Coupling Of Grains Research Articles

Investigation of the Usability of Travertine Quarry Wastes As Concrete Aggregate

Denizli basin is one of the most important travertine deposition area around the world. Kaklik, Kocabas, Honaz, Asagidagdere, Akkoy, Civril are major production areas. Tectonic activity in the region has been negatively affected the block production efficiency in the quarries. Quarry wastes by wire cutting are collected in waste disposal areas and these materials do not use for any other purposes. In this study, investigation of usability of different sized travertine quarry wastes in concrete as aggregate is aimed. For this purpose, physico mechanical properties of aggregates were investigated and concrete mixtures were prepared. Strength of 7, 28 and 56 days concrete samples which are targeted of C30 concrete class were also determined. Strength of prepared travertine aggregated concrete compared to still being produced concrete. For comparison, all procedures were performed for limestone aggregated concrete. Thin sections were prepared for investigations of distribution and coupling of grains in cement material under polarizan microscope. 32.80-42.70 MPa and 39.34-46.04 MPa strengths were determined for travertine aggregated and limestone aggregated concretes respectively. By using travertine aggregate, limit values for C30 concrete class have been reached. This study demonstrates that the travertine quarry wastes can be used as aggregates in concrete. In Denizli region there is no concrete firm use travertine wastes as aggregate. It is thought that, as an alternative building material travertine can be used in production of brick, curbs, keystone and various materials. For this aim more detailed experimental studies on travertines are needed.

Enhanced transport critical current density in textured Sr0.6K0.4Fe2As2 + Ag tapes by an intermediate sintering process of precursors

Fe-clad textured Sr0.6K0.4Fe2As2+Ag tapes have been fabricated through an intermediate sintering process of precursors. Comparing with the conventional fabrication process, it yields much better phase formation and denser microstructure, resulting in improved coupling of grains. Furthermore, the superconducting cores exhibit homogeneous element distribution and well developed c-axis texture. As a result, sharper resistive transition and larger transport critical current density were achieved. The transport Jc values in the fields of 0T and 10T are 1.5×104A/cm2 and 2.3×103A/cm2 at 4.2K, respectively, which are the highest values reported so far for the pnitide wires and tapes with Ag addition.

Development of High Performance MgB2Tapes and Wires

MgB 2 shows no weak coupling of grains, indicating that grain alignment is not essential to obtain large current transfer from one grain to adjacent grain. This is a very big advantage because a simple process, such as powder-in-tube (PIT) method, can be applied for the fabrication of wires and tapes. Here, we report the recent progress of our MgB 2 tapes and wires. There are two PIT methods. One is an in situ method, in which a powder mixture of Mg and B is used as a starting material. The other is an ex situ method, in which reacted MgB 2 powder is used. Superconducting properties are much sensitive to the quality of the starting powder, the porosity of the MgB 2 core and the heat-treatment conditions. Impurity addition to the mixture of starting powder is an effective method to improve J c of MgB 2 tapes and wires. Among the impurities nano-SiC powder is the most popular additive. SiC addition introduces C substitution for B site in MgB 2 and enhances H c2 , thus leading to the increase of J c in high ...

Improved critical current densities in MgB2 tapes fabricated through an ex situ process using powders treated in various chemical solutions

We have studied grain coupling and critical current density (Jc) in ex situ processed Fe-sheathed MgB2 tapes fabricated by a powder-in-tube (PIT) technique, using MgB2 powder soaked in various chemical solutions. The grain coupling and the Jc are strongly influenced by the chemical solutions. Compared with the chemical treatment in a benzene solution of benzoic acid, the use of a cyclohexane solution of benzoic acid doubles the Jc value. Cyclohexane is less stable and hence effectively removed from the surface of MgB2 grains, bringing about the improved coupling of grains and the Jc enhancement.

