Effect of Na substitution and Ag addition on the superconducting properties of Bi-2212 textured materials

The effect of Ag addition on the structural and superconducting properties of Bi2Sr2Ca0.925Na0.075Cu2Oy + x wt% Ag materials with x = 0, 1, 3, and 5 prepared by sol–gel method, followed by directional growth using the laser floating zone (LFZ) technique has been investigated. Powder XRD measurements demonstrate that all samples are composed by nearly single Bi-2212 phase. SEM micrographs and EDX results confirms that Bi-2212 is the major phase with minor amounts of a Bi-free one (Sr, Ca)CuO2 and metallic Ag. According to the magnetic results, Ag does not modify the superconducting critical temperature, Tc, but significantly increases the magnetic hysteresis loop area. The magnetic critical current density is maximum in the 3 wt% Ag containing samples, due to the larger grain clusters found in these samples, reaching 2.1 × 105 A/cm2 at 10 K. Moreover, Ag addition also improves the rods mechanical properties, determined by three point bending tests. The highest bending stress is also reached in the 3 wt% Ag samples

Improved copper–epoxy adhesion by laser micro- and nano-structuring of copper surface for thermal applications

The objective of this work is the enhancement of metal-to-metal bonding to provide high thermal conductivity together with electrical insulation, to be used as heat sinks at room and cryo-genic temperatures. High thermal conductive metal (copper) and epoxy resin (Stycast 2850FT) were used in this study, with the latter also providing the required electrical insulation. The copper surface was irradiated with laser to induce micro- and nano-patterned structures that result in an improvement of the adhesion between the epoxy and the copper. Thus, copper-to-copper bonding strength was characterized by means of mechanical tensile shear tests. The effect of the laser processing on the thermal conductivity properties of the Cu/epoxy/Cu joint at different temperatures, from 10 to 300 K, is also reported. Using adequate laser parameters, it is possible to obtain high bonding strength values limited by cohesive epoxy fracture, together with good thermal conductivity at ambient and cryogenic temperatures. © 2021 by the authors. Licensee MDPI, Basel, Switzerland

Tuning Ca3Co4O9 thermal and transport properties by TiC nanoparticles addition

Ca3Co4O9 + xwt.% TiC (x = 0, 0.25, 0.5, 0.75, and 1.0) polycrystalline thermoelectric ceramics have been prepared through the classical ceramic route. XRD characterization has demonstrated that all samples are mainly composed by the Ca3Co4O9 phase, while microstructural observation has shown that no reaction between both components has been produced. Moreover, TiC particles are well distributed for small additions, and start to agglomerate from 0.75 wt.% content. Density measurements showed that nearly no changes have been produced by TiC addition and nearly the whole porosity appears as open one. Furthermore, electrical resistivity decreases up to 0.75 wt.% addition, increasing for higher content. On the other hand, Seebeck coefficient has been maintained unchanged in all samples. In spite of an irregular behaviour of thermal conductivity with temperature, it tends to decrease when the temperature is raised, displaying the lowest values for the 0.25 wt.% samples. These data led to an increase of about 40% in ZT values at 800 °C for samples with 0.25 wt.% addition, when compared with the pure ones. Finally, linear thermal expansion coefficient is decreased when TiC content is increased, which can be exploited to fit the thermal expansion coefficients of all the components used to build a power generation thermoelectric module

Collagen-laponite nanoclay hydrogels for tumor spheroid growth

The extracellular matrix (ECM) plays an important regulatory role in the development and progression of tumoral tissue. Its functions and properties are crucial in determining tumor cell behavior such as invasion, migration, and malignancy development. Our study explores the role of collagen type I in cancer development and spread using engineered tumor models like multicellular spheroids grown in collagen-based hydrogels to simulate early tumor formation. We employ microfluidic techniques to test the hypothesis that (i) adding Laponite nanoclay to collagen hydrogels modifies mechanical and rheological properties and (ii) changing the stiffness of the collagen microenvironment affects tumor spheroid growth. Our findings support our theories and suggest the use of ECM components and engineered tumor models in cancer research, offering a biocompatible and biomimetic method to tailor the mechanical properties of conventional collagen hydrogels

Effect of pb doping on the electrical properties of textured Bi-2212 superconductors

Bi2-xPbxSr2CaCu2Oy textured materials (x=0.0, 0.2, 0.4, and 0.6) have been successfully prepared by the laser floating zone technique. Microstructure and electrical properties (JC and TC) have been clearly affected by Pb addition. From the E-I curves, slope of the transition between the superconducting and the normal state (n) at 77K reaches a maximum of about 16 for the 0.4 Pb doped samples. This value is much higher than the typical ones for the Bi-2212 materials. Moreover, when the electrical properties of the 0.4 Pb doped samples are measured at lower temperatures (between 65 and 77K), n values increase when the temperature is decreased. A maximum n value of 32 has been reached at 65K which makes this material very attractive for its use as resistive fault current limiters. © 2014 Elsevier Ltd.The authors wish to thank the Gobierno de Aragón (Research Groups T12 and T87) for financial support. M. A. Madre and H. Amaveda acknowledge the Spanish MINECO-FEDER (Project MAT2011-22719) for funding. The technical contributions of C. Estepa and C. Gallego are also acknowledged. Sh. Rasekh acknowledges a JAE-PreDoc2010 grant from CSIC.Peer Reviewe

Desarrollo de materiales compuestos cerámicos reforzados con fibras de sílice

Resumen del póster presentado a la 6ª Jornada de Jóvenes Investigadores en Física y Química de Aragón celebrada en Zaragoza el 20 de noviembre de 2014.Peer reviewe

