Search

Terra preta is a fertile anthropogenic soil found in the Amazon basin. One of the most significant differences between the terra preta and surrounding soils is that terra preta is rich in aromatic carbons. Previous infrared investigations of terra preta were reported at energies above 1000 cm−1 where many other forms of carbon also have absorption lines. No measurements have been reported below 800 cm−1, where many absorptions associated with aromatic carbons occur in the absence of aliphatic carbon lines. We employ Fourier transform infrared spectroscopy between 150 cm−1 and 500 cm−1. A comparison was made between the spectra of terra preta, several pure aromatic compounds, organic fertilizers developed to replicate terra preta and several Australian soils, some of which containing char from bushfires. The spectra in the 150–500 cm−1 range were very similar between terra preta and the organic fertilizers, while they were very different for the natural soils. These findings indicate that the content of aromatic carbons in terra preta and organic fertilizers is different than in natural soils containing the bushfire chars, but also soils produced entirely by bacterial and fungal activities. This point to the importance of the preparation conditions of the biochars, which are essential ingredients of terra preta and organic fertilizers used in this study.

Fourier transform infrared spectroscopy was performed for two structurally similar liquid crystals: 4-octyloxybenzoic acid and 4-decyloxybenzoic acid in 6–15 THz range. Density functional theory modeling at PBE0/def2-TZVPP level was used to assign the vibrational modes to each observed absorption peak. We observed six peaks that are common for both liquid crystals, assigned to the same vibrational modes and appearing at similar energies. Each of the samples also had additional peaks unique to itself. A strong absorption peak appears at about 280 cm–1 for both samples; however, it corresponds to different vibrational modes for the two samples. This work shows that the spectroscopy in this often-neglected frequency range can easily distinguish structurally similar compounds.

The resistivity of the undoped In0.53Ga0.47As photoconductive semiconductor switch has been measured over a temperature range of 5–300 K. The experimental temperature characteristic curve of the resistivity exhibits unimodality, and the peak resistivity is more than two orders of magnitude greater than that at 300 K. Based on the two-level model (shallow donor impurities level and deep defect level), we analyzed carrier relaxation rate due to scattering by ionized impurity and polar optical phonons. It is found that the unusually thermal characteristics is dominated by scattering of donor states by ionized impurity scattering and polar optical phonon scattering. The theoretical result is in very good agreement with the experimental results. Our results provide quantitative understanding of charge relaxation in InxGa1−xAs-based devices. Understanding of thermal properties of dark resistance can be used to optimize InxGa1−xAs-based electronic and photonic devices performance in different temperature regimes.

Radom su obuhvaćene teorijske osnove grejanja i klimatizacije kao i proračuni svih elemenata vezanih za ovu oblast kao i praktična provera zamenjenog kotla, provera postojećeg dimnjaka zahtevima novo instalisanog kotla, kao i tehničko rešenje grejanja i klimatizacije manipulativne zgrade na naftnoj sabirnoj stanici uz pomoć toplotne pumpe „vazduh – voda“.

Biochars have been recognized as an important material to improve soil properties. In a number of studies their beneficial properties have been found to increase with residence time in soil and during the composting process. The beneficial properties have been correlated with surface functional groups resulting from the interactions between char particles, inorganic and organic matter in the soil and soil biota. These interactions result in the formation of organo-mineral phases on the internal and external surfaces of the biochar. A paucity of information exists, particularly from longer-term field trials, on organo-mineral phases present on both the internal and external surfaces of the biochar. To characterize the structure of, and interface between, the carbon and mineral phases, we examined biochars recovered from two field trials and after composting from different countries using high resolution scanning electron microscopy (SEM), atomic resolution transmission electron microscopy (TEM) and scanning TEM (STEM), energy electron loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy (EDS) at resolutions of 1–20 nm. The work revealed the formation of porous agglomerates of different minerals/inorganic compounds bound together with organic compounds on the surfaces of the biochar. In some cases, these agglomerates were bound together to form organo-mineral associations. The analyses also showed that the organic compounds containing both N and C functional groups and mixed valence iron oxide nanoparticles are possibly interacting with the organic compounds. The analysis also showed the formation of pores at the interface of the carbon matrix and organo-mineral aggregates.

The low-energy lattice dynamics of the quasiperiodic spin-ladder cuprate ${\mathrm{Sr}}_{14\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$ are investigated using terahertz frequency synchrotron radiation. A high density of low-lying optical excitations is present in the 1--3 THz energy range, while at least two highly absorbing excitations stemming from long-wavelength acoustic oscillations of the incommensurate chain and ladder sublattices, are observed at subterahertz frequencies. The effects of Ca substitution on the subterahertz quasiacoustic sliding mode gaps is investigated using coherent synchrotron radiation. Analysis of the results suggest increasing substitution of Sr for Ca is accompanied by a transfer of spectral weight between sliding modes associated with different chain-ladder dynamics. The observation is consistent with a transfer of hole charges from the chains to the ladders and modification of the sublattice dimensions following Ca substitution. The results are discussed in context to the significance of low-lying vibrational dynamics and electron-phonon coupling in the superconducting state of certain quasiperiodic systems.

We have measured the spectrum of L-alanine in the terahertz region over a wide temperature range. At the lowest temperature, the fundamental line at 2.21 THz is very sharp, 0.02 THz (0.8 cm-1). As temperature increases, the line broadens, undergoes a non-uniform red shift, becomes more asymmetrical and decreases in oscillator strength.

