Study of the growth process of magnetic nanoparticles obtained via the non-aqueous sol–gel method

The growth process of iron oxide nanoparticles during synthesis via the non-aqueous sol–gel method has been analyzed. Samples obtained after reaching reaction temperature T _R = 200 °C and during the following 23 h of synthesis were characterized in detail by X-ray diffraction measurements, transmission electron microscopy, X-ray photoelectron spectroscopy, thermogravimetric analysis, Fourier transform infrared spectroscopy, Mössbauer spectroscopy, and magnetization measurements. The results show that the iron oxide nanocrystals are majorly composed of maghemite and grow in a faceted shape. The particle size increases gradually during the synthesis, and the structural and magnetic properties improve primarily during the first 6 h of reaction.     

Structural and luminescent properties of Eu3+-doped GdSrAl3O7 nanophosphor

Eu³⁺-doped GdSrAl₃O₇ nanophosphor with promising luminescent properties has been synthesized by low-temperature solution combustion synthesis. The structural properties examined by X-ray diffractometer, scanning electron microscopy, and transmission electron microscopy showed that pure tetragonal GdSrAl₃O₇: Eu³⁺ red nanophosphor having narrow size distribution in 50–55 nm range could be readily obtained at low temperature 550 °C. The photoluminescent excitation and emission spectra, life time, and concentration effect were studied in detail. Under excitation at 266 nm, Eu³⁺-doped GdSrAl₃O₇ nanophosphor revealed weak green emission and strong red emission attributed to ⁵D₁ → ⁷F_1–2 and ⁵D₀ → ⁷F_0–3 transitions of Eu³⁺ ion, respectively in the region of 525–700 nm. The red emission from ⁵D₀ → ⁷F₂ transition at 616 nm exhibits the highest intensity under the optimized concentration of 10 mol% after which the quenching mechanism became relevant. Quenching behavior of the europium in the GdSrAl₃O₇ host was explained by nonradiative cross-relaxation phenomenon. Moreover, Eu³⁺-doped GdSrAl₃O₇ nanophosphor can generate light from orange to deeper red by properly tuning the concentration of europium ions based on the energy transfer principle.     

Fabrication and characterization of superparamagnetic nanocomposites based on epoxy resin and surface-modified γ-Fe2O3 by epoxide functionalization

In this study, the effect of modified epoxide-terminated γ-Fe₂O₃ on the magnetic, mechanical, and thermal properties of epoxy nanocomposite was investigated. The γ-Fe₂O₃ nanoparticles were prepared via a wet chemical approach, surface modified with 3-glycidoxypropyltrimethoxysilane (GPTMS), and characterized by particle size analyzer, XRD, FT-IR, and TGA techniques. The catalytic effect of γ-Fe₂O₃ on the cure reaction temperature of epoxy/triethylenetetramine (TETA) was determined by differential scanning calorimeter (DSC). The glass transition temperature (T _g) of nanocomposite containing 5 wt% modified γ-Fe₂O₃ increased slightly (12 °C), while the initial decomposition temperature (T _ID) did not show improvement. Transmission electron microscopy (TEM) showed improvement in dispersion of surface-modified γ-Fe₂O₃ nanoparticles in the resin matrix. The effect of interfacial bonding between modified γ-Fe₂O₃ and epoxy resin, via crosslink reactions, on the mechanical properties of nanocomposite such as flexural and tensile strength was studied, and the fractured surface of samples was investigated by scanning electron microscopy (SEM). Comparing with the mechanical properties of neat epoxy resin, tensile, and flexural strength of 10 wt% modified γ-Fe₂O₃/epoxy nanocomposite increased 20 and 19 %, respectively, while tensile and flexural strength of 10 wt% unmodified/epoxy nanocomposite decreased slightly. The saturation magnetization (M _s) of 5 wt% modified γ-Fe₂O₃/epoxy nanocomposites with superparamagnetic property was approximately 80 % greater than that of unmodified γ-Fe₂O₃/epoxy nanocomposites.     

