Liquid Infiltration Process Research Articles

XRD analysis of superconducting MgB2prepared by reactive Mg liquid infiltration process

XRD analysis of superconducting MgB₂prepared by reactive Mg liquid infiltration process

Magnetic lenses using different MgB2 bulk superconductors

A magnetic lens allows the concentration of magnetic fields using the diamagnetism of superconductors. The important features of the magnetic lens are a tapered inner diameter from which the magnetic flux is extruded and a slit to suppress the circumference current that shields the magnetic flux. This concept was experimentally confirmed through the use of GdBaCuO bulks and a stack of NbTi/Nb/Cu sheets. We refer to this arrangement as a magnetic lens. The Mg-reactive liquid infiltration (Mg-RLI) process developed by Edison SpA is suitable for the production of large and high-density MgB2 bulks. Three MgB2 bulk magnetic lenses, each with a different microstructure, were fabricated following the Mg-RLI process. The properties of the MgB2 magnetic lenses were measured in a cryocooler system as well as in liquid helium. The results confirmed that the MgB2 bulk magnetic lenses could concentrate a magnetic field and that their field concentration properties were greatly affected by the temperature and the external field. In addition, the microstructure of the MgB2 bulk also had an influence on the magnetic properties at different external fields. The results indicated that the MgB2 lens might be utilized as a field amplifier in intermediate fields.

Fabrication of C/SiC composites by combining liquid infiltration process and spark plasma sintering technique

Carbon fibre-reinforced silicon carbide composites (C-SiC) were fabricated combining, for the first time, a liquid infiltration process (LI) of a mesophase pitch doped with silicon carbide nanoparticles followed by reactive liquid silicon infiltration using Spark Plasma Sintering (SPS) technique. A graphitization step was applied in order to improve the effectiveness of the processing. Up to three different morphologies of SiC particles were identified with a noticeable influence on the preliminary oxidation tests carried out. The presence of SiC nanoparticles added to the carbon matrix affects the morphology of the SiC obtained by in situ reaction of silicon and carbon during the LI process by SPS and it leads to an improvement of the material oxidation resistance. The results show that SPS is a promising method to develop C-SiC composites in a short time and with a high efficiency in the liquid silicon infiltration process.

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.

Magnetic shielding capability ofMgB2 cylinders

The shielding of a DC magnetic field has been demonstrated to becomplete at 4.2 K, at least up to 2 T, inside the 18 mm bore of a thickMgB2 bulk cylinder, 70 mm tall. No hysteresis has been detected along the cycle of thefield application and removal, and no flux jumps have been detected either. TheMgB2 cylinderwas manufactured by the Reactive Mg Liquid Infiltration process, an ‘in situ’ technology able to give verydense MgB2 manufacts. According to the previously measuredJc(B, T) characteristicsof a MgB2 bulk superconductor of the same origin and quality, a critical current density of about30 kA cm − 2 can be assumed at the fields of 2–3 T and at the temperature of 4.2 K of thereported shielding experiment. Accordingly, we estimate that the thickness of theMgB2 cylinder filled by the shielding currents is about 5.75 mm, lower than the present cylinderthickness of 8.77 mm.

Investigation of thermal management materials for automotive electronic control units

Today’s electronics packages are smaller and more powerful than ever before. This leads to ever increasing thermal challenges for the systems designer. The automotive electronic control unit (ECU) package faces the same challenge of thermal management as the industry in general. This is coupled with the latest European Union legislation (Euro 6 standard) which forced the ECU manufacturers to completely re-design their ECU platform with improved hardware and software capability. This will result in increased power densities and therefore, the ability to dissipate heat will be a key factor. A higher thermal conductivity (TC) material for the ECU housing (than the currently used Aluminium) could improve heat dissipation from the ECU. This paper critically reviews the state-of-the-art in thermal management materials which may be applicable to an automotive ECU. This review shows that of the different materials currently available, the Al/SiC composites in particular have very good potential for automotive ECU application. In terms of metal composites processing, the liquid metal infiltration process is recommended as it has a lower processing cost and it also has the ability to produce near net-shape materials.

