Defect structure of superconducting YBa2Cu3O7−δ

The superconducting material Y₁Ba₂Cu₃O_7?δ has been investigated by electron microscopy. Special attention has been paid to the defects occurring in the material. Twinning on (110) or (11 0) planes is intrinsically related to the orthorhombicity, and when cooled slowly the twin bands are pseudoperiodic with an average width of ? 50 nm. The orthorhombic-tetragonal transition is reversible and diffusion controlled. Under particular conditions and in slightly reduced material a 2a₀ X b₀ superstructure resulting from vacancy ordering is formed. When kept in air under powder form the material seems to be unstable. Planar defects along (001) accompany the degeneration of the material.     

Oxygen ordering and twinning in YBa2Cu3O7−x

Direct structure images of the YBa₂Cu₃O_7−x high T_c superconducting ceramic (also called the 1–2–3 compound) at 1.7 Å resolution have been obtained for the [100] and [001] orientations. It was found that for the purposes of studying oxygen ordering in this compound it is better to use lattice images of lower resolution. The oxygen ordering was studied via the measurement of the bending of (100) and (11 0) lattice planes on crossing the (110) twin boundaries in crystals oriented in the [001] zone. Significant variations were found in the b/a ratios, owning to a variation in oxygen ordering, between different crystal grains, and between different regions in the same grain. For the three different 1–2–2 samples studied, the average b/a ration was 1.016, the same value as was found in neutron diffraction studies. The twin boundaries in the orthorhombic 1–2–3 phase are sharp and planar. It seems likely that the transformation from the high-temperature tetragonal phase to the lower-temperature orthorhombic phase is martensitic in nature. A new phase has been discovered on some of the twin boundaries. The new phase can be indexed as tetragonal with a = 7.5 ± 0.2 Å, and c = 6.8 ± 0.2 Å. It is possible that the new phase is stabilized by the stress which occurs at the twin boundaries.     

Surface structures of YBa2Cu3O7-x,BiCa1·7Sr0·7Cu2Ox and TlCaBaCuO4·5±x investigated by scanning tunnelling microscopy

Surface structures of the high-T_c superconductors YBa₂Cu₃O_7-x, BiCa_1·7Sr_0·7Cu₂O_x and TlCaBaCuO_4·5±x have been investigated with scanning tunnelling microscopy. The observed features include well-organized stripe corrugations in Y-Ba-Cu-O as well as orientated flake-like structures and steps with various heights in the Bi- and Tl-compounds. These observations suggest strong local variations of the elastic and electronic properties of the investigated materials.     

Preliminary study of electron-energy-loss spectra of YBa2 Cu3O7−δ

We have obtained the electron-energy-loss spectra below 1,000 eV for YBa₂Cu₃O_7?δ. The spectra show contributions from all elemental constituents, although the Y-signal is very weak owing to its small concentration. The low-loss region and the Ba-core losses are very similar to those obtained for BaO. The Cu absorption is similar to that seen in CuO with minor differences relating to Cu concentration and anisotropy. The oxygen 1s absorption has a shape that is quite different from that obtained in CuO and shows striking orientational anisotropy. Radiation damage in the microscope is a problem and leads to changes in the shape of the oxygen edge.     

Structure of superconducting thin films of YBa2Cu3O7−x grown on SrTiO3 and cubic zirconia

Thin films of the superconductive oxide YBa₂Cu₃O_7?x have been made by electron-beam coevaporation of the metals in an oxygen atmosphere onto single-crystal {001}-oriented SrTiO₃ and yttria-stabilized zirconia (YSZ) substrates. The oxide films were superconducting in the as-deposited state (T_c = 81–83K, J_c = 10⁶ A/cm² at 4.2K). Bright-field imaging, selectedarea diffraction (SAD), and high-resolution imaging in the transmission electron microscope were used to characterize the microstructure of these films. All of the films were polycrystalline. On SrTiO₃ the films were oriented, for the most part, with {110} parallel to the substrate surface. On YSZ, two microstructures were observed: one with smaller rectangular grains oriented with (100) or (010) parallel to the substrate surface and the other with (001) parallel to the surface (i.e., c-axis up).     

