High-yield synthesis of single-crystal short Eu2O3 nanorods through a facile sol–gel template approach

High-yield Eu₂O₃ short nanorods have been prepared by a facile sol-gel method with polystyrene/polyelectrolyte (PS/PE) microreactor as template in an aqueous solution of europium nitrate in the presence of ammonia and urea. The properties of Eu₂O₃ nanorods were characterized by powder X-ray diffraction, thermogravimetric analysis, transmission electron microscopy (TEM), high-resolution transmission electron microscopy, field emission scanning electron microscopy (FESEM), and photoluminescence spectroscopy. The particle sizes measured from TEM and FESEM are about 200 nm×500 nm (W×L). A possible mechanism for the formation of such high-yield oxide nanorods is discussed.     

Phase composition,microstructure and conductivity of (85-α)VO2–15(Vanadium–Phosphate–Glass)–αCu glass–ceramics

Electric conductivity, microstructure and phase composition of (85-α)VO₂–15VPG–αCu glass–ceramics (VPG = Vanadium–Phosphate–Glass) with copper content in the interval 0 wt% ⩽ α ⩽ 15 wt% were investigated. VPG is the glass (molar %) 80V₂O₅–20P₂O₅. Only two phases: VO₂ and VPG were identified when α ⩽ 5 wt%. VO₂ crystallites, VPG and pores are observed in these ceramic microstructures. Glass forms layers 1–2 μm thick in the space between VO₂ crystallites. The copper is dissolved in VPG during glass–ceramic synthesis. It increases the electric conductivity of the glass and provides improvement of electrical bonds between VO₂ crystallites. Therefore glass–ceramics conductivity exhibits an abrupt change of approximately 100× in the vicinity of the phase transition temperature, T_t, in VO₂. A new crystalline phase appears in (85-α)VO₂–15VPG–αCu ceramics when α ∼ 6 wt%. This phase is observed as small crystallites with the sizes of 1–5 μm. The increase in such phase content, with an increase in copper content is accompanied by a decrease in the content of VO₂. Percolation along the new phase is a primary contributor to electric conductivity when α ⩾ 8 wt%. In this case the conductivity exhibits no abrupt change in the vicinity of the temperature T_t. The new phase is probably the bronze Cu_xV₂O₄. It crystallizes from a liquid phase during glass–ceramics synthesis.     

Bulk-quantity SnO2 nanorods synthesized from simple calcining process based on annealing precursor powders

Tin dioxide (SnO₂) nanorods have been successfully synthesized in bulk quantity by a calcining process based on annealing precursor powders in which sodium chloride, sodium carbonate, and stannic chloride were homogeneously mixed. Transmission electron microscopy shows that the as-prepared nanorods are structurally perfect and uniform, with widths of 10–25 nm, and lengths of several hundreds nanometers to a few micrometers. X-ray diffraction and energy-dispersive X-ray spectroscopy analysis indicate that the as-prepared nanorods have the same crystal structure and chemical composition found in the tetragonal rutile form of SnO₂. Selected area electron diffraction and high-resolution transmission electron microscopy reveal that the as-prepared nanorods grow along the [1 1 0] crystal direction. We found that the calcined temperature has a strong influence on the size and morphology of SnO₂ nanorods. The growth process of SnO₂ nanorods is suggested to follow an Ostwald ripening mechanism. Our findings indicate that other nanorods or nanowires may be manipulated by using this technique, and might provide insight into the new opportunities to control materials fabrication.     

Conductivity stabilization by metal and oxide additives in ceramics on the basis of VO2 and glass V2O5–P2O5

Conductivity behavior during the temperature cycling through the phase transition temperature of VO₂ (T_t = 68 °C) was investigated in glass-ceramics based on VO₂ and vanadium phosphate glass (VPG) for compositions without and with Cu and SnO₂ additives. Copper and SnO₂ additives stabilize the conductivity of glass-ceramics at temperature cycling. For ceramics (wt%) (80 − y)VO₂–5Cu–15VPG–ySnO₂ the best stabilizing effect takes place when SnO₂ content is in the interval 35 wt% < y < 50 wt%. Ceramics with such SnO₂ content keeps a stable value of the conductivity change (∼10²) in the vicinity of VO₂ phase transition temperature and shows the conductivity decrease no more than of 2.5 times after 3000 thermal cycles. The reasons of conductivity stabilizing in VO₂-based glass-ceramics with additives of Cu and SnO₂ are being discussed. The analysis resting on the percolation theory has shown the increase of conductivity stability in VO₂-based glass-ceramics when the VO₂ volume fraction and the average size of VO₂ crystallites decrease and the ceramics surface tension increases.     

