Effect of contacts between γ-Fe2O3 nanoparticles on their phase-transition temperature

The effect of mechanical contacts between γ-Fe₂O₃ particles on the temperature of the γ-α structural transition in them is established by magnetic studies and differential thermal analysis (DTA). The sample in which γ-Fe₂O₃ particles had no mechanical contacts with one another remained in the ferromagnetic state up to T _C = 630°C and had two exothermal DTA peaks. The first peak almost coincided with the Curie temperature, while the second peak attributed to the γ → α structural transition corresponded to 760°C. The magnetic transition for particles with a larger number of contacts was shadowed by the γ → α structural transition with a temperature lowered to 550°C.     

Soft chemistry routes for synthesis of rare earth oxide nanoparticles with well defined morphological and structural characteristics

Phosphors of (Y_0.75Gd_0.25)₂O₃:Eu³⁺ (5 at.%) have been prepared through soft chemistry routes. Conversion of the starting nitrates mixture into oxide is performed through two approaches: (a) hydrothermal treatment (HT) at 200 °C/3 h of an ammonium hydrogen carbonate precipitated mixture and (b) by thermally decomposition of pure nitrate precursor solution at 900 °C in dispersed phase (aerosol) within a tubular flow reactor by spray pyrolysis process (SP). The powders are additionally thermally treated at different temperatures: 600, 1000, and 1100 °C for either 3 or 12 h. HT—derived particles present exclusively one-dimensional morphology (nanorods) up to the temperatures of 600 °C, while the leaf-like particles start to grow afterward. SP—derived particles maintain their spherical shape up to the temperatures of 1100 °C. These submicron sized spheres were actually composed of randomly aggregated nanoparticles. All powders exhibits cubic Ia-3 structure (Y_0.75Gd_0.25)₂O₃:Eu and have improved optical characteristics due to their nanocrystalline nature. The detailed study of the influence of structural and morphological powder characteristics on their emission properties is performed based on the results of X-ray powder diffractometry, scanning electron microscopy, X-ray energy dispersive spectroscopy, transmission electron microscopy, and photoluminescence measurements.     

Dielectric relaxation study of sulfolane in carbon tetrachloride solution from microwave absorption data

The dielectric constant (ɛ′) and dielectric loss (ɛ″) for dilute solutions of sulfolane in carbon tetrachloride solution have been measured at 9.885 GHz at different temperatures viz. 25°C, 30°C, 35°C and 40°C by using standard microwave techniques. Following the single frequency concentration variational method, the dielectric relaxation time (τ) and dipole moment (μ) have been calculated. It is found that dielectric relaxation process can be treated as the rate process, just like the viscous flow. Based on the above studies, monomer structure of sulfolane in carbon tetrachloride has been inferred. Energy parameters (ΔH _g , ΔF _g , ΔS _g ) for dielectric relaxation process of sulfolane in carbon tetrachloride at 25°C, 30°C, 35°C and 40°C have been calculated and compared with the corresponding energy parameters (ΔH _η , ΔF _η , ΔS _η ) for the viscous flow.     

Potentiometric DO detection in water by ceramic sensor based on sub-micron RuO2 sensing electrode

An alumina sensor using sub-micron RuO₂ sensing electrode (SE) was fabricated and examined for potentiometric dissolved oxygen (DO) detection in water at a temperature range of 9–35 °C. The electromotive force (emf) response at these temperatures was linear to the logarithm of DO concentration in the range from 0.6 to 8.0 ppm (log[O₂], −4.71 to −3.59). RuO₂-SE displays a Nernstian slope of −41 mV per decade at pH 8.0. It was also found that the response/recovery time to DO changes were sluggish as the water temperature cools down. Response time T ₉₀ to DO changes increased from 8 min at a temperature of 23 °C to about 30 min at a temperature of 9 °C. The proton conductivity of hydrous RuO₂ appears to be due to the dissociative adsorption of water and the formation of acidic OH groups in Ru (III,IV) cluster ions. In strong alkaline solutions, the sensor’s emf exhibited a mixed potential of fast and slow electrochemical reactions involving DO, RuO₄ ²⁻ and OH⁻ ions. The results also revealed that as pH of the solution increases to pH 10.0–13.0, the response/recovery rate becomes faster, stabilizing more or less quickly depending upon the solution alkalinity. Scanning electron microscopy, energy dispersive X-ray-analysis and impedance spectroscopy techniques were used to examine respectively the morphology, crystalline structure and electrochemical behaviour of sub-micron RuO₂ oxides.     

