Intergrain Material Research Articles

Explaining Browns paradox in NdFeB magnets from micromagnetic simulations

We present the results of a systematic investigation into how the coercivity and maximum energy product of NdFeB permanent magnets are affected by magnetic and geometric microscopic properties. The results are based on numerical micromagnetic simulations carried out with the open source numerical framework MagTense. We considered artificially generated realistic microstructures for which we can control the number of crystal grains as well as the thickness of the intergrain region. Based on variations of the exchange constant, easy axis orientation, grain boundary width and intergrain material properties, the results indicate that while all of these can contribute to a reduction of coercivity, the easy axis orientation has the largest influence. For this, if the easy axis orientation is distributed within a cone with an opening angle of 15°, that is enough to reduce the coercivity by 1 T. Regarding the maximum energy product, the width of the grain boundary layer as well as the easy axis orientation were seen to have the largest influence, with the exchange constant only very weakly influencing the maximum energy product. Our analysis thus methodically clarifies the different factors contributing to the reduction of the value of the coercivity and maximum energy product when compared to the theoretical limit, a discrepancy known as Brown’s paradox.

Single and multi-walled carbon nanotubes doped (Cu0.5Tl0.5)Ba2Ca2Cu3O10−δ superconductors

Carbon nanotubes (CNTs) added polycrystalline (CNT)y(Cu0.5Tl0.5)Ba2Ca2Cu3O10−δ (y=0, 0.25%, 1.0% and 7%) superconductor samples have been synthesized at normal pressure. The X-ray diffraction (XRD) studies of these samples have shown an orthorhombic crystal structure following PMMM space group. The added CNTs have been found to diffuse at the inter-grain sites and act as cementing material. SEM images show that CNTs are forming an electrical-network between grains. These observations suggest that the CNT addition to the CuTl-1223 compounds improve the electrical connection between superconducting grains which in turn increase the Jc of the final compound. The room temperature resistivity of as-prepared samples decreases with the addition of CNT. The onset temperature of superconductivity [Tc(onset)] and zero resistivity critical temperature [Tc(R=0)] are found to decrease with the CNT additions. Except for the samples with CNT additions of 0.25%, the density [mass/volume] of final compound after CNT addition is decreased. Magnitude of diamagnetism is also suppressed with the addition of CNT which suggests that some of carbon atoms diffuse into the unit cells at the intra-grain sites. The Tc(R=0) becomes quite stable after post-annealing in oxygen atmosphere; the Tc(R=0) varies between 98 and 96K in polycrystalline (CNT)y(Cu0.5Tl0.5)Ba2Ca2Cu3O10−δ (y=0, 0.25%, 1.0% and 7%) samples. The out of phase component of magnetic susceptibility observed as a broad peak in the as-prepared samples, is transformed to a sharp peaks after post-annealing of the samples in oxygen atmosphere. This shows that oxygen diffusion converts the inter-grain material to respective oxides and carbonates that increase the surface areas of the current flow around inter-grain loops that reduce the hysteresis losses. The electron-microscopy of these samples has shown that inter-grain voids population is suppressed with the addition of CNTs. FTIR absorption measurements of (CNT)y(Cu0.5Tl0.5)Ba2Ca2Cu3O10−δ (y=0, 0.25%, 1.0% and 7%) samples have shown apical and planar oxygen modes at their usual positions.

Interaction of vortices with different types of pinning centers in MgB2 superconducting films

This contribution reports on the magnetic response of two MgB2 films with 600 nm of thickness. These films were grown using the hybrid physical chemical vapor deposition (HPCVD) method under different temperatures. One of the films, grown at 580 °C, has a micro structure with columnar grains, a rough surface and small grain size. The other, grown at 650 °C, has a smooth surface and larger grains. A double transition is present in the columnar sample. The lower transition temperature is due to currents tunneling through the intergrain material and the higher one is associated with the transition of the grains to the normal state. Magnetic phase diagrams in tilted geometries, with the applied fields (AC and DC) forming angles in the range (0°, 90°) with the plane of the film, were determined to verify the influence of the perpendicular component of the AC field on pinned vortices. Hysteresis loops of magnetization versus applied field were also measured for both samples. The columnar sample exhibits jumps in the decreasing field branch of the magnetization, indicating that vortices pinned by the columnar microstructures leave the sample in bundles.