Effect of continuous layer in CGC perpendicular recording media

The effect of continuous layer on CoCrPt–SiO 2 granular layer is studied in coupled granular continuous (CGC) perpendicular recording media. In the cross-section transmission electron microscope (TEM) observation, magnetic grain in the granular layer shows columnar structure, while Co/Pd multilayer shows continuous layer. The plane-view TEM image of the granular layer shows well-isolated grain structure with average grain size of around 6 nm, and grain-to-grain separation width of around 2 nm. Therefore, the interactions among the grains are negligible ( J∼0). By depositing a continuous layer on a CoCrPt–SiO 2 granular layer, the grains in the granular layer are magnetically coupled through capping layer that leads to the suppression of magnetic anisotropy dispersion. This CGC structure reduces the coercivity dispersion ( Δ H c / H c ) from 0.26 to 0.15 and saturation field ( H s) from 10.4 to 6.7 kOe. The reduction of H s and Δ H c / H c improves the OW by 21.3 dB. The small Δ H c / H c also maintains SNR of CGC media with strong magnetic exchange coupling. Furthermore, the coupling of grains through continuous layer enlarges the magnetic nucleation field ( H n) from 0.4 to −1.7 kOe. Consequently, CGC media shows better thermal stability compared to non-CGC media.

The effect of oxygen partial pressure and cooling rate during processing on the criticalcurrent density and grain coupling of Bi-2212/Ag tapes

We discuss the reason why high oxygen partial pressure(PO2) heat treatment is effective in increasing the critical current density(Jc)of Bi2Sr2CaCu2Oy (Bi-2212)/Ag tapes prepared by a partial melting and subsequent slow cooling method. The highJc values obtainedby the high PO2 heat treatment is attributed to the strong coupling of grains due to the suppression of theformation of non-superconducting phases at grain boundaries and the decrease ofanisotropy due to the high oxygen content of the Bi-2212.

Effect of hot uniaxial pressing on the microstructure and critical current density of (Bi, Pb)-2223 tapes

Pressures up to 10 MPa were applied uniaxially to (Bi, Pb)2Sr2Ca2Cu3Oy ((Bi, Pb)-2223) tapes during either the first or second stage of the heat treatment process in air. After both annealing stages, an increase of the oxide filament density of the tapes was observed when applying pressure. For the first stage, the critical current (Ic) of the tapes was very sensitive to the applied pressure, and the reproducibility was poor. This behaviour is related to the large amount of liquid involved in the reaction. This liquid was easily pushed out from the centre to the edges of the tapes by pressure, which caused local deviations of the composition and hence, a decrease of volume fraction of (Bi, Pb)-2223. In contrast, applying pressure only during the second stage when a moderate amount of liquid is produced brought about the high densification and the reproducible enhancement of Ic and critical current density (Jc) values by about 40 and 100%, respectively. This enhancement is attributed to the high densification and improved coupling of grains due to the uniaxial pressure.

Exchange coupling and magnetic properties of Nd 2Fe 14B/Co nanocomposite thin films

The structure, exchange coupling interactions and magnetic properties of magnetron sputtered untextured Nd 2Fe 14B/Co thin films with thickness in the range 700–840 nm were studied. Energy products as high as 106 kJ/m 3 (13.3 MG Oe) were obtained for rapidly annealed thin films and are comparable to values obtained in bulk polycrystalline NdFeB alloys containing Co. The exchange coupling interactions between the soft and hard phases were evaluated by recoil demagnetization, irreversible magnetization, and spin-reorientation temperature changes. We find evidence for stronger exchange coupling of grains after a rapid anneal (RA) as compared to a longer time anneal. We suggest that this stronger exchange coupling is due to a uniform nanostructure promoted by the RA.

Multilayer Co/Pd films with nanocrystalline and amorphous Co layers: Coercive force, random anisotropy, and exchange coupling of grains