Irreversible increase in the temperature range of existence of the orthorhombic antiferroelectric phase in PbZr1 ? xTixO3 ceramics (0.02 ≤ x ≤ 0.05)

The influence of thermostating at 450°C on the dielectric and pyroelectric properties of PbZr_{1 − x} Ti _x O₃ polarized ferroelectric specimens the range 0.02 ≤ x ≤ 0.05 has been investigated. An irreversible increase in the temperature of the ferroelectric-antiferroelectric phase transition was observed in all specimens upon their cooling after exposure to an elevated temperature for more than 1 h. The original position of the phase-transition point on the temperature scale was not recovered as a result of afterpoling upon cooling through the Curie temperature. Heating of the same specimens in the bridged mode up to 650°C with subsequent abrupt immersion in silicone oil at room temperature (hardening) is accompanied by an additional increase in the range of existence of the antiferroelectric phase from 3 to 15°C with an increase in the x concentration.     

Quantitative evaluation of substrate temperature dependence of Ge incorporation in GaAs during molecular beam epitaxy

Combined Hall effect and low-temperature photoluminescence measurements have been used to perform a thorough evaluation of the growth temperature dependence of Ge incorporation in GaAs during molecular beam epitaxy (MBE) over the entire substrate temperature range (400°≦T _s ≦600[°C]) practicable forn-type layer growth. Using a constant As₄ to Ga flux ratio of two, growth below 500°C yieldsn-GaAs: Ge films having electrical and optical properties rapidly deteriorating with decreasingT _s . Growth at 500° ≦T _s ≦600[°C] produces high-qualityn-GaAs: Ge films (N _D /N _A ≈4) with C as well as Ge residual acceptors competing on the available As sites. The amount of Ge atoms on As sites [Ge_As] increases with substrate temperature, whereas simultaneously the amount of C atoms on As sites [C_As] decreases thus leading to the well-establishednonlinear behaviour of the (N _A /N _D vs. 1/T _s plot. Counting the incorporated Ge impurities separately, however, yields alinear behaviour of the ([Ge_As]/[Ge_Ga]) vs. 1/T _s plot which has exactly the same slope as the (P _{As 2}/P _Ga) vs. 1/T _s plot derived from vapour pressure data of As₂ and Ga over solid GaAs surfaces. The important result is, therefore, that the incorporation behaviour of Ge in GaAs during molecular beam epitaxy is directly correlated with theevaporation behaviour of the growing GaAs surface.     

Small angle neutron scattering study of two nonionic surfactants in water micellar solutions

Two classic nonionic surfactants — C₁₄E₇ (heptaethylene glycol monotetradecyl ether) and C₁₀E₇ (heptaethylene glycol monodecyl ether) were investigated in heavy water solution for concentration c = 0.17% (dilute regime) at different temperatures in the range t = 10–35°C by small angle neutron scattering (SANS) method. In the case of C₁₄E₇ surfactant — for all temperatures at c = 0.17% there are two axial ellipsoidal micelles with longer axis 15 nm at 10°C and 49.5 nm at 35°C in investigated solutions. For C₁₀E₇ surfactant at the same concentration of solution and temperature — two axial ellipsoidal micelles were observed, too. The longer axis is equal to 7.5 nm at 10°C, 9 nm at 20°C and at 35°C this axis is equal to 12 nm. Micelles of C₁₀E₇ nonionic surfactant are smaller than those of C₁₄E₇ surfactant in the same experimental conditions.      