Preparation of porous SiC ceramics from waste cotton linter by reactive liquid Si infiltration technique

Porous carbon and porous silicon carbide with homogenous microstructure are prepared from cotton linter and phenolic resin through mold-stacking, solidification, carbonization and liquid Si infiltration process. SEM observation reveals that the irregular interconnected pores of carbon preparation were originated from the random arrangement of cotton linter fiber and different carbonization shrinkage. After the liquid Si infiltration process, the porous carbon was converted into dense SiC/Si composites. The porous SiC was formed during the Si-removing process at higher temperature. The result shows that open porosity of porous silicon carbide increases as Si-removing time increase, while the bending strength and the fracture toughness decrease.

Boron Purity Effects on Structural Properties of the ${\rm MgB}_{2}$ Obtained by Mg-Reactive Liquid Infiltration

The Mg-Reactive Liquid Infiltration (Mg-RLI) process allows to obtain well densified MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> bulks by using crystalline boron powders up to 100 micron in size: natural liquid Mg percolation within the boron powders preform proceeds for depths of about 50 mm. Much less effective percolation is obtained by using micronic size boron powders, commercially defined "amorphous", of nominal purity of about 96%. The aim of our research is to enlighten the effects of the purity of boron powders as well as of their granularity on the Mg-RLI process. The opportunity of using micronic boron, in order to produce massive superconducting bodies, is particularly appealing due to the low cost of the powders and to the superior superconducting properties of the resulting MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , as it has been already tested.

Analysis of Various Dopants on the ${\rm MgB}_{2}$ Superconducting Properties

The doping of MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> superconductor has been approached with the addition of carbon atoms into the boron sites and metallic elements (i.e. Zn,Cu,Co) into the MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> lattice. Here we use the Mg-Reactive Liquid Infiltration process in an attempt to promote the dopant insertion, either by using Mg based alloys and mixing the dopants (i.e. nano-size SiC in the case of C doping) with the boron powders. The efficiency of the MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> crystal lattice doping via the infiltration process is monitored by measuring the lattice parameters variation by X-ray diffraction and by microanalytical fluorescence scanning spectroscopy. The superconducting properties have been measured magnetically at high fields and at various temperatures, up to about the critical temperature. The magnetization cycles have shown a substantial increase of the irreversibility field, <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">irr</sub> , with doping. The performances of the MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> with different dopants are compared and discussed.

Superconducting Permanent Magnet Made by a Bulk ${\rm MgB}_{2}$ Ring

The bulk superconductors can be used as permanent magnets of very high magnetic induction, provided that a feasible way of magnetization is identified and that the superconducting material can withstand the magnetic forces induced by high trapped fields. Here it has been considered as superconducting permanent magnet a bulk MgB2 ring, obtained by the Reactive Liquid Infiltration (RLI) process. This material offers several advantages with respect to the well known YBCO textured material, even if its application temperature will be far lower, around 20-30 K. First of all, MgB2 does not require grain orientation to reach enough current density, it can be produced in a quite straight forward way, it is lightweight and it is mechanically robust. To magnetize the MgB2 ring, we have applied a DC magnetic field with a superconducting magnet, using a step wise, field cooling type, procedure. The induced magnetization of the bulk MgB2 ring, at 4 K, is around 2 T in its vicinity. It has been verified that the magnetization is very stable during the time and it can withstand also AC disturbances of several Hz in frequency.

Elaboration and characterization of metallic foams based on tin–lead

This study is devoted to the fabrication of metallic foams based on tin–lead of various relative densities and pore sizes by means of the liquid alloy infiltration process and its characterization (mechanical behavior and microstructure). Room temperature uniaxial compression tests were carried out in order to study the influence of the size of cell and of the relative density on the behavior in compression and to interpret these relations within a framework. A characterization on a microscopic scale (metallography and hardness) is achieved in order to link the morphological and mechanical characteristics of the constitutive phases, the parameters of the process and the macroscopic mechanical behavior.

Improvement of Chemical Liquid Vaporized Infiltration Process for Fast Densification of Carbon/Carbon Composites

An improved chemical liquid vaporized infiltration process was developed to fast densify carbon/carbon (C/C) composites. A disc-shaped carbon felt was chosen as preform, whose upper and lower sides were fixed and heated simultaneously by two flat surfaces of two heat sources, and the precursor was heated by the third heat source separated. By this method, carbon felts (bulk density ~0.2 g/cm3) were densified to C/C composites with density of 1.29, 1.61 and 1.72 g/cm3 when prepared for 3h at 900°C, 1000°C and 1100°C, respectively. Scanning electron microscopy (SEM) reveals that the carbon fibers of the composite are surrounded by ring-shaped pyrocarbon. The deposition process is analyzed by dividing the reactor into four regions associated with specific functions and the reasons for the rapid fabrication are proposed as the short convection and diffusion path for the precursor and the existing thermal gradients across the preform.

