Preparation and calibration of the NBS SRM767: A superconductive temperature fixed point device

More than 100 cryogenic-temperature reference-point devices based upon the reproducible superconductive transition temperatures of five elements have been distributed through the NBS Office of Standard Reference Materials as SRM 767. The fixed-point temperatures of the device have been noted as reference temperatures of the Provisional Temperature Scale 0.5 K to 30 K. The methods of preparation of the device, the procedures for its calibration, and a summary of the results of the calibration experiments are briefly presented. Suggestions are offered for verification of the device temperatures and for improvements in the device performance.     

Theoretical and experimental study of the saturation of a superconducting composite under fast changing magnetic field

The behaviour of a superconducting composite subjected to a transverse time-varying magnetic field has been simulated with an electrical network. The problem of the current distribution inside the composite has been solved with a computer code.The theoretical results clearly show the existence of different zones (saturated, intermediate, and saturation-free zones) as well as their evolutions when the field variation rate ? increases. The loss power has been also calculated.We found that the penetration of the saturated zones in the middle plane of the composite is relatively easy, and that the centre is reached for a finite value of ? However, the total saturation of the whole composite is achieved asymptotically only when ? becomes infinite. Experiments using high values of ? have confirmed the theoretical predictions about the evolution of the saturated zones inside the composite.     

The superconducting transition temperature of multilayer superconductors: The frustratedXY-Josephson model

The superconducting transition temperature of multilayered superconducting structures of the S-N(D)-S type in the RVB theory is defined taking into account the resonance pair tunneling between superconducting layers. The Josephson array defectivity is considered with the frustration parameters.     

Control of ductile to brittle transition temperature in ferritic pressure vessel steel

Abstract

Controlled amounts of cold work are shown to cause a minimum in the ductile to brittle transition temperature (DBTT) in a ferritic steel at a critical level of ～1·5%. Mechanical property assessments show that the hardness values exhibit the same trend. A theory is advanced for explanation of these effects, based on work hardening and Cottrell–Bilby locking models. Consideration is given to an alternative Ashby–Embury model, but it is concluded that the former approach is most successful in predicting the observed DBTT shift behaviour. Although independent of fracture surface type, the degree of plastic deformation shows some dependency on the grain boundary character. This leads to the conclusion that the matrix yield strength is the primary factor in determining the DBTT in these steels. Discussion focuses on methods for exploiting the effect to give higher toughness steels utilising knowledge of how to control matrix hardening and cleavage fracture strength.     

Electrical conductivity and transition temperature of NbN thin films

Niobium nitride thin films are grown using reactive RF sputtering technique for four different partial nitrogen pressures in argon atmosphere. The superconducting transition temperature of the films has been measured. The films exhibit a negative temperature coefficient of resistivity. The electrical characterization of the films has been carried out and the conductivity measured between room temperature and liquid nitrogen temperature. This is fitted usingT ^P law.     

Composition dependence of transition temperature in some ceramic superconductors

The composition dependence of transition temperature in some ceramic superconductors (La_2−x (Ba, Sr) _x CuO₄) was studied by modifying our earlier approach and developing a Fourier-transformed effective potential which involves the effect of two-dimensional (2D) acoustic plasmons. This potential was used to obtain the pairing (electron-electron attraction) parameter (λ), the averaged Coulomb repulsive parameter (μ*) and the cut-off 2D acoustic plasmon frequency (ω _c) required to compute the superconducting transition temperature (T _c) from the strong coupling theory. The variations ofT _c with compositions (x) obtained for La_2−x (Ba, Sr) _x CuO₄ show reasonably good agreement with experimental data.     

Direct observation of the transition point between quasi-spherical and cubic nanoparticles in a two-step seed-mediated growth method

An aqueous seed-mediated growth method is adapted to explore the shape transformation of quasi-spherical Au seeds to nanocubes in a direct and continuous manner. Quenching the growth process at varied reaction-duration times and exploring the intermediate products by high-resolution transmission electron microscopy and UV/vis spectroscopy shows an abrupt cuboctahedral-to-nanocube transition at 25-27 nm without any change in the nanoparticle size. The size of the obtained nanocubes remains constant (25-27 nm) until most (>90%) of the cuboctahedral nanoparticles are transformed to nanocubes. At this point, the (25-27 nm) nanocubes initiate further continuous and homogeneous growth until they reach 50-nm Au cubes. These observations are ascribed to a scenario in which the kinetically controlled growth mode of the nanoparticle is significantly affected by the surface self-diffusion of metal adatoms, especially when the adatom's self-diffusion distance is comparable with the nanoparticle's size.     

