Searcharxiv⌕ Search

arXiv subjects

R. Cloots

Publications and source records attributed to R. Cloots.

At least 19 recordsLinked to original sources
arXiv2013

AC vs. DC Electrophoretic Deposition of Hydroxyapatite on Titanium

The bio-inertness of titanium and its alloys attracts their use as bone implants. However a bioactive coating is usually necessary for improving the bone bonding of such implants. In this study, electrophoretic deposition(EPD) of hydroxyapatite (HA) powder on titanium plate was performed using butanol as solvent under direct current (DC) and alternating current (AC) fields. The zeta potential of the suspensions was measured to understand their stability and the charge on the particles. Coating thickness was varied by adjusting the voltage and time of deposition. Surface morphology and cross section thickness were studied using scanning electron microscopy and image analysis software. Surface crack density was calculated from the micrographs. The results showed that the samples of similar thickness have higher grain density when coated using AC as compared to DC EPD. This facile but novel test proves the capability of AC-EPD to attain denser and uniform HA coatings from non-aqueous medium.

cond-mat.mtrl-sci↗
arXiv2011

Microwave properties of DyBCO monodomain in the mixed state and comparison with other RE-BCO systems

We report on microwave measurements on DyBa$_2$Cu$_3$O$_{7-\rmδ}$ monodomains grown by the top-seeded melt-textured technique. We measured the field increase of the surface resistance $R_{\rm s}(H)$ in the a-b plane at 48.3 GHz. Measurements were performed at fixed temperatures in the range 70 K - $T_{\rm c}$ with a static magnetic field $μ_0H<0.8$ T parallel to the c-axis. Low field steep increase of the dissipation, typical signature of the presence of weak links, is absent, thus indicating the single-domain behaviour of the sample under study. The magnetic field dependence of $R_{\rm s}(H)$ is ascribed to the dissipation caused by vortex motion. The analysis of $X_{\rm s}(H)$ points to a free-flow regime, thus allowing to obtain the vortex viscosity as a function of temperature. We compare the results with those obtained on RE-BCO systems. In particular, we consider strongly pinned films of YBa$_2$Cu$_3$O$_{7-\rmδ}$ with nanometric BaZrO$_3$ inclusions.

cond-mat.supr-con↗
arXiv2008

Microwave properties of DyBa_2Cu_3O_(7-x) monodomains and related compounds in magnetic fields

We present a microwave characterization of a DyBa$_{2}$Cu$_{3}$O$_{7-x}$ single domain, grown by the top-seeded melt-textured technique. We report the (a,b) plane field-induced surface resistance, $ΔR_s(H)$, at 48.3 GHz, measured by means of a cylindrical metal cavity in the end-wall-replacement configuration. Changes in the cavity quality factor Q against the applied magnetic field yield $ΔR_s(H)$ at fixed temperatures. The temperature range [70 K ; T_c] was explored. The magnetic field $μ_0 H <$ 0.8 T was applied along the c axis. The field dependence of $ΔR_s(H)$ does not exhibit the steep, step-like increase at low fields typical of weak-links. This result indicates the single-domain character of the sample under investigation. $ΔR_s(H)$ exhibits a nearly square-root dependence on H, as expected for fluxon motion. From the analysis of the data in terms of motion of Abrikosov vortices we estimate the temperature dependences of the London penetration depth $λ$ and the vortex viscosity $η$, and their zero-temperature values $λ(0)=$165 nm and $η(0)=$ 3 10$^{-7}$ Nsm$^{-2}$, which are found in excellent agreement with reported data in YBa$_{2}$Cu$_{3}$O$_{7-x}$ single crystals. Comparison of microwave properties with those of related samples indicate the need for reporting data as a function of T/T_c in order to obtain universal laws.

cond-mat.supr-con↗
arXiv2007

Electrical transport and percolation in magnetoresistive manganite / insulating oxide composites: case of La0.7Ca0.3MnO3 / Mn3O4

