Bulk Discs Research Articles

Avalanche dynamics of magnetic flux in the Nb-Ti superconducting ring

We report observations of the spatial and temporal variations in flux penetration into doubly connected bulk superconductors (hollow cylinder, ring) and discs (singly connected) from thermomagnetic avalanches. Dramatic changes were established in the stability of the critical state of the disc when it was transformed into a ring: the region of instability in the magnetic field in the ring doubled, and the number of thermomagnetic avalanches increased from 9 to 60. The duration of the flux jump, the amount of flux entering the sample, and its dependence on the magnetic field changed significantly, and the weak linear field dependence in the disc was inversely proportional to the magnetic field in the ring. A complicated fine structure of the magnetic flux penetrating the ring hole was detected. It was experimentally determined that the speed of the avalanche flux front throughout the sample increased sharply when magnetic flux appeared in the hole. This study also presents other aspects of the dynamic response of a superconducting ring that are discussed in detail.

Partially-oriented MgB2 superconducting bulks with addition of B4C and cubic BN obtained by slip casting under high magnetic field and spark plasma sintering

Partially-oriented MgB2 bulk discs (13 and 9 %) with the starting compositions of (MgB2)0.99(B4C)0.01 and (MgB2)0.99(c-BN)0.01 were fabricated by slip casting under an H0 = 12 T magnetic field (perpendicular to the disc surface) and subsequent spark plasma sintering. The maximum critical current density and irreversibility field are for H//H0 (H=applied field). These values are higher or similar to the randomly-oriented samples with the same composition. The maximum volume pinning force (Fp) is lower in the partially-oriented ones than in the randomly-oriented samples. The pinning-force-related parameters depend on the additive and orientation. Assessment of the major pinning mechanism within the scaling and percolation models considering these parameters shows significant limitations. A method to scale Fp is proposed; for the randomly and partially-oriented samples (that show an extra peak in Fp), the single and double Gaussian functions fit well. The results suggest an anisotropic influence of carbon substituting for boron in the MgB2.

Reproducibility of small Ge2C6H10O7-added MgB2 bulks fabricated by ex situ Spark Plasma Sintering used in compound bulk magnets with a trapped magnetic field above 5\xa0T

Bulk discs (20 mm diameter and 4.3 mm thickness) of MgB2 added with Ge2C6H10O7 were obtained by Spark Plasma Sintering. Six samples with composition MgB2(Ge2C6H10O7)0.0014 and one undoped sample were fabricated under similar conditions and were magnetically characterized in order to determine the scattering of properties and reproducibility. The main source of the scattering of the properties is the decomposition of the additive due to elimination of the organic part in gas form, which occurs stepwise with intensive vacuum drops at around ~ 560 and ~ 740 °C. A third drop, which is sometimes not well resolved being part of the second peak at 740 °C, occurs at ~ 820 °C. The critical temperature at the midpoint of the transition, Tc, shows only a relatively small variation between 37.4 and 38 K, and the irreversibility field at a low temperature of 5 K takes values between 8 and 10 T. The pinning force and pinning force related parameters do not correlate with the carbon substituting for boron in MgB2 and suggest a synergetic influence of the microstructural details and carbon. Overall, despite the superconducting properties scattering, the samples are of high quality. Stacked into a column of six samples, they can trap at the center and on the surface of the column a magnetic field of 6.78 and 5.19 T at 12 K, 5.20 and 3.98 T at 20 K and 2.39, and 1.96 T at 30 K. These promising values, combined with facile fabrication of the samples with relatively high quality and reproducibility, show the feasibility of their use in building complex and large compound arrangements for bulk magnets and other applications.

Inorganic Porous Bulk Discs as a Matrix for Thin-Layer Chromatography and Translucent Hard Composite Materials.

Magnesium-stabilized amorphous calcium carbonate (Mg-ACC), amorphous magnesium calcium silicate hydrate (MCSH), and hydroxyapatite (HAp) are prepared by a precipitation method. By cold-pressing these particles, it is possible to produce porous bulk discs with a narrow pore size distribution. These porous inorganic discs (Mg-ACC, MCSH, and HAp) are investigated as stationary phases to study the chromatographic behavior and adsorption ability of rhodamine B, methylene blue, and ribonuclease. The adsorption affinities of different biomolecules can be easily observed and evaluated through this method. Furthermore, by infiltrating fabricated opaque porous discs with benzyl ether, which has a similar refractive index as the used inorganic particles (Mg-ACC, MCSH, and HAp), their optical properties significantly change and the discs become translucent. Moreover, by infiltrating the MCSH discs with a light-curing polymer, translucent composites with good surface hardness are fabricated. By doping particles with ions such as Ni2+, Co2+, Fe3+, and Eu3+, the color and UV-visible spectrum of the bulk discs can be adjusted. Typically, by using iron-doped MCSH particles as the inorganic matrix, nanocomposites, which show a steep UV-absorption edge at 400 nm, are fabricated. Our work provides a simple and economical method to evaluate the affinity of biomolecules to inorganic materials and a novel way to fabricate translucent hard composite materials. The fabricated nanocomposite discs show a great UV shielding effect and superior surface hardness compared to polymethyl methacrylate and commercial sunglasses, suggesting their potential as new sunglass materials.