Process of epitaxial crystal growth for CaSO4 · 0.5H2O on a surface of dissolving fluorapatite crystals studied by scanning electron microscopy

A process for the epitaxial crystal growth for CaSO₄· 0.5H₂O on a surface of dissolving natural fluorapatite (FAP) crystals under conditions of wet-process phosphoric acid production has been studied by scanning electron microscopy (SEM). With the SEM working in a secondary electron mode, we were able to establish the main stages of the coating formation, including direct measurements of its thickness and growing rate values for individual needle-like CaSO₄· 0.5H₂O crystals. Confirmation of an aggregate mechanism for CaSO₄· 0.5H₂O epitaxial crystals growth, established earlier under conditions of chemical crystallization, was achieved for the first time. The mechanism includes the following steps: (1) ultramicrocrystals of CaSO₄· 0.5H₂O were formed, (2) they coalesced into needle-like crystals by means of mutual aggregation, (3) the needle-like crystals united into star-like and besom-like aggregates, and (4) the latter formed macro-aggregates. We have found that these steps occur simultaneously, resulting in multilayer coating formation. During the investigation, the phenomenon of FAP crystals aggregation by means of mutual intergrowing of epitaxial CaSO₄· 0.5H₂O coatings and the peculiarities of the etching process for the FAP crystal surface under conditions of CaSO₄· 0.5H₂O coatings formation were also studied.     

Analytical electron microscopy study of high Tc superconductor YBa2Cu3O7

The high T_c superconducting material YBa₂Cu₃O₇ shows a complex relationship between microstructure and oxygen content, which are controlled by length of heat treatment, atmosphere, and quench rate. An AEM investigation studying changes in the oxygen near edge features was undertaken. Electron energy loss spectroscopy (EELS) measurements have the important advantage that they can be made on single crystal grains, allowing orentation-dependent studies. Both ion-milled and crushed samples with varying O₂ content were analyzed. The structure of YBaCu₃O₇ was determined by neutron diffraction to be orthorhombic with distinct Cu-O chains along the b-axis as well as Cu-O planes in the a—b plane. Therefore, by looking for a crystallographic dependence of the oxygen K-edge one might be able to distinguish inequivalent oxygen atoms by their core level binding energy and correlate site occupancy with varying O₂ content. The EELS results on the oxygen K-edge are strongly dependent on oxygen content, most noticeably when the c-axis is parallel to the electron beam.     

An electron microscopy study of the phase Al3Fe

The phase Al₃Fe (monoclinic C2/m, a = 1·549 nm, b = 0·808 nm, c = 1·248 nm, β = 107·8°) has been studied by transmission electron microscopy (TEM) and high resolution electron microscopy (HREM). Crystals were obtained from a direct chill-cast ingot of an Al-0·25 wt% Fe-0·13 wt% Si alloy. Extracted crystals were prepared by dissolving the aluminium phase in butanol and filtering off the particles. The extracted Al₃Fe crystals were mainly (100) platelets. The monclinic lattice was confirmed by tilt experiments and the mirror plane was confirmed by convergent beam electron diffraction. Experimental HREM images from the [100] and [110] projection agreed with images calculated by the multislice method. The interpretation of images in terms of a projected crystal structure is discussed. Common defects in Al₃Fe crystals were: twins on (100) and faults on (001). The (001) faults could be described by a displacement 1/2·[100] on a fault plane at z = 0·5 in the unit cell. A model for (001) faults, based on multiple twinning, is proposed.     