Effect of growth temperature on polytype transition of GaN from zincblende to wurtzite

We have investigated effect of growth temperature on the polytype conversion of cubic GaN (c-GaN) grown on GaAs (001) substrates by MOVPE. It was found that the polytype transition of GaN from zincblende (cubic) to wurtzite (hexagonal) structures is much dependent on the growth temperature. Transmission electron microscopy (TEM) observations demonstrate that the GaN grown layers have the cubic structure (c-GaN) and contain bands of stacking faults (SFs) parallels to {111} planes. For low growth temperatures (∼ 900 °C), XRD results demonstrate that the GaN grown layers with the cubic phase purity higher than 85% were obtained. No different types of single diffraction spots, indicating the incorporation of single-crystal h-GaN, on the selected area diffraction (SAD) pattern was observed. It is also found that a density of SFs decreases with the distance from the interface of c-GaN/GaAs. On the other hand, GaN layers exhibited a transition from cubic to mixed cubic/hexagonal phase under conditions of increasing growth temperature (∼ 960 °C) as determined using TEM-SAD technique with complementary XRD and PL observations. In addition, the optical characteristics of c-GaN layers are shown to be very sensitive to the presence of the single-crystal h-GaN.     

Relationship between transition of fracture mode of carbon fiber-reinforced plastic and glass transition temperature of its resin

In this study, the correlation between the stress–strain behavior of a carbon fiber-reinforced plastic (CFRP) and the temperature at which the heat-affected zone (HAZ) is generated is investigated. First, CFRP ([?45/45]_2s laminate) specimens were heated at several temperatures to induce thermal damage, i.e. a HAZ. Subsequently, tensile tests were conducted on the specimens with thermal damage. Then, microscopy and X-ray measurements were carried out to discuss the stress–strain responses from a microscopic viewpoint. The results of strain measurement during thermal treatment indicated that the strain increases with increasing temperature. The tensile tests showed that the CFRP specimens subjected to thermal damage during heating at a high temperature fractured in the ductile mode, whereas the fracture mode of the CFRP specimens with low-temperature thermal damage was discontinuous. Microstructure observation using X-ray tomography showed that the debonding between the carbon fibers and the resin matrix induced by heating to above the glass transition temperature was responsible for the continuous fracture mode.     

On the use of Charpy transition temperature as reference temperature for the choice of a pipe steel

Transition temperature is not intrinsic to material but depends on specimens and mode of loading used for tests. Here, the linear dependence of transition temperature with constraint is shown. Constraint is evaluated by the effective T stress which is the value of the stress difference distribution for the effective distance provided by the Volumetric method.Application of this approach gives the best choice of the reference transition temperature by reducing conservatism when comparing with intrinsic transition temperature of the studied structure.     

氢稀释比与衬底温度对硅基薄膜相变及其光电性能影响的研究

采用热丝辅助微波电子回旋共振化学气相沉积法(HWAMWECR-CVD),通过改变衬底温度及氢稀释比制备了系列硅基薄膜,研究了衬底温度及氢稀释比对薄膜由非晶相转晶相相变及其光电性能的影响。研究结果表明,当采用低温制备硅基薄膜时,衬底温度和氢稀释比的提高都有利于非晶相向晶相的转变,但提高氢稀释比对相变的影响更为显著;晶化比越高并不代表薄膜光电性能越好,95%氢稀释比条件下制备的微晶硅薄膜具有优良的光电性能。     

Synthesis of carbazole derivatives with high quantum yield and high glass transition temperature

A series of fluorescent compounds derived from carbazole were synthesized via the Wittig–Horner reaction. The compounds had high glass transition temperatures (T_g), in the region of 172–232 °C, and high decomposition temperatures (T_d), ranging from 456 to 491 °C. The derivatives showed very high fluorescence efficiency in solution, with fluorescence quantum yields in the range of 0.88–1.00. The substituent effects on fluorescence emission differed between solution and solid state. In dichloromethane solutions, the substituents had only a minimal effect on the maximum emission wavelength of the compounds with the same bridge.     

An X-ray spectral study of the influence on the electronic structure of highT c superconductors of the parameters determining the superconducting transition temperature

CuL _α and OK _α X-ray fluorescence spectra, corresponding to the Cd3d-Cu2p and O2p-O1s transitions, and XANES and EXAFS spectra (CuK, YK, BiL, PbL) have been obtained for the following series of samples: (i) YBa₂Cu₃O_7−δ (0<δ<0·8), (ii) La_2−x Sr _x CuO₄ (0<x<0·3) and (iii) Bi₂(Sr, Ca) _n+1 Cu _n O_2n+4 (n=1,2,3), Bi_1·8Pb_0·3Sr_1·9Ca₂Cu₃O _y . A correlation has been found between the decrease in the superconducting transition temperature (T _c ) with variation in the stoichiometric parametersδ andx in series (i) and (ii), and the shift of the maximum in their CuL _α spectra and the relative increase in the number of occupied states Cu3d in the upper part of the valence band. The changes in the distribution of the Cu3d and O2 _p densities and in the spectral parameters of the samples are discussed.     