Nanoparticles have a great potential to increase the therapeutic efficiency of several cancer therapies. This research examines the potential for silver-doped lanthanum manganite nanoparticles to enhance radiation therapy to target radioresistant brain cancer cells, and their potential in combinational therapy with magnetic hyperthermia. Magnetic and structural characterisation found all dopings of nanoparticles (NPs) to be pure and single phase with an average crystallite size of approximately 15 nm for undoped NPs and 20 nm for silver doped NPs. Additionally, neutron diffraction reveals that La0.9Ag0.1MnO3 (10%-LAGMO) NPs exhibit residual ferromagnetism at 300 K that is not present in lower doped NPs studied in this work, indicating that the Curie temperature may be manipulated according to silver doping. This radiobiological study reveals a completely cancer-cell selective treatment for LaMnO3, La0.975Ag0.025MnO3 and La0.95Ag0.05MnO3 (0, 2.5 and 5%-LAGMO) and also uncovers a potent combination of undoped lanthanum manganite with orthovoltage radiation. Cell viability assays and real time imaging results indicated that a concentration of 50 μg/mL of the aforementioned nanoparticles do not affect the growth of Madin-Darby Canine Kidney (MDCK) non-cancerous cells over time, but stimulate its metabolism for overgrowth, while being highly toxic to 9L gliosarcoma (9LGS). This is not the case for 10%-LAGMO nanoparticles, which were toxic to both non-cancerous and cancer cell lines. The nanoparticles also exhibited a level of toxicity that was regulated by the overproduction of free radicals, such as reactive oxygen species, amplified when silver ions are involved. With the aid of fluorescent imaging, the drastic effects of these reactive oxygen species were visualised, where nucleus cleavage (an apoptotic indicator) was identified as a major consequence. The genotoxic response of this effect for 9LGS and MDCK due to 10%-LAGMO NPs indicates that it is also causing DNA double strand breaks within the cell nucleus. Using 125 kVp orthovoltage radiation, in combination with an appropriate amount of NP-induced cell death, identified undoped lanthanum manganite as the most ideal treatment. Real-time imaging following the combination treatment of undoped lanthanum manganite nanoparticles and radiation, highlighted a hinderance of growth for 9LGS, while MDCK growth was boosted. The clonogenic assay following incubation with undoped lanthanum manganite nanoparticles combined with a relatively low dose of radiation (2 Gy) decreased the surviving fraction to an exceptionally low (0.6 ± 6.7)%. To our knowledge, these results present the first biological in-depth analysis on silver-doped lanthanum manganite as a brain cancer selective chemotherapeutic and radiation dose enhancer and as a result will propel its first in vivo investigation.

For the first time, large single crystals of the simplest amino acid, glycine, have been used to determine the temperature dependence of its terahertz spectrum. High-quality spectra with very sharp absorption features are observed at cryogenic temperatures. The α-glycine structure and the purity of the crystals were verified via Raman spectroscopy and X-ray diffraction. Spectral redshift with increasing temperature was observed for all absorption bands in the terahertz region (10–250 cm−1, or 1–8 THz) over the temperature range of 20–300 K. X-ray diffraction revealed expansion in all planes of the crystal lattice over the same temperature range. A Bose-Einstein distribution was used to model the frequency position shift of the two lowest-energy fundamental modes at 50 cm−1 and 69 cm−1. On this basis, we attribute the observed redshift and broadening with increasing temperature to the anharmonic potential associated with the phonon bath.

A recent application of terahertz spectroscopy is to biochar, the agricultural charcoal produced by pyrolysis of various organic materials. Biochars simultaneously improve soil fertility and assist in carbon sequestration. Terahertz spectroscopy allows different biochars to be distinguished. However, the origin of the absorption features observed has not been clear. Given that biochar-based fertilizers are rich in aromatic compounds, we have investigated simple aromatic compounds as an approach to unravelling the complex biochar spectrum.

Rare earth orthoferrites demonstrate great application potentials in spintronics and optical devices due to their multiferroic and magnetooptical properties. In RFeO3, magnetic R3+ undergo a spontaneous spin reorientation in a temperature range determined by R (rare earth), where the magnetic structure changes from Γ2 to Γ4. The b-axis component of their magnetic moment, Mb, is reported in numerous neutron diffraction studies to remain zero at all temperatures. More sensitive magnetometer measurements reveal a small non-zero Mb, which is minute above ∼200 K. Mb increases with cooling and reaches values of ∼10–3 μB/f.u. at temperatures within or below the spin reorientation temperatures. Our results can be explained by assuming the Fe3+ spins as the origin of non-zero Mb, while R3+ spins suppress Mb. The representation analysis of point groups shows that non-zero Mb is associated with a small admixture of the Γ3 phase to Γ2 or Γ4. Such a mixing of the three magnetic phases requires at least a fourth order of the spin Hamiltonian for RFeO3 to describe the non-zero Mb.

The terahertz absorption spectra of aspirin and benzoic acid have been measured in the range 200-500 cm-1 (6-15 THz). Density-functional theory (DFT) modeling has assigned fundamental vibrational modes to the observed absorption bands. Hydrogen bonds between the crystalline planes of aspirin resulted in better agreement between the experimental and modeled spectra than for benzoic acid. The similar structure of these two molecules suggests a similar absorption spectrum, which indeed was obtained experimentally. However, the detailed crystal structure and molecular differences result in some of the apparently common absorption bands being assigned to different vibrational modes through the DFT modeling. Thus, our study importantly reveals that even though crystalline forms of two similar molecules may have similar experimental terahertz spectra, the resemblance may be superficial rather than fundamental.