STM and AFM investigations of high-Tc superconductors

We have studied the (001) surface of single crystal YBa₂Cu₃O_7-x high-T_c superconductors using scanning tunnelling microscopy (STM) and atomic force microscopy (AFM) at room temperature at ambient pressure. Both methods show flat terraces with steps which are multiples of the c-axis lattice constant (of 1·17 nm) high. Our results show that the bulk crystal structure extends to the surface and that the crystals were formed by island growth. Only occasionally tunnelling was possible with sample bias voltages below +1·0 V. We interpret the observed voltage dependence and the difficulty to get good STM images to be due to the presence of a less-conducting surface layer. Auger spectroscopy indicates that carbon is present at the surface, which is probably related to a contamination layer.     

STM study of the structure of the sulphur (1×2) overlayer on molybdenum (001) in air: ordered domains,phase boundaries and defects

A (001) surface of molybdenum, covered by one monolayer of sulphur was prepared in UHV and characterized by LEED, Auger and XPS. This surface was found to be stable in air for periods of several days. STM images of the surface, obtained in air in the topographic and local barrier height modes, revealed the atomic arrangement of sulphur atoms in domains with 1×2 and 2×1 periodicities. Boundaries between domains, adsorbate and substrate defect structures and crystallites formed during the initial oxidation of the Mo substrate were observed.     

Anomalous voltage dependence of tunnelling microscopy in WSe2

We present scanning tunnelling microscopy (STM) investigations of the layered semiconductor WSe₂. The tunnelling experiments were performed in air and under silicone oil with markedly different results. In air, atomic resolution images of the hexagonally structured surface could be obtained for sample-to-tip voltages of both negative and positive polarities, from ?1·5 to ?0·3 V for negative sample and from +0·6 to +1·6 V for positive sample, respectively. Under silicone oil, however, good atomic images could be seen for negative sample biases down to at least ?14 V, while for positive sample biases no difference with respect to the tunnelling in air was found.     

Multiferroic behavior of the functionalized surface of a flexible substrate by deposition of Bi2O3 and Fe2O3

Thin films of bismuth and iron oxides were obtained by atomic layer deposition (ALD) on the surface of a flexible substrate poly(4,4′-oxydiphenylene-pyromellitimide) (Kapton) at a temperature of 250°C. The layer thickness was 50 nm. The samples were examined by secondary-ion mass spectrometry, and uniform distribution of elements in the film layer was observed. Surface morphology, electrical polarization, and mechanical properties were investigated by atomic force microscope, piezoelectric force microscopy, and force modulation microscopy. The values of current in the near-surface layer varied in the range of ±80 pA when a potential of 5 V was applied. Chemical analysis was performed by X-ray photoelectron spectroscopy, where the formation of Bi₂O₃ and Fe₂O₃ phases, as well as intermediate phases in the Bi–Fe–O system, was observed. Magnetic measurements were carried out by a vibrating sample magnetometer that showed a ferromagnetic response. The low-temperature method of functionalization of the Kapton surface with bismuth and iron oxides will make it possible to adapt the Bi–Fe–O system to flexible electronics.     

A spectroscopic study of some high-temperature superconductors using a low-temperature STM

We used a scanning tunnelling microscope (STM) to measure both the tunnel current, I, and the dynamic conductance, dI/dV, at 4·2 K for a number of high-transition temperature oxide superconductors. Large spatial variations in the tunnelling characteristics are observed. At low tunnel resistances, all samples show evidence of single electron tunnelling and incremental charging. Results on BiSrCaCu₂O_x show the coexistence of charging with Josephson coupling between grains within the sample. Results on both the Bi sample and a single crystal of YBa₂Cu₃O_6·5+x reveal possible energy gap (2A) values of 17 and 20 meV, respectively. A very sharp 5 meV gap, observed in ceramic samples of YBa₂Cu₃O_6·5+x and Y_0·5Al_0·05Ba₂Cu₃O_6·5+x, may indicate the presence of a lower temperature phase in these samples.     