The magnetic structure of nanocrystalline and amorphous ferromagnets is determined by their random magnetic anisotropy (RMA) and results from a competition of the ordering action of exchange interactions and the disordering effect of the random local anisotropy K breaking the long-range ferromagnetic order [1]. In such materials, the ferromagnetic order is established at a characteristic length R f = R c ( H ex / D 1/2 H a ) 2 , where R c is the random anisotropy correlation radius, H ex is the exchange field, D is a numerical coefficient of the symmetry (equal to 1/15 in the case of a uniaxial anisotropy), and M s is the saturation magnetization. The resulting magnetic structure can be described in terms of the ensemble of weakly coupled magnetic blocks [1, 2] with an average dimension of 2 R f , the average block anisotropy 〈 K 〉 = K / N 1/2 = K ( R c / R f ) 3/2 , and randomly oriented unit anisotropy vectors n . To the present, researchers engaged in the magnetic properties of such materials believed that this type of magnetic structure is just what accounts for the observed values of characteristics of the magnetically soft amorphous and nanocrystalline alloys [2‐4]. It should be noted that an analysis of the behavior of the coercive force H c within the framework of the RMA model used in [2‐4] is directed mostly to the study of H c as a function of the nanograin size, H c ( R c ), thereby assuming that the values of anisotropy K ( R c ), exchange coupling A ( R c ), and saturation magnetization M s ( R c ) are constant (or their changes can be ignored). However, now there are prerequisites for undertaking complex measurements of all the parameters entering into theoretical expressions for H c ( R c , H a , A , and M s ) and revealing the corresponding dependences. In particular, the RMA and magnetic block structure can be studied using small-angle neutron diffraction [5‐7] and the measurements of magnetization curves in the region of magnetizations close to saturation [8‐10]. The exchange coupling constants are widely determined using methods based on the spin- wave resonance (SWR) [11] and Bloch’s T 3/2 law [12]. The results of our preliminary investigation of the magnetization behavior M ( H ) in multilayer Co/Pd film structures [9] showed that these films are characterized by a two-dimensional inhomogeneity of the magnetic anisotropy. In particular, it was found that the experimental curve of M ( H ) measured in the range of field strengths from 5 to 25 kOe can be described by the expression

Study on the heat treatment condition to improve coupling of grains in Bi 2− xPb xSr 2CaCu 2O y/Ag tapes

We initially investigated the stable region of Ca 2PbO 4 impurity phase in Bi 2− x Pb x Sr 2CaCu 2O y ((Bi,Pb)-2212)/Ag tapes, in the oxygen partial pressure ( P O 2 )–temperature ( T) diagram using bulk (Bi,Pb)-2212 samples. With increasing temperature and decreasing P O 2 , the volume fraction of Ca 2PbO 4 decreased and Pb in Ca 2PbO 4 dissolved in Bi-2212 phase. These results indicate that rapid cooling from high temperature under low P O 2 is effective to avoid the segregation of Ca 2PbO 4. Taking this result into account, we prepared (Bi,Pb)-2212/Ag tapes applying rapid cooling. We found that rapid cooling is more effective to improve the coupling of grains and bring about high critical current density ( J c) values than slow cooling in (Bi,Pb)-2212/Ag tapes of x=0.2. This is due to the reduction of the segregation of Ca 2PbO 4 at the grain boundaries of (Bi,Pb)-2212 by the high cooling rate.

種々の酸素分圧下で作製したBi-2212/Agテープの粒界構造

Grain boundary structure of Bi-2212/Ag tapes processed in various oxgen partial pressures (PO2) was investigated by high-resolution transmission electron microscopy (HRTEM). Most frequently observed boundaries for each tape were (001) twist boundaries. Two kinds of twist boundaries were mainly observed. One has an amorphous region at the interface and the other has no amorphous region (clean boundary). The population of clean boundary increased with increasing PO2. The real part of AC susceptibility showed smaller amplitude dependence for the tapes processed in higher PO2, suggesting improved coupling of grains.

高Ｔｃ酸化物超電導線材

Present status of R & D on high-Tc oxide superconducting tape and wire conductors is reviewed. For Bi-based oxide, tape and wire conductors are fabricated by powder methods such as Ag sheath method, dip-coating method and jelly-roll method. Grain orientated microstructure is easily obtained by these methods because of strong two-dimensionality of the Bi-based oxide and coupling of grains is much improved. Transport Jc of -105A/cm2 is attained in high magnetic fields over 20T at-4.2K. Recently, small coils are fabricated using Bi-based oxides and promising results are obtained. However, Jc in fields decreases rapidly with increasing temperatures. This is also due to the strong two-dimensionality. Y-123 and Tl-1223 show less strong two-dimensionality and, hence, more excellent flux pinning characteristics at high temperatures. For these oxides, however, it is difficult to obtain grain oriented microstructure by powder methods. This results in weak coupling of grains and small transport Jc. For Y-123 and Tl-1223, excellent transport Jc can be obtained in films. Biaxially grain oriented Y-123 short tapes prepared by laser ablation technique and Tl-1223 thick films prepared by spray coating method show excellent Jc-B characteristics at 77K.