We report the results of electrical resistivity measurements carried out on well-sintered La0.7Ca0.3MnO3 / Mn3O4 composite samples with almost constant composition of the magnetoresistive manganite phase (La0.7Ca0.3MnO3). A percolation threshold (fc) occurs when the La0.7Ca0.3MnO3 volume fraction is ~ 0.19. The dependence of the electrical resistivity as a function of La0.7Ca0.3MnO3 volume fraction (fLCMO) can be described by percolation-like phenomenological equations. Fitting the conducting regime (fLCMO > fc) by the percolation power law returns a critical exponent t value of 2.0 +/- 0.2 at room temperature and 2.6 +/-0.2 at 5 K. The increase of t is ascribed to the influence of the grain boundaries on the electrical conduction process at low temperature.

cond-mat.mtrl-sci↗
arXiv2005

Synthesis and characterization of Bi2Sr2CaCu2O8 ceramics prepared in presence of sodium

This paper aims to study Na doping in Bi-2212 superconducting ceramics. Na-doped Bi-2212 samples powders with a Bi2-uSr2-vCa1-wCu2-xNayOz stoichiometry are prepared by means of thermal treatments. The intended Na substitution level, y, is fixed at 0.3. The initial content of the other ions is adopted so that Na substitution on one crystallographic site only is expected. Samples are characterized by electrical properties, X-ray diffraction analysis (XRD) and Energy Dispersive X-ray analysis (EDX). The influence of Na on the crystal chemistry and phases compatibility for 2212 materials is emphasized. Electrical resistivity results show that the critical temperature is independent of the sample initial stoichiometry. In some cases the production of a Bi-2223 phase is however enhanced.

cond-mat.supr-con↗
arXiv2005

Influence of Na-doping and sintering temperature on increasing Bi2Sr2CaCu2O8 superconducting phase content in powder form materials

This paper proposes a systematic way to explore the effects of Na on Bi-2212 phase formation. The influence of sodium metal is correlated with the 2212 fraction formed in the sample. The effect on the sintering temperature and the importance of the substitution site is studied. Samples are characterized by X-ray diffraction analysis (XRD). Results clearly show that the 2212 phase content is enhanced and sintering temperatures can be lowered with regard to the undoped samples.

cond-mat.supr-con↗
arXiv2005

Silver paint as a soldering agent for DyBaCuO single-domains welding

Silver paint has been tested as a soldering agent for DyBaCuO single-domain welding. Junctions have been manufactured on Dy-Ba-Cu-O single-domains cut either along planes parallel to the c-axis or along the ab-planes. Microstructural and superconducting characterisations of the samples have been performed. For both types of junctions, the microstructure in the joined area is very clean: no secondary phase or Ag particles segregation has been observed. Electrical and magnetic measurements for all configurations of interest are reported $ρ(T)$ curves, and Hall probe mapping). The narrow resistive superconducting transition reported for all configurations shows that the artificial junction does not affect significantly the measured superconducting properties of the material.

cond-mat.mtrl-sci↗
arXiv2004

Unusual thermoelectric behavior of packed crystalline granular metals

Loosely packed granular materials are intensively studied nowadays. Electrical and thermal transport properties should reflect the granular structure as well as intrinsic properties. We have compacted crystalline $CaAl$ based metallic grains and studied the electrical resistivity and the thermoelectric power as a function of temperature ($T$) from 15 to 300K. Both properties show three regimes as a function of temperature. It should be pointed out : (i) The electrical resistivity continuously decreases between 15 and 235 K (ii) with various dependences, e.g. $\simeq$ $T^{-3/4}$ at low $T$, while (iii) the thermoelectric power (TEP) is positive, (iv) shows a bump near 60K, and (v) presents a rather unusual square root of temperature dependence at low temperature. It is argued that these three regimes indicate a competition between geometric and thermal processes, - for which a theory seems to be missing in the case of TEP. The microchemical analysis results are also reported indicating a complex microstructure inherent to the phase diagram peritectic intricacies of this binary alloy.

cond-mat.mtrl-sci↗
arXiv2004

From RE-211 to RE-123. How to control the final microstructure of superconducting single-domains