Screened remote plasma-enhanced atomic vapor deposition of Sb–Te thin film for the improvement of trench-covering ability

A new screened remote plasma-enhanced atomic vapor deposition (SPEAVD) technique was studied for depositing Sb–Te phase-change materials inside trench structures with high aspect ratios. The theoretical model of the screening mechanism included the concepts of a plasma sheath and a sticking coefficient. Plasma’s limitation of poor step coverage was overcome by filtering its high-energy ions and electrons. Cross-sectional observation was conducted using field emission scanning electron microscopy (SEM). The trench-covering ability of SPEAVD was much better than that of direct remote PEAVD under broad chamber conditions. Surface morphology was observed by SEM and atomic force microscopy. The surface chemical state of the deposited film was measured by X-ray photoelectron spectroscopy. The crystallinity in various deposition temperatures was analyzed by X-ray diffraction measurements. The electrical characteristics of the films were measured by a four-point probe. We expect this research to provide a new deposition method that will allow fine control of step coverage and other characteristics.     

The influence of triangular silver nanoplates on antimicrobial activity and color of cotton fabrics pretreated with chitosan

The effect of cotton fabric pretreatment with biopolymer chitosan (CHT) on deposition of colloidal triangular silver nanoplates was studied. Also, the influence of deposited silver nanoparticles on color and antimicrobial activity of cotton fabrics was evaluated. Characterization of colloidal silver nanoparticles as well as silver nanoparticles deposited on cotton fabrics was performed using electron microscopy (TEM and FESEM), XRD analysis, atomic absorption spectroscopy, UV–Vis absorption, and reflectance spectroscopy. The cotton fabric turned from white to blue color upon deposition of triangular silver nanoplates. Antimicrobial activity of CHT pretreated cotton fabric impregnated with silver nanoparticles was tested against Gram-negative bacteria Escherichia coli, Gram-positive bacteria Staphylococcus aureus, and fungi Candida albicans. Deposited silver nanoparticles imparted excellent antimicrobial properties to cotton fabric. The standard sterilization procedure of cotton fabric for antimicrobial activity testing resulted in color change of the fabric from blue to yellow. This color change is most likely consequence of transformation of triangular silver nanoplates into nanodiscs and/or their agglomeration into spheroids.     

Synthesis and characterization of novel PtSe2/graphene nanocomposites and its visible light driven catalytic properties

In this study, we report a novel PtSe₂/graphene nanocomposite by facile ultrasonic-assisted techniques. The “as-prepared” nanocomposites were further characterized by various techniques such as X-ray diffraction, scanning electron microscopy with an energy dispersive X-ray analysis, transmission electron microscopy, UV–Vis absorbance spectra analysis, diffuse reflectance analysis, and Raman spectroscopic analysis. The photocatalytic activities of the composites were investigated by the degradation of rhudamine B and methylene blue as a standard dyes. The photodegradation rates of organic dyes by the nanocomposites are found to be markedly high. This study suggests that the as-prepared PtSe₂/graphene composite can be utilized as highly efficient photocatalyst materials that employed visible light as an energy source.     

Critical conditions for the occurrence of quench cracking in an Al–Zn–Mg–Cu alloy

The occurrence of quench cracking in small cuboidal samples of aluminium alloy AA7150 was determined to be related to the maximum temperature difference (?T _max) between various locations within samples during quenching. When ?T _max between different locations is between 96 and 124 °C, there is some risk of quench cracking under various quenching conditions. When the ?T _max value is higher than 124 °C, quench cracks cannot be avoided. Quench cracks preferentially occur at sample corners and edges and are preferentially propagating in the short transverse direction–long transverse direction plane. Finite element modelling results indirectly indicate that the quench cracking should occur at the very early stages of the quenching process. Microscopy reveals that the quench cracking mode is intergranular, and cracks preferentially occur at high-angle grain boundaries with an average misorientation angle of ~42°. Moreover, quench cracks can penetrate through the whole thickness of a sample quenched from 495 into 20 °C water. Fractography reveals that no constituent particles exist in the quench fracture region, indicating that, unlike impact fracture, the occurrence of quench cracks is not dependent on the presence of coarse particles.     

Synthesis and characterization of SiO2–La2O3 gels obtained in a water-free environment

SiO₂–La₂O₃ binary oxide system was prepared by the sol–gel method without addition of strong polar solvents (e.g. water, alcohol) to the initial system. Changes in the surface and gel network structure were described as a function of the type and concentration of the second component that is lanthanum oxide. A series of samples with Si to La molar ratios of 1:0.1, 1:0.25, 1:0.5, and 1:1 were characterised by XRD, TG–TA, FTIR, and low-temperature nitrogen adsorption measurements. Results of the investigation showed that higher amounts of the lanthanum component in the binary oxide gel system led to a completely changed morphology of the porous structure of the surface gels obtained in a non-aqueous medium.     