Numerical simulation of magnetization reversal in hard magnetic materials using a finite element method

The presented numerical approach to coercivity analysis combines a nucleation model of coercivity with the finite element method. The simulation program offers the opportunity to investigate the effect of material properties, the microstructure and the orientation of the applied field on the coercivity of an ensemble of hard magnetic grains. The influence of grain alignment and non-magnetic intergranular phases on the angular dependence of coercivity confirms the coupling of grains by dipole interactions. The stray field interaction between misaligned grains explains the deviation of the angular dependence of the coercive field from its theoretical values. In hard magnetic materials the interplay between microstructure and material properties is responsible for the coercivity. Stray field terms and material constants represent the microstructure and the material properties in the micromagnetic equation. To calculate the magnetic stray field of a given microstructure a finite element program has been developed. An algorithm for automatic mesh refinement adapts the finite element mesh to the given microstructure.

High-field magnetization of YBa 2Cu 3−xM xO 7 (M≠Pb, Sn) at 4.2 K

Magnetization measurements on lead- or tin-doped YBa 2Cu 3O 7 have been performed at 4.2 K in magnetic fields up to 29 T. The critical current densities are deduced from the irreversible magnetization and the pinning forces are evaluated. The effect of both lead and tin doping on the critical current density is quite appreciable. In particular, the critical current density of the sample doped with lead is more than twice that of the undoped sample. This effect is thought to be associated with the enhancement of the intergranular coupling of grains through the formation of BaPbO 3 phase at the interface of the 123 superconducting phase. The relationship between the critical current density and a micrograph of the tin-doped sample is also discussed.

Superconductivity in Bi Pb Ca Sr Cu O thin films

Abstract Thin films of Bi Pb Ca Sr Cu O sample were prepared by RF sputtering from sintered ceramic targets. Single crystal of MgO(100) was selected as substrate. The sputtering was held at room temperature. Different annealing conditions were carried out to obtain optimum conditions. High temperature resistivity was measured in air to study the thermodynamic reaction of the sintered films. An resistivity anomaly was found in the first heating cycle which suggests a thermodynamic reaction. A temperature dependence of Ic was measured to study the coupling of grains in the granular films in different temperature ranges and the results will be discussed.

Intergrain coupling and flux creep in textured Bi-based oxide superconductor

Abstract A.C. susceptibility and D.C. magnetization measurements were carried out for textured Bi(Pb) Sr Ca Cu O tape in order to investigate the coupling nature of grains and flux creep. Coupling of grains and intergrain critical current density were significantly improved by the grain alignment. Activation energy of flux creep was also increased by texturing.

On the Coupling of Grains to the Gas in Circumstellar Envelopes

On the Coupling of Grains to the Gas in Circumstellar Envelopes

Theoretical Description of the Interstellar Medium

As excellent reviews have appeared recently (Spitzer, 1968a; Pikel'ner, 1968a) we shall consider here only some special topics where progress is being made – thermal properties of Hi and Hii clouds, and shock waves. The components of the medium which are dynamically important are neutral atoms, charged ions and electrons, grains, cosmic rays, magnetic field, and light. In Table I we indicate the degree of momentum coupling among these components. The main mass of the medium, in ions and atoms, is well coupled through collisions, with a mean free path λ ≈ 10−3 pc, and may usually be treated hydrodynamically. Parker (1965) has shown that for many purposes the cosmic-rays can be considered as a separate gas, strongly coupled to the magnetic field. The coupling of the atom-ion gas to the magnetic field is through the magnetic forces on the ions, which transmit the forces to the neutral atoms by collisions unless the differential stresses on the two components are unusually large. Grains are not coupled strongly to any other component, as their stopping distances are of the order of parsecs and their gyro-periods of the order of 104 yr. The coupling of grains to radiation pressure can lead to interesting differential effects. Light is important in carrying away thermal energy but does not critically affect the momentum balance of the gas. A possible exception is the Ly-α radiation field discovered by Kurt and Sunyaev (1967) which has a pressure of 10−13 dyne cm−2. If this phenomenon is common in interstellar space, it could have effects.