The effect of grain boundaries and adsorbates on the electrical properties of hydrogenated ultra nano crystalline diamond

The results of a comprehensive study on the temperature dependence of the electrical properties of hydrogenated and air exposed undoped UNCD layers following heating/cooling cycles are presented. The results clearly show that, in contrast to hydrogenated and air exposed single crystal type IIa diamond, which exhibits a clear highly conductive p-type surface layer, the electrical properties of hydrogen and H 2O exposure of UNCD are determined by the properties of the entire layer. The changes in the electrical conductivity of UNCD as a result of heating are governed by two different processes: (i) Loss of water from the external surface that takes place at about 150 °C. This process is reversible, reviving the electrical properties upon exposure to humidity, just like in single crystalline diamond.(ii) Modification of the inter-grain material, which occurs at higher temperatures possibly due to H diffusion and passivation of some dangling bonds in the inter-grain material. This increases the resistivity in an irreversible manner. The conduction mechanism in the inter-grain material is characterized by variable range hopping in band tails thus indirectly proving that the material between the grains is some kind of amorphous carbon.

Bulk and surface‐enhanced Raman spectroscopy of nitrogen‐doped ultrananocrystalline diamond films

AbstractUltrananocrystalline diamond films (UNCD) grown in an Ar‐rich microwave plasma with nitrogen gas added in amounts of 0–25% were studied by Raman spectroscopy using 514.5 and 413.1 nm laser excitation. Besides the Raman spectra of diamond, the first and second order Raman scattering of disordered graphite and polyacetylene were detected and analyzed for samples with different nitrogen content. With surface enhanced Raman scattering (SERS) a variety of surface vibrational modes was observed: (i) CHx stretching vibrations of sp3 and sp2 hybridized carbon in the range of 2800–3100 cm–1; (ii) the sp2 and sp1 hybridized CC(CN) stretching modes in the range of 1600–2300 cm–1; and (iii) lines in the range of 200–1250 cm–1 related to different kinds of vibrations of substituted aromatic rings. These results indicate that in N‐doped UNCD films the intergrain material is a well connected mixture of predominantly sp2 phase with some amount of sp1 phase of carbon and nitrogen that is suggested to provide high electrical conductivity. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Particle size effect of a.c. superconducting properties in compound

Using a new starting material of Ba2Cu3O5 and a three step heat treatment, single phase Tl2Ca1Ba2Cu2O8 high- superconducting samples have been prepared, possessing the onset- and critical temperatures K and K. The morphology dependent value of is 17 Oe, 8 Oe and 5 Oe at 77 K in the case of bulk, crushed and powdered materials, respectively. The a.c. susceptibility, r.f. susceptibility and microwave absorption properties show a significant dependence on the particle size with a sharp change in the interval between 750 μm and 1200 μm. These experiments provide characteristic parameters for intergrain material (treated as 3D Josephson network) as mm, Oe and A/cm2 at 77 K. The data are controlled by modulated microwave absorption measurements. The results obtained can be explained well both by the finite size junction model and cavity mode absorption model. The Josephson network is determined unambiguously by metallic S-N-S weak links.

Relaxation of La 0.67 Ca 0.33 MnO 3 Films Resistance in Pulsed High Magnetic Fields

The resistance relaxation after switch-off of high magnetic field pulses of 20-40 T was studied for two series of La0.67Ca0.33MnO3thin films. The response dynamics was obtained for temperature range 77-300 K. It is shown that the relaxation time is sensitive to the temperature of the resistivity maximum and to magnetoresistive properties of a film. The observed relaxation times are assumed to be related to existence of magnetic domains or intergrain material.

Superconducting properties of Pb-free and Pb-substituted bulk ceramics of Bi-2212 cuprates

Superconducting properties of Pb-free and partially Pb-substituted bulk ceramics of the Bi 2- x Pb x Sr 2CaCu 2O z system are studied under the influence of various methods of post-treatment. It is shown that the shift of T c from the maximum of about 95 K to lower values is due to an intercalation of oxygen in the perovskite slabs, most probably resulting in the creation of localized carriers in form of Cu 3+. The substitution Pb→Bi displays a beneficial role in the improvement of the superconducting properties of the intergrain material of ceramics as well as in the creation of pinning centres with an optimal Pb concentration of x=0.4. For this value the average distance of the Pb pinning centres of about 10 Å correlates with the coherence length of 2212 in the a-b plane. As possible pinning centres lattice defects created by the intergrowth of 2223 in 2212 are considered.

