In-field Conductivity Fluctuations in Ba<Subscript>0.72</Subscript>K<Subscript>0.28</Subscript>Fe<Subscript>2</Subscript>As<Subscript>2</Subscript> Single Crystals

Fluctuations in the conductivity of Ba_0.72K_0.28Fe₂As₂ single crystal are studied systematically by resistance measurements as a function of temperature and magnetic field. A clear Maki?Thompson and Aslamakov?Larkin (MT–AL) two- to three-dimensional (2D–3D) crossover is found on the excess conductivity (Δσ) curves as the temperature approaches the superconducting critical temperature, T _c. 3D fluctuations in superconductivity are realized near T _c that are well fitted to experimental data by the 3D Aslamazov–Larkin theory. The Maki–Thompson model shows a 2D conductivity fluctuation above the 2D-3D temperature transition, T ₀, which depends on magnetic field. Results show that the 2D-3D dimensional crossover moves to lower temperature with increasing magnetic field. The values of the transition temperature and the crossover in the reduced temperature, ln(ε ₀), as functions of magnetic field were used to determine the coherence length and the lifetime, τ _φ , of the fluctuational pairs at the temperature of 35 K. Analysis of the Ba_0.72K_0.28Fe₂As₂ single crystal gives a value of 3.76 × 10^??12 s for the τ _φ in the absence of magnetic field and it decreases to 2.4 × 10^??12 s in magnetic field of 13 T.     

Memory characteristics of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/LaAlO<Subscript>3</Subscript>/SiO<Subscript>2</Subscript> multilayer structures with tunnel oxide thickness variation

Charge trap flash (CTF) memory devices are candidates to replace NAND flash devices. In this study, Pt/Al₂O₃/LaAlO₃/SiO₂/Si multilayer structures with lanthanum aluminate charge traps were fabricated for nonvolatile memory device applications. An aluminum oxide film was used as a blocking oxide for low power consumption in the program/erase operation and to minimize charge transport through the blocking oxide layer. The thickness of SiO₂ as tunnel oxide layer was varied from 30 to 50 Å. Thicknesses of oxide layers were confirmed by high resolution transmission electron microscopy (HRTEM) and all the samples showed amorphous structure. From the C–V measurement, a maximum memory window of 3.4 V was observed when tunnel oxide thickness was 40 Å. In the cycling test for reliability, the 30 Å tunnel oxide sample showed a relatively large memory window reduction by repeated program/erase operations due to the high electric field of ~10 MV/cm through tunnel oxide. The other samples showed less than 10% loss of memory window during 10⁴ cycles.     

Anisotropy of dielectric losses in Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and SiO<Subscript>2</Subscript> single crystals

The procedure and results of measurements of the dielectric loss tangent using the dielectric resonator technique on azimuthal modes of the HE (quasi-E) and EH (quasi-H) types are considered. The measurements were performed for uniaxial anisotropic single crystals of Al₂O₃ (at a frequency of 11 GHz) and SiO₂ (at 39 GHz) in a temperature range of 80–373 K and for an isotropic single crystal of Y₃Al₅O₂ (YAG) at room temperature in a frequency range of 9–15 GHz. The proposed method revealed the anisotropy of dielectric losses in Al₂O₃ and SiO₂ single crystals in the temperature range studied. According to this, losses along the optical axis of these crystals are lower than in the transverse plane. In the YAG crystal, the Q values for modes of the two types with the same frequency are close, which corresponds to isotropic losses. The dielectric losses in YAG increase in proportion to the frequency.     

Microwave dielectric properties of Bi(Sc<Subscript>1/3</Subscript>Mo<Subscript>2/3</Subscript>)O<Subscript>4</Subscript> ceramics for LTCC applications

A novel microwave dielectric ceramics Bi(Sc_1/3Mo_2/3)O₄ with low firing temperature were prepared via the solid reaction method. The specimens have been characterized using scanning electron microscopy, X-ray diffraction, Raman spectroscopy and DC conductivity. The Bi(Sc_1/3Mo_2/3)O₄ ceramics showed B-site ordered Scheelite-type structure with space group C2/c. Raman analysis indicated that prominent bands were attributed to the normal modes of vibration of MoO₄ ^2? tetrahedra. The dielectric loss of Bi(Sc_1/3Mo_2/3)O₄ ceramics can be depended strongly the bulk conductivity by DC measurement. The superior microwave dielectric properties are achieved in the Bi(Sc_1/3Mo_2/3)O₄ ceramic sintered at 875 °C/4 h, with dielectric constant?~?25, Q?×?f ~?51,716 GHz at 6.4522 GHz and temperature coefficient of resonance frequency ~???70.4 ppm/°C. It is a promising microwave dielectric material for low-temperature co-fired ceramics technology.     