Photocatalytic oxidation of butane by titania after reductive annealing

Highly photoactive anatase and rutile modifications of titania were prepared by annealing the product of thermal hydrolysis of titania peroxo-complexes under different atmosphere (hydrogen, nitrogen, and oxygen) at 950 °C. The prepared samples were characterized by X-ray powder diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, electron spin resonance, scanning electron microscopy, and Brunauer–Emmett–Teller surface area and porosity measurement. The individual samples differ with phase composition (ratio of anatase and rutile) and color that determines the rate of reduction of TiO₂ and is for reduced form of TiO₂ characterized. The UV/Vis diffuse reflectance spectroscopy was employed to estimate band-gap energies. Annealed samples were deposited as a 300 μm thick layer on a glass plate (10 cm × 15 cm) for assessment of the kinetics of a photocatalytic degradation of butane in a gas phase.     

Influence of TiN-nanolayered insertions on microstructure and mechanical properties of TiSiN nanocomposite film

TiN nanolayers with different thicknesses were inserted in TiSiN nanocomposite film by magnetron-sputtering technique. The influences of TiN insertion nanolayers with different thicknesses on microstructure and mechanical properties of TiSiN film were investigated X-ray diffraction, high-resolution transmission electron microscopy, scanning electron microscopy, and nanoindentation techniques. When the TiN insertion layer thickness is <0.5 nm, TiN nanolayers can coordinate the misorientations between TiN nanocrystallites in adjacent TiSiN layers, leading to the transformation from the nanocomposite structure with TiN nanocrystallites encapsulated by SiN _x interfacial phase into columnar crystal structure, and disappearance of the strengthening effect from the nanocomposite structure. When the TiN insertion layer thickness increases to 1.0 nm, the film is strengthened with the epitaxial growth structures between TiSiN and TiN layers. As the TiN insertion layers further thicken, the hardness and elastic modulus evidently decrease, which can be attributed to the breakage of epitaxial growth structures between TiSiN and TiN layers.     

Fullerene-modified polyamide 6 by in situ anionic polymerization in the presence of PCBM

Activated anionic ring-opening polymerization of ε-caprolactam (ECL) was carried out for the first time in the presence of [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) to prepare polyamide 6 (PA6)-based composites comprising up to 3 wt% of this fullerene derivative. This in situ polymerization process produced high molecular weight composites containing 52–80 % of gel fraction at PCBM concentration ≥0.5 wt%. Spectral, thermo-mechanical, synchrotron X-ray, and scanning electron microscopy data were used to elucidate the structure and morphology of the PA6/PCBM composites. A mechanism of the chemical structure evolution was proposed starting with incipient complexation between ECL and PCBM, via subsequent chemical linking of ECL moieties on the C₆₀ spheroid and final formation of star-burst and cross-linked morphologies. PCBM amounts of 0.1 wt% and more decreased the volume resistivity from 10¹² Ω cm (neat PA6) to 10⁹–10⁷ Ω cm, thus opening the way for new applications of anionic PA6.     

Structural,dielectric, magnetic,and optical properties of Ni0.75Zn0.25Fe2O4–BiFeO3 composites

The magnetoelectric composites of spinel ferrite Ni_0.75Zn_0.25Fe₂O₄ (NZF) and BiFeO₃ (BFO) with general formula xNi_0.75Zn_0.25Fe₂O₄ + (1 ? x)BiFeO₃ (x = 0, 0.10, 0.20, and 0.30) have been prepared via hybrid processing route. Subsequently, the effects of addition of NZF on structural, dielectric, magnetic, magnetoelectric, and optical properties of BFO have been investigated, and significant enhancements have been observed in physical observables such as grain size, dielectric constant, magnetization, and polarization in ferroelectric hysteresis loops. The variation of magnetization with temperature indicates the presence of spin glass behavior along with the ferromagnetic component. The magnetoelectric coupling is found with a value of magnetocapacitance to be 4.6 % for 30 mol% addition of NZF. The optical properties of the composites are also studied using UV–Vis diffuse reflectance spectroscopy, Photoluminescence spectroscopy, and FTIR spectroscopy.     