The Magnetic Characterization of a ${\rm MgB}_{2}$ Superconducting Hollow Cylinder, at Variable Temperatures up to ${\rm T}_{\rm c}$

DC magnetic fields have been applied to a superconducting hollow MgB2 cylinder of relatively large dimensions (dext/int = 48/44 mm, h = 31 mm), produced by reactive Mg liquid infiltration (RLI) process. Such a kind of full dense cylinder has no discontinuity in its shape, at difference with other HTS textured cylindrical objects, and it represents a prototype of a larger one that can be applied in many electromechanical applications, like bearing, or pure magnetic application, like shielding. The aim of the experiments is to verify its shielding ability at low fields and the maximum trapped fields, when higher field are applied. The measurements are performed at variable temperatures from 13 K up to Tc . The magnetic field values, measured by Hall probe located at the center of the cylinder, dependent on the various cooling regimes (either zero field cooling (ZFC) and field cooling (FC)) and on the presence of a background external magnetic field. The trapped fields dependence from the temperature and background field is reproduced by a simple model of the current distribution and with an analytical expression of Jc (B,T). It was possible also to describe the transition of the superconducting cylinder to the normal state, induced by flux jumping, on a base of a phenomenological model with two process having different time scale.

Effect of alloying elements on the interfacial bonding strength and electric conductivity of carbon nano-fiber reinforced Cu matrix composites

The effect of strong carbide forming elements (Ti, Zr) in the Cu matrix on interfacial bonding and electric conductivity of CNF/Cu composites has been studied. The chemical bonding states of C/Cu interfaces were calculated by using the first-principles molecular orbital method, which showed that the presence of Ti or Zr in the interface layer strongly increases chemical bonding strength across the interface. The CNF/Cu composites with the alloying elements have been fabricated successfully by liquid infiltration process. Tensile strength of composites with additional element has been increased. However, Ti addition compared with Zr addition resulted in great loss of electric conductivity.

OPTIMIZING THE EFFECTIVE PARAMETERS OF TUNGSTEN – COPPER COMPOSITES

Tungsten – Copper composites are generally used for electrical contact materials. These composites are suitable for hard working conditions such as intensive electrical sparks, gouging spark erosion, surface melting, welding, material transfer etc. The aim of the present article is to determine optimum processing conditions to improve the mechanical and physical properties of W-Cu composites with the view to increase their lifetime. W-Cu specimens are produced using powder material and the liquid infiltration process. Chemical composition, pressing machine pressure, infiltration time and temperature are variable parameters for specimen production. By optimizing the amount of cobalt addition, shaping pressure, time and temperature of production process, optimum values for mechanical and electrical properties such as density, hardness and resistivity are obtained.

Analysis of the minority crystalline phases in bulk superconducting MgB 2 obtained by reactive liquid Mg infiltration

The preparation of MgB 2 starting from the elements (in situ process), when performed in conjunction with the Mg liquid infiltration process, gives rise to very high dense manufacts, with outstanding mechanical and superconducting characteristics. The infiltration technique is particularly suitable when realized into preforms of crystalline boron powders of grain size up to about 100 μm; however, the resulting products typically present a microstructure characterized by a composite morphology, indicating a inhomogeneous distribution of different crystalline phases. In this paper the composition and the morphology of typical products has been analyzed with several techniques, including X-ray powder diffraction (XRPD), scanning electron microscopy and X-ray fluorescence microanalysis, aiming to quantify the (spatial) distribution of crystalline phases and the presence of minority phases different from the MgB 2. The minority phases that are commonly present in the MgB 2 infiltration preparation are residual elemental magnesium and a new boron-rich magnesium boride phase (Mg 2B 25), recently discovered and structurally characterized (by unconventional XRPD methods) by our group. We have analyzed the common morphological features of typical MgB 2 samples, and cross-checked the analytical results obtained from different experimental techniques. Our results show that the new Mg 2B 25 phase amounts to less than 10 molar % of the MgB 2 entire product, and that it is mainly concentrated, as submicronic inclusions (inside the larger crystalline MgB 2 grains) where it can reach concentration values up to 40 molar %.