The effect of cooling rate during hydrothermal synthesis of ZnO nanorods

The hydrothermal method was employed in order to obtain zinc oxide nanorods directly on Si/SiO₂/Ti/Zn substrates forming brush-like layers. In the final stages of synthesis, the reaction vessel was naturally cooled or submitted to a quenching process. X-ray diffraction results showed that all the nanostructures grew [0 0 0 1] oriented perpendicular to the substrate. The influence of the cooling process over the morphology and dimensions of the nanorods was studied by scanning electron microscopy. High-resolution transmission electron microscopy images of the quenched samples showed that the zinc oxide (ZnO) crystal surfaces exhibit a thin-layered coating surrounding the crystal with a high degree of defects, as confirmed by Raman spectroscopy results. Photodetectors made from these samples exhibited enhanced UV photoresponses when compared to the ones based on naturally cooled nanorods.     

Characterization of deep levels in a-plane GaN epi-layers grown using various growth techniques

To study the correlation between defects and deep levels in a-plane GaN films grown on r-plane sapphire substrates, transmission electron microscopy (TEM) and deep level transient spectroscopy (DLTS) have been performed on three types of a-plane GaN samples grown using modified two-step growth (sample I), SiN_x interlayer (sample II), and patterned insulator on sapphire substrate (sample III). From the microstructure evolution in cross-sectional TEM images, it was shown that combination of growth techniques is highly efficient in the reduction of dislocation densities. Average dislocation densities of samples I, II, and III were about 2.2×10⁹ cm^?2, 1.1×10⁹ cm^?2, and 3.4×10⁸ cm^?2, respectively. The trap a_t E_c–E_t～0.13 eV (E1) was observed in only sample I, and three electron traps at 0.28–0.33 eV (E2), 0.52–0.58 eV (E3), and 0.89–0.95 eV (E4) from the conduction band edge were measured common to all the samples. The analysis of trap properties indicated that E2 and E3 trap levels are strongly associated with the partial dislocations in a-plane GaN films.     

Preparation and the third-order optical nonlinearities of the sodium borosilicate glass doped with Cu7.2S4 quantum dots

The sodium borosilicate glass doped with Cu_7.2S₄ quantum dots was prepared by using both sol–gel and atmosphere control methods. The formation mechanism and the microstructure of the glass were examined using differential thermal analysis and thermal gravimeter (TG-DTA), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), energy dispersive X-ray spectra (EDX), high-resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED). The results revealed that Cu_7.2S₄ quantum dots in orthorhombic crystal system had formed in the glass, and the size ranged from 9 nm to 21 nm. In addition, Z-scan technique was used to measure the third-order optical nonlinearities of the glass. The results indicated that the third-order optical nonlinear refractive index γ, the absorption coefficient β, and the susceptibility χ⁽³⁾ of the glass were 1.11 × 10^? 15 m²/W, 8.91 × 10^? 9 m/W, and 6.91 × 10^? 10 esu, respectively.     

Spectral investigations on VO2+ ion doped in CaO–SrO–Na2O–B2O3 glass systems

Glasses of the (20 ? x)CaO–xSrO–(20 ? y)Na₂O–60B₂O₃ ? y (CSNB) system with (5 ≤ x ≤ 15) mol% and y = 0.1 mol% of V₂O₅ were characterized by X-ray diffraction (XRD), EPR (Electron Paramagnetic Resonance), Optical absorption Spectra and FT-IR (Fourier transform Infrared Spectroscopy) studies. EPR spectra of all the glass samples exhibit resonance signals characterstic of VO²⁺ ions. The values of spin-Hamiltonian parameters indicate that the VO²⁺ ions in CSNB glasses were present in octahedral sites with tetragonal compression and belong to C_4v symmetry. Spin-Hamiltonian parameters ‘g’ and ‘A’ were evaluated. The Optical band energy (E_opt) and Urbach energy (ΔE) were calculated from their ultra violet edges. By correlating EPR and Optical data the molecular orbital coefficients have been evaluated. IR spectra of these glasses were analyzed in order to identify the contribution of each component to the local structure that determines the physical properties of these glasses.     

