The microwave response of MgB2 /Al2O3 superconducting thin films

Microwave characteristics of MgB2/Al2O3 superconducting thin films were investigated by coplanar resonator technique. The thin films studied have different grain sizes resulting from different growth techniques. The experimental results can be described very well by a grain-size model which combines coplanar resonator theory and Josephson junction network model. It was found that the penetration depth and surface resistance of thin films with smaller grain sizes are larger than those of thin films with larger grain sizes.     

Impedance behavior of excess CaO type non-stoichiometric Ca1−xZrO3−δ perovskite ceramic

Excess CaO type non-stoichiometric Ca_1?xZrO_3?δ (x = ?0.2, ?0.23) was prepared to investigate their impedance behavior in 500 °C–1100 °C temperature range. It was confirmed that grain plays predominantly role in total resistance and relaxation process. The grain boundary resistance decreases more rapidly than that of grain with the rising of temperature. The relaxation time was found reduces with temperature increases. The activation energies are calculated to be 1.19 eV of C1.2Z and 1.53 eV of C1.23Z. The excess CaO is suggested can enhance the resistance and relaxation process of grain boundary and gives rise to a higher activation energy of C1.23Z. By analyzing dielectric permittivity, polarization effect was verified works more seriously at higher temperatures especially in C1.2Z. The excess CaO was proved can relieve the polarization effect in C1.23Z.     

Relaxation studies of spray deposited Bi2Co0.1V0.9O5.35 solid electrolyte thin films on stainless steel substrate

The room temperature-stabilized γ-phase ion conducting Bi₂Co_0.1V_0.9O_5.35 thin film was deposited by spray pyrolysis on stainless steel substrate. The intra-grain and grain interior charge transfer was confirmed in these films with impedance spectroscopic measurements done from 1 Hz to 10 MHz in the temperature range 550 K to 741 K. Impedance data further revealed non-Debye kind of relaxation dispersion in the film. The relaxation frequency ranges from 10 to 100 Hz, which predicts long range jumping of the oxygen ions with chemical diffusivity. The Arrhenius plots for relaxation frequency and grain boundary conductivity were found to have same activation energy revealing grain boundary dominant conduction. The effect of polycrystallinity on relaxation dispersion is investigated for the first time on Bi₂Co_0.1V_0.9O_5.35 thin films using impedance formalism.     

Microstructures and electrical conductivity of nanocrystalline ceria-based thin films

Ceria-based thin films are potential materials for use as gas-sensing layers and electrolytes in micro-solid oxide fuel cells. Since the average grain sizes of these films are on the nanocrystalline scale (< 150 nm), it is of fundamental interest whether the electrical conductivity might differ from microcrystalline ceria-based ceramics. In this study, CeO₂ and Ce_0.8Gd_0.2O_1.9−x thin films have been fabrication by spray pyrolysis and pulsed laser deposition, and the influence of the ambient average grain size on the total DC conductivity is investigated. Dense and crack-free CeO₂ and Ce_0.8Gd_0.2O_1.9−x thin films were produced that withstand annealing up to temperatures of 1100 °C. The dopant concentration and annealing temperature affect highly the grain growth kinetics of ceria-based thin films. Large concentrations of dopant exert Zener drag on grain growth and result in retarded grain growth. An increased total DC conductivity and decreased activation energy was observed when the average grain size of a CeO₂ or Ce_0.8Gd_0.2O_1.9−x thin film was decreased.     

Brownian relaxation of magnetic nanoparticles in fluid: the effect of the solvent

A simple, environmentally friendly hydrothermal stripping route for synthesizing highly size-controlled spherical ferric oxide nanoparticles was developed that involved stripping ferric ion from iron-loaded organic phase. The particle size was found to be influenced by the initial concentration of iron in organic phase, stripping temperature and time, and a mathematical model about the size was established based on the experimental results and theoretical analysis. The model suggests that the process apparent activation energy of stripping iron from organic phase is 97.3? kJ·mol^?1, the energy of crystal growth is 51.6? kJ·mol^?1, and the synthesis of Fe₂O₃ is controlled by the crystal growth of embryo at low temperature (T < 590?K). The sizes calculated by the model comparatively accord with the experimental data and this provides a method for controlling the sizes.     