Directional solidification and growth characteristics of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Er<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>/ZrO<Subscript>2</Subscript> ternary eutectic ceramic by laser floating zone melting

Directionally solidified Al₂O₃/Er₃Al₅O₁₂/ZrO₂ ternary eutectic ceramic in situ composite rods with length of 110 mm have been fabricated by laser floating zone melting. The microstructural characteristics of steady growth zone, initial growth zone and solid/liquid interface are investigated under high temperature gradient. In the steady growth zone, the eutectic spacing (λ) is rapidly decreased as increasing the growth rate (V), and the corresponding relationship between growth rate and eutectic spacing is determined to be λ = 11.14 × V ^?1/2. The temperature gradient has been measured to be about 5.3 × 10³ K/cm. In the initial growth zone, the melting process and temperature distribution are recorded by infrared thermal imager, and several unstable complex microstructures are observed. In the quenched zone, the regular eutectics with minimum eutectic spacing of 200 nm are obtained. Moreover, the solid/liquid interface during solidification shows convex interface morphology and the interface height is gradually decreased as increasing the growth rate. The eutectic growth behaviors at the center and edge of the as-grown rod are compared and discussed.     

Investigation of Er-doped Sc<Subscript>2</Subscript>O<Subscript>3</Subscript> transparent ceramics by positron annihilation spectroscopy

0.25 at.% Er-doped Sc₂O₃ transparent ceramics fabricated using the two-step sintering method with different combinations of sintering temperatures were investigated by positron annihilation spectroscopy. Analysis of the broadening of the annihilation photopeak revealed the presence of the same type of defect in all samples. The lack of long lifetimes (τ ≥ 2 ns) suggested no positronium formation or the lack of trapping sites large enough to trap positronium for long enough time for the annihilation to be observed. Analysis of positron annihilation lifetime revealed the presence of a single lifetime that ranged from 208 to 219 ps, depending on the sintering conditions. These results also suggest the absence of a significant presence of vacancy clusters and other larger open-volume defects, and that the dominant open-volume defect corresponds to monovacancies and/or complex defects associated with monovacancies. The bulk lifetime of Er-doped scandia is estimated to be equal or lower than 208 ps.     

Tailoring the tensile/compressive response of magnesium alloy ZK60A using Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoparticles

ZK60A nanocomposites containing Al₂O₃ nanoparticle reinforcement were fabricated using solidification processing followed by hot extrusion and T5 heat treatment. Agglomeration of Al₂O₃ nanoparticles was observed in the nanocomposites. However, in the case of ZK60A/1.0 vol%Al₂O₃ nanocomposite (compared to monolithic ZK60A), increase in tensile strength (up to 14%) without significant decrease in ductility and simultaneous increase in compressive strength (up to 12%) and ductility (+23%) were observed. Here, the strength of ZK60A was increased without significant decrease in ductility. On the other hand, in the case of ZK60A/1.5 vol%Al₂O₃ nanocomposite (compared to monolithic ZK60A), simultaneous increase in tensile strength (up to 6%) and ductility (+26%), but decrease in compressive strength (up to 40%) with increase in ductility (+43%) were observed. Here, the ductility of ZK60A was significantly increased without significant increase in strength. This tailoring of tensile and compressive properties of ZK60A via integration with Al₂O₃ nanoparticles are investigated in this article.     

Structural phase transitions and piezoelectric anomalies in ordered Sc<Subscript>0·5</Subscript>Ga<Subscript>0·5</Subscript>N alloys

Local-density approximation calculations (LDA) within density functional theory (DFT) and Berry phase approach within modern theory of polarization are performed to predict the structural and piezoelectric properties of ordered Sc_0·5Ga_0·5N alloys under compressive and tensile in-plane strain. This alloy is found to exhibit a tremendous piezoelectric response, associated with a phase transition from nonpolar p6₃/mcc(D _6h) space group to a polar p6₃ mc(C _6v) structure, at fixed Ga and Sc compositions when continuously changing the experimental accessible parameters (i.e. the compressive and tensile strain). The mechanism of the effects behind such anomalous behaviour is revealed and explained.     

Sintering temperature depression in Al<Subscript>2</Subscript>O<Subscript>3</Subscript> by mechanical milling

High-energy milling of Al₂O₃ with hardened steel milling media has confirmed that nanocrystalline powders are readily formed. At a ball to charge mass-ratio of 20:1, the crystallite size falls below 30 nm in just 2 h and below 15 nm in 4 h. The as-milled powders are contaminated with Fe which increases linearly with increased milling time, reaching ∼10 wt% after 16 h. The HCl leaching process of Karagedov and Lyakhov [Karagedov and Lyakhov (1999) Nanostruct Mater 11(5):559] was found to remove a large proportion of the Fe, but residual Fe was found with XRF analysis. Milled and leached samples show significant sintering temperature depression to approximately 1100 °C and produce sintered densities greater than 94% without the application of pressure. Milling induced lattice expansion of the Al₂O₃ is observed which we posit to be due to defect formation rather than Fe absorption. The respective roles of small crystallite size and lattice defects in reducing the sintering temperature are discussed.     