The influence of different boron precursor powders on the critical current density of MgB2 superconductor with added Ni– Co–B nanoparticles is investigated. Different types of boron have different morphologies, particle sizes and impurity content, all of which have a strong impact on the critical current density (Jc). The boron samples investigated in this study include three different boron powders: semicrystalline boron (denoted as SC), with nanosized particles and high purity (99%) supplied from Specialty Materials, Inc. (USA) ; amorphous boron (denoted as AB), with nanosized particles and high purity (99%) supplied from Russia ; and finally pure crystalline boron (denoted as CB), with micron-sized particles and 96% purity supplied from Tangshan WeiHao (China). The MgB2 samples made from AB showed the best improvement of critical current with 2.5 wt.% Ni–Co–B doping ; Jc was above 100, 000 A cm−2 at 4 T and 5 K. Under the same conditions, Jc for MgB2 made from SC was slightly worse than for the MgB2 made from AB, ∼100, 000 A cm −2. On the other hand, MgB2 made from CB presented sharply reduced Jc with Ni–Co–B doping at both measured temperatures, 5 K and 20 K.

We investigate carbon-based materials (graphite, graphite nanofibers and multi-walled carbon nanotubes) using terahertz time-domain spectroscopy (THz-TDS). The THz-TDS of carbon-based materials has been especially useful for understanding of many fundamental physical properties (e.g. the electronic structure and electron-phonon interactions, etc.) of all sp2 carbons, given that carbon-based materials can be either semiconducting or metallic depending on their geometric structures. A significant difference was obtained in the spectra of graphite nanofibers as compared to the multi-walled carbon nanotubes and graphite.

Herein we investigate a (001)-oriented GaAs1−xBix/GaAs structure possessing Bi surface droplets capable of catalysing the formation of nanostructures during Bi-rich growth, through the vapour-liquid-solid mechanism. Specifically, self-aligned “nanotracks” are found to exist trailing the Bi droplets on the sample surface. Through cross-sectional high-resolution transmission electron microscopy the nanotracks are revealed to in fact be elevated above surface by the formation of a subsurface planar nanowire, a structure initiated mid-way through the molecular-beam-epitaxy growth and embedded into the epilayer, via epitaxial overgrowth. Electron microscopy studies also yield the morphological, structural and chemical properties of the nanostructures. Through a combination of Bi determination methods the compositional profile of the film is shown to be graded and inhomogeneous. Furthermore, the coherent and pure zincblende phase property of the film is detailed. Optical characterisation of features on the sample surface is carried out using polarised micro-Raman and micro-photoluminescence spectroscopies. The important light producing properties of the surface nanostructures are investigated through pump intensity-dependent micro-PL measurements, whereby relatively large local inhomogeneities are revealed to exist on the epitaxial surface for important optical parameters. We conclude that such surface effects must be considered when designing and fabricating optical devices based on GaAsBi alloys.

Polyhedral magnetite nanocrystals with multiple facets were synthesised by a low temperature hydrothermal method. Atomistic simulation and calculations on surface attachment energy successfully predicted the polyhedral structure of magnetite nanocrystals with multiple facets. X-ray diffraction, field emission scanning electron microscopy, and high resolution transmission microscopy confirmed the crystal structure of magnetite, which is consistent with the theoretical modelling. The magnetic property measurements show the superspin glass state of the polyhedral nanocrystals, which could originate from the nanometer size of individual single crystals. When applied as an anode material in lithium ion cells, magnetite nanocrystals demonstrated an outstanding electrochemical performance with a high lithium storage capacity, a satisfactory cyclability, and an excellent high rate capacity. Crystal class: Polyhedral magnetite nanocrystals were prepared by a low temperature hydrothermal method, in which multiple facets can be theoretically predicated (see figure). The as-prepared magnetite nanocrystals show a magnetic property of superspin glass state, and demonstrated an outstanding electrochemical performance for lithium storage in lithium ion cells. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We report a facile and efficient synthesis technique for the preparation of iron oxide semiconductor nanowires in large quantity by using nitrilotriacetic acid (NTA) as a chelating agent to form polymeric chains, followed by heat treatment. This technique can also be applied for preparing other transition metal oxide nanowires such as MnO2 nanowires and NiO nanowires. The as-prepared α-Fe2O3 nanowires have exhibited a blue shift of bandgap, peculiar magnetic properties, and high sensitivities toward ethanol and acetic acid gases. © 2008 American Chemical Society.

We present a detailed analysis of the effects of excess Mg on the lattice parameters, the critical temperature (Tc), the critical current (Jc), the irreversibility field (Hirr), and the upper critical field (Hc2) of MgB2 bulks and wires produced by an in situ solid-state reaction method and the powder-in-tube method, respectively. It was found that Jc, Hirr, and Hc2 were significantly enhanced for Mg excess samples. All these properties were highly sensitive to the processing temperature for Mg excess samples, while there was only a weak dependence on processing temperature for normal ones. The Tc variation for the 10% Mg excess sample was 1.6 K (36.3–37.9 K) when the sintering temperature changed from 650 to 850 °C while it only varied by 0.5 K (37.2–37.7 K) for the normal sample. The low-field Jc for the 10% Mg excess samples sintered at 750 °C increased by a factor of 3, compared to that for the normal MgB2 samples, while the Hc2 for the 10% Mg excess samples sintered at 650 °C reached 8.7 T at 25 K, compared to 6.6 T for the normal sample. Rietveld refinement x-ray diffraction (XRD) analysis showed that the MgO content was reduced in the 10% excess Mg samples, leading to an increase in the effective cross section of the superconductor.