Electronic conductivity and chemical diffusion in n-conducting barium titanate ceramics at high temperatures

The electronic conductivity as well as the chemical diffusion coefficient of barium titanate ceramics doped with Y and Mn (donor-doped and acceptor co-doped) have been determined by application of conductivity relaxation experiments. The equilibrium values of the electronic conductivity of n-conducting BaTiO₃ have been analyzed by application of a defect chemical model involving electrons and cation vacancies as the predominant defect species at oxidizing conditions (fairly high oxygen partial pressures). The relaxation curves of the electronic conductivity yield the chemical diffusion coefficient of the bulk by employing a spherical grain model where the appropriate diffusion length is the radius of grains (average grain size). The conductivity relaxation experiments have been performed as a function of temperature ranging from 1100 to 1250 °C at oxygen partial pressures between 0.01 and 1 bar. The kinetics of the oxygen exchange process can be interpreted in terms of extremely fast diffusion of oxygen via oxygen vacancies along the grain boundaries and slow diffusion of Ti (cation)-vacancies from the grain boundaries into the grains. The Ti-vacancy diffusion coefficients were extracted from the chemical diffusion coefficients as a function of temperature. Typical values for the Ti-vacancy diffusivity are around 10^− 15 cm² s^− 1 with an activation energy of 3.9 ± 0.7 eV.     

Al-Zn合金复相界面线性稳态弛豫

研究了复相共析Ａｌ－Ｚｎ合金中低频稳态阻尼特征．实验表明：在一低温、低振幅范围内，阻尼呈显著的线性粘滞特征，并遵守规律Ｑ^－１＝（Ｂ／ｆⁿ）ｅｘｐ（－ｎＨ／ｋＴ），其中Ｈ是真实激活能，Ｂ和ｎ（＝０．２１）是两个实验参数，ｋ是玻耳兹曼常数，并测得Ｈ值为０．７４ｅＶ，该值与界面运动粘滞性紧密相关，文中提出了界面线性粘滞运动模型，并解释了实验结果。     

Studies on grain boundary effects in spray deposited BICOVOX 0.1 films on platinum-coated stainless steel substrate

Thin film of Bi₂Co_0.1V_0.9O_5.35 is deposited by using spray pyrolysis technique on platinum-coated stainless steel substrate. Impedance measurements done in the frequency range 1 to 10 MHz and in the temperature range 502 to 720 K revealed two relaxation processes with distinguishable time constants. The first corresponds to the grain interior charge transfer while the second could be due to grain boundary. The change in polarization seems to be associated with hopping of charge carriers showing Arrhenius behavior with increase in temperature. The relaxation frequency of grain interior transport for the thin film ranges from 96 kHz to 2.59 MHz. The blocking factor was found to be increasing with increase in temperature at low temperature region from 502 to 640 K. At higher temperature above 640 K, diffusive nature of grain boundaries is inferred with the decrease in blocking factor. The same inference is derived by specific grain boundary conductivity calculations since specific grain boundary conductivity decreases in low temperature region while it increases rapidly at higher temperature. These observations prove the grain boundaries to be blocking in lower temperatures while at higher temperatures above 640 K they turn diffusive. These changes are attributed to structural phase transition, or ordering of vacancies in Bi₂Co_0.1V_0.9O_5.35 films.     

Al—Zn合金复相界面线性稳态弛豫

研究了复相共折Ａｌ－Ｚｎ合金中低频稳态阻尼特征，实验表明：在一低温，低振幅范围内，阻尼呈显著的线性粘滞特征，并遵守规律Ｑ＾－＾１＝（Ｂ／ｆ＾ｎ）ｅｘｐ（－ｎＨ／ｋＴ），其中Ｈ是真实激活能，Ｂ和ｎ（＝０．２１）是两个实验能数，ｋ是玻耳兹曼常数，并测得Ｈ值为０．７４ｅＶ，该值与界面运动粘滞性紧密相关，文中提出了界面线性粘滞运动模型，并解释了实验结果。     

Structural, thermal and optical studies of mechanical alloyed Ga40Se60 mixture

A mixture of elemental Ga and Se with the nominal composition Ga₄₀Se₆₀ was submitted to the Mechanical alloying technique and their structural, thermal and optical properties were followed by X-ray diffraction, differential thermal analysis, photoacoustic spectroscopy, UV-VIS‐NIR absorbance spectroscopy and Raman spectroscopy techniques. After 10 h of milling the X-ray pattern showed monoclinic Ga₂Se₃ phase nucleation, which is in the nanometric form, and also a minority amorphous phase. The DSC results showed exothermic reactions between 430 and 720 K attributed to amorphous-crystalline phase transition and structural relaxation of Ga₂Se₃ phase. Based on this a small amount of the as-milled sample was annealed at 723 K. Its XRD pattern showed evidences of grain growth, reduction of the interfacial component, as well as, disappearance of the amorphous phase. The annealing process induced thermal diffusivity increasing, while the optical band gap energy and Raman profile remained practically unchanged.     