Fabrication of Cr<Subscript>2</Subscript>O<Subscript>3</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> composite nanofibers by electrospinning

PVA(Polyvinyl alcohol)/chromium nitrate/aluminum nitrate composite nanofibers were prepared by using sol–gel processing and electrospinning technique. By high temperature calcinating the above precursor fibers, Cr₂O₃/Al₂O₃ composite nanofibers were successfully obtained. The fibers were characterized by XRD, IR, and SEM, respectively. The results showed that the crystalline phase and the morphology of the fibers depended on the calcination temperatures.     

Effects of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> additive on sintering behavior and microwave dielectric properties of ZnTa<Subscript>2</Subscript>O<Subscript>6</Subscript> ceramics

ZnTa₂O₆ ceramics with various amount of Al₂O₃ additive were synthesized by a conventional mixed-oxide route. The grain growth of ZnTa₂O₆ ceramics was accelerated with Al₂O₃ additive. However, excessive addition (>1.0 wt%) of Al₂O₃ leaded to abnormal grain growth. With Al₂O₃ addition, the Al₂O₃ additive did not solubilized into ZnTa₂O₆ structure but resulted in forming the second phase. The Al₂O₃ addition resulted in the lower sintering temperature of ZnTa₂O₆ ceramics and improved microwave dielectric properties. The dielectric constant (ε_r) of the samples did not change much and ranged from 32.41 to 34.33 with different amount of Al₂O₃ addition. The optimized quality factor (Q × f) was higher than 70,000 GHz as a result of the denser ceramics. The temperature coefficient of resonant frequency (τ _f ) of the doped ZnTa₂O₆ ceramics could be optimized to near-zero.     

Crack propagation in layered Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/ZrO<Subscript>2</Subscript> composites prepared by electrophoretic deposition

Crack propagation through layered Al₂O₃/ZrO₂ composites was studied. The specimens were prepared via electrophoretic deposition of alumina and zirconia powders from suspensions with monochloroacetic acid and isopropanol. The kinetics of electrophoretic deposition could be described fully if the electrophoretic mobility and conductivity of suspensions were known. The conductivity of suspensions increased in the course of deposition. Adjusting to properly controlled kinetics of deposition and sintering, deposits were prepared with strongly bonded layers of different pre-defined thicknesses and, consequently, with different magnitudes of residual stress. Cracks, produced by an indentation technique, propagated askew towards layer interfaces deflected towards the interface in the Al₂O₃ layers and away from the interface in the ZrO₂ layers. Changes in the direction of crack propagation were described for the whole range of angles of incidence (0°–90°). The biggest change in the crack propagation was observed for the angle of incidence 45° and was ca. 15°, irrespective of the magnitude of residual stress in the layers.     

Nanostructured phase formation in CeO<Subscript>2</Subscript>/LaAlO<Subscript>3</Subscript> and CeO<Subscript>2</Subscript>/(Ni-W) prepared by deposition from metalorganic solutions

We have studied nanoscale structural evolution processes and phase formation in barrier cerium oxide layers grown from methacrylate solutions on lanthanum aluminate and a nickel-tungsten tape by dip coating. The layers were characterized by atomic force microscopy, scanning electron microscopy, precision electron diffraction, X-ray microanalysis, and X-ray diffraction (including texture analysis). We have investigated nanostructured morphological forms of buffer cerium oxide layers in all synthesis steps: liquid-phase, pyrolytic, and crystalline. We have established a granular character of the nanostructure and determined the type of orientation of the films: 〈111〉 texture with a 〈200〉 additional orientation. Optimal conditions for the synthesis of a barrier layer several tens of nanometers in thickness are proposed.     

Role of Ti diffusion on the formation of phases in the Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> brazed interface

Diffusivities of Ti, Cu, Al and Ag in the interface of Al₂O₃–Al₂O₃ braze joints using Ag–Cu–Ti active filler alloy, have been calculated by Matano–Boltzman method. The Matano plane has been identified for each elemental diffusion at various brazing temperatures. The diffusivities of Ag, Cu and Al are almost insignificant on formation of interface during brazing, whereas the diffusivity of Ti changes significantly with the brazing temperature and controls the formation of different reaction product in the interface. Presence of TiO and Ti₃Cu₃O phases in the interface has been confirmed by transmission electron microscopy (TEM).     