Hybrid silicon evanescent approach to optical interconnects

We discuss the recently developed hybrid silicon evanescent platform (HSEP), and its application as a promising candidate for optical interconnects in silicon. A number of key discrete components and a wafer-scale integration process are reviewed. The motivation behind this work is to realize silicon-based photonic integrated circuits possessing unique advantages of III–V materials and silicon-on-insulator waveguides simultaneously through a complementary metal-oxide semiconductor fabrication process. Electrically pumped hybrid silicon distributed feedback and distributed Bragg reflector lasers with integrated hybrid silicon photodetectors are demonstrated coupled to SOI waveguides, serving as the reliable on-chip single-frequency light sources. For the external signal processing, Mach–Zehnder interferometer modulators are demonstrated, showing a resistance-capacitance-limited, 3 dB electrical bandwidth up to 8 GHz and a modulation efficiency of 1.5 V mm. The successful implementation of quantum well intermixing technique opens up the possibility to realize multiple III–V bandgaps in this platform. Sampled grating DBR devices integrated with electroabsorption modulators (EAM) are fabricated, where the bandgaps in gain, mirror, and EAM regions are 1520, 1440 and 1480 nm, respectively. The high-temperature operation characteristics of the HSEP are studied experimentally and theoretically. An overall characteristic temperature (T ₀) of 51°C, an above threshold characteristic temperature (T ₁) of 100°C, and a thermal impedance (Z _T ) of 41.8°C/W, which agrees with the theoretical prediction of 43.5°C/W, are extracted from the Fabry–Perot devices. Scaling this platform to larger dimensions is demonstrated up to 150 mm wafer diameter. A vertical outgassing channel design is developed to accomplish high-quality III–V epitaxial transfer to silicon in a timely and dimension-independent fashion.     

Ac-conductivity and dielectric relaxations above glass transition temperature for parylene-C thin films

45% semi-crystalline parylene-C (–H₂C–C₆H₃Cl–CH₂–) _n thin films (5.8 μm) polymers have been investigated by broadband dielectric spectroscopy for temperatures above the glass transition (T _g =90°C). Good insulating properties of parylene-C were obtained until operating temperatures as high as 200°C. Thus, low-frequency conductivities from 10⁻¹⁵ to 10⁻¹² S/cm were obtained for temperatures varying from 90 to 185°C, respectively. This conductivity is at the origin of a significant increase in the dielectric constant at low frequency and at high temperature. As a consequence, Maxwell–Wagner–Sillars (MWS) polarization at the amorphous/crystalline interfaces is put in evidence with activation energy of 1.5 eV. Coupled TGA (Thermogravimetric analysis) and DTA (differential thermal analysis) revealed that the material is stable up to 400°C. This is particularly interesting to integrate this material for new applications as organic field effect transistors (OFETs). Electric conductivity measured at temperatures up to 200°C obeys to the well-known Jonscher law. The plateau observed in the low frequency part of this conductivity is temperature-dependent and follows Arrhenius behavior with activation energy of 0.97 eV (deep traps).     

Structure,thermal properties,conductivity and electrochemical stability of di-urethanesil hybrids doped with LiCF<Subscript>3</Subscript>SO<Subscript>3</Subscript>

Variable chain length di-urethane cross-linked poly(oxyethylene) (POE)/siloxane hybrid networks were prepared by application of a sol-gel strategy. These materials, designated as di-urethanesils (represented as d-Ut(Y′), where Y′ indicates the average molecular weight of the polymer segment), were doped with lithium triflate (LiCF₃SO₃). The two host hybrid matrices used, d-Ut(300) and d-Ut(600), incorporate POE chains with approximately 6 and 13 (OCH₂CH₂) repeat units, respectively. All the samples studied, with compositions ∞ > n ≥ 1 (where n is the molar ratio of (OCH₂CH₂) repeat units per Li⁺), are entirely amorphous. The di-urethanesils are thermally stable up to at least 200 °C. At room temperature the conductivity maxima of the d-Ut(300)- and d-Ut(600)-based di-urethanesil families are located at n = 1 (approximately 2.0 × 10⁻⁶ and 7.4 × 10⁻⁵ Scm⁻¹, respectively). At about 100 °C, both these samples also exhibit the highest conductivity of the two electrolyte systems (approximately 1.6 × 10⁻⁴ and 1.0 × 10⁻³ Scm⁻¹, respectively). The d-Ut(600)-based xerogel with n = 1 displays excellent redox stability.     