The use of fixed points based on metal-carbon eutectic alloys to increase the accuracy of measurements in the temperature range above 1357.77 K

It is shown that it is possible in practice to use high-temperature models of black bodies based on phase transitions of metal-carbon eutectics for radiometry, radiation thermometry, and photometry. __________ Translated from Izmeritel’naya Tekhnika, No. 2, pp. 51–54, February, 2007.     

热氧化法制备二氧化钒薄膜及其相变温度研究

采用直流磁控溅射在普通玻璃衬底上沉积金属V膜,然后在空气中热氧化制备VO2薄膜.用X射线衍射仪(XRD)、原子力显微镜(AFM)、傅里叶变换红外光谱仪(FT-IR)和X射线衍射光电子能谱仪(XPS)分析氧化温度对薄膜的微观结构、光学透过率、相变温度及其组分的影响.结果表明:金属V膜在空气中400℃热氧化1 h得到相变温...     

A method for preparation of a high-quality high temperature superconducting ceramics

An improved method for hot pressing of high temperature superconducting (HTSC) powders prepared by conventional solid phase synthesis of the initial BaCO₃, CuO and Ln₂O₃ allows one to obtain HTSC targets and magnetic shields possessing a high degree of homogeneity. By sputtering such targets HTSC films with critical current density of 3.3·10⁶ A/cm² have been deposited. Shields, prepared according to this method, show a shielding coefficient of 10⁵ andH _c of 79 Öe in the constant magnetic field and in the alternating magnetic field the amplitude is 90 Öe in the frequency range of 70–3000 Hz.     

Determination of the temperature corrections to the readings of measuring instruments under dynamic temperature conditions

A method of obtaining the temperature corrections to readings of measuring instruments, taking their thermal inertia into account, when the temperature of the medium changes is proposed. The procedure was tested under laboratory conditions on static nonthermostated gravimeters. The results of the investigations, which confirm the effectiveness of the proposed procedure, are presented. __________ Translated from Izmeritel’naya Tekhnika, No. 1, pp. 50–52, January, 2007.     

Effect of oxygen deficiency (δ) on transition temperature of yttrium cuprate superconductors

The nature of pairing mechanism as well as transition temperature of yttrium cuprates is discussed using the strong coupling theory. An interaction potential has been developed for the layered structure with two conducting CuO₂(a–b) layers in a unit cell. The interaction potential properly takes care of electron-electron, electron-phonon and electron-plasmon interactions. Furthermore, the electron-phonon coupling parameter (λ), the modified Coulomb repulsive parameter (μ*) and the 2D acoustic phonon (plasmon) energy as a function of oxygen deficiency is worked out. Finally, the superconducting transition temperature (T _c) is then evaluated by using these coupling parameters and obtainedT _c = 95(92)K for Y(Yb)Ba₂Cu₃O_7−δ superconductors withδ = 0·0. The model parameters estimated from the layered structure approach are consistent with the strong coupling theory. The result deduced on the variation ofT _c withδ are in fair agreement with the earlier reported data on yttrium cuprates. The analysis of the above results are discussed.     

Realization of laser textured brass surface via temperature tuning for surface wettability transition

Superhydrophobic surfaces have attracted extensive interests and researches into their fundamentals and potential applications. Laser texturing provides the convenience to fabricate the hierarchical micro/nanostructures for superhydrophobicity. However, after laser texturing, long wettability transition time from superhydrophilicity to superhydrophobicity is a barrier to mass production and practical industrial applications. External stimuli have been applied to change the surface composition and/or the surface morphology to reduce wettability transition time. Herein, by temperature tuning, wettability transition of laser textured brass surfaces is investigated. Scanning electron microscopy and surface contact angle measurement are employed to characterize the surface morphology and wettability behavior of the textured brass surfaces. By low-temperature heating (100 ℃~150 ℃), partial deoxidation of the top CuO layer occurs to form hydrophobic Cu2O. Therefore, superhydrophobicity without any chemical coating and surface modification could be achieved in a short time. Furthermore, after low-temperature heating, the low adhesive force between the water droplet and the sample surface is demonstrated for the laser textured brass surface. This study provides a simple method to fabricate the micro/nanostructure surfaces with controllable wettability for the potential applications.