Ca_xWO₃ transforms from orthorhombic (0.01 ≤ x ≤ 0.02) to tetragonal (0.03 ≤ x ≤ 0.11) and then to cubic (0.12 ≤ x ≤ 0.15). The average size of crystallites is in the range 1–5 μm. All samples show semiconductor behaviour in their temperature dependence of resistivity. The electrical conduction mechanism changes from variable-range hopping to the thermally activated mechanism when x > 0.12.     

Evidence for microwave-induced recrystallization in NiZn ferrites

A series of ferrites was prepared using the microwave method, starting with hematite as precursor in the Ni_1−xZn_xFe₂O₄ (x = 0–1) system. After synthesis, the NiZn ferrite samples were irradiated with a microwave field for 90 s. X-ray diffraction (XRD) measurements were performed to yield the lattice constant as function of the amount x of Zn substitution, before and after irradiation. The lattice parameter was found to increase much slower with increasing x for the microwave irradiated specimens. Mössbauer spectroscopy was performed at room temperature and spectra were analyzed using the binomial distribution. For x = 0.4, 0.5 and 0.6 the resonance lines are much sharper for the irradiated samples and indicate that a microwave-induced recrystallization of the NiZn ferrites occurs in the composition range specified.