Plastic deformation behavior of ultrafine-grained Al–Mg–Sc alloy

Ultrafine-grained (UFG) Al–Mg–Sc alloy was obtained by friction stir processing. The UFG alloy was subjected to uniaxial tensile testing to study the tensile deformation behavior of the alloy. An inhomogeneous yielding (Lüdering phenomenon) was observed in the stress–strain curves of UFG alloy. This deformation behavior was absent in the coarse-grained alloy. The Lüdering phenomenon in UFG alloy was attributed to the lack of dislocations in UFG microstructure. A strong dependence of uniform ductility on the average grain size was exhibited by the UFG alloy. Below a critical grain size (0.5 μm), ductility was very limited. Also, with the decrease in grain size, most of the plastic deformation was observed to be localized in necked region of the tensile samples. The negative strain rate sensitivity (SRS) observed for the UFG alloy was opposite of the SRS values reported for UFG alloys in the literature. Based on activation volume measurement, grain boundary mediated dislocation-based plasticity was concluded to be the micro-mechanism operative during plastic deformation of UFG Al–Mg–Sc alloy.     

Thermoelectric performance of tellurium and sulfur double-substituted skutterudite materials

Two series of skutterudite materials, Co₄Sb_11.3Te_0.7?x S _x (x = 0.07–0.2) and Co₄Sb_12?x S _x (x = 0.07–0.15), were synthesized through solid state reaction and consolidated by spark plasma sintering. The samples were characterized by powder X-ray diffraction, electron probe analysis, and measurements of electrical conductivity, Hall coefficient, Seebeck coefficient, and thermal conductivity. The results indicate that sulfur in Co₄Sb_12?x S _x most likely forms the CoSbS compound and is unlikely to get into the CoSb₃ lattice, while it can dissolve in Co₄Sb_12?x Te _x compounds due to the radius compensation when fabricated by the methods in this study. The lattice thermal conductivity decreases from 2.07 Wm^?1 K^?1 for tellurium single-doped Co₄Sb_11.3Te_0.7 to 1.46–1.67 Wm^?1 K^?1 for Co₄Sb_11.3Te_0.7?x S _x (x = 0.07–0.20) at 800 K. The thermoelectric performance is significantly enhanced by tellurium–sulfur co-doping in Co₄Sb_11.3Te_0.7?x S _x compounds, and a peak dimensionless figure of merit ~1.1 is achieved in Co₄Sb_11.3Te_0.63S_0.07. The enhancement is mainly attributed to the great reduction of the lattice thermal conductivity due to the increased phonon scattering by the sulfur defect.     

Low-temperature preparation of transparent conductive Al-doped ZnO thin films by a novel sol–gel method

We developed a novel sol–gel method to prepare transparent conductive Al-doped ZnO (AZO) thin film at low temperature. The AZO nanocrystals were prepared by a solvothermal method and then they were dispersed in the monoethanolamine and methanol to form AZO colloids. A (002)-oriented ZnO thin film was used as a nucleation layer to induce the (002)-oriented growth of AZO thin films. The AZO thin films were prepared on Si(100) and fused quartz glass substrates with the (002)-oriented ZnO nucleation layer and annealed at 400 °C for 60 min. All AZO thin films showed (002) orientation. For electrical and optical measurements, the films deposited on glass substrates were post-annealed at 400 °C for 30 min in forming gas (100 % H₂) to improve their conductivity. These samples had high transparency in the visible wavelength range, and also showed good conductivity. A 0.2 mol L^?1 AZO solution with 3 at.% Al content was heated in a Teflon autoclave at 160 °C for 30 min to form AZO nanocrystals, and then the AZO nanocrystals were suspended in the MEA and methanol to obtain the stable AZO colloid. The Al content in the AZO nanocrystals was 2.7 at.%, and the high Al doping coefficient was mainly attributed to the formation of AZO nanocrystals in the autoclave. The AZO thin film using this colloid had the lowest resistivity of 3.89 × 10^?3 Ω cm due to its high carrier concentration of 3.29 × 10²⁰ cm^?3.     

Solution of regular second- and fourth-order Sturm–Liouville problems by exact dynamic stiffness method analogy

This paper treats all regular second- or fourth-order Sturm–Liouville (SL) problems as generalised vibration problems of non-uniform structural members on elastic foundations. Hence such SL problems can be solved by the Wittrick–Williams (WW) algorithm and the authors’ recursive second-order exact dynamic stiffness vibration method. The coefficients of the mathematical SL problems range more widely than for structural vibration problems, and so the method must account for additional possibilities, e.g. the equivalent of large negative stiffness continuous elastic supports. The method computes exact dynamic stiffnesses and their derivatives with respect to the eigenparameter accurately by solving the associated linear boundary-value problems using a standard adaptive solver. The difficulty of calculating the number of exact fixed-end eigenvalues below any trial eigenvalue needed by the WW algorithm is overcome by dividing the whole SL problem domain into an appropriate mesh, in which each element is guaranteed to have all fixed-end eigenvalues above the current trial eigenvalue. Results for second- and fourth-order SL problems shown in the literature to be particularly challenging demonstrate the effectiveness, efficiency, accuracy and reliability of the proposed method.     