Intergrain flux-pinning in relation to structural phase transformation and tweed formation in YBa 2(Cu 1- xFe x) 3O 7- y and NdBa 2(Cu 1- xFe x) 3O 7- y

Fe-doped superconductors REBa 2(Cu 1- x Fe x ) 3O 7- y (RE=Y, Nd; 0≤x≤0.1) were investigated by conventional and synchrotron X-ray diffraction, thermogravimetric analysis and AC susceptibility measurements. The transition from orthorhombic to macroscopically tetragonal symmetry is analogous for both systems and occurs at x c≈0.030 for YBa 2(Cu 1- x Fe x ) 3O 7- y and at x c≈0.021 for NdBa 2(Cu 1- x Fe x ) 3O 7- y . The spontaneous strain ϵ=2( b- a)/( a+ b) shows tricritical-like scaling as ϵ∼| x- x c| 1 4 , and tweed formation is indicated by diffraction-line-broadening effects. For YBa 2(Cu 1- x Fe x ) 3O 7- y the onset temperature T c of superconductivity is almost constant below x=0.03 (i.e., in the orthorhombic regime) and falls linearly for x > 0.03 in the tweed regime, extrapolating to zero near x=0.2. Samples which were treated twice in flowing O 2 at 400°C for 10 h show a significantly steeper linear decrease of T c with x in the tweed regime. Contrary to the Y-compound, NdBa 2(Cu 1- x Fe x ) 3O 7- y shows no plateau of T c in the orthorhombic regime, and T c drops rapidly with dopant concentration x. The low-field AC susceptibility of each sintered sample is strongly dependent upon applied AC RMS and superimposed DC bias fields, owing to the weak-link network of the superconducting grains. Critical transport current densities, J 0, and pinning force densities α J for the intergranular vortices were estimated from AC susceptibility measurements. Both quantities are strongly suppressed by Fe-doping and the substitution of Nd ions for Y ions. A distinct change of the slope dα J /d x at the critical composition x c for both the YBCO and the NdBCO series indicates a softening of the free energy gradient between superconducting grains and non-superconducting intergrain material due to the formation of twin-domain boundaries in the grains.

SUPERCONDUCTING PROPERTIES OF INTERGRAIN AND INTRAGRAIN MATERIAL IN POLYCRYSTALLINE GdBa2Cu3O7

High quality polycrystalline GdBa 2 Cu 3 O 7−x samples with oxygen content equal to 7.0 were synthesized. Sensitive magnetization measurements were performed at low temperatures and in vicinity of T c . From these measurements the intergrain material volume was calculated. The existence of two first penetration fields was demonstrated.

COMPLEX SUSCEPTIBILITY AND CRITICAL CURRENT OF HIGH-TEMPERATURE SUPERCONDUCTORS

The ac complex susceptibility χ′−iχ″ of several bulk samples with very high critical current density Jc have been measured. The results show that these samples are all polycrystalline bulk material, the peak of χ″ still related to losses within “the weak link structure” of the intergrain material. But the intergrain coupling strength of the high Jc samples is much stronger than ordinary high-Tc superconductors. A parameter β=(Tc2(0) −Tc2(h0))/h02/3 has been proposed to characterize the coupling strength.

COMPLEX SUSCEPTIBILITY, MAGNETIZATION AND ELECTRICAL RESISTIVITY OF HIGH TEMPERATURE SUPERCONDUCTORS

ac complex susceptibility, χ′ and χ″, dc magnetization and electrical resistivity of sintered superconductors Bi–Pb–Sr–Ca–Cu–O and Y–Ba–Cu–O have been measured. The results show two non-linear 2/3 power behaviors: field dependence of χ″ peak temperature, a quasi de Almeida-Thouless line, and a irreversibility line. The former corresponds to the property of intergrain material measured at low field, it can be interpreted by superconductive glass state in high-T c superconductor and/or the weakly connected loop model. The latter corresponds to the property of intragrain material measured at higher field, it can be interpreted by the thermally activated flux creep model. Moreover, the field dependences of width and height of χ″ peak are also exhibited. Finally, we have measured the I – V characteristics at 103.8 K for Bi–Pb–Sr–Ca–Cu–O sample and suggested the relation between the χ″ peak temperature and the zero dissipation temperature.

Comparative study of hydrogen evolution from amorphous hydrogenated silicon films

The results of hydrogen evolution from amorphous hydrogenated silicon (a-Si:H) films prepared by glow discharge decomposition of SiH 4, sputtering in an ArH 2 plasma and vacuum evaporation with the addition of hydrogen are compared. Two evolution processes can be distinguished: a low temperature evolution due to the release of molecular hydrogen through voids or cracks and an evolution at high temperatures due to effusion from compact a-Si:H material, which is dominated by diffusion of atomic hydrogen. The low temperature evolution for sputtered and evaporated films is related to molecular hydrogen that is adsorbed dissociatively on internal surfaces during film growth and for glow discharge samples to the presence of polysilane-like intergrain material. Differences in the position of the high temperature evolution peaks are attributed to a higher degree of disorder in evaporated and sputtered films compared with that in glow discharge samples.