This paper reviews the usual techniques for producing YBCO-type single-domains and the microstructure of the as-obtained samples. The problems of seed dissolution and parasite nucleations are discussed in details. Formation of microstructural defects, such as pores and cracks, are examined. An important part of this review is devoted to the study of the influence of RE-211 particles [RE2BaCuO5 where RE denotes Y, Yb, Nd, Sm, Dy, Gd, Eu or a mixture of them. Generally Nd4Ba2Cu2O10 is preferred to Nd2BaCuO5] on the microstructure and properties of RE-Ba-Cu-O single-domains. Trapping/Pushing theory is described in order to explain the spatial distribution of RE-211 particles in the RE-123 [(RE)Ba2Cu3O7-d] monoliths. Formation of RE-211-free regions is discussed. Different ways to limit the RE-211 coarsening are reviewed. Microstructural defects in the RE-123 matrix caused by the RE-211 particles are presented. It is also shown that RE-211 particles play a significant role on the mechanical properties of single-domain samples. We finish this review by discussing the Infiltration and Growth process as a good technique to control the microstructure.

cond-mat.supr-con↗
arXiv2004

Magneto - transport characterization of Dy123 monodomain superconductors

The following report considers textured materials of the DyBa2Cu3O7 type seeded with a Nd123 seed as initiator. They are grown with an excess 20% Dy211 phase on a Dy2O3 substrate. We report chemical characterizations, electrical resistivity, thermoelectric power and thermal conductivity over a broad temperature range as a function of an applied magnetic field up to 6 T. We show that specific features appear on the magneto thermal transport properties different in these materials from those found in single crystals and polycrystalline samples. We propose that two vortex regimes can be distinguished in the mixed phase, - due to the intrinsic microstructure. We calculate the viscosity, entropy and figure of merit of the samples.

cond-mat.supr-con↗
arXiv2004

Exponential and power law distribution of mass clusters in a (magnetic-like) deposition model of elongated grains in 2D piles

A generalized so called magnetically controlled ballistic rain-like deposition (MBD) model of granular piles has been numerically investigated in 2D. The grains are taken to be elongated disks whence characterized by a two-state scalar degree of freedom, called ''nip'', their interaction being described through a Hamiltonian. Results are discussed in order to search for the effect of nip flip (or grain rotation from vertical to horizontal and conversely) probability in building a granular pile. The characteristics of creation of + (or $-$) nip's clusters and clusters of holes (missing nips) are analyzed. Two different cluster-mass regimes have been identified, through the cluster-mass distribution function which can be exponential or have a power law form depending on whether the nip flip (or grain rotation) probability is large or small. Analytical forms of the exponent are empirically found in terms of the Hamiltonian parameters.

cond-mat.stat-mech↗
arXiv2003

AC magnetic behavior of large grain magneto-resistive La0.78Ca0.22Mn0.90Ox materials

We report a detailed set of AC magnetic measurements carried out on bulk large grain La-Ca-Mn-O samples extracted from a floating zone method-grown rod. Three samples with $La_{0.78}Ca_{0.22}Mn_{0.90}O_{x}$ stoichiometry but differing in their microstructure were investigated by electrical resistivity and AC susceptibility measurements: (i) a single grain sample, (ii) a sample containing two grains and (iii) a polycrystalline sample. We show that the superimposition of DC magnetic fields during AC magnetic susceptibility measurements is an efficient way for characterizing the magnetic transition of samples with different microstructures. Whereas both single grain and polycrystalline samples display a single susceptibility peak, an additional kink structure is observed in the case of the double grain sample. The temperature dependence of the AC susceptibility measured with superimposed DC magnetic fields is analyzed in the framework of second-order phase transition ideas. The relations between the critical exponents ($β+ γ$ ~ 1.5, $δ$ ~ 2.5) are found to be close to those of the mean-field model for all samples. This is attributed to the disordering caused by unoccupied Mn sites.

cond-mat.supr-con↗
arXiv2003

The contribution of 211 particles to the mechanical reinforcement mechanism of 123 superconducting single domains

Hardness and fracture toughness of Dy-123 single-domains were studied by Vickers micro-indentation. A significant anisotropy of the mechanical properties was observed. Hardness tests give higher values when performed in (001) planes rather than in planes parallel to the c-axis. Moreover cracks pattern around the indentation follows preferential orientation in planes parallel to the c-axis whereas a classical ''four-cracks'' pattern is observed in the (001) planes. It has been possible to show the crucial role played by the 211-particles in the deviating mechanism of cracks and the relevance of the 211-particle distribution high homogeneity in the material.

cond-mat.supr-con↗
arXiv2003

Intergranular and intragranular properties of superconducting multifilamentary Bi 2223 Ag tapes : comparison of electrical transport and magnetic measurement methods