The preferential orientation of the directionally solidified Si–TaSi2 eutectic in situ composite

The Si–TaSi₂ eutectic in situ composite is a favorable field emission material due to relatively low work function, good electron conductivity, and three-dimensional array of Schottky junctions grown in the composite spontaneously. The preferential orientation during directional solidification is determined by the growth anisotropy. In order to obtain the preferential direction of the steady-state crystal growth, the transmission electron microscopy (TEM) is used for analysis. It is found that the preferential orientation of the Si-TaSi₂ eutectic in situ composite prepared by Czochralski (CZ) technique is [3 3¯ 2¯] Si∥[0 0 0 1] TaSi₂, (2 2 0)Si∥(2 2¯ 0 0) TaSi₂. Whereas the preferential orientation of the Si–TaSi₂ eutectic in situ composite prepared by electron beam floating zone melting (EBFZM) technique is [0 1¯ 1¯ ]] Si∥[0 0 0 1] TaSi₂,(0 1¯ 1) Si∥(0 1¯ 1 1)TaSi₂. The preferential directions of the Si-TaSi₂ eutectic in situ composites prepared by two kinds of crystal growth techniques are distinctly different from each other, which results from different solid–liquid interface temperatures on account of the different crystal growth conditions, e.g. different solidification rate, different temperature gradient, different solid–liquid interface curvature and different kinetic undercooling.     

Dielectric relaxation and morphologic properties of CaCu3Ti4O12 doped with GeO2

CaCu₃Ti₄O₁₂ (CCTO) is a material with giant dielectric constant, presenting good stability over a wide temperature and frequency ranges. The preparation method and doping has a great influence on the microstructure and dielectric properties of this material. In this work, doping CCTO with 2–10 wt% GeO₂ has been shown to increase the dielectric constant. We studied the prepared samples by X-ray diffraction (XRD), scanning electron microscopy (SEM) and impedance spectroscopy. X-ray diffraction shows the presence of nanocristals. Grains and grain boundaries compositions have been observed by scanning electron microscopy with energy dispersive X-ray spectrometry mapping. Impedance spectroscopy measurements, in the frequency range from 75 kHz to 30 MHz, and temperature from 250 to 325 K, have been performed. The data were analyzed using the Cole–Cole model of dielectric relaxation.     

Crystallization of NANOPERM-type amorphous alloys

In the present work, we have investigated progress of crystallization in the Fe_91?xMo₈Cu₁B_x (x = 12, 15, 17, 20) alloy as a function of annealing temperature. This material belongs to the family of the Fe–M–B–(Cu) alloys (transition metal M = Zr, Nb, Hf, Mo, …) called also NANOPERM. The alloy was found to contain small amounts (<5%) of bcc Fe(Mo) and Mo₂FeB₂ nanocrystallites (<2 nm) located on the surface of the ribbon-shaped samples already in the as-quenched state. Depending upon composition, the nanocrystallites are formed on the air and/or wheel side of the ribbon. They are characterized by atomic force microscopy, conversion electron Mössbauer spectroscopy, and by X-ray diffraction of synchrotron radiation. Fast detection (every 10 s) of the latter during continuous heating of as-quenched specimens enabled an in situ observation of the evolution of the crystallization. For x = 12, the crystallization starts earlier at the wheel side of the ribbon but its progress is more rapid at the opposite, i.e. air side.     

Microstructures of Mg–Zn–Nd alloy including small quasicrystalline grains

Two kinds of Mg-rich and low neodymium Mg–Zn–Nd alloys including icosahedral quasicrystal phase (I-phase) were prepared under conventional casting conditions. The microstructures and phases of Mg–Zn–Nd quasicrystal alloys were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and transmission electron microscopy (TEM). The results indicate that the I-phase in Mg–Zn–Nd quasicrystal alloy is a simple icosahedral quasicrystal with quasi-lattice of a_R = 0.525 nm. It has been proved that the as-cast Mg_70.8Zn₂₈Nd_1.2 quasicrystal alloy mainly consisted of I-phase and Mg₇Zn₃ matrix phase. While the as-cast Mg_70.5Zn_28.5Nd₁ alloy mainly consisted of I-phase, Mg₇Zn₃ matrix phase, dendrite α-Mg phase and a new rod-like hexagonal phase.     