Study of air oxidation of amorphous Zr<Subscript>65</Subscript>Cu<Subscript>17.5</Subscript>Ni<Subscript>10</Subscript>Al<Subscript>7.5</Subscript> by X-ray photoelectron spectroscopy (XPS)

The oxidation of the bulk amorphous alloy Zr₆₅Cu_17.5Ni₁₀Al_7.5 in air in its amorphous and the supercooled liquid states was studied in the temperature range 573–663 K using X-ray photoelectron spectroscopy (XPS). The oxide film mainly consisted of the oxides of Zr (as ZrO₂) and Al (as Al₂O₃). No Cu or Ni was found in the oxide film formed on the amorphous state of the alloy while significant Cu (as CuO) was present in the oxide film formed on the alloy in its supercooled liquid state. The role of the various alloying elements during oxidation at high temperatures in air is discussed in the paper. The XPS data from oxide film support the previously suggested mechanism for oxidation of this alloy, i.e. the rate controlling process during oxidation of the alloy at low temperatures (in the amorphous state) is the back-diffusion of Ni and Cu, while the oxidation at high temperatures (in the supercooled liquid state) is dominated by the inward diffusion of oxygen.     

Preparation of fine-grained ceramics by hot-pressing of Ce<Subscript>0.09</Subscript>Zr<Subscript>0.91</Subscript>O<Subscript>2</Subscript>/MgO/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanopowder

We have studied the effect of hot-pressing conditions (different pressure rise rates and isothermal holding temperatures in the range 1450–1550°C) on the microstructure of ceramics produced from nanopowder with the composition Ce_0.09Zr_0.91O₂/MgAl₆O₁₀/γ-Al₂O₃ (20.6, 37.4, and 42.0 wt %, respectively). Firing at 1450°C for 1 h made it possible to obtain fine-grained ceramics with less than 3 μm in grain size. The compaction pressure rise rate was shown to be a key parameter under such thermal conditions (20 + 10°C/min → 1450°C). Grain growth was prevented most effectively when the maximum load, 30 MPa, was reached at a temperature of 1000°C. Under such conditions, the grain size was 0.4–0.8 μm and the relative density reached 98.8%.     

Influence of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> buffer layer on catalyst morphology and spinnability of carbon nanotube arrays

Carbon nanotube (CNT) fibers spun from vertically aligned CNT arrays hold great promise in promoting CNT’s practical applications. Their production and properties strongly depend on the spinnability of the arrays. Herein, we study the influence of Al₂O₃ buffer layer on catalyst morphology and the spinnability of CNT arrays. Long and vertically aligned CNT arrays have been obtained from a wide range of Al₂O₃ buffer layer thickness, but the spinnable ones have only derived from a narrow range of the thickness. It is further found that the Al₂O₃ buffer layer can regulate the size and size distribution of the catalyst particles through balancing surface diffusion and inter-layer diffusion. Small, dense, and uniform-distributed nanoparticles are fingerprinted as the optimal catalyst for growing long and spinnable CNT arrays and can be obtained at a proper thickness of buffer layer. By using a tailored tri-layered Fe/Al₂O₃/SiO₂ catalyst, the obtained CNT arrays could reach a height of 500–800 µm and are highly spinnable.     

Tailoring magnetic and dielectric properties of Co<Subscript>0.9</Subscript>Cu<Subscript>0.1</Subscript>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript> with substitution of small fractions of Gd<Superscript>3+</Superscript> ions

This paper describes the structural, magnetic, and dielectric properties of Gd³⁺ substituted cobalt–copper ferrite. The influence of Gd³⁺ substitution on the structural, magnetic and electrical properties of cobalt–copper ferrite was investigated through various characterization techniques. Thermal analysis was carried out on the prepared gel to know the combustion and calcination temperature. The detailed structural analysis suggests that the substitution of a Fe³⁺ ion with a Gd³⁺ ion at B site results in lattice distortion, modification in crystallite size and grain size of the material. X-ray photoelectron spectroscopy confirmed the oxidation states of the elements present. Magnetic measurement performed at 300 and 50 K depicts the decrease in saturation magnetization (Ms) and increase in coercivity (Hc) with Gd³⁺ substitution in the cobalt–copper spinel ferrite. The dielectric measurements acquired over a wide range of frequencies and temperature showed an increase in dielectric constant with increasing Gd³⁺ concentration.     

Wetting of grain boundaries in Al by the solid Al<Subscript>3</Subscript>Mg<Subscript>2</Subscript> phase

The microstructure of binary Al_100−x –Mg _x (x = 10, 15, 18 and 25 wt%) alloys after long anneals (600–4000 h) was studied between 210 and 440 °C. The transition from incomplete to complete wetting of Al/Al grain boundaries (GBs) by the second solid phase Al₃Mg₂ has been observed. The portion of completely wetted GBs increases with increasing temperature beginning from T _wsmin = 220 °C. Above T _wsmax = 410 °C all Al/Al GBs are completely wetted by the Al₃Mg₂ phase.