Several experimental techniques have been used in order to characterize the properties of multifilamentary Bi-2223 / Ag tapes. Pristine samples were investigated by electrical resistivity, current-voltage characteristics and DC magnetic moment measurements. Much emphasis is placed on comparing transport (direct) and magnetic (indirect) methods for determining the critical current density as well as the irreversibility line and resolving usual lacks of consistency due to the difference in measurement techniques and data analysis. The effect of an applied magnetic field, with various strengths and directions, is also studied and discussed. Next, the same combination of experiments was performed on bent tapes in order to bring out relevant information regarding the intergranular coupling. A modified Brandt model taking into account different types of defects within the superconducting filaments is proposed to reconciliate magnetic and transport data.

cond-mat.supr-con↗
arXiv2003

Anisotropic AC Behavior of Multifilamentary Bi-2223/Ag Tapes

In this communication, we report on the anisotropy of the superconducting properties of multifilamentary Bi-based tapes experimentally investigated by AC magnetic susceptibility measurements. The susceptibility $χ= χ' - j χ''$ was measured using a commercial system and a couple of orthogonal pick-up coils. The $χ''$ vs. temperature curves were shown to exhibit two peaks. The smaller of the peaks, occurring near T = 72K, was only visible for particular field directions and within a given frequency window. Such results point out the role played by the phase difference between the applied magnetic field and the internal magnetic field seen by the filaments.

cond-mat.supr-con↗
arXiv2002

Magnon-polaron and Spin-polaron Signatures in the Specific Heat and Electrical Resistivity of $La_{0.6}Y_{0.1}Ca_{0.3}MnO_3$ in Zero Magnetic Field, and the Effect of $Mn-O-Mn$ Bond Environment

$La_{0.6}Y_{0.1}Ca_{0.3}MnO_{3}$, an $ABO_{3}$ perovskite manganite oxide, exhibits a non trivial behavior in the vicinity of the sharp peak found in the resistivity $ρ$ as a function of temperature $T$ in zero magnetic field. The various features seen on $dρ/dT$ are discussed in terms of competing phase transitions. They are related to the $Mn-O-Mn$ bond environment depending on the content of the $A$ crystallographic site. A Ginzburg-Landau type theory is presented for incorporating concurrent phase transitions. The specific heat $C$ of such a compound is also examined from 50 till 200 K. A log-log analysis indicates different regimes. In the low temperature conducting ferromagnetic phase, a collective magnon signature ($C \simeq T^{3/2}$) is found as for what are called magnon-polaron excitations. A $C \simeq T^{2/3}$ law is found at high temperature and discussed in terms of the fractal dimension of the conducting network of the weakly conducting (so-called insulating) phase and Orbach estimate of the excitation spectral behaviors. The need of considering both independent spin scattering and collective spin scattering is thus emphasized. The report indicates a remarkable agreement for the Fisher-Langer formula, i.e. $C$ $\sim$ $dρ/dT$ at second order phase transitions. Within the Attfield model, we find an inverse square root relationship between the critical temperature(s) and the total local $Mn-O-Mn$ strain.

cond-mat↗
arXiv2001

Magnetic properties of magnetically textured Bi-2212 ceramics

This paper aims at reporting magnetic properties of bulk polycrystalline Bi2Sr2Ca0.8Dy0.2Cu2O8-y samples textured under a magnetic field. The microstructure of these materials is highly anisotropic and exhibits particular features needed to be taken into account in order to interpret their magnetic and electrical properties. First the AC magnetic susceptibility c = c ' - j c" has been measured for several magnetic fields (H // ab and H // c) and compared to the electrical resistivity data. The structure of the c" peak is shown to be related to the chemical content distribution of the superconducting grains. Next, the magnetic flux profiles have been extracted from the magnetic measurements using the Campbell - Rollins procedure. The anisotropy of the flux profiles and their peculiar curvature behaviour for H // c point out the role of both grain platelet structure and the presence of secondary phases. From these results, we conclude that the magnetic properties of such magnetically textured materials do not allow for a reliable extraction of the critical current density Jc but essentially probe geometric effects. Such effects have to be taken into account for improving the manufacture of attractive high-Tc materials.

cond-mat.supr-con↗