Sol–gel derived organic–inorganic hybrid electrolytes for thin film electrochromic devices

Sol–gel derived, lithium ion conducting organic–inorganic hybrid electrolytes for ambient temperatures applications, have been synthesized from tetraethyl orthosilicate (TEOS), poly(ethylene oxide) (PEO), propylene carbonate (PC), propylene oxide, butyl acrylate, butyl methacrylate, ethyl acetoacetate and LiClO₄ precursors. Mass fractions of the organic additions in the gels were of ca 30 mass% for gels 0/B, F–H and 40 mass% for gel J. The colorless transparent or translucent hybrid materials obtained in this work were aged at room temperature for at least three weeks and then dried at 80 °C for 3 h. The morphology and structure of all compositions were investigated by scanning electron microscopy equipped with energy dispersive X-ray spectroscopy (SEM/EDX), Fourier transform infrared spectroscopy and ²⁹Si MAS nuclear magnetic resonance. Amorphous nature of the hybrids was confirmed by X-ray diffraction. SEM, FTIR and NMR analysis showed structural properties and [SiO₄] tetrahedrons poly-condensation process to be strongly influenced by organic additives have been employed. Room temperature ionic conductivities of the hybrid electrolytes were in a range of 9.84 × 10⁻⁴–1.56 × 10⁻³ Ω⁻¹ cm⁻¹.     

Correlation between the method of preparation and the properties of the sol–gel HfO2 thin films

This work describes the preparation of HfO₂ thin films by the sol–gel method, starting with different precursors such as hafnium ethoxide, hafnium 2,4-pentadionate and hafnium chloride. From the solution prepared as mentioned above, thin films on silicon wafer substrates have been realized by ‘dip-coating’ with a pulling out speed of 5 cm min^?1. The films densification was achieved by thermal treatment for 10 min at 100 °C and 30 min at 450 °C or 600 °C, with a heating rate of 1 °C min^?1. The structural and optical properties of the films are determined employing spectroellipsometric (SE) measurements in the visible range (0.4–0.7 μm), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The main objective of this paper was to establish a correlation between the method of preparation (precursor, annealing temperature) and the properties of the obtained films. The samples prepared from pentadionate and ethoxide precursors are homogenous and uniform in thickness. The samples prepared starting from chloride precursor are thicker and proved to be less uniform in thickness. Higher non-uniformity develops in multi-deposition films or in crystallized films. A nano-porosity is present in the quasi-amorphous films as well in the crystallized one. For the samples deposited on silicon wafer, the thermal treatment induced the formation of a SiO₂ layer at the coating–substrate interface.     

Growth mechanisms and defect structures of B12As2 epilayers grown on 4 H-SiC substrates

Epitaxial growth of icosahedral B₁₂As₂ on c-plane 4 H-SiC substrates has been analyzed. On on-axis c-plane 4 H-SiC substrates, Synchrotron white beam x-ray topography (SWBXT) revealed the presence of a homogenous solid solution of twin and matrix B₁₂As₂ epilayer domains. High resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy (STEM) both revealed the presence of an ～20 nm thick, disordered transition layer at the interface. (0003) twin boundaries are shown to possess fault vectors such as 1/3[1–100]_B12As2, which originate from the mutual shift between the nucleation sites. On the contrary, B₁₂As₂ epilayers grown on c-plane 4 H-SiC substrates intentionally misoriented from (0001) towards [1–100] is shown to be free of rotational twinning. SWBXT, HRTEM and STEM all confirmed the single crystalline nature and much higher quality of the films. In addition, no intermediate layer between the epilayer and the substrate was observed. It is proposed that the vicinal steps formed by hydrogen etching on the off-axis 4 H-SiC substrate surface before deposition cause the film to adopt a single orientation during nucleation process. This work also demonstrates that c-plane 4 H-SiC with offcut toward [1–100] is potentially a good substrate choice for the growth of high-quality, single crystalline B₁₂As₂ epilayers for future device applications.     

Structure and electrical properties of nitrided NbN–TiN sol–gel derived films

The results of investigations of electrical conductivity and the structure of NbN–TiN thin films in a different NbN/TiN molar ratio are presented in this work. Sol–gel derived xNb₂O₅?(100?x)TiO₂ coatings (where x = 100, 90, 80, 70, 60, 50, 40, 0 mol%) were nitrided at 1200 °C to obtain NbN–TiN films. The structural transformations occurring in the films as a result of ammonolysis were studied using X-ray diffraction (XRD), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The electrical conductivity was measured with a conventional four-terminal method in the temperature range of 5–280 K. The NbN–TiN samples exhibited a negative temperature coefficient of resistivity. The positive temperature coefficient of resistivity was observed only for the x = 0 sample. The results of conductivity versus temperature may be described on the grounds of a model proposed for a weakly disordered system. The film thickness effect on the superconducting properties was studied for x = 80 and x = 100 samples. The superconducting transition was not observed in all samples, the exception was x = 80 sample, 1050 nm in thickness. It is not clear, why all x = 100 samples do not exhibit superconducting transition in resistivity measurements. It seems to be possible, that the Josephson junction formation between NbN grains could be blocked by non-superconducting phases present in these samples.     