Growth and properties analysis of metal-organic chemical vapor deposited MgxZn1-xO films on C-Al2O3 substrates

Mg_xZn_1−xO (0 〈 x ⩽ 0.12) thin films with the wurtzite structure have been successfully grown on c-Al₂O₃ substrates by metal-organic chemical vapor deposition (MOCVD). X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), photoluminescence (PL) spectrometry, and transmission measurements are performed to study the characteristics of Mg _x Zn_1−x O thin films. Results show that with increasing Mg content, the diffraction peak of Mg _x Zn_1−x O thin films shifts towards a higher diffraction angle (the biggest shift is 0.22°), and the full width at half maximum (FWHM) of the diffraction peak is broadened. Meanwhile, a blue-shift occurs at the near-band-edge (NBE) emission peak and the largest blue-shift of the band gap of the Mg _x Zn_1−x O films is 113 meV with Mg content x50.12. Therefore, the energy band gap of the Mg _x Zn_1−x O films is determined by Mg content in the thin films and the energy band gap increases with an increase of Mg content.     

Arsenic-terminated silicon and germanium surfaces studied by scanning tunnelling microscopy

The epitaxial growth of As on the (111) and (100) faces of Si and the (111) face of Ge has been studied with vacuum tunnelling microscopy. The (111) faces of both semiconductors display a principally 1×1 termination, but differ with the presence of point defects on the Si(111): As-1×1 surface and trenches separating large (～ 100 Å) domains on the Ge(111): As-1×1 surface. I-V characteristics of the tunnel junction show a surface energy gap of approximately 1·9 eV for the Si(111): As-1×1 surface and 0·9 eV for the Ge(111): As-1×1 surface, in good agreement with recent theoretical calculations for these systems. As deposition on Si(001) results in a nominal 2×1 reconstruction of symmetric As dimers and elimination of missing dimer defects characteristic of the native Si(001) 2×1 surface. Further studies on vicinal, double-stepped substrates shows the orientation of the dimers with respect to the substrate depends critically on the substrate temperature during the growth phase, with destruction of the single principle domain surface order occurring at temperatures in excess of 700°C.     

Experimental investigation on the electrochemical machining of 00Cr12Ni9Mo4Cu2 material and multi-objective parameters optimization

Special stainless steel 00Cr12Ni9Mo4Cu2 has multiple composition and inhomogeneous tissues; short circuiting will frequently occur when using conventional electrolyte processing. This article analyzes the reason why the process of machining is difficult from the material composition and structure. We used the NaNO₃ and NaClO₃ electrolyte composite to select the appropriate concentration, and then by using the orthogonal experiment and gray relational analysis method, we discussed how the voltage, feed speed, and electrolyte pressure solved the problem of the material removal rate (MRR), surface roughness (SR), and side gap. Under optimal conditions of 20 V, an electrolyte composite concentration of 178 g/l NaNO₃ and 41 g/l NaClO₃, a feed rate of 0.7 mm/min, and an electrolyte pressure of 0.8 MPa, a material removal rate of 100.8 mm3/min, a surface roughness of Ra 0.8 μm, and a side gap of 0.16 mm were produced. Given the same voltage, with an increasing cathode feed rate, the MRR was shown to increase while the surface roughness value and the side gap decreased. Under the same cathode feed rate, the MRR decreases, while the side gap and the surface roughness increase as the electrochemical machining application voltage increases. This study proves that using a certain concentration of electrolyte composite is a simple, low-cost, and feasible approach in improving efficiency and quality.     

Surface micromorphology of dental composites [CE‐TZP]‐[Al2O3] with Ca+2 modifier

The objective of this study was to characterize the three‐dimensional (3D) surface micromorphology of the ceramics produced from nanoparticles of alumina and tetragonal zirconia (t‐ZrO₂) with addition of Ca⁺² for sintering improvement. The 3D surface roughness of samples was studied by atomic force microscopy (AFM), fractal analysis of the 3D AFM‐images, and statistical analysis of surface roughness parameters. Cube counting method, based on the linear interpolation type, applied for AFM data was used for fractal analysis. The morphology of non‐modified ceramic sample was characterized by the rather big (1–2 μm) grains of α‐Al₂O₃ phase with a habit close to hexagonal drowned in solid solution of t‐ZrO₂ with smooth surface. The pattern surfaces of modified composite content a little amount of elongated prismatic grains with composition close to the phase of СаСеAl₃О₇ as well as hexahedral α‐Al₂O₃‐grains. Fractal dimension, D, as well as height values distribution have been determined for the surfaces of the samples with and without modifying. It can be concluded that the smoothest surface is of the modified samples with Ca⁺² modifier but the most regular one is of the non‐modified samples. A connection was observed between the surface morphology and the physical properties as assessed in previous works. Microsc. Res. Tech. 78:840–846, 2015. © 2015 Wiley Periodicals, Inc.     