Crystal structure of ethyl (2Z)-2-(3-methoxy-4-acetyloxy benzylidene)-7-methyl-3-oxo-5-phenyl-2,3-dihydro-5H-[1,3]thiazolo[3,2-a]pyrimidine-6-carboxylate

The title compound, C₂₆H₂₄N₂O₆S, (I), crystallizes in the monoclinic space group, P2₁/c, with cell parameters a = 16.248(1), b = 7.927(1), c = 19.371(4) ?, β = 105.295(2)°, Z = 4. The central pyrimidine ring in the compound (I) is significantly puckered, assuming a screw-boat conformation. The C11–C16 benzene ring stands vertical while thiazole and C18–C23 benzene rings are coplanar to the mean plane of pyrimidine ring having dihedral angles of 87.48(12), 3.63(11) and 0.94(12)°, respectively. In the absence of potential hydrogen bonding interaction, the crystal packing is influenced by intramolecular C-H…S interaction and intermolecular C-H…π interactions.     

Internal friction in directed-crystallization (Cu-Sn)-Nb alloys

The distinctive features of the low-frequency internal friction Q ⁻¹(T) of (Cu-Sn)-Nb composites at high temperatures (up to 400°C) are investigated for strains in the range 10⁻⁵–10⁻⁴. Considerable hysteresis of Q ⁻¹(T) in the heating-cooling cycle is recorded, including the presence of a minimum at ∼175°C when the sample is heated to 400°C and two peaks P ₂ (at 280°C) and P ₁ (at ∼100°C) when the sample is cooled from 400°C. The activation energy of the anomalous internal friction background (up to 175°C), the oxygen diffusion parameters, and the oxygen concentration in the niobium fibers (all of which govern the peak P ₂) are calculated, and the value and temperature dependence of the yield point of the bronze matrix (which govern the peak P ₁) are estimated. Zh. Tekh. Fiz. 68, 114–117 (November 1998)     

Coincidence of rotational energy of H<Subscript>2</Subscript>O ortho-para molecules and translation energy near specific temperatures in water and ice

A hypothesis of the quantum nature of the specific temperatures T _s of water and ice, whose values is not random, was formulated. It was found that the quantum energy hΩ _mn of closely located rotational transitions in the ortho and para spin isomers of H₂O molecules coincides with the translation energy kT near the well-known specific temperatures T _s in ice and water. On the basis of this fact it was suggested that ortho-para conversion occurs at temperatures close to T _s upon inelastic collisions and resonance energy exchange kT _s ↔ hΩ _mn in the rotation-translation-rotation (RTR) processes. Such conversion can induce rearrangement of the H-bond set structure and repacking of H₂O molecules. The coincidence kT _s ≈ hΩ _mn was checked for ice and water at 12 known T _s, as well as for heavy water D₂O near T _s = 11.2°C (maximum density) and −140°C (glassy transition). The previously observe strong deformation of the OH Raman band near T _s = 4, 19, 36, and 76°C (maximum density, maximum surface tension, minimum heat capacity, and maximum speed of sound, respectively) was interpreted as a manifestation of the water structure rearrangement induced by H₂O ortho-para conversion.     

Specific features of the structural states in Pb2CdWO6 in the temperature range 15 ≤ T ≤ 770°C

In the temperature range 15–770°C, a correlation between the dielectric and structural specific features of lead cadmium tungstate is established. When studying the structure of Pb₂CdWO₆ in the temperature range 15 ≤ T ≤ 770°C, the unit-cell parameters and atomic parameters of the orthorhombic phase with space group Pmc2₁ (15 ≤ T ≤ 380°C) and cubic phase with space group Fm3m (380 ≤ T ≤ 770°C) are determined. It is revealed that the antiferroelectric state observed in of Pb₂CdWO₆ in the orthorhombic phase is associated with antiparallel displacements of Pb atoms. In the cubic phase, the Cd atoms are found to have disordered static displacements along [100]-type directions.     