Terahertz spectroscopy of benzoic acid (BA) and two of its derivatives, 2-hydroxybenzoic acid (2OH-BA) and 3-hydroxyben-zoic acid (3OH-BA), has been investigated in the spectral region 200 to 500 cm−1 (6.06 to 15.15 THz). The spectra show distinct absorption features. There is agreement between some of the absorption lines observed for these compounds, despite a shift in energy, which is attributed to the differences in the molecular structures. Numerical modeling gave corresponding absorption lines, and this helped in the assignment. Temperature dependence studies revealed that most of the absorption lines are composite lines in this frequency region.

Previous THz spectroscopy of the TNT explosive precursor, 2,4-dinitrotoluene (DNT), has been restricted to room temperature (apart from one set of data at 11 K). Here, for the first time, we investigate the spectrum as the temperature is systematically varied, from 7 to 245 K. Many new features appear in the spectrum on cooling below room temperature. As well as the five absorption lines observed previously, we observe five additional lines. In addition, a new room-temperature line at 8.52 THz (281 cm(-1)) is observed. Six of the lines red-shift with temperature and four of them blue-shift. The blue shift is explained by interplay between intramolecular and intermolecular hydrogen bonds. The variation in line width and line intensity with temperature is not systematic, although a conspicuous decrease in line intensity with temperature is observed in all cases. Modeling with hybrid PBE0 and TPSSh functionals helps identify absorption modes.

Grafička proizvodnja danas se sve više okreće funkcionalnom tisku koji obuhvaća široki spektar tiskanih komponenti koje su najčešće dio drugih proizvoda. Takav funkcionalni tisak je svoje mjesto pronašao u različitim industrijama, a najčešće je korišten u tiskanoj elektronici. Tiskana elektronika podrazumijeva tiskanje vodljivih ili optičkih tiskarskih bojila na podlogu kako bi se stvorio aktivni ili pasivni uređaj. Pri tome elektrovodljiva tiskarska bojila uvijek sadrže vodljive čestice nekog metala, dok podloga može biti od različitih krutih ili fleksibilnih materijala. U tu svrhu se najčešće primjenjuju različiti karbonski i polimerni filmovi, no sve je češća uporaba papira zbog njegove cijene i ekološke prihvatljivosti. Stoga je cilj ovog rada bio istražiti mogućnost primjene elektrovodljivih na tri različite vrste papira. U eksperimentu su korišteni nepremazani premazani i reciklirani papir, a otisci su napravljeni tehnikom sitotiska s bojilom koje sadrži elektrovodljive čestice srebra. U radu se nastojalo definirati promjene u električnom otporu otisnutih elemenata s obzirom na vrstu tiskovne podloge tj. vrste papira i dimenzija otisnutih elemenata. Rezultati istraživanja ukazuju na razlike u jačini električnog otpora na različitim vrstama papira, ali i ovisnost jačine električnog otpora o dimenzijama otisnutih elemenata. Također se utvrdilo da je otpor otisnutih elemenata na premazanom papiru veći u odnosu na nepremazani, dok je otpor elemenata otisnutih na recikliranom papiru veći u odnosu na nepremazani i premazani papir. Električna otpornost otisnutih elemenata se ponašala linearno, tj. s jednolikim smanjenjem ili povećanjem dimenzija otisnutih elemenata došlo je do približno jednake promjene vrijednosti električnog otpora na sve tri vrste papira. Iz dobivenih rezultata istraživanja može se zaključiti da postoji veliki prostor za primjenu papira u funkcionalnom tisku jer svojstva otisnutih elemenata pokazuju zadovoljavajuće fizikalno svojstvo električne vodljivosti. To je važna značajka za primjenu papira u onim segmentima funkcionalnog tiska gdje nije potreban dugi rok uporabe gotovog proizvoda kao što je npr. industrija oglašavanja i ambalažna industrija. U takvim industrijama primjena papira kao podloge za elektrovodljive elemente je financijski i ekološki prihvatljivija od najčešće primjenjivanih podloga od različitih polimernih materijala.

The effects of graphene oxide (GO) addition on the dispersion of nanosilver (Ag) in an MgB2 matrix were studied using bulk samples prepared through a diffusion process. The influence of the dispersion of Ag and Ag/GO particles on the critical current density (Jc) of MgB2 was also investigated. GO has emerged as an excellent dopant which can significantly improve both the low- and high-field performance of MgB2 due to its capability to improve intergrain connectivity (GO) and inter- and intragrain pinning (GO and AgMg). The addition of nanosize Ag particles also results in an improvement of vortex pinning, and at the same time, it offers the advantage of preventing the loss of Mg during the sintering process. It is found that the dispersion of nanosilver in the presence of GO results in significant improvements in the critical current density in MgB2, particularly at high magnetic fields, due to improved intergrain connectivity and flux pinning. The use of the GO net as a platform for doping MgB2 in our case with Ag yielded a 10-fold-better critical current density (Jc) than standard Ag doping at 9 T and 5 K. Even without sophisticated processes, we obtained a Jc result of 104 A/cm2 at 9 T and 5 K, which is one of the best ever achieved