Effect of Al2O3 Particle Size on Electrical Wear Performance of Al2O3/Cu Composites

Al₂O₃/Cu composites (1.0 vol%) reinforced with different size of α-Al₂O₃ particles were fabricated by a powder metallurgy method and electrical sliding wear tests were performed on a self-made pin-on-disk electrical wear tester. The effect of Al₂O₃ particle size on electrical wear performance of the Al₂O₃/Cu composite was studied, and the wear mechanism of the Al₂O₃/Cu composite was also discussed based on worn surface observations. The results show that the tribological properties of A1₂O₃/Cu composite are closely related to the mechanical properties. With an increase in Al₂O₃ particle size, the wear rates of A1₂O₃/Cu composites have a reverse variation with hardness of A1₂O₃/Cu composites. In the range of 50–100 nm, Al₂O₃/Cu composites have the highest wear resistance and mechanical properties. Microstructural observation revealed that the wear mechanisms of Al₂O₃/Cu composites were mainly adhesive wear and plastic deformation accompanied by a small amount of arc damage. In addition, the plastic deformation area on the pin sample of the frictional end depends on the electrical wear resistance of A1₂O₃/Cu composites.     

A small scanning tunnelling microscope with large scan range for biological studies

We have developed a scanning tunnelling microscope specially designed for biological applications presenting some new features: the scanner tube is mounted parallel to the surface of the sample which enables a high resolution optical microscope to be brought close to the sample when working in air or liquids. The maximum scan range is 5×20 μm with a vertical range of 20 μm and the total size of the system does not exceed 10×40 mm. The piezo-sensitivity of the scanner tube versus applied voltage was analysed by interferometry measurements and by using scanning tunnelling microscopes. We found a value for the piezoelectric constant d₁₃ of ?1·71 Å/V at low voltages (under a few volts) going up to ?2 Å/V for higher voltages. Large-scale images of a carbon grid showed a surprisingly good linearity of the scanner tube.     

Automated Microscope-Absorption-Spectrophotometry Of Rock-Forming Minerals In The Range 40,000–5000 Cm−1 (250–2000 Nm)

To measure polarized absorption spectra of microcrystals of 3dⁿ-ion bearing silicate minerals, computer processed, microscope-spectrophotometric methods have been developed. Absorbance, log (I₀/I), can be measured with high relative accuracy (near u.v. and vis: ±0·002 to 0·001; n.i.r.: ±0·004 to 0·002), and relatively small spectral band widths are available. Hence, weak spin-forbidden dd-bands of 3dⁿ-ions can be recognized alongside spin-allowed dd-transitions without artificial broadening of absorption bands due to finite resolution. The smallest area from which absorption spectra can be taken is 8 μm in diameter. As one example of the many applications in mineralogy and material sciences, absorption spectra of a natural spessartine garnet, Spess_69·7Alm_30·0Gross_0·05, containing Mn²⁺, Fe²⁺, and traces of Fe³⁺ as 3dⁿ-ions, and of a pure Mn²⁺-garnet, Spess₆₇Gross₃₃, are presented. From these it is evident that bands in natural spessartines at ～ 26,900, 23,200 cm^?1 which were assigned to dd-transitions in Fe³⁺⁽⁶⁾, have to be reassigned to Mn²⁺⁽⁸⁾. Comparison of spectra obtained with the microscopic equipment described with those obtained by means of conventional macroscopic equipment prove that the methods described produce true spectra.