Anhydrous proton-conducting organic–inorganic hybrid membranes synthesized from tetramethoxysilane/methyltrimethoxysilane/diisopropyl phosphite and ionic liquid

Inorganic–organic hybrid membranes were prepared by sol–gel process with tetramethoxysilane/methyltrimethoxysilane/diisopropyl phosphite and 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF₄) ionic liquid as precursors. The Fourier transform infrared spectroscopy (FT-IR) and ³¹P, ²⁹Si, ¹H, ¹³C, and ¹⁹F nuclear magnetic resonance measurements have shown good chemical stability and complexation of (POH[(CH₃)₂CHO]₂) with [BMIMBF₄] ionic liquid in the fabricated hybrid membranes. The influence of the textural properties of all the prepared composite membranes could be interpreted from nitrogen adsorption–desorption measurements. The average pore size was increased proportionally with the ionic liquid weight percent ratio in the host phosphosilicate matrix from 2.59 to 11.71 nm, respectively. Thermogravimetric analysis and differential thermal analysis measurements confirmed that the hybrid membranes were thermally stable up to 260 °C. Thermal stability of the hybrid membranes was significantly enhanced by the presence of inorganic SiO₂ framework and high stability of [BF₄] anion. For all the composite membranes, the conductivities were measured within the temperature range (−30 °C) to 150 °C, and a maximum conductivity of 7 × 10⁻³ S/cm at 150 °C was achieved for 40 wt.% ionic liquid-based composite membrane under nonhumidified conditions.     

Genesis of supported carbon-coated Co nanoparticles with controlled magnetic properties,prepared by decomposition of chelate complexes

Following procedures formerly developed for the preparation of supported heterogeneous catalysts, carbon-coated cobalt nanoparticles dispersed on porous alumina have been prepared by impregnation of γ-Al₂O₃ with (NH₄)₂[Co(EDTA)] and thermal decomposition in inert atmosphere. Below 350 °C, Co(II) ions are complexed in a hexa-coordinated way by the EDTA ligand. The thermal treatment at 400–900 °C leads to the EDTA ligand decomposition and recovering of the support porosity, initially clogged by the impregnated salt. According to X-ray absorption spectroscopy, and due to in situ redox reactions between the organic ligand and Co(II), both oxidic and metallic cobalt phases are formed. Characterisation by transmission electron microscopy, X-ray diffraction and magnetic measurements reveals that an increase in the treatment temperature leads to an increase of the degree of cobalt reduction as well as to a growth of the cobalt metal particles. As a consequence, the samples prepared at 400–700 °C exhibit superparamagnetism and a saturation magnetisation of 1.7–6.5 emu g⁻¹ at room temperature, whilst the sample prepared at 900 °C has a weak coercivity (0.1 kOe) and a saturation magnetisation of 12 emu g⁻¹. Metal particles are homogeneously dispersed on the support and appear to be protected by carbon; its elimination by a heating in H₂ at 400 °C is demonstrated to cause sintering of the metal particles. The route investigated here can be of interest for obtaining porous magnetic adsorbents or carriers with high magnetic moments and low coercivities, in which the magnetic nanoparticles are protected from chemical aggression and sintering by their coating.     

Orientation dependence of electrical properties of 0.96Na<Subscript>0.5</Subscript>Bi<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript>-0.04BaTiO<Subscript>3</Subscript> lead-free piezoelectric single crystal