The effects on transition critical temperature, lattice parameters, critical current density, and flux pinning of doping MgB2 with carbon nanoparticles, were studied for bulk, wire and tape under a wide range of processing conditions. Under the optimum conditions, magnetic Jc was enhanced by two orders of magnitude at 5 K for a field of 8 T, and by a factor of 33 at 20 K for a field of 5 T for bulk samples, whereas enhancement by a factor of 5.7 was observed in the transport Ic at 12 T and 4.2 K for a wire sample. Samples sintered at high temperature (900 and 1000 °C) exhibited excellent Jc, approximately 10 000 A cm−2 in fields up to 8 T at 5 K. This result indicates that flux pinning was enhanced by the carbon substitution for B with increasing sintering temperature. Highly dispersed nanoparticles are believed to enhance the flux pinning directly, in addition to the introduction of pinning centres by carbon substitution. Nano-C is proposed to be one of the most promising dopants besides SiC and CNT for the enhancement of flux pinning for MgB2 in high fields.

Carbon nanotubes (CNTs) are an excellent candidate for introducing effective pinning centres and at the same time enhancing the upper critical field of MgB2. We report on the use of a low intensity ultrasonication as a method of dispersion of CNTs into precursor magnesium and boron powder. The ultrasonication improved homogenous mixing of CNTs with the MgB2 matrix. Ultrasonication of CNT doped MgB2 resulted in a significant enhancement in the field dependence of critical current density. The density of the sample increased due to the improved adherence between CNTs and MgB2 matrix. CNTs donate carbon that is substituted for boron in MgB2.

We have examined the structure and phase behaviour of strontium-doped Ce 1− x Sr x CoO 3− δ and found that the perovskite form is stabilised over a relatively narrow solid solution range ( x > 0.85). A combination of electron, powder X-ray and neutron diffraction has revealed tetragonal superstructures of the basic perovskite unit; ( I 4/ mmm ) 2 a p × 2 a p × 4 a p ( x = 0.90) and ( P 4/ mmm ) a p × a p × 2 a p ( x = 0.95). Magnetisation measurements show ferromagnetic behaviour under applied magnetic fields. Low temperature neutron diffraction of Ce 0.10 Sr 0.90 CoO 2.80 in zero field reveals a magnetic cell of dimension 2 a p × 2 a p × 4 a p with an ordered cobalt moment of 1.7 B.M. at 25 K.

Significant differences in superconducting and microstructural properties between two types of MgB/sub 2/ films prepared by pulsed laser deposition were determined. A very high H/sub c2/-T slope of 1.1 T/K was achieved in the in situ film. The J/sub c/-H curves of the in situ film also show a much weaker field dependence than that of the ex situ film. The magneto-optical (MO) images show that at 4 K the flux penetrates the in situ MgB/sub 2/ film through random paths, while for the ex situ film, the flux penetration pattern is mostly repeatable, indicating a defect-controlled flux penetration. Microstructural study (transmission electron microscopy and atomic force microscopy) revealed a relatively big grain size in the ex situ film. The correlation between the superconducting properties, microstructure and preparation conditions is discussed with regard to the two types of films.

Af erromagnetic sheath around a superconducting wire results in an unusual transport Jc(H) .F or the f iel dp erpendicular to the current, there is a plateau in Jc(H) at high temperatures and intermediate fields. This plateau develops into a peak at lower temperatures—resembling a ‘peak effect’. A model based on cancellation of the self-fiel do f the current and the external field within the iron sheath was proposed for the explanation of the plateau in Jc(H) .W e test this model in three key experiments. Firstly, we show that the form of Jc(H) fo rr ound MgB2/Fe wires is strongly temperature dependent .T hi si si nc ontradiction with the model, because the properties of the iron sheath do not change in the measured temperature range. However, the temperature dependence of Jc might still account fo rt he change of Jc(H) .S econdly, the model requires a substantial component of the self-field to be parallel to the external field. Our measurements of Jc(H) for afi eld parallel to the current show a peak in Jc(H) at high temperatures and ap ronounced plateau at low temperatures. The model cannot explain this because the self-field and external field are perpendicular in this experiment. Thirdly, the iron sheath was made thinner on one side of the wire, which should produce an asymmetry in Jc(H) in the model for two different orientations of the external field. Such asymmetry was not observed. These experiments show that the effect of the self-field is of much lower importance than an as yet unknown effect that results in the observed plateau and peak in Jc(H) .S uch an effect is likely to be based on a specific interaction between the superconductor and ferromagnet, perhaps similar to the overcritical state effect.

We report a systematic study on the effect of sintering temperature on the phase formation, critical current density, upper critical field and irreversibility field of nanoscale SiC doped MgB2. Bulk and Fe sheathed wires doped with different nano-SiC particle sizes have been made and heat treated at temperatures ranging from 580 to 1000 °C. A systematic correlation between the sintering temperature, normal state resistivity, RRR, Jc, Hc2, and Hirr has been found in all samples of each batch. Samples sintered at a lower temperature have a very fine and well consolidated grain structure while samples sintered at a high temperature contain large grains with easily distinguishable grain boundaries. Low temperature sintering resulted in a higher concentration of impurity precipitates, larger resistivity, higher Jc up to 15 T and lower Tc values. These samples show higher Hc2 and Hirr at T near Tc but lower Hc2 near T = 0 than samples sintered at high temperature. It is proposed that huge local strains produced by nano-precipitates and grain boundary structure are the dominant mechanism responsible for higher Hc2 at T near Tc. However, higher impurity scattering due to C substitution is responsible for higher Hc2 in the low temperature regime for samples sintered at a higher temperature. In addition to high Hc2, it is also proposed that the large number of nano-impurities serve as pinning centres and improve the flux pinning, resulting in higher Jc values at high magnetic fields up to 15 T.