The Influence of Microporosity on the Hydrogen Storage Capacity of Ordered Mesoporous Carbons

The adsorption of hydrogen on nanostructured carbons prepared by templating of mesoporous silicas has been studied at 77 K and between 0.1 and 1 MPa. The maximum capacity was obtained with a carbon synthetized by a sucrose liquid infiltration process in MCM-48 silica. It was observed that the hydrogen weight percentage which can be adsorbed on a carbon can be correlated to its microporous volume determined by CO2 adsorption and Dubinin-Radushkevich equation.

Fabrication of carbon nano-sized fiber reinforced copper composite using liquid infiltration process

In the present study, the unidirectionally aligned carbon nano-sized fiber (CNF) reinforced Cu composite was fabricated by the liquid infiltration process, and the tensile behavior of the composite was evaluated. For the unidirectional alignment of the severely entangled CNFs, the mechanical drawing process was utilized. The bundle of drawn Cu tube, compacted with the unidirectionally aligned CNFs, was placed on the specially designed mold, which was subsequently canned, degassed, evacuated, heated at 1100 °C for 10 min in the electric furnace to liquefy Cu and finally transferred to the press. Tensile test results showed that approximately twofold increase in tensile strength was obtained with 13% CNF reinforcement, despite the weak chemical bonding between C and Cu. Based on the micrographic and fractographic observations, the strengthening mechanism of the CNF reinforced Cu composite fabricated by the liquid infiltration process was discussed.

Tensile behavior of carbon nano-fiber-reinforced Cu composite using the liquid infiltration process

A carbon nano-fiber (CNF)-reinforced Cu composite was fabricated by the liquid infiltration process, and the tensile behavior of the fabricated composite was evaluated. For the unidirectional alignment of the severely entangled CNFs, the mechanical drawing process was utilized. The bundle of drawn Cu tubes, compacted with unidirectionally aligned CNFs, was placed on the specially designed mold. The mold was subsequently canned, degassed, evacuated, heated at 1100 °C for 10 minutes in the electric furnace to liquefy the Cu, and, finally, transferred to the press. The optical and scanning electron microscope (SEM) micrographic observations of the final product demonstrated that, even though the CNFs were not uniformly distributed throughout the specimen due to the molten metal flow during the melting process, the local unidirectionality was preserved in those areas where CNFs were densely packed. Tensile test results showed that a twofold increase in tensile strength was obtained with an approximately 13 pct CNF reinforcement, despite there being no known bonding between CNFs and Cu. The transmission electron microscope (TEM) observations demonstrated that the C atoms in the CNF were forced into the Cu matrix, which caused the roughening of the CNF surface and the supersaturated solutionization of the Cu matrix in the vicinity of the CNF during liquid processing. A strong mechanical interlocking was believed to exist between the CNF and Cu due to the roughened surface of the CNF and the stress field developed around the CNF as a result of the supersaturated solutionization of the Cu matrix. This mechanical bonding mechanism appeared to be unique in the composite system reinforced with nano fibers produced by the liquid infiltration process.

On the identification of the failure mechanisms in oxide/oxide composites using acoustic emission

In the present work, a new class of oxide/oxide composites made of Nextel™ 720 fibre reinforcement and a mullite-based matrix, fabricated by using liquid polymer infiltration process, were studied. A fibre coating was applied via sol–gel in order to achieve improved damage tolerant behaviour. Mechanical properties were investigated at ambient temperature under quasi-static loading in the presence of continuous Acoustic Emission (AE) monitoring. Statistical pattern recognition analysis is the proposed tool for the classification of the monitored AE events. Lacking an a priori knowledge of different signal classes, unsupervised pattern recognition algorithms were used. A complete methodology including descriptor selection methods, procedures for numerical verification and cluster validity criteria is followed. Cluster analysis of AE data was achieved and the resulted clusters were correlated to the damage mechanisms of the material under investigation. This process was assisted by systematic microscopic examination. Furthermore, the initiation and evolution of each mechanism is described by plotting the cumulative hits of each class as a function of the applied load.