Impulsional excitation system for resonator characterisation in atomization applications

[ES] El desarrollo de materiales piezocerámicos ha despertado un renovado interés en el campo de la atomización para generar gotas de tamaño microscópico, difíciles de conseguir por los medios mecánicos habituales. En este trabajo se presenta un sistema de excitación de cerámicas piezoeléctricas encaminado a su caracterización, basado en un convertidor DC/AC resonante con topología de puente completo que permite trabajar hasta 2 MHz. La salida del convertidor, aplica una onda de tensión cuadrada bajo la forma de ráfagas de impulsos, limitando el autocalentamiento de las cerámicas testeadas. Las señales de control permiten fijar todos los parámetros de test: frecuencia de resonancia, número de impulsos de salida así como frecuencia de repetición, ciclo de trabajo y tensión pico a pico de los mismos. Los primeros ensayos realizados sobre una cerámica de 1,668 MHz han permitido observar la lógica dependencia de la eficacia de atomización con la frecuencia de excitación y han puesto de manifiesto la importancia del ciclo de trabajo, del número mínimo de impulsos para iniciar la atomización y de su valor pico a pico de tensión que genera distintas distribuciones en diámetro de gota, aunque siempre con un máximo situado alrededor de 9 μm.[EN] Piezoelectric ceramic materials are of main interest in the field of spray-dryng applications, in order to generate microscopic droplets which are difficult to obtain by mechanical systems. In this work an excitation electronic system for piezoelectric transducers is presented. The system is oriented to device characterization with resonance frequencies up to 2MHz and is based on a full bridge DC – AC resonant voltage converter. The converter applies a square voltage waveform burst, limiting the device under test self-heating. The following test parameters can be adjusted: frequency, duty cycle and peak-to-peak voltage of the output waveform, number of output pulses constituting the burst and its repetition rate. First results with a 1.668MHz ceramic disc show the spray-dryng efficiency dependence on the excitation frequency, but also on the duty cycle, minimum number of pulses to start the spray-dryng process and peak-to-peak voltage. This last variable modifies the droplet diameter distribution but not the peak value, centered at 9μm.Peer reviewe

Transient High-frequency Ultrasonic Water Atomization

An experimental study was performed to improve the understanding of the characteristics of ultrasonic water atomization when excited with waves in the MHz range. In the present experiments, small volumes of water were atomized, observing the temporal evolution of the process. Typical diameters of the resulting droplets are of the order of a few microns. To visualize them, images were acquired with very high magnification. Appropriate lenses were used to enable high resolution at a distance from the flow. Droplet size distributions were also calculated with a Malvern diffractometer. Droplet exit velocity was measured using particle image velocimetry. It was noticeable that, as the remaining liquid mass deposited over the ultrasonic transducer decreased, the atomization characteristics changed, and a second peak of larger droplets appeared in the size distribution function. This phenomenon is related to the change in the curvature of the liquid surface. Although results are not conclusive, it appears that, under the conditions in this study, some observations about droplet formation are better described by cavitation phenomena rather than by the simplified surface wave theory usually invoked to explain these processesThis project was partially supported by the Diputación General de Aragón, under contract P104/97.Peer reviewe

Barras texturadas de (Bi1.6Pb0.4)Sr2CaCu2O8+δ dopadas con Ag

[EN] In this work, superconducting samples of (Bi1.6Pb0.4)Sr2CaCu2O8+δ with Ag additions have been studied. (Bi1.6Pb0.4)Sr2CaCu2O8+δ + x wt.% Ag (with x = 0, 1 and 3) powders were synthesized using a sol-gel method. The obtained powders were used as precursors to fabricate long textured cylindrical bars through a floating zone melting method. A drastic change on the microstructure has been found when comparing with undoped Bi2Sr2CaCu2O8+δ samples. The results showed that electrical resistivity at room temperature, critical current as well as flexural strength are improved when Ag is added to these Pb doped samples, while critical temperature does not change. On the other hand, it has been found that samples with composition (Bi1.6Pb0.4)Sr2CaCu2O8+δ + Ag shown E-I curves with very high sharpness values on the zone of the superconducting to normal transition, reaching n-values (E∼In) as high as 45 at 65K.[ES] Se han preparado polvos cerámicos de composición (Bi1.6Pb0.4)Sr2CaCu2O8+δ + x % Ag en peso (con x = 0, 1 y 3) mediante un proceso sol-gel. Estos polvos se han utilizado para fabricar precursores que se texturaron por medio del método de fusión zonal flotante. Se ha encontrado un gran cambio en la microestructura cuando se compara con muestras de composición pura Bi2Sr2CaCu2O8+δ. Tanto la resistividad eléctrica a temperatura ambiente, como la corriente crítica, así como la resistencia a flexión se mejoran cuando la Ag se adiciona a estas muestras dopadas con Pb, mientras que no se observa cambio en la temperatura crítica. Por otra parte, se ha encontrado que las muestras de composición (Bi1.6Pb0.4)Sr2CaCu2O8+δ + Ag presentan una gran pendiente de la curva E-I en la zona de transición entre el estado superconductor y el estado normal. Con estas composiciones, se han encontrado valores de n (E∼In) de hasta 45 a 65K.The authors are indebted the Spanish Ministry for Education and Science (MAT 2005-06279-C03-01) and to the Aragón Regional Government (Consolidated Research Groups E03 and T12) for financial support.Peer reviewe