Highly smooth and c-axis oriented superconducting MgB2 thin films were prepared by pulsed laser deposition (PLD) with off-axis geometry. The films were deposited on Al2O3-C substrates perpendicularly aligned to a stoichiometric MgB2 target in a 120 mTorr high purity Ar background gas. An in situ annealing was carried out at 650C for 1 min in a 760 Torr Ar atmosphere. Despite the short annealing time, an x-ray theta-2 theta scan shows fairly good crystallization, according to the clear c-axis oriented peaks for the films. Both atomic force microscopy and the x-ray diffraction results indicate that the crystallite size is less than 50nm. The root mean square roughness of our off-axis film is ~4 nm in a 5x5 square micron area. The Tc onset value of the best off-axis film reaches 33.1 K with a narrow transition width of 0.9 K. The films showed no anisotropy in Hc2-T curves when parallel and perpendicular fields were applied. The slope of Hc2-T curves in low field regime is 1 T/K, which is among the highest reported values., Comment: 7 pages 7 figures

Various amounts of Si up to a level of 18 wt% were added into MgB2 thin films fabricated by pulsed laser deposition (PLD). Si was introduced into the PLD MgB2 films by sequential ablation of a stoichiometric MgB2 target and an Si target. The precursor films were annealed in situ at 685 °C for 1 min. Up to an Si level of , the superconducting transition temperature (Tc) of the film does not change significantly, as compared to the undoped film. The magnetic critical current density (Jc) of the film at 5 K was increased by 50% with Si addition, as compared to the undoped film. The slope of Hirr(T) and Hc2(T) curves of the 3.5 wt% Si added MgB2 film is slightly higher than that for the undoped film.

Different precursor powders and procedures were used to fabricate MgB2 superconductor. It was found that the purity of the B powder had strong effects on the Tc and Jc of MgB2. The Jc of the sample made from 90% B powder decreased 40 times at 3 T and 20 K compared with the 99% B powder samples. The impurity phases might be the reason for the suppression of the Tc and the Jc. Variation of the Mg:B ratio also influenced the MgB2 properties. Jc decreased with fluctuations from the normal rate (Mg:B = 1:2). It was found that the effects of the oxidization of the Mg powder on Jc were not strong when the Mg powder was only slightly oxidized. However, when the Mg powder was severely oxidized, the Jc decreased significantly. A two-step sintering process was adopted to make MgB2. In this process, the first sintering stage was at a lower temperature. It was found that this pre-sintering did not have a good effect on the properties, despite the fact that the mass density was improved. The phase composition and microstructure were analysed to explain the different effects.

Polycrystalline samples of multi-walled carbon nanotube (CNT) doped MgB2 superconductors were synthesized by solid state reaction. The carbon nanotube substitution results in a small decrease in lattice parameter and Tc, as compared to ordinary carbon substitution. Enhancement of Hc2 and Hirr shown by CNT doped samples show that the effect of CNT doping is two-fold. For sintering temperatures higher than 900 °C, the CNTs partially decompose, allowing substitution of boron for carbon. This results in an increased Hc2. However, remains of the CNTs are incorporated into the crystal matrix of MgB2, and they act as pinning centres. For lower sintering temperatures, there is little substitution of boron for carbon, and CNTs are incorporated into the crystal matrix as a whole, acting as pinning centres without affecting Hc2.

We have demonstrated that SrCoO3−δ can be stabilised into phase pure perovskite forms by the introduction of small amounts ∼5% of certain rare earth ions (Sm3+Yb3+). At the same doping levels, La3+ and Pr3+ crystallise with the same isostructural trigonal structure as Sr6Co5O15; while the Nd3+ composition shows a mixture of both structure types. Powder X-ray diffraction showed only a simple cubic perovskite structure, however, a combination of electron and neutron diffraction has revealed a tetragonal (P4/mmm) ap×ap×2ap superstructure. Strontium and the rare earth ions are disordered over a single site, while the oxygen vacancies are localised on the apical O2 sites. Magnetisation measurements show that these materials undergo transitions to a spin-glass state at temperatures below 150 K, and that significant coupling occurs between the rare earth ions and the mixed Co3+/4+ ions. Magnetisation measurements as a function of applied field reveals that below the transition temperature ferromagnetic ordering takes place at relatively large fields.

The U/n method is a well-established means of improving flux pinning and critical current performance in cuprate superconductors. The method involves the doping of the superconductor with 235U followed by irradiation with thermal neutrons to promote fission. The resultant columnar damage tracks produced by the energetic fission products pin flux vortices and improve critical current performance in magnetic fields. No such improvement could be observed when the U/n method was applied to MgB2 superconductor. No fission tracks could be observed in TEM, even for samples that were irradiated at the highest fluence. Gamma-ray spectroscopy indicated that fission had occurred in the expected way.