Current Sensing Noise Thermometry: A Fast Practical Solution to Low Temperature Measurement

We describe the design and performance of a series of fast, precise current sensing noise thermometers. The thermometers have been fabricated with a range of resistances from 1.290  $\Omega $ down to 0.2 m $\Omega $ . This results in either a thermometer that has been optimised for speed, taking advantage of the improvements in superconducting quantum interference device noise and bandwidth, or a thermometer optimised for ultra-low temperature measurement, minimising the system noise temperature. With a single temperature calibration point, we show that noise thermometers can be used for accurate measurements over a wide range of temperatures below 4 K. Comparisons with a melting curve thermometer, a calibrated germanium thermometer and a pulsed platinum nuclear magnetic resonance thermometer are presented. For the 1.290  $\Omega $ resistance we measure a 1 % precision in just 100 ms, and have shown this to be independent of temperature.     

Onset conditions for a Rayleigh–Taylor instability with step function density profiles

The stability of several density profiles constructed from a linear combination of step functions in a vertically orientated two-dimensional porous medium are considered. A quasi-steady-state approximation is made so that the initial stability of the system can be approximated. Using a similar approach to that of Tan and Homsy (Phys Fluids 29:3549–3556, 1986) a dispersion equation for each density profile is obtained analytically at time zero. Neutral stability curves are then obtained to allow the regions of the parameter space to be divided into stable and unstable regions.     

Ca2SiO4:Ln (Ln = Ce3+, Eu2+, Sm3+) tricolor emission phosphors and their application for near-UV white light-emitting diode

Tricolor emission Ca₂SiO₄:Ln (Ln = Ce³⁺, Eu²⁺, Sm³⁺) phosphors were synthesized by the conventional solid-state reaction method, and their photoluminescence properties were investigated. Ce³⁺-, Eu²⁺-, or Sm³⁺-doped Ca₂SiO₄ phosphors showed typical blue, green, or red luminescence in the CIE1931 chromaticity diagram, respectively. In addition, the luminescence efficiency of the tricolor emission Ca₂SiO₄:Ln (Ln = Ce³⁺, Eu²⁺, Sm³⁺) phosphors was evaluated. A series of white light-emitting diode (LED) prototypes were fabricated by combining near-UV LED chip and the as-prepared tricolor emission phosphors with various ratios in weight. White LED prototypes with tunable correlated color temperature and color-rendering index values were realized by controlling the amount of phosphors. The presented results indicated the potential application of Ca₂SiO₄:Ln (Ln = Ce³⁺, Eu²⁺, Sm³⁺) phosphors in near-UV white LED.     

On thermal resistance in concentric residential geothermal heat exchangers

Residential geothermal ground-source heat pumps have been used for nearly 30 years as a low-cost, environmentally friendly alternative to traditional fossil-fuel systems. However, the limitation on a wider range of acceptance of the technology is the cost of the installation of a piping network through which the energy is transferred between the soil and the coolant. This cost is proportional to the piping length. We formulate a new mathematical modeling framework that calculates a characteristic streamwise length based on the geometry of the system, the operating conditions, and the material properties of the system materials and effective properties of the surrounding soil using a vertical concentric geothermal heat exchanger as an example. These concentric systems consist of a core flow (from the residence), which flows from the ground surface to the base of the well, and an annular return region in which the heat exchange between the fluid and the soil is expected to take place. Two modeling scenarios are considered: steady-state temperature profiles in the annular fluid region if the radial thermal resistance between the fluid and soil is fixed; a quasi-steady fluid temperature that captures the radial heat transfer from the fluid to the soil. For the first case, we find that the characteristic length is determined by the smallest eigenvalue of the separable thermal problem, where the velocity profile is laminar and there is no thermal transport between the core and the fluid. When this core-annular heat transfer is possible, the eigenvalue problem no longer satisfies the conditions for Sturm–Liouville theory, and through direct computation we find that energy transferred from the annular flow to the core reduces the temperature change. In the second case, we find that the temperature change is reduced over time, as the soil temperature near the exchanger responds to the energy transport. In both cases, the best thermal transport takes place when the annular gap is small. The impact of these results on system design considerations is discussed.