Studies of dielectric and impedance properties of KCa2V5O15 ceramics

A polycrystalline sample, KCa₂V₅O₁₅, with tungsten bronze structure was prepared by a mixed-oxide method at low temperature (i.e., at 630 °C). A preliminary structural analysis of the compound showed an orthorhombic crystal structure at room temperature. Surface morphology of the compound was studied by scanning electron microscopy (SEM). Two dielectric anomalies at 131 and 275 °C were observed in the temperature dependency of dielectric response at various frequencies, which may be attributed to the ferroelastic-ferroelectric and ferroelectric-paraelectric transitions, respectively. The nature of variation of the electrical conductivity, and value of activation energy of different temperature regions, suggest that the conduction process is of mixed-type (i.e., ionic-polaronic and space charge generated from the oxygen ion vacancies). The impedance plots showed only bulk contributions, and non-Debye type of relaxation process occurs in the material. A hopping mechanism of electrical transport processes in the system is evident from the modulus analysis. The activation energy of the compound (calculated both from loss and modulus spectrum) is same, and hence the relaxation process may be attributed to the same type of charge carriers.     

Numerical simulations of hydrogen detonations with detailed chemical kinetics

Time-dependent, multidimensional simulations of unstable propagating detonations were performed using a detailed thermochemical reaction model for a stoichiometric argon-diluted hydrogen–oxygen mixture at low pressures and a hydrogen–air mixture at atmospheric pressure. Detonation cells computed for the low-pressure, dilute H₂–O₂–Ar systems were regular in shape, and their sizes compared reasonably well with experimental observations. The computed H₂–air cells at atmospheric conditions were qualitatively different from those observed in experiments, and their widths range from less than 1 mm to nearly 5 mm with multilevel hierarchal structures. The effective activation energy of the H₂–air mixture, based on constant-volume ignition delay times computed using the detailed thermochemical model, varies between 5 and 40 over the range of post-shock temperatures and pressures in the simulations and is, on average, significantly larger than expected based on the regularity of experimental cellular patterns. Analysis of the simulations suggests that vibrational relaxation of the gas molecules, a process which is ignored when calibrating detailed chemical reaction models, occurs on time scales similar to the ignition delay times for the detonations and may be a source of discrepancy between numerical and experimental results.     

Impedance response of polycrystalline tungsten oxide

Polycrystalline tungsten oxide (WO₃) pellets were prepared by conventional ceramic processing technology. The ac small-signal electrical data acquired in the frequency (f) range 100 Hz≤f≤1 MHz at temperature (T) ranging the 31-100 °C revealed distinct semicircular relaxation in the impedance plane. This relaxation indicates device behavior originating from the grain boundaries. The lumped grain impedance associated with the device action remained too small to detect when the large resistance scale is realized. The semicircular relaxation is thermally activated indicating 0.58 eV as the activation energy for the relaxation time.     

Growth kinetics of antiphase domains in the alloy Ni4Mo

Using the methods of transmission and diffraction electron microscopy, we analyze the laws governing the growth kinetics of antiphase domains in the alloy Ni₄Mo during its isothermal ordering at 840, 800, and 700°C. The size distributions of the antiphase domains as well as the mean size and central moments of the distributions are determined. It is shown that the growth of the antiphase domains obeys the equation Dⁿ-D ₀ ⁿ =Ae^–Q/kT. The value of the parameter n is 2.5 for 840 and 800°C, and 5 for 700°C. By considering the isochronous cross section of the kinetic curves, we calculate the activation energy for the growth of antiphase domains; it equals 65 kcal/mole.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 84–89, January, 1977.     