Pure and doped MgB 2 thick films were fabricated on Ni substrates by applying a coating mixture of powders of elemental magnesium and boron with varying amounts of elemental Cu and nano-SiC powders, followed by a high pressure press, and sintering at 840 or 900 °C for just a few minutes, and then quenching in liquid nitrogen. For films sintered at 900 °C, critical current densities J c were achieved as high as 1.4 × 10 6 A/cm 2 at 20 K and 2.3 × 10 5 A/cm 2 at 20 K and 2 T for the pure and SiC added films. Films doped with 5 wt.% of Cu powders were observed to have better adherence to the Ni substrate without degradation in T c , and J c was found to be slightly decreased, but still remains as high as 7 × 10 5 A/cm 2 at 20 K in zero field. It was observed that J c and irreversibility field increase with an increasing sintering temperature up to 900 °C. Furthermore, nano-SiC addition has significantly improved the irreversibility field compared to undoped MgB 2 films.

Pure and 10 wt% nano-SiC doped MgB2/Cu tapes were fabricated using the coating and pressing method. Samples were sintered by an in situ reaction process. It was observed that the nano-SiC doped tapes were significantly reacted with the Cu sheath at 700 °C, while pure samples have less reactivity with Cu under the same conditions. However, for sintering at 667 °C for just 6 min, the reaction with Cu was significantly reduced for the nano-SiC doped samples and led to very high critical current densities of more than 1 MA cm−2 in zero field at K. The Jc values exceed 105 A cm−2 for 30 K in zero field, 20 K in 2 T and K in 4 T. These Jc values are of one to two orders of magnitude higher than those of the pure MgB2/Cu short tapes, are the best reported Jc values for Cu sheathed wires and tapes and are comparable to the Jc values reported for MgB2/Fe tapes. These nano-SiC doped MgB2/Cu tapes also exhibited very small flux jumping at 5 K. Such a high Jc value and its field performance together with its possible high thermal stability make the Cu-sheathed MgB2 tapes an attractive candidate for large scale applications.

Pure and Cu doped MgB 2 think films have been prepared on Cu, Ni and stainless steel substrates using a short-time sintering method. Results showed that single MgB 2 phase films can be easily formed in a short period of time (3 min) at temperatures above 700 °C. Un-doped MgB 2 films were found to be loosely attached to the Ni and stainless steel substrates. However, the MgB 2 with Cu powders addition adhered well to the substrates without serious degradation of T c and flux pinning. The J c increased one order of magnitude and irreversibility field determined from M–H loops also increased when sintering temperature increased from 745 to 900 °C. J c values in the range of 1–9×10 5 A/cm 2 at 15 K have been achieved for both doped and un-doped films sintered at 900 °C for 3 min.

Transport critical current (Ic) was measured for MgB2/Fe round wires, with magnetic field oriented perpendicular to the wire and parallel to it. Measurements were made on a wire with pure MgB2 core and another wire where MgB2 core was doped with nano-size SiC. This doping strongly improved the vortex pinning in MgB2. The field dependence of Ic was strongly improved due to the presence of the iron sheath. At 30K, Ic did not depend on the field for fields between 0.09 and 0.7T. At lower temperatures, Ic increased with the field, after initial decrease, resembling a "peak effect". This effect was extended to higher fields as the temperature went down: at 10K the peak appeared at 3.5T. This improvement was not due to mere magnetic shielding by iron, but more likely to an interaction between the iron sheath and superconductor. Improvement of vortex pinning did not affect the range of fields within which this effect was observed.

Magnetic hysteresis and magnetic relaxation measurements have been performed to study vortex pinning behaviors for pure, Fe doped and heavily Pb doped Bi2212 single crystals. Unlike pure and Fe doped Bi2212 crystals, heavily Pb doped crystal showed strong vortex pinning behavior. We interpret the strong pinning in heavily Pb doped Bi2212 single crystals as arising from the improved Josephson coupling in Bi2212 single crystal after heavy Pb doping. In heavily Pb doped single Bi2212 crystals, H/sub dis/(T) was observed to decrease with increasing T. Here, H/sub dis/(T) is an order-disorder field that separates a weakly elastically disordered vortex lattice from a plastically disordered vortex solid. However, in pure and Fe doped Bi2212 single crystals, H/sub dis/(T) was observed to be temperature independent. We also report a significant shift of T/sub CR/, a crossover temperature separating two pinning regimes, toward higher temperatures with heavy Pb doping of Bi2212 single crystals. On the other hand T/sub CR/ did not shift with Fe doping of Bi2212 single crystals. It is argued that the temperature dependence of H/sub dis/(T) and the shift of T/sub CR/ in heavily Pb doped Bi2212 crystals was related to the enhanced c-axis conductivity caused by the Pb situated between the CuO/sub 2/ layers and imposing a 3D characteristic on the vortex lattice.

Ag and Ag-alloy sheathed Bi-2223 tapes were fabricated by a powder-in-tube technique with different configurations of the precursor and restack sheath materials: Ag, AgAu7 wt%, AgSb0.6 wt%, AgMg0.2 wt%. Analysis of the I/sub c/ and volume fractions of the Bi-2223, Bi-2212, Bi-2201 and Bi-3221 phases indicated that volume fractions of Bi-2223>90%, Bi-2212/spl sim/5%, Bi-2201/spl sim/0% and Bi-3221

Mg 1-x Ti x B 2 /Fe (x = 0, 0.1 and 0.2) wires were made by Powder-In-Tube method. XRD patterns indicate the main phase is MgB 2 and there are Ti particles in the Ti doped wires and no other Ti based impurities in the cores. SEM images show that the MgB 2 particles are distributed evenly in porous core. The J c of the wire (Mg 1-x Ti x B 2 /Fe, x = 0.1) measured magnetically is higher than that of those wires (Mg 1-x Ti x B 2 /Fe, x = 0 and 0.2), while the H irr are almost the same for three wires.