Direct measurements of laser absorptivity during metal melt pool formation associated with powder bed fusion additive manufacturing processes

Direct calorimetric measurements are used to study the effective optical absorptivity at 1070 nm laser wavelength for bulk and powder-coated discs of industrially relevant metals. Effective absorptivity is plotted as a function of nominal laser power from 30 up to 600 W for scanning velocities of 100, 500, and 1500 mm/s. The absorptivity versus power curves of the bulk materials typically shows a reduction in effective absorptivity until the beginning of the formation of a keyhole-type surface depression that is associated with an increased absorption of the laser light in the growing keyhole until a saturation value is reached. For powders, an additional plateau of higher absorptivity can be observed for low laser power, until the curves qualitatively collapse when full melting of the powder tracks is achieved. It is shown that, under conditions associated with laser powder-bed fusion additive manufacturing, absorptivity values can vary greatly, and differ from both room temperature powder layer measurements and liquid metal estimates from the literature.

Trapped magnetic field in a (NdFeB)–(MgB2) pair-type bulk magnet

Superconducting bulk discs, S, of 20mm in diameter and 3.5 or 3.3mm thickness of MgB2 (pristine or added with cubic BN, respectively) with density above 97% were prepared by Spark Plasma Sintering. Discs were combined in a pair-type sandwich-like arrangement with a permanent NdFeB axially magnetised magnet, PM (∼0.5 T). Measurement of the trapped field, Btr, with temperature, time, and the reduction rate of the applied magnetic field was performed using a Hall sensor positioned at the centre between the superconductor and the permanent magnet. It is shown that the permanent magnet with certain polarity favors higher trapped field of the superconductor owing to suppression of flux jumps specific for high density MgB2 samples. The Btr of the PM–S pair was 2.45 T (20K) and 3.3 T (12K).

Study of Sol-Gel/Powder Metallurgy Technique for Processing Titanium Parts

The powder metallurgy processing of titanium devices for biomedical applications has complex steps. In order to introduce a new processing route, this work studied a sol-gel technique combined with powder metallurgy for producing porous titanium samples. The process involves the mixture of titanium powders with sodium alginate suspension, which undergoes reticulation by calcium salt solution contact, forming a titanium/calcium alginate hydrogel in granule shape. Later, the hydrogel granules were dried and sintered in a high vacuum furnace for titanium particles consolidation and calcium alginate removal. The samples characterization was performed by scanning electron microscopy, optical microscopy, metallographic analysis, semi-quantitative X-ray fluorescence spectroscopy and X-ray diffraction. The results showed that the methodology used is adequate for producing porous titanium parts, since the samples presented no contamination, a uniform shape, particle consolidation and interconnected porosity. The research continues aiming to obtain samples with different bulk morphology, like, discs or bars for surgical implant applications.

Magnetic Field Trapping in MgB$_{2}$ Bulks and Inserts

In order to realize superconductive permanent magnets to be used in power applications like the magnetic levitation or electrical motors, we have studied the magnetic field trapping capability of MgB2 discs of different shapes, at temperatures >;10 K. In particular we have compared MgB2 bulk discs with MgB2 Superconductive Inserts in Metallic Substrates (SIMS) of diameters of 70 mm. Both superconductive devices have been produced by the Reactive Mg Liquid Infiltration (Mg-RLI) process. The magnetization was performed by the application and removal of an external magnetic field up to 2 T, produced by a superconducting magnet, or by Field Cooling of the superconductive devices nearby NdFeB permanent magnets. The SIMS devices showed a higher stability of the trapped fields with respect to the bulk discs. Typical trapped fields, measured at 1 mm from the surface of the device, are of the order of 1 T. The density distribution of the supercurrents has been estimated by measuring the trapped field at various temperatures up to Tc and performing magnetic levitation forces measurements.

High-Pressure Torsion of Machining Chips and Bulk Discs of Amorphous Zr<SUB>50</SUB>Cu<SUB>30</SUB>Al<SUB>10</SUB>Ni<SUB>10</SUB>

alloywereprocessedbyhigh-pressuretorsion(HPT).ItwasconfirmedthatthemachiningchipscanbeconsolidatedusingHPTatroomtemperaturetodisc-shapedsampleswith10mmdiameterand0.6mmthicknesshavinga relative densityof 96.7%.The hardnessforthe consolidatedchips was wellcomparableto thoseobtainedusingthe bulksampleswhenthe imposed strain is large. No crystallization was detected after HPT processing of chips and bulk samples.[doi:10.2320/matertrans.MB200914](Received August 24, 2009; Accepted October 20, 2009; Published December 2, 2009)Keywords: high-pressure torsion, severe plastic deformation, amorphous, bulk metallic glasses, machining chips

Laser interferometric measurement of displacement-field characteristics of piezoelectric actuators and actuator materials

The objective of this study was to provide an accurate and robust technique for the measurement of the displacement-field responses of piezoceramic actuator materials. Both “hard” and “soft” piezoceramic materials as bulk discs and multilayer devices were characterized. The main goal was to reduce the errors of measured displacements to less than 5%. The developed interferometric measurement system enabled the point-to-point scanning of multilayer actuators in which the interest was to study the edge effects in the interface of the high electric field loaded active and the passive regions. In addition to the developed Michelson interferometer system Moiré interferometer measurements were also carried out for multilayer device characterization.

Autoclaving of Porous Silicon within a Hospital Environment: Potential Benefits and Problems

The microstructure and chemical composition of silicon surfaces subjected to standard sterilization protocols in hospital autoclaves has been examined. Both metallic contamination and structural changes can accompany porous Si hydrolysis. The in-vitro incubation of bulk and porous Si discs with human lymphocytes reveals potential sources of cytoxicity such as silane emission, and the supersaturation of media with eluted Si.