Efficient approach to metal/metal oxide interfaces within variable charge model

A modified procedure of calculating the energy of metal/oxide interfaces and surfaces in the frame of the CTIP + EAM model (charge transfer ionic potential + embedded atom method) has been developed. According to the proposed approach, local charges and positions of atoms are determined only in a restricted zone surrounding the interface, while in the remaining region they are fixed. As a result, the number of variables undergoes a significant reduction, which enables carrying out efficient calculations for metal/oxide systems. The modified procedure has been applied to studying the relaxation of the α-Al₂O₃ surface. Using three different forms of the CTIP + EAM model present in literature, it has been shown that the correctness of the obtained results is conditioned by the appropriate relation between the CTIP and EAM components. Finally, the relaxation of the Ni/α-Al₂O₃ interface has been examined.     

Numerical simulation of energy relaxation processes in a ZnMoO<Subscript>4</Subscript> single crystal

We present the results of our experimental investigation and numerical simulation of excitation-energy relaxation processes in zinc molybdate crystals. We show that our kinetic model of the energy relaxation makes it possible to describe basic features of experimental results. Using this model, we estimate the trap concentration in ZnMoO₄ upon irradiation of the crystal by VUV synchrotron radiation and X-ray radiation. We conclude that prolonged phosphorescence of ZnMoO₄ that is observed after irradiation of the crystal by X-ray radiation can be caused by the occurrence of additional traps with a low activation energy.     

Surface layer self diffusion in icosahedral Al-Pd-Mn quasicrystals

The self diffusion of Mn and Pd in a single grain icosahedral Al_69.9Pd_20.5Mn_9.6 quasicrystal has been determined by low energy ion scattering (LEIS). The diffusion was determined by depositing different elements (Pd, Mn) on the surface and measuring the rate of change in surface composition as a function of temperature by LEIS. The surface composition was monitored over the temperature range of 355-575 K for Mn and 440-745 K for Pd and compared to model calculations to allow the activation energy for diffusion to be determined. Activation energies of 0.20 ± 0.01 eV for Mn and 0.64 ± 0.03 eV for Pd have then been measured for self diffusion in i-Al-Pd-Mn, respectively. No deviation from Arrhenius behavior was detected in the temperature range covered by the present experiments. From the low values of activation energy we propose that this range of diffusion is phason related, reflecting the specific nature of the icosahedral structure.     

Microstructure and dielectric study of pure BST and doped BSTF ceramic materials by broadband dielectric spectroscopy

Pure BST and doped BSTF (with BSTF2: Fe₂O₃ 2 wt % and BSTF4: Fe₂O₃ 4 wt %) ceramics were prepared by solid state reaction. XRD pattern showed the different phases were formed depend on the weight percent of Fe₂O₃. The crystal size and lattice parameters increased while the lattice strain decreased. The topography of the sintered samples shows increase of the grain size with increasing Fe₂O₃ ratio and hence enhances the compaction of ceramics. Broadband dielectric spectroscopy was employed to investigate the effect of magnetite nanoparticle on the dielectric properties of the pure BST ceramic. The interfacial polarization and the conductivity contribution reflect the high values of permittivity and its gradual increase as frequency decreases. The two BSTF samples show relaxation peak dynamic originated from presence of immobile species/electrons at low temperatures and defects/vacancies results from the formation of oxygen vacancies originates from the spontaneous change in oxidation states of Fe ions (Fe ^3+/Fe²⁺) at high temperatures. The relaxation rate obeys Arrhenius law at high temperatures in case of BST sample with activation energy 225 kJ/mol. This high value of activation energy at higher temperatures reflects and confirms the slowed down of the dynamics at the interphase and the decoupling nature of the OH-dynamic and the interfacial polarization.     

Creep strain recovery of Fe-Ni-B amorphous metallic ribbon

Creep strain recovery and structural relaxation of the amorphous metallic glass Fe₄₀Ni₄₁B₁₉ after longtime loading at different annealing temperatures below the glass transition temperature have been studied using anisothermal differential scanning calorimetry (DSC) and dilatometry (TMA). It has been demonstrated that structural relaxation effects depend on the stress-annealing temperature of the amorphous ribbon. The structural relaxation states of the amorphous ribbon annealed at different temperatures under and without applied stress have been compared. The activation energy spectra were calculated from the anisothermal dilatometric measurements using the modern method based on the Fourier transformation technique. The influence of the annealing temperature on the shape of creep strain recovery spectra has been analyzed.