Hydrogenation of commercial MgB 2 powder (superconducting transition temperature 38.2 K) at temperatures ⩾600 °C and hydrogen pressures up to 10 bar has lead to an increase in T c , as determined by ac susceptibility measurements. Applied dc fields reduced the transition temperature by a slightly larger value than that for the pure powder. Heating the MgB 2 powder for 2 h at 600 °C in 10 bar of either helium or argon gas has also lead to an increase in the transition temperature.

The isostructural polymeric compounds Co(thiazole)2X2 (X=Cl (1), Br(2)) have been synthesised by the addition of thiazole to an ethanolic solution of the corresponding anhydrous cobalt halide. Powder X-ray and neutron diffraction measurements were used for structural determination. The structures were determined using powder neutron diffraction data and Rietveld techniques: (1) C2/c, a=17.806(2) A, b=3.6806(6) A, c=14.807(3) A, β=94.78(1)°, V=967.1(3) A3, Z=4; (2) C2/c, a=18.079(3) A, b=3.8138(8) A, c=15.022(4) A, β=92.71(1)°, V=1034.6(4) A3, Z=4. Each linear polymer chain is composed of pseudo-octahedral, high-spin Co2+ centres, doubly linked by halide bridges. Magnetisation measurements of 1 and 2 at 5 K between 0 and 10 kG reveals a metamagnetic transition between antiferromagnetic and ferromagnetic states. Low temperature susceptibility data have been fitted to a one-dimensional Ising model with a mean field correction and were found to be anisotropic with ferromagnetic intrachain interactions along the b-axis and weaker antiferromagnetic interchain interactions.

Yb 2 CoMnO 6 perovskite manganite was synthesised. Rietveld refinement of powder X-ray diffraction data indicated that this compound crystallised with a GdFeO 3 -type orthorhombic structure containing distorted (Co/Mn)O 6 octahedra (Pnma; a =5.5593(2) A; b =7.4154(2) A; c =5.1825(1) A; V =213.65(1) A 3 ; Z =2). The DC magnetisation and frequency dependence of real part of AC susceptibility was measured over a wide temperature range. A spin glass ground state at 40 K was observed to arise from a ferromagnetic state and caused by competition between the ferromagnetic and antiferromagnetic interaction.

La2CoMnO6 perovskite manganite was synthesized by solid-state reaction. Rietveld refinement of powder x-ray diffraction data indicated that this compound crystallized with an orthorhombic structure containing distorted (Co/Mn)O6 octahedra (Pnma; a = 5.5 A; b = 7.2 A; c = 5.4 A. The dc magnetization and real part (x') of the ac susceptibility were measured at dc fields up to 1 T and an ac field of 0.1 Oe at frequencies of 21, 217, 600 and 2000 Hz over a wide range of temperatures from 300 K down to 4.2 K. A ferromagnetic transition was observed at Tc = 220 K. A spin glass ground state was also determined from the shift of a peak at 220 K in the real part of the ac susceptibility with frequency and slow spin relaxation for T < Tc as indicated by the time dependence of the remanent magnetization. The spin glass state arises from a ferromagnetic state and might be caused by competition between the ferromagnetic and antiferromagnetic interaction.

La0.8Sr0.2Mn1−xZnxO3 perovskite manganite with x = 0, 0.01, 0.03, 0.05, 0.1, 0.15, 0.3, 0.5 and 0.75 has been prepared by the solid state reaction and investigated by structural, transport and magnetic measurements. The structural characterization of the samples has been done by x-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements. The x-ray powder diffraction results indicate a single phase for 0 0.1 decreases with increasing zinc doping levels. The transport measurements show that the insulator-to-metal transition temperature, Tc, decreases with increasing zinc substitution on the Mn site, and the resistivity increases. The Tc variations agree with the magnetic measurement data for x 0.3. The possible mechanism behind the effects of zinc doping into the La0.8Sr0.2Mn1−xZnxO3 system involves degradation of the double exchange between Mn3+–O–Mn4+ in this compound.

The peak effect (PE) in Fe-doped Bi2.1Sr1.9Ca1.0(Cu1−yFey)2Ox crystals with different nominal y values of 0, 0.005, 0.013 and 0.022 grown by the floating zone method was studied by measuring M–H loops. The peak effect appears over a narrow temperature range between 20 and 40 K for y up to 2.2%, but with a small peak field of about 200 Oe. The appearance of the PE in all the Fe-doped Bi-2212 can be explained by the existence of Bi5+ cluster regions in the Bi3+ 2212 matrix. ΔMmax/ΔM0 versus T/Tc and (Hmax − Hmin)/Hmax versus T/Tc were used to characterize the peak effect. ΔMmax and ΔM0 represent the width of the hysteresis loop at the secondary peak (Hmax) and the central peak H0 (H = 0). Hmin represents the field at which the magnetization starts to increase, giving the anomalous peak. The secondary peak moves away from the central peak in a linear way as the temperature increases as indicated by (Hmax − Hmin)/Hmax versus T/Tc, in agreement with what is found in pure Bi-2212 single crystals.