Network structure of multi-component sodium borosilicate glasses by neutron diffraction

Neutron diffraction structure study has been performed on multi-component sodium borosilicate based waste glasses with the composition of (65 − x)SiO_2. · xB₂O₃ · 25Na₂O · 5BaO · 5ZrO₂, x = 5–15 mol%. The maximum momentum transfer of the experimental structure factor was 30 Å⁻¹, which made available to determine the distribution function with high r-space resolution. Reverse Monte Carlo modelling was applied to calculate several partial atomic pair correlation functions, nearest neighbor distances and coordination numbers have been revealed. The characteristic features of Si–O and Si–Si distributions are similar for all glassy samples, suggesting that the Si–O network consisting of tetrahedral SiO₄ units is highly stable even in the multi-component glasses. The B–O correlations proved to be fairly complex, two distinct first neighbor distances are present at 1.40 Å and 1.60 Å, the latter equals the Si–O distance. Coordination number distribution analyzes has revealed 3 and four-coordinated boron atoms. The O–O distribution suggests a network configuration consisting of boron rich and silicon rich regions. Our findings are consistent with a structure model where the boron rich network contains mostly trigonal BO₃ units, and the silicon rich network is formed by a mixed continuous network of ^[4]Si–O–Si^[4] with several different ^[4]B–O–Si^[4] and ^[3]B–O–Si^[4] linkages.     

Tetracycline and propolis incorporation and release by bioactive glassy compounds

This study compares the release of tetracycline and propolis incorporated into four silica-based bioactive glassy systems. The bioactive glasses, with composition (SiO₂)x(P₂O₅)y(CaO)z, were prepared using a sol–gel process at room temperature. Tetraethoxysilane (TEOS), triethylphosphate, and calcium chloride were used as Si, P, and Ca precursors, respectively. The quantities of tetracycline and propolis incorporated were 2% in weight. For delivery assays, the samples were individually immersed in deionized water and buffered with tris-hydroxymethyl amino methane, pH 7.4, and kept in water bath (37 °C) for thirty days. Aliquots were withdrawn and analyzed by ultraviolet spectrophotometry in the tetracycline (270 nm) and propolis (420 nm) wavelengths. For the glass–tetracycline compounds, it was observed that four days after release had started all samples had released about 90% of the total tetracycline concentration. In contrast, 90% of the propolis was released in about 30 days’ time. Sample characterization was made using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), Fourier transform infrared spectroscopic (FTIR), and thermogravimetry (TG).     

Diamond film growth on the Mo–Re alloy foil

Nanocrystalline diamond film was deposited on the substrate of Mo–Re alloy foil by using a hot filament chemical vapor deposition (HFCVD) method. The morphology, band structures and crystalline structure of the film were analysed by scanning electron microscopy (SEM), Raman spectroscopy and X-ray diffractometer (XRD), respectively. The results show that the thickness of the diamond film is about 300 nm after 1 h deposition. There is a 2H-Mo₂C layer between the diamond film and the Mo–Re substrate. The values of a and the ratio c/a of Mo₂C are 3.003 and 1.579 Å, respectively. This Mo₂C layer might be formed due to carbon atoms in the gas phase diffusing into the Mo–Re alloy.     

Investigation of glass forming ability in Ce–Al–Ni alloys

The compositional dependence of the glass forming ability (GFA), the correlation between their GFA and the GFA related parameters, and the thermal stability of the Ce–Al–Ni alloys were investigated. Rapidly quenched Ce₆₅Al_xNi_35 ? x (x = 2, 5, 10, 17, 20) and Ce₇₀Al_xNi_30 ? x (x = 2, 5, 10, 15, 20) ribbons were prepared by melt spinning, and their phase transformations were studied by X-ray diffraction (XRD), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). The experimental results indicated that the GFA of Ce₆₅Al_xNi_35 ? x (x = 2, 5, 10, 17, 20) and Ce₇₀Al_xNi_30 ? x (x = 2, 5, 10, 15, 20) alloys increased firstly and then decreased with the increasing of the Al content up to 20 at.%, respectively. It was found that only one parameter, F₁, in evaluated currently available empirical GFA parameters searching for metallic glasses with a good GFA, can reflect the GFA of the Ce–Al–Ni alloys. It was indicated that the thermal stability of alloy with fully amorphous maybe lower than that of alloy with partial amorphous.