In this paper, a single crystal of 0.96Na_0.5Bi_0.5TiO₃-0.04BaTiO₃ with dimensions of Φ 30×10 mm was grown by the top-seeded-solution growth method. X-ray powder diffraction results show that the as-grown crystal possesses the rhombohedral perovskite-type structure. The dielectric, piezoelectric and electrical conductivity properties were systematically investigated with 〈001〉, 〈110〉 and 〈111〉 oriented crystal samples. The room-temperature dielectric constants for the 〈001〉, 〈110〉 and 〈111〉 oriented crystal samples are found to be 650, 740 and 400 at 1 kHz. The (T _m, ε _m) values of the dielectric temperature spectra are almost independent of the crystal orientations; they are (306°C, 3718), (305°C, 3613) and (307°C, 3600) at 1 kHz for the 〈001〉, 〈110〉 and 〈111〉 oriented crystal. The optimum poling conditions were obtained by investigating the piezoelectric constants d ₃₃ as a function of poling temperature and poling electric field. For the 〈001〉 and 〈110〉 crystal samples, the maximum d ₃₃ values of 146 and 117 pC/N are obtained when a poling electric field of 3.5 kV/mm and a poling temperature of 80°C were applied during the poling process. The as-grown 0.96Na_0.5Bi_0.5TiO₃-0.04BaTiO₃ crystal possesses a relatively large dc electrical conductivity, especially at higher temperature, having a value of 1.98×10⁻¹¹ Ω⁻¹⋅m⁻¹ and 3.95×10⁻⁹ Ω⁻¹⋅m⁻¹ at 25°C and 150°C for the 〈001〉 oriented crystal sample.     

Synthesis,structural and electrical properties of double perovskite Sr<Subscript>2</Subscript>NiMoO<Subscript>6</Subscript> ceramics

The double perovskite Sr₂NiMoO₆ powders and ceramics were prepared by two different (conventional and precursor) solid-state reaction methods. The phase structure was characterized by XRD and TEM techniques. It has been indicated that single-phase perovskite powders were obtained when calcined in air at 1300°C. However, nano-particles of the size 30–60 nm have been found in powders prepared with the precursor method, while those from the conventional route exhibit large irregular shaped particles with aggregation. The dielectric properties (ε _r and tanδ) were also examined in the sintered ceramics. The results showed the transition point at 280°C for conventional route, while no clear phase change was observed in ceramics from the precursor route. These observations clearly indicate that the different starting processes affected the phase formation behavior and the electrical properties of Sr₂NiMoO₆ ceramics.     

Solid-precursor vaporizer for aerosol reactors: Synthesis of magnesium aluminate nanoparticles by thermal decomposition of magnesium aluminum tert-butoxide

A solid-precursor vaporizer for laboratory-scale aerosol reactors has been developed and successfully tested on the synthesis of magnesium aluminate (MgAl₂O₄) nanoparticles by thermal decomposition of magnesium aluminum tert-butoxide (Mg[Al(O^tBu)₄]₂), a single-source precursor, in a furnace aerosol reactor. The reactor temperature was varied from 600 to 1000°C with the precursor concentration at 6.4 × 10⁻⁶ mol/l. The atomic ratio of Al to Mg of produced particles was determined by XPS to be 2:1 and the primary particle size ranged from 28 to 55 nm. As-produced particles were all amorphous, but the particles further heated to 1000°C exhibited crystalline MgAl₂O₄ structures.     

Synthesis of SmAlO<Subscript>3</Subscript> nanocrystalline powders by polymeric precursor method

SmAlO₃ nanocrystalline powders were successfully synthesized by the polymeric precursor method using ethylenediaminetetraacetic acid as a chelating agent. The precursor and the derived powders were characterized by thermogravimetry analysis (TG) and differential scanning calorimetry analysis (DSC), infrared spectroscopy (IR), X-ray diffractometry (XRD) and transmission electron microscopy (TEM). The results showed that pure SmAlO₃ powder with orthorhombic perovskite structure could be synthesized at 800°C for 2 h without formation of any intermediate phase. The average particle size of the powder synthesized at 900°C was as low as 28 nm. Subsequently, the bulk SmAlO₃ ceramics were prepared at various sintering temperatures using the synthesized powders calcined at 900°C for 2 h as starting materials. The sintering experiments indicated that the sample sintered at 1550°C for 2 h exhibited the highest relative density of 97.2% and possessed the best microwave dielectric properties of ε _r=20.94, Q×f=78600 GHz and τ _f